Synthesis of new rhodium complexes with a hemilabile nitrogen-containing bis(phosphinite) or bis(phosphine) ligand. Application to hydroformylation of styrene

Article abstract of DOI:10.1016/S0022-328X(01)00701-X

Two new cationic rhodium(I) complexes with a hemilabile nitrogen-containing bis(phosphinite) or bis(phosphine) ligand have been prepared. ³¹P-NMR studies provide evidence that the ligands are coordinated to the metal in P,P-bidentate mode, wher

Full text of DOI:10.1016/S0022-328X(01)00701-X

Journal of Organometallic Chemistry 626 (2001) 221–226
www.elsevier.nl/locate/jorganchem
Synthesis of new rhodium complexes with a hemilabile
nitrogen-containing bis(phosphinite) or bis(phosphine) ligand.
Application to hydroformylation of styrene
Ioannis D. Kostas *
National Hellenic Research Foundation, Institute of Organic and Pharmaceutical Chemistry, Vas. Constantinou 48, 116 35 Athens, Greece
Received 4 January 2001; received in revised form 22 January 2001
Abstract
Two new cationic rhodium(I) complexes with a hemilabile nitrogen-containing bis(phosphinite) or bis(phosphine) ligand have
been prepared. 31P-NMR studies provide evidence that the ligands are coordinated to the metal in P,P-bidentate mode, whereas
for the bis(phoshine)-based ligand, a P,N,P-tridentate mode is also possible to be found at low temperatures. The complexes were
applied to the hydroformylation of styrene and displayed a high chemoselectivity for aldehydes with a very good branched/linear
ratio. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Bisphosphine; Bisphosphinite; P,N,P-Ligand; Hemilabile ligand; Rhodium complex; Hydroformylation
1. Introduction
containing bis(phosphinites) have been less studied and
refer to ligands based on pyridine or pyrrolidine moi-
eties [5,6,19]. The enormous interest in hydroformyla-
tion, one of the most important industrial applications
of homogeneous catalysis, is well known [22,23]. Up to
now, the use of such ligands in hydroformylation has
been limited [19]. The application of nitrogen-contain-
ing bis(phosphines) (e.g. ligands with aminomethyl-
phosphine moieties) in hydroformylation has also been
reported [24–27].
The importance of ligands possessing both ‘soft’
phosphorus and ‘hard’ nitrogen atoms as chelating
reagents is well known, and the use of these ligands can
be expected to improve the catalytic activity of some
complexes [1–13]. This class of compounds includes
P,N,P-ligands containing sp3, sp2 or sp nitrogen
donors, whose coordination behavior towards transi-
tion metals has been widely studied [4–11].
In the last few years, interest in bis(phosphinites) as
ligands in homogeneously catalytic processes, mostly in
hydrogenation, has increased considerably [14–21].
This increase is due to their facile preparation by the
reaction of readily available diols with chlorophos-
phines in the presence of a base. The catalytic activity
of bis(phosphinites) versus bis(phosphines) has been
studied and, according to a recent report, rhodium
complexes of achiral bis(phosphinites) exhibit lower
reactivity than bis(phosphines) in the hydrogenation of
imines while the situation is reversed for the chiral
analogues [20]. In contrast to the significant number of
publications concerning bis(phosphinites), nitrogen-
Recently, we reported the synthesis of new P,N-lig-
ands and their application in rhodium-catalyzed hydro-
formylation and domino hydroformylation-reductive
amination reactions [28,29]. As an extension of the
ongoing research, in this paper the synthesis and char-
acterization of new cationic rhodium complexes with
the hemilabile nitrogen-substituted bis(phosphinite)
PhN(CH2CH2OPPh2)2 (2) and the bis(phosphine)
PhN(CH2CH2PPh2)2 (5), are described. The coordina-
tion behavior of the latter ligand towards palladium
and rhenium has been reported previously [10,30]. The
catalytic activity of the complexes was tested on the
hydroformylation of styrene, under varying conditions.
* Tel.: +30-1-7273878; fax: +30-1-7273877.
