Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Interligand Charge Transfer in a Complex of Deprotonated cisIndigo Dianions and Tin(II) Phthalocyanine Radical Anions with Cp*IrIII Dmitri V. Konarev,*,† Leokadiya V. Zorina,‡ Salavat S. Khasanov,‡ Alexander F. Shestakov,† Alexey M. Fatalov,†,§ Akihiro Otsuka,∥,⊥ Hideki Yamochi,∥,⊥ Hiroshi Kitagawa,∥ and Rimma N. Lyubovskaya† †
Institute of Problems of Chemical Physics RAS, Chernogolovka, Moscow region 142432, Russia Institute of Solid State Physics RAS, Chernogolovka, Moscow region 142432, Russia § Moscow State University, Leninskie Gory,119991 Moscow, Russia ∥ Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ⊥ Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ‡
S Supporting Information *
ABSTRACT: A diamagnetic complex, {(cis-indigoN,N)2−(Cp*IrIII)} (1), in which deprotonated cis-indigo dianions coordinate an iridium center through two nitrogen atoms was obtained. By employment of the ability of the iridium center in 1 to coordinate an additional ligand, the complex [(Bu 4 N + ) 2 {[Sn II (Pc •3− )](cis-indigo-N,N) 2− Cp*Ir III } •− 2 ·0.5(H2Indigo)·2.5C6H4Cl2 (2), which has two functional ligands coordinating an IrIII center, was obtained. This complex has a magnetic moment of 1.71 μB at 300 K, in accordance with an S = 1/2 spin state. The spin density is mainly delocalized over the Pc•3− macrocycle and partially on (cis-indigo-N,N)2−. Due to an effective π−π interaction, a thermally activated charge transfer from [SnII(Pc•3−)]•− to (cis-indigo-N,N)2− is observed, with an estimated Gibbs energy (−ΔG°) of 9.27 ± 0.18 kJ/mol. The deprotonation of indigo associated with the coordination of IrIII by the indigo releases H+ ions, which protonate noncoordinating indigo molecules to produce leuco cis-indigo (H2Indigo). One H2indigo links two (cis-indigo-N,N)2− dianions in 2 to produce strong N−H···OC and O−H···OC hydrogen-bonding interactions.
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INTRODUCTION Indigo is a well-known dye produced on the ton scale.1 In recent years, this compound has found new applications in rechargeable batteries and as an electronic material.2 In addition to chemical modification by different substituents,3 indigo’s properties can be modified by its coordination of transition metals.4 In fact, the formation of coordination complexes is made possible by the ability of indigo to undergo deprotonation and form mono- and dianions. These species can then coordinate one or two metal centers in a trans conformation using both the ligand’s nitrogen and carbonyl oxygen atoms. All obtained coordination complexes of indigo thus contain deprotonated trans-indigo anions.4 The corresponding cis conformation was identified only in the coordination complex with RuII(acac)25 and the complexes of some N-indigo derivatives.6 Recently, it was shown that the preparation of coordination complexes with the radical anion of indigo enables the transition of indigo from a trans to a cis conformation. In particular, in the case of (cis-indigoO,O)(Cp*CrIICl), the coordination of both of indigo’s carbonyl oxygens to chromium was observed.7 © XXXX American Chemical Society
Transition-metal complexes of deprotonated cis-indigo dianions (Scheme 1) are rare. We hereby report the Scheme 1
preparation of the complex of this ligand with iridium(III) pentamethylcyclopentadienyl {(cis-indigo-N,N)2−(Cp*IrIII)} (1). Development of a strategy to design coordination complexes with two functional ligands that can communicate with each other is a very important research goal in material chemistry, since these complexes can be used as sensors or Received: September 13, 2017
A
DOI: 10.1021/acs.inorgchem.7b02351 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry detectors.8 In this work, both the cis-indigo dianion and the paramagnetic [SnII(Pc•3−)]•− radical anion were used to coordinate Cp*IrIII to form (Bu4N+)2{[SnII(Pc•3−)](cisindigo-N,N)2−Cp*IrIII}•−2·0.5(H2Indigo)·2.5C6H4Cl2 (2). The effective π−π interaction between the two ligands enables communication between them, accompanied by a temperatureinduced interligand charge transfer, which influences the ligand geometry and properties. In addition to the coordinating cisindigo dianions, complex 2 also contains a protonated leuco cis-indigo with as yet unknown structure. This species forms strong hydrogen-bonding interactions with the cis-indigo dianions, thus binding two {[Sn II (Pc •3− )](cis-indigoN,N)2−Cp*IrIII}•− anions in one assembly. Below are discussed the crystal structures and optical and magnetic properties of complexes 1 and 2, as well as those of two forms of indigo.
