Synthesis and Characterization of Paramagnetic Tungsten Imido

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Synthesis and Characterization of Paramagnetic Tungsten Imido Complexes Bearing α‑Diimine Ligands Hiromasa Tanahashi, Hideaki Ikeda, Hayato Tsurugi,* and Kazushi Mashima* Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

ABSTRACT: Tungsten imido complexes bearing a redox-active ligand, such as N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (L1), N,N′-bis(2,6-diisopropylphenyl)-1,4diaza-1,3-butadiene (L2), and 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (L3), were prepared by salt-free reduction of W(NC 6 H 3 -2,6- i Pr 2 )Cl 4 (1) using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) followed by addition of the corresponding redox-active ligands. In the initial stage, reaction of W(NC6H3-2,6-iPr2)Cl4 with MBTCD afforded a tetranuclear W(V) imido cluster, [W(NC6H3-2,6-iPr2)Cl3]4 (2), which served as a unique precursor for introducing redox-active ligands to the tungsten center to give the corresponding mononuclear complexes with a general formula of W(NC6H32,6-iPr2)Cl3(L) (3, L = L1; 4, L = L2; and 6, L = L3). X-ray analyses of complexes 3 and 6 revealed a neutral coordination mode of L1 and L3 to the tungsten in solid state, while the electron paramagnetic resonance (EPR) spectra of 3 and 4 clarified that a radical was predominantly located on the tungsten center supported by neutral L1 or L2, and the EPR spectra of complex 6 indicated that a radical was delocalized over both the tungsten center and the monoanionic redox-active ligand L3.

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haloalkanes on the d0 tantalum center with a dianionic αdiimine ligand, in which one-electron transfer proceeded from the dianionic α-diimine ligand to the haloalkanes through the redox-innocent metal center, and further treatment of (αdiimine)TaCl4 with NaBPh4 produced (α-diimine)TaCl3, NaCl, BPh3, and biphenyl through a one-electron reduction of the monoanionic α-diimine ligand (Scheme 1c).7 As a part of our continuing interest in the combination of early transition metals and α-diimine ligands,7,8 we prepared paramagnetic mononuclear imido−tungsten complexes with a general formula of W(NC6H3-2,6-iPr2)Cl3(L) [3, L = N,N′-bis(2,6diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (L1); 4, L = N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L2); and 6, L = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (L3)]. We herein report the incorporation of three types of redox-active α-diimine skeletons L1L3 into an imido−tungsten(V) fragment, W(NC6H3-2,6-iPr2)Cl3 (Figure 1). The X-ray diffraction studies of 3 and 6 clarified the neutral coordination mode of the α-diimine ligands L1 and L3 to tungsten, while electron paramagnetic resonance (EPR) measurement of 3, 4, and 6 elucidated the spin densities of the ligands over the d1 metal center in solution.

INTRODUCTION A recent trend is to synthesize transition metal complexes bearing redox-active ligands, in which the electronic interactions between the metal center and ligand play an important role in controlling the redox events over the metal center through oxidation and reduction of the supporting ligands.1,2 Much attention has been directed to early transition metals with a high oxidation state, as suitable redox-active ligands could introduce any redox reactions on such redox-innocent high oxidation state early transition metal centers.3,4 The first report on this topic was presented by Heyduk, demonstrating that oxidative addition of halogens to a d0 zirconium center having two dianionic o-amidophenolato (tBu-NOcat) ligands was compensated for the two ligand-centered one-electron oxidation processes, and that reductive elimination proceeded on the d0 diphenylzirconium center having two monoanionic oiminosemiquinonate (tBu-NOsq) ligands by accepting two electrons from the metal center to the ligand, giving biphenyl as the organic product (Scheme 1a).5 Another attractive example was a d0 zirconium complex with two 1,4-diaza-1,3butadiene (α-diimine, DAD) ligands that controlled a multielectron transfer between the ligand and the coordinated dioxygen via ligand-based oxidation and reduction: the bis(αdiimine)zirconium(IV) complex exhibited reversible dioxygen uptake/release behavior, which was attributed to the redox processes of the α-diimine ligands, giving bis(α-diimine)Zr(η2peroxo)2 as the oxygen uptake product (Scheme 1b).6 We also reported reductive cleavage of a carbon−halogen bond of © XXXX American Chemical Society

