Stabilising Rh-P coordination by phosphanylalkylcyclopentadienyl ligands

Article abstract of DOI:10.1039/b211287f

(Phosphanylalkyl/aryl)cyclopentadienyl complexes have been synthesized from lithiated 1,2,3,4-tetramethyl-5-(2-P-dialkyl/arylphosphinoethyl)cyclopentadiene and [RhCl(CO)₂]₂; the electron donating properties of these ligands have been

Full text of DOI:10.1039/b211287f

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Stabilising Rh–P coordination by phosphanylalkylcyclopentadienyl
ligands
Ann E. C. McConnell,a Douglas F. Foster,b Peter Pogorzelec,b Alexandra M. Z. Slawin,a
David J. Lawc and David J. Cole-Hamilton*a
a School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST.
E-mail: djc@st-and.ac.uk
b Catalyst Evaluation and Optimisation Service, School of Chemistry,
University of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST
c Hull Research and Technology Centre, BP Chemicals, Salt End, Hull, UK HU12 8DS
Received 15th November 2002, Accepted 10th January 2003
First published as an Advance Article on the web 23rd January 2003
Table 1 Properties of phosphanylethylcyclopentadienyl complexes of
rhodium and the related cyclopentadienyl phosphine complexes
(Phosphanylalkyl/aryl)cyclopentadienyl complexes have
been synthesised from lithiated 1,2,3,4-tetramethyl-5-
(2-P,P-dialkyl/arylphosphinoethyl)cyclopentadiene
and
Compound
νCO/cmϪ1
C–O/pm
Rh–P/pm
[RhCl(CO)2]2; the electron donating properties of these
ligands have been compared and the P–Rh interaction is
shown to be stable under methanol carbonylation reaction
conditions.
1
1924
1933
1938
1947
1920
1947
1944
1953
116.3(8)
115.7(5)
117.1(6)
221.6(2)
223.06(10)
218.38(11)
2
3
410
511
611
711
812
Electron rich metal complexes have recently been shown to have
a number of important uses in catalysis,1 with considerable rate
enhancements being observed in reactions such as methanol
carbonylation, where nucleophilic attack of the metal centre on
methyl iodide is the rate determining step.2,3 In particular, we
have recently shown that rhodium-PEt3 complexes are more
active than the industrially used catalyst, [RhI2(CO)2]Ϫ,3
although its lifetime is short because the PEt3 ligand is lost as
Et3PO. We have also shown that a rhodium complex containing
the electron donating pentamethylcyclopentadienyl (Cp*) lig-
and is highly active for the carbonylation of methyl acetate to
acetic anhydride in the absence of added hydrogen.4 A related
cobalt complex containing two electron donating ligands,
[Cp*Co(CO)PEt3], has a higher intrinsic activity for methanol
carbonylation than does [RhI2(CO)2]Ϫ.5 These observations of
the beneficial effects of pentamethylcyclopentadienyl ligands
and trialkylphosphines has led us to investigate the possibility
of stabilising the coordination of both ligands using the chelate
effect. To do this we now report the synthesis of a ligand in
which a trialkylphosphine is attached to a pentamethylcyclo-
pentadienyl ligand via a hydrocarbon bridge together with the
properties of rhodium complexes of this and related ligands.
Many ligands of this type have been reported,6,7 but none that
contain a permethylated cyclopentadienyl ring and ethyl groups
on phosphorus. The chemical properties of the two groups are
quite different and binding these onto a metal may lead to some
interesting chemical reactivities.8
phide in high yield. Tetramethyl-spiro-[2,4]-hepta-4,6-diene was
synthesised via direct alkylation of lithium tetramethylcyclo-
pentadienide with 1-bromo-2-chloroethane and subsequent
reaction with butyl lithium.7 †
The ligands were then reacted with [RhCl(CO)2]2 to give the
complexes 1–3 (Fig. 1) as air stable red solids, which were fully
characterised analytically and spectroscopically and by X-ray
structural determinations. These data are deposited, but for
compound 1 the structure is shown in Fig. 2.‡ They all show
The lithiated ligand and some related ones that are already
known,7,10,11 were synthesised by reactions of the spirohydro-
carbons shown in Fig. 1 with lithium diethyl/diphenyl phos-
Fig. 2 X-Ray crystal structure and numbering scheme for compound
1. Selected bond angles (Њ): Rh(1)–P(1)–C(1) 103.2(2), P(1)–C(1)–C(2)
112.6(3), C(1)–C(2)–C(3) 112.7(6), C(2)–C(3)–C(4) 126.1(6), C(2)–
C(3)–C(7) 126.1(5), C(4)–C(3)–C(7) 107.8(4).
