Cyclopentanone Insertion into η9-Indenyl Rings of Zirconium

Mar 17, 2010 - Doris Pun, Emil Lobkovsky, Ivan Keresztes, and Paul J. Chirik*. Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell...
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Organometallics 2010, 29, 1789–1796 DOI: 10.1021/om100096g

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Cyclopentanone Insertion into η9-Indenyl Rings of Zirconium Sandwich Complexes Doris Pun, Emil Lobkovsky, Ivan Keresztes, and Paul J. Chirik* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853 Received February 6, 2010

Addition of two equivalents of cyclopentanone to the bis(indenyl)zirconium sandwich compound (η9-C9H5-1,3-iPr2)(η5-C9H5-1,3-iPr2)Zr resulted in selective insertion of two equivalents of the ketone into the 1 and 4 positions of one of the indenyl ligands, and the resulting product was characterized by NMR spectroscopy and X-ray diffraction. For the mixed ring compound, (η5-C5Me5)(η9-C9H51,3-iPr2)Zr, two isomeric double-insertion products were observed where C-C bond formation took place at the 1 and 4 (major) and 1 and 6 (minor) positions of the indenyl. The major product was characterized by X-ray diffraction. Deuterium labeling experiments and addition of substoichiometric amounts of ketones were used to gain insight into the preferred sites of insertion, and mono(ketone) intermediates were observed and characterized by NMR spectroscopy.

Introduction Cyclopentadienyl (“Cp”) and related indenyl (“Ind”) rings are ubiquitous supporting ligands in organometallic chemistry.1 The utility of group 4 metallocenes as catalysts for olefin polymerization and as reagents in organic synthesis has motivated numerous studies focused on establishing structure-reactivity relationships between ring substitution and stereo- and regiocontrol, catalyst productivity, and the electronic properties of the metal center.2,3 In most reactions, the Cp and Ind rings are robust, chemically inert supporting ligands that do not directly participate in the observed chemistry, although examples to the contrary have become more common.4 Gleiter and Wittwer reported unexpected participation of the cyclopentadienyl ligands in the reaction chemistry upon addition of ketones to (η5-C5H5)2Ti(PMe3)2.5 Instead of the anticipated pinacol products, fulvene compounds derived from ketone-cyclopentadienyl coupling were isolated. Organometallic intermediates in these reactions were not observed or characterized, so little structural evidence is available. Crowe and Vu have also reported an unexpected functionalization of the cyclopentadienyl ligand upon addition of isocyanates to metallocyclic titanocene compounds, and crystallographic characterization of a tBuNC insertion product was described.6 Beckhaus and co-workers have also observed ketone insertion into

cyclometalated cyclopentadienyl substituents in chemistry reminiscent of group 4 metallocene alkyl compounds.7 Insertions of ketones into related titanium pentadienyl compounds have also been reported.8 Rosenthal’s seminal discoveries of the coupling of titanocyclopentadienes with the “ancillary” η5-C5H5 rings to form titanium dihydroindene complexes opened new opportunities for Cp-based group 4 metallocene reactivity.9 Xi and Takahashi have since reported liberation of benzene and pyridine from titanocyclopentadiene compounds where the Cp ligand serves as a source of 2 and 3 [CH] units, respectively.10 This approach has since been evolved to include the synthesis of indenes using the Cp ligands as a [CH] source.11-13 While similar chemistry is not yet as mature for zirconium, Takahashi has recently reported a versatile TiCl4-promoted synthesis of substituted indenes from zirconocyclopentadienes14 involving coupling of the substituted Cp or Ind ring with the metallocycle. As in titanium chemistry, zirconium pentadienyl compounds exhibit a rich reaction chemistry involving ligand modification upon treatment with unsaturated organic molecules.8

*Corresponding author. E-mail: [email protected].  epniccka, P. Ferrocenes: Ligands, Materials and Biomole(1) (a) St cules; John Wiley & Sons Ltd: Chichester, 2008. (b) Togni, A.; Hayashi, T. Ferrocenes: Homogeneous Catalysis, Organic Synthesis, Materials; Wiley-VCH: New York, 1994. (2) (a) Coates, G. W. Chem. Rev. 2000, 100, 1223. (b) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124. (3) Busico, V. Dalton Trans. 2009, 41, 8794. (4) For reviews of cyclopentadienyl reactivity in metallocenes see: (a) Liu, R.; Zhou, X. J. Organomet. Chem. 2007, 692, 4424. (b) Takahashi, T.; Kanno, K. Top. Organomet. Chem. 2005, 8, 217. (5) Gleiter, R.; Wittwer, W. Chem. Ber. 1994, 127, 1797. (6) Crowe, W. E.; Vu, A. T. J. Am. Chem. Soc. 1996, 118, 5508.

(7) Stroot, J.; Beckhaus, R.; Saak, W.; Haase, D.; L€ utzen, A. Eur. J. Inorg. Chem. 2002, 7, 1729. (8) Stahl, L.; Ernst, R. D. Adv. Organomet. Chem. 2008, 55, 137. (9) (a) Tillack, A.; Baumann, W.; Ohff, A.; Lefeber, C.; Spannenberg, A.; Kempe, R.; Rosenthal, U. J. Organomet. Chem. 1996, 520, 187. (b) Rosenthal, U.; Lefeber, C.; Arndt, P.; Tillack, A.; Baumann, W.; Kempe, R.; Burlakov, V. V. J. Organomet. Chem. 1995, 503, 221. (c) Thomas, D.; Peulecke, N.; Burlakov, V. V.; Heller, B.; Baumann, W.; Spannenberg, A.; Kempe, R.; Rosenthal, U.; Beckhaus, R. Z. Anorg. Allg. Chem. 1998, 624, 919. (10) Xi, Z.; Sato, K.; Gao, Y.; Lu, J.; Takahashi, T. J. Am. Chem. Soc. 2003, 125, 9568. (11) Takahashi, T.; Kuzuba, Y.; Kong, F.; Nakajima, K.; Xi, Z. J. Am. Chem. Soc. 2005, 127, 17188. (12) Takahashi, T.; Song, Z.; Sato, K.; Kuzuba, Y.; Nakajima, K.; Kanno, K. J. Am. Chem. Soc. 2007, 129, 11678. (13) Takahashi, T.; Song, Z.; Hsieh, Y.-F.; Nakajima, K.; Kanno, K. J. Am. Chem. Soc. 2008, 130, 15236. (14) Ren, S.; Igarashi, E.; Nakajima, K.; Kanno, K.; Takahashi, T. J. Am. Chem. Soc. 2009, 131, 7492.

