Diversities of Coordination Geometry at Cu2+ Center in the Bis

Crystal Growth & Design , 2006, 6 (11), pp 2530–2537 ... A highly distorted square-planar coordination geometry at the Cu2+center was observed in tw...
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CRYSTAL GROWTH & DESIGN

Diversities of Coordination Geometry at Cu2+ Center in the Bis(maleonitriledithiolato)cuprate Complexes: Syntheses, Magnetic Properties, X-ray Crystal Structural Analyses, and DFT Calculations

2006 VOL. 6, NO. 11 2530-2537

X. M. Ren,*,†,‡ Z. P. Ni,§ S. Noro,† T. Akutagawa,† S. Nishihara,⊥ T. Nakamura,*,† Y. X. Sui,# and Y. Song§ Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 060-0812, Japan, CREST, Japan Science and Technology Corporation (JST), Kawaguchi 332-0012, Japan, College of Science, Nanjing UniVersity of Technology, Nanjing 210009, P. R. China, Coordination Chemistry Institute & State Key Lab, Nanjing UniVersity, Nanjing 210093, P. R. China, Department of Physical Science, Graduate School of Science, Osaka Prefecture UniVersity, Sakai, Osaka 599-8531, Japan, and Center of Modern Analysis, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed May 20, 2006; ReVised Manuscript ReceiVed August 11, 2006

ABSTRACT: Four new ion-pair complexes, consisting of bis(maleonitriledithiolato)cuprate dianion with the derivatives of benzylpyridinium (1-3) or monoprotonated 1,4-diazabicyclo-[2.2.2]octane (4), have been prepared and characterized structurally and magnetically, and a diversity of the coordination geometry at Cu2+ center of [Cu(mnt)2]2- was observed. Highly distorted square-planar geometry at the tetracoordination Cu2+ center of [Cu(mnt)2]2- was found in 1 and 4. Two chelate rings make a dihedral angle of 27.2° in 1 and 20.5, 34.3, and 19.7° for three crystallographically independent [Cu(mnt)2]2- molecules in 4. Interestingly, the distortion is not ascribed to steric repulsion between ligands or weakly coordinating interaction from the nearest anions, countercations, or solvent molecules but to the weak supramolecular interactions between anion and cations [such as S‚‚‚H, N‚‚‚H, or S‚‚‚I]. In 2, the Cu2+ center in [Cu(mnt)2]2- possesses approximate square-planar coordination geometry, and the dihedral angle between two chelate rings is 5.8°. In 3, the Cu2+ ion of [Cu(mnt)2]2- lies on a symmetric center (2/m), and the anion has C2h symmetry. The coordination S atoms and central Cu2+ ion of [Cu(mnt)2]2- are strictly coplanar with perfect square-planar coordination geometry because of the symmetric constraint. However, the anion is nonplanar because the ligand fragment is bent away from the CuS4 plane with a dihedral angle of 13.8°. X-band EPR spectra at 293 and 110 K failed to give any available information about the electronic structural difference between the anions of square-planar and distorted square-planar geometries because no hyperfine splitting was detected. Density functional theory (DFT) has been implemented to calculate the single-point energy for each [Cu(mnt)2]2in 1-4 and optimize the molecular geometry of [Cu(mnt)2]2-. On the basis of these theoretic analyses, the reasons for the structural fluctuations between square-planar and distorted square-planar coordination geometry were explored. Introduction Much effort has been devoted to the study of bis(1,2dithiolato) transition metal complexes in the areas of conducting and magnetic materials, dyes, nonlinear optics, and catalysis.1,2 For the transition metal ion with an open-shell electronic structure, it is common for the metal ion to possess a planar coordination environment; however, the distorted square-planar coordination geometry around the metal center has also been observed in bis(1,2-dithiolato) transition metal complexes.3,4 In the cases of the metal ions being Mn, Fe, and Co (for example, in the complexes [RBzPy]2[Fe2(mnt)4],5 [Bu4N]2[Fe2(mnt)4],6 [Bu4N]2[Fe2(edt)4],7 [Et4N]2[Mn2(edt)4],8 Co2[S2C2(CF3)2],9 and [Bu4N]2[Co2(S2C6Cl4)4]10), the deviation from planarity is caused by dimerization via M‚‚‚S interaction. Surprisingly, for the bis(1,2-dithiolato)cuprate complex, in the few cases in which there is a significantly nonplanar environment around a tetracoordinate Cu2+ center, it is a result of the intermolecular interactions between S atoms of the dithiolate ligands and the metal ion in the neighboring counterions.11 Mostly, the causes that result in the coordination geometry distorting at a Cu2+ center of * To whom correspondence should be addressed. Tel: 81 11-706-2849. Fax: 81 11-706-4972. E-mail: [email protected] (X.M.R.); tnaka@ imd.es.hokudai.ac.jp (T.N.). † Hokkaido University and Japan Science and Technology Corporation. ‡ Nanjing University of Technology. § Coordination Chemistry Institute & State Key Lab, Nanjing University. ⊥ Osaka Prefecture University. # Center of Modern Analysis, Nanjing University.

