Trifluoromethyl-Substituted Benzocyclobutenone and

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Trifluoromethyl-Substituted Benzocyclobutenone and Benzocyclobutenedione: The Structure Anomaly of (Benzocyclobutenedione)tricarbonylchromium Complexes Monika Böning née Pfennig,† Krishna Gopal Dongol,† Geanne Marize Romero Boston,† Stefan Schmitz,† Rudolf Wartchow,‡ Juan D. Samaniego-Rojas,§ Andreas M. Köster,*,§ and Holger Butenschön*,† Downloaded via GUILFORD COLG on August 1, 2019 at 16:18:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Institut für Organische Chemie, Gottfried Wilhelm Leibniz Universität Hannover, Schneiderberg 1B, D-30167 Hannover, Germany Institut für Anorganische Chemie, Gottfried Wilhelm Leibniz Universität Hannover, Callinstraße 9, D-30167 Hannover, Germany § Departamento de Química, Centro de Investigación y de Estudios Avanzados, Av. Instituto Politécnico Nacional 2508, Col. San Pedro Zacatenco, CDMX, C.P. 07360, México ‡

S Supporting Information *

ABSTRACT: The syntheses of methoxy- and trifluoromethylsubstituted benzocyclobutenone and benzocyclobutenedione tricarbonylchromium complexes are described. As with the unsubstituted complex, a route via the respective acetals was used. However, attempts to hydrolyze rac-tricarbonyl[1,2-bis(ethylenedioxy)-3-(trifluoromethyl)benzocyclobutene]chromium(0) (rac-12) resulted in only a single hydrolysis and led to rac-tricarbonyl{η6-[2-(ethylendioxy)-3-(trifluoromethyl)benzocyclobutenone]}chromium(0) (rac-14) with different regioselectivity in comparison to the respective reaction of the methoxy-substituted derivative. The synthesis of the desired ractricarbonyl[3-(trifluoromethyl)benzocyclobutenedione]chromium(0) (rac-13) was finally achieved by a route via an acyclic diacetal. Compounds were characterized spectroscopically and in a number of cases also by crystal structure analyses. The unusual bending of the annelated cyclobutenedione ring toward the tricarbonylchromium moiety was observed for ractricarbonyl-[η6-(methoxybenzocyclobutenedione)chromium(0) (rac-4) as well. To gain more insight into the unusually large bending of the annelated cyclobutenedione or cyclobutenone rings toward the tricarbonylchromium group in some of the studied compounds, we also performed density functional theory (DFT) calculations. In general, the gas-phase DFT optimized structure parameters show good agreement with the crystal structure data, indicating that the cyclobutenedione or cyclobutenone ring bending is a molecular rather than a crystal-packing effect. The DFT optimized structure data also show that annelated cyclobutenedione rings bend more strongly toward the tricarbonylchromium group than do their cyclobutenone analogues. Moreover, the staggered conformation of the tricarbonylchromium group favors larger bending angles. Topological analyses of the electron density of the studied (arene)tricarbonylchromium complexes suggest that the cyclobutenedione ring bending originates from the bending of π orbitals of the arene ring toward the tricarbonylchromium group.

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benzocyclobutenedione (Scheme 1), were disclosed.16−18 These include the anion-driven ring opening of benzocyclobutenol complexes to the corresponding o-quinodimethane intermediates, which can be trapped by [4 + 2] cycloadditions or electrocyclic processes,19−23 and the dianionic oxy-Cope rearrangement, which occurs upon nucleophilic syn diaddition

INTRODUCTION

(Arene)tricarbonylchromium complexes have attracted interest, because their chemical properties significantly differ from those of the uncoordinated arenes.1 The most prominent reasons for this are a strong electron withdrawal by the tricarbonylchromium group, which is comparable to that of a p-nitro substituent at an arene,2−6 and the facial differentiation, which eliminates one plane of symmetry, allowing for stereoselective applications of (arene)tricarbonylchromium complexes in stoichiometric as well as catalytic asymmetric natural product synthesis.7−15 Within our investigations in the field of (arene)tricarbonylchromium complexes we concentrated on ligand systems with annelated functionalized rings. As results of this research a number of oxyanion accelerated reactions which take place at low temperatures, starting from the tricarbonylchromium complexes rac-1 and 2 of benzocyclobutenone and © XXXX American Chemical Society

Scheme 1

Received: June 3, 2019

A

DOI: 10.1021/acs.organomet.9b00370 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics of alkenyllithium reagents at 2.24−29 More recently, we found the first examples of an anionic thia-Fries rearrangement at (arene)tricarbonylchromium complexes.30,31 (Benzocyclobutenedione)tricarbonylchromium (2) is a highly strained complex, which deserves interest in many aspects. In contrast to the uncoordinated benzocyclobutenedione, which can be obtained in 49% overall yield from ninhydrin by photodecarbonylation of the 1,3-diacetal followed by hydrolysis,32 complex 2 for obvious steric reasons allows for a syn-1,2-diaddition of nucleophiles at the face opposite to the tricarbonylchromium moiety, a reaction that results in decomposition when it is tried with uncoordinated benzocyclobutenedione.33,34 In the mass spectrum of 2 the base peak is that of the (η6-benzyne)chromium cation, which results from the fragmentation of the three CO ligands followed by two molecules of CO from the dione moiety.35 Finally, the crystal structure analysis of 2 shows that, although all carbon atoms in the ligand system are sp2 hybridized, the annelated ring is bent toward the tricarbonylchromium moiety by ca. 8°.29 While this observation was intuitively explained as a result of the ring strain of the annelated ring causing an enhanced complexation of the C2a−C6a bond at chromium, we felt that the phenomenon has not yet been fully explained. Here we report the syntheses and some structures of related, mostly trifluoromethyl substituted complexes in combination with a theoretical study of the effect targeting at a more profound understanding of this structural anomaly.

transformed to the diacetal 11 in 65% yield under standard reaction conditions. Although 11 is electron poor, its reaction with Cr(CO)6 gave the complex rac-12 in 30% yield containing some decoordinated ligand. Application of the usual hydrolysis conditions successful in the syntheses of 2 and rac-4, however, failed and resulted in decomplexation of the deprotected ligand, affording 10 in 92% yield. However, when trifluoroacetic acid was used for hydrolysis, a partial hydrolysis was achieved, giving monoacetal rac-14 in 59% yield. rac-14 was characterized by spectroscopy as well as by an X-ray crystal structure analysis (Figure 4). The formation of rac-14 is remarkable, because the partial hydrolysis of the corresponding methoxy-substituted complex rac-15 gave the alternative regioisomer rac-16 (Scheme 3).24 The difference in regioselectivity of the partial hydrolyses of the trifluoromethyl- and the methoxy-substituted derivatives rac-12 and rac-15 can be explained by considering the cationic intermediates rac-17 and rac-21 and their resonance formulas rac-18−rac-20 and rac-22−rac-24, respectively (Scheme 4). For the partial hydrolysis of rac-12 at C-2 these contain the formula rac-18, in which the positive charge is unfavorably located next to the trifluoromethyl substituent. Presumably, as this is not the case for a hydrolysis at C-1, this is favored. The preferred hydrolysis at C-2 in methoxy derivative rac-15 is explained by charge delocalization in 25 not only into the arene ring as in rac-26 but also to the electron-donating methoxy group in resonance formula rac-27 (Scheme 5). This stabilization, which is not given for a hydrolysis at C-1, goes back not only to a more extended charge delocalization but also to a decrease of positive charge density at the arene ring, which is coordinated to the highly electron withdrawing tricarbonylchromium group. As a consequence of the incomplete acetal hydrolysis of the cyclic ethylene acetal rac-12, use of a less stable, noncyclic acetal was envisaged. Attempts to obtain 1,1,2,2-tetramethoxy3-(trifluoromethyl)benzocyclobutene (32) from 10 by treatment with trimethoxymethane in boiling methanol in the presence of a catalytic amount of trifluoromethylsulfonic acid40 afforded only ring-opened products 28 and 29 in 10% and 9% yields, respectively. Replacing the trifluoromethylsulfonic acid by p-toluenesulfonic acid (PTSA) resulted in the isolation of only the starting material. Treatment of 10 with trimethoxymethane in the presence of a catalytic amount of PTSA in the absence of a solvent gave only partial acetalization with formation of 30 and 31 in only 9% and 6% yields, respectively (Scheme 6). Finally, the reaction of 10 with methoxytrimethylsilane in the presence of 0.1 equiv of (trimethylsilyl)trifluoromethanesulfonate (Me3SiOTf)41 was successful and afforded the desired acyclic acetal 32 in 64% yield. Subsequent complexation under standard reaction conditions with hexacarbonylchromium in dibutyl ether/THF (10/1) at reflux afforded tricarbonylchromium complex rac-33 in 50% yield in addition to 34% of starting material 32, which was recovered. However, as indicated by the 13C NMR spectrum, the isolated rac-33 was contaminated by hexacarbonylchromium, which could not easily be removed. The same reaction under microwave heating gave only a 20% yield of rac-33,42 and complexation with triamminetricarbonylchromium afforded rac-33 in only 7% yield. Finally we were pleased to find that the reaction of 32 with tricarbonyl(naphthalene)chromium(0) (Kündig reagent)43 in THF afforded rac-33 in 41% yield in addition to

