Activation of a Nonstrained C(sp2)âC(sp2) Single Bond in a Vaska

Organometallics 2011, 30, 171–186 DOI: 10.1021/om100985r

171

Activation of a Nonstrained C(sp2)-C(sp2) Single Bond in a Vaska Complex Analogue Using Chelating Assistance Andreas Obenhuber† and Klaus Ruhland*,‡ †

Lehrstuhl f€ ur Anorganische Chemie, Department Chemie, Technische Universit€ at M€ unchen, Lichtenbergstrasse 4, D-85747 Garching bei M€ unchen, Germany, and ‡Lehrstuhl f€ ur Chemische Physik und Materialwissenschaften, Institut f€ ur Physik, Universit€ at Augsburg, Universit€ atsstrasse 1, D-86159 Augsburg, Germany Received October 15, 2010

The reaction of Vaska’s complex with the ligand P(iPr)2-O-Ar-CO-Ar-O-P(iPr)2 (L1, p-H; L2. p-OMe) containing a benzophenone moiety, resulted in the quantitative oxidative addition of the PhC-CO bond leading to 1. For 1 three consecutive reactions can be observed: upon loss of CO trans-2 is formed, which further isomerizes first to cis-3 and finally to cis0 -4. The mechanism of the oxidative addition, and the CO extrusion as well as the trans/cis/cis0 isomerization were examined, activation parameters determined, and trapping experiments performed. If 3 is placed under a 4 bar atmosphere of CO at 373 K for several days, the slow re-formation of 1 (20% after 5 days) can be observed. Labeling studies with 13CO revealed no scrambling of the 13CO into the acyl position. When the oxidative addition was examined, starting with the 13CO-labeled Vaska complex, the label remained exclusively at the metal and no carbonyl scrambling was observed during CO extrusion. Heating 3 to temperatures higher than 423 K resulted in the formation of 4. Performing this reaction under an atmosphere of 13CO revealed an incorporation of 13CO into 4. Halide abstraction from 3 failed. Using AgBF4 in chloroform resulted in the formation of the Ag-bridged dimer 6. Molecular structures of the new complexes 1a, 2-4, and 6 have been determined by single-crystal X-ray diffraction studies. DFT calculations on the major pathways have been performed.

Introduction The development and understanding of controlled C-C single-bond activation processes still remains a great challenge in current organometallic chemistry.1 Several C-C single bonds have already been successfully cleaved.2-4 Only a few examples can be found for metal insertion into C-C single bonds using Ir complexes. Among the most prominent examples is Milstein’s PCP pincer type system.2f-s Iridium and other metals (Ru,2q Os,2r,s Rh,2g-o Ni,2p Pt2q) have been shown to insert into the unstrained C(sp2)-C(sp3) single bond of these ligands.2f Crabtree employed [IrH2(Me2CO)2{(p-FC6H4)3P}2]SbF6 as precursor to cleave the CH3(sp3)-C(sp3) bond in 5,5-dimethylcyclopentadiene in an intramolecular reaction using aromatization as the driving force.2e Additionally, the strained C(sp2)-C(sp2) single bond of biphenylene was cleaved using [(COD)IrCl]2 as the metal source.3a In addition to the potential of Ir compounds to activate carbon-carbon bonds, since the early work of Vaska on trans-IrCl(CO)(PPh3)2 this iridium compound is known to oxidatively add a broad range of molecules such as H2, O2, Cl2, HCl, CH3I,5 whereas no example exists for an oxidative addition of a C-C single bond to it. Recently we reported the successful cleavage of nonstrained C(sp2)-C(sp2) single bonds by a Ni(0) center using

chelating assistance.6a One of the investigated ligands (L1, P(iPr)2-O-Ph-CO-Ph-O-P(iPr)2) contained a benzophenone moiety. We extended our investigations with this ligand architecture into group 9 metals. We herein report our latest results concerning the oxidative addition of a nonstrained C(sp2)-C(sp2) single bond in a Vaska complex analogue. We have already reported on the reactivity of Vaskaanalogue complexes in connection with strained C-C single bonds.6b

Results and Discussion

(1) (a) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (b) Chul-Ho, J. Chem. Soc. Rev. 2004, 33, 610.

For our investigations ligands L1 and L2 were used (Scheme 1). To probe electronic influences we introduced a methoxy group para to the carbonyl function. The þM effect of the OMe groups in L2 (ν(CO) 1603 cm-1) leads to a more electron-rich CdO group in comparison to L1 (ν(CO) 1655 cm-1). The ligand architecture employed comprises several advantages: • Once a C-C single bond activation occurs, two strainfree (five- and six-membered) rings are formed. • Metal-C(sp2) bonds are generally considered stronger as compared to M-C(sp3) bonds.7 • The usage of oxo linkers between the phosphorus groups and the phenyl rings help to provide the shortest possible chelate backbone, forcing and fixing the C-C single bond under focus in close proximity to the metal center.

