Metal-oxo complexes play an essential role in oxidative processes in biological and synthetic systems.  In proteins, earth abundant metal ions (Mn, Fe) react with dioxygen to produce highly oxidized M–oxo species that have an array of functional properties.  For instance, non-heme oxygenases form FeIV–oxo centers that cleave a diverse set of unactivated C–H bonds, whereas MnV–oxo complexes have been implicated in the conversion of water to dioxygen.  We are developing synthetic systems in which high valent Mn and Fe–oxo species are observable.  Our studies are geared toward generating structure-function relationships in order to understand the underlining features that lead to formation of these species and how their subsequent reactivity can be modulated.  A variety of spectroscopic, analytical, structural, and electrochemical tools are used to define the properties of the complexes, which are correlated to their activity. 

 

Metal complexes with discrete heterobimetallic cores have important functional consequences in chemical and biological systems.  In proteins, several examples have been discovered in which two different types of metal centers are needed to perform a specific chemical transformation.  In addition, numerous synthetic complexes have been proposed to function through heterobimetallic species, especially those that contain one main group metal ion and one transition metal ion.  We are developing general synthetic protocols for the preparation of heterobimetallic complexes: one class contains a group 2 metal ion and one transition metal ion to emulate the oxygen-evolving complex (OEC) active site (an example is shown below) and another class contains two different transition metal ions.  We have shown that we can tune both physical and chemical properties of these systems to target specific reactions. 

Development of Heterobimetallic Complexes

High-Spin Metal–Oxo Complexes

C–H Bond Functionalization

C–H bond functionalization is one of the most difficult chemical transformations, yet it is an essential biological process for survival.  One of the cleanest (and greenest) methods to promote C–H cleavage is to utilize earth abundant metal complexes that react with dioxygen to form M–O(H) intermediates.  We have been exploring the properties of reactive M–O(H) complexes derived from dioxygen to understand the underlying features that promote efficient and selective C–H bond activation. 

Oxygen Evolving Center

(OEC; Mn4Ca cluster)

Nature Chem. 2013, 5, 259.

J. Am. Chem. Soc. 2010, 132, 12188.

J. Am. Chem. Soc. 2012, 134, 1996.

High-spin FeIV=O complex

High-spin MnV=O complex

J. Am. Chem. Soc. 2011, 133, 9258.

MnIIICaII bimetallic complex

FeIIIMnII bimetallic complex

Inorg. Chem. 2013, 52, 10229.

New Complexes with a Redox Active Framework

Chem. Sci. 2018, 9, 6540.

The project seeks to stabilize high-valent and otherwise highly reactive mononuclear metal complexes. Key in accessing high valency is the redox active backbone of the ligand which can work in conjunction with the redox ability of the metal ion to which it is bound to, Isolation of mononuclear metal complexes is enforced by steric bulk of the ligand’s trisubstituted phenyl arms. Through stabilization of mononuclear complexes with enhanced redox activity, the chemistry related to intermediates found or proposed within catalytic cycles of metalloenzymes can be evaluated.

J. Am. Chem. Soc. 2019, 141, 11142.