Hexahomotrioxacalix[3]arene: A scaffold for a C3-symmetric phosphine ligand that traps a hydrido-rhodium fragment inside a molecular funnel

Article abstract of DOI:10.1039/a905677g

Stereoselectivity around an H-Rh-CO fragment has been realized by reaction of the hexahomotrioxacalix[3]arene-derived triphosphine 4 with [Rh(acac)(CO)₂]; the resultant trigonal bipyramidal complex has the Rh-H bond directed inside the cavity.

Full text of DOI:10.1039/a905677g

Hexahomotrioxacalix[3]arene: a scaffold for a C3-symmetric phosphine ligand
that traps a hydrido-rhodium fragment inside a molecular funnel
Cedric B. Dieleman,a Dominique Matt,*a Ion Neda,b Reinhard Schmutzler,b Anthony Harrimanc and Reza
Yaftiand
a
Groupe de Chimie Inorganique Moléculaire, UMR 7513 CNRS, 1 rue Blaise Pascal, F-67008 Strasbourg Cedex,
France. E-mail: dmatt@chimie.u-strasbg.fr
Institut für Anorganische Chemie und Analytische Chemie der Universität, Postfach 3329, D-38023 Braunschweig,
b
Germany
c
Ecole Européenne de Chimie, Polymères et Matériaux, 1 rue Blaise Pascal, F-67008 Strasbourg Cedex, France
Department of Chemistry, University of Zandjan, PO box 313, 45195 Zandjan, Iran
d
Received (in Cambridge, UK) 13th July 1999, Accepted 6th August 1999
Stereoselectivity around an H–Rh–CO fragment has been
realized by reaction of the hexahomotrioxacalix[3]arene-
derived triphosphine 4 with [Rh(acac)(CO)2]; the resultant
trigonal bipyramidal complex has the Rh–H bond directed
inside the cavity.
corresponding 1H NMR spectrum displays a unique AB system
for the ArCH2 methylene groups. Compound 3 adopts a partial
cone conformation as deduced from the presence of two singlets
of intensity 2+1 in the 31P NMR spectrum, together with three
1
AB patterns for the ArCH2 groups in the H NMR spectrum.
The phosphine oxides 2 and 3 could be reduced with PhSiH3 in
refluxing toluene, affording the triphosphines 4 and 5,†
respectively, in quantitative yield. The partial cone structure of
5 (Fig. 1) was confirmed by an X-ray diffraction study.‡ It
should be emphasised that compounds 2–5 are highly soluble in
alkanes.
The triphosphorylated calixarene 2 was found to be an active
phase-transfer agent for rare-earth picrates (from water to
CH2Cl2).§ Extraction experiments carried out with seven rare-
earth cations (Y, La, Pr, Nd, Sm, Eu and Gd) showed the
extraction percentages to be significantly higher than those
obtained with conventional phosphine oxides, e.g. trioctylphos-
phine oxide, or the phosphato-calix[4]arenes reported recently
by Harrowfield et al.9 Interestingly, the highest selectivity was
found for those elements with an ionic radius close to that of
praseodymium; similar peak selectivity was observed recently
with a cone-calix[4]arene substituted by four CH2P(O)Ph2
phosphine oxide arms tethered at the lower rim.10 Our findings
confirm previous observations showing that phosphorus-based
ligands built on calixarenes may display different extraction
properties than do their acyclic counterparts.11
Phosphorus-bearing calixarenes have received considerable
attention both for their ionophoric receptor properties and their
facility for localizing reactive transition metal centres adjacent
to the cavity.1 To date, most studies have been concerned with
functionalized calix[4]arenes,2 while the corresponding hex-
ahomotrioxacalix[3]arenes 1 remain relatively unexplored,
Ligand 4 looks to be an ideal scaffold for the formation of C3-
symmetrical complexes. Thus, treatment of 4 with [Mo(CO)3-
(cycloheptatriene)] in refluxing THF afforded the colourless
complex 6† in high yield (Scheme 1). The carbonyl region of
the IR spectrum displays two strong absorption bands showing
the presence of an Mo(CO)3 unit with local C3v symmetry.12
Tridentate fac-coordination of the tripod was inferred from the
appearance of a singlet at dP 25.7 in the 31P NMR spectrum. In
6, the tripodal arms retain the metal centre beneath the cavity.
Likewise, complex 7,† formed in quantitative yield on reaction
of 4 with [Au(THF)(THT)]PF6 (THT = tetrahydrothiophene),
although such receptors have been equipped with PIII- or PV-
based binding sites.3,4 The larger macrocyclic unit provided by
1 looks to be particularly appropriate for use as a platform on
which to build C3-symmetrical ligands able to operate as conical
receptors.5–8 We now describe the synthesis and properties of
the first triphosphine ligands, and their corresponding oxides,
formed by anchoring CH2P(O)Ph2 groups to the phenolic
oxygen atoms of a calix[3] matrix.
Complete phosphorylation was achieved by reacting 1 with
NaH and Ph2P(O)CH2OTs in toluene at 90 °C for three days.
The reaction resulted in formation of a 4+1 mixture of the (non-
interconverting) phosphine oxides 2 and 3.† Separation by
column chromatography afforded pure 2 in 72% yield, but the
isolated samples of 3 always contained small amounts of 2.
Consistent with a C3-symmetrical structure, the 31P NMR
spectrum of 2 shows a single peak at dP 24.9 while the
Fig. 1 PLUTON view of triphosphine 5. For clarity, only the ipso carbon
atoms of the PPh groups are shown.
Chem. Commun., 1999, 1911–1912
1911