chelate binding of the ligands through the cyclopentadienyl
ring and the phosphorus atom with little strain in the bridge
and planarity at C(3). The IR stretching frequencies (Table 1)
of these complexes were used as an indication of the electron
donating properties of the ligands and are compared with
those of the known complex 410 and complexes in which the Cp
ring is not attached to the phosphine, [(C5R5)Rh(CO)(PRЈ3)],
(5 R = Me, RЈ = Et;11 6 R = Me, RЈ = Ph;11 7 R = H, RЈ = Et11
and 8 R = H, RЈ = Ph12). A low νCO indicates a high electron
density on the metal centre because of increased back bonding
into the π* orbitals of the CO.
Fig. 1 Synthesis of chelating ligands and their rhodium complexes.
D a l t o n T r a n s . , 2 0 0 3 , 5 1 0 – 5 1 2
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 3
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Table 2 Carbonylation of methanol catalysed by rhodium complexes
From comparison of 1 with 5, it appears that there is similar
electron density on the metal for the chelating ligand to that in
the analogous complex containing Cp* and PEt3 although
comparison of 3 with 7 shows that the extra methylene substit-
uent on the ring in 3 increases the electron density at rhodium.
For the other pairs of complexes (2 and 6, 4 and 8), the electron
density on rhodium is enhanced by the presence of the chelate
backbone because a phenyl group on phosphorus is replaced by
the alkyl backbone chain. As expected, the compounds with
RЈ = Et (1, 3, 5, 7) are significantly more electron rich then their
analogues with RЈ = Ph (2, 4, 6, 8) and those with R = Me (1, 2,
5, 6) are significantly more electron rich than if R = H (3, 4, 7,
8), so that the most electron rich of the complexes containing
Entry
Complex
Rate/mol (dm solution)Ϫ3 hϪ1
1
2
3
4
5
1a
0.4
0.4
5a, b
a
[RhCl(CO)2]2
0.8
1c
11.4, 2d
10.9, 0f
3e
a [Rh] = 1.25 mmol dmϪ3, [MeI] = 1.61 mol dmϪ3, [MeOAc] = 2.52 mol
dmϪ3, [HOAc] = 10.5 mol dmϪ3, [H2O] = 5.55 mol dmϪ3, solution
volume = 10 cm3, 150 ЊC, 27 bar. b PEt3 (1.25 mmol dmϪ3) added.
c [Rh] = 0.11 mol dmϪ3, [MeI] = 1.37 mol dmϪ3, [MeOH] = 21 mol dmϪ3
,
[H2O] = 3.32 mol dmϪ3, solution volume = 5.85 cm3, 150 ЊC, 27 bar.
d Rate drops after 30 min (20 bar consumed, 26% conversion). e As c,
the chelate ligands is 1 with R = Me and RЈ = Et.
except [Rh] = 0.078 mol dmϪ3 f Reaction stops after 13 min (5 bar
.
᎐
The C᎐O bond lengths for complexes 1–3 are not signifi-
᎐
consumed, 6.5% conversion).
cantly different. However, the Rh–P bond lengths are in the
order 2 > 1 > 3, suggesting that, at least for the PEt2 complexes,
the Rh–P bond has little π-back bonding component. Signifi-
cant back binding should make Rh–P in 1 shorter than that
in 3.
[Rh(Et2PCH2CH2C5Me4)(C(O)Me)I] and [Rh(Et2PCH2CH2-
C5Me4)I2], respectively, whilst the remaining one (δ 71.1, JRhP
=
134 Hz) is from an unidentified complex. For this complex, the
coupling to rhodium confirms that the PEt2 unit is still bound,
whilst the chemical shift indicates that the chelate is still in tact.
