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

Irradiation of CpRh­(PMe₃)­(C₂H₄) (<b>1</b>; Cp = η⁵-C₅H₅) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η²-C<i>,C-</i>coordinated pentafluoropyridine complex, CpRh­(PMe₃)­(η²-<i>C</i>,<i>C-</i>C₅NF₄) (<b>2</b>). The molecular structure of <b>2</b> 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(PMe₃)­(C₅NF₄)H (<b>3</b>). In contrast, UV irradiation of <b>1</b> in hexane, in the presence of 4-substituted tetrafluoropyridines C₅NF₄X, where X = NMe₂, OMe, results in elimination of C₂H₄ and HF to form the metallacycles CpRh­(PMe₃)­(κ²-<i>C</i>,<i>C</i>-<i>C</i>H₂N­(CH₃)­C₅NF₃) (<b>4</b>) and CpRh­(PMe₃)­(κ²-<i>C</i>,<i>C</i>-<i>C</i>H₂OC₅NF₃) (<b>5</b>), respectively. The X-ray structure of <b>4</b> shows a planar RhCCNC-five-membered ring. Complexes <b>2</b>–<b>5</b> may also be formed by thermal reaction of CpRh­(PMe₃)­(Ph)H with the respective pyridines at 50 °C