Polyhedron 29 (2010) 553–563

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Polyhedron

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The next generation of C2-symmetric ligands: A di-N-heterocyclic carbene (NHC)ligand and the synthesis and X-ray characterization of mono- and dinuclearrhodium(I) and iridium(I) complexes

Roxy J. Lowry, Muhammad T. Jan, Khalil A. Abboud, Ion Ghiviriga, Adam S. Veige *

Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, FL 32611, United States

a r t i c l e i n f o

Article history:Available online 3 July 2009

Keywords:CarbeneCatalysisBimetallicNHCChiralEnantioselective

0277-5387/$ - see front matter Published by Elsevierdoi:10.1016/j.poly.2009.06.075

* Corresponding author.E-mail address: veige@chem.ufl.edu (A.S. Veige).

a b s t r a c t

A new C2-symmetric chelating di-N-heterocyclic carbene (NHC) ligand is reported. The stable free di-car-bene (+/�)[DEAM-BY] (3) forms upon treating the imidazolium salt (+/�)[DEAM-BI][OTf]2 (2) with potas-sium bis-trimethylsilylamide (where DEAM-BY = trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzyl)imidaz-2-ylidene and DEAM-BI = trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzyl)imidazolium). Metalation reactions of 2 with [Rh(COD)Cl]2 and [Ir(COD)Cl]2 are carried out undermild conditions to produce either mono- or bimetallic complexes. Each compound is characterized byNMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography.

Published by Elsevier Ltd.

1. Introduction

Establishing fundamental relationships between ligand struc-ture and metal–catalyst activity and selectivity are vital to reactionoptimization and predictability. Solid-state structural analysis ofmetal–catalysts and their organic ligand precursors is one ap-proach used to extract potential structure/activity relationships.A relatively modern development in metal-mediated catalysis isthe employment of diamino-stabilized carbenes as supporting li-gands [1]. Of these, N-heterocyclic carbenes (NHCs) are the mostprolific. Two nitrogen atoms within the heterocycle stabilize thecarbene carbon through inductive and mesomeric effects [2], allo-cating strong r-donation to a metal ion. As a result, NHC–M com-plexes are often thermally robust and electron-rich, ideal forcatalytic application. NHCs are considered electronically analogousto long-established phosphorus based ligands. However, the shapedisparity, planar (NHC) versus conical (PR3), of the ligands allowfew direct correlations [3].

Monodentate NHC ligands are effective in a wide range of metal-catalyzed transformations including enantioselective reactions [4].Recently, catalysts bearing chelating di-NHCs are receiving consider-able interest [5], but few demonstrate asymmetric transformationsresulting in high enantiomeric excesses [6]. Trudell [5a] and Rajanb-abu [5b] first reported the chiral binapthyl-di-NHC derivative, butsince then only six distinct architectures have emerged and onlyone version is commercially available [7]. With this in mind, we de-

Ltd.

signed a family of C2-symmetric di-NHCs derived from a dihydroet-hanoanthracene unit. This report will highlight the synthesis andcharacterization of the free di-N-heterocyclic carbene DEAM-BY(DEAM-BY = trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis-(1-benzyl)imidaz-2-ylidene) and the efficient synthesis of mono-and bimetallic Rh(I) and Ir(I) complexes. A detailed X-ray structuralanalysis of all newly synthesized compounds will highlight uniquecharacteristics of these rare di-NHC ligands.

2. Experimental

2.1. Materials and characterization

Unless specified otherwise, all manipulations were performedunder an inert atmosphere using standard Schlenk or gloveboxtechniques. Glassware was oven dried before use. Pentane, toluene,diethyl ether (Et2O), tetrahydrofuran (THF), and 1,2-dimethoxyeth-ane (DME) were dried using a GlassContours drying column. Allother solvents were purchased from Fisher Scientific and used with-out further purification. C6D6 (Cambridge Isotopes) was dried oversodium-benzophenone ketyl and distilled or vacuum transferredand stored over 4 Å molecular sieves. CDCl3 (Cambridge Isotopes)was dried over calcium hydride, distilled or vacuum transferredand stored over 4 Å molecular sieves. Bisnorbornadienerhodium(I)tetrafluoroborate ([Rh(nbd)2][BF4]), and chloro (1,5-cyclooctadi-ene)M(I) dimer ([M(COD)Cl]2; M = Ir, Rh) were purchased fromStrem Chemicals and used without further purification. 1-Benzyl-imidazole, styrene and vinyl acetate were purchased from Sigma Al-drich and used without further purification. KN(SiMe3)2 was

554 R.J. Lowry et al. / Polyhedron 29 (2010) 553–563

purchased from Fisher Scientific and used without further purifica-tion. NMR spectra were obtained on Varian INOVA 500 MHz, VarianMercury Broadband 300 MHz, or Varian Mercury 300 MHz spec-trometers. Chemical shifts are reported in d (ppm). For 1H NMRspectra, the residual protio solvent peak was referenced as an inter-nal reference. GC spectra were recorded on a HP 5890 series II withan HP 3396 integrator and mass spectrometry was performed at thein-house facility of the Department of Chemistry at the University ofFlorida. Elemental analyses were performed at either the in-housefacility of the Department of Chemistry at the University of Floridaor by Robertson Microlit Laboratories Inc., Madison, NJ.

2.2. Synthesis of (+/�) trans-1,10-[9,10-dihydro-9,10-ethanoanthracene-11,12-diyldimethanediyl]bis(benzylimidazole)bis(trifluoromethansulfonate) [DEAM-BI][OTf]2 (2)

