"Giant" gyroscope-like molecules consisting of dipolar Cl-Rh-CO rotators encased in three-spoke stators that define 25-27-membered macrocycles

Article abstract of DOI:10.1002/anie.200601191

(Chemical Equation Presented) Yet another feat for the Crubbs catalyst: Reactions with 1a,b followed by hydrogenations give 2a,b. The sizeable cavities of 2, which were characterized crystallographically, allow for rapid Cl-Rh-CO rotation in solution. Reactions with ArC≡CLi give ArC≡C-Rh-CO species; when the aryl group is sufficiently large, it is confined between two macrocycles, blocking rotation.

Full text of DOI:10.1002/anie.200601191

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rotation of the {MLm} moietyis rapid on the NMR time scale
at room temperature. Although the iron complexes could be
converted into cationic [Fe(CO)2(NO)]+ and [HFe(CO)3]+
analogues, which feature dipolar rotators, the accompanying
anions are undesirable: theyare capable of dipole–dipole
interactions with the rotator, which can increase the rota-
tional energybarriers.
Another challenge associated with such molecules is the
construction of larger stators that would accommodate
extended rotators. Thus, we sought in this studyto 1) probe
whether significantlylonger bridges might be accessible by
alkene metathesis, 2) introduce more rigid architectural units
that would enforce a minimum dimensionality, 3) apply this
chemistryto neutral complexes with dipolar rotors, and
4) map the effective size of the stator through ligand
substitution and NMR spectroscopyexperiments.
DOI: 10.1002/anie.200601191
“Giant” Gyroscope-Like Molecules Consisting of
Dipolar Cl-Rh-CO Rotators Encased in Three-
Spoke Stators That Define 25–27-Membered
Macrocycles**
Leyong Wang, Frank Hampel, and John A. Gladysz*
There is intense current interest in molecular rotors,[1] which
are comprised of a “stator”, a “rotator”, and a connecting
axis. One important objective is the synthesis of assemblies in
which the rotators are partially“enclosed” or sterically
shielded from their environments.[1–6] This approach follows
byanalogyto macroscopic machines in which rotating
components are often for safetyor mechanical reasons
encased in protective housings. Two prominent classes of
molecular rotors are compasses and gyroscopes.[3,4] When
rotators feature dipole moments, theycan be aligned in static
Pursuant the first two objectives, alkene-containing triar-
ylphosphine ligands were sought. Accordingly, 4-bromophe-
=
nol, the a,w-bromoalkenes Br(CH2)nCH CH2 (a n = 4, b n =
5, c n = 6), and the base K2CO3 (1:1:1.5) were combined in
DMF at 558C. Workup gave the corresponding ethers p-
=
BrC6H4O(CH2)nCH CH2 (1a–c) in 92–95% yields. As shown
electric fields, similar to compasses, or forced to turn
in Scheme 1, Grignard reagents were subsequentlygenerated,
and reactions with PCl3 gave the phosphines P{p-C6H4O-
[1]
unidirectionallyin rotating electric fields,
similar to gyro-
=
scopes. In all cases, it is desirable that rotation be as
“frictionless” as possible, without interference from nearby
molecules or ions.
(CH2)nCH CH2}3 (2a–c) in 84–92% yields. The new com-
pounds 1–2 and all the others below were characterized by
microanalyses and the usual spectroscopic methods, as
summarized in the Supporting Information.[7]
We have found that the trigonal-bipyramidal iron tricar-
=
bonyl complexes trans-[Fe(CO)3(P{(CH2)nCH CH2}3)2] (n =
Square-planar complexes featuring two trans-disposed
ligands 2 and two unlike ligands L’/L’’ were sought. Using a
standard procedure,[8] the rhodium chloride [{(cod)Rh(m-
Cl)}2], 2a–c, and CO were combined. As shown in Scheme 1,
workup gave the bis(phosphine) carbonyl chloride complexes
3a–c in 88–82% yields. The NMR spectra showed the usual
103Rh and virtual couplings associated with such compounds.[8]
Next, 3b (ca. 0.001m in CH2Cl2) was treated with the Grubbs
4–6) and square-planar palladium and platinum dichloride
complexes trans-[MCl2(P{(CH2)nCH CH2}3)2] (n = 6, M = Pd,
Pt) undergo threefold intramolecular alkene metatheses that
couple the trans phosphine ligands. Hydrogenations afford
cage molecules of the types I and II, which bear a clear
=
=
catalyst (15 mol%, or 5%/new C C linkage). Chromatogra-
phygave the intramolecular metathesis product 4b, which
features three 25-membered macrocycles, in 45% yield. The
31
=
P NMR spectrum indicated a 60:30:10 mixture of C C
isomers. The sample was hydrogenated (4 atm H2, 15 mol%
PtO2), giving the target molecule 5b in 91% yield.
A similar sequence with 3c gave 4c (49%) and then 5c
(87% with a purityof 90% or 53% with a purityof > 98%).
These complexes feature 27-membered macrocycles. How-
ever, 3a did not afford detectable quantities of monorhodium
products. The 13C NMR spectra of 5b,c exhibited a single set
of signals for the CH2 groups, indicating rapid rotation of the
Cl-Rh-CO moieties about the P-Rh-P axes on the NMR time
scale. No decoalescence was observed at À1208C in CDFCl2,
although the peaks became severelybroadened. Also, the
PC6H4O moieties gave rise to onlytwo CH 13C and 1H NMR
signals (o, m), indicating rapid interconversion of the meso
[5,6]
resemblance to toygryoscopes.
The three (CH2)2n+2
“spokes” or bridges provide a high degree of steric insulation.
When these define seventeen-membered macrocycles (n = 6),
[*] Dr. L. Wang, Dr. F. Hampel, Prof. Dr. J. A. Gladysz
Institut für Organische Chemie
Friedrich-Alexander-Universität Erlangen-Nürnberg
Henkestrasse 42, 91054 Erlangen (Germany)
Fax: (+49)9131-85-26865
and rac PAr3 propeller diastereomers.[9] Importantly, the 31
P
E-mail: gladysz@organik.uni-erlangen.de
signal and CO 13C signal remained coupled to 103Rh at room
temperature, excluding dissociative mechanisms for render-
ing the above-mentioned groups equivalent.
[**] We thank the Deutsche Forschungsgemeinschaft (DFG, GL 300/1-
3), the Humboldt Foundation (Fellowship to L.W.), and Johnson
Matthey PMC (rhodium loan) for support.
To help visualize the spatial relationships in these
molecules, the crystal structures of the solvates 5b·(C6H6)1.5·
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angew. Chem. Int. Ed. 2006, 45, 4372 –4375

