Synthesis of N-heterocyclic carbene rhenium(i) carbonyl complexes

Abstract

Reaction of aminophosphinimine [RHN(CH₂)₂NPPh₃] (R = H, Et) with Re₂(CO)₁₀ provided the NH-functionalized carbene rhenium complex [Re₂(CNHCH₂CH₂NR)(CO)₉] (3a, R = H, 3b, R = Et). Treatment of 3 with Br₂ provided the mono nuclear [Re(CNHCH₂CH₂NR)(CO)₄Br] (1, R = H, 2, R = Et). However, NH-functionalized carbene complexes 1–3 did not undergo N-alkylation with alkyl halides to yield the N-substituted NHC complexes. The direct ligand substitution of [Re(CO)₅Br] with a carbene donor was employed to prepare [Re(IMes₂)(CO)₄Br] (6a, IMes₂ = 1,3-di-mesitylimidazol-2-ylidene; 6b, IMes₂ = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene). Analyses of spectroscopic and crystal data of 6a and 6b show similar corresponding data among these complexes, suggesting the saturated and unsaturated NHCs have similar bonding with Re^I metal centers. Reduction of 6a and 6b with LiEt₃BH yielded the corresponding hydrido complexes 7a–b [ReH(CO)₄(IMes₂)], but not 1 and 2. Ligand substitution of 1, 6a and 6b toward 2,2′-bipyridine (bipy) was investigated. Crystal structures of 1, 3a–b, 6a–b and 7b were determined for characterization and comparison.