DOI: 10.1002/chem.201403581
Full Paper
& Molecular Magnetism
Hetero-tri-spin [2p-3d-4f] Chain Compounds Based on Nitronyl Nitroxide Lanthanide Metallo-Ligands: Synthesis, Structure, and Magnetic Properties Mei Zhu,[a] Peng Hu,[a] Yungai Li,[a] Xiufeng Wang,[a] Licun Li,*[a] Daizheng Liao,[a] V. M. L. Durga Prasad Goli,[b] S. Ramasesha,*[b] and Jean-Pascal Sutter*[c, d] Dedicated to Prof. Marius Andruh on the occasion of his 60th birthday
Abstract: Employing nitronyl nitroxide lanthanide(III) complexes as metallo-ligands allowed the efficient and highly selective preparation of three series of unprecedented heterotri-spin (CuLn-radical) one-dimensional compounds. These 2p–3d–4f spin systems, namely [Ln3Cu(hfac)11(NitPhOAll)4] (LnIII = Gd 1Gd, Tb 1Tb, Dy 1Dy ; NitPhOAll = 2-(4’-allyloxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), [Ln3Cu(hfac)11(NitPhOPr)4] (LnIII = Gd 2Gd, Tb 2Tb, Dy 2Dy, Ho 2Ho, Yb 2Yb ; NitPhOPr = 2-(4’-propoxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and [Ln3Cu(hfac)11(NitPhOBz)4] (LnIII = Gd 3Gd, Tb 3Tb, Dy 3Dy ; NitPhOBz = 2-(4’-benzyloxy-
Introduction Molecular nanomagnets have become a prominent research topic in the field of molecular magnetism owing to their unique magnetic properties and their relevance in information storage and quantum computation at a molecular level.[1] Among these, 1D compounds such as the CoII-radical helical chain[2] showing slow magnetization relaxation and behaving as a magnet have led to a new class of compounds called [a] M. Zhu,+ P. Hu,+ Y. Li, X. Wang, Prof. L. Li, Prof. D. Liao Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry, Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Nankai University, Tianjin 300071 (China) E-mail: [email protected] [b] V. M. L. Durga Prasad Goli, Prof. S. Ramasesha Solid State & Structural Chemistry Unit Indian Institute of Science, Bangalore 560012 (India) E-mail: [email protected] [c] Dr. J.-P. Sutter LCC (Laboratoire de Chimie de Coordination), CNRS F-31077 Toulouse (France) E-mail: [email protected] [d] Dr. J.-P. Sutter UPS, INPT, LCC Universit de Toulouse, 31077 Toulouse (France) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403581. Chem. Eur. J. 2014, 20, 13356 – 13365
phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) involve O-bound nitronyl nitroxide radicals as bridging ligands in chain structures with a [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] repeating unit. The dc magnetic studies show that ferromagnetic metal–radical interactions take place in these heterotri-spin chain complexes, these and the next-neighbor interactions have been quantified for the Gd derivatives. Complexes 1Tb and 2Tb exhibit frequency dependence of ac magnetic susceptibilities, indicating single-chain magnet behavior.
single-chain magnets (SCMs).[3] A characteristic feature of a SCM is to exhibit blocking of the magnetization below a critical temperature, which relies on a large Ising anisotropy of the magnetic center, strong intrachain magnetic coupling without spin compensation between the magnetic units, and very weak interchain interaction.[3, 4] An effective strategy for improving the blocking temperature or the energy barrier is to increase the intrachain magnetic interaction and/or to enhance the magnetic anisotropy by using rare-earth ions with stronger spin–orbit coupling. This strategy has led to various bi- and even trimetallic SCMs, such as 3d–4f,[5] 3d–3d’–4f,[6] and 3d– 4d–4f.[7] However, the exchange interaction with Ln ions is always small because the singly occupied f-orbitals are shielded. A well-established approach to reach fair magnetic communication with Ln ions is to involve organic radicals, such as aminoxyl derivatives, acting as ligands. For these, the moiety bearing the unpaired electron directly coordinates to the metal ion leading thus to the strongest possible exchange interaction.[8] For chain compounds, radical ligands can be involved as bridge between the Ln centers, thus yielding enhanced intrachain magnetic coupling between the spin carriers. An illustration of it was given by a series of nitronyl nitroxide–lanthanide 1D compounds featuring different relaxation features.[9] Pursuing these clues and with the aim of exploring new routes for the design of such materials, we focused our study on hetero-tri-spin (2p–3d–4f) systems. To date, only two examples of compounds combining a radical ligand and a 3d and a 4f ion have been reported.[10] Very recently, we have dis-
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Full Paper closed preliminary results on a novel approach towards the preparation of hetero-tri-spin 1D coordination polymers involving the nitronyl nitroxide radical as bridging ligand.[11] In an extension of this work, we have investigated the effect of chemical changes at periphery of the radical molecule on the assembling process as well as on the magnetic features of the resulting chain compounds. A special motivation was the control of the inter-chain interactions by varying the steric demand of the R group of the phenyl ether moiety (Scheme 1). Herein we confirm that the construction strategy consisting in using preformed Ln–nitronyl nitroxide complexes as metallo-ligand is a very efficient and versatile approach toward mixed Cu-radical-Ln compounds.
by sequential addition of the reagents. There are no differences in the assembling process leading to these hetero-tri-spin assemblies, but the one-step procedure is simpler because [Ln(hfac)3(NitPhOR)2] need not be prepared in advance. In all cases, a single product, with a spin carrier ratio of {CuLn3Rad4}, was isolated. All compounds are homologous to the heterotrispin chain characterized with the ethyl substituted radical molecule (R = Et)[11] but for the latter this chain formed concomitantly to another chain with a {CuLnRad2} composition. It is satisfying to see that the selectivity of the hetero-tri-spin assembling process is greatly improved for the series of R groups considered herein. The absence of ligand scrambling and reorganization of the complexes (to cationic and anionic species for instance) can be ascribed to the non-polar medium (heptane) used for these preparations. Moreover, the affinity of Ln ions for oxygen ligands certainly explains the formation of the intermediate [Ln(hfac)3(NitPhOR)2] complexes when the procedure consisting in sequential addition of the reagents is applied (that is, for series 2Ln and 3Ln). Crystal structures [Ln3Cu(hfac)11(NitPhOAll)4], 1Ln (Ln = GdIII, 1Gd ; TbIII, 1Tb ; DyIII, 1Dy)
Scheme 1. Nitronyl nitroxide radical derivatives.
A highly selective assembling process was obtained in all cases and the nature of R we have considered does not modify the assemblage motif. We report on three families of chains of general formula [{Ln(hfac)3}3{Cu(hfac)2}(NitPhOR)4] (Nit stands for nitronyl nitroxide, that is, 4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, and R groups are allyl, propyl, and benzyl, respectively) that all exhibit the same assembling motif but differ by the radical molecules. Each family comprises several Ln derivatives. A total of eleven novel 1D compounds, namely [Ln3Cu(hfac)11(NitPhOAll)4] (LnIII = Gd, 1Gd ; Tb, 1Tb ; Dy, 1Dy and hfac = hexafluoroacetylacetonate), [Ln3Cu(hfac)11(NitPhOPr)4] (LnIII = Gd, 2Gd ; Tb, 2Tb ; Dy, 2Dy ; Ho, 2Ho ; Yb, 2Yb), and [Ln3Cu(hfac)11(NitPhOBz)4] (LnIII = Gd, 3Gd ; Tb, 3Tb ; Dy, 3Dy), have been investigated. Their preparations, crystal structures, and magnetic behavior are described. The modeling of the magnetic behavior undertaken for the Gd derivatives, including for those reported earlier, are also discussed. Finally, we show that complexes 1Tb and 2Tb exhibit clear frequency-dependence of ac magnetic susceptibilities, suggesting that they behave as SCMs.
Results and Discussion Synthesis All of the 1D coordination polymers were formed by reacting the appropriate [Ln(hfac)3(NitPhOR)2] complex with Cu(hfac)2 in a 1:1 ratio in heptane. Two procedures were used. Complexes [Ln3Cu(hfac)11(NitPhOAll)4], 1Ln, were obtained by reacting preformed [Ln(hfac)3(NitPhOAll)2] with the Cu(hfac)2, while for the series 2Ln and 3Ln the Ln metallo-ligand was generated in situ Chem. Eur. J. 2014, 20, 13356 – 13365
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The three compounds are isomorphous and exhibit a chain structure that develops from {Ln(hfac)3} units and {Cu(hfac)2} units bridged by NitPhOAll radicals coordinated by their NO groups. Their molecular structures are shown in Figure 1 and
Figure 1. Crystal structure of complex 1Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOAll)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
the Supporting Information, Figures S1 and S2; selected bond lengths and angles are compiled in Table 1. The three compounds crystallize in triclinic space group P1¯ and their asymmetric unit consists of a [{Cu(hfac)2}{Ln(hfac)3}3(NitPhOAll)4] fragment. The supramolecular 1D organization is made up with a repeating [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] sequence. During the assembling process, one of the Ln complexes formally loses its radical ligands and its coordination sphere is completed by two [Ln(hfac)3(NitPhOAll)2] metallo-ligands. The latter fragment assembles with CuII centers developing the extended coordination polymer. In these three complexes, the CuII ion exhibits an elongated octahedral geometry, where four oxygen atoms from two hfac ligands occupy the equatorial positions of CuII while the two axial positions are occupied by two oxygen atoms arising from
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Full Paper Table 1. Selected bond lengths [] and angles [8] for complexes 1Gd–2Dy.
