Synthesis and Reactivity of an Iridium (I) Acetonyl PNP Complex

Aug 9, 2010 - D labeling and DFT studies indicate that the H−H and C−H bond activation ..... Jacorien Coetzee , Graham R. Eastham , Alexandra M. Z...
3 downloads 0 Views 3MB Size
Download PDF
Organometallics 2010, 29, 3817–3827 DOI: 10.1021/om1004435

3817

Synthesis and Reactivity of an Iridium(I) Acetonyl PNP Complex. Experimental and Computational Study of Metal-Ligand Cooperation in H-H and C-H Bond Activation via Reversible Ligand Dearomatization Leonid Schwartsburd,† Mark A. Iron,‡ Leonid Konstantinovski,‡ Yael Diskin-Posner,‡ Gregory Leitus,‡ Linda J. W. Shimon,‡ and David Milstein*,† †

Department of Organic Chemistry and ‡Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot 76100, Israel Received May 9, 2010

The complex (PNP)IrI(CH2COCH3) 2 (PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine) was prepared by reaction of the dearomatized, electron-rich complex (PNP*)IrI(COE) (1; PNP* = deprotonated PNP, COE = cyclooctene) with acetone. Upon treatment with CO, complex 2 undergoes a surprising elimination of acetone to form the dearomatized species (PNP*)IrI(CO) (4), involving proton migration from the ligand “arm” to the acetonyl moiety. DFT studies reveal that this process occurs via the squarepyramidal intermediate 2þCO, formed upon CO coordination to 2, in which the acetonyl moiety is located at the apical position prior to proton migration. Reaction of 2 with H2 (D2) indicates an equilibrium between complex 2 and the nonaromatic (PNP*)IrIII(H)(CH2COCH3) complex 2b, which is the species that actually activates H2 to exclusively form the trans-dihydride (PNP)IrIII(H)2(CH2COCH3) (5a) and activates D2 to form the trans-hydride-deuteride 5b with benzylic-D incorporation, as also corroborated by DFT studies. Interestingly, benzene C-H activation by complex 2 results in formation of the complex (PNP)IrI(C6H5) (6a) and elimination of acetone. DFT studies show that the benzene C-H bond is actually activated by the dearomatized “bare” (PNP*)IrI intermediate 2c, formed upon acetone elimination from 2.

Introduction Metal-ligand cooperation can play an important role in homogeneous catalysis by metal complexes.1 For instance, amido ligands effectively cooperate with the metal center in *To whom correspondence should be addressed. E-mail: david. [email protected]. Fax: 972-8-9346569. (1) (a) Grutzmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814 and references therein. (b) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40. (c) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201. (d) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (e) Maire, P.; Buttner, T.; Breher, F.; Le Floch, P.; Grutzmacher, H. Angew. Chem., Int. Ed. 2005, 44, 6318. (f) Fryzuk, M. D.; MacNeil, P. A. Organometallics 1983, 2, 682. (g) Samec, J. S. M.; Backvall, J. E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237. (h) Casey, C. P.; Beetner, S. E.; Johnson, J. B. J. Am. Chem. Soc. 2008, 130, 2285. (i) Shvo, Y.; Czarkie, D.; Rachamim, Y. J. Am. Chem. Soc. 1986, 108, 7400. (j) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366. (2) For PCP and PCN systems see: (a) Ashkenazi, N.; Vigalok, A.; Parthiban, S.; Ben-David, Y.; Shimon, L. J. W.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2000, 122, 8797. (b) Frech, C. M.; BenDavid, Y.; Weiner, L.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 7128. (c) Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Organometallics 2005, 24, 1082. (3) For a review on PNP systems and their analogues see: (a) van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009, 48, 8832. Cooperativity in an aliphatic PNP-Ru complex: (b) Friedrich, A.; Drees, M.; Gunne, J. S.; Schneider, S. J. Am. Chem. Soc. 2009, 131, 17552. (4) (a) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840. (b) Zhang, J.; Shimon, L. J. W.; Milstein, D. Dalton Trans. 2006, 107. (c) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2006, 45, 1113. (d) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790. (e) Gunanathan, C.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2009, 131, 3146. (f) Gunanathan, C.; Milstein, D. Angew. Chem., Int. Ed. 2008, 47, 8661. (g) Gnanaprakasam, B.; Zhang, J.; Milstein, D. Angew. Chem., Int. Ed. 2010, 49, 1468. r 2010 American Chemical Society

Noyori’s enantioselective catalytic hydrogenation of ketones by chiral RuII amido complexes.1b,c Pincer ligands, although frequently considered as spectator platforms for very stable metal complexes, can adapt to changes at the metal center and thereby exhibit cooperative reactivity modes.2-7 PNP and PNN pincer type systems, in which the metal is anchored to a pyridine-based moiety, actively interact with the metal center, resulting in facile, reversible dearomatization reactions.3-7 Metal-ligand cooperation via reversible dearomatization can play key roles in the catalytic O-H bond activation of alcohols,4 exemplified by the direct coupling of alcohols to form esters with liberation of H2,4a,b the hydrogenation of esters to alcohols under mild pressure,4c and the dehydrogenative coupling of alcohols with amines to produce amides,4d catalyzed by the nonaromatic (PNN*)RuII(H)(CO) complex (PNN = 2-((di-tert-butylphosphino)methyl)-6-((diethylamino)methyl)pyridine; PNN* = deprotonated PNN). Furthermore, this powerful complex promotes splitting of water to H2 and O2 in consecutive heat- and light-induced steps.5 Recently, we communicated benzene C-H activation processes involving metal-ligand cooperation by reversible dearomatization of the ligand.6 In particular, the nonaromatic (5) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74. (6) Ben Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 15390. (7) (a) Iron, M. A.; Ben-Ari, E.; Cohen, R.; Milstein, D. Dalton Trans. 2009, 9433. (b) Zeng, G.; Guo, Y.; Li, S. Inorg. Chem. 2009, 48, 10257. Published on Web 08/09/2010

pubs.acs.org/Organometallics

3818

Organometallics, Vol. 29, No. 17, 2010

(PNP*)IrI(COE) complex C-H activates benzene to yield the aromatic (PNP)IrI(C6H5) complex with no overall change in the formal metal oxidation state6 (PNP = 2,6-bis((di-tert-butylphosphino)methyl)pyridine; PNP* = deprotonated PNP; COE = cyclooctene). The (PNP)IrI(C6H5) complex reacts with H2 to exclusively form the trans-dihydride complex (PNP)IrIII(H)2(C6H5), and upon reaction with D2 the trans hydride-deuteride complex (PNP)IrIII(H)(D)(C6H5) is formed, with one deuterium atom incorporated into a benzylic position.6 Computational studies corroborate an unobserved equilibrium between the aromatic (PNP)IrI(C6H5) complex and the nonaromatic (PNP*)IrIII(H)(C6H5) complex, which is the species that actually activates dihydrogen.7 The nonaromatic (PNP*)IrIII(H)(C6H5) was prepared independently at -78 °C, by deprotonation of a cationic [(PNP)IrIII(H)(C6H5)]BF4 complex, and was trapped with CO to give the nonaromatic species (PNP*)IrIII(CO)(H)(C6H5).6 Following our recent observation of acetone C-H activation by a cationic [(PNP)IrI(COE)]BF4 complex to yield the cationic acetonyl-hydride [(PNP)IrIII(H)(CH2COCH3)]BF4,8a we decided to study metal-ligand cooperation in the C-H bond activation of acetone. Relatively few studies have been done on the properties of acetonyl transition-metal complexes.8 In this paper, we report the preparation and unusual reactivity of a PNP-based IrI-acetonyl complex. The PNP ligand and the metal center act jointly in (i) elimination of acetone from the (PNP)IrI-acetonyl complex upon treatment with CO, forming the nonaromatic (PNP*)IrI(CO) complex, (ii) oxidative addition of H2 to the acetonyl complex, giving exclusively the transdihydride acetonyl (PNP)IrIII(H)2(CH2COCH3) complex, and (iii) benzene C-H activation by the (PNP)IrI-acetonyl complex, resulting in substitution of the acetonyl group by a phenyl group. The experimental results are supported by a DFT study.

