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

Dimetallic 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 nBu2SnS2C6H2S2SnnBu2, which are protected forms of 1,2,4,5-benzenetetrathiolate. Selective monodeprotections of OCS2C6H2S2CO or nBu2SnS2C6H2S2SnnBu2 lead to [(P−P)Ni(S2C6H2S2CO)] or [(P−P)Ni(S2C6H2S2SnnBu2)]; 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 ΔE1/2 = 0.66 V, corresponding to Kc of 1.6 × 1011 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(μ-Ag2)][BArF24]2, [4c][BArF24]2, in which no redox chemistry has occurred. Crystal structures of bis(disulfide)-linked compounds [(P−P)Ni(S2C6H2(μ-S2)2C6H2S2)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 π-type orbitals that have significant contributions from the sulfur p orbitals.