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 (S. Mebs, J. Beckmann, Physical Chemistry Chemical Physics 2022, 24, 20953–20967).

In Silico Activation of CO2, NO2, and SO2 with Light Atom Molecules and Stepwise Conversion of CO2 into Methanol and Water

“…However, once being embedded within the active site, all three EO 2 molecules are bent towards close-to-tetrahedral angles (Figure S4b) and the EÀ O bonds are stretched without any further barrier, which is thoroughly explained by the electric field being exerted to the target molecule by the frustrated Lewis pair(s). Similar observations have been made for H 2 [9] and N 2 [19,20] before. In the EO 2complexes, the ligand strain rises to 380.8 kJ mol À 1 for 1-CO 2 and 280.7 kJ mol À 1 for 2-SO 2 , but remains almost unchanged at 251.2 kJ mol À 1 for 2-NO 2 .…”

“…The EO 2 ‐fragments within the EO 2 ‐complexes are reduced by about 2e (CO 2 ), 1.5 e (NO 2 ), and 1.0 e (SO 2 ) with respect to their unbound counterparts, the majority of which accumulates at the central E atom (−1.7 e in CO 2 , −0.8 e in NO 2 or SO 2 ); 0.1–0.4 e are located at the O atoms. Equally strong charge accumulation effects have been obtained in the recently published works on N 2 ‐activation with related ligand molecules [19,20] . The vast majority of this charge stems from the P atoms lone pairs, LP(P), of the ligand molecules, minor contributions come from the acenaphthene backbones.…”

“…Equally strong charge accumulation effects have been obtained in the recently published works on N 2 -activation with related ligand molecules. [19,20] The vast majority of this charge stems from the P atoms lone pairs, LP(P), of the ligand molecules, minor contributions come from the acenaphthene backbones. Surprisingly, the À BF 2 fragments, although being involved in polar-covalent OÀ B bonds in the adducts, show negligible changes.…”

“…In a screening attempt, we employ three different peri ‐substituted scaffolds, (6‐Ph 2 P‐ ace ‐5−)P(−5′‐ ace ‐6′‐BF 2 ) 2 ( 1 ), P(−5‐ ace ‐6‐BF 2 ) 3 ( 2 ), and PhP(−5‐ ace ‐6‐BF 2 ) 2 ( 3 ) and test their abilities to capture and activate CO 2 , NO 2 , and SO 2 . In recent publications we showed that variations of such tripodal ligands are able to activate N 2 [19,20] and capture noble gas atoms [21] and methane [22] . Crucial parameters to accomplish fixation and activation are the number and type of Lewis acidic and basic sites, and their relative orientation, respectively, as well as the size of the spacer molecule (e. g. naphthalene, acenaphthene, biphenylene, xanthene) to form frustrated Lewis pairs which do not quench intra ‐molecularly in the initial starting state.…”