Synthesis, structures, and redox properties of octa(mu(3)-sulfido)hexarhenium(III) complexes having terminal pyridine ligands

A series of mu(3)-sulfido Re-Re bonded octahedral hexarhenium(III) clusters having mixed chloride-pyridine (py) or -4-cyanopyridine (cpy) terminal lig ands, [Bu4N](2)[trans-{Re6S8Cl4(py)(2)}] (Bu4N+ = tetra-n-butylammonium cat ion) (1a), [Bu4N](2)[cis-{Re6S8Cl4(py)(2)}] (1b), [Bu4N](2)[trans-{Re6S8Cl4 (cpy)(2)}] (2a), [Bu4N]2[cis-{Re(6)S(8)Cl4-(cpy)(2)}] (2b), and [Bu4N] [mer -{Re6S8Cl3(py)(3)}] (3), and their one-electron-oxidized (Re5ReIV)-Re-III s pecies, [Bu4N]-[trans-{Re6S8Cl4(py)(2)}] (1a'), [Bu4N][trans-{Re6S8Cl4(cpy) (2)}] (2a'), and mer-[Re6S8Cl4(py)(3)] (3'), have been prepared and charact erized by several physical methods. X-ray crystallographic studies for 1a, 2a, and 3 showed that the Re6S8 core structures are not significantly affec ted by the type and number of pyridyl ligands. The mixed valent cluster 1a' is of a structurally delocalized type, structural parameters being very si milar to those of 1a. Cyclic voltammograms in acetonitrile showed that ther e is no distinct difference in the redox potentials (Re-6(III)/(Re5ReIV)-Re -III) between the cis and the corresponding trans isomers. Both la and Ib s how a reversible redox wave at 0.77 V vs Ag/AgCl. Redox potentials are more positive for 2a and 2b (0.83 V) and 3 (0.97 V). Clusters 2a and 2b show tw o-step ligand-centered redox waves at -1.19 and -1.28 V, and -1.18 and -1.2 9 V, respectively. Temperature-dependent magnetic susceptibility measuremen ts have revealed that 1a' and 3' have an S = 1/2 ground state. The electron self-exchange rate constant for the reaction of 2a with 2a' in dichloromet hane as obtained by H-1 NMR line-broadening method is 1.2 x 10(9) M-1 s(-1) (298.2 K) with Delta H double dagger = 30.2 +/- 2.1 kJ mol(-1) and Delta S double dagger = 30 +/- 8 J mol(-1) K-1. It has been suggested that the pre viously reported protonated species [Re6S7(SH)Cl-6](3-) would actually be a one-electron-oxidized [Re6S8Cl6](3-). Crystal data: [Bu4N](2)[trans-{Re6S8 Cl4(py)(2)}] (1a), monoclinic, space group C2/c, a = 24.693(8) Angstrom, b = 19.494(4) Angstrom, c = 18.592(4) Angstrom, beta = 115.76(2)degrees, Z = 4; [Bu4N](2)[trans-(Re6S8Cl4(cpy)(2)}] (2a), orthorhombic, space group Cmca , a = 19.304(3) Angstrom, b = 17.894(7) Angstrom, c = 18.773(4) Angstrom, Z = 4; [Bu4N] [mer-{Re6S8Cl3(py)3)] (3), monoclinic, space group P2(1)/n, a = 16.156(5) Angstrom, b = 19.760(5) Angstrom, c = 18.895(4) Angstrom, beta = 108.94(2)degrees, Z = 4; [Bu4N][trans-{Re6S8Cl4(py)(2)}] (1a'), monoclini c, space group C2/c, a = 20.524(5) Angstrom, b = 13.794(4) Angstrom, c = 16 .399(4) Angstrom, beta = 109.72(2)degrees, Z = 4.