Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted R-diimine-Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [(N ∧ N)Pd(CH 3 )(OEt 2 )]BAr′ 4 (N ∧ N ) aryl-substituted R-diimine, Ar′ ) 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArNd C(H)C(H)dNAr]Pd(CH 3 )(Cl) and [ArNdC(Me)C(Me)dNAr]Pd(CH 3 ) 2 (Ar ) 2,6-(iPr) 2 C 6 H 3 ) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, (N ∧ N)Pd(R)(olefin) + , are the catalyst resting states and that the barriers to migratory insertions lie in the range 17-19 kcal/mol. Following migratory insertion, the cationic palladium alkyl complexes (N ∧ N)Pd(alkyl) + formed are β-agostic species which exhibit facile metal migration along the chain ("chain walking") via β-hydride elimination/readdition reactions. Model studies using palladiumn-propyl and -isopropyl systems provide mechanistic details of this process, which is responsible for introducing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN ∧ NAr)Pd(CH 3 )(OEt 2 ) + (Ar ) 2,6-(iPr) 2 C 6 H 3 , 2-tBuC 6 H 4 ) occurs by C-H activation of β-C-H bonds of the ortho isopropyl and tert-butyl substituents and loss of methane. The rate of associative exchange of free ethylene with bound ethylene in (N ∧ N)Pd(CH 3 )(C 2 H 4 ) + is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.

The mechanistic details of the cobalt-catalyzed intermolecular hydroacylation reaction have been investigated using kinetic, spectroscopic, and crystallographic methods. The Co(I) bisolefin complex 1, [C5Me5Co(C2H3SiMe3)2], was shown to catalyze the addition of a series of alkyl aldehydes (2a−l) to vinylsilanes to give the corresponding ketones with exclusive anti-Markovnikov selectivity under mild conditions. The catalytic cycle exhibits two resting states, complex 1 and a bisalkyl carbonyl complex, [C5Me5Co(CO)(R)(R‘)], 4a−l which are in equilibrium. Kinetic investigations along with low-temperature NMR spectroscopy establish a sensitive balance between resting states during catalysis which is strongly dependent on substrate structure. The turnover-limiting step was established as the reductive elimination of ketone from intermediate 4. Using ferrocenecarboxaldehyde (Fc-C(O)H), 2l, as substrate, the intermediate 4l [C5Me5Co(CO)(Fc)(CH2CH2SiMe3)] was isolated at low temperatures and characterized by X-ray crystallography. Complex 4l was used to study the carbon−carbon bond-forming step directly by thermolysis in the presence of a trapping ligand L (P(OMe)3, PMe3). Kinetic analysis showed competitive ligand dependent and ligand independent pathways for ketone formation. Deuterium scrambling, isomerization of aldehydes prior to ketone formation, and production of isomeric ketones in certain cases establish that complex isomerization processes occur prior to productive ketone elimination from 4. A detailed mechanism accounting for all observations is proposed. Catalyst deactivation was shown to involve primarily decarbonylation to yield [C5Me5Co(CO)]2 and [C5Me5Co(CO)(C2H3SiMe3)]. When excess aldehyde is present, catalytic aldehyde dimerization occurs to give esters.

The dimerization of olefins by well-defined cationic η3-allyl−palladium complexes of the type [(C3H5)Pd(L)(PR3)]+[BAr‘4]- (Ar‘ = [3,5-C6H3(CF3)2]; L = OEt2, H2O; R = cyclohexyl (Cy), n-butyl (nBu)) has been studied. These complexes react with ethylene or methyl acrylate at −80 °C with loss of L to form the η2-olefin complexes [(C3H5)Pd(η2-olefin)(PR3)]+[BAr‘4]- (olefin = H2CCH2, CH2CHC(O)OCH3). Upon warming, allyl−olefin coupling occurs. The dimerization of ethylene occurs rapidly at 0 °C with an observable ethyl−ethylene intermediate [(C2H5)Pd(C2H4)2(PCy3)]+[BAr‘4]-. Methyl acrylate reacts to form a stable acrylate chelate complex, [(CH3O(O)CCH2CH2)Pd(CH2CHC(O)OCH3)(PR3)]+[BAr‘4]-, which is the catalyst resting state for methyl acrylate dimerization which occurs at room temperature to give predominantly trans-dimethyl-2-hexenedioate.

Co(III) agostic alkyl complexes [Cp*Co(L)(CH2CH2-μ-H)][B(ArF)4] (Cp* = C5(CH3)5, L = P(OCH3)3, 1a, or P(CH3)3, 1b; ArF = 3,5-(CF3)2C6H3) react with H2 to yield ethane and trihydride complexes characterized as η2-dihydrogen hydride species [Cp*Co(L)(H)(η2-H2)][B(ArF)], 2a and 2b, in which there is rapid scrambling between the η2-H2 ligand and the terminal Co−H. Complexes 2a and 2b react with a variety of neutral donor ligands (L‘ = RCN, PMe3, P(OMe)3, H2O, CH3OH) to yield [Cp*Co(L)(L‘)(H)][B(ArF)4] complexes. Reaction of silanes with either 1a,b in the presence of traces of water or 2a,b yields η2-silane hydrides, [Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]. Analysis of the dynamics of these species by NMR spectroscopy provides evidence for an extremely rapid process involving silyl migration between hydrogens and a slower process in which a cobalt-silyl η2-H2 complex is formed as an intermediate and results in hydrogen scrambling between the two diastereomers of [Cp*Co(L)(H)(η2-H-SiHMePh)][B(ArF)4]. The structures and dynamics of 2a,b and the η2-silane complexes imply that cleavage of the cobalt−alkyl bonds of 1a,b in hydrogenation and hydrosilation catalytic cycles occurs by the σ-CAM (σ-complex-assisted metathesis) process.