The crystal structures of iron and cobalt pyridine (py)–sulfates, [Fe(SO4)(py)4]n and [Co3(SO4)3(py)11]n

The crystal structures of two first-row transition metal (Fe and Co) pyridine–sulfate complexes are presented. The compounds demonstrate infinite chains of metal pyridine units connected by bridging sulfate anions.

The solid-state structures of two metal-pyridine-sulfate compounds, namely catena-poly [[tetrakis(pyridine-N) (SO 4 ) 3 (C 5 H 5 N) 11 ] n , (2), are reported. The iron compound (1) displays a polymeric structure, with infinite chains of Fe II atoms adopting octahedral N 4 O 2 coordination environments that involve four pyridine ligands and two bridging sulfate ligands. The cobalt compound (2) displays a polymeric structure, with infinite chains of Co II atoms. Two of the three Co centers have an octahedral N 4 O 2 coordination environment that involves four pyridine ligands and two bridging sulfate ligands. The third Co center has an octahedral N 3 O 3 coordination environment that involves three pyridine ligands, and two bridging sulfate ligands with one sulfate chelating the cobalt atom.

Chemical context
The first reports of a pyridine-sulfato-metal complex were in the late 19th century (Reitzenstein, 1894;Reitzenstein, 1898), and this work played a significant role in the Werner-Jørgensen controversy (Howe, 1898). While most early work in coordination chemistry was based upon ammonia complexes, the demonstration of the existence of similar complexes with other organic bases such as pyridine was an important contribution to the field. Despite the long history of these complexes, and their contributing role in the development of coordination chemistry, their crystallographic characterization is limited.

Figure 2
The molecular structure of compound (2), including (a) the asymmetric unit, (b) the coordination environment of Co1, (c) the coordination environment of Co2 and (d) the coordination environment of Co3. Displacement ellipsoids are drawn at the 50% probability level. H atoms are drawn as spheres of arbitrary radius. C-HÁ Á ÁO interactions (Table 2) are shown as dashed lines. [Symmetry codes: (i) 1 2 À x, 1 À y, À 1 2 + z]

Database survey
Though complexes of this form have been known for more than a century, their crystallographic characterization has been limited. Prior to our report earlier this year, there were only two structures in the literature of metal-pyridine-sulfates with no other ligands or components (Cotton & Reid, 1984;Memon et al., 2006). There are a number of closely related structures that have been reported, particularly transition-metal-aqua-pyridine-sulfate complexes. Six of these are found in the literature (Ali et al., 2005;Castiñ eiras & García-Santos, 2008;Cotton et al., 1994;Kožíšek et al., 1989;Shi et al., 2009;Zhang, 2004). The metrical parameters in the reported structures are consistent with those seen in the metal-pyridine-triflates (Haynes et al., 1986).
In a report earlier this year, we presented the structures of the metal-pyridine-sulfates of nickel, copper and zinc. It was of note that these three structures exhibited different coordination geometries, consistent with the crystal field stabilization energies (CFSE) associated with their d-electron count: d 8 nickel is octahedral, d 9 copper is square pyramidal, and d 10 zinc is both tetrahedral and octahedral. The structures reported here both exhibit octahedral coordination environments. For d 6 iron, the observed octahedral environment gives a CFSE of 4 Dq, while the preferred geometry might be square pyramidal with a CFSE of 4.67 Dq. Similarly for d 7 cobalt, the observed octahedral environment gives a CFSE of 8 Dq, while the preferred geometry might once again be square pyramidal with a CFSE of 9.14 Dq. The difference between octahedral and square pyramidal in these two compounds is small compared to the 3.14 Dq difference for d 9 copper, where a square-pyramidal geometry is observed. With such small electronic preferences, the impact of weaker interactions (and C-HÁ Á ÁO) and steric effects could play significant roles in determining the observed coordination environments.

Synthesis and crystallization
Approximately 25 mg of each metal sulfate [iron sulfate heptahydrate (J. T. Baker), cobalt sulfate heptahydrate (J. T. Baker)] were dissolved in pyridine (3 mL, Fisher Chemical) in a 20 mL vial under an atmosphere of dinitrogen. In the cobalt case, 0.1 mL of water was also added. The vials were heated to 353 K for 24-48 h, after which single crystals suitable for X-ray diffraction studies were isolated.

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. (2). For both structures, program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al. 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al. 2009) and

catena-Poly[[tetrakis(pyridine-κN)iron(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.

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.