Rh(acac)(CO)(PR3) and Rh(oxinate)(CO)(PR3) complexes - Substitution chemistry and structural aspects

Article abstract of DOI:10.1016/S0022-328X(00)00118-2

The substitution of CO in Rh(acac)(CO)₂ by the phosphorus ligands P(OPh)₃, P(NC₄H₄)₃, and PPh₂(NC₄H₄) has been studied kinetically by stopped-flow spectrophotometry as a function of temperature. With P(OPh)₃ and P(NC₄H₄)₃, both CO ligands are replaced in a stepwise fashion via the intermediate Rh(acac)(CO)(PR₃). However, the disubstituted complexes Rh(acac)(PR₃)₂ are thermodynamically unstable. Judged from the activation parameters, the individual steps are associative processes. In the case of PPh₂(NC₄H₄) only the monosubstituted complex is formed. The differences in the substitution rates as well as the stability of the various products are largely dominated by electronic (e.g. basicity) effects. X-ray structures of some of the mono-substituted complexes are given. In addition, also the reaction of Rh(oxinate)(CO)₂ with P(OPh)₃ has been studied kinetically showing that oxinate has a labilizing effect relative to acetylacetonate

Full text of DOI:10.1016/S0022-328X(00)00118-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 602 (2000) 59–64
Rh(acac)(CO)(PR3) and Rh(oxinate)(CO)(PR3)
complexes—substitution chemistry and structural aspects
Walter Simanko a, Kurt Mereiter b, Roland Schmid a, Karl Kirchner a,*1,
Anna M. Trzeciak c, Jozef J. Zio*lkowski c,*2
a Institute of Inorganic Chemistry, Technical Uni6ersity of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria
b Institute of Mineralogy, Crystallography, and Structural Chemistry, Technical Uni6ersity of Vienna, Getreidemarkt 9, A-1060 Vienna, Austria
c Faculty of Chemistry, Uni6ersity of Wroc*law, 14 F. Joliot-Curie str., 50-383 Wroc*law, Poland
Received 29 November 1999; received in revised form 7 February 2000; accepted 14 February 2000
Abstract
The substitution of CO in Rh(acac)(CO)2 by the phosphorus ligands P(OPh)3, P(NC4H4)3, and PPh2(NC4H4) has been studied
kinetically by stopped-flow spectrophotometry as a function of temperature. With P(OPh)3 and P(NC4H4)3, both CO ligands are
replaced in a stepwise fashion via the intermediate Rh(acac)(CO)(PR3). However, the disubstituted complexes Rh(acac)(PR3)2 are
thermodynamically unstable. Judged from the activation parameters, the individual steps are associative processes. In the case of
PPh2(NC4H4) only the monosubstituted complex is formed. The differences in the substitution rates as well as the stability of the
various products are largely dominated by electronic (e.g. basicity) effects. X-ray structures of some of the mono-substituted
complexes are given. In addition, also the reaction of Rh(oxinate)(CO)2 with P(OPh)3 has been studied kinetically showing that
oxinate has a labilizing effect relative to acetylacetonate © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Rhodium; Ligand substitution; Tertiary phosphines; Kinetics; Acetylacetonate
1. Introduction
to obtain di-substituted Rh(acac)(PR3)2. Weak p-accep-
tors, even when used in large excess, afford only mono-
substituted products [4,6,7].
