Rhenabenzenes and Unexpected Coupling Products from the

Dec 24, 2014 - Chemistry of Metallacyclobutadienes. Mingxu Cui , Ran Lin , Guochen Jia. Chemistry - An Asian Journal 2018 13 (8), 895-912 ...
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Rhenabenzenes and Unexpected Coupling Products from the Reactions of Rhenacyclobutadienes with Ethoxyethyne Ran Lin, Ka-Ho Lee, Herman H. Y. Sung, Ian D. Williams,* Zhenyang Lin,* and Guochen Jia* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: Treatment of Na[Re(CO)5] with methyl 3(naphthalen-1-yl)propiolate (NpCCCO2Me) followed by acetyl chloride and alcohols ROH (R = Me, nPr) afforded the rhenacyclobutadiene complexes Re{-C(Np)C(CO2Me)C(OR)}(CO)4. Reactions of these rhenacyclobutadiene complexes with HCCOEt produced the rhenabenzene complexes Re{-C(Np)C(CO 2 Me)C(OR)CHC(OEt)}(CO)4 and new rhenacyclobutadienes with a pendant vinyl substituent Re{-C(Np)C(C(OR)CH(CO2Et))C(OMe)}(CO)4. In the vinyl-substituted rhenacyclobutadiene products, the ethyl of the ester group is from the alkyne HCCOEt, the alkyl of the ReC(OR) group is from the ester group of the starting rhenacyclobutadienes, and the alkyl group of the OR of the vinyl substituent is from the ReC(OR) group of the starting rhenacyclobutadienes. A plausible mechanism for the formation of the vinyl-substituted rhenacyclobutadienes is discussed.

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(CF3)2]3.3 The reaction has been suggested as the key step in alkyne metathesis.4,5 Formation of cyclopentadienyl complexes has been reported for the reactions of alkynes with metallacyclobutadiene complexes such as Re(CPhCPhCPh)(CO)4,6 Re(C(R)C(CO2Me)C(X))(CO)4 (R = Me, X = OEt, NEt2; R = Ph, X = OEt),7 Ir[C3H-(CHC{Ph}S-1)(Ph-3)]Cl(PPh3)2,8 W(CEtCEtCEt)(OCMe 3 )(OCMe 2 CMe 2 O), 9 and W(C(CMe3)CRCR)Cl3 (R = Et, Me).9 The reactions leading to the formation of cyclopentadienyl complexes could proceed via metallabenzene intermediates.10,11 Formation of stable metallabenzene complexes has been reported for reactions of alkynes with complexes such as Ir[C3 H(CHC{CO 2 Me}S-1)(CO 2 Me-2)]I(PPh 3 ) 2 8 and Re{-C(Ph)C(CO 2 Me)C(OEt)}(CO)4.12 We have recently found that reactions of rhenacyclobutadienes 1 with HCCOEt produced a mixture of species, from which we were able to isolate two types of compounds, one is the rhenabenzene complexes 2 and the other one is the vinylsubstituted rhenacyclobutadiene complexes 3 (Scheme 2).13 Complexes 2 are formed by formal insertion of HCCOEt into Re−C bonds, the mechanism of which has been described in detail in our previous report.13 At first sight, complexes 3 can be thought as being formed by formal insertion of HCCOEt into the C-C(CO2Et) bonds of 1. The formation of 3 from the reaction of 1 with HCCOEt is somewhat surprising and unexpected, considering reactivities previously reported for metallacyclobutadiene complexes toward alkynes. We have therefore carried out additional experiments and theoretical studies to define the mechanism of this interesting trans-

INTRODUCTION Metallacyclobutadienes are an interesting class of organometallic compounds that can play an important role in catalysis and can serve as useful starting materials for organometallic synthesis.1 Therefore, it is of interest to investigate their chemical reactivity in order to better understand reaction mechanisms and to design new organometallic synthesis involving such species. This article concerns the reactivity of metallacyclobutadienes toward alkynes. Previous studies have demonstrated that reactions of alkynes with metallacyclobutadienes can give rise to several possible outcomes, including the formation of new metallacyclobutadiene complexes through metathesis, formation of cyclopentadienyl complexes, or metallabenzenes through insertion of alkynes into metal−carbon bonds (Scheme 1). Metathesis reactions of metallacyclobutadiene complexes with alkynes have been reported for complexes such as W(CRCRCR)[O-2,6C6H3(iPr)2]3 (R = Et, nPr)2 and W(CMeCMeCMe)[OCMeScheme 1

Received: October 10, 2014

© XXXX American Chemical Society

A

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a

Scheme 3

R = naphthalen-1-yl (a), phenanthren-9-yl (b), and pyren-1-yl (c).

formation. These studies reveal that complexes 3 are not formed by simple insertion of HCCOEt into the CC(CO2Et) bonds of 1 but proceed through initial attack on the α-carbon of the metallacycles by HCCOEt, followed by a series of interesting rearrangement reactions. During the course of this work, we have also isolated two new rhenabenzenes, which have been characterized structurally. In this article, we report the details of these findings.

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RESULTS AND DISCUSSION Experimental Observations. The simplest pathway for the formation of complexes 3 is through the formal insertion of HCCOEt into the C−C(CO2Et) bonds of 1. To examine this possibility, we have prepared rhenacyclobutadiene 5 (which contains an OMe group at the α-carbon and a CO2Me group at the β-carbon) and studied its reaction with HCCOEt. If HCCOEt inserts into the C−C(CO2Me) bond of 5, one would expect that the reaction would produce a vinylsubstituted metallacyclobutadiene complex analogous to 3 having an OMe group attached to the α-carbon of the metallacyclobutadiene and an OEt group and a CO2Me group attached to the vinyl carbons. Rhenacyclobutadiene 5 was prepared from the reaction of Na[Re(CO)5] (4) with methyl 3-(naphthalen-1-yl)propiolate followed by treatment with acetyl chloride and methanol (Scheme 3).14 A pure sample of 5 can be obtained in 55% yield after purification by column chromatography. Treatment of 5 with HCCOEt produced a mixture of species. From the reaction mixture, we were able to isolate two pure compounds, one in the form of an orange solid and the other one in the form of a yellow oil, in 12.5% and 14.3% isolated yields, respectively, as was also the case of the reaction of 1 with HC COEt. The in situ 1H NMR suggests that 6 and 7 were produced in a molar ratio of 1:1. The orange solid compound is identified as the expected rhenabenzene 6 derived from an insertion of HCCOEt into the Re−C(OMe) bond in 5. Its structure has been confirmed by X-ray diffraction (Figure 1). The structural parameters associated with the metallacycle are similar to those of other reported rhenabenzenes.12,13 Consistent with the solid state structure, the 1H NMR spectrum of 6 displayed a characteristic 1 H signal at 6.31 ppm due to the proton on the metallacyclic ring. The 13C{1H} NMR spectrum showed ReC(OEt) and ReC(Np) signals at 261.1 and 205.4 ppm, respectively. The other 13C signals of the metallacycle were found at 104.2 (CH), 137.8 (C(CO2Me)), and 188.6 (C(OMe)) ppm. As expected, the NMR data associated with the metallacycle of complex 6 are also similar to those of reported rhenabenzenes.12,13

