A new template for the synthesis of triphosphorus macrocycles

Article abstract of DOI:10.1039/b002174l

A new template synthesis of triphosphorus macrocycles has been developed based upon the intramolecular hydrophosphination of allylphosphine coordinated to the (η⁵-cyclopentadienyl)iron(II) cation; the resulting tri- secondary macrocycle 1,5,9-triphosphacyclododecane is readily alkylated with ethene to give (η³-1,5,9-[12]aneP₃Et₃)Fe(η⁵-Cp)⁺ which is in turn liberated as the free ligand stereospecifically as the syn-syn isomer; related reactions with phenyl(allyl)phosphine lead directly to the triphenyl macrocycle (1,5,9-[12]aneP₃Ph₃) which is also liberated stereospecifically.

Full text of DOI:10.1039/b002174l

A new template for the synthesis of triphosphorus macrocycles
Andrew J. Price and Peter G. Edwards*
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff, UK CF10 3TB.
Email: edwardspg@cardiff.ac.uk
Received (in Cambridge, UK) 17th March 2000, Accepted 10th April 2000
Published on the Web 9th May 2000
A new template synthesis of triphosphorus macrocycles has
been developed based upon the intramolecular hydro-
tion. Complex 2 is readily soluble in most polar solvents, such
as THF and CH2Cl2, but insoluble in hydrocarbons and other
non-polar solvents. The 31P {1H} NMR and 31P NMR spectra
consist of a singlet and triplet, respectively (d 29.0, 1JP–H 328
Hz), consistent with magnetically equivalent coordinated
allylphosphine ligands (cf. for free allylphosphine, d2134.4,
1JP–H 195 Hz).
Cyclotrimerisation of 2 has been attempted in tetrahy-
drofuran (THF), chlorobenzene and anisole, using AIBN
[azobis(isobutyronitrile)] as radical initiator (ca. 1–2% mol
equivalents). In THF, both the cyclisation intermediates and
final macrocyclic product slowly precipitate during the reaction
which remains incomplete as a result, unless performed in dilute
solution. Attempts to separate and characterise these inter-
mediates have not been successful.
5
phosphination of allylphosphine coordinated to the (h -
cyclopentadienyl)iron(II) cation; the resulting tri-secondary
macrocycle 1,5,9-triphosphacyclododecane is readily alky-
3
5
lated with ethene to give (h -1,5,9-[12]aneP3Et3)Fe(h -Cp)+
which is in turn liberated as the free ligand stereospecifically
as the syn–syn isomer; related reactions with phenyl-
(allyl)phosphine lead directly to the triphenyl macrocycle
(1,5,9-[12]aneP3Ph3) which is also liberated stereospecif-
ically.
The first examples of C3 symmetrical triphosphamacrocycles
were those based upon the 1,5,9-[12]aneP3 core prepared by a
stereoselective metal template assisted synthesis using a
Mo(CO)3 template.1,2 Early results confirm that they support
relatively very stable facially capped complexes for a range of
transition metals,3 they may also impose unusual structural
distortions on metal centres (e.g. for metals that prefer square
planar geometries).4 These observations highlight distinct
differences in comparison to the behaviour of analogous
nitrogen or sulfur ligands or of related tripodal phosphines. In
view of their influences on structure and reactivity and their
ability to stabilise low oxidation states, the study of their
complexes is of substantial interest.
In chlorobenzene the cyclisation is more efficient, forming
5
3
[(h -C5H5)Fe(h -1,5,9-[12]aneP3H3)]+[PF6]2 3, in moderate
yield. Complex 3 has a band due to the P–H stretch in its IR
spectrum (nP–H 2303 cm21) and a doublet in the 31P NMR
spectrum (d 12.7, 1JP–H 336 Hz) assigned to phosphorus atoms
in the coordinated macrocycle. This increase in chemical shift
(Dd 21.8 ppm) and coupling constant is similar to that observed
in the analogous Mo(0) complexes, (H2PC3H5)3Mo(CO)3 and
3
(h -1,5,9-[12]aneP3H3)Mo(CO)3, (Dd 27.0 ppm, for the latter
1JP–H 318 Hz).