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Scheme 1. Dynamic Behavior of η9,η5-Bis(indenyl)zirconium Compounds

The discovery of bis(indenyl)zirconium15,16 and related mixed ring cyclopentadienyl-indenyl zirconium sandwich17 compounds has allowed fundamental spectroscopic and reactivity studies of an isolable source of “zirconocene”.18 One unusual structural feature of these compounds is the presence of an η9-indenyl ring, where all nine carbon atoms are bound to the metal. For the bis(indenyl) examples, variable-temperature NMR data,16 in combination with computational studies,19 established facile exchange of ring hapticity in solution that proceeds through a traditional η5,η5-zirconium sandwich intermediate (Scheme 1). Reactivity studies support this view, as facile oxidative addition of C-H,15 H-H,20 and C-O21 bonds has been observed as well as N2 activation22 and alkyne coupling.16 As part of our continuing investigations into the utility of zirconium sandwich derivatives as two electron reductants for organic synthesis, the reactivity of these unique compounds with ketones was explored. Here we describe a new carbon-carbon bond forming reaction for zirconiumbound indenyl ligands arising from the insertion of cyclopentanone into the metal-carbon bonds of both the cyclopentadienyl and benzo portion of the carbocycle. Ketone insertion into the zirconium-carbon bonds is reversible, as migration of the inserted carbonyl was observed.

Results and Discussion Addition of two equivalents of cyclopentanone to a pentane solution of the isopropyl-substituted bis(indenyl)zirconium sandwich (η9-C9H5-1,3-iPr2)(η5-C9H5-1,3-iPr2)Zr (1) furnished a new C1 symmetric product identified as 1-(OcPent)2-1,4, arising from insertion of two equivalents of ketone into the 1 and 4 positions of the indenyl ring (Scheme 2). Under these reaction conditions, there was no evidence for enolization of the C-H bonds adjacent to the carbonyl. The positions of the insertion into the zirconium-indenyl and resulting C-C bond formation were established by a series of multinuclear and two-dimensional NMR experiments and X-ray diffraction. The IUPAC numbering system for a 1,3-disubstituted indenyl ligand is presented in Chart 1. Treatment of a pentane solution of the mixed ring zirconium sandwich (η5-C5Me5)(η9-C9H5-1,3-iPr2)Zr (2)17 with (15) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125, 8110. (16) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 16937. (17) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Organometallics 2006, 25, 2080. (18) Chirik, P. J. Organometallics 2010, 29, ASAP. (19) Veiros, L. F. Chem.-Eur. J. 2005, 11, 2505. (20) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 6454. (21) Bradley, C. A.; Veiros, L. F.; Pun, D.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 16600. (22) Pun, D.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2008, 130, 6047.

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two equivalents of cyclopentanone furnished a 1.0:0.7 ratio of two C1 symmetric products identified as two isomers of the double insertion compound, 2-(OcPent)2-1,4 (major) and 2-(OcPent)2-1,6 (minor) (Scheme 2). In all three cases, NMR spectroscopy has proven particularly useful in establishing the site of cyclopentanone insertion and confirming C-C bond formation. Complete 1H and 13C NMR peak listings for all cyclopentanone insertion products are reported in Tables 1 and 2, respectively. The observation of the 1,6product for double insertion from 2 but not 1 may be a result of the difference in steric environments between the (η5C5Me5) supporting ligand and a 1,3-disubstituted indenyl. The most dramatic upfield shifts for the functionalized indenyl positions were observed by 13C NMR spectroscopy. For example, for 1-OcPent-1,4 in benzene-d6, the C1 carbon appears at 67.27 ppm, while the related C3 position remains downfield at 153.93 ppm. Likewise, the C4 carbon, also a product of C-C formation, is shifted upfield to 49.17 ppm, while the related C7 position appears at 73.87 ppm, which is also atypically upfield for an indenyl and is likely a result of the change in ring hapticity. Similar upfield shifts are observed for C1 in both 2-OcPent-1,4 and 2-OcPent-1,6. In the latter compound, the C6 carbon is shifted to 51.46 ppm. Recrystallization of the insertion products from pentane at -35 °C produced crystals suitable for single-crystal X-ray diffraction. For the mixture of products obtained from 2, the major isomer, 2-(OcPent)2-1,4, was selectively isolated. Representations of the solid state structures of 1-(OcPent)2-1,4 and 2-(OcPent)2-1,4 are presented in Figure 1 and confirm the sites of insertion proposed on the basis of NMR spectroscopy. Selected bond distances and angles are presented in Table 3. Idealized piano stool geometries are observed in both compounds, where the coordination sphere of the zirconium consists of an η5-indenyl or pentamethylcyclopentadienyl ligand, two alkoxides arising from cyclopentanone insertion, and a Zr-C bond from an η1-interaction with the modified indenyl. The bond lengths in each modified indenyl ligand demonstrate disruption of the aromaticity of the ligand and bond localization. For example, C(2)-C(3) distances of 1.346(3) (1) and 1.351(7) (2) A˚ are consistent with CdC bonds, while the C(3)-C(4) lengths of 1.516(3) (1) and 1.529(7) (2) A˚ indicate carbon-carbon single bonds. The assignment of formal η1-hapticity of the indenyl is supported by the observed zirconium-carbon bond distances. The Zr(1)-C(1) distances of 2.323(2) and 2.341(5) A˚ in 1-(OcPent)2-1,4 and 2-(OcPent)2-1,4, respectively, are comparable to the analogous zirconium-carbon distances in the corresponding sandwich compounds. The remaining Zr-C distances in the modified indenyls are significantly longer and outside the range typically ascribed to zirconium-carbon bonds. The Zr(1)-C(2) bonds are exceptions and are between those found for C(1) and the other positions and may be indicative of a weak interaction or simply may be a consequence of the tilt of the ring following the doubleinsertion event. Additional studies were conducted to determine the kinetic and thermodynamic stabilities of the observed ketone insertion products. One question concerns the reversibility of the C-C bond-forming reaction. To probe this possibility, crossover experiments were conducted. For ease of analysis by 1H, 13C HSQC NMR spectroscopy, 1-(OcPent-d4)2-1,4 was prepared from addition of 2 equiv of 2,2,5,5-cyclopentanone-d4 to 1. Allowing a benzene-d6 solution of this compound to stand at 23 °C in the presence of 2.4 equiv of natural abundance

