Article pubs.acs.org/Organometallics
Theoretical Study on Internal Alkyne/Vinylidene Isomerization in Group 8 Transition-Metal Complexes Miho Otsuka,† Noriko Tsuchida,*,†,§ Yousuke Ikeda,‡ Natacha Lambert,‡ Rina Nakamura,‡ Yuichiro Mutoh,‡,∥ Youichi Ishii,‡ and Keiko Takano*,† †
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Department of Chemistry and Biochemistry, Graduate School of Humanities and Sciences, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *
ABSTRACT: DFT calculations on the transformation reaction of the internal alkynes [CpM(PhCCC6H4R-p)(dppe)]+ (M = Fe, Os; R = OMe, CO2Et, Cl) to the corresponding vinylidenes were carried out. It was found that the isomerization of all complexes of group 8 elements studied in the present work, as well as the Ru complex that had already been reported in our previous study, proceeds via a direct 1,2aryl shift. For the Fe complex, two types of direct 1,2-aryl shifts (paths 1 and 2), which depend on the orientation of the alkyne/vinylidene parts, were found, and the activation free energy of path 2 is smaller than that of path 1. As for the Os complex, path 1 and another direct 1,2-shift, path 3, were obtained, and path 3 has smaller activation free energy, which is the case for the Ru complex. Therefore, the isomerization reaction of internal alkynes to vinylidenes proceeds through path 2 for the Fe complexes and path 3 for the Ru and Os complexes. The 1,2-migration reactions via path 2 for the Fe complex and path 3 for the Os complex were found to be nucleophilic, which is based on an orbital interaction corresponding to an electron transfer from a carbon on the migrating group to the atom being migrated, as well as for the Ru complexes. To evaluate the stability of the alkyne and vinylidene complexes, orbital interaction energies between an organometallic complex part and an internal alkyne or a vinylidene moiety were calculated by natural bond orbital (NBO) analysis. It was revealed that the Os complex has the strongest interaction, followed by the Ru and Fe complexes. Namely, both the internal alkyne complexes and the vinylidene complexes are more stabilized in a heavier metal complex. The activation free energy for migration of the aryl or phenyl group is actually the lowest for the Fe complex among the three metals. These findings contribute to the development of the synthetic strategy of vinylidenes from internal alkynes.
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INTRODUCTION Vinylidene is a high-energy tautomer of an alkyne and exhibits higher reactivity in comparison to the parent alkyne. This reactivity facilitates the use of vinylidene as intermediates in catalytic alkyne transformations. It is well-known that vinylidene is stabilized by coordination to a transition metal, and the relative stability of alkyne and vinylidene isomers in the coordination sphere of a transition metal is reversed in many cases.1 The alkyne/vinylidene isomerization on a transitionmetal complex has been exploited in the synthesis of a wide range of vinylidene complexes.2 For the development of the use of vinylidene, it is essential to establish a method for synthesizing various vinylidene complexes and to elucidate reaction mechanisms. Great effort has been devoted to both experimental and theoretical approaches to determine the mechanism underlying the transformation of terminal alkynes into the corresponding vinylidene complexes.3 Three general pathways are suggested for the terminal alkyne/vinylidene rearrangement (Scheme 1). © 2015 American Chemical Society
Scheme 1. Conversion Pathways of an Alkyne Ligand to a Vinylidene Isomer for Terminal Alkynes
Received: December 15, 2014 Published: August 3, 2015 3934
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Organometallics The initial formation of a transition-metal complex with an alkyne is in η2 binding mode, which is common among all pathways. After the formation of an alkyne complex, the reaction pathways are different depending on the type of the central metal. The d6 metal systems such as Ru(II) and Mn(I) complexes proceed via either a direct 1,2-hydrogen shift (path a in Scheme 1) or an indirect 1,2-hydrogen shift which has a σ complex as a well-defined intermediate (path b in Scheme 1).4,5 In contrast, the isomerization of the d8 metal systems such as Co(I) and Rh(I) complexes progresses via a 1,3-hydrogen shift with a hydride alkynyl intermediate (path c in Scheme 1).6 For the three pathways in Scheme 1, most theoretical calculations concluded that the hydrogen migrations are electrophilic reactions, while there has been a report that hydrogen migrates as a hydride toward the electron-deficient Cα for the terminal alkyne.7 Whereas the terminal alkyne/vinylidene rearrangement has been investigated extensively, conversion reactions from internal alkynes to vinylidenes are still uncommon processes. Although the reaction of heteroatom-substituted alkynes such as trimethylsilyl species8 has appeared in the literature, it is only recently that the migration of a carbon-based group on alkynes has been demonstrated to give the corresponding vinylidenes.9−11 Shaw et al. described that internal alkynones lead to the disubstituted vinylidene complexes [CpRu{C CR1(COR2)}(PPh3)2]+.9 Valerga et al. also succeeded in the synthesis of vinylidenes from internal alkynone complexes.10 Density functional theory (DFT) calculations were applied to their isomerization reactions from [TpRu{η1-OC(R)C CPh}(κ2P,N-iPr2PXPy)] (Tp = tris(pyrazolyl)borate; X = CH2, S; R = Me, Ph), in which they concluded that the isomerization reactions of internal alkynone/vinylidene are electrophilic on the basis of the natural bond orbital (NBO) charges. This is in the same manner as isomerization reactions of the terminal alkyne/vinylidene.10b According to the reports by some of the present authors, aryl- or alkyl-disubstituted vinylidene compounds have been obtained by the reaction of [CpRu(dppe)]+ and [CpFe(dppe)]+ with PhCCC6H4R-p (R = OMe, Me, H, Cl, CO2Et), revealing the migratory activity of the substituents with 13C NMR.11b,c The relative migratory aptitude increases in the order CO2Et > Cl > H > Me > OMe (see Schemes 2 and 3),11b,c which suggests that aryl 1,2-migration proceeds as an electrophilic reaction. However, our recent DFT calculation studies on [CpRu(PhCCC6H4R-p)(dppe)]+ (R = OMe, Cl, CO2Et) clarified that the migration reaction is not electrophilic
Scheme 3. Migratory Aptitude for Alkyne Substituents on an Fe Complexa
a
The asterisks represent 13C-enriched carbon atoms.