LnO(rad) Ln-O-N O(rad)-Ln- O(rad)
CuO(rad) Cu-O-N O(rad)-Cu-O(rad)
1Gd
1Tb
1Dy
2Gd
2Tb
2Dy
2.351(3)–2.419(3) 137.3(3)–147.2(3) 137.57(11) 141.16(11) 136.48(12) 2.456(4) 2.493(4) 128.2(3) 133.1(3) 176.60(12)
2.343(4)–2.400(4) 137.1(4)–147.2(4) 137.18(14) 136.14(15) 140.91(15) 2.444(4) 2.491(5) 127.6(4) 133.1(4) 176.55(14)
2.337(3)–2.397(3) 137.5(3)–147.8(3) 137.56(11) 136.31(12) 141.00(12) 2.452(3) 2.489(4) 128.4(3) 132.6(3) 176.40(12)
2.344(3)–2.408(3) 136.1(3)–146.2(3) 137.31(11) 138.28(12) 140.90(12) 2.418(3) 2.523(3) 129.7(3) 131.3(3) 177.92(13)
2.349(4)–2.410(4) 136.4(4)–146.0(4) 138.25(15) 140.54(15) 136.63(15) 2.414(4) 2.524(4) 130.3(4) 131.2(4) 178.14(15)
2.326(3)–2.386(3) 137.1(3)–146.5(3) 136.68(12) 138.20(12) 140.72(13) 2.408(4) 2.527(4) 129.9(3) 131.9(3) 178.12(13)
two radicals. The CuO (hfac) distances are in the range of 1.926(4)–1.941(3) while the CuOapical distances are found between 2.444(4) and 2.493(4) , which indicates that the CuII ions display a Jahn– Teller elongation.[12] The Cu-O-N angles are found in the range of 127.6(4)–133.1(4)8. Each LnIII ion is surrounded by eight oxygen atoms coming from three bidentate hfac ligands and two nitroxide groups. Continuous shape measures have been performed with SHAPE to evaluate the actual shape of the coordination spheres of the Ln centers.[13] The Ln surroundings have been found to exhibit either distorted square pyramidal geometry or distorted dodecahedral geometries (Supporting Information, Table S12). For instance, for derivative 1Gd, Gd1 and Gd3 are located in a distorted dodecahedral environment while Gd2 has distorted square pyramidal surrounding. For compound 1Tb it is Tb3 that exhibits a distorted square pyramidal coordination sphere and the two other Ln centers show distorted dodecahedral geometries. Finally for 1Dy, all Dy exhibit distorted dodecahedral environments. The LnO(radical) distances fall within the range 2.337(3)– 2.419(3) , which are comparable to values that have been observed in other nitronyl nitroxide–Ln(hfac)3 Figure 2. a), c) Packing of the chains in crystal for complex 1Gd ; b) detail of the diamagcomplexes.[9, 14] The average ORad-Ln-ORad angle is netic surrounding of the magnetic array (F atoms are not depicted). Color pictures can be found in Supporting Information, Figure S1. 138.408 for 1Gd, 138.088 for 1Tb, 138.298 for 1Dy, while the ORad-Cu-ORad angle is 176.60(12)8 for 1Gd, for complex 1Gd, 9.787 for complex 1Tb, and 9.782 for com176.55(14)8 for 1Tb, 176.40(12)8 for 1Dy. The Ln-O-N angles are in the range of 137.1(4)–147.8(3)8. The average distances of inplex 1Dy. The nearest interchain Ln···Ln distance is 10.718 for trachain Cu···Ln and Ln···Ln are 8.418 and 8.600 for complex 1Gd, 10.719 for 1Tb, and 10.729 for 1Dy. The closest contacts 1Gd, 8.402 and 8.580 for complex 1Tb, 8.392 and 8.568 for between the chains are established by CH···F contacts (Supporting Information, Tables S1–S3). complex 1Dy. The chain packing diagrams are shown in Figure 2 for 1Gd, and the Supporting Information, Figures S1 and S2 for 1Tb, and 1Dy, respectively. [Ln3Cu(hfac)11(NitPhOPr)4], 2 Ln (LnIII = GdIII, 2Gd ; TbIII, 2Tb ; DyIII, A salient feature of these chains is the position of the mag2Dy ; HoIII, 2Ho ; YbIII, 2Yb) netic centers. These are located at the middle of a cylinder and are wrapped by the ligands and organic moieties, which form These five compounds also crystallize in the triclinic space a diamagnetic shell surrounding the magnetic array (Figgroup P1¯ and exhibit a chain organization very similar to that ure 2 b). These cylinders are further decorated by the allyl of the previous three compounds. Here NitPhOPr radicals groups that point towards the neighboring chains. As a result (Scheme 1) act as bridges between the metal centers (Figure 3; the magnetic centers are well-shielded. The closest interchain Supporting Information, Figures S3–S6). The LnO(radical) M···M distances are provided by Cu···Ln separations of 9.801 bond lengths span from 2.305(3) to 2.410(4) (Table 1 and 2), Chem. Eur. J. 2014, 20, 13356 – 13365
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Full Paper Table 2. Selected bond lengths [] and angles [8] for complexes 2Ho–3Dy.
LnO(rad) Ln-O-N O(rad)-Ln-O(rad)
CuO(rad) Cu-O-N O(rad)-Cu-O(rad)
2Ho
2Yb
3Gd
3Tb
3Dy
2.313(3)–2.376(3) 136.6(3)–146.0(3) 136.29(13) 138.13(13) 140.27(13) 2.412(4) 2.529(4) 130.1(3) 131.6(3) 178.07(14)
2.305(3)–2.364(3) 137.0(3)–146.2(3) 135.98(12) 138.05(11) 139.95(12) 2.424(4) 2.538(4) 131.2(3) 131.7(3) 178.43(14)
2.350(3)–2.418(3) 134.9(3)–143.3(3) 138.39(12) 140.05(13) 136.51(11) 2.431(3) 2.574(4)
2.349(3)–2.407(3) 135.1(2)–143.2(3) 137.65(10) 140.07(12) 138.43(10) 2.433(3) 2.574(3) 131.3(3) 132.7(3) 167.47(11)
2.334(4)–2.404(4) 135.2(3)–143.4(3) 138.56(13) 140.13(15) 137.96(13) 2.438(4) 2.578(4) 132.2(3) 132.7(3) 166.92(13)
131.6(3) 132.7(3) 167.52(13)
Figure 3. Crystal structure for 2Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOPr)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
and compare well with the Ln-O(rad) bond lengths for compounds [Ln3Cu(hfac)11(NitPhOAll)4]. The Ln-O-N(aminoxyl) angles are found between 136.1(3) and 146.5(4)8. Shape analysis of the coordination polyhedron of the Ln ions revealed distorted dodecahedral geometry for all lanthanides (Supporting Information, Table S12). The coordination environment for the Cu ion has elongated octahedral geometry. The CuO(rad) bond distances are in the range of 2.408(4)–2.538(4) , which is slightly longer than the four other CuO bonds, in agreement with Jahn–Teller distortion. The Cu-O-N(rad) angles vary from 129.7(3) to 131.9(3)8. The packing of the chains is depicted in the Supporting Information, Figure S7. The shortest interchain Cu···Ln separations are 10.046 for 2Gd, 10.050 for 2Tb, 10.024 for 2Dy, 10.022 for 2Ho, and 10.007 for 2Yb, which are slightly longer than those found for [Ln3Cu(hfac)11(NitPhOAll)4]. The closest interchain Ln···Ln distances are 10.719 for 2Gd, 10.732 for 2Tb, 10.770 for 2Dy, 10.752 for 2Ho, and 10.771 for 2Yb. In the crystal packing, the chains are connected through weak CH···F hydrogen bonds (Supporting Information, Tables S4–S8). [Ln3Cu(hfac)11(NitPhOBz)4], 3 Ln (Ln = GdIII, 3Gd ; TbIII, 3Tb ; DyIII, 3Dy) Compounds 3Ln exhibit one-dimensional chain structures with a supramolecular organization reminiscent to that found for 1Ln and 2Ln (Figure 4; Supporting Information, Figures S8 and S9), but they crystallize in the monoclinic space group P21/c. Within the repeating [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] sequence, the CuII ion adopts an elongated octahedral surrounding. Geometry analysis by SHAPE revealed that two of the three Ln Chem. Eur. J. 2014, 20, 13356 – 13365
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Figure 4. Crystal structure of complex 3Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOBz)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
ions sit in distorted dodecahedral environments while for one (Gd3, Tb1, Dy3), the coordination sphere exhibits a shape midway between dodecahedral and square antiprism geometry (Supporting Information, Table S12). The LnO(radical) distances are in the range of 2.334(3)– 2.418(3) and the axial CuO(radical) bond lengths are found between 2.431(3) and 2.578(4) . The Cu-O-N(aminoxyl) bond angles lie between 131.3(3) and 132.7(3)8 and the Ln-O-N angles span from 134.9(3) to 143.4(3)8 (Table 2). The average distances of intrachain Cu···Ln and Ln···Ln are 8.352 and 8.55 for complex 3Gd, 8.35 and 8.548 for complex 3Tb, and 8.363 and 8.576 for complex 3Dy. These bond parameters are close to those found for previous two series. The packing of the chains in crystal is shown in the Supporting Information, Figure S10. The closest interchain metal–metal separation is provided by Cu···Ln distance of 10.501 for 3Gd, 10.500 for 3Tb, and 10.484 for 3Dy. The nearest interchain Ln···Ln distance is 10.895, 10.895, and 10.879 for complexes 3Gd, 3Tb and 3Dy, respectively. CH···F hydrogen bonds take place between the chains (Supporting Information, Tables S9–S11). At this stage, it is interesting to compare the interchain metal to metal distances as a function of the R groups. Starting from a Cu to Ln distance of 9.78 for the R = Et (Ln = Gd) chains,[11] this interchain distance increases with introduction on larger R groups to reach 10.50 for the benzyl substituent. However, the Ln···Ln interchain separations are hardly modified with an increase of just about 0.15 from smallest to largest R group. These inter-chain separations are quite large and added to the diamagnetic shell wrapping the magnetic centers the
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Full Paper magnetic chains appear well-shield against inter-chain magnetic interactions.
Magnetic properties The magnetic behaviors for all compounds have been investigated. The molar magnetic susceptibilities, cM, have been recorded both in dc and ac mode, and the field dependences of the magnetizations were measured at 2 K. To avoid torque effects with anisotropic Ln derivatives, the samples have been blocked (see the Experimental Section). Below we briefly describe the results for each family of compounds. The analysis of the magnetic behavior undertaken for the Gd derivatives will be discussed in the ensuing section.
[Ln3Cu(hfac)11(NitPhOAll)4] The magnetic susceptibilities for 1Gd, 1Tb, and 1Dy are plotted as cMT versus T in Figure 5. Relevant values characterizing the
behaviors of these derivatives have been collected in the table included in the figure. These are the experimental value at 300 K (cMT300K), the anticipated value for the non-coupled spin system (cMTtheo), the value reached for 2 K (cMT2K), the value for the peak (if any) exhibited by the curve (cMTmax), and the temperature for this maximum (Tmax). From these data, it can be seen that the experimental values found at 300 K are slightly larger than the anticipated contributions of the non-interacting magnetic centers. For the Gd and Tb derivatives, an increase of cMT is observed from 300 K on; first smoothly, then faster as T is lowered. But divergence of cMT is not observed at lower temperature; instead a maximum is reached at 6 K and below cMT falls steeply. Such a behavior is in agreement with the ferromagnetic interactions anticipated for radical–Cu (axially coordinated)[15] and radical–Ln (heavy lanthanides)[16] interactions; it also reveals the occurrence of antiferromagnetic contributions from interchain or next nearest neighbor interactions. The Dy derivative shows a very gradual increase of cMT when T is lowered, with a broad maximum at 90 K followed by a slight decrease. While such a behavior may look puzzling, it is nothing but the result of a very pronounced variation with T of the paramagnetic contribution of Dy coming from crystal-field effect.[16, 17] Obviously for 1Dy, the paramagnetism of the Dy ions and the ferromagnetic Rad–M interactions contribute concomitantly to cMT from 300 K but with opposite effects. These contributions almost balance each other resulting in the observed experimental behavior. The same phenomenon takes place for 1Tb but for TbIII the paramagnetic contribution varies less with T. It can be noticed that for 1Dy cMT does not either diverge at low T. The field dependences of the magnetization for the three derivatives are also given in Figure 5. The Gd derivative reaches a value of 24.85 mB for a field of 5 tesla, which is in agreement with ferromagnetic interactions among the spin carriers. However, the saturation of the magnetization is not reached, which highlights the existence of antiferromagnetic contributions in this system also. Similar M versus H behaviors are found for 1Tb and 1Dy (with 20.0 mB and 19.9 mB respectively, for 5 tesla); here the magnetic anisotropy of the Tb and Dy ions may contribute to it. The magnetic susceptibility in ac mode has been recorded for 1Tb and 1Dy. The onset of a signal for the out-of-phase susceptibility (cM’’) was found for the Tb derivative below 4 K (frequency 1 kHz), but no maximum was seen. Application of a static magnetic field did not change the observed behavior (Supporting Information, Figure S11). [Ln3Cu(hfac)11(NitPhOPr)4]
Figure 5. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOAll)4] derivatives 1Ln. Some relevant values for this series of compounds are compiled in the table. The best fit of the calculated cMT versus T for 1Gd is shown as a full line. The solid line for the magnetization curves is provided to guide the eye. Chem. Eur. J. 2014, 20, 13356 – 13365
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The magnetic susceptibilities recorded in dc mode for compounds 2Gd, 2Tb, 2Dy, 2Ho, and 2Yb are given Figure 6 as cMT versus T plots. They exhibit a behavior reminiscent of the preceding series of compounds. However, it can be noticed that for the Dy derivative, cMT goes through a minimum at 10 K (with 41.5 c m3 mol1 K) and sharply increases for lower temperatures. This is the signature of a raise of the magnetic moment
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Full Paper weak; the ratio cM’/cM” is about 400 (Supporting Information, Figure S15). However, for 2Tb (Figure 7) a clear frequency dependent out-of-phase signal is observed, in agreement with a slow relaxation process for the magnetization.