the preferential entrapment of Rh–H vs. Rh–CO will promote
selective insertion reactions with appropriately sized substrates
that enter the cavity.
Notes and references
† Satisfactory elemental analyses were obtained for all new compounds.
2
Selected data for 2: dH(CDCl3) 6.74 (s, 6H, m-ArH), 4.61 (d, JPH 3, 3H
each, PCH2O), 4.61 and 4.31 (AB q, 2J 13, 6H each, ArCH2), 1.42 (s, 27H,
2
But); dP(CD2Cl2) 24.9. For 3: dH(CDCl3) 4.75 and 4.43 (AB q, J 10, 2H
each, PCH2O), 4.56 (s, 2H, PCH2O), 4.66 and 4.13 (AB q, 2J 13, 2H each,
ArCH2), 4.53 and 4.06 (AB q, 2J 13, 2H each, ArCH2), 3.95 and 3.79 (AB
q, 2J 13, 2H each, ArCH2), 0.94 and 0.84 (2s, 18H + 9H, But); dP(CDCl3)
26.3 and 25.7. For 4: dH(CDCl3): 6.89 (s, 6H, m-ArH), 4.51 (s, 6H, PCH2O),
4.51 and 4.47 (AB q, 2J 4, 6H each, ArCH2), 1.06 (s, 27H, But); dP(CDCl3)
2
220.0. For 5: dH(CDCl3) 4.67 and 4.53 (AB q, J 10, 2H each, PCH2O),
4.62 (s, 2H, PCH2O), 4.61 and 4.01 (AB q, 2J 13, 2H each, ArCH2), 4.42 and
4.12 (AB q, 2J 13, 2H each, ArCH2), 3.85 and 3.74 (AB q, 2J 13, 2H each,
ArCH2), 1.02 and 0.91 (2s, 18H + 9H, But); dP(CDCl3) 217.4 and 219.9.
For 6: nmax (KBr)/cm21 1946s, 1854s (CNO); dH(CD2Cl2) 6.82 (s, 6H, m-
ArH), 5.17 (s, 6H, PCH2O), 4.10 and 3.84 (AB q, 2J 4, 6H each, ArCH2),
1.08 (s, 27H, But); dP(CDCl3) 25.7. For 7: dH(CDCl3) 6.75 (s, 6H, m-ArH),
5.05 (s, 6H, PCH2O), 4.47 and 4.25 (AB q, 2J 4, 6H each, ArCH2), 1.03 (s,
27H, But); dP(CDCl3) 29.7. For 8: dH(CD2Cl2) 6.95 (s, 6H, m-ArH), 5.22 (s,
6H, PCH2O), 4.50 and 4.02 (AB q, 2J 4, 6H each, ArCH2), 1.13 (s, 27H,
But); dP(CDCl3) 6.1 (2d, J107AgP 312, J109AgP 360). For 9: nmax(KBr)/cm21
1977s (CNO); dH(C6D6) 5.15 (s, 6H, PCH2O), 4.70 (HA) and 4.12 (HB) (AB
q, 2J 4, 6H each, ArCH2), 1.13 (s, 27H, But), 29.70 (q, 2JPH 14, 1H, Rh-H);
dP(C6D6) 33.4 (d, JPRh 153).
Scheme 1 Reagents and conditions: i, [Mo(CO)3(cycloheptatriene)], THF,
reflux; ii, [Au(THT)(THF)]PF6, CH2Cl2; iii, [Rh(acac)(CO)2], 20 bar H2/
CO, toluene, 70 °C.
‡ Crystal data for 5 C154H180O17P6•3CH3OH•H2O, M 2488.98, triclinic,
space group P1, colourless prisms, a 13.0120(7), b 13.3008(4), c 21.468(1)
positions a metal centre across one entrance of the cone. In this
case, the trigonal AuP3 fragment strapped across the lower rim
exaggerates the conical nature of the calix[3] unit. A similar
funnel-shaped silver complex 8† (not drawn) was also obtained
quantitatively from reaction with AgBF4. The room temperature
NMR data of both these cationic complexes are in full accord
with a C3v symmetrical structure. As deduced from a variable
temperature NMR study made with 8, the threefold symmetry of
this complex is only virtual and corresponds in fact to an
averaged structure. Indeed, the A3X (X = Ag) pattern observed
at 25 °C in the 31P NMR (202.45 MHz) spectrum broadens on
decreasing the temperature and eventually changes to an A2BX
system, while maintaining finite AgP coupling constants. The
Cs symmetry observed at low temperature probably minimizes
the strain within two of the three metallomacrocycles formed by
complexation, but the exact structure of the complex is not
known. Loss of C3 symmetry at low temperature has previously
been observed in a titanium-capped cone homooxacalix[3]ar-
ene, but in this case the structural modification appears to be
controlled by a stereodynamic process occurring at the metal
