Synthesis, Structures, and Properties of 1,2,4,5-Benzenetetrathiolate Linked Group 10 Metal Complexes

Dimetaillic compounds [(P-P)M(S2C6H2S2)M(P-P)] (M=Ni, Pd; P-P=chelating bis(phosphine), 3a-3f) are prepared from O=CS2C6H2S2C=O or (Bu2SnS2C6H2S2SnBu2)-Bu-n-Bu-n, which are protected forms of 1,2,4,5-benzenetetrathiolate. Selective monodeprotections of O=CS2C6H2S2C=O or (Bu2SnS2C6H2S2SnBu2)-Bu-n-Bu-n lead to [(P-P)Ni(S2C6-H2S2C=O)] or [(P-P)Ni((S2C6H2S2SnBu2)-Bu-n)]; the former is used to prepare trimetallic compounds [(dcpe)-Ni(S2C6H2S2)M(S2C6H2S2)Ni(dcpe)] (M = Ni (6a) or Pt (6b); dcpe = 1,2-bis(dicyclohexylphosphino)ethane). Compounds 3a-3f are redox active and display two oxidation processes, of which the first is generally reversible. Dinickel compound [(dcpe)Ni(S2C6H2S2)Ni(dcpe)] (3d) reveals two reversible oxidation waves with Delta E-1/2=0.66 V, corresponding to K-c of 1.6 x 10(11) for the mixed valence species. Electrochemical behavior is unstable to repeated scanning in the presence of [Bu4N][PF6] electrolyte but indefinitely stable with Na[BArF24] (BArF24 = tetrakis(3, 5-bis(trifluoromethyl)phenyl)borate), suggesting that the radical cation generated by oxidation is vulnerable to reaction with PF6-. Chemical oxidation of 3d with [Cp2Fe][BArF24] leads to formation of [3d][BArF24]. Structural identification of [3d][BArF24] reveals appreciable shortening and lengthening of C-S and C-C bond distances, respectively, within the tetrathioarene fragment compared to charge-neutral 3d, indicating this to be the redox active moiety. Attempted oxidation of [(dppb)Ni(S2C6H2S2)Ni(dppb)] (3c) (dppb = 1,2-bis(diphenylphosphino)benzene) with AgBArF24 produces [[(dppb)Ni(S2C6H2S2)Ni(dppb)](2) (mu-Ag-2)][BArF24](2), [4c][BArF24](2), in which no redox chemistry has occurred. Crystal structures of bis(disulfide)-linked compounds [(P-P)Ni(S2C6H2(mu-S-2)(2)C6H2S2)Ni(P-P)] are reported. Near IR spectroscopy upon cationic [3d](+) and neutral 6a reveals multiple intense absorptions in the 950-1400 nm region. Time-dependent density functional theory (DFT) calculations on a 6a model compound indicate that these absorptions are transitions between ligand-based pi-type orbitals that have significant contributions from the sulfur p orbitals.