An orbitally adapted push–pull template for N2 activation and reduction to diazene-diide

Abstract

A Lewis superacidic bis(borane) C₆F₄{B(C₆F₅)₂}₂ was reacted with tungsten N₂-complexes [W(N₂)₂(R₂PCH₂CH₂PR₂)₂] (R = Ph or Et), affording zwitterionic boryldiazenido W(II) complexes trans-[W(L)(R₂PCH₂CH₂PR₂)₂(N₂{B(C₆F₅)₂(C₆F₄B(C₆F₅)₃})] (L = ø, N₂ or THF). These compounds feature only one N–B linkage of the covalent type, as a result of intramolecular boron-to-boron C₆F₅ transfer. Complex trans-[W(THF)(Et₂PCH₂CH₂PEt₂)₂(N₂{B(C₆F₅)₂C₆F₄B(C₆F₅)₃})] (5) was shown to split H₂, leading to a seven-coordinate complex [W(H)₂(Et₂PCH₂CH₂PEt₂)₂(N₂{B(C₆F₅)₂}₂C₆F₄)] (7). Interestingly, hydride storage at the metal triggers backward C₆F₅ transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N₂ complex [W(H)₂(Et₂PCH₂CH₂PEt₂)₂(N₂{B(C₆F₅)₃}] (10) differing only in the Lewis acid (LA), namely B(C₆F₅)₃, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N₂ activation up to the diazene-diide (N₂²⁻) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N₂.