Cationic Iridium Complexes Containing Anionic Iridium Counterions

Article pubs.acs.org/Organometallics

Cationic Iridium Complexes Containing Anionic Iridium Counterions Supported by Redox-Active N‑Heterocyclic Carbenes Kuppuswamy Arumugam, Jinho Chang, Vincent M. Lynch, and Christopher W. Bielawski* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-0165, United States S Supporting Information *

ABSTRACT: The first cationic iridium complex supported by two bulky, ferrocenylated N-heterocyclic carbenes, [(1)2Ir(COD)]+[Cl]− ([2]+[Cl]−; 1 = 1-ferrocenylmethyl-3-mesitylimidazol-2-ylidene, Fc-NHC; COD = cis,cis-1,5-cyclooctadiene), was synthesized and characterized. Treatment of the aforementioned complex with a mixture of [N(nBu)4]+[Cl]− and [Ir(COD)(Cl)]2 afforded [(1)2Ir(COD)]+[Ir(COD)(Cl)2]− ([2]+[Ir(COD)Cl2]−), which featured cationic as well as anionic Ir centers. Electrochemical analysis of [2]+[Ir(COD)Cl2]− revealed that the complex displayed two reversible (iron- and iridium-centered) and two irreversible (iridium-centered) redox processes, which were assigned to the cationic and anionic components, respectively. Stirring [2]+[Ir(COD)Cl2]− under an atmosphere of carbon monoxide generated the corresponding Ir carbonyl complex [(1)2Ir(CO)2]+[Ir(CO)2(Cl)2]− ([3]+[Ir(CO)2Cl2]−). Cyclic voltammetry (CV) measurements of the aforementioned complexes containing [(1)2Ir(COD)]+ showed that the iron-centered oxidations were concurrent, reflective of limited electronic coupling between the two ferrocenyl moieties. However, spectroelectrochemical analysis of [(1)2Ir(CO)2]+[BArF4]− revealed that the electron density at the Ir centers supported by the Fc-NHC ligands decreased upon oxidation of the ferrocenyl groups, as evidenced by a ca. 10 cm−1 increase in the recorded νCO bands.

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INTRODUCTION N-heterocyclic carbene (NHC) supported transition-metal complexes have found utility as catalysts for promoting a variety of synthetically useful transformations.1 Prominent examples of reactions that have benefitted from NHCs include olefin metatheses,2 cross-couplings,3 cycloisomerizations,4 transfer hydrogenations,5,6 and hydrosilylations.6,7 In many cases, the respective catalytic processes are facilitated by the strong σ-donating character and unique steric properties displayed by NHCs.8 An ability to externally modulate9 these properties using redox chemistry may provide a means to alter the activity and/or selectivity displayed by the respective catalyst over the course of the reaction.10−13 In a seminal example, Wrighton et al. demonstrated that a Rh complex facilitated hydrogenation or hydrosilylation reactions depending on the oxidation state of a diphenylphosphinocobaltocene ligand.11,14,15 Building on the pioneering work of Bildstein (A),16 Arduengo (B),17 Plenio (C),10 and others (D and E),18 our group (F, G, and 1)13,19 has studied a broad range of transitionmetal complexes coordinated to redox-active NHCs containing metallocenes (Figure 1). For example, spectroelectrochemical IR analysis of Ir carbonyl complexes supported by ferrocenylated NHCs revealed that the oxidation of the redox-active group resulted in a change in the carbonyl stretching frequency (ΔνCO) of +10−11 cm−1, regardless of the attachment point of the redox-active group (i.e., via the NHC nitrogen atom or via the backbone),20 reflecting the attenuation of the donating ability of the NHC upon oxidation. Moreover, the oxidation of © 2013 American Chemical Society

NHCs bearing one ferrocenyl unit gave results similar to those bearing two units,20 indicating an apparent limitation to modify the electronic properties at the ligated metal center beyond a certain threshold. We reasoned that the restriction may be overcome and greater electronic control may be realized through the ligation of two independently ferrocenylated NHCs. Herein, we report the synthesis of a new class of Ir cationic complexes bearing two 1-ferrocenylmethyl-3-mesitylimidazol-2-ylidene (1) ligands. The counteranions to these complexes were also Ir-based (i.e., [Ir(COD)Cl2]− (COD = cis,cis-1,5-cyclooctadiene) or [Ir(CO)2Cl2]−), which enabled a direct comparison of the electrochemical potentials at which oppositely charged iridium complexes undergo oxidation. The aforementioned Ir complexes were studied using a variety of electrochemical and spectroelectrochemical techniques and compared to mono-NHC-supported analogues.

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RESULTS AND DISCUSSION Prior reported methods to access [(NHC)2Ir(COD)]+ type complexes bearing N(R1),N′(R2)-imidazol-2-ylidenes (R1, R2 = methyl, ethyl, cyclohexyl, tert-butyl) were limited to the treatment of [Ir(COD)(OEt)]2 with the corresponding NHC21 or treatment of appropriate metal precursors with NHC-supported silver salts ([(NHC)2Ag]+) (R1, R2 = methyl, ethyl, methyl, pentafluorobenzyl, p-tolylmethyl).22 Unfortunately, the synthetic routes that utilize Ag complexes as Received: May 29, 2013 Published: July 22, 2013 4334

dx.doi.org/10.1021/om4004829 | Organometallics 2013, 32, 4334−4341

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Figure 1. Representative N-heterocyclic carbenes containing redox-active units (n = 0−2, X = donor group).