Rh(acac)(CO)2 (acac=acetylacetonate) in the pres-
ence of tertiary phosphines or phosphites (PR3) is a well
established catalytic system for hydroformylation, hy-
drogenation, and isomerization reactions of olefins [1–
5]. For the hydroformylation process, several reaction
sequences take place wherein Rh(acac)(CO)2 is eventu-
ally converted into the catalytically active hydrido com-
plex HRh(CO)(PR3)3. In the first reaction step, CO is
replaced by PR3 giving Rh(acac)(CO)(PR3) [6,7]. The
substitution of the second CO ligand is rather sluggish
requiring the removal of free CO by purging with an
inert gas. Even then, however, strong p-acceptors such
as PR3=P(OPh)3 or P(NC4H4)3 are necessary in order
In the present paper we report on the synthesis and
structure of Rh(acac)(CO)(PR3) and Rh(oxi-
nate)(CO)(PR3) (oxinate=quinolin-8-olate) complexes
as well as their substitution kinetics with some tertiary
phosphines and phosphites. The kinetic and thermody-
namic analyses will aid in understanding the factors
that govern the formation of Rh(acac)(CO)(PR3) and
Rh(acac)(PR3)2 and their oxinate analogs. Note that
the reaction of Rh(acac)(CO)2 with P(OPh)3 had al-
ready been partially investigated previously by some of
us [8]. Furthermore, the substitution of CO in Rh(b-
diketone)(CO)2 complexes by 1,5-cyclooctadiene (COD)
[9] and P(OPh)3 [8] has been reported. Studies of COD
substitution in Rh(b-diketone)(COD) by P(OPh)3 have
also shown that the reaction rate is strongly dependent
on the electronic properties of the substituents at the
b-diketone ligand [10].
1 *Corresponding author. Tel.: +43-1-5880115341; fax: +43-1-
5880115399; e-mail: kkirch@mail.zserv.tuwien.ac.at
2 *Corresponding author. Tel.: +48-71-3204254; fax: +48-71-
3282348.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 0 ) 0 0 1 1 8 - 2

60
W. Simanko et al. / Journal of Organometallic Chemistry 602 (2000) 59–64
(3a). Typical time profiles as a function of the concen-
tration of P(OPh)3 are depicted in Fig. 1. Thus, the
second reaction does not go to completion under the
selected experimental conditions. The kinetic results are
summarized in Table 1. The individual steps (k1, k2,
k
−2) are characterized by large and negative entropies
of activation and are thus associative in nature. It is
further seen that the barrier of CO substitution is
somewhat higher in 2a compared to 1. This is inter-
preted in terms of the higher s-donor strength of the
phosphite ligand compared to CO, thereby strengthen-
ing the remaining RhꢀCO bond via an increased back-
donation. In this case the higher barrier is
predominantly electronic in origin. The small equi-
librium constant K2,−2 obtained from combining the
rate constants k2 and k−2 shows that the equilibrium is
shifted towards the monosubstituted complex 2a even
at high phosphite concentrations. Due to the reversibil-
ity of the second step, the overall reaction rate becomes
a function of the total Rh complex concentration as a
result of its effect on the CO concentration as men-
tioned elsewhere [8]. The k−2 path has also been stud-
ied by treatment of 3a with CO dissolved in toluene.
These CO solutions have been prepared from a stock
solution of CO in toluene (solubility of CO at 25°C is
ca. 6.87 mmol l−1). The second order rate constant of
2.2×104 M−1 s−1 obtained from the concentration
time curves is in good agreement with k−2 in Table 1,
considering the rather little dependability of the CO
concentrations.
Fig. 1. Typical biphasic time profiles for the reaction of
Rh(acac)(CO)2 with P(OPh)3 in toluene at 25°C. The concentrations
are [Rh]: 0.05 mM, [P(OPh)3]: 0.0 (A), 0.25 (B), 0.5 (C), 0.75 (D), 1.0
(E), 1.5 (F), and 2.5 mM (G).
2. Results and discussion
2.1. Rh(acac)(CO)2
The reactions shown in Eqs. (1) and (2) have been
studied by means of stopped flow spectrophotometry.
The details of data collection and establishment of the
rate law are presented in the Experimental Section. For
P(OPh)3 the data fit a two-term rate law involving
irreversible formation
1
As has been established by H- and 31P{1H}-NMR
studies [4], P(NC4H4)3 reacts analogously to P(OPh)3
replacing consecutively both CO ligands. Note the
slowing down of the k1 step by a factor of about 55
compared to P(OPh)3. Unfortunately, the second reac-
tion step could not be measured by photometric meth-
ods since 2b and 3b are too similar with respect to their
absorption spectra in the UV–vis range.
