Electrocrystallization And Structural And Physical Properties of Charge-transfer Complexes Derived From [η ⁶-c₆me₆)₂m]²⁺(m = Fe, Ru) And Tcnq (tcnq = Tetracyanoquinodimethane)

Charge-transfer complexes [07⁶-C₆Me₆)₂M] [TCNQ], (M = Fe, Ru; x = 2, 4; TCNQ = tetracyanoquinodimethane) and [(17⁶-C₆Me₆)₂M][TCNQ']₂(M = Fe, Ru; TCNQ' = (TCNQ)C1₂, (TCNQ)F₄) are prepared with electrocrystallization techniques by reduction of TCNQ or TCNQ' in the presence of [0;⁶-C₆Me₆)₂M]²⁺. Selectivity toward the TCNQ phases can be directed electrochemically by control of the electrode potential during electrocrystallization. At very negative potentials a poorly conducting 1:2 phase is formed as a deep purple crystalline solid, whereas more positive potentials favor a black, conducting 1:4 phase. The selectivity is determined by the potential-dependent concentrations of TCNQ and TCNQ” at the electrode. Complexes with anions possessing halogen substituents (e.g., (TCNQ)C1₂, (TCNQ)F₄) only crystallize as 1:2 phases, presumably due to decreased Coulombic repulsion between associated anions. Conversely, only conducting phases are observed with derivatives possessing electron-donating substituents (e.g., (TCNQ)Me₂, TCNQ(OMe)₂). The single-crystal X-ray structures of the 1:2 phases, which all crystallize in the triclinic PT space group, are reported. For [(η⁶-QMe₆)₂M][TCNQ]₂, for M = Fe [Ru], a = 10.167 (2) [11.350 (7)] A, b = 11.281 (3) [9.163 (3)] A, c = 9.187 (1) [10.184 (5)] A, a = 111.08 (2) [91.82 (5)]°, β = 98.10 (1) [110.89 (4)]°, 7 = 92.37 (2) [98.19 (3)]°, K= 968.5 [976 (2)] A³, Z = 1 [1], d = 1.35 [1.42] g cm”³, R_u= 0.046 [0.039], and R_w= 0.052 [0.043]. For [η⁶-C₆Me₆)₂M][(TCNQ)F₄]₂, for M = Fe [Ru], a = 10.140 (3) [10.143 (8)] A, b = 10.907 (4) [10.830 (2)] A, c = 10.056 (4) [10.092 (5)] A, a = 95.74 (3) [92.11 (4)]°,β = 93.72 (3) [96.40 (3)]°, 7 = 92.35 (3) [94.40 (5)]°, V = 1103.0 [1097 (2)] A³, Z = 1 [1], d = 1.50 [1.57] g cm”³, R_u= 0.068 [0.065], and R_w= 0.082 [0.073]. For [(η⁶-C₆Me₆)₂M][(TCNQ)Cl₂]₂, for M = Fe [Ru], a = 9.985 (6) [9.993 (11)] k, b= 11.116(5) [11.225 (10)] A, c = 9.725 (7) [9.838 (6)] A, a = 104.28 (7) [89.21 (8)]°, β = 98.07 (5) [75.36 (7)]°, 7 = 89.88 (6) [81.98 (7)]°, V= 1035 (2) [1057 (2)] A³, Z = 1 [1], d = 1.49 [1.42] g cm”³, R_u= 0.078 [0.069], and Rp, = 0.097 [0.075]. The structures of the [(η⁶-C₆Me₆)₂M]²⁺cations are reported here for the first time; the average Fe-C, C-C, and C-Me distances are 2.156, 1.411, and 1.505 A, respectively, whereas the average Ru-C, C-C, and C-Me distances are 2.257,1.426, and 1.501 A, respectively. The 1:2 phases possess dimer dianions that exhibit ring-ring overlap and intradimer separations (A) decreasing in the order TCNQ (3.23) > (TCNQ)C1₂(3.21) > (TCNQ)F₄(3.17). The [TCNQ]₂²~ dimer forms mixed-stack linear chains with the organometallic dications, i.e.,.daadaadaa.,with the molecular planes of the [TCNQ]₂^2-dimer parallel to the hexamethylbenzene rings of the cations. The interplanar separations between the hexamethylbenzene ligand and the nearest TCNQ anion are greater than 3.6 A. Extended mixed-stack linear chains are also observed when TCNQ' = (TCNQ)C1₂or (TCNQ)F₄; however, the [TCNQ']₂^2-dimer is stacked “end on” between the cations with the long molecular axes of the TCNQ anions roughly parallel to the linear-chain axes. Close intermolecular contacts between the cyano nitrogen atoms and the ring carbons of the cation that are significantly less than the sum of the van der Waals radii are observed, suggesting a unique type of donor-acceptor interaction. The compounds with 1:2 stoichiometry have relatively low conductivities (0–300k <10^-78Q^-1cm^-1) and diamagnetic, temperature-independent magnetic susceptibilities. In contrast, the 1:4 phases exhibit much higher conductivity (<r_300K- 0.1 Ω^-1cm^-1) with temperature dependence indicating semiconducting behavior(ea= 0.06 eV), which is explained in terms of electron localization along extended TCNQ arrays as a result of Coulombic repulsion. The 1:4 phases exhibit temperature-dependent paramagnetic susceptibilities consistent with the random exchange Heisenberg antiferromagnetic exchange (REHAC) model.