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Cite this: DOI: 10.1039/c5cc03944d Received 12th May 2015, Accepted 5th June 2015
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Bi- and trimetallic rare-earth–palladium complexes ligated by phosphinoamides† a a b b Franziska Vo ¨ lcker, Felix M. Mu ¨ ck, Konstantinos D. Vogiatzis, Karin Fink* and a Peter W. Roesky*
DOI: 10.1039/c5cc03944d www.rsc.org/chemcomm
The synthesis of the heterometallic early–late 4d/4f bi- and trinuclear phosphinoamido Ln–Pd(0) complexes [(Ph2PNHPh)Pd{l-(Ph2PNPh)}3Ln(l-Cl)Li(THF)3] (Ln = Y, Lu) and [Li(THF)4][{(Ph2PNHPh)Pd}2{l-(Ph2PNPh)}4Ln] (Ln = Y, Lu) is described. The latter compounds are the first early–late trimetallic phosphinoamido complexes. Although the metal atoms are forced into close proximity by the phosphinoamido ligands, quantum chemical calculations show only weak metal-to-metal interactions.
Heterobi- or multimetallic complexes have been studied over recent years in terms of the possible cooperative and synergistic effects that can arise from the simultaneous or consecutive action of different metal centers in different media.1 The application of heterobimetallic complexes in catalysis has been reviewed recently.1 One of the big challenges that still remain is the synthesis of heterobimetallic early–late complexes2 containing rare-earth metals. For the rare-earth elements only a few complexes with non-supported metal-to-metal bonds to a transition metal are known.3 Examples include complexes with Lu–Ru,4 Ln–Re (Ln = La, Sm, Yb, Lu),5 Nd–Fe,6 and Yb–Fe bonds.7 Bimetallic complexes which have a rare-earth metal atom and a rhodium, palladium, or platinum atom in close proximity (distance of less than 3.5 Å) are very rare and to the best of our knowledge trimetallic compounds are unknown. Kempe et al. reported a few Nd–Rh and Nd–Pd complexes about 15 years ago.8 In these compounds, the metals are brought close together by bis(aminopyridinato) ligands. Hou et al. recently published rare-earth–platinum heterobinuclear half-sandwich rare-earth– metal alkyl complexes bearing Cp ligands with a phosphine side arm.9 a
¨r Anorganische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu Engesserstr. 15, Geb. 30.45, 76131 Karlsruhe, Germany. E-mail: [email protected] b ¨r Nanotechnologie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu Postfach 3640, 76021 Karlsruhe, Germany. E-mail: [email protected] † Electronic supplementary information (ESI) available: Details of experimental and crystallographic details, NMR spectra and DFT calculations. CCDC 1400333– 1400339. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc03944d
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It was recently shown by Thomas et al. that different heterobimetallic transition metal complexes with metal-to-metal bonds can be formed when they are supported by phosphinoamido ligands.10 The known combinations of metals realized by this concept are bimetallic complexes, e.g. Co–Ti,11 Co–Zr,10,12 Co–Hf,3 Pt–Zr,13 V–Fe,14 Nb–Co,5b Ta–Co,5b and Co–U.15 By using this approach, complexes with higher nuclearity could not be obtained. More than a decade ago, we started working on mononuclear phosphinoamido complexes of the lanthanides. The ionic complexes [Li(THF)4][(Ph2PNPh)4Ln] (Ln = Y, Yb, Lu)16 were reported by us in 1999. Recently, we also reported the analogous dysprosium complex [Li(THF)4][(Ph2PNPh)4Dy] and investigated its magnetic properties.17 The occurrence of slow magnetic relaxation, which is likely to be related to the individual ion anisotropy of the Dy(III) center, was observed. In general, [Li(THF)4][(Ph2PNPh)4Ln] complexes are obtained by reactions of LiNPhPPh2 with anhydrous LnCl3 in a 4 : 1 molar ratio.16,17 [Li(THF)4][(Ph2PNPh)4Ln] are, in contrast to the transition metal phosphinoamido compounds, ionic compounds. Nevertheless, we were interested in using them as starting materials for the synthesis of heterobimetallic early–late rare-earth metal complexes. Reactions of [Li(THF)4][(Ph2PNPh)4Ln] (Ln = Y, Lu) with the palladium allyl complex [Pd2(C3H5)2Cl2] did not give the expected Ln–Pd(II) species, but rather the fully reproducible Ln–Pd(0) complexes [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3Ln(m-Cl)Li(THF)3] (Ln = Y (1a), Lu (1b)) in 22% and 45% yield (Scheme 1). A related reduction was observed upon the formation of bimetallic Co–Zr phosphinoamido complexes.18 Treatment of (Ph2PNiPr)3ZrCl, (iPr2PNMes)3ZrCl or (iPr2PNiPr)3ZrCl with Co(II)I2 resulted in Co(I)–Zr compounds in each case.18 Single crystals of 1a and 1b suitable for X-ray diffraction were obtained by crystallization from THF/pentane (Fig. 1). In both compounds, the Li atom is coordinated to the Lu atom via a m-Cl bridge. Under the same conditions, an ionic modification of 1b, [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3LuCl][Li(THF)4] (1b 0 ), in which the lithium atom is coordinated by four THF molecules, was also obtained on one occasion (Fig. 2). Due to the similar composition of 1b and 1b 0 we could not control which product was formed.