Figure 7. Temperature-dependent ac magnetic susceptibility of 2Tb with an oscillation of 3.0 Oe in zero dc field.
Figure 6. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOPr)4] derivatives 2Ln. Some relevant values for this series of compounds are compiled in the table. The best fit of the calculated cMT versus T for 2Gd is shown as a full line. The solid line for the magnetization curves are provided to guide the eye.
of the compound owing to ferromagnetic interactions among the magnetic centers. The Ho and Yb derivatives exhibit a gradual decrease of their cMT values as T is lowered, for 2Ho the decrease is more pronounced below 30 K. The behavior for the Ho and Yb derivatives do not preclude the occurrence of ferromagnetic Ln-radical contributions. For these ions they are weaker[16a] and their effect on cMT are just overwhelmed by the stronger contributions of the crystal field effect as compared to Tb or Dy.[17b] The field dependence of the magnetizations recorded at 2 K for the five derivatives (Figure 6) are in agreement with the behaviors found for the homologous compounds 1Ln, with values of 25.2 mB (2Gd), 18.1 mB (2Tb), 19.1 mB (2Dy), 19.7 mB, (2Ho), and 6.1 mB (2Yb) reached for 5 tesla. The possibility of slow relaxing magnetization for these compounds has been evaluated by ac magnetic susceptibility in the temperature range 2–20 K; the Gd derivative was used to confirm the absence of magnetic ordering. For compounds 2Gd, 2Ho, and 2Yb, no out-of-phase signals (cM’’) are observed, indicating absence of magnetic ordering or magnetic relaxation above 2 K (Supporting Information, Figures S12–S14). For 2Dy only onsets of signals of cM“ are found and these are very Chem. Eur. J. 2014, 20, 13356 – 13365
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The possibility for a spin glass behavior was ruled out by the shift of peak temperature (Tp) of the in-phase signal (cM’), which yielded the spin glass parameter f= (DTp/Tp)/D(log f) = 0.322 (for spin-glass behavior 0.01 < f< 0.08),[18] thus suggesting a slow magnetic relaxation process of molecular origin. The Arrhenius equation, t = t0exp(Dt/kBT), was employed to extract the effective energy barrier for the magnetization reversal, Dt/kB, and the pre-exponential factor, t0. The peak temperatures for the cM“T curves that exhibit a maximum was obtained by the Lorentzian fitting; the derived ln t versus T1 plot was analyzed by Arrhenius equation to afford t0 = 4.02 109 s, which is consistent with observed values of SCMs[3] and Dt/ kB = 21.4 K. Although this latter value is not remarkable, these data suggest that 2Tb is the first example of heterotri-(2p–3d– 4f)-spin SCM. [Ln3Cu(hfac)11(NitPhOBz)4] The cMT = f(T) and M = f(H) behaviors for compounds 3Gd, 3Tb, and 3Dy are plotted in Figure 8 together with pertinent values. They very much follow the behavior described earlier; however, a salient feature is a continuous increase of cMT at low T for the Gd and Tb derivatives. No maximum is observed down to 2 K. The ac susceptibility investigations indicated the absence of signals for cM’’, either with or without an applied external field (Supporting Information, Figure S17). Modeling of the cMT versus T behavior for the Gd derivatives The cMT behavior for the Gd derivatives have been modeled to extract information on the exchange interactions taking place within these species. The direct Cu–radical[15] and Gd–
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Full Paper Equation (1) represents the interaction of each spin with external magnetic field (H) that is applied along z-axis. The last term represents the intermolecular interaction of z nearest-neighbors with interaction strength J ’ in mean field approximation. Positive (negative) zJ ’ represents ferromagnetic (antiferromagnetic) behavior of intermolecular interaction. The mean value of Sz in Equation (1) can be obtained from the following equation: P 0 MS exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ hSZ iMS g i P h Sz i ¼ P P exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ0 hSZ iMS g PP S
MS
S
MS
i
where E0(S,MS) are the eigenvalues of the H0. The eigenvalues are obtained by solving the Hamiltonian in Equation (1) for a system with two formula units. This unit consists of eight magnetic centers of which six have s = 1/2 and two have s = 7/ 2 for the alternating [-Cu-Rad-Gd-Rad-]n chain (Rad stands for the nitronyl nitroxide molecule with R = Et;[11] see below) or five s = 1/2 and three s = 7/2 for derivatives 1Gd–3Gd. We solve for all the eigenstates of these systems, numbering 4096 states for the former and 16 384 states for remaining compounds in all MS sectors. < mi > is the expectation value of Szi and b = 1/(KBT). The magnetization (M) can be obtained from < Sz > and is given by: P PP P 0 ð gi hmi iÞ exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ hSZ iMS g S M i i Figure 8. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOBz)4] derivatives S PP P M¼ 3Ln. Some relevant values for this series of compounds are compiled in the exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ0 hSZ iMS g table. The best fit of the calculated cMT versus T for 3Gd is shown as a full line. The solid line for the magnetization curves are provided to guide the eye.
S
X
0
gi siz zJ hSz i
i
H0 ¼
X i
X
siz
ð1Þ
i
X ! ! ! ! J1i S i S iþ1 þ J2i S i S iþ2 i
where J1i and J2i are the nearest and next-nearest neighbor interactions in Heisenberg Hamiltonian H0. The second term in Chem. Eur. J. 2014, 20, 13356 – 13365
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i
The cMT value can be calculated from the following equation:
radical[14a–c, 16c] interactions are known to be ferromagnetic, but for spin systems such as those reported, next-neighbor interactions (that is, radical–radical, GdGd, CuGd) cannot be ignored, and these are usually antiferromagnetic.[16a, c, 19] Occurrence of such competitive intra-chain interactions is suggested by the magnetization behaviors we have described above. Furthermore, we do not expect strong spin–orbit interactions because the Gd ion has a half-filled 4f shell and the radicals are built only with first-row elements. The g-factors all are expected to be close to 2 and magnetic anisotropy can be neglected. Therefore, we use the following Hamiltonian [Eq. (1)] to model these systems:
H ¼ H0 þ mB H
MS
c ¼ NmB T
@M @H
ð2Þ
We use Equation (2) to calculate cMT. In this calculation, we have taken two unit cells for the [-Cu-Rad-Gd-Rad-]n chain and for remaining compounds the number of sites and site spins per unit cell limit us to take one unit cell. In the thermodynamic limit, long range correlations exist in these compounds. Notwithstanding this, our calculated cMT versus T plots are in good agreement with experiments, indicating that the correlation lengths are small in the temperature range over which we have studied. A first set of information was deduced from the fit to the behavior of the alternating [-Cu-Rad-Gd-Rad-]n chain we have reported recently (Rad stands for the nitronyl nitroxide molecule with R = Et).[11] For this system, the JGdGd interaction is absent, which reduces the number of parameters to be considered. Experimental and calculated behaviors are shown Figure 9, the best-fit parameters are JCuRad = 24.5 cm1, JGdRad = 5.26 cm1, JRadRad = 6.8 cm1, JCuGd = 1.28 cm1, zJ’ = 0.007 cm1, gRad = 2, gGd = 2.09, and gCu = 2.17. These values confirm the anticipated ferromagnetic Cu–radical and Gd–radical interactions. The con-
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Figure 9. Experimental (*) and calculated (c) cMT = f(T) for the [CuGd(NitPhOEt)2]n[11] chain compound.
sideration of antiferromagnetic next-neighbor interaction was essential to accurately reproduce the experimental behavior; the JRadRad value obtained is in agreement with values found in the literature.[16a, c] Related modeling has been performed for the CuGd3(NitPhO-R)4 derivatives 1Gd, 2Gd, and 3Gd, as well as for the homologous R = Et derivative.[11] Comparison of experimental and simulated behaviors are given in Figures 5, 6, 8, and the Supporting Information, Figure S18, and best fit parameters are compiled in Table 3. Very close exchange parameters were found for the four derivatives and these values compare well to those obtained for the related [CuGdRad2] derivative. It can be noticed that slightly larger Ln–radical interactions are found for 2Gd and 3Gd. These belong to the series for which the related Tb derivatives reach large cMT values at 2 K, thus confirming the stronger contribution from the ferromagnetic components. Finally, in all cases the intermolecular interactions, zJ’, are very small which suggests that the magnetic chains are well-isolated.
Table 3. Best fit parameters for the [CuGd3(radical-R)4] derivatives.
JRad-Cu[a] JRad-Gd[a] JRad-Rad[a] JGd-Gd[a] zJ’[a] gCu gGd gRad
The results presented in this report illustrate the possibility offered by the novel construction strategy that consists in using preformed LnNit complexes as metallo-ligands. Following this approach, a series of unprecedented hetero-tri-(2p-3d-4f)-spin chain compounds have been obtained with high selectivity and good yields. The alteration at the periphery of the phenylNit core did not modify the assembling process, but this substituent was found to have an effect on the magnetic properties. Especially the occurrence of slow relaxing magnetization for the systems involving anisotropic Ln ions was found to depend on the R substituent. This could be related to the actual shape of the coordination polyhedron for the Ln centers, which were found to be either distorted square antiprisms or dodecahedrons, one or both geometries occurring in the chains as a function of the radical substituent and the nature of the Ln ion. While the compounds described herein can be considered as a proof-of-concept, the chemical versatility of the molecular building units opens many opportunities to design novel hetero-tri-spin materials.
