Structure of 3(5)-[(4-diphenylphosphinoyl)phenyl]pyrazole in the solidstate (X-ray and CPMAS NMR) and in solution (NMR): Tautomerism and hydrogenbonds

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 13C, 31P and 15N 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 3-tautomer 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.