Ionic rotors. Preparation, structure, and dynamic solid-state 2D NMR study of the 1,4-diethynylbenzenebis(triphenylborate) dianion

Article abstract of DOI:10.1021/ja053256j

The reaction between p-(LiC₂)₂C₆H₄ (generated in situ from butyllithium and dialkynylbenzene) and 2 equiv of BPh₃ affords high yields of [Li(THF)₄]₂[p-(Ph₃BC₂

Full text of DOI:10.1021/ja053256j

Published on Web 08/18/2005
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Ionic Rotors. Preparation, Structure, and Dynamic Solid-State D NMR Study
of the 1,4-Diethynylbenzenebis(triphenylborate) Dianion
James R. Gardinier,*,† Perry J. Pellechia,‡ and Mark D. Smith‡
Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201, and Department of Chemistry and
Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208
Received May 18, 2005; E-mail: james.gardinier@marquette.edu
The incorporation of moving parts into molecular compounds is
a topic of current interest with regard to the development of
molecular machines.1 This interest is fueled in part by the desire
to manipulate matter on the small scale. Since macroscale gyro-
scopes can be used in guidance systems, similar functions can be
envisioned in micro- or nanoscale devices. Therefore, research into
developing molecular “gyroscopes” has attracted considerable
interest as of late.2 Molecular gyroscopes consist of rotors encased
in a protective scaffold to allow freedom of motion to the internal
component. Recently, elegant work by Garcia-Garibay’s group on
Figure 1. Isoelectronic rotors.
systems with a diethynylphenylene rotor connected between two
trityl groups (middle of Figure 1) has shown that it is possible to
control the rate that the central phenyl ring rotates in the solid state
by appropriately substituting groups on the ligand periphery.3
Crystal packing interactions (such as CH-π and π-π interactions)
involving these charge-neutral compounds or their solvates dictate
the rotational velocity of the central phenylene ring; the more
substituted the trityl/trypticenyl end groups, the more isolated the
central rotors, and the faster the central phenylene rotation.4 This
is clearly evident in the 105 increase in room temperature phenylene
rotation rates between the p-(Ph3CC2)2C6H4 and p-[(3,5-tBu2C6H3)3-
CC2)2C6H4 derivatives.3
In the late 1970s, the tin and lead congeners of the type p-(Ph3-
MC2)2C6H4 (M ) Sn, Pb) were reported but for purposes other
than studying the rotation of the central phenylene spacer.5
Likewise, in the early 1990s, Stang reported the preparation of the
cationic species based on phosphonium derivatives (Figure 1, above)
starting from the bis(phenyliodonium)acetylene triflate.6 Surpris-
ingly, the isoelectronic borate counterpart (Figure 1, right) has not
yet been reported. Our interest in the synthesis and supramolecular
chemistry of boron alkynyl compounds led us to consider this
promising class of ionic derivatives as alternatives to the charge-
neutral carbon-based rotors for the design of electromagnetically
responsive materials. Ionic-based compounds may allow for better
separation of rotors due to Coulombic repulsion, and ultimately,
the crystal structure of such species may be fine-tuned by
incorporating different cations that will serve to direct the supramo-
lecular structure of the resulting solids. We now wish to com-
municate our initial endeavors in preparing a compound that
contains the p-(Ph3BC2)2C6H4 dianion, the heretofore missing
member of the isoelectronic series of molecular rotors (Figure 1,
right).
Figure 2. ORTEP drawing of the [p-(Ph3BC2)2C6H4]2- dianion. Thermal
ellipsoids are drawn at the 50% probability level.
with 2 equiv of triphenylborane, according to eq 1. The diborate
salt is soluble in polar solvents
+ 2 BPh3
2 LinBu + p-(HC2)2C6H4 THF,toluene8
reflux
[Li(THF)4]2[p-(Ph3BC2)2C6H4] + BuH (1)
such as acetone or acetonitrile, moderately soluble in THF, but is
insoluble in hydrocarbons and diethyl ether, and decomposes in
chlorinated solvents.
An ORTEP diagram of the dianion in [Li(THF)4]2[p-(Ph3-
BC2)2C6H4] is given in Figure 2. The dianion is well-behaved with
a typical sp3-boron-alkynyl carbon bond distance of 1.60 Å,7 but
one of the THF molecules in the Li(THF)4 cation suffers a disorder,
as outlined in the Supporting Information.
As expected, the Coulombic repulsion of dianions and the
presence of the atoms of the cation leads to a greater separation of
rotors in [Li(THF)4]2[p-(Ph3BC2)2C6H4] in the solid state compared
to those found in the structures of p-(Ph3CC2)2C6H4 or its benzene
solvate.3 As detailed in Figure 3 and elaborated on in the discussion
of the supramolecular structure found in the Supporting Information,
the rotors of the borate derivative are well separated from one
