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DOI: 10.1039/B107858E (Paper) CrystEngComm, 2001, 3, 255-257

An infinite supramolecular array structure in metal dithiolate complexes: crystal structure of K(dibenzo-18-crown-6)[M(dmit)2](CH3CN)2 (M = Ni, Au)

Kozo Shitagami a, Tomoyuki Akutagawa abc, Tatsuo Hasegawa ab, Takayoshi Nakamura *ab and Neil Robertson d
aGraduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
bResearch Institute for Electronic Science, Hokkaido University, Sapporo 060-0810, Japan. E-mail: tnaka@imd.es.hokudai.ac.jp
cPRESTO, Japan Science and Technology Corporation (JST), Japan
dDepartment of Chemistry, University of Edinburgh, Edinburgh, UK EH9 3JJ

Received 31st August 2001 , Accepted 15th November 2001

Abstract

K(dibenzo-18-crown-6)[M(dmit)2](CH3CN)2 (M = Ni, Au), which have a one-dimensional supramolecular array, have been prepared. These salts are isomorphous and each K+(dibenzo-18-crown-6) suparamolecular cation unit is connected through the interaction of two CH3CN molecules. The [Ni(dmit)2]− molecules exist as isolated monomers, and the magnetic susceptibility of the salt exhibits Curie-type behavior.

Introduction

The nickel dithiolate complex [Ni(dmit)2] (dmit2− = 2-thioxo-1,3-dithiole-4,5-dithiolate) is a planar π conjugated molecule having an open-shell electronic structure, and a variety of [Ni(dmit)2] salts with characteristic electronic and magnetic properties have been reported.1 In its partially oxidized state, [Ni(dmit)2] gives organic metals such as TTF[Ni(dmit)2]2, which exhibits a superconducting transition at 1.62 K (7 kbar).2 On the other hand, monovalent [Ni(dmit)2]− bears an S = 1/2 spin and can be a useful building block for molecular magnets, including spin-ladders.3

Recently, supramolecular structures formed through non-covalent interactions have increasingly attracted interest in view of crystal engineering.4 Weak interactions such as hydrogen bonding and π–π interactions are utilized to construct infinite columns and chiral double-helices, which are important models in biological systems.5–8

The cation structure largely affects [Ni(dmit)2]− arrangements within the crystals. We introduced supramolecular cation structures composed of inorganic metal cations–crown ethers as a counter cation for [Ni(dmit)2]− anionic species. In the crystals, [Ni(dmit)2]− arrangements are regulated by supramolecular cation structures, showing a large diversity in intermolecular interaction modes between [Ni(dmit)2]− as well as in electrical and magnetic properties determined by these interactions.9 In addition, supramolecular cations exhibited specific structures including ionic channels.10 In this paper, we report the crystal structure of K(DB18c6)[M(dmit)2](CH3CN)2 (DB18c6 = dibenzo-18-crown-6; M = Ni, Au) (Scheme 1), in which K+(DB18c6)(CH3CN)2 supramolecular cations form a one-dimensional array structure and [M(dmit)2] exists as an isolated monomer.


scheme, filename = b107858e-s1.gif
Scheme 1

Experimental

Preparation of the salts 1 and 2

Single crystals of K(DB18c6)[Ni(dmit)2](CH3CN)21 and K(DB18c6)[Au(dmit)2](CH3CN)22 were prepared by slow diffusion between (n-tetrabutylammonium)[M(dmit)2] and DB18c6 with KClO4 in CH3CN for one week in an H-shaped glass cell.

Crystallographic data collection and structure determination

A black, block crystal of dimensions 0.50 × 0.60 × 0.45 mm (1) and a brown plate crystal of dimensions 0.40 × 0.60 × 0.02 mm (2) were used for data collection on a Rigaku R-AXIS RAPID imaging plate-type diffractometer. Data sets were collected at 100 and 296 K for 1 and 2, respectively, using graphite-monochromatized MoKα radiation (λ = 0.71069 Å) and ω-scan techniques. Calculations were performed using the teXsan crystallographic software packages.11 Both crystal structures were solved using SIR92.12 Lorentz polarization and absorption corrections were applied in the refinements. The structure refinements were performed by a full-matrix, least squares method. In the final least squares cycles all non-hydrogen atoms were allowed to vibrate anisotropically. Hydrogen atoms were placed at idealized geometries for 1 and 2.

