Triple-decker complexes formed via the weak link approach

Article abstract of DOI:10.1021/om0602310

Through the weak link approach and a halideinduced ligand rearrangement process, semi-open and condensed triple-decker complexes (TDCs) were prepared and fully characterized. These triple-decker structures with tailorable layers through choice of hemilabile ligand starting materials can be chemically opened and closed to expose the interior layer in a reversible fashion using small-molecule and elemental anion ligand substitution reactions.

Full text of DOI:10.1021/om0602310

Organometallics 2006, 25, 2729-2732
2729
Triple-Decker Complexes Formed via the Weak Link Approach
You-Moon Jeon, Jungseok Heo, Aaron M. Brown, and Chad A. Mirkin*
Department of Chemistry and the Institute for Nanotechnology, 2145 Sheridan Road,
EVanston, Illinois 60208-3113
ReceiVed March 13, 2006
Scheme 1. On/Off Regulation of the Central Functional
Group via a Blocking Mechanism
Summary: Through the weak link approach and a halide-
induced ligand rearrangement process, semi-open and con-
densed triple-decker complexes (TDCs) were prepared and fully
characterized. These triple-decker structures with tailorable
layers through choice of hemilabile ligand starting materials
can be chemically opened and closed to expose the interior layer
in a reVersible fashion using small-molecule and elemental
anion ligand substitution reactions.
Supramolecular cyclophanes and tweezer complexes have
received a significant amount of attention, due to their encap-
sulating properties and potential applications in catalysis,
sensing, mixture separation, molecular electronics, and facilitated
small-molecule transport.1,2 Recently, we have shown that one
can prepare mimics of allosteric enzymes by creating multi-
metallic structures with distinct and chemically addressable
shape-regulating and catalytic sites.3 These systems typically
rely on the use of macrocycles or tweezers with catalytic pockets
that can be turned on and off through the reaction of small
molecules or anions with sites that open or close the supra-
molecular entity. These systems all rely on catalysts that function
in a bimetallic fashion. An alternative strategy to realizing
allosteric enzyme mimics would be to create a reactive site that
is blocked above and below with chemically inert entities in
the form of a triple-decker complex with chemically displaceable
top and bottom layers. At present, there are no methods for
reliably synthesizing such structures. Herein, we describe an
approach to such complexes that utilizes the weak link approach
(WLA) for synthesizing supramolecular tweezer complexes2-5
and a halide-induced ligand rearrangement process6 that allows
one to realize triple-decker structures with chemically tailorable
layers through the choice of hemilabile ligand starting materials.
Importantly, these structures can be chemically opened and
closed to expose the interior layer in a reversible fashion using
small-molecule and elemental anion ligand substitution reactions
(Scheme 1). This work opens the door to synthesizing allosteric
structures with single sites that can be reversibly activated and
(5) Farrell, J. R.; Mirkin, C. A.; Liable-Sands, L. M.; Rheingold, A. L.
J. Am. Chem. Soc. 1998, 120, 11834.
(6) (a) Brown, A. M.; Ovchinnikov, M. V.; Mirkin, C. A. Angew. Chem.,
Int. Ed. 2005, 44, 4207. (b) Brown, A. M.; Ovchinnikov, M. V.; Stern, C.
L.; Mirkin, C. A. J. Am. Chem. Soc. 2004, 126, 14316. (c) 3a: 1,4-
(Ph2PCH2CH2S)2C6H4 (150 mg, 0.264 mmol), Ph2PCH2CH2O-2,3,5,6-
C6(CH3)4H (192 mg, 0.530 mmol), and [Rh(COD)Cl]2 (130 mg, 0.263
mmol) were dissolved in dichloromethane (20 mL) and stirred overnight at
room temperature. Solvent was evaporated at reduced pressure and sonicated
for 30 min in diethyl ether (10 mL). The yellow precipitate was isolated by
* To whom correspondence should be addressed. Fax: (1) 847-467-5123.
E-mail: chadnano@northwestern.edu.
1
(1) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100,
853. (b) Thanasekaran, P.; Liao, R. T.; Liu, Y. H.; Rajendran, T.; Rajagopal,
S.; Lu, K. L. Coord. Chem. ReV. 2005, 249, 1085. (c) Seidel, S. R.; Stang,
P. J. Acc. Chem. Res. 2002, 35, 972. (d) Fujita, M. Acc. Chem. Res. 1999,
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36, 621. (i) Kovbasyuk, L.; Kramer, R. Chem. ReV. 2004, 104, 3161. (j)
Klarner, F. G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919. (k) Slone, R.