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Scheme 1
Formation of 1a,b (1b 0 ) and 2a,b.
Fig. 1 Solid-state structure of 1a. Hydrogen atoms bound to carbon are omitted for clarity. Compound 1b is similar. Selected bond lengths [Å], angles [1]: 1a: Pd–Y 2.9898(6), Y–Cl 2.6548(15), Y–O1 2.478(4), Y–N1 2.288(4), Y–N2 2.353(4), Y–N3 2.311(4), Pd–P1 2.3689(14), Pd–P2 2.3622(12), Pd– P3 2.3934(14), Pd–P4 2.3595(14); O1–Y–Cl 80.80(10), N1–Y–Cl 114.00(11), N1–Y–O1 81.55(13), N1–Y–N2 132.63(14), N1–Y–N3 98.37(15), N2–Y–Cl 106.29(10), N2–Y–O1 81.32(14), N2–Y–N3 108.48(15), N3–Y–Cl 86.04(11), N3–Y–O1 165.47(15), P1–Pd–P2 117.43(5), P1–Pd–P3 106.00(5), P1–Pd–P4 114.39(5), P2–Pd–P3 105.20(5), P2–Pd–P4 104.22(5), P3–Pd–P4 109.10(5). 1b: Lu–Pd 2.9031(11), Lu–Cl 2.621(3), Lu–O1 2.449(7), Lu–N1 2.304(9), Lu– N2 2.224(9), Lu–N3 2.260(7), Pd–P1 2.358(3), Pd–P2 2.418(3), Pd–P3 2.377(3), Pd–P4 2.352(3), O1–Lu–Cl 81.2(2), N1–Lu–Cl 103.0(2), N1–Lu–O1 83.1(3), N1–Lu–N2 108.6(3), N1–Lu–N3 137.8(3), N2–Lu–Cl 85.1(2), N2–Lu–O1 163.7(3), N2–Lu–N3 97.9(3), N3–Lu–Cl 111.6(2), N3–Lu–O1 79.3(3), P1– Pd–P2 104.66(10), P1–Pd–P3 119.39(11), P1–Pd–P4 104.60(13), P2–Pd–P3 105.47(11), P2–Pd–P4 108.34(12), P3–Pd–P4 113.67(11).
In one case we even co-crystallized both compounds in one single crystal consisting of 1b and 1b 0 in a 2 : 3 ratio (Fig. S2, ESI†). Interestingly, an yttrium analogue of 1b 0 was not obtained. In order to compare the structures, we also established the solidstate structure of the starting compound [Li(THF)4][(Ph2PNPh)4Y] by single crystal X-ray diffraction, which was not reported in the original publication16 (Fig. S1, ESI†). In each of the new bimetallic compounds the metal atoms are in close proximity having Ln–Pd distances of 2.9898(6) Å (1a), 2.9031(11) Å (1b), and 2.9712(8) Å (1b 0 ). Whereas the Ph2PNPh ligands are Z2-coordinated in [Li(THF)4][(Ph2PNPh)4Ln], the soft phosphorus atoms are no longer bound to the lanthanide atom in 1a, 1b and 1b 0 respectively, but instead they are coordinated to
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Fig. 2 Solid-state structure of the anion of 1b0 . Hydrogen atoms bound to carbon are omitted for clarity. Selected bond lengths [Å], angles [1]: Lu–Pd 2.9712(8), Lu–Cl 2.5700(12), Lu–O1 2.441(3), Lu–N1 2.315(3), Lu–N2 2.259(3), Lu–N3 2.271(3), Pd–P1 2.3570(11), Pd–P2 2.3551(11), Pd–P3 2.3913(10), Pd– P4 2.3573(10); O1–Lu–Cl 81.20(7), N1–Lu–Cl 105.59(8), N1–Lu–O1 80.67(10), N1–Lu–N2 135.10(10), N1–Lu–N3 109.11(11), N2–Lu–Cl 112.18(8), N2–Lu–O1 81.98(10), N2–Lu–N3 96.53(11), N3–Lu–Cl 86.92(8), N3–Lu–O1 166.39(9), P1– Pd–P2 116.88(4), P1–Pd–P3 105.02(4), P1–Pd–P4 105.03(4), P2–Pd–P3 105.58(4), P2–Pd–P4 113.83(4), P4–Pd–P3 110.13(4).
