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Tertiary phosphines are ubiquitous in inorganic chemistry. They play important roles as ligands in coordination chemistry and catalysis. Furthermore, they act as surface acidity probes for oxide surfaces. However, only volatile phosphines, such as PH3 have been applied in this function so far. Here we demonstrate for the first time that the triaryl- and trialkylphosphines PPh3 and PCy3 with high melting points self-adsorb readily onto a silica surface even in the absence of a solvent. The self-adsorption takes place within days when both solid components are mixed and then left undisturbed. The phosphines form well-defined monolayers on the surface and the transition from monolayer to left-over polycrystalline phosphine is abrupt. Therefore, the maximal surface coverage with a monolayer can be easily determined. When the phosphines are adsorbed from solutions, the same maximal surface coverage is found. Solid-state NMR spectroscopy provides a unique analytical tool for studying the structure and dynamics of phosphines in different environments. 31P and 2H solid-state NMR measurements are successfully applied for characterizing the adsorption process and the mobilities of the adsorbed phosphines across the silica surface. Furthermore, using (Ph3P)2Ni(CO)2 as a representative, it is demonstrated that the silica surface has a hitherto unrecognized impact on immobilized and surface-residing catalysts because it competes for phosphine ligands coordinated to a metal center. This competition manifests as one more factor leading to the loss of phosphine ligands and ultimately leaching of immobilized metal complexes or nanoparticle formation. Besides the increase of fundamental knowledge about adsorption processes, the presented results have implications for chromatographic separations of metal complexes and for the lifetime of immobilized and other types of surface-residing catalysts. 

Oxidations represent important reactions that are ubiquitous in academia and industry. Hydrogen peroxide (H2O2) is a common source of active oxygen for oxidations. H2O2 is usually diluted in water as it is too unstable to be used in pure form. However, the presence of water can complicate reactions because biphasic mixtures with organic solvents form. Furthermore, secondary reactions with water may lead to side products. Therefore, alternative forms of H2O2, such as peroxide adducts, are an active area of research. Di(hydroperoxy)alkane adducts of phosphine oxides are one attractive solution because they are soluble in organic solvents, crystallizable, shelf-stable and active towards a variety of oxidation reactions. The only drawback is that the phosphine oxide carrier has to be removed after the reaction. In this contribution, the bifunctional ligand (EtO)3Si(CH2)2PPh2 is immobilized on a silica (SiO2) support which is subsequently end-capped with EtOSi(CH3)3. The new surface-bound di(hydroperoxy)propane adduct is then generated with the immobilized phosphine oxide as carrier. The adduct and a deuterated analog are characterized with solid-state and solution NMR spectroscopy. It has been demonstrated that substrates in organic solvents easily access the surface-bound peroxide and are oxidized quantitatively. The phosphine oxide carrier remains bound to the surface and can be removed easily by settling of the silica. Using the oxidative esterification of nonyl aldehyde it is proven that the immobilized peroxide adduct does not leach from the silica support and is active and reusable over multiple cycles. 

Many applications of catalysts immobilized on solid supports like silica via bifunctional phosphine linkers are still hampered by their decomposition, leaching, agglomeration, and uncontrolled nanoparticle formation, all of which change their activities and selectivities. In general, the success of an immobilized catalyst is crucially dependent on the linker and its attachment to the oxide support. In this contribution, an improved method for covalently binding phosphine linkers to silica via ethoxysilane groups is described. This method leads to well-defined sub-monolayers of linkers on silica surfaces without cross-linking of the linkers, which typically leads to clogged pores and metal agglomeration during catalysis, thus entailing less active and selective catalysts. The novel immobilization method has been supported by multinuclear classical CP/MAS solid-state NMR spectroscopy, as well as suspension NMR of slurries. It has been demonstrated by TEM that nickel complexes coordinated by immobilized phosphine linkers in a well-defined sub-monolayer coverage do not form larger aggregates or nanoparticles during the catalytic cyclotrimerization of phenylacetylene under various conditions, in contrast to analogous complexes in homogeneous catalytic runs. 

The di(hydroperoxy)adamantane adducts of water (1) and phosphine oxides p -Tol 3 PO·(HOO) 2 C(C 9 H 14 ) (2), o -Tol 3 PO·(HOO) 2 C(C 9 H 14 ) (3), and Cy 3 PO·(HOO) 2 C(C 9 H 14 ) (4), as well as a CH 2 Cl 2 adduct of a phosphole oxide dimer (8), have been created and investigated by multinuclear NMR spectroscopy, and by Raman and IR spectroscopy. The single crystal X-ray structures for 1–4 and 8 are reported. The IR and 31 P NMR data are in accordance with strong hydrogen bonding of the di(hydroperoxy)adamantane adducts. The Raman ν (O–O) stretching bands of 1–4 prove that the peroxo groups are present in the solids. Selected di(hydroperoxy)alkane adducts, in combination with AlCl 3 as catalyst, have been applied for the direct oxidative esterification of n -nonyl aldehyde, benzaldehyde, p -methylbenzaldehyde, p -bromobenzaldehyde, and o -hydroxybenzaldehyde to the corresponding methyl esters. The esterification takes place in an inert atmosphere, under anhydrous and oxygen-free conditions, within a time frame of 45 minutes to 5 hours at room temperature. Hereby, two oxygen atoms per adduct assembly are active with respect to the quantitative transformation of the aldehyde into the ester.