Coordination chemistry and hydroformylation activity of platinum complexes containing 1-aryl-phospholes

Article abstract of DOI:10.1016/S0022-328X(99)00156-4

The reaction of the sterically crowded 1-aryl-phospholes (1-(2′,4′,6′-tri-isopropylphenyl)-3-methylphosphole (1), 1-(2′,4′,6′-tri-tert-butylphenyl)-3-methyl-phosphole (2) and 1-(2′,4′-di-tert-butyl-6′-methylphenyl)-3-methylphosphole (3)) with PtCl₂(PhCN)₂ was investigated by NMR spectroscopy. Significant differences in their reactivity towards PtCl₂(PhCN)₂ have been observed. Both cis-PtCl₂(phosphole)(PhCN) and trans-PtCl₂(phosphole)₂ complexes were formed under normal reaction conditions. The corresponding reaction with the more basic 1-phenyl-3,4-dimethyl-phosphole (4) resulted in the immediate formation of cis-PtCl₂(4)₂ as the major product accompanied by ≈ 3% trans-PtCl₂(4)₂. The structure of the two types of platinum complexes in solution has been confirmed by ¹J(¹⁹⁵Pt,³¹P) coupling constants in ³¹P-NMR and several shielding effects in ¹H-NMR spectrometry. The reduced aromatic character of the coordinated phosphole ligands has been confirmed by the pyramidalization of the phosphorus and the loss of the perpendicular arrangement of the phosphole and aryl rings. While the reaction of the sterically congested phospholes (1, 2) with [Rh(nbd)Cl]₂ resulted in the formation of Rh(nbd)(phosphole)Cl complexes, the most basic 4 brought about the [Rh(nbd)(4)₂]⁺ cation. Both the conversion and the regioselectivity of platinum- and rhodium-catalysed hydroformylation of styrene show strong dependence on the basicity of the phosphole ligand.

Full text of DOI:10.1016/S0022-328X(99)00156-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 586 (1999) 79–84
Coordination chemistry and hydroformylation activity of platinum
complexes containing 1-aryl-phospholes
Zsolt Cso´k a, Gyo
9
rgy Keglevich b, Gyo¨rgy Petocz c, La´szlo´ Kolla´r c,d,
*
3
a Uni6ersity of Veszpre´m, Department of Silicate Technology and Materials Science, H-8201 Veszpre´m, PO Box 158, Hungary
b Technical Uni6ersity of Budapest, Department of Organic Chemical Technology, H-1521 Budapest, Hungary
c Janus Pannonius Uni6ersity, Department of Inorganic Chemistry, H-7624 Pe´cs, PO Box 266, Hungary
d Research Group for Chemical Sensors of the Hungarian Academy of Sciences, H-7624 Pe´cs, Hungary
Received 16 December 1998; received in revised form 1 March 1999
Dedicated to: Professor La´szlo´ Marko´ on the occasion of his 70th birthday.
Abstract
The reaction of the sterically crowded 1-aryl-phospholes (1-(2%,4%,6%-tri-isopropylphenyl)-3-methylphosphole (1), 1-(2%,4%,6%-tri-
tert-butylphenyl)-3-methyl-phosphole (2) and 1-(2%,4%-di-tert-butyl-6%-methylphenyl)-3-methylphosphole (3)) with PtCl2(PhCN)2
was investigated by NMR spectroscopy. Significant differences in their reactivity towards PtCl2(PhCN)2 have been observed. Both
cis-PtCl2(phosphole)(PhCN) and trans-PtCl2(phosphole)2 complexes were formed under normal reaction conditions. The corre-
sponding reaction with the more basic 1-phenyl-3,4-dimethyl-phosphole (4) resulted in the immediate formation of cis-PtCl2(4)2
as the major product accompanied by :3% trans-PtCl2(4)2. The structure of the two types of platinum complexes in solution has
been confirmed by 1J(195Pt,31P) coupling constants in 31P-NMR and several shielding effects in 1H-NMR spectrometry. The
reduced aromatic character of the coordinated phosphole ligands has been confirmed by the pyramidalization of the phosphorus
and the loss of the perpendicular arrangement of the phosphole and aryl rings. While the reaction of the sterically congested
phospholes (1, 2) with [Rh(nbd)Cl]2 resulted in the formation of Rh(nbd)(phosphole)Cl complexes, the most basic 4 brought about
the [Rh(nbd)(4)2]+ cation. Both the conversion and the regioselectivity of platinum- and rhodium-catalysed hydroformylation of
styrene show strong dependence on the basicity of the phosphole ligand. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Platinum; Phosphole; NMR; Hydroformylation; Steric congestion
phospholes possessing basically pyramidal phosphorus
donor atoms show similar behaviour [13,14].