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RESULTS AND DISCUSSION Complexes rac-1 and 2 are accessible in high yields by complexation of the acetal-protected ligands and subsequent acetal hydrolysis.20,21,27,29 Enantiopure 1 was obtained by classical or chromatographic resolution20 or by diastereoselective complexation of the enantiopure ligand.36 The more electron rich methoxy-substituted derivatives rac-3 and rac-4 (Scheme 1) have been prepared in a similar way.37 In order to obtain a less electron rich derivative, we attempted to prepare the corresponding trifluoromethyl-substituted complexes rac-7 and rac-13. As the direct complexation of 6-(trifluoromethyl)benzocyclobutenone38 failed, 1,1-diethoxy-6-(trifluoromethyl)benzocyclobutene (5)38 was coordinated at Cr(CO)3 under standard reaction conditions, affording complex rac-6 in 62% yield. Subsequent acetal hydrolysis with 50% hydrochloric acid gave the desired 6-(trifluoromethyl)benzocyclobutenone complex rac-7 in 95% yield (Scheme 2). Scheme 2

In an attempt to prepare the corresponding (trifluoromethyl)-substituted benzocyclobutenedione complex rac-13, we resorted to the procedure originally reported by Liebeskind.39 6-(Trifluoromethyl)benzocyclobutenone (8)38 was treated with N-bromosuccinimide (NBS) under radical reaction conditions, affording 2,2-dibromo-6-(trifluoromethyl)benzocyclobutenone (9) in 46% yield. Subsequent hydrolysis with 50% sulfuric acid gave 6-(trifluoromethyl)benzocyclobutenedione (10) in 56% yield, which was B


Article

Organometallics Scheme 3

Scheme 4

dione 10 in quantitative yield, deacetalization by formic acid was successful and afforded the desired tricarbonyl[3(trifluoromethyl)benzocyclobutenedione]chromium(0) (rac13) in 20% yield in addition to some unconsumed starting material rac-33 and ligand 10. Unfortunately, rac-13 could so far not be crystallized and was therefore characterized spectroscopically. The complex showed diagnostic infrared carbonyl ligand absorptions at ν̃ 1993 and 1917 cm−1 as well as keto carbonyl absorptions at ν̃ 1782 and 1737 cm−1. In the 13C NMR spectrum signals of the keto carbonyl carbon atoms were observed at δ 186.8 and 184.9 ppm. The EI mass spectrum of rac-13 deserves some comment: in the mass spectra of the closely related compounds 2 and rac-4 the fragmentation pattern is characterized by the dissociation of the three carbonyl ligands followed by two CO molecules from the keto groups, leading to (η6-benzyne)chromium cations 34 and rac35 as the base peaks in the mass spectra.35,37 While this fragmentation pattern is observed for rac-13 as well, the relative abundance of rac-36 (m/z 196) is only 10% (Scheme 8). In the mass spectrum of rac-13 the base peak is observed at m/z 172, which corresponds to the fragment formed by loss of

Scheme 5

Scheme 6

40% of unconsumed 32 with no hexacarbonylchromium impurity (Scheme 7). Several attempts to hydrolyze the acyclic acetal moieties in rac-33 were undertaken. While treatment of rac-33 with hydrochloric acid resulted in the isolation of decomplexed C


Article

Organometallics Scheme 7

Scheme 8

Cr(CO)3 and another CO. Remarkably, m/z 172 is also the base peak in the mass spectrum of 3-(trifluoromethyl)benzocyclobutenedione (10), thus indicating a very dominant fragmentation path involving the dissociation of the ligand, which is not observed for 2 and rac-4. Decarbonylation is the most important fragmentation for benzocyclobutenediones. Prior to the respective measurement the absence of decoordinated 10 has been checked by NMR. The observation is in accord with the sensitivity of rac-13 to decomplexation, which precluded its synthesis for quite some time. In order to estimate the effect of the different substituents (H, OMe, CF3), crystal structure analyses were performed (Figures 1−3). The most important geometric parameters are

Figure 2. Structure of rac-3 in the crystal.44


the experimentally obtained crystal structure and the DFT optimized gas-phase structure (see also Figure 7). Experimentally, a staggered conformation with carbonyl ligands anti to the C2a−C6a, C3−C4, and C5−C6 bonds is found for rac7, whereas the DFT optimized gas-phase conformation is only slightly distorted from the eclipsed conformation of rac-3 with carbonyl ligands synperiplanar to C2a, C4, and C6. As a consequence, the electron withdrawal of the carbonyl ligands in rac-7 reduces the back-bonding into the respective antibonding ligand orbital to such an extent that the shorter bonds C2a−C6a, C3−C4, and C5−C6 are less tightly coordinated. This effect is quite pronounced for the bond C2a−C6a (136.7 pm), which is much shorter than the other C−C bonds. The dihedral angles given in Table 1 indicate that the ligands are practically planar, as expected for fully sp2 hybridized systems. Only rac-7 slightly deviates from planarity,

44

Figure 1. Structure of rac-1 in the crystal.

given in Table 1 for comparison. However, it has to be taken into account that the quality of the crystals of rac-1 was rather poor, so that these data are of limited significance. In fact, the experimentally assigned geometric parameters for rac-1, given in Table 1, differ significantly from those obtained by DFT structure optimization, as a comparison with Table S2 of the Supporting Information shows. Whereas this difference can be straightforwardly attributed to the poor quality of the analyzed crystals, the origin for the difference between the experimental and theoretical C2a−C6a and C3−C4 bond lengths in rac-7 is less obvious. A possible explanation might be the variation in the conformation of the tricarbonylchromium group between D


Article

Organometallics Table 1. Comparison of Structures in the Crystal: Selected Bond Lengths (pm), Atom Distances (pm), and Torsion Angles (deg) of complexes 1 and rac-3 (Crystallized from Diethyl Ether/Pentane) and rac-7 (Crystallized from Dichloromethane/Hexane)

bond

rac-1a

C1−C2 C1−C6a C2−C2a C2a−C3 C2a−C6a C3−C4 C4−C5 C5−C6 C6−C6a C1−O1 C6−OCH3 C6−CF3 C2a−Cr C3−Cr C4−Cr C5−Cr C6−Cr C6a-Cr C1···Cr C2···Cr O1···Cr C1−C6a−C2a−C3 C2−C2a−C6a−C6

152.3(16) 150.6(17) 153.5(16) 135.4(15) 142.4(14) 135.1(16) 142.3(16) 135.8(15) 135.6(14) 118.7(13)

217.3(10) 218.4(13) 218.9(13) 220.6(12) 218.4(11) 216.7(10) 322.1(14) 328.6(13) 418.8(10) −179.0(9) 179.6(9)

rac-3 155.6(4) 150.2(3) 152.3(3) 138.9(4) 142.8(3) 141.1(3) 141.5(3) 141.0(3) 141.7(3) 118.8(3) 133.8(3) 218.1(2) 223.4(2) 217.3(2) 222.1(2) 231.1(2) 222.5(2) 327.6(2) 329.6(2) 425.6(2) −179.5(2) 177.7(2)

rac-7 156.6(12) 150.6(13) 152.8(9) 140.0(11) 136.7(12) 137.0(12) 141.7(11) 139.2(12) 141.9(15) 117.1(11)


Table 2. Comparison of Structures in the Crystal: Selected Bond Lengths (pm), Atom Distances (pm), and Torsion Angles (deg) of Complexes rac-14 and rac-1624 a

150.1(16) 218.3(6) 222.5(8) 217.4(8) 218.4(6) 217.0(9) 217.6(9) 325.7(9) 328.6(6) 416.8(6) −177.2(6) 177.5(8)

a

Data for one of the two independent molecules present in the asymmetric unit. Data for the other one see SI.

as is also indicated by the shorter distance O1−Cr (416.8 pm) in comparison to the corresponding distance in rac-3 (425.6 pm). While the crystal structure analysis of rac-16 has already been reported,24 here a structure analysis of the closely related rac-14 has been performed, which was crystallized from dichloromethane/hexane (Figure 4). A comparison of relevant structural parameters is given in Table 2. A comparison of the two related structures reveals significant differences: the distances Cr−O1, Cr−C1, and Cr−C2 are smaller in the methoxy-substituted complex rac-16 than the corresponding distances in the trifluoromethyl-substituted rac14, indicating that the carbonyl region of the annelated cyclobutane ring in rac-16 is bent toward the tricarbonylchromium moiety to a larger extent, while this effect is somewhat less pronounced for rac-14. This observation is confirmed by the dihedral angles given in Table 2. On the other hand, the theoretical gas-phase calculations of rac-14 and rac-16 (Tables S8 and S9) show exactly the opposite trend: i.e., a slightly stronger bending of the annelated cyclobutane ring in rac-14 than in rac-16. The comparison between experimental and theoretically optimized PBE0 bond distances reveal unusually large discrepancies for the Cr−O1, Cr−C1, and Cr−C2 distances in rac-16. Therefore, it seems reasonable to attribute

a

bond in rac-14

rac-14

rac-1624

bond in rac-16

C1−C2 C2−C2a C1−C6a C6a−C6 C2a−C6a C5−C6 C4−C5 C3−C4 C2a−C3 C1−O1 C3−CF3 C6a−Cr C6−Cr C5−Cr C4−Cr C3−Cr C2a−Cr C2···Cr C1···Cr O1···Cr C2−C2a−C6a−C6 C1−C6a−C2a−C3

157.8(6) 155.0(5) 149.2(6) 141.4(6) 139.7(6) 139.1(7) 138.9(7) 143.6(6) 140.2(5) 119.1(5) 147.4(7) 218.3(3) 220.7(4) 222.4(5) 219.3(5) 221.0(4) 218.1(3) 331.4(3) 319.8(4) 412.6(3) −179.1(3) 174.8(3)

