Identifying and Rationalizing the Conditions for the Isomerization of 1

Article pubs.acs.org/Organometallics

Identifying and Rationalizing the Conditions for the Isomerization of 1,5-Cyclooctadiene in Iridium Complexes by Experimental and Theoretical Mechanistic Studies Benjamin Raible, Verena Gierz, and Doris Kunz* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany S Supporting Information *

ABSTRACT: The independent synthesis of the biscarbene complexes [Ir(cod)(vegiR)]PF6 (2) (cod =1,5-cyclooctadiene, vegiR = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir(1-κ-4,5,6-η-C8H12)(NCCH3)(vegiR)]PF6 (3) is described. We elucidated acetic acid as the catalyst and coordinated acetonitrile as the thermodynamic driving force for this cod-isomerization. By using the stronger trifluoroacetic acid, we isolated complex [Ir(cod)(F3CCO2)(H)(veginPr)]PF6 (7a) as an intermediate of the isomerization. From H/D exchange experiments as well as DFT calculations, we conclude that after formation of the Ir−H complex, an olefin insertion, followed by a concerted metalation-deprotonation step and a coordination of acetonitrile, is the mechanistic pathway. On the basis of our findings, we were able to carry out the cod-isomerization for the first time also for the less-electron-rich complex [Ir(2,2′-bipy)(cod)]PF6 (8) (2,2-bipy = 2,2′-bipyridine). o our knowledge, isomerization of a cod ligand was first reported by Cotton et al. in 1971 for Ru complexes1 and later by Bönnemann et al. for cobalt complexes.2 In 1992 Esteruelas et al. found the first isomerization of the cod ligand in an iridium complex and showed that this isomerization occurs via the hydride complex [Ir(η4-C8H12)(dppm)(H)].3 Upon insertion of one of the double bonds into the Ir−H bond, double bond migration, and subsequent C−H activation, the κ1,η3-C8H12-moiety in A (Figure 1) is formed. Following reports of η4-bound 1,5-cod acting as a noninnocent ligand accounted for different conditions for the isomerization: Martı ́n et al. observed the cod-activation by addition of acetonitrile or olefins to [Ir(η4-C8H12)(NCCH3)(PMe3)]BF4 leading to

T

[Ir(κ1,η3-C8H12)(NCCH3)2(PMe3)]BF4 (B)4 and showed that acetonitrile contributes to this transformation by thermodynamic as well as kinetic means.5 In contrast, Merola and Franks found an acid-catalyzed isomerization of the cod ligand in water leading to C.6 Moreover, Bera and co-workers concluded that metal-bound acetate promotes the rearrangement of the cod ligand in the formation of [Ir(κ1,η3-C8H12)(κ2C,N-1-mesityl-3(pyrid-2-yl)imidazolin-2-ylidene)]Br (D).7 The conditions and postulated mechanisms of the isomerization vary strongly, as a hydride ligand,3 Lewis base,4,5 Brønsted acid,6 and a basic Ir-OAc complex 7 are all documented to evoke formation of the κ1,η3-C8H12 ligand. In all cases, the isomerization of the cod ligand had not been expected initially. This reveals that the reactivity of the cod ligand in such complexes is not yet fully understood. Recently, we introduced the bis(NHC) ligand “vegi” as an analogue of 2,2′-bipyridine and 1,10-phenanthroline and showed that its respective Rh(cod) complex E is active in transfer hydrogenation of ketones.8,9 Therefore, we were interested in the synthesis and reactivity of the analogous iridium complex.9b However, transferring the synthesis protocol of the Rh complex to the iridium case led to formation of the cod-isomerized complex bearing a κ1,η3-coordinating C8H12ligand, as shown below. Therefore, we decided to investigate the cod-isomerization in more detail in order to shed light onto

Figure 1. Literature examples3−8 for κ1,η3-C8H12 and cod complexes.

Received: March 27, 2015 Published: May 5, 2015

© 2015 American Chemical Society

2018

DOI: 10.1021/acs.organomet.5b00267 Organometallics 2015, 34, 2018−2027

Article

Organometallics Scheme 1. Synthesis and Reactivity of [Ir(vegi)(cod)]+ Complexes: Isomerization of the Cod Ligand

the partly contradictory conditions given in the literature.3−7,10,11 In the following, we describe the selective synthesis of the nonisomerized Ir(cod) complexes 2 as well as the isomerized iridium complexes 3, and we clarify the factors that govern the isomerization. After elucidating the mechanism of this reaction by experimental and theoretical studies, we validate our conclusions by applying them in the first cod-isomerization of a bipyridine Ir-complex. Synthesis and Characterization. In the attempt to synthesize complex 2a by reacting the bisimidazolium salt veginPr·2HPF6 (1a) with KOAc and [Ir(μ-Cl)(η4-C8H12)]2 in acetonitrile, we obtained the C−H activation product [Ir(1-κ4,5,6-η-C8H12)(NCCH3)(κ2-veginPr)]PF6 (3a) in good yields (Scheme 1). The 1H NMR spectrum of 3a shows two signals for the bis(NHC) ligand, indicating a Cs symmetry of the complex. The isomerized C8H12 fragment shows characteristic multiplets at 5.24 and 4.16 ppm for the η3-allyl and at 0.86 ppm for the κ1-alkyl protons, while for a (nonisomerized) cod ligand, two broad signals around 5−6 ppm and 2−3 ppm (free cod: 5.58 and 2.36 ppm12) would be expected. In the 13C NMR spectrum of 3a, the signal of the carbene C atoms is detected at 156.4 ppm. The signal of the central allyl atom is observed at 94.7 ppm, while the signals of the adjacent carbon atoms are shifted significantly downfield to 63.8 ppm. The κ1-bound alkyl C atom leads to a peak at 20.3 ppm. The analogous reactivity and NMR spectra were obtained when using the tBu substituted bisimidazolium salt 1b. Crystals of the tBu-substituted complex 3b suitable for X-ray diffraction analysis were obtained (Figure 2). The iridium center exhibits a slightly distorted octahedral coordination environment with the vegitBu ligand and the allyl moiety in equatorial position, while acetonitrile and the alkyl moiety occupy the apical coordination sites. The vegi ligand shows a bite angle of 79.8(1)° and the Ir−C bonds measure 2.096(3) Å and 2.092(3) Å. This is in accordance with our previously reported Rh(I)-cod complex E.9a A striking difference, however, is the concave shape of the anellated ring system of the vegi ligand. We ascribe this to the steric influence of the demanding tBu substituents rather than to crystal packing effects as this feature is also observed in DFT calculations (see Supporting Information).

