Spectroscopic, Thermal, and Magnetic Properties of Metal/TCNQ Network Polymers with Extensive Supramolecular Interactions between Layers

Reactions of [M(MeCN)_n][BF₄]₂ salts with Bu₄N(TCNQ) in MeOH yield M(TCNQ)₂(MeOH)_x (M = Mn, 1; Fe, 2; Co, 3; Ni, 4, x = 2−4), and reactions of MnCl₂, FeSO₄·7H₂O, CoSO₄·7H₂O and NiCl₂·6H₂O in H₂O produce M(TCNQ)₂(H₂O)₂ (M = Mn, 5; Fe, 6; Co, 7; Ni, 8). Infrared spectroscopy, powder X-ray diffraction and thermogravimetric analyses of 1−8 indicate that the products prepared in the same solvent constitute isostructural families of compounds. [Mn(TCNQ−TCNQ)(MeOH)₄]_∞ (9) crystallizes in the triclinic space group P1̄, a = 7.2966(8) Å, b = 7.4289(8) Å, c = 14.060(2) Å, α = 76.112(2)° β = 87.242(2)°, γ = 71.891(2)°, V = 702.91(13) ų, Z = 1; [Mn(TCNQ)(TCNQ−TCNQ)_0.5(MeOH)₂]_∞ (10) crystallizes in the monoclinic space group C2/c, a = 14.4378(5) Å, b = 27.3067(11) Å, c = 13.1238(5) Å, β = 90.057(1)°, V = 5174.0(3) ų, Z = 8. Compound 9 contains TCNQ^- ligands that have undergone an unusual σ-dimerization to [TCNQ−TCNQ]^2- that acts as a tetradentate ligand to Mn(II) ions to give a 2-D staircase polymeric motif. The layers are connected by hydrogen-bonds between axially coordinated MeOH from adjacent layers and MeOH located between the layers. Compound 10 exhibits a zigzag polymeric motif with equatorially bound TCNQ-derived ligands of two types; the edges of the layers consist of cis-μ-TCNQ^- molecules involved in π-stacking with TCNQ^- units from another layer and σ-[TCNQ−TCNQ]^2- acting as a tetradentate bridging ligand. Axial MeOH ligands are hydrogen-bonded to dangling nitrile groups of cis-μ-TCNQ^- ligands in the next layer. Mn(TCNQ)₂(H₂O)₂ (11) crystallizes in the monoclinic space group I2/a, a = 12.5843(7) Å, b = 13.7147(7) Å, c = 13.3525(70 Å, β = 92.887(1)°, V = 2301.58 ų, Z = 4. This material adopts a double-layer motif consisting of Mn(II) ions bonded to syn-μ₂-TCNQ^- equatorial ligands and axial H₂O molecules. Compounds 1−11 exhibit Curie−Weiss behavior with weak antiferromagnetic coupling being observed at low temperatures. The Zn(II) analogue of 10, [Zn(TCNQ)(TCNQ−TCNQ)_0.5(MeOH)₂]_∞ (12) was also prepared:  space group C2/c, a = 14.2252(1) Å, b = 27.3290(4) Å, c = 13.1177(2) Å, β = 90.074(1)°, V = 5099.64(11) ų, Z = 8. Powder X-ray diffraction was used to probe structures 1−11, and it was found that 9 converts to a new phase with heating or exposure to X-rays that is related to disruption of the σ-dimer (TCNQ−TCNQ)^2- ligands and loss of MeOH. The new phase, whose powder pattern is identical with that of a phase prepared from MeCN, exhibits ferromagnetic behavior.