REDUCTION OF WATER TO H-2 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(CH2CH2CH2CH2){(pz)(3)BH-N,N'}](-) with concomitant formation of th e palladium(IV) complex Pd(CH2CH2CH2CH2)(OH){(pz)(3)BH-N,N',N ''} has been studied theoretically at the MP2//SCF level using [PdMe2{(H2C=N-N H)(3)BH}](-) as a model for the pallada(II)cyclopentane reagent. The c alculations 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 coord inated group in the palladium(IV) product. Thus, initial protonation l eads to formation of a N-protonated palladium(II) species Pd(CH2CH2CH2 CH2){(pz)(2)(pzH)BH-N,N'} containing a ''Pd ... H-N'' interaction, fol lowed by hydroxide coordination and hydride ligand formation to give a palladium(IV) species s-[Pd(CH2CH2CH2CH2)(H)(OH)(2){(pz)(3)BH-N,N'}]( -), a second protonation to form trans-Pd(CH2CH2CH2CH2)(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 pyrazol e group to form Pd(CH2CH2CH2CH2)(OH){(pz)(3)BH-N,N',N ''}.