1. Introduction
In the present work the coordination chemistry of
various sterically congested P-aryl-phospholes of a
novel type towards platinum is described. The effect of
the 2,4,6-substituents of the aryl ring on the ligand
reactivity, product selectivity and type of coordination,
as well as the carbonylation activity of some of these
platinum–phosphole systems will be discussed.
Besides the transition metal complexes of optically
active ditertiary phosphines, those of chelating diphos-
pholes are of increasing importance in asymmetric
catalysis [1–4]. The optically active chelating ligands
containing both dibenzophospholyl [5–8] and binaph-
thophospholyl [9–11] moieties have been used success-
fully in asymmetric carbonylation reactions, among
them enantioselective platinum-catalysed hydroformyl-
ation. Although the coordination of analogous diphos-
phines and diphospholes to platinum(II) is remarkably
different [9,12], monophosphines and simple mono-
2. Results and discussion
2.1. The reaction of phosphole ligands (1, 2, 3 and 4) with
PtCl2(PhCN)2
* Corresponding author. Tel.: +36-7232-7622/4153; fax: +36-
7232-7622/4680.
E-mail address: kollar@ttk.jpte.hu (L. Kolla´r)
In spite of the considerable lone-pair delocalization
within the phosphole ring, and as a consequence of
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00156-4

80
Z. Cso´k et al. / Journal of Organometallic Chemistry 586 (1999) 79–84
Table 1
Ratio of cis-PtCl2(L)(PhCN)/trans-PtCl2(L)2 (a/b) in the reaction of
PtCl2(PhCN)2+2L (L= monodentate phosphole ligand)a
L
Reaction time
a
b
1
0.5 h
1 h
170 h
78
49
45
22
51
55
Fig. 1. Reaction of 4 [16] with PtCl2(PhCN)2
.
2
3
0.5 h
28 days
–
62
–
38b
that, the relatively large Bird index [15] and small pKa
values compared with phosphines, it has been shown
that the coordination chemistry of monophospholes
investigated to date is very similar to that of much
more basic monotertiary phosphines.
For example, the well-known 1-phenyl-3,4-dimethyl-
phosphole (4) has the most pronounced basic character
(possesses the ‘most pyramidal’ phosphorus) of the
monophospholes investigated in this study. It behaves
principally like monotertiary phosphines: its reaction
with PtCl2(PhCN)2 gave the expected mixture of cis-
and trans-PtCl2(4)2 complexes, which was converted
quantitatively to trans-PtCl(SnCl3)(4)2 upon addition of
SnCl2 as has been found earlier (Fig. 1, [16]).
However, the phospholes of reduced pyramidal char-
acter from steric crowding (1 [17], 2 [18], 3 [19]) due to
the steric congestion of the ortho-substituents of the
phenyl ring with the atoms of the phosphole ring, show
significant difference in reactivity towards
PtCl2(PhCN)2 (Fig. 2, Table 1).
0.5 h
7 days
38 days
100
88
83
0c
12
17
a The ratios and the composition of the reaction mixtures are
determined by 31P-NMR.
b Due to oxidation of 2 and its consecutive dimerization 23% of the
total phosphorus is in the form of 2%.
c 3b can be found in the reaction mixture only in traces (less than
0.5%).
