Structure and Bonding Nature of the Strained Lewis Acid 3-Methyl-1-boraadamantane: A Case Study Employing a New Data-Analysis Procedure in Gas Electron Diffraction

Graphical Abstract

Basis for bonding: Base-free 3-methyl-1-boraadamantane was synthesized and studied by gas-phase electron diffraction at 100 °C. The configuration of the boron atom is pyramidal with (C-B-C)=349.4(4)°. The nature of bonding was analyzed by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The shortening of the BC bonds and elongation of the adjacent CC bonds is explained by the σ(C-C)→p(B) hyperconjugation (see figure).

Abstract

Base-free 3-methyl-1-boraadamantane was synthesized by starting from its known THF adduct, transforming it to a butylate-complex with n-butyllithium, cleaving the cage with acetyl chloride to give 3-n-butyl-5-methyl-7-methylene-3-borabicyclo[3.3.1]nonane and closing the cage again by reacting the latter with dicyclohexylborane. The identity of 3-methyl-1-boraadamantane was proven by ¹H, ¹¹B and ¹³C NMR spectroscopy and elemental analysis. The experimental equilibrium structure of the free 3-methyl-1-boraadamantane molecules has been determined at 100 °C by using gas-phase electron diffraction. For this structure determination, an improved method for data analysis has been introduced and tested: the structural refinement versus gas-phase electron diffraction data (in terms of Cartesian coordinates) with a set of quantum-chemically derived regularization constraints for the complete structure under optimization of a regularization constant, which maximizes the contribution of experimental data while retaining a stable refinement. The detailed analysis of parameter errors shows that the new approach allows obtaining more reliable results. The most important structural parameters are: r_e(B-C)_av=1.556(5) Å, _e(C-B-C)_av=116.5(2)°. The configuration of the boron atom is pyramidal with (C-B-C)=349.4(4)°. The nature of bonding was analyzed further by applying the natural bond orbital (NBO) and atoms in molecules (AIM) approaches. The experimentally observed shortening of the BC bonds and elongation of the adjacent CC bonds can be explained by the σ(C-C)→p(B) hyperconjugation model. Both NBO and AIM analyses predict that the BC bonds are significantly bent in the direction out of the cage.