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Scheme 2. Cyclopentanone Insertion with 1 and 2

Chart 1. Indenyl Labeling Scheme

Table 1. 1H NMR Shifts (ppm) of Each Zirconium Compound Recorded in Benzene-d6 at 23 °Ca hydrogen attachment

1-(OcPent)2-1,4

2-(OcPent)2-1,4

2-(OcPent)2-1,6

C(2) C(4) C(5) C(6) C(7)

5.91 3.70 4.78 5.23 3.97

6.02 3.85 5.04 5.24 3.98

5.10 5.53 3.90 3.81 5.38

a The IUPAC numbering system presented in Chart 1 for indenyl is used.

Table 2. 13C NMR Shifts (ppm) of Each Zirconium Compound Recorded in Benzene-d6 at 23 Ca carbon

1-(OcPent)2-1,4

2-(OcPent)2-1,4

2-(OcPent)2-1,6

C(1) C(2) C(3) C(3a) C(4) C(5) C(6) C(7) C(7a) CO

67.27 130.78 153.93 137.50 49.17 120.86 119.67 73.87 152.30 97.71 133.12

67.82 131.39 153.74 137.18 49.59 118.29 119.86 72.65 152.52 97.08 131.62

66.10 114.74 149.16 97.95 119.44 95.36 51.46 112.26 155.02 120.6

a

The IUPAC numbering system presented in Chart 1 is used.

cyclopentanone for 24 h resulted in formation of the mixed isotopologue where the protio ketone had exchanged solely into the 1 position of the indenyl (Scheme 3).

A second crossover experiment was conducted whereby a benzene-d6 solution containing a mixture of the mixed ring insertion products 2-(OcPent)2-1,4 and 2-(OcPent)2-1,6 was allowed to stand at 23 °C in the presence of 2 equiv of free sandwich compound 1. Over the course of 24 h, 2-(OcPent)2-1,4 was completely consumed, the concentration of 2-(OcPent)2-1,6 remained unchanged, and two new zirconium products were observed (Scheme 3). The first, accounting for 57% of the new products, was identified as the C1 symmetric mono(ketone) compound 2-(OcPent)-4, with the cyclopentanone inserted exclusively at the 4 postion of the indenyl ring. The second was a Cs symmetric compound identified as the zirconocene enolate hydride 1-(OcPentenyl)H, arising from C-H activation of the position adjacent to the ketone by 1. Both the isotopic labeling studies and capture of cyclopentanone by 1 from a double-insertion product established the reversibility of C-C bond formation. Substoichiometric amounts of cyclopentanone were also added to both zirconium sandwich compounds in an attempt to observe mono(ketone) products and to gain insight into the initial site(s) of insertion. Addition of 0.5 equiv of cyclopentanone to a benzene-d6 solution of 1 furnished a 0.9:1 ratio of 1-(OcPent)2-1,4 to the zirconocene enolate hydride, 1-(OcPentenyl)H. The products were formed in 35% yield, with the balance of the material remaining as 1. Notably, no mono(ketone) products were detected by 1H NMR spectroscopy under these conditions. Repeating the addition with 0.5 equiv of 2,2,5,5-cyclopentanone-d4 resulted in immediate conversion to 25% of 1-(OcPent-d4)2-1,4 with no evidence for any other zirconium products. After 2 days at 23 °C, this mixture of 1 and 1-(OcPent-d4)2-1,4 converted to 27% 1-(OcPentenyl-d3)D and 27% of a new C1 symmetric product identified as 1-(OcPent-d4)-4, arising from formal selective ketone deinsertion from the 1 position of the modified indenyl (Scheme 4). These experiments demonstrate that enolization becomes competitive with insertion at low concentrations of cyclopentanone, and a deuterium kinetic isotope effect can be used to initially suppress this side reaction. Unfortunately we are unable to distinguish if

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Figure 1. Solid-state structures of 1-(OcPent)2-1,4 (left) and 2-(OcPent)2-1,4 (right) with 30% probability ellipsoids. Table 3. Selected Bond Distances (A˚) for 1-(OcPent)2-1,4 and 2-(OcPent)2-1,4

Zr(1)-O(1) Zr(1)-O(2) Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(3) Zr(1)-C(4) Zr(1)-C(5) Zr(1)-C(9) O(1)-C(25) O(1)-C(30) C(8)-C(25) C(4)-C(30) C(1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(6)-C(7) C(7)-C(8) C(8)-C(9) C(1)-C(9) C(5)-C(9)

1-(OcPent)2-1,4

2-(OcPent)2-1,4

1.9655(13) 1.9688(13) 2.323(2) 2.609(2) 2.864(2) 3.089(2) 2.936(2) 2.5762(19) 1.433(2) 1.419(2) 1.589(3) 1.567(3) 1.431(3) 1.346(3) 1.516(3) 1.516(3) 1.490(3) 1.341(3) 1.505(3) 1.526(3) 1.445(3) 1.365(3)