(1)
(2)
Finally, with PPh2(NC4H4) as the reagent, the overall
reaction becomes very unfavorable. The second step is
of the intermediate Rh(acac)(CO)(P(OPh)3) (2a) fol-
lowed by reversible formation of Rh(acac)(P(OPh)3)2
Table 1
Kinetic and equilibrium dataa for the conversion of 1 to 3 in toluene at 25°C
PR3
Step
DH‡ or DH° (kcal mol−1
)
DS‡ or DS° (cal K−1 mol−1
)
k (M−1 −1) or K
s
P(OPh)3
k1
k2
k−2
K2,−2
291
692
693
0.2
−2792
−2296
−19910
−3.1
2.6×105
4.5×103
2.8×104
0.2
b
P(NC4H4)3
P(NC4H4)Ph2
k1 c
291
−3092
4.7×104
2.4×103
3.6×104
k1
c
k−1
K1,−1
6.7×10−2
c
a The large error limits are due to the small absorbance changes during the reactions.
b The k2,k−2 path was not observable because of insufficient absorbance changes.
c Insufficient absorbance change at higher temperatures.

W. Simanko et al. / Journal of Organometallic Chemistry 602 (2000) 59–64
61
Table 2
Selected properties of the Rh(acac)(CO)(PR3) and Rh(oxinate)(CO)(P(OPh)3) complexes
a,b
c
31P a (ppm) 1J(RhꢀP) a
Cone angle c
(°)
RhꢀP (A)
RhꢀO (A)
k1 (25°C)
DHrxn
,
,
PR3
wCO
(cm−1
)
(Hz)
(trans to P)
(M−1 s−1
)
(kcal mol−1
)
P(NC4H4)3
P(OPh)3
P(NC4H4)2Ph 2002
2012
2006
102.5
212.1
104.7
90.0
48.6
126.3
251
293
218
194
180
281
141
136
150
154
145
136
2.166(1)
2.170(1)
2.054(2)
2.063(2)
4.7×104
2.6×105
3.6
4.2
3.8
4.1
4.8
P(NC4H4)Ph2 1990
2.223(1)
2.243(2)
2.186(1)
2.078(2)
2.087(4)
2.4×103
\3×105
d
PPh3
1975
1983
e
P(OPh)3
a Ref. [4].
b Ref. [20]
c Ref. [21]
d Ref. [22]
e Rh(oxinate)(CO)(P(OPh)3)
not observed at all, and the first one is largely shifted to
the educts. The much smaller rate constant k1 is plausible
since PPh2(NC4H4) is a weaker p-acceptor and, concomi-
tantly, a stronger s-donor. No temperature dependence
study has been undertaken in view of the small ab-
sorbance changes in the UV–vis region. This situation
could not be remedied by the insertion of CF3 or phenyl
groups in the acac moiety. Qualitatively, as judged from
NMR spectroscopy, the reaction rates of the first substi-
tution step were similar to that of the parent complex
The differences in the rate of CO substitution by the
series of phosphorus ligands as well as in the thermo-
dynamic stability of the products is remarkable. As
noted above, this result appears to be largely an elec-
tronic, i.e. basicity, effect. Possible steric components
remain masked in view of the similar sizes, expressible
by their cone angles, of the ligands used (Table 2). It
should be noted that the rate constants k1 correlate
neither with DHrxn nor wCO but to some extent with the
31P{1H}-NMR shifts and the 1J(RhꢀP) coupling constants.
However, there is no a priori reason for an intrinsic
relationship between NMR chemical shifts or coupling
constants and chemical reactivity (Direct relationships
between chemical shifts and rate constants have been
described elsewhere [11,12]). On the other hand, there is
a correlation between the wCO values and the RhꢀP and
RhꢀO (trans to P) distances of complexes 2 (Table 2).