Figure 1. Molecular structure of 6 (thermal ellipsoids are set at 30% probability). Selected bond lengths [Å] and angles [deg]. Re(1)−C(1) 2.183(2), Re(1)−C(5) 2.147(3), C(1)−C(2) 1.352(3), C(2)−C(3) 1.451(3), C(3)−C(4) 1.380(3), C(4)−C(5) 1.401(4); C(5)−Re(1)− C(1) 85.91(3), C(2)−C(1)−Re(1) 127.66(5), C(1)−C(2)−C(3) 125.0(2), C(4)−C(3)−C(2) 128.0(2), C(3)−C(4)−C(5) 124.0(2), and C(4)−C(5)−Re(1) 129.02(18).

Surprisingly, the yellow oily compound was identified to be vinyl-substituted metallacyclobutadiene complex 7, which contains an OMe group attached to the α-carbon of the metallacyclobutadiene, an OMe group (rather than OEt), and a CO2Et (rather than CO2Me) group attached to vinyl carbons. The structure of this compound can be deduced from its NMR data (Table S1, Supporting Information). The presence of the CH group is indicated by the appearance of a characteristic 1H signal at 4.98 ppm in the 1H NMR spectrum. The presence of the metallacyclobutadiene ring and the vinyl group C(OMe)CHCO2Et is indicated by the 13C{1H} NMR spectrum, which shows signals of the skeletal carbons at 244.1 (ReC(OMe)), 213.7 (ReC(Np), 159.6 (β-C), 161.4 (C(OMe)) and 93.4 (CH) ppm. The presence of CO2Et rather than CO2Me is indicated by the 13C chemical shifts of the B

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group of the starting rhenacyclobutadienes. For this purpose, we have prepared rhenacyclobutadiene 9, which contains an OnPr group at the α-carbon and a CO2Me group at the βcarbon, and studied its reaction with HCCOEt. The required rhenacyclobutadiene 9 was prepared from the reaction of Na[Re(CO)5] (4) with methyl 3-(naphthalen-1yl)propiolate followed by treatment with acetyl chloride and 1propanol. Complex 9 was isolated as a bright orange solid in 71% yield. From the reaction of 9 with HCCOEt, we have isolated the analogous rhenabenzene complex 10 (in 9% yield) and vinyl substituted rhenacyclobutadiene 11 (in 12.8% yield) (Scheme 4). The in situ 1H NMR shows that 10 and 11 were produced in a molar ratio of 1:1.2.

OCH2CH3 group, which appear at 58.7 (OCH2) and 13.2 (CH3) ppm. The chemical shifts are identical to those of the CO2Et group (58.7 (OCH2) and 13.2 (CH3) ppm) rather than the C(OEt) group (79.1 (OCH2) and 13.5 (CH3) ppm) of Re{C(Np)C(C(OEt)CH(CO2Et))C(OEt)}(CO)4 (3a). Furthermore, the cis arrangement of OMe and H in the vinyl group C(OMe)CHCO2 Et is confirmed by an NOE experiment. When the 1H signal of CH at 4.98 ppm was irradiated, the OMe signal at 3.07 ppm was enhanced by 4.44%. To further confirm its structure, the yellow oily compound was treated with 4-methoxyaniline. The reaction produced the crystalline rhenacyclobutadiene complex 8, the structure of which has been confirmed by X-ray diffraction (Figure 2). As

Scheme 4

Figure 2. Molecular structure of 8 (thermal ellipsoids are set at 30% probability). Selected bond lengths [Å] and angles [deg]: Re(1)−C(1) 2.176(4), Re(1)−C(3) 2.184(4), C(1)−C(2) 1.368(6), C(2)−C(3) 1.464(5), C(2)−C(4) 1.482(6), C(4)−C(5) 1.344(6); C(1)−Re(1)− C(3) 61.26(15), Re(1)−C(3)−C(2) 95.9(3), C(2)−C(1)−Re(1) 99.4(3), and C(1)−C(2)−C(3) 103.3(3).

shown in Figure 2, the complex contains a four-membered metallacycle with the vinyl substituent C(OMe)CHCO2Et at the β-carbon. The X-ray diffraction study confirms that the OMe and CO2Et groups in the vinyl substituent are trans to each other. The structural parameters associated with the fourmembered metallacycle are similar to those of related rhenacyclobutadiene complexes.6,12−14 In agreement with the solid-state structure, the 1H NMR spectrum of 8 displayed a characteristic 1H signal at 5.04 ppm for the CH proton. The 13C{1H} NMR showed two ReC signals at 197.5 (ReC(NHAr)) and 196.1 (ReC(Np) ppm, a CH signal at 95.2, a C(OMe) signal at 162.7, and the β-C signal at 152.9 ppm (Table S1, Supporting Information). The cis arrangement of OMe and H in the vinyl group C(OMe) CHCO2Et is also confirmed by an NOE experiment. When the 1 H signal of CH at 5.04 ppm was irradiated, the OMe signal at 3.24 ppm was enhanced by 4.79%. The result suggests that the ethyl group in the ester group of rhenacyclobutadiene 3 or 7 is from the alkyne HCCOEt. Therefore, complexes 3 and 7 are not formed by simple insertion of HCCOEt into the C−C(CO2R) (R = Et or Me) bonds of 1 and 5, respectively. In order to understand how complexes 3 and 7 might be formed, it is obviously necessary to establish the final location of the alkyl group from the ester

