The varied structures of cobalt(II)–pyridine (py)–sulfate: [Co(SO4)(py)4]n, [Co2(SO4)2(py)6]n, and [Co3(SO4)3(py)11]n

The crystal structures of two new forms of cobalt–pyridine–sulfate complexes are presented. The feature infinite chains of metal–pyridine units connected by bridging sulfate anions, which are distinct from the only previously reported structure of a cobalt–pyridine–sulfate compound.

The solid-state structures of two cobalt-pyridine-sulfate compounds, namely catena-poly [[tetrakis(pyridine-N) [Co 2 (SO 4 ) 2 -(C 5 H 5 N) 6 ] n , (2), are reported. Compound (1) displays a polymeric structure, with infinite chains of Co II cations adopting octahedral N 4 O 2 coordination environments that involve four pyridine ligands and two bridging sulfate ions. Compound (2) is also polymeric with infinite chains of Co II cations. The first Co center has an octahedral N 4 O 2 coordination environment that involves four pyridine ligands and two bridging sulfate ligands. The second Co center has an octahedral N 2 O 4 coordination environment that involves two pyridine ligands and two bridging sulfate ions that chelate the Co atom. The structure of (2) was refined as a two-component inversion twin.

Chemical context
The synthesis of metal-pyrdine-sulfates has been reported since the 19th century, when Jørgensen's chain theory was still the prevailing hypothesis (Reitzenstein, 1894;Howe, 1898). Since that time, the structural understanding of metal complexes has greatly increased, first with the acceptance of Werner's coordination theory (Werner, 1893), with crystal field theory from Bethe in 1929(Bethe, 1929, and the modifications of theory in the ninety years since. Despite the long history of these compounds, their crystallographic study is rather limited. Before we began a crystallographic examination of metal-pyridine-sulfates in 2018, there were only two examples of such complexes without other ligands or components reported in the literature (Cotton & Reid, 1984;Memon et al., 2006).
Since we began studying the structural chemistry of metalpyridine-sulfates, we have observed many different structural motifs in the complexes. The coordination environment of each compound can usually be predicted with crystal field theory, although the exact nature is dependent upon the number of pyridines bound and the binding mode of the sulfate anion. The sulfate anion can have a number of different coordination modes, including -sulfato-2 -O:O, -sulfato-2 -O:O 0 and -sulfato-3 -O:O 0 :O". Herein we report two new structures of cobalt-pyridine-sulfates formed by altering the growth conditions and compare these structures with the previously reported structure of a cobalt-pyridinesulfate and the structures of related complexes. ISSN 2056-9890

Structural commentary
The asymmetric unit of the pink crystals of (1) consists of two pyridine molecules and one half of a sulfate anion coordinated to a cobalt atom sitting on an inversion center (Fig. 1a). When grown out, the cobalt ion shows an octahedral coordination environment (Fig. 1b). The equatorial positions of the octahedron are occupied by four pyridine ligands in a squareplanar arrangement. The CoN 4 unit exhibits planarity enforced by symmetry, with cis N-Co-N angles of 86.45 (6) and 93.55 (6) . To complete the octahedron, the axial positions are occupied by two sulfate ions, with an inversion enforced O-Co-O angle of 180 and cis O-Co-N angles of 88.87 (6) and 91.67 (6) . The pyridine rings are rotated from the CoN 4 plane by dihedral angles of 47.30 (10) and 78.33 (9) . The 78.33 (9) angles are constrained by two C-HÁ Á ÁO interactions between the ortho hydrogen atoms and the two trans sulfates (Table 1).
The asymmetric unit of the purple crystals of (2) consists of two cobalt atoms, six coordinated pyridines and two sulfate anions (Fig. 2a). There are two crystallographically unique cobalt atoms, with Co1 ( Fig. 2b) displaying an octahedral N 4 O 2 coordination environment and Co2 (Fig. 2c) exhibiting an octahedral N 2 O 4 coordination geometry.
Co1 has four pyridine ligands occupying the equatorial positions of an octahedron, with the CoN 4 plane showing a maximum deviation from planarity of 0.018 Å . Two sulfate anions occupy the axial positions to complete the octahedral coordination. The cis N-Co-N angles have values ranging from 87.48 (13) to 93.18 (12) , and the trans O-Co-O angle is 173.43 (12) . The planes of the four pyridine rings are rotated from the equatorial CoN 4 plane by dihedral angles of 58.6 (2), 64.6 (2), 65.6 (2), and 73.1 (2) . Two of the rings show one C-HÁ Á ÁO interaction with an ortho hydrogen atom, one ring shows two C-HÁ Á ÁO interactions with two ortho hydrogen atoms, and the fourth ring shows no C-HÁ Á ÁO interactions (Table 2).

