Differentially Substituted Acyclic Diaminocarbene Ligands Display

Jun 9, 2010 - Hwimin Seo , Benjamin P. Roberts , Khalil A. Abboud , Kenneth M. Merz , Jr. , and Sukwon Hong. Organic Letters 2010 12 (21), 4860- .... ...
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Organometallics 2010, 29, 3047–3053 DOI: 10.1021/om1004226

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Differentially Substituted Acyclic Diaminocarbene Ligands Display Conformation-Dependent Donicities Mary S. Collins, Evelyn L. Rosen, Vincent M. Lynch, and Christopher W. Bielawski* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 Received May 5, 2010

Complexes of the type [(L)Ir(COD)Cl] and [(L)Ir(CO)2Cl] (L = N,N0 -dimesityl-N,N0 -dimethylformamidin-2-ylidene (3) and N,N0 -bis(2,6-di-isopropylphenyl)-N,N0 -dimethylformamidin-2-ylidene (4); COD = cis,cis-1,5-cyclooctadiene) were synthesized and studied in solution as well as in the solid state. While the acyclic diaminocarbene (ADC) ligand in [(3)Ir(COD)Cl] adopted a conformation in which the N-aryl substituents were anti with respect to the coordinated metal, the respective Ir carbonyl complex was prepared as separable isomers ([(anti-3)Ir(CO)2Cl] and [(amphi-3)Ir(CO)2Cl]). The ADC ligands in [(4)Ir(COD)Cl] and [(4)Ir(CO)2Cl] adopted exclusively amphi conformations, where one Naryl substituent was oriented toward the coordinated metal and the other was oriented away. The Tolman electronic parameter (TEP) for anti-3 (2047.8 cm-1) was derived from the carbonyl stretching energy (νCO) of the aforementioned Ir(CO)2Cl complex and was found to be larger than the TEPs calculated for amphi-3 (2044.4 cm-1) and 4 (2044.0 cm-1). Likewise, the oxidation potential of [(anti-3)Ir(CO)2Cl], as measured by cyclic voltammetry, was found to be significantly higher (1.57 V) than the analogous oxidation potentials measured for [(amphi-3)Ir(CO)2Cl] (1.26 V) and [(4)Ir(CO)2Cl] (1.24 V). Introduction N-Heterocyclic carbenes (NHCs)1 have proven to be useful ligands for a broad range of transition metals, particularly for applications in catalysis.2 Much of their widespread success can be attributed to their remarkable steric and electronic properties, both of which may be modified using established substitution or functionalization methodologies. Although NHCs with a wide variety of N-substituents are known,1,2 those featuring bulky 2,4,6-trimethylphenyl (mesityl) or 2,6-di-isopropylphenyl (Dipp) groups (e.g., 1 and 2 in Figure 1) are common because they provide steric protection to metals upon coordination, are often kinetically stable, and, in some cases, are commercially available.3 NHCs are also strong electron donors, a feature that frequently enhances the catalytic activities of metal complexes that utilize these ligands, especially when compared to analogous catalysts that contain phosphines.2 Consequently, numerous reports have sought to quantify, both experimentally4,5 and

computationally,6 the electron-donating abilities of NHCs to better predict and to understand their unique characteristics.

*To whom correspondence should be addressed. E-mail: bielawski@ cm.utexas.edu. (1) (a) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913–921. (b) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247–2273. (c) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (2) For reviews of catalytically active transition metal complexes containing NHCs, see: (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (b) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239–2246. (c) Cavell, K. J.; McGuiness, D. S. Coord. Chem. Rev. 2004, 248, 671–681. (d) Kantchev, E. A. B.; O'Briend, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768–2813. (e) Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. Rev. 2007, 251, 726–764. (f) Díez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676. (g) Samojzowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708–3742. (3) (a) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 10, 1815– 1828. (b) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet. Chem. 2005, 690, 5407–5413. (c) Díez-Gonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874–883.

(4) For recent studies of the electron-donating abilities of NHCs, see: (a) Frey, G. D.; Rentzsch, C. F.; Preysing, D. V.; Scherg, T.; M€ uhlhofer, M.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem. 2006, 691, 5725–5738. (b) Leuth€aeusser, S.; Schwarz, D.; Plenio, H. Chem.;Eur. J. 2007, 13, 7195–7203. (c) F€urstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129, 12676–12677. (d) Khramov, D. M.; Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch, V. M.; Bielawski, C. W. Tetrahedron 2008, 64, 6853–6862. (e) Song, G.; Zhang, Y.; Li, X. Organometallics 2008, 27, 1936–1943. (f) Rosen, E. L.; Varnado, C. D., Jr.; Tennyson, A. G.; Khramov, D. M.; Kamplain, J. W.; Sung, D. H.; Cresswell, P. T.; Lynch, V. M.; Bielawski, C. W. Organometallics 2009, 28, 6695–6706. (g) Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Eur. J. Inorg. Chem. 2009, 1913–1919. (h) Benhamou, L.; Vujkovic, N.; Cesar, V.; Gornitzka, H.; Lugan, N.; Lavigne, G. Organometallics 2010, 29, DOI: 10.1021/om100233y. (5) Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202–210. (6) For recent reports, see: (a) Gusev, D. G. Organometallics 2009, 28, 763–770. (b) Tonner, R.; Frenking, G. Organometallics 2009, 28, 3901– 3905. (c) Gusev, D. G. Organometallics 2009, 28, 6458–6461. (d) Fey, N.; Haddow, M. F.; Harvey, J. N.; McMullin, C. L.; Orpen, A. G. Dalton Trans. 2009, 8183–8196. (7) (a) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1121–1123. (b) Alder, R. W.; Blake, M. E. Chem. Commun. 1997, 1513–1514. (c) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, € M. J. Chem. Commun. 1999, 241–242. (d) Herrmann, W. A.; Ofele, K.; Preysing, D. V.; Herdtweck, J. J. Organomet. Chem. 2003, 684, 235–248. (e) Otto, M.; Conejero, S.; Canac, Y.; Romanenko, V. D.; Rudzevitch, V.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 1016–1017. (f) Frey, G. D.; Herrmann, W. A. J. Organomet. Chem. 2005, 690, 5876–5880. (g) Frey, G. D.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem. 2006, 691, 2465–2478. (h) Wanniarachchi, Y. A.; Slaughter, L. M. Chem. Commun. 2007, 3294–3296. (i) Wanniarachchi, Y. A.; Slaughter, L. M. Organometallics 2008, 27, 1055–1062. (j) Bartolome, C.; Carrasco-Rando, M.; Coco, S.; Cordovilla, C.; Martín-Alvarez, J. M.; Espinet, P. Inorg. Chem. 2008, 47, 1616–1624. (k) Snead, D. R.; Ghiviriga, I.; Abboud, K. A.; Hong, S. Org. Lett. 2009, 11, 3274–3277. (l) Ruiz, J.; Perandones, B. F. Organometallics 2009, 28, 830–836.

r 2010 American Chemical Society

Published on Web 06/09/2010

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Figure 1. Structures of NHCs 1 and 2 as well as ADCs 3 and 4. The conformations shown for the ADCs are consistent with their solution and solid-state structures (see text).