The structures of 9 and 10 can be readily assigned based on their NMR data. The structure of rhenabenzene 10 has also been confirmed by X-ray diffraction (Figure 3). The structure of 11 can be deduced from its NMR data (Table S1, Supporting Information). The presence of the CH group is indicated by the appearance of a characteristic 1H signal at 4.95 ppm in the 1H NMR spectrum. The presence of the metallacyclobutadiene ring is indicated by the 13C{1H} NMR spectrum, which shows signals at 244.1 (ReC(OMe)), 213.9 (ReC(Np), 160.1 (β-C), 161.7 (C(OnPr)) and 93.4 (CH) ppm. The presence of CO2Et is indicated by the 13C chemical shifts of the OCH2CH3 group, which appear at 58.8 (OCH2) and 13.3 (CH3) ppm. The chemical shifts are similar to those of 7 (58.7 (OCH2) and 13.2 (CH3) ppm). The presence of a methoxy group at the α-carbon is indicated by the chemical shift of the methyl signal (68.9 ppm), which is similar to that of ReC(OMe) (68.8 ppm) rather than the C(OMe) (54.8 ppm) of complex 7. Furthermore, when the 1H signal of CH at 4.95 ppm was irradiated, the OCH2 signal of the propyl group at 3.12 ppm was enhanced by 5.50% in an NOE experiment. Further support for the structure of 11 is from its reaction with 4-methoxyaniline, which produced the vinyl-substituted rhenacyclobutadiene complex 12, the structure of which has been confirmed by X-ray diffraction (Figure 4). The NMR data C

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substituted rhenacyclobutadiene products are from the ester group of the starting rhenacyclobutadienes, and the alkyl group of the OR of the vinyl substituents is from the ReC(OR) group of the starting rhenacyclobutadienes. Proposed Mechanism for the Reactions. Scheme 5 shows a plausible general mechanism for the formation of the Scheme 5. Proposed Mechanism for the Formation of Rhenabenzenes and Vinyl-Substituted Rhenacyclobutadienes

Figure 3. Molecular structure of 10 (thermal ellipsoids are set at 30% probability). Selected bond lengths [Å] and angles [deg]. Re(1)−C(1) 2.178(5), Re(1)−C(5) 2.149(4), C(1)−C(2) 1.353(6), C(2)−C(3) 1.453(6), C(3)−C(4) 1.384(6), C(4)−C(5) 1.391(6); C(5)−Re(1)− C(1) 86.07(17), C(2)−C(1)−Re(1) 127.4(3), C(1)−C(2)−C(3) 125.4(4), C(4)−C(3)−C(2) 127.5(4), C(3)−C(4)−C(5) 124.2(4), and C(4)−C(5)−Re(1) 128.7(3).

rhenabenzene complexes and vinyl-substituted rhenacyclobutadienes from the reactions of rhenacyclobutadienes with HC COEt, using complex 13 as a representative of the starting rhenacyclobutadienes. As we discussed previously, rhenabenzenes can be produced by initial direct nucleophilic attack of the β-carbon of HCCOEt on the carbene carbon of the rhenacyclobutadienes. The reaction is possible because the βcarbon of an alkoxyalkyne HCCOR is nucleophilic, and a Fischer carbene carbon is electrophilic. Thus, direct nucleophilic attack of the β-carbon of HCCOEt on the carbene carbon of the metallacycle 13 could give the zwitterionic metallacycle 14, which could rearrange to the rhenabenzene 16, via the five-membered metallacycle 15, which contains a freecarbene group stabilized by the OEt substituent. It is likely that the vinyl-substituted metallacyclobutadienes are also formed by initial direct nucleophilic attack of the βcarbon of HCCOEt on the carbene carbon of the rhenacyclobutadienes. Thus, the zwitterionic complex 14 formed by nucleophilic addition of the terminal carbon of HCCOEt to the C(OR) carbene carbon of the rhenacyclobutadiene 13 may undergo a cyclization reaction to give the bicyclic metallacycle 17 by nucleophilic addition of the carbonyl oxygen to the [COEt]+ carbon. Complex 17 could evolve into the zwitterionic carbene complex 18 (three resonance forms are shown in Scheme 5) by detaching the C(OR) carbon from the metal center. Recoordination of the C(OR′) carbon would produce the bicyclic metallacycle 19. Ring opening of complex 19 would eventually produce the vinyl-substituted rhenacyclobutadiene complex 20.

Figure 4. Molecular structure of 12 (thermal ellipsoids are set at 30% probability). Selected bond lengths [Å] and angles [deg]: Re(1)−C(1) 2.246(14), Re(1)−C(3) 2.201(9), C(1)−C(2) 1.33(2), C(2)−C(3) 1.478(13), C(2)−C(4) 1.467(17), C(4)−C(5) 1.352(18); C(1)− Re(1)−C(3) 59.3(5), Re(1)−C(3)−C(2) 97.5(6), C(2)−C(1)− Re(1) 100.2(10), and C(1)−C(2)−C(3) 103.0(11).

are consistent with the structure. In particular, the 1H NMR spectrum of 12 displayed a characteristic 1H signal at 5.02 ppm for the CH proton. The 13C{1H} NMR spectrum showed two ReC signals at 197.6 (ReC(NHAr)) and 195.8 (ReC(Np) ppm, a CH signal at 94.9, a C(OnPr) signal at 162.2, and the βC signal at 153.6 ppm (Table S1, Supporting Information). The cis arrangement of OnPr and H in the vinyl group C(OnPr) CHCO2Et is confirmed by an NOE experiment. When the 1H signal of CH at 5.02 ppm was irradiated, the OCH2 signal of the propyl group at 3.25 ppm was enhanced by 7.41%. The result from the reaction of 9 with HCCOEt suggests that the ethyl of the ester group of the vinyl-substituted rhenacyclobutadiene products is from the alkyne HCCOEt, the alkyl groups R of the ReC(OR) fragments of the vinylD

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Figure 5. Energy profiles calculated for the reaction of the model complex 21 with HCCOMe to give metallabenzene complex 24 and rhenacyclobutadiene complex 28. The relative Gibbs free energies and electronic energies (in parentheses) at 298 K are given in kcal/mol.