1-Benzylimidazole (3.13 g, 19.8 mmol) was added to a 250 mLflask containing (+/�) 9,10-dihydro-9,10-ethanoanthracene-11,12-diyldimethanediyl bis(trifluoromethanesulfonate) [8](5.00 g, 9.40 mmol) in dry DME (100 mL). After refluxing under ar-gon for 2 h, the solvent was removed leaving a yellow powder. Thepowder was suspended in ethyl acetate and sonicated to produce awhite powder which was isolated by filtration as a white micro-crystalline solid. Yield 7.63 g, 9.02 mmol, 96%. 1H NMR (299 MHz,DMSO-d6) d (ppm): 9.27 (m, 2 H, 10), 7.83 (m, 2 H, 8 or 9), 7.78(m, 2 H, 8 or 9), 7.45–7.51 (m, 8H, Ar), 7.34–7.45 (m, 4H, Ar), 7.31(m, 2 H, Ar), 7.15–7.23 (m, 4H, Ar), 5.37–5.57 (m, 4 H, 11), 4.08 (s,2 H, 5), 3.92 (dd, J = 13.9, 4.2 Hz, 2 H, 7 or 70), 3.62 (dd, J = 13.7,8.9 Hz, 2 H, 7 or 70), 2.04–2.18 (m, 2 H, 6). 13C{1H} NMR (75 MHz,DMSO-d6) d (ppm): 142.5 (2C, Ar), 139.3 (2C, Ar), 136.6 (2C, 10),134.8 (2C, Ar), 129.1 (4C, Ar), 128.9 (2C, Ar), 128.4 (4C, Ar), 126.5(2C, Ar), 126.3 (2C, Ar), 125.7 (2C, Ar), 123.9 (2C, Ar), 123.0 (s, 2C,9 or 8), 122.6 (s, 2C, 9 or 8), 120.7 (q, J = 322 Hz, –CF3), 52.2 (s, 2C,7 or 11), 51.9 (s, 2C, 7 or 11), 44.4 (s, 2C, 5), 43.1 (s, 2C, 6).MS(HR�ESI+): Calc. for [C40H36N4S2O6F6]: m/z 869.8450 [M+Na]+,found m/z 869.1876. Anal. Calc. for C40H36N4S2O6F6: C, 56.73; H,4.29; N, 6.62. Found: C, 56.68; H, 4.35; N, 6.50%.

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N10N

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15

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122 OTf

+/-

2.3. Synthesis of (+/�) trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzylimidazolidine-2-ylidene, DEAM-BY (3)

To a 100 mL flask containing (+/�) [DEAM-BI][OTf]2 (2) (800 mg,0.95 mmol) in THF (10 mL) was added KN(SiMe3)2 (395 mg,1.98 mmol in 5 mL THF) at �35 �C. After 1 h, the solution was al-lowed to warm to ambient temperature while stirring. The solventwas then removed in vacuo producing an orange–yellow powder.After trituration with diethyl ether (2 � 5 mL) and pentanes

(2 � 5 mL) the solid was taken up in pentanes and filtered. The pre-cipitate was then washed with diethyl ether and extracted withTHF. The THF was removed to produce a peach colored powder.Yield 366 mg, 0.67 mmol, 71%. 1H NMR (299 MHz, C6D6) d (ppm):7.57 (dd, J = 7.4, 1.1 Hz, 2H, 14), 7.04–7.27 (m, 16H, Ar), 6.47–6.52(overlapping d, 4H, 8 & 9), 5.17 (s, 4H, 10), 4.38 (d, J = 1.1 Hz, 2H,5), 3.77 (dd, J = 13.2, 8.4 Hz, 2H, 7 or 70), 3.47 (dd, J = 13.3, 5.9 Hz,2H, 7 or 70), 2.28–2.39 (m, 2H, 6). 13C NMR (75 MHz, C6D6) d(ppm): 216.2 (s, 17), 144.6 (s, Ar), 141.5 (s, Ar), 139.7 (s, Ar), 129.1(s, Ar), 128.3 (s, Ar), 126.9 (s, Ar), 126.8 (s, Ar), 126.4 (s, Ar), 124.0(s, Ar), 120.7 (s, 8 or 9), 119.1 (s, 8 or 9), 55.4 (overlapping singlets,7 & 10), 47.1 (overlapping singlets, 5 & 6). MS(DIP-CI): Calc. for[C38H34N4]: m/z 546.2929 [M]+, found m/z 546.2783.

5

6

2

3 4

1

7/ 7′

NN

NN

9

8

10

111615

1413

12

+/-

17

2.4. Synthesis of the (+/�) rhodium(I) trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzylimidazolidine-2-ylidenecyclooctadiene triflate, [(DEAM-BY)Rh(COD)][OTf] (4)

(a) To a solution of (+/�) DEAM-BY (3) (500 mg, 0.92 mmol) inTHF (5 mL) was added a solution of [Rh(COD)Cl]2 (226 mg,0.458 mmol) in THF (5 mL). The reaction was stirred for 2 h duringwhich time a yellow precipitate formed. The precipitate was filteredand washed with cold THF (2� 3 mL) to provide [(DEAM-BY)Rh(CO-D)][OTf] (4) as a bright yellow solid. Yield 417 mg, 0.46 mmol, 50%.

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N

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N

Rh

16

N23

24

25

OTf

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1819

2021

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2928

27

22/22′13/13′

(b) At �100 �C, a solution of KN(SiMe3)2 (50 mg, 0.25 mmol) in dryTHF (3 mL) was added to a solution of (+/�)[DEAM-BI][OTf]2

(100 mg, 0.118 mmol) in THF (5 mL) and allowed to warm to ambi-ent temperature while stirring vigorously. After 45 min, the solution

R.J. Lowry et al. / Polyhedron 29 (2010) 553–563 555

was again cooled to �100 �C. After 15 min, a solution of[Rh(COD)Cl]2 (28 mg, 0.057 mmol) in THF (5 mL) was added to themixture and allowed to warm to ambient temperature and stirredovernight to provide a yellow precipitate. The yellow solid was col-lected by filtration, washed with Et2O (10 mL) and THF (2 mL), andresidual solvent removed in vacuo. Yield 97 mg, 0.11 mmol, 90%.

1H NMR (299 MHz, CDCl3) d (ppm): 7.78 (d, J = 1.7 Hz, 1H, 14),7.34–7.58 (4H, 1,4,5,8), 7.28–7.34 (3H, 28,29,30), 7.1–7.25 (7H,2,3,6,7,18,19,20), 6.88 (overlapping d and m, for d J = 2.0 Hz, 3H,23,27,31), 6.58 (m, 2H, 17,21), 6.68 (d, J = 1.7 Hz, 1H, 15), 6.56 (d,J = 2.0 Hz, 1H, 24), 5.97 (d, J = 15.9 Hz, 1H, 250), 5.36 (d, J = 15.6 Hz,1H, 160), 5.17 (d, J = 16.1 Hz, 1H, 25), 5.09 (d, J = 16.1 Hz, 1H, 16),4.69 (dd, J = 13.3, 3.1 Hz, 1H, 130), 4.62 (d, J = 1.1 Hz, 1H, 10), 4.47–4.58 (m, 1H, 40), 4.43 (d, J = 0.8 Hz, 1H, 9), 4.37–4.45 (m, 1H, 11),4.27–4.34 (m, 1H, 44), 4.19–4.30 (dd, 1H, 13), 4.01–4.09 (m, 1H, 39),3.97 (dd, J = 14.0, 7.8 Hz, 1H, 220), 3.61 (m, 1H, 43), 3.12 (dd, J = 14.2,2.0 Hz, 1H, 22), 1.23–2.39(m, 9H, 12, 41, 42, 45, 46). 13C NMR(75 MHz, CDCl3) d (ppm): 181.8 (d, J = 54.1 Hz, 20 or 31), 179.4 (d,J = 52.7 Hz, 20 or 31), 144.4 (s, 12), 144.2 (s, 1), 139.6 (s, 6), 138.9 (s,7), 135.6 (s, 33 or 22), 135.6 (s, 33 or 22), 129.1 (s, 35 and 37),129.1(s, 26 and 24), 128.3 (s, 36), 128.1 (s, 25), 126.7 (s, 10), 126.7(s, 3), 126.4 (s, 4), 126.33 (s, 38 and 34), 126.2 (s, 9), 126.1 (s, 23 and27), 125.9 (s, 5), 125.6 (s, 8), 125.5 (s, 18), 124.7 (s, 29), 123.7 (s, 11),123.1 (s, 2), 121.3 (s, 30), 121.1 (s, 19), 92.0 (d, J = 8.2 Hz, 40), 89.7(d, J = 7.3 Hz, 39), 88.7 (d, J = 9.6 Hz, 44), 84.8 (d, J = 8.2 Hz, 73), 57.2(m, 17), 52.81 (m, 32), 55.0 (m, 28), 54.1 (m, 21), 48.14 (s, 13), 47.8(s, 15), 47.6 (s, 14), 47.4 (s, 16), 32.0 (s, 45), 31.3 (s, 41), 29.4 (s, 42),29.1 (s, 46). MS(HR�ESI+): Calc. for [C47H46N4Rh]+: m/z757.7874M+, found m/z 757.2773. Anal. Calc. for C47H46N4SO3F3Rhplus one molecule CH3Cl: C, 58.12; H, 4.87; N, 5.65%. Found: C,57.88; H, 4.834; N, 5.50%.

