Probing Edge/Support Electronic Cooperativity in Single Edge Fe/Co6Se8 Clusters.

This study provides insights into the electronic structure of an atomically precise Fe/Co6Se8 cluster and the extent of redox cooperativity between the Fe active site and the noninnocent Co6Se8 support. Chemical oxidation studies enable the isolation of two types of oxidized Fe/Co6Se8 clusters, in which the nature of the counterion (I- or OTf-) significantly impacts the structural interactions between Fe and the Co6Se8 unit. Experimental characterization by single crystal X-ray diffraction, 57Fe Mössbauer spectroscopy, and 31P{1H} NMR spectroscopy is complemented by computational analysis. In aggregate, the study reveals that upon oxidation, the charge is shared between the Fe edge site and the Co6Se8 core.

Metal/Support Interactions Regulate Substrate Binding in Fe/Co/Se Cluster Catalysts.

Here, we investigate the stereoelectronic requirements of a family of Fe/Co6Se8 molecular clusters to achieve a Goldilocks regime of substrate affinity for the catalytic coupling of tosyl azide and tert-butyl isocyanide. The reactivity of a catalytically competent iron-nitrenoid intermediate, observed in situ, is explored toward nitrene transfer and hydrogen-atom abstraction. The dual role of isocyanide, which, on the one hand, prevents catalyst degradation but, in large amounts, slows down reactivity, is exposed. The impact of distal changes (the number of neighboring active sites and the identity of supporting ligands) on the substrate affinity, electronic properties, and catalytic activity is investigated. Overall, the study reveals that the dynamic, push-pull interactions between the substrate (tBuNC), active site (Fe), and support (Co6Se8) create a regime where increased substrate activation occurs with facile dissociation.

Metal-Support Interactions in Molecular Single-Site Cluster Catalysts.

This study provides atomistic insights into the interface between a single-site catalyst and a transition metal chalcogenide support and reveals that peak catalytic activity occurs when edge/support redox cooperativity is maximized. A molecular platform MCo6Se8(PEt3)4(L)2 (1-M, M = Cr, Mn, Fe, Co, Cu, and Zn) was designed in which the active site (M)/support (Co6Se8) interactions are interrogated by systematically probing the electronic and structural changes that occur as the identity of the metal varies. All 3d transition metal 1-M clusters display remarkable catalytic activity for coupling tosyl azide and tert-butyl isocyanide, with Mn and Co derivatives showing the fastest turnover in the series. Structural, electronic, and magnetic characterization of the clusters was performed using single crystal X-ray diffraction, 1H and 31P nuclear magnetic resonance spectroscopy, electronic absorption spectroscopy, cyclic voltammetry, and computational methods. Distinct metal/support redox regimes can be accessed in 1-M based on the energy of the edge metal's frontier orbitals with respect to those of the cluster support. As the degree of electronic interaction between the edge and the support increases, a cooperative regime is reached wherein the support can deliver electrons to the catalytic site, increasing the reactivity of key metal-nitrenoid intermediates.

Multi-active Site Dynamics on a Molecular Cr/Co/Se Cluster Catalyst.

This study uncovers the interconnected reactivity of the three catalytically active sites of an atomically precise nanocluster Cr3(py)3Co6Se8L6 (1(py)3, L = Ph2PNTol-, Ph = phenyl, Tol = 4-tolyl). Catalytic and stoichiometric studies into tosyl azide activation and carbodiimide formation enabled the isolation and crystallographic characterization of key catalytically competent metal-imido intermediates, including the tris(imido) cluster 1(NTs)3, the catalytic resting state 1(NTs)3(CNtBu)3, and the site-differentiated mono(imido) cluster 1(NTs)(CNtBu)2. In the stoichiometric regime, nitrene transfer proceeds via a stepwise mechanism, with the three active sites engaging sequentially to produce carbodiimide. Moreover, the chemical state of neighboring active sites was found to regulate the affinity for substrates of an individual Cr-imido edge site, as revealed by comparative structural analysis and CNtBu binding studies.