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was also confirmed via elemental analysis. Anal. Calcd for C26H23IrN2O2, Mr = 587.66: C, 53.14; H, 3.91; N, 4.76. Found: C, 52.86; H, 3.73; N, 4.65. The crystals of (Bu4N+)2{[SnII(Pc•3−)]•−(indigo-N,N)2−Cp*IrIII}2· 0.5(H2Indigo)·2.5C6H4Cl2 (2) were obtained by the following procedure. An 11 mg portion of trans-indigo (0.042 mmol) and 26.6 mg of tin(II) phthalocyanine (SnII(Pc2−), 0.042 mmol) were reduced by a slight excess of sodium fluorenone ketyl10 (20 mg, 0.098 mmol) in the presence of Bu4NBr (28 mg, 0.087 mmol) in 16 mL of o-dichlorobenzene. After stirring of the solution for 24 h at 80 °C a deep blue solution formed and all reagents were dissolved. The obtained hot solution was filtered into a flask containing 24.2 mg (0.042 mmol) of Cp*IrI2 in the form of a dimer, and the solution was stirred additionally for 1 day at 80 °C with preservation of the deep blue color. The solution was cooled to room temperature and filtered into a 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. The crystals precipitated on the walls of the tube over 1.5 months. Then the solvent was decanted from the crystals and they were washed with nhexane to yield black prisms with the characteristic for phthalocyanines copper luster in 67% yield. The composition of the crystals was determined from X-ray diffraction analysis on single crystals. Several crystals tested from the synthesis showed the same unit cell parameters. Therefore, they belonged to one crystal phase. X-ray Crystal Structure Determination. Crystal data for 1: C26H23IrN2O2, formula weight 587.66, deep red block, 0.17 × 0.07 × 0.02 mm3, T = 115.0(2) K, monoclinic, space group P21/n, a = 12.5472(2) Å, b = 12.8679(2) Å, c = 14.2551(2) Å, β = 111.192(2)°, V = 2145.93(6) Å3, Z = 4, dcalcd = 1.819 Mg m−3, μ = 6.248 mm−1, F(000) = 1144, 2θmax = 60.992°, 23325 reflections collected, 6202 independent, R1 = 0.0165 for 5830 observed data (>2σ(F)) with 109 restraints and 282 parameters, wR2 = 0.0404 (all data), final GOF = 1.077, CCDC 1572270. Crystal data for 2: C171H166Cl5Ir2N23O5Sn2, formula weight 3422.29, black prism, 0.328 × 0.061 × 0.034 mm3, T = 100.0(2) K, triclinic, space group P1̅, a = 12.1404(6) Å, b = 23.6181(5) Å, c = 27.3935(9) Å, α = 108.206(2)°, β = 94.485(4)°, γ = 96.124(3)°, V = 7366.9(5) Å3, Z = 2, dcalcd = 1.543 Mg m−3, μ = 2.294 mm−1, F(000) = 3464, 2θmax = 50.054°, 72148 reflections collected, R1 = 0.0503 for 35134 observed data (>2σ(F)) with 3811 restraints and 2422 parameters, wR2 = 0.0836 (all data); final GOF = 0.804, refined as a two-component twin with HKLF5 SHELXL instruction (twin law is 180° rotation about a*, twin fraction is 0.4772(3)), CCDC 1572271. The intensity data for the structural analysis of 1 and 2 were collected on an Oxford Diffraction “Gemini-R” CCD diffractometer with graphite-monochromated Mo Kα radiation using an Oxford Instrument Cryojet system. Raw data reduction to F2 was carried out using CrysAlisPro from Oxford Diffraction Ltd. The structures were solved by direct methods and refined by full-matrix least-squares methods against F2 using SHELX-2013 and -2016/6.11 Non-hydrogen atoms were refined in the anisotropic approximation. Positions of hydrogen atoms were calculated geometrically. Methyl groups of Cp* in 1 are rotationally disordered between two orientations with 0.5/0.5 occupancies. There is a strong disorder in the crystal structure of 2. The leuco cis-indigo molecule is positioned near the inversion center and statistically disordered between two orientations of 0.5 occupancy. One of two coordination [SnII(Pc•3−)]•−(indigoN,N)2−(Cp*IrIII) units contains coordinated cis-indigo dianions statistically disordered between two orientations. As a result, all distances presented in the article were calculated for the second unit containing ordered cis-indigo dianion. Three end ethyl groups of the Bu4N+ cations are disordered between two orientations with the 0.52/ 0.48, 0.60/0.40, and 0.54/0.46 occupancies. There are three positions of disordered solvent C6H4Cl2 molecules in 2. In two fully occupied positions these molecules are disordered between two and three orientations with 0.75/0.25 and 0.45/0.30/0.25 occupancies. C6H4Cl2 molecules positioned in the inversion center are disordered between two orientations with 0.28/0.22 occupancies. To keep the anisotropic thermal parameters of the disordered fragments within reasonable limits, the displacement components were restrained using ISOR,
EXPERIMENTAL SECTION
Materials. trans-Indigo (>97%) and tin(II) phthalocyanine were purchased from TCI. While two crystal phases are reported, indigo shows the trans conformations in both cases.9 Tetrabutylammonium bromide (Bu4NBr, >99%), decamethylcobaltocene (Cp*2Co), and (pentamethylcyclopentadienyl)iridium(III) diiodide dimer ((Cp*IrIIII2)2) were purchased from Aldrich. Sodium fluorenone ketyl was obtained as described.10 o-Dichlorobenzene (C6H4Cl2) was distilled over CaH2 under reduced pressure; n-hexane was distilled over Na/benzophenone. All operations on the synthesis of 1 and 2 and their storage were carried out in a MBraun 150B-G glovebox with controlled atmosphere and water and oxygen content less than 1 ppm. The solvents were degassed and stored in the glovebox. KBr pellets for IR and UV−visible−NIR measurements were prepared in the glovebox. Polycrystalline samples of 1 and 2 were placed in 2 mm quartz tubes under anaerobic conditions for EPR and SQUID measurements and sealed under umbient pressure. General Measurements. UV−visible−NIR spectra were measured in KBr pellets on a PerkinElmer Lambda 1050 spectrometer in the 250−2500 nm range. FT-IR spectra (400−7800 cm−1) were measured in KBr pellets with a PerkinElmer Spectrum 400 spectrometer. EPR spectra were recorded for polycrystalline samples of 1 and 2 with a JEOL