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

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