Article pubs.acs.org/JPCA
Theoretical Investigation of the Mechanisms for the Reaction of Fused Tricyclic Dimetallenes Containing Highly Strained EE (E = C, Si, Ge, Sn, and Pb) Double Bonds Bo-Ying Li and Ming-Der Su* Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan S Supporting Information *
ABSTRACT: The potential energy surfaces for the reactions of fused tricyclic dimetallenes that feature a highly strained EE double bond, Rea-EE, where E = C, Si, Ge, Sn, and Pb, were studied using density functional theory (B3LYP/ LANL2DZ). Three types of chemical reactions (i.e., a self-isomerization reaction, a [2 + 2] cycloaddition with a ketone and a methanol 1,2-addition reaction) were used to determine the reactivity of the Rea-EE molecules. The theoretical findings reveal that the smaller the singlet−triplet splitting of the Rea-EE, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions with other chemical molecules. Theoretical observations suggest that the relative reactivity increases in the following order: CC ≪ SiSi < GeGe < SnSn < PbPb. Namely, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its fused tricyclic Rea-EE to chemical reaction. It is thus predicted that the fused tricyclic Rea-CC and Rea-SiSi molecules should be stable and readily synthesized and isolated at room temperature. The computational results show good agreement with the available experimental observations. The theoretical results obtained from this work allow a number of predictions to be made.
I. INTRODUCTION The synthesis and characterization of molecules with multiple bonds that contain heavier group 14 elements has been the subject of many studies because of their unique bonding and structures. These differ significantly from those of their corresponding carbon compounds.1−5 In particular, considerable interest has been shown in the nature of alkene analogues of the heavier group 14 elements1,2 because of their unusual structures and interesting bonding, since the isolation of a stable tetramesityldisilene by West and co-workers in 1981.2a Many experimental studies have reported that the π bond of the disilenes displays an increased reactivity toward many reagents compared with that of alkenes because of their relatively small HOMO−LUMO energy gaps.1 For instance, unlike the ethylene species, disilenes undergo smooth [2 + 2] cycloadditions with alkenes and alkynes to produce the disilacyclobutane and disilacyclobutene derivatives, respectively.1 In spite of the fact that there has been a dramatic increase in the understanding of the chemistry of these heavy, doubly bonded compounds, the intrinsic reactivities and related reaction mechanisms of the EE (E = group 14 elements) double bond have not been fully determined. Such studies are important to the systematic understanding of the characteristics of group 14 homopolar double bonds, including CC bonds. Recently, Iwamoto, Kira and co-workers have reported two isolable, fused tricyclic disilenes that are tethered by two dialkyl silene moieties, that is, cis- and trans-1.6 According to the experimental evidence,6 neither cis-1 nor trans-1 dissociates into © 2012 American Chemical Society
the corresponding dialkyl silylenes. Conversely, when heated, trans-1 undergoes unprecedented intramolecular [2s + 2a] cycloaddition of the Si−Si single bond to the Si−Si double bond to produce a tetracyclic compound (2);6 see Scheme 1. Nevertheless, attempts to synthesize and isolate the other fused tricyclic heavy dimetallene analogues, such as >GeGeSnSnPbPb Rea-SiSi (64.3) > Rea-GeGe (56.3) > Rea-Sn Sn (53.6) > Rea-PbPb (52.8). This strongly implies that the corresponding singlet−triplet energy gap should also decrease as the E atoms become heavier. Indeed, this prediction agrees well with what is observed in the theoretical computations. Namely, the DFT computations reveal that the singlet−triplet energy splitting, ΔEst (kcal/mol), of Rea-EE are calculated to be; 48 (Rea-CC) > 16 (Rea-SiSi) > 14 (Rea-GeGe) > 12 (Rea-SnSn) > 11 (Rea-PbPb). These results will be used in the following sections to account for the origin of barrier heights and reaction enthalpies for the three chemical reactions (eqs 1−3), as well as the reactivity of the Rea-EE molecules. (3). Intramolecular Isomerization Reactions of Fused Tricyclic Dimetallenes. According to the experimental observations,6 in spite of the fact that both the cis-1 and trans-1 species are intramolecular dimers of two isolable dialkyl silylene moieties, they do not dissociate into two corresponding dialkyl silylene molecules upon heating to 100 °C. Nevertheless, if trans-1 is heated to 110 °C in [D10]p-xylene or in the solid state, this compound isomerizes stereospecifically into a tetracyclic molecule (2; Scheme 1) with a quantitative yield.6 As far as the authors are aware, this intramolecular cycloaddition reaction represents the first experimental evidence of
Figure 4. B3LYP/LANL2DZ optimized geometries of the stationary points for intramolecular isomerization reactions of Rea-EE (E = C, Si, Ge, Sn, and Pb) molecules. The relative energies for each species see Table 1. The heavy arrows indicate the main atomic motions in the transition state eigenvector. Some methyl groups and hydrogens are omitted for clarity. 4225
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