Experimental Section Materials and physical measurements All of the reagents and chemicals were purchased from commercial sources and used as received. The radical ligands NitPhOAll, NitPhOBz, and NitPhOPr were prepared by previously reported methods.[20] Mononuclear complexes [Ln(hfac)3(NitPhOAll)2] (Ln = Gd, Tb, Dy) were synthesized according to the literature.[21] Elemental analysis for C, H, and N were carried out on a PerkinElmer elemental analyzer model 240. Magnetic measurements were carried out with a Quantum design MPMS 5S SQUID magnetometer in the temperature domain 2–300 K. The measurements were performed on microcrystalline samples mixed to grease and put in gelatin capsules. The magnetic susceptibilities were measured in an applied field of 1000 Oe. The molar susceptibility (cM) was corrected for sample holder and for the diamagnetic contribution of all the atoms by using Pascal’s tables. The ac susceptibility was measured with a MPMS 5S SQUID magnetometer with an oscillating ac field of 3 Oe and frequency between 10 to 1500 Hz. For complexes [Ln3Cu(hfac)11(NitPhOPr)4] 2 Ln (Ln = Gd, Tb, Dy, Ho, Yb), ac susceptibilities were carried out on a Quantum Design PPMS-9.
R = Et[11]
R = All
R = Prop
R = Bz
Synthesis
25.0 6.05 6.0 0.51 0.009 2.1 2.0 2.01
25.0 6.0 6.0 0.25 0.045 2.15 2.05 2.0
25.0 7.0 5.0 0.35 0.032 2.15 2.04 2.0
25.0 7.0 6.0 0.9 0.057 2.1 2.06 2.0
[Gd3Cu(hfac)11(NitPhOAll)4] 1Gd : A solution of Cu(hfac)2 (0.008 g, 0.02 mmol) in dry boiling heptane (20 mL) was heated to 90 8C and a dry CH2Cl2 solution (5 mL) of [Gd(hfac)3(NITPhOAll)2] (0.027 g, 0.02 mmol) was added. The resulting mixture was stirred for 15 min at this temperature, then cooled to room temperature and filtered off. The filtrate was kept at room temperature for two days and green crystals of 1Gd were obtained. Yield: 51 %. Elemental analysis calcd (%) for C119H95CuF66Gd3N8O34 : C 36.00, H 2.41, N 2.82; found: C 36.48, H 2.75, N 2.98. IR (KBr): n = 1653 s, 1504 m, 1257 s, 1211 s, 1147 s, 800 m cm1. [Tb3Cu(hfac)11(NitPhOAll)4] 1Tb : As for 1Gd but [Tb(hfac)3(NitPhOAll)2] was used in place of [Gd(hfac)3(NitPhOAll)2]. Yield: 55 %. Elemental analysis calcd (%)for C119H95CuF66Tb3N8O34 :
[a] In cm1; positive (negative) values represents ferro-(antiferro-)magnetic interactions.
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Full Paper C 35.95, H 2.41, N 2.82; found: C 35.67, H 2.94, N 3.01. IR (KBr): n = 1653 s, 1504 m, 1257 s, 1210 s, 1147 s, 800 m, 662 m cm1. [Dy3Cu(hfac)11(NitPhOAll)4] 1Dy : As for 1Gd but [Dy(hfac)3(NitPhOAll)2] was used in place of [Gd(hfac)3(NitPhOAll)2]. Yield: 61 %. Elemental analysis calcd (%) for C119H95CuF66Dy3N8O34 : C 35.86, H 2.40, N 2.81; found: C 35.53, H 2.63, N 2.91. IR (KBr): n = 1654 s, 1505 m, 1257 s, 1210 s, 1147 s, 800 m, 662 m cm1. [Gd3Cu(hfac)11(NitPhOPr)4] 2Gd : A solution of Gd(hfac)3·2 H2O (0.016 g, 0.02 mmol) in dry boiling heptane (20 mL) was heated to reflux for 3 h and then cooled to 90 8C, 3 mL of a dichloromethane solution of NitPhOPr (0.012 g, 0.04 mmol) was slowly added with stirring about 10 min. Then the solid of Cu(hfac)2 (0.008 g, 0.02 mmol) was added. The resulting green solution was stirred for another 10 min at this temperature, then cooled to room temperature and filtered. After two days, green elongated crystals suitable for single crystal diffraction were obtained. Yield 47 %. Elemental analysis calcd (%)for C119H103N8O34F66CuGd3 : C 35.93, H 2.61, N 2.82; found: C 36.35, H 2.87, N 3.10. IR (KBr): n = 3442 m, 1652 s, 1501 m, 1342 m, 1256 s, 1211 s, 1148 s, 799 w, 662 w cm1. [Tb3Cu(hfac)11(NitPhOPr)4] 2Tb : This complex was synthesized using same procedure as for 2Gd. Yield 51 %. Elemental analysis calcd (%) for C119H103N8O34F66CuTb3 : C 35.88, H 2.81, N 2.84; found: C 35.76, H 2.55, N 3.02. IR (KBr): n = 3447 m, 1652 s, 1500 m, 1344 m, 1256 s, 1210 s, 1148 s, 780 w, 661 w cm1. [Dy3Cu(hfac)11(NitPhOPr)4] 2Dy : This complex was synthesized using same procedure as for 2Gd. Yield 44 %. Elemental analysis calcd (%) for C119H103N8O34F66CuDy3 : C 35.78, H 2.60, N 2.80; found: C 36.01, H 2.79, N 3.03. IR (KBr): n = 3443 m, 1652 s, 1502 m, 1342 w, 1256 s, 1211 s, 1147 s, 780 w, 660 w, 584 w cm1. [Ho3Cu(hfac)11(NitPhOPr)4] 2Ho : This complex was synthesized using same procedure as for 2Gd. Yield 52 %. Elemental analysis calcd (%) for C119H103N8O34F66CuHo3 : C 35.72, H 2.59, N 2.80; found: C 35.83, H 3.51, N 3.12. IR (KBr): n = 3443 m, 1654 s, 1607 w, 1506 m), 1342 w, 1256 s, 1209 s, 1147 s, 799 w, 662 w, 587 w cm1. [Yb3Cu(hfac)11(NitPhOPr)4] 2Yb : This complex was synthesized using same procedure as for 2Gd. Yield 52 %. Elemental analysis calcd (%) for C119H103N8O34F66CuYb3 : C 35.50, H 2.58, N 2.78; found: C 35.72, H 3.11, N 3.12. IR (KBr): n = 3445 m, 1654 s, 1604 w, 1503 m, 13 423 w, 1255 s, 1203 s, 1145 s, 799 w, 660 w, 586 w cm1. [Gd3Cu(hfac)11(NitPhOBz)4] 3Gd : This complex was prepared by same procedure as for 2Gd but NitPhOBz was used. Yield: 53 %. Elemental analysis calcd (%) for C135H103N8O34F66CuGd3 : C 38.88, H 2.49, N 2.69; found: C 38.35, H 2.57, N 2.60. IR (KBr): n = 1654 s, 1505 s, 1148 s, 1343 m, 1256 m, 1210 m, 1211 m, 800 m, 662 m, 586 m cm1. [Tb3Cu(hfac)11(NitPhOBz)4] 3Tb : The complex was prepared by same procedure as for 3Gd. Yield: 56 %. Elemental analysis calcd (%) for C135H103N8O34F66CuTb3 : C 38.83, H 2.49, N 2.68; found: C 38.56, H 2.55, N 2.52. IR (KBr): n = 1654 s, 1506 s, 1257 s, 1210 s, 1148 s, 800 m, 662 m, 586 m cm1. [Dy3Cu(hfac)11(NitPhOBz)4] 3Dy : The complex was prepared by same procedure as for 3Gd. Yield: 50 %. Elemental analysis calcd (%) for C135H103N8O34F66CuDy3 : C 38.73, H 2.48, N 2.68; found: C 39.01, H 2.59, N 2.73. IR (KBr): n = 1652 s, 1502 s, 1257 s, 1210 s, 1147 s, 799 m, 661 m, 585 m cm1.
X-ray crystallography Diffraction data for eleven complexes were recorded on a Rigaku Saturn CCD diffractometer with graphite-monochromated Mo Ka radiation (l = 0.71073 ) at 113 K. In each case, absorption corrections were applied. The structure was solved by direct methods Chem. Eur. J. 2014, 20, 13356 – 13365
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using the SHELXS-97 program[22] and all non-hydrogen atoms were refined anisotropically by least-squares methods on F 2 using the SHELXL program.[23] Hydrogen atoms were added theoretically and refined isotropically using a riding mode. Disorders were observed in these complexes for some fluorine atoms. Detailed data collection and refinement of these eleven compounds are summarized in Table S13 and S14. Selected bond lengths and angles are listed in Table 1 and 2. CCDC-1000884 (1Dy), CCDC-1000885 (1Gd),CCDC1000886 (1Tb), CCDC-1000887 (3Dy), CCDC-1000888 (3Gd), CCDC1000889 (3Tb), CCDC-1000890 (2Dy), CCDC-1000891 (2Gd), CCDC1000892 (2Ho), CCDC-1000893 (2Tb), and CCDC-1000894 (2Yb) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 91122013, 90922032) and MOE Innovation Team of China (IRT13022). S.R. and J.P.S. are grateful to CEFIPRA/IFCPAR (Indo-French Center for the Promotion of Advanced Research) for support. S.R. is also thankful to DST(India) for support through various grants. J.P.S. is grateful to J.F. Meunier and L. Rchignat (LCC) for assistance in magnetic data collection. Keywords: chain compounds · lanthanides · molecular magnetism · nitronyl nitroxide · radicals [1] a) D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, Oxford, 2006; b) M. N. Leuenberger, D. Loss, Nature 2001, 410, 789 – 793. [2] A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M. G. Pini, M. A. Novak, Angew. Chem. 2001, 113, 1810 – 1813; Angew. Chem. Int. Ed. 2001, 40, 1760 – 1763. [3] a) L. Bogani, A. Vindigni, R. Sessoli, D. Gatteschi, J. Mater. Chem. 2008, 18, 4750 – 4758; b) R. Clrac, H. Miyasaka, M. Yamashita, C. Coulon, J. Am. Chem. Soc. 2002, 124, 12837 – 12844; c) C. Coulon, H. Miyasaka, R. Clrac in Single-Molecule Magnets and Related Phenomena, Vol. 122 (Ed.: R. Winpenny), Springer, Berlin, 2006, pp. 163 – 206; d) H. L. Sun, Z. M. Wang, S. Gao, Coord. Chem. Rev. 2010, 254, 1081 – 1100. [4] a) H. Miyasaka, M. Julve, M. Yamashita, R. Clrac, Inorg. Chem. 2009, 48, 3420 – 3437; b) T. S. Venkatakrishnan, S. Sahoo, N. Brfuel, C. Duhayon, C. Paulsen, A.-L. Barra, S. Ramasesha, J.-P. Sutter, J. Am. Chem. Soc. 2010, 132, 6047 – 6056; c) W. X. Zhang, R. Ishikawa, B. Breedlove, M. Yamashita, RSC Adv. 2013, 3, 3772 – 3798. [5] a) J. P. Costes, J. M. Clemente-Juan, F. Dahan, J. Milon, Inorg. Chem. 2004, 43, 8200 – 8202; b) Y.-G. Huang, X.-T. Wang, F.-L. Jiang, S. Gao, M.Y. Wu, Q. Gao, W. Wei, M.-C. Hong, Chem. Eur. J. 2008, 14, 10340 – 10347. [6] a) R. Gheorghe, A. M. Madalan, J. P. Costes, W. Wernsdorfer, M. Andruh, Dalton Trans. 2010, 39, 4734 – 4736; b) M.-X. Yao, Q. Zheng, K. Qian, Y. Song, S. Gao, J.-L. Zuo, Chem. Eur. J. 2013, 19, 294 – 303. [7] D. Visinescu, A. M. Madalan, M. Andruh, C. Duhayon, J.-P. Sutter, L. Ungur, W. Van den Heuvel, L. F. Chibotaru, Chem. Eur. J. 2009, 15, 11808 – 11814. [8] a) C. Benelli, D. Gatteschi, Chem. Rev. 2002, 102, 2369 – 2387; b) A. Caneschi, D. Gatteschi, P. Rey, Prog. Inorg. Chem. 1991, 39, 331 – 429; c) A. Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 1989, 22, 392 – 398. [9] a) K. Bernot, L. Bogani, A. Caneschi, D. Gatteschi, R. Sessoli, J. Am. Chem. Soc. 2006, 128, 7947 – 7956; b) L. Bogani, C. Sangregorio, R. Sessoli, D. Gatteschi, Angew. Chem. 2005, 117, 5967 – 5971; Angew. Chem. Int. Ed. 2005, 44, 5817 – 5821; c) R. Liu, Y. Ma, P. Yang, X. Song, G. Xu, J. Tang, L.
