Phosphorus-phosphorus coupling in a diphosphine with a ten bond P...P separation

Article abstract of DOI:10.1039/b602605b

The large, unsymmetrical diphosphine 5,11,17,23-tetra-tert-butyl-25,26- bis(diphenylphosphinomethoxy)-27(or 28)-benzoyloxy-28(or 27)-hydroxycalix[4] arene, in which the phosphorus atoms are separated by ten bonds, was prepared by monobenzoylation of 5,11,

Full text of DOI:10.1039/b602605b

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www.rsc.org/dalton | Dalton Transactions
Phosphorus–phosphorus coupling in a diphosphine with a ten bond
P · · · P separation†
P. Kuhn,a C. Jeunesse,a D. Matt,*a J. Harrowfieldb and L. Ricardc
Received 21st February 2006, Accepted 24th March 2006
First published as an Advance Article on the web 20th April 2006
DOI: 10.1039/b602605b
The large, unsymmetrical diphosphine 5,11,17,23-tetra-tert-butyl-25,26-bis(diphenylphosphino-
methoxy)-27(or 28)-benzoyloxy-28(or 27)-hydroxycalix[4]arene, in which the phosphorus atoms are
separated by ten bonds, was prepared by monobenzoylation of 5,11,17,23-tetra-tert-butyl-25,26-
bis(diphenylphosphinoylmethoxy)-27,28-bis(hydroxy)calix[4]arene, followed by reduction with phenyl
silane. Its molecular structure has been determined by X-ray crystallography. In the solid state, the
˚
phosphorus atoms are separated by 5.333(1) A. NMR investigations reveal an unexpected
“through-space” JPP’ coupling constant of 8.0 Hz and also show that, in solution, the calixarene is
conformationally mobile, the phenoxy ring flipping rapidly through the calixarene annulus. The spatial
proximity of the two phosphorus atoms was further demonstrated by the ease of obtaining the
3
cis-chelate complex [Pd(g -C3H4Me)(THF)2]BF4 (THF = tetrahydrofuran).
substituted by –CH2PPh2 ligands, the two P atoms being separated
Introduction
1
by ten bonds. The H, 31P and 13C spectra all reveal long range
Long range coupling constants, which conventionally are related
to systems where an interacting pair of nuclei is separated by
more than four bonds, are often used as a source of information
concerning a molecular structure.1,2 There has been continuous
interest in P(III)-containing ligands in organometallic chemistry
and catalysis for over 50 years,3,4 however, relatively few data
are available for JP(III)P(III) long range coupling constants even
though many diphosphines consisting of phosphorus centres in
remote sites on large molecular frameworks are known.5,6 Thus,
for example, the question whether rigidification of a backbone
allows for very remote phosphorus–phosphorus coupling remains
open. One reason for this lack of information arises from the
fact that the observation of long range couplings requires the
phosphorus atoms to be anisochronous, i.e. to occupy inequivalent
couplings involving both phosphorus atoms.
Results
The diphosphine 3 was obtained from 5,11,17,23-tetra-tert-butyl-
25,26-bis(diphenylphosphinoylmethoxy)-27,28-bis(hydroxy)calix-
[4]arene, 1, according to the two-step synthesis shown below.
Mono-acylation of 1 with ClC(O)Ph in excess afforded 2 in 68%
yield as a racemic mixture of 2a and 2b, which differ only by the
position of the benzoyl substituent on the calixarene skeleton.
Subsequent reduction with PhSiH3/toluene gave 3 (racemic
mixture of 3a and 3b) in 87% yield.
The solid state structure of 3 was determined from X-ray diffrac-
tion data (Fig. 1). The space group is such that both enantiomeric
forms of 3 are present in the unit cell. The calixarene skeleton
adopts an elliptically distorted (“pinched”) cone conformation in
which the acylated phenol ring and its distal counterpart (i.e. the
ring bearing P(1)) are almost parallel. The centroid separations
8
7,8
9
9
ꢀ
ꢀ
molecular sites. While both JPP and JPP coupling constants
(27.5 Hz and 10.3 Hz, respectively) were recently reported for
new diphosphanes, the lack of accompanying crystal structure
determinations prevented any assessment of possible correlations
of the coupling constant values with particular structural features
of the molecule, such as, for example, a constrained spatial
proximity which would favor direct “through-space” coupling.
This issue is a focus of the present discussion.
As part of an ongoing programme aimed at the synthe-
sis of calixarene P(III)-podands,10,11 we have prepared a new,
inherently chiral calix[4]arene, 5,11,17,23-tetra-tert-butyl-25,26-
bis(diphenylphosphinomethoxy)-27(or 28)-benzoyloxy-28(or 27)-
hydroxycalix[4]arene, 3, in which two proximal phenol rings are
aLaboratoire de Chimie Inorganique Mole´culaire, LC 3 CNRS-Universite´
Louis Pasteur, 1 rue Blaise Pascal, F-67008 Strasbourg cedex, France.
