Testing the polynuclear hypothesis: multi-electron reduction of small molecules by triiron reaction sites

摘要

High-spin trinuclear iron complex (^tbsL)Fe3(thf) ([^tbsL]⁶⁻ = [1,3,5- C6H9(NPh-o-NSi^tBuMe2)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 counter cation, 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–N_imido and Fe–Fe bond lengths of 1.941(6) Å 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 bisimido 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 bisimido 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 [(^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.