Di- and Trinuclear Iridium(III) Complexes with Poly-Mesoionic

Article pubs.acs.org/Organometallics

Di- and Trinuclear Iridium(III) Complexes with Poly-Mesoionic Carbenes Synthesized through Selective Base-Dependent Metalation Ramananda Maity, Margarethe van der Meer, Stephan Hohloch, and Biprajit Sarkar* Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195, Berlin, Germany S Supporting Information *

ABSTRACT: Mutidentate carbene ligands based on a rigid aromatic platform are valuable synthons for generating carbene complexes with higher nuclearity. We present here the selective, base-dependent synthesis of a dinuclear or a trinuclear IrIII complex from the 1,3,5-substituted benzene derived tris-triazolium salt. The dinuclear IrIII complex features an unreacted triazolium unit which enables us to compare the metric parameters between the bonded 1,2,3-triazol-5-ylidene to their parent triazolium salt present in the same molecule. Single crystal X-ray diffraction studies confirm the di- and trinuclear nature of the complexes and establish their configuration and conformation. Both the di- and trinuclear IrIII complexes have been used for catalytic transfer hydrogenation, and these complexes are potent precatalysts delivering good to excellent yields for the reduction of benzaldehyde, acetophenone, benzophenone, and cyclohexanone. Furthermore, they show a preference for reducing nitrobenzene to either azoxybenzene or azobenzene. Mercury poisoning tests conclusively prove the homogeneous nature of the reported catalysis. The lack of orthometalation in these complexes and the possible effect thereof on catalysis are discussed.

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INTRODUCTION 1,2,3-Triazol-5-ylidenes, a type of carbene belonging to the socalled mesoionic carbene (MIC) class, are fast establishing themselves as privileged ligands in organometallic chemistry1 and in homogeneous catalysis.2 Transition metal complexes of such ligands have been used as catalysts for the click reaction,3 in oxidation reactions,4 in hydroarylation of alkynes,5 in Suzuki−Miyaura cross-coupling reactions,6 and for transfer hydrogenation reactions.7 Some other recent uses of such complexes are related to their photophysical applications.8 1,2,3-Triazol-5-ylidenes have been postulated as even better ligands in catalysis owing to their superior donor ability over their classical N-heterocyclic carbene (NHC) counterparts.9 Applications for MICs may be further augmented through the use of poly-MIC ligands to generate multinuclear complexes.10 Whereas monodentate triazolylidenes have been the most used ligands of this type, some modifications have been performed by incorporating MIC type triazolylidene ligands into a bis-chelate4e,f,7a or a tridentate8,11 kind of framework. However, the majority of such reported complexes are mononuclear.1,8b Only few dinuclear complexes bearing metal centers at each MIC donor have been reported in the literature.2c,6a,10,12 In all of those cases (except A),10 ligands based on a 1,3-substituted-phenylene (meta) backbone (see C, Figure 1) were used where the triazolylidene donors are in the 1,3 positions. Polynuclear complexes with 1,2,3-triazol-5ylidenes or other MIC ligands are rare.13 Considering the importance of NHC and MIC type ligands in various fields of chemistry generating ligand platforms for synthesizing polynuclear complexes with multidentate MIC or NHC donors is an important goal.14 A few ligand platforms for generating di© XXXX American Chemical Society

Figure 1. An orthometalated dinuclear IrIII (A) complex with a MIC ligand. Examples of di- and trinuclear complexes with MIC ligands (C and B). A related orthometalated trinuclear complex with NHC donors (D). All complexes were reported in the literature.

and trinuclear complexes with multiple MIC donors have been introduced by us in the last years.10,13 Recently, we described the synthesis of a doubly orthometalated dinuclear iridium(III) complex A,10 where each iridium(III) atom is coordinated to a MIC donor and additionally orthometalates the central aryl ring Received: May 11, 2015

A

DOI: 10.1021/acs.organomet.5b00365 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