2.5. Synthesis of (+/�) [l2-DEAM-BY][Rh(COD)Cl]2 (5)

A solution of KN(SiMe3)2 (50 mg, 0.250 mmol) in dry THF (5 mL)was added to a solution of (+/�) [DEAM-BI][OTf]2 (100 mg,0.118 mmol) in THF (5 mL) at �100 �C. After stirring at ambienttemperature for 45 min, the solution was cooled to �100 �C andadded slowly to a cold solution of [Rh(COD)Cl]2 (58 mg,0.117 mmol) in THF (5 mL). The reaction was stirred overnight pro-viding an orange solution. After the volatiles were removed, theresulting powder was washed with Et2O and extracted with THF.The THF was removed and the product was washed with a smallamount of Et2O and pentane to provide a yellow powder. Yield116 mg, 0.112 mmol, 94%. 1H NMR (300 MHz, CDCl3) d (ppm):7.49 (2H, d, J = 7.0 Hz, Ar), 7.29–7.39 (4H, m, Ar), 7.18–7.26 (8H,m, Ar), 7.06–7.19 (4H, m, Ar), 6.90 (2H, J = 1.9 Hz, 8 or 9), 5.88(2H, d, J = 13.6 Hz, 10 or 100), 4.77–4.88 (2H, m, COD–CH), 4.57(2H, d, J = 1.9 Hz, 8 or 9), 4.56 (2H, d, J = 13.7 Hz, 10 or 100),4.42–4.54 (2H, m, COD–CH), 4.23 (2H, d, J = 2.8 Hz, 5), 4.10 (2H,overlapping dd, J = 12.6 Hz, 7 or 70), 3.57 (2H, dd, J = 13.3, 2.6 Hz,6), 3.19–3.30 (2H, m, 7 or 70), 2.72–2.84 (2H, m, COD–CH), 2.39–2.50 (2H, m, COD–CH), 2.16–2.34 (2H, m, COD–CH2), 1.89–2.05(2H, m, COD–CH2), 1.56–1.89 (8H, m, COD–CH2), 1.33–1.56 (4H,m, COD–CH2). 13C NMR (75 MHz, CDCl3) d (ppm): 179.5 (d,J = 51.2 Hz, NCN), 142.5 (s, Ar), 142.3 (s, Ar), 136.2 (s, Ar), 130.0(s, Ar), 128.7 (s, Ar), 128.1 (s, Ar), 126.0 (s, Ar), 125.7 (s, Ar), 125.3(s, Ar), 124.3 (s, Ar), 120.7 (s, NCH@CHN), 119.9 (s, NCH@CHN),98.8 (d, J = 6.6 Hz, COD–CH), 98.0 (d, J = 7.2 Hz, COD–CH), 68.6 (d,J = 14.3 Hz, COD–CH), 67.4 (d, J = 14.9 Hz, COD–CH), 56.1 (s, N–CH2–C), 53.9 (s, CH–CH2–N), 48.7 (s, CCHCHCH2), 39.7 (s, CCHC),33.1 (s, COD–CH2), 31.6 (s, COD–CH2), 29.5 (s, COD–CH2), 27.3 (s,COD–CH2). MS(HR�SI+): Calc. for [C47H46N4Rh]+: m/z757.7874M+, found m/z 757.2773. Anal. Calc. for C54H58N4Rh2Cl2:C, 62.37; H, 5.62; N, 5.39. Found: C, 62.17; H, 5.57; N, 5.18%.

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8

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2.6. Synthesis of the (+/�) Iridium(I) trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzylimidazolidine-2-ylidenecyclooctadiene triflate, [(DEAM-BY)Ir(COD)][OTf] (6)

