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Jun 27, 2011 - corresponding substituent(s) (Scheme 1).4 It is believed that the formation of a cobaltacycle ... stand out as one of the most versatil...
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DFT Studies on the Reaction of CpCo(PPh3)2 with Diphenylphosphinoalkynes: Formation of Cobaltacycles and Cyclobutadiene-Substituted CpCoCb Diphosphines Chia-Ming Weng and Fung-E Hong* Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40227, Taiwan

bS Supporting Information ABSTRACT: Several adiabatic potential energy surfaces (PES) were employed for probing the processes of the formation of the cobaltacycles (η5-C5H5)(PMe3-kP)Co(1,4-C4(PPh2)2R2-1kC,4kC0 ) (3P) and their conversion to CpCoCb (4; (η5-C5H5)Co(η4-1,2-C4(PPh2)2R2)). The former species were prepared from the reactions of (η5-C5H5)Co(PMe3)2 (0_Me) with various diphenylphosphino-substituted alkynes, RCtCPPh2 (R = H, F, CF3, CH3, Ph, iPr). The conversion of 3P to the corresponding species 4 is more feasible for an alkyne with a bulky substituent (R = iPr, Ph) than with R = H. In contrast, two conformers, 3P and 4, are energetically quite different in the case of R = H. The former is energetically more stable than the latter: 60.2 vs 46.8 kcal/mol. Moreover, a high energy barrier (ca. 67.6 kcal/mol) is present which might retard the conversion rate for 3P to 4. Likewise, it is also true for the cases with strongly electron-withdrawing groups such as R = F, CF3. This mechanistic study has been done by using DFT methods, which provides an explanation for the experimental observation that showed the conversion of (η5-C5H5)(PPh3-kP)Co(1,4-C4(PPh2)2H2-1kC,4kC0 ) (3P_n_H) to (η5-C5H5)Co(η4-1,2-C4(PPh2)2H2) (4_n_H) was hardly achieved thermally.

1. INTRODUCTION The chemistry of sandwich-type compounds has been extensively studied since the discovery of ferrocene (η5-C5H5)2Fe (abbreviated Cp2Fe) in 1951.1 Later, the cobalt-containing counterpart sandwich compound (η5-C5H5)Co(η4-C4Ph4) (abbreviated CpCoCb), with each carbon of the four-membered ring being attached by a phenyl group, was first reported in 1961.2 As is known, one of the most common approaches for the synthesis of cyclobutadiene-substituted CpCoCb complexes is through the reaction of the air-sensitive half-sandwich compound (η5-C5H5)CoL2 (L= PPh3, CO)3 with alkyne(s) bearing the corresponding substituent(s) (Scheme 1).4 It is believed that the formation of a cobaltacycle (I) is through the oxidative coupling of two alkynes followed by the coordination of a PPh3 ligand to the cobalt center.5 Further reactions of the cobaltacycle with heteroatom sources lead to the formation of various cobaltcontaining sandwich compounds. Experimentally, four types of frequently observed cobalt complexes derived from cobaltacycle I are by way of (a) thermal conversion to CpCoCb (II),4 (b) treatment with carbon monoxide leading to the formation of the cyclopentadienone cobalt complex III,6 (c) reaction with another 1 equiv of alkyne, thereby yielding CpCo(η4-arene) complex IV7 (these types of complexes are believed to be the intermediates of the [2 + 2 + 2] cyclotrimerization of alkynes8), and (d) formation of the cobalt boryl complex V through a reaction with borane.9 For the past few decades, organophosphine ligands with various shapes and functions have been widely used in transition r 2011 American Chemical Society

Scheme 1. Reaction of (η5-C5H5)CoL2 (L= PPh3, CO) with Alkyne and Further Treatments with Heteroatom Sources

metal complex catalyzed cross-coupling reactions.10 1,10 -Bis(diphenylphosphino)ferrocene (1,10 -dppf) and its derivatives stand out as one of the most versatile and employed categories of metal-containing bidentate phosphine ligands.11 Recently, we were able to prepare a cobalt-containing phosphine ligand Received: January 3, 2011 Published: June 27, 2011 3740

