Ion Pair Charge‐Transfer Complexes of Dithiolene Metalates with Diquaternary 2,2′‐Biimidazoles

Cycloalkylated biimidazolium dications (A2+) of reduction potentials from -0.4 to -1.4 V form ion pair charge-transfer complexes {A2+ [M(mnt)2]2−} with dithiolene metalates, M = Zn, Ni, Pd, Pt, mnt2− = maleonitrile-2,3-dithiolate. X-ray analyses of {P2BBIm2+[Ni(mnt)2]2−} [P2BBIm2+ = 1,1′,3,3′-bis(propane-1,3-diyl)bibenzimidazolium] and {B2Blm2+ [Pd(mnt)2]2−} [B2BIm2+ = 1,1′,3,3′-bis(butane-1,4-diyl)biimidazolium] reveal that the solid-state structure is largely determined by the geometry of the acceptor. When the latter is strongly twisted, the usually observed mixed donor-acceptor columns are modified to a chain-like arrangement. In the case of the bibenzimidazolium dication P2BBIm2+ the otherwise planar [Ni(mnt)2]2− becomes tetrahedrally distorted. By the application of the Hush model a reorganization energy of about 67 kJ/mol is estimated for the thermal electron transfer from [M(mnt)2]2− to A2+ when M = Ni, Pd, Pt, but considerable deviations from this model are observed when M = Zn. Irradiation of the free biimidazolium acceptors in the presence of EDTA affords the strongly reducing radical cations which reduce water to hydrogen in the presence of colloidal platinum. Attempts to sensitize this reaction by irradiating into the charge-transfer band of {A2+ [M(mnt)2]2−} have failed until now. The molar absorptivity of one biimidazolium radical cation is measured by spectroelectrochemistry.