Novel palladium complexes employing mixed phosphine phosphonates and phosphine phosphinates as anionic chelating [P,O] ligands

Article abstract of DOI:10.1039/b612694d

A route to various substituted phosphine phosphonic acid compounds of the general form Ar₂PC₆H₄PO(OH)₂ (Ar = Ph, o-MeC₆H₄, o-MeOC₆H₄) has been investigated. These comp

Full text of DOI:10.1039/b612694d

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www.rsc.org/dalton | Dalton Transactions
Novel palladium complexes employing mixed phosphine phosphonates and
phosphine phosphinates as anionic chelating [P,O] ligands†
Corinna M. Reisinger, Ru¨diger J. Nowack, Dirk Volkmer and Bernhard Rieger*
Received 13th September 2006, Accepted 30th October 2006
First published as an Advance Article on the web 14th November 2006
DOI: 10.1039/b612694d
A route to various substituted phosphine phosphonic acid compounds of the general form
Ar2PC6H4PO(OH)2 (Ar = Ph, o-MeC6H4, o-MeOC6H4) has been investigated. These compounds were
employed as bidentate anionic [P,O] ligands in neutral palladium complexes. The [P,O] chelating
coordination was determined by X-ray crystallography of a representative palladium complex.
Furthermore, the bifunctional ligand Ph2PC6H4PO(OH)Ph represents the first example of a chelating
anionic [P,O] ligand resulting from the combination of a phosphine and a phosphinate moiety.
In view of the interesting properties of the single-site palladium
Introduction
catalysts based on phosphine sulfonate ligands, we designed a
In the early 1980s, nickel complexes bearing anionic chelating
new class of chelating anionic [P,O] ligands by replacing the
[P,O] ligands were already being successfully applied as cata-
sulfonate moiety by the related phosphonate or phosphinate group
lysts in the Shell Higher Olefin Process (SHOP).1 Nevertheless,
(Scheme 1). Similar tertiary phosphines containing phosphate or
research activities concerning the use of late transition metal
phosphonate ester functionalities have recently attracted attention
catalysts, especially in polymerisation reactions, increased slowly.2
as neutral [P,O] ligands owing to their well investigated hemilabile
behaviour in transition metal complexes.7 Furthermore, the im-
provement in water-solubility of phosphine ligands due to included
phosphonate groups has also been demonstrated.8
By employing anionic ligands, the generally low oxophilicity
of late transition metal systems can be further decreased. This
may lead to catalyst systems with enhanced functional group
tolerance, thereby offering the attractive possibility of being able
to copolymerise ethylene with polar comonomers,3 or even to run
reactions in aqueous solution.4
So far, the most commonly employed anionic ligands are
bidentate [P,O] and [N,O] compounds. However, there are notable
examples, in which other types of chelating anionic ligands, e.g.
[N,N] or [P,N], have been used.3a
In 2002, Drent et al. reported the use of alkoxyarylphosphine
ligands bearing sulfonic acid groups that, when combined in situ
with palladium acetate or Pd2(dba)3, were able to promote the
formation of non-alternating polyketones from carbon monoxide
and ethylene. In the copolymerisation of ethylene and methyl
acrylate, copolymers with the acrylate units statistically built into
the linear polyethylene chain were produced.5 In fact, this was the
first reported example of transition metal catalysis providing a
low-temperature/low-pressure route to this particular subclass of
copolymers.
Scheme 1 Phosphine phosphonate and phosphine phosphinate ligands
analogue to Drent’s phosphine sulfonate ligands.
In this paper, we report the synthesis and characterisation of
various mixed phosphine phosphonate and phosphine phosphi-
nate ligands. In addition, we focus on their use as novel anionic
chelating [P,O] ligands in palladium complexes.
To gain a deeper insight into the structure of the catalytically
active palladium species, recent work in our group was focused
on the development of defined single-site catalysts containing
bisarylphosphinobenzene sulfonic acids in deprotonated form as
anionic bidentate [P,O] ligands. In the copolymerisation of carbon
monoxide and ethylene, these single-site catalysts proved to be
superior compared to the palladium species formed in situ. Higher
percentages of additional ethylene incorporation were observed,
as well as higher turnover numbers.6
Results and discussion
Preparation of 2-diphenylphosphinophenylphosphonic acid was
carried out according to the procedure reported by Knight and
co-workers.8a In a similar manner, the ortho-substituted derivatives
Ar2PC6H4PO(OH)2 (Ar = Me, OMe) were obtained.
Initially, we prepared ortho-substituted bromo-bisaryl-
phosphinobenzenes by two different strategies (Schemes 2 and
3), followed by the conversion into the corresponding phosphine-
diethylphosphonates (Scheme 4). Afterwards, the phosphine
phosphonic acids (7–9) were obtained by cleavage of the ester
groups.
Department of Materials Science and Catalyst Design, University of Ulm,
Albert Einstein Allee 11, D-89069, Ulm, Germany. E-mail: bernhard.rieger@
uni-ulm.de; Fax: +49(0)731/50-23039; Tel: +49(0)731/50-22575
After deprotonation of the phosphonic acid moiety by Na2CO3,
the phosphine phosphonates were employed as anionic ligands,
† Dedicated to Professor Heinz Berke on the occasion of his 60th birthday.
272 | Dalton Trans., 2007, 272–278
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◦
˚
Table 1 Selected bond lengths (A) and bond angles ( ) for complex 10
Pd(1)–O(1)
Pd(1)–P(2)
Pd(1)–C(19)
Pd(1)–C(20)
Pd(1)–C(24)
P(1)–O(1)
1.508(7)
2.280(2)
2.269(10)
2.395(13)
2.039(9)
1.508(7)
P(1)–O(2)
P(1)–O(3)
P(1)–C(1)
P(2)–C(6)
P(2)–C(7)
P(2)–C(13)
1.542(6)
1.508(6)
1.805(7)
1.836(9)
1.816(8)
1.815(8)
O(1)–Pd(1)–P(2)
P(2)–Pd(1)–C(24)
C(24)–Pd(1)–C(19)
C(24)–Pd(1)–C(20)
C(19)–Pd(1)–C(20)
C(19)–Pd(1)–O(1)
C(20)–Pd(1)–O(1)
P(1)–O(1)–Pd(1)
O(1)–P(1)–O(2)
94.28(17)
91.3(3)
81.5(4)
94.6(5)
34.6(5)
95.5(4)
79.8(4)
120.8(4)
110.3(4)
113.3(3)
O(1)–P(1)–C(1)
O(2)–P(1)–O(3)
O(2)–P(1)–C(1)
O(3)–P(1)–C(1)
C(6)–P(2)–Pd(1)
C(6)–P(2)–C(7)
C(6)–P(2)–C(13)
C(7)–P(2)–Pd(1)
C(7)–P(2)–C(13)
C(13)–P(2)–Pd(1)
106.6(3)
112.3(3)
106.6(3)
107.2(4)
112.0(2)
105.5(4)
103.2(4)
118.7(3)
102.8(4)
113.0(3)
Scheme 2 Synthesis of 1-bromo-2-bis(2-methylphenyl)phosphinoben-
zene (1).