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Chemie
occupies a cleft. The closest
rhodium–carbon distance is
5.87 Š, or 4.17 Š after sub-
tracting the van der Waals
radius of carbon. Figure 1D
illustrates how a bridge of one
molecule nestles quite close to
the rotator of another in the
crystal. The closest intermolec-
ular rhodium–carbon distances
are 4.84 Š (5b) and 4.80 Š
(5c), or 3.14 Š and 3.10 Š
after subtracting the van der
Waals radius of carbon. This
proximityshould severely
impede Cl-Rh-CO rotation in
the crystalline state.
Given the rapid rotation of
the Cl-Rh-CO moieties of 5b,c
in solution, we sought to
replace the chloride bya bulk-
ier ligand that would function
as a “brake”. Reactions of
[Rh(CO)(Cl)(L)2] species and
acetylide nucleophiles have
been reported.[11] Thus, LiC
CC6H5 and 5b were combined
ꢀ
in THF at À808C. 31P NMR
spectra indicated
quantitative
a
nearly
conversion.
Workup gave the phenylacety-
lide complex (Scheme 1),
6
which was somewhat labile in
all solvents investigated, in
22% yield. The NMR spectro-
scopic properties of 6 were
verysimilar to those of the
triphenylphosphine analogue
Scheme 1. Synthesis and reactions of “giant” rhodium gyroscope-like molecules; cod=cycloocta-1,5-
diene.
ꢀ
trans-[Rh(CO)(C CC6H5)-
(H2O)0.5 and 5c·(C6H6)1.5·(CH3OH) were determined (see
Supporting Information). As shown in Figure 1A and B, the
molecular structures are similar, with P-Rh-P angles of 176–
1798 and nearlyeclipsed C ipso-P-P-Cipso moieties (torsion
angles 4.8–7.68). In the horizontal dimension of Figure 1A
and B both molecules exhibit a large amount void space
between the Cl-Rh-CO rotators and the diphosphine stators,
enforced in part bythe rigid arene spacers; in the vertical
dimension, there are veryslight van der Waals contacts
(PPh3)2].[12] In the crystal structure of the latter,[12] the
distance from rhodium to the p-C6H5 hydrogen is 8.32 Š.
When the van der Waals radius of hydrogen is added, the
distance is similar to the horizontal clearance offered bythe
stator in 5b (9.52 vs. 8.54–9.02 Š). However, NMR spectra of
6 in CD2Cl2 at À708C showed onlyone bridge (e.g., six signals
for CH2 groups in the 13C NMR spectrum), indicating rapid
ꢀ
rotation of the C6H5C C-Rh-CO moietyon the NMR time
scale.
À
À
(Figure 1C). The Rh Cl and Rh CO distances (2.36 Š and
2.94 Š) can be compared with those from the rhodium to the
distal carbon atoms of the bridges (10.24–10.72 Š for 5b;
10.46–12.05 Š for 5c). When the van der Waals radius of
A similar reaction was conducted with a slightly“longer”
ꢀ
acetylide, LiC CC6H4-p-CH3. Workup gave the correspond-
ing complex 7 (37%, Scheme 1), for which a distance of
8.89 Š from rhodium to the p-methyl carbon atom can be
estimated. When the van der Waals radius of carbon is added
(10.59 Š), this becomes distinctlygreater than the clearance
offered bythe stator. Accordingly, NMR spectra in CD 2Cl2 at
room temperature showed two types of bridges. As illustrated
in Figure 2, twelve signals arising from CH2 groups were
