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DOI: 10.1039/C5DT00550G
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Structural, Photophysical and Magnetic Properties of Transition Metal
Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
Iuliia Nazarenko,a Flavia Pop,a Qinchao Sun,b Andreas Hauser,b Francesc Lloret,c Miguel Julve,c Abdelkrim El-Ghayoury,a and Narcis Avarvari*a a
Laboratoire MOLTECH Anjou, UMR 6200, UFR Sciences, CNRS, Université d’Angers,
Bât. K, 2 Bd. Lavoisier, 49045 Angers Cedex, France. E-mail: [email protected] b
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211
Geneva, Switzerland c
Instituto de Ciencia Molecular (ICMol)/Departament de Química Inorgànica, Universitat de
València, C/ Catedrático José Beltrán 2, 46980 Paterna, València, Spain
I.N. and F.P. contributed equally to this work.
† Electronic supplementary information (ESI) available: X-ray crystallographic file in CIF format, structural details, photophysics, additional Figures as mentioned in the text. CCDC 1045093, 1045094, 1045095, 1045096, 1045097.
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Complexes Based on the Dipicolylamino-chloro-1,2,4,5-tetrazine Ligand†
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Abstract:
The ligand
3-chloro-6-dipicolylamino-1,2,4,5-tetrazine
(Cl-TTZ-dipica) 1,
prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-
corresponding metal chlorides. The dinuclear structure of the isostructural complexes was Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
disclosed by single crystal X-ray analysis, clearly indicating the formation of [MII-(µ-Cl)2MII] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as substituent on the tetrazine ring, engage respectively in specific anion-π intramolecular and intermolecular interactions with the electron poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm-1) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm-1), the spin Hamiltonian being defined as H = -JSA.SB.
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dipica-MCl2]2, with M = Zn(II), Cd(II), Mn(II) and Co(II), when reacted with the
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Introduction Incorporation of the 1,2,4,5-tetrazine (TTZ) unit, an electron deficient heterocyle,1 in the dicarboxymethyl ester,3 or, more recently, 3,6-dibenzocarboxylates,4 allowed the preparation Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
of extended series of bimetallic complexes or polymetallic coordination networks.5 More generally, tetrazine derivatives, beside their interest as high energetic compounds,6 have been extensively studied especially in Diels-Alder cycloaddition reactions with inverse demand,7 thanks to the low lying LUMO of π type,1b which is also responsible for their facile reversible reduction to radical anions.8 This feature, combined with the luminescent properties of halogen and oxygen substituted derivatives,9 arising from a symmetry forbidden n-π* excitation, motivated their incorporation in π-conjugated systems for photovoltaic applications.10 Alternatively, from a crystal engineering point of view, the π acidity of the tetrazine ring11 favors the establishment of anion-π interactions12 in the solid state, which possibly interplay with other weak intermolecular forces such as hydrogen or halogen bonding, π-π interactions, etc., to provide original supramolecular architectures.13 In the case of the tetrazine based coordination complexes, the properties of the metal center, e.g. luminescence, magnetism, redox activity, may coexist or interplay with the ones inherent to the tetrazine unit.1b,5 The ditopic ligands mentioned above are symmetric since they are prepared through a modified Pinner type synthesis starting from the corresponding nitriles.1b For the synthesis of monotopic ligands, the use of 3,6-dichloro-1,2,4,5-tetrazine (Cl2TTZ),1b,7a as starting reagent in nucleophilic substitution reactions to provide Cl-TTZ-L (L = ligand) derivatives, seems more suitable. Moreover, the presence of the electron withdrawing Cl substituent on the TTZ ring was shown to be beneficial for the emissive properties.1b Nevertheless, to the best of our knowledge, no monotopic chelating ligands but one, containing the TTZ platform, have been used so far in metal complexes, although this is highly attractive in view of the modulation of the TTZ properties with the coordinating fragment, and vice-versa. Indeed, very recently, we have reported the synthesis of the redox active ligand tetrathiafulvalene-TTZ-2,2’-dipicolylamine (TTF-TTZ-dipica), starting from the Cl-TTZ-dipica precursor 1 (Chart 1).14 In this case, the luminescence of TTZ was quenched by the strong electron donor TTF. As the same quenching was also observed in the chloro derivative Cl-TTZ-TTF, the phenomenon is thus independent of the presence of the coordinating dipica unit and has therefore been attributed to intramolecular electron transfer
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structure of ditopic ligands, such as 3,6-dipyridyl and analogous derivatives,2 3,6-
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quenching. Nevertheless, as discussed above, the emission of TTZ derivatives is highly dependent on the substitution scheme. For example, the replacement of one chlorine substituent of Cl2-TTZ by an amine, such as in compound 1, provokes a massive decrease of amine through the conjugation of the nitrogen lone pair with the TTZ ring.1b Thus, a Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
modulation of the luminescence of TTZ by coordination of the dipica chelating unit may be envisaged,15 since the lone pair of the amino nitrogen atom is often involved in (semi)coordination with the metal center.16 We describe herein the detailed synthesis and structural characterization of the Cl-TTZ-dipica ligand 1, together with MCl2 (M = Zn, Cd, Co, Mn) complexes, with an emphasis on the anion-π interactions in the solid state. The photophysical and magnetic properties of the complexes are thoroughly discussed.
