Tetrahedral Copper(II) Complexes with a Labile Coordination Site

Article pubs.acs.org/IC

Tetrahedral Copper(II) Complexes with a Labile Coordination Site Supported by a Tris-tetramethylguanidinato Ligand Ikuma Shimizu,† Yuma Morimoto,† Dieter Faltermeier,‡ Marion Kerscher,‡ Sayantan Paria,† Tsukasa Abe,† Hideki Sugimoto,† Nobutaka Fujieda,† Kaori Asano,§ Takeyuki Suzuki,§ Peter Comba,*,‡ and Shinobu Itoh*,† †

Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ‡ Anorganisch-Chemisches Institut and Interdisciplinary Center for Scientific Computing, Universität Heidelberg, INF 270, 69120 Heidelberg, Germany § Comprehensive Analysis Center, The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0057, Japan S Supporting Information *

ABSTRACT: A new tridentate N3 ligand (TMG3tach) consisting of cis,cis1,3,5-triaminocyclohexane (tach) and three N,N,N′,N′-tetramethylguanidino (TMG) groups has been developed to prepare copper complexes with a tetrahedral geometry and a labile coordination site. Treatment of the ligand with CuIIX2 (X = Cl and Br) gave copper(II)-halide complexes, [CuII(TMG3tach)Cl]+ (2Cl) and [CuII(TMG3tach)Br]+ (2Br), the structures of which have been determined by X-ray crystallographic analysis. The complexes exhibit a four-coordinate structure with C3v symmetry, where the labile halide ligand (X) occupies a position on the trigonal axis. 2Br was converted to a methoxido-copper(II) complex [CuII(TMG3tach)(OMe)](OTf) (2OMe), also having a similar four-coordinate geometry, by treating it with an equimolar amount of tetrabutylammonium hydroxide in methanol. The methoxido-complex 2OMe was further converted to the corresponding phenolato-copper(II) (2OAr) and thiophenolato-copper(II) (2SAr) complexes by ligand exchange reactions with the neutral phenol and thiophenol derivatives, respectively. The electronic structures of the copper(II) complexes with different axial ligands are discussed on the basis of EPR spectroscopy and DFT calculations.

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dimethyl-2,2′-bipyridine)2]2+ is a well-known example, in which a highly distorted tetrahedral geometry of the copper(II) center (the dihedral angle of the two N−Cu−N planes is 62.8°) is enforced by steric repulsion between the two rigid bidentate ligands having methyl groups at the 6- and 6′-positions.4 In this case, however, it is impossible to accommodate a labile coordination site while retaining the tetrahedral geometry, which is necessary to examine the reactivity toward external substrates in such a coordination environment. In this respect, the hydrotris(pyrazolyl)borate (Tp) ligand system developed by Kitajima, Tolman, and others is a rare example that can impose a tetrahedral geometry on the copper(II) center with a labile external ligand.5−11 However, the tetrahedral structure of the Tp complexes is easily converted to a square pyramidal structure in the presence of a small amount of coordinating solvent such as dimethylformamide (DMF).5 In this study, we present a new N3-tridentate ligand, TMG3tach, consisting of a rigid cis,cis-1,3,5-triaminocyclohex-

INTRODUCTION Physicochemical properties and reactivities of copper complexes are largely controlled by the coordination geometry of the metal centers. For example, blue copper proteins, responsible for biological electron transfer, are able to tune the redox potential as well as the electron transfer rate constant by subtle changes of the geometric alignment of the donor groups as well as the kind of donor atoms (nitrogen vs sulfur vs oxygen).1 Geometric effects on the reactivity of copper reaction centers (especially small molecule activation) in biological systems have also attracted considerable attention in synthetic bioinorganic chemistry.2,3 A large number of copper(II) model complexes with square planar, square pyramidal, or trigonal bipyramidal structures have been developed to examine the geometric effects on the physicochemical properties and reactivity. However, little is known about the chemistry of copper(II) complexes with a tetrahedral geometry, since such complexes easily undergo ligand rearrangement and/or solvent or counteranion coordination, giving thermodynamically more stable compounds with square pyramidal or trigonal bipyramidal structure. The tetrahedral copper(II) complex [Cu(6,6′© 2017 American Chemical Society

