Novel unsymmetrical P/O substituted ferrocene ligands and the first structurally characterised hydroxyferrocene derivative

Article abstract of DOI:10.1039/b403862b

Two new unsymmetrical 1′-substituted hydroxyferrocene ligands featuring either phosphine or phosphine oxide substituents have been synthesised and the phosphine oxide derivative has been structurally characterised. A nickel complex of the hydroxyl/phosphine ligand has been formed, along with preliminary evaluation of the complex for catalysis of ethylene polymerisation. The Royal Society of Chemistry 2004.

Full text of DOI:10.1039/b403862b

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F U L L P A P E R
Novel unsymmetrical P/O substituted ferrocene ligands and the
first structurally characterised hydroxyferrocene derivative†
Robert C. J. Atkinson, Vernon C. Gibson,* Nicholas J. Long,* Andrew J. P. White and
David J. Williams
Department of Chemistry, Imperial College London, South Kensington, London, UK SW7 2AZ.
E-mail: n.long@imperial.ac.uk; Fax: +44 (0)20 7594 5804; Tel: +44 (0)20 7594 5781
Received 12th March 2004, Accepted 5th May 2004
First published as an Advance Article on the web 19th May 2004
Two new unsymmetrical 1′-substituted hydroxyferrocene ligands featuring either phosphine or phosphine oxide
substituents have been synthesised and the phosphine oxide derivative has been structurally characterised. A nickel
complex of the hydroxyl/phosphine ligand has been formed, along with preliminary evaluation of the complex for catalysis
of ethylene polymerisation.
singlet at approximately 3.50 ppm (depending on concentration and
solvent) was confirmed as being due to the O–H proton by a D2O
exchange experiment.
Introduction
Asymmetric, multidentate ligands that are either unsymmetrical or
potentially hemilabile1 in character have found numerous applica-
tions in catalysis. Chelating ligands with a ferrocene backbone are
of particular interest and extensive studies into their coordination
chemistry and applications within catalysis have been undertaken.2,3
Symmetrical ferrocene species, with the same donor groups sub-
stituted onto the cyclopentadienyl rings, are well known, as are
unsymmetrical 1,2-disubstituted ferrocenes. Much rarer are the
more synthetically challenging unsymmetrical 1,1′-disubstituted
ferrocenes featuring hetero-combinations of N, P or S atoms, usu-
ally formed via bromo,4,5 lithio6 or stannyl7 intermediates. We have
recently published the synthesis of both unsymmetrical thioether-,
thiolate-6 and phosphine-, thiolate-8 neutral-anionic ferrocenediyl
ligands featuring 1,1′-unsymmetrical disubstitution. These ligands
are analogous to the industrially important heteroditopic (P/O−)9–16
and (N/O−)17–19 “SHOP”-based catalysts. (PO)/O− ligands are
scarce but have recently been used in Rh(I)-catalysed hydrofor-
mylation of alkenes.20 So far as we are aware, however, similar
1,1′-P/O− or (PO)/O− ferrocenediyl species are hitherto unknown.
Ferrocene compounds containing hydroxyl-substituents and their
application in coordination chemistry21 are rare: their tendency to
decompose in air to cyclopentenone species makes their synthesis
and handling a challenge. Herein, we report the synthesis of the first
unsymmetrical 1,1′-ferrocenediyl compound featuring a hydroxyl
substituent in combination with another donor group (in this case
either a phosphine or phosphine oxide). Preliminary studies into the
ligands’ coordination chemistry and ability to polymerise ethylene
are also undertaken.
Scheme 1 Reagents and conditions: (i) n-BuLi, THF, −25 °C,
30 min; ClPPh2, THF, rt, 20 h; (ii) n-BuLi, THF, −78 °C, 10 min;
TMSOOTMS, −78 °C, 1 h; (iii) N2 sat. H2O; (iv) air sat. H2O; (v) NaH,
THF, 60 °C, 15 h. (vi) trans-[Ni(PPh3)2PhCl], toluene, 20 h.