JES-TE 200 X-band ESR spectrometer equipped with a JEOL ES-CT470 cryostat working between room and liquid helium temperatures. A Quantum Design MPMS-XL SQUID magnetometer was used to measure the static magnetic susceptibility of 2 at 100 mT magnetic field under cooling and heating conditions in the 300−1.9 K range. A sample holder contribution and core temperature independent diamagnetic susceptibility (χd) were subtracted from the experimental values. The χd values were estimated by the extrapolation of the data in the high-temperature range by fitting the data with the following expression: χM = C/(T − Θ) + χd, where C is the Curie constant and Θ is Weiss temperature. Effective magnetic moments (μeff) were calculated with the following formula: μeff = (8χMT)1/2. Synthesis. Crystals of {(cis-indigo-N,N)2−(Cp*IrIII)} (1) were obtained by the following procedure. An 11 mg portion of transindigo (0.042 mmol) was reduced by Cp*2Co (13.6 mg, 0.042 mmol) in 8 mL of o-dichlorobenzene, and separately 24.2 mg (0.042 mmol) of Cp*IrI2 in the form of a dimer was reduced by Cp*2Co (13.6 mg, 0.042 mmol) in 8 mL of o-dichlorobenzene. After stirring of both solutions for 1 h, they were filtered in one flask and stirred additionally for 4 h at 80 °C to yield a deep red solution. The solution was cooled to room temperature and filtered into a 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. The crystals precipitated on the walls of the tube over 1 month. Then the solvent was decanted from the crystals and they were washed with n-hexane to yield deep red blocks in 76% yield. Composition of the crystals was determined from X-ray diffraction analysis on single crystals. We tested several crystals from the synthesis, and all of them showed the same unit cell parameters. Therefore, they belonged to one crystal phase. The composition of 1 B
DOI: 10.1021/acs.inorgchem.7b02351 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry SIMU, and DELU SHELXL instructions. This resulted in 109 and 3811 restraints used for the refinement of the crystal structures of 1 and 2, respectively. CCDC 1572270−1572271 contain supplementary crystallographic data for this paper.
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RESULTS AND DISCUSSION
Initially, we tried to obtain a coordination complex between indigo and Cp*IrIII. trans-Indigo reduced by decamethylcobaltocene (Cp*2Co) was mixed with 1 equiv of reduced Cp*IrIIII2. After these reagents were mixed, a deep red solution was obtained, the slow mixing of which with n-hexane over a 1 month period led to the formation of red crystals in good yield. X-ray diffraction analysis of a single crystal provided evidence of the composition {(cis-indigo-N,N)(Cp*Ir)} (1), which was confirmed by elemental analysis. In contrast to the indigo complex with chromium,7 both deprotonated nitrogen atoms of indigo coordinate the iridium center in 1. Therefore, iridium has strong affinity for the nitrogen atoms of indigo, whereas chromium has affinity for the carbonyl oxygen atoms. Such coordination of the indigo ligand is rare. According to DFT calculations the isomeric form of 1 with the indigo ligand coordinated by O atoms of carbonyl groups has 40.5 kcal/mol higher energy (see the Supporting Information). Previously only the complex (cis-indigo){RuII(acac)2} with deprotonated cis-indigo was obtained.5 In the case of 1 coordination of CpIrIII is realized with the reduced form of indigo, which makes the transition of indigo from the trans to the cis conformation possible. Previously, we have shown that reduction of indigo decreases the energy barrier of such a transition.7 In a study described in a 2015 report, we showed that Cp*IrIIII2 coordinates tin(II) phthalocyanine, which leads to the formation of a single Sn−Ir bond in the {SnII(Pc2−)(Cp*IrIIII2)}·2C6H4Cl2 complex.12 Therefore, we used the tin(II) phthalocyanine radical anion as an additional paramagnetic ligand for the iridium center in 1. Mixing of (Bu4N+)(indigo)•− and (Bu4N+)[SnII(Pc•3−)]•− salts with 1 equiv of Cp*IrIIII2 in dimeric form led to the formation of a deep blue solution, which confirms the preservation of [SnII(Pc•3−)]•− in the reaction solution. Slow mixing of this solution with n-hexane over a 1.5 months period led to the formation of black prismatic crystals of 2. Their composition was determined from single-crystal X-ray diffraction analysis to be (Bu4N+)2{[SnII(Pc•3−)](cis-indigo-N,N)2−Cp*IrIII}•−2·0.5(H2Indigo)·2.5C6H4Cl2 (2). Several single crystals that had precipitated following the synthetic approach just described showed the same unit cell parameters, an observation indicating that only one crystal phase had formed. Elemental analysis cannot be used to confirm the composition of 2 due to the high sensitivity of the material to air (most probably due to the presence of [SnII(Pc•3−)]•−). The structure of the coordination unit in 1 is depicted in Figure 1a. Both N(indigo)−Ir bonds are short: 1.993(2) and 2.001(2) Å. The Cp* ligand binds the iridium ion through η5type coordination, where the average length of the C(Cp*)−Ir bond is 2.187(2) Å. These bond lengths are reproduced well by DFT calculations, being 2.019 and 2.015 Å, respectively, and the geometry of the indigo ligand is also reproduced well (see Table 1). The lengths of the bonds of the deprotonated cis-indigo dianions in 1 are presented in Table 1. Notably, these bonds are similar in length to those of pristine transindigo13 Thus, deprotonation and transition to the cis
Figure 1. Molecular structures of the coordination units discussed in the text: (a) (cis-indigo-N,N)2−(Cp*IrIII) in 1 and (b) [(cis-indigoN,N)2−{SnII(Pc•3−)}(Cp*IrIII)]•− anion in 2. Carbon atoms are shown in brown, nitrogen atoms in blue, oxygen atoms in red, iridium ions in violet, and the tin center in pink. The ellipsoid probability is 30%.