PtCl2(1)2 (1b and 1b%) showing very similar NMR
characteristics has been observed.
The use of the sterically most congested ligand 2
resulted in mono- and bis-phosphole complexes in long
reaction times, only (Table 1). The highly unreactive
feature of this ligand was also shown by the fact, that
in spite of the ligand excess, some unreacted 2 (5%)
and, as a consequence of the 2/PtCl2(PhCN)2=2/1
ratio, also some unreacted platinum precursor
(PtCl2(PhCN)2) were detected even after 1 month.
It is worth noting that cyclodimerization of the pri-
marily formed phosphole oxide took place to a great
extent during this prolonged reaction. The characteris-
tic pair of doublets in the 31P-NMR spectrum (AX spin
system; 59.0 and 85.0 ppm, 3J(31P,31P)=40 Hz) has
been assigned to the Diels–Alder cycloadduct, 2% (Fig.
3). In accordance with earlier findings, the monomeric
phosphole oxide could not be detected due to its fast
dimerization.
a
By allowing 1 to react with PtCl2(PhCN)2 in CDCl3,
the immediate formation of 1a possessing unprece-
dented structure and two diastereomers of the trans-
Fig. 2. Reaction of sterically congested phospholes (1–3) with
PtCl2(PhCN)2
Fig. 3. The dimers of phosphole oxides of 1 and 2.
.

Z. Cso´k et al. / Journal of Organometallic Chemistry 586 (1999) 79–84
81
Table 2
been obtained for a number of cis-PtCl2(P)2-type com-
plexes [20]. However, in the case of 1a, 2a and 3a, the
coordination of a benzonitrile ligand is unambiguously
confirmed by NMR (Table 3). Therefore, the coordina-
tion of only one phosphole ligand is shown in the
primary products, the rather unexpected cis-
PtCl2(L)(PhCN) type complexes. In the case of a two-
fold excess of 3, it was clearly demonstrated that a
single product (3a) was formed, half of the ligand
remained unreacted and the total amount of
PtCl2(PhCN)2 was consumed.
31P-NMR data of Pt complexes containing phosphole ligands, 1, 2, 3
and 4a
Complexes
lP (ppm)
1J(195Pt,31P) (Hz)
cis-PtCl2(1)(PhCN)
trans-PtCl2(1)2
(1a)
(1b)
(1b%)
(2a)
(2b)
(2b%)
(3a)
(3b)
5.0
3160
2347
2347
3302
2494
2490
3217
2390
3350
2029
2156
23.45
23.50
9.0
24.2
23.8
9.5
26.0
8.8
41.7
16.2
cis-PtCl2(2)(PhCN)
trans-PtCl2(2)2
cis-PtCl2(3)(PhCN)
trans-PtCl2(3)2
cis-PtCl2(4)2
trans-PtCl2(4)2
trans-PtCl(SnCl3)(4)2
1
However, in the case of 4, on the basis of H-NMR
evidence (no traces of coordinated benzonitrile), the
1J(Pt,P) coupling constants of 3350 and 2029 Hz are
assigned unambiguously to cis- and trans-PtCl2(4)2,
respectively.
b
a All spectra were measured in CDCl3 at 121.4 MHz at 298 K.
b 2J(117,119Sn,31P)=233 Hz (the 117Sn and 119Sn satellites coincide).
(ii) The 1J(195Pt,31P) coupling constants of about
2500 Hz are typical for phosphorus donors trans to a
phosphorus ligand and are of diagnostic value. In the
case of 3 it was shown that the cis-PtCl2(3)(PhCN)
complex formed primarily reacts further with 3, result-
ing in the trans-complex (3b). A similar reaction se-
quence was assumed for the other two ligands (1 and
2). The cis-monophosphole-complexes (1a and 2a) and
the corresponding trans-diphosphole complexes (1b and
2b) are present in similar amounts at the end of the
reaction when no further changes were detected (Table
1).
