1. Pursuit of a Bidentate Ligand for an Eight-Coordinate,
Dodecahedral Rhenium(V) Center
Alexis G. Scorzelli, Georga S. Torres, Brian E. Macalush, and Gregory A. Moehring
Department of Chemistry and Physics, Monmouth University, West Long Branch, New Jersey 07764
Abstract
This work tests our model of pseudorotational rearrangement of eight coordinate dodecahedral rhenium(V) complexes. Pseudorotation is the rearrangement of atoms in a
molecule, without breaking bonds, and changes the steric relationship of atoms. Pseudorotational rearrangement is equivalent to rotating a molecule in space. Pseudorotations
have been a topic of interest because of their potential use as catalysts, molecular motors, and molecular switches. There are currently three established classes of pseudorotations.
Our work focuses on the establishment of a fourth class. In order to establish this fourth class, we prepared and investigated new eight coordinate dodecahedral rhenium(V)
complexes. The variable temperature NMR of one such compound, ReH5(PPh3)2(en), revealed that this molecule’s rearrangement is consistent with a pseudorotation. Other new
molecules prepared included ReH5(PPh3)2(4-Hq) and ReH4(PPh3)2(naphthimine), which both contain bidentate chelating ligands and exhibited two isomers at all temperatures
investigated; also consistent with a pseudorotational rearrangement for these new molecules.
Introduction
Previously Walton et al. published the crystal structures of two separate isomers with the formula
[ReH2(mhp)2(PPh3)2]+, in which mhp is the bidentate chelating anion of 6-methyl-2-hydroxypyridine (Figure 1). The
isomers have eight-coordinate dodecahedral structures at a rhenium center. Following this work, Crabtree et al.
proposed that certain eight-coordinate rhenium(V) complexes undergo pseudorotation (Figure 2). Pseudorotation
is described as the rearrangement of atoms in a molecule without breaking bonds and is equivalent to rotating a
molecule in space, but changes the steric relationship of atoms within the molecule. Our work focuses on better
establishing the ubiquity of pseudorotations in eight-coordinate rhenium(V) complexes. These complexes adopt a
pseudododecahedral coordination around a rhenium center that consists of two orthogonal trapezoids of ligands
(Figure 3). Each trapezoid consists of two equivalent A sites and two equivalent B sites, in which the A sites are
more sterically demanding.
In 1998, our group published the structure of ReH4[o-(C6H4)CH=NPh](PPh3)2, the first transition metal polyhydride
center supported by a sigma bond between an aromatic carbon atom and a transition metal center (Figure 4). This
structure is consistent with the described eight-coordinate complexes, in which one plane of the molecule
contains two PPh3 groups in B sites and two hydride ligands in A sites, and a second plane contains a bound imine
nitrogen atom in one B site, a carbon atom in an A site, and two hydride ligands in the remaining A and B sites.
Previously, our group also published the first report that identified isomers of certain rhenium(V) polyhydride
complexes with the formula ReH5(PPh3)2L, in which L represents an unsymmetrically substituted aromatic amine,
such as pyrimidine in ReH5(PPh3)2(pyr) (Figure 5). Our current work expands upon previous results we obtained,
which reveal the presence of isomers in ReH5(PPh3)2(pyr) that we believe interconvert through pseudorotation.
The purpose of this project was to gain a greater understanding of eight-coordinate rhenium(V) complexes that
pseudorotate.
Acknowledgements
Summer Research Program, Monmouth University
Department of Chemistry and Physics
School of Science
REFERENCES
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in a Dodecahedral Transition Metal Polyhydride. R. Bosque, F. Maseras, O. Eisenstein, B. P. Patel, W. Yao, and R. H.
Crabtree, Inorg. Chem., 1997, 36, 5505-5511.
5. Synthesis and structural characterization of eight-coordinate geometrical isomers of [ReH2(mhp)2(PPh3)2]PF6
that retain their structural identity in solution. M. Leeaphon, P. E. Fanwick, and R. A. Walton, J. Am. Chem. Soc.,
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Results and Discussion
Results and Discussion (cont.)
ReH5(PPh3)2(en)
While unsymmetrical aromatic amine-supported rhenium(V) pentahydride compounds exhibit isomers in their
low temperature NMR spectra, such complexes supported by primary amines have never exhibited the presence
of isomers. Because of its capacity for free rotation of the Re-N bound and lack of a chiral center, the
ethylenediamine-supported complex, ReH5(PPh3)2(en) (Figure 9), was not expected to exhibit properties
associated with isomers. Thus, the data collected for its variable temperature 31
P-{1
H} NMR spectrum was
expected (Figure 10). The peaks were about the same size at each temperature and were located around the
same frequency. Slight changes between peak size at differing temperatures are due to the low temperature
lowering the solubility of ReH5(PPh3)2(en) in the solvent and, thus, its concentration in the solution. In the low
temperature 1
H NMR hydride region, unlike phosphorus, multiple signals were expected to be present when the
temperature of the solution was lowered. This is due to slowing of the fluxional exchange of hydride ligands
among the various coordination sites taking place in the compound. At higher temperatures ligand exchanges
occur fast enough for hydride signals to be measured as one signal. However, once the temperature is lowered,
pseudorotations slow down enough for individual hydride resonances to be measured. These results were
observed in the 1
H NMR spectrum for ReH5(PPh3)2(en) (Figure 11). At temperatures above 250 K, the
coalescence point for this substance, one peak is observed. Below 250 K, four individual peaks are observed.
