Formation of a nonanuclear copper(II) cluster with 3,5-dimethylpyrazolate starting from an NHC complex of copper(I) chloride

The complete nonanuclear cluster in bis[1,3-bis(2,6-dimethylphenyl)imidazolium] di-μ-chlorido-tetrachlorido-octakis(μ-3,5-dimethylpyrazolato)hexa-μ3-hydroxido-nonacopper(II) chloroform disolvate, [HIXy]2[Cu9(μ-pz*)8(μ3-OH)6(μ2-Cl)2Cl4]·2CHCl3 or (C19H21N2)2[Cu9(C5H7N2)8Cl6(OH)6]·2CHCl3, where pz* is the 3,5-dimethylpyrazolyl anion, C5H7N2 −, and HIXy is the 1,3-bis(2,6-dimethylphenyl)imidazolium cation, C19H21N2 +, is generated by a crystallographic centre of symmetry with a square-planar CuII ion bound to four μ3-OH ions lying on the inversion centre.


Chemical context
The study of N-heterocyclic carbene (NHC) complexes of the group 11 metals has proven fruitful for researchers active in this field. Copper (Egbert et al., 2013) and gold (Díez-Gonzá lez et al., 2009) complexes have proven particularly useful in catalysis while silver complexes are routinely used as NHC transfer reagents in addition to finding applications as pharmaceutical species (Garrison & Youngs, 2005). Our interest has been the study of the structural chemistry of copper(I) NHC species and, in particular, the replacement of the chloride ligand in [Cu(NHC)Cl] with a variety of pseudohalides, including thiocyanate and cyanate (Dodds & Kennedy, 2014;Dodds et al., 2019). In addition, we have been keen to highlight novel copper(II) species that can form when exploring copper(I) NHC complexes, such as the curious [(1,3dimesityl-1H-imidazol-3-ium-2-yl)methanolato]copper(II) chloride dimer that formed when formaldehyde was inserted into a copper-carbene bond (Dodds & Kennedy, 2018).
We sought to extend our studies through the reaction of [Cu(NHC)Cl] with the scorpionate ligand hydrotris(3,5-dimethylpyrazolyl)borate (Tp*), hoping to replace the chloride ligand with Tp*. The reaction of [Cu(IXy)Cl] [IXy = 1,3bis(2,6-dimethylphenyl)imidazol-2-ylidene] with an impure batch of NaTp* (predominant contaminant unreacted 3,5-dimethylpyrazole) in chloroform at room temperature resulted in the isolation of a blue solution, which yielded a pale-red powder. Vapour diffusion of diethyl ether into a chloroform solution of this powder generated both colourless and green crystals. The colourless crystals were analysed by X-ray diffraction and were identified as unreacted [Cu(IXy)Cl].
The green crystals were also suitable for X-ray diffraction studies and were identified as the title ionic species [HIXy] 2 [Cu 9 (-pz*) 8 ( 3 -OH) 6 ( 2 -Cl) 2 Cl 4 ]Á2CHCl 3 (I) (where pz* is 3,5-dimethylpyrazolyl, C 5 H 7 N 2 À ), with the dianion being an unusual nonanuclear copper(II) cluster. Subsequent attempts to rationally prepare this species have proven unsuccessful to date, and consequently the mechanism for the formation of this species is unknown. There are a large number of examples in the Cambridge Structural Database (CSD) of complexes containing trinuclear triangular 3 -OH capped copper(II) clusters (Groom et al., 2016). On searching the CSD for structures containing a central Cu 9 O 6 core identical to the structure reported, no exact matches were found. The closest match found was the nonanuclear Cu II complex [Cu 9 (L) 4 ( 3 -OH) 4 (MeOH) 2 ] (L = pentadentate trianionic Schiff-base ligand with N 2 O 3 donor atoms) (Khanra et al., 2009). This complex consists of a central copper(II) atom, which resides in a Jahn-Teller-distorted octahedral geometry, coordinated by six oxygen atoms. The remaining Cu II atoms are in distorted square-based pyramidal coordination environments, with each Cu II ion coordinated by one nitrogen atom and four oxygen atoms. The imidazolium cation, [HIXy] + , has been structurally characterized previously, with two entries in the CSD (Ilyakina et al., 2012;Bortoluzzi et al., 2016).