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Received: May 19, 2014 Published online on August 28, 2014
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& Molecular Magnetism
Hetero-tri-spin [2p-3d-4f] Chain Compounds Based on Nitronyl Nitroxide Lanthanide Metallo-Ligands: Synthesis, Structure, and Magnetic Properties Mei Zhu,[a] Peng Hu,[a] Yungai Li,[a] Xiufeng Wang,[a] Licun Li,*[a] Daizheng Liao,[a] V. M. L. Durga Prasad Goli,[b] S. Ramasesha,*[b] and Jean-Pascal Sutter*[c, d] Dedicated to Prof. Marius Andruh on the occasion of his 60th birthday
Abstract: Employing nitronyl nitroxide lanthanide(III) complexes as metallo-ligands allowed the efficient and highly selective preparation of three series of unprecedented heterotri-spin (CuLn-radical) one-dimensional compounds. These 2p–3d–4f spin systems, namely [Ln3Cu(hfac)11(NitPhOAll)4] (LnIII = Gd 1Gd, Tb 1Tb, Dy 1Dy ; NitPhOAll = 2-(4’-allyloxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), [Ln3Cu(hfac)11(NitPhOPr)4] (LnIII = Gd 2Gd, Tb 2Tb, Dy 2Dy, Ho 2Ho, Yb 2Yb ; NitPhOPr = 2-(4’-propoxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and [Ln3Cu(hfac)11(NitPhOBz)4] (LnIII = Gd 3Gd, Tb 3Tb, Dy 3Dy ; NitPhOBz = 2-(4’-benzyloxy-
Introduction Molecular nanomagnets have become a prominent research topic in the field of molecular magnetism owing to their unique magnetic properties and their relevance in information storage and quantum computation at a molecular level.[1] Among these, 1D compounds such as the CoII-radical helical chain[2] showing slow magnetization relaxation and behaving as a magnet have led to a new class of compounds called [a] M. Zhu,+ P. Hu,+ Y. Li, X. Wang, Prof. L. Li, Prof. D. Liao Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry, Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Nankai University, Tianjin 300071 (China) E-mail: [email protected] [b] V. M. L. Durga Prasad Goli, Prof. S. Ramasesha Solid State & Structural Chemistry Unit Indian Institute of Science, Bangalore 560012 (India) E-mail: [email protected] [c] Dr. J.-P. Sutter LCC (Laboratoire de Chimie de Coordination), CNRS F-31077 Toulouse (France) E-mail: [email protected] [d] Dr. J.-P. Sutter UPS, INPT, LCC Universit de Toulouse, 31077 Toulouse (France) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403581. Chem. Eur. J. 2014, 20, 13356 – 13365
phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) involve O-bound nitronyl nitroxide radicals as bridging ligands in chain structures with a [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] repeating unit. The dc magnetic studies show that ferromagnetic metal–radical interactions take place in these heterotri-spin chain complexes, these and the next-neighbor interactions have been quantified for the Gd derivatives. Complexes 1Tb and 2Tb exhibit frequency dependence of ac magnetic susceptibilities, indicating single-chain magnet behavior.
single-chain magnets (SCMs).[3] A characteristic feature of a SCM is to exhibit blocking of the magnetization below a critical temperature, which relies on a large Ising anisotropy of the magnetic center, strong intrachain magnetic coupling without spin compensation between the magnetic units, and very weak interchain interaction.[3, 4] An effective strategy for improving the blocking temperature or the energy barrier is to increase the intrachain magnetic interaction and/or to enhance the magnetic anisotropy by using rare-earth ions with stronger spin–orbit coupling. This strategy has led to various bi- and even trimetallic SCMs, such as 3d–4f,[5] 3d–3d’–4f,[6] and 3d– 4d–4f.[7] However, the exchange interaction with Ln ions is always small because the singly occupied f-orbitals are shielded. A well-established approach to reach fair magnetic communication with Ln ions is to involve organic radicals, such as aminoxyl derivatives, acting as ligands. For these, the moiety bearing the unpaired electron directly coordinates to the metal ion leading thus to the strongest possible exchange interaction.[8] For chain compounds, radical ligands can be involved as bridge between the Ln centers, thus yielding enhanced intrachain magnetic coupling between the spin carriers. An illustration of it was given by a series of nitronyl nitroxide–lanthanide 1D compounds featuring different relaxation features.[9] Pursuing these clues and with the aim of exploring new routes for the design of such materials, we focused our study on hetero-tri-spin (2p–3d–4f) systems. To date, only two examples of compounds combining a radical ligand and a 3d and a 4f ion have been reported.[10] Very recently, we have dis-
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Full Paper closed preliminary results on a novel approach towards the preparation of hetero-tri-spin 1D coordination polymers involving the nitronyl nitroxide radical as bridging ligand.[11] In an extension of this work, we have investigated the effect of chemical changes at periphery of the radical molecule on the assembling process as well as on the magnetic features of the resulting chain compounds. A special motivation was the control of the inter-chain interactions by varying the steric demand of the R group of the phenyl ether moiety (Scheme 1). Herein we confirm that the construction strategy consisting in using preformed Ln–nitronyl nitroxide complexes as metallo-ligand is a very efficient and versatile approach toward mixed Cu-radical-Ln compounds.
by sequential addition of the reagents. There are no differences in the assembling process leading to these hetero-tri-spin assemblies, but the one-step procedure is simpler because [Ln(hfac)3(NitPhOR)2] need not be prepared in advance. In all cases, a single product, with a spin carrier ratio of {CuLn3Rad4}, was isolated. All compounds are homologous to the heterotrispin chain characterized with the ethyl substituted radical molecule (R = Et)[11] but for the latter this chain formed concomitantly to another chain with a {CuLnRad2} composition. It is satisfying to see that the selectivity of the hetero-tri-spin assembling process is greatly improved for the series of R groups considered herein. The absence of ligand scrambling and reorganization of the complexes (to cationic and anionic species for instance) can be ascribed to the non-polar medium (heptane) used for these preparations. Moreover, the affinity of Ln ions for oxygen ligands certainly explains the formation of the intermediate [Ln(hfac)3(NitPhOR)2] complexes when the procedure consisting in sequential addition of the reagents is applied (that is, for series 2Ln and 3Ln). Crystal structures [Ln3Cu(hfac)11(NitPhOAll)4], 1Ln (Ln = GdIII, 1Gd ; TbIII, 1Tb ; DyIII, 1Dy)
Scheme 1. Nitronyl nitroxide radical derivatives.
A highly selective assembling process was obtained in all cases and the nature of R we have considered does not modify the assemblage motif. We report on three families of chains of general formula [{Ln(hfac)3}3{Cu(hfac)2}(NitPhOR)4] (Nit stands for nitronyl nitroxide, that is, 4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, and R groups are allyl, propyl, and benzyl, respectively) that all exhibit the same assembling motif but differ by the radical molecules. Each family comprises several Ln derivatives. A total of eleven novel 1D compounds, namely [Ln3Cu(hfac)11(NitPhOAll)4] (LnIII = Gd, 1Gd ; Tb, 1Tb ; Dy, 1Dy and hfac = hexafluoroacetylacetonate), [Ln3Cu(hfac)11(NitPhOPr)4] (LnIII = Gd, 2Gd ; Tb, 2Tb ; Dy, 2Dy ; Ho, 2Ho ; Yb, 2Yb), and [Ln3Cu(hfac)11(NitPhOBz)4] (LnIII = Gd, 3Gd ; Tb, 3Tb ; Dy, 3Dy), have been investigated. Their preparations, crystal structures, and magnetic behavior are described. The modeling of the magnetic behavior undertaken for the Gd derivatives, including for those reported earlier, are also discussed. Finally, we show that complexes 1Tb and 2Tb exhibit clear frequency-dependence of ac magnetic susceptibilities, suggesting that they behave as SCMs.
Results and Discussion Synthesis All of the 1D coordination polymers were formed by reacting the appropriate [Ln(hfac)3(NitPhOR)2] complex with Cu(hfac)2 in a 1:1 ratio in heptane. Two procedures were used. Complexes [Ln3Cu(hfac)11(NitPhOAll)4], 1Ln, were obtained by reacting preformed [Ln(hfac)3(NitPhOAll)2] with the Cu(hfac)2, while for the series 2Ln and 3Ln the Ln metallo-ligand was generated in situ Chem. Eur. J. 2014, 20, 13356 – 13365
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The three compounds are isomorphous and exhibit a chain structure that develops from {Ln(hfac)3} units and {Cu(hfac)2} units bridged by NitPhOAll radicals coordinated by their NO groups. Their molecular structures are shown in Figure 1 and
Figure 1. Crystal structure of complex 1Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOAll)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
the Supporting Information, Figures S1 and S2; selected bond lengths and angles are compiled in Table 1. The three compounds crystallize in triclinic space group P1¯ and their asymmetric unit consists of a [{Cu(hfac)2}{Ln(hfac)3}3(NitPhOAll)4] fragment. The supramolecular 1D organization is made up with a repeating [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] sequence. During the assembling process, one of the Ln complexes formally loses its radical ligands and its coordination sphere is completed by two [Ln(hfac)3(NitPhOAll)2] metallo-ligands. The latter fragment assembles with CuII centers developing the extended coordination polymer. In these three complexes, the CuII ion exhibits an elongated octahedral geometry, where four oxygen atoms from two hfac ligands occupy the equatorial positions of CuII while the two axial positions are occupied by two oxygen atoms arising from
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Full Paper Table 1. Selected bond lengths [] and angles [8] for complexes 1Gd–2Dy.
