Dinuclear zinc complexes using pentadentate phenolate ligands.

The synthesis and characterization of 15 dinuclear zinc complexes are reported, including the X-ray crystal structures of 5 complexes. The ligand motif utilizing p-cresol as a bridging unit between the two zinc centers and a set of three related ligands has been synthesized; 2,6-bis(R)-p-cresol (where R is CH(2)NMe(CH(2))(2)NMe(2) = L(1), CH=N(CH(2))(2)NMe(2) = L(2), and CH(2)NH(CH(2))(2)NMe(2) = L(3)). Dizinc trihalide complexes [L(n)Zn(2)(mu-X)X(2)] (where X = Cl, Br, I) have been prepared. The trihalide complexes were treated with potassium ethoxide, resulting in quantitative substitution of the bridging halide group to give [L(n)Zn(2)(mu-OEt)X(2)]. The dizinc ethoxide complexes were tested in situ as initiators for lactide ring opening polymerization. Dizinc triacetate complexes have also been synthesized [L(n)Zn(2)(OAc)(3)], as well as cationic diacetate species containing two bridging acetate groups, [L(n)Zn(2)(mu-OAc)(2)][BPh(4)]. Structural differences between complexes of the three ligands are discussed.

Zirconium catalysed enantioselective hydroamination/cyclisation.

A chiral zirconium alkyl cation catalyses the cyclisation of certain aminoalkenes with enantioselectivity up to 82%, the highest thus far observed for such a process.

Problems and solutions for alkene polymerisation catalysts incorporating Schiff-bases; migratory insertion and radical mechanisms of catalyst deactivation.

Steric blocking of an intramolecular 1,2-migratory insertion reaction of a zirconium salicylaldiminato complex leads to a long-lived catalyst for ethene polymerisation, but promotes a new radical catalyst decomposition mechanism in certain instances; kinetic and thermodynamic parameters for both pathways have been established.

Group 4 catalysts for ethene polymerization containing tetradentate salicylaldiminato ligands.

The structural properties of three new classes of titanium and zirconium complex bearing tetradentate salicylaldiminato proligands are elucidated using X-ray diffraction and NMR spectroscopy. On activation with MAO co-catalyst, their behaviour in ethene polymerization depends strongly on the nature of the structure and the substitution pattern. One titanium complex based on a 2-aminobenzylamine (C3-chain) backbone has a trans arrangement of the co-ligand sites and, unsurprisingly, does not polymerize ethene. The 1,8-diaminonaphthalene (C3-chain) backbone gives a rather ring-strained cis complex, but was also unproductive. A range of cis complexes of zirconium with the 2,2'-diaminobibenzyl (C6-chain) backbone give low to moderate productivities of multimodal poly(ethene), while in contrast the structurally analogous titanium compounds provide highly active, single site catalysts. Thermal degradation of these catalysts is slowed significantly by a substitution pattern on the phenolate unit which sterically protects the imine donor unit; a phenomenon which has been previously observed in much lower activity catalysts based on 2,2'-diaminobiphenyl (C4-chain) but which does not improve the stability of the very highly active unbridged systems.

Chiral biarylamido/anisole complexes of yttrium in enantioselective aminoalkene hydaroamination/cyclisation.

A group of chiral, dibasic, biaryl-bridged amido proligands containing peripheral methoxyphenyl (anisole) ligation are developed for the synthesis of new amide complexes of yttrium and lanthanum. A potentially tetradentate bis(amidoanisole) system gives, on reaction with [Y[N(SiMe(2)H)(2)](3)(THF)] a crystallographically-characterised bis complex [Y(H)] presumably as a result of low steric demand, since a more bulky version gives the target [Y[N(SiMe(2)H)(2)](THF)]. The molecular structure of the latter reveals a similar cis-alpha structure to our recently reported Schiff-base analogue. Variable-temperature NMR studies are consistent with low rigidity in the molecular structure. A potentially tridentate, amidoanisolyl/amido proligand gives complexes [M[N(SiMe(2)H)(2)](THF)(n)](M = Y, n= 1; M = La, n= 2). Chiral non-racemic versions of the above complexes were tested in the hydroamination/cyclisation of 2,2'-dimethylaminopentane to the corresponding pyrrolidine. Activities were relatively low compared to recently reported examples, and ee values were in the range 20-40% despite the well-expressed chirality of the catalysts.