The current and only route to this class of ligands, by radical
initiated hydrophosphination, does however have significant
preparative limitations. The tris-allylphosphine precursor com-
plexes required for this route are very sensitive to oxygen,
moisture and large scale reactions require fastidious care in
order to be reproducible. Of greater significance is that this
method is currently limited to a minimum ring size of twelve
atoms. We have previously commented upon the inability of the
Mo(CO)3 template, and its smaller chromium analogue, to
support the cyclisation of vinyl phosphines in attempts to
achieve smaller ring sizes.5 These templates lack any possible
steric control over the trans-coordinated precursor phosphines
precluding any freedom to force the ring closure of vinyl
phosphines by compression of the non-bonded P–P distances. In
order to address these problems and to develop a more reliable
synthetic route to tri-phosphines of this nature, we have
undertaken a study of alternative methods. For these reasons we
Hydrophosphination of ethene by 3 gives the tris-ethylated
5
3
macrocyclic
complex
[(h -C5H5)Fe(h -1,5,9-[12]ane-
P3Et3)]+[PF6]2 4 which is characterised by a singlet in the 31
P
5
6
have chosen to investigate the [(h -Cp)Fe(h -arene)]+ class of
complexes which readily form tris-phosphine adducts6 includ-
ing those of acyclic triphosphines (linear6b and tripodal6c) and
of primary phosphines,6d all of which are of course restricted to
5
facial coordination by the h -Cp group. This approach has led
us to an entirely new template method for the stereoselective
synthesis of 1,5,9-[12]aneP3R3 triphosphorus macrocycles;
preliminary results of which are presented herein.
The photolytically activated substitution of the aryl ligand in
5
6
[(h -C5H5)Fe(h -C6H6)]+ (1) in the presence of allylphosphine
5
gives
the
tris-primary
phosphine
complex
[(h -
C5H5)Fe(H2PC3H5)3]+ 2, as the hexafluorophosphate salt, in
quantitative yield (Scheme 1). In the IR spectrum, a peak due to
nP–H is observed (2318 cm21). The microcrystalline solid is
indefinitely air-stable, although in CH2Cl2 solution the complex
decomposes over several days to give a green, NMR silent and
presumably paramagnetic species of undetermined composi-
Scheme 1 Reagents and conditions: i > 3 allylphosphine for 2a or > 3
phenyl(allyl)phosphine for 2b, CH2Cl2, hn ( > 400 nm); ii, chlorobenzene,
AIBN (2 mol%), 90 °C; iii, ethene (2 atm), chlorobenzene, AIBN (2 mol%);
iv, [12]aneP3Et3, CH2Cl2, hn ( > 400 nm); v, Na/NH3(l); 5a, R = Et; 5b, R
= Ph.
DOI: 10.1039/b002174l
Chem. Commun., 2000, 899–900
This journal is © The Royal Society of Chemistry 2000
899

{1H} NMR spectrum (d 38.1) and the absence of a band due to
nP–H in its IR spectrum. The identity of 4 is further supported by
its preparation in THF directly from the template precursor
complex 1 by addition of free 1,5,9-[12]aneP3Et3 obtained by
our previously reported route.2 The products obtained by either
method have identical spectroscopic, analytical and physical
properties. This independent confirmation of the nature of 4, in
turn confirms the characterisation of 3 as the secondary
phosphine macrocycle complex. Both 3 and 4 are air stable for
extended periods in both solution and the solid state.
sizes as well as different ‘backbone’ bridging functions;
preliminary results are encouraging.
We thank the EPSRC for a studentship (A. J. P.).