The experimental procedure for 3 was adapted to synthesise
4: crude 2 was dissolved in diethyl ether, air-saturated water was
added and the mixture vigorously stirred for 15 h under nitrogen.
The layers were separated and the aqueous layer was acidified to
pH = 1 and the resulting yellow precipitate extracted into diethyl
ether, dried (Na2SO4) and the solvent removed yielding 4 as an or-
ange crystalline solid in 21% yield. 1H NMR spectroscopy showed
four pseudo-triplets for the C5H4 ring protons together with signals
for the phenyl groups, shifted downfield compared to 3, and a sharp
singlet at 9.89 ppm which was assigned to the O–H proton. The
ability of the phosphine oxide group to act as a hydrogen bond
acceptor is thought to promote intermolecular hydrogen bond-
ing (this was later confirmed by X-ray analysis) hence the higher
chemical shift and sharper peak compared to 3. The 31P{1H} NMR
spectra of both 3 and 4 show singlets, at −18.0 and +33.2 ppm re-
spectively. Although solution studies indicated that the species was
not stable, a single crystal X-ray analysis confirmed the presence
in the solid state of a stable (in air for at least one month) form
of hydroxyferrocenediyldiphenylphosphine oxide (4), Fig. 1. The
geometry at phosphorus is distorted tetrahedral with angles in the
Results and discussion
The use of bis(trimethylsilyl)peroxide22 in the preparation of fer-
rocenyl trimethylsilyl ethers23 and ferrocenyl hydroxide deriva-
tives24 (via hydrolysis) prompted its application in the synthesis of
3 and 4 (Scheme 1). Selective lithium–halogen exchange was
used with 1,1′-dibromoferrocene to synthesise 1 as described by
Dong et al.5 and Butler et al.25 1 was reacted with one equiv. of
n-butyllithium in a cooled THF solution, followed by addition of
bis(trimethylsilyl)-peroxide. 2 was immediately hydrolysed in-situ
with nitrogen-saturated water to give 3 as a yellow, air-sensitive
1
powder, after purification, in 45% yield. H NMR spectroscopy
showed the unsymmetrical substitution of the cyclopentadienyl
rings with the presence of four pseudo triplets for the C5H4 ring
protons together with signals due to the phenyl groups. A broad
† Electronic supplementary information (ESI) available: Fig. S1: Molecular
structure of 3; Fig. S2: Molecular structure of 4 (50% probability ellipsoids).
See http://www.rsc.org/suppdata/dt/b4/b403862b/
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 8 2 3 – 1 8 2 6
1 8 2 3

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range 103.5(2)–115.5(2)°; the bond lengths are typical of cyclo-
pentadienyl-substituted diphenylphosphine oxides26 and emphasise
the double bond nature of the PO linkage [1.498(4) Å]. The two
cyclopentadienyl rings are inclined by only ca. 2° and staggered by
ca. 11°. The P–C5H4 and O–C5H4 vectors are inclined by ca. 156°,
a geometry very similar to that observed in the related carboxylic
acid analogue.26 There have been to our knowledge no previously
reported examples of structurally characterised hydroxyferrocene
complexes, the only remotely related compound27 being one con-
taining a bis(trimethylsilyl)hydroxycyclopentadienyl iron unit
where the C5–OH bond length is 1.363 Å, a value equivalent to that
which we observe in 4 [1.360(6) Å].Afeature of the structure which
contributes to its stability is the presence in the solid state of an
intermolecular chain of O–HO hydrogen bonds between the hy-
droxyl and the phosphine oxide groups of screw related molecules
(Fig. 2); OO 2.69 Å, HO 1.86 Å, O–HO 152°.