E-mail address: ikostas@eie.gr (I.D. Kostas).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00701-X

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I.D. Kostas / Journal of Organometallic Chemistry 626 (2001) 221–226
2. Results and discussion
values of large size chelate rhodium complexes with
bis(phosphinites), indicating the absence of RhꢀN coor-
dination, which would create a six-membered chelate
ring [18]. In the H-NMR spectrum of 3, the NCH2
2.1. Synthesis of the ligands
1
resonance is slightly shifted to a high field compared to
the free ligand, contrary to that expected for the case of
RhꢀN coordination. This observation is additional evi-
dence that the NCH2 protons are situated away from
the coordination sphere around the metal. A P,P-
bidentate coordination mode was also observed for
complex 6, at room temperature, according to the
31P-NMR spectrum. In the 31P-NMR spectrum of five-
membered chelate rhodium complexes with analogous
P,N-ligands, e.g. Ph2PCH2CH2NMe2, the phosphine
resonance is observed at l ca. 40, with a 31P-coordina-
tion chemical shift l (lP(coord)−lP(free)) of around 60
ppm [12,28]. The resonance at l 17.33 with a coordina-
tion chemical shift=38.55 ppm in complex 6 is quite
different from these values, indicating the absence of a
five-membered P,N-chelate ring, at room temperature.
The absence of RhꢀN coordination has been reported
in the solid state for the similar rhodium complex
[(PhN(CH2PPh2)2)Rh(COD)]BF4 [26].
The synthetic sequence for the preparation of the
ligands and their complexes in the present work is
outlined in Scheme 1. N-Phenyldiethanolamine (1) was
used as starting material for their synthesis.
Bis(phosphinite) 2 was prepared in 89% yield by
hydrogen abstraction of 1, using two equivalents of
n-BuLi and subsequent reaction of the resulting
bis(alkoxide) with two equivalents of Ph2PCl. Replace-
ment of the hydroxyl groups in 1 by the diphenylphos-
phino group for the synthesis of ligand 5 was achieved
via its bis(tosylate) [10].
2.2. Synthesis and characterization of the rhodium
complexes
Treatment of (COD)2Rh+BF−4 in dichloromethane
solution with one equivalent of ligand 2 or 5, yielded
the cationic rhodium complexes 3 or 6, respectively, in
high yield (Scheme 1).
In the variable-temperature 31P-NMR spectra, com-
plexes 3 and 6 show different behavior (Fig. 1). The
31P-NMR spectrum of 3 did not significantly change
even at temperatures down to 213 K, indicating that the
room temperature conformation and coordination
mode is retained at low temperatures. For complex 6
on the other hand, the doublet at l 17.33, observed at
300 K, on cooling starts to become broader at 233 K
and merges to a broad singlet at 213 K. On further
cooling to 183 K two broad signals of different inten-
sity are observed at approximately 10 and 22 ppm
which at even lower temperature (173 K) become a
well-resolved doublet at 10.18 ppm (JRhP=137.3 Hz)
and two broad signals at approximately 17.5 and 27.5
ppm. A possible explanation for this behavior is the
presence of different coordination modes or conforma-
tions of the ligand at low temperature, one of which,
corresponding to the signal at 17.5 ppm, is probably the
same as that at 300 K. The additional resonances may
possibly be due to an alternative P,N,P-coordination
mode of the ligand.
The coordination mode in the complexes was deter-
mined by spectroscopic techniques. The observation of
the [LRh(COD)]+ ion (L=2 or 5) and the absence of
higher aggregates in the ESI MS spectra suggests the
monomeric nature of the complexes. The 31P-NMR
spectra clearly indicate that both phosphorus atoms are
bonded to the metal. In CD2Cl2 at room temperature,
complex 3 shows a doublet at l 120.00 (JRhP=181.8
Hz) and for complex 6 a doublet at l 17.33 (JRhP
=
142.6 Hz). Lack of coupling between the two phospho-
rus atoms indicates their equivalence. The chemical
shift and the RhꢀP coupling constant in the 31P-NMR
of 3 are in full accordance with the corresponding
Despite the absence or presence of RhꢀN coordina-
tion in complexes 3 and 6, the existence of an addi-
tional coordination site as a stabilizing group during
the course of a metal-mediated reaction could improve
the catalytic efficiency of the complexes, as shown
below for hydroformylation. Indeed, in complexes of
the type [(R(NCH2PPh2)2)Rh(COD)]BF4, where RhꢀN
coordination is absent, the presence of a nitrogen in the
backbone improves considerably the catalytic activities
as opposed to analogous ligands with the all-carbon
bridge [26].
