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

Clayton J Curtis,Abhik Ghosh,Andrei V Astashkin,Elisa Tomat,Jeanet Conradie

doi:10.1021/acs.inorgchem.1c01691

Clayton J Curtis, Abhik Ghosh + Show 3 more

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https://doi.org/10.1021/acs.inorgchem.1c01691

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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.

Highlights

Coordination compounds of redox-active ligands, in which oxidizing and/or reducing equivalents can be stored on the ligand π system, have generated broad interest owing to their potential applications in areas including homogeneous catalysis,[1] molecular magnetism,[2] photovoltaic devices,[3] and quantum information processing.[4]
The reaction progress was accompanied by a color change from yellow to deep red over the course of 3−4 h at room temperature: in particular, we observed the gradual decrease of the single absorption band of Hpdp·MeOH at 280 nm and the growth of two main bands at 382 and 545 nm, consistent with our previous reports on dipyrrindione complexes.[22,24]
In our previous investigations of palladium(II) dipyrrindione complexes with primary amine ligands (e.g., [Pd(en)(pdp)]+, Chart 2d),[24] we found that the intramolecular hydrogen bonds are important for complex stability

Summary

Introduction

Coordination compounds of redox-active ligands, in which oxidizing and/or reducing equivalents can be stored on the ligand π system, have generated broad interest owing to their potential applications in areas including homogeneous catalysis,[1] molecular magnetism,[2] photovoltaic devices,[3] and quantum information processing (e.g., spin qubits).[4]. Early examples of linear oligopyrrolic radicals in metal complexes were found in the study of bilindiones (Chart 1a),[8,9] but more recent reports highlighted the formation of stable ligand-based radicals in metal complexes of tetradentate bis(phenolate)dipyrrins (Chart 1b),[10,11] bis(2-aminophenyl)-dipyrrins,[12] and diimino-dipyrrins,[13] as well as tridentate tripyrrindione (Chart 1c)[14−18] and dihydrazonopyrrole[19] scaffolds. Contracting the conjugated π system from tetradentate to tridentate to bidentate oligopyrroles, the observation of stable ligand-centered radicals becomes less common. Within the large family of bidentate dipyrrins,[20] ligand-centered redox chemistry is observed with varying degrees of success depending on substituents;[21] the bidentate dipyrrin system has not been generally employed for the stabilization of unpaired electrons.

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