Received: May 6, 2017 Published: July 28, 2017 9634

DOI: 10.1021/acs.inorgchem.7b01154 Inorg. Chem. 2017, 56, 9634−9645

Article

Inorganic Chemistry

carbon working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to Ag/AgNO3 (1.0 × 10−2 M). All electrochemical measurements of the copper complexes were carried out under a nitrogen atmosphere. The oneelectron oxidation potential values (Eox’s) of Cu(I) complexes (vs Ag/ 10.0 mM AgNO3) were converted to those versus SCE by adding 0.298 V.62 Synthesis. 2,2′,2″-((1s,3s,5s)-Cyclohexane-1,3,5-triyl)tris(1,1,3,3tetramethyl guanidine) (TMG3tach). The compound was prepared by referring to a reported procedure.29 An aqueous solution (2.5 mL) containing (1s,3s,5s)-cyclohexane-1,3,5-triamine trihydrobromide (1.0 g, 2.7 mmol) and NaOH (0.46 mg, 8.1 mmol) was stirred overnight at room temperature. Removal of the solvent by a rotary evaporator gave a white solid, which was dried further at 100 °C under vacuum for 5 h. Then, under a nitrogen atmosphere, a solution of chloro-N,N,N′,N′tetramethylformamidinium chloride (1.5 g, 8.8 mmol) in dry acetonitrile (MeCN, 30 mL) was added dropwise to a suspension of the white solid in Et3N (1.1 mL, 8.1 mmol) with vigorous stirring at 0 °C (ice bath). After 3 h of reflux, a solution of NaOH (0.33 g, 8.1 mmol) in H2O (1.0 mL) was added. Solvents and Et3N were then removed under vacuum. The residue was treated with 50% KOH (10 mL) to deprotonate the product, and the organic product was extracted with MeCN (3 × 20 mL). The combined organic layer was concentrated under vacuum, and the resulting brown solid was dissolved in hot n-hexane. The resulting solution was dried with Na2SO4 and stirred with activated charcoal while still warm to eliminate impurities. Then, the solution was filtered through Celite. Finally, recrystallization from n-hexane gave the ligand as a slightly yellow crystal in 42% yield. 1H NMR (400 MHz, 298 K, CDCl3): δ = 1.40 (dd, 3 H, CH2, J = 23.4, 11.9 Hz) 1.61 (m, 3 H, CH2), 2.66 (s, 18 H, CH3), 2.73 (s, 18 H, CH3), 3.28 (m, 3H, CH). 13C NMR (400 MHz, 298 K, CDCl3): δ = 38.8 (CH3), 40.1 (CH3), 44.9 (CH2), 54.2 (CH), 159 (NCNMe2). HRMS (FAB+) m/z = 424.3856, calcd for C21H46N9 = 424.3871. Anal. Calcd for L·0.2H2O (C21H46.4N9O0.2): C, 59.03; H, 10.71; N, 29.51. Found: C, 59.22; H, 10.82; N, 29.32. [CuI3(TMG3tach)2](OTf)3 (1). TMG3tach (20 mg, 47 μmol) was treated with 1.5 equiv of [CuI(MeCN)4](OTf) (OTf: trifluoromethanesulfonate) (26.6 mg, 71 μmol) in MeCN (1.0 mL) under a N2 atmosphere in a glovebox. After the solution stirred for 30 min at ambient temperature, addition of the resulting solution to ether (50 mL) gave a white precipitate, which was collected by filtration to give [CuI3(TMG3tach)2](OTf)3 (1) as a white powder in 79% yield. 1H NMR (400 MHz, 298 K, CDCl3): δ = 2.07 (dd, 3 H, CH2, J = 24.0, 11.6 Hz), 2.22 (m, 3 H, CH2), 2.89 (br s, 18 H, CH3), 2.92 (br s, 18 H, CH3), 3.60 (m, 3H, CH). ESI-MS (pos): m/z = 1333.4354, calcd for C44H90Cu3F6N18O6S2 1333.4524. Anal. Calcd for [CuI3(TMG3tach)2](OTf)3 (C36H90Cu3F9N18O9S3): C, 36.39; H, 6.11; N, 16.98. Found: C, 36.18; H, 6.16; N, 17.00. [CuII(TMG3tach)Cl](OTf) (2Cl). TMG3tach (20 mg, 47 μmol) was treated with an equimolar amount of CuIICl2 (6.4 mg, 47 μmol) in CH2Cl2/THF (1.0 mL/1.0 mL) under a N2 atmosphere in a glovebox. After the solution stirred for 30 min at ambient temperature, NaOTf (8.0 mg, 47.2 μmol) was added to the mixture to give a red suspension. After for another 30 min of stirring, insoluble material was removed by filtration. Addition of the resulting dark red solution to ether (50 mL) gave a red precipitation, which was collected by filtration to give 2Cl as a red powder in 57% yield. Single crystals suitable for the X-ray crystallography were obtained by slow diffusion of ether into a saturated THF solution of the complex. ESI-MS (pos): m/z = 521.3442, calcd for C21H45ClCuN9 521.2782. Anal. Calcd for [CuII(TMG3tach)Cl](OTf)·0.5H2O (C22H46ClCuF3N9O3.5S): C, 39.34; H, 6.75; N, 18.77. Found: C, 39.20; H, 6.86; N, 18.84. IR spectrum showed existence of water molecules in a powder sample: ν(H2O) = 3440 cm−1 (Figure S3). [CuII(TMG3tach)Br](OTf) (2Br). TMG3tach (20 mg, 47 μmol) was treated with an equimolar amount of CuIIBr2 (11 mg, 47 μmol) in CH2Cl2/THF (1.0 mL/1.0 mL) under a N2 atmosphere in a glovebox. After the solution stirred for 30 min at ambient temperature, NaOTf (8.0 mg, 47.2 μmol) was added to the mixture to give a red suspension. After another 30 min of stirring, insoluble material was

ane (tach) framework and three TMG (N,N,N′N′-tetramethylguanidino) donor groups (Scheme 1). This tach derivative Scheme 1. TMG3tach Ligand