1.838(3) Å] cf. their values in 4 [1.784(5), 1.815(2) and 1.817(3) Å
respectively]. Comparable longer P–C bond lengths are also ob-
served in the sulfur analogue of 3.8
A preliminary investigation into the application of 3 and 4 in
coordination chemistry and catalysis has been undertaken. 3 and
4 were screened with Ni precursors for ethylene oligomerisation
by dissolving 10 mol of either 3 or 4 in toluene with an excess of
Ni(COD)2 (in order to prevent formation of an inactive bis-chelate
species). The reaction mixture was stirred at 40 °C under an ethyl-
ene pressure of 1 bar for 1 h but unfortunately, no polymer or oligo-
mer formation was observed. The reaction was repeated at 70 °C
under an ethylene pressure of 50 bar for 1 h but again no polymer or
oligomer formation was observed. Peuckert and Keim reported that
upon the reaction of diphenylphosphinoacetic acid with an excess
of Ni(COD)2, both a 4-enyl and a 3-allyl complex was formed in
the ratio 3:228 so in order to form a single well-defined Ni complex,
a complex of 3 analogous to Keim’s original SHOP catalyst was
synthesised. The sodium salt of 3 was formed in situ by stirring the
ligand with 3 equiv. of sodium hydride and then added to a toluene
solution of trans-bis(diphenylphosphine)phenylchloronickel(II).
After stirring at room temperature for 20 h, the solvent was removed
in vacuo and the coffee-coloured residue washed with pentane and
dried in vacuo to give 5 in 45% yield. We suggest that the complex
has a square planar structure due to its characteristic colour (tetra-
hedral Ni(II) complexes are usually blue or green) and the lack of
broadening in the NMR spectra (i.e. diamagnetic metal centre). 1H
NMR shows that a single isomer is formed. Three multiplets are ob-
served corresponding to the C5H4 ring protons together with peaks
for the Ph, PPh3 and PPh2 protons. The positive FAB mass spectrum
gives fragmentation peaks due to loss of Ph and PPh3 and the mi-
croanalytical data are consistent with the predicted formula. Inter-
estingly, the 31P{1H} NMR exhibits only one peak at +24.0 ppm
(shifted downfield with respect to 3). We propose that this is due
to the PPh3 and C5H4–PPh2 environments having the same chemi-
cal shift, thus giving a single peak. This prediction was confirmed
by the reaction of 5 with 3 equiv. of tricyclohexylphosphine in d8-
toluene in order to substitute the PPh3 moiety for the more strongly
donating PCy3 in an attempt to differentiate the two P environments.
After heating the mixture at 40 °C for 20 h, a peak for the liberated
PPh3 was observed together with two doublets (2JP–P = 295 Hz) at
14.27 and 6.50 ppm corresponding to the PCy3 analogue of 5 and
unreacted PCy3. The observed coupling constant indicates a trans
configuration and is similar to the 2JP–P values previously reported
for trans-[P,O]Ni(Ph)(PR3) complexes.10,12,13 We therefore assume
that 5 has the same structure.
Fig. 1 The molecular structure of 4.
5 was also tested as a precatalyst for ethylene oligomerisation or
polymerisation. The complex was dissolved in toluene with 6 equiv.
of Ni(COD)2 to act as a phosphine scavenger. The reaction mixture
was subjected to 1 bar ethylene pressure for 1 h at 70 °C. Unfortu-
nately, no polymer or oligomer formation was observed.
Work is in progress to further explore the diverse coordination
chemistry expected for 3 and 4 with other metal precursors and the
application of these complexes in homogeneous catalysis. The ac-
tivity of modified versions of 3 and 4 with a variety of Ni precursors
is also currently under study to further investigate their potential use
in Ni-catalysed ethylene polymerisation catalysis.
Experimental
General procedures
Fig. 2 The linking of screw-related molecules of 4 via O–HO hydrogen
bonds (a) with OO 2.69 Å, HO 1.86 Å and O–HO 152°.
All preparations were carried out using standard Schlenk tech-
niques.29 All solvents were distilled over standard drying agents
under nitrogen directly before use, and all reactions were carried
out under an atmosphere of nitrogen. Chromatographic separa-
tions were carried out on silica gel (Kieselgel 60, 70–230 mesh).
