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Computational Investigation on the Role of Disilene Substituents Toward N2O Activation Bholanath Maity and Debasis Koley* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India S Supporting Information *

ABSTRACT: The effect of substituents in disilene mediated N2O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (IA−Ft) in the presence of four different substituents (−NMe2, −Cl, −Me, −SiMe3) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decomposition followed by N2O coordination and Pathway II constituting the N2O attack without Si−Si bond cleavage. Depending on the fashion of N2O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the “end-on” and “side-on” attack to the disilene scaffold. Interestingly, the lone pair containing substituents (−NMe2, −Cl,) facilitates disilene → silylene dissociation; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N2O attack. In the absence of any lone-pair influence from substituents (−Me, −SiMe3), the decomposition of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N2O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calculations reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19−26 kcal/mol) is Pathway IIb, which involves successive addition of two N2O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estimated in terms of the first activation barrier of N2O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si−Si bond strength, indicating toward the versatility of the reaction route.

1. INTRODUCTION During the last few decades, remarkable progress has been made in the chemistry of compounds with heavier Group 14 elements.1−16 Following the successful synthesis and isolation of the first stable disilene in 1981,17 these classes of compounds have attracted serious attention in chemical community.18−28 Thereafter, numerous disilenes were synthesized and characterized in terms of their structure, bonding, and reactivity.29−31 The wide variety of reactions of disilenes not only attained curiosity in main group chemistry but also made an indispensable building block in organosilicon chemistry and organic synthesis.32,33 The fundamental reactivity of disilenes, particularly the addition reactions of water,34,35 alcohol and substituted phenols,36,37 HX (X = F, Cl)38,39 and other small molecules40−42 were extensively studied over the last three decades. Recently, Jutzi, Scholler et al. successfully isolated a disilene compound, trans-[(TMS)2N(η1-Me5C5)SiSi(η1Me5C5)N(TMS)2] (It) stable under ambient reaction conditions.43 A year later, Roesky and co-workers were successful in establishing a new method for synthesizing the same compound with enhanced yield.44 Furthermore, the same authors have extended their study in demonstrating the reactivity of It toward small molecules activation, viz., nitrous © 2016 American Chemical Society

oxide (N2O), elemental sulfur (S8), and white phosphorus (P4).45,46 It was surprising to note that N2O addition to transdisilene, It afforded a combination of isomeric cis- and transdioxadisiletane (Pc and Pt; Scheme 1) products. This is the first such report where both the isomers were isolated and characterized from the single disilene precursor. The cyclodisiloxanes, the product of the present reaction (Scheme 1), exhibit commercial importance as precursors to high molecular weight silicone polymers.47 These products can also be easily obtained by the reaction of molecular oxygen with disilene.48−53 Another most significant aspect of this reaction is the room temperature decomposition of N2O, which is a long-lived atmospheric trace gas. With a global warming potential of nearly 300 times that of carbon dioxide, N2O is the third most significant greenhouse after carbon dioxide and methane.54−56 In addition, N2O is now the dominant stratospheric ozone-depleting substance, because restrictions on the use of chlorofluorocarbons (CFCs) have greatly reduced their emissions to the atmosphere.57 The concentration of N2O Received: December 1, 2016 Published: December 7, 2016 401

DOI: 10.1021/acs.jpca.6b11988 J. Phys. Chem. A 2017, 121, 401−417

Article

The Journal of Physical Chemistry A Scheme 1. Nitrous Oxide Addition to trans-Disilene It

Scheme 2. Favorable Pathway in N2O Addition to the Ita

a

Energy values over the arrows are relative Gibbs free energy change (ΔGsolL in kcal/mol) for each steps at BP86(SMD)/TZVP//BP86/SVP level.60

Figure 1. Isolated trans-disilenes with different substituents: (a) −Mes (2,4,6-trimethylphenyl) and −N(SiMe3)2;4 (b) −Mes and tBu; (c) −SiMe3 and 2,4,6-triisopropylphenyl;76 (d) −Cl and −SiMe(SitBu3)2.77

undergoes facile transformation to transient silanone intermediate 2 (Scheme 2). The silanone will further react with another unit, leading to either intermediate 3c or 3t. Intermediates 3c and 3t are loosely bound complexes with −Cp* and −N(Me3Si)2 substituents oriented either trans or cis to each other. Subsequent combination of the silanone fragments in 3c and 3t will give rise to the product mixture. Moreover, from energetic grounds the formation of transdioxadisiletane product (Pt) is more favorable than the isomeric cis-dioxadisiletane (Pc), which is in good agreement with the experimental findings. In this present study, we have revisited the mechanism of thermal reaction of disilenes with N2O by density functional theory (DFT) calculations for a systematic understanding of substituent effects on the reactivity of disilene, because the substituent plays an important role on the geometries and energies of SiSi double bonds.68,69 We have collected possible substituents in the present study from different isolated disilenes reported in the literature (Figure 1). Some of substituents are truncated to a smaller model to reduce the computational cost while the rest remain unchanged to examine the steric effect exhibited throughout the reaction. Herein our aim is to explore the effect of different substituents in modulating the reaction pathways in N2O addition.

in Earth’s atmosphere is steadily increasing and is currently 19% higher than preindustrial levels.58,59 The mechanistic study of reactions of the compounds containing double bonded silicon is still in its infancy. It is a great challenge to interpret the interesting reactivity in terms of electronic structure and bonding. In the last year, we have employed our effort to illustrate the mechanism of the above reaction (Scheme 1) by computational study.60 We have reported that in an energetically favorable route, the disilene would be dissociated to respective silylene (S) units prior to N2O attack (Scheme 2). Such dissociation equilibrium of disilene and silylene was the subject of interest over the last few decades.61−63 Okazaki et al. first reported the thermal dissociation of extremely hindered and kinetically stable disilene to silylene under mild conditions.40,64,65 Tsutsui et al. further supported the existence of thermal dissociation by performing trapping experiments and DFT calculations.66 Kira and co-workers reported spectroscopic evidence for the existence of the silylene in equilibrium with the corresponding disilene at low temperature.67 Later, West and Apeloig reported the existence of dynamic equilibrium between Z-diaminodisilyldisilene and diaminosilylene by NMR and UV−vis spectroscopy.61 The same authors have shown that two molecules of the saturated diaminosilylene initially undergo an insertion reaction leading to an intermediate silylene, which later dimerizes to the novel Z-diaminodisilyldisilene. However, in the presence of N2O, the monomeric silylene species S 402


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The Journal of Physical Chemistry A

Table 1. Energy Changes and Required Activation Barriers for Disilene Dissociation into Silylenes (It → S; Scheme 2)a and Singlet−Triplet Energy Difference (ΔESTωB97xD) for the Respective Silylenes step

ΔHgωB97xD

ΔGgωB97xD

ΔGLωB97xD

ΔGLM06‑2X

Δ‡GLωB97xD

Δ‡GLM06‑2X

→ 2SA → 2SB → 2SC → 2SD → 2SE → 2SF IR → 2SR SA

7.2 20.0 30.3 34.0 52.8 70.7 60.0 SB

−2.9 8.8 20.0 21.1 40.6 60.2 50.1 SC

−3.9 10.0 21.0 23.1 41.7 62.0 51.5 SD

−3.9 7.3 17.7 19.2 37.8 57.5 48.2 SE

3.0 15.5

1.8 12.5

SF

SR

36.3

23.2

16.9

11.6

19.9

IAt IBt I Ct I Dt IEt IFt

53.5 a

41.2

The energy terms (in kcal/mol) are described in the Computational Details.

