Hydroquinoid Chromium Complexes Bearing an Acyclic Conjugated

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Organometallics 2010, 29, 6172–6185 DOI: 10.1021/om100257x

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Hydroquinoid Chromium Complexes Bearing an Acyclic Conjugated Bridge: Chromium-Templated Synthesis, Molecular Structure, and Haptotropic Metal Migration Peter Hegele,† Bindu Santhamma,†,4 Gregor Schnakenburg,§ Roland Fr€ ohlich, Olga Kataeva, Martin Nieger,§,^ Konstantinos Kotsis,‡ Frank Neese,*,‡ and Karl Heinz D€ otz*,† †

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Kekul e-Institut f€ ur Organische Chemie und Biochemie, Rheinische Friedrich-Wilhelms-Universit€ at Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany, ‡Institut f€ ur Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universit€ at Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany, § Institut f€ ur Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universit€ at Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany, ^Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014, Finland, Organisch-Chemisches Institut, Westf€ alische Wilhelms-Universit€ at M€ unster, Corrensstrasse 40, D-48149 M€ unster, Germany, and 4 Cellular and Structural Biology, UT Health Science Center, San Antonio, Texas 78229, United States Received April 1, 2010

The naphthohydroquinoid tricarbonyl chromium complexes 3 and 6, bearing a styryl or phenylazo moiety, have been synthesized and studied for the haptotropic metal migration along the extended π-system. Quantum chemical calculations suggested a feasible stepwise rearrangement of the Cr(CO)3 fragment from the hydroquinoid to the other terminal phenyl ring for the azo- rather than for the ethenebridged system. An experimental and kinetic study of the ethene-bridged complex 3 revealed a haptotropic metal shift onto the adjacent naphthalene ring to give isomer 7 and suggested a competing intermolecular decomplexation-recomplexation pathway for the coordination of the terminal phenyl ring, affording bismetalated complexes 8 and 9. Attempts of a controlled metal migration in the azo complex analogue 6 under similar conditions were unsuccessful and resulted in partial decomposition.

Introduction Metal complexes bearing a ligand that offers more than a single coordination mode may undergo an intramolecular metal shift along the ligand platform. This process in which the metal center remains permanently coordinated to the ligand during the metal relocation is referred to as haptotropic metal migration.1 Since the first report of a reversible shift of a Cr(CO)3 fragment along the platform of 2,3-dimethylnaphthalene,2 the rearrangement of chromium fragments along arene platforms has *To whom correspondence should be addressed. E-mail: doetz@ uni-bonn.de; [email protected]. (1) Anh, N. T.; Elian, M.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 110. € (2) Deubzer, B.; Fritz, H. P.; Kreiter, C. G.; Ofele, K. J. Organomet. Chem. 1967, 7, 289. (3) (a) D€ otz, K. H. Angew. Chem., Int. Ed. Engl. 1975, 14, 644; Angew. Chem. 1975, 87, 672. (b) D€ otz, K. H.; Dietz, R. Chem. Ber. 1977, € 1555. (c) Cunningham, S. D.; Ofele, K.; Willeford, B. J. Am. Chem. Soc. 1983, 105, 3724. (d) Kirss, R. U.; Treichel, P. M., Jr. J. Am. Chem. Soc. 1986, 108, 853. (e) K€ undig, E. P.; Desobry, V.; Rivet, C.; Rudolph, B.; Splicher, S. Organometallics 1987, 6, 1173. (f ) Oprunenko, Y. F.; Malyugina, S. G.; Ustynyuk, Y. A.; Ustynyuk, N. A.; Kravtsov, D. N. J. Organomet. Chem. 1988, 338, 357. (g) D€ otz, K. H.; Stinner, C. Tetrahedron: Asymmetry 1997, 8, 1751. (h) Oprunenko, Y. F.; Malyugina, S.; Nesterenko, P.; Mityuk, D.; Malyshev, O. J. Organomet. Chem. 2000, 597, 42. (i) Jahr, H. C.; Nieger, M.; D€ otz, K. H. J. Organomet. Chem. 2002, 641, 185. (j) D€otz, K. H.; Szesni, N.; Nieger, M.; N€attinen, K. J. Organomet. Chem. 2003, 671, 58. (k) Jahr, H. C.; D€ otz, K. H. Chem. Rec. 2004, 4, 61. (l) Jahr, H. C.; Nieger, M.; otz, K. H. Chem.;Eur. J. 2005, 11, 5333. (m) D€otz, K. H.; Stendel, J., Jr.; D€ M€ uller, S.; Nieger, M.; Ketrat, S.; Dolg, M. Organometallics 2005, 24, 3219. (n) Dubarle Offner, J.; Fr€ohlich, R.; Kataeva, O.; Rose-Munch, F.; Rose, E.; D€ otz, K. H. Organometallics 2009, 28, 3004. pubs.acs.org/Organometallics

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attracted increasing interest.3,4 Experimental and theoretical investigations have been undertaken to elucidate the mechanism of this process. Theoretical studies on naphthalene tricarbonyl chromium complexes suggested a metal shift along the periphery of the π-system via a η4-coordinated trimethylene methane-like transition state.5 An alternative scenario potentially leading to (4) For reviews on the haptotropic migration, see: (a) D€ otz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587; Angew. Chem. 1984, 96, 573. (b) Ustynyuk, N. A. Organomet. Chem. USSR 1989, 2, 20; Metalloorg. Khim. 1989, 2, 43; Chem. Abstr. 1989, 111, 115236. (c) Semmelhack, M. F. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: Oxford, UK, 1995; Vol. 12, Chapter 9, p 979. (d) Wulff, W. D. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: Oxford, UK, 1995; Vol. 12, pp 469. (e) Morris, M. J. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G.,Eds.; Pergamon: Oxford, UK, 1995; Vol. 5, p 501. (f ) D€ otz, K. H.; Tomuschat, P. Chem. Soc. Rev. 1999, 28, 187. (g) Oprunenko, Y. F. Russ. Chem. Rev. 2000, 69, 683; Usp. Khim. 2000, 69, 744; Chem. Abstr. 2000, 134, 178576. (h) D€ otz, K. H.; Stendel, Jr., J. In Modern Arene Chemistry; Astruc, D., Ed.; Wiley-VCH: Weinheim, 2002; p 250. (i) Minatti, A.; D€otz, K. H. Top. Organomet. Chem. 2004, 13, 123. (j) D€otz, K. H.; Wenzel, B.; Jahr, H. C. Top. Curr. Chem. 2004, 248, 63. (k) D€otz, K. H.; Stendel J. Chem. Rev. 2009, 109, 3227. (5) (a) Albright, T. A.; Hofmann, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P. A. J. Am. Chem. Soc. 1983, 105, 3396. (b) Oprunenko, Y. F.; Akhmedov, N. G.; Laikov, D. N.; Malyugina, S. G.; Mstislavsky, V. I.; Roznyatovsky, V. A.; Ustynyuk, Y. A.; Ustynyuk, N. A. J. Organomet. Chem. 1999, 583, 136. (c) Oprunenko, Y. F.; Malyugina, S.; Nesterenko, P.; Mityuk, D.; Malyshev, O. J. Organomet. Chem. 2000, 597, 42. (d) Ketrat, S.; Mueller, S.; Dolg, M. J. Phys. Chem. A 2007, 111, 6094. (e) Jimenez-Halla, J. O. C.; Robles, J.; Sola, M. Organometallics 2008, 27, 5230. (f ) Pfletschinger, A.; Dolg, M. J. Organomet. Chem. 2009, 694, 3338. (g) Joistgen, O.; Pfletschinger, A.; Ciupka, J.; Dolg, M.; Nieger, M.; Schnakenburg, G.; Fr€ohlich, R.; Kataeva, O.; D€otz, K. H. Organometallics 2009, 28, 3473. r 2010 American Chemical Society

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the same final rearrangement products may follow an intermolecular decoordination-recoordination mechanism during which the Cr(CO)3 fragment is a stabilized by a coordinating solvent. Whereas an intermolecular reaction is favored by high temperatures and by coordinating solvents such as THF, DMSO, and toluene, an intramolecular pathway typically results from kinetic reaction control using noncoordinating or only moderately coordinating solvents such as alkanes, dialkyl ethers, or octafluorotoluene.4c,j,k An intramolecular haptotropic metal migration is expected to reveal first-order kinetics and, moreover, can be corroborated by stereochemical criteria, i.e., conservation of enantiomeric or diastereomeric purity in the resulting haptotropomer resulting from a complex precursor bearing a chiral plane. Among a variety of transition metal complexes studied in this context, most reports concentrated on arene chromium complexes, reflecting their ease of preparation. Beyond the direct complexation of the arene by Cr(CO)6 or more elaborate Cr(CO)3 transfer reagents such as Cr(NH3)3(CO)3,6 which typically results in the formation of regioisomeric complexes, the chromium-templated benzannulation of alkoxy(aryl)carbene chromium complexes provides a regioselective labeling of a specific benzene ring in densely substituted fused hydroquinoid tricarbonyl chromium complexes under mild conditions4a,f,i,h,k,7 Apart from naphthalene complexes, more extended π-platforms such as phenanthrenes, naphthobenzofuranes, and triphenylenes bearing various substitution patterns have been investigated for their tendency of haptotropic metal migration.3i,m,n A remarkable example is presented by an organometallic switch in which a reversible haptotropic migration was tuned by an appropriate coligand sphere.3k,l,8 Besides polycyclic arene platforms, metal shifts have also been observed along nonaromatic conjugated π-systems for several transition metals, including η2- and η4-iron complexes.9 Extending our work on fused aromatic platforms we turned our attention to acyclic π-linkers bridging two arene skeletons. Herein, we report on benzene and hydroquinoid naphthalene platforms connected by an ethene (-CHdCH-) or an azo (-NdN-) moiety and their tendency for haptotropic migration of a Cr(CO)3 fragment.