The monophosphole–platinum complex (3a) was
formed selectively in a few minutes after starting the
reaction, when 3 was used as the sterically congested
monophosphole. Although the substitution of both
benzonitrile ligands also took place in part in a consec-
utive reaction yielding 3b, the mixture of 3a and 3b was
formed in a few hours. The selective formation of 3b by
complete substitution has not been achieved even under
long reaction times.
2.2. NMR characterisation of platinum–phosphole
complexes
The bis-monophosphole complexes containing 1 and
2 gave two sets of signals of very similar 31P-NMR
characteristics assigned to 1b, 1b% and 2b, 2b%, respec-
tively. This phenomenon can be explained by the pres-
ence of an sp3 phosphorus stereogenic centre upon
coordination resulting in the formation of racemic
(R,R/S,S) and ‘meso’ (R,S) trans-PtCl2(L)2 complexes.
It must be noted that the nearly 1/1 diastereomeric
(i) The reaction of 1, 2 and 3 with PtCl2(PhCN)2
1
resulted in platinum complexes possessing J(195Pt,31P)
coupling constants of 3160, 3302 and 3217 Hz (Table
2). These coupling constants are of diagnostic value for
a P–Pt–Cl trans arrangement and similar values have
Table 3
1H-NMR data for phosphole 3 and for its platinum-complexes, 3a and 3b
3
3aa
3ba
PtCl2(PhCN)2
l (ppm)
J(31P,1H) (Hz)
J(1H,1H) (Hz)
l (ppm)
J(31P,1H) (Hz)
l (ppm)
J(31P,1H) (Hz)
l (ppm)
H-2
H-4
H-5
6.46
6.84
6.94
2.24
1.72
6.94
37.4
16.7
37.4
6.0
1.6; ca. 1.5
7.0; 1.6
7.0; 2.4
ca. 1.5
–
2.3
–
2.3
–
6.56
6.70
6.98
2.13
3.28
6.93 (brs)
1.24
35
6.51
ca. 34
–
–
–
–
–
–
–
–
b
b
3-CH3
8-CH3
H-9
10-C(CH3)3 1.28
H-11 7.41
12-C(CH3)3 1.73
Ph(o)
Ph(m)
B2
–
2.09
ca. 0.5
3.15
–
–
–
b
–
1.23
b
6.0
ca. 2
7.11
ca. 4
–
1.48
1.38
–
6.84c
7.32c
7.59c
7.80
7.55
7.73
Ph(p)
a All the J(1H,1H) values are smaller than 1.5 Hz in 3a and 3b.
b The multiplets of the complex overlap with those of the non-coordinating ligand.
c The chemical shifts of the corresponding Ph(o), Ph(m), Ph(p) protons in ‘free’ PhCN are at 7.64, 7.46, 7.59.

82
Z. Cso´k et al. / Journal of Organometallic Chemistry 586 (1999) 79–84
Table 4
ratio of 1b/1b% determined on the ‘in situ’ mixture by
NMR measurements was shifted to approximately 2/1
when the reaction of 1 with PtCl2(PhCN)2 was carried
out in benzene (See Section 3) and crystallised from the
reaction mixture. This phenomenon can be explained
both by solvent effects on the substitution of the second
benzonitrile ligand and by the different solubilities of
the diastereomers.
(iii) The change of the shielding effect on some
protons of both the phosphole and the benzonitrile
ligands proved to be extremely useful from the point of
view of structural characterization. The largest effects
have been observed in 3a and 3b (Table 3). (The
numbering of the phosphole positions is the same as
described earlier [19].) The largest effects have been
observed for 8-CH3 (ortho methyl substituent of the
aryl ring) and 12-C(CH3)3 group (ortho tert-butyl sub-
stituent of the aryl ring) both in 3a and 3b. The
deshielding effect on 8-CH3 and the loss of the deshield-
ing effect in the case of the ortho tert-butyl group are
accompanied by a decrease in the magnitude of
J(31P,1H) coupling constants to the C-3 methyl sub-
stituent of the phosphole ring and to H-11 (one of the
meta protons of the aryl ring).
