Tetrabromoethane as σ -Hole Donor toward Bromide Ligands: Halogen Bonding between C 2 H 2 Br 4 and Bromide Dialkylcyanamide Platinum(II) Complexes

: The complexes trans -[PtBr 2 (NCNR 2 ) 2 ] (R 2 = Me 2 1 , (CH 2 ) 5 2 ) were cocrystallized with 1,1,2,2-tetrabromoethane ( tbe ) in CH 2 Cl 2 forming solvates 1 · tbe and 2 · tbe , respectively. In both solvates, tbe involved halogen bonding, viz. the C–Br ··· Br–Pt interactions, were detected by single-crystal X-ray diffractions experiments. Appropriate density functional theory calculations (M06/def2-TZVP) performed for isolated molecules and complex- tbe clusters, where the existence of the interactions and their noncovalent nature were conﬁrmed by electrostatic potential surfaces ( ρ = 0.001 a.u.) for isolated molecules, topology analysis of electron density, electron localization function and HOMO-LUMO overlap projections for clusters. ESP calculations show the formation of and is


Introduction
In the past decade, the halogen bonding (XB) concept [1] has attracted a considerable attention as a new type of intermolecular interaction and has now become an important tool for XB involving crystal engineering. The expressed directionality of XB was subsequently used in biology and materials sciences to create functional systems with a wide range of applications spanning from supramolecular chemistry, to crystal engineering, catalysis, electrochemistry, etc. [2][3][4][5][6]. Although the vast majority of XB studies do not utilize organometallic building blocks, these metal-containing species functioning as XB participants are very useful for the design of new supramolecular systems [7][8][9][10].
We report herein our data on cocrystallization of tbe with dialkylcyanamide bromide complexes of platinum (II). In 1:1 solvates 1·tbe and 2·tbe, the first examples of tbe involved XBs and hydrogen bonds (HBs) were found. We report herein our data on cocrystallization of tbe with dialkylcyanamide complexes of platinum (II). In 1:1 solvates 1·tbe and 2·tbe, the first exampl involved XBs and hydrogen bonds (HBs) were found.

Synthesis of Complex Trans-[PtBr2(NCN(CH2)5)2]
To K2[PtCl4] (0.1 g, 0.24 mmol) in water (1 mL) added a 10-fold excess of K g, 2.4 mmol), whereupon the solution was heated with mixing for 2 h at 70 heating the solution became maroon. A 5-fold excess of NCNC5H10 (0.140 mL, 1 was added to this solution. Oil formed at the bottom of the vessel after 24 h. The oil was powdered with diethyl ether and held under ultrasound. The resulting p (a mixture of cis-and trans-isomers) was washed with three portions of 3 mL of w diethyl ether, and then dried in air at room temperature. To isolate the pure tran the resulting mixture was dissolved in CHCl3 and was refluxed for 3 h. Yield NMR (400 MHz, CDCl3) δ = 3.32 (m, NCH2), 1.62-1.67 (m, NCH2CH2) and 1.52 NCH2CH2CH2) ppm ( Figure S5). 13 Figure S4). Single crystals of 2 were prepare dichloromethane at RT by slow evaporation ( Figure S1). Comparison of the PXRD data showed that the phases coincide ( Figure S8).

Crystallization
Single crystals of 1·tbe and 2·tbe were obtained by slow evaporat dichloromethane solution (1 mL) of a mixture of the corresponding 1 (0.002 m (0.002 mmol) and tbe taken in an excess (0.87 mmol) at RT. Yellow crystals 2, 2·tbe of suitable for XRD were released after 3−4 d.

Synthesis of Complex Trans-[PtBr 2 (NCN(CH 2 ) 5 ) 2 ]
To K 2 [PtCl 4 ] (0.1 g, 0.24 mmol) in water (1 mL) added a 10-fold excess of KBr (0.284 g, 2.4 mmol), whereupon the solution was heated with mixing for 2 h at 70 • C. After heating the solution became maroon. A 5-fold excess of NCNC 5 H 10 (0.140 mL, 1.2 mmol) was added to this solution. Oil formed at the bottom of the vessel after 24 h. The resulting oil was powdered with diethyl ether and held under ultrasound. The resulting precipitate (a mixture of cisand trans-isomers) was washed with three portions of 3 mL of water and diethyl ether, and then dried in air at room temperature. To isolate the pure trans-isomer, the resulting mixture was dissolved in CHCl 3 and was refluxed for 3 h. Yield: 71%. 1 H NMR (400 MHz, CDCl 3 ) δ = 3.32 (m, NCH 2 ), 1.62-1.67 (m, NCH 2 CH 2 ) and 1.52-1.59 (m, NCH 2 CH 2 CH 2 ) ppm ( Figure S5). 13 Figure S4). Single crystals of 2 were prepared from a dichloromethane at RT by slow evaporation ( Figure S1). Comparison of the XRD and PXRD data showed that the phases coincide ( Figure S8).