Figure 2. Molecular structure of 3b. The PF6− anion and cocrystallized Et2O are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1−C1 = 2.096(3), Ir1−C8 = 2.092(3), C1−C8 = 2.686(5), Ir1−C19 = 2.085(4), Ir1−C22 = 2.210(3), Ir1−C23 = 2.140(3), Ir1− C24 = 2.226(3), Ir1−N27 = 2.125(3); C1−Ir1−C8 = 79.8(1), N2− C1−N10 = 101.1(3), N7−C8−N9 = 100.6(3), C19−Ir1−N27 = 179.1(1), C19−Ir1−C23 = 96.5(1).

The central atom C23 of the allyl moiety is bound significantly closer to Ir than the adjacent atoms C22 and C24 (2.140(3) Å vs 2.210(3)/2.226(3) Å), which is a typical feature of allyl-ligands.3 Due to the large trans-influence13 of the σ-bound alkyl carbon C19, the Ir−N bond distance of the acetonitrile ligand is considerably elongated (2.125(3) Å), which indicates facile substitution of this ligand as observed in related complexes.4 Dissolving 3b in DMSO leads to a rapid exchange of the acetonitrile ligand by the sulfoxide, yielding 3c. It was obvious from the observed coordination of acetonitrile in 3a and 3b that the η4-cod complex 2 should be synthesized in the absence of this solvent. In addition, acetonitrile was proposed in the literature to play an important role in the isomerization of the η4-1,5-cod-ligand in iridium complexes for thermodynamic as well as kinetic reasons.4,5 Therefore, we reacted our bis(NHC) precursor 1a, KOAc, and [Ir(μ-Cl)(η4C8H12)]2 in tetrahydrofuran as a solvent and obtained the initially desired cod complex [Ir(η4-C8H12)(κ2-veginPr)]PF6 (2a). The 1H NMR spectrum of 2a shows two signals for the bis(NHC)-backbone as well as three signals for the cod-ligand indicating a C2v-symmetry of 2a. The structural assignment is 2019


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decomposition products could be observed in the NMR spectrum, but there were no signals of the isomer 3a. In accordance with Bera and co-workers,7 addition of KOAc to isolated 2a in acetonitrile did not lead to an isomerization either. However, addition of a catalytic amount of acetic acid to a solution of 2a in acetonitrile led to decoloration of the red solution within a few hours and formation of the κ1,η3-C8H12 complex 3a, as it was reported in a similar way for complex C.6 We observed this reactivity only in acetonitrile, while in dichloromethane and dimethyl sulfoxide, complex 2a remained stable. The observation that acetic acid catalyzes the transformation of isolated 2a into 3a can also explain the outcome of the reaction of imidazolium salt 1a, KOAc, and the iridium precursor in acetonitrile. As acetic acid is formed upon deprotonation of the imidazolium moieties, it leads to the aforementioned isomerization of the in situ formed complex 2a under these conditions. Deprotonation of 1a with stronger bases like MeLi or nBuLi and subsequent reaction with [Ir(μCl)(η4-C8H12)]2 led solely to the nonisomerized (η4-C8H12)complex 2a, even in the presence of acetonitrile. In a further isomerization experiment, two identical samples of 2a in acetonitrile were each mixed with 30 mol % of acetic acid. Then an additional 70 mol % of KOAc were added to only one of the samples. Direct comparison of the reaction rate for the isomerization by 1H NMR showed that additional KOAc slows down the reaction rate. This result can be rationalized by assuming a pre-equilibrium of the starting material 2a with a protonated iridium complex 6a (vide infra). Additional acetate enhances the back reaction and increases the concentration of 2a, thus slowing down the overall reaction rate. The assumption that the coordination of acetonitrile leads to a thermodynamic stabilization of isomer 3 can also be confirmed: upon concentrating solutions of 3a in CH3CN or CH2Cl2 in vacuo, a shift of the equilibrium to the side of the η4cod complex 2a is observed due to dissociation and removal of the coordinated acetonitrile. However, this reversibility is only confirmed for the complex 3a bearing the veginPr ligand, whereas for complex 3b containing the vegitBu ligand the back reaction was not observed. This removal of the coordinated acetonitrile impeded isolation of pure 3a. Reactivity of 2a toward Acids. On the basis of the literature reports,6 we assumed that addition of acetic acid to the electron-rich complex 2a would lead to protonation of the iridium center, yielding the formally oxidized Ir(III) complex 6a. Its preferred octahedral coordination sphere would be realized by coordination of the acetate counterion. In a 1H NMR experiment, we observed only a very weak hydride signal at −15.35 ppm when reacting complex 2a with an excess of acetic acid in dichloromethane. This suggests that complex 2a is only a slightly weaker base than the acetate ion (which is in accordance to the literature known complex [Ir(η4-C8H12)(H)(PMe3)3]2+ (pKa = 4.2 in H2O)6). As the equilibrium lies in favor of complex 2a, no attempts for the isolation of 6a were made. Consequently, we chose the stronger trifluoroacetic acid to alleviate the oxidative addition and to shift the equilibrium to the side of the hydride complex 7a. Indeed, when using trifluoroacetic acid, we obtained complex 7a in high yields from a dichloromethane solution (Scheme 3). 7a was characterized by NMR and IR spectroscopy as well as elemental and X-ray diffraction analysis. In the 1 H NMR spectrum, the hydride signal is observed at −18.63 ppm, and the characteristic Ir−H stretching frequency in the IR

also confirmed by X-ray structure analysis (Figure 3) showing that complex 2a is almost isostructural to our analogous Rh complex E.9a

Figure 3. Molecular structure of 2a. Hydrogen atoms and the PF6− anion are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1−C1 = 2.102(7), Ir1−C8 = 2.068(7), C1−C8 = 2.667(9), C17−C18 = 1.41(1), C21−C22 = 1.38(1), C17/C18/C21/C22−Ir (mean) = 2.161(8); C1−Ir1−C8 = 79.5(3), N2−C1−N10 = 100.5(6), N7−C8−N9 = 100.0(6).