Figure 2. Possible conformations adopted by an N,N0 -diphenylN,N0 -dimethyl ADC ligated to a transition metal complex ([M]). The nomenclature used to describe each conformation shown was based on the relationship between the coordinated metal and the N-substituents, in accord with literature precedence for related aminidium ions and other ADCs.7i,14

A rapidly emerging class of ligands that may expand the favorable characteristics of NHCs is acyclic diaminocarbenes (ADCs) (e.g., 3 and 4).7,8 Due to their relatively wide N-C-N bond angles, the steric properties of ADCs may be considered to be more encumbering than their NHC analogues. Likewise, ADCs may also be regarded as stronger donors than NHCs on account of their high basicities,9 although reports that comprehensively compare the steric and electronic properties of these two classes of ligands are relatively scarce.10 Another potentially useful feature of ADCs, particularly when compared to NHCs, is rotational freedom.11 As illustrated in Figure 2, differentially substituted ADCs are capable of adopting multiple conformations,12 each of which may place ligated metal centers in distinct steric environments or exhibit different electronic properties.13

Herein we study how the electron-donating abilities of differentially substituted ADCs are influenced by their conformations. These efforts were prompted by the recent discovery12b that, depending on the metal center to which it was coordinated, ADC 3 was capable of adopting either anti or amphi conformations (the nomenclature used was based on the relationship of the N-aryl substituents and the ligated metal; see Figure 2).7i,14 To facilitate comparison with other ligands, particularly NHCs, reported in the literature,4,5 Ir(COD)Cl (COD=cis,cis-1,5-cyclooctadiene) and Ir(CO)2Cl complexes containing 3 were synthesized and studied using a variety of spectroscopic and electrochemical methods. Analogous complexes containing 4, an ADC that displays restricted conformational freedom relative to 3, presumably due to its increased steric bulk (vide infra), were also evaluated as comparative controls.

(8) For examples and discussions of ADC ligands used in catalytically active metal complexes, see: (a) Kremzow, D.; Seidel, G.; Lehmann, C.; F€ urstner, A. Chem.;Eur. J. 2005, 11, 1833–1853. (b) Moncada, A. I.; Manne, S.; Tanski, J. M.; Slaughter, L. M. Organometallics 2006, 25, 491– 505. (c) Dhudshia, B.; Thadani, A. N. Chem. Commun. 2006, 668–670. (d) Wanniarachchi, Y. A.; Kogiso, Y.; Slaughter, L. M. Organometallics 2008, 27, 21–24. (e) Slaughter, L. M. Comments Inorg. Chem. 2008, 29, 46– 72. (f) Bartolome, C.; Ramiro, Z.; Perez-Galan, P.; Bour, C.; Raducan, M.; Echavarren, A. M.; Espinet, P.; Palape, I. Inorg. Chem. 2008, 47, 11391– 11397. (g) Hirsch-Weil, D.; Snead, D. R.; Inagaki, S.; Seo, H.; Abboud, K. A.; Hong, S. Chem. Commun. 2009, 18, 2475–2477. (h) Wanniarachchi, Y. A.; Subramanium, S. S.; Slaughter, L. M. J. Organomet. Chem. 2009, 694, 3297–3305. (i) Luzyanin, K. V.; Tskhovrebov, A. G.; Carias, M. C.; da Silva, M. F. C. G.; Pombeiro, A. J. L.; Kukushkin, V. Y. Organometallics 2009, 28, 6559–6566. (j) Bartolome, C.; Ramiro, Z.; Garcí-Cuadrado, D.; Perez-Galan, P.; Raducan, M.; Bour, C.; Echavarren, A. M.; Espinet, P. Organometallics 2010, 29, 951–956. (9) (a) Denk, K.; Sirsch, P.; Herrmann, W. A. J. Organomet. Chem. 2002, 649, 219–224. (b) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717–8724. (c) Lee, M.-T.; Hu, C.-H. Organometallics 2004, 23, 976–983. (10) (a) Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F.; Sch€ utz, J. Angew. Chem., Int. Ed. 2004, 43, 5896–5911. (b) Vignolle, J.; Catto€en, X.; Bourissou, D. Chem. Rev. 2009, 109, 3333– 3384. (c) Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385–3407. (11) Alder, R. W.; Blake, M. E. J. Phys. Chem. A 1999, 103, 11200– 11211. (12) (a) Rosen, E. L.; Sanderson, M. D.; Saravanakumar, S.; Bielawski, C. W. Organometallics 2007, 26, 5774–5777. (b) Rosen, E. L.; Sung, D. H.; Chen, Z.; Lynch, V. M.; Bielawski, C. W. Organometallics 2010, 29, 250–256. (13) For applications of conformationally labile NHCs and ADCs, see: (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. Angew. Chem., Int. Ed. 2003, 42, 3690–3693. (b) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am. Chem. Soc. 2004, 126, 15195– 15201. (c) Vieille-Petit, L.; Luan, X.; Mariz, R.; Blumentrii, S.; Linden, A.; Dorta, R. Eur. J. Inorg. Chem. 2009, 1861–1870. (d) Snead, D.; Inagaki, S.; Abboud, K. A.; Hong, S. Organometallics 2010, 29, 1729–1739.

