Dmitri V.
Konarev
*a,
Salavat S.
Khasanov
b,
Alexander F.
Shestakov
a,
Alexey M.
Fatalov
ac,
Mikhail S.
Batov
ac,
Akihiro
Otsuka
de,
Hideki
Yamochi
de,
Hiroshi
Kitagawa
d and
Rimma N.
Lyubovskaya
a
aInstitute of Problems of Chemical Physics RAS, Chernogolovka, 142432 Russia. E-mail: konarev3@yandex.ru
bInstitute of Solid State Physics RAS, Chernogolovka, 142432 Russia
cLomonosov Moscow State University, Leninskie Gory, Moscow, 119991 Russia
dDivision of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
eResearch Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
First published on 25th September 2017
Reaction of decamethylchromocene (Cp*2Cr) with thioindigo (TI) yields a coordination complex {[TI-(μ2-O), (μ-O)]Cp*Cr}2·C6H14 (1) in which one Cp* ligand in Cp*2Cr is substituted by TI. TI adopts cis-conformation in 1 allowing the coordination of both carbonyl groups to chromium. Additionally, one oxygen atom of TI becomes a μ2-bridge for two chromium atoms to form {[TI-(μ2-O), (μ-O)]Cp*Cr}2 dimers with a Cr⋯Cr distance of 3.12 Å. According to magnetic data, diamagnetic TI2− dianions and two Cr3+ atoms with a high S = 3/2 spin state are present in a dimer allowing strong antiferromagnetic coupling between two Cr3+ spins with an exchange interaction of −35.4 K and the decrease of molar magnetic susceptibility below 140 K. Paramagnetic TI˙− radical anions with the S = 1/2 spin state have also been obtained and studied in crystalline {cryptand[2,2,2](Na+)}(TI˙−) (2) salt showing that both radical anion and dianion states are accessible for TI.
Thioindigo (TI) is also a well-known dye5 related to indigo but containing sulfur atoms instead of the N–H groups (Fig. 1). TI shows intense absorption in the visible range and a color in solution and solid state. Potentially it is also able to form stable complexes with transition metals by employing the carbonyl groups. However, in contrast to indigo, no coordination complexes of TI are known. Any information about the structure and properties of the TI radical anions or dianions in solid state is also absent.
Fig. 1 Schematic presentation of the molecular structures of pristine trans-TI, cis-TI2− dianion in 1 and cis- and trans-forms of the TI˙− radical anion in 2. |
In this work we studied the reaction of TI with decamethylchromocene (Cp*2Cr) and obtained the first transition metal complex of thioindigo {[TI-(μ2-O), (μ-O)]Cp*Cr}2·C6H14 (1). We also synthesized {cryptand[2,2,2](Na+)}(TI˙−) (2) containing the TI˙− radical anions to understand the charged state of TI in 1. Crystal structures, and optical and magnetic properties of 1 and 2 are discussed and a possible mechanism for the formation of 1 is presented.
Similar to indigo, TI also substitutes the Cp* ligand at the chromium atom of Cp*2Cr. However, indigo and TI form different complexes with Cp*Cr and have different charges in these complexes. The different features of TI and indigo are understood mainly by the variation of their redox properties and the magnitude of energy difference between the cis- and trans-isomers. TI shows essentially stronger acceptor properties with first and second reduction potentials Ered = −0.305 and −0.97 V vs. Ag/AgClO4 in 0.1 M solution of TBAClO4 in DMF or −0.01 and −0.67 V vs. SCE, respectively.6 The first reduction potential of indigo is essentially more negative (−0.75 V vs. Ag/AgCl or −0.795 and vs. SCE).7 As a result, deeper reduction of TI is possible in the complexes in comparison with indigo. Unlike indigo, TI has a smaller energy difference between ground state trans- and cis-forms. The calculated values are 11.6 and 13.4 kcal mol−1 for neutral and negatively charged TI molecules, respectively. The corresponding values for indigo are 16.7 and 20.6 kcal mol−1, respectively. The reason is the absence of short H⋯H contacts for cis-TI.
Since Cp*2Cr is a strong donor with the first oxidation potential of −1.04 V vs. SCE,8 it can reduce TI to form a rather stable outer-sphere charge transfer complex (Cp*2Cr+)(TI˙−). With the aid of quantum chemical calculations, the reaction mechanism to produce 1 is inferred as follows. The formation of (Cp*2Cr+)(TI˙−) is accompanied by an energy decrease of 8.5 kcal mol−1 (Fig. 2). The color change of reaction solution mentioned above is most probably caused by the formation of a coordination complex. This complex can form via an intermediate (trans-TI-O)Cp*2Cr complex with the formation of an isomeric (cis-TI-O,O)Cp*2Cr complex accompanied by the subsequent η5–η1 shift of the Cp* ring (see the ESI† for a more detailed mechanism). This leads to a slight increase in energy by 2.5 kcal mol−1 (Fig. 2). Therefore, the equilibrium concentration of the (cis-TI-O,O)Cp*2Cr complex is expected to be sufficient for bimolecular reaction of transformation of this complex into binuclear complex 1 with the subtraction of two Cp* rings in the form of the (Cp*)2 dimer. This process proceeds with a noticeable energy gain of 33.6 kcal mol−1 explaining the formation of 1.