Scheme 1.

I.D. Kostas / Journal of Organometallic Chemistry 626 (2001) 221–226
223
Fig. 1. Variable-temperature 31P{1H}-NMR spectra for the complexes: (a) 3 and (b) 6. The sharp signal at approximately 13.5 ppm, which
although of very small peak area compared to the adjacent peaks, is prominent in the low temperature spectra, is due to a small amount of
impurity which is barely observable in the room temperature spectrum.
respectively, while at 60°C, both complexes yielded an
almost complete conversion of styrene after 1 h (entries
11 and 12). At 30°C, the conversion of styrene was not
complete even after 22 h (entries 1 and 2).
According to the above mentioned results, the
rhodium complexes 3 and 6 are very active and selective
catalysts for the hydroformylation of styrene. It is of
interest to note that, under identical reaction condi-
tions, their reactivity is higher than five-membered
chelate rhodium(I) complexes with P,N-ligands previ-
ously reported by our group [28]. At a pressure of 100
bar, after 4 h at 40°C, for instance, the TON’s using 3
and 6 are 1305 and 1107, respectively, in contrast to the
2.3. Hydroformylation
The catalytic activity of the rhodium complexes 3
and 6 was tested on the hydroformylation of styrene,
under variable conditions of pressure and temperature
(Scheme 2; Table 1). In general, they display a high
chemoselectivity for aldehydes (over 98%) with a very
good branched/linear ratio (up to 96%). In all experi-
ments, ethylbenzene as hydrogenation product was
present only in traces (0.1–0.7%). For both complexes,
at 100 bar total pressure of syngas, the regioselectivity
towards 2-phenyl-propanal (8) (branched aldehyde) was
found to be about 96, 95, and 87% at 30, 40, and 60°C,
respectively. Lowering the syngas pressure to 30 or 20
bar, at 40°C, the regioselectivity was decreased to about
88 or 87%, respectively. The increase of the temperature
not only yields a lower regioselectivity but dramatically
effects the reaction rate. At a pressure of 100 bar, after
1 h at 40°C, the conversion of styrene was 17.6 and
11.9% using the complexes 3 and 6 (entries 7 and 8),
Scheme 2.

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I.D. Kostas / Journal of Organometallic Chemistry 626 (2001) 221–226
Table 1
Hydroformylation of styrene catalyzed by rhodium complex 3 or 6 a
b
c
d
e
Entry
Catalyst
T (°C)
P
(bar)
Time (h)
Conversion (%)
Rc (%)
Rbr (%)
TON f
1
2
3
4
5
6
7
8
3
6
3
6
3
6
3
6
3
6
3
6
3
6
3
6
30
30
40
40
40
40
40
40
60
60
60
60
40
40
40
40
100
100
100
100
100
100
100
100
100
100
100
100
30
22
22
22
22
4
4
1
1
4
4
1
1
68
68
72
72
93.1
85.9
99.9
99.9
90.7
76.8
17.6
11.9
99.9
99.8
99.4
98.6
99.5
98.5
99.9
98.8
99.1
98.9
98.5
98.8
98.9
99.1
98.1
98.8
99.4
98.2
98.1
98.6
99.2
99.7
99.2
99.5
96.1
96.0
95.3
95.1
94.9
95.0
94.8
94.7
87.2
87.0
87.1
87.0
88.3
87.9
87.3
87.0
1342
1236
1432
1436
1305
1107
251
171
9
1445
1426
1419
1415
1436
1429
720
10
11
12
13
14
15 g
16 g
30
20
20
715
a Styrene/catalyst=1455:1.
b A 4 mM solution in CH2Cl2.
c P=initial total pressure of CO/H2 (1:1).