LnO(rad) Ln-O-N O(rad)-Ln- O(rad)
CuO(rad) Cu-O-N O(rad)-Cu-O(rad)
1Gd
1Tb
1Dy
2Gd
2Tb
2Dy
2.351(3)–2.419(3) 137.3(3)–147.2(3) 137.57(11) 141.16(11) 136.48(12) 2.456(4) 2.493(4) 128.2(3) 133.1(3) 176.60(12)
2.343(4)–2.400(4) 137.1(4)–147.2(4) 137.18(14) 136.14(15) 140.91(15) 2.444(4) 2.491(5) 127.6(4) 133.1(4) 176.55(14)
2.337(3)–2.397(3) 137.5(3)–147.8(3) 137.56(11) 136.31(12) 141.00(12) 2.452(3) 2.489(4) 128.4(3) 132.6(3) 176.40(12)
2.344(3)–2.408(3) 136.1(3)–146.2(3) 137.31(11) 138.28(12) 140.90(12) 2.418(3) 2.523(3) 129.7(3) 131.3(3) 177.92(13)
2.349(4)–2.410(4) 136.4(4)–146.0(4) 138.25(15) 140.54(15) 136.63(15) 2.414(4) 2.524(4) 130.3(4) 131.2(4) 178.14(15)
2.326(3)–2.386(3) 137.1(3)–146.5(3) 136.68(12) 138.20(12) 140.72(13) 2.408(4) 2.527(4) 129.9(3) 131.9(3) 178.12(13)
two radicals. The CuO (hfac) distances are in the range of 1.926(4)–1.941(3) while the CuOapical distances are found between 2.444(4) and 2.493(4) , which indicates that the CuII ions display a Jahn– Teller elongation.[12] The Cu-O-N angles are found in the range of 127.6(4)–133.1(4)8. Each LnIII ion is surrounded by eight oxygen atoms coming from three bidentate hfac ligands and two nitroxide groups. Continuous shape measures have been performed with SHAPE to evaluate the actual shape of the coordination spheres of the Ln centers.[13] The Ln surroundings have been found to exhibit either distorted square pyramidal geometry or distorted dodecahedral geometries (Supporting Information, Table S12). For instance, for derivative 1Gd, Gd1 and Gd3 are located in a distorted dodecahedral environment while Gd2 has distorted square pyramidal surrounding. For compound 1Tb it is Tb3 that exhibits a distorted square pyramidal coordination sphere and the two other Ln centers show distorted dodecahedral geometries. Finally for 1Dy, all Dy exhibit distorted dodecahedral environments. The LnO(radical) distances fall within the range 2.337(3)– 2.419(3) , which are comparable to values that have been observed in other nitronyl nitroxide–Ln(hfac)3 Figure 2. a), c) Packing of the chains in crystal for complex 1Gd ; b) detail of the diamagcomplexes.[9, 14] The average ORad-Ln-ORad angle is netic surrounding of the magnetic array (F atoms are not depicted). Color pictures can be found in Supporting Information, Figure S1. 138.408 for 1Gd, 138.088 for 1Tb, 138.298 for 1Dy, while the ORad-Cu-ORad angle is 176.60(12)8 for 1Gd, for complex 1Gd, 9.787 for complex 1Tb, and 9.782 for com176.55(14)8 for 1Tb, 176.40(12)8 for 1Dy. The Ln-O-N angles are in the range of 137.1(4)–147.8(3)8. The average distances of inplex 1Dy. The nearest interchain Ln···Ln distance is 10.718 for trachain Cu···Ln and Ln···Ln are 8.418 and 8.600 for complex 1Gd, 10.719 for 1Tb, and 10.729 for 1Dy. The closest contacts 1Gd, 8.402 and 8.580 for complex 1Tb, 8.392 and 8.568 for between the chains are established by CH···F contacts (Supporting Information, Tables S1–S3). complex 1Dy. The chain packing diagrams are shown in Figure 2 for 1Gd, and the Supporting Information, Figures S1 and S2 for 1Tb, and 1Dy, respectively. [Ln3Cu(hfac)11(NitPhOPr)4], 2 Ln (LnIII = GdIII, 2Gd ; TbIII, 2Tb ; DyIII, A salient feature of these chains is the position of the mag2Dy ; HoIII, 2Ho ; YbIII, 2Yb) netic centers. These are located at the middle of a cylinder and are wrapped by the ligands and organic moieties, which form These five compounds also crystallize in the triclinic space a diamagnetic shell surrounding the magnetic array (Figgroup P1¯ and exhibit a chain organization very similar to that ure 2 b). These cylinders are further decorated by the allyl of the previous three compounds. Here NitPhOPr radicals groups that point towards the neighboring chains. As a result (Scheme 1) act as bridges between the metal centers (Figure 3; the magnetic centers are well-shielded. The closest interchain Supporting Information, Figures S3–S6). The LnO(radical) M···M distances are provided by Cu···Ln separations of 9.801 bond lengths span from 2.305(3) to 2.410(4) (Table 1 and 2), Chem. Eur. J. 2014, 20, 13356 – 13365
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Full Paper Table 2. Selected bond lengths [] and angles [8] for complexes 2Ho–3Dy.
LnO(rad) Ln-O-N O(rad)-Ln-O(rad)
CuO(rad) Cu-O-N O(rad)-Cu-O(rad)
2Ho
2Yb
3Gd
3Tb
3Dy
2.313(3)–2.376(3) 136.6(3)–146.0(3) 136.29(13) 138.13(13) 140.27(13) 2.412(4) 2.529(4) 130.1(3) 131.6(3) 178.07(14)
2.305(3)–2.364(3) 137.0(3)–146.2(3) 135.98(12) 138.05(11) 139.95(12) 2.424(4) 2.538(4) 131.2(3) 131.7(3) 178.43(14)
2.350(3)–2.418(3) 134.9(3)–143.3(3) 138.39(12) 140.05(13) 136.51(11) 2.431(3) 2.574(4)
2.349(3)–2.407(3) 135.1(2)–143.2(3) 137.65(10) 140.07(12) 138.43(10) 2.433(3) 2.574(3) 131.3(3) 132.7(3) 167.47(11)
2.334(4)–2.404(4) 135.2(3)–143.4(3) 138.56(13) 140.13(15) 137.96(13) 2.438(4) 2.578(4) 132.2(3) 132.7(3) 166.92(13)
131.6(3) 132.7(3) 167.52(13)
Figure 3. Crystal structure for 2Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOPr)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
and compare well with the Ln-O(rad) bond lengths for compounds [Ln3Cu(hfac)11(NitPhOAll)4]. The Ln-O-N(aminoxyl) angles are found between 136.1(3) and 146.5(4)8. Shape analysis of the coordination polyhedron of the Ln ions revealed distorted dodecahedral geometry for all lanthanides (Supporting Information, Table S12). The coordination environment for the Cu ion has elongated octahedral geometry. The CuO(rad) bond distances are in the range of 2.408(4)–2.538(4) , which is slightly longer than the four other CuO bonds, in agreement with Jahn–Teller distortion. The Cu-O-N(rad) angles vary from 129.7(3) to 131.9(3)8. The packing of the chains is depicted in the Supporting Information, Figure S7. The shortest interchain Cu···Ln separations are 10.046 for 2Gd, 10.050 for 2Tb, 10.024 for 2Dy, 10.022 for 2Ho, and 10.007 for 2Yb, which are slightly longer than those found for [Ln3Cu(hfac)11(NitPhOAll)4]. The closest interchain Ln···Ln distances are 10.719 for 2Gd, 10.732 for 2Tb, 10.770 for 2Dy, 10.752 for 2Ho, and 10.771 for 2Yb. In the crystal packing, the chains are connected through weak CH···F hydrogen bonds (Supporting Information, Tables S4–S8). [Ln3Cu(hfac)11(NitPhOBz)4], 3 Ln (Ln = GdIII, 3Gd ; TbIII, 3Tb ; DyIII, 3Dy) Compounds 3Ln exhibit one-dimensional chain structures with a supramolecular organization reminiscent to that found for 1Ln and 2Ln (Figure 4; Supporting Information, Figures S8 and S9), but they crystallize in the monoclinic space group P21/c. Within the repeating [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] sequence, the CuII ion adopts an elongated octahedral surrounding. Geometry analysis by SHAPE revealed that two of the three Ln Chem. Eur. J. 2014, 20, 13356 – 13365
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Figure 4. Crystal structure of complex 3Gd ; one [{Cu(hfac)2}{Gd(hfac)3}3(NitPhOBz)4] repeating unit is depicted. All of the hydrogen and fluorine atoms are omitted for clarity.
ions sit in distorted dodecahedral environments while for one (Gd3, Tb1, Dy3), the coordination sphere exhibits a shape midway between dodecahedral and square antiprism geometry (Supporting Information, Table S12). The LnO(radical) distances are in the range of 2.334(3)– 2.418(3) and the axial CuO(radical) bond lengths are found between 2.431(3) and 2.578(4) . The Cu-O-N(aminoxyl) bond angles lie between 131.3(3) and 132.7(3)8 and the Ln-O-N angles span from 134.9(3) to 143.4(3)8 (Table 2). The average distances of intrachain Cu···Ln and Ln···Ln are 8.352 and 8.55 for complex 3Gd, 8.35 and 8.548 for complex 3Tb, and 8.363 and 8.576 for complex 3Dy. These bond parameters are close to those found for previous two series. The packing of the chains in crystal is shown in the Supporting Information, Figure S10. The closest interchain metal–metal separation is provided by Cu···Ln distance of 10.501 for 3Gd, 10.500 for 3Tb, and 10.484 for 3Dy. The nearest interchain Ln···Ln distance is 10.895, 10.895, and 10.879 for complexes 3Gd, 3Tb and 3Dy, respectively. CH···F hydrogen bonds take place between the chains (Supporting Information, Tables S9–S11). At this stage, it is interesting to compare the interchain metal to metal distances as a function of the R groups. Starting from a Cu to Ln distance of 9.78 for the R = Et (Ln = Gd) chains,[11] this interchain distance increases with introduction on larger R groups to reach 10.50 for the benzyl substituent. However, the Ln···Ln interchain separations are hardly modified with an increase of just about 0.15 from smallest to largest R group. These inter-chain separations are quite large and added to the diamagnetic shell wrapping the magnetic centers the
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Full Paper magnetic chains appear well-shield against inter-chain magnetic interactions.
Magnetic properties The magnetic behaviors for all compounds have been investigated. The molar magnetic susceptibilities, cM, have been recorded both in dc and ac mode, and the field dependences of the magnetizations were measured at 2 K. To avoid torque effects with anisotropic Ln derivatives, the samples have been blocked (see the Experimental Section). Below we briefly describe the results for each family of compounds. The analysis of the magnetic behavior undertaken for the Gd derivatives will be discussed in the ensuing section.
