Crystal structure of fac-tricarbonyl(cyclohexyl isocyanide-κC)(quinoline-2-carboxylato-κ2 N,O)rhenium(I)

The ReI atom in the molecule of the title compound has a distorted C4NO coordination sphere defined by three carbonyl ligands, one chelating quinaldate anion and one isocyanide ligand. As a result of the trans effect of the isocyanide derivative, one Re—CO bond is elongated.

In the title compound, [Re(C 10 H 6 NO 2 )(C 7 H 11 N)(CO) 3 ], the Re I atom is coordinated by three carbonyl ligands in a facial arrangement and by the N, O and C atoms from a chelating quinaldate anion and a monodentate isocyanide ligand, respectively. The resultant C 4 NO coordination sphere is distorted octahedral. A lengthening of the axial Re-CO bond trans to the isocyanide ligand is indicative of the trans effect. Individual complexes are stacked into rods parallel to [001] through displacedinteractions. Weak C-HÁ Á ÁO hydrogenbonding interactions between the rods lead to the formation of layers parallel to (010). These layers are stacked along [010] by C-HÁ Á ÁH-C van der Waals contacts.

Chemical context
Tricarbonylrhenium(I) compounds are being explored as luminescent probes for cell imaging, photosensitizers in photocatalysis (Lyczko et al., 2015;Bertrand et al., 2014), and as potential radiopharmaceuticals based on the already extensive use of radioactive 186/188 Re compounds in nuclear medicine for pain palliation and radiosynovectomy (Schneider et al., 2005;Bodei et al., 2008). Recent studies have also revealed the potential of cold tricarbonylrhenium(I) complexes as anticancer agents (Leodinova & Gasser, 2014).
As part of our ongoing research in the field of Re/Tc coordination compounds, the crystal structure of a new '2 + 1' tricarbonyl rhenium complex, fac-[M(CO) 3 (L)(QA-NO)], where M is Re,Tc, L is the monodentate ligand cyclohexylisocyanide, and QA-NO is deprotonated quinaldic acid, is presented. As a result of of the versatility of the '2 + 1' system, fac-[M(CO) 3 (L)(QA-NO)] complexes can be used as model compounds in the development of targeted radiopharmaceuticals or anticancer agents by suitable replacement of either the bidentate or monodentate ligand. For example, the monodentate ligand may be the isocyanide derivative of a pharmacophore with affinity for a certain receptor. Alternatively, the bidentate ligand may be a more extensive conjugated system to act as a DNA intercalator. Both quinaldate-and isocyanide-based ligands have been used as possible DNA intercalators (Li et al., 2009;Agorastos et al., 2007).

Structural commentary
The molecular structure of the title compound, [Re(C 10 H 6 NO 2 )(C 7 H 11 N)(CO) 3 ], is shown in Fig. 1. The Re I atom is six-coordinated by four C, one N and one O atoms in a distorted octahedral coordination sphere. The carbonyl C atoms are in a facial arrangement, with distances in the range 1.903 (8)-1.960 (8) Å , resulting in a cis arrangement of the biand monodentate ligands. The longest distance involving the carbonyl ligands [1.960 (8) Å ; Re-C11] corresponds to the ligand trans to the isocyanide cyclohexyl ligand, defining the axial direction of the octahedral complex. The Re I atom almost lies in the equatorial plane (deviation, 0.006 Å ) defined by the C12, C13, O1 and N1 atoms. The bite angle (N1-Re-O1) of the chelating ligand, corresponding to a five-membered ring, has a typical value of 75.2 (2) (Lyczko et al., 2015). The Re-N1 and Re-O1 bond lengths are 2.273 (5) and 2.149 (5) Å , respectively. The isocyanide carbon atom, C14, is at a distance of 2.107 (8) Å from the metal site. All these values are close to those of a complex with the same core (Agorastos et al., 2007). The isocyanide group is oriented within the equatorial plane of the cyclohexyl ring which exhibits a chair conformation. The molecular structure and atom-labelling scheme of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
A rod of complexes extending parallel to [001] throughinteractions. The colour code is as in Fig. 2.

NMR investigation
In the solution NMR spectra of the complex, both the quinaldate and isocyanocyclohexane moieties are distinguishable. Coordination by the quinaldate is evident from the downfield shifts of all its protons ranging from 0.10 to 0.44 p.p.m. compared to free quinaldic acid under the same conditions (our data). Downfield shifts are also recorded for most of the C atoms of quinaldic acid, the most notable one (4.8 p.p.m.) being the one of the carboxylate carbonyl carbon. For the isocyanocyclohexane moiety, downfield shifts are recorded for the C atom (2.7 p.p.m.) bearing the isocyanide group and for its H atom (0.31 p.p.m.) compared to the free ligand. The most characteristic sign of coordination of the isocyanocyclohexane moiety is the sizable upfield shift of the isocyanide C atom of 15.5 p.p.m., attributed to an increased carbene character upon coordination (Stephany et al., 1974;Sagnou et al., 2010Sagnou et al., , 2011. In the 13 C NMR spectrum of the complex, one of the carbonyl ligands of the Re(CO) 3 + core appears shielded (by 2.8 p.p.m. on average) compared to the other two, an observation that may also be attributed to the trans effect of the isocyanide ligand.  (Benny et al., 2009;Mundwiler et al., 2004). Finally, the respective bond length, 1.947 Å , is longer than both Re-C bonds trans to equatorial O (1.912 Å ) and N (1.914 Å ) atoms if the Re atom is bonded to a P atom of a phosphine ligand (Hayes et al., 2014). In the case of the isocyanide group trans to the axial Re-C bond (Agorastos et al., 2007), the results are indistinct. In one case (XIDPUW), the axial bond length (1.756 Å ) is shorter than the equatorial one (1.849 Å trans to O and 1.901 Å trans to N) whereas in the other case (XIDQAD), the corresponding length (1.914 Å ) is longer than the equatorial one (1.495 Å trans to O and 1.885 Å trans to N). In the present structure, the Re-C11 bond (1.960 Å ), is longer than the Re-C13 (1.903 Å , trans to O) and Re-C12 (1.912 Å , trans to N) bonds. This result is supported by the NMR analysis and is indicative of the structural trans effect (Coe & Glenwright, 2000).

Synthesis and crystallization
To a stirred solution of quinaldic acid (17.3 mg, 0.1 mmol) in 5 ml methanol, a solution of [NEt 4 ] 2 [ReBr 3 (CO) 3 ] (77 mg, 0.1 mmol) in 5 ml methanol was added. The mixture was heated at 333 K, and after 30 min a solution of cyclohexyl isocyanide (0.1 mmol) in 3 ml methanol was added. The mixture was stirred at room temperature for 2 h and the reaction progress was monitored by HPLC. The solvent was removed under reduced pressure and the solid residue was recrystallized from dichloromethane/hexane. The resulting solid was redissolved in a minimum volume of dichloro-  methane, layered with hexane and left to stand at room temperature. After a few days crystals suitable for X-ray analysis were isolated (yield: 44 mg, 80%  193.65, 193.12, 190.54, 172.06, 152.63, 146.23, 142.09, 138.85, 133.04, 130.47, 129.67, 129.61, 127.78, 122.78, 53.72, 30.48, 23.91, 20.70.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry.