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

Rh(I) and Ir(I) complexes of the type [Rh(cod)(eta(2)-TMPP)](1+) (1) and M(cod)(eta(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'BF4 (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 (H-1 and P-31) spectroscopies as well as by cyclic voltammetry. The H-1 NMR spectrum of [Rh(cod)(eta(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 P-31{H-1} NMR spectrum of the neutral species 2 exhibits a significant upheld 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, results in the isolation of the monocarbonyl species [Rh(TMPP)(eta(2)-TMPP)(CO)][BF4] (5) and the stable dicarbonyl compound [Ir(TMPP)(2)(CO)(2)][BF4] (4), respectively. Single crystal X-ray data for 4 CH2Cl2: <P(1)over bar>, a=13.512(2), b=18.348(3), c=13.358(2) Angstrom, alpha=97.26(1), beta=90.55(1), gamma=95.02(1)degrees, V=3272(2) Angstrom(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 H-1 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 pi-acceptor ligands such as carbon monoxide.