(ARYNE)METALLOCENE-DERIVED AND (ALKYNE)METALLOCENE-DERIVED DIMETALLIC ZIRCONIUM ALUMINUM COMPLEXES CONTAINING PLANAR-TETRACOORDINATE CARBON

Compounds X-[M2] of Lewis acidic main group metals can be added to very reactive alkyne-transition metal complexes (eta2-RC=CR)[M1] to form dimetallabicyclic organometallic compounds [M1](mu-,eta1:eta2-RCCR)(mu-X)[M2] containing a planar-tetracoordinate carbon atom, which is stabilized by the combined sigma-donor/pi-acceptor properties of the specific metal/ligand combination. This general synthetic scheme has been used to prepare such planar-tetracoordinate carbon complexes by reacting e.g. (eta2-aryne)-(PMe3)Z4Cp2 (12) with 2 molar equiv of diisobutylaluminum hydride. Initial Me3P.HAI(iBU)2 adduct formation generates the reactive (eta2-aryne)ZrCP2 intermediate, which is then trapped by additional hydridoaluminum reagent to yield the thermodynamically stable complex Cp2Zr(mu-eta1:eta2-C6H4)(mu-H)Al(iBU)2 (14a). Complex 14a crystallizes in space group P2(1)/n with cell parameters a = 16.749 (3) angstrom, b = 13.833 (1) angstrom, c = 20.178 (2) angstrom, beta = 90.74 (1)-degrees, R = 0.088, and R(w) = 0.093. It contains a planar-tetracoordinate carbon center [C(2)] at the bridgehead position of the dimetallabicyclic framework which is bonded to two carbon atoms [d(C(2)-C(l)) = 1.37 (2) angstrom, d(C(2)-C(3)) = 1.41 (1) angstrom], the zirconium [d(C(2)-Zr) = 2.430 (8) angstrom], and the aluminum atom [d(C(2)-Al) = 2.09 (1) angstrom, all values averaged over the two independent molecules, the sum of bonding angles around C(2) is 360-degrees]. The reaction between 12 and trimethylaluminum gave the analogously structured CP2Zr(mu-eta1:eta2-CrH4)(mu-CH3)AlMe2 COMPleX 14b; treatment of 12 with triethylaluminum furnished CP2Zr(mu-eta1:eta2-C6H4))(mu-CH2CH3)AlEt2 (14c). Complex 14b was characterized by X-ray diffraction (space group P2(1)/n, a = 9.151 (1) angstrom, b = 14.022 (1) angstrom, c = 14.415 (1) angstrom, beta = 104.56 (1)-degrees, R = 0.042, R(w) = 0.057). Again, carbon atom C(2) is planar-tetracoordinate [d(C(2)-C(l)) = 1.383 (4), d(C(2)-C(3)) = 1.423 (5), d(C(2)-Zr) = 2.481 (3), d(C(2)-Al) = 2.082 (3) angstrom; the sum of bonding angles at C(2) is 360-degrees]. The general synthetic scheme for the preparation of dimetallabicyclic "anti-van't Hoff/LeBel compounds" can also be applied to reaction sequences starting from stable (eta2-alkyne)metallocene complexes. Thus, the reaction of (eta2-cyclohexyne)(PMe3)ZrCp2 (11a) or (eta2-diphenylacetylene)(PMeOZrCp2 (11b) with excess trimethylaluminum gave the respective CP2Zr(mu-eta1:eta2-RCCR) (mu-CH3)AlMe2 products (7d,e). The reaction between the (tolane)metallocene complex 11b with a mixture of 9-BBN and triethylboron produced Cp2Zr(mu-eta1:eta2-PhCCPh)(mu-H)BEt2 (15) in low yield.