Upper-rim monofunctionalization of calix[4]arene by organometallic diphenylphosphinorhodium complexes

Article abstract of DOI:10.1039/b001485k

Monofunctionalization of the calix[4]arene upper-rim is achieved by lithiation with Bu(n)Li, followed by phosphination with Ph₂PCl of monobromotetra-n-propoxycalix[4]arene; subsequent reactions with [Cp*Rh(III)Cl₂]₂ and Cp

Full text of DOI:10.1039/b001485k

Upper-rim monofunctionalization of calix[4]arene by organometallic
diphenylphosphinorhodium complexes†
Martin Vézina, Jonathan Gagnon, Karine Villeneuve, Marc Drouin and Pierre D. Harvey*
Département de Chimie, Université de Sherbrooke, Sherbrooke, P.Q. Canada, J1K 2R1. E-mail:
pharvey@courrier.usherb.ca
Received (in Irvine, CA, USA) 17th February 2000, Accepted 1st May 2000
Monofunctionalization of the calix[4]arene upper-rim is
achieved by lithiation with BunLi, followed by phosphina-
tion with Ph2PCl of monobromotetra-n-propoxycalix[4]ar-
ene; subsequent reactions with [Cp*RhIIICl2]2 and
Cp*RhI(CO)2 provide the corresponding Cp*[Ph2(ca-
lix)P]RhIIICl2, which can be converted to Cp*[Ph2(ca-
lix)P]RhIII(H)2 and Cp*[Ph2(calix)P]RhI(CO), respectively.
and has been structurally characterized by X-ray methods.‡ Fig.
1 shows the X-ray structure and confirms the expected
monofunctionalization of the calix[4]arene. The coordination
environment of the RhIII metal center is relatively similar to that
Substituted derivatives of the bowl-like calix[4]arene are easy
to synthesize and extremely rich chemistry has emerged from
this simple molecule, primarily due to the pioneering work of
Gutsche.1 Many applications in the area of cation binding and
transport are now known, and highly selective receptors and
novel sensors for polyanionic species have been discovered
with this host.2 Recently, several researchers have prepared
transition metal complexes of this platform molecule, mainly
via the lower-rim section,3 but some rare upper-rim functional-
izations have also appeared in the literature.4 Of particular
interest, Matt and coworkers synthesized conic calix[4]arene
complexes using two anchoring phosphine groups placed
opposite to each other in the upper-rim.5 Complexation induced
a loop structure where the calix[4]arene cavity was blocked by
the transition metal fragment, hence, losing its important
hydrophobic hosting function. We now report the first mono-
functionalization of the calix[4]arene upper-rim, using the Ph2P
complexing fragment, along with some of its Rh complexes.
The complexation of an Rh residue near a hosting device offers
the hope of convenient regioselective reactivity in situations
where the reacting molecule reversibly binds the calix[4]arene
cavity. Many Cp*Rh complexes are known to activate C–H
bonds,6 and anchoring this fragment onto the supramolecular
calix[4]arene represents a unique opportunity of potential
regioselective C–H bond activation. This work describes the
5
syntheses and characterization of three (h -pentamethylcyclo-
Scheme 1 Synthesis of 2–5. Reagents and conditions: i, BuLi/THF/278 °C;
ii, ClPPh2/THF/278 °C; iii, [Cp*RhCl2]2/EtOH/reflux 5 h; iv, NaBH4/
EtOH/reflux 1 h; v, Cp*Rh(CO)2/benzene/reflux 24 h.
pentadienyl)rhodium complexes of diphenylphosphinocalix-
[4]arene.
The entry into the monofunctionalization proceeds via the use
of the known monobromotetra-n-propoxycalix[4]arene 1.7 In
this case, simultaneous standard lithiation and phosphination
procedures produce the monophosphine ligand: [eqn. (1) and
(2)].
(PrO)4calix–Br + BunLi ? (PrO)4calix–Li + BunBr
(1)
(2)
(PrO)4calix–Li + Ph2PCl ? (PrO)4calix–PPh2 + LiCl
The latter is easily oxidized to its oxo form in the presence of
air in solutions as evidenced from the X-ray structure determi-
nation to be described elsewhere, and therefore inert conditions
must be used at all times.
The complexation routes of 2 with Cp*Rh complexes are
shown in Scheme 1. Complex 3 is conveniently synthesized by
directly reacting the known dimeric [Cp*RhCl2]28 with 2, and is
obtained in 85% isolated yield. Complex 3 is relatively stable
Fig. 1 ORTEP drawing of 3. Ellipsoids are shown at 10% probability and H
atoms are not shown for clarity. Selected bond and distances (Å) and angles
(°): Rh–C11 2.3835(18), Rh–C12 2.4053(20), Rh–P 2.3354(18), av. Rh–Cl
2.19(3); C11–Rh–C12 93.54(8), C11–Rh–P 85.71(6), P–Rh–C12
93.50(7).
† Electronic supplementary information (ESI) available: synthesis and
characterisation of compounds 2–5. See http://www.rsc.org/suppdata/cc/
b0/b001485k/
DOI: 10.1039/b001485k
Chem. Commun., 2000, 1073–1074
This journal is © The Royal Society of Chemistry 2000
1073