Received: September 17, 2015

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DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Representative Examples for Early Transition Metal Complexes with Redox-Active Ligands To Support Metal-Centered Redox Reactions by Ligand-Centered Oxidation/Reduction

Figure 2. Molecular structure of complex 2 with 30% thermal ellipsoids. The hydrogen atoms are omitted for clarity.

clear structure of 2, and the bond lengths and angles are summarized in Table 1. Four tungsten atoms are bridged as the Table 1. Selected Bond Distances (Å) and Angles (deg) for Complex 2 W1−N1 W1−Cl1 W1−Cl3 W1−Cl5 W2−Cl4 W2−Cl6 W1−W2 W1−N1−C1

Figure 1. Three α-diimine ligands with different substituents on the ligand backbone used for the complexation to a W(NC6H32,6-iPr2)Cl3 fragment (Ar = 2,6-iPr2C6H3).

1.730(6) 2.346(2) 2.453(2) 2.614(2) 2.393(2) 2.461(2) 2.7796(9) 176.9(5)

W2−N2 W1−Cl2 W1−Cl4 W2−Cl3 W2−Cl5 W2*−Cl6 W2···W2* W2−N2−C13

1.735(6) 2.340(2) 2.465(2) 2.401(2) 2.5215(19) 2.460(2) 3.768(1) 172.2(5)

dimer of the dinuclear imido tungsten(V) fragment of “W2( NC6H3-2,6-iPr2)2Cl6”. Within each dimer unit, two tungsten atoms are connected by a face-sharing bioctahedron with a triply chloride bridging (W−Cl = 2.393(2)−2.614(2) Å). One tungsten atom of the dinuclear unit has two terminal chloride atoms (W1−Cl1 = 2.346(2) Å and W1−Cl2 = 2.340(2) Å), and the other has two additional bridged chloride atoms (W2− Cl6 = 2.461(2) Å and W2*−Cl6 = 2.460(2) Å). The facesharing bioctahedron allows for the shorter bond distance of W1−W2 (2.7796(9) Å), suggesting the presence of a single bond,10 whereas there is no direct bonding interaction between W2 and W2* (3.768(1) Å). The bond distances of W1−N1 (1.730(6) Å) and W2−N2 (1.735(6) Å) are regarded to have a typical triple bond characteristic in good accordance with the bond angles of W1−N1−C1 (176.9(5)°) and W2−N2−C13 (172.2(5)°) donating six electrons of the nitrogen atom to the tungsten center.11 Thus, the salt-free reduction enabled us to isolate the neutral tetranuclear imido tungsten(V) cluster 2, being notably different from the known anionic dinuclear imido−tungsten complexes, [P(CH2Ph)Ph3][W2(NR)2Cl7] (R = Et and Ph), which were derived from the reaction of W(NR)Cl4 with sodium amalgam in the presence of [P(CH2Ph)Ph3][Cl] as an anion source.12 Complex 2 was a tetramer of the “W(NC6H3-2,6-iPr2)Cl3” fragment involving the W(V) center, and, hence, 2 served as a precursor of “W(NC6H3-2,6-iPr2)Cl3” upon treatment with redox-active ligands L1L3 to give a series of the corresponding mononuclear tungsten imido complexes W(

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RESULTS AND DISCUSSION Reduction of W(NC6H3-2,6-iPr2)Cl4 (1) by MBTCD7,9 at 80 °C gave a tetrameric imido−tungsten(V) complex, [W( NC6H3-2,6-iPr2)Cl3]4 (2), in 72% yield along with the formation of Me3SiCl (1 equiv) and C6H5CH3 (0.5 equiv) in C6D6, which was monitored by 1H NMR spectroscopy, confirming the consumption of 0.5 equiv of MBTCD to 1 (eq 1). Because complex 2 was less soluble in nonpolar

solvents, its composition was clarified by combustion analysis and the structure was revealed by an X-ray diffraction study. Although the complex 2 was soluble in THF, we did not observe any resonances in the 1H NMR spectrum, and a broad signal was detected in the EPR measurement, probably due to the formation of paramagnetic and monomeric W(NC6H32,6-iPr2)Cl3(THF)n (vide inf ra). Figure 2 shows the tetranuB

DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

presumably due to the π-donor character of the chloride ligand at the trans position to N2; π-back-donation to the vacant π* orbital of N2−C2 shortens the W−N2 and elongates the N2− C2 bonds. Based on the geometrical parameters of the αdiimine ligand moiety, ligand L1 coordinates to the metal center in a neutral mode. When L2 was used as a supporting ligand to the metal center, the reaction of 2 and L2 in toluene at 80 °C for 8 h produced a paramagnetic complex 4 in 89% yield (eq 2). Although we did not obtain any single crystals of 4 suitable for X-ray diffraction study, we reduced complex 4 using 0.5 equiv of 2,3,5,6tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene13 as a reducing reagent in toluene at 80 °C for 2 h to give the known complex W(NC6H3-2,6-iPr2)Cl2(L2) (5)8i in 75% yield along with 1 equiv of Me3SiCl and 0.5 equiv of 2,3,5,6tetramethylpyrazine (abbreviated Me4pyrazine) (eq 3). The formation of 5 thus suggested that complex 4 contained one imido ligand, one L2, and three chlorides, similar to complex 3.

NC6H3-2,6-iPr2)Cl3(L) (3, L = L1; 4, L = L2; 6, L = L3). We first examined the reaction of a tungsten complex with αdiimine ligand L1 having the most negative redox potential. Treatment of 2 with 4 equiv of L1 in toluene at 80 °C for 5 days gave trichlorotungsten complex 3 in 59% yield as a brown powder (eq 2). The resonances in the 1H and 13C NMR

spectra did not appear in the normal regions, suggesting that complex 3 had a paramagnetic nature. The overall molecular structure was clarified by the X-ray diffraction study, and the ORTEP drawing and geometrical parameters are shown in Figure 3 and Table 2, respectively. Complex 3 adopts a six-

By treating complex 2 with 4 equiv of L3 in toluene at 80 °C for 8 h, complex 6 was obtained in 88% yield (eq 4). The

structure of complex 6 was determined by combustion analysis and an X-ray diffraction study (Figure 4), and the geometrical

Figure 3. Molecular structure of complex 3 with 30% thermal ellipsoids. The hydrogen atoms are omitted for clarity.

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 3 W−N1 W−N3 W−Cl2 N1−C1 C1−C2

2.336(5) 1.754(5) 2.3707(18) 1.295(9) 1.484(9)

W−N2 W−Cl1 W−Cl3 N2−C2 W−N3−C29

2.046(5) 2.3567(18) 2.3878(14) 1.388(7) 177.4(5)

coordinated octahedral geometry with one of two nitrogen atoms of L1 cis and the other trans to the imido ligand. The bond distance of W−N3 (1.754(5) Å) and the angle of W− N3−C29 (177.4(5)°) are reasonable for a six-electron donor imido ligand. Because of the strong trans influence of the 6edonating imido ligand, the distance of W−N1 is much longer (0.290 Å) than that of W−N2. The α-diimine skeleton possesses a short−long−short sequence, in which the distance of N2−C2 is 0.093 Å longer than that of N1−C1. The short distance of W−N2 and the long distance of N2−C2 are

Figure 4. Molecular structure of complex 6 with 30% thermal ellipsoids. The hydrogen atoms are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry parameters of 6 are summarized in Table 3. Similar to complex 3, the tungsten center possesses a six-coordinated octahedral

calized over the five-membered metallacycle), as shown in Chart 1. It was noteworthy that a very weak multiline splitting

