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Dispersion Stabilized Molecules

Dispersion Stabilized Molecules are molecules where the London Dispersion Force (LDF), a non-covalent attractive force between atoms and molecules, plays a significant role in promoting the molecule's stability. Distinct from steric-hindrance, dispersion stabilization has only recently been considered in depth by organic and inorganic chemists after earlier gaining prominence in protein science and supramolecular chemistry. Although usually weaker than covalent bonding and other forms of non-covalent interactions hydrogen bonding, dispersion forces are known to be a significant if not dominating stabilizing force in certain organic, inorganic, and main group molecules with otherwise reactive moieties and exotic bonding.

Stabilization Through Dispersion

Dispersion interactions are a stabilizing force arising from quantum mechanical electron correlation. Although quantum mechanical in nature, the energy of dispersion interactions can be approximated classically showing a R^-6 dependence on the distance between two atoms. This distance dependence helps to make dispersion interactions weak for individual atoms and has lead to dispersion effects being historically neglected in molecular chemistry. However, in larger molecules dispersion effects can become significant. Given the right molecular structure, dispersion forces have been identified to stabilize unique geometries and bonding patterns. Dispersion stabilization is often signified by atomic contacts below their van der Waals radii in a molecule's crystal structure. Especially for short H•••H contacts between bulky, rigid, polarizable groups, short contacts may indicate that a dispersion force is overcoming the Pauli repulsion present between the two H atoms.

Advances in quantum computational chemistry methods have allowed for faster theoretical examination of dispersion effects in molecular chemistry. Standard density functional theory (DFT) does not account well for dispersion effects, but corrections like the popular -D3 correction can be used with DFT to provide efficient dispersion energy corrections. The -D3 correction is a force field type correction that does not take into account electronic structure, but nonetheless the popular correction works with many functionals and produces values that often fall within 5-10% of more sophisticated calculations. The "gold standard" computational method are coupled-cluster methods like CCSD(T) that account for the electron correlation origin of dispersion interactions.

Dispersion Stabilization Compared to Steric Bulk

Dispersion forces stabilizing a reactive moiety within a molecule is distinct from using standard steric bulk to protect that reactive moiety. Adding "steric hindrance" to a molecule's reactive site through bulky ligands has been a common strategy in molecular chemistry to stabilize reactive moieties within a molecule. In this case bulky ligands like terphenyls, bulky alkoides, aryl-substituted NHCs, etc. serve as a protective wrapper on the molecule, promoting kinetic stability. Steric protection differs from dispersion stabilization because under the steric hindrance model the bulky ligands are assumed to repel each other through Pauli repulsion. Dispersion stabilization occurs when these bulky ligands form favorable non-covalent interactions and the magnitude of this stabilization is enough to overcome some Pauli replusion. Although the two effects are distinct, dispersion stabilized molecules frequently also benefit from steric protection of the molecule's reactive moiety.

Organic Chemistry

Formation of substituted hexaphenylethane from radical monomers. Increased steric bulk promotes head-to-head addition.

Dispersion stabilization explains the reactivity patterns of bulky hydrocarbon radicals. •CPh3 radicals have been known since 1900. In the mid-1960s, the dimer form of the radical was observed, and instead of forming the expected Ph₃CCPh₃ product, the radical instead undergoes head to tail addition. By contrast, adding two ^tBu groups to the meta positions on the phenyl rings will cause the radical to readily dimerize to (3,5-^tBu₂H₃C₆)₃C–C(C₆H₃-3,5-^tBu₂)₃. Initially this discovery puzzled researchers because •CPh3 head-to-tail addition seemed to suggest steric repulsion disfavored direct addition, but the more sterically crowded molecule underwent head-to-head addition. More recently, computational analysis has shown the stabilization of the tBu substituted molecule to be through dispersion interactions. The study suggests that dispersion interactions between tBu groups provides ~60kcal/mol of stabilization to the molecule, enough to overcome the unfavorable steric interactions from the additional ^tBu groups.

The bond critical points in (C^tBu)₄ based on Bader AIM analysis. The critical points between H atoms on the ^tBu groups are possible evidence for dispersion stabilization.

Exploiting the lessons from hexaphenylethane, Schriener and coworkers have synthesized new molecules with "dispersion energy donors" to form both long C-C bonds and short H•••H contacts. The (3,5-^tBu₂H₃C₆)₃C–C(C₆H₃-3,5-^tBu₂)₃ contains a 1.67Å C-C single bond, much longer than the canonical 1.54Å C-C single bond. Despite the long C-C bond, the molecule remains stable at room temperature because of dispersion based stability. The effect of dispersion stabilization was further probed with a series of of meta-substituted hexaphenylethane molecules substitued with Me, ⁱPr, ^tBu, Cy, and adamantene. Of these molecules, only ^tBu and adamantene were observed to form the head to head dimer, showing the sensitivity of dispersion stablization to rigid, polarizable substituents. Dispersion stabilization has additionally been used to stabilize intermolecular contacts. When the (3,5-^tBu₂H₃C₆)₃CH molecule dimerizes to form [(3,5-^tBu₂H₃C₆)₃CH]₂, stabilizing interactions between ^tBu groups bring the the central pair of hydrogens hydrogens to a contact distance of 1.566Å as determined by neutron diffraction; well within their van der Waals radii and at the time the shortest reported H•••H contact.

