Emission and metal- and ligand-centered-redox characteristics of the hexarhenium(III) clusters trans- and cis-[Re6(μ3-S)8Cl4(L)2]2-, where L is a pyridine derivative or pyrazine

Preparations of a series of face-capped octahedral hexarhenium(III) clusters having two N-heterocyclic ligands, [Bu4N](2)[trans-{Res(6)(mu(3)-S)(8)Cl-4(L)(2)}] (Bu4N+ = tetra-n-butylammonium cation; L = pyrazine (1a), 4,4'-bipyridine (3a), 4-methylpyridine (5a), 4-(dimethylamino)pyridine (6a)) and their cis analogues (Ib, 3b, 5b, and 6b, respectively), and their electrochemical and photophysical properties have been reported. An X-ray crystal structure determination has been carried out for la to confirm the trans configuration (C40H80N6S8Cl4Re6, orthorhombic, space group Cmca (No. 64), a = 19.560(5) Angstrom, b = 19.494(4) Angstrom, c = 18.592(4) Angstrom, beta = 115.76(2)degrees Z = 4). The redox potential of the reversible Re-6(III)/(Re5ReIV)-Re-III process of these complexes and previously reported [Bu4N](2)-[trans- and cis-{Re-6(mu(3)-S)(8)Cl-4(4-cyanopyridine)(2)}] (2a and 2b, respectively) and [Bu4N](2)[trans- and cis-{Re-6(mu(3)-S)(8)Cl-4(pyridine)(2)}] (4a and 4b, respectively) in acetonitrile depends linearly on the pK(a) of the N-heterocylic ligands, with the potentials being more negative with basic ligands. The ligand-centered-redox waves for la, Ib, 2a, and 2b were observed as split waves (Delta E-1/2 = 90-140 mV), the extent of the splitting being larger for the cis isomer and largest for the pyrazine complexes. Electronic interaction between the two ligands through the [Re-6(mu(3)-S)(8)](2+) core has been suggested. The second ligand-reduction wave was also observed for 3a and 3b, the potential being shifted positively to coalesce with the first reduction wave on addition of the weak proton donor imidazole. This is accounted for by the proton-coupled redox reaction at the free pyridyl site of the 4,4'-bipyridine ligands. All of the complexes show luminescence in acetonitrile at room temperature. While the complexes of pyridine and 4-methylpyridine show photophysical characteristics (lambda(em) 740-750 nm, phi(em) 0.031-0.057, tau(em) 4.2-6.2 mu s) similar to those (770 nm, 0.039, and 6.3 mu s, respectively) of [Re-6(mu(3)-S)(8)Cl-6](4-), emissions of other complexes are significantly weak with lambda(em), phi(em), and tau(em) values in the ranges 763-785 nm, 0.0010-0.0017, and 0.013-0.029 mu s, respectively. Suggestions are given for the excited states localized on the cluster core and the ligand pi* orbitals.