N-Heterocyclic Dicarbene Iridium(III) Pincer Complexes Featuring

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N-Heterocyclic Dicarbene Iridium(III) Pincer Complexes Featuring Mixed NHC/Abnormal NHC Ligands and Their Applications in the Transfer Dehydrogenation of Cyclooctane† Weiwei Zuo and Pierre Braunstein* Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universit
e de Strasbourg, 4 rue Blaise Pascal, F-67081 Strasbourg Cedex, France

bS Supporting Information ABSTRACT: The reaction of 1,3-bis(imidazolyl)benzene with excess 1-bromoadamantane at 170 °C for 14 h afforded 1,3-bis(1adamantylimidazolium)benzene dibromide (1), which was characterized by IR, NMR, and X-ray diffraction. Metathetical anion exchange with excess sodium iodide yielded the N-heterocyclic dicarbene precursor 1,3-bis(1-adamantylimidazolium)benzene diiodide, (CHimidCHCHimid)I2 (2), in high yield. The reaction of [Ir(μ-Cl)2(cod)]2 (cod = 1,5-cyclooctadiene) with 2 in the presence of 2.2 equiv of Cs2CO3 in refluxing acetonitrile led to the formation of the unsymmetrical iridium(III) hydride pincer complex [Ir(H)I(CNHCCCaNHC)(NCMe)] (3), which contains a C2bound NHC ligand and a C5-bound NHC ligand. Recrystallization of 3 from ClCH2CH2Cl/Et2O generated the dihalide Ir(III) pincer complex [IrX2(CNHCCCaNHC)(NCMe)], where X = mixture of I and Cl (4). Its formation probably involves initial formation of an iridium chloro iodo intermediate via solvent-induced substitution of the hydride ligand of 3 by chloride, followed by partial chloride/iodide anion exchanges at both the Ir
Cl and Ir
I sites. Treatment of 4 with excess KI in acetonitrile afforded the diiodide Ir(III) pincer complex [IrI2(CNHCCCaNHC)(NCMe)] (5), which was characterized by IR, NMR, and X-ray diffraction. Dehydroiodination of the monohydride complex 3 with sodium tert-butoxide (NaO-t-Bu) in the presence of hydrogen led to the formation of the new hydrido iridium pincer complex 6, which shows detectable catalytic activity toward the transfer dehydrogenation reaction of cyclooctane.

’ INTRODUCTION N-heterocyclic carbene (NHC) ligands have been extensively studied during the past decade since the isolation of free NHCs by Arduengo and co-workers1,2 and continue to be of considerable interest,3
8 in particular because NHC metal complexes are often more efficient than the corresponding phosphine complexes in several homogeneous catalytic reactions.9,10 This is related to the lesser tendency of the NHC ligands to dissociate in solution from the metal center and their ability to often enhance the catalytic reactivity and selectivity of their metal complexes.11
18 Imidazolium-derived NHC ligands typically coordinate to the metal center via the C2 carbon. However, since C4(or C5)bound NHC or “abnormal” carbenes have been reported in 2001,19
24 much effort has been devoted to the synthesis and reactivity of their metal complexes.25,26 Theoretical calculations27 and experimental results both indicate that these abnormal carbenes are stronger donors than their C2-bound analogues.28
32 Catalytic reactions have also demonstrated that abnormal NHC complexes often display better performances than their C2bound analogues32 or promote reactions that are not accessible by using normal NHC complexes under otherwise identical conditions.23,33 r 2011 American Chemical Society

To date, iridium complexes with mono-,30,34
36 bis-,37
40 pincer-type,5,41
46 and polydentate NHC ligands47 or with functional NHC ligands bearing pendant coordinating groups (pyridine,48 phosphine,49 pentamethylcyclopentadienyl,50
55 etc.) have found a variety of catalytic applications such as C
H bond activation,46,48,53,56 hydrogenation,36,38 transfer hydrogenation,55,57 oxidation reactions,51 and alkylation of secondary alcohols.55 Introduction of NHC ligands into iridium complexes allows the synthesis of complexes with improved air and thermal stability, which often display interesting catalytic results in terms of conversions, range of applicability, and catalyst stability. We recently studied the reactivity of CimidCCimid bis(imidazolium) salts toward Ir(I) complexes with the aim of forming new Ir(III) pincer complexes, and we found that the nature of the spacer between the two imidazolium rings played a critical role in determining the structure of the final products.41,42,58,59 Whereas Special Issue: F. Gordon A. Stone Commemorative Issue Received: May 25, 2011 Published: July 29, 2011 2606

