Synthesis and Structure of Methylsulfanyl Derivatives of Nickel Bis(Dicarbollide) †

Symmetrically and unsymmetrically substituted methylsulfanyl derivatives of nickel(III) bis(dicarbollide) (Bu4N)[8,8′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2], (Bu4N)[4,4′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2], and (Bu4N)[4,7′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2] were synthesized, starting from [Ni(acac)2]3 and the corresponding methylsulfanyl derivatives of nido-carborane (Bu4N)[10-MeS-7,8-C2B9H11] and (Bu4N)[10-MeS-7,8-C2B9H11]. Structures of the synthesized metallacarboranes were studied by single-crystal X-ray diffraction and quantum chemical calculations. The symmetrically substituted 8,8′-isomer adopts transoid conformation stabilized by two pairs of intramolecular C–H···S hydrogen bonds between the dicarbollide ligands. The unsymmetrically substituted 4,7′-isomer adopts gauche conformation, which is stabilized by two nonequivalent C–H···S hydrogen bonds and one short chalcogen B–H···S bond (2.53 Å, −1.4 kcal/mol). The gauche conformation was found to be also preferred for the 4,7′-isomer.


Introduction
Molecular switches are molecules or supramolecular assemblies that can exist in two or more stable states that differ in the mutual orientation of the components and which can be transformed from one state to another by means of various external stimuli via rotation of these components relative to each other [1]. Molecular switches are the basic structural elements of any molecular electronic devices, in particular, molecular logic gates, which are capable of performing all the simplest logical and arithmetic operations, such as s summation and subtraction, encoding and decoding functions, and so on [2][3][4][5][6].
Fifteen years ago, nickel bis(dicarbollides) [3,3 -Ni(1,2-C 2 B 9 H 11 ) 2 ] −1/0 were proposed as the main construction element of molecular switches in which switching between two stable forms is achieved by reversible change of the nickel oxidation degree [7,8]. The cisoid conformation with the dicarbollide ligands turned by~36 degree relative each other rotation angle is most favorable for the nickel(IV) bis(dicarbollide) complex, whereas the transoid conformation with ligand rotation angle 180 degree is preferred for nickel(III) bis(dicarbollide) [7][8][9][10]. Somewhat later, several such molecular switches were Molecules 2019, 24, 4449 2 of 12 synthesized [11,12]; however, it was found that the difference in energies between the transoid and gauche rotamers in the synthesized nickel(III) complexes are negligibly low (~0.1 kcal/mol) [11]. In addition, it was found that nickel(IV) bis(dicarbollide) complexes are unstable in a number of solvents and are easily reduced to the corresponding nickel(III) complexes [13,14]. Thus, nickel bis(dicarbollide) complexes cannot be used to create effective electrochemically controlled molecular switches.
An alternative option involves the use of transition metal bis(dicarbollide) derivatives as structural elements in design of the complexation-driven molecular switches [15]. Additional stabilization of certain rotamers can be achieved by an introduction of appropriate substituents in the upper belt of the dicarbollide ligand, which results in the appearance of stabilizing interactions between the ligands. For instance, the introduction of iodine atoms to positions 8 and 8 of the cobalt bis(dicarbollide) anion increases the energy difference between the transoid and gauche and the transoid and cisoid rotamers of [8,8 -I 2 -3,3 -Co(1,2-C 2 B 9 H 10 ) 2 ] − in comparison with the parent anion from 0.4 to 4.7 kcal/mol and from 2.5 to 15.7 kcal/mol, respectively, due to intramolecular C-H···I-B hydrogen bonding between the ligands [16,17]. The bistability of molecular switches can be achieved by using substituents that are capable to form weak intramolecular hydrogen bonds between the dicarbollide ligands in the transoid conformation and stronger dative bonds with external transition metals in the cisoid conformation. Indeed, we found that the methylsulfanyl groups are able to stabilize the transoid conformation of the corresponding derivatives of cobalt and iron bis(dicarbollides) [8, 2 -3,3 -M(1,2-C 2 B 9 H 10 ) 2 ] − (M = Co, Fe) due to formation of intramolecular C-H···S(Me)B hydrogen bonds [18,19]. On the other hand, in the presence of external transition metals or labile metal complexes, the methylsulfanyl groups form complexes with them, resulting in transformation of the transoid conformation of cobalt bis(dicarbollide) to the cisoid one [20,21].
In this contribution, we describe the synthesis and structure of the methylsulfanyl derivatives of nickel bis(dicarbollide).
