Triphenyl-phosphine and -arsine analogues which facilitate the electrospray mass spectrometric analysis of neutral metal complexes

Article abstract of DOI:10.1039/a906010c

The six triarylphosphines PPh_n(C₆H₄OMe-p)_3-n and PPh_n(C₆H₄NMe₂-p)_3-n (n = 0-3) and the arsine As(C₆H₄OMe-p)₃ (L) have been synthesized and examined for their use in the electrospray mass spectrometric (ESMS) study of metal complexes. This has been tested with selected examples of the complexes [Mo(CO)₄L₂], [Fe(CO)₃L₂], [Fe(CO)₄L], [Ru₃(CO)₉L₃], cis-[PtCl₂L₂], [PdCl₂L₂] and [AuCl(L)]. All of the metal carbonyl complexes of these ligands gave [M + H]⁺ ions in their spectra, while in contrast the analogous PPh₃ complexes do not, suggesting that these electrospray-friendly ligands should be useful for the characterisation of a wide range of complexes by ESMS. The incorporation of the ligands into metal halide complexes however does not allow the observation of [M + H]⁺ ions, with ions formed by the previously reported halide-loss mechanism being the only ones observed. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a906010c

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Triphenyl-phosphine and -arsine analogues which facilitate the
electrospray mass spectrometric analysis of neutral metal
complexes
Corry Decker, William Henderson* and Brian K. Nicholson
Department of Chemistry, The University of Waikato, Private Bag 3105, Hamilton,
New Zealand
Received 26th July 1999, Accepted 20th August 1999
The six triarylphosphines PPhn(C6H4OMe-p)3Ϫn and PPhn(C6H4NMe2-p)3Ϫn (n = 0–3) and the arsine As(C6H4OMe-
p)3 (L) have been synthesized and examined for their use in the electrospray mass spectrometric (ESMS) study of
metal complexes. This has been tested with selected examples of the complexes [Mo(CO)4L2], [Fe(CO)3L2],
[Fe(CO)4L], [Ru3(CO)9L3], cis-[PtCl2L2], [PdCl2L2] and [AuCl(L)]. All of the metal carbonyl complexes of these
ligands gave [M ϩ H]ϩ ions in their spectra, while in contrast the analogous PPh3 complexes do not, suggesting
that these electrospray-friendly ligands should be useful for the characterisation of a wide range of complexes by
ESMS. The incorporation of the ligands into metal halide complexes however does not allow the observation of
[M ϩ H]ϩ ions, with ions formed by the previously reported halide-loss mechanism being the only ones observed.
bromide in the detection of intermediates (by ESMS) in the
Introduction
Suzuki coupling of phenylboronic acids and aryl halides.11
However, the routine use of electrospray-friendly ligands to
facilitate mass spectrometric analysis is rare in inorganic
chemistry.
Electrospray (ES) is a relatively recent mass spectrometric ion-
isation technique, which is proving to be invaluable in the
armoury of the co-ordination and organometallic chemist.1 In
general, the only proviso for the successful detection of ions
using this method is solubility in an appropriate solvent,
together with a means by which the parent molecule, if
uncharged, can become ionised. Often, this is by attachment
In comparison with the chemistry of phosphines, that of
related arsines (and especially stibines) is far less well
developed, and in part this is likely to be due to the lack of
readily accessible NMR-active nuclei for As and Sb when
compared to the 31P nucleus.12 A new method of monitoring
reactions involving arsine and stibine ligands should encourage
further development of their chemistry.
of a proton (or another cation such as NH4 or Naϩ) to some
ϩ
basic site (typically an oxygen or nitrogen atom with a lone
pair). In some cases, compounds do not readily ionise using
these “standard” ESMS conditions, in which case it is necessary
to perform some chemical derivatisation step in order to gener-
ate charged species. This is well illustrated by the addition of
either alkoxide2 or azide3 ions to neutral transition metal
carbonyl complexes; the resulting ions, formed by nucleophilic
attack of the anion at a co-ordinated CO, are readily analysed
by ESMS. Other pathways by which ionisation of transition
metal complexes can occur are electrochemical oxidation in the
metal capillary of the instrument,4 and by loss of an anionic
ligand, typically a halide.5,6
Results and discussion
Ligand choice and synthesis
The “electrospray-friendly” ligands chosen for this study are
derived from the very widely employed PPh3 and AsPh3. Func-
tionalised analogues, containing OMe or NMe2 groups in para
positions on one to three phenyl rings, were chosen, because of
their overall similarity to the parent ligand. Structures of the
ligands are shown below.
Since the protonation pathway for ionisation is potentially
the most general, and is gentle in nature, we wished to extend its
usefulness by the development of ligands which contain suit-
able basic substituents. In this paper we describe our studies
on tertiary phosphines and arsines. Tertiary phosphines are
ubiquitous in organometallic and co-ordination chemistry, and
their complexes find numerous applications, ranging from
catalysis7 to medicine.8 Triphenylphosphine is perhaps the
archetypal phosphine ligand, yet its presence in a complex
offers no pathway for ionisation, which must instead rely on
ionisation from other parts of the complex, by protonation,
oxidation, halide loss, etc. We reasoned that the use of
protonatable methoxy or dimethylamino substituted triphenyl-
phosphines, -arsines, and -stibines would facilitate the proton-
ation ionisation pathway. The derivatisation of molecules with
readily ionisable substituents is not a new concept, for example
the incorporation of ferrocene substituents for electrochemical
ionisation,9 the use of 4-aminobenzoic acid 2-(diethylamino)-
ethyl ester for the derivatisation of oligosaccharides and anal-
ysis by protonation10 and the use of a protonatable pyridyl
A key aim of this project was to use ligands which are similar
in most respects to the parent PPh3 or AsPh3 ligands, so that the
fundamental chemistry is essentially unchanged. Accordingly,
the para isomers were chosen (instead of the ortho or meta) so
that the steric properties of the ligands are very similar. Thus,
the steric properties of PPh3 and P(C6H4OMe-p)3 are identi-
cal,13 both having cone angles of 145Њ while their electronic
properties are reasonably similar, as usually expressed by the
pKa values of the protonated phosphines of 2.73 and 4.59
respectively.14 Some related triphenylphosphine-derived ligands,
containing OMe and/or SMe groups in ortho and para posi-
tions, have recently been reported, though no detailed MS
studies were carried out.15
The P(C6H4OMe-p)3 and P(C6H4NMe2-p)3 ligands, and their
metal complexes, are generally expected to be more soluble in
organic solvents than their PPh3 analogues, so if a very close
similarity is desired, then PPh2(C6H4OMe-p) and PPh2(C6H4N-
Me2-p) are more appropriate. On the other hand, it
was anticipated that the ligands (and complexes) bearing the
J. Chem. Soc., Dalton Trans., 1999, 3507–3513
This journal is © The Royal Society of Chemistry 1999
3507

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nucleus occurs on incorporation of OMe or NMe2 groups.
