74836-68-3Relevant academic research and scientific papers
Intramolecular bridge/terminal oxo exchange within [Mov2O3]4+ complexes containing linear oxo bridges
Thompson, Robert L.,Lee, Samkeun,Geib, Steven J.,Cooper, N. John
, p. 6067 - 6075 (2008/10/08)
Room-temperature 13C NMR spectra of the d1-d1 dimer [Mo2O3{S213CN(CH2Ph) 2}4] (1) exhibit a broad singlet for the thiocarboxylate ligand. The molecule is fluxional, and in the low-temperature limit this singlet is replaced by two assymetrical doublets (with components at δ 207.4, 207.2 and δ 200.7, 199.7) assigned to the thiocarboxylate ligands with one S trans to the bridging oxo group and one S trans to the terminal oxo group respectively of the syn and the anti isomers of 1. The barrier to exchange has been determined between 320 and 335 K from the width of the resonance in the fast exchange region, and the small entropic contribution to the barrier (ΔH≠ = 11.9 ± 0.4 kcal mol-1, ΔS≠ = -2.0 ± 0.6 cal K-1 mol-1) argues in favor of an intramolecular mechanism for the exchange reaction in which the thiocarbamate ligands are rendered equivalent by exchange of bridge and terminal oxo groups through a transition state in which the linear oxo bridge has been replaced by two bent oxo bridges. This interpretation is supported by the observation that the related xanthate complexes [Mo2O3(S213COEt)4] (4) and [Mo2O3(S213COiPr) 4] (5) participate in similar exchange processes with ΔH≠ = 11.1 ± 0.3 and 12.9 ± 0.4 kcal mol-1 and ΔS≠ = -3.9 ± 1.2 and 1.9 ± 0.6 cal K-1 mol-1, respectively. The small entropic contributions are again consistent with an intramolecular mechanism, and the similarity in enthalpic parameters for 1, 4, and 5 argues against a mechanism involving dissociation of the [MoV2O3]4+ dimers into [MoVIO2]2+ and [MoVIO]2+ containing monomers since such disproportionations are ligand sensitive. Single crystal diffraction studies have established that 1 (monoclinic space group P21/n, with a = 11.604(4) A?, b = 21.81(2) A?, c = 13.85(1) A?, β = 99.90(2)°, Z = 2, and R = 4.37%) and 5 (monoclinic space group C2/c, with a = 25.508(5) A?, b = 9.659(2) A?, c = 14.986(4) A?, β = 101.96(2)°, Z = 4, and R = 5.52%) have anti and syn orientations respectively for the terminal oxo groups in the solid state. All three complexes exist as a mixture of syn and anti isomers in solution, establishing that there is only a small free energy difference between these geometries.