Computational Studies. To verify the proposed mechanism, we have carried out DFT B3LYP calculations. Figure 5 shows the calculated energy profile for the reaction of the model rhenacyclobutadiene complex 21 with HCCOMe. As shown in Figure 5, the nucleophilic addition of the terminal carbon of HCCOMe to the C(OMe) carbene carbon of the metallacycle 21, giving the zwitterionic complex 22 via TS21−22, has a reasonable free energy barrier of 25.3 kcal/mol, which is the rate-determination step for the reaction. Our calculation confirms that the zwitterionic complex 22 can evolve into both the thermodynamically more stable metallabenzene 24 and vinyl-substituted metallacyclobutadiene 28 with a similar reaction barrier. In the formation of the vinylsubstituted metallacyclobutadiene 28, the zwitterionic complex 22 is first converted to the bicyclic metallacycle 25. The process is thermodynamically favored by ca. 22.9 kcal/mol with a very small free energy barrier of 2.5 kcal/mol. Conversion of the bicyclic metallacycle 25 to the more stable (by 25.6 kcal/mol) isomer 27 can be achieved through the zwitterionic carbene complex 26. Interestingly, 26 is more stable than complex 25 by 10.4 kcal/mol, but less stable than 27 by 15.2 kcal/mol. The rearrangement of the bicyclic metallacycle 27 to give the vinylsubstituted metallacyclobutadiene 28 is almost barrierless. In the formation of the metallabenzene 24, the zwitterionic complex 22 is first converted to the five-membered metallacycle 23, also with a very small barrier of 1.5 kcal/mol. Formation of the metallabenzene complex 24 from 23 occurs via TS23−24 with a barrier of 7.0 kcal/mol. We have also considered the possibility of forming the rhenabenzene 24 via a rhenium Dewar benzene intermediate derived from formal cycloaddition of the CC triple bond of the alkyne with the ReC(OMe) carbene bond. This process was found to be kinetically less

favorable as the transition state leading to the rhenium Dewar benzene intermediate from intermediate 22 is 4 kcal/mol higher in energy (see Figure S1a in the Supporting Information) than the transition state TS22−23 (Figure 5) and as the overall barrier for the formation of 24 is even higher if it proceeds through rhenium Dewar benzene intermediate involving prior CO dissociation (see Figure S1(b) in the Supporting Information). The low barriers for the formation of metallabenzene 24 and vinyl-substituted metallacyclobutadiene 28 via the pathways shown in Figure 5 as well as the similarity in the overall barriers are in agreement with the experimental observation that both metallabenzenes and vinyl-substituted metallacyclobutadienes were produced from the reactions of HCCOEt with the starting metallacyclobutadienes at room temperature. The structures of the intermediates presented in Figure 5 were presented based on their calculated structural parameters [see Figure S2 in the Supporting Information for details]. The most interesting and unusual species presented in Figure 5 are complexes 22, 23, and 26. The zwitterionic complex 22 has a negative rhenium center and a positively charged oxonium fragment [CCOMe]+. The presence of the oxonium fragment [CCOMe]+ is indicated by the short C−C (1.288 Å) and C−O (1.237 Å) distances. A reported example of complexes with a coordination sphere similar to that of 22 is the anionic complex [Re(Ph)(COPh)(CO)4]−.15 Related oxonium cations are also known, for example, [CH3CH2O CH2]SbF6.16 Complex 23 contains a carbene carbon stabilized by a OMe group. The carbene intermediate is structurally related to reported aminoalkylcarbenes R2NCOR17 and haloaminocarbenes R2NCX.18 The zwitterionic carbene complex 26 has formally a negative rhenenium center and a E

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Figure 6. Energy profiles calculated for the nucleophilic addition reactions of HCCOMe with 21 and 28. The relative Gibbs free energies and electronic energies (in parentheses) at 298 K are given in kcal/mol.

positively charged pyrylium group. The presence of the Re−C carbene bond is indicated by the short Re−C(Np) bond distance of 2.034 Å. The presence of the pyrylium ring is supported by C−C (1.370−1.436 Å) and C−O (1.340−1.365 Å) associated with the six-membered heterocycle. Related fivecoordinated monoanionic rhenium complexes are known, for example, [Re(CO)5]−.14b Pyrylium salts are also well-known organic compounds.19 The complexes we isolated from the reactions of the naphthalenyl (Np) rhenacyclobutadienes complexes 1a, 5, and 9 with HCCOEt are those derived from nucleophilic attack of HCCOEt on the C(OR) (R = Et, Me and nPr) carbene carbon of the rhenacyclobutadienes complexes. One may wonder whether the C(Np) carbon could also be attacked nucleophilically by HCCOEt. To answer this question, we have calculated the energy profile for the addition of HC COMe to the C(Np) carbene carbon of the model complex 21. As shown in Figure 6, the addition of HCCOMe to the C(Np) carbon is 3.0 kcal/mol less favorable than that on the C(OMe) carbene carbon, suggesting that the attack of HC COR on C(Np) carbon is kinetically less favorable. To understand the different reactivity of the C(OMe) and C(Np) carbons of complex 21, we have examined the lowest unoccupied molecular orbital (LUMO) and NBO charges of the Re-bonded carbons of the model complex 21, which are expected to play an important role in the reaction of the rhenabutadiene with the nucleophile HCCOMe. As shown in Figure 7, the LUMO of 21 is mainly composed of the pπ orbitals of the two α-carbons. The contribution of the pπ orbitals of C(Np) and C(OMe) to the LUMO are 32% and 23%, respectively. If the addition reaction is LUMO-orbital controlled, one would expect that the C(Np) carbon is more reactive than the carbene C(OMe) carbon. Therefore, the different reactivities of C(Np) and C(OMe) carbons toward HCCOMe appear not to be related to the contributions of the pπ orbitals of the two α-carbons to the LUMO. However, the NBO charges on the C(OMe) and C(Np) carbons are

Figure 7. Plots of LUMOs of the model complexes 21 and 28 and their orbital energies. The percentage contributions of pπ atomic orbitals and the NBO charges of selected carbons are given.