2. COMPUTATIONAL DETAILS The Gaussian09 program package 70 was used for all calculations. Geometry optimizations and vibrational frequency calculations of all saddle points were performed at the DFT level, using the long-range corrected hybrid density functional, ωB97xD, with damped atom−atom dispersion corrections.71 All the atoms were treated with Ahlrich’s triple-ζ quality split valence plus polarization (TZVP) basis sets.72,73 The geometries were optimized without any symmetry constraints. Harmonic force constants were computed at the optimized geometries to characterize the stationary points as minima or first-order saddle points. Intermediates and transition states were discerned by the presence of 0 or 1 imaginary frequency, respectively. Transition states were located from a linear transit scan where the reaction coordinate was kept fixed at different distances while all other degrees of freedom were relaxed. After the linear transit search, the transition states were optimized using the default Berny algorithm implemented in the Gaussian quantum code.70 All the transition states were further validated by IRC (intrinsic reaction coordinate) calculations. Zero-point vibrational corrections were determined from the harmonic vibrational frequencies to convert the total energies (Ee) to ground state energies E0. The rigid-rotor harmonic-oscillator approximation was applied to evaluate the thermal and entropic contribution terms, which are used to derive the enthalpies (Hg) and Gibbs free energies (Gg) at 298 K. For further validation of the energetics, and to investigate the effect of basis set expansion, single-point calculations were performed using ωB97xD and M06-2X functionals on ωB97xD/TZVP optimized structures employing a valence quadruple-ζ quality of basis set (QZVP).74,75 Single-point solvent calculations at M062X/QZVP//ωB97xD/TZVP levels were performed for all the intermediates and transitions states, using the SMD continuum solvation model78 implemented in Gaussian09. Toluene was chosen as a solvent (dielectric constant ε = 2.374) with SMDintrinsic Coulomb radii for the respective atoms.45,46 All singlepoint calculations were performed with tight wave function convergence criteria and “ultrafine” (99 950) grid was used in numerical integration. The different energy terms, ΔHgωB97xD, ΔGgωB97xD, ΔGLωB97xD, and ΔGLM06‑2X, are collected in Table 1a, whereas only ΔGLM06‑2X values are used in other tables and text, if otherwise not mentioned. The first two terms are gas-phase enthalpy and Gibbs free energy changes at ωB97xD/TZVP level and last two terms are Gibbs free energy changes with the calculated electronic energies at ωB97xD/QZVP and M06-2X/ QZVP levels augmented with the gas-phase free energy correction at ωB97xD/TZVP level.79 The charge distribution

was analyzed using the Weinhold’s NPA (natural population analysis) approach.80 The wave function file generated at the M06-2X/6-311++G(2d,2p)//ωB97xD/TZVP81 level was used to perform QTAIM82 analysis in the AIMALL program suite to investigate Si−Si bonding scenario in disilenes. We have applied Bader’s AIM (atoms-in-molecule)83,84 concept to characterize the electron distributions in IA−Ft. Any bonded pair of atoms has a bond path, i.e., a connecting line with maximum electron density. The bond critical point (BCP) is a point on this line where the gradient ∇ρ(r) of the density is equal to zero. The magnitude of the electron density, ρ(r), and its Laplacian, ∇2ρ(r), at the BCP provide information about the strength and type of bond. The Laplacian indicates whether the density is locally concentrated (∇2ρ < 0) or depleted (∇2ρ > 0). To get insight into the bonding interactions in disilenes and charge displacement in some transition states, we have performed the energy decomposition analysis (EDA) using the program package ADF 2013.01.85 All analyses were performed by employing the BP8686 functional in conjunction with a triple-ξ-quality basis set using uncontracted Slater-type orbitals (STOs) augmented by two sets of polarization functions (TZ2P) for all atoms, with no frozen-core approximation for inner core electrons.87−90 To account for the scalar relativistic effects, zeroth-order regular approximation (ZORA) was applied. 91 The EDA method, developed independently by Morokuma92,93 and Ziegler and Rauk,94,95 gives a quantitative description of the chemical bonds in molecules.96−99 In this method the bond dissociation energy De of a molecule AB is divided into the instantaneous interaction energy ΔEint and the preparation energy ΔEprep [eq 1]: ΔE(=−De) = ΔE int + ΔEprep

(1)

The preparation energy ΔEprep is the energy that is required to promote fragments A and B from their equilibrium geometries in the electronic ground state to the geometries in the corresponding intermediates/complexes, whereas ΔEint is the actual interaction energy between the prepared fragments and form the respective intermediates. ΔEint can be further divided into three main components: ΔE int = ΔEelstat + ΔE Pauli + ΔEorb

(2)

In eq 2, ΔEelstat is the quasiclassical electrostatic interaction energy between the fragments. ΔEPauli refers to the repulsive interactions between the fragments, which are caused by the fact that two electrons with the same spin cannot occupy the same region in space and can be calculated by enforcing the Kohn−Sham determinant on the superimposed fragments to 403


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Figure 2. Optimized geometries of substituted disilenes at the ωB97xD/TZVP level. All bond lengths are in Å.

will give rise to cis- and trans-dioxadisiletane products, respectively. The above reaction route is studied for all the substituted disilenes in the current manuscript and is referred as Pathway I, which is discussed in section 3.1 (vide inf ra). The second mechanistic route, named as Pathway II, which is discussed in section 3.2 considers the direct N2O attack to the disilene precursor. 3.1. Pathway I. We have first decided to study the energetics of disilene to silylene dissociations for all substituted disilenes, IA−Ft,R as collected in Table 1a. Before discussing the relative energy changes for different disilenes, we have illustrated the factors responsible for the dissociation to occur. Kira et al. clearly mentioned that the bond dissociation energy effectively depends upon the π-bond energy gained in the individual silylene compounds upon dissociation. The Carter−Goddard−Malrieu−Trinquier (CGMT) model explains the theoretical understanding of this proposition.106−111 According to this model, the structural conformation adapted by the disilene and its corresponding silylene depends upon the stable electronic ground state of silylene monomer. For example, a silylene with lower singlet−triplet energy difference (ΔEST) gives rise to mostly planar disilene with a short SiSi double bond. A silylene featuring a high ΔEST value leads to the formation of a more pronounced trans bent disilene with a comparatively longer SiSi bond, possessing double dative type bond character. The disilene → silylene dissociation energies follow the order IAt < IBt < ICt < IDt < IEt < IFt, showing good correlation with the ΔEST values of respective silylenes (Table 1b). Goddard et al. explained the physical effects that can control the ΔEST value of silylene species.106 According to their argument, the bond between the electronegative atom and the Si center in silylene exhibit higher p-character, subsequently affecting the nonbonding lone pair with more s-character. In another fact, if the substituents have π-symmetric lone pairs, which can donate electron density to the Si p-orbital, then the singlet state becomes more stabilized. It is also worthwhile to mention that the disilene-containing lone pair donor substituents can also adapt a heterobridged conformer as a thermodynamically stable form.112,113 Considering this situation, we have optimized the disilene species in their respective bridging geometries (RA‑E, Figure S1 in the Supporting Information). Interestingly, all the bridging forms