The latter have been taken from the X-ray structures of
2a and 2c depicted in Figs. 2 and 3 with selected bond
distances and angles reported in the captions. Summing
up, the PR3 ligands can be arranged in the following
order of decreasing p and increasing s-donor properties
(and trans influence of the ligands): P(NC4H4)3\
P(OPh)3\PPh2(NC4H4)\PPh3. Based on our kinetic
studies, qualitatively CO substitution is faster in the case
of strong p-acceptor ligands forming stronger RhꢀP
bonds, while an increased s-donor strength of
the entering ligand causes the rate to decrease. At
present, however, we cannot explain the fact that
Fig. 2. Structural view of Rh(acac)(CO)(P(OPh)3) (2a) showing 20%
,
thermal ellipsoids. Selected bond lengths (A) and angles (°): RhꢀC(6)
1.823(3), RhꢀO(1) 2.040(2), RhꢀO(2) 2.063(2), RhꢀP 2.170(1),
C(6)ꢀRhꢀO(1) 178.55(9), O(2)ꢀRhꢀP 177.54(5).
Fig. 3. Structural view of Rh(acac)(CO)(PPh2(NC4H4)) (2c) showing
,
20% thermal ellipsoids. Selected bond lengths (A) and angles (°):
RhꢀC(6) 1.817(2), RhꢀO(1) 2.033(2). RhꢀO(2) 2.078(2), RhꢀP
2.223(1), C(6)ꢀRhꢀO(1) 177.26(9), O(2)ꢀRhꢀP 179.58(4).

62
W. Simanko et al. / Journal of Organometallic Chemistry 602 (2000) 59–64
coupling constant are similar to those in analogous
b-diketone complexes.
In order to ascertain the ligand disposition in 5 the
compound has been recrystallized from MeOH and
subjected to an X-ray structure determination. A view
of the molecular structure is given in Fig. 4 with
important bond lengths and angles given in the caption.
Thus, rhodium adopts a slightly distorted square planar
coordination with a r.m.s. deviation from planarity of
,
0.030 A and with the phosphite ligand trans to oxinate-
N. The same feature has been encountered in a number
of related compounds and the PPh3 analogue of Rh(oxi-
nate)(CO)(PPh3), for which bond lengths of RhꢀN=
2.098, RhꢀO=2.041, RhꢀC=1.785, and RhꢀP=2.261
,
A were reported [13]. The very significant difference in
the RhꢀP bond length relative to that of the phosphite
,
complex 5 (RhꢀP=2.186 A) is a consequence of
Fig. 4. Structural view of Rh(oxinate)(CO)(P(OPh)3) (5) showing 20%
,
thermal ellipsoids. Selected bond lengths (A) and angles (°): RhꢀC(28)
P(OPh)3 being the weaker s-donor but a much better
p-acceptor than PPh3, resulting in a relatively strong
RhꢀP bond. Similar observations have been made for a
1.813(3), RhꢀO(1) 2.022(2). RhꢀN 2.097(2), RhꢀP 2.186(1), O(1)ꢀC(8)
1.326(3), C(28)ꢀO(5) 1.146(3), C(28)ꢀRhꢀO(1) 179.6(1), NꢀRhꢀP
169.9(1), RhꢀO(1)ꢀC(8) 113.3(1), RhꢀNꢀC(1) 130.8(1), RhꢀNꢀC(9)
110.4(1).
pair
of
carbonyl-(2-quinolinecarboxylato-N,O)-
triphenylphosphine/triphenylphosphite Rh(I) complexes
[14].
Table 3
Kinetic and equilibrium data for the conversion of 4 to 6 in acetone
at 25°C
The reactions shown in Eqs. (3) and (4) have been
studied by means of stopped flow spectrophotometry.
In contrast to Rh(acac)(CO)2, the substitution of the
first CO ligand by P(OPh)3 was too fast to measure
with our stopped-flow equipment.
Step DH‡ or DH°
DS‡ or DS°
k (M−1 −1) or K
s
(kcal mol−1
)
(cal K−1 mol−1
)
a
k1
k2
\3×105
1.6×103
5.0×104
0.03
1.890.2
4.490.7
−3896
−2294
−16
k−2
K2,−2 −2.6
(3)
(4)
a Too fast to measure by the stopped-flow method.
