Halogen-bonded network of trinuclear copper(II) 4-iodopyrazolate complexes formed by mutual breakdown of chloroform and nanojars

Acidity created by the decomposition of chloroform solvent leads to breakdown of (Bu4N)2[{CuII(μ-OH)(μ-4-I-pz)}nCO3] (n = 27–31) nanojars in a chloroform/1,4-dioxane solution to the trinuclear complex (Bu4N)2[Cu3(μ 3-Cl)2(μ-4-I-pz)3Cl3]·0.5dioxane, which forms extended sheets based on C—I⋯Cl—Cu halogen bonding and C—H⋯Cl—Cu hydrogen bonding.


Chemical context
Nanojars, supramolecular coordination complexes of the formula [{Cu(-OH)(-pz)} n anion] (pz = pyrazolate anion; n = 27-36), have emerged as a new class of anion encapsulation agents of unparalleled efficiency, which allow the extraction of anions with large hydration energies, such as phosphate, carbonate and sulfate, from water into organic solvents (Mezei, Baran et al., 2004;Fernando et al., 2012;Mezei, 2015;Ahmed, Szymczyna et al., 2016;Ahmed, Calco & Mezei, 2016;Ahmed, Hartman & Mezei, 2016). Trinuclear copper pyrazolate complexes have been identified as key intermediates in the self-assembly mechanism of nanojars from copper(II) nitrate, pyrazole and NaOH (1:1:2 molar ratio) in the presence of carbonate . The trinuclear intermediate can be isolated if the amount of available base is reduced (copper:pyrazole:base molar ratio 3:3:4), and can subsequently be converted to nanojars by adding an additional amount of base to reach a 1:1:2 molar ratio. Moreover, nanojars can be broken down to the trinuclear complex by acids, which easily protonate the OH groups of the nanojar. As a consequence, nanojars and the trinuclear pyrazolate complex are in a pH-dependent equilibrium. The sensitivity of nanojars to even very weak acids is further demonstrated by the fact that a weak base, such as Et 3 N, is unable to convert the trinuclear complex to nanojars in solution (e.g., DMF, THF), despite its ability to provide the hydroxide ions needed by the nanojar, in the presence of moisture (Et 3 N + H 2 O Ð Et 3 NH + + HO À ). This is due to the acidity of the conjugate acid, the triethylammonium cation (pK a = 10.75 in H 2 O), which would form in the process (Mezei, 2016). Nevertheless, nanojars can be obtained using Et 3 N if the solution is diluted with excess water, which leads to the precipitation of hydrophobic nanojars (Fernando et al., 2012).
New evidence supporting the vulnerability of nanojars to acids emerges from an unexpected source. An attempt to grow single crystals from a solution of (Bu 4 N) 2 [{Cu(-OH)(-4-Ipz)} n CO 3 ] (n = 27-31) (Ahmed, Calco et al., 2016) in chloroform/1,4-dioxane provided, instead of the expected nanojars, crystals of (Bu 4 N) 2 [Cu 3 ( 3 -Cl) 2 (-4-I-pz) 3 Cl 3 ]Á0.5dioxane , accompanied by a color change of the solution from blue to green. The chloride ions originating from CHCl 3 is not surprising, as chloroform has long been known to slowly decompose in the presence of air and moisture producing HCl and phosgene (CHCl 3 + 1 2 O 2 ! COCl 2 + HCl) (Baskerville & Hamor, 1912). The latter can hydrolyze to provide further amounts of HCl, and CO 2 (COCl 2 + H 2 O ! 2HCl + CO 2 ). What is surprising though is the large amount of chloride formed in a relatively short period of time (ca 48 chloride ions per nanojar). Chloroform preserved with ethanol (0.5-1%), such as the one used here for crystal growing, is much more stable than the pure form and it does not decompose at a significant rate. This points to a decomposition catalyzed by the dissolved nanojars, possibly aided by light. A search of the literature shows that various classes of compounds have been found to catalyze the photodecomposition of chloroform (Semeluk & Unger, 1963  . A balanced equation of the reaction between nanojars of different sizes and HCl, producing the title trinuclear complex, is given below: 3[{Cu(-OH)(-4-Ipz) n CO 3 ] 2-+ 5nHCl ! n[Cu 3 ( 3 -Cl) 2 -(-4-Ipz) 3 Cl 3 ] 2-+ (2n À 6) H 3 O + + (n + 9) H 2 O + 3CO 2 (n = 27-31).

Supramolecular features
The intermolecular distances between iodine substituents of the pyrazole units and the terminal chlorine atoms of adjacent complexes are less than the sum of the van der Waals radii (Bondi, 1964) of iodine and chlorine atoms (3.73 Å ). Thus, a halogen-bonded (Cavallo et al., 2016;Gilday et al., 2015) sheet based on C-IÁ Á ÁCl-Cu interactions (Fig. 2) is generated parallel to the (110) plane (and c axis); IÁ Á ÁCl distances and C-IÁ Á ÁCl angles are shown in Table 1. Bifurcated halogen bonds are noted between Cl1A/Cl1B and I1 0 and I3 0 . The formation of the extended halogen-bonded network might account for the near-planarity of the title complex, as opposed to related complexes with unsubstituted or differently substituted 4-Rpyrazoles (R = H, Cl, Br, Me; Angaridis et al., 2002;, which do not form intermolecular halogen bonds and are severely distorted from planarity. Additionally, the dioxane solvent molecule, which is located around an inversion center, forms C-HÁ Á ÁCl hydrogen bonds with terminal chlorido ligands of the trinuclear complex [C43Á Á ÁCl2: 3.751 (10); H43BÁ Á ÁCl2: 2.83; C43-H43B: 0.97 Å ; C43-H43bÁ Á ÁCl2: 160 (5) ], creating further bridges within the two-dimensional framework.

Database survey
A search of the Cambridge Structural Database (Groom et al., 2016) reveals only seven metal complexes that contain a 4-iodopyrazole moiety, either in its neutral, monodentate form (Guzei & Winter, 1997;Govor et al., 2012;Song et al., 2013;da Silva et al., 2015), or in its deprotonated, bidentate form (Heeg et al., 2010;Song et al., 2013). Of these, only one is a Cu II complex (Song et al., 2013). Hence, the crystal structure presented here offers the first solid-state structural description  Two-dimensional sheet [along (110)] formed by intermolecular iodine-chlorine halogen bonding (only one dioxane solvent molecule and no counterions are shown). Halogen bonds and C-HÁ Á ÁCl hydrogen bonds are indicated by dotted lines. Table 1 Halogen-bond geometry (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-H hydrogen atoms were placed in idealized positions and refined using a riding model. One of the three terminal Cl-atoms is disordered over two positions (60/40). Two terminal CH 2 CH 3 groups of one tetrabutylammonium counter-ion and another CH 2 CH 3 group of the other counter-ion are disordered over two positions (60/40); C-H bond-length restraints were used for the disordered C atoms. Residual electron density of 3.52 eÅ À3 is found at 0.83 Å from heavy atom I3, due to Fourier truncation ripples.   Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/6 (Sheldrick, 2015); molecular graphics: CrystalMaker (Palmer, 2014); software used to prepare material for publication: CrystalMaker (Palmer, 2014).

Bis(tetrabutylammonium) di-µ 3 -chlorido-tris(µ-4-iodopyrazolato-κ 2 N:N′)tris[chloridocuprate(II)] 1,4-dioxane hemisolvate
Crystal data (C 16   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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq Occ.