[Ln3Cu(hfac)11(NitPhOAll)4] The magnetic susceptibilities for 1Gd, 1Tb, and 1Dy are plotted as cMT versus T in Figure 5. Relevant values characterizing the
behaviors of these derivatives have been collected in the table included in the figure. These are the experimental value at 300 K (cMT300K), the anticipated value for the non-coupled spin system (cMTtheo), the value reached for 2 K (cMT2K), the value for the peak (if any) exhibited by the curve (cMTmax), and the temperature for this maximum (Tmax). From these data, it can be seen that the experimental values found at 300 K are slightly larger than the anticipated contributions of the non-interacting magnetic centers. For the Gd and Tb derivatives, an increase of cMT is observed from 300 K on; first smoothly, then faster as T is lowered. But divergence of cMT is not observed at lower temperature; instead a maximum is reached at 6 K and below cMT falls steeply. Such a behavior is in agreement with the ferromagnetic interactions anticipated for radical–Cu (axially coordinated)[15] and radical–Ln (heavy lanthanides)[16] interactions; it also reveals the occurrence of antiferromagnetic contributions from interchain or next nearest neighbor interactions. The Dy derivative shows a very gradual increase of cMT when T is lowered, with a broad maximum at 90 K followed by a slight decrease. While such a behavior may look puzzling, it is nothing but the result of a very pronounced variation with T of the paramagnetic contribution of Dy coming from crystal-field effect.[16, 17] Obviously for 1Dy, the paramagnetism of the Dy ions and the ferromagnetic Rad–M interactions contribute concomitantly to cMT from 300 K but with opposite effects. These contributions almost balance each other resulting in the observed experimental behavior. The same phenomenon takes place for 1Tb but for TbIII the paramagnetic contribution varies less with T. It can be noticed that for 1Dy cMT does not either diverge at low T. The field dependences of the magnetization for the three derivatives are also given in Figure 5. The Gd derivative reaches a value of 24.85 mB for a field of 5 tesla, which is in agreement with ferromagnetic interactions among the spin carriers. However, the saturation of the magnetization is not reached, which highlights the existence of antiferromagnetic contributions in this system also. Similar M versus H behaviors are found for 1Tb and 1Dy (with 20.0 mB and 19.9 mB respectively, for 5 tesla); here the magnetic anisotropy of the Tb and Dy ions may contribute to it. The magnetic susceptibility in ac mode has been recorded for 1Tb and 1Dy. The onset of a signal for the out-of-phase susceptibility (cM’’) was found for the Tb derivative below 4 K (frequency 1 kHz), but no maximum was seen. Application of a static magnetic field did not change the observed behavior (Supporting Information, Figure S11). [Ln3Cu(hfac)11(NitPhOPr)4]
Figure 5. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOAll)4] derivatives 1Ln. Some relevant values for this series of compounds are compiled in the table. The best fit of the calculated cMT versus T for 1Gd is shown as a full line. The solid line for the magnetization curves is provided to guide the eye. Chem. Eur. J. 2014, 20, 13356 – 13365
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The magnetic susceptibilities recorded in dc mode for compounds 2Gd, 2Tb, 2Dy, 2Ho, and 2Yb are given Figure 6 as cMT versus T plots. They exhibit a behavior reminiscent of the preceding series of compounds. However, it can be noticed that for the Dy derivative, cMT goes through a minimum at 10 K (with 41.5 c m3 mol1 K) and sharply increases for lower temperatures. This is the signature of a raise of the magnetic moment
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Full Paper weak; the ratio cM’/cM” is about 400 (Supporting Information, Figure S15). However, for 2Tb (Figure 7) a clear frequency dependent out-of-phase signal is observed, in agreement with a slow relaxation process for the magnetization.
Figure 7. Temperature-dependent ac magnetic susceptibility of 2Tb with an oscillation of 3.0 Oe in zero dc field.
Figure 6. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOPr)4] derivatives 2Ln. Some relevant values for this series of compounds are compiled in the table. The best fit of the calculated cMT versus T for 2Gd is shown as a full line. The solid line for the magnetization curves are provided to guide the eye.
of the compound owing to ferromagnetic interactions among the magnetic centers. The Ho and Yb derivatives exhibit a gradual decrease of their cMT values as T is lowered, for 2Ho the decrease is more pronounced below 30 K. The behavior for the Ho and Yb derivatives do not preclude the occurrence of ferromagnetic Ln-radical contributions. For these ions they are weaker[16a] and their effect on cMT are just overwhelmed by the stronger contributions of the crystal field effect as compared to Tb or Dy.[17b] The field dependence of the magnetizations recorded at 2 K for the five derivatives (Figure 6) are in agreement with the behaviors found for the homologous compounds 1Ln, with values of 25.2 mB (2Gd), 18.1 mB (2Tb), 19.1 mB (2Dy), 19.7 mB, (2Ho), and 6.1 mB (2Yb) reached for 5 tesla. The possibility of slow relaxing magnetization for these compounds has been evaluated by ac magnetic susceptibility in the temperature range 2–20 K; the Gd derivative was used to confirm the absence of magnetic ordering. For compounds 2Gd, 2Ho, and 2Yb, no out-of-phase signals (cM’’) are observed, indicating absence of magnetic ordering or magnetic relaxation above 2 K (Supporting Information, Figures S12–S14). For 2Dy only onsets of signals of cM“ are found and these are very Chem. Eur. J. 2014, 20, 13356 – 13365
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The possibility for a spin glass behavior was ruled out by the shift of peak temperature (Tp) of the in-phase signal (cM’), which yielded the spin glass parameter f= (DTp/Tp)/D(log f) = 0.322 (for spin-glass behavior 0.01 < f< 0.08),[18] thus suggesting a slow magnetic relaxation process of molecular origin. The Arrhenius equation, t = t0exp(Dt/kBT), was employed to extract the effective energy barrier for the magnetization reversal, Dt/kB, and the pre-exponential factor, t0. The peak temperatures for the cM“T curves that exhibit a maximum was obtained by the Lorentzian fitting; the derived ln t versus T1 plot was analyzed by Arrhenius equation to afford t0 = 4.02 109 s, which is consistent with observed values of SCMs[3] and Dt/ kB = 21.4 K. Although this latter value is not remarkable, these data suggest that 2Tb is the first example of heterotri-(2p–3d– 4f)-spin SCM. [Ln3Cu(hfac)11(NitPhOBz)4] The cMT = f(T) and M = f(H) behaviors for compounds 3Gd, 3Tb, and 3Dy are plotted in Figure 8 together with pertinent values. They very much follow the behavior described earlier; however, a salient feature is a continuous increase of cMT at low T for the Gd and Tb derivatives. No maximum is observed down to 2 K. The ac susceptibility investigations indicated the absence of signals for cM’’, either with or without an applied external field (Supporting Information, Figure S17). Modeling of the cMT versus T behavior for the Gd derivatives The cMT behavior for the Gd derivatives have been modeled to extract information on the exchange interactions taking place within these species. The direct Cu–radical[15] and Gd–
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Full Paper Equation (1) represents the interaction of each spin with external magnetic field (H) that is applied along z-axis. The last term represents the intermolecular interaction of z nearest-neighbors with interaction strength J ’ in mean field approximation. Positive (negative) zJ ’ represents ferromagnetic (antiferromagnetic) behavior of intermolecular interaction. The mean value of Sz in Equation (1) can be obtained from the following equation: P 0 MS exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ hSZ iMS g i P h Sz i ¼ P P exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ0 hSZ iMS g PP S
MS
S
MS
i
where E0(S,MS) are the eigenvalues of the H0. The eigenvalues are obtained by solving the Hamiltonian in Equation (1) for a system with two formula units. This unit consists of eight magnetic centers of which six have s = 1/2 and two have s = 7/ 2 for the alternating [-Cu-Rad-Gd-Rad-]n chain (Rad stands for the nitronyl nitroxide molecule with R = Et;[11] see below) or five s = 1/2 and three s = 7/2 for derivatives 1Gd–3Gd. We solve for all the eigenstates of these systems, numbering 4096 states for the former and 16 384 states for remaining compounds in all MS sectors. < mi > is the expectation value of Szi and b = 1/(KBT). The magnetization (M) can be obtained from < Sz > and is given by: P PP P 0 ð gi hmi iÞ exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ hSZ iMS g S M i i Figure 8. cMT = f(T) and M = f(H) behaviors for [Ln3Cu(NitPhOBz)4] derivatives S PP P M¼ 3Ln. Some relevant values for this series of compounds are compiled in the exp½bfE0 ðS; MS Þ þ mB H gi hmi i zJ0 hSZ iMS g table. The best fit of the calculated cMT versus T for 3Gd is shown as a full line. The solid line for the magnetization curves are provided to guide the eye.
S
X
0
gi siz zJ hSz i
i
H0 ¼
X i
X
siz
ð1Þ
i
X ! ! ! ! J1i S i S iþ1 þ J2i S i S iþ2 i
where J1i and J2i are the nearest and next-nearest neighbor interactions in Heisenberg Hamiltonian H0. The second term in Chem. Eur. J. 2014, 20, 13356 – 13365
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i
The cMT value can be calculated from the following equation:
radical[14a–c, 16c] interactions are known to be ferromagnetic, but for spin systems such as those reported, next-neighbor interactions (that is, radical–radical, GdGd, CuGd) cannot be ignored, and these are usually antiferromagnetic.[16a, c, 19] Occurrence of such competitive intra-chain interactions is suggested by the magnetization behaviors we have described above. Furthermore, we do not expect strong spin–orbit interactions because the Gd ion has a half-filled 4f shell and the radicals are built only with first-row elements. The g-factors all are expected to be close to 2 and magnetic anisotropy can be neglected. Therefore, we use the following Hamiltonian [Eq. (1)] to model these systems:
H ¼ H0 þ mB H
MS
c ¼ NmB T
@M @H
ð2Þ
We use Equation (2) to calculate cMT. In this calculation, we have taken two unit cells for the [-Cu-Rad-Gd-Rad-]n chain and for remaining compounds the number of sites and site spins per unit cell limit us to take one unit cell. In the thermodynamic limit, long range correlations exist in these compounds. Notwithstanding this, our calculated cMT versus T plots are in good agreement with experiments, indicating that the correlation lengths are small in the temperature range over which we have studied. A first set of information was deduced from the fit to the behavior of the alternating [-Cu-Rad-Gd-Rad-]n chain we have reported recently (Rad stands for the nitronyl nitroxide molecule with R = Et).[11] For this system, the JGdGd interaction is absent, which reduces the number of parameters to be considered. Experimental and calculated behaviors are shown Figure 9, the best-fit parameters are JCuRad = 24.5 cm1, JGdRad = 5.26 cm1, JRadRad = 6.8 cm1, JCuGd = 1.28 cm1, zJ’ = 0.007 cm1, gRad = 2, gGd = 2.09, and gCu = 2.17. These values confirm the anticipated ferromagnetic Cu–radical and Gd–radical interactions. The con-
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Figure 9. Experimental (*) and calculated (c) cMT = f(T) for the [CuGd(NitPhOEt)2]n[11] chain compound.
sideration of antiferromagnetic next-neighbor interaction was essential to accurately reproduce the experimental behavior; the JRadRad value obtained is in agreement with values found in the literature.[16a, c] Related modeling has been performed for the CuGd3(NitPhO-R)4 derivatives 1Gd, 2Gd, and 3Gd, as well as for the homologous R = Et derivative.[11] Comparison of experimental and simulated behaviors are given in Figures 5, 6, 8, and the Supporting Information, Figure S18, and best fit parameters are compiled in Table 3. Very close exchange parameters were found for the four derivatives and these values compare well to those obtained for the related [CuGdRad2] derivative. It can be noticed that slightly larger Ln–radical interactions are found for 2Gd and 3Gd. These belong to the series for which the related Tb derivatives reach large cMT values at 2 K, thus confirming the stronger contribution from the ferromagnetic components. Finally, in all cases the intermolecular interactions, zJ’, are very small which suggests that the magnetic chains are well-isolated.