It should be noted that the intramolecular isomerization of a fused tricyclic dimetallene with a highly strained EE double bond should produce a four-membered ring cycloadduct compound, in a single step (i.e., in a concerted manner) and stereospecifically. Such intramolecular cycloaddition reactions should favor the production of stereoretention products. Moreover, the examples of intramolecular rearrangements studied are in agreement with the available experimental observations, suggesting that thermodynamic rather than kinetic control occurs during high temperature reaction in the Rea-SiSi (trans-1) species.6,19 (iii) To ascertain the key factors that determine the general features of these intramolecular isomerization reactions (eq 1), a configuration mixing (CM) model, which was developed by Pross and Shaik,20,21 was used to gain a better understanding of the reactivity of the various reactants. According to the conclusions provided by this model, the energy barriers governing processes, as well as the reaction enthalpies, should be proportional to the energy gap, ΔEst (= Etriplet − Esinglet), between the singlet and the triplet states of a fused tricyclic dimetallene that bears a highly strained EE double bond (i.e., Rea-EE). In other words, the smaller the value of ΔEst for Rea-EE, the lower is the barrier height and the more exothermic and, in turn, the faster is the intramolecular rearrangement reaction. Keeping this conclusion in mind, the origin of the observed trend can be explained: The reason why the intramolecular isomerization of Rea-EE possessing heavier group 14 atoms (E) is more easily achieved than for heavier group 14 elements can be traced back to the singlet− triplet energy gap (ΔEst) of a Rea-EE. It was proven earlier that a fused tricyclic dimetallene featuring two heavier group 14 atoms (i.e., EE) should have a smaller ΔEst value than one containing lighter group 14 atoms. Namely, the energy of the ΔEst was found to decrease as the atomic mass of the group 14 elements increases in the following order: Rea-CC > ReaSiSi > Rea-GeGe > Rea-SnSn > Rea-PbPb. As already detailed in Table 1, this result is in accordance with the trends in activation energy and enthalpy (ΔE‡ and ΔH) for the intramolecular isomerization of a fused tricyclic dimetallene. To the authors’ best knowledge, this is the first theoretical proof for the mechanism of the intramolecular isomerization of a fused tricyclic disilene to the corresponding tetracyclic compound, which was discovered experimentally by Iwamoto, Kira et al.6 (iv) After submitting this paper, one reviewer indicated that such fused tricyclic dimetallenes containing highly strained E E (E = C, Si, Ge, Sn, and Pb) double bonds would be easily dissociated into correspondingly different compounds. We, thus, calculated the dissociation energies for these reactant (Rea-EE) species using the B3LYP/LANL2DZ level of theory. Our computational results show that the dissociation energy for these molecules is estimated to be 35 (Rea-CC), 53 (Rea-SiSi), 29 (Rea-GeGe), 21 (Rea-SnSn), and 4.8 (Rea-PbPb) kcal/mol. Compared with the activation barriers for intramolecular isomerization reactions (eq 1) given in Table 1, our theoretical findings strongly suggest that the dissociation of the Rea-EE (E = C, Ge, Sn, and Pb) molecules, except for Rea-SiSi, should be much easier than their isomerization into the tetracyclic compounds. We eagerly await the experimental facts to confirm our predictions. (4). [2 + 2] Cycloaddition Reactions of Fused Tricyclic Dimetallenes with Ketone. It is well-known that ketones add to dimetallenes to produce dimetalloxetanes.22 These dimetalloxetanes are of considerable chemical interest as reactive
energies relative to the reactant molecule, that is, Rea-EE, are also summarized in Table 1. Four points are noteworthy. Table 1. Relative Energies for the Intramolecular Isomerization on Rea-EE Process: Reactant (Rea-EE) → Transition State (TS-Iso-E-E) → Isomerization Product (Pro-Iso-E-E)a system Rea-CC Rea-SiSi Rea-GeGe Rea-SnSn Rea-PbPb
ΔE‡b (kcal mol−1) 78.2 50.2 39.3 34.4 31.0
[73.1] [50.9] [38.4] [33.6] [27.8]
ΔHc (kcal mol−1) −11.5 −10.9 −13.1 −17.7 −20.1
[−10.6] [−13.0] [−18. 9] [−20.2] [−22.7]
a
All were calculated at the B3LYP/LANL2DZ level of theory. For the B3LYP optimized structures of the stationary points see Figure 4. The Gibbs free energies are given in the square bracket. bThe activation energy of the transition state, relative to the corresponding reactant. c The reaction enthalpy of the product, relative to the corresponding reactant.
(i) When heat is applied to the reactants (Rea-EE), these molecules absorb thermal energy and a six-membered, ringopening reaction occurs to form a final tetracyclic product (Pro-Iso-E-E) with an E−E single bond.17 The transition state is thus located for each Rea-EE species (TS-Iso−C-C, TSIso−Si-Si, TS-Iso−Ge-Ge, TS-Iso−Sn-Sn, and TS-Iso−PbPb) at the DFT. Examination of the single imaginary frequency for each transition state provides excellent confirmation of the isomerization process, the silicon substituent group attacking the group 14 E elements to form a cyclic ring. The DFT computations given in Figure 2 (Rea-EE) and Figure 4 (TSIso-E-E) indicate that the breaking Si−Si single bond is stretched by 78, 30, 20, 8.7, and 12% for TS-Iso−C-C, TS-Iso− Si-Si, TS-Iso−Ge-Ge, TS-Iso−Sn-Sn, and TS-Iso−Pb-Pb, respectively, relative to its corresponding Rea-EE. This strongly implies that, according to the Hammond’s postulate,17 the transition state for a fused tricyclic dimetallene with atoms, E, of a lower atomic weight should take on a more product-like character and that the barrier should be encountered later than for a dimetallene analogue with atoms, E, of a greater atomic weight. The calculations using the theoretical model confirm this prediction, as demonstrated in Table 1. Namely, the calculated results reveal that the intramolecular isomerization (eq 1) undergoes an antarafacial [2 + 2] addition of the Si−Si single bond to the EE double bond.6 (ii) The key geometrical parameters for the intramolecular isomerization products (Pro-Iso−C-C, Pro-Iso−Si-Si, ProIso−Ge-Ge, Pro-Iso−Sn-Sn, and Pro-Iso−Pb-Pb) are also shown in Figure 4. As seen in this figure, the two terminal fivemembered rings in Pro-Iso-E-E are fused to the central bicyclo[2.2.0] hexane skeleton in an anti fashion. As expected, the E− E single bond length in the tetracyclic product (Pro-Iso-E-E) increases in the order: 2.234 Å (Pro-Iso−C-C) < 2.482 Å (ProIso−Si-Si) < 2.559 Å (Pro-Iso−Ge-Ge) < 2.828 Å (Pro-Iso− Sn-Sn) < 2.897 Å (Pro-Iso−Pb-Pb). It is also apparent that the order of the reaction enthalpy follows a similar trend to that of the activation energy, as shown in Table 1.18 That is to say, from both a kinetic and a thermodynamic viewpoint, the chemical reactivity for the intramolecular rearrangement reaction of fused tricyclic dimetallene increases in the following order: Rea-CC < Rea-SiSi < Rea-GeGe < Rea-SnSn < Rea-PbPb. 4226