O(1)–P(1)–O(3)
confirmed by single-crystal X-ray diffraction studies. The solid-
state structure clearly provides evidence for the phosphine phos-
phonate ligand binding as an anionic bidentate [P,O] ligand to the
palladium centre (Fig. 1). It is interesting to note that in previous
reports it was claimed that such ligands are unable to coordinate
in a chelating manner.8a
Scheme 3 Synthesis of 1-bromo-2-bis(2-methoxyphenyl)phosphinoben-
zene (4).
Fig. 1 ORTEP plot of the solid-state structure of complex 10 (ellipsoids
with 50% probability; hydrogen atoms are omitted for clarity).
In the solid state, the second hydroxyl group of the phosphonic
acid moiety, which remains free after coordination, is involved in
an intermolecular hydrogen bonding system building up dimeric
structures of high symmetry (Fig. 2). A similar behaviour is already
known to exist in the phenylphosphonic acid skeleton.9
Variable-temperature NMR studies of the palladium complex
11 in solution indicated a dynamic coordination mode of the
phosphine phosphonate ligand.
In Fig. 3, the 31P NMR spectra of complex 11 within a temper-
ature range from 20 to −20 ◦C are shown. During the cooling and
heating process, the phosphorus signal of the phosphine moiety
underwent remarkable changes in shape. The well defined doublet
became a broad singlet by lowering the temperature from 20 to
Scheme 4 Synthesis of phosphine phosphonate ligands 7–9 and the
corresponding palladium complexes 10–13 (R = H, Rꢀ = Me 10, R =
H, Rꢀ = Et 11, R = Me, Rꢀ = Et 12, R = OMe, Rꢀ = Me 13).
generating neutral palladium complexes (Scheme 4). The struc-
ture of complex 10 was tentatively assigned based on NMR
spectroscopy, mass spectrometry and elemental analysis, and
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Fig. 2 Excerpt from the structure of complex 10: intermolecular hydrogen
bonding between two molecules of the asymmetric unit.
Scheme 5 Synthesis of phosphine phosphinate ligand 14 and the corre-
sponding palladium complex 15.
◦
◦
structure of one representative palladium complex. In addition, we
report the synthesis of the first bidentate [P,O] ligand combining
a phosphine moiety and a phosphinic acid functionality. Studies
concerning the use of the neutral palladium complexes as single-
site catalysts in different polymerisation reactions are currently
under way.
10 C. At 0 C, the signal appeared as a quasi-triplet. On closer
inspection, the signal shape can be explained by the partial overlap
of two separate doublets, which can be recognised by the significant
hump in the triplet structure at −10 ◦C. This reveals that there are
two slightly different complex structures existing in a dynamic
equilibrium. Either the second hydroxyl group (P–OH) or the
=
doubly-bonded oxygen (P O) could, to some extent, be involved
in bonding to the palladium centre.
Experimental
In one step, 2-diphenylphosphinophenylphosphinic acid
was prepared from 1-bromo-2-diphenylphosphinobenzene and
dichlorophenylphosphine oxide (Scheme 5). After deprotonation
of the phosphinic acid moiety by Na2CO3, the phosphine phos-
phinate could be introduced as an anionic chelating [P,O] ligand
in a palladium complex.
Compared to complex 11, complex 15 showed a reduced
dynamic behaviour in the variable-temperature NMR spectra.
This difference may be attributed to the lack of a second, free
hydroxyl group in the case of the phosphinate instead of the
phosphonate moiety.
General considerations
All reactions were carried out under a dry argon atmosphere
using standard Schlenk line techniques. Solvents were obtained
from commercial sources, dried (as appropriate, tert-butyl methyl
ether, diethyl ether, hexane, pentane, tetrahydrofuran over sodium
(benzophenone) and dichloromethane over calcium hydride) and
deoxygenated prior to use. Deuterated solvents were dried over the
appropriate drying agents (chloroform-d1 and dichloromethane-
d2 over calcium hydride). The NMR solvents were deoxygenated
using three freeze–thaw pump cycles and transferred using a high
vacuum line. N,N-Diethylaminodichlorophosphine, 1-bromo-2-
diphenylphosphinobenzene, bis(2-methylphenyl)chlorophosphine
and 1-diethylphosphonato-2-diphenylphosphinobenzene were
prepared according to literature procedures,7c,8a,10 [(Dcp-
ORꢀ)PdCl]2 (Rꢀ = Me, Et) was prepared as previously reported.6
Other chemicals were purchased from commercial sources and
Conclusion
In summary, we have prepared three different phosphine phospho-
nate ligands and the corresponding neutral palladium complexes.
For the first time, the chelating coordination mode of this class of
anionic [P,O] ligands has been clearly confirmed by a solid-state
used without further purification. 1H, 13C{ H} and 31P{ H} NMR
1
1
Fig. 3 Variable-temperature NMR study of the palladium complex 11 in CD2Cl2 solution within the temperature range from 20 to −20 ◦C.
274 | Dalton Trans., 2007, 272–278
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spectra were recorded on a Bruker Avance 400 MHz spectrometer.
