Dinuclear cerium tetraphenolate complexes: from Lewis acid catalysts to redox chemistry

Abstract

Molecular cerium complexes provide opportunities to take advantage of intrinsic characteristics of cerium, such as its strong Lewis acidity and redox chemistry for homogenous catalysis, and its high earth abundance has sustainability benefits. Cerium chemistry and general f-block chemistry are introduced in order to outline the objectives and challenges that arise due to the unique properties of these elements. A review of cerium(IV) complexes is provided, focusing on interesting synthetic routes and reported applications in homogenous catalysis. Dinuclear complexes relevant to this work are also discussed.

Synthetic studies into homodinuclear tetraphenolate (TP) complexes of cerium(III) and cerium(IV), both of the metal’s easily accessible oxidation states, are presented. The TP ligands used are of the general formula [{2-(OC6H2R1R2-2,4)2CH}2-Ar]4-, where the aryl spacer (Ar) is either a para- or meta-substituted phenyl ring, or a terphenyl bridge. New TP ligands (where R1 = Adamantyl; R2 = Me) have been prepared and along with existing ligands (R1 = R2 = tBu), used to synthesise and characterise various new dinuclear cerium(IV) complexes, where the two metal centres are linked together by one TP ligand. A general synthetic route to cerium-siloxide complexes is proposed, by forming an in situ salt-metathesis/protonolysis reagent by addition of sodium trimethylsiloxide to tetraethylammonium hexachlorocerate(IV). The stabilisation of the +4 oxidation state is shown to be very strong, e.g. Epc = -2.47 V vs. Fc/Fc+ for [NEt4]2[Ce2Cl2(OSiMe3)4(pTPtBu)].

The new di-Ce(IV) complexes have been tested for their suitability as catalysts for anhydride/epoxide ring-opening copolymerisation (ROCOP). The molecular structure – controlled by TP and other ligands – has a direct effect on catalytic activity, which is probed in detail for phthalic anhydride/cyclohexene oxide ROCOP. The data suggest that complexes with the closest cerium-cerium distances – those with the meta¬-substituted phenyl spacer – are the best initiators, alluding to the presence of a cooperative effect. Kinetic and computational studies investigate this further, indicating a first order dependence on catalyst concentration and that dinuclear reaction pathways are accessible for both para¬- and meta-spacer complexes. The fastest catalysts are highly active, the complex [NEt4]2[Ce2Cl2(OSiMe3)4(mTPAd)] achieved a TOF = 246 h-1, all have generally good control over polyester formation (ester linkage selectivity >95%), and are compared to current state-of-the-art systems. More challenging monomers were also handled successfully and an end-group analysis of polymers using MALDI-ToF mass spectrometry has been carried out, showing that chloride ligands are the most probable initiating group. Also discussed, is exploratory work into complexes with both [Ce2TP] and [Ce2TP2] motifs. Various cerium starting materials and oxidising agents are used with the objective of identifying simple ways to switch between the cerium(III) and cerium(IV) oxidation states, and the resulting potential effects on reactivity.

At the end of the thesis, a summary of the work is presented, conclusions offered and suggestions on where further progress on these projects may be made. Experimental details, procedures and characterisation data are also provided.