³¹P CP/MAS NMR of polycrystalline and immobilized phosphines and catalysts with fast sample spinning

Abstract

Cross-polarization (CP) at fast magic angle spinning (MAS) frequencies leads to a splitting of the Hartmann-Hahn (HH) matching profile into a centerband and additional bands of higher orders. The matching profiles differ with the substance categories. Therefore, signal intensity is usually lost, when e.g. the routine standard NH 4 H 2 PO 4 is used for optimizing the 1 H-31 P HH match prior to measuring phosphines and their metal complexes in polycrystalline or immobilized form. Here, a variety of model compounds, such as Ph 2 PCH 2 CH 2 PPh 2 and (CO) 2 Ni(PPh 3 / 2 , which can be used as 31 P CP standards for analogous substances or materials are presented. Investigating the influences of MAS frequency, contact time, 1 H pulse power and sample volume on the matching profiles of the model compounds leads to general trends. Thereby, a new strategy for measuring difficult samples with CP at high MAS rates has been developed: their optimum CP parameters are derived from the most intense maxima in the HH matching profiles of the corresponding model compounds. This new strategy is compared with variations of a conventional ramp sequence. Although the latter generally provide smaller signal half-widths, the new strategy leads to higher signal intensities. The new method was successfully applied to polycrystalline and immobilized phosphines and catalysts.

New Silica‐Immobilized Nickel Catalysts for Cyclotrimerizations of Acetylenes

“…However, this result is in accord, for example, with the finding of Fyfes group [58] for the analogous complex [(EtO) 3 SiCH 2 CH 2 PPh 2 ] 2 NiCl 2 , which was reported not to give a 31 P NMR signal at room temperature. Obviously, these complexes are tetrahedral, although some other dichloronickel complexes with diarylalkylphosphine ligands that we and others have investigated so far, [19,59] including 15 (see below) are planar and diamagnetic, and they give 31 P and 13 C CP/MAS signals with small halfwidths. [19,59] A more detailed overview of diamagnetic versus paramagnetic Ni(II) diphosphine complexes has been given by Booth.…”

“…Obviously, these complexes are tetrahedral, although some other dichloronickel complexes with diarylalkylphosphine ligands that we and others have investigated so far, [19,59] including 15 (see below) are planar and diamagnetic, and they give 31 P and 13 C CP/MAS signals with small halfwidths. [19,59] A more detailed overview of diamagnetic versus paramagnetic Ni(II) diphosphine complexes has been given by Booth. [60] All the other diamagnetic nickel complexes have been fully characterized by the usual analytical methods, and the NMR signal assignments are based upon former results, [9,10,13,17] comparisons with values from the literature [61] and 13 …”

“…[19] The linewidths of these polycrystalline samples are rather small, with about 100 to 150 Hz, depending on the method of measurement. All polycrystalline dicarbonyland dichloronickel complexes with chelating phosphines show two well-resolved and narrow 31 P NMR resonances [19] in the solid state, indicative of two magnetically non-equivalent phosphorus nuclei in the elemental cell.…”

“…Nowadays, the method of choice to study the immobilized species is classical solid-state NMR, which we have optimized especially for the measurement of immobilized phosphines and molecular catalysts. [19] As a complementary method, suspension NMR spectroscopy [15,20] allows the rapid check of a great number of routine samples, and also helps to detect merely adsorbed in contrast to covalently bound species. [15,18] The cyclotrimerization of alkynes represents one of the most useful methods for the selective synthesis of various substituted aromatic compounds.…”

“…However, as soon as the phosphines are immobilized on silica, they can be fully exposed to the air for hours, until measurable amounts of phosphine oxide appear in the 31 P NMR spectra. [19] The methods of choice for purifying the phosphines is Kugelrohr distillation or reversed phase chromatography. …”