Crystal structures of the gold NHC complex bis(4-bromo-1,3-diethylimidazol-2-ylidene)gold(I) iodide and its 1:1 adduct with trans-bis(4-bromo-1,3-diethyl-imidazol-2-ylidene)diiodidogold(III) iodide

In the first title compound, [Au(C7H11BrN2)2]I, the cations and anions form chains via halogen bond linkages Br⋯I⋯Br. The second title compound, [Au(C7H11BrN2)2][AuI2(C7H11BrN2)2]I2, forms a layer structure involving Br⋯I⋯Br and I⋯I⋯Au linkages.

The first title compound, [Au(C 7 H 11 BrN 2 ) 2 ]I, crystallizes in the space group P1 without imposed symmetry. The cations and anions are linked to form chains by BrÁ Á ÁIÁ Á ÁBr halogen-bond linkages. The second title compound, [Au(C 7 H 11 BrN 2 ) 2 ][AuI 2 (C 7 H 11 BrN 2 ) 2 ]I 2 , is an adduct of the first and its formally I 2 -oxidized Au III analogue. It also crystallizes in space group P1, whereby both gold atoms occupy inversion centres. The extended structure is a reticular layer involving BrÁ Á ÁIÁ Á ÁBr and IÁ Á ÁIÁ Á ÁAu linkages.

Chemical context
Gold complexes have been used in medicine since ancient times and have been applied as drugs for the treatment of rheumatoid arthritis since the 1930s. Currently, gold species are being actively investigated in inorganic medicinal chemistry as possible anticancer agents or anti-infectives (Mora et al., 2019). Some of the existing therapeutics have reached the clinical trial stage as a result of drug repurposing efforts. Metal N-heterocyclic carbene (NHC) complexes in general have also proved to be biologically and medicinally active compounds (Ott, 2020); in particular, gold complexes with NHC ligands are often synthesized and investigated because of the high stability of the gold-carbon bonds and the convenient synthetic access to a broad variety of structurally diverse NHC structures (Nahra et al., 2021). We have reported on the synthesis, characterization and biological effects of [bis(4bromo-1,3-diethyl-imidazol-2-ylidene)gold(I)] iodide (3) (Schmidt et al., 2017a) (Fig. 1). Notably, this complex and related derivatives triggered cytotoxicity against cancer cells, showed a low serum protein binding, and inhibited growth of some pathogenic bacteria. Furthermore, we have recently investigated various gold NHC complexes as antibacterial agents and inhibitors of bacterial thioredoxin reductase (Bü ssing et al., 2021).
Here we report the structure of 3, together with that of its 1:1 complex (4) with trans-[bis(4-bromo-1,3-diethyl-imidazol-2-ylidene)diiodidogold(III)] iodide, formally its I 2 -oxidized Au III analogue; the latter was formed in small quantities when 3 was recrystallized. Further studies on the bioinorganic and medicinal chemistry of 3 and related derivatives are the subject of ongoing projects.

Structural commentary
The structure of the asymmetric unit of 3 is shown in Fig. 2. All atoms lie on general positions in space group P1. Selected intra-and intermolecular dimensions (including contact distances) are presented in Table 1. The gold atom is, as expected, linearly coordinated. The NHC planes subtend an interplanar angle of 78.74 (10) . The short contact Br2Á Á ÁI1 seen in Fig. 2 is one of two such contacts that determine the crystal packing (see next section).
The structure of compound 4 is shown in Fig. 3. Selected metrical parameters for intra-and intermolecular interactions (including contact distances) are presented in Table 2. Both gold atoms lie on inversion centres; the C-Au-C and I-Au-I angles are thus exactly linear, and the NHC planes of both cations are exactly coplanar. The gold(III) centre displays the expected square planar geometry. The Au-C bond is slightly longer than in 3. For further discussion, see Database survey below.  Structure of the asymmetric unit of compound 3; ellipsoids represent 50% probability levels. The dashed line indicates a halogen bond.

Figure 3
Structure of compound 4; the asymmetric unit has been extended by symmetry to show complete cations. Ellipsoids represent 50% probability levels. The dashed lines indicate halogen bonds.

Supramolecular features
The packing of compound 3 is shown in Fig. 4. It is dominated by short BrÁ Á ÁI contacts ( Table 1) that may be considered as halogen bonds (for reviews, see Metrangelo, 2008 andCavallo et al., 2016). The C-BrÁ Á ÁI angles are approximately linear, whereas BrÁ Á ÁIÁ Á ÁBr is approximately a right angle. The anions and cations are connected to form chains with overall direction parallel to [111]. The chains are in turn connected in pairs by the contact AuÁ Á ÁBr2 [3.8033 (3) Å , operator 1 À x, 1 À y, 1 À z]. Within the double chains, the intercentroid distance between the carbene rings based on N1 and N2 is 3.5265 (14) Å , and between the double chains the intercentroid distance between the rings based on N3 and N4 (operator 1 À x, 2 À y, Àz) is 3.6187 (14) Å ; these offset contacts may represent Á Á Á interactions. The packing of compound 4 ( Fig. 5) also involves halogen bonds. The cations are connected to form chains parallel to [331] (horizontal in Fig. 5) by contacts between each bromine atom and the iodide I2. As in 3, the C-BrÁ Á ÁI angles are approximately linear. The Au III cations are further connected in the [111] direction (vertical in Fig. 4) by a very short I1Á Á ÁI2 contact and a long I2Á Á ÁAu2 contact. The result is a reticular layer structure parallel to (110), in which the iodide anion I2 is four-coordinate. The angle between the two chain directions is 76.4 . There are no short contacts between ring centroids.
Contact distances and angles involving the heavy atoms are included in Tables 1 and 2. Some C-HÁ Á ÁBr and C-HÁ Á ÁI contacts are listed in the supporting information; these might be considered as borderline hydrogen bonds.