Chart 1
Results and Discussion Synthesis and solid-state structures of Cl-TTZ-dipica (1) and its metal complexes. The ligand Cl-TTZ-dipica (1) was prepared, as previously described,14 by the direct reaction of Cl2-TTZ with di(2-picolyl)-amine in methyl-t-butylether (MTBE), leading to the selective substitution of only one Cl substituent. Single crystals of 1 were obtained by slow evaporation of a dichloromethane solution. The ligand crystallizes in the monoclinic system, space group P21/n, with one independent molecule in the unit cell (Figure 1, left). The stack is formed by superposed layers of molecules with pyridyl units of adjacent molecules disposed parallel, with shortest inter-planar distances of about 3.85 Å (Figure 1, right).
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the emission intensity of TTZ as a consequence of the electron donating character of the
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Fig. 1 Molecular structure of Cl-TTZ-dipica ligand 1 together with atom numbering scheme (left) and view of the packing in the ac plane (right).
Furthermore, we envisaged the use of Cl-TTZ-dipica (1) as chelating unit towards the diamagnetic centers Zn(II) and Cd(II), in order to investigate the modulation of the luminescence properties upon coordination, as well as towards the paramagnetic centers Co(II) and Mn(II), with a focus on the magnetic properties of the complexes. Accordingly, solutions of 1 in dichloromethane were mixed with dichloride salts MCl2 (M = Zn, Cd) or MCl2*xH2O (M = Mn, Co) in acetonitrile, affording dinuclear complexes of type [Cl-TTZdipica-MCl2]2 (2a-d) (Scheme 1), as demonstrated by the solid state structures, in which the “degree” of dimerization depends on the nature of the metal (vide infra). Electrospray ionization mass spectra of the four complexes show signals corresponding to both monomeric and dimeric species minus a Cl ligand, suggesting that the chloro-bridged complexes could be already present in solution.
N MCl2 or MCl2 *xH 2o N
N
N N
1
N
M = Zn, Cd, Mn, Co
N Cl
N N Cl
N N
N N 2a, M = Zn 2b, M = Cd 2c, M = Mn 2d, M = Co
Cl M
N
N
Cl M Cl
Cl
N N N
N
Cl N N
Scheme 1 Synthesis of complexes 2a-d.
Single crystals of the complexes were obtained by slow diffusion of diethyl ether into a solution of the corresponding complex in dichloromethane/acetonitrile 1/1. The four compounds are isostructural and crystallize in the monoclinic system, centrosymmetric space group C2/m, with half of ligand and MCl2 fragment as independent motif in the unit cell (Figure 2); thus the chlorine atoms, the amino nitrogen and the metal have a site occupancy
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factor of 0.5. The above mentioned apparent dimerization is asymmetric with alternate short
Fig. 2 Independent motif in the unit cell of compounds 2a-d (left) and molecular structure of [(Cl-TTZdipica)CoCl2]2 (2d) (right) together with atom numbering scheme and M–X (Cl, N) distances.
In order to analyze the bonding properties in more detail, Figure 3 shows the first coordination sphere of the four metal ions in the solid state including not only the two strongly bonded N1 (pyridine) and the strongly bonded Cl1 and Cl2 atoms but also the N2 (amine) and Cl1’ atoms, the latter coming from the other fragment of the dimer. Despite being isostructural, the metal-chlorine bridging distances observed in the X-ray structure of the complexes (Table 1) suggest that they can be described as either effectively mononuclear (Zn), because the Zn-Cl1’ (symmetry code: (’) = 1-x, y, 1-z) distance of 3.228 Å is far too long for chemical bonding, or indeed as dichloro-bridged dinuclear species for the other metal ions with M-Cl1’ distances of less than 2.8 Å (Mn, Co) and 2.922 Å (Cd), respectively. If considered as mononuclear species the zinc center lies in a distorted tetrahedral geometry (Figure S1) formed by the two chlorine atoms Cl1 and Cl2 with Zn-Cl bond lengths of 2.282 and 2.314 Å and the two Npy atoms with Zn-Npy bond lengths of 2.060 Å (Table 1). However, it could also be regarded as five-coordinated with the participation of the Namine atom (Figure S1). The longer Zn-Namine distance of 2.697 Å in complex 2a corresponds to a semicoordination, in analogy to the one previously observed with the TTF-TTZ-dipica ligand.14 Thus, the coordination geometry of Zn(II) ion is different in this respect from most of the other ZnCl2 complexes with similar tridentate dipica ligands, which show a true fivecoordinated geometry due to the strong interaction with the Namine atom.15a,17
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and long bonds between the two metal ions and the µ2-dichloro bridge.
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Fig. 3 Distorted octahedral environment for complexes 2a-d. The Zn complex 2a shows a pronounced tendency towards penta-coordination. Symmetry codes are listed in Table 1.
