Dr John Errington

john.errington@ncl.ac.uk

Dr. Errington's interests lie at the interface between coordination/organometallic and solid state chemistry, where effects arising from the transition from small molecules to extended solid systems provide opportunities for advances in catalysis and materials science. Fundamental studies of solution reactivity, dynamics and aggregation processes underpin a "bottom-up" approach to the design and synthesis of metal-oxide based functional molecules and materials, where a key challenge is the control of aggregation so that the composition, size and morphology of large, polynuclear species and extended lattices can be engineered to give well defined reactive sites, surfaces and bulk properties. Metal alkoxides and polyoxometalates are used extensively as versatile procatalysts and/or molecular building blocks and their chemistry is under extensive investigation.

General design principles for co-operative dinuclear catalysis were established in studies of aluminium complexes of N-alkoxy-β-ketoimines, whereby treatment of inactive chloro complexes with one equivalent of epoxide produced efficient initiators for living lactide polymerisation. Insertion of the epoxide into one Al—Cl bond generates a nucleophilic alkoxide ligand on one metal adjacent to the electrophilic binding site for the lactide substrate, creating the optimum arrangement for ring-opening. The remaining chloride can shuttle between metals on the opposite face of the complex as the roles of the metals at each insertion cycle are reversed.

A major challenge in metal alkoxide chemistry is the controlled assembly of heterometallic aggregates. We have been able to promote metal alkoxide aggregation by the strategic introduction of bridging methoxide ligands, as demonstrated by the preparation of the serpentine Ti4(OMe)6(OPri)10from Ti(OPri)4 as an intermediate en route to the fully condensed Ti4(OMe)16. This principle was successfully applied to the systematic preparation of a series of mixed-metal alkoxides [{(L)M'M(OMe)(OPri)4}2] for exploration of Lewis acid catalytic activity (M = Ti, Zr, Hf and M' = Li, Zn), which were shown to be active initiators for living ring-opening polymerisation of ε-caprolactone. These compounds are dynamic in solution and detailed NMR studies of the Zn2Zr2 compound revealed a concerted 'flip' of the Zn centres from one side of the molecule to the other via a symmetric transition state.

Polyoxometalates possess an unrivalled range of properties and interest in these molecular metal-oxide semiconductors is growing rapidly due to potential applications in catalysis, materials, bioscience and medicine. Through the development of synthetic methodology involving the controlled hydrolysis of metal alkoxides in the presence of oxoanions we have been able systematically to manipulate surface ligands and heterometal for the first time in a range of molecular oxides [(X)M'M5O18]n- (M = Mo, W; M' = Ti, Zr, Hf, V, Nb). 17O enrichment is routine with this methodology and changes to the oxide framework are readily monitored by 17O NMR.

Protonolysis studies of [(MeO)TiW5O18]3- and [(μ-MeO)2(ZrW5O18)2]6- revealed that Ti remains 6-coordinate, while Zr adopts 7- or 8-coordination unless steric pressure between organic groups and the oxide surface prevents coordination expansion, as in aryloxide derivatives [(ArO)ZrW5O18]3- which do not dimerise. An important finding in the zirconium system was that dissociation of weakly binding ligands provides a pathway for aggregation of ZrW5 building blocks. The conversion of the structurally characterised [(μ-Ph2PO2)2(ZrW5O18)2]6- to [(μ-HO)2(ZrW5O18)3H]7- suggested that large clusters should be accessible by teatment of [(-MeO)2(ZrW5O18)2]6- with acids of non-coordinating anions. Preliminary experiments support this proposal and we are developing this approach as a rational method for polyoxometalate aggregation. We have already established the linkage of hexametalate units through oxo or diolate bridges in [(μ-O)(MW5O18)2]n- (M = Ti, n = 6; M = Nb, n = 4) and [(μ-1,4-OC6H4O)(TiW5O18)2]6- respectively.

Analogies between polyoxometalates and solid oxides suggest that heterogeneous catalytic processes may be modelled using polyoxometalates. The structural characterisation of protonated ZrW5 derivatives [(μ-HO)2(ZrW5O18)2]6- and [(μ-HO)2(ZrW5O18)3H]7- provides insight into the strong acid behaviour of tungstated zirconia and titania, which have been proposed to contain polynuclear structures at their surfaces, whilst NMR studies of site selectivity in protonation and methylation reactions of these hexametalates are revealing interesting kinetic effects. The introduction of bulky alkoxides onto the TiW5O18 framework provided evidence for the formation of Ti=O via C—H bond activation at the oxide surface, by analogy with the reverse reaction, alkene activation, on solid acid polyoxometalates. We are now exploring the introduction of metal-bound hydrocarbyl groups into polyoxometalates, a long-standing challenge which should shed light on fundamental transformations involved in Fischer-Tropsch and related heterogeneous processes.

The incorporation of functional molecular oxides into materials to exploit their electronic, photoactive, magnetic or chemical properties requires the development of strategies for immobilisation and organisation. We have demonstrated the covalent linkage of TiW5O18 units to single-crystal and porous silicon by protonolysis of the metal-alkoxide bond in [(MeO)TiW5O18]3-.

Isolated clusters of oxometalates were formed rather than monolayer coverage, and competitive hydrolysis of Ti—O linkages occurred over time. We have extended our studies to other structural units and linkages that are more resistant to hydrolysis.