Mechanistic Studies on Cyclopalladation of the Solvated Palladium(II) Complexes with N-Benzyl Triamine Ligands in Various Solvents. Crystal Structures of [Pd(Sol)(Bn2Medptn)](BF4)2 (Sol = Acetonitrile and N,N-Dimethylformamide; Bn2Medptn = N,N′-Dibenzyl-4-methyl-4-azaheptane-1,7-diamine) and [Pd(H−1Bn2Medptn-C,N,N′,N″)]CF3SO3

Abstract

Abstract Several solvated palladium(II) complexes with the potentially cyclopalladating dibenzyl ligand have been synthesized. These include [Pd(CH3CN)(Bn2Medptn)](BF4)2 (1) (Bn2Medptn = N, N′-dibenzyl-4-methyl-4-azaheptane-1,7-diamine), [Pd(dmf)(Bn2Medptn)](BF4)2 (2) (dmf = N,N-dimethylformamide), and [Pd(dmso)(Bn2Medptn)](BF4)2 (3) (dmso = dimethyl sulfoxide), their cyclopalladated complex, [Pd(H−1Bn2Medptn-C,N,N′,N″)]CF3SO3 (4), the solvated monobenzyl complex, [Pd(CH3CN)(BnMedptn)](BF4)2 (5) (BnMedptn = N-(3-aminopropyl)-N′-benzyl-N-methyl-1,3-propanediamine), and its deuterated complex, [Pd(CH3CN)(BnMedptn-d7)](BF4)2 (6) (BnMedptn-d7 = N-(3-aminopropyl)-N′-heptadeuteriobenzyl-N-methyl-1,3-propanediamine). The crystal structures of 1·CH3CN·CH2Cl2, 2, and 4 have been determined by X-ray structure analysis to characterize the reactant and the product for the cyclopalladation of the solvated complexes, where one of the ortho carbons of 1 is directed toward the palladium(II) center (Pd···C(1) = 3.513(9) Å). The rate constants for the cyclopalladation of 1 at 25 °C in various solvents increase in the order DMF < DMSO << pyridine, but the reaction does not proceed in acetonitrile or nitromethane. The activation parameters for the cyclopalladation in neat solvent have been obtained as follows: k298 = 5.74 × 10−6 s−1, ΔH‡ = 104.0 ± 1.2 kJ mol−1 and ΔS‡ = 3.5 ± 3.9 J K−1 mol−1 for 1 in DMF, k298 = 3.13 × 10−4 s−1, ΔH‡ = 83.8 ± 2.6 kJ mol−1 and ΔS‡ = −31.0 ± 8.8 J K−1 mol−1 for 1 in DMSO, k298 = 1.30 × 10−4 s−1, ΔH‡ = 81.2 ± 0.5 kJ mol−1 and ΔS‡ = −47.0 ± 1.8 J K−1 mol−1 for 5 in DMF, k298 = 1.76 × 10−3 s−1 for 5 in DMSO, k298 = 1.26 × 10−5s−1, ΔH‡ = 92.8 ± 1.4 kJ mol−1 and ΔS‡ = −27.5 ± 4.4 J K−1 mol−1 for 6 in DMF and k298 = 2.69 × 10−4 s−1 for 6 in DMSO. The activation enthalpy is reduced as the solvent basicity increases. The kinetic isotope effects (kH/kD) for the cyclopalladation of the monobenzyl complex at 25 °C are calculated to be 10.3 in DMF and 6.5 in DMSO using the rate constants for 5 and 6. It is confirmed from the kinetic results obtained that the nucleophilic attack of the basic solvent on the ortho proton is essential for the C–H bond cleavage observed in the activation process. In addition, the fact that the rate constant for the cyclopalladation is proportionally dependent on the concentration of DMSO in nitromethane strongly suggests that the solvent-dissociation pre-equilibrium is negligible in neat basic solvent.