0.449 and 0.081, respectively. The NBO charge distribution correlates well with the reactivity of C(Np) and C(OMe) carbons toward HCCOMe. The difference in the steric effect of Np and OMe groups may also play an important role in causing the higher reactivity of the C(OMe) carbon when compared with that of C(Np). Structurally, the vinyl-substituted rhenacyclobutadiene products are similar to the starting rhenacyclobutadiene complexes. Thus, one might expect that these vinyl-substituted rhenacyclobutadiene products may further react with HCCOEt. However, such side products were not detected in our experiments. To understand why the vinyl-substituted rhenacyclobutadiene products are less reactive than the starting rhenacyclobutadiene complexes, we have calculated the barrier for the addition of HCCOMe to the model complex Re{C(Np)C(C(OMe)CH(CO 2 Me))C(OMe)}(CO) 4 (28). Our calculation shows that nucleophilic addition of the terminal carbon of HCCOMe to the C(OMe) carbon of the metallacycle 28 have a noticeable higher barrier of 31.2 kcal/ mol, suggesting that the lower reactivity of the vinylrhenacyclobutadienes toward HCCOEt is of kinetic origin F

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Article

Organometallics

General Procedure for the Preparation of Rhenacyclobutadiene Complexes. To a solution of Na[Re(CO)5] (0.500 g, 1.43 mmol) in THF (20 mL) was slowly added one equivalent of methyl 3(naphthalen-1-yl)propiolate (0.30 g, 1.43 mmol) in THF (20 mL) at RT. The mixture was stirred for 12 h to give a red brown solution. The solvent of the mixture was removed under vacuum to give an orange brown sticky residue which was subsequently dissolved in CH2Cl2. The CH2Cl2 solution was cooled to −15 °C and treated with acetyl chloride (0.15 mL, 2.10 mmol). The mixture turned dark red immediately. After 20 min of stirring, 10 equiv of methanol or 1propanol was added, and the contents were allowed to warm to room temperature. A yellow brown residue was obtained after removal of the solvent under reduced pressure. The product was purified by a silica gel column chromatography to give the desired complex as a bright orange solid. The eluents for column chromatography used for purification were mixtures of n-hexane and diethyl ether [v/v = 7:1]. Re{-C(Np)C(CO2Me)C(OMe)}(CO)4 (5). Na[Re(CO)5], 0.50 g, 1.43 mmol; methanol, 0.60 mL, 14.8 mmol; yield, 0.43 g, 55%; bright orange solid. 1H NMR (400.1 MHz, CD3COCD3): δ 3.22 (s, 3H, CO2CH3), 4.70 (s, 3H, OCH3), 7.38 (d, J = 6.8 Hz, 1H, Np), 7.53 (m, 2H, Np), 7.61 (t, J = 7.6 Hz, 1H, Np), 7.81−7.97 (m, 3H, Np). 13 C{1H} NMR (100.6 MHz, CD3COCD3): δ 51.4 (s, CO2CH3), 71.8 (s, OCH3), 124.0, 126.6, 126.8, 127.5, 127.6, 127.7, 129.9, 130.1, 135.0, 148.2 (all s, C of Np), 156.9 (s, C(CO2Me)), 160.2 (s, CO2Me), 192.0 (s, trans-Re(CO)), 193.1 (s, Re(CO)), 194.8 (s, Re(CO)), 230.5 (s, ReC(Np)), 249.6 (s, ReC(OMe)). Elemental analysis (%) calcd for C20H13O7Re·H2O: C, 42.18; H 2.65. Found: C, 42.20; H, 2.72. Re{-C(Np)C(CO2Me)C(OnPr)}(CO)4 (9). Na[Re(CO)5], 0.50 g, 1.43 mmol; 1-propanol, 1.0 mL, 14.9 mmol; yield, 0.59 g, 71%; bright orange solid. 1H NMR (400.1 MHz, CD3COCD3): 1.14 (t, J = 6.8 Hz, 3H, OCH 2 CH 2 CH 3 ), 2.06 (q, J = 6.8 Hz, 2H, OCH2CH2CH3), 3.20 (s, 3H, CO2CH3), 4.85 (t, J = 6.8 Hz, 2H, OCH2CH2CH3), 7.35 (d, J = 7.2 Hz, 1H, Np), 7.50 (m, 2H, Np), 7.57 (m, 1H, Np), 7.80 (m, 1H, Np), 7.86 (m, 3H, Np). 13C{1H} NMR (100.6 MHz, CD3COCD3): δ 8.72 (s, OCH2CH2CH3), 21.8 (s, OCH2CH2CH3), 49.2 (s, OCH3), 85.2 (s, OCH2CH2CH3), 121.9, 124.4, 126.6, 125.4, 125.5, 125.6, 127.7, 127.9, 132.8, 146.0 (all s, C of Np), 155.0 (s, C(CO2Me)), 158.1 (s, CO2Me), 190.0 (s, transRe(CO)), 190.9 (s, Re(CO)), 192.6 (s, Re(CO)), 227.5 (s, ReC(Np)), 244.8 (s, ReC(OnPr)). Elemental analysis (%) calcd for C22H17O7Re: C, 45.59; H 2.96. Found: C, 45.45; H, 3.02. General Procedure for the Reactions of Rhenacyclobutadiene Complexes with HCCOEt. A solution of HCCOEt (40% solution in hexanes, 1.32 mL, 5.42 mmol) was added to a solution of rhenacyclobutadiene (1.80 mmol) in acetonitrile (80 mL). The resulting mixture was stirred in an ice bath for 1 h, then warmed slowly to room temperature and was stirred at room temperature for ca. 12 h to give a dark red brown suspension. A dark brown residue was obtained after removal of the solvent under reduced pressure. The mixture was purified by column chromatography on silica gel. The column was first eluted with hexane/Et2O (10:1) followed by hexane/ Et2O (1:5) to afford two fractions. The first fraction was a light yellow solution. The second fraction was collected as a light orange solution. Removal of the solvents under reduced pressure afforded the desired complexes as a yellow oil and a bright orange solid, respectively. Re{-C(Np)C(CO2Me)C(OMe)CHC(OEt)}(CO)4 (6). Prepared from Re{-C(Np)C(CO2Me)C(OMe)}(CO)4 (5), 0.99 g, 1.80 mmol; yield, 140 mg, 12.5%; bright orange solid. The eluents for column chromatography used for purification were mixtures of nhexane and diethyl ether [v/v = 1:5]. The fraction was a light orange solution. A bright orange solid was formed after refrigerating a solution of the crude mixture in n-hexanes overnight. Product was collected by filtration and dried under vacuum. 1H NMR (400.1 MHz, CD3CN): δ 1.56 (t, J = 6.8 Hz, 3H, OCH2CH3), 3.08 (s, 3H, CO2CH3), 4.08 (s, 3H, OCH3), 4.51 (q, J = 6.8 Hz, 2H, OCH2CH3), 6.31 (s, 1H, CH), 6.83 (d, J = 7.2 Hz, 1H, Np), 7.41−7.50 (m, 3H, Np), 7.57 (d, J = 8.4 Hz, 1H, Np), 7.70 (d, J = 8.0 Hz, 1H, Np), 7.85 (d, J = 8.4 Hz, 1H, Np). 13C{1H} NMR (100.6 MHz, CD3CN): δ 14.5 (s, OCH2CH3), 51.3 (s, CO2CH3), 58.6 (s, OCH3), 70.0 (s, OCH2CH3), 104.2 (s, CH), 117.9, 125.5, 125.6, 126.0, 126.6, 127.6, 127.8, 129.0, 134.4,

(Figure 6). The lower reactivity of the vinyl-substituted rhenacyclobutadiene products when compared with the starting rhenacyclobutadiene complexes appears to be related to the relative energies of LUMO of the metallacycle. As shown in Figure 7, compositions of the LUMOs as well as the NBO charges on the α-carbene carbons calculated for the complexes 21 and 28 are similar. However, the energy of the LUMO of 21 (−2.49 eV) is appreciably lower than that of 28 (−2.37 eV).

