1,2-Dibenzyl- and 1,2-Diaryltetrakis(dimethylamido)dimolybdenum and -ditungsten Compounds: M2R2(NMe2)4 (MM). Structural Effects of Me2N-to-M ? Bonding

Article abstract of DOI:10.1021/ja00343a015

From the reactions between RMgCl (R = CH2C6H5 and CH2-p-tolyl) or LiR (R = C6H5, p- and o-tolyl) (2 equiv.) and 1,2-M2Cl2(NMe2)4 compounds in hydrocarbon solvents, the new compounds 1,2-M2R2(NMe2)4 (M<*>M), where M = Mo and W, have been isolated and characterized by a variety of physicochemical techniques.The new compounds, which are air sensitive, hydrocarbon soluble, and diamagnetic, are related to the ethane-like dimers previously characterized for R = alkyl.The compound 1,2-Mo2(CH2H6H5)2(NMe2)4 crystallizes in the space group P2₁/a with a = 17.595 (7) Angstroem, b = 16.038 (6) Angstroem, c = 10.542 (4) Angstroem, β = 122.11 (2) deg, with Z = 4.Pertinent bond distances and bond angles (averaged) are Mo-Mo = 2.200 (1) Angstroem, Mo-N = 1.95 (1) Angstroem, Mo-C = 2.19 (1) Angstroem, Mo-Mo-N 104 (1) deg, Mo-Mo-C = 100 (1) deg.The central Mo2N4C2 skeleton has virtual C₂ symmetry (gauche rotamer).The benzyl ligand is ? bonded, not ? bonded, as evidenced by equivalent Mo...C_γ distances = 3.7 Angstroem.The compound Mo2(p-tolyl)2(NMe2)4 crystallizes in the anti rotamer: the central Mo2N4C2 unit has virtual C_2h symmetry.Crystal data are a = 8.046 (2) Angstroem, b = 17.319 (7) Angstroem, c = 18.179 (8) Angstroem, with Z = 4 in the space group Pcan.Pertinent bond distances and angles (averaged) are Mo-Mo = 2.196 (1) Angstroem, Mo-N = 1.95 (1) Angstroem, Mo-C = 2.156 (4) Angstroem, Mo-Mo-N = 104 (1) deg, Mo-Mo-C = 101.6 (1) deg.The compound 1,2-Mo2(o-tolyl)2(NMe2)4 crystallizes in the gauche rotamer and has crystallographically imposed C₂ symmetry.Crystal data are a = 16.845 (4) Angstroem, b = 17.651 (5) Angstroem, c = 8.451 ( 2) Angstroem, β = 102.74 (1) deg, with Z = 4 and space group A2/a.Pertinent bond distances and angles are Mo-Mo = 2.226 (1) Angstroem, Mo-N = 1.994 (4) Angstroem, Mo-C = 2.169 (4) Angstroem, Mo-Mo-N = 104 (2) deg, Mo-Mo-C = 105.0 (1) deg.In all three compounds, the Mo-NC2 units are planar and the NC2 blades are aligned along the Mo-Mo axis leading to proximal and distal methyl groups.As a result of forming a Mo<*>Mo bond, 3 Mo-L ? bonds and 2 Me2N-to-Mo ? bonds, each molybdenium attains 16 valence shell electrons.The preferred alignment of the NC2 blades along the Mo-Mo axis is determined by the fact that only the in-plane d orbitals (d_(x²-y²), d_xy) are available for ligand-to-metal ? bonding: the d_(z2₎, d_xz, and d_yz are used to form the Mo<*>Mo bond.The ?, rather than ?-benzyl, ligand coordination reflects the importance of in-plane Me2N-to-Mo bonding, as do the relatively high energy barriers observed in solution for rotation about Mo-N bonds, E_act ca. 14 kcal mol^-1.The lack of ?-benzyl coordination along the Mo-Mo axis is also noteworthy and supports the previous suggestion that axial ligation to the (Mo<*>Mo)⁶⁺ unit is not favored.In Mo2(p-tolyl)2(NMe2)4, the aryl ring is aligned along the Mo-Mo axis, but in solution rotation about the Mo-C bond is not frozen out even at -90 deg C, ...