Reversible CO-induced chloride shuttling in RhI tweezer complexes containing urea-functionalized hemilabile ligands

Article abstract of DOI:10.1021/ic8008909

The urea moiety, which acts as a good hydrogen-bond donor, has been incorporated into a hemilabile phosphinoalkyl thioether ligand. Upon reaction of the ligand with a Rh^I precursor, a tweezer complex with near-parallel planar urea moieties 2 forms. The host-guest interaction of 2 with Cl^- has been characterized in solution and in the solid state. Cl^- binding with the urea groups in 2 is retained under CO in nonpolar solvents to give a five-coordinate CO adduct 3. In polar solvents, CO binding to Rh^I results in a Cl^- shift from the urea host site to the Rh^I metal center with a concomitant breaking of the Rh-S bonds. This is an unusual example of how two types of different interactions important in molecular recognition (ligand coordination to a metal and hydrogen bonding) can be regulated within one molecule through small-molecule coordination chemistry.

Full text of DOI:10.1021/ic8008909

Inorg. Chem. 2008, 47, 9727-9729
Reversible CO-Induced Chloride Shuttling in RhI Tweezer Complexes
Containing Urea-Functionalized Hemilabile Ligands
Hyojong Yoo,† Chad A. Mirkin,*,† Antonio G. DiPasquale,‡ Arnold L. Rheingold,‡
and Charlotte L. Stern†
Department of Chemistry and the International Institute for Nanotechnology, Northwestern
UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113, and Department of Chemistry and
Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe, MC 0358,
La Jolla, California 92093-0358
Received May 15, 2008
The urea moiety, which acts as a good hydrogen-bond donor,
has been incorporated into a hemilabile phosphinoalkyl thioether
ligand. Upon reaction of the ligand with a RhI precursor, a tweezer
complex with near-parallel planar urea moieties 2 forms. The
host-guest interaction of 2 with Cl- has been characterized in
solution and in the solid state. Cl- binding with the urea groups in
2 is retained under CO in nonpolar solvents to give a five-coordinate
CO adduct 3. In polar solvents, CO binding to RhI results in a Cl-
shift from the urea host site to the RhI metal center with a
concomitant breaking of the Rh-S bonds. This is an unusual
example of how two types of different interactions important in
molecular recognition (ligand coordination to a metal and hydrogen
bonding) can be regulated within one molecule through small-
molecule coordination chemistry.
binding group of the hemilabile ligand at metal hinge sites.
This approach and these structures have allowed us to design
a wide variety of molecules with stoichiometric and catalytic
chemistries that can be turned on and off or up and down
using the concept of allosteric regulation.3-5
Urea is a common motif used in molecular recognition
chemistry, and it has an especially high affinity for anions
under the appropriate conditions.6 We hypothesized that the
WLA could be used to assemble hemilabile ligands based
upon urea groups and phosphines into tweezer complexes
that would position the urea moieties to cooperatively
recognize anionic analytes such as Cl-. Moreover, we
thought the chemistry that occurs at the metal hinge in these
complexes could be used to subsequently alter the pocket
and perhaps the chemistry that occurs between the urea-based
host and Cl-. We have designed one such ligand, 1, which
when reacted with a RhI precursor forms the desired tweezer
complex 2 (eq 1). This complex indeed binds Cl- through
hydrogen bonding with the urea moieties and undergoes a
Two general approaches have been used to develop
recognition properties in supramolecular complexes: those
that involve cumulative weak interactions such as hydrogen
bonding and others that involve coordination bonds.1,2 Many
novel metallosupramolecular complexes have now been
prepared via the weak-link approach (WLA).3-5 These
structures are based upon hemilabile ligands and allow one
to build macrocycle,3 triple decker,4 and tweezer-based
complexes5 with chemistry that can be regulated based upon
structural changes induced by the displacement of the weak
(2) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853.
(b) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483. (c)
Kovbasyuk, L.; Kra¨mer, R. Chem. ReV. 2004, 104, 3161. (d) Seidel,
S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (e) Fujita, M. Acc.
Chem. Res. 1999, 32, 53. (f) Holliday, B. J.; Mirkin, C. A. Angew.
Chem., Int. Ed. 2001, 40, 2022. (g) Cotton, F. A.; Lin, C.; Murillo,
C. A. Acc. Chem. Res. 2001, 34, 759. (h) Thanasekaran, P.; Liao, R.-
T.; Liu, Y.-H.; Rajendran, T.; Rajagopal, S.; Lu, K.-L. Coord. Chem.