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• Side reactions such as β-H elimination and CH activation are made impossible or less likely by the ligand architecture employed. Both ligands L1 and L2, when reacted with Vaska’s complex (or IrCl(CO)(coe)3), gave a quantitative oxidative (2) C(sp3)-C(sp3): (a) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. J. Am. Chem. Soc. 1992, 114, 1025–1032. (b) Evans, W. J.; Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 1068–1069. (c) Etienne, M.; Mathieu, R.; Donnadieu, B. J. Am. Chem. Soc. 1997, 119, 3218–3228. (d) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471–6473. (e) Crabtree, R. H.; Dion, R. P. Chem. Commun. 1984, 1260. (f) Crabtree, R. H.; Dion, R. P.; Gibboni, D. J.; McCrath, D. V.; Holt, E. M. J. Am. Chem. Soc. 1986, 108, 7222. C(sp3)-C(sp2): (g) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 1996, 118, 12406. (h) Rybtchinski, B.; Oevers, S.; Montag, M.; Vigalok, A.; Rozenberg, J. M. L.; Martin, H.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 9064. (i) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. L. J. Am. Chem. Soc. 2000, 122, 7095. (j) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993, 364, 699. (k) Rytchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 1996, 118, 12406. (l) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 9774. (m) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 895–905. (n) Vigalok, A.; Milstein, D. Organometallics 2000, 19, 2061. (o) Vigalok, A.; Milstein, D. Organometallics 2000, 19, 2341. (p) Salem, H.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. Organometallics 2006, 25, 2292. (q) van der Boom, M. E.; Liou, S.-Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2004, 357, 4015–4023. (r) van der Boom, M. E.; Kraatz, H. B.; Hassner, L.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 3873. (s) Gauvin, R. M.; Rozenberg, H.; Shimon, L. J. W.; Milstein, D. Organometallics 2001, 20, 1719. (t) Gauvin, R. M.; Rozenberg, H.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Chem. Eur. J. 2007, 13, 1382. (u) Suggs, J. W.; Jun, C.-H. J. Am. Chem. Soc. 1984, 106, 3054–3056. (v) Suggs, J. W.; Jun, C.-H. J. Am. Chem. Soc. 1986, 108, 4679–4681. (w) Jun, C.-H.; Lee, H.; Lim, S.G. J. Am. Chem. Soc. 2001, 123, 751–752. (x) Jun, C.-H.; Lee, H. J. Am. Chem. Soc. 1999, 121, 880–881. C(sp3)-C(sp): (y) Acosta-Ramirez, A.; Munoz-Hernandez, M.; Jones, W. D.; Garcia, J. J. Organometallics 2007, 26, 5766–5769. (z) Garcia, J. J.; Arevalo, A.; Brunkan, N. M.; Jones, W. D. Organometallics 2004, 23, 3997–4002. (3) C(sp2)-C(sp2): (a) Lu, Z.; Jun, C. H.; de Gala, S. R.; Sigalas, M. P.; Eisenstein, O.; Crabtree, R. H. Organometallics 1995, 14, 1168– 1175. (b) Chantani, N.; Ie, Y.; Kakiuchi, F.; Murai, S. J. Am. Chem. Soc. 1999, 121, 8645–8646. (c) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843–2853. (d) Perthuisot, C.; Edelbach, B. J.; Zubirs, D. L.; Simhai, N.; Iverson, C. N.; M€uller, C.; Satoh, T.; Jones, W. D. J. Mol. Catal. A: Chem. 2002, 189, 157–168. (e) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18, 4040–4049. (f) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. Organometallics 1999, 18, 4660–4668. (g) M€ uller, C.; Lachicotte, R. J; Jones, W. D. Organometallics 2002, 21, 1975–1981. (h) Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J.; Jones, W. D. Organometallics 1998, 17, 4784–4794. (i) Ujaque, G.; Maseras, Z.; Eisenstein, O.; Liable-Sands, L.; Rheingold, A. L.; Yao, W.; Crabtree, R. H. New J. Chem. 1998, 1493–1498. (j) Iverson, C. N.; Jones, W. D. Organometallics 2001, 20, 5745–5750. (k) Zhang, X.; Carpenter, G. B.; Sweigart, D. A. Organometallics 1999, 18, 4887–4888. (l) Schwager, H.; Spyroudis, S.; Vollhardt, K. P. C. J. Organomet. Chem. 1990, 382, 191–200. (m) Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Kr€uger, C.; Tsay, Y. H. Organometallics 1985, 4, 224–231. (n) Shaltout, R. M.; Sygula, R.; Sygula, A.; Fronczek, F. R.; Stanley, G. G; Rabideau, P. W. J. Am. Chem. Soc. 1998, 120, 835–836. (o) Schaub, T.; Radius, U. Chem. Eur. J. 2005, 11, 5024– 5030. (p) Daugulis, O.; Brookhart, M. Organometallics 2004, 23, 527. uller, C.; Iverson, C. N.; Lachicotte, R. J.; Jones, W. D. C(sp2)-C(sp): (q) M€ J. Am. Chem. Soc. 2001, 123, 9718–9719. (r) Garcia, J. J.; Arevalo, A.; Brunkan, N. M.; Jones, W. D. Organometallics 2004, 23, 3997–4002. (s) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544–5545. (t) Garcia, J. J.; Brunkan, N. M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 9547– 9555. (u) Li, T.; Garcia, J.; Brennessel, W., W.; Jones, W. D. Organometallics 2010, 29, 2430. (4) C(sp)-C(sp): (a) Rosenthal, U.; Arndt, P.; Baumann, W.; Burlakov, V. V.; Spannenberg, A. J. J. Organomet. Chem. 2003, 670, 84. (b) Rosenthal, U.; G€ orls, H. J. Organomet. Chem. 1992, 439, C36. (5) (a) Vaska, L. Science 1963, 140, 809. (b) Vaska, L.; Bath, S. S. J. Am. Chem. Soc. 1966, 88, 1333. (c) Chock, P. B.; Halpern, J. J. Am. Chem. Soc. 1966, 88, 3511. (d) Vaska, L. Acc. Chem. Res. 1968, 1, 335. (6) (a) Ruhland, K.; Obenhuber, A.; Hoffmann, S. D. Organometallics 2008, 27, 3482. (b) Ruhland, K.; Herdtweck, E. Adv. Synth. Catal. 2005, 347, 398. (7) Martinho, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629–688.