Isomerization and Reactivity. To obtain insights into the isomerization mechanism we monitored the formation of 3a in acetonitrile-d3 by 1H NMR spectroscopy at −40 °C. Three intermediates could be observed: at first an unsymmetrical Irvegi complex with one remaining imidazolium proton was formed. This species 4a could independently be synthesized by using only one equivalent of base and is analogous to the known Rh complex.9a The second intermediate showed two signals for the vegi ligand and two signals for the olefinic protons of the cod ligand, indicating Cs symmetry of the complex. Based on a literature report14 as well as on the fact that no hydride signal could be detected, it was obvious that this species is the square-pyramidal five-coordinate Ir(I) complex 5a with an apical Cl-ligand. The third intermediate was identified as the Ir(I)-cod complex 2a. During the course of the reaction, the concentration of 4a and 5a decreased while complex 2a was formed almost entirely, which is finally isomerized to 3a. A reasonable reaction sequence is depicted in Scheme 2. To obtain further information on the reaction conditions, we studied the cod-isomerization starting from isolated 2a. Unexpectedly, pure 2a remained inert in acetonitrile, even at 50 °C. After several days at this temperature small peaks of Scheme 2. Stepwise Reaction in Synthesis of 3a As Observed in 1H NMR Spectra

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In order to get deeper insights into the role of coordinating (trifluoro)acetate for the cod-isomerization, we tried to isolate an iridium-hydride complex with the weakly coordinating BF4− anion. The reaction of 2a with HBF4·Et2O in dichloromethane at first led to the desired protonation of the iridium center yielding a hydride complex with a signal at −16.03 ppm in the 1 H NMR spectrum. However, consecutive reactions hampered isolation of the pure hydride complex. Addition of acetonitrile to the in situ generated BF4−-complex led to disappearance of the hydride signal in the 1H NMR spectrum in combination with a transformation of the cod ligand revoking any symmetry of the complex. The same complex could be independently noticed after several days in the reaction of bisimidazolium salt 1a with [Ir(μ-Cl)(η4-C8H12)]2 in acetonitrile, when DIPEA was used as a base instead of KOAc. We assume that a hydrido complex is formed by protonation of Ir(I) complex 2a with DIPEA-H+ that undergoes the analogous transformation. We were not yet able to identify this reaction product unequivocally, but preliminary results suggest an insertion of one double bond of the cod into the Ir−H bond. It can be concluded that the formation of the κ1,η3-C8H12 ligand can only be observed in acetonitrile with concomitant presence of a suitable acid that generates a hydrido complex in situ in which the acid anion is coordinated trans to the hydrido ligand. None of these two factors on its own is able to isomerize the 1,5-cod ligand. Deuterium Exchange Experiments. To elucidate the mechanistic pathway of the isomerization, we carried out the oxidative addition of the acid to 2a with deuterated trifluoroacetic acid (TFA-d1). As expected, this led to deuterated 7a-d1 in CD2Cl2. However, several minutes after the addition of TFA-d1 to 2a, a significant hydride signal could be detected by 1H NMR spectroscopy. As perdeuterated solvent was used, this 1H atom must stem from the cod ligand. Consistently, 2H NMR spectroscopy showed incorporation of deuterium into the CH2−groups of the η4-cyclooctadiene ligand (Scheme 3). Comparable H/D scrambling in an Ir-cod complex could be observed by Kimmich et al.18 In addition, the NMR spectra of 7a-dn reveal that only the axial hydrogen atoms of all CH2−groups of the cod ligand were exchanged, while no H/D scrambling was observed for the equatorial as well as for the olefinic hydrogen atoms. The H/D-scrambling can be interpreted as an evidence for the insertion of an olefin into the Ir-D bond followed by β-Helimination of a different hydrogen atom. However, due to the restraints of the coordinated cod ligand, a mere reversible insertion/β-H-elimination process at the same carbon atom would not lead to 2H incorporation, as only the newly formed axial deuterium atom could be involved in the back reaction. βhydride elimination of the endo hydrogen atom of the second neighboring CH2 group (blue in IIIa, Scheme 5) would lead to migration of the double bond and formation of a 1,4cyclooctadiene ligand as proposed by Esteruelas et al.3 In this case, the scrambling would have to proceed via double bond migration of the second double bond to obtain back the thermodynamically more stable 1,5-cod ligand and 7a-dn. However, due to the ease of the isomerization of the cod ligand in 2 to the κ1,η3-C8H12 ligand in 3, we favor the H/D scrambling in the hydrido complex 7a to proceed via a different mechanism that also involves a symmetric κ1,η3-C8H12 alkylallyl-species IV as an intermediate. Here, in order to obtain solely axial H/D-exchange, the deprotonation and protonation of the allylic position must occur intramolecularly by a

Scheme 3. Equilibrium of the Oxidative Addition of Acids to 2aa

a

In CH2Cl2, for X = H, the equilibrium lies in favor of 2a, while for X = F, the equilibrium is on the product side.

spectrum (KBr) measures 2238 cm−1. This high frequency gives an indication for the protic nature of the hydride ligand.15 As shown in the molecular structure of 7a in Figure 4, the Ir(III) center is coordinated in an octahedral fashion with the

Figure 4. Molecular structure of 7a. Hydrogen atoms (except H1) as well as the PF6− counterion are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1−C1 = 2.080(4), Ir1−C8 = 2.088(2), Ir1−O25 = 2.215(2), C1−C8 = 2.671(4), C17/C18/C21/C22−Ir1 (mean) = 2.246(2), C17−C18 = 1.386(4), C21−C22 = 1.386(4); C1−Ir1−C8 = 79.7(1), H1−Ir1−O25 = 172.8(9), N2−C1−N10 = 101.3(2), N7−C8−N9 = 101.7(2). The distance H1−Ir1 = 1.580 Å was fixed for structure refinement (value from DFT calculations, see Supporting Information).

carbene ligand veginPr and the η4-cod ligand in equatorial positions. The hydride ligand coordinates trans to the trifluoroacetate ligand indicating an SN2-type oxidative addition of the acid. Such reactions are well-known in the literature for Vaska’s complex16 as well as for other group 9 M(I) complexes.17 As it is in general difficult to find hydrogen atoms next to heavy atoms in X-ray diffraction experiments, the Ir−H distance was calculated for the gas phase using density functional theory (vide infra), and it was fixed for the structure refinement. Complex 7a can be deprotonated quantitatively with NEt3 in dichloromethane leading to 2a without any decomposition. Furthermore, after dissolving colorless 7a in THF, the solution turns dark red after seconds, and 2a starts to precipitate. This supports the coexistence of 2a and 7a in equilibrium in solution, which can be shifted to 2a and free trifluoroacetic acid by precipitating the red Ir(I) complex 2a. The formal reductive elimination is presumably alleviated by the strong trans influence of the hydrido ligand. Upon addition of acetonitrile to a solution of the hydride complex 7a in CD2Cl2 slow formation of the κ1,η3-C8H12 complex 3a can be observed. However, in contrast to the acetic-acid-catalyzed isomerization, this reaction yielded several unknown byproducts. 2021