Thus, as shown in Table 1, there is a small stepwise decrease
in the δ(31P) value from [Mo(CO)4(PPh3)2] (38.9), to [Mo-
(CO)4{PPh2(C6H4OMe-p)}2] (37.1) to [Mo(CO)4{PPh(C6H4-
OMe-p)}2] (35.5) to [Mo(CO)4{P(C6H4OMe-p)3}2] (34.0). Very
approximately, the presence of two NMe2 groups (in 1d) has
around the same effect on the 31P NMR shift as four OMe
groups (in 1b). Similar trends in δ(31P) were observed for the
other series of complexes, with NMe2 substituted complexes
having lower values than the analogous complexes with OMe
substituents.
Electrospray mass spectrometry
(a) Metal carbonyl complexes. Table 2 lists the ions observed
in the ES spectra of the metal carbonyl complexes 1–3 and 7–8.
The neutral complexes [Mo(CO)4(PPh3)2] 1, [Fe(CO)3(PPh3)2]
2, [Ru3(CO)9(PPh3)3] 3, [Mo(CO)4(AsPh3)2] 7 and [Fe(CO)3-
(AsPh3)2] 8 did not give ions in their ES spectra, as expected
with the absence of basic sites for protonation. In contrast, all
of the neutral metal carbonyl complexes which contain the
amine- or methoxy-modified ligands ionise readily, and give
good ES spectra with strong [M ϩ H]ϩ ions. In some cases,
[M ϩ NH4]ϩ ions were also observed, but their intensities were
generally low. The crude product from the attempted synthesis
of [Fe(CO)3{As(C6H4OMe-p)3}2] was found by ESMS to be a
mixture of this complex together with the monosubstituted
complex [Fe(CO)4{As(C6H4OMe-p)3}], demonstrating the
usefulness of the technique in determining product formation
in such ligand substitution reactions. However, upon recrystal-
lisation, only the monosubstituted complex was obtained, and
analytical data are reported for this complex only.
At low skimmer cone voltages, between 5 and 20 V, there is
no fragmentation of the parent ions, so the parent [M ϩ H]ϩ
ions can readily be assigned by comparison of experimental
and calculated isotope distribution patterns. Such lack of frag-
mentation is typical for the ES process when low cone voltages
are used. On increasing the cone voltage, carbonyl ligands are
lost sequentially, as illustrated in Fig. 1 for [Ru3(CO)9{PPh2-
(C6H4OMe-p)}3] 3a. Similar loss of carbonyl ligands has been
observed previously, e.g. in studies of neutral metal carbonyl
complexes investigated using alkoxide2 or azide3 ionisation.
Having established the generality of the use of the derivatised
phosphines, it was of interest to carry out a comparison of the
ionisation efficiencies of the different ligands, to compare the
effect of an oxygen with a more basic nitrogen, and of one
substituent per ligand versus two or three. An equimolar
mixture of the complexes [Mo(CO)4{PPh2(C6H4OMe-p)}2] 1a,
[Mo(CO)4{PPh(C6H4OMe-p)2}2] 1b and [Mo(CO)4{P(C6H4-
OMe-p)3}2] 1c was prepared in methanol solution and analysed
by ESMS. The relative intensities of the [M ϩ H]ϩ ions increase
(24:51:100) directly in proportion to the increasing number of
MeO groups in the complex from 2 to 4 to 6, as shown in Fig. 2.
In a separate experiment to compare the ionisation efficiencies
of OMe substituted complexes versus NMe2 substituted com-
plexes, a 1:1 mixture of 1b and [Mo(CO)4{PPh(C6H4NMe2-
p)2}2] 1e was analysed. In this case, the NMe2-substituted
complex showed a much greater ionisation efficiency, with the
methoxy analogue appearing at only 20% relative intensity to
the base peak of [Mo(CO)4{PPh(C6H4NMe2-p)2}2 ϩ H]ϩ.
The ESMS analysis using the substituted ligands appears
to be very sensitive; using a series of dilutions, the complex
[Ru3(CO)9{PPh2(C6H4NMe2-p)3}3] 3d was able to be detected
down to concentrations of ca. 7 × 10Ϫ8 mol LϪ1, without any
special attempts to optimise signal intensity. For routine char-
acterisation, however, we typically use more concentrated
solutions (ca. 1 mg mLϪ1).
greatest number of OMe or NMe2 substituents might provide
the best ES spectra. A comparison of the ligands in terms of
their ionisation efficiencies and electronic similarities was car-
ried out, and results are discussed later in this paper.
The ligands are all known from previous studies and were
readily prepared by reaction of the appropriate Grignard
reagent with PCl3, PhPCl2, Ph2PCl or AsCl3 as appropriate. All
are air-stable crystalline solids, similar to PPh3 and AsPh3.
While the ligands can easily be prepared from inexpensive start-
ing materials, some are also commercially available.
Synthesis and characterisation of metal complexes
A wide range of metal complexes of the methoxy- and amino-
substituted phosphines has been prepared following procedures
previously published for the PPh3 and AsPh3 analogues. These
are listed in Table 1. The examples were chosen to cover two
main types of complex. Neutral metal phosphine(arsine) carb-
onyl complexes of molybdenum, iron and ruthenium were
examined, since the PPh3 analogues do not give ES spectra. The
second group involved metal phosphine halide complexes of
platinum, palladium and gold, chosen to evaluate the tendency
(if any) of these complexes to undergo ionisation by proton-
ation compared to ionisation by halide loss, which is the ionis-
ation pathway previously observed for other complexes of this
type.5,6 Selected PPh3 and AsPh3 complexes were also syn-
thesized for comparative purposes. Complexes 2c, 4f, 5b and 6f
have been synthesized previously (see Experimental section),
but the others appear to be new. Characterisation data for the
new complexes are given in Table 1.
