Issue 29, 2024

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

Abstract

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-η11-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-η11-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.

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

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Article information

Article type
Edge Article
Submitted
24 apr 2024
Accepted
14 jun 2024
First published
17 jun 2024
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2024,15, 11321-11336

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

L. Escomel, F. F. Martins, L. Vendier, A. Coffinet, N. Queyriaux, V. Krewald and A. Simonneau, Chem. Sci., 2024, 15, 11321 DOI: 10.1039/D4SC02713B

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