Comparative Studies of Preferential Binding of Group Nine Metalloporphyrins (M = Co, Rh, Ir) with Methoxide/Methanol in Competition with Hydroxide/Water in Aqueous Solution

Abstract

Aqueous solutions of iridium(III) tetra-(p-sulfonatophenyl)porphyrin [(TSPP)Ir(III)] form a hydrogen ion dependent equilibrium distribution of bisaquo ([(TSPP)Ir(III)(OD(2))(2)](3-)), monoaquo/monohydroxo ([(TSPP)Ir(III)(OD(2))(OD)](4-)) and bishydroxo ([(TSPP)Ir(III)(OD)(2)](5-)) complexes. Comparison of acid dissociation constants of group nine ([(TSPP)M(III)(OD(2))(2)](3-)) (M = Co, Rh, Ir) complexes show that the extent of proton dissociation in water increases regularly on moving down the group from cobalt to iridium consistent with increasing metal ligand bond strength. Addition of small quantities of methanol to aqueous solutions of [(TSPP)Ir(III)] results in the formation of methanol and methoxide complexes in equilibria with aquo and hydroxo complexes that are observed by (1)H NMR. Direct quantitative evaluation of competitive equilibria of [(TSPP)Ir(III)] complexes reveals a remarkable thermodynamic preference for methanol binding over that of water (DeltaG degrees (298 K) = -5.2 kcal mol(-1)) and methoxide binding over that of hydroxide (DeltaG degrees (298 K) = -6.1 kcal mol(-1)) in aqueous media. A comparison of equilibrium thermodynamic values for displacement of hydroxide by methoxide for group nine (TSPP)M(III) (M = Co, Rh, Ir) complexes in aqueous media are also reported.