Crystallization
Single crystals of 1·tbe and 2·tbe were obtained by slow evaporation of a dichloromethane solution (1 mL) of a mixture of the corresponding 1 (0.002 mmol) or 2 (0.002 mmol) and tbe taken in an excess (0.87 mmol) at RT. Yellow crystals 2, 1·tbe and 2·tbe of suitable for XRD were released after 3−4 d.

Analytic Methods
The MS data were obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source. The NMR spectra were recorded on a Bruker AVANCE III 400 spectrometer at ambient temperature in CDCl 3 (at 400, 101, 86 MHz for 1 H, 13 C{ 1 H} and 195 Pt NMR spectra, respectively). IR spectra were recorded on a Bruker TENSOR 27 FT-IR spectrometer (4000-200 cm −1 , KBr pellets) ( Figure S3). Powder X-ray diffraction (PXRD) data were measured at room temperature using a Bruker D2 Phaser Desktop X-ray diffractometer equipped with a CuKα1 + 2 source; the data were collected in the range of 2θ = 5-80 • with a step size of 0.02 • (2θ).

X-ray Structure Determination and Refinement
Suitable single crystals were studied on a SuperNova Duo CCD diffractometer (Cu Kα (λ = 1.54184), mirror monochromator, ω-scan). Crystals were incubated at 100 K during data collection. All structures were deciphered by direct methods using SHELXT [30] and refined using SHELXL [31]. All non-hydrogen atoms were refined with individual parameters of anisotropic displacement. Hydrogen atoms in all structures are placed in ideal calculated positions and refined as colliding atoms with parameters of relative isotropic displacement. The main data of crystallography and details of refinement are given in Table S1 in Supporting Information. CCDC numbers 2094282-2094284 contain all supporting structural and refinement data.

Computational Details
The energy characteristics of the complexes, tbe and clusters included in this study were calculated by the DFT method with the M06/def2-TZVP [32,33] theory using atom coordinates obtained from crystal structures. The GAUSSIAN-09 [34] program was used for calculations. The MEP surfaces [35] were calculated at the same theoretical level and presented using 0.001 a.u. isosurfaces. The color scheme is a red-white-blue scale with red for ρ+ cut (repulsive) and blue for ρ− cut (attractive). White isosurfaces correspond to weakly repulsive and attractive interactions, respectively. 3D-surfaces were visualized using the VMD 1.9.3 [36] program. ELF projections and QTAIM analysis was performed in Multiwfn 3.7 [37,38] software. The QTAIM analysis was performed using the program at the same level of theory. Visualization of the projections of boundary orbitals was carried out using the program Multiwfn 3.7.

Electrostatic Surface Potentials
Electrostatic potentials (ESP) on surface (ρ = 0.001 a.u.) were calculated (M06/def2-TZVP) for tbe, 1 and 2 isolated molecules. Donor XB has a σ-hole on the surface of the bromine atom, which we see for tbe (positive potential, Figure 2). XB acceptors are nucleophiles, and bromide ligands demonstrate significant negative potential on all sides ( Figure 3). The scale was selected in such a way as to convey as much information as possible about the distribution of the electrostatic potential in the molecules [39,40]. Thus, the ESP calculations show that the formation of XB and HB is promising for their joint crystallization.
given in Table S1 in Supporting Information. CCDC numbers 2094282supporting structural and refinement data.