V.; Benkstein, K. D.; Belanger, S.; Hupp, J. T.; Guzei, I. A.; Rheingold,
A. L. Coord. Chem. ReV. 1998, 171, 221. (l) Cacciapaglia, R.; Di Stefano,
S.; Mandolini, L. Acc. Chem. Res. 2004, 37, 113. (m) Pease, A. R.; Jeppesen,
J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res.
2001, 34, 433. (n) Amijs, C. H. M.; van Klink, G. P. M.; van Koten, G.
Dalton Trans. 2006, 308.
(2) (a) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40,
2022. (b) Gianneschi, N. C.; Masar, M. S.; Mirkin, C. A. Acc. Chem. Res.
2005, 38, 825.
(3) (a) Gianneschi, N. C.; Bertin, P. A.; Nguyen, S. T.; Mirkin, C. A.;
Zakharov, L. N.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 10508.
(b) Gianneschi, N. C.; Cho, S. H.; Nguyen, S. T.; Mirkin, C. A. Angew.
Chem., Int. Ed. 2004, 43, 5503. (c) Gianneschi, N. C.; Nguyen, S. T.; Mirkin,
C. A. J. Am. Chem. Soc. 2005, 127, 1644.
filtration and dried in Vacuo (325 mg, 77%). H NMR (CD2Cl2): δ 2.01
(s, 12H, Ar m-CH3), 2.18 (s, 12H, Ar o-CH3), 2.26 (m, 4H, -SCH2CH2-
PPh2), 2.50 (br t, 2H, -SCH2CH2PPh2), 2.58 (br t, 2H, -SCH2CH2PPh2),
2.73 (br m, 4H, -OCH2CH2PPh2), 4.00 (br m, 4H, -OCH2CH2PPh2), 6.74
(s, 2H, Ar H), 7.08-7.58 (m, 40H, -P(C6H5)2), 8.07 (s, 4H, -SC6H4S-).
31P{1H} NMR (CD2Cl2): δ 73.4 (dd, -SCH2CH2PPh2, JRh-P ) 183 Hz,
JP-P ) 41 Hz), 25.8 (dd, -OCH2CH2PPh2, JRh-P ) 167 Hz, JP-P ) 41
Hz). MS (ES, m/z): [M - Cl-]+ 1531.1 (calcd for [C82H86ClO2P4Rh2S2]+
1531.2), [M - 2Cl-]2+ 749.0 (calcd for [C82H86O2P4Rh2S2]2+ ) 748.1).
Anal. Calcd for C82H86Cl2O2P4Rh2S2‚1/2CH2Cl2: C, 59.71; H, 4.78.
Found: C, 60.08; H, 4.88. (d) 4a: to a dichloromethane solution (10 mL)
of complex 3a (45 mg, 0.028 mmol) was added LiB(C6F5)4‚Et2O (50 mg,
0.066 mmol) dropwise, and the mixture was stirred for 1 h at room
temperature. The solution was filtered through Celite. Solvent was
evaporated at reduced pressure and the residue was sonicated for 30 min in
dry benzene (10 mL). The yellow precipitate was isolated by filtration and
washed with benzene. The precipitate was redissolved in dichloromethane
and dried again in vacuo. Hexanes (10 mL) were added, and the system
was sonicated for 10 min to generate a fine powder, which was isolated by
filtration and dried under vacuum (52 mg, 64%). 1H NMR (CD2Cl2): δ
1.83 (s, 12H, Ar m-CH3), 1.94 (s, 12H, Ar o-CH3), 2.02-2.65 (m, 12H,
-SCH2CH2PPh2, -OCH2CH2PPh2), 3.68 (dt, 4H, -OCH2CH2PPh2), 6.50
(s, 2H, -C6(CH3)4H), 6.78 (s, 4H, -SC6H4S-), 7.27-7.45 (m, 40H,
-P(C6H5)2). 31P{1H} NMR (CD2Cl2): δ 72.7 (dd, -SCH2CH2PPh2, JRh-P
) 197 Hz, JP-P ) 42 Hz), 53.0 (dd, -OCH2CH2PPh2, JRh-P ) 170 Hz,
(4) (a) Farrell, J. R.; Mirkin, C. A.; Guzei, I. A.; Liable-Sands, L. M.;
Rheingold, A. L. Angew. Chem., Int. Ed. 1998, 37, 465. (b) Holliday, B.