Results and Discussion Synthesis and Characterization of (PNP)IrI(CH2COCH3) (2). Heating a concentrated acetone solution (130 mM) of the nonaromatic complex (PNP*)IrI(COE) (16) at 60 °C for 6 h resulted in C-H activation of acetone to form the complex (PNP)IrI(CH2COCH3) (2; 83%), accompanied by intramolecular C-H activation of COE to form the cyclooctenyl complex (PNP)IrI(C8H13) (3; 17%) (Scheme 1). A color change from dark purple to dark violet was observed during the reaction. Heating a 20-fold-diluted acetone solution (6.5 mM) of 1 under the same conditions resulted in formation of complex 2 in 95% yield by 31P{1H} NMR, with only traces of complex 3 (5%) present in the reaction solution (Scheme 1). Thus, dilution increases intermolecular acetone C-H activation versus intramolecular vinyl C-H activation. (8) For acetonyl complexes of iridium see: (a) Feller, M.; Karton, A.; Leitus, G.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 12400. (b) Arndtsen, B. A.; Bergman, R. G. J. Organomet. Chem. 1995, 504, 143. (c) Milstein, D. Acc. Chem. Res. 1984, 17, 221. (d) Milstein, D.; Calabrese, J. C. J. Am. Chem. Soc. 1982, 104, 3773. For acetonyl complexes of rhodium see: (e) de Pater, B. C.; Fruhauf, H. W.; Goubitz, K.; Fraanje, J.; Budzelaar, P. H. M.; Gal, A. W.; Vrieze, K. Inorg. Chim. Acta 2005, 358, 431. (f) Milstein, D. J. Am. Chem. Soc. 1982, 104, 5227. For acetonyl complexes of palladium see: (g) Vicente, J.; Arcas, A.; Fernandez-Hernandez, J. M.; Bautista, D. Organometallics 2001, 20, 2767. (h) Vicente, J.; Arcas, A.; Fernandez-Hernandez, J. M.; Bautista, D. Organometallics 2008, 27, 3978. For acetonyl complexes of platinum see: (i) Falvello, L. R.; Garde, R.; Miqueleiz, E. M.; Tomas, M.; Urriolabeitia, E. P. Inorg. Chim. Acta 1997, 264, 297. (j) Vicente, J.; Arcas, A.; Fernndez-Hernndez, J. M.; Aulln, G.; Bautista, D. Organometallics 2007, 26, 6155.

Schwartsburd et al. Scheme 1

Scheme 2

Complex 2 was characterized by 31P, 1H, and 13C NMR spectroscopy. The 31P{1H} NMR spectrum of 2 exhibits a singlet at 50.1 ppm, indicating equivalent phosphorus donor atoms, while the benzylic protons give rise to one virtual triplet at 2.2 ppm (JP-H = 3.5 Hz) in the 1H NMR spectrum, implying rearomatization of the PNP ligand. The aromaticity of the ligand backbone is further indicated by the 1H and 13 C{1H} NMR spectra. The acetonyl moiety exhibits a triplet resonance at 4.4 ppm (JP-H = 5.6 Hz), attributed to the methylene protons of the Ir-CH2 group, and a singlet resonance at 2.4 ppm, attributed to the methyl protons. The carbonyl group gives rise to a resonance at 219.9 ppm in the 13 C{1H} NMR spectrum and to absorption at 1700 cm-1 in the IR spectrum. From a mechanistic point of view, the acetonyl complex 2 is probably formed by substitution of a COE ligand by an acetone molecule, followed by acetone C-H bond cleavage to yield complex 2b,9 with concomitant proton migration to the side arm and aromatization (Scheme 2). Note that the overall process involves cooperation between the metal and the ligand and no change in the formal metal oxidation state. The cyclooctenyl complex 3 was prepared independently by heating complex 1 at 60 °C for 4 h in the inert solvent heptane (rather than acetone) (Scheme 3). The reaction was accompanied by a color change from dark purple to dark brown. Complex 3 is most likely formed by a facile intramolecular C-H bond activation of COE to give the hydridecyclooctenyl (PNP*)IrIII intermediate 1a, followed by proton migration to the side arm and aromatization (Scheme 3).10 (9) Attempts to prepare 2b independently at -78 °C by deprotonation of the cationic [(PNP)IrIII(H)(CH2COCH3)]BF4 resulted in a mixture of products. (10) The facile room-temperature insertion of iridium into a Csp2-H bond of cyclooctene to give the hydride-octenyl complex (PNP)IrIIIH(C8H13)Cl, upon reaction of the iridium dimer [(COE)2IrCl]2 with the PNP ligand, was described: Hermann, D.; Gandelman, M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. Organometallics 2002, 21, 812.

Article

Organometallics, Vol. 29, No. 17, 2010

3819

Scheme 3

Scheme 4

Figure 1. Structure of complex 4 (ellipsoids shown at the 50% probability level). Hydrogen atoms, except for the benzylic positions, are omitted for clarity.

Complex 3 was characterized by multinuclear NMR spectroscopy. The 31P{1H} NMR spectrum exhibits a singlet at 36.3 ppm, indicating equivalent phosphorus donor atoms, while the benzylic protons give rise to one doublet at 3.1 ppm (JP-H = 2.8 Hz) in the 1H NMR spectrum, implying rearomatization of the PNP ligand. Additionally, the 1H and 13C{1H} NMR spectra indicate aromaticity of the ligand backbone. The cyclooctenyl ligand gives rise to a characteristic set of signals in the 1H and 13 C{1H} NMR spectra (see the Experimental Section). Elimination of Acetone from (PNP)IrI(CH2COCH3) (2) upon Treatment with CO. Formation of the Nonaromatic Complex (PNP*)IrI(CO) (4). Surprisingly, treatment of the IrI acetonyl complex 2 with 1 equiv of CO in benzene-d6 at room temperature resulted in elimination of acetone to quantitatively form the nonaromatic (PNP*)IrI carbonyl complex 4 (Scheme 4). A dramatic color change from dark violet to bright pink-red was observed during the reaction. Complex 4 was prepared independently from 1 by a facile substitution of the COE ligand by CO (Scheme 4), accompanied by a color change from dark purple to bright pink-red. The resulting complex 4 was characterized by 31P, 1H, and 13 C NMR spectroscopy. The 31P{1H} NMR spectrum of 4 exhibits an AB doublet of doublets centered at 69.8 ppm (JP-P= 247 Hz), indicating nonequivalent phosphorus donor atoms. A one-proton virtual triplet at 3.99 ppm (JP-H=7.1 Hz), assigned to the Py-CH-P group in the 1H NMR spectrum, and a doublet at 97.30 ppm (JP-C=11.2 Hz), assigned to this group in the 13C{1H} NMR spectrum, indicate formation of a dearomatized PNP* system. The IR spectrum of the complex shows a sharp νCO band at 1932 cm-1, indicating a greater degree of back-bonding to CO than in the aromatic cationic [(PNP)IrI(CO)]PF6 complex (νCO 1962 cm-1), as expected.11 X-ray-quality crystals of complex 4 were obtained by slow evaporation of a pentane solution at room temperature. The single-crystal X-ray study reveals a slightly distorted squareplanar structure with the CO ligand coordinated trans to the pyridine nitrogen atom (Figure 1). Selected bond lengths and bond angles are given in Table 1. The bond length of C(9)-C(10) is shorter than that of C(4)-C(5) by ∼0.09 A˚, (11) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2006, 25, 3007.

