Perimidin-2-ylidene rhodium(I) complexes; unexpected halogen exchange and catalytic activities in transfer hydrogenation reaction

Perimidin-2-ylidene rhodium(I) complexes; unexpected halogen exchange and catalytic activities in transfer hydrogenation reaction

Journal of Organometallic Chemistry 765 (2014) 23e30 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage:...

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Journal of Organometallic Chemistry 765 (2014) 23e30

Contents lists available at ScienceDirect

Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Perimidin-2-ylidene rhodium(I) complexes; unexpected halogen exchange and catalytic activities in transfer hydrogenation reaction P. Arzu Akıncı a, Süleyman Gülcemal a, *, Olga N. Kazheva b, Grigorii G. Alexandrov c, Oleg A. Dyachenko b, Engin Çetinkaya a, Bekir Çetinkaya a a b c

Ege University, Department of Chemistry, Catalyst Research Laboratory, 35100 Bornova, Izmir, Turkey Institute of Problems of Chemical Physics, Russian Academy of Sciences, Semenov Av. 1, Chernogolovka, 142432 Moscow Region, Russia N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Prosp. 31, 119991 Moscow, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2014 Received in revised form 24 April 2014 Accepted 29 April 2014

Four new [(NHC)RhCl(COD)] (NHC ¼ perimidin-2-ylidene; COD ¼ 1,5-cyclooctadiene) rhodium complexes (4aec and 5a) have been prepared by the reaction of [Rh(m-OMe)(COD)]2 with perimidinium bromides (2aec and 3a). The halogen exchange, abstracted from 1,2-dichloroethane was observed during the reaction and [(NHC)RhCl(COD)] complexes were obtained instead of [(NHC)RhBr(COD)]. All ligand precursors and complexes have been fully characterized by 1H, 13C NMR, IR spectroscopy, elemental analysis and mass spectroscopy. X-ray diffraction studies on single crystals of 4a and 5a confirm the square planar geometry. The y(CO) values of [(NHC)RhCl(CO)2] complexes (6aec and 7a) revealed that the NHC ligands are not good s-donors. Transfer hydrogenation (TH) reaction of aliphatic and aromatic ketones has been comparatively studied by using complexes 4aec and 5a as catalysts. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: Perimidine N-Heterocyclic carbenes Rh(I) complexes Transfer hydrogenation

Introduction N-Heterocyclic carbenes (NHCs) exhibit good s-donor and weak p-acceptor electronic properties and have emerged as a versatile class of ligands since they often give more stable organometallic complexes due to the strength of the metal-NHC bond [1]. This strong metaleNHC bond avoids decomposition to free and inactive metal under catalytic conditions in many cases [2]. Additionally, their tunable nature allows for the control of the electronic and steric properties at the metal center [3]. In this respect, the most popular NHC scaffolds derived from five-membered heterocycles such as imidazol-2-ylidenes, imidazolin-2-ylidenes and benzimidazol-2-ylidenes have been successfully applied as ligands in transition metal-catalyzed reactions [4]. Perimidine-based NHCs and their metal complexes have first been investigated by Richeson [5] and other research groups [6]. In some of these studies s-donation of perimidin-2-ylidenes were reported to be stronger than their five-membered analogs. Primarily, from these observations it is expected that this donor property should be reflected in the catalytic activity of derived complexes. In the other hand, there are some recent examples

* Corresponding author. Tel.: þ90 232 3111581; fax: þ90 232 3888264. E-mail address: [email protected] (S. Gülcemal). http://dx.doi.org/10.1016/j.jorganchem.2014.04.033 0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

against this s-donation ability [6a,b]. Therefore, we wished to pay more attention to investigate synthesis, characterization and properties of new perimidin-2-ylidene complexes bearing alkylated benzyl substituent(s) on the nitrogen atom(s) and their catalytic properties. Results and discussion Syntheses of NHC precursors and NHCeRh(I) complexes The synthesis of the desired dissymmetric perimidinium salts (2aec and 3a) was carried out by stepwise substitution reactions. In the first step, N-alkylperimidines (2 and 3) were prepared from literature-known 1H-perimidine [7] and alkyl halides in the presence of NaH and obtained as air stable yellow solids (Scheme 1). Then, N-alkylperimidines (2 and 3) were treated with 2-bromopropane, 2-bromoethyl methyl ether or 2,4,6-trimethylbenzyl bromide to give corresponding perimidinium bromides (2aec and 3a) as air and moisture stable yellow solids in good yields (Scheme 1). All these ligand precursors were spectroscopically characterized. The IR data clearly indicate the presence of the C]N group with a y(C]N) vibrations between 1655 and 1664 cm1, and the 1H NMR spectra were consistent with the proposed structures; characteristic NCHN proton resonance at d ¼ 6.69e8.78 ppm as sharp singlets.

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Scheme 1. Synthesis of 1-alkylperimidine, perimidinium salts and NHCeRh(I) complexes: i) NaH, ReBr, THF, reflux; ii) R0 eBr, PhMe, 60  C or reflux; iii) [Rh(m-OMe)(COD)]2, ClCH2CH2Cl, reflux.

[(NHC)RhCl(COD)] complexes (4aec and 5a) were obtained, by deprotonation of two equivalents of perimidinium salt (2aec and 3a) with [Rh(m-OMe)(COD)]2 in 1,2-dicholoroethane, as yellow solids in moderate yields (Scheme 1). All new [(NHC)RhCl(COD)] complexes (4aec and 5a) with perimidin-2-ylidene ligands were characterized by NMR spectroscopy and elemental analysis. The characteristic downfield signals for the NCHN protons of the corresponding perimidinium salts disappeared in the 1H NMR spectra of complexes 4aec and 5a. In the 13C NMR spectra, chemical shifts showed that Ccarbene is substantially deshielded. Values of Ccarbene were in the d ¼ 214.4e215.2 ppm range and coupling constants JRhe C were in the range 47.8e49.1 Hz for the new complexes and these values were similar to those found for other perimidin-2-ylidene rhodium(I) complexes [5,6a,b,d,i,j]. The carbon atoms in the two cyclooctadiene double bonds are coupled with the rhodium center differently (JRheC ¼ w6.8 and w14.5 Hz), which is consistent with their position trans to NHC and the halide atom, respectively. In principle, deprotonation of azolium bromides with basic [Rh(m-OMe)(COD)]2 give the corresponding bromide complexes [(NHC)RhBr(COD)] with MeOH elimination [8]. However, in this work the presence of chloride coordinated to the Rh atom in complexes 4aec and 5a and occurred (Scheme 1). The coordination

of chloride instead of bromide clearly has proven by elemental analysis and X-ray crystallography (for 4a and 5a). In a detailed study on the AgeNHC complexes by Jin, it has been proposed that the source of chloride is the chlorinated solvents (1,2dichloroethane or dichloromethane) and the halogen exchange reaction occurred unambiguously during the synthesis and not during recrystallization [9]. As far as we are aware this type of halogen exchange for Rh(I)eNHC complexes have not been observed so far. Measuring of carbonyl stretching frequencies in [(NHC) M(X)(CO)2] (M ¼ Rh or Ir, X ¼ halogen) complexes with IR spectroscopy allows us to compare donor properties of NHC ligands, which is an important criteria for homogeneous catalysis [10]. Thus, the corresponding [(NHC)RhCl(CO)2] complexes (6aec and 7a) were prepared in order to compare the electronic properties of the NHC ligands. The complexes 6aec and 7a were easily obtained by passing carbon monoxide gas through a dichloromethane solution of the 4aec and 5a at room temperature (Scheme 2). These reactions resulted in almost quantitative substitution of COD by CO ligands. The cis conformation of the CO ligands in complex 6aec and 7a were confirmed by NMR and IR spectroscopies. 13C NMR spectra exhibit three signals at w182, w185.4 and w204 ppm as

