Office: 2103 Hahn Hall South
Group Web Pages: http://www.files.chem.vt.edu/chem-dept/yee/YeeGroup.htm
BS Chemistry, University of California, Berkeley, CA, 1983
PhD Physical Chemistry, Stanford University, Stanford, CA, 1990
Postdoctoral Researcher, DuPont, 1990-1991
Assistant Professor of Chemistry, University of Colorado, Boulder, 1992-2000
Research Associate, Visiting Fellow and Instructor, Univ. of Colorado, Boulder, 2000-2001
Associate Professor of Chemistry, Virginia Tech, 2001-present
Honors and Awards:
Certificate of Teaching Excellence Virginia Tech, College of Science, 2010
E. Gary Cook Faculty Teaching Award Virginia Tech, Department of Chemistry, 2008
Research Award E. I. DuPont, Wilmington, DE, Merit award for co-discovery of the first room temperature molecule-based ferromagnet, 1991
We have previously published our work on a coordination polymer that results from the reaction of V(CO)6 and pentafluorophenyltricyanoethylene (1), which becomes magnetically ordered at 245 K. The compound is analogous to the first room temperature molecule-based magnet, V[TCNE]2, where TCNE = tetracyanoethylene. These solids are believed to be a network of V2+ cations bridged through the lone pairs on two or more nitrile nitrogen atoms on the organic radical anions to give a ferrimagnet. Motivation for trying to discover other organic acceptor bridging ligands comes from the observation that V[TCNE]2 and all other V-containing magnets found to date are air-sensitive, presumably because low valent vanadium is very oxophilic. However, analogous Mn, Fe, Co and Ni phases with TCNE are only magnetic at low temperatures, necessitating a search for tunable TCNE replacements.
In unpublished work, we have found that the corresponding unfluorinated acceptor, 2, also supports magnetic order, albeit at the lower temperature of about 210 K. Although we expected to observe a smooth trend in Tc, the critical ordering temperature, with mono and poly fluorine substitution between these two extremes of 1 and 2, this prediction has proven to be incorrect. In particular, the 2,6-difluorophenyl compound (3) results in a magnetically ordered solid at 300 K (manuscript in preparation). Further examination of all of the mono- and di- fluorine substituted compounds, as well as their chlorine analogs, seems to indicate that both resonance and inductive effects from the halogen atoms play a role, but steric effects cannot be ruled out.
Such steric effects are not understood because structural information in the above class of magnetic solids is lacking. To address this, we have synthesized and crystallized complexes that are possible models of the local structure and bonding in the magnetic phases. These molecules feature polydentate capping ligands that limit the dimensionality of the network to mono- and di-nuclear species that can be analyzed by single crystal X-ray diffraction. In our first publication (#5 below) we reported the structure and magnetic properties of Mn(amtp)(CH3CN)(TCNE)2 and [Mn(amtp)(CH3CN)(µ-TCNE)]2(ClO4)2, where amtp = tris(pyrazol-1-ylmethyl)amine, which formally contain two cis- TCNE radical anionic ligands and two bridging TCNE radical anionic ligands, respectively. The images below show that the TCNE anions are dimerized to form a [TCNE]22-, which by SQUID magnetometry, we have determined to be diamagnetic and totally ineffective at mediating magnetic communication between the two manganese(II) centers. One possible explanation, then, for low Tc's in these coordination polymer magnets is the presence of "dead link" [TCNE]22- instead of the magnetically "active" TCNE radical anions.