ReV. 2005, 249, 1085.
(3) (a) Gianneschi, N. C.; Masar, M. S., III; Mirkin, C. A. Acc. Chem.
Res. 2005, 38, 825, and references cited therein. (b) Oliveri, C. G.;
Gianneschi, N. C.; Nguyen, S. T.; Mirkin, C. A.; Stern, C. L.; Wawrzak,
Z.; Pink, M. J. Am. Chem. Soc. 2006, 128, 16286. (c) Kuwabara, J.;
Stern, C. L.; Mirkin, C. A. J. Am. Chem. Soc. 2007, 129, 10074. (d)
Yoon, H. J.; Heo, J.; Mirkin, C. A. J. Am. Chem. Soc. 2007, 129, 14182.
(4) Jeon, Y.-M.; Heo, J.; Brown, A. M.; Mirkin, C. A. Organometallics
2006, 25, 2729.
(5) (a) Brown, A. M.; Ovchinnikov, M. V.; Mirkin, C. A. Angew. Chem.,
Int. Ed. 2005, 44, 4207. (b) Gianneschi, N. C.; Cho, S.-H.; Nguyen,
S. T.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 5503.
(6) (a) Gale, P. A. Encyclopedia of Supramolecular Chemistry; Marcel
Dekker: New York, 2004; pp 31-41. (b) Amendola, V.; Esteban-
Go´mez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res. 2006, 39, 343.
(c) Hay, B. P.; Firman, T. K.; Moyer, B. A. J. Am. Chem. Soc. 2005,
127, 1810. (d) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001,
40, 486,and references cited therein.
* To whom correspondence should be addressed. E-mail: chadnano@
northwestern.edu.
†
Northwestern University.
University of California, San Diego.
‡
(1) (a) Lehn, J.-M. Supramolecular Chemistry, Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995. (b) Knight, L. K.; Freixa, Z.; van
Leeuwen, P. W. N. M.; Reek, J. N. H. Organometallics 2006, 25, 954.
(c) Duckmanton, P. A.; Blake, A. J.; Love, J. B. Inorg. Chem. 2005,
44, 7708. (d) Sandee, A. J.; van der Burg, A. M.; Reek, J. N. H. Chem.
Commun. 2007, 864. (e) Lu, X.-X.; Tang, H.-S.; Ko, C.-C.; Wong,
J. K.-Y.; Zhu, N.; Yam, V. W.-W. Chem. Commun. 2005, 1572. (f)
Breit, B.; Seiche, W. Angew. Chem., Int. Ed. 2005, 44, 1640. (g) Coxall,
R. A.; Lindoy, L. F.; Miller, H. A.; Parkin, A.; Parsons, S.; Tasker,
P. A.; White, D. J. Dalton Trans. 2003, 55. (h) Bondy, C. R.; Gale,
P. A.; Loeb, S. J. Chem. Commun. 2001, 729.
10.1021/ic8008909 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 21, 2008 9727
Published on Web 10/04/2008

COMMUNICATION
novel solvent-assisted CO-induced intramolecular shuttling
of the Cl- guest between the urea tweezer pocket and the
Rh hinge in a reversible fashion (eq 2). Although the
irreversible migration of Cl- assisted by CO in a transition-
metal complex with urea groups has been reported by other
research groups,1b two of the most interesting aspects of the
system that we are reporting are the reversibility and the
corresponding ability to regulate the position of the two urea
groups (and the pendant R groups) through small-molecule
coordination chemistry within one complex.
Figure 1. Binding of Cl- in complex 2·BF4 monitored by 1H NMR
spectroscopy (CD2Cl2): (A) a solution of 2·BF4; (B) a solution of 2·BF4
with 0.5 equiv of nBu4NCl; (C) a solution of 2 · BF4 with 5.0 equiv of
nBu4NCl.
The urea-based hemilabile ligand, 1 (PS ligand), was
synthesized by the reaction of 4-aminothiophenol with
1-isocyanato-3,5-bis(trifluoromethyl)benzene to generate the
urea functionality, followed by the reaction with
PPh2(CH2)2Cl over K2CO3. The electron-withdrawing 3,5-
bis(trifluoromethyl)benzyl group was used to enhance the
acidity of the urea groups, thus generating stronger hydrogen
bonds with anions, a strategy utilized by others.6b The
tweezer RhI complexes with Cl- and BF4- counterions, 2·Cl
and 2 · BF4, respectively, were synthesized from stoichio-
metric amounts of the ligand 1 and the RhI precursors
([Rh(cod)Cl]2 or [Rh(cod)2]BF4 (cod ) 1,5-cyclooctadiene))
in CH2Cl2 at room temperature (eq 1).
n
Figure 2. Job plot from the addition of Bu4NCl to the complex 2·BF4.
