Coordination chemistry of rationally designed, multidentate phosphine ligands

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Abstract

Rational ligand design is a powerful strategy in organometallic chemistry. By carefully considering what features a ligand will need to possess for certain applications, drawing on information in the literature and, perhaps, computational insight, appropriate systems can be designed, enabling more efficient experiments to be conducted. This could have wide-ranging application in coordination chemistry and catalysis, with the potential to influence metal-based reactivity through modification of the coordination spheres. In this thesis, the application of ligand design strategies to three projects is presented. The roles of the primary and secondary coordination spheres are evaluated and the group 9 coordination chemistry of three multidentate phosphine ligands is investigated. These ligands were designed or chosen to achieve particular goals.

By considering the role of pincer ligands in the primary coordination sphere, [M(PNP-Np)(biph)][BArF4] complexes were synthesised (biph = 2,2’-biphenyl, M = Rh/Ir, BArF4 = B[3,5-(CF3)2C6H3]4), exploiting the flexibility of the neopentyl chains to investigate agostic interactions in these complexes. The results are in contrast to PNP-tBu analogues, which do not possess significant agostic interactions.

The versatility of pincers is also utilised, in combination with secondary coordination sphere considerations, for the design and synthesis of a resorcinarene-derived pincer ligand. The overarching aim was to design a system that could stabilise an alkane ligand in the solid and solution states. Whilst not yet achieved, this synthesis is described, and rhodium carbonyl complexes of this ligand are presented.

Finally, the focus was shifted to reaction control in the secondary coordination sphere, and a resorcinarene-derived phosphine-phosphite, previously synthesised in the Chaplin group, was investigated in the rhodium-catalysed hydroformylation of alkyl alkenes. The system showed branched selectivity that increased significantly with increasing alkene chain length, attributed to cavity effects.

Overall, this work highlights how the implementation of rational ligand design approaches can be a useful strategy for chemists.

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