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Organometallics analogous to dppf, (η5-cyclopentadienyl)(η4-1,2-bis(diphenylphosphino)-3,4-diphenylcyclobutadiene)cobalt (1,2-dppc) and its isomeric form 1,3-dppc (Figure 1). Both complexes can be regarded as derivatives of CpCoCb bearing phosphine ligands. Their capacity as legitimate and efficient bidentate ligands in several cross-coupling reactions was evaluated.12 Our previous work had demonstrated that the reaction of (η5cyclopentadienyl)bis(triphenylphosphine)cobalt (0_Ph) with phenyl(diphenylphosphino)acetylene (PhCtCPPh2) gave the cobaltacycles (η5-C5H5)(PPh3-kP)Co(1,4-C4(PPh2)2Ph2-1kC,4kC0 ) (3P_n_Ph) and (η5-C5H5)(PPh3-kP)Co(1,3-C4(PPh2)2Ph2-1kC,4kC0 ) (3P_a_Ph) (Scheme 2). Conformers with less hindered substituents on neighboring R2 and R3 positions are believed to be energetically more favorable. Thermolysis of 3P_n_Ph or 3P_a_Ph yielded the corresponding sandwich-type cyclobutadiene-substituted CpCoCb diphosphine (η5-C5H5)Co(η4-1,2-C4(PPh2)2Ph2) (4_n_Ph or 1,2-dppc) or (η5-C5H5)Co(η4-1,3-C4(PPh2)2Ph2) (4_a_Ph or 1,3-dppc), respectively. In contrast, the reaction of 0_Ph with a less bulky alkyne, HCtCPPh2, only led to the formation of the cobaltacycle (η5-C5H5)(PPh3-kP)Co(1,4-C4(PPh2)2H2-1kC,4kC0 )

Figure 1

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(3P_n_H). Unexpectedly, thermolysis of the cobaltacycle 3P_n_H did not yield the desired cis cyclobutadiene-substituted CpCoCb diphosphine, (η5-C5H5)Co(η4-1,3-C4(PPh2)2H2) (4_n_H), even at high temperature and prolonged reaction time. Rather, the oxidized products (η5-C5H5)(PPh3-kP)Co(η4-1,4-C4(OdPPh2)(PPh2)H2-1kC,4kC0 ) (3PO_n_H) and (η5-C5H5)(PPh3-kP)Co(η4-1,4-C4(OdPPh2)2H2-1kC,4kC0 ) (3POO_n_H) were observed as the major products accompanied by uncharacterized decomposition products. The first two compounds were isolated by chromatography and characterized by spectroscopic means as well as X-ray diffraction methods. Previously, similar queries concerning the formations of 3Plike derivatives and the mechanism for the conversion from 3P to 4 derivatives were raised and tackled by both experimental and theoretical methods. Nevertheless, to the best of our knowledge, none had mentioned this unexpected result: the thermal conversion of 3P_n_H to 4_n_H is almost inaccessible by only simply changing the substituent from Ph to H at the alkyne. It is of interest to us to investigate this distinct disparity from the subtle differences in the substituent of alkyne from different perspectives and methods, particularly by computational methods. The density functional theory method (DFT) has been proven repeatedly to be useful as a complementary tool with the experimental investigation in providing reliable results on the studies of the mechanisms of transition metal complex catalyzed reactions. It has also been widely employed in the investigation of catalysis executed by cobalt complexes. For example, Albright studied cobalt-catalyzed alkyne trimerization13 and Gandon investigated alkyne and alkene trimerization as well as hydrosilylation of alkenes.14 Koga examined the formation of 2-methylpyridine from alkyne and acetonitrile catalyzed by CpCoL2

Scheme 2. Preparation of Cobaltacycles 3P and Cyclobutadiene-Substituted CpCoCb Complexes 4 from the Reactions of CpCo(PPh3)2 (0_Ph) with Substituted Alkynesa

a

The terms “n” and “a” stand for conformers with two identical substituents in neighboring or alternate positions, respectively. 3741

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Scheme 3. Proposed Reaction Pathways for the Formation of Cobaltacycle 3P and Cyclobutadiene-Substituted CpCoCb 4 from 0_Me

(L= PPh3, CO).15 In the realm of the formation of cobalt complexes, Koga also probed the mechanism of formation of metal cyclopentadiene and cobalt benzene complexes.16 Moreover, the formation of Co and Ru metallocene complexes was also examined by Kirchner, Calhorda, and Veiros.17 In addition, Gleiter investigated the pathway to the formation of a CpCocapped benzoquinone complex.6b,18 Since DFT methods had been successfully employed in formerly known cobalt complexes, it seems to be appropriate for us to employ this broadly trusted technique in this study.

