| Abstract |
The reaction between the binuclear dicationic compound [(PPh3)(2)Ni\mu-(eta-C5H4)CMe2(eta-C5H4)\Ni(PPh3)(2)][PF6](2) (1) and either 2 or 4 equivalents of isocyanides leads to the substitution of a single PPh3 ligand per nickel atom. The new binuclear compounds [(CNR)(PPh3)Ni\mu-(eta-C5H4)CMe2(eta-C5H4)\Ni(PPh3)(CNR)][PF6](2) (R = S-(-)-CH(Ph)Me (2), cyclohexyl (3), tert-butyl (4) and mesityl (5)) were prepared in almost quantitative yields and were fully characterised by elemental analysis, FT-IR and NMR spectroscopies. In the X-ray crystal structure of 4 two crystallographically inequivalent CpNi(PPh3)(CN Bu) moieties are observed, where the Ni(PPh3)(CN Bu) fragments are positioned on the C5H4 faces of the bridging ligand to minimise inter-ring repulsions. Addition of an excess of S-(-)-CNCH(Ph)Me to 1 results in the catalytic polymerisation of the isocyanide, yielding polymers of increasing molecular weights when the ratio [CNR](o)/[Ni](o) is increased, consistent with non-living polymerisation. The molecular weights of the poly(S-(-)-1-phenylethyl isocyanide) (6) obtained with complex I are higher than those obtained with the related mononuclear complex [Ni(eta-C5H5)(PPh3)(2)]PF6 (8), which suggests an initiation step involving migratory insertion of isocyanide into the Ni-cyclopentadienyl bonds. An EPR spectrum of the reaction mixture acquired at 77 K, after 10 min of polymerisation at 20degreesC, reveals the existence of two pentacoordinated Ni(I) 3d(9) chain propagating species with trigonal bipyramidal geometry (d(z2) ground state). One of the Ni species contains two PPh3 ligands occupying both axial positions of an axially distorted trigonal bipyramid, whereas the other species contains an axially coordinated PPh3 and either a PPh3 coordinated in the equatorial plane or dissociated from the nickel atom. (C) 2004 Elsevier Ltd. All rights reserved. |