Table 3. Selected Bond Distances (Å) and Angles (deg) for Complex 6

Chart 1. Different Electronic Structures in W(NC6H32,6-iPr2)Cl3(L) (3, L = L1; 4, L = L2; and 6, L = L3): Two Resonance Forms of RLM and RDM in W(V) Species and Their Electronic Isomer of RDL for W(VI) Species

W−N1 W−N3 W−Cl2 N1−C1 C1−C2

2.383(4) 1.742(4) 2.3665(10) 1.290(6) 1.481(6)

W−N2 W−Cl1 W−Cl3 N2−C2 W−N3−C37

2.098(3) 2.3583(12) 2.3880(12) 1.333(7) 178.6(3)

geometry where one nitrogen atom (N1) of L3 occupies a position trans to the imido ligand and the other nitrogen atom (N2) is located in a position cis to the imido ligand, and, accordingly, the three chlorine atoms are meridional and cis to the imido ligand. The 6e-donating imido ligand is confirmed by the structural features of the bond distance of W−N3 (1.742(4) Å) and the angle of W−N3−C37 (178.6(3)°). The distance of W−N1 (2.383(4) Å) is much longer than that of W−N2 (2.098(3) Å) due to the trans influence of the imido ligand. The distances of N1−C1 (1.290(6) Å) and N2−C2 (1.333(7) Å) are longer than those of normal NC bonds of L3, but shorter than those of N−C bonds of L3 coordinating to a metal center in dianionic coordination mode.14 The longer W−N2 and shorter N2−C2 distances in 6, upon comparison with the corresponding bond distances for complex 3, might be due to the weaker π-back-bonding to the L3 ligand. Overall, the geometrical properties of complexes 3 and 6 suggested that the coordination mode of α-diimine ligands to tungsten(V) is predominantly neutral in the solid state. Spin density in these paramagnetic tungsten complexes 3, 4, and 6 in solution was estimated based on their EPR measurements (Figure 5). The EPR spectra of 3 and 4

signal (g = 2.004, Aiso = 6.2 G)16 was observed for complex 4, which corresponded to a ligand-localized organic radical (RDL, a radical delocalized in the ligands), as an electronic isomer to RLM/RDM, represented in Chart 1. A similar signal for complex 3 was negligible, because the positive redox potential of L2 over L1 (L1, E1/2 = −2.48 V; L2, E1/2 = −2.13 V), ascribed to the reduction ease of L2 by the “(ArN)WCl3” fragment, led to the observation of an RDL-derived signal in the EPR spectrum for complex 4. In addition, the EPR spectrum of complex 3 in THF showed two signals: one was the same signal to complex 3 in toluene, and the other was assignable to THF-coordinated (ArN)WCl3(THF)n species,16 indicating that the L1 coordinated to the metal center as a neutral ligand and was reversibly exchanged to the coordinating solvent compared with L2. In sharp contrast, complex 6 concurrently showed two strong signals: a highly broadened three-line signal (g = 1.903, Aiso = 17 G) for the resonance forms of RLM/RDM and a seven-line splitting signal at a typical g value for organic radicals (g = 2.005, Aiso = 5.1 G) corresponding to the electronic isomer represented as RDL.16 The relative intensity of the two signals in toluene is ca. 3:1, and the signal assignable to RLM/RDM was the major. The stronger signal for the organic radical in 6 compared with 4 for RDL is due to the most positive redox potential of L3 (L3: E1/2 = −1.68 V) among the α-diimine ligands. The hyperfine coupling of the organic radical was supported by coupling of the radical with two p-H of N-aromatic substituents and two equivalent nitrogen atoms in L3,7 but the radical did not couple with protons on the acenaphthene backbone, indicating that the radical was mainly delocalized over the bis(N-aryl)diimine moiety.17 Thus, the coordination mode of L3 was assignable as both a neutral and monoanionic ligand to the tungsten in solution. We previously observed the same phenomena that the complexation of d1 transition metal species, TaCl4, with L1 L3 gave TaCl4(L), in which the strength of the signal due to RDL similarly depended on the redox property of the L1L3.7 Based on the above findings, we concluded that the spin density at the metal, the metallacycle, and the supporting ligand were tunable by changing the redox potential of the supporting redox-active ligands. Because α-diimine ligands L1−L3 are redox-active ligands, the electrochemical properties of complexes 3, 4, and 6 were measured by cyclic voltammetry (CV) in CH2Cl2 containing 0.1 M [nBu4N][BF4] with a scan rate of 100 mV/s, and results are shown in Figure 6. The CV measurement of 4 and 6 revealed two reversible (for 4) and two quasi-reversible (for 6) one-electron redox processes corresponding to one-electron reduction (4/4−, E1/2 = −0.61 V; 6/6−, E1/2 = −0.72 V vs