Results and Discussion Synthesis of Hydroquinoid Benzostilbene and Benzoazobenzene Cr(CO)3 Complexes. The regioselective labeling of hydroquinoid benzostilbene and benzoazobenzene platforms by a Cr(CO)3 fragment is based on the chromium-templated benzannulation of stilbene and azobenzene chromium carbenes. Lithiation of (E)-1-bromo-4-styrylbenzene (1), which is accessible in a Wittig reaction of benzyltriphenylphosphonium bromide and 4-bromobenzaldehyde,10 followed by addition to hexacarbonyl chromium and O-methylation of the acyl chromate intermediate afforded pentacarbonyl[methoxy(4-{(E)-20 -phenylethenyl}phenyl)carbene]chromium(0) (2) in 82% yield (Scheme 1). Its (6) (a) Rausch, M. D.; Moser, G. A.; Zaiko, E. J.; Lipman, A. L., Jr. J. Organomet. Chem. 1970, 23, 185. (b) Knox, G. R.; Leppard, D. G.; Pauson, P. L.; Watts, W. E. J. Organomet. Chem. 1972, 34, 347. (c) Moser, G. A.; Rausch, M. D. Synth. React. Inorg. Met.-Org. Chem. 1974, 4, 37. (d) Top, S.; Jaouen, G. J. Organomet. Chem. 1979, 182, 381. (7) For another recent review, see: Waters, M. L.; Wulff, W. D. In Organic Reactions; Overman, L. E., Ed.; Hoboken, NJ, 2008; Vol. 70, pp 121. (8) Jahr, H. C.; Nieger, M.; D€ otz, K. H. Chem. Commun. 2003, 2866. (9) (a) Foxman, B.; Marten, D.; Rosan, A.; Raghu, S.; Rosenblum, M. J. Am. Chem. Soc. 1977, 99, 2160. (b) Whitlock, H. W., Jr.; Reich, C.; Woessner, W. D. J. Am. Chem. Soc. 1971, 9, 2483. (c) Hafner, A.; von Philipsborn, W.; Salzer, A. Angew. Chem., Int. Ed. 1985, 24, 126; Angew. Chem. 1985, 97, 136. (10) Leung, S. H.; Angel, S. A. J. Chem. Educ. 2004, 81, 1492.

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Figure 1. Molecular structure of stilbene chromium carbene 2. The numbering of atoms differs from that used in the NMR characterization. Selected bond lengths (A˚) and angles: C(11)-Cr(1) = 2.083(7); {O(1)-C(11)-C(1)}-{C(16)-Cr(1)-C(17)} = 36, (phenyl)-(phenylene)=4, {O(1)-C(11)-C(1)}-(phenylene)=27. Scheme 1. Synthesis of Hydroquinoid Benzostilbene Cr(CO)3 Complex 3

molecular structure reveals a nearly coplanar stilbene moiety with a torsion angle of 4.3 between the arene rings (Figure 1). Upon warming with 3-hexyne in tert-butyl methyl ether to 60 C, chromium carbene 2 underwent a clean benzannulation to give tricarbonyl{[η6-1,2,3,4,4a,8a]-1-tert-butyldimethylsilyloxy-2,3-diethyl-4-methoxy-7-[(E)-20 -phenylethenyl]naphthalene}chromium (3) as a single regioisomer in 92% yield after O-silylation (Scheme 1). The synthesis of the azobenzene chromium carbene analogues required a modification of the standard protocol due to the thermolability of lithioazobenzenes above -78 C.11 This problem was overcome by a low-temperature (-100 C) transmetalation by anhydrous zinc bromide to generate the organozinc derivative.12,13 This protocol is also compatible with alkyl and amino donor substituents in the arene, which may compensate in part the electron deficiency caused by the azo moiety. Addition of hexacarbonyl chromium and subsequent O-alkylation of the resulting acyl chromate intermediate by the more soluble triethyloxonium tetrafluoroborate afforded the azobenzene-functionalized chromium carbenes 5a-c in moderate to good yields (Scheme 2). (11) (a) Katritzky, A. R.; Wu, J.; Verin, S. V. Synthesis 1995, 651. (b) Kozlecki, T.; Syper, L.; Wilk, K. A. Synthesis 1997, 681. (12) Santhamma, B. Unpublished results, 2007. (13) Garlichs-Zschoche, F. A.; D€ otz, K. H. Organometallics 2007, 26, 4535.

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Scheme 2. Synthesis of Azobenzene Chromium Carbenes 5a-c

Benzannulation of azobenzene-based chromium carbenes 5a-c by 3-hexyne under our standard conditions followed by naphthol protection afforded the hydroquinoid diazoarene Cr(CO)3 complexes 6a-c in moderate to good yields (Scheme 3). Thermal Metal Rearrangement: Haptotropic Metal Migration versus Intermolecular Metal Shift. Benzoazobenzene Cr(CO)3 Complex. The intramolecular mobility of the Cr(CO)3 fragment was first studied for the azobenzene benzannulation product 6a. A degassed solution of complex 6a in di-n-butyl ether was first warmed to 70 C under an argon atmosphere. Then the temperature was increased by 10 C after every 30 min, and the progress of the reaction was monitored by IR spectroscopy. No metal migration was observed even after a total reaction time of 5 h while the temperature was increased to 120 C. We speculated that the incorporation of the electronwithdrawing azo group deactivates the naphthalene system by increasing the activation barrier for haptotropic migration of the Cr(CO)3 fragment. Thus, we aimed at a compensation of the electron deficiency imposed by the azo group by donor substitution of the terminal phenyl ring. However, no indication of a metal shift was observed either for the methyl or the dimethylamino derivative 6b or 6c under the aforementioned conditions. Instead, all these reactions resulted in decomplexation and further decomposition. Benzostilbene Cr(CO)3 Complexes. The haptotropic migration studies were then extended to the stilbene benzannulation product. In a separate series of experiments, solutions of complex 3 in di-n-butyl ether were warmed within a temperature range from 65 to 90 C, while the metal shift was continuously monitored by IR spectroscopy over a period of 3 h. We identified only a small temperature window of 75-80 C in which metal migration occurred reasonably fast without dominating decomposition, as indicated by increasing formation of hexacarbonyl chromium. Even within this temperature window we observed the formation of a complex product pattern consisting of two pairs of mono- and bis-Cr(CO)3 complexes (Scheme 4). At 75 C unreacted starting material 3 and its rearrangement product 7 bearing the metal fragment on the adjacent naphthalene ring appeared as the major products. Isomer 8 with the coordinated terminal phenyl ring was formed in up to 10% yield. Surprisingly, also small amounts (4-5%) of binuclear complexes 9 and 10 were observed bearing chromium fragments on both the naphthalene moiety and the phenyl ring. In an attempt to rationalize these results, we modified the reaction

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conditions by variation of temperature, time, and concentration. As expected, the conversion of the “kinetic” starting material 3 increased with increasing temperature (80 C) and prolonged reaction time in favor of mononuclear complexes 7 and 8; however, this is accompanied by increased decomposition, as indicated by formation of Cr(CO)6. The formation of the naphthalene rearrangement product 7 (60% yield) could be optimized by a combination of relatively short reaction time (1 h) and slightly increased temperature (80 C). An increase of concentration did not reveal significant changes of the product distribution. The parallel formation of mono- and dinuclear rearrangement products indicates that competing mechanisms are operative. In order to examine the influence of Cr(CO)3 coordination of the terminal phenyl ring on the metal shift within the naphthalene moiety, we first concentrated on binuclear complex 9. Sufficient amounts of this compound were obtained by direct complexation of the kinetic benzannulation product 3 by the Cr(CO)3 transfer reagent Cr(CO)3(NH3)3 in diethyl ether using boron trifluoride etherate as an assisting Lewis acid. After chromatographic purification complex 9 was isolated in moderate yield of 54% along with its regioisomer 10 formed as a minor product (36%). Attempts to induce a haptotropic metal shift within the naphthalene skeleton of complex 9 failed under our standard conditions (di-n-butyl ether, 80 C) and resulted in predominating decomplexation. While the formation of mononuclear complexes 7 and 8 might a priori be compatible with an intramolecular haptotropic metal migration, the minor binuclear byproducts 9 and 10 formed along with significant amounts of hexacarbonyl chromium are indicative for a competing intermolecular scenario such as a decomplexation-recomplexation sequence. The product distribution outlined in Scheme 4 suggests that intermolecular reactions gain importance with longer reaction times, higher temperatures, and higher concentrations. Due to the diversity of the product pattern, we tried to separate the overall reaction into individual steps of metal migration and to study them in more detail. On the basis of previous observations4,5 the first metal shift is expected to occur within the naphthalene moiety, modifying the “kinetic” benzannulation product 3 into its “thermodynamic” haptotropomer 7. This idea was corroborated by a kinetic 1H NMR study carried out at 70 C to minimize subsequent rearrangement or decomposition. Octafluorotoluene was chosen as an electron-poor solvent unable to stabilize Cr(CO)3 fragments and thus to assist an intermolecular metal shift. The relative concentrations of the haptotropomers were determined on the basis of the resonance integrals of the naphthalene hydrogen atoms of complex 3 relative to those of complex 7, which appear shifted 2.0-2.5 ppm to higher field (Scheme 5). The ln[c/c0] versus time plot reveals a first-order kinetics consistent with an intramolecular haptotropic migration of the Cr(CO)3 fragment. The resulting rate constant of k = 5.6 10-3 s-1 exceeds that observed for comparable naphthalene complexes3l,14 by 1 order of magnitude, resulting in a free enthalpy of activation of ΔG‡ = 23.7 kcal/mol, which is lowered by ca. 10%, respectively. Both parameters resemble those obtained for phenanthrene complex analogues,5d revealing the influence of the more extended π-system. To get insight into the putative second individual migration step, the rearrangement from haptotropomer 7 to complex 8, (14) D€ otz, K. H.; Szesni, N.; Nieger, M.; N€attinen, K. J. Organomet. Chem. 2003, 671, 58.

