Article pubs.acs.org/Organometallics
Optimized Structures and Ring Strain Energies of Isoelectronic Homo- and Heterosiliranes c‑AX2SiR2SiR2q (A/q = Al/1−, Si/0, P/1+): Unexpected Effects of Charge and Size Christina L. Allard, Philippe Gauthier, Austin L. Gille, Gerald E. Thomas, and Thomas M. Gilbert* Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States S Supporting Information *
ABSTRACT: Ring strain energies (RSEs) estimated for a series of isoelectronic three-membered ring homo- and heterosiliranes c-AX2SiR2SiR2q (A/q = Al/1−, Si/0, P/1+; X = H, Me, F, CF3; R = H, Me) fall into two categories. Those where the X substituent is electron-withdrawing show RSE trends where the aluminatadisilirane and trisilirane differ only slightly, while that for the phosphoniodisilirane is significantly larger. This arises because the positive charge/small size of the phosphorus in the last species is poorly accommodated in the ring and more readily accommodated in one of the chain molecules used to calculate RSE. In contrast, rings where the X substituent is neutral or an electron donor show V-shaped trends in RSE. This holds despite the fact that the bond distances and angles exhibit expected linear trends. Quantum theory of atoms in molecules (QTAIM) calculations suggest that this arises because the charged rings have the greatest difference between bond lengths and bond path distances, the latter being the paths of most significant electron density.
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the RSE, while changing from SiH2 to PH to S decreases it. It is apparent that explaining these trends presents a challenge. Moreover, for a given apical group, the RSEs for rings with H2Si−SiH2 bases are larger than those for rings with H2C−CH2 bases. This runs counter to the general concept that larger atoms in a ring decrease strain because they accommodate smaller bond angles. Cremer et al.6 showed quantitatively that Baeyer angle strain (the strain associated with the angles forced on the ring atoms)7 is indeed ca. 8.6 kcal mol−1 smaller in trisilirane, c-SiH 2 SiH 2 SiH 2 , than in cyclopropane, cCH2CH2CH2. They argued that the RSE is larger in the former than in the latter mainly because trisilirane has Si−Si and Si−H bonds that are weaker than the C−C and C−H bonds in cyclopropane, a result they ascribe to the lowered ability of silicon to form spn hybrid orbitals with large p orbital character and to decreased electron delocalization across the ring surface in polysilicon rings vs polycarbon rings. This presents an interesting perspective, but it fails to explain why, as one proceeds from c-CH2CH2CH2 to c-SiH2CH2CH2 to cCH2SiH2SiH2 to c-SiH2SiH2SiH2, the RSE increases and then falls. One difficulty associated with comparing species across a row in Table 1 is that the apical atoms are not isocoordinated. This could lead to invalid theories for the underlying causes of differences. For example, Bachrach suggested that the RSE of phosphirane, c-PHCH2CH2, is smaller than that of cyclopropane because of the longer P−C distances in the former and the greater ability of P to accommodate smaller angles than C.8 This may be correct, but it is an apples-to-oranges comparison, because it neglects the steric consequences (or lack thereof) of the lone pair in c-PHCH2CH2 vs a C−H bond in cCH2CH2CH2. This difference might have a profound effect
ing strain energies (RSEs) represent useful parameters for considering electronic distribution and thermodynamics in cyclic organic and inorganic systems. Among many issues, ring strain accounts for the exceptional exothermicities of combustion of cyclopropane and cyclobutane, as well as their enhanced reactivities,1 product ring size in ring-opening metathesis polymerization (ROMP) processes,2 NMR chemical shifts in norbornanes,3 changes in cyclopentadienyl ring hapticity in metallocenophanes,4 and the preference for linear polymercury cations over aromatic polymercury rings.5 Consequently, many computational studies have explored how RSEs change with ring member count, which atoms comprise the ring, and the nature of the peripheral substituents. Our interest in examining the RSEs of three-membered rings developed from inspection of the data in Table 1, collected from several sources. For the “base” H2C−CH2 of the threemembered ring, as the apical group changes from CH2 to NH to O among 2p atoms, the RSE hardly changes. However, as the apical group changes from SiH2 to PH to S, the RSE drops markedly, particularly between the first two. In contrast, for the H2Si−SiH2 “base”, changing from CH2 to NH to O increases Table 1. Ring Strain Energies (RSEs, kcal mol−1) for Representative Three-Membered Rings Estimated at Various Levels Using Various Desmotic Methods c-CH2CH2CH2 27.5,35 27.61 c-SiH2CH2CH2 35.9,35 37.1,36 39.037 c-CH2SiH2SiH2 38.7,35 40.5,6 38.437 c-SiH2SiH2SiH2 37.3,6 31.4,37 36.135
