STRUCTURAL, SPECTROSCOPIC, AND CHIROPTICAL PROPERTIES OF THE CHIRAL QUADRUPLE-BONDED DIMOLYBDENUM COMPLEXES MO2CL4[(R,R)-DIOP]2 AND MO2CL4[(S,S)-DIOP]2

By reaction of K4Mo2Cl8 with (R, R)- or (S, S)-DIOP, (-)- or (+)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis-(diphenylphosphino)butane, two complexes of Mo2Cl4(DIOP)2 were prepared. Their UV-vis, CD, and NMR spectra have been recorded, and the crystal structure of Mo2Cl4[(R, R)-DIOP]2 has been determined. Crystal data: space group P1, a = 13.406 (8) Å, b= 13.187(3) Å, c= 11.855 (4) Å, α = 116.34 (2)°,β= 109.69 (3)°, Γ = 100.70 (3)°, V= 1623 (3) Å3, Z = 1. Final residuals: R = 0.0525 and Rw = 0.0728. The structure is disordered so that while only one set of ligand atoms can be resolved, there are two incompletely occupied sets of metal atom positions. In effect there are two different molecules, which have opposite chiralities about the Mo-Mo axis but very similar packing requirements, that randomly occupy the crystal sites. The primary form (P), which is Ʌ-Mo2Cl4[(R, R)-DIOP]2, occupies 89% of the sites, while the Δ molecules (S) occupy the remaining 11%. The P molecules have a twist angle of -78° and the S molecules an angle of 87°. The 1H NMR spectrum of a solution in CH2Cl2 shows that both molecules are again present but now in a P/S ratio of 1.7. Assignment of the various signals to the P and S molecules was possible on an a priori basis because the large diamagnetic anisotropy of the Mo-Mo quadruple bond affects corresponding hydrogen atoms in the two molecules in predictably different ways. An anisotropy of (5700 ± 1200) X 10-36 m3 molecule-1 5is estimated. The CD spectra of both the R, R and S, S complexes are found to be fully in accord with the theoretical prediction that a A molecule with a twist angle 90°-ϰ will have the same signs for the CD bands as a Δ molecule with a twist angle ϰ. The correct absolute sign is predicted in all cases. The UV-vis spectrum is in accord with the twist angle. © 1990, American Chemical Society. All rights reserved.