J.; Farrell, J. R.; Mirkin, C. A.; Lam, K. C.; Rheingold, A. L. J. Am. Chem.
Soc. 1999, 121, 6316.
-
2+
JP-P ) 42 Hz). MS (ES, m/z): [M - 2B(C6F5)4
]
748.5 (calcd for
[C82H86O2P4S2Rh2]2+ 748.1). Anal. Calcd for C130H86B2F40O2P4Rh2S2‚
CH2Cl2: C, 53.51; H, 3.02. Found: C, 53.31; H, 2.79.
10.1021/om0602310 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/22/2006

2730 Organometallics, Vol. 25, No. 11, 2006
Scheme 2. Synthesis of Triple-Decker Complexes
Communications
deactivated through small-molecule reactions that assemble and
disassemble the novel trilayer structures.
reside above and below the plane of the central aromatic bridge,
most likely due to the steric limitations of having a cofacial
arrangement. The Rh-Rh distance is 9.31 Å.
The neutral heteroligated bimetallic semi-open triple-decker
complexes (TDCs) were synthesized according to the procedure
for preparing heteroligated monometallic Rh(I) tweezers, from
stoichiometric amounts of the corresponding bidentate and
monodentate hemilabile ligands Ph2P(CH2)2X(C6(R1)4)X-
(CH2)2PPh2 and Ph2P(CH2)2YC6H(R2)4 and [Rh(COD)Cl]2
(COD ) cyclooctadiene) in CH2Cl2 at room temperature
(Scheme 2).6
All spectroscopic data, including 1H NMR spectroscopy,
31P{1H} NMR spectroscopy, and ES-MS, are in full agreement
with the proposed formulations. For example, the 31P{1H} NMR
spectrum of 3a exhibits highly diagnostic resonances at δ 73.4
(dd, η2-PS, JRh-P ) 183 Hz, JP-P ) 41 Hz) and δ 25.8 (dd,
η1-PO, JRh-P ) 167 Hz, JP-P ) 41 Hz), indicative of the two
inequivalent phosphorus atoms in the semi-open Rh(I) com-
plex.6,7 The solid-state structure of 3a, determined by single-
crystal X-ray diffraction, is consistent with its proposed solution
structure (Figure 1).8 Each of the Rh(I) centers is coordinated
by η2-PS, η1-PO, and Cl- moieties in a distorted-square-planar
geometry with Cl(1)-Rh(1)-P(2) and Cl(1)-Rh(1)-S(1)
angles of 89.5 and 86.0°, respectively. This coordination
geometry is very similar to that of previously reported semi-
open Rh(I) macrocycles and tweezers.6 The two Rh(I) metals
The condensed cationic Rh(I) TDCs 4a-c were synthesized
by the abstraction of Cl- ions from the corresponding complexes
3a-c with a small excess of LiB(C6F5)4 in CH2Cl2 (Scheme
2).6 The remaining LiB(C6F5)4 was removed by washing with
dry benzene. All data for these compounds are consistent with
1
the proposed formulations as well. H NMR spectroscopy is
diagnostic of Cl- abstraction and the trilayer structure formation.
For example, the resonance due to the aryl protons of the central
arene and the resonance for the p-H atoms of the upper and
lower aromatic rings of the trilayer were shifted upfield by 1.29
and 0.24 ppm, respectively. These upfield shifts are diagnostic
of aromatic π-π stacking.9 Furthermore, a 2D NOESY experi-
ment of 4a in CD2Cl2 clearly shows NOE cross-peaks between
Ha and Hb (Ha ) protons of the central aromatic ring, Hb )
methyl protons of the upper/lower methylated aromatic rings)
(8) (a) Crystal data for 3a (CCDC-287009): C90H106Cl2O4P4Rh2S2,
monoclinic, space group P21/c, a ) 17.497(5) Å, b ) 14.530(4) Å, c )
16.851(5) Å, â ) 104.004(5)°, V ) 4157(2) Å3, Z ) 2, T ) 153(2) K,
2θmax ) 57.7°, Mo KR (λ ) 0.710 73 Å). R1 ) 0.0443 (for 6715 reflections
with I > 4σ(I)), wR2 ) 0.1018, and S ) 1.012 for 475 parameters and
9840 unique reflections. (b) Crystal data for 4a (CCDC-287010): C130H86B2-
F40O2P4Rh2S2, triclinic, space group P1h, a ) 12.6015(10) Å, b )
16.0906(13) Å, c )17.2636(14) Å, R ) 110.401(1)°, â ) 105.726(1)°, γ
) 102.030(1)°, V ) 2974.8(4) Å3, Z ) 1, T ) 153 K, 2θmax ) 57.84°, Mo
KR (λ ) 0.710 73 Å). R1 ) 0.0501 (for 9401 reflections with I > 2σ(I)),
wR2 ) 0.1106, and S ) 0.964 for 820 parameters and 14 003 unique
reflections.