All NMR spectra were recorded using a Delta upgrade on a JEOL
EX270 MHz spectrometer operating at 270.1 MHz (1H) and 101.3
MHz (31P{1H}). Chemical shifts () are reported in ppm using C6D6
(1H, 7.15 ppm) as the reference, while 31P{1H} spectra were refer-
enced to H3PO4. Mass spectra were recorded using positive FAB
In attempts to determine the solid state structure of 3, a single
crystal X-ray analysis revealed the presence in the solid state of
a co-crystallised 75:25 mixture of the simple phosphine hydroxy
complex 3 and its phosphine oxide analogue 4. The structure is
further complicated by the presence of at least four identifiable ori-
entations for the C5H4–OH unit (Fig. S1: see ESI†). A noteworthy
feature of this structure is a counter-intuitive significant lengthen-
ing of the three P–C bonds [P–C5H4 1.805(6) Å, P–Ph 1.832(3),
1 8 2 4
D a l t o n T r a n s . , 2 0 0 4 , 1 8 2 3 – 1 8 2 6

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methods, on anAutospec Q mass spectrometer. Microanalyses were
carried out at SACS, University of North London. trans-Bis(diph
enylphosphine)phenylchloronickel(II) was prepared according to
literature methods.30
Anal. Calc. for C46H38FeNiOP2: C 70.54, H 4.89%. Found: C
70.13, H 4.92%; H NMR (C6D6) ppm: 3.40 (t, 2H, C5H4), 4.06
1
(m, 4H, C5H4), 4.55 (t, 2H, C5H4), 6.55 (m, 5H, C6H5), 7.00 (m, 15H,
PC6H5), 7.44 (m, 4H, FcPC6H5), 7.94 (m, 6H, FcPC6H5); 13C{1H}
NMR (C6D6) ppm: 63.24 (C5H4), 64.37 (C5H4), 70.79 (d, o-C5H4),
74.42 (d, ipso-P–C5H4, 1JP–C = 11 Hz), 84.96 (C5H4), 121.90 (ipso-
O–C5H4), 122.00 (NiC6H5), 125.45 (NiC6H5), 128.00 (C6H5 obsc. by
C6D6), 128.79 (P(C6H5)3), 129.75 (P(C6H5)3), 130.46 (C6H5), 131.4
(br d, ipso-C6H5), 131.8 (br d, ipso-C6H5), 132.66 (d, ipso-C6H5),
133.16 (C6H5), 134.75 (d, o-FcP(C6H5)2), 137.21 (FcP(C6H5)2);
31P{1H} NMR (C6D6) ppm: 24.0; m/z: 705 (M+–C6H5), 520 (M+–
P(C6H5)3), 386 (3+).
Syntheses: 3. Compound 1 (0.92 g, 2.05 mmol) was dissolved in
dry THF (40 cm3) and cooled to −78 °C. To this solution was added
n-butyllithium (1.6 M solution in hexane, 1.3 cm3, 2.05 mmol) and
the solution stirred at −78 °C for 10 min. Bis(trimethylsilyl)peroxide
(0.34 cm3, 3.08 mmol) was added and the solution stirred at −78 °C
for 1 h before being allowed to warm to room temperature. Nitro-
gen-saturated water was added (10 cm3) and the solution was stirred
vigorously. Nitrogen-saturated diethyl ether was added (40 cm3)
and the layers were separated. The aqueous layer was re-extracted
with diethyl ether (20 cm3) and the combined organic layers dried
(Na2SO4) and the solvent removed in vacuo yielding a yellow solid.
The crude mixture was purified by column chromatography (silica,
hexane:EtOAc 2:1) yielding 3 as a yellow, air sensitive powder
(0.36 g, 45%).
Crystal data for 3. (C22H19FeOP)0.75(C22H19FeO2P)0.25, M =
390.2, monoclinic, P21/c (no. 14), a = 14.145(2), b = 10.461(2),
c = 11.997(2) Å,  = 90.32(1)°, V = 1775.2(4) Å3, Z = 4, Dc =
1.460 g cm−3, (Cu K) = 7.72 mm−1, T = 203 K, orange needles;
2588 independent measured reflections, F2 refinement, R1 = 0.054,
wR2 = 0.129, 1813 independent observed absorption corrected re-
flections [|Fo| > 4(|Fo|), 2max = 120°], 218 parameters.