but rather nucleophilic.12 Whereas these recent findings of the internal alkyne/vinylidene isomerizations have been demonstrated to be effective in the synthesis of various vinylidenes, the effects of a central metal on the reaction remain unrevealed. Some of the present authors already reported differences in reaction time between the Ru and Fe complexes. In particular, the migration reaction proceeds more quickly for the Ru complex with Me and CO2Et substituents than for the corresponding Fe complex (Schemes 2 and 3).11b,c This could be due to the metal dependence of reaction pathways. In fact, the metal affects a reaction pathway of the terminal alkyne/vinylidene isomerization, as the fundamental intention for using a metal is to stabilize the vinylidene.1,4−6 Therefore, it is almost certain that an interaction between a metal and a vinylidene (or an alkyne moiety) depends on the kind of metal. Recently, the effect of metal on energy balance between transition-metal-coordinated alkynes and vinylidenes was investigated systematically, and it was reported that group 7− 9 transition metals in low oxidation states are most likely to stabilize the vinylidene form, whereas group 10 and 11 transition metals favor the alkyne form.13 To enhance the availability of uncommon internal alkyne/ vinylidene isomerization, the effects of a transition metal (M) and a substituent (R) on the reaction should be revealed. We report the computational results of DFT calculations on the transformation reaction of internal alkyne complexes having group 8 transition metals [CpM(PhCCC6H4R-p)(dppe)]+ (M = Fe, Ru, Os; R = OMe, CO2Et, Cl). The metal dependence of the reaction pathways and interactions between a metal and the alkyne/vinylidene moiety is addressed. The transition state structures and activation energies are also discussed to investigate the reaction mechanism and the substituent (R) effect.
Scheme 2. Migratory Aptitude for Alkyne Substituents on an Ru Complex Reported in Ref 11ba
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a
RESULTS AND DISCUSSION Geometries and Energy Profiles for Three Reaction Paths. Two kinds of reaction pathways were found for each metal complex. For Ru complexes, paths 1 and 3 in Figure 1 were found at the B3PW91/SDD+6-31G(d) level in our previous study.12 The structures obtained for the Ru complexes were used as initial geometries for the transition state (TS) search calculation on the Os and Fe complexes. For the Os complexes, paths 1 and 3 were obtained, as expected. In the case of the Fe complexes, paths 1 and 2 were found, but path 3 was not found. A given initial geometry for the TS of path 3 led to path 2. All paths (paths 1−3) proceed through the direct 1,2-
The asterisks represent 13C-enriched carbon atoms. 3935
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Figure 1. Optimized structures of the stationary points in each reaction path at the B3PW91/SDD+6-31G(d) level (paths 1 and 3, M = Os, R = OMe; path 2, M = Fe, R = OMe). Hydrogen atoms are omitted for clarity.
stable than those of path 1 (perpendicular type) by 2.5−5.0 kcal/mol (see GR in Table 1). Therefore, it is difficult to compare ΔGR⧧ values of the Fe complexes in a straightforward manner between paths 1 and 2 as a reaction route. If the rotation barrier from perpendicular to coplanar is higher than the barrier of aromatic ring transfer, path 2 is unfavorable. Rotation barriers of alkyne on the Fe complexes using the scan calculations depending on the dihedral angle were estimated to be 9.3 kcal/mol for R = OMe and 9.4 kcal/mol for R = CO2Et, respectively (see Figure S1 in the Supporting Information), and these values are lower than the activation energies of path 1, which has a perpendicular type reactant (OMe, 17.6 kcal/mol; CO2Et, 17.6 kcal/mol). Thus, it should be considered that path 2 is the most plausible reaction pathway for the Fe complexes. For the Ru and Os complexes, path 3 is the most plausible pathway, since paths 1 and 3 have the same perpendicular orientation as a reactant. The path with the smallest ΔGR⧧ value is the Ph migration for R = OMe (Fe, 6.9 kcal/mol; Ru, 13.8 kcal/mol; Os, 12.6 kcal/mol) and the Ar migration for R = CO2Et (Fe, 7.5 kcal/ mol; Ru, 15.1 kcal/mol; Os, 16.3 kcal/mol) (Table 1). These calculation results are consistent with the experimental findings that Ph migration is kinetically more favorable than Ar migration for R = OMe (Ar:Ph = 2:98 (Fe), 6:94 (Ru), 3:97 (Os)) and less favorable for R = CO2Et (Ar:Ph = 90:10 (Fe), 86:14 (Ru), N/A (Os)) (see Schemes 2 and 3, and the description for compound 2 in the Experimental Section). Furthermore, the ΔΔGR⧧ values calculated for Ar and Ph migration provide more information about the preference of the migration. The ΔΔGR⧧ values calculated from the ΔGR⧧ values in Table 1 indicate that Ph migration has an advantage over Ar migration for R = OMe by 3.8 kcal/mol (= 10.7 − 6.9, Fe), 1.9 kcal/mol (= 15.7 − 13.8, Ru), and 4.3 kcal/mol (= 16.9 − 12.6, Os), which is consistent with the smaller difference of migratory aptitude for M = Ru. Furthermore, Ar migration has an advantage over Ph migration for R = CO2Et by 2.5 kcal/ mol (= 10.0 −7.5 , Fe) and 1.8 kcal/mol (= 16.9 − 15.1, Ru), and experimental results show that the Fe complex has a greater