was acquired to construct transition metal complexes with a tetrahedral geometry to mimic reaction centers with a tetrahedral coordination environment for biological small molecule activation.12−26 The TMG group has attracted much recent attention as a strongly electron-donating group in coordination chemistry.27−35 Notable recent examples are the successful generation and characterization of Cu, Fe, and Ni complexes of active oxygen species (superoxide, peroxide, and oxide), relevant to reactive intermediates in biological oxidation/oxygenation chemistry.36−48 Steric effects of the Nalkyl groups on the electron donor ability of the guanidino group have also been examined by changing the N-methyl groups to larger alkyl groups.38,49−51 In addition, TMGcontaining aromatic compounds have been developed as noninnocent ligands, to study the electronic communication between the supported transition metal ions and extended πconjugation systems, consisting of an aromatic ring and the attached guanidino substituents.52−60 We herein report the synthesis, characterization, and some reactivity studies of copper(II)-TMG3tach complexes exhibiting a four-coordinate core structure with C3v symmetry, where a labile external ligand X lies on the symmetry axis, [CuII(TMG3tach)(X)]+ (X = Cl−, Br−, RO−, and RS−). The complexes can be regarded as rare examples of copper(II) complexes with a nearly perfect C3v arrangement of the nitrogen donor groups, with a labile coordination site on the C3 rotation axis. Comparison of the chemical features of the present copper(II)-TMG3tach complexes with those of the copper(II)-Tp complexes5−10 will provide further insights into the electronic and steric effects of the TMG3tach ligand.

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EXPERIMENTAL SECTION

General. The reagents and solvents used in this study, except the ligand and the copper complexes, were commercial products of the highest available purity and used as received without further purification, unless otherwise noted. Chloro-N,N,N′,N′-tetramethylformamidinium chloride was prepared according to the reported method.61 All reactions were carried out under an N2 atmosphere using standard Schlenk techniques or a glovebox (KK-011-AS, KOREA KIYON product). UV−visible spectra were taken on a Jasco V-570 or a Hewlett-Packard 8453 photodiode array spectrophotometer, equipped with a Unisoku thermostated cryostat cell holder USP-203. 1H NMR and 13C NMR spectra were recorded on a JEOL ECP400 or a JEOL ECS400. Elemental analyses were performed on a Yanaco New Science Inc. CHN order MT-5 or a J-SCIENCE LAB Co., Ltd. MICRO CORDER JM10. EPR measurements were carried out on a JEOL JES-TE200 or BRUKER EMX-micro instrument. CSIMS (cold-electrospray ionization mass spectra) measurements were performed on a BRUKER cryospray micrOTOFII. Electrochemical measurements (cyclic voltammetry) were performed at 298 K using an Automatic Polarization System HZ-7000 or an HZ-3000 HOKUTODENKO in deaerated acetonitrile (MeCN) containing TBAPF6 (tetran-butylammonium hexafluorophosphate, 0.10 M) as supporting electrolyte. A conventional three-electrode cell was used with a glassy 9635


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Inorganic Chemistry removed by filtration. Addition of the resulting dark red solution to ether (50 mL) gave a red precipitate, which was collected by filtration to give 2Br as a red powder in 71% yield. Single crystals suitable for the X-ray crystallography were obtained by slow diffusion of ether into a saturated THF solution of the complex. ESI-MS (pos): m/z = 564.8194, calcd for C21H45BrCuN9 565.2277. Anal. Calcd for [CuII(TMG3tach)Br](OTf)·0.5H2O (C22H46BrCuF3N9O3.5S): C, 36.44; H, 6.39; N, 17.38. Found: C, 36.15; H, 6.38; N, 17.36. IR spectrum showed existence of water molecules in a powder sample: ν(H2O) = 3450 cm−1 (Figure S4). X-ray Crystallography. All single crystals obtained were mounted on a CryoLoop (Hamptom Research Co.) with mineral oil, and all Xray data were collected at −170 °C on a Rigaku R-AXIS RAPID diffractmeter using filtered Mo Kα radiation. The structures were solved by direct methods (SIR2008) and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically by fullmatrix least-squares on F2. Hydrogen atoms were attached at idealized positions on carbon atoms and were not refined. All structures in the final stages of refinement showed no movement in the atom positions. The calculations were performed using the Single-Crystal Structure Analysis Software, version 3.8 (Rigaku Corporation: The Woodlands, TX, 2000−2006). DFT Calculations. Initial structure optimizations by DFT for the time-dependent DFT calculations were done with Gaussian 09 (revision D.01; Gaussian, Inc.).63 Molecular structures were optimized by using the UB3LYP functional with the 6-311+g(d) basis set.64,65 For the optimized geometry, normal coordinate analyses for energy minima were performed to confirm no imaginary frequency. Electronic excitation energies and intensities were computed by TD-DFT calculations at the same level, and the first 30 excited states were calculated.66,67 Graphical outputs of the computational results were generated with the Gauss View software (ver. 5.0.8) developed by Semichem, Inc.68 Structures used for the simulation of the EPR spectra were optimized with Gaussian, using the Def2tzvp69,70 basis set, and the PCM71−73 model with CH2Cl2 as solvent. This level of theory was validated with the experimental structure of 2Cl and 2Br (see Supporting Information). For the computation of spin Hamiltonian parameters, ORCA (3.0.3)74 was used with the B3LYP functional and the Def2tzvp basis set to compute the g-tensor parameters. It is known that the accurate computation of spin Hamiltonian parameters, in particular for CuII complexes, strongly depends on the level of theory used,75,76 and for specific classes of compounds, this has been evaluated in some detail.77,78 For the current set of compounds, a benchmark study involving four functionals and up to five basis sets has been performed on the well-resolved EPR spectrum of 2OMe (see Supporting Information, Table S4). The copper hyperfine splitting parameters A were computed with a ligand-field-based model, using the computed g values and orbital eigenvalues (see Supporting Information for details).76,79,80

