Synthesis and crystal structure of 1,3-bis(4-hydroxyphenyl)-1H-imidazol-3-ium chloride

The title compound, 1,3-bis(4-hydroxyphenyl)-1H-imidazol-3-ium chloride (IOH·Cl) is a new imidazolium salt with a hydroxy functionality.


Chemical context
N-Heterocyclic carbenes (NHCs) represent a versatile class of ligand systems for metal-center activation or stabilization in modern organic synthesis (Arduengo et al., 1999;Benhamou et al., 2011). Chemically, carbenes are nucleophilic 'phosphine mimics' that are high in the order of the Tolman electronic and steric parameter scales, which influences their reactivity. Metal complexes bearing NHC ligands are found in many catalytic reactions (Flanigan et al., 2015;Hopkinson et al., 2014;Huynh, 2018;Marion & Nolan, 2008;Scholl et al., 1999;Velazquez & Verpoort, 2012;Wang et al., 2018), and recently have shown promise as cytotoxic agents (Garrison & Youngs, 2005;Lam et al., 2018;Liu & Gust, 2013;Mora et al., 2019;Riener et al., 2014;Zou et al., 2018). Imidazolium salts, which are simple salts of the free carbene, are commonly used in many systems in preference to their free carbene counterparts due to their high stability. Unlike the free carbenes, which readily react with water or oxygen (Alder et al., 1995), imidazolium salts are indefinitely stable. Use of the imidazolium salt does not require Schlenk techniques and the corresponding 'free' carbene can be prepared in situ via deprotonation with a strong base (e.g. NaO t Bu and NaH) (Arduengo et al., 1991;McGuinness et al., 2001;Hauwert et al., 2008;Voutchkova et al., 2005). Expanding the functional diversity of NHC ligands will broaden their utility. The synthesis of the novel imidazolium salt in this report offers a unique extension of previously reported imidazolium salts through the addition of phenolic groups, herein referred to as IOHÁCl, for functionalization (see Scheme). The hydroxyl functional group presents the possibility of tethering other chemical groups for varied applications, including catalysis, materials, and biomedicine. The synthesis of IOHÁCl ( Fig. 1) does not require Schlenk techniques and the product is isolated as an air-stable solid that can be stored indefinitely without decomposition. The synthesis is part of a study to develop reaction methods for C-N bond formation from high-oxidation-state transition metals.

Figure 3
A plot of the O-HÁ Á ÁCl hydrogen bonds in crystals of IOHÁCl. These interactions, drawn as dashed solid lines, link molecules into head-to-tail zigzag chains that extend parallel to the b axis. The unlabelled molecule is related to its labelled counterpart by the crystallographic 2 1 screw axis (Àx + 1 2 , y À 1 2 , Àz + 1 2 ).

Synthesis and crystallization
The overall reaction for the synthesis of the title compound is depicted in Fig. 1. Step 1, Synthesis of the precursor N,N 0 -bis(4hydroxyphenyl)-1,4-diazabutadiene (1)

Refinement
Crystal data, data collection, and structure refinement details are given in Table 2. All H atoms were found in difference Fourier maps. Hydroxy H-atom coordinates were refined freely, with U iso (H) = 1.5U eq (O). Carbon-bound H atoms were included in calculated positions and refined using a standard riding model, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C). Refinement progress was checked using an R-tensor (Parkin, 2000), PLATON (Spek, 2009), and checkCIF (https://checkcif.iucr.org/).

1,3-Bis(4-hydroxyphenyl)-1H-imidazol-3-ium chloride
Crystal data Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2009) and by an R-tensor (Parkin, 2000 Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x−1/2, −y+1/2, z−1/2; (iii) −x, −y+1, −z; (iv) −x, −y+1, −z+1; (v) −x−1/2, y+1/2, −z+1/2.