| dc.description.abstract | Polynuclear active sites are a common motif employed by enzymes to facilitate cooperative redox chemistry. For instance, diiron enzymes serve a variety of purposes in nature, including the promotion of C–H functionalization (soluble methane monooxygenase) and small molecule activation (ribonucleotide reductase) reactivity in biological systems. Inspired by the cooperativity of these and similar enzymes, we aim to derive a functional mimic utilizing a bimetallic scaffold to promote metal–metal interaction, isolate reactive intermediates, and ultimately perform challenging multi-electron reactions. Specifically, we propose the use of dipyrrinato Pacman structures, wherein the dinucleating architecture can support two metals proximally oriented to maximize their ability to perform cooperative chemistry. To this end, we have developed strategies to tether two dipyrromethene motifs in a cofacial manner utilizing organic backbones and have incorporated late transition metals into this framework.
With this platform in hand, we demonstrate the accessibility of a series of diiron bridging hydroxo and oxo complexes in multiple oxidation states, analogous to the reactive intermediates observed in enzymatic active sites. This includes a rare diiron(II) μ-oxo with distinct bond metrics from the few other reported examples. Spectroscopic, crystallographic, and computational methods have been employed to examine the effect of molecular oxidation state on the acid-base reactivity of these motifs. The concurrent findings that the basicity of the bridging oxo unit decreases upon oxidation are consistent with observations that hemerythrin, a diiron dioxygen transport enzyme, undergoes a decrease in the pKa of the bridging hydroxide proton concomitant with oxidation.
To gain insight into the effect of the metal identity on the reactivity of these complexes, we extende this system to cobalt, targeting the analogous dicobalt bridging hydroxo species. As such, we have synthesized dicobalt(II) bridging mono- and bis-hydroxide species that display unique reactivity from the diiron system. Specifically, lability of the hydroxide bridges of the bis-hydroxo complex by acid-base or redox reactions allows for interconversion between these species. Furthermore, both hydroxide bridges are expelled when the bis-hydroxide species undergoes two-electron oxidation by an inner-sphere oxidant, such as Ph3CCl, to yield the corresponding dicobalt(II) dichloride species and generate Ph3COH as the organic byproduct. Computational studies are utilized to model potential higher valent intermediates operative in this reaction, which demonstrate the roles of ligand/metal oxidation and spin accumulation at the bridging hydroxides that likely contribute to this unique reactivity.
Finally, we hypothesize that these bimetallic units can also be used to perform cooperative redox chemistry, in which the proximal metal centers work together to manage the redox load of multielectron reactions. As such, we demonstrate the capability of this dinuclear system to mediate the reduction of nitrite to nitric oxide without the requirement of external substrates (e.g., reductants, proton sources, or oxophillic substrates) to help initiate the reduction. The major iron-containing products of this transformation include the first reported four-coordinate diiron bis-nitrosyl species, as well as a diiron(III) μ-oxo organometallic species that is unique within the dipyrrin framework. We have also explored the facilitation of multielectron reductions, to which we observe promising evidence of the deoxygenation chemistry of pollutants such as perchlorate and periodate and the reduction of aryl-nitro substrates to the corresponding aniline complexes with the dipyrrin Pacman complexes. | |
