Halide-induced supramolecular ligand rearrangement

Article abstract of DOI:10.1021/ja045316b

A novel reaction involving the halide-induced rearrangement of ligands within supramolecular Rh(I) complexes containing hemilabile ligands is presented. Three analogous bis- and trishemilabile ligands have been synthesized to construct bi- and trimetallic Rh(I) macrocyclic complexes. An intentionally added halide source results in the formal rotation of only one hemilabile ligand along the axis that is perpendicular to the plane defined by the aryl backbone of the hemilabile ligands. X-ray structures, as determined by X-ray crystallography, of key intermediates and products are presented. Copyright

Full text of DOI:10.1021/ja045316b

Published on Web 10/13/2004
Halide-Induced Supramolecular Ligand Rearrangement
Aaron M. Brown, Maxim V. Ovchinnikov, Charlotte L. Stern, and Chad A. Mirkin*
Department of Chemistry and the Institute for Nanotechnology, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois, 60201-3113
Received August 3, 2004; E-mail: camirkin@northwestern.edu
Scheme 1. Synthesis of Rearranged Macrocyclesa
There are now a variety of coordination chemistry approaches
that allow one to construct two- and three-dimensional supramo-
lecular structures using appropriate ligand design and coordination
chemistry principles.1 The weak-link approach (WLA) allows one
to rationally template the formation of flexible, supramolecular
macrocycles in nearly quantitative yields by utilizing appropriate
transition-metal precursors and hemilabile ligands.1c,2-4 Recently,
we reported the synthesis of ligand 1-(Ph2PCH2CH2O)-4-(Ph2PCH2-
CH2S)C6H4 (1a) and its ability to facilitate the formation of a variety
of heterobimetallic macrocycles.5 Through the course of our
investigation into the WLA, we have discovered a fascinating and
what appears to be unprecedented halide-induced rearrangement
of macrocycles containing Rh(I) cis-thioether/cis-phosphine and Rh-
(I) NBD/cis-phosphine (NBD is norbornadiene) coordination
environments (Scheme 1). An intentionally added halide or a halide
a Reagents and solvents: (i) CH2Cl2; (ii) CH2Cl2, 2 (3a and 3b) or 3
which acts as a counterion within a preformed complex (2a-c)
equiv (3c) of AgBF4; (iii) benzyltriethylammonium chloride.
will facilitate the conversion of such complexes to condensed
binuclear macrocyles 4a-c. This intramolecular process involves
the formal rotation of only one hemilabile ligand along the axis
that is perpendicular to the plane defined by the aryl backbone of
the hemilabile ligands. The reaction is general, high yielding, and
allows one to prepare structures not attainable via any other known
synthetic route, providing access to a series of heteroligated
macrocycles that potentially can be used as receptors in chemical
sensors and as flexible allosteric catalysts.3
Bimetallic Rh(I) macrocycles 4a and 4b, as well as trimetallic
macrocycle 4c, were synthesized in two-step procedures from the
corresponding hemilabile ligands, 1a, 4′-(2-Ph2PCH2CH2O)-4-(2-
Figure 1. ORTEP diagram of 3b (C40H36ClOP2RhS)2 with thermal
ellipsoids drawn to 30% probability. Hydrogen atoms and solvent molecules
have been omitted for clarity. Labeling and coloring scheme is as follows:
Rh (orange), P (green), O (red), S (yellow), and Cl (blue).
Ph2PCH2CH2S)biphenyl (1b), and (Ph2PCH2CH2O)(Ph2PCH2-
CH2S)2TPB (TPB is 1,3,5-triphenylbenzene) (1c), respectively, and
stoichiometric amounts of the Rh(I) precursor, [Rh(NBD)Cl]2.
Complexes 2a,b were prepared by treating 1 equiv of [Rh(NBD)-
Cl]2 with a CH2Cl2 solution containing 2 equiv of ligands 1a and
1b, respectively, at room temperature. Trimetallic complex 4c was
made in an analogous manner; however, 1.5 equiv of [Rh(NBD)-
Cl]2 to 2 equiv of ligand 1c were used. All three NBD-containing
evidence for the formation of these intermediate complexes was
obtained from a single-crystal X-ray analysis of complex 3b (Figure
1). The Rh(I) centers in 3b exhibit square planar geometries, with
Cl1-Rh1-P2 and Cl1-Rh1-S1 angles of 88.15(3) and 87.16-
(3)°, respectively. The cavity distance between the centroids defined
by the phenyl rings of each ligand is 6.225 Å, while the distance
between Rh1 and Rh2 is 13.659 Å. X-ray quality crystals of
complex 3a were grown in a similar fashion; however, refinement
efforts yielded a disordered structure (R1 ) 13%), which supports
chemical connectivity similar to that observed for 3b (Supporting