158.2(6) 150.1(7) 151.7(6) 138.6(6) 143.0(6) 139.7(6) 138.7(6) 140.6(6) 140.1(9) 117.1(5) 135.0(6) 216.7(5) 222.2(5) 217.9(5) 222.0(5) 227.3(5) 218.6(5) 313.6(6) 324.5(5) 402.1(4) −171.3(5) 177.3(5)

C1−C2 C1−C6a C2−C2a C2a-C3 C2a−C6a C3−C4 C4−C5 C5−C6 C6−C6a C1−O1 C6−OMe C2a−Cr C3−Cr C4−Cr C5−Cr C6−Cr C6a−Cr C1···Cr C2···Cr O1···Cr C1−C6a−C2a−C3 C2−C2a−C6a−-C6

Bonds corresponding to one another are listed in the same line.

the larger bending experimentally observed in rac-16 to crystalpacking effects. This is further supported by the fact that bond length trends in the substituent are in good qualitative agreement between theory and experiment. As an example, the bond length C2a−C6a in rac-14 is shorter than the corresponding bond length in rac-16, indicating a weaker backE


Article

Organometallics bonding from the metal. The distances Cr−C3 and Cr−C4 in rac-14 are significantly shorter than the corresponding distances in rac-16, and the bond C3−C4 in rac-14 is longer than the respective bond in rac-16, which is in accord with a stronger back-bonding in rac-14 between chromium and the bond C3−C4, which is in close proximity to the electronwithdrawing trifluoromethyl group. It was also possible to grow crystals of rac-4 (from dichloromethane/diethyl ether) that were suitable for crystal structure analyses (Figure 5).

Table 3. Comparison of Structures in the Crystal: Selected Bond Lengths (pm), Atom Distances (pm), and Torsion Angles (deg) of Complexes 229 and rac-4 bond C1−C2 C1−C6a C2−C2a C2a−C3 C2a−C6a C3−C4 C4−C5 C5−C6 C6−C6a C1−O1 C2−O2 C3−OCH3 C2a−Cr C3−Cr C4−Cr C5−Cr C6−Cr C6a−Cr C1···Cr C2···Cr O1···Cr O2···Cr C1−C6a−C2a−C3 C2−C2a−C6a−C6


229

rac-4

156.6(7) 150.7(6) 150.4(6) 140.7(3) 141.8(3) 139.9(4) 141.0(4) 139.9(4) 140.7(3) 118.9(8) 118.9(8) 215.2(23) 222.1(4) 220.8(8) 220.5(14) 222.2(2) 215.9(16) 312.7(23) 312.7(23) 408(6) 408(6) 171.2(2) −174.6(2)

156.1(4) 149.8(4) 151.0(4) 141.2(3) 143.1(4) 139.8(4) 140.9(4) 140.3(4) 139.3(4) 119.4(4) 118.8(4) 133.7(3) 217.7(3) 231.1(3) 223.9(3) 219.6(3) 221.5(3) 212.2(3) 303.2(3) 309.6(3) 395.8(2) 406.7(2) 172.2(2) −171.8(3)

tetramethylcyclooctatetraenyl)uranium(IV) to be bent toward the metal atom by an average of 4.1°. The authors exclude intermolecular effects and suggest a bending of the cyclooctatetraeneyl π orbitals toward the uranium atom for better orbital overlap, causing the methyl groups to bend toward the uranium atom. Alternatively the bending might be caused by polarization and contraction of the ligand π system by the highly charged metal atom.46 In the same year, Rees and Coppens published a low-temperature X-ray and neutron diffraction study of benzenetricarbonylchromium. A slight bending of the hydrogen atoms toward the tricarbonylchromium group was also explained by a bending of the π orbitals toward the metal for better π orbital overlap.47 In contrast, the structure of (hexamethylbenzene)tricarbonylchromium does not show a significant bending of the methyl groups toward the chromium atom.48 There are some structures derived from ferrocene that show a deviation of the cyclopentadienyl (Cp) substituents from the Cp plane. In ferrocene in the gas phase an electron diffraction study revealed the C−H bonds to be bent about 5° out of the Cp plane toward the iron atom.49 Cais and Herbstein have investigated the structures of ferrocenylmethyl carbenium ions. These are rather stable, and with a charge delocalization to the iron atom a resonance formula with a fulvene ligand can be formulated. The diferrocenylmethylium ion has been structurally investigated, showing a very significant bending of the exocyclic C−C bonds toward the iron atoms (17.7−19.9°).50 This is in accord with structures of fulvene complexes,51 in which the exocyclic double bond is coordinated at the metal atom:52−54 for example, tricarbonyl(fulvene)chromium(0).55 In this context a recent structural investigation of ferrocenylboranes, which are isoelectronic with ferrocenylcarbenium ions, revealed a corresponding bending of the C−B bonds toward the iron atom.56 Unfortunately, the

The structure of the 3-methoxy-substituted benzocyclobutenedione complex rac-4 is rather similar to that of the unsubstituted complex 2 (Table 3). The asymmetry imposed by the methoxy substituent in rac-4 is reflected by a small bond length difference between C1−C6a and C2−C2a (149.8 vs 151.0 pm); however, this difference must not be overestimated. In both compounds the rather long bond length C1−C2 (2, 156.6 pm; rac-4, 156.1 pm) causes the annelated ring to adopt a distorted-rectangular geometry. As in 3 and in rac-16,24 the methoxy substituent adopts a conformation with the methyl group directed to the keto function. This conformation is presumably preferred for electrostatic reasons, because the lone electron pairs of the methoxy and those of the keto oxygen atoms point away from each other. As in 2, the tricarbonylchromium group in rac-4 adopts a conformation with no carbonyl ligand located below the annelated cyclobutane ring. Likewise, the bond lengths C2a−Cr and C6a−Cr are significantly shorter than those between the other arene carbon atoms and the chromium atom reflecting the ring strain of the annelated cyclobutenedione. This closer coordination of the C2a−C6a bond at the chromium atom comes along with a significant bending of the annelated cyclobutenedione ring toward the tricarbonylchromium moiety by 8.2° (2, 8 ± 1°29). This observation clearly indicates that the bending of the annelated ring is an intrinsic property of the (benzocyclobutenedione)tricarbonylchromium class of compounds. It should be mentioned that the uncoordinated ligand benzocyclobutenedione adopts a slightly bent structure in the solid state with an interplanar angle of 1−3°.45 Deviations of the structures of π ligands from planarity in sandwich as well as in half-sandwich complexes have been reported previously. Raymond observed the methyl substituents in bis(1,3,5,7F


Article

Organometallics trifluoromethyl-substituted complex rac-13 did not give crystals suitable for an X-ray crystal structure analysis. In order to gain further insight into the bending of the annelated cyclobutenedione ring toward the tricarbonylchromium moiety, we performed gas-phase density functional theory (DFT) calculations on the discussed tricarbonylchromium complexes. We started our theoretical studies with a validation of different DFT approaches for the structure optimization of compound 2. To this end we employed the local density approximation (LDA) with the Dirac exchange57 and VWN correlation58 functional, the generalized gradient approximation (GGA) in the form of the PBE59 functional, and the B3LYP60 and PBE061 hybrid functionals. Table 4 compares selected optimized structure parameters obtained from these four functionals with the corresponding experimental crystal structure data given in Table 3.

with the crystal structure data. In particular, the PBE0 optimized bond lengths are in good agreement with the experimental data. This holds even for the very long distances between the chromium atom and the carbon and oxygen atoms of the annelated cyclobutenedione ring. With regard to the bending of the cyclobutenedione ring, all four functionals indicate a significant bending toward the tricarbonylchromium group. Again, the best quantitative agreements with experiment are obtained with the PBE and PBE0 functionals. Thus, all theoretical methodologies employed here clearly indicate that the bending of the cyclobutenedione ring toward the tricarbonylchromium moiety is a molecular effect. On the basis of this analysis we have performed structure optimizations of all compounds discussed here at the VWN, PBE, and PBE0 levels of theory. The optimized structure data of the different theoretical methodologies are compared to each other and the available crystal structure data in Tables S1−S8 in the Supporting Information. As an illustrative example, Figure 6 depicts the correlation between the calculated and measured Cr−C6a and Cr−C2a bond lengths in the compounds rac-1, 2, rac-3, rac-4, rac-7, rac-14, and rac16. As Figure 6 shows, the optimized VWN bond lengths (red dots) are systematically too short, whereas the optimized PBE bond lengths (green dots) are systematically too long. This is in agreement with common experiences: namely, that LDA

Table 4. Comparison of DFT Optimized Gas-Phase Structure Parameters with Experimental Crystal Structure Data for Compound 2a bond

VWN

PBE

B3LYP

PBE0

Experiment

C1−C2 C1−C6a C2−C2a C2a−C3 C2a−C6a C3−C4 C4−C5 C5−C6 C6−C6a C1−O1 C2−O2 C2a-Cr C3−Cr C4−Cr C5−Cr C6−Cr C6a-Cr C1···Cr C2···Cr O1···Cr O2···Cr C1−C6a−C2a−C3 C2−C2a−C6a−C6

158.4 150.6 150.6 140.3 142.2 140.6 142.0 140.6 140.3 119.2 119.2 211.9 217.6 215.8 215.8 217.6 211.8 305.9 305.9 400.6 400.6 168.9 −168.9

160.2 152.0 152.0 141.5 143.1 141.7 143.2 141.7 141.5 120.1 120.1 217.9 223.1 221.0 221.1 223.1 217.8 317.6 317.6 414.0 414.0 172.7 −172.8

159.5 151.3 151.3 140.8 141.7 140.7 142.4 140.7 140.8 119.0 119.0 220.8 225.6 223.7 223.7 225.6 220.8 322.9 322.9 418.6 418.6 175.5 −175.4

158.3 150.7 150.7 140.6 141.7 140.6 142.2 140.6 140.6 118.7 118.7 215.6 220.5 218.7 218.7 220.5 215.6 314.9 314.9 410.1 410.1 172.6 −172.7

156.6(7) 150.7(6) 150.4(6) 140.7(3) 141.8(3) 139.9(4) 141.0(4) 139.9(4) 140.7(3) 118.9(8) 118.9(8) 215.2(23) 222.1(4) 220.8(8) 220.5(14) 222.2(2) 215.9(16) 312.7(23) 312.7(23) 408(6) 408(6) 171.2(2) −174.6(2)

a

Bond lengths and atom distances in pm; dihedral angles in deg.