We believe that ReH5(PPh3)2(en) undergoes pseudorotation. It was expected that the pseudorotations occur
rapidly enough for a single [average] hydride resonance to appear in the NMR data. However, if the
pseudorotations are slowed enough, we expected that four separate signals could be measured. Our data
collected shows that above 250 K, this single hydride resonance was measured. Below 250 K, four signals
appeared, supporting the theory of pseudorotations taking place in the molecule. The phosphorus NMR
spectrum was expected to show a different pattern, due to the structure of the molecule. Since the molecule is
able to pseudorotate freely and the resultant stable phosphorous coordination sites are equivalent,
pseudorotation does not affect phosphorus signals. Thus, it was expected that the NMR spectrum would show
only one resonance for phosphorus with varied temperatures.
ReH4[o-(C6H4)CH=NPh](PPh3)2
In order to determine the reaction mechanism of ReH4[o-(C6H4)CH=NPh](PPh3)2 converting between two
isomers, we looked at the entropy of activation, ∆S‡, and enthalpy of activation, ∆H‡, for the isomer
interconversion. By simulating previously collected 31P-{1H} NMR spectra of ReH4[o-(C6H4)CH=NPh](PPh3)2 at
various temperatures, we were able to determine the rate constants of the conversion between isomers of this
complex and create an Eyring plot (Figure 12). From this plot, we were able to calculate ∆H‡, 8.9 kcal/mol, and
∆S‡, -22 cal/mol∙K. This small, negative value of ∆S‡ indicates that there is no breaking of bonds when converting
from one isomer to another. In a dissociation reaction, bonds in the reactant break, forming two or more
products, which is favorable in respect to entropy. Using the equation ∆G‡=∆H‡-T∆S‡, where a negative value for
∆G‡ is favorable, T is always positive, and ∆H‡ is negative, we can see that ∆S‡ would have to be positive for the
overall reaction to be favorable. Since dissociations are favorable in respect to entropy, a fairly large positive
value for ∆S‡ of a reaction is indicative of a dissociation reaction. Therefore, the small, negative value we
obtained for ∆S‡ indicates that no dissociation takes place when converting between isomers, further supporting
a pseudorotational mechanism.
Concluding Remarks
Our data support the notion that pseudorotation is the cause of the observed different isomers. In doing so we
have also collected information that does not support other theories as to how the interconversion of the
molecules is achieved. Using the NMR data we collected of non-symmetric rhenium compounds we can
determine that the molecule’s atoms are experiencing different environments which would mean that the
molecule is not simply rotating in a static position but in fact, is staying bound together and the atoms attached
to rhenium center are rotating around each other causing the different environments that are seen on the NMR
results. From the information gathered from the Eyring plot we can also conclude that there are no bonds
breaking during the rotation because the ∆S‡ value is a negative and if the molecule was breaking into two
different parts then the value would be positive. By that information we can be sure that while these
interconversions are taking place the material is not breaking apart, rotating then rejoining back together in
different geometric relationships.Experimental
Preparation of Rhenium(V) pentahydridobis(triphenylphosphine)(4-hydroxyquinazoline). 100 mg of ReH7(PPh3)2
and 50 mg of 4-Hydroxyquinazoline (4Hq) were added to a round-bottomed flask, deoxygenated, and put under a
nitrogen atmosphere. 5 mL of deoxygenated THF were added to the round-bottomed flask. The solution was
refluxed for 30 minutes and then cooled to room temperature. The solution was transferred to a filter flask where
the solvent was removed under vacuum. The residue was then dissolved in acetone. Methanol was added to form
a precipitate, which was then filtered and washed with a copious amount of MTBE to acquire a light orange solid.
Preparation of Rhenium Oxytrichlorobistriphenylphosphine. To a refluxing solution of 1620 mg of Ammonium
perrhenate, 10 mL of concentrated HCl, and 50 ml Ethanol in a round-bottomed flask a warm solution of 9000 mg
of Triphenylphosphine was added. After 30 minutes of additional reflux, the mixture was cooled to room
temperature. A bright green precipitate was filtered and washed with Ethanol and air dried to acquire a yellow
solid.
