Synthesis and structure of rhodium(i) silyl carbonyl complexes: photochemical C–F and C–H bond activation of fluorinated aromatic compounds

Abstract

The rhodium(I) silyl carbonyl complexes [Rh{Si(OEt)₃}(CO)(dippp)] (1) and [Rh{Si(OEt)₃}(CO)(dippe)] (2) (dippp = 1,3-bis(diisopropylphosphino)propane, dippe = 1,2-bis-(diisopropylphosphino)ethane) were synthesized on treatment of the methyl compounds [Rh(CH₃)(CO)(dippp)] (3) or [Rh(CH₃)(CO)(dippe)] (4) with HSi(OEt)₃ at low temperature. The methyl complexes 3 and 4 were prepared starting from the binuclear complexes [{Rh(μ-Cl)(dippp)}₂] (5) and [{Rh(μ-Cl)(dippe)}₂] (8), respectively. The silyl complexes 1 and 2 as well as the precursors [{Rh(μ-I)(dippp)}₂] (6), [Rh(X)(CO)(dippp)] (3: X = CH₃, 7: X = I) and [Rh(X)(CO)(dippe)] (4: X = CH₃, 9: X = Cl) were characterized by NMR and IR spectroscopy and the structures in the solid state were determined by X-ray crystallography. The silyl complex 1 converts into the carbonyl-bridged complex [{Rh(μ-CO)(dippp)}₂] (10) above temperatures of −30 °C by loss of the silyl ligand, whereas 2 is more thermally stable and a reaction to the binuclear complex [{Rh(μ-CO)(dippe)}₂] (11) was observed at 50 °C. The silyl complex 2 reacted under irradiation with hexafluorobenzene and pentafluoropyridine to give the C–F activation products [Rh(C₆F₅)(CO)(dippe)] (12) and [Rh(2-C₅F₄N)(CO)(dippe)] (17), respectively. As additional products the silyl dicarbonyl complex [Rh{Si(OEt)₃}(CO)₂(dippe)] (13) and the cationic complex [Rh₂(μ-H)(μ-CO)₂(dippe)₂]⁺[SiF₅]⁻ (14) were identified. Compound 13 was synthesized independently by treatment of 2 with gaseous CO. In a similar manner, the dippp analogue [Rh{Si(OEt)₃}(CO)₂(dippp)] (15) was also prepared starting from 1. Photochemical reaction of 2 with pentafluorobenzene and 2,3,5,6-tetrafluoropyridine resulted selectively in C–H bond activation to afford 12 and [Rh(4-C₅F₄N)(CO)(dippe)] (18), respectively.