Chart 1

Chart 2

[Cu(dithiolato)2]2- are not ascribed to M‚‚‚S interaction between the neighboring molecules but to the weak supramolecular interactions between the cation and anion through S‚‚‚H or N‚‚‚H interactions.12-15 These observations lead to some interesting questions, for example, whether the configuration energy gap between the distorted square-planar and squareplanar coordination geometries in a bis(1,2-dithiolato)cuprate dianion is comparable to the weak intermolecular packing interaction (such as H-bonding or van der Waals force), so that a tunable system for controlling the coordination geometry of [Cu(mnt)2]2- can later be designed. In this paper, we present four new complexes based on [Cu(mnt)2]2- (their cations are schematically illustrated in Chart

10.1021/cg0602966 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006

Coordination Geometries in Bis(maleonitriledithiolato)cuprates

Crystal Growth & Design, Vol. 6, No. 11, 2006 2531

Table 1. Crystallographic Data for 1-5

T (K) mol. formula CCDC no. mol. mass space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z µ (mm-1) F (g cm-3) R1b wR2b a

1

2

3

4

5a,17

293 C32H22Cl2N6S4Cu CCDC-282932 753.24 P1h 11.309(2) 13.025(3) 13.279(3) 113.67(3) 95.36(3) 105.41(3) 1682.2(6) 2 1.089 1.487 0.0314 0.0870

293 C32H22I2N6S4Cu CCDC-282933 936.14 P1h 11.175(2) 12.045(2) 15.013(3) 86.38(3) 70.02(3) 65.19(3) 1716.2(6) 2 2.711 1.812 0.0310 0.0686

293 C32H22F2N6S4Cu CCDC-282934 720.34 C2/m 13.846(7) 12.157(5) 10.303(3) 90 106.90(2) 90 1659.4(12) 2 0.953 1.442 0.0270 0.0713

223 C24H32N10S4Cu CCDC-282935 652.38 P1h 12.704(3) 17.768(4) 21.609(4) 108.07(3) 96.90(3) 95.44(3) 4558.5(16) 6 1.027 1.426 0.0529 0.1209

293 C32H22N8O4S4Cu CCDC-169994 774.36 P1 6.9660(14) 10.574(2) 12.026(2) 72.84(3) 85.48(3) 78.49(3) 829.2(3) 1 0.962 1.551 0.0309 0.0815

Complex 5 is reported in the literature.17

b

R1 )∑(||F0| - |Fc||)/∑|F0|, wR2 )∑w(|F0|2 - |Fc|2)2/∑w(|F0|2)2]1/2.