Scheme 1. Syntheses of Iridium Compounds with Various Counteranionsa

Abbreviations: Fc, ferrocenyl; Mes, mesityl; BArF4, [B(C6H3(CF3)2)4]−. Conditions: (a) (i) NaHMDS (1 equiv), toluene, 25 °C, 0.5 h, (ii) [Ir(COD)Cl]2 (0.25 equiv), 60 °C, 2 h, 93% yield; (b) [Ir(COD)Cl]2 (0.5 equiv), [N(nBu)4]+[Cl]−, CH2Cl2, 30 °C, 12 h, 91% yield; (c) CO (1 atm), CH2Cl2, 25 °C, 2 h, 87% yield; (d) NaBArF4 (1 equiv), CH2Cl2, 25 °C, 12 h, 90% yield; (e) CO (1 atm), CH2Cl2, 25 °C, 2 h, 91% yield. a

intermediates are not viable in the presence of ferrocene, as they promote Fe2+ → Fe3+ oxidation. To overcome the aforementioned restrictions and to expand the flexibility associated with the preparation of [(NHC)2Ir(COD)]+ type complexes, an alternative synthetic route was developed. Treatment of [Fe(η5-C5H4CH2(C3H3N2)(Mes))Cp]+[I]−13 with NaHMDS in toluene followed by filtration and subsequent addition of [Ir(COD)Cl]2 (0.25 equiv) at 60 °C for 2 h led to the formation of [2]+[Cl]−, which was subsequently isolated as an orange precipitate in 93% yield (Scheme 1). The 1H NMR spectrum recorded for [2]+[Cl]− was similar to that recorded for the monosubstituted [(1)Ir(COD)Cl] in the same solvent (CDCl3),13 except that the former displayed four (rather than eight) signals attributed to the COD ligand on account of symmetry. The structural assignment of [2]+[Cl]− was unambiguously confirmed by X-ray diffraction analysis of crystals grown by diffusing pentane into a saturated CH2Cl2

solution. The solid-state structure revealed that the complex adopted a square-planar geometry with two NHCs oriented in a cis geometry (Figure 2, left). The average N−C−N bond angles and Ir−C bond lengths were measured to be 104.8(5)° and 2.094(6) Å, respectively, which were in good agreement with those reported for [(1)Ir(COD)Cl]13 and other analogous complexes.20 The ability to incorporate two NHC ligands23 into Ir(COD)Cl complexes in comparison to analogues that are supported by only one NHC ligand may be due to the conformational flexibility of the N-ferrocenylmethyl group. We reasoned that [2]+[Cl]− may be converted to [2]+[Ir(COD)Cl2]− via anion metathesis using [N(nBu)4]+[Ir(COD)Cl2]−.24 After a mixture of [2]+[Cl]−, [Ir(COD)Cl]2, and [N(nBu)4]+[Cl]− in CH2Cl2 was stirred at 30 °C for 12 h, the solvent was removed under reduced pressure and the residue was washed with cold THF, followed by Et2O. 1H NMR analysis of the resulting orange solid indicated that a single 4335


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Figure 2. (left) ORTEP diagram of [2]+[Cl]− rendered using POV-Ray. Thermal ellipsoid plots are drawn at the 50% probability level. For clarity, the hydrogen atoms and solvent molecules are omitted and the cis,cis-1,5-cyclooctadiene ligands are represented as lines. Selected bond lengths (Å) and angles (deg): C1−N1, 1.359(8); C1−N2, 1.351(8); C2−N1, 1.392(8); C3−N2, 1.419(8); C2−C3, 1.340(9); C24−N3, 1.370(8); C24−N4, 1.354(8); C1−Ir1, 2.105(6); C24−Ir1, 2.084(6); C25−C26, 1.351(9); C25−N3, 1.382(8); C26−N4, 1.398(8); N1−C1−N2, 105.2(5); N1−C1− Ir1, 122.9(5); N2−C1−Ir1, 131.8(5); C3−C2−N1, 107.5(6); C2−C3−N2, 106.1(6); N3−C24−N4, 104.3(5); N1−C13−C14, 112.8(5); N4− C27−C28, 113.0(5); C1−Ir1−C24, 94.5(2). (right) ORTEP diagram of [2]+[Ir(COD)Cl2]− rendered using POV-Ray. For clarity, the hydrogen atoms are omitted and the cis,cis-1,5-cyclooctadiene ligands are represented as lines. Selected bond lengths (Å) and angles (deg): C1−N1, 1.360(8); C1−N2, 1.351(8); C2−N1, 1.369(8); C3−N2, 1.387(9); C2−C3, 1.306(10); C4−N1, 1.462(8); C13−N2, 1.474(9); C1−Ir1, 2.085(7); Cl1−Ir2, 2.352 (2); N1−C1−N2, 103.8(6); N2−C13−C14, 111.4(6).

Figure 3. Overlaid 1H NMR spectra (CD2Cl2) of [2]+[Cl]− (top), [2]+[Ir(COD)Cl2]− (center), and [N(nBu)4]+[Ir(COD)Cl2]− (bottom).

contained within [2]+[Ir(COD)Cl2]− matched those collectively displayed by [2]+[Cl]− and [N(nBu)4]+[Ir(COD)Cl2]− and supported the aforementioned structural assignment. To gain additional support, X-ray diffraction quality crystals were grown by the diffusion of n-pentane into a saturated CH2Cl2 solution. The solid-state structure of [2]+[Ir(COD)Cl2]− revealed that the two Fc-NHC ligands were cis with

product was present and displayed spectroscopic features that were consistent with those expected from [2]+[Cl]−, except for the additional signals corresponding to the [Ir(COD)Cl2]− counteranion. To confirm the identity of the anionic fragment, [N(nBu)4]+[(Ir(COD)Cl2]− was independently prepared24 and spectroscopically compared to the aforementioned complex (Figure 3). The 1H NMR signals assigned to the COD ligand 4336


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distances found in the [(1)2Ir(CO)]+ component (1.873(10) Å) were elongated when compared to the analogous average distances measured in the [Ir(CO)2Cl2]− (1.820(14) Å) component; collectively, these results were attributed to the relatively strong trans effect of NHC ligands. Next, a series of electrochemical analyses were performed to investigate the effect of ligand oxidation on the respective iridium metal; key data are summarized in Table 1. Due to the