1.989(3) 1.980(13) 2.341(5) 2.574(5) 2.739(5) 3.004(5) 2.861(5) 2.556(5) 1.417(6) 1.426(6) 1.603(8) 1.570(7) 1.426(7) 1.351(7) 1.529(7) 1.502(7) 1.493(7) 1.317(7) 1.508(6) 1.524(8) 1.429(7) 1.372(7)

enolization occurs directly from the addition of the ketone to the sandwich or via deinsertion of the carbonyl from a double-inserted product. However, the cyclopentanone inserted at the 1 position of the indenyl is labile. Analogous experiments were conducted with sandwich 2. Treatment of a benzene-d6 solution of 2 with 0.5 equiv of cyclopentanone initially formed a mixture of compounds containing a 1.0:0.7 ratio of the previously observed double-insertion products 2-(OcPent)2-1,4 and 2-(OcPent)2-1,6 (24% conversion, 66% remaining 2). A new C1 symmetric compound, accounting for 10% of the mixture, arising from a single cyclopentanone insertion, was also observed. Two-dimensional NMR experiments established exclusive insertion into the 1 position of the indenyl ring to yield 2-(OcPent)-1 (Scheme 5). Allowing this mixture to stand in benzene-d6 at 23 °C in the presence of an (Me3Si)2O internal standard demonstrated gradual disappearance of both 2-(OcPent)-1 and 2-(OcPent)2-1,4 with concomitant growth of the mono(ketone) product, 2-(OcPent)-4. Also during this time, the

zirconocene enolate hydride, 2-(OcPentenyl)H, was also observed. Notably, the absolute quantity of the other bis(ketone) product, 2-(OcPent)2-1,6, did not change over time, and no mono(ketone) product at the 6 position of the indenyl was observed. The concentrations of the various zirconium compounds measured as a function of time are presented in Table 4. With the observation of mono(ketone) products arising from 2, reaction with additional cyclopentanone was studied. Addition of excess ketone to a benzene-d6 solution containing 2-(OcPent)-4 resulted in an increase of 2-(OcPent)2-1,4, again demonstrating the 1 position of the indenyl as an available site for ketone insertion. It is important to note that in this experiment 2 was present and also served as a source of 2-(OcPent)2-1,4. However, the ratio of the newly formed 2-(OcPent)2-1,4:2-(OcPent)2-1,6 was larger than the usual ratio of 1.0:0.7, demonstrating that the excess product was indeed derived from 2-(OcPent)-4. The bonding and ring hapticity in both 2-(OcPent)-1 and 2-(OcPent)-4 are worthy of comment. In 2-(OcPent)-1, cyclopentanone insertion at the 1 position induces haptotropic rearrangement of the indenyl ring, forming an η6-benzo in the final product. As established previously,23 zirconium η6benzo interactions are best described as two-electron-reduced L2X2 rings, resulting in a Zr(IV) oxidation state in 2-(OcPent)-1. To determine if 1-(OcPent)2-1,4, 2-(OcPent)2-1,4, and 2-(OcPent)2-1,6 were kinetic or thermodynamic products of cyclopentanone insertion, thermolysis experiments were conducted. Warming a benzene-d6 solution of 1-(OcPent)21,4 to 90 °C for one week resulted in conversion to an equimolar mixture of Cs and C1 symmetric products. Significant quantities (∼50%) of free indene accompanied this transformation. Multinuclear and multidimensional NMR spectroscopy identified the Cs symmetric product as the 1-(OcPent)2-4,7, arising from formal migration of the ketone in 1-(OcPent)2-1,4. The C1 symmetric product exhibits 1H and 13 C NMR shifts similar to Cs symmetric 1-(OcPent)2-4,7, supporting cyclopentanone insertion at the same positions (23) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P J. J. Am. Chem. Soc. 2005, 127, 10291.

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Scheme 3. Crossover Experiments to Probe the Reversibility of C-C Bond Formation and Cyclopentanone Insertion

Scheme 4. Products Observed from Addition of 0.5 equiv of Cyclopentanone-d4 to 1

Scheme 5. Products Observed from Addition of 0.5 equiv of Cyclopentanone to 2

of the indenyl ring but implying a different conformation that lowers the overall molecular symmetry. Unfortunately, we were unable to definitively assign the identity of this compound. Warming a benzene-d6 solution of

pure 2-(OcPent)2-1,4 to 65 °C for 16 h resulted in decomposition. By contrast, 2-(OcPent)2-1,6 (a mixture with trace quantities of 2-(OcPent)2-1,4) was stable upon heating to 105 °C for 3 days.

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Table 4. Relative Amounts of Each Zirconium Product As a Function of Time (or addition of excess cyclopentanone) Arising from Treatment of 2 with 0.5 equiv of Cyclopentanone time (h)a

2

2-(OcPent)2-1,4

2-(OcPent)2-1,6

2-(OcPent)2-1

0.08 2 4 8 12 24 þ xs ketone

0.58 0.55 0.52 0.50 0.49 0.47

0.12 0.10 0.07 0.06 0.05 0.03 0.30

0.09 0.09 0.09 0.09 0.09 0.09 0.32

0.09 0.05 0.03 0.01

a

2-(OcPent)2-4

2-(OcPentenyl)H

0.03 0.06 0.07 0.09

0.05 0.08 0.10 0.13 0.15 0.21

6% free indene was observed after two hours and remained constant.

The results of the above experiments provide insights into the nature and preferences of cyclopentanone insertion into η9-indenyl ligands. In both mono- and bis(ketone) products, the C-C and Zr-O bond-forming events are reversible, as exchange with free ketone and migrations were observed for both sandwich motifs. For 2, the 1 position of the indenyl ring was shown to be the initial site of cyclopentanone insertion, and the resulting mono(ketone) product is labile, undergoing isomerization to the 4 position and to zirconocene enolate hydride. It is likely that haptotropic rearrangement from an η6-benzo compound to an η5-cyclopentadienyl derivative provides the driving force for the observed isomerizations. The 1,4-bis(insertion) products derived from 1 and 2 were shown to be substitutionally labile at the 1 position, while the 4 position was inert. The studies also highlight the importance of the haptotropic flexibility of the indenyl ligand to enable new reactivity in zirconium sandwich chemistry. Investigations into the scope of these new transformations with various ketones and different sandwich structures as well as additional mechanistic investigations are currently in progress in our laboratory.