Scheme 2. Synthesis of TMG3tach

Figure 1. ORTEP drawing of TMG3tach showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity.

coordinate tetrahedral geometry as seen in the reported tachbased ligand systems.19,22,23,25,26 However, treatment of TMG3tach with [CuI(MeCN)4](OTf) in any copper(I)/ligand ratio always gave the trinuclear copper(I) complex [CuI3(TMG3tach)2](OTf)3 (1) as confirmed by the elemental analysis and ESI-MS (see Experimental Section). A relatively simple 1H NMR spectrum of 1 (Experimental Section) suggested that the trinuclear copper(I) complex has a C3 symmetry as illustrated in Scheme 3, where each copper(I) ion takes a linear two-coordinate structure ligated by two TMG donor groups from different ligands. For the formation of a mononuclear copper(I) complex with a tetrahedral geometry, all TMG substituents need to be axially disposed at the cyclohexane ring. However, such a conformation may not be sufficiently stabilized due to weak coordinative interactions between copper(I) and the nitrogen atoms of TMG, thus affording the trinuclear copper(I) complex with the TMG groups occupying the equatorial positions. The resulting twocoordinate linear geometry at the metal center is also one of the common geometries of copper(I) complexes. Thus, we have not yet succeeded in isolating the expected mononuclear copper(I) complex with a tetrahedral structure. Nonetheless, a mononuclear copper(I) complex was suggested to be formed in solution by 1H NMR and ESI-MS, when the ligand and [CuI(CH3CN)4](OTf) were mixed in CD3Cl in a 1:1 ratio (Figures S1 and S2). However, instability of the mononuclear copper(I) complex hampered further characterization of its solution structure. Copper(II) Complexes 2. Since copper(II) complexes with tetrahedral geometry are very rare, we subsequently tried to prepare a series of copper(II) complexes using TMG3tach. Copper(II) Halide Complexes 2X. In contrast to the case of copper(I), simple mononuclear copper(II)-halide complexes

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RESULTS AND DISCUSSION Synthesis and Characterization. Ligand. The tridentate ligand, TMG3tach, was prepared according to the synthetic procedure outlined in Scheme 2. (1s,3s,5s)-Cyclohexane-1,3,5triamine trihydrobromide was deprotonated by treating it with 3 equiv of NaOH(aq) to give the free base, to which TMG groups were introduced by the reaction with chloro-N,N,N′,N′tetramethylformamidinium chloride in the presence of triethylamine in MeCN. The crystal structure shown in Figure 1 confirms the formation of TMG3tach, in which all TMG groups are in the free base form situated at equatorial positions of the cyclohexane framework. Representative bond lengths, bond angles, and dihedral angles of the TMG moiety are listed in Table 1. Copper(I) Complex 1. First of all, we attempted to get a mononuclear copper(I) complex using TMG3tach, since the ligand might be suited to stabilize a metal complex with a four9636


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Inorganic Chemistry Table 1. Selected Bond Lengths [Å], Angles [deg], and Dihedral Angles [deg] of Ligand (TMG3tach), [CuII(TMG3tach)Cl](OTf) (2Cl), and [CuII(TMG3tach)Br](OTf) (2Br) TMG3tach Cu−X Cu−N(1) Cu−N(4) Cu−N(7) N(1)−C(7) C(7)−N(2) C(7)−N(3) N(4)−C(12) C(12)−N(5) C(12)−N(6) N(7)−C(17) C(17)−N(8) C(17)−N(9) X−Cu−N(1) X−Cu−N(4) X−Cu−N(7) N(1)−Cu−N(4) N(4)−Cu−N(7) N(7)−Cu−N(1) C(7)N(2)C(9)−C(7)N(3)C(10) C(12)N(5)C(14)−C(12)N(6)C(15) C(17)N(8)C(19)−C(17)N(9)C(20)

1.283(3) 1.399(3) 1.395(2) 1.284(3) 1.3946(18) 1.399(3) 1.285(2) 1.393(3) 1.413(2)

73.82 69.45 69.38

2Cl

2Br

2.2542(10) 1.953(3) 1.937(3) 2.066(3) 1.316(4) 1.364(4) 1.357(4) 1.321(4) 1.357(4) 1.362(5) 1.308(4) 1.383(4) 1.356(5) 114.85(8) 129.64(9) 117.08(8) 93.74(11) 98.75(10) 96.66(10) 62.79 61.37 59.57

2.3852(16) 1.946(8) 1.939(8) 2.042(6) 1.341(11) 1.357(12) 1.352(11) 1.330(11) 1.364(11) 1.323(12) 1.309(10) 1.367(10) 1.377(13) 115.8(2) 128.2(3) 116.8(3) 94.0(3) 99.2(3) 97.1(3) 63.85 62.08 59.38

