Preparation and Characterization of 1,2-Dialkyl Compounds of Dimolybdenum and Ditungsten of Formula M2R2(NMe2)4 (M=-M)

Article abstract of DOI:10.1021/ja00404a012

1,2-M2Cl2(NMe2)4 compounds, where M = Mo and W, and alkyllithium reagents react in hydrocarbon solvents to give 1,2-M2R2(NMe2)4 compounds (M=-M), where R = -CH2CH3, -CH2CD3, -CD2CH3, -CH2CH2CH2CH3, -CH(CH3)3,- CH(CH3)(C2H5), -CH2C(CH3)3, -CH2Si(CH3)3, and -C(CH3)3.These new compounds have been characterized by a combination of physicochemical studies, which include elemental analyses, mass spectroscopy, infrared spectroscopy, and NMR spectroscopy.The molybdenum compounds where R = CH2CH3 and -CH(CH3)2 have been examined by single-crystal X-ray diffraction studies.The characterization data establish that all compounds have a staggered ethanelike M2N4C2 core.In solution, NMR studies reveal the existance of a mixture of anti and gauche rotamers: as the bulkiness of the alkyl group increases, the gauche rotamer becomes increasingly favored.In the crystalline state, a gauche Mo2(C2H5)2(NMe2)4 molecule has been fully characterized by X-ray studies.The molecule has a virtual C₂ axis of symmetry with the following important structural parameters: Mo-Mo = 2.203(1) Angstroem, Mo-N = 1.96(1) Angstroem (average), M-C = 2.16(1) Angstroem (average), gauche isomerization is slow.An X-ray study on crystalline samples of Mo22(NMe2)4 revealed disordered Mo2N4C2 units: it was not possible to distinguish NMe2 and CH(CH3)2 groups. 1H NMR studies revealed that the anti rotamer of Mo22(NMe2)4 showed frozen out conformers at low temperatures arising from restricted rotations about the Mo-C bonds.The isolation of the isopropyl, sec- and tert-butyl, and labeled ethyl compounds (R = CD2CH3 and CH2CD3) demonstrates that β-hydrogen elimination does not occur readily in these compounds.The origin of the barrier to β-hydrogen elimination is discussed and these results are compared and contrasted with previous findings for mononuclear transition metal alkyl compounds.The crystal data for Mo2(C2H5)2(NMe2)4 are: space group = P1/, a = 11.730(2) Angstroem, b = 8.402(1) Angstroem, c = 9.840(1) Angstroem, α = 98.09(0) deg, β = 98.43(1) deg, γ = 82.74(0) deg, with Z = 2.