Metal-Ligand Multiple Bonds

We are interested in complexes with a multiple bond between a transition metal and C (“carbene”), N (“imido”), or O (“oxo”). With metals in groups 7-10, the M=L functionality is typically electrophilic, enabling reactions that form new bonds to difficult substrates like hydrocarbons. However, chemists have found it difficult to isolate and characterize compounds that are so reactive. The low coordination number of our compounds stabilizes metal-ligand multiple bonds, leading to reactive complexes with oxo and imido groups that can be isolated and studied in detail. For example, we reported the first isolable diiron(II) complex with an oxo ligand (illustrated below).

We have also characterized a reactive imido complex of iron(III). The planar geometry and the S=3/2 ground state give a relatively weak π-bond. When pyridine is added, the iron(III) imido complex abstracts hydrogen atoms from hydrocarbons (below, right), the first step toward amination of organic compounds. This C-H activation reaction is selective for sterically unhindered C-H bonds.

This compound is also a catalyst for formation of isocyanates from azides and CO, and for carbodiimides from isocyanides (see illustration). Therefore, it has an ideal balance between high reactivity for catalysis, and stability for isolation and spectroscopic study.

These studies have also led to complexes in which the unpaired electrons move from the metal to the ligand (“redox non-innocence”). As a result, we have isolated interesting iron(II) complexes of a tetrazene radical (below, left) and the first complex of the hexazene ligand (below, right).

Relevant Publications

Megan E. Reesbeck, Katarzyna Grubel, Daniel Kim, William W. Brennessel, Brandon Q. Mercado, Patrick L. Holland
Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)
Inorg. Chem.  201756, 1019-1022.  
Ryan E. Cowley, Matthew R. Golder, Nathan A. Eckert, Malik H. Al-Afyouni, Patrick L. Holland
Mechanism of Catalytic Nitrene Transfer using Iron(I)-Isocyanide Complexes
Organometallics  201332, 5289-5298.  
Ryan E. Cowley, Patrick L. Holland
Ligand Effects on Hydrogen Atom Transfer from Hydrocarbons to Three-Coordinate Iron Imides
Inorg. Chem.   201251, 8352-8361.  
Ryan E. Cowley, Nathan A. Eckert, Sridhar Vaddadi, Travis M. Figg, Thomas R. Cundari, Patrick L. Holland
Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron(III) Complex
J. Am. Chem. Soc.   2011133, 9796-9811.  
Ryan E. Cowley, Patrick L. Holland
C-H activation by a terminal imidoiron(III) complex to form a cyclopentadienyliron(II) product
Inorg. Chim. Acta   2011369, 40-44.  
Ryan E. Cowley, Nathan J. DeYonker, Nathan A. Eckert, Thomas R. Cundari, Serena DeBeer, Eckhard Bill, Xavier Ottenwaelder, Christine Flaschenriem, Patrick L. Holland
Three-Coordinate Terminal Imidoiron(III) Complexes: Structure, Spectroscopy, and Mechanism of Formation
Inorg. Chem.  201049, 6172-6187.  
Ryan E. Cowley, Eckhard Bill, Frank Neese, William W. Brennessel, Patrick L. Holland
Iron(II) Complexes With Redox-Active Tetrazene (RNNNNR) Ligands
Inorg. Chem.  200948, 4828-4836.  
Ryan E. Cowley, Nathan A. Eckert, Jerome E. Elhaïk, Patrick L. Holland
Catalytic nitrene transfer from an imidoiron(III) complex to form carbodiimides and isocyanates
Chem. Commun.   2009, 1760-1762.  
Nathan A. Eckert, Sridhar Vaddadi, Sebastian Stoian, Rene J. Lachicotte, Thomas R. Cundari, and Patrick L. Holland
Coordination Number Dependence of Reactivity in an Imidoiron(III) Complex
Angew. Chem., Int. Ed. Engl.   200645, 6868-6871.