p-Substituent effects on the redox chemistry of the diaryltriazenido-bridged dirhodium complexes [Rh2(CO)4−n(PPh3)n(μ-p-XC6H4NNNC6H4X′-p)2] (n = 0–2)

Abstract

The complexes [Rh₂(CO)₄(μ-p-XC₆H₄NNNC₆H₄X′-p)₂] (X = X′ = H, Me, Et, OMe, CN, F, Cl or Br; X = H, X′ = OMe or NO₂) were prepared in a two-step reaction involving the cleavage of [{Rh(μ-Cl)(CO)₂}₂] with the diaryltriazene p-XC₆H₄NNNHC₆H₄X′-p followed by the deprotonation of the resulting mononuclear triazene complex [RhCl(CO)₂{N(C₆H₄X-p)NNHC₆H₄X′-p}] with NEt₃. Yields of the dimeric products were maximised by carefully controlling the reaction time for each step. Reaction of the tetracarbonyls with PPh₃ gave the mono- and di-substituted species [Rh₂(CO)_4−n(PPh₃)_n(μ-p-XC₆H₄NNNC₆H₄X′-p)₂] (n = 1 or 2), the reaction times again depending on the substituents X and X′. Each binuclear complex undergoes at least one reversible one-electron oxidation reaction at a platinum electrode in CH₂Cl₂. In some cases, e.g. X = X′ = OMe, as many as three oxidation waves are observed; for X = H, X′ = NO₂, n = 1 or 2, well-defined reduction waves are apparent. The oxidation potential depends on the extent of carbonyl substitution (for each incremental increase in n the potential is decreased by ca. 300 mV) and on the triazenide ligand substituent such that E°′ for the first oxidation wave can be varied systematically over a range of 800 mV. There is a linear relationship between E°′ for the first oxidation step and the Hammett parameter σ_p but a poorer correlation for the second oxidation process.