SQUID measurements

The temperature dependent magnetic susceptibility was measured using a Quantum Design MPMS-XL7 SQUID magnetometer for polycrystalline samples. The magnetic field applied was 1 T for all measurements.

Results and discussion

In the crystals 1 and 2, [M(dmit)2] is in the monovalent state according to the compositions determined by X-ray crystallography. The salts 1 and 2 are isomorphous and cell parameters are summarized in Table 1. Fig. 1 shows the ORTEP-III13 drawing of the crystal structure of 1. Within the crystal, arrays of {K+(DB18c6)(CH3CN)2}n and [M(dmit)2] layers are alternately arranged along the c-axis. K+(DB18c6)(CH3CN)2 cation units form a one-dimensional supramolecular array structure along the b-axis and K+ is incorporated at the center of the DB18c6 cavity. The K+ ions lie on two-fold axes and the DB18c6 ligands also have two-fold crystallographic symmetry. The anionic species have crystallographic inversion symmetry with the metal atoms on the inversion centers.
Crystal structure of K(DB18c6)[Ni(dmit)2](CH3CN)21 viewed along the b-axis. Solvent molecules and H atoms are omitted for clarity. Click image or here to access a 3D representation.
Fig. 1 Crystal structure of K(DB18c6)[Ni(dmit)2](CH3CN)21 viewed along the b-axis. Solvent molecules and H atoms are omitted for clarity. Click image or 1.htm to access a 3D representation.
Table 1 Crystal data and structure refinement details for K(DB18c6)[M(dmit)2](CH3CN)2a,b
Parameter 1 2
a Full-matrix, least square refinement on F2. b Click b107858e.txt for full crystallographic data (CCDC 169867 and 171909).
Empirical formula C30H30N2O6S10KNi C30H30N2O6S10KAu
M 932.98 1071.24
Crystal dimensions/mm 0.50 × 0.60 × 0.45 0.40 × 0.60 × 0.02
Crystal system Monoclinic Monoclinic
Space group C2/c (no. 15) C2/c (no. 15)
a/Å 22.923(1) 23.084(3)
b/Å 8.2033(4) 8.475(2)
c/Å 21.2915(9) 21.267(4)
α/° 90 90
β/° 101.4911(9) 99.524(6)
γ/° 90 90
V/Å3 3923.4(3) 4103(1)
Z 4 4
T/K 100 296
D c/Mg m−3 1.579 1.734
μ/mm−1 1.176 4.250
λ/Å 0.71069 0.71069
Independent reflections 3690 2431
Parameters 255 255
F(000) 1916.00 2120.00
R 0. 0290 0.0582
wR 2 0. 0742 0.1371

Fig. 2 shows the cation units viewed along the c-axis. DB18c6 molecules have a V-shaped structure and stack regularly along the b-axis. Two disordered CH3CN molecules are located between DB18c6 molecules. There are four possible CH3CN sites (A)–(D) between K+(DB18c6) units. The population of each CH3CN was determined as 0.5 because of a crystallographic two-fold axis running at K+ along the b-axis. Not only (A) and (B) but also (C) and (D) are crystallographically equivalent. Therefore, the coordination number of K+ is eight.


Cation units viewed along the c-axis. Population of each CH3CN is 0.5. Not only (A) and (B) but also (C) and (D) are crystallographically equivalent. The direction of (A) and (C) or (B) and (D) are parallel with the direction of the CN group opposite to each other. Click image or here to access a 3D representation.
Fig. 2 Cation units viewed along the c-axis. Population of each CH3CN is 0.5. Not only (A) and (B) but also (C) and (D) are crystallographically equivalent. The direction of (A) and (C) or (B) and (D) are parallel with the direction of the CN group opposite to each other. Click image or 2.htm to access a 3D representation.

The intermolecular distances (A)–(D) and (B)–(C) [C15 in (A)–C16 in (D) = 0.479 Å; C15 in (A)–C17 in (D) = 1.337 Å] are not structurally reasonable. On the other hand, the intermolecular distances N1 in (A)–C17 in (C) = 3.195 Å and C15 in (A)–N2 in (C) = 3.276 Å are comparable to the C–N van der Waals contact distance (3.25 Å).14 Accordingly, the pair (A)–(C) or (B)–(D) should stably exist within the crystal. Two CH3CN molecules are aligned parallel with opposite dipole directions.

