Iron complexes of a bidentate picolyl-NHC ligand: synthesis, structure and reactivity.

The synthesis, structure and reactivity of bidentate picolyl N-heterocyclic carbene (NHC) iron compounds were studied. Compounds [FeBr(HL)2]Br (1), [FeBr(HL)(HMDS)] (2) and [FeBr2(HL)] (3) (HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-1-ylidene, HMDS = hexamethyldisilazide) were prepared from H2LBr with suitable amounts of Fe(HMDS)2 or in situ prepared [Fe(HMDS)Br]. The deprotonation of 1 with 2 eq. of LiHMDS gave [FeL2] (4), featuring dearomatized pyridine moieties with exocyclic C-C double bonds. The protonation of 4 with 2 eq. of PPh3·HBr results in the formation of 1. Attempted deprotonation of 3 using benzyl Grignard as the base resulted in transmetalation products [FeBnBr(HL)] (5) and [FeBn2(HL)] (6). Exposure of 6 to CO resulted in the formation of diamagnetic compound [Fe(CO)3(HL)] (7) and dibenzyl ketone. Prolonged exposure of 7 to CO with heating induces pyridine dissociation, affording [Fe(CO)4(HL-κC)] (8). Treatment of compound 6 with an equimolar amount of p-methoxybenzyl bromide yielded homo- and cross-coupling products.