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Crystal structure of the [(1,3-dimesityl-1H-imidazol-3-ium-2-yl)methanolato]copper(II) chloride dimer: insertion of formaldehyde into a copper–carbene bond

The title complex is assumed to have formed via the insertion of formaldehyde into the copper–carbon bond in an N-heterocyclic carbene complex of copper(I) chloride·The Cu—O bond lengths are shorter than those previously reported in structures with the same central Cu2O2 motif. The complex displays C—H⋯Cl interactions involving the H atoms of the heterocycle backbone and the chloride ligands of a neighbouring molecule.


Chemical context
The chemistry of N-heterocyclic carbene (NHC) ligands is prominent within the landscape of inorganic and organometallic chemistry and now, more than 25 years on from Arduengo's land-mark paper (Arduengo et al., 1991), this prominence looks set to continue. One area that has proven fruitful is the chemistry of NHCs with the group 11 transition metal elements. In particular, the chemistry with copper has resulted in an abundance of complexes that have proven to be effective catalysts for a range of organic transformations, including conjugate addition and carboxylation (Egbert et al., 2013). The preparation of neutral mono-NHC complexes with the general formulae [Cu(NHC)Cl] is routine and straightforward with a variety of synthetic routes to such species available (McLean et al., 2010;Santoro et al., 2013;Gibard et al., 2013;Lake et al., 2012). One of the simplest routes is the reaction of imidazol(in)ium chloride with Cu 2 O under reflux conditions with no requirement for the exclusion of air and water. The species formed are stable when they are isolated in the solid state but solutions show signs of oxidation upon standing for prolonged periods, especially when they are prepared in coordinating solvents such as THF or acetonitrile. The tell-tale green colour, which indicates the formation of copper(II) species, is surely a common observation for chemists who work with copper(I)-NHC species, but surprisingly the literature offers little on the identification of these species and corresponding reaction pathways. This is most likely a consequence of the inherent difficulty in character-izing the paramagnetic copper(II) species formed. Our interest in this area has previously revolved around the modification of [Cu(NHC)Cl] complexes through the replacement of the chloride ligand with the thiocyanate ligand (Dodds & Kennedy, 2014). Herein we report the formation of the unusual (1,3-dimesityl-1H-imidazol-3-ium-2-yl) copper(II) chloride dimer, formed presumably from the insertion of formaldehyde into the Cu-NHC bond, upon prolonged standing of a THF solution containing [Cu(IMes)Cl] [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] and trace amounts of formaldehyde at 255 K. The presence of formaldehyde was a result of the preparation of the imidazolium chloride precursor, which utilises paraformaldehyde. Evidently trace amounts of paraformaldehyde have been present during the reflux of Cu 2 O with imidazolium chloride, with the resulting solution generating the reported complex upon prolonged standing. This is the first structurally characterized example of a species formed through the insertion of a small molecule into a copper-NHC bond. To date, all attempts to prepare the complex rationally have proven unsuccessful.

Structural commentary
The structure of the binuclear molecule (I) consists of a Cu 2 O 2 Cl 4 central core, possessing a Cu 2 O 2 four-membered ring with each copper centre further coordinated by two chloride ligands. Each dimer is sited around a crystallographic centre of symmetry and thus Z 0 = 0.5. The structure of the asymmetric unit, with atom labels, is given in Fig. 1 and the dimeric unit is shown in Fig. 2. The copper centres reside in a distorted square-planar environment, as can be evidenced by the O-Cu-O and Cl-Cu-Cl bond angles [74.10 (11) and 97.58 (5) respectively], which both deviate markedly from 90 . This distortion from ideal square planar geometry is further illustrated by the trans O-Cu-Cl bond angles [162.46 (10) and 162.36 (9) ], which also deviate noticeably from the expected 180 . Similar Cu 2 O 2 Cl 4 central cores have been observed previously by a number of groups (Schä fer et al., 1965;Sager et al., 1967;Watson & Johnson, 1971;Ivashevskaja et al., 2002)   View of the contents of the asymmetric unit of (I). Non-H atoms are drawn as 50% probability ellipsoids and H atoms as small spheres of arbitrary size.

Figure 2
Molecular structure of dimeric (I). The two halves of the dimer are related by Àx, Ày + 1, Àz + 2. H atoms are omitted for clarity. above and below the Cu 2 O 2 ring, as shown in Fig. 2. The distance between the centroids of the Cu 2 O 2 ring and the mesityl ring is 3.390 (2) Å .

Supramolecular features
The complex exhibits intermolecular C-HÁ Á ÁCl interactions, specifically two short interactions between the H atoms on the unsaturated backbone of the heterocycle and the chloride ligands of a neighbouring molecule at position Àx À 1 2 , y + 1 2 , Àz + 3 2 . The intermolecular HÁ Á ÁCl distances measure 2.51 and 2.76 Å . These interactions combine to give a two-dimensional supramolecular motif than propagates parallel to the (101) plane. Fig. 3 illustrates the C-HÁ Á ÁCl intermolecular interactions and numerical details are given in Table 1.

Database survey
Outside the complex reported herein, there are eight structures reported in the Cambridge Structural Database (CSD, Version 5.39, update No. 2, February 2018;Groom et al., 2016) that contain a Cu 2 O 2 Cl 4 core and in which there is no additional coordination to the Cu II atoms. The majority of structures reported contain pyridine N-oxide ligands (Schä fer et al., 1965: refcodes CUCPYO, CUCPYO11 and CUCPYO13;Sager et al., 1967: refcodes QQQBWD, QQQBWG and QQQBWJ;Watson & Johnson, 1971: refcode PHPYOC). The lone example that does not include a pyridine N-oxide ligand instead contains the related quinoline N-oxide ligand (Ivashevskaja et al., 2002;refcode HULZOD). We are aware of no previous examples of ligands formed from NHC by an insertion reaction similar to the one reported herein.

Synthesis and crystallization
[Cu(IMes)Cl] was prepared according to literature procedures outlined by Abernethy and co-workers (McLean et al., 2010). After isolation of an initial crop of [Cu(IMes)Cl], the filtrate was placed in the freezer (255 K) and left standing for $6 months. After this time the pale-orange THF solution had changed to a deep green and a small amount of green crystalline solid had precipitated alongside some green powder. This solid was isolated by filtration, yielding 34 mg of solid. The crystalline material isolated was suitable for single crystal X-ray diffraction. Additionally the isolated product was characterized by elemental analysis and ATR FT-IR. Analysis

Bis[µ-(1,3-dimesityl-1H-imidazol-3-ium-2-yl)methanolato-κ 2 O:O]bis[dichloridocopper(II)]
Crystal data Special details 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.