Chemistry of tris(2,4,6-trimethoxyphenyl)phosphine with rhodium(I) and iridium(I) olefin complexes

Abstract

Rh(I) and Ir(I) complexes of the type [Rh(cod)( η 2-TMPP)] 1+ ( 1) and M(cod)( η 2-TMPP- O) (M = Rh ( 2), Ir ( 3); cod = cyclooctadiene; TMPP = tris(2,4,6-trimethoxyphenyl)phosphine; TMPP- O = mono-demethylated form of TMPP) have been isolated from reactions of [M(cod)Cl] 2 with M′BF 4 (M′ = Ag +, K +, Na +) followed by addition of the tertiary phosphine ligand. This chemistry is dependent on the identity of the metal, as both the cationic phosphine complex and the neutral phosphino-phenoxide compound are stable for Rh(I), whereas only the latter is stable for Ir(I). The three complexes have been characterized by IR and NMR ( 1H and 31P) spectroscopies as well as by cyclic voltammetry. The 1H NMR spectrum of [Rh(cod)( η 2-TMPP)] 1+ ( 1) is in accord with the formula and reveals that the TMPP phenyl rings are undergoing rapid exchange between coordinated and non-coordinated modes; the corresponding spectra of 2 and 3 support free rotation about the PC bonds of the unbound phenyl rings with no fluxionality of the bound demethylated ring. The 31P{ 1H} NMR spectrum of the neutral species 2 exhibits a significant upfield shift with respect to the analogous cationic compound 1. This shielding is the result of improved electron donation to the metal from a phenoxide group as compared to an ether substituent. In situ addition of CO to the reaction between TMPP and [Rh(cod)Cl] 2 or [Ir(cod)Cl] 2 in the presence of M′BF 4 results in the isolation of the monocarbonyl species [Rh(TMPP)( η 2-TMPP)(CO)][BF 4] ( 5) and the stable dicarbonyl compound [Ir(TMPP) 2(CO) 2][BF 4] ( 4), respectively. Single crystal X-ray data for 4· CH 2 Cl 2:P 1, a = 13.512(2), b = 18.348(3), c = 13.358(2) A ̊ , α = 97.26(1), β = 90.55(1), γ = 95.02(1)°, V = 3272(2) A ̊ 3, Z = 2, R = 0.051, R w = 0.065 . The geometry of 4 is square planar, with essentially ideal angles for the mutually trans disposed phosphine and carbonyl ligands, as found in earlier studies for the analogous Rh dicarbonyl compound. The 1H NMR spectrum of 4 supports the assignment of magnetically equivalent phosphorus nuclei in solution. The results of this study indicate that cyclooctadiene is a particularly strong ligand for monovalent late transition metals ligated by TMPP, to the extent that it is inert with respect to substitution in the absence of π-acceptor ligands such as carbon monoxide.