Synthesis and Reactivity of Cyclopentadienyl Ruthenium(II) Complexes with Tris(alkylthio)benzenes: Transformation between Dinuclear and Sandwich-Type Complexes

To explore the structural transformation of cyclopentadienyl ruthenium (CpRu) complexes in response to external stimuli, the reaction of [RuCp(MeCN)3][X] (X = PF6, (FSO2)2N [= FSA]) and tris(alkylthio)benzenes (1,3,5-C6H3(SR)3; L1: R = Pr, L2: R = Me) was investigated, and the crystal structures and thermal properties of the products were examined. The reaction produced the sandwich complexes [RuCpLn][X] or dinuclear complexes [Ru2Cp2(μ-Ln)2(CH3CN)m][X]2 (X = PF6, FSA) depending on the reaction conditions. The sandwich complex [RuCpL1][FSA] was an ionic liquid. The solids of dinuclear complexes transformed into the thermodynamically stable sandwich complexes upon heating accompanied by acetonitrile loss. This change resulted in a transformation from crystal to ionic liquid for complexes with the FSA anion. UV irradiation of the sandwich complex [RuCpL1][PF6] in methanol produced the dinuclear complex [Ru2Cp2(μ-L1)2L12][PF6]2. The complex transformed into the sandwich complex upon heating.


■ INTRODUCTION
Many cyclopentadienyl ruthenium (CpRu) complexes have been synthesized to date because of the interest in their chemical reactivities and catalytic activities. 1−15 They are also used for the construction of various supramolecular assemblies. 16−18 A versatile precursor for their production is the triacetonitrile complex [RuCp(MeCN) 3 ] + , 9−13 which produces cationic sandwich-type Ru complexes upon reaction with arene ligands. 13−15 Ionic liquids are salts with melting points below 100°C, 23,24 and we have synthesized a variety of organometallic ionic liquids containing cationic sandwich complexes. 19 −22 In particular, ionic liquids with the formula [RuCp(arene)][X] (X = fluorinated anion) have been prepared using the reaction of [RuCp(MeCN) 3 ] + with arene ligands. 25−28 As part of this investigation, we previously found that the reaction with orthosubstituted benzenes afforded either sandwich-type or chelate complexes depending on the reaction conditions (Figure 1), and their interconversion in solution was possible upon application of light and heat. 29 Based on this mechanism, organometallic ionic liquids that transform into amorphous coordination polymers upon photo-irradiation were designed. 30,31 In this study, to further explore the structural transformations of CpRu complexes via external stimuli, the reaction of the triacetonitrile complex with meta-substituted ligands was performed because they cannot form chelate complexes unlike the ortho-substituted benzenes. The reaction with 1,3,5-C 6 H 3 (SR) 3 ligands (L 1 : R = Pr; L 2 : R = Me) produced sandwich-type or dinuclear complexes depending on the reaction condition ( Figure 2). The hexafluorophosphate (PF 6 − ) and bis(fluorosulfonyl) amide ((FSO 2 ) 2 N − ; FSA) anions were used as counter anions. The FSA anion is often used for the preparation of ionic liquids. 24 The thermal properties of the products were investigated using differential scanning calorimetry (DSC), along with crystal structure determination. Furthermore, the thermal reactivity of the dinuclear complexes and the photochemical reactivity of the sandwich-type complexes were investigated.  6 ] were determined at 90 K, crystallizing in space groups P2 1 2 1 2 and P2 1 /c, respectively. The packing diagrams are provided in Figure S1 (Supporting Information), and the structures of the cations are shown in Figure 4. One of the four crystallo-    graphically independent cations is shown for [1a][PF 6 ], while the other cations are provided in Figure S2 (Supporting Information) and exhibit different substituent conformations. The Ru−Cp centroid and Ru−Arene centroid distances in these cations are 1.82 and 1.71 Å, respectively, which are usual values for CpRu complexes. 28  [2a][PF 6 ] 2 exhibited melting and acetonitrile loss simultaneously, which was observed as an endothermic peak at approximately 152°C (onset: 141°C) in the differential thermal analysis (DTA) curve (Figure 5a), whereas [2b][PF 6 ] melted at a higher temperature than the acetonitrile loss, exhibiting two peaks at approximately 168 and 202°C (onset) in the DTA curve ( Figure 5b). Their thermal properties were also investigated using DSC (see below), which revealed that the loss of acetonitrile was accompanied by a structural transformation to the sandwich complexes.
Crystal  6 ] were determined at 90 K, and the dinuclear cationic complex structures are shown in Figure 6. The two Ru ions in each cation are bridged by two arene ligands, where each Ru ion is coordinated with an acetonitrile and the two sulfur atoms of the arene ligands. Therefore, one of the three sulfur atoms in each arene ligand remained uncoordinated.  6 ] 2 were oriented in a parallel manner because the cation is located on the inversion center. The centroid− centroid distances between the two arene rings in these complexes were 3.65 and 3.53 Å, respectively, and were likely stabilized by intramolecular π−π interactions. The dinuclear structure resembles the structural motif of the previously reported Ag I coordination polymer [Ag 2 (bsb) 2 (ClO 4 ) 2 ] n [bsb = 1,3,5-tris(benzylsulfanyl)benzene]. 33    6 ] 2 (see below).
Thermal Conversion from a Dinuclear to Sandwich-Type Complex. The formation of the sandwich-type and dinuclear complexes by the reactions at 90°C and room temperature, respectively, indicates that they are the thermodynamic and kinetic products, respectively. DFT calculations also indicated that the sandwich-type complex is thermodynamically more stable ( Figure S3, Supporting Information). Based on this feature, we could observe a thermal transformation from the dinuclear complex to the sandwich-type complex.
The photographs taken upon heating the crystals of   6 ] 2 ) in 28% yield. The structure of the cation was determined by X-ray crystallography at 90 K and is shown in Figure 9 (space group

