Dimethylphosphinato and dimethylarsinato complexes of palladium(II), [Pd(Me2PO2)2]3and Pd(Me 2AsO2)2, and their adducts

Article abstract of DOI:10.1002/zaac.200900532

The complexes [Pd(Me₂PO₂)₂]₃ and Pd(Me₂AsO₂)₂ were prepared from the corresponding acids and palladium(II) acetate. Their structures were deduced by IR and NMR spectroscopy. Addition of pyridine and 2,2'-bipyridine to [Pd(Me ₂PO₂)₂]₃ gave the adducts Pd(Me ₂PO₂)₂py₂ and Pd(Me ₂PO₂)₂bipy, which were characterized by ¹H NMR spectroscopy. Addition of nicotinic acid and nicotinamide in water gave the adducts Pd(Me₂PO₂)₂L ₂, whereas in methanol the adducts Pd(Me₂PO ₂)₂L were obtained. The cacodylate containing complex formed the adducts Pd(Me₂AsO₂)₂py and Pd(Me₂AsO₂)₂bipy1/2, which are unstable in CDCl³. Triphenylphosphine deoxygenated both Pd(Me₂MO ₂)₂ complexes, but the palladium(II) containing products could not be isolated, The expected Pd(Me₂P-O)₂ reacted further and gave many products, whereas the anticipated Pd(Me ₂As-O)₂ did not bind triphenylphosphine.

Full text of DOI:10.1002/zaac.200900532

ARTICLE
DOI: 10.1002/zaac.200900532
Dimethylphosphinato and Dimethylarsinato Complexes of Palladium(II),
[Pd(Me2PO2)2]3 and Pd(Me2AsO2)2, and their Adducts
Panayiotis V. Ioannou*[a]
Keywords: Dimethylphosphinic acid; Dimethylarsinic acid; Palladium; Dimethylphosphinates; Dimethylarsinates; Arsenic
Abstract. The complexes [Pd(Me2PO2)2]3 and Pd(Me2AsO2)2 were ducts Pd(Me2PO2)2L were obtained. The cacodylate containing com-
prepared from the corresponding acids and palladium(II) acetate. Their plex formed the adducts Pd(Me2AsO2)2py and Pd(Me2AsO2)2bipy1/2
,
structures were deduced by IR and NMR spectroscopy. Addition of which are unstable in CDCl3. Triphenylphosphine deoxygenated both
pyridine and 2,2'-bipyridine to [Pd(Me2PO2)2]3 gave the adducts Pd(Me2MO2)2 complexes, but the palladium(II) containing products
Pd(Me2PO2)2py2 and Pd(Me2PO2)2bipy, which were characterized by could not be isolated. The expected Pd(Me2P–O)2 reacted further and
1H NMR spectroscopy. Addition of nicotinic acid and nicotinamide in gave many products, whereas the anticipated Pd(Me2As–O)2 did not
water gave the adducts Pd(Me2PO2)2L2, whereas in methanol the ad- bind triphenylphosphine.
the shifting of the Me2P signal relative to the parent complex
Introduction
2 from the 1H NMR spectra (Table 1). Adducts of 2 with nico-
There are few reports on the preparation and reactions of
tinic acid and nicotinamide were also prepared, the stoichiome-
PdII complexes with the ligands Me2P(S)S– [1–3] and
try of which was found to be solvent-dependent. Complex 10
Me2As(S)S– [4]. The complex Pd(Me2PS2)2 was not desulfu-
produced different and quite labile adducts with py and bipy.
rized by phosphines but added to give Pd(Me2PS2)2PIII (y = 1
or 2) [2, 3], whereas the complex Pd(Me2AsS2)2 shoywed an
Finally, triphenylphosphine, Ph3P, deoxygenated both com-
plexes 2 and 10 but the palladium-containing products could
unexpected behavior towards Ph3P and PdII, e.g. with Ph3P the
not be isolated. Moreover, the expected product Pd(Me2AsO)2
initial desulfurization was followed by other unknown reac-
in solution did not bind Ph3P.
tions [4].
The oxygen analogue of Me2PS2H, dimethylphosphinic acid
Me2P(O)OH (1) reacts with metal cations to give “metal-phos-
Results and Discussion
phinate-bridged coordination polymers”, e.g. with BeII [5],
CrIII [6], MnII [7], CuII [8]. It coordinates to metal carbonyls
(to yield e.g. [{μ-Me2PO2}Re(CO)3]2 [9]) and to organometal-
lic moieties e.g. Me3Sb- [10]. In all cited complexes the phos-
phinate group acted as an O,O' bridge.
Preparation and Reactions of [Pd(Me2PO2)2]3 (2)
The oxygen analogue of the unknown Me2AsS2H, dimethy-
larsinic acid (cacodylic acid) Me2As(O)OH (5), which is
known since 1843 [11], forms simple salts and complexes
with, e.g. Na+, group 2 cations [12], rare earths [13], FeIII [14],
AgI [15], CdII [16], ZnII [17], and organometallic moieties such
–
as Ph2Sb [18] and R2Sn< [19]. Me2AsO2 acts as an O,O'
bridging ligand [19], whereas for Ph2SbO2AsMe2 the IR spec-
troscopic data could not distinguish between the chelating and
the bridging modes [18].