Table 1. Selected Bond Lengths (A˚) and Angles (deg) of Complex 4 Bond Lengths Ir(1)-N(1) Ir(1)-P(1) Ir(1)-P(2) Ir(1)-C(1) C(1)-O(1)

2.083(2) 2.296(1) 2.308(1) 1.818(2) 1.162(3)

P(1)-C(4) P(2)-C(10) C(4)-C(5) C(9)-C(10)

1.820(3) 1.774(3) 1.480(4) 1.394(3)

Bond Angles Ir(1)-N(1)-C(5) N(1)-Ir(1)-P(1) P(1)-Ir(1)-P(2) C(1)-Ir(1)-P(2) O(1)-C(1)-Ir(1)

121.9(2) 83.1(1) 166.1(1) 96.4(1) 179.4(3)

P(1)-C(4)-C(5) P(2)-C(10)-C(9) C(4)-C(5)-N(1) C(10)-C(9)-N(1)

113.4(2) 116.2(2) 117.7(2) 119.7(2)

while the bond length of P(2)-C(10) is shorter than that of P(1)-C(4) by ∼0.05 A˚, indicating that in the solid state the nonaromatic configuration 4 contributes more than the aromatic phosphor-ylide configuration 40 (Scheme 5). Oxidative Addition of H2 to the (PNP)IrI(CH2COCH3) Complex 2. Exclusive Formation of the trans-Dihydride Acetonyl Complex (PNP)IrIII(H)2(CH2COCH3) (5a). Remarkably, upon reaction of complex 2 with 1 equiv of H2 in benzened6 only the trans-dihydride acetonyl complex (PNP)IrIII(H)2(CH2COCH3) (5a) was observed (Scheme 6). The 31P{1H} NMR spectrum of 5a shows a singlet at 48.6 ppm, arising from the two equivalent phosphorus heteroatoms. The 1H NMR spectrum shows a single virtual triplet for the tert-butyl protons on the ligand at 1.40 ppm (JP-H = 6.2 Hz), indicating an environment of C2v symmetry. This symmetry is also evident in the hydride resonance, which appears as a triplet at -9.35 ppm (JP-H=14.8 Hz) integrating to 2H. There is no evidence for the formation of a cis-dihydride complex. A number of transdihydride IrIII complexes have been reported to date.1f,6,11,12 (12) (a) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem. Soc. 2010, 132, 4534. (b) Li, S. H.; Hall, M. B. Organometallics 1999, 18, 5682. (c) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16, 3786. (d) Laporte, C.; Buttner, T.; Ruegger, H.; Geier, J.; Schonberg, H.; Grutzmacher, H. Inorg. Chim. Acta 2004, 357, 1931. (e) Dahlenburg, L.; Gotz, R. Inorg. Chim. Acta 2004, 357, 2875. (f) Nemeh, S.; Flesher, R. J.; Gierling, K.; Maichle-Moessmer, C.; Mayer, H. A.; Kaska, W. C. Organometallics 1998, 17, 2003. (g) Brown, J. M.; Dayrit, F. M.; Lightowler, D. J. Chem. Soc., Chem. Commun. 1983, 8, 414.

3820

Organometallics, Vol. 29, No. 17, 2010

Schwartsburd et al. Table 2. Selected Bond Lengths (A˚) and Angles (deg) of Complex 5a Bond Lengths Ir(1)-N(1) Ir(1)-P(1) Ir(1)-P(2)

2.114(2) 2.291(1) 2.297(1)

Ir(1)-H(1) Ir(1)-C(1)

1.65(3) 2.150(2)

Bond Angles N(1)-Ir(1)-P(1) P(1)-Ir(1)-P(2) C(1)-Ir(1)-P(2)

Figure 2. Structure of complex 5a (ellipsoids shown at the 50% probability level). Hydrogen atoms, except for the hydrides, are omitted for clarity. Scheme 5

Scheme 6

An X-ray structure analysis of 5a (Figure 2) shows a slightly distorted octahedral geometry with the trans-dihydride ligands perpendicular to the PNP plane. The quality of the data set allowed for the location of both hydrides. The acetyl group of the acetonyl moiety is aligned at about 120° to the PNP plane. Selected bond lengths and bond angles are given in Table 2. Significantly, reaction of complex 2 with D2 in benzene yielded the trans-Ir(H)(D) complex 5b, with incorporation of one deuterium atom into the benzylic position (23% deuterium incorporation by 1H NMR integration) (Scheme 6). The 2 H{1H} NMR spectrum of the freshly prepared 5b in benzene revealed an Ir-D signal at -9.15 ppm and a benzylic-D signal at 2.95 ppm; the 1H NMR spectrum revealed a hydride triplet at -9.35 ppm. The 31P{1H} NMR spectrum exhibited a multiplet pattern, centered at 49.2 ppm (JD-P =16.2 Hz),

83.3(1) 163.0(1) 96.9(1)

N(1)-Ir(1)-H(1) Ir(1)-C(1)-C(2)

89.3(1) 119.7(2)

resulting from superposition between the benzylic-D coupled phosphorus atom and the second uncoupled phosphorus atom. The reaction of 2 with H2 (D2) likely involves an unobserved equilibrium between (PNP)IrI(CH2COCH3) (2) and the nonaromatic (PNP*)IrIII(H)(CH2COCH3) (2b) (Scheme 6), which is the species that actually activates H2 (D2) to exclusively form complex 5a (5b) (Scheme 6). Thus, an apparently simple oxidative addition of H2 to IrI actually involves H2 activation by IrIII, as observed with the (PNP)IrI(C6H5) complex.6,7 C-H Bond Activation of Cyclooctene and Benzene by (PNP)IrI(CH2COCH3) (2). Formation of (PNP)IrI(C8H13) and (PNP)IrI(C6H5). Heating the dark violet complex 2 in a cyclooctene solution at 100 °C for 18 h resulted in acetonyl group substitution by a cyclooctenyl group, yielding the dark brown cyclooctenyl IrI complex 3 (Scheme 7). Likewise, when complex 2 was heated in benzene at 80 °C for 9 h, the acetonyl group was replaced by a phenyl group to give the known phenyl IrI complex 6a6 (Scheme 7). In order to study the benzene C-H bond activation mechanism, the acetonyl complex 2 was reacted with C6D6 under the same conditions, resulting in formation of the known complex 6b6 (Scheme 7). Deuterium incorporation into the benzylic position of 6b was observed (on the basis of the integration of benzylic protons relative to the pyridine ligand protons in the 1H NMR spectrum), revealing the ligand “arm” involvement in the benzene C-H bond activation process.13 DFT Study of H2 and Benzene Activation by the (PNP)IrI(CH2COCH3) (2). Reaction of complex 2 with H2 and with benzene seems to occur via two comparable pathways. The reaction with H2 involves proton transfer to and from the methylene arm, as evident from the corresponding reaction with D2. The reaction with benzene also involves the methylene arm, as deuterium incorporation into the arm was observed in the reaction with benzene-d6. In order to better understand the mechanism, the reactivity of 2 was studied computationally (see Computational Methods for full details; all relative energies;with respect to 2;are given in Table 3). The reactivity of 2 with H2 was found to mirror that of its previously reported phenyl analogue (PNP)IrI(C6H5) (6a).7a The initial step is the equilibrium of 2 with its dearomatized counterpart 2b (Scheme 6). This complex is trapped by H2, which binds to the site trans to the hydride. Finally, complex 5a is obtained by transfer of one proton from the η2-dihydrogen ligand to the methylene arm (Scheme 6). In both hydrogen-transfer (13) Since H/D scrambling at the benzylic positions might take place upon prolonged heating in the presence of the acetone generated in the reaction, the reaction solution was heated for the minimal period required for full conversion.

