Heteroligated RhI tweezer complexes

Article abstract of DOI:10.1002/anie.200500689

(Chemical Equation Presented) A halide-induced ligand rearrangement around a Rh^I center can be used for the preparation of heteroligated tweezer-type complexes. By using a variety of P,S- and P,O-hemilabile ligands, the complexes can be interco

Full text of DOI:10.1002/anie.200500689

Angewandte
Chemie
Coordination Chemistry
Heteroligated RhI Tweezer Complexes**
Aaron M. Brown, Maxim V. Ovchinnikov, and
Chad A. Mirkin*
The unique combination of the coordination geometry of
transition-metal centers with the structural and geometric
properties of organic ligands provides the basis for the
synthesis and design of metallosupramolecular complexes[1]
and metal–organic frameworks.[2] Indeed, the ability to
incorporate multiple types of building blocks (namely, metal
centers and ligands) into metallosupramolecular structures is
a key requirement for the rational tailoring of their structural
and electronic properties. Currently, the impetus for this
research is shifting away from the synthesis of structurally
complex and aesthetically pleasing structures towards the
preparation of functional metallosupramolecular entities with
preprogrammed catalytic and molecular-sensing properties.[3]
We have previously demonstrated that tweezer-type salen-
containing (salen = N,N’-bis(salicylidene)cyclohexyldiamine)
complexes have improved catalytic rates and selectivities
relative to the corresponding bimetallic macrocyclic ana-
logues.[3b,c] These results motivated us to develop a general
synthesis of “heteroligated” tweezer-type complexes.
Recently, we reported a novel halide-induced ligand-
rearrangement process that was observed in multimetallic
RhI metallomacrocycles and which results in the formation of
two- and three-dimensional complexes that contain hetero-
ligated coordination environments.[4] Herein, we report a
general high-yielding methodology for the preparation and
postsynthetic modification of heteroligated monometallic
RhI tweezer-type complexes (Scheme 1). This methodology
is based on a unique reaction that allows the preparation of
condensed complexes 4b–f which contain two distinct phos-
phine-substituted ligands or heteroligated systems that can
undergo stepwise substitution reactions with ClÀ ions and/or
CO to result in four unique geometries with respect to the
orientation of each S- or O-appended aromatic group.
The neutral heteroligated “semiopen” tweezer complexes
3a–f were synthesized in one step from stoichiometric
amounts of the corresponding hemilabile ligands Ph2P-
(CH2)2SAr (Ar= aryl), Ph2P(CH2)2XC6H5 (X = O, CH2),
and [{RhCl(nbd)}2] (nbd = norbornadiene) in CH2Cl2. Ini-
tially, the kinetic products 1a–f, which have cis-thioether and
cis-phosphine ligands around a RhI center, and 2a–f, which
have nbd and cis-phosphine ligands around a RhI center, are
[*] A. M. Brown, M. V. Ovchinnikov, Prof. C. A. Mirkin
Department of Chemistry and the Institute for Nanotechnology
2145 Sheridan Road
Evanston, IL 60208-3113 (USA)
Fax: (1)847-467-5123
E-mail: chadnano@northwestern.edu
[**] C.A.M. acknowledges the NSF and AFOSR for support of this work.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 4207 –4209
DOI: 10.1002/anie.200500689
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4207

