Formation, reactivity and redox properties of carbon- and sulfur-bridged diiron complexes derived from dibenzothienyl Schiff bases: effect of N,N- and N,P-chelating moieties

Abstract

Dibenzothienyl Schiff bases, DBT-NN and DBT-NP, were derived from condensation of 4-formyldibenzothiophene with N,N-dimethylethylenediamine and 2-(diphenylphosphino)ethylamine, respectively. A photochemical reaction of [Fe(CO)₅] with DBT-NN produced diiron complexes, [Fe₂(μ-DBT-NN)(CO)₆] (1) and [{Fe(μ-BPT-NN-κ³S,C,N)(CO)₂}Fe(CO)₃] (2). Complex 1 has π coordination of a Schiff base CN bond and an N,N-chelate, while 2 has an S,C,N-tridentate ligand having an uncoordinated dimethylamino group. The corresponding reaction with DBT-NP produced a diiron complex of an S,C,N,P-tetradentate ligand, [{Fe(μ-BPT-NP-κ⁴S,C,N,P)(CO)}Fe(CO)₃] (3), which has an N,P-chelate, via the DBT-NP complex [Fe(DBT-NP-κP)(CO)₄] (4). Reactions of 2 and 3 with one equivalent of PMe₂Ph gave the monosubstituted complexes [{Fe(μ-BPT-NN-κ³S,C,N)(CO)₂}Fe(CO)₂(PMe₂Ph)] (5) and [{Fe(μ-BPT-NP-κ⁴S,C,N,P)(CO)}Fe(CO)₂(PMe₂Ph)] (6), respectively. The corresponding reactions with an excess of PMe₂Ph produced mononuclear complexes, trans-[Fe(BPT-NN-κ³S,C,N)(CO)(PMe₂Ph)₂] (7) and [Fe(BPT-NP-κ⁴S,C,N,P)(CO)(PMe₂Ph)] (8), respectively. Complexes 1–3 and 5–8 were structurally characterized by X-ray crystallography. Complexes 2, 3, 5 and 6 have similar dinuclear structures with different carbonyl/phosphine substitution patterns. Cyclic voltammograms of 2, 3 and 5 showed two one-electron reduction processes, and more negative potentials were observed for 6. The shift of the redox potentials are rationalized by the electron-donating character of the phosphine ligands, which suggests that the reduction occurs at the asymmetrically bridged diiron core.