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Scheme 3. Benzannulation of Azobenzene Chromium Carbenes 5a-c

Scheme 4. Product Distribution of the Thermal Metal Rearrangement of Stilbene Benzannulation Product 3

bearing the Cr(CO)3 fragment on the terminal phenyl ring, we warmed complex 7 both in di-n-butyl ether and in octafluorotoluene and monitored both processes by IR and NMR spectroscopy, respectively, in a temperature range of 70-80 C. Both experiments did not reveal any indication of a controlled metal migration and, instead, finally resulted in decomposition of the starting material. We speculate that the chromium arene interaction is destabilized when the metal fragment leaves its position above the center of the less substituted naphthalene ring, moving toward the π-periphery, and that the further migration to the olefinic CdC bond cannot compete with alternative pathways finally resulting in decomplexation. Molecular Structures of Mono- and Bimetallic Benzostilbene Complexes 3 and 7-10 and Azobenzene Complex 6b. The molecular structures of both the benzannulation products and

their rearrangement products arising from a shift of the Cr(CO)3 fragment have been established by X-ray diffraction. The “kinetic” benzostilbene and azobenzene complexes 3 and 6b reveal an eclipsed conformation of the Cr(CO)3 tripod that is slightly shifted toward the periphery of the hydroquinoid ring. A similar, but more pronounced dislocation is observed for haptotropomer 7, in which the metal fragment distinctly points away from the ring junction and, moreover, adopts a staggered conformation, which also occurs in the phenyl-coordinated isomer 8. While an only minor deviation from coplanarity of the naphthalene and benzene platforms (ca. 8) is found for complexes 6b, 7, and 8, the distortion of the extended π-system increases to 30 in the “kinetic” benzannulation product 3. A considerable deviation of the skeleton of fused arenes from coplanarity has been found to hamper efficient haptotropic

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Scheme 5. Kinetic Plots for the Rearrangement of Complex 3 to Haptotropomer 7

Figure 2. Molecular structure of benzostilbene complex 3. The numbering of atoms differs from that used in the NMR characterization. Selected bond lengths (A˚) and angles (deg) (323 K): CrC(1)=2.2465(18), Cr-C(2)=2.2501(18), Cr-C(3)=2.2268(18), Cr-C(4) = 2.2570(19), Cr-C(5) = 2.2985(18), Cr-C(10) = 2.2525(18); plane (phenyl)-(naphthalene) = 30.

metal migration and to favor decomplexation; however, this argument does not hold for arene complexes bearing a more flexible alkene or azo π-bridge. The molecular structures of mononuclear arene complexes 3, 6b, 7, and 8 are shown in Figures 2-5. In the binuclear complex 9 the angle between the two arene platforms (11) resembles that observed for the mononuclear complexes 6b, 7, and 8. The Cr(CO)3 fragments reveal an anti conformation and an eclipsed conformation for the Cr1 tripod within the naphthalene and a staggered one for the Cr2 tripod within the benzene moieties, as encountered separately in complexes 3 and 8, respectively. In complex 10 the syn conformation of both staggered metal fragments results in a considerable distortion of the extended π-skeleton, as indicated by an angle of 42 between the benzene and naphthalene planes. The molecular structures of the binuclear arene complexes 9 and 10 are depicted in Figures 6 and 7.

Figure 3. Molecular structure of benzoazobenzene complex 6b. Selected bond lengths (A˚) and angles (deg): C(1)-Cr=2.262(3), C(2)-Cr = 2.276(3), C(3)-Cr = 2.235(2), C(4)-Cr = 2.248(2), C(4A)-Cr = 2.272(3), C(8A)-Cr = 2.239(3); plane (phenyl)(naphthalene) = 8.

Quantum Chemical Calculations. Calibration. To ensure the reliability of density functional theory (DFT) and, in particular, the BP86 functional15 for the study of haptotropic rearrangements in arene Cr(CO)3 complexes, single-point calculations of the reaction system shown in Scheme 6 have been performed using different quantum chemical ab initio approaches ranging from the SCF to the highly correlated and accurate CCSD(T) level.16 The DFT hybrid and double-hybrid functionals B3LYP17 and B2PLYP18 were also tested for their ability to reproduce the CCSD(T) results. (15) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (b) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (16) Wennmohs, F.; Neese, F. Chem. Phys. 2008, 343, 217. (17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (c) Lee, C; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (18) Grimme, S. J. Chem. Phys. 2006, 124, 34108.

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Figure 4. Molecular structure of benzostilbene complex 7. The numbering of atoms differs from that used in the NMR characterization. Selected bond lengths (A˚) and angles (deg): Cr-C(5) = 2.325(3), Cr-C(6) = 2.205(3), Cr-C(7) = 2.213(3), Cr-C(8) = 2.238(3), Cr-C(9) = 2.209(3), Cr-C(10) = 2.322(3); plane (phenyl)-(naphthalene) = 9.

Figure 5. Molecular structure of benzostilbene complex 8. Selected bond lengths (A˚) and angles (deg): C(100 )-Cr(1) = 2.227(2), C(200 )-Cr(1)=2.211(3), C(300 )-Cr(1)=2.208(3), C(400 )-Cr(1)= 2.210(3), C(500 )-Cr(1)=2.203(3), C(600 )-Cr(1)=2.194(3) ; plane (phenyl)-(naphthalene) = 9.

The DFT calculations are in excellent agreement with the ab initio wave function-based quantum chemical calculations (Table 1). The difference between BP8615 and B3LYP17 is marginal. B2PLYP18 performs also well but, as expected, reveals a more pronounced basis set dependence than the standard DFT functionals. As the error of the reaction enthalpy, calculated with the BP86 functional15 relative to the ab initio methods, is very small, we felt confident with the DFT results and applied this method for the following study. Geometric Structures and Comparison to the Experiments. For the computational study of the target systems we have made a few minor approximations to the computational model. The hydroquinoid ring has been substituted with two methoxy and two methyl groups, while more bulky substituents (a methoxy, a OTBDMS group, and two ethyl groups) have been used in the experimental study. Moreover, we optimized the structures of the most stable conformation of the Cr(CO)3 tripod with respect to the central bond C4a-C8a, consistent with the previous study.5d In the experimental study, exclusively the E-derivatives


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Figure 6. Molecular structure of benzostilbene complex 9. The numbering of atoms differs from that used in the NMR characterization. Selected bond lengths (A˚) and angles (deg): C(1)-Cr(1) = 2.269(3), C(2)-Cr(1) = 2.258(3), C(3)-Cr(1) = 2.233(3), C(4)Cr(1)=2.243(3), C(5)-Cr(1)=2.280(3), C(10)-Cr(1) = 2.273(3), C(24)-Cr(2)=2.241(3), C(25)-Cr(2)=2.223(3), C(26)-Cr(2) = 2.221(4), C(27)-Cr(2)=2.199(4), C(28)-Cr(2)=2.215(3), C(29)Cr(2) = 2.206(3); (phenyl)-(naphthalene) = 11.

Figure 7. Molecular structure of benzostilbene complex 10. Selected bond lengths (A˚) and angles (deg): C(4A)-Cr(1)=2.284(3), C(5)Cr(1) = 2.224(3), C(6)-Cr(1) = 2.233(4), C(7)-Cr(1) = 2.239(4), C(8)-Cr(1) = 2.202(4), C(8A)-Cr(1) = 2.275(3), C(100 )-Cr(2) = 2.238(3), C(200 )-Cr(2) = 2.200(4), C(300 )-Cr(2) 2.211(4), C(400 )Cr(2)=2.225(4), C(500 )-Cr(2)=2.209(4), C(600 )-Cr(2)=2.210(3); (phenyl)-(naphthalene)=42. Scheme 6. Haptotropic Migration of the Cr(CO)3 Fragment in a Model Naphthohydroquinone Complex (BP86/TZVP structures)

of the benzostilbene and benzoazobenzene Cr(CO)3 complexes were observed. In agreement with this finding, their calculated structures are 4.6-18.7 and 0.6-16.0 kcal/mol lower in energy than their respective Z-isomers. For both the benzostilbene (I, II, III, IV) and the benzoazobenzene complex (V, VI, VII, VIII) four minima along the haptotropic migration pathway could be located, as shown in Figures 8 and 9, respectively. These four structures correspond to the coordination of the Cr(CO)3 fragment to one of the three

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aromatic rings or to the bridging CdC or NdN double bonds. Experimental X-ray data are available for isomers I, II, IV, and V but not for III and VII, where the metal fragment is located above the central CdC or NdN bond, respectively. The calculated Cr-Carene, Cr-Ccarbonyl, and C-O bond lengths for the Cr(CO)3 fragment of the benzostilbene and benzoazobenzene complexes for all minimum energy structures are compiled in Tables 2 and 3 and compared to the available experimental data. The calculated Cr-C4a and Cr-C8a distances (Table 2) are longer than the other Cr-C distances in the same arene ring (a difference of 0.05-0.08 A˚ for ring A and 0.08-0.15 A˚ for ring B). The calculated structure of ring B of the Cr(CO)3benzostilbene derivative is in excellent agreement with the experimental data. The structure of ring A agrees well with the calculated structure; however small differences between experimentally and theoretically determined distances are observed. For Cr(CO)3-benzostilbene and Cr(CO)3-benzoazobenzene complexes 3 and 6b the experimental Cr-C8a distance is shorter than the calculated one (experimental 2.25 and 2.24 A˚ and theory 2.32 and 2.30 A˚, respectively) and is similar to most other C-C bond lengths in the arene ring. In the experimental Table 1. Comparison of Reaction Energies, ΔE, Calculated by ab Initio and DFT Quantum Chemical Methods for the Migration of the Cr(CO)3 Fragment along the Model Naphthohydroquinone Skeletona,b method

