Multiple Surface Site Three-Dimensional Structure Determination of a Supported Molecular Catalyst

Abstract

The structural characterization of supported molecular catalysts is challenging due to the low density of active sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in an N-heterocyclic carbene based on iridium and supported on silica, at all stages of its synthesis. By combining a suitable isotope labeling strategy with the implementation of multinuclear dipolar recoupling DNP-enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multistep synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting EXAFS spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multisite environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC-supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Speciation and Structures in Pt Surface Sites Stabilized by N-Heterocyclic Carbene Ligands Revealed by Dynamic Nuclear Polarization Enhanced Indirectly Detected 195Pt NMR Spectroscopic Signatures and Fingerprint Analysis

“…21 Furthermore, metal sites may be bonded to one or several organic ligands as recently shown on a supported Ir-NHC complex where two types of surface organometallic sites were identified, with the Ir metal centers bound to one or two NHC surface ligands. 22 Yet, despite the evidence of various surface sites, obtaining a precise understanding of the structure and coordination environment of metal sites in supported catalysts remains a challenge and a critical aim. Thus, developing methodologies that enable both identifying speciation and describing the coordination environment of metal sites is of prime importance to understand and further improve the catalytic performance of supported catalysts, paralleling the realm of molecular chemistry and homogeneous catalysis.…”

“…For example, interactions may occur between the metal centers and the surface functionalities of the solid support that modify the coordination environment around the active sites as observed for an immobilized Ru-NHC catalyst . Furthermore, metal sites may be bonded to one or several organic ligands as recently shown on a supported Ir-NHC complex where two types of surface organometallic sites were identified, with the Ir metal centers bound to one or two NHC surface ligands . Yet, despite the evidence of various surface sites, obtaining a precise understanding of the structure and coordination environment of metal sites in supported catalysts remains a challenge and a critical aim.…”

Double-resonance17O NMR experiments reveal unique configurational information for surface organometallic complexes

“…Recent years have seen what is arguably the greatest leap in our understanding of SOMC catalyst structure with the development and generalization of dynamic nuclear polarization (DNP)-enhanced NMR distance measurements. 8,[10][11][12][13] Either alone, or in combination with XAS, these methods have been used to elucidate the first threedimensional (3D) configurational structures of SOMC complexes. We, for instance, showed that 13 C{ 27 Al} distance measurements in an Al 2 O 3 -supported Ir pincer complex could be used to distinguish between a dozen different potential grafting configurations, 12 and that 13 C{ 29 Si} distance measurements in a 29 SiO 2 -supported Sc amidinate complex were able to differentiate different coordination geometries.…”

“…While the ultimate goal is synthesizing sites with well-defined and uniform structures that will have identical reactivities, we and others have underlined that the chemistry is far more complex than is often implied, sometimes leading to many supported species. [2][3][4][5][6][7][8][9][10] Of course, understanding the reactivity of these sites requires knowledge of their structure, however, the most typical characterization tools, such as 1D nuclear magnetic resonance (NMR), infrared spectroscopy, and X-ray absorption spectroscopy (XAS), are insensitive to subtle changes in configuration.…”

Spatial arrangement of dynamic surface species from solid-state NMR and machine learning-accelerated MD simulations

“…With general interest in developing methods to determine inter-site distances on surfaces and gain insights into cooperativity and isolation, it is necessary to find ways to recover accurate inter-site distances from NMR-based approaches. 15,[32][33][34][35] One potential approach for the accurate measurement of internuclear distances using NMR spectroscopy is the application of molecular dynamics (MD) simulations to analyze molecular motions and estimate their impacts on dipolar couplings, as originally introduced by Ishii et al 19 In the context of surface species, Paterson et al combined dipolar coupling measurement and applied direct density functional theory (DFT) MD simulations to gain insights into the dynamics of grafted metal complexes. 36 While their approach was only partly successful in explaining the observed motions, this combination of accurate DFT MD methods for calculating dynamically-averaged dipolar coupling constants may well enable the use of NMR to analyze the structure and arrangement of mobile molecules on solid surfaces.…”