the Pd atom. The vacant coordination sites of the lanthanide atom are occupied by one chloride ion (from [Pd2(C3H5)2Cl2] or LiCl (see below)) and one THF molecule. Thus, the lanthanide atoms each show the rather low coordination number of five. A distorted square pyramid is formed with the chlorine atom at the apex, three nitrogen atoms from three Ph2PNPh ligands and one THF molecule at the base. The Ln–Cl bond lengths are 2.6548(15) Å (1a), 2.621(3) Å (1b) and 2.5700(12) Å (1b 0 ). Although the lanthanide atoms each have a relatively low coordination number, the Ln–N bonds (av. 2.317 (1a) Å, 2.262 Å (1b) and av. 2.281 (1b 0 ) Å) do not deviate significantly from the starting material [Li(THF)4][(Ph2PNPh)4Ln] (av. 2.309 Å (Ln = Y) and av. 2.262 Å (Ln = Y)).16 The fourth Ph2PNPh ligand, which in [Li(THF)4][(Ph2PNPh)4Ln] was coordinated to the lanthanide atom, is now in its protonated form and is coordinated to the palladium atom. The zero-valent palladium atom is four-coordinate with a distorted tetrahedral coordination geometry. This tetrahedron is formed by the phosphorus atoms of three m-(Ph2PNPh) anions and one terminal Ph2PNHPh ligand. As mentioned above a similar reduction was observed upon the formation of bimetallic Co–Zr phosphinoamido complexes.18 We do not know the source of reduction of the palladium atom. We presume that [Li(THF)4][(Ph2PNPh)4Ln] partly decomposes and Ph2PN(H)Ph is formed as a side-product. Using a suitable Pd(0) source such as [Pd(PPh3)4] instead of [Pd2(C3H5)2Cl2] did not result in the formation of detectable amounts of complex. The difference between 1b and 1b0 is only in the coordination of the lithium atom. In 1b it is coordinated by three THF molecules and the (m-Cl), whereas in 1b an ion pair composed of a [Li(THF)4]+ cation and a [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3LnCl] anion is formed (Fig. 2). The 1H and 31P{1H} NMR spectra show fast decomposition of 1a,b in various solvents (C6D6, d8-THF, CD2Cl2). Two major signals with a ratio of about 1 : 3 were observed in the 31P{1H} NMR spectra of both compounds (Fig. S3–S6, ESI†). These signals can be assigned to the coordinated Ph2PNHPh and Ph2PNPh ligands. For 1b they are broad, whereas for 1a higher-order
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fine structures are seen. However, due to fast decomposition there remain significant uncertainties of the correct assignment of the signals. Furthermore, some smaller broader signals, which could not be further resolved with low temperature VT NMR experiments, were also detected. During the synthesis of 1a and 1b, the ionic trimetallic sideproducts [Li(THF)4][{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] (Ln = Y (2a), Lu (2b)) were formed (Scheme 1). Although 2a,b were obtained several times, we did not observe their formation each time the experiments were performed. Whereas 2b was obtained in significant quantities and a full characterization could be performed 2a, was obtained only a few times in tiny amounts. Up to now, we have not been able to rationalize the seemingly random formation of 2a,b. However, by adding LiCl we were able to hamper the formation of 2a,b. Formally, the formation of 2a,b can be rationalized as the insertion of two {(Ph2PNHPh)Pd} fragments into the Ln–P bonds of [Li(THF)4][(Ph2PNPh)4Ln]. To the best of our knowledge, the insertion of two metals into phosphinoamido complexes is unique. As seen for 1a,b, the substitution of [Pd2(C3H5)2Cl2] by a chlorine-free Pd(0) precursor did not result in any detectable products. In contrast to the numerous phosphinoamide-bridged bimetallic d/d- and 5f/d-metal complexes, 2a,b are the first examples of trimetallic species. The solid-state structures of 2a,b were established by single crystal X-ray diffraction (Fig. 3). Compounds 2a,b are composed of a [Li(THF)4]+ cation and a [{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] anion. In compound 2a,b, the [Li(THF)4]+ cation is disordered. As observed for 1a,b, the palladium atoms are zero-valent, but in 2a,b they are only coordinated by two {m-(Ph2PNPh)} anions and one terminal Ph2PNHPh ligand forming a distorted triangular arrangement of the ligands around the palladium atoms. The lanthanide atoms show the low coordination number of four. A distorted coordination tetrahedron, which is formed by four nitrogen atoms from the Ph2PNPh ligands, is formed around the lanthanide atom. The Ln–N bonds of av. 2.325 (2a) Å, av. 2.286 (2b) Å are in the range of 1a,b. The Pd–Ln–Pd angle of 160.67(3)1 (2a) and 160.403(12)1 (2b) shows a slight bending, which might be the result of packing effects. The metal atoms are in close proximity (Ln–Pd 3.1410(13) Å and 3.1860(12) Å (2a), 3.1063(4) Å and 3.1574(4) Å (2b)) due to the rigid ligand scaffold. Compared to compounds 1a, 1b and 1b0 , the metal-to-metal distances are significantly longer. Obviously, the reduced number of phosphinoamido ligands (two instead of three for each Ln–Pd contact), which cannot hold the metals tightly together, is the reason for the longer metal-to-metal distances. Since all phosphinoamide bridged bimetallic d/d-metal and 5f/d-metal complexes reported so far show significant metal-tometal bonds, we were interested in studying the Ln–Pd interaction in more detail. Therefore, compound 1b was theoretically investigated by quantum chemical calculations. The quantum chemical calculations were performed with TURBOMOLE V6.219 using density functional theory (D F T). The def2-TZVP basis set was used throughout.20 As a first step, the structure of 1b (the counter ion was neglected, therefore the calculated structure has to be compared to 1b0 ) was optimized with different functionals with and without the addition of dispersion corrections,21 see Table S2 in the ESI.† Good agreement was found between the calculated and measured structure for