■ CONCLUSIONS
The reaction of the triacetonitrile CpRu complex [RuCp-(MeCN) 3 ] + with 1,3,5-tris(alkylthio)benzenes produced either sandwich-type complexes or dinuclear complexes depending on reaction conditions. Dinuclear complex formation from the meta-substituted ligands is a striking contrast to the formation of mononuclear chelate complexes from the ortho-substituted ligands. The dinuclear complexes exhibited a novel structural type and are interesting from a supramolecular perspective. Furthermore, the dinuclear complexes transformed to thermodynamically stable sandwich complexes upon heating concomitant with the loss of acetonitrile. The sandwich complex photochemically produced a dinuclear complex with four ligands in solution.
We explored the phase change phenomenon coupled with structural changes of metal complexes upon application of external stimuli based on the unique reactivities of CpRu complexes. The transformation between the ionic liquid and amorphous solid upon application of light and heat was previously reported. In this study, a thermal conversion from the crystal to ionic liquid was achieved, adding versatility to the material transformation phenomenon of CpRu complexes.

■ EXPERIMENTAL SECTION
General. 1 H NMR spectra were recorded using a Bruker AVANCE 400 spectrometer, and IR spectra were recorded using a Thermo Nicolet Avatar 360 FT-IR spectrometer (ATR method). Elemental analyses were performed using a PerkinElmer 2400II elemental analyzer. DSC measurements were performed using a TA instruments Q100 differential scanning calorimeter at a 10°C min −1 scan rate. TG analyses were performed under a nitrogen atmosphere at a 10°C min −1 heating rate using a Rigaku TG8120 TG analyzer. DFT calculations were performed at the ωB97-D/LanL2DZ level using Spartan′18 software. Single-crystal X-ray diffraction data were collected using a Bruker APEX II Ultra diffractometer with Mo Kα radiation at 90 K. The structures were solved using SHELXS, 34 and the crystallographic parameters are provided in Tables S1 and S2 (Supporting Information).
Synthesis of the Sandwich-Type Complexes. The resulting solution was evaporated under reduced pressure, and the residue was dissolved in a small amount of dichloromethane before being subjected to column chromatography (alumina, eluent: diethyl ether and then acetonitrile). The fraction containing the desired salt was collected and evaporated. The residue was dissolved in a small amount of acetone, to which excess of diethyl ether was added to precipitate the product as a white powder (65 mg, 83%). Pale yellow crystals were obtained upon recrystallization from acetone−diethyl ether at −40°C (36 mg, 46%  (20 mL), and the organic layer was dried over MgSO 4 before being evaporated. The procedure was repeated seven times until the complete disappearance of PF 6 − was confirmed by 19 F NMR spectra (CD 3 CN). The resulting yellow oil was purified via column chromatography (activated alumina, dichloromethane (R f = 0.1)/acetonitrile (R f = 0.9), gradient from 1:0 to 0:1). The solution was evaporated and dried in vacuo for 6 h to afford a yellow liquid (32 mg, 28%