Triflate salt 2 (100 mg, 0.118 mmol), KN(SiMe3)2 (50 mg,0.25 mmol), and [Ir(COD)Cl]2 (39 mg, 0.058 mmol) were eachweighed into separate scintillation vials and suspended in dryTHF (5 mL). At �100 �C, the KN(SiMe3)2 solution was added drop-wise to the salt suspension. The reaction mixture was allowed towarm to ambient temperature while stirring, forming a brilliantyellow color after 45 min. The mixture was again cooled to�100 �C followed by dropwise addition of the [Ir(COD)Cl]2 solu-tion. The reaction was stirred overnight at ambient temperature.Mononuclear 6 formed as an orange precipitate, was isolated by fil-tration and washed with THF (2 mL) and Et2O (5 mL) to remove ex-cess base. The metal complex was extracted from any remainingstarting materials with CH2Cl2, and upon solvent removal was iso-lated as a brilliant orange powder. Yield 57 mg, 0.0572 mmol, 48%.1H NMR (300 MHz, CD2Cl2) d (ppm): 7.41–7.52 (2H, m, Ar), 7.29–7.40 (6H, m, Ar), 7.19–7.29 (5H, d overlapped by m, 14 and Ar),7.09–7.18 (2H, m, Ar), 6.81–6.88 (2H, m, Ar), 6.74 (2H, 2 overlap-ping doublets, J = 1.6 Hz, 23 and 15), 6.63–6.69 (2H, m, Ar), 6.56(1H, d, J = 2.0 Hz, 24), 5.76 (1H, d, J = 15.9 Hz, 25 or 250), 5.35(2H, d, J = 15.5 Hz,, 16 or 160), 5.09 (1H, d, J = 15.8 Hz, 250 or 25),4.82 (1H, d, J = 15.6 Hz, 16 or 160), 4.38 (2H, 2 overlapping doublets,J = 1.9 Hz and J = 1.9 Hz, 10 and 9), 4.18–4.26 (2H, m, overlapping13 or 130 and 22 or 220), 4.13 (2H, dd, J = 14.3, 8.0 Hz, 13 or 130),4.02–4.14 (2H, m, COD–CH), 3.85–3.98 (1H, m, COD–CH), 3.64–3.75 (4H [3 are from THF], m overlapping THF, COD–CH), 3.54–3.64 (1H, m, COD–CH), 3.26–3.35 (1H, m, 11), 3.22 (1H, dd,J = 14.4, 1.7 Hz, 22 or 220), 1.95–2.15 (2H, m, COD–CH2), 1.76–1.89 (5H [3 are from THF], m, COD–CH2), 1.51–1.76 (4H, m, 12and COD–CH2), 1.30–1.50 (1H, m, COD–CH2). 13C NMR (75 MHz,CD2Cl2) d (ppm): 179.5 (NCN), 176.6 (NCN), 145.0 (C, Ar), 144.9(C, Ar), 139.6 (C, Ar), 139.5 (C, Ar), 136.2 (C, Ar), 135.9 (C, Ar),129.6 (2C, Ar), 129.6 (2C, Ar), 128.9 (C, Ar), 128.8 (C, Ar), 127.5 (C,Ar), 127.3 (C, Ar), 127.0 (C, Ar), 126.9 (C, Ar), 126.9 (2C, Ar), 126.8(2C, Ar), 126.2 (2C, Ar), 124.8 (N–CH@CH–N), 124.2 (N–CH@CH–N) 123.7 (N–CH@CH–N), 123.7 (N–CH@CH–N), 121.6 (2C, Ar),79.4 (COD–CH), 78.0 (COD–CH), 77.1 (COD–CH), 73.1 (COD–CH),57.4 (CH–CH2–N), 55.7 (N–CH2–C), 55.1 (CH–CH2–N), 54.5 (N–CH2–C), 49.0 (C–CH–C), 48.8 (CH–CH–CH2), 48.2 (C–CH–C), 47.9(CH–CH–CH2), 32.7 (COD–CH2), 32.3 (COD–CH2), 30.9 (COD–CH2),30.8 (COD–CH2). MS(TOF�SI+): Calc. for [C46H46N4Ir]+: m/z847.6958M+, found m/z 847.3350. Anal. Calc. for C47H46N4IrO3F3S:C, 56.67; H, 4.65; N, 5.62. Found: C, 56.37; H, 4.51; N, 5.45%.

556 R.J. Lowry et al. / Polyhedron 29 (2010) 553–563

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2.7. Synthesis of (+/�) [l DEAM-BY][Ir(COD)Cl]2 (7)

Triflate salt 7 (100 mg, 0.118 mmol), KN(SiMe3)2 (50 mg,0.25 mmol), and [Ir(COD)Cl]2 (79 mg, 0.117 mmol) were eachweighed into separate scintillation vials and suspended in dry THF(4 mL). At �100 �C, the KN(SiMe3)2 solution was added dropwiseto the salt suspension; the reaction mixture was stirred vigorouslywhile warming to ambient temperature. After 45 min, the mixturewas again cooled to�100 �C and added dropwise to the [Ir(COD)Cl]2

solution. The reaction was allowed to warm to ambient temperatureovernight. The solvent was removed and the resulting residue waswashed with Et2O (10 mL) and extracted with THF. Evaporation invacuo of all volatiles provides the dinuclear (+/�) [l2-DEAM-BY][Ir(-COD)Cl]2 (7) as a yellow solid. Yield 71 mg, 0.0582 mmol, 49%. 1HNMR (300 MHz, CDCl3) d (ppm): 7.09–7.58 (18H, m, Ar), 6.96 (2H,d, J = 2.1 Hz, 8 or 9), 5.87 (2H, d, J = 13.5 Hz, 10 or 100), 4.67 (2H, d,J = 1.8 Hz, 8 or 9), 4.57 (2H, d, J = 13.8 Hz, 10 or 100), 4.49 (2H, m,COD–CH), 4.24 (2H, d, J = 2.1 Hz, 5), 4.08–4.20 (2H, m, COD–CH),3.91–4.08 (2H, overlapping dd, 7 or 70), 3.61 (2H, dd, J = 13.8, 1.0,0.7 Hz, 7 or 70), 3.22 (2H, br. d, J = 10.8 Hz, 6), 2.39–2.56 (2H, m,COD–CH), 2.23 (2H, m, COD–CH), 2.07–2.21 (2H, m, COD–CH2),1.90 (2H, m, COD–CH2), 1.70 (5H, m, COD–CH2), 1.35–1.51 (2H, m,COD–CH2), 1.06–1.35 (5H, m, COD–CH2). 13C NMR (75 MHz, CDCl3)d (ppm): 177.3 (NCN), 142.2 (C, Ar), 142.0 (C, Ar), 136.1 (C, Ar),130.0 (C, Ar), 128.7 (C, Ar), 128.7 (C, Ar), 126.0 (C, Ar), 125.7 (C, Ar),125.2 (C, Ar), 124.2 (C, Ar), 119.9 (NCN@CHN), 119.5 (NCN@CHN),84.9 (COD–CH), 83.8 (COD–CH), 55.7 (COD–CH), 53.6 (COD–CH),52.2 (CH2), 50.8 (CH2), 48.6 (CHCHCH2), 39.4 (CCHC), 33.9 (COD–CH2), 31.9 (COD–CH2), 30.4 (COD–CH2), 27.7 (COD–CH2).MS(TOF�ESI+): Calc. for [C54H58N4Ir2Cl]+: m/z 1183.967 (MCl)+,found m/z 1183.3595. Anal. Calc. for C54H58N4Ir2Cl2: C, 53.23; H,4.80; N, 4.60. Found: C, 51.53; H, 4.08; N, 4.90%.

