Crystal structure of cis-bis(μ-β-alanine-κ2 O:O′)bis[trichloridorhenium(III)](Re–Re) sesquihydrate

A dirhenium(III) cis-dicarboxylate complex is reported, which is representative of a small class of zwitterionic ammoniacarboxylato complexes involving a quadruple metal–metal bond.


Chemical context
Investigations of complex compounds with multiple metalmetal bonds, which exhibit biological activity, generate great interest at the present stage of development of coordination chemistry (Jung & Lippard, 2007;Shtemenko et al., 2013). Binuclear clusters of rhenium(III) are the classical complexes with a unique quadruple metal-metal bond (Cotton et al., 2005;Golichenko & Shtemenko, 2006). In our previous studies, we have shown that these compounds can be used in medical practice as antitumor, antiradical, and hepato-and nephro-protective substances with low toxicity (Dimitrov & Eastland, 1978;Shtemenko et al., 2007Shtemenko et al., , 2008Shtemenko et al., , 2009Shtemenko et al., , 2013. Labile axial ligands and equatorial chloride groups are the reactive centres not only for other substances in vitro, but also in interactions with biological macromolecules, such as proteins, DNA, and others in vivo (Shtemenko et al., 2013). In this context, we present the synthesis and crystal structure of a new complex compound of dirhenium(III) with -alanine as biologically active substance, which can exhibit antitumor activity (Shtemenko et al., 2009).

Structural commentary
It is well known that -alanine and other amino acids are able to coordinate to a variety of transition metals (Korp et al., ISSN 2056-9890 1981; Shtemenko et al., 2009). The quadruple Re-Re bond [2.2494 (3) Å ] is typical of related dicarboxylato clusters (Cotton et al., 2005;Shtemenko et al., 2009). The octahedral coordination environment of each rhenium ion in the title compound ( Fig. 1) also comprises two chloride anions and two oxygen atoms of zwitterionic alanine ligands. The distorted octahedral coordination of the metals is completed by weakly bonded chloride ions [Re1-Cl3 = 2.6766 (16) and Re2-Cl6 = 2.7501 (14) Å ], in a trans-position to the Re-Re bond. This may be compared with the similar weak binding of N-or O-donors, which is characteristic of dicarboxylatodirhenium compounds (Bera et al., 2003;Shtemenko et al., 2009) and is even more appreciable for cationic tetracarboxylatodirhenium species commonly accommodating a pair of chloride anions at both axial sites (Re-Cl = 2.48-2.52 Å ; Shtemenko et al., 2001).

Supramolecular features
The title compound displays a three-dimensional structure dominated by weak hydrogen bonds of the O-HÁ Á ÁCl, N-HÁ Á ÁCl, C-HÁ Á ÁO and C-HÁ Á ÁCl types (Table 1). The primary supramolecular motif consists of centrosymmetric dimers (symmetry code: Àx, Ày + 1, Àz) incorporating two complex moieties and two water molecules (Fig. 2), with a typical hydrogen-bonding geometry [OÁ Á ÁCl = 3.342 (6) and 3.360 (6) Å ], while an extensive hydrogen-bonding network involving the ammonium groups and chloride acceptors assembles the dimers into a three-dimensional framework. One of these N-HÁ Á ÁCl bonds is bifurcated and one is trifurcated (Table 1). It is worth noting that most of the N-HÁ Á ÁCl interactions are observed for the Cl3 and Cl6 acceptors. Such selectivity is likely predetermined by the steric accessibility and relative negative charge located at the Cl atoms, since these distal 'axial' chloride ligands Cl3 and Cl6 are the most underbonded and highly nucleophilic. The disordered water molecules reside in the framework cages and adopt a series of short contacts, which may be attributed to weak hydrogen bonding [OÁ Á ÁCl = 3.07 (2)-3.42 (4) Å ]. The molecular structure of the title complex, with displacement ellipsoids drawn at the 40% probability level. Solvent water molecules have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The crystal structure of the title complex viewed down the a axis, with the C-H hydrogens and disordered water molecules omitted for clarity. Dotted lines indicate hydrogen bonds involving the OH and NH groups. Note the assembly of the hydrogen-bonded dimers constituted by two complex molecules and two water molecules. [Symmetry codes: (ii) Àx + 1, Ày + 1, Àz; (iii) x + 1 2 , Ày + 1 2 , z + 1 2 ; (v) Àx + 3 2 , y + 1 2 , Àz + 1 2 ; (viii) Àx, Ày + 1, Àz; (ix) Àx + 1, Ày + 1, Àz + 1.] initial volume using a rotary evaporator. A new portion (10 ml) of the solvent was added and the solution was evaporated to half of the initial volume. This procedure was repeated five times. The dark-green crystals obtained were filtered, washed with two 5 ml portions of cold acetonitrile and diethyl ether and dried under vacuum at 353 K. The product (0.77 g) was recrystallized from acetone, yielding the title complex in 81% yield.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were refined using a riding model, with O-H = 0.85, N-H = 0.90, C-H = 0.98 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (N,O). One of the solvate water molecules is disordered over two unequal contributions, which are further disordered about an inversion centre. The refined partial occupancies for this oxygen atom (O6A and O6B) are 0.3 and 0.2, respectively. Both sites were refined anisotropically. The H atoms of the partially occupied water molecule could not be located and were omitted from the final refinement. Computer programs: SMART and SAINT (Bruker, 2008), SHELXS97 and SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012 program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012).

cis-Bis(µ-β-alanine-κ 2 O:O′)bis[trichloridorhenium(III)](Re-Re) sesquihydrate
Crystal data [Re 2 Cl 6 (C 3 H 7 NO 2 ) 2 ]·1. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.70 e Å −3 Δρ min = −1.62 e Å −3 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. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. One of the solvate water molecules is disordered over center of inversion. Moreover, judging by the high anisotropy of thermal motion for this oxygen atom, two contributions of the disorder were considered and the refined partial occupancy factors were 0.20 and 0.30. Both of this contributions were refined anisotropically. However, the hydrogen atoms were not added for this disordered molecule.