SYNTHESES, STRUCTURES AND SPECTRA OF BETA-MO2X4(DPCP)2 (X = CL, BR, I), WHERE DPCP IS (+/-)-TRANS-1,2-BIS(DIPHENYLPHOSPHINO)CYCLOPENTANE - A STUDY OF SEVERAL INTERESTING EFFECTS OF HALOGEN ATOMS ON THE MO-MO QUADRUPLE BOND

The complexes beta-Mo2-X4(dpcp)2 (X = Br, I) were prepared by reaction of Mo2(O2CCF3)4, (CH3)3SiX (X = Br, I) and (+/-)-trans-1,2-bis(diphenylphosphino)cyclopentane, dpcp, in THF at room temperature. beta-Mo2Cl4(dpcp)2 was prepared by reaction of K4Mo2Cl8 and dpcp in boiling methanol. Their UV-Vis and NMR spectra have been recorded and the structures have been determined. The crystals of these three compounds are isotypic in the space group C2/c with four molecules per unit cell. They show no disorder of the Mo2 units. Crystallographic data: beta-Mo2Cl4(dpcp)2: a = 19.598 (4), b = 20.142(3), c = 21.186(7) angstrom, beta = 112.12(2)-degrees, V = 7748(6) angstrom-3; final residuals: R = 0.0686, R(w) = 0.0934. beta-Mo2Br4(dpcp)2: a = 19.772(7), b = 20.094(6), c = 21.154(6) angstrom, beta = 112.09(3)-degrees, V = 7788(8) angstrom-3; final residuals: R = 0.0694, R(w) = 0.0904. beta-Mo2I4(dpcp)2: a = 20.141(12), b = 20.140(10), c = 21.543(9) angstrom, beta = 112.10(4)-degrees, V = 8097(14) angstrom-3; final residuals: R = 0.0562, R(w) = 0.0844. The structures of these three complexes have the same twist angle (P-Mo-Mo-P) of about 22-degrees, and in all these the Mo-Mo distance is 2.155[5] angstrom. Some generalizations can be made for the series of complexes beta-Mo2Cl4(dpcp)2, beta-Mo2Br4(dpcp)2 and beta-Mo2I4(dpcp)2: (i) the Mo-X and Mo-P distances increase in the order Cl < Br < I, but the Mo-Mo distances and torsional angles are essentially unaffected: (ii) the absorption bands display marked sensitivities to the nature of the halogen atoms the energies are red-shifted in the order Cl < Br < I; (iii) the diamagnetic anisotropies experienced by the cyclopentane protons appear to increase from X = Cl to X = I although detailed or quantitative analysis is not possible.