We are very interested in "three-coordinate complexes," in which the metal has only three bonds. These complexes are a springboard into new chemistry for several reasons. First, three-coordinate complexes have not been studied as extensively as those with more bonds to the metal (a typical number of bonds to the metal is six!). Three-coordinate complexes are electronically unsaturated: whereas most complexes prefer an 18-electron configuration, the iron chloride complex pictured below has only 12 valence electrons. This leads to unusual reactions, and the third ligand may be varied for electronic and steric control of the reactions. Finally, the potential for low coordination numbers in enzyme active sites and on surfaces suggests that we can gain insights into these catalysts using our compounds.
Because the three-coordinate complexes are
high-spin and paramagnetic, we use EPR, magnetic
susceptibility, and other magnetic techniques for understanding their
properties. Mössbauer spectroscopy in collaboration with Eckard Münck
(Carnegie Mellon University) has taught us that diketiminate-iron
complexes have an unusual electronic structure with a very low-lying
excited state. As a result, the compounds have highly anisotropic
magnetic properties from unusually strong zero-field splitting.
Publications:
Jeremy M. Smith, Rene J. Lachicotte, Patrick L. Holland, "Tuning Metal Coordination Number by Ancillary Ligand Steric Effects: Synthesis of a Three-Coordinate Iron(II) Complex," Chem. Commun. 2001, 1542-1543. (PDF)
Hanspeter Andres, Emile M. Bominaar, Jeremy M. Smith, Nathan A. Eckert, Patrick L. Holland, Eckard Münck, "Planar Three-Coordinate High-Spin Fe(II) Complexes with Large Orbital Angular Momentum: Mössbauer, Electron Paramagnetic Resonance, and Electronic Structure Studies," J. Am. Chem. Soc. 2002, 124, 3012-3025. (PDF)
Patrick L. Holland, Thomas R. Cundari, Lanyn L. Perez, Nathan A. Eckert, Rene J. Lachicotte, "Electronically Unsaturated Three-Coordinate Chloride and Methyl Complexes of Iron, Cobalt, and Nickel," J. Am. Chem. Soc. 2002, 124, 14416-14424. (PDF)Nathan A. Eckert, Emily M. Bones, Rene J. Lachicotte, Patrick L.
Holland, "Nickel Complexes of a Bulky β-Diketiminate Ligand," Inorg.
Chem. 2003, 42, 1720-1725. (PDF)
Nathan A. Eckert, Rene J. Lachicotte, Patrick L.
Holland, "Low-Coordinate Fe(II) Amido Complexes of β-Diketiminates:
Synthesis, Structure and Reactivity,"
Inorg. Chem. 2004, 43, 3306-3321. (DOI)
Elizabeth A. Gregory, Rene J. Lachicotte, Patrick L. Holland, "A Cationic Three-Coordinate Iron(II) Complex and the Reaction of Diketiminate with Ethyl Diazoacetate," Organometallics 2005, 24, 1803-1805. (DOI)
Javier Vela, Jeremy M. Smith, Ying Yu, Nicole A. Ketterer,
Christine J. Flaschenriem, Rene J. Lachicotte, Patrick L. Holland,
"Synthesis and Reactivity of Low-Coordinate Iron(II) Fluoride Complexes
and Their Use in the Catalytic Hydrodefluorination of Fluorocarbons," J. Am. Chem. Soc. 2005,
Sebastian Stoian, Ying Yu, Jeremy M. Smith, Patrick L. Holland, Emile L. Bominaar, Eckard Münck, "Mössbauer, EPR and Crystallographic Characterization of a High-Spin Fe(I) Diketiminate Complex with Orbital Degeneracy," Inorg. Chem. 2005, 44, 4915-4922. (DOI)
Javier Vela, Sridhar Vaddadi, Savariraj Kingsley, Christine J. Flaschenriem, Rene J. Lachicotte, Thomas R. Cundari, Patrick L. Holland, "Bidentate Coordination of Pyrazolate in Low-Coordinate Iron(II) and Nickel(II) Complexes," Angew. Chem., Int. Ed. Engl. 2006, 45, 1607-1611. (online)
Patrick L. Holland, "Electronic Structure
and Reactivity of Three-Coordinate Iron Complexes," Acc. Chem. Res. 2008, 41, 905-914. (DOI)
Karen P. Chiang, Pamela M. Barrett, Feizhi Ding, Jeremy M. Smith, Savariraj Kingsley, William W. Brennessel, Meghan M. Clark, Rene J. Lachicotte, Patrick L. Holland, "Ligand Dependence of Binding to Three-Coordinate Fe(II) Complexes," Inorg. Chem. 2009, 48, 5106-5116. (DOI)
Daniel J. Mindiola, Patrick L. Holland, Timothy H. Warren, "Complexes of Bulky Beta-Diketiminate Ligands: Introduction," Inorg. Synth., in press.