Cyclophanes as Platforms for Reactive Multimetallic Complexes

• Published in: Accounts of chemical research 2019-02, Vol.52 (2), p.447-455
• Main Authors: Ferreira, Ricardo B, Murray, Leslie J
• Format: Article
• Language: English
• Subjects: Bridged-Ring Compounds - chemical synthesis; Bridged-Ring Compounds - chemistry; Carbon Dioxide - chemistry; Catalysis; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Macrocyclic Compounds - chemical synthesis; Macrocyclic Compounds - chemistry; Metals, Heavy - chemistry; Nitrogen - chemistry; Oxidation-Reduction; Oxygen - chemistry
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.8b00559

Conspectus Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N2, O2, CO2) with prominent roles in geochemical element cycles or detoxification. Notable examples include the iron–molybdenum cofactor of the molybdenum-dependent nitrogenases, which catalyze N2 fixation, and the NiFe4S4 cluster and the Mo­(O)­SCu site in various carbon monoxide dehydrogenases. The prevailing proposed reaction mechanisms for these multimetallic cofactors relies on a cooperative pathway, in which the oxidation state changes are distributed over the aggregate coupled with orbital overlap between the substrate and more than one metal ion within the cluster. Such cooperativity has also been proposed for chemical transformations at the surfaces of heterogeneous catalysts. However, the design details that afford cooperative effects and allow such reactivity to be harnessed effectively in homogeneous synthetic systems remain unclear. Relatedly, hydride donors ligated to these metal cluster cofactors are suggested as precursors to the state that reacts with substrates; here too, however, the reactivity of hydride-decorated clusters supported by weak-field ligands is underexplored. Inspired by the reactivity potential of multimetallic assemblies evidenced in biological systems, approaches to design, synthesize, and evaluate reactivity of polynuclear metal compounds have been actively explored. In a similar vein to the templating function afforded by enzyme active sites, a carefully engineered organic ligand can be employed to control metal nuclearity of the complex and the local coordination environment of each metal center. This Account presents our efforts within this field, beginning with ligand design considerations followed by a survey of observed small molecule activation by trimetallic cyclophanates. We highlight the distinct reactivity outcomes accessed by multimetallic compounds as compared to aggregates that assemble in reaction mixtures from monometallic precursors. Contributing to the opportunity for programmed cooperativity in these designed multimetallic compounds, the cyclophane also dictates the orientation of substrate binding and metal–substrate interactions, which has a prominent influence on reactivity. For example, the dinitrogen-tricopper­(I) cyclophanate reacts with dioxygen with markedly different results as compared to monocopper