respect to each other on the cationic Ir(COD) fragment with a [Ir(COD)Cl2]− unit serving as the counteranion (Figure 2, right).25 The Ir1−C1 bond length (2.085(7) Å) and N1−C1− N2 bond angle (103.8(6)°) values were in agreement with those measured in the solid-state structures of [(1)Ir(COD)Cl]13 and analogous iridium complexes (1.99−2.091 Å).20 While stable in the solid state, prolonged standing of [2]+[Ir(COD)Cl2]− in CDCl3 resulted in decomposition, as observed by the loss of the 1H NMR signals assigned to the COD ligand found in the counteranion ([Ir(COD)Cl2]−). Since ligated carbonyl groups have been shown to be excellent IR handles for probing the electronic properties at the respective iridium metal centers,26,27 subsequent efforts shifted toward the synthesis and study of [(1) 2Ir(CO) 2 ]+[Ir(CO)2Cl2]− ([3]+[Ir(CO)2Cl2]−), primarily to independently interrogate the electronics at the cationic and anionic iridium centers. Stirring a CH2Cl2 solution of [2]+[Ir(COD)Cl2]− under an atmosphere of carbon monoxide for 2 h followed by purification using a series of n-pentane washes afforded [3]+[Ir(CO)2Cl2]−, which was subsequently isolated in 87% yield. The structure of this complex was supported by the disappearance of the COD signals in the 1H NMR spectrum (CD2Cl2) recorded for the product and the appearance of a new 13C NMR signals at 167.4 and 168.6 ppm which were attributed to the CO ligands. Diagnostic νCO values (1973, 2003, 2055, and 2068 cm−1) corresponding to symmetric and asymmetric stretches of the carbonyls on the cationic and anionic units were also observed. Diffusion of n-pentane into a saturated CH2Cl2 solution afforded single crystals of [3]+[Ir(CO)2Cl2]− that were suitable for X-ray diffraction analysis (Figure 4). The solid-state structure of [3]+[Ir(CO)2Cl2]− adopted a square-planar geometry with the carbene ligands oriented in a cis geometry. The average Ir−NHC bond length measured in the solid-state structure of [3]+[Ir(CO)2Cl2]− (2.086(8) Å) was comparable to those measured in the solidstate structures of analogous (NHC)Ir(CO)2Cl complexes (2.065−2.122 Å).20,26,28 Moreover, the average Ir−CO bond

Table 1. Electrochemical Data Recorded for Various Ir Complexesa E1/2 (V)

complex +

−

[2] [Ir(COD)Cl2] [N(nBu)4]+[Ir(COD)Cl2]− [2]+[BArF4]− [3]+[BArF4]− [3]+[Ir(CO)2Cl2]−

0.53 0.53 0.68 0.64 0.53

b

(ir), 0.68 (r),c 0.90 (ir),b 1.36 (r)d (ir),b 0.88 (ir)b (r),c 1.36 (r)d (r)c (ir),b 0.63 (r)c

a

Potentials obtained from DPV measurements in CH2Cl2 with 0.10 M [N(nBu)4]+[PF6]− electrolyte, 0.1 mM analyte, and referenced vs SCE. See Figure 5 and Supporting Information for the corresponding cyclic voltammograms and differential pulse voltammograms. Abbreviations: r, reversible; ir, irreversible. bAssigned to an anionic Ir-centered oxidation process. cAssigned to an Fe-centered oxidation process. d Assigned to a cationic Ir-centered oxidation process.

presence of the redox-active counterions in [2]+[Cl]−, [2]+[Ir(COD)Cl2]−, and [3]+[Ir(CO)2Cl2]−, multiple oxidation processes were observed. For example, the cyclic voltammogram (CV) of [2]+[Ir(COD)Cl2]− in CH2Cl2 with 0.1 M [N(nBu)4]+[PF6]− electrolyte exhibited four oxidation processes at 0.53, 0.68, 0.90 and 1.36 V vs SCE. The processes measured at 0.53 and 0.90 V appeared to be irreversible and were assigned to iridium-centered processes that originated from the [Ir(COD)Cl2]− component. In contrast, the processes observed at 0.68 and 1.36 V appeared to be reversible and were assigned to iron- and iridium-centered oxidations, respectively, originating from the cationic component (i.e., [(1)2Ir(COD)]+) after comparison to the CV recorded for [N(nBu)4]+[Ir(COD)Cl2]− under similar conditions.29 To gain support for these assignments, [(1) 2Ir(COD)]+[BArF4 ]− ([2]+[BArF4]−)30 was synthesized (Scheme 1) and found to display two reversible oxidations at 0.68 and 1.36 V, which were attributed to iron- and iridium-centered oxidations, respectively (Figure 5). Complex [3]+[BArF4]− ([(1)2Ir(CO)2]+[BArF4]−), which does not contain a [Ir(CO)2Cl2]− group, exhibited only one reversible oxidation at 0.64 V and was assigned to a FeII/ FeIII couple.31 For comparison, the CV recorded for the carbonyl complex [3]+[Ir(CO)2Cl2]− revealed two redox processes at 0.53 and 0.63 V, which were assigned to the anionic ([Ir(CO)2Cl2]−) and cationic ([(1)2Ir(CO) 2]+) components, respectively, with the latter process being Febased. To quantify the number of electrons involved in the redox process measured at 0.68 V for [2]+[BArF4]−, chronoamperometry (CA) using a platinum ultra micro electrode (Pt UME, a = 12.5 μm) was performed.32 The diffusion coefficient (D) for the aforementioned complex was calculated by potentialstep chronoamperometry in CH2Cl2 with [N(nBu)4]+[PF6]− (0.1 M) as the electrolyte. The time-dependent and steady-state currents, i(t) and iss, respectively, were measured by increasing the potential from 0 to 0.4 V (holding at 0.4 V for 10 s). The slope and the intercept of the line defined by i(t)/iss vs t−1/2

Figure 4. ORTEP diagram of [3]+[Ir(CO)2Cl2]− rendered using POV-Ray. Thermal ellipsoid plots are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−N1, 1.377(11); C1−N2, 1.341(11); N1− C2, 1.389(11); C3−N2, 1.383(11); C2−C3, 1.342(13); C1−Ir1, 2.075(8); C24−Ir1, 2.097(8); Ir2−Cl1, 2.331(4); Ir2−Cl2, 2.312(4); Ir2−C49, 1.809(14); Ir2−C50, 1.831(15); Ir1−C47, 1.873(10); Ir1− C48, 1.873(11); C47−O1, 1.135(12); C48−O2, 1.127(12); N1−C1− N2, 104.5(7); C1−Ir1−C24, 90.6(3); N3−C24−N4, 105.2(7); N2− C13−C14, 114.5(7); N3−C24−N4, 105.2(7); N3−C25−C26, 106.4(9); N4−C26−C25, 106.7(9); C47−Ir1−C48, 90.0(5). 4337


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Figure 5. Cyclic voltammogram (left) and the corresponding differential pulse voltammogram (right; 50 mV pulse amplitude) of [2]+[BArF4]− in CH2Cl2 (1 mM) with 0.1 M [N(nBu4]+[PF6]− at a scan rate of 100 mV s−1.

