Crystal structure of the cis and trans polymorphs of bis[μ-2-(1,3-benzothiazol-2-yl)phenolato]-κ3 N,O:O;κ3 O:N,O-bis[fac-tricarbonylrhenium(I)]

The molecular structure of the title compounds may be visualized as two octahedral metal coordinated units, fused through μ-oxide bridges across symmetry centres, leading to edge-sharing dimers.


Chemical context
Organometallic complexes are regarded as interesting and important compounds owing to their versatile photophysical, photochemical and biological properties. In particular, the importance of the use of metal complexes in medicine began with the discovery of the anti-cancer activity of cis-platin (Rosenberg et al., 1965). Since then, attempts to synthesize and characterize novel organometallics with potential pharmaceutical applications remains the main focus of anticancer drug discovery.
While it has been discovered recently that some rheniumindolato complexes exhibit light-induced anti-cancer activity (Kastl et al., 2013), a number of tricarbonyl-rhenium complexes are well known agents in the field of biomedical imaging (Lo et al., 2010(Lo et al., , 2011. Several rhenium(I) tricarbonyl heterocyclic complexes are known to exhibit intense luminescence in the visible region and, owing to their stability to photodecomposition, are promising candidates for solar energy conversion applications (Wallace & Rillema, 1993). In the context of earlier works (Shi et al., 1996;Bradshaw & Westwell, 2004;Potgieter et al., 2012) suggesting benzothiazole derivatives to be promising ligands for rhenium which possess potential usefulness in radiotherapy, the intra-and intermolecular features of the crystal structures of the title compound may well be regarded as relevant. More recently, a host of rhenium-tricarbonyl complexes containing heterocyclic derivatives have been shown to exhibit antimicrobial properties (Kumar et al., 2016). In a recent review, a systematic evaluation of neutral Re I tricarbonyl complexes was undertaken for their suitability as organic light-emitting diodes (Zhao et al., 2016). The molecular structure of II, with displacement ellipsoids drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by (Àx, y, 1 2 À z). H atoms have been omitted for clarity.

Figure 1
The molecular structure of I, with displacement ellipsoids drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by (1 À x, 1 À y, 1 À z). H atoms have been omitted for clarity.

Supramolecular features
The crystal structures of I and II are governed by C-HÁ Á ÁO hydrogen bonds which significantly differ in their strengths and the mode of participation of the carbonyl O atoms. In I, the O3 atom of the apical carbonyl group C16 O3 plays a role in connecting the molecules across inversion centres into a chain along the c axis (Fig. 3, Table 1). In addition, a short O4Á Á ÁO4 iii contact [symmetry code: (iii) -x + 1, -y + 2, -z + 1] involving centrosymmetrcally related carbonyl groups C15 O4 [2.792 (10) Å ] is present, linking the chains along the b axis to form layers parallel to the bc plane.
In II, the oxygen atom of the equatorial carbonyl group C14 O2 links the molecules across the glide planes into a three-dimensional network (Fig. 4, Table 2). Similarly to that observed in I, a C-HÁ Á ÁO hydrogen bond involving the O3 atom of the apical carbonyl group C16 O3 is present, which extends along the b axis through translation. Therefore it may be concluded that in both the trans and cis polymorphs, the mode of participation to the hydrogen-bonding network of the O atom of the apical carbonyl group is through simple translation, while there is a significant 'switching' in the choice of the O atoms of the equatorial carbonyl groups. A common feature between the two structures is that one of the three carbonyl groups, namely C14 O2 in I and C15 O4 in II, forbids its O atom from participating in the intermolecular interactions.

Database survey
A search in the Cambridge structural Database (Version 5.35, November 2014 update; Groom et al., 2016) for -oxido bridging dinuclear complexes of rhenium having an octahedral coordination environment similar to that observed in the title compounds (i.e. involving three carbonyl C atoms, two oxygens and a nitrogen) was made. The search returned 45 crystal structures with three-dimensional coordinates determined, excluding duplicate structure determinations and having an R factor less than 0.075. Out of these 45 crystal structures, 25 crystallize in the monoclinic, nine in the triclinic, eight in the orthorhombic and three in the trigonal systems. In these compounds, the ReÁ Á ÁRe distance ranges from 3.330 to 3.501 Å , the OÁ Á ÁO separation within the Re 2 O 2 core ranges from 2.485 to 2.701 Å , and Re-O bond lengths from 2.065 to 2.215 Å .  Table 1 Hydrogen-bond geometry (Å , ) for I. Symmetry code: (i) x; y; z À 1.

Figure 3
Teflon flask was placed in a steel bomb. The bomb was placed in an oven maintained at 433 K for 48 h and then cooled to 298 K. Pale-yellow crystals were obtained and separated by filtration.