of the reported less encumbered [Cp*Rh(Cl)2P(Me)2CH2]2
complex.9
has been successful. Their structural properties and reactivities
are under investigation, and will be published in due course.
We acknowledge NSERC (Natural Sciences and Engineering
Research Council), FCAR (Fonds Concertés pour l’Avance-
ment de la Recherche) and le Bureau de la Recherche de
l’Université de Sherbrooke for funding and graduate scholar-
ships.
The key feature of this structure is that 3 exhibits calix C–P
and P–Rh single bonds and at least 12 conformations are
possible. Rotation around this C–P bond provides two rotamers
where part of the cavity opening is covered by phenyl groups,
and one where the metallic residue occupies this specific above
position (Cs point group). The single crystal data shows one of
the former described rotamers (Ph group on top of the cavity),
and the Cp*RhCl2 fragment adopts a conformation exhibiting a
minimum of steric hindrance between Cp* and Ph, as shown in
Notes and references
‡ Crystal data for C62H72Cl2O4PRh 3: M = 1085.98, monoclinic, space
group P21/c, a = 18.009(7), b = 15.401(2), c = 21.996 (5) Å, b =
110.34(2)°, U = 5720(3) Å3, T = 293 K, Z = 4, m(Cu-Ka) = 1.54060 Å,
18919 reflections measured, 9703 unique (Rint = 0.07) which were used in
all calculations. The final wR(F2) was 0.1897 (all data). Single crystals of
Cp*[Ph2P(calix)]RhCl2 3 were obtained from recrystallization in ethanol.
These dark red crystals were air-stable and one of them was mounted at 298
K on an Enraf-Nonius CAD-4 automatic diffractometer. The full structure
was solved using direct methods and refined by full matrix least-squares on
F2.
1
Fig. 1. H NMR spectroscopy clearly established the presence
of fluxionality in 3. Between 218 and 343 K, the spectra exhibit
three main regions including the aromatic, methylene and
propyl 1H region, which all undergo coalescence processes. The
most striking observation is that both the PPh2 proton and
aromatic signals coalesce at the same temperature of ca. 268 K,
corresponding to  2.2 kJ mol21. This result strongly indicates
that the fluxion must involve cooperative motions of the Ph
groups and the calix[4]arene aromatic fragments. The most
probable motion is rotation of the Cp*(PPh2)RhCl2 group
around the calix C–P single bond, where the Ph groups also
rotate to pass over the bowlic structure of the calix[4]arene
residue. Computer modellings clearly demonstrate that indeed
rotations around the calix C–P bond must be accompanied by
cooperative rotations of the Ph–P bond, somewhat similar to a
‘merry-go-round’ motion, allowing the Ph substituents to hop
over the calix[4]arene ‘walls’.
CCDC 182/1634.
1 C. D. Gutsche, Calixarenes, Royal Society of Chemistry, Cambridge,
UK, 1989.
2 Calixarenes: a versatile class of macrocyclic compounds, ed. J. Vicens
and V. Böhmer, Kluwer Academic Publishers, Dordrecht, 1991;
Calixarenes 50th anniversary: commemorative issue, ed. J. Vicens, Z.