d Rc=chemoselectivity towards aldehydes 8 and 9.
e Rbr=regioselectivity towards branched aldehyde 8.
f Turnover no. (TON)=aldehydes fraction×substrate/catalyst ratio.
g Styrene/catalyst=727:1.
rhodium(I) complex containing a methoxy amino phos-
phine, which led to a TON of 821, under the same
reaction conditions [28]. The catalytic activity of 3 in
the hydroformylation of styrene was also compared to
that of the analogue rhodium complex with a nitrogen-
containing bis(phosphinite) ligand based on a pyrro-
lidine moiety [19]. Under similar conditions
(styrene/catalyst=727:1, P(CO/H2)=20 bar, T=
40°C) complex 3 led to almost quantitative conversion
of styrene after 72 h, yielding the desired aldehydes
with a regioselectivity of 87% towards the branched
aldehyde 8. In contrast to these experimental results,
hydroformylation of styrene by the rhodium complex
with the pyrrolidine-based bis(phosphinite) ligand, led
to only 10% conversion into aldehydes after 72 h, with
a 77% regioselectivity towards the branched aldehyde
[19]. The much lower activity and selectivity of this
system, in contrast to the bis(phosphinite)-based
rhodium complex reported in this paper, could be due
to the rigid pyrrolidine moiety in the backbone which
restricts the conformational flexibility of the chelate.
The improved reactivity of rhodium complexes by in-
creasing the flexibility of the metal chelate is also
known for the hydrogenation of imines [20].
and, respectively, have been synthesized. The
bis(phosphinite)-based ligand is bonded to the metal
exlucively in
a P,P-bidentate coordination mode,
whereas for the bis(phoshine)-based ligand, a P,N,P-tri-
dentate mode is also possible together with the most
favorable P,P-bidentate mode. The complexes were
examined as catalysts for the hydroformylation of sty-
rene, providing a very good activity and selectivity.
4. Experimental
4.1. General
N-Phenyldiethanolamine (1) was commercially avail-
able. (COD)2RhBF4 was prepared from rhodium
trichloride hydrate according to a literature procedure
[31–33]. n-BuLi was prepared from lithium metal and
n-BuCl in methylcyclohexane. All preparations and hy-
droformylations were carried out under argon by using
dry and degassed solvents. THF and diethylether were
distilled from 9-fluorenylpotassium and Na/benzophe-
none, respectively. Styrene was distilled from CaH2 and
kept under argon. Hydroformylation studies were per-
formed in a stainless steel autoclave (300 ml) with
magnetic stirring. The syngas CO/H2 (1:1) (CO, 1.8; H2,
3.0) was purchased from Messer Griesheim GmbH.
NMR: Bruker AC 300 (300.13, 75.47, and 121.50 MHz
3. Conclusions
1
1
Two new cationic rhodium complexes with a nitro-
gen-substituted bis(phosphinite) or bis(phosphine) lig-
for H, 13C and 31P, respectively); H- and 13C-NMR
shifts were referenced to the solvents, the 31P-NMR

I.D. Kostas / Journal of Organometallic Chemistry 626 (2001) 221–226
225
shifts were referenced to external 85% H3PO4 in D2O.
ESI MS: Finnigan MAT TSQ 7000. GC–MS (EI):
Varian Saturn 2000 with a 30 m×0.25 mm DB5-MS
column. GC: Varian Star 3400 CX with a 30 m×0.53
mm DB5 column.
2.75. Calc. for C24H27NO6S2 (489.60): C, 58.88; H, 5.56;
N, 2.86%.
4.4. N,N-bis[2-(Diphenylphosphino)ethyl]-benzenamine
(5) [10,30]
4.2. N,N-bis[2-[(Diphenylphosphino)oxy]ethyl]-
benzenamine (2)
A solution of lithium diphenylphosphide, freshly pre-
pared from lithium (0.9 g, 128.6 mmol) and
chlodiphenylphosphine (1.85 ml, 10.3 mmol) in THF
(100 ml), was added to a solution of 4 (2.45 g, 5.0
mmol) in THF (40 ml) at room temperature and stirred
for 1 h. Then methanol (4 ml) was added and the
solution was evaporated under vacuum. The residue
was suspended in toluene (100 ml) and filtered. The
solution was passed through a short column of Celite
and alumina and the solvent was evaporated under
reduced pressure, yielding 2.12 g of the crude product
as a white viscous oil, which was solidified by ether at
−20°C. The resulting solid was washed once more with
cold ether, yielding 5 (1.91 g, 74%) as a white solid,
m.p. 102–104°C, which was identified by 1H-, 13C-,
31P-NMR and MS spectra [10].
