Examination of catalytically relevant palladium `N`-Heterocyclic carbene complexes
Creators: name: Vanston, CR
Publisher: name: University Of Tasmania
Year: 2023

description: Palladium complexes bearing N-heterocyclic carbene (NHC) ligands, and their applications in catalysis have remained topical in the literature for the past two decades. Their versatility is due to a vast array of structural modifications possible that afford influences on metal complex geometry and reactivity, and alternative ligand binding modes. These have been extensively developed due to the wide range of catalytic applications for palladium NHC complexes and continue to remain of interest. This thesis describes investigations into several aspects of palladium NHC complexes with potential or known catalytic applications. The synthesis of the novel bis(NHC) dipalladium(I) hydride complex `[˜í¬¿-{(MesIm)_2CH_2}_2Pd_2H][PF_6]` has been reported. \\(^1H\\) `NMR` spectroscopic studies and preliminary single crystal X-ray and neutron diffraction studies on a THF solvate of this complex were indicative of possible solid state hydride dynamics. This hypothesis was probed by further variable temperature and low temperature single crystal neutron diffraction experiments, and by incoherent inelastic neutron scattering (IINS) experiments coupled with DFT-MD simulations. The simulations allowed us to approximate the rate of hydride transfer in the solid state. Laue single crystal neutron diffraction was also employed to examine the propylene-linked bis(NHC) complexes `[˜í¬¿-{(MesIm)_2(CH_2)_3}_2Pd_2H][PF_6]` and `[˜í¬¿-{(MesIm)_2(CH_2)_3}{(PdH)(MesIm)_2(CH_2)_3}_2][PF_6]_2`. The synthesis of chelated bis(NHC) palladium(II) dihalide complexes has been established to proceed via a pendant imidazolium mono(NHC) palladium(II) dihalide acetate intermediate. This intermediate was isolable for the `t`-`butyl` `N`-substituent and a series of complexes sharing the motif `[{(tBuIm)(tBuImH)CH_2}PdX_2CO_2R]` were prepared with variations to the ancillary halide ligands and acetate substituent. The hydrogen bonding between the imidazolium C-2 proton and the acetate ligand was examined in solution by \\(^1H\\) `NMR` spectroscopy and in the solid state by single crystal X-ray and Laue neutron diffraction. The `N`-mesityl pendant imidazolium mono(NHC) palladium(II) trihalide complexes of the motif `[{(MesIm)(MesImH)(CH_2)_n}PdX_3]` were prepared and used to access the mono(NHC) palladium dihalide acetate intermediates of the form `[{(MesIm)(MesImH)(CH_2)_n}PdX_2CO_2R]`. We expanded previous studies on chelated bis(NHC) palladium(II) complexes to include saturated NHC species through the preparation of complexes \\([\\{(^sMesIm)_2CH_2\\} PdBr_2]\\) and \\([{(^SMesIm)_2CH_2}Pd(NCMe)_2][PF_6]2\\) and examined the catalytic activity of the latter towards ethylene and carbon monoxide copolymerisation, similar to studies undertaken on the unsaturated analogue. The reactivity towards formation of the dipalladium(I) hydride complex from the dicationic palladium(II) precursor under basic conditions was also examined. Expansion of these saturated NHC complexes to include extended alkyl linkers was limited by the formation solely of the pendant imidazolinium mono(NHC) palladium(II) tribromide complexes \\([{(^SMesIm)(^SMesImH)(CH_2)_n}PdBr_3]\\) upon reaction of the iimidazolinium salts with palladium acetate. The bis(NHC) disilver(I) complexes \\([{(^SMesIm)_2(CH_2)_n}_2Ag_2][PF_6]_2\\) bearing methylene and propylene-linked imidazoline ligands were prepared, though transmetallation via PdBrMe(COD) to form reactive palladium(II) complexes for catalysis was not successful. There has been growing interest in the study of abnormal or remote NHC ligands (aNHCs and rNHCs, respectively) due to their apparent increase in ˜ìvâ-donor strength and reported improvement in catalytic activity `cf.` their normal NHC (nNHC) counterparts. We prepared an aNHC ligand precursor containing a 5-(4-pyridinium) substituent which, if aligned coplanar to the NHC ring, could allow conjugation through the biaryl C-C bond, and show varying contributions from aNHC and partially normalised‚ÄövÑvp NHC resonance forms. The complexes of the motif \\([1,2,3‚Äöv†v¿trimethyl‚Äöv†v¿5‚Äöv†v¿(N‚Äöv†v¿methylpyridinium)ImPdL_n][PF_6]_y\\) were prepared by oxidative addition of the 4-brominated ligand precursor to palladium(0) and structurally characterised by X-ray crystallography. The degree of contribution from the desired resonance form was probed by examination of key C-C bond lengths and by the angle between the NHC and pyridinium ring planes.