Scheme 3. Reaction of TMG3tach and [CuI(MeCN)4]+

Figure 2. (A) ORTEP drawings of [CuII(TMG3tach)Cl](OTf) (2Cl) (left) and [CuII(TMG3tach)Br](OTf) (2Br) (right) showing 50% probability thermal ellipsoids. The counteranions and hydrogen atoms are omitted for clarity. (B) Core structure (top view, cyclohexane omitted) of 2Cl (left) and 2Br (right). The counteranions, hydrogen atoms, and halide ions are omitted for clarity.

were obtained by treating TMG3tach with an equimolar amount of CuIIX2 (X = Cl, Br) in CH2Cl2/THF (1:1 = v:v)

under anaerobic conditions. Plots of crystal structure analyses of [CuII(TMG3tach)Cl]+ (2Cl) and [CuII(TMG3tach)Br]+ (2Br) 9637


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Figure 3. UV−vis−NIR spectra of 2Cl in CH2Cl2 (left) and 2Br in CH2Cl2 (right) at 25 °C.

Figure 4. (A) Schematic presentation of the strong electronic interaction between the TMG substituents and the halide ligand (Br) through the copper(II) ion in 2Br. (B) KS-HOMO − 1 (147b) and (C) KS-LUMO (149b), calculated by TD-DFT with UB3LYP/6-311+G(D).

that the degree of conjugation between the p orbitals of the NC group and those of the terminal C−N(Me)2 groups increases upon complexation with copper(II). This is reflected by the fact that the dihedral angles between the NC3 planes in each TMG substituent get smaller by about 10° in the copper(II) complexes as compared to those of free ligand (Table 1). The copper(II)-halide complexes exhibit a relatively intense and broad absorption band around 1100 nm (ε ≈ 250 M−1 cm−1) as shown in Figure 3. Together with the significantly broadened EPR spectrum of 2X (see Supporting Information, Figures S5 and S6), it is concluded that there is a strong electron delocalization between the TMG substituents and the halide ligand X through the copper(II) ion as presented in Figure 4A, and this is as expected from experimental and computational studies of other transition metal complexes (specifically also CuII) of guanidine-based ligands.53,56,58,81 The electronic structure of 2Br was further investigated by DFT calculations. The crystal structure of the complex was well-reproduced by a UB3LYP/6-311+G(D) level calculation. With the same functional and basis set, the TD-DFT calculation reproduced its electronic absorption spectrum rather accurately (see Supporting Information, Figure S7). The characteristic absorption band around 1100 nm can be assigned to a transition from 147b (β KS-HOMO − 1; KSHOMO = highest occupied Kohn−Sham orbital) to 149b (β KS-LUMO; KS-LUMO = lowest unoccupied Kohn−Sham orbital); both orbitals are highly delocalized over the CuII center and the ligand, including bromide and TMG3tach ([147b] Cu, 4.2%; Br, 11.3%; TMG3tach, 84.5%; [149b] Cu, 30.8; Br, 5.7; TMG3tach, 63.5; see Figure 4B,C; more details are given in the Supporting Information, Figure S8). 2Br gave a reduction peak at −0.30 V versus SCE in the cyclic voltammetry measurement in MeCN, and an oxidation peak

are shown in Figure 2; selected bond lengths and bond and dihedral angles around the TMG moiety are listed in Table 1 (the X-ray crystallographic data are presented in Table S1). The copper(II) centers of both complexes have a similar tetrahedral geometry with the three nitrogen atoms N(1), N(4), and N(7), and the halide ligand, Cl− or Br−: τ4 = 0.80 for 2Cl, τ4 = 0.82 for 2Br (τ4 = [360° − (α + β)]/141°; α and β represent the two largest torsional angles θ around the four-coordinate metal center; the value of τ4 ranges from 1 for a perfect tetrahedral geometry to 0 for a perfect square planar geometry).75 The dihedral angles around the metal centers (planeN−Cu−N vs planeN−Cu−X, averaged values of three dihedral angles), which are 90° in ideal tetrahedral geometry, are 87.07° in 2Cl and 87.05° in 2Br. Although many copper(II) complexes with a highly distorted tetrahedral geometry are known, 2X demonstrate rare examples of copper(II) complexes with a nearly perfect C3v symmetry. Kitajima and co-workers reported the copper(II)-chloride complex supported by a hydrotrispyrazolylborate ligand (Tp) with a tetrahedral geometry.5 Their copper(II)-chloride complex was, however, easily converted to a five-coordinate copper(II) complex with a square pyramidal geometry, when it was treated with a small amount of coordinating solvent such as DMF in CH2Cl2.5 In contrast to the Tp system, our copper(II) halide complexes 2X did not undergo such a solvent-addition reaction in polar solvents (vide inf ra), demonstrating the structural rigidity of the TMG3tach ligand system, maintaining tetrahedral geometry. Additionally, the TMG groups surrounding the CuII center may also prohibit the approach of solvent molecules. It should also be noted that all NC bonds of the TMG moieties (N(1)−C(7), N(4)−C(12), and N(7)−C(17)) in 2X are elongated as compared with those in the free ligand, while the C−N(Me)2 bonds of the TMG moiety are shortened upon complex formation (Table 1). These results may indicate 9638


