Triarylboron-functionalized Cu(II) carboxylate paddlewheel complexes

Article abstract of DOI:10.1021/ic202521z

The assembly of two copper(II)-carboxylate dimer complexes appended with four peripheral triarylborane functionalities has been achieved. Complex stabilities in the presence of fluoride are examined.

Full text of DOI:10.1021/ic202521z

Communication
pubs.acs.org/IC
Triarylboron-Functionalized Cu(II) Carboxylate Paddlewheel
Complexes
Barry A. Blight, Alexander F. Stewart, Nan Wang, Jia-sheng Lu, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
S
* Supporting Information
Scheme 1. Synthetic Pathways to 2, 3, and 4
ABSTRACT: The assembly of two copper(II)−carbox-
ylate dimer complexes appended with four peripheral
triarylborane functionalities has been achieved. Complex
stabilities in the presence of fluoride are examined.
ecently, triarylboron functionalized metal complexes have
contributed significantly to the development of organic
R
light emitting diodes (OLEDs)1 as emitters for electron
transport materials,2 and as colorimetric or fluorescent sensors
for selective detection of anions such as fluoride and cyanide.3
Appended to the periphery of ligand sets such as 2,2′-
bipyridine,4 2-phenyl-pyridine,5 or 2-(2-pyridyl)thiophene,6
the triarylboron moiety often provides enhanced photo-
luminescence and electron transport properties relative to
those of the parent boron-free compounds due to the
introduction of the low-lying empty pπ orbital of the boron
center. These ligand sets, however, are charge neutral and thus
not ideal candidates for the functionalization of surfaces or
nanoparticles. On the other hand, carboxylate-terminated
ligands have been shown to be extremely effective in this
regard.7,8 In our pursuit of such conjugated boron-functional-
ized materials we have synthesized a versatile ligand motif,
which in addition to surface modification (to be reported in due
course), can act as a bridging ligand in the assembly of metal−
carboxylate dimeric complexes. Some recent reports on this
class of dinuclear species have investigated molecular
magnetics,9 catalysis,10 and of course repeating nodes in
metal−organic frameworks (MOFs).11 Here, we report the
synthesis of two new discrete triarylboron-containing copper-
(II)−carboxylate dimer complexes as model compounds
toward the development of their metal−carboxylate MOF
counterparts. Synthesis of the ligand can be achieved in two
steps from commercially available starting materials, followed
by the self-assembly step of the metal-containing products,
making this a versatile approach to luminescent paddlewheel-
shaped complexes.
(NO3)2·2.5H2O (2:1 ratio) in ethanol (EtOH) at room
temperature resulted in the immediate precipitation of the
copper(II) dimer 3 as a blue solid in 69% yield. Finally, copper
complex 3 was converted to 4 by the addition of two
equivalents of pyridine in CH2Cl2. The blue-green compound 4
was isolated by precipitation of the crude solid with EtOH at
room temperature in 94% yield. All compounds synthesized are
air -stable and fully characterized by NMR, elemental analysis,
UV−vis, and fluorescence spectroscopy.
The structures of ligand 2 and Cu dimers 3 and 4 were
determined using single crystal X-ray diffraction analysis. The
crystal morphology of all three compounds resembled very thin
needles and diffracted only weakly (see Supporting Information
(SI)), which led to the poor quality of refinements and
structural data. Nonetheless, the key structural features of these
compounds were established unequivocally. Ligand 2 crystal-
lizes in the monoclinic space group P21/c. As one would expect
for carboxylic acids, ligand 2 forms a head-to-head hydrogen-
bonded pair with an average H-bond distance of ∼2.62 Å
(O···O), as shown in Figure 1.