shift of the aromatic carbon substituent and have only one TS, as is the case with the previous studies.10b,12 The alkyne moiety (ArCCPh) of the reactant for path 2 and TS for paths 2 and 3 has coplanar coordination to the Cp−M bond, while it has perpendicular coordination in the other structures, as shown in Figure 1. These orientations are denoted as coplanar and perpendicular types, respectively. It is shown in Figure 1 that paths 2 and 3 differ in the reactant structures, which have the coplanar and the perpendicular types, respectively. The dihedral angles γ(PMC1C2) of the reactant structure for each path are given in the Supporting Information. The coplanar type complexes in paths 2 and 3 have structural isomers that depend on the positions of the aryl and phenyl groups against the cyclopentadienyl (Cp) ring. The letters “P” and “A” represent phenyl group positions that are on the same and opposite sides of the Cp, respectively (see Scheme 4). These results indicate that the reactant structures, or the reaction pathways, can differ according to the kind of metal. Scheme 4. Two Geometric Isomers for the Alkyne Complex, Denoted as “P” and “A”, Which Correspond to the Positions of Ph and Ar Groups with Respect to the Cp Group
Table 1 gives relative energies to the most stable reactant complex in each substituent and Gibbs free energy barriers, ΔGR⧧ = GTS − GR, for each path. On comparison of the energy barriers (ΔGR⧧), path 2 for the Fe complexes and path 3 for the Ru and Os complexes have ΔGR⧧ values lower than that of path 1 for any substituents (R). For the Fe complexes, the reactants have different orientations of the alkyne between paths 1 and 2, as shown in Figure 1. The reactants of path 2 (coplanar type) are less 3936
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Table 1. Relative Energies to the Most Stable Reactant Complex (GR, GTS, GP) and Gibbs Free Energy Barriers (kcal/mol) (ΔGR⧧ = GTS − GR) for Each Path in Three Metal Complexes at the B3PW91/SDD+6-31G(d) Level Ar migration R OMe
path
GR
GTS
GP
ΔGR⧧
path
Fe
1 2P 2A 1 3P 3A 1 3P 3A 1 2P 2A 1 3P 3A 1 3P 3A 1 2P 2A 1 3P 3A 1 3P 3A
0.0 3.1 2.5 0.6 0.0 0.6 0.6 0.6 0.0 0.0 4.4 5.0 1.0 1.4 1.8 1.3 0.6 0.6 0.0 3.8 4.4 0.6 0.6 1.3 0.6 0.0 0.0
17.6 13.8 13.2 22.0 18.2 16.3 23.8 20.1 16.9 17.6 14.4 12.6 22.6 20.1 16.9 23.8 22.0 16.9 18.2 13.2 13.2 22.6 18.8 17.6 24.5 20.7 18.2
−9.4 −9.4 −8.8 −7.5 −8.8 −8.8 −10.7 −11.3 −11.3 −8.2 −8.2 −8.2 −7.8 −7.2 −7.8 −11.9 −11.3 −12.6 −8.8 −8.8 −8.8 −6.9 −8.2 −6.9 −11.9 −12.6 −11.3
17.6 10.7 10.7 21.3 18.2 15.7 23.2 19.5 16.9 17.6 10.0 7.5 21.6 18.6 15.1 22.6 21.3 16.3 18.2 9.4 8.8 22.0 18.2 16.3 23.8 20.7 18.2
1 2P 2A 1 3P 3A 1 3P 3A 1 2P 2A 1 3P 3A 1 3P 3A 1 2P 2A 1 3P 3A 1 3P 3A
Ru12
Os
CO2Et
Fe
Ru12
Os
Cl
Ph migration
metal
Fe
Ru12
Os
GR
GTS
2.5 3.1
9.4 11.9
0.0 0.6
13.8 16.3
1.3 0.6
13.8 16.9
3.8 5.0 0.0 1.3 1.9
13.8 15.7 23.8 18.2 20.7
0.0 0.6
19.5 22.6
3.8 4.4 0.0 0.6 1.3
12.6 14.4 23.8 17.6 20.1
1.9 0.6
19.5 20.7
GP
ΔGR⧧
−8.8 −8.8
6.9 8.8
−7.5 −6.9
13.8 15.7
−10.0 −10.7
12.6 16.3
−8.2 −8.2 −6.9 −8.2 −7.5
10.0 10.7 23.8 16.9 18.8
−11.9 −11.9
19.5 22.0
−8.8 −8.8 −8.2 −8.2 −6.9
8.8 10.0 23.8 16.9 18.8
−11.9 −11.3
17.6 20.1
N/A
N/A
N/A
N/A
N/A
N/A
N/A
cannot be directly compared to those in the reaction time reported experimentally. In the previous study on the Ru complexes, it was noted that whether the reactant structure is P or A could be a causative factor in the preference of the reaction paths, Ph migration and Ar migration.12 Similar tendencies were found in the case of Fe and Os complexes, namely, the difference in ΔGR⧧ values between Ph migration and Ar migration depends on the reactant structure. In most cases, reactant structures P and A show a preference, respectively, to Ph migration and Ar migration (see Table 1). As for the Os complexes, for example, with the CO2Et substituent, the ΔGR⧧ values indicate that Ph migration has an advantage over Ar migration by 1.8 kcal/mol (= 21.3 − 19.5) in path 3P, while Ar migration has a smaller ΔGR⧧ value than Ph migration in path 3A by 5.7 kcal/mol (= 22.0 − 16.3). In the case of R = Cl, Ph migration has an advantage over Ar migration by 3.1 kcal/mol (= 20.7 − 17.6) in path 3P and Ar migration has an advantage over Ph migration by 1.9 kacl/mol (= 20.1 − 18.2). This is the case for the Fe complexes. Even though there are a small number of exceptions such as path 3A for Os complex with OMe and path 2P for Fe complex with CO2Et, the preference of the reaction pathway, eventually, the ratio of Ph migration to Ar migration, depends on whether the geometric isomer of the reactant is P or A for the Fe and Os complexes as well as for the Ru complex Electronic Features of the 1,2-Migration. In a previous study, NBO analysis14 for the TS structure of the Ru complexes revealed that the migration reaction is nucleophilic, and whether the preferable migration group is Ph or Ar is dependent on the interaction energy between donor and