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Fig. 3 Solid state structure of the anion of 2a. (The [Li(THF)4]+ cation is disordered.) Hydrogen atoms are omitted for clarity. Compound 2b is isostructural. Selected bond lengths [Å], angles [1]: 2a: Pd1–Y 3.1410(13), Pd1–P1 2.310(2), Pd1–P2 2.295(2), Pd1–P5 2.360(2), Pd2–Y 3.1860(12), Pd2–P3 2.303(2), Pd2–P4 2.291(2), Pd2–P6 2.334(2), Y–N1 2.327(5), Y–N2 2.329(6), Y–N3 2.314(6), Y–N4 2.333(6)1; Pd1–Y–Pd2-160.67(3), P1–Pd1–P2 134.02(7), P1–Pd1–P5 116.04(7), P2–Pd1–P5 108.57(7), P3–Pd2– P4 132.93(8), P3–Pd2–P6 115.38(8), P4–Pd2–P6 111.07(8), N1–Y–N2 140.0(2), N1–Y–N3 106.3(2), N1–Y–N4 92.8(2), N2–Y–N3 93.5(2), N2–Y–N4 93.5(2), N3– Y–N4 140.3(2). 2b: Pd1–Lu-3.1063(4), Pd1–P1 2.3025(13), Pd1–P2 2.2888(13), Pd1–P5 2.3636(14), Pd2–Lu-3.1574(4), Pd2–P3 2.2939(14), Pd2–P4 2.2813(15), Pd2–P6 2.335(2), Lu–N1 2.290(4), Lu–N2 2.285(4), Lu–N3 2.276(4), Lu–N4 2.291(4)1; P1–Pd1–P2 133.62(5), P1–Pd1–P5 116.37(5), P2–Pd1– P5 108.59(5), P3–Pd2–P4 132.38(6), P3–Pd2–P6 115.58(6), P4–Pd2–P6 111.30(6), N1–Lu–N2 141.49(15), N1–Lu–N3 106.29(15), N1–Lu–N4 92.52(2), N2–Lu–N3 93.21(2), N2–Lu–N4 92.3(2), N3–Lu–N4 141.27(15).
all functionals, with PBE022 showing the best overall agreement. For all functionals, the calculated Lu–Pd distance was slightly longer (3.011 Å for PBE0) than the experimental distance. After addition of Grimme’s D3 dispersion correction, the distance shortened to 2.985 Å, in very good agreement with the experimental value of 2.9712 Å for 1b 0 . In all further DFT calculations, the PBE0-D3 functional was used. The electronic structure was analysed by a natural population analysis.23 The natural charges for Lu and Pd are +1.55 and +0.03, respectively. The Mayer bond order24 between Lu and Pd amounts to 0.30. For further analysis, two model systems were set up (Fig. 4). In model a, the phenyl rings were substituted by hydrogen atoms and enabled an analysis of the interaction of the phenyl rings. In model b, the P–N bridge was removed. With this model, it was possible to analyse the long-range electrostatic interactions in the complex. The optimized structure of model a predicts a Lu–Pd distance of 3.103 Å. This value is on the order of the sum of the metallic radii, which highlights the stabilizing role of the phosphinoamine ligands in 1b. For each of these bridging ligands, a phenyl ring connected to P and the phenyl ring connected to N are in a similar configuration as in the T-shaped benzene dimer, which has an interaction energy of 2.66 kcal mol1 at CCSD(T) level in the basis set limit.25 We conclude that the short Lu–Pd distance in 1b is caused by the ligands and not by a metal–metal bond. In model b, the optimized Lu–Pd distance amounts to 3.15 Å. At long distances, model b dissociates into an uncharged Pd complex and a Lu complex with a charge of 1. Therefore, the
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Fig. 4 Model systems (a) without phenyl rings (b) unbridged system. Grey: Pd, black: Lu, magenta: P, green: N, red: O, small black: C, blue: Cl, white: H, dotted line in (b) indicates the orientation only. There exists no bond between P and N in model (b).
long range interaction shows R2 behaviour (R = Lu–Pd distance), which is typical for a charge–dipole interaction (see Fig. S7, ESI†). The formation and the structures of heterobimetallic Ln–Pd complexes supported by phosphinoamido ligands are significantly different from the well-established family of heterobimetallic transition metal complexes with phosphinoamido ligands. The formation of all Ln–Pd complexes is accompanied by reduction of the palladium precursor from Pd(II) to Pd(0). The 4d/4f bi- and trinuclear phosphinoamido Ln–Pd(0) complexes [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3Ln(m-Cl)Li(THF)3] (Ln = Y, Lu) and [Li(THF)4][{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] (Ln = Y, Lu) have short metal-to-metal distances. As shown by quantum chemical calculations for 1b these short distances are a result of ligand effects, rather than metal–metal bonds. This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 ‘‘3MET’’ Project B1 and B3. A. T. Wagner is acknowledged for designing the cover image.
Notes and references ´ and P. Braunstein, Chem. Rev., 2015, 115, 28. 1 P. Buchwalter, J. Rose 2 (a) L. H. Gade, Angew. Chem., Int. Ed., 2000, 39, 2658; (b) N. Wheatley and P. Kalck, Chem. Rev., 1999, 99, 3379. 3 B. Oelkers, M. V. Butovskii and R. Kempe, Chem. – Eur. J., 2012, 18, 13566. 4 I. P. Beletskaya, A. Z. Voskoboynikov, E. B. Chuklanova, N. I. Kirillova, A. K. Shestakova, I. N. Parshina, A. I. Gusev and G. K. I. Magomedov, J. Am. Chem. Soc., 1993, 115, 3156.
