A computational and synthetic study of poly(benzyl phenyl ether) dendrimers

Abstract

Organic and organochromium poly(benzyl phenyl ether) dendrimers have been investigated by synthetic · and computational methods. The first generation organic benzyl alcohol and bromide wedges have been prepared, and a first generation organic dendrimer was synthesised. The preparation of novel organochromium dendrimers has been investigated. Several chromium arene complexes were prepared by different routes, and the halogenation methodology was thoroughly explored with various reagents. The target compound, a chromium complexed first generation dendritic wedge, was not accessible by any of the methods investigated. The novel compound (dibenzyl ether)bis[tricarbonylchromium(O)] has been prepared by two routes and the crystal structure of this complex was determined. Several other chromium arene complexes were synthesised, and the crystal structure of one of these, (benzyl methyl ether)tricarbonylchromium(O), was determined. The structure and properties of the organic and organochromium dendrimers have been investigated by molecular mechanics and molecular dynamics techniques. The existing CHARMm polymer force field was extended to include parameters for the tricarbonylchromium moiety. The two crystal structures solved as part of the synthetic effort, were used in this regard. The accuracy of the new parameters was assessed by simulation of the crystal structure of ( dibenzyl ether)bis[tricarbonylchromium(O)]. The important ether linkage torsion angle parameter, which plays a significant role in the topology of the dendrimer, was singled out for refinement. The torsion angle was parameterised with the model compound benzyl phenyl ether, by fitting the CHARMm results for rotation about the dihedral, to ab initio torsional data. Molecular dynamics simulations have been performed on generations one through five of the organic and organochromium dendrimers. The radius of gyration and RMS variation were investigated as a function of generation for both dendrimer series. The radius of gyration was found to increase exponentially with generation, whereas the relationship of RMS variation to generation was not as well defined. Density distributions were calculated for all five generations of the organic and organometallic dendrimers. In addition, the distribution of the monomers from each generation within the dendrimer was analysed. In the later generation dendrimers, the monomers belonging to the earlier generations are extended, while the terminal groups are mobile and found in all regions of the dendrimer. The solvent accessible surface was calculated for the organochromium dendrimers. It was concluded from these results that although the terminal groups are mobile, the majority of these monomers remain on the periphery of the dendrimer, or in solvent accessible areas.