Information). Neutral intermediate complexes 3a and 3b are
sparingly soluble in common organic solvents (CH2Cl2, THF, and
1,2-dichlorobenzene) and could be in equilibrium with oligomeric
species via a (µ-Cl)2Rh2 motif. This makes monitoring the conver-
sion from 2a,b to intermediates 3a,b by NMR spectroscopy difficult
as the products precipitate during the reaction. On the other hand,
trimetallic complex 3c is soluble in common organic solvents;
therefore, it was possible to follow the rearrangement from 2c to
3c by 31P NMR spectroscopy in CD2Cl2. Phosphorus atoms in the
complexes 2a-c exhibit highly diagnostic resonances6 in their 31
P
{1H} NMR spectra in the δ 65.3-64.6 range (d, JRh-P ) 162 Hz),
corresponding to the Rh(I) centers with cis-thioether/cis-phosphine
coordination environments, and δ 13.5 (br m), corresponding to
the Rh(I) centers with cis-phosphine/NBD coordination environ-
ments. Elemental analysis, 1H NMR, and ES-MS are in full
agreement with the proposed formulations.
When Cl- is the counterion, compounds 2a-c slowly convert
to intermediate complexes 3a-c, where 1 equiv of ligand has
formally rotated and the Cl- ion has moved to the inner coordination
sphere (Scheme 1). These processes, which are rare examples of
molecular rotations within metallosupramolecular structures,7 have
been studied by 31P NMR spectroscopy under ambient conditions
in CD2Cl2, and corresponding half-lives have been measured (t1/2
) 1.5 h (2b), 4 h (2a), and 6 h (2c), 20mM [Rh(NBD)Cl]2). Further
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10.1021/ja045316b CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. ORTEP diagram of the product obtained from the reaction of
CO with 4a. Thermal ellipsoids are drawn to 30% probability. Hydrogen
atoms, solvent molecules, and counterions have been omitted for clarity.
Labeling and coloring scheme is as follows: Rh (orange), P (green), O
(red), and S (yellow).
the stronger Rh(I)-S bonds intact. This control is due to the lower
affinity of Rh(I) for the O donor as compared to that of the S donor.
For example, reactivity studies were conducted with the condensed
complex 4a. The weaker Rh-O bonds can be selectively cleaved
upon the addition of carbon monoxide (Figure 3) and nitriles,
leaving the stronger Rh-S links intact.
Figure 2. Rearrangement from complex 2c to 3c monitoring the 31P {1H}
NMR resonances of the Rh(I) cis-thioether/cis-phosphine coordination site.
The intensity of each spectrum has been normalized.
In conclusion, we have discovered an unusual, but general,
halide-induced ligand rearrangement in multimetallic Rh(I) mac-
rocycles containing hemilabile ligands. This approach provides
condensed macrocycles containing individually addressable metal
centers which will be useful as building blocks for more sophis-
ticated, heteroligated, three-dimensional architectures.
additional Rh(I) cis-thioether/cis-phosphine coordination environ-
ment that are present in both 2c and 3c are useful tags for
monitoring the rearrangement by 31P NMR spectroscopy (Figure
2). For example, complex 2c exhibits a 31P {1H} NMR signal at δ
65.3 (d, JRh-P ) 162 Hz), corresponding to the magnetically
equivalent P atoms that make up the two Rh(I) cis-thioether/cis-
phosphine coordination environments. As the complex is trans-
formed into 3c, this signal decreases in intensity with a concomitant
increase in intensity for the resonance associated with 3c (δ 64.7,
d, JRh-P ) 162 Hz).
Acknowledgment. This work was supported by the AFSOR
and the NSF.
Supporting Information Available: Detailed synthetic procedures,
characterization, ORTEP diagram of 3b, and CIF files for complexes
3a, 3b, and the product obtained from the reaction of CO with 4a (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Abstraction of the Cl- ions from 3a-c (3a,b suspensions in CH2-
Cl2, 3c soluble) with stoichiometric quantities of AgBF4 results in
the clean formation of cationic macrocycles 4a-c. Each of these
complexes exhibits similar characteristic 31P {1H} NMR resonances
at δ 74.1 (dd, JP-P ) 41 Hz, JRh-P ) 201 Hz), corresponding to
the Rh(I) phosphine/ether environment, and δ 51.0 (dd, JP-P ) 41
Hz, JRh-P ) 170 Hz), corresponding to the Rh(I) phosphine/
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The result of this rearrangement of the hemilabile ligand and
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the ability to selectively cleave the Rh(I)-O bonds while leaving
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