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Chart 1

Scheme 1. Postulated Mechanism for the Formation of Pincer Iridium(III) Hydride Complexes42

the dicarbene ligands containing a xylylene spacer tend to produce dinuclear Ir(I) complexes bridged by a biscarbene ligand (complex I, Chart 1), metalation of the central aryl ligand was observed with the m-phenylene-derived bis(imidazolium) carbene precursors, which results in the formation of pincer Ir(III) complexes (complexes II and III, Chart 1).41,42 The influence of the experimental conditions (solvent, nature of the weak base, temperature, etc.) and of the nature of the imidazolium counteranions on these metalation reactions was also investigated.42,59 Starting from the bis(imidazolium) salts of the type (CHimidCHCHimid)I2 and [Ir(μ-Cl)(cod)]2 in the presence of a weak base, we developed a direct method for the synthesis of hydrido and diiodo CNHCCCNHC iridium(III) pincer complexes.41,42 On the basis of the isolation and characterization of a series of reaction intermediates, a possible mechanism for the formation of the pincer iridium(III) hydride complex was suggested, which includes four steps: (a) formation of a mono-NHC Ir(I) complex, (b) oxidative addition at the aromatic C2 position of the C
H bond to the Ir(I) center, (c) base-assisted HI elimination from the resulting Ir(III) complex, and (d) oxidative addition of the second imidazolium C
H to the Ir(I) center (Scheme 1).42 Herein, we report extensions of this route to the adamantylsubstituted bis(imidazolium) diiodide ligand and evaluate the consequences of increased steric hindrance in comparison to the i-Pr or n-Bu N substituents used previously. Introduction of adamantyl substituents on the NHC ligand was expected to increase the thermal stability of the resulting metal complexes, which should be beneficial for catalytic applications, such as dehydrogenation reactions, which require thermal activation. This has been recently observed in the case of PCP pincer complexes of iridium.60,61

’ RESULTS AND DISCUSSION 1. Synthesis and Characterization of the Bis(imidazolium) Salts. Following the literature or slightly modified procedures,62

1,3-bis(1-adamantylimidazolium)benzene dibromide (CHimidCHCHimid)Br2 (1) was prepared by direct adamantylation of 1,3bis(imidazolyl)benzene in neat 1-bromoadamantane at 170 °C for 14 h. The bromide anions were readily exchanged with iodides using excess sodium iodide in ethanol to afford the new bis(imidazolium) diiodide (CHimidCHCHimid)I2 (2) (Scheme 2).41
43,62
64 In contrast to a previously reported procedure,65 we did not succeed in obtaining the desired bis(imidazolium) salt by the reaction of 1,3-bis(imidazolyl)benzene with 1-bromoadamantane in solvents such as DMF and acetic acid. The NMR spectroscopic data of 1 agree with the proposed structure and indicate a symmetrical structure in solution. In the 1H NMR spectra of 1, characteristic signals of the imidazolium protons were observed at δ 8.66, 8.88, and 10.40 ppm, respectively, while the 13C NMR resonances of the imidazolium carbon atoms appear between δ 120.5 and 133.8 ppm. Single crystals of the salt 1 suitable for X-ray diffraction were grown by slow evaporation of Et2O into a CH2Cl2/CH3OH solution of 1 (Table 1). An ORTEP drawing of salt 1 is shown in Figure 1, with selected bond distances and angles. The orientation of the two heterocycles is such that their imidazolium C
H bonds point toward the same bromide anion with which they form hydrogen bonds, with C(1)
Br(1) and C(2)
Br(1) separations of 3.54 and 3.48 Å, respectively. 2. Synthesis and Characterization of Iridium(III) Pincer Complexes. Refluxing a mixture of [Ir(μ-Cl)(cod)]2, 2, and Cs2CO3 (molar ratio 0.5:1:2.2) in MeCN for 8.5 h afforded [Ir(H)I(CNHCCCaNHC)(NCMe)] (3) as the major product (about 90% of the crude product based on 1H NMR analysis) (Scheme 3). The identity of this product was first suggested by 1 H NMR spectroscopy in CD2Cl2, where a low-field doublet (8.54 ppm) is assigned to the imidazole 2-proton, while a doublet at higher field (6.96 ppm) is assigned to the 4-proton and the coupling constant between these two protons is 4J(HH) = 1.5 Hz. Related iridium complexes featuring an abnormal carbene coordination show similar NMR spectroscopic features.19,21 The normal NHC ligand gives rise to signals at 7.26 and 7.49 ppm with the coupling constant 3J(HH) = 2.2 Hz. In agreement with our previous observations,42 the metal-bound hydride shows a high-field resonance at
23.4 ppm. The presence of the hydride ligand was also confirmed in IR spectroscopy by the ν(IrH) vibration band at 2116 cm
1. These observations, together with the chemical formula given by the ESI-MS analysis (peak at m/z 712.3 [M
I]+) and elemental analysis, lead us to propose the structure shown for 3 in Scheme 3. 2607

