Ring-Expanded N‑Heterocyclic Carbene Complexes of Rhodium with Bifluoride, Fluoride, and Fluoroaryl Ligands

Thermolysis of Rh­(PPh₃)₄H in the presence of the six-membered N-heterocyclic carbene 1,3-bis­(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-ⁱPr) gave the monocarbene complex Rh­(6-ⁱPr)­(PPh₃)₂H as a 1:2 mixture of the cis- and trans-phosphine isomers 1a and 1b. This same isomeric mixture was formed as the ultimate product from treating Rh­(PPh₃)₃(CO)H with 6-ⁱPr at room temperature, although pathways involving both CO and PPh₃ loss were observed at initial times. Treatment of 1a/1b with Et₃N·3HF generated the bifluoride complex cis-Rh­(6-ⁱPr)­(PPh₃)₂(FHF) (2a), which upon stirring with anhydrous Me₄NF was converted to the rhodium fluoride complex cis-Rh­(6-ⁱPr)­(PPh₃)₂F (3a). Thermolysis of 1a/1b with C₆F₆ resulted in C–F bond activation to afford a mixture of 3a and the pentafluorophenyl complex trans-Rh­(6-ⁱPr)­(PPh₃)₂(C₆F₅) (5b). Complexes 1b, 2a, 3a, and 5b were structurally characterized.