Values of coupling constants are given in Hz. Shift references are
CHCl3 or CH2Cl2 as internal standards in 1H and 13C NMR and
H3PO4 (85%) as external standard in 31P NMR spectra. Elemental
analyses were performed on a VARIO EL by the analytical
department, University of Ulm. Mass spectra were recorded on a
Finnigan MAT, SSQ-7000 and TSQ-7000 or a Bruker Daltonics
REFLEX III spectrometer.
chlorophosphine (13.4 g, 63.6 mmol) was added dropwise at the
same temperature. The resulting mixture was allowed to warm to
room temperature and stirred for a further 16 h. After hydrolysis
with water (200 mL), the aqueous phase was extracted with diethyl
ether (2 × 25 mL), the combined organic phases were washed
with water (2 × 25 mL), dried (Na2SO4) and evaporated. Pure
compound 2 was isolated as a colourless oil (17.4 g, 83%) after
◦
distillation (110 C, 10−2 mbar) of the crude product (Found: C,
50.6; H, 7.2; N, 8.9. C14H24BrN2P requires C, 50.8; H, 7.3; N, 8.5%);
dH (400 MHz; CDCl3; CHCl3) 1.13 (12 H, t, J 7.1, 2 × CH3), 3.03–
3.19 (8 H, m, 2 × CH2), 7.11–7.15 (1 H, m), 7.30–7.33 (1 H, m),
7.47–7.50 (1 H, m), 7.53–7.56 (1 H, m); dC (100 MHz; CDCl3) 14.9
(d, J 3.7), 43.6 (d, J 18.3), 126.7, 126.9 (d, J 30.0), 129.0 (d, J 1.5),
132.3 (d, J 5.9), 133.4, 142.5 (d, J 11.7); dP (162 MHz; CDCl3;
H3PO4) 96.4 (s).
Bis(N,N-diethylamino)chlorophosphine
A 1 L Schlenk flask was charged with diethyl ether (300 mL) and
PCl3 (68.0 g, 0.50 mol) and cooled to −50 ◦C. Diethylamine (138 g,
1.88 mol) in 150 mL diethyl ether was added dropwise over a period
of 2 h with vigorous stirring. The reaction mixture was allowed
to warm to room temperature and stirred for an additional hour.
Diethyl ether was removed by distillation under argon at normal
pressure and the product was distilled in vacuo from the inorganic
salts. The crude product was purified by fractional distillation
1-Bromo-2-dichlorophosphinobenzene
(3). Compound
2
(9.59 g, 28.95 mmol) was dissolved in diethyl ether (400 mL).
Dry HCl (g) was passed through this solution for 30 min. The
HCl-saturated solution was stirred for a further 2.5 h, and then the
solvent was removed in vacuo. The residue was taken up in diethyl
ether (300 mL) and the resulting suspension filtered to separate
the insoluble diethylamine hydrochloride. After concentration
of the filtrate in vacuo, the resulting crude oil was purified by
distillation (65 ◦C, 10−2 mbar) to yield compound 3 (6.25 g, 84%)
as a colourless liquid (Found: C, 28.1; H, 1.7. C6H4BrCl2P requires
C, 27.9; H, 1.6%); dH (400 MHz; CDCl3; CHCl3) 7.40–7.44 (1 H,
m), 7.54–7.57 (1 H, m), 7.61–7.65 (1 H, m), 8.13–8.15 (1 H, m); dC
(100 MHz; CDCl3) 126.4 (d, J 46.1), 128.4, 131.8 (d, J 4.4), 132.9,
133.7, 139.5 (d, J 56.4); dP (162 MHz; CDCl3; H3PO4) 154.6 (s);
m/z (CI) 221 (M+ − Cl, 100%), 223 (94).
◦
(49 C, 10−2 mbar) and bis(N,N-diethylamino)chlorophosphine
was obtained as a colourless liquid (39.5 g, 38%). dH(400 MHz;
CDCl3; CHCl3) 1.12 (12 H, t, J 7.2, 4 × CH3), 3.11–3.19 (8 H, m,
4 × CH2), dC (100 MHz; CDCl3) 13.7 (2 × d, J 5.1), 41.0 (2 × d,
J 17.6), dP (162 MHz; CDCl3; H3PO4) 160.0 (s). CAUTION: the
solid residue is pyrophoric and should be destroyed carefully by
slow addition of water.
1-Bromo-2-bis(2-methylphenyl)phosphinobenzene
Dibromobenzene (1.99 g, 8.44 mmol) was dissolved in a mixture
of diethyl ether and THF (30 mL–30 mL). At −110 C, n-BuLi
(1). 1,2-
◦
(5.0 mL, 8.00 mmol, 1.6 M solution in hexanes) was added
over a period of 40 min. The resulting mixture was stirred for
further 30 min at −110 ◦C followed by the dropwise addition
of bis(2-methylphenyl)chlorophosphine (2.00 g, 8.04 mmol)
dissolved in diethyl ether–THF (10 mL–10 mL). The reaction
mixture was then allowed to warm to room temperature over
20 h. After hydrolysis with saturated NH4Cl solution (80 mL), the
aqueous phase was extracted with diethyl ether (2 × 25 mL). The
combined organic phases were washed with water (2 × 25 mL),
dried (Na2SO4) and evaporated. The crude product was dissolved
in dichloromethane (30 mL) and filtered over a small pad of
silica. Then the filtrate was evaporated to dryness and the residue
washed with small amounts of pentane (5 × 5 mL) to yield
compound 1 (1.72 g, 59%) as a white solid (Found: C, 64.9; H,
4.9. C20H18BrP requires C, 65.1; H, 4.9%); dH (400 MHz; CDCl3;
CHCl3) 2.48 (6 H, s, 2 × CH3), 6.79–6.82 (3 H, m), 7.13–7.16
(2 H, m), 7.22–7.27 (2 H, m) 7.28–7.36 (4 H, m), 7.64–7.69
(1 H, m); dC (100 MHz; CDCl3) 21.2 (2 × d, J 22.0), 126.2 (2 ×
s), 127.5, 129.0 (2 × s), 130.1, 130.2 (2 × d, J 5.1), 130.5 (d, J
32.2), 133.0 (d, J 2.2), 133.2 (2 × s), 134.0 (2 × d, J 11.7), 134.6
(d, J 1.5), 137.7 (d, J 11.0), 142.8 (2 × d, J 27.1); dP (162 MHz;
CDCl3; H3PO4) −19.0 (s); m/z (CI) 370 (MH+, 100%), 371 (66),
369 (67), 368 (99), 289 (94, M+ − Br).
