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Toward the Mechanism of Perchlorinated Cyclopentasilane (SiCl ) Ring Flattening in the [SiCl ·2Cl] Dianion 5

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Konstantin Pokhodnya, Kenneth J. Anderson, Svetlana V Kilina, and Philip Boudjouk J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12938 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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The Journal of Physical Chemistry

Toward the Mechanism of Perchlorinated Cyclopentasilane (Si5Cl10) Ring Flattening in the [Si5Cl10·2Cl]2- Dianion Konstantin Pokhodnya*, Kenneth Anderson, Svetlana Kilina and Philip Boudjouk* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND 58108 Abstract We report the detailed computational study of flattening of the puckered Si5 ring by suppression of the pseudo-Jahn-Teller (PJT) effect through coordination of two Cl− anions to the molecule forming an inverse sandwich dianion [Si5Cl10·2Cl]2- complex. The PJT effect which causes nonplanarity of the Si5Cl10 structure (Cs) results from vibronic coupling of pairs of occupied molecular orbitals (OMOs) and unoccupied molecular orbitals (UMOs). It was shown that filling the intervenient molecular orbitals of puckered Si5Cl10 with valent electron pairs of Cl− donors suppresses the PJT effect, with the Si5 ring becoming planar (D5h) upon complex formation. In this paper the stabilization energy E(2) associated with donor-acceptor charge transfer (delocalization) was estimated using NBO analysis for all studied inverse sandwich compounds [Si5Cl10·2X]2- (where X= F, Cl, Br). It was found that the polarizability of donor ion might significantly affect the stabilization energy value and should be taken into account when choosing the ligands suitable for forming Si-based one-dimensional compounds and other nanoscale materials.

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Introduction Cyclopentasilane (Si5H10, CPS), cyclohexasilane (Si6H12, CHS) and other liquid oligosilanes are useful precursors for the fabrication of silicon based materials that have the potential to reduce the cost of manufacturing of solar cells, thin film transistors and lightweight batteries.1-11 For CHS, the attraction to its practical applications is complemented by the interest generated in the structural and bonding features of its precursor, the complex dianion, [Si6Cl12·2Cl]2-, which consists of a planar Si6 ring (in contrast to neutral Si6Cl12 possessing D3h symmetry12-13) with two µ6 coordinated chloride ions forming the apical vertices of a hexagonal bipyramid. This dianion, which is the major product of the reaction of trichlorosilane and tertiary polyamines,14-15 is also produced by the addition of chloride ions to dodecachlorocyclohexasilane, Si6Cl12.12 We have demonstrated that Si6Cl12 is a Lewis acid that is sufficiently strong to form complexes with neutral donors such as nitriles.16-17 We have also shown that the Si5Cl10 ring will form similar complexes with nitriles.18 In addition to the common property of Lewis acidity for the halogenated Si6 and Si5 rings, there is the similar structural change both rings experience when exposed to donors, i.e., ring flattening upon complexation. The nature of hyper-coordination in cyclic silanes as well as suppression of their ring puckering effects has been studied extensively by several groups.19-22 Using the density functional theory (DFT) approach it was shown that the D3d symmetry for the “chair” and C2v for “boat” distortion of the Si6 ring in Si6X12 is a manifestation of a pseudo-Jahn–Teller (PJT) that occurs due to a vibronic coupling between pairs of occupied (OMOs) and unoccupied molecular orbitals (UMOs). Additionally, calculations have shown that the PJT effect can be completely suppressed (flat Si6 ring of D6h symmetry) when the unoccupied orbitals of Si6Cl12 engaged in vibronic coupling are filled with electron pairs of donors (e.g. Cl- and R-C≡N) resulting in