An alternative option involves the use of transition metal bis(dicarbollide) derivatives as structural elements in design of the complexation-driven molecular switches [15]. Additional stabilization of certain rotamers can be achieved by an introduction of appropriate substituents in the upper belt of the dicarbollide ligand, which results in the appearance of stabilizing interactions between the ligands. For instance, the introduction of iodine atoms to positions 8 and 8′ of the cobalt bis(dicarbollide) anion increases the energy difference between the transoid and gauche and the transoid and cisoid rotamers of [8,8′-I2-3,3′-Co(1,2-C2B9H10)2] − in comparison with the parent anion from 0.4 to 4.7 kcal/mol and from 2.5 to 15.7 kcal/mol, respectively, due to intramolecular C-H···I-B hydrogen bonding between the ligands [16,17]. The bistability of molecular switches can be achieved by using substituents that are capable to form weak intramolecular hydrogen bonds between the dicarbollide ligands in the transoid conformation and stronger dative bonds with external transition metals in the cisoid conformation. Indeed, we found that the methylsulfanyl groups are able to stabilize the transoid conformation of the corresponding derivatives of cobalt and iron bis(dicarbollides) [8,8′-(MeS)2-3,3′-M(1,2-C2B9H10)2] − (M = Co, Fe) due to formation of intramolecular C-H···S(Me)B hydrogen bonds [18,19]. On the other hand, in the presence of external transition metals or labile metal complexes, the methylsulfanyl groups form complexes with them, resulting in transformation of the transoid conformation of cobalt bis(dicarbollide) to the cisoid one [20,21].
In this contribution, we describe the synthesis and structure of the methylsulfanyl derivatives of nickel bis(dicarbollide).

X-Ray Diffraction Study
Since the chemistry of nickel bis(dicarbollides) is much less studied than the chemistry of its iron and cobalt analogues, only a limited number of X-ray structures of its derivatives are known. Like the parent nickel bis(dicarbollide), its derivatives contain substituents in the lower belt of the dicarbollide ligand adopt transoid conformation in the nickel(III) complexes and cisoid conformation in the nickel(IV) complexes [8,11,23]. In the case of derivatives containing substituents in the upper belt of the ligand, the structure of the complexes depends not only on the oxidation state of the metal, but also on the type and position of the substituents. The transoid conformation was found both in 1,1′-Me2and 1,2′-Me2-derivatives of nickel(IV) bis(dicarbollide), whereas the corresponding nickel(III) complexes adopt gauche and transoid conformations, respectively [25]. The gauche conformation stabilized by two intramolecular C-H···OMe hydrogen bonds was found in the 8,8′-(MeO)2-derivative of nickel(IV) bis(dicarbollide) [28]. The same conformation stabilized by two intramolecular C-H···SMe2 bonds was found also in the 8,8′-(Me2S)2-derivative of nickel(II) bis(dicarbollide) [29]. The latter examples demonstrate the role of substituents capable of forming intramolecular hydrogen bonds in the stabilization of rotamers, which are generally less preferred in the absence of such substituents.
An asymmetric unit cell contains three structural units: one cation and two halves of anion (A and A'). The crystal structure is formed by alternate layers parallel to the bc crystallographic plane ( Figure S4). Most of intermolecular interactions between different structural units are of van-der-Waals-type. However, it is interesting to note that, in anionic layers, interactions between A and A' are somewhat more pronounced (Figure 2), and that might be the reason of formation of two independent anions. Such conclusion can be supported by many literature examples in which interactions between different structural units appear to be stronger [30][31][32].

X-Ray Diffraction Study
Since the chemistry of nickel bis(dicarbollides) is much less studied than the chemistry of its iron and cobalt analogues, only a limited number of X-ray structures of its derivatives are known. Like the parent nickel bis(dicarbollide), its derivatives contain substituents in the lower belt of the dicarbollide ligand adopt transoid conformation in the nickel(III) complexes and cisoid conformation in the nickel(IV) complexes [8,11,23]. In the case of derivatives containing substituents in the upper belt of the ligand, the structure of the complexes depends not only on the oxidation state of the metal, but also on the type and position of the substituents. The transoid conformation was found both in 1,1 -Me 2 -and 1,2 -Me 2 -derivatives of nickel(IV) bis(dicarbollide), whereas the corresponding nickel(III) complexes adopt gauche and transoid conformations, respectively [25]. The gauche conformation stabilized by two intramolecular C-H···OMe hydrogen bonds was found in the 8,8 -(MeO) 2 -derivative of nickel(IV) bis(dicarbollide) [28]. The same conformation stabilized by two intramolecular C-H···SMe 2 bonds was found also in the 8,8 -(Me 2 S) 2 -derivative of nickel(II) bis(dicarbollide) [29]. The latter examples demonstrate the role of substituents capable of forming intramolecular hydrogen bonds in the stabilization of rotamers, which are generally less preferred in the absence of such substituents.
