Directory: Faculty
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Richard D. ErnstINORGANIC CHEMISTRYProfessor B.S.,University of California, Berkeley, 1973 Phone: (801) 581-8639 Office: 2166 HEB-N Email: ernst@chem.utah.edu |
Activities & Awards
- NSF Creativity Award, 1987-89
- AAAS Fellow, 1991
- ASUU Student Choice Awards for Excellence in Teaching, 1997 and 2002
- University of Utah Distinguished Teaching Award, 2003
- Robert W. Parry Teaching Award, 2003
Research Interests
Our most intensive efforts are being applied to the synthesis, reaction studies and structural characterization of transition metal complexes containing pentadienyl ligands. These ligands seem interesting as they can be considered to be "opened" cyclopentadienyl ligands. One might therefore expect that a large area of chemistry would exist wherein a pentadienyl ligand replaces its more familiar cyclic counterpart (C5H5). A likely advantage of such a ligand should be a greater tendency to exist in monohapto (I) and trihapto (II) bonding configurations as well as pentahapto (III). Isomerizations
between these forms can be very beneficial chemically as well as catalytically, as a mechanism then exists for either bringing on additional ligands or expelling others. For these reasons one can expect metal pentadienyl complexes to be chemically very reactive. Further, since η5-bound pentadienyl is in essence a better chelate than allyl, a wide variety of metal-pentadienyl complexes should be thermally stable. Thus, a major goal of our work in this area is to investigate both the physical and chemical natures of these systems and thereby gain a more unified understanding of our ligands.
The potential of these ligands can be appreciated from the complex Ti(C5H5) (2,4-C7H11)(PEt3) (C5H5 = cyclopentadienyl; C7H11= dimethylpentadienyl). A structural study has revealed the Ti-C bonds for the open pentadienyl ligand to be much shorter than those for the C5H5 ligand, ca. 2.24 vs. 2.35 Å. Nevertheless, this compound reacts readily with a wide variety of unsaturated organic molecules (e.g., imines, ketones, alkynes, nitriles, and isonitriles), leading to selective coupling between the unsaturated molecule and the open pentadienyl ligand. Thus not only is pentadienyl more strongly bound than cyclopentadienyl, it is also more reactive.
The coupling reactions of the above half-open titanocene with alkynes have led to some particularly notable results. With a,w-diynes, for example, the reactions yield complexes containing nine-membered rings fused to other rings, as in 1 . These species, however, subsequently rearrange, with the selective cleavage of a C-C single bond. In this respect, the cage-like complex 1 mimics a carborane, in that the electrophilic titanium center can interact with, and even break, saturated C-C bonds.
Even stronger evidence of an interaction between a metal center and saturated C-C bonds has been discovered in a cage-like coupling product isolated from the reaction of three equivalents of PhC2SiMe3 with the half-open titanocene, Ti(C5H5)(C8H11)(PEt3) (C8H11=cyclooctadienyl). In this product, one observes the titanium center making a closer approach to a saturated carbon atom than to seven Ti-bonded unsaturated carbon atoms. Theoretical studies, as well as remarkably low 13C-13C coupling constants, also point to the presence of direct interactions of the metal center with several C-C bonds. Naturally it would be of interest to enhance the interactions, and the use of zirconium or hafnium would seem to be the approach to take, as they tend to form stronger bonds. Indeed, recent studies have now identified additional examples of complexes containing these interactions, and for zirconium, C-C and C-Si bond cleavage reactions are being observed more frequently in the initial alkyne coupling products than had been observed in similar reactions of titanium complexes.
We employ a number of techniques for characterizing our compounds, including NMR, ESR, IR, and mass spectroscopies as well as magnetic and X-ray diffraction studies.
Selected Publications
- A. Rajapakshe, N.E. Gruhn, D.L. Lichtenberger, R. Basta, A.M. Arif, and R.D. Ernst, "Pentadienyls vs. Cyclopentadienyls and Reversal of Bonding Affinity with Metal Oxidation State: Synthesis, Molecular Structures, and Electronic Structures of High-Valent Zirconium Pentadienyl Complexes,"J. Am. Chem. Soc, submitted ( 2004 ).
- S. Pillet, G. Wu, V. Kulsomphob, B.G. Harvey, R.D. Ernst, and P. Coppens, "Investigation of Zr-C, Zr-N, and Potential Agostic Interactions in an Organozirconium Complex by Experimental Electron Density Analysis,"J. Am. Chem. Soc. 125, 1937 ( 2003 ).
- R. Basta, A.M. Arif, and R.D. Ernst, "Reaction of Chromium(III) Chloride with the Cycloheptadienyl Anion: Susceptibility of Edge-Bridges to C-H Activation Reactions," Organometallics 22 , 812 ( 2003 ).
- V. Kulsomphob, A.M. Arif, and R.D. Ernst, "Syntheses, Characterization, and Structural Studies of Half-Open Zirconocenes,"Organometallics 21, 3182 ( 2002 ).
- R. Tomaszewski, A.M. Arif, and R.D. Ernst, "Tandem Couplings of Imines and Other Unsaturated Organic Compounds with a Half-Open Titanocene,"J. Chem. Soc., Dalton Trans. 1883 ( 1999 ).
- R.D. Ernst, "Pentadienyl Ligands: Their Properties, Potential, and Contributions to Inorganic and Organometallic Chemistry," Comments Inorg. Chem. 21 , 285 ( 1999 ).
- R. Tomaszewski, I. Hyla-Kryspin, C.L. Mayne, A.M. Arif, R. Gleiter and R.D. Ernst, "Shorter Nonbonded than Bonded Contacts or Nonclassical Metal-to-Saturated Carbon Atom Interactions?,"J. Am. Chem. Soc. 120 , 2959 ( 1998 ).
- I. Hyla-Kryspin, T.E. Waldman, E. Meléndez, W. Trakarnpruk, A.M. Arif, M.L. Ziegler, R.D. Ernst and R. Gleiter, "Pentadienyl as a Stronger Binding but more Reactive Ligand than Cyclopentadienyl: Syntheses, Reactions, and MO Studies of Half-Open Titanocenes,"Organometallics 14 , 5030 ( 1995 ).
- R.D. Ernst, J.W. Freeman, L. Stahl, D.R. Wilson, A.M. Arif, B. Nuber and M.L. Ziegler, "Longer but Stronger Bonds: Structures of PF 3 , P(OEt) 3 , and PMe 3 Adducts of an Open Titanocene," J. Am. Chem. Soc. 117 , 5075 ( 1995 ).