Synthesis and structure of an arylselenenium(II) cation, [C34H41N4Se+]2[Hg(SeCN)4]2−, based on a 5-tert-butyl-1,3-bis(1-pentyl-1H-benzimidazol-2-yl)benzene scaffold

In [C34H41N4Se+]2[Hg(SeCN)4]2−, the arylselenenium cations, [C34H41N4Se]+, and [Hg(SeCN)4]2− anions are linked by C—H⋯N hydrogen bonds. In the cation, the geometry around the Se atom is T-shaped, resulting from the coordination of Se by the C atom of the central aromatic ring and the N atoms of the benzimidazolyl moieties.

Our group has been active in the area of synthesis and isolation of novel, unstable arylchalcogen derivatives featuring intramolecular interactions (EÁ Á ÁD; E = S, Se, Te and D = N, O) between chalcogen heteroatoms by using either one or two coordinating groups (Zade et al., 2004a,b;Selvakumar et al., 2011a,b,c,d;Singh et al., 2011;Prasad et al., 2016). Recently, and for the first time, we have shown the use of the bis-benzimidazole group to isolate an organometallic deriva- ISSN 2056-9890 tive of a non-transition metal where 1,3-bis(N-substituted benzimidazol-2 0 -yl)benzene has been used as a pincer ligand with chalcogens (Rani et al., 2018a).
As far as the synthesis of transition metal complexes with the bis-benzimidazole group is concerned, there are several reports in the literature for platinum(II) pincer complexes with similar kinds of scaffolds. Some of these were investigated for their photoluminescence properties (Wang et al., 2014;Dorazco-Gonzá lez, 2014;Chan et al., 2016). Recently, we also reported some palladium(II) pincer complexes with a 1,3bis(N-substituted benzimidazol-2 0 -yl)benzene-based ligand. In all the cases, we found that the transition metal complexes were quite stable and in no case was auto-ionization observed (Rani et al., 2018b).
In an attempt to synthesize {4-(tert-butyl)-2,6-bis(1-pentyl-1H-benzo[d]imidazol-2-yl)phenyl}(selenocyanato)mercury (3), [4-tert-butyl-2,6-bis(1-pentyl-1H-benzimidazol-2-yl)phenyl]mercury(II) chloride (1) was reacted with potassium selenocyanate in 1,4-dioxane under reflux conditions. It was observed that, instead of the formation of the desired compound, the reaction leads to the isolation of an arylselenenium(II) cation via auto-ionization (Scheme 1). The procedure for the synthesis of complex 1 will be reported elsewhere. A plausible mechanism for the formation of complex 2 is shown in Scheme 2. Organomercury complex 1 reacts with potassium selenocyanate to form the desired product 3 with potassium chloride as a by-product. However, if complex II is unstable, mercury may be eliminated in elemental form via a reductive elimination pathway to form intermediate III. Strong secondary bonding interactions between SeÁ Á ÁN atoms may facilitate auto-ionization and the formation of an arylselenenium cation with CN À as the counter-anion IV. In the presence of a polar protic solvent, there is the possibility of decomposition of organomercury complex 1 to give the free ligand along with HgCl 2 and Hg(OMe) 2 as by-products.
HgCl 2 reacts with an excess of KSeCN to form K 2 [Hg(SeCN) 4 ] (Space & Armeanu, 1930). Two selenenium cations can then associate with the [Hg(SeCN) 4 ] 2À anion to form complex 2. Since we only used one equivalent of potassium selenocyanate for the reaction, the product was obtained in low yield (11%).