Fig. 2 Scheme of the possible reaction of TI with decamethylchromocene. All energies below the structures are given relatively to one level – free Cp*2Cr and trans-thioindigo. |
In previous work4 we showed that the complex (indigo-O,O)(Cp*CrIICl) is formed only in the presence of chloride anions the source of which is the chloro(1,5-cyclooctadiene)rhodium(I) dimer, {RhI(cod)Cl}2. The {RhI(cod)Cl}2 can accept also the Cp* ligand leaving from Cp*2Cr. Without the addition of the {RhI(cod)Cl}2 the coordination complex of indigo is not formed.4 The TI reaction is realized without {RhI(cod)Cl}2. Since TI is negatively charged in 1, it is possible that chloride anions are not needed to compensate for the positive charge of chromium atoms. The absence of a chloride substituent at the chromium atom in 1 opens a position for coordination of the third oxygen atom of TI which becomes a μ2-bridge bonding two [TI-O,O]Cp*Cr units into a dimer (Fig. 3a). For the (cis-indigo-O,O)Cp*2Cr complex, a similar process accompanied by the formation of an {[indigo-(μ2-O),(μ-O)]Cp*Cr}2 dimer, which is an analog of complex 1, has also a significant energy gain. The absence of such a product is caused by the thermodynamically unfavorable formation of an intermediate (cis-indigo-O,O)Cp*2Cr complex. As a result, its effective concentration is low, and only bimolecular processes of substitution of the Cp* ligand by the chloride anion are possible in the presence of {RhI(cod)Cl}2 which is present in high concentration.
Fig. 3 View on the {[TI-(μ2-O),(μ-O)]Cp*Cr}2 dimers in 1 (carbon is brown, chromium is green, oxygen is red and sulfur is yellow) (a); packing of the dimers in the crystal structure of 1 (b). |
The geometry of TI in 1 is listed in Table 1. The distinct modulation of bond lengths is observed in TI at the formation of 1. The central CC bond and the CO bonds elongate and become close to the single C–C and C–O bonds, whereas the C(O)–C(C) bonds are shortened to the length of the double CC bond. Such bond length variation indicates the dianion state of TI in 1 and agrees well with the molecular formula proposed for the indigo and TI dianions (Fig. 1). To distinguish the geometry of the TI dianion from that of the radical anion we also synthesized crystalline salt {cryptand[2,2,2](Na+)}(TI˙−) (2).‡ Unfortunately, disorder appears in the structure of 2 due to the presence of both trans- and cis-conformations for two independent TI with orientational disorder of the conformations. One of the independent TI molecules has the occupancy for the disordered S and CO groups of 0.607/0.393 and 0.861/0.139. These values of occupancy lead to 0.254 for trans-conformation, at least, and 0.468 for cis-conformation, at least (Fig. S13†). The second independent TI molecule has higher scattering of trans/cis ratio, again the lowest possible occupancies are 0.114 and 0.124 for the trans- and cis-conformations, respectively. We discuss the molecular structure of the first TI˙− in 2 (Table 1). The central CC bond in TI˙− has an intermediate length between those for neutral and dianion TI. At the same time both CO bonds are elongated but both C(O)–C() bonds are shortened in TI˙− in comparison with trans-TI (Table 1). Such geometry change agrees with the delocalized feature of one negative charge over the two oxygen atoms of TI˙−, one of the canonical structures of which is shown in Fig. 1. DFT calculations of the molecular geometry of TI˙− in cis- and trans-conformations (Fig. S1†) also support delocalization of negative charge over two oxygen atoms of TI˙− (Table 1). Pristine trans-TI adopts a planar conformation around the central CC bond.11,12 An increased length of the CC bond in the radical anion and dianion states reduces the double bond nature of this bond. As a result, the dihedral angle between two planar C6H4SC(O)C fragments increases to 6–7° for the radical anion in 2 and up to 19.7° for the dianion in 1 which nearly corresponds to the central C–C single bond. The S–C bonds are not so sensitive to the charged state of TI (Table 1).