Database survey
Using version 2.0.5 of the CSD (Groom et al., 2016), a ConQuest search (Bruno et al., 2002) for bis(carbene)gold(I) cations gave 355 hits, with an average Au-C bond length of 2.023 Å . For Au III cations of the form [(carbene) 2 AuX 2 ] + (X = halogen), only 38 hits were recorded, and only six of these involved iodine as the halogen [refcodes: ANUJIE (Baron et al., 2016), CIVMOK (Jothibasu et al., 2008), MEZZOI (Gil-Rubio et al., 2013), POYHOB (Ghosh & Catalano, 2009), XOMFIR and XONCAH (Holthoff et al., 2019)]. XOMFIR presents a rare example of a non-cyclic carbene ligand. The average Au-C and Au-I bond lengths are 2.034 and 2.614 Å , respectively. The Au-C bond lengths of 3 and 4 may thus be considered normal, whereas the Au-I bond of 4 is longer than all those previously reported. It is tempting to suggest that this is associated with the halogen bonding, but MEZZOI and POYHOB also display short IÁ Á ÁI contacts (3.680 and 3.478 Å , respectively), while XONCAH has a short AuÁ Á ÁI contact of 3.438 Å . Short halogenÁ Á Áhalogen contacts between Au III species are relatively frequent; we recently drew attention to such contacts in AuCl 4 À and AuBr 4 À salts with protonated amine cations (Dö ring & Jones, 2016) but we did not include AuI 4 À salts because these are far more difficult to access.

Synthesis and crystallization
We have described the syntheses of compounds 1, 2 (Schmidt et al., 2017b) and 3 (Schmidt et al., 2017a) elsewhere, but give a brief summary here. The reagents were purchased from Sigma-Aldrich, Alfa Aesar or TCI and used without additional purification steps. All reactions were performed without precautions to exclude air or moisture. In the first step, 4-bromoimidazole was reacted with ethyl iodide in the presence of potassium carbonate to yield the bisalkylated imidazolium iodide (1) (Fig. 1). Compound 1 was then transformed in a two-step procedure by reaction with Ag 2 O and chlorido(dimethylsulfide)gold(I) to the gold(I) NHC complex 2. The biscarbene complex [(NHC) 2 Au] + I À (3) was obtained by further reaction of 2 with 1.
Single crystals of complex 3 were obtained by diffusion of nhexane into a solution of 3 in chloroform/deuterochloroform. A few crystals of the mixed-valence complex 4 also formed, for reasons that are not clear, and the compound was identified by X-ray analysis as reported here.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both structures, the methyl groups were refined as idealized rigid groups allowed to rotate but not tip (AFIX 137; C-H 0.98 Å , H-C-H 109.5 ). The   Packing diagram of compound 3 viewed perpendicular to (011). Hydrogen atoms are omitted. Dashed lines indicate halogen bonds or AuÁ Á ÁBr interactions. Atom labels correspond to the asymmetric unit methylene and NHC ring hydrogens were included using a riding model starting from calculated positions (C-H = 0.99 or 0.95 Å respectively). The U iso (H) values were fixed at 1.2 (for methylene groups) or 1.5 (for methyl groups) times the U eq value of the parent carbon atoms.
The asymmetric unit of 3 was chosen to include the short Br2Á Á ÁI1 contact. This means that the iodide lies outside the reference unit cell. Similarly, the asymmetric unit of 4 was chosen as a central I2 anion coordinated by two cations (Fig. 2). The long and narrow shape of this unit means that the centroid of the Au III cation does not lie within the reference cell. In both cases, this leads to a CheckCIF Alert G.
The large difference peaks close to Au2 and I2 of structure 4 may be a consequence of its moderate crystal quality (somewhat irregular and diffuse reflection shapes) and/or residual absorption errors. The peaks can of course be made smaller by cutting the data to a lower 2 max value, but we prefer not to do this because the mean I/(I) value at highest resolution (0.74-0.71 Å ) is still quite high at 8.4. Acta Cryst. (2021). E77, 1249-1252 research communications Computer programs: CrysAlis PRO (Rigaku OD, 2021), SHELXS (Sheldrick, 2008), SHELXL2017 (Sheldrick, 2015) and XP (Siemens, 1994). program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: Siemens XP (Siemens, 1994); software used to prepare material for publication: SHELXL2017 (Sheldrick, 2015).

Bis(4-bromo-1,3-diethylimidazol-2-ylidene)gold(I) iodide (3)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.53 e Å −3 Δρ min = −2.02 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.

ylidene)diiodidogold(III) diiodide (4)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 3.97 e Å −3 Δρ min = −2.66 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.

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