As this comparison shows, VWN, B3LYP, and PBE0 reproduce rather well the experimental bond lengths in the benzocyclobutenedione substituent. On the other hand, the PBE optimized bond lengths are systematically too long with respect to the experimental reference. This agrees with the common experience that GGA optimized bond lengths usually overestimate their experimental counterparts. Whereas VWN, B3LYP, and PBE0 deliver reasonable optimized structure parameters for the benzocyclobutenedione substituent, larger deviations are found for the bond lengths between the chromium atom and the carbon atoms of the arene ring. As expected, the LDA optimized bond lengths with the VWN functional are significantly too short, whereas the B3LYP optimized lengths are too long by up to 5 pm. Both the PBE and, particularly, the PBE0 functional improve this situation and yield optimized bond lengths that are in good agreement

Figure 6. Correlation between the calculated gas-phase and experimentally measured crystal structure Cr−C6a (top) and Cr− C2a (bottom) bond lengths in rac-1, 2, rac-3, rac-4, rac-7, rac-14 and rac-16. The diagonal lines indicate perfect correlation between theory and experiment. G


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Figure 7. Superimposed experimental (red) and PBE0 (black) optimized structures.

significantly shorter than in all other compounds. On the other hand, the PBE0 optimized C2a−C6a bond length in rac-7 is around 141 pm, which is in the same range as for the other compounds and practically identical with the PBE0 optimized C2a−C6a bond length in its monoacetal analogue, rac-14. As a consequence, the experimentally observed bond length alternation in the arene ring of rac-7 is not found in the DFT optimized structures (see Table S6 in the Supporting Information). Thus, no significant difference in the π system conjugations are found in the theoretical studies between rac-3 and rac-7. We find, however, a slightly stronger bending of the cyclobutenone ring toward the tricarbonylchromium group in rac-7. Also, in rac-16 the significant differences between the DFT optimized structures and their experimental counterparts are very much localized. Here, the experimentally observed strong bending through the C1−C6a−C2a−C3 dihedral angle is not confirmed by our DFT calculations. In fact, in the DFT optimized structures the bending through this dihedral angle is even more pronounced in rac-14 than in rac-16. Figure 7

underestimates and GGA overestimates bond lengths. Note that the PBE0 optimized bond lengths are always between these two limits and are stochastically distributed around the lines of perfect correlation with experiment. Even though these examples are for specific bonds, the trend observed here is rather general, as Tables S1−S8 show. Particularly large deviations from this trend, i.e. experimental bond lengths that are significantly shorter than the VWN optimized lengths or significantly longer than the PBE optimized lengths, are found in rac-1 (C1−C2, C2a−C3, C3−C4, C5−C6, and C6−C6a), rac-7 (C2a−C6a and C3− C4), and rac-16 (C1−Cr and O1−Cr). The rather large deviations between theory and experiment of up to 5 pm in some arene bond lengths of rac-1 (see Table S2) are most certainly rooted in the poor quality of the crystals available for the structure analysis. In rac-7 significant differences between theory and experiment are only observed for two bond lengths. Most pronounced is this discrepancy for the C2a−-C6a bond, which is experimentally determined to 136.7 pm and therefore H


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Organometallics

In both structures, bond paths between the Cr atom and the two carbon bridgehead atoms C2a and C6a are found. This is a marked difference from all other studied structures, where only one bond path between the chromium and one of the bridgehead atoms exists. Thus, the topological analysis of the electron density suggests that the bending of the annelated cyclobutenedione ring toward the tricarbonylchromium moiety arises from the direct interaction between the Cr atom and the π system of the substituent. The large curvature of the bond paths between the chromium atom and the bridgehead carbon atoms further supports this interpretation. A reasonable explanation for this bond path curving is a local deformation of the π system due to the bending of the π orbitals on C2a and C6a toward the chromium atom. Note also the bond path bending in the cyclobutenedione ring that reflects the large strain in this system. The other bond path from the chromium atom lies in the symmetry plane of the system and connects to the C4−C5 arene bond.

graphically summarizes the differences between the PBE0 optimized structures (black) and their crystal structure counterparts (red) by superimposing them with each other. For comparison we also added benzenetricarbonylchromium(0), where the experimental structure had been obtained from electron diffraction data in the gas phase.62 As Figure 7 shows, the most significant differences between theory and experiment are in soft modes: i.e., in dihedral angles. Thus, it seems reasonable to assume that most of these differences arise from crystal-packing effects. For the bending of the annelated cyclobutenedione or cyclobutenone ring toward the tricarbonylchromium group, we obtained the following ordering from the PBE0 optimized structures: 2 ≥ rac‐13 ≥ rac‐4 ≫ rac‐14 ≈ rac‐1 ≈ rac‐7 ≥ rac‐16 ≥ rac‐3

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This ordering shows that the bending is strongest in the cyclobutenedione complexes. In the DFT optimized structures we find a significant gap between the bending dihedral angles in the cyclobutenedione complexes and those in the other studied compounds. From this we conclude that the annelated cyclobutenedione is the primary cause for the observed large bending toward the tricarbonylchromium group. Surprisingly, the unsubstituted compound 2 is heading this group and, thus, shows the largest bending of all studied structures. Note that in 2 the tricarbonylchromium group adopts a perfectly staggered conformation with respect to the arene ring. Also, rac-13 and rac-4 show staggered conformations; however, these are distorted by the CF3 and OMe groups. This observation suggests that the staggered conformation of the tricarbonylchromium group might have an additional effect on the cyclobutenedione bending. To further investigate this, we performed a topological analysis63 of the PBE0 electronic density of compounds 2 and rac-13. The molecular graphs obtained along with the bond critical points are depicted in Figure 8.

CONCLUSIONS This work reports a combined experimental and theoretical study of the structure anomaly in (benzocyclobutenedione)tricarbonylchromium complexes. The validation of gas-phase optimized structure data of the parent compound with corresponding crystal structure data reveals that the PBE0/ aug-cc-pVTZ methodology is best suited for the quantitative prediction of structural data. At this level of theory, optimized and measured bond lengths usually differ by less than 2 pm. Because this agreement between theory and experiment holds for most of the studied systems, we can unequivocally identify the bending of the cyclobutenedione ring as a molecular effect. For this bending, the theoretical study in gas phase yields the ordering 2 ≥ rac‐13 ≥ rac‐4 ≫ rac‐14 ≈ rac‐1 ≈ rac‐7 ≥ rac‐16 ≥ rac‐3

Thus, the bending is strongest in the benzocyclobutenedione complexes 2, rac-13, and rac-4. This finding is in excellent agreement with the available crystal structure data. A rationalization for this trend is given by the topological analysis of the electronic density, which revealed curved bond paths between the Cr atom and both bridgehead carbon atoms in the two compounds (2 and rac-13) with the largest bending angles of the cyclobutenedione ring.

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EXPERIMENTAL SECTION

General Considerations. All operations were performed in flame-dried reaction vessels under an argon atmosphere using Schlenk techniques. tert-Butyl methyl ether (TBME) and tetrahydrofuran (THF) were distilled from sodium−potassium alloy/benzophenone. Petroleum ether (PE) and chlorinated solvents were distilled from P4O10. 1H NMR spectra were collected on AVS 200 (200.1 MHz) and AM 400 (400.1 MHz) instruments. 13C NMR spectra were collected on Bruker AVS 200 (50.3 MHz), AM 400 (100.1 MHz), and DRX 500 (125.8 MHz) instruments. Signal multiplicities were determined with APT and DEPT techniques; signals with negative phase are labeled with − and those with positive phase with + . Chemical shifts refer to δTMS 0, to residual solvent signals (1H, 13C), or to F3CPh (19F). Air-sensitive samples were prepared and sealed under an argon atmosphere. IR spectra were collected on PerkinElmer FT-IR 580 and 1710 instruments, with the ATR (attenuated total reflection) technique. Only selected diagnostic bands are reported. MS measurements were obtained on Finnigan MAT 112 and 312, AM 400, and SSQ 7000 instruments at 70 eV. HRMS data were collected

Figure 8. Molecular graphs and bond critical points (blue) of the PBE0 electron densities of compound 2 (top) and rac-13 (bottom). I


