Reactivity of Mn(I) Picolyl-NHC Complexes Toward H₂ and CO₂ Activation and the Catalytic Dehydrogenation of Formic Acid

We report the synthesis and reactivity of [MnBr(HL)(CO)3], 1 (where HL = 3,4,5-trimethyl-1-(2-picolyl)-imidazol-2-ylidene). The reaction of 1 with a base leads to the deprotonation of the methylene group of the ligand backbone and the formation of two isomeric dimers [MnL(CO)3]2, 2 and 2’. The deprotonation of 1 under an H2 atmosphere gives the hydrido complex [Mn(HL)(CO)3H], 3. Alternatively, 3 can be obtained as the major product from the reaction of 1 with KHBEt3 or the reaction of 2 and 2’ with H2. The reaction of 3 with carbon dioxide yields the O-bound formate complex [Mn(HL)(CO)3(OOCH)], 4, which can be converted back to 3 upon deprotonation under an H2 atmosphere or heating under vacuum. Complexes 1, 3, and 4 show catalytic activity toward the dehydrogenation of formic acid.