ΔE (kcal/mol)

restricted Hartree-Fock (RHF) NCPF/116 CPF/119 QCISD(T)20 CCSD(T) BP86 B3LYP B2PLYP

-6.9 -6.3 -6.3 -6.0 -6.0 -6.3 -6.4 (-6.3) -5.5 (-6.6)

a A def2-TZVP basis set21 is applied at the metal atom and the newly developed bn-ANO-DZP basis set22 on all the other atoms. b The values with the larger def2-TZVPP basis set21 are given in parentheses.

structure of Cr(CO)3-benzoazobenzene complex 6b the Cr-C4a bond is shorter than calculated (2.27 to 2.30 A˚) and the Cr-C2 distance is longer compared to the calculated Cr-C2 distance in Table 2. Calculated (I, II, III, IV) and Experimentally Observed (3, 6b, 7, 8) Cr-Carene Bond Lengths (in A˚) of the Benzostilbene and the Benzoazobenzene Complexes (Calculated and Experimental Minimum Structures) bond

Cr(CO)3-benzostilbene

Cr(CO)3-benzoazobenzene

ring A

calcd (I)

exptl (3)

calcd (V)

exptl (6b)

Cr-C1 Cr-C2 Cr-C3 Cr-C4 Cr-C4a Cr-C8a

2.24 2.24 2.24 2.24 2.31 2.32

2.25 2.25 2.23 2.26 2.30 2.25

2.23 2.24 2.24 2.25 2.30 2.30

2.26 2.28 2.23 2.24 2.27 2.24

a

ring Bb Cr-C4a Cr-C5 Cr-C6 Cr-C7 Cr-C8 Cr-C8a DBc Cr - C10 Cr - C20 Cr - N10 Cr - N20 C10 - C20 N10 - N20 ring Cd

calcd (II) 2.35 2.22 2.21 2.26 2.21 2.34 calcd (III)

exptl (7) 2.32 2.20 2.21 2.24 2.21 2.32

calcd (VI) 2.35 2.23 2.21 2.23 2.20 2.35 calcd (VII)

2.30 2.20 1.91 2.28 1.41 calcd (IV)

exptl (8)

1.34 calcd (VIII)

Cr-C100 2.29 2.24 2.26 2.22 2.21 2.22 Cr-C200 2.24 2.21 2.24 Cr-C300 00 2.22 2.20 2.22 Cr-C4 2.24 2.21 2.24 Cr-C500 2.22 2.21 2.22 Cr-C600 a Complexation of ring A. b Complexation of ring B. c Complexation of the double bond. d Complexation of ring C.

Figure 8. Calculated minimum energy structures for the haptotropic metal migration of the Cr(CO)3 benzostilbene complex.

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Figure 9. Calculated minimum energy structures for the haptotropic metal migration of the Cr(CO)3 benzoazobenzene complex. Table 3. Calculated and Experimental Cr-Ccarbonyl and C-O Bond Lengths (in A˚) for the Cr(CO)3 Fragment Coordinated to Rings A, B, and C or to the Central CdC and the NdN Bridges, Respectively Cr(CO)3-benzostilbene calcd

exptl

average Cr-C (CO) ring A ring B ring C CdC NdN

1.84 (I) 1.84 (II) 1.84 (IV) 1.83 (III)

1.83 (3) 1.84 (7) 1.83 (8)

calcd

exptl

average Cr-C (CO) 1.84 (V) 1.85 (VI) 1.85 (VIII)

1.84 (6b)

1.84 (VII) average C-O

ring A ring B ring C CdC NdN


1.17 (I) 1.17 (II) 1.17 (IV) 1.17 (III)

1.15 (3) 1.15 (7) 1.16 (8)

average C-O 1.16 (V) 1.16 (VI) 1.16 (VIII)

1.16 (6b)

1.17 (VII)

ring A (2.28 to 2.24 A˚), shifting the metal fragment from above the center toward the periphery (toward above bond C2-C3) of the coordinated hydroquinoid ring. A similar dislocation (toward above bond C6-C7 of ring B; see Figure 9) is calculated and experimentally observed for haptotropomer 7. In contrast, the Cr-Carene distances within ring C are almost equal, except the Cr-C100 distance of 2.29 A˚, which exceeds the other Cr-C bond lengths (average value 2.23 A˚) for the benzostilbene, and of 2.26 A˚ (compared to a mean distance of 2.23 A˚) for the azobenzene system (Table 3). The Cr-Carene bonds in ring C are, on average, shorter than those in rings A and B, which is (19) (a) Ahlrichs, R.; Scharf, P.; Ehrhardt, C. J. Chem. Phys. 1985, 82, 890. (b) Ahlrichs, R.; Scharf, P. In Ab Initio Methods in Quantum Chemistry; Lawley, K. P., Ed.; Wiley: New York, 1987; Vol. 1, p 501. (20) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968. (21) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (22) Neese, F. Unpublished.

in accordance with the larger complexation (binding) energy of the Cr(CO)3 fragment to the benzene system (∼45 kcal/ mol, BSSE corrected) compared to the hydroquinone system (42 kcal/mol). The calculated Cr-Ccarbonyl and C-O bond distances within the Cr(CO)3 fragment are in excellent agreement with the available X-ray data (Table 3). These distances are essentially independent of the positioning of the Cr(CO)3 fragment on the arene skeleton. The extended aromatic skeletons in all calculated structures are essentially planar except for structures III and VII, in which the Cr(CO)3 fragment is located above the CdC or NdN double bonds, respectively. The available experimental molecular structures of complexes 7 and 8 reveal essentially coplanar arene platforms with the benzene ring twisted by ca. 8 relative to the naphthalene moiety. In contrast, however, the X-ray structure of complex 3, bearing the metal-coordinated hydroquinoid ring, shows a considerably increased angle of 30 between both aromatic ring systems. Consequently, the incorporation of another Cr(CO)3 fragment coordinated to ring C further increases the twisting of the extended π-skeleton by ca. 12, as observed for the binuclear complex 10 (for the crystal structures of 3 and 10, see Figures 2 and 6). Transition States. The calculated transition states for the metal migration in the benzostilbene and the benzoazobenzene complexes are represented in Figures 10 and 11, respectively. In general, both systems behave analogously, revealing three transition states separating four minima. As previously reported for the naphthalene Cr(CO)3 system,3m,5 the migration of the chromium fragment from ring A to ring B does not take a least motion pathway (crossing the middle of the carboncarbon bond common to two fused six-membered rings) but rather proceeds along the π-ligand periphery with a η4trimethylenemethane-like complex transition state, IX and XII, respectively. Both transition states reveal one short Cr-C8a (or Cr-C4a) distance of 2.17 (or 2.16) A˚ (in agreement with previous studies3) along with the other three Cr-C distances of 2.50-2.66 A˚ for Cr-C4a, Cr-C1, Cr-C8 and Cr-C4, Cr-C5, Cr-C8a, respectively.

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Figure 10. Calculated transition-state structures IX-XI for the tricarbonyl chromium benzostilbene system.

Figure 11. Calculated transition state structures XII-XIV for the tricarbonyl chromium benzoazobenzene system.

In all calculated transition-state structures, the extended π-arene skeletons are essentially planar except for XIII, characterizing the migration of the metal fragment from ring B to the NdN double bond, where a significant twist has been calculated. This transition-state structure has the metal much closer to the NdN bond than observed for the corresponding transition state of the benzostilbene system (compare XIII in Figure 11 with X in Figure 10 and see Table 4 for the bond distances between the metal and the carbon or nitrogen atoms of the CdC or NdN bonds). In contrast, the structures calculated for the final migration step from the double bond to ring C are very similar for the benzostilbene and benzoazobenzene systems. Again the Cr-N20

distance is smaller than the corresponding Cr-C20 distance in the stilbene system. The Cr-CO bonds apparently become slightly stronger in the transition states, as indicated by the shorter calculated Cr-C distances (average values 1.82 A˚ in the transition states of both systems (Table 5) in comparison to 1.84 A˚ for benzostilbene and 1.85 A˚ for benzoazobenzene (Table 3)). Energetics. According to our calculations the overall haptotropic migration of the Cr(CO)3 fragment along the extended arene platform is endothermic by 6.7 kcal/mol for the benzostilbene system, but exothermic (-11.0 kcal/mol) for the benzoazobenzene complex, suggesting that the nature of the acyclic bridge has a major impact on the course of the

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Table 4. Cr-C Bond Lengths (in A˚) of the Calculated TransitionState Structures in the Benzostilbene and the Benzoazobenzene Series bond a

TS

Cr(CO)3-benzostilbene


2.17 2.66 2.56 2.50

2.62 2.16

ring A f ring B Cr-C8a Cr-C4a Cr-C1 Cr-C8 Cr-C4 Cr-C5 ring B f CdC Cr-C7 Cr-C8 Cr-C6 Cr-C10 ring B f NdN Cr-C7 Cr-C6 Cr-N10 Cr-N20 CdC f C Cr-C100 Cr-C20 Cr-C600 Cr-C200 C10 -C20 NdN f C Cr-C100 Cr-N20 Cr-C600 Cr-C200 N10 -N20 a

2.53 2.55

2.34 2.27 2.39 2.04 2.18 2.76 2.34 2.54 1.38 2.16 2.66 2.38 2.55 1.29

calculated bond lengths Cr(CO)3-benzostilbene


average Cr-C (CO) ring A f ring B ring B f CdC ring B f NdN CdC f C NdN f C

1.81 1.82

1.82 1.83

1.82 1.82 average C-O

ring A f ring B ring B f CdC ring B f NdN CdC f C NdN f C a TS = transition state.

