Reduction of Water to H2 by Diorganopalladium(II) Complexes of Tris(pyrazol-1-yl)borate: Ab-Initio Theoretical Study of the Mechanism

The reduction of water to H 2 by the tris(pyrazol-1-yl)borate complex [Pd(CH 2 CH 2 CH 2 CH 2 ){(pz) 3 BH- N,N ‘}] - with concomitant formation of the palladium(IV) complex Pd(CH 2 CH 2 CH 2 CH 2 )(OH){(pz) 3 BH- N,N ‘,N ‘‘} has been studied theoretically at the MP2//SCF level using [PdMe 2 {(H 2 C N NH) 3 BH}] - as a model for the pallada(II)cyclopentane reagent. The calculations suggest that the uncoordinated pyrazole group has a major role as an intramolecular nucleophile in delivering 2H + (per mole of H 2 formed) to the palladium center, with an eventual role as a coordinated group in the palladium(IV) product. Thus, initial protonation leads to formation of a N-protonated palladium(II) species Pd(CH 2 CH 2 CH 2 CH 2 ){(pz) 2 (pzH)BH- N,N ‘} containing a “Pd···H−N” interaction, followed by hydroxide coordination and hydrido ligand formation to give a palladium(IV) species trans -[Pd(CH 2 CH 2 CH 2 CH 2 )(H)(OH){(pz) 3 BH− N,N ‘}] -, a second protonation to form trans -Pd(CH 2 CH 2 CH 2 CH 2 )(H)(OH){(pz) 2 (pzH)BH− N,N ‘} prepared for a dihydrogen bond interaction “Pd−H···H−N”, and finally by elimination of H 2 and coordination of the pyrazole group to form Pd(CH 2 CH 2 CH 2 CH 2 )(OH){(pz) 3 BH- N,N ‘,N ‘‘}.