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
the CO double bond in the ketone, it is expected that the [2 + 2] cycloaddition reactions of fused tricyclic dimetallenes (Rea-EE) with a ketone should proceed by a concerted [2πs + 2πs] mechanism, which is made kinetically favorable by the relaxation of orbital symmetry restrictions.25 As a consequence, their bimolecular pericyclic reaction is no longer forbidden (see below). As seen in Figure 5, this [2 + 2] formation leads to a fourmembered-ring transition state structure with different E−O and E−C bond distances. The transition state (TS-Ketone−CC, TS-Ketone−Si-Si, TS-Ketone−Ge-Ge, TS-Ketone−Sn-Sn and TS-Ketone−Pb-Pb) have been located for each EE species at DFT, along with the imaginary frequency eigenvector. According to the computations, only one transition state has been located for each reaction and it is confirmed that it is a true transition state, on the basis of frequency analysis. It is anticipated that these cycloaddition reactions are concerted. It is interesting to note that the four-membered-ring structure in the TS is nearly planar,26 thus confirming the concept that the Rea-EE and ketone [2 + 2] cycloaddition reaction proceeds through a concerted [2πs + 2πs] mechanism. Moreover, comparing the EE and CO bond lengths in these TS structures with the corresponding distances in the reactants, it is found that the E−E single bond in the transition structures (TS-Ketone-E-E) is stretched by 14.3, 30.5, 22.2, 10.8, and 10.2%, relative to its respective value in the corresponding Rea-EE reactant. The DFT calculations also indicate that the CO bond in the carbonyl unit is longer by 6.5 (TS-Ketone−C-C), 5.7 (TS-Ketone−Si-Si), 4.2 (TSKetone−Ge-Ge), 3.3 (TS-Ketone−Sn-Sn), and 3.8% (TSKetone−Pb-Pb) than that in the primitive ketone. All of these theoretical data demonstrate that the structures of the transition states are much as can be expected from Hammond’s postulate.17 Namely, the [2 + 2] reaction for the fused tricyclic dimetallenes (Rea-EE) that possess heavier group 14 elements, E, have lower activation energy, because the TS resembles the corresponding reactants more closely than the final products. Consequently, the barrier for the [2 + 2] cycloaddition process is encountered earlier for the heavier group 14 elements, E, as already confirmed by the B3LYP calculations detailed in Figure 5. (ii) Again, the optimized geometries for the pericyclic [2 + 2] products (i.e., Pro-Ketone−C-C, Pro-Ketone−Si-Si, ProKetone−Ge-Ge, Pro-Ketone−Sn-Sn, and Pro-Ketone−PbPb) are presented in Figure 5. These theoretical results show that all of the [2 + 2] products (Pro-Ketone-E-E) adopt a fourcyclic-ring geometry. Unfortunately, the experimental structures for these cycloaddition products are not yet known. As already stated, a fused tricyclic dimetallene (Rea-EE) with an element, E, of heavier atomic weight reaches the transition state relatively early, whereas one with an element, E, of less atomic weight arrives relatively late. The former is therefore predicted to undergo a more exothermic cycloaddition, which is confirmed by the theoretical calculations as shown in Figure 5. Consequently, considering both the calculated activation barriers and reaction enthalpies, it is concluded that, for the [2 + 2] pericyclic reaction of the Rea-EE molecule, the order of reactivity is Rea-CC ≪ Rea-SiSi < Rea-GeGe < ReaSnSn < Rea-PbPb. This can be explained by an examination of the E−E bond strength. In other words, the heavier the group 14 atom, E, the larger is the atomic radius of E and the smaller is the EE bond dissociation energy. This, in turn, results in a decrease in the barrier height and a more
intermediates, since they undergo metathesis reactions. These reactions are synthetically valuable because they may produce compounds containing EC and EO (E = C, Si, Ge, Sn and Pb) bonds. Although the regio- and stereochemistry of these reactions have been studied experimentally,22 their mechanisms have been much less extensively investigated and are rather poorly understood. Therefore, to examine the factors that control the reactivity of fused tricyclic dimetallenes that bear the EE double bond, the [2 + 2] cycloaddition reaction of ketone toward such substituted doubly bonded molecules, which proceeds via eq 2, is considered. As a result, the cycloaddition mechanisms can be thought to proceed as follows: reactants (Rea-EE + ketone) → transition state (TS-Ketone-E-E) → cycloaddition product (Pro-Ketone-E-E), which is schematically outlined in Figure 5. Selected geo-
Figure 5. B3LYP/LANL2DZ optimized geometries (in Å) of the transition state (TS-Ketone-E-E) and cycloaddition product (ProKetone-E-E) for the [2 + 2] reaction between reactants Rea-EE (E C, Si, Ge, Sn, and Pb) and ketone. Selected geometrical parameters and relative energies for each species (energy relative to the corresponding reactants) are given as well. The heavy arrows indicate the main atomic motions in the transition state eigenvector. Some methyl groups and hydrogens are omitted for clarity.