The compound [Li(THF)4]2[p-(Ph3BC2)2C6H4] and its deuterio
counterpart were prepared in 65-70% yield by reacting p-(LiC2)2-
C6H4 (generated in situ from butyllithium and dialkynylbenzene)
† Marquette University.
‡ University of South Carolina.
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10.1021/ja053256j CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
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Figure 4. Experimental (left) and calculated (right) solid-state D NMR
Figure 3. Packing diagram of [Li(THF)4]2[p-(Ph3BC2)2C6H4] viewed into
the bc plane.
spectra for central phenylene rotation in the [p-(Ph3BC2)2C6D4] dianion at
various temperatures. The calculated spectra on the right were obtained by
considering a two-site model with fast (g10 MHz) and slow (10 kHz-1
MHz) moving components.
another, while those in the charge-neutral derivatives are interpen-
etrated to different extents. It is anticipated that the isolation of the
rotors in the title compound should lead to high rates of phenylene
rotation.
than that of the carbon analogue. The two-site system indicates
the complex structural nature of the desolvated compound: either
different domains with both compact (slow rotation) and more-
separated (fast rotation) local environments coexist or the desolvated
compound has a new supramolecular structure with symmetry
independent rotors. These details will be addressed further in the
full report and in future reports, where we will delineate the role
of cation substitution on (1) increasing the air stability of the rotor,
and (2) how the supramolecular organization of the borate rotors
effects phenylene rotation rates. It is hoped that we will learn how
to control the supramolecular organization of these potentially
electroresponsive borate-based solid architectures.
Crystals of the diborate salt rapidly (minutes) turn opaque when
removed from the mother liquor at room temperature. Integration
1
of the resonances obtained in the solution H NMR spectrum of
the opaque samples dissolved in acetone-d6 confirms that the salt
readily loses its coordinated THF. The 1H NMR spectra for samples
of the diborate salt that were dried under vacuum for 1 h consistently
revealed the presence of only 4 of the expected 8 equiv of THF,
indicating that [Li(THF)2]2[p-(Ph3BC2)2C6H4] was sufficiently stable
for further analyses, although the exact structural nature of the
desolvated form remains unclear. The samples of the borate salts
lose all coordinated THF at the expense of coordinating water over
the period of weeks when kept in air, as indicated by NMR
spectroscopy and elemental analyses. The hydrated salt is insoluble
in most weakly coordinating solvents and is prone to hydrolytic
decomposition in solution. Thus, this hygroscopic diborate salt is
best stored protected from atmospheric moisture.
A sample of desolvated diborate salt with the composition [Li-
(THF)2]2[p-(Ph3BC2)2C6D4] (NMR) that was protected from atmo-
spheric moisture (by manipulation in a nitrogen-filled drybox) was
subjected to a variable-temperature solid-state 2D spin-echo NMR
spectroscopic study to determine whether and at what rate the
central phenylene ring rotates (via 180° ring flips)8 in the solid
state. The results of the NMR study are summarized in Figure 4,
full details are provided in the Supporting Information. The
molecular motion of the central phenylene of the diborate dianion
can best be modeled as a two-site system that contains a fast (g10
MHz) and a slower (10 kHz to 1 MHz) moving component. In
general, the slower component of phenylene rotation in the borate
derivative is faster than, but comparable to, that of the carbon-
based analogues over comparable temperature range. For instance,
the rotation in the borate has a rate of 300 kHz at 294 K versus 15
kHz at 297 K for its ditrityl counterpart. The slower component of
phenylene rotation reaches the MHz regime at only 308 K in the
borate complex, whereas the carbon analogue was reported to
achieve a rate of 1.3 MHz at 348 K.2a,b Thus, even with the
structural collapse involving the desolvation of [Li(THF)4]2[p-(Ph3-
BC2)2C6D4], the Coulombic repulsion of anions and the mere
presence of cation atoms likely afford greater separation between
rotors than that in the trityl system and allows for faster rotation
Acknowledgment. J.R.G. thanks Marquette University and the
Petroleum Research Fund for support.
Supporting Information Available: Crystallographic information
file (CIF), additional structural information and figures, including the
supramolecular organization, and experimental procedures. This material
is available free of charge via the Internet at http://pubs.acs.org.
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