Recognition of SÂ·Â·Â·Cl Chalcogen Bonding in Metal-Bound

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Recognition of S•••Cl Chalcogen Bonding in Metal-bound Alkylthiocyanates Ekaterina S. Yandanova, Daniil M. Ivanov, Maxim L. Kuznetsov, Andrey G. Starikov, Galina L. Starova, and Vadim Yu. Kukushkin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00346 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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Crystal Growth & Design

Recognition of S•••Cl Chalcogen Bonding in Metal-bound Alkylthiocyanates Ekaterina S. Yandanova,a‡ Daniil M. Ivanov,a‡ Maxim L. Kuznetsov,b* Andrey G. Starikov,c Galina L. Starova,a and Vadim Yu. Kukushkina* a

Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab. 7/9, 199034 Saint Petersburg, Russian Federation b Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c Institute of Physical and Organic Chemistry at Southern Federal University, Stachka Av. 194/2, 344090 Rostov-on-Don, Russian Federation ABSTRACT: Reaction of K2[PtCl4] with excess AlkSCN in water gives the alkylthiocyanate complexes trans-[PtCl2(AlkSCN)2] (Alk = Et 1, nPr 2; 80–85%). These species were studied, in particular, by X-ray crystallography. In the solid state, both 1 and 2 exhibit the previously unreported S•••Cl chalcogen bonding, which consolidates the complexes into networks and leads to layered structures. Theoretical density functional theory (DFT) calculations and Bader’s atoms in molecules (AIM) analysis demonstrated two types of intermolecular interactions in tetramer (1)4, viz. the S•••Cl chalcogen and the H•••Cl hydrogen bonds. Despite each particular S•••Cl or H•••Cl bonding is weak with the estimated energy of 1–2 kcal/mol, altogether they play a crucial role in the stabilization of the S2Cl2 fragment in (1)4, the basic set superposition error corrected interaction energy being –12.8 kcal/mol per monomer complex molecule. The chalcogen bonding and the rhomboidal structure of the S2Cl2 fragment can be interpreted in terms of electrostatic arguments as a result of the interaction between the belt of negative electrostatic potential around the Cl atoms and the sulfur σ-holes. The natural bond orbital (NBO) analysis revealed that both LP(S) → LP*(Pt)/σ*(Pt–N)/σ*(Pt–Cl) and LP(Cl) → σ*(S–C) types of hyperconjugative charge transfers are important in the chalcogen bonding.

1. INTRODUCTION In the past decades, chemical crystallography in general and crystal engineering in particular allowed for a deeper and more subtle understanding of noncovalent weak interactions, their nature, and their roles. The noncovalent weak interactions such as, e.g., hydrogen bonds,1 halogen bonds,2 and metallophilic interactions3 are already well recognized as useful tools for crystal growth and design. Similarly to halogen bond,4 the chalcogen atom (Chal) may have an electropositive region at its outermost end (the socalled σ-hole)5 and it can interact with an electron donor forming a noncovalent chalcogen bond.6 Many types of the Chal•••D (where D is donor) contacts are known (for reviews see Refs.4,7 and for recent works see Refs.8-10), especially for the heavy chalcogens such as selenium and tellurium.4,7,9,11,12 Corresponding contacts for sulfur are substantially less common, but nevertheless the S•••O, S•••N, and S•••S weak interactions already found their applications in crystal engineering.8,10,11,13-15 Although Cl•••S contacts are vaguely studied, two types of such contacts were recognized. The first type includes the Cl•••S halogen bond, when σ-hole of chlorine interacts with lone pairs of sulfur (Chart 1).10 Another type is the S•••Cl chalcogen bond, when chlorine behaves as donor. The latter type of weak interaction is so far represented by only few examples.6,16-24 In particular, the S•••Cl short contacts were rec-

ognized between chloride ligand and sulfur(IV)21,22,24 in SCl3+ or in metal-coordinated SOCl2, between chlorine and sulfur in uncomplexed S2Cl216 and SOCl2,21 between chloride ligand and thioether20 or dithiocarbamate18 ligands in metal species, and between a chloride ligand in metal complexes and sulfur in heterocyclic rings.6,19,23 Noteworthy that the only one NCS– •••Cl contact involving anionic thiocyanate ligand was reported and it includes interaction between two dianions [Se2(µCl)2(SCN)2]2–.17 Chart 1. The Cl•••S halogen and S•••Cl chalcogen bonds.

