A cavity-shaped diphosphane displaying Oschelating behavior

Article abstract of DOI:10.1002/anie.201005169

A molecular balance wheel: Transition metals form β-cyclodextrin- derived diphosphane chelate complexes (picture: ligand structure) in which a fast oscillatory motion about the metal ion takes place ("oschelating behavior"). The observed movement occurs w

Full text of DOI:10.1002/anie.201005169

Communications
DOI: 10.1002/anie.201005169
Coordination Chemistry
A Cavity-Shaped Diphosphane Displaying
“Oschelating” Behavior**
Rafael Gramage-Doria, Dominique Armspach,* Dominique Matt,* and
Loꢀc Toupet
Angewandte
Chemie
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Molecules that combine the properties of a transition metal
with those of an appended cavity have attracted a great deal
of attention.[1–3] So far, research in this area has focused on
four main objectives: 1) the design and synthesis of systems
exploiting the binding properties of a receptor unit linked to a
transition-metal center, with the aim of producing catalysts
that mimic an enzyme;[4–8] 2) the study of metal-centered
reactions taking place in a confined environment, thereby
favoring highly regio-, stereo-, and shape-selective reac-
tions;[9–13] 3) the metal-assisted entrapment and recognition
of ionic species;[14–16] 4) the construction of sensors capitaliz-
ing on an electro- or photoactive metal unit covalently
attached to a close, cavity-shaped receptor.[17,18]
While the coordination chemistry of many multitopic
ligands giving rise to 3D architectures such as capsules, cages,
bowls, and boxes has been thoroughly investigated,[19–25] the
use of such ligands for generating oscillatory motion about a
metal ion has not been considered yet, although examples of
transition metals moving around the periphery of such an
object are known.[26] Herein we show how a cavity bearing
two introverted donor atoms may behave as a balance wheel
swinging about a central metal unit. Our approach is based on
the use of a rigid, cyclodextrin-based diphosphane charac-
terized by a long P···P separation, which results in highly
unsymmetrical chelation.
Diphosphane 6 was obtained in 20% overall yield
according to Scheme 1. Its synthesis began with a regioselec-
tive double capping of native b-cyclodextrin (b-CD) using the
bulky dialkylating reagent 1.[27] The non-alkylated hydroxy
groups were subsequently methylated with NaH/MeI, result-
ing in the ABDE-functionalized intermediate 2 (yield 50%).
Deprotection with HBF4, leading to tetrol 3, followed by
reaction with mesyl chloride in pyridine afforded tetramesy-
late 4. Reaction of the latter with Li2PPh in THF gave
diphosphane 6 in approximately 70% yield along with two
other, unidentified products. Workup required the prepara-
tion of the diborane adduct 5, which was separated chromato-
graphically. Finally, 5 was treated with HNEt2 to afford 6
quantitatively. As expected, the 31P NMR spectrum of 6 in
C6D6 shows two very close singlets, seen at d = À15.0 and
À15.2 ppm, respectively (in CDCl3 the spectrum showed only
a unique peak). Prolonged standing in air of a solution of 6 in
MeOH produced the di(phosphane oxide) 7, the structure of
which was determined by an X-ray diffraction study
Figure 1. X-ray structure of the di(phosphane oxide) 7 (view from the
secondary face). Solvent molecules have been omitted. Selected
distances [ꢀ]: P1–P2 6.91; O1–O2 4.32.
=
(Figure 1). In the solid state, the two P O vectors of 7 point
towards the interior of the cavity, the P···P separation being
6.91 ꢀ. Two nonbridged glucose units are tipped towards the
CD axis, reflecting some strain within the CD.
Despite the long separation between the two phosphorus
atoms, diphosphane 6 turned out to be suitable for chelation.
Thus, for example, reaction with [Au(tht)(thf)]PF6 (tht = tet-
rahydrothiophene; thf = tetrahydrofuran) led quantitatively
to complex 8 (Scheme 2). The mass spectrum of 8 showed an
intense peak at m/z 1717.62 having exactly the isotopic profile
expected for [Au·6]+. Furthermore, the 31P NMR (CD2Cl2,
258C) spectrum displayed an AB pattern with a J(PP)
coupling constant of 326 Hz, which is in accord with a very
large bite angle. In fact, molecular models indicate that the
bite angle is close to 1608; in other terms the ligand cannot
behave as a perfect trans-chelator. The good chelating
properties of 6 were further confirmed by its reaction with
[PdCl2(PhCN)2], [PtCl2(PhCN)2], and [{RhCl(CO)2}2], lead-
ing to the chelate complexes 9–11, respectively, in 100% yield
(Scheme 2 and the Supporting Information). As in 8, the
corresponding large J(PP’) coupling constants (see the
Supporting Information) reflect the large bite angle of the
ligand.[28,29]
As shown by a variable-temperature NMR study, complex
10 displays fluxional behavior in solution. The 31P NMR
(CD2Cl2) spectrum of 10, measured at À808C, revealed the
presence of two species (10a and 10b) present in a 1:1 ratio,
each characterized by an ABX pattern (2J(AB) = 492 and
476 Hz, respectively; 1J(PPt) coupling poorly resolved;
Figure 2, bottom). Upon raising the temperature, the signals
first broadened, then coalesced near À158C, and finally
merged into a single ABX spectrum (2J(AB) = 496 Hz,
1J(PPt) = 2510 Hz; Figure 2, top). The observed data are
consistent with exchange between two complexes both of
which contain a close to linear P-Pt-P unit. A variable-
temperature 1H NMR study was also carried out which
confirmed the 1:1 stoichiometry of the equilibrating species.