Scheme 2. Synthesis of [(NHC)RhCl(CO)2] complexes (6aec and 7a).

P.A. Akıncı et al. / Journal of Organometallic Chemistry 765 (2014) 23e30

doublets in 6aec and 7a for the two CO and Ccarbene ligands. The average carbonyl stretching frequency in the IR spectra was found to be nCO av ¼ 2047, 2045, 2045 and 2047 cm1 for complexes 6aec and 7a respectively. The Tolman electronic parameters (TEP) [11] could also be used to compare the donor strength of a NHC ligand. By applying the modified relationship introduced by Plenio and Nolan [12] the CO stretching frequencies of [(NHC)MX(CO)2] complexes (M ¼ Ir or Rh) can be converted to the corresponding TEP value. The calculated TEP value was found to be 2054, 2056, 2056 and 2054 cm1 (Nolan’s equation: TEP ¼ 0.847  nCO avþ336; Plenio’s equation: nIr av ¼ 0.8695  nRh avþ250.7) for complexes 6aec and 7a respectively (Table 1). There are number of reports on IR data for related neutral perimidin-2-ylidene complexes carrying various alkyl groups attached to the nitrogen atoms [5,6a,b,i,j]. Comparison of the CO stretching frequencies (nCO av) and the subsequent TEP values are not consistent. Furthermore, the complex 6aec and 7a are weaker s-donors. Molecular structures of complexes 4a and 5a The molecular view and selected bond lengths of 4a and 5a are presented in Fig. 1 and Table 2 respectively. The structures of 4a and 5a show a three-coordinate environment for C(1), a planar geometry around the N atoms and carbene center, short NeCcarbene distances with an average of 1.354(2) and 1.359(2)  A for 4a and 5a, respectively, which are consistent with a carbene structure. The maximal deviations of N(1), N(2) and C(1) atoms out of the bonded atoms’ plane don’t exceed 0.03  A for 4a and 0.04  A for 5a. The sums of valent angles of the corresponding atoms are extremely close to 360 . The longer NeCnaphth bond lengths (average 1.412(2) and 1.417(3)  A) suggest that N atoms lone pairs are more involved with bonding to Ccarbene than to the naphthyl ring, as described in work [5]. The NeCcarbeneeN angles of 117 also close to the previously described analogs [5]. Perimidine tricycles are approximate planar, maximal deviation of non-hydrogen atoms out of their planes don’t exceed 0.18  A for 4a (N(2) atom) and 0.09  A for 5a (N(2) atom). Rh(1) coordination geometry is a distorted square plane, perpendicularly oriented to the carbene plane, as in work [5]. Distances Rh(1)eC(1) are 2.060(2)e2.062(2)  A, which corresponds to the wide range of 2.04e2.09  A values reported in similar rhodium

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compounds [5,6a,i,j,13]. Rh(1)eCl(1) bond lengths are 2.3599(6) and 2.3709(6)  A for 4a and 5a respectively, which is a bit shorter than in works [14]. Crystal structures of 4a and 5a are presented if Figs. 2 and 3. In the crystal structure of 4a water molecules as solvent were discovered. Catalysis Transfer hydrogenation (TH) is a metal-catalyzed process that requires a hydrogen donor atom, typically 2-propanol, in combination with a strong base. This process is preferred for large-scale industrial use in the hope of developing a greener process by reducing waste production, energy use and lowering toxicity. TH is a safer and more valuable atom-efficient method when compared with the conventional hydrogenation reaction using the highly flammable H2 gas [4b]. In this study, Rh(I)eNHC complexes with a perimidin-2-ylidene scaffold has been used in such reduction reaction for the first time. For this purpose, [(NHC)RhCl(COD)] complexes (4aec and 5a) were tested as catalysts for TH of cyclohexanone to cyclohexanol using 2-propanol as a hydrogen donor in the presence of KOH. The catalytic experiments were carried out using 0.5 mmol of ketone, 0.005 mmol (1 mol%) of 4aec or 5a 0.025 mmol of KOH (5 mol%), 2.5 mL of 2-propanol, with a catalyst/base/substrate ratio of 1:5:100. The catalyst was added to a solution of 2-propanol containing KOH, which was kept at 82  C for 5 min and corresponding ketone was added into this solution. Percentage conversions were screened by GC analysis and results were summarized in Table 3. The catalytic activity of the complexes 4aec and 5a decreases in order 5a > 4a > 4b > 4c (Table 3, entries 1e4). The catalyst 5a with i Pr and 2,4,6-triisopropylbenzyl groups attached to the nitrogen atoms of perimidine scaffold was found to be the most active catalyst within all complexes examined and reaches to 83% conversion in 4 h (Table 3, entry 4) for the TH of cyclohexanone to cyclohexanol. The most active catalyst 5a was also examined for TH of different substituted aromatic and aliphatic ketones (Table 3, entries 5e9). In the presence of acetophenone 68% conversion was achieved in 4 h with the catalyst 5a (Table 3, entry 5). When compared with the other [(NHC)RhX(COD)] analogs with a fivemembered imidazol-2-ylidene or benzimidazol-2-ylidene ligand, catalytic activity of perimidine-2-ylidene ligand in TH reaction was

Table 1 Stretching frequencies and TEP values of the [(NHC)RhX(CO)2] complexes.

