This thesis details the design of a novel ortho-substituted bispyridyl-pyrazole ligand, Ph2Hbpp(3,5-bis(6-phenylpyridin-2-yl)pyrazole) and the synthesis, structure and reactivity of its mono-,di-, and multinuclear late-3d and late-4d metal coordination complexes.To give context to this work, Chapter 1 introduces the design aspects of dinucleating ligandframeworks, with a focus on the versatile pyrazolate bridging moiety and its 3,5-bis(pyridyl)derivatives known as Hbpp ligands. A review of ortho-substituted Hbpp ligands is presented,which discusses the range of reported modifications with additional donor atoms and tuning ofsteric properties to result in various di- and multinuclear metal complex with uniquearchitectures. The limitations in some of these ligands, and the unexplored aspects of theirdesign and application are also discussed.Results Chapter 2 presents the design and synthesis of the novel ortho-substituted Hbppderivative Ph2Hbpp (3,5-bis(6-phenylpyridin-2-yl)pyrazole), and its versatility in the selectivesynthesis of homometallic mono-, di- and tetranuclear Pd(II) complexes, such as[(Ph2Hbpp)PdII(MeCN)]BF4, [(Ph2bpp)PdII2(MeCN)2]BF4 and [(Ph2bpp)PdII2(μ-OAc)]2. Theseinclude the first example of a dinuclear (or multinuclear) Pd complex based on the bppframework.These complexes were spectroscopically and crystallographically characterised, and theirstructures compared with respect to nuclearity and M···M distances, with [(Ph2bpp)PdII2(μ-OAc)]2containing an interesting ‘bis-clam-shell’ motif with two parallel pairs of co-facial Pd(II) centresexhibiting closed-shell interactions.Chapter 3 assesses the electrochemical properties of the ligand and its Pd complexes by cyclicvoltammetry, and chemical oxidation of the Pd complexes. Ph2Hbpp was found to be redox inert(between –2.75 V and 1.10 V (vs Fc/Fc+)), and the Pd complex generally resistant to reductionowing to their cyclometalated structures. The mono- and dinuclear complexes oxidised from 0.9V onwards, while the tetranuclear [(Ph2bpp)PdII2(μ-OAc)]2 displayed significantly lower oxidationpotentials owing to dinuclear redox synergy between pairs of axially interacting Pd centres. Broadpseudo-reversible oxidation peaks at 0.58 V and 0.69 V were assigned as each 2e– events for anoverall 4e– oxidation to an unprecedented Pd(III)4 state with a proposed structure of[(Ph2bpp)PdIII2(μ-OAc)]24+. This theory was corroborated by modelling the two proposed 2e–processes in simulated CVs that fitted well to the experimental data, and the results of chemicaloxidation experiments that required two equivalents of two electron oxidant for full conversion.The robustness of the C–Pd bonds in these complexes from in the N,Nʹ,C-trichelate was alsoevident in its inertness towards C–C bond reductive elimination or protonation by a strong acid.Preliminary tests in catalytic activity in the oxidation of alkenes and alcohols was assessed, withthe tetranuclear [(Ph2bpp)PdII2(OAc)]2 catalysing the aerobic oxidation of alcohols with air,attributed to the accessible high oxidation states afforded by Pd-Pd closed-shell interactions.Chapter 4 demonstrates the ability of Ph2Hbpp to support non-cyclometalated homodinuclear[LM2] complexes based on square-planar (d8) metals Rh and Ir, with [(Ph2bpp)(RhI(CO)2)2Cl],[(Ph2bpp)(MI(COD))2Cl] (MI = Rh(I), Ir(I)) and their cationic analogues [(Ph2bpp)(RhI(CO)2)2]PF6and [(Ph2bpp)(MI(COD))2]BF4. The complexes are complimentary to the cyclometalated Pd(II)complexes presented in Chapter 2, and are an expansion to the hitherto sparingly reportedexamples of 4d bpp complexes. All but one of these complexes were crystallographicallycharacterised, in which the influence of the Ph2bpp– phenyl groups on the metal coordinationsphere was clear by significant steric interactions with proximal co-ligands, resulting in a largerRh···Rh distance that in the structurally characterised analogue [(bpp)(RhI(COD)2)2Cl] withunsubstituted bpp–. The ability of Ph2bpp–to be hemi-labile was also demonstrated in the neutralCOD complexes in solution.Finally, Chapter 5 expands the use of Ph2Hbpp to constructing 3d metal (Cu, Ni) homometallicmultinuclear complexes, and reports the attempted syntheses of heterometallic Pd-3dcomplexes from the mononuclear precursor [(Ph2Hbpp)PdIICl]. In transmetallation reactions ofK[Ph2bpp] with various Cu(I) precursors, the coordinating strength of the solvent, and the sizeand coordinating strength of co-ligands on Cu were found to significantly affect the outcome, andthe exploitation of these factors in the synthesis of proposed dinuclear [(Ph2bpp)CuI2]-typecomplexes is discussed. Synthesis and spectroscopic and crystallographic characterisation ofhexanuclear multi-ligand cluster [(Ph2bpp)4(CuI6)](BF4)2 and the [2x2]-grid complex[(Ph2bpp)4CuI4] and its mixed-valence congener [(Ph2bpp)4CuI2CuII2](PF6)2 are presented. Closelygrouped Cu centres and a flexible metallic core made accessible by ligand-defined ‘clefts’ in thesupramolecular structure of [(Ph2bpp)4CuI4] and [(Ph2bpp)4CuI2CuII2](PF6)2 were analysed withthe aid of structurally XRD data and van der Waal’s surface modelling. The new mononuclearNHC complex [(bpp)(IPr)PdIICl] based on the unsubstituted bpp scaffold is also presented, andattempts to access heterodinuclear complexes Pd-M' from it are discussed with respect to itsisomerisation behaviour.The main findings and conclusions of this thesis are summarised in Chapter 6, alongsidediscussion of promising avenues of future work with the Ph2Hbpp ligand and its variety ofcoordination complexes, and the how the unique ligand synthesis route that has been developedmight be exploited to afford further homo- or heteroditopic ortho-substituted Hbpp ligands.
Synthesis and Reactivity of Di- and Multinuclear Palladium and Late Transition Metal Complexes with Bipyridyl-pyrazolate Ligands: (Alternative Format Thesis)
Coles, A. (Author). 25 Mar 2026
Student thesis: Doctoral Thesis › PhD