Table 1 Bond lengths (Å) and angles (°) together with the trigonality parameters τ for complexes 2a-d Compound 2a, M = Zn
2b, M = Cd
2c, M = Mn
2d, M = Co
M-Cl/N (Å) Zn1 – Cl1 Zn1 – Cl2 Zn1 – N1 Zn1••••N2 Zn1••••Cl1’
2.2824(11) 2.3143(10) 2.060(2) 2.6972 3.2280
(’) = 1-x, y, 1-z (”) = x, -y, z Cd1 – Cl1 Cd1 – Cl2 Cd1 – N1 Cd1••••N2 Cd1••••Cl1’
2.4781(18) 2.5108(18) 2.262(4) 2.710 2.9218
(’) = 1-x, y, 1-z (”) = x, -y, z Mn1 – Cl1 Mn1 – Cl2 Mn1 – N1 Mn1••••N2 Mn1••••Cl1’
2.4149(12) 2.4228(11) 2.178(2) 2.6167 2.7683(11)
(’) = 1-x, y, -z (”) = x, -y, z Co1 – Cl1 Co1 – Cl2 Co1 – N1 Co1••••N2 Co1 – Cl1’
2.3415(12) 2.3564(12) 2.063(3) 2.521 2.7316(12)
(’) = -x, y, -z (”) = x, -y, z
Angles (o) N1 Zn1 N1” N1 Zn1 Cl1 N1 Zn1 Cl2 N1 Zn1 Cl1’ Cl1 Zn1 Cl1’ Cl1 Zn1 Cl2 Cl2 Zn1 Cl1’ τ N1 Cd1 N1” N1 Cd1 Cl1 N1 Cd1 Cl2 N1 Cd1 Cl1’ Cl1 Cd1 Cl1’ Cl1 Cd1 Cl2 Cl1’ Cd1 Cl2 τ N1 Mn1 N1” N1 Mn1 Cl1 N1 Mn1 Cl2 N1 Mn1 Cl1’ Cl1 Mn1 Cl1’ Cl1 Mn1 Cl2 Cl2 Mn1 Cl1’ τ N1 Co1 N1 N1 Co1 Cl1 N1 Co1 Cl2 N1 Co1 Cl1’ Cl1 Co1 Cl1’ Cl1 Co1 Cl2 Cl2 Co1 Cl1’ τ
139.61(12) 103.33(6) 100.14(6) 78.88(6) 78.02(3) 107.45(4) 173.66(4) 0.567 138.0(2) 107.33(10) 96.78(10) 81.33(10) 81.20(5) 104.44(6) 174.35(6) 0.605 142.10(13) 106.48(7) 95.61(6) 83.45(6) 81.46(4) 101.59(4) 176.95(5) 0.580 148.23(16) 103.33(7) 95.96(7) 83.69(7) 81.08(4) 100.33(5) 178.60(5) 0.506
(Cl1,N1,N1)-M (Å) 0.4301
0.3494
0.2721
0.2514
When considering the dinuclear structure, the metal atom can be regarded as either fivecoordinate by two pyridine nitrogen and three chlorine atoms or six-coordinate with the contribution of the Namine atom (Figure 3). Two of the chlorine ligands act as µ2-bridges, with
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M-Cl1 bond lengths ranging between 2.342 Å for 2d and 2.479 Å for 2b, while the M-Cl1’ distances are much longer with 2.732 Å for 2d and 2.922 Å for 2b (Table 1). For the fivecoordination the geometry around the metal is intermediate between trigonal bipyramidal,
Cl1 atom defining the apical position (Figure S2), and characterized by the trigonality Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
parameter τ in the range of 0.506 – 0.605 (Table 1). As stated above, comparatively, the Zn···Cl1’ distance (3.228 Å) is much longer than any of the other bridging M–Cl lengths in 2b-d, therefore complex 2a can be considered more mononuclear than dinuclear. Furthermore the distance between the plane defined by the chlorine and Npy atoms (Cl1, N1, N1’) and the metal center is larger for the Zn complex (0.430 Å) than for the other three (0.251 – 0.349 Å) suggesting a more distorted tetrahedral coordination geometry for the former (Table 1). In order to substantiate the above, Table 2 shows the contribution of the valence of a bond to the total valence of the metal calculated using the bond valence parameters and the distances between the metal and the different heteroatoms.18 Interestingly, the valence bond contribution of the Zn1–Cl1’ interaction amounts to only 0.037 (1.9%), which is much lower than any of the other M–Cl1’ contributions (6.47 – 8.60%), whereas the contribution of the semi-coordination of the M-Namine is of the same order for all four metal ions. Table 2 Contributions of the bond valences (ν νij) to the total metal valence (ν ν) for complexes 2a-d
dij (Å)
Compound 2a, M = Zn (II)
2b, M = Cd (II)
2c, M = Mn (II)
2d, M = Co (II)
Rij (Å)
Zn1 – Cl1 Zn1 – Cl2 Zn1••••Cl1’ Zn1 – N1 Zn1••••N2
2.2824(11) 2.3143(10) 3.2280 2.060(2) 2.6972
RZnCl = 2.01
Cd1 – Cl1 Cd1 – Cl2 Cd1••••Cl1’ Cd1 – N1 Cd1••••N2
2.4781(18) 2.5108(18) 2.9218 2.262(4) 2.710
RCdCl = 2.23
Mn1 – Cl1 Mn1 – Cl2 Mn1 – Cl1’ Mn1 – N1 Mn1••••N2
2.4149(12) 2.4228(11) 2.7683(11) 2.178(2) 2.6167
RMnCl = 2.14
Co1 – Cl1 Co1 – Cl2 Co1 – Cl1’ Co1 – N1 Co1••••N2
2.3415(12) 2.3564(12) 2.7316(12) 2.063(3) 2.521
RCoCl = 2.01
Rij = bond valence parameter; νij = valence of a bond;
RZnN = 1.77
RCdN = 1.96
RMnN = 1.87
RCoN = 1.84
ν = total valence