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The alkylthiocyanate species, AlkSCN, may also form the NC–S•••D chalcogen bonds. Our inspection of CCDC data showed that 51 of 63 structures with AlkSCN fragments demonstrate such NC–S•••D short contacts, which can, in principle, be interpreted as chalcogen bonds. The high tendency of the alkylthiocyanate moiety to form the chalcogen bonds was also predicted by theoretical studies.25 However, only few NC–S•••F,26,27 NC–S•••O,28-30 and NC–S•••N28,31,32 short contacts were mentioned in the literature, and no NC(Alk)S•••Cl short contacts were noted. Herein we report on recognition of the S•••Cl chalcogen bonding found in the solid trans-[PtCl2(EtSCN)2] (1) and trans-[PtCl2(nPrSCN)2] (2) species, which consolidates the complexes into networks and leads to layered structures.

2. RESULTS AND DISCUSSION Structures of 1 and 2 in the solid state. Both 1 and 2 demonstrate square planar environment of the platinum atoms and trans-configuration of the ligands (Figure 1). The Pt–Cl, Pt–N, C–N, and S–CN bond lengths in both complexes agree well with each other (Table 1). However, orientation of the alkyl substituents, as expected, is different and minimal torsion angles Cl–Pt–S–CH2 for 1 [41.18(16)°] and 2 [74.5(5)°] are not equal due to packing and weak interactions effects (see next sections).

Figure 1. Molecular structures of 1 (top) and 2 (bottom) with the atomic numbering scheme. Hereinafter thermal ellipsoids are shown with the 50% probability.

Table 1. Selected bond lengths (Å) of 1 and 2. Bonds Pt1–Cl1 Pt1–N1 N1–C1 C1–S1

1 2.2990(5) 1.951(2) 1.145(3) 1.682(2)

2 2.301(4) 1.953(12) 1.132(18) 1.710(15)

Both 1 and 2 demonstrate the anisotropic packing. Crystals are formed by layers, and interactions between molecules in and out of these layers are different (for more information see SI). In 1, each molecule is linked with eight neighbors forming eight chalcogen bonds and four hydrogen bonds (Figure 2). Involvement of 1 in the net of short contacts affects solubility