1 for 1-3 and Chart 2 for 4), in which a diversity of coordination geometry at the Cu2+ center of [Cu(mnt)2]2-, square-planar and distorted square-planar, was observed. In order to get more insight into such structural fluctuation between square-planar and distorted square-planar coordination geometry, we calculated the single-point energy for [Cu(mnt)2]2- in each complex and performed an unrestricted optimization for the [Cu(mnt)2]2- molecular geometry in the framework of density functional theory (DFT). Experimental Section Preparations. Na2mnt16 and [1-(4′-chlorobenzyl)pyridinium]2[Cu(mnt)2] (1), [1-(4′-iodobenzyl)pyridinium]2[Cu(mnt)2] (2), and [1-(4′-fluorobenzyl)pyridinium]2[Cu(mnt)2] (3) were prepared in accordance with published procedures.17 Elemental anal. Calcd for C32H22Cl2N10S4Cu (1): C, 51.0; H, 2.94; N, 11.2. Found: C, 50.9; H, 2.98; N, 10.8. Calcd for C32H22I2N10S4Cu (2): C, 41.1; H, 2.37; N, 8.98. Found: C, 41.2; H, 2.39; N, 8.95. Calcd for C32H22F2N10S4Cu (3): C, 53.4; H, 3.08; N, 11.7. Found: C, 53.2; H, 3.11; N, 11.8. [HDABCO]2[Cu(mnt)2]‚2MeCN (4). Na2mnt, CuCl2‚2H2O, and monoprotonated 1,4-diazabicyclo-[2.2.2]octane tetrafluoroboric ([HDABCO]BF4) were mixed in the molar ratio of 2:1:2 in a methanol solution and stirred at room temperature. The shiny crystalline product appeared about 10 min later. This mixture stood for ∼12 and was then collected by filtration and washed with methanol. Crystals of [HDABCO]2[Cu(mnt)2]‚2MeCN (4) suitable for X-ray structure analysis were obtained by dispersing diethyl ether into an acetonitrile solution of 4. Elemental anal. for [HDABCO]2[Cu(mnt)2]:18 Calcd for C20H26N8S4Cu: C, 42.1; H, 4.60; N, 19.7. Found: C, 41.6; H, 4.91; N, 19.3. EPR Spectra and Magnetic Susceptibility. X-band EPR spectra were recorded on a Bruker EMX spectrometer near 9 GHz on polycrystalline samples at 293 and 110 K. The measurement of temperature dependence of magnetic susceptibility for the polycrystalline sample of each complex was carried out using a Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer at 2-350 K under 1.0 T. X-ray Single-Crystal Structure. Crystallographic data were collected using a Rigaku Raxis-Rapid diffractometer with Mo KR (λ ) 0.71073 Å) radiation from a graphite monochromator. Structures were solved by direct methods using the SHELXL-97 software package;19 The non-H atoms were refined anisotropically using the full-matrix least-squares method on F2. All H atoms were placed at calculated positions (C-H ) 0.930 Å for benzene or pyridine rings and 0.970 Å for methylene) and refined riding on the parent atoms with U(H) ) 1.2 × Ueq (bonded C or N atom). Details of the crystal parameters, data collection, and refinement are summarized in Table 1. The bond lengths in a [Cu(mnt)2]2- with their estimated standard deviations are listed in Table S1 of the Supporting Information. Details of DFT Calculations. All density functional theory (DFT) calculations were carried out utilizing the GAUSSIAN 98 program20

on the SGI 3800 workstation. The whole nonmodelized molecular structure of the real complexes 1-4 as well as 5, [NO2BzPy]2[Cu(mnt)2] (which was reported earlier; its Cu2+ centre in [Cu(mnt)2]2possesses an almost perfect square-planar coordination geometry),17 were taken directly from X-ray crystallography complete structures. The single-point energy calculations for [Cu(mnt)2]2- in each complex were performed at the UB3LYP/LANL2DZ level, and the convergence criterion of SCF was 1 × 10-8. The restricted geometric optimization for [Cu(mnt)2]2- was performed by using the opt ) modredundant keyword, and the hybrid functional UB3LYP with the LANL2DZ basis set for the Cu atom and 6-31G* basis set for C, N, and S atoms were employed. Corresponding to increasing distorted values, the different imposed dihedral angles, which are defined as those between the planes of S(1)-Cu(1)-S(2) and S(3)-Cu(1)-S(4), range between 0 and 90° with steps of 10°, see the inset in Figure 7. For each fixed dihedral angle value, all other structural parameters were fully optimized without any symmetric constraints, and the convergence criterion of SCF was 1 × 10-4.

