p-Substituent effects on the redox chemistry of the diaryltriazenido-bridged dirhodium complexes [Rh-2(CO)(4-n)(PPh3)(n-)(mu-p-XC6H4NNNC6H4X '-p)(2)] (n = 0-2)

摘要

The complexes [Rh-2(CO)(4)(mu-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(mu-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 [Rh-2(CO)(4-n)(PPh3)(n)(mu-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 degrees' for the first oxidation wave can be varied systematically over a range of 800 mV. There is a linear relationship between E degrees' for the first oxidation step and the Hammett parameter sigma(p) but a poorer correlation for the second oxidation process. [References: 22]