Complex

Solvent

nCO (cm1)

1 nav ) CO (cm

TEP (cm1)

Reference

8A (X ¼ Cl) 8A (X ¼ I) 8B (X ¼ Cl) 8B (X ¼ Cl) 8C (X ¼ Cl) 8D (X ¼ I) 8E (X ¼ I) 8F (X ¼ I) 6a 6b 6c 7a

CH2Cl2 CH2Cl2 No info No info No info CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

2007, 2004, 1985, 1989, 2000, 2000, 1999, 2001, 2006, 2008, 2008, 2006,

2046 2040 2029 2031 2040 2037 2036 2038 2045 2047 2047 2045

2055 2051 2043 2044 2051 2049 2048 2049 2054 2056 2056 2054

[6a] [6j] [5] [6i] [6b] [6j] [6j] [6j] This This This This

2085 2075 2073 2073 2079 2073 2072 2075 2084 2085 2086 2083

study study study study

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P.A. Akıncı et al. / Journal of Organometallic Chemistry 765 (2014) 23e30

Fig. 1. Molecular view of complexes 4a and 5a with the atom labeling scheme.

found to be lower even with a higher catalyst loading [8a,b,e]. A black precipitation was observed during the catalytic process in all cases and this observation could be attributed to the low stability of catalysts in these reaction conditions. Conclusions This report was devoted to the design of perimidine derivatives and their NHC-metal complexes, we presented here simple synthetic strategies for the construction of new type of 1,3disubstituted perimidine salts and perimidin-2-ylidenes. Despite the acidity of C2eH (d ¼ 6.69e8.78 ppm), perimidinium architecture has a dramatic effect on the deprotonation behavior and is base selective. The deprotonation/coordination was successfully achieved only by [Rh(m-OMe)(COD)]2 in boiling 1,2-dichloroethane. The halogen exchange reaction of Rh(I)eNHC complexes from a chlorinated solvent during the synthetic process was observed for the first time. The catalytic activities for rhodium(I) complexes with a perimidine scaffold were evaluated for TH reaction of cyclohexanone and acetophenone. The catalytic efficiency of complexes is low, which may be attributed to the weak s-donocity of 1,3disubstituted perimidin-2-ylidene ligands. Experimental General considerations

hexane and toluene over Na and dichloromethane over CaH2). The reagents were purchased from SigmaeAldrich, Merck, Alfa Aesar and Acros Organics. 1H-perimidine [7], [Rh(m-OMe)(COD)]2 [15], 2,4,6-trimethylbenzyl bromide and 2,4,6-triisopropylbenzyl bromide [16] were prepared according to the published procedures. 1H (400 MHz) and 13C NMR (100.6 MHz) spectra were acquired on a Varian AS 400 Mercury spectrometer with CDCl3 as solvent and tetramethylsilane (TMS) as internal standard. Chemical shifts (d) were reported in units (ppm) by assigning TMS resonance in the 1H spectrum as 0.00 ppm and CDCl3 resonance in the 13C spectrum as 77.0 ppm. All coupling constants (J values) were reported in Hertz (Hz). Melting points were recorded with Gallenkamp electrothermal melting point apparatus. FTIR spectra were recorded on a Perkin Elmer Spectrum 100 series. Elemental analyses were performed on a PerkineElmer PE 2400 elemental analyzer. The chromatographic analyses (GC) were performed with an Agilent 7820A instrument equipped with a flame ionization detector and an Agilent 19091J-413 column (30 m  320 mm  0.5 mm). Synthesis of 1-alkylperimidine derivatives (2 and 3) A suspension of NaH (0.86 g; 35.7 mmol) in THF (15 mL) was transferred drop wise into the solution of 1H-perimidine (5.0 g; 29.7 mmol) in 60 mL of THF and the resulting mixture was refluxed for 24 h. Then 2,4,6-trimethylbenzyl bromide or 2,4,6triisopropylbenzyl bromide (35.7 mmol) was added and the

Unless otherwise noted all manipulations were carried out under an argon atmosphere by using Schlenk techniques. All solvents were dried rigorously, freshly distilled prior to use and stored under argon atmosphere (tehrahydrofuran over Na/benzophenone, nTable 2 Selected bond lengths [ A] for 4a and 5a. 4a Rh(1)eC(1) Rh(1)eC(25) Rh(1)eC(32) Rh(1)eC(29) Rh(1)eC(28) Rh(1)eCl(1) N(1)eC(1) N(2)eC(1)

5a 2.062(2) 2.107(2) 2.130(2) 2.185(2) 2.215(2) 2.3599(6) 1.349(2) 1.360(2)

Rh(1)eC(1) Rh(1)eC(31) Rh(1)eC(38) Rh(1)eC(35) Rh(1)eC(34) Rh(1)eCl(1) N(1)eC(1) N(2)eC(1)

2.060(2) 2.114(2) 2.138(2) 2.188(2) 2.225(2) 2.3709(6) 1.359(2) 1.359(2)

Fig. 2. Crystal structure of 4a.

P.A. Akıncı et al. / Journal of Organometallic Chemistry 765 (2014) 23e30

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24.5, 24.2 (CH3). Elemental analysis (%) calc. for C27H32N2: C, 84.33; H, 8.39; N, 7.28; found: C, 84.28; H, 8.42; N, 7.25. Synthesis of perimidinium salts (2aec and 3a)

Fig. 3. Crystal structure of 5a.

mixture was refluxed for additional 24 h. The solvent was removed under reduced pressure. The oily residue was treated with CH2Cl2 and filtered off, then was purified by column chromatography over silica gel using CH2Cl2/MeOH (98:2) as the eluent to give pure compounds as yellow solid. 2: 6.0 g (67%), mp 159e160  C. FTIR (film) n(C]N) 1628 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.28e7.16 (m, 4H, CHarom), 6.94 (s, 2H, CH2C6H2(CH3)3-2,4,6), 6.81 (d, J ¼ 6.8 Hz, 1H, CHarom), 6.77 (s, 1H, NCHN), 6.37 (d, J ¼ 6.8 Hz, 1H, CHarom), 4.39 (s, 2H, CH2C6H2(CH3)32,4,6), 2.31, 2.28 (s, 9H, CH2C6H2(CH3)3-2,4,6). 13C NMR (CDCl3): d/ ppm ¼ 145.3 (NCHN), 143.6 (AreC), 139.3 (AreC), 139.1 (AreC), 138.4 (AreC), 135.7 (AreC), 129.9 (AreC), 129.1 (AreC), 127.7 (Are C), 126.0 (AreC), 123.2 (AreC), 120.5 (AreC), 120.0 (AreC), 115.4 (AreC), 100.3 (AreC), 46.6 (CH2C6H2(CH3)3-2,4,6), 21.3, 19.9 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C21H20N2: C, 83.96; H, 6.71; N, 9.33; found: C, 83.99; H, 6.70; N, 9.30. 3: 9.9 g (87%), mp 167  C. FTIR (film) n(C]N) 1629 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.29e7.19 (m, 4H, CHarom), 7.10 (s, 2H, CH2C6H2(CHMe2)3-2,4,6), 6.83 (s, 1H, NCHN), 6.82 (d, J ¼ 6.8 Hz, 1H, CHarom), 6.43 (d, J ¼ 7.2 Hz, 1H, CHarom), 4.43 (s, 2H, CH2C6H2(CHMe2)3-2,4,6), 2.96 (sept, J ¼ 7.2 Hz, 3H, CH2C6H2(CHMe2)3-2,4,6), 1.28 (d, J ¼ 6.4 Hz, 6H, CH3), 1.24 (d, J ¼ 6.8 Hz, 12H, CH3). 13C NMR (CDCl3): d/ppm ¼ 150.8 (NCHN), 149.0 (AreC), 145.6 (AreC), 143.6 (AreC), 139.0 (AreC), 135.7 (Are C), 129.1 (AreC), 127.7 (AreC), 123.2 (AreC), 122.2 (AreC), 120.4 (AreC), 120.1(AreC), 115.5 (AreC), 100.2 (AreC), 44.6 (CH2C6H2(CHMe2)3-2,4,6), 34.7, 30.1 (CH2C6H2(CHMe2)3-2,4,6),