Both series of experiments led to a free energy of activation
DG° = (11.3 Æ 0.2) kcalmolÀ1. Interestingly, the low-temper-
[*] R. Gramage-Doria, Prof. D. Armspach, Dr. D. Matt
Laboratoire de Chimie Inorganique Molꢁculaire et Catalyse
Institut de Chimie UMR 7177 CNRS, Universitꢁ de Strasbourg
67008 Strasbourg Cedex (France)
E-mail: d.armspach@unistra.fr
dmatt@unistra.fr
Dr. L. Toupet
Groupe Matiꢂre Condensꢁe et Matꢁriaux, UMR 6626 CNRS
Universitꢁ de Rennes 1
35042 Rennes Cedex (France)
[**] Oschelating is a contraction of oscillating and chelating. This work
was supported by the CNRS and the Rꢁgion Alsace (grant to
R.G.D.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005169.
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Scheme 1. Stepwise buildup of 6. DMAP=4-dimethylaminopyridine; Ms=methanesulfonyl.
ature 31P NMR spectra revealed two AB patterns with a large
separation between the A and B parts (ca. 12 ppm), indicating
that the two P atoms of each complex are coordinated to the
platinum atom with unequal strength. Note that one of the
phosphorus signals appears near the midpoint between the
signal of the free ligand and that of the other signal. As shown
by an off-resonance 31P{1H}-31P{1H} ROESY NMR experi-
ment (see the Supporting Information), the “strongly”
coordinated phosphorus atom of each species is in exchange
with the “weakly” coordinated phosphorus atom of the other
isomer.
Overall, the best way to describe these findings is to
consider that the dx2Ày2 orbital involved in formation of the
À
M P bonds changes its orientation in a pendular fashion so as
to adopt in alternation different overlaps with each of the two
convergent, but nonaligned phosphorus lone pairs. Conse-
quently, metal binding to the two phosphorus donors is
nonequivalent in a given species (Scheme 3). The observed
isomerization is probably also accompanied by a slight
displacement of the metal center. Thus, being unable to
form an authentic trans complex (i.e. with a P-M-P angle of
1808), diphosphane 6 may be regarded as a frustrated
chelator, which compensates the metal electron deficiency
generated at one coordination site by oscillating about the
complexed metal ion. We propose to term this type of
bidentate ligand an oschelating (contraction of “oscillating”
and “chelating”) species. It is worth mentioning here that the
Figure 2. Variable-temperature 31P{1H} NMR study of platinum com-
plex 10. The AB patterns of the two equilibrating species are
represented by dots and squares. Filled symbols are for the A parts,
open symbols for the B parts.
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resulting complex 12 crystallized with a water
molecule coordinated to one of the palladium
atoms (Scheme 4, Figure 3). It is likely that owing
to its rigidity, diphosphane 6 cannot adapt to a
{Pd2Cl2(m-Cl)2} fragment having the usual flat or roof
structure and therefore prefers to cap a {Pd2Cl4} unit
having only a single chlorido bridge. Another reac-
tion directly related to the oschelating behavior of 6
is the reaction between 9 and O2/H2O in methanol,
leading to equimolar amounts of complexes 13 and
14 (Scheme 4). Complex 13, which was characterized
by an X-ray diffraction study, is seemingly formed by
À
cleavage of one of the Pd P bonds and its substitu-
tion by a bond to H2O, followed by oxidation of the
dissociated P atom. Complex 14, the structure of
which was unambiguously determined by multidi-
mensional NMR analysis,[36] corresponds to a com-
À
plex in which the other Pd P bond has been cleaved,
but in this case the phosphane oxide moiety formed is
coordinated to the metal atom, as deduced from the
3
existence of a J(PP’) coupling constant (4.4 Hz).