conformation do not appear to have a strong effect on bond lengths in indigo. Coordination to iridium only affects the planarity of the molecule. Initially, trans-indigo is nearly planar,13 but as part of 1, the cis-indigo dianion adopts a concave conformation whereby bending occurs near the central CC bond (the dihedral angle between the two C6H4C(O)CN planes is 11.17°). However, this bending is not due to the rotation of the mentioned planes around the central CC bond. Though both 1 and (cis-indigo){RuII(acac)2}5 contain cis-indigo ligands, the lengths of some bonds in them are different. cis-Indigo in 1 has shorter central C−C bond and essentially longer (C)C−N bonds (Table 1). The reason for this could be the different charge state of cis-indigo in these complexes. cis-Indigo in 1 obtained under the reduction conditions has a dianionic state with central C−C and (C)C−N bonds closer to double and single bonds, respectively. The formation of cis-indigo in (cis-indigo){RuII(acac)2} is accompanied by oxidation of cis-indigo dianion and the formation of a neutral cis-indigo ligand.5 This provides elongation of the central C−C bond and shortening of (C)C−N bonds, which become closer to single and double bonds, respectively. The geometry of the [(cis-indigo-N,N)2−{SnII(Pc•3−)} (Cp*IrIII)]•− anions in 2 is depicted in Figure 1b. The tin(II) phthalocyanine radical anions coordinate the iridium ion to form a single Sn−Ir bond of 2.5764(7) Å in length. The presence of this additional ligand affects the geometry of (cisindigo-N,N)2−(Cp*IrIII). The mean plane of Cp* in 1 is nearly perpendicular to that defined by the iridium center and the two coordinating nitrogen atoms (the angle between these planes is 87.06°). This angle decreases to 54.21° in 2 (Figure 1b), most likely due to steric reasons. This geometric change results in a noticeable elongation of the Ir−N bonds in 2, whose lengths increase up to 2.071(7) and 2.102(7) Å. At the same time, the average length of the C(Cp*)−Ir bonds, 2.193(8) Å, is nearly the same as that of the analogous parameter in1. Noticeable changes in the bond lengths are observed for cis-indigo dianions in 2 in comparison with pristine trans-indigo13 and cis-indigo dianions in 1 (Table 1). The central CC and carbonyl CO bonds are noticeably elongated in 2, but the (C)C−C(O) bonds are shortened. Such changes indicate an increase in the effective negative charge on the cis-indigo dianions in 2 in comparison with 1. Similar bond length changes are observed in the transition of the related thioindigo species from the neutral form to the radical anion and, finally, to the dianion state.14 We presume that the geometric changes observed in 2 may be the result of the combination of two effects: first, the highly electron donating properties of the C
DOI: 10.1021/acs.inorgchem.7b02351 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Average Indigo Bond Lengths (Å) in the Presented Complexes (1 and 2) and in Reference Compounds indigo moleculeb unit
central CC bond
carbonyl CO bond, av
single (C)C−N bond, av
single (C)C−C(O) bond
trans-indigo13 (cis-indigo)2−(Cp*IrIII) in 1 calcd structure of 1 [(cis-indigo)2−{SnII(Pc•3−)}(Cp*IrIII)]•− in 2a free leuco cis-indigo in 2 {[cis-thioindigo-(μ2-O),(μ-O)]2− Cp*CrIII}214 (cis-indigo)RuII(acac)25
1.343(2) 1.370(2) 1.381 1.382(11) 1.42(4) 1.445(2) 1.391(6) 1.404(6)
1.239(2) 1.223(2) 1.226 1.299(10) 1.35(4) 1.332(2) 1.214(5) 1.212(6)
1.381(2) 1.385(2) 1.385 1.394(10) 1.40(3)
1.460(2) 1.484(2) 1.483 1.421(12) 1.40(3) 1.377(2) 1.495(6) 1.501(6)
1.338(6) 1.339(6)
a
The geometry of only one of two units with ordered cis-indigo dianions is considered. bFor the position of the bonds see Scheme 1.