1-Bromo-2-bis(2-methoxyphenyl)phosphinobenzene (4). To a
solution of anisole (2.10 g, 19.39 mmol) and TMEDA (200 mg,
1.72 mmol) in tert-butyl methyl ether (120 mL) at 0 ◦C was
added n-BuLi (10.7 mL, 17.2 mmol, 1.6 M solution in hexanes)
dropwise. Upon completion of the addition, the resulting mixture
was refluxed for 24 h. The reaction was then cooled to 0 ◦C and a
solution of compound 3 (2.00 g, 7.76 mmol) in tert-butyl methyl
ether (25 mL) was added over the period of 1 h. The reaction
mixture was stirred for 1 h at room temperature and then for a
further 24 h under reflux conditions. After hydrolysis with water
(120 mL) at room temperature, the suspension was filtered and
the resulting solid dissolved in dichloromethane (100 mL). The
organic phase was washed with water (2 × 50 mL), dried (Na2SO4)
and evaporated to yield compound 4 (2.19 g, 62%) as a white solid
(Found: C, 59.5; H, 4.6. C20H18BrO2P requires C, 59.9; H, 4.5%);
dH (400 MHz; CDCl3; CHCl3) 3.78 (6 H, s, 2 × OCH3), 6.71–6.75
(2 H, m), 6.83–6.86 (1 H, m), 6.88–6.91 (2 H, m), 6.93–6.96 (2 H,
m), 7.18–7.23 (2 H, m), 7.37–7.41 (2 H, m), 7.59–7.64 (1 H, m); dC
(100 MHz; CDCl3) 55.7 (2 × s), 110.4 (2 × d, J 1.5), 121.1 (2 ×
s), 123.9 (2 × d, J 13.2), 127.1, 129.8, 130.3 (d, J 33.7), 130.4 (2 ×
s), 132.7 (d, J 2.9), 134.0 (2 × d, J 1.5), 134.6, 138.2 (d, J 12.4),
161.6 (2 × d, J 16.8); dP (162 MHz; CDCl3; H3PO4) −25.4 (s); m/z
(CI) 402 (MH+, 100%), 404 (16), 403 (86), 401 (96), 400 (87), 321
(42, M+ − Br).
1-Bromo-2-bis(diethylamino)phosphinobenzene (2). To a solu-
tion of 1,2-dibromobenzene (15.0 g, 63.3 mmol) in a mixture of
diethyl ether and THF (120 mL–120 mL) at −110 ◦C n-BuLi
(39.7 mL, 63.6 mmol, 1.6 M solution in hexanes) was added over
a period of 4 h. Afterwards, the reaction mixture was stirred
for further 45 min at −110 ◦C, before bis(N,N-diethylamino)-
1-Diethylphosphonato-2-bis(2-methylphenyl)phosphinobenzene
(5). To a solution of compound 1 (1.20 g, 3.25 mmol) in THF
(75 mL) was added n-BuLi (2.1 mL, 3.25 mmol, 1.6 M solution
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in hexanes) dropwise at −78 ◦C. The resulting solution was
stirred for 1 h, before diethylchlorophosphate (0.5 mL, 560 mg,
3.25 mmol) was added dropwise at the same temperature. The
reaction was allowed to warm to room temperature and stirred for
an additional 16 h. After concentrating under reduced pressure,
the residue was dissolved in diethyl ether (40 mL) and hydrolysed
with water (40 mL). The aqueous phase was extracted with diethyl
ether (2 × 30 mL). The combined organic phases were washed
with water (2 × 50 mL), dried (Na2SO4) and evaporated. The
crude product was washed with pentane (5 × 7 mL) to yield
compound 5 (1.31 g, 95%) as a white solid (Found: C, 67.3; H,
6.6. C24H28O3P2 requires C, 67.6; H, 6.6%); dH (400 MHz; CDCl3;
CHCl3) 1.11 (6 H, t, J 6.8, 2 × OCH2CH3), 2.39 (6 H, s, 2 × CH3),
3.85–3.93 (2 H, m, CHH), 4.07–4.16 (2 H, m, CHH), 6.66–6.69
(2 H, m), 7.05–7.11 (3 H, m), 7.20–7.28 (4 H, m), 7.39–7.42 (2 H,
m), 8.17–8.23 (1 H, m); dC (100 MHz; CDCl3) 15.9 (2 × d, J 7.3),
21.1 (2 × d, J 22.0), 62.0 (2 × dd, J 5.9 and 2.2), 125.9 (2 × s),
128.4 (d, J 14.6), 128.5 (2 × s), 130.0 (2 × d, J 4.4), 132.1 (d, J 2.9),
133.0 (2 × s), 133.7 (d, J 32.9), 134.8 (dd, J 11.0 and 8.1), 135.6
(2 × d, J 13.2), 135.9 (dd, J 15.4 and 1.5), 140.4 (dd, J 23.4 and
12.4), 142.2 (2 × d, J 27.1); dP (162 MHz; CDCl3; H3PO4) −22.5
(s, P), 18.7 (s, PO); m/z (CI) 427 (MH+, 100%), 428 (25), 426 (26).
ether/pentane to obtain compound 7 (4.04 g, 92%) as a white
solid; dH (400 MHz; CDCl3; CHCl3) 7.22–7.27 (1 H, m), 7.33–7.39
(4 H, m), 7.42–7.50 (7 H, m), 7.53–7.56 (1 H, m), 8.05–8.11 (1 H,
m), 9.12 (2 H, br s, 2 × OH); dC (100 MHz; CDCl3) 126.9 (d, J
42.4), 129.2 (4 × d, J 10.3), 131.6 (2 × s), 131.8, 131.9 (dd, J 24.2
and 2.9), 133.7 (4 × d, J 14.6), 134.1 (dd, J 9.5 and 9.5), 135.6
(dd, J 13.9 and 5.1), 137.1 (d, J 21.2), 138.9 (2 × d, J 21.2); dP
(162 MHz; CDCl3; H3PO4) −3.3 (s, P), 14.6 (s, PO); m/z (CI) 341
(MH+, 100%), 343 (51), 342 (23), 340 (8), 339 (23), 325 (60, M −
H2O), 311 (50).
2-Bis(2-methylphenyl)phosphinophenylphosphonic acid (8).
This ligand was obtained as a white solid (623 mg, 68%), in a
manner similar to ligand 7 from compound 5 (1.06 g, 2.47 mmol)
and bromotrimethylsilane (1.2 mL, 8.65 mmol, 3.5 equiv.); dH
(400 MHz; CDCl3; CHCl3) 2.47 (6 H, s, 2 × CH3), 6.99–7.05 (2 H,
m), 7.18–7.22 (1 H, m), 7.31–7.34 (2 H, m), 7.41–7.44 (2 H, m),
7.59–7.67 (3 H, m), 7.83–7.86 (1 H, m), 8.15–8.21 (1 H, m), 9.88
(2 H, br s, 2 × OH); dC (100 MHz; CDCl3) 21.2 (2 × d, J 19.0),
117.2 (2 × d, J 18.9), 126.0 (2 × d, J 2.9), 129.4 (2 × s), 129.7,
130.4 (2 × d, J 5.9), 131.6, 132.9 (2 × d, J 2.9), 133.8 (dd, J 9.5
and 9.5), 136.1 (d, J 15.4), 136.9 (dd, J 193.2 and 29.8), 138.0
(dd, J 176.4 and 9.5), 142.5 (2 × d, J 23.4); dP (162 MHz; CDCl3;
H3PO4) −8.4 (d, J 15.1, P), 11.7 (d, J 15.1, PO); m/z (CI) 371
(MH+, 100%), 372 (21), 370 (17).