Complexes 2·Cl and 2·BF4 were characterized by multi-
nuclear NMR and FT-IR spectroscopies, ESI-MS, and
elemental analysis. The 1H and 31P{1H} NMR spectra of 2·Cl
are almost identical with analogous spectra for 2·BF4, except
N-H resonances assigned to the urea moiety in 2·Cl [δ 9.85
(Ha) and 9.36 (Hb)], which are significantly downfield from
those observed for 2 · BF4 [δ 8.04 (Ha) and 7.83 (Hb)]. This
observation is consistent with the conclusion that Cl- is
bound to the urea groups in solution. The sequestration of
Figure 3. X-ray crystal structure of complex 2·Cl with thermal ellipsoids
set at 50% probability. For clarity, all H atoms except those on the urea N
atoms are omitted. Dotted lines indicate the intermolecular hydrogen-
bonding interactions between the Cl atoms and the urea H atoms. Symmetry
code: ′ ) 1 - x, 1 - y, -z.
1
Cl- by 2 · BF4 can be monitored by H NMR spectroscopy,
by titrating a CD2Cl2 solution of 2 · BF4 with tetrabutylam-
monium chloride (nBu4NCl), which results in a gradual
downfield shift of the urea N-H resonances as a function
of the Cl- concentration (Figure 1). The 31P{1H} NMR
spectra of these solutions did not significantly change (δ 65,
d, JRh-P ) 163 Hz in CD2Cl2) during the titration. A Job
plot was also used to characterize the host-guest interactions
and confirms a 1:1 stoichiometery between the coordination
complex host and the Cl- guest anion (Figure 2). Finally,
gel-permeation chromatography (GPC) of 2·Cl is consistent
with the molecular weight of a monomer rather than an
oligomer (Supporting Information). This is important because
the structure forms a dimer in the solid state (vide infra).
Single crystals of 2 were grown in two different ways: by
slow evaporation of a CH2Cl2 solution saturated with 2 and by
slow diffusion of hexane into such a solution. The two methods
yielded two different crystals, 2·Cl and 2·BF4, respectively.
Both structures were determined by single-crystal X-ray dif-
fraction studies, which show dimers held together by either Cl-
(2·Cl) or BF4- (2·BF4) (Supporting Information). The RhI
centers in both complexes are coordinated by two κ2-PS ligands,
and each shows a Rh center with a four-coordinate, distorted
square-planar geometry with a trans arrangement of phosphines
and thioether ligands. This geometry and ligand coordination
9728 Inorganic Chemistry, Vol. 47, No. 21, 2008

COMMUNICATION
a
Table 1. 31P{1H} NMR Resonances for 3·BF4 as a Function of Added
Scheme 1. Systematic Structural Changes of 2·Cl That Accompany
a
Cl-
the Addition and Removal of CO/Cl-
Cl-(equiv)
δ (ppm)b
a
0
0.5
1
2
3
4
5
10
42.7 42.6 42.3 38.6 36.8 30.8 28.9 26.7
31P{1H} NMR spectra were taken at 20 °C in CD2Cl2 (external ref
85% H3PO4). b Doublets (JRh-P ) 106-120 Hz).
mode are very similar to those observed for previously reported
RhI macrocycles and tweezers.3,5
In 2·Cl, the urea moieties are approximately parallel to each
other and form hydrogen bonds to the Cl- counterions. In the
solid state, Cl- does not reside within an intramolecular bis-
urea pocket. Instead, the complex forms a 2:2 structure with
two pairs of ureas from two different RhI complexes creating
intermolecular pockets, each with one Cl-. The average distance
between the Cl- and H atoms of urea is 2.55 Å. It is interesting
to note that intermolecular hydrogen bonding with Cl- is more
favorable in the solid state than the intramolecular hydrogen
bonding observed in solution. The solid-state structure of 2·BF4
also shows intermolecular hydrogen bonding between urea
groups and BF4- (Supporting Information).