Structural commentary
The molecular structure of (I) consists of a nonanuclear dianion and two imidazolium cations: two solvent CHCl 3

Figure 1
Contents of the asymmetric unit of (I) with non-H atoms shown as 50% probability ellipsoids and H atoms as spheres of arbitrary size.

Figure 2
Structure of the centrosymmetric nonanuclear anion in (I). The symmetry-equivalent atoms are generated by the symmetry operationx, -y, -z. molecules complete the structure. The dianion is crystallographically centrosymmetric (Z 0 = 0.5) with Cu1 occupying the centre of symmetry. The dianion can thus be best thought of as two [Cu 4 (-pz*) 4 ( 3 -OH) 3 ( 2 -Cl)Cl 2 ] moieties with each connected to a Cu II centre via two 3 -OH groups (Figs. 1 and 2). This central Cu II ion resides in a square-planar geometry, as evidenced by the O-Cu1-O bond angles (Table 1). The eight outer Cu II ions are found in two different coordination environments. Cu5 and Cu5 i [symmetry code: (i) -x, -y, -z] can be described as residing in flattened tetrahedral geometries (sum of bond angles = 666.32 ) and each of these Cu centres bonds to a single N atom of each of two pz* ligands, to one 3 -OH ligand and to a terminal chloride ligand. The N-Cu-N and O-Cu-Cl bond angles have widened to 150.43 (14) and 133.07 (7) , respectively, with the remaining angles compressed to between 92.56 (10) and 98.93 (9) , see Table 1. The Cu5-O3 bond length is 2.029 (2) Å , which is similar to the values of other reported Cu-O bond lengths between Cu II ions and 3 -OH groups (Casarin et al., 2005;Khanra et al., 2009). The two Cu5-N bond lengths are statistically identical at 1.924 (3) and 1.927 (3) Å and finally the Cu5-Cl bond length is 2.2466 (19) Å . The remaining six Cu II centres (Cu2, Cu3 and Cu4 and their symmetry clones) reside in distorted square-based pyramidal geometries. Each of these metal ions is coordinated to a single N atom from each of two pz* ligands, to two 3 -OH ligands and to a chloride ligand (either terminal or bridging). The cis-N 2 O 2 basal planes are comprised of the 3 -hydroxo oxygen atoms and pz* nitrogen atoms with the chloride ligands occupying the apical positions. Details of coordination bond lengths and angles are given in Table 1, with some pertinent features highlighted below. The Cu-N bond length range is 1.943 (3) to 1.979 (3) Å while the Cu-O bond length range is 1.986 (2) to 2.115 (2) Å with both sets of values comparing well to previously reported examples of multinuclear copper(II) complexes containing both 3 -OH groups and pyrazolate ligands (Casarin et al., 2005). The Cu-Cl bond lengths vary as expected, depending on whether the chloride is bonding via bridging or terminal modes. The Cu4-Cl2 bond length for the terminal chloride anion is 2.5191 (9) Å while the bridging chloride ions have longer Cu-Cl bond lengths of 2.5755 (8) and 2.6282 (9) Å . Note that all these Cu-Cl and Cu-N distances are longer than those found for four-coordinate Cu5, but that the Cu5-O3 distance fits within the range given above. These interactions combine to give a nonanuclear dianion whose core can be envisioned as a linear Cu ( (7) Symmetry code: (i) Àx; Ày; Àz.