obey the Pauli principle by antisymmetrization and renormalization. The stabilizing orbital interaction term ΔEorb is calculated in the final step of the energy partitioning analysis when the Kohn−Sham orbitals relax to their optimal form. Additional information on the EDA method and its application can be found elsewhere.100 Furthermore, to gain deeper insight on the orbital information in chemical bond, we have performed EDA-NOCV (EDA with natural orbitals for chemical valence) calculations. The EDA-NOCV101−103 method combines charge and energy partitioning schemes to decompose the deformation density (Δρ), which is associated with the bond formation, into different components of the chemical bond. The EDA-NOCV scheme provides pairwise energy contributions for each pair of interacting orbitals to the total bond energy. All chemical structures are drawn using ISIS-draw. Ball and stick models are made from Chemcraft and CYLview visualization programs.104,105

3. RESULTS AND DISCUSSION Our interest is to explore both electronic and steric effects of substituents in disilenes toward the reactivity with N2O species. Therefore, we have chosen four different types of substituents in this current study; two lone-pair-containing groups, viz., −NMe2 and −Cl, and two others without lone pairs, −Me and −SiMe3. −Cl is selected as it can exhibit both −I and +R effects whereas −SiMe3 is considered a substituent that will impose steric influence around the disilene scaffold. As a starting point of our mechanistic endeavor, we have considered only the trans isomers of disilene moieties. In total there are six compounds, IXt (X = A−F) possible by taking combinations from four substituents (Figure 2). For further comparative understanding in terms of bonding and reactivity, we have included the smallest disilene Si2H4 (IR) as a reference molecule in our computational study. According to our previous report, the energetically facile route of N2O addition to the silylene (S) followed the formation of transient silanone species 2 (Scheme 2).60 This mechanism requires dissociation of disilene into silylenes prior to N2O attack. However, intermediate 2 will further react with another silanone unit, to afford loosely bound adducts 3c or 3t. Subsequent combination of the silanone fragments in 3c and 3t 404


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Figure 3. KS-HOMO of disilenes at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The trans bent angle is shown in the left corner and the respective magnitudes (Φ in degrees) are in parentheses.

Figure 4. (a) 2D contour plot of Laplacian, ∇2ρ(r) at M06-2X/6-311++G(2d,2p)//ωB97xD/TZVP. Blue lines indicate the charge depletion (∇2ρ(r) > 0), and red lines indicate charge concentration (∇2ρ(r) < 0). The black lines connecting the nuclei are the bond paths; green dots are BCPs. (b) 1D Laplacian profile along the Si1−Si2 bond path in IAt (top) and IFt molecule (down).

directionality (Figure 3). Therefore, the greater the trans bent geometry, the weaker will be the Si−Si bond, leading to a facile dissociation to monomeric silylenes (Figure 3 and Table 1a). To elucidate the bonding nature of Si−Si bond with varying substituents, we have calculated the bonding parameters (ρ(r), ∇2ρ(r); Computational Details) at the BCP for disilenes IA−Ft (Figure 4). In general, for a covalent bond, the magnitude of ρ(r) is greater than 0.20 au and less than 0.10 au for closedshell interaction.83,84 Therefore, all the ρ(r) ≤ 0.10, indicates Si−Si bonds to have closed-shell character. Moreover, the ρ(r) values smoothly increase from IAt → IFt, exhibiting close resemblance to the Si−Si bond strength. Interestingly, the negative values of ∇2ρ(r) reveal that electronic charges are concentrated in the interatomic path, which is characteristic of a covalent interaction. This is not surprising as the Laplacian values at the BCP reside on the region of charge accumulation for all disilenes. However, it is also interesting to note that the Laplacian values become more negative from IAt → IFt, indicating an enhancement of covalent character.114−116 In this cases it is more informative to draw a 2D graph of the Laplacian (∇2ρ(r)) along the Si−Si bond axis, showing the

are energetically less stable than the parent disilenes except for I A t , where −NMe 2 and −Cl substituents are present [ΔG(IA−E→RA−E) = −7.4, 0.6, 20.0, 8.5, and 37.3 kcal/mol at the ωB97xD/TZVP level, Figure S1]. However, we have considered the trans bent form of IAt in this study to be consistent in comparison with other disilenes. According to the CGMT model disilenes having −NMe2 and −Cl groups should exhibit more trans bent geometry, which is substantiated with the highest value reported in Figure 3. Compound IAt containing two nonbonding donor substituents shows a maximum trans bent angle (Φ = 34.4°), whereas IFt is completely planar (Φ = 0°) due to the absence of any lone pair donor groups. Trinquier et al. proposed that the trans bent geometry of disilene arises from the interaction between singlet silylene species (Figure S1).107−111 The nonbonding electron pair of one silylene species is partly delocalized into the empty p-orbital of another silylene moiety. This type of donor− acceptor bond between two Si atoms leads to the distorted geometry as well as longer Si−Si bond length. The donor− acceptor type of bonding is further supported by the HOMO of IXt, where the lone pair lobes on the Si atoms displays opposite 405


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The Journal of Physical Chemistry A Table 2. Energy Decomposition Analysis (kcal/mol) for Disilenesa

a

molecule

ΔEint

ΔEPauli

IAt IBt ICt IDt IEt IFt IR

−8.7 −24.7 −39.2 −39.8 −59.6 −81.9 −80.3

81.3 169.9 197.4 215.8 245.5 439.9 291.8

ΔEelstat −48.0 −102.5 −114.2 −130.0 −145.8 −229.4 −176.9

(53.4%) (52.7%) (48.3%) (50.9%) (47.8%) (44%) (47.6)

ΔEorb −41.9 −92.1 −122.4 −125.6 −159.4 −292.4 −195.2

(46.6%) (47.3%) (51.7%) (49.1%) (52.2%) (56.0%) (52.4%)

ΔEattr −89.9 −194.6 −236.6 −255.6 −305.2 −521.8 −372.1

ΔEattr represents the total stabilizing terms (ΔEelstat + ΔEorb). The values in parentheses represent the percentage contribution to the ΔEattr.

Figure 5. Pathway I energy profile for IAt. Red and green lines represent the trans and cis addition of silanone.

