Rh–FHF and Rh–F Complexes Containing SmallN-Alkyl Substituted Six-Membered Ring N-Heterocyclic Carbenes

Abstract

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), an...