Figure 6. Normalized IR difference spectra showing the shift in carbonyl stretching frequency to higher wavenumbers upon oxidation (Eapp = +0.75 V) of (left) [3]+[Ir(CO)2Cl2]− and (right) [3]+[BArF4]−. Conditions: 1 mM analyte in CH2Cl2 with [N(nBu)4]+[PF6]− as the supporting electrolyte (0.1 M). The arrows indicate the direction of the spectral changes over time.

enabled the determination of the diffusion coefficient, D0 = 6.22 × 10−6 cm2/s and n = 2 (see the Supporting Information for additional details). The value of n = 2 calculated for the oxidation process observed at 0.68 V was consistent with two concurrent Fe2+/Fe3+ oxidations deriving from the Fc-NHC units. The simultaneous oxidations reflected minimal electronic communication between the corresponding ferrocenyl units. To further examine the ligand donating abilities and to explore the electron density at the iridium metal center upon oxidation of the ferrocenyl moieties, the metal-bound carbonyl stretching frequencies were analyzed by FT-IR spectroscopy for [3]+[Ir(CO)2Cl2]− and [3]+[BArF4]−. Consistent with its structure, [3]+[Ir(CO)2Cl2]− exhibited four carbonyl stretching frequencies at 1973, 2003, 2055, and 2068 cm−1 in CH2Cl2. Furthermore, IR analysis revealed that the asymmetric and symmetric carbonyl stretches of the [(1)2Ir(CO)2] fragment occurred at 2068 and 2003 cm−1, while the asymmetric and symmetric stretches at 2055 and 1973 cm−1 originated from the [Ir(CO)2Cl2]− fragment.33 The lower carbonyl stretching frequencies observed for the anionic fragment reflected a higher degree of back-bonding in comparison to the cationic

fragment, and were consistent with the average Ir−CO bond lengths measured in the solid-state structure of [3]+[Ir(CO2)Cl2]−: 1.873(10) Å for [(1)2Ir(CO)2]+ vs 1.820(14) Å for [Ir(CO2)Cl2]−. IR spectroelectrochemistry was utilized to quantify the change in electron density at the iridium centers upon oxidation of the ferrocenyl groups in [3]+[Ir(CO)2Cl2]− and [3]+[BArF4]−. Bulk oxidation (electrolysis) of [3]+[Ir(CO)2Cl2]− at 0.75 V vs SCE effectively increased the νCO of the cationic fragment by ∼10 cm−1 and the ΔνCO of the anionic component by >70 cm−1 (Figure 6). Similar IR spectroelectrochemical measurements with [3]+[BArF4]− also resulted in ΔνCO values of ∼10 cm−1 upon oxidation. Since the measured ΔνCO values for [(1)2Ir(CO)2]+ were comparable to those observed with [(1)Ir(CO)2Cl]13 under similar conditions, the oxidation of metal complexes supported by ferrocenylated NHCs appears to be independent of the number of redox-active NHC units. In other words, despite decorating the iridium metal center with either one or two independent redox-active NHCs, an additive effect was not observed. Moreover, these observations indicate that introducing a point positive charge near an iridium metal results in a 4338


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ΔνCO of ∼10 cm−1 irrespective of its position of attachment or magnitude of overall charge.

