5- tert -Butyl-4-nitro-1 H -pyrazol-3-ol

The structure of the title compound, C7H11N3O3, consists of mol­ecules that pack in a linear hydrogen-bonded ribbon motif. This hydrogen-bonding arrangement is constructed through two dimer formations, one that is atypical of pyrazoles (N—H⋯N) and the other via an inter­action from the hydr­oxy OH group to one of the nitro O atoms.

The structure of the title compound, C 7 H 11 N 3 O 3 , consists of molecules that pack in a linear hydrogen-bonded ribbon motif. This hydrogen-bonding arrangement is constructed through two dimer formations, one that is atypical of pyrazoles (N-HÁ Á ÁN) and the other via an interaction from the hydroxy OH group to one of the nitro O atoms.

Comment
Pyrazoles and related compounds are common molecules used in coordination or organometallic chemistry as bridging ligands, utilizing the ring positions of the two N atoms. There are 1388 structures in the Cambridge Structural Database (CSD; Version 5. 26, November 2004;Allen, 2002) that contain a pyrazole ring with the extra search constraints 'no extra cyclic routes' and 'require 3D coordinates'. This number reduces to 23 for 4-nitropyrazoles, 80 for 5-tert-butylpyrazoles, and 15 for 3-hydroxypyrazoles. Interestingly, there is only one structure (CSD refcode: WILBAU), that of 3,5-di-tert-butyl-4nitropyrazole (Llamas-Saiz et al., 1994), which contains two of the three mentioned substituents.
In a series of studies on the preparation and hydrogenbonding properties of 3,4,5-trisubstituted pyrazoles, we now report 5-tert-butyl-4-nitro-1H-pyrazol-3-ol, (I). The structure of (I) (Fig. 1) consists of molecules that pack to form a linear hydrogen-bonded ribbon motif (Fig. 2). The hydrogenbonding arrangement can be described by two centrosymmetric dimer formations (Table 1). The first of these dimer formations is atypical of pyrazoles and involves an N1-HÁ Á ÁN2 interaction, centred at ( 1 2 ,0,0) described by an R 2 2 (6) graph set (Etter, 1990), while the second dimer formation, centred at (0,1,0), involves one intramolecular hydrogenbonding association from O3-H to O42, forming an S(6) graph-set motif, and an R 2 2 (4) graph-set motif arising from the three-centre association involving H3 and two O42 atoms. The other O atom (O41) of the nitro group is not involved in the hydrogen-bond network. The ribbon motifs are stacked in the a-axis direction, the perpendicular distances between ribbon planes being 3.263 (2) and 3.195 (2) Å (calculated with PLATON; Spek, 2003).

Experimental
Synthetically, (I) originated from 3,5-di-tert-butylpyrazole, being produced by gently warming this compound in concentrated nitric acid. In this reaction, 3,5-di-tert-butylpyrazole is attacked by nitric acid to form the onium species, which then displaces one tert-butyl group. The subsequent vacant position is then filled by an OH group that does not tautomerize to form the pyrazolone. The title compound was obtained from Key Organics Ltd and crystals were grown from ethanol solution.

D-HÁ
All tert-butyl H atoms were included in the refinement at calculated positions, in the riding-model approximation, with C-H distances of 0.98 Å . All H atoms involved in the hydrogen-bonding associations were located in Fourier syntheses and positional parameters were refined. The isotropic displacement parameters for all H atoms were set equal to 1.25U eq of the carrier atom.