Syntheses and Properties of Dinuclear Group 6 Metal Complexes

Article pubs.acs.org/Organometallics

Syntheses and Properties of Dinuclear Group 6 Metal Complexes with the Zwitterionic Sulfur Donor Ligand Bis(N,N‑diethylamino)carbeniumdithiocarboxylate Tomoaki Sugaya,†,∥ Takeshi Ohba,† Fumiya Sai,† Shigeru Mashima,† Takashi Fujihara,*,‡ Kei Unoura,§ and Akira Nagasawa*,† †

Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Saitama 338-8570, Japan Comprehensive Analysis Center for Science, Saitama University, Saitama 338-8570, Japan § Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan ‡

S Supporting Information *

ABSTRACT: A class of dinuclear group 6 metal complexes [{M0(CO)5}2(μ-S,S′-EtL)] (M = Cr, Mo, W) with the asymmetrically bridged zwitterionic sulfur donor bis(N,N-diethylamino)carbeniumdithiocarboxylate (EtL) was prepared by photoreaction of [M0(CO)6] with EtL in hexane−CH2Cl2 for Cr and Mo or THF for W. The same structure in a syn−anti coordination mode was revealed by X-ray analyses for chromium(0) and tungsten(0) complexes, 1 and 3, respectively, and speculated from various analytical data for the molybdenum(0) complex 2. The pertinent criteria for the coordination mode are found to be the wavenumber of the asymmetric stretching vibration of the −CS2 moiety in the solid state and the ligand-based reduction potential in solution. Complexes 1 and 3 showed quasi-reversible reduction waves, which are assigned to a two-step, one-electron reduction process derived from the bridging EtL. In a series of [Cr(CO)5L] complexes (L = monodentate ligand), the 13C NMR chemical shifts of the trans-to-L carbonyl group showed the order of increasing σ-donor/π-acceptor ratio of the ligands. The sulfur donor ligands, including EtL and thione, are positioned in the middle of this range. All complexes exhibited negative solvatochromism: the wavelength of the absorption maximum shifted to the blue side in the range 500−600 nm with an increase in the Reichardt solvent polarity parameters ET(30), except for protic solvents. Quantum chemical calculations by time-dependent density functional theory− polarized continuum model were employed for understanding the excited states and solvatochromic properties of complex 3. The calculated vertical excitation energies in solution are consistent with the experimental data, suggesting that the transition is a metal-to-ligand charge-transfer transition. In addition, UV−vis, NMR, and cyclic voltammetry data showed that complex 3 dissociates into two mononuclear species in polar solvents: [W(CO)5(EtL)] and [W(CO)5(solvent)].

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INTRODUCTION The first transition-metal carbonyl complex, [{Pt(CO)2}2(μCl)]2, was reported by Schützenberger in 1868,1 after which [Ni(CO)4] was synthesized by Mond in 1890.2 Since that time, a large number of metal carbonyls have been synthesized and studied extensively. Metal carbonyls are readily decarbonylated by photoreaction followed by substitution with another ligand. In particular, tungsten(0) carbene complexes were used as the activated species in the catalytic reaction by Fischer in 1964.3 Bis(N,N-disubstituted amino)carbeniumdithiocarboxylates4 (abbreviated as RL) were synthesized for the first time in 1965 by Winberg and Coffman,5 and they have since been investigated with respect to synthesis, structure, and reactivity.6 These species can exist in both zwitterionic (inner salt) form and neutral forms as canonical structures (Scheme 1). In fact, the compound is found to exist in the zwitterionic form both in the solid state and in solution, even in CHCl3, by X-ray structural analysis and 13C NMR spectra, respectively.6d In the solid state, the N2C− and −CS2 planes are nearly perpendicular to each other, with a dihedral angle of 82.0° for EtL (R = Et). © XXXX American Chemical Society

Scheme 1. Canonical Structure between a Zwitterionic Form and a Neutral Form of Bis(N,N-disubstituted amino)carbeniumdithiocarboxylate (RL)

The 13C NMR chemical shifts of signals suggest that the positive and negative charges are localized on N2C− and −CS2, respectively. Various uses of RLs have been reported, such as use as pathogenic agents,7 catalysts,8 and potential materials for gold−thiolate-based self-assembled monolayers (SAMs).9 These compounds can bind to metals via the dithiocarboxylate moiety, although this type of complex has not yet been sufficiently explored. Several types of metal complexes have been prepared, including mononuclear [Mo(C 5 H 5 )(CO)2(RL)](PF6) (R2N = (CH3)2N, piperidinyl)10 and NiII, Received: August 30, 2012

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dx.doi.org/10.1021/om300831y | Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Syntheses of the Complexes

electrons is the most suitable for enhanced π back-donation to the ligand. In the complexes of group 12 (ZnII, CdII, HgII) and 10 (PtII) metals, filled t2g orbitals have sufficient electrons to back-donate to the ligand.12,15 Dithiocarbamates (R2dtc−), analogous sulfur-donating bidentate ligands, form complexes with group 6 metals in various oxidation states (M0−MVI); these complexes have been prepared and studied extensively. For example, higher-valent chromium (CrIII−CrVI) complexes tend to be formed rather than those of lower-valent chromium (Cr0 or CrI), and only [Et4N][Cr0(η2-S2CNC5H10)(CO)4] has been reported for chromium(0).18 Thus, EtL and R2dtc− have preferences for different metal ion valencies, probably because of the difference in the electron-donating and -accepting abilities of the dithiocarboxylate moiety, which results from differences in the backbone. In complexes with EtL, a stronger π acid and a weaker σ base than R2dtc−, the π back-donation from the metal plays an important role in the stabilization of the metal complexes, and lower-valent metal complexes are available. Molecular Structure in the Solid State. All of the prepared complexes are solids that are unstable in air. Single crystals of all the complexes were obtained by vapor diffusion of hexane into a CH2Cl2 solution of each complex. None of the crystals contained solvent molecules. The molecular structures of 1 and 3 were determined by single crystal X-ray analysis. Chromium(0) Complex. An ORTEP drawing of 1 is shown in Figure 1. Selected bond lengths and angles are given in Table 1. Each of the sulfur atoms in EtL is bonded to each Cr atom to form a dinuclear structure [{Cr0(CO)5}2(μ-S,S′-EtL)], but the dithiocarboxylate moiety asymmetrically bridges between two Cr centers in a syn−anti coordination mode. The length of Cr(1)−S(1) (2.3692(9) Å) for syn coordination is shorter than that of Cr(2)−S(2) (2.4243(9) Å) for anti coordination. These