Experimental Section Materials and Methods. Benzene was distilled from sodium and benzophenone ketyl under an atmosphere of nitrogen and then degassed by three consecutive freeze-pump-thaw cycles. Toluene and CH2Cl2 were dried and degassed using a Vacuum Atmospheres solvent purification system and then subsequently stored over 3 A˚ molecular sieves. The following compounds were synthesized according to literature procedures: N,N0 -dimesityl-N,N0 -dimethylformamidin-2-ylidene (3),12b N,N0 -bis(2,6-di-isopropylphenyl)-N,N0 -dimethylformamidin-2-ylidene (4),12a N,N0 -dimesityl-N,N0 -dimethylformamidinium iodide ([3H][I]),12b N,N0 -bis(2,6-di-isopropylphenyl)-N,N0 -dimethylformamidinium iodide ([4H][I]),12a and [Ir(COD)Cl]2.15 All other materials and solvents were of reagent quality and were used as received from commercial sources. Unless otherwise noted, all manipulations were performed under an atmosphere of nitrogen using drybox or Schlenk techniques. Instrumentation. 1H and 13C{1H} NMR spectra were recorded using a Varian 300, 400, 500, or 600 MHz spectrometer. Chemical shifts δ (in ppm) were referenced to tetramethylsilane using the residual protio solvent as an internal standard (1H: CDCl3, 7.24 ppm; toluene-d8, 2.09 ppm; 13C: CDCl3, 77.0 ppm). Coupling constants (J) are expressed in hertz (Hz). Decomposition temperatures (Td) were determined by thermogravimetric analyses (TGA) using a Mettler Toledo TGA/SDTA851e instrument at a scan rate of 25 °C/min under an atmosphere of air. (14) Butler, W. M.; Enemark, J. H.; Parks, J.; Balch, A. L. Inorg. Chem. 1973, 12, 451–457. (15) Yang, D.; Long, Y.; Wang, H.; Zhang, Z. Org. Lett. 2008, 10, 4723–4726.

Article Electrochemical experiments were conducted on CH Instruments electrochemical workstations (series 660D and 700B) using a gastight, three-electrode cell under an atmosphere of dry nitrogen. The cell was equipped with gold working, tungsten counter, and silver quasi-reference electrodes. Measurements were performed in dry CH2Cl2 with 0.1 M [n-Bu4N][PF6] as the electrolyte and [(Me5Cp)2Fe] (Fc*) (Cp = cyclopentadienyl) as the internal standard. All potentials noted were determined at 100 mV s-1 scan rates and were referenced to saturated calomel electrode (SCE) by shifting (Fc*)0/þ to -0.057 V (CH2Cl2).16 IR spectra were recorded using a Perkin-Elmer Spectrum BX FTIR instrument. High-resolution mass spectra (HRMS) were obtained with a VG analytical ZAB2-E or a Karatos MS9 instrument (ESI or CI) and are reported as m/z (relative intensity). Elemental analyses were performed by Midwest Microlabs, LLC (Indianapolis, IN). [(3)Ir(COD)Cl] (3a). A 7.5 mL borosilicate glass vial equipped with a Teflon-coated magnetic stir bar was charged with [3H][I] (991 mg, 2.30 mmol), and a separate 7.5 mL glass vial equipped with a stir bar was charged with NaHMDS (420 mg, 2.30 mmol). After adding toluene (3 mL) to each vial, they were placed in a -25 °C freezer for 15 min. The toluene mixture containing NaHMDS was then added dropwise to the toluene mixture containing [3H][I] with stirring. After the addition was complete, stirring was continued at ambient temperature for 1 h. The resulting mixture was then filtered through a 0.20 μm PTFE filter into a separate 7.5 mL glass vial. The filtered solution was then charged with [Ir(COD)Cl]2 (763 mg, 1.14 mmol), and the resulting mixture was stirred at ambient temperature for 36 h. A yellow solid precipitated, which was collected via filtration and dried under vacuum. The filtrate was concentrated by evaporation under reduced pressure and purified by column chromatography (media SiO2; eluent 1:1 v/v hexanes/chloroform until bis(trimethylsilyl)amine eluted followed by chloroform). A yellow solution was obtained and subsequently concentrated under reduced pressure to recover additional product. The initial precipitate and chromatographically purified material were combined to afford the desired product as a bright yellow solid (590 mg, 40% yield). Td = 240 °C. 1H NMR (400 MHz, CDCl3): δ 6.40 (s, 2H, mesityl aryl), 6.36 (s, 2H, mesityl aryl), 4.54-4.52 (m, 2H, COD olefin trans relative to the ADC ligand), 4.00 (s, 6H, N-CH3), 3.43-3.42 (m, 2H, COD olefin cis relative to the ADC ligand), 2.30-2.26 (m, 2H, COD CH2), 2.18 (s, 6H, mesityl CH3), 2.05 (s, 6H, mesityl CH3), 1.97 (s, 6H, mesityl CH3), 1.71-1.54 (m, 6H, COD CH2). 13C NMR (100 MHz, CDCl3): δ 209.6, 142.0, 136.1, 134.8, 133.2, 128.7, 128.1, 81.1, 51.8, 46.7, 33.4, 29.2, 20.6, 19.5, 19.0. HRMS: [Mþ] calcd for C29H40N2193IrCl 644.2509; found 644.2499. Anal. Calcd (%) for C29H40N2O2IrCl: C, 54.06; H, 6.26; N, 4.35. Found: C, 53.99; H, 6.04; N, 4.32. [(4)Ir(COD)Cl] (4a). A 7.5 mL borosilicate glass vial equipped with a Teflon-coated magnetic stir bar was charged with [4H][I] (95.5 mg, 0.183 mmol), NaHMDS (34.3 mg, 0.183 mmol), and toluene (4 mL). The toluene mixture was stirred for 30 min at ambient temperature. The resulting mixture was then filtered through a 0.20 μm PTFE filter into a separate 7.5 mL glass vial containing [Ir(COD)Cl]2 (61.5 mg, 0.0898 mmol) and a stir bar. The combined mixture was stirred at ambient temperature for 24 h. Removal of solvent under reduced pressure afforded a dark red solid, which was then purified by column chromatography (media: silica gel; eluent: hexanes followed by 20:1 v/v hexanes/ethyl acetate) to afford the desired product as a bright yellow solid (65 mg, 50% yield). Td =225 °C. 1H NMR (400 MHz, CDCl3): δ 7.34-7.28 (m, 2H, Dipp aryl), 7.25-7.23 (m, 1H, Dipp aryl), 7.17-7.09 (m, 3H, Dipp aryl), 4.26-4.23 (m, 1H, COD olefin trans relative to the ADC ligand), 4.15 (s, 3H, N-CH3), 4.12 (sep, 1H, J = 6.8, Dipp CH), 4.01-3.97 (m, 1H, (16) (a) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910. (b) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713–6722.

