In Silico Partial N2 to NH3 Conversion with a Light Atom Molecule

Abstract

N2 can be stepwise converted in silico into one molecule NH3 and a secondary amide with a bond activator molecule consisting only of light main group elements. The proposed N2-activating pincer-related compound carries a silyl ion (Si(+)) center as well as three Lewis acidic (–BF2) and three Lewis basic (–PMe2) sites, providing an efficient binding pocket for gaseous N2 within the framework of intramolecular frustrated Lewis pairs (FLP). In addition, it exhibits supportive secondary P–B and F…B contacts, which stabilize the structure. In the PSi(+)–N–N–BP environment the N≡N triple bond is extended from 1.09 Å to remarkable 1.43 Å, resembling a N–N single bond. The strongly activated N–N-fragment is prone to subsequent hydride addition and protonation steps, resulting in the energy efficient transfer of two hydrogen equivalents. The next hydride added causes the release of one molecule NH3, but leaves the ligand system as poisoned R3Si(+)–NH2–PMe2 or R3Si(+)-NH3 dead-end states behind. The study indicates that approximately tetrahedral constrained SiBP2-pockets are capable to activate N2, whereas the acid-rich SiB3- and SiB2P-pocktes, as well as the base-rich SiP3-pockets fail, hinting towards the high relevance of the acid-base proportion and relative orientation. The electronic structure of the N2-activated state is compared to the corresponding state of a recently published peri-substituted bond activator molecule featuring a PSi(+)–N–N–Si(+)P site (Mebs & Beckmann 2022).