1.17 1.17

ΔE

-1.5 14.1

13.3 5.5

-19.3

-12.1

ring A f ring B 27.5 ring B f CdC 20.8 ring B f NdN CdC f C 10.2 NdN f C a ΔE1 and Ea1 denote the reaction tively; the zero vibrational energies energies.

2.20 2.54 2.19 2.93

1.17 1.17

1.17 1.17

reaction. In the benzostilbene series the intermediates B and III are energetically uphill with respect to both the starting material A and the putative final rearrangement product C. In the benzoazobenzene series, however, the energies of the respective intermediates range between those of the starting and the final complexes A and C. The metal shift involves two major steps: (1) The migration within the naphthalene part is endothermic by 13.3 kcal/mol in the benzostilbene system but exothermic by -6.4 kcal/mol for the benzoazobenzene complex. The activation barrier for the haptotropic metal migration from ring A to ring B amounts to 35.1 kcal/ mol for the benzostilbene complex and 27.5 kcal/mol for the


1

Ea

Table 5. Cr-C and C-O Bond Lengths (in A˚) of the Calculated Transition-State Structures within the Cr(CO)3 Fragment

TS

ΔE ring A f ring B ring B f CdC ring B f NdN CdC f C NdN f C

TS = transition state.

a

Table 6. Reaction and Activation Energies (in kcal/mol) for the Haptotropic Cr(CO)3 Migration in Benzostilbene and Benzoazobenzene Complexesa

calculated bond lengths Cr(CO)3-benzostilbene

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ΔE

ΔE1

-2.1

-6.4

5.5

4.3

-11.3

-8.9

Ea1

Ea

Ea1

35.1 15.1

27.5

26.3

13.7

12.5

8.2 21.2 24.1 and activation energies, respec(ZVE) are added to the DFT

azobenzene complex (see Table 7 and Figure 12), suggesting that the NdN bridge is suited to favor the metal shift within the naphthalene moiety over the CdC linker. The calculated ΔG‡ value for the benzostilbene system, corrected to the experimental temperature, is 26.8 kcal/mol, which is in good agreement with the experimental value of ΔG‡ = 23.7 kcal/mol obtained from the kinetic study at 70 C in C6F5CF3. Solvent effects have not been considered in this study. The barrier for the naphthalene complex (calculated value ΔG‡ ∼30 kcal/mol) is in the same range, indicating an only minor influence of the styryl extension on the activation barrier. (2) The putative metal shift from the non-hydroquinoid naphthalene ring to the terminal benzene ring is supposed to proceed via intermediates III (VII), which are 9.6 kcal/mol (8.2 kcal/mol) lower in energy in the benzostilbene (benzoazobenzene) series than the calculated transition-state energies for the metal shift from ring B to the respective double bond (see Figure 12 and Table 6). Importantly, the highest barrier in each series refers to the initial metal migration from ring A to ring B. Thus, the calculations suggest that, if the initial migration is kinetically feasible, the subsequent steps should be feasible as well, resulting in a complete intramolecular migration along the extended π-platform. The absence of any detected intermediate with the metal fragment coordinated to the double-bond bridge might be explained by (a) a low transition state for the final metal shift onto ring C, which is energetically uphill with respect to the starting material in the benzostilbene case, or (b) alternative competing pathways characterized by lower barriers, which have not been included in the calculations but might be operative especially in the benzoazobenzene system, revealing an activation energy, Ea1, of 24.1 kcal/mol for the metal migration onto ring C (see Table 6 and Figure 12). From an experimental point of view, the reaction produces not only mononuclear but also binuclear complexes that were trapped and structurally investigated. The bimetallic complexes may be generally formed in two ways: (a) by initial dissociation of the Cr(CO)3 fragment from one of the intermediates, temporary stabilization by the solvent or another donor ligand, and subsequent recoordination to another aromatic ring or (b) by direct bimolecular transfer of the Cr(CO)3 fragment between two monometallic complexes. No experimental evidence is presently available to favor one mechanism over the other. However, calculations of the disproportionation reaction indicate for the benzostilbene system that a reaction between two molecules of

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Table 7. Crystallographic Data and Details of Refinement for 2, 3(223 K), 3 (123 K), and 7

empirical formula fw [g/mol] wavelength T [K] cryst syst space group a [A˚] b [A˚] c [A˚] R [deg] β [deg] γ [deg] volume [A˚3] Z calcd density [g/cm3] absorp coeff [mm-1] F(000) cryst size [mm3] θ range [deg] limiting indices collected reflns unique reflns R (int) params/restraints R indices for (I > 2σ(I )) R indices (all data) goodness-of-fit on F2 largest diff peak and hole [e A˚-3]

2

3(223K)

3(123 K)

7

C21H14CrO6 414.32 0.71073 123 orthorhombic P212121 (#19) 7.3508(13) 11.578(5) 44.049(8) 90 90 90 3748.9(19) 8 1.468 0.646 1696 0.50 0.14 0.03 1.85 to 28.00 -9 e h e 9 -15 e k e 15 -58 e l e 58 22 481 8556 0.2056 505/1 R1 = 0.0658 wR2 = 0.1340 R1 = 0.1445 wR2 = 0.1536 0.759 0.65/-1.287

C32H38CrO5Si 582.71 0.71073 223 monoclinic P2/c (#13) 14.898(1) 12.022(1) 17.710(1) 90 104.62(1) 90 3069.2(4) 4 1.261 0.449 1232 0.40 0.30 0.10 1.69 to 30.49 -19 e h e 21 -17 e k e 15 -25 e l e 19 22 032 9300 0.058 360/0 R1 = 0.0515 wR2 = 0.1118 R1 = 0.1180 wR2 = 0.1328 1.017 0.434/-0.385

C32H38CrO5Si 582.71 0.71073 123 monoclinic P2/c (#13) 14.8276(4) 11.9847(3) 17.5574(5) 90 104.178(2) 90 3024.99(14) 4 1.280 0.455 1232 0.40 0.30 0.10 2.94 to 27.47 -17 e h e 19 -15 e k e 15 -22 e l e 22 17 938 6738 0.0279 353/0 R1 = 0.0329 wR2 = 0.0795 R1 = 0.0544 wR2 = 0.0855 0.963 0.308/-0.298

C34H43CrO5.5Si 619.77 0.71073 223 monoclinic P21/c (#14) 9.006(1) 15.916(1) 23.474(1) 90 100.45(1) 90 3308.9(4) 4 1.244 0.421 1316 0.25 0.25 0.05 1.55 to 27.84 -9 e h e 11 -17 e k e 20 -30 e l e 23 21 567 7812 0.077 395/3 R1 = 0.0606 wR2 = 0.1083 R1 = 0.1305 wR2 = 0.1268 1.010 0.373/-0.410

Figure 12. Energy diagram for the haptotropic rearrangement in Cr(CO)3-benzostilbene and Cr(CO)3-benzoazobenzene complexes. The zero vibrational energies (ZVE) are added to the DFT energies. All energies are relative to the energy of the complex bearing the metal fragment coordinated to ring A. A: Coordination via hydroquinoid naphthalene ring. B: Coordination via nonhydroquinoid naphthalene ring. C: Coordination via benzene ring.

3 (metal above ring A) or 7 (metal above ring B) leading to a metal-free arene ligand and a bimetallic complex would be energetically downhill by 7-8 kcal/mol, while disproportionation of complex 8 bearing the Cr(CO)3 fragment above ring C results in a thermoneutral process. Thus, the computational study is consistent with the experiment demonstrating that the disproportionation reactions are energetically feasible.

Conclusions Hydroquinoid benzostilbene and benzoazobenzene Cr(CO)3 complexes bearing the metal label selectively

on the hydroquinoid ring are readily accessible via chromiumtemplated benzannulation of stilbene and azobenzene chromium carbenes. A quantum chemical study suggests for the benzoazobenzene system that a stepwise haptotropic migration of the Cr(CO)3 fragment along the extended π-system to the other terminal phenyl ring may be feasible according to the calculated activation barriers provided that the initial metal shift within the naphthalene moiety (V f VI) characterized by the highest activation barrier in the metal shift sequence can be realized under the reaction conditions. In contrast, the respective rearrangements in the benzostilbene series result in transition states, intermediates, and a product higher in energy than the starting material. The experimental study within the benzostilbene series reveals that the initial metal migration requires a small temperature window of 75-80 C and is complicated by competing pathways even under these optimized conditions. It affords the regioisomers expected for an intramolecular sliding of the Cr(CO)3 fragment along the conjugated π-platform. An intramolecular mechanism for the initial haptotropic metal migration from the hydroquinoid to the adjacent naphthalene ring (3 f 7) is supported by a kinetic NMR study. The product pattern experimentally observed for this process also includes minor amounts of complex 8 bearing a coordinated terminal phenyl ring as well as of binuclear complexes 10 and 11, which indicates competing intermolecular pathways. A subsequent controlled haptotropic metal migration from the naphthalene to the terminal phenyl ring via the acyclic conjugated bridge could not be confirmed experimentally. Quantum chemical calculations indicate that the metal fragments become kinetically labile while traversing