Results and Discussion Crystal Structure. The asymmetric unit within a cell of 1 is illustrated in Figure 1 that is comprised of one [Cu(mnt)2]2and two ClBzPy+. For [Cu(mnt)2]2-, the bond lengths of Cu-S are all different, even if they are expected to be chemically equivalent, and they range from 2.2601(8) to 2.2779(8) Å; the bond angles in a chelate ring of S-Cu-S are 91.96(3) and 91.43(3)°. These values are comparable with other reported [Cu(mnt)2]2- complexes.12,13,17 To simplify the description, we represent two chemically equivalent but crystallographic distinct [ClBzPy]+ cations as units A (containing Cl(1)) and B (containing Cl(2)), respectively. Within the A and B units, both bond lengths and angles are almost identical, but the molecular conformations are different. Pyridine and benzene rings make a dihedral angle with the reference plane CAr-CH2-NPy of 90.9 and 72.6° in A versus 126.5 and 83.1° in B, and the dihedral angle between pyridine and benzene rings is 66.5° in A and 73.7° in B. The striking structural feature of 1 is a highly twisted coordination environment around the tetracoordinate Cu2+ center in which the Cu2+ ion deviates from each mean-molecular-plane of two mnt2- ligands by 0.167 and 0.176 Å, respectively. The angles of S(1)-Cu(1)-S(4) and S(2)-Cu(1)-S(3) (cf. Figure 1) are 159.83(3) and 162.88(3)°, much less than the linear 180°, and the dihedral angle between two chelate rings is 27.2°. It is comparatively rare that the tetracoordinate Cu2+ ion possesses a pronounced nonplanar coordination geometry,21 and distortion is generally caused by either the steric repulsions between ligands22 or weakly coordinating interaction from the counterions or solvent molecules.23 However, the above-mentioned pos-

2532 Crystal Growth & Design, Vol. 6, No. 11, 2006

Ren et al.

Table 2. Interatomic Contacts (Å) (less than the sum of the van der Waals radii) and Angles of D-H‚‚‚A (deg) 1a atomic pair S‚‚‚X

AB S(1)‚‚‚H(13A)#1 ) 2.838 (∠C(13)#1-H(13A)#1‚‚‚S(1) ) 140.6°) S(1)‚‚‚H(26B)#2 ) 2.849 (∠C(26)#2-H(26B)#2‚‚‚S(1) ) 162.9°)

2b

3c

AB S(2)‚‚‚I(2)#1 ) 3.747

4d

AB

AB

N(1)‚‚‚H(5A)#1 ) 2.382 (∠C(5)#1-H(5A)#1‚‚‚N(1) ) 143.3°)

N(7)‚‚‚H(38B)#1 ) 2.521 (∠C(38)#1-H(38B)#1‚‚‚N(7) ) 151.4°)

S(2)‚‚‚I(1)#2 ) 3.566 S(4)‚‚‚H(25A)#2 ) 2.771 (∠C(25)#2-H(25A)#2‚‚‚S(4) ) 165.3°)

N‚‚‚X

a Symmetry application applied on the second atom for AB: #1 ) x, y, z; #2 ) 1 - x, -y, -z. b Symmetry application applied on the second atom for AB: #1 ) x, y, z; #2 ) 1 - x, 1 + y, z. c Symmetry application applied on the second atom for AB: #1 ) 0.5 + x, -0.5 + y, z. d Symmetry application applied on the second atom for AB: #1 ) x, y, z; #2 ) x, -1 + y, z.

Figure 3. (a) Molecular structure with displacement ellipsoids at the 30% probability level; H atoms omitted for clarity. (b) Side view of the anionic structure of 3. Figure 1. (a) ORTEP view of 1 with ellipsoids at the 30% probability level; H atoms omitted for clarity. (b) The anionic configuration shows a distorted geometry.