Table 3. Best fit parameters for the [CuGd3(radical-R)4] derivatives.
JRad-Cu[a] JRad-Gd[a] JRad-Rad[a] JGd-Gd[a] zJ’[a] gCu gGd gRad
The results presented in this report illustrate the possibility offered by the novel construction strategy that consists in using preformed LnNit complexes as metallo-ligands. Following this approach, a series of unprecedented hetero-tri-(2p-3d-4f)-spin chain compounds have been obtained with high selectivity and good yields. The alteration at the periphery of the phenylNit core did not modify the assembling process, but this substituent was found to have an effect on the magnetic properties. Especially the occurrence of slow relaxing magnetization for the systems involving anisotropic Ln ions was found to depend on the R substituent. This could be related to the actual shape of the coordination polyhedron for the Ln centers, which were found to be either distorted square antiprisms or dodecahedrons, one or both geometries occurring in the chains as a function of the radical substituent and the nature of the Ln ion. While the compounds described herein can be considered as a proof-of-concept, the chemical versatility of the molecular building units opens many opportunities to design novel hetero-tri-spin materials.
Experimental Section Materials and physical measurements All of the reagents and chemicals were purchased from commercial sources and used as received. The radical ligands NitPhOAll, NitPhOBz, and NitPhOPr were prepared by previously reported methods.[20] Mononuclear complexes [Ln(hfac)3(NitPhOAll)2] (Ln = Gd, Tb, Dy) were synthesized according to the literature.[21] Elemental analysis for C, H, and N were carried out on a PerkinElmer elemental analyzer model 240. Magnetic measurements were carried out with a Quantum design MPMS 5S SQUID magnetometer in the temperature domain 2–300 K. The measurements were performed on microcrystalline samples mixed to grease and put in gelatin capsules. The magnetic susceptibilities were measured in an applied field of 1000 Oe. The molar susceptibility (cM) was corrected for sample holder and for the diamagnetic contribution of all the atoms by using Pascal’s tables. The ac susceptibility was measured with a MPMS 5S SQUID magnetometer with an oscillating ac field of 3 Oe and frequency between 10 to 1500 Hz. For complexes [Ln3Cu(hfac)11(NitPhOPr)4] 2 Ln (Ln = Gd, Tb, Dy, Ho, Yb), ac susceptibilities were carried out on a Quantum Design PPMS-9.
R = Et[11]
R = All
R = Prop
R = Bz
Synthesis
25.0 6.05 6.0 0.51 0.009 2.1 2.0 2.01
25.0 6.0 6.0 0.25 0.045 2.15 2.05 2.0
25.0 7.0 5.0 0.35 0.032 2.15 2.04 2.0
25.0 7.0 6.0 0.9 0.057 2.1 2.06 2.0
[Gd3Cu(hfac)11(NitPhOAll)4] 1Gd : A solution of Cu(hfac)2 (0.008 g, 0.02 mmol) in dry boiling heptane (20 mL) was heated to 90 8C and a dry CH2Cl2 solution (5 mL) of [Gd(hfac)3(NITPhOAll)2] (0.027 g, 0.02 mmol) was added. The resulting mixture was stirred for 15 min at this temperature, then cooled to room temperature and filtered off. The filtrate was kept at room temperature for two days and green crystals of 1Gd were obtained. Yield: 51 %. Elemental analysis calcd (%) for C119H95CuF66Gd3N8O34 : C 36.00, H 2.41, N 2.82; found: C 36.48, H 2.75, N 2.98. IR (KBr): n = 1653 s, 1504 m, 1257 s, 1211 s, 1147 s, 800 m cm1. [Tb3Cu(hfac)11(NitPhOAll)4] 1Tb : As for 1Gd but [Tb(hfac)3(NitPhOAll)2] was used in place of [Gd(hfac)3(NitPhOAll)2]. Yield: 55 %. Elemental analysis calcd (%)for C119H95CuF66Tb3N8O34 :
[a] In cm1; positive (negative) values represents ferro-(antiferro-)magnetic interactions.
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Full Paper C 35.95, H 2.41, N 2.82; found: C 35.67, H 2.94, N 3.01. IR (KBr): n = 1653 s, 1504 m, 1257 s, 1210 s, 1147 s, 800 m, 662 m cm1. [Dy3Cu(hfac)11(NitPhOAll)4] 1Dy : As for 1Gd but [Dy(hfac)3(NitPhOAll)2] was used in place of [Gd(hfac)3(NitPhOAll)2]. Yield: 61 %. Elemental analysis calcd (%) for C119H95CuF66Dy3N8O34 : C 35.86, H 2.40, N 2.81; found: C 35.53, H 2.63, N 2.91. IR (KBr): n = 1654 s, 1505 m, 1257 s, 1210 s, 1147 s, 800 m, 662 m cm1. [Gd3Cu(hfac)11(NitPhOPr)4] 2Gd : A solution of Gd(hfac)3·2 H2O (0.016 g, 0.02 mmol) in dry boiling heptane (20 mL) was heated to reflux for 3 h and then cooled to 90 8C, 3 mL of a dichloromethane solution of NitPhOPr (0.012 g, 0.04 mmol) was slowly added with stirring about 10 min. Then the solid of Cu(hfac)2 (0.008 g, 0.02 mmol) was added. The resulting green solution was stirred for another 10 min at this temperature, then cooled to room temperature and filtered. After two days, green elongated crystals suitable for single crystal diffraction were obtained. Yield 47 %. Elemental analysis calcd (%)for C119H103N8O34F66CuGd3 : C 35.93, H 2.61, N 2.82; found: C 36.35, H 2.87, N 3.10. IR (KBr): n = 3442 m, 1652 s, 1501 m, 1342 m, 1256 s, 1211 s, 1148 s, 799 w, 662 w cm1. [Tb3Cu(hfac)11(NitPhOPr)4] 2Tb : This complex was synthesized using same procedure as for 2Gd. Yield 51 %. Elemental analysis calcd (%) for C119H103N8O34F66CuTb3 : C 35.88, H 2.81, N 2.84; found: C 35.76, H 2.55, N 3.02. IR (KBr): n = 3447 m, 1652 s, 1500 m, 1344 m, 1256 s, 1210 s, 1148 s, 780 w, 661 w cm1. [Dy3Cu(hfac)11(NitPhOPr)4] 2Dy : This complex was synthesized using same procedure as for 2Gd. Yield 44 %. Elemental analysis calcd (%) for C119H103N8O34F66CuDy3 : C 35.78, H 2.60, N 2.80; found: C 36.01, H 2.79, N 3.03. IR (KBr): n = 3443 m, 1652 s, 1502 m, 1342 w, 1256 s, 1211 s, 1147 s, 780 w, 660 w, 584 w cm1. [Ho3Cu(hfac)11(NitPhOPr)4] 2Ho : This complex was synthesized using same procedure as for 2Gd. Yield 52 %. Elemental analysis calcd (%) for C119H103N8O34F66CuHo3 : C 35.72, H 2.59, N 2.80; found: C 35.83, H 3.51, N 3.12. IR (KBr): n = 3443 m, 1654 s, 1607 w, 1506 m), 1342 w, 1256 s, 1209 s, 1147 s, 799 w, 662 w, 587 w cm1. [Yb3Cu(hfac)11(NitPhOPr)4] 2Yb : This complex was synthesized using same procedure as for 2Gd. Yield 52 %. Elemental analysis calcd (%) for C119H103N8O34F66CuYb3 : C 35.50, H 2.58, N 2.78; found: C 35.72, H 3.11, N 3.12. IR (KBr): n = 3445 m, 1654 s, 1604 w, 1503 m, 13 423 w, 1255 s, 1203 s, 1145 s, 799 w, 660 w, 586 w cm1. [Gd3Cu(hfac)11(NitPhOBz)4] 3Gd : This complex was prepared by same procedure as for 2Gd but NitPhOBz was used. Yield: 53 %. Elemental analysis calcd (%) for C135H103N8O34F66CuGd3 : C 38.88, H 2.49, N 2.69; found: C 38.35, H 2.57, N 2.60. IR (KBr): n = 1654 s, 1505 s, 1148 s, 1343 m, 1256 m, 1210 m, 1211 m, 800 m, 662 m, 586 m cm1. [Tb3Cu(hfac)11(NitPhOBz)4] 3Tb : The complex was prepared by same procedure as for 3Gd. Yield: 56 %. Elemental analysis calcd (%) for C135H103N8O34F66CuTb3 : C 38.83, H 2.49, N 2.68; found: C 38.56, H 2.55, N 2.52. IR (KBr): n = 1654 s, 1506 s, 1257 s, 1210 s, 1148 s, 800 m, 662 m, 586 m cm1. [Dy3Cu(hfac)11(NitPhOBz)4] 3Dy : The complex was prepared by same procedure as for 3Gd. Yield: 50 %. Elemental analysis calcd (%) for C135H103N8O34F66CuDy3 : C 38.73, H 2.48, N 2.68; found: C 39.01, H 2.59, N 2.73. IR (KBr): n = 1652 s, 1502 s, 1257 s, 1210 s, 1147 s, 799 m, 661 m, 585 m cm1.
X-ray crystallography Diffraction data for eleven complexes were recorded on a Rigaku Saturn CCD diffractometer with graphite-monochromated Mo Ka radiation (l = 0.71073 ) at 113 K. In each case, absorption corrections were applied. The structure was solved by direct methods Chem. Eur. J. 2014, 20, 13356 – 13365
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using the SHELXS-97 program[22] and all non-hydrogen atoms were refined anisotropically by least-squares methods on F 2 using the SHELXL program.[23] Hydrogen atoms were added theoretically and refined isotropically using a riding mode. Disorders were observed in these complexes for some fluorine atoms. Detailed data collection and refinement of these eleven compounds are summarized in Table S13 and S14. Selected bond lengths and angles are listed in Table 1 and 2. CCDC-1000884 (1Dy), CCDC-1000885 (1Gd),CCDC1000886 (1Tb), CCDC-1000887 (3Dy), CCDC-1000888 (3Gd), CCDC1000889 (3Tb), CCDC-1000890 (2Dy), CCDC-1000891 (2Gd), CCDC1000892 (2Ho), CCDC-1000893 (2Tb), and CCDC-1000894 (2Yb) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Received: May 19, 2014 Published online on August 28, 2014
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