{SnII(Pc•3−)}•− ligand induce a decrease in the electrophilicity of the IrIII center and therefore an increase in the effective negative charge on (cis-indigo-N,N)2−, and second, the formation of strong hydrogen bonds between the O atoms of the carbonyl groups of (cis-indigo-N,N)2− and the OH group of leuco cis-indigo, which results in an elongation of the C−O bonds of the carbonyl groups (see the Supporting Information). The cis-indigo dianions also have a bent conformation in 2, with an angle between the two C6H4C(O)CN planes of 11.11°. The geometry of {SnII(Pc•3−)}•−(Cp*IrIII) in 2 is rather close to that of {SnII(Pc2−)}(Cp*IrIIII2).12 The only difference is the alternation of the N(imine)−C bonds in 2, which is characteristic of Pc•3− 15 (the difference between short and long bonds is 0.022 Å). Since both {SnII(Pc•3−)}•− and cis-indigo dianions coordinate the same metal center, they come into close proximity with each other. Their molecular planes are nearly parallel, and 12 short C,N,O(indigo)−C,N(Pc) contacts are formed with lengths in the 3.10−3.34 Å range, which result in an effective π−π interaction between the two ligands. Free cis-indigo is positioned near the inversion center of 2 and is disordered between two orientations of 0.5 occupancy. In spite of the poor accuracy of the bond lengths due to the mentioned disorder (Table 1), it is obvious that the CC and CO bonds are elongated up to the length of single bonds, whereas the single (C)C−C(O) bond is shortened (Table 1). Such a geometry is close to that of related dianions (e.g., to that of the thioindigo dianion,14 Table 1). The large dihedral angle (12.53°) between the two C6H4C(O)CN planes in cis-indigo also supports the formation of a single central C− C bond characteristic of the leuco form. In 2, free cis-indigo forms short C−O···OC contacts of 2.4 Å in length with carbonyl oxygen atoms of the neighboring cis-indigo 2− dianions. The length of this contact indicates the formation of a strong hydrogen-bonding interaction, and it simultaneously requires the presence of hydrogen atoms attached to the oxygen atoms of free cis-indigo. Therefore, we suppose that a protonated leuco cis-indigo (Scheme 1) is present in 2. This scenario is made possible by the fact that coordination of iridium deprotonates indigo and releases free H+ ions that can protonate noncoordinating indigo anions. The association of three indigo units through hydrogen-bonding interactions is depicted in Figure 2b. Hydrogen-bonding interactions occur between the N−H and O−H groups of the leuco form and the carbonyl oxygens of (cis-indigo)2−. Similar contacts are formed in another orientation of leuco cis-indigo. We made efforts to model hydrogen bonds formed in 2 using several approaches. In the separate complexes [Sn II (Pc •3− )]{(cis-indigoN,N)2−Cp*IrIII}•−(H2indigo) (I) and (H2indigo){cis-indigo-
Figure 2. (a) Fragment of the crystal structure of 2 showing the packing of the {[SnII(Pc•3−)](cis-indigo-N,N)Cp*IrIII}•− anions. Only the major occupied orientations are shown for the disordered components. Solvent C6H4Cl2 molecules are not shown for clarity. (b) Hydrogen bonds formed between (cis-indigo)2− and one orientation of leuco cis-indigo. The black and (parenthetical) green numerical figures represent the lengths (in Å) of the hydrogen bonds and of the N,O···N,O contacts, respectively.
N,N)2−Cp*IrIII}[SnII(Pc•3−)]•− (II) (Figure S4 in the Supporting Information), which are different in the orientation of leuco cis-indigo relative to the cis-indigo2− ligand, good correspondence of calculated and experimental (in parentheses) contacts, O−O 2.531 (2.437) and 2.698 (2.712) and N−O 2.749 (2.787) and 3.150 (2.962) Å, is observed (for more details see the Supporting Information). In both cases the interaction energy with H2indigo is almost the same, ∼25 kcal/mol. A strong hydrogen bond with proton localization on the cis-indigo2− ligand forms in the neutral trimolecular complex {(cis-indigo-N,N)2−Cp*IrIII}(H2indigo){cis-indigoN,N)2−Cp*IrIII} (III, Figure S5a in the Supporting Information) without the [SnII(Pc•3−)]•− units. In the more complicated model {[SnII(Pc•3−)](cis-indigoN,N) 2− Cp*Ir III } •− }(H 2 indigo){cis-indigo-N,N) 2− Cp*Ir III [SnII(Pc•3−)]}•− (IV, Figure S5b) the effect of noncompensated Coulomb repulsion is observed. This can be seen from the elongation of the Sn−Sn distance of 20.5 Å with respect to the experimental distance of 19.7 Å (for more details see the Supporting Information). These results demonstrate that leuco cis-indigo species stabilized the structure of 2 through the formation of strong hydrogen bonds. The (cis-indigo-N,N)2−(Cp*IrIII) units form π-stacking dimers in 1 packed in such a way that the Cp* ligands from the neighboring dimers form several short C(Cp*)−C,O(indigo) contacts with them (Figure 3). Figure 2a gives a view D
DOI: 10.1021/acs.inorgchem.7b02351 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
IR spectra of pristine trans-indigo and complexes 1 and 2 are presented in Figures S7−S9 in the Supporting Information, and the wavenumbers of the absorption maxima are given in Table S1 in the Supporting Information. The absorption bands of the CO vibration modes are very sensitive to the charge state of indigo and are shifted to smaller wavenumbers on reduction, since double CO bonds are elongated and become similar in length to a single C−O bond. In the spectrum of trans-indigo, the most intense band is observed at 1627 cm−1. Deprotonation with the formation of cis-indigo dianions and their coordination of IrIII in 1 shift this band to 1658 cm−1. Therefore, in 1, CO bonds are shorter than they are in pristine trans-indigo, a scenario confirmed by X-ray diffraction analysis (Table 1). The position of this band at 1604 cm−1 in the spectrum of 2 confirms the elongation of the CO bonds in comparison with trans-indigo and 1. We explain this elongation by an additional transfer of electron density from [SnII(Pc•3−)]•− to cis-indigo dianions. Complex 1 is electron paramagnetic resonance (EPR) silent down to 4 K, indicating that IrIII and cis-indigo dianions are in a diamagnetic state. In contrast, complex 2 is paramagnetic. An effective magnetic moment of 1.71 μB at 300 K per half of the formula unit (Figure S12a in the Supporting Information) indicates contribution of one noninteracting S = 1/2 spin (the calculated value is 1.73 μB). The temperature dependence of the reciprocal molar magnetic susceptibility of 2 is linear, which allows determination of a Weiss temperature of −2 K (Figure S12b). This value shows that only a weak