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making this compound almost insoluble in common organic solvents and in water. Most likely the intramolecular interactions increase crystal lattice energy and, consequently, determine poor solubility of 1. By contrast to 1, complex 2 is sparingly soluble in CH2Cl2. A possible explanation of the difference in solubility can be found upon inspection of the environment of 1 and 2 in the solid state (Figure 3). Each molecule is surrounded by only four neighbors forming four chalcogen bonds and four hydrogen bonds. These interactions are considered to be weaker (see next section) than those in 1, which can be rationalized by both packing features and larger steric bulk of n-propyl than that of the ethyl substituents. Consequently, the weaker interactions between the molecules of 2 in the solid state lead to a higher solubility. Recognition of novel type chalcogen bonding. As indicated in Introduction, alkylthiocyanates are so far poorly studied as chalcogen bond donors and only short contacts of the alkylthiocyanate sulfur with fluorine,26,27 oxygen,28-30 and nitrogen28,31,32 were identified, whereas the NC–(Alk)S•••Cl short contacts were not previously reported. Meanwhile, the NC– S•••Cl–Se short contact were noted for S-coordinated anionic thiocyanate ligand.17 Complexes 1 and 2 were found to form previously unrecognized NC–(Alk)S•••Cl short contacts, which can be interpreted as chalcogen bonds (see theoretical consideration later). First type of the intermolecular chalcogen bond is realized in both 1 and 2, viz. the short contact between alkyl thiocyanate ligand from one molecule of complex and chloride ligand from another molecule (Figure 4). The S1•••Cl1 distances 3.2987(7) (1) or 3.471(5) Å (2) are shorter than sum of chlorine and sulfur Rowland’s33 vdW radii (3.57 Å). The values of angles of these contacts give an idea that chlorine atoms are donors (∠(S•••Cl–Pt) = 118.63(2)° in 1 and ∠(S•••Cl–Pt) = 114.50(15)° in 2) delivering their lone pairs, and sulfur atoms are acceptors (∠(C–S•••Cl) = 168.12(9)° in 1 and ∠(C–S•••Cl) = 161.3(6)° in 2) providing σ-holes (Table 2). All these issues were confirmed by theoretical calculations. In 2, each molecule of the complex forms pair of the NC–S•••Cl contacts with its neighbor. Another type of the chalcogen bond was identified only in 1 (Figure 5). These contacts are formed opposite to the S–Et covalent bonds (corresponding angle is 168.78(11)°) and it means ethyl thiocyanate moiety serves as a bifunctional chalcogen bond donor. The corresponding S1•••Cl1 distance in 2 (Figure 5) is longer (3.582(5) Å) than the sum of Rowland’s vdw radii (3.57 Å), and n-proryl thiocyanate moiety behaves as monofunctional chalcogen bond donor. In both cases, the C1•••Cl1 distances are less than the corresponding sum of vdW radii (3.345(5) and 3.314(15) vs. 3.53 Å), however, our calculations performed for 1 (see next section) did not show any weak interactions between these atoms apart from electrostatic. Besides chalcogen bonding, both structures exhibit hydrogen bonds between the polarized methylene groups S–CH2 of one molecule and a chloride ligand from another molecule (Figure 6); parameters of HB’s are compiled in Table 3. Although the geometric parameters of the S•••Cl short contacts reflect the availability of chalcogen bonding, we decided to confirm this assumption by DFT calculations to exclude packing effects and determine energy of corresponding weak interactions.


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Figure 2. The neighbors of one molecule in the packing of 1.

Figure 3. The neighbors of one molecule in the packing of 2.

Table 2. The characteristic parameters of the chalcogen bonds in 1 and 2. Compound 1

C–S•••Cl–Pt d(S•••Cl), Å RSCl* ∠(C–S•••Cl),° C1–S1•••Cl1–Pt1 3.2987(7) 0.92 168.12(9) C2–S1•••Cl1’–Pt1’ 3.4775(8) 0.97 168.78(11) C1–S1•••Cl1’–Pt1’ 3.471(5) 0.97 161.3(6) 2 Comparison** 3.57 1.00 180 * RSCl = d(Cl•••S)/(RvdW(S)+RvdW(Cl) **Comparison is the sum of Rowland’s vdW radii for distance and classic chalcogen bond values for angles.


∠(S•••Cl–Pt),°

118.63(2) 98.976(18) 114.50(15) 90


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Figure 6. The C–H•••Cl HB’s in 1 and 2. Figure 4. Comparison of the NC–S•••Cl short contacts in 1 and 2.

Figure 5. The Et–S•••Cl short contacts in 1 and the analogous fragment in 2.

