Bis‑N‑Heterocyclic Carbene (NHC) Stabilized η6‑Arene Iron(0) Complexes: Synthesis, Structure, Reactivity, and Catalytic Activity

Reaction of FeCl2 with the chelating bis-N-heterocyclic carbene (NHC) bis-(N-Dipp-imidazole-2-ylidene)­methylene (abbreviated {(DippC:)2CH2}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl2{(DippC:)2CH2}] (1) in high yield. Reduction of complex 1 with excess KC8 with a 10-fold molar excess of PMe3 affords the Fe­(II) complex [FeH­{(DippC:)2CH2}­(PMe3)­(η2-PMe2CH2)] (2) as a mixture of three stereoisomers. Complex 2, the first example of any iron­(II) complex bearing mutually an NHC and PMe3 ligand, is likely obtained from the in situ, reductively generated 16 VE Fe(0) complex, [Fe­{(DippC:)2CH2}­(PMe3)2] (2′), following intramolecular C–H activation of one of the phosphorus-bound CH3 groups. Complex 2 is unstable in aromatic solvents and forms, via a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe3 elimination, the Fe (0) arene complex [Fe­{(DippC:)2CH2}­(η6-C6D6)] (4-d6) in C6D6. Complex 4-d6 represents the first example of an NHC stabilized iron (0) arene complex. The transformation from 2 to 4-d6 can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of 1 in the presence of toluene or benzene affords the complexes [Fe­{(DippC:)2CH2}­(η6-C7H8)] (3) and [Fe­{(DippC:)2CH2}­(η6-C6H6)] (4), selectively and in good yields. DFT calculations characterizing the bonding situation in 3 and 4 reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a π-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the 57Fe Mössbauer spectrum of 4 at 80K exhibits parameters (δ = 0.43 mm·s–1; ΔEQ = 1.37 mm·s–1) which are consistent with a five-coordinate Fe(0) system, rendering 3 and 4 the first examples of well-defined authentic Fe(0)-η6-arene complexes of the type [Fe­(η6-arene)­L2] (L = η1 or 2 neutral ligand, mono or bidentate). Some reactivitiy studies of 3 are also reported: The reaction of 3 with excess CO selectively yields the five-coordinate piano-stool complex [Fe­{(DippC:)2CH2}­(CO)3] (6) in near quantitative yields, while the reaction of complex 3 with C6D6 under heating affords by toluene elimination 4-d6. The catalytic ability of 4 was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph2SiH2 as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of 4 as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process.