Ion Exchange of Protons by Coinage Metals to Give Gold and Silver Encapsulation within a Pseudo‐D_2d Distorted Face‐Capped Pd₁₄ Cubic Kernel: [(μ₁₄‐M)Pd₂₂(CO)₂₀(PEt₃)₈]⁺ (M=Au, Ag)

Abstract

Dedicated to Professor Mike MingosDuring the last 10-15 years, bimetallic gold-palladium nanosized clusters have attracted considerable attention owing to their greater stabilities [1a] and enhanced catalytic performances compared to their single-metal counterparts. [1b,c] Likewise, a recent report [1d] revealed that bimetallic Ag-Pd nanoparticles had higher catalytic activity and selectivity for oxidation of propene to propene oxide with molecular oxygen (in methanol) than did Pd salts and Pd and Ag monometallic nanoparticles. However, syntheses of stoichiometrically defined nanosized bimetallic palladium composites with Group 11 coinage-metal elements still remain challenging: [2a] there have been no previous publications of nanosized Ag-Pd clusters. [2b] Recently we reported the synthesis of the bicuboctahedral Au-Pd Au 2 Pd 28 (CO) 26 (PEt 3 ) 10 , [2c] which, unlike its structurally analogous non-isovalent homopalladium Pd 30 cluster, was obtained under CO atmosphere by reactions of coordinatively unsaturated Pd n (CO) x (PR 3 ) y species (resulting from CO-induced fragmentations of either of two structurally dissimilar homopalladium Pd n clusters, n = 23, 38) with the CO-tolerant Au-centered cuboctahedral cluster precursor, Au 2 Pd 21 (CO) 20 (PEt 3 ) 10 . Based upon both direct crystallographic evidence and DFT calculations, [2c] the higher stability to CO of the Au 2 Pd 28 cluster was attributed to electron delocalization of each Au 6s valence electron for the two adjacent interior Au atoms instead of the formation of a localized electron-pair AuÀAu bond. This important stereochemical implication has particular relevance to the recent report [1e] that the presence of Au in Au-Pd nanoparticle catalysts, used for the complete conversion of formic acid into high-purity hydrogen (and CO 2 ) for chemical hydrogen storage, suppresses CO poisoning, in sharp contrast to early deactivation of the corresponding Pd counterpart material by carbon monoxide.Herein we present a different synthetic approach to obtain a gold-encapsulated nanosized Au-Pd cluster; appli-cation of this method is also shown to provide preparation of the first corresponding high-nuclearity Ag-Pd cluster. The resulting two remarkable isostructural nanosized M-centered cluster monocations, [(m 14 -M)Pd 22 (CO) 20 (PEt 3 ) 8 ] + , M= Au (1), Ag (2), with [(CF 3 CO 2 ) 2 H] À counterions, were obtained by two-step/one-pot reactions of Pd 10 (CO) 12 (PEt 3 ) 6 with CF 3 CO 2 H (in acetone at 50 8C) followed by coinage-metal ion exchange of protons by the addition of either Au(SMe 2 )Cl (45-60 % yields) or Ag(CF 3 SO 3 ) (28 % yield), respectively. A similar attempt to isolate the Cu-Pd analogue by incorporation of Cu + metal ion was unsuccessful. This preparative method outlined in the reaction sequence (1) is based upon: a) prior treatment of the homopalladium precursor, Pd 10 (CO) 12 (PEt 3 ) 6 (4), with CF 3 CO 2 H acid in the presence of Me 3 NO that affords a non-isolated protonated species (for example, monoprotonated [HPd n (CO) x (...