1. This is a surprising result because the corresponding trisimidazolium salt based on a phenyl backbone was shown to undergo triple orthometalation,14a,b and complex A (Figure 1) also has orthometalated iridium centers.10 In view of the observations made in the past,10,14a−d,15 we believe that charge and electronic factors probably are not responsible for the lack of orthometalation in [2]I and [3]. It is likely the steric crowding at the central phenyl ring that makes orthometalation difficult in the present cases. Both di- and trinuclear complexes ([2]I and [3]) are well soluble in solvents like dichloromethane, chloroform, dimethyl sulfoxide, and acetonitrile. The formation of these complexes was confirmed by 1H NMR spectroscopy, mass spectrometry, and X-ray crystallography. The 1H NMR spectrum of complex [2]I shows the resonance for the unreacted triazolium salt as a singlet at δ 9.46 ppm. The resonance for three chemically different aryl C− H protons appeared also as singlets at δ 7.87, 8.30, and 8.38 ppm. The signals observed for [2]I in its NMR spectrum are broad, pointing to the existence of restricted rotation in that complex. Various rotamers are possible for a complex such as [2]I. Cooling down the sample delivered more than one set of signals which we were not able to confidently assign. However, mass spectrometry and single crystal X-ray diffraction studies clearly establish the dinuclear nature of this complex (see below), and elemental analysis proves its purity. The formation of complex [3] was easily monitored by the 1H NMR spectrum, which shows the disappearance of the triazolium C−H proton signal (δ 9.27 ppm) for the original tris-triazolium salt 1.13 Two sets of broad signals appeared in the 1H NMR spectrum of complex [3] at room temperature. However, three distinct sets of signals were observed both in the 1H and 13C{1H} NMR spectra of complex [3] at −70 °C (Figure 2 and the Supporting Information). This observation is possibly due to the restricted rotation of the [IrIII(Cl)2Cp*(MIC)] moieties around the Caryl−Ctrz bonds and points to the existence of three different rotamers at −70 °C. The ratio of the two sets of signals was 1:2 at room temperature and became 1:1:1 (three sets of signals) at −70 °C. The resonance for the aryl C−H protons appeared at δ 7.91 ppm as a singlet with an additional broad singlet at δ 8.10−8.91 ppm in the 1H NMR spectrum of complex [3] at ambient temperature. Two singlets at δ 1.45 and 1.55 ppm attributed to the Cp*−CH3 protons were observed, and these resonances fall in the range reported for the iridium(III) mesoionic carbene complexes.16 The 13C{1H} NMR spectrum at room temperature shows only one set of signals with the exception of aryl C−H carbon atoms, which appeared at δ 138.8 and 138.2 ppm. The resonance for the characteristic carbene carbon atoms was observed at δ 144.2 ppm. The carbon signal for the Ctrz−Ar atoms (δ 146.3 ppm) appeared more downfield shifted compared to the carbene carbon resonance (δ 144.2 ppm). This is in accordance with other reports observed for mesoionic iridium(III) carbene16 complexes and also with the related trinuclear palladium(II)13 complex. The formation of di- and trinuclear IrIII complexes was further supported by ESI mass spectrometry. The ESI spectra (positive ions) show peaks at m/z = 1568.0309 (calcd for [[2]I−I]+ 1568.0230) and 1564.3281 (calcd for [[3]−Cl]+ 1564.3193) as the strongest signals. X-ray Crystal Structures of Complexes. Single crystals suitable for X-ray diffraction study were obtained for both complexes by the slow diffusion of pentane into a

of the ligand. We have also demonstrated the preparation of a trinuclear palladium(II) complex B with a tris-1,2,3-triazol-5ylidene ligand possessing a 1,3,5-substitution pattern of the central phenyl ring.13 In the following, results are presented on di- and triiridium complexes with the ligand platform that was used for synthesizing the tripalladium(II) complex B (Figure 1). Synthesis and crystal structures of these complexes are presented below where we discuss the lack of orthometalation in the new iridium complexes compared to complex A. Additionally, we also present results from catalytic investigations of these complexes in various transfer hydrogenation processes.

