November 20, 2003
Experimental Electron Density Study of an Organozirconium Compound
S. Pillet1, G. Wu1, V. Kulsomphob2, B.G. Harvey2, R.D. Ernst2, and P. Coppens1
1State University of New York - Buffalo, Buffalo, NY;
2University of Utah, Salt Lake City, UT
Scientists at the State University of New York in Buffalo and at the University of Utah have synthesized a novel organozirconium complex and subjected it to a careful experimental electron density study at 16 K. The study demonstrates that it is now possible to achieve accurate results for such heavier atom-containing species when high intensity sources and small crystal sizes are employed. The analysis of the electron density distribution for the organozirconium complex has revealed unusual 4d orbital populations and provided insight both into the nature of the bonding of the zirconium center to its various ligands, and into a number of interatomic contacts between formally nonbonded atoms.
In electron deficient organometallic compounds, it is not
uncommon for empty orbitals on a metal center to interact with
electron pairs in nearby C-H bonds. That was certainly what was
expected to be found in Complex 1, in which a very small Zr-N-C
angle indicated there was some sort of interaction between Zr and
the isopropyl group. However, spectroscopic data did not give any
indication of such an interaction. This led the researchers to
undertake an experimental electron density study of the compound,
even though a truly accurate study of a compound with an atom as
heavy as zirconium had never been achieved before. The high
intensity X-rays provided by the NSLS and high-resolution
single-crystal facilities at the SUNY X3 beamline were indispensable
in this undertaking.
Despite the inherent obstacles to overcome in obtaining accurate
data, the researchers were rewarded with a variety of interesting
results, although ironically they ultimately did not find an
unambiguous answer to the question that had prompted the study,
which concerned the putative agostic interaction between the metal
center and a proximal C,H region. To begin with, the d orbitals on
zirconium were found to exhibit unusual populations based on ligand
field considerations. Three orbitals (dz2, dx2-y2, and dxz) were
found to be relatively highly populated, indicating α donor
interactions from the organic ligands, and both α and π donor
interactions from the nitrogen center, respectively.
The study also revealed significant details concerning the
bonding electrons in the compound. A topological analysis of these
electrons showed “bond paths” for all the expected bonds between the
lighter atoms. However, bond paths were only found between the
zirconium atom and three ligand atoms (N and two C). Many of the
carbon atoms to which Zr is presumably bonded were not found to be
connected to the Zr atom by bond paths. Thus, not only were there no
bond paths connecting the Zr with the isopropyl group, but there
were also no bond paths to four of the five dienyl carbon atoms
(Figure 1), and three of the four diene carbon atoms. Nonetheless,
the electron density distributions and ellipticities, together with
the actual placement of the zirconium atom, provided clear evidence
for the presence of interactions between these ligand atoms and the
zirconium center. As topological analyses have only recently begun
to be employed to characterize the bonding in key types of organometallic compounds, this result provides guidance for further
studies using this method.
An unexpected bonus was provided by the observation that the p-orbital electron densities of the diene and dienyl π systems were not oriented perpendicular to their atomic planes, but rather experienced “tilts” or rehybridizations in order to be directed more toward the zirconium center. Such tilts had previously been inferred from distortions exhibited by π ligand substituents, but this is the first direct experimental evidence for these reorientations. As noted above, the charge density analysis did not provide direct evidence for the existence of an agostic interaction (Figure 2).
The topological analyses also revealed interesting interactions between some formally nonbonded atoms. Intermolecular CH/π and π/π interactions were observed between dienyl ligands in adjacent complexes, while a more unusual bond path was observed between opposing hydrogen atoms present on the dienyl ligand termini. This interaction may account for the observed tendency of pentadienyl ligands under some conditions to undergo loss of H2, yielding the more common cyclopentadienyl ligand.
While this study has yielded substantial insight into the bonding of this compound, very few electron density studies have been carried out on organometallic compounds in general. One can therefore expect that a wealth of additional new information will be achieved through similar studies of a wide variety of organometallics.
BEAMLINE
X3A1
FUNDING
U.S. Department of Energy
National Science Foundation
PUBLICATION
“Investigation of Zr-C, Zr-N, and Potential Agostic Interactions in
an Organozirconium Complex by Experimental Electron Density
Analysis”, S. Pillet, G. Wu, V. Kulsomphob, B.G. Harvey, R.D. Ernst,
and P. Coppens, J. Am. Chem. Soc., 125, 1937-1949
(2003).
FOR MORE INFORMATION
Philip Coppens
Department of Chemistry
State University of New York
Buffalo, NY
Email: coppens@acsu.buffalo.edu
Richard D. Ernst
Department of Chemistry
University of Utah
Salt Lake City, UT
Email: ernst@chem.utah.edu