Bimetallic Complexes for Cooperative Redox Chemistry

 

Introduction. Polynuclear active sites are a common motif employed by enzymes to facilitate cooperative redox chemistry. One class involves bimetallic active sites, in which two metal centers are poised to facilitate a variety of important functions, ranging from C–H functionalization to small molecule activation. Inspired by the cooperativity of such enzymes, we designed a bimetallic ligand architecture that promotes metal–metal interaction to enable the isolation of reactive metal species and multi-electron reductive processes.

 

Synthesis. For this purpose, we demonstrate the use of dipyrinnato Pacman structures, wherein the dinucleating architecture can support two metals that are proximally oriented in order to maximize their ability to perform cooperative chemistry. Inspired by the principles our group has established for dipyrrin ligand platforms (i.e., modifiability, open coordinate environments, high-spin late transition metal complexes capable for redox chemistry), we sought to tether two dipyrromethene motifs in a cofacial manner utilizing organic dibenzofuran (dbf) and dimethylxanthene (dmx) backbones. With these ligands in hand, we have been able to install late transition metals (iron, cobalt, nickel, and copper to date) and have sought to study their coordination chemistry, electronic structure, and reactivity.

 

 

 

 

 

 

 

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Selected compounds. As the nomenclature suggests, Pacman compounds exhibit immense flexibility and can support a wide range of metal–metal distances by changing the backbone of the Pacman architecture and the coordination environment at the metal centers. One example featuring diiron bridging hydroxo and oxo complexes on the dmx ligand is highlighted here. We found that the diiron platform supports bridging oxo complexes in multiple oxidation states. Beginning with a diiron(II) organometallic starting material, we can use a protonlysis strategy to afford a diiron(II) hydroxo complex with a stark change in the iron–iron distance from 6.7 Å to 3.7 Å. Heating this complex results in a diiron(II) oxo complex with an even shorter iron–iron distance of 3.0 Å. We can also access a diiron(III) oxo species via oxidation, with a 3.5 Å iron–iron distance, and a mixed-valent iron(II) iron(III) oxo species (not shown). With these complexes, we were able to study the acid-base reactivity of diiron oxo complexes as a function of oxidation state and iron–oxo covalency.

 

 

 

 

 

 

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Similar trends were observed on the dicobalt, dicopper, and dinickel systems; for instance, we have synthesized a dicopper(I) acetonitrile starting material with a copper–copper distance of 6.4 Å that reacts with aryl azides to afford dicopper(II) imido complexes with shorter copper–copper distances of 2.8 Å.

 

Future Directions. In addition to utilizing the bimetallic scaffold to isolate reactive bridging species, we hypothesize that this dinuclear unit could further be used to perform challenging multielectron reactions, thereby taking advantage of the flexibility of the Pacman structure to allow substrates to fit into the cavity and the proximal metal centers to perform cooperative redox chemistry.