νij 0.478 (24.5%) 0.439 (22.5%) 0.037 (1.9%) 0.456 (23.42) (x2) 0.0816 (4.19%) ν = 1.947 0.511 (23.78%) 0.468 (21.78%) 0.154 (7.17%) 0.442 (20.57%) (x2) 0.1317 (6.13%) ν = 2.148 0.475 (22.34%) 0.465 (21.87%) 0.183 (8.60%) 0.435 (20.46%) (x2) 0.133 (6.25%) ν = 2.126 0.408 (18.59%) 0.392 (17.86%) 0.142 (6.47%) 0.547 (24.93%) (x2) 0.158 (7.20%) ν = 2.194
Dalton Transactions Accepted Manuscript
with the Cl1’ and Cl2 chlorine atoms occupying the axial sites, and square pyramidal, with the
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All the complexes are stabilized by intramolecular interactions between one of the coordinated chlorine atoms (Cl1) and ortho H-atoms of the pyridine rings, with CH···Cl 118° because of the geometrical constraints,19 and anion-π interactions between the other Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
chlorine atom (Cl2) and the TTZ ring (3.325 – 3.456 Å), that is in the usual range for this type of interactions,12 due to the π-acidity of the latter (Figure 4 for 2a and 2d and Table 3).
Fig. 4 Molecular structures of 2a (left) and 2d (right) with an emphasis on the intramolecular Cl···H–CPy
hydrogen bonding (2.964 Å and 3.034 Å) and Cl···TTZ anion-π (3.325 Å and 3.329 Å) interactions.
Table 3 Short intra- and intermolecular distances for complexes 2a-d Compound
Cl···TTZ
Cl···H
[(1)ZnCl2]2 (2a)
3.325 (intra) 3.325 (inter) 3.456 (intra) 3.531 (inter) 3.412 (intra) 3.474 (inter) 3.329 (intra) 3.381 (inter)
2.963 (intra) 2.785 (inter) 3.415 (intra) 2.747 (inter) 3.276 (intra) 2.740 (inter) 3.034 (intra) 2.734 (inter)
[(1)CdCl2]2 (2b) [(1)MnCl2]2 (2c) [(1)CoCl2]2 (2d)
The molecules further pack via TTZ-Cl···π(TTZ) contacts, reminiscent of anion-π interactions, and Cl···H(CH2-Py) hydrogen bonding along the b axis forming a 2D structure in the ac plane (Figure 5 for 2d, Figures S3 for 2a, S4-S5 for 2b, and S6-S7 for 2c). Note that PXRD measurements clearly indicate the existence of only one crystalline phase for all four complexes (see SI).
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distances ranging between 2.964 Å for 2a and 3.416 Å for 2d and C-H···Cl angles of 117-
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Fig. 5 2D network in the structure of complex 2d in the ac plane via anion-π and Cl···H interactions.
Luminescence properties of 1 and 2a-b. As mentioned above, the emission of Cl-TTZ derivatives substituted by an amino group, as in ligand 1, is generally extremely weak because of the electron donating character of the amine through conjugation with the TTZ ring.1b By coordination with diamagnetic centers such as Zn (2a) and Cd (2b) the structure of the ligand is rigidified concomitantly with the involvement of the amino nitrogen lone pair in semicoordination with the metal, and thus switch-on of the emission can be expected. The absorption spectrum of Cl-TTZ-dipica (1) measured in acetonitrile shows two absorption bands in the UV-vis region: one centered at around 410 nm (24390 cm-1) with an extinction coefficient ε ≈ 1000 M-1 cm-1, and the other one at around 515 nm (19420 cm-1) with an extinction coefficient ε ≈ 500 M-1 cm-1 (Figure 6). The former corresponds to a π-π* transition essentially of the tetrazine unit but with a strong contribution due to the conjugation by the amine nitrogen of the dipica fragment,9b while the latter corresponds to a n-π* transition centered on the tetrazine unit.14 The emission of the free ligand 1 in the same acetonitrile solution is, as expected, extremely weak (quantum yield less than 0.01%), but real as confirmed by the corresponding excitation spectrum (Figure 6, Table 4).
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Fig. 6 Absorption (light blue), emission (red) and excitation (purple) spectra of ligand Cl-TTZ-dipica (1) in
acetonitrile (c = 10-4 M. The emission spectrum was recorded with an excitation wavelength of λex = 400 nm, and the excitation spectrum was recorded with an emission wavelength λem = 580 nm.
