Synthesis and crystal structure of tricarbonylchlorido{1-[(pyridin-2-ylmethylidene)amino]adamantane}rhenium(I)

The compound [ReCl(pyAm)(CO)3], where pyAm is 1-[(pyridin-2-ylmethylidene)amino]adamantane, comprises an ReI atom with an octahedral C3ClN2 coordination set.


Chemical context
The diverse photophysical and photochemical properties of tricarbonylrhenium(I) complexes make them invaluable for a range of applications, such as light-emitting devices, nonlinear optical materials, radiopharmaceuticals, reagents for COreduction chemistry and photopolymerization (Kumar et al., 2010). As a consequence, among organometallic complexes, tricarbonylrhenium(I) compounds have received considerable attention. Facile synthesis and previously available knowledge of their photophysics (Stufkens & Vlcek, 1998) encouraged us to design new photo-active carbonylrhenium complexes as CO-donating molecules. Photo-active metal-carbonyl complexes (photoCORMs) have been utilized as more controllable CO donors to exploit various salutary effects in mammalian pathophysiology when administered in moderate concentrations (Gonzalez & Mascharak, 2014;Romao et al., 2012;Schatzschneider, 2015). We (Carrington et al., 2016) and others (Zobi et al., 2012) have shown applications of rhenium carbonyl-based photoCORMs towards the eradication of aggressive malignant cells, as well as oxidatively damaged cell restoration through light-induced CO delivery. Along the line of developing metal-carbonyl complex-based photoCORMs (Chakraborty et al., 2014), we report herein the synthesis and structural characterization of a carbonylrhenium complex, [ReCl(pyAm)(CO) 3 ], where pyAm is 1-[(pyridin-2-ylmethylidene)amino]adamantane. In this design of pyAm ligand, the adamantyl moiety has been included beacuse of its wellknown pharmacokinetic properties (Wanka et al., 2013).

Structural commentary
The molecular structure of the title complex is shown in Fig. 1. The coordination geometry of Re I in the complex is distorted octahedral ( Table 1). The pyAm ligand binds the metal in a bidenate fashion, while the three CO ligands reside in a facial disposition. The distortion from ideal values is reflected by the N1-Re1-N2 bite angle of 75.41 (9) . The sixth site is occupied by a chloride ligand. The equatorial plane composed of atoms N1, N2, C2 and C3 is satisfactorily planar, with a mean deviation of 0.034 Å . In this complex, the chelate ring composed of atoms Re1, N1, C8, C9 and N2 is almost planar, with a mean deviation of 0.007 Å . The Re-Cl bond is considerably longer [1.963 (4) Å ] compared to the other two Re-C bonds [1.918 (4) and 1.920 (3) Å ], which can be attributed to the trans-labilizing effect arising from the chloride ligand across this bond.

Supramolecular features
The crystal packing of the title complex reveals few nonclassical hydrogen-bonding interactions of the C-HÁ Á ÁCl type (Table 2 and Fig. 2), leading to a three-dimensional network structure. The arrangement of molecules along the c axis is shown in Fig. 3.

Database survey
A search of the Cambridge Structural Database (Groom et al., 2016) revealed only a few structurally similar complexes, with a general formula of [ReCl(pyR)(CO) 3 ], where R represents substituted or unsubstituted aromatic amines. The complex [ReCl(2-PP)(CO) 3 ] [where 2-PP = N-(pyridin-2-ylmethylidene)aniline] has space-group symmetry P2 1 /n (Dominey et al., 1991) and exhibits comparable metric parameters as the title complex. However, careful scrunity reveals that in this case the trans-influence of the chloride ligand is not reflected as in the title complex. Later, the same complex was found to adopt also triclinic symmetry in the P1 space group (Hasheminasab et al., 2014). Another complex, [ReCl(L 1 )(CO) 3 ] {where L 1 = 4-[(pyridin-2-ylmethylidene)amino]phenol} has P2 1 /n space-group symmetry, with unit-cell dimensions close to those of [ReCl(2-PP)(CO) 3 ] (Liu & Heinze, 2010). In another report, two rhenium complexes of the general formula The molecular structure of the title complex. Displacement ellipsoids correspond to 50% probability levels. Table 2 Hydrogen-bond geometry (Å , ).
171.19 (12) [ReCl(pyca-C 6 H 4 OH)(CO) 3 ] (where pyca = pyridine-2carbaldehydeimine) were structurally characterized (Chanawanno et al., 2013). In this case, the two complexes can be differentiated on the basis of the position of the -OH group on the arene ring. The complex with the -OH group at the meta position was described in the P2 1 /c space group, while that with the -OH group in the ortho position of the arene ring was described in the setting P2 1 /n. In a relatively recent report, another rhenium complex, namely [ReCl(pyca-2,6-iPr 2 C 6 H 3 )(CO) 3 ], was synthesized (C2/c; Kianfar et al., 2015). However, no such rhenium complex incorporating an aliphatic amine in the Schiff base ligand has been structurally characterized so far.

Synthesis and crystallization
A slurry of 50 mg (0.138 mmol) of [ReCl(CO) 5 ] and 33 mg of pyAm (0.138 mmol) were added in a mixture of 15 ml of methanol and 5 ml of chloroform and allowed to reflux for 24 h. After this time, the reaction mixture was allowed to cool to room temperature, whereupon an orange precipitate was observed. The orange solid was collected by filtration and dried under vacuum to obtain 44.2 mg (55%) of the title complex. Single crystals were obtained by layering hexanes over a dichloromethane solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were included in calculated positions on the C atoms to which they are bonded, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). One reflection (i.e. 101) was removed from the refinement because it was partly obscured by the beam stop. Packing diagram of the title complex along the c axis.  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and CrystalMaker (Palmer, 2014)′; software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
[ReCl(C 16 H 20 N 2 )(CO) 3 ] M r = 546.02 Monoclinic, P2 1 /n a = 6.9550 (6)  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.