Monomeric Fe(iii) half-sandwich complexes [Cp′FeX2] - synthesis, properties and electronic structureElectronic supplementary information (ESI) available: Additional experimental details, crystallographic data (cif) and details, VT 1H NMR studies, solid-state magnetic susceptibility studies, additional Mössbauer spectroscopy data, electrochemical studies, and computational details. CCDC 1838543-1838547. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt01570

The half-sandwich complex [Cp′Fe(μ-I)] 2 ( 1 ; Cp′ = η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ) is cleaved when heated in toluene to form a cation-anion pair [{Cp′Fe(η 6 -toluene)} + {Cp′FeI 2 } − ] ( 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 C 2 H 4 I 2 , the thermally stable 15VE species [Cp′FeI 2 ] ( 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(CH 2 SiMe 3 ) 2 to form the unprecedented Fe( iii ) ( S = 3/2) bis(alkyl) complex [Cp′Fe(CH 2 SiMe 3 ) 2 ] ( 4 ). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field 57 Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin ( S = 2) Fe( ii ) alkyl species [Cp′FeCH(SiMe 3 ) 2 ], which resists the reaction with H 2 as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H 2 (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(η 6 -C 6 H 6 )] ( A ) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp′Fe) 2 (μ 2 -η 5 :η 5 -C 12 H 12 )] ( 7 ). Further evidence for the intermediacy of [Cp′Fe(η 6 -C 6 H 6 )] ( A ) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF 6 proceeded via C-C bond cleavage instead of metal oxidation to form [Cp′Fe(C 6 H 6 )][SbF 6 ] ( 8 ). Bis(alkyl)iron( iii ) half-sandwich complexes exhibit solvent dependent reactivity when treated with H 2 .