Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 22. Blue five-co-ordinate rhodium(III) complexes derived from catechol and related compounds

Abstract

Reaction of [Rh₂(η⁵-C₅Me₅)₂Cl₄] with 4-substituted catechols 4-XC₆H₃(OH)₂ in aqueous base gives dark blue [Rh(C₅Me₅)(4-XC₆H₃O₂)]·H₂O (X = H, Cl, Me, or Bu^t). The crystal structure of the complex [Rh(C₅Me₅)(C₆H₄O₂)]·2C₆H₄(OH)₂ has been determined by X-ray diffractometry and shown to contain a η⁵-pentamethylcyclopentadienylrhodium(III)OO′-bonded to the catecholato-ligand, which is itself further hydrogen-bonded to the two other catechols present in the lattice. The long C–O distances [av. 1.386(18)Å] indicate that the ligand is in the catecholato- and not in the o-quinonoid form and that the Rh is in the +III oxidation state. These catecholato-complexes show strong colours arising from (π→d) ligand-to-metal charge-transfer bands; these bands are shifted to higher energy in more polar (better co-ordinating) solvents. The complexes also form adducts with phosphines; those with triphenylphosphine are labile and dissociate readily. K_e for [Rh(C₅Me₅)(4-XC₆H₃O₂)]+ PPh₃⇌[Rh(C₅Me₅)(4-XC₆H₃O₂)(PPh₃)] have been found to be 3.4 × 10³(X = H), 1.6 × 10³(X = Me), and 2.45 × 10⁴ dm³ mol^–1(X = Cl) in CH₂Cl₂ solution. The preparation and properties of [Rh(C₅Me₅){o-C₆H₄(NH)Y}](Y = O or NH) derived from o-aminophenol or o-phenylenediamine are also reported.