Table 3 Catalytic TH of ketones with catalysts 4aec and 5a.a

Entry

Catalyst

Ketone

Time/conversion [%]b 0.5 h

1 2 3 4 5 6 7 8 9 10c 11d

4a 4b 4c 5a 5a 5a 5a 5a 5a RheNHC [8a] RheNHC [8a]

Cyclohexanone Cyclohexanone Cyclohexanone Cyclohexanone Acetophenone 20 -Chloroacetophenone 40 -Bromoacetophenone Propiophenone 2-Heptanone Acetophenone Acetophenone

22 26 18 29 28 <1 5 12 20 e e

1h 39 41 24 54 42 5 11 23 42 e >99

1.5 h

2h

4h

51 46 29 66 49 8 18 29 61 e e

55 49 32 78 64 12 21 33 77 >99 e

58 51 36 83 68 14 27 36 79 e e

Ketone (0.5 mmol), KOH (5 mol%), catalyst (1 mol%), IPA (2.5 mL), 82  C. Determined by using GC from an average of at least two runs. c RheNHC ¼ bromo(h4-cycloocta-1,5-diene)-1,3-bis(2,4,6-trimethylbenzyl)imidazol-2-ylidene rhodium(I) (0.5 mol%). d RheNHC ¼ bromo(h4-cycloocta-1,5-diene)-1,3-bis(2,4,6-trimethylbenzyl)imidazolin-2-ylidene rhodium(I) (0.5 mol%). a

b

To a solution of 2 or 3 (5 mmol) in toluene (10 mL), 1.1 eq. of 2bromopropane (for 2a and 3a), 2-bromoethyl methyl ether (for 2b) or 2,4,6-trimethylbenzyl bromide (for 2c) was added. The mixture was stirred for 24 h at 60  C (for 2a and 3a) or 110  C (for 2b and 2c). Following the completion of the process, the solid formed was filtered off and the resulting precipitate was recrystallized from CH2Cl2/Et2O as yellow solid. 2a: 1.79 g (85%), mp 189  C. FTIR (film) n(C]N) 1655 cm1. 1H NMR (CDCl3): d/ppm ¼ 8.78 (s, 1H, NCHNþ), 7.42e7.36 (m, 3H, CHarom), 7.26 (t, J ¼ 8.4 Hz, 1H, CHarom), 7.07 (d, J ¼ 7.2 Hz, 1H, CHarom), 6.85 (s, 2H, CH2C6H2(CH3)3-2,4,6), 6.84 (d, J ¼ 6.8 Hz, 1H, CHarom), 5.40 (s, 2H, CH2C6H2(CH3)3-2,4,6), 4.67 (sept, J ¼ 6.4 Hz, 1H, CH(CH3)2), 2.34, 2.20 (s, 9H, CH2C6H2(CH3)3-2,4,6), 1.52 (d, J ¼ 4.0 Hz, 6H, CH(CH3)2). 13C NMR (CDCl3): d/ppm ¼ 148.8 (NCHNþ), 139.8 (AreC), 137.8 (AreC), 135.2 (AreC), 132.1 (AreC), 131.2 (AreC), 130.7 (AreC), 128.6 (AreC), 128.5 (AreC), 124.9 (Are C), 124.2 (AreC), 121.8 (AreC), 108.8 (AreC), 108.6 (AreC), 53.8 (CH(CH3)2), 51.7 (CH2C6H2(CH3)3-2,4,6), 21.1, 20.9 (CH2C6H2(CH3)32,4,6), 20.8 (CH(CH3)2). Elemental analysis (%) calc. for C24H27BrN2: C, 68.08; H, 6.43; N, 6.62; found: C, 68.12; H, 6.46; N, 6.61. 2b: 1.67 g (76%), mp 209e210  C. FTIR (film) n(C]) 1659 cm1. 1H NMR (CDCl3): d/ppm ¼ 8.57 (s, 1H, NCHNþ), 7.50e7.37 (m, 4H, CHarom), 7.07 (d, J ¼ 7.6 Hz, 1H, CHarom), 6.97 (s, 2H, CH2C6H2(CH3)32,4,6), 6.93 (d, J ¼ 7.6 Hz, 1H, CHarom), 5.13 (s, 2H, CH2C6H2(CH3)32,4,6), 4.44 (t, J ¼ 4.4 Hz, 2H, CH2CH2OCH3), 3.73 (t, J ¼ 5.2 Hz, 2H, CH2CH2OCH3), 3.23 (s, 3H, CH2CH2OCH3), 2.44, 2.31 (s, 9H, CH2C6H2(CH3)3-2,4,6). 13C NMR (CDCl3): d/ppm ¼ 151.2 (NCHNþ), 140.3 (AreC), 138.6 (AreC), 135.2 (AreC), 132.6 (AreC), 131.4 (Are C),130.6 (AreC), 128.7 (AreC), 128.5 (AreC), 124.9 (AreC), 124.6 (AreC), 123.3 (AreC), 121.6 (AreC), 108.6 (AreC), 108.2 (AreC), 66.5 (CH2C6H2(CH3)3-2,4,6), 58.9 (CH2CH2OCH3), 52.0 (CH2CH2OCH3), 50.8 (CH2CH2OCH3), 21.3, 20.6 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C24H27BrN2O: C, 65.60; H, 6.19; N, 6.38; found: C, 65.65; H, 6.21; N, 6.36. 2c: 2.20 g (86%), mp 217e218  C. FTIR (film) n(C]N) 1664 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.60e7.53 (m, 4H, CHarom), 7.39 (d, J ¼ 7.2 Hz, 2H, CHarom), 6.73 (s, 4H, CH2C6H2(CH3)3-2,4,6), 6.69 (s, 1H, NCHNþ), 4.92 (s, 4H, CH2C6H2(CH3)3-2,4,6), 2.32, 2.11 (s, 18H, CH2C6H2(CH3)32,4,6). 13C NMR (CDCl3): d/ppm ¼ 145.7 (NCHNþ), 140.4 (AreC), 138.4 (AreC), 134.9 (AreC), 132.4 (AreC), 129.9 (AreC), 129.0 (Are C), 125.2 (AreC), 122.7 (AreC), 121.2 (AreC), 109.2 (AreC), 49.9 (CH2C6H2(CH3)3-2,4,6), 21.4, 19.7 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C31H33BrN2: C, 72.51; H, 6.48; N, 5.46; found: C, 72.46; H, 6.50; N, 5.45. 3a: 2.13 g (84%), mp 199  C. FTIR (film) n(C]N) 1662 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.53e7.45 (m, 4H, CHarom), 7.43 (s, 1H, NCHNþ), 7.35 (d, J ¼ 6.6 Hz, 1H, CHarom), 7.30 (d, J ¼ 7.6 Hz, 1H, CHarom), 7.14 (s, 2H, CH2C6H2(CHMe2)3-2,4,6), 5.13 (s, 2H, CH2C6H2(CHMe2)3-2,4,6), 4.87 (sept, J ¼ 6.4 Hz, 1H, CHMe2), 3.00 (sept, J ¼ 7.2 Hz, 2H, CH2C6H2(CHMe2)3-2,4,6), 2.91 (sept, J ¼ 7.2 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 1.24 (d, J ¼ 6.4 Hz, 6H, CH3), 1.23 (d, J ¼ 7.6 Hz, 12H, CH3), 1.21 (d, J ¼ 6.4 Hz, 6H, CH(CH3)2). 13C NMR (CDCl3): d/ppm ¼ 152.7 (NCHNþ), 149.4 (AreC), 145.6 (AreC), 139.6 (AreC), 135.2 (AreC), 132.1 (AreC), 131.0 (AreC), 128.9 (AreC), 128.8 (AreC), 125.4 (AreC), 125.1 (AreC), 122.9 (AreC), 121.5 (AreC), 120.4 (AreC), 109.6 (AreC), 108.9 (AreC), 94.3 (AreC), 53.3 (CH(CH3)2), 48.4 (CH2C6H2(CHMe2)3-2,4,6), 34.5, 30.1 (CH2C6H2(CHMe2)3-2,4,6), 24.6, 24.0 (CH3), 20.8 (CH(CH3)2). Elemental analysis (%) calc. for C30H39BrN2: C, 70.99; H, 7.75; N, 5.52; found: C, 71.04; H, 7.79; N, 5.49.

