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