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. 1,3,5Tris[(trimethylsilyl)ethynyl]benzene was converted to the corresponding tris-triazole compound under typical Cu(I)catalyzed click conditions by using a reported procedure.13 The tris-triazolium salt 1 was prepared from the corresponding tristriazole by using methyl iodide as a methylating agent under heating condition by reported procedures.13 The reaction of the tris-triazolium salt 1 with 1 equiv of [IrCl2(Cp*)]2 in the presence of K2CO3 resulted in the formation of a dinuclear diiridium(III) complex [2]I (Scheme 1). However, the reaction Scheme 1. Synthesis of the Dinuclear and Trinuclear IrIII Complexes [2]I and [3]

of the same tris-triazolium salt 1 with Ag2O and the subsequent transmetalataion to [IrCl2(Cp*)]2 yielded a trinuclear iridium(III) complex [3] (Scheme 1). Both complexes were obtained in reasonable yield. The change in either the amount of base or the amount of metal precursor did not lead to the formation of trinuclear complex [3] under the reaction condition mentioned for the synthesis of [2]I. All the attempts ended up with the formation of dinuclear complex [2]I. Thus, it is seen that the use of a particular base leads to the selective formation of complexes with a particular nuclearity. Surprisingly, no orthometalated complexes were observed under the reaction conditions mentioned in Scheme B


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Organometallics

Figure 2. Stacking plot of 1H NMR spectra of [3] in CD2Cl2 at different temperatures.

dichloromethane:chloroform:acetonitrile solution mixture of each complex ([2]I or [3]) at ambient temperature. The molecular structure analysis confirmed the formation of halfsandwich type di- and trinuclear iridium(III) complexes [2]I (Figure 3) and [3] (Figure 4), respectively.

Figure 4. ORTEP plot of [3] in [3]·4CHCl3·CH2Cl2·CH3CN. Ellipsoids are drawn at 50% probability. Hydrogen atoms have been omitted, and only the first atom of each of the N-ethyl substituents is shown for clarity. Figure 3. ORTEP plot of [2]I in [2]I·2CHCl3·0.5CH2Cl2. Ellipsoids are drawn at 50% probability. Hydrogen atoms except H30, solvent molecules, and the counteranion have been omitted for clarity. Only the first atom of each of the N-ethyl substituents is shown.

The triazolylidene moieties are rotated out of the central aryl ring plane in both complexes. The torsion angles (Caryl−Caryl− Ctrz−CMIC) measure 80.5 and 77.4° in complex [2]I and 77.2, 91.3, and 120.0° in complex [3]. The torsion angle associated with the free triazolium unit in complex [2]I (Caryl−Caryl−Ctrz− Ctrazolium) measures 34.6°. The metric parameters of the coordinated 1,2,3-triazolin-5ylidenes can be directly compared to their triazolium precursor present in the same molecule in [2]I·2CHCl3·0.5CH2Cl2 (Figure 2). The transformation of a triazolium cation into a MIC leads to an expansion of the intraring NEt−Ctrz and Ctrz− Ctrz bond distances (NEt−Ctrz, Δd ≈ 0.037; Ctrz−Ctrz, 0.023 Å) with a consequent decrease of the NEt−CMIC−Ctrz angles by about 5.7°. A similar observation was also reported with the iridium(III) complexes bearing orthometalated classical NHC ligands.14b,d The expansion of the intraring N−CMIC bond distances (Δd ≈ 0.025 Å) or the decrease of the N−CMIC−N

The coordination geometry around the iridium(III) center in both complexes [2]I and [3] can be described as a three-legged piano stool,17 where the three legs are one MIC donor and two iodide donors for complex [2]I or two chloride donors for complex [3]. The distance between the iridium center and the centroid of the Cp* ring measures 1.848 Å (average) in [2]I and 1.806 Å (average) in complex [3]. The Ir−Ctrz distances ([2]I, 2.043(15) and 2.052(15) Å; [3], 2.035(7)−2.055(6) Å) are almost equidistant within the experimental error in both complexes. These bond distances along with the Ir−I/Cl distances fall in the range reported for iridium(III)− triazolylidene complexes.4a,16,18 C