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P.A. Akıncı et al. / Journal of Organometallic Chemistry 765 (2014) 23e30

Synthesis of the [(NHC)RhCl(COD)] complexes (4aec and 5a) To a solution of perimidinium salt (2aee or 3a) (0.5 mmol) in 10 mL of 1,2-dicholoroethane, [Rh(m-OMe)(COD)]2 (121 mg; 0.25 mmol) was added under argon atmosphere. The reaction mixture was stirred 1 h at 25  C and then refluxed for 48 h. Meanwhile the reaction progress was monitored with thin layer chromatography. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel using CH2Cl2/acetone (98:2) as the eluent to give a yellow solid. 4a: 124 mg (42%). 1H NMR (CDCl3): d/ppm ¼ 7.72 (sept, J ¼ 7.2 Hz, 1H, CH(CH3)2), 7.32e7.26 (m, 2H, CHarom), 7.19 (d, J ¼ 8.8 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 7.13 (s, 1H, CHarom), 7.06e7.01 (m, 2H, CHarom), 6.78 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.71 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.49 (d, J ¼ 16.8 Hz, 1H, CHarom), 6.16 (d, J ¼ 8.4 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 5.09e5.04 (m, 1H, COD-CH), 5.01e4.96 (m, 1H, COD-CH), 3.65e3.61 (m, 1H, COD-CH), 3.48e3.42 (m, 1H, COD-CH), 2.51e2.38 (m, 4H, COD-CH2), 2.32 (d, J ¼ 9.2 Hz, 6H, CH(CH3)2), 2.03e1.95 (m, 4H, COD-CH2), 1.91, 1.89, 1.86 (s, 3H, CH2C6H2(CH3)3-2,4,6). 13C NMR (CDCl3): d/ppm ¼ 214.4 (d, JRhe Carbene ¼ 48.1 Hz, Ccarbene), 136.7, 136.4, 135.1, 134.7, 133.7, 132.2, 131.2, 130.6, 127.7, 127.6, 127.2, 121.5, 121.3, 121.0, 108.0, 105.6 (Are C), 97.9 (d, J ¼ 6.8 Hz, COD-CH), 97.1 (d, J ¼ 6.9 Hz, COD-CH), 71.5 (d, J ¼ 14.5 Hz, COD-CH), 70.0 (d, J ¼ 14.5 Hz, COD-CH), 61.9 (CH(CH3)2), 33.3 (s, COD-CH2), 29.6 (s, COD-CH2), 21.4, 21.1 (CH(CH3)2), 20.9, 20.2, 19.0 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C32H38ClN2Rh: C, 62.25; H, 6.50; N, 4.76; found: C, 62.19; H, 6.47; N, 4.79. 4b: 169 mg (56%). 1H NMR (CDCl3): d/ppm ¼ 7.37e7.30 (m, 2H, CHarom), 7.23 (d, J ¼ 15.2 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 7.12e7.04 (m, 2H, CHarom), 6.91 (d, J ¼ 7.2 Hz, 1H, CHarom), 6.80 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.72 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.65e6.49 (br, 1H, CH2CH2OCH3), 6.38 (d, J ¼ 16.4 Hz, 1H, CH2C6H2(CH3)32,4,6), 6.20 (d, J ¼ 8.0 Hz, 1H, CHarom), 5.07 (s, 2H, COD-CH), 4.88e 4.65 (br, 1H, CH2CH2OCH3), 4.30e4.21 (m, 1H, CH2CH2OCH3), 3.88e 3.82 (m, 1H, CH2CH2OCH3), 3.52 (s, 3H, CH2CH2OCH3), 3.47e3.39 (m, 2H, COD-CH), 2.51e2.36 (m, 4H, COD-CH2), 2.33 (s, 3H, CH2C6H2(CH3)3-2,4,6), 2.30 (s, 3H, CH2C6H2(CH3)3-2,4,6), 2.22 (s, 3H, CH2C6H2(CH3)3-2,4,6), 1.99e1.88 (m, 4H, COD-CH2). 13C NMR (CDCl3): d/ppm ¼ 215.2 (d, JRheCarbene ¼ 49.1 Hz, Ccarbene), 136.5, 136.4, 134.7, 133.9, 133.8, 131.2, 130.7, 127.9, 127.8, 127.5, 121.6, 121.3, 120.3, 105.9, 105.6 (AreC), 98.8 (d, J ¼ 6.9 Hz, COD-CH), 98.3 (d, J ¼ 6.9 Hz, COD-CH), 71.5 (d, J ¼ 14.6 Hz, COD-CH), 71.2 (d, J ¼ 14.6 Hz, COD-CH), 68.6 (CH2CH2OCH3), 59.5 (CH2CH2OCH3), 58.5 (s, CH2C6H2(CH3)3-2,4,6), 54.3 (CH2CH2OCH3), 32.8 (d, J ¼ 7.6 Hz, COD-CH2), 29.0 (d, J ¼ 9.1 Hz, COD-CH2), 21.5, 21.2, 21.0 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C32H38ClN2ORh: C, 63.53; H, 6.33; N, 4.63; found: C, 63.49; H, 6.34; N, 4.65. 4c: 156 mg (46%). 1H NMR (CDCl3): d/ppm ¼ 7.26 (d, J ¼ 8.4 Hz, 2H, CHarom), 7.10 (t, J ¼ 8.0 Hz, 2H, CHarom), 6.92 (d, J ¼ 8.0 Hz, 4H, CH2C6H2(CH3)3-2,4,6), 6.85 (d, J ¼ 4.8 Hz, 4H, CH2C6H2(CH3)3-2,4,6), 6.40 (d, J ¼ 8.0 Hz, 2H, CHarom), 5.08 (s, 2H, COD-CH), 3.62 (s, 2H, COD-CH), 2.52 (s, 6H, CH2C6H2(CH3)3-2,4,6), 2.34 (s, 6H, CH2C6H2(CH3)3-2,4,6), 2.28 (s, 6H, CH2C6H2(CH3)3-2,4,6), 2.22e2.10 (br, 4H, COD-CH2), 1.90e1.84 (m, 4H, COD-CH2). 13C NMR (CDCl3): d/ ppm ¼ 214.8 (d, JRheCarbene ¼ 49.1 Hz, Ccarbene), 135.7, 135.5, 135.3, 135.2, 135.0, 133.5, 133.3, 133.1, 129.7, 129.6, 129.0, 128.6, 127.1, 126.6, 126.4, 120.2, 118.8, 118.4, 104.7, 104.1 (AreC), 96.8 (d, J ¼ 6.9 Hz, COD-CH), 69.9 (d, J ¼ 13.7 Hz, COD-CH), 58.4 (CH2C6H2(CH3)3-2,4,6), 31.50, 27.7 (COD-CH2), 20.0, 19.7, 19.5 (CH2C6H2(CH3)3-2,4,6). Elemental analysis (%) calc. for C39H44ClN2Rh: C, 68.97; H, 6.53; N, 4.12; found: C, 69.03; H, 6.55; N, 4.10.