E-mail: dmatt@chimie.u-strasbg.fr
bISIS, alle´e Gaspard Monge BP 70028, F-67083 Strasbourg Cedex, France
cEcole Polytechnique, DCPH, F-91128, Palaiseau Cedex, France
† Electronic supplementary material (ESI) available: Solid state NMR for
3. See DOI: 10.1039/b602605b
Fig. 1 Solid state structure of 3.
3454 | Dalton Trans., 2006, 3454–3457
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The 31P{ H} NMR spectrum (121.5 Hz, CDCl3, 20 ◦C) of 3
displays two doublets, one centered at −20.1 ppm and the other
at −22.2 ppm, with a ten-bond P–P coupling of 8.0 Hz. The
existence of coupling between the two P atoms was demonstrated
by measuring the spectrum in CDCl3 at different fields and by
carrying out a 2D homonuclear 31P COSY experiment (Fig. 2).
When the experiments were repeated in CD3OD, the magnitude of
the coupling did not change. While this coupling, in principle,
1
might arise from either “through-space” or “through-bond”
10
ꢀ
coupling in a non-
interactions, a pure “through-bond”
J
PP
conjugated system such as 3 is expected to be nearly zero. Even in a
highly conjugated naphthyl-derived diphosphine, a 9JPP of barely
ꢀ
4.0 Hz has been reported.12 If “through-bond” interactions are
negligible, significant coupling is still possible via “through-space”
effects involving transmission of spin–spin interactions via the lone
pairs of electrons on the phosphorus atoms.13 As pendent groups
on the calixarene skeleton, the phosphine substituents are brought
into relatively close proximity, although interactions involving the
P centres must depend strongly on the ligand conformations and
their dynamics. As revealed by a 2D ROESY experiment, the
calixarene core displays high flexibility, the unsubstituted phenolic
ring flipping rapidly through the calixarene annulus, while there
is clearly considerable flexibility in the pendent arms. Thus, it is
certainly possible for the phosphorus centres to come into close
contact temporarily. The spatial proximity of the two phosphorus
atoms was further demonstrated by the ease of forming cis-chelate
complexes with 3, such as 4, which was obtained quantitatively
3
by reacting 3 with [Pd(g -C3H4Me)(THF)2]BF4 (THF = tetrahy-
drofuran) (see Experimental). Finally, a variable temperature
ꢀ
NMR study revealed that the JPP coupling constant increases
significantly when lowering the temperature (varying from 8.0 Hz
at 283 K to 10.5 Hz at 183 K), an observation that is consistent
with a longer period of overlap of the lone pairs as a result of
slower molecular dynamics.13
˚
between opposed rings of the macrocyclic unit are 5.62(1) A and
˚
7.48(1) A. The O(4) · · · O(5) and O(4) · · · O(2) distances, 3.070(1)
˚
and 2.909(2) A, respectively, indicate that the hydroxy group is
hydrogen-bonded to one of the neighbouring phenoxy oxygen
atoms. In fact, the position of H(4) could be inferred from the X-
˚
ray data, and was found to be 2.26(1) A from O(2). The phosphorus
lone pairs are not directed towards the centre of the ligand cavity
formed by the pendent substituents, although P(1) is closer to
an appropriate orientation than is P(2). The separation between
˚
the two P atoms is 5.333(1) A, with an approximate alignment of
the two P-(lone pair) dipoles. Another factor possibly influencing
the solid state orientation of P(1) is a p–p interaction between the
phenacyl ring and the C(60)-phenyl group, these rings being nearly
˚
parallel and separated by only 3.7 A. Other structural parameters
can be inferred from Table 1.
In the solid state, the separation between the two phosphorus
atoms is well-defined. Unfortunately, solid state NMR required
the use of a powdered sample, which led to the desolvation of the
crystals and detection of several species; coupling between the P
atoms within the resolution of the measurements (<150 Hz) was
undetectable.