(7) (a) Bonuzzi, C.; Bressan, M.; Morandini, F.; Morvillo, A. Inorg. Chim.
Acta 1988, 154, 41. (b) Lindner, E.; Wang, Q. Y.; Mayer, H. A.; Bader,
A.; Kuhbauch, H.; Wegner, P. Organometallics 1993, 12, 3291. (c) Teixidor,
F.; Benakki, R.; Vinas, C.; Kivekas, R.; Sillanpaa, R. Organometallics 1998,
17, 4630.
(9) Chang, K. J.; Kang, B. N.; Lee, M. H.; Jeong, K. S. J. Am. Chem.
Soc. 2005, 127, 12214.

Communications
Organometallics, Vol. 25, No. 11, 2006 2731
Figure 1. Stick representation for the crystal structure of 3a.
Hydrogen atoms and a solvent molecule (diethyl ether) have been
omitted for clarity. Selected bond distances (Å) and angles (deg):
Rh(1)-P(1) ) 2.1964(10), Rh(1)-P(2) ) 2.2527(9), Rh(1)-S(1)
) 2.3419(10), Rh(1)-Cl(1) ) 2.3945(9), Rh‚‚‚Rh ) 9.313(6);
P(1)-Rh(1)-P(2) ) 98.50(4), P(1)-Rh(1)-S(1) ) 86.46(4), P(2)-
Rh(1)-Cl(1) ) 89.49(4), S(1)-Rh(1)-Cl(1) ) 85.99(4), P(2)-
Rh(1)-S(1) ) 173.65(3), P(1)-Rh(1)-Cl(1) ) 170.26(3).
Figure 3. Stick representations for the crystal structure of 4a: (a)
side view; (b) top view. Hydrogen atoms and counteranions
(B(C6F5)4-) have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Rh(1)-P(1) ) 2.1814(9), Rh(1)-P(2) )
2.2356(8), Rh(1)-S(1) ) 2.3488(8), Rh(1)-O(1) ) 2.161(2),
Rh‚‚‚Rh ) 9.154(8); P(1)-Rh(1)-P(2) ) 98.66(3), P(1)-Rh(1)-
S(1) ) 86.46(3), P(2)-Rh(1)-O(1) ) 82.49(6), O(1)-Rh(1)-S(1)
) 92.40(6), P(1)-Rh(1)-O(1) ) 178.84(6), P(2)-Rh(1)-S(1) )
174.56(3).
amounts of tetrabutylammonium chloride in CD2Cl2, the cor-
responding semi-open complexes 3a-c were formed in quan-
titative yields, as evidenced by 1H and 31P{1H} NMR spectros-
copy.6,7
Figure 2. NOESY spectrum of 4a in CD2Cl2 (Ha ) the protons
of the central aromatic ring, Hb ) methyl protons of the upper/
lower methylated aromatic rings, Hc ) methylene protons R to
sulfur).
For 4a, the trilayer structure was confirmed by a single-crystal
X-ray diffraction study (Figure 3).8 As indicated by the solution
NMR data, each of the Rh(I) metal centers is coordinated by
η2-PS and η2-PO ligands in a distorted-square-planar geometry
with P(1)-Rh(1)-S(1) and P(2)-Rh(1)-O(1) angles of 86.5
and 82.5°, respectively. In addition, the intermetallic distance,
9.15 Å, is slightly shorter than that of the corresponding semi-
open Rh(I) TDC 3a, 9.31 Å. While the oxygen center has a
distorted-trigonal-planar geometry, the sum of the bond angles
around the coordinating oxygen is 358.7°, indicating that Rh(1),
O(1), C(18), and C(28) constitute a common plane. The
coordinating sulfur atom has a trigonal-pyramidal geometry with
C(2)-S(1)-Rh(1), C(2)-S(1)-C(4), and C(4)-S(1)-Rh(1)
angles of 110.5, 100.3, and 100.5° respectively. Therefore, in
the solid state, the three aromatic rings that make up the trilayer
are in a three-step configuration with a torsion angle of 60.9°.