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Organometallics concerted metalation-deprotonation (CMD)19,20 or a direct oxidative addition step. A comparable stereoselectivity in deuteration experiments was found by Mønsted et al. upon formation of their Ir-κ1,η3-C8H12 complex.11 In order to obtain further insights into the scrambling mechanism of 7a, we carried out VT 1H NMR experiments. However, in a temperature range from −90 to 40 °C, no intermediates could be observed. Addition of stoichiometric amounts of acetic acid-d1 to the Ir(I) complex 2a in acetonitrile led to the known isomerization yielding 3a-d. 2H NMR spectroscopy revealed that the deuterium atom is selectively incorporated at the endopositions at C12 next to the iridium-bound alkyl-moiety (Scheme 4). As

side. However, as we observed a strong solvent dependence for this reaction, we included solvent effects (acetonitrile) in the calculations of 2b and I applying COSMO. In this case, we found complex I by 8 kJ/mol higher in energy than 2b (+AcOH). This explains why the hydrido-acetato complex 6a could not be observed in acetonitrile even at low temperatures. Pathway A. Starting from the deuterido complex I, we investigated a direct insertion of one of the double bonds into the Ir-D bond, but no geometrically and energetically reasonable transition state could be found. The isomerized cis-complex IIa, however, allows an insertion reaction with an activation barrier of only 43 kJ/mol (Figure 5). The transition state TS(IIa-IIIa) of this migratory insertion shows an Ir···H distance of 1.677 Å and a C···H distance of 1.388 Å. The insertion leads to a five-coordinate κ1,η2-C8H13 species from which intermediate IIIa is formed. This intermediate has an energy of 22 kJ/mol relative to complex 2b and shows a strong agostic interaction21 of the allylic H atom (in red, Scheme 5) with a C−H···Ir angle of 99.4°. The C−H bond is significantly elongated to 1.205 Å, and the Ir···H distance measures 1.855 Å. From IIIa, only a small conformational change is necessary to reach the transition state of a typical CMD20 process TS(IIIaIV), also featuring an agostic interaction (C···H 1.460 Å; O···H 1.562 Å; Ir···H 1.880 Å). This CMD shows an activation barrier of 11 kJ/mol and leads in an exothermic reaction to the symmetric κ1,η3-C8H12 alkyl-allyl complex IV with coordinated acetic acid. From a thermodynamic point of view, this acid is likely to be dissociated in the presence of acetonitrile and replaced by it. The resulting complex 3b is by 57 kJ/mol more stable than the initial complex 2b. A back reaction from here would require an activation energy of 98 kJ/mol and is therefore unlikely. This explains the selectivity of the H/D scrambling for H-12endo in our AcOD induced isomerization experiment. However, in absence of a coordinating solvent like acetonitrile, the reprotonation of the allyl moiety in symmetric IV (L ≠ CH3CN) could proceed at the other side of the allyl moiety. This step could initiate the formal back reaction, however, now leading to the 2-fold deuterated species 7b-d2. Apart from the now deuterated species, this reaction shows qualitatively the same energy profile. The isomerization cycle could then start anew until a substantial amount of axial CH2 hydrogen atoms are exchanged by deuterium. Due to the selective H/D exchange of the axial protons in 7a, we can exclude an intermolecular deprotonation from IIIa to IV by trifluoroacetate. In the case of acetic acid and acetonitrile as a solvent, the thermodynamic driving force by coordination of acetonitrile as in complex 3a hampers any detectable reprotonation by AcOD. As no net olefin migration and further H/D exchange is observed, it cannot be clarified experimentally, if the allyl-formation occurs intramolecularly by coordinated acetate or intermolecularly by free acetate, which is a reasonably strong base in acetonitrile (pKa(AcOH) = 23.5122). However, we expect the reaction to proceed via an intramolecular CMD process, too. Pathway B. The second pathway B involves the reverse order of the key steps as suggested by Bera and co-workers for complex D.7 At first, the intramolecular deprotonation of an allylic position in I by coordinated acetate proceeds without any agostic interaction (C···H 1.309 Å, O···H 1.320 Å, Ir···H 3.274 Å). This transition state TS(I-IIb) has a very high energy of 107 kJ/mol relative to 2b and leads to formation of an allylolefin species, which coordinates in a κ1,η2-fashion as shown in

Scheme 4. Deuteration Experiments Starting from 2a

no deuteration at C13 is observed, these results indicate that there is no back reaction of the symmetric κ1,η3-C8H12 alkylallyl-species 3a-d1 in acetonitrile. Presumably, coordination of solvent to the iridium center stabilizes this species thermodynamically and prevents back reaction with double bond migration, which would lead to deuterium scrambling over both the positions C12 and C13. Further Mechanistic Investigations. By addition of acetic acid to 2a in dichloromethane, we could observe a hydride peak in the 1H NMR spectrum (vide supra). In addition, our deuteration experiments showed that in hydrido complexes like 7a, some isomerization steps can proceed. Furthermore, we found that addition of KOAc slows down the isomerization reaction from 2a to 3a. From these three experimental results, we conclude that the isomerization starts from an Ir-hydride species, which is formed in a fast pre-equilibrium upon oxidative addition of acetic acid to 2a. Starting from the Ir−H complex I, the formation of the symmetric κ1,η3-C8H12 alkyl-allyl-species IV could proceed via pathway A or pathway B (Scheme 5). They differ by the sequence of the olefin insertion and the deprotonation step at the allylic position. Both mechanisms are suitable to explain cod isomerization as well as the H/D scrambling. As we could not find any experimental hints to distinguish between these two mechanisms, we examined the reaction by DFT calculations (BP86-D3/def2-TZVPP//BP86-D3/def2-SVP) for the vegitBu complexes and acetic acid (X = H). The energy profile is shown in Figure 5. At first the formation of the hydrido complex I was examined. The computational results show that complex I is by 12 kJ/mol more stable than the Ir(I) complex 2b. This seems to be in contrast to our experimental finding that the equilibrium of the reaction of 2a with AcOH lies on the educt 2022


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Scheme 5. Proposed Reaction Pathways A and B for Deuterium Exchange in 7a-d, Differing in the Sequence of the Insertion and the CMD step

for the olefin insertion is reached and lies 12 kJ/mol below that for the CMD step. TS(IIIb-IV) shows C···H and Ir···H distances of 1.528 and 1.657 Å. As in pathway A, a formal back reaction with TFA-d1 from the resulting complex IV would lead to 7b-d2 as well and could explain the observed H/D scrambling of axial CH2 positions. However, the very high activation barrier makes this pathway even less likely. Comparison of the Two Mechanisms. In mechanism B, the intramolecular deprotonation of the allylic position constitutes the rate-determining step with an activation barrier of 107 kJ/ mol (26 kcal/mol) starting from 2b, while a CMD process after the olefin insertion as in mechanism A is energetically much more favorable, possibly enhanced by agostic interactions. The rate-determining step of mechanism A is the insertion of the olefin, which has an activation barrier of only 41 kJ/mol (10 kcal/mol) starting from 2b. Therefore, pathway B seems very unlikely. This is in accordance with Bera and co-workers, who found high activation barriers in a similar mechanism with chelating cod.7 By the release of one of the double bonds of the cod from the Ir center, they found a reaction mechanism for cod isomerization, which follows the CMD−insertion sequence and has reasonable activation barriers.7 Considering our pathway A, however, we could show that the cod isomerization can proceed via an insertion−CMD

Figure 5. Energy profiles ([kJ/mol]) of the postulated mechanisms A (black) and B (red) for cod isomerization by acetic acid and H/D scrambling.

