Back donation, intramolecular electron transfer and N–O bond scission targeting nitrogen oxyanion reduction: how can a metal complex assist?

Abstract

A density functional theory exploration studies a range of ancillary coordinated ligands accompanying nitrogen oxyanions with the goal of promoting back donation towards varied nitrogen oxidation states. Evaluation of a suite of Ru and Rh metal complexes reveals minimum back donation to the κ¹-nitrogen oxyanion ligand, even upon one-electron reduction. This reveals some surprising consequences of reduction, including redox activity at pyridine and nitrogen oxyanion dissociation. Bidentate nitrate was therefore considered, where ancillary ligands enforce geometries that maximize M–NO_x orbital overlap. This strategy is successful and leads to full electron transfer in several cases to form a pyramidal radical NO₃²⁻ ligand. The impact of ancillary ligand on degree of nitrate reduction is probed by comparing the powerful o-donor tris-carbene borate (TCB) to a milder donor, tris-pyrazolyl borate (Tp). This reveals that with the milder Tp donor, nitrate reduction is only seen upon addition of a Lewis base. Protonation of neutral and anionic (TCB)Ru(κ²-NO₃) at both terminal and internal oxygens reveals exergonic N–O bond cleavage for the reduced species, with one electron coming from Ru, yielding a Ru^III hydroxide product. Comparison of H⁺ to Na⁺ electrophile shows weaker progress towards N–O bond scission. Finally, calculations on (TCB)Fe(κ²-NO₃) and [(TCB)Fe(κ²-NO₃)]^– show that electron transfer to nitrate is possible even with an earth abundant 3d metal.