Electronic And Spin-State Effects On Dinitrogen Splitting To Nitrides In A Rhenium Pincer System

Bimetallic nitrogen (N-2) splitting to form metal nitrides is an attractive method for N-2 fixation. Although a growing number of pincer-supported systems can bind and split N-2, the precise relationship between the ligand properties and N-2 binding/ splitting remains elusive. Here we report the first example of an N-2-bridged rhenium(III) complex, [(trans-P(2)(tBu)Pyrr)ReCl2](2)(mu-eta(1):eta(1)-N-2) (P-2 (tBu)Pyrr = [2,5-((CH2PBu2)-Bu-t)(2)C4H2N](-)). In this case, N-2 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 pi-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N-2 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 (P(2)(tBu)Pyrr)ReNCl.