Crystal structure of fac-aquatricarbonyl[(S)-valinato-κ2 N,O]rhenium(I)

The title molecule consists of an Re(CO)3 + fragment, an aqua ligand and one N,O-chelating valinate anion to complete a slightly distorted coordination sphere.

In the molecule of the title compound, [Re(C 5 H 10 NO 2 )(CO) 3 (H 2 O)], the Re I atom adopts a distorted octahedral coordination sphere defined by one aqua and three carbonyl ligands as well as one amino N and one carboxylate O atom of the chelating valinate anion. The carbonyl ligands are arranged in a facconfiguration around the Re I ion. In the crystal, an intricate hydrogen-bonding system under participation of two O-H, two N-H and one C-H donor groups and the carboxylate and carbonyl O atoms as acceptor groups contribute to the formation of a three-dimensional supramolecular network.

Chemical context
The syntheses of metal-organic compounds, which are capable of visualization of biomolecules, is receiving growing interest in biocoordination chemistry (Coogan & Ferná ndez-Moreira, 2014). For the labeling of biomolecules, octahedral fac-tricarbonyl complexes of Tc and Re are the most promising compounds (Alberto, 2007;. The compact M(CO) 3 -core (M = Tc, Re) allows labeling of low molecular weight substrates under retention of activity and specificity. In this context, Re(CO) 3 + compounds are of interest as the closest non-radioactive analogs of 99m Tc-based systems, which could be particularly important for visualization and immunotherapy. Studies of the cytotoxicity of rhenium carbonyl complexes also suggest their specific anticancer activity (Leonidova & Gasser, 2014).
Most of the known Re(CO) 3 + complexes with biologically essential substrates comprise tridentate co-ligands, e.g. histidinato-O,N,N 0 (Alberto et al., 1999), methioninato-N,O,S (He et al., 2005, 2,3-diaminopropionato-N,N 0 ,O (Liu et al., 2006), completing the coordination octahedra of the central ions. At the same time, coordinatively unsaturated complexes of bidentate aminocarboxylates could be suited for interactions with additional ligands, such as guanine bases (Zobi et al. 2005a), thus allowing an attractive scenario for the assembly of mixed-ligand systems.
In this communication, we report the synthesis and crystal structure of a novel Re(CO) 3 + complex with valine and water as co-ligands. Following the findings of Zobi et al. (2005b), sufficient reactivity of this compound towards DNA may be anticipated.

Structural commentary
In the molecule of the title compound ( Fig. 1), the Re 1 ion resides in a slightly distorted octahedral coordination environment, with a facial arrangement of three nearly equidistant carbonyl ligands [Re1-C bond lengths are in the range 1.881 (7)-1.909 (7) Å ]. The compound crystallizes in the chiral space group P2 1 2 1 2 1 , with the S-enantiomer of the valinate anion present in the selected crystal. The anion coordinates in a bidentate-chelating fashion through the amino N and one carboxylate O atoms, with Re1-N1 and Re1-O4 bond lengths of 2.195 (5) and 2.122 (4) Å , respectively. The five-membered chelate ring [bite angle N1-Re1-O4 = 74.62 (18) ] has the expected envelope conformation, with the atoms of the Re1-O4-C4-C5 fragment being coplanar within 0.035 (3) Å and the N1 flap atom deviating from the given mean plane by 0.547 (6) Å . The Re1-O6 bond involving the aqua ligand [2.175 (5) Å ] is slightly longer than the one with the carboxyl O atom. The CO ligands coordinate in an almost linear fashion, with O-C-Re bond angles spanning a range from 175.5 (7) to 179.9 (8) , while the corresponding C-Re1-C angles are within 87.1 (3)-89.8 (2) . All other bond length and angles are comparable to those found for related Re I complexes (Rajendran et al., 2000).

Supramolecular features
In the crystal, the packing of the molecules is governed by an intricate system of hydrogen bonds, including classical The molecular structure of the title complex, with displacement ellipsoids drawn at the 40% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
The crystal structure of the title complex showing all hydrogen-bonding interactions (O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO) as dashed lines. The isopropyl CH-hydrogen atoms were omitted for clarity. [Symmetry codes: O-HÁ Á ÁO and N-HÁ Á ÁO bonds and weaker C-HÁ Á ÁO interactions (Table 1). Two rather strong and nearly linear O-HÁ Á ÁO bonds are observed between the aqua ligand and the non-coordinating carboxylate O atoms of two symmetryrelated neighbouring molecules. The amino group forms two weaker N-HÁ Á ÁO bonds to carbonyl O atom acceptor groups of two neighbouring molecules. Each non-coordinating carboxylate O atom accepts two such bonds, yielding hydrogen-bonded chains parallel to the a-axis direction (Fig. 2), whereas the N-HÁ Á ÁO bonds expand the hydrogenbonding system into a three-dimensional network. Additional C-HÁ Á ÁO interactions consolidate this arrangement (Fig. 3). The combination of O-HÁ Á ÁO and C-HÁ Á ÁO (involving the chiral C5 atom) bonds may be important for the observed enantioselective packing of the chiral moieties (Petkova et al., 2001).

Synthesis and crystallization
To a solution of dl-valine (0.116 g, 0.984 mmol) in 5 ml of water, a solution of triaquatricarbonylrhenium(I) bromide (0.100 g, 0.246 mmol) in 10 ml of methanol was added. The reaction mixture was heated and stirred at 343 K under a steady stream of argon for 4 h. After cooling to room temperature, the solution was left to evaporate in air for a period of a few days. After removal of the methanol cosolvent, a colourless crystalline product formed. The precipitate was collected by suction filtration, washed with water and then with a 50 ml portion of petroleum ether and dried (yield: 0.068 g, 68%). Suitable single crystals were obtained by slow diffusion of hexane vapor into a methanol solution of the complex. IR (KBr, cm À1 ): as (CO) 2028 (s), s (CO) 1905 (s).