IIb. Here replacement of the formed acid could take place in acetonitrile, but it was not examined computationally because of the high activation barrier to form IIb. An η2,η3-coordination mode of the carbocyclic ligand in IIb would lead to a sevencoordinated 20-electron species, so that this seems only achievable after dissociation of the acid. Prior to the insertion step, the intermediate IIb has to isomerize to IIIb as the insertion has to take place two C atoms remote from the CMDinvolved position. From there, the transition state TS(IIIb-IV) 2023


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could achieve the isomerization of the 1,5-cod ligand in the more-electron-deficient 2,2′-bipyridin-complex 8 for the first time. Therefore, our experimental and computational findings on the mechanism and the conditions of the cod isomerization give now a complete picture of this reaction, which allows us to make predictions and to explain the reactivity of other cod-Ir(I) complexes.

sequence without necessity of a double bond dissociation. In addition, we find such a dissociation implausible in the presence of the very weakly coordinating acetic acid still coordinated to the iridium center. Isomerization of [Ir(2,2′-bipy)(η4-cod)]PF6 (8). In order to demonstrate the generality of our results, we wanted to apply the reaction conditions on other Ir-cod complexes. We chose [Ir(2,2′-bipyridine)(η4-cod)]PF6 (8) as a challenging substrate, as the isomerization of the cod ligand had been attempted unsuccessfully for this complex before.7 Based on our experimental and computational results, we checked the reactivity of complex 8 under acidic conditions. At first, only traces of isomerization product 9 could be observed after prolonged heating (18 days, 80 °C) of a solution of [Ir(2,2′-bipyridine)(η4-cod)]PF6 (8) with 50 mol % of acetic acid in acetonitrile. However, in the presence of the stronger trifluoroacetic acid under identical conditions, the isomerization of 8 to the κ1,η3-cod complex 9 could be achieved in good yields (Scheme 6). Upon addition of stoichiometric amounts of

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EXPERIMENTAL SECTION

Unless noted otherwise, all reactions were carried out under an atmosphere of argon in dried and degassed solvents using the Schlenk technique or an argon-filled glovebox (MBraun). Degassed acetonitrile, tetrahydrofuran, dichloromethane, toluene, and n-pentane were purchased from Sigma-Aldrich and dried using the solvent purification system SPS-800 (MBraun). Deuterated solvents were purchased from Deutero, refluxed over CaH2 (CD3CN), Na (THF-d8), or P2O5 (CD2Cl2), distilled, and degassed. DMSO-d6 was degassed and dried using several batches of molecular sieves (3 Å). CD3CN, CD2Cl2, and DMSO-d6 were stored over molecular sieves (3 Å). VeginPr·2HPF6 (1a) and vegitBu·2HPF6 (1b) were prepared according to literature.9 All other reagents were purchased from commercial suppliers and used without further purification. NMR spectra were recorded with a Bruker AVII+400 or a Bruker DRX 250 MHz spectrometer. 1H, 2H, and 13C{1H} NMR spectra were referenced to (residual) solvent signals as the internal standard. Signals were assigned using 2D experiments and numbered according to Schemes 1 and 6. Elemental analyses were determined using a varioMICRO cube by the elemental analysis section of the Institut für Anorganische Chemie at the University of Tübingen. HR-ESI mass spectra were measured using a Bruker Daltonics APEX II FT-ICR or a Bruker Daltonics maXis 4G UHR-TOF instrument by the MS section of the Institut für Organische Chemie at the University of Tübingen. IR spectra were recorded using a Bruker Vertex 70 spectrometer. Melting points were determined using the Büchi Melting Point apparatus M-560. Calculations. All calculations were performed on the basis of density functional theory at the BP86/def2-TZVPP//BP86/def2SVP23 level implemented in Turbomole,24 which proved suitable for transition metal complexes before.25 The RI-approximation,26 D3correction27 and def2-ecp28 for Ir were used all over. Convergence criteria were set to 10−7 H (energy) and 10−5 H/Bohr (gradient) for ground states and to 10−6 H (energy) and 10−3 H/Bohr (gradient) for transition states (Turbomole default). All structures were verified to be minimum structures or transition states by calculating Hessian matrices and ensuring that they have zero (ground state) or only one (transition state) imaginary frequency. For transition states, the MEP was followed to check the rationality of the structures. Thermodynamic data were calculated at 298.15 K and 0.1 kPa assuming a scaling factor of 0.9914 (Turbomole default). Graphics of the calculated structures were prepared using Mercury CSD 3.3.1.29 Crystallographic Data. Data collection was carried out on a Bruker Smart APEX II Duo diffractometer using Mo Kα radiation. Absorption corrections were applied using SADABS.30 Structure solutions by direct methods and structure refinements were performed with SHELX-201331 and the gui ShelXle.32 Further details can be found in the Supporting Information. Graphics of the molecular structures were prepared using Mercury CSD 3.3.129 [Ir(η4-cod)(veginPr-κ2C,C′)]PF6 (2a). VeginPr·2HPF6 (1a; 100 mg, 187 μmol), [Ir(μ−Cl)(η4-cod)]2 (62.9 mg, 93.6 μmol) and KOAc (36.7 mg, 374 μmol) were suspended in THF (10 mL) and the mixture was stirred at 50 °C for 24 h. The dark red suspension was filtered and the residue washed three times with THF (2 mL each) until the solvent turned colorless. The residual solid was partly dissolved in 20 mL of CH2Cl2, and the suspension was filtered. The filtrate was concentrated to dryness in vacuo. The obtained red solid was washed three times with n-pentane (4 mL each) and dried in vacuo (69 mg, 54%). Crystals suitable for X-ray diffraction analysis

Scheme 6. Isomerization of 1,5-Cod in 8

trifluoroacetic acid to 8 the quantitative formation of [Ir(2,2′bipyridine)(κ1,η3-C8H12) (NCCH3)]PF6 (9) was achieved after 8 days at 80 °C. Taking into account that 2,2′-bipyridine is a less-electrondonating ligand than N-heterocyclic carbenes, complex 8 is expected to be less basic than complex 2. As a consequence the oxidative addition of the acid to complex 8 to form the hydrido complex in a pre-equilibrium becomes more difficult. This can be overcome by using the stronger trifluoroacetic acid, although trifluoroacetate is the weaker corresponding base to alleviate the allyl-formation. Thus, the experimental results for the isomerization of the 2,2′-bipyridine complex 8 are in good agreement with our mechanistic finding for complex 2.