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11

2.8. Catalytic hydroformylation

The hydroformylation experiments were carried out in a100 mL stainless steel Parr reactor heated in a sand bath. The reac-tor was charged with 50 mg of the substrate, 0.1 mol% of the car-bene rhodium complex (4) and 1.5 mL of solvent. Before startingthe catalytic reactions, the charged reactor was purged three timeswith 10–20 bar of syngas (CO/H2 = 1/1) and then pressurized to100 bar. The reaction mixture was stirred at 800 rpm at 50 �C forthe appropriate reaction time (typically 24 h). Once complete, thereactor was cooled to room temperature, the pressure was reducedto 1.0 bar in a well-ventilated hood, and the reaction mixture wascollected in a vial. The reaction mixtures were analyzed directlywithout further purification. The % conversion and regioselectivitywere determined by 1H NMR spectroscopy and gas chromatogra-phy. The enantiomeric purity was determined by GC using Supe-lco’s Beta Dex 225 column. Temperature program: 100 �C for5 min, then 4 �C/min to 160 �C. Retention times: 4.8 min for vinylacetate, 11 (S) and 12.8 (R) min for the enantiomers of methyl 2-methyl-3-oxopropanoate (branched regioisomer), 16 min formethyl 4-oxobutanoate (linear regioisomer), 9 min for styrene,18 (S) and 18.2 (R) min for the enantiomers of 2-phenylpropanal(branched regioisomer), and 22 min for 3-phenylpropanal (linearregioisomer).

3. Results and discussion

3.1. Preparation and solution-state analysis

3.1.1. [DEAM-BI](OTf)2 (2)Scheme 1 illustrates the synthesis of compound 2. The trifluoro-

methanesulfonate (triflate) of compound 1 [9] is replaced in a sub-stitution reaction by 1-benzylimidazole, a commercially availablesubstrate. Unlike other derivatives of 2, such as the methylimida-zolium salt [DEAM-MBI](OTf)2 [7a], [DEAM-BI] does not precipitatefrom the reaction medium. However, removal of all volatiles pro-vides 2 in sufficient purity for subsequent reactions. Obtaining ana-lytically pure 2 requires sonication in ethyl acetate or diffusioncrystallization followed by filtration.

NMR spectroscopy, high resolution mass spectrometry (HRMS),and combustion analysis confirms the identity of 2. A 1H NMRspectrum of 2 reveals a signature downfield imidazolium protonresonance (N–CH@N) at 9.27 ppm. Coupling between the imidazo-lium proton and the imidazole olefinic protons (N–CH@CH–N)should form a doublet of doublets; however, relatively weak cou-pling causes the signal to appear as an unresolved multiplet (1HNMR 300 MHz). Two additional unresolved multiplets appear at7.83 and 7.78 ppm corresponding to the olefinic protons. Upfield,the backbone bridgehead (C–CH–CH) and bridge (CH–CH–CH2)protons appear as a singlet at 4.08 ppm and a multiplet at2.10 ppm, respectively. The diastereotopic methylene protons onthe ligand arms resonate as a pair of doublet of doublets at 3.92(J = 13.9, 4.2 Hz) and 3.62 (J = 13.7, 8.9 Hz). In the 13C{1H} NMRspectrum, the imidazolium carbon (N–CH@N) resonates at136.7 ppm. It is also worth noting that the most downfield signalappears at 142.5 ppm, corresponding to an aromatic carbon.

3.1.2. [DEAM-BY] (3)Scheme 2 depicts the synthesis of the free di-NHC ligand

[DEAM-BY] (3). Deprotonation of 2 at �35 �C occurs immediatelyupon addition of the base, potassium bis(trimethylsilyl)amide(KN(SiMe3)2), forming a brilliant yellow solution in 71% yield.

The proton at 9.27 ppm for the imidazolium salt 2 is not presentin the 1H NMR spectrum of 3. This observation alone, however,does not provide conclusive evidence for the formation of a free

3

3

Δ

Scheme 1. Synthesis of [DEAM-BI](OTf)2 (2).

3 2

Scheme 2. Synthetic scheme for [DEAM-BIY] (3).

R.J. Lowry et al. / Polyhedron 29 (2010) 553–563 557

di-NHC. Related benzimidazole derivatives prefer formation ofenetetraamines, which result in a similar 1H NMR spectrum [7].Increased electron density within the heterocycles of 3 shifts theolefinic signals upfield, from 7.83 and 7.78 ppm in 2, to 6.47–6.51 ppm in 3. A diagnostic carbene–carbon resonance at216.2 ppm in the 13C{1H} spectrum verifies the identity of 3 as afree carbene species. Free N-heterocyclic carbene carbon atomsresonate between 205 and 245 ppm, [2b] whereas enetetraaminecarbons are shifted well upfield to approximately 145 ppm [9].

3.1.3. [(DEAM-BY)Rh(COD)]OTf (4)The mononuclear, cationic Rh(I) complex [(DEAM-BY)Rh(CO-

D]OTf (4) readily forms by combining solutions of the free di-

Scheme 3. Synthesis of [(DE

NHC 3 and [Rh(COD)Cl]2. Alternatively, (see experimental detail)in situ deprotonation of 2 followed by reaction with [Rh(COD)Cl]2

is a more convenient and higher yielding route (Scheme 3). Com-plex 4 forms as a yellow–orange precipitate and is isolated via fil-tration in 90% yield.

Chelation to a single rhodium center creates a C1 symmetriccompound. The lower symmetry causes each proton and carbonto become chemically and magnetically unique resulting in com-plex 1H and 13C{1H} NMR spectra. Despite the complexity, NOESYand HMBC experiments enable the absolute assignment of mostresonances. Between 5 and 6 ppm are four distinct doublets attrib-uted to the diastereotopic methylene protons of the N-benzyl sub-stituent (5.97, d, J = 15.9 Hz, 5.36, d, J = 15.6 Hz, 5.17, d, J = 16.1 Hz,

2

2

A-BY)Rh(COD)]OTf (4).

558 R.J. Lowry et al. / Polyhedron 29 (2010) 553–563

and 5.09 ppm; d, J = 16.1 Hz). The other diastereotopic protons(CH–CH2–N) appear as four doublet of doublets at 4.69 (J = 13.3,3.1 Hz), 4.19–4.30 (dd, overlapping with a COD–CH), 3.97(J = 14.0, 7.8 Hz), and 3.12 (J = 14.2, 2.0 Hz) ppm. The bridge pro-tons, that resonate as singlets in compounds 2 and 3, now appearas doublets at 4.62 (d, J = 1.1 Hz) and 4.43 (d, J = 0.8 Hz) ppm.The most striking feature of the 1H NMR spectrum is the large sep-aration between the two bridgehead protons, appearing at 4.40and 1.87 ppm. The 13C{1H} NMR spectrum of 3 displays two dis-tinct doublets for the carbene carbons at 181.8 (J = 54.1 Hz) and179.4 ppm (J = 52.7 Hz), and provide further evidence of an unsym-metrical Rh center.