Theoretical calculations. With aim to interpret additionally the bonding nature in 1, quantum chemical calculations of the isolated tetramer cluster (1)4 have been carried out without boundary constrains. The general structure of the crystal fragment was not changed significantly during the geometry optimization (Figure 7). The maximum difference between the main calculated and experimental bond lengths was found for the Pt–Cl bonds (0.06 Å) and does not exceed 0.02 Å for other bonds often falling within 3σ interval of the experimental data. The calculations indicate that the >S•••Cl–Pt distances in tetramer (1)4 (3.235–3.596 Å) are similar to those found by Xray crystallography in the solid state (3.299 and 3.477 Å), and three of them are lower than the sum of Rowland’s vdW radii of S and Cl (3.57 Å). The central S2Cl2 framework has a rhomboidal structure with the ∠(Cl•••S•••Cl) and ∠(S•••Cl•••S) angles of 114–115° and 64–66°, respectively, what is also close to the corresponding experimental values of 109° and 71°. The topological analysis of the electron density distribution (AIM) revealed the existence of the bond critical points (BCPs) associated with the S•••Cl contacts (Figure 8). The calculated ρ, ∇2ρ, and Hb values for these BCPs are 0.047– 0.083 e/Å3, 0.565–0.971 e/Å5, and 0.008–0.010 Hartree/Å3, respectively. The low magnitude of the electron density and the positive values of its Laplacian and the energy density at the BCPs are typical for halogen and chalcogen bondings5,34-37 and indicate that the interaction is weak.38,39 Meanwhile, the negative values of the second eigenvalue of the Hessian matrix λ2 (–0.001 to –0.008 a.u.) and of Sign(λ2)ρ (–0.007 to –0.012 a.u.) show that the interactions are binding and they fall into the van der Waals domain.40


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Table 3. The characteristic parameters of the H•••Cl HB’s.

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Compound 1 2

C–H•••Cl C2–H2B•••Cl1 C2–H2B•••Cl1 Comparison*

d(H•••Cl), Å 2.8475(10) 2.754(4) 2.86

d(C•••Cl), Å 3.498(5) 3.571(17) 3.53

∠(C–H•••Cl),°

125.2(3) 142.5(8) 120

*Comparison is the corresponding sums of Rowland’s vdW radii and minimal hydrogen bond angle.

Figure 7. Calculated equilibrium structure of tetramer (1)4 with the S•••Cl and H•••Cl short contacts indicated.

Figure 8. Contour line diagrams of the Laplacian distibution ∇2ρ(r), bond paths, zero flux surfaces (A) and atomic basin paths (B) projected on the S2Cl2 plane. The Cl and S atoms are shown by green and yellow, respectively, dashed red lines of the Laplacian indicate charge depletion (∇2ρ(r) > 0), solid blue lines indicate charge concentration (∇2ρ(r) < 0).

Additionally, BCP between two S atoms in the S2Cl2 fragment was found. However, low ρ, ∇2ρ, and Hb values (0.038 e/Å3, 0.441 e/Å5 and 0.007 Hartree/Å3) as well as the significant S•••S internuclear distance (3.683 Å) indicate that this interaction is much less important in the S2Cl2 fragment than the S•••Cl interactions. As was indicated in Introduction (Chart 1), two schemes of the Hal•••S bonding are usually considered in terms of electrostatic arguments, i.e. (i) the halogen bonding when the lone

electron pairs of the S atoms interact with the σ-hole of halogen and (ii) the chalcogen bonding when the halogen atom acts as a donor due to the presence of a belt with negative electrostatic potential (ESP) around halogen.5 Analysis of the ESP distribution in monomer (1)1 demonstrates that the chloride ligands in trans-[PtCl2(EtSCN)2] has both the σ-hole at the outermost end of Cl and the belt with negative ESP due to three lone electron pairs of the Cl atom (Figure 9, A). The most positive regions of ESP near the S atoms (sulfur σ-holes)