2. RESULTS AND DISCUSSION 2.1. Computational Methods Selected. All the calculations carried out in this work employed the Gaussian 09 package.19 Various kinds of functionals (B3LYP,20 BP86, B3P86,21 B3PW91,22 and M0623) were selected to evaluate the adequacy of the computation level. As revealed in Table S1 (Supporting Information), bond lengths calculated by the M06 method match up well with previously reported crystal data. Hereafter, the geometries of molecules were fully optimized by this method with the basis set LACVP(d,p), in which LANL2DZ24 including the double-ζ basis sets for the valence and outermost electrons and effective core potential for core electrons was used for Co and 6-31G(d)25 for the rest of the atoms, under C1 symmetry. For computing efficiency, PMe3 rather than PPh3 was selected as the acting phosphine. All the stationary points found were characterized via harmonic vibrational frequency analysis as minima (number of imaginary frequencies Nimag = 0) and transition states (Nimag = 1) and to obtain zero-point energies (ZEPs). The intrinsic reaction coordinate (IRC)26 is employed to ensure

the connectivity between the transition states and local minima. The molecular structures were optimized first. Subsequently, the optimized geometries were used for single-point calculations at the M06/6-311+G(2d,2p) level and the solvent effect (toluene) was taken into account using the polarized continuum model (PCM).27 Further, natural bond orbital analysis (NBO)28 and Wiberg indices (WI)29 were used for the population analysis. It should be noted that during the conversion of different conformations of cobalt complexes, the possibility of generating a triplet state in addition to the singlet state should not be overlooked. Many studies have shown that using the minimum energy crossing point (MECP) technique in examining the proposed mechanism involving state changes makes the results more reliable.30 Here, the MECP codes developed by Harvey and co-workers were employed.31 For computational efficiency, the conformations of molecules in MECP at the M06/LACVP(d,p) level were located first; then, single-point calculations at the M06/6-311+G(2d,2p) level were pursued. Throughout this article the abbreviations of the species involved in the reaction are as follows: C for cobalt complex and TS for transition state. In addition, superscripts 1 and 3 after the symbol of a conformation denote singlet and triplet states, respectively. 2.2. Mechanistic Study on the Reaction of (η5-C5H5)Co(PMe3)2 (0_Me) with Substituted Alkynes. The reactions of 0-derived complexes with alkynes which led to the formation of 1- and 2-related complexes have been extensively studied. First, it is believed that the substitution reaction of 0_Me with alkyne which yields 1 most likely undergoes a dissociative mechanism, since it is an electronically saturated compound (Scheme 3).32 Second, the replacement of another phosphine by alkyne which 3742

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Figure 2. Optimized structures of intermediates and transition states during the formation of 3P1 and 41 from 2. Selected bond lengths (Å) and angles (deg) are shown.

leads to the formation of 2 is also achieved through a similar mechanism. Spectroscopic data as well as crystal structures for compounds closely related to 1 and 2 were reported which substantiated the reliability of the reaction processes.33 Therefore, the geometrically optimized 2 has been deliberately designated as the starting conformer and as the energetic standard hereafter. The formation of intermediate 31 is believed to occur through oxidative coupling of two alkynes. As is known, conformers with less hindered substituents as the two R groups are more energetically favorable. Subsequently, the coordination of a PMe3 ligand to the cobalt atom of 31 leads to the formation of cobaltacycle 3P1 (Scheme 3). The cyclobutadiene-substituted CpCoCb complex 41 can be obtained from 31 by reductive transition. The possibility of generating intermediates with triplet states such as 33, 3P3, and 43 should not be overlooked and was examined as well. Indeed, conformers with different spin states can be converted to each other through minimum energy crossing points (MECPs) XP1, XP2, and XP3. 2.3. Detailed Description of the Molecular Conformations during the Process of Forming 31. The optimized conformation of 2_n_H shows that the average distances between cobalt and the carbons C(H) and C(PMe2) of alkynes are about 1.974 and 2.029 Å, respectively (Figure 2). The bond strengths of two triple bonds are reduce,d as is evidenced from the increase of the bond lengths to about 1.260 Å. In the transition state 2TS_n_H, the two hydrogen-attached carbons become closer