In order to confirm the close similarity of the functionalised
ligands compared to their PPh3 or AsPh3 analogues a 31P NMR
and IR spectroscopic survey has been carried out. Generally,
increasing numbers of OMe or NMe2 groups in the ligand
cause a slight shift of CO stretching bands to lower wavenum-
bers, due to increased back donation with a slightly more
electron-rich phosphine. However, the values are overall similar
to those of the PPh3 analogues. This is well illustrated by the
series of [Mo(CO)4L2] complexes, where there is a ca. 10 cmϪ1
shift in CO stretching frequency going from [Mo(CO)4(PPh3)2]
to [Mo(CO)4{P(C6H4NMe2-p)3}2]. Similar trends are observed
for the iron carbonyl complexes and the stronger bands of the
ruthenium carbonyl complexes. These observations are paral-
For these complexes where competing ionisation is not pos-
sible, even one OMe or NMe2 group is sufficient to provide ions
with a good signal to noise ratio. In protic solvents such as
leled by the 31P NMR chemical shifts, where shielding of the 31
P
3508 J. Chem. Soc., Dalton Trans., 1999, 3507–3513

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Table 1 Characterisation data for the complexesa
Elemental analysis (%)b
31P NMRc
Complex
Colour
mp/ЊC
C
H
N
(δ)
IR CO ν˜ max
/cmϪ1
1 [Mo(CO)4(PPh3)2]
Bright yellow
38.9
2023s, 1927 (sh), 1908s,
1897 (sh)d
1a [Mo(CO)4{PPh2(C6H4OMe-p)}2]
1b [Mo(CO)4{PPh(C6H4OMe-p)2}2]
1c [Mo(CO)4{P(C6H4OMe-p)3}2]
Bright yellow 149–150 63.36 (63.64) 4.25 (4.33) 0.00 (0.00) 37.1
Bright yellow 144–145 61.66 (61.98) 4.46 (4.50) 0.00 (0.00) 35.5
Bright yellow 142–143 60.08 (60.53) 4.66 (4.65) 0.00 (0.00) 34.0
2021s, 1919 (sh), 1906s,
1879 (sh)d
2020s, 1918 (sh), 1905s,
1876 (sh)d
2018s, 1917 (sh), 1904s,
1874 (sh)d
1d [Mo(CO)4{PPh2(C6H4NMe2-p)}2] Bright yellow 142–148 64.37 (64.56) 4.75 (4.89) 3.37 (3.42) 36.0
1e [Mo(CO)4{PPh(C6H4NMe2-p)2}2] Bright yellow 166–167 62.92 (63.72) 5.79 (5.53) 6.20 (6.20) 33.3
2019s, 1920 (sh), 1903s,
1875 (sh)d
2016s, 1913 (sh), 1899s,
1872 (sh)d
1f [Mo(CO)4{P(C6H4NMe2-p)3}2]
Bright yellow 155–160 62.93 (63.04) 6.18 (6.06) 8.59 (8.49) 30.8
2013s, 1907 (sh), 1900s,
1873 (sh)d
2 [Fe(CO)3(PPh3)2]
Dark yellow
Dark yellow
Dark yellow
Dark yellow
Deep red
83.1
1884d
2a [Fe(CO)3{PPh2(C6H4OMe-p)}2]
2b [Fe(CO)3{PPh(C6H4OMe-p)2}2]
2c [Fe(CO)3{P(C6H4OMe-p)3}2]
3 [Ru3(CO)9(PPh3)3]
220–221 67.16 (67.97) 4.60 (4.74) 0.00 (0.00) 81.3
222–223 65.08 (65.83) 4.82 (4.89) 0.00 (0.00) 79.5
1882d
1880d
77.8
38.0
1874d
2044w, 1979 (sh),
1967s (br)e
3a [Ru3(CO)9{PPh2(C6H4OMe-p)}3]
3b [Ru3(CO)9{PPh(C6H4OMe-p)2}3]
3c [Ru3(CO)9{P(C6H4OMe-p)3}3]
3d [Ru3(CO)9{PPh2(C6H4NMe2-p)}3] Deep red
3e [Ru3(CO)9{PPh(C6H4NMe2-p)2}3] Deep red
Deep red
Deep red
Deep red
138–144 54.73 (55.35) 4.03 (3.06) 0.00 (0.00) 36.7
134–142 53.82 (54.44) 3.89 (3.78) 0.00 (0.00) 35.8
118–120 53.62 (53.63) 4.36 (3.95) 0.61 (0.00) 34.6
2043w, 1979 (sh), 1967s f
2047w, 1978 (sh), 1967s f
2054w, 1977 (sh), 1966s f
2054w, 1977 (sh), 1965s f
2055w, 1972 (sh), 1963s f
2054w, 1970 (sh), 1959s f
140–142 55.65 (56.32) 3.89 (4.08)
148–150 56.21 (56.27) 4.86 (4.69)
160–165 54.71 (56.24) 5.44 (5.21)
36.0
34.3
32.7
15.1 (3673)
3f [Ru3(CO)9{P(C6H4NMe2-p)3}3]
4 cis-[PtCl2(PPh3)2]
4a cis-[PtCl2{PPh2(C6H4OMe-p)}2]
4b cis-[PtCl2{PPh(C6H4OMe-p)2}2]
4c cis-[PtCl2{P(C6H4OMe-p)3}2]
4f cis-[PtCl2{P(C6H4NMe2-p)3}2]
5 [PdCl2(PPh3)2]
5a [PdCl2{PPh2(C6H4OMe-p)}2]
5b [PdCl2{PPh(C6H4OMe-p)2}2]
5c [PdCl2{P(C6H4OMe-p)3}2]
6 [AuCl(PPh3)]
6a [AuCl{PPh2(C6H4OMe-p)}]
6b [AuCl{PPh(C6H4OMe-p)2}]
6c [AuCl{P(C6H4OMe-p)3}]
6f [AuCl{P(C6H4NMe2-p)3}]
7 [Mo(CO)4(AsPh3)2]
Deep red
White
White
White
258–259 51.52 (53.65) 4.00 (4.04) 0.00 (0.00) 13.8 (3684)
242–243 52.29 (52.75) 4.16 (4.21) 0.00 (0.00) 12.7 (3695)
White
White
11.5 (3705)
9.7 (3758)
Deep yellow
Deep yellow
Deep yellow
Deep yellow
White
White
White
White
White
155–156 43.59 (43.13) 3.49 (3.63) 0.00 (0.00) 30.0
28.3
Deep yellow
2025s, 1921 (sh), 1913s,
1881 (sh)d
7c [Mo(CO)4{As(C6H4OMe-p)3}2]
Deep yellow
147–148 55.12 (55.22) 4.23 (4.20)
2022s, 1919 (sh), 1909s,
1876 (sh)d
8 [Fe(CO)3(AsPh3)2]
8c [Fe(CO)4{As(C6H4OMe-p)3}]
Dark yellow
Dark yellow
2049s, 1972 (sh), 1940sd
2047s, 1970 (sh), 1938sd
148–149 53.13 (53.19) 3.45 (3.72) 0.00 (0.00)
1
a Data for the known PPh3 and AsPh3 complexes restricted to 31P NMR and IR. b Calculated values given in parentheses. c The J(PtP) coupling
constant (in Hz) is given in parentheses. d In CH2Cl2. e Values according to the literature.24 f In CH2Cl2–hexane (1:1).
methanol there is no need to add extra acid to encourage ion
formation, so the conditions for analysis are relatively mild.