Figure 5. EPR spectra (X-band, 9.812 GHz) of (a) 3, (b) 4, and (c) 6 in toluene at −40 °C.

displayed a highly broad three-line patterned signal (g = 1.888 (Aiso = 22 G) for 3 and 1.919 (Aiso = 19 G) for 4) assignable to a radical at the tungsten atom, based on the g value and the three-line patterned hyperfine coupling.15 The g value was higher and the isotropic hyperfine coupling constant was lower for 3 and 4 than for those of the purely tungsten-centered localized radical (g = 1.798 for complex 2 in THF;16 g = 1.878, Aiso = 46 G for W(O)(TPP)X (TPP = tetraphenylporphyrin)15a) due to the two resonance forms of tungsten-localized radical (RLM, a radical localized at the metal center) and WN2C2 metallacycle-delocalized one (RDM, a radical deloD

DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Anhydrous hexane, toluene, and THF were purchased from Kanto Chemical and further purified by passage through activated alumina under positive argon pressure as described by Grubbs et al.21 Benzened6 was distilled over CaH2 and degassed before use. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were measured on BRUKER AVANCEIII-400 spectrometers. The EPR spectra were recorded on a BRUKER EMX-10/12 spectrometer. The elemental analyses were recorded by using PerkinElmer 2400 at the Faculty of Engineering Science, Osaka University. All melting points were measured in sealed tubes under argon atmosphere. UV−vis spectra were recorded on Agilent 8453. Cyclic voltammograms were recorded in a glovebox at room temperature in CH2Cl2 solution containing 0.1 M [nBu4N][BF4] as the supporting electrolyte. Synthesis of [W(NC6H3-2,6-iPr2)Cl3]4 (2). MBTCD (0.58 mL, 2.0 mmol) in toluene (5.0 mL) was added to a suspension of W( NC6H3-2,6-iPr2)Cl4 (1.00 g, 2.00 mmol) in toluene (30 mL) at room temperature. The reaction mixture was stirred for 48 h at 80 °C. The supernatant was removed by decantation, and the precipitate was washed with hexane (3 × 10 mL) and dried under reduced pressure to give 2 (720 mg, 0.387 mmol) as a brown powder in 72% yield, mp >300 °C. Unreacted MBTCD was removed by the workup process. Because of the low solubility of the complex 2, we could not observe any signals in the 1H NMR spectrum. When complex 2 was dissolved in coordination solvents such as THF, a broad signal was observed in the EPR spectrum (g = 1.798) due to the formation of W(NC6H32,6-iPr2)Cl3(THF)n. Anal. Calcd for C48H68Cl12N4W4: C, 30.96; H, 3.68; N, 3.01; Found: C, 31.34; H, 3.91; N, 3.04. Synthesis of W(NC6H3-2,6-iPr2)Cl3(L1) (3). N,N′-Bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (217 mg, 0.536 mmol) and 2 (247 mg, 0.133 mmol) were suspended in toluene (40 mL), and the reaction mixture was heated for 5 days at 80 °C. After filtration of the reaction mixture, solvent was evaporated and the residue was washed with hexane (3 × 7 mL). Drying in vacuo gave 3 as a brown powder in 59% yield (274 mg, 0.315 mmol), mp 190 °C (dec). Anal. Calcd for C40H57Cl3N3W: C, 55.22; H, 6.60; N, 4.83; Found: C, 55.30; H, 6.19; N, 4.91. Synthesis of W(NC6H3-2,6-iPr2)Cl3(L2) (4). To a solution of N,N’bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (243 mg, 0.645 mmol) in toluene (5.0 mL) was added a suspension of complex 2 (300 mg, 0.161 mmol) in toluene (5.0 mL) at room temperature. The reaction mixture was stirred for 8 h at 80 °C to give a dark brown solution. The solvent was removed under reduced pressure. The residue was washed with hexane (3 × 5 mL) to give 4 (480 mg, 0.570 mmol) as a pale brown powder in 89% yield, mp 273 °C (dec). Anal. Calcd for C38H53Cl3N3W: C, 54.20; H, 6.34; N, 4.99; Found: C, 54.13; H, 6.50; N, 4.66. Synthesis of W(NC6H3-2,6-iPr2)Cl2(L2) (5).8i A solution of 2,3,5,6tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (33.6 mg, 0.119 mmol) in toluene (5 mL) was added to a solution of complex 4 (200 mg, 0.238 mg) in toluene (5 mL) at room temperature, and then the reaction mixture was stirred for 2 h at 80 °C. After removal of all the volatiles under reduced pressure, the resulting product was extracted with hexane (2 × 10 mL). Hexane was removed under reduced pressure to leave a mixture of complex 5 and 2,3,5,6-tetramethylpyrazine, the latter which was removed under reduced pressure for 3 h at 80 °C. Complex 5 (144 mg, 0.179 mmol) was obtained as a brown powder in 75% yield. Complex 5 was superimposed to the NMR data with that in the literature.8i Synthesis of W(NC6H3-2,6-iPr2)Cl3(L3) (6). Complex 2 (140 mg, 0.0752 mmol) was suspended in toluene (10 mL), and then 1,2bis[(2,6-diisopropylphenyl)imino]acenaphthene (150 mg, 0.300 mmol) in toluene (5.0 mL) was added at room temperature. The reaction mixture was stirred for 8 h at 80 °C to change the color to dark red. All the volatiles were removed under reduced pressure, and the residure was washed with hexane (3 × 5 mL) to give 6 (257 mg, 0.266 mmol) as a dark powder in 88% yield, mp 284 °C (dec). Anal. Calcd for C48H57Cl3N3W: C, 59.67; H, 5.95; N, 4.35; Found: C, 54.34; H, 6.19; N, 4.21. X-ray Diffraction Study. All crystals were handled similarly. The crystals were mounted on the CryoLoop (Hampton Research Corp.)

