Unusual 1,1-2,2-isomers of Mo2Cl4(PR3)(2)(R'CN)(2). Compounds with R = CH2CH2CN and R' = CH3, CH3CH2 and CH(CH3)(2)

Synthesis, characterization and structure are reported for the first complexes of tris(beta-cyanoethyl) phosphine (CEP) with an M-2(4+) core containing a quadruple bond. The first complex is a blue precipitate (1) obtained by reacting Mo-2(O2CCH3)(4) with 2 or more equivalents of CEP and 4 or more equivalents of(CH3)(3)SiCl at room temperature in toluene or tetrahydrofuran. This blue material, characterized by elemental analysis, infrared and visible spectroscopy and by its reactivity, is a three-dimensional polymer with the formula [Mo2Cl4(CEP)2](x), in which each CEP ligand binds to one molybdenum center via the phosphorus atom and bridges to a different molybdenum center via one of its nitrile groups. This blue material is amorphous and extremely insoluble in most organic solvents, but it dissolves readily in nitrile solvents. From these solutions the highly crystalline complexes Mo2Cl4(CEP)(2)(RCN)(2) . xRCN (x=0,1,2) can be obtained. Crystal and molecular structures are reported for the derivatives with R=CH2, CH3CH2 and CH(CH3)(2); all three are dark green, robust compounds, and they contain an unexpected geometric isomer of the Mo(2)X(4)L(2)L(?)2 core. In all three structures, both of the CEP ligands are coordinated in a trans manner to one molybdenum atom and the nitrile ligands in a trans fashion to the other one. The CEP ligand withdraws some electron density from the Mo-Mo quadruple bond manifold and thus differs slightly from other trialkyl phosphines in its bonding to the Mo-2(4+) core. Crystal data for complexes 2-4 are: Mo2Cl4(CEP)(2)(NCCH3)(2) . 2CH(3)CN (2), orthorhombic, space group Pbcn (No. 60), a=15.866(1), b=14.530(1), c=116.462(3) Angstrom, V=3795.0(6) Angstrom(3), Z=4, structure refined to R=0.033, wR2=0.081 for 2380 reflections with I> 2 sigma(I) and R=0.065 for all 3332 data; Mo2Cl4(CEP)(2)(NCCH2CH3)2 (3), orthorhombic, space group Cmcm (No. 63), a=16.470(3), b=14.039(3), c=16.787(3) Angstrom, V=3881(2) Angstrom(3), Z=4, structure refined to R=0.041, wR2=0.101 for 929 reflections with I > 2 sigma(1) and R=0.047 for all 1017 data; Mo2Cl4(CEP)(2)(NCCH(CH3)(2))(2) .(CH3)(2)CHCN (4), monoclinic, space group P2(1)/c (No. 14), a=11.686(1), b=16.750(3), c=22.297(2) Angstrom, beta=100.10(1)degrees; V=4297(2) Angstrom(3), Z=4, structure refined to R=0.039, wR2=0.096 for 4155 reflections with I> 2 sigma(I) and R=0.061 for all 5643 data. The structures consist of discrete Mo2Cl4P2N2 molecular units in which the two ML4 halves of the dinuclear units are eclipsed. The crystal packing may be influenced in each case by dipole-dipole interactions between CH2CN units. No coordination of the CEP nitrile groups to molybdenum is observed in the crystals of 2-4.