Figure 4. Weak H-bonding interactions between anions and cations in 3 (H-bonding pair: N(1)‚‚‚H(5A)i-C(5)i, symmetric code i ) 0.5 + x, -0.5 + y, z; corresponding H-bonding parameters: N(1)‚‚‚H(5A)i ) 2.382 Å, N(1)‚‚‚C(5)i ) 3.177 Å, and ∠N(1)-H(5A)i-C(5)i ) 143.3°. Figure 2. Schematic illustration for intermolecular interactions of S‚‚‚H in 1 (symmetry code A ) 1 - x, -y, -z).

sibilities that lead to nonplanarity of [Cu(mnt)2]2- are obviously excluded from the view point of the structure (the nearest Cu‚‚‚Cu and Cu‚‚‚S distances are 6.438 and 4.910 Å, respectively), and the weak supramolecular interactions between the

cation and anion, such as S‚‚‚H or N‚‚‚H interactions, are possibly responsible for the coordination geometry distortion at Cu2+ center.12,13 Therefore, the shorter interatomic contacts (less than the sum of the van der Waals radii24) between anion and cations were inspected. The weak intermolecular interactions are depicted schematically in Figure 2, and their interatomic contacts are listed in Table 2.

Coordination Geometries in Bis(maleonitriledithiolato)cuprates

Crystal Growth & Design, Vol. 6, No. 11, 2006 2533

Figure 5. (a) Top and (b) side views of anionic molecular structures; (c) cationic molecular structures in an asymmetric unit for 4 (with displacement ellipsoids at the 30% probability level; H atoms and solvents omitted for clarity).

Figure 6. (a) Adjacent cations arranged in a staggered fashion within a chain and (b) molecular sheets in a crystal of 4 that are parallel to the ab-plane (H atoms omitted for clarity).

2 crystallizes in a triclinic system with space group P1h, and an asymmetric unit consists of one anion together with two cations, which is similar to 1 (the molecular structures refer to Figure S1 of the Supporting Information). The geometric details (bond lengths and bond angle of S-Cu-S in a chelate ring) in [Cu(mnt)2]2- are in fair agreement with the literature values and 1. The Cu2+ ion deviates from each mean-molecular-plane of two mnt2- ligands by 0.031 and 0.148 Å. The angles of S(1)-Cu(1)-S(4) and S(2)-Cu(1)-S(3) (cf. Figure S2 of the Supporting Information) are 174.99(4) and 176.79(4)°, which

are close to the linear 180°. The dihedral angle of 5.8° between two chelate rings is much smaller than the observation in 1, thus the anion possesses a smaller distortion (cf. Figure S2 of the Supporting Information). The nearest Cu‚‚‚Cu and Cu‚‚‚S distances in 2 are 8.162 and 7.521 Å. The shorter interatomic contacts that are less than the sum of the van de Waals radii are found between cations and anions that involve the atomic pairs S‚‚‚I and S‚‚‚H and are reported in Table 2. The crystal of 3 is a monoclinic system with space group C2/m, and the unit cell contains two anions and four cations.