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Organometallics angles (3−5°) are slightly larger here as compared to the classical NHC complexes. These values are also within the range of previous DFT-based predictions.19 Thus, it is seen that for both [2]I and [3] one of the several possible conformers as observed by NMR spectroscopy preferentially crystallizes out of solution. Catalytic Transfer Hydrogenation of Aldehyde and Ketones. Metal-catalyzed transfer hydrogenation of aldehyde and ketones (and the reverse reaction) has been extensively studied during the last couple of decades,20 and complexes of NHC ligands have also been used for this purpose.21 The transfer hydrogenation method excludes the necessity of using dihydrogen gas under high pressures, and thus has certain advantages over direct hydrogenation reactions.20 Reports on catalytic transfer hydrogenation reactions with complexes bearing triazolylidene (MIC) type ligands are rare.7,10 For this work, we used both di- and trinuclear IrIII complexes as precatalysts for the transfer hydrogenation reactions of aldehyde and ketones. We have also compared the catalytic activities of these complexes with orthometalated IrIII compexes bearing MIC donors.10 Isopropanol was used as a hydrogen source for the transfer hydrogenation reactions. The reactions were carried out at 100 °C with 20 mol % of KOH as a base. Both di- and trinuclear IrIII complexes were active precatalysts in the transfer hydrogenation of benzaldehyde, cyclohexanone, acetophenone, and benzophenone. A full conversion of benzaldehyde and cyclohexanone to their corresponding alcohols was achieved in 1 h upon using 0.25 mol % of dinuclear or 0.16 mol % of trinuclear IrIII complexes (same amount of iridium for both types of complexes, Table 1). Both di- and trinuclear IrIII

complexes were more active compared to the orthometalated IrIII complexes of type A (Figure 1) reported by us previously.10 Those complexes show 40−87% conversions of benzaldehyde to benzyl alcohol under identical reaction conditions after 1 h.10 Using acetophenone as a substrate, the conversions were lower under identical reaction conditions. Both [2]I and [3] gave more than 75% conversions after 3 h. The di-iridium complex [2]I (3 h: 83%) appeared to be slightly more active than the trinuclear counterpart [3] (3 h: 77%). However, full conversion was achieved with both complexes on carrying out the reaction for 6 h. Following these results, we decided to try the transfer hydrogenation of benzophenone which is a more sterically demanding substrate compared to the other ketones described above. Only low conversions were achieved after 3 h of reaction time. However, the conversions drastically improved on performing the reaction for 6 h. The dinuclear IrIII complex [2]I (6 h: 89%) again acted as a slightly better precatalyst compared to the trinuclear complex [3] (6 h: 80%) toward benzophenone reduction. Catalytic Transfer Hydrogenation of Nitrobenzene. Catalytic transfer hydrogenation of nitrobenzene has received interest not only due to their mechanistic investigation but also for the valuable hydrogenated products formed during this reaction.22 Recently, a series of chelating IrIII complexes with 1,2,3-traizol-5-ylidenes ligands have been reported as precatalysts for the nitrobenzene reduction.7a All of these complexes yielded a mixture of hydrogenated products (aniline, azobenzene, and azoxybenzene) with azobenzene and azoxybenzene as major products.7a Both complexes [2]I and [3] have been tested for the catalytic transfer hydrogenation of nitrobenzene. An initial screening test was performed with 0.75 mol % of [2]I or 0.5 mol % of [3] and 0.5 equiv of KOH in 2propanol at 80 °C (Scheme 2). Both complexes showed very little reactivity toward nitrobenzene reduction at 80 °C. However, the trinuclear IrIII complex [3] as a precatalyst at 100 °C showed a complete conversion of nitrobenzene to aniline, azobenzene, and azoxybenzene in amounts of 11, 75, and 14%, respectively. The dinuclear complex [2]I showed 18, 7, and 60% conversions to aniline, azobenzene, and azoxybenzene, respectively, with 15% nitrobenzene remaining unreacted. The complex [3] thus appears to be a good catalyst for the majority transformation to azobenzene at 100 °C. The dinuclear complex [2]I on the other hand is a better precatalyst for preferential azoxybenzene formation under these conditions (Table 2, entry 3). These results thus point to the importance of iridium(III) complexes with MIC ligands for the conversion of nitrobenzene to valuable products such as azobenzene or azoxybenzene. An initial indication that catalysis under the conditions presented here works in a homogeneous way was the clear nature of the solution after catalysis was over. In order to address the issue of the possible formation of colloidal iridium particles under catalytic conditions, mercury poisoning tests were carried out for the catalytic reactions with both [2]I and [3]. For both complexes [2]I and [3], two parallel catalytic

