University of Tasmania

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Structural and mechanistic investigations of systematically modified bis(N-heterocyclic carbene) palladium complexes

posted on 2023-05-27, 10:44 authored by Curtis HoCurtis Ho
Alkylene-linked bis(imidazol-2-ylidene) based N-heterocyclic carbene (NHC) ligands of general formula [{(RIm)2CnH2n}2], where R = alkyl or aryl, n = 1, 2 or 3, remain topical in contemporary organometallic chemistry due to their extensive catalytic applications as transition metal catalyst ligands and also utility in advanced materials science. Their versatility is a result of the vast array of structural modifications possible that afford influences on metal complex geometry, alternate ligand binding mode preferences and metal complex reactivity. This thesis describes the synthesis, structure and reactivity of a series of systematically modified alkylene-linked bis(imidazol-2-ylidene) based NHC palladium complexes. We also present preliminary CO/ethylene copolymerisation catalytic studies and discussion on possible catalyst deactivation pathways relevant to base-mediated catalytic processes. A series of bis(NHC) palladium(II) complexes with different N-substituents, [{(RIm)2CH2}Pd(NCMe)2][PF6]2, where R = Me, i-Pr and 2,4-Me2Ph, were synthesised to investigate steric bulk influences of the bis(NHC) ligand motif on the reduction chemistry of palladium(II) complexes and structural effects on bimetallic bonding arrangements and Pd-Pd interactions in dipalladium(I) derivatives. These aims were justified with the literature synthesis of novel dipalladium(I) hydride complexes [{bis(˜í¬¿-carbene)}2Pd2H][PF6] under basic conditions, where 1,1'-di(aryl)-3,3'-methylenediimidazol-2,2'-diylidenes are used as the bis(NHC) ligand scaffold (aryl = 2,4,6-trimethylphenyl and 2,6-diisopropylphenyl). Unique Pd2H cores are exhibited between the two ligand systems in these dipalladium(I) complexes, with a Pd-Pd-H interaction for the former and Pd-H-Pd arrangement for the latter. A number of structural transformations are apparent in the overall reaction processes leading to the formation of the dipalladium(I) hydride complexes; namely; i, mononuclear to dinuclear, ii, bis(NHC) chelating to bridging, iii, palladium reduction and, iv, hydride formation. Comparative analysis of a range of isolated reactive intermediates, coupled with DFT computational studies aided in identifying many mechanistic steps. Of key importance is the steric congestion in a series of dimeric chelating bis(NHC) palladium(II) methoxide complexes as early stage intermediates. A series of variously contorted isomeric structures were identified as features that act as a reactivity trigger resulting in palladium complex structural transformation and reduction. These dimers are less favoured relative to the mononuclear complexes for the bulkier systems, which react via ˜í‚â§-hydrogen elimination giving Pd(II) hydride complexes leading to the observed final Pd(I) products. N,N'-asymmetrically substituted, methylene-linked bis(imidazol-2-ylidene) complexes have been prepared subsequent to a selective synthesis of the bis(imidazolium) salt precursors involving the quartenisation of N-alkyl and -aryl imidazoles with N-halomethyl imidazolium salts. The adaptability of the ligand precursor synthesis is illustrated through access to the N-Me/Nvçvªv•-Mes and N-Mes/Nvçvªv•-2,6-(i-Pr)2Ph systems, leading to the Pd(II) complexes [{(MeIm)(MesIm)CH2}Pd(L)2]n+, L = Cl/I (n = 0) and NCMe (n = 2), and [{(MesIm)[2,6-(i-Pr)2PhIm]CH2}Pd(L)2], L = Cl/I. The dicationic hybrid N,Nvçvªv•-alkyl/aryl complex was inactive in the copolymerisation of CO/ethylene, displaying reactivity akin to N,Nvçvªv•-dialkyl analogues. Base-assisted reactivity studies were also conducted for synthesised dicationic chelating bis(NHC) palladium(II) bis(acetonitrile) complexes and further extended the reactivity trend established in the previous studies, resulting in a dimeric palladium(II) methoxide complex for the N-Me/Nvçvªv•-Mes system and a reduced dipalladium(I) hydride species for the bulkier N-Mes/Nvçvªv•-2,6-(i-Pr)2Ph hybrid. The structural and reactivity consequences presented by the compounded effect of bis(NHC) alkylene linker length extension and the presence of bulky wingtip N-substituents was demonstrated by the synthesis of chelating ethylene- and propylene-linked bis(NHC) palladium(II) complexes [{(MesIm)2CnH2n}Pd(L)2]m+, n = 2 and 3, L = Br (m = 0) and NCMe (m = 2). Comparative structural analysis revealed systematic variation of key geometric parameters such as increased NHC to metal coordination plane angles and C-Pd-C bite angles with longer linker lengths. More interesting was the discovery of a novel on-metal bis(NHC) ligand transformation process unique to the ethylene-linked bis(NHC) ligand motif affording a carbene-carbene coupled tricyclic product. DFT computational studies aided in the formulation of a possible reaction pathway revealing an unusual chelating bis(nNHC/aNHC) palladium(II) intermediate crucial to the on-metal bis(NHC) ligand transformation. Preliminary experimental investigations were also conducted to support the proposed mechanism with the synthesis of metal precursors that may allow access to calculated reaction intermediates. CO/ethylene copolymerisation catalytic studies of prepared dicationic chelating bis(NHC) palladium(II) bis(acetonitrile) adducts and selected dimeric palladium(II) methoxide complexes are also discussed. Catalysis trials showed a general trend of increased activity with increased steric bulk, with the palladium(II) methoxide dimeric complexes outperforming their precursor dicationic bis(acetonitrile) adducts. An alternate synthetic route towards the generation of a monocationic chelating bis(NHC) palladium(II) methyl complex [{(MesIm)2CH2}Pd(Me)(NCMe)][PF6] is also presented via the formation of a bis(NHC) disilver(I) intermediate followed by transmetallation with [(COD)PdBrMe]. This pre-activated palladium(II) methyl complex proved to be the most active catalyst in CO/ethylene copolymerisation within the scope of this study. The various deactivation pathways possible during CO/ethylene copolymerisation catalysis are also discussed, with evidence suggesting non-innocent solvent interactions along with side reactions with catalysis substrates.


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Copyright 2014 the author

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