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are located at the extensions of the C–S bonds (Figure 9, B). However, there are no regions of the negative ESP at the lateral sides of the C–S–C fragments, and the presence of the CN group in the thiocyanate ligand is responsible for this effect. Indeed, in uncomplexed Me2S, ESP is clearly negative in the regions lateral to the C–S bonds owing to the lone electron pairs of S, whereas in uncomplexed methyl thiocyanate MeS– C≡N, ESP in those regions is already positive (Figure 9, C). Thus, from the electrostatic point of view, only the second type of interaction (i.e. the chalcogen bonding) is possible. This conclusion is confirmed by the structure of the S2Cl2 fragment in (1)4 with the ∠(S•••Cl–Pt) angles of 71–108° and the ∠trans-(C–S•••Cl) angles of 153–167°, the S•••Cl bond lines directing toward the belt with negative ESP at the Cl atoms. The charge transfer (CT) is another important factor determining the intermolecular interactions in (1)4. The natural bond orbital (NBO) analysis revealed two types of the intermolecular CTs in (1)4 involving the sulfur atoms. The first one is a charge transfer from the S atoms of the S2Cl2 fragment to the {PtClN2} moieties (Figure 10, A). The second-order perturbation theory analysis indicates that the intermolecular CT from LP(S) is mainly associated with the hyperconjugative transitions LP(S) → LP*(Pt), LP(S) → σ*(Pt–N) and LP(S) → σ*(Pt–Cl) with the total E(2) values of 10.7, 3.0, and 1.9 kcal/mol, respectively (Table 4). Correspondingly, the total occupancy of all LP(S) in both S atoms decreases by 92 me on going from monomer (1)1 to tetramer (1)4, while the total occupancies of the LP*(Pt), σ*(Pt–N) and σ*(Pt–Cl) NBOs increase by 28, 27, and 30 me, correspondingly (note that the main contribution in the intermolecular CT to the LP*(Pt), σ*(Pt–N) and σ*(Pt–Cl) NBOs comes from LP(S)). As a result of the charge transfer to the σ*(Pt–Cl) and σ*(Pt–N) NBOs, lengths of the Pt–Cl bonds interacting with S increase noticeably upon the tetramer formation (by 0.03 Å), whereas an elongation of the Pt–N bonds does not exceed 0.01 Å. The second type of CT in (1)4 is the back-donation from LP(Cl) to the σ*(S–C) NBOs with the total E(2) value of 6.0 kcal/mol (Figure 10, B). The overall occupancy of LP(Cl) drops by 55 me from (1)1 to (1)4 and the main reason of such a decrease is the LP(Cl) → σ*(S–C) transition. The total occupancy of the σ*(S–C) NBOs increases by 32 me. The resulting effect of these oppositely directed CTs leads to an enhancement of the effective atomic NBO charge on each S atom of the S2Cl2 moiety by 50 me upon formation of (1)4 from the monomers. Thus, the sulfur atoms predominantly act as electron donors in the intermolecular interactions in (1)4. Finally, the structure of tetramer (1)4 is additionally stabilized by six C–H•••Cl short contacts with the H•••Cl lengths of 2.618–2.781 Å (Figure 7). These values are lower than the sum of Rowland’s vdW radii for the H and Cl atoms (2.86 Å). The AIM analysis shows the presence of BCPs for the H•••Cl contacts. The calculated ρ, ∇2ρ, and Hb values for these BCPs are 0.060–0.079 e/Å3, 0.654–0.901 e/Å5, and 0.009–0.011 Hartree/Å3, respectively, and they are typical for weak Hbonds.41,42 The total E(2) energy of the LP(Cl) → σ*(C–H) charge transfers responsible for this type of interactions is 5.4 kcal/mol. This CT adds to the effect of depopulation of LP(Cl) mentioned above.

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Figure 9. ESP distribution in (1)1 (A, B), Me2S, and MeSC≡N (C).

Table 4. Electron density (ρ ρ, in e/Å3), Laplacian (∇ ∇2ρ, in 5 3 e/Å ) and energy density (Hb, in Hartree/Å ) at the S•••Cl and H•••Cl BCPs, second-order perturbation energies of the intermolecular CTs (E(2), in kcal/mol) and change of the occupancies of selected NBOs (∆ ∆occ., in me).a ρ (S•••Cl/H•••Cl) ∇2ρ (S•••Cl/H•••Cl) Hb (S•••Cl/H•••Cl) E(2) LP(S) → LP*(Pt) E(2) LP(S) → σ*(Pt–N) E(2) LP(S) → σ*(Pt–Cl) E(2) LP(Cl) → σ*(S–C) E(2) LP(Cl) → σ*(C–H) ∆occ. LP(S) ∆occ. LP*(Pt) ∆occ. σ*(Pt–N) ∆occ. σ*(Pt–Cl) ∆occ. LP(Cl) ∆occ. σ*(S–C) a

(0.047–0.083)/(0.060–0.079) (0.565–0.971)/(0.654–0.901) (0.008–0.010)/(0.009–0.011) 10.7 3.0 1.9 6.0 5.4 –92 28 27 30 –55 32

Total E(2) and ∆occ. values for all NBOs involved in the discussed CTs are provided.