Figure 6. Cyclic voltammograms of (a) 3, (b) 4, and (c) 6 in CH2Cl2 (5 mM of samples, 0.1 M of [nBu4N][BF4]) at room temperature. Scan rate: 100 mV/s.

[Cp2Fe]+/0) and one-electron oxidation (4+/4, E1/2 = 0.34 V; 6+/6, E1/2 = 0.25 V vs [Cp2Fe]+/0) (Figures 6b and 6c). Complex 3 also had a reversible one-electron reduction process (3/3−: E1/2 = −0.88 V vs [Cp2Fe]+/0), but an irreversible oxidation event was observed (Figure 6a). The difference in the irreversible peak in 3, 4, and 6 was ascribed to the stability of the one-electron oxidized species, [W(NC6H3-2,6-iPr2)Cl3(L)][BF4], where ligand L1 might be easily dissociated from the metal center as a neutral ligand to be decomposed, which was consistent with the observation of (ArN)WCl3(THF)n-derived signal in the EPR spectrum of complex 3 in THF.

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CONCLUSION We prepared paramagnetic imido−tungsten complexes 3, 4, and 6 bearing a redox-active α-diimine ligand (L1−L3) using [W(NC6H3-2,6-iPr2)Cl3]4 (2) as a tungsten(V) source. Based on EPR measurements of tungsten complexes 3, 4, and 6, the spin density of the ligands over the d1 metal center was tunable by the redox potentials of the supporting redoxactive ligands; complexes 3 and 4 having L1 and L2, respectively, had a spin density preferentially at the metallacycle moiety, whereas complex 6, having L3 with a positively shifted redox potential compared with L1 and L2, had an organic πradical feature on the ligand. Redox behavior of the α-diimine ligands attached to the tungsten center was clarified by the CV measurements, which indicated that ligands L2 and L3 preferentially bound to the tungsten center in one-electron redox processes.