Complex 3 also belongs to the monoclinic P21/c space
group. The structure of 3 possesses a crystallographically
imposed inversion center of symmetry (Figure 2). The Cu···Cu
separation distance, 2.61 Å, is similar to those of previously
known Cu(II) carboxylate dimers.9a,13 The Cu(II) centers
adopt a square-pyramidal geometry with an average Cu−O
(carboxylate) bond length of 1.95 Å, while the axial Cu−O
Ligand 2 was successfully synthesized by two sequential
lithium−halogen exchange reactions. The reaction protocol
proceeds by treating 4,4′-dibromobiphenyl with one equivalent
of butyllithium at −78 °C in tetrahydrofuran (THF), followed
by quenching of the aryl-lithiate with dimesitylboronfluoride to
afford 1.12 Subsequent lithiation of 1 (Scheme 1) under the
same conditions followed by bubbling of the reaction solution
with CO2 results in the formation of carboxylate 2-Li, which,
after acidification, is isolated as the carboxylic acid 2 in 72%
yield. The reaction of its sodium salt (2-Na) with Cu-
Received: November 22, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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dx.doi.org/10.1021/ic202521z | Inorg. Chem. 2012, 51, 778−780

Inorganic Chemistry
Communication
complexes.14 This indicates that the higher energy transitions
are dominantly ligand-based. Emission spectra of all three
compounds are similar, exhibiting purple luminescence with
broad emission profiles, characteristic of triarylboron-contain-
ing species,2 with maxima between 400 and 410 nm. Metal
complexes 3 and 4 show little or no shift in the emission
spectra. Again, this indicates that emission is a ligand-based
process with little contribution from the metal centers.
However, quantum efficiencies of the metal complexes (3, Φ
= 0.05; 4, Φ = 0.06) are much lower compared to ligand 2 (Φ
= 0.18), which is likely due to either an increase in vibrational
modes for energy dissipation in the larger complexes relative to
the smaller 2, energy transfer to the lower energy d−d excited
state, or a combination of both. Solid-state emission spectra for
2 and its methyl-ester (Figure S5) were also recorded, which
are similar to the solution spectra. Complexes 3 and 4 were not
observably emissive in the solid state. Cyclic voltammetry was
also performed on complex 4 (which lacks protic solvent co-
ligands) to examine the reduction potential of the boryl ligand.
Two reduction peaks similar to the methyl ester of ligand 2
were observed for the complex, attributable to the reduction of
the carbonyl and the boron center, respectively (Figure S22). In
addition, the complex shows an irreversible oxidation peak at
∼0.24 V (relative to that of FeCp2+/0, similar to the
electrochemical profile of anhydrous copper(II) acetate (see
the SI).
Figure 1. The crystal structure of 2, illustrating the head-to-head
hydrogen-bonding geometry with 35% thermal ellipsoids. Boron,
yellow; oxygen, red.
The Lewis acidity of triarylboranes makes them susceptible
to complexation by small anions,3 and thus examining the
stability of the CuII moiety in the presence of fluoride anions
was of interest. Ligand 2 was not subjected to fluoride binding
experiments (as a standard) due to interference from the
carboxylate proton. Since the high energy transitions of
complexes 3 and 4 seem to be ligand centered, we employed
the methylester of 2 and titrated it with tetra-n-butylammonium
fluoride (TBAF, monitored by both absorption and fluo-
rescence spectroscopy, see the ESI) in order to better model
the interaction of the fluoride anion with the boron center of
the ligands. The UV−vis absorption profile of the titration
experiment is comparable to that of trimesitylborane;15 the
transition at 333 nm quenches upon the addition of multiple
equivalents of fluoride. The same is true when 3 and 4 are
subjected to excess TBAF (Figure 3). Initially, we were
concerned about the stability of 3 and 4 in the presence of
fluoride. Inspection of the d−d transitions (675 nm) of the
Figure 2. Crystal structures of 3 (top) and 4 (bottom) with 35%
thermal ellipsoids. The carbon atoms in the disordered ethanol/
pyridine ligands and all methyl groups are shown as stick models for
clarity. Boron, yellow; oxygen, red; nitrogen, blue; copper, sky blue.