Anal. Calc. for C22H19FeOP: C 68.42, H 4.96%. Found: C 68.54,
H 5.09%; 1H NMR (C6D6) ppm: 3.50 (br s, 1H OH), 3.60 (t, 2H,
C5H4), 3.94 (t, 2H, C5H4), 4.07 (t, 2H, C5H4), 4.14 (t, 2H, C5H4), 7.06
(m, 6H, PC6H5), 7.53 (m, 4H, PC6H5); 13C{1H} NMR (C6D6) ppm:
59.59 (C5H4), 63.68 (C5H4), 71.95 (C5H4), 73.80 (d, ipso-P–C5H4,
1JP–C = 14 Hz), 75.88 (C5H4), 121.15 (ipso-O–C5H4), 128.47 (C6H5),
Crystal data for 4. C22H19FeO2P, M = 402.2, orthorhombic,
Pna21 (no. 33), a = 13.822(2), b = 14.044(2), c = 9.254(2) Å, V =
1796.2(4) Å3, Z = 4, Dc = 1.487 g cm−3, (Mo K) = 0.94 mm−1, T =
293 K, yellow blocky needles; 2179 independent measured reflec-
tions, F2 refinement, R1 = 0.039, wR2 = 0.088, 1875 independent
1
128.82 (C6H5), 134.02 (d, o-C6H5, JP–C = 19 Hz), 139.31 (br d,
ipso-C6H5); 31P{1H} NMR (C6D6) ppm: −18.0; m/z: 386 (M+), 309
observed absorption corrected reflections [|Fo| > 4(|Fo|), 2max
=
(M+–Ph), 305 (M+–C5H4OH), 202 (M+–PPh2).
55°], 215 parameters. The absolute structure of 4 was determined by
a combination of R-factor tests [R1+ = 0.0390, R1− = 0.0410] and by
use of the Flack parameter [x+ = +0.05(6), x− = +0.95(6)].
CCDC reference numbers 214905 (3) and 214906 (4).
See http://www.rsc.org/suppdata/dt/b4/b403862b/ for crystal-
lographic data in CIF or other electronic format.
Syntheses: 4. Compound 1 (0.50 g, 1.11 mmol) was dissolved in
dry THF (25 cm3) and cooled to −78 °C. To this solution was added
n-butyllithium (1.6 M solution in hexane, 0.69 cm3, 1.11 mmol) and
the solution stirred at −78 °C for 10 min. Bis(trimethylsilyl)peroxide
(0.18 cm3, 1.67 mmol) was added and the solution stirred at −78 °C
for 1 h before being allowed to warm to room temperature. The
solvent was removed in vacuo and the residue redissolved in air-
saturated diethyl ether (30 cm3). Air-saturated water was added
(30 cm3) and the solution was stirred vigorously for 15 h under
nitrogen, after which time the organic layer was dark green and the
aqueous layer was dark brown. The layers were separated and the
organic layer extracted with water (30 cm3). The combined aqueous
layers were acidified to pH = 1 with concentrated HCl yielding a
yellow precipitate which was extracted into diethyl ether (80 cm3).
The layers were separated and the organic layer was washed with
water (20 cm3), dried (Na2SO4) and the solvent removed in vacuo
yielding 4 as an orange crystalline solid (0.09 g, 21%). Crystals
suitable for X-ray diffraction were grown by slow evaporation from
diethyl ether solution.
Acknowledgements
We acknowledge financial support from the Department of Chem-
istry, Imperial College London.
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D a l t o n T r a n s . , 2 0 0 4 , 1 8 2 3 – 1 8 2 6
1 8 2 5

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D a l t o n T r a n s . , 2 0 0 4 , 1 8 2 3 – 1 8 2 6