An asymmetric unit cell contains three structural units: one cation and two halves of anion (A and A'). The crystal structure is formed by alternate layers parallel to the bc crystallographic plane ( Figure S4). Most of intermolecular interactions between different structural units are of van-der-Waals-type. However, it is interesting to note that, in anionic layers, interactions between A and A' are somewhat more pronounced (Figure 2), and that might be the reason of formation of two independent anions. Such conclusion can be supported by many literature examples in which interactions between different structural units appear to be stronger [30][31][32].  Single-crystal X-ray diffraction study of the tetrabutylammonium salt of the meso-diastereomer (Bu4N)[4,7′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2] revealed that it is isostructural with the corresponding iron [19] and cobalt [18] complexes. Similarly to the corresponding derivatives of iron(III) and cobalt(III) bis(dicarbollides), the dicarbollide ligands in the [4,7′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2] − anion adopt gauche configuration stabilized by two CH···S and one BH···S intramolecular contacts ( Figure 3).  Single-crystal X-ray diffraction study of the tetrabutylammonium salt of the meso-diastereomer (Bu4N)[4,7′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2] revealed that it is isostructural with the corresponding iron [19] and cobalt [18] complexes. Similarly to the corresponding derivatives of iron(III) and cobalt(III) bis(dicarbollides), the dicarbollide ligands in the [4,7′-(MeS)2-3,3′-Ni(1,2-C2B9H10)2] − anion adopt gauche configuration stabilized by two CH···S and one BH···S intramolecular contacts (Figure 3). Single-crystal X-ray diffraction study of the tetrabutylammonium salt of the meso-diastereomer (Bu 4 N)[4,7 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] revealed that it is isostructural with the corresponding iron [19] and cobalt [18] complexes. Similarly to the corresponding derivatives of iron(III) and cobalt(III) bis(dicarbollides), the dicarbollide ligands in the [4,7 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] − anion adopt gauche configuration stabilized by two CH···S and one BH···S intramolecular contacts (Figure 3).      Despite that 4,7 -bis(methylsulfanyl) derivatives of iron, cobalt, and nickel bis(dicarbollides) are isostructural, a thorough analysis of the structural features of these complexes (Table 1) revealed that, despite the largest size of the nickel cation [33] that clearly reflects at the metal···dicarbollide ligand distance, the BH···S chalcogen bond in the nickel complex is much shorter (2.53 Å) than in other complexes (2.61 and 2.70 Å for the iron and cobalt complexes, respectively) and probably plays a larger role in stabilization of the gauche conformation. Generally, the formation of a chalcogen bond is explained in terms of the positive electrostatic potential present on the outermost portion of the chalcogen's surface, opposite to the R-Ch bonds. This region of positive electrostatic potential, which is called the "σ-hole", can interact attractively with negative sites of the same molecule or other molecules. Due to the presence of lone pairs of the chalcogen atom, a σ-hole is usually surrounded by a region of negative molecular electrostatic potential, and this determines a high directionality of the chalcogen bonds [34][35][36]. The acceptors of the chalcogen bond may be atoms with an unshared electron pair (e.g., pyridine or amine nitrogen atom, and ether or carbonyl group oxygen atom), π-system (e.g., double or triple bond and arene moiety), or anion (e.g., halide anion and polyatomic oxyanion) [37]. To the best of our knowledge, there is only one reference in the literature to boron hydrides as acceptors of chalcogen bonds [38], and examples of chalcogen bonds with other hydrides are quite rare, as well [39,40].

Quantum Chemical Calculations
The parent nickel(III) bis(dicarbollide) [3,3 -Ni(1,2-C 2 B 9 H 11 ) 2 ] − is known to prefer the transoid configuration, while the gauche form is only slightly less (~1 kcal/mol) preferred [7,41]. Our DFT calculations using the BP86 functional with cc-pvdz basis set gave the similar result with the transoid configuration preferred over the gauche one by only 0.4 kcal/mol, whereas the cisoid-rotamer is energetically disfavored by as much as 4 kcal/mol. These allow us to suppose that weak noncovalent intramolecular interactions could switch preferability of the transoid and gauche rotamers.
