Stepwise formation of heterobimetallic macrocycles synthesized via the weak-link approach

Article abstract of DOI:10.1021/ja027936n

A new dissymmetric bis-hemilabile ligand with strong binding phosphorus moieties, weak binding thioether moieties, and weaker binding ether moieties has been synthesized to construct heterobimetallic Rh(I)- and Pd(II)-containing macrocycles. The metals are placed in either the phosphorus/thioether or the phosphorus/ether coordination pocket of the dissymmetric ligand by taking advantage of the stepwise synthetic control offered by the weak-link approach. The weak bonds of these isomeric intermediates are systematically broken through ligand substitution reactions to cleanly and selectively generate a variety of open, macrocyclic architectures. Copyright

Full text of DOI:10.1021/ja027936n

Published on Web 02/14/2003
Stepwise Formation of Heterobimetallic Macrocycles Synthesized via the
Weak-Link Approach
Adam H. Eisenberg, Maxim V. Ovchinnikov, and Chad A. Mirkin*
Department of Chemistry and the Center for Nanofabrication and Molecular Self-Assembly,
Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113
Received July 30, 2002 ; E-mail: camirkin@chem.northwestern.edu
Architecturally sophisticated metallomacrocycles are important
for their potential in a variety of molecular recognition-based
chemical applications, including chemical sensing, molecular
electronics, and catalysis.1 Several groups have employed a variety
of strategies, such as the “directional-bonding” or “symmetry-
interaction” approaches for building such structures.2 Our group
has developed a methodology now known as the weak-link
approach for the synthesis of homomultimetallic macrocycles from
elementary metal and ligand precursors.3-6 This approach has
further utilized flexible and symmetric hemilabile7 multidentate
ligands to yield metastable intermediates that contain both strong
Figure 1. 31P{1H} NMR spectra of compounds 1 (A), 2 (B), 3 (C), 4 (D),
and 5 (E). The spectrum of 3 is shown at -50 °C.
and weak bonds. The weak bonds in these condensed intermediates
are subsequently broken to create the desired supramolecular
complexes in very high yields, Scheme 1. In this manner, the
thermodynamic products generated when directly targeting the
flexible macrocyclic products are avoided.
Scheme 1
Using this approach, we have synthesized over 50 examples of
metallomacrocycles which vary in metal, hemilable ligand, and
ancillary ligand attached to the metal that make up the macrocycle.4,5
Herein, we report the synthesis of the first heterobimetallic
macrocycles prepared via this approach. These macrocycles incor-
porate a novel dissymmetric ligand that contains both ether and
thioether weak links. This allows one to exploit the difference in
lability between metal-ether and metal-thioether bonds to form
a variety of heterobimetallic architectures, Scheme 2. Because of
the relative differences in the binding strengths of these relatively
weak binding moieties toward late transition metals,6,7 heterobi-
metallic macrocyclic intermediates with metals placed selectively
in either coordination pocket can be synthesized through a judicious
choice of reaction conditions, Scheme 2. The weak bonds of these
intermediates can be sequentially broken through the appropriate
ligand substitution reactions to generate open, macrocyclic archi-
tectures 8 and 9, Scheme 2.
Scheme 2
The moderately air-stable dissymmetric ligand, 1, was synthe-
sized based, in part, on a modified literature procedure8 (see
Supporting Information for details). The 31P{1H} NMR spectrum
of 1 contains two singlets at -15.4 and -20.5 ppm assigned to the
two inequivalent phosphine moieties, Figure 1A. These two
resonances compare well with those for the related symmetric ether
or thioether ligands.4,6 Upon addition of 2 equiv of ligand 1 to 1
equiv of a Pd(II)9 or Rh(I)10 starting material, monometallic
intermediates 211 and 312 were synthesized in nearly quantitative
yields, respectively.13
The metals in these two intermediates selectively coordinate to
the thioether ends of 1 to yield structures that contain (1) strong
phosphorus-metal bonds, (2) relatively weak sulfur-metal bonds,
and (3) uncoordinated phosphine and ether moieties (2 and 3). The
31P{1H} NMR spectra of 2 and 3, Figure 1B and C, are consistent
with their proposed structures showing preferential coordination
of the thioether to the metal center. Notably, no mixed (thioether/
ether) products are observed. This preferred coordination environ-
ment can be attributed to the stronger binding affinity of the
thioether groups to either Rh(I) or Pd(II) than that for ether
Complexes 2 and 3 are stable with respect to
isomerization and decomposition when heated in CD2Cl2 solution
functionalities.14
under N2 at 60 °C for 3 days. When an additional equivalent of a
9
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J. AM. CHEM. SOC. 2003, 125, 2836-2837