The addition of CO (1 atm) to a CD2Cl2 solution of 2 · Cl
and 2 · BF4 at room temperature leads to the formation of
five-coordinate CO adducts, 3 · Cl and 3 · BF4, which have
1
been characterized by H NMR, 31P{1H} NMR, and FT-IR
spectroscopy. The 31P{1H} NMR spectra show doublets at
δ 42.3 (3 · Cl, JRh-P ) 107 Hz) and δ 42.7 (3 · BF4, JRh-P
)
a
Hydrogen bonding between the urea functionality and Cl- is denoted
107 Hz) in CD2Cl2, which allow us to assign these complexes
as five-coordinate CO adducts, based upon a comparison to
literature values.7 Upon the addition of successively increas-
with a dotted line.
in quantitative yield. The solvent dependence of the Cl-
shuttling reaction is a result of the competition of the polar
solvent with Cl- for the urea pocket because DMSO and DMF,
compared to CH2Cl2, are good hydrogen-bond acceptors.
In conclusion, we have synthesized new coordination RhI
tweezer complexes with near-parallel urea moieties, 2 · Cl
and 2 · BF4. These compounds complex anions (Cl- and
n
ing amounts of Bu4NCl to a CD2Cl2 solution of 3 · BF4 (or
3 · Cl), the 31P{1H} NMR resonances gradually shift upfield
(Table 1). This shift is consistent with a change in the
coordination environment, due to the cleavage of the Rh-S
bonds by coordination of Cl- and generation of the four-
coordinate fully open complex 4 · Cl (Scheme 1).
In more polar solvents such as dimethyl sulfoxide (DMSO)
or N,N-dimethylformamide (DMF), a slightly different reac-
tion occurs. CO and Cl- coordinate to the Rh center in 2,
forming a four-coordinate adduct, 4, but a Cl- anion does
not reside in the urea pocket as in 4 · Cl (Scheme 1). This
Cl--free urea pocket is evidenced by 1H and 31P{1H} NMR
-
BF4 ) in the pockets formed by the urea moieties. CO
binding to the Rh atom can be used as a switch to effect the
intramolecular shuttling of the Cl moiety from the urea
pocket to the Rh center in a somewhat solvent-dependent
manner. These types of processes may be useful in regulating
the reactivity of supramolecular complexes where small
molecules can be used to induce structural rearrangements
that lead to the conversion of the complexes from inactive
to active states in the context of both stoichiometric and
catalytic reactions.
1
spectroscopy. Indeed, significant upfield H NMR shifts of
the N-H resonances in the urea moiety are observed (δ 9.83
and 9.48 for 2 · Cl in DMSO-d6; δ 9.57 and 9.19 for 4 in
DMSO-d6), indicating a Cl--free urea pocket. The 31P{1H}
NMR spectra of 4 in DMSO-d6 and DMF-d7 at room
temperature show broad resonances at δ 32.5 and 30.0,
respectively, which are far upfield compared to the analogous
resonances for 2·Cl and 3·BF4.8 When CO is removed under
vacuum, the open complex 4 reforms the closed complex 2·Cl
Acknowledgment. H.Y. acknowledges Dee Dee Smith for
her help with the GPC data. C.A.M. acknowledges the AFOSR
(DDRE-MURI), ARO, and NSF for support of this work.
Supporting Information Available: Details of experimental
procedures, synthesis and characterization of each compound,
additional key NMR spectra, Job plot, binding constant measure-
ments, GPC traces, and CIF files giving crystallographic data for
2·Cl and 2·BF4. This material is available free of charge via the
Internet at http://pubs.acs.org.
(7) Kourkine, I. V.; Slone, C. S.; Mirkin, C. A.; Liable-Sands, L. M.;
Rheingold, A. L. Inorg. Chem. 1999, 38, 2758. Unlike model complexes
studied previously, the isomers with a square-pyramidal coordination
geometry for 3 are favored based upon variable-temperature NMR
assignments. See the Supporting Information.
(8) The 31P{1H} NMR resonances for 3·BF4 in CD2Cl2, DMSO-d6, and
DMF-d7 [under a CO atmosphere (1 atm)] are δ 42.7 (d, JRh-P ) 108
Hz), 49.9 (d, JRh-P ) 105 Hz), and 49.1 (d, JRh-P ) 108 Hz),
respectively.
IC8008909
Inorganic Chemistry, Vol. 47, No. 21, 2008 9729