Reactivity of Fe and Ru Complexes of Picolyl-Substituted N-Heterocyclic Carbene Ligand: Diverse Coordination Modes and Small Molecule Binding

Complexes [MC1Cp*(HL)] (1[Fe]/1[Ru]) (where HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-2-ylidene) were synthesized from the reaction of in situ generated HL ligand and [FeC1Cp*(TMEDA)] (where TMEDA is N,N,N',N'-tetramethylethylenediamine) or [RuC1Cp*](4,) respectively. The deprotonation of 1[Fe]/1[Ru] with 1 equiv of LiHMDS led to cyclometalation through the 0-Me of the mesityl group, forming [MCp*(L-kappa C,kappa C',kappa N)] 2[Fe]/ 2[Ru]. The coordinatively unsaturated compounds [MCp*(HL)]BPh4 3[Fe]/3[Ru] were prepared from 1[Fe]/1[Ru] and halide scavenger NaBPh4. Complex 3[Ru] showed agostic interactions between the 0-Me group of the mesityl moiety and the metal center in solution and the solid state. When the vacant coordination site of 3[Fe]/3[Ru] is occupied by CO, the resulting [MCp*(CO)(HL)]BPh4 4[Fe]/4[Ru] can be deprotonated with 1 equiv of KHMDS at the pyridylic position to afford complexes [MCp*L' (CO)] 5[Fe]/5 [Ru], where the L'(-) ligand chelates to the metal center through the nitrogen donor atom of the dearomatized pyridine ring and the carbene carbon. Complex 2[Fe] reacted rapidly with CO to afford the simple ligand substitution product [FeCp*(L-kappa C,kappa C')(CO)] 6[Fe], where the L- acts as a bidentate chelating ligand through the carbene carbon and benzylic carbon. Under the same condition, the reaction of 2[Ru] with CO forms [RuCp*L(CO)] 7[Ru], where the L(-) ligand (an isomer of L- and L'(-)) chelates to the metal center through the carbene carbon and a pyridyl carbon. Complexes 3[Fe]/3[Ru] reversibly bind dinitrogen to form [MCp*(HL)(N-2)]BPh4 8[Fe]/8[Ru]. 3[Ru] reversibly binds dihydrogen to give [MCp*(H-2)(HL)]BPh4 9[Ru], while no reaction was observed between 3[Fe] and H-2. The reaction of 3[Ru] with dioxygen led to the isolation of a stable side-on O-2 complex [RuCp*(HL)(O-2)BPh4 10[Ru], while the reaction of 3[Fe] with dioxygen led to an intractable mixture of products.