Mechanism of Generation of closo-Decaborato Amidrazones. Intramolecular Non-covalent B–H•••π(Ph) Interaction Determines Stabilization of the Configuration around the Amidrazone C=N Bond
Abstract
Three types of N(H)-nucleophiles, viz. hydrazine, acetyl hydrazide, and a set of hydrazones, were used to study the nucleophilic addition to the C≡N group of the 2-propanenitrilium closo-decaborate cluster (Ph3PCH2Ph)[B10H9NCEt], giving N-closo-decaborato amidrazones. A systematic mechanistic study of the nucleophilic addition is provided and included detailed synthetic, crystallographic, computational and kinetic work. As a result, two possible mechanisms have been proposed, which consist of firstly a consecutive incorporation of two Nu(H) nucleophiles, with the second responsible for a subsequent rapid proton exchange. The second possible mechanism assumes a pre-formation of a dinuclear [Nu(H)]2 species which subsequently proceeds with the nucleophilic attack on the boron cluster. The activation parameters for hydrazones indicate a small dependence on bond formation [ΔH≠ = 6.8–15 kJ⋅mol–1], but significantly negative entropies of activation [ΔS≠ ranges from –139 to –164 J⋅K–1⋅mol–1] with the latter contributing some 70–80% of the total Gibbs free energy of activation, ΔG≠. In the X-ray structure of (Z)-(Ph3PCH2Ph)[B10H9N(H)=C(Et)NHN=CPh2], very rare intramolecular non-covalent interactions B–H•••π(Ph) were detected and studied by DFT calculations (M06-2X/6-311++G** level of theory) and topological analysis of the electron density distribution within the framework of Bader’s theory (QTAIM method). Estimated strength of these non-covalent interactions is 0.8–1.4 kcal/mol.