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COD olefin cis relative to the ADC ligand), 3.87 (sep, 1H, J = 6.8, Dipp CH), 3.45-3.42 (m, 1H, COD olefin), 3.18 (sep, 1H, J=6.8, Dipp CH), 2.89 (sep, 1H, J=6.8, Dipp CH), 2.65 (s, 3H, N-CH3), 2.36-2.31 (m, 1H, COD olefin), 2.25-2.19 (m, 1H, COD CH2), 1.76-1.63 (m, 3H, COD CH2), 1.53-1.46 (m, 1H, COD CH2), 1.41 (d, 3H, J=6.4, Dipp CH3), 1.37-1.28 (5 overlapping d, 15H, N-CH3), 1.02-0.92 (m, 8H, Dipp CH3 and COD CH2), 0.78-0.70 (m, 1H, COD CH2). 13C NMR (100 MHz, CDCl3): δ 202.7, 145.6, 145.0, 143.6, 142.6, 142.5, 142.0, 127.4, 126.9, 123.5, 123.2, 122.2, 121.9, 81.4, 53.3, 52.3, 50.3, 46.9, 37.8, 32.5, 30.1, 30.0, 29.9, 29.7, 28.5, 28.3, 27.9, 27.4, 27.3, 27.1, 24.9, 24.2, 24.1, 23.9. HRMS: [Mþ] calcd for C35H52N2193IrCl 728.3448; found 728.3446. Anal. Calcd (%) for C35H52N2IrCl: C, 57.71; H, 7.19; N, 3.85. Found: C, 58.01; H, 7.19; N, 4.05. anti-[(3)Ir(CO)2Cl] (anti-3b). Under ambient atmosphere, a 20 mL borosilicate glass vial equipped with a Teflon-coated magnetic stir bar was charged with 3a (99.3 mg, 0.154 mmol) and CH2Cl2 (3 mL), and then sealed with a rubber septum. The reaction was placed in an ice bath, stirred for 5 min, and then purged with CO for 30 min at 0 °C. Residual COD was removed by the repeated addition of a minimal amount of cold pentane followed by evaporation under reduced pressure. The resulting solid was dried under reduced pressure to afford the desired product as a pale yellow powder (88 mg, 96% yield). The product contained greater than 98% of the anti isomer as determined by 1H NMR spectroscopy. Td=275 °C. 1H NMR (500 MHz, CDCl3, -30 °C): δ 6.44 (s, 4H, mesityl aryl), 3.78 (s, 6H, N-CH3), 2.13 (s, 6H, mesityl CH3), 2.07 (s, 6H, mesityl CH3), 2.05 (s, 6H, mesityl CH3). 13C NMR (125 MHz, CDCl3, -30 °C): δ 202.1, 180.2, 168.6, 141.3, 136.6, 133.8, 132.9, 128.5, 128.2, 47.6, 20.7, 19.3, 19.1. HRMS: [Mþ] calcd for C23H28N2O2IrCl 592.1469; found 592.1464. IR (CH2Cl2): 2063, 1979 cm-1. Anal. Calcd (%) for C23H28N2O2IrCl: C, 46.65; H, 4.77; N, 4.73. Found: C, 46.65; H, 4.71; N, 4.59. amphi-[(3)Ir(CO)2Cl] (amphi-3b). Under ambient atmosphere, a 7.5 mL borosilicate glass vial equipped with a Teflon-coated magnetic stir bar was charged with 3a (158.2 mg, 0.2455 mmol) and CH2Cl2 (5 mL), and then sealed with a septum. While stirring at ambient temperature, the resulting solution was purged with CO until the solvent had evaporated, which required approximately 3 h. Residual COD was removed by repeatedly dissolving the yellow solid in a minimal amount of pentane followed by evaporation under reduced pressure. The resulting solid was then dried under reduced pressure to afford a yellow powder, which was a mixture of both anti-3b and amphi-3b. The desired isomer was obtained as a yellow solid by gently washing the mixture with hexanes (3  2 mL) followed by drying under reduced pressure (55 mg, 38% yield). Td = 277 °C. 1H NMR (400 MHz, CDCl3): δ 6.91 (s, 1H, mesityl aryl), 6.90 (s, 1H, mesityl aryl), 6.88 (s, 1H, mesityl aryl), 6.86 (s, 1H, mesityl aryl), 3.84 (s, 3H, N-CH3), 3.84 (s, 3H, N-CH3), 2.51 (s, 3H, mesityl CH3), 2.48 (s, 3H, mesityl CH3), 2.37 (s, 3H, mesityl CH3), 2.28 (s, 9H, mesityl CH3), 2.26 (s, 3H, mesityl CH3). 13C NMR (100 MHz, CDCl3): δ 198.1, 179.8, 168.7, 144.8, 142.0, 138.6, 137.7, 135.7, 135.1, 134.0, 133.2, 129.7, 129.4, 129.03, 128.95, 49.5, 40.7, 21.02, 20.99, 19.4, 18.6, 18.1, 17.8. HRMS: [Mþ] calcd for C23H28N2O2IrCl 592.1469; found 592.1459. IR (CH2Cl2): 2059, 1975 cm-1. Anal. Calcd (%) for C23H28N2O2IrCl: C, 46.65; H, 4.77; N, 4.73. Found: C, 46.81; H, 4.86; N, 4.60. [(4)Ir(CO)2Cl] (4b). Under ambient atmosphere, a 5 mL flask was charged with 4a (32.8 mg, 0.0450 mmol), CH2Cl2 (2 mL), and a Teflon-coated magnetic stir bar. After stirring the resulting reaction mixture under a slight pressure of CO (balloon) for 24 h, the solvent was removed by evaporation under reduced pressure. Subsequent removal of residual COD was aided by the repeated addition of a minimal amount of pentane followed by evaporation under reduced pressure to afford the desired product as a pale yellow powder (29.4 mg, 97% yield). Td =264 °C. 1 H NMR (400 MHz, CDCl3): δ 7.38-7.31 (m, 2H, Dipp aryl), 7.20-7.17 (m, 4H, Dipp aryl), 3.96 (s, 1H, N-CH3), 3.89 (sep, 1H, J = 6.8, Dipp CH), 3.37 (sep, 1H, J = 6.8, Dipp CH), 3.25

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Scheme 1. Synthesis of Ir(COD)Cl and Ir(CO)2Cl Complexes Containing ADCs 3 and 4

(i) (a) NaHMDS (1.0 equiv), toluene, ambient temperature, 30 min. (b) [Ir(COD)Cl]2 (0.5 equiv), ambient temperature, toluene, 24-36 h. (ii) For anti3b (from 3a): CO (1 atm), CH2Cl2, 0 °C, 0.5 h. For amphi-3b or 4b (from 3a or 4a, respectively): CO (1 atm), CH2Cl2, ambient temperature, 24 h.