It is interesting to note that complexes 12 and 13
both contain a “{(phosphane)PdX2(H2O)}” unit, thus
constituting rare examples of [LL’PdX2] complexes
containing a unique monophosphane ligand. Overall,
the coordinative properties of 6 are markedly differ-
ent from those of the previously reported a-CD
analogue (TRANSDIP).[37]
In conclusion, by using a capping methodology
that relies on the use of the bulky dialkylating agent
1, we have synthesized the first large-bite-angle
diphosphane (6) based on a b-CD backbone. Owing
to the rigidity of this ligand as well as the large
Scheme 2. Synthesis of complexes 8–11.
observed fluxionality is different from
that found in complexes containing
hemilabile ligands, the latter leading to
intermediates in which one end is totally
dissociated.[30–33] It should be empha-
sized that similar dynamics (with com-
parable activation energies) were
observed for all three oschelated com-
plexes 8, 9, and 11. Furthermore, for all
four complexes investigated, the J(PP’)
coupling constants persisted over the
temperature range À808C to + 808C.
In other words, the observed dynamic
behavior occurs without dissociation of
À
the M P bonds. This phenomenon may
be regarded as a variant of bond-stretch
isomerism.[34,35]
We anticipated that the dynamic
nature of 6 in the chelate complexes
À
would weaken the P M bonds. In fact,
treatment of 9 with [PdCl2(PhCN)2] led
Scheme 3. Balance wheel movement of ligand 6 in complex 10 (view along the CD axis). The
formally to insertion of a PdCl2 fragment
green arrows in the lower part indicate the orientations of the dx2 orbital involved in formation
Ày2
À
À
into one of the Pd P bonds of 9. The of the two M P bonds.
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Communications
separation between the phosphorus
atoms, 6 behaves towards transition-
metal ions as an unsymmetrical
chelator, inducing rapid oscillation
of the chelate about the coordinated
metal center. This phenomenon
occurs without dissociation of the
phosphorus atoms. The unprece-
dented chelating behavior of 6 ena-
bles the formation of monophos-
phane complexes located inside a
CD, thereby opening a way to the
further study of organometallic cat-
alysts operating in a confined envi-
ronment.
Experimental Section
Full experimental details including X-ray
structural data are given in the Support-
ing Information. CCDC 756653 (7),
758028 (12), and 770728 (13) contain
the supplementary crystallographic data
for this paper. These data can be
obtained free of charge from The Cam-
bridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
6: 31P{1H} NMR (202.5 MHz,
À
Scheme 4. Breaking the P Pd bonds of 9.
C6D6,
258C):
d = À15.0
(s),
À15.2 ppm (s).
8: 31P{1H} NMR (121.5 MHz,
2
CD2Cl2, À808C): d = 40.6 and 31.6 (AB system, JP1,P2
=
326 Hz), 38.0 and 33.8 (AB system, 2JP1,P2 = 326 Hz),
À144.3 ppm (hept, 1JP, F = 716 Hz).
9: 31P{1H} NMR (121.5 MHz, CD2Cl2, À808C) = 21.7 and
2
11.4 (2d, AB system, JP1,P2 = 564 Hz), 17.7 and 5.6 ppm (2d,
AB system, 2JP1,P2 = 549 Hz).
10: 31P{1H} NMR (121.5 MHz, CDCl3, 258C): d = 8.9 ppm
1
(brs with br Pt satellites, JP, Pt ꢀ 2500 Hz); 31P{1H} NMR
(121.5 MHz, CD2Cl2, À808C) = 0.97 and 12.20 (2 d, AB
system, 2JP1,P2 = 476 Hz), 5.02 and 17.76 ppm (2d, AB system,
2JP1,P2 = 492 Hz) (1JPt,P poorly resolved); 31P{1H} NMR
(202.5 MHz, C2D2Cl4, + 808C) = 7.04 and 7.35 ppm (2d with
1
1
Pt satellites, ABX system, JPt,P1 = 2507 Hz, JPt,P2 = 2481 Hz,
2JP1,P2 = 496 Hz).
11: 31P{1H} NMR (162.0 MHz, CD2Cl2, À808C) = 28.0 and
9.4 (2 dd, ABX system, 2JP1,P2 = 354 Hz, 1JP, Rh = 118 and
2
118 Hz), 16.4 and 9.4 ppm (2 dd, ABX system, JP1,P2
=
354 Hz, 1JP, Rh = 118 and 118 Hz).
Received: August 18, 2010
Revised: November 12, 2010
Published online: January 26, 2011
Keywords: cavitands · chelates · cyclodextrins ·
.
molecular dynamics · P ligands
Figure 3. X-ray crystal structures of dinuclear complex 12 (top) and the
monophosphane Pd complex 13 (bottom). Distances between the
phosphorus atoms [ꢀ]: 6.29 (12) and 6.60 (13) (cf. 6.91 ꢀ in 7). The
short Owater···O(P) separation (2.61 ꢀ) in 13 is indicative of a hydrogen
bond between the coordinated water molecule and the phosphoryl
group.
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