metrical parameters for these stationary points and their relative energies calculated at the B3LYP/LANL2DZ level of theory are also listed in Figure 5. Figure 5 has several noteworthy features. (i) Based on the Woodward−Hoffmann rules for [2 + 2] cycloaddition reactions,23 a concerted supra−supra process ([2πs + 2πs]) is thermally forbidden. It is, however, generally believed that the polarization of the double bond results in a relaxation of these rules.24 In fact, due to the bond polarity of 4227
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
exothermic reaction of its bimolecular pericyclic with a ketone (eq 2). (iii) The calculated results for [2 + 2] can be well understood with reference to the CM model stated previously.20,21 Using this model, it is seen that the barrier height (ΔE‡), as well as the reaction enthalpy (ΔH), can be expressed in terms of the singlet−triplet splitting, ΔEst (= Etriplet − Esinglet), of the reactants. Accordingly, the smaller the value of ΔEst of the ReaEE reactant, the lower is its barrier height, the more exothermic is the reaction, and the faster is the [2 + 2] reaction with a ketone. With reference to the B3LYP Gibbs free energy calculations for the aforementioned five systems, detailed in Figure 5, the following correlations are evident (units in kcal/ mol; r2 is the correction coefficient): ΔE ≠ = 3.33ΔEst − 3.79 (r 2 = 0.977)
(a)
ΔG = 2.45ΔEst − 8.00 (r 2 = 0.901)
(b)
As can be seen in eqs a and b, a linear correlation exists between ΔEst and ΔE‡ (the activation energy), as well as with ΔG (the Gibbs reaction enthalpy). The calculations using the model, thus, provide strong evidence that electronic factors in the group 14 element, E, play a decisive role in determining the reactivity of the fused tricyclic dimetallene Rea-EE species. (5). 1,2-Addition Reactions of Fused Tricyclic Dimetallenes with MeOH. The addition of alcohols to the EE double bond of dimetallenes is one of the most extensively studied chemical reactions of dimetallenes, because of the rich chemistry of the 1,2-addition products formed in these reactions.27 Although various reaction mechanisms have been suggested and theoretically investigated for the addition reactions of aliphatic alcohols to unsaturated dimetallene compounds,27 their detailed mechanisms are not fully understood. Recently, Leigh, Bendikov, and co-workers reported in a combined experimental and computational study that alcohol addition to silenes progresses by a mechanism that involves alcohol dimers rather than monomers.27 On the basis of a previous study, it is therefore supposed that reagents in dimeric or oligomeric forms may react with a lower activation barrier than in the monomeric form. As a result, this study examines the 1,2-addition reactions of Rea-EE with methanols (eq 3). For comparison, two addition pathways are feasible: one proceeds via monomeric methanol addition (path I), while the other proceeds via dimeric methanol addition (path II). Figure 6 illustrates the optimized geometries of the reactants (ReaEE and MeOH) and the final product (Pro-MeOH-E-E), together with the transition state (TS-MeOH-E-E and TS2MeOH-E-E) that connects the reactants and the product.28 Selected optimized geometrical parameters for the critical points of eq 3 and their relative energies, based on B3LYP/ LANL2DZ calculations, can be seen in Table 2. The major conclusions that can be drawn from the current study can be summarized as follows: (i) For path I (the normal 1,2-addition), the transition state (TS-MeOH-E-E) has been located for each of the group 14 elements, E, at the DFT level of theory. The DFT frequency calculations for the transition states TS-MeOH−C-C, TSMeOH−Si-Si, TS-MeOH−Ge-Ge, TS-MeOH−Sn-Sn, and TS-MeOH−Pb-Pb suggest that the single imaginary frequency values are 1252i, 198i, 145i, 127i, and 183i cm−1, respectively. All TS-MeOH-E-E geometries are quite similar and they all have a four-center-like structure that includes the E−E bond, an oxygen atom, and one hydrogen atom from methanol. These
Figure 6. B3LYP/LANL2DZ optimized geometries of the transition states (TS-MeOH-E-E and TS-2MeOH-E-E) and product (ProMeOH-E-E) for the methanol addition reaction between reactants Rea-EE (E = C, Si, Ge, Sn, and Pb) and a MeOH (path I) or two MeOHs (path II). Selected geometrical parameters and relative energies for each species see Table 2. Some methyl groups and hydrogens are omitted for clarity.
optimized TS-MeOH-E-E geometries indicate that electrons flow from the lone pair orbital of the oxygen atom into the π*(EE) antibonding orbital. This results in a longer E−E bond distance in the TS-MeOH-E-E. For instance, the theoretical calculations predict distances of 1.678 Å (TSMeOH−C-C), 2.416 Å (TS-MeOH−Si-Si), 2.548 Å (TSMeOH−Ge-Ge), 2.864 Å (TS-MeOH−Sn-Sn), and 2.918 Å (TS-MeOH−Pb-Pb). These theoretical data also show that the heavier the group 14 element, E, involved in the Rea-EE molecule, the greater is the E−E bond length in the TS. The dimeric methanol addition pathway (path II) was also considered, using the same level of theory. The B3LYP frequency calculations for the transition states TS-2MeOH−CC, TS-2MeOH−Si-Si, TS-2MeOH−Ge-Ge, TS-2MeOH−SnSn, and TS-2MeOH−Pb-Pb predict that the single imaginary frequency values are 1223i, 963i, 840i, 982i, and 772i cm−1, respectively. Again, because the lone pair of electrons of the oxygen atom interact and are donated to the antibonding π orbital of the EE double bond in the dimeric TS (TS2MeOH-E-E), as mentioned previously, their E−E bond lengths are expected to be longer than that of the corresponding reactant, as shown in Table 2. Further, from Table 2 it is obvious that the activation barrier for path I is much larger than that for path II. Because the overall activation energy for path II (dimeric form) is always much lower than that for path I (monomeric form), the former is concluded to be the preferred pathway for the methanol 1,2addition reactions to fused tricyclic dimetallenes. These computations reveal that, for both monomeric and dimeric MeOH addition reactions, the activation energies decrease as the group 14 atoms, E, become heavier. 4228
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
Table 2. Selected Geometrical Parameters (Bond Distances in Å), Relative Energies (Zero-Point Corrected; kcal mol−1), and Relative Gibbs Free Energies (kcal mol−1) at 298 K at the B3LYP/LANL2DZ Level of Theory for the Optimized Stationary Points on the Studied 1,2-Addition Channels (Paths I and II)a,b geometrical parameters systems
EE
Rea-CC TS-MeOH−C-C TS-2MeOH−C-C Pro-MeOH−C-C Rea-SiSi TS-MeOH−Si-Si TS-2MeOH−Si-Si Pro-MeOH−Si-Si Rea-GeGe TS-MeOH−Ge-Ge TS-2MeOH−Ge-Ge Pro-MeOH−Ge-Ge Rea-SnSn TS-MeOH−Sn-Sn TS-2MeOH−Sn-Sn Pro- MeOH−Sn-Sn Rea-PbPb TS-MeOH−Pb-Pb TS-2MeOH−Pb-Pb Pro-MeOH−Pb-Pb
1.415 1.678 1.540 1.807 2.334 2.416 2.518 2.521 2.540 2.548 2.622 2.617 2.824 2.864 2.842 2.2.872 2.817 2.918 3.005 2.949
E-O1
E-H
O1-H1
O2-H2
1.934 2.724 1.480
1.609c 1364d 1.100
1.133 1.274 2.395
1.038
2.722 2.082 1.732
2.863c 2.112d 1.501
1.006 1.063 3.023
1.062
3.297 2.067 1.828
3.180c 1.924d 1.567
0.9895 1.167 3.147
1.164
3.248 2.150 1.965
3.328c 2.028d 1.743
0.9893 1.264 3.491
1.203
3.308 2.206 2.058
3.385c 1.917d 1.789
0.9885 1.407 3.594
1.394
ΔE
ΔG
0.0 +95.32 +88.61 +27.30 0.0 +43.78 +14.10 −18.37 0.0 +35.81 +9.125 −28.24 0.0 +22.76 +5.310 −28.24 0.0 +25.10 +4.083 −32.01
0.0 +92.85 +113.7 +38.36 0.0 +43.91 +41.10 −12.67 0.0 +36.46 +34.85 −15.61 0.0 +23.85 +31.52 −16.97 0.0 +24.19 +28.41 −17.41
a
The C−O and O−H bond lengths in parent MeOH were calculated to be 1.460 and 0.9793 Å, respectively. bFor structures, see Figure 6. cFor E− H1. dFor E−H2.
the height of the reaction barrier, as well as a more exothermic reaction. The calculations using the model, shown in Table 2, confirm this prediction. That is, from the theoretical and kinetic viewpoints, irrespective of whether path I or path II is chosen, the greater the atomic weight of the group 14 element, E, involved in the Rea-EE species, the smaller is the activation energy, and the larger is the reaction enthalpy for the final additional product.