Experimental Section General Considerations. All air- and moisture-sensitive manipulations were carried out using standard vacuum line, Schlenk, or cannula techniques or in an M. Braun inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents for airand moisture-sensitive manipulations were initially dried and deoxygenated using literature procedures.24 Benzene-d6 was distilled from sodium metal under an atmosphere of argon and stored over 4 A˚ molecular sieves or titanocene. Argon and hydrogen gas were purchased from Airgas Incorporated and passed through a column containing manganese oxide on vermiculite and 4 A˚ molecular sieves before admission to the highvacuum line. Carbon monoxide was passed through a liquid nitrogen cooled trap immediately before use. 1 H NMR spectra were recorded on a Varian Inova 400 spectrometer operating at 399.779 MHz (1H), while 13C spectra were collected on a Varian Inova 500 spectrometer operating at 125.704 MHz. All chemical shifts are reported relative to SiMe4 using 1H (residual) or 13C NMR chemical shifts of the solvent as a secondary standard. NOESY experiments were recorded on a Varian 500 Inova spectrometer operating at 499.920 MHz for 1H. HSQC and HMBC experiments were recorded on a Varian 600 Inova spectrometer operating at 599.773 MHz for 1H and 150.811 MHz for 13C. 2 H NMR spectra were recorded on a Varian Inova 500 spectrometer operating at 76.740 MHz, and the spectra were referenced using an internal benzene-d6 standard. Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil in a drybox and were quickly transferred to the goniometer head of a Siemens SMART CCD Area detector system equipped with a molybdenum (24) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.

X-ray tube (δ = 0.71073 A˚). Preliminary data revealed the crystal system. A hemisphere routine was used for data collection and determination of lattice constants. The space group was identified, and the data were processed using the Bruker SAINT program and corrected for absorption using SADABS. The structures were solved using direct methods (SHELXS) completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures. Infrared spectra were collected on a Thermo Nicolet spectrometer. Elemental analyses were performed at Robertson Microlit Laboratories, Inc., in Madison, NJ. The following compounds were prepared according to literature procedures: 1,16 2,17 and cyclopentanone-d4.25 Cyclopentanone was purchased from Aldrich and distilled from CaH2 prior to use. Preparation of 1-(OcPent)2-1,4. In a drybox, a 20 mL scintillation vial was charged with 0.032 g (0.07 mmol) of 1 and approximately 4 mL of pentane. To the resulting burgundy solution was added 0.011 g (0.13 mmol) of cyclopentanone by syringe with stirring. A yellow solution formed and was stirred for 1 h at ambient temperature. The solvent was removed in vacuo, and subsequent recrystallization from pentane at -35 °C furnished 0.030 g (70%) of a yellow foam identified as 1-(OcPent)2-1,4. Anal. Calcd for C40H54O2Zr: C, 73.01; H, 8.27. Found: C, 72.64; H, 8.34. 1H NMR (benzene-d6): δ 0.78 (d, 8 Hz, 3H, CHMe2), 0.86 (d, 8 Hz, 3H, CHMe2), 0.88 (d, 8 Hz, 3H, CHMe2), 0.91 (d, 8 Hz, 3H, CHMe2), 0.92 (d, 8 Hz, 3H, CHMe2), 1.15 (m, 4H, CHMe2), 1.18 (d, 8 Hz, 3H, CHMe2), 1.20 (m, 1H, cyclopentanone CH2), 1.23 (d, 8 Hz, 3H, CHMe2), 1.33 (m, 1H, cyclopentanone CH2), 1.37 (m, 4H, CHMe2/cyclopentanone CH2), 1.53 (d, 8 Hz, 3H, CHMe2), 1.57 (m, 6H, CHMe2/three cyclopentanone CH2), 1.61 (d, 8 Hz, 3H, CHMe2), 1.74 (m, 1H, cyclopentanone CH2), 1.81 (m, 6H, cyclopentanone CH2), 2.03 (m, 1H, CHMe2), 2.08 (m, 1H, cyclopentanone CH2), 2.71 (m, 1H, CHMe2), 3.19 (m, 1H, CHMe2), 3.33 (m, 1H, CHMe2), 3.70 (dd, 4 Hz, 1 Hz, 1H, C4-H), 3.97 (d, 6 Hz, 1H, C7-H), 4.78 (dd, 9 Hz, 4 Hz, 1H, C5-H), 5.23 (dd, 9 Hz, 6 Hz, 1H, C6-H), 5.91 (s, 1H, C2-H), 6.43 (s, 1H, CpH), 6.90 (m, 1H, η5 Benzo), 6.98 (m, 1H, η5 Benzo), 7.26 (m, 1H, η5 Benzo), 7.56 (m, 1H, η5 Benzo). 2H NMR of (1-(OcPent)-1-(OcPent-d4)-4) (benzene-d0): 1.20, 1.34, 1.43 (cyclopentanone R CH2 on Ind C4), 1.56, 1.60, 1.78 (cyclopentanone R CH2 on Ind C1), 1.79 (cyclopentanone R CH2 on Ind C4), 1.80 (cyclopentanone R CH2 on Ind C1). 13 C NMR (benzene-d6): δ 18.63, 20.28, 20.98, 21.90 (CHMe2), 21.93, 22.40 (cyclopentanone CH2), 22.67 (CHMe2), 23.17, 24.13 (cyclopentanone CH2), 24.90, 25.62, 26.09 (CHMe2), 27.52 (CHMe2 on η5 Ind), 27.61 (CHMe2 on η1 Ind), 27.90 (CHMe2 on η5 Ind), 31.22 (CHMe2 on η1 Ind), 37.06 (cyclopentanone R CH2 on Ind C4), 37.82 (cyclopentanone R CH2 on Ind C1), 40.60 (cyclopentanone R CH2 on Ind C4), 42.52 (cyclopentanone R CH2 on Ind C1), 49.17 (C4), 67.27 (C1), 73.87 (C7), 97.71 (C-O insert at C1), 112.18 (CpH), 118.53, 119.02 (Cp), 119.67 (C6), 120.86 (C5), 121.83, 122.33, 123.68, 124.18, 124.60, 126.49 (Benzo), 130.78 (C2), 133.12 (C-O insert at C4), 137.50 (C3a), 152.30 (C7a), 153.93 (C3). Preparation of (η5-C9H5-1,3-(CHMe2)2)(η1-C9H5-1,3-(CHMe2)2-4-C(K1-O)(CH2)4-7-C(K1-O)(CH2)4)Zr (1-(OcPent)2-4,7). In a J. Young NMR tube charged with 0.010 g (0.02 mmol) of (25) Wanat, R. A.; Collum, D. B. Organometallics 1986, 5, 120.