Researchers have posited that the stability of the bulky hydrocarbon tetra(tert-butyl)tetrahedrane is in part from dispersion forces. Originally, the molecule's thermal stability in air up to 135°C was attributed to the corset effect, wherein any stability gained by elongating a C-C bond to release ring strain would be countered by increased repulsion between the remaining ^tBu groups. Although the corset effect is the dominant force driving stability, this explanation has been recently supplemented with calculations that show the ^tBu groups provide 3.1 kcal/mol of stability to the molecule.

Inorganic Chemistry

Molecular structure of Fe(nor)₄ with hydrogens omitted. The short contacts (less than sum of van der Waal radii) between carbon atoms that could indicate dispersion stabilization shown in blue.

Dispersion forces can stabilize reactive organometallic molecules by using bulky ligands in metal complexes. The first two coordinate Cu(II) complex, Cu{N(SiMe₃)Dipp}₂ (Dipp = C₆H₅-2,6ⁱPr₂), was prepared by combining a 1:1 ratio of LiN(SiMe₃)Dipp to CuCl. The mechanism of this reaction is unknown, but the net result is Cu disproportionation to form the Cu(II) complex and Cu metal. The authors suggest that a dispersion stabilization of about 25 kcal/mol from the bulky ligands assists in the initial disproportionation reaction but prevents crystals of the complex from decomposing via further disporportionation to DippN=NDipp, N-(SiMe₃)₂Dipp, and Cu metal. The complex's eclipsed ligand conformation further suggests that dispersion stabilizes the low coordinate complex.

Metal tetranorbornyls, M(nor)₄ (1-nor = 4bicyclo[2.2.1]hept-1-yl, M = Fe,Co), benefit from stability imparted by dispersion. The compounds display high stability for a formally 4⁺ oxidation state metal center, which has traditionally been attributed to unfavorable β-elimination. Computational work has determined that the close norbornyl contacts are worth -45.9 kcal/mol of energy, providing significant stabilization to the molecule.

Main Group Chemistry

Three representations of the plumbylene dimer. a) A cannonical Lewis structure with two resonance contributors. b) A ball and stick diagram showing the asymmetric geometries about the Pb atoms c) A spacefill model showing how the proximity of bulky trimethylsilyl groups

Researches have used dispersion stabilization to promote unusual main group bonding. Dispersion forces have been shown to stabilize bonding in a cyclic silylated plumbylene dimer. The crystal structure of the plumbylene dimer has different arrangements about each Pb atom, indicating the molecule forms a singular donor-acceptor interaction instead of double donor-acceptor interaction. Computational investigations reveal that the dimer has roughly the twice bond dissociation energy of the trans-bent parent diplumbene (Pb₂H₄) despite the parent diplumbene having a higher Wiberg Bond Index. The authors suggest through DFT calculations that dispersion forces between bulky trimethylsilyl groups are the determine the dimer's conformation.

Dispersion has been implicated in stabilizing a Ga-substituted doubly bonded dipnictenes of the form [L(X)Ga]₂E₂ where E = As, Sb, Bi and L = C[C(Me)N(2,6-ⁱPr₂-C₆H₃). Researchers synthesized the As version of the molecule and computationally analyzed the full series. By calculating the energy of the molecules with and without dispersion, it was determined that dispersion nearly doubles the dimer's enthalpic stabilization when formed from the monomer. In this case, dispersion couples with electronics to provide stability to the molecule.

The thermal stability of an extended Si-Si bond in ^tBu₃SiSi^tBu₃, or superdisilane, has also been attributed to dispersion interactions. Superdisilane has a Si-Si bond length of 2.697Å in the solid state, significantly extended compared to the gas phase Si-Si bond length of 2.331Å in the parent disilane H₃SiSiH₃. Despite the long bond length, superdisilane exhibits thermal stability up to 323K. Dispersion forces keep the molecule inert even while its core Si-Si bond lengthens. Similarly, the longest known Ge-Ge bond is found in ^tBu₃GeGe^tBu₃ and is also facilitated by dispersion stabilization.

Lewis diagrams showing the lengthened Si-Si bond in superdisilane.

Dispersion stabilization has also been invoked for (^tBuC)₃P, a main group analog of a hydrocarbon tetrahedrane. (^tBuC)₃P displays greater thermal stability than a similar tetrahedrane (^tBuCP)₂, which decomposes at just -32°C. The additional stability has been attributed to dispersion interactions between the bulky ^tBu groups beneath the tetrahedrane. The authors through computational analysis identified 9 H-H interactions that each provide -0.7 kcal/mol of energy, overcoming the steric penalty of bringing the ^tBu groups together. Geometry optimizations with ^tBu groups swapped for less bulky hydrogens cause significant molecule shape change, indicating that dispersion plays a significant role in stabilizing the molecule's structure.

Applications

Dispersion stabilized molecules are an active area of research. Presently dispersion interactions have mostly been examined after the synthesis of molecules, but molecular design with dispersion effects in mind has generated some excitement for potential future applications. The dispersion interaction effect has been compared to the emerging work on frustrated Lewis pairs, and there is speculation that dispersion could be useful in future catalysis.