Synthesis, structure and reactivity of bicycloheptatrienyl-bridged dimolybdenum complexes: crystal structure of [Mo2(CO)4(µ-I)(µ-η7 : η7-C14H12)][PF6]

Abstract

Thermal rearrangement of bicycloheptatriene, C₁₄H₁₄, followed by reaction with 1 equivalent of [CPh₃][PF₆] at –20 °C afforded the cycloheptatriene-substituted cycloheptatrienyl cation, [C₁₄H₁₃][PF₆] as a mixture of two isomers differing in the position of the CH₂ group on the cycloheptatriene ring. Treatment of [C₁₄H₁₃][PF₆] with a further equivalent of [CPh₃][PF₆] afforded the bicycloheptatrienyl dication [C₁₄H₁₂][PF₆]₂ which reacts with [Mo(CO)₃(NCMe)₃] to give the bicycloheptatrienyl bridged bimetallic [Mo₂(CO)₆(µ-η⁷ : η⁷-C₁₄H₁₂)][PF₆]₂1. Reaction of [C₁₄H₁₃][PF₆] with [Mo(CO)₃(NCMe)₃] leads to co-ordination at the cycloheptatrienyl ring and formation of [Mo(CO)₃(η⁷-C₁₄H₁₃)][PF₆]2 which is the precursor to [MoI(CO)₂(η⁷-C₁₄H₁₂)][PF₆]5 which contains an unco-ordinated, pendant cycloheptatrienyl ring. Complex 5 reacts with [Mo(CO)₃(NCMe)₃] to give doubly bridged [Mo₂(CO)₄(µ-I)(µ-η⁷ : η⁷-C₁₄H₁₂)][PF₆]7. X-Ray crystallographic characterisation of 7 reveals two crystallographically distinct cations both of which exhibit a twisted conformation of the bicycloheptatrienyl ligand. Reaction of 1 with Na[Mn(CO)₅] results in conversion of the carbon–carbon bond which links the bridgehead carbons, from a single to a double bond and isolation of heptafulvalene-bridged [Mo₂(CO)₆(µ-η⁶ : η⁶-C₁₄H₁₂)]8. The acetonitrile derivative [Mo₂(CO)₄(NCMe)₂(µ-η⁷ : η⁷-C₁₄H₁₂)][BF₄]₂11, formed in a two-step synthesis from 1, is activated towards η⁷→η³ hapticity interconversion by comparison with [Mo(CO)₂(NCMe)(η⁷-C₇H₇)][BF₄], and when 11 is dissolved in acetonitrile at room temperature, the major product is [Mo₂-(CO)₄(NCMe)₆(µ-η³ : η³-C₁₄H₁₂)][BF₄]₂13. Cyclic voltammetric investigations on [Mo₂(NCMe)₂(Ph₂PCH₂CH₂PPh₂)₂(µ-η⁷ : η⁷-C₁₄H₁₂)][PF₆]₂15 reveal two discrete, reversible oxidations with ΔE°= 130 mV, indicating a limited degree of intermetallic electronic communication through the bicycloheptatrienyl ligand.