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Crystal Growth & Design Pt

N Pt

N

N

N Cl

Cl S ..

S ..

A Cl N

Pt

N

. . Cl C

S C B

Figure 10. The main types of intermolecular charge transfer in (1)4 involving the S atoms.

The overall adiabatic BSSE corrected interaction energy of four monomers to give (1)4 is –51.2 kcal/mol. This corresponds to the value of –12.8 kcal/mol per complex molecule and indicates a considerable stabilization of the system upon formation of (1)4 from monomers. Apparently, each interaction described above should contribute to this energy. The binding energies of the individual S•••Cl and H•••Cl contacts may be approximately estimated using of formula proposed by Espinosa et al.:43 EHB = –½V(rb), where V(rb) is potential energy density at a BCP. The calculated EHB values for the S•••Cl and H•••Cl contacts are 1.1–2.3 and 1.3–1.9 kcal/mol, respectively, confirming that these interactions are weak and comparable in strength. Both S•••Cl and H•••Cl interactions play a crucial role in the stabilization of the crystal structure. Indeed, the geometry optimization of the model system (trans-[PtCl2(MeCN)2]•Me2S)2 initially bearing the same S2Cl2 fragment but without the H•••Cl contacts resulted in the collapse of the S2Cl2 framework (Figure 3S, A, Supporting Information). Similarly, the structure of the tetramer (trans-[PtCl2(MeCN)2])4 initially corresponding to that of (1)4, but without any S atoms collapsed as a result of the optimization (Figure 3S, B). The NC–S•••Cl short contacts verified upon inspection of CCDC data. We processed available CCDC data and found only nine structures (Table 5) displaying short contacts (less than RvdW(Cl)+ RvdW(S) = 3.57 Å) between Cl atoms and the thiocyanate sulfur in both alkylthiocyanates (four structures) or in S-bound thiocyanate metal and selenium(II) species (six structures). Only in one study this short contact was indicated, whereas in the other eight reports the bonding between Cl and S was not even noted.17 Interestingly that in most cases values of corresponding ∠(C–S•••Cl) angles are in range 150–180°, and ∠(S•••Cl–RCl) are close to 90–140°, and these short contacts can also be interpreted as chalcogen bonding.