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nm and a weak d−d transition at 720 (150) nm with clear isosbestic points at 330, 385, 645, and 920 nm. The spectral change ended, when an equimolar amount of MeOH/nBu4N· OH was added (inset of Figure 6A), indicating the formation of a 1:1 adduct. The CSI-MS of the final solution of the titration showed a major peak cluster at m/z = 517.33; the peak position as well as the peak distribution pattern are consistent with the molecular formula of the mononuclear copper(II) methoxide complex [CuII(TMG3tach)(OMe)]+ (2OMe) (Figure 6C). In this titration, a small peak cluster corresponding to the mononuclear copper(II) hydroxide complex, [CuII(TMG3tach)(OH)]+ (2OH), was also observed at 503.32, suggesting the formation of 2OH as a minor product. The EPR spectrum of the final solution of the titration is shown in Figure 6B, which is a typical EPR spectrum for a copper(II) complex with trigonal pyramidal (“tetrahedral”) geometry and a dz2 ground state. It is interesting to note that this spectrum is very similar to those reported for tris-pyrazolylborate-based “tetrahedral” CuII complexes.5 Due to the relative instability of this and other complexes reported in this paper, it is inevitable that the species trapped and spectroscopically characterized in solution also contain minor impurities. This is especially evident from the EPR spectra (see extra peaks in the spectrum presented in Figure 6B and in other spectra shown in the Supporting Information). Spin quantification indicates that the sample consists of nearly 100% of this trigonal pyramidal (“tetrahedral”) copper(II) species. Thus, it can be concluded that the bromide complex 2Br undergoes a ligand exchange reaction with methoxide anion to give 2OMe having a similar trigonal pyramidal geometry as the crystallographically characterized precursor (Figure 2), and this is a rare example of a mononuclear copper(II)-alkoxide complex with a trigonal pyramidal (“tetrahedral”) geometry. It should be noted that the broad absorption band at 1120 nm of the bromide complex 2Br disappeared, when it was converted to 2OMe (see Supporting Information, Figure S12), even though both copper(II) complexes have a similar tetrahedral geometry. Thus, the band at 1120 nm of 2Br is attributed to the strong electronic interaction between the TMG substituents and the halide ligand (Br) through the copper(II) ion, as discussed above (Figure 4). Copper(II) Phenolate and Thiophenolate Complexes (2OAr and 2SAr). The methoxide complex 2OMe can be further converted to copper(II) complexes with different external ligands (Scheme 5). Addition of an equimolar amount of pentafluorophenol (ArOH; Ar = C6F5) to 2OMe at −60 °C in CH2Cl2 gave the spectral changes shown in Figure 7A. The absorption bands due to 2OMe disappeared immediately with concomitant increase of new absorption bands at 270 nm (ε = 4600 M−1 cm−1), 300 (4600), 400 (1500), and 500 (1000). The CSI-MS spectrum of the resulting solution gave a major peak cluster at m/z = 669.30, the peak position as well as the peak distribution pattern of which were consistent with the molecular formula of the corresponding mononuclear copper(II) phenolate complex 2OAr (Figure 7C). The optical spectral change for the formation of thiophenolate complex 2SAr (ArSH; Ar = C6F5) is also shown in Figure 7B, in which more intense LMCT bands due to the thiolate to copper(II) interaction are observed at 363 nm (ε = 5280 M−1 cm−1), 440 (4000), and 540 (4000). It is worth comparing some features of 2OAr and 2SAr to those of the copper(II)-phenolate and thiophenolate complexes

appeared at +0.76 V (Figure 5). A similar electrochemical behavior was observed with 2Cl as presented in the Supporting

Figure 5. Cyclic voltammogram of 2Br (1.0 mM) in dry MeCN containing TBAPF6 (0.10 M) under N2: working electrode glassy carbon, counter electrode Pt, reference electrode Ag/10.0 mM AgNO3. Scan rate is 100 mV s−1.

Information (Figure S9). Due to the strong electron-donating nature of the TMG groups and also due to the strong interaction with the bromide (chloride) ligand (as mentioned above, see Figure 4), the reduction peak potential of the copper(II) complex 2X at −0.30 V (vs SCE) was much more negative than those of the copper-halide(X) complexes supported by a similar tach-based ligand carrying an imino group instead of TMG (X = Br, E1/2 = +0.57 V; X = Cl, E1/2 = +0.51 V vs Ag/AgCl).19 The irreversibility of the CVs of the copper(II) complexes could be ascribed to the structural change from the mononuclear structure of the copper(II) complexes 2X to oligonuclear copper(I) complexes as the [CuI4(TMG3tach)2X2]2+ complex exemplified in Figure S11 (Tables S2 and S3). Such a conversion of a mononuclear copper(II) to a tetranuclear copper(I) complex (the molar ratio of CuI: X− is 4:2) might release free X− in the solution. Importantly, a CV scan of 2Br gave an irreversible oxidation peak at 1.18 V versus SCE, which corresponds to the electrochemical oxidation of Br− to Br• (Figure S10). The very positive oxidation peak potential at +0.76 V is also consistent with a higher stability of the copper(I) oxidation state in the two-coordinate linear structure seen in the tetranuclear copper(I) complex shown in Figure S11. Copper(II) Methoxide Complex 2OMe. The bromide complex Br 2 can be converted to a methoxide complex 2OMe by ligand exchange reaction (Scheme 4). In Figure 6A are shown the Scheme 4. Ligand Exchange Reaction of 2Br with MeOH

spectral changes observed when a MeOH solution containing 10% tetrabutylammonium hydroxide (Bu4N·OH) was added to a CH2Cl2 solution of [CuII(TMG3tach)Br](OTf) (2Br) (0.25 mM) at −60 °C. The bands at 310 (ε = 4630 M−1 cm−1), 420 (2130), and 560 (1830) nm due to 2Br decreased with concomitant increase of a new absorption band at 347 (3730) 9639