Table 4 Absorption and emission properties of the ligand 1 and complexes 2a and 2b, measured at room
temperature in deoxygenated acetonitrile
a
Sample
Absorption/nm
Emission/nm
Quantum yielda (%)
Ligand 1
410/515
575
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This article can be cited before page numbers have been issued, to do this please use: I. Nazarenko, F. Pop, Q. Sun, A. Hauser, F. Lloret, M. Julve, A. El-Ghayoury and N. Avarvari, Dalton Trans., 2015, DOI:
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/dalton
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Structural, Photophysical and Magnetic Properties of Transition Metal
Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
Iuliia Nazarenko,a Flavia Pop,a Qinchao Sun,b Andreas Hauser,b Francesc Lloret,c Miguel Julve,c Abdelkrim El-Ghayoury,a and Narcis Avarvari*a a
Laboratoire MOLTECH Anjou, UMR 6200, UFR Sciences, CNRS, Université d’Angers,
Bât. K, 2 Bd. Lavoisier, 49045 Angers Cedex, France. E-mail: [email protected] b
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211
Geneva, Switzerland c
Instituto de Ciencia Molecular (ICMol)/Departament de Química Inorgànica, Universitat de
València, C/ Catedrático José Beltrán 2, 46980 Paterna, València, Spain
I.N. and F.P. contributed equally to this work.
† Electronic supplementary information (ESI) available: X-ray crystallographic file in CIF format, structural details, photophysics, additional Figures as mentioned in the text. CCDC 1045093, 1045094, 1045095, 1045096, 1045097.
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Complexes Based on the Dipicolylamino-chloro-1,2,4,5-tetrazine Ligand†
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Abstract:
The ligand
3-chloro-6-dipicolylamino-1,2,4,5-tetrazine
(Cl-TTZ-dipica) 1,
prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-
corresponding metal chlorides. The dinuclear structure of the isostructural complexes was Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
disclosed by single crystal X-ray analysis, clearly indicating the formation of [MII-(µ-Cl)2MII] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as substituent on the tetrazine ring, engage respectively in specific anion-π intramolecular and intermolecular interactions with the electron poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm-1) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm-1), the spin Hamiltonian being defined as H = -JSA.SB.
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dipica-MCl2]2, with M = Zn(II), Cd(II), Mn(II) and Co(II), when reacted with the
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Introduction Incorporation of the 1,2,4,5-tetrazine (TTZ) unit, an electron deficient heterocyle,1 in the dicarboxymethyl ester,3 or, more recently, 3,6-dibenzocarboxylates,4 allowed the preparation Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
of extended series of bimetallic complexes or polymetallic coordination networks.5 More generally, tetrazine derivatives, beside their interest as high energetic compounds,6 have been extensively studied especially in Diels-Alder cycloaddition reactions with inverse demand,7 thanks to the low lying LUMO of π type,1b which is also responsible for their facile reversible reduction to radical anions.8 This feature, combined with the luminescent properties of halogen and oxygen substituted derivatives,9 arising from a symmetry forbidden n-π* excitation, motivated their incorporation in π-conjugated systems for photovoltaic applications.10 Alternatively, from a crystal engineering point of view, the π acidity of the tetrazine ring11 favors the establishment of anion-π interactions12 in the solid state, which possibly interplay with other weak intermolecular forces such as hydrogen or halogen bonding, π-π interactions, etc., to provide original supramolecular architectures.13 In the case of the tetrazine based coordination complexes, the properties of the metal center, e.g. luminescence, magnetism, redox activity, may coexist or interplay with the ones inherent to the tetrazine unit.1b,5 The ditopic ligands mentioned above are symmetric since they are prepared through a modified Pinner type synthesis starting from the corresponding nitriles.1b For the synthesis of monotopic ligands, the use of 3,6-dichloro-1,2,4,5-tetrazine (Cl2TTZ),1b,7a as starting reagent in nucleophilic substitution reactions to provide Cl-TTZ-L (L = ligand) derivatives, seems more suitable. Moreover, the presence of the electron withdrawing Cl substituent on the TTZ ring was shown to be beneficial for the emissive properties.1b Nevertheless, to the best of our knowledge, no monotopic chelating ligands but one, containing the TTZ platform, have been used so far in metal complexes, although this is highly attractive in view of the modulation of the TTZ properties with the coordinating fragment, and vice-versa. Indeed, very recently, we have reported the synthesis of the redox active ligand tetrathiafulvalene-TTZ-2,2’-dipicolylamine (TTF-TTZ-dipica), starting from the Cl-TTZ-dipica precursor 1 (Chart 1).14 In this case, the luminescence of TTZ was quenched by the strong electron donor TTF. As the same quenching was also observed in the chloro derivative Cl-TTZ-TTF, the phenomenon is thus independent of the presence of the coordinating dipica unit and has therefore been attributed to intramolecular electron transfer
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structure of ditopic ligands, such as 3,6-dipyridyl and analogous derivatives,2 3,6-
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quenching. Nevertheless, as discussed above, the emission of TTZ derivatives is highly dependent on the substitution scheme. For example, the replacement of one chlorine substituent of Cl2-TTZ by an amine, such as in compound 1, provokes a massive decrease of amine through the conjugation of the nitrogen lone pair with the TTZ ring.1b Thus, a Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
modulation of the luminescence of TTZ by coordination of the dipica chelating unit may be envisaged,15 since the lone pair of the amino nitrogen atom is often involved in (semi)coordination with the metal center.16 We describe herein the detailed synthesis and structural characterization of the Cl-TTZ-dipica ligand 1, together with MCl2 (M = Zn, Cd, Co, Mn) complexes, with an emphasis on the anion-π interactions in the solid state. The photophysical and magnetic properties of the complexes are thoroughly discussed.