1-Diethylphosphonato-2-bis(2-methoxyphenyl)phosphinobenzene
(6). To a solution of compound 4 (1.31 g, 3.25 mmol) in THF
(75 mL) was added n-BuLi (2.1 mL, 3.25 mmol, 1.6 M solution
in hexanes) dropwise at −78 ◦C. The resulting solution was
stirred for 1 h, before diethylchlorophosphate (0.5 mL, 560 mg,
3.25 mmol) was added dropwise at the same temperature. The
reaction was allowed to warm to room temperature and then
stirred for 16 h. After concentrating in vacuo, the residue was
dissolved in dichloromethane (80 mL) and hydrolysed with water
(80 mL). The aqueous phase was extracted with dichloromethane
(2 × 20 mL). The combined organic phases were washed with
water (2 × 20 mL), dried (Na2SO4) and evaporated. Compound
6 was obtained as a white solid (1.20 g, 80%) from the crude oil
by the addition of diethyl ether (10 mL) (Found: C, 62.9; H, 6.1.
C24H28O5P2 requires C, 62.9; H, 6.2%); dH (400 MHz; CDCl3;
CHCl3) 1.10 (6 H, t, J 7.0, 2 × OCH2CH3), 3.71 (6 H, s, 2 ×
OCH3), 3.93–4.03 (2 H, m, CHH), 4.08–4.17 (2 H, m, CHH),
6.58–6.62 (2 H, m), 6.82–6.86 (2 H, m), 6.87–6.91 (2 H, m),
7.08–7.12 (1 H, m), 7.31–7.35 (2 H, m), 7.38–7.42 (1 H, m),
7.44–7.48 (1 H, m), 8.20–8.26 (1 H, m); dC (100 MHz; CDCl3)
15.9 (2 × d, J 7.3), 55.5 (2 × s), 61.9 (2 × dd, J 5.9 and 2.2),
110.1 (2 × d, J 1.5), 120.8 (2 × s), 125.6 (2 × d, J 15.4), 128.1 (d,
J 14.6), 129.9 (2 × s), 131.8 (d, J 2.9), 133.9 (2 × s), 134.9 (dd, J
11.0 and 8.1), 135.1 (d, J 32.9), 135.3 (dd, J 16.1 and 1.5), 141.7
(dd, J 24.9 and 12.4), 161.0 (2 × d, J 16.1); dP (162 MHz; CDCl3;
H3PO4) −26.9 (s, P), 19.1 (s, PO); m/z (CI) 459 (MH+, 100%),
460 (34), 458 (34).
2-Bis(2-methoxyphenyl)phosphinophenylphosphonic acid (9).
This ligand was obtained as a white solid (649 mg, 93%), in a
manner similar to ligand 7 from compound 6 (795 mg, 1.73 mmol)
and bromotrimethylsilane (0.8 mL, 6.06 mmol, 3.5 equiv.); dH
(400 MHz; CDCl3; CHCl3) 3.85 (6 H, s, 2 × OCH3), 7.11–7.19
(6 H, m), 7.27–7.34 (1 H, m), 7.61–7.64 (1 H, m), 7.73–7.77 (2 H,
m), 7.82–7.85 (1 H, m), 8.27–8.33 (1 H, m), 9.25 (2 H, br s, 2 ×
OH); dC (100 MHz; CDCl3) 56.9 (2 × s), 104.8 (2 × d, J 95.1),
112.3 (2 × d, J 5.8), 118.3 (dd, J 103.9 and 11.0), 122.1 (2 × d, J
13.2), 131.8 (dd, J 13.2 and 2.2), 134.1–134.4 (2 × m), 134.7 (2 ×
d, J 8.8), 135.0 (dd, J 9.5 and 9.5), 137.2 (2 × s), 137.3 (dd, J 186.6
and 11.0), 161.4 (2 × d, J 2.2); dP (162 MHz; CDCl3; H3PO4) −9.9
(br s, P), 11.8 (d, J 6.8, PO); m/z (CI) 95 (C6H7O+, 100%), 403 (38,
MH+), 402 (6), 401 (9), 385 (49, M − H2O).
Complex 10. In a Schlenk flask, a solution of 7 (500 mg,
1.46 mmol) and Na2CO3 (170 mg, 1.61 mmol, 1.1 equiv.) in
dichloromethane (20 mL) was stirred for 20 h. During this time, the
white monosodium salt of 7 precipitated and [(Dcp-OMe)PdCl]2
(446 mg, 0.73 mmol) was added with stirring at −18 ◦C. The
resulting solution was allowed to stir for further 6 h at room
temperature. Before concentrating in vacuo, the complex solution
was filtered to separate the NaCl formed during the reaction. The
crude product obtained was washed with small amounts of diethyl
ether (5 × 7 mL) to yield complex 10 (698 mg, 78%) as a pure pale
yellow solid (Found: C, 56.7; H, 5.2. C29H30O4P2Pd requires C,
57.0; H, 5.0%); dH (400 MHz; CD2Cl2; CH2Cl2) 1.07 (1 H, d, J
9.9, CHH), 1.26 (1 H, d, J 9.6, CHH), 2.09 (1 H, dd, J 14.0 and
3.9, CH), 2.32–2.35 (2 H, m, CH, CHH), 2.61–2.72 (2 H, m, CH,
CHH), 2.79–2.83 (1 H, m, CH), 2.91–2.97 (1 H, m, CH), 3.01 (3 H,
s, OCH3), 3.47–3.49 (1 H, m, CH), 6.46 (1 H, br s, CH), 6.89–6.95
(1 H, m, CH), 7.21–7.25 (1 H, m, CH), 7.28–7.34 (1 H, m, CH),
7.37–7.40 (1 H, br s, CH), 7.41–7.59 (10 H, m, CH), 7.95–8.00 (1
H, m, CH), 8.60 (1 H, br s, OH); dC (100 MHz; CD2Cl2) 31.8, 35.7,
2-Diphenylphosphinophenylphosphonic acid (7). In a Schlenk
flask,
1-diethylphosphonato-2-diphenylphosphinobenzene
(5.12 g, 12.8 mmol) was dissolved in dichloromethane (100 mL)
and bromotrimethylsilane (5.9 mL, 44.8 mmol, 3.5 equiv.)
