fac-Acetonitriletricarbonyl(dimethylcarbamodithioato-κ2 S,S′)rhenium(I): crystal structure and Hirshfeld surface analysis

An octahedral fac-C3NS2 coordination geometry is found for the ReI atom in the title compound; the dithiocarbamate ligand forms symmetric Re—S bonds. In the crystal, supramolecular layers are formed via weak C—H⋯O interactions.


Chemical context
The reaction between a secondary amine and carbon disulfide in the presence of an alkali metal hydroxide yields a class of ligands, the dithiocarbamates, (À) S 2 CNRR 0 . These ligands have long attracted the attention of coordination chemists owing to their high affinity for heavy-atom centres drawn from transition metals, main group elements as well as lanthanides and actinides. The motivation for their study ranges across various disciplines and in the present time focuses upon their development as drugs (Hogarth, 2012;Bertrand & Casini, 2014), as chelating agents for the removal of toxic levels of metals in bio-remediation, etc. (Gallagher & Vo, 2015), as imaging/ radio-pharmaceutical agents (Berry et al., 2012) and as synthetic precursors for metal sulfide nanoparticles (Lewis et al., 2015;Knapp & Carmalt, 2016). In terms of crystal engineering endeavours, dithiocarbamates are not nearly as well studied as carboxylates. This partly arises as a result of the greater chelating ability of dithiocarbamate by virtue of the significant contribution of the (2À) S 2 CN (+) RR 0 canonical form to the electronic structure of the anion, i.e. with formal negative charges on each of the sulfur atoms. This has the consequence of reducing the Lewis acidity of the metal atom, often precluding additional donor atoms from entering the coordination sphere. Main group element dithiocarbamate compounds are more likely to feature bridging ligands, often through secondary MÁ Á ÁS interactions which may be mitigated by steric effects associated with the R,R 0 groups or, in cases of organometallic derivatives, metal-bound substituents (Tiekink, 2006;Tiekink & Zukerman-Schpector, 2010). Another consequence of the tight chelating mode of the dithiocarbamate ligands is the formation of aromatic MS 2 C chelate rings that can function as acceptors for C-HÁ Á Á interactions, i.e. C-HÁ Á Á(chelate) interactions (Tiekink & Zukerman-Schpector, 2011;Jotani et al., 2016). As a result of the above, a very large number of crystal structure determinations have been reported in the literature, with the last systematic reviews published over a decade ago (Heard, 2005;Hogarth, 2005).
Reflecting the wealth of structural information on metal dithiocarbamates, a search of the Cambridge Crystallographic Database (Groom et al., 2016) for rhenium dithiocarbamate structures reveals over 70 'hits'. One structure that attracted the attention of the authors was that of twofold symmetric, binuclear [(CO) 3 Re(S 2 CNEt 2 )] 2 , whereby each dithiocarbamate ligand is 2 -tridentate, simultaneously chelating one Re I atom while bridging a second (Flö rke, 2014). The unusual feature of the structure is that the dithiocarbamate ligands lie to one side of the molecule and might be described as being syn. This arrangement is the same as that found in analogous, isoelectronic Pt IV complexes (Heard et al., 2000), but contradicts the observations seen in the overwhelming majority of the binary dithiocarbamates of the zinc triad elements, a focus of present research, whereby binuclear molecules with equal numbers of chelating and 2 -tridentate ligands lead to binuclear molecules of the general formula, {M(S 2 CNRR') 2 } 2 (Cox & Tiekink, 2009;Tiekink, 2003;Jotani et al., 2016). This disparity lead to the attempted synthesis of the dimethyldithiocarbamate analogue of [(CO) 3 Re(S 2 CNEt 2 )] 2 , which when recrystallized from acetonitrile resulted in the isolation of mononuclear (CO) 3 Re(S 2 CNMe 2 )(N CMe), (I). Herein, the molecular and crystal structures of (I) are described along with a detailed analysis of the self-assembly via a Hirshfeld surface analysis.

Structural commentary
The molecular structure of (I) is shown in Fig. 1 and selected geometric parameters are collected in Table 1. The Re I atom is coordinated by three facially-orientated carbonyl ligands, two dithiocarbamate-S atoms and an acetonitrile-N atom. The dithiocarbamate ligand is chelating in a symmetric mode with the difference between the long and short Re-S bond lengths being less than 0.01 Å . This mode of coordination is reflected in the equivalence of the associated C-S bond lengths and a relatively short C1-N1 bond length, Table 1, all pointing to a significant contribution of the (2À) S 2 C N (+) Me 2 canonical form to the overall electronic structure of the dithiocarbamate ligand. From the geometric data collected in Table 1, there is evidence that the shortest Re-CO bond length is formed by the carbonyl trans to the acetonitrile-N atom as opposed to those trans to the dithiocarbamate-S atoms. However, the experimental errors do not allow definitive conclusions to be made. This point is discussed further in Database survey below.

Figure 1
The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.
of the van der Waals radii, each with an acetonitrile-C-H atom, Table 2. The combination of these weak interactions leads to supramolecular layers in the ab plane, Fig. 2b. The two other potentially basic sites, namely the dithiocarbamate-S atoms, form intramolecular interactions with dithiocarbamatemethyl-H atoms, Table 2. The layers stack along the c axis as shown in Fig. 2c, i.e. without directional interactions between them.

Hirshfeld surface analysis
The protocols for the Hirshfeld surface analysis were as described recently (Yeo et al., 2016). In general, the Hirshfeld surface of (I) features some close interaction contacts as evidenced from the intense-red spots, Fig. 3a, being indicative of d norm contact distances shorter than the sum of van der Waals radii (McKinnon et al., 2007). The combination of the d i and d e , in intervals of 0.01 Å , resulted in the sparrow-like twodimensional fingerprint plot. This has been decomposed into several close contacts as shown in Fig. 3b-f. Specifically, the intense-red spots resulting from OÁ Á ÁH/HÁ Á ÁO as well as CÁ Á ÁO/OÁ Á ÁC contacts give bat-and scarab-shaped fingerprint profiles with corresponding d e + d i contact distances tipped at ca 2.5 and 3.0 Å , respectively; Fig. 3b and f. These contact distances are approximately 0.25 Å shorter than the sum of the respective van der Waals radii (Batsanov, 2001) and constitute about 33.8 and 5.7%, respectively, of the overall Hirshfeld surface contacts for the molecule.

Synthesis and crystallization
All chemicals and solvents were used as purchased without purification, and all reactions were carried out under ambient conditions. The melting point was determined using an Electrothermal digital melting point apparatus and was uncorrected. The IR spectra were obtained on a Perkin Elmer Spectrum 400 FT Mid-IR/Far-IR spectrophotometer from 4000 to 400 cm À1 (abbreviations: vs, very strong; s, strong). 1 H NMR spectra were recorded at room temperature in DMSO-d 6 solution on a Bruker AVANCE-400 MHz instrument.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Carbon-bound H atoms were placed in calculated positions (C-H = 0.98 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The maximum and minimum residual electron density peaks of 0.80 and 1.21 e Å À3 were located 0.87 and 0.91 Å , respectively, from the Re atom.  SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.80 e Å −3 Δρ min = −1.21 e Å −3 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.