PdII, PtII, AgI, and AuI complexes with RL ((R2N)2C = 1,3dimethylimidazolium).11 Structures of group 12 metal complexes have been determined to be mononuclear, but the composition of the metal and RL and the coordination mode significantly depend on the substituent R on the diaminocarbenium moiety.12 Petz et al. reported transition-metal complexes with similar ligands, known as “carbodiphosphorane−CS2 adducts.”13 For group 6 metals (M = Cr, Mo, W) mononuclear-chelating complexes [M(CO)4(S2CC(PPh3)2)] are synthesized by photoreaction of [M0(CO)6] with the ligand. Recently, Delaude and Wilton-Ely et al. reported the syntheses and catalytic evaluation of ruthenium(II) and palladium(II) complexes with RL.14 Similar to the case for carboxylate, an oxygen analogue of dithiocarboxylate, RL acts as a bridging ligand. The platinum(II) complex has a dinuclear structure bridged by two μ-S,S′-EtL ligands,15 while rhenium(I) and dirhenium(I) carbonyl complexes have chelate and μ-S,S′bridged EtL structures, respectively.16 The EtL ligand bridges two metal centers in a syn−syn coordination mode in these complexes. No other coordination mode has been reported for metal complexes with a bridging dithiocarboxylate. On the other hand, carboxylate has several bridging modes, such as syn−syn, anti−anti, and syn−anti. Physical properties that depend on those coordination modes have been reported, such as magnetism.17 In this study, we describe the crystal structures, spectroscopic, and electrochemical properties of asymmetrically bridged dinuclear zerovalent group 6 metal complexes with EtL, [{M0(CO)5}2(μ-S,S′-EtL)] (M = Cr (1), Mo (2), W (3)), in the solid state and in solution. We also evaluate the correlation among the coordination mode and spectroscopic and electrochemical properties. The electronic spectra of complex 3 in various solvents exhibit a distinct solvatochromic effect, details of which have been examined using a time-dependent density functional theory (TD-DFT) approach coupled with a polarized continuum model (PCM) algorithm. A combination of experimental and theoretical approaches was used for investigating the influence of electronic factors on the solvatochromic properties of complex 3. In addition, we report the dynamic behavior of complex 3 in solution: i.e., solvolysis in a polar solvent.

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RESULTS AND DISCUSSION Syntheses. Complexes of the type [{M0(CO)5}2(μ-S,S′EtL)] (M = Cr (1), Mo (2), W (3)) incorporating the same asymmetrically bridged dinuclear structure were prepared by photoreaction of [M(CO)6] with EtL in hexane−CH2Cl2 for complexes 1 and 2 and THF for complex 3 (Scheme 2). To the best of our knowledge, the chromium(0) complex 1 was exclusively formed with EtL, with no higher-valent metal complexes (e.g., CrII or CrIII) formed. Low-valent rhenium(I) and dirhenium(I) complexes with carbonyls were also reported for group 7 metals.16 As a result, these early transition metals tend to form complexes with EtL bearing a low-spin d6 electron configuration, suggested by diamagnetism, as exhibited by the NMR spectra and the total charge of the complex. The electron configuration in which metal t2g orbitals are filled with six

Figure 1. ORTEP drawing of 1. Ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity. B


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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1 C(1)−S(1) C(1)−S(2) C(1)−C(2) C(2)−N(1) C(2)−N(2) Cr(1)−S(1) Cr(1)−C(11) Cr(1)−C(12) Cr(1)−C(13) S(1)−C(1)−S(2) N(1)−C(2)−N(2) C(1)−S(1)−Cr(1) C(1)−S(2)−Cr(2) C(11)−Cr(1)−S(1) S(1)C(1)S(2)−N(1)C(2)N(2) C(1)S(1)Cr(1)−S(1)Cr(1)C(12) C(1)S(1)Cr(1)−S(1)Cr(1)C(13)

1.670(3) 1.682(3) 1.501(4) 1.332(3) 1.322(4) 2.3692(9) 1.837(3) 1.911(4) 1.928(4) 126.19(17) 125.1(2) 117.03(10) 120.41(10) 171.17(14) 81.03 54.75 36.71