Compound | Central CC bond | Average CO bond | Average C(O)–C() bond | Average S–C() bond | Average S–C(Phen) bond | Dihedral angleb (°) |
---|---|---|---|---|---|---|
a Geometry of the more ordered TI˙− radical anion among the two crystallographically unique ones is considered. b Dihedral angle around the central CC bond between two planes defined by SCC(O) atoms. | ||||||
Tetrachlorothioindigo11 | ||||||
trans-Conformation (0.65) | 1.345(2) | 1.200(2) | 1.447(2) | 1.783(3) | 1.766(3) | 0.01 |
cis-Conformation (0.35) | 1.345(2) | 1.204(2) | 1.452(2) | 1.799(3) | 1.783(3) | 1.01 |
trans-TI12 | 1.34(3) | 1.21(3) | 1.54(3) | 1.78(3) | 1.74(3) | 0 |
TI˙− radical anion in 2a | ||||||
trans-Conformation | 1.411(8) | 1.270(18) | 1.356(15) | 1.860(9) | 1.726(8) | 6.40 |
1.34(3) | 1.331(11) | |||||
cis-Conformation | 1.411(8) | 1.330(8) | 1.305(12) | 1.803(5) | 1.730(6) | 7.46 |
1.377(12) | 1.331(8) | |||||
Calculated cis-TI˙− radical anion | 1.404 | 1.242 | 1.474 | 1.781 | 1.753 | — |
Calculated trans-TI˙− radical anion | 1.388 | 1.256 | 1.454 | 1.768 | 1.765 | — |
cis-TI2− dianion in 1 | 1.445(2) | 1.332(2) | 1.377(2) | 1.762(2) | 1.731(2) | 19.7 |
The packing mode of the {[TI-(μ2-O),(μ-O)]Cp*Cr}2 dimers in 1 is shown in Fig. 3b. There are short S, C(TI)–C(Cp*) contacts in the 3.20–3.46 Å range between TI and Cp* ligands but the TI and Cp* fragments are arranged nonparallel to each other. Therefore, intermolecular π–π interactions between the dimers are weak.
IR spectra of TI are shown in Fig. S7–S9† and listed in Table S1.† Reduction of TI affects the CO stretching mode. Neutral TI manifests two intense bands at 1588 and 1657 cm−1 attributed to vibrations of the double CO bond (Fig. S7†). Since according to the calculations trans- and cis-TI should have the position of these bands in the IR-spectra at 1584, 1655 cm−1 and 1582, 1713 cm−1, respectively, we can conclude that mainly trans-TI is present in the starting compound. Reduction decreases strongly the intensity of these bands in the spectrum of 2 (only band at 1586 cm−1 is retained) and a new band appears at 1503 cm−1 (Fig. S8†). The bands in the 1550–1600 cm−1 region are very weak in the spectrum of 1 indicating disappearance of double CO bonds in the dianion state of TI (Fig. S8†). Calculations show that bands of the C–O vibrations together with the contribution from the valent C–C vibrations are manifested mainly at 1464, 1409, 1369 and 1350 cm−1 (see ESI†).
The magnetic properties of the Cr(III) dimers14 were modeled by including biquadratic exchange interactions (j) according to the general exchange Hamiltonian H = −2JS1·S2 − j(S1·S2)2, where j is the biquadratic exchange constant.15 This leads to the following expression:
Using this model with J = −24.6 cm−1 or −35.4 K, j = 1.5 cm−1, g = 1.98 the experimental data (Fig. 5d, black squares) can be fitted well (Fig. 5d, red curve). The g-factor nearly 1.98 is typical of Cr(III) complexes.15 The value of magnetic exchange interaction of J = −35.4 K reflects very effective magnetic coupling between two Cr(III) atoms through two oxygen bridges of TI2−. The exchange coupling can be estimated based on the Weiss constant. In the mean field approximation the Weiss constant relates to the exchange coupling by: Θ = 2zJS(S + 1)/3k, where z = number of the nearest neighbors (z = 1 for a dimer). Using Θ = −250 K an initial estimate gives J/k = −33 K based on S = 3/2. This value is in good agreement with that derived from the Cr-dimer model. To estimate the contribution from the TI ligand we studied magnetic behavior of 2. This salt contains the TI˙− radical anions with the S = 1/2 spin state providing an effective magnetic moment of 1.71μB at 300 K (Fig. S14†) characteristic of one noninteracting S = 1/2 spin (1.73μB). The salt shows nearly paramagnetic behavior with a Weiss temperature of −1 K (Fig. S15†) due to the absence of π–π interactions between TI˙− (Fig. S12†). Salt 2 manifests an intense EPR signal with a nearly temperature independent g-factor of 2.0051 and the linewidth of 0.6–0.7 mT (Fig. S16–S18†). Observation of this signal allows one to determine unambiguously the presence of TI˙− in the compound. Magnetic moment of 1 indicates the absence of contribution from TI˙− since higher magnetic moment is expected in this case. EPR spectra of polycrystalline 1 at 140, 59 and 6.5 K are shown in Fig. S20–S22.