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formation of an inverse sandwich complex.19 It was presumed that the formation of a pair of Lewis acid sites above and below the Si6 plane, on which the apical donors’ electrons are partially accommodated, occurs due to a withdrawal of a significant portion of electron density from Si atoms of the ring by 12 halogens.12, 19 In the computational study of [Si6X12·2Y]2- type dianions (X = Cl, Br, I and Y = Cl, Br) it was shown that the Si6X12 ring interaction with halides (Y-) is of a donor-acceptor type, stabilized by the lone pair donation of the anion (Y-) onto the antibonding Si-X (n(Y) → σ*(Si-X)) orbitals.21 A similar type of bonding most likely occurs when the lone pair of nitrile ligands (R-C≡N) are donated to the Si6X12 ring Lewis acid sites.20 Utilization of ligands that can be coordinated to multiple Si rings opens the possibility of building Si-based multidimensional coordination compounds that may have a wide variety of applications as synthons in the material science field. The first example of this strategy was shown recently where substituted 1,4-dicyanobenzenes were used to link Si6Cl12 rings, forming one-dimensional coordination polymers.17 The mechanism of the observed Si ring puckering suppression in the nitrile coordinated compounds18, however, has not been studied. Here, we computationally investigate parental Si5Cl10 as well as not yet synthesized [Si5Cl10·2X]2- (X=F, Cl, Br) inverse sandwich compounds. Our results show that the ring puckering in Si5Cl10 occurs due to orbital vibronic coupling (PJT effect) similar to that in Si6Cl12. Additionally, the corresponding vibrational modes as well as the character of MOs involved were identified and the stabilization energy resulting from the donors’ lone pair delocalization was evaluated. Computational Method Different conformations of Si5H10 have been studied computationally using various levels of theory including ab-initio (MP2 and CCSD) and DFT approaches.23 The average computed


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Si5H10 Si-Si bond length was found to be 2.356 (CCSD/6-31G*), which is about 0.02 Å larger than the experimental value derived from electron diffraction (2.342 Å)24 and the recently reported single crystal X-ray diffraction data (2.336 Å)4. The Si-Si bond length of 2.368 Å computed at hybrid B3LYP/B2 level is about 0.03 Å larger than experimental values. Recently, it was pointed out that long range dispersion corrections added to hybrid functionals (HFs) such as in wB97XD25 are important for accurate modeling of methyl or hydrogen substituted cyclohexasilane geometries.20 To systematically investigate the effect of HF inclusion and long range corrections in the density functionals on Si5X10 (X=H, Cl) geometries, we performed DFT calculations using representative GGA (PBE)26, GGA/HF hybrid (PBE1)27, and long-range corrected functions, i.e., wB97XD25 and LC-wPBE28. It is known that with increasing basis set the calculated bond lengths may shrink.23 Therefore, we have tested the performance of the relatively short basis set 6-31G*29 as well as longer ones, i.e., 6-311G++(3df,3pd)30 and Def2TZVP 31, that may be especially useful for Si5Cl10 modeling. Since the crystal structure of Si5Cl10 is not available, structural parameters of the flat Si5Cl10 fragment of Si5Cl10·2(p-MeC6H4CN) (D5h symmetry) were used as a benchmark for choosing the best DFT functional set. The geometries of the studied cyclopentasilanes were optimized within the given symmetry point group with the Gaussian-09 DFT software package.32 Analysis of the second order perturbation of the Fock matrix, LCNBO-MO expansion and charge distribution have been performed with the NBO 6.0 program.33-34

Results and Discussion The Si5H10 molecule has an envelope conformation with perfect Cs symmetry and Si-Si bond lengths between 2.3353(1) and 2.3377(7) Å.4 The geometry optimization was performed using different combinations of the DFT functionals and basis sets. The results are summarized in


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Table 1S together with atom labels provided in Fig. 2S. Application of GGA PBE and hybrid PBE1 functionals overestimates the average Si-Si bond length by almost 0.05 Å. In contrast, the implementation of the long range part in the functional exchange component significantly improves the obtained results, especially for LC-wPBE. The Si-Si bond length discrepancy decreased to 0.003 Å when 6-31G* basis set was used. As expected, for Si5H10 the usage of larger basis sets had very little effect. Similar to Si5H10, three conformers of Si5Cl10 were evaluated, i.e., planar D5h, twist C2, and envelope Cs. The benchmarking was performed for Si5Cl10 of D5h symmetry since experimental data is available only for this conformer. The results summarized in Table 1 show that the LC-wPBE functional accurately describes the Si-Si bond and underestimates the Si-Cl bond insignificantly. Overall, LC-wPBE/6-31G* is the most optimal choice of the methodology providing reasonably accurate geometries both for Si5H10 and Si5Cl10. Despite being slightly less accurate in Si-Cl bond length evaluation ( 0.8 eV/Å the Si5Cl10 becomes unstable vs. dihedral angle perturbation. Recently the vibronic coupling parameters were estimated for Si4F4 allowing us to assume that this obtained threshold value is quite reasonable.39


