Selective molecular recognition by nanoscale environments in a supported iridium cluster catalyst.

The active sites of enzymes are contained within nanoscale environments that exhibit exquisite levels of specificity to particular molecules. The development of such nanoscale environments on synthetic surfaces, which would be capable of discriminating between molecules that would nominally bind in a similar way to the surface, could be of use in nanosensing, selective catalysis and gas separation. However, mimicking such subtle behaviour, even crudely, with a synthetic system remains a significant challenge. Here, we show that the reactive sites on the surface of a tetrairidium cluster can be controlled by using three calixarene-phosphine ligands to create a selective nanoscale environment at the metal surface. Each ligand is 1.4 nm in length and envelopes the cluster core in a manner that discriminates between the reactivities of the basal-plane and apical iridium atoms. CO ligands are initially present on the clusters and can be selectively removed from the basal-plane sites by thermal dissociation and from the apical sites by reactive decarbonylation with the bulky reactant trimethylamine-N-oxide. Both steps lead to the creation of metal sites that can bind CO molecules, but only the reactive decarbonylation step creates vacancies that are also able to bond to ethylene, and catalyse its hydrogenation.

C-H activation by multiply bonded complexes with potentially noninnocent ligands: a computational study.

Second- and third-row (typically precious metals) transition metal complexes are known to possess certain electronic features that define their structure and reactivity and are usually not observed in their first-row (base metal) congeners. Can these electronic features be conferred onto first-row transition metals with the aid of noninnocent and/or very high-field ligands? In this research, the impact upon methane C-H bond activation was modeled using the dipyridylazaallyl (smif) supporting ligand for late, first-row transition metal (M) imide, oxo, and carbene complexes (M = Fe, Co, Ni, or Cu; E = O, NMe, or CMe2). Density functional theory calculations suggest that the combination of smif with iron and the oxo activating ligand is the most energetically favorable complex for methane C-H activation. A change in the preferred transition state for methane C-H activation from [2+2] addition to hydrogen atom abstraction was observed upon going from Fe to Cu and for Fe as compared to precious metals. Contrary to expectations, it was the imide ligand rather than the dipyridylazaallyl ligand that was found to possess redox "noninnocent" characteristics.