Are cationic [eta]3-allyl complexes general precursors to metallacyclobutanes? Synthesis and structure of [mu]-chloro-bis[([eta]3-allyl)-2,2'-bipyridyldicarbonylmolybdenum(II)] tetrafluoroborate

Abstract

Silver tetrafluoroborate reacts with Cl(bipy)(CO)2([eta]3-C3H5)Mo in CH2Cl2 to give the dimeric cation, ([mu]-Cl)[(bipy)(CO)2([eta]3-C3H5)Mo]2+, isolated as the crystalline BF4- salt with CH2Cl2 of solvation (5). Complex 5 crystallizes in the triclinic system (space group P, No. 2) with cell parameters: a 11.831(2), b 10.142(3), c 15.618(3) A; [alpha] 93.96(2), [beta] 104.33(2), [gamma] 91.41(2)[deg], V 1809.5(8) A3 Z = 2, [varrho]calc 1.60 g cm-3. Full matrix refinement with all but three atoms anisotropic converged at R1 = 0.057 and R2 = 0.073 based on 3742 reflections with I > 30(1). The two halves of the dimer are connected by a single chloride bridge (Mo-Cl 2.554(3), 2.519(3) A, Mo-Cl-Mo 134.0(1)[deg]). The [eta]3-allyl group is oriented so that its open face points toward the two cis-carbonyl groups, a feature common to all L2X(CO)2Mo([eta]3-allyl) structures determined to date. A molecular orbital analysis shows that this rotational preference of the allyl group has its roots in the strong [pi]-bonding character of the carbonyls. The MO analysis also provides a rationale for regioselectivity observed in the reactions of various [eta]3-allyl complexes with nucleophiles. In particular, the factors which determine whether the terminal carbons are attacked (giving olefin) or whether the central carbon is attacked (giving a metallacyclobutane) are exposed.