Bis(dicarbollide) Complexes of Transition Metals as a Platform for Molecular Switches. Study of Complexation of 8,8′-Bis(methylsulfanyl) Derivatives of Cobalt and Iron Bis(dicarbollides)

Complexation of the 8,8′-bis(methylsulfanyl) derivatives of cobalt and iron bis(dicarbollides) [8,8′-(MeS)2-3,3′-M(1,2-C2B9H10)2]− (M = Co, Fe) with copper, silver, palladium and rhodium leads to the formation of the corresponding chelate complexes, which is accompanied by a transition from the transoid to the cisoid conformation of the bis(dicarbollide) complex. This transition is reversible and can be used in design of coordination-driven molecular switches based on transition metal bis(dicarbollide) complexes. The solid-state structures of {(Ph3P)ClPd[8,8′- (MeS)2-3,3′-Co(1,2-C2B9H10)2-κ2-S,S′]} and {(COD)Rh[8,8′-(MeS)2-3,3′-Co(1,2-C2B9H10)2-κ2-S,S′]} were determined by single crystal X-ray diffraction.


Introduction
The design of molecular machines-molecules or molecular complexes that are capable of performing quasi-mechanical movement to do useful work-is one of the most attractive and rapidly developing areas of modern chemistry [1][2][3][4]. The importance and topicality of this area was recognized by the 2016 Nobel Prize in Chemistry awarded to Jean-Pierre Sauvage, Bernard Feringa, and Fraser Stoddart "for the design and synthesis of molecular machines" [5][6][7]. Depending on the type of motion, all molecular machines can be divided into two main groups-performing linear or rotational motion. In turn, molecular machines that carry out rotational motion can also be divided into two types-molecular motors (rotors) and molecular switches [8]. Molecular switches are molecules or supramolecular complexes that can exist in two or more stable forms, differing in the mutual orientation of their components, and which, under the influence of external factors, can be converted from one form to another due to the rotation (rotation) of these components relative to each other. Currently, the most studied are molecular switches based on organic molecules, including photochromic molecular switches based on azobenzenes, stilbenes, dithienylethenes, spiropyrans, and spirooxazines, as well as switches based on mechanically interlocked molecular systems, rotoxanes and catenanes, in which the