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CONCLUSIONS

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EXPERIMENTAL SECTION

Reactions of rhenacyclobutadienes Re{-C(Np)C(CO2R)C(OR′)}(CO)4 with HCCOEt produced both rhenabenzene complexes Re{-C(Np)C(CO 2 R)C(OR′)CHC(OEt)}(CO)4 and the new rhenacyclobutadienes with a pendant alkenyl substituent Re{-C(Np)C(C(OR′)CH(CO2Et))C(OR)}(CO)4. Formation of the vinyl-substituted rhenacyclobutadiene products is especially interesting, as the ethyl of the ester group in the products is from the alkyne HCCOEt, whereas the alkyl of the ReC(OR) group of the vinyl-substituted rhenacyclobutadiene products is from the ester group of the starting rhenacyclobutadienes, and the alkyl group in OR of the vinyl substituents of rhenacyclobutadiene products originates from the ReC(OR) group of the starting rhenacyclobutadienes. A theoretical study suggests that the vinyl-substituted metallacyclobutadienes are formed by initial direct nucleophilic attack of the β-carbon of HCCOEt on the carbene carbon of the rhenacyclobutadienes, which rearrange to vinyl-substituted rhenacyclobutadiene products via zwitterionic carbene intermediates. The reaction represents a new reactivity of metallacyclobutadienes. The structurally characterized rhenabenzenes reported in this work are also interesting as well-characterized metallabenzenes of group 7 metals are still rare.

General Comments. All manipulations were carried out at room temperature under a nitrogen atmosphere using standard Schlenk techniques, unless otherwise stated. Solvents were distilled under nitrogen from sodium benzophenone (n-hexane, cyclohexane, diethyl ether, and tetrahydrofuran), or calcium hydride (acetonitrile). The starting material Na[Re(CO)5] was synthesized by a literature procedure.14b Microanalyses were performed by M-H-W Laboratories (Phoenix, AZ). NMR spectra were collected with a Bruker AV-400 spectrometer (1H 400.1 MHz; 13C 100.6 MHz). Preparation of Methyl 3-(naphthalen-1-yl)propiolate. To a solution of 1-ethynylnaphthalene (2.10 g, 13.8 mmol) in THF (50 mL) was slowly added a solution of n-butyllithium (15.2 mmol, 2.0 M solution in cyclohexane solution] at −78 °C. After 1 h of stirring, methyl chloroformate (1.07 mL, 13.8 mmol) was added, and the mixture turned dark green immediately. The reaction mixture was stirred at this temperature for 30 min and was stirred for an additional 3 h at room temperature to give a brown suspension. This suspension was poured into a separating funnel and washed successively with water and brine. The organic layer was collected, dried with MgSO4, and filtered. The filtrate was concentrated under reduced pressure to give a dark brown oil which was subjected to chromatography with a silica gel column. The column was eluted with a mixture of n-hexane and dichloromethane [v/v = 5:1]. The fraction was a light yellow solution. A light yellow oil was obtained after removal of the solvents. Yield: 1.74 g, 60%. 1H NMR (400.1 MHz, CD3Cl): δ 3.89 (s, 3H, CH3), 7.30−8.33 (m, 7H, Np). 13C{1H} NMR (100.6 MHz, CD3Cl): δ 52.2 (s, CH3), 84.2 (s, CC), 84.5 (s, CC), 116.4, 124.4, 125.0, 126.3, 127.1, 127.9, 130.8, 132.3, 132.5, 133.0 (all s, C of Np), 154.0 (s, CO2Me). The spectroscopic data are in accordance with the published data.20 G