Article

Organometallics, Vol. 29, No. 17, 2010

3821

Scheme 7

Table 3. Relative Free Energies in Solution (ΔG298, kcal/mol) of All Calculated (DFT) Complexes complex

ΔG298

2 TS(2-2b)(anti) 2b(anti) TS(2b anti-syn) 2b(syn) 2bþH2(anti) 2bþH2(syn) TS(2bþH2-5a)(anti) TS(2bþH2-5a)(syn) 5a 2þ2H2O TS(2-2b)þ2H2O(anti) 2bþ2H2O(anti) TS(2bþH2-5a)þH2O(anti) TS(2bþH2 - 5a)þH2O(syn)

0.0 33.0 10.4 13.5 8.9 12.7 19.4 30.0 30.7 -0.5 -7.9 18.5 5.3 26.5 28.8

TS(2b-2c)(anti) 2c 2e TS(2e-7) 7 7 þ 2H2O TS(7 - 6a) TS(7 - 6a)þ2H2O 6aþ2H2Oþacetone 6a 8 2d 2d0 2d00 (syn) 2d00 (anti)

24.4 22.4 18.1 20.1 2.7 -3.3 28.4 11.3 -12.7 -7.4 38.7 32.7 9.0 17.4 22.0

2þCOapic 2þCOequa 2þCOalt 2þCOequaþH2O TS(2þCOequa-4) TS(2þCOequa-4)þH2O 4þH2Oþacetone 4 TS(2-2b)þCO 2b(anti)þCO TS(2b-2c)þCO

1.5 -9.8 -13.7 -9.1 24.9 -7.1 -36.7 -43.8 26.5 -12.0 27.1

steps, the barriers can be lowered by water forming a bridge between the two sites, resulting in catalysis by adventitious trace water.14 (In both studies, two molecules of water, likely part of a small cluster, were found to be sufficient to lower the reaction barrier enough for the reaction to proceed at room temperature. In the case of TS(2bþH2-5a), one water molecule is sufficient). Assistance of H2 activation by trace water was concluded also for the Ir(I) complexes 6a7a and (PONOP)IrICH3 (PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine)12a and for an aliphatic (PNP)Ru complex.3b (14) Since trace amounts of adventitious water are sufficient for proton transfer catalysis, experiments in the complete absence of water were not practical. Experiments aimed at exploring the rate effect of controlled concentrations of water in a water-miscible solvent are planned.

Figure 3. Reaction profile for the reaction of 2 with H2. The reactions via the syn and anti isomers are depicted in red and green, respectively, while common parts of the profile are shown in blue. Solid lines denote the reaction without additional water molecules, while the dotted lines denote the reaction with added water molecule(s).

In contrast to the case for (PNP*)IrIII(H)(C6H5), in complex 2b there are two possible orientations of the acetonyl ligand, with the carbonyl oxygen either on the same side (i.e., syn) of the P-N-P-Ir plane as the hydride or on the opposite side (i.e., anti); the barrier for the interconversion was found to be less than 5 kcal/mol. The profile for the reaction of 2 with H2 is shown in Figure 3, while the key transition state TS(2bþH2-5a)þH2O(anti) is depicted in Figure 4 (due to its similarity with the phenyl analogue,7a TS(2-2b)þ 2H2O(anti) is not shown). There are three potential reaction pathways for the reaction of 2 with benzene to give the phenyl complex 6a with concomitant liberation of acetone (Scheme 8 and Figures 4 and 5): (a) complex 2 can directly undergo C-H oxidative addition of benzene, forming the complex (PNP)IrIII(CH2COCH3)(H)(C6H5) (2d), which in turn reductively eliminates acetone; (b) proton migration occurs from the benzylic position to the metal accompanied by dearomatization of 2 to yield 2b, which then activates a benzene C-H bond directly across the Ir-methylene distance to give 2d00 , which in turn reductively eliminates acetone; (c) formation of 2b, reductive elimination of acetone to give 2c, and subsequent C-H activation of benzene take place to give 6a. On the basis of the observed deuterium incorporation into the methylene arm, route (a) seems unlikely. There are three isomers for (PNP)IrIII(CH2COCH3)(H)(C6H5), but this reaction pathway would require that the hydride be cis to both the acetonyl and phenyl ligands: that is, in the free equatorial site with axial phenyl and acetonyl ligands (i.e., complex 2d). This is because the complex is formed by the oxidative addition of benzene, while the product 6a is formed by reductive elimination of acetone, and both steps require the two participating components to be mutually cis. However,

3822

Organometallics, Vol. 29, No. 17, 2010

Schwartsburd et al.

Figure 4. DFT structures of (clockwise from top left) TS(2bþH2-5a)þH2O(anti), TS(2b-2c)(anti), TS(2e-7), and TS(7-6a)þ2H2O. Hydrogen atoms of the tert-butyl groups have been omitted for clarity. Color scheme: H, white; C, gray; N, dark blue; O, red; P, yellow; Ir, blue. Scheme 8

Figure 5. Reaction profile for the reaction of 2 with benzene. Routes (a) and (c) are depicted in red and blue, respectively (see Scheme 8). Solid lines denote the reaction without additional water molecules, while the dotted lines denote the reaction with added water molecule(s). Due to the high energies of the intermediates along the red route, the associated transition states were not located.

this isomer is the least stable, due to the significant steric repulsion between the large acetonyl and even larger phenyl

ligands with the phosphine tBu substituents. Thus, the oxidative addition of benzene is more likely to result in 2d0 , the isomer with phenyl in the equatorial position. This isomer is more stable by 23.8 kcal/mol but precludes the reductive elimination of acetone from occurring due to the trans orientation of the two ligands. Route (b) is very unlikely. Although direct proton transfer from a coordinated ligand has been shown computationally to occur in related systems involving addition of dihydrogen7 or water,5 attempts at finding a transition state for this proposed route were unsuccessful. Route (c) was found to be the most likely. With two water molecules forming a proton bridge, the barrier for