Results and Discussion
Our previous attempt to obtain a chelate complex of 8,8 -bis(methylsulfanyl) cobalt bis(dicarbollide) [8, 2 2 [8, 2 -3,3 -Co(1,2-C 2 B 9 H 10 ) 2 -κ 2 -S,S ]} − separated as the tetrabutylammonium salt [22]. However, this study gave us a proper understanding of how the coordination is reflected in the NMR spectra of the complexes (See Supplementary Materials NMR spectra of compounds 2-6 and 8). In addition to a downfield shift of the signal of the methylsulfanyl group in the 1 H and 13 C NMR spectra, the value of which to some extent reflects the strength of the sulfur-metal bond, the most characteristic is a strong narrowing of the signal of the substituted boron atom in the 11 B NMR spectra together with the broadening of the signals of unsubstituted boron atoms. The last one makes it possible to observe the metal coordination in the reaction mixture, while the shift of the SMe group signal in the case of weakly bound complexes can be insignificant [23][24][25][26], and the signal itself can be "masked" by the signals of the solvent, ligand, or accidental water.
In this study, the singly charged diamagnetic cations of copper(I) and silver(I) were chosen to study the complexation of [8,8 -(MeS) 2 -3,3 -Co(1,2-C 2 B 9 H 10 ) 2 ] − in order to facilitate the isolation and purification of the resulting complexes, as well as their characterization using NMR spectroscopy.
The reaction of (Me 3 NH) [ [19], to the cisoid one, stabilized by dative bonds to the metal (Scheme 1). The complexation results in the downfield shift of the signal of the MeS-substituted boron atoms in the 11 B NMR spectrum from 11.3 to 15.8 ppm, while the signal of the MeS groups in the 1 H NMR spectrum demonstrate the downfield shift from 1.87 to 2.21 ppm. Despite the fact that the coordination of copper with anion 1 is retained during chromatographic purification, our numerous attempts to obtain crystals of complex 2 suitable for X-ray diffraction study were unsuccessful, which may be due to both weak coordination of solvent molecules (acetone or acetonitrile) as well as due to the possibility of the copper coordination by B-H groups of the anion, similar to that previously found in {(Ph 3 P)Cu[3,3 -Co(1,2-C 2 B 9 H 11 ) 2 ]} [27] and some other complexes with polyhedral boron hydride anions [28][29][30]. At the same time, attempts to replace the solvent molecules in the coordination sphere of copper with stronger ligands, such as 1,2-bis(diphenylphosphino)ethane or triphenylphosphine, lead to the destruction of complex 2 with the release of free thioether ligand 1 (Scheme 1). 1.87 to 2.21 ppm. Despite the fact that the coordination of copper with anion 1 is retained during chromatographic purification, our numerous attempts to obtain crystals of complex 2 suitable for Xray diffraction study were unsuccessful, which may be due to both weak coordination of solvent molecules (acetone or acetonitrile) as well as due to the possibility of the copper coordination by B-H groups of the anion, similar to that previously found in {(Ph3P)Cu[3,3′-Co(1,2-C2B9H11)2]} [27] and some other complexes with polyhedral boron hydride anions [28][29][30]. At the same time, attempts to replace the solvent molecules in the coordination sphere of copper with stronger ligands, such as 1,2bis(diphenylphosphino)ethane or triphenylphosphine, lead to the destruction of complex 2 with the release of free thioether ligand 1 (Scheme 1). In a similar way, the reaction of ((Me3NH) [1]) with AgNO3 in acetonitrile leads to the corresponding silver complex {LAg[8,8′-(MeS)2-3,3′-Co(1,2-C2B9H10)2-κ 2 -S,S']} (3) (Scheme 2). The complexation results in the downfield shift of signals of the substituted boron atoms and the MeS groups in the 11 B NMR and 1 H spectra to 13.9 and 2.19 ppm, respectively. In the silver complex 3, as in the case of the copper complex 2, the coordination sphere of the metal can be completed by both solvent molecules and the BH groups of cobalt bis(dicarbollide) [31][32][33]. Silver can be easily removed from the complex 3 by the treatment with benzyltriethylammonium chloride, which leads to the reverse transformation of the cisoid conformation to the transoid one. In contrast to the complex 2, the reaction of complex 3 with triphenylphosphine proceeds without breaking the silver-sulfur bonds resulting in {(Ph3P)Ag[8,8′-(MeS)2-3,3′-Co(1,2-C2B9H10)2-κ 2 -S,S']} (4) (Scheme 3). The complexation with such a strong ligand as triphenylphosphine leads to some weakening of the bond of silver with the methylsulfide ligand, which is reflected in the high field shift of signals of the substituted boron atoms and the MeS groups in the 11 B NMR and 1 H spectra to 12.8 and 1.97 ppm, respectively. It should be noted that, unlike the copper complex 2, the silver complexes 3 and 4 are destroyed on a chromatographic column. . The complexation results in the downfield shift of signals of the substituted boron atoms and the MeS groups in the 11 B NMR and 1 H spectra to 13.9 and 2.19 ppm, respectively. In the silver complex 3, as in the case of the copper complex 2, the coordination sphere of the metal can be completed by both solvent molecules and the BH groups of cobalt bis(dicarbollide) [31][32][33]. Silver can be easily removed from the complex 3 by the treatment with benzyltriethylammonium chloride, which leads to the reverse transformation of the cisoid conformation to the transoid one. In contrast to the complex 2, the reaction of complex 3 with triphenylphosphine proceeds without breaking the silver-sulfur bonds resulting in {(Ph 3 P)Ag [8,8 -

