Charged ligands for direct ESI-MS analysis of catalytic reactions

Date

2011-05-30

Authors

Chisholm, Danielle Marie

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Abstract

Electrospray ionization mass spectrometry (ESI-MS) is well-established in the detection of large fragile organic molecules such as polymers, peptides and proteins. The study of catalysis by transition metal complexes is complicated by difficulties including ligand lability, complex neutrality and air- and moisture-sensitivity. This work is focused on establishing methods to solve these problems and to apply them to well-understood systems in order to establish credibility before applying them to new systems. Attempts to synthesize a 2,2’-bipyridine (bipy)-type ligand designed to have proton sponge-like properties after binding to a metal are presented. The synthesis of 3,3’-N,N’-bis(dimethylamino)-2,2’-bipyridine was stymied by the formation of two strong intramolecular hydrogen bonds, which are clearly evident in the X-ray crystal structure of the isolated dimethylated 3,3’-bis(methylamino)-2,2’-bipyridine. A simple, one step synthesis of a charge-tagged phosphine from commercially available precursors was developed. Monoalkylation of bisphosphines is a highly convenient approach to such ligands, avoiding the multi-step routes demanded for phosphine/ammonium ligands. 4-diphenylphosphino-1-benzyldiphenylphosphonium-butane tetrafluoroborate was used for the investigation of hydrogenation of olefins by RhCl(PPh3)3 (Wilkinson’s catalyst) by ESI-MS. The results obtained by ESI-MS and ESI-MS/MS on the speciation of the reaction as well as the potential reactivity of select species are in agreement with results obtained by traditional techniques. This work serves as a proof of principle that the methodology employed in our lab is suited to these investigations. The same ligand was used to examine the poorly understood dehydrocoupling of di(n-hexyl)silane by the same catalyst. Continuous monitoring of the reaction over 48 minutes added the time dimension to the data, and insight into the dynamics of the reaction was obtained. Key intermediates were observed, along with decomposition products and circumstantial evidence supporting the formation of a silylene intermediate was also obtained. Lastly, some collaborative work is presented in which some of the techniques and methods developed in our laboratory were applied to problems of interest to other scientists. The formation of a heteronuclear ruthenium-gold cluster is monitored by ESI-MS and further analyzed by ESI-MS/MS. The characteristics that affect the surface activity of an ion are discussed and solutions of a dication with two different anions are examined. Biologically active ruthenium trimers were studied by EDESI-MS/MS, and their fragmentation behaviour shown to be analogous to their properties as CO-releasing molecules.

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Keywords

ligand, catalysis, synthesis

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