However, for some complexes, protic solvents may be incompat-
ible with chemical stability. Preliminary studies show that the
electrospray-friendly complexes can also be analysed in 1,2-
dimethoxyethane (dme) or thf solutions with added NaBPh4 as
the cation source. The use of potassium salts dissolved in apro-
tic solvents in ESMS analyses has been reported previously.16
Thus, analysis of compounds 1e and 1f in thf solution with
a small quantity of added Na[BPh4] as an ionisation source
yielded [M ϩ Na]ϩ ions as the base peaks for both compounds,
at m/z 928 and 1014 respectively. The use of a more strongly co-
ordinating solvent such as dme is less preferred, since this will
compete with the analyte for the Naϩ ions; analysis of complex
1c in dme gave the solvated species [M ϩ Na(dme)]ϩ at m/z
1026. These preliminary results suggest that the concept of
electrospray-friendly ligands should be applicable to complexes
with a wide range of stabilities.
Thus, the bidentate phosphine (p-MeOC6H4)2CH2CH2P(C6H4-
OMe-p)2 (L–L), when treated with [Mo(CO)4(pip)2] (pip =
piperidine), yields [Mo(CO)4(L–L)] which has not been fully
characterised but gives the expected [M ϩ H]ϩ ion (m/z 727) as
the only peak in the ESMS spectrum.
(b) Metal halide complexes. Table 3 summarises the ESMS
data for metal halide complexes of the electrospray-friendly lig-
ands. Previously, we carried out a study on a wide range of
transition metal complexes which contain halide ligands, in
addition to other neutral ancillary donor ligands, such as phos-
phines, etc.6 In almost all cases, loss of a halide ligand, with
solvation of the resulting cation by a solvent molecule at low
cone voltages, provides the dominant ionisation pathway. For
the complexes containing MeO- or NMe2-substituted phos-
phine ligands, the protonation mechanism, forming [M ϩ H]ϩ
ions, might become competitive with halide loss. However, even
for the complex cis-[PtCl2{P(C6H4NMe2-p)3}2] spectra were
dominated by the ions [M Ϫ Cl]ϩ and [M Ϫ Cl ϩ solvent]ϩ. For
the platinum complexes 4a–4c the intensities of the ions
appeared to depend on the number of OMe groups present. In
The incorporation of OMe and NMe2 groups into other
types of ligands should provide a general means of facilitating
mass spectrometric characterisation of complexes thereof.
J. Chem. Soc., Dalton Trans., 1999, 3507–3513
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Table 2 Positive-ion ESMS data for the transition-metal carbonyl complexes, recorded in either MeCN–water (a) or MeOH (b) solution
Compound
Solvent
a
Cone voltage/V
Ions observed (m/z, %)
1a
20
60
[M ϩ H]ϩ (795, 100), [M ϩ NH4]ϩ (812, 20)
[M ϩ H]ϩ (795, 69), [M ϩ H Ϫ CO]ϩ (767, 12), [M ϩ H Ϫ 2CO]ϩ (739, 100), [M ϩ H Ϫ
3CO]ϩ (711, 48), [M ϩ H Ϫ 4CO]ϩ (683, 49)
80
[M ϩ H]ϩ (795, 90), [M ϩ H Ϫ CO]ϩ (767, 19), [M ϩ H Ϫ 2CO]ϩ (739, 20), [M ϩ H Ϫ
3CO]ϩ (711, 13), [M ϩ H Ϫ 4CO]ϩ (683, 100)
1b
1c
a
b
20
20
60
[M ϩ H]ϩ (855, 100), [M ϩ NH4]ϩ (872, 4)
[M ϩ H]ϩ (915, 100)
[M ϩ H]ϩ (915, 15), [M ϩH Ϫ 2CO]ϩ (859, 100), [M ϩ H Ϫ 3CO]ϩ (831, 41), [M ϩ H Ϫ
4CO]ϩ (803, 18)
1d
1e
1f
2a
2b
a
b
b
b
a
20
20
20
20
20
60
80
20
60
80
20
60
[M ϩ H]ϩ (821, 100)
[M ϩ H]ϩ (907, 100)
[M ϩ H]ϩ (993, 100)
[M ϩ H]ϩ (725, 53), [M ϩ NH4]ϩ (742, 100)
[M ϩ H]ϩ (785, 100)
[M ϩ H]ϩ (785, 100), [M ϩ H Ϫ 3CO]ϩ (701, 60)
