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

Abstract

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{(H2CNNH)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‘‘}.

Synthesis and Decomposition Behavior of Pallada(IV)cyclopentane Complexes

“…Complexes 2 − 5 adopt configuration B in solution. Configurations A and B are assignable from integrations and by comparison with spectra of related complexes such as PdX(C 4 H 8 ){(pz) 3 BH} (X = Br, I) 12a…”

“…For palladium, an element of particular importance in organic synthesis and catalysis, decomposition of hydrocarbylpalladium(IV) complexes is characterized by C···C bond formation in reductive elimination reactions, − although detailed mechanistic studies to date have been confined to PdIMe 3 (bpy) (bpy = 2,2‘-bipyridine), which undergoes facile reductive elimination of ethane . The work reported herein was prompted by several proposals that pallada(IV)cyclic complexes may be involved in catalysis, demonstration of this by detection of a palladium(IV) intermediate for a palladacyclopentadiene system, the remarkable reactivity of some pallada(II)cyclopentane complexes to reduce water to hydrogen with concomitant oxidation of palladium(II) to palladium(IV), and observations in preliminary studies showing that the decomposition of Pd IV Br(C 4 H 8 )(CH 2 Ph)(bpy) [Pd(C 4 H 8 ) occurs under mild conditions in solution …”

On the assistance of the heterolytic cleavage of H2 by an external base: a theoretical assessment

“…26 Work along these lines for reactions of palladium-() and -() complexes is currently underway. 27,28…”

Oxidative addition of water to transition metal complexes

“…Dedieu and Canty used computational studies on a simplified model to propose a mechanism for the reduction of water to H 2 by 27 (Scheme 7). 23 Their proposal involves stepwise, pyrazolylassisted addition of a proton and hydroxide to the Pd(II) center to give the Pd(IV) hydride/hydroxide 29 (Scheme 7). The dangling pyrazolyl ligand also participates in the H 2 formation (30 -31) upon protonation of 29.…”