Article

dearomatization (i.e., 2 to 2b) is ΔGq298 = 26.4 kcal/mol. This step is mildly endergonic by ΔG298 = 10.4 kcal/mol. This reaction leads to the anti isomer of 2b because of the orientation of the acetonyl ligand required for the subsequent C-H coupling. The syn isomer would have the carbonyl blocking the methylene carbon from the hydride; the syn isomer, however, is slightly more stable by 1.5 kcal/mol. The transition state TS(2b-2c)(anti) (shown in Figure 4) for the loss of acetone was located and leads to a reaction barrier of ΔG‡298 =14.0 kcal/mol, which is ΔG‡298=24.4 kcal/mol relative to 2. The overall reaction energy for the loss of acetone from 2 to give 2c is ΔG298 =22.4 kcal/mol. This is reasonable for a reaction carried out at 80 °C. Formation of the benzene coordination complex 2e is favorable by ΔGq298=-4.3 kcal/mol. The barrier for C6H5-H activation is ΔGq298=1.9 kcal/mol, ΔGq298=20.1 kcal/mol relative to 2; the transition state TS(2e-7) is shown in Figure 4. Rearomatization, aided by the two water molecules, to give the final product 6a has a barrier of ΔGq298=14.6 kcal/mol; the transition state TS(7-6a)þ2H2O is shown in Figure 4. Overall, the reaction of 2 with benzene to give 6a and acetone is exergonic: ΔGq298 = -7.4 kcal/mol. This reaction pathway corroborates the observed deuterium incorporation into the methylene arm of 6b upon the reaction of 2 with benzene-d6. Comparing the reactivity of 2 toward dihydrogen and benzene, one notes that the former is activated by the dearomatized IrIII hydride-acetonyl complex 2b while the latter is activated by the “bare” dearomatized IrI complex 2c. The involvement of 2c might be a bit surprising, as one would have expected such a complex, which is formally 14-electron IrI, to be too high in energy to be feasible. Nonetheless, due to its much larger size, benzene cannot react with 2b due to severe steric interactions and must therefore resort to more drastic measures. Unlike the reaction with H2, the reaction with benzene requires heating, making the formation of 2c possible, and this highly reactive species instantly reacts with the benzene solvent. DFT Study of Acetone Elimination from (PNP)IrI(CH2COCH3) (2) upon Treatment with CO and Formation of (PNP*)IrI(CO) (4). The reaction of 2 with CO (Scheme 4) to give 4 and acetone was investigated by DFT (see Computational Methods for full details; all relative energies;with respect to 2;are given in Table 3). There are three potential reaction pathways (Scheme 9): (a) proton migration from the benzylic position to the metal, accompanied by dearomatization of 2 to form the 16-electron IrIII intermediate 2b, with subsequent reductive elimination of acetone to form the 14-electron IrI species 2c and concomitant trapping by CO; (b) formation of 2b and coordination of CO to form the 6-coordinate 18-electron IrIII species 2bþCO followed by reductive elimination of acetone;15 (c) coordination of CO to 2 to form the 18-electron IrI species 2þCO followed by elimination of acetone, with concomitant dearomatization. Overall, the reaction 2 þ CO f 4 þ acetone is very exergonic (ΔG298 = -43.8 kcal/mol). The first two pathways involve intermediate 2b, which has already been examined. The formation of 2b involves a barrier of ΔGq298 =26.4 kcal/mol and is endergonic by ΔG298 =10.4 kcal/mol. However, the formation of the square-pyramidal complex 2þCOapic, where the CO is in the formerly vacant (15) Reductive elimination of acetone from the 6-coordinate 18electron IrIII complex 2bþCO is unlikely. It was shown that the C-H reductive elimination of benzene from the 6-coordinate 18-electron IrIII complex Ir(PiPr3)2(CO)Cl(C6H5)H is inhibited by the coordination of CO. See: Rosini, G. P.; Wang, K.; Patel, B.; Goldman, A. S. Inorg. Chim. Acta 1998, 270, 537.

Organometallics, Vol. 29, No. 17, 2010

3823

Figure 6. Reaction profile for the reaction of 2 with CO, route (c) (see Scheme 9). Solid lines denote the reaction without additional water molecules, while the dotted lines denote the reaction with a bridging water molecule. Scheme 9

apical position, is practically isoergonic (ΔG298 = 1.5 kcal/mol) (Scheme 9 and Figure 6). Rearrangement of this complex, which is unlikely to involve a significant reaction barrier, to give the analogue 2þCOequa, where the acetonyl ligand is in the apical position, results in a species that is ΔG298 = -9.8 kcal/mol more stable than 2 and free CO (Scheme 9 and Figure 6). The Ir-CR bond in the 18-electron IrI complex 2þCOequa is significantly elongated (2.397 A˚ compared to 2.177 A˚ in 2þCOapic and 2.170 A˚ in 2). The transition state TS(2þCOequa-4) (shown in Figure 7) for the coupling of the acetonyl R-carbon with a benzylic proton was found and results in a barrier of ΔGq298 = 34.7 kcal/mol (24.9 kcal/mol relative to 2 and free CO) (Figure 6). However, a bridging water molecule significantly lowers this barrier to 2.7 kcal/mol, which is -7.1 kcal/mol below the reactants (Figure 6). Thus, clearly route (c) is the most favorable, as it does not involve any high-energy intermediates or barriers. It should be noted that an alternate isomer of 2þCOequa was located where the acetonyl oxygen is located directly

3824

Organometallics, Vol. 29, No. 17, 2010

Schwartsburd et al.

followed by benzene C-H activation. The role of water molecules in significantly lowering the transition state energy for proton migration from the “arm” to the metal is indicated by DFT. Further experimental and theoretical investigations on metal-ligand cooperation via aromatizationdearomatization, and its implications for catalytic design, are in progress.

Experimental Section

Figure 7. DFT structure for TS(2þCOequa-4)þH2O. Hydrogen atoms of the tert-butyl groups have been omitted for clarity. Color scheme: H, white; C, gray; N, dark blue; O, red; P, yellow; Ir, light blue.

over the benzylic position (Scheme 9); the distance between the acetonyl oxygen and the benzylic proton is 1.986 A˚, and the Ir-CR bond length is 2.473 A˚. This isomer is slightly more stable (ΔG298 =-13.7 kcal/mol relative to 2 and free CO). Thus, one could conceive of a variant of pathway c where the proton is transferred to the acetonyl oxygen instead of the R-carbon, resulting in enol rather than acetone. This would initially result in 4 þ enol, which has ΔG298 = -27.6 kcal/mol relative to 2 and free CO. The keto-enol rearrangement is known to be facile and rapid, and thus this pathway could be viable. Nonetheless, despite repeated attempts, a transition state for proton transfer to the acetonyl oxygen could not be located.

Summary The nonaromatic electron-rich (PNP*)IrI(COE) (1) undergoes facile C-H activation of acetone by cooperation between the metal center and the pincer ligand, with aromatization of the ligand, to form the aromatic (PNP)IrI(CH2COCH3) (2). This reaction is accompanied by a minor intramolecular Csp2-H activation of COE, giving the complex (PNP)IrI(C8H13) (3), formation of which is diminished when lower concentration of 1 in acetone is utilized. Complex 2 exhibits unusual reactivity, in which metal-ligand cooperation via reversible dearomatization plays a key role. First, treatment of 2 with CO results in a surprising acetone elimination to form the dearomatized (PNP*)IrICO (4), involving proton migration from the ligand “arm” to the acetonyl moiety, with concomitant dearomatization. DFT studies reveal that this reaction involves the square-pyramidal intermediate 2þCO, formed upon CO coordination to 2, in which the acetonyl moiety moves to the apical position prior to the proton migration. The transition state for proton migration from the arm to the metal is significantly lowered in energy by a bridging water molecule. Second, reaction of 2 with D2 sheds light on an unobserved equilibrium between 2 and the dearomatized complex (PNP*)IrIII(H)(CH2COCH3) (2b), which activates D2 to exclusively form the trans-hydridedeuteride complex (PNP)IrIII(H)(D)(CH2COCH3) (5b) with benzylic-D incorporation, as also corroborated by the DFT studies. And third, (PNP)IrI(CH2COCH3) (2) activates C6D6, forming (PNP)IrI(C6D5) (6b) with benzylic-D incorporation, accompanied by acetone elimination. DFT studies show that acetone elimination from 2 to form the dearomatized “bare” (PNP*)IrI intermediate 2c takes place first,