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Scheme 2. Synthesis of Bis(imidazolium) Salts 1 and 2

Table 1. Crystal Data and Structure Refinement Details for 1 and 5 1 3 0.5CH2Cl2

5

chem formula

C32H40N4 3 2Br 3 0.5CH2Cl2

C34H40I2IrN5

formula mass/amu

682.96

967.73

cryst syst

monoclinic

triclinic

a/Å

17.3057(10)

8.865(6)

b/Å c/Å

14.5628(9) 12.9205(5)

14.884(7) 15.122(7)

R/deg

90.00

92.39(3)

β/deg

104.184(3)

100.23(4)

γ/deg

90.00

101.12(4)

V/Å3

3156.9(3)

1920.7(18)

T/K

173(2)

173(2)

space group

P21/c

P1

Z μ/mm
1

4 2.681

2 5.110

no. of rflns measd

9934

11 981

no. of indep rflns

5821

7794

Rint

0.0341

0.0407

final R1 (I > 2σ(I))

0.0492

0.0527

final wR2(F2) (I > 2σ(I))

0.1229

0.1425

final R1 (all data)

0.0785

0.1125

final wR(F2) (all data) goodness of fit on F2

0.1316 1.054

0.1629 0.976

Moreover, the structural characterization of its diiodide analogue (see below) confirmed the presence of an unsymmetrical pincer-type ligand featuring a normal and an abnormal NHC ligand. The 1H NMR spectrum of 3 in CD3CN (a solvent more polar than CH2Cl2) also revealed the presence of a minor complex having similar NMR resonances. This latter species is suggested to be a cationic complex resulting from 3 by displacement, in solution, of one iodide ligand by an acetonitrile molecule (see NMR spectra in the Supporting Information, Figure S-1). If the sample is kept in CD3CN overnight, the signals of the neutral complex 3 and of the bis-acetonitrile solvento complex [Ir(H)(CNHCCCaNHC)(NCMe)2]I, in which a second acetonitrile molecule is coordinated to the metal center as a result of iodide dissociation, correspond now to a ca. 1:1 ratio (see the Supporting Information, Figure S-2). Similar coordination of MeCN by displacement of an iodide ligand has been recently observed with related Ir(III) pincer complexes.42 Whereas in the case of the ligands with bis-(i-Pr) or bis-(n-Bu) N-substituents the metalation took place exclusively at the 2-H position to afford bis(NHC)
Ir(III) pincer hydride complexes,42 incorporation of the larger adamantyl N substituents changes the

course of the reaction and allows the smooth synthesis of the mixed NHC/abnormal NHC complex 3. This further demonstrates the profound impact of the nature of the nitrogen substituents, in particular their steric bulk, on the metalation of such bis(imidazolium) salts. Calculations and experimental results have suggested that formation of abnormal carbene complexes proceeds via a C4 (or C5)
H oxidative addition pathway, leading to C4 (or C5) bonding to the metal.66 Steric factors have previously been shown to be of importance in the synthesis of abnormal NHC complexes, as this coordination mode reduces the steric pressure at the metal center.20,21,25,67 We have previously suggested that the generation of the pincer iridium(III) hydride complexes occurs via a mono-NHC Ir(I) complex as a kinetic intermediate, which undergoes subsequent oxidative addition of the second imidazolium ligand to the Ir(I) center to form the CNHCCCNHC chelate.42 We have also found that the reaction of the 4,6-dimethyl analogue of the bis(imidazolium) species 2 with [Ir(μ-Cl)(cod)]2 and Cs2CO3 first gives a monoNHC Ir(I) complex, which contains a normal NHC ligand and an unreacted imidazolium arm.68 Subsequent reaction of this complex with Cs2CO3 affords a mixed NHC/abnormal NHC hydrido Ir(III) pincer complex, which is structurally similar to complex 3.69 On the basis of these results, we propose that a similar mono-NHC Ir(I) complex was first formed and then oxidative addition of the C5
H bond (instead of the C2
H one) of the second imidazolium ligand to the Ir(I) center would afford the mixed NHC/abnormal NHC Ir(III) complex 3. The selectivity for C5
H vs C2
H activation is clearly controlled by the steric bulk of the adamantyl groups. Recrystallization of 3 from ClCH2CH2Cl/Et2O for 2 weeks generated the dihalide pincer Ir(III) complex [IrX2(CNHCCCaNHC)(NCMe)] (X = mixture of I and Cl) (4). Attempts were made to characterize this complex by X-ray diffraction analysis, but unfortunately the quality of the crystals was not good enough for a satisfactory refinement (see the Experimental Section). However, the overall atom connectivity in this complex was ascertained and a disorder was found to involve the two halide ligands, with partial occupancy by chloride and iodide at each X position (see below). For this reason, complex 4 was treated with excess KI in MeCN at room temperature and the diiodide Ir(III) pincer complex [IrI2(CaNHCCCNHC)(NCMe)] (5) could then be fully characterized spectroscopically and crystallographically. Single crystals of complex 5 suitable for X-ray analysis were obtained by slow diffusion of Et2O into its ClCH2CH2Cl solution for 12 h (Figure 2, Table 1). The iridium center (Ir(1)) is coordinated by the central metalated aryl ring and a normal and an abnormal NHC ligand through the C(7), C(1), and C(5) atoms, respectively. Two mutually trans iodides and an acetonitrile molecule complete the distorted, meridional octahedral metal coordination.41 The most remarkable structural feature 2608