(ethanol) bond distances are much longer (2.17 Å). The four
boron centers have a rectangular arrangement with B···B
separation distances of 16.7 Å and 19.0 Å, respectively, along
the edge of the rectangle. The diagonal B···B separation
distance is 25.3 Å. Copper complex 4 crystallized in the triclinic
P1 space group with a Cu···Cu distance of 2.65 Å. Again, the
̅
paddlewheel geometry is obtained with pyridine at the axial
positions, resulting in a square-pyramidal geometry about each
Cu center with an average Cu−O (carboxylate) bond length of
1.96 Å. In this case, the average axial Cu−N (pyridine) distance
is measured to be 2.16 Å. The four boron centers, again, have a
rectangular arrangement with B···B separation distances of 16.2
Å and 19.3 Å, respectively, along the edge of the rectangle.
Here, the diagonal B···B separation distance is approximately
25.2 Å
In THF, the free ligand 2 shows two strong absorption peaks
(Figure S1, λmax = 328 and 292 nm) likely caused by π−pπ and
π−π* transitions, respectively. The absorption spectra of
complexes 3 and 4 exhibit very little deviation from that of 2,
with the exception of a very weak d−d transition at
approximately 675 nm, common for blue copper(II)-dimer
Figure 3. UV−vis absorption titration of copper dimer 3 (1 × 10−5 M)
with TBAF (0−10 equiv) in THF at 298 K. (Inset: UV−vis absorption
titration of copper dimer 3 (1 × 10−4 M) with TBAF (0−10 equiv) in
THF at 298 K focusing on the d−d transition of copper dimer 3).
Titration experiments with 4 show similar trends.
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Inorganic Chemistry
Communication
(3) (a) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P.
Chem. Rev. 2010, 110, 3958 andreferences therein. (b) Hudnall, T. W.;
Chiu, C. W.; Gabbaï, F. P. Acc. Chem. Res. 2009, 42, 388. (c) Kubo, Y.;
Yamamoto.; Ikeda, M.; Takeuchi, M.; Shinkai, S.; Yamaguchi, S.;
Tamao, K. Angew. Chem., Int. Ed. 2003, 42, 2036. (d) Yamaguchi, S.;
Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001, 123, 11372.
(4) (a) Sun, Y.; Wang, S. Inorg. Chem. 2009, 48, 3755. (b) Sun, Y.;
Ross, B.; Wang, R.-Y.; Wang, S. Can. J. Chem. 2009, 87, 188. (c) Sun,
Y.; Ross, N.; Zhao, Z.-B.; Huszarik, K.; Jia, W.-L.; Wang, R.-Y.; Wang,
S. J. Am. Chem. Soc. 2007, 129, 7510.
Cu−Cu dimer upon titration with TBAF (Figure 3, inset),
however, revealed that the complexes remained intact after four
equivalents of fluoride were added (the stoichiometry of F−
required for saturation of the four boron sites) but were fully
quenched above six equivalents of TBAF. Fluorescence
experiments where copper complexes 3 and 4 were titrated
with TBAF confirmed this observation, as the ligand-centered
emission for both complexes exhibits almost complete
quenching at approximately four equivalents of TBAF.
In summary, we have described the synthesis of a new
triarylboron-appended carboxylic acid, which acts as a suitable
ligand in the facile assembly of copper(II)−carboxylate dimer
complexes. Spectroscopic characterization revealed that the
luminescent properties of these complexes are predominantly
ligand-centered. This observation was confirmed by fluoride
binding experiments, where boron-centered emission was
quenched at elevated concentrations of the fluoride anion. In
addition, dissociation of the ligand from the complexes was not
observed below four equivalents of fluoride anion, suggesting
that all four boron sites become occupied by fluoride before
dissociation occurs. These studies also suggest that the Lewis
acidity of triarylboron ligand systems in the presence of
carboxylate−metal dimers is such that the boron sites will
associate small guests well before complex degradation occurs,
making this class of metal−organic nodes an interesting
candidate for guest binding in an extended network. As such,
we have started to expand these metal−triarylborane systems
into three dimensions by employing a C3 symmetric tris-
carboxylate analogue of 2 toward the self-assembly of new
borane-containing MOFs, in addition to studying the use of 2
and other related molecules in surface functionalization.