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Fig. 1. Interaction of the pairs of occupied and unoccupied molecular orbitals of Si5Cl10 (D5h, A1’) responsible for the PJT effect: distortion toward the “envelope” and “twist” structures upon following the doubly degenerate ω1,2 (e2”) imaginary frequency mode. Molecular orbital analysis of both Si5Cl10 and the [Si5Cl10·2Cl]2- dianion (Fig. 2) has revealed the shape resemblance between the Si5 ring fragments of the empty molecular orbitals in the neutral Si5Cl10, i.e., LUMO, LUMO+1 and LUMO+4/+5, and those orbitals occupied by valent lone pair electrons of Cl- in the [Si5Cl10·2Cl]2- dianion, i.e., HOMO-7, HOMO and HOMO-4/-3, respectively. Natural Bond Orbital (NBO) analysis may provide indispensable information about the nature of these valent orbitals as well as the details of Cl- donor


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coordination.21,

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As was pointed out earlier, NBO software “optimally expresses numerical

solutions of Schrödinger's wave equation in the chemically intuitive language of Lewis-like bonding patterns (two-center bond or lone pair) and associated resonance-type 'donor-acceptor' interactions”.41

Fig. 2. Close resemblance of the Si5 ring related fragments of the unoccupied molecular orbitals of Si5Cl10 (highlighted in yellow) to those of [Si5Cl10·2Cl]2- (highlighted in green). Occupation by Cl- donors’ lone pair electrons of the former MOs results in the suppression of the PJT effect in the latter compound. NBO analysis of the planar Si5Cl10 molecule revealed two groups of antibonding BD*(1) type non-Lewis NBOs, i.e., five orbitals originating from the single bonds between adjacent Si atoms, as well as ten degenerate orbitals originating from the single bonds between Si and the exocyclic Cl atoms. The energies of these NBOs are right above the energies of similarly degenerate Lewis bonding BD(1) type NBOs corresponding to Si-Si and Si-Cl single bonds. The DFT derived CMOs, LUMO (A2”), LUMO+1 (A1’), and LUMO+4/+5 (E1”), lack probability


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density at Si-Cl bonds; in contrast, both degenerate LUMO+2/+3 (E1’, not shown) and LUMO+6/+7 (E2’, not shown) have deficiency of density probability at Si-Si bonds, suggesting that these CMOs possess σ*(Si−Cl) and σ*(Si−Si) character, respectively (Fig. 2, top row). Further analysis of the canonical LUMOs using information from the LCNBO-MO expansion provides more details regarding the nature of the canonical LUMOs of Si5Cl10 (D5h). LUMO, LUMO+1 and LUMO+4/5 have zero contributions from BD*(Si-Si) antibonding NBOs and consist primarily of BD*(Si-Cl) (65-70%) and nonbonding character. Conversely, LUMO+2/+3 and LUMO+6/+7 have contributions from both BD*(Si-Si) (40-50%) and BD*(Si-Cl) (25-30%) with the remainder again being nonbonding. LUMO-9/-10 have the least antibonding character of the first ten LUMOs with roughly 12% BD*(Si-Si) and 1% BD*(Si-Cl). It should be noted that NBO analysis predicts substantial interactions of the filled BD(Si-Si) single bond Lewistype NBOs with “empty” BD*(Si-Cl) NBOs (non-zero occupancy) which suggests some delocalization associated with these orbitals (canonical HOMO and LUMO). In addition, interaction of nonbonding p-type lone pairs on the ring chlorines with BD*(Si-Si) and BD*(Si−Cl) orbitals are also predicted (vide infra) and, therefore, delocalization of these orbitals (possibly via negative hyperconjugation) cannot be ruled out.42 From the shape of the HOMO of [Si5Cl10·2Cl]2- dianion it is conceivable that the lone pair on the apical Cl- anions directed toward the Si5 ring plane interact with the σ*(Si−Cl) orbitals (former LUMO and LUMO+1 of planar Si5Cl10) causing stabilization of the dianion. Four NBOs associated with lone pairs (LP) are available on the closed shell Cl- anion. One of them LP(1) is mostly of s-character (sp0.45) and exhibits occupancy close to 2 (1.996); two others, LP(2) and LP(3), are of pure p character with an equal reduced occupancy of 1.901. The lowest occupancy of 1.785 for LP(4) sp2.22 hybrid suggests a substantial delocalization of LP(4)


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electron density (see Table 6S). The NBO analysis also provides information about the Lewis and non-Lewis occupancies of core, valence, and Rydberg shell contributions. Since the natural Lewis structure description for [Si5Cl10·2Cl]2- covers 99.1% of the total electron density, the donor-acceptor (σ→σ*) interactions in the NBO basis can be treated as a second-order perturbation. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with delocalization i→j is estimated as 2 = ∆ =

