Mixed-valence dinuclear molybdenum complexes with benzenediamido and dianilido bridges: comparison with related phenolato and dipyridyl species, and with their pentammineruthenium analogues

Abstract

The monoanionic mixed-valence benzenediamido complexes ortho- and para-[{Mo(NO)[HB(dmpz) ₃ ]X} ₂ (Z ₂ C ₆ H ₄ )] ^- A [HB(dmpz) ₃ = tris(3,5-dimethylpyrazolyl) hydroborate; Z = NH; X = Cl, Br or I] have been characterised by electronic, EPR and IR spectroscopic and electrochemical techniques. They are delocalised at room temperature on the EPR time-scale but valence-trapped on the IR time-scale, whereas the meta isomer exhibits valence-trapped behaviour according to both techniques. The electrochemical properties of these species and of related dianilido complexes [{Mo(NO)[HB(dmpz) ₃ ]X} ₂ (ZC ₆ H ₄ EC ₆ H ₄ Z)] B [Z = NH; E = CH ₂ , O, SO ₂ , CO or OC ₆ H ₄ (SO ₂ )C ₆ H ₄ O] are dependent on X and E. These results are compared with those obtained from related diphenolato species A (Z = O) and B (Z = O) and dipyridyl analogues [{Mo(NO)[HB(dmpz) ₃ ]X} ₂ {(NC ₅ H ₄ )E′(C ₅ H ₄ N)}] C (E′ = nothing, CH ₂ CH ₂ , CHCH or dimethyldecapentene). Comproportionation constants and related data for the redox equilibria A + A ^2- ⇌ 2A ^- , B + B ^2- ⇌; 2B ^- and C + C ^2- ⇌ 2C ^- show that interaction between the molybdenum-based redox centres increases significantly as the distance between them decreases and as a function of the group connecting the bridge to the molybdenum, in the order O < NH < N (pyridine). The data are generally consistent with those obtained from related dinuclear pentammineruthenium complexes [{Ru(NH ₃ ) ₅ } ₂ Q] ^{n +} (Q = bridging group) but the interaction between the metal centres is much greater in the molybdenum species than in the ruthenium complexes, and a simple explanation for this behaviour is given.