† The complex pattern at all these temperatures most likely arises from zero field splitting. We used the parameters J and j of spin Hamiltonian, which allows the magnetic susceptibility of 1 to be described well, to calculate the temperature dependence of the population of different spin states S = 1, 2 and 3 of the dimer. The population of all magnetic states is very low at 6.5 K, and the EPR spectrum is mainly defined by the Curie impurities (Fig. S22†). At the same time at 59 K populations of these states became 0.40, 0.05 and 0.002, respectively. Therefore, the spectrum of a nearly pure triplet state is observed at 59 K (Fig. S21†). The spectrum at 140 K is similar to that at 59 K at high and low magnetic fields but additional signals appear at 320–420 mT (Fig. S20†). Estimation gives the following population of the S = 1, 2 and 3 states, namely, 0.43, 0.23 and 0.08, respectively. The population of quintet states increases at 140 K and the admixture of the septet state appears. Therefore, additional signals at intermediate fields (320–420 mT) can most probably be attributed to the quintet state. The absence of narrow signals from TI˙− in the EPR spectrum of 1 supports the formation of diamagnetic and EPR silent TI2− dianions. All these data indicate the ionic formula of 1 as {[TI2−-(μ2-O),(μ-O)](Cp*−)(CrIII)}2·C6H14 and that agrees with the redox potential of TI. In contrast, in the case of indigo no charge transfer was found from chromium(II) to indigo in the ground state of the (indigo-O,O)0[(Cp*−)CrII(Cl−)] complex.4
The crystals of {cryptand[2,2,2](Na+)}(TI˙−) (2) were obtained by the following procedure. 12.5 mg of thioindigo (0.042 mmol) was reduced by excess of sodium fluorenone ketyl (12 mg, 0.059 mmol) in the presence of one equivalent of cryptand[2,2,2] (16 mg, 0.042 mmol) in 16 mL of o-dichlorobenzene. Intense stirring at 80 °C for 4 hours provided the formation of red-violet solution. The solution was cooled down to room temperature and filtered into the 50 mL glass tube of 1.8 cm diameter with a ground glass plug, and 30 mL of n-hexane was layered over the solution. After slow mixing of two solvents for 1 month the crystals were precipitated on the walls of the tube. Then the solvent was decanted from the crystals and they were washed with n-hexane to yield elongated violet parallelepipeds up to 0.8 × 0.2 × 0.2 mm3 in size in 74% yield. The composition of the crystals was determined from X-ray diffraction analysis on single crystals. We tested several crystals from the synthesis which showed the same unit cell parameters and belonged to one crystal phase.
Elemental analysis cannot be used to confirm the composition of these compounds due to their air-sensitivity.
Footnotes |
† Electronic supplementary information (ESI) available: IR spectra of starting compounds and 1, 2, mechanism of the formation of 1, crystal structures and magnetic properties of 1 and 2. CCDC 1561767 and 1561770. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02878d |
‡ Crystal data for 1: C58H60Cr2O4S4, F.W. 1053.30, black plate, 0.30 × 0.30 × 0.050 mm3; 200.0(2) K: triclinic, space group P, a = 10.3104(4), b = 10.9762(4), c = 12.2084(2) Å, α = 67.979(2), β = 82.612(6), γ = 83.122(3)°, V = 1266.33(7) Å3, Z = 1, dcalcd = 1.368 Mg m−3, μ = 0.641 mm−1, F(000) = 552, 2θmax = 56.560°; 22567 reflections collected, 6189 independent; R1 = 0.0293 for 5495 observed data [>2σ(F)] with 182 restraints and 409 parameters; wR2 = 0.0828 (all data); final G.o.F. = 1.048. CCDC 1561767.† Crystal data for 2: C34H44N2NaO8S2, F.W. 695.82, black prism, 0.350 × 0.187 × 0.122 mm3; 110(2) K, monoclinic, space group P21, a = 12.2584(3), b = 14.4597(3), c = 19.1968(5) Å, β = 96.589(2)°, V = 3380.21(14) Å3, Z = 4, dcalcd = 1.422 Mg m−3, μ = 0.225 mm−1, F(000) = 1476, 2θmax = 54.202°; 29401 reflections collected, 13553 independent; R1 = 0.0401 for 10803 observed data [>2σ(F)] with 436 restraints and 960 parameters; wR2 = 0.0884 (all data); final G.o.F. = 1.028. CCDC 1561770.† |
This journal is © The Royal Society of Chemistry 2017 |