(ii) A comparison of the two pathways involving the reaction of the Rea-EE double bond molecules with MeOH and 2MeOH leads to the same products, that is, Pro-MeOH−C-C, Pro-MeOH−Si-Si, Pro-MeOH−Ge-Ge, Pro-MeOH−Sn-Sn, and Pro-MeOH−Pb-Pb, whose significant geometrical parameters are shown in Table 2. It is interesting to observe that their six-membered rings all adopt a chair conformation to minimize steric repulsion from the substituent groups. In addition, the DFT computations show that the E−E bond distance of the final product is longer than that of the corresponding reactant and that this bond distance increases along the group 14 family, from carbon to lead. More importantly, the B3LYP theoretical results demonstrate that the order of reaction enthalpy follows the same trend as that for the activation energy. That is to say, the heavier the atomic weight of the group 14 atoms (E) contained in the ReaEE reactant, the lower is the barrier height, the more exothermic is the reaction enthalpy and, in turn, the easier is the methanol 1,2-addition reaction. In particular, the addition reactions with the dimeric form of MeOH have significantly lower barriers than those for monomeric MeOH. This theoretical observation is consistent with the results of earlier work.27 It is thus anticipated that the addition of alcohols to the EE bond of the Rea-EE reactants should involve the polymeric form (at least the dimeric form) of alcohols, rather than the monomeric form. As there are no relevant experimental or theoretical data for the addition of alcohols to the Rea-EE systems, this result is a prediction. (iii) As demonstrated previously,20,21 it is apparent that the reactivity of these 1,2-addition reactions with alcohols is governed by the singlet−triplet excitation energies for each of the reactants, that is, ΔEst (= Etriplet − Esinglet for Rea-EE) and ΔEnσ* (= Etriplet − Esinglet for MeOH). Accordingly, if Δ Enσ* is a constant, then a smaller value for ΔEst leads to a reduction in
IV. CONCLUSION Using the B3LYP level of theory, this work studied the mechanisms for three kinds of chemical reactions of fused tricyclic dimetallene Rea-EE (E = C, Si, Ge, Sn, and Pb) species that feature a highly strained EE double bond. It should be mentioned that this study provides the first theoretical demonstration of the reaction trajectory and theoretical estimation of the activation energy and reaction enthalpy for these chemical processes. The theoretical findings show that the chemical reactivity of Rea-EE molecules increases in the following order: Rea-C C ≪ Rea-SiSi < Rea-GeGe < Rea-SnSn < Rea-Pb Pb. From a mechanistic viewpoint, the theoretical observations confirm a general belief that one of the important influences on the isolability of a Rea-EE molecule is its centric EE double bond.1 That is to say, the fused tricyclic dimetallene, Rea-EE, that bears a highly strained EE double bond with a lighter main group element (such as E = C and Si) should be stable and can be readily synthesized and isolated at room temperature. In brief, electronic as well as steric factors play a dominant role in determining the reactivity of the group 14 Rea-EE molecules, both kinetically and thermodynamically. It is hoped that this study can stimulate further research into the subject. 4229
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
■
Article
Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1999, 121, 886. (t) Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Müller, T.; Bukalov, S.; West, R. J. Am. Chem. Soc. 1999, 121, 9479. (u) Grybat, A.; Boomgaarden, S.; Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999, 38, 2010. (v) Ichinohe, M.; Matsuno, T.; Sekiguchi, A. Angew. Chem., Int. Ed. 1999, 38, 2194. (w) Lee, V. Y.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2000, 122, 9034. (x) Kira, M.; Ohya, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C. Organometallics 2000, 19, 1817. (y) Takahashi, M.; Veszprémi, T.; Hajgató, B.; Kira, M. Organometallics 2000, 19, 4660. (z) Wiberg, N.; Niedermayer, W.; Nöth, H.; Warchhold, M. Z. Anorg. Allg. Chem. 2001, 627, 1717. (aa) Sekiguchi, A.; Matsuno, T.; Ichinohe, M. J. Am. Chem. Soc. 2001, 123, 12436. (bb) Wiberg, N.; Niedermayer, W.; Polborn, K.; Mayer, P. Chem.Eur. J. 2002, 8, 2730. (cc) Lee, V. Y.; Takanashi, K.; Matsuno, T.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2004, 126, 4758. (dd) Wiberg, N.; Vasisht, S. K.; Fischer, G.; Mayer, P. Z. Anorg. Allg. Chem. 2004, 630, 1823. (ee) Tanaka, R.; Iwamoto, T.; Kira, M. Angew. Chem., Int. Ed. 2006, 45, 6371. (ff) Kinjo, R.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Sumimoto, M.; Nagase, S. J. Am. Chem. Soc. 2007, 129, 7766. (gg) Iwamoto, T.; Kobayashi, M.; Uchiyama, K.; Sasaki, S.; Nagendran, S.; Isobe, H.; Kira, M. J. Am. Chem. Soc. 2009, 131, 3156. (3) For digermenes, see: (a) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.; Thorne, A. J. Chem. Commun. 1984, 480. (b) Snow, J. T.; Murakami, S.; Masamune, S.; Williams, D. J. Tetrahedron Lett. 1984, 25, 4191. (c) Goldberg, D. E.; Hitchcock, P. B.; Lappert, M. F.; Thomas, K. M.; Thorne, A. J.; Fjelberg, T.; Haaland, A.; Schilling, B. E. R. Dalton Trans. 