DOI: 10.1021/om501034e Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 156.7 (all s, C of Np), 137.8 (s, C(CO2Me)), 168.7 (s, CO2Me), 188.5 (s, trans-Re(CO)), 188.6 (s, C(OMe)), 188.9 (s, Re(CO)), 191.2 (s, Re(CO)), 205.4 (s, ReC(Np)), 261.1 (s, ReC(OEt)). Elemental analysis (%) calcd for C24H19O8Re: C, 46.37; H 3.08. Found: C 46.41, H 3.25. Re{-C(Np)C(C(OMe)CH(CO2Et))C(OMe)}(CO)4 (7). Re{C(Np)C(CO2Me)C(OMe)}(CO)4 (5), 0.99 g, 1.80 mmol; yield, 160 mg, 14.3%; yellow oil. The eluents for column chromatography used for purification were mixtures of n-hexane and diethyl ether [v/v = 10:1]. The fraction was a light yellow solution. A yellow oil was obtained after removal of the solvents. 1H NMR (400.1 MHz, CD3CN): δ 1.22 (t, J = 7.2 Hz, 3H, CO2CH2CH3), 3.07 (s, 3H, C(OCH3)), 4.06 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 4.47 (s, 3H, ReC(OCH3)), 4.98 (s, 1H, CH), 7.35 (d, J = 8.1 Hz, 1H, Np), 7.45− 7.55 (m, 3H, Np), 7.84 (d, J = 8.1 Hz, 1H, Np), 7.91 (d, J = 8.1 Hz, 1H, Np), 8.02 (d, J = 8.1 Hz, 1H, Np). 13C{1H} NMR (100.6 MHz, CD3CN): δ 13.2 (s, CO2CH2CH3), 54.8 (s, C(OCH3)), 58.7 (s, CO2CH2CH3), 68.8 (s, ReC(OCH3)), 93.4 (s, CH), 122.5, 124,8, 125.0, 125,1, 125.6, 126.2, 127.5, 127.8, 132.8, 145.1 (all s, C of Np), 159.6 (s, C(C(OMe)), 161.4 (s, C(OMe)), 165.9 (s, CO2Et), 190.0 (s, trans-Re(CO)), 191.3 (s, Re(CO)), 193.7 (s, Re(CO)), 213.7 (s, ReC(Np)), 244.1 (s, ReC(OMe)). Elemental analysis (%) calcd for C24H19O8Re: C, 46.37; H, 3.08. Found: C 46.40, H 3.20. Re{-C(Np)C(CO2Me)C(OnPr)CHC(OEt)}(CO)4 (10). Prepared from Re{-C(Np)C(CO2Me)C(OnPr)}(CO)4 (9), 0.95 g, 1.80 mmol; yield, 100 mg, 9%; bright orange solid. The eluents for column chromatography used for purification were mixtures of nhexane and diethyl ether [v/v = 1:5]. The fraction was a light orange solution. A bright orange solid was formed after refrigerating a solution of the crude mixture in n-hexanes overnight. The product was collected by filtration and dried under vacuum. 1H NMR (400.1 MHz, CD3CN): δ 1.00 (t, J = 6.8 Hz, 3H, OCH2CH2CH3), 1.55 (t, J = 6.8 Hz, 3H, OCH2CH3), 1.80 (q, J = 6.8 Hz, 2H, OCH2CH2CH3), 3.12 (s, 3H, CO2CH3), 4.26 (t, J = 6.8 Hz, 2H, OCH2CH2CH3), 4.47 (q, J = 6.8 Hz, 2H, OCH2CH3), 6.29 (s, 1H, CH), 6.83 (d, J = 7.2 Hz, 1H, Np), 7.43−7.50 (m, 3H, Np), 7.57 (d, J = 8.4 Hz, 1H, Np), 7.70 (d, J = 8.0 Hz, 1H, Np), 7.85 (d, J = 8.4 Hz, 1H, Np). 13C{1H} NMR (100.6 MHz, CD3CN): δ 11.1 (s, OCH2CH2CH3), 15.1 (s, OCH2CH3), 23.2 (s, OCH2CH2CH3), 51.8 (s, OCH3), 70.4 (s, OCH2CH3), 73.7 (s, OCH2CH2CH3), 105.2 (s, CH), 118.4, 126.0, 126.1, 126.5, 127.1, 128.1, 128.3, 129.5, 134.9, 157.2 (all s, C of Np), 138.5 (s, C(CO2Me)), 169.3 (s, CO2Me), 188.5 (s, C(OnPr)), 189.2 (s, Re(CO)), 189.5 (s, Re(CO)), 191.8 (s, trans-Re(CO)), 205.4 (s, ReC(Np)), 260.7 (s, ReC(OEt)). Elemental analysis (%) calcd for C26H23O8Re: C, 48.07; H 3.57. Found: C 47.80, H 3.95. Re{-C(Np)C(C(OnPr)CH(CO2Et))C(OMe)}(CO)4 (11). Re{C(Np)C(CO2Me)C(OnPr)}(CO)4 (9), 0.95 g, 1.80 mmol; yield, 150 mg, 12.8%; yellow oil. The eluents for column chromatography used for purification were mixtures of n-hexane and diethyl ether [v/v = 10:1]. The fraction was a light yellow solution. A yellow oil was obtained after removal of the solvents. 1H NMR (400.1 MHz, CD3CN): δ 0.17 (t, J = 6.8 Hz, 3H, OCH2CH2CH3), 0.91 (q, J = 6.8 Hz, 2H, OCH2CH2CH3), 1.23 (t, J = 6.8 Hz, 3H, OCH2CH3), 3.12 (m, 2H, OCH2CH2CH3), 4.10 (q, J = 6.8 Hz, 2H, OCH2CH3), 4.47 (s, 3H, CO2CH3), 4.95 (s, 1H, CH), 7.38 (d, J = 8.1 Hz, 1H, Np), 7.42−7.55 (m, 3H, Np), 7.82 (d, J = 8.1 Hz, 1H, Np), 7.89 (d, J = 8.1 Hz, 1H, Np), 8.00 (d, J = 8.1 Hz, 1H, Np). 13C{1H} NMR (100.6 MHz, CD3CN): 8.34 (s, OCH2CH2CH3), 13.3 (s, OCH2CH3), 20.5 (s, OCH2CH2CH3), 58.8 (s, OCH2CH3), 68.9 (s, OCH3), 69.4 (s, OCH2CH2CH3), 93.4 (s, CH), 122.9, 124.8, 125.1, 125.2, 125.6, 126.1, 127.4, 127.6, 133.0, 145.4 (all s, C of Np), 160.1 (s, C(C(OnPr)), 160.7 (s, C(OnPr)), 166.2 (s, CO2Et), 190.1 (s, trans-Re(CO)), 191.4 (s, Re(CO)), 193.7 (s, Re(CO)), 213.9 (s, ReC(Np)), 244.1 (s, ReC(OMe)). Elemental analysis (%) calcd for C26H23O8Re: C, 48.07; H, 3.57. Found: C 48.25, H 3.09. General Procedure for the Isolation of Amino-rhenacyclobutadiene Complexes. A mixture of rhenacyclobutadiene with pNH2C6H4OMe in THF was stirred at room temperature for 12 h to give a brown yellow solution. The solvent of the reaction mixture was removed under vacuum. A yellow solid was formed after refrigerating a