General Procedures. All experiments with metal complexes and the phosphine ligand were carried out under an atmosphere of purified argon in an MBRAUN Unilab glovebox or using standard Schlenk techniques. The complex (PNP*)Ir(COE) (1) was prepared according to the literature procedure.6 All solvents were reagent grade or better. All nondeuterated solvents were refluxed over sodium/benzophenone ketyl and distilled under an argon atmosphere. Deuterated solvents were used as received. All the solvents were degassed with argon and kept in the glovebox over 4 A˚ molecular sieves (except for acetone, which was dried with Drierite). Commercially available reagents were used as received. Analysis. The NMR spectra were recorded at 400 MHz (1H), 100 MHz (13C), and 162 MHz (31P) using a Bruker Avance-400 NMR spectrometer and at 500 MHz (1H), 126 MHz (13C), and 202 MHz (31P) using a Bruker Avance-500 NMR spectrometer. All spectra were recorded at 23 °C. 1H NMR and 13C{1H} NMR chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane. 1H NMR chemical shifts were referenced to the residual hydrogen signal of C6D6 (7.15 ppm). In 13 C{1H} NMR measurements the signal of C6D6 (128.0 ppm) was used as a reference. 31P NMR chemical shifts are reported in ppm downfield from H3PO4 and referenced to an external 85% solution of phosphoric acid in D2O. Abbreviations used in the description of NMR data are as follows: b, broad; s, singlet; d, doublet; t, triplet; m, multiplet; v, virtual; Py, pyridine. IR spectra were measured with a Nicolet-6700 FT-IR spectrometer. Elemental analyses were performed by the Unit of Chemical Research Support, Weizmann Institute of Science. Reaction of (PNP*)IrI(COE) (1) with Acetone. Formation of (PNP)IrI(CH2COCH3) (2). (a). A concentrated acetone solution (0.5 mL) of complex 1 (45.0 mg, 0.065 mmol) was heated at 60 °C for 6 h, accompanied by a color change from dark purple to dark violet. The 31P{1H} NMR spectrum revealed formation of compounds 2 (83%) and 3 (17%). The ratio was determined by integration. (b). A dilute acetone solution (10 mL) of complex 1 (45.0 mg, 0.065 mmol) was heated at 60 °C for 6 h, accompanied by a color change from dark purple to dark violet. The 31P{1H} NMR spectrum revealed formation of complex 2, which was present in 95% purity, together with 5% of complex 3. The acetone solvent was evaporated, and the residue was dissolved in pentane. Complex 2 was precipitated from the pentane solution at -35 °C. The precipitate was separated by decantation and dried under vacuum, giving 36.0 mg (0.056 mmol, 86% yield) of the pure 2 as a black-violet solid. 31 P{1H} NMR (C6D6): 50.1 (s). 1H NMR (C6D6): 7.71 (t, JH-H = 7.5 Hz, 1H, Py-H4), 6.29 (d, JH-H = 7.5 Hz, 2H, PyH3,5), 4.40 (t, JP-H =5.6 Hz, 2H, Ir-CH2), 2.39 (s, 3H, C(O)CH3), 2.20 (vt, JP-H = 3.5 Hz, 4H, Py-CH2-P), 1.36 (vt, JP-H = 6.4 Hz, 36H, P-t-Bu). 13C{1H} NMR (C6D6): 219.89 (s, CO), 162.38 (t, JP-C = 4.3 Hz, Py-C2,6), 126.31 (s, Py-C4), 120.26 (t, JP-C=4.7 Hz, Py-C3,5), 40.33 (vt, JP-C = 9.4 Hz, Py-CH2-P), 35.43 (vt, JP-C=9.1 Hz, PC(CH3)3), 33.03 (s, C(O)CH3), 29.39 (vt, JP-C=3.0 Hz, P-C(CH3)3), 8.77 (t, JP-C=5.0 Hz, Ir-CH2). Assignment of 13C{1H} NMR signals was confirmed by 13C DEPT and by 13C-1H HMQC correlation. IR (thin film): νCO 1700 cm-1. Anal. Found (calcd for C26H48IrNP2): C, 48.47 (48.43); H, 7.41 (7.50).

Article Transformation of (PNP*)IrI(COE) (1) to (PNP)IrI(C8H13) (3). A heptane solution (0.5 mL) of complex 1 (20.0 mg, 0.029 mmol) was heated at 60 °C for 4 h, accompanied by a color change from dark purple to dark brown. The 31P{1H} NMR spectrum revealed formation of complex 3. The reaction solution was evaporated, leaving a dark brown residue. The residue was dissolved in pentane and the solution filtered through a cotton pad. The pentane was then evaporated to dryness, giving 18.0 mg (90% yield) of 3 as a dark brown solid. 31 P{1H} NMR (C6D6): 36.3 (s). 1H NMR (C6D6): 7.24 (d, JH-H = 7.5 Hz, 2H, Py-H3,5), 7.17 (vt, JH-H=7.5 Hz, 2H, PyH4, partially obscured by the solvent signal), 5.64 (m, JP-H=2.0 Hz, JHa-H = 5.2 Hz, JHb-H = 7.2 Hz, 1H, Ir-CdCHCHaHb), 3.08 (d, JP-H = 2.8 Hz, 4H, Py-CH2-P), 2.58 (m, 1H, Ir-Cd CHCHaHbCH2), 2.36 (m, 1H, Ir-CdCHCHaHbCH2), 2.06 (m, 2H, Ir-CCH2), 1.47-1.37 (m, 8H, COE-CH2), 1.12 (d, JP-H = 10.7 Hz 36H, P-t-Bu). 13C{1H} NMR (C6D6): 161.56 (d, JP-C = 14.7 Hz, Py-C2,6), 135.84 (s, Ir-CdCH), 130.28 (s, Ir-CdCH), Py-C4 signal obscured by the solvent signal, 120.81 (d, JP-C = 9.7 Hz, Py-C3,5), 32.37 (d, JP-C = 25.9 Hz, Py-CH2P), 31.84 (d, JP-C = 24.1 Hz, P-C(CH3)3), 30.44 (s, COE-CH2), 30.16 (s, COE-CH2), 29.88 (d, JP-C = 13.8 Hz, P-C(CH3)3), 29.47 (s, IrCdCHCH2), 29.06 (s, Ir-CCH2), 26.41 (s, COE-CH2), 25.74 (s, COE-CH2). Assignment of 13C{1H} NMR signals was confirmed by 13C DEPT, by 13C-1H HMQC correlation, and by 13 C-1H HMBC correlation). Anal. Found (calcd for C31H56IrNP2): C, 53.41 (53.42); H, 8.10 (8.10). Reaction of (PNP)IrI(CH2COCH3) (2) with CO. Formation of (PNP*)IrI(CO) (4). In an NMR tube containing a benzene-d6 solution of 2 (14.0 mg, 0.022 mmol) was injected 1 equiv of CO (0.49 mL, 0.022 mmol). The solution was shaken until the color changed from dark violet to bright pink-red. The 31P{1H} and 1 H NMR spectra revealed formation of compound 4. In addition, the 1H NMR spectrum revealed the presence of acetone in the reaction solution (1 equiv relative to 4), which was not present before the injection of CO. The solvent was evaporated. The residue was dissolved in pentane and the solution filtered through a cotton pad. The pentane was then evaporated to dryness, giving 12.6 mg (95% yield) of 4 as a pink-red solid. Preparation of (PNP*)IrI(CO) (4) from (PNP*)IrI(COE) (1). In an NMR tube, containing a benzene solution of 1 (25.6 mg, 0.037 mmol), was injected 1 equiv of CO (0.82 mL, 0.037 mmol). The solution was shaken until the color changed from dark purple to bright pink-red. The 31P{1H} and 1H NMR spectra revealed formation of complex 4. The solvent was evaporated, the residue was dissolved in pentane, and this solution was filtered and evaporated to dryness, giving 20.7 mg (92% yield) of 4 as a pink-red solid. Crystals suitable for X-ray analysis were obtained by slow evaporation of the pentane solution of 4. 31 P{1H} NMR (C6D6): 69.8 (dd (AB), JP-P = 247 Hz). 1H NMR (C6D6): 6.38 (m, 2H, Py-H3,4), 5.37 (d, JH-H = 4.5 Hz, 1H, Py-H5), 3.99 (vt, JP-H = 7.1 Hz, 1H, Py-CHP), 2.77 (d, JP-H = 9.0 Hz, 2H, Py-CH2P), 1.48 (d, JP-H = 13.3 Hz, 18H, P-t-Bu), 1.12 (m, 18H, P-t-Bu). 13C{1H} NMR (C6D6): 188.23 (m, CO), 174.18 (m, JP-C = 5.8 Hz, Py-C2,6), 135.84 (s, Py-C3 or 4), 131.91 (s, Py-C3 or 4), 114.60 (d, JP-C=17.6 Hz, Py-C5), 97.30 (d, JP-C = 11.2 Hz, Py-CHP), 37.73 (dd, JP-C=28.3 Hz, JP-C=2.1 Hz, PC(CH3)3), 35.76 (d, JP-C=22.5 Hz, Py-CH2P), 35.65 (dd, JP-C = 21.3 Hz, JP-C = 1.9 Hz, PC(CH3)3), 29.84 (d, JP-C=4.8 Hz, PC(CH3)3), 29.04 (d, JP-C=4.3 Hz, PC(CH3)3). Assignment of 13C{1H} NMR signals was confirmed by 13C DEPT. IR (thin film): νCO 1932 cm-1. Anal. Found (calcd for C24H42IrNOP2): C, 46.72 (46.89); H, 7.05 (6.89). X-ray Structural Analysis of 4. Crystal data: C24H42IrNOP2, red chunk, 0.55  0.47  0.14 mm3, orthorhombic, space group Pbca, a=11.8998(3) A˚, b=15.5324(4) A˚, c=27.9034(7) A˚, from 20 degrees of data, T = 100(2) K, V=5157.5(2) A˚3, Z=8, fw= 614.73, Dc = 1.583 Mg/m3, μ = 5.316 mm-1. Data collection and treatment: Bruker APEX-II Kappa CCD diffractometer, Mo KR (λ = 0.710 73 A˚), graphite monochromator, 73 519

