Professor and past Director, MSU Center for Fundamental Materials Research. A.B., 1977, Harvard University; Ph.D., 1987, Princeton University; Postdoctoral Fellow, 1986-88, Ohio State University. Physical Organic Chemistry. Reactive intermediates and reaction mechanisms; hydridic-to-protonic hydrogen bonding (AKA "dihydrogen bonding" organic magnetic and conductive materials; catalytic pathways to chemical intermediates.
Tel. (517)-355-9715 ext. 141.
Hydrogen-Hydrogen Hydrogen Bonding
In work started in summer of 1994, and primarily carried forward by summer high school students and undergraduates, we demonstrated an interaction between the electron pair of a B-H bond and a traditional H-X (X=N, O, Halogen) hydrogen bonding partner. Single crystal X-ray and neutron diffraction, solution spectroscopies (IR, NMR), and ab initio calculations all indicate that such relationships are best understood as a novel type of hydrogen bonding.
While our own studies have focused on the borohydrides,
related examples of close interactions between hydridic and protonic hydrogen
sites in organoiridium complexes have been reported from the Crabtree group
at Yale. [Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold,
A. L.; Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348-354.]
The new associative effect has implications for regiochemical control of organic reactions and for the rational assembly of crystalline, covalent materials. Consistent with calculations (shown at left) Sterling Gatling has uncovered strong, controllable directing and rate effects in borohydride reduction of hydroxyketones under appropriate conditions. The idea of chemical "basting stitches"-weak linkages that can organize and hold a structure's form while it is more firmly sewn together-is also very appealing. Hydrogen-hydrogen H-bonding represents just such an interaction, and Radu Custelcean's work in this area amply demonstrated the idea. He established that loss of H2 from HHH-bonds in the solid state can lead to products distinct from those in fluid phases (melt or solution) and that in favorable cases, crystallinity can be maintained throughout the reaction despite some geometry change and the release of gas inside the crystals [45,47]. It is now clear that HHH-bonding can be used as a key element in crystal engineering. Because of their reliable structural preferences and compactness, HHH-bonds may serve as key elements in strategies aimed at synthesis of new covalent linkages in solid crystal lattices [52,54]. We have recently reviewed the chemistry of this interesting interaction , and presented our first efforts to assemble extended covalently bonded networks via the above HHH-bond-directed pathways.[60, 62].
The hydridic-to-protonic hydrogen bonding
project, begun in the 1994 with high school student Mani Sharma,
has benefited from the participation of several talented and hardworking
high school students--Mani Sharma ('94), Charley Wilson ('95), Susanna
Bass ('96), and Jennifer Hung ('97). Three of the four entered the Westinghouse
Science Talent Search competition, and two (Sharma and Bass) were semifinalists,
among their many awards. The X-ray structure of guanidinium
borohydride, an interesting example of multipoint HHH-bonding, came
out of Susanna Bass's studies in the summer of 1996. After Drs. Gatling
and Custelcean moved on to new horizons, Simona Marincean, aided by such talented undergraduate
researchers as Gates Cambridge Scholar Robin Stein,
Melinda Baker, and Justin Roberts (all now attending top ten graduate schools in Chemistry),
uncovered new segments of this unusual story. Simona, now Dr. Marincean, used ab initio methods
to study the dynamics of proton transfer in the reaction of various acids with
BH4 and AlH4 anions. 
Together with Custelcean and Stein, she also reexamined the crystal structure of NH4H2PO2, a salt
whose crystal structure, reported in 1934 before the "classical" hydrogen bond had even been
recognized as a common structural theme. This compound's crystal structure had been analyzed
in terms which implied hydridic-to-protonic hydrogen bonding, but though their insight turned out to
be propetic, our modern structural reanalysis found no such bonding in this compound. 
Oxygen Atom Transfer Reactions of Carbenes:
Carbenes' most familiar reactions are insertions into C=C pi and R-H sigma bonds, and H or halogen abstractions. However, we have recently begun to systematically examine O atom abstraction (see below) by the reactive carbenes fluorenylidene (Fl:) and methylene (H2C:, the simplest carbene).
:C: + R2C=O --> CO + R2C:
R2C: + O=CR'2 --> R2C=O + :CR'2
That C atoms could abstract oxygen from nearly anywhere has been known for some time. Although the reaction is far less exothermic, we now know that fluorenylidene and methylene (Fl: and H2C:) are similarly able to deoxygenate suitable carbonyl compounds to yield a new carbene and the carbonyl products fluorenone or formaldehyde (Fl=O or H2C=O). This atom transfer, which breaks and forms C=O double bonds, represents a rare reaction path for carbenes. The overall process may vary from near thermoneutral to highly exothermic, and may have an observable intermediate or none. In strongly exothermic cases, as with the tetramethyl urea (TMU) substrate shown above, it can be very rapid (4 x 108 l/mol.sec). We are pursuing experimental and theoretical studies to establish the scope, kinetics, and thermodynamics of these reactions and to ask whether they are one- or two-step processes. Recent results of Dalila Kovacs' ab initio calculations (MP2/6-31G* level) on this system point to the formation of an ylide intermediate as shown at right, despite the large exothermicity of the overall reaction. Thus, it appears that even this rather favorable case proceeds stepwise.