Hydroformylation of styrene in the presence of platinum and
rhodium catalystsa
b
c
Catalyst
R. time (h)
Conversion
(%)
RC (%) Rbr (%)
PtCl2(PhCN)2 130
+1+SnCl2
PtCl2(PhCN)2 130
+2+SnCl2
PtCl2(PhCN)2 130
+4+SnCl2
PtCl2(PhCN)2 130
+9+SnCl2
[Rh(nbd)Cl]2 20
+1
[Rh(nbd)Cl]2 10
+4
21
11
48
3
81
28.5
63
81
90
85.5
70
87
99
61
99
80.5
58
98.5
a Reaction conditions: 0.05 mmol catalyst, Pt/P/SnCl2=1/2/1; Rh/
P=1/2; 100 mmol styrene; solvent: toluene; p(CO)=p(H2)=40 bar;
reaction temperature: 100°C.
b Chemoselectivity; (mol 6+mol 7)/(mol 6+mol 7+mol 8)×100.
c Regioselectivity; (mol 6)/(mol 6+mol 7)×100.
was the only detectable species a few minutes after
starting the reaction. It was assigned to the
[Rh(nbd)(4)2]+ cation. This result is in agreement with
the earlier findings obtained with monophosphines.
On the basis of the changing of the shielding effects
and the decrease of the J(P,H) coupling constants
(Table 3), both the change of the nearly planar charac-
ter of the phosphorus towards a pyramidal one and the
loss of perpendicular arrangement of phosphole and
aryl rings upon coordination can be envisaged.
2.4. Homogeneous hydroformylation of styrene with
platinum- and rhodium-containing catalysts
Styrene (5) as a model substrate was allowed to react
in the presence of ‘in situ’ platinum- and rhodium-con-
taining catalysts prepared from PtCl2(PhCN)2 and the
corresponding ligand (Table 4).
2.3. NMR characterisation of rhodium–phosphole com-
plexes
The most aromatic phosphole (2) reacted slowly with
[Rh(nbd)Cl]2 in CDCl3 and Rh(nbd)(2)Cl (2c) (l, 30.8
ppm, 1J(Rh–P)=125 Hz) was formed with :13%
conversion. Upon prolonged reaction (more than 10
days) the characteristic pair of doublets assigned to the
cyclodimerization product of the phosphole oxide (2%,
Fig. 3) was detected but no further reaction towards 2c
was observed. Surprisingly, no formation of Rh–phos-
phole complexes has taken place even in traces when
Rh(CO)2(acac) complex was used as starting material.
The reaction of the triisopropyl-phenyl derivative (1)
with [Rh(nbd)Cl]2 resulted in the formation of
PhCHꢀCH2 (5)CO“/H2PhCH(CH3)CHO (6)
+Ph(CH2)2CHO (7)+PhC2H5 (8)
In addition to the formyl regioisomers 6 and 7, the
hydrogenation product 8 was also formed in all cases.
Although the catalytic activity of the above platinum
catalysts containing monophospholes and especially
that of the in situ system containing the cyclodimer
diphosphine (9 [21], Fig. 4) is very low, their tests are of
theoretical importance, since the use of sterically hin-
dered monophospholes and 7-phospha-norbornene type
ligands in homogeneous catalysis is unprecedented.
It is worth noting that both the chemo- and regiose-
lectivities of hydroformylation are influenced by the
basicity of the phosphole ligands. The use of the more
basic ligand (1) in platinum-containing catalyst results
in low chemo- and regioselectivities, 81 and 28.5%,
respectively (regioselectivity towards branched aldehyde
regioisomer was defined). While the chemoselectivity is
nearly the same in the case of 2, the regioselectivity is
substantially improved, i.e. the formation of the
1
Rh(nbd)(1)Cl (1c) (l, 32.7 ppm, J(Rh–P)=138 Hz).