If the –C5Me4 moiety were no longer bound, a chemical shift of
ca. 20–30 ppm would be expected.3
Additional evidence for the large electron density on, and
hence high nucleophilicity of, the rhodium centre for complex 1
comes from its reaction with MeI at room temperature to give
[Rh(Et2PCH2CH2C5Me4)(C(O)Me)I], (9, 31P δ 65.2 JRhP
=
Complex 5 also gave a rate similar to that obtained using
complex 1, but in this case 31P NMR studies showed almost
complete conversion of Et3P to Et3PO (δ 82.0) even at 90 ЊC
(there was a small doublet at δ 26.2, JRhP = 103 Hz, accounting
for about 10% of the P atoms). Interestingly, a similar reaction
using complex 3 was much less successful than that with 1, with
the reaction halting after 6.5% conversion. This suggests that
the methyl groups on the Cp ring are essential for catalyst
stability.
165 Hz, Fig. 3) and with deuteriated methylene chloride at
We conclude that binding of both Cp* and PEt3 to rhodium
can be sufficiently stabilised by chelation that complexes con-
taining [Et2PCH2CH2C5Me4]Ϫ are stable under the aggressive
conditions used for methanol carbonylation.
Fig. 3 Reaction of 1 with MeI.
room temperature to give [Rh(Et2PCH2CH2C5Me4)(CO)Cl2]
(10, 31P δ 76.3 JRhP = 189 Hz). This reaction was carried out at
room temperature in an NMR tube with CD2Cl2 that had been
pretreated with NaHCO3 to remove any HCl that may have
been present. After 18 h the reaction had gone halfway to com-
pletion and after 48 h all of complex 1 had reacted. These
products were characterised by 1H NMR, 13C NMR, 31P NMR
and X-Ray crystallography (deposited). We have previously
reported that electron rich rhodium complexes can oxidatively
add dihaloalkanes13 and shown that the haloalkyl complexes
formed react with CO to give dihalo complexes, probably with
the elimination of ketene.14
We thank BP Chemicals and the EPSRC for a CASE
studentship (A. E. C. M.).
Notes and references
† Synthesis: There is some controversy in the literature about the exact
regioselectivity of the reaction with 1-bromo-2-chloroethane,7,9 but we
find that the method described by van Beek and Gruter gives the
desired spirohydrocarbon in 50% yield.7
Compound 1. LiPEt2 (1.15 g, 12.0 mmol) was stirred with 4,5,6,7-
tetramethyl-spiro-[2,4]-hepta-2,4-diene (2.0 g, 13.5 mmol) in THF
(50 cm3) at room temperature for 3 d. The THF was removed in vacuo
and the residue washed with light petroleum (bp 40–60 ЊC, 2 × 50 cm3)
to leave a white solid (LiC5Me4CH2CH2PEt2, 2.55 g, 87%). THF
(10 cm3) containing this white solid (0.18 g, 0.72 mmol) was
added dropwise to THF (50 cm3) containing [RhCl(CO)2]2 (0.11 g,
0.28 mmol). The colour changed immediately from pale yellow to dark
red and the solution was refluxed for 8 h. The solvent was removed in
vacuo and light petroleum (50 cm3) added. After filtration, of a white
solid, the red solution was concentrated to half volume and cooled to
Ϫ78 ЊC to yield red crystals (0.12 g, 58%). Compounds 2 and 3 were
similarly prepared but using LiPPh2 and spiro-[2,4]-hepta-2,4-diene
respectively.
In an attempt to discover whether the chelate stabilisation of
the bonding of the phosphine and the Cp* ring is effective
under methanol carbonylation conditions, complex 1 was used
for methanol carbonylation in the presence of methyl iodide
(Entry 1, Table 2). Zero order kinetics were obtained through-
out the 4 h reaction at a rate approximately half that obtained
using [RhCl(CO)2]2, which gives the Monsanto catalyst under
the reaction conditions. A 31P NMR spectrum of the final
solution from reaction 4, Table 2, showed that most of the P
atoms were coupled to rhodium (no oxidation had occurred,
but 31P resonances at δ 36.1 and 33.7 may indicate small
amounts of quaternised phosphine (<5%)§. Most of the rho-
dium precipitated from the final solution as pure [Rh(Et2-
PCH2CH2C5Me4)I2] (11, 31P δ 60.5, JRhP = 147 Hz), which was
crystallographically characterised.
