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Heating the six-membered ring N-heterocyclic carbenes 6-Me and 6-Et with Rh(PPh)H afforded the rhodium monocarbene hydride complexes Rh(6-NHC)(PPh)H 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 EtN·3HF gave only the trans-P,P isomer of the bifluoride complex Rh(6-Me)(PPh)(FHF) (6b), whereas 5a/b reacted to form Rh(6-Et)(PPh)(FHF) as a mixture of cis- and trans-phosphine isomers (7a/b). Variable temperature H and F NMR spectroscopy showed that 6b and the previously reported 6-Pr carbene analogue cis-Rh(6- Pr)(PPh)(FHF) (2a; Organometallics 2012, 41, 8584) were fluxional in solution. F Magnetization transfer experiments revealed F exchange in both compounds and afforded similar ΛH values (2a, 51 ± 5 kJ mol ; 6b, 60 ± 6 kJ mol) but somewhat different values of ΛS (2a, -70 ± 17 J mol K; 6b, -27 ± 18 J mol K ). The fluoride complexes cis-Rh(6-Me)(PPh) F (8a), cis-/trans-Rh(6-Et)(PPh)F (9a/b), and the previously reported 6-Pr analogue 3a could be formed upon C-F activation of CFCF=CF by 4a/b, 5a/b, and Rh(6- Pr)(PPh)H (1a/b), respectively. Complex 3a reacted slowly with H to partially reform 1a/b but rapidly with CO to give Rh(6-Pr)(PPh)(CO)F (10) and Rh(PPh )(CO)F, and also quickly with MeSiCF to form cis-Rh(6-Pr)(PPh) (CF) (11a). Complexes 4b, 5b, 6b, 7b, and 11a were structurally characterized.
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- 2 Finished
12/06/06 → 11/06/09
Project: Research council