10.1021/ja027936n CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Detailed X-ray structural data including a summary of crystallographic
parameters, select bond angles and distances, an ORTEP diagram, and
CIF file for 1. Correlation table between structural and spectroscopic
properties of selected Rh(I) and Pd(II) macrocycles (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
metal precursor is added to 2 or 3, the heterobimetallic condensed
intermediates 4 and 5 are formed in nearly quantitative yields
(>95%) and contain (1) two different metal atoms, (2) strong
metal-phosphorus bonds, (3) weak metal-thioether bonds, (4)
weaker metal-ether bonds, and (5) substitutionally labile metal
centers. The spectra indicate that there is a set of equivalent
phosphine moieties coordinated to each metal center. The 31P{1H}
NMR spectrum of complex 4 contains a singlet at 65.0 ppm, and
this singlet lies on top of a buried doublet, Figure 1D.15 Complex
5 displays a singlet at 65.4 ppm and a doublet at 67.3 ppm
(JP-Rh ) 161 Hz) in its 31P{1H} NMR spectrum, Figure 1E. The
31P{1H} NMR chemical shifts and coupling constants for all four
of these intermediates, 2-5, are consistent with their structural
formulations and compare well with those for the symmetric,
homobimetallic model complexes.4-6
The weak ether and thioether bonds can be selectively broken
through simple substitution chemistry. By varying the reaction
conditions and incoming ligands, we synthesized fully and partially
opened macrocycles (with respect to the coordination environment
around the metal center) with 0, +1, +2, or +3 charges. Two
examples are shown in Scheme 2. In the first example, a charged
macrocycle, 8, is synthesized by first adding excess acetonitrile in
the presence of CO (1 atm). These weak ligands selectively break
the weak metal-ether bond to yield the half-opened intermediate
6.16 The thioether bonds are then broken by adding 2 equiv of a
methanol solution of KCN to yield the fully opened macrocycle
8.17 Following a similar route to synthesize the neutral macrocycle
9, 2 equiv of a methanol solution of KCN are added to a methanol
suspension of 5. The cyanide selectively breaks the palladium-
ether bonds to yield 7 as a solid that can be dissolved in methylene
chloride.18 The remaining metal-thioether bonds are then broken
by adding excess [(CH3)4N]Cl and charging the resulting solution
with CO (1 atm) to yield the fully opened, neutral macrocycle 9.19
Both sets of reactions are nearly quantitative.13 The 31P{1H} NMR
chemical shifts and coupling constants of these complexes cor-
respond to the trans position of the phosphine ligands and compare
well with the analogous symmetric, homobimetallic complexes.4-6
In conclusion, by taking advantage of the stepwise synthetic
control offered by the weak-link approach, we have prepared
partially opened and structurally flexible, fully opened heterobi-
metallic macrocycles in high yields. The mechanistic distinction
offered by this method not only provides a synthetic route to
structures unobtainable via other, thermodynamically driven, ap-
proaches, but also substantially increases the scope of the types of
structures that can be targeted via the weak-link approach.
References
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(4) Farrell, J. R.; Eisenberg, A. H.; Mirkin, C. A.; Guzei, I. A.; Liable-Sands,
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(5) (a) Eisenberg, A. H.; Dixon, F. M.; Mirkin, C. A.; Stern, C. L.; Incarvito,
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(9) [Pd(NCCH3)4][BF4]2.
(10) Generated from [RhCl(COE)2]x via reaction with AgBF4 in CH2Cl2
(COE ) cyclooctene); see Supporting Information.
(11) 2: 31P{1H} NMR (CD2Cl2) δ 63.7 (s, Ph2PCH2CH2S), -21.5 (s, Ph2PCH2-
CH2O).
(12) 3: 31P{1H} NMR (CD2Cl2) δ 66.1 (d, JRh-P ) 162 Hz, Ph2PCH2CH2S),
-20.3 (s, Ph2PCH2CH2O).
(13) As determined by 31P{1H} NMR spectroscopy.
(14) Hutton, A. T. In ComprehensiVe Coordination Chemistry; Wilkinson, G.,
Ed.; Pergamon: New York, 1987; Vol. 5, pp 1131-1155.
(15) This doublet can be see upon opening the Pd side of the intermediate as
the prominent Pd-P resonance shifts upon addition of KCN.
(16) 6: 31P{1H} NMR (CD2Cl2) δ 63.2 (s, Ph2PCH2CH2S), 22.0 (d, Ph2PCH2-
CH2O, JRh-P ) 120 Hz).
(17) 8: 31P{1H} NMR (CD2Cl2) δ 17.9 (s, Ph2PCH2CH2S), 16.9 (d, Ph2PCH2-
CH2O, JRh-P ) 122 Hz).
Acknowledgment. C.A.M. acknowledges the NSF (CHE-
9625391) for support of this research and Charlotte L. Stern for
solving the structure of 1.
(18) 7: 31P{1H} NMR (CD2Cl2) δ 64.2 (d, Ph2PCH2CH2S, JRh-P ) 164 Hz),
17.3 (s, Ph2PCH2CH2O).
(19) 9: 31P{1H} NMR (CD2Cl2) δ 21.4 (d, Ph2PCH2CH2S, JRh-P ) 122 Hz),
16.6 (s, Ph2PCH2CH2O).
Supporting Information Available: Detailed experimental pro-
cedures and data for the syntheses of 1-9 and the precursors to 1.
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