2,6‐Bis(diazaboryl)pyridine: A Superbasic Sterically Demanding Pyridine Ligand.

Abstract

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol , which outperforms the "proton sponge" 1,8-bis(dimethylamino)naphthalene (996 kJ mol ). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.

2,6‐Bis(diazaboryl)pyridine – A Ligand with Hemilabile Donor and Lewis Acid Functionalities

“…When the analogous reaction of dab 2 py was conducted with GaCl 3 as the Lewis‐acid, no reaction was observed at room temperature, as monitored on a CD 2 Cl 2 solution by multinuclear NMR spectroscopy. This was unexpected, because dab 2 py readily forms a 1:1 adduct with BCl 3 , but GaCl 3 is the stronger Lewis‐acid (FIA [kJ/mol –1 ]: BCl 3 = 405; GaCl 3 = 432) . Neither did the ligand induced ionization process proceed, as observed for the analogous reaction of dab 2 py with AlBr 3 .…”

“…Geometry optimization for the calculated 1:1 adduct dab 2 py→GaCl 3 clearly showed that the adduct formation is sterically hindered (TPSS‐D3(BJ)/def2‐TZVPP level of theory, see ESI). The gallium atom is displaced out of the pyridine plane with an angle between the N–Ga bond and the pyridine plane of 21.4° (the corresponding angle found in the crystal structure of dab 2 py→BCl 3 is only 6.0°) . The inability of the adduct formation of the Lewis‐base dab 2 py and the Lewis‐acid GaCl 3 due to steric hindrance is making this system by definition a Frustrated Lewis Pair (FLP)…”

“…Recently, our group utilized a diazaboryl‐substituent for the synthesis of the pyridine ligand 2,6‐bis(diazaboryl)pyridine (dab 2 py; dab = –B(N i PrCH 2 ) 2 ) . The electron density at the pyridine's nitrogen atom is increased significantly by the strong +I‐effect of the diazaboryl group, making it a superbase.…”

“…Our group recently reported the addition of dab 2 py to BF 3 and BCl 3 . The corresponding 1:1 adducts were obtained with boron being coordinated by the pyridine's nitrogen atom only.…”

[4‐(Ph3B)‐2,6‐Mes2Py]−: A Sterically Demanding Anionic Pyridine

“…On the other hand, the reactions carried out with n BuLi/TMEDA and t BuLi yielded Mes 2 Py; however, the reactions were not selective as Mes 2 Py was accompanied by significant amounts of byproducts, thereby rendering this route unfeasible. Recently, our group reported a one‐pot procedure for the borylation of pyridine at the 2,6‐postions by the addition of lithium to a solution of 2,6‐dibromopyridine and B(O i Pr) 3 . This protocol was adopted for the direct borylation of 2 ‐Cl.…”