lengths fall within the range (2.36−2.50 Å) found for related complexes bearing an octahedral {Cr0(CO)5S} core. The transto-sulfur Cr−CO bonds (1.837(3), 1.844(4) Å) are significantly shorter than the four cis-to-sulfur Cr−CO bonds (average 1.915, 1.905 Å). These two types of Cr−CO bonds are similar in length to those observed in the related octahedral core {Cr0(CO)5S} in [Cr(CO)5(SCN2-iPr2C2Me2)],19 [Cr(CO)5(SNNR2)] (R = methyl, phenyl),20 [Cr(CO)5(S CMe2)],21 and [Cr(CO)5{SCHCHC(SEt2)].22 The geometries around the two chromium centers are both distorted octahedral, where the S(1)−Cr(1)−C(11) and S(2)−Cr(2)− C(16) angles are 171.17(14) and 169.65(10)°, respectively. Distortions of the coordination octahedrons appear to be due to steric repulsion between the EtL and CO ligands. The dihedral angle between the N2C− and −CS2 planes of 1 (81.0°) is almost equal to that of the free ligand EtL (82.0°), suggesting a zwitterionic structure with a C−C single bond in the backbone. Tungsten(0) Complex. An ORTEP drawing of 3 is shown in Figure 2. Selected bond lengths and angles are given in Table 2. Complex 3 is isostructural with complex 1. All bonds exhibited the same trends in their lengths and angles as those described for complex 1. The syn-coordinated W(1)−S(1) bond (2.5048(13) Å) is shorter than the anti-coordinated W(2)− S(2) bond (2.5443(13) Å), but both of these lengths fall within the range (2.48−2.61 Å) found for related complexes having an octahedral {W0(CO)5S} core.23 IR Spectra in the Solid State. Strong absorptions are observed at 1560 and 1050 cm−1 for the free RL in the solid state, which were assigned to the asymmetric stretching vibrations of N2C− and −CS2 moieties, respectively.6d On complexation, these peaks shifted to higher (N2C−) or lower (−CS2) wavenumbers, respectively, than the original positions. The wavenumber of the asymmetric stretching vibration (νasym) of the −CS2 moiety was found to be a useful criterion for the coordination mode. Previously, we reported that the νasym of −CS2 shifted to a higher wavenumber according to the following sequence: chelate < bridge ≤ free RL < monodentate.12b A similar tendency was found for metal carboxylate (RCO2−) complexes, and this observation was utilized for the criterion of the coordination mode. The difference in wavenumbers between symmetric and asymmetric −CO2 stretching vibrations (Δν = νasym − νsym) was used for

Cr(1)−C(14) Cr(1)−C(15) Cr(2)−S(2) Cr(2)−C(16) Cr(2)−C(17) Cr(2)−C(18) Cr(2)−C(19) Cr(2)−C(20)

1.919(4) 1.902(4) 2.4243(9) 1.844(4) 1.923(3) 1.922(4) 1.885(3) 1.891(4)

C(12)−Cr(1)−C(14) C(13)−Cr(1)−C(15) C(16)−Cr(2)−S(2) C(17)−Cr(2)−C(19) C(18)−Cr(2)−C(20) C(1)S(2)Cr(2)−S(2)Cr(2)C(19) C(1)S(2)Cr(2)−S(2)Cr(2)C(20)

178.18(15) 177.29(14) 169.65(10) 171.69(15) 175.61(15) 18.33 68.36

Figure 2. ORTEP drawing of 3. Ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity.

the identification of the coordination mode.24 The stretching wavenumbers for −CS2, N2C−, and C−H of CH3 are summarized in Table 3. The dependence of νasym (−CS2) on the coordination mode is the same as in the previous study. Hence, νasym (−CS2) may be a useful probe for the coordination modes in the various dithiocarboxylate complexes. In the cyclopentadienylchromium dithiocarbamate complexes, the ν asym(−CS2) values for monodentate R2dtc− ([Cr(Cp)(CO)3(Me2dtc)], 970 cm−1) and chelated R2dtc− ([Cr(Cp)(CO)2(Me2dtc)], 821 cm−1) are quite different.25 It is suggested that this trend holds for the related dithiocarbamate complexes. The mononuclear complex [M(CO)5L] (L = monodentate ligand) exhibits four ν(CO) peaks in the range 1800−2400 cm−1 in the IR spectrum, whereas the present complex shows many peaks, suggesting that each metal center has a unique environment. νasym(−CS2) and the six strong sharp −CO bands are observed at 1030 and ca. 2100−1800 cm−1, respectively, for 2 in the solid state, suggesting that the coordination mode is same as those of 1 and 3, since the pattern and positions of IR peaks are similar to those of 2. C


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Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3 C(1)−S(1) C(1)−S(2) C(1)−C(2) C(2)−N(1) C(2)−N(2) W(1)−S(1) W(1)−C(11) W(1)−C(12) W(1)−C(13) S(1)−C(1)−S(2) N(1)−C(2)−N(2) C(1)−S(1)−W(1) C(1)−S(2)−W(2) C(11)−W(1)−S(1) S(1)C(1)S(2)−N(1)C(2)N(2) C(1)S(1)W(1)−S(1)W(1)C(12) C(1)S(1)W(1)−S(1)W(1)C(13)

1.675(5) 1.676(5) 1.509(6) 1.331(6) 1.326(6) 2.5048(13) 1.987(6) 2.061(6) 2.055(5) 126.4(3) 124.8(4) 114.43(17) 120.66(17) 175.40(16) 84.17 35.64 54.2

W(1)−C(14) W(1)−C(15) W(2)−S(2) W(2)−C(16) W(2)−C(17) W(2)−C(18) W(2)−C(19) W(2)−C(20)

2.033(5) 2.030(5) 2.5443(13) 1.966(6) 2.039(6) 2.044(6) 2.048(6) 2.040(6)

C(12)−W(1)−C(14) C(13)−W(1)−C(15) C(16)−W(2)−S(2) C(17)−W(2)−C(19) C(18)−W(2)−C(20) C(1)S(2)W(2)−S(2)W(2)C(17) C(1)S(2)W(2)−S(2)W(2)C(20)

175.12(19) 178.2(2) 172.9(2) 174.9(2) 175.5(2) 59.68 31.36

Table 3. IR Absorption Bands (cm−1) for EtL Complexes EtL Ru−EtLa 1 2 3 Re−EtLb Pt−EtLc Zn−EtLd Cd−EtLe Hg−EtLf

−CS2

N2C−

C−H of CH3

coord mode

1048 1020 1032 1030 1030 1046 1072 1077 1076, 1036 1077, 1033

1558 1584 1577 1581 1581 1593 1590 1583 1574 1574

2970 2976 2970 2976 2985

free chelate bridge bridgeg bridge bridge bridge monodentate monodentate/chelate monodentate/chelate

2976 2982 2982 2979

Figure 3. 13C NMR chemical shifts (δ vs TMS) of the carbonyl carbon at the position trans to a substituent L.