We have explored the mechanistic pathway for all the silylene (SA−F,R) species (Table 1a). Herein, the fundamental steps involve in Pathway I are described for SA (Figure 5) whereas for other analogues (SB−F,R) they are collected in the Supporting Information (Figures S2−S7). According to our previous report the terminal N atom of N2O binds to the silylene S leading to a stable intermediate 1, as displayed in Scheme 2.60 In search of a similar S → 1 step, the N2O was allowed to react with the silylene SA, leading to the transition state [SA-1A]‡, accompanied by an activation barrier of 29.0 kcal/mol (Figure 5). The imaginary eigenmode corresponds to the incoming motion of terminal atoms (N and O) to the Si center. Unfortunately, we were unable to optimize a Si−N bonded intermediate similar to 1 despite repeated attempts; instead, facile liberation of N2 from the transition state occurs, directly leading to the silanone intermediate 2A. To our surprise, analogous species 1B−E,R exist as true intermediates in the energy surface, and apart from 1R, we surmise that the influence of a single lone-pair-containing substituent might play a role for their existence (Figures S2− S7). It is important to mention about recent experimental evidence regarding isolation and characterization of similar N2O adducts with various substituted N-heterocyclic carbenes by Severin et al.117,118 Existence of silanone, in general, as a transient species eludes its isolation and further characterization. The high reactivity of silanone is attributed to the strongly polarized SiO bond, which is considered to have zwitterionic character with an electron deficient Si+ center and a negatively charged oxygen atom.119−121 Driess and co-workers have recently stabilized and

valence shell charge concentrations (VSCC, Figure 4a). The shape of VSCC (red line) in IAt indicates the presence of lone pair (LP) charges residing at Si centers. There is no charge delocalization observed between Si1 and Si2, which reveals very weak Si−Si interaction, whereas from IAt → IFt the extent of electron sharing is gradually developed and finally in IFt a σ−π type interaction dominates. Furthermore, we have monitored the changes in Laplacian values along the Si1−Si2 bond path (Figure 4b). For IAt, two clear VSCC minima residing in each atomic basins are observed in line with the 2D contour plots, whereas in the case of IFt, only one minimum at the BCP is obtained, suggesting a better covalent interaction. The bonding interaction between Si centers in disilenes was further interpreted by using the energy decomposition analysis (EDA) technique. The EDA results (Table 2) show that the bonding energy (ΔEint) gradually increases from IAt → IFt, a trend well in accordance with the bond dissociation energies reported in Table 1a. Surprisingly, the destabilizing term (ΔEPauli) also increases along with the ΔEint values for the disilenes. This intriguing pattern can be justified after considering the total attractive interaction energies, ΔEattr (ΔEelstat + ΔEorb) between the silylene fragments. From IAt to IFt, the magnitude of ΔEattr becomes more dominant over the repulsive ΔEPauli. Interestingly, apart from disilene IDt, the percentage of orbital interaction (ΔEorb) gradually increases in comparison to the electrostatic component ΔEelstat. Therefore, in general, we can state that the orbital interaction plays a profound role in estimating the Si−Si bond stability in disilenes. 406


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The Journal of Physical Chemistry A Table 3. Energies (kcal/mol) for All the Steps (Scheme 2) Involved in Pathway I steps

IAt

IBt

I Ct

IDt

IEt

[IXt-SX]‡ ‡

1.8 −5.3 29.0

12.5 −5.2 22.9 −21.2 −213.7 4.1 −93.5 3.6 −93.7

17.7a 21.7 −16.2 −206.9 4.6c −111.2 3.8 −111.1c

19.2a 19.2 −61.9 −175.4 11.0c −91.3 10.1 −90.8c

37.8a 13.6 −16.7 −212.3

−230.8b

48.2a 2.6 −92.6 −120.6

−92.9d

−98.5d

−108.0d

−95.5c

−99.2c

→ [IXt-SX] → 2(SX) SX+N2O → [SX-1X]‡ [SX-1X]‡ → 1X 1X → 2X + N2 2(2X) → [2X-PXc]‡ [2X-PXc]‡ → PXc 2(2X) → [2X-PXt]‡ [2X-PXt]‡ → PXt IXt

−210.9b 2.6 −98.2 2.3 −97.9 ‡

IFt 57.5a 6.8

IR

‡

Transition state [IX -SX] (X = C to F and R) does not exist. Energy change for step [SX-1X] → 2X + N2, in the absence of intermediate 1x. Loosely bound intermediates 3Xt/3Xc (X = C and D) present rather than the corresponding transition state. dNeither transition state nor loosely bound intermediate exist in the case of 2X (X = E, F, and R) recombination. a

t

b

c

Figure 6. (a) Relative frontier orbital energies of SA and N2O from NBO study; (b) orbital interaction of N2O with SA in transition state [SA-1A]‡; (c) plot of deformation densities (Δρ) of the pairwise orbital interaction between SA and N2O in [SA-1A]‡. All energy terms presented are in kcal/ mol. The color code of charge flow is sky-blue → green and ν represenst eigenvalues.

silylene dissociation is easier in the presence of lone pair donor substituent’s having higher ΔEST values (vide supra). From Table 3 we have observed that the activation barriers gradually decrease from IA → IF in N2O attack step (SX + N2O → 1X). This interesting aspect can be addressed by employing the frontier molecular orbital approach. The lesser energy gap between SA-HOMO and N2O-LUMO (9.0 eV, Figure 6a, S8) will allow easy donation of the lone pair residing at the Si center to π* orbital of N2O unit (Figure 6b). Thus, during the step SA → 1A, the electron density is reduced on the Si atom that is confirmed by the significant increase of positive NPA charge on Si center in [SA-1A]‡ [|ΔqSi(SA → [SA-1A]‡)| = 0.244]. Therefore, we anticipate that activation energy will gradually decrease with increase in NPA charge on silicon center in silylene SX. However, the decreasing order of the NPA charge [qSi: 1.015 (SB) > 1.007 (SA) > 0.966 (SC) > 0.654 (SD) > 0.627 (SF) > 0.557 (SE)] on Si centers does not comply with the activation barrier trend for the SX → 1X step (Table 3). For deeper understanding we have performed EDA-NOCV analysis for transition state [SA-1A]‡. Energy terms associated with the interaction of disilene fragment with N2O are shown in Table S1. The plot of deformation density of the pairwise orbital interactions is presented in Figure 6c. The major contribution to the orbital interaction (ΔEorb = −67.1 kcal/mol) arises from the σ-donation of SA fragment into the anti-bonding π*-orbital of N2O fragment. Another significant contribution arises due to π-back-donation from N2O to vacant p-orbital on Si center. The former interaction depends on the electronegativity of the

isolated silanone as Lewis base adducts, in which the electrondeficient Si+ center is stabilized efficiently by coordination of a Lewis base.122,123 However, the highly reactive silanone 2A readily undergoes a [2+2] cycloaddition type process with another 2A to form cyclic dioxane products (Figure 5). To investigate the proper potential energy surface (PES), we allowed the approach of two SiO in a fashion where the substituents are oriented either trans or cis to each other. This reaction step requires very low activation barriers of 2.3 and 2.6 kcal/mol for transition states [2A-PAt]‡ and [2A-PAc]‡, respectively (Figure 5). In general, it is not always possible to optimize a true transition state for the silanone recombination step. From intermediates 2C and 2D we could only optimize loosely bound intermediates (Figure S3, S4) similar to 3c and 3t, as reported for the [Si(Cp*)N(TMS)2] system (Scheme 2). In the case of 2E, 2F, and 2R neither the transition state nor the unstable intermediates were converged.124 The relative energies of the transition states or loosely bound intermediates are responsible to control the product selectivity of the reaction. Table 3 represents the relative energy changes in Pathway I for the disilenes IA−Ft,R. We noted an interesting observation during the disilene decomposition step; in teh case of disilenes IAt and IBt moderately low activation barriers (1.8 and 12.5 kcal/mol) are required, whereas the remaining disilenes (IC−Ft,R) decomposed without any barrier possibly due to the high endothermicity of the dissociation step. This, in fact, gets support from the previous explanation that the disilene to 407