Organometallics, Vol. 29, No. 17, 2010

3825

reflections collected, -14 e h e 19, -25 e k e 25, -45 e l e 45, frame scan width 0.5°, scan speed 1° per 20 s, typical peak mosaicity 0.70°, 17 516 independent reflections (Rint = 0.0437). The data were processed with APEX-II. Solution and refinement: structure solved by direct methods with Autosolve, fullmatrix least-squares refinement based on F2 with SHELXL-97, 286 parameters with no restraints, final R1 = 0.0293 (based on F2) for data with I > 2σ(I) and R1 = 0.0498 on 12 476 reflections, goodness of fit on F2 1.062, largest electron density peak 2.314 e/A˚3, largest hole -3.700 e/A˚3. Reaction of (PNP)IrI(CH2COCH3) (2) with H2. Formation of the Acetonyl Complex trans-(PNP)IrIII(H)2(CH2COCH3) (5a). In an NMR tube containing a benzene-d6 solution of 2 (10.0 mg, 0.016 mmol) was injected 1 equiv of H2 (0.35 mL, 0.016 mmol). The solution was shaken until the color changed from dark violet to bright brown-yellow. The 31P{1H} and 1H NMR spectra revealed formation of compound 5a. The solvent was evaporated. The residue was dissolved in pentane and the solution filtered through a cotton pad. The pentane was then evaporated to dryness, giving 8 mg (80% yield) of 5a as a beige solid. Crystals suitable for X-ray analysis were obtained by slow evaporation of the pentane/ether solution of 5a. 31 P{1H} NMR (C6D6): 48.6 (s). 1H NMR (C6D6): 6.69 (t, JH-H = 8.0 Hz, 1H, Py-H4), 6.33 (d, JH-H = 8.0 Hz, 2H, PyH3,5), 3.45 (t, JP-H = 5.6 Hz, 2H, Ir-CH2), 2.98 (vt, JP-H = 3.2 Hz, 4H, Py-CH2P), 2.60 (s, 3H, C(O)CH3), 1.40 (vt, JP-H = 6.2 Hz, 36H, P-t-Bu), -9.35 (t, JP-H = 14.8 Hz, 2H, Ir-H). 13C{1H} NMR (C6D6): 219.56 (s, CO), 163.22 (t, JP-C = 2.9 Hz, PyC2,6), 132.74 (s, Py-C4), 117.94 (t, JP-C = 4.2 Hz, Py-C3,5), 42.95 (vt, JP-C = 11.0 Hz, Py-CH2P), 34.94 (vt, JP-C = 10.3 Hz, PC(CH3)3), 29.29 (vt, JP-C = 2.5 Hz, PC(CH3)3), 28.46 (s, C(O)CH3), -13.40 (t, JP-C = 4.8 Hz, Ir-CH2). Assignment of 13 C{1H} NMR signals was confirmed by 13C DEPT. IR (thin film): νCO 1706 cm-1. Anal. Found (calcd for C26H50IrNOP2): C, 48.41 (48.28); H, 7.74 (7.79). X-ray Structural Analysis of 5a. Crystal data: C26H50IrNOP2, orange plate, 0.2  0.1  0.1 mm3, monoclinic, P21/n, a = 8.5056(3) A˚, b=20.0077(7) A˚, c=16.4676(6) A˚, β=101.969(2)° from 20 degrees of data, T = 100(2) K, V = 2741.49(17) A˚3, Z = 4, fw = 646.81, Dc = 1.567 Mg/m3, μ = 5.005 mm-1. Data collection and treatment: Bruker APEX-II Kappa CCD diffractometer, Mo KR (λ = 0.710 73 A˚), graphite monochromator, 52 340 reflections collected, -13 e h e 12, -30 e k e 31, -25 e l e 25, frame scan width 0.5°, scan speed 1° per 60 s, typical peak mosaicity 0.65°, 10 907 independent reflections (Rint = 0.0349). The data were processed with APEX-II. Solution and refinement: structure solved by direct methods with SHELXS, full-matrix least-squares refinement based on F2 with SHELXL-97, 301 parameters with no restraints, final R1 = 0.0246 (based on F2) for data with I > 2σ(I) and R1 = 0.0384 on 10 907 reflections, goodness of fit on F2 1.016, largest electron density peak 1.522 e/A˚3, deepest hole -0.670 e/A˚3. Reaction of (PNP)IrI(CH2COCH3) (2) with D2. Formation of trans-(PNP)IrIII(H)(D)(CH2COCH3) (5b). In an NMR tube containing a benzene solution of 2 (10.0 mg, 0.016 mmol) was injected 1 equiv of D2 (0.35 mL, 0.016 mmol). The solution was shaken until the color changed from dark violet to bright brownyellow. The 31P{1H}, 2H{1H}, and 1H NMR spectra of the reaction solution revealed the formation of compound 5b. The solvent was evaporated. The residue was dissolved in pentane and the solution filtered through a cotton pad. The pentane was then evaporated to dryness, giving 7.7 mg (77% yield) of 5b as a beige solid. 2 H{1H} NMR (C6H6; measured without 2H lock): 2.95 (bs, Py-CDHP), -9.15 (bs, Ir-D). 31P{1H} NMR (C6H6): 49.2 (m, JD-P = 16.2 Hz). 1H NMR (C6H6): -9.35 (t, JP-H = 14.3 Hz, IrH). 1H NMR (C6D6): 6.69 (t, JH-H = 8.0 Hz, 1H, Py-H4), 6.33 (d, JH-H = 8.0 Hz, 2H, Py-H3,5), 3.45 (t, JP-H = 5.6 Hz, 2H, IrCH2), 2.98 (distorted vt, JP-H = 3.7 Hz, 3H, Py-CH2P, broadening at one side of the pattern due to coupling with 2H at

