The career of Bruce Gates has placed a strong emphasis on structure-function relationships and is most realized when the catalyst is molecular. With collaboration of the Katz group, a single-site silica-supported Ir4L3(CO)9, L = tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl and Pr = propyl) cluster catalyst was synthesized. The three calixarene ligands are bonded to 3 of the 4 iridium atoms at the basal plane and the ligands stabilize the cluster both mechanically and electronically. An apical Ir atom, without calixarenes, can react with olefins. The number of catalytically active iridium sites was controlled by selectively decarbonylating apical Ir sites by reaction with a mild oxidant, trimethylamine N-oxide. The number of carbonyl ligands removed is dependent on the atmosphere where the decarbonylation reaction occurs. In the presence of Ar, 0.5 apical carbonyls are removed but in the presence of ethylene 1.1 apical carbonyls are removed. The cluster’s activity for ethylene hydrogenation is controlled by the number of apical carbonyls removed from the cluster’s apical site (see figure). The data demonstrates single-site behavior by the linear relationship with activity and the number of apical active sites available. The cluster’s olefin hydrogenation activity is further improved by the addition of 2 peroxo ligands to the supported single-site Ir4L3(CO)9 cluster catalyst. Oxygen is ubiquitous in catalysis by supported metals, often originating from the treatment of precursors in synthesis steps such as calcination. The role of oxygen at metal-support interface sites has been invoked to account for promotion of noble-metal catalysts in reactions involving hydrogen transfer, such as hydrogenation, dehydrogenation, and hydrogenolysis. Yet, to date, there has been no molecular foundation (i.e., structure-function connection) to explain the promoting effect of oxygen on hydrogen transfer in a supported metal. From electronic structure calculations, measurements of a kinetic isotope effect, and a lower measured activation (15.3 to 13.8 kcal/mol), the role of oxygen as a ligand changes the mechanism for ethylene hydrogenation by the formation of a bridging H atom between the basal and apical plane. Without peroxo ligands no bridging H is observed.

Dialing in single-site behavior and activity on a single-site silica-supported cluster catalyst

ACS Spring 2022

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