was added dropwise at room temperature. After stirring for
24 h, dichloromethane and the excess bromotrimethylsilane was
removed in vacuo. The residue was dissolved in methanol (100 mL)
to hydrolyse the silyl esters formed and stirred for further 4 h
before evaporating. The crude product was recrystallised from
276 | Dalton Trans., 2007, 272–278
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39.2, 40.9, 51.5, 53.9, 55.9, 57.4, 82.6, 127.3 (d, J 6.6), 128.6 (2 ×
d, J 11.0), 128.7 (2 × d, J 11.0), 129.2 (d, J 5.1), 129.7 (d, J 8.8),
130.2 (2 × d, J 46.8), 130.3 (dd, J 11.3 and 1.1), 130.6 (d, J 2.2),
130.9 (d, J 2.2), 131.1 (d, J 49.8), 131.5 (dd, J 45.0 and 8.0), 133.2
(dd, J 10.2 and 8.8), 133.9 (2 × d, J 12.4), 134.2 (2 × d, J 12.4),
134.5 (dd, J 11.0 and 2.2); dP (162 MHz; CD2Cl2; H3PO4) 11.6
(d, J 13.7, P), 19.9 (d, J 13.7, PO); m/z (MALDI): found: 610.5.
C29H30O4P2Pd requires 610.1 (M+).
(1H, s, CH), 6.93–6.96 (4 H, m, CH), 7.08 (1 H, m, CH), 7.16 (1
H, m, CH), 7.28 (3 H, br s, CH), 7.36–7.38 (1 H, m, CH), 7.49
(2 H, m, CH), 7.93–8.00 (1 H, br s, CH), 11.18 (1 H, br s, OH); dC
(100 MHz; CD2Cl2) 32.1, 36.2, 39.7, 40.1, 41.9, 43.5, 55.2, 55.0,
55.7, 55.9, 83.1, 111.8 (d, J 54.8), 121.1 (d, J 11.0), 125.5 (d, J 7.3),
125.9 (d, J 8.7), 128.8, 129.9 (d, J 13.1), 133.3 (d, J 14.6), 137.3,
161.1 (d, J 51.3); dP (162 MHz; CD2Cl2; H3PO4) 12.3 (br s, P), 12.7
(br s, PO); m/z (MALDI): found: 669.4. C31H34O6P2Pd requires
670.9 (M+).
Complex 11. This complex was obtained as a pale yellow solid
(343 mg, 63%), in a manner similar to complex 10 from 7 (300 mg,
0.79 mmol), Na2CO3 (112 mg, 1.06 mmol, 1.1 equiv.) and [(Dcp-
OEt)PdCl]2 (280 mg, 0.38 mmol); dH (400 MHz; CD2Cl2; CH2Cl2)
1.00 (3 H, t, J 7.0, CH2CH3), 1.09 (1 H, d, J 9.8, CHH), 1.32
(1 H, d, J 9.8, CHH), 1.76–1.77 (1 H, m, CH), 2.33–2.34 (2 H,
m, CH, CHH), 2.67–2.72 (2 H, m, CH, CHH), 2.82–2.83 (1 H,
m, CH), 2.96–3.02 (1 H, m, CH), 3.10–3.17 (1 H, m, CHHCH3),
3.32–3.40 (1 H, m, CHHCH3), 3.64–3.67 (1 H, m, CH), 6.46 (1 H,
br s, CH), 6.85–6.91 (1 H, m, CH), 7.22–7.26 (1 H, m, CH), 7.34–
7.37 (1 H, m, CH), 7.41–7.45 (2 H, m, CH), 7.46–7.47 (1 H, m,
CH), 7.48–7.55 (8 H, m, CH), 8.00–8.03 (1 H, m, CH), 12.31 (1 H,
br s, OH); dC (100 MHz; CD2Cl2) 15.6, 31.8, 35.8, 39.3, 41.8, 52.4,
53.9, 57.4, 63.6, 80.7, 127.5 (d, J 7.3), 128.6 (2 × d, J 11.0), 128.8
(2 × d, J 11.0), 129.2–129.4 (2 × m), 130.2 (2 × d, J 46.8), 130.3
(dd, J 11.7 and 2.2), 130.7 (d, J 2.2), 130.8 (d, J 2.2), 131.3 (d, J
49.8), 131.5 (dd, J 45.0 and 8.1), 133.2 (dd, J 11.0 and 8.8), 134.0
(2 × d, J 12.4), 134.1 (2 × d, J 12.4), 134.5 (dd, J 11.0 and 2.9);
dP (162 MHz; CD2Cl2; H3PO4) 11.6 (d, J 13.7, P), 20.0 (d, J 13.7,
PO); m/z (MALDI): found: 623.1. C30H32O4P2Pd requires 624.1
(M+).
2-Diphenylphosphinophenylphosphinic acid (14). In a Schlenk
flask, 1-bromo-2-diphenylphosphinobenzene (1.00 g, 2.93 mmol)
was dissolved in THF (40 mL). At −78 ◦C, n-BuLi (1.8 mL,
2.93 mmol, 1.6 M solution in hexanes) was added dropwise
with stirring over a period of 40 min. The resulting solution
was slowly transferred via cannula to a stirred solution of
dichlorophenylphosphonate (0.63 g, 3.22 mmol, 1.1 equiv.) in
THF (50 mL) at −78 ◦C. After completion of the addition,
the solution was stirred for a further 18 h and allowed to
warm to room temperature. Then, the solvent was removed
in vacuo and the residue dissolved in diethyl ether (100 mL). Water
(100 mL) was added to hydrolyse the monochlorophosphonate
formed. The phases were separated and the aqueous phase was
extracted with diethyl ether (2 × 50 mL). The combined organic
phases were washed with water (50 mL), dried (Na2SO4) and
evaporated. The crude product was purified by recrystallisation
from dichloromethane/pentane to obtain the desired compound
14 (246 mg, 21%) as a white solid (Found: C, 71.2; H, 5.2.