Figure 3
Simplified diagram of the coordination bonds within the anion in (I). The outer ring is a 24-membered [CuN 2 ] 8 unit that contains a Cu-and O-based core.
Of the 3 -OH groups, atom O3 is situated 0.364 (2) Å out of the plane defined by the three copper atoms (Cu3 i /Cu4/Cu5) whilst O1 and O2 adopt more pyramidal geometries and are situated out of the planes defined by the copper atoms (Cu1 i / Cu2 i /Cu3 i and Cu1/Cu2/Cu4) by 0.651 (2) and 0.758 (2) Å , respectively.
The structural parameters of the imidazolium cation, [HIXy] + , in (I) compare well to the previously reported structures (Ilyakina et al., 2012;Bortoluzzi et al., 2016). The C1-N bond lengths of the heterocycle are slightly shorter at 1.322 (5) and 1.334 (5) Å compared to the 1.333-1.357 Å range in the previously reported structures. The N1-C1-N2 bond angle of the heterocycle is 109.5 (3) compared to 108.6 for both of the previously reported structures. Table 2 shows the short hydrogen-bonding contacts of the structure. All three classical hydrogen bonds are intramolecular O-HÁ Á ÁCl contacts and the intermolecular contacts are thus non-classical interactions involving C atoms. The main interactions observed between the anion and the cation involve the labile C1-H1 group of the imidazolium cation. This interacts with two Cl ligands of the anion through the C1-H1Á Á ÁCl1 hydrogen bond and through a geometry C-H to Cl3 i interaction [CÁ Á ÁCl = 3.093 (2) Å ]. The other interactions of Table 2 are all internal to the [HIXy] 2 [Cu 9 (pz*) 8 ( 3 -OH) 6 ( 2 -Cl) 2 Cl 4 ]Á2CHCl 3 unit, except for the C2-H2Á Á ÁCl3 ii contact [symmetry code: (ii) -x + 1, -y, -z]. This short contact exists between an H atom of the unsaturated backbone of the imidazolium cation and a chloride ligand of a neighbouring anion and connects anions and cations by translation along the a-axis direction.

Database survey
Outside the complex reported herein there are eleven structures reported in the CSD (Version 5.41, update no. 1, March 2020; Groom et al., 2016) that contain a Cu 9 O 6 core as observed in the complex reported. Of these, only one structure is truly a nonanuclear copper(II) cluster (Khanra et al., 2009: refcode DUGLOH). There are two reports in the CSD of structures that contain the imidazolium cation

Synthesis and crystallization
[Cu(IXy)Cl] (234 mg, 0.625 mmol) was dissolved in chloroform (5 ml) and NaTp* (200 mg, 0.625 mmol) dissolved in chloroform (5 ml) was added. (Retrospectively it was found that the NaTp* used was not pure, containing significant quantities of unreacted 3,5-dimethylpyrazole.) The initially paleyellow solution turned pale green and the solution was left stirring for 24 h. After this time, the solution had turned blue and it appeared as though a small amount of white precipitate had formed. The mixture was filtered through Celite and the solvent was removed in vacuo. During the removal of the solvent, the colour changed from blue to deep red-brown, resulting in the isolation of a deep red-brown oil. Diethyl ether was added, which resulted in the precipitation of a pale- Central Cu 9 O 6 core in (I) viewed (a) from above and (b) from the side. Table 2 Hydrogen-bond geometry (Å , ). (4) 3.098 (2)  red solid, which was isolated by filtration and dried, yielding 180 mg of solid. In an effort to grow crystals suitable for single-crystal X-ray diffraction studies, 19 mg of solid was dissolved in chloroform (0.5 ml) and vapour diffused with diethyl ether. The majority of the crystals that formed were colourless and analysed as unreacted [Cu(IXy)Cl]. The green crystals that were isolated analysed as the reported [HIXy] 2 [Cu 9 (-pz*) 8 ( 3 -OH) 6 ( 2 -Cl) 2 Cl 4 ]Á2CHCl 3 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Data were measured by the EPSRC National Crystallography Service (Coles & Gale, 2012). All H atoms bound to C were geometrically placed and modelled in riding mode with C-H distances of 0.95, 0.98 and 1.00 Å for sp 2 CH, methyl, and sp 3 CH groups, respectively. For methyl groups, the constraint U iso (H) = 1.5U eq (C) was applied and elsewhere U iso (H) = 1.2U eq (C). The H atoms of the OH groups were positioned as found in a difference map and refined isotropically with the O-H distance restrained to 0.88 (1) Å . Displacement ellipsoids show a relatively high amount of motion in the Cl atoms of the solvent CHCl 3 molecule, and the highest residual electron density lies close to this feature. Disordered models were constructed, but were not as satisfactory as the ordered model presented.   (Rigaku, 2012); cell refinement: CrystalClear-SM Expert (Rigaku, 2012); data reduction: CrystalClear-SM Expert (Rigaku, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2020), OLEX2 Dolomanov et al., 2003); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

Bis[1,3-bis(2,6-dimethylphenyl)imidazolium] di-µ-chlorido-tetrachloridooctakis(µ-3,5-dimethylpyrazolato)hexaµ 3 -hydroxido-nonacopper(II) chloroform disolvate
Crystal data (C 19  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.