ALKYLIDENES BY ALPHA-H ABSTRACTION FROM METALLACYCLES - SYNTHESIS AND CHARACTERIZATION OF ALKYLIDENE-BRIDGED COMPLEXES OF NICKEL

The 16e- pseudoallylic complex Ni(eta3-CH2C6H4-o-Me)CI(PMe3) effects alpha-C-H abstraction from the benzoannelated nickelacycle Ni(CH2CMe2-o-C6H4)(PMe3)2 (1) under very mild conditions, with formation of the tetrametallic species [Ni2(mu2-CHCMe2-o-C6H4)(PMe3)2]2(mu-Cl)2 (2), in which two identical metal-metal-bonded bimetallic subunits are held together by chloride bridges. Each of these moieties incorporates a bifunctional organic fragment that possesses alkylidene and aryl termini that bridge the two metal atoms. Rupture of the (Ni2)2-(mu-Cl)2 bridges by PMe3 produces a binuclear nonsymmetric alkylidene, Ni2(mu2-CHCMe2-o-C6H4)Cl(PMe3)3 (3), also exhibiting alkylidene and aryl bridges. A similar reaction of 2 with Me2PCH2CH2PMe2 (dmpe) furnishes the related, albeit cationic, alkylidene species [Ni2(mu2-CHCMe2-o-C6H4)(dmpe)2]Cl(4). Interaction of the tetranuclear alkylidene 2 with NaCp produces the bimetallic product CpNi(mu2-CHCMe2-0-C6H4)Ni(PMe3)2 (5). This complex can also be obtained by alpha-H abstraction from the methylene group of the metallacycle 1 by the 18-electron allyl derivative CpNi(eta3-C3H5). The former reaction route also allows the synthesis of the analogous derivatives (C5H4R)Ni(mu2-CHCMe2-0-C6H4)Ni(PMe3)2 (R = Me(6), i-Pr(7)). Different kinds of mu-alkylidene compounds are furnished by the reaction of 2 and the poly(pyrazolyl)-borate salts K[HB(pz)3] (Pz = pyrazolyl) and TI[H2B(3-t-BU-PZ)2]. These new complexes have eta2-bonded poly(pyrazolyl)borate ligands that bridge the two Ni atoms, i.e. Ni2(mu2-CHCMe2-0-C6H4)[mu2-eta1:eta1-HB(PZ)3](PMe3)2 (8) and Ni2(mu2-CHCMe2-0-C6H4)[mu2-eta1:eta1-H2B(3-t-Bu-pz)2]-(PMe3)2 (9), respectively. Complexes 2 and 3 have been characterized by X-ray crystallography. Compound 2 crystallizes in the monoclinic space group P2(1)/n with a = 9.815(4) angstrom, b = 10.550(4) angstrom, c = 19.669(7) angstrom, beta = 104.39(3)-degrees, and Z = 4. Crystals of 3 are orthorhombic, Pbca, with a = 17.152(4) angstrom, b = 18.431(3) angstrom, c = 32.577(5) angstrom, and Z = 16. Both display short metal-metal contacts (2.384(3) and 2.393(3) angstrom, respectively) suggestive of strong Ni-Ni bonding interactions. The alkylidene fragment symmetrically bridges the two Ni atoms in 2 and 3, but while in 2 the aryl bridges are also symmetric, in 3 one of the Ni-C(aryl) distances at 2.17(1) angstrom is perceivably longer than the other (1.99(2) angstrom). Compound 5 has also been characterized by X-ray crystallography. It crystallizes in the monoclinic space group P2(1)/n, with unit cell constants a = 9.013(2) angstrom, b = 16.675(2) angstrom, c = 15.291(7) angstrom, beta = 91.04(3)-degrees, and Z = 4.