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Figure 6. (A) UV−vis spectral changes observed upon addition of a MeOH solution containing 10% (nBu)4NOH to a CH2Cl2 solution of 2Br (0.25 mM) at −60 °C. Inset: Titration plot based on the absorption change at 560 nm. (B) X-band EPR spectrum of the titration product in CH2Cl2 measured at 77 K (top experimental, bottom simulation); microwave frequency, 9.116 GHz; g1 = 2.36, g2 = 2.17, g3 = 2.03, A3 = 90 G. (C) CSI-MS of the titration product in CH2Cl2 measured at −60 °C. Inset: Expanded spectrum (EXP) and its simulation spectrum (SIM) of 2OMe.

Scheme 5. Ligand Exchange Reactions of 2OMe with C6F5OH and C6F5SH Giving 2OAr and 2SAr, Respectively

of the Tp complexes (see Figure 7A). Notably, the copper(II)phenolate complexes of the Tp ligand were relatively stable and slowly induced intramolecular electron transfer from phenolate to copper(II) to provide phenoxyl radical derived C−C coupling products.9 However, the stability of 2OAr was much lower even at low temperature. Furthermore, the reactivity of 2OMe toward the bulky 2,6-di-tert-butylphenol was negligibly slow in contrast to the Tp system, which may be due to higher steric hindrance in the second coordination sphere in the TMG3tach system, prohibiting the direct coordination of the bulky phenolate. A copper(II)-SC6F5 (thiophenolate) complex, supported by the same Tp ligand, which also showed a distorted tetrahedral structure with an absorption spectrum (λmax = 420 nm (ε = 630 M−1 cm−1), 665 (5960), and 1020 (1200)) similar to those of blue copper proteins with trigonal pyramidal structures, was also reported.7 The thiophenolate to copper(II) LMCT band at 540 nm in 2SAr is significantly blue-shifted as compared to that of the Tp complex (665 nm), demonstrating a strong Cu−S bonding interaction in 2SAr. When the temperature of the solutions of 2OAr and 2SAr was increased above −40 °C, the UV−vis spectra turned back to that of the original bromide complex 2Br. Thus, it is apparent that the binding of Br− to the copper(II) ion in 2Br is stronger than those of ArO− and ArS− in 2OAr and 2SAr.

supported by a Tp ligand. Fujisawa and co-workers reported that the di(μ-hydroxido)dicopper(II) complex of the 3,5diisopropyl derivative of Tp reacted with 4-fluorophenol, 2,6dimethylphenol, and 2,6-di-tert-butylphenol to give the corresponding copper(II)-phenolate complexes, exhibiting LMCT bands at 300−400 and 600−700 nm.9 2OAr also shows an absorption band around 400 nm, but the band at the higher wavelength region is blue-shifted as compared with that 9640


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Figure 7. (A) UV−vis spectral change for the reaction of 2OMe (0.25 mM) with pentafluorophenol (ArOH) in CH2Cl2 at −60 °C. (B) UV−vis spectral change for the reaction of 2OMe (0.25 mM) with pentafluorobenzenethiol (ArSH) in CH2Cl2 at −80 °C. (C) CSI-MS of 2OAr in CH2Cl2 measured at −60 °C. Inset: Expanded spectrum (EXP) and its simulation spectrum (SIM) of 2OAr.

The EPR spectra of 2OAr and 2SAr are shown in the Supporting Information (Figure S13). 2OAr has an EPR spectrum that is similar to those of 2Cl, 2Br, and 2OMe, suggesting the smilar trigonal pyramidal (“tetrahedtral”) geometry of the copper(II) center as experimentally observed for 2Cl and 2Br with a dz2 electronic ground state (Figure 6 and Figures S5 and S6). The EPR spectrum of 2 SAr is unambiguously due to a different electronic ground state. Quite similar effects have been observed in trispyrazolylborate complexes.5 A thorough structural analysis (see Supporting Information, Table S6, and discussion above) indicates that there is no significant structural difference between 2Cl, 2Br, 2OMe, and 2OAr on one hand and 2SAr on the other: the major structural parameters defining the tetrahedral coordination geometry and the metal−ligand orientation are similar (distance of the CuII center from the trigonal plane provided by the CuII center and the N-donor atoms, angles between the N3−Cu, the Cu−X, and the X−R vectors, where X is Cl, Br, O, or S). Also, the (computed) Cu−SAr distance is as expected from other CuII complexes of SC6F5 and the other 2X complexes. That is, the differences in electronic properties are primarily the result of the CuII axial ligand bonding, and this is expected from a thorough analysis of the electronics of the CuII−Scysteinate bonding in blue copper proteins and that of CuII−SAr in model systems.82−88 The Sthiolate → CuII bonding is highly covalent, and the S(thiolate)−d(CuII) charge transfer (related

