Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System
Jeremy E. Weber, Faraj Hasanayn, Majed S. Fataftah, Brandon Q. Mercado, Robert H. Crabtree, Patrick L. Holland
Abstract
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-P2tBuPyrr)ReCl2]2(μ-η1:η1-N2) (P2tBuPyrr = [2,5-(CH2PtBu2)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 (P2tBuPyrr)ReNCl.