reflects the order of increasing σ-donor/π-acceptor ratio in these ligands. Thus, the ylide ligand at the right of the series is a stronger σ donor but a weaker π acceptor. The sulfur donor ligands, including EtL and thione, are located in the middle region. The same order of decreased shielding was observed for complexes 2 and 3. The peak of the trans-to-L carbonyl for each [M(CO)5L] complex was shifted to the more highly shielded side depending on the metal center in the sequence chromium(0) (δ 224) < molybdenum(0) (δ 214) < tungsten(0) (δ 203), which probably reflects the levels of the highest-occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) frontier orbitals; i.e., the electronegativity of the central metal. Electrochemical Properties. Electrochemical data obtained by cyclic voltammetry in CH2Cl2 are summarized in Table 4, and voltammograms (CVs) are shown in Figure 4. The free ligand RL shows a reversible reduction wave, representing one two-electron process or two consecutive one-electron processes depending on R (for example, EtL: E1/2 = −1.93 V vs Fc/Fc+, difference in potentials between anodic and cathodic

a

[Ru(EtL)3](PF6)2. b[Re2(CO)6(μ-Cl)2(μ-EtL)]. c[Pt2Cl4(μ-EtL)2]. [Zn(EtL)4](PF6)2. e[Cd(EtL)4](PF6)2. f[Hg(EtL)3](PF6)2. gThe coordination mode was speculated by the IR spectrum. d

NMR Spectra in Solution. For all compounds, coordination had a marginal effect on the chemical shift of EtL, as evidenced by the 1H and 13C NMR signals (1H NMR, chemical shift difference between free and coordinated EtLs Δδ < 0.2; 13 C{1H} NMR, Δδ < 3), except for the 13C signal of the −CS2 moiety, which was shifted to a more shielded region by Δδ = 9−13. These observations suggest that the positively charged N2C− moiety and the alkyl substituents are marginally affected electronically by ligation to the metal ion, while −CS2 is affected to some extent, because the clear charge separation and the C−C bond prevent the influence of a decrease in the negative charge density extending onto the −CS2 moiety on coordination. All complexes with EtL exhibit two broad signals in 1H NMR, which are attributed to a rapid cooperative interchange between syn−anti and anti−syn conformations in solution, because the signals remain almost unchanged even at low temperature (230 K). The carbonyl carbon 13C NMR signals of a class of substituted metal carbonyl complexes, [M(CO)5L] (M = Cr0, Mo0, W0), have been studied extensively.26 Several trends can be seen in carbonyl carbon shielding. The carbonyl signals of [M(CO)5L] are located at a lower field than those of the parent metal hexacarbonyls. The carbonyl at the position trans to L is more deshielded than that at the cis position. Figure 3 gives a comparison of the 13C NMR chemical shifts of the trans-to-L carbonyl in a series of [Cr(CO)5L] complexes, showing the order of decreased shielding. It is suggested that this trend also

Table 4. Redox Potentials for 1 and 3 redox potentiala complex

reduction process

1b

−1.52 (150), −1.65 (130), −1.84 (140)

3b

−1.49 (190), −1.61 (180)

oxidation process 0.35 (180), 0.52 (130) 0.67 (Epa)

a Redox potentials: E1/2/V vs Fc/Fc+ [0.1 M Bu4NClO4] (ΔE (=Epa − Epc)/mV)). bSolution in CH2Cl2 (5 mM) containing Bu4NClO4 (0.1 M). Scan rate: 0.1 V s−1.

D


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a metal-to-ligand charge transfer (MLCT) process. The magnitude of the transition energy, EMLCT, obtained from the wavenumbers of this absorption maximum follows the order Mo > W > Cr. The observed tendency is very similar to that of the analogous compounds [M(CO)4(diimine)] (M = Cr, Mo, W).32 In addition, all the present complexes were soluble in an exceptionally wide range of solvents and exhibited solvatochromism: i.e., both the spectrum pattern and the intensity of the absorption significantly depend upon the solvent. However, because of solvolysis, the spectra of complexes 1 and 2 rapidly changed with time after dissolution. Thus, we examined the solvatochromic behavior of complex 3 in detail, because it is relatively stable. There have been many investigations of the solvatochromism of group 6 carbonyl complexes since the earliest report by Saito et al.33 The correlation between the wavelength of the MLCT band maximum for those complexes and several parameters of the solvent have been investigated.34 The UV−vis spectra of 3 in various solvents are shown in Figure 5, and the wavelengths

Figure 4. Cyclic voltammograms of EtL, 1, and 3.

peaks ΔE (=Epa − Epc) = 90 mV) and an irreversible oneelectron oxidation wave (EtL: Epa = 0.17 V vs Fc/Fc+) in CH3CN). It is noted that the alignment between the N2C− and −CS2 planes in RL is almost perpendicular in the solid state and in solution.6d The electronic structure of RL, according to the results of DFT calculations, is such that the sulfur lone pairs interact with carbon σ* and π* orbitals in the N2C− plane.27 In the reduction process, the dihedral angle between the N2C− and −CS2 planes in RL decreases from 84 to 6° (for EtL28) and the N2C−CS2 bond length decreases (1.52 to 1.39 Å), allowing the C−C moiety of the RL to accommodate two additional electrons to form a double bond.29 The two-electron-reduced species were actually isolated in the solid state, and the structures were determined by X-ray analysis.30 On the other hand, in the oxidation process, the RL dimerizes to give the disulfide intermediate, through which a one-sulfur-eliminated species was finally obtained.31 Complexes 1 and 3 respectively show two or three quasireversible reduction waves in the range E1/2 = −1.5 to ∼ −1.8 V vs Fc/Fc+ (ΔE = 130−190 mV), which are assigned to the processes of bridging EtL0/EtL−, bridging EtL−/EtL2−, and monodentate EtL0/EtL2−. Complexes 1 and 2 are substitution labile in solution; in particular, 2 is readily decomposed in solution. Thus, the CV of complex 1 shows waves from the original dinuclear species as well as the solvolyzed mononuclear species, whereas that of complex 3 shows only waves from the original dinuclear species in CH2Cl2. However, the waves of the mononuclear species gradually appeared, in addition to the waves of the dinuclear species in CH3CN. This behavior may be due to solvolysis, details of which are described below. The reduction waves derived from EtL are assigned to a two-step, one-electron reduction process, similar to those of the analogous rhenium(I) complex [Re2(CO)6(μ-Cl)2(μ-EtL)], which has a dinuclear triply bridged structure. On the other hand, the reduction potential of RL coordinated in a monodentate mode (e.g., [Zn(EtL)4](ClO4)2) is very close to that of the free RL: ca. −2.1 to ca. −2.3 V.12b It is shown that the reduction waves of the bridging and the chelate ligands are 1 V higher in potential than those of the monodentate or free RL. The difference may be due to the stabilization of the reduced form. UV−Vis Absorption Spectra. A prominent band is observed at 500−600 nm for all complexes; it is assigned to

Figure 5. UV−vis absorption spectra of 3 in various solvents.

of the absorption maximum and the transition energies are summarized in Table 5. The higher the relative permittivity, εr, of the solvent, the higher the EMLCT, until it becomes nearly constant when εr > 20. The EMLCT value increases almost linearly with the Reichardt solvent polarity parameter ET(30),35 except for protic solvents, including EtOH and MeOH (Figure 6). It is clear that the energy of the MLCT transition correlates with the electron-accepting ability of the solvents.36 For complex 3, the MLCT band shifted toward the shorter wavelength by 45 nm on increasing the polarity of the solvent. This is the so-called absorption hypsochromic effect (i.e., “negative” solvatochromism), which was interpreted as a greater ground-state stabilization by solvation in comparison with the excited state with increasing solvent polarity. Reaction with Polar Solvent. The deep purple color of complex 3 gradually changed to red at room temperature in polar solvents, including CH3CN and DMSO. The spectrum of complex 3 in CH3CN is shown in Figure 7. In 1 day, the absorption maximum at 553 nm shifted to a shorter wavelength of 467 nm with isosbestic points. During a further 2 weeks, the spectrum of this solution changed to an even shorter E


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Table 5. Solvent Dependence of MLCT Absorption Maxima of 3 solvent

εr

ET(30)

hexane CCl4 toluene benzene Et2O 1,4-dioxane THF CHCl3 CH2Cl2 acetone EtOH MeOH CH3CN DMSO propylene carbonate

1.9 2.2 2.4 2.3 4.2 2.2 7.4 4.7 8.9 20.7 24.3 32.6 37.5 48.9 65.1

31 32.4 33.9 34.3 34.5 36 37.4 39.1 40.7 42.2 51.9 55.4 45.6 45.1 46

DN

0.1 19.2 14.8 20 0 17 20 19.1 14.1 29.8 15.1

λmax/nm

EMLCT/eV

597 596 585 584 577 575 566 581 572 557 565 561 553 555 556

2.076 2.080 2.119 2.122 2.148 2.156 2.190 2.134 2.167 2.225 2.195 2.210 2.242 2.234 2.230

Figure 7. Time dependence of UV−vis spectra for 3 in CH3CN at room temperature: (solid line) first step; (dashed line) second step.

−1.27 V vs Fc/Fc+ (ΔE = 80 mV) and −1.37 V (70 mV), respectively. Next, two new reduction waves appeared at −1.6 to −1.7 V, and their intensities increased with time; finally, waves were observed only at E1/2 = −1.61 V (130 mV) and Epc = −1.74 V. These reduction waves were positioned in the lower-potential regions in comparison with those from bridging ligands and at higher potentials in comparison with those of monodentate or free RL. Metal complexes with RL coordinated in a monodentate mode only have a d10 electron configuration.12b The degree of reduction of 3, which has a d6 electron configuration, is easier than that of complexes with a d10 electron configuration. The final solution was dried in vacuo, and a red powder was obtained. FAB-MS of the red powder revealed a peak at m/z 557, ascribed to [[W(CO)5(EtL)] + H+]. Therefore, the final absorption maxima in the UV−vis spectrum and reduction waves in the CV are attributed to the monodentate EtL derived from [W(CO)5(EtL)]. The reaction scheme of complex 3 in polar solvents is suggested in Scheme 3. In the first step, the dinuclear complex 3 is dissociated into two mononuclear species, [W(CO)5(EtL)] and [W(CO)5(solvent)], followed by gradual decomposition of

wavelength of 366 nm, and the resulting spectrum coincided with that of the corresponding free EtL. The spectrum in DMSO changed more rapidly (first step, 2 h; second step, 10 days). The 1H and 13C NMR spectra for 3 in CD3CN are shown in Figure 8. The 1H NMR spectrum exhibited two broad signals for the EtL protons (−CH3, δ 1.35; −CH2−, δ 3.63) shortly after dissolution in CD3CN. After several minutes, new sharp signals at δ 1.23 (triplet) and 3.51 (quartet) appeared and then grew gradually with the gradual disappearance of the original signals within 1 day. Simultaneously in the 13C NMR spectrum, the −CS2 carbon signal at δ 225.4 shifted to the unshielded region at δ 231.4 within 1 day. The final position of this signal is different from that of free EtL (δ 239), whereas the ethyl and N2C− signals remained almost unshifted. The carbonyl carbons of the coordinated CO at δ 198.0 and 203.4 were shifted slightly to the deshielded region (δ 198.9, 204.3) after 1 day, and new peaks appeared at δ 197.4 and 201.2, the chemical shifts of which are similar to those of [W(CO)5(CH3CN)].38 Cyclic voltammograms for complex 3 in CH3CN are shown in Figure 9. Immediately after it was mixed with CH3CN, complex 3 showed two reversible reduction waves at E1/2 =

Figure 6. MLCT absorption maximum of 3 in solution: (a) versus the relative permittivity of the solvent εr; (b) versus Reichardt’s solvent parameters ET(30), Open circles are EtOH and MeOH; except for these data, the line is fitted by leastsquares. F


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Table 6. Comparison of Calculated Selected Bond Lengths (Å) and Angles (deg) for 3 in the Gas Phase with Experimental Values from X-ray Analysis W(1)−S(1) W(1)−C(trans)a W(1)−C(cis)b,c W(2)−S(2) W(2)−C(trans)a W(2)−C(cis)b,c C(1)−Sc C(1)−C(2) C(2)−Nc S(1)−C(1)−S(2) N(1)−C(2)−N(2) S(1)C(1)S(2)−N(1)C(2)N(2)

exptl

calcd gas phase

2.5048(13) 1.987(6) 2.044 2.5443(13) 1.966(6) 2.042 1.676 1.509(6) 1.329 126.4(3) 124.8(4) 84.17

2.469 1.995 2.039 2.520 1.990 2.033 1.681 1.479 1.335 125.39 123.80 73.69

a

trans-to-sulfur W−CO bond. bcis-to-sulfur W−CO bond. cAverage value.

between N2C− and −CS2 planes (73.69°) was smaller in comparison with the dihedral angle obtained by experiment (84.17°). The orbital energies and compositions of selected molecular orbitals are given in Table 7. Surface plots of some of the frontier molecular orbitals are shown in Figures S2 and S3 (Supporting Information). Inspecting the composition of the frontier orbitals, it is observed that throughout HOMO to HOMO-5, the major contribution comes from the d orbitals of the tungsten centers and π* orbitals of CO moieties, which is consistent with a 5d6 electron configuration of the metal ion. In addition, the high contribution of the −CS2 moiety in the HOMO describes the W−EtL π back-bonding, indicating that the HOMO is spectroscopically the most important member of the HOMO manifold. In general, the group 6 mononuclear complexes ([M(CO)4L]39 and [M(CO)5L]40) have the same electron configuration, and the three HOMOs have similar energies. In complex 3, the HOMO to HOMO-5 energy levels are broadened because of the contributions of the asymmetric metal centers and EtL. The contributions of asymmetric tungsten centers and the coordinated COs are not the same. The HOMO and HOMO-1 are dominated by the syncoordinated tungsten atom, while HOMO-5 is dominated by the anti configuration. The contribution of the trans-to-EtL carbonyl is greater than that of the cis-to EtL carbonyl in the vicinity of the HOMO (W(1), HOMO, HOMO-1; W(2), HOMO, HOMO-1, -2). The LUMO is mainly localized on the N2C−CS2 fragment. LUMO+1 and LUMO+2 are very close in energy and are localized on EtL. In particular, although LUMO+1 lies on the W(2) and EtL, LUMO+2 equivalently lies on W(1) and W(2) centers and EtL. In contrast, both LUMO+3 and LUMO+4 characters arise from the anti conformation of W(2) and the cisto EtL carbonyls. To investigate the lowest-lying singlet states of complex 3, TD-DFT calculations were performed. Selected calculated states together with their vertical excitation energies and

Figure 8. 1H and 13C NMR spectra of 3 in CD3CN: (a) shortly after dissolution; (b) after 1 day.

Figure 9. Cyclic voltammograms of 3 in CH3CN (5 mM) containing Bu4NClO4 (0.1 M): (a) shortly after dissolution; (b) after 2 h; (c) after 12 h; (d) after 2 days. Scan rate: 0.1 V s−1.

these complexes. It is suggested that solvolysis of both complexes 1 and 2 occurs in a similar fashion, but more rapidly than in the case of complex 3. Theoretical Results. A DFT calculation was performed with complex 3 starting from crystallographic data in the gas phase. Selected bond lengths and angles of the optimized geometries for complex 3 are presented in Table 6. Almost all the calculated bond lengths and angles were consistent with the experimental results determined by single-crystal X-ray structural analysis. However, the calculated dihedral angle Scheme 3. Solvolysis of 3 in a Polar Solvent

G


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Table 7. Selected Orbital Energies (eV) and Orbital Compositions (%) for 3 in the Gas Phase

a

MO

E

W(1)

CO(cis)a

CO(trans)a

W(2)

CO(cis)b

CO(trans)b

EtL

152 151 150 149 148 147 146 145 144 143 142 141

−2.96 −2.99 −3.20 −3.42 −4.20 −5.76 −6.06 −6.37 −6.41 −6.45 −6.80 −7.29

3.07 2.16 18.34 1.09 4.15 21.80 53.89 26.85 27.09 31.40 0.54 2.31

2.44 1.90 11.64 0.87 4.19 13.05 17.13 13.23 12.80 18.58 0.29 1.80

0.71 0.38 3.00 0.17 2.07 6.53 8.82 2.88 2.10 0.16 0.07 0.83

18.04 23.25 13.23 41.89 2.69 12.43 5.78 24.93 25.89 26.79 55.68 9.74

66.86 64.29 15.23 20.97 4.83 7.98 2.45 10.26 23.37 22.32 48.56 1.64

0.29 0.95 2.90 8.06 1.08 3.02 1.21 5.66 5.88 5.23 0.08 1.52

8.60 7.07 35.67 26.96 80.99 34.29 10.73 16.19 13.96 6.63 4.90 82.16

Bound for W(1). bBound for W(2).

predict negative solvatochromism and the magnitude of the solvent shift for the lowest-energy MLCT band in the electronic spectra as well as in a similar mononuclear system. To establish the usefulness of DFT calculations for understanding solvatochromism, more systematic comparisons between theory and experiment should be undertaken. Syntheses of other metal complexes with RL are in progress for clarifying the correlation of the coordination mode with spectroscopic and electrochemical properties.

oscillator strengths are displayed in Table S2 (Supporting Information). The lowest-energy highly solvatochromic transition of complex 3 can be assigned predominantly to the HOMO → LUMO transition, since the contributions from other transitions to the final state are minor. The solvent effects in complex 3 were modeled by the PCM approach. In an early stage of our DFT investigation, we tried to directly optimize the geometries of complex 3 using these solvation models; however, unfortunately, we failed to obtain optimized geometries. Thus, the relative energies including solvation effects were estimated from single-point calculations with geometries optimized in the gas phase. The orbital energies and compositions of selected molecular orbitals are given in Tables S3 and S4 (Supporting Information). It is apparent that the solvent electric field stabilizes the HOMO, destabilizing the LUMO and ΔE gs > ΔE es (ΔE gs = EHOMO(solvent) − EHOMO(gas), ΔEes = ELUMO(solvent) − ELUMO(gas)). This trend enhances the HOMO−LUMO energy difference, in complete accordance with the experimental results of negative solvatochromism.

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EXPERIMENTAL SECTION

Materials. All of the reactions were carried out under an argon atmosphere by using standard Schlenk tube techniques. All of the solvents were dehydrated and purified by distillation: Et2O and THF were refluxed over Na and distilled, and CH2Cl2 and hexane were refluxed over CaH2 and distilled. The solvent CH3CN was refluxed over P2O5 following the distillation and then refluxed over CaH2 and finally distilled. These purified solvents were stored under an argon atmosphere. Other reagents employed in the research were of reagent grade and used as received without further purification. The ligand EtL6 was prepared according to the literature method. Syntheses of Complexes. [{Cr0(CO)5}2(μ-S,S′-EtL)] (1). A solution of [Cr(CO)6] (0.44 g, 2.0 mmol) and EtL (0.23 g, 1.0 mmol) in a hexane−CH2Cl2 mixture (65 mL/15 mL) was irradiated by UV light at 313 nm (from a Pyrex-filtered high-pressure Hg discharge lamp) for 1 h under an Ar atmosphere. During irradiation the red-brown color of the solution gradually changed to deep purple. The solvent was removed by evaporation under reduced pressure, and the residual powder was dissolved in the smallest amount of CH2Cl2 and loaded onto a silica gel column. A blue-purple fraction eluted by hexane/ CH2Cl2 (2/1 v/v) was collected, and again the solvents were removed by evaporation under reduced pressure to obtain powdery products: purple powder, yield 0.35 g (29% from the precursor Cr compound). Anal. Found (calcd for C20Cr2H20N2O10S2): C, 38.59 (38.96); H, 3.30 (3.27); N, 4.31 (4.54). Mass spectrum (FAB-MS): m/z 616 (calcd for M+). IR (KBr disk, ν/cm−1): 2992 (C−H), 2067, 2052, 1997, 1966, 1941, 1926, 1903, 1883 (C−O), 1577 (C−N), 1032 (C− S) (cf. free ligand EtL (KBr disk): 2970 (C−H), 1558 (C−N); 1048 (C−S)). UV−vis (CH2Cl2, λmax/nm (ε/103 cm2 mol−1)): 395 (5300), 574 (9460) (cf. free ligand EtL (CH2Cl2): 271 (7800), 283 (sh, 6500), 368 (10000), 436 (250), 509 (130)). 1H NMR (CD2Cl2): δ 1.40 (br, 12H, CH3), 3.61 (br, 8H, CH2). (cf. free ligand EtL (CD3CN): δ 1.22 (t, 3H, CH3), 3.56 (q, 2H, CH2)12b). 13C NMR (CD2Cl2): δ 12.9 (CH3), 47.0 (CH2), 170.4 (NCN), 215.2 (CrCO(cis)), 223.7 (CrCO(trans)), 227.1 (SCS). (cf. free ligand EtL (CD3CN): δ 13.2 (CH3), 47.7 (CH2), 169.6 (NCN), 238.9 (SCS)). [{Mo0(CO)5 }2(μ-S,S′-EtL)] (2). The procedure used for the preparation of the chromium(0) complex 1 was applied. A mixture of [Mo(CO)6] (0.55 g, 2.0 mmol) and EtL (0.23 g, 1.0 mmol) in hexane/CH2Cl2 (80 mL, 13/3 v/v) was irradiated at 313 nm through a

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CONCLUSION Novel group 6 metal complexes having a zwitterionic sulfur donor ligand (EtL) with an asymmetrically bridged (syn−anti coordination) dinuclear structure were prepared. We found that the π back-donation from the metal center to the sulfur atoms in EtL plays a more important role in the stabilization of the complex than that with R2dtc−. The present results suggest that an appropriate oxidation state of the metal ion is essential for forming complexes with RL. Although all the complexes are soluble in an exceptionally wide range of solvents, decomposition took place except in the case of the tungsten(0) complex 3, which is stable in solution and exhibits negative solvatochromism. Particularly in polar solvents, it is found that complex 3 dissociated into two mononuclear species with monodentate EtL and the solvent molecule, [W(CO)5(EtL)] and [W(CO)5(solvent)], respectively, by UV−vis, NMR, and reduction potential measurements. The electronic structure of the ground state of complex 3 has been fully elucidated by DFT calculations. Thus, in six electron-filled orbitals including HOMO to HOMO-5, the major contribution comes from the d orbitals of both of the tungsten centers and π* of the CO moieties, and the high contribution of the −CS2 moiety on EtL in the HOMO describes the W−EtL π back-bonding. The LUMO is mainly localized on the N2C−CS2 fragment. TDDFT-PCM calculations for the dinuclear complex 3 correctly H