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Cite this: DOI: 10.1039/c5cc03944d Received 12th May 2015, Accepted 5th June 2015
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Bi- and trimetallic rare-earth–palladium complexes ligated by phosphinoamides† a a b b Franziska Vo ¨ lcker, Felix M. Mu ¨ ck, Konstantinos D. Vogiatzis, Karin Fink* and a Peter W. Roesky*
DOI: 10.1039/c5cc03944d www.rsc.org/chemcomm
The synthesis of the heterometallic early–late 4d/4f bi- and trinuclear phosphinoamido Ln–Pd(0) complexes [(Ph2PNHPh)Pd{l-(Ph2PNPh)}3Ln(l-Cl)Li(THF)3] (Ln = Y, Lu) and [Li(THF)4][{(Ph2PNHPh)Pd}2{l-(Ph2PNPh)}4Ln] (Ln = Y, Lu) is described. The latter compounds are the first early–late trimetallic phosphinoamido complexes. Although the metal atoms are forced into close proximity by the phosphinoamido ligands, quantum chemical calculations show only weak metal-to-metal interactions.
Heterobi- or multimetallic complexes have been studied over recent years in terms of the possible cooperative and synergistic effects that can arise from the simultaneous or consecutive action of different metal centers in different media.1 The application of heterobimetallic complexes in catalysis has been reviewed recently.1 One of the big challenges that still remain is the synthesis of heterobimetallic early–late complexes2 containing rare-earth metals. For the rare-earth elements only a few complexes with non-supported metal-to-metal bonds to a transition metal are known.3 Examples include complexes with Lu–Ru,4 Ln–Re (Ln = La, Sm, Yb, Lu),5 Nd–Fe,6 and Yb–Fe bonds.7 Bimetallic complexes which have a rare-earth metal atom and a rhodium, palladium, or platinum atom in close proximity (distance of less than 3.5 Å) are very rare and to the best of our knowledge trimetallic compounds are unknown. Kempe et al. reported a few Nd–Rh and Nd–Pd complexes about 15 years ago.8 In these compounds, the metals are brought close together by bis(aminopyridinato) ligands. Hou et al. recently published rare-earth–platinum heterobinuclear half-sandwich rare-earth– metal alkyl complexes bearing Cp ligands with a phosphine side arm.9 a
¨r Anorganische Chemie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu Engesserstr. 15, Geb. 30.45, 76131 Karlsruhe, Germany. E-mail: [email protected] b ¨r Nanotechnologie, Karlsruher Institut fu ¨r Technologie (KIT), Institut fu Postfach 3640, 76021 Karlsruhe, Germany. E-mail: [email protected] † Electronic supplementary information (ESI) available: Details of experimental and crystallographic details, NMR spectra and DFT calculations. CCDC 1400333– 1400339. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc03944d
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It was recently shown by Thomas et al. that different heterobimetallic transition metal complexes with metal-to-metal bonds can be formed when they are supported by phosphinoamido ligands.10 The known combinations of metals realized by this concept are bimetallic complexes, e.g. Co–Ti,11 Co–Zr,10,12 Co–Hf,3 Pt–Zr,13 V–Fe,14 Nb–Co,5b Ta–Co,5b and Co–U.15 By using this approach, complexes with higher nuclearity could not be obtained. More than a decade ago, we started working on mononuclear phosphinoamido complexes of the lanthanides. The ionic complexes [Li(THF)4][(Ph2PNPh)4Ln] (Ln = Y, Yb, Lu)16 were reported by us in 1999. Recently, we also reported the analogous dysprosium complex [Li(THF)4][(Ph2PNPh)4Dy] and investigated its magnetic properties.17 The occurrence of slow magnetic relaxation, which is likely to be related to the individual ion anisotropy of the Dy(III) center, was observed. In general, [Li(THF)4][(Ph2PNPh)4Ln] complexes are obtained by reactions of LiNPhPPh2 with anhydrous LnCl3 in a 4 : 1 molar ratio.16,17 [Li(THF)4][(Ph2PNPh)4Ln] are, in contrast to the transition metal phosphinoamido compounds, ionic compounds. Nevertheless, we were interested in using them as starting materials for the synthesis of heterobimetallic early–late rare-earth metal complexes. Reactions of [Li(THF)4][(Ph2PNPh)4Ln] (Ln = Y, Lu) with the palladium allyl complex [Pd2(C3H5)2Cl2] did not give the expected Ln–Pd(II) species, but rather the fully reproducible Ln–Pd(0) complexes [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3Ln(m-Cl)Li(THF)3] (Ln = Y (1a), Lu (1b)) in 22% and 45% yield (Scheme 1). A related reduction was observed upon the formation of bimetallic Co–Zr phosphinoamido complexes.18 Treatment of (Ph2PNiPr)3ZrCl, (iPr2PNMes)3ZrCl or (iPr2PNiPr)3ZrCl with Co(II)I2 resulted in Co(I)–Zr compounds in each case.18 Single crystals of 1a and 1b suitable for X-ray diffraction were obtained by crystallization from THF/pentane (Fig. 1). In both compounds, the Li atom is coordinated to the Lu atom via a m-Cl bridge. Under the same conditions, an ionic modification of 1b, [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3LuCl][Li(THF)4] (1b 0 ), in which the lithium atom is coordinated by four THF molecules, was also obtained on one occasion (Fig. 2). Due to the similar composition of 1b and 1b 0 we could not control which product was formed.