detected in the 13C NMR spectrum. These signals consist of
chlorine or oxygen[10] is added to the Rh Cl and Rh CO
distances, and that of carbon is subtracted from the rhodium
to the distal carbon distance, substantial clearance remains
(4.11 Š and 4.46 Š versus 8.54–9.02 Š and 8.76–10.35 Š).
However, in contrast to the smaller iron-based systems I,
the stators in 5b,c are “porous”, or capable of intercalation.
For example, for 5b·(C6H6)1.5·(H2O)0.5, one benzene molecule
À
À
two sets in an approximately2:1 ratio. Two sets of P C6H4O 13
C
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Figure 2. NMR spectra of 7 (top, CD2Cl2) and 5b (bottom, CDFCl2,
À208C): a) 1H NMR (400 MHz), aryl region; b) and c) 13C NMR
ꢀ
(100 MHz). Signals associated with the C C moiety and solvent are
indicated by arrows and asterisks, respectively. The red signals
originate from the acetylide ligand.
Cl, the rotators rapidlyrotate within the 25 to 27-membered
macrocycles in solution. However, when the Rh–X length
Figure 1. A,B) ORTEP (thermal ellipsoids set at 50% probability) and
ꢀ À
becomes greater than 10 Š, such as with p-CH3C6H4C C Rh,
C,D) space-filling representations of the crystal structures of 5b·
(C6H6)1.5·(H2O)0.5 and 5c·(C6H6)1.5·(CH3OH): A) 5b and B) 5c with
solvate molecules omitted; C) 5b with solvate molecules omitted; D)
packing view for 5c showing intercalation of neighboring molecules.
Selected bond lengths [Š] and angles [8], 5b/5c: Rh1–C1 1.790(5)/
1.791(8), Rh1–P1 2.3223(11)/2.3251(14), Rh1–Cl1 2.3646(10)/
2.3651(15), C1–O1 1.153(5)/1.148(9); C1-Rh1-P1 91.00(15)/90.7(2),
C1-Rh1-Cl1 174.73(17)/178.0(3), P1-Rh1-Cl1 88.18(4)/89.02(5), P1-Rh1-
P2 175.87(4)/179.26(6), O1-C1-Rh1 176.4(5)/178.8(8).
this motion is effectively“braked”. These assemblies are
structurallyporous, such that solvent or neighboring mole-
cules can intercalate between the spokes. This intercalation
blocks rotation in the solid state. Future efforts will include
restricting access to this space through the introduction of
bulkygroups and/or interbridge connections, and exploiting
the dipole moments of the rotators to achieve directed rotary
motion.
and 1H signals (2:1) were also observed. These features
persisted in spectra recorded in C6D5Cl at 1208C, under which
conditions 7 concurrentlydecomposed. These data are best
modeled bya rapid equilibrium involving the quasi-enantio-
meric structures III and IV and the transition state V.[13] This
process entails passage of the shorter CO ligand of the rotator
Received: March 25, 2006
Published online: May26, 2006
Keywords: alkynyl ligands · hydrogenation · macrocycles ·
.
metathesis · rhodium
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In summary, we have demonstrated that it is possible to
synthesize “giant” gyroscope-like molecules that feature
dipolar X-Rh-CO rotators and three-spoke stators that
incorporate rigid p-disubstituted benzene rings. When X =
4374
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