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for 3
P1–C60
P1–C54
P1–C29
P2–C72
P2–C66
1.828 (2)
1.843(2)
1.873(2)
1.824(2)
1.832(2)
P2–C30
O2–O4
O1–O4
O4–O5
C60–C32
1.853 (2)
2.909 (2)
3.194 (2)
3.070 (2)
3.871(2)
ꢀ
Overall the present study describes the first example of JPP
Dihedral angles between phenoxy rings
O1/O4 77.2(1) O2/O5
coupling in a diphosphine where the phosphorus atoms are
separated by ten chemical bonds. In view of the absence of any
conjugation between the two phosphorus centres, it is reasonable
12.0(1)
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2
4.55 and 3.23 (AB system, 2H, ArCH2Ar, JAB = 13.4 Hz), 4.38
2
and 3.19 (AB system, 2H, ArCH2Ar, JAB = 13.2 Hz), 4.19 and
2
3.19 (AB system, 2H, ArCH2Ar, JAB = 13.2 Hz), 1.31 and 1.23
1
(2s, 2 × 9H, C(CH3)3), 0.80 (s, 18H, C(CH3)3). 13C{ H} NMR
(75.5 MHz, CDCl3): d 166.15 (s, C(O)), 152.26–124.97 (aryl C),
1
1
74.24 (d, P(O)CH2, JPC = 80.2 Hz), 70.73 (d, P(O)CH2, JPC
=
73.4 Hz), 33.98, 33.91, 33.75 and 33.71 (4s, C(CH3)3), 33.10, 31.81
and 31.67 (3s, ArCH2Ar), 33.10, 31.81 and 31.67 (3s, C(CH3)3).
31P{ H} NMR (121.5 MHz, CDCl3): d 25.9 (s), 23.2 (s). Found: C,
1
78.19; H, 6.96%. Calc. for C77H82O7P2 (Mr = 1181.46): C, 78.28;
H, 7.00%.
5,11,17,23-Tetra-tert-butyl-25,26-bis(diphenylphosphinometh-
oxy)-27(or 28)-benzoyloxy-28(or 27)-hydroxycalix[4]arene (race-
mate 3a + 3b) (3). A mixture of 2 (0.300 g, 0.25 mmol) and
PhSiH3 (0.230 g, 2.12 mmol) in toluene (10 mL) was stirred for 7 d
at 90 ◦C. The solution was evaporated to dryness, upon which the
residue was taken up in CH2Cl2. Addition of methanol followed
by slow evaporation of CH2Cl2 afforded 3 as a white precipitate.
Fig. 2 COSY 31P–31P spectrum of 3.
◦
−1
=
Yield: 0.260 g, 87%; mp > 240 C; IR (KBr, cm ): m(C O) =
1726 s; 1H NMR (300.1 MHz, CDCl3): d 8.59–8.57 (m, 2H, ArH),
to regard this interaction as a “through-space” coupling. Further
studies aimed at the understanding of the mechanism of such
remote coupling, in particular their dependency on internal
backbone dynamics, are currently under way.
7.56–7.10 (m, 20H, ArH), 7.03–6.89 (m, 5H, ArH), 6.77–6.75 (m,
2H, ArH), 6.60–6.55 (m, 4H, ArH), 5.75 (s, 1H, OH), 5.55 and
2
4.90 (ABX system with X P, 2H, PCH2, JAB = 12.6 Hz, 2JAX
=
=
4.0 Hz, 2JBX = 0 Hz), 4.75 and 4.58 (ABX system with X P, 2H,
=
2
2
2
PCH2, JAB = 11.7 Hz, JAX = 5.5 Hz, JBX = 0 Hz), 4.58 and
Experimental
2
3.01 (AB system, 2H, ArCH2Ar, JAB = 12.8 Hz), 4.29 and 3.21
2
(AB system, 2H, ArCH2Ar, JAB = 13.0 Hz), 4.18 and 3.23 (AB
All manipulations were performed in Schlenk-type flasks under
dry nitrogen. Solvents were dried by conventional methods and
distilled immediately prior to use. CDCl3 was passed down a
5 cm diameter alumina column and stored under nitrogen over
system, 2H, ArCH2Ar, 2JAB = 13.5 Hz), 4.14 and 3.13 (AB system,
2H, ArCH2Ar, 2JAB = 13.4 Hz), 1.32, 1.30, 0.87 and 0.81 (4s, 4 ×
9H, C(CH3)3. 13C{ H} NMR (75.5 MHz, CDCl3): d 166.41 (s,
1
1
=
C O), 152.89–124.90 (Aryl C), 78.74 (d, PCH2, JPC = 7.4 Hz),
1
1
molecular sieves (4 A). The 1H, 13C{ H} and 31P{ H} spectra
were recorded using Bruker FT instruments (AC-300, ARX-500).
1H chemical shifts are referenced to residual protonated CDCl3
(7.26 ppm), 13C chemical shifts are referenced to CDCl3 (77.0 ppm)
and the 31P NMR data are referenced to external H3PO4. The mass
spectrum of 4 was recorded on a MicroTOF Bruker Daltonic
spectrometer (MALDI) using CH2Cl2. 1 was prepared using the
procedure reported in the literature.14
˚
1
ꢀ
75.79 (dd, PCH2, JPC = 11.2 Hz, JP C = 3.6 Hz), 34.23, 34.04,
33.93 and 33.87 (4s, C(CH3)3), 33.21 (d, ArCH2Ar, 5JPC = 7.2 Hz),
32.51 (dd, ArCH2Ar, 5JPC = 6.8 Hz, 5J(PꢀC) = 2.4 Hz), 32.24 (d,
ArCH2Ar, 5JPC = 2.8 Hz), 31.90 (s, ArCH2Ar), 31.86 and 31.78 (2s,
C(CH3)3), 31.09 (broad, C(CH3)3). 31P{ H} NMR (121.5 MHz,
1
ꢀ
CDCl3): −20.1 and −22.2 (AB system, JPP = 8.0 Hz). Found: C,
80.34; H, 7.10%. Calc. for C77H82O5P2 (Mr = 1149.46): C, 80.46;
H, 7.19%.
