Multi-Electron Reduction of Small Molecules by Triiron Reaction Sites

Abstract

The observation that multi-electron activation of small molecule substrates occurs at polynuclear reaction sites, common to both metalloenzymes and heterogeneous catalysts, has led to the articulation of the polynuclear hypothesis - the idea that the expanded redox reservoir afforded by M-M interactions in polynuclear systems stabilizes multiple oxidation states and facilitates multi-electron transformations. Currently, examples of synthetic clusters that test the viability of polynuclear reaction sites towards effecting multi-electron activation of small molecule substrates are lacking. To test the polynuclear hypothesis, we targeted a system that embodies design elements common to metaloenzyme cofactors: polynuclear reaction sites that feature high-spin, coordinatively unsaturated metal centers. Metallation of tbsLH6 [tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3] yields high-spin trinuclear Fe(II) complex (tbsL)Fe3(THF). The filled anti-bonding orbitals in high-spin cluster (tbsL)Fe3(THF) renders ligand reorganization facile, which allows for a range of metal-substrate binding modes. The polynuclear site within the (tbsL)Fe3(THF) cluster cooperatively binds anionic donors and allows 2e- reduction of substrates including inorganic azide and hydrazines, yielding μ3-nitrido and μ3-imido products, respectively. The 4e- reductive N=N bond cleavage of azobenzene is also achieved in the presence of (tbsL)Fe3(THF) to yield Fe3 bis-imido complex ((tbsL)Fe3(μ3-NPh)(μ2-NPh), which has been structurally characterized. Cyclic voltammograms of a series of selected triiron imido and nitrido clusters suggest that oxidation states up to [Fe(IV)][Fe(III)]2 are electrochemically accessible. Addition of neutral pi-acidic molecules including tert-butylisonitrile (tBuNC) and carbon monoxode (CO) to trinuclear cluster (tbsL)Fe3(THF) led to the formation of a new series of coordination compounds, where binding to a single metal center is favored over cooperative substrate binding. Coordinated substrates are activated toward further reactivity, highlighted by the reductive coupling of isonitriles by (tbsL)Fe3(μ1-CNtBu)3 in the presence of phenylsilane. Finally, efforts to synthesize of a family of mixed Fe-Mn clusters that differ by single metal-site substitutions are presented. Substitutionally homogeneous (tbsL)Fe2Mn(THF) cluster is accessed from binuclear complex (tbsLH2 )Fe2. Attempts to synthesize similar Mn2Fe clusters results in isolation of a mixture of heterotrinuclear species. In conjunction with NMR, EPR, Mössbauer, and X-ray fluorescence spectroscopies, anomalous scattering measurements were critical for the unambiguous assignment of the metal substitution products that were synthesized.