Reductive Elimination of Anhydrides from Anionic Iodo Acetyl Carboxylato Rhodium Complexes

The first part of this study is devoted to the search for improved and highly efficient synthetic procedures for the preparation of rhodium carbonyl precursors, and the second part to the production of acetyl derivatives. Microwave heating of a methanol solution of RhCl3·xH2O for 10 min under 5–10 bar CO produces the complex [H]+[RhCl2(CO)2]–, which is quantitatively converted into the dimeric complex [Rh2(μ‐Cl)2(CO)4] (1) by rapid solvent evaporation under a stream of CO. The treatment of 1 with NaI or AgOAc under a CO atmosphere gives [Rh2(μ‐I)2(CO)4] (2) and [Rh2(μ‐OAc)2(CO)4] (3), respectively. The complex cis‐[PPN][RhI2(CO)2] (4) is prepared in high yield by the reaction of RhI3 with N,N‐dimethylformamide (DMF) at 160 °C under CO followed by metathesis of the [NH2Me2]+ cation with [bis(triphenylphosphoranylidene)ammonium]chloride ([PPN]Cl). The analogue cis‐[PPN][Rh(OAc)2(CO)2] (5) is obtained by reaction of 4 with AgOAc. Complex 4 reacts with neat CH3I to afford quantitatively the dimeric rhodium(III) complex [PPN]2[Rh2(μ‐I)2I4(COMe)2(CO)2] (7). The cleavage of the iodo bridging ligands in 7 occurs readily with CO, pyridine or 1,2‐dimethylimidazole to give [PPN][RhI3(COMe)(CO)2] (8), [PPN][RhI3(COMe)(CO)(Py)] (9) and [PPN][RhI3(COMe)(CO)(1,2‐MeIm)] (10), respectively, whereas the reactions with the chelating 2‐acetylpyridine and methyl‐2‐picolinate ligands give the neutral [Rh(COMe)I2(CO)(κ2‐2‐acetylpyridine)] (11) and [Rh(COMe)I2(CO)(κ2‐methyl‐2‐picolinate)] (12) complexes, respectively. The X‐ray crystal structures of 4, 7 (centrosymmetric isomer), 8, 11 and 12 have been solved. The reductive elimination from 7 and 8 in the presence of various carboxylato sources to provide acid anhydrides is described. In the presence of significant amounts of acetate ions, the reductive elimination reaction from the anionic species [RhI3(COCH3)(CO)2]– does not provide CH3COI, but after substitution of an iodo ligand by CH3COO–, produces the anhydride (CH3CO)2O from [RhI2(COCH3)(OCOCH3)(CO)2]–. This reaction can be extended to various carboxylates to give the corresponding (CH3CO)O(OCOR) mixed anhydrides.