and the distance between the two carbons of the two alkynes is 1.861 Å. Subsequently, chemical bonding with a bond length of 1.473 Å is formed through the coupling of two carbons in 31_n_H. This cobalt cyclopentadiene complex conformer, with C1 symmetry, has all four carbons of the coupled butadiene moiety in nonequivalent environments. It is observed that one of the carbons, C(H), has an agostic interaction with the cobalt center. The bond distance between Co and C(H) is 2.120 Å, indicating that an extra covalent bond is beneficial to the stability of the presumably unsaturated 16e species 31_n_H. The PES for the conversion of 2 to 31 is shown in Figure 3. As revealed, the conversion from 2 to 2TS is an energy-demanding process when the substituents are on alternate positions in both cases for R = H, Ph. 2.4. Detailed Description of the Molecular Conformations in the Process of Forming Singlet State 3P1 or 41. The PES for the conversion of 31 to either 3P1 or 41 is shown in Figure 4. For clarity, only those cases with R groups in neighboring positions are depicted. For the case of R = H, 31TS_n_H is proposed as the transition state to the formation of 41_n_H from 31_n_H (Figure 2). First, a chemical bond is about to be formed between two carbons C(PMe2) in the transition state 31TS_n_H. The distance between these two carbons is 1.978 Å. The energy barrier for the conversion is 11.9 kcal/mol. Subsequently, this leads to the formation of 41_n_H, which has a four-electrondonating cyclobutadiene ring and is 42.3 kcal/mol more stable 3743

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Organometallics than 31_n_H. The average CC bond lengths for five- and fourmembered rings are 1.425 and 1.457 Å, respectively. Meanwhile, the average CoC(η5-C5H5) and CoC(η4-C4H2(PPMe2)2) bond distances are 2.058 and 1.961 Å, respectively. Alternatively, 31_n_H might be transformed to another product, 3P1_n_H, through the coordination of a PMe3 ligand to the Co center of the former. It is a greatly exothermic process (ca. 55.7 kcal/mol). The five atoms in the cobaltacyclopentadienyl ring are almost coplanar. The bond lengths of CoCR, CRCβ, and CβCβ are 1.947, 1.355, and 1.445 Å, respectively. This cobaltacyclic complex exhibits a quasi-Cs-symmetric geometry. Notably, 3P1_n_H is more stable than 41_n_H by the amount of 13.4 kcal/mol. This

Figure 3. Energy profiles for the formation of 31 from 2.

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could be one of the reasons that the conformer 3P1_n_H, a cobalt cyclopentadiene complex, was the dominating product from thermal reaction. 2.5. Detailed Description of the Molecular Conformations in the Process of Forming Triplet State 3P3 or 43. The accessibility for the conversion of a conformer from its singlet to triplet state was examined. As shown in Figure 4, the triplet state species 33_n_H is 12.1 kcal more stable than the singlet state species 31_n_H (4.5 vs 16.6 kcal/mol). The crossing point XP1_H, which connects these two states, is located slightly above 31_n_H by 1.7 kcal/mol (2.8 vs 4.5 kcal/mol). Thereby, it is almost effortless to overcome the energy barrier for the singlet state species 31_n_H to its corresponding triplet state species 33_n_H. The latter is found to have a virtual Cssymmetry geometry. Subsequently, the attack of PMe3 on the Co of 33_n_H leads to the formation of the metallacycle 3P3_n_H. Here in this stage, the triplet state is energetically more unfavorable (ca. 26.7 kcal/mol) than its corresponding singlet state. It is easy for the triplet state 3P3_n_H to convert to the singlet state 3P1_n_H through the crossing point XP2_n_H, since the energy gap is as low as 1.0 kcal/mol. The most distinct differences between the conformers of 3P1_n_H, XP2_n_H, and 3P3_n_H are the bond distances between cobalt atoms and the coordinated phosphines; they are 2.152, 2.568, and 2.476 Å, respectively. The Cs symmetry is removed in both 3P3_n_H and XP2_n_H as a result of the rotation of the methyl group on the trimethylphosphine ligand. The formation of 4 3 _n_H is through 33 TS_n_H from 3 3 _n_H. The CRCR bond distance is 2.132 Å in the transition state (Figure 5). This process is relatively more unfavorable than that of the singlet state in terms of higher energetic requirements. The conformation of 43_n_H exhibits cyclobutadiene with η2coordination toward the cobalt atom. The bond lengths between

Figure 4. Energy profiles for the reaction of 0_Me with substituted alkynes (RCtCPPh2; R = H, Ph) and the arrangements of substituents computed with the PCM model (toluene). The term “XPn” stands for the crossing point. 3744

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Figure 5. Optimized structures of intermediates and transition states for the formation of 3P3 and 43 from 33 and crossing points. Selected bond lengths (Å) and angles (deg) are shown.

