Unexpected reactions of NHC*—CuI and —AgI bromides with potassium thio- or selenocyanate

This article reports the unexpected reactions of Cu and Ag NHC bromides with potassium thio- or selenocyanates. It contains the first report of the boomerang-shaped [Ag(SCN)3]2− ion.


Figure 1
Lewis structures of the ligands NHC* and NHC*Se.
angles ranging from 104.96 (2) to 127.68 (2) ( Table 1). As expected, all three SCN À ligands are virtually linear. The sum of the S-Ag-S bond angles (Table 1) indicates that the anion is almost planar [the deviation of the Ag3 from the leastsquares plane of the three S atoms is 0.1270 (5) Å ]. The [Ag(SCN) 3 ] 2À anion is situated between the two crystallographically independent cations, but not in the middle (Fig. 2): cation 1 (Ag1) has a shortest distance of 4.161 (2) Å from N3 to Ag3 (line 2 in Fig. 2), whereas cation 2 (Ag2) has a shortest distance of 3.069 (2) Å from C66 to Ag3 (line 1 in Fig. 2). As a consequence of this close association, the benzyl groups in cation 2 are all aligned away from the anion. Due to its greater distance from the anion, the benzyl groups of cation 1 have greater flexibility, allowing it to take a shape suitable to fill gaps in the packing caused by the constraint on cation 2 (Fig. 3). The remaining gaps are filled by two noncoordinating diethyl ether molecules, one of which is highly disordered and could not be refined in terms of atomic sites. The SQUEEZE option (Spek, 2015) in PLATON was used to compensate for the ill-defined electron density.
2.2. (NHC*Se) 4 Ag 2 Br 2 Á6CH 2 Cl 2 , (2) Compound (2) is characterized by a molecular structure complemented by dichloromethane solvent molecules. Two Ag I cations and two bridging NHC*Se ligands build up a centrosymmetric four-membered Ag 2 Se 2 ring. Each silver cation carries a further terminal NHC*Se ligand and a terminal bromide ligand, in each case leading to a coordination number of 4 in the shape of a distorted tetrahedron (Table 1 and Fig. 4)  The [Ag(SCN) 3 ] 2À anion in (1) with the two closest [Ag(NHC*) 2 ] + cations. Substituents on the imidazole moiety have been omitted for clarity and displacement ellipsoids are drawn at the 50% probability level.

Figure 4
The molecular structure of (2), with phenyl groups represented by their ipso carbon atoms only. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (I) Àx + 1, Ày + 1, Àz + 1.] or the terminal one [2.6899 (4) Å ], suggesting that two AgBr(NHC*Se) 2 moieties are weakly attached to each other. The Ag-Se-C angles are all strongly bent (Table 1), as one would expect. The bridging and terminal NHC*Se ligand pairs, as well as the two bromide ligands, end up in pseudotrans positions with respect to each other, allowing an overall compact shape of the uncharged [AgBr(NHC*Se) 2 ] 2 complex. Gaps in the packing are filled by dichloromethane solvent molecules, two of which were treated with the SQUEEZE option in PLATON.

NHC*Se-CuCNÁCH 3 CN, (3)
The structure of (3) is polymeric in nature and contains two distinct Cu I atoms. The backbone of the structure is a linear copper-cyanide polymer 1 1 [Cu1-C N-Cu2-], where every Cu I atom is also coordinated by selenium from a terminal NHC*Se ligand (Fig. 5). The Cu-Se-C angles are in the same region as those in (2) ( Table 1). The carbon and nitrogen atoms of the two cyanide anions can be distinguished, not only by their electron densities, but also by their different bond lengths to Cu I atoms, with Cu-N shorter by ' 0.04 Å ( Table 1). The relatively bulky NHC*Se ligands, which lead to the rare coordination number of 3 of the two Cu I cations, move to opposite positions with respect to the copper cyanide polymer, allowing better packing for the overall structure ( Fig. 5). The sum of the three angles at Cu1 and Cu2 (Table 1) indicate that the coordination is practically planar at the central copper(I) atoms (the displacement of Cu1 from the least-squares Se1/N6/C5 plane is 0.099 Å and of Cu2 from the least-squares Se2/N5/C60 plane is 0.054 Å ). Acetonitrile solvent molecules fill voids in the crystal packing.

Figure 7
View of the crystal structure of (1) along [100]. Displacement ellipsoids are drawn at the 50% probability level and hydrogen-bonding interactions are shown as pink dashed lines.

Figure 5
Section of the polymeric structure of (3), with displacement ellipsoids drawn at the 50% probability level. Phenyl groups are represented by their ipso carbons only and acetonitrile solvent molecules have been omitted for clarity. [Symmetry codes: (I) Àx + 2, y + 1 2 , Àz + 3 2 ; (II) Àx + 2, y À 1 2 , Àz + 3 2 .] NHC*-M-SeCN complexes are not very stable in solution, but are liable to Schlenk-type equilibrium exchange. It is worth mentioning that the syntheses of (2) and (3) require SeCN À acting as a selenating agent similar to selenium powder (Verlinden et al., 2015) or Woollins Reagent (Bockfeld et al., 2017), even under the relatively mild conditions reported here. Thus, neither thiocyanate nor selenocyanate take up their roles as unreactive substituents in these planned substitution reactions. These findings raise questions about the suitability of NHC-M-X (M = Cu and Ag; X = pseudohalide) as drugs because drug molecules need to be stable in solution under ambient conditions.