5a: 175 mg (52%). 1H NMR (CDCl3): d/ppm ¼ 7.70 (sept, J ¼ 6.8 Hz, 1H, CH(CH3)2), 7.43 (d, J ¼ 15.2 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 7.29e7.21 (m, 2H, CHarom), 7.10 (d, J ¼ 8.4 Hz, 1H, CHarom), 7.00 (d, J ¼ 7.2 Hz, 1H, CH2C6H2(CHMe2)32,4,6), 6.94 (t, J ¼ 8.0 Hz, 2H, CHarom), 6.90 (s, 1H, CH2C6H2(CHMe2)3-2,4,6), 6.28 (d, J ¼ 15.6 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 6.21 (d, J ¼ 7.6 Hz, 1H, CHarom), 5.10e5.08 (m, 1H, COD-CH), 5.00e4.95 (m, 1H, COD-CH), 3.62e3.54 (m, 1H, COD-CH), 3.43 (br, 1H, COD-CH), 2.81 (sept, J ¼ 6.8 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 2.55e2.45 (m, 2H, CH2C6H2(CHMe2)32,4,6), 2.42e2.28 (m, 2H, COD-CH2), 2.08e2.04 (m, 2H, COD-CH2), 1.90 (d, J ¼ 5.2 Hz, 3H, CH(CH3)2), 1.88 (d, J ¼ 5.2 Hz, 3H, CH(CH3)2), 1.91e1.83 (m, 4H, COD-CH2), 1.21, 1.19 (d, J ¼ 4.4 Hz, 18H, CH3). 13C NMR (CDCl3): d/ppm ¼ 215.1 (d, JRheCarbene ¼ 47.8 Hz, Ccarbene), 147.8, 146.8, 145.1, 133.6, 132.4, 130.9, 125.9, 125.7, 124.1, 121.7, 121.1, 120.2, 119.9, 119.3, 106.6, 105.2 (AreC), 97.0 (d, J ¼ 6.9 Hz, COD-CH), 95.8 (d, J ¼ 6.9 Hz, COD-CH), 69.9 (d, J ¼ 14.5 Hz, COD-CH), 69.2 (d, J ¼ 14.5 Hz, COD-CH), 60.9 (CH(CH3)2), 54.9 (CH2C6H2(CHMe2)32,4,6), 32.9 (s, COD-CH2), 32.4 (CH2C6H2(CHMe2)3-2,4,6), 29.0 (s, COD-CH2), 24.7, 24.4 (CH2C6H2(CHMe2)3-2,4,6), 22.9 (d, J ¼ 9.1 Hz, CH3), 22.5, 21.9 (CH3), 18.6, 18.2 (CH(CH3)2). Elemental analysis (%) calc. for C38H50ClN2Rh: C, 67.80; H, 7.49; N, 4.16; found: C, 67.75; H, 7.53; N, 4.17. Synthesis of the [(NHC)RhCl(CO)2] complexes (6aec and 7a) Complexes 4aec or 5a (0.1 mmol) was dissolved in CH2Cl2 (5 mL) and carbon monoxide was bubbled through the solution for 1 h. Color of the solution changed from yellow to pale yellow. The solution was concentrated ca 1 mL and pentane was added. The pale yellow solid that separated out was filtered, washed with pentane and discarded. 6a: 51 mg (95%). FTIR (CH2Cl2) nCO 2006 and 2084 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.35e7.26 (m, 3H, CHarom), 7.11e7.05 (m, 2H, CHarom), 6.91 (s, 1H, CHarom), 6.83 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.82 (d, J ¼ 16.8 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 6.70 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.66 (sept, J ¼ 6.8 Hz, 1H, CH(CH3)2), 6.27 (d, J ¼ 7.6 Hz, 1H, CHarom), 6.16 (d, J ¼ 16.8 Hz, 1H, CH2C6H2(CH3)32,4,6), 2.48 (s, 3H, CH2C6H2(CH3)3-2,4,6), 2.24 (s, 3H, CH2C6H2(CH3)3-2,4,6), 2.21 (s, 3H, CH2C6H2(CH3)3-2,4,6), 1.80 (d, J ¼ 7.6 Hz, 3H, CH(CH3)2), 1.74 (d, J ¼ 7.6 Hz, 3H, CH(CH3)2). 13C NMR (CDCl3): d/ppm ¼ 202.6 (d, JRheCarbene ¼ 41.3 Hz, Ccarbene), 185.5 (d, JRheC ¼ 55.2 Hz, CO), 182.4 (d, JRheC ¼ 75.9 Hz, CO), 136.9, 134.9, 133.4, 131.6, 131.0, 130.4, 128.7, 127.5, 127.0, 126.9, 122.1, 121.9, 121.8, 108.7, 106.7 (AreC), 63.0 (CH(CH3)2), 58.3 (CH2C6H2(CH3)3-2,4,6), 28.5 (CH2C6H2(CH3)3-2,4,6), 21.1, 20.7 (CH2C6H2(CH3)3-2,4,6), 19.0, 17.6 (CH(CH3)2). 6b: 50 mg (91%). FTIR (CH2Cl2) nCO 2008 and 2085 cm1. 1H NMR (CDCl3) d/ppm ¼ 7.39e7.36 (m, 2H, CHarom), 7.32 (d, J ¼ 8.0 Hz, 1H, CHarom), 7.12 (t, J ¼ 4.0 Hz, 1H, CHarom), 6.89 (d, J ¼ 16.0 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 6.78 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.74 (s, 1H, CH2C6H2(CH3)3-2,4,6), 6.32 (d, J ¼ 8.0 Hz, 1H, CHarom), 5.49 (d, J ¼ 16.4 Hz, 1H, CH2C6H2(CH3)3-2,4,6), 5.51e5.34 (br, 1H, CH2CH2OCH3), 4.90e4.78 (br, 1H, CH2CH2OCH3), 3.96e3.86 (m, 2H, CH2CH2OCH3), 3.42 (s, 3H, CH2CH2OCH3), 2.47 (s, 3H, CH2C6H2(CH3)3-2,4,6), 2.25 (s, 6H, CH2C6H2(CH3)3-2,4,6). 13C NMR (CDCl3): d/ppm ¼ 204.3 (d, JRheCarbene ¼ 42.9 Hz, Ccarbene), 185.4 (d, JRheC ¼ 54.4 Hz, CO), 182.4 (d, JRheC ¼ 75.9 Hz, CO), 136.9, 136.0, 135.1, 134.7, 133.7, 133.3, 131.1, 130.4, 127.8, 127.6, 126.9, 122.3, 122.0, 120.8, 106.9, 106.1 (AreC), 67.6 (CH2CH2OCH3), 59.1 (CH2CH2OCH3), 58.8 (CH2C6H2(CH3)3-2,4,6), 55.1 (CH2CH2OCH3), 21.2, 20.7 (CH2C6H2(CH3)3-2,4,6). 6c: 60 mg (96%). FTIR (CH2Cl2) nCO 2008 and 2086 cm1. 1H NMR (CDCl3) d/ppm ¼ 7.27 (d, J ¼ 2.11 Hz, 2H, CHarom), 7.09 (t, J ¼ 2.11 Hz, 2H, CHarom), 6.82e6.93 (br, 2H, CH2C6H2(CH3)3-2,4,6), 6.75e6.82