Chart 1
Results and Discussion Synthesis and solid-state structures of Cl-TTZ-dipica (1) and its metal complexes. The ligand Cl-TTZ-dipica (1) was prepared, as previously described,14 by the direct reaction of Cl2-TTZ with di(2-picolyl)-amine in methyl-t-butylether (MTBE), leading to the selective substitution of only one Cl substituent. Single crystals of 1 were obtained by slow evaporation of a dichloromethane solution. The ligand crystallizes in the monoclinic system, space group P21/n, with one independent molecule in the unit cell (Figure 1, left). The stack is formed by superposed layers of molecules with pyridyl units of adjacent molecules disposed parallel, with shortest inter-planar distances of about 3.85 Å (Figure 1, right).
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the emission intensity of TTZ as a consequence of the electron donating character of the
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Fig. 1 Molecular structure of Cl-TTZ-dipica ligand 1 together with atom numbering scheme (left) and view of the packing in the ac plane (right).
Furthermore, we envisaged the use of Cl-TTZ-dipica (1) as chelating unit towards the diamagnetic centers Zn(II) and Cd(II), in order to investigate the modulation of the luminescence properties upon coordination, as well as towards the paramagnetic centers Co(II) and Mn(II), with a focus on the magnetic properties of the complexes. Accordingly, solutions of 1 in dichloromethane were mixed with dichloride salts MCl2 (M = Zn, Cd) or MCl2*xH2O (M = Mn, Co) in acetonitrile, affording dinuclear complexes of type [Cl-TTZdipica-MCl2]2 (2a-d) (Scheme 1), as demonstrated by the solid state structures, in which the “degree” of dimerization depends on the nature of the metal (vide infra). Electrospray ionization mass spectra of the four complexes show signals corresponding to both monomeric and dimeric species minus a Cl ligand, suggesting that the chloro-bridged complexes could be already present in solution.
N MCl2 or MCl2 *xH 2o N
N
N N
1
N
M = Zn, Cd, Mn, Co
N Cl
N N Cl
N N
N N 2a, M = Zn 2b, M = Cd 2c, M = Mn 2d, M = Co
Cl M
N
N
Cl M Cl
Cl
N N N
N
Cl N N
Scheme 1 Synthesis of complexes 2a-d.
Single crystals of the complexes were obtained by slow diffusion of diethyl ether into a solution of the corresponding complex in dichloromethane/acetonitrile 1/1. The four compounds are isostructural and crystallize in the monoclinic system, centrosymmetric space group C2/m, with half of ligand and MCl2 fragment as independent motif in the unit cell (Figure 2); thus the chlorine atoms, the amino nitrogen and the metal have a site occupancy
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factor of 0.5. The above mentioned apparent dimerization is asymmetric with alternate short
Fig. 2 Independent motif in the unit cell of compounds 2a-d (left) and molecular structure of [(Cl-TTZdipica)CoCl2]2 (2d) (right) together with atom numbering scheme and M–X (Cl, N) distances.
In order to analyze the bonding properties in more detail, Figure 3 shows the first coordination sphere of the four metal ions in the solid state including not only the two strongly bonded N1 (pyridine) and the strongly bonded Cl1 and Cl2 atoms but also the N2 (amine) and Cl1’ atoms, the latter coming from the other fragment of the dimer. Despite being isostructural, the metal-chlorine bridging distances observed in the X-ray structure of the complexes (Table 1) suggest that they can be described as either effectively mononuclear (Zn), because the Zn-Cl1’ (symmetry code: (’) = 1-x, y, 1-z) distance of 3.228 Å is far too long for chemical bonding, or indeed as dichloro-bridged dinuclear species for the other metal ions with M-Cl1’ distances of less than 2.8 Å (Mn, Co) and 2.922 Å (Cd), respectively. If considered as mononuclear species the zinc center lies in a distorted tetrahedral geometry (Figure S1) formed by the two chlorine atoms Cl1 and Cl2 with Zn-Cl bond lengths of 2.282 and 2.314 Å and the two Npy atoms with Zn-Npy bond lengths of 2.060 Å (Table 1). However, it could also be regarded as five-coordinated with the participation of the Namine atom (Figure S1). The longer Zn-Namine distance of 2.697 Å in complex 2a corresponds to a semicoordination, in analogy to the one previously observed with the TTF-TTZ-dipica ligand.14 Thus, the coordination geometry of Zn(II) ion is different in this respect from most of the other ZnCl2 complexes with similar tridentate dipica ligands, which show a true fivecoordinated geometry due to the strong interaction with the Namine atom.15a,17
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and long bonds between the two metal ions and the µ2-dichloro bridge.
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Fig. 3 Distorted octahedral environment for complexes 2a-d. The Zn complex 2a shows a pronounced tendency towards penta-coordination. Symmetry codes are listed in Table 1.
