Electrochemical behavior of S,S′-bridged adducts of square planar metalladithiolene complexes [M(S2C2Ph2)2](M=Ni, Pd, and Pt)

The electrochemical behavior of the S,S′-bridged adducts of square planar metalladithiolene complexes was investigated by using cyclic voltammetry and electrochemical spectroscopies (visible, near-IR, and ESR). The norbornene-bridged S,S′-adduct [Ni(S2C2Ph2)2(C7H8)] (2a; C7H8=norbornene) formed by [Ni(S2C2Ph2)2] (1a) and quadricyclane (Q) was dissociated by an electrochemical reduction, and anion 1a− and norbornadiene (NBD) were formed. Q was isomerized to NBD in the overall reaction. The o-xylyl-bridged S,S′-adduct [Ni(S2C2Ph2)2(CH2)2(C6H4)] (3a; (CH2)2(C6H4)=o-xylyl) was also dissociated by an electrochemical reduction, and this reaction gave the o-xylyl radical (o-quinodimethane). The reduction of complex 3a in the presence of excess o-xylylene dibromide underwent the catalytic formation of o-quinodimethane. The butylene-bridged S,S′-adduct [Ni(S2C2Ph2)2(CH2)4] (4a; (CH2)4=butylene) was stable on an electrochemical reduction. The lifetimes of reduced species of these adducts 2a–4a were influenced by the stability of the eliminated group (stability: NBD > o-xylyl radical (o-quinodimethane) > butylene radical). Therefore, the reduced species are stable in the sequence 4a− > 3a− > 2a−. Although the palladium complex [Pd(S2C2Ph2)2] (1b) was easier to reduce than the nickel complex 1a or the platinum complex [Pt(S2C2Ph2)2] (1c), their S,S′-adducts were easier to reduce in the order of Ni adduct > Pd adduct > Pt adduct.