Structural commentary
The title compound, 2, crystallizes in the monoclinic space group C2/c. The asymmetric unit contains a selenenium cation along with half of a [Hg(SeCN) 4 ] 2À anion with the Hg atom located on a crystallographic twofold axis (Fig. 1). In the cation, the coordination geometry around Se is T-shaped with each Se atom bonded to the central carbon atom of the aromatic ring and intramolecularly coordinated to the two N atoms. This coordination gives rise to a heptacyclic framework. The tetracyanoselenomercurate anion [Hg(SeCN) 4 ] 2À is sandwiched between two arylselenenium cationic units. The observed Se-C bond length is 1.886 (3) Å , which is comparable with that found for a NCN pincer-based selenenium cation [2,6-(Me 2 NCH 2 ) 2 C 6 H 3 Se] + [PF 6 ] À (1.874 Å ; , and an OCN pincer-based selenenium cation [2-NO 2 ,6-(C 6 H 5 N=CH)C 6 H 3 Se] + [Br 3 ] À (1.84 Å ). The Se3-N1 and Se3-N2 bond lengths are almost equal [2.087 (3) and 2.099 (3) Å ]. The Se-N distances are shorter than the sum of the van der Waals radii for Se and N [AE rvdw (Se,N) 3.45 Å ] and longer than the covalent radii [AE rcov (Se,N) 1.91 Å ] (Bondi, 1964). This implies stronger intramolecular SeÁ Á ÁN interactions in the selenenium cation. The N1-Se3-N2 bond angle is found to be 159.29 (11) . In related molecules (Rani et al., 2017a,b,c), in the absence of coordinated Hg or Se atoms, the benzimidazole arms are twisted significantly out of the plane of the central phenyl ring. However, in the present structure, as a result of the interaction with Se, the two benzimidazole arms are almost in the plane of the central phenyl ring [dihedral angles of 3.10 (16) and 7. 18 (19) ]. The Se atom is displaced by 0.116 (4) Å from the plane of the central phenyl ring. The atoms involved in the chelating system (N2, C11, C6, C1, C2, C11A, N1) form a plane (r.m.s deviation for fitted atoms of 0.0182 Å ) with the Se in this plane [deviation from the plane of 0.011 (2) Å ].
In the anion, the mercury atom is coordinated by four selenocyanate anions (two are crystallographically unique) and the geometry around the mercury atom is distorted tetrahedral with Se-Hg-Se angles ranging from 88.78 (3) to 126.64 (2) .

Supramolecular features
In the crystal, the molecules are arranged in a parallel fashion along the b-axis direction as shown in Fig  A view of the structure of the title compound, showing the atom-labelling scheme and the disorder in the pentyl side chain. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code for generating equivalent atoms: 1 À x, y, 3 H18CÁ Á ÁN2S interactions (numerical details are given in Table 1) andstacking interactions between the benzimidazole rings (centroid-centroid distances = 3.535 Å ).

Synthesis and crystallization
To a solution of 1 (0.2 g, 0.269 mmol) in 1,4-dioxane (30 ml) was added potassium selenocyanate (0.039 g, 0.270 mmol) dissolved in MeOH. The reaction mixture was stirred for 6 h under a nitrogen atmosphere and refluxed. The reaction mixture was filtered and the precipitate was washed with dioxane. Colourless prism-shaped crystals of 2 were obtained by layering a MeOH solution with diethyl ether at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H distances ranging from 0.95 to 0.99 Å . U iso (H) = xU eq (C), where x = 1.5 for methyl H atoms and 1.2 for all other C-bound H atoms. One of the pentyl substituents is disordered with an occupancy ratio of 0.852 (8):0.148 (8). It was refined as two equivalent conformations using SAME and SIMU instructions (SAME 0.01 and SIMU 0.01).

Funding information
RJB is grateful for NSF award 1205608, to the Partnership for Reduced Dimensional Materials for partial funding of this research, to Howard University's Nanoscience Facility for access to liquid nitrogen, and the NSF-MRI program (grant No. CHE0619278) for funds to purchase the X-ray diffractometer. HBS is grateful to the DST, New Delhi, for a J. C. Bose National Fellowship. VR gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, for a Senior Research Fellowship. Table 1 Hydrogen-bond geometry (Å , ).  Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT, SADABS and XPREP (Bruker, 2002); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Bis{[5-tert-butyl-1,3-bis(1-pentyl-1H-benzimidazol-2-yl)benzene]selenium} tetrakis(selenocyanato)mercury
Crystal data (C 34  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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (