Crystal structure of tris(4,7-diphenyl-1,10-phenanthroline-κ2 N,N′)cobalt(III) tris(hexafluorophosphate) monohydrate

The title compound acts as a potential photosensitizer in photodynamic therapy against tumoral cells. It co-crystallizes with one solvent molecule of water. In the crystal, intermolecular π–π stacking interactions, C—H⋯π, C—H⋯F, and O—H⋯F and interactions are present.

The title compound, [Co(C 72 H 48 N 6 )](PF 6 ) 3 ÁH 2 O, crystallizes with one tripositive complex molecule, three hexafluorophosphate anions and one solvent molecule of water in the asymmetric unit. The N 6 coordination set around the central Co III atom defines a distorted octahedral environment. Four fluorine atoms of one hexafluorophosphate anion are disordered over two sets of positions with siteoccupancy factors of 0.697 (5) and 0.303 (5). In the crystal, intermolecularstacking interactions, C-HÁ Á Á, C-HÁ Á ÁF and O-HÁ Á ÁF and interactions are present.

Chemical context
Over the years, metal complexes with polypyridyl ligands have been investigated as photosensitizers in photodynamic therapy (PDT) against cancer. Ru II remains undoubtedly the most studied metal for this purpose due to its tunable photophysical properties (Caspar et al., 2006;Howerton et al., 2012;Heinemann et al., 2017;Monro et al., 2019;McFarland et al., 2020).
Inspired by the exciting results reported with Ru II , we were motivated to develop new metal-based complexes with similar structures. Among the transition metals, cobalt is commonly known for its potential to coordinate with chelate ligands like amino-acid compounds (Otter & Hartshorn, 2004) and polypyridyl derivative ligands. The resulting compounds were used in different fields of research. A series of Co III complexes based on substituted 3-(pyridine-2-yl)-triazine ligands (Wang et al., 2004), or bis(1,10-phenanthroline), bis(2,2 0 -bipyridine) and derivatized imidazole-phenanthroline ligands were developed (Nagababu et al., 2008). These compounds were found to cleave calf thymus DNA (Zhang et al., 2001).
Cobalt complexes are not only used for biological purposes. For example, a series of substituted polypyridine ligands, acting in a bidentate or tridentate manner, coordinating to Co II were investigated as electron-transfer mediators in dyesensitized solar cells (Sapp et al., 2002). Tris(2,2 0 -bipyridyl)based ligands were also used to design redox stable Co II/III complexes for redox flow batteries (Yang et al., 2018).

Structural commentary
The shape of the cobalt complex in the title compound is pseudooctahedral (Fig. 1). The cobalt(III) atom is coordinated by six nitrogen atoms from three dip ligands (dip = 4,7diphenyl-1,10-phenanthroline). The Co-N bond lengths are in the range 1.934 (3)-1.954 (3) Å (Table 1) and correlate well with literature values observed for Co III species. Indeed, the average Co-N bond length is 2.128 Å in Co I cations (three hits in the Cambridge Structural Database (CSD; Groom et al., 2016), 2.115 Å in Co II cations (106 hits), and 1.952 Å in Co III cations (28 hits) in reported Co(phen) 3 n+ (phen = phenanthroline) species. The bond angles between the axially bound ligand atoms are in the range 175.62 (13)

Figure 1
The molecular structure of the tris(4,7-diphenyl-1,10-phenanthroline)cobalt(III) cation of the title compound with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Figure 2
A view of the crystal packing showingstacking interactions forming chains extending perpendicular to the b axis. molecular hydrogen bonds link the anionic hexafluorophosphate species (acceptors) to the tricationic molecules and to the solvent water molecules (donors). These interactions form chains along the a axis (Fig. 4). The most significant interactions for which CÁ Á ÁF < 3.35 Å and C-HÁ Á ÁF > 125 , and OÁ Á ÁF < 3.00 Å and O-HÁ Á ÁF > 125 are complied in Table 2.

Synthesis and crystallization
[Tris(4,7-diphenyl-1,10-phenanthroline)cobalt(III)] tris(hexafluorophosphate) was obtained following the procedure previously described (McLaurin et al., 2009). The experimental protocol used for the synthesis has two steps: Firstly, the synthesis of the [bis(4,7-diphenyl-1,10-phenanthroline)cobalt(III) dichloride] chloride was carried out by the reaction of (4,7-diphenyl-1,10-phenanthroline) with cobalt(II) dichloride in methanol at reflux. The obtained compound was oxidized with chlorine gas made in situ to convert Co II to Co III . Finally, the substitution of the dichloride group for the bidentate ligand (4,7-diphenyl-1,10-phenanthroline) was performed in ethylene glycol at reflux. After cooling to room temperature, ammonium hexafluorophosphate was added to obtain a dark-brown precipitate. The final complex was then isolated by filtration, washed with water and diethyl ether and dried under vacuum. Slow diffusion between methanol and diethyl ether of the acetonitrile solution of the obtained powder gave orange needles of the title compound suitable for X-ray diffraction.

Tris(4,7-diphenyl-1,10-phenanthroline-κ 2 N,N′)cobalt(III) tris(hexafluorophosphate) monohydrate
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.