Basicity and Hydride-Donating Ability of Palladium(II) Hydride Complex with Diarylamido-bis-phosphine Pincer Ligand

The hydride ion transfer and proton transfer are the key steps in the reactions of (de)hydrogenation, dehydrocoupling, production of H-2, and reduction of CO2 with the participation of transition metal hydrides; complexes with bifunctional ligands often act as catalysts for these transformations. The aim of this work was to study the hydride-donating properties of pincer palladium(II) hydride (PNP)PdH (1; PNP is bis(2-diisopropylphosphino-4-methylphenyl)amide). For this purpose, its reaction with Lewis acids (BF3 center dot Et2O, B(C6F5)(3)) was studied using IR and NMR spectroscopies combined with quantum chemical calculations (DFT/M06/def2-TZVP). Correlations between electrochemical reduction potentials of the corresponding cations and thermodynamic hydridity of the metal hydrides proposed in the literature were also applied. [(PNP)Pd(MeCN)][BF4] undergoes an irreversible two-electron reduction in acetonitrile (E-p= -1.82 V). The use of the obtained potential in correlations gives an overestimated value of the hydride donating ability Delta G(H-)(o). It was found that the reaction of 1 with boron-containing Lewis acids unexpectedly leads to the protonation of the nitrogen atom of the pincer ligand with an impurity of water, rather than the reaction with the hydride ligand. According to DFT calculations, the proton affinity of the nitrogen atom is much higher than that of PdH, which determines its higher activity in protonation processes.