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chloric acid (150 mL) was added to rac-6 (4.0 g, 10.1 mol) in the dark. The mixture was stirred at 25 °C for 2 h and became orange. The mixture was extracted twice with TMBE (100 mL each), and the collected organic layers were washed twice with water (100 mL each). After the collected organic layers were dried over magnesium sulfate, the solvent was removed at reduced pressure. rac-7 (3.09 g, 9.6 mmol, 95%) was obtained as an orange-red solid (mp 65 °C). IR (ATR): ν̃ 1987 (s, CO), 1897 (s, CO), 1767 (s, CO), 1524 (m), 1496 (m), 1447 (m), 1412 (w), 1369 (m), 1314 (s, C−F), 1220 (s), 1182 (m), 1169 (m), 1130 (s), 1103 (s, C−F), 976 (m), 958 (m), 919 (m), 851 (m) cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 3.94 (d, 1H, 2 Jexo‑2, endo‑2 = −16.7 Hz, 2-H), 4.32 (d, 1H, 2Jendo‑2, exo‑2 = −16.8 Hz, 2H), 5.89 (d, 1H, 3J = 6.5 Hz, 5-H), 6.15 (dd, 1H, 3J = 6.4 Hz, 3J = 6.4 Hz, 4-H), 6.36 (d, 1H, 3J = 6.5 Hz, 3-H). 13C NMR (100.6 MHz, acetone-d6, APT): δ 53.2 (+, C-2), 87.0 (q, −, 3JC,F = 3.3 Hz, C-5), 89.1 (−, C-4), 93.4 (d, +, 2JC,F = 39 Hz, C-6), 93.8 (−, C-3), 117.4 (+, C-2a), 123.2 (q, +, 1JC,F = 273 Hz, C-7), 140.7 (+, C-6a), 180.6 (+, C-1), 229.1 (+, CO). MS (70 eV, 70 °C): m/z (%) 322 (65) [M+], 266 (20) [M − 2CO], 238 (57) [M − 3CO], 218 (100) [M − 3CO − HF], 190 (3), 167 (3), 148 (7), 120 (88), 101 (9), 75 (8), 52 (13) [Cr]. Crystal Structure Analysis of rac-7:44 C12H5CrF3O4, molecular weight 322.16, crystal system monoclinic, space group P21/c (No. 14), a = 11.787(4) Å, b = 7.915(2) Å, c = 13.183(6) Å, α = 90°, β = 93.22(5)°, γ = 90°, V = 1228.0(8) Å3, Z = 4, dcalcd = 1.743 g cm−3, F(000) = 640e, μ = 9.8 cm−1, crystal orange-red plate∥(001), size 0.15 × 0.26 × 0.04 mm, Stoe IPDS (imaging plate) diffractometer, T = 300 K, Mo Kα = 0.71073 Å, 2θmin = 6.2°, 2θmax = 48.5°, scan type 100 exposure, ΔΦ = 1.8°, 5301 measured reflections (±13, ±8, ±15), 1098 independent (R(I)int = 0.144) and 420 observed reflection (It > 2.0 σ(I)), completeness of data 55.3%, no absorption correction, no extinction correction, structure solution with direct methods with SHELXS-86, refinement with SHELXL-93, hydrogen atoms in geometrically calculated positions, Nref = 1098, Npar = 121, R = 0.0384, Rw = 0.0563 (w = 1/σ2(Fo2)), S = 0.62, minimal and maximal residual electron density −0.22/0.18 e Å−3. 2,2-Dibromo-6-(trifluoromethyl)benzocyclobutenone (9). 6Trifluoromethylbenzocyclobutenone (8; 8.00 g, 43.0 mmol)),38 Nbromosuccinimide (19.14 g, 108.0 mmol), and dibenzoyl peroxide (1.3 g, 5.4 mmol) in carbon tetrachloride (200 mL) were heated at reflux for 5 days. The solution was cooled to 20 °C, and PE (100 mL) was added to precipitate the succinimide formed. The solid was filtered off with a Büchner funnel and washed with PE. The filtrate was concentrated and filtered through a short silica column, with methylene chloride as eluent. After solvent removal the residue was purified by column chromatography (SiO2, 400 × 30 mm, TMBE/PE 1/4). 9 (6.80 g, 19.8 mmol, 46%) was obtained as a yellow solid (mp 83 °C). IR: (ATR): ν̃ 1798 (s, CO), 1324 (s, C−F), 1181 (s), 1138 (s, C−F), 1106 (s), 988 (m), 919 (m), 793 (m), 685 (m) cm−1. 1 H NMR (400.1 MHz, CDCl3): δ 7.87 (d, 1H, 3J = 7.7 Hz, 5-H), 7.90 (dd, 1H, 3J = 8.0 Hz, 3J = 7.6 Hz, 4-H), 7.65 (d, 1H, 3J = 7.6 Hz, 3H). 13C NMR (100.6 MHz, acetone-d6): δ 57.8 (+, C-2), 122.3 (q, +, 1 JC,F = 273 Hz, C-7), 125.7 (−, C-3), 126.2 (d, +, 2JC,F = 37.6 Hz, C6), 130.3 (q, −, 3JC,F = 4.0 Hz, 5-C), 136.6 (+, C-2a), 137.9 (−, C-4), 159.2 (+, C-6a), 174.3 (+, C-1). MS (70 eV, 168 °C): m/z (%) 344 (100) [M+], 316 (8), 298 (6), 265 (34) [M − Br], 235 (81), 216 (2), 191 (9), 156 (62), 137 (6), 106 (13), 78 (20). HRMS (C9H379Br2F3O): calcd 341.8504, found 341.8504. 3-(Trifluoromethyl)benzocyclobutenedione (10). At 0 °C sulfuric acid (100 mL, 50%) was added to 9 (6.00 g, 17.0 mmol), and the mixture was heated at reflux for 24 h, with the color changing to dark brown. The mixture was cooled to 25 °C and was extracted twice with dichloromethane (100 mL each). The collected organic layers were washed twice with water (100 mL each). After the collected organic layers were dried over MgSO4, the solvent was removed at reduced pressure, and the residue was purified by column chromatography (SiO2, 400 × 30 mm, TMBE/PE 1/2). 10 (1.88 g, 9.4 mmol, 56%) was obtained as a yellow solid (mp 84 °C). IR: (ATR): ν̃ 1773 (s, CO), 1602 (m), 1359 (m, C−F), 1317 (s), 1237 (m), 1169 (s), 1130 (s, C−F), 1083 (s), 1012 (m), 955 (m),

with Finnigan MAT 312 VG Autospec instruments, with peak matching with PFK, Micromass LCT with lock-spray unit (ESI). Combustion analyses were carried out with Heraeus CHN Rapid and Elementar Vario EL instruments (Analysensysteme GmbH). Melting points were obtained on a Thermal Sciences PL Gold DSC instrument. For column chromatography, silica gel was degassed by heating it with a heat gun at reduced pressure followed by setting it under normal pressure with argon. This sequence was repeated five times. Crystal Structure Analysis of rac-Tricarbonyl[η 6 -(6methoxybenzocyclobutenone)chromium(0) (rac-3): 44 C12H8CrO5, molecular weight 284.19, crystal system orthorhombic, space group P212121 (No. 19), a = 7.917(1) Å, b = 9.096(1) Å, c = 16.203(2) Å, α = 90°, β = 90°, γ = 90°, V = 1166.8(2) Å3, Z = 4, dcalcd = 1.618 g cm−3, F(000) = 576e, μ = 9.9 cm−1, crystal red rod∥[100], size 0.67 × 0.18 × 0.12 mm, Stoe IPDS (imaging plate) diffractometer, T = 300 K, Mo Kα = 0.71073 Å, 2θmin = 5.0°, 2θmax = 56.5°, scan type 220 exposure, ΔΦ = 1.3°, 16334 measured reflections (±10, −11, +12, ± 21), 2840 independent (R(I)int = 0.062) and 2329 observed reflection (It > 2.0σ(I)), completeness of data 100%, no absorption correction, no extinction correction, structure solution with direct methods with SHELXS-86, refinement with SHELXL-93, hydrogen atoms in geometrically calculated positions, Nref = 2840, Npar = 163, R = 0.0398, Rw = 0.0656 (w = 1/σ2(Fo2)), S = 1.09, minimal and maximal residual electron density −0.34/0.32 e Å−3. Crystal Structure Analysis of rac-Tricarbonyl[η 6 -(3methoxybenzocyclobutenedione)chromium(0) (rac-4). 44 C12H6CrO6, molecular weight 298.17, crystal system monoclinic, space group P21/c (No. 14), a = 8.475(1) Å, b = 7.946(1) Å, c = 17.616(3) Å, α = 90°, β = 95.63(2)°, γ = 90°, V = 1180.6(3) Å3, Z = 4, dcalcd = 1.678 g cm−3, F(000) = 600e, μ = 9.9 cm−1, crystal redbrown prism||(010), size 0.15 × 0.67 × 0.11 mm, Stoe IPDS (imaging plate) diffractometer, T = 300 K, Mo Kα = 0.71073 Å, 2θmin = 4.6°, 2θmax = 48.1°, scan type 160 exposure, ΔΦ = 1.4°, 8555 measured reflections (±9, ±8, ±20), 1847 independent (R(I)int = 0.039) and 1321 observed reflection (It > 2.0σ(I)), completeness of data 99%, no absorption correction, no extinction correction, structure solution with direct methods with SHELXS-86, refinement with SHELXL-93, hydrogen atoms in geometrically calculated positions, Nref = 1847, Npar = 172, R = 0.0288, Rw = 0.0646 (w = 1/σ2(Fo2)), S = 1.04, minimal and maximal residual electron density −0.23/0.24 e Å−3. rac-Tricarbonyl[{η 6 -[1,1-diethoxy-6-(trifluoromethyl)benzocyclobutene]}chromium(0) (rac-6). 1,1-Diethoxy-6(trifluoromethyl)benzocyclobutene (5;38 8.00 g, 30.8 mmol) and hexacarbonylchromium (8.12 g, 36.9 mmol) in a mixture of dibutyl ether (200 mL) and THF (20 mL) were heated at reflux for 20 h. After the solution was cooled to 25 °C, it was filtered through a 5 mm thick layer of silica gel with THF as eluent. After solvent removal at reduced pressure the residue was purified by column chromatography (SiO2, 400 × 30 mm, TMBE/PE, 2/1), giving rac-6 (7.6 g, 19.1 mmol, 62%) as a yellow solid (mp 46 °C). IR (ATR): ν̃ 1982 (s, CO), 1909 (s, CO), 1318 (s, C−O), 1240 (s, C−F), 1173 (s), 1128 (s), 1085 (m), 1060 (s) cm−1. 1H NMR (400.1 MHz, acetone- d6): δ 1.24 (t, 6H, 3J = 7.0 Hz, CH3), 3.31 (AB line system, 2H, endo-2-H, exo-2-H, 2Jendo‑2,exo‑2 = −13.8 Hz), 3.67 (m, 4H, 3J = 7.3 Hz, 2 CH2CH3), 5.58 (d, 1H, 3J = 6.5 Hz, 3-H), 5.64 (dd, 1H, 3J = 6.4 Hz, 4-H), 6.05 (d, 1H, 3J = 6.2 Hz, 5-H). 13C NMR (100.6 MHz, acetoned6, APT): δ 14.1 (−, CH3), 14.3 (−, CH3), 43.6 (+, C-2), 59.0 (+, CH2CH3), 59.6 (+, CH2CH3), 86.1(q, −, 4JC,F = 3.3 Hz, C-5), 89.1(−, C-4), 91.6 (−, C-3), 94.1 (d, +, 2JC,F = 37.6 Hz, C-6), 104.8 (+, C-1), 110.8 (+, C-2a), 127.2 (q, +, 1JC,F = 273 Hz, CF3), 143.2 (+, C-6a), 230.9 (+, CO). MS (70 eV): m/z (%) 396 (44) [M+], 340 (33) [M − 2CO], 312 (72) [M − CO], 267 (24), 235 (49), 215 (18), 196 (100) [M − 3CO − C2H4O], 167 (53), 145 (30), 120 (27), 102 (14), 75 (6), 52 (12) [Cr]. HRMS: calcd for C16H15CrF3O5 396.0277, found 396.0275. Anal. Calcd for C16H15CrF3O5 (396.9): C, 48.49; H 3.82. Found: C, 48.43; H, 3.85. rac-Tricarbonyl{η 6 -[6-(trifluoromethyl)benzocyclobutenone]chromium(0) (7). At 0 °C half-concentrated hydroJ