Scheme 4. Synthesis of [l-DE

Scheme 5. Synthesis of [(DEAM-BY)Ir(COD)]O

3.1.4. [l-DEAM-BY][Rh(COD)Cl]2 (5)The dimeric starting material [Rh(COD)Cl]2 used in the synthe-

sis of 4 is also utilized to create the bimetallic [l-DEAM-BY][-Rh(COD)Cl]2 (5). Deprotonation of 2 followed by dropwiseaddition into a cold THF solution containing [Rh(COD)Cl]2 forms5 in 94% yield (Scheme 4). Compound 5 is significantly more solu-ble than its monometallic counterpart 4. To obtain pure 5, the yel-low–brown powder obtained after removal of volatiles must befurther washed with cold ether and extracted into THF.

The two Rh(I) centers of 5 are bridged by the ethanoanthraceneligand through coordination of one NHC moiety to each metal cen-ter. Unlike monometallic 4, 5 possesses C2 symmetry. The 1H and

2

2

AM-BY][Rh(COD)Cl]2 (5).

2

2

2

2

Tf (6) and [l-DEAM-BY][Ir(COD)Cl]2 (7).

R.J. Lowry et al. / Polyhedron 29 (2010) 553–563 559

13C{1H} NMR spectra of 5 clearly reveal higher symmetry. The ben-zyl methylene protons, corresponding to four individual doubletsin 4, collapse to two doublets at 5.88 (J = 13.6 Hz) and 4.56(J = 13.7 Hz) ppm in the 1H NMR spectrum of 5. The olefinic imid-azole protons (N–CH@CH–N) appear as two doublets at 6.90(J = 1.9 Hz) and 4.57 (J = 1.9 Hz) ppm, and the bridge and bridge-head protons as a doublet at 4.23 (J = 2.8 Hz) and doublet of dou-blets at 3.57 (J = 13.3, 2.6 Hz) ppm, respectively. A single distinctdoublet at 179.5 (d, J = 51.2 Hz) ppm in the 13C{1H} NMR spectrumdemonstrates the two Rh–C bonds are equivalent.

Synthesis of the mono- and dinuclear Ir(I) complexes [(DEAM-BY)Ir(COD)]OTf (6) and [l-DEAM-BY][Ir(COD)Cl]2 (7) follows thesame synthetic methods as the Rh(I) counterparts, by replacing[Rh(COD)Cl]2 with [Ir(COD)Cl2] (Scheme 5). NMR spectra of mono-nuclear 6 are, as expected similar to the spectra of 4, excluding afew key features. Instead of the doublets in the 13C{1H} NMR spec-trum of 4, the iridium-bound carbons resonate as singlets at 179.5and 176.6 ppm. Unlike 103Rh, the predominant iridium isotopes,191Ir and 193Ir, have quadrapolar nuclei with spin of 3/2, therebynegating M–C spin–spin coupling in the spectrum of both 6 and 7.

3.2. X-ray structural analysis and comparisons of compounds 2–7

To obtain a more complete understanding of molecules 2–7structural studies were performed by single-crystal X-ray diffrac-tion. Solid-state investigations of these molecules allow compari-son of structural variations between analogs.

3.2.1. Comparison of ligand precursors 2 and 3Diffusion crystallization provides suitable crystals for X-ray

diffraction studies of both 2 and 3. Compound 2 crystallizes bydiffusion of pentane into an ethanol solution containing the salt,

Fig. 1. Molecular structure of [DEAM-BI](OTf)2 (2) and [DEAM-BY] (3) with displacem

and crystals of the free di-carbene are similarly grown by diffu-sion of pentane into a saturated benzene solution of 3. Fig. 1 dis-plays the solid-state structure of the dicationic imidazolium salt2 and the neutral free di-carbene 3. Table 1 lists the crystallo-graphic data and Table 2 contains pertinent bond lengths andangles.

The asymmetric unit of 2 contains one ligand moleculeand two triflate anions in a monoclinic space group. Investi-gation of the packing structure reveals an intermolecularattraction between the olefin of the cationic imidazole andthe electron-rich benzyl ring of its neighboring molecule(see Supporting data). The shortest distance between a carbonon the olefin and the aryl ring is 3.72 Å, whereas the distancebetween the olefin and the plane of the aromatic ring is3.42 Å.

Unlike 2, the neutral free di-NHC (3) forms an orthorhombiccrystal system with no discernible intermolecular attractions.Overall, the metric parameters for both molecules correspond tothose expected for imidazolium salts and their corresponding freecarbenes [10,2b]. Deprotonation increases the electron density atthe carbene center, causing the N–C–N angles to contract from108.8(2)� and 107.9(2)� in 2 to 102.0(3)� and 101.8(3)� in 3. TheN–Ccarbene bond lengths in 3 (d(N1–C20) = 1.370(4), d(N2–C20) =1.352(5), d(N3–C31) = 1.378(5), and d(N4–C31) = 1.360(4) Å) arenotably longer than in 2 (d(N1–C20) = 1.322(3), d(N2–C20) =1.334(3), d(N3–C31) = 1.328(3), and d(N4–C31) = 1.330(3) Å).Although both imidazolium p-systems obey the 4n+2 aspect ofHückel’s rule [11] of aromaticity, the alterations caused by in-creased electron density does not affect the imidazole olefin(C@C) bond distances. No appreciable bond length alteration ofthe N–Calkene or C@C bonds is apparent between compounds 2and 3.

ent ellipsoids drawn at the 50% probability level and protons removed for clarity.

Table 1X-ray crystallographic structure parameters and refinement data for 2, 3, 4, 5, 6, and 7.

2 3 4 5 6 7

Empirical formula C40H36F6N4O6S2 C38H34N4 C48H48Cl2F3 N4O3RhS C60H64Cl2N4Rh2 C53H52F3IrN4O3S C60H64Cl2Ir2N4

Formula weight 846.85 546.69 991.77 1117.87 1074.25 1296.45Crystal system monoclinic orthorhombic monoclinic triclinic monoclinic monoclinicSpace group P21/c Pna21 P21/n P�1 P21/n P�1Crystal dimensions (mm) 0.29 � 0.26 � 0.22 0.18 � 0.10 � 0.05 0.16 � 0.16 � 0.10 0.13 � 0.05 � 0.02 0.09 � 0.02 � 0.02 0.15 � 0.12 � 0.04a (Å) 12.5937(8) 10.3902(9) 9.8237(13) 13.839(2) 10.1135(17) 13.8318(17)b (Å) 9.3303(11) 21.6719(19) 38.633(3) 13.995(2) 17.996(3) 13.9515(17)c (Å) 24.6077(16) 13.1662(11) 24.957(3) 14.479(2) 24.932(4) 14.5432(18)a (�) 90 90 90 80.356(3) 90 80.640(2)b (�) 97.815(1) 90 92.909(2) 70.239(3) 92.221(4) 70.355(2)c (�) 90 90 90 75.220(3) 90 75.494(2)V (Å3) 3985.1(5) 2964.7(4) 4418.9(10) 2541.5(7) 4534.3(13) 2549.4(5)Z (Å) 4 4 4 2 4 2Absorption coefficient (mm�1) 0.214 0.072 0.615 0.798 3.053 5.363F (0 0 0) 1752 1160 2040 1152 2168 1280Dcalcd (g cm�3) 1.411 1.225 1.491 1.461 1.574 1.689c (Mo Ka) (Å) 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073T (K) 173(2) 173(2) 173(2) 173(2) 173(2) 173(2)h Range (�) 1.63–24.99 1.81–25.00 1.39–27.50 1.50–27.50 1.40–27.50 1.49–27.50Completeness to hmax (%) 93 100 99.7 97.8 99.3 97.5Index ranges �8 6 h 6 14,�29 6 k 6 15,