1986, 2387. (d) Batcheller, S. A.; Tsumuraya, T.; Tempkin, O.; Davis, W. M.; Masamune, S. J. Am. Chem. Soc. 1990, 112, 9394. (e) Kira, M.; Iwamoto, T.; Maruyama, T.; Kabuto, C.; Sakurai, H. Organometallics 1996, 15, 3767. (f) Weidenbruch, M.; Stürmann, M.; Kilian, H.; Pohl, S.; Saak, W. Chem. Ber., Recl. 1997, 130, 735. (g) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics 1997, 16, 1920. (h) Schäfer, A.; Saak, W.; Weidenbruch, M.; Marsmann, H.; Henkel, G. Chem. Ber., Recl. 1997, 130, 1733. (i) Schäfer, A.; Saak, W.; Weidenbruch, M. Z. Anorg. Allg. Chem. 1998, 624, 1405. (j) Schäfer, H.; Saak, W.; Weidenbruch, M. Organometallics 1999, 18, 3159. (4) For distannenes, see: (a) Lay, U.; Pritzkow, H.; Grützmacher, H. Chem. Commun. 1992, 260. (b) Weidenbruch, M.; Kilian, H.; Peters, H.; Schnering, H. G. V.; Marsmann, H. Chem. Ber. 1995, 128, 983. (c) Klinkhammer, K. W.; Schwarz, W. Angew. Chem., Int. Ed. 1995, 34, 1334. (d) Leung, W. P.; Kwok, W.-H.; Xue, F.; Mak, T. C. W. J. Am. Chem. Soc. 1997, 119, 1145. (e) Klinkhammer, K. W.; Fässler, T. F.; Grützmacher, H. Angew. Chem., Int. Ed. 1998, 37, 124. (f) Drost, C.; Hitchcock, P. B.; Lappert, M. F. Angew. Chem., Int. Ed. 1999, 38, 1113. (5) For diplumbene, see: (a) Stürmann, M.; Weidenbruch, M.; Klinkhammer, K. W.; Lissner, F.; Marsmann, H. Organometallics 1998, 17, 4425. (b) Stü r mann, M.; Saak, W.; Weidenbruch, M.; Klinkhammer, K. W. Eur. J. Inorg. Chem. 1999, 579. (c) Stürmann, M.; Saak, W.; Marsmann, H.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999, 38, 187. (6) Tanaka, R.; Iwamoto, T.; Kira, M. Angew. Chem., Int. Ed. 2006, 45, 6371. (7) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (8) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (9) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1976; pp1−28. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) ibid. 1985, 82, 284. (d) ibid. 1985, 82, 299. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr., J. A.; Stratmann, R. E.; Burant, J. C. et al. Gaussian 03; Gaussian, Inc.: Pittsburgh PA, 2003. (11) According to both the experimental observations and our theoretical computations, the geometry around the SiSi double bond in trans-1 is more highly distorted than that in cis-1. In fact, the SiSi double bond distance of trans-1 is observed to be the second largest among all the reported disilenes; the largest was found in (Z)-
ASSOCIATED CONTENT
S Supporting Information *
Additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful to the National Center for HighPerformance Computing of Taiwan for generous amounts of computing time. They also thank the National Science Council of Taiwan for the financial support. Special thanks are also due to Referees 1 and 2 for very helpful suggestions and comments.
■
REFERENCES
(1) For reviews with leading references, see: (a) Brook, A. G.; Brook, M. A. Adv. Organomet. Chem. 1996, 39, 71. (b) Okazaki, R.; West, R. Adv. Organomet. Chem. 1996, 39, 231. (c) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (d) Power, P. P. Chem. Rev. 1999, 99, 3463. (e) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704. (f) Kira, M.; Iwamoto, T. J. Organomet. Chem. 2000, 611, 236. (g) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209. (h) Weidenbruch, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: Chichester, U.K., 2001; Vol. 3, Chapter 5. (i) West, R. Polyhedron 2002, 21, 467. (j) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39. (k) Weidenbruch, M. Organometallics 2003, 22, 4348. (l) Power, P. P. Chem. Commun. 2003, 2091. (m) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (n) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (o) Lee, V. Ya; Sekiguchi, A. Organometallics 2004, 23, 2822. (p) Kira, M.; Iwamoto, T.; Ishida, S. In Organosilicon Chemistry VI − From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley- VCH: Weinheim, 2005; p 25. (q) Kira, M.; Iwamoto, T. Adv. Organomet. Chem. 2006, 54, 73. (r) Gehrhus, B.; Hitchcock, P. B.; Pongtavornpinyo, R.; Zhang, L. Dalton Trans. 2006, 15, 1847. (s) Sekiguchi, A.; Ichinohe, M.; Kinjo, R. Bull. Chem. Soc. Jpn. 2006, 79, 825. (t) Lee, V. Y.; Sekiguchi, A. Angew. Chem., Int. Ed. 2007, 46, 6596. (u) Kira, M.; Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007, 80, 258. (v) Kira, M. Chem. Commun. 2010, 46, 2893. (w) Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877. (2) For disilenes, see: (a) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343. (b) Masamune, S.; Hanzawa, Y.; Murakami, S.; Bally, T.; Blount, J. F. J. Am. Chem. Soc. 1982, 104, 1150. (c) Fink, M. J.; Michalczyk, M. J.; Haller, K. J.; Michl, J.; West, R. Organometallics 1984, 3, 793. (d) Michalczyk, M. J.; West, R.; Michl, J. J. Am. Chem. Soc. 1984, 106, 821. (e) Michalczyk, M. J.; West, R.; Michl, J. Organometallics 1985, 4, 826. (f) Yokelson, H. B.; Maxka, J.; Siegel, D. A.; West, R. J. Am. Chem. Soc. 1986, 108, 4239. (g) Watanabe, H.; Takeuchi, K.; Fukawa, N.; Kato, M.; Goto, M.; Nagai, Y. Chem. Lett. 1987, 1341. (h) Masamune, S.; Eriyama, Y.; Kawase, T. Angew. Chem., Int. Ed. 1987, 26, 584. (i) Shepherd, B. D.; Powell, D. R.; West, R. Organometallics 1989, 8, 2664. (j) Tokitoh, N.; Suzuki, H.; Okazaki, R. J. Am. Chem. Soc. 1993, 115, 10428. (k) Sekiguchi, A.; Maruki, I.; Sakurai, H. J. Am. Chem. Soc. 1993, 115, 11460. (l) Kira, M.; Maruyama, T.; Kabuto, C.; Ebata, K.; Sakurai, H. Angew. Chem., Int. Ed. 1994, 33, 1489. (m) Apeloig, Y.; Nakash, M. J. Am. Chem. Soc. 1996, 118, 9798. (n) Kira, M.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1996, 118, 10303. (o) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem., Int. Ed. 1997, 36, 2503. (p) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 1260. (q) ibid., Organometallics, 1998, 17, 2307. (r) Wiberg, N.; Auer, H.; Nöth, H.; Knizek, J.; Polborn, K. Angew. Chem., Int. Ed. 1998, 37, 2869. (s) Iwamoto, T.; 4230