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CONCLUSION We were able to selectively synthesize the bis(NHC)(cod)iridium(I) complexes 2a and 2b as well as the complexes 3a, 3b, and 3c, bearing a κ1,η3-C8H12 moiety resulting from cod isomerization by choice of the solvents. We found that this isomerization is acid-catalyzed (e.g., by acetic acid or trifluoroacetic acid). However, the equilibrium lies only on the product side, if donating solvents like acetonitrile are present to stabilize the product complex by additional coordination, as we could confirm by H/D exchange experiments and DFT calculations. The mechanism involves an Ir(III) hydrido complex resulting from oxidative addition of the acid to the iridium(I) complex 2. This species 7a was isolated in the case of trifluoroacetic acid. DFT calculations also reveal that the isomerization proceeds first by rearrangement of COD ligand into the cis-Ir(III) hydrido complex followed by olefin insertion of the cod ligand into the Ir−H bond, which is the rate-determining step. Subsequent formation of the allyl moiety proceeds via an intramolecular concerted metalation deprotonation transition state by the coordinated acid anion. Based on our results, we 2024


Article

Organometallics

14), 4.57 (t, 1H, 3J = 8.1 Hz, H-15), 1.90−1.87 (m, 2H, H-13endo), 1.85 (s, 18H, tBu), 1.59−1.49 (m, 2H, H-12endo), 1.28−1.19 (m, 2H, H-13exo), 1.07−1.02 (m, 2H, H-12exo), 0.94 (t, 1H, 3J = 6.3 Hz, H-11). Signals of coordinated acetonitrile could not be detected due to a fast exchange with CD3CN. 1H NMR (CD2Cl2, 400.11 MHz): δ 7.45 (s, 2H, H-3/6), 7.05 (s, 2H, H-4/5), 4.95 (dt, 2H, 3J = 3.9 Hz, 3J = 8.4 Hz, H-14), 4.53 (t, 1H, 3J = 8.0 Hz, H-15), 1.95 (s, 3H, NCCH3), 1.94−1.88 (m, 2H, H-13endo), 1.87 (s, 18H, tBu), 1.58−1.48 (m, 2H, H-12endo), 1.30−1.21 (m, 2H, H-13exo), 1.07−1.03 (dd, 2H, 3J = 6.4 Hz, 3J = 12.5 Hz, H-12exo), 0.97 (t, 1H, 3J = 6.4 Hz, H-11). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 155.6 (C1/8), 123.0 (C3a/5a), 117.2 (C3/6), 115.3 (C4/5), 95.8 (C15), 67.5 (C14), 61.8 (C(CH3)3), 51.7 (C12), 31.7 (C(CH3)3), 26.1 (C13), 22.8 (C11). Signals of coordinated acetonitrile could not be detected due to a fast exchange with CD3CN. Anal. Calcd (%) for C26H37N5IrPF6: C, 41.26; H, 4.93; N, 9.25. Found: C, 41.44; H, 4.83; N, 9.08. HR-ESI+ (TOF, CH3CN): m/z 571.240 97 [M − NCCH3]+, calcd 571.240 81. IR (KBr) [cm−1]: ν 3183 (s), 2981 (vs), 2931 (vs), 2870 (vs), 2827 (s), 1616 (m), 1370 (s), 1307 (s), 1198 (vs), 1145 (m), 839 (vs), 558 (vs). [Ir(1-κ-4,5,6-η-C8H12)(DMSO-d6)(vegitBu-κ2C,C′)]PF6 (3c). Complex 3b (10 mg, 0.013 mmol) was dissolved in 0.5 mL DMSO-d6 and the solution was concentrated to dryness in vacuo. The NMR spectrum in CD2Cl2 shows quantitative conversion into 3c. 1 H NMR (CD2Cl2, 400.11 MHz): δ 7.72 (s, 2H, H-3/6), 7.14 (s, 2H, H-4/5), 4.99−4.95 (m, 1H, H-15), 4.82 (dt, 2H, 3J = 3.9 Hz, 3J = 8.5 Hz, H-14), 2.07−1.96 (m, 2H, H-13endo), 1.90 (s, 18H, tBu), 1.68− 1.58 (m, 2H, H-12endo), 1.45−1.37 (m, 2H, H-13exo), 1.32−1.27 (m, 2H, H-12exo), 1.12 (t, 1H, 3J = 6.1 Hz, H-11). 13C{1H} NMR (CD2Cl2, 100.61 MHz): δ 148.3 (C1/8), 123.1 (C3a/5a), 118.1 (C3/6), 114.5 (C4/5), 93.6 (C15), 70.9 (C14), 62.0 (C(CH3)3), 50.8 (C12), 32.1 (C(CH3)3), 31.0 (C11), 25.6 (C13). Coordinated DMSO-d6 could not be detected most probably due to the low intensity (13C,2H coupling) of the signal. [Ir(η4-cod)(F3CCO2)(H)(veginPr-κ2C,C′)]PF6 (7a). [Ir(η4-cod)(veginPrκ2C,C′)]PF6 (2a; 100 mg, 145 μmol) was dissolved in 10 mL of CH2Cl2. Under vigorous stirring, trifluoroacetic acid (22.4 μL, 33.2 mg, 291 μmol) was added. After stirring the suspension for 5 min at rt, the solvent was concentrated to 5 mL in vacuo. The colorless precipitate was filtered and washed three times with n-pentane (3 mL each). After drying in vacuo, complex 7a (96.2 mg, 83%) was obtained as colorless powder. Crystals suitable for X-ray diffraction analysis could be obtained by slow diffusion of n-pentane into a solution of the complex in CH2Cl2. mp 213−215 °C (dec). 1H NMR (CD2Cl2, 400.11 MHz): δ 7.30 (s, 2H, H-3/6), 7.14 (s, 2H, H-4/5), 5.81 (s, br, 2H, H-12), 5.37 (s, br, 2H, H-11), 4.11−4.04 (m, 2H, nPr), 3.92−3.84 (m, 2H, nPr), 2.70− 2.66 (m, 2H, H-14ax), 2.59−2.49 (m, 4H, H-13), 2.30−2.24 (m, 2H, H-14eq), 1.95−1.86 (m, 4H, nPr), 0.97 (t, 6H, 3J = 7.4 Hz, nPr), −18.63 (s, 1H, Ir−H). 13C{1H} NMR (CD2Cl2, 100.61 MHz): δ 140.2 (C1/8), 122.6 (C3a/5a), 117.7 (C3/6), 115.4 (C4/5), 90.6 (C11), 89.3 (C12), 52.6 (nPr), 33.8 (C13), 28.6 (C14), 25.3 (nPr), 11.1 (nPr). The signals of coordinated trifluoroacetate could not be detected. 19F{1H} NMR (CD2Cl2, 376.48 MHz): δ −73.5 (d, 1JPF = 710.7 Hz, PF 6 − ), −75.4 (s, −CF 3 ). Anal. Calcd (%) for C24H31N4O2IrPF9: C, 35.96; H, 3.90; N, 6.99. Found: C, 35.85; H, 3.75; N, 6.96. IR (KBr) [cm−1]: ν 3168 (m), 2967 (m), 2238 (w, νIrH), 1693 (vs), 1405 (m), 1197 (vs), 1131 (s), 836 (vs), 558 (s). [Ir(2,2′-bipyridine)(η4-cod)]PF6 (8). The title compound was prepared following literature instructions for the analogous BF4−salt.33 [Ir(μ-Cl)(η4-cod)]2 (20.0 mg, 29.8 μmol) was dissolved in 0.5 mL of CH2Cl2, and 2,2′-bipyridine (9.5 mg, 60 μmol) was added. To the resulting dark purple solution KPF6 was added, and the mixture was refluxed for 2 h. The dark purple suspension was diluted with 4.5 mL of CH2Cl2, filtered to remove KCl, and the residue extracted with CH2Cl2 (three times with 1 mL). Et2O (20 mL) was added to the vigorously stirred combined CH2Cl2 solutions. The resulting suspension was filtered, and the residue was washed three times with Et2O (2 mL each) and dried in vacuo, yielding 8 as a microcrystalline solid (30 mg, 83%).