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Figure 1. ORTEP drawing of the molecular structure of 1. H atoms (in calculated positions) are omitted for clarity, except the imidazolium protons. Ellipsoids include 40% of the electron density. Selected bond distances (Å) and angles (deg): C(1)
N(1) = 1.335(5), C(1)
N(2) = 1.343(5), C(2)
N(3) = 1.345(5), C(2)
N(4) = 1.329(5); N(1)
C(1)
N(2) = 108.6(4), N(3)
C(2)
N(4) = 108.4(4).

Scheme 3. Synthesis of Iridium(III) Pincer Complexes 3
5

of this complex is the occurrence of two NHC ligands adopting different coordination modes to the same metal center.23,70
73 In Figure 2, the left NHC ligand is attached to iridium through the imidazole 5-carbon (labeled C5), whereas the second NHC ligand is bonded in the usual way through the imidazole 2-carbon (labeled C1). The bite angles between the donor atoms of the pincer ligand and the metal are 79.3(5)° for C(1)
Ir(1)
C(7) and 79.1(5)° for C(7)
Ir(1)
C(5), which are involved in fivemembered rings, and 158.4(4)° for C(1)
Ir(1)
C(5). The Ir
C bond length associated with the abnormal carbene ligand (Ir(1)
C(5) = 2.08(2) Å) is marginally shorter than the normal linkage (Ir(1)
C(1) = 2.11(2) Å), in contrast to related structural studies which usually revealed longer iridium
abnormal NHC bond distances.21,22,71,73 The bond lengths in the two imidazoyl heterocycles show large variations, with N(3)
C(2) (1.33(2) Å) being significantly shorter than N(1)
C(1) (1.42(2) Å) and N(3)
C(5) (1.43 (2) Å) being much longer

than N(1)
C(3) (1.35(2) Å). The CdC double bond in the abnormally bound imidazole ring (C(5)
C(6) = 1.35(2) Å) is similar to that (C(3)
C(4) = 1.31(2) Å) in the normal NHC ligand. It is worth mentioning that the abnormal coordination mode of the carbene ligand has led the adamantyl group to take an orientation away from the metal center, and this arrangement has released the steric constraints that would have been imposed on the metal center if both adamantyl groups had been attached to normal NHC ligands. The reduced steric bulk around the metal center resulting from this abnormally bound carbene ligand is expected to facilitate the approach of the metal to the C
H bond to be activated and also to increase the subsequent reactivity of the metal center, which has become more electronrich. These aspects are especially important for catalytic reactions. It should be also mentioned that although NMR analysis has confirmed the absence of chloride in complex 5, the X-ray analysis of the single crystal grown from ClCH2CH2Cl solution 2609

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Figure 2. ORTEP drawing of the molecular structure of 5. H atoms (in calculated positions) are omitted for clarity. Ellipsoids include 40% of the electron density. Selected bond distances (Å) and angles (deg): Ir(1)
C(1) = 2.11(2), Ir(1)
C(5) = 2.08(2), Ir(1)
C(7) = 1.98(2), Ir(1)
I(1) = 2.66(1), Ir(1)
I(2) = 2.65(1), Ir(1)
N(5) = 2.08(1), N(1)
C(1) = 1.42(2), N(1)
C(3) = 1.35(2), C(3)
C(4) = 1.31(2), N(3)
C(2) = 1.33(2), N(3)
C(5) = 1.43 (2), C(5)
C(6) = 1.35(2); C(1)
Ir(1)
C(7) = 79.3(5), C(7)
Ir(1)
C(5) = 79.1(5), C(1)
Ir(1)
C(5) = 158.4(4).