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
(24) However, as reported in the previous work (ref 19), the mechanism of the [2 + 2] addition of aldehydes to group 14 dimetallenes without substituents has been investigated through a theoretical study of the addition of formaldehyde to CSi, CGe, SiSi, SiGe, and GeGe bonds using several sophisticated levels of theory. The reaction pathways located can be grouped as either involving the formation of singlet diradical or zwitterionic intermediates or as concerted processes. As a result, this paper (ref 19) showed that different heteronuclear doubly bonded molecules may adopt various reaction pathways. So far, no conclusive results have been obtained. In fact, this paper used the simple group 14 dimetallenes (i.e., the substituents are hydrogens) as the model molecules. Therefore, it is believed that the substituent effects should play a significant role on their reaction routes so that mechanism of the addition of ketone to group 14 dimetallenes needs to be further examined carefully. (25) Moreover, according to the Woodward−Hoffmann rules, there is another possible thermally allowed [2 + 2] cycloaddition reaction mechanism, namely, the [2πs + 2πa] mechanism. Nevertheless, this reaction mechanism would be too sterically hindered, even in the case of the parent homonuclear doubly bonded compounds, needless to mention the sterically congested fused tricyclic dimetallenes studied in this work. In fact, we had tried every possibility to obtain the barrier height for the [2πs + 2πa] mechanism using the B3LYP method. However, it always failed. The reason for this is simply because of the steric effects, as mentioned above. (26) The supporting evidence comes from the fact that the ∠C−O− E−E dihedral angle was calculated to be 41, 33, 36, and 27° for TSKetone-C-C, TS-Ketone-Si-Si, TS-Ketone-Ge-Ge, TS-Ketone-SnSn, and TS-Ketone-Pb-Pb, respectively, based on the B3LYP/ LANL2DZ level of theory. (27) The addition of alcohols to dimetallenes: (a) Nagase, S.; Kudo, T. J. Chem. Soc., Chem. Commun. 1983, 363. (b) Wiberg, N. J. Organomet. Chem. 1984, 273, 141. (c) Apeloig, Y.; Karni, M. J. Am. Chem. Soc. 1984, 106, 6676. (d) Brook, A. G.; Safa, K. D.; Lickiss, P. D.; Baines, K. M. J. Am. Chem. Soc. 1985, 107, 4338. (e) Nagase, S.; Kudo, T.; Ito, K. In Applied Quantum Chemistry; Smith, V. H., Jr, Schaefer, H. F., III, Morokuma, K., Eds.; John Wiley & Sons: New York, 1986; pp 249−267. (f) De Young, D. J.; Fink, M. J.; West, J.; Michel, J. Main Group Met. Chem. 1987, 10, 19. (g) Jones, P. R.; Bates, T. F. J. Am. Chem. Soc. 1987, 109, 913. (h) Steinmetz, M. G.; Udayakumar, B. S.; Gordon, M. S. Organometallics 1989, 8, 530. (j) Kira, M.; Maruyama, T.; Sakurai, H. J. Am. Chem. Soc. 1991, 113, 3986. (k) Sluggett, G. M.; Leigh, W. J. J. Am. Chem. Soc. 1992, 114, 1195. (l) Sekiguchi, A.; Maruki, I.; Sakurki, H. J. Am. Chem. Soc. 1993, 115, 11460. (m) Leigh, W. J.; Sluggett, G. M. J. Am. Chem. Soc. 1994, 116, 10468. (n) Apeloig, Y.; Nakash, M. J. Am. Chem. Soc. 1996, 118, 9798. (o) Leigh, W. J.; Bradaric, C. J.; Kerst, C.; Banisch, J. H. Organometallics 1996, 15, 2246. (p) Baradarci, C. J.; Leigh, W. J. J. Am. Chem. Soc. 1996, 118, 8971. (q) Baradarci, C. J.; Leigh, W. J. Can. J. Chem. 1997, 75, 1393. (r) Kerst, C.; Rogers, C. W.; Ruffolo, R.; Leigh, W. J. J. Am. Chem. Soc. 1997, 119, 466. (s) Kerst, C.; Ruffolo, R.; Leigh, W. J. Organometallics 1997, 16, 5804. (t) Kerst, C.; Boukherroub, R.; Leigh, W. J. J. Photochem. Photobiol., A 1997, 110, 243. (u) Veszprémi, T.; Takahashi, M.; Ogasawara, J.; Sakamoto, K.; Kira, M. J. Am. Chem. Soc. 1998, 120, 2408. (v) Leigh, W. J.; Boukherroub, R.; Kerst, C. J. Am. Chem. Soc. 1998, 120, 9504. (w) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 1260. (x) Apeloig, Y.; Nakash, M. Organometallics 1998, 17, 2307. (y) Leigh, W. J.; Kerst, C.; Boukherroub, R.; Morkin, T. L.; Jenkins, S. L.; Sung, K.; Tidwell, T. T. J. Am. Chem. Soc. 1999, 121, 4744. (z) Leigh, W. J.; Toltl, N. P.; Apodaca, P.; Castruita, M.; Pannell, K. H. Organometallics 2000, 19, 3232. (aa) Kira, M. Pure Appl. Chem. 2000, 72, 2333. (bb) Takahashi, M.; Veszprémi, T.; Kira, M. Int. J. Quantum Chem. 2001, 84, 192. (cc) Takahashi, M.; Veszprémi, T.; Sakamoto, K.; Kira, M. Mol. Phys. 2002, 100, 1703. (dd) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3, Chapter 9. (ee) Morkin, T. L.; Leigh, W. J.; Tidwell, T. T.; Allen, A. D.