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Figure 7. (a) Relative frontier orbital energies of IAt, IFt, and N2O; (b) orbital interaction of N2O with disilene during “side-on” attack, where E1X = | E(N2O-HOMO) − E(IXt-LUMO)| and E2X = |E(IXt-HOMO) − E(N2O-LUMO)| in eV.

addition route, the SiSi double bond is initially activated to furnish mono-oxa cyclic intermediate and then a second N2O addition leads to dioxa cyclic products (Figure S9).60 To trace the potential energy surface for step It → 5t, the N2O molecule was allowed to advance uniformly toward both the silicon centers in It. However, during the transition state optimization of [It-5t]II‡ the O atom moved closer to one of the Si center (Figure S9). Such a preferential approach of the O atom toward a particular Si center is also observed in [5t-Pt]II‡, which leads to the trans isomeric product Pt (red line in Figure S9). The cis product formation route was calculated where the initial isomerization of 5t to 5c occurs and then subsequent N2O addition generates the cis isomer (green line). Calculations reveal that high transition barriers are required in both steps (Δ‡GsolL(It→5t/5t→Pt) = 32.8/39.7 kcal/mol), suggesting the direct N2O attack route to be least favorable. Even though, in this current study, we have examined a similar, uniform approach of N2O to the Si centers for all the reported disilenes, unfortunately, we were unable to optimize transition states similar to [It-5t]II‡ for disilenes IBt, IDt, and IFt. To seek a proper explanation for the addition step, we resorted to frontier molecular orbital (FMO) analysis of the reactants. Interestingly, we noticed that the energy gap between IA/FtHOMO and N2O-LUMO is lower than that between IA/FtLUMO and N2O-HOMO orbitals (Figure 7a). Therefore, the electron density of IXt can be easily transferred to the π* orbital of N2O (green line, Figure 7a), which is distributed over the entire molecule. With this notion, we have investigated two different modes of N2O approach to the disilene scaffold. In one case, the O atom will advance uniformly to the Si centers similar to an “end-on” type of attack and will be discussed as Pathway IIa. Alternatively, the terminal N and O atoms of the

substituent’s atom whereas later it is controlled by the lone pair donor ability of the substituents. Therefore, substituents having a lone pair electron as well as the −I effect will increase the activation barrier. The relative activation barriers for the steps SA/B→1A/B and SA/C→1A/C will roughly quantify the influence of electronegative substituent (Cl) and lone pair (NMe2) in the N2O addition step. Considering only the electronic effect, energy differences (ΔΔG‡) of 6.1 and 7.3 kcal/mol for the former and the latter pair of steps signify a higher contribution from the lone pair donor substituents. From the above findings it is worthwhile to note that the lone pair donor substituents exhibit antagonist influence on the reaction steps: on one hand it accelerates the IXt → SX dissociation whereas on the other hand it retards the SA + N2O → 1A step. However, in our parent silylene S (Scheme 2) the N lone pair is no longer delocalized toward the Si center, because the filled orbital remains orthogonal to the vacant p-orbital on Si, mostly due to the steric bulk from TMS group.60 In that case the silylene moiety is stabilized by the augmentation of coordination number on silicon center by the −Cp* group. 3.2. Pathway II. Allen et al. reported that the σ and π-bond strengths are largely determined by the π−σ* energy gap of a double bond and therefore related to the HOMO−LUMO separation of the molecule.125 The calculated HOMO−LUMO energy gap in H2SiSiH2 is much smaller (7.4 eV) than H2CCH2 (11.8 eV), indicating a weaker SiSi bond than the prototypical CC double bond.126 This result implies that in contrast to CC bonds, SiSi bonds are well-known to react readily with water and alcohols in the absence of any catalytic intervention.34−37 Hence there remains an alternative possibility of direct N2O insertion into the reactive π-bond of disilenes.127 As reported earlier, following the direct N2O 408


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Figure 8. Pathway IIa energy profile diagram for IEt.

Figure 9. Frontier orbitals of trans-monooxadisilirane 5A−Ft,R at the M06-2X/QZVP//ωB97xD/TZVP level of theory. H is KS-HOMO and L is KSLUMO respectively. The orbital eigenvalues (in eV) are given in parentheses, where E1X = |E(N2O-HOMO) − E(IXt-LUMO)| and E2X = |E(IXtHOMO) − E(N2O-LUMO)| in eV.

the transition state [IEt-5Et]IIa‡ directly affords the monooxadisilirane 5Et instead of any intermediate having a loosely bound N2 unit similar to 4Ft (vide inf ra). In 5Et the Si−Si bond shows a covalent σ-bond character and it is true for all other computed 5Xt’s (Figure 9). Frontier molecular orbitals allow us to envisage the reactivity between the N2O-HOMO with the 5Et-LUMO, signifying a similar “end-on” type attack to the Si− Si σ-bond. It is evident that the second N2O addition will entail a higher activation energy than the previous one [Δ‡GLM06‑2X([IEt-5Et]IIa‡/[5Et-PEt]IIa‡ = 27.0/36.2 kcal/mol, Figure 8] because the rupture of the Si−Si σ-bond takes place unlike the π-cleavage in the initial case. This energy trend is consistent with the other disilenes, as reported in Figure 8. In this regard it is worthwhile to mention that the second N2O attack via the transition state [5Xt-PXt]IIa‡ is observed for all substituted 5A−Ft due to their very similar frontier orbital shape (Figure 9. Table S2). Therefore, Pathway IIa can be invoked for the second oxygenation step once the mono-oxacyclic intermediates 5B,D,Ft get formed via some other mechanistic route, viz., Pathway IIb (vide inf ra). 3.2.2. Pathway IIb. As an alternative route of N2O coordination, the “side-on” attack is discussed in this section.