Supramolecular features
In (1), there are some weak nonclassical hydrogen bonds between the cations and the solvent molecules, as detailed in Table 2. One anion has connections to the two cations closest to it, as described above, and one to the cation of an adjacent ion triplet (Fig. 6), linking the ion triplets into a one-dimensional chain. Fig. 7 shows a view of (1) along [100] with these contacts shown as dashed lines.
All nonclassical hydrogen bonds in (2) and (3) are intramolecular, and we are not aware of any other noteworthy intermolecular features in these structures.

Database survey
To the best of our knowledge, the crystal structure of (1) is the first report of a salt with the tri-blade boomerang-shaped [Ag(SCN) 3 ] 2À ion. The alkaline metals salts Rb 2 Ag(SCN) 3 and Rb 2 Ag(SCN) 3 have one-dimensional polymeric chains as anions rather than isolated [Ag(SCN) 3 ] 2À (Thiele & Kehr, 1984). Hathaway et al. (1970) reported the spectroscopic properties of [Cu(NH 3 ) 2 Ag(SCN) 3 ] and indicated that they had determined its crystal structure as well. However, in this article, only the space group type (P62c) and the number of formula units (Z = 2) were given, not the crystal structure itself. From what is reported it can be gleaned that the anion must be situated on a 6 position, i.e. it is planar and adhering to threefold rotation symmetry. If this is true then this is in stark contrast to the [Ag(SCN) 3 ] 2À anion reported here, where the Ag-S bond lengths and S-Ag-S and Ag-S-C bond angles cover a wide range. However, since together with the information above only a schematic drawing of the surrounding of the Cu II atom was given, we cannot establish   (2) fall well within the region reported for similar compounds (Perras et al., 2018;Nahra et al., 2018). Remarkably, at least for the neutral compounds, the distances do not depend on whether the coordination number around the silver is 3 or 4: In N,N-dimesitylselenoimidazolesilver nitrate, the Ag-Se bond lengths range from 2.65 to 2.68 Å for the four-coordinate atom and from 2.63 to 2.71 for the three-coordinate atom (Perras et al., 2018).
A comparison with the compounds reported by Kimani et al. (2011) shows a remarkable impact of the cyanide anion on the Cu-Se bond length compared with the corresponding halides. For threefold-coordinated Cu, the distances between Cu and nonbridging Se range from 2.33 to 2.35 Å , whereas both of them in (3) are about 2.39 Å (Table 1). This is closer to the Cu--Se distances (2.41-2.42 Å ) reported by Kimani et al. (2011). In other words, cyanide is the better ligand for Cu I when compared with halides, and as a result relatively long Cu-Se distances are observed for cyanide derivative (3).
A saturated solution of the compound in dichloromethane was prepared at 313 K; from this solution, that was kept for 7 d at 253 K, the title compound (NHC*Se) 4 Ag 2 Br 2 crystallized as clear pale-yellow block-like prisms of the dichloromethane hexasolvate.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed at calculated positions and treated as riders, with U iso values set at 1.2U eq or 1.5U eq of their respective bonding partners.
In the crystal structure of (1), one of the phenyl groups was refined as partially disordered over two positions rotated against each other around the ipso-para axis [occupancy ratio 0.55 (2):0.45 (2)]. In the crystal structure of (2), one of the dichoromethane solvent molecules was refined over two positions [ratio 0.898 (4):0.102 (4)] due to positional disorder around one C-Cl bond.
The SQUEEZE procedure (Spek, 2015) in PLATON was used to treat regions of disordered solvent molecules in (1) and (2) which could not be modelled in terms of atomic sites. In (1), the number of electrons found in these regions in the unit cell, 82, was assigned to two diethyl ether solvent molecules (ideal 84 electrons). In (2), 84 electrons were found and assigned to two solvent molecules of dichloromethane in the unit cell (ideal 84 electrons). Since Z = 2 for (1) and Z = 1 for (2), one solvent molecule of diethyl ether in (1) and two solvent molecules of dichloromethane in (2) are missing in the final models and the given chemical formulae and other crystal data given in Table 3 take into account these solvent molecules. For all structures, data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
[Ag (C 29  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.003 Δρ max = 0.98 e Å −3 Δρ min = −1.13 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. Refinement. The structure was first solved by the Patterson method as implemented in SHELXS. The three silver atoms and one sulfur atom were located. The result was subjected to the Tangens Expansion formula as implemented in SHELXS. Almost all non-hydrogen atoms were located. The remainder was found in the difference fourier map from SHELXL refinements. The PLATON SQUEEZE procedure (A. L. Spek. Acta Cryst. C71, 2015, 9-18) was used to treat regions of disordered solvent which could not be modelled in terms of atomic sites. The number of electrons found in these regions, 82, was assigned to 2 molecules of diethylether. 2 diethylethers would give 84 electrons.

selenoimidazole-κSe)silver(I)] dichloromethane hexasolvate (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.05 e Å −3 Δρ min = −1.48 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. Refinement. The PLATON SQUEEZE procedure (A. L. Spek. Acta Cryst. C71, 2015, 9-18) was used to treat regions of disordered solvent which could not be modelled in terms of atomic sites. The number of electrons found in these regions, 84, was assigned to 2 molecules of dichloromethane. 2 dichloromethanes would give 84 electrons.

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