Obenhuber and Ruhland

addition of the PhC-CO bond to the metal center at 348 K. To the best of our knowledge, this is the first time that a C-C single bond was cleaved using a Vaska-type complex. Upon further heating, three consecutive reactions were observed. Scheme 2 summarizes the results obtained with L1 and L2. Oxidative Addition. For L2 the reaction with Vaska’s complex (cVaska=10 mM) was followed by 31P{1H} (Figure 1) and 1H NMR spectroscopy in toluene. At room temperature L2 coordinates immediately via the P atom to the Ir center and a mixture of three compounds together with free PPh3 (broadened) is observed. Scheme 3 summarizes the proposed reaction network leading finally to the C-C single-bond activation (compound 1a). Compounds b and c can be assigned to three doublets (25.5 (3 PPh3), 143.4 (2 P(iPr)2) and 143.6 ppm (1 P(iPr)2)), all showing a 2JPP coupling constant of 362 Hz, proving the trans configuration of the two coupling phosphorus nuclei. Two further nearly overlapping signals at 148 ppm are assigned to dimer a and the noncoordinated phosphinite of b (chemical shift of the free ligand L2 149 ppm). X-ray structures analogous to dimer a are known with several bisphosphine ligands.8 We can provide X-ray structures for compounds similar to a and d with an innocent but still comparable chelate ligand bearing a CH2-O- linker instead of just an oxo between the P atom and the biphenyl backbone without a CO moiety (a0 Rh and d0 ; Figure 2). Evaluation of the signal integrals confirms the composition of the reaction solution. A ratio of 7/8/1 is found for a/b/ c. Addition of PPh3 shifts the equilibrium at room temperature and increases the amounts of b and c. When the solution was heated at 348 K, oxidative addition (activation of the C-C single bond) slowly proceeded. The development of two new doublets at 111 and 145 ppm with a shared coupling constant of 2JPP = 318 Hz could be observed, which are assigned to 1a (proof by X-ray analysis). The oxidative addition proceeds via the OC-Ir-Cl axis with the phosphorus groups located trans to one another. Concerning the possible diastereomers (1a vs 1a0 ) only one is observed. After 75 min all starting material (a-c) is consumed. No hydride signals were detected in the 1H NMR spectrum (0 to -40 ppm) during the course of the oxidative addition reaction. As d is not observed, we propose that the rate-determining step for the whole process is not metal insertion into the C-C bond but either the formation of b (from a and/or c) or PPh3 substitution in b to form d. Crystals suitable for X-ray analysis of 1a were grown from chloroform/ pentane, and a molecule in the solid state is depicted in Figure 3. The coordination around the Ir center shows nearly no distortion from an ideal octahedral geometry. As X-ray diffraction revealed, the only diastereomer being formed is the one with the metal-bonded CO and the acyl group of the ligand orientated cis to one another. With a distance of d(C-C) = 3.032 A˚ the PhC-CO bond has been completely cleaved (d(Caryl-Cacyl)=1.47 A˚ in 2,20 dihydroxybenzophenone).9 Although compound 1a does not give rise to any dynamic behavior on the spectral time scale of the employed analytical methods at room temperature, the possibility of carbonyl scrambling during the oxidative addition process could not be excluded, and it was further examined. Therefore, (8) (a) Sanger, A. R. Dalton Trans. 1977, 20, 1971. (b) McDonald, R.; Sutherland, B. R.; Cowie, M. Inorg. Chem. 1987, 26, 3333. (9) Schlemper, E. O. Acta Crystallogr. 1982, B38, 1619. (10) Benett, M. A.; Miller, D. L. J. Am. Chem. Soc. 1969, 91, 6983.