Table 1. Catalytic Transfer Hydrogenation of Aldehyde and Ketonesa

entry

substrate

cat.

conversion (%)

time (h)

1 2 3 4 5 6 7 8 9 10 11 12

benzaldehyde

[2]I [3] [2]I [3] [2]I [3] [2]I [3] [2]I [3] [2]I [3]

99 99 99 99 83 77 99 99 23 24 89 80

1 1 1 1 3 3 6 6 3 3 6 6

cyclohexanone acetophenone

benzophenone

a Reactions conditions: 0.5 mol % IrIII (0.25 mol % for the di- and 0.166 mol % for the trinuclear complexes), 20 mol % KOH, isopropanol, 100 °C.

Scheme 2. Conversion of Nitrobenzene to Various Products

D


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could be assigned at −70 °C. Structural characterization of the two complexes confirmed their nuclearity and also clearly showed the lack of orthometalation in these complexes. This result is contrary to complex A10 and to what has been observed with the corresponding tris-imidazolium salts.14a−c The lack of orthometalation is likely related to steric factors on the ligand backbone. Both di- and trinuclear complexes are active precatalysts for transfer hydrogenation of aldehyde and ketones. However, the dinuclear IrIII complex seems to be a slightly better precatalyst compared to the trinuclear complex for such transfer hydrogenation reactions (albeit with different halide ligands). Both complexes show different reactivity toward the reduction of nitrobenzene. The dinuclear IrIII complex is more selective toward azoxybenzene formation, whereas the trinuclear complex shows preferential formation of azobenzene in the transfer hydrogenation of nitrobenzene. The complexes described here are more potent transfer hydrogenation catalysts compared to their orthometalated counterpart A. However, no cooperativity in catalysis has been observed for the present cases, a result that is contrary to what was observed for A. As steric factors seem to be the primary reason for the lack of orthometalation in [2]I and [3], it will be intriguing to use a larger central platform to avoid steric crowding and hence make orthometalation possible. The dinuclear complex [2]I contains an unreacted triazolium unit. This free triazolium unit can also be accessible for coordination to a different metal center under suitable reaction conditions to generate heteromultinuclear complexes. Further research in our laboratory is directed toward those directions.

Table 2. Catalytic Transfer Hydrogenation of Nitrobenzenea,b,c entry 1 2 3 4 5 6

cat. a

[2]I [3]a [2]Ib [3]b [2]Ic [3]c

aniline

azobenzene

azoxybenzene

2 2 18 11 9 30

1 0 7 75 0 52

0 0 60 14 32 18

a

Reactions conditions: 1.5 mol % IrIII (1.5 mol % IrIII: 0.75 mol % for the di- or 0.5 mol % for the trinuclear complexes), 0.5 equiv KOH, isopropanol, 80 °C, 24 h. bReactions conditions: 1.5 mol % IrIII, 0.5 equiv of KOH, isopropanol, 100 °C, 24 h. c5 mol % IrIII, 0.5 equiv of KOH, isopropanol, 100 °C, 24 h.

reactions were set up which were allowed to run under the conditions shown in Table 2 initially for 6 h. After 6 h, an excess of mercury was added to one reaction mixture, whereas no mercury was added to the second reaction mixture. Reaction controls were performed after 15 and 24 h, respectively. After both of these times, the conversions from reaction mixtures with and without mercury were found to be identical, and these match the values reported in Table 2. These results thus prove that addition of mercury has no influence on the catalytic properties of the presented metal complexes. The negative mercury poisoning test thus is an indication that the catalysts presented here work under homogeneous conditions. We were also able to detect metal-hydride signals in the 1H NMR spectrum by carrying out the reactions without the addition of the substrate (Figure S6, Supporting Information). Such metal hydrides have been postulated as key intermediates in the catalytic reduction of nitroarenes under homogeneous conditions. We would like to note that reduction of nitroarenes has been previously reported to work under homogeneous catalytic conditions.7a,22e,f As has been discussed above, the non-orthometalated iridium complexes presented here are more active precatalysts for transfer hydrogenation reactions compared to their chelating orthometalated counterparts such as A (Figure 1).10 However, in the case of A, cooperativity was observed in catalysis, with the complex A delivering conversions that were more than double of its mononuclear orthometalated counterparts. Such a cooperative effect is missing for the present set of complexes (see above). We believe that the presence of cooperativity in A is related to the orthometalated nature of that complex which makes electronic interactions between the two IrIII centers through the orthometalated phenyl ring possible. Such an effect is missing in the non-orthometalated examples presented here, leading to a lack of cooperativity. Nevertheless, the complexes [2]I and [3] are far more potent transfer hydrogenation catalysts compared to their orthometalated counterparts if conversion per iridium center is considered.