yellow solution to dark orange was observed. The resulting mixture was stirred at 60 °C for 2 h, during which time a bright orange precipitate was observed. The precipitate was collected by gravity filtration, washed with 3 × 2 mL portions of Et2O, and dried under reduced pressure to afford the desired compound as a bright orange solid. Yield: 0.035 g, 93%. 1H NMR (δ, ppm in CDCl3): 7.16−7.15 (m, 2H), 7.14 (br s, 2H), 7.08 (br s, 2H), 6.765−6.761 (m, 2H), 5.87−5.84 (d, J = 14.5, 2H), 4.71−4.68 (m, 2H), 4.38−4.37 (m, 2H), 4.30−4.28 (m, 2H), 4.23−4.22 (m, 2H), 4.10−4.09 (m, 2H), 4.06 (s, 10H), 3.96−3.93 (d, J = 15, 2H), 3.04−3.00 (m, 2H), 2.44 (s, 6H), 2.10−1.97 (m, 8H), 1.85 (s, 6H), 1.57 (s, 6H), 1.54−1.49 (m, 2H). 13 C NMR (δ, ppm in CDCl3): 17.8, 17.9, 21.3, 27.2, 35.9, 50.4, 68.1, 68.8, 69.2, 69.3, 70.0, 70.6, 79.0, 79.8, 121.1, 124.2, 129.6, 135.3, 135.5, 135.9, 140.0, 176.6. HRMS (ESI): [M − Cl]+ calcd for C54H60N4Fe2Ir, 1069.3146; found, 1069.3152. Anal. Calcd for C54H60N4ClFe2Ir: C, 58.70; H, 5.48; N, 5.07. Found: C, 57.89; H, 5.68; N, 4.95. [Bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)iridium(I)(1,5-COD)][dichloro(1,5-COD)iridium(I)] ([2]+[Ir(COD)Cl2]−). A 10 mL vial equipped with a stir bar was charged with [2]+[Cl]− (0.022 g, 0.0199 mmol), [Ir(COD)Cl]2 (0.0066 g, 0.00996 mmol), [N(nBu)4]+[Cl]− (0.0055 g, 0.0199 mmol), and 3 mL of dry CH2Cl2. The resulting mixture was stirred at 30 °C for 12 h, which resulted in the formation of a bright orange solution. The resulting mixture was taken to dryness under reduced pressure to yield an orange residue. The resulting residue was washed with 3 × 2 mL portions of cold THF, followed by 3 × 2 mL portions of Et2O. The resulting orange solid was dried under reduced pressure to afford the desired complex as a bright orange solid. Yield: 0.026 g, 91%. 1H NMR (δ, ppm in CD2Cl2): 7.17−7.15 (m, 4H), 7.13 (br s, 2H), 6.78 (br s, 2H), 5.95−5.91 (d, J = 14.5, 2H), 4.77−4.74 (m, 2H), 4.38−4.35 (m, 4H), 4.29 (br s, 2H), 4.19 (br s, 2H), 4.13 (s, 10H), 3.95−3.92 (d, J = 14.5, 2H), 3.77 (b, 4H), 3.11−3.06 (m, 2H), 2.49 (s, 6H), 2.13−2.02 (m, 10H), 1.89 (s, 6H), 1.60−1.44 (m, 10H), 1.28−1.26 (m, 4H). 13C NMR (δ, ppm in CD2Cl2): 18.1, 18.2, 21.5, 27.5, 32.3, 36.4, 50.8, 59.0, 68.7, 69.3, 69.7, 69.9, 70.4, 71.0, 79.7, 80.3, 121.2, 124.5, 129.8, 130.1, 135.7, 135.8, 136.5, 140.4, 177.3. HRMS (ESI): [M − Ir(COD)2Cl2]+ calcd for C54H60N4Fe2Ir, 1069.3146; found, 1069.3155. Anal. Calcd for C62H72N4Cl2Fe2Ir2: C, 51.68; H, 5.04; N, 3.89. Found: C, 51.81; H, 5.15; N, 3.61. [Bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)iridium(I)(dicarbonyl)][dichloro(dicarbonyl)iridium(I)] ([3]+[Ir(CO)2Cl2]−). A 10 mL Schlenk flash was charged with a stir bar, [2]+[Ir(COD)Cl2]− (0.020 g, 0.0139 mmol), and 2 mL of dry CH2Cl2. The resulting solution was stirred under 1 atm of carbon monoxide for 2 h, during which time a color change from orange-red to yellow was observed. After 2 mL of CH2Cl2 was added, the resulting mixture was filtered through Celite. The volatiles were then removed under reduced pressure, and the resulting crude residue was washed with 3 × 2 mL of cold pentane to afford the desired complex as a yellow powder. Yield: 0.0165 g, 87%. 1H NMR (δ, ppm in CD2Cl2): 7.26 (s, 2H), 7.20−7.18 (m, 4H), 6.94 (s, 2H), 4.69−4.65 (d, J = 15, 2H), 4.30−4.27 (m, 4H), 4.22−4.19 (m, 4H), 4.16 (s, 10H), 3.84−3.80 (d, J = 15, 2H), 2.49 (s, 6H), 2.05 (s, 6H), 1.62 (s, 6H). 13C NMR (δ, ppm in CD2Cl2) 18.0, 18.4, 21.4, 50.8, 69.2, 69.4, 69.6, 69.9, 80.1, 122.5, 125.6, 130.1, 130.3, 134.6, 134.7, 137.0, 140.7, 167.4, 168.6, 180.4. HRMS (ESI): [M − Ir(CO)2Cl2]+ calcd for C48H48N4Fe2IrO2, 1017.2105; found, 1017.2137; [M − Ir(1)2(CO)2]− calcd for C 2 IrCl 2 O 2 318.8895; found, 318.8892. Anal. Calcd for C50H48N4Cl2O4Fe2Ir2: C, 44.93; H, 3.62; N, 4.19. Found: C, 44.90; H, 3.61; N, 3.96. Bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)iridium(I)(1,5-COD) Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ([2]+[BArF4]−). A 10 mL vial equipped with a stir bar was charged with [2]+[Cl]− (0.050 g, 0.045 mmol), NaBArF4 (0.039 g, 0.045 mmol), and 3 mL of CH2Cl2. The resulting heterogeneous mixture was stirred at 25 °C for 12 h, followed by filtration through a PTFE filter. The volatiles were removed under reduced pressure to yield a crude residue, which was dissolved in Et2O and filtered through Celite. Subsequent removal of the residual solvent afforded the desired complex as an orange-red powder. Yield: 0.078 g; 90%. 1H NMR (δ,

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CONCLUSIONS In summation, we demonstrated that iridium complexes bearing two bulky, ferrocenylated NHC ligands may be prepared via the treatment of 1, generated in situ, with [Ir(COD)Cl]2. The chronoamperometric response for the iron-centered oxidation measured at 0.68 V reflected concurrent Fe2+/Fe3+ oxidations at the two Fc-NHC units, which indicated that the iron centers were electronically decoupled. Spectroelectrochemical IR analysis of [3]+[BArF4]− revealed that the change in average carbonyl stretching frequency was similar to that observed for [(1)Ir(CO)2Cl]. Moreover, the ΔνCO values displayed by [3]+[Ir(CO)2Cl2]− upon oxidation enabled the quantification of the electronic perturbations at the iridium metal centers: the Fe2+ → Fe3+ oxidations at the cationic component increased the measured νCO by ca. 10 cm−1, whereas the Ir1+ → Ir2+ oxidations at the anionic component increased the νCO by >70 cm−1. Collectively, these results have important design considerations: (1) the magnitude of the modulation of the ligated metal upon the oxidation of NHCs bearing ferrocenyl groups appears to be independent of the number of redoxactive groups and (2) installing one redox-active Fc-NHC (vs two) may be sufficient to achieve maximal redox-switching properties. We expect our findings to expedite the rational design of redox-active complexes and catalysts supported by ferrocenylated NHCs.

