Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

Abstract

Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic ( J ≈ −2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.

Biopyrrin Pigments: From Heme Metabolites to Redox-Active Ligands and Luminescent Radicals

“…V.ConradieJ.GhoshA.TomatE. Curtis, C. J. Astashkin, A. V. Conradie, J. Ghosh, A. Tomat, E. 2021601245712466 . The delocalization of an unpaired electron on the compact propentdyopent scaffold is showcased in this study of a binuclear hydroxo-bridged Pd­(II) complex featuring a bidentate dipyrrindione on each metal center.…”

“…2021, 60, 12457−12466. 4 The delocalization of an unpaired electron on the compact propentdyopent scaffold is showcased in this study of a binuclear hydroxo-bridged Pd(II) complex featuring a bidentate dipyrrindione on each metal center. A two-electron reduction produced a dianionic diradical species, with two ligand-centered unpaired spins coupled by a weak antiferromagnetic exchange interaction.…”

“…Indeed, both the monomeric and dimeric complexes present essentially pdp-based LUMOs (Figure 9C). 4 The neutral diamagnetic complex [Pd(μ-OH)(pdp)] 2 undergoes two reversible one-electron reductions at potentials comparable to neutral homoleptic propentdyopent complexes. The two-electron reduction product was synthesized upon treatment with two equivalents of cobaltocene and isolated as an air-sensitive cobaltocenium salt.…”

“…Although the availability of low-lying frontier orbitals of predominantly ligand character had been shown experimentally in our dipyrrindione complexes, this study was the first to include a computational analysis of this ligand. Indeed, both the monomeric and dimeric complexes present essentially pdp-based LUMOs (Figure C) …”

“…(F) Spin density plot of Cs 2 [Pd­(μ-OH)­(pdp) • ] 2 from DFT (OLYP-D3/TZ2P) analysis. Adapted with permission from ref . Copyright 2021 American Chemical Society.…”