could be obtained by slow diffusion of Et2O into a solution of the complex in CH3CN. mp 272 °C (dec). 1H NMR (CD2Cl2, 400.11 MHz): δ 7.40 (s, 2H, H-3/6), 7.23 (s, 2H, H-4/5), 5.10 (s, br, 4H, CHcod), 4.10 (t, 4H, 3J = 7.5 Hz, nPr), 2.23−2.19 (m, 4H, CH2 cod), 2.09−2.01 (m, 4H, CH2 cod), 1.93−1.86 (m, 4H, nPr), 1.05 (t, 6H, 3J = 7.4 Hz, nPr). 1H NMR (CD3CN, 400.11 MHz): δ 7.44 (s, 2H, H-3/6), 7.04 (s, 2H, H4/5), 5.00 (s, br, 4H, CHcod), 4.03 (t, 4H, 3J = 9.1 Hz, nPr), 2.23−2.17 (m, 4H, CH2 cod), 2.09−2.02 (m, 4H, CH2 cod), 1.93−1.86 (m, 4H, nPr), 0.98 (t, 6H, 3J = 7.4 Hz, nPr). 13C{1H} NMR (CD2Cl2, 62.90 MHz): δ 159.9 (C1/8), 122.0 (C3a/5a), 117.8 (C3/6), 114.8 (C4/5), 74.5 (CHcod), 52.7 (nPr), 32.5 (CH2 cod), 25.8 (nPr), 11.4 (nPr). Anal. Calcd (%) for C22H30N4IrPF6: C, 38.42; H, 4.40; N, 8.15. Found: C, 38.36; H, 4.19; N, 8.30. HR-ESI+ (TOF, CH2Cl2): m/z 543.209 53 [C22H30N4Ir]+, calcd 543.209 49. IR (KBr) [cm−1]: ν 3147 (m), 2964 (s), 2924 (s), 2879 (m), 1606 (m), 1470 (s), 1407 (m), 1261 (m), 1173 (s), 1088 (s, br), 839 (vs), 559 (vs). In situ generation of [Ir(η4-cod)(vegitBu-κ2C,C′)]PF6 (2b). VegitBu· 2HPF6 (1b) (200 mg, 356 μmol) was dissolved in 6 mL of acetonitrile and cooled to −40 °C. A solution of n-butyllithium in hexane (2.5 M, 0.28 mL, 0.71 mmol) was added slowly. After stirring for 5 min at −40 °C, the solution was transferred via cannula to a precooled solution of [Ir(μ−Cl)(η4-cod)]2 (119.5 mg, 177.8 μmol) in 6 mL of THF at −40 °C. The resulting solution was stirred for 10 min at −40 °C and 1 h at rt. After solvent exchange, NMR spectra were recorded. 1 H NMR (CD3CN, 400.11 MHz): δ 7.69 (s, 2H, H-3/6), 7.00 (s, 2H, H-4/5), 2.83 (s, br, 4H, CHcod), 2.47−2.44 (m, 4H, CH2 cod), 1.72 (s, 4H, CH2 cod), 1.97 (s, 18H, tBu). 13C{1H} NMR (CD3CN, 100.61 MHz): δ 122.6 (C3a/5a), 115.7 (C3/6), 113.7 (C4/5), 59.9 (C(CH3)3), 55.8 (CHcod), 33.1 (CH2 cod), 30.6 (C(CH3)3). The signal of the carbene C atoms could not be observed. [Ir(1-κ-4,5,6-η-C8H12)(NCCH3)(veginPr-κ2C,C′)]PF6 (3a). [Ir(η4-cod)(veginPr-κ2C,C′)]PF6 (2a; 41.0 mg, 59.6 μmol) was dissolved in 4 mL of CH3CN under an inert atmosphere. After addition of acetic acid (3.5 μL, 3.7 mg, 61 μmol), the solution was heated at 50 °C for 18 h. After cooling to rt, 1 mL of n-pentane was added, the emulsion was vigorously stirred for 1 min, and the n-pentane phase containing some acetonitrile was separated. This procedure for removing acetonitrile was repeated until the product precipitated and remained as a slightly off-white solid. The product was dried in vacuo for only 15 min to avoid its back reaction by loss of acetonitrile. 1 H NMR (CD3CN, 400.11 MHz): δ 7.43 (s, 2H, H-3/6), 7.15 (s, 2H, H-4/5), 5.25 (ddd, 2H, 3J = 8.0 Hz, 3J = 8.8 Hz, 3J = 3.8 Hz, H14), 4.61−4.54 (m, 2H, nPr), 4.41−3.34 (m, 2H, nPr), 4.16 (t, 1H, 3J = 8.0 Hz, H-15), 2.04−1.95 (m, 4H, nPr), 1.91−1.85 (m, 2H, H13endo), 1.43−1.31 (m, 2H, H-12endo), 1.29−1.19 (m, 2H, H-13exo), 1.15−1.10 (m, 2H, H-12exo), 0.94 (t, 6H, 3J = 7.4 Hz, nPr), 0.83 (t, 1H, 3J = 6.7 Hz, H-11). Signals of coordinated acetonitrile could not be detected due to a fast exchange with CD3CN. 13C{1H} NMR (CD3CN, 100.61 MHz): δ 156.4 (C1/8), 123.0 (C3a/5a), 118.3 (C3/ 6), 115.7 (C4/5), 94.7 (C15), 63.8 (C14), 54.2 (nPr), 51.7 (C12), 26.4 (C13), 25.7 (nPr), 20.3 (C11), 11.5 (nPr). Signals of coordinated acetonitrile could not be detected due to a fast exchange with CD3CN. [Ir(1-κ-4,5,6-η-C8H12)(NCCH3)(vegitBu-κ2C,C′)]PF6 (3b). VegitBu· 2HPF6 (1b; 150 mg, 267 μmol), [Ir(μ-Cl)(η4-cod)]2 (89.6 mg, 133 μmol) and KOAc (52.4 mg, 534 μmol) were suspended in CH3CN (8 mL) under an inert atmosphere. The orange mixture was stirred at 60 °C for 5 days. The white precipitate was filtered and extracted two times with CH3CN (3 mL each). The combined solutions were evaporated to dryness and the residue was dried 1 h at 50 °C in vacuo. CH2Cl2 (4 mL) was added to the light yellow solid, and the resulting suspension was filtered. The residue was again extracted twice with CH2Cl2 (2 mL each). The combined solutions were evaporated to dryness. The resulting solid was washed 7 times with THF (1 mL each) until the solvent became colorless, 2 times with pentane (4 mL each), and dried in vacuo (137 mg, 68%). Crystals suitable for X-ray diffraction analysis could be obtained by slow diffusion of Et2O into a solution of the complex in CH3CN. mp 149 °C (dec). 1H NMR (CD3CN, 400.11 MHz): δ 7.56 (s, 2H, H-3/6), 7.07 (s, 2H, H-4/5), 4.92 (dt, 2H, 3J = 4.1 Hz, 3J = 8.1 Hz, H2025