N2O molecule will interact separately with the two Si atoms replicating a “side-on” attack. This latter route, referred to as Pathway IIb, will be mentioned in detail in the forthcoming sections. 3.2.1. Pathway IIa. In our previous calculations, it was observed that “end-on” attack of N2O to 1t resulted in Pt and Pc ring products with activation barrier as high as 41 kcal/mol (ΔGsolL; BP86(SMD)/TZVP//BP86/SVP level of theory). In this current study, similar “end-on” attack of N2O is feasible for disilenes IAt, ICt and IEt respectively. This section describes the mechanism of N2O addition in the presence of IEt (Figure 8), and the energy profiles for IAt and ICt are collected in the Supporting Information (Figures S10 and S11). For IEt the first N2O attack step via the transition step [IEtt 5E ]IIa‡ requires an activation barrier of 27.0 kcal/mol. The transition eigenmode involves asynchronous coordination of the O atom toward the Si centers (Si−O = 1.958/3.295 Å); a similar observation was reported for N2O addition with It. Additionally, an analogous off-center type of ethylene coordination to substituted germylene (GeR2) was calculated by Su and Chu using state-of-the-art computational methods.128 Surprisingly, due to the high feasibility of N2 evolution, 409


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Figure 10. Pathway IIb energy profile diagram for IFt.

Figure 11. Plot of deformation densities of the pairwise orbital interaction between disilene fragment and N2O fragment in [IAt-4At]‡ (left) and [IFt4Ft]‡ (right), respectively. For other convention, refer to Figure 6.

Unlike in Pathway IIa, the “side-on” attack of N2O is favored for all the disilenes presented in Figure 1. Herein, we have considered the mechanistic route for disilenes IFt (Figure 10) whereas pathways concerning other disilene are described in the Supporting Information (Figures S12−S16). To develop this reaction channel, we have first allowed the approach of terminal N, O atoms of N2O toward different Si centers of IFt. During such progress, bending of the linear angular arrangement of the N2O fragment occurs, indicating more toward the formation of N−Si1 and O−Si2 bonds. In the transition state [IFt-4Ft]‡ (Figure 10) the ∠N−N−O bond angle becomes 155.2° and the imaginary frequency involves simultaneous approach of terminal N, O toward the Si centers. However, it is worthwhile to mention that a similar eigenmode depicting the uniform approach of N and O centers to the different Si atoms is not observed in the case of other disilenes because the shape of the KS-HOMOs changes with the increase in trans bent angles, as displayed in Figure 3. For example, in the case of

disilene, IAt, with the most trans bent geometry, the transition state [IAt-4At]‡ shows the approach of N2O to one of the Si centers (for details refer to Figure S12). To get further support from the frontier orbital explanation (vide supra, Figure 7b), we have performed EDA-NOCV analysis for transition states [IAt4At]‡ and [IFt-4Ft]‡, respectively. According to deformation density plots (Figure 11), the major contribution to the ΔEorb (−44.1 kcal/mol ([IAt-4At]‡)/−22.5 kcal/mol ([IFt-4Ft]‡), Table S1) arises due to the interaction accompanying charge transfer from IA/Ft-HOMO to N2O-LUMO (61.7/66.2%). On the contrary, the minor contribution shows back-donation type charge transfer from N2O-HOMO → IA/Ft-LUMO (27/22.7%). These observations further support the crucial role of both σdonation and π-back-donation in the context of mechanistic interpretation of “side-on” attack route. The first “side-on” attack to IFt requires an activation barrier of 15.1 kcal/mol (Figure 10). The resulting intermediate 4Ft shows an interesting geometry where the N2 is already formed 410


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The Journal of Physical Chemistry A Table 4. Energies (in kcal/mol) for All the Steps Involved in the Pathway IIb for trans-Disilenes steps IX + N2O → [IX -4Xt]‡ [IXt-4Xt]‡ → 4Xt 4Xt → [4Xt-5Xt]‡ [4Xt-5Xt]‡ → 5Xt + N2 5Xt+ N2O→ [5Xt-PXt]‡ [5Xt-PXt]‡ → PXt + N2 t

a

t

IAt

IBt

ICt

26.0

19.7

17.3

−154.3a 22.5 −175.1

−155.6a 19.3 −178.2

I Dt 14.1 −103.9

−153.7a 22.1 −176.6

−38.2b 19.2 −170.8

IEt 14.9

−140.2a 21.8 −169.5

IFt 15.1 −79.2 0.4 −54.6 19.7 −167.1

IR 15.7

−136.4a 21.2 −170.8

Relative energies for [IXt-4Xt]‡ → 5Xt. bEnergy change due to 4Dt → 5Dt.

Figure 12. Cis product formation route in Pathway IIb for IFt.

bond strength in 5Xt’s that is somehow reflected on their bond distances [rSi−Si (5A−Ft and 5R) = 2.211, 2.225, 2.197, 2.243, 2.218, 2.229, and 2.202 Å]. Thus, the Si−Si bond strength in monooxadisilirane, 5Xt, is invariant to the change in substituents and may only differ from the +I/−I effect and that too for a lesser extent. Therefore, in Pathway IIb, the second N2O addition step (5Xt → PXt) is considered as rate limiting although in the case of IAt and IBt, the first step (IA/Bt → 5A/Bt) requires higher activation barriers that are in line with the largest trans bent angle (Table 4). It is also necessary to point out that the second oxygenation step (5Xt → PXt) is more favorable with “side-on” rather than “end-on” attack of N2O unit as apparent after comparing the energy profiles of Pathway IIa and Pathway IIb for steps 5A,C,Et → PA,C,Et (Figures 8 and 10). To further investigate the possibility of disilene oxygenation in a single step, we have considered the approach of two N2O molecules from either the −syn or −anti direction with respect to the Si1−Si2 bond. All disilenes, IA−Ft, show the uniform result that is the cleavage of the Si1−Si2 bond when two N2O species attack the individual Si centers from the same side. Interestingly, in the case of −anti attack disilenes IAt and IBt showed similar Si1−Si2 bond dissociation. On the contrary, for the remaining disilenes (IC−Ft) the N2O addition does lead to the dioxadisiletane product; however, a proper IRC validated transition state was not obtained even after repeated attempts. The transition state was dissociative with respect to one of the N2O units leading directly to the monooxadisilirane intermediate. Thus, the above results indicate the absence of any favorable single step pathway for disilene oxygenation in the

and is weakly bound to the Si1 and O atoms. The N−N distance in 4Ft is slightly elongated (1.100 Å) than in free N2 molecule (1.089 Å). Subsequent liberation of N2 from 4Ft furnished the mono-oxacyclic intermediate 5Ft, which is −118.3 kcal/mol more stable than the disilene IFt. This step is highly facile with very low activation barrier of 0.4 kcal/mol and the accompanying transition state [4Ft-5Ft]‡ involves the twisting approach of O to Si1 with concomitant N2 liberation. In a similar fashion a second N2O molecule will attack 5Ft from opposite direction to the Si−O−Si bridge. This transformation occurs via [5Ft-PFt]‡, manifesting a similar transition vector corresponding to [IFt-4Ft]‡. It is obvious that the relative activation barrier of the second N2O attack is higher than the first by 4.6 kcal/mol (ΔΔGLM06‑2X) because disruption of the Si−Si σ-bond occurs in the latter case. However, the second N2O attack directly leads to the final dioxadisiletane product PFt instead of any intermediate species similar to 4Ft. As expected from our previous discussion, the “side-on” attack of N2O requires high activation barrier for disilenes having larger trans bent geometry (vide supra). The respective activation barriers for the step IXt → 4Xt are in the following order: IAt > IBt > ICt > IEt ≈ IFt > IDt (Table 4). Another interesting point to note is the existence of intermediate 4Xt along the potential energy surface of N2O addition. In this transient intermediate, the N−O bond of N2O has already cleaved while the N2 unit is still loosely bound to the Si center. Unfortunately, this type of intermediate exists only for IDt and IFt disilenes but is nonexistent for other combination of substituents. In the case of the second N2O attack step, the associated activation barriers are almost equal for all disilenes (Table 4). The only reason that emerges is about similar Si−Si 411