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EXPERIMENTAL SECTION

General. All manipulations involving air- and moisture-sensitive compounds were carried out under argon using the standard Schlenk technique or argon-filled glovebox. 1-Methyl-3,6-bis(trimethylsilyl)1,4-cyclohexadiene (MBTCD),18 N,N′-bis(2,6-diisopropylphenyl)-1,4diaza-2,3-dimethyl-1,3-butadiene (L1),19 N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L2),19 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene (L3),19 and W(NC6H32,6-iPr2)Cl420 were prepared according to the literature procedures. E

DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

N. H.; Siegler, M. A.; van der Vlugt, J. I. Angew. Chem., Int. Ed. 2015, 54, 1516. (3) Recent reviews for early transition metal complexes with redoxactive ligands: (a) Munha, R. F.; Zarkesh, R. A.; Heyduk, A. F. Dalton Trans. 2013, 42, 3751. (b) O’Reilly, M. E.; Veige, A. S. Chem. Soc. Rev. 2014, 43, 6325. (4) Recent examples for early transition metal complexes with redoxactive ligands: (a) Zarkesh, R. A.; Ziller, J. W.; Heyduk, A. F. Angew. Chem., Int. Ed. 2008, 47, 4715. (b) Blackmore, K. J.; Sly, M. B.; Haneline, M. R.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2008, 47, 10522. (c) Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci. 2011, 2, 166. (d) Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorg. Chem. 2011, 50, 9849. (e) Lu, F.; Zarkesh, R. A.; Heyduk, A. F. Eur. J. Inorg. Chem. 2012, 2012 (3), 467. (f) Munhá, R. F.; Zarkesh, R. A.; Heyduk, A. F. Inorg. Chem. 2013, 52, 11244. (5) (a) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005, 44, 5559. (b) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 2728. (6) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400. (7) Tsurugi, H.; Saito, T.; Tanahashi, H.; Arnold, J.; Mashima, K. J. Am. Chem. Soc. 2011, 133, 18673. (8) (a) Mashima, K.; Matsuo, Y.; Tani, K. Chem. Lett. 1997, 767. (b) Mashima, K.; Matsuo, Y.; Tani, K. Organometallics 1999, 18, 1471. (c) Matsuo, Y.; Mashima, K.; Tani, K. Angew. Chem., Int. Ed. 2001, 40, 960. (d) Nakamura, A.; Mashima, K. J. Organomet. Chem. 2001, 621, 224. (e) Tsurugi, H.; Ohno, T.; Kanayama, T.; Arteaga-Müller, R. A.; Mashima, K. Organometallics 2009, 28, 1950. (f) Panda, T. K.; Kaneko, H.; Pal, K.; Tsurugi, H.; Mashima, K. Organometallics 2010, 29, 2610. (g) Kaneko, H.; Nagae, H.; Tsurugi, H.; Mashima, K. J. Am. Chem. Soc. 2011, 133, 19626. (h) Panda, T. K.; Kaneko, H.; Michel, O.; Pal, K.; Tsurugi, H.; Törnroos, K. W.; Anwander, R.; Mashima, K. Organometallics 2012, 31, 3178. (i) Tanahashi, H.; Tsurugi, H.; Mashima, K. Organometallics 2015, 34, 731. (9) (a) Tsurugi, H.; Tanahashi, H.; Nishiyama, H.; Fegler, W.; Saito, T.; Sauer, A.; Okuda, J.; Mashima, K. J. Am. Chem. Soc. 2013, 135, 5986. (b) Arteaga-Müller, R. A.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. J. Am. Chem. Soc. 2009, 131, 5370. (10) (a) Cotton, F. A.; Demarco, D.; Kolthammer, B. W. S.; Walton, R. A. Inorg. Chem. 1981, 20, 3048. (b) Barder, T. J.; Cotton, F. A.; Lewis, D.; Schwotzer, W.; Tetrick, S. M.; Walton, R. A. J. Am. Chem. Soc. 1984, 106, 2882. (c) Bradley, D. C.; Errington, R. J.; Hursthouse, M. B.; Short, R. L. J. Chem. Soc., Dalton Trans. 1990, 1043. (11) (a) Bradley, D. C.; Errington, R. J.; Hursthouse, M. B.; Short, R. L. J. Chem. Soc., Dalton Trans. 1987, 2067. (b) Nugent, W. A. Inorg. Chem. 1983, 22, 965. (c) Nugent, W. A. Coord. Chem. Rev. 1980, 31, 123. (12) Bradley, D. C.; Errington, R. J.; Hursthouse, M. B.; Short, R. L. J. Chem. Soc., Dalton Trans. 1990, 1043. (13) Saito, T.; Nishiyama, H.; Tanahashi, H.; Kawakita, K.; Tsurugi, H.; Mashima, K. J. Am. Chem. Soc. 2014, 136, 5161. (14) El-Ayaan, U.; Paulovicova, A.; Fukuda, Y. J. Mol. Struct. 2003, 645, 205. (15) (a) Nandi, G.; Sarker, S. Eur. J. Inorg. Chem. 2013, 2013, 3518. (b) Laye, R. H.; Bell, Z. R.; Ward, M. D. New J. Chem. 2003, 27, 684. (16) See the Supporting Information. (17) (a) Mondal, P.; Agarwala, H.; Jana, R. D.; Plebst, S.; Grupp, A.; Ehret, F.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2014, 53, 7389. (b) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Parsons, S.; Yellowlees, L. J. Chem. Commun. 2005, 4563. (18) Laguerre, M.; Dunogues, J.; Calas, R.; Duffaut, N. J. Organomet. Chem. 1976, 112, 49. (19) (a) Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. B 1981, 36, 823. (b) van Asselt, R.; Elsevier, C. J.; Smeets, W. J.; Spek, A. L.; Benedix, R. Recl. Trav. Chim. Pays-Bas 1994, 113, 88. (20) Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park, L.; Dimare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.; Krüger, C.; Betz, P. Organometallics 1990, 9, 2262.