antiferromagnetic coupling of spins exists in 2, due to the large distance separating Pc•3− moieties. The decrease in value of the magnetic moment of 2 below 80 K (Figure S12a) can be explained by antiferromagnetic intermolecular interactions. According to DFT calculations {[SnII(Pc•3−)](cis-indigoN,N)2−Cp*IrIII}•−}(H2Indigo){cis-indigoN,N)2−Cp*IrIII[SnII(Pc•3−)]}•− (IV) has the triplet ground state located slightly below the singlet state. This corresponds well to the experimental observations. The EPR signal of 2 is well reproduced with three Lorentzian lines, and such a close reproduction is observed throughout the temperature range studied (4−325 K). These components have g1 = 2.0026 and line width (ΔH) of 0.88 mT, g2 = 1.9995 and ΔH = 2.82 mT, and g3 = 1.9806 and ΔH = 13.88 mT at 162 K (Figure 5a). Integral intensities of these signals are related as 2/9/89% at 162 K, respectively. EPR signals due to the [SnII(Pc•3−)]•− radical anions coordinating transition metals are generally broad and have g factors smaller than 2.000. These signals have temperature-dependent g factors and line widths.16 Solid-state EPR spectra of the thioindigo radical anion are known14 and display EPR signals that are essentially narrower (0.5−0.8 mT) and have g factors larger than 2.000. On the basis of this consideration, narrower signals with g1 and g2 can be attributed to cis-indigo dianions displaying additional spin density due to spin donation from [SnII(Pc•3−)]•−. Both spectra also show similar temperature dependence of the g factors and line widths (Figure 6). The main signal with g3 can be unambiguously attributed to [SnII(Pc•3−)]•−. It is a very broad signal, and as the temperature decreases, it shows a narrowing and a decrease in the value of the g factor (Figure 6). Narrow and broad signals show different saturation behavior, since narrow signals are suppressed at high microwave power. Thus, EPR evidence supports the idea that spin density in 2 is delocalized on both ligands, due to electron and spin density transfer from
Figure 3. View of the packing of the {(cis-indigo-N,N)(Cp*IrIII)} units in 1. Short van der Waals contacts between the units are shown by green dashed lines.
of the crystal structure of 2. The main structural motif is the pairs of {[SnII(Pc•3−)]•−(indigo-N,N)Cp*IrIII}•− anions linked to leuco cis-indigo through hydrogen bonding. These pairs are isolated by Bu4N+ cations and solvent molecules. As a result, the Pc•3− macrocycles have no short contacts with each other. Pristine trans-indigo shows an intense band in the visible range at 668 nm and weaker bands at 335 and 288 nm (Figure 4, curve a). Deprotonation and the cis conformation of indigo
Figure 4. UV−visible−NIR spectra of (a) trans-indigo, (b) complex 1, (c) salt {cryptand(Na+)}[SnII(Pc•3−)]•−·C6H4Cl2 with noncoordinated [SnII(Pc•3−)]•− radical anions,16 and (d) compound 2. Spectra were measured in KBr pellets prepared under anaerobic conditions.
in 1 result in a strong blue shift of the lowest energy band and its splitting into two bands with maxima at 552 and 467 nm. In the spectrum of 1, the band in the UV range retains its position at 298 nm (Figure 4, curve b). Complex 2 shows two intense bands in the visible range at 626 and 726 nm (Figure 4, curve d). The spectrum of the reference complex {SnII(Pc2−)}(Cp*IrIIII2)·2C6H4Cl2 is characterized by a single intense band at 734 nm attributed to SnII(Pc2−);12 therefore, the band at 726 nm in the spectrum of 2 can be attributed to [SnII(Pc•3−)]•−. The band at 626 nm is probably due to the coordinated cis-indigo dianions, to which, in the spectrum of 1, is attributed the lowest energy band at 552 nm. Therefore, a red shift of this band is observed in the spectrum of 2. Bands in the near-IR (NIR) range at 895 and 1043 nm and in the UV range at 330 nm (Figure 4, curve d) can be ascribed to [SnII(Pc•3−)]•−. The position of these bands is similar to those observed in the spectrum of {cryptand(Na+)}[SnII(Pc•3−)]•−· C6H4Cl2, with maxima at 858, 1038, and 335 nm (Figure 4, curve c).16 E
DOI: 10.1021/acs.inorgchem.7b02351 Inorg. Chem. XXXX, XXX, XXX−XXX
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[SnII(Pc•3−)]•− to (cis-indigo)2−. This hypothesis is also confirmed by the changes in bond lengths and the shift of the absorption band of the CO vibration modes for (cisindigo)2− observed in 2. Although the deprotonated indigo dianion is weak acceptor, its further reduction is also possible: for example, by KC8 in tetrahydrofuran.4e The relative intensities of narrow signals are only 6% of those of the broad signals at 120 K, and they increase up to about 28% at 300 K (Figure 5b), showing that this charge transfer is a thermally activated process. The relative integral intensities of the signals enable determination of the equilibrium constants (K) of the electron transfer: [SnII(Pc•3−)]•− + (cis-indigo)2− ⇄ [SnII(Pc2−)]0 + (cis-indigo)•3−. Plotting ln K versus 1/T (Figure S13 in the Supporting Information) yields the Gibbs energy (−ΔG°) of the reaction, which is equal to 9.27 ± 0.18 kJ/mol. However, it should be noted that the observed charge transfer does not change the magnetic moment of 2 and only affects the distribution of spin density between two ligands. To model electron transfer between ligands, the coordination center {[SnII(Pc•3−)]•−(indigo-N,N)Cp*IrIII}•− (V) was considered. Its optimized structure is given in Figure S6 in the Supporting Information. In this complex, the charges of Pc and indigo ligands are −0.84 and −0.79, respectively, but the spin density, 0.98, is almost completely localized on Pc•3−. The [SnII(Pc•3−)]•− and (cis-indigo-N,N)Cp*IrIII fragments have charges of −0.40 and −0.60, respectively. The breakdown of this complex into these two fragments with frozen geometric parameters shows that the electron is localized on the Pc ligand, but its localization on the (cis-indigo)2− ligand requires insignificant energy expense, 0.44 eV for intermolecular electron transfer. As TDDFT calculations of the spectrum of excited states of the complex {[SnII(Pc•3−)]•−(indigo-N,N)Cp*IrIII}•− show, the energy of the corresponding interligand transfer becomes 0.29 eV (28 kJ/mol). The low-lying excited state with a relative energy of 0.15 eV (14 kJ/mol) is completely localized on the Pc ligand because of the quasidegenerate nature of the Pc2− LUMO. This energy almost coincides with the energy of the first excited state in the [SnII(Pc•3−)]•− fragment with frozen geometry. This quasidegeneracy causes the temperature dependence of the EPR signal in systems containing the Pc•3− radical anions.
