Biomedical subjects
Publications and source records attributed to Vernon C Gibson.
In an unprecedented transformation, amide ligands are found to attack the imine carbon centers of tridentate Schiff base ligands attached to tin. The process is reversible, and the resultant (masked) amide species can be exploited as latent single-site initiators for the controlled polymerization of rac-lactide.
Bis(imino)pyridine iron complexes bearing ether and thioether backbone substituents have been synthesized and evaluated for the polymerization of ethylene. The methoxy derivative is inactive whereas bulky phenoxides or thioether derivatives afford activities as high as the most active systems reported to date.
The bis(imino)pyridine iron complex, [[2,6-(MeC=N-2,6-iPr2C6H3)2C5H)N]FeCl2] (1), in combination with MAO and ZnEt2 (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR2 product, a Poisson distribution of linear alkanes is obtained; linear alpha-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe2 and ZniPr2 also show catalyzed chain growth; in the case of ZnMe2 a slight broadening of the product distribution is observed. The products obtained from Zn(CH2Ph)2 show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh2 shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR3 (R = Me, Et, octyl, IBu) and GaR3 (R = Et, nBu) act as highly efficient chain transfer agents, whereas GaMe3 exhibits behavior close to catalyzed chain growth. LinBu, MgnBu2 and BEt3 result in very low activity catalyst systems. SnMe4 and PbEt4 give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt2 compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn-C bond, and (4) a reasonably close match in Zn-C and Fe-C bond strengths.
Group 4 metal complexes containing phenoxy-amide ligands bearing soft pendant donors are shown to give more highly active ethylene polymerisation catalysts than counterparts containing hard donors or systems without a pendant donor.
Vanadium complexes containing bis(benzimidazole)amine ligands, upon activation by simple alkylaluminium reagents, give unusually robust, single-site, catalysts for olefin polymerisation/co-polymerisation.
Interest in the chemistry of ferrocene remains intense, largely due to applications within catalysis. New synthetic routes to unsymmetrical ferrocene ligands have provided another facet to this area, as substituents can be designed to be electronically- and/or sterically-distinct in order to affect the environment around the catalytically-active metal centre. This critical review provides a concise summary of the synthetic routes that have been applied to the synthesis of unsymmetrical ferrocene ligands, along with a systematic survey of the applications of these ligands in homogeneous catalysis. The aim is to help the reader select a suitable ferrocenediyl ligand for a particular synthetic application, and in the synthesis of ligands that require particular structural and/or electronic features.
Two new unsymmetrical 1'-substituted hydroxyferrocene ligands featuring either phosphine or phosphine oxide substituents have been synthesised and the phosphine oxide derivative has been structurally characterised. A nickel complex of the hydroxyl/phosphine ligand has been formed, along with preliminary evaluation of the complex for catalysis of ethylene polymerisation.
Reaction of Me(3)Al (one equivalent) with the bis(imino)phenol, [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] (I)(Ar = 2,6-Pr(i)(2)C(6)H(3)) in toluene at ambient temperature yields the yellow complex [Me(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](1). Interaction of two equivalents of Me(3)Al in refluxing toluene affords the red complex [(Me(2)Al)(2)[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](2). Similar interaction (two equivalents, refluxing toluene) of MeAlCl(2) or (i)Bu(3)Al with [2,6-(ArNCH)(2)-4-MeC(6)H(2)OH] affords [ClAl[2,6-(ArNCH)(2)-4-MeC(6)H(2)O](2)](3) or [(i)Bu(2)Al[2,6-(ArNCH)(2)-4-MeC(6)H(2)O]](4), respectively. Hydrolysis of 2 readily affords the iminoaminophenol ligand [2-(ArN=CH)-6-ArNHCH(Me)-4-MeC(6)H(2)OH](II), which reacts further with Me(3)Al to afford [Me(2)Al[2-ArNCH(Me)-6-(ArNCH)-4-MeC(6)H(2)O]](5). An X-ray study on reveals bidentate imino-alkoxide ligation about the distorted aluminium centre, whereas is a binuclear structure with tetrahedral aluminiums ligated by imino-alkoxide and amido-alkoxide ligand fragments, respectively. For and bidentate imino-alkoxide ligation is observed.
A new family of aluminum complexes bearing tetradentate bis(aminophenoxide) ligands is reported and shown to initiate the living ring-opening polymerization of rac-lactide. The microstructures of the polylactide products are found to be highly dependent upon the ancillary ligand substituents, ranging from highly isotactic (Pm = 0.79) to very highly heterotactic (Pr = 0.96).
The synthesis of the unsymmetrically substituted beta-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N'-bis(2,6-diisopropylphenyl) beta-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex, (BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds, and reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).
Pincer-heterocyclic carbene complexes of Cr(III), of the form [2,6-(1-alkylimidazol-2-ylidene)pyridine]CrCl3, have been prepared and evaluated as catalysts for the oligomerization of ethylene to alpha-olefins. In combination with methylaluminoxane cocatalyst, exceptionally high activities are obtained, ranging up to ca. 40 000 g mmol-1 bar-1 h-1.
Iron(II) complexes bearing monoanionic tridentate salicyladiminato ligands are shown to be highly efficient catalysts for atom transfer radical polymerization (ATRP). Polymerization rates for styrene are among the highest reported for iron-mediated ATRP in nonpolar media, correlating well with E1/2 potentials and DeltaEp values for the complexes. The rigidity of the tridentate ligands, combined with ample space around the metal center to accommodate a halogen atom, we believe to be an important factor in the efficient ATRP behavior of these systems.
Three classes of ligands, designed to explore the effect of variations on the central pyridine donor core in bis(imino)pyridine iron and cobalt ethylene polymerization catalysts of the general formula [LMCl(2)] (M = Fe or Co), have been prepared. The first class comprises six-membered N-heterocycles (pyrimidine and triazine) and the second class five-membered heterocycles (furan and thiophene) as the central donor core. In the third class of ligands, the imine donor arm has been extended by one carbon to give anionic tridentate ligands based on carbazole and neutral analogues based on dibenzofuran and dibenzothiophene. The coordination behavior of these ligands upon reaction with FeCl(2) or CoCl(2) has been investigated, whereby only in the case of the neutral pyrimidine or the anionic carbazolide unit as the central donor core have stable complexes been obtained. Ethylene polymerization results are compared with the parent bis(imino)pyridine iron and cobalt catalyst systems.
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Bis(imino)pyridine cobalt(I) n-alkyl complexes react with ethylene by beta-hydrogen transfer, allowing direct study of a termination step commonly encountered in polymerisation and oligomerisation catalysis.
Four-coordinate iron(II) complexes bearing alpha-diimine ligands with alkyl substituents are shown to be efficient catalysts for the well-controlled atom transfer radical polymerisation of styrene; catalysts containing aryldiimine ligands support competitive beta-hydrogen chain transfer processes.
Addition of MeLi to bis(imino)pyridines results in an unprecedented nucleophilic attack at pyridine nitrogen to afford novel mono-anionic [N,N,N] ligands: their treatment with FeCl3, followed by MAO activation, affords highly active ethylene polymerisation catalysts.
p-tert-Butylcalix[8]areneH8 reacts with WCl6 forming a number of new metallocalix[8]arenes, one of which is reduced to give the first example of a metal-metal triple bond supported by a large calixarene ligand; the degree of metallation and conformations adopted by the newly generated metallocalix[8]arene systems are determined by single crystal X-ray diffraction.