To a solution of 1 (5.2 g, 28.69 mmol) in THF (50
ml) under argon, n-butyllithium (1.79 M in methylcy-
clohexane, 33 ml, 59.07 mmol) was added at −78°C.
The reaction mixture was stirred at room temperature
for 20 min and subsequently
a
solution of
chlorodiphenylphosphine (10.7 ml, 59.60 mmol) in
THF (30 ml) was added dropwise, and the mixture then
stirred overnight. The volatile materials were removed
by evaporation, the residue was suspended in toluene
(100 ml) and filtered to remove the inorganic salts. The
solution was then passed through a short column of
basic alumina to remove the phosphorus impurities.
Evaporation of the solvent under reduced pressure af-
forded 2 as a viscous oil (14.05 g, 89%), which turned to
1
a white solid upon standing, m.p. 63–66°C. H-NMR
4.5. [Rh(COD)-2]BF4 (3)
(CHCl3-d, l ppm): 7.51–7.35 (m, 20H, Ar); 7.23–7.18
(m, 2H, Ar); 6.71 (obscured with the doublet at 6.70
A solution of the bis(phosphinite) 2 (0.2830 g, 0.52
mmol) in CH2Cl2 (10 ml) was added dropwise to the
dark red solution of (COD)2Rh+BF4− (0.2093 g, 0.52
mmol) in CH2Cl2 (10 ml) at −78°C. The reaction
mixture was warmed slowly to room temperature
within 1 h, and stirred at this temperature for an
additional 2 h. The resulting orange solution was evap-
orated under reduced pressure to 2 ml, and addition of
ether (20 ml) caused the precipitation of a dark orange
solid. The supernatant solution was decanted, the solid
was washed with ether (10 ml) and dried by vacuum,
3
ppm, 1H, Ar); 6.70 (d, J=7.8 Hz, 2H, Ar); 3.97 (dt,
3JPH=9.0 Hz, 3JHH=6.5 Hz, 4H, CH2O); 3.63 (t,
3J=6.5 Hz, 4H, CH2N). 13C{1H}-NMR (CHCl3-d, l
ppm): 147.31–112.00 (Ar); 66.48 (d, 2JPC=16.9 Hz,
3
CH2O); 52.03 (d, JPC=7.7 Hz, CH2N). 31P{1H}-NMR
(CHCl3-d, l ppm): 114.63 (s). ESI MS: m/z 550 ([M+
H]+). Anal. Found: C, 74.35; H, 6.06; N, 2.34. Calc. for
C34H33NO2P2 (549.59): C, 74.31; H, 6.05; N, 2.55%.
4.3. N,N-bis[2-(p-Tolylsulfonoxy)ethyl]-benzenamine (4)
[10]
1
yielding rhodium complex 3 (0.4145 g, 94%). H-NMR
(CH2Cl2-d2, l ppm): 7.73–7.33 (m, 20H, Ar); 7.09–7.04
(m, 2H, Ar); 6.63 (t, 3J=7.1 Hz, 1H, Ar); 6.26 (d,
3J=8.3 Hz, 2H, Ar); 4.49 (sl br s, 4H, CODꢀCH);
4.04–3.98 (m, 4H, CH2O); 3.18 (t, 3J=4.1 Hz, 4H,
CH2N); 2.25 (sl br s, 8H, CODꢀCH2). 13C{1H}-NMR
(CH2Cl2-d2, l ppm): 146.74–111.74 (Ar); 102.77–
102.56 (m, CODꢀCH); 66.64–66.51 (m, CH2O); 53.96
(s, CH2N); 30.43 (s, CODꢀCH2). 31P{1H}-NMR
(CH2Cl2-d2, l ppm): 120.00 (d, JRhP=181.8 Hz). ESI
MS: m/z 760 ([M−BF4]+). Anal. Found: C, 58.98; H,
5.39; N, 1.55. Calc. for C42H45BF4NO2P2Rh (847.48):
C, 59.52; H, 5.35; N, 1.65%.