Figure 5. (a) EPR signal of polycrystalline 2 at 162 K. Fitting of the signal by three Lorentzian lines is shown below. (b) Temperature dependence of the content of [SnII(Pc•3−)]•− and (cis-indigo-N,N)•3− according to EPR measurements.
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CONCLUSION
In conclusion, a complex of deprotonated cis-indigo dianions with iridium pentamethylcyclopentadienyl was obtained. Such a conformation allows the coordination of both nitrogen atoms of indigo to the metal center. This complex coordinates the tin(II) atom of the [SnII(Pc•3−)]•− radical anions via the iridium center. The Pc•3− radical trianions are paramagnetic and insert a magnetic component in the complex. The [SnII(Pc•3−)]•− radical anion also has strong donor properties, and evidence shows that it donates electron and spin density to the cis-indigo dianion moiety through an effective π−π interaction between (cis-indigo-N,N)2− and Pc•3−. The approach developed in this study enables the design of coordination complexes with two functional ligands that can introduce different functionalities into the complexes: for example, optical and magnetic components. The development of other complexes of this type is in progress.
Figure 6. Temperature dependence of g factor (a) and line width (b) of three Lorentzian lines composing the EPR signal of 2. Curves indicated by filled squares, triangles, and circles present temperature dependences for the lines with g1, g2, and g3, respectively.
F
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dehydroindigo anions as bridging electron-donor ligands in diruthenium complexes. Inorg. Chem. 2016, 55, 3105−3116. (e) Guo, F.-S.; Layfield, R. A. Strong direct exchange coupling and single-molecule magnetism in indigo-bridged lanthanide dimers. Chem. Commun. 2017, 53, 3130−3133. (5) Mondal, P.; Das, A.; Lahiri, G. K. The electron-rich {Ru(acac)2} directed varying configuration of the deprotonated indigo and evidence for its bidirectional noninnocence. Inorg. Chem. 2016, 55, 1208−1218. (6) (a) Nawn, G.; Waldie, K. M.; Oakley, S. R.; Peters, B. D.; Mandel, D.; Patrick, B. O.; McDonald, R.; Hicks, R. G. Redox-active bridging ligands based on indigo diimine (’Ninidigo’) derivatives. Inorg. Chem. 2011, 50, 9826−9837. (b) Nawn, G.; Waldie, K. M.; Oakley, S. R.; Peters, B. D.; Mandel, D.; Patrick, B. O.; McDonald, R.; Hicks, R. G. Redox-active bridging ligands based on indigo diimine (“Nindigo”) derivatives. Inorg. Chem. 2011, 50, 9826−9837. (c) Mondal, P.; Plebst, S.; Ray, R.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Uncommon cis configuration of a metal−metal bridging noninnocent nindigo ligand. Inorg. Chem. 2014, 53, 9348−9356. (7) Konarev, D. V.; Khasanov, S. S.; Kuzmin, A. V.; Shestakov, A. F.; Otsuka, A.; Yamochi, H.; Saito, G.; Lyubovskaya, R. N. Cisconformation of indigo in coordination complex (indigo-O,O)(Cp*CrIICl). Dalton Trans. 2016, 45, 17095−17099. (8) (a) Carter, K. P.; Young, A. M.; Palmer, A. E. Fluorescent sensors for measuring metal ions in living systems. Chem. Rev. 2014, 114, 4564−4601. (b) Suzuki, Y.; Yokoyama, K. Development of functional fluorescent molecular probes for the detection of biological substances. Biosensors 2015, 5, 337−363. (c) Albrecht, M.; van Koten, G. Platinum group organometallics based on pincer complexes: sensors, switches, and catalysts. Angew. Chem., Int. Ed. 2001, 40, 3750−3781. (9) Süsse, P.; Wolf, A. A new crystalline phase of indigo. Naturwissenschaften 1980, 67, 453. (10) Konarev, D. V.; Khasanov, S. S.; Yudanova, E. I.; Lyubovskaya, R. N. The η2-complex of nickel bis(diphenylphosphino)propane with fullerene: (Ni(dppp)·(η2-C60)·(solvent) obtained by reduction. Eur. J. Inorg. Chem. 2011, 816−820. (11) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (12) Konarev, D. V.; Troyanov, S. I.; Kuzmin, A. V.; Nakano, Y.; Khasanov, S. S.; Otsuka, A.; Yamochi, H.; Saito, G.; Lyubovskaya, R. N. Coordination complexes of pentamethylcyclopentadienyl iridium(III) diiodide with tin(II) phthalocyanine and pentamethylcyclopentadienyl