Article


Vaska’s complex with 13C-labeled CO was prepared.10 Examining the oxidative addition reaction as described above revealed no exchange between the carbonyl at the metal center (13C{1H} NMR: 173 ppm (pseudo t, 2JCPA,cis = 2JCPB,cis = 6.9 Hz)) and the acyl position of the ligand (13C{1H} NMR: 193 ppm (dd, 2JCPA,cis = 8.1 Hz, 2JCPB,cis = 4.4 Hz). This enabled us to prepare 1a* selectively, which was used in elucidating the mechanism of the consecutive CO extrusion (vide infra). In the case of L1 the reaction with Vaska’s complex in toluene (cVaska = 10 mM) resulted in the formation of an insoluble coordination polymer at room temperature which upon heating (378 K, 45 min) was completely converted to the oxidative addition product 1. All spectroscopic results Scheme 1

173

speak in favor of the same regioselectivity in 1 as was shown for 1a. Separation of 1 from the products of the consecutive reactions is achieved through column chromatography. Replacing the PPh3 substituents through the more labile coe ligands using IrCl(CO)(coe)3 did not facilitate the formation of monomeric species (analogously to b and d). Dilution (cIr=1mM) and changing the solvent to chloroform avoided the formation of an insoluble coordination polymer. A slightly broadened singlet in the 31P NMR spectrum at 148 ppm is observed, which is assigned to the ligand-bridged dimer analogous to a.8 Ir-bonded coe can no longer be detected in the 1H NMR spectrum, with the free coe being slightly broadened. The complete reaction, including an oxidative addition reaction at room temperature, was not finished after days and suffered from decomposition at higher temperatures, most likely as a result of reaction with the solvent. DFT Calculations on the C-C Single-Bond Activation. DFT calculations on the B3PW91/LanL2DZ level support the experimental findings. We calculated the pathways for both possible diastereomers concerning the C-C singlebond activation and the Cl-Ir-CO axis and assuming the formation of d (Scheme 4).

Scheme 2

Figure 1. Development of the reaction of 1 equiv of L2 with Vaska’s complex at 348 K in toluene-d8 with time, followed by 31P{1H} NMR spectroscopy (excerpt). NMR spectra were recorded in 5 min intervals.

174


Obenhuber and Ruhland Scheme 3

These calculations show that the precoordinated complex is thermodynamically unstable against C-C single-bond activation (Kactiv,298 K = 3204 f quantitative formation of 1). Also, the exclusively found diastereomer is predicted correctly by the calculations. The formation of it is favored over the alternative diastereomer by about 60 kJ/mol with regard to the activation enthalpy. Thus, according to the calculations the diastereoselectivity is unequivocally controlled kinetically. We propose that a more favorable trans effect (phenyl/CO and acyl/Cl in comparison to phenyl/Cl and acyl/CO) in the transition state of the preferred pathway explains this finding chemically. The calculated small activation enthalpy of 82 kJ/mol supports the notion that the activation of the C-C single bond is not the rate-determining step in the reaction cascade. Notably, according to the calculations a precoordination of the keto group prior to the activation does not take place, in contrast with all other examples in the literature. If the CO moiety is placed at

distances smaller than 2 A˚ for C and O to the Ir center (forcing a CO-η2 coordination), during the structure optimization the CO group develops away from the Ir center, ending in structure d without the CO being bonded to Ir. The same happens if an O-η starting geometry is applied with a O 3 3 3 Ir distance smaller than 2 A˚. CO Extrusion. The fate of 1/1a upon further heating was examined. The development of the reaction at 368 K in toluene for 1 is shown in Figure 4 (31P{1H} NMR spectra). As the intensity of the doublets of 1 located at 108 and 143 ppm starts to diminish, a new signal at 132 ppm appears. This singlet is assigned to 2, which is formed through loss of CO from 1 (proof by X-ray analysis). We were able to isolate 2 through adjustment of reaction conditions (time and temperature) and solubility properties. Crystals suitable for X-ray analysis were grown from chloroform/pentane, and a molecule in the solid state is shown in Figure 5.

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Figure 2. ORTEP representations of molecules in the solid state of compounds a0 Rh (left) and d0 (right). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity.

Figure 3. ORTEP representation of a molecule in the solid state of compound 1a. Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg): d(P1-Ir) = 2.3457(6), d(C1-Ir) = 1.924(2), d(C3-Ir) = 2.119(2), d(C22-Ir) = 2.037(2), d(C3-C22) = 3.032(3); R(C1-Ir1-Cl1) = 91.87(7), R(C3-Ir1-C22) = 93.65(9), R(P2-Ir1-P1) = 171.89(2), γ(O4-C22-Ir1-C1)=-35.4(2).