In general, since CH···S hydrogen bonds are formed with lone pairs of sulfur atoms, the formation of two such interactions per sulfur atom in methylsulfanyl derivatives is allowed. This results in stabilization of gauche1- [4,4′- In contrast, only one intramolecular chalcogen bond can be effectively formed, since only one σ-hole in the extension of the CH3-S bond is pointed in the appropriate direction. The formation of the chalcogen bond with hydride ligand requires rotation of the SMe group to form a linear fragment of C-S···H-B, which, in turn, leads to disruption of the CH Me ···HB interactions. As a result, the transoid rotamer of the 4,7′ isomer with strongest chalcogen bonds have only two interactions of the Me group with BH (one per the Me group). Notable that the least favored among all conformers cisoid-rotamer of the 4,4′-isomer featuring four S···HB interactions has no interactions of the Me groups with the boron cage at all. Therefore, with a small difference in energy between gauche and transoid rotamers, the possibility of formation of four stabilizing CH···S interactions unambiguously leads to formation of the corresponding rotamer. Such a conformer is For the [4,4 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] − isomer, the transoid rotamer should have two sulfur atoms close to each other. Despite the possibility of stabilizing CH···S interactions in this rotamer sulfur atoms repulsion leads to the mutual rotation of the dicarbollide ligands, pushing molecules to the gauche configuration. Thus, no transoid rotamer was found on the PES, but two gauche and two cisoid rotamers were found (due to molecular symmetry). At that, the most favored one is a gauche1 rotamer, which features four CH···S interactions (r(H···S) = 2.60-2.90 Å, E BCP = −2.2 ÷ −1.3 kcal/mol, Figure 5). The another gauche2 rotamer is unfavored by ∆E = +5.4 kcal/mol due to only two BH···S and S···S stabilizing interactions. Both the cisoid rotamers remain highly unfavorable (∆E = +8.2 and +6.0 kcal/mol), despite the presence of four BH···S (cisoid1) and two BH···S with two CH···S (cisoid2) interactions. At that, the cisoid2 rotamer featuring CH···S interactions is slightly preferred.
In general, since CH···S hydrogen bonds are formed with lone pairs of sulfur atoms, the formation of two such interactions per sulfur atom in methylsulfanyl derivatives is allowed. This results in stabilization of gauche1-[4,4 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] − and transoid-[8,8 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] − rotamers. In contrast, only one intramolecular chalcogen bond can be effectively formed, since only one σ-hole in the extension of the CH 3 -S bond is pointed in the appropriate direction. The formation of the chalcogen bond with hydride ligand requires rotation of the SMe group to form a linear fragment of C-S···H-B, which, in turn, leads to disruption of the CH Me ···HB interactions. As a result, the transoid rotamer of the 4,7 isomer with strongest chalcogen bonds have only two interactions of the Me group with BH (one per the Me group). Notable that the least favored among all conformers cisoid-rotamer of the 4,4 -isomer featuring four S···HB interactions has no interactions of the Me groups with the boron cage at all. Therefore, with a small difference in energy between gauche and transoid rotamers, the possibility of formation of four stabilizing CH···S interactions unambiguously leads to formation of the corresponding rotamer. Such a conformer is further stabilized due to a larger amount of CH Me ···HB interactions. When the atomic configuration does not allow such interactions, chalcogen bonds affect the rotamer stabilization.