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EXPERIMENTAL SECTION

Literature procedures were used to synthesize [Ir(COD)Cl]2 (COD = cis,cis-1,5-cyclooctadiene),34 [Fe(η5-C5H4CH2(C3H3N2)(Mes))Cp][I] (Mes = mesityl, Cp = cyclopentadienyl),13 and NaBArF4.35 All other reagents were purchased from commercial sources and used as received. CD2Cl2 and CDCl3 (99.9%) were dried over 3 Å molecular sieves and degassed using three consecutive freeze−pump−thaw cycles. Solvents were dried and degassed using a solvent purification system (CH2Cl2, Et2O, and toluene). All reactions and manipulations were conducted under an atmosphere of N2 unless otherwise indicated. IR spectra were recorded using a FT-IR spectrometer in the absorption mode. NMR spectra were recorded at 25 °C and referenced to the residual solvent: for 1H, CDCl3 7.24 ppm, CD2Cl2 5.32 ppm; for 13C, CDCl3 77.0 ppm; CD2Cl2 53.8 ppm. Coupling constants (J) are expressed in hertz (Hz). High-resolution mass spectra (HRMS) are reported as m/z (relative intensity). Electrochemical measurements were performed using a silver-wire quasireference electrode, a platinum-disk working electrode, and a Pt wire as the auxiliary electrode, in a gastight three-electrode cell under an atmosphere of nitrogen. Unless specified otherwise, measurements were performed using 1.0 mM solutions of analyte in dry CH2Cl2 with 0.1 M [N(nBu)4]+[PF6]− as the electrolyte and decamethylferrocene (Fc*) as the internal standard. Differential pulse voltammetry measurements were performed with 50 mV pulse amplitudes and 2 mV data intervals. All potentials listed were determined by cyclic voltammetry at 100 mV s−1 scan rates and referenced to a saturated calomel electrode (SCE) by shifting (Fc*)0/+ to −0.057 V (CH2Cl2).36 Bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)iridium(I)(1,5-COD) Chloride ([2]+[Cl]−). A 10 mL vial equipped with a stir bar was charged with [Fe(η5-C5H4CH2(C3H3N2)(Mes))Cp]+[I]−13 (0.035 g, 0.068 mmol), NaN(SiMe3)2 (0.0125 g, 0.068 mmol), and 2 mL of dry toluene. The resulting mixture was stirred at 25 °C for 0.5 h, which resulted in the formation of a yellow solution containing a white suspension. The heterogeneous mixture was filtered into a clean 10 mL vial containing [Ir(COD)Cl]2 (0.0114 g, 0.0170 mmol) using a PTFE filter. An immediate color change from a bright 4339


Organometallics

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1999, 1, 953. (e) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247. (f) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2000, 34, 18. (g) Weskamp, T.; Kohl, F. J.; Herrmann, W. A. J. Organomet. Chem. 1999, 582, 362. (h) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem., Int. Ed. 1999, 38, 2416. (i) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann, W. A. Angew. Chem., Int. Ed. 1998, 37, 2490. Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Arlt, D.; Grela, K. J. Am. Chem. Soc. 2006, 128, 13652. (3) (a) Valente, C.; Ç alimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.; Organ, M. G. Angew. Chem., Int. Ed. 2012, 51, 3314. (b) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass, G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem. Eur. J. 2006, 12, 4743. (c) Organ, M. G.; Abdel-Hadi, M.; Avola, S.; Hadei, N.; Nasielski, J.; O’Brien, C. J.; Valente, C. Chem. Eur. J. 2007, 13, 150. (d) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (4) (a) Amijs, C. H. M.; Ferrer, C.; Echavarren, A. M. Chem. Commun. 2007, 698. (b) Amijs, C. H. M.; López-Carrillo, V.; Raducan, M.; Pérez-Galán, P.; Ferrer, C.; Echavarren, A. M. J. Org. Chem. 2008, 73, 7721. (c) Marion, N.; de Fremont, P.; Lemiere, G.; Stevens, E. D.; Fensterbank, L.; Malacria, M.; Nolan, S. P. Chem. Commun. 2006, 2048. (d) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. (e) Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am. Chem. Soc. 2005, 127, 6178. (f) Nolan, S. P. Acc. Chem. Res. 2010, 44, 91. (g) Ricard, L.; Gagosz, F. Organometallics 2007, 26, 4704. (h) Seo, H.; Roberts, B. P.; Abboud, K. A.; Merz, K. M., Jr.; Hong, S. Org. Lett. 2010, 12, 4860. (5) (a) Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem. Commun. 2002, 32. (b) Albrecht, M.; Miecznikowski, J. R.; Samuel, A.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 3596. (c) Diez, C.; Nagel, U. Appl. Organomet. Chem. 2010, 24, 509. (d) Kuhl, S.; Schneider, R.; Fort, Y. Organometallics 2003, 22, 4184. (e) Mas-Marzá, E.; Mata, J. A.; Peris, E. Angew. Chem., Int. Ed. 2007, 46, 3729. (f) MasMarzá, E.; Poyatos, M.; Sanaú, M.; Peris, E. Organometallics 2003, 23, 323. (6) Diez, C.; Nagel, U. Appl. Organomet. Chem. 2010, 24, 509. (7) (a) Anderson, P. G.; Munslow, I. J. Modern Reduction Methods; Wiley-VCH: Weinheim, Germany, 2008. (b) Buitrago, E.; Zani, L.; Adolfsson, H. Appl. Organomet. Chem. 2011, 25, 748. (c) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157. (8) (a) Jafarpour, L.; Nolan, S. P. J. Organomet. Chem. 2001, 617− 618, 17. (b) Scott, N. M.; Clavier, H.; Mahjoor, P.; Stevens, E. D.; Nolan, S. P. Organometallics 2008, 27, 3181. (9) Leibfarth, F. A.; Mattson, K. M.; Fors, B. P.; Collins, H. A.; Hawker, C. J. Angew. Chem., Int. Ed. 2013, 52, 199. (10) (a) Leuthäußer, S.; Schwarz, D.; Plenio, H. Chem. Eur. J. 2007, 13, 7195. (b) Süßner, M.; Plenio, H. Angew. Chem., Int. Ed. 2005, 44, 6885. (11) Lorkovic, I. M.; Duff, R. R.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 3617. (12) (a) Broderick, E. M.; Guo, N.; Vogel, C. S.; Xu, C.; Sutter, J.; Miller, J. T.; Meyer, K.; Mehrkhodavandi, P.; Diaconescu, P. L. J. Am. Chem. Soc. 2011, 133, 9278. (b) Broderick, E. M.; Guo, N.; Wu, T.; Vogel, C. S.; Xu, C.; Sutter, J.; Miller, J. T.; Meyer, K.; Cantat, T.; Diaconescu, P. L. Chem. Commun. 2011, 47, 9897. (c) Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.; Oxford, P. J.; White, A. J. P. J. Am. Chem. Soc. 2006, 128, 7410. (d) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 1998, 37, 894. (13) Arumugam, K.; Varnado, C. D.; Sproules, S.; Lynch, V. M.; Bielawski, C. W. Chem. Eur. J. 2013, DOI: 10.1002/chem.201301247. (14) Lorkovic, I. M.; Wrighton, M. S.; Davis, W. M. J. Am. Chem. Soc. 1994, 116, 6220. (15) More recently, we reported that the rates of Kumada coupling reactions catalyzed by group 10 metal complexes were dependent on the oxidation state of a 1,3-dimesitylnapthoquinimidazol-2-ylidene (NQMes) ligand; see: Tennyson, A. G.; Lynch, V. M.; Bielawski, C. W. J. Am. Chem. Soc. 2010, 132, 9420. See also: (a) Sanderson, M. D.;