Article

Organometallics 855 (m), 843 (m), 831 (m), 806 (s), 719 (m) cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 8.18 (dd, 1H, 3J = 7.5 Hz, 3J = 8.2 Hz, 5-H), 8.25 (d, 1H, 3J = 8.0 Hz, 6-H), 8.45 (d, 1H, 3J = 7.7 Hz, 4-H). 13C NMR (100.6 MHz, acetone-d6, APT): δ 120.7 (q, +, 1JC,F = 272.2 Hz, C-7), 124.5 (q, +, 2JC,F = 37.4 Hz, C-3), 125.8 (−, C-5), 132.0 (q, −, 3JC,F = 4.2 Hz, C-4), 136.2 (−, C-6), 186.8 (+, C-2a), 172.6 (+, C-6a), 190.0 (+, C-1), 192.5 (+, C-2). 19F NMR (376.5 MHz, acetone-d6): δ −63.7 (s, CF3) ppm. MS (70 eV): m/z (%) 200 (20) [M+], 172 (100) [M − CO], 144 (91) [M − 2 CO], 125 (23) [M − 2CO − F], 94 (10), 85 (2), 75 (16). HRMS for C9H3F3O2: calcd 200.0085; found 200.0086. Anal. Calcd for C9H3F3O2 (200.11): C, 54.01; H, 1.51. Found: C, 54.17; H, 1.63. 1,2-Bis(ethylenedioxy)-3-(trifluoromethyl)benzocyclobutene (11). 3-(Trifluoromethyl)benzocyclobutenedione (10; 3.40 g, 17.0 mmol), 1,2-ethanediol (8.00 g, 51.0 mmol), and p-toluenesulfonic acid (200 mg, 1.2 mmol) in benzene (150 mL) were heated at reflux with azeotropic water removal. When no more water was separated, the solvent and excess 1,2-ethanediol were removed at reduced pressure, and the residue was crystallized from ethyl acetate, giving 11 (3.2 g, 11.0 mol, 65%) as colorless crystals (mp 154 °C). IR: (ATR): ν̃ 2975 (w), 2902 (w), 1615 (w), 1528 (w), 1345 (s, C−F), 1269 (s, C−O), 1222 (s, C−O), 1166 (m), 1126 (s, C−F), 1043 (s), 756 (m), 687 (m) cm−1. 1H NMR(400.1 MHz, acetone-d6): δ 4.04 (m, 4H, 2 CH2), 4.18 (m, 4H, 2 CH2), 7.48 (d, 1H, 3J = 7.9 Hz, 6-H), 7.72 (m, 1H, 5-H), 7.80 (m, 1H, 4-H). 13C NMR (100.6 MHz, acetone-d6, APT): δ 59.2 (+, C-2), 64.9 (+, 2 CH2), 65.4(+, 2 CH2), 71.8 (+, C-1), 121.2 (q, +, 1JC,F = 271 Hz, CF3), 126.2 (d, +, 2JC,F = 38 Hz, C-3), 126.3 (−, C-6), 127.5 (−, C-5), 129.5(q, −, 3JC,F = 4.0 Hz, C-4), 133.1 (+, C-2a), 146.9 (−, C-6a). MS (70 eV, 60 °C): m/z (%) 287 (3) [M+], 261 (10), 244 (10), 216 (100) [M − CF3 − 2H], 197 (15), 172 (76), 144 (40), 125 (11), 108 (1), 91 (19), 75 (7). Anal. Calcd for C13H11F3O4 (288.2): C, 54.17; H, 3.85. Found: C, 54.19; H, 4.39. rac-{η 6 -[1,2-Bis(ethylendioxy)-3-(trifluoromethyl)benzocyclobutene]}tricarbonylchromium(0) (12). 11 (3.00 g, 10.0 mmol) and hexacarbonylchromium (4.83 g, 22.00 mmol) in dibutyl ether (150 mL) and THF (15 mL) were heated at reflux for 20 h. After the mixture was cooled to 25 °C and filtered through a 5 mm thick layer of silica gel with THF as eluent, the solvent was removed at reduced pressure. The residue was purified by column chromatography (SiO2, 400 × 30 mm, TMBE/PE, 1/2), yielding rac12 (1.3 g, 3.0 mmol, 30%) as a yellow solid accompanied by some decomplexed ligand. IR: (ATR): ν̃ 2975 (w), 2902 (w), 1990 (s, CO), 1976 (s, CO), 1906 (s, CO), 1345 (s, C−F), 1269 (s), 1222 (s), 1166 (m), 1126 (s, C−F), 1043 (s), 949 (m), 756 (m), 687 (m) cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 4.05 (m, 4H, 2 CH2), 4.18 (m, 4H, 2 CH2), 5.54 (dd, 1H, 3J = 6.4 Hz, 3J = 5.6 Hz, 5-H), 5.92 (d, 1H, 3J = 5.6 Hz, 4-H), 6.13 (d, 1H, 3J = 6.4 Hz, 6-H). 13C NMR (100.6 MHz, acetone-d6): δ 65.4 (+, CH2), 65.86 (+,CH2), 65.93 (+, CH2), 66.2 (+,CH2), 87.7 (−, C-6), 88.9 (−, C-5), 89.5 (q, −, 3JC,F = 3.3 Hz, C-4), 110.9 (+, C-1 or C-2), 111.5 (+, C-1 or C-2), 123.9 (q, +, 1JC,F = 272 Hz, CF3), 125.2 (+,d, +, 2JC,F = 37 Hz, C-3), 143.1 (+, C-2a), 146.8 (+, C-6a), 229.1 (+, CO). MS (70 eV, 80 °C): m/z (%) 424 (5) [M+], 368 (2) [M+ − 2CO], 340 (8) [M+ − 3CO], 269 (3), 244 (6), 216 (100) [M − Cr(CO)3 − C2H4 − C2H4O], 197 (11), 172 (63), 144 (26), 125 (8), 100 (7), 72(5) [Cr]. - HRMS for C16H11CrF3O7: calcd 423.9862; found 423.9861. rac-Tricarbonyl{η6-[2-(ethylendioxy)-3-(trifluoromethyl)benzocyclobutenone]}chromium(0) (rac-14). At 0 °C {η6-[1,2bis(ethylendioxy)-3-(trifluoromethyl)benzocyclobutene]}tricarbonylchromium(0) (rac-12; 0.15 g, 0.40 mmol) was added to trifluoroacetic acid (10 mL). The solution was stirred at 25 °C for 22 h. After addition of water (20 mL) the mixture was extracted five times with dichloromethane (8 mL each). The collected organic layers were dried over MgSO4. Solvent removal at reduced pressure gave a red solid, which was purified by column chromatography (SiO2, 30 × 2 cm, PE/TBME 2/1), giving rac-14 (0.05 g, 0.1 mmol, 33%) as a red solid. IR (ATR): ν̃ 2908 (w), 2002 (s, CO), 1933 (s, CO), 1866 (s, CO), 1782 (s, CO), 1604 (w), 1318 (s), 1267 (m), 1227 (m), 1160 (s), 1134 (s, C−F), 1083 (m), 1010 (m), 952 (m)

cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 4.24−4.38 (br m, 4H, CH2CH2), 5.81 (dd, 1H, 3J = 6.4 Hz, 3J = 6.4 Hz, 5-H), 6.33 (d, 1H, 3 J = 6.5 Hz, 6-H), 6.37 (d, 1H, 3J = 6.3 Hz, 4-H) ppm. 13C NMR [125.8 MHz, acetone-d6]: δ 67.9 (CH2), 68.8 (CH2), 88.3 (C-6), 92.4 (d, 3JC,F = 3.0 Hz, C-4), 92.9 (d, 2JC,F = 39.5 Hz, C-3), 106.4 (C6a), 121.7 (C-2), 123.6 (d, 1JC,F = 272.1 Hz, CF3), 124.3 (d, 4JC,F = 1.4 Hz, C-5), 134.8 (C-2a), 190.5 (C-1), 228.2 (CO) ppm. MS: m/z (%) 380 (13) [M+], 352 (19) [M+ − CO], 324 (16) [M+ − 2CO], 296 (73) [M+ − 3CO], 277 (10) [M+ − 3CO − F], 268 (37) [M+ − 4CO], 225 (12), 216 (17) [M+ − 4CO − Cr], 196 (100) [(F3CC6H3Cr)+], 172 (36), 162 (38), 155 (11), 150 (54), 144 (38), 134 (24), 125 (56), 115 (19), 106 (95) [(FCC6H3)+], 103 (50), 87 (78) [C7H3)+], 75 (53) [(C6H3)+], 52 (83) [Cr+]. HRMS for C13H7CrFO5 [M+ − CO]: calcd 351.9651; found 351.9649. Crystal Structure Analysis of rac-14.44 C14H7CrF3O6, molecular weight M = 380.20 g/mol, orange block, size 0.25 × 0.19 × 0.14 mm, monoclinic, space group P21/c, (No. 14), a = 10.736(4) Å, b = 9.154(3) Å, c = 14.961(5) Å, α = 90.00°, β = 91.41(5)°, γ = 90.00°, V = 1469.9(9) Å3, Z = 4, dcalcd = 1.718 g/cm3, T = 293(2) K, Stoe IPDS diffractometer, Mo Kα = 0.71073 Å, 2θmin = 2.61°, 2θmax = 26.14°, scan type 219 exposures, ΔΦ = 1.2°, h,k,l ±13, ±11, ±17, 14846 measured, 2779 unique (Rint = 0.1404), and 1779 observed (I > 2σ(I)) reflections, Npar = 217, R1 = 0.06, wR2 = 0.13, S = 1.11, hydrogen atoms in geometrically calculated positions, minimal and maximal residual electron density −0.6,0.5 e/Å3. Unsuccessful Attempts toward 1,1,2,2-Tetramethoxy-3(trifluoromethyl)benzocyclobutene (32). (a) At 0 °C trimethoxymethane (0.6 mL, 5.0 mmol) was added to 6-(trifluoromethyl)benzocyclobutenedione (10; 0.10 g, 0.5 mmol) in methanol (1 mL). After addition of trifluoromethanesulfonic acid (0.1 mL, 1.0 mmol) the mixture was heated at reflux for 48 h. After addition of a saturated aqueous solution of NaHCO3 (5 mL) the mixture was extracted with diethyl ether (2 × 5 mL). After drying over MgSO4, filtration, and solvent removal at reduced pressure, the product mixture was separated by column chromatography (SiO2, 10 × 1.5 cm, PE/TBME 4/1), affording methyl 2-(dimethoxymethyl)-6-(trifluoromethyl)benzoate (28; 0.014 g, 0.5 mmol, 10%) and 2-(methoxycarbonyl)3-(trifluoromethyl)benzoic acid (29; 0.011 g, 0.4 mmol, 9%) as pale yellow oils. Data for 28 are as follows. 1H NMR (400.1 MHz, CDCl3): δ 3.31 (s, 6H, 2 OCH3), 3.93 (s, 3H, OCH3), 5.61 (s, 1H, 9-H), 7.67 (d, 1H, 3 J = 7.1 Hz, 5-H), 7.75 (dd, 1H, 3J = 7.7 Hz, 4-H), 7.85 (d, 1H, 3J = 7.7 Hz, 3-H), ppm. 13C NMR (100.1 MHz, CDCl3): δ 52.7 (OCH3), 53.0 (OCH3), 100.1 (C-9), 123.4 (q, 1JC,F = 273.8 Hz, C-7), 126.3 (q, 3 JC,F = 4.7 Hz, C-1), 127.9 (q, 2JC,F = 31.9 Hz, C-6), 129.4 (ArC), 130.5 (q, 4JC,F = 0.8 Hz, C-2), 137.2 (ArC), 167.6 (C-8), ppm. HRMS (ESI) for C12H13F3O4Na: calcd 301.0664; found 301.0663. Data for 29 are as follows. 1H NMR (400.1 MHz, CDCl3): δ 4.02 (s, 3H, OCH3), 7.75 (dd, 1H, 3J = 7.6 Hz, 3J = 7.9 Hz, 4-H), 7.96 (d, 1H, 3J = 7.9 Hz, 5-H), 8.12 (d, 1H, 3J = 7.6 Hz, 3-H), 10.49 (s, 1H, COOH) ppm. 13C NMR (100.1 MHz, CDCl3): δ 53.4 (OCH3), 122.9 (q, 1JC,F = 273.8 Hz, C-7), 128.7 (q, 2JC,F = 32.1 Hz, C-6), 129.6 (ArC), 130.3 (ArC), 131.4 (q, 3JC,F = 4.5 Hz, C-1), 133.8 (q, 4JC,F = 0.8 Hz, C-2), 133.9 (ArC), 166.3 (C-8), 188.9 (C-9), ppm. HRMS (ESI) for C10H7F3O4Na: calcd 271.0194; found 271.0201. (b) p-Toluenesulfonic acid (0.19 g, 1.0 mmol) was added to 6(trifluoromethyl)benzocyclobutenedione (10; 0.10 g, 0.5 mmol) in trimethoxymethane (0.6 mL, 5.0 mmol). The reaction mixture was heated at reflux for 4 h. After addition of saturated aqueous NaHCO3 (5 mL) the mixture was extracted with diethyl ether (2 × 5 mL). After drying over MgSO4, filtration, and solvent removal at reduced pressure the mixture was separated by column chromatography (SiO2, 10 × 1 cm, PE/TBME 9/1 → 6/1). 2,2-dimethoxy-3(trifluoromethyl)benzocyclobutenone (30; 0.013 g, 0.04 mmol, 9%) and 2,2-dimethoxy-6-(trifluoromethyl)benzocyclobutenone (31; 0.007 g, 0.03 mmol, 6%) were isolated as pale yellow oils. Data for 30 are as follows. IR (ATR): ν̃ 2957 (w, OCH3), 1782 (s, CO), 1591 (w), 1435 (w), 1323 (s, CF3), 1273 (s), 1211 (m), 1171 (s, CF3), 1128 (s, C−O−C), 1084 (s), 1068 (s), 945 (m), 858 (m), 821 (m), 754 (s), 713 (m), 663 (m), 596 (m) cm−1. 1H NMR K