with a layer of light mineral oil and placed in a nitrogen stream at 113(1) K. Measurements were made on Rigaku R-AXIS RAPID imaging plate area detector Rigaku AFC7R/Mercury CCD detector with graphite-monochromated Mo Kα (1.71075 Å) radiation. Crystal data and structure refinement parameters are listed in Table S1. The structure of complexes 2 and 3 were was solved by SHELXS97.22 The structure of complex 5 was solved by SIR92.23 The structures were refined on F2 by full-matrix least-squares method, using SHELXL-97.22 Non-hydrogen atoms were anisotropically refined. Hydrogen atoms were included in the refinement on calculated positions riding on their carrier atoms. The function minimized was [∑w(Fo2 − Fc2)2] (w = 1/[σ2(Fo2) + (aP)2 + bP]), where P = (Max(Fo2,0) + 2Fc2)/3 with σ2(Fo2) from counting statistics. The functions R1 and wR2 were (∑||Fo| - |Fc||)/∑|Fo| and [∑w(Fo2 − Fc2)2/∑(wFo4)]1/2, respectively. The ORTEP-3 program was used to draw the molecules.24

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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.5b02145. EPR spectra of 2, 3, 4, and 6 (PDF) Crystallographic data for 2, 3, and 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*H.T.: e-mail, [email protected]; tel, +81-6-68506247. *K.M.: e-mail, [email protected], tel/fax +81-66850-6245. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H. Tanahashi expresses his special thanks for the financial support provided by the JSPS Research Fellowships for Young Scientists. H. Tsurugi acknowledges the financial support by a Grant-in-Aid for Young Scientists (A) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (22) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112. (23) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435. (24) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.

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DOI: 10.1021/acs.inorgchem.5b02145 Inorg. Chem. XXXX, XXX, XXX−XXX