In this paper we present our results on the preparation of
[Pd(Me2PO2)2]3 (2) and Pd(Me2AsO2)2 (10). With the Lewis
bases pyridine (py) and 2,2'-bipyridine (bipy), complex 2 gave
adducts, the coordination mode of which can be inferred from
Complex 2 could not be prepared from dimethylphosphinic
acid (1) and palladium(II) acetate in toluene (at room tempera-
ture or under reflux), in methanol/acetone/benzene, or in ace-
tone/methanol; in all cases black solids (Pd and/or PdO [20])
were obtained. It was, however, prepared in good yields in
dry acetone by portion-wise addition of diethyl ether to 1 plus
palladium(II) acetate in a 2:1 mole ratio. In palladium(II) ace-
tate the exchange of the AcO– group by another oxoanion
seems to depend on the strength of the oxoanions as bases.
* Prof. Dr. P. V. Ioannou
E-Mail: ioannou@chemistry.upatras.gr
[a] Department of Chemistry
University of Patras
Patras, GR 26504, Greece
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1347

P. V. Ioannou
ARTICLE
Table 1. Analytical and spectroscopic data of the complexes 2 and 10 and their adducts. Nac = nicotinic acid; Nam = nicotinamide; s = singlet;
d = doublet. All complexes and adducts are yellow (except 11 and 12 which are brown). For the IR spectroscopic data of 3–8 and 10 see text.
Compound
Formula
Elemental analyses
IR (KBr) /cm–1
1H NMR for Me2M in
31P NMR
Found (Calcd)
all bands very strong
CDCl3 D2O
CDCl3 D2O
Mr
C /%
–
H /% N /%
νas (MO2) νs (MO2) Δν /cm–1 δ /ppm
JH,P /Hz δ /ppm JH,P /Hz δ /ppm δ /ppm
Me2PO2H
[Pd(Me2PO2)2]3
1 –
–
–
–
1150
1156
982
168
82
132
98
1.525 d
1.170 d
1.133 d
0.946 d
14.0
14.8
14.8
13.2
1.539 d 14.8
1.455 d 14.4
54.65 56.30
74.84 53.20
2 C4H12O4P2Pd
292.49
16.58 3.96
(16.42) (4.14)
1074
1024
1042
Pd(Me2PO2)2·2py 3 C14H22N2O4P2Pd
37.63 4.85 5.69
(37.31) (4.92) (6.22)
37.64 4.30 5.95
(37.48) (4.49) (6.25)
35.89 3.95 5.19
(35.67) (4.12) (5.20)
35.65 4.37 10.46 1136
(35.80) (4.51) (10.44)
29.15 4.15 3.40
(28.90) (4.12) (3.37)
29.07 4.53 7.14
(28.97) (4.38) (6.76)
1140
1138
1136
–
–
–
–
47.38
–
450.69
Pd(Me2PO2)2·1bipy 4 C14H20N2O4P2Pd
1040
1038
1040
1038
1038
98
98
96
96
96
–
1.543 d
13.6
–
48.12
–
448.67
Pd(Me2PO2)2·2Nac 5 C16H22N2O8P2Pd
–
–
–
–
1.432 d 14.0
1.394 d 14.0
–
–
–
–
–
–
–
–
44.70
538.71
Pd(Me2PO2)2·2Nam 6 C16H24N4O6P2Pd
536.73
Pd(Me2PO2)2·1Nac 7 C10H17NO6P2Pd
415.60
Pd(Me2PO2)2·1Nam 8 C10H18N2O5P2Pd
414.61
–
44.58
1134
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1134
–
Me2AsO2H
Pd(Me2AsO2)2
10·1py
9 –
–
–
–
918, 894 750
866, 832
916, 894 750
864, 838
1.956 s
sat d
1.867 s
–
10C4H12O4As2Pd
380.38
11 C9H17NO4As2Pd
459.48
12C8H16NO4As2Pd
446.46
12.43 3.10
(12.63) (3.18)
23.93 3.48 2.97
(23.52) (3.73) (3.01)
21.35 3.74 3.02
(21.52) (3.61) (3.14)
–
–
–
870, 850 760
–
Figure 1 b
Figure 1 c
–
10·½bipy
900, 848 764
–
–
Thus, with acids stronger than acetic acid, e.g. Me2PO2H (pKa
~3) [21], the exchange is easy and excess of the incoming acid
is not required. For acids with a strength similar to acetic acid,
e.g. PhCOOH or weaker than acetic acid (pKa 4.76), e.g.
Me2AsO2H (pKa ~6.2) [22a], an excess of the incoming acid is
required [20, this work] to push the equilibrium to the desired
product. Complex 2 was not stable in CDCl3: after 24 h it gave
The reaction of the complex 2 with bipy in a 1:1.5 mole ratio
proved that bipy binds with a 1:1 stoichiometry to give the
adduct 4. The reaction is much slower compared with py. In
1
the H NMR spectrum is no hint for a fast exchange between
free and bound bipy. The solubility of 4 in chloroform indi-
cates that it is not polymeric.