ppm in CDCl3): 7.69−7.68 (m, 8H), 7.49 (s, 4H), 7.10 (br s, 2H), 7.06 (br s, 2H), 7.04−7.03 (m, 2H), 6.66−6.65 (m, 2H), 5.92−5.89 (d, J = 14.5, 2H), 4.74−4.70 (m, 2H), 4.30−4.29 (m, 2H), 4.25−4.24 (m, 4H), 4.13−4.12 (m, 2H), 4.07 (s, 10H), 3.90−3.87 (d, J = 14.5, 2H), 3.07−3.02 (m, 2H), 2.44 (s, 6H), 2.12−2.01 (m, 6H), 1.85 (s, 6H), 1.53 (s, 6H). 13C NMR (δ, ppm in CDCl3): 17.7, 17.8, 21.2, 27.2, 35.9, 50.4, 68.2, 68.9, 69.3, 69.6, 70.2, 70.3, 79.3, 79.5, 117.4, 120.7, 123.9, 124.5 (q, 1JC−F = 270 Hz), 128.85 (q, 2JC−F = 31.5 Hz), 129.4, 129.8, 134.8, 135.21, 135.27, 136.0, 140.2, 161.68 (q, 1JB−C = 50.0 Hz), 177.1. 19F NMR (δ, ppm in CDCl3): −62.7. 11B NMR (δ, ppm in CDCl3): −6.4. HRMS (ESI): [M − BArF4]+ calcd for C54H60N4Fe2Ir, 1069.3146; found, 1069.3154. Anal. Calcd for C86H72N4Fe2IrBF24: C, 53.46; H, 3.76; N, 2.90. Found: C, 53.51; H, 3.98; N, 2.68. [Bis(1-(ferrocenylmethyl)-3-mesitylimidazol-2-ylidene)iridium(I)(dicarbonyl)][dichloro(dicarbonyl)iridium(I)] Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ([3]+[BArF4]−). A 10 mL Schlenk flash was charged with a stir bar, [2]+[BArF4]− (0.030 g, 0.0157 mmol), and 2 mL of dry CH2Cl2. The resulting solution was stirred under 1 atm of carbon monoxide for 2 h, during which time a color change from orange-red to yellow was observed. After 2 mL of Et2O was added, the resulting mixture was filtered through Celite. The volatiles were removed under reduced pressure, and the resulting crude residue was washed with 3 × 2 mL of pentane to afford the desired complex as a yellow powder. Yield: 0.026 g, 91%. 1H NMR (δ, ppm in CDCl3): 7.69−7.68 (m, 8H), 7.50 (s, 4H), 7.14 (br, 2H), 7.05−7.04 (d, J = 2, 2H), 7.03 (br, 2H), 6.78−6.77 (d, J = 2, 2H), 4.73−4.70 (d, J = 14.5, 2H), 4.24−4.21 (m, 2H), 4.15−4.14 (m, 2H), 4.11 (s, 10H), 3.68−3.65 (d, J = 14.5, 2H), 2.44 (s, 6H), 2.02 (s, 6H), 1.49 (s, 6H). 13C NMR (δ, ppm in CDCl3): 17.3, 18.1, 21.1, 50.6, 68.7, 69.0, 69.3, 69.4, 70.0, 79.1, 117.4, 121.5, 124.5 (q, 1JC−F = 270 Hz), 125.0, 128.8 (q, 2JC−F = 31.5 Hz), 129.5, 130.2, 133.9, 134.2, 134.8, 136.7, 140.6, 161.8 (q, 1JB−C = 50.0 Hz), 167.3, 179.7. 19F NMR (δ, ppm in CDCl3): −62.3. HRMS (ESI): [M − BArF4]+ calcd for C48H48N4Fe2IrO2, 1017.2105; found, 1017.2123. Anal. Calcd. for C80H60N4O2Fe2IrBF24: C, 51.08; H, 3.22; N, 2.98. Found: C, 50.99; H, 3.00; N, 2.33.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving additional NMR and electrochemical data and a table and CIF file giving crystallographic data. 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 for C.W.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported in part by the U.S. Army Research Office under grant number W911NF-09-10446, the National Science Foundation (CHE-1266323 to C.W.B. and CHE-0741973 for an X-ray diffractometer), and the Robert A. Welch Foundation (F-0046).

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REFERENCES

(1) (a) Boeda, F.; Nolan, S. P. Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2008, 104, 184. (b) Glorius, F. Top. Organomet. Chem. 2007, 21, 1. (c) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (d) Nolan, S. P., N-Heterocyclic Carbenes in Synthesis; WileyVCH: Weinheim, Germany, 2006. (2) (a) Fürstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012. (b) Grubbs, R. H.; Trnka, T. M., Ruthenium-Catalyzed Olefin Metathesis; Wiley-VCH: Weinheim, Germany, 2005. (c) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674. (d) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 4340