Article

Organometallics H NMR (CH2Cl2, 400.11 MHz): δ 8.36 (dm, 3J = 8.1 Hz, 2H, H3), 8.30 (ddd, 3J = 8.1 Hz, 3J = 7.6 Hz, 4J = 1.5 Hz, 2H, H-4), 8.22 (dm, 3J = 5.6 Hz, 2H, H-6), 7.74 (ddd, 3J = 7.6 Hz, 3J = 5.6 Hz, 4J = 1.5 Hz, 2H, H-5), 4.44−4.39 (m, 4H, CHcod), 2.44−2.34 (m, 4H, CH2 cod), 2.05−1.99 (m, 4H, CH2 cod). Anal. Calcd (%) for C18H20N2IrPF6: C, 35.94; H, 3.35; N, 4.66. Found: C, 35.76; H, 3.01; N, 4.78. [Ir(2,2′-bipyridine)(κ1,η3-C8H12)(NCCH3)]PF6 (9). Under an inert atmosphere, [Ir(2,2′-bipyridine)(η4-cod)]PF6 (8; 5.0 mg, 8.3 μmol) was dissolved in 0.5 mL of CD3CN in a J. Young NMR tube. After addition of trifluoroacetic acid (0.6 μL, 8 μmol), the dark red reaction mixture was heated at 80 °C for 8 days, leading to a yellow solution. The NMR spectrum shows 93% conversion of 8 and formation of 9 as the only product. 1 H NMR (CD3CN, 400.11 MHz): δ 9.57 (ddd, 2H, 3J = 5.6 Hz, 4J = 1.5 Hz, 5J = 0.9 Hz, H-6), 8.45 (dm, 2H, 3J = 8.2 Hz, H-3), 8.20 (ddd, 2H, 3J = 8.2 Hz, 3J = 7.6 Hz, 4J = 1.5 Hz, H-4), 7.66 (ddd, 2H, 3J = 7.6 Hz, 3J = 5.6 Hz, 4J = 1.5 Hz, H-5), 4.79 (ddd, 2H, 3J = 9.1 Hz, 3J = 7.5 Hz, 3J = 4.6 Hz, H-10), 4.62 (t, 1H, 3J = 7.5 Hz, H-11), 2.01− 1.95 (m, 2H, H-9endo), 1.65−1.55 (m, 2H, H-8endo), 1.19−1.09 (m, 2H, H-9exo), 1.06 (t, 1H, 3J = 6.7 Hz, H-7), 0.86−0.80 (m, 2H, H-8exo). From an experiment, in which a mixture of 1 eq AcOH and 1 eq TFA was used the 13C NMR shifts were obtained: 13C{1H} NMR (CD3CN, 100.61 MHz): δ 157.5 (C2), 156.8 (C6), 140.6 (C4), 129.3 (C5), 125.0 (C3), 90.0 (C11), 58.4 (C10), 45.2 (C8), 29.2 (C7), 27.3 (C9). 1

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S Supporting Information *

CIF file providing experimental and crystal data for 2a, 3b, and 7a, NMR spectra of all new compounds as well as the graphics of all calculated structures and a text file of their Cartesian coordinates in xyz-format. CCDC 1018025 (2a), 1018026 (3b), and 1018027 (7a) contain the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc. cam.ac.uk/data_request/cif. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00267.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 7071 2972063. Fax: +49 7071 29-2436. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to Professor Peter Hofmann on the occasion of his retirement to honor his distinguished contributions to organometallic chemistry and the chemistry education and research at Heidelberg University. We are grateful to the BMBF and MWK-BW (Professorinnenprogramm) as well as the Hambrecht-Voscherau-Stiftung (fellowship for B.R.) for financial support. We thank the bwGRiD project34 for computational resources and Prof. Dr. R. Fink for help with the calculations. We gratefully acknowledge Natalie Denninger for providing starting material 1b as well as Prof. Dr. Karl W. Törnroos for help with the X-ray data analysis of compound 2a.

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REFERENCES

(1) Cotton, F. A.; Deeming, A. J.; Josty, P. L.; Ullah, S. S.; Domingos, A. J. P.; Johnson, B. F. G.; Lewis, J. J. Am. Chem. Soc. 1971, 93, 4624− 4626. (2) Bönnemann, H.; Goddard, R.; Grub, J.; Mynott, R.; Raabe, E.; Wendel, S. Organometallics 1989, 8, 1941−1958. 2026


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Identifying and Rationalizing the Conditions for the Isomerization of 1

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