1
numerous signals in the region 0.8–1.8 ppm in the H NMR
spectrum. In D2O, however, the spectrum did not change with
time.
By reaction of nicotinic acid or nicotinamide with compound
2 in water in a 2:1 mole ratio, the off-yellow adducts 5 and 6
were prepared as soft solids, which were soluble in methanol
and water. However, in a 1:1 mole ratio in methanol, com-
pounds 7 and 8 were obtained as brown hard solids, which
were insoluble in methanol, DMF, DMSO, and water manifest-
ing a different polymeric association.
Addition of triphenylphosphine to the complex 2 deoxygen-
ated the P=O group in less than 30 min giving Ph3P=O and
numerous other compounds, which could not be identified by
Titration of a suspension of 2 in CDCl3 with pyridine (py)
showed that with an equivalent quantity of py, all py was
bound to give the adduct 3. With more than 1:2 py, compound
3 was formed leaving the excess py non complexed (see Fig-
ure 1B) and the sharpness of the signals excluded a fast ex-
change between free and bound py. Compound 3 is better pre-
pared in chloroform than in pyridine (as solvent and reagent
[20]) and from its solubility in chloroform it can be inferred
that it is not polymeric.
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Dimethylphosphinato and Dimethylarsinato Complexes of Palladium(II)
1
Figure 1. H NMR spectra in CDCl3 (TMS 0.000 ppm). (A) saturated [Pd(Me2PO2)2]3 (2); the spectrum did not change from +25 to –50 °C.
(B) Pd(Me2PO2)2py2 (3) in the presence of free pyridine. The sharpness of the signals excludes a fast exchange between free and bound pyridine.
(C) Pd(Me2PO2)2bipy (4). (a) Pd(Me2AsO2)2 (10). (b) Pd(Me2AsO2)2py (11) decomposed in CDCl3; free pyridine is seen; the spectrum did not
change from +25 to –50 °C. (c) Pd(Me2AsO2)2bipy1/2 (12) decomposed in CDCl3 free bipy is not seen. (d) Pd(Me2AsO2)2 (10) and Ph3P (1:4
mole ratio): two molecules of free Ph3P per one molecule of reduced 10 are seen. The aromatic regions are vertically expanded and the 7.267 ppm
singlet is due to CHCl3.
1H and 31P NMR spectroscopy. Control experiments in CDCl3
Dimethylphosphinic acid (1) in CDCl3 and in D2O gave a
showed that Ph3P deoxygenated 1 very slowly (23 % Ph3P=O doublet for Me2P hydrogen atoms in the 1H NMR and a singlet
in 4 days confirmed by 1H and 31P NMR spectroscopy) giving in the 31P NMR spectra (Table 1). The H NMR spectrum of
the CDCl3-insoluble Me2HP=O.
1
a freshly prepared solution of 2 in CDCl3 gave an apparent
Dimethylphosphinic acid (1) forms dimers with strong hy- triplet signal, Figure 1A, upfield from the doublet of 1. The
drogen bonding in the solid state [23] and the bands in its IR triplet did not change when the measurements were performed
spectrum have been assigned previously [24]. The phosphinate at 25, 0, –25, and –50 °C, and it were in fact two doublets
group is able to coordinate to a metal cation in different ways (Table 1), which suggest two environments for the Me2P pro-
and the differences in the frequencies of the PO2 stretching tons. The proton-decoupled 13C NMR spectrum showed 4 lines
bands in the IR spectra can give information concerning its at 17.22, 17.64, 18.19 and 18.73 ppm corresponding to two
coordination mode [25]. For [Pd(Me2PO2)2]3 (2) in the solid different methyl carbons (17.49 ppm, JC,P 53.4 Hz and
state, the asymmetric PO2 stretching vibration did not change 18.46 ppm, JC,P 54.7 Hz). The proton-decoupled 31P NMR
(1156 cm–1) compared to the dimeric acid 1 (1150 cm–1) but spectrum revealed one singlet signal whereas without decou-
the symmetric PO2 stretching moved from 982 cm–1 to 1024 pling an heptaplet was obtained, which indicated one kind of
and 1074 cm–1 (Table 1). A decrease in the separation between phosphorus atoms. However, in D2O the 1H NMR spectrum of
the two frequencies suggests a more symmetrical binding of complex 2 had only one doublet and the proton-decoupled 13
C
the PO2 group to PdII which can be chelating or bridging [25]. NMR spectrum showed two lines (14.78 and 15.58 ppm) due
The IR (KBr) spectra of 3 and 4 showed strong absorptions at to phosphorus–carbon coupling (JC,P 80.0 Hz), which implies
~3390 cm–1, somewhat broader than the ~3430 cm–1 band due the same environment for the Me2P groups. A singlet signal
to traces of water in dry KBr, which imply that these adducts was observed in the proton-decoupled 31P NMR spectrum far
sorbed water from air during grinding. At the same time, in away from the 31P singlet in CDCl3 (Table 1) and without de-
the adducts 3–8 the νas(PO2) vibration gave a signal at 1134– coupling a heptaplet was obtained thus denoting only one kind
1140 cm–1 whereas the νs(PO2) signal was observed at 1038– of phosphorus atoms. These facts mean that complex 2 in
1042 cm–1 (Table 1), which suggests a more symmetric PO2 CDCl3 and D2O adopt different structures. It is not unreasona-
group than in 2; this should be attributed to hydrogen bonding ble to assume that complete hydrolysis of 2 in D2O has taken
between P=O and sorbed water/C–OH/NH2.
