Issue 15, 2025

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

Abstract

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 (bipyBr), 4,4′-di(tert-butyl)-2,2′-bipyridine (bipyt-Bu)). Collision-induced dissociation of ion pairs {[Fe(N^N)3]X}+ (X = BPh4 and BArF4; ArF = 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(bipyt-Bu)3]2+ > [Fe(phen)3]2+ ≫ [Fe(bipyBr)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(bipyt-Bu)3]2+ > [Fe(bipyBr)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(bipyt-Bu)3]+ ≈ [Fe(phen)3]+ > [Fe(bipy)(phen)2]+ > [Fe(bipy)2(phen)]+ > [Fe(bipyBr)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.

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

Supplementary files

Article information

Article type
Paper
Submitted
15 Jan 2025
Accepted
24 Mar 2025
First published
25 Mar 2025
This article is Open Access
Creative Commons BY-NC license

Phys. Chem. Chem. Phys., 2025,27, 7882-7892

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

D. Schilter, U. Terranova, C. B. Summers and R. R. Robinson, Phys. Chem. Chem. Phys., 2025, 27, 7882 DOI: 10.1039/D5CP00199D

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