Computational Details
The energy characteristics of the complexes, tbe and clusters inc were calculated by the DFT method with the M06/def2-TZVP [32,33] coordinates obtained from crystal structures. The GAUSSIAN-09 [34] for calculations. The MEP surfaces [35] were calculated at the same t presented using 0.001 a.u. isosurfaces. The color scheme is a red-white for ρ+ cut (repulsive) and blue for ρ− cut (attractive). White isosurf weakly repulsive and attractive interactions, respectively. 3D-surfac using the VMD 1.9.3 [36] program. ELF projections and QTAIM analy in Multiwfn 3.7 [37,38] software. The QTAIM analysis was performed at the same level of theory. Visualization of the projections of bou carried out using the program Multiwfn 3.7.

Electrostatic Surface Potentials
Electrostatic potentials (ESP) on surface (ρ = 0.001 a.u.) were cal TZVP) for tbe, 1 and 2 isolated molecules. Donor XB has a σ-hole on bromine atom, which we see for tbe (positive potential, Figure 2) nucleophiles, and bromide ligands demonstrate significant negative p ( Figure 3). The scale was selected in such a way as to convey as m possible about the distribution of the electrostatic potential in the mole the ESP calculations show that the formation of XB and HB is prom crystallization.

Single-Crystal X-Ray Diffraction Data for Solvates
The structure of 1·tbe consists of one molecule of the complex trans-[PtBr2(NCNMe2)2] and one molecule of tbe; the same is observed in the structure of adduct 2·tbe. In both cases, the complex molecule is surrounded by 4 tbe molecules with the formation of bromine-bromine short contacts.
In 1·tbe, when the halogen bonds are stronger, a significant (within 3σ) elongation of the Pt-Br coordination bonds (2.4340(6) Å and 2.4555(9) Å) is observed, which can be explained by the redistribution of the electron density during the formation of these halogen bonds. In 2·tbe, there is no difference within 3σ in the isolated molecule (2.4382(6) Å) and in the solvate (2.4357(4) Å). However, there is a difference in the piperidyl conformations in the solvent-free crystal 2 and in 2·tbe, which can be explained by the crystal packing effects.

Single-Crystal X-ray Diffraction Data for Solvates
The structure of 1·tbe consists of one molecule of the complex trans-[PtBr 2 (NCNMe 2 ) 2 ] and one molecule of tbe; the same is observed in the structure of adduct 2·tbe. In both cases, the complex molecule is surrounded by 4 tbe molecules with the formation of bromine-bromine short contacts.

Theoretical Consideration
By analogy with the formation of two XBs with a bromide ligand, the formation of two HBs with sterically available oxygen in the composition of phosphine oxide is possible, which shows the general behavior between different supramolecular synthons [43].
The formation of XBs leads to the 2D supramolecular layers in both cases ( Figure S2). In 1·tbe, the additional Br 2 CHCBr 2 -H···Br-Pt hydrogen bonds were detected ( Figure S9 and Table S2). The hydrogen bonds together with XBs allow the 3D scaffold supramolecular structure of 1·tbe.

