Rh–FHF and Rh–F Complexes Containing Small <i>N</i>‑Alkyl Substituted Six-Membered Ring N‑Heterocyclic Carbenes

Heating the six-membered ring N-heterocyclic carbenes 6-Me and 6-Et with Rh­(PPh₃)₄H afforded the rhodium monocarbene hydride complexes Rh­(6-NHC)­(PPh₃)₂H as a mixture of cis- and trans-P,P isomers (<b>4a</b>/<b>b</b>, NHC = 6-Me; ratio = 1:20; <b>5a</b>/<b>b</b>, NHC = 6-Et; ratio = 1:9). Reaction of <b>4a</b>/<b>b</b> with Et₃N·3HF gave only the trans-P,P isomer of the bifluoride complex Rh­(6-Me)­(PPh₃)₂(FHF) (<b>6b</b>), whereas <b>5a</b>/<b>b</b> reacted to form Rh­(6-Et)­(PPh₃)₂(FHF) as a mixture of cis- and trans-phosphine isomers (<b>7a</b>/<b>b</b>). Variable temperature ¹H and ¹⁹F NMR spectroscopy showed that <b>6b</b> and the previously reported 6-ⁱPr carbene analogue cis-Rh­(6-ⁱPr)­(PPh₃)₂(FHF) (<b>2a</b>; Organometallics 2012, 41, 8584) were fluxional in solution. ¹⁹F Magnetization transfer experiments revealed F exchange in both compounds and afforded similar Δ<i>H</i>^⧧ values (<b>2a</b>, 51 ± 5 kJ mol^–1; <b>6b</b>, 60 ± 6 kJ mol^–1) but somewhat different values of Δ<i>S</i>^⧧ (<b>2a</b>, −70 ± 17 J mol^–1 K^–1; <b>6b</b>, −27 ± 18 J mol^–1 K^–1). The fluoride complexes cis-Rh­(6-Me)­(PPh₃)₂F (<b>8a</b>), cis-/trans-Rh­(6-Et)­(PPh₃)₂F (<b>9a</b>/<b>b</b>), and the previously reported 6-ⁱPr analogue <b>3a</b> could be formed upon C–F activation of CF₃CFCF₂ by <b>4a</b>/<b>b</b>, <b>5a</b>/<b>b</b>, and Rh­(6-ⁱPr)­(PPh₃)₂H (<b>1a</b>/<b>b</b>), respectively. Complex <b>3a</b> reacted slowly with H₂ to partially reform <b>1a</b>/<b>b</b> but rapidly with CO to give Rh­(6-ⁱPr)­(PPh₃)­(CO)F (<b>10</b>) and Rh­(PPh₃)₂(CO)­F, and also quickly with Me₃SiCF₃ to form cis-Rh­(6-ⁱPr)­(PPh₃)₂(CF₃) (<b>11a</b>). Complexes <b>4b</b>, <b>5b</b>, <b>6b</b>, <b>7b</b>, and <b>11a</b> were structurally characterized