Crystal structure of a second polymorph of tricarbonyl(N-methylpyridine-2-carboxamide-κ2 N 1,O)(thiocyanato-κN)rhenium(I)

The second polymorph (monoclinic form) of the [Re(NCS){LH(Me) NO}(CO)3] complex, where LH(Me) NO is N-methylpyridine-2-carboxamide, has been obtained and structurally characterized by X-ray diffraction and supported by DFT calculations.

In the current study, a second polymorph of this compound crystallizing in the monoclinic space group P2 1 /n has been obtained and its structure is reported here, including a comparison of the triclinic and monoclinic polymorphs. ISSN 2056-9890

Structural commentary
The molecular structure of the monoclinic polymorph of the studied tricarbonylrhenium(I) complex with a bidentate ligand and a pseudohalide anion is presented in Fig. 1. The metal ion is surrounded in a slightly distorted octahedral coordination environment by six donor atoms, including three carbon atoms of the carbonyl groups, two nitrogen atoms and one oxygen atom. The three CO ligands occupy the facial positions of this octahedron. The Re-C bond lengths are in the range 1.9028 (16)-1.9201 (16) Å . The three remaining positions in the fac-[Re(CO) 3 ] + core are occupied by one bidentate ligand and one monodentate ligand, which results in a so called '2 + 1' system. N-methylpyridine-2-carboxyamide behaves in the complex as a neutral ligand and chelates the rhenium(I) ion by means of oxygen and nitrogen atoms with bond lengths of 2.1583 (10) and 2.1836 (13) Å , respectively, forming a five-membered ring. The N1-Re1-O4 bite angle of 74.33 (4) is typical for that type of chelate ring. The sixth coordination position of the metal ion is occupied by the N atom of the thiocyanate anion. The use of the NCS À ion in the reaction mixture together with an LH(Me) NO ligand leads to the formation of a neutral complex. This pseudohalide ion, which can exhibit an ambidentate character acting with the central metal cation either by its sulfur or nitrogen atom, coordinates in the present complex through the N atom, which is generally typical for hard metal ions using the 'hard and soft acids and bases' (HSAB) concept. All of the structural parameters mentioned above are very similar to those previously reported for the triclinic polymorph of the title compound (see Table 1). The molecular structures of the two polymorphic forms are compared in Fig. 2.
It can be ruled out that the use of AgBF 4 for precipitation of Cl À ions during the synthesis of the title complex (see Section 5) leads to the crystallization of its monoclinic polymorph, while the presence of PF 6 À anions, originating from the silver salt, contributes to the formation of its triclinic form (Lyczko et al., 2015).

DFT calculations
The bond lengths and angles for the present complex originating from the crystal structure determination are in good agreement with DFT calculations (see Table 1) performed by means of the B3LYP functional and three different basis sets for non-metallic atoms (the Re atom was described by the LANL2DZ basis set) using the GAUSSIAN09 software The molecular structure of the title compound, with displacement ellipsoids for the non-H atoms drawn at the 50% probability level. Table 1 Comparison of selected bond lengths, distances (Å ) and angles ( ) between the experiments and calculations from three different basis sets for the studied complex (a) .

Database survey
The triclinic polymorph of the title complex has been presented recently (Lyczko et al., 2015). Only a few crystal structures in which the thiocyanate ion coordinates to a tricarbonylrhenium (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms bonded to C atoms were inserted in calculated positions with C-H = 0.98 (methyl) or 0.95 Å (aromatic) and refined isotropically using a riding model with U iso (H) equal to 1.5U eq (C) or 1.2U eq (C) for methyl and aromatic H atoms, respectively. In turn, the H atom of the NH pair was located in a difference Fourier map and its position was freely refined.  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.