Table 1 Bond lengths (Å) and angles (°) together with the trigonality parameters τ for complexes 2a-d Compound 2a, M = Zn
2b, M = Cd
2c, M = Mn
2d, M = Co
M-Cl/N (Å) Zn1 – Cl1 Zn1 – Cl2 Zn1 – N1 Zn1••••N2 Zn1••••Cl1’
2.2824(11) 2.3143(10) 2.060(2) 2.6972 3.2280
(’) = 1-x, y, 1-z (”) = x, -y, z Cd1 – Cl1 Cd1 – Cl2 Cd1 – N1 Cd1••••N2 Cd1••••Cl1’
2.4781(18) 2.5108(18) 2.262(4) 2.710 2.9218
(’) = 1-x, y, 1-z (”) = x, -y, z Mn1 – Cl1 Mn1 – Cl2 Mn1 – N1 Mn1••••N2 Mn1••••Cl1’
2.4149(12) 2.4228(11) 2.178(2) 2.6167 2.7683(11)
(’) = 1-x, y, -z (”) = x, -y, z Co1 – Cl1 Co1 – Cl2 Co1 – N1 Co1••••N2 Co1 – Cl1’
2.3415(12) 2.3564(12) 2.063(3) 2.521 2.7316(12)
(’) = -x, y, -z (”) = x, -y, z
Angles (o) N1 Zn1 N1” N1 Zn1 Cl1 N1 Zn1 Cl2 N1 Zn1 Cl1’ Cl1 Zn1 Cl1’ Cl1 Zn1 Cl2 Cl2 Zn1 Cl1’ τ N1 Cd1 N1” N1 Cd1 Cl1 N1 Cd1 Cl2 N1 Cd1 Cl1’ Cl1 Cd1 Cl1’ Cl1 Cd1 Cl2 Cl1’ Cd1 Cl2 τ N1 Mn1 N1” N1 Mn1 Cl1 N1 Mn1 Cl2 N1 Mn1 Cl1’ Cl1 Mn1 Cl1’ Cl1 Mn1 Cl2 Cl2 Mn1 Cl1’ τ N1 Co1 N1 N1 Co1 Cl1 N1 Co1 Cl2 N1 Co1 Cl1’ Cl1 Co1 Cl1’ Cl1 Co1 Cl2 Cl2 Co1 Cl1’ τ
139.61(12) 103.33(6) 100.14(6) 78.88(6) 78.02(3) 107.45(4) 173.66(4) 0.567 138.0(2) 107.33(10) 96.78(10) 81.33(10) 81.20(5) 104.44(6) 174.35(6) 0.605 142.10(13) 106.48(7) 95.61(6) 83.45(6) 81.46(4) 101.59(4) 176.95(5) 0.580 148.23(16) 103.33(7) 95.96(7) 83.69(7) 81.08(4) 100.33(5) 178.60(5) 0.506
(Cl1,N1,N1)-M (Å) 0.4301
0.3494
0.2721
0.2514
When considering the dinuclear structure, the metal atom can be regarded as either fivecoordinate by two pyridine nitrogen and three chlorine atoms or six-coordinate with the contribution of the Namine atom (Figure 3). Two of the chlorine ligands act as µ2-bridges, with
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M-Cl1 bond lengths ranging between 2.342 Å for 2d and 2.479 Å for 2b, while the M-Cl1’ distances are much longer with 2.732 Å for 2d and 2.922 Å for 2b (Table 1). For the fivecoordination the geometry around the metal is intermediate between trigonal bipyramidal,
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parameter τ in the range of 0.506 – 0.605 (Table 1). As stated above, comparatively, the Zn···Cl1’ distance (3.228 Å) is much longer than any of the other bridging M–Cl lengths in 2b-d, therefore complex 2a can be considered more mononuclear than dinuclear. Furthermore the distance between the plane defined by the chlorine and Npy atoms (Cl1, N1, N1’) and the metal center is larger for the Zn complex (0.430 Å) than for the other three (0.251 – 0.349 Å) suggesting a more distorted tetrahedral coordination geometry for the former (Table 1). In order to substantiate the above, Table 2 shows the contribution of the valence of a bond to the total valence of the metal calculated using the bond valence parameters and the distances between the metal and the different heteroatoms.18 Interestingly, the valence bond contribution of the Zn1–Cl1’ interaction amounts to only 0.037 (1.9%), which is much lower than any of the other M–Cl1’ contributions (6.47 – 8.60%), whereas the contribution of the semi-coordination of the M-Namine is of the same order for all four metal ions. Table 2 Contributions of the bond valences (ν νij) to the total metal valence (ν ν) for complexes 2a-d
dij (Å)
Compound 2a, M = Zn (II)
2b, M = Cd (II)
2c, M = Mn (II)
2d, M = Co (II)
Rij (Å)
Zn1 – Cl1 Zn1 – Cl2 Zn1••••Cl1’ Zn1 – N1 Zn1••••N2
2.2824(11) 2.3143(10) 3.2280 2.060(2) 2.6972
RZnCl = 2.01
Cd1 – Cl1 Cd1 – Cl2 Cd1••••Cl1’ Cd1 – N1 Cd1••••N2
2.4781(18) 2.5108(18) 2.9218 2.262(4) 2.710
RCdCl = 2.23
Mn1 – Cl1 Mn1 – Cl2 Mn1 – Cl1’ Mn1 – N1 Mn1••••N2
2.4149(12) 2.4228(11) 2.7683(11) 2.178(2) 2.6167
RMnCl = 2.14
Co1 – Cl1 Co1 – Cl2 Co1 – Cl1’ Co1 – N1 Co1••••N2
2.3415(12) 2.3564(12) 2.7316(12) 2.063(3) 2.521
RCoCl = 2.01
Rij = bond valence parameter; νij = valence of a bond;
RZnN = 1.77
RCdN = 1.96
RMnN = 1.87
RCoN = 1.84
ν = total valence
νij 0.478 (24.5%) 0.439 (22.5%) 0.037 (1.9%) 0.456 (23.42) (x2) 0.0816 (4.19%) ν = 1.947 0.511 (23.78%) 0.468 (21.78%) 0.154 (7.17%) 0.442 (20.57%) (x2) 0.1317 (6.13%) ν = 2.148 0.475 (22.34%) 0.465 (21.87%) 0.183 (8.60%) 0.435 (20.46%) (x2) 0.133 (6.25%) ν = 2.126 0.408 (18.59%) 0.392 (17.86%) 0.142 (6.47%) 0.547 (24.93%) (x2) 0.158 (7.20%) ν = 2.194