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compared to CC bond rotation in olefins.130 In our previous report we have optimized the rotational transition state connecting the conformational isomers of disilene It.60 Our calculations showed a quasi-perpendicular orientation of the Si(Cp*)N(TMS)2 units (N−Si−Si−N = 68° at the uBP86/ SVP level) in the transition state with a higher stability at the triplet state. Very recently, Sekiguchi, Apeloig, and co-workers, on the basis of temperature-dependent EPR spectroscopy and theoretical investigations, reported the preference of a triplet rotational excited state for disilene isomerization.131 Therefore, similar single step isomerization is studied by optimization of the rotational triplet electronic state for all disilenes. The energy values for the steps IA−Ft → IA−Fc are 25.9, 18.7, 13.2, 13.0, 15.0, and 21.3 kcal/mol, respectively, with corresponding rotational triplet states [IXt-IXc]T. The second route, R2, occurs via dissociation−recombination mechanism between disilene and the corresponding silylenes.132,133 However, this route is only feasible for IA−Ct, whereas for the remaining disilenes ID−Ft the initial dissociation step is highly unfavorable, as discussed in the previous section (vide supra, refer to Table 1a). The third possibility of isomerization requires a disilene− (silyl)silylene rearrangement route referred to as R3 (Scheme 3). Migration leading to the (silyl)silylene intermediate was first reported by Sakurai et al. while investigating the rearrangement of transient hexamethyltrisilene to its corresponding trisilane1,1-diyl at 200 °C.133 We have briefly discussed this step for IAt (Figure 13), whereas for other disilenes the energy profiles are

presence of N2O species. This observation is in agreement with our previous computational findings.60 3.3. Cis-Product Formation. Another interesting aim of this study is to explore different pathways for generation of cisdioxadisiletane isomers from the trans-disilenes IA−Ft. In Pathway I (vide supra) the transient silanone intermediate 2X will further combine with another entity to afford either cis or trans product (Figure 5), depending on the respective orientation of their association. Therefore, this particular silanone combination step (2X → PXt/PXc) will be responsible for controlling the selectivity of dioxadisiletane ring product formation. In the case of Pathway IIb, the isomeric product formation route is generated from intermediate 4Ft (Figure 10). The isomerization from 4Ft occurs via simple rotation around the Si1−Si2 single bond (rSi1−Si2 = 2.462 Å) as schematically represented in Figure 12. Interestingly, during such rotation, the N2 gets gradually decoordinated from the Si1 center with simultaneous progress of the O atom to Si1 to afford the bridging Si−O−Si intermediate 5Fc. This rotational transformation is energetically facile not only due to the smaller activation barrier (5.2 kcal/mol) but also due to the high exothermicity associated with monooxadisilirane intermediate formation (Figure 12). In a successive step the cismonooxadisilirane, 5Fc, undergoes similar N2O addition to furnish the cis-dioxadisiletane PFc. The activation barrier required for the step 4Ft → 5Ft (0.4 kcal/mol, Figure 10) is relatively lower than the corresponding cis product formation route 4Ft → 5Fc (5.2 kcal/mol), indicating a strong preference for trans isomer. This finding gains further support from experimental findings on N2O addition to It. Unfortunately, this particular route for cis product formation is feasible only for IA,Dt because the loosely bound adduct analogous to 4Ft does not exist (vide supra) for remaining disilenes. West et al. proposed that E/Z isomerization is thermally accessible for highly hindered 1,2-di( L -adamantyl)dimesityldisilene, with an activation energy of 28.8 kcal/mol on the basis of kinetic study using variable temperature NMR spectroscopy.129 This experimental finding suggests a clear possibility of disilene isomerization prior to N2O addition. Based on experimental and theoretical evidence, three possible mechanisms (Scheme 3) for the cis/trans isomerization of disilenes are currently reported in the literature.64,65,130−133 The first route (R1; Scheme 3) requires simple rotation around the SiSi bond in disilene.130,131 Kira et al. have reported that isomerization of disilenes are indeed facile

Figure 13. Isomerization of IAt → IAc via disilene−(silyl)silylene rearrangement.

Scheme 3. Possible Isomerization Routes of trans-Disilene to cis-Disilenea

a

collected in the Supporting Information (Figure S17−S21, Table 5). In the first step one Cl atom transfers from Si2 → Si1 to generate (silyl)silylene intermediate 6At. The activation barrier for the Cl shift is quite low as 4.9 kcal/mol. The accompanying transition state [IAt-6At]‡ animates the desirable migration of Cl from Si2 to Si1 center. The resulting intermediate 6At possess an anti orientation of the −NMe2 groups at the Si centers. In the next step, isomerization of 6At → 6Ac takes place by simple rotation around Si−Si single bond. Finally, from the cis intermediate 6Ac, analogous −Cl migration from Si1 → Si2 will furnish the cis-disilene, IAc. After careful inspection of the energetics for E/Z isomerization, we can suggest that for disilenes IAt and IBt, the dissociation−recombination (R2) route will be preferred. On the contrary, for disilenes IC,Dt the more favorable step will

R and S represent two different substituents of disilene. 412


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The Journal of Physical Chemistry A Table 5. Energies (in kcal/mol) for All the Steps Involved in the Disilene Isomerization along the R3 Route steps

IAt

IBt

I Ct

I Dt

IEt

IFt

→ [IXt-6Xt]‡ t [IX -6Xt]‡ → 6Xt 6Xt → 6Xc 6Xc → [6Xc-IXc]‡ [6Xc-IXc]‡ → Xc

4.9 −11.4 −0.4 13.6 −5.3

13.7 −22.9 0.3 26.2 −16.6

10.4 −14.6 0.4 15.5 −11.1

6.1 −12.2 −2.6 13.4 −5.8

6.1 −8.9 2.3 8.6 −6.3

15.0 −5.0 −1.1 6.5 −13.3

IXt

Figure 14. Flowchart for all possible pathways of N2O addition to disilenes. R and S represent two different substituents of disilene.