sources and used as received. The high resolution ESI massspectra were measured on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source; CH2Cl2 or MeOH were used as the solvents. Infrared spectra (4000–400 cm–1) were recorded on a Shimadzu FTIR 8400S instrument in KBr pellets. 1H, 13C{1H}, and 195Pt{1H} NMR spectra of the complexes in CDCl3 were acquired on a Bruker 400 MHz Avance spectrometer (400.130 MHz for 1H NMR, 100.613 MHz for 13C{1H} NMR and 86.015 MHz for 195Pt{1H} NMR) at ambient temperature. 195Pt{1H} chemical shifts are given relative to aqueous K2[PtCl4] = –1630 ppm. Computational Details. The full geometry optimization of (1)4 and the model structures has been carried out with the help of the Gaussian-0945 program package at the DFT level of theory using the B3LYP,46,47 CAM-B3LYP,48 and M06-2X49 functionals and the def2-TZVP basis set as implemented in Gaussian. Results obtained with the M06-2X functional are in the best agreement with the experimental X-ray data and they are discussed in the text. No symmetry operations have been applied. The Hessian matrix was calculated analytically in order to prove the location of correct minima (no imaginary frequencies). The topological analysis of the electron density distribution with the help of the AIM method of Bader50 was performed using the programs Multiwfn51 and AIMAll.52 The atomic charges and bond orbital nature were analyzed by using the natural bond orbital (NBO) partitioning scheme.53 The adiabatic energy of the tetramer (1)4 formation from monomers was calculated with the counterpoise estimates of the basis set superposition error (BSSE)54,55 for (1)4 with the geometry relaxation. Synthesis of trans-[PtCl2(AlkSCN)2]. AlkSCN (0.730 mmol) was added to a solution of K2[PtCl4] (0.100 g, 0.241 mmol) in water (1 mL). Reaction mixture was stirred for 3 d at 20–25 °C. The yellow amorphous precipitate formed was filtered off, washed with three 3-mL portions of water and one 3mL portion of diethyl ether, and dried in air at RT. Yields 80– 85%. 1. Found: C, 16.53; H, 2.41; N, 6.48 (Calc. for C6H10N2Cl2PtS2: C, 16.37; H, 2.29; N, 6.36); m/z (HRESI+) 462.9174 ([M + Na]+, requires 462.9180); IR (KBr, selected bands, cm–1): 2966 m, 2925 m, (C–H), 2223 m, (C≡N); δH: 1.59 (3H, t, CH3), 2.15 (2H, q, CH2); δC: 15.65 (CH3), 30.10(CH2), and 112.13 (NCS); δPt: –2280.73. Crystals of 1 suitable for X-ray study were obtained by the slow evaporation of a dichloromethane–hexane solution of the complex at room temperature in air. 2. Found: C, 20.63; H, 3.09; N, 6.05 (Calc. for C8H14N2Cl2PtS2: C, 20.52; H, 3.01; N, 5.98); m/z (HRESI+) 490.9498 ([M + Na]+, requires 490.9494); TLC, Rf 0.63 (eluent CHCl2); IR (KBr, selected bands, cm–1): 2960 m, 2926 m, 2870 m, (C–H), 2227 m, (C≡N); δH: 1.09 (3H, t, CH3), 1.87– 1.97 (2H, m, CH2), 3.12 (2H, t, CH2); δC: 12.74 (CH3), 23.57 (CH2), 37.35 (CH2), and 113.21 (NCS); δPt: –2281.79. Crystals of 2 suitable for X-ray study were obtained by the slow evaporation of a dichloromethane–diethyl ether solution of the complex at room temperature in air.

3. EXPERIMENTAL SECTION Materials and Instrumentation. EtSCN was purchased from Aldrich, whereas nPrSCN was prepared in accord with the published method,44 but using propyl bromide instead of propyl iodide. Solvents were obtained from commercial



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Table 5. The characteristic parameters of the chalcogen bonds obtained in this work by processing available CCDC data.

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RS N

C

S

Cl RCl

Structure CEWRAY CEWREC JAJTAQ JASHUH MURXIG MURXUS NEKTEC RIWKAL UCEYUX YARNIP

RS RCl* Alk – Alk AuIII HgII HgII Alk Ar SeII SeII SeII SeII IV Pt PtIV 2 Csp Alk HgII HgII HgII Ar Comparison**

d(S•••Cl), Å 3.2418(9) 3.3141(12) 3.508(2) 3.5436(15) 3.3748(13) 3.5317(12) 3.318(4) 3.566(2) 3.449(2) 3.341(3) 3.57

∠(C–S•••Cl),°

∠(S•••Cl–RCl),°

174.03(7) 160.81(15) 164.9(2) 163.80(16) 151.92(13) 151.91(13) 80.6(5) 80.9(2) 167.1(2) 157.55(19) 180

– 135.09(4) 92.62(5) 105.25(15) 123.58(3) 114.75(3) 155.64(14) 81.5(2) 142.38(3) 99.6(2) 90

*If none, free chloride anion is a donor. **Comparison is the sum of Rowland’s vdW radii and classic chalcogen bond angles.