2534 Crystal Growth & Design, Vol. 6, No. 11, 2006

The molecular structures of the anion and cation are displayed in Figure 3a. The Cu2+ ion lies on the inversion centre (2/m), and the molecular point group of the anion is C2h. As expected, all Cu-S bond lengths are identical (2.2793(7) Å), and two bond angles of S-Cu-S in a chelate ring are the same (90.80(3)°). These values are comparable to those in 1 and 2. The coordination S atoms and central Cu2+ ion are strictly coplanar because of the symmetric constraint; however, the anion still exhibits nonplanarity. As shown in Figure 3b, the Cu2+ ion deviates from each mean-molecular-plane of two mnt2- ligands by 0.396 Å. As a result, the ligand mnt2fragments are bent away from the plane of CuS4, and a meanmolecular-plane of mnt2- fragment has a dihedral angle of 13.8° with the CuS4 plane. The cation possesses Cs symmetry, the symmetric plane coincides with the reference plane that is defined by CAr-CH2-NPy [N(2)-C(6)-C(7)], and the bond lengths in a cationic moiety are in good agreement with the literature.25 The lattice structure of the crystal of 3 is stabilized by weak H-bonding interactions between the anions and cations that extend from the H atoms of the benzene ring to the N atoms of CN group in the ligands of mnt2- (represented in Figure 4). The cations separating the neighboring anions lead to a nearest Cu‚‚‚Cu distance of 9.213 Å, which is too large. The crystal of 4 was determined at 223 K. Its space group is P1h, and an asymmetric unit includes three anions and six cations together with six solvent molecules of MeCN. Panels a and c of Figure 5 illustrate crystallographically independent [Cu(mnt)2]2and DABCOH+ cations in an asymmetric unit, respectively. The bond lengths of Cu-S and angles of S-Cu-S in a chelate ring for each anion range from 2.2519(12)-2.2884(12) Å and 90.51(5)-91.40(5)° in the Cu(1) moiety, 2.2491(11)-2.2684(11) Å and 91.84(5)-91.48(5)° in the Cu(2) moiety, and 2.2614(13)2.2693(13) Å and 91.36(5)-91.50(5)° in the Cu(3) moiety. These results are in agreement with the values in 1-3. The distorted square-planar coordination geometry was observed in three crystallographically independent [Cu(mnt)2]2-, and the angles of S(1)-Cu(1)-S(4) and S(2)-Cu(1)-S(3) (cf. panels a and b of Figure 5) are 163.57(5) and 167.37(4)°; S(5)-Cu(2)-S(8) and S(6)-Cu(2)-S(7), 156.14(4) and 155.01(4)°; S(9)-Cu(3)-S(12) and S(10)-Cu(3)-S(11), 165.77(5) and 166.14(5)°. These values deviate much from the linear 180°. The dihedral angles between two chelate rings in a [Cu(mnt)2]2- are 20.5, 34.3, and 19.7° for the Cu(1), Cu(2), and Cu(3) moieties. The interatomic distances between each Cu2+ ion and other atoms in an adjacent anion, countercation, or solvent molecule are much greater than their sums of van der Waals radii. For example, the nearest distances of Cu‚‚‚Cu and Cu‚‚‚S are 5.186 and 5.551 Å; therefore, the possibility that there is a weakly coordinating interaction around each Cu2+ center from other molecules is excluded. Other types of supramolecular interactions between anion and cations (or solvents) were inspected, and the corresponding interatomic contacts are summarized in Table 2. For the six crystallographically independent cations, the protons in N-H of DABCOH+ are disordered with two positions and their occupied factor was fixed at 0.5 for each position in the structural refinement process. The popular chain arrangement of DABCOH+ cations was observed,26 and each chain runs parallel to the direction of a + b. Within a chain, the adjacent cations are in a staggered manner, as shown in Figure 6a. The anions and solvents occupying the interstices between cationic chains construct a sheet that is parallel to the ab-plane (Figure 6b), and the adjacent sheets are held together via van der Waals interactions.

Ren et al. Table 3. Calculated Energy Difference for [Cu(mnt)2]2- Structure between 1-4 and 5 (kJ mol-1) 4a ∆E θb

1

2

3

Cu(1)

Cu(2)

Cu(3)

5

-19.165 27.2

-11.89 0

-1.8 5.8

-19.605 20.5

-25.599 34.3

-19.605 20.5

0 0

a There exist three crystallographically independent [Cu(mnt) ]2- mol2 ecules in an symmetric unit for 4, and each one was identified by its Cu2+ ion. b The dihedral angle between two chelate ring planes (deg).

Figure 7. Potential energy surfaces for [Cu(mnt)2]2- showing the changes in relative energies (kJ mol-1) with respect to the dihedral angle θ.

