Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System

Bimetallic nitrogen (N₂) splitting to form metal nitrides is an attractive method for N₂ fixation. Although a growing number of pincer-supported systems can bind and split N₂, the precise relationship between the ligand properties and N₂ binding/splitting remains elusive. Here we report the first example of an N₂-bridged rhenium(III) complex, [(trans-P₂Pyrr)ReCl₂]₂(μ-η¹:η¹-N₂) (P₂Pyrr = [2,5-(CH₂P^tBu₂)₂C₄H₂N]^-). In this case, N₂ binding occurs at a higher oxidation level than that in other reported pincer analogues. Analysis of the electronic structure through computational studies shows that the weakly π-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N₂ in the bridged complex. Utilizing SQUID magnetometry, we demonstrate a singlet ground state for this Re-N-N-Re complex, and we offer tentative explanations for antiferromagnetic coupling of the two local S = 1 sites. Reduction and subsequent heating of the rhenium(III)-dinitrogen complex leads to chloride loss and cleavage of the N-N bond with isolation of the terminal rhenium(V) nitride complex (P₂Pyrr)ReNCl.