UNPRECEDENTED TWO-DIMENSIONAL POLYMERS OF MN(II) WITH TCNQ-. (TCNQ = 7,7,8,8-TETRACYANOQUINODIMETHANE)

The concept of combining in one “hybrid” the special properties of individual inorganic and organic components is a topic of major interest in the materials community.1 In one approach, researchers are coassembling transition metal spin centers and organonitrile radicals into covalent networks,2,3 efforts that have unearthed extraordinary materials including highly metallic polymers of Cu with DCNQI (DCNQI ) N,N′-dicyanoquinonediimine),4 an electrically bistable material of Cu with TCNQ (TCNQ ) 7,7,8,8-tetracyanoquinodimethane),5 and a bulk ferromagnet of V with TCNE (TCNE ) tetracyanoethylene).6 With the exception of the DCNQI systems, however, X-ray structures of these intriguing compounds have not been obtained. Clearly, the main obstacle to be overcome in this chemistry is rapid precipitation of the products, which are typically far too insoluble to allow for recrystallization. As part of our investigation into the use of homoleptic acetonitrile cations as precursors for new materials, we discovered that the compounds [M(CH3CN)6][BF4]2 (M ) Fe, Mn, Co, and Ni) react with 2 equiv of [Bu4N][TCNQ] in CH3CN to yield microcrystalline products.7 While attempting to grow single crystals of the Mn compound, it was found that two entirely different products are formed in the presence of MeOH. Herein we report the X-ray structures, infrared spectra, and magnetism of two novel metallopolymers in which manganese ions are involved in unprecedented arrangements with ligands derived from [TCNQ]-•. Rapid mixing of methanolic solutions of [Mn(MeCN)4][BF4]2 and [Bu4N][TCNQ] instantaneously yields a purple microcrystalline solid formulated as Mn(TCNQ)2(MeOH)2. Slow diffusion of the parent compounds in MeOH leads to blue triclinic parallelepipeds of [Mn(TCNQ-TCNQ)(MeOH)2‚MeOH]∞ (1) (TCNQ-TCNQ ) σ-dimerized [TCNQ]-•) are formed.9a A crystallographic study revealed that 1 is composed of a 2-D network of six-coordinate Mn(II) ions equatorially bound to the four outer nitrile groups of the unusual [TCNQTCNQ]2ligand (Figure 1). The tetradentate dianion is nonplanar which leads to a staircase motif, with each “step” consisting of infinite chains of Mn(II) ions joined by 12membered metallacyclic rings involving cis-nitriles that lie in the same plane. A labeled diagram of a portion of 1 is given in Figure 2a, and a packing diagram of the layers is depicted in Figure 2b. The novel 2-D structure is stabilized by extensive interlayer hydrogen bonding that involves axial MeOH ligands and interstitial MeOH molecules situated between the layers (linear O1‚‚‚O2 ) 2.629 A and bent O1‚‚‚O2′ ) 2.983 A). Purple monoclinic needles of [Mn(TCNQ)(TCNQ-TCNQ)0.5(MeOH)2]∞ (2) are formed by slow diffusion of the reactants in a MeOH/MeCN mixture.10a Crystals of 2 also exhibit a 2-D structure, but in this case the Mn(II) centers are coordinated to tetradentate [TCNQ-TCNQ]2and bidentate [TCNQ]-• ligands (Figure 3a); the latter act as bridging ligands through the 1,2dicyano positions with the two unligated NtCgroups pointing outward from the edges of the zig-zag layers towards axial MeOH ligands in the adjacent layers (N3‚‚‚O2A ) 2.834 (8) A and N4‚‚‚O1A ) 2.846 (8) A). These hydrogen bonds serve to stabilize a densely packed, interdigitated arrangement of layers as shown in Figure 3b. The importance of the current X-ray structures lies in the fact that they are only the second and third crystallographically determined binary materials of TCNQ since their discovery in 1962.11a Prior to the present results, the only related structurally characterized example is [Ag(μ4-TCNQ)]∞, which consists of tetrahedral Ag(I) ions bridged by spiral columns of μ4-[TCNQ] groups that stack at ∼3.5 A along the a axis.12a There are several known complexes of Mn(II) with TCNQ behaving as a ligand,13,14 but the only extended structure is a 1-D polymer of Mn with [TCNQ]2bridges and Schiff base coligands.12b To our knowledge, however, there are no structures of coordination compounds with [TCNQ-TCNQ]2ligands. In fact, that [TCNQ-TCNQ]2entity has only been documented three previous times and in these instances it behaves as an outersphere anion.14 The C-C bonds of 1.659(10) and 1.635(10) A (f-f′ in Figure 1) that join the [TCNQ]-• units in 1 and 2 are long, but obviously significant since the C atoms involved are