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Table 8. Crystallographic Data and Details of Refinement for 6b, 8, 9, and 10

empirical formula fw [g/mol] wavelength T [K] cryst syst space group a [A˚] b [A˚] c [A˚] R [deg] β [deg] γ [deg] volume [A˚3] Z calcd density [g/cm3] absorp coeff [mm-1] F(000) cryst size [mm3] θ range [deg] limiting indices collected reflns unique reflns R(int) params/restraints R indices for (I > 2σ(I )) R indices (all data) goodness-of-fit on F2 largest diff peak and hole [e A˚-3]

6b

8

9

10

C32H40CrN2O5Si 612.75 0.71073 123 triclinic P1 (#2) 9.3080(6) 9.9851(6) 18.7963(14) 91.533(5) 92.188(6) 111.237(5) 1625.58(19) 2 1.252 0.429 648 0.40 0.20 0.10 2.17 to 26.00 -12eh e 11-12ek e 12-23el e 23 12 138 6099 0.0389 386/5 R1 = 0.0441 wR2 = 0.1190 R1 = 0.0605 wR2 = 0.1235 1.090 0.540/-0.557

C32H38CrO5Si 582.71 0.71073 123 triclinic P1 (#2) 11.5676(4) 13.7999(4) 21.5818(7) 88.1781(16) 75.4222(16) 67.0976(17) 3062.92(17) 4 1.264 0.450 1232 0.44 0.40 0.36 1.61 to 27.50 -15eh e 15-17ek e 17-27el e 28 35 335 14 035 0.0630 719/5 R1 = 0.0517 wR2 = 0.1259 R1 = 0.0969 wR2 = 0.1409 0.960 0.761/-0.661

C35H38Cr2O8Si 718.74 0.71073 123 triclinic P1 (#2) 8.5910(2) 9.8163(3) 20.8240(7) 80.0285(10) 86.8668(10) 81.2150(17) 1708.59(9) 2 1.397 0.721 748 0.32 0.24 0.08 2.62 to 28.00 -10eh e 11-12ek e 12-27el e 26 17 188 8213 0.0558 423/26 R1 = 0.0515 wR2 = 0.0962 R1 = 0.1140 wR2 = 0.1206 0.902 0.566/-0.767

C78H94Cr4O17Si2 1567.71 0.71073 123 monoclinic P21/c (#14) 15.2728(6) 11.1828(7) 23.2046(11) 90 102.001(3) 90 3876.6(3) 2 1.343 0.642 1644 0.24 0.12 0.08 2.72 to 27.49 -19eh e 19-13ek e 14-21el e 30 21 437 8858 0.0724 496/57 R1 = 0.0503 wR2 = 0.1094 R1 = 0.1336 wR2 = 0.1299 0.889 0.508/-0.760

the CdC or NdN double bonds. They may be cleaved under the reaction conditions and coordinated to mononuclear complexes, resulting in minor amounts of binuclear byproducts. Despite encouraging theoretical studies, no controlled metal migration was observed for the respective benzoazobenzene Cr(CO)3 complex 6b, which may reflect the electrondeficient azobenzene moiety.

Experimental Section All operations involving organometallic compounds were carried out under argon using round-bottom flasks (rearrangement of tricarbonylchromium complexes in preparative scale). Solvents were predistilled, dried using standard methods, saturated, and stored under argon. The solvent used for the kinetic NMR study of the haptotropic rearrangement (octafluorotoluene, Acros) was degassed by three freeze, pump, and thaw cycles. Merck silica gel (0.040-0.063 mm) was used for chromatographic purification. Given yields refer to pure products. 1H and 13C NMR spectra of the purified complexes were recorded at 25 C (298 K) using Bruker DRX-300, -400, and -500 instruments. FT-IR: Nicolet Magna 550. MS (EI): Thermo Finnigan MAT 95 XL. Pentacarbonyl[methoxy(4-{(E)-2 0 -phenylethenyl}phenyl)carbene]chromium(0) (2). A 2.2 mL (5.5 mmol) portion of an n-butyllithium (2.5 mol in n-hexane) solution was slowly added to a solution of 1.3 g (5 mmol) of 4-bromostilbene in 80 mL of diethyl ether precooled to 0 C. The reaction mixture was stirred for 60 min. Then 1.54 g (7 mmol) of hexacarbonylchromium was added, and the mixture was stirred for a further 90 min. After addition of 1 mL (8 mmol) of methyl trifluoromethanesulfonate the solvent was removed under reduced pressure after 15 min, and the residue was purified by column chromatography (petroleum ether/dichloromethane, 2:1) to yield 1.69 g (4.1 mmol, 81.6%) of 2 as a brown solid. 1 H NMR (300 MHz, CDCl3): δ 4.82 (s, 3H, OCH3), 7.13 (d, 3 J=16.52 Hz, 1H, CdC-H), 7.25 (d, 3J=16.52 Hz, 1H, CdC-H), 7.32 (t, 3J = 7.08 Hz, 1H, H-10), 7.41 (m, 2H, H-9/90 ), 7.50 (d,

J = 7.55 Hz, 2H, H-8/80 ), 7.56 (m, 4H, H-2/20 /3/30 ). 13C NMR (DEPT 135, 75 MHz, CDCl3): δ 67.18 (OCH3), 125.32, 126.08, 126.79, 127.34, 128.25, 128.79, 131.16 (Ar-CH, CdCH), 136.78, 140.22, 151.98 (Ar-C), 216.39 (cis-CO), 224.00 (trans-CO), 345.14 (CrdC). FT-IR (cm-1, PE): νCO 2062 (m, A1), 1954 (vs, E). MS (EI): (Mþ) m/z (%) 414.0 (8), 386.0 (29), 358.0 (25), 330.0 (19), 302.0 (4), 274.0 (17), 219.9 (100). HR-MS: calcd for C32H38CrO5Si - 4CO 302.0399, found 302.0405. Tricarbonyl{[η6-1,2,3,4,4a,8a]-1-tert-butyldimethylsilyloxy-2, 3-diethyl-4-methoxy-7-[(E)-20 -phenylethenyl]naphthalene}chromium(0) (3). A solution of 2 (420 mg, 1 mmol) and 3-hexyne (0.45 mL, 4 mmol) in 40 mL of tert-butylmethyl ether was degassed by three freeze-pump-thaw cycles and warmed to 55 C for 80 min. After cooling to 0 C, triethylamine (0.28 mL, 2 mmol) followed by tertbutyldimethylsilyl trifluoromethanesulfonate (0.35 mL, 2 mmol) were added, and the reaction mixture was stirred for a further 60 min while the temperature was allowed to warm to room temperature. The solvent was removed under reduced pressure, and the residue was purified by column chromatography (petroleum ether/dichloromethane, 2:1) to yield 540 mg (0.93 mmol, 91.8%) of 3 as a red solid. 1 H NMR (300 MHz, CDCl3): δ 0.30 (s, 3H, SiCH3), 0.35 (s, 3H, SiCH3), 1.06 (s, 9H, SiC(CH3)3), 1.21-1.27 (m, 6H, CH2CH3), 2.50-2.80 (m, 4H, CH2CH3), 3.90 (s, 3H, OCH3), 7.00 (d, 3J = 16.42 Hz, 1H, CdC-H), 7.11 (d, 3J = 16.42 Hz, 1H, CdC-H), 7.21 (t, 3J = 7.08 Hz, 1H, H-400 ), 7.30 (“t”, 3J = 7.46 Hz, 2H, H-300 /500 ), 7.45 (d, 3J = 7.37 Hz, 2H, H-200 /600 ), 7.62 (d, 3J = 9.06 Hz, 1H, H-5), 7.70 (d, 3J = 9.06 Hz, 1H, H-6), 7.77 (s, 1H, H-8). 13C NMR (DEPT 135, 75 MHz, CDCl3): δ -0.00 (SiCH3), -0.87 (SiCH3), 17.46 (CH2CH3), 18.72 (CH2CH3), 20.99 (SiC(CH3)3), 22.19 (CH2CH3), 23.29 (CH2CH3), 28.02 (SiC(CH3)3), 65.76 (OCH3), 98.09, 101.56, 104.19, 109.18 (Ar-C), 123.99, 124.12, 126.32, 126.73, 127.28, 128.19, 128.77, 130.59 (Ar-CH, CdCH), 130.65, 131.10, 135.17, 136.68 (Ar-C), 234.03 (CO). FT-IR (cm-1, PE): νCO 1959 (vs, A1), 1897, 1884 (s, E). MS (EI): (Mþ) m/z (%) 582.1 (5), 526.1 (5), 498.1 (65), 446.2 (100). HR-MS: calcd for C32H38CrO5Si 582.1894, found 582.1896. 3

