Synthesis, structure, electrostatic properties and spectroscopy of 3-methyl-4,5,6,7-tetrafluoro-1H-indazole. An experimental and ab initio computational study†

Abstract

The azine of pentafluorobenzaldehyde had been previously prepared from pentafluorobenzaldehyde and hydrazine. However, the analogous reaction of 2,3,4,5,6-pentafluoroacetophenone 1 with hydrazine did not result in the formation of azine 3 but resulted instead in the formation of 3-methyl-4,5,6,7-tetrafluoro-1H-indazole, 4, via the hydrazone 2. The resulting indazole was characterized by high resolution mass spectroscopy and ¹H-, ¹³C-, and ¹⁹F-NMR spectroscopy. The geometry and electrostatic properties of the parent indazole and its derivative, 4, were studied with ab initio quantum theory and density functional methods. Our optimized structure of the parent indazole computed at the MP2(fc)/6-311G** level is presumably more accurate than the structure derived from microwave measurements. The preferred conformer of 4 was determined from RHF/6-31G* energies and full normal mode analyses were used to characterize both conformers. The minimum structure of 4 was refined at the MP2(fc)/6-311G** level of theory and compared to the unsubstituted structure. The electrostatic properties of the parent indazole and 4 are discussed and compared to those of benzene and hexafluorobenzene calculated at the same level. Natural bond order (NBO) calculations were performed to rationalize the difference in direction of the dipole moments of the parent indazole and 4. The gauge-invariant atomic orbital (GIAO) method was employed to calculate atomic shielding tensors of the indazoles using density functional theory at the B3LYP/6-311+G(2d,p) level. The calculated chemical shifts were used to aid in assigning peaks in the NMR spectra.