Gas‐Phase Synthesis of [Ag₄H]⁺ and Its Mediation of the CC Coupling of Allyl Bromide

Abstract

The formation of carbon-carbon bonds mediated by metal surfaces and nanoparticles continues to attract considerable attention. [1, 2] Recent work on the interaction of allyl halides (C 3 H 5 X) with a range of metal surfaces revealed different reactivity that depends on both the metal as well as the halide. [3, 4] Of all the metal surfaces examined, silver surfaces appear to be the most effective at mediating carbon-carbon bond coupling of allyl halides to give 1,5-hexadiene. Two distinct pathways have been observed:[4a] 1) dissociative adsorption of C 3 H 5 X to give surface-bound allyl groups, which then undergo coupling, and 2) a coupling reaction between C 3 H 5 X and a surface-bound allyl group. Surface defects play a significant role in the selectivity of 1,5-hexadiene formation which increases from 20 % to greater than 60 % when chlorine atoms are preadsorbed on silver surfaces.[4b] Also of interest are the largely forgotten results of Tamura and Kochi, [5] who synthesized silver nanoparticles over 30 years ago and showed that they reacted with allyl bromide to give 1,5-hexadiene and silver bromide [Eq. (1)]. Here we describe the gas-phase assembly of the subvalent silver hydride cluster [Ag 4 H] + [6] and its subsequent ionmolecule reactions with allyl bromide.[Ag 4 H] + was "synthesized" in the gas phase by collisioninduced dissociation (CID) of a silver-amino acid precursor in a quadrupole ion trap (QIT) mass spectrometer.[7] The precursor was the silver ion cluster [(M + Ag-H) 3 + Ag] + (M = N,N-dimethylglycine), which was formed through electrospray ionization (ESI) of a mixture of silver nitrate (AgNO 3 ) and the amino acid.[ clusters (M = glycine or N,N-dimethylglycine). These reactions represent the first gas-phase assembly of silver clusters mediated by simple biomolecules. [9, 10] Interestingly, the anionic amino acid ligands act as the reductants; this is in contrast to the condensed-phase assembly of silver clusters and nanoparticles on biomolecular templates which requires the addition of reductants.[11]The ion-molecule reactions of these silver clusters with the allyl halides, CH 2 =CHCH 2 X (X = Cl, Br, and I) were examined. [12, 13] [Ag 4 H] + (m/z = 433) [14] exhibits the highest selectivity for CÀC bond coupling with allyl bromide, CH 2 = CHCH 2 Br (Figure 1 (m/z = 189) [14] in combination with the neutral cluster [Ag 3 Br 3 ][15] (Figure 1 c, Equation (4)). Overall, these equations combine to give a rare example of a metal-mediated CÀ C bond-coupling reaction [Eq. (5)] in the gas phase. [16] Additional CID studies and DFT (density functional theory) calculations [17] were carried out to gain further support that CÀC bond coupling had occurred. CID of [Ag(C 3

Gas-phase reactivity of ruthenium carbonyl cluster anions

“…The compound we chose to examine was the anionic ruthenium cluster [H 3 Ru 4 (CO) 12 ] Ϫ (1), prepared as the [PPh 3 NPPh 3 ] ϩ (PPN) salt [27]. Cluster 1 has several desirable features; it is easily prepared, charged (and hence highly amenable to analysis by ESI-MS), contains both hydride and carbonyl ligands (thus may undergo interesting reactivity involving both types of ligand), and is a catalyst for reactions including the water gas shift reaction [50,51], hydrogenation [52], and coupling reactions [53].…”

“…Cluster 1 has several desirable features; it is easily prepared, charged (and hence highly amenable to analysis by ESI-MS), contains both hydride and carbonyl ligands (thus may undergo interesting reactivity involving both types of ligand), and is a catalyst for reactions including the water gas shift reaction [50,51], hydrogenation [52], and coupling reactions [53]. The parent compound, H 4 Ru 4 (CO) 12 , is also a well-known catalyst for a range of similar transformations [54 -57].…”

“…The three conventional mass spectra at the right provide snapshots of the ligand stripping process, at 10, 80, and 150 V; note the number of product ions missing from this portrayal. Inset: structure of the anionic ruthenium carbonyl cluster [H 3 Ru 4 (CO) 12 ] Ϫ (1) [27]. The Ru atoms describe a tetrahedron; each Ru atom has three terminally-bound CO ligands and hydride ligands bridge three of the six Ru-Ru bonds.…”

“…Just how good the correspondence is between gas-phase and solution results is subject to active debate [5][6][7], but there is little argument that the gas phase benefits from the reduction in the number of variables, especially when dealing with complicated systems. The gas-phase reactivity of a wide variety of partially ligated clusters have been studied, including oxides [8 -11], hydrides [12], sulfides [13][14][15], carbides [16], carbenes [17], phosphines [18]. and even some mixed species [19,20].…”

“…We 12 ] salt was dissolved in dichloromethane and was infused via syringe pump at 5-10 L min Ϫ1 . EDESI-MS(/MS) spectra were collected using published procedures [28,29], and processed using the program EDit [30].…”