solution of the crude mixture in tetrahydrofuran/cyclohexane (1:30) for 2 days. The product was collected by filtration, washed with nhexane, and dried under vacuum. Re{-C(Np)C(C(OMe)CH(CO2Et))C(p-NHC6H4OMe)}(CO)4 (8). Re{-C(Np)C(C(OMe)CH(CO2Et))C(OMe)}(CO) 4 (7), 50 mg, 0.08 mmol; p-NH2C6H4OMe, 12 mg, 0.10 mmol; yield, 41 mg, 72%. 1H NMR (400.1 MHz, CD2Cl2): δ 1.20 (t, J = 7.2 Hz, 3H, CO 2 CH 2 CH 3 ), 3.24 (s, 3H, OCH 3 ), 3.87 (s, 3H, pNHC6H4OCH3), 4.08 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 5.04 (s, 1H, CH), 7.05 (d, J = 8.8 Hz, 2H, C6H4), 7.30 (d, J = 6.8 Hz, 1H, Np), 7.41−7.51 (m, 5H, Np and C6H4), 7.73 (d, J = 8.4 Hz, 1H, Np), 7.86 (d, J = 8.4 Hz, 1H, Np), 8.03 (d, J = 8.4 Hz, 1H, Np), 9.77 (s, 1H, NH). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 15.0 (s, CO2CH2CH3), 56.4 (s, p-NHC6H4OCH3), 56.6 (s, OCH3), 60.6 (s, CO2CH2CH3), 95.2 (s, CH), 115.5, 123.7, 125.8, 126.0, 126.1, 126.5, 127.0, 127.8, 128.3, 128.7, 134.3, 134.4, 146.3, 160.3 (all s, C of Np and Ph), 152.9 (s, C(C(OMe)), 162.7 (s, C(OMe)), 166.9 (s, CO2Et), 192.2 (s, Re(CO)), 192.3 (s, trans-Re(CO)), 195.1 (s, Re(CO)), 196.1 (s, ReC(Np)), 197.5 (s, ReC(p-NHC6H4OMe)). Elemental analysis (%) calcd for C30H24NO8Re·0.5C4H8O: C, 51.33; H, 3.77; N, 1.87. Found: C, 51.73; H, 4.18; N, 1.91. Re{-C(Np)C(C(OnPr)CH(CO2Et))C(pNHC6H4OMe)}(CO)4 (12). Re{-C(Np)C(C(OnPr)CH(CO2Et))C(OMe)}(CO)4 (11), 52 mg, 0.08 mmol; p-NH2C6H4OMe, 12 mg, 0.10 mmol; yield, 45 mg, 76%. 1H NMR (400.1 MHz, CD2Cl2): δ 0.35 (t, J = 7.2 Hz, 3H, OCH2CH2CH3), 1.11 (q, J = 7.2 Hz, 2H, OCH2CH2CH3), 1.23 (t, J = 7.2 Hz, 3H, CO2CH2CH3), 3.25 (t, J = 7.2 Hz, 2H, OCH2CH2CH3), 3.87 (s, 3H, p-NHC6H4OCH3), 4.12 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 5.02 (s, 1H, CH), 7.05 (d, J = 8.8 Hz, 2H, C6H4), 7.32 (d, J = 6.8 Hz, 1H, Np), 7.39−7.51 (m, 5H, Np and C6H4), 7.71 (d, J = 8.4 Hz, 1H, Np), 7.84 (d, J = 8.4 Hz, 1H, Np), 8.00 (d, J = 8.4 Hz, 1H, Np), 9.77 (s, 1H, NH). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 10.4 (s, OCH2CH2CH3), 15.1 (s, CO2CH2CH3), 22.3 (s, OCH2CH2CH3), 56.4 (s, p-NHC6H4OCH3), 60.6 (s, CO2CH2CH3), 71.2 (s, OCH2CH2CH3), 94.9 (s, CH), 115.4, 123.9, 125.8, 126.0, 126.1, 126.5, 126.9, 127.6, 128.3, 128.7, 134.2, 134.3, 146.6, 160.3 (all s, C of Np and Ph), 153.6 (s, C(C(OnPr)), 162.2 (s, C(OnPr)), 167.1 (s, CO2Et), 192.3 (s, Re(CO)), 192.4 (s, trans-Re(CO)), 195.1 (s, Re(CO)), 195.8 (s, ReC(Np)), 197.6 (s, ReC(p-NHC6H4OMe)). Elemental analysis (%) calcd for C31H26NO8Re: C, 51.88; H, 3.81; N, 1.89. Found: C, 51.90; H, 3.74; N, 2.08. X-ray Crystallography. Crystals of 6 and 10 were obtained by refrigerating a solution of the rhenabenzene in tetrahydrofuran/ cyclohexane (1:30) for 1 week, whereas those of 8 and 12 were grown by slowly evaporating the dichloromethane solvent from its saturated solution. The diffraction intensity data of 6, 8, 10, and 12 were collected with an Oxford Diffraction Gemini S Ultra X-ray Diffractometer with monochromatized Cu−Kα radiation (λ = 1.54178 Å) at 100 K. Diffraction data were collected and processed using the CrysAlisPro software (version 1.171.35.19). Empirical absorption corrections were performed using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm in the CrysAlisPro software suite. Structure solution and refinement for all compounds were performed using the Olex2 software package21 (which embedded SHELXTL).22 All the structures were solved by direct methods, expanded by difference Fourier syntheses and refined by full matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically with a riding model for the hydrogen atoms except those noted separately. The vinyl group C(OnPr)CH(CO2Et) in complex 12 is separated into two parts with a site occupancy ratio of 0.75:0.25. The aryl group Np in complex 12 is separated into two parts with a site occupancy ratio of 0.5:0.5. The deposition numbers CCDC-1009443 (6), 1009444 (8), 1009445 (10), and 1009446 (12) contain the supplementary crystallographic data for this paper. Further details of crystal data and refinements are given in Table S2 (Supporting Information). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Details. All structures were optimized in solvent without any constraint at the B3LYP level of density functional H

DOI: 10.1021/om501034e Organometallics XXXX, XXX, XXX−XXX

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Organometallics theory.23 Acetonitrile was employed as the solvent, according to the reaction conditions. The solute surface was defined by UAKS radii on the conductor polarizable continuum model (CPCM).24 The standard 6-31G* basis set was used for C, N, O, and H atoms. The effective core potentials (ECPs) of Lanl2dz was used to describe Re,25 with polarization functions Re(ζ( f) = 0.869) being added.26 Frequency calculations were also performed to identify all the stationary points as minima (zero imaginary frequency) or transition states (one imaginary frequency) and to provide free energies at 298.15 K. To reduce the overestimation of the entropy contribution for 2-to-1 transformations due to the overestimation of the calculated gas-phrase free energy, corrections of −2.6 kcal/mol in free energies were made base on the theory of free volume.27 This correction was employed in a number of earlier theoretical studies.28 All of the calculations were performed with the Gaussian 03 software package.29

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ASSOCIATED CONTENT

S Supporting Information *

Selected NMR chemical shifts for rhenacyclobutadiene complexes 7, 8, 11, and 12; details of X-ray crystallography of compounds 6, 8, 10, and 12; and details of Cartesian coordinates and electronic energies of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(I.D.W.) E-mail: [email protected]. *(Z.L.) E-mail: [email protected]. *(G.J.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Hong Kong Research Grant Council (Project Nos.: 602611, 601812, and CUHK7/CRF/ 12G-2).

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