Synthesis and structural characterization of hexa-μ2-chlorido-μ4-oxido-tetrakis{[4-(phenylethynyl)pyridine-κN]copper(II)} dichloromethane monosolvate

The title compound, [Cu4Cl6O(C13H9N)4], was obtained by the reaction of CuCl with 4-phenylethynylpyridine in dichloromethane. The complex contains a tetrahedron of four CuII cations coordinated to a central μ4-O atom, with the six edges of the Cu4 tetrahedron bridged by Cl atoms. The Cu—O distances average 1.905 Å and Cu—Cl 2.418 Å.


Chemical context
Polynuclear Cu II complexes with various bridges between the metal centres have attracted much attention in the past decade, from both an experimental and a theoretical point of view, and a significant amount of research has been devoted to analysing their structural and magnetic properties (Bertrand & Kelley, 1966). Copper complexes that form clusters of the type Cu 4 OX 6 L 4 (X = halogen, L = ligand or X) are known (Bertrand et al., 1968;Dey et al., 2002;Mukherjee et al., 2007;Thakurta et al., 2009;Wegner et al., 2001). In our studies on dimeric, tetrameric, and polymeric Cu complexes supported by ethynylpyridine-based ligands, we have obtained Cu 4 OX 6 L 4 complexes where a central oxide ion is tetrahedrally coordinated to four copper ions, which are in turn bridged in pairs by six chloride ions, and the L groups complete the trigonal-bipyramidal coordination of the copper centres. The structural complexity of these [Cu 4 OX 6 L 4 ] systems, as well as their challenging magnetic properties, has promoted sustained structural work on the subject (Atria et al., 1999), where the magnetic properties exhibited by the compound were successfully modelled in a rather simple and elegant fashion. We report herein the synthesis of the title complex 4 -oxo-hexa-2 -chlorido-tetrakis[(4-phenylethynylpyridine)copper(II)] dichloromethane solvate (1) from 4-(2phenylethynyl)pyridine and CuCl in dichloromethane. It is well known that CuX (X = Cl, Br, I) salts react with ethynylpyridine-based ligands in dichloromethane to form ISSN 2056-9890 coordination-driven self-assembled tetrahedral Cu I complexes; however, oxidation to form Cu II species is also possible. We have a long-standing interest in the design and development of functional ethynyl-based carbocyclic and heterocyclic ligands and their transition metal complexes Haque et al., 2019a). In the past, we have reported several dimeric, tetrameric, and polymeric Cu I complexes supported by ethynylpyridine-based ligands. (Al-Balushi et al., 2016a,b;Ilmi et al., 2018). In the quest for new dimeric halide-bridged Cu complexes, we obtained an oxidized Cu II product, compound 1. Our experience suggests that the chloride-containing Cu I complexes are somewhat less stable and oxidize easily (in situ or during crystallization), leading to the formation of multiple products. The crystal structure, as well as Hirshfeld surface analysis, indicate that the most important contributions to the packing arrangement within are from HÁ Á ÁH and CÁ Á ÁH/HÁ Á ÁC interactions.

Supramolecular features
The crystal structure of the title compound is consolidated by several inter-and intramolecular interactions, the presence of which are supported by a Hirshfeld surface analysis. In the crystal, the presence of several C-HÁ Á ÁCl (Fig. 2 The molecular structure of the title compound with atom labelling and displacement ellipsoids drawn at the 40% probability level.

Figure 2
Crystal packing of the title compound showing the C-HÁ Á ÁCl interactions.

Hirshfeld surface analysis
In order to better visualize and analyse the role of weak intermolecular contacts in the crystal, a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) generated using CrystalExplorer17.5 (Turner et al., 2017). The white surface indicates contacts with distances equal to the sum of van der Waals radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distant contact) than the sum of the van der Waals radii, respectively (Venkatesan et al., 2016). The dark-red spots on the d norm surface arise as a result of short interatomic contacts (Fig. 4), while the other weaker intermolecular interactions appear as light-red spots. The red points, which represent close contacts and negative d norm values on the surface, correspond to the C-HÁ Á ÁCl interactions. The shape-index of the Hirshfeld surface is a tool for visualizing thestacking by the presence of adjacent red and blue triangles; if these triangles do not appear, then there are nointeractions. The plot of the Hirshfeld surface mapped over shape-index shown in Hirshfeld surfaces of the title molecule plotted over (a) d norm (b) shapeindex showing thestacking and (c) curvedness.

Figure 3
Crystal packing of the title compound showing the C41-H41Á Á ÁCg1 and C43-H43Á Á ÁCg2 interactions viewed along the b-axis direction.

Synthesis and crystallization
The ligand L was prepared by adapting a previously reported procedure (Haque et al., 2019b). 1-Ethynylbenzene (0.33 g, 3.23 mmol) and 4-iodopyridine (0.66 g, 3.23 mmol) were dissolved in a i Pr 2 NH/THF mixture (1:2, 60 mL) under an argon atmosphere. Catalytic amounts of Pd(OAc) 2 (3 mg), CuI (3 mg), and PPh 3 (10 mg) were added to the mixture and it was refluxed overnight. The solvent was then removed under vacuum and the residue was dissolved in dichloromethane (100 mL), washed with water and extracted with dichloromethane. The combined organic layers were washed with water and brine and then dried over anhydrous magnesium sulfate. The solution was concentrated under vacuum, and the crude product was chromatographed on a silica column using a mixture of hexane:dichloromethane (1:1, v/v). The ligand was obtained as an orange/pale-brown powder (0.51g, 88% yield). IR ( max ) cm À1 : 2185 (-C C The title complex 1 was obtained by the reaction of the ethynylpyridine-based ligand with Cu I Cl due to partial oxidation under the reaction conditions employed. The methodology for the synthesis of the complex is as follows: L (0.050 g, 0.24 mmol) and CuCl (0.024 g, 0.24 mmol) were dissolved in dichloromethane (50 mL). The reaction mixture was stirred at room temperature under a partial argon atmosphere for 24 h, after which period the solvent was removed under reduced pressure. The crude product was dissolved in dichloromethane and filtered through a pad of celite using dichloromethane giving the final product as an orange powder (0.057 g, 79% yield). Diffusion of hexane to a dichloromethane solution gave the final product as orange crystals.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq