Accessing Sodium Ferrate complexes containing neutral and anionic N-heterocyclic carbene ligands: structural, synthetic, and magnetic insights

This study reports the synthesis and single-crystal X-ray crystallographic, NMR spectroscopic, and magnetic characterization of a series of sodium ferrates using bis(amide) Fe(HMDS)<inf>2</inf> as a precursor (HMDS = 1,1,1,3,3,3-hexamethyldisilazide). Reaction with sodium reagents NaHMDS and NaCH<inf>2</inf>SiMe<inf>3</inf> in hexane afforded donor-solvent-free sodium ferrates [{NaFe(HMDS)<inf>3</inf>}<inf>∞</inf>] (1) and [{NaFe(HMDS)<inf>2</inf>(CH<inf>2</inf>SiMe<inf>3</inf>)}<inf>∞</inf>] (2), respectively, which exhibit contacted ion pair structures, giving rise to new polymeric chain arrangements made up of a combination of inter- and intramolecular Na···Me(HMDS) electrostatic interactions. Addition of the unsaturated NHC IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to 1 and 2 caused deaggregation of their polymeric structures to form discrete NHC-stabilized solvent-separated ion pairs [Na(IPr)<inf>2</inf>]⁺[Fe(HMDS)<inf>3</inf>]^- (3) and [(THF)<inf>3</inf>·NaIPr]⁺[Fe(HMDS)<inf>2</inf>CH<inf>2</inf>SiMe<inf>3</inf>]^- (4), where in both cases, the NHC ligand coordinates preferentially to Na. In contrast, when IPr is sequentially reacted with the single-metal reagents NaCH<inf>2</inf>SiMe<inf>3</inf> and Fe(HMDS)<inf>2</inf>, the novel heteroleptic ferrate (THF)<inf>3</inf>Na[:C{[N(2,6-ⁱPr<inf>2</inf>C<inf>6</inf>H<inf>3</inf>)]<inf>2</inf>CHCFe(HMDS)<inf>2</inf>}] (5) is obtained. This contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Fe and its normal C2 position to Na. The formation of 5 can be described as an indirect ferration process where IPr is first metalated at the C4 position by the polar sodium alkyl reagent, which in turn undergoes transmetalation to the more electronegative Fe(HMDS)<inf>2</inf> fragment. Treatment of 5 with 1 molar equiv of methyl triflate (MeOTf) led to the isolation and structural elucidation of the neutral abnormal NHC (aNHC) tricoordinate iron complex [CH<inf>3</inf>C{[N(2,6-iPr<inf>2</inf>C<inf>6</inf>H<inf>3</inf>)]<inf>2</inf>CHCFe(HMDS)<inf>2</inf>}] (6) with the subsequent elimination of NaOTf, disclosing the selectivity of complex 5 to react with this electrophile via its C2 position, leaving its Fe-C4 and Fe-N bonds intact. The magnetic susceptibility properties of compounds 1-6 have been examined. This study revealed a drastic change of magnetic susceptibility in replacing a pure σ donor from an idealized trigonal coordination environment by an NHC π donating character.