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General Procedure for the Haptotropic Rearrangement. A solution of 3 (405 mg, 0.7 mmol) in 45 mL of di-n-butyl ether was degassed by three freeze-pump-thaw cycles and was warmed for 3 h at 80 C. The solvent was removed at reduced pressure, and the residue was purified by column chromatography (petroleum ether/dichloromethane, 1:1) to obtain a mixture of 3 (24 mg, 0.04 mmol, 6%), 7 (85 mg, 0.15 mmol, 21%), 8 (113 mg, 0.19 mmol, 28%), 9 (40 mg, 0.06 mmol, 8%), and 10 (35 mg, 0.05 mmol, 7%). Tricarbonyl{[η6-4a,5,6,7,8,8a]-1-tert-butyldimethylsilyloxy-2, 3-diethyl-4-methoxy-7-[(E)-20 -phenylethenyl]naphthalene}chromium(0) (7). 1H NMR (500 MHz, CDCl3): δ 0.31 (s, 3H, SiCH3), 0.32 (s, 3H, SiCH3), 1.06 (t, 3J = 7.25 Hz, 3H, CH2CH3), 1.18 (s, 9H, SiC(CH3)3), 1.20 (t, 3J = 7.33 Hz, 3H, CH2CH3), 2.58-2.70 (m, 2H, CH2CH3), 2.84-2.92 (m, 2H, CH2CH3), 4.00 (s, 3H, OCH3), 5.74 (d, 3J = 6.94 Hz, H-6), 6.49 (s, 1H, H-8), 6.50 (m, 1H, H-5), 6.80 (d, 3J = 16.16 Hz, 1H, CdC-H), 7.09 (d, 3J = 16.16 Hz, 1H, CdC-H), 7.31 (t, 3J = 7.41 Hz, 1H, H-400 ), 7.39 (“t”, 3J = 7. 41 Hz, 2H, H-300 ,500 ), 7.51 (d, 3J = 7.41 Hz, 2H, H-200 /600 ). 13C NMR (DEPT 135, 125 MHz, CDCl3): δ -3.22 (SiCH3), -2.64 (SiCH3), 14.33 (CH2CH3), 15.59 (CH2CH3), 18.83 (SiC(CH3)3), 20.17 (CH2CH3), 20.50 (CH2CH3), 26.02 (SiC(CH3)3), 62.03 (OCH3), 84.00, 86.45, 90.09 (Ar-CH), 100.04, 100.95, 103.98 (Ar-C), 125.77, 126.89, 128.50, 128.78, 131.01 (Ar-CH, CdCH), 132.01 (Ar-C), 135.26 (Ar-C), 135.99 (Ar-C), 144.06 (Ar-C), 146.77 (Ar-C), 232.53 (CO). FT-IR (cm-1, PE): νCO 1964 (vs, A1), 1901, 1889 (s, E). MS (EI): (Mþ) m/z (%) 582.2 (32), 526.2 (14), 498.2 (17), 446.2 (100). HR-MS: calcd for C32H38CrO5Si 582.1894, found 582.1891. Tricarbonyl{[η6-100 ,200 ,300 ,400 ,500 ,600 ]-1-tert-butyldimethylsilyloxy2,3-diethyl-4-methoxy-7-[(E)-2 0 -phenylethenyl]naphthalene} chromium(0) (8). 1H NMR (500 MHz, CDCl3): δ 0.22 (s, 6H, SiCH3), 1.12 (m, 3H, CH2CH3), 1.15 (s, 9H, SiC(CH3)3), 1.24 (m, 3H, CH2CH3), 2.81-2.89 (m, 4H, CH2CH3), 3.92 (s, 3H, OCH3), 5.31 (t, 3J = 6.23 Hz, 1H, H-400 ), 5.47 (“t”, 3J = 6.46 Hz, 2H, H-300 / 500 ), 5.59 (d, 3J = 5.86 Hz, 2H, H-200 /600 ), 6.72 (d, 3J = 16.14 Hz, 1H, CdC-H), 7.10 (d, 3J = 16.14 Hz, 1H, CdC-H), 7.66 (dd, 3 J = 8.84 Hz, 4J = 1.64 Hz, 1H, H-6), 7.97 (d, 3J = 8.84 Hz, 1H, H-5), 8.04 (d, 4J = 1.64 Hz, 1H, H-8). 13C NMR (125 MHz, CDCl3): δ -3.08 (SiCH3), -3.03 (SiCH3), 14.80 (CH2CH3), 14.83 (CH2CH3), 15.83 (CH2CH3), 15.87 (CH2CH3), 20.49 (SiC(CH3)3), 26.17 (SiC(CH3)3), 62.30 (OCH3), 90.65, 91.01, 92.77, 122.31, 122.54, 122.63, 122.79, 123.22,124.27,126.46,127.50, 128.22, 128.69, 129.20, 131.76, 137.51 (Ar-CH, CdCH), 232.93 (CO). FT-IR (cm-1, PE): νCO 1971 (vs, A1), 1900 (s, E). MS (EI): (Mþ) m/ z (%) 582.2 (1), 498.2 (17), 446.2 (100). Synthesis of Bischromium Complexes 9 and 10 by Direct Complexation. A suspension of benzostilbene complex 3 (180 mg, 0.31 mmol) and boron trifluoride etherate (%, 0.13 mL, 0.34 mmol) in 20 mL of diethyl ether was stirred at ambient temperature for 1 day. The solvent was removed at reduced pressure, and the residue was purified by column chromatography (petroleum ether/dichloromethane, 1:1) to obtain a mixture of 8 (120 mg, 0.17 mmol, 54%) and 9 (80 mg, 0.11 mmol, 36%). Hexacarbonyl{η6:η6-[1,2,3,4,4a,8a:100 ,200 ,300 ,400 ,500 ,600 ]-1-tertbutyldimethylsilyloxy-2,3-diethyl-4-methoxy-7-[(E)-20 -phenylethenyl]naphthalene}bischromium(0) (9). 1H NMR (500 MHz, CDCl3): δ 0.36 (3H, SiCH3), 0.42 (3H, SiCH3), 1.15 (s, 9H, SiC(CH3)3), 1.34-1.30 (m, 6H, CH2CH3), 2.88-2.56 (m, 4H, CH2CH3), 3.98 (s, 3H, OCH3), 5.34 (“t”, 3J = 5.96 Hz, 1H, H-400 ), 5.45-5.48 (m, 2H, H-300 /500 ), 5.54 (d, 3J = 6.56 Hz, 1H, H-200 /600 ), 5.60 (d, 3J = 6.66 Hz, 1H, H-200 /600 ), 6.68 (d, 3J = 16.24 Hz, CdC-H, 1H), 6.95 (d, 3 J = 16.24 Hz, CdC-H, 1H), 7.60 (d, 3J = 9.14 Hz, H-6, 1H), 7.77 (d, 3J = 9.14 Hz, H-5, 1H), 7.84 (s, H-8, 1H). 13C NMR (DEPT 135, 125 MHz, CDCl3): δ -2.88 (SiCH3), -2.05 (SiCH3), 15.38 (CH2CH3), 16.89 (CH2CH3), 19.02 (SiC(CH3)3), 20.11 (CH2CH3), 21.40 (CH2CH3), 26.01 (SiC(CH3)3), 66.05 (OCH3), 90.60, 91.52, 92.40 (Ar-CH), 96.93, 102.12, 104.10, 104.64, 109.76 (Ar-C), 124.18, 125.43, 126.23, 126.75, 129.65 (Ar-CH, CdCH), 130.11, 131.73, 133.26 (Ar-C), 232.70 (CO), 233.81 (CO). FT-IR (cm-1,