Article 1-(OcPent)2-1,4 0.5 mL of benzene-d6 was added. The tube was placed in a 65 °C oil bath for 8 days, producing two isomers of 1-(OcPent)2-4,7 in a 1:1 ratio. Cs symmetric isomer: 1H NMR (benzene-d6): δ 1.02 (d, 8 Hz, 6H, CHMe2), 1.16 (d, 8 Hz, 6H, CHMe2), 1.18 (d, 8 Hz, 6H, CHMe2), 1.56 (d, 8 Hz, 6H, CHMe2), 1.55 (m, 2H, cyclopentanone CH2), 1.65 (m, 2H, cyclopentanone CH2), 1.77 (m, 2H, cyclopentanone CH2), 2.00 (m, 2H, cyclopentanone CH2), 2.75 (m, 2H, CHMe2), 3.46 (m, 2H, CHMe2), 3.63 (m, 2H, C4-H/C7-H), 5.53 (s, 1H, C2-H), 5.83 (m, 2H, C5-H/C6-H), 6.54 (s, 1H, CpH), 6.93 (m, 2H, η5 Benzo), 7.42 (m, 2H, η5 Benzo). Eight cyclopentanone CH2 resonances not located. 13C NMR (benzene-d6): δ 20.64 (CHMe2), 21.93, 22.40 (cyclopentanone CH2), 22.93, 24.04, 25.00 (CHMe2), 26.60, 26.70 (CHMe2), 39.72, 44.11 (cyclopentanone CH2), 44.94 (C4/C7), 104.76 (C2), 112.35 (C-O insert at C4/C7), 116.53 (CpH), 119.59 (Cp), 121.81 (Benzo), 122.29 (Cp), 124.13 (C1/C3), 124.62 (Benzo), 126.77 (C5/C6), 135.29 (C3a/C7a). Two cyclopentanone CH2 resonances not located. C1 symmetric isomer: 1H NMR (benzene-d6): δ 0.97 (d, 8 Hz, 3H, CHMe2), 1.06 (d, 8 Hz, 3H, CHMe2), 1.17 (d, 8 Hz, 3H, CHMe2), 1.25 (d, 8 Hz, 6H, CHMe2), 1.40 (d, 8 Hz, 3H, CHMe2), 1.46 (d, 8 Hz, 3H, CHMe2), 1.61 (d, 8 Hz, 3H, CHMe2), 1.40 - 2.39 (m, 16H, cyclopentanone CH2), 2.97 (m, 1H, CHMe2), 3.16 (m, 1H, CHMe2), 3.41 (m, 1H, C7-H),3.48 (m, 1H, CHMe2), 3.49 (m, 1H, CHMe2), 3.76 (m, 1H, C4-H), 5.66 (s, 1H, C2-H), 5.77 (m, C5-H), 5.78 (m, 2H, C6-H), 6.40 (s, 1H, CpH), 6.89 (m, 1H, η5 Benzo), 6.93 (m, 1H, η5 Benzo), 7.48 (m, 1H, η5 Benzo), 7.54 (m, 1H, η5 Benzo). 13C NMR (benzened6): δ 21.11, 21.14, 22.00, 22.80, 22.99, 24.27, 25.16, 25.62 (CHMe2), 27.21, 27.24, 27.95 (CHMe2), 42.01 (cyclopentanone CH2), 44.76 (C7), 45.78 (C4), 106.86 (C2), 110.82 (CpH), 113.81 (C-O insert at C7), 117.82 (Cp), 121.78 (2) (Benzo), 122.58, 122.94 (Cp), 123.10, 123.46 (Benzo), 124.50 (Cp), 124.86 (C3), 125.82 (C6), 126.15 (C1), 126.96 (C5), 131.90 (C3a), 135.74 (C7a). One CHMe2, eight cyclopentanone CH2, and one C-O resonance not located. Addition of 0.5 equiv of Cyclopentanone to 1. A J. Young NMR tube was charged with 0.008 g (0.02 mmol) of 1 and approximately 0.5 mL of benzene-d6. Using a 10 μL syringe, 0.7 μL (0.01 mmol) of cyclopentanone was added. After 1 day at 22 °C, both 1-(OcPent)2-1,4 and 1-(OcPentenyl)H, as well as starting 1, were observed in a 0.9:1.0:3.5 ratio by 1H NMR spectroscopy. Addition of 0.5 equiv of Cyclopentanone-d4 to 1. A J. Young NMR tube was charged with 0.008 g (0.02 mmol) of 1 and approximately 0.5 mL of benzene-d6. Using a 10 μL syringe, 0.8 μL (0.01 mmol) of cyclopentanone-d4 was added. After 1 day at 22 °C, 1-(OcPent-d4)2-1,4, 1-(OcPentenyl-d3)D, and 1-(OcPentd4)-4, as well as starting 1, were observed in a 1.1:1.0:0.7:1.3 ratio by 1H NMR spectroscopy. Spectroscopic Data for (η5-C9H5-1,3-(CHMe2)2)Zr(OCCHCH2CH2CH2)H (1-(OcPentenyl)H). 1H NMR (benzene-d6): δ 1.11 (d, 8 Hz, 3H, CHMe2), 1.15 (d, 8 Hz, 3H, CHMe2), 1.24 (d, 8 Hz, 3H, CHMe2), 1.26 (d, 8 Hz, 3H, CHMe2), 1.81 (m, 2H, cyclopentanone CH2), 2.16 (m, 2H, cyclopentanone CH2), 2.35 (m, 2H, cyclopentanone CH2), 2.98 (m, 1H, CHMe2), 3.06 (m, 1H, CHMe2), 4.52 (pseudo s, 1H, CH), 5.95 (s, 2H, CpH), 6.09 (s, 1H, Zr-H), 6.91 (m, 2H, Benzo), 6.94 (m, 2H, Benzo), 7.28 (m, 2H, Benzo), 7.34 (m, 2H, Benzo). 13C NMR (benzene-d6): δ 22.68, 24.93, 24.99, 25.28 (CHMe2), 25.43, 26.67 (CHMe2), 28.68, 34.72 (cyclopentanone CH2), 98.95 (olefinic CH), 112.22 (CpH), 119.67, 121.93 (Cp), 122.42, 123.02 (Benzo), 123.35 (Cp), 124.61 (Benzo), 162.93 (C-O). One cyclopentanone CH2, one Cp, and one Benzo resonance not located. Characterization of (η5-C9H5-1,3-(CHMe2)2)(η5-C9H5-1,3(CHMe2)2-4-C(K1-O)(CH2)4)Zr (1-(OcPent-d4)-4). 1H NMR (benzene-d6): δ 0.74 (d, 8 Hz, 3H, CHMe2), 1.02 (d, 8 Hz, 3H, CHMe2), 1.06 (d, 8 Hz, 3H, CHMe2), 1.17 (d, 8 Hz, 3H, CHMe2), 1.18 (d, 8 Hz, 3H, CHMe2), 1.24 (d, 8 Hz, 3H, CHMe2), 1.46 (d, 8 Hz, 3H, CHMe2), 1.47 (d, 8 Hz, 3H, CHMe2), 1.49 (m, 1H, cyclopentanone CH2), 1.64 (m, 1H, cyclopentanone CH2),