[M ϩ H]ϩ (785, 32), [M ϩ H Ϫ 3CO]ϩ (701, 100)
2c
3a
a
[M ϩ H]ϩ (845, 100)
[M ϩ H]ϩ (845, 28), [M ϩ H Ϫ 3CO]ϩ (761, 100)
[M ϩ H]ϩ (845, 18), [M ϩ H Ϫ 3CO]ϩ (761, 100)
b
[M ϩ H]ϩ (1434, 100)
[M ϩ H]ϩ (1434, 45), [M ϩ H Ϫ CO]ϩ (1406, 100), [M ϩ H Ϫ 2CO]ϩ (1378, 36),
[M ϩ H Ϫ 3CO]ϩ (1350, 23), [M ϩ H Ϫ 4CO]ϩ (1322, 12), [M ϩ H Ϫ 5CO]ϩ (1294, 5)
[M ϩ H]ϩ (1434, 88), [M ϩ H Ϫ CO]ϩ (1406, 69), [M ϩ H Ϫ 2CO]ϩ (1378, 99),
[M ϩ H Ϫ 3CO]ϩ (1350, 77), [M ϩ H Ϫ 4CO]ϩ (1322, 100), [M ϩ H Ϫ 5CO]ϩ (1294, 93),
[M ϩ H Ϫ 6CO]ϩ (1266, 47)
80
3b
3c
b
b
20
60
[M ϩ H]ϩ (1524, 100)
[M ϩ H]ϩ (1524, 100), [M ϩ H Ϫ CO]ϩ (1496, 88), [M ϩ H Ϫ 2CO]ϩ (1468, 20),
[M ϩ H Ϫ 3CO]ϩ (1440, 9), [M ϩ H Ϫ 4CO]ϩ (1412, 4)
[M ϩ H]ϩ (1614, 100)
20
60
[M ϩ H]ϩ (1614, 100), [M ϩ H Ϫ CO]ϩ (1586, 90), [M ϩ H Ϫ 2CO]ϩ (1558, 28),
[M ϩ H Ϫ 3CO]ϩ (1530, 11), [M ϩ H Ϫ 4CO]ϩ (1502, 4)
[M ϩ H]ϩ (1614, 18), [M ϩ H Ϫ CO]ϩ (1586, 95), [M ϩ H Ϫ 2CO]ϩ (1558, 63),
[M ϩ H Ϫ 3CO]ϩ (1530, 68), [M ϩ H Ϫ 4CO]ϩ (1502, 100), [M ϩ H Ϫ 5CO]ϩ (1474, 49),
[M ϩ H Ϫ 6CO]ϩ (1446, 9)
80
3d
3e
3f
a
a
a
20
60
[M ϩ H]ϩ (1473, 100), [M ϩ Na]ϩ (1495, 10)
[M ϩ H]ϩ (1473, 25), [M ϩ H Ϫ CO]ϩ (1445, 92), [M ϩ H Ϫ 2CO]ϩ (1417, 100),
[M ϩ H Ϫ 3CO]ϩ (1389, 69), [M ϩ H Ϫ 4CO]ϩ (1361, 56), [M ϩ H Ϫ 5CO]ϩ (1333, 29)
[M ϩ H]ϩ (1473, 18), [M ϩ H Ϫ CO]ϩ (1445, 29), [M ϩ H Ϫ 2CO]ϩ (1417, 50),
[M ϩ H Ϫ 3CO]ϩ (1389, 14), [M ϩ H Ϫ 4CO]ϩ (1361, 40), [M ϩ H Ϫ 5CO]ϩ (1333, 79),
[M ϩ H Ϫ 6CO]ϩ (1305, 100)
80
20
60
[M ϩ H]ϩ (1602, 100), [M ϩ Na]ϩ (1624, 29)
[M ϩ H]ϩ (1602, 70), [M ϩ Na]ϩ (1624, 20), [M ϩ H Ϫ CO]ϩ (1574, 100),
[M ϩ H Ϫ 2CO]ϩ (1546, 65), [M ϩ H Ϫ 3CO]ϩ (1518, 30), [M ϩ H Ϫ 4CO]ϩ (1490, 27)
[M ϩ H]ϩ (1602, 59), [M ϩ H Ϫ CO]ϩ (1574, 48), [M ϩ H Ϫ 2CO]ϩ (1546, 68),
[M ϩ H Ϫ 3CO]ϩ (1518, 35), [M ϩ H Ϫ 4CO]ϩ (1490, 78), [M ϩ H Ϫ 5CO]ϩ (1362, 79),
[M ϩ H Ϫ 6CO]ϩ (1434, 100)
80
20
60
[M ϩ H]ϩ (1731, 100), [M ϩ Na]ϩ (1753, 30)
[M ϩ H]ϩ (1731, 100), [M ϩ Na]ϩ (1753, 16), [M ϩ H Ϫ CO]ϩ (1703, 65),
[M ϩ H Ϫ 2CO]ϩ (1675, 20), [M ϩ H Ϫ 3CO]ϩ (1647, 8), [M ϩ H Ϫ 4CO]ϩ (1619, 8),
[M ϩ H Ϫ 5CO]ϩ (1582, 8), [M ϩ H Ϫ 6CO]ϩ (1552, 10)
[M ϩ H]ϩ (1003, 100)
7c
8c
a
a
20
20
[M ϩ H]ϩ (564, 100)
m/z values quoted represent the peak of greatest intensity in the isotope distribution pattern of that ion, verified with a simulated isotope pattern.
the gold compounds 6 and 6a–6c the bis(phosphine)gold cation
Conclusion
[L2Au]ϩ dominated both the high and low cone voltage spectra,
but smaller solvated [M Ϫ Cl ϩ MeCN]ϩ ions were seen. This
behaviour is typical for gold(I) phosphine complexes which
tend to be labile in solution.17 For complex 6f the base peak was
interestingly the ammonia-containing ion [M Ϫ Cl ϩ NH3]ϩ,
though the bis(phosphine)gold cation was still observed.
Dilute HCl was added to try and suppress the loss of chloride
from the platinum centre in complexes 4a–4f, and to promote
protonation, however the spectra were basically unchanged,
with the exception that the [M Ϫ Cl ϩ NH3]ϩ ions observed in
MeCN–water solution had disappeared. No [M ϩ H]ϩ ions
were observed. Addition of pyridine to the analyte solution
resulted in observation of the expected [M Ϫ Cl ϩ py]ϩ ions, as
observed previously.6 Thus, the use of electrospray-friendly lig-
ands appears to offer no advantages over the simple triphenyl-
phosphine, when ionisable halide ligands are present in the
complex.
