FORMATION AND REACTIVITY OF (TETRAARYLPORPHYRINATO)RHODIUM(II) MONOCARBONYLS - BENT RH(II)CO COMPLEXES THAT REACT LIKE ACYL RADICALS

Reactions for a series of rhodium(II) porphyrins with CO are used in illustrating the use of ligand steric effects in both promoting and inhibiting CO coupling to form alpha-diketone complexes ((por)RhC(O)C(O)Rh(por)). [Tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), (TMP)Rh., reacts with CO to produce an equilibrium between a C-C-bonded diamagnetic dimer (TMP)RhC(O)C(O)Rh(TMP), with a small amount of a paramagnetic (S = 1/2) monomer, (TMP)RhCO, but only the monocarbonyl complex is observed for the most sterically demanding porphyrin complex examined: [tetrakis(2,4,6-triisopropylphenyl)porphyrinato]rhodium(II), (TTiPP)Rh.. EPR spectra for ((TMP)RhCO)-C-13 and ((TTiPP)RhCO)-C-13 display three distinct g transitions which indicate the presence of nonlinear RhCO units. (TMP)RhCO reacts like an acyl radical, as evidenced by dimerization through C-C bonding to form (TMP)RhC(O)C(O)Rh(TMP), reactions with (Bu)3SnH and H-2 that produce the formyl complex (TMP)RhCHO, and reaction with styrene to form (TMP)RhC(O)CH2CH(C6H5)C(O)Rh(TMP). Thermodynamic and activation parameters for the dimerization of (TMP)RhCO through C-C bonding provides an enthalpy profile for the carbon monoxide coupling reaction.