Interactions of Metal-Based and Ligand-Based Electronic Spins in Neutral Tripyrrindione π Dimers

Abstract

The ability of tetrapyrrolic macrocycles to stabilize unpaired electrons and engage in π–π interactions is essential for many electron-transfer processes in biology and materials engineering. Herein, we demonstrate that the formation of π dimers is recapitulated in complexes of a linear tripyrrolic analog of naturally occurring pigments derived from heme decomposition. Hexaethyltripyrrindione (H3TD1) coordinates divalent transition metals (i.e., Pd, Cu, Ni) as a stable dianionic radical and was recently described as a robust redox-active ligand. The resulting planar complexes, which feature a delocalized ligand-based electronic spin, are stable at room temperature in air and support ligand-based one-electron processes. We detail the dimerization of neutral tripyrrindione complexes in solution through electron paramagnetic resonance (EPR) and visible absorption spectroscopic methods. Variable-temperature measurements using both EPR and absorption techniques allowed determination of the thermodynamic parameters of π dimerization, which resemble those previously reported for porphyrin radical cations. The inferred electronic structure, featuring coupling of ligand-based electronic spins in the π dimers, is supported by density functional theory (DFT) calculations.

Redox‐Switchable Cyan Fluorescence of a BODIPY Analog Inspired by Propentdyopent Pigments

“…Upon controlled‐potential electrolysis (–1.12 V, 200 s), the two visible absorption bands (at 380 and 445 nm) of (pdp)BF 2 are replaced by a broad band at 419 nm, and a new band appears at 725 nm. This long‐wavelength feature is characteristic of propentdyopent radical anions and other ligand‐based π radicals in oligopyrroles , , . Consistent with reversible redox chemistry, exposure to air of the electrochemical reduction product leads to recovery of the parent complex (pdp)BF 2 with minimal degradation (Figure S8).…”

“…This long-wavelength feature is characteristic of propentdyopent radical anions [21] and other ligand-based π radicals in oligopyrroles. [20,39,40] Consistent with reversible redox chemistry, exposure to air of the electrochemical reduction product leads to recovery of the parent complex (pdp)BF 2 with minimal degradation ( Figure S8). These observations, along with the clear isosbesticity of the spectroelectrochemical conversion, are consistent with the formation of radical anion [(pdp)BF 2 ] ·carrying a ligand-based unpaired spin on the dipyrrindione framework.…”

“…The scaffolds of tripyrrolic and dipyrrolic heme metabolites are currently being investigated as platforms for ligand‐based redox processes in coordination chemistry . We have shown that tetraethyl propentdyopent Hpdp coordinates divalent transition metals and that the resulting homoleptic pseudo‐tetrahedral complexes [M(pdp) 2 , Scheme ] host stable ligand‐based radicals upon one‐electron reduction .…”

“…[17] The scaffolds of tripyrrolic and dipyrrolic heme metabolites are currently being investigated as platforms for ligand-based redox processes in coordination chemistry. [18][19][20] We have shown that tetraethyl propentdyopent Hpdp coordinates divalent transition metals and that the resulting homoleptic pseudo-tetrahedral complexes [M(pdp) 2 , Scheme 1] host stable ligand-based radicals upon one-electron reduction. [21] Although convenient syntheses of dipyrrindione ligands were first reported in the 1980s, [22][23][24] their boron coordination has not been investigated to the best of our knowledge.…”

Using X-ray Diffraction Techniques for Biomimetic Drug Development, Formulation, and Polymorphic Characterization

“…Tripyrrindione is an analogue of the naturally occurring urinary pigment, uroerythrine. The structural characterization of tripyrrindione and its Pd(II), Cu(II), and Zn(II) complexes in different redox states were thoroughly studied by Tomat et al ( Figure 5 ) [ 20 , 21 ]. The X-ray structure determination establishes (a) stabilization of unpaired electrons that participates in π−π interactions and is vital for several electron-transfer biological processes (b) planar geometry with three pyrrolic nitrogens bound to the metal center and the fourth coordination sphere occupied by an aqua ligand and (c) dimerization in the solid state.…”