Reductionoxidation properties of organo-transition metal complexes: XXXIV. Superoxide and semi-quinone derivatives of the triazenido-bridged [Rh2]4+ unit☆

Abstract The [Rh 2 ] 3+ -containing complex [Rh 2 (CO) 2 (bipy)(μ-RNNNR) 2 ] + ( 1 ; R  p -tolyl) undergoes carbonyl substitution with molecular oxygen to give [Rh 2 (CO)(O 2 )(bipy)(μ-RNNNR) 2 ] + ( 4 ), the ESR spectrum of which shows a superoxide ligand bound to an [Rh 2 ] 4+ core. By contrast, the complex [Rh 2 (RO)(PPh 3 )(bipy)(μ-RNNNR) 2 ] + ( 2 ) binds O 2 , again as a superoxide ligand, in the adduct [Rh 2 (CO)(PPh 3 )(O 2 )(bipy)(μ-RNNNR) 2 ] + ( 3 ). In a similar reaction involving co-ordination of a redox-active ligand and intramolecular metal-ligand electron-transfer, 1 and the o -quinone o -O 2 C 6 Cl 4 ( o -chloranil, tetrachloro-1,2-benzoquinone) give [Rh 2 (CO)( o -O 2 C 6 Cl 4 )(bipy)(μ-RNNNR) 2 ] + ( 5 ) in which the O,O-chelate, in the o -benzosemiquinone form, is bonded to an [Rh 2 ] 4+ centre. The cyclic voltammogram of 5 shows two oxidation and two reduction waves at a platinum electrode in CH 2 Cl 2 , and electrochemical or chemical (with ferrocene) reduction yields the diamagnetic catecholate-[Rh 2 ] 4+ complex 6 .