centre.13
¯
Å, V 3687.2(5) Å3, Z 1, Dc 1.12 g cm23, m 0.133 mm21, F(000) 1330. Data
were collected on a Nonius KappaCCD diffractometer (graphite Mo-Ka
radiation, l = 0.71073 Å) at 2100 °C. 27813 reflections collected (2.5 @
q @ 30.5°), 7778 data with I > 3s(I). The structure was solved using the
Nonius (ref. 14) package and refined by full-matrix least-squares with
anisotropic thermal parameters for all non-hydrogen atoms except for the
phenyl rings attached to P(3), H2O and two MeOH molecules which were
refined isotropically. Some disorder was found for the P(3)Ph2 group
adopting two possible orientations with equal occupancy. In Fig. 1 one of
the two possible orientations is shown. Final results: R(F) 0.091, wR(F)
0.122, GOF 1.065, 791 parameters, largest difference peak 1.339 e Å23
.
CCDC 182/1368. See http://www.rsc.org/suppdata/cc/1999/1911/ for crys-
tallographic data in .cif format.
§ Extraction percentage of rare-earth picrates by 2 (the rare-earths are
ranged according to increasing atomic number) : Y, 4.0; La, 42.0; Pr, 43.0;
Nd, 42.5; Sm, 38.5; Eu, 33.5; Gd, 31.5. Conditions: [M(Pic)3]initial
=
[Ligand]initial 2.5 3 1024 M, T 293 K, solvent: CH2Cl2. A separate
experiment was performed for each metal.
¶ The ArCH2 protons appear as an AB system, with dA 4.70 and dB 4.12.
Only the signal at d 4.70 correlates with the hydride.
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White, J. Chem. Soc., Dalton Trans., 1996, 1687.
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Solvent Extr. Ion Exch., 1997, 15, 975.
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A remarkable feature of triphosphine 4 concerns its ability to
trap and orientate a linear H–Rh–CO fragment. Thus, treatment
of a toluene solution of 4 and [Rh(acac)(CO)2] with 20 bar of
H2/CO at 70 °C afforded complex 9† in quantitative yield
(Scheme 1). The latter species is characterized by a strong
carbonyl absorption band at 1977 cm21 while all NMR data are
consistent with a threefold symmetry axis. The 31P NMR
spectrum, for example, shows a doublet at dP 36.4 (JRhP = 153
1
Hz) due to the phosphorus atoms with the H NMR spectrum
displaying a quartet at dH 29.70 for the hydrido ligand. The
small JPH value (14 Hz) is in full agreement with a trigonal
bipyramidal geometry. Two-dimensional ROESY (500 MHz)
experiments unambiguously revealed that the hydride lies close
to the methylenic ArCH2O groups¶ and to the PCH2 protons.
This is a clear indication that the Rh–H bond is directed inside
the funnel. This particular orientation of the organometallic
fragment is presumably stabilised by weak interactions between
the hydride group and the phenolic oxygen atoms that also serve
to enlarge the open mouth of the cavity.
As exemplified by complexes 7–9, the tridentate homooxa-
calix[3]arene 4 facilitates formation of chelate complexes
where the phosphine ligands occupy three equatorial sites. This
strategy positions a transition metal centre at the bottom of a
cone-shaped molecule and, in the case of Rh, leaves an axial
coordination site trapped inside the cavity. It is anticipated that
14 OpenMoleN, Interactive Structure Solution, Nonius B.V., Delft, The
Netherlands, 1997.
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Chem. Commun., 1999, 1911–1912