Synthesis and characterisation of organometallic charge-transfer salts containing the bis(benzene)molybdenum radical cation

Abstract

The salt [Mo(η-C₆H₆)₂][PF₆] has been prepared by iodine oxidation of [Mo(η-C₆H₆)₂] followed by metathesis with NaPF₆. The cyclic voltammogram of [Mo(η-C₆H₆)₂][PF₆] in MeCN exhibits a reversible redox couple (E_½) at –730 mV vs. a saturated calomel electrode (SCE). The solid-state room-temperature EPR spectrum exhibits an isotropic resonance centred at g= 1.993 which is consistent with an assignment of a ²A_1g ground state for the 17-electron [Mo(η-C₆H₆)₂]⁺ radical cation. The salt [Mo(η-C₆H₆)₂][FeBr₄] has been synthesised from [NEt₄][FeBr₄] and [Mo(η-C₆H₆)₂][PF₆]. It crystallises in the orthorhombic space group Pbca, a= 18.527(17), b= 12.521(5), c= 14.915(6)Å; R and R′= 0.051 and 0.059. The salt [Mo(η-C₆H₆)₂][FeBr₄] consists of segregated chains of cations and anions. The solid-state room-temperature EPR spectrum exhibits two resonances centred at g= 1.987 and 2.008 assignable to the cation and anion respectively. The solid-state magnetic susceptibility data for [Mo(η-C₆H₆)₂][FeBr₄] can be fitted to the Curie–Weiss law between 40 and 300 K with C= 3.93 and θ=–5.4 K. Below 15 K the [FeBr₄]^– anion sub-lattice orders antiferromagnetically leaving a magnetic susceptibility which is dominated by the [Mo(η-C₆H₆)₂]⁺ cations.