P.A. Akıncı et al. / Journal of Organometallic Chemistry 765 (2014) 23e30

(br, 2H, CH2C6H2(CH3)3-2,4,6), 6.71 (d, J ¼ 4.22 Hz, 2H, CH2C6H2(CH3)3-2,4,6), 6.39 (d, J ¼ 2.01 Hz, 2H, CHarom), 5.69 (d, J ¼ 4.22 Hz, 2H, CH2C6H2(CH3)3-2,4,6), 2.40e2.56 (br, 6H, CH2C6H2(CH3)3-2,4,6), 2.29e2.40 (br, 6H, CH2C6H2(CH3)3-2,4,6), 2.25 (s, 6H, CH2C6H2(CH3)3-2,4,6). 13C NMR (CDCl3) d/ppm ¼ 205.6 (d, JRheCarbene ¼ 42.9 Hz, Ccarbene), 185.4 (d, JRheC ¼ 54.4 Hz, CO), 182.6 (d, JRheC ¼ 75.9 Hz, CO), 137.3, 136.4, 134.6, 133.8, 130.8, 127.7, 127.5, 122.4, 121.1, 107.4 (AreC), 59.8 (CH2C6H2(CH3)3-2,4,6), 21.0, 20.7 (CH2C6H2(CH3)3-2,4,6). 7a: 58 mg (94%). FTIR (CH2Cl2) nCO 2006 and 2083 cm1. 1H NMR (CDCl3): d/ppm ¼ 7.31e7.29 (m, 2H, CHarom), 7.20 (d, J ¼ 8.4 Hz, 1H, CHarom), 7.08 (s, 1H, CH2C6H2(CHMe2)3-2,4,6), 7.04e6.98 (m, 2H, CHarom), 6.95 (s, 1H, CH2C6H2(CHMe2)3-2,4,6), 6.85 (d, J ¼ 15.6 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 6.73 (sept, J ¼ 6.8 Hz, 1H, CH(CH3)2), 6.44 (d, J ¼ 7.6 Hz, 1H, CHarom), 5.50 (d, J ¼ 15.6 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 3.68 (sept, J ¼ 6.8 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 3.35 (sept, J ¼ 6.8 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 2.85 (sept, J ¼ 6.8 Hz, 1H, CH2C6H2(CHMe2)3-2,4,6), 1.81 (d, J ¼ 6.8 Hz, 3H, CH(CH3)2), 1.68 (d, J ¼ 6.8 Hz, 3H, CH(CH3)2), 1.38, 1.31, 1.22, 1.19 (4  d, J ¼ 6.4 Hz, 18H, CH2C6H2(CH(CH3)2)3-2,4,6). 13C NMR (CDCl3): d/ ppm ¼ 203.9 (d, JRheCarbene ¼ 41.4 Hz, Ccarbene), 185.7 (d, JRhe C ¼ 54.5 Hz, CO), 182.1 (d, JRheC ¼ 75.9 Hz, CO), 148.6, 147.6, 147.2, 134.7, 133.7, 131.5, 128.7, 127.1, 126.7, 125.4, 122.5, 122.3, 121.9, 121.8, 121.5, 108.5, 107.4 (AreC), 63.1 (CH(CH3)2), 56.4 (CH2C6H2(CHMe2)3-2,4,6), 34.1 (CH2C6H2(CHMe2)3-2,4,6), 30.2, 30.0 (CH2C6H2(CHMe2)3-2,4,6), 28.0, 25.5, 24.5, 23.9, 23.6, 23.2 (CH2C6H2(CH(CH3)2)3-2,4,6), 19.2, 17.4 (CH(CH3)2). General procedure for the transfer hydrogenation reaction Under inert atmosphere, in a vial fitted with a screw cap, an aliquot of the tested complex (1 mol%), from a stock solution in dichloromethane was added. The solvent was removed under vacuum and solution of KOH (0.25 mmol, 5 mol%) in 2-propanol (2.5 mL) was added. Mesitylene (0.1 mmol) was added to the solution as internal standard. The solution was heated to 82  C for 5 min. Subsequently, cyclohexanone (0.5 mmol) was added. After the desired reaction time an aliquot (10 mL) of the mixture was taken and quenched with 2 mL of dichloromethane. The resulting solution was filtered to remove insoluble inorganic material and the reaction progress was monitored by GC. The results for each experiment are averages over two runs. X-ray diffraction studies X-ray diffraction studies were carried out on a Bruker SMART APEX2 CCD diffractometer at room temperature, Mo-Ka radiation. The crystal structure was solved by direct methods and following Fourier synthesis using SHELXS-97 [17]. The structure was refined by full-matrix least squares procedures using an anisotropic approximation for all non-hydrogen atoms with the SHELXL-97 program [18]. Hydrogen atoms were put in idealized positions and treated as riding models in the calculations. Crystallographic data for 4a: C32H40ClN2ORh, M ¼ 605.00, monoclinic, space group P21/c, a ¼ 15.720(1), b ¼ 9.7813(9) and c ¼ 19.672(2)  A, b ¼ 103.050(1) , V ¼ 2946.6(5)  A3, Z ¼ 4, 3 1  l ¼ 0.7107 A, dcalc ¼ 1.36 g cm , m ¼ 0.697 mm . The number of measured reflections, 34,320; the number of independent reflections, 8905; the number of refined parameters, 334; R-factor 0.039 for 7877 reflections with [F0 > 4s(F0)]. Crystallographic data for 5a: C38H50ClN2Rh, M ¼ 673.16, monoclinic, space group P21/c, a ¼ 10.7829(6), b ¼ 15.6629(9) and c ¼ 20.467(1)  A, b ¼ 104.103(1) , V ¼ 3352.6(3)  A3, Z ¼ 4, l ¼ 0.7107  A, dcalc ¼ 1.33 g cm3, m ¼ 0.617 mme1. The number of