DFT Calculation for Anionic Single Energy. In crystals of 1 and 4, the weak supramolecular interactions between the cation and anion cause a significant distortion of the coordination geometry at the Cu2+ center, which means the energy gap between square-planar and distorted square-planar geometries should be smaller so that the conversion between the different coordination geometries can be triggered by weak intermolecular interactions. Theoretical analysis is necessary in order to gain more insight into the observations. In the most recent studies,27 some of the present authors theoretically investigated the spin distributions in the spin systems [Ni(mnt)2]- utilizing a series of functionals and basis sets in the framework of DFT and found that the calculating spin populations depend on the method. Among them, the values of spin density distributions at the UBPW91/LANL2DZ level matches with the EPR experimental results well.27,28 In this paper, all single-energy calculations were performed in the framework of DFT using the three-parameter functional developed by Becke,29 which combines the Becke’s gradient-corrected exchange functional and the Lee-YangParr and Vosko-Wilk-Nusair correlation functionals30 with part of the exact Hartree-Fock exchange energy, and LANL2DZ basis sets have been employed. The final molecular energies of [Cu(mnt)2]2- in 1-4 together with 5 (for its structure, see Figure S3 of the Supporting Information) were obtained though their single-point energy (E) calculations, and the configuration energy gap Ei - E5 (i ) 1-4) as well as the twisted angle (the dihedral angle between two chelate rings) are represented in Table 3. The results indicate that (1) the distorted square-planar coordination geometry is a little bit more stable than the squareplanar coordination geometry and (2) the energy gap between the two configurations is smaller and comparable to the weak intermolecular interaction energy (such as weak H-bonding interaction), so that the coordination geometry at the Cu2+ center in [Cu(mnt)2]2- is possibly tuned by weak intermolecular packing interactions. Additionally, the restricted optimization for [Cu(mnt)2]2- was further performed. The different imposed dihedral angles (θ) defined as that between the planes of S(1)-Cu(1)-S(2) and S(3)-Cu(1)-S(4) varied from 0 to 90°

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Crystal Growth & Design, Vol. 6, No. 11, 2006 2535

Figure 8. EPR spectra of 1-4 at 293 (left) and 110 K (right). Table 4. Parameters of EPR Spectra at 293 and 110 K for 1-4 1

2

3

4

293 K 110 K 293 K 110 K 293 K 110 K 293 K 110 K g1 g2 g3 gav ∆H1/Gs ∆H2/Gs ∆H3/Gs

2.045 2.045 2.065 2.052 17 17 65

2.027 2.042 2.11 2.06 12 6 55

2.032 2.032 2.091 2.052 9 9 42

2.025 2.025 2.10 2.05 19 19 40

2.035 2.035 2.10 2.057 12 12 60

2.026 2.040 2.09 2.052 12 7 45

2.023 2.023 2.05 2.032 35 35 80

2.035 2.035 2.080 2.05 31 31 80

(see the inset in Figure 7), and the rest of the geometry was fully optimized. The dihedral angle dependencies of relative energy are depicted in Figure 7, and the calculated bond lengths are summarized in Table S3 of the Supporting Information. The optimization results indicated that the averaged bond lengths of C-C are comparable with the experimental data and that others are slight longer than the experimental values (the averaged Cu-S, S-C, CdC, and CtN bond lengths are longer

than the corresponding experimental values by about 0.1, 0.02, 0.03, and 0.03 Å, respectively). It is worth noting that the relative energy decreases with the anionic geometry distortion increase and achieves a minimum around θ ) 30°; hereafter, the value increases gradually as the dihedral angle increases again. Because of the diversity in the basis sets adopted for C, N, and S (LanL2DZ + RECP for single-point energies; 6-31G* for geometry optimizations) and the difference in the SCF threshold criteria adopted, the energy-gap values between two configurations (for example, between the configurations θ ) 0 and 30°) from the restricted geometric optimization and the single-energy calculation are significantly different from each other (the former is about four times larger than the latter). However, both results predict that the Cu2+ center in [Cu(mnt)2]2- prefers the distorted square-planar coordinate environment. EPR Spectra and Magnetic Susceptibilities. Theoretical analysis revealed that the bonding in [Cu(mnt)2]2- is strongly covalent, and the HOMO of [Cu(mnt)2]2- consists of the 3dxy

Figure 9. Temperature dependencies of the magnetic susceptibilities for 1-4 (open circles, experimental data; solid lines, fits).