diaminodisilyl disilene (2.289 Å). See Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Müller, T.; Bukalov, S.; West, R. J. Am. Chem. Soc. 1999, 121, 9479. (12) For acyclic alkenes, the trans-disilene is more stable than the cisisomer due to the steric congestion in the latter. However, it is wrong for cyclic alkenes. For small cyclic alkenes, the cis-isomer is more stable due to the less significant ring strain. We thank one reviewer for this comment. (13) Basically, these observations can be explained in terms of the expected atomic size of the central atom E, which increases as E changes from carbon to lead. (14) It was experimentally reported that the EE double bond lengths are 1.356 Å (CC; ref 14), 2.139−2.360 Å (SiSi; ref 2), 2.212−2.509 Å (GeGe; ref 3), 2.601−2.961 Å (SnSn; ref 4), and 2.990−3.537 Å (PbPb; ref 5). (15) (a) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (b) Brook, A. G.; Baines, K. M. Adv. Organomet. Chem. 1986, 25, 1. (c) West, R. Angew. Chem., Int. Ed. 1987, 26, 1201. (d) Tsumuraya, T.; Batcheller, S. A.; Masamune, A. Angew. Chem., Int. Ed. 1991, 30, 902. (e) Wade, Jr. L. G. In Organic Chemistry; Pearson Education Inc.: New York, 2009; p 282. (16) Indeed, we tried very hard to search for the intermediate between the reactant (Rea-EE) and the corresponding transition state (TS-Iso-EE) using different sophisticated theoretical methods (such as, automatic search and manual exploration) as well as the larger basis sets (e.g., LANL2DZdp). However, these theoretical computations always failed. We thus believe that there should be no local minimum point between the reactant and its intramolecular transition state. (17) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334. (18) It has to be mentioned that Kira et al. (ref 6) have shown that the activation enthalpy for the isomerization is only 19 kcal/mol (351 K). Because experimental ΔH‡ and ΔS‡ were known, it is easy to evaluate the ΔG‡ at 298 K using ΔG1 = ΔH‡ − TΔS‡ to be 25.2 kcal mol−1. However, our theoretical data given in Table 1 is too high compared with this value. Thus, one reviewer indicates that the theoretical level used here may be insufficient to discuss the reactions. Nevertheless, because it is well-known that the entropy (ΔS‡) is the function of temperature, the value of ΔG‡ should be changed once the temperature factor is considered. (19) However, this experimental study was based on the temperature 351 K. Our Gibbs free energy given in Table 1 is shown to be 51 kcal/ mol, but at 298 K. Nevertheless, It is believed that using more sophisticated theoretical methods should greatly reduce the barrier height. (20) For details, see: (a) Shaik, S.; Schlegel, H. B.; Wolfe, S. Theoretical Aspects of Physical Organic Chemistry; John Wiley & Sons Inc.: New York, 1992. (b) Pross, A. Theoretical and Physical Principles of Organic Reactivity; John Wiley & Sons Inc.: New York, 1995. (c) Shaik, S. Prog. Phys. Org. Chem. 1985, 15, 197. (d) Shaik, S.; Hiberty, P. C. In A Chemist’s Guide to Valence Bond Theory; Wiley, Interscience: New York, 2008. (21) (a) The first paper that originated the CM model: Shaik, S. J. Am. Chem. Soc. 1981, 103, 3692. (b) About the most updated review of the CM model, one can see: Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. 1999, 38, 586. (22) The addition of ketone to dimetallenes, see: (a) Barrau, J.; Escudié, J.; Satgé, J. Chem. Rev. 1990, 90, 283. (b) Wiberg, N.; Link, M. Chem. Ber. 1995, 128, 1231. (c) Boomgaarden, S.; Saak, W.; Weidenbruch, M.; Marsmann, H. Organometallics 2001, 20, 2451. (d) Wakita, K.; Tokitoh, N.; Okazaki, R.; Nagase, S.; von Schleyer, P.; Jiao, H. J. Am. Chem. Soc. 1999, 121, 11336. (e) Leigh, W. J. Pure Appl. Chem. 1999, 71, 453. (f) Mosey, N. J.; Baines, K. M.; Woo, T. K. J. Am. Chem. Soc. 2002, 124, 13306 and related references therein.. (g) Gusel’nikov, L. E. Coord. Chem. Rev. 2003, 244, 149. (h) Li, B.Y.; Su, M.-D. Organometallics 2011, 30, 6189. (23) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; John Wiley & Sons: New York, 1985; pp164. 4231
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
The Journal of Physical Chemistry A
Article
Organometallics 2001, 20, 5707. (ff) Veszprémi, T.; Takahashi, M.; Hajgato, B.; Kira, M. J. Am. Chem. Soc. 2001, 123, 6629. (gg) Takahashi, M.; Sakamoto, K.; Kira, M. Int. J. Quantum Chem. 2001, 84, 198. (hh) Schmohl, K.; Reinke, H.; Oehme, H. Eur. J. Inorg. Chem. 2001, 2, 481. (ii) Leigh, W. J.; Li, X. Organometallics 2002, 21, 1197. (jj) Bendikov, M.; Quadt, S. R.; Rabin, O.; Apeloig, Y. Organometallics 2002, 21, 3930. (kk) Owens, T. R.; Harrington, C. R.; Pace, T. C. S.; Leigh, W. J. Organometallics 2003, 22, 5518. (ll) Oláh, J.; Veszprémi, T. Organometallics 2008, 27, 2723. (mm) Takahashi, M.; Veszprémi, T.; Kira, M. Organometallics 2004, 23, 5768. (nn) Leigh, W. J.; Owens, T. R.; Bendikov, M.; Zade, S. S.; Apleoig, Y. J. Am. Chem. Soc. 2006, 128, 10772. (oo) Yamabe, S.; Mizukami, N.; Tsuchida, N.; Yamazaki, S. J. Organomet. Chem. 2008, 693, 1335. (pp) Guliashvili, T.; Tibbelin, J.; Ryu, J.; Ottosson, H. Dalton Trans. 2010, 39, 9379. (28) We also searched for dimetallene−MeOH and dimetallene− 2MeOH precursor complexes for such addition reactions at the B3LYP level using numerous starting geometries with various conformations. However, our computational results always showed that these were unsuccessful. We thus anticipate that these precursor complexes do not exist based on the present level of theoretical calculations.
4232
dx.doi.org/10.1021/jp3018138 | J. Phys. Chem. A 2012, 116, 4222−4232
