Abstract
The transformation of aromatic nitrocompounds into amines by CO/H2O is catalyzed by palladium(II) complexes. Recently, we have proposed that the catalytic cycle includes Pd0 as the transient intermediate and herein, for the first time, we describe the application of palladium nanoparticles (PdNPs) stabilized by monodentate N-heterocyclic ligands as nanocatalysts facilitating the reduction of Ar–NO2 into Ar–NH2 by CO/H2O. Among the series—Pd(II) complexes, PdNPs and commercial Pdblack—the highest catalytic activity was observed for PdNPs (3.0 ± 0.5 nm) stabilized by 4-Me-pyridine in the presence of 2-Cl-pyridine. The results may be helpful for mechanistic considerations on the role of metallic nanoparticles as active species in other organic processes.
Highlights
We did not succeed with this ligand as a stabilizing agent [18], probably due to its low nucleophilicity [12]. This observation does not refute the hypothesis that Pd0 is the active species in the system catalyzed with PdCl2(2ClPy)2 because in such a case, Pd0 generated in situ may be instantaneously stabilized by traces of aniline
Comparison of the experiments with Pd nanoparticles (PdNPs) with experiments with Pd(II) complexes, applied respectively as pre-catalysts, showed that Pd0 is a real active species responsible for the observed catalytic effect, confirming the mechanism proposed in Scheme 1
We suggest that Pd nanoparticles stabilized by pyridine ligands are the most similar to catalytically active reaction intermediates or that Pd nanoparticles act as reservoirs for the formation of homogeneous complexes which are the real catalysts
Summary
Te hpearintitarlopdruecctiipointaotifoPndo0ftPodthbleacrke[a1c6t]io. nCosymsptelmexwPdou0(lXdniPnycr)mea(sCeOth)zefroartme,eads itnheresatcetpioonf (n3u) crleeaacttisonwaitnhdnfiotrrombeantizoennoefaPndd0c(aRrebaocntiomnosn(o2x) iadneda(n3d)), aist othmisitmteodm, aenndt, itthceacnagtaivlyetiac pcyrocloef stthaarttsP;dseNePSscahreemeess1e.ntial for the catalytic process described by Scheme 1
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