Preparation of [ReH4(PPh3)2(4-Methyl-1,10-phenanthroline)](BF4). This compound was prepared by adding 100
mg of ReH7(PPh3)2 and 42mg of 4-Methyl-1, 10-phenanthroline to a small round-bottomed flask. The flask was
attached to a condenser, deoxygenated, and filled with an atmosphere of nitrogen. Next, 5 ml of deoxygenated
THF was added to the flask and the blue solution was refluxed for an hour. After cooling the solution to room
temperature, 0.5 ml of Tetrafluoroboric acid was added and allowed to react for five minutes. The resultant
solution was added to 50 ml of ether and stirred until a good amount of purple precipitate had formed. The
precipitate was filtered, washed with ether, and dried.
Preparation of ReH4(PPh3)2(4-tolyl, naphthalenecarbaldehyde imine). was prepared by adding 50 mg of 4-tolyl,
naphthalenecarbaldehyde imine and 100 mg of ReH7(PPh3)2 to a 5ml round-bottomed flask which was then
attached to a condenser. The system was deoxygenated and put under nitrogen. Next, 5 ml of deoxygenated
benzene was added to the flask and the resultant solution was refluxed for 15 minutes. After cooling, 50 ml of
ether was added and the solution was stirred. The mixture was filtered before adding 15 ml of hexanes to the
filtrate. Then, the solvent was stripped off until only the product, an oil, remained.
Preparation of ReH5(PPh3)2(pyr). The compound ReH5(PPh3)2(pyr) was produced by refluxing 100 mg of
ReH7(PPh3)2, under a nitrogen atmosphere, in 5 ml of de-oxygenated tetrahydrofuran and 0.5 ml of pyrimidine for
one hour. After the mixture was cooled to room temperature, 50 ml of diethyl ether was added and then after 5
minutes 10 ml of hexane was added and 2 minutes later the mixture was filtered. After filtering, the solution was
moved to another Erlenmeyer flask and allowed to mix while 125 ml of hexane was added evenly over a period of
45 minutes. After this time the mixture was filtered and the yellow precipitate was collected, and dried.
Preparation of ReH7(PPh3)2. ReH7(PPh3)2 was produced by combining 410 mg of ReOCl3(PPh3)2 with 675 mg of
NaBH4 under an atmosphere of nitrogen. The round-bottomed flask was then placed over a stirrer inside an ice
bath. With stirring 8 ml of deoxygenated water and 8 ml of deoxygenated tetrahydrofuran was added to the
mixture. The chemicals were then allowed to react for 15 minutes under chilled conditions, after this time the
reaction was removed from the ice bath and stirred for another 15 minutes. If the reaction was not fully reacted
an extra 75mg of NaBH4 was added to the reaction mixture and allowed to react for another 5 minutes or until the
reaction turned a tan color. Once the reaction was complete, the reaction was checked for homogeneity. If the
reaction separated 5 ml of methanol was added and the reaction mixed for an additional 2 minutes. The material
was then filtered, washed with water twice, methanol twice, and diethyl ether three times.
Figure 1. Two [ReH2(mhp)2(PPh3)2]+
isomers.
Figure 2. A generic pseudorotation in
an eight-coordinate rhenium(V)
complex.
Figure 3. Two representations of an
eight-coordinate rhenium(V) complex.
Figure 4. The ORTEP structure of
ReH4[o-(C6H4)CH=NPh](PPh3)2.
Figure 5. Two
ReH5(PPh3)2(pyr) isomers.
Figure 11. Hydride region of the 1H NMR spectrum of
the complex ReH5(PPh3)2(en) at several
temperatures.
Figure 10. 31P-{1H} NMR spectrum of the complex
ReH5(PPh3)2(en) at several temperatures.
Figure 9.
ReH5(PPh3)2(en)
Figure 12. An Eyring plot of ReH4[o-(C6H4)CH=NPh](PPh3)2.
ReH5(PPh3)2(4-Hq)
ReH5(PPh3)2(4-Hq) (Figure 6) disproves the rotational exchange mechanism due
to its bidentate chelating nature. The 1H NMR spectrum is expected to have
pseudorotations slowed down to lower temperatures revealing two isomers.
Peaks in both the 1H NMR and 31P NMR spectrum complexes have slight changes
in size and frequency at differing temperatures. The 1H NMR for ReH5(PPh3)2(4-
Hq) (Figure 7) at several temperatures shows no individual resonances at high
temperatures. However, at low temperatures the exchange rate slows downs
displaying two isomers. In theory, the 31P NMR spectrum is expected to also have
two isomers as well. The 31P NMR spectrum of the complex ReH5(PPh3)2(4-Hq)
(Figure 8) shows that there are two isomers at all temperatures. These NMR
spectra, confirm that the bidentate ligand in ReH5(PPh3)2(4-Hq) only binds
through the oxygen and nitrogen making a rotational exchange mechanism very
unlikely and supporting a pseudorotational exchange mechanism.
Figure 8.31P-{1H} NMR spectrum of the complex
ReH5(PPh3)2(4-Hq) at several temperatures.
Figure 7. Hydride region of the 1H NMR spectrum of the
complex ReH5(PPh3)2(4-Hq) at several temperatures.
Figure 6.
ReH5(PPh3)2(4Hq)