(sep, 1H, J = 6.8, Dipp CH), 3.12 (sep, 1H, J = 6.4, Dipp CH), 2.69 (s, 3H, N-CH3), 1.45 (d, 3H, J=6.8, Dipp CH3), 1.41-1.39 (2 overlapping d, 6H, J=6.8, Dipp CH3), 1.36-1.33 (2 overlapping d, 6H, J=6.8, Dipp CH3), 1.27 (d, 3H, J=6.8, Dipp CH3), 1.04 (d, 3H, J=6.8, Dipp CH3), 0.97 (d, 3H, J=6.8, Dipp CH3). 13 C NMR (100 MHz, CDCl3): δ 195.8, 177.2, 166.7, 144.40, 144.35, 143.0, 142.8, 142.4, 140.9, 128.1, 127.8, 123.4, 123.2, 122.7, 122.6, 53.1, 46.4, 30.4, 30.0, 29.9, 28.8, 28.4, 28.0, 27.4, 27.1, 24.7, 24.4, 24.1, 23.9. HRMS: [Mþ] calcd for C29H40N2O2IrCl 676.2408; found 676.2408. IR (CH2Cl2): 2059, 1974 cm-1. Anal. Calcd (%) for C29H40N2O2IrCl: C, 51.50; H, 5.96; N, 4.14. Found: C, 51.86; H, 6.05; N, 4.25.

Results and Discussion As summarized in Scheme 1, known12 formamidinium salts [3H][I] and [4H][I] were independently treated with NaHMDS (HMDS = hexamethyldisilazide) in toluene (to generate the respective free ADCs in situ)17 followed by the addition of [Ir(COD)Cl]2.18 The desired Ir complexes 3a and 4a were isolated in 40% and 50% yield, respectively, as bright yellow solids after purification via column chromatography. The 1H NMR spectrum (CDCl3) of 3a revealed a singlet at δ = 4.00 ppm, which was assigned to chemically equivalent N-methyl groups, consistent with the ADC adopting either an anti or syn conformation (e.g., see Figure 2). To elucidate the ground-state structure of 3a, a NOESY NMR experiment was performed; as summarized in Figure S6, the signals assigned to the N-methyl groups correlated with signals attributed to the Ir-bound olefin moieties (δ = 4.53 and 3.42 ppm). Hence, we believe that the ADC ligand in 3a adopted an anti conformation in solution, where its N-mesityl groups were oriented away from the coordinated metal center. In contrast, the ADC in 4a adopted an amphi conformation, as chemically inequivalent signals assigned to its N-methyl (δ = 4.15 and 2.65 ppm) and methine groups (δ=4.12, 3.87, 3.18, and 2.89 ppm) were observed in the 1H NMR spectrum (CDCl3) recorded for this complex.19 To support the aforementioned solution-state structure assignments, X-ray quality crystals of 3a and 4a were obtained independently by slow evaporation of their concentrated solutions in CH2Cl2 or hexanes, respectively. As shown in Figure 3 (left), the ADC ligand in 3a adopted an anti (17) The solid-state structures of 3 and 4 exhibited amphi conformations and were consistent with their solution-state structures (see Figures S8 and S9, respectively). To the best of our knowledge, only one other solid-state structure of an ADC has been previously reported.7a (18) For an earlier report of preparing formamidinium salts as precursors to ADCs, see: Alder, R. W.; Blake, M. E.; Bufali, S.; Butts, C. P.; Orpen, A. G.; Sch€ utz, J.; Williams, S. J. J. Chem. Soc., Perkin Trans. 1 2001, 1586–1593. (19) Heating a solution of 3a in toluene-d8 for 24 h at 100 °C did not result in any measurable formation of the amphi isomer by 1H NMR spectroscopy.

Figure 3. (left) ORTEP diagram of 3a showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Key distances (A˚), angles (deg), and torsions (deg): Ir-C1, 2.061(4); Ir-C6, 2.119(4); Ir-C7, 2.109(4); Ir-C8, 2.162(4); Ir-C9, 2.194(5); Ir-Cl, 2.363(1); N1-C1, 1.471(5); N2-C1, 1.350(5); Cl-Ir-C1, 89.1(1); N1-C1-N2, 118.9(4); C2-N1-C1-N2, -170.3(4); C3-N2-C1-N1, 178.9(4); C4-N1-C1-N2, 19.5(6); C5-N2-C1-N1, 17.8(6). (right) ORTEP diagram of 4a showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Key distances (A˚), angles (deg), and torsions (deg): Ir-C1, 2.082(4); Ir-C6, 2.106(6); Ir-C7, 2.125(6); IrC8, 2.173(5); Ir-C9, 2.178(5); Ir-Cl, 2.400(1); N1-C1, 1.344(5); N2-C1, 1.364(5); Cl-Ir-C1, 88.3(1); N1-C1-N2, 119.1(4); C2-N1-C1-N2, -7.3(6); C3-N2-C1-N1, 178.3(3); C4-N1C1-N2, 168.2(4); C5-N2-C1-N1, 2.8(7).

conformation, which was consistent with the solution-based structural analysis described above. More specifically, the N-mesityl substituents in 3a were orthogonal to the N-C-N plane, oriented away from the coordinated metal and in close proximity to each other.20 The amphi conformation adopted by the ADC ligand in 4a was also consistent with its solutionderived assignment (see Figure 3, right). Although the Ccarbene-Ir distance observed in 3a was shorter (2.061(4) A˚) than the analogous distance observed in 4a (2.082(4) A˚), the measured values compared favorably with those reported for related Ir complexes containing NHCs (2.041(3)2.090(13) A˚).5,21 Additionally, the average distances of the Ir-olefin moieties trans (2.178(5) A˚ for both 3a and 4a) and cis (2.114(4)-2.116(6) A˚) to the ADC ligands in 3a and 4a were within the corresponding ranges observed for previously reported NHC-containing Ir(COD)Cl complexes (trans: 2.1635(7)-2.1845(8) A˚; cis: 2.0995(8)-2.125(7) A˚).5,21 The (20) The centroids of the arene faces were slightly offset and measured to be 3.55 A˚ apart, a value that is in accord with a displaced π-π interaction. See: (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–5534. (b) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885–3896. (21) (a) Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.; Fallis, A. A.; Cavell, K. J. Dalton Trans. 2009, 7099–7112. (b) Iglesias, M.; Beetstra, D. J.; Stasch, A.; Horton, P. N.; Hursthouse, M. B.; Coles, S. J.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2007, 26, 4800–4809.

