Experimental and theoretical aspects of the haptotropic rearrangement of diiron and diruthenium carbonyl complexes bound to 4,6,8-trimethylazulene

The haptotropic rearrangement of dinuclear metal carbonyl species on the conjugate π-ligand of (μ₂,η³:η⁵- 4,6,8-trimethylazulene)M₂(CO)₅ [M = Fe (3) and Ru (4)] was investigated in detail both experimentally and theoretically. The complexes, 3 and 4, were synthesized and characterized by spectroscopy and crystallography. The spin saturation transfer technique of ¹H NMR was used to measure the rate constant k of the haptotropic isomerization between the two enantiomers of 3 and 4, from which thermodynamic parameters were determined: (3; ΔS^‡ = -7 ± 1 cal K^-1 mol^-1, ΔH^‡ = 22 ± 1 cal mol^-1, ΔG ^‡₃₇₃ = 25 ± 1 cal mol^-1), (4; ΔS^‡ = 7 ± 1 cal K^-1 mol^-1, ΔH^‡ = 25 ± 1 cal mol^-1, ΔG ^‡₃₇₃ = 23 ± 1 cal mol^-1). DFT calculations (the B3LYP, B1B95 and PBE1PBE methods) were also carried out using the CEP-31G and cc-pVDZ as the basis set of the transition metal and other elements, respectively, by which both ground state and transition state structures were optimized for the haptotropic rearrangement of 3 and 4. The potential energy surface for these reactions suggests that the reaction involves the conversion of the coordination mode from μ₂, η³,η⁵- (ground state) to μ₂, η¹,η⁵- (transition state). Mechanistic consideration, in particular that of differences in transition states between the diiron and diruthenium complexes, is also described.