2536 Crystal Growth & Design, Vol. 6, No. 11, 2006

Ren et al.

orbital of copper and hybrids of 3s, 3px, and 3py orbitals of sulfur atoms. As the molecular geometry of the anion changes from square-planar to distorted square-planar, the abovementioned atomic orbitals are mixed with the pz orbitals of copper and sulfur, and such mixing has a direct effect on the copper hyperfine splitting.12 Therefore, the EPR spectra are expected to give more insight into the difference between the bonding properties of an anion in the square-planar and distorted square-planar geometries. Figure 8 displays typical X-band EPR spectra of 1-4 that were carried out on polycrystalline samples at 293 and 110 K. At 293 K, the EPR spectra of 1-4 exhibit the axial symmetrical signals and smaller line widths even if the g| and g⊥ are not well resolved in the spectra of 1, 3, and 4. At 110 K, the axial symmetrical signals in the spectra of 2 and 4 become clearer, and the spectra of 1 and 3 are split into a rhombic signal with three different g-factor values, but no resolved hyperfine coupling to the Cu2+ ion has been observed in the spectra of 1-4 yet. Unfortunately, the measurements of EPR spectra for 1-4 did not provide the expected information. We take into consideration the field derivative of absorption signal and Lorentzian line shape to simulate the EPR spectra of 1-4, and the corresponding parameters, g-factor, and the half-width at half-height, from the best fits are summarized in Table 4. The magnetic susceptibilities of 1-4 have been determined on polycrystalline samples in the range of 2-350 K. The results are given in Figure 9 as χm ) f(T). As is shown in these figures, χm ) f(T) follows the Curie-Weiss law for all complexes; therefore, the temperature dependence of the magnetic susceptibility was fit to eq 1

χm )

C + χ0 T-θ

(1)

where χ0 is the contribution of the core diamagnetism and possible van Vleck paramagnetism. The fits yielded C ) 0.403 emu K mol-1, θ ) -0.16 K, and χ0 ) -1.8 × 10-4 emu mol-1 for 1; C ) 0.421 emu K mol-1, θ ) -2.13 K and χ0 ) -4.0 × 10-4 emu mol-1 for 2; C ) 0.387 emu K mol-1, θ ) -0.21 K and χ0 ) -3.5 × 10-4 emu mol-1 for 3; C ) 0.398 emu K mol-1, θ ) -3.33 K and χ0 ) -3.3 × 10-4 emu mol-1 for 4. The values of the g-factor (2.073 for 1, 2.119 for 2, 2.032 for 3, and 2.060 for 4) are close to that found from the corresponding EPR spectra measurements. Small Weiss constants indicate that each complex is almost an isolated spin system, which is consistent with the crystal structural analyses (existence of far distances of Cu‚‚‚Cu and Cu‚‚‚S between the nearest anions).31 Conclusions and Remarks In conclusion, the structures and magnetic properties were presented for four new [Cu(mnt)2]2- complexes, and the highly distorted square-planar coordination geometry at the Cu2+ center of [Cu(mnt)2]2- was observed in 1 and 4. Theoretically, the molecular geometry of the anion changing influences directly on the nature of its HOMO, which is probably reflected in the hyperfine structure of EPR spectra. X-band EPR spectra for polycrystalline samples of 1-4 at 293 and 110 K indicated that no hyperfine splitting was detected, thus such a measurement did not give more insightful information about the electronic structural difference between the anions of square-planar and distorted square-planar geometries. DFT calculations of configuration energy for every [Cu(mnt)2]2- structure in 1-5, as well as the restricted optimization for [Cu(mnt)2]2-, further revealed that the Cu2+ center in [Cu(mnt)2]2- prefers the

distorted square-planar coordinate environment. On the basis of single-point energy calculations, we determined that the energy gap between the distorted square-planar and squareplanar coordination geometry is not much higher than the realms of crystal packing forces. As a result, it can be expected that the configuration preference for the [Cu(mnt)2]2- complexes will be governed by the combination of all kinds of intermolecular interactions. Acknowledgment. This work was partly supported by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. X.M.R. (JSPS P03271) thanks the Japan Society for the Promotion of Science for financial support. Supporting Information Available: X-ray crystallographic files (CIF) of 1-4, tables of the bonding lengths in an anionic moiety for 1-5, and XRD patterns both from single-crystal data at 223 K and powder diffraction at room temperature for 4. This materials is available free of charge via the Internet at http://pubs.acs.org.

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