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(CH2SiMe3)2, have been developed. The first route consists of addition of 2 equiv of LiCH2SiMe3 to the iron dihalide complex (CNC)­FeBr2, while the second relies on addition of the free CNC ligand to the readily prepared (py)2Fe­(CH2SiMe3)2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)­Fe­(CH2SiMe3)2(N2) ( 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 (tBuACNC)­Fe­(CH2SiMe3)2 (tBuACNC = 2,6-(tBu-imidazol-2-ylidene)2pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)­Fe­(CH2SiMe3)2(N2) derivatives to an H2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)­FeH4. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H2, generating isotopologues and isotopomers of ( Ar CNC)­Fe­(H n )­(D4–n ) (n = 0–4). When (3,5-Me2 MesCNC)­Fe­(CH2SiMe3)2(N2) (3,5-Me2 MesCNC = 2,6-(2,4,6-Me3-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) was treated successively with H2 and then N2, the corresponding reduced dinitrogen complex (3,5-Me2 MesCNC)­Fe­(N2)2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me2 MesCNC)­Fe­(CH2SiMe3)2(N2).