Coordination of Al(C₆F₅)₃ vs. B(C₆F₅)₃ on group 6 end-on dinitrogen complexes: chemical and structural divergences

The coordination of the Lewis superacid tris(pentafluorophenyl)alane (AlCF) to phosphine-supported, group 6 bis(dinitrogen) complexes [ML₂(N₂)₂] is explored, with M = Cr, Mo or W and L = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), dmpe (1,2-bis(dimethylphosphino)ethane) or 2 × PMe₂Ph. Akin to tris(pentafluorophenyl)borane (BCF), AlCF can form 1 : 1 adducts by coordination to one distal nitrogen of general formula trans-[ML₂(N₂){(μ-η¹:η¹-N₂)Al(C₆F₅)₃}]. The boron and aluminium adducts are structurally similar, showing a comparable level of N₂ push-pull activation. A notable exception is a bent (BCF adducts) vs. linear (AlCF adducts) M-N-N-LA motif (LA = Lewis acid), explained computationally as the result of steric repulsion. A striking difference arose when the formation of two-fold adducts was conducted. While in the case of BCF the 2 : 1 Lewis pairs could be observed in equilibrium with the 1 : 1 adduct and free borane but resisted isolation, AlCF forms robust 2 : 1 adducts trans-[ML₂{(μ-η¹:η¹-N₂)Al(C₆F₅)₃}₂] that isomerise into a more stable cis configuration. These compounds could be isolated and structurally characterized, and represent the first examples of trinuclear heterometallic complexes formed by Lewis acid-base interaction exhibiting p and d elements. Calculations also demonstrate that from the bare complex to the two-fold aluminium adduct, substantial decrease of the HOMO-LUMO gap is observed, and, unlike the trans adducts (1 : 1 and 1 : 2) for which the HOMO was computed to be a pure d orbital, the one of the cis-trinuclear compounds mixes a d orbital with a π* one of each N₂ ligands. This may translate into a more favourable electrophilic attack on the N₂ ligands instead of the metal centre, while a stabilized N₂-centered LUMO should ease electron transfer, suggesting Lewis acids could be co-activators for electro-catalysed N₂ reduction. Experimental UV-vis spectra for the tungsten family of compounds were compared with TD-DFT calculations (CAM-B3LYP/def2-TZVP), allowing to assign the low extinction bands found in the visible spectrum to unusual low-lying MLCT involving N₂-centered orbitals. As significant red-shifts are observed upon LA coordination, this could have important implications for the development of visible light-driven nitrogen fixation.