The CH3CN molecule has a large dipole moment (3.92 D),15 and the stabilizing energy due to the dipole–dipole interaction between them was estimated at ca. 25.8 kJ mol−1,16 which is of the order of ordinary hydrogen bonds.17 The stabilization energy between CH3CN molecules plays an important role for the formation of the one-dimensional supramolecular array structure along the b-axis. Kiviniemi et al. reported polar crystal structures in which imidazolium and pyrazolium cations coordinated to DB18c6 and formed one-dimensional arrays.6 In these crystals, guest molecules connect with DB18c6 molecules through N–H⋯O hydrogen bonds and through aromatic–aromatic interactions between the benzene ring of DB18c6 and heteroaromatic ring. As a result, one-dimensional polar crystals are formed. Reid et al. reported the crystal of [Mn2+(H2O)2(15-crown-5)]Br2, in which [Mn2+(H2O)2(15-crown-5)] units are connected by H⋯Br bonds and they form an infinite columnar structure.7

In the present crystal, the nitrogen atoms of CH3CN interact with the K+, and each SC+ (supramolecular cation) unit associates with an adjacent SC+ unit through the CH3CN–CH3CN dipole–dipole interaction. In each column, V-shaped SC+ units are aligned in the same direction along the b-axis one-dimensionally with the two benzene rings towards the a-axis. The direction of the V-shaped SC+ units is reversed every c/2 due to an inversion center, eliminating the dipole of the whole crystal.

The centroid–centroid distance of the benzene rings between adjacent DB18c6 columns is 4.747 Å. K–O and K–N distances are ca. 2.709(1)–2.803(1) Å and ca. 2.684(6)–2.81(3) Å in 1 and 2, respectively. The six O atoms in DB18c6 are almost coplanar as is usually observed.18 Dihedral angles between benzene rings in DB18c6 are 115.87 and 114.11° in 1 and 2, respectively.

Since [Ni(dmit)2]− is an anion radical, it is likely that it forms a dimer structure because of the transfer energy gain between [Ni(dmit)2]− anions.3 However, [Ni(dmit)2]− exists as an isolated monomer entity within the crystal, and the magnetic susceptibility of 1 exhibits Curie behavior with a Curie constant of 0.388 emu mol−1 K. As shown in Fig. 3, there are two interactions between adjacent [Ni(dmit)2]− units. The transfer integrals (t) estimated by extended Hückel molecular orbital calculation19 are t1 = −8.96 × 10−3 eV and t2 = 1.75 × 10−3 eV. The interactions t are smaller by one order of magnitude than those usually observed for [Ni(dmit)2]− dimers. [Au(dmit)2]− has a completely filled HOMO, thus the monomeric structure in the crystal is reasonable. Imai et al. reported that when a [p-EPYNN]+ (p-EPYNN =  p-N-ethylpyridinium α-nitronyl nitroxide) was combined with [Ni(dmit)2]− and [Au(dmit)2]−, [Ni(dmit)]− forms a dimer (ladder) structure whereas [Au(dmit)]− gives a monomer structure in the crystal.3 Despite the difference in dimer-formation energy between [Ni(dmit)2]− and [Au(dmit)2]−, crystals 1 and 2 are isomorphous. The self-assembly of large K+(DB18c6)(CH3CN)2 units to the infinite column SC+ structure may prevent the dimer structure of [Ni(dmit)2]−. The counter cations strongly affect both the crystal structure and electronic structure of [M(dmit)2]− in the crystal. The results show that appropriate design of the SC+ structure will enable us to control the arrangement of functional anionic species possessing π-electron systems in view of crystal engineering and solid state chemistry.


[Ni(dmit)2]− arrangement viewed along the c-axis. Each [Ni(dmit)2]− is isolated as a monomer. Click image or here to access a 3D representation.
Fig. 3 [Ni(dmit)2]− arrangement viewed along the c-axis. Each [Ni(dmit)2]− is isolated as a monomer. Click image or 3.htm to access a 3D representation.

Acknowledgements

The authors thank Dr K. Ichimura and Professor K. Nomura for use of the SQUID magnetometer. 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.

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