two carbons of the η2-coordinated cyclobutadiene and the cobalt atom are about 2.002 Å. The environments of the four carbons of the cyclobutadiene are similar. The conversion of 33TS_n_H to 43_n_H is achieved by releasing 24.9 kcal/mol. Moreover, the change from a triplet state to a singlet state is readily accessible via XP3_n_H, since the energy gap between the two states is as low as 0.6 kcal/mol. As shown in Figure 4, the formation of 41 is much accessible from the singlet state 31 than from the triplet state 33, due to the lower energy barrier. By comparison, conversion to 3P1 from either the singlet or triplet state of 3 is readily accessible. 2.6. Variations of Energetic States Caused by Substituents R = H, Ph. As mentioned, the metallacyclic conformer 3P is the major product when R = H. This is a reasonable observation, due to the fact that 3P is more stable than its corresponding CpCoCb species 4 in terms of energetic stability (60.2 vs

46.8 kcal/mol). For comparison, in the case of R = Ph, both products, 3P and 4, are at rather similar energetic states (43.6 and 43.9 kcal/mol, respectively) (Figure 4). The formation of the product 41_n_Ph is readily accessible, since 31TS_n_Ph is merely 4.5 kcal/mol higher than 31_n_Ph. For the formation of 3P1_n_Ph, the first and most straightforward pathway is through the direct attack of 31_n_Ph by PMe3 at the cobalt center. For the secondary pathway, the singlet state 31_n_Ph is designated to convert to its corresponding triplet state 33_n_Ph through the connection of the cross point XP1_n_Ph. The conversion is readily achieved, since the energetic states for these three conformers are close to each other (1.5, 1.6, and 1.0 kcal/mol, respectively). Subsequently, the formation of 3P3_n_Ph is through the coordination of PMe3 to the cobalt center of 33_n_Ph. It is accompanied by an energy release of 16.6 kcal/mol. Finally, a direct transformation of the conformer from its triplet to its 3745

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Table 1. Calculated Energies (kcal mol1) of Various Conformers with Different Substituentsac

F CF3 H Me Ph i

Pr

XP3

4

3TS

3

XP1

3P

XP2

22.4 (23.6)

-56.7 (56.6)

-10.9 (11.5)

-24.9 (25.4)

24.8 (24.3)

-78.9 (80.9)

52.6 (52.5)

-22.0 (23.2)

-5.2 (11.5)

-32.9 (35.6)

-49.4 (50.3)

4.4 (19.1)

-3.9 (2.6)

-14.0 (14.2)

6.0 (20.4)

2.5 (0.1)

13.5 (12.9) 12.0 (12.1)

-46.8 (47.5)

7.4 (15.1)

-14.2 (13.2)

10.7 (13.9)

8.8 (8.2)

-46.3 (45.5)

11.2 (12.7)

3.9 (3.0)

9.1 (6.5)

-9.8 (9.4) -43.9 (41.6)

12.7 (12.0) 3.0 (12.1)

-2.3 (0.9) -1.5 (2.7)

-9.7 (7.5)

7.0 (10.0)

-1.6 (6.0)

-42.9 (45.6)

9.2 (12.4)

4.1 (7.6)

-9.5 (12.4)

7.6 (12.5)

1.9 (6.5)

6.7 (9.9)

-4.5 (3.8)

-59.5 (53.7) 4.5 (5.6)

-44.1 (46.6)

17.2 (18.6)

-18.1 (19.7) 2.8 (0.3)

-16.6 (13.2)

-60.2 (58.6)

32.5 (28.9)

-33.5 (30.3) 11.4 (7.1)

-43.8 (42.0)

18.2 (16.1)

1.0 (6.5)

-18.6 (16.6) -43.6 (40.2)

15.9 (4.9)

-18.2 (9.3) 6.4 (8.3)

-29.5 (27.4)

6.1 (11.0)

-6.4 (9.8)

a Relative energies are referenced to the corresponding reactants. b Values for conformers with two substituents on the alternate positions are shown in parentheses. c Values for singlet states are given in boldface, while those for triplet states are given in italics.