Redox-Switchable Allosteric Effects in Molecular Clusters.

We demonstrate that allosteric effects and redox state changes can be harnessed to create a switch that selectively and reversibly regulates the coordination chemistry of a single site on the surface of a molecular cluster. This redox-switchable allostery is employed as a guiding force to assemble the molecular clusters Zn3Co6Se8L'6 (L' = Ph2PN(H)Tol, Ph = phenyl, Tol = 4-tolyl) into materials of predetermined dimensionality (1- or 2-D) and to encode them with emissive properties. This work paves the path to program the assembly and function of inorganic clusters into stimuli-responsive, atomically precise materials.

Tuning the Electronic Structure of Atomically Precise Sn/Co/Se Nanoclusters via Redox Matching of Tin(IV) Surface Sites.

A new strategy is reported to tailor the electronic properties of a superatomic metal chalcogenide cluster by redox matching the cluster core with surface tin(IV) sites. Two ternary clusters (SnR2)3Co6Se8L6 (R = Me, nBu) are synthesized by salt metathesis from the hexalithiated salt [Li2(py)2]3Co6Se8L6 and R2SnCl2. Cyclic and differential-pulse voltammetry studies reveal that the tristannylated clusters feature two new, near-degenerate, electronic states within the highest occupied molecular orbital-lowest unoccupied molecular orbital gap of the Co6Se8 core, which are attributed to the reduction of a surface tin site. Single-crystal X-ray diffraction analysis reveals that no Sn···Se coordination is present in the solid state. The single-crystal X-ray structure of the hexalithiated salt starting material is reported for the tetrahydrofuran (THF) adduct variant [Li2(THF)2]6Co6Se8L6.

Atomically Defined Nanopropeller Fe3Co6Se8(Ph2PNTol)6: Functional Model for the Electronic Metal-Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation.

Atomically defined interfaces that maximize the density of active sites and harness the electronic metal-support interaction are desirable to facilitate challenging multielectron transformations, but their synthesis remains a considerable challenge. We report the rational synthesis of the atomically defined metal chalcogenide nanopropeller Fe3Co6Se8L6 (L = Ph2PNTol) featuring three Fe edge sites, and its ensuing catalytic activity for carbodiimide formation. The complex interaction between the Fe edges and Co6Se8 support, including the interplay between oxidation state, substrate coordination, and metal-support interaction, is probed in detail using chemical and electrochemical methods, extensive single crystal X-ray diffraction, and electronic absorption and Mössbauer spectroscopy.

Catalytic intramolecular hydroamination of aminoallenes using titanium complexes of chiral, tridentate, dianionic imine-diol ligands.

Alkylation of d- or l-phenylalanine or valine alkyl esters was carried out using methyl or phenyl Grignard reagents. Subsequent condensation with salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, or 5-fluorosalicylaldehyde formed tridentate, X2L type, Schiff base ligands. Chiral shift NMR confirmed retention of stereochemistry during synthesis. X-ray crystal structures of four of the ligands show either inter- or intramolecular hydrogen bonding interactions. The ligands coordinate to the titanium reagents Ti(NMe2)4 or TiCl(NMe2)3 by protonolysis and displacement of two equivalents of HNMe2. The crystal structure of one example of Ti(X2L)Cl(NMe2) was determined and the complex has a distorted square pyramidal geometry with an axial NMe2 ligand. The bis-dimethylamide complexes are active catalysts for the ring closing hydroamination of di- and trisubstituted aminoallenes. The reaction of hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives a mixture of 6-ethyl-2,3,4,5-tetrahydropyridine (40-72%) and both Z- and E-2-propenyl-pyrrolidine (25-52%). The ring closing reaction of 6-methyl-hepta-4,5-dienylamine at 135 °C with 5 mol% catalyst gives exclusively 2-(2-methyl-propenyl)-pyrrolidine. The pyrrolidine products are obtained with enantiomeric excesses up to 17%.