The formation of 2/2a can be quantified in the temperature range 348-378 K through evaluating the integrals of the different species in 31P{1H} NMR spectroscopy. The formation of 2a with three different starting concentrations of 1a was examined at 363 K, assuming a first-order rate law (9, 18, 36 mg/mL). As 2/2a reacts further in the examined temperature range (vide infra), this assumption is only valid for 20-25% conversion (depending on the temperature). No dependence on the concentration for the process was found (Figure 6). We therefore conclude that the rate-determining step is a monomolecular process. The activation parameters for the CO extrusion were determined using an Eyring plot, shown in Figure 7. The kinetic data are summarized in Table 1. For 1 f 2 an activation enthalpy of ΔHq = 121 ( 2 kJ/mol

and an activation entropy of ΔSq = 4 ( 6 J/(K mol) were extracted. For the OMe-substituted 1a values of ΔHq = 123 ( 5 kJ/mol and ΔSq = 10 ( 12 J/(K mol) were found. The entropy values mirror the intramolecular character of the rate-determining step. Scheme 5 provides three different pathways for the formation of the trans isomer (2/2a). The lost CO can in principle originate from the carbonyl at the metal, from the acyl position of the ligand, or from both. Pathway I demands the exclusive loss of the metal-bonded CO as the first step. A migratory extrusion in the 5-fold-coordinated 5a would then yield 2a (pathway IB). Although 5a was not observed, there are several examples of isolable 5-fold-coordinated acyl complexes of IrIII. These complexes are known to isomerize to alkyl or aryl carbonyl complexes. The reaction mechanism of those has also been studied.11 We can provide an X-ray structure for a complex analogous to 5a with Rh instead of Ir as metal center which is obtained by reaction of L1 with [(coe)2RhCl]2, but it will be reported in the context of another paper. Pathway II would include the possibility of 12/13CO scrambling. The acyl-bonded aryl group must in this case migrate to the cis metal-bonded carbonyl. Two consecutive ligand rearrangements (“T-flip”, migratory aryl extrusion) would lead to 2a*. We were able to differentiate between the two basic options I and II by examining the formation of 2a starting with 1a* (13CO-labeled 1a) (vide supra). As 31P and 13 C NMR spectroscopy revealed, the only process taking place is the loss of the 13CO bonded to the metal center (pathway I). The doublet of doublets at 111 and 145 ppm (2JPP = 318 Hz, 2JCP = 7.4 Hz) in the 31P NMR spectrum vanished at 378 K within 120 min, together with the pseudotriplet at 173 ppm in the 13C NMR spectrum for the metalbonded CO. No 13C-labeled CO was incorporated into 2a (31P{1H} NMR 134 ppm (s); 13C NMR 166.3 ppm (t, 2JCP = 8.05 Hz, IrCO). A comparison of the activation enthalpies (about 122 kJ/mol) with literature values for IrIII-CO bond dissociation enthalpies (D(IrIII-CO) ≈ 138 kJ/mol)12 supports (11) (a) Kubota, M.; Blake, D. M.; Smith, S. A. Inorg. Chem. 1971, 10, 1430. (b) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16, 299. (12) Rosini, G. P.; Liu, F; Krogh-Jespersen, K.; Goldman, A. S.; Li, C.; Nolan, S. P. J. Am. Chem. Soc. 1998, 120, 9256.

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Figure 4. Development of the CO extrusion and trans/cis isomerization starting from 1 at 368 K in toluene-d8 with time followed by 31P NMR spectroscopy. NMR spectra were recorded in 0.5 h (first three spectra) and 1.0 h (remaining spectra) intervals. Scheme 4

that the rate-determining step is the loss of CO (not aryl migration). We therefore propose a transition state with a partially cleaved Ir-CO bond. The electron-releasing character of the OMe groups increases the metal to carbonyl back-bonding (ν(CO) 2049 cm-1 for 1 vs ν(CO) 2041 cm-1 for 1a) and causes a slight deceleration of CO loss. The presence of free PPh3 (2 equiv) has nearly no influence on the

kinetics of the formation of 2a, as was determined at 378 K (probe for temporary available coordination sites). DFT Calculations Concerning the CO Extrusion. The results of the DFT calculations on the loss of CO are shown in Scheme 6. The calculated activation parameters (ΔHqcalcd = 123 kJ/ mol vs ΔHqexptl = 121 kJ/mol; ΔSqcalcd = 6 J/(mol K) vs