(5) (a) Sun, Y; Hudson, Z. M.; Wang, R.-Y.; Wang, S. Inorg. Chem.
2011, 50, 3373. (b) Wang, Z.-B.; Helander, M. G.; Hudson, Z. M.;
Qiu, J.; Wang, S.; Lu, Z.-H. App. Phys. Lett. 2011, 98, 213301−1.
(c) Hudson, Z. M.; Helander, M. G.; Lu, Z.-H.; Wang, S. Chem.
Commun. 2011, 47, 755. (d) Hudson, Z. M.; Sun, C.; Helander, M. G.;
Amarne, H.; Lu, Z.-H.; Wang, S. Adv. Funct. Mater. 2010, 20, 3426.
Rao, Y.-L.; Wang, S. Inorg. Chem. 2009, 48, 7698. Rao, Y.-L.; Amarne,
H; Zhao, S.-B.; McCormick, T.; Martic, S.; Sun, Y.; Wang, R.-Y.;
Wang, S. J. Am. Chem. Soc. 2008, 130, 12898.
(6) (a) Li, H.; Lalancette, R. A.; Jakle, F. Chem. Commun. 2011,
̈
[Online] DOI: 10.1039/C1CC13287C. (b) Zlojutro, V.; Sun, Y.;
Hudson, Z. M.; Wang, S. Chem. Commun. 2011, 47, 3837.
(7) (a) Vallee, A.; Humblot, V.; Pradier, C.-M. Acc. Chem. Res. 2010,
43, 1297. (b) Spitler, M. T.; Parkinson, B. A. Acc. Chem. Res. 2009, 42,
2017. (c) Astruc, D.; Ornelas, C.; Ruiz, J. Chem.Eur. J. 2009, 15,
8936. (d) Barth, J. V. Surf. Sci. 2009, 603, 1533. (e) Koziej, D.; Barsan,
N.; Weimar, U. Sensor Lett. 2008, 6, 817.
(8) (a) Kolemen, S.; Bozdemir, O. A.; Cakmak, Y.; Barin, G.; Erten-
Ela, S.; Marszalek, M.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin,
M. K.; Gratzel, M.; Akkaya, E. U. Chem. Sci. 2011, 2, 949. (b) Kim, Y.-
̈
Y.; Meldrum, F. C.; Walsh, D. Polymer Chemistry 2011, 2, 1375.
(c) Muhammad, F.; Guo, M.; Qi, W.; Sun, F.; Wang, A.; Guo, Y.; Zhu,
G. J. Am. Chem. Soc. 2011, 133, 8778. (d) Estruga, M.; Domingo, C.;
Ayllon, J. A. J. Mat. Chem. 2011, 21, 4408.
(9) (a) Reger, D. L.; Debreczeni, A.; Bryn, R.; Vitaly, R.; Smith, M.
D. Inorg. Chem. 2009, 48, 8911. (b) Shi, Q.; Sun, Y.; Sheng, L.; Ma, K.;
Cai, X.; Liu, D. Inorg. Chim. Acta 2009, 362, 4167. (c) Motokawa, N.;
Miyasaka, H.; Yamashita, M.; Dunbar, K. R. Angew. Chem., Int. Ed.
2008, 47, 7760. (d) Liu, C.-S.; Wang, J.-J.; Yan, L.-F.; Chang, Z.; Bu,
X.-H.; Sanudo, E. C.; Ribas, J. Inorg. Chem. 2007, 46, 6299.
(10) Sarma, D.; Ramanujachary, K. V.; Stock, N.; Natarajan, S. Cryst.
Growth Des. 2011, 11, 1357. Hansen, J.; Davies, H. M. L. Coord. Chem.
Rev. 2008, 252, 545. Nichols, J. M.; Liu, Y.; Zavalij, P.; Isaacs, L.;
Doyle, M. P. Inorg. Chim. Acta 2008, 361, 3309. Buijs, W.; Comba, P.;
Corneli, D.; Mengerink, Y.; Pritzkow, H.; Schickedanz, M. Eur. J. Inorg.