Notes and references
1 B. N. Diel, P. F. Brandt, R. C. Haltiwanger and A. D. Norman, J. Am.
Chem. Soc., 1982, 104, 4700; B. N. Diel, P. F. Brandt, R. C. Haltiwanger,
M. L. J. Hackney and A. D. Norman, Inorg. Chem., 1989, 28, 2811.
2 P. G. Edwards, J. S. Fleming and S. S. Liyanage, Inorg. Chem., 1996, 35,
4563.
3 P. G. Edwards, J. S. Fleming, S. J. Coles and M. B. Hursthouse, J. Chem.
Soc., Dalton Trans., 1997, 3201.
4 P. G. Edwards, F. Ingold, S. J. Coles and M. B. Hursthouse, Chem.
Commun., 1998, 45.
The reaction of [(h -C5H5)Fe(h -C6H6)]+ with phenyl-
(allyl)phosphine in a manner similar to that for the formation of
2a, leads to the tri-secondary phosphine complex, 2b, in
excellent yield (99%). Complex 2b is characterised by its
31P{1H} NMR spectrum which has two multiplets from the
presence of both diastereomers and enantiomers arising from
the three chiral phosphines in 2b (d 55.8). The radical induced
cyclisation of the coordinated secondary phosphines in 2b
proceeds to the tri-tertiary phosphine macrocycle complex, 4b.
Intermediates which do not readily cyclise are also observed
after extended reaction times; these are readily separated from
4b upon work up. The resistance of these intermediates to
cyclisation is presumably dependant upon the relative chirality
at adjacent phosphines. Although we have recently reported the
synthesis of the triphenyl macrocycle, 5b,7 this is the first
example of the cyclotrimerisation of a secondary phosphine to
a tri-tertiary triphosphorus macrocycle and may offer direct
routes to tri-tertiary phosphines which might otherwise be
difficult to obtain.
5
6
5 P. G. Edwards, J. S. Fleming, S. S. Liyanage, S. J. Coles and M. B.
Hursthouse, J. Chem. Soc., Dalton Trans., 1996, 1801.
6 (a) H. Schumann, L. Eguren and J. W. Ziller, J. Organomet. Chem., 1991,
408, 361; (b) D. Catheline and D. Astruc, Organometallics, 1984, 3,
1094; (c) G. T. Crisp, G. Salem and S. B. Wild, Organometallics, 1989,
8, 2360; (d) D. C. R. Hockless, Y. B. Kang, M. A. McDonald, M. Pabel,
A. C. Willis and S. B. Wild, Organometallics, 1996, 15, 1301; (e) 4b:
R. B. King, P. N. Kapoor and R. N. Kapoor, Inorg. Chem., 1971, 10,
1841; (f) S. G. Davies, H. Felkin and D. Watts, J. Chem. Soc., Chem.
Commun., 1980, 159; (g) 4c: R. B. King, L. W. Houk and K. H. Pannell,
Inorg. Chem., 1969, 8, 1042; (h) J. D. Goodrich and J. P. Selegue,
Organometallics, 1989, 8, 2360.
7 D. J. Jones, P. G. Edwards and R. P. Tooze, J. Chem. Soc., Dalton Trans.,
1999, 1045.
8 Selected data: for 2a: yield: 99%. Anal. Found (calc.): C, 34.5 (34.4); H,
5.3 (5.4)%. MS(ES), m/z 343 (100%, [M2PF6]+). IR (KBr disc): n(P–H),
The macrocycle complexes, 4a and 4b, are digested by
sodium in liquid ammonia resulting in the high yield liberation
of the free macrocyclic ligands 5a and 5b stereoselectively as
the syn–syn isomers. The identities and stereochemistries of
both the triphosphines 5a and 5b, are confirmed by singlets in
their 31P {1H} NMR spectra (d 231.7 and 234.6, respectively);
their spectra are identical to those previously reported.2,7 In all
2319.4 cm21. LM = 25 Ω cm2 mol21 31P NMR: d 29.1 (t, 1JP–H = 328
.
