The first structurally characterized, quadruple-bonded complexes containing chiral amine ligands; syntheses, structures and optical activities of [Mo2Cl4(S-chea)4] and [Mo2Cl4(R-chea)4] (chea = 1-cyclohexylethylamine)†

Abstract

The complexes [Mo₂Cl₄(S-chea)₄] 1 (chea = 1-cyclohexylethylamine) and [Mo₂Cl₄(R-chea)₄] 2 were prepared by reactions of [Mo₂Cl₄(PPh₃)₂(MeOH)₂] with S- and R-chea, respectively, in acetone. Their NMR, UV/VIS and circular dichroism spectra have been recorded and the structures of both 1 and 2 have been determined. The molybdenum atoms in both crystal structures were found to be disordered, so that while only one set of ligand atoms can be resolved, there are two incompletely occupied sets of metal atoms. The ratios of the primary to the secondary form are 97∶3 and 94∶6 for 1 and 2, respectively. The lowest-energy bands of the absorption spectra at 531 nm for both 1 and 2 can be assigned to δ_xy → δ_xy* transitions. The solid-state CD spectra of these two complexes show two prominent bands at 520 and 455 nm and form mirror images of each other. The variable-temperature CD spectra of the complexes in toluene intersect in isosbestic points, indicating temperature-dependent equilibria between pairs of conformers for each complex. The Wood–Fickett–Kirkwood method has been used to calculate the free-energy changes of the Δ and Λ conformers and their populations in solution.