�15 6 l 6 15�11 6 h 6 12,�25 6 k 6 25,�15 6 l 6 14

�12 6 h 6 12, �22 6 k 6 23,�25 6 l 6 32

�17 6 h 6 17, �18 6 k 6 12,�18 6 l 6 18

�13 6 h 6 13,�22 6 k 6 23,�19 6 l 6 32

�17 6 h 6 17,�18 6 k 6 16,�18 6 l 6 18

Reflections collected 12 702 15 241 29 619 17 430 29 378 17 315Independent reflections [Rint] 6516[0.0349] 5055[0.0624] 10 128[0.0483] 11 405[0.0811] 10 343[0.1461] 11 384[0.0589]Maximum, minimum

transmission0.9582, 0.9433 0.9966, 0.9890 0.9441, 0.9039 0.9844, 0.9236 0.9453, 0.8086 0.8217, 0.4259

Data/restraints/parameters 6516/0/509 5055/1/379 10 128/1/570 11 405/0/613 10 343/1/571 11 384/0/613Final R1 indices [I >2r(I)] R1 = 0.0467, wR2 = 0.1194

[5128]R1 = 0.0570, wR2 = 0.1523[4229]

R1 = 0.0359,wR2 = 0.0988[8289]

R1 = 0.0538, wR2= 0.1185[6208]

R1 = 0.0872, wR2 = 0.0965[5988]

R1 = 0.0293, wR2 = 0.0760[9922]

R indices (all data) R1 = 0.0467, wR2 = 0.1279 R1 = 0.0700, wR2 = 0.1775 R1 = 0.0459, wR2 = 0.1024 R1 = 0.1125, wR2 = 0.1376 R1 = 0.1676, wR2 = 0.1127 R1 = 0.0348, wR2 = 0.0791Largest difference in peak and

hole (e �3)0.411/�0.361 0.161/�0.268 0.801/�1.078 0.554/�0.672 1.085/ �1.791 1.769/�1.397

Goodness-of-fit (GOF) on F2 1.035 1.075 1.077 0.881 1.034 1.007

560R

.J.Lowry

etal./Polyhedron

29(2010)

553–563

Table 2Selected bond lengths (Å) and angles (�) for 2, 3, 4, 5, 6, and 7.

2 3 4

Bond lengths C18–C19 1.343(4) C18–C19 1.337(5) Rh–C20 2.068(2)C29–C30 1.344(3) C29–C30 1.346(5) Rh–C31 2.037(2)N3–C31 1.328(3) N1–C18 1.369(5) N1–C18 1.384(3)N3–C29 1.374(3) N1–C20 1.370(4) N1–C20 1.367(3)N3–C28 1.473(3) N1–C17 1.458(5) N1–C17 1.468(3)N4–C31 1.330(3) N2–C20 1.352(5) N2–C20 1.359(3)N4–C30 1.369(3) N2–C19 1.382(5) N2–C19 1.387(3)

Angles N1–C20–N2 108.8(2) N1–C20–N2 101.8(3) C20–Rh–C31 89.93(8)N3–C31–N4 107.9(2) N3–C31–N4 102.0(3) N1–C20–N2 103.82(18)

N3–C31–N4 104.13(18)

Torsion angles H14A–C14–C16–H16A 58.96 H14A–C14–C16–H16A 65.54 H14A–C14–C16–H16A 70.99H13A–C13–C15–H15A 59.44 H13A–C13–C15–H15A 66.23 H13A–C13–C15–H15A 71.01C28–C15–C16–C17 119.12 C17–C16–C15–C28 108.96 C28–C15–C16–C17 95.8

5 6 7

Bond lengths Rh1–C20 2.021(5) Ir–C20 2.031(9) Ir1–C20 2.033(3)Rh2–C31 2.024(5) Ir–C31 2.059(8) Ir2–C31 2.033(3)N1–C20 1.364(6) N1–C18 1.382(10) N1–C20 1.355(4)N1–C18 1.368(6) N1–C20 1.351(10) N1–C18 1.382(4)N1–C17 1.463(6) N1–C17 1.467(10) N1–C17 1.475(4)N2–C20 1.362(6) N2–C20 1.361(10) N2–C20 1.365(4)N2–C19 1.376(6) N2–C19 1.375(11) N2–C19 1.384(4)N2–C21 1.460(6) N2–C21 1.469(10) N2–C21 1.457(4)C18–C19 1.354(7) C18–C19 1.349(5)C29–C30 1.340(7) C29–C30 1.353(5)Rh2–Cl2 2.3741(15) Ir1–Cl1 2.3673(9)Rh1–Cl1 2.3622(15) Ir2–Cl2 2.3530(10)

Angles N1–C20–N2 104.5(4) C20–Ir–C31 91.2(3) N1–C20–N2 103.9(3)N3–C31–N4 103.1(4) N1–C20–N2 103.7(7) N3–C31–N4 104.2(3)

N3–C31–N4 104.1(7)

Torsion angles H16A–C16–C14–H14A 58.84 H16A–C16–C14–H14A 70.77 H14A–C14–C16–H16A 58.71H15A–C15–C13–H13A 58.27 H15–C15–C13–H13A 69.92 H15A–C15–C13–H13A 58.51C28–C15–C16–C17 125.29 C28–C15–C16–C17 96.17 C17–C15–C16–C28 125.6

Fig. 2. Molecular structure of [(DEAM-BY)Rh(COD)]OTf (4) and [(DEAM-BY)Ir(COD)]OTf (6) with displacement ellipsoids drawn at the 50% probability level and protonsremoved for clarity.