This is clearly shown in a top view of 4a (Figure 3b). This is
in contrast with the solution structure of 4a, which indicates
fast rehybridization and coordination to the Rh centers resulting
in an average structure characterized by three symmetrical
stacking aromatic rings (e.g., the two methyl groups in the outer
and between Hb and Hc (Hc ) methylene protons R to sulfur),
showing that the three aromatic rings are stacked on top of one
another (Figure 2).10 Consistent with this conclusion, the
corresponding semi-open complex 3a, which does not have
closely spaced aromatic rings, does not show a similar NOE
correlation. The 31P{1H} NMR spectrum of complex 4a exhibits
two resonances at δ 72.7 (dd, JRh-P ) 197 Hz, JP-P ) 42 Hz)
and δ 53.0 (dd, JRh-P ) 170 Hz, JP-P ) 42 Hz), corresponding
to the P atom of the η2-PS ligand and the P atom of the η2-PO
chelating ligand, respectively.6,7
The reactions to form the trilayer structures are completely
reversible. Indeed, when 4a-c were treated with stoichiometric
(10) (a) Smurnyy, Y. D.; Elyashberg, M. E.; Blinov, K. A.; Lefebvre,
B. A.; Martin, G. E.; Williams, A. J. Tetrahedron 2005, 61, 9980. (b)
Neuhaus, D.; Williamson, M. P. The Nuclear OVerhauser Effect in Structural
and Conformational Analysis; Wiley: New York, 2000.

2732 Organometallics, Vol. 25, No. 11, 2006
Communications
aromatic rings are in an equivalent chemical environment, as
determined by NMR spectroscopy). This is a fast process where
the interconverting structure could not be observed by low-
temperature 1H NMR spectroscopy. Indeed, the signals for the
two methyl groups at -60 °C only broaden, with no definitive
identification of the two exchanging isomers. In the solid-state
structure of 4a, the two outer oxy-aromatic rings are parallel to
each other and are separated by ∼7.73 Å. The central aromatic
ring is equidistant to the other two rings and 16.7° from being
parallel planar with them. The average interplane distance in
4a is ∼3.86 Å, which is ∼0.34 Å shorter than that of the
coordination-mediated triple-decker Rh(I) metallacyclophanes,
∼4.20 Å,4 but slightly longer than those of the corresponding
condensed Rh(I) macrocycles, 3.51 and 3.59 Å, respectively.11
This work is important for the following reasons. First, it
demonstrates that one can construct triple-decker complexes via
the WLA in a few steps and in high yield in a chemically
reversible fashion. Second, the ability to systematically and
convergently introduce different functional groups into an
A-B-A triple-layered structure via coordination chemistry
provides an advantage over serial organic chemistry approaches
to analogous structures. Third, the metal centers and hemilabile
ligands in such structures allow one to chemically address the
metal sites in a way that allows one to controllably protect and
expose the central aromatic ring of the trilayer structure.
Therefore, this approach provides a blueprint for synthesizing
and evaluating a new class of trilayered structures with chemical
and physical (electron transfer and optical) properties that can
be selectively turned on and off through the reversible formation
of the trilayer unit. Note that it has been shown that one can
build hemilabile ligands with catalytic salen complexes in place
of the bridging aromatic ring used in these studies.
Acknowledgment. C.A.M. acknowledges the NSF and ARO
for support of this work. He is also grateful for a NIH Director’s
Pioneer Award.
Supporting Information Available: Text giving details of the
synthesis and characterization of the compounds prepared in this
paper and a CIF file giving crystallographic data for 3a and 4a.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) (a) Dixon, F. M.; Eisenberg, A. H.; Farrell, J. R.; Mirkin, C. A.;
Liable-Sands, L. M.; Rheingold, A. L. Inorg. Chem. 2000, 39, 3432. (b)
Ovchinnikov, M. V.; Brown, A. M.; Liu, X. G.; Mirkin, C. A.; Zakharov,
L. N.; Rheingold, A. L. Inorg. Chem. 2004, 43, 8233.
OM0602310