Structure of 3(5)-[(4-diphenylphosphinoyl)phenyl]pyrazole in the solid state (X-ray and CPMAS NMR) and in solution (NMR): Tautomerism and hydrogen bonds

Rosa M Claramunt,Lev N Zakharov,Arnold L Rheingold,Concepción López,José Elguero,Swiatoslaw Trofimenko,Paul Krapcho

doi:10.3998/ark.5550190.0004.a21

Abstract

The structure of 3(5) -[(4-diphenylphosphinoyl)phenyl]pyrazole has been determined by X -ray crystallography. The compound crystallizes from DMF as the 1H-3-substituted tautomer (monoclinic, space group P21/c). The complete characterization by means of 13 C, 31 P and 15 N CPMAS NMR allowed us to confirm that at the solid state only such tautomer is found, with strong intermolecular hydrogen bonds between the pyrazole-NH donor and the phosphine oxide group acceptor. At 300 K in DMSO-d6 solution, the two tautomers were detected in a ratio 3tautomer versus 5-tautomer of 86:14. According to DFT calculations (B3LYP/6-31G** + ZPE), the 3-[(4-diphenylphosphinoyl)phenyl]-1H-pyrazole tautomer is more stable by about 3.8 kJ mol –1 .

Highlights

Our research on supramolecular architectures formed by NH-azoles in the solid state,[1,2,3,4,5,6,7,8,9,10] and most pyrazoles,[2,3,4,5,6] led us to try to establish several rules capable of predicting annular tautomerism, which nitrogen bears the proton, and the network formed by the intermolecular N-H···N hydrogen bonds
NMR spectroscopy in solution and in solid state When the 1H NMR spectra of compound 3 was recorded in DMSO-d6 solution the two tautomers were detected as proved by the two pyrazole-NH signals appearing at 13.50 ppm and 13.05 ppm with a relative intensity of 14/86
Concerning tautomerism, 3(5)-[(4-diphenylphosphinoyl)phenyl]pyrazole exists in the solid state as the 3-substituted tautomer 3a, the same that is predominant in DMSO solution (86%)

Summary

Introduction

Our research on supramolecular architectures formed by NH-azoles in the solid state,[1,2,3,4,5,6,7,8,9,10] and most pyrazoles,[2,3,4,5,6] led us to try to establish several rules capable of predicting annular tautomerism, which nitrogen bears the proton, and the network formed by the intermolecular N-H···N hydrogen bonds. The secondary structure, resulting from the hydrogen bonds between molecules in the crystal, leads to the formation of dimers or tetramers in alkyl substituted pyrazoles and trimers or catemers for the aryl ones. 3(5)-phenylpyrazole (2) where it is possible to isolate the two independent tautomers in the solid state, the most stable one 5-phenyl-1H-pyrazole (2b) forming a catemer of order 2,5 and the 3phenyl-1H-pyrazole (2a) hexamers.[6].

Results and Discussion

Method

Conclusions