propensity for Ar migration. In the case of R = Cl, there is only a slight difference in the migration aptitude between the Ar migration and Ph migration (Ar:Ph = 59:41 (Fe), 55:45 (Ru), 51:49 (Os)). The ΔGR⧧ values of paths 2 and 3 for R = Cl are almost the same between Ar migration (Fe, 8.8 kcal/mol; Ru, 16.3 kcal/mol; Os, 18.2 kcal/mol) and Ph migration (Fe, 8.8 kcal/mol; Ru, 16.9 kcal/mol; Os, 17.6 kcal/mol), which are also in good agreement with the laboratory findings. These correspondences ensure the present calculation results that the isomerization reactions of the internal alkynes to vinylidenes proceed through path 2 for the Fe complexes and path 3 for the Ru and Os complexes. On comparison of the lowest GTS (or ΔGR⧧) in each metal complex, the energies are similar between the Ru complexes and Os complexes, while the Fe complexes have a lower value for any substituent. As for the Gibbs free energy of products (GP), it is shown that the Fe and Ru complexes have comparable values and the Os complexes have the most negative product energy, indicating that they are thermodynamically most favorable (Table 1). As shown in Schemes 2 and 3, the migration reaction proceeds faster for the Ru complex with an CO2Et substituent than the corresponding Fe complex (Fe, 24 h; Ru, 12 h), but the calculated values show the lower energy barrier for the Fe complex. This inconsistency can be attributed to the fact that the reactant in the present computational study is the alkyne complex, and the formation process of an alkyne complex from an alkyne ligand and transition-metal complex is not considered. Therefore, the differences in calculated reaction energy barriers among the different kinds of metal complexes 3937
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Table 2. Orbital Interaction Energies Based on the Second-Order Perturbation Theory Analysis in the NBO Basis and Orbital Compositions (s, p, d) of the Acceptor Carbon Orbital, LP*(C1), for TS Structures R2
entry
donor
acceptor
metal
ΔEi→j (kcal/mol)
εi − εj (au)
F(i,j)
H
OMe
1
Ar
OMe
H
2
Ph
H
CO2Et
3
Ar
CO2Et
H
4
Ph
H
Cl
5
Ar
Cl
H
6
C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3 C2−C3
C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1
Fe Ru Os Fe Ru Os Fe Ru Os Fe Ru Os Fe Ru Os Fe Ru Os
230.2 232.0 207.0 44.0 39.3 26.4 76.5 63.9 39.4 126.6 144.3 118.1 94.9 94.9 39.2 75.0 103.3 85.6
0.61 0.63 0.66 0.81 0.83 0.86 0.77 0.80 0.84 0.72 0.72 0.74 0.75 0.77 0.83 0.77 0.76 0.78
0.36 0.37 0.35 0.18 0.18 0.15 0.23 0.22 0.18 0.29 0.31 0.28 0.26 0.26 0.18 0.23 0.27 0.25
R
migration group
OMe
Ph
CO2Et
Cl
R1
hybrids for LP*(C1) s, p, d (%) 7.67, 7.63, 6.02, 0.26, 0.26, 0.12, 0.90, 0.67, 0.21, 2.48, 3.37, 2.15, 1.50, 1.65, 0.20, 0.80, 1.96, 1.26,
92.18, 92.22, 93.85, 99.71, 99.72, 99.88, 99.03, 99.28, 99.77, 97.41, 96.52, 97.76, 98.42, 98.27, 99.79, 99.15, 97.96, 98.69,
0.15 0.15 0.13 0.03 0.02 0.00 0.07 0.05 0.02 0.11 0.12 0.09 0.08 0.08 0.01 0.06 0.08 0.05
Scheme 5. Numbering for Each Carbon Atom in the Reaction Center
Table 3. Orbital Interaction Energies Based on the Donor−Acceptor Analysis in NBO Basis between an Organometallic Complex Part and an Internal Alkyne Moiety at the Reactant Structure ΔEi→j (kcal/mol) R
donor
acceptor
OMe
LP(M) BD(C1−C2)
BD*(C1−C2) LP*(M)
BD(C1−C2)
BD*(M−P)
BD(C1−C2) LP(M) BD(C1−C2)
BD*(M−Cp) BD*(C1−C2) LP*(M)
BD(C1−C2)
BD*(M−P)
BD(C1−C2) LP(M) BD(C1−C2)
BD*(M−Cp) BD*(C1−C2) LP*(M)
BD(C1−C2)
BD*(M−P)
BD(C1−C2)
BD*(M−Cp)
CO2Et
Cl
εi − εj (au)
Fe
Ru
Os
132.8
72.4 156.4 21.7
111.7 314.5 72.0 44.8 40.8 139.0 120.4 323.5 73.8 47.0 42.5 144.0 121.5 324.5 72.3 47.7 44.3 147.6
159.1
157.3 35.3
69.7 76.6 159.9 21.5
70.6 76.0 160.7 21.8
70.5
acceptor and on the s orbital component of the acceptor carbon.12 In the case of the Ru complexes, the path with higher interaction energy and greater s character has higher migration ability in comparison to the other path, since s character increases the spatial freedom of the reaction. To elucidate the
F(i,j)
Fe
Ru
Os
0.85
0.62 0.88 1.06
0.56 1.40 1.40 2.22 2.26 1.01 0.55 1.41 1.40 2.17 2.25 1.00 0.55 1.47 1.36 2.15 2.27 0.98
0.89
0.89 0.75
0.71 0.61 0.89 1.07
0.71 0.61 0.89 1.07
0.71
Fe
Ru
Os
0.302
0.191 0.346 0.144
0.224 0.619 0.302 0.308 0.296 0.353 0.230 0.630 0.306 0.312 0.302 0.358 0.231 0.644 0.298 0.313 0.310 0.359
0.340
0.336 0.149
0.204 0.195 0.352 0.144
0.206 0.194 0.352 0.145
0.206
features of the migration reaction in the Fe and Os complexes, an orbital interaction analysis was performed for the TS structure of the plausible reaction pathways of each metal complex. Table 2 gives orbital interaction energies based on a second-order perturbation theory analysis in the NBO basis and 3938
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Table 4. Orbital Interaction Energies Based on the Donor−Acceptor Analysis in NBO Basis between an Organometallic Complex Part and a Vinylidene Moiety at the Product Structure ΔEi→j (kcal/mol)
εi − εj (au)
F(i,j)
R
donor
acceptor
Fe
Ru
Os
Fe
Ru
Os
Fe
Ru
Os
OMe
LP(M) BD(M−C2) BD(M−P)
LP*(C2) LP*(M) BD*(M−C2)
34.1
88.2
0.60
0.42
0.176
BD(M−C2)
BD*(M−P)
BD(M−C2) LP(M) BD(M−C2) BD(M−P)
BD*(M−C2) LP*(C2) LP*(M) BD*(M−C2)
36.5
0.59
1.40 1.40 1.46 1.48 1.53 0.41
0.37 1.45 2.59 2.55 2.47 2.41 2.74 0.36
0.130