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under a nitrogen atmosphere using standard Schlenk techniques. Glassware was oven-dried at 130 °C. Solvents were distilled by standard procedures prior to use. 1H and 13C{1H} NMR spectra were recorded on a JOEL ECP 500 spectrometer or a Jeol ECS 400 spectrometer. Chemical shifts (δ) are expressed in ppm downfield from tetramethylsilane using the residual protonated solvent as an internal standard. All coupling constants are expressed in Hertz and only given for 1H,1H couplings unless mentioned otherwise. Mass spectra were obtained with an Agilent 6210 ESI-TOF instrument. The tristriazolium salt 113 and [Ir(Cl)2(Cp*)]223 were synthesized as described in the literature. K2CO3, Ag2O, KI, NaCl, and IrCl3·xH2O were purchased from commercial sources and were used as received without further purification. Compound [2]I. To a mixture of the tristriazolium salt 113 (0.020 g, 0.025 mmol), K2CO3 (0.016 g, 0.116 mmol), [Ir(Cp*)(Cl)2]223 (0.022g, 0.028 mmol), and KI (excess) was added acetonitrile (10 mL). The resulting suspension was stirred for 20 h at 84 °C. The acetonitrile was removed in vacuo, and the crude mixture was extracted with dichloromethane (15 mL). The solvent was removed, and the orange-red residue was loaded onto a silica gel column. Elution with a dichloromethane:methanol (98:2, v:v) mixture gave compound [2]I as an orange-red solid. Yield: 0.018 g (0.011 mmol, 44%). 1H NMR (400 MHz, CDCl3): δ = 9.46 (s, br, 1H, Htriazolium), 8.38 (s, br, 1H, HAr), 8.30 (s, br, 1H, HAr), 7.87 (s, br, 1H, HAr), 5.18− 5.44 (m, 3H, N−CH2), 5.0 (s, br, 1H, N−CH2), 4.81 (s, br, 2H, N− CH2), 4.44 (s, 3H, N−CH3), 4.12 (s, 6H, N−CH3), 1.53−1.74 [m, br, 39H, (30H, Cp*−CH3), (9H, N−CH2−CH3)] ppm. HRMS (ESI, positive ions): m/z = 1568.0309 (calcd for [[2]I−I]+ 1568.0230). Anal. Calcd for [2]I·2CH3OH: C, 29.34; H, 3.78; N, 7.16. Found: C, 29.32; H, 3.80; N, 7.03. Compound [3]. To a mixture of the tristriazolium salt 113 (0.050 g, 0.063 mmol) and Ag2O (0.045 g, 0.194 mmol) was added dichloromethane (10 mL). The resulting mixture was stirred at ambient temperature for 18 h under the exclusion of light. To the

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CONCLUSIONS Two IrIII complexes differing in nuclearity have been synthesized selectively starting from the same tris-triazolium ligand precursor 1 depending on the external base employed. [3] is the first example of a trinuclear IrIII complex bearing three MIC donors as ligands. NMR spectroscopy shows the formation of various rotamers for the two complexes due to hindered rotations around C−C bonds. For the trinuclear complex [3], signals corresponding to three different rotamers E


