Linkage isomerization in M(CO)₅(DHF) complexes (M = Cr, Mo, W; DHF = 2,3-dihydrofuran, 2,5-dihydrofuran) studied by time-resolved infrared absorption spectroscopy

Photolysis of a cyclohexane (CyH) solution of M(CO)₆ (M = Cr, Mo, W) in the presence of excess 2,3- or 2,5-dihydrofuran (DHF) yields the complex M(CO)₅(DHF). The solvated complex M(CO)₅(CyH) formed upon photolysis reacts with DHF to form, with a second-order rate constant of 10⁶-10⁷ L mol^-1 s^-1, a single product, identified as the O-bound complex M(CO)₅(η¹-DHF). On a longer time scale (milliseconds to seconds), equilibrium is established between this kinetic product and a second product, M(CO)₅(n²-DHF), in which the ligand is bound to the metal through the DHF C=C double bond, via an intramolecular linkage isomerization reaction. Time-resolved infrared absorption spectroscopy (TRIR) is used to monitor the kinetics of this linkage isomerization, and activation and equilibrium parameters for the isomerization are determined. For all six systems, ΔH^‡ ≈ 15 kcal mol^-1, while the isomerization is thermoneutral or slightly exothermic (|ΔH°| ≈ 0-2 kcal mol^-1). Trends in reactivity as a function of the metal and of the ligand are discussed, and the utility of TRIR as a method for investigating intramolecular linkage isomerization in carbonyl complexes is assessed.