A cuboidal [Cu4(SO4)4] structure supported by β-picoline ligands

A cobalt sulfate complex supported by β-picoline ligands produces a unique cuboidal tetramer.

The solid-state structure of the cobalt--picoline-sulfate complex tetra-3sulfato-tetrakis[bis(3-methylpyridine)cobalt(II)], [Co 4 (SO 4 ) 4 (C 6 H 7 N) 8 ], is reported. The tetrameric cobalt cluster contains a cuboidal core comprised of four cobalt(II) cations and four sulfate anions at alternate cube vertices. The cobalt corners are each capped with two -picoline ligands. The sulfate anions adopt a rare [3.2110] bridging motif, and the cuboidal cluster is unprecedented in coordination chemistry.

Chemical context
For the past few years, our lab has examined the solid-state structures of first-row transition-metal-pyridine-sulfate complexes (Park et al., 2019;Pham et al., 2018;Roy et al., 2018). Despite the first such compound being reported in 1886 (Jørgensen, 1886;Manke, 2021), the structures of only two had been described in the literature when we started exploring this class of compounds. A series of these structures including Fe, Co, Ni, and Zn, showed one-dimensional coordination polymers exhibiting sulfate dianions bridging in -sulfato-2 O:O 0 modes. Interestingly, by modifying growth conditions, cobalt demonstrated two additional crystalline forms with variation in the bridging mode of sulfate ions that was not observed for the other metals. We have also explored the structural chemistry of such complexes with substituted pyridines, including -picoline, which showed similar structural chemistry to that observed with the pyridine ligand . When we looked at the reaction of cobalt sulfate with -picoline, a unique structure was obtained, a tetramer exhibiting an unprecedented cuboidal Cu 4 (SO 4 ) 4 core, described herein. ISSN 2056-9890

Structural commentary
The asymmetric unit of the title compound contains one cobalt cation, one sulfate anion, and two -picoline ligands (Fig. 1). When grown out, the cobalt center demonstrates a pseudooctahedral coordination environment. This consists of twopicoline nitrogen atoms, two oxygen atoms of a chelating sulfate ligand, one oxygen atom of a second sulfate anion, which bridges to another metal, and one terminal oxygen atom of a third sulfate ligand. The grown-out structure forms a tetramer of (-pic) 2 CoSO 4 units, demonstrating a cuboidal core in which four vertices are occupied by cobalt cations, and the other four vertices are occupied by sulfate anions (Fig. 2). The sulfate anions all bridge three Co 2+ cations, demonstrating [3.2110] bridging by Harris notation (Fig. 3). Harris notation is written as [XÁYYYY] where X is the number of metals that a ligand bridges, and the Ys are the number of metals connected to each donor atom in the ligand (Papatriantafyllopoulou et al., 2009). The [3.2110] bridging motif is rare in sulfates and has only been observed in 1D coordination polymers of copper (Li et al., 2008) and lanthanide/iron mixed-metal 3D coordination polymers (He et al., 2017). There are two C-HÁ Á ÁO interactions between the ortho hydrogens of one -picoline ligand and the oxygens of two sulfate ions (Table 1). This results in a plane-to-plane angle between the CoN 3 O plane and the pyridine ring of 16.25 (9) . These interactions are not present in the second unique picoline ligand, giving a larger plane-to-plane angle of 26.95 (9) .

Supramolecular features
The crystal packing for the compound is shown in Fig. 4. The are weak C-HÁ Á ÁO interactions between the trans-hydrogen atom of one picoline ligand and one of the terminal sulfate oxygens of a neighboring cuboid [C3-H3Á Á ÁO2 ii ; symmetry code: (ii) 1 2 + x, 3 2 À y, 1 2 À z, Table 1). This interaction might Figure 2 The [3.2110] coordination mode of sulfate in the title compound.

Figure 1
The asymmetric unit of the title compound showing the atomic labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines.
assist in the interdigitation of the cuboids in the structure. No significantinteractions are observed.

Database survey
The reported structures demonstrating sulfate ions with

Synthesis and crystallization
32 mg of CoSO 4 Á7H 2 O were dissolved in 2.0 mL of 3-methylpyridine (Aldrich) and heated at 343 K for 24 h. Dark-pink crystals suitable for X-ray analysis were obtained.

Tetra-µ 3 -sulfato-tetrakis[bis(3-methylpyridine)cobalt(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.