Evaluating iron diimines: ion-pairing, lability and the reduced state

Tris(diimine)iron(II) complexes are aspirational photosensitizers but their small ligand fields confer lability and distinct redox properties. We study these aspects in the gas phase using mass spectrometry and density-functional theory of [Fe(N^N)₃]²⁺ dications (N^N = 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), 4,4'-dibromo-2,2'-bipyridine (bipy^Br), 4,4'-di(<i>tert</i>-butyl)-2,2'-bipyridine (bipy^<i>t</i>-Bu)). Collision-induced dissociation of ion pairs {[Fe(N^N)₃]X}⁺ (X = BPh₄^- and BAr^F₄^-; Ar^F = 3,5-bis(trifluoromethyl)phenyl) requires high energies, not because of strong ion pairing but because the tetraarylborates are poor ligands, such that the lowest-energy pathway requires ligand dissociation. Dissociation of dications reveals contrasting thermal stabilities ([Fe(bipy^<i>t</i>-Bu)₃]²⁺ > [Fe(phen)₃]²⁺ ≫ [Fe(bipy^Br)₃]²⁺ > [Fe(bipy)(phen)₂]²⁺ > [Fe(bipy)₂(phen)]²⁺ > [Fe(bipy)₃]²⁺), while ion-mobility spectrometry reveals their relative collision cross-sections ([Fe(bipy^<i>t</i>-Bu)₃]²⁺ > [Fe(bipy^Br)₃]²⁺ > [Fe(phen)₃]²⁺ > [Fe(bipy)(phen)₂]²⁺ > [Fe(bipy)₂(phen)]²⁺ > [Fe(bipy)₃]²⁺). Dications can be reduced to their respective monocations with [1,3-dicyanobenzene]^-, and the extent of reaction increases with calculated redox potentials for [Fe(N^N)₃]^2+/+ couples. Despite the ligand-centered nature of the redox processes, the stabilities of the radical monocations ([Fe(bipy^<i>t</i>-Bu)₃]⁺ ≈ [Fe(phen)₃]⁺ > [Fe(bipy)(phen)₂]⁺ > [Fe(bipy)₂(phen)]⁺ > [Fe(bipy^Br)₃]⁺ > [Fe(bipy)₃]⁺) follow a similar order to the dications. This suggests that the π-donor and -acceptor properties of diimines are apt to stabilize both charge states, as would be present in photoredox catalysis.