Theoretical Consideration
In addition to ESP surfaces, the presence and nature of intermolecular interactions were studied using the complex·tbe clusters with each type (see Supplementary Materials for details) of the interactions under consideration. The calculation (M06/def2-TZVP) was carried out on the experimentally obtained atomic coordinates. To obtain more information about the weak interactions under study, a topological analysis of the electron density distribution was carried out within the framework of the Bader QTAIM method [44,45].
The QTAIM analysis demonstrates the presence of a bond critical point (3, −1) (BCP) between the bromine and the bromide ligand (Table 2), as well as between the bromine in tbe and the hydrogen atoms in the complexes. Negative [46] and small values of sign(λ 2 )ρ at the BCPs confirm the attractive and noncovalent nature of the interactions. They can also be treated as typically noncovalent due to close to zero positive energy density (0.001-0.002 a.u.) and the balance of the Lagrangian kinetic energy G(r) and the potential energy density V(r) (−G(r)/V(r) > 1) on the corresponding BCPs. The same calculations for the same clusters were performed in natural atomic partitioning scheme. The sums of natural population analysis (NPA) atomic charges are negative on the tbe molecule in each cluster, so the interaction between halogens also occurs due to the charge transfer from the complex molecules to tbe. Wiberg bond indices in NPA can be interpreted as chemical bond indices (orders). In all clusters, the Wiberg bond indices of the Br···Br XBs are very small but not zero (0.02-0.04) which point to the small covalent contribution to these interactions.
The ELF is a derivative of the electron density, which allows the location of areas of shared and unshared electron pairs [18,[47][48][49]. A combination of ELF and QTAIM methods is represented in Figure 5, where ELF projections were plotted together with bond (3, −1) critical points (BCPs, blue), nuclear (3, −3) critical points (NCPs, brown), ring (3, +1) critical points (RCPs, orange), and bond paths (white lines). Intermolecular halogen bonds are always directed towards their bromide ligands should be noted that the HOMO's of the complex are located mainly on bromide ligan The superposition of the boundary orbitals of donors and acceptors of halogen bonds the crystal structures of their adducts demonstrates that all associates exhi HOMO/LUMO overlaps ( Figure 6) [12,16,50]. To construct the projections, HOMO-1 w taken, since the lone pairs of the bromine atom are in the plane of the complex, and HOMO they are perpendicular to the plane. HOMO-LUMO overlap projections were bu along with the construction of connection paths. This suggests that the arrangement Br···Br contacts is associated with molecular orbital interactions between donors a acceptors of halogen bonds, which indicates the importance of the covalent component In (1)·(tbe) clusters, the Br···Br bond paths (top on Figure 5) pass through the lone pairs of bromide and through the depletion ELF areas of the on bromine atoms in tbe. These observations confirm the XB nature of the Br···Br interactions, where bromide ligands are nucleophiles and tbe molecules are electrophiles.
The same observations were performed for (2)·(tbe) clusters (bottom on Figure 5) halogen-bonding bond paths go through lone pairs on the halide ligands and the σ-holes on Br atoms in XB donors.
It is noteworthy that, in the case of these clusters the ELF regions for halogen bonding Br1A···Br1 and Br2A···Br1, the critical points of bonds and the arrangement of bond paths are the same for both clusters, which indicates the similarity in the nature of noncovalent interactions.
Intermolecular halogen bonds are always directed towards their bromide ligands. It should be noted that the HOMO's of the complex are located mainly on bromide ligands. The superposition of the boundary orbitals of donors and acceptors of halogen bonds on the crystal structures of their adducts demonstrates that all associates exhibit HOMO/LUMO overlaps ( Figure 6) [12,16,50]. To construct the projections, HOMO-1 was taken, since the lone pairs of the bromine atom are in the plane of the complex, and for HOMO they are perpendicular to the plane. HOMO-LUMO overlap projections were built along with the construction of connection paths. This suggests that the arrangement of Br···Br contacts is associated with molecular orbital interactions between donors and acceptors of halogen bonds, which indicates the importance of the covalent component in the binding of halogens.

Conclusions
In this work, we firstly found the formation of XBs with 1,1,2,2-tetrabromoethane as XB donor, which were shown in the formation of 1:1 solvates with platinum(II) bromide dialkylcyanamide complexes. The nature of halogen bonds was investigated experimentally by single-crystal X-ray diffraction analysis of the solvates. Further theoretical calculations, including topological analysis of electron density, ESP surfaces, ELF projections, HOMO/LUMO overlaps, Wiberg bond indices and natural population charge analysis, confirmed tetrabromoethane is indeed an electrophile toward bromide ligand due to the presence of σ-holes on bromine atoms. As other bromoalkanes, it can be used as electrophilic supramolecular synthon (donor of four σ-holes) together with bromide complexes. Our inspection of literature data for substances with the general formula RCHBr2 show that they may also be studied as potential XB donors [51][52][53][54][55][56], among dibromomethane, bromoform and 1,1,2,2-tetrabromoethane.

Conclusions
In this work, we firstly found the formation of XBs with 1,1,2,2-tetrabromoethane as XB donor, which were shown in the formation of 1:1 solvates with platinum(II) bromide dialkylcyanamide complexes. The nature of halogen bonds was investigated experimentally by single-crystal X-ray diffraction analysis of the solvates. Further theoretical calculations, including topological analysis of electron density, ESP surfaces, ELF projections, HOMO/LUMO overlaps, Wiberg bond indices and natural population charge analysis, confirmed tetrabromoethane is indeed an electrophile toward bromide ligand due to the presence of σ-holes on bromine atoms. As other bromoalkanes, it can be used as electrophilic supramolecular synthon (donor of four σ-holes) together with bromide complexes.