Monomeric Fe(iii) half-sandwich complexes [Cp′FeX2] – synthesis, properties and electronic structure

Abstract

The half-sandwich complex [Cp′Fe(μ-I)]₂ (1; Cp′ = η⁵-1,2,4-(Me₃C)₃C₅H₂) is cleaved when heated in toluene to form a cation–anion pair [{Cp′Fe(η⁶-toluene)}⁺{Cp′FeI₂}⁻] (2), in which the two Fe(II) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C₂H₄I₂, the thermally stable 15VE species [Cp′FeI₂] (3) can be isolated, in which the Fe(III) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH₂SiMe₃)₂ to form the unprecedented Fe(III) (S = 3/2) bis(alkyl) complex [Cp′Fe(CH₂SiMe₃)₂] (4). The respective spin states of complexes 2–4 are confirmed by single-crystal X-ray crystallography, zero-field ⁵⁷Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(II) alkyl species [Cp′FeCH(SiMe₃)₂], which resists the reaction with H₂ as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H₂ (8 bar) in cyclohexane to yield iron hydrides [{Cp′Fe}₂(μ-H)₃] (5) and [Cp′Fe(μ-H)₂]₂ (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp′Fe(η⁶-C₆H₆)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp′Fe)₂(μ₂–η⁵:η⁵-C₁₂H₁₂)] (7). Further evidence for the intermediacy of [Cp′Fe(η⁶-C₆H₆)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF₆ proceeded via C–C bond cleavage instead of metal oxidation to form [Cp′Fe(C₆H₆)][SbF₆] (8).