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CO-Induced Formation of an Interpenetrating Bicuboctahedral Au2Pd18 Kernel in Nanosized Au2Pd28(CO)26(PEt3)10: Formal Replacement of an Interior (μ12-Pd)2 Fragment in the Corresponding Known Isostructural Homopalladium Pd30(CO)26(PEt3)10 with Nonisovalent (μ12-Au)2 and Resulting Experimental/Theoretical Implications

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journal contribution
posted on 21.11.2011, 00:00 by Evgueni G. Mednikov, Sergei A. Ivanov, Lawrence F. Dahl
Initially isolated from Pd10(CO)12(PEt3)6 (5) and Au(SMe2)Cl precursors in a two-step carbon monoxide (CO)-involved procedure, the nanosized interpenetrating bicuboctahedral gold (Au)–palladium (Pd) Au2Pd28(CO)26(PEt3)10 (1) was then directly obtained in 25–30% yield from the CO-induced reaction of the CO-stable Au-centered cuboctahedral Au2Pd21(CO)20(PEt3)10 (3) with the structurally analogous CO-unstable Pd23(CO)20(PEt3)10 (4). Our hypothesis that this latter synthesis is initiated by the reaction of 3 with coordinatively unsaturated homopalladium species resulting from CO-induced fragmentation of 4 was subsequently substantiated by the alternatively designed synthesis of 1 (∼25% yield) from the CO-induced reaction of 3 with the structurally dissimilar CO-unstable Pd38(CO)28(PEt3)12 (6). The composition of 1, unambiguously established from a 100 K CCD X-ray diffractometry study, is in accordance with single-crystal X-ray Au–Pd field-emission microanalysis. The pseudo-C2h 30-atom Au2Pd28 geometry of 1 may be formally derived via substitution of the interior12-Pd)2 moiety in the interpenetrating bicuboctahedral Pd20 kernel of the known isostructural Pd30(CO)26(PEt3)10 (2) with the corresponding interior12-Au)2 moiety, in which the otherwise entire metal-core geometry and CO/PR3-ligated environment are essentially not altered. Of major significance is that this interior nonisovalent Pd-by-Au replacement in 2 produces CO-stable 1, whereas nanosized CO/PR3-ligated homopalladium Pdn clusters with n > 10 are generally unstable under CO. Because the two adjacent encapsulated Au atoms of 2.811(1) Å separation are not present on the metal surface, isolation of 1 under CO is ascribed to an electronic property. The virtually ideal geometrical site-occupancy fit between 1 and 2 provides definite crystallographic evidence for extensive delocalization in 1 of the two valence Au 6s electrons over the entire cluster (instead of a “localized” covalent Au–Au electron-pair interaction). Gradient-corrected (pseudo-scalar-relativistic) density functional theory (DFT) calculations were performed on the isostructural Au2Pd28(CO)26(PH3)10 (1-H) and Pd30(CO)26(PH3)10 (2-H) model clusters along with hypothetical [Au2Pd28(1-H)]2+ and [Pd30(2-H)]2– analogues (with phosphine ethyl substituents replaced by hydrogen ones). Natural population analysis of these four model clusters revealed similar highly positively charged metal surfaces of 28 Pd atoms relative to the two negatively charged interior metal atoms, which reflect a partially oxidized metal surface due to dominant CO back-bonding. The surprising observation that each less electronegative interior Pd atom in 2-H is more negatively charged by 0.30e than each interior Au atom in 1-H points to a more cationic Au in 1 than interior Pd in 2; this unexpected (opposite) charge difference is consistent with delocalization of each Au 6s valence electron toward a Au+ configuration. This premise is in agreement with the calculated Wiberg bond index (WBI) value of 0.055 for the Au–Au bond order in 1-H versus the WBI single-bond value of 1.01 obtained from analogous DFT calculations for the bare, neutral Au2 dimer, which has a much shorter spectroscopically determined gas-phase distance of 2.472 Å (that corresponds to a “localized” electron-pair interaction). Isolation of 1 under CO is of prime importance in nanoscience/nanotechnology in establishing relative stabilizations toward CO in well-defined CO/PEt3-ligated nonisovalent Pd2-by-Au2-substituted Au2Pdn–2 clusters [namely, n = 30 (1) and 23 (3)]. These important stereochemical implications have a direct relevance to the recent report of the higher tolerance to CO poisoning of highly active Au–Pd nanoparticle catalysts used for the complete conversion of formic acid into high-purity hydrogen (and CO2) for chemical hydrogen storage.