Article

N-C-N angles measured in the solid-state structures of 3a and 4a were nearly identical (118.9(4)° and 119.1(4)°, respectively) to each other as well as to their free ADCs17 3 and 4 (119.4(2)° and 120.3(1)°, respectively) but contracted when compared to the analogous angles observed in the solid-state structures of their formamidinium salt precursors [3H][I] and [4H][I] (129.5(2)° and 129.6(3)°, respectively).12 These structural features may be rationalized by the relative amount of p-character formed at the free carbene versus formamidinium nuclei. Independently stirring CH2Cl2 solutions of complexes 3a and 4a under an atmosphere of carbon monoxide (CO) afforded carbonyl complexes 3b and 4b, respectively. The outcome of aforementioned carbonylation involving 3a was found to be strongly dependent on the temperature at which the reaction was performed. For example, a mixture of anti3b (δ=3.80 ppm) and amphi-3b (δ=3.84, 2.51 ppm; CDCl3) was obtained in a 1:4 molar ratio when the carbonylation reaction was conducted at ambient temperature, as determined by 1H NMR spectroscopy using the signals attributed to the N-methyl substituents. However, when the same carbonylation reaction was repeated at 0 °C, the formation of anti-3b was strongly favored.22 The solution structure of anti-3b was confirmed by a NOESY NMR experiment (see Figure S6) and found to be similar to that of 3a, where both of the N-mesityl groups of the ADC ligand were oriented away from the Ir center. It is also noteworthy that anti-3b was found to slowly isomerize to its amphi isomer over a period of days in solution; however, heating this complex in toluene at 100 °C for 48 h afforded a mixture of the anti and amphi isomers in a 1:4 molar ratio, respectively, as determined by 1H NMR spectroscopy.23 Washing this mixture with cold hexanes was found to selectively remove the minor isomer and facilitated separation of amphi-3b. The 1H NMR spectrum of this isolated compound exhibited N-methyl signals at δ = 3.84 and 2.51 ppm (CDCl3), which supported the preliminary assignments of the mixtures of isomers described above. Consistent with the results obtained for 4a, the ADC ligand in 4b adopted an amphi conformation in solution, as evidenced by 1H NMR signals attributed to its inequivalent N-methyl substituents (δ = 3.96 and 2.69 ppm; CDCl3). X-ray quality crystals of anti-3b, amphi-3b, and 4b24 were obtained independently by slow evaporation of saturated solutions of the aforementioned complexes in dry hexanes. The N-C-N bond angles for anti-3b (120.4(2)°), amphi-3b (122.0(2)°), and 4b (122.2(2)°) as well as their Ccarbene-Ir distances (2.119(2), 2.117(3), and 2.115(2) A˚, respectively) (22) Negligible amounts (less than 2%) of amphi-3b were visible by 1H NMR spectroscopy. (23) Elucidation of activation parameters for the conversion of anti-3a to amphi-3a in toluene-d8 revealed that the isomerization process was entropically driven (ΔHq =80.1 kJ mol-1; ΔSq =68.0 J mol-1 K-1, ΔGq = 59.5 kJ mol-1; Figure S7), a likely consequence of the decreased symmetry in the amphi isomer. (24) A unique bimetallic decomposition product (4c) was obtained while attempting to grow X-ray quality crystals of 4b by slow diffusion of methanol into a saturated ethyl acetate solution under ambient conditions. As shown in Figure S10, the Ir center inserted into a C-H bond of an N-methyl group oriented toward the metal, which was accompanied with loss of the trans CO ligand (relative to the ADC) and concomitant bridging of two molecules of hydroxide. Some [Ir(CO)2Cl] complexes supported by NHCs have been reported to form chloro-bridged dimers with concomitant loss of CO. For examples, see: (a) Sanderson, M. D.; Kamplain, J. W.; Bielawski, C. W. J. Am. Chem. Soc. 2006, 128, 16514– 16515. (b) Barluenga, J.; Vicente, R.; Lopez, A.; Tomas, M. J. Organomet. Chem. 2006, 691, 5642–5647. (c) Bittermann, A.; H€arter, P.; Herdtweck, E.; Hoffmann, S. D.; Herrmann, W. A. J. Organomet. Chem. 2008, 693, 2079– 2090.

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Figure 4. (top left) ORTEP diagram of anti-3b showing 50% probability ellipsoids. Hydrogen atoms and lower occupancy atoms of the disordered carbonyl and chloro groups have been omitted for clarity. Key distances (A˚), angles (deg), and torsions (deg): Ir-C1, 2.119(2); Ir-C6, 1.863(7); Ir-C7, 1.896(3); Ir-Cl, 2.351(2); N1-C1, 1.346(3); N2-C1, 1.344(3); Cl-Ir-C1, 88.70(8); Cl-Ir-C6, 177.7(3); N1-C1-N2, 120.4(2); C2-N1-C1-N2, -175.4(2); C3-N2-C1-N1, -178.7(2); C4-N1-C1-N2, 22.2(3); C5-N2-C1-N1, 19.2(3). (top right) ORTEP diagram of amphi-3b showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Key distances (A˚), angles (deg), and torsions (deg): Ir-C1, 2.115(2); Ir-C6, 1.867(4); Ir-C7, 1.888(3); Ir-Cl, 2.321(9); N1-C1, 1.339(3); N2-C1, 1.338(4); Cl-Ir-C1, 87.66(7); Cl-Ir-C6, 172.1(1); N1-C1-N2, 122.0 (2); C2-N1-C1-N2, -1.3(4); C3-N2-C1-N1, 176.3(2); C4-N1-C1-N2, 179.3(2); C5-N2-C1-N1, 0.4(4). (bottom) ORTEP diagram of 4b showing 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Key distances (A˚), angles (deg), and torsions (deg): Ir-C1, 2.117(3); Ir-C6, 1.907(4); Ir-C7, 1.885(3); Ir-Cl, 2.378(1); N1-C1, 1.349(3); N2-C1, 1.336(3); Cl-Ir-C1, 87.15(8); Cl-Ir-C6, 169.7(1); N1C1-N2, 122.2(2); C2-N1-C1-N2, -4.8(4); C3-N2-C1-N1, -177.7(2); C4-N1-C1-N2, 178.0(2); C5-N2-C1-N1, 2.7(4).

