Metal–Support Interactions in Molecular Single-Site Cluster Catalysts

Abstract

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 MCo 6 Se 8 (PEt 3 ) 4 (L) 2 (1-M, M = Cr, Mn, Fe, Co, Cu, and Zn) was designed in which the active site (M)/support (Co 6 Se 8 ) 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, 1 H and 31 P 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.

Metal-support interactions regulate substrate binding in Fe/Co/Se cluster catalysts

“…40% of the catalyst is consumed in the off-cycle formation of the amido complex 1-Fe(NHTs). 15 It is noteworthy that under similar conditions the triiron cluster 2-Fe3 also degrades, possibly undergoing fragmentation, 16 whereas decomposition is shut off when a large excess of a coordinating ligand (tert-butyl isocyanide or pyridine) is present (Figure S10 and S12). 16 To uncover how the number of edge sites and the isocyanide concentration modulate the catalytic activity, the 1-Fe and 2-Fe3 clusters are compared.…”

“…[11][12][13][14] To this end, our group introduced a molecular construct in which the identity and number of the active sites installed on an atomically precise inorganic cluster support can be systematically tuned. [15][16][17][18][19] This synthetic innovation has enabled systematic investigations into the nature of the active site/support cooperativity 15,16 and the role of multi-active site dynamics in modulating catalytic activity for azide activation and asymmetric carbodiimide synthesis. 18 In this study we investigate the stereo-electronic requirements of the active site to access a Goldilocks regime of substrate affinity at a family of Fe/Co6Se8 molecular cluster catalysts (Figure 1).…”

“…Connectivity and first coordination sphere of the Fe edge sites in 2-Fe3, 2′-Fe3, and 1-Fe from previously reported single crystal X-ray structures. 15,16,30 substrate/active site/support interactions coalesce to maximize catalytic activity. Strong metal/support interactions promote substrate (isocyanide) dissociation, while simultaneously enabling π-backbonding and increased substrate activation.…”

“…18 In contrast, identification of iron nitrenoid intermediates for the triiron Fe3Co6Se8L6 (2-Fe3) or monoiron derivative FeCo6Se8(PEt3)4(L)2 (1-Fe; Et = ethyl) has been elusive. 15,16 We hypothesized that the presence of a single active site could make observing intermediates and interpreting the reactivity of 1-Fe more accessible than for the triiron congener 2-Fe3. Indeed, at 10% 1-Fe loading, we have previously observed the formation of two paramagnetic species by 1 H NMR spectroscopy during the course of the carbodiimide formation.…”

“…Indeed, at 10% 1-Fe loading, we have previously observed the formation of two paramagnetic species by 1 H NMR spectroscopy during the course of the carbodiimide formation. 15 To probe the identity of these species, here, the reaction between 1-Fe and TsN3 was evaluated first under stoichiometric conditions (Scheme 1). Slow addition of the azide to a thawing solution of the iron complex leads to rapid effervescence and the consumption of the azide, as evinced by the disappearance of the diagnostic tosyl azide stretch (2123 cm -1 ; Figure S2) in the associated infrared spectrum.…”