Article

Organometallics (400.1 MHz, CDCl3): δ 3.55 [s, 6H, OCH3], 7.73 (dd, 1H, 3J = 7.4 Hz, 5-H), 7.77 (d, 1H, 3J = 6.5 Hz, 6-H), 7.84 (d, 1H, 3J = 7.1 Hz, 4H) ppm. 13C NMR (100.1 MHz, CDCl3): δ 53.4 [OCH3], 122.4 (q, 1 JC,F = 276.7 Hz, CF3), 125.1 (q, 4JC,F = 1.0 Hz, C-5), 127.3 (q, 2JC,F = 36.0 Hz, C-3), 130.8 (C-6a), 132.1 (q, 3JC,F = 4.2 Hz, C-4), 132.4 (C6), 149.1 (C-2a), 156.8 (q, 3JC,F = 2.5 Hz, C-2), 191.1 (C-1), ppm. 19 F NMR (376.5 MHz, CDCl3): δ −62.11 (s, CF3) ppm. HRMS (ESI) for C11H10F3O3: calcd 247.0582; found 247.0583. Data for 31 are as follows. IR (ATR): ν̃ 2916 (w, OCH3), 2849 (w), 1724 (s, CO), 1591 (w), 1435 (w), 1325 (s, CF3), 1273 (s), 1213 (m), 1172 (s, CF3), 1130 (s, C−O−C), 1090 (s), 1069 (s), 941 (m), 858 (m), 831 (m), 770 (s), 754 (s), 716 (w), 678 (m), 553 (w) cm−1. 1H NMR (400.1 MHz, CDCl3): δ 3.60 [s, 6H, OCH3], 7.75 (t, 1H, 3J = 7.5 Hz, 4-H), 7.83 (d, 1H, 3J = 7.9 Hz, 3-H), 7.91 (d, 1H, 3J = 7.5 Hz, 5-H) ppm. 13C NMR (100.1 MHz, CDCl3): δ 185.4 (C-1), 159.0 (C-2), 144.9 (C-2a), 135.6 (C-3), 130.8 (C-6a), 128.7 (q, 3JC,F = 4.3 Hz, C-5), 125.3 (q, 2JC,F = 36.5 Hz, C-6), 126.0 (q, 4JC,F = 1.0 Hz, C-4), 121.9 (q, 1JC,F = 272.6 Hz, CF3), 52.8 (OCH3) ppm. 19F NMR (376.5 MHz, CDCl3): δ −62.11 (s, CF3) ppm. HRMS (ESI) for C11H10F3O3: calcd 247.0582; found 247.0583. 1,1,2,2-Tetramethoxy-3-(trifluoromethyl)benzocyclobutene (32). At 28 °C (trimethylsilyl)trifluoromethanesulfonate (0.07 g, 0.3 mmol) was added to 6-(trifluoromethyl)benzocyclobutenedione (10; 0.73 g, 3.7 mmol) in methoxytrimethylsilane (15.2 mL). After the mixture was stirred for 72 h at 28 °C, a saturated aqueous solution of sodium hydrogencarbonate (30 mL) was added, and the mixture was extracted with diethyl ether (3 × 50 mL). The collected organic layers were dried over magnesium sulfate and filtered, and after solvent removal at reduced pressure the product was purified by column chromatography (SiO2, 35 × 2 cm, PE/TBME 6/1). 32 (0.69 g, 2.3 mmol, 64%) was obtained as a pale yellow oil. IR (ATR): ν̃ 2946 (w), 2837 (w, OCH3), 1320 (s, CF3), 1260 (m), 1237 (m), 1209 (s), 1172 (s, CF3), 1124 (s, C−O−C), 1078 (s), 1033 (s), 1002 (s), 984 (s), 911 (m), 812 (m), 799 (m), 758 (m), 731 (s), 657 (m) cm−1. 1H NMR (400.1 MHz, CDCl3): δ 3.51 (s, 6H, OCH3], 3.53 (s, 6H, OCH3), 7.52 (t, 1H, 3J = 7.7 Hz, 5-H), 7.61 (d, 1H, 3J = 7.4 Hz, 6-H), 7.63 (d, 1H, 3J = 7.9 Hz, 4-H) ppm. 13C NMR (100.1 MHz, CDCl3): δ 51.99 (s, 2 OCH3), 51.95 (q, 6JC,F = 1.5 Hz, 2 OCH3), 109.0 (C2a), 109.9 (C-6a), 123.0 (q, 1JC,F = 272.5 Hz, CF3), 126.0 (C-5), 126.5 (q, 2JC,F = 34.8 Hz, C-3), 127.6 (q, 3JC,F = 4.4 Hz, C-4), 130.4 (C-6), 141.1 (q, 4JC,F = 2.4 Hz, C-2), 144.9 (C-1) ppm. HRMS (ESI) for C13H15F3O4Na: calcd 315.0820; found 315.0819. rac-Tricarbonyl[1,1,2,2-tetramethoxy-3-(trifluoromethyl)benzocyclobutene]chromium(0) (rac-33). Tricarbonyl(η 6 naphthalene)chromium(0)64,65 (0.98 g, 4.4 mmol) and 1,1,2,2tetramethoxy-3-(trifluoromethyl)benzocyclobutene (32, 1.18 g, 4.0 mmol) in THF (20 mL) were heated at reflux for 24 h. After it was cooled to 25 °C, the mixture was filtered through a layer of silica gel (1 cm), and the solvent was removed at reduced pressure. The residue was separated by column chromatography (SiO2, 30 × 1.5 cm, PE/ TBME 4/1 → 2/1 → TBME). Data for 32 are as follows. Yield: 0.47 g, 1.6 mmol, 40%. Data for rac-33 are as follows. Yield: 0.70 g, 1.7 mmol, 41%, yellow solid. IR (ATR): ν̃ = 2943 (w, ArH), 2841 (w, OMe), 1977 (s, C O), 1911 (s, CO), 1846 (s, CO), 1524 (s), 1445 (s), 1318 (s, CF3), 1250 (s), 1200 (s), 1174 (s, CF3), 1126 (s), 1083 (s), 1023 (s), 1000 (s), 840 (m), 801 (m), 718 (w), 654 (m), 634 (m), 611 (m) cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 3.47 (br. s, 12H, OCH3), 5.45 (t, 1H, 3J = 6.4 Hz, 5-H), 5.90 (d, 1H, 3J = 6.6 Hz, 6-H), 6.33 (d, 1H, 3J = 6.3 Hz, 4-H) ppm. 13C NMR (125.7 MHz, acetone-d6): δ 52.16 (s, OCH3), 52.20 (br s, OCH3) 52.3 (br. s, OCH3), 52.47 (s, OCH3), 89.0 (C-5), 91.0 (C-6), 91.0 (q, 2JC,F = 38.1 Hz, C-3), 91.3 (q, 3JC,F = 3.2 Hz, C-4), 110.0 (C-1 or C-2), 111.4 (C-1 or C-2), 124.1 (q, 1JC,F = 271.9 Hz, CF3), 127.6 (q, 4JC,F = 1.0 Hz, C-2a), 128.3 (q, 3JC,F = 4.4 Hz, C-6a), 230.9 (CO) ppm. HRMS (ESI) for C16H15F3O7CrNa: calcd 451.0073; found 451.0074. rac-Tricarbonyl[(3-trifluormethyl)benzocyclobutenedione]chromium(0) (rac-13). rac-Tricarbonyl[1,1,2,2-tetramethoxy-3(trifluormethyl)benzocyclobutene]chromium(0) (rac-33; 0.70 g, 1.7 mmol) in formic acid (5 mL) was stirred at 25 °C for 4 h. After

addition of water (40 mL) the mixture was extracted with dichloromethane (4 × 15 mL). After drying of the collected organic layers over magnesium sulfate, filtration through a glass frit, and solvent removal at reduced pressure, the crude red product was purified by column chromatography in order to remove some rac-33 and ligand 10 (SiO2, 30 × 1.5 cm, PE → PE/TBME 6/1 → 4/1 → 2/ 1 → 1/1 → 1/2 → TBME) and gave rac-13 (0.11 g, 0.3 mmol, 20%) as a red solid. IR (ATR): ν̃ 1993 (s, CO), 1917 (s, CO), 1782 (m, CO), 1737 (m, CO), 1578 (m), 1445 (s), 1304 (s, CF3), 1172 (s, CF3), 1133 (s), 1082 (s), 1053 (s), 1017 (s), 806 (m), 655 (m), 639 (m), 601 (m) cm−1. 1H NMR (400.1 MHz, acetone-d6): δ 6.19 (t, 1H, 3J = 6.2 Hz, H-5) 6.59 (d, 1H, 3J = 6.2 Hz, H-4), 6.79 (d, 1H, 3J = 6.4 Hz, H-6) ppm. 13C NMR (125.7 MHz, acetone-d6): δ 91.3 (C-6), 92.9 (q, 3JC,F = 3.3 Hz, C-4), 92.9 (C-5), 94.8 (q, 2JC,F = 40.3 Hz, C-3), 122.9 (q, 4JC,F = 1.0 Hz, C-2a), 123.4 (q, 1JC,F = 272.9 Hz, CF3), 133.2 (q, 3JC,F = 3.4 Hz, C-6a), 184.9 (C-1 or C-2), 186.8 (C-1 or C-2), 226.3 (CO) ppm. 19F NMR (376.5 MHz, acetone-d6): δ −60.1 (s, CF3) ppm. MS (70 eV): m/z (%) 336 (1.4) [M+], 280 (1.6) [M+ − 2CO], 252 (3.8) [M+ − 3CO], 224 (0.6) [M+ − 4CO], 219 (1.2), 196 (10.0) [M+ − 5 CO], 172 (100.0) [M − Cr(CO)3 − CO], 144 (78.8) [C6H3CF3], 125 (26.3), 106 (11.9) [CrC7H3F], 87 (4.3) [CrC7H3], 75 (6.9) [CrC6H3]. HRMS for C12H3O5F3Cr: calcd 335.9338; found 335.9333.

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THEORETICAL SECTION

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ASSOCIATED CONTENT

All calculations were performed with the quantum chemistry code deMon2k.66 For the linear combination of Gaussian type orbital (LCGTO) expansions the correlation consistent aug-cc-pVTZ67 basis set was used. The calculation of four-center ERIs was avoided by the variational fitting of the Coulomb68 and Fock69 potentials. For the expansion of the auxiliary density and orbital product densities the automatically generated GEN-A2* auxiliary function set70 was used. It contains s, p, d, f, and g auxiliary functions that are grouped together into sets with common exponents. The auxiliary density was also used for the calculation of the exchange-correlation energy and potential: i.e., the auxiliary density functional theory (ADFT) approach71 was employed. All structures were fully optimized and characterized by frequency analyses at the corresponding level of theory. For superimposing the optimized and crystal structures the internal alignment algorithm of deMon2k was used.72 In addition, the topological analysis of the electron density was performed with the deMon2k internal algorithm.73 For all other keywords the default settings were employed.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00370. 1

H, 13C, and 19F NMR spectra of new compounds, details of the crystal structure analysis of 1 including bond lengths and torsional angles in rac-1, and comparison of VWN, PBE, and PBE0 optimized structure parameters with experimental crystal structure data (PDF) Accession Codes

CCDC 1574684, 1575733, 1575735, 1575738, and 1575740 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. L


Article

Organometallics

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.M.K.: [email protected]. *E-mail for H.B.: [email protected]. de. ORCID

Holger Butenschön: 0000-0002-7689-1992 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was kindly supported by the Gottlieb Daimlerand Karl Benz-Foundation (doctoral fellowship to K.G.D.) and by the Deutsche Forschungsgemeinschaft (BU 814/7-1, 7-2). We are indebted to Dr. Michael Wiebcke, Institut für Anorganische Chemie, Leibniz Universität Hannover, for help with a crystallographic analysis. J.D.S.-R. gratefully acknowledges a CONACyT Ph.D. fellowship (490270). The research at Cinvestav was supported by the SEP-Cinvestav project 65 and the CONACyT project GIC 268251.

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