Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH 2 SiMe 3 ) 2 , have been developed. The first route consists of addition of two equivalents of LiCH 2 SiMe 3 to the iron dihalide complex, (CNC)FeBr 2 , while the second relies on addition of the free CNC ligand to readily-prepared (py) 2 Fe(CH 2 SiMe 3 ) 2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ) ( Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, ( tBu ACNC)Fe(CH 2 SiMe 3 ) 2 ( tBu ACNC = 2,6-( t Bu-imidazol-2-ylidene) 2 pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ) derivatives to an H 2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)FeH 4 . These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H 2 , generating isotopologues and isotopomers of ( Ar CNC)Fe(H n )(D 4- n ) ( n = 0-4). When (3,5-Me 2 Mes CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ) (3,5-Me 2 Mes CNC = 2,6-(2,4,6-Me 3 -C6H2-imidazol-2-ylidene) 2 -3,5-Me 2 -pyridine) was treated successively with H 2 and then N 2 , the corresponding reduced dinitrogen complex (3,5-Me 2 Mes CNC)Fe(N 2 ) 2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me 2 Mes CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ).