ARTICLE pubs.acs.org/Organometallics
Reversible Intramolecular Single-Electron Oxidative Addition Involving a Hemilabile Noninnocent Ligand Ralph H€ubner,† Sebastian Weber,† Sabine Strobel,† Biprajit Sarkar,† Stanislav Zalis,‡ and Wolfgang Kaim*,† † ‡
Institut f€ur Anorganische Chemie, Universit€at Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany J. Heyrovsky Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-18223 Prague, Czech Republic
bS Supporting Information ABSTRACT: Using the noninnocent ligand Q [= 4,6-di-tertbutyl-(2-methylthiophenylimino)-o-benzoquinone] with a thioether group as potential coordination function, it has been possible to substantiate a single-electron transfer induced oxidative addition within the complex [IrCp*Q]0/þ (Cp* = C5Me5) via structural characterization (catecholato f semiquinonato transition coupled with reversible SfIr coordination), via cyclic voltammetry, EPR, and DFT (semiquinone formulation with about 8% Ir participation). The intramolecular rearrangement of the 16-electron precursor [IrCp*Q] triggered by electron removal illuminates the complementary activities of the substrate binding metal and the electron-buffering ligand as was recently employed by Ringenberg et al. in dihydrogen activation (Organometallics 2010, 29, 1956).
’ INTRODUCTION The oxidative addition-reductive elimination sequence continues to be one of the most useful and most frequently applied reaction patterns in transition metal chemistry.1 Metal-based electron configuration changes such as the d8/d6 two-electron interchange have been employed to provide very different coordination situations for substrate binding, conversion, and release.2 Noninnocent3 ligands, which provide an electron reservoir, have been employed lately not only for spectroscopic studies4 but also in schemes involving chemical transformations.5,6 Among such ligands, the 1,2-dioxolene (o-quinone/catecholate) redox system has been favored due to its stability (via chelate binding) and variability (e.g., substitution of O by NR functions and of H through substituents).4-6 While the aromatic catecholate forms are σ- and π-electron rich, the semiquinones and especially the quinones are π-electron accepting. Herein we describe an established6,7 organometallic species, pentamethylcyclopentadienyliridium(III), [IrCp*]2þ, in connection with a potentially tridentate noninnocent ligand Q [= 4, 6-di-tert-butyl-(2-methylthiophenylimino)-o-benzoquinone]. This ligand was shown8 to form a semiquinone that can provide a thioether sulfur donor for additional coordination. In a bis(semiquinone)copper(II) complex Cu(Q)2 this has led to structural effects modulating the intramolecular three-spin interaction.8 The related 4,6-di-tert-butyl-(2-trifluoromethylphenylimino)o-benzoquinone Q0 was presented recently in connection with [IrCp*]2þ in a system displaying redox-switched oxidation of H2.6 That reversibly oxidizable system [IrCp*(Q0 )]nþ was shown r 2011 American Chemical Society
to react in the radical state (n = 1) with H2 to produce Hþ. In contrast, the related complex [IrCp*(RNCHCHNR)], R = 2,6dimethylphenyl, cannot be oxidized completely reversibly under these conditions, i.e., in CH2Cl2.9 To understand this reactivity, we have now prepared [IrCp*Q] and its oxidized form [IrCp*Q](PF6) and studied their structures and spectroscopic and electrochemical properties.
’ RESULTS AND DISCUSSION Separately prepared and characterized [IrCp*Q] and its oxidized form [IrCp*Q](PF6) were found to be connected via an irreversible processs before a reversible second oxidation for the couple [IrCp*Q]2þ/þ takes place (Figures 1 and S1, S2). Structural evidence, supported by DFT calculations, was then obtained to clarify this reactivity: The neutral [IrCp*Q] (Figure 2) exhibits coordinative unsaturation (16 valence electron situation) because the electron deficit at the trivalent metal is mitigated by strong σ- and π-electron donation from what is clearly an iminocatecholate ligand with average and little variable C-C distances of about 1.39 Å in the aromatic ring (Table 1). Such coordinative unsaturation for d6 systems has been found also for the related [IrCp*Q0 ]6 and [IrCp*(RNCHCHNR)]9 as well as for RhCp*(cat), [Cr(CO)3(cat)]2-, and [Mn(CO)3(cat)](cat = catecholate).10 Consequently, there is no bonding interaction between Ir and S at 4.0033(7) Å distance, and the metric Received: October 6, 2010 Published: February 22, 2011 1414
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Figure 1. Cyclic voltammograms of [IrCp*Q] (ca. 10-3 M) in 0.1 M Bu4NPF6/CH2Cl2 at 15, 25, 50, 75, 100, 125, 150, 175, 200, 225, and 250 mV/s scan rate vs Fcþ/0 (quasi-steady-state voltammograms). Peak potentials in V vs Fcþ/0 at 100 mV/s: Epa = -0.06, þ0.30; Epc = þ0.23, -0.50 V.
Figure 3. Molecular structure of the cation in the crystal of [IrCp*Q](PF6) 3 CH2Cl2.
Figure 4. Electronic paramagnetic resonance spectrum and simulation of [IrCp*Q](PF6) in CH2Cl2 at 110 K (9.5 GHz). Simulation was carried out using the data from the text.
Figure 2. Molecular structure of [IrCp*Q] in the crystal.
Table 1. Selected Atom-Atom Distances [Å] for Complexes [IrCp*Q]
a
[IrCp*Q]þa
Ir-S
4.0033(7)
2.3754(15)
Ir-N
1.947(2)
2.062(4)
Ir-O
2.010(2)
2.072(4)
N-C6
1.396(4)
1.366(7)
O-C1
1.337(3)
1.318(6)
C-Cb
1.386(4)-1.407(4)
1.364(7)-1.448(8)
In the crystal of [IrCp*Q](PF6) 3 CH2Cl2. b Intraring distances of Q.
parameters of the ligand reveal not only an aromatic ring but also C-O and C-N single bonds (Table 1). On one-electron oxidation, the structure rearranges characteristically (Figure 3): Iridium-sulfur coordination occurs with a standard11 bond length of 2.354(15) Å, leading to coordinative saturation (piano-stool configuration). The dihedral angle between the two C6 rings decreases from 89° to 56° in the cation. The metric parameters of the noninnocent ligand reveal bond alternacy within the benzo-semiquinone ring as well as shortened C-O and C-N bonds (Table 1). ESR spectroscopy shows DFT confirmed g tensor components at g1 = 1.996, g2 = 1.985, and
g3 = 1.951, the latter with notable 14N hyperfine splitting of 1.7 mT (Figure 4). This observation reveals non-negligible metal contributions (DFT: about 8%, see below) from iridium with its high spin-orbit coupling constant, but otherwise an anion radical complex,12 corresponding to a semiquinone/IrIII description.9 UV-vis spectroelectrochemistry shows absorption features (Figure S3) that agree with those of previously described analogues.6,9 Apparently, the one-electron oxidation of the iminocatecholate to the iminobenzosemiquinonate is already sufficient to remove the tolerance of coordinative unsaturation in the 16 valence electron species, resulting in binding of either weakly donating thioether-S, dihydrogen,6 or halogen species.9 Scheme 1 illustrates the square scheme for the present case, showing calculated structures and energy differences. The calculations confirm that this indirect oxidative addition at the metal involving one-electron exchange1,13 on the ligand is made possible through the participation of a noninnocently behaving3b ligand while maintaining a formally invariant metal oxidation state. Two stable configurations, A, corresponding to N,O coordination, and B, characterized by N,O,S coordination, were found by DFT (G09/PBE0) geometry optimization for both neutral and oxidized [IrCp*Q]nþ (Scheme 1). The forms corresponding to the experimental structures (A, Bþ) have the lowest energies. The free energy differences, ΔG, are 0.500 and 0.191 eV in the case of the neutral (A, B) and oxidized forms (Aþ, Bþ), respectively. The DFT-optimized bond lengths and angles of neutral [IrCp*Q] and its radical cation agree with the experimental crystal structures (Table 2). 1415
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Scheme 1. DFT-Calculated Structural Changes Involving Electron Transfer Effected S(thioether) Coordination (Hysteresis)a
Figure 5. Spin density plots for the two configurations Bþ (left) and Aþ (right) of the radical complex cation [IrCp*Q]þ.
The ΔG differences between A and B configurations are 0.500 eV (neutral forms) and 0.191 eV (cations). a
Table 2. Comparison of Selected Experimental and DFTCalculated Bond Lengths (Å) and Angles (deg) for [IrCp*Q]nþ, n = 0, 1a n=0 exptl
calcd
n=1 calcd
exptl
conf A conf B
calcd
calcd
conf B conf A
Ir-N
1.947(2)
1.951
2.056
2.062(4)
2.064
2.003
Ir-O
2.010(2)
1.987
2.072
2.072(4)
2.062
2.026
Ir-S
4.0033(7)
3.885
2.339
2.3754(15)
2.406
3.939
Ir-Ccpb
2.153
2.155
2.171
2.173
2.166
2.150
N-Ir-O
79.24(8)
78.74
77.65
77.61(17)
76.72
78.42
83.20 92.48
77.47(14) 97.85(11)
77.30 97.93
N-Ir-S O-Ir-S relative free
0.000
the paramagnetic intermediate can bind not only weakly donating thioether-S as shown here but also H26 or halogen species.9 Under electrochemical aspects, the different reversibility patterns for the two-step oxidation processes are remarkable: While the [IrCp*Q0 ]n system exhibits two conventional waves in the cyclic voltammogram,6 the [IrCp*(RNCHCHNR)]n redox system with R = 2,6-dimethylphenyl showed a reversible first but irreversible second oxidation in propylene carbonate.9 In yet another variant, the example [IrCp*Q]0/þ/2þ described here is distinguished by an irreversible first oxidation process involving fast intramolecular thioether coordination after initial electron transfer. On the return scan in the cyclic voltammogram, the wave at about -0.5 V is due to reduction of N,O,S-coordinated [IrCp*Q]þ, which can also undergo a fully reversible oneelectron oxidation (Bþ/B2þ) to the [IrCp*Q]2þ state at 0.26 V.
0.500
0.000
0.191
energy (eV) a Configuration A corresponds to N,O coordination and configuration B to N,O,S coordination. b Symmetry averaged.
Figure 5 depicts spin densities for configurations Aþ and Bþ of [IrCp*Q]þ. ADF/BP86 calculations for B yield a metal spin density of 0.084 and negligible spin density at Cp*. This approach yields g tensor components of g1 = 2.0028, g2 = 1.9878, and g3 = 1.9541, in good agreement with experimental values of g1 = 1.996, g2 = 1.985, and g3 = 1.951. Calculated hyperfine coupling values for 14N are A1 = 0.02 mT, A2 = 0.03 mT, and A3 = 1.47 mT. Summarizing, the results observed for the structurally characterized redox pair [IrCp*Q]0/þ, connected via an irreversible processs, have demonstrated how an indirect oxidative addition at the metal coupled with a one-electron exchange on the ligand can occur via participation of a noninnocently behaving ligand while the metal oxidation state remains invariant. Remarkably,
’ EXPERIMENTAL SECTION Materials. CH2Cl2 and CD2Cl2 were dried over CaH2; hexane was dried over sodium. All solvents were degassed and placed under vacuum before use. [Cp*IrCl2]2 and AgPF6 were purchased from Strem; 2-methylthioaniline and 3,5-di-tert-butylcatechol were purchased from Aldrich and used without further purification. 3,5-Di-tert-butyl-2-hydroxy-1-(2-methylthioanilido)benzene (H2Q) was prepared from 2-methylthioaniline and 3,5-di-tert-butylcatechol.14 Instrumentation. UV/vis spectra including spectroelectrochemistry were recorded on a TIDAS diode array spectrometer using an OTTLE cell. 1H NMR spectra were measured on a Bruker AV 250 spectrometer. EPR spectra were measured on a Bruker ESP 300 system at 9.5 GHz. Cyclic voltammograms were recorded as described below. Syntheses. [IrCp*Q]. A solution of 80 mg (0.1 mmol) of [Cp*IrCl2]2 and 68 mg (0.2 mmol) of H2Q26 in 25 mL of CH2Cl2 was stirred. After 5 min an excess of K2CO3 (500 mg) was added, and the reaction was stirred for 48 h at room temperature, during which the solution turned from yellow to orange. The orange solution was filtered to remove excess K2CO3, and the solvent was removed under reduced pressure, resulting in an orange solid. The air-stable solid was triturated with 10 mL of hexane, and the mixture was filtered through a mediumporosity frit. This process was repeated four times in order to ensure removal of excess H2Q. Residual solvent was removed by evacuation at 0.01 mmHg for 12 h at 80 °C. Yield: 120 mg (1.80 mmol, 90%). Anal. Calcd for C31H42IrNOS (found): C, 55.66 (55.59); H, 6.33 (6.28); N, 2.09 (2.05). 1H NMR (CD2Cl2): δ 1.14 (s, 9H), 1.54 (s, 9H), 1.71 (s, 15H), 6.28 (d, 1H), 6.75 (d, 1H), 7.08 (d, 1H), 7.23 (t, 1H), 7.26 (t, 1H), 7.30 (d, 1H). ESI-MS: m/z = 669.12 (Mþ). UV-vis (CH2Cl2): 450 (ε = 1.4 104 M-1 cm-1), 291 (sh) nm. 1416
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Table 3. Crystallographic Parameters [IrCp*Qy]
[IrCp*Q](PF6) 3 CH2Cl2
formula
C31H42IrNOS
C32H44Cl2F6IrNOPS
fw
668.92
898.81
cryst syst
monoclinic
monoclinic
space group
P21/c
P21/n
a (Å)
10.71340(10)
9.67850(10)
b (Å)
22.2774(3)
11.6660(4)
c (Å)
13.2353(2)
31.090(2)
β (deg) V (Å3)
113.096(1) 2905.64(6)
96.01(1) 3491.1(2)
Z
4
4
F(000)
1344
1788
Dcalced (g cm-3)
1.529
1.710
μ(Mo KR) (mm-1)
4.690
4.142
cryst size (mm)
0.09 0.06 0.04
0.35 0.33 0.1
temp (K)
100(2)
100(2)
θ limits (deg) index ranges
2.28 - 28.38 -14 e h e 14
1.32 - 27.83 -12 e h e 12
-29 e k e 29
-15 e k e 15
-17 e k e 17
-40 e k e 40
no. of collected reflns
14 102
15 187
no. of unique reflns
7238
8201
no. of reflns with I > 2σ(I) 5917
5499
no. of params/restraints
317/0
407/0
GOF (on F2) R1 (on F, I > 2σ(I))
1.039 0.0255
0.986 0.0469
wR2 (on F2, all data)
0.0512
0.1096
max./min. ΔF (e Å-3)
1.411/-0.980
1.311/-2.679
[IrCp*(Q)](PF6). A solution of 100 mg (0.15 mmol) of [IrCp*Q] in 5 mL of CH2Cl2 was added to a suspension of 38 mg (0.15 mmol) of AgPF6 in 15 mL of CH2Cl2. An immediate color change from orange to dark was observed. After 3 h of stirring, the reaction mixture was filtered through Celite 500 to remove precipitated silver. The solvent was evaporated from the filtrate, and the resulting solid triturated with 20 mL of hexane. The hexane was then removed via a filter cannula fitted with a G6 glass fiber filter. This precipitation process was repeated until the oily residue converted to a brown powder. Residual solvent was removed by evacuation at 0.01 mmHg for 12 h. Yield: 105 mg (85%). Anal. Calcd for C31H42F6IrNOPS (found): C, 45.75 (45.65); H, 5.20 (5.23); N, 1.72 (1.74). ESI-MS: m/z = 813.95 (Mþ). UV-vis (CH2Cl2): 850 (ε = 0.8 103 M-1 cm-1), 620 (sh) 500 (5 103), 355 (5.5 103) 300 (6 103) nm. Single crystals were obtained by slow evaporation of solutions in CH2Cl2 ([IrCp*Q]) or CH2Cl2/hexane (3:1) ([IrCp*Q](PF6) 3 CH2Cl2). Electrochemical Experiments. A 10-3 M solution of [IrCp*Q] in 5 mL of CH2Cl2 was prepared and saturated with Ar for cyclic voltammetry analysis using a M 273 A potentiostat from EG&G. The setup was a platinum working electrode, a platinum wire counter electrode, a Ag/ Agþ reference electrode, and 0.1 M Bu4NPF6 as a supporting electrolyte. The Fc/Fcþ couple was used as internal standard. Spectroelectrochemistry. UV-vis spectroelectrochemistry was measured in an OTTLE15 (optically transparent thin-layer electrolytic cell) using CH2Cl2 and Bu4NPF6 as supporting electrolyte. The measurements were carried out with a TIDAS diode array spectrometer. Crystal Structure Determination. The samples were studied on a Bruker-Nonius Kappa CCD with graphite-monochromatized Mo KR (0.71073 Å) radiation. The structures were solved by direct methods using SHELXL-9716 and refined using full-matix least-squares on F2
(SHELXL-97).17 All non-hydrogen atoms in the structures were refined with anisotropic displacement parameters. Selected crystal data and structure refinement details for both compounds are given in Table 3. Quantum Chemical Calculations. The electronic structures of [IrCp*Q] and of its oxidized form were calculated by density functional theory (DFT) methods using the Gaussian 0918 and Amsterdam Density Functional (ADF2009.01)19,20 program packages. G09 calculations employed the Perdew, Burke, Ernzerhof21,22 PBE0 hybrid functional (G09/PBE0). The geometry of the cationic form was calculated by the UKS approach. Geometry optimization was followed by vibrational analysis. For H, C, O, S, and N atoms polarized triple-ζ basis sets 6-311G(d) were used,23 together with quasirelativistic effective core pseudopotentials and a corresponding optimized set of basis functions for Ir.24,25 Within the ADF program Slater-type orbital (STO) basis sets of triple-ζ quality with two polarization functions for Ir, C, N, O, and S atoms and of double-ζ quality with one polarization function for the H atoms were employed. Inner shells were represented by the frozen core approximation (1 s for C, N, and O, 2p for S, and 1s-4d for Ir were kept frozen) within calculations of the g tensor; core electrons were included in the calculation of the A tensor. Within ADF the functional including Becke’s gradient correction to the local exchange expression in conjunction with Perdew’s gradient correction to local density approximation (LDA) with VWN parametrization of electron gas data was used (ADF/ BP86).26,27 A and g tensors were obtained by first-order perturbation theory from a ZORA Hamiltonian in the presence of a time-independent magnetic field.28,29 The g tensor was obtained from a spin-nonpolarized wave function after incorporating the spin-orbit (SO) coupling.
’ ASSOCIATED CONTENT
bS
Supporting Information. X-ray crystallographic files in CIF format for [IrCp*Q] and [IrCp*Q](PF6) 3 CH2Cl2, cyclovoltammograms, and spectroelectrochemical results. This material is available free of charge via the Internet at http://pubs.acs. org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Support from the Deutsche Forschungsgemeinschaft, the EU (COST D35), the Grant Agency of the Academy of Sciences of the Czech Republic (KAN 100400702), and the Ministry of Education of the Czech Republic (Grant No. LD 11086) is gratefully acknowledged. ’ REFERENCES (1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: Weinheim, 2009; Chapter 6. (2) Cornils, B.; Herrmann, W. A., Eds. Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed.; Wiley-VCH: Weinheim (Germany), 2002. (3) (a) Jorgensen, C. K. Coord. Chem. Rev. 1966, 1, 164. (b) Ward, M. D.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 2002, 275. (4) (a) Kaim, W.; Wanner, M.; Kn€odler, A.; Zalis, S. Inorg. Chim. Acta 2002, 337, 163. (b) J€ustel, T.; Bendix, J.; Metzler-Nolte, N.; Weyherm€uller, T.; Nuber, B.; Wieghardt, K. Inorg. Chem. 1998, 37, 35. (c) Haga, M.; Dodsworth, E. S.; Lever, A. B. P. Inorg. Chem. 1986, 25, 447. 1417
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