Communications
Figure 1. ORTEP diagram of 3e (C42H42ClOP2RhS) with thermal
ellipsoids set at 50% probability. Hydrogen atoms and solvent
molecules have been omitted for clarity.[6]
consistent with the proposed formulations. For example,
complex 4b exhibits highly diagnostic resonances in its
31P{1H} NMR spectrum at d = 73.5 ppm (dd, J(Rh-P) =
200 Hz, J(P-P) = 41 Hz) and d = 51.8 ppm (dd, J(Rh-P) =
168 Hz, J(P-P) = 41 Hz), which are characteristic of P centers
in a coordination environment in which a RhI center has cis-
phosphine, cis-thioether, and ether ligands. Furthermore, a
signal at m/z 731.2 is observed in its ESIMS spectra that
corresponds to a [C40H38P2S2Rh]+ ion. Complexes 4b–f react
Scheme 1. Synthesis of the heteroligated RhI tweezer complexes 3a–f,
and the stepwise synthesis of closed complexes 4b–f, neutral cis semi-
open complexes 3a–f, cationic trans semiopen complexes 5a–f, and
À
open tweezer complexes 6a–f. BArF=B[3,5-(CF3)2C6H3]4
with
stoichiometric
quantities
of
benzyltriethyl
formed, but convert over 18 h into the semiopen neutral
ammonium chloride in CH2Cl2 to regenerate the correspond-
ing chloride-ligated semiopen complexes 3b–f. Abstraction of
the ClÀ ion from 3a results in a mixture of products because
there is no O atom to which the RhI center can coordinate.
However, the addition of one equivalent of benzyltriethy-
lammonium chloride in solution or excess CO to 4a in CH2Cl2
results in the formation of complexes 3a or 5a, respectively.
The addition of CO to complexes 4b–f selectively cleaves
1
complexes 3a–f (Scheme 1). Elemental analysis, H, 19F, and
31P NMR spectroscopic, and ESIMS data are in full agree-
ment with the proposed formulations. For example, complex
3b exhibits highly diagnostic resonances in its 31P{1H} NMR
spectrum at d = 73.61 ppm (dd, J(Rh-P) = 185 Hz, J(P-P) =
41 Hz), which corresponds to the P center of the h2-P,S
chelating ligand, and d = 30.1 ppm (dd, J(Rh-P) = 168 Hz,
J(P-P) = 41 Hz), which corresponds to the P center of the h1-
P,O ligand.[4,5] Further evidence for the formation of these
neutral complexes was obtained from a single-crystal X-ray
study of complex 3e (Figure 1).[6] The Rhi center in 3e
exhibits a square-planar geometry with Cl1-Rh1-P2 and Cl1-
Rh1-S1 angles of 87.8 and 86.88, respectively.
À
the Rh O bond and results in the formation of cationic
semiopen tweezer-type complexes 5b–f, in which the P atoms
are trans to one another. Again, all the data collected are
consistent with the proposed formulations. For example,
semiopen complex 5b exhibits resonances in its 31P{1H} NMR
spectrum at d = 63.1 ppm (dd, J(Rh-P) = 116 Hz, J(P-P) =
268 Hz), which corresponds to a P center coordinated to a
RhI center with phosphine and thioether ligands, and d =
19.2 ppm (dd, J(Rh-P) = 121 Hz, J(P-P) = 268 Hz), which
corresponds to a P center coordinated to a Rhi center with
phosphine and CO ligands. Alternatively, complexes 5a–f
may also be synthesized from complexes 6a–f and stoichio-
metric quantities of NaBArF in diethyl ether. In this case, the
lack of an O atom in 6a does not affect the abstraction of the
ClÀ ion and results in the clean formation of 5a.
The rate of these conversions was also studied by
31P NMR spectroscopy at 258C in CD2Cl2, and the corre-
sponding half-lives of the reactions were measured (3a: t1/2
=
180 min, 3b: 95 min, 3c: 65 min, 3d: 40 min, 3e: 152 min, 3 f:
95 min; 20 mm [{RhCl(nbd)}2]). These rates appear to be
inversely proportional to the electron density of the aryl
groups tethered to the S atom: As the electron density of the
À
aryl subsitutent in the P,S ligand decreases, the Rh S bonds in
1 weaken and the rate of the formation of 3 increases. This
trend in the reaction rates reflects the increase in the strength
The neutral open complexes 6a–f are synthesized from
5a–f and stoichiometric quantities of benzyltriethylammo-
nium chloride in CH2Cl2. Alternatively, the addition of CO to
À
of the Rh S bond as a function of the increase in electron
density of the aromatic group appended to the S atom.
The reversible abstraction of ClÀ ions from 3b–f with one
equivalent of NaBArF results in the clean formation of
condensed cationic complexes 4b–f in which the RhI center
coordinates to the O atom. All the data obtained are
À
complexes 3a–f cleaves the Rh S bonds and results in the
fully open tweezers 6a–f in which the phosphines are trans to
one another, thus resulting in further separation of the
phosphine substituents. Each of these complexes exhibits
4208
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Angewandte
Chemie
overlapping doublets of doublets in their 31P{1H} NMR
spectra at approximately d = 24 ppm, which corresponds to
two inequivalent phosphines.
[4] A. M. Brown, M. V. Ovchinnikov, C. L. Stern, C. A. Mirkin, J.
Am. Chem. Soc. 2004, 126, 14316 – 14317.
[5] C. Bonuzzi, M. Bressan, F. Morandini, A. Morvillo, Inorg. Chim.
Acta 1988, 154, 41 – 43.
The ClÀ ion is an integral component in these trans-
formations. For example, the independently synthesized
intermediate RhI complexes 1b and 2b were stirred together
for 18 hours and the semiopen complex 3b was formed.
[6] Crystal data for 3e: C42H42ClOP2RhS: Monoclinic space group
P21/c, a = 9.7234(9), b = 18.5595(2), c = 21.697(2) ꢁ, a = 90.00,
b = 102.686(2), g = 908, V= 3819.9(6) ꢁ3, Z = 4, T= 153 K,
2qmax = 57.848, MoKal = 0.71073 ꢁ, R1 = 0.0318 (for 8061 reflec-
tions with I > 2s(I)) wR2 = 0.0787 and S = 1.070 for 434 param-
eters and 9263 unique reflections. Further details on the crystal
structure investigations may be obtained from the Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Ger-
many (fax: (+ 49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.
de) on quoting the depository number CSD-415095.
À
However, this reaction was repeated with the BF4 salt
instead of the chloride salt and resulted in no ligand
rearrangement, as determined by 31P NMR spectroscopy.
The interconversion between complexes 3–6 allows the
spatial separation between the aryl groups and the overall
flexibility of these complexes to be systematically controlled
(Scheme 1). The synthetic approach described herein is
amenable to the inclusion of a variety of aromatic groups,
including free-base salen functionalities, as demonstrated in
3 f–6 f. With this approach, we now have the ability to
synthesize designer catalytic intramolecular systems, which
could be used to incorporate a catalytic functionality into one
arm of a tweezer and add a separate cocatalyst, activator, or
enantioselectivity-influencing agent on the other arm, with a
precise control over spatial separation and flexibility. These
types of structures are becoming important as allosteric
catalysts[3b,c] and sensors,[3a] and the ability to modulate their
properties through this type of chemistry will help researchers
to finetune the reactivity of these complexes.
Received: February 23, 2005
Published online: June 7, 2005
Keywords: coordination modes · nanostructures ·
.
supramolecular chemistry · synthetic methods · transition
metals
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