R.J. Lowry et al. / Polyhedron 29 (2010) 553–563 561

Although the only chemical alteration between 2 and 3 occurson the heterocycle, the largest difference in their solid state struc-tures is the conformation of the ligand arms and benzyl groups.Torsion angles across the anthracene bridge differ by �10� (for 3

\C28–C15–C16–C17 = 108.96� and for 2 \C28–C15–C16–C17 = 119.12�). The larger torsion angle is due to the electrostaticrepulsion between the two positively charged heterocycles of 2.Formation of a neutral heterocycle reduces this repulsion in 3.

Table 3Catalytic hydroformylation 4.

Substrate Catalysta Solvent Conversion % b:lb % e.e.c

Styrene ±4 chloroform 80 94:6Styrene ±4 toluene 100 93:7

562 R.J. Lowry et al. / Polyhedron 29 (2010) 553–563

3.2.2. Comparison of mononuclear [(DEAM-BY)M(COD)]OTf complexes(where M = Rh (4), Ir (6)

X-ray diffraction studies confirm the structure and orientationof the C1 symmetric complexes 4 and 6 (Fig. 2). Not-surprisingly,the two mononuclear complexes form similar crystals structures.The crystal lattice of both complexes form monoclinic cells withP2(1)/n space groups. Each asymmetric unit contains a cationicmolecule, its triflate counter ion, and a solvent molecule. BothM(I) ions sit in a slightly distorted square–planar coordinationsphere. The NHCs occupy cis coordination sites opposite a chelatedcyclooctadiene (COD). Examination of the molecular structure sug-gests chelation forces the ligand to adopt a strained geometry. Thedihedral angles \C28–C15–C16–C17 of 95.8� for 4 and 96.17� for 6are significantly smaller than those for the organic precursor mol-ecules 2 and 3. Although ligand bite angles correspond to valuesexpected for square–planar complexes (\C20–Rh–C31 of89.93(8)� and \C20–Ir–C31 of 91.2(3)�), conformational strain in-duced by chelation is evident from unsymmetrical NHC–M bonds.This phenomenon is significant in the solid state structure of Rh(I)4. One NHC–M bond length, C31–Rh, is 2.037(2) Å correspondingclosely to the average Rh–NHC bond length found in the Cam-bridge Structural Database by Baba et al. [12]. The second Rh–NHC bond is longer at 2.068(2) Å for C20–Rh, well over 3r.Although the trend appears to continue in 6 the difference lieswithin the error of the data (2.031(9) Å for Ir–C20 and 2.059(8) Åfor Ir–C31).

Binding of NHCs to metal centers is believed to be highly elec-trostatic in nature, allowing the NHC to retain much of its free car-bene character. Supporting this assertion, the N–C–N bond lengthsand angles of 4 and 6 correspond closely to those noted for freecarbene 3. The \N–C–N of 3 is slightly smaller than in the metalcomplexes whereas the \N–C–N of 2 is approximately 5� widerthan in 4 or 6. The N–Ccarbene bond lengths of each chelate complexresemble the slightly longer bonds of compound 3 rather than thebonds of 2 (Table 2).

Styrene �4 toluene 100 93:7 0Vinyl acetate ±4 chloroform 21 96:4Vinyl acetate �4 toluene 45 95:5 0

a Catalyst = 0.1 mol%, substrate = 50 mg, substrate/Rh = 1000, pressure = 100 bar(CO/H2 = 1/1), temperature = 50 �C, time = 24 h.

b Ratio of branched to linear determined by GC.c % e.e. determined by GC.

3.2.3. Comparison of mononuclear [l-DEAM-BY][M(COD)]2 complexes(where M = Rh (5) and Ir (7)

Fig. 3 displays the X-ray crystal structures of bimetallic com-plexes 5 and 7. As with the monometallic compounds, 5 and 7 formcrystal lattices with similar characteristics. Both molecules crystal-

Fig. 3. Molecular structure of [l-DEAM-BY][M(COD)]2 complexes (where M = Rh (5) andremoved for clarity.

lize in the P�1 space group. The solution-state assignment of C2

symmetry for the dinuclear complexes is retained in the solid-statestructures. The C2 axis bisects the C15–C16 bond, passing throughthe center of the dihydroethanoanthracene backbone. Two neutralM(I) centers are bridged by the chiral ligand, each NHC acting as amonodentate ligand in a distorted square–planar coordinationsphere that includes a chloride and a chelating COD. The NHC moi-eties parallel each other but face opposite directions, situating themetal centers directly above opposing backbone aryl rings. Thebenzyl substituents are twisted in such a manner to allow closecontact (�2.7 Å) to the heterocyclic protons. The absence of strainfrom chelation permits M–CNHC bond lengths (d(Rh1–C20) = 2.021(5), d(Rh2–C31) = 2.024(5), and 2.033(3) Å for bothIr1–C20 and Ir2–C31) that correspond to reported literature values[12]. The large torsion angles of \C28–C15–C16–C17 of 125.29�and 125.6� for 5 and 7, respectively, could be the consequence ofa weak p–p interaction between the imidazole rings, and again sig-nifies the apparent flexibility of the ligand.

3.3. Hydroformylation of styrene and vinyl acetate using 4 as theprecatalyst

Complex 4 was chosen to test for hydroformylation activitywith the substrates styrene and vinyl acetate. Table 3 lists the cat-alytic results. Each reaction entry describes a reaction with 50 mgof substrate, 0.1 mol% catalyst loading, and 1:1 CO/H2 gas(100 bar). With styrene as substrate, the product aldehydes form

Ir (7). Displacement ellipsoids are drawn at the 50% probability level and protons

R.J. Lowry et al. / Polyhedron 29 (2010) 553–563 563

in 80% and 100% conversion when chloroform and toluene are thesolvents, respectively. The branched to linear ratio of product alde-hydes are surprisingly high (94:6), though when enatiopure 4 isused no detectable enantiomeric excess (e.e.) is found in thebranched aldehyde 2-phenylpropanal. For vinyl acetate the resultsare similar including 0% e.e. when enantiopure 4 is the precatalyst.These result mirror previous attempts to use di-NHC ligated Rh(I)complexes [7c] and other Rh(I) precatalysts under hydroformyla-tion conditions [13]. One plausible explanation is the catalyst de-grades to RhH(CO)4 via di-NHC substitution by CO which isknown to provide identical b:l ratios. Alternatively, as a conse-quence of performing the reaction at 50 �C, the intact catalystmay not provide chiral induction due to ligand flexibility.

4. Supplementary data

CCDC 730377, 730378, 730379, 730380, 70381, and 730382contain the supplementary crystallographic data for compounds5, 4, 3, 2, 7, and 6, respectively. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:deposit@ccdc.cam.ac.uk.

Acknowledgements

The authors thank the University of Florida for financial supportand K.A.A. thanks the National Science Foundation for funding thepurchase of X-ray equipment.

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