Organometallics

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Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514. (b) Tennyson, A. G.; Ono, R. J.; Hudnall, T. W.; Khramov, D. M.; Er, J. A. V.; Kamplain, J. W.; Lynch, V. M.; Sessler, J. L.; Bielawski, C. W. Chem. Eur. J. 2010, 16, 304. (c) Ono, R. J.; Suzuki, Y.; Khramov, D. M.; Ueda, M.; Sessler, J. L.; Bielawski, C. W. J. Org. Chem. 2011, 76, 3239. (16) (a) Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.; Ongania, K.-H. J. Organomet. Chem. 1997, 540, 127. (b) Bildstein, B.; Malaun, M.; Kopacka, H.; Ongania, K.-H.; Wurst, K. J. Organomet. Chem. 1998, 552, 45. (c) Bildstein, B.; Malaun, M.; Kopacka, H.; Ongania, K.-H.; Wurst, K. J. Organomet. Chem. 1999, 572, 177. (d) Bildstein, B.; Malaun, M.; Kopacka, H.; Wurst, K.; Mitterbock, M.; Ongania, K.-H.; Opromolla, G.; Zanello, P. Organometallics 1999, 18, 4325. (17) (a) Arduengo, A. J.; Bannenberg, T. P.; Tapu, D.; Marshall, W. J. Tetrahedron Lett. 2005, 46, 6847. (b) Arduengo, A. J.; Bannenberg, T. P.; Tapu, D.; Marshall, W. J. Chem. Lett. 2005, 34, 1010. (c) Arduengo, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530. (d) Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027. (e) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (f) Arduengo, A. J.; Tapu, D.; Marshall, W. J. J. Am. Chem. Soc. 2005, 127, 16400. (g) Arduengo, A. J.; Tapu, D.; Marshall, W. J. Angew. Chem., Int. Ed. 2005, 44, 7240. (h) Arduengo, A. J.; Tapu, D.; Marshall, W. J. Angew. Chem. 2005, 117, 7406. (18) (a) Gischig, S.; Togni, A. Organometallics 2004, 23, 2479. (b) Seo, H.; Park, H.-j.; Kim, B. Y.; Lee, J. H.; Son, S. U.; Chung, Y. K. Organometallics 2003, 22, 618. (c) Siemeling, U. Eur. J. Inorg. Chem. 2012, 3523. (d) Siemeling, U.; Farber, C.; Bruhn, C. Chem. Commun. 2009, 98. (e) Siemeling, U.; Farber, C.; Bruhn, C.; Leibold, M.; Selent, D.; Baumann, W.; von Hopffgarten, M.; Goedecke, C.; Frenking, G. Chem. Sci. 2010, 7, 697. (f) Labande, A.; Debono, N.; Sournia-Saquet, A.; Daran, J.-C.; Poli, R. Dalton Trans. 2013, 42, 6531. (g) Debono, N.; Labande, A.; Manoury, E.; Daran, J.-C.; Poli, R. Organometallics 2010, 29, 1879. (h) Gülcemal, N.; Labande, A.; Daran, J.-C.; Ç etinkaya, B.; Poli, R. Eur. J. Inorg. Chem. 2009, 1806. (i) Labande, A.; Daran, J.-C.; Manoury, E.; Poli, R. Eur. J. Inorg. Chem. 2007, 1205. (19) (a) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem., Int. Ed. 2006, 45, 6186. (b) Khramov, D. M.; Rosen, E. L.; Lynch, V. M.; Bielawski, C. W. Angew. Chem., Int. Ed. 2008, 47, 2267. (c) Varnado, C. D., Jr; Lynch, V. M.; Bielawski, C. W. Dalton Trans. 2009, 7253. (20) Rosen, E. L.; Varnado, C. D.; Tennyson, A. G.; Khramov, D. M.; Kamplain, J. W.; Sung, D. H.; Cresswell, P. T.; Lynch, V. M.; Bielawski, C. W. Organometallics 2009, 28, 6695. (21) (a) Herrmann, W. A.; Fischer, J.; Ö fele, K.; Artus, G. R. J. J. Organomet. Chem. 1997, 530, 259. (b) Herrmann, W. A.; Gooen, L. J.; Spiegler, M. J. Organomet. Chem. 1997, 547, 357. (22) (a) Hintermair, U.; Englert, U.; Leitner, W. Organometallics 2011, 30, 3726. (b) Burling, S.; Mahon, M. F.; Reade, S. P.; Whittlesey, M. K. Organometallics 2006, 25, 3761. (23) Frey, G. D.; Rentzsch, C. F.; von Preysing, D.; Scherg, T.; Mühlhofer, M.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem. 2006, 691, 5725. (24) Weng, W.; Guo, C.; Moura, C.; Yang, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2005, 24, 3487. (25) Although complexes containing anionic Ir counteranions associated with cationic Ir complexes supported by NHCs could not be found in the Cambridge Structural Database, Rh analogues are known; see: (a) Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.; Fallis, I. A.; Cavell, K. J. Dalton Trans. 2009, 7099. (b) Stylianides, N.; Danopoulos, A. A.; Tsoureas, N. J. Organomet. Chem. 2005, 690, 5948. (26) Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202. (27) Wolf, S.; Plenio, H. J. Organomet. Chem. 2009, 694, 1487. (28) (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am. Chem. Soc. 2004, 126, 15195. (b) Iglesias, M.; Beetstra, D. J.;

Stasch, A.; Horton, P. N.; Hursthouse, M. B.; Coles, S. J.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2007, 26, 4800. (29) To assist with the electrochemical assignments [(1)2Ir(COD)]+[BArF4]− and [N(nBu)4]+[Ir(COD)Cl2]− were independently evaluated using CV and DPV. (30) The electrochemical assignments of [2]+[Cl]−, [2]+[Ir(COD) Cl2]−, and [3]+[Ir(CO)2Cl2]− were complicated by the [Cl]− and [Ir(COD)Cl2]−, [Ir(CO)2Cl2]− counteranions, respectively. As such, [2]+[BArF4]− and [3]+[BArF4]−, which feature inert counteranions, were synthesized and studied. (31) Oxidation of the Ir center was not observed within the solvent window (−2 to +2 V, CH2Cl2). (32) Denuault, G.; Mirkin, M. V.; Bard, A. J. J. Electroanal. Chem. 1991, 308, 27. (33) (a) Schütz, J.; Herrmann, W. A. J. Organomet. Chem. 2004, 689, 2995. (b) Vickers, P. W.; Pearson, J. M.; Ghaffar, T.; Adams, H.; Haynes, A. J. Phys. Org. Chem. 2004, 17, 1007. (c) Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2008, 693, 899. (34) Komiya, S. Synthesis of Organometallic Compounds: A Practical Guide; Wiley: New York, 1997. (35) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579. (36) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713.

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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper published on July 22, 2013, some works were not cited in reference 18. The version of this paper that appears as of July 25, 2013, has these works cited.

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Cationic Iridium Complexes Containing Anionic Iridium Counterions

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