place to give
a
new complex, most likely trans-
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P. V. Ioannou
ARTICLE
Pd(Me2PO2)2(H2O)2. It is known that the trimeric [26, 28] 3, 5, and 6 should be trans whereas compound 4 should be cis
[Pd(OAc)2]3 in non-polar solvents partially hydrolyses in the with monodentate phosphinate group. The placement of the
presence of small amounts of water, PdII retains its square- ligands around the square planar PdII in 3 and 4 can be inferred
planar coordination [26].
from the 1H NMR spectra in CDCl3, whereas the structures of
Two pyridine molecules bind to complex 2 to give the adduct 5 and 6 cannot be established unequivocally from the 1H NMR
3 as was shown by the downfield shifts of pyridine's protons spectra in D2O. The brown adducts 7 and 8, which are insolu-
[27] and the Me2P signal, now a single doublet, moved upfield ble in most organic solvents and water, should be polymeric
relative to the Me2P signals of 2 in CDCl3 (Figure 1B). One but from the IR spectroscopic data is not certain which ligand
–
bipy molecule binds to complex 2 producing the adduct 4 as (nicotinic acid/nicotinamide or Me2PO2 ) bridges the square
shown by the downfield shift of the bipy hydrogen atoms [27] planar PdII atoms.
but now the Me2PV single doublet moved downfield relative
to the Me2P signals of 2 (Figure 1C) nearly at the same posi-
tion as that of 1. If the adducts 3 and 4 were monomeric, bipy
should be cis and the two pyridines in 3 should be trans coor-
dinated. Therefore, these shifts in the 1H NMR spectra in
CDCl3 may be a useful criterion of coordination and could
provide that the spectra are run under the same conditions. In
order to check the above hypothesis we note that Wilkinson
and co-workers [20] found that the adduct Pd(MeCOO)2bipy
is cis, whereas the adduct Pd(MeCOO)2py2 is trans and the
acetate groups were monodentate. We found that the methyl
protons of Pd(MeCOO)2 in CDCl3 appear as a singlet at
2.005 ppm, whereas in the adducts Pd(MeCOO)2py2 and
Pd(MeCOO)2bipy the singlet signals moved to 1.857 and
Preparation and Reactions of Pd(Me2AsO2)2 (10)
2.142 ppm, respectively. Thus, it seems that there is a trend
with these amine ligands: trans coordination causes an upfield
The preparation of Pd(MeAsO2)2 (10) from cacodylic acid
shift of the Me2PO2 and MeCO2 protons whereas cis coordina-
(9) and palladium(II) acetate proved to be quite difficult be-
tion results in a downfield shift. The origin of the shift of the
methyl protons probably is due to the electronic charge on the
binding oxygen in the ligands: the higher the charge on the
oxygen atom, the more upfield the shifting. In the adducts 5
and 6 in D2O the resonances of the aromatic protons moved
downfield [27] and gave a similar but very complex pattern of
peaks, the Me2P doublets moved slightly upfield compared to
the doublet of 2 (Table 1).
cause cacodylic acid is less soluble in acetone than dimethyl-
phosphinic acid (1) and it is a weaker acid than dimethylphos-
1
phinic acid and acetic acid. The H NMR spectra of isolated
crude 10 showed singlet signals both upfield and downfield
from the singlet of the pure product 10 (Table 1 and Fig-
ure 1a), which probably indicates a stepwise replacement of
the acetate groups from the trimeric [26, 28] palladium(II) ace-
tate. Using 2:1 stoichiometry, by using an inconveniently large
volume of acetone, crude 10 was obtained in 98 % yield,
whereas with less acetone the replacement was incomplete.
Better yields and a purer product were obtained by increasing
the amount of cacodylic acid (9) to 3:1 or better 4:1 mole ratio.
With the latter ratio ~30 % of the excess 9 was recovered. The
solubility of the complex points towards to be monomeric and
not polymeric.
Attempts to obtain X-ray quality crystals of 2–6 were unsuc-
cessful. From the above described data, it seemed that complex
2 has different structures in the solid state and in solution. In
the solid state, a dimeric structure with both chelating and
–
bridging Me2PO2 ligands is not likely based on the NMR
spectroscopic data and because PdII forms bridging oligonu-
clear complexes [26, 28]. In the dimethylphosphinato com-
–
plexes known so far (see Introduction), Me2PO2 acts as a
Following the reaction of 10 in CDCl3 with py in 1:2 mole
–
1
bridging ligand. With a bridging Me2PO2 ligand, a dimeric
ratio by H NMR spectroscopy, the spectra showed free and
structure is conceivable but
a trimeric (analogous to
bound py and methyl singlet signals at 1.248, 1.768, 1.844
(main), 2.172, and 2.373 ppm, which imply that the adduct,
whichever it was, was not stable in CDCl3 solution. Therefore,
the adduct was prepared using pyridine as solvent and reagent,
by a procedure used to prepare the adduct Pd(AcO)2py2 [20].