singlet state is linked by the cross point XP2_n_Ph. This species is in an energetic state (2.4 kcal/mol) slightly higher than that of 3P3_n_Ph. As revealed from the calculations, the energy barriers for the conversion of 31_n_Ph to 3P1_n_Ph or 41_n_Ph are similar and small. In addition, both of the products, 3P1_n_Ph and 41_n_Ph, are in closely energetic states. Therefore, it is predicted from the computational study that both conformers can be obtained in comparable quantities. Although crystal data for the conformer 3P1_n_Ph are not available, the fact that several closely related compounds have been isolated and structures determined by X-ray diffraction methods provides a strong support for this assertion.34 In contrast, the energy barrier for the conversion of 31_n_H to 41_n_H is higher than that to 3P1_n_H. In addition, the latter is more stable than 41_n_H. Thereby, it is likely that metallacycle 3P1 is the only dominating product other than CpCoCb 41. 2.7. Comparison of the Effects Caused by Different Substituents on the Alkynes. For the purpose of finding the factors that affect the stability of the product, substituents (R = F, CF3, Me, iPr) on the alkynes with varied electronic and steric effects were screened (Table 1). First, conformers 41_n_F and 41_n_H were chosen for a comparison of electronic effects. The former is much stable than the latter by 9.9 kcal/mol. This is obviously due to the electron-withdrawing capacity of F. In terms of steric effects, there is not much difference between 41_n_Me and 41_n_iPr, since these bulky substituents are separated from each other in the conformation of 41. Subsequently, the influence of electronic effects on 3P1 was examined by using either R = F or R = H. The electron-withdrawing group F on 3P1 causes the stability of 3P1_n_F to be as much as 18.7 kcal/mol greater than that of 3P1_n_H. In terms of steric effects, higher energetic states are found when the bulky substituent iPr is employed. They are 29.5 and 27.4 kcal/mol for 3P1_n_iPr and 3P1_a_iPr, respectively. These observations indicate that the types of substituents have more influence on both electronic and steric effects in conformer 3P1 than in 41. For all of the conformers, the lowest energetic states are obtained by using the electron-withdrawing group F as the substituent (Table 1). The conversion from 31_F to 3P1_F by the attack of PMe3 at the cobalt center of the former leads to a great release of energy: 78.9 and 80.9 kcal/mol for 3P1_n_F

and 3P1_a_F, respectively. Almost no energy barrier is required to be surmounted. In contrast, although it is an exothermic process, the conversion from 31_F to 41_F still requires an energy barrier of 14.0 kcal/mol to be overcome. In addition, the conformer 3P1_F is much more stable than 41_F energetically. The energy differences are 22.2 and 24.3 kcal/mol for nei- and alt-, respectively. Therefore, it seems reasonable to predict that 3P1_F rather than 41_F will be the major product under these conditions. As was mentioned, the steric effect is more severe in 3P1_iPr than in 41_iPr. In contrast to the previous cases, it takes only 5.24.8 kcal/mol to overcome the energy barrier for the conversion of 31_iPr to 41_iPr. In addition, conformer 41_iPr is even more stable than 3P1_iPr. Therefore, it is predicted that the former is the major product in the reaction rather than the latter. Notably, in the case of R = CF3, a strongly withdrawing and bulky group, the energetic states of both 41_CF3 and 3P1_CF3 are alike. It seems reasonable to expect that both conformers can be obtained in similar quantities. On the basis of the above observations, it seems reasonable to conclude that 3P1 is the major product when a less bulky and a more weakly electron withdrawing group at the alkyne is employed; in contrast, 41 is the dominating product when a bulky group is used at the alkyne. When a simultaneously bulky and a more strongly electron withdrawing group such as CF3 is used at the alkyne, both conformers might be obtained in similar quantities.

3. SUMMARY The reactions of CpCo(PMe3)2 with alkynes RCtCPPh2 (R = H, Ph) to give metallacyclic (3P1) and/or CpCoCb (41) derivatives have been examined by DFT methods. In the case of R = H, the metallacycle is greatly favored over CpCoCb in production. In contrast, both conformers can be obtained in comparable quantities for R = Ph, since the energetic states are close to each other. This theoretical examination is consistent with our previous experimental work. In addition, the influences of employing various substituted alkynes, RCtCPPh2 (R = F, CF3, Me, iPr), were also taken into account. This study shows that the reaction leads to the formation of metallacyclic derivatives with electron-withdrawing substituents and to the formation of CpCoCb derivatives with bulky groups. 3746

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’ ASSOCIATED CONTENT

bS Supporting Information. Structural parameters of two crystals compared with the results from the calculations via different levels of techniques selected (Table S1), a comparison of the relative energies of 31, 3P1 ,and 41 by two different theoretical methods, M06 and B3LYP (Table S2), and a table giving selected Cartesian coordinates for optimized geometries at the M06/ LACVP(d,p) level and single-point energies at the M06/6-311+G(2d,2p) level. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Fax: 886-4-22862547. E-mail: [email protected].

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