64.8 65.5 62.9 63.4 30.8 93.2
125.0 46.9 233.7 267.3 192.7 190.1 149.2 138.8
0.134
0.276 0.278 0.281 0.284 0.201 0.179
0.194 0.243 0.711 0.754 0.635 0.624 0.590 0.202
BD(M−C2)
BD*(M−P)
BD(M−C2) LP(M) BD(M−C2) BD(M−P)
BD*(M−C2) LP*(C2) LP*(M) BD*(M−C2)
BD(M−C2)
BD*(M−P)
BD(M−C2)
BD*(M−C2)
CO2Et
Cl
65.7 66.2
36.7
30.8 92.5 66.2
30.8
94.2 283.8 179.2 217.6 170.4 130.2 46.5 228.7 261.5 190.1 186.7 147.5
orbital compositions (s, p, d) of the acceptor carbon orbital for TS structures. C1 denotes the carbon to which the migration group moves, and C3 denotes the carbon that migrates (for the numbering in detail, see Scheme 5). There is an interaction between C2−C3 as an electron donor and C1 as an electron acceptor for all the reactions, although the energy values are different from each other. This orbital interaction corresponds to the electron transfer from a carbon on the migrating group to the atom being migrated. Thus, the migration reaction for the Fe and Os complexes is also nucleophilic. Furthermore, in analogy with the Ru complex, the migratory aptitude is correlated to the magnitude of the interaction energy and the s orbital component. In the case of R = OMe, the energy of charge transfer from the donor (C2−C3) on the phenyl side to the acceptor C1, 207.0−232.0 kcal/mol (Table 2, entry 1), is much larger than that for Ar migration (entry 2, 26.4−44.0 kcal/mol). The amount of s orbital component of the acceptor carbon in Ph migration (6.02− 7.67) is also higher than that in Ar migration (0.12−0.26). Greater interaction energies and s characters of C1 atom are advantageous for Ph migration. For the complex with R = CO2Et, entry 4 is dominant. This result is consistent with the experimental results that the ratio of Ar migration is higher than that of Ph. In the case of R = Cl, which has almost the same aptitude for Ar and Ph migrations, there is not much difference in the orbital interaction energies as well as in the s character. However, it does not seem that there is any relationship between these characters, orbital interaction energy and the orbital composition, and the kind of metal. Further details of the calculation results and descriptions of the TS structures are reported in the Supporting Information (Tables S2 and S3). Orbital Interactions between Complexes and Internal Alkynes or Vinylidenes. To estimate the stability of the alkyne and vinylidene complexes, interactions between an internal alkyne or a vinylidene moiety and the other parts of organometallic complexes have been investigated for the
1.40 1.40
0.59
1.54 0.41 1.40
1.53
2.83 2.71 2.23 2.48 2.89 0.36 1.45 2.58 2.55 2.44 2.37 2.74
0.278 0.279
0.134
0.201 0.178 0.279
0.201
0.465 0.793 0.565 0.669 0.640 0.196 0.242 0.703 0.745 0.626 0.612 0.586
reactant and product complexes of the most favorable pathway. The calculation results for internal alkyne complexes and vinylidene complexes are given in Tables 3 and 4, respectively. Numbering for each atom refers to that shown in Scheme 5 (reactant or product), and the interaction energies (ΔEi→j) more than 20 kcal/mol are given in the tables. As shown in Table 3, quite a number of interactions are given for the Os complexes among the three kinds of complexes. Furthermore, the Os complexes have the largest values among the same type of orbital interactions and are followed by Ru and Fe complexes in all types of orbital interactions. For example, the interaction energies between BD(C1−C2) as a donor and LP*(M) as an acceptor in the complex, which correspond to π donation, are as follows: 314.5 kcal/mol for Os, 156.4 kcal/mol for Ru, and 132.8 kcal/mol for Fe in the case of R = OMe. The interactions corresponding to π back-donation, LP(M) as a donor and BD*(C1−C2) as an acceptor, are also in the same order: 111.7 kcal/mol for Os, 72.4 kcal/mol for Ru, and 0.08 kcal/mol (not given in Table 3) for Fe. The extremely small value for Fe (0.08 kcal/mol) is due to the small orbital overlap between LP(M) and BD*(C1−C2), which should be related to different reactant structures, namely coplanar (Fe) and perpendicular (Ru and Os) (Figure S2 in the Supporting Information). The tendency that the heavier metal has a stronger interaction for the same donor−acceptor combination agrees with the previous studies by Pandey et al.15 and by Zeng and his coworkers.16 Pandey et al. revealed by energy decomposition analysis that the π back-bonding of M−PNR2 (M = Fe, Ru, Os; R = Me, iPr) increases upon going from Fe to Os in the electrophilic phosphinidene complexes.15 Zeng et al. reported that the interaction energies between [MBrCl(CO)(PMe3)2]+ and BO− are 154.7, 149.1, 141.7, 130.9, and 123.6 kcal/mol for the Pt−BO, Ir−BO, Rh−BO, Pd−BO, and Ni−BO bonds, respectively, by using DFT calculations.16 Stronger interactions between the complex and the alkyne ligand contribute to stabilize the reactant complex. These results are consistent with 3939
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Organometallics the reaction energy barriers (ΔGR⧧) in the present study, which increase upon going from Fe to Os (see Table 1). The orbital energy differences (εi − εj) increase for heavier metal complexes, and the overlap (F(i,j)) also increases in the same order. As indicated by the equation of second-order perturbation to evaluate interaction energy14 E = ΔEij = qi
