Optical activity of dimolybdenum complex induced by chiral, chelating diamine ligand; syntheses and structures of [Mo 2(O 2CCF 3) 2( S, S-dach) 2(CH 3CN) 2][BF 4] 2 and [Mo 2(O 2CCF 3) 2( R, R-dach) 2(CH 3CN) 2][BF 4] 2 (dach=1,2-diaminocyclohexane)

Abstract

The first structurally characterized, quadruply bonded complexes containing chiral diamine ligands, [Mo 2(O 2CCF 3) 2( S, S-dach) 2(CH 3CN) 2][BF 4] 2 ( 1), and [Mo 2(O 2CCF 3) 2( R, R-dach) 2(CH 3CN) 2][BF 4] 2 ( 2); (dach=1,2-diaminocyclohexane) were prepared by reactions of [Mo 2(O 2CCF 3) 2(CH 3CN) 6][BF 4] 2 with S, S-dach and R, R-dach, respectively, in CH 3CN. Their UV–Vis and circular dichroism (CD) spectra have been recorded and their structures determined by X-ray crystallography. Crystals of complexes 1 and 2 conform to the space groups P2 with two independent half molecules in the asymmetric unit. The two molecules have a similar structure consisting of a Mo 2 unit bridged by two cis-trifluoroacetate ligands and chelated by two dach ligands. Two acetonitrile molecules are coordinated to the Mo centers along the MoMo bond. The absorption wavelength at 507 nm for both 1 and 2 can be assigned to δ xy →δ xy * transitions. The solution CD spectra of these two complexes show two prominent bands at 525 and 385 nm and form mirror images of each other. The solid CD spectra of complexes 1 and 2 show marked red-shift in the absorption energies as compared with those measured in solution. The one-electron static coupling mechanism was invoked to explain the CD spectra for these complexes and the second lowest energy bands were assigned to be δ xy →δ x 2− y 2 transitions.