C24H20O2P2 requires C, 71.6; H, 5.0%); dH (400 MHz; CDCl3;
CHCl3) 7.03–7.06 (4 H, m), 7.18–7.27 (9 H, m), 7.31–7.35 (1 H,
m), 7.37–7.40 (2 H, m), 7.67–7.72 (2 H, m), 8.13–8.19 (1 H, m),
11.01 (1 H, br s, OH); dC (100 MHz; CDCl3) 127.7 (2 × d, J 13.9),
128.1 (4 × d, J 2.9), 128.2 (2 × s), 128.6 (dd, J 17.6 and 12.4),
131.5 (dd, J 56.0 and 2.6), 131.6 (2 × dd, J 11.0 and 3.7), 132.1 (d,
J 10.2), 133.3 (4 × d, J 19.8), 133.6 (dd, J 9.5 and 9.5), 136.3 (dd,
J 13.2 and 1.5), 137.3 (2 × d, J 13.2), 138.0 (d, J 32.9), 139.3 (d, J
32.9), 141.3 (dd, J 23.4 and 14.6); dP (162 MHz; CDCl3; H3PO4)
−12.2 (d, J 9.1, P), 34.7 (d, J 9.1, PO); m/z (CI) 403 (MH+, 100%),
404 (25), 402 (12).
Complex 12. This complex was obtained as a pale yellow solid
(246 mg, 55%), in a manner similar to complex 10 from 8 (250 mg,
0.68 mmol), Na2CO3 (80 mg, 0.75 mmol, 1.1 equiv.) and [(Dcp-
OEt)PdCl]2 (205 mg, 0.34 mmol); dH (400 MHz; CD2Cl2; CH2Cl2)
0.91 (3 H, t, J 6.9, CH2CH3), 1.06 (1 H, d, J 9.9, CHH), 1.35 (1 H,
d, J 9.6, CHH), 1.35–1.37 (1 H, m, CH), 2.23–2.34 (2 H, m, CH,
CHH), 2.52 (3 H, s, CH3), 2.65–2.70 (5 H, m, CH, CHH, CH3),
2.87–2.89 (1 H, m, CH), 2.91–2.94 (1 H, m, CH), 2.97–3.03 (1 H,
m, CHHCH3), 3.32–3.36 (1 H, m, CHHCH3), 3.54–3.55 (1 H, m,
CH), 6.40 (1 H, br s, CH), 6.85–6.92 (3 H, m, CH), 7.13–7.19
(2 H, m, CH), 7.24–7.28 (1 H, m, CH), 7.34–7.37 (2 H, m, CH),
7.40–7.46 (3 H, m, CH), 7.49–7.55 (1 H, m, CH), 8.17–8.19 (1 H,
m, CH), 10.41 (1 H, br s, OH); dC (100 MHz; CD2Cl2) 15.5, 22.9,
23.1, 31.7, 35.9, 39.3, 41.3, 54.3, 58.3, 63.2, 81.7, 126.3 (2 × d, J
8.8), 126.4 (2 × d, J 8.8), 129.4–129.5 (2 × m), 130.7 (d, J 10.3),
131.0 (2 × s), 131.5 (dd, J 6.6 and 1.5), 131.7 (2 × d, J 10.3), 133.42
(m), 133.7 (2 × d, J 7.3), 134.5–134.7 (2 × m), 142.4 (d, J 11.7),
142.9 (d, J 12.4); dP (162 MHz; CD2Cl2; H3PO4) 12.7 (br s, P, PO);
m/z (MALDI): found: 651.7. C32H36O4P2Pd requires 652.1 (M+).
Complex 15. In a Schlenk flask, a solution of 14 (179 mg,
0.45 mmol) and Na2CO3 (57 mg, 0.53 mmol, 1.1 equiv.) in
dichloromethane (10 mL) was stirred for 40 h. During this time,
the white sodium salt of 14 precipitated and [(Dcp-OEt)PdCl]2
(142 mg, 0.22 mmol) was added with stirring at −18 ◦C. The
resulting solution was allowed to stir for further 20 h at room
temperature. Before concentrating in vacuo, the complex solution
was filtered to separate the NaCl formed during the reaction. The
crude product obtained was washed with small amounts of diethyl
ether (5 × 5 mL) to yield complex 15 (170 mg, 61%) as a pure yellow
solid; dH (400 MHz; CD2Cl2; CH2Cl2) 0.97 (3 H, t, J 7.0, CH3),
1.10 (1 H, d, J 9.9, CHH), 1.31 (1 H, d, J 9.9, CHH), 1.71 (1 H,
dd, J 13.6 and 4.8, CH), 2.29–2.39 (2 H, m, CH, CHH), 2.70–2.74
(2 H, m, CH, CHH), 2.82–2.83 (1 H, m, CH), 2.97–3.02 (1 H, m,
CH), 3.07–3.14 (1 H, m, CHH), 3.32–3.39 (1 H, m, CHH), 3.75–
3.76 (1 H, m, CH), 6.46–6.48 (1 H, br s, CH), 6.89–6.94 (1 H, m,
CH), 7.11–7.15 (2 H, m, CH), 7.21–7.26 (2 H, m, CH), 7.27–7.29
(1 H, m, CH), 7.36–7.45 (11 H, m, CH), 7.47–7.52 (2 H, m, CH),
7.82–7.89 (1 H, m, CH); dC (100 MHz; CD2Cl2) 15.5, 31.9, 35.9,
39.3, 42.2, 54.0, 57.6, 63.7, 80.9, 126.9 (d, J 57.1), 127.6 (2 × d,
Complex 13. This complex was obtained as a pale yellow
solid (327 mg, 78%), in a manner similar to complex 10 from
9 (250 mg, 0.62 mmol), Na2CO3 (72 mg, 0.68 mmol, 1.1 equiv.)
and [(Dcp-OMe)PdCl]2 (189 mg, 0.31 mmol), (Found: C, 55.0;
H, 5.2. C31H34O6P2Pd requires C, 55.5; H, 5.1%); dH (400 MHz;
CD2Cl2; CH2Cl2) 1.03 (1 H, m, CHH), 1.21 (1 H, m, CHH), 1.41–
1.59 (1 H, m, CH), 2.20–2.35 (2 H, m, CH, CHH), 2.61–2.79 (2 H,
m, CH, CHH), 2.72 (3 H, s, OCH3), 3.28 (1 H, s, CH), 3.36–3.56
(2 H, m, CH), 3.62 (3 H, s, OCH3), 3.68 (3 H, s, OCH3), 6.35
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Table 2 Selected crystallographic data for complex 10·2/3C7H8
group warrants an even distribution. The compound crystallises
as the (non-chiral) racemate.