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Figure 5. ORTEP representations of molecules in the solid state of compounds 2 (left) and 3 (right). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (A˚) and angles (deg) for 2: d(P1-Ir1) = 2.352(2), d(C2-Ir1) = 2.133(6), d(Cl1-Ir1) = 2.430(7), d(C1-Ir1) = 1.78(2), d(C2-C1) = 2.73(2); R(C2-Ir1-C1) = 88.1(7), R(C2-Ir1-P1a) = 100.9(2), R(C2-Ir1-P1) = 79.1(2). Selected bond distances (A˚) and angles (deg) for 3: d(P1-Ir1) = 2.358(2), d(C13-Ir1) = 2.121(7), d(Cl-Ir1) = 2.401(2), d(C13-C7) = 3.19(1), d(C1-Ir1) = 1.853(8), d(C1-C7) = 2.74(1); R(C7-Ir1-C1) = 86.3(3), R(P2-Ir1-P1) = 109.56(6), R(C13-Ir1-P2) = 77.4(2), R(C7-Ir1-C13) = 96.7(3). Table 1. Rate Constants for the CO Extrusion of Compounds 1 and 1a at Different Temperatures

Figure 6. Evaluation of the CO extrusion assuming a first-order rate law with three different starting concentrations of 1a at 363 K: (O) 9 mg/mL; (4) 18 mg/mL; (0) 36 mg/mL

Figure 7. Eyring plot for the CO extrusion of compounds 1 (]) and 1a (0).

T (K)

104kCOex(1) (s-1)

104kCOex(1a) (s-1)

348 353 358 363 368 373 378

0.09(2) 0.18(5) 0.31(6) 0.52(2) 1.01(3) 1.70(2) 2.65(6)

0.13(4) 0.25(7) 0.45(1) 0.91(3) 1.34(2) 2.23(6)

ΔSqexptl = 4 J/(mol K)) for the nonconcerted pathway IB in Scheme 5 are in very good agreement with the measured values. A concerted CO loss/migratory CO extrusion, which was taken into account because of the near-zero activation entropy found experimentally, and the nonobservance of 5a, including the inability to trap it with PPh3, can be ruled out according to the calculations because the activation enthalpy is far too high. (It must be mentioned, though, that 2 and free CO according to the calculations should be thermodynamically decisively less stable than 1, which contradicts our experimental results. This finding does not change when a more powerful basis set was applied (G6-311 for C, H, O, P, Cl; LanL2DZ for Ir). We assume that this is most likely because the calculations are made for both the complex and CO being in the gas phase; thus, the release of gaseous CO from the complex in the solid or in solution is entropically underestimated since the complex in reality is not in the gas phase.) Trans/Cis/Cis0 Isomerization. If 2/2a is heated further, an isomerization to the cis isomer (3/3a) takes place. The development of the isomerization can be followed by 31 P{1H} NMR spectroscopy. Figure 4 shows the formation of 3 at 368 K, which appears as a singlet at 142 ppm. As the cis isomer does not react further at T < 423 K, isolation was achieved after prolonged heating in toluene or xylene. Crystals suitable for X-ray analysis of 3 were grown from toluene/pentane, and a molecule in the solid state is shown in Figure 5.

178


Obenhuber and Ruhland Scheme 5

Scheme 6

The formation of 3/3a was quantified in the temperature range 363-393 K through evaluating the integrals of the different species in the 31P{1H} NMR spectrum. The concentration dependence of the process was examined. Three different starting concentrations of 2 (9, 18, 36 mg/mL) were evaluated at 368 K and at 393 K assuming a first-order rate law (Figure 8). No concentration dependence was detected, and we conclude that within the examined temperature and concentration range the rate-determining step is a monomolecular process. The activation parameters for the isomerization were determined using an Eyring plot, shown in Figure 9. The kinetic data are summarized in Table 2. For 2 f 3 an activation enthalpy of ΔHq = 110 ( 7 kJ/mol and an activation entropy of ΔSq = -34 ( 19 J/(K mol) were extracted. For the OMe-substituted 2a values of ΔHq = 106 ( 5 kJ/mol and ΔSq = -42 ( 14 J/(K mol) were obtained. The isomerization to 3 is irreversible. If pure 3 is heated for several days at 353 K, no formation of 2 is observed.