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Organometallics Notes

reaction mixture was added [Ir(Cl)2(Cp*)]2 (0.076 g, 0.095 mmol) and NaCl (excess). The reaction mixture was stirred at ambient temperature for an additional 18 h. The mixture was then filtered through a pad of Celite to obtain a clear solution. The solvent was removed, and the crude mixture was loaded onto a silica gel column. Elution with dichloromethane:methanol (98:2, v:v) gave [3] as a yellow solid. Yield: 0.070 g (0.044 mmol, 70%). 1H NMR (500 MHz, CD2Cl2, rt): 7.91 (s, 3H, HAr), 5.10−5.17 (m, 6H, N−CH2), 4.11 (s, 9H, N−CH3), 1.64−1.74 (m, 6H, N−CH2−CH3), 1.45 (s, 45H, Cp*−CH3) ppm. Other fluxional isomer 1H NMR (500 MHz, CD2Cl2): δ = 8.10−8.91 (s, br, 3H, HAr), 5.10−5.17 (m, 1H, N−CH2), 4.41−4.53 (m, 5H, N−CH2), 3.97 (s, 9H, N−CH3), 1.64−1.74 (m, 9H, N−CH2−CH3), 1.55 (s, 45H, Cp*−CH3) ppm. 13C{1H} NMR (100 MHz, CD2Cl2, rt): δ = 146.3 (Ctrz−Ar), 144.2 (Ctrz−Pd), 138.8/ 138.2 (CAr−H), 127.4 (CAr−Ctrz), 88.0 (Cp*), 49.5 (N−CH2−CH3), 38.3 (N−CH3), 16.4 (N−CH2−CH3), 9.1 (Cp*−CH3) ppm. HRMS (ESI, positive ions): m/z = 1564.3281 (calcd for [[3]−Cl] + 1564.3193). Anal. Calcd for [3]·3CH3OH·CH2Cl2: C, 37.08; H, 4.87; N, 7.08. Found: C, 37.07; H, 5.06; N, 7.14. X-ray Crystallography. Single crystals suitable for X-ray diffraction studies were obtained for the complexes [2]I·2CHCl3· 0.5CH2Cl2 and [3]·4CHCl3·CH2Cl2·CH3CN by slow diffussion of pentane into a concentrated dichloromethane:chloroform:acetonitrile mixture of the corresponding complexes at room temperature. X-ray diffraction data were collected at T = 140 K with a Bruker Smart AXS diffractometer equipped with a rotation anode using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The strategy for the data collection was evaluated by using the Smart software. The data were collected by the standard “omega scan technique”, and were scaled and reduced using the Saint+ software. The structures were solved by direct methods using SHELXS-97 and refined by full matrix least-squares with SHELXL-97, refining on F2.24 Crystallographic details are given in Table S1 (Supporting Information). CCDC 1019283 and 1019284 contain the cif files of complexes [2]· 2CHCl3·0.5CH2Cl2 and [3]·4CHCl3·CH2Cl2·CH3CN. These data can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif. General Procedure for Transfer Hydrogenation Catalysis. The respective aldehyde/ketones (0.5 mmol) and the corresponding IrIII complexes were mixed with KOH (6 mg, 0.1 mmol) in a Schlenk tube under an inert gas atmosphere. To this was added dry iPrOH (4 mL). The resulting mixture were heated at 100 °C for the mentioned time and cooled to ambient temperature. The reaction mixture was then filtered through a small pad of silica using iPrOH and analyzed by either GC-MS chromatography using hexadecane as an internal standard or NMR spectroscopy using hexamethylbenzene as an internal standard. General Procedure for Transfer Hydrogenation of Nitrobenzene. Nitrobenzene (0.0205 g, 0.167 mmol) and the corresponding IrIII complexes were mixed with KOH (5 mg, 0.089 mmol) in a Schlenk tube under an inert gas atmosphere. To this was added dry iPrOH (4 mL). The resulting mixture was heated at the mentioned temperature and cooled to ambient temperature. The reaction mixture was then filtered through a small pad of silica using iPrOH and analyzed by GC-MS chromatography using hexadecane as an internal standard.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Fonds der Chemischen Industrie (FCI) and the Freie Universität Berlin are kindly acknowledged for financial support.

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ASSOCIATED CONTENT

* Supporting Information S

Spectral data for complexes [2]I and [3]. Hydride signals for reaction of [3] with NaOH and iPrOH. Selected crystallographic data for the complexes and their cif files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00365.

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Corresponding Author

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