CCDC reference number 616052.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b612694d
Empirical formula
Mr
3(C29H30O4P2Pd)·2(C7H8)
2016.88
Crystal system
Space group
Monoclinic
P21/c (no. 14)
16.955(3)
33.696(7)
18.155(4)
116.24(3)
9303(3)
193(2)
4
0.738
˚
a/A
˚
b/A
Acknowledgements
˚
c/A
b/◦
V/A
T/K
Z
We would like to thank Deutsche Forschungs Gemeinschaft
(DFG) and Fond der Chemischen Industrie for funding. C. R.
and R. N. would like to express thanks to Mr B. Mu¨ller for X-ray
data collection and Mr U. Ziegler for NMR measurements.
3
˚
l(Mo-Ka)/mm−1
Total reflections
Independent reflections
Rint
72578
17313
0.0882
0.0909, 0.2620
References
R1, wR2 [I > 2r(I)]
1 (a) W. Keim, F. H. Kowaldt, R. Goddard and C. Kru¨ger, Angew. Chem.,
Int. Ed. Engl., 1978, 17, 466; (b) W. Keim, A. Behr and G. Kraus,
J. Organomet. Chem., 1983, 251, 377; (c) M. Peuckert and W. Keim,
Organometallics, 1983, 2, 594; (d) W. Keim, in Industrial Applications of
Homogenous Catalysis, ed. A. Mortreux and F. Petit, Dordrecht, 1988,
pp. 335–347.
2 V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283.
3 (a) S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev., 2000, 100,
1169; (b) L. S. Boffa and B. M. Novak, Chem. Rev., 2000, 100, 1479;
(c) particular case of palladium-catalysed alternating olefin–carbon
monoxide copolymerisation: E. Drent and P. H. M. Budzelaar, Chem.
Rev., 1996, 96, 663.
J 12.4), 128.8 (2 × d, J 9.5), 128.9 (2 × d, J 11.0), 129.5 (2 ×
d, J 46.8), 129.6 (dd, J 7.3 and 2.2), 129.9 (d, J 2.9), 130.4 (d, J
2.2), 130.5 (d, J 2.2), 130.6 (d, J 44.6), 130.9 (2 × m), 131.3 (2 ×
d, J 9.5), 131.7 (m), 133.9 (2 × d, J 12.4), 134.3 (2 × d, J 12.4),
134.5 (dd, J 9.1 and 3.3), 135.1 (dd, J 10.3 and 10.3); dP (162 MHz;
CD2Cl2; H3PO4) 19.1 (br s, P), 23.2 (d, J 11.4, PO); m/z (MALDI):
found: 683.3. C36H36O3P2Pd requires 684.1 (M+).
4 A. Held, F. M. Bauers and S. Mecking, Chem. Commun., 2000, 301.
5 (a) E. Drent and D. H. L. Pello, Eur. Pat. Appl., 1995, EP0632084 (to
Shell); (b) E. Drent, R. van Dijk, R. van Ginkel, B. van Oort and R. I.
Pugh, Chem. Commun., 2002, 744; (c) E. Drent, R. van Dijk, R. van
Ginkel, B. van Oort and R. I. Pugh, Chem. Commun., 2002, 964.
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24, 2755; (b) J. Chatt, L. M. Vallarino and L. M. Venanzi, J. Chem.
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7 (a) N. Oberbeckmann-Winter, X. Morise, P. Braunstein and R. Welter,
Inorg. Chem., 2005, 44, 1391; (b) X. Morise, P. Braunstein and R. Welter,
Inorg. Chem., 2003, 42, 7752; (c) D. D. Ellis, G. Harrison, A. G. Orpen,
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Soc., Dalton Trans., 2000, 671; (d) A. Weigt and S. Bischoff, Phosphorus,
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Mießner and B. Lu¨cke, J. Mol. Catal., 1996, 107, 339; (f) I. Le Gall,
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X-Ray crystallography
Single crystals of complex 10 were recrystallised from toluene.
Data of complex 10 was collected on a STOE-IPDS diffractometer
using graphite-monochromated Mo-Ka radiation. Table 2 sum-
marises the crystallographic data of complex 10. The structure
was solved by direct methods with the program SHELXS-97
[Sheldrick, 1997]11 and refined by full-matrix least-squares based
on F2 using SHELXL-9712 to give a final R1 = 0.0872 for 10260
reflections with I > 2r(I). All hydrogen atoms were placed in
calculated positions with the following parameters: d(Car–H) =
˚
˚
0.95 A, U(H) = 1.2Ueq(C); d(Cmethylene–H) = 0.99 A, U(H) =
˚
1.2Ueq(C); d(Cmethyl–H) = 0.98 A, U(H) = 1.5Ueq(C); d(O–H) =
˚
0.84 A, U(H) = 1.5Ueq(O). For atoms C(48)–C(58) (corresponding
8 (a) T. L. Schull, J. C. Fettinger and D. A. Knight, Inorg. Chem., 1996,
35, 6717; (b) S. Lelie`vre, F. Mercier and F. Mathey, J. Org. Chem., 1996,
61, 3531.
to the dicyclopentadienyl ligand in coordination unit 2), we had
to fix the individual bond distances for individual bonds using a
set of DFIX instructions. The electron density in this region of the
asymmetric unit is not sharply defined to allow an unrestrained
refinement of atomic positions. We have to point out that the
absolute configuration of the coordinated dicyclopentadienyl
ligand in this coordination unit therefore is uncertain. However,
the refinement of a split model placing both stereoisomers at the
palladium centre (Pd(2)) gave rise to an unstable refinement. This
refinement strategy does not influence the absolute ratio of chiral
isomers in the crystal lattice, since the centrosymmetric space
9 (a) A. H. Mahmoudkhani and V. Langer, J. Mol. Struct., 2002, 609,
97; (b) M. C. Aragoni, M. Arca, A. J. Blake, V. Lippolis, M. Schro¨der
and C. Wilson, Acta Crystallogr., Sect. C, 2002, 58, o260.
10 (a) S. Harder, L. Brandsma, J. A. Kanters, A. Duisenberg and J. H. v.
Lenthe, J. Organomet. Chem., 1991, 420, 143; (b) A. L. Casalnuovo,
T. V. RajanBabu, T. A. Ayers and T. H. Warren, J. Am. Chem. Soc.,
1994, 116, 9869; (c) D. Quintard, M. Keller and B. Breit, Synthesis,
2004, 6, 905; (d) D. Meinhard, F. Hollmann, W. Huhn, U. Thewalt, M.
Klinga and B. Rieger, Organometallics, 2004, 23, 5637.
11 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
12 G. M. Sheldrick, SHELXL-97, University of Go¨ttingen, Germany,
1997.
278 | Dalton Trans., 2007, 272–278
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The Royal Society of Chemistry 2007
©