Scheme 7 provides three different possibilities for the mechanisms of the trans/cis isomerization. All three pathways contain as the first step the formation of a five-coordinated Ir center that rearranges through internal rotation (“T-flip”) in order to form the cis isomer. Pathway A starts with a migratory carbonyl insertion, whereas an Ir-P bond dissociation is the first step in pathway B. Pathway C differs from pathways A and B through the temporary formation of charged species. As the trans/cis isomerization is performed in toluene or xylene, contact ion pairs rather than solvent-separated ions seem plausible. To differentiate between the three proposed pathways, several reagents were added to the isomerization reaction mixture in order to trap anticipated intermediates or to influence the kinetics of the process. The addition of PPh3 or pyridine decelerated the isomerization, but no intermediate could by trapped. This observation does not exclude one of the proposed pathways, since all three proceed via under-coordinated species. If 3 is dissolved in toluene or xylene and placed under a 4 bar atmosphere of CO at 373 K for several days, the slow re-formation of 1 (20% after 5 days) together with 2 (16% after 5 days) can be observed. When the experiment is repeated with 13C-labeled CO, the 13CO is exclusively found at the metal center of 1*. A related reaction can be found in the literature.13 When trans-Ir(Cl)2(CO)Me(PPh3)2 was placed under a 1.4 bar atmosphere of CO in methanol/dcm at room temperature for 12 h, the quantitative formation of trans-Ir(Cl)2(CO)(COMe)(PPh3)2 was observed. No labeling studies were done in order to follow the pathway of CO incorporation in that work. No reaction with CO was observed in benzene under the same reaction conditions, and the authors proposed a reaction mechanism proceeding via chloride dissociation in that case. As the ionic pathway C requires Cl dissociation, the halide abstraction was examined. Although the isomerization was run in nitromethane (which did not decisively accelerate the process) to promote the formation of charged compounds, (13) Blake, D. M.; Kubota, M. J. Am. Chem. Soc. 1971, 93, 1368.

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179

Figure 8. Evaluation of the trans/cis isomerzation assuming a first-order rate law with three different starting concentrations of 2 at 368 K (left) and 393 K (right): (O) 9 mg/mL; (4) 18 mg/mL; (0) 36 mg/mL

Figure 9. Eyring plot for the trans/cis isomerization of compounds 2 (]) and 2a (0). Table 2. Rate Constants for the Trans/Cis Isomerization of Compounds 2 and 2a at Different Temperatures T (°C)

104kiso(2) (s-1)

363 368 373 378 382 388 393

0.27(1) 0.48(1) 0.72(2) 1.39(7) 1.89(7) 2.82(9)

104kiso(2a) (s-1) 0.26(1) 0.34(1) 0.61(1) 1.06(2) 1.37(2) 2.53(6)

no intermediate could be trapped with Na[BPh]4 at 363 K over several hours. Also, the synthesis of a charged species from 3 was tested. TlPF6 in chloroform had no effect on 3 even after prolonged heating to 333 K. The reaction between AgBF4 and 3 in chloroform caused the immediate formation of the new compound 6 at room temperature. Crystals suitable for X-ray analysis of 6 were grown from chloroform, and a molecule in the solid state is shown in Figure 10. The Ir center and the silver cation are competing for the chloride ion. 6 can be viewed as an intermediate on the reaction path of Cl abstraction.14 The transfer of the chlorine from the IrIII center to the AgI center is arrested in an early stage, as can be judged by the bond lengths (Ag 3 3 3 Cl in 6 2.561(1) A˚ vs Ag 3 3 3 Cl in AgCl(g) 2.281 A˚ (microwave spectroscopy17); Ir 3 3 3 Cl in 6 2.4306(9) A˚ vs Ir 3 3 3 Cl in 3 (14) (a) Liston, D. J.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A. J. Am. Chem. Soc. 1989, 111, 6643. (b) Mattson, B. M.; Graham, W. A. G. Inorg. Chem. 1981, 20, 3186. (15) Hargittai, M. Chem. Rev. 2000, 100, 2233.

2.400(2) A˚). Reaction of 2 with AgBF4 in chloroform let to unidentified decomposition of 2. The failure to trap or synthesize ionic compounds disfavors pathway C. The parallel course of pathways A and B can be analytically described under steady-state conditions for 5a (Scheme 8). It is clearly seen that the overall activation parameters are an average of several elementary steps. The rate constants for the different steps k1, k2/k3, k-2, and k4 can be estimated by a least-squares fit of the equations in Scheme 8 for the isomerization of 2 to 3. Table 3 summarizes the results of this evaluation for two temperatures. Whereas for the isomerization 2 to 3 the pathway A contributes significantly, in the case of the reaction 2a to 3a pathway A seems to be negligible. At least in that case no parallel formation of 2a and 3a starting from 1a is observed; only a consecutive reaction 1a to 2a to 3a is found. Pathways A and B for the reaction 2 to 3 have been further examined by DFT calculations, and this will be discussed below. When 3 is heated to temperatures higher than 423 K either in the solid or in solution, the formation of 4 (at least 90% of product spectrum) can be detected (no 2 is found in the course of the reaction, which supports again the irreversibility of the formation of 3 from 2). An X-ray structure analysis of 4 (Figure 11) reveals it to be another cis0 isomer with the CO group and one P arm having changed positions. If the heating is performed under an atmosphere of 13CO, the incorporation of 13CO into 4 is observed. We explain the further isomerization behavior as shown in Scheme 9. The first experiments concerning the cis/cis0 isomerization of 3 to 4 in nitromethane and acetonitrile demonstrate that the mechanism in highly polar and coordinating solvents changes toward an ionic pathway with a cationic Ir center. We conclude this from the following observations: • The cis/cis0 isomerization takes place already at 353 K for 3a in acetonitrile (dielectricity constant 37.5 (20 °C); donor number 14.1), in contrast to 3, which under the same conditions barely shows any reactivity. • If instead of acetonitrile as solvent nitromethane is used (dielectricity constant 36 (30 °C); donor number 2.7), both 3a and 3 do not isomerize at 373 K. DFT Calculations on the Trans/Cis/Cis0 Isomerization. Schemes 10 and 11 (trans/cis) and Scheme 12 (cis/cis0 ) summarize the results of the DFT calculations concerning the trans/cis/cis0 isomerization.

