Department of Chemistry,
University of Rochester

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"). Metal-ligand multiple bonds of this type are well-known in complexes of the "early" transition metals (groups 4-7), but are rare for the "late" transition metals (groups 8-10) in low oxidation states. Using late metals, the M=L functionality is typically a strong electrophile, enabling formation of new bonds to difficult substrates like hydrocarbons. However, chemists have found it difficult to find complexes that are highly reactive yet possible to characterize in detail. The low coordination number of our compounds stabilizes metal-ligand multiple bonds, leading to exciting new complexes with oxo and imido ligands. For example, we reported the first isolable diiron(II) complex with an oxo ligand (illustrated below).

We have also characterized an imido complex of iron(III) using a range of spectroscopic and crystallographic methods. The predominant form in solution has a planar geometry that gives differential occupation of π-orbitals (shown below, left) and an S=3/2 ground state. This compound is a catalyst for formation of isocyanates from azides and CO, and for carbodiimides from isocyanides (see illustration). Therefore, it has an ideal balance where it is active as a catalyst, yet can be isolated and studied using spectroscopy.

When pyridine is added, the complex abstracts hydrogen atoms from hydrocarbons (below, right), the first step toward amination of organic compounds. This C-H activation reaction has a giant kinetic isotope effect, and is very selective for sterically unhindered C-H bonds.

Consistent with the surprising oxidizing ability of the iron(III) imido complex, the iron-diketiminate unit has a overriding tendency to keep the iron in the +2 oxidation state. As a result, we have isolated interesting iron(II) complexes of a tetrazene radical (below, left) and of the previously unknown hexazene ligand (below, right).

Funding: National Science Foundation

Publications:

Nathan A. Eckert, Sebastian Stoian, Jeremy M. Smith, Emile L. Bominaar, Eckard Münck, Patrick L. Holland, "Synthesis, Structure, and Spectroscopy of an Oxodiiron(II) Complex," J. Am. Chem. Soc. 2005, 127, 9344-9345. (DOI)

Nathan A. Eckert, Sridhar Vaddadi, Sebastian Stoian, Rene J. Lachicotte, Thomas R. Cundari, Patrick L. Holland, "Coordination Number Dependence of Reactivity in an Imidoiron(III) Complex," Angew. Chem., Int. Ed. Engl. 2006, 45, 6868-6871. (online)

Ryan E. Cowley, Jerome Elhaik, Nathan A. Eckert, William W. Brennessel, Eckhard Bill, Patrick L. Holland, "A bridging hexazene (RNNNNNNR) ligand from reductive coupling of azides," J. Am. Chem. Soc. 2008, 130, 6074-6075. (DOI) (Highlight in Chemical & Engineering News)

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. (DOI)

Ryan E. Cowley, Eckhard Bill, Frank Neese, William W. Brennessel, Patrick L. Holland, "Iron(II) Complexes With Redox-Active Tetrazene (RNNNNR) Ligands," Inorg. Chem. 2009, 48, 4828-4836. (DOI)

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. 2010, 49, 6172-6187.(DOI)

Ryan E. Cowley, Patrick L. Holland, "C-H activation by a terminal imidoiron(III) complex to form a cyclopentadienyliron(II) product," Inorg. Chim. Acta 2011, 369, 40-44. (Special Issue for Robert G. Bergman) (DOI)

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. 2011, 133, 9796-9811. (DOI)