Dalila's hig level ab initio theoretical studies of the closely
related reaction H2C: + CO2 --> H2C=O
+ CO have uncovered a single-step oxygen abstraction pathway with an activation
barrier of 24 kcal/mol on the singlet potential energy surface.
However, in the most favorable path, methylene attacks CO2 without
an enthalpic barrier to form oxiranone (aka alpha-lactone), a 47 kcal/mol
exothermic reaction. This product then traverses a second barrier, 20 kcal/mol
below the energy of the starting species, to fragment into CO and CH2O,
for an overall release of 61 kcal/mol. We believe that the initial addition
reaction produces "hot" alpha lactone with its vibrational energy in just
the right modes to rapidly continue over the fragmentation barrier to form
CH2O + CO. The dynamics of such a process would be interesting and amenable
to computational modeling, which is currently getting underway. Here is
a "movie" from
the dynamic reaction coordinate (i.e. total energy fixed oscillating back
and forth between kinetic and potential forms) for CH2+CO2
--> CH2O + CO as calculated at the relatively primitive HF/STO-3G
level. Some of the frames from the movie may be viewed as an animated
Symmetry Analysis of the Carbene-Olefin Cycloaddition Transition
|As noted above, one of the most familiar reactions of carbenes is insertion into C=C double bonds to form cyclopropanes. Based on the theoretical studies by Hoffmann and Dewar in the early '70s, this process has long been "known" to occur via an unsymmetrical reaction path wherein the carbene tilts to orient its empty 2p orbital to attack the electron pair of the pi bond, while the carbene's sp2 lone pair remains "out of the way" until the reaction has progressed well along the reaction coordinate. This view of the reaction, founded on orbital symmetry arguments, remained experimentally unexplored for roughly two decades until Julius Remenar's studies here at MSU. Julius applied "Tolbert analysis" [ Tolbert, L. M.; and Ali, B., J. Am. Chem. Soc. 1981, 103, 2104] to the reaction of :CCl2 with three dialkyl (diethyl-; l-bornyl, ethyl-; and di-l-bornyl-) fumarates, uncovering evidence that supports the asymmetric trajectory.||
Self-assembly of Magnetic Materials:
Magnetism arises from the cumulative behavior of
unpaired electrons, so design of a molecular magnetic material requires
understanding and ultimately control of the system's spin-couplings. One
ion-binding strategy for self-assembly that defines structural and magnetic
relationships between organic radicals is shown below, followed by pieces--molecular
models we have built--of the (RM)n magnetically coupled chain.
In the left X-ray structure, two radicals flank a bound K+ ion
("RMR"); in the structure at right, from pulsed EPR spectroscopy and computer
modeling, two Li+ cations flank a radical ("MRM").
From detailed structural and spectroscopic studies of diamagnetic analogues
and by examining related systems' magnetic behaviors [30,
we are building an understanding of the rules relating structure to electron
coupling. We have shown that new couplings can be introduced by ion binding;
continuing work in new structural motifs has begun to target extended chains
and control of interchain coupling in "simple" salts 
and hydrogen bonded systems 
as well as magneto-structural correlations in layered vanadyl phosphate
and organophosphonate magnetic materials.[32,
We are also interested in the magnetic characteristics of alkali metal
salts of organic radical anions, of which we have synthesized several sets.[46,53]
After careful consideration, we have designed a new diradical dianion building
block for organic magnetic materials related to ortho-semiquinone ligands.
This work is largely carried out in the context of a rich collaboration
with Prof. James L. Dye, the discoverer of alkalides and electrides, salts
in which the anions are alkali metal anions or simply electrons, charge-balanced
by alkali metal cations bound in neutral complexants such as crown ethers
or cryptands. Such compounds are notoriously sensitive to heat and air,
and we have recently been working to develop ligands that were more highly
resistant to reduction than the crown ether/cryptand classes. Our efforts
have paid off in the synthesis of the first room temperature stable alkalides , the first
alkalide in which the cation is a complexed proton (H+) , and a novel six-coordinate Li complex in a peraza cryptand.
Mechanisms of Catalytic Processes:
Other interests include:
Computational Modeling of Carbene and Radical Reaction Paths;
Solid State Reaction Mechanisms;