The product, Pd(Me2AsO2)2py (11), is a mono adduct accord-
Pd3(CH3COO)6 [26, 28]) or trimetallic unit in [Cu3(Me2PO2)6]x
[8] is considered to be more probable. The two PO2 symmetric
stretchings in the solid 2 (Table 1) may indicate two different
phosphinato groups. Upon dissolution in CDCl3, all phospho-
rus atoms are equivalent but their methyl groups are not, half
of them (facing the PdII atoms) are in a different environment
than the other half (facing each other). When the solid 2 is
dissolved in D2O, its trimeric structure is not preserved and it
most likely gives monomeric trans-Pd(Me2PO2)2(H2O)2 as the
31P NMR shift indicates (Table 1).
1
ing to elemental analysis and integration of the H NMR spec-
trum, although the latter indicated that pure 11 was not stable
in CDCl3 giving numerous singlet signals (Figure 1b).
The reaction of 10 with bipy was slower than that with py
(1H NMR spectroscopic data). The product precipitated from
By analogy to the adducts trans-Pd(MeCO2)2py2 and cis- chloroform with ether as a gum was transformed to a powder
Pd(MeCO2)2bipy with monodentate acetate [20], the adducts very slowly by triturating with acetone. It had the formula
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Dimethylphosphinato and Dimethylarsinato Complexes of Palladium(II)
Pd(Me2AsO2)2bipy1/2 (12) according to elemental analysis and Pd(Me2As-O)2 in CDCl3 that will satisfy the square planarity
1H NMR spectroscopy. The latter (Figure 1c) indicated that of PdII and inability to bind Ph3P.
compound 12 is not stable in CDCl3. It also decomposed as
solid at –20 °C.
Experimental Section
Triphenylphosphine cleanly deoxygenated complex 10 to
give free Ph3P=O and only one product with a singlet signal
at 1.850 ppm, which should be due to the salt of dimethylarsi-
nous acid Pd(Me2AsO)2. Since PdII prefers square-planar coor-
dination and since Ph3P=O was not bound, we added more
Ph3P in order to get Pd(Me2AsO)2(Ph3P)2. However this excess
Ph3P also remained free at 7.322 ppm and the Me2AsIII singlet
remained at 1.846 ppm (Figure 1d). Attempts to isolate
Pd(Me2AsO)2 resulted in ~50 % recovery of the starting com-
plex 10. This reoxidation by dioxygen in air may have been
catalyzed by palladium(II) itself because it is known that PdII
is able to catalyze the air oxidation of alcohols to ketones [29].
The reduction as a means for the preparation of Me2As–OH
(unknown) or cacodylic oxide, Me2As–O–AsMe2, is not prac-
tical compared to other methods [22b] and was not studied
further.
General
Dimethylphosphinic acid, 2,2'-bipyridine, and nicotinamide (Aldrich),
dimethylarsinic and nicotinic acids (Serva), palladium(II) acetate
(Merck), were used as received, with the published melting points.
However, as analyzed by 1H NMR spectroscopy, palladium(II) acetate
in non-dried CDCl3 had traces of “impurities” as published [26]. Tri-
phenylphosphine (Aldrich) was recrystallized from methanol. Pyridine
was distilled from sodium hydroxide and kept over A4 molecular
sieves. Acetone was kept over A4 molecular sieves.
Silica gel 60 H (Merck) was used for thin layer chromatography (TLC)
and spots were made visible by iodine vapors. IR spectra were obtained
1
on a Perkin–Elmer model 16PC FT-IR spectrometer. H NMR spectra
(400 MHz) (TMS or DSS 0.000 ppm), 13C NMR spectra (100 Mz),
and 31P NMR spectra (162 MHz) (85 % aqueous H3PO4 0.00 ppm)
were recorded with a Bruker DPX Avance spectrometer. In all cases
downfield shifts are positive. Elemental analyses were obtained
through the Center of Instrumental Analyses, University of Patras, Pat-
ras, Greece.
Cacodylic acid (9) in the solid state forms hydrogen bonded
dimers [30] and its bands in the IR spectrum have been as-
signed previously [31]. Arsinate, in principle, is able to coordi-
nate as the phosphinate group [25] but only two IR spectro-
scopic studies were published, which indicate bidentate
coordination of Me2AsO2– (see Introduction). The dark yellow
complex 10 decomposed to cacodylic acid on attempts to get
its IR spectrum in KBr (Table 1) and in Nujol. The IR spectra
of 11 and 12 were quite complicated but the presence of two
asymmetric AsO2 bands at 870/900 and 850 cm–1 and one
Syntheses
Bis(dimethylphosphinato)palladium(II) (2): To a saturated solution
of dimethylphosphinic acid (1) (176 mg, 1.87 mmol) in acetone
(8 mL) was added solid palladium(II) acetate (210 mg, 0.935 mmol)
and stirred at room temp. for 15 min. To the clear orange solution,
symmetric AsO2 band at ~760 cm–1 (Table 1) may suggest two ether (8 mL) was added and stirred at room temp. for 8 h. Afterwards,
–
more ether (20 mL) was added and the yellow suspension was stirred
at room temp. for 2 days. Centrifugation and washing with acetone/
ether 1:2 (1 × 3 mL) gave 2 as a yellow powder (215 mg, 79 %),
which was insoluble in ether, sparingly soluble in acetone, moderately
soluble in chloroform and in methanol, and soluble in water. M.p.