vinylidenes were performed to clarify the reaction mechanism and to discuss the effects of transition metals (M) and substituents (R) on the reaction. Two types of 1,2-migration pathways were obtained for each metal complex, which are paths 1 and 2 in the Fe complexes and paths 1 and 3 in the Ru and Os complexes, which differ from each other in their TS structures. The activation free energy, ΔGR⧧, of paths 2 and 3 is smaller than that of path 1 for any substituent (R). Therefore, the isomerization reaction of internal alkynes to vinylidenes proceeds through a direct 1,2shift. Moreover, it was clarified that the activation free energy for migration is lower for the Fe complexes. This is due to the differences in the reactant structures, which are coplanar type for the Fe complexes and perpendicular type for the Ru and Os complexes. The NBO analysis of the TS structure unveiled the reaction mechanism. There is an orbital interaction between C2−C3 as an electron donor and C1 as an electron acceptor, which corresponds to the electron transfer from a carbon on the migrating group to the atom being migrated. Thus, the migration reaction for the Fe and Os complexes is found to be nucleophilic, as well as the Ru complexes in our previous study.12 Orbital interaction energies between the internal alkyne moiety and the other parts of organometallic complexes estimated by NBO analysis show that the Os complexes have the strongest interaction, followed by the Ru and Fe complexes. Stronger interactions between the complex and the alkyne ligands contribute to stabilize the reactant complex. The interaction energies between the vinylidene moiety and the other parts of organometallic complexes were also evaluated by NBO analysis, and a similar tendency for the reactant complex was observed. Therefore, it was clarified that both the internal alkyne and the vinylidene moieties are stabilized well with a heavier metal complex. The vinylidene complexes are key products for a variety of organic compounds, and the development of novel complexes with new methodologies is of great importance. The present work gives a new way for the synthetic strategy of vinylidenes.
F(i , j)2 εj − εi
where F(i,j)2 is the Fock matrix element between the ith and jth NBO orbitals, εi and εj are the NBO orbital energies, and qi is the population of the donor orbital, a greater difference in the orbital energies contributes to a decrease of the interaction energy. Therefore, the stronger interactions between the metal complex and the alkyne in the Os complexes are due to the larger orbital overlap, not the orbital energy difference. This can be understood by one’s intuition, because the extents of valence d orbitals in each metal are different (Fe, 3d; Ru, 4d; Os, 5d), and that of Os is the largest. Table 4 gives the interaction energies between the complex and the vinylidene moieties for the product complexes. A similar tendency with the reactant complex was observed. The interactions between the complex and the vinylidene moieties are strongest for the Os complexes due to both the greater number and greater values of orbital interactions. These results also agree with the relative energy values of the product complexes given in Table 1. All of the metal complexes have large interaction energies (ΔEi→j) between LP(M) as a donor and LP*(C2) as an acceptor, corresponding to π backdonation, whereas the Fe complexes show no significant interactions corresponding to π donation from the vinylidenes. For the LP(M)−LP*(C2) interaction, the Os complexes have the largest value, followed by the Ru and Fe complexes, for any substituent. A heavier metal has a smaller orbital energy difference (εj − εi) and a larger orbital overlap (F(i,j)2) in the LP(M)−LP*(C2) interaction. Thus, a heavier metal has a greater donor−acceptor interaction of LP(M)−LP*(C2). In contrast to the alkyne complexes (reactants), the structures of vinylidene complexes (products) are not significantly different among the different metals and are of the perpendicular type (Figure 1). The LP(M)−LP*(C2) interaction in the Ru and Os complexes becomes larger in the following order for the substituents R, OMe < Cl < CO2Et, while Fe complexes have the same order but the complexes with R = Cl, CO2Et have comparable values. This can attribute to the substituent effect: namely, OMe (electron-donating group) prevents LP*(C2) from accepting electrons from the metal and CO2Et (electronwithdrawing group) promotes this. As for the opposite donor−acceptor interaction between the donor BD(M−C2) and the acceptor LP*(M), i.e., between C2 as a donor and M as an acceptor, only the Os complexes with R = OMe (46.9 kcal/mol), Cl (46.5 kcal/mol) have interaction energies greater than 20 kcal/mol. This suggests that the electron-withdrawing CO2Et substituent affects C2 and lowers the donating ability of BD(M−C2). Therefore, both the internal alkyne and the vinylidene moieties are stabilized well with a heavier metal complex.
