Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin–Spin Interaction

Abstract

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1•-)RhIII(1•-)]+ ([2]+), starting from the low-valent electron reservoir [RhI]. The less stable monoradical [RhIII(1•-)Cl2(PPh3)3] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]+, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t26) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]+ lies near ∼2.3 μB over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1•-)RhIII(1•-)]+ have been found experimentally (J ≈ -5 cm-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.