The microscopic steps responsible for the perfectly alternating copolymerization of ethylene and CO catalyzed by 1,10-phenanthroline (phen) based palladium complexes have been studied. Palladium carbonyl alkyl, carbonyl acyl, ethylene alkyl, and ethylene acyl complexes [(phen)Pd(R)(L)+Ar‘4B- (Ar‘ = 3,5-(CF3)2C6H3; R, L = CH3, CO (2); CH3, C2H4 (3); CH2CH3, C2H4 (7); C(O)CH3, CO (8); C(O)CH3, C2H4 (13); CH2CH2C(O)CH3, C2H4 (15); CH2CH2C(O)CH3, CO (16); C(O)CH2CH2C(O)CH3, C2H4 (17); C(O)CH2CH2C(O)CH3, CO (18)); and the β- and γ-keto chelate complexes (phen)PdCH2CH2C(O)CH3 + (14) and (phen)PdC(O)CH2CH2C(O)CH3 + (19)] have been prepared. An X-ray structure of the carbonyl acyl complex (phen)Pd(C(O)CH3)(CO)+Ar‘4B-·CH2Cl2 (8·CH2Cl2) has been obtained. The migratory insertion reactions of 2, 3, 7, 13, 16, and 17 have been studied by low-temperature NMR techniques. The barriers for insertion increase in the following order:  ΔG ⧧ R → CO ≈ 15 kcal/mol (−66 °C) < ΔG ⧧ Ac → C 2 H 4 ≈ 17 kcal/mol (ca. −45 °C) < ΔG ⧧ R → C 2 H 4 ≈ 19 kcal/mol (−25 °C). The relative binding affinities of ethylene and CO to Pd methyl, acyl, and chelate complexes have been determined by combining stepwise measurements of the binding affinities of ligands with intermediate strength to (phen)Pd(CH3)(L)+ (CO > MeSPh > CH3CN ≈ C2H4 > C6H5CN ≫ OEt2) with relative equilibrium constants for ethylene/CO binding between acyl and alkyl complexes. The copolymerization mechanism has been determined from the kinetic and thermodynamic data. The catalyst resting state is a carbonyl acyl complex which is in equilibrium (K 5(25 °C) = (7.1 ± 3.5) × 10-4) with a less stable ethylene acyl intermediate which undergoes β-acyl migratory insertion to generate a Pd alkyl species followed by rapid reaction with 2 equiv of CO to reform the resting state. This model is tested by comparing calculated and experimental turnover frequencies.

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The cationic Pd(II) complexes, [(phen)Pd(CH3)(L)]+[BAr‘4]- phen = 1,10−phenanthroline; L = Et2O, Me3SiC⋮CSiMe3; Ar‘ = 3,5-(CF3)2C6H3) catalyze the hydrosilation and dehydrogenative silation of olefins. Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyclohexene by HSiR3 (R = CH2CH3, C6H5) occurs in the presence of 1 mol % [(phen)Pd(CH3)(L)]+[BAr‘4]-. The reaction of tert-butylethylene with HSi(i-Pr)3 in the presence of [(phen)Pd(CH3)(L)]+[BAr‘4]- yields neohexane and t-BuCHCHSi(i-Pr)3. Low-temperature NMR experiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is [(phen)Pd(SiR3)(η2-H2CCHR‘)]+[BAr‘4]-. Evidence for rapid, reversible silyl migration at −70 °C was observed by 1H NMR spectroscopy. Deuterium labeling studies show that the intermediate Pd(II) alkyl complexes can isomerize via a series of β-hydride eliminations followed by reinsertions of olefin prior to reaction with DSiEt3. Styrene undergoes both hydrosilation and dehydrogenative silation in the presence of [(phen)Pd(CH3)(L)]+[BAr‘4]- or [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]- yielding ethylbenzene, R3SiCH2CH2C6H5 and trans-R3SiCHCHPh (R = CH2CH3, CH(CH3)2). 1H NMR spectroscopy revealed that the π-benzyl complexes [(phen)Pd(η3-CH(CH2SiR3)C6H5)]+[BAr‘4]- and [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]- are the catalyst resting states for the silation reactions of styrene.

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The β-methyl migratory insertion chemistry of a series of cis-coordinated styrene methyl complexes of palladium [(phen)Pd(CH3)(p-X-C6H4CHCH2)+Ar‘4B- (2X) (phen = 1,10-phenanthroline; X = CF3, Cl, H, CH3, OCH3; Ar‘ = 3,5-(CF3)2C6H3)] has been investigated. Complexes 2X are prepared in situ from the addition of p-X-styrene to CD2Cl2 solutions of (phen)Pd(CH3)(OEt2)+Ar‘4B- (1). The X-ray structure of 1 has been determined [P21/c; a = 16.460(4) Å, b = 18.911(3) Å, c = 17.374(3) Å; β = 117.996(14)°; V = 4775.2(15) Å3; Z = 4] at −78 °C. The rearrangement of 2X, via β-CH3 migratory insertion then arene coordination, to yield (phen)Pd(η3-CH(CH2CH3)(C6H4-p-X)+Ar‘4B- (3X) has been studied by 1H NMR spectroscopy. The rearrangement is accelerated by electron-withdrawing groups; a Hammett analysis at −29.2 °C reveals that log k values are best fit by σp parameters:  ρp = 1.1 ± 0.1, r = 0.992. The anti-isomer of 3H has been structurally characterized:  P21/n; a = 13.950(4) Å, b = 15.582(5) Å, c = 24.004(8) Å; β = 101.961(24)°; V = 5105(3) Å3; Z = 4. Complex 3H reacts further with styrene to form (phen)Pd(η3-CH(CH3)(C6H5)+Ar‘4B- (4) and (E)-β-methylstyrene; kinetic and isotopic labeling experiments have been employed to determine the mechanism of this benzyl exchange reaction. The relative binding affinities of the p-X-styrenes of 2X have been determined for the equilibria p-X-C6H4CHCH2 + 2H ⇌ C6H5CHCH2 + 2X at −66 °C. The electron-rich styrenes bind tightest to Pd; a Hammett plot of these equilibrium constants yields ρp = −2.2 ± 0.1 and r = 0.999. The kinetic and thermodynamic data indicate that both the ground and transition-states are stabilized by electron-rich styrenes and, of the two, the substituent effects are greatest in the ground-states. Since the relative rates are determined by the relative differences in energy of the ground and transition states, the migratory insertion reaction of 2X is accelerated by styrenes bearing electron-withdrawing substituents.