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EXPERIMENTAL SECTION
Computational Details. DFT calculations were performed to study the isomerization pathway for six kinds of complexes having group 8 transition metals (i.e., [CpM(PhCCC6H4R-p)(dppe)]+ (M = Fe, Os; R = OMe, CO2Et, Cl)). The optimized geometries for Ru complexes obtained from our previous study12 were used as initial geometries for the Fe and Os complexes. Geometry optimization, vibrational frequency calculations, and intrinsic reaction coordinate (IRC) calculations17 were carried out using the B3PW91 functional18 with the basis sets as follows: the Stuttgart/Dresden (SDD) ECP and corresponding basis set19 for the metals and the 6-31G(d) basis set20 for the remaining nonmetal atoms. The combined basis set is denoted as SDD+6-31G(d) in the present study. We used the B3PW91/SDD +6-31G(d) method, since it showed good correspondence between the calculation results and the experimental results with reasonable computational cost in our previous study.12 All of the stationary points were characterized by vibrational analysis as local minima (LM), transition states (TS), or higher-order saddle points. Natural bond orbital (NBO) analysis14 was performed by Hartree−Fock calculations with the same basis set (SDD+6-31G(d)). To evaluate the solvent effect on energy profiles, single-point energy calculations with the polarizable continuum model (PCM)21 with benzene for Fe and Os complexes and dichloroethane for Ru complexes were performed. The solvent effect was insignificant (see Table 1 and Table S4 (Supporting Information)). Concerning dispersion effects, we considered the D3 version of Grimme’s dispersion, which is a method for dispersion
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CONCLUSION In the present study, DFT calculations on the transformation reaction of internal alkynes, [CpM(PhCCC6H4R-p)(dppe)]+ (M = Fe, Ru, Os; R = OMe, CO2Et, Cl), to the corresponding 3940
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[CpOs{CC(Ph)C6H4Cl-p}(dppe)][BArF4] (3). This compound was synthesized from [CpOsCl(dppe)]·(acetone) (20.6 mg, 0.028 mmol), NaBArF4·2H2O (31.3 mg, 0.034 mmol), and 1-chloro-4-(2phenylethynyl)benzene (30.0 mg, 0.141 mmol) by a procedure similar to that adopted for the synthesis of 2, except that the reaction was performed for 16 h. Orange crystals were obtained (42.5 mg, 0.025 mmol, 89% yield). 31P{1H} NMR (CDCl3): δ 38.3 (s, dppe). 1H NMR (CDCl3): δ 7.75−6.37 (m, 41H, Ar), 5.59 (s, 5H, Cp), 3.06−3.03 (m, 4H, CH2 of dppe). Selected 13C{1H} NMR data (CDCl3): δ 307.5 (t, 2 JPC = 10.1 Hz, OsC), 131.4 (s, OsCC), 89.5 (s, C5H5), 29.3 (dd, 1JPC = 40.4 Hz, 2JPC = 9.1 Hz, CH2 of dppe). IR (cm−1): 1638 (s, νCC). Anal. Calcd for C77H50BClF24OsP2: C, 53.47; H, 2.91. Found: C, 53.77; H, 2.81. The migratory aptitude of aryl groups was calculated similarly; the ratio of Ph and C6H4Cl-p migration was found to be 49:51.
