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). 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 its scope is under
active investigation.
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).
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)