Bis[O-methyl (S)-penicillaminato]-cis-dioxomolybdenum(VI), [MoO2{(S)-pen-OMe}2]; structure and spectroscopic studies

Abstract

The complex [MoO₂{(S)-pen-OMe}₂] crystallises in the monoclinic space group P2₁, with a= 11.794(2), b= 12.519(3), c= 13.040(2)Å, β= 106.32(2)°, and Z= 4. The structure was solved from 5 502 unique observed reflections, and refinement gave a final R of 0.039; anomalous dispersion was employed to verify the absolute configuration at molybdenum and at the chiral carbon of the ligands. The two crystallographically independent molecules have opposite absolute configurations (Λ and Δ) for the ligand chelate rings about the metal; the expected cis-dioxo-stereochemistry is confirmed, with normal bond lengths [average Mo–O 1.712(7), Mo–S 2.407(5), and Mo–N 2.362(26)Å]. Proton, ¹³C, and ⁹⁵Mo n.m.r. spectra are consistent with these two diastereoisomeric molecules persisting in solution, and together with c.d. data are taken to indicate that, for the (S)-ligand complex in (CD₃)₂SO at ca. 298 K, the Λ isomer is present in a ca. 2:1 excess over the Δ isomer, the reverse being the case for the (R)-ligand complex. The two diastereoisomers are in dynamic equilibrium and ¹H n.m.r. data collected over the temperature range 297–385 K indicate that the activation parameters for the Λ→Δ interconversion are ΔH^‡= 44 ± 6 kJ mol^–1 and ΔS^‡=–110 ± 17 J K^–1 mol^–1. The mechanism for the inversion of configuration at molybdenum is considered to proceed by an intramolecular process, involving Mo–N bond rupture followed by rotation of the ligand atoms of the subsequent intermediate and ring closure. C.d. spectra resolve each of the u.v.–visible absorption bands centred at ca. 264 and 352 nm into two components. The i.r. active ν(Mo–O_t) stretching vibrations of cis-[MoO₂{(S)-pen-OMe}₂] are manifest as a pair of doublets at 918/913 and 878/875 cm^–1, presumably due to the Λ and Δ isomers having slightly different ν(Mo–O_t) values.