c-NHCH2CH2 26.8,35 27.71 c-PHCH2CH2 20.1,8 20.236 c-NHSiH2SiH2 39.0,35 39.5,6 37.937 c-PHSiH2SiH2 29.5,6 24.337
c-OCH2CH2 24.9,35 27.61 c-SCH2CH2 17.7,1 17.336 c-OSiH2SiH2 43.3,35 47.7,6 45.037 c-SSiH2SiH2
Received: January 17, 2013 Published: April 11, 2013
25.4,6 22.937
© 2013 American Chemical Society
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dx.doi.org/10.1021/om400038z | Organometallics 2013, 32, 2558−2566
Organometallics
Article
Choosing the production-quality model chemistry required examination of several variants. On the basis of prior work,18 the MP2 model19 was selected as one that captured a reasonable amount of electronic correlation and dispersion energy, while still requiring modest computational resources. That this model predicted RSEs in excellent agreement with those from higher level MP4//MP2 and CCSD(T)//MP2 computations was confirmed for several examples (see the Supporting Information, Table S1). To examine the effects of different basis sets, structures of several small rings were optimized using the MP2/aug-cc-pVTZ model chemistry. This model chemistry was too resource-intensive for use for all rings examined but provided baselines for comparisons using model chemistries involving smaller basis sets. It was discovered that optimizations using the MP2/aug-ccpVTZ model chemistry predicted unusual conformations for several rings (see the Supporting Information, Table S2, and the associated graphics; and see also below). These provided useful baselines for selecting an acceptable basis set. In the end, the MP2/6-311+ +G(3df,2p) model chemistry best matched the MP2/aug-cc-pVTZ model chemistry in terms of both observation of conformers and of root-mean-square deviations for structural parameters. This model chemistry was selected for all final structural optimizations and RSE calculations. The issue of conformer observation merited further investigation, as it had not been previously reported. Two types of conformers were observed: tilted vs symmetric and staggered vs eclipsed. Optimizations of difluorophosphoniodisilirane rings 9/12 at the MP2/6-311+ +G(3df,2p) level provided structures where the fluorine substituents were “tilted” toward one face of the ring as opposed to being symmetric (see the Supporting Information for pictures). Frequency calculations carried out at this level on both conformers of 9/12 established that the former were minima and the latter transition states. That the transition states connected the conformational minima was confirmed by visualizing the imaginary vibrations using GaussView.20 The energy differences between the stationary points were sufficiently small to suggest artifactual results. This was tested by reoptimizing 9 using the higher level, extremely resource-intensive CCSD/6-311++G(3df,2p) and CCSD/aug-cc-pVTZ model chemistries. As the coupled cluster methodology incorporates a number of configurations beyond those employed in the MP2 model, observation of both conformers as stationary points here would have supported the MP2 results. In any event, both optimizations led to symmetric structures. We therefore believe that observation of tilted ground states and symmetric transition states for 9/12 using the MP2 model is artifactual and that the true ground states of 9/12 are symmetric. Optimizations of all the aluminatadisiliranes and some of the trisiliranes at the MP2/6-311++G(3df,2p) level led to structures where the basal substituents adopted staggered geometries rather than eclipsed ones (see the Supporting Information for pictures defining staggered and eclipsed). Frequency calculations carried out at this level on these established that the staggered structures were minima and the eclipsed structures transition states. That the transition states connected the minima was confirmed as above.20 The associated barriers were small, from 0.1 to 0.9 kcal mol−1, again suggesting artifactual differences (see the Supporting Information, Table S3). Accordingly, we reoptimized rings 1, 7, 8, and 16, spanning neutral, electron-withdrawing, and electron-donating apical substituents, using the two CCSD-based model chemistries. Parent aluminatadisilirane ring 1 and trisilirane 8 optimized from staggered to eclipsed conformations at the CCSD levels, while difluoro- and dimethylaluminatadisiliranes 7 and 16 maintained staggered conformations, but with notably reduced basal torsion angles. Ring 16 showed particularly noteworthy behavior. Optimization at the MP2/6-311++G(3df,2p) level gave a significantly staggered ring, with a basal H−Si−Si−H torsion angle of 26.5°. Using this structure as a starting point, optimization at the CCSD/6-311++G(3df,2p) level gave a much less staggered ring, with a basal torsion angle of 8.5°. The CCSD energy difference between this ring and an eclipsed structure optimized at the same level was
