The reduction of water to H2 by the
tris(pyrazol-1-yl)borate complex
[Pd(CH2CH2CH2CH2){(pz)3BH-N,N‘}]-
with concomitant formation of the palladium(IV) complex
Pd(CH2CH2CH2CH2)(OH){(pz)3BH-N,N‘,N‘‘}
has been studied theoretically at the MP2//SCF level
using
[PdMe2{(H2CNNH)3BH}]-
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
H2 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(CH2CH2CH2CH2){(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(CH2CH2CH2CH2)(H)(OH){(pz)3BH−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 H2 and coordination of the
pyrazole group to form
Pd(CH2CH2CH2CH2)(OH){(pz)3BH-N,N‘,N‘‘}.