The OMe and NMe2 substituted triphenylphosphines, and the
arsine As(C6H4OMe-p)3 appear to be good ligands for the
ESMS characterisation of neutral transition metal complexes,
especially carbonyls (and other complexes without labile
anionic ligands) where the analogous PPh3 or AsPh3 com-
pounds are invisible. The NMR and IR studies confirm that the
electrospray-friendly ligands are similar to the parent PPh3 and
AsPh3 ligands. For complexes which contain a halide ligand,
loss of this halide remains the dominant ionisation pathway. A
single OMe group appears to be sufficient to allow analysis by
ESMS, but greater ionisation efficiency is achieved by higher
numbers of OMe groups, and particularly by the use of the
more basic NMe2 group. The use of these ligands may play a
very useful role in transition metal chemistry, allowing the
detection of products and intermediates not previously char-
acterised. In this respect, the ability to sample directly from
3510
J. Chem. Soc., Dalton Trans., 1999, 3507–3513

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Table 3 Positive-ion ESMS dataa for the transition-metal halide complexes, recorded in MeCN–water solution at a cone voltage of 20 V
Compound
Ions observed (m/z, %)b
4a
4b
4c
4f
5a
5b
5c
6a
6b
6c
6f
[M Ϫ Cl ϩ MeCN]ϩ (856, 100), [M Ϫ Cl ϩ NH3]ϩ (832, 9), [M Ϫ Cl]ϩ (815, 52)
[M Ϫ Cl ϩ MeCN]ϩ (916, 99), [M Ϫ Cl]ϩ (875, 100)
[M Ϫ Cl ϩ MeCN]ϩ (976, 30), [M Ϫ Cl ϩ NH3]ϩ (952, 10), [M ϪCl]ϩ (935, 100)
[M Ϫ Cl ϩ NH3]ϩ (1030, 100), [M Ϫ Cl]ϩ (1013, 84)
[M Ϫ Cl ϩ MeCN]ϩ (766/768, 40), [M Ϫ Cl ϩ NH3]ϩ (742/744, 18), [M Ϫ Cl]ϩ (725/727, 100)
[M Ϫ Cl ϩ MeCN]ϩ (826/828, 26), [M Ϫ Cl ϩ NH3]ϩ (802/804, 13), [M Ϫ Cl]ϩ (785/787, 100)
[M Ϫ Cl]ϩ (845/847, 100)
[L2Au]ϩ (781, 100), [M Ϫ Cl ϩ MeCN]ϩ (530, 7)
[L2Au]ϩ (841, 100), [M Ϫ Cl ϩ MeCN]ϩ (560, 8)
[L2Au]ϩ (901, 100), [M Ϫ Cl ϩ MeCN]ϩ (590, 18)
[L2Au]ϩ (979, 15), [M Ϫ Cl ϩ MeCN]ϩ (629, 50), [M Ϫ Cl ϩ NH3]ϩ (605, 100)
a m/z values quoted represent the peak of greatest intensity in the isotope distribution pattern of that ion, verified with a simulated isotope pattern.
b M refers to the parent complex, L to the phosphine ligand therein.
Fig. 2 Positive-ion ES spectrum of a 1:1:1 molar mixture of [Mo-
(CO)4{PPh2(C6H4OMe-p)}2] 1a, [Mo(CO)4{PPh(C6H4OMe-p)2}2] 1b
and [Mo(CO)4{P(C6H4OMe-p)3}2] 1c in methanol solution, at a cone
voltage of 20 V, showing the increased ionisation efficiency with
increased numbers of OMe groups.
at 300.13, 75.47 and 121.5 Hz respectively. All NMR spectra
were recorded in CDCl3 solution, 1H and 13C referenced to
residual CHCl3 and 31P to an external standard of 85% H3PO4.
Elemental microanalyses were carried out by the Campbell
Microanalytical Laboratory, University of Otago.
Fig. 1 Positive-ion ES spectra of [Ru3(CO)9{PPh2(C6H4OMe-p)}3] 3a
in methanol, at cone voltages of 20 (upper spectrum) and 60 V (lower
spectrum), showing the assignment of the ions.
Electrospray mass spectra were recorded in positive-ion
mode on a VG Platform II mass spectrometer. Compounds
were dissolved in 1:1 v/v acetonitrile–water or methanol and
injected directly via a Rheodyne injector with a 10 µl sample
loop. A SpectraPhysics isocratic LC Spectra System P1000
pump delivered the solution (typically 1 mg mLϪ1) at 0.02 ml
minϪ1 to the mass spectrometer source (60 ЊC), and nitrogen
was employed both as a drying and nebulising gas. Skimmer
cone voltages were varied between 20 and 80 V, in order to
investigate the effects of higher cone voltages on fragmentation
of the parent ions. Theoretical isotope distribution patterns
were calculated using the ISOTOPE computer program18 and
used to aid in assignment. Peaks in the mass spectra are listed in
Tables 2 and 3 by the most intense m/z value in the isotopic
mass distribution.
reaction solutions carried out on a very small scale is a major
advantage of ESMS over other methods of monitoring.
Assignment of peaks is simplified by the lack of fragmentation
of parent ions, compared with other MS techniques.
Experimental
Unless otherwise specified, all reactions were performed under
a dinitrogen atmosphere using standard Schlenk techniques.
The solvents dichloromethane and light petroleum (bp 40–
60 ЊC) were distilled from calcium hydride, tetrahydrofuran and
diethyl ether from sodium–benzophenone. Other reagent grade
solvents were used without purification.
Melting points were recorded on a Reichert Thermopan
apparatus and are uncorrected. Infrared spectra were obtained
in solution on a BioRad FTS-40 instrument, and 1H, 13C-{1H}
and 31P-{1H} NMR spectra on a Bruker AC300P spectrometer
The phosphine ligands PPh2(C6H4OMe-p), PPh(C6H4-
OMe-p)2 and P(C6H4OMe-p)3 were prepared as reported by
Schiemenz.19 The other ligands were prepared following the
J. Chem. Soc., Dalton Trans., 1999, 3507–3513
3511

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same general procedure. The preparations of PPh2(C6H4NMe2-
the reaction mixture was placed in a freezer for 12 h. The pre-
p),20 PPh(C6H4NMe2-p)2 and P(C6H4NMe2-p)3 have been
reported before. Complexes 1,22 2,23 3,24 4f20 and 625 were pre-
pared by the published methods. The complexes [PtCl2(cod)]26
and [PdCl2(cod)]27 (cod = 1,5-cyclooctadiene) were prepared by
the literature procedures, and the phosphine complexes formed
from them by ligand displacement.28 The complexes [Mo(CO)4-
(pip)2]22 (pip = piperidine) and HAuCl4 29 were prepared by the
literature procedures. The compound PhPCl2, Ph2PCl, [Ru3-
(CO)12] and [Fe(CO)5] (Strem Chemicals), p-bromoanisole and
p-bromo-N,N-dimethylaniline (Aldrich), PCl3 and PPh3 (BDH)
were used as supplied.
cipitate was collected by filtration and washed with CH3OH
(3 × 5 mL). The complex was purified by dissolving the product
in CH2Cl2 (5 mL) and filtering it into a flask containing 10 mL
of ice-cold CH3OH. The precipitate and solution were cooled
to 5 ЊC for 12 h. The product was collected by filtration, washed
with three 5 mL portions of ice-cold CH3OH, and dried under
vacuum. Yield: 78 mg, 39%.