3826

Organometallics, Vol. 29, No. 17, 2010

benzylic position), 2.60 (s, 3H, C(O)CH3), 1.40 (vt, JP-H = 6.2 Hz, 36H, P-t-Bu), -9.35 (t, JP-H = 14.3 Hz, 1H, Ir-H). Reaction of (PNP)IrI(CH2COCH3) (2) with COE. Formation of (PNP)IrI(C8H13) (3). A cyclooctene solution (1.0 mL) of complex 2 (17.0 mg, 0.026 mmol) was heated at 100 °C for 18 h, accompanied by a color change from dark violet to dark brown. The 31P{1H} NMR spectrum revealed formation of complex 3, present in 94% purity. The cyclooctene was evaporated, resulting in a dark brown residue. The residue was dissolved in pentane and the solution filtered through a cotton pad. The pentane was then evaporated to dryness, giving 13.8 mg (75% yield) of 3 as a dark brown solid. Reaction of (PNP)IrI(CH2COCH3) (2) with C6H6 (C6D6). Formation of (PNP)IrI(C6H5) (6a) and (PNP)IrI(C6D5) (6b). (a). A benzene solution (1.0 mL) of complex 2 (6.5 mg, 0.010 mmol) was heated in a screw-cap NMR tube at 80 °C for 9 h (the minimum period required for the full conversion to take place), accompanied by a color change from dark violet to black. The 31P{1H} NMR spectrum revealed full consumption of 2 and formation of the previously reported complex 6a.6 The solvent was evaporated. The residue was dissolved in pentane and the solution filtered through a cotton pad. Evaporation of pentane gave 5.7 mg (85% yield) of 6a as a black solid. 6a was identified by its 31P{1H} and 1H NMR spectra in benzene-d6. (b). A benzene-d6 solution (1.0 mL) of complex 2 (6.5 mg, 0.010 mmol) was heated in a screw-cap NMR tube at 80 °C for 9 h (the minimum period required for the full conversion to take place), accompanied by a color change from dark violet to black. The 31P{1H} and 1H NMR spectra revealed formation of the previously reported compound 6b.6 Deuterium incorporation into the benzylic position of 6b was observed, on the basis of the integration of the benzylic protons to 3H in the 1H NMR spectrum (the spectrum was measured with a time delay of 20 s; the protons were integrated relative to the aromatic ligand protons). Product acetone was observed, but only ∼50% of the expected amount was formed, due to losses upon heating. Computational Methods. All calculations were carried out using Gaussian 09, Revision A.02.16 Two members of the M06-family of DFT functionals17 were used: M06, a meta-hybrid functional containing 27% HF exchange,18a and M06-L, its local (nonhybrid) variant.18b With these functionals, two basis set-RECP (relativistic effective core potential) combinations were used. The first, denoted SDD(d), is the combination of the Huzinaga-Dunning double-ζ (D95V) basis set19 on lighter elements with the Stuttgart-Dresden basis set-RECP combination20 on transition metals; extra polarization functions (i.e, the D95(d) basis set) were added to phosphorus. The (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; , Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (17) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157. (18) (a) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (b) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (19) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry 3: Methods of Electronic Structure Theory; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1977; Vol. 3, pp 1-28. (20) Dolg, M. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J., Ed.; John von Neumann Institute for Computing: J€ ulich, Germany, 2000; Vol. 3, pp 507-540.

Schwartsburd et al. second, denoted SDB-cc-pVDZ, combines the Dunning cc-pVDZ basis set21 on the main group elements and the Stuttgart-Dresden basis set-RECP20 on the transition metals with an added f-type polarization exponent taken as the geometric average of the two f exponents given in the appendix of ref 22. In order to improve the efficiency of the calculations, density-fitting basis sets (DFBS) were employed during the calculation of the Coulomb interaction. The automatic DFBS generation algorithm as implemented in Gaussian09 was employed.23 The accuracy of the DFT method was improved by adding an empirical dispersion correction as recommended by Grimme.24 This correction is added to the M06-L and M06 DFT functionals and is included during the geometry optimizations and the frequency, energy, and solvation calculations; this is indicated by appending þd to the functional acronym. Our local version of Gaussian09 has been locally modified to allow the inclusion of dispersion for any functional for which the parameters are available. Briefly, the dispersion energy is equal to24

Edisp ¼ - s6

NX Nat at - 1 X i

fdmp ðRij Þ ¼

j

C6ij fdmp ðRij Þ R6ij

    - 1 Rij 1 þ exp - d -1 Rr

C6ij ¼

qffiffiffiffiffiffiffiffiffiffiffi C6i C6j

where Rr is the sum of van der Waal radii (rvdW) of the two atoms in question and Ci6 is an empirical constant; these values for H-Xe have been determined by Grimme.25 s6 is an empirical scaling factor unique for each DFT functional. For M06-L and M06, it has been determined to be 0.20 and 0.25, respectively.26

The rvdW and Ci6 values are missing for the third-row transition metals (i.e., the entire sixth row of the periodic table). These values for the first- and second-row transition metals were determined by Grimme by averaging the values for the group 2 and 13 elements of the same row.25 Thus, the rvdW and Ci6 parameters for the third-row transition metals were determined by averaging the parameters for Ba and Tl; the lanthanides were assigned parameters in the same manner. The parameters for these two elements, as well as the rest of the sixth row of the periodic table, were determined by a geometric extrapolation of the parameters for the preceding two rows. Bulk solvent effects were approximated by single-point energy calculations using a polarizable continuum model (PCM),27 specifically the integral equation formalism model (IEF-PCM)27a,b,28 with benzene as the solvent as in the experiments. In the PCM (21) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (22) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408. (23) (a) Dunlap, B. I. J. Chem. Phys. 1983, 78, 3140–3142. (b) Dunlap, B. I. J. Mol. Struct. (THEOCHEM) 2000, 529, 37. (24) (a) Schwabe, T.; Grimme, S. Phys. Chem. Chem. Phys. 2007, 9, 3397. (b) Schwabe, T.; Grimme, S. Acc. Chem. Res. 2008, 41, 569. (25) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (26) Karton, A.; Tarnopolsky, A.; Lamere, J.-F.; Schatz, G. C.; Martin, J. M. L. J. Phys. Chem. A 2008, 112, 12868. (27) (a) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (b) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (c) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253. (d) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43. (28) (a) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506. (b) Tomasi, J.; Mennucci, B.; Cances, E. J. Mol. Struct. (THEOCHEM) 1999, 464, 211.

Article

Organometallics, Vol. 29, No. 17, 2010

3827

model, the united atom topological model was used with the atomic radii from the UFF force field with explicit spheres on the hydrogen atoms. Geometries were optimized using the default pruned (75,302) grid, while the “ultrafine” (i.e., a pruned (99,590)) grid was used for energy and solvation calculations, especially essential for calculations with the M06 family of functionals.29 Geometry optimizations and frequency calculations were performed in the gas phase at the M06-Lþd/SDD(d)/DFBS level of theory. The energies in solution (benzene) were determined at the PCM(C6H6)-M06þd/SDB-cc-pVDZ level of theory. This com-

bined level of theory is conventionally denoted as PCM(C6H6)M06þd/SDB-cc-pVDZ//M06-Lþd/SDD(d)/DFBS. The connectivities of all transition states were confirmed by performing intrinsic reaction coordinate (IRC) calculations.30

(29) Wheeler, S. E.; Houk, K. N. J. Chem. Theory Comput. 2010, 6, 395. (30) (a) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (b) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154. (c) Fukui, K. Acc. Chem. Res. 1981, 14, 363.

Supporting Information Available: CIF files giving X-ray data for 4 and 5a and tables giving Cartesian coordinates (XYZ format) of all DFT-optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. This research was supported by the Israel Science Foundation, by the European Research Council under the FP7 framework (ERC No. 246837), and by the Helen and Martin Kimmel Center for Molecular Design. D.M. is the holder of the Israel Matz Professorial Chair of Organic Chemistry.