General Considerations
All reactions were performed in an inert nitrogen atmosphere while using standard Schlenk techniques. (Bu 4 N)[10-MeS-7,8-C 2 B 9 H 11 ], (Bu 4 N)[9-MeS-7,8-C 2 B 9 H 11 ], and [Ni(acac) 2 ] 3 were prepared as described in the literature [42][43][44]. Anhydrous tetrahydrofuran was prepared by using the standard procedure [45]. Acros Organics silica gel (0.060-0.200 mm) was used for column chromatography. Thin-layer chromatograms (DC Kieselgel F 254 silica gel on aluminum plates, Merck) were visualized, using 0.1% PdCl 2 in 3 M of HCl(aq). The 11 B NMR spectra at 128.4 MHz were recorded with a Bruker Avance-400 spectrometer (Bruker Biospin AG, Fällanden, Switzerland) and referenced by using BF 3 ·Et 2 O as the external standard. High-resolution mass spectra (HRMS) were measured on a Bruker micrOTOF II instrument (Bruker Daltonics, Bremen, Germany), using electrospray ionization (ESI). The measurements were done in a negative ion mode (3200 V); mass range from m/z 50 to m/z 3000; external or internal calibration was done with ESI Tuning Mix, Agilent (Santa Clara, CA, USA). A syringe injection was used for solutions in acetonitrile (flow rate 3 mL/min). Nitrogen was applied as a dry gas; the interface temperature was set at 180 • C.  3 was added, and the reaction mixture was heated under reflux for 24 h. The reaction mixture was allowed to cool to room temperature and concentrated to dryness in vacuo. The residue was treated with 50 mL of dichloromethane and 50 mL of water. Organic layer was separated, dried over Na 2 SO 4 , filtered, and evaporated in vacuo. The crude product was purified by column chromatography on silica, using dichloromethane as eluent to obtain 0.20 g (55%) of brown product. 11  The reaction mixture was allowed to cool to room temperature and concentrated to dryness in vacuo. The residue was treated with 50 mL of dichloromethane and 50 mL of water. The organic layer was separated, washed over Na 2 SO 4 , filtered, and evaporated in vacuo. The mixture of isomers was separated by column chromatography on silica with the mixture of dichloromethane and chloroform (4:1, v/v) as eluent to obtain 0.26 g (30%, R f = 0.75) of rac-isomer and 0.21 g (24%, R f = 0.50) of meso-isomer as brown solids. Rac-isomer: 11 [4,7 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] were carried out by using SMART APEX2 CCD diffractometer (λ(Mo-Kα) = 0.71073 Å, graphite monochromator, ω-scans) at 120 K. Collected data were processed by the SAINT and SADABS programs incorporated into the APEX2 program package [46]. The structures were solved by the direct methods and refined by the full-matrix least-squares procedure against F2 in anisotropic approximation. The refinement was carried out with the SHELXTL program [47]. In the case of (Bu 4 N)[8,8 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ], all carborane hydrogen atoms were refined isotropically, without constraints. In contrast, reflection ability of (Bu 4 N)[8,8 -(MeS) 2 -3,3 -Ni(1,2-C 2 B 9 H 10 ) 2 ] was extremely weak; therefore, all hydrogen atoms were refined within the riding model. The details of data collection and crystal structures refinement are summarized in Table S1.

Quantum Chemical Calculations
The geometry optimizations were performed with the BP86 functional [48,49] with cc-pvdz basis set [50][51][52][53], without any symmetry restrictions, using Gaussain09 [54]. The tight SCF convergence criteria and ultrafine grid were applied. All minima on the PES were confirmed to have no negative values in their diagonalized force constants matrix. The QTAIM analysis was performed by using the AIMALL [55] and MULTIWFN [56] programs package based on the wave function obtained by the BP86 calculations. The energies of noncovalent intramolecular interactions were calculated by using the correlation between the interaction energy (E BCP ) and the value of the potential energy density function V C (r) at the corresponding critical point (3, −1): E BCP = 0.5·V C (r) (Table S2) [57,58]. Such approach is widely used for energetic analysis of intermolecular interactions of different types [42,43,59,60].

Conclusions
Symmetrically and unsymmetrically substituted methylsulfanyl derivatives of nickel(III) bis(dicarbollide) (Bu 4 N) [8,8 - Structures of the synthesized metallacarboranes were studied by single-crystal X-ray diffraction and quantum chemical calculations. The symmetrically substituted 8,8 -isomer was found to adopt transoid conformation stabilized by two pairs of intramolecular C-H···S hydrogen bonds between the dicarbollide ligands. The unsymmetrically substituted 4,7 -isomer adopts gauche conformation, which is stabilized by two nonequivalent C-H···S hydrogen bonds and one short chalcogen B-H···S bond (2.53 Å, −1.4 kcal/mol). The gauche conformation stabilized by two pairs of C-H···S hydrogen bonds was found to be also preferred for the 4,7 -isomer. At a small difference in energy between the gauche and transoid rotamers, the possibility of formation of four stabilizing CH···S interactions unambiguously leads to the formation of the corresponding rotamers. When the atomic configuration does not allow such interactions, the formation of BH···S chalcogen bonds affects the rotamer stabilization.
Author Contributions: S.A.A., synthesis of nickel bis(dicarbollide) derivatives; K.Y.S., single-crystal X-ray diffraction study; O.A.F., quantum chemical calculations; I.B.S., experiment planning, data analysis, and manuscript conception. All authors were involved in preparation of the manuscript.
Funding: This work was supported by the Russian Science Foundation (Grant No. 16-13-10331).