Understanding the Nature and Properties of Hydrogen–Hydrogen Bonds: The Stability of a Bulky Phosphatetrahedrane as a Case Study

Abstract

Recently, the first mixed C/P phosphatetrahedranes ( t BuC)3P and ( t BuCP)2 were reported. Unlike ( t BuCP)2, ( t BuC)3P exhibits remarkable thermal stability, which can be partially attributed to a network of nine hydrogen–hydrogen bonds (HHBs) localized between the tert-butyl substituents. The stabilizing contribution arising from this network of HHBs was obtained from local energy decomposition (LED) analysis calculated at the domain-based local pair natural orbital CCSD­(T) (DLPNO-CCSD­(T)) level of theory. These calculations suggest that each HHB contributes approximately −0.7 kcal/mol of stabilization; however, the net stabilization energy likely lies between −0.25 and −0.5 kcal/mol because of steric repulsion. Spatial analysis of the London dispersion energy via a dispersion interaction density (DID) plot reveals that the DID surface is localized at key C–H groups involved in HHBs, consistent with London dispersion interactions predominantly arising from HHBs. In addition, we present a computed mechanism that supports a phosphinidenoid species as a key reaction intermediate in the synthesis of ( t BuC)3P.

London Dispersion Helps Refine Steric A-Values: Dispersion Energy Donor Scales

“…Note that all H···H distances are slightly above ca. 2.20 Å, the distance at which the interaction between hydrogens of two hydrogen–hydrogen bound methane molecules becomes attractive . The C 3 ···C­(R) distances are shorter than the equilibrium distance of a methane dimer (3.70 or 4.02 Å), a distance in which both repulsion and LD are active, but repulsion is expected to be more significant.…”

“…2.20 Å, the distance at which the interaction between hydrogens of two hydrogen− hydrogen bound methane molecules becomes attractive. 34 The C 3 •••C(R) distances are shorter than the equilibrium distance of a methane dimer (3.70 55 or 4.02 Å 56 ), a distance in which both repulsion and LD are active, but repulsion is expected to be more significant. However, the reduction in distances and angles could also result from the fact that the CH 3 hydrogens of Et and iPr can experience repulsion from the H eq on C 1 .…”

“…In a similar fashion, other unstable bonds or fragments can be stabilized inside a dispersion-held shell, 28 such as very long C−C bonds, 29,30 a SiSi bond with a formal zero oxidation state for silicon, 31,32 and tetrahedranes. 27,33,34 As a force that operates between induced dipoles, the strength of LD interactions depends on the size and number of the interacting groups (or their number of electrons in a particular volume element) and on their polarizabilities. For this reason, LD is usually considered to be important mainly in large molecules or in the bulk phase.…”

“…For example, bulky tert -butyl groups at the meta positions can stabilize highly unstable hexaphenyl­ethane (HPE), , and they can also stabilize dimers of tri­(3,5- tert -butylphenyl)­methane, in which the central covalent C–C bond of HPE was replaced by two C–H bonds. , The attractive interactions between the tert -butyl groups form a dispersion-held shell (or “corset”), which causes the formation of the shortest reported C–H···H–C distance to date (1.566(5) Å). In a similar fashion, other unstable bonds or fragments can be stabilized inside a dispersion-held shell, such as very long C–C bonds, , a SiSi bond with a formal zero oxidation state for silicon, , and tetrahedranes. ,, …”

Accurate Crystal Structures of C12H9CN, C12H8(CN)2, and C16H11CN Valence Isomers Using Nonspherical Atomic Scattering Factors

“…The promise of routinely refining H-atom parameters is especially welcome with the increased recognition of the importance of London dispersion interactions (the attractive component of the van der Waals potential) on many aspects of chemical interaction and structure. These are overwhelmingly hydrocarbon interactions, ranging from small molecules, ,− crystal structure directors, chemical reaction mechanisms, and biological macromolecular interaction mechanisms . A dispersion energy donor scale has recently been proposed .…”