to the 550−650 nm CT transition, see above) is known to contribute to a significant extent to the ground state wave function: for plastocyanin, the delocalization of electron density on the thiolate ligand was estimated to be 38%.84 A quantitative analysis of the g- and A-parameters of 2SAr versus 2OAr may be based on quantum chemical or ligand field calculations,76,86,89−91 but both are problematic in the current case, where both the guanidine-based tripodal ligand platform and the aromatic thiolate coligand lead to substantial delocalization of the unpaired electron (see Supporting Information, Tables S8 and S9).82,92 With DFT it is in general difficult to correctly estimate covalency in CuII coordination compounds.76,89−91 That the unpaired spin is not well-localized in specific d orbitals also emerges from the computed Mulliken charges, spin distributions, and orbital eigenvalues (see Supporting Information, Tables S5 and S7 and Figure S14). Also, a thorough analysis of the EPR spectra is precluded by the ill-resolved spectra of 2Cl, 2Br, and 2OAr (Figures S5, S6, and S13). From the structural analysis (see Supporting Information, Table S6) it appears that the change of electronic ground state is primarily related to changes of the bonding of the coligand, and the π-donation of C6F5S− into the dπ orbitals must be responsible for the drastic spectroscopic changes.

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SUMMARY We have developed a new N3-tridentate ligand, TMG3tach, containing three TMG donor groups connected to the rigid 9641


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cis,cis-1,3,5-triaminocyclohexane (tach) framework. The ligand is designed to produce copper complexes with a tetrahedral geometry, and to examine the effects of the strongly electrondonating TMG group on the structure and reactivity of the copper complexes. In the reaction with [CuI(MeCN)4]+, the most stable product was the trinuclear copper(I) complex [CuI3(TMG3tach)2](OTf)3 (1), in which each copper(I) ion is bound to two TMG donor groups from different ligands, leading to a linear two-coordinate copper(I) coordination geometry. It can be concluded that the copper(I) ion ligated by the strongly electron-donating TMG group favors the twocoordinate linear structure rather than a typical tetrahedral geometry of copper(I). On the other hand, treatment of the ligand with CuIIX2 (X = Cl, Br) gave the expected copper(II)halide complexes 2X with a tetrahedral geometry and C3v symmetry. Therefore, the structural feature of the TMG3tach ligand is quite different from that of other tach-based ligands, which generally stabilize the tetrahedral geometry in the CuI but not in the CuII oxidation state. This may be attributed to the bulkiness and strong electron donor ability of the TMG donor groups. There is a strong electronic interaction between the axial ligand and the TMG substituents through the copper(II) ion, which is revealed by a broad absorption band around 1100 nm and a significantly broadened EPR signal, and also supported by DFT calculations. A comparable electronic communication between a halide ion and the TMG donor group through the copper(II) ion has recently been reported in tetrakisguanidine compounds with aromatic spacers like benzene or naphthalene.57 The rigid tach backbone and the bulky TMG substituent stabilize the tetrahedral copper(II) complexes by prohibiting a fifth ligand coordination. Instead, a ligand exchange reaction of 2Br with methanol easily takes place in the presence of an equimolar amount of a strong base such as tetrabutylammonium hydroxide to give the copper(II)-methoxide complex (2OMe), which also exhibits a tetrahedral geometry. Furthermore, the methoxide complex 2OMe can be converted to the corresponding copper(II)-phenolate (2OAr) and copper(II)thiophenolate (2SAr) complexes by reaction with the corresponding phenol and thiophenol ligands, respectively. In these reactions, however, Br− in solution, which originated from 2Br, tends to rebind to the copper(II) center due to the stronger binding, hampering further detailed characterization of the copper(II) products 2OAr and 2SAr. Kitajima and co-workers reported similar tetrahedral copper(II) complexes with halide, phenolate, and thiophenolate ligands, using the hydrotrispyrazolylborate (Tp) tridentate ligand. Their tetrahedral copper(II) complexes are fragile for further ligation; i.e., even a solvent molecule can disturb its tetracoordinate geometry. However, our CuII system with TMG3tach maintains its tetrahedral geometry even in the presence of ligands such as methoxide or hydroxide. Tetrahedral copper(II) complexes that can undergo ligand exchange reactions with maintaing the tetrahedral geometry are rare. Thus, the present system is an excellent model system to understand the reactivity of copper(II) complexes with tetrahedral geometry, which is frequently found in copper proteins and the surface of solid catalysts. Structure and reactivity of CuII-alkylperoxide complexes supported by TMG3tach are now under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01154. Figures S1−S14 and Tables S1−S15, and additional computational information (PDF) Accession Codes

CCDC 1457022−1457024 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sayantan Paria: 0000-0001-5476-8259 Nobutaka Fujieda: 0000-0003-1045-6063 Shinobu Itoh: 0000-0002-3711-2378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST CREST (JPMJCR16P1), Japan, and a Grant-in-Aid for Challenging Exploratory Research (16K13963) from JSPS. We are grateful for computational resources provided by the bwForCluster JUSTUS, funded by the Ministry of Science, Research and Arts and the Universities of the State of Baden-Württemberg, Germany, within the framework program bwHPC-C5.

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REFERENCES

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Synthesis, Electronic Structure, and Magnetic Coupling. Inorg. Chem. 2016, 55, 1683−1696.

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Tetrahedral Copper(II) Complexes with a Labile Coordination Site

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