Testing the Polynuclear Hypothesis: Multielectron Reduction of Small Molecules by Triiron Reaction Sites

High-spin trinuclear iron complex (L-tbs)Fe-3(thf) ([L-tbs](6-) = [1,3,5-C6H9(NC6H4-o-(NSiBuMe2)-Bu-t)(3)](6-)) (S = 6) facilitates 2 and 4e(-) reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenyl-hydrazine with (L-tbs)Fe-3(thf) yields triiron mu(3)-imido cluster (L-tbs)Fe-3(mu(3)-NH) and ammonia or aniline, respectively. (L-tbs)Fe-3(mu(3)-NH) has a similar zero-field Fe-57 Mossbauer spectrum compared to previously reported [(L-tbs)Fe-3(mu N-3)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido (L-tbs)Fe-3(mu(3)-NH) with lithium bis(trimethylsily)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [(L-tbs)Fe-3(mu(3)-N)]Li(thf)(3). The lithium capped nitrido, structurally similar to the pseudo C-3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C-3-symmetric featuring intracore distances of Fe-Fe 2.4802(5) angstrom, Fe-N-(nitride) 1.877(2) angstrom, and N-(nitride)-Li 1.990(6) angstrom. A similar 2e(-) reduction of 1,2-diphenylhydrazine by (L-tbs)Fe-3(thf) affords (L-tbs)Fe-3(mu(3)-NPh) and aniline. The solid state structure of (L-tbs)Fe-3(mu(3)-NPh) is similar to the series of mu(3)-nitrido and -imido triiron complexes synthesized in this work with average Fe-N-imido and Fe-Fe bond lengths of 1.941(6) and 2.530(1) angstrom, respectively. Reductive N=N bond cleavage of azobenzene is also achieved in the presence of (L-tbs)Fe-3(thf) to yield triiron bis-imido complex (L-tbs)Fe-3(mu(3)-NPh)(mu(2)-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e(-) metal based oxidation resulting in an (Fe-IV)(Fe-III)(2) formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (Fe-IV)(Fe-III)(2) (in the case of [(L-tbs)Fe-3(mu(3)-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.