29

measured reflections, 16,674; the number of independent reflections, 7648; the number of refined parameters, 379; R-factor 0.032 for 6013 reflections with [F0 > 4s(F0)]. Acknowledgments We thank to Ege University Research Fund (Project Number: 11FEN-048) for financial support of this work and Mr. Salih Günnaz for NMR analyses. Appendix A. Supplementary material CCDC 985038 and 985039 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. References [1] (a) S. Gaillard, J.L. Renaud, Dalton Trans. 42 (2013) 7255; (b) H.D. Velazquez, F. Verpoort, Chem. Soc. Rev. 41 (2012) 7032; (c) S. Díez-González, N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools, RSC Catalysis Series No. 6, Royal Society of Chemistry, Cambridge, 2011; (d) L. Benhamou, E. Chardon, G. Lavigne, S. Bellemin-Laponnaz, V. César, Chem. Rev. 111 (2011) 2705; (e) D. Tapu, D.A. Dixon, C. Roe, Chem. Rev. 109 (2009) 3385; (f) O. Schuster, L. Yang, H.G. Raubenheimer, M. Albrecht, Chem. Rev. 109 (2009) 3445; (g) J.C.Y. Lin, R.T.W. Huang, C.S. Lee, A. Bhattacharyya, W.S. Hwang, I.J.B. Lin, Chem. Rev. 109 (2009) 3561; (h) A.T. Normand, K.J. Cavell, Eur. J. Inorg. Chem. (2008) 2781; (i) F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122; (j) F.E. Hahn, Angew. Chem. Int. Ed. 45 (2006) 1348; (k) V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619; (l) K.J. Cavell, D.S. McGuinness, Coord. Chem. Rev. 248 (2004) 671; (m) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290; (n) K. Denk, J. Fridgen, W.A. Herrmann, Adv. Synth. Catal. 344 (2002) 666; (o) D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000) 39; (p) J.K. Huang, H.J. Schanz, E.D. Stevens, S.P. Nolan, Organometallics 18 (1999) 2370; (q) W.A. Herrmann, C. Köcher, Angew. Chem. Int. Ed. 36 (1997) 2162. [2] R. Dorta, E.D. Stevens, C.D. Hoff, S.P. Nolan, J. Am. Chem. Soc. 125 (2003) 10490. [3] (a) W.D. Jones, J. Am. Chem. Soc. 131 (2009) 15075; (b) F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis, Springer, Berlin, 2007; (c) C.S.J. Cazin, N-Hetrocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, Catalysis by Metal Complexes 32, Springer ScienceþBusiness Media B.V., New York, 2011. [4] (a) V. César, L.H. Gade, S. Bellemin-Laponnaz, in: S. Díez-González (Ed.), NHeterocyclic carbenes: From Laboratory Curiosities to Efficient Synthetic Tools, RSC Catalysis Series No. 6, Royal Society of Chemistry, Cambridge, 2011, pp. 228e251; (b) B. Çetinkaya, in: S. Díez-González (Ed.), N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools RSC Catalysis Series No. 6, Royal Society of Chemistry, Cambridge, 2011, pp. 366e398; (c) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612; (d) X. Bantreil, J. Broggi, S.P. Nolan, Annu. Rep. Prog. Chem. Sect. B 105 (2009) 232; (e) J. Dupont, C.S. Consorti, J. Spencer, Chem. Rev. 105 (2005) 2527; (f) M.E. van der Boom, D. Milstein, Chem. Rev. 103 (2003) 1759; (g) M. Albrecht, G. van Koten, Angew. Chem. Int. Ed. 40 (2001) 3750. [5] P. Bazinet, P.A. Glenn, D.S. Richeson, J. Am. Chem. Soc. 125 (2003) 13314. [6] (a) K. Verlinden, C. Ganter, J. Organomet. Chem. 750 (2014) 23; (b) L. Oehninger, L. Nadine Küster, C. Schmidt, A. Muñoz-Castro, A. Prokop, I. Ott, Chem. Eur. J. 19 (2013) 17871; (c) G. Choi, H. Tsurugi, K. Mashima, J. Am. Chem. Soc. 135 (2013) 13149; (d) A.F. Hill, C.M.A. McQueen, Organometallics 31 (2012) 8051; (e) H. Tsurugi, S. Fujita, G. Choi, T. Yamagata, S. Ito, H. Miyasaka, K. Mashima, Organometallics 29 (2010) 4120; (f) T. Tu, J. Malineni, X. Bao, K.H. Dötz, Adv. Synth. Catal. 351 (2009) 1029; (g) D.G. Gusev, Organometallics 28 (2009) 6458; (h) N. Fey, M.F. Haddow, J.N. Harvey, C.L. McMullin, A.G. Orpen, Dalton Trans. (2009) 8183; (i) P. Bazinet, T.G. Ong, J.S. O’Brien, N. Lavoie, E. Bell, G.P.A. Yap, I. Korobkov, D.S. Richeson, Organometallics 26 (2007) 2885; (j) W.A. Herrmann, J. Schütz, G.D. Frey, E. Herdtweck, Organometallics 25

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