Compound 9. 1 (0.1 g, 0.27 mmol) in THF (15 cm3) was strirred with
MeI (2 cm3, 32.1 mmol) at room temperature for 4 h. The solvent was
removed in vacuo and the red solid recrystallised from light petroleum
(25 cm3). Yield 0.1 g, 75%.
Compound 10. 1 (0.05 g, 0.14 mmol) was stirred at room temperature
for 24 h in CD2Cl2 (5 cm3), which had previously been treated with
Na2CO3. After evaporation to dryness, the product was recrystallised
from benzene to give bright orange crystals (0.05 g, 87%).
Compound 11. 1 (0.23 g,0.61 mmol) was dissolved in methanol
(4 cm3) and charged into a mechanically stirred autoclave, which had
previously been flushed with CO. Water (0.35 cm3) was added, the
reactor pressurised to 25 bar and the temperature raised to 150 ЊC.
After 15 min, methanol (1 cm3) containing MeI (0.5 cm3) was injected
from a substrate injector and the pressure adjusted to 27 bar. CO was
An in situ 31P NMR study carried out under similar condi-
tions to those used for entry 1, Table 2, but using a higher
concentration of rhodium and 180 ЊC, also showed that all the
P was coordinated to Rh throughout the reaction. Three doub-
lets (δ 71.1, JRhP = 134 Hz, 64.6, JRhP = 166 Hz and 61.0, JRhP
=
150 Hz) in a ratio 20 : 3 : 5 were the only resonances observed at
the end of the reaction. The last two of these can be assigned to
D a l t o n T r a n s . , 2 0 0 3 , 5 1 0 – 5 1 2
511

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then fed into the reactor from a ballast vessel through a mass flow
controller, which maintained the pressure within the reactor at 27 bar.
The pressure in the ballast vessel was monitored every 5 s. After 2 h, the
reactor was cooled and vented. The red crystals which separated from
the deep red solution were collected and identified as 11. Catalytic reac-
tions were carried out as described above, but using the conditions
described in Table 2, with rates being calculated from the pressure drop
within the ballast vessel.
5421 observed, R1 = 0.0766, wR2 = 0.1568. CCDC reference number
197740.
For 11, [C15H26I2PRh], Mr = 594.04, monoclinic, space group Cc,
a = 12.1284(13), b = 13.5763(15), c = 11.9212(13) Å, β = 92.718(8)Њ,
U = 1960.7(4) Å3, Z = 4, F(000) = 1128, 0.18 × 0.1 × 0.1 mm, 4132
reflections collected, 2662 observed, R1 = 0.0316, wR2 = 0.0842. CCDC
reference number 197741. See http://www.rsc.org/suppdata/dt/b2/
b211287f/ for crystallographic data in CIF or other electronic format.
§ Doublets were observed at δ 76.5 (JRhP = 117.1 Hz), 63.9 (JRhP = 145.3
Hz) and 36.3 (JRhP = 68 Hz) and broad singlets at δ 59 and 62 Hz). The
phosphine oxide is expected to resonate near δ 70 and the quaternised
phosphine near δ 40.3
‡ All data were collected on a Bruker SMART CCD diffractometer
at 293(2) K using monochromated radiation [µ(Mo-Kα) = 24.58 cmϪ1
,
λ = 0.71073 Å]. The Co, I, P and O atoms were refined anisotropically,
the C atoms isotropically (any exceptions are noted). Structures were
solved by direct methods and expanded using Fourier techniques.
Crystal data for 1, [C16H26OPRh]: Mr
=
368.25, monoclinic,
space group P21/c, a = 7.2206(6), b = 16.6210(13), c = 14.3525(12) Å,
1 M. C. Simpson and D. J. Cole-Hamilton, Coord. Chem. Rev., 1996,
155, 163–207.
