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

High-spin
trinuclear iron complex (tbsL)­Fe3(thf) ([tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–) (S =
6) facilitates 2 and 4e– reduction of NxHy type substrates
to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine
with (tbsL)­Fe3(thf) yields triiron μ3-imido cluster (tbsL)­Fe3(μ3-NH) and ammonia or aniline, respectively. (tbsL)­Fe3(μ3-NH) has a similar zero-field 57Fe Mössbauer spectrum compared to previously reported
[(tbsL)­Fe3(μ3-N)]­NBu4, and can be directly synthesized by protonation of the anionic triiron
nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron
parent imido (tbsL)­Fe3(μ3-NH)
with lithium bis­(trimethylsilyl)­amide results in regeneration of the
triiron nitrido complex capped with a thf-solvated Li cation [(tbsL)­Fe3(μ3-N)]­Li­(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium
countercation, is rigorously C3-symmetric
featuring intracore distances of Fe–Fe 2.4802(5) Å, Fe–N(nitride) 1.877(2) Å, and N(nitride)–Li
1.990­(6) Å. A similar 2e– reduction of 1,2-diphenylhydrazine
by (tbsL)­Fe3(thf) affords (tbsL)­Fe3(μ3-NPh) and aniline. The solid state structure
of (tbsL)­Fe3(μ3-NPh) is similar
to the series of μ3-nitrido and -imido triiron complexes
synthesized in this work with average Fe–Nimido and
Fe–Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively.
Reductive NN bond cleavage of azobenzene is also achieved
in the presence of (tbsL)­Fe3(thf) to yield triiron
bis-imido complex (tbsL)­Fe3(μ3-NPh)­(μ2-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 (FeIV)­(FeIII)2 formulation. Cyclic voltammograms
of the series of triiron clusters presented herein demonstrate that
oxidation states up to (FeIV)­(FeIII)2 (in the case of [(tbsL)­Fe3(μ3-N)]­NBu4) are electrochemically accessible. These results
highlight the efficacy of high-spin, polynuclear reaction sites to
cooperatively mediate small molecule activation.