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)2 as a precursor (HMDS
= 1,1,1,3,3,3-hexamethyldisilazide). Reaction with sodium reagents
NaHMDS and NaCH2SiMe3 in hexane afforded donor-solvent-free
sodium ferrates [{NaFe­(HMDS)3}∞] (1) and [{NaFe­(HMDS)2(CH2SiMe3)}∞] (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)2]+[Fe­(HMDS)3]− (3) and [(THF)3·NaIPr]+[Fe­(HMDS)2CH2SiMe3]− (4), where in both cases, the NHC ligand coordinates
preferentially to Na. In contrast, when IPr is sequentially
reacted with the single-metal reagents NaCH2SiMe3 and Fe­(HMDS)2, the novel heteroleptic ferrate (THF)3Na­[:C­{[N­(2,6-iPr2C6H3)]2CHCFe­(HMDS)2}] (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)2 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 [CH3C­{[N­(2,6-iPr2C6H3)]2CHCFe­(HMDS)2}] (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.