Hz, cation), 2145.0 (septet, 1JP—F 711 Hz, anion). 1H NMR, d 2.54 (m,
H2PCH2CHCH2), 4.12, 5.00 (d m, 1JP–H 328 Hz, H2PCH2CHCH2), 4.55
3
(q, JP–H
2 Hz, Cp-H); 5.20 (m, H2PCH2CHCH2), 5.82 (m,
H2PCH2CHCH2). 13C{1H} NMR:
d 28.2 (d, JP–C = 40 Hz,
1
H2PCH2CHCH2), 80.1 (s, Cp), 119.7 (s, H2PCH2CHCH2), 133.9 (s,
H2PCH2CHCH2). For 2b: yield: 95%. Anal. Found (calc.): C, 53.6
(53.7); H, 5.3 (5.4)%; LM = 39 Ω cm2 mol21. IR (KBr disc): d(P–H),
1
cases analytical, conductivity, H and 13C{1H} NMR data are
5
2303 cm21. MS(ES), m/z 571(100%) [(h -C5H5)Fe(PhPHC3H5)3]+.
consistent with the formulations.8. In addition to the 31P NMR
data discussed above, a resonance due to PF62 is also observed
in each case (d 2144.0, 1JP–F 711 Hz).
1
31P{1H} NMR (CDCl3, 36.23 MHz), d 56.5 (m); 60.2 (m). H NMR: d
4.30 (q), 4.55 (s), 4.69 (s), 4.78 (br m), 4.93 (s), 5.08 (br m), 5.24 (br m),
5.35 (br m), 5.52 (br m), 5.76 (br m), (15H, allyl groups). For 3: yield:
40%. Anal. Found (calc.): C, 33.5 (34.4); H, 4.8 (5.4)%. MS(ES), m/z 343
(100%, [M2PF6]+). IR (KBr disc): d(P–H), 2302.6 cm21. LM = 32 Ω
This new route to the coordinated secondary phosphine
ligands, the facile alkylation of the secondary phosphine
macrocycle complex 3 to the corresponding tertiary phosphines
and the successful and stereospecific liberation of the free
ligands offers an alternative preparation based upon a more
readily accessible template than the sole current literature route
and via relatively air-stable precursor and intermediate com-
plexes. We are currently investigating the influences of
variations in substituents on the Cp ring upon the course of the
macrocyclisation reactions. We are also investigating the
application of this method to the cyclisation of alternative
phosphine precursors in order to achieve both different ring
1
cm2 mol21
.
31P NMR: d 12.7 (d, JP–H 336 Hz, cation), 2145 (septet,
1JP–F = 711 Hz, anion). For 4a. Yield: 95%. Anal. Found (calc): C, 47.0
(46.7); H, 7.6 (7.4)%. MS(ES), m/z 427 (100 %, [M2PF6]+). LM = 40 Ω
cm2 mol21 31P NMR: d 38.1 (s, cation), 2145 (septet, 1JP–F = 711 Hz,
.
anion). 1H NMR: d 1.28 (br m, PCH2CH3), 1.61 (br m, PCH2CH3 and
PCH2CH3CH2P); 1.87 (br m, PCH2CH2CH2P), 4.26 (q, 3JP–H 2 Hz, Cp-
H). For 4b: yield: 30%. Anal. found (calc.): C, 53.6 (53.6); H, 5.4 (5.3)%.
MS (ES), m/z 571 (100%, [M2PF6]+).31P{1H} NMR: d 35.0 (s). 1H
NMR: d 1.97, 2.10, 2.20, 2.33, 2.46, 2.60 (all br m, PCH2 CH2 CH2P),
3.99 (q, 3JP–H 2 Hz, Cp-H), 7.19 (s, phenyl-H), 7.46 (m, phenyl-H), 7.56
(br m, phenyl-H), 7.66 (br m, phenyl-H).
900
Chem. Commun., 2000, 899–900