Polynuclear complexes for cooperative redox chemistry
The development of polynuclear coordination complexes has given rise to structurally and electronically well-defined species possessing redox-flexibility with the potential to perform multi-electron transformations. The successful construction of these materials has allowed us to understand their redox behavior and target small molecule activation pathways reminiscent of naturally occurring enzymatic function.
We have developed hexadentate, amine-based ligand scaffolds for the construction of trinuclear complexes. Using these ligands, we have synthesized trinuclear manganese, iron, cobalt, and copper complexes, featuring variable M-M interactions. In addition, hexanuclear Mn, Fe, and Co complexes were fashioned from the dimerization of two trinuclear complexes.
Modifications of the ligand enable tuning of the electronic structure of these trinuclear complexes. Despite open-shell configurations, it is the direct M-M interactions, through orbital overlap, that dictate the changes in electronic structure. The tri-iron complexes in particular can achieve spin states of S = 1, 2, 4, 6,with spin-crossover allowing intermediate spin states to be accessed. The hexanuclear Fe complex has been isolated in eight different oxidation states, and characterized by 1H NMR, 57Fe Mössbauer, SQUID magnetometry, and X-ray diffraction.
Cooperative redox and small molecule activation.
Chemical oxidation in low-spin complexes gives rise to a delocalized core, where the trinuclear core is sharing the resultant hole. Chemical oxidation in intermediate- or high-spin complexes occurs site-isolated redox and coordination changes. Tri-manganese complexes in particular have been seen to extrude oxygen from O2, N2O and CO2.
We are working to modify our ligand framework to prevent strong M-M bonding, rendering the resultant trinuclear cores more reactive. We also hope to incorporate the tri- and hexanuclear clusters into 1D chains, 2D sheets, and 3D coordination networks with each polynuclear node functioning as a building unit and a reaction site. Using these novel polynuclear platforms, we will probe small molecule activation processes.
Iron-mediated, catalytic C-H bond functionalization
An unprecedented high-spin iron imido has been synthesized and characterized, displaying reactivity towards unactivated C-H bonds. Catalysis for C-H bond functionalization has been realized with second generation dipyrromethene platforms.
We have developed pyrrole-based ligand platforms, featuring monoanionic dipyrromethene, dianionic dipyrromethane and trianionic tris(pyrrolyl)ethane. Using these ligands, we have synthesized manganese, iron, cobalt, and nickel complexes.
The dipyrromethane and tris(pyrrolyl)ethane metal complexes have exhibited an inverted electronic structure where doubly-populated pyrrole π-electrons are higher in energy than 3d electrons on the transition metal ion. In addition, dipyrromethene complexes feature the same photo-physical properties as porphyrin analogues, including intense ligand π→π* transitions with molar absorptivities reaching up to 140,000 M-1cm-1.
Atom transfer and group reactivity.
Dipyrromethene iron complexes react with organic azides, O-atom donors, or diazoalkanes to form new C-N, O, or C bonds from unactivated C-H bonds. Conversely, dipyrromethene iron complexes lacking reactive C-H bonds react with aryl azides to afford stable, high spin (S = 2) iron imido complexes, which are competent to deliver the “NAr” fragment intermolecularly.> 57Fe Mössbauer, X-ray diffraction analysis, and DFT studies indicate the imido is best formulated as and FeIII (S = 5/2) antiferromagnetically coupled to an imido radical (S = 1/2) to give the overall S = 2 assignment.
Our iron complexes catalytically aziridinate olefins and intermolecularly aminate C-H bonds with alkyl azides. Linear alkanes are functionalized with primary C-H bond selectivity, occurring as a function of ligand sterics.
We will tune the ligand design to target more catalytically stable species, and extend this catalysis more generally to introduce functionality (NR, O, S, CR2) into unactivated C-H bonds.