180



Schemes 10 and 11 show that pathways A and B can both contribute to the formation of 3. The activation enthalpy for pathway B is well predicted by the calculations (experimental, 131 kJ/mol; calculated, ∼128 kJ/mol). It has to be mentioned, though, that compound 5a is predicted to be too stable by the calculations, according to which it should be observable, in contrast to our experimental findings. The calculations predict correctly the quantitative formation of the cis isomer starting from the trans isomer (K298 K,cis/trans = 190). Also, the final thermodynamic product 4 and the high activation enthalpy connected with its formation are predicted correctly (K298 K,cis0 /cis = 7.5; Scheme 12). The formation of only one diastereomer for the cis0 compound must be kinetically controlled according to the calculations. The competing diastereomer in which the Cl ligand T-flips instead of the CO is calculated to be more stable than that found experimentally. An interesting feature of the calculations

Figure 10. ORTEP representation of a molecule in the solid state of compound 6. Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity. The BF4- anions are not shown. Selected bond distances (A˚) and angles (deg): d(P1-Ir) = 2.387(1), d(C1-Ir) = 1.845(4), d(Cl1-Ir) = 2.431(1), d(Ag1-Cl1) = 2.561(1); R(Cl1-Ir1C1)=171.8(1), R(Ag1-Cl1-Ir)=106.38(4).

is that the oxo linker between the phosphorus and the aryl of the ligand shows an interaction with the metal center in some intermediates (M 3 3 3 O distances shorter than 2.45 A˚, bending of the phenyl moiety toward the metal) and also lowers the activation energy of the transition states connected with these intermediates. There was found no evidence for a secondary interaction of the oxo linker or the decoordinated P atom with the CO carbon in the most decisive transition states for the trans/cis as well as for the cis/cis0 isomerization, in spite of an intensive search by DFT calculations, including frozen coordinates, to force this kind of interaction. Scheme 13 summarizes the proposed reaction network for CO extrusion/insertion and trans/cis/cis0 isomerization.

Conclusion We were able to show that Vaska’s complex can serve as an Ir precursor, cleaving an unstrained PhC-CO bond when chelating assistance is employed. Mechanistic examinations using labeling studies and determination of activation parameters elucidated the ongoing chemistry in the reaction sequence “oxidative addition-CO loss-CO extrusiontrans/cis isomerization-cis/cis0 isomerization”. The reversibility of this sequence under CO pressure was shown. DFT calculations helped to explain the mechanism and to demonstrate the decisive role of secondary interactions concerning the phosphinite-oxo linker. Notably, the activation of the C-C single bond is not the rate-determining step and also a precoordination of the carbonyl moiety prior to the activation is not necessary according to the DFT calculations. Interesting reactivity with the employed ligand architecture was also found with further transition metals. Exploration of the reactivity of our ligand system with Ru and Os complexes is currently in progress. Also, a comparison with the analogous Rh chemistry is under construction.

Scheme 7

Article


Experimental Section General Procedures. Manipulations and experiments were performed under an argon atmosphere using standard Schlenk techniques and in an argon-filled glovebox if not otherwise stated. Diethyl ether, pentane, dichloromethane, and toluene were dried and degassed using a two-column drying system (MBraun) and stored under an argon atmosphere over molecular sieves. Deuterated solvents used in NMR studies, including CDCl3, CD2Cl2, benzene-d6, toluene-d8, and nitromethane-d3, were stored under argon over molecular sieves. IrCl3 3 xH2O was purchased from Strem. Vaska’s complex,16 trans-IrCl(PPh3)213CO,10 Table 3. Rate Constants for the Elemental Steps and the Pure Activation Enthalpies Received from Them for the Isomerization of 2 to 3 T (°C)

k2/k3

104k-2 (s-1)a

104k4 (s-1)b

104k1 (s-1)

368 378

4.9 5.1

0.37 1.09

0.87 2.62

1.21 4.02

a

ΔHq ≈ 128 kJ/mol. b ΔHq ≈ 131 kJ/mol.

181

IrCl(PPh3)3,11 and [IrCl(coe)2]217 were prepared according to literature methods. Carbon monoxide 2.5 was purchased from Messer Griessheim. 13C-labeled CO (13CO, 99%;

Activation of a Nonstrained C(sp2)âC(sp2) Single Bond in a Vaska

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