correction as an add-on to standard DFT with less empiricism. However, Grimme et al. reported that the dispersion correction on chemical reaction energies is beyond the scope of this method.22 Therefore, consideration of dispersion effects on the present chemical reaction is not practical at the moment. All of the calculations in the present study were conducted using the Gaussian 09 program.23 Linux PC cluster machines at Ochanomizu University and the computer facilities at the Research Center for Computational Science in Okazaki, Japan, were used. General Considerations of Synthesis. All manipulations were carried out under an argon atmosphere by using standard Schlenk techniques unless otherwise stated. The solvents (anhydrous grade) were purchased from Sigma-Aldrich and purged with argon before use. Phenylacetylene-2-13C (PhC13CH) was purchased from Taiyo Nippon Sanso Co. and used as a ca. 26% 13C-enriched reagent. 13Cenriched substituted diphenylacetylenes PhC13CC6H4OMe-p, PhC13CC6H4Cl-p, and PhC13CC6H4CO2Et-p were prepared from PhC13CH by the literature methods.24 [CpFeCl(dppe)],11b [CpOsCl(dppe)],25 and NaBArF4·2H2O26 were synthesized according to the literature. 1H (500 MHz), 13C{1H} (126 MHz), and 31P{1H} (202 MHz) NMR spectra were recorded on a JEOL ECA-500 spectrometer. Chemical shifts are reported in δ, referenced to residual 1 H and 13C signals of deuterated solvents as internal standards or to the 31P signal of PPh3 (δ −5.65) as an external standard. IR spectra were recorded on a JASCO FT/IR-4200 spectrometer by using KBr pellets. Elemental analyses were performed on a PerkinElmer 2400 series II CHN analyzer. Amounts of the solvent molecules in the crystals were determined not only by elemental analyses but also by 1H NMR spectroscopy. [CpFe{CC(Ph)C6H4CO2Et-p}(dppe)][BArF4] (1). A mixture of [CpFeCl(dppe)] (39.1 mg, 0.071 mmol), NaBArF4·2H2O (78.0 mg, 0.085 mmol), and ethyl 4-(2-phenylethynyl)benzoate (75.2 mg, 0.300 mmol) in benzene (3 mL) was stirred at 70 °C for 24 h. The resulting brown suspension was filtered through a plug of Celite, and the plug was rinsed with benzene. The combined filtrate was dried in vacuo, and the residue was chromatographed on silica gel (eluent Et2O/ CH2Cl2 2/1; Rf = 0.7). The red fraction was collected and dried in vacuo, and the residue was recrystallized from Et2O/hexane to afford the desired compound (49.5 mg, 0.030 mmol, 42% yield) as a red powder. 31P{1H} NMR (CD2Cl2): δ 93.7 (s, dppe). 1H NMR (CD2Cl2): δ 7.73−6.56 (m, 41H, Ar), 5.25 (s, 5H, Cp), 4.33 (q, 3JHH = 7.1 Hz, 2H, OCH2CH3), 3.28−3.24 (m, 2H, CH2 of dppe), 3.05−3.02 (m, 2H, CH2 of dppe), 1.15 (t, 3JHH = 7.1 Hz, 3H, OCH2CH3). Selected 13C{1H} NMR data (CD2Cl2): δ 358.0 (t, 2JPC = 33.0 Hz, FeC), 144.0 (s, FeCC), 89.5 (s, C5H5), 61.5 (s, OCH2CH3), 27.9 (pseudo t, 1JPC = 23.2 Hz, CH2 of dppe), 14.0 (s, OCH2CH3). IR (cm −1 ): 1712 (s, νCO), 1594 (s, νCC). Anal. Calcd for C80H55BF24FeO2P2: C, 58.85; H, 3.40. Found: C, 58.58; H, 3.09. The migratory aptitude of aryl groups was calculated in a manner similar to our previous method using 13C{1H} NMR data of [CpFe{ CC(Ph)Ar}(dppe)][BArF4] synthesized from [CpFeCl(dppe)] and PhC13CAr (ca. 26% enriched).11b [CpOs{CC(Ph)C6H4OMe-p}(dppe)][BArF4] (2). A mixture of [CpOsCl(dppe)]·(acetone) (21.0 mg, 0.028 mmol), NaBArF4·2H2O (28.5 mg, 0.031 mmol), and 1-methoxy-4-(2-phenylethynyl)benzene (29.0 mg, 0.139 mmol) in benzene (2 mL) was stirred at 70 °C for 1 h. The resulting red suspension was filtered through a plug of Celite, and the plug was rinsed with benzene. The combined filtrate was dried in vacuo and recrystallized from Et2O/hexane to afford the desired compound (37.9 mg, 0.022 mmol, 79% yield) as a red solid. 31P{1H} NMR (CDCl3): δ 38.9 (s, dppe). 1H NMR (CDCl3): δ 7.71−6.42 (m, 41H, Ar), 5.55 (s, 5H, Cp), 3.07 (s, 3H, OCH3), 3.05−2.99 (m, 4H, CH2 of dppe). Selected 13C{1H} NMR data (CDCl3): δ 309.6 (t, 2JPC = 10.6 Hz, OsC), 131.7 (s, OsCC), 89.4 (s, C5H5), 55.4 (s, OCH3), 29.6 (dd, 1JPC = 40.4 Hz, 2JPC = 9.1 Hz, CH2 of dppe). IR (cm−1): 1639 (s, νCC). Anal. Calcd for C78H53BF24OOsP2: C, 54.30; H, 3.10. Found: C, 54.56; H, 2.97. The migratory aptitude of aryl groups was calculated similarly; the ratio of Ph and C6H4OMe-p migration was found to be 97:3.
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ASSOCIATED CONTENT
S Supporting Information *
Tables and figures giving dihedral angles γ(PMC2C1) of the reactant structures, energy profiles for scan calculations from the perpendicular type to the coplanar type on the Fe complexes, selected structural parameters and imaginary frequencies in the TS structures, orbital interaction energies between C4−C6 or C4 as a donor and C1 as an acceptor for TS structures, natural bond orbitals and orbital energies for the orbital interaction between LP(M) and BD*(C1−C2) in the reactant, and energy relative to the most stable reactant complex calculated with solvent effects using PCM. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/om501280c.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for N.T.:
[email protected]. *E-mail for K.T.:
[email protected]. Present Addresses §
Department of Liberal Arts, Faculty of Medicine, Saitama Medical University, 38 Morohongo, Moroyama-machi, Irumagun, Saitama 350-0495, Japan. ∥ Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Research Center for Computational Science in Okazaki, Japan, for the use of the computer facilities. This research was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” (23105543 and 25105747) from the MEXT of Japan. The authors also thank FIST (Foundation for Interaction in Science and Technology, Japan) for the financial support for this research.
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DOI: 10.1021/om501280c Organometallics 2015, 34, 3934−3943