revealed a small chloride contribution at each site occupied by the halide ligands (see the Experimental Section). The chloride percentage is, however, much lower than that in complex 4. We attribute this observation to an iodide for chloride exchange reaction induced by the chlorinated solvent during the crystallization process of 5, in contrast to the observations made with silver NHC complexes, where halogen exchange reactions occur during their synthesis.74 The NMR spectrum (CD2Cl2) of 5 is similar to that of 3, except for the absence of hydride signals in 5. The imidazoyl protons of the NHC ligand form an AB spin system, giving rise to two mutually coupled doublets at δ 7.30 and 7.54 ppm (3J(HH) = 2.2 Hz). The protons at C2 and C4 of the abnormal carbene ligand of 5 resonate at δ 6.96 and 8.48 ppm and show a 4J(HH) coupling constant of 1.5 Hz. In the 13C{1H} NMR spectrum, two 13 C NMR resonances are observed at δ 173.7 and 172.2 ppm, which are assigned to the corresponding Ir
bound C2 and C5 carbons, respectively. These data confirm that the structure found for 5 in the solid state is retained in solution and that 5 and 3 have similar structures in solution. The structure of 5 being clearly established, we can now return to complex 4. Its 1H NMR spectrum in CD3CN is almost identical with that of 5 in the same solvent, which is consistent with the displacement of one X ligand by acetonitrile. The major component in this solvent is the neutral complex 4, and the minor component is the cationic complex in which one of the X ligands has been displaced by acetonitrile (see the Experimental Section). We propose that complex 3 first undergoes a chlorinated solvent-induced hydride for chloride exchange reaction to afford an iridium(III) intermediate which contains one chloride and one iodide ligand and that subsequent partial chloride/ iodide and iodide/chloride anion exchanges occur at the Ir
Cl and Ir
I sites, respectively, resulting in the corresponding iridium mixed-dihalogen complex 4. Similar replacements of a

hydride by a chloride ligand have already been reported with some iridium hydride complexes, which react for example with chloroform to give the corresponding chloride derivative and dichloromethane.75
78 We have also recently observed that ligand exchange reactions between iridium-bound iodide/chloride ligands can easily take place in NHC complexes.68,69 3. Transfer Dehydrogenation of Cyclooctane Using Complex 3 as Precatalyst. Complexes of the type [(PCP)IrH2] (PCP = C6H3(CH2PR2)2-2,6; C6H3(OPR2)2-2,6 or anthracene1,8-diphosphanes)58,79
8182
88 have shown considerable promise for alkane C
H bond activation, due to their high thermal stability and high efficiency in such catalytic reactions. NHC ligands, which behave as phosphine mimics, have recently been shown to be able to enhance the thermal stability of the complexes and increase the electronic density around the metal center.11
18 These observations prompted us to synthesize the NHC-based Ir(III) pincer complexes and to investigate their applications as thermostable homogeneous catalysts for alkane dehydrogenation reactions. We have recently reported the synthesis of pincer iridium(III) hydride complexes featuring two chelating normal NHC ligands and the use of the corresponding polyhydride NHC iridium complexes as precatalysts for cyclooctane transfer dehydrogenation reactions, although initial experiments revealed no catalytic activity.42 The stronger donor ability of the abnormal NHC ligand is expected to further increase the electron density at the metal center, thus opening the way to new reactivity patterns.22,26,32,33 It has been observed that some NHC C4bound complexes allow catalytic reactions not available with their normal NHC counterparts or show substantially better catalytic performances than their C2-bound analogues under otherwise identical conditions.23,26,32,33,89 We hoped that the special bonding arrangement found in complex 3 may positively alter the electronic properties and resulting reactivity patterns of the 2610

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Scheme 4. Dehydroiodination of the Monohydride Complex 3

Table 2. Catalytic Transfer Dehydrogenation of Cyclooctane in the Presence of tert-Butylethene with 6a entry

temp/°C

time/h

amt of tbe/mL

TOF/h
1

1

200

10

0.23

0.36

2

200

3

0.23

0.25

3

200

10

0.10

4 5

200 150

10 10

0.46 0.23