S
The complexation with such a strong ligand as triphenylphosphine leads to some weakening of the bond of silver with the methylsulfide ligand, which is reflected in the high field shift of signals of the substituted boron atoms and the MeS groups in the 11 B NMR and 1 H spectra to 12.8 and 1.97 ppm, respectively. It should be noted that, unlike the copper complex 2, the silver complexes 3 and 4 are destroyed on a chromatographic column.
The complexation with such a strong ligand as triphenylphosphine leads to some weakening of the bond of silver with the methylsulfide ligand, which is reflected in the high field shift of signals of the substituted boron atoms and the MeS groups in the 11 B NMR and 1 H spectra to 12.8 and 1.97 ppm, respectively. It should be noted that, unlike the copper complex 2, the silver complexes 3 and 4 are destroyed on a chromatographic column. . It should be noted that the direct complexation reaction of 1 with [(Ph3P)2PdCl2] does not proceed, and the formation of complex 5 occurs only upon the addition of silver nitrate. The signals of two non-equivalent SMe groups in the 1 H NMR spectrum of 5 is located at 2.18 and 1.91 ppm (See ESI). The asymmetry of the palladium coordination environment is reflected also in the 11 B NMR spectrum of 5 which demonstrates two narrow singlets at 15.1 and 11.2 ppm corresponding two substituted boron atoms with different ligands in the trans-position.
The difference in the binding strength of the palladium atom with the MeS groups, which is reflected in different Pd-S bond lengths and positions of the signals of the substituted boron atoms and the MeS groups in the NMR spectra, is in good agreement with the greater trans influence of the PPh3 ligand [34,35]. . It should be noted that the direct complexation reaction of 1 with [(Ph 3 P) 2 PdCl 2 ] does not proceed, and the formation of complex 5 occurs only upon the addition of silver nitrate. The signals of two non-equivalent SMe groups in the 1 H NMR spectrum of 5 is located at 2.18 and 1.91 ppm (See ESI). The asymmetry of the palladium coordination environment is reflected also in the 11 B NMR spectrum of 5 which demonstrates two narrow singlets at 15.1 and 11.2 ppm corresponding two substituted boron atoms with different ligands in the trans-position.
The difference in the binding strength of the palladium atom with the MeS groups, which is reflected in different Pd-S bond lengths and positions of the signals of the substituted boron atoms and the MeS groups in the NMR spectra, is in good agreement with the greater trans influence of the PPh 3 ligand [34,35]. The solid-state structure of complex 6 was determined by single crystal X-ray diffraction ( Figure  2). Complex 6 crystallizes in the form of solvate with chloroform. An asymmetric unit cell contains two independent Co-Rh-complexes and two chloroform molecules. The dicarbollide ligands in 6 adopt the cisoid conformation with the pseudotorsion angles B8…Centroid(C1-C2-B7-B8-B4)… Centroid(C1′-C2′-B7′-B8′-B4′)…B8′ equal to -38.1(7)° and 39.4(7)° for two independent molecules. The short contacts C3-H3A…H12-B12 for two independent molecules correspond to 2.39 and 2.37 Å. The solid-state structure of complex 6 was determined by single crystal X-ray diffraction ( Figure  2). Complex 6 crystallizes in the form of solvate with chloroform. An asymmetric unit cell contains two independent Co-Rh-complexes and two chloroform molecules. The dicarbollide ligands in 6 adopt the cisoid conformation with the pseudotorsion angles B8…Centroid(C1-C2-B7-B8-B4)… Centroid(C1′-C2′-B7′-B8′-B4′)…B8′ equal to -38.1(7)° and 39.4(7)° for two independent molecules. The short contacts C3-H3A…H12-B12 for two independent molecules correspond to 2.39 and 2.37 Å.
Single crystal X-ray diffraction experiments for compounds 5 and 6 were carried out 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 [39]. The structures were solved by the direct methods and refined by the full-matrix least-squares procedure against F 2 in anisotropic approximation. The refinement was carried out with the SHELXTL program [40]. The CCDC numbers (2045460, 2045461, for 5 and 6, respectively) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of Complex 2
Solution of copper(I) iodide (40 mg, 0.21 mmol) in 20 mL of acetonitrile was added to orange solution of ((Me 3 NH) [1]) (100 mg, 0.21 mmol) in 10 mL of acetonitrile and the reaction mixture was stirred at room temperature overnight. Thereafter, the purple reaction mixture was filtered and concentrated under reduced pressure. The crude product was purified by column chromatography on silica using a mixture of dichloromethane and acetone (1:1, v/v) as eluent to give an purple solid 2 (76 mg, yield 65%). 1