After 4 days a pair of doublets (vide supra) (l, 57.3
ppm, 80.3 ppm, 37.8 Hz) appeared (even under very
inert conditions) which was assigned to the analogous
dimer dioxide, 1% (Fig. 3).
The most basic 4 shows different coordination chem-
istry towards [Rh(nbd)Cl]2. Neither unreacted phosp-
hole ligand nor cyclodimerization products could be
observed when 4/Rh=2/1 ratio was used. A complex
1
showing a doublet (l, 26.6 ppm, J(Rh–P)=114 Hz)

Z. Cso´k et al. / Journal of Organometallic Chemistry 586 (1999) 79–84
83
branched aldehyde is favoured. A further increase in
3.3. Preparation of trans-PtCl2(1)2
both selectivities was achieved by the application of 4
resulting in 90% and 85.5% chemo- and regioselectiv-
ity, respectively.
In a typical experiment 0.1 mmol of PtCl2(PhCN)2
was dissolved in 8 ml refluxing benzene, and a solu-
tion of 0.1 mmol 1 (or 3) in 4 ml of benzene was
added. The experiment was conducted for 6 h. A pale
yellow solid was formed, which was isolated by filtra-
tion, after the mixture was cooled to room tempera-
ture. The product was recrystallysed from benzene.
Analysis for PtCl2(1)2: Anal. Calc: C, 55.42; H, 6.74;
Cl, 8.18; Found: C, 55.77; H, 6.91; Cl, 7.93.
The use of the diphosphine 9 (Fig. 4) resulted in
negligible activity (less than 3% conversion in 130
hours) in platinum-catalysed hydroformylation.
However, the highly active rhodium-containing in
situ systems containing both 1 and 4 provide the ex-
pected high chemoselectivity. The ‘branched selectiv-
ity’ decreased dramatically when 4 was substituted for
1.
3.4. Hydroformylation experiments
In a typical experiment a solution of 0.025 mmol
of PtCl2(PhCN)2, 0.05 mmol of monophosphole and
0.05 mmol of SnCl2 in 30 ml toluene containing 0.1
mole of styrene was transferred under argon into a
150 ml stainless steel autoclave. The reaction vessel
was pressurized to 80 bar total pressure (CO/H2=1/
1) and placed in an oil bath and the mixture was
stirred with a magnetic stirrer. The pressure was mon-
itored throughout the reaction. After cooling and
venting of the autoclave, the pale yellow solution was
removed and immediately analysed by GC.
3. Experimental
3.1. Chemicals
The 1-arylphosphole ligands (1, 2 and 3) were syn-
thesised as described earlier [17–19]. The synthesis of
9
has been published recently [21]. Platinum
(II)chloride was purchased from Aldrich. Benzene was
distilled under argon from sodium in the presence of
benzophenone.
The PtCl2(PhCN)2 precursor was prepared as de-
scribed previously [22]. The PtCl2(L2) complexes were
prepared in refluxing benzene by the widely used
‘benzonitrile method’ [22].
Acknowledgements
The authors wish to thank the Hungarian National
Science Foundation for financial support (OTKA
T23525, T029039 and T016260) and the Ministry of
Education for grant FKFP 0818.
3.2. Instrumentation
1H- and 31P-NMR spectra were recorded in CDCl3
on a Varian Unity 300 spectrometer at 300 and 121.4
MHz, respectively. Chemical shifts are reported in l
ppm, referred to TMS (tetramethylsilane) as internal
standard and to orthophosphoric acid (85%, higher
fields refer to lower chemical shifts) as external
standard.
All experiments were carried out under an argon
atmosphere by using standard inert Schlenk tech-
niques.
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