The following were prepared similarly: [Fe(CO)3{PPh(C6H4-
OMe-p)2}] 2b, from [Fe(CO)5] (0.1 mL, 0.149 g, 0.76 mmol),
PPh(C6H4OMe-p)2 (0.515 g, 1.6 mmol) and NaBH4 (0.058 g,
0.53 mmol), yield 92 mg (46%); [Fe(CO)3{P(C6H4OMe-p)3}2]
2c, from [Fe(CO)5] (0.1 mL, 0.149 g, 0.76 mmol), P(C6H4-
OMe-p)3 (0.563 g, 1.6 mmol) and NaBH4 (0.058 g, 0.53 mmol),
yield 71 mg (36%); previously by alternative methods;30
[Fe(CO)3(AsPh3)2] 8, from [Fe(CO)5] (0.53 mL, 0.078 g, 0.4
mmol), AsPh3 (0.245 g, 0.8 mmol) and NaBH4 (0.03 g, 0.8
mmol), yield 150 mg (50%); [Fe(CO)4{As(C6H4OMe-p)3}] 8c,
from [Fe(CO)5] (0.53 mL, 0.078 g, 0.4 mmol), As(C6H4OMe-p)3
(0.317 g, 0.8 mmol) and NaBH4 (0.03 g, 0.8 mmol), yield 85 mg
(23%); analysis of the crude reaction mixture showed a mixture
of the mono- and di-substituted products.
21
20
Syntheses
PPh(C6H4NMe2-p)2. A solution of the Grignard BrMgC6H4-
NMe2-p was prepared from Mg (1.04 g, 43.2 mmol) and
p-bromo-N,N-dimethylaniline (8.64 g, 43.2 mmol) in thf (40
mL). A solution of PhPCl2 (2.53 g, 1.95 mL; 14.4 mmol) in thf
(40 mL) was added slowly at 0 ЊC. The reaction mixture was
refluxed for 1 h, cooled to room temperature and hydrolysed
with strong (ca. 10%) NH4Cl solution at 0 ЊC. The phosphine
was then extracted with toluene (3 × 100 mL), evaporated to
dryness under reduced pressure and recrystallised from
methanol. Yield: 4.00 g, 80%.
[Ru3(CO)9{PPh2(C6H4OMe-p)}3] 3a. This complex was pre-
pared by a modification of the literature procedure.24 A solu-
tion of [Ru3(CO)12] (61 mg, 0.14 mmol) and PPh2(C6H4OMe-p)
(123 mg, 0.42 mmol) in toluene (5 mL) was refluxed for 30 min.
The pure product was obtained by chromatography on silica,
eluting with light petroleum–CH2Cl2 (1:4). Yield: 160 mg, 80%.
The following complexes were similarly prepared: [Ru3(CO)9-
{PPh(C6H4OMe-p)2}3] 3b, from [Ru3(CO)12] (85.5, 0.20 mmol)
and PPh(C6H4OMe-p)2 (190 mg, 0.60 mmol), yield 218 mg
(73%); [Ru3(CO)9{P(C6H4OMe-p)3}3] 3c, from [Ru3(CO)12] (54
mg, 0.124 mmol) and P(C6H4OMe-p)3 (131 mg, 0.372 mmol),
yield 172 mg (86%); [Ru3(CO)9{PPh2(C6H4NMe2-p)}3] 3d, from
[Ru3(CO)12] (29 mg, 0.066 mmol) and PPh2(C6H4NMe2-p) (60
mg, 0.20 mmol), yield 53 mg (53%); [Ru3(CO)9{PPh(C6H4-
NMe2-p)2}3] 3e, from [Ru3(CO)12] (22 mg, 0.05 mmol) and
PPh(C6H4NMe2-p)2 (50 mg, 0.14 mmol), yield 39 mg (49%);
[Ru3(CO)9{P(C6H4NMe2-p)3}3] 3f, from [Ru3(CO)12] (29 mg,
0.066 mmol) and P(C6H4NMe2-p)3 (78 mg, 0.20 mmol), yield
39 mg (34%).
As(C6H4OMe-p)3. Using the same method as above, the
Grignard from Mg (907 mg, 37.8 mmol), and p-bromoanisole
(7.07 g, 4.7 mL; 37.8 mmol) was treated with AsCl3 (2.27 g, 1.1
mL; 12.6 mmol) to give As(C6H4OMe-p)3 (3.2 g, 64%).
[Mo(CO)4{PPh2(C6H4OMe-p)}2] 1a. The complex [Mo(CO)4-
(pip)2] (95 mg, 0.25 mmol) was partially dissolved in CH2Cl2 (10
mL) and PPh2(C6H4OMe-p) (146 mg, 0.5 mmol) added. The
reaction mixture was heated to reflux whereupon the [Mo(CO)4-
(pip)2] fully dissolved. Reflux was maintained for 15 min. The
reaction solution was allowed to cool to room temperature and
the orange solution filtered. The filtrate was reduced in volume
to ca. 4 mL (rotary evaporator) and methanol (10 mL) added.
The solution was cooled in a freezer overnight and the pale
yellow product crystallised. It was collected by filtration and air
dried. The complex was purified by dissolving in hot methanol,
and crystallising at Ϫ20 ЊC. Yield 202 mg, 81%.
Similarly prepared were: [Mo(CO)4{PPh(C6H4OMe-p)}2] 1b,
from [Mo(CO)4(pip)2] (132 mg, 0.35 mmol) and PPh(C6H4-
OMe-p)2 (225 mg, 0.7 mmol), yield 261 mg (87%); [Mo(CO)4-
{P(C6H4OMe-p)3}2] 1c, from [Mo(CO)4(pip)2] (144 mg, 0.38
mmol) and P(C6H4OMe-p)3 (268 mg, 0.76 mmol), yield 242
mg (69%); [Mo(CO)4{PPh2(C6H4NMe2-p)}2] 1d, from
[Mo(CO)4(pip)2] (312 mg, 0.825 mmol) and PPh2(C6H4NMe2-p)
(500 mg, 1.65 mmol), yield 415 mg (62%); [Mo(CO)4{PPh(C6H4-
NMe2-p)}2] 1e, from [Mo(CO)4(pip)2] (272 mg, 0.72 mmol) and
PPh(C6H4NMe2-p)2 (500 mg, 1.44 mmol), yield 507 mg (78%);
[Mo(CO)4{P(C6H4NMe2-p)3}2] 1f, from [Mo(CO)4(pip)2] (242
mg, 0.64 mmol) and P(C6H4NMe2-p)3 (500 mg, 1.28 mmol),
yield 258 mg (41%); [Mo(CO)4(AsPh3)2] 7, from [Mo(CO)4-
(pip)2] (150 mg, 0.4 mmol) and AsPh3 (245 mg, 0.8 mmol),
yield 190 mg (58%); [Mo(CO)4{As(C6H4OMe-p)3}2] 7c, from
[Mo(CO)4(pip)2] (150 mg, 0.4 mmol) and As(C6H4OMe-p)3
(317 mg, 0.8 mmol), yield 114 mg (29%).