Synthesis of Complex 3
Solution of silver(I) nitrate (36 mg, 0.21 mmol) in 20 mL of acetonitrile was added to orange solution of (Me 3 NH) [1] (100 mg, 0.21 mmol) in 10 mL of acetonitrile and the reaction mixture was stirred at room temperature overnight. Thereafter, the reaction mixture was filtered through layer of silica and concentrated under reduced pressure to give an orange solid 3 (90 mg, yield 82%). 1

Synthesis of Complex 4
Solution of silver(I) nitrate (36 mg, 0.21 mmol) in 20 mL of acetonitrile was added to orange solution of (Me 3 NH) [1] (100 mg, 0.21 mmol) in 10 mL of acetonitrile and the reaction mixture was stirred at ambient temperature overnight. Thereafter, solution of triphenyl phosphine (55 mg, 0.21 mmol) in 10 mL of acetonitrile was added and the reaction mixture was stirred at room temperature for 1 h. Then, the mixture was filtered through layer of silica and concentrated under reduced pressure to give an orange solid 4 (156 mg, yield 92%). 1  in 10 mL of acetonitrile was added and the reaction mixture was stirred at room temperature for 1 h. Then, the reaction mixture was filtered and concentrated under reduced pressure. The crude product was purified by column chromatography on silica using dichloromethane as eluent to give a dark orange solid 5 (70 mg, yield 41%). 1 1B, s, B-S

Synthesis of Complex 6
[(COD) 2 Rh](BF 4 ) (80 mg, 0.20 mmol) was added to Cs [1] (100 mg, 0.18 mmol) in 4 mL of dichloromethane and the reaction mixture was stirred at room temperature overnight. Thereafter, the red reaction mixture was filtered and concentrated under reduced pressure. The crude product was purified by column chromatography on silica using of chloroform as eluent to give a red solid 6 (90 mg, yield 80%). 1

Synthesis of Complex 8
[(COD) 2 Rh](BF 4 ) (200 mg, 0.49 mmol) was added to Cs [7] (270 mg, 0.49 mmol) in 10 mL of dichloromethane and the reaction mixture was stirred at room temperature overnight. Thereafter, the brown mixture was filtered and concentrated under reduced pressure. The crude product was purified twice by column chromatography on silica with use of chloroform (first column) and toluene (second column) as eluent to give a brown solid 8 (190 mg, yield 62%). 1

Conclusions
The complexation of the 8,8 -bis(methylsulfanyl) derivatives of cobalt and iron bis(dicarbollides) [8,8 -(MeS) 2 -3,3 -M(1,2-C 2 B 9 H 10 ) 2 ] − (M = Co, Fe) with various transition metals (copper, silver, palladium, rhodium) leads to the formation of the corresponding chelate complexes, which is accompanied by a transition from the transoid to the cisoid conformation of the bis(dicarbollide) complex. This transition is reversible on removal of the external transition metal using stronger ligands or precipitating agents and can be used in design of coordination-driven molecular switches based on transition metal bis(dicarbollide) complexes.