137 °C (dec.). IR (KBr): ν = 2984 (m), 2918 (m), 1550 (w), 1420 (m),
1294 (vs), 1156 (vs), 1074 (vs), 1024 (vs), 874 (vs), 748 (m), 536 (s),
504 (s) cm–1. For further data see Table 1.
different modes of coordination of the two Me2AsO2 ligands.
A saturated solution of cacodylic acid (9) in CDCl3 shows a
singlet at 1.956 ppm which is further downfield from the dou-
blet of dimethylphosphinic acid (1) (1.525 ppm) despite ar-
senic is less electronegative than phosphorus. Complex 10 in
CDCl3 had one Me2AsV singlet signal at 1.867 ppm (Fig-
ure 1a) and one singlet signal at 18.22 ppm in the 13C NMR
spectrum, which implies equivalent methyl groups. As noted
above, dissolution of the pure adducts 11 and 12 in CDCl3
resulted in irreversible production of more than one species
(Figure 1b and Figure 1c). The irreversibility was deduced by
Bis(dimethylphosphinato)bis(pyridine)palladium(II) (3): In an
NMR tube, to a suspension of complex 2 (12.6 mg, 43 μmol) in CDCl3
(0.6 mL) was added dry pyridine (10.5 μL, 129 μmol) and the tube
was tumbled. Its 1H NMR spectrum, recorded 15 min after mixing,
showed complete reaction of the complex to yield the adduct 3
(0.956 ppm, d, JH,P 13.2 Hz, 12 H for Me2P) with two bound pyridine
molecules (7.43 ppm, t, J = 7.2 Hz, 4 H, meta-H; 7.87 ppm, t, J =
7.6 Hz, 2 H, para-H; 8.85 ppm, d, J = 5.2 Hz, ortho-H), and one free
pyridine molecule (at 7.30, 7.69, and 8.62 ppm) (Figure 1B). The spec-
1
variable H NMR spectra of 11 in CDCl3 at 25, 0, –25, and –
50 °C, where the same number (> 6) of Me2As singlet signals
with the same relative intensities was recorded.
The solid-state structure of complex 10 may contain two che-
lating Me2AsO2 molecules, the chelation could be permitted
–
by the greater size of arsenic compared to phosphorus. This trum did not change after 24 h tumbling of the tube. The slightly opal-
chelation can explain the singlet signals in the H and 13C
escent yellow solution was filtered (Pasteur pipette plugged with cot-
1
ton), washed with chloroform (0.5 mL), and the filtrate was evaporated
NMR spectra and its instability during grinding in KBr and
and dried to give a yellow solid (17.2 mg). It was recrystallized from
minimum amount of chloroform (0.3 mL) (orange solution) by adding
ether (10 mL). After 12 h at room temp., centrifugation and drying
gave the adduct 3 (15.5 mg, 80 %) as yellow powder. It was very
soluble in chloroform, soluble in water, and insoluble in ether and in
acetone. M.p.: at 103 °C darkened slowly and at ~150 °C turned
brown/black. IR (KBr): ν = 3386 (s, broad), 2978 (w), 2924 (w), 1606
Nujol. Attempts to prepare crystals of 11 and 12 suitable for
X-ray analyses were not successful. Solid state structures of 11
and 12 cannot be proposed with certainty from their IR spectra.
Because both compounds are soluble in chloroform, 11 and 12
should not be polymeric and it is possible that they have one
–
chelating and one monodentate Me2AsO2 groups. Also, we
cannot offer a probable structure for the deoxygenated product (m), 1484 (w), 1452 (s), 1426 (w), 1296 (ms), 1140 (vs), 1042 (vs),
Z. Anorg. Allg. Chem. 2010, 1347–1353
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1351

P. V. Ioannou
ARTICLE
982 (w), 866 (s), 764 (s), 738 (mw), 690 (s) cm–1. For further data see brown product 8 (30.0 mg, 72.5 %) was insoluble in ether, chloroform,
Table 1.
acetone, methanol, hot DMF, hot DMSO, and water. M. p.: at ~180 °C
turned black. IR (KBr): see 7, but with a broader band centered at
3400 cm–1.
When the preparation was done in dry pyridine as solvent and reagent
(0.2 mL for a scale of 50 μmol of complex 2), after 24 h at –20 °C,
work up as above gave only 7 mg (31 %) of the adduct 3 as an off-
white solid with the same IR and melting behavior as the yellow vari-
ety, but it was somewhat less soluble in chloroform.