Hegele et al. PE): νCO 1978, 1957 (vs, A1), 1916, 1896, 1889, 1883 (s, E). MS (EI): (Mþ) m/z (%) 718.1 (9), 634.1 (25), 582.1 (9), 498.1 (100), 446.2 (67). HR-MS: calcd for C35H39Cr2O8Si 718.1146, found 718.1140. Hexacarbonyl{η6:η6-[4a,5,6,7,8,8a:100 ,200 ,300 ,400 ,500 ,600 ]-1-tertbutyldimethylsilyloxy-2,3-diethyl-4-methoxy-7-[(E)-20 -phenylethenyl]naphthalene}bischromium(0) (10). 1H NMR (500 MHz, CDCl3): δ 0.29 (3H, SiCH3), 0.32 (3H, SiCH3), 1.06 (t, 3J = 7.33 Hz, 3H, CH2CH3), 1.18 (s, 9H, SiC(CH3)3), 1.21 (t, 3J = 6.86 Hz, 3H, CH2CH3), 2.52-2.67 (m, 2H, CH2CH3), 2.80-2.88 (m, 2H, CH2CH3), 3.99 (s, 3H, OCH3), 5.35 (“t”, J = 5.91 Hz, 1H, H-400 ), 5.43 (d, 3J = 6.46 Hz, 1H, H-200 /600 ), 5.47 (“t”, 3J = 7.17 Hz, 1H, H-300 ,H-500 ), 5.53 (d, 3J = 6.46 Hz, 1H, H-200 /600 ), 5.58 (“t”, 3J = 7.41 Hz, 1H, H-300 ,H-500 ), 5.68 (d, 3J = 6.86 Hz, 1H, H-6), 6.44 (s, 1H, H-8), 6.48 (d, 3J = 6.86 Hz, 1H, H-5), 6.60 (d, 3 J = 16.00 Hz, 1H, CdC-H), 6.68 (d, 3J = 16.00 Hz, 1H, CdC-H). 13C NMR (125 MHz, CDCl3): δ -3.25 (SiCH3), -2.74 (SiCH3), 14.28 (CH2CH3), 15.58 (CH2CH3), 18.83 (SiC(CH3)3), 20.21 (CH2CH3), 20.49 (CH2CH3), 25.98 (SiC(CH3)3), 62.06 (OCH3), 84.51, 86.21, 90.07, 90.66, 90.88, 91.50, 91.60, 92.28, 92.76, 122.65, 124.31, 127.14, 128.43, 132.26, 133.86, 135.76 (Ar-CH, CdCH), 232.13 (CO), 232.51 (CO). FT-IR (cm-1, PE): νCO 1976, 1969 (vs, A1), 1917, 1909, 1886, 1896 (s, E). MS (EI): (Mþ) m/z (%) 718.0 (9), 634.1 (22), 582.1 (11), 498.1 (100), 446.2 (67). HR-MS: calcd for C35H39Cr2O8Si 718.1146, found 718.1131. General Procedure for the Benzannulation of Azobenzene Carbene Complexes. A solution of 4a (300 mg, 0.71 mmol) and 3-hexyne (230 mg, 2.84 mmol) in 10 mL of tert-butylmethyl ether was degassed by three freeze-pump-thaw cycles and warmed at 60 C for 60 min. After cooling the reaction mixture to -20 C, triethylamine (0.12 g, 1.27 mmol) and then tert-butyldimethylsilyl trifluoromethanesulfonate (330 mg, 1.27 mmol) were added, and the solution was stirred for 60 min while the temperature was allowed to warm to room temperature. The solvent was removed under reduced pressure, and the residue was purified by chromatography on silica gel (petroleum ether/CH2Cl2, 2:1), affording 6a as a dark brown solid (290 mg, 70%). Tricarbonyl{[η6-1,2,3,4,4a,8a]-1-tert-butyldimethylsilyloxy2,3-diethyl-4-ethoxy-7-[(E)-2 0 -phenyldiazenyl]naphthalene}chromium(0) (6a). 1H NMR (300 MHz, CDCl3): δ 0.37 (s, 3H, SiCH3), 0.41 (s, 3H, SiCH3), 1.16 (s, 9H, SiC(CH3)3), 1.38-1.32 (m, 6H, CH2CH3), 1.56 (t, 3J = 7.0 Hz, 3H, CH3), 2.96-2.51 (m, 4H, CH2CH3), 4.06-3.98 (m, 1H, OCH2), 4.28-4.21 (m, 1H, OCH2), 7.56-7.48 (m, 3H, Ar-CH), 7.80 (d, 3J = 9.4 Hz, 1H, H-5), 7.96 (d, J = 7.1 Hz, 2H, Ar-CH), 8.07 (dd, 3J = 9.4 Hz, 4J = 1.5 Hz, 1H, H-6), 8.55 (d, 4J = 1.5 Hz, 1H, H-8). 13C NMR (DEPT 135, 75 MHz, CDCl3): δ -2.63 (SiCH3), -2.21 (SiCH3), 15.80 (CH2CH3), 14.91 (CH2CH3), 17.25 (OCH2CH3), 19.04 (SiC(CH3)3), 20.02 (CH2CH3), 21.78 (CH2CH3), 26.06 (SiC(CH3)3), 75.80 (OCH2CH3), 94.92, 103.66, 104.20, 110.98 (Ar-C), 121.31, 123.03, 124.33, 127.63 (Ar-CH), 127.85 (Ar-C), 129.14, 131.30 (Ar-CH), 133.89, 148.71, 152.31 (Ar-C), 233.58 (CO). FT-IR (cm-1, PE): νCO = 1957 (s, A1), 1894, 1886 (s, E). MS (EI): (Mþ) m/z (%) 598.1 (14), 514.1 (10), 462.2 (100). HR-MS (ESI pos.): calcd for C31H38CrN2O5SiHþ 599.2028, found 599.2028. Tricarbonyl{[η6-1,2,3,4,4a,8a]-1-tert-butyldimethylsilyloxy2,3-diethyl-4-ethoxy-7-[(E)-20 -(400 -methyl)phenyldiazenyl]naphthalene}chromium(0) (6b). Purification by chromatography on silica gel (petroleum ether/CH2Cl2, 2:1) afforded 6b as a dark brown solid in 84% yield. 1 H NMR (300 MHz, CDCl3): δ 0.36 (s, 3H, -CH3), 0.41 (s, 3H, -CH3), 1.15 (s, 9H, -CH3), 1.38-1.31 (m, 6H, -CH3), 1.56 (t, J = 6.98 Hz, 3H, -CH3), 2.45 (s, 3H, --OCH3), 2.96-2.52 (m, 4H, -CH2), 4.05-3.95 (m, 1H, -OCH2), 4.27-4.19 (m, 1H, -OCH2), 7.33 (m, 2H, ArH), 7.80 (d, 3J = 9.4 Hz, 1H, H-5), 7.86 (m, 2H, ArH), 8.07 (dd, 3J = 9.4 Hz, 4J = 1.7 Hz, 1H, H-6), 8.15 (d, 4J = 1.7 Hz, 1H, H-8). 13C NMR (DEPT 135, 75 MHz, CDCl3): δ -2.63 (SiCH3), -2.19 (SiCH3), 14.92 (CH2CH3), 15.80 (CH2CH3), 17.23 (OCH2CH3), 19.05 (SiC(CH3)3), 20.05 (CH2CH3), 21.58 (-CH3), 21.76 (CH2CH3), 26.06 (SiC(CH3)3),

Article 75.75 (OCH2), 95.21, 103.66, 104.12, 110.86 (Ar-C), 121.51, 123.05, 124.28, 126.90 (Ar-CH), 127.95 (Ar-C), 129.81 (ArCH), 133.79, 141.98, 148.93, 150.68 (Ar-C), 233.66 (CO). FT-IR (cm-1, PE): νCO 1961 (s, A1), 1901, 1888 (s, E). MS (EI): (Mþ) m/z (%) 612.2 (20), 528.2 (30), 476.3 (100). HR-MS: calcd for C32H40CrN2O5Si 612.2112, found 612.2136. Tricarbonyl{[η6-1,2,3,4,4a,8a]-1-tert-butyldimethylsilyloxy2,3-diethyl-4-methoxy-7-[(E)-20 -(400 -N,N-dimethylamino)phenyldiazenyl]naphthalene}chromium(0) (6c). Purification by chromatography on silica gel (petroleum ether/CH2Cl2, 2:1) afforded 6c as a dark brown solid in 50% yield. 1 H NMR (300 MHz, CDCl3): δ 0.38 (s, 3H, SiCH3), 0.43 (s, 3H, SiCH3), 1.15 (s, 9H, -CH3), 1.34 (q, J = 7.3 Hz, 6H, -CH3), 1.56 (t, J = 7 Hz, 3H, -CH3), 2.74-2.54 (m, 3H, -CH2), 2.952.81 (m, 1H, -CH2), 3.11 (s, 6H, -N(CH3)2), 4.08-3.98 (m, 1H, -CH2), 4.30-4.20 (m, 1H, -CH2), 6.80-6.75 (m, 2H, Ar-CH), 7.79 (d, J = 9.40 Hz, 1H, H-5), 7.93-7.88 (m, 2H, Ar-CH), 8.07 (dd, J = 9.40 Hz, J = 1.60 Hz, 1H, H-6), 8.37 (d, J = 1.60 Hz, 1H, H-8). 13C NMR (DEPT 135, 75 MHz, CDCl3): δ -2.67 (SiCH3), -2.09 (SiCH3), 15.01 (CH2CH3), 15.80 CH2CH3), 17.06 OCH2CH3), 19.06 (SiC(CH3)3), 20.14 CH2CH3), 21.68 CH2CH3), 26.11 SiC(CH3)3), 40.29 (-N(CH3)2), 75.50 (OCH2), 96.63, 103.56, 103.66, 110.20 (Ar-C), 111.52, 122.43, 123.64, 124.13, 125.27 (ArCH), 128.46, 133.25, 143.66, 149.81, 152.67 (Ar-C), 234(CO). FTIR (cm-1, PE): νCO 1959 (s, A1), 1899, 1884 (s, E). MS (EI): (Mþ) m/z (%) 641.2 (2), 557.2 (24), 505.2 (100), 476.2 (30). HR-MS (ESI pos.): calcd for C33H43CrN3O5SiHþ 642.2450, found 642.2450. Crystallography. The crystal structure determinations were performed on a Nonius KappaCCD diffractometer at 123(2) K/ 223(2) K for 3(223K), 3(123K), 7, 8, 9, and 10 and on a STOE IPDS 2T diffractometer at 123(2) K for 2 and 6b using Mo KR radiation (λ = 0.71073 A˚). Crystal data, data collection parameters, and results of the analyses are listed in Tables 7 and 8. Direct methods (SHELXS-97)23 were used for structure solution, and refinement was carried out using SHELXL-97 (full-matrix least-squares on F2).24 Hydrogen atoms were refined using a riding model. (23) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (24) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (25) Neese, F. ORCA-an ab Initio Density Functional and Semiempirical Program Package, version 2.5-20.2007; Universit€at Bonn: Bonn, Germany, 2007.


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CCDC-769717 (2), CCDC-770726 (3(223K)), CCDC-717393 (3(123K)), CCDC-769720 (6b), CCDC-770727 (7), CCDC769718 (8), CCDC-769716 (9), and CCDC-769719 (10) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. The ORCA electronic structure program package25 was used for all quantum chemical calculations. Minimum energy structures and transition states were optimized with the BP86 functional11 in combination with the Ahlrichs TZVP basis set for the Cr(CO)3 template and the smaller SV(P) basis set for the arene system. The resolutionof-the-identity RI approximation26 was used in the Split-RI-J variant27 with the appropriate Coulomb fitting sets. The recently implemented transition-state searcher of the ORCA package is based on eigenvector following28 and is very efficient due to the use of partial Hessians. Transition states and minima were verified through frequency calculations that were performed by two-sided differentiation of analytic gradients.

Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 624 “Templates”) and the Fonds der Chemischen Industrie is gratefully acknowledged. B.S. thanks the Alexander-von-Humboldt Foundation for a postdoctoral research fellowship. Supporting Information Available: 1H and 13C NMR spectra of all complexes and crystallographic data of compounds 2, 3, 6b, 7, 8, 9, and 10 (CIF format) are available free of charge via the Internet at http://pubs.acs.org.

Note Added after ASAP Publication. This paper was published on the Web on Nov 1, 2010, with errors in Table 7 and the caption for Figure 2. The corrected version was reposted on Nov 5, 2010. (26) (a) Vahtras, O.; Alml€ of, J.; Feyereisen, M. W. Chem. Phys. Lett. 1993, 213, 514. (b) Weigend, F.; H€aser, M. Theor. Chem. Acc. 1997, 97, 331. (c) Bernholdt, D. E.; Harrison, R. J. Chem. Phys. Lett. 1996, 250, 477. (27) Neese, F. J. Comput. Chem. 2003, 24, 1740. (28) Baker, J. J. Comput. Chem. 1986, 7, 385.

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