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Scheme 1
Formation of 1a,b (1b 0 ) and 2a,b.
Fig. 1 Solid-state structure of 1a. Hydrogen atoms bound to carbon are omitted for clarity. Compound 1b is similar. Selected bond lengths [Å], angles [1]: 1a: Pd–Y 2.9898(6), Y–Cl 2.6548(15), Y–O1 2.478(4), Y–N1 2.288(4), Y–N2 2.353(4), Y–N3 2.311(4), Pd–P1 2.3689(14), Pd–P2 2.3622(12), Pd– P3 2.3934(14), Pd–P4 2.3595(14); O1–Y–Cl 80.80(10), N1–Y–Cl 114.00(11), N1–Y–O1 81.55(13), N1–Y–N2 132.63(14), N1–Y–N3 98.37(15), N2–Y–Cl 106.29(10), N2–Y–O1 81.32(14), N2–Y–N3 108.48(15), N3–Y–Cl 86.04(11), N3–Y–O1 165.47(15), P1–Pd–P2 117.43(5), P1–Pd–P3 106.00(5), P1–Pd–P4 114.39(5), P2–Pd–P3 105.20(5), P2–Pd–P4 104.22(5), P3–Pd–P4 109.10(5). 1b: Lu–Pd 2.9031(11), Lu–Cl 2.621(3), Lu–O1 2.449(7), Lu–N1 2.304(9), Lu– N2 2.224(9), Lu–N3 2.260(7), Pd–P1 2.358(3), Pd–P2 2.418(3), Pd–P3 2.377(3), Pd–P4 2.352(3), O1–Lu–Cl 81.2(2), N1–Lu–Cl 103.0(2), N1–Lu–O1 83.1(3), N1–Lu–N2 108.6(3), N1–Lu–N3 137.8(3), N2–Lu–Cl 85.1(2), N2–Lu–O1 163.7(3), N2–Lu–N3 97.9(3), N3–Lu–Cl 111.6(2), N3–Lu–O1 79.3(3), P1– Pd–P2 104.66(10), P1–Pd–P3 119.39(11), P1–Pd–P4 104.60(13), P2–Pd–P3 105.47(11), P2–Pd–P4 108.34(12), P3–Pd–P4 113.67(11).
In one case we even co-crystallized both compounds in one single crystal consisting of 1b and 1b 0 in a 2 : 3 ratio (Fig. S2, ESI†). Interestingly, an yttrium analogue of 1b 0 was not obtained. In order to compare the structures, we also established the solidstate structure of the starting compound [Li(THF)4][(Ph2PNPh)4Y] by single crystal X-ray diffraction, which was not reported in the original publication16 (Fig. S1, ESI†). In each of the new bimetallic compounds the metal atoms are in close proximity having Ln–Pd distances of 2.9898(6) Å (1a), 2.9031(11) Å (1b), and 2.9712(8) Å (1b 0 ). Whereas the Ph2PNPh ligands are Z2-coordinated in [Li(THF)4][(Ph2PNPh)4Ln], the soft phosphorus atoms are no longer bound to the lanthanide atom in 1a, 1b and 1b 0 respectively, but instead they are coordinated to
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Fig. 2 Solid-state structure of the anion of 1b0 . Hydrogen atoms bound to carbon are omitted for clarity. Selected bond lengths [Å], angles [1]: Lu–Pd 2.9712(8), Lu–Cl 2.5700(12), Lu–O1 2.441(3), Lu–N1 2.315(3), Lu–N2 2.259(3), Lu–N3 2.271(3), Pd–P1 2.3570(11), Pd–P2 2.3551(11), Pd–P3 2.3913(10), Pd– P4 2.3573(10); O1–Lu–Cl 81.20(7), N1–Lu–Cl 105.59(8), N1–Lu–O1 80.67(10), N1–Lu–N2 135.10(10), N1–Lu–N3 109.11(11), N2–Lu–Cl 112.18(8), N2–Lu–O1 81.98(10), N2–Lu–N3 96.53(11), N3–Lu–Cl 86.92(8), N3–Lu–O1 166.39(9), P1– Pd–P2 116.88(4), P1–Pd–P3 105.02(4), P1–Pd–P4 105.03(4), P2–Pd–P3 105.58(4), P2–Pd–P4 113.83(4), P4–Pd–P3 110.13(4).
