Dancing at the Interface: Surface-Mediated Active Site Dynamics within Molecular Nanoclusters

Abstract

The key to accessing next-generation heterogeneous catalysts, which lie at the heart of many industrial manufacturing and energy conversion processes, is the identification and detailed surveillance of the catalytic active site. However, owing in part to the myriad interfacial dynamics that dictate active site identity, reactivity and selectivity, these catalytic systems are notoriously challenging to study. As Muetterties proposed years ago via the cluster-surface analogy, polynuclear molecular clusters provide an environment to study and harness these surface processes with atomic precision. Through this work, we introduce a new family of M/Co/Se nanoclusters that not only enables detailed investigations of active site identity and mechanism, but also reveals the presence of multi-site allosteric effects that orchestrate surface dynamics.Herein, homoleptic Co6Se8LR,H6 clusters decorated with bifunctional aminophosphine ligands (LR,H = Ph2PN(H)R; R = 4-tolyl, isopropyl) serve as redox-active, inorganic platforms that are pre-templated to host three substrate-accessible “active sites” at their surface. After deprotonation of the pendent amino groups, this cluster-ligand is trimetallated with a wide array of first-row transition metal ions to afford a family of propeller-shaped M/Co/Se nanoclusters, M3Co6Se8LTol6 (M3; LTol = Ph2PN(–)Tol; M = Cr2+, Mn2+, Fe2+, Co2+). Owing to their unique physicochemical properties, these nanoclusters provide an atomically defined, molecular environment to study the metal-support interactions that govern heterogeneous catalytic interfaces. In Chapter 1, we find the surface Fe centers of the triiron nanocluster, Fe3, engage in dynamic coordinative and electronic interactions with the Co6Se8 support, “dancing” with one another on this well-defined inorganic surface. These interactions are facilitated by hemilabile Fe···Se bonding, the strength of which exhibits a clear redox-state dependence. The electronic structure of Fe3 was investigated in detail using a suite of synthetic and electrochemical methods, extensive single crystal X-ray diffraction analysis, electronic absorption and infrared spectroscopy, and 57Fe Mössbauer spectroscopy. We find that the Fe edge sites communicate electronically with the Co6Se8 core and that these interactions are responsive to ligand binding. In turn, the Co6Se8 core is demonstrated as a redox non-innocent ligand platform. Ultimately, Fe3 serves as an excellent catalyst for the conversion of tosyl azide (TsN3) and tert-butyl isocyanide (CNtBu) to the asymmetric carbodiimide, TsN=C=NtBu, via a multi-electron redox transformation. In Chapter 2, detailed synthetic investigations reveal how supporting aminophosphine ligands impact product distribution in the synthesis of the related cluster-ligand, Co6Se8LiPr,H6, granting access to a series of smaller Co/Se cluster intermediates. Introduction of more sterically demanding isopropyl substituents at the edge sites enables isolation of pseudo-D3 symmetric trimetallated clusters, M3Co6Se8L iPr6 (M3iPr; LiPr = Ph2PN(–)iPr; M = Fe2+, Zn2+), wherein no ancillary ligands are bound at the edge metals. In Chapter 3, the binding of surface ligands is found to have a collective impact on the surface sites of Fe¬3, as multi-active site domino interactions promote allosteric ligand substitution at discrete Fe centers on the cluster surface. In an unprecedented molecular example of the inter-adsorbate effect, we resolve these inter-adsorbate dynamics at the cluster surface crystallographically. By combining a battery of crystallographic, spectroscopic, and computational techniques, we demonstrate how delocalized Fe/Se/Co bonding interactions promote ligand exchange at Fe sites positioned on the same ⍺/β cluster face. In Chapter 4, these allosteric effects are harnessed to encode structural anisotropy within a family of redox-active two-dimensional nanosheets built via self-assembly of Co3. In Chapter 5, we return to the catalytic synthesis of asymmetric carbodiimide to conduct a thorough investigation of active site identity and reaction mechanism. Following our initial discovery of this reactivity using Fe¬3, a trichromium derivative, Cr3(py)3, is developed to conduct a detailed analysis of catalytic azide activation and nitrene transfer through a series of stoichiometric and catalytic reactivity studies. Through the isolation of key mono-, bis- and tris(imido) intermediates, we pin-point the identity of the catalytic active site within Cr3(py)3 and discover the presence of multi-active site interactions that regulate nitrene transfer kinetics. In Chapter 6, a comparative crystallographic study of Cr3 clusters unearths counter-intuitive bonding trends in the structural site-differentiation of the Cr edge sites. Building upon this crystallographic analysis, we apply a combination of computational, electrochemical, spectroscopic, and synthetic techniques to explore the mechanism of active site speciation within the trichromium nanocluster. Through this study, we establish a molecular orbital basis for electronic active site differentiation at within these M/Co/Se nanocluster.