Abstract
The solid state 13C and 15N CPMAS NMR spectra of 3,5‐di‐tert‐butyl‐1,2,4‐diazaphosphole 4 and 3,5‐diphenyl‐1,2,4‐diazaphosphole 5 have been recorded. The X‐ray structure of the first compound was already known (it is a cyclic dimer with localized N–H protons) while the structure of the second cannot be determined due to the difficulty to grow suitable single crystals. NMR results pointed out that 4 is a “classical” compound while 5 is probably a tetramer showing Intermolecular Solid‐State Proton Transfer (ISSPT). GIAO/ab initio calculations have been carried out to estimate the absolute 1H, 13C and 15N shieldings. The agreement with the experimental chemical shifts is good enough to assign the signals of carbons C‐3 and C‐5.
Highlights
The solid state 13C and 15N CPMAS NMR spectra of 3,5-di-tert-butyl-1,2,4-diazaphosphole 4 and 3,5-diphenyl1,2,4-diazaphosphole 5 have been recorded
CPMAS NMR experiments: the solid state 13C and 15N NMR spectra were obtained at 300 K on a Bruker AC-200 spectrometer operating at 50.32 MHz (13C) and 20.28 MHz (15N) under cross polarization (CP) and magic angle spinning (MAS) conditions, using a 7 mm Bruker DAB 7 probehead that achieves rotational frequencies of about 3.5–4.5 kHz
The standard CPMAS pulse sequence was applied with 1H-90◦ pulse width of 7 μs for 13C and of 10.5 μs for 15N; 1 ms contact pulses and 5 s repetition time, the spectral width being of 15 000–17 000 Hz
Summary
1,2,4-Diazaphospholes 1 are related to pyrazoles 2 since they can be considered 4-phosphapyrazoles. Since 3,5-diphenylpyrazole 8 (2, R = C6H5) crystallizes in cyclic tetramers [6] we decided to explore the structure of 3,5-diphenyl-1,2,4-diazaphosphole 5 (1, R = C6H5) [1] in order to extend the comparison. As we could not obtain crystals of 5 suitable for X-ray analysis, we decided to investigate the structure of 1H-1,2,4-diazaphospholes 3, 4 and 5 using solid state NMR spectroscopy.
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