How Solid Surfaces Control Stability and Interactions of Supported Cationic Cu^I(dppf) Complexes─A Solid-State NMR Study

Abstract

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex−solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(L x ) + (dppf = 1,1′-bis(diphenylphosphino)ferrocene, L x = monoand bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by 31 P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al 2 O 3 resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of 31 P nuclei verified by 31 P-31 P RFDR and 1 H-31 P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad 31 P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(L x ) + complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

Solid-State NMR Probe Molecules for Catalysts and Adsorbents: Concepts, Quantification, Accessibility, and Spatial Distribution

“…Last, but not least, molecular heterogeneous catalysts, a support with immobilized, metal–organic complexes as active sites, become increasingly important for research on catalysis . These complexes are immobilized on surfaces via linkers or inside pores, to benefit from confinement effects inside these. − A quantitative investigation of accessibility and spatial location is, however, complicated by the fact that most commonly applied probe molecules required too harsh loading conditions, since already at room temperature surface sites can decompose these molecular complexes. , This short survey on the various types of active sites present demonstrates that each catalyst type, even each scientific problem, requires dedicated probe molecules that all have their advantages and disadvantages . Thus, for the quantitative evaluation of accessibility and spatial distribution, each time a dedicated probe molecule approach must be established.…”

“…Acetone-2- 13 C has in particular been loaded on zeolites and zeotypes ZSM-5, ,,, Y, ,,, MOR, ZSM­12, ZSM-22, MAPO-5, MAPO-34, − amorphous silica-alumina, ,, [Al]­MCM-41, , [Sn]­MCM-41, [Sn]­Beta, [Sn]­SSZ-13, on γ-Al 2 O 3 , , on TiO 2 (anatase), on heteropoly acid H 3 PW 12 O 40 , ,, on pure and sulfated SnO 2 and ZrO 2 , on MoO 3 /SnO 2 , on pure, MoO x - and WO x -loaded ZrO 2 , on acidic metal salts (mainly halogenides at low temperature), and on UiO-66 hydroxyl groups. , Acetone-2- 13 C reacts at room temperature on acid sites mainly via aldol condensation. ,,,,,, The 13 C MAS NMR peaks caused by the reaction products are usually of no further relevance for the interpretation; however, they cause often confusion and shall thus be addressed. In particular, NMR peaks of diacetone were identified around δ 13C = 209 and 73 ppm, of mesityl oxide at δ 13C = 210 and 188 ppm, and of isophorone and mesitylene around δ 13C = 210, 180, 138, and 34 ppm. , On [Sn]­Beta, supported by 119 Sn MAS NMR spectra, gem -diol-type intermediates giving rise to a slim peak at δ 13C = 76 ppm were reported .…”

“…Evaluating the CSA properties of adsorbates is then a way to elucidate the binding geometry and available space around adsorbed probe molecules. For example, the CSA properties of supported metalorganic phosphine complexes change, depending on the way of deposition and the support . Also the CSA tensors of 13 CO and 13 CO 2 were investigated, and, by the help of DFT calculations, the binding geometries were discussed. , In confined spaces, found at certain site locations, binding geometries might be different, which would in reverse change the CSA properties.…”

“…Thus, reasonable delays can be used for direct excitation spectra of the metal-bound phosphorus. A typical example for the use of phosphines for complexation of cations would be cationic copper complexes in ion exchangers …”

“…Complicating the situation, some LAS tend to transform into BAS upon contact with water traces, which may result in a misleading peak in the range of BAS sites if the transformation was irreversible. For example, BAS on γ-Al 2 O 3 that resulted in a peak at 222 ppm after loading acetone-2-­ 13 C were assigned as such by performing a loading with ammonia as a secondary reference . Thus, if the nature of acid sites on the probed material is unclear, a cross-check with a second probe molecule is strongly advised.…”