cis-[PtCl2(PPh3)2] 4. Without regard for exclusion of air,
PPh3 (142 mg, 0.54 mmol) was added to a dichloromethane
solution (5 mL) of [PtCl2(cod)] (100 mg, 0.27 mmol). The reac-
tion mixture was stirred at room temperature for 10 min. Addi-
tion of light petroleum induced precipitation of 4 (164 mg,
77%), which was filtered off and washed with diethyl ether.
The following complexes were prepared similarly: cis-[PtCl2-
{PPh2(C6H4OMe-p)}2] 4a, PPh2(C6H4OMe-p) (158 mg, 0.54
mmol) and [PtCl2(cod)] (100 mg, 0.27 mmol) gave 152 mg
(66%); cis-[PtCl2{PPh(C6H4OMe-p)2}2] 4b, PPh(C6H4OMe-p)2
(174 mg, 0.54 mmol) and [PtCl2(cod)] (100 mg, 0.27 mmol) gave
198 mg (80.5%); cis-[PtCl2{P(C6H4NMe2-p)3}2] 4f, P(C6H4-
NMe2-p)3 (211 mg, 0.54 mmol) and [PtCl2(cod)] (100 mg, 0.27
mmol) gave 105 mg (39%); [PdCl2(PPh3)2] 5, PPh3 (183 mg, 0.7
mmol) and [PdCl2(cod)] (100 mg, 0.35 mmol) gave 225 mg
(92%); [PdCl2{PPh2(C6H4OMe-p)}2] 5a, PPh2(C6H4OMe-p)
(204 mg, 0.7 mmol) and [PdCl2(cod)] (100 mg, 0.35 mmol) gave
106 mg (40%); [PdCl2{PPh(C6H4OMe-p)2}2] 5b, PPh(C6H4-
OMe-p)2 (225 mg, 0.7 mmol) and [PdCl2(cod)] (100 mg, 0.35
mmol) gave 170 mg (60%); this complex has been reported pre-
viously;31 [PdCl2{P(C6H4OMe-p)3}2] 5c, P(C6H4OMe-p)3 (246
mg, 0.7 mmol) and [PdCl2(cod)] (100 mg, 0.35 mmol) gave 200
mg (65%).
[Fe(CO)3{PPh2(C6H4OMe-p)}2] 2a. The reaction was carried
out following a modification of the literature procedure for
complex 223 with [Fe(CO)5] (0.1 mL, 0.149 g, 0.76 mmol),
PPh2(C6H4OMe-p) (0.467 g, 1.6 mmol) and NaBH4 (0.059 g,
1.53 mmol). The NaBH4 was placed in the reaction flask,
followed by 20 ml of ethanol. The solution was purged with
nitrogen for 20 min and the phosphine ligand added. Penta-
carbonyliron(0) was then added dropwise by a syringe. Precipi-
tation began during the course of the reaction. After cooling,
[AuCl{PPh2(C6H4OMe-p)}] 6a. Without regard for the exclu-
sion of air, PPh2(C6H4OMe-p) (172 mg, 0.59 mmol) was added
to an ethanol solution (5 mL) of HAuCl4 (100 mg, 0.29 mmol).
3512
J. Chem. Soc., Dalton Trans., 1999, 3507–3513

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J. Organomet. Chem., 1995, 502, 177; P. Kalck and F. Monteil, Adv.
The reaction mixture was stirred at room temperature for 10
min. Addition of several mL of water induced precipitation of
complex 6a. The solution was cooled in a refrigerator over-
night and the product was filtered off and dried (63 mg, 41%).
An analytically pure sample was prepared by recrystallisation
from hot methanol.
The following complexes were prepared similarly: [AuCl-
{PPh(C6H4OMe-p)2} 6b, PPh(C6H4OMe-p)2 (189 mg, 0.59
mmol) and HAuCl4 (100 mg, 0.29 mmol) gave 74 mg (45%);
[AuCl{P(C6H4OMe-p)3} 6c, P(C6H4OMe-p)3 (207 mg, 0.59
mmol) and HAuCl4 (100 mg, 0.29 mmol) gave 154 mg (90%);
[AuCl{P(C6H4NMe2-p)3}] 6f, P(C6H4NMe2-p)3 (227 mg, 0.58
mmol) and HAuCl4 (100 mg, 0.29 mmol) gave 95 mg (53%),
reported previously by a different route.32
Organomet. Chem., 1992, 34, 219; W. A. Herrmann and C. W.
Kohlpaintner, Angew. Chem., Int. Ed. Engl., 1993, 32, 1524;
Advanced Inorganic Chemistry, eds. F. A. Cotton, G. Wilkinson,
C. A. Murillo and M. Bochmann, 6th edn., Wiley, New York,
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8 S. J. Berners-Price and P. J. Sadler, Chem. Br., 1987, 541.
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12 N. R. Champness and W. Levason, Coord. Chem. Rev., 1994, 133,
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13 T. L. Brown and K. J. Lee, Coord. Chem. Rev., 1993, 128, 89.
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Acknowledgements
We thank the University of Waikato for financial support of
this work, the New Zealand Lottery Grants Board for grants-
in-aid for the mass spectrometer and precious metals purchase,
and Wendy Jackson for technical assistance. The generous loan
of platinum from Johnson-Matthey plc is acknowledged. We
thank Dr Elisabeth Bouwmann of Leiden University for the
generous gift of the diphosphine.
18 L. J. Arnold, J. Chem. Educ., 1992, 69, 811.
19 G. P. Schiemenz, Liebigs Ann. Chem., 1971, 752, 30.
20 H. A. Brune, M. Falck, R. Hemmer and G. Schmidtberg, Chem.
Ber., 1984, 117, 2791.
21 G. P. Schiemenz, Chem. Ber., 1965, 98, 65.
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