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 ₂C₆H₄)]^- 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(ZC₆H ₄EC₆H₄Z)] B [Z = NH; E = CH₂, O, SO₂, CO or OC₆H₄(SO₂)C₆H₄O] 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{(NC₅ H₄)E′(C₅H₄N)}] C (E′ = nothing, CH₂CH₂, CH=CH 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(NH₃)₅}₂Q]^{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.

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