Syntheses and Reactivity of Piano-Stool Iron Complexes of Picolyl-Functionalized N‑Heterocyclic Carbene Ligands

The complex [FeClCp*­(HL)] (1; where HL = 3-methyl-1-(2-picolyl)-imidazol-2-ylidene) was synthesized from the reaction of the in situ generated HL ligand and [FeClCp*­(TMEDA)] (where TMEDA is N,N,N′,N′-tetramethylethylenediamine). The deprotonation of 1 with KHMDS led to the removal of a pyridylic proton and the dearomatization of the pyridine ring of the HL ligand, forming [Cp*­(L)­Fe­(μ-N2)­FeCp*­(L)] (2) under N2 or [(FeCp*)2(μ-H)­(μ-L)] (3) under Ar. Complex 2 splits H2 across the L– ligands and the iron centers to give [FeCp*­(H)­(HL)] (4). Complex 4 readily converts to [Cp*­(L″)­Fe­(μ-N2)­FeCp*­(L″)] (5) under N2, where the L″– ligand chelates to the metal center through the carbene carbon and a pyridyl carbon. The reactions of 2 with PhSiH3 and Ph2SiH2 give silyl complexes 6 and 7, respectively. The compounds 2, 4, and 5 are active (pre)­catalysts for the dehydrogenative coupling of dimethylamine–borane.