2 M. J. Howard, G. J. Sunley, A. D. Poole, R. J. Watt and B. K.
Sharmas, Science Technol. Catal., 1998, 61.
3 J. Rankin, A. C. Benyei, A. D. Poole and D. J. Cole-Hamilton,
J. Chem. Soc., Dalton Trans., 1999, 3771.
4 A. C. Marr, P. Lightfoot, E. J. Ditzel, A. D. Poole, G. P. Schwarz,
D. F. Foster and D.J. Cole-Hamilton, Inorg. Chem. Commun., 2000,
3, 617.
5 A. C. Marr, E. J. Ditzel, A. C. Benyei, P. Lightfoot and D. J.
Cole-Hamilton, Chem. Commun., 1999, 1379.
6 H. Butenschon, Chem. Rev., 2000, 100, 1527.
7 J. A. M. van Beek and G. J. M.Gruter, Eur. Pat. Appl., 1997,
97201270.2.
8 C. Muller, D. Vos and P. Jutzi, J. Organomet. Chem., 2000, 600, 127.
9 D. P. Krut’ko, M. V .Borzov, E. N. Veksler, R. S. Kirsanov and
A. V. Churakov, Eur. J. Inorg. Chem., 1999, 1973.
10 I. Lee, F. Dahan, A. Maisonnat and R. Poilblanc, Organometallics,
1994, 13, 2743.
β = 102.160(2)Њ, U = 1683.8(2) Å3, Z = 4, F(000) = 760, 0.1 × 0.03 × 0.03
mm, 7070 reflections collected, 2382 observed, R1 = 0.0440, wR2
=
0.0838. CCDC reference number 197744.
For 2, [C24H26OPRh]: Mr = 464.33, monoclinic, space group P21/n,
a = 8.8431(9), b = 24.718(3), c = 9.8846(10) Å, β = 101.548(2)Њ, U =
2116.9(4) Å3, Z = 4, F(000) = 952, 0.1 × 0.1 × 0.03 mm, 10327 reflections
collected, 2989 observed. R1 = 0.0297, wR2 = 0.0632. CCDC reference
number 197742.
For 3, [C12H18OPRh]: Mr = 312.14, orthorhombic, space group
Pna2(1), a = 12.3029(4), b = 9.5500(3), c = 11.1065(4) Å, U = 1304.93(8)
Å3, Z = 4, F(000) = 632, 0.08 × 0.08 × 0.1 mm, 5187 reflections
collected, 1841 observed, R1 = 0.0229, wR2 = 0.0562. CCDC reference
number 197743.
For 9, [C17H29IOPRh], Mr = 510.18, monoclinic, space group P21/c,
a = 8.0159(2), b = 17.77960(10), c = 13.6522(3) Å, β = 94.2430(10) Њ,
U = 1940.37(7) Å3, Z = 4, F(000) = 1008, 0.2 × 0.2 × 0.03 mm, 7999
reflections collected, 2694 observed. C(21), C(22) and O(1) were refined
as a rigid body as this proved the best way to overcome the slight
disorder in the structure, R1 = 0.0520, wR2 = 0.1336. CCDC reference
number 197739.
For 10, [C16.5H27.5Cl2PRh], Mr = 430.67, orthorhombic, space group
Pbcn, a = 30.6515(13), b = 8.6919(4), c = 28.3619(7) Å, U = 7556.2(5) Å3,
Z = 16, F(000) = 3528, 0.1 × 0.1 × 0.03 mm, 31133 reflections collected,
11 S. Doring and G. Erker, Synthesis (Stuttgart), 2001, 1, 43.
12 H. Werner and B. Klingert, J. Organomet. Chem., 1982, 233, 365.
13 R. C. Gash, D. J. Cole-Hamilton, R. Whyman, J. C. Barnes and
M. C. Simpson, J. Chem. Soc., Dalton Trans., 1994, 1963.
14 W. Weston and D. J. Cole-Hamilton, Inorg. Chim. Acta, 1998, 280,
99–117.
D a l t o n T r a n s . , 2 0 0 3 , 5 1 0 – 5 1 2
512