Neutral and Cationic Mono- and Bis-N-heterocyclic Carbene Complexes Derived From Manganese and Rhenium Carbonyl Precursors

Synthesis of the manganese and rhenium bis-NHC complexes fac-M(IⁱPr₂Me₂)₂(CO)₃X (IⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; M = Mn, X = Br, 1; M = Re, X = Cl, 2) is achieved by treatment of M(CO)₅X with 2 equiv of the free carbene. Halide abstraction by NaBAr₄^F (BAr₄^F = B{C₆H₃(3,5-CF₃)₂}₄) yields the spectroscopically characterized agostic species [M(IⁱPr₂Me₂)₂(CO)₃]BAr₄^F (M = Mn, 3; M = Re, 4). These react rapidly with CO to form the tetracarbonyl compounds cis-[M(IⁱPr₂Me₂)₂(CO)₄]BAr₄^F (M = Mn, 5; M = Re, 6). Addition of 1 equiv of IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to M(CO)₅X yields the monocarbene complexes M(IPr)(CO)₄X (M = Mn, 8; M = Re, 9), which, in the case of Re, affords the dichloromethane complex [Re(IPr)(CO)₄(CH₂Cl₂)]BAr₄^F (11) upon reaction with AgOTf/NaBAr₄^F. Removal of halide from 8 and 9 by AgOTf/NaBAr₄^F in the presence of CO affords the cationic pentacarbonyl species [M(IPr)(CO)₅]BAr₄^F (M = Mn, 13; M = Re, 14). Complexes 1, 2, 5, 6, 8, 9, 11, 13, and 14 have all been structurally characterized.