the Pd atom. The vacant coordination sites of the lanthanide atom are occupied by one chloride ion (from [Pd2(C3H5)2Cl2] or LiCl (see below)) and one THF molecule. Thus, the lanthanide atoms each show the rather low coordination number of five. A distorted square pyramid is formed with the chlorine atom at the apex, three nitrogen atoms from three Ph2PNPh ligands and one THF molecule at the base. The Ln–Cl bond lengths are 2.6548(15) Å (1a), 2.621(3) Å (1b) and 2.5700(12) Å (1b 0 ). Although the lanthanide atoms each have a relatively low coordination number, the Ln–N bonds (av. 2.317 (1a) Å, 2.262 Å (1b) and av. 2.281 (1b 0 ) Å) do not deviate significantly from the starting material [Li(THF)4][(Ph2PNPh)4Ln] (av. 2.309 Å (Ln = Y) and av. 2.262 Å (Ln = Y)).16 The fourth Ph2PNPh ligand, which in [Li(THF)4][(Ph2PNPh)4Ln] was coordinated to the lanthanide atom, is now in its protonated form and is coordinated to the palladium atom. The zero-valent palladium atom is four-coordinate with a distorted tetrahedral coordination geometry. This tetrahedron is formed by the phosphorus atoms of three m-(Ph2PNPh) anions and one terminal Ph2PNHPh ligand. As mentioned above a similar reduction was observed upon the formation of bimetallic Co–Zr phosphinoamido complexes.18 We do not know the source of reduction of the palladium atom. We presume that [Li(THF)4][(Ph2PNPh)4Ln] partly decomposes and Ph2PN(H)Ph is formed as a side-product. Using a suitable Pd(0) source such as [Pd(PPh3)4] instead of [Pd2(C3H5)2Cl2] did not result in the formation of detectable amounts of complex. The difference between 1b and 1b0 is only in the coordination of the lithium atom. In 1b it is coordinated by three THF molecules and the (m-Cl), whereas in 1b an ion pair composed of a [Li(THF)4]+ cation and a [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3LnCl] anion is formed (Fig. 2). The 1H and 31P{1H} NMR spectra show fast decomposition of 1a,b in various solvents (C6D6, d8-THF, CD2Cl2). Two major signals with a ratio of about 1 : 3 were observed in the 31P{1H} NMR spectra of both compounds (Fig. S3–S6, ESI†). These signals can be assigned to the coordinated Ph2PNHPh and Ph2PNPh ligands. For 1b they are broad, whereas for 1a higher-order
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fine structures are seen. However, due to fast decomposition there remain significant uncertainties of the correct assignment of the signals. Furthermore, some smaller broader signals, which could not be further resolved with low temperature VT NMR experiments, were also detected. During the synthesis of 1a and 1b, the ionic trimetallic sideproducts [Li(THF)4][{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] (Ln = Y (2a), Lu (2b)) were formed (Scheme 1). Although 2a,b were obtained several times, we did not observe their formation each time the experiments were performed. Whereas 2b was obtained in significant quantities and a full characterization could be performed 2a, was obtained only a few times in tiny amounts. Up to now, we have not been able to rationalize the seemingly random formation of 2a,b. However, by adding LiCl we were able to hamper the formation of 2a,b. Formally, the formation of 2a,b can be rationalized as the insertion of two {(Ph2PNHPh)Pd} fragments into the Ln–P bonds of [Li(THF)4][(Ph2PNPh)4Ln]. To the best of our knowledge, the insertion of two metals into phosphinoamido complexes is unique. As seen for 1a,b, the substitution of [Pd2(C3H5)2Cl2] by a chlorine-free Pd(0) precursor did not result in any detectable products. In contrast to the numerous phosphinoamide-bridged bimetallic d/d- and 5f/d-metal complexes, 2a,b are the first examples of trimetallic species. The solid-state structures of 2a,b were established by single crystal X-ray diffraction (Fig. 3). Compounds 2a,b are composed of a [Li(THF)4]+ cation and a [{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] anion. In compound 2a,b, the [Li(THF)4]+ cation is disordered. As observed for 1a,b, the palladium atoms are zero-valent, but in 2a,b they are only coordinated by two {m-(Ph2PNPh)} anions and one terminal Ph2PNHPh ligand forming a distorted triangular arrangement of the ligands around the palladium atoms. The lanthanide atoms show the low coordination number of four. A distorted coordination tetrahedron, which is formed by four nitrogen atoms from the Ph2PNPh ligands, is formed around the lanthanide atom. The Ln–N bonds of av. 2.325 (2a) Å, av. 2.286 (2b) Å are in the range of 1a,b. The Pd–Ln–Pd angle of 160.67(3)1 (2a) and 160.403(12)1 (2b) shows a slight bending, which might be the result of packing effects. The metal atoms are in close proximity (Ln–Pd 3.1410(13) Å and 3.1860(12) Å (2a), 3.1063(4) Å and 3.1574(4) Å (2b)) due to the rigid ligand scaffold. Compared to compounds 1a, 1b and 1b0 , the metal-to-metal distances are significantly longer. Obviously, the reduced number of phosphinoamido ligands (two instead of three for each Ln–Pd contact), which cannot hold the metals tightly together, is the reason for the longer metal-to-metal distances. Since all phosphinoamide bridged bimetallic d/d-metal and 5f/d-metal complexes reported so far show significant metal-tometal bonds, we were interested in studying the Ln–Pd interaction in more detail. Therefore, compound 1b was theoretically investigated by quantum chemical calculations. The quantum chemical calculations were performed with TURBOMOLE V6.219 using density functional theory (D F T). The def2-TZVP basis set was used throughout.20 As a first step, the structure of 1b (the counter ion was neglected, therefore the calculated structure has to be compared to 1b0 ) was optimized with different functionals with and without the addition of dispersion corrections,21 see Table S2 in the ESI.