were comparable to each other. However, the Ir-Ccarbene distances measured in these structures were relatively long when compared to those typically observed for analogous complexes containing NHCs (2.071(4)-2.121(14) A˚)5,21b presumably due to the wide N-C-N bond angles of ADCs and, consequently, increased steric interactions with the ligated metal centers. The cis and trans (relative to the ADC ligands) Ir-CO distances observed in anti-3b, amphi-3b, and 4b (cis: 1.863(7)-1.907(4) A˚; trans: 1.888(3)-1.896(3) A˚) were comparable to the corresponding ranges observed in analogous complexes containing NHCs (1.611(6)-1.893(4) and 1.847(6)-1.959(4) A˚, respectively).5,21b With the aforementioned Ir olefin and carbonyl complexes in hand, attention shifted toward evaluating the electronic properties of these systems. Initial efforts were directed toward determining the Tolman electronic parameters (TEPs) of 3 and 4 as such values are commonly used to compare the electron-donating abilities of various ligands.4,6 The TEP was initially defined as the A1 stretching mode of the carbonyls in various [(PR3)Ni(CO)3] (R=aryl, alkyl) complexes25 (25) Tolman, C. A. Chem. Rev. 1977, 77, 313–348.

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Collins et al.

Table 1. Carbonyl Stretching Energies and Derived TEPs of Selected (L)Ir(CO)2Cl Complexesa (L)Ir(CO)2Cl, where L = anti-3 amphi-3 4 15 25 55 628 79a

νCO (Ir) (cm-1)

νav (Ir) (cm-1)

TEP (cm-1)

2063, 1979 2059, 1975 2059, 1974 2066, 1980 2067, 1981 2068, 1981

2021.0 2017.0 2016.5 2023.0 2024.0 2024.5 2006.2c 2007.5c

2047.8 2044.4 2044.0 2049.5 (2050.7)b 2050.3 (2051.5)b 2050.8 2035.3 2036.4

a IR data obtained in CH2Cl2. TEP of L = 0.847  νCOav(Ir) + 336 cm-1.5 b Values in parentheses were determined from the A1 stretching mode in the respective [(NHC)Ni(CO)3] complexes for comparison.5 c Values of νCOav(Rh) were first converted to νCOav(Ir) using the equation27 νCOav(Ir) = 0.8695  νCOav(Rh) + 250.7 cm-1.

and was designed to provide a sensitive measure of a phosphine’s donating ability independent of steric effects. To circumvent handling of toxic Ni(CO)4, Crabtree26 developed a linear correlation between the TEP and the average IR stretching frequency observed in complexes of the type [(PR3)Ir(CO)2Cl] (R = aryl, alkyl), which was later extended to include common NHCs by Nolan5 and analogous Rh carbonyl complexes by Plenio.27 The νCO for complexes 3b and 4b were measured in CH2Cl2 by IR spectroscopy, and the TEPs of the respective ADCs were determined using aforementioned methods; the results are summarized in Table 1. The TEP for amphi-3 (2044.4 cm-1) was found to be significantly different than that of anti-3 (2047.8 cm-1), a result that suggested differentially substituted ADCs display conformation-dependent electron donicities. For comparison, the difference in the TEPs derived for NHC 1 and its saturated analogue (1,3-dimesitylimidazolin-2ylidene (5); structure not shown) or for a series of diaminocarbenes containing N-mesityl versus N-Dipp substituents (i.e., amphi-3b versus 4b or 1 versus 2) is only 0.9 cm-1 or less.5 Regardless of conformation, ADCs 3 and 4 appeared to be stronger donors than their five-membered NHC analogues5 1 and 2 but weaker than the six-membered NHC analogue 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene (6)28 and bis(di-isopropylamino)carbene (7)9a (structures not shown). Considering that the oxidation potentials of transition metal complexes are influenced by the donating abilities of their ligands, Ir(COD)Cl complexes 3a and 4a as well as Ir (CO)2Cl complexes 3b and 4b were further evaluated using cyclic voltammetry (CV); the results are summarized in Table 2. The IrI/II redox couples for 3a and 4a were measured at 0.82 and 0.69 V (versus SCE), respectively, values that were within the range of those reported for analogous Ir (COD)Cl complexes containing NHCs (0.59-1.04 V).4b,f,29 Considering that the calculated TEPs of 3 and 4 varied by only 0.4 cm-1 when these ADCs were in the same amphi-conformation, (26) (a) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (b) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461– 2468. (27) Wolf, S.; Plenio, H. J. Organomet. Chem. 2009, 694, 1487–1492. (28) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R. Chem.;Eur. J. 2004, 10, 1256–1266. (29) (a) Vorfalt, T.; Leuth€ausser, S.; Plenio, H. Angew. Chem., Int. Ed. 2009, 48, 5191–5194. (b) Tennyson, A. G.; Rosen, E. L.; Collins, M. S.; Lynch, V. M.; Bielawski, C. W. Inorg. Chem. 2009, 48, 6924–6933. (c) Tennyson, A. G.; Ono, R. J.; Hudnall, T. W.; Khramov, D. M.; Er, J. A. V.; Kamplain, J. W.; Lynch, V. M.; Sessler, J. L.; Bielawski, C. W. Chem.;Eur. J. 2010, 16, 304–315.