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Pyrex filter under an Ar atmosphere for 1 h. The resulting deep purple solution was evaporated to dryness under reduced pressure, and the residual powder was dissolved in CH2Cl2 and loaded onto a silica gel column. A blue-purple fraction was eluted by hexane/CH2Cl2 (2/1 v/ v), and for the solutions the solvents were removed by evaporation under reduced pressure to obtain powdery products: purple powder, yield 0.25 g (18% from the precursor Mo compound). Anal. Found (calcd for C20H20Mo2N2O10S2): C, 34.21 (34.10); H, 2.64 (2.86); N, 3.54 (3.98). IR (KBr disk, ν/cm−1): 2976 (C−H), 2074, 2063, 1995, 1930, 1911, 1892 (C−O), 1581 (C−N), 1030 (C− S). UV−vis (CH2Cl2, λmax/nm (ε/103 cm2 mol−1)): 367 (sh), 547 (7500). 1H NMR (CD2Cl2): δ 1.23 (t, 12H, CH3), 3.41 (q, 8H, CH2). 13 C NMR (CD2Cl2): δ 12.0 (CH3), 46.4 (CH2), 169.2 (NCN), 205.0 (MoCO(cis)), 214.2 (MoCO(trans)), 228.8 (SCS). 95Mo NMR (CDCl3): δ −1870. [{W0(CO)5}2(μ-S,S′-EtL)] (3). The procedure used for the preparation of 1 and 2 was applied. [W(CO)6] (0.71 g, 2.0 mmol) and EtL (0.23 g, 1.0 mmol) in THF (80 mL) were irradiated at 313 nm. After irradiation for 4 h, the red color changed to deep purple. The solvent was evaporated, and the residual powder was dissolved in CH2Cl2 and loaded onto a silica gel column. A blue-purple fraction eluted by hexane/CH2Cl2 (2/1 v/v) was evaporated under reduced pressure to obtain powdery products: purple powder, yield 0.64 g (36% from the precursor W compound). Anal. Found (calcd for C20H20N2O10S2W2): C, 27.34 (27.29); H, 2.12 (2.29); N, 3.02 (3.18). Mass spectrum (FAB-MS): m/z 880 (calcd for M+). IR (KBr disk, ν/cm−1): 2985 (C−H), 2073, 2060, 1988, 1920, 1905, 1888 (C−O), 1581 (C−N), 1030 (C−S). UV−vis (CH2Cl2, λmax/nm (ε/103 cm2 mol−1)): 419 (3700), 572 (12200). 1H NMR (CD2Cl2): δ 1.39 (br, 12H, CH3), 3.61 (br, 8H, CH2). 13C NMR (CD2Cl2): δ 13.2 (CH3), 47.6 (CH2), 171.3 (NCN), 197.7 (WCO(cis)), 202.7 (WCO(trans)), 223.1 (SCS). Physical Measurements. The UV−vis spectra were recorded on Shimadzu UV-160A and JASCO V-530 spectrophotometers. The IR spectra in KBr disks were recorded on Perkin-Elmer System 2000 and JASCO FT/IR-660 Plus spectrophotometers. The 1H and 13C NMR spectra were recorded on Bruker ARX400, DRX400, DPX400, and AV500 FT-NMR spectrometers with an external reference (CH3)4Si (δ 0) in CD2Cl2. 95Mo NMR spectra (97.9 MHz for 95Mo) were recorded on Bruker ARX400 and DRX400 NMR spectrometers with the external reference 1.0 M Na2MoO4 in D2O (δ 0). Cyclic voltammetry was carried out using an ALS Electrochemical System 660A with a glassy-carbon working electrode, a Pt-coil counter electrode, and an Ag/Ag+ (0.1 M [AgNO3]) reference electrode in 0.1 M tetrabutyammonium perchlorate in CH2Cl2 or CH3CN. The potential was calibrated with the ferrocene/ferrocenium couple (Fc/ Fc+) as an external reference in CH2Cl2 or CH3CN. FAB-mass spectra were recorded on a JEOL JMS-700AM instrument. The elemental analyses were carried out using FISONS EA 1112 and FISONS EA 1108 instruments at the Comprehensive Analysis Center for Science, Saitama University. X-ray Crystallography. All of the single crystals for X-ray crystallography were obtained by vapor diffusion of hexane into a CH2Cl2 solution at 273 K. Crystallographic data and structure refinement for 1 and 3 are given in Table S5 (Supporting Information). In addition, the molecular structure of free EtL was reanalyzed by single crystal X-ray analysis (Figure S4 and Tables S6 and S7, Supporting Information)6d The X-ray diffraction data for all the compounds were collected on a Bruker SMART APEX diffractometer equipped with a CCD area detector. The frame data were acquired with SMART-W2K/NT41 software using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The observed frames were processed using SAINT-WK2/NT42 software to give the hkl data set. The crystals used for the diffraction measurements showed no decomposition during the data collection. Absorption correction was performed using the SADABS program.43 The structures were solved by direct methods44 or Patterson methods45 and refined by least-squares methods on F2 using SHELXTL-NT46 software. All non-hydrogen atoms for all the compounds were refined anisotropically. Hydrogen atoms were included and refined using a

riding model for all the compounds. CCDC 829454, 829455, and 830126 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Computational Details. All DFT calculations of 3 were carried out using the GAUSSIAN03 package program.47 The functional used throughout this study is SVWN. Pople’s standard basis sets, 6-31G(d), were employed for H, C, N, and O, while the 6-31+G(2d) basis sets were used for S. For W atoms, we employed the Stuttgart− Dresden−Bonn quasi-relativistic ECP28MWB (SDD) effective core potential48 and pseudopotential in all complexes. The ground-state geometries were obtained in the gas phase by full geometry optimization, starting from structural data. The vibrational frequency was calculated and compared with experimental data to ensure that the optimized geometry represented a local minimum. To assign the lowlying electronic transition in the experimental spectra, TD-DFT calculations of the complex were carried out. We computed the 60 lowest singlet−singlet transitions. In order to estimate the possible response of electronic structure due to the solvation, the solvent was modeled by the polarizable continuum model (PCM).49 The chosen solvents were CCl4 and DMSO.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving crystallographic data for EtL and complexes 1 and 3, figures and tables giving structures, frontier orbitals, and Cartesian coordinates of the EtL and complex 3, and a table giving crystallographic data and experimental parameters for the structure analysis of EtL and complexes 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.F.); nagasawa@mail. saitama-u.ac.jp (A.N.). Present Address ∥

Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Tokyo 169-8555, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research(C) (23510115 to T.F.) from the Japan Society for the Promotion of Science (JSPS). We thank Enago (www. enago.jp) for the English language review.

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