Periodic Trends in Late First-Row Transition Metal Complexes

Abstract

The ability of oxidative metalloenzymes to efficiently drive desirable reactions (e.g., C–H functionalization) under mild conditions motivates the isolation of key reactive intermediates in molecular environments for systematic study. Specifically, late first-row transition metals are present in various enzymatic active sites, and numerous bio-inspired organometallic complexes have been reported in the past few decades. However, the impact of metal identity on electronic configuration, which in turn affects reactivity profile, is obfuscated by the use of different ligand scaffolds in accessing these reactive motifs. Here, we report the synthesis and characterization of two series of reactive intermediates spanning mid to late transition metals on a sterically encumbered dipyrrin ligand. Dioxygen-bound metal complexes are of interest both as precursors to the formation of oxenoid complexes that may participate in hydroxylation reactions, and as intermediates in air separation; in Chapter 2, we access an isostructural series of side-bound metal-dioxygen adducts (cobalt, nickel, copper). While vibrational data, redox chemistry, and structural metrics favor greater activation of dioxygen at Co versus Ni or Cu, the combination of soft and hard X-ray absorption spectroscopy data suggests that a common superoxide antiferromagnetically coupled to an open-shell M(II) center is the dominant configuration for all three complexes. Chapter 3 describes the synthesis of isostructural iron, cobalt, nickel, and copper nitrenoid complexes that are revealed via spectroscopic characterization to span imido, imidyl, and nitrene configurations. Redox reactivity studies show that there is significant and metal-dependent redox flexibility across the series, where the metal, nitrenoid fragment, and ligand may be redox active.