Asfari and J. M. Harrowfield, Kluwer Academic Publishers, Dordrecht,
1994.
3 C. Wieser, C. B. Dieleman and D. Matt, Coord. Chem. Rev., 1997, 165,
93.
4 P. D. Beer, D. Hesek, K. C. Nam and M. G. B. Drew, Organometallics,
1999, 18, 3933; B. R. Cameron and S. J. Loeb, Chem. Commun., 1996,
2003.
5 I. A. Bagati, D. Matt, H. Thonnessen and P. G. Jones, Inorg. Chem.,
1999, 38, 1585; C. Wieser-Jeunesse, D. Matt and A. De Cian, Angew.
Chem., Int. Ed., 1998, 37, 2861.
6 A. E. Shilov, Activation of Saturated Hydrocarbons by Transition Metal
Complexes, D. Riedel, Hingham, MA, 1984.
7 A. Ikeda, M. Yoshimura, P. Lhotak and S. Shinkai, J. Chem. Soc.,
Perkin Trans. 1, 1996, 1045.
8 J. W. Kang, K. Moseley and P. M. Maitlis, J. Am. Chem. Soc., 1969, 91,
5970.
9 W. Keim, P. Kraneburg, G. Dahmen, G. Deckers, U. Englert, K. Linn,
T. P. Spaniol, G. Raabe and C. Kruger, Organometallics, 1994, 13,
3085.
10 D. P. Drolet and A. J. Lees, J. Am. Chem. Soc., 1992, 114, 4186.
11 W. D. Jones and F. J. Feher, J. Am. Chem. Soc., 1986, 108, 4814; W. D.
Jones and F. J. Feher, J. Am. Chem. Soc., 1985, 107, 620; W. D. Jones
and F. J. Feher, J. Am. Chem. Soc., 1984, 106, 1650; R. A. Periana and
R. G. Bergman, J. Am. Chem. Soc., 1984, 106, 7272; M. J. Wax, J. M.
Stryker, J. M. Buchanan, C. A. Kovac and R. G. Bergman, J. Am. Chem.
Soc., 1984, 106, 1121.
Complex 3 predictably reacts with NaBH4 in refluxing
ethanol to produce the corresponding dihydride 4. The latter is
very reactive, particularly in the presence of light and air, and
must be freshly prepared prior to further studies. The presence
of hydride groups in 4 is readily illustrated from its 1H NMR and
IR spectra which exhibit a characteristic resonance at d –13.07
1
(1JHRh = 28 Hz, JHP = 38 Hz ), and an absorption at 2080
cm21 (nRhH), respectively. Complexation of monophosphine
ligand 2 can also be performed with the mononuclear
Cp*RhI(CO)2 complex10 via a simple thermally induced CO
substitution, to form 5. A single and characteristic nCO
absorption is indeed observed at 1960 cm21 in the solid state,
and a complete characterization by standard methods (NMR,
FAB mass, chemical analysis), confirms the identity and purity
of this novel complex. Complex 5 is somewhat more stable than
the dihydride 4, but decomposition can also be observed upon
UV–VIS light irradiation. Preliminary results show that indeed
both 4 and 5 either thermally or photochemically eliminate H2
and CO, respectively, to generate the very reactive species
Cp*RhIL (L = phosphine ligand, here 2).11 This important
intermediate postulated as Cp*Rh[Ph2P(calix)], is anticipated to
activate C–H bonds,12 and regioselectivity would be an
unprecedented asset. Preparation of the more basic and less
encumbered R2P–calix ligand (R = Pri) and its Cp*Rh complex
12 B. K. McNamara, J. S. Yeston, R. G. Bergman and C. B. Moore, J. Am.
Chem. Soc., 1999, 121, 6437; A. A. Bengali, R. H. Schultz, C. B. Moore
and R. G. Bergman, J. Am. Chem. Soc., 1994, 116, 9585.
1074
Chem. Commun., 2000, 1073–1074