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1.85 (m, 1H, cyclopentanone CH2), 2.07 (m, 1H, cyclopentanone CH2), 3.00 (m, 1H, CHMe2), 3.29 (m, 1H, CHMe2), 3.34 (m, 1H, CHMe2), 3.35 (dd, 9 Hz, 6 Hz, 1 Hz, 1H, C4-H), 3.52 (dd, 9 Hz, 6 Hz, 1H, C7-H), 5.17 (dd, 9 Hz, 6 Hz, 1H, C5-H), 5.62 (s, 1H, C2-H), 6.51 (dd, 9 Hz, 6 Hz, 1H, C6-H), 6.82 (m, 1H, Benzo), 6.89 (m, 1H, Benzo), 7.36 (m, 2H, Benzo). One CHMe2 resonance not located. 13C NMR (benzene-d6): δ 20.86, 21.91, 21.99 (CHMe2), 22.36 (cyclopentanone CH2), 23.13 (2) (CHMe2), 24.01 (cyclopentanone CH2), 24.88, 25.31, 26.54 (CHMe2), 26.62, 27.32, 27.49 (CHMe2), 46.56 (C-4), 73.45 (C-7), 108.06 (Cp), 108.38 (C-2), 114.40 (Cp-H), 116.83 (Cp), 117.21 (C-5), 121.79, 122.38, 122.84 (2) (Benzo), 123.92 (Cp), 123.99 (C1), 123.92 (Cp), 126.53 (C-7a), 126.78 (Cp), 129.77 (C-3a), 129.18 (C-6), 134.96 (C-3). One CHMe2, two cyclopentanone CD2, and one C-O resonance not located. Preparation of (η5-C5Me5)(η1-C9H5-1,3-(CHMe2)2-1-C(K1O)(CH2)4-4-C(K1-O) (CH2)4)Zr (2-(OcPent)2-1,4). This compound was prepared in a similar manner to 1-(OcPent)2 using 0.109 g (0.26 mmol) of 2 dissolved in approximately 10 mL of pentane and 43 mg (0.51 mmol) of cyclopentanone. Recrystallization from pentane at -35 °C afforded 52 mg (34%) of a yellow solid identified as 2-(OcPent)2-1,4 without contamination from the other products. Anal. Calcd for C35H50O2Zr: C, 70.77; H, 8.48. Found: C, 70.49; H, 8.76. 1H NMR (benzene-d6): δ 0.83 (d, 8 Hz, 3H, CHMe2), 0.95 (d, 8 Hz, 3H, CHMe2), 1.00 (d, 8 Hz, 3H, CHMe2), 1.22 (d, 8 Hz, 3H, CHMe2), 1.56 (m, 8 H, cyclopentanone CH2), 1.84 (m, 2H, cyclopentanone CH2), 1.94 (s, 15H, Cp* Me), 2.06 (m, 7H, CHMe2/cyclopentanone CH2), 2.83 (m, 1H, CHMe2), 3.85 (dd, 4 Hz, 1H, C4-H), 3.98 (dd, 6 Hz, 1H, C7-H), 5.04 (ddd, 9 Hz, 4 Hz, 1H, C5-H), 5.24 (ddd, 9 Hz, 6 Hz, 1H, C6-H), 6.02 (s, 1H, C2-H). 13C NMR (benzene-d6): δ 11.35 (Cp* Me), 18.66, 20.34, 21.23, 21.95, 23.38, 23.82, 24.54, 26.31, 27.65, 31.71, 37.17, 37.66, 40.74, 43.63 (CHMe2/cyclopentanone CH2), 49.59 (C4), 67.82 (C1), 72.65 (C7), 97.08 (C-O), 118.29 (C5), 118.59 (Cp), 119.86 (C6), 131.39 (C2), 131.62 (C-O), 137.18 (C3a), 152.52 (C-7a), 153.74 (C3). Addition of 0.5 equiv of Cyclopentanone to 2. A J. Young NMR tube was charged with 0.008 g (0.02 mmol) of 2 and approximately 0.5 mL of benzene-d6. Using a 10 μL syringe, 0.8 μL (0.01 mmol) of cyclopentanone was added. Monitoring the reaction by 1H NMR spectroscopy revealed an initial ratio of remaining 2, the bis(insertion) products 2-(OcPent)2-1,4 and 2-(OcPent)2-1,6, and the mono(insertion) product 2-(OcPent)-1 of 1.0:0.21:0.16:0.16. Over the course of 1 day at 22 °C, 2-(OcPent)-1 was consumed and 2-(OcPent)H and 2-(OcPent)-4 formed in the ratio of 1:0.06:0.19: 0.32:0.19 for 2:2-(OcPent)2-1,4:2-(OcPent)2-1,6:2-(OcPentenyl)H: 2-(OcPent)-4. Characterization of (η5-C5Me5)(η1-C9H5-1,3-(CHMe2)2-1C(K1-O)(CH2)4-6-C(K1-O) (CH2)4)Zr (2-(OcPent)2-1,6). 1H NMR (benzene-d6): δ 0.81 (d, 8 Hz, 3H, CHMe2), 0.93 (d, 8 Hz, 3H, CHMe2), 1.23 (d, 8 Hz, 3H, CHMe2), 1.27 (d, 8 Hz, 3H, CHMe2), 1.94 (s, 15H, Cp* Me), 1.40-2.10 (m, 16H, cyclopentanone CH2), 1.98 (m, 1H, CHMe2), 2.60 (m, 1H, CHMe2), 3.81 (dd, 5 Hz, 4 Hz, 1 H, C6-H), 3.90 (ddd, 9 Hz, 4 Hz, 2 Hz, 1 H, C5H), 5.10 (s, 1H, C2-H), 5.38 (d, 5 Hz, 1 H, C7-H), 5.53 (d, 9 Hz, 1 H, C4-H). 13C NMR (benzene-d6): δ 11.43 (Cp* Me), 19.41, 20.24, 20.85, 21.72, 22.82, 22.92, 23.18, 23.22, 23.62, 24.07, 24.23, 24.32, 27.37, 31.26 (CHMe2/cyclopentanone CH2), 51.46 (C-6), 66.10 (C-1), 95.36 (C-5), 97.95 (C-3a), 112.26 (C7), 114.74 (C-2), 118.56 (Cp*), 119.44 (C-4), 120.61 (C-O), 149.16 (C-3), 155.02 (C-7a). One C-O resonance not located. Characterization of (η5-C5Me5)(η5-C9H5-1,3-(CHMe2)2)Zr(OCCHCH2CH2CH2)H (2-(OcPentenyl)H). 1H NMR (benzene-d6): δ 1.20 (d, 8 Hz, 3H, CHMe2), 1.23 (d, 8 Hz, 3H, CHMe2), 1.42 (d, 8 Hz, 3H, CHMe2), 1.45 (d, 8 Hz, 3H, CHMe2), 1.77 (s, 15H, Cp* Me), 1.40 - 2.10 (m, 6H, cyclopentanone CH2), 3.29 (m, 1H, CHMe2), 3.60 (m, 1H, CHMe2), 4.52 (pseudo s, 1H, CH), 6.05 (s, 1H, Zr-H), 6.35 (s, 1H, CpH), 6.87 (m, 2H, Benzo), 7.36 (m, 1H, Benzo), 7.42 (m, 1H, Benzo). 13C NMR (benzene-d6): δ 11.46 (Cp* Me), 22.15, 22.17, 25.40, 25.44