Syntheses
(g3 -2-Methylallyl)-(P,Pꢀ )-{5,11,17,23-tetra-tert-butyl-25,26-
bis(diphenylphosphinomethoxy)-27 (or 28)-benzoyl-28 (or 27-
hydroxy)-calix[4]arene}palladium(II) tetrafluoroborate (4). To a
5,11,17,23-Tetra-tert-butyl-25,26-bis(diphenylphosphinoylmeth-
oxy)-27(or 28)-benzoyloxy-28(or 27)-hydroxycalix[4]arene (2).
A
3
solution containing 1 (3.230 g, 3.00 mmol) and benzoyl chloride
(0.878 g, 6.20 mmol) in pyridine (100 mL) was stirred for 15 h
at room temperature. After evaporation of the solvent, the solid
residue was taken up in CH2Cl2. Addition of MeOH and partial
removal of CH2Cl2 under vacuo afforded 2 as a white precipitate.
solution of [Pd(g -C4H7)Cl]2 (0.037 g, 0.093 mmol) in CH2Cl2
(10 mL) was added a solution of AgBF4 (0.036 g, 0.186 mmol)
in THF (1 mL). After 5 min, the solution was decanted in order
to eliminate AgCl. The supernatant was filtered through Celite
and added to a solution of 3 (0.214 g, 0.186 mmol) in CH2Cl2
(300 mL). After 1 h, the solution was concentrated to ca. 5 mL and
addition of pentane afforded a yellow precipitate. Yield: 0.207 g,
85%; mp > 199 ◦C (dec.). In keeping with two possible orientations
of the Me-allyl group, the 1H and 31P NMR spectra revealed
◦
−1
=
Yield: 2.410 g, 68%; mp > 240 C; IR (KBr, cm ): m(C O) 1726s;
1H NMR (300.1 MHz, CDCl3): d 8.47–8.44 (m, 2H, ArH), 7.95–
7.85 (m, 4H, ArH), 7.60–7.10 (m, 19H, ArH), 7.07 and 7.02 (AB
system, 2H, m-ArH, 4J = 2.2 Hz), 6.87 and 6.66 (AB system, 2H,
m-ArH, 4J = 2.4 Hz), 6.54 (s br, 2H, m-ArH), 6.45 and 6.42 (AB
1
the presence of two isomers present in a ca. 1 : 3 ratio. 31P{ H}
4
NMR (162 MHz, CDCl3): d 20.47 and 12.90 (AB system, 2JPP
=
ꢀ
system, 2H, m-ArH, J = 2.3 Hz), 5.65 and 4.75 (ABX system
2
2
37.9 Hz), 19.50 and 10.93 (AB system, 2JPP = 37.8 Hz). ES mass
spectrum m/z 1309.6 [M−BF4]+. Found: C, 65.21; H, 5.97%. Calc.
for C81H89O5P2PdBF41.5 CH2Cl2 (Mr = 1397.76 + 127.39): C,
64.97; H, 6.08%.
ꢀ
with X = P, 2H, P(O)CH2, JAB = 14.8 Hz, JAX = 4.1 Hz,
2JBX = 0 Hz), 5.27 (s, 1H, OH), 5.22 and 4.67 (ABX system
with X P, 2H, P(O)CH2, JAB = 13.9 Hz, 2JAX = 4.1 Hz, 2JBX
=
2
=
0 Hz), 4.79 and 2.91 (AB system, 2H, ArCH2Ar, 2JAB = 13.0 Hz),
3456 | Dalton Trans., 2006, 3454–3457
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Single-crystal X ray diffraction study
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methanol into a CH2Cl2 solution of the disphosphine 3.
C77H82O5P21/2CH2Cl2, Mr = 1191.83, triclinic, P1, a = 10.637(1),
¯
˚
b = 15.234(1), c = 21.083(1) A, a = 99.840(1), b = 96.1300(10),
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−3
˚
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−1
˚
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rameters; maximum/minimum residual density 0.76(6)/−0.77(6)
−3
15
˚
e A . The crystal structure was solved with SIR97 and refined
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thermal displacement parameters for all non-hydrogen atoms;
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Acknowledgements
We are grateful to Johnson Matthey for a generous loan of
palladium chloride.
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