Coordination Chemistry from Trigonally Coordinated Iron Platforms: Chemistry Relevant to Dinitrogen Reduction

Abstract

Use of the sterically-encumbering tris(phosphino)borate ligand, 1;PhB(CH_2P^iPr_2)_33;-, enabled isolation of tetrahedral iron species spanning five oxidn. states, all featuring nitrogenous ligands in the fourth coordination site. Low oxidn. state iron species (e.g. Fe^0, Fe^I) readily bind weakly-coordinating, π-acidic ligands (e.g. N_2). The bound dinitrogen can then be further reduced and even functionalized to yield diazenido products (e.g. Fe^(II)(N_2R)). High oxidn. state iron complexes were fashioned through oxidative group transfer reactions to yield imido/nitrene products (e.g. Fe^(III)(NR)), while decompn. of an iron-ligated, dibenzoazabicycloheptadiene amide yielded a room temp. stable Fe^(IV) nitride complex. The ability to mediate multi-electron redox transformations demonstrates the redox flexibility inherent to these iron complexes. We have demonstrated that a singular iron site is capable of stabilizing the span of oxidn. states required for dinitrogen redn. (i.e., Fe^I in Fe(N_2) and Fe^(IV) in Fe≡N) and have isolated and characterized several of the potential intermediates relevant to dinitrogen redn.