iridium(II) halide with fullerene C60− anions. Organometallics 2015, 34, 879−889. (13) Kettner, F.; Schäfer, L. J.; Röder, K.; Purgahn, U.; Krautscheid, H. Selective crystallization of indigo B by a modified sublimation method and its redetermined structure. Acta Crystallogr., Sect. E: Struct. Rep. Online 2011, 67, o2867. (14) Konarev, D. V.; Khasanov, S. S.; Shestakov, A. F.; Fatalov, A. M.; Otsuka, A.; Yamochi, H.; Kitagawa, H.; Lyubovskaya, R. N. Cisthioindigo (TI) − new ligand with accessible radical anion and dianion states. Strong magnetic coupling in the {[TI-(μ2-O),(μO)]Cp*Cr}2 dimers. Dalton. Trans. 2017, 46, 14365−14372. (15) Konarev, D. V.; Kuzmin, A. V.; Faraonov, M. A.; Ishikawa, M.; Nakano, Y.; Khasanov, S. S.; Otsuka, A.; Yamochi, H.; Saito, G.; Lyubovskaya, R. N. Synthesis, crystal structures, optical and magnetic properties of crystalline salts with radical anions of metal-containing and metal-free phthalocyanines. Chem. - Eur. J. 2015, 21, 1014−1028. (16) Konarev, D. V.; Kuzmin, A. V.; Nakano, Y.; Faraonov, M. A.; Khasanov, S. S.; Otsuka, A.; Yamochi, H.; Saito, G.; Lyubovskaya, R. N. Coordination complexes of transition metals (M = Mo, Fe, Rh and Ru) with tin(II) phthalocyanine in neutral, radical anion and dianionic states. Inorg. Chem. 2016, 55, 1390−1402.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02351. IR spectra of starting compounds and 1 and 2, EPR spectra and SQUID data of 2, and details of theoretical calculations (PDF) Accession Codes
CCDC 1572270−1572271 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, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for D.V.K.:
[email protected]. ORCID
Dmitri V. Konarev: 0000-0002-7326-8118 Hiroshi Kitagawa: 0000-0001-6955-3015 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by Russian Science Foundation Grant No. 17-13-01215 and by JSPS KAKENHI Grant Number JP26288035 and the JST (ACCEL) 27 (100150500010) project.
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REFERENCES
(1) (a) Cooksey, C. J. Indigo: an annotated bibliography. Biotech. Histochem. 2007, 82, 105−125. (b) Stasiak, N.; Kukuła-Koch, W.; Głowniak, K. Modern industrial and pharmacological applications of indigo dye and its derivatives - a review. Acta Polym. Pharm. 2014, 71, 215−221. (2) (a) Yao, M.; Sano, H.; Ando, H.; Kiyobayashi, T. Molecular ion battery: a rechargeable system without using any elemental ions as a charge carrier. Sci. Rep. 2015, 5, 10962. (b) Irimia-Vladu, M.; Głowacki, E. D.; Troshin, P. A.; Schwabegger, G.; Leonat, L.; Susarova, D. K.; Krystal, O.; Ullah, M.; Kanbur, Y.; Bodea, M. A.; Razumov, V. F.; Sitter, H.; Bauer, S.; Sariciftci, N. S. Indigo - a natural pigment for high performance ambipolar organic field effect transistors and circuits. Adv. Mater. 2012, 24, 375−380. (3) Klimovich, I. V.; Leshanskaya, L. I.; Troyanov, S. I.; Anokhin, D. V.; Novikov, D. V.; Piryazev, A. A.; Ivanov, D. A.; Dremova, N. N.; Troshin, P. A. Design of indigo derivatives as environment-friendly organic semiconductors for sustainable organic electronics. J. Mater. Chem. C 2014, 2, 7621−7631. (4) (a) Beck, W.; Schmidt, C.; Wienold, R.; Steinmann, M.; Wagner, B. Indigo-metal complexes: synthesis and structure of PdII and PtII compounds containing the anions of indigo and octahydroindigo as mono- and bis-chelate ligands. Angew. Chem., Int. Ed. Engl. 1989, 28, 1529−1531. (b) Lenz, A.; Schmidt, C.; Lehmann, A.; Wagner, B.; Beck, W. Metallkomplexe von Farbstoffen, VIII [1]. Ein- und zweikernige Palladium- und Platin(II)-Chelat-Komplexe mit dem Mono- und Dianion von Indigo und Indigoderivaten. Z. Naturforsch. B, Chem. Sci. 1997, 52, 474−484. (c) Wu, J. Y.; Chang, C. H.; Thanasekaran, P.; Tsai, C. C.; Tseng, T. W.; Lee, G. H.; Peng, S. M.; Lu, K. L. Unusual face-to-face π−π stacking interactions within an indigo-pillared M3(tpt)-based triangular metalloprism. Dalton Trans. 2008, 6110−6112. (d) Mondal, P.; Chatterjee, M.; Paretzki, A.; Beyer, K.; Kaim, W.; Lahiri, G. K. Noninnocence of indigo: G
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