Table 2. Summary of Electrochemical Properties for Selected Complexesa complex

E1/2 (V)b

complex

Epa (V)c

3a

0.82, 0.60d

4a

0.69

anti-3b amphi-3b 4b

1.57 1.26 1.24

a

Measurements were performed in CH2Cl2 containing 0.1 M [Bu4N][PF6] at 100 mV s-1 scan rate and were referenced to Fc* as an internal standard adjusted to -0.057 V vs SCE.16 b The Ir(COD)Cl complexes exhibited quasi-reversible oxidation processes and are reported as halfwave potentials (E1/2). c The [Ir(CO)2Cl] complexes exhibited irreversible oxidation processes and are reported as anodic peak potentials (Epa). d A minor quasi-reversible feature was observed at the noted potential.

the difference in the oxidation potentials exhibited by 3a and 4a appeared significant. This difference was attributed primarily to the conformational-dependent donicities of the corresponding ADCs as opposed to their differing N-aryl substituents (mesityl versus Dipp). In contrast, the Epa values measured for the respective Ir carbonyl complexes containing ADCs that adopted amphi conformations were similar (compare 1.26 V for amphi-3b versus 1.24 V for 4b), although their absolute potentials were significantly higher than their COD derivatives, as expected.4f,29b,c Remarkably, the oxidation potential measured for anti-3b (1.57 V) was over 300 mV higher than that measured for amphi-3b. This result indicated that the former complex possessed a relatively electron-deficient Ir center and was consistent with the decreased ligand donicity observed in anti-3 versus amphi-3, as described above. Collectively, the IR spectroscopy and electrochemical results suggested to us that diaminocarbenes may be more than pure σ-donors.24a,30 Recent theoretical studies have claimed that bulky NHCs display decreased MfCcarbene π-back-bonding interactions relative to smaller analogues.9c Hence, compared to its amphi isomer, ADC 3 may facilitate such interactions in its metal complexes when in an anti conformation and, as a result of decreased electron density, explain why anti-3b was observed to oxidize at a higher potential than its isomer amphi-3b. To quantify the steric properties in the aforementioned complexes, the percent buried volume (%VBur), which measures the theoretical spherical volume occupied by a ligand coordinated to a metal center,31 was calculated for the ADC ligands in anti-3b, amphi-3b, and 4b using the method reported by Cavallo.32 As expected and in (30) For discussions of π-back-bonding interactions in NHC-M complexes, see: (a) Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755–764. (b) Nemcsok, D.; Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640–3646. (c) Jacobsen, H.; Correa, A.; Costabile, C.; Cavallo, L. J. Organomet. Chem. 2006, 691, 4350–4358. (d) Mercs, L; Labat, G.; Neels, A.; Ehlers, A.; Albrecht, M. Organometallics 2006, 25, 5648–5656. (e) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organometallics 2007, 26, 6042–6049. (f) Radius, U.; Bickelhaupt, M. Organometallics 2008, 27, 3410–3414. (g) Srebro, M.; Michalak, A. Inorg. Chem. 2009, 48, 5361–5369. (h) Antonova, N. S.; Carbo, J. J.; Poblet, J. M. Organometallics 2009, 28, 4283–4287. (i) Jacobsena, H.; Correab, A.; Poaterb, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687–703. (31) (a) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. Organometallics 2003, 22, 4322–4326. (b) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A., III; Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P. Organometallics 2004, 23, 1629– 1635. (c) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485–2495. (d) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880–5889. (e) Clavier, H.; Correa, A.; Cavallo, L.; Escudero-Adan, E. C.; Benet-Buchholz, J.; Slawin, A. M. Z.; Nolan, S. P. Eur. J. Inorg. Chem. 2009, 1767–1773. (32) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759–1766.

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support of our rationalization, the ADC ligand in anti-3b was considerably less bulky (%VBur = 30.7) than the ADC ligands in amphi-3b and 4b (%VBur = 35.6 and 34.9, respectively).33 However, we acknowledge that the Ir-Ccarbene distances measured in the X-ray crystal structures of anti-3b, amphi-3b, and 4b were nearly identical, and therefore an alternative rationale explaining the different donicities observed may stem from through-space or “π-face donor” interactions, as described by Plenio,34 between the N-arene moieties and their juxtaposed metal centers in amphi-3b and 4b.35,36

Conclusions In conclusion, five new Ir complexes containing differentially substituted ADCs 3 and 4 were synthesized and studied in solution as well as in the solid state. Depending on the N-substituents and the ancillary ligands coordinated to the Ir centers, different ADC conformations were observed. When in an amphi conformation, the aforementioned ADC ligands appeared to exhibit similar donicities. However, the anti conformation of 3, where both of its N-mesityl substituents were oriented away from the coordinated metal center, (33) The %VBur values were calculated with Bondi radii scaled by 1.17, 3.5 A˚ radius of the sphere, and 2.10 A˚ distance of the ligand from the sphere. (34) (a) S€ ussner, M.; Plenio, H. Chem. Commun. 2005, 5417–5419. (b) Leuth€aeusser, S.; Schmidts, V.; Thiele, C. M.; Plenio, H. Chem.;Eur. J. 2008, 14, 5465–5481. (35) The distances between the Ir atoms and the centroids of neighboring arene rings in amphi-3b and 4b were measured to be 3.87 and 3.95 A˚, respectively. For additional information on cation-π interactions, see: Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303–1324. (36) Anchimeric assistance and rehybridization effects at the carbene nucleus may also contribute to the conformation-dependent donicities observed.

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appeared to be a weaker donor than its amphi conformer, which may be due to increased MfCcarbene π-back-bonding or through-space, arene-metal interactions. These results, in accordance with the growing evidence presented by others,3a,4b,6a,d,31c,37 suggest that TEPs may not adequately describe the electron-donating properties of diaminocarbenes. While cyclic voltammetry proved to be a very sensitive measure of the electronic nature of the ligated metals, we appreciate that this technique may not be general, as metal carbonyl complexes often have inaccessible oxidation potentials.4f,29b,c Collectively, these results establish a new design parameter for fine-tuning the electronic properties of transition metal complexes that contain differentially substituted ADCs and may unlock novel opportunities for switching the activities or selectivities of metal-based catalysts that contain them. Moreover, these results indicate that changes in the steric properties of conformationally fluxional diaminocarbenes may significantly affect their electron-donating abilities and thus alter the expected outcomes of catalysts designed around such types of ligands.13

Acknowledgment. We are grateful to the National Science Foundation (CHE-0645563) and the Robert A. Welch Foundation (F-1621) for their generous financial support. We would like to thank Steve Sorey for his help with variable-temperature NMR studies and NOESY experiments. Supporting Information Available: Cyclic voltammograms, NMR spectra, kinetics data, and crystallography data are available free of charge via the Internet at http://pubs.acs.org. (37) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451–5457.