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with the Cl1’ and Cl2 chlorine atoms occupying the axial sites, and square pyramidal, with the
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All the complexes are stabilized by intramolecular interactions between one of the coordinated chlorine atoms (Cl1) and ortho H-atoms of the pyridine rings, with CH···Cl 118° because of the geometrical constraints,19 and anion-π interactions between the other Published on 02 April 2015. Downloaded by Seton Hall University on 03/04/2015 02:11:53.
chlorine atom (Cl2) and the TTZ ring (3.325 – 3.456 Å), that is in the usual range for this type of interactions,12 due to the π-acidity of the latter (Figure 4 for 2a and 2d and Table 3).
Fig. 4 Molecular structures of 2a (left) and 2d (right) with an emphasis on the intramolecular Cl···H–CPy
hydrogen bonding (2.964 Å and 3.034 Å) and Cl···TTZ anion-π (3.325 Å and 3.329 Å) interactions.
Table 3 Short intra- and intermolecular distances for complexes 2a-d Compound
Cl···TTZ
Cl···H
[(1)ZnCl2]2 (2a)
3.325 (intra) 3.325 (inter) 3.456 (intra) 3.531 (inter) 3.412 (intra) 3.474 (inter) 3.329 (intra) 3.381 (inter)
2.963 (intra) 2.785 (inter) 3.415 (intra) 2.747 (inter) 3.276 (intra) 2.740 (inter) 3.034 (intra) 2.734 (inter)
[(1)CdCl2]2 (2b) [(1)MnCl2]2 (2c) [(1)CoCl2]2 (2d)
The molecules further pack via TTZ-Cl···π(TTZ) contacts, reminiscent of anion-π interactions, and Cl···H(CH2-Py) hydrogen bonding along the b axis forming a 2D structure in the ac plane (Figure 5 for 2d, Figures S3 for 2a, S4-S5 for 2b, and S6-S7 for 2c). Note that PXRD measurements clearly indicate the existence of only one crystalline phase for all four complexes (see SI).
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distances ranging between 2.964 Å for 2a and 3.416 Å for 2d and C-H···Cl angles of 117-
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Fig. 5 2D network in the structure of complex 2d in the ac plane via anion-π and Cl···H interactions.
Luminescence properties of 1 and 2a-b. As mentioned above, the emission of Cl-TTZ derivatives substituted by an amino group, as in ligand 1, is generally extremely weak because of the electron donating character of the amine through conjugation with the TTZ ring.1b By coordination with diamagnetic centers such as Zn (2a) and Cd (2b) the structure of the ligand is rigidified concomitantly with the involvement of the amino nitrogen lone pair in semicoordination with the metal, and thus switch-on of the emission can be expected. The absorption spectrum of Cl-TTZ-dipica (1) measured in acetonitrile shows two absorption bands in the UV-vis region: one centered at around 410 nm (24390 cm-1) with an extinction coefficient ε ≈ 1000 M-1 cm-1, and the other one at around 515 nm (19420 cm-1) with an extinction coefficient ε ≈ 500 M-1 cm-1 (Figure 6). The former corresponds to a π-π* transition essentially of the tetrazine unit but with a strong contribution due to the conjugation by the amine nitrogen of the dipica fragment,9b while the latter corresponds to a n-π* transition centered on the tetrazine unit.14 The emission of the free ligand 1 in the same acetonitrile solution is, as expected, extremely weak (quantum yield less than 0.01%), but real as confirmed by the corresponding excitation spectrum (Figure 6, Table 4).
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Fig. 6 Absorption (light blue), emission (red) and excitation (purple) spectra of ligand Cl-TTZ-dipica (1) in
acetonitrile (c = 10-4 M. The emission spectrum was recorded with an excitation wavelength of λex = 400 nm, and the excitation spectrum was recorded with an emission wavelength λem = 580 nm.
Table 4 Absorption and emission properties of the ligand 1 and complexes 2a and 2b, measured at room
temperature in deoxygenated acetonitrile
a
Sample
Absorption/nm
Emission/nm
Quantum yielda (%)
Ligand 1
410/515
575