† Good agreement was found between the calculated and measured structure for
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Fig. 3 Solid state structure of the anion of 2a. (The [Li(THF)4]+ cation is disordered.) Hydrogen atoms are omitted for clarity. Compound 2b is isostructural. Selected bond lengths [Å], angles [1]: 2a: Pd1–Y 3.1410(13), Pd1–P1 2.310(2), Pd1–P2 2.295(2), Pd1–P5 2.360(2), Pd2–Y 3.1860(12), Pd2–P3 2.303(2), Pd2–P4 2.291(2), Pd2–P6 2.334(2), Y–N1 2.327(5), Y–N2 2.329(6), Y–N3 2.314(6), Y–N4 2.333(6)1; Pd1–Y–Pd2-160.67(3), P1–Pd1–P2 134.02(7), P1–Pd1–P5 116.04(7), P2–Pd1–P5 108.57(7), P3–Pd2– P4 132.93(8), P3–Pd2–P6 115.38(8), P4–Pd2–P6 111.07(8), N1–Y–N2 140.0(2), N1–Y–N3 106.3(2), N1–Y–N4 92.8(2), N2–Y–N3 93.5(2), N2–Y–N4 93.5(2), N3– Y–N4 140.3(2). 2b: Pd1–Lu-3.1063(4), Pd1–P1 2.3025(13), Pd1–P2 2.2888(13), Pd1–P5 2.3636(14), Pd2–Lu-3.1574(4), Pd2–P3 2.2939(14), Pd2–P4 2.2813(15), Pd2–P6 2.335(2), Lu–N1 2.290(4), Lu–N2 2.285(4), Lu–N3 2.276(4), Lu–N4 2.291(4)1; P1–Pd1–P2 133.62(5), P1–Pd1–P5 116.37(5), P2–Pd1– P5 108.59(5), P3–Pd2–P4 132.38(6), P3–Pd2–P6 115.58(6), P4–Pd2–P6 111.30(6), N1–Lu–N2 141.49(15), N1–Lu–N3 106.29(15), N1–Lu–N4 92.52(2), N2–Lu–N3 93.21(2), N2–Lu–N4 92.3(2), N3–Lu–N4 141.27(15).
all functionals, with PBE022 showing the best overall agreement. For all functionals, the calculated Lu–Pd distance was slightly longer (3.011 Å for PBE0) than the experimental distance. After addition of Grimme’s D3 dispersion correction, the distance shortened to 2.985 Å, in very good agreement with the experimental value of 2.9712 Å for 1b 0 . In all further DFT calculations, the PBE0-D3 functional was used. The electronic structure was analysed by a natural population analysis.23 The natural charges for Lu and Pd are +1.55 and +0.03, respectively. The Mayer bond order24 between Lu and Pd amounts to 0.30. For further analysis, two model systems were set up (Fig. 4). In model a, the phenyl rings were substituted by hydrogen atoms and enabled an analysis of the interaction of the phenyl rings. In model b, the P–N bridge was removed. With this model, it was possible to analyse the long-range electrostatic interactions in the complex. The optimized structure of model a predicts a Lu–Pd distance of 3.103 Å. This value is on the order of the sum of the metallic radii, which highlights the stabilizing role of the phosphinoamine ligands in 1b. For each of these bridging ligands, a phenyl ring connected to P and the phenyl ring connected to N are in a similar configuration as in the T-shaped benzene dimer, which has an interaction energy of 2.66 kcal mol1 at CCSD(T) level in the basis set limit.25 We conclude that the short Lu–Pd distance in 1b is caused by the ligands and not by a metal–metal bond. In model b, the optimized Lu–Pd distance amounts to 3.15 Å. At long distances, model b dissociates into an uncharged Pd complex and a Lu complex with a charge of 1. Therefore, the
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Fig. 4 Model systems (a) without phenyl rings (b) unbridged system. Grey: Pd, black: Lu, magenta: P, green: N, red: O, small black: C, blue: Cl, white: H, dotted line in (b) indicates the orientation only. There exists no bond between P and N in model (b).
long range interaction shows R2 behaviour (R = Lu–Pd distance), which is typical for a charge–dipole interaction (see Fig. S7, ESI†). The formation and the structures of heterobimetallic Ln–Pd complexes supported by phosphinoamido ligands are significantly different from the well-established family of heterobimetallic transition metal complexes with phosphinoamido ligands. The formation of all Ln–Pd complexes is accompanied by reduction of the palladium precursor from Pd(II) to Pd(0). The 4d/4f bi- and trinuclear phosphinoamido Ln–Pd(0) complexes [(Ph2PNHPh)Pd{m-(Ph2PNPh)}3Ln(m-Cl)Li(THF)3] (Ln = Y, Lu) and [Li(THF)4][{(Ph2PNHPh)Pd}2{m-(Ph2PNPh)}4Ln] (Ln = Y, Lu) have short metal-to-metal distances. As shown by quantum chemical calculations for 1b these short distances are a result of ligand effects, rather than metal–metal bonds. This work was supported by the DFG-funded transregional collaborative research center SFB/TRR 88 ‘‘3MET’’ Project B1 and B3. A. T. Wagner is acknowledged for designing the cover image.
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