Figure 2
The molecular structure of compound (2), including (a) the asymmetric unit, (b) the coordination environment around Co1, and (c) the coordination environment around Co2. Displacement ellipsoids are drawn at the 50% probability level. H atoms are drawn as spheres of arbitrary radius. C-HÁ Á ÁO interactions ( Co2 is bound by two pyridine ligands and two chelating sulfate anions to give an octahedral coordination environment. The pyridine rings adopt a cis configuration, with an N-Co-N angle of 93.63 (13) . The two sulfate ligands exhibit O-Co-O bite angles of 65.90 (10) and 66.37 (10) . The other cis O-Co-O angles are 86.87 (11), 98.98 (11), and 102.84 (11) , and the six cis N-Co-O angles range from 92.49 (12) to 98.33 (13) . Each pyridine ring is involved in ortho C-HÁ Á ÁO interactions (Table 2).

Supramolecular features
The Co II atoms in compound (1)

Database survey
In a prior publication, we reported the structure of another cobalt-pyridine-sulfate [Co 3 (SO 4 ) 3 (C 5 H 5 N) 11 )] n , which was grown at a lower concentration of cobalt. This structure shows two successive octahedral cobalt atoms with N 4 O 2 coordination, where each atom is coordinated to four pyridines and two bridging sulfates. The third cobalt atom in the chain shows N 3 O 3 coordination where three pyridines are bound and there are two sulfates bound, one of which is chelating to the cobalt . Fig. 3 compares the chain structure of this complex with those of compounds (1) and (2). In compound (1), every cobalt atom possesses an octahedral N 4 O 2 coordination. This complex is isostructural with the structure observed for the iron and nickel pyridine-sulfate complexes . This structural motif is also consistent with that observed for the 4-picoline-sulfate structures of iron, cobalt, nickel and cadmium (Pham et al., 2019). In compound (2), the cobalt atoms alternate between N 4 O 2 coordination and N 2 O 4 coordination. This tetrapyridine/bipyridine alternation is similar to what is observed in the zinc-pyridinesulfate structure, which alternates between octahedral and tetrahedral zinc centers. In the case of cobalt, the bis(pyridine) cobalt center is still octahedral because the two coordinated sulfates both chelate to the cobalt. The end result is an infinite chain of octahedral cobalt atoms, which is true in compound (1) and the previously reported cobalt-pyridine-sulfate complex. The methanesulfato complexes of cobalt (II) have also been reported as octahedral tetrakis(pyridine), [Co(SO 3 CH 3 ) 2 (py) 4 ], and octahedral bis(pyridine), [Co(SO 3 CH 3 ) 2 (py) 2 ], compounds, consistent with the two independent cobalt centers observed in (2) Table 2 Hydrogen-bond geometry (Å , ) for (2).

Figure 4
The packing of (a) compound (1) along the c-axis and (b) compound (2) along the b-axis. Displacement ellipsoids are drawn at the 50% probability level. H atoms are omitted for clarity.

Synthesis and crystallization
For compound (1), 40 mg of cobalt sulfate heptahydrate (J. T. Baker) was dissolved in pyridine (2 mL, Fischer Chemical) and distilled water (100 mL) in a 20 mL vial. The vial was heated to 338 K for 48 h, after which single crystals suitable for X-ray diffraction studies were isolated from the reaction mixture.
For compound (2), 48 mg of cobalt sulfate heptahydrate (J. T. Baker) was dissolved in pyridine (2 mL, Fischer Chemical) and distilled water (30 mL) in a 20 mL vial. The vial was heated to 358 K for 48 h, after which single crystals suitable for X-ray diffraction studies were isolated from the reaction mixture.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All structure solutions were obtained by intrinsic phasing. All non-hydrogen atoms were refined anisotropically (SHELXL) by full-matrix least squares on F 2 . Hydrogen atoms were placed in calculated positions and then refined with a riding model with C-H bond lengths of 0.95 Å and with isotropic displacement parameters set to 1.20 U eq of the parent C atom. The structre of (2) was refined as a two-component inversion twin, BASF = 0.165 (13).

catena-Poly[[tetrakis(pyridine-κN)cobalt(II)]-µ-sulfato-κ 2 O:O′] (1)
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.