Studies in alkylpalladium chemistry, including development of the new oxidation state +IV

This thesis describes the development of convenient and systematic routes to a wide range of monomethylpalladium(II) and dimethylpalladium(II) complexes of nitrogen donor ligands, and the first development of an extensive chemistry of alkylpalladium(IV) complexes. The work reported here has important implications for catalytic processes employing palladium, and for the future development of the new oxidation state, +Iv, in organopalladium chemistry. The synthesis and characterisation of (PdMe2(Ln )) and PdMeX(Ln) } complexes is described, with 2,2'-bipyridyl (bipy), 1,10-phenanthroline (phen), and alkane bridged bidentate ligands as L2 and tridentate ligands as L3, where L2 and L3 contain combinations of the N-donor heterocycles N-methylimidazol-2-yl, pyridin-2-y1 and pyrazol- 1-yl. Tridentate ligands were initially employed to assess the ability of palladium(H) to adopt coordination numbers greater than four. The reaction of (PdMe2(Ln)) with various organohalides (RX) has been studied, and has led to the detection, isolation and characterisation of the first hydrocarbylpalladium(IV) complexes. The complexes (PdMe2(Ln)) and (PdMeI(Ln)) have been synthesised using methyllithium. During development of these routes, the complexes (PdMe2(11- PYridazine)) n and [PdMe(-1-X)(SMe2))2 (X=C1,Br,I) were also isolated, and reaction of these complexes with ligands (La ) give dimethylpalladium(ii) and methylpalladium(ii) complexes, respectively. The iodo-bridged dimer (PdMe(gI)(SMe2)) 2 has also been used to prepare cationic complexes by reaction with AgBF4 followed by addition of bipy, or by direct reaction with 2,2':6',2\-terpyridyl (terpy). The halogeno-dimers (PdMe(1.1-x)(SMe2))2 with square planar geometry for Pd(i1) have been characterised by near and far infrared and 1H NMR spectroscopy molecular weight determinations and an X-ray structure analysis for (PdMe4tC1)(SMe2)) 2. The dimethylpalladium00 complex (PdMe2(g-pyridazine)) n is unstable at ambient temperature and was characterised only by 1H NMR spectroscopy. The neutral complexes (PdMeX(Ln)) (X=Me halide) and cationic complexes [PdMe(S)(hiPY)]BF4 (S=SMe2 CH3CN y-pic) and [PdMe(terpy)]I were characterised by microanalysis 1 H NMR spectroscopy and for square planar [PdMe(y-Pic)(biPY)]BF4 an X-ray structure analysis. 1H NMR spectra for the alkane bridged bidentate ligand complexes (PdMeX(12)) (X=Me halide) at ambient temperature were consistent with rapid boat to boat inversion of the chelate ring while spectra for the tridentate ligand complexes (PdMeX(L3)) (X=Me halide) were consistent with rapid exchange between free and bound donor groups. Both processes could be resolved upon cooling and low temperature solution state conformations determined. 1H NMR spectra of the cationic complexes [PdMe(S)(hiPY)JBF4 also displayed variable temperature behaviour consistent with site exchange of the Pd-S and Pd-Me groups. Synthesis of the complexes (PdMeI(L2)) was also accomplished by reaction of Mel with the corresponding dimethylpalladium(II) complex (PdMe2(1-2)) and led to the in situ detection of palladium(IV) intermediates and to the subsequent isolation and characterisation of the first hydrocarbylpalladium(IV) complexes. Reaction of Mel with (PdMe2(L2)) gave the isolable complexes (pdivme3I(L2)) for L2=bipy and phen only although neutral and in some instances cationic complexes were spectroscopically detected for most of the other bidentate ligand complexes studied. The reaction of Mel with the tridentate ligand complexes (PdMe2(L3)) on the other hand produced the cations (PdIvMe3(L3))I which could be isolated for all tridentate ligands studied. The reaction of organohalides (RX) with (PdMe2((pyridin-2-yl)bis(N-methylimidazol-2-yOmethane)) to give the stable cations [Pd1 vMe2R(pymim2CH)]X was also studied and the reaction of PhCH2Br with (PdMe2(bipy)) produced the neutral complex (PdIvMe2(CH2Ph)Br(bipy)). The neutral complexes (PdMe2RX(L2)) were characterised by microanalysis 1H NMR spectroscopy molecular weight determinations and for (PdMe3I(bipy)) by an X-ray structure determination. The kinetics of oxidative addition of Mel to (PdMe2(hiPY)) and reductive elimination of ethane from (PdMe3I(bipY)) have been studied and in both cases reaction proceeds via a polar intermediate and an estimate of the Pd(IV)-Me bond strength has been determined. The cationic complexes [PdMe2R(L3)]X were characterised by microanalysis 1H NMR spectroscopy and by an X-ray crystallographic study of the isostructural cations [MMe3(pz3CH)]I (M=Pd Pt). For both isolated and in situ detected neutral and cationic complexes a facial arrangement of the organogroups is proposed."