!, # $ − $

where qi is the donor orbital occupancy, εi and εj are diagonal elements (orbital energies) and F(i,j) is the off-diagonal NBO Fock matrix element.40 In the case of [Si5Cl10·2Cl]2- we calculated the stabilization energy related to the donor lone pairs LP(1)-LP(4) interaction with acceptor non-Lewis BD*(Si-Si) and BD*(Si-Cl) NBOs. The results are summarized in Table 3S in relation to the atom labels given in Fig. 3S, right. It is clear that LP(1) (sp0.45 hybrid) with small p-orbital contribution does not add significantly to the stabilization energy. LP(2) and LP(3) lying nearly parallel to the Si5 ring plane (see Fig. 2, HOMO-3/-4 of [Si5Cl10·2Cl]2-) each interact with ten BD*(Si-Cl) NBOs but for only one of the interacting NBO pairs does E(2) exceed 6 kcal/mol. Interestingly, these LP NBOs also interact with BD*(Si-Si) NBOs, albeit with substantially smaller associated stabilization energy of less than 1 kcal/mol. The LP(4) NBO is of sp2.22 character with its axis directed towards and oriented perpendicular to the Si5 ring plane (see Fig. 2, HOMO and HOMO-7 of [Si5Cl10·2Cl]2-) which promotes the most favorable interaction with five BD*(Si-Cl) NBOs resulting in the highest E(2) values of ~12 kcal/mol. It should be noted that the LP(4) of the Cl- donor located above the Si5 plane most strongly interact with BD*(Si-Cl) NBOs in which the associated Cl atom is located below the ring plane, and vice


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versa. Most probably it reflects the fact that electron density in this antibonding orbital is pushed away from chlorine in the Si-Cl bond. Since the interaction between the donor halide and Si ring is of the donor/acceptor type, the donors’ valent p-orbital size as well as its distance to the Si5 ring Lewis acid sites should influence the stability of the inverse sandwich compounds. To elucidate these effects on stabilization energy the NBO calculations were also performed for [Si5Cl10·2F]2- and [Si5Cl10·2Br]2- dianions. The computed structures of both compounds (see Fig. 4S) are very similar to that of [Si5Cl10·2Cl]2-. They adopt D5h symmetry and flattened Si rings that are typical characteristics of the inverse sandwich compounds. The calculated bond length parameters (in Å) and natural population analysis (NPA) computed charges (in a.u.) of Si5Cl10 and [Si5Cl10·2X]2(X=F, Cl, Br) are shown in Table 2. The calculated Si−X bond lengths in all studied [Si5Cl10·2X]2- complexes were found to be substantially shorter than the sum of their van der Waals radii (RVdW) but longer than the sum of their covalent radii, suggesting that despite the stabilizing effect, Si-X interactions are weaker than the covalent Si-Si and Si-Cl bonding within the Si5 ring. The distances between the ring Cl and X donor are also shorter than the sum of their RVdW for X=Cl and Br but not for F (see Table 2 and Fig. 4S). This contrast is likely a result of fluorides small size which allows both donors to more easily penetrate the Si5Cl10 ring and possibly even interact with each other, since the F-F′ distance of 2.995 Å is only slightly larger than the sum of two RVdW of fluorine (2.94 Å). The calculated stabilization energies, E(2), due to fluoride, chloride and bromide coordination are summarized for BD*(Si-Si) and BD*(Si-Cl) acceptor orbitals in Figures 6S and 7S, respectively. E(2) stabilization energies of individual NBO donor-acceptor interactions for fluoride, chloride and bromide adducts are shown in Tables 4S, 3S and 5S, respectively, with


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corresponding atom labels given in Fig. 5S for fluoride and bromide adducts and Fig. 3S for the chloride adduct. As mentioned above, the E(2) calculation confirmed a substantial Si5Cl10 ring stabilization due to the exocyclic chlorines’ lone pairs (LP(2) and LP(3)) coupling with BD*(SiSi) and BD*(Si-Cl) NBOs. However, the variation of their E(2) values upon donor change is small (Cl>Br) implying an increase of electron density on the Si atoms. The charges on exocyclic Cl atoms as well as Si-Cl bond lengths remain essentially unchanged. It is known that, for donor-acceptor complexes the bonding is defined by the delicate balance between electron pair donation and back-donation. It was shown that in the Si6-based halogen substituted cyclic silanes a significant back-donation charge component from the Si6 ring to donor is present.21 Using Charge Decomposition Analysis (CDA) for the series of inverse sandwich compounds [Si6Cl12·2X]2- (where X= F, Cl, Br) it was found that both donation of the electron density from the donor to Si6 ring and a reverse back-donation increase with the donor atomic number (F

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