High-valent dinuclear bis-μ-hydroxo complexes for PCET reactivity

Abstract

High valent dinuclear bis-μ-hydroxo transition metals complexes are hypothesised to play a potentially crucial role in several enzymatic mechanism involving their high-valent dinuclear bis-μ-oxo counterparts such as soluble Methane Monooxygenase (sMMO) and Tyrosinase. However, even though late transition metals such as Ni and Cu, are postulated to be extremely powerful oxidants, an overall dearth of high-valent late transition metals μ-hydroxo model systems is present in literature. Herein we report the synthesis and characterization of the first example of high valent bis(μ-hydroxo)NiIII2 and the mixed valent bis(μ-hydroxo)MIIMIII complexes (M = Ni, Cu). The complexes were prepared from low-valent precursors supported by a novel dicarboxamidate ligand and were characterised using X-ray absorption and electron paramagnetic resonance spectroscopies, mass spectrometry, and DFT calculations. These techniques confirmed the oxidation states of the metal sites in the complexes and showed the retention of the bis(μ-hydroxide) diamond core in the high valent species. Reactivity studies were performed, demonstrating that both high valent bis(μ-hydroxo)NiIII2 and the mixed valent bis(μ-hydroxo)MIIMIII (M = Ni, Cu) complexes were powerful oxidants towards oxidative activation of phenolic O-H bonds. Thorough kinetic analysis of these reactions showed that the high-valent complexes were able to react through a hydrogen atom transfer mechanism.