XRD experiments and refinement. Suitable crystals of 1 and 2 were selected and fixed on a micro mount and placed on a SuperNova, Dual, Cu at zero, Atlas (1) or on a Agilent Technologies Excalibur Eos (2) diffractometer and measured at a temperature of 100K using monochromated CuKα (1) or MoKα (2) radiation. Using Olex2,56 the structure was solved with the ShelXS57 structure solution program using Direct Methods and refined with the ShelXL57 refinement package using Least Squares minimization. The carbon-bound H atoms were placed in calculated positions and were included in the refinement in the ‘riding’ model approximation, with C–H 0.96 Å for the CH3 groups and Uiso(H) set to 1.2Ueq(C) and C– H 0.97 Å for the CH2 groups. Empirical absorption correction was applied in CrysAlisPro58 program complex using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. High values of the refinement parameters and rather low bonds precision in the structural model of 2 are due to the small size and low quality of the crystals. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

vides a significant stabilization of (1)4, the interaction energy being –12.8 kcal/mol per monomer complex molecule. The chalcogen bonding is interpreted in terms of electrostatic arguments (as a result of the interaction between the belt of negative EPS around the Cl atoms and the sulfur σ-holes) and charge transfer arguments, both LP(S) → LP*(Pt)/σ*(Pt– N)/σ*(Pt–Cl) and LP(Cl) → σ*(S–C) types of CT playing an important role in the intermolecular S•••Cl interactions. The observation of NC(Alk)S•••Cl short contacts indicates that the available CCDC data for metal-bound alkylthiocyanates may potentially require additional attention (in particular, theoretical calculations) to address the possibility of the chalcogen bonding.

ASSOCIATED CONTENT Supporting Information. Views of 1 and 2 demonstrating their layered structures in the solid state. Additional details for the model systems (trans-[PtCl2(MeCN)2]•Me2S)2 and (trans[PtCl2(MeCN)2])4. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

4. CONCLUSION Although the ability of the alkylthiocyanate moiety to form the chalcogen bonds was predicted by a theoretical study almost a decade ago,25 only few NC–S•••F,26,27 NC–S•••O,28-30 and NC–S•••N28,31,32 short contacts were observed and no NC(Alk)S•••Cl weak interactions were reported. In this work, we synthesized the alkylthiocyanate complexes trans[PtCl2(AlkSCN)2] (Alk = Et, nPr) and in their solid state structures we found the chalcogen bond between the thiocyanate S atom and a chloride ligand from the neighboring molecule. Our data provide the first experimental evidence of the NC(Alk)S•••Cl chalcogen bonding. Theoretical DFT calculations and topological analysis of the electron density distribution (AIM) revealed the existence of two types of intermolecular interactions in the tetrameric cluster (1)4, viz. the S•••Cl chalcogen and the H•••Cl hydrogen bonds. Each of these interactions is weak with the estimated energies of 1–2 kcal/mol. However, their overall effect pro-

Corresponding Authors *email: [email protected] and [email protected]

Author Contributions All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT Financial support from the Russian Science Foundation (project 14-43-00017) is gratefully acknowledged. Physicochemical studies were performed at the Magnetic Resonance Research Center, Center for X-ray Diffraction Studies, and Center for Chemical Analysis and Materials Research (all belong to Saint Petersburg State University).

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For Table of Contents Use Only Recognition of S•••Cl Chalcogen Bonding in Metal-bound Alkylthiocyanates Ekaterina S. Yandanova, Daniil M. Ivanov, Galina L. Starova, and Vadim Yu. Kukushkin*

Maxim

L.

Kuznetsov,*

Andrey

G.

Starikov,

Two platinum(II) alkylthiocyanate complexes trans-[PtCl2(AlkSCN)2] (Alk = Et, nPr) demonstrate nets of the S•••Cl chalcogen bonding in the solid state. The nature of weak interactions was investigated by XRD experiments and theoretical calculations.


Recognition of SÂ·Â·Â·Cl Chalcogen Bonding in Metal-Bound

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