To a solution of 1 (9.7 g, 53.5 mmol) in pyridine (80
ml), p-toluenesulfonyl chloride (20.4 g, 107.0 mmol)
was added slowly at 0°C, and the reaction mixture was
then stirred at this temperature for 3 h. 1 M HCl (300
ml) was added, and the solution was extracted with
CH2Cl2 (5×100 ml). The combined organic layers were
washed with 200 ml of saturated brine, dried over
Na2SO4, filtered and evaporated to dryness, yielding
24.8 g of the crude product as a brown very viscous oil,
which turned to a yellow solid by addition of 20 ml of
cold methanol. The resulting solid was washed once
more with 20 ml of cold methanol, yielding 4 (13.75 g,
1
53%) as a white solid, m.p. 89–91°C. The H-NMR
4.6. [Rh(COD)-5]BF4 (6)
spectrum has previously been reported [10]. 13C{1H}-
NMR (CHCl3-d, l ppm): 145.61, 144.92, 132.48,
129.81, 129.36, 127.72, 117.51 and 111.96 (Ar); 66.53
and 50.07 (OCH2CH2N). ESI MS: m/z 490 ([M+H]+),
318 ([M−OTs]+). Anal. Found: C, 58.46; H, 5.67; N,
According to the procedure described for the synthe-
sis of 3, rhodium complex 6 was prepared from the
bis(phosphine)
5
(0.2476 g, 0.48 mmol) and
(COD)2Rh+BF−4 (0.1944 g, 0.48 mmol), as an orange

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I.D. Kostas / Journal of Organometallic Chemistry 626 (2001) 221–226
1
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37 (1996) 797.
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solid (0.3646 g, 93%). H-NMR (CH2Cl2-d2, l ppm):
3
7.50–7.34 (m, 20H, Ar); 7.28 (t, J=7.7 Hz, 2H, Ar);
6.91 (t, J=7.0 Hz, 1H, Ar); 6.70 (m, 2H, Ar); 4.53 (sl
3
br s, 4H, CODꢀCH); 3.68 (sl br m, 4H, CH2); 2.60 (sl
br s, 4H, CH2); 2.35 (sl br s, 4H, CODꢀCH2); 2.19–2.12
(m, 4H, CODꢀCH2). 13C{1H}-NMR (CH2Cl2-d2, l
ppm): 149.03–115.94 (Ar); 99.14–98.59 (m, CODꢀCH);
50.75–50.68 (m, CH2); 30.93 (s, CODꢀCH2); 28.53–
28.46 (m, CH2). 31P{1H}-NMR (CH2Cl2-d2, l ppm):
17.33 (d, JRhP=142.6 Hz). ESI MS: m/z 728 ([M−
BF4]+), 620 ([M−COD−BF4]+). Anal. Found: C,
62.58; H, 6.05; N, 1.44. Calc. for C42H45BF4NP2Rh
(815.48): C, 61.86; H, 5.56; N, 1.72%.
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4.7. Hydroformylation
In a typical experiment, styrene (2 ml, 17.456 mmol)
and
a 4 mM solution of rhodium complex in
dichloromethane (3 ml, 0.012 mmol) were placed under
argon in an oven-dried autoclave, which was then
closed, pressurized with syngas (CO/H2=1:1) to 100
bar and brought to the required temperature. After the
required reaction time, the autoclave was cooled to
room temperature, the pressure was carefully released
and the solution was passed through Celite and ana-
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1
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Acknowledgements
The investigation was supported by the National
Hellenic Research Foundation. The author thanks Dr
B.R. Steele for helpful discussions and Miss M. Zervou
and Miss I. Kyrikou for technical assistance in the
variable-temperature 31P{1H}-NMR experiments.
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