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(CHMe2), 26.08 (CHMe2), 28.91, 36.26 (cyclopentanone CH2), 97.64 (olefinic CH), 107.04, 108.70 (Cp), 113.69 (CpH), 118.51, 118.73 (Cp), 121.58, 122.42, 122.84 (Benzo), 163.26 (C-O). One CHMe2, one cyclopentanone CH2, one Cp, and one Benzo resonance not located. Characterization of (η5-C5Me5)(η4-C9H5-1,3-(CHMe2)2-1C(K1-O)(CH2)4)Zr (2-(OcPent)-1). 1H NMR (benzene-d6): δ 0.72 (d, 8 Hz, 3H, CHMe2), 0.75 (d, 8 Hz, 3H, CHMe2), 1.05 (d, 8 Hz, 3H, CHMe2), 1.06 (d, 8 Hz, 3H, CHMe2), 1.40-2.10 (m, 8H, cyclopentanone CH2), 1.79 (m, 1H, CHMe2), 2.18 (m, 1H, CHMe2), 2.89 (ddd, 9 Hz, 6 Hz, 1 Hz, 1H, C5-H), 3.37 (ddd, 9 Hz, 6 Hz, 1 Hz, 1H, C6-H), 4.46 (dd, 6 Hz, 1 Hz, 1H, C7-H), 4.74 (dd, 6 Hz, 1 Hz, 1H, C4-H), 5.32 (s, 1H, C2-H). One Cp* Me resonance not located. 13C NMR (benzene-d6): δ 11.67 (Cp* Me), 16.42, 18.95, 20.08, 22.11 (CHMe2), 26.21, 27.06 (CHMe2), 70.91 (C-1), 75.32 (C-4), 78.76 (C-7), 113.81 (C-5), 115.74 (C-6), 131.63 (C-2), 139.64 (C-3a), 143.50 (C-7a), 148.72 (C-3). Four cyclopentanone CH2, one Cp, and one C-O resonance not located. Characterization of (η5-C5Me5)(η5-C9H5-1,3-(CHMe2)2-4C(K1-O)(CH2)4)Zr (2-(OcPent)-4). 1H NMR (benzene-d6): δ 0.89 (d, 8 Hz, 3H, CHMe2), 1.16 (d, 8 Hz, 3H, CHMe2), 1.20

Pun et al. (d, 8 Hz, 3H, CHMe2), 1.22 (d, 8 Hz, 3H, CHMe2), 1.17 - 2.10 (m, 8H, cyclopentanone CH2), 1.71 (m, 1H, CHMe2), 1.94 (s, 1H, Cp* Me), 3.06 (m, 1H, CHMe2), 3.47 (dd, 9 Hz, 6 Hz, 1 Hz, 1H, C4-H), 3.69 (dd, 9 Hz, 6 Hz, 1H, C7-H), 5.21 (dd, 9 Hz, 6 Hz, 1H, C5-H), 5.70 (s, 1H, C2-H), 6.53 (dd, 9 Hz, 6 Hz, 1H, C6-H). 13 C NMR (benzene-d6): δ 11.56 (Cp* Me), 21.73, 23.56 (CHMe2), 24.91 (CHMe2), 25.95, 26.48 (CHMe2), 26.58 (CHMe2), 38.14, 40.27, 44.09 (cyclopentanone CH2), 46.79 (C-4), 72.31 (C7), 106.91 (C-O), 108.43 (C-2), 116.75 (C-5), 117.05 (Cp*), 123.37 (C-1), 126.97 (C-7a), 128.95 (C-3a), 129.13 (C-6), 134.89 (C-3). One cyclopentanone CH2 resonance not located.

Acknowledgment. We thank the Director, Office of Basic Energy Sciences, Chemical Science Division of the U.S. Department of Energy (DE-FG02ER15659), for financial support. Supporting Information Available: Representative NMR spectra and crystallographic data in cif format. This material is available free of charge via the Internet at http://pubs.acs.org.