Photochemical Reactions of Fluorinated Pyridines at Half-Sandwich Rhodium Complexes: Competing Pathways of Reaction

Irradiation of CpRh(PMe 3 )(C 2 H 4 ) ( 1 ; Cp = η 5 -C 5 H 5 ) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η 2 -C ,C- coordinated pentafluoropyridine complex, CpRh(PMe 3 )(η 2 - C , C- C 5 NF 4 ) ( 2 ). The molecular structure of 2 was determined by single-crystal X-ray diffraction, showing coordination by C3-C4, unlike previous structures of pentafluoropyridine complexes that show N-coordination. Corresponding experiments with 2,3,5,6-tetrafluoropyridine yield the C-H oxidative addition product CpRh(PMe 3 )(C 5 NF 4 )H ( 3 ). In contrast, UV irradiation of 1 in hexane, in the presence of 4-substituted tetrafluoropyridines C 5 NF 4 X, where X = NMe 2 , OMe, results in elimination of C 2 H 4 and HF to form the metallacycles CpRh(PMe 3 )(κ 2 - C , C - C H 2 N(CH 3 )C 5 NF 3 ) ( 4 ) and CpRh(PMe 3 )(κ 2 - C , C - C H 2 OC 5 NF 3 ) ( 5 ), respectively. The X-ray structure of 4 shows a planar RhCCNC-five-membered ring. Complexes 2 - 5 may also be formed by thermal reaction of CpRh(PMe 3 )(Ph)H with the respective pyridines at 50 °C.