Chem. 2001, 3143.
(11) (a) Koeberl, M.; Cokoja, M.; Herrmann, W. A.; Kuehn, F. E.
Dalton Trans. 2011, 40, 6834. (b) Ling, Y.; Chen, Z.; Zheng, H.; Zhou,
Y.; Weng, L.; Zhao, D. Cryst. Growth Des. 2011, 11, 2811. (c) Seo, J.;
Bonneau, C.; Matsuda, R.; Takata, M.; Kitagawa, S. J. Am. Chem. Soc.
2011, 133, 9005. (d) Arslan, H. K.; Shekhah, O.; Wieland, D. C. F.;
Paulus, M.; Sternemann, C.; Schroer, M. A.; Tiemeyer, S.; Tolan, M.;
Fischer, R. A.; Woll, C. J. Am. Chem. Soc. 2011, 133, 8158. (e) Chen,
B.; Xiang, S.; Qian, G. Acc. Chem. Res. 2010, 43, 1115. (f) Vagin, S. I.;
Ott, A. K.; Rieger, B. Chem. Ing. Tech. 2007, 79, 767.
ASSOCIATED CONTENT
* Supporting Information
■
S
All experimental details for 2−4 and 2-OMe; absorption and
emission titration data for 2-OMe, 3, and 4; cyclic voltammetry
for 4; and single crystal X-ray diffraction data for 2, 3, and 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: suning.wang@chem.queensu.ca.
■
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Council of
Canada (NSERC) for financial support. B.A.B. is grateful for an
NSERC Postdoctoral Fellowship.
REFERENCES
■
(12) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45,
5475.
(13) (a) Reger, D. L.; Horger, J. J.; Debreczeni, A.; Smith, M. D.
Inorg. Chem. 2011, 50, 10225. (b) Bronner, C.; Baudron, S. A.;
Hosseini, M. W. Inorg. Chem. 2010, 49, 8659.
(14) (a) Inoue, M.; Kishita, M.; Kubo, M. Inorg. Chem. 1964, 3, 239.
(b) Lewis, J.; Mabbs, F. J. Chem. Soc. 1965, 3894. (c) Francois, M. A.;
Crivelli, I.; Andrade, C. Bol. Soc. Chil. Quim. 1982, 27, 11.
(1) (a) Jakle, F. Chem. Rev. 2010, 110, 3985. (b) Hudson, Z. M.;
̈
Wang, S. Acc. Chem. Res. 2009, 42, 1584. (c) Yamaguchi, S.;
Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413. (d) Entwistle, C. D.;
Marder, T. B. Chem. Mater. 2004, 16, 4574. (e) Entwistle, C. D.;
Marder, T. B. Angew. Chem., Int. Ed. 2002, 41, 2927. (f) Noda, T.;
Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.
(2) (a) Li, F.-H.; Jia, W. L.; Wang, S.; Zhao, Y.-Q.; Lu, Z.-H. J. Appl.
Phys. 2008, 103, 034509. (b) Jia, W. L.; Feng, X. D.; Bai, D. R.; Lu, Z.
H.; Wang, S.; Vamvounis, G. Chem. Mater. 2005, 17, 164. (c) Jia, W.
L.; Moran, M. J.; Yuan, Y. Y.; Lu, Z. H.; Wang, S. J. Mater. Chem. 2005,
15, 3326. (d) Jia, W. L.; Bai, D. R.; McCormick, T.; Liu, Q. D.; Motala,
M.; Wang, R.; Seward, C.; Tao, Y.; Wang, S. Chem.Eur. J. 2004, 10,
994. (e) Doi, H.; Kinoshita, M.; Okumoto, K.; Shirota, Y. Chem. Mater.
2003, 15, 1080.
(15) Sole,
́
S.; Gabbaï, F. P. Chem. Commun. 2004, 40, 1284.
780
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