Bis(dimethylarsinato)palladium(II) (10): To a suspension of crystal-
line cacodylic acid (9) (221 mg, 1.6 mmol) in dry acetone (8 mL),
solid palladium(II) acetate (90 mg, 0.4 mmol) was added and the sys-
tem was stirred vigorously for 48 h. After 15 min the product started
to precipitate and some cacodylic acid was stuck on the wall of the
flask. Centrifugation and washing with boiling acetone (2 mL) gave a
dark red supernatant (from which 4 mg of cacodylic acid were recov-
ered, leaving 120 mg of a dichloromethane-soluble red solid with a
complex 1H NMR spectrum in the region 1.26–2.64 ppm) and the
crude beige product. The crude product was extracted with dichloro-
methane (4 × 10 mL) by centrifugation. The residue (32 mg) was
somewhat impure cacodylic acid. The yellow extracts were combined,
evaporated, and dried in vacuo to give the product 10 (149 mg, 98 %)
as a dark yellow solid, which was soluble in chloroform and in metha-
nol, sparingly soluble in acetone, and insoluble in ether. M.p.: at
112 °C started turning brown/black, at 185 °C swelled. IR (KBr): hy-
drolyzed on grinding to give bands of cacodylic acid (9) (Table 1). IR
(Nujol): decomposed to 9. For further data see Table 1.
Bis(dimethylphosphinato)(2,2'-bipyridine)palladium(II) (4): Com-
plex 2 (15.4 mg, 53 μmol) and 2,2'-bipyridine (11 mg, 71 μmol) were
placed in an NMR tube. CDCl3 (0.6 mL) was added and the tube was
tumbled overnight in order to obtain complete reaction of the complex.
The 1H NMR spectrum showed the Me2P signal of the adduct
(1.536 ppm, d, JP–H 13.6 Hz, 12 H), one molecule bipy bound per
complex molecule (at 7.46, 8.08, 8.38, and 8.83 ppm) and excess of
bipy (at 7.31, 7.82, 8.38, and 8.68 ppm). Filtration (Pasteur pipette
plugged with cotton), washing with chloroform (1 mL), evaporation
and drying gave a yellow film (19 mg), which was recrystallized from
warm chloroform (0.8 mL) by adding ether (10 mL). After 12 h at
room temp., centrifugation and drying gave the adduct 4 (16 mg,
66 %) as a yellow powder, which was less soluble in chloroform than
3 and insoluble in ether. M.p.: at ~140 °C started darkening very
slowly until 200 °C where it turned black. IR (KBr): ν = 3386 (s,
broad), 2978 (w), 2924 (w), 1602 (m), 1466 (w), 1446 (m), 1298 (m),
1244 (w), 1138 (vs), 1040 (vs), 866 (s), 766 (s), 736 (mw) cm–1. For
further data see Table 1.
Bis(dimethylarsinato)(pyridine)palladium(II) (11): To complex 10
(120 mg, 0.32 mmol), dry pyridine (2 mL) was added and the mixture
was stirred at room temp. The initially formed brown gum dissolved
and the yellow product started precipitating. After 3 h it was warmed
(hair drier) until dissolution, filtered hot (Pasteur pipette plugged with
cotton), washed with warm pyridine (0.5 mL), and cooled to –20 °C
for 2 days. Centrifugation and washing with diethyl ether (2 × 2 mL)
gave 11 as yellow hygroscopic crystals (110 mg, 76 %), which were
soluble in chloroform, acetone, methanol, and water, slightly soluble
in pyridine, and insoluble in ether. M.p.: at 111 °C turned light orange
and at 119–121 °C melted to an orange oil. IR (KBr): ν = 1670 (s),
1646 (s), 1600 (s), 1560 (s), 1450 (s), 1410 (s), 1274 (m), 1212 (m),
1152 (m), 1070 (m), 870 (vs), 850 (vs), 760 (vs), 690 (vs), 606 (s),
548 (s), 498 (m) cm–1. For further data see Table 1. Compound 11 was
stable at –20 °C for at least 3 months.
Bis(dimethylphosphinato)bis(nicotinic acid)palladium(II) (5): In a
centrifuge tube, complex 2 (29.2 mg, 0.1 mmol) and nicotinic acid
(25.0 mg, 0.2 mmol) were suspended in water (0.25 mL) and stirred
at room temp. for 4 h. To the brown solution, acetone (10 mL) was
added and stirred to precipitate the adduct 5 as an off-yellow powder
(46.5 mg, 86 %). It was insoluble in ether, chloroform, and acetone,
moderately soluble in methanol, and soluble in water to give a yellow
solution. M. p.: 140–200 °C gradually turned darker, 200 °C turned
black. IR (KBr): ν = 3380 (vs, broad), 3214 (s, broad), 1694 (sh), 1664
(vs), 1620 (s), 1570 (m), 1449 (m), 1396 (m), 1296 (m), 1198 (w),
1136 (vs), 1038 (vs), 866 (m), 830 (w), 736 (m), 694 (m), 668 (m),
636 (m) cm–1. For further data see Table 1.