refrain from considering this isomerization route for further investigation. To progress further to form cis-dioxadisiletanes PA−Fc, we have performed the successive N2O additions to cis-disilenes IA−Fc. Pathway IIb was considered to evaluate structures and energetics for this addition because it remains the most favorable route among all studied pathways. Energy profiles for IA−Fc → PA−Fc transformations are displayed in the Supporting Information (Figure S22−S27). Transition states [IXc-4Xc]‡ and [5Xc-PXc]‡ corresponding to two individual N2O addition steps shows close resemblance with [IXt-4Xt]‡ and [5Xt-PXt]‡, respectively. Similar to the observations drawn from transdisilenes (vide supra, Table 4), the second N2O addition step is

follow the bond rotation (R1) route, whereas route R3 will be populated by IEt and IFt, respectively. Another route of isomerization may originate from a facile transformation of trans intermediate 5Xt to its cis variant 5Xc. Subsequent N2O addition to 5Xc will lead to the cis-product PXc. It has been reported that isomerization of 5t → 5c will require the dissociation of the Si−Si bond to generate free Si−O−Si species 7t (Figure S9). The intermediate 7t is stabilized by an augmented coordination number at silicon due to the presence of a Cp* group as substituent. Unfortunately, a similar type of intermediate does not exist for disilenes IA−Ft probably due to unsaturation in covalency at the Si1/Si2 atoms. Therefore, we 413


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The Journal of Physical Chemistry A rate determining for cis isomers IC−Fc whereas for IAc and IBc the first N2O addition requires a higher activation barrier (Table S3). To predict cis/trans product selectivity, the Curtin−Hammett principle134,135 is employed to calculate the product ratios for Pathway IIb, which are collected in Table S4 in the Supporting Information. Calculations show that the product ratios are limited within the short range of 1.0−11.0, which are high compared with the experimental findings.45,46 From these calculated values we surmise that the trans isomeric product will be dominated for the disilene IXt (X = A, B, D, and F), whereas for the other two IYt (Y = C and E) the cis product will be populated. In Figure 14, we have presented various pathways for disilene oxygenation with N2O in a single energy profile for different disilenes. We have observed that, for the disilenes with greater trans bent angles, e.g., IAt and IBt, the initial dissociation step is very facile (Pathway I, vide supra) but still is not the favorable pathway because the subsequent silanone formation step requires larger energetics. It is obvious from the profiles that Pathway IIb, constituting the “side-on” attack of N2O units, shows a minimum energy route for all disilenes investigated in this current study. Calculations reveal that the second step (5Xt + N2O → PXt) is found to be the rate limiting step to attain the final dioxadisiletane in contrast to the greater trans bent disilene, IA,Bt where the trend is reversed. However, one can compare the reactivity of disilene toward N2O by the activation barrier required for the first N2O addition step (IXt+N2O → 4Xt) because of the very high exergonicity (Figure 14) involved in this step. In concurrence with the above statement, we can presume that disilenes with strong Si−Si bonds along with low trans bent angles exhibit low oxygenation barriers (ID,E,Ft → 4D,E,Ft: 14.1 (X = D), 14.9 (X = E) and 15.1 (X = F) kcal/mol) and hence are more reactive.

However, in Pathway IIb, N2O acts as an electrophile and leads to the energetically more dominating interaction of N2OLUMO with IXt-HOMO. In this case, the preferable approach of N2O to disilene is along the “side-on” fashion. However, our calculated results reveal that Pathway IIb will undergo energetically facile transformation in comparison to other reaction routes and probably be effective under mild reaction conditions. Two successive oxygenation steps are involved in this pathway. In general, the second N2O addition requires higher activation barrier than the first attack, except for disilenes with greater trans pyramidalization, e.g., IA,Bt. If we consider the activation barrier for first oxygenation step (IXt → 4Xt) as an important parameter for disilene reactivity, then the corresponding N2O addition is less influenced by the substituents on disilenes because all the activation barriers are restricted between 26.0 and 14.1 kcal/mol. One interesting point to mention that the disilenes with strong Si−Si bonds (ID,E,Ft) are considered to be more reactive toward N2O, obvious from the lower oxygenation barrier. Therefore, at this point we hope that our computational study will stimulate further experimental research into the subject.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b11988. Complete Gaussian reference, discussion of choice of functionals, disilene conformations, energy profiles for Pathway I for IB−Ft,R, EDA-NOCV calculation results, frontier orbitals of silylene, Pathway IIa for It, IAt, and ICt, Pathway IIb for IA−Et and IA−Ec, energy profile for isomerization of IB−Ft to IB−Fc, population comparisons, computed energy profile of Pathway I and Pathway IIb for IA−Ft at different levels of theory, Cartesian coordinates and absolute energies of all the studied intermediates and transition states (PDF)

4. CONCLUSION The present study illustrates a detail investigation in terms of reactivity and mechanistic aspects of substituent effects in disilene toward N2O activation. For the first time, both the electronic and steric influences of the substituents are carefully considered while addressing the N2O addition steps. Three different pathways (Pathways I, IIa, and IIb) are investigated for disilene oxidation resulting in the formation of both trans- and cis-dioxadisiletane products PXt and PXc respectively. In summary, Pathway I consist of three fundamental steps: disilene dissociation, nucleophilic attack of N2O to monomeric silylene and dimerization of silanone units to furnish cis and trans products. In the case of lone-pair-containing substituents (−NMe2, −Cl) the disilene dissociation step (IXt → SX) is energetically more favored and the subsequent N2O attack step becomes unfavorable. On the contrary, opposite observation was observed for substituents without lone pair, e.g., −Me, −SiMe3. In alternative pathways, the N2O reacts directly with disilenes due to favorable frontier molecular orbital (FMO) interactions. In Pathway IIa, N2O acts as a nucleophile leading to the interaction between N2O-HOMO with IXt-LUMO. This kind of interaction manifests toward the “end-on” attack of N2O to Si1/ Si2 centers. Although this pathway does exist only for disilenes IA,C,Et, the accompanying activation barriers are very high (IXt → 5Xt/5Xt → PXt: 29.9/35.8 (X = A), 22.8/35.4 (X = C), and 27.0/36.2 (X = E) kcal/mol) because large energy differences exist between the FMO

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AUTHOR INFORMATION

Corresponding Author

*D. Koley. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.M. acknowledges CSIR for the SRF fellowship. The authors acknowledge IISER-Kolkata for the computational facility. D.K. is grateful to the DST-SERB for the fast-track fellowship (No: SR/FT/CS-72/2011) and IISER-Kolkata for the start-up grant.

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DEDICATION This article is dedicated to Professor Herbert W. Roesky for his outstanding contribution in silicon chemistry. REFERENCES

(1) West, R. The Disilenes: Chemistry of Silicon-Silicon Double Bonds. Pure Appl. Chem. 1984, 56, 163−173. (2) Satgé, J. Reactive Intermediates in Organogermanium Chemistry. Pure Appl. Chem. 1984, 56, 137−150. (3) Raabe, G.; Michl, J. Multiple Bonding to Silicon. Chem. Rev. 1985, 85, 419−509. (4) West, R. Chemistry of the Silicon-Silicon Double Bond. Angew. Chem., Int. Ed. Engl. 1987, 26, 1201−1211.

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