Tetra(dimethylarsinato)(2,2'-bipyridine)dipalladium(II) (12): To a
solution of complex 10 (124 mg, 0.33 mmol) in chloroform (2 mL)
solid 2,2'-bipyridine (51 mg, 0.33 mmol) was added and the mixture
stirred at room temp. for 24 h. Filtration (Pasteur pipette plugged with
cotton) and washing with chloroform (0.5 mL) gave a yellow filtrate
into which ether (8 mL) was added whilst stirring to precipitate the
product as a gum. Centrifugation gave a supernatant [from which
slightly impure bipy (35 mg) was recovered] and the product as a
sticky yellow-orange gum (107 mg). Triturating the gum with acetone
(5 mL) for 2 days gave 12 (76 mg, 52 %) as a yellowish powder,
which was very soluble in methanol, soluble in chloroform, moderately
soluble in dichloromethane, and insoluble in ether and in acetone.
M.p.: at 142 °C darkened slowly and at 165 °C turned black. IR
(KBr): ν = 1602 (s), 1496 (w), 1468 (m), 1448 (s), 1416 (s), 1312 (m),
1270 (m), 1160 (w), 1108 (w), 1072 (w), 1022 (w), 900 (vs), 848 (vs),
764 (vs), 730 (s), 648 (s), 606 (m) cm–1. For further data see Table 1.
Compound 12 was not stable when stored at –20 °C for 3 months
being partially soluble in chloroform.
Bis(dimethylphosphinato)bis(nicotinamide)palladium(II) (6): It
was prepared in water following the same procedure as applied for 5
in 87 % yield as an off-yellow powder, which was insoluble in ether,
chloroform, and acetone; soluble in methanol and in water to give
yellow solutions. M. p.: 130–190 °C gradually turned light orange,
190 °C turned black. IR (KBr): see 5, but the band at 3380 was now
at 3322 cm–1. For further data see Table 1.
Bis(dimethylphosphinato)(nicotinic acid)palladium(II) (7): In a
centrifuge tube, complex 2 (29.2 mg, 0.1 mmol) and nicotinic acid
(12.5 mg, 0.1 mmol) were suspended in methanol (1 mL) and stirred
at room temp. for 1 h. The initial khaki suspension became brown and
after 2 h a brown solution was formed. Addition of ether (10 mL) gave
a voluminous brown solid which, after 2 days at +4 °C, was centri-
fuged to give a faint yellow supernatant and a precipitate. The latter
gave 7 as a hard, brown solid (30.0 mg, 72 %) on drying in vacuo. It
was insoluble in ether, chloroform, acetone, methanol, hot DMF, hot
DMSO, and water. M. p.: at ~180 °C turned black. IR (KBr): ν = 3382
(vs, broad), 1680 (ms), 1612 (ms), 1564 (m), 1453 (mw), 1422 (m),
1298 (m), 1246 (w), 1134 (vs), 1038 (vs), 866 (ms), 826 (w), 738
(mw), 694 (m), 620 (w) cm–1. For further data see Table 1.
Reaction of (Me2PO2)2Pd (2) with Triphenylphosphine
Bis(dimethylphosphinato)(nicotinamide)palladium(II) (8): It was The reaction of compound 2 with triphenylphosphine in CDCl3 re-
prepared following the same procedure as applied for 7. The hard, vealed that under 1:1 stoichiometry all Ph3P reacted in 30 min to give
1352
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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2010, 1347–1353

Dimethylphosphinato and Dimethylarsinato Complexes of Palladium(II)
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a black solid (with very broad IR bands) and a brown supernatant
containing Ph3P=O (TLC: Et2O, Rf 0.16. 1H NMR: three multiplet
signals in the region 7.3–7.7 ppm, 31P NMR: 29.79 ppm, pure Ph3P=
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1
by H NMR (in the region 0.7–1.8 ppm) and 31P NMR spectroscopy
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Reaction of (Me2AsO2)2Pd (10) with Triphenylphosphine
In an NMR tube, complex 10 (11.3 mg, 30 μmol) and triphenylphos-
phine (15.6 mg, 60 μmol) were dissolved in CDCl3 (0.6 mL) and the
tube was tumbled. After 2 h the yellow solution became orange and
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1939, 6, 638.
1
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the H NMR spectrum showed the presence of free Ph3P=O [verified
by 31P NMR: 29.82 ppm (pure Ph3P=O 29.85 ppm) and TLC in ether,
Rf 0.20], traces of Ph3P, and a singlet at 1.850 ppm virtually free of
other singlet signals in this region. After addition of more Ph3P
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1
(15.6 mg) and 24 h tumbling of the tube, the H NMR spectrum (Fig-
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G. Wilkinson, J. Chem. Soc. 1965, 3632.
ure 1d) showed that this additional triphenylphosphine did not react
1
(singlet at 7.322 ppm in the H NMR spectrum and –4.9 ppm in the
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1
31P NMR spectrum) (pure Ph3P: H 7.30 ppm, 31P –8 ppm in benzene
[32]). In order to isolate the product with the 1.850 ppm singlet, ether
(3 mL) was added and the precipitated pink solid (5.0 mg) was the
starting complex 10, which was confirmed by 1H NMR (singlet at
1.867 ppm) and IR spectroscopy.
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Acknowledgement
We are truly indebted to the two referees for their very useful sugges-
tions in improving the quality of this work.
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Received: November 24, 2009
Published Online: March 18, 2010
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Z. Anorg. Allg. Chem. 2010, 1347–1353
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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