P(OPh)3 reacts faster than P(NC4H4)3 and apparently
more subtle differences between these ligands have to
be taken into account.
Thus only a lower limiting value equal to about 3×105
M−1 s−1 can be given. In line with the Rh(acac)
system, the second substitution step is reversible. The
kinetic results are summarized in Table 3. Again, the
formation of the bisphosphite rhodium complex 6 is
unfavorable. The Rh(oxinate)(P(OPh)3))2 complex was
isolated from the CO-free solution.
2.2. Rh(oxinate)(CO)2
For comparison with the acac system, we have also
studied the CO substitution in Rh(oxinate)(CO)2 (4) by
P(OPh)3. Complex 4 reacts with P(OPh)3 to give the
Rh(oxinate)(CO)(P(OPh)3) (5). Of the two possible iso-
mers only the one with P(OPh)3 trans to the nitrogen
atom of the oxinate ligand is formed. Noteworthy, the
same isomer has been found for the analogous complex
Rh(oxinate)(CO)(PPh3). This may be explained by the
stronger trans influence of nitrogen. The nCO band in
the IR spectrum of 5 was found at 1983 cm−1 and is
shifted to higher frequencies by about 20 cm−1 com-
pared to Rh(b-diketone)(CO)(P(OPh)3). This may be
attributed to the stronger donor properties of the oxi-
nate ligand. The 31P{1H}-NMR data, however, remain
3. Experimental
3.1. General methods
All reactions were performed under an inert atmos-
phere of purified argon by using Schlenk techniques
and/or a glove box. All chemicals were standard
reagent grade and used without further purification.
The solvents were purified and dried according to stan-
1
,
unchanged. Both the chemical shift and the J(RhꢀP)
dard procedures and stored over 4 A molecular sieves.

W. Simanko et al. / Journal of Organometallic Chemistry 602 (2000) 59–64
63
The deuterated solvents were purchased from Aldrich
3.3. X-ray structure determination
X-ray data were collected on a Siemens Smart CCD
,
and dried over 4 A molecular sieves. Rh(acac)(CO)2 (1)
[15], Rh(acac)(CO)(P(OPh)3) (2a) [7], Rh(acac)-
(P(OPh)3)2 (3a) [16], Rh(acac)(CO)(P(NC4H4)3) (2b) [4],
Rh(acac)(P(NC4H4)3)2 (3b) [4], [Rh(acac)(CO)(PPh2-
(NC4H4)) (2c) [4] and Rh(oxinate)(CO)2 (4) [15] were
prepared according to the literature. Pyrrolylphos-
,
area detector diffractometer (Mo–Ka, u=0.71073 A,
absorption corrections by multi-scan method). All
structures were solved by direct methods using the
program SHELXS-97 [18]. Structure refinement on F2
with program SHELXL-97 [19] Rh(acac)(CO)(P(OPh)3)
2a. ꢀC24H22O6PRh, M=540.30, orthorhombic, space
1
phines were obtained as described elsewhere [17]. H-,
13C{1H}-, and 31P{1H}-NMR spectra were recorded on
a Bruker AC-250 spectrometer operating at 250.13,
62.86, and 101.26 MHz, respectively, and were refer-
enced to SiMe4 and H3PO4 (85%).
group Pbcn (No 60), a=18.120(5), b=9.709(3), c=
3
,
,
26.173(8) A, V=4605(2) A , T=223(2) K, Z=8, v=
0.849 mm−1, 48834 reflections (q527°), 4991 unique
reflections (Rint=0.039) which were used in all calcula-
tions. The final wR(F2) was 0.064, R1=0.043.
Rh(acac)(CO)(PPh2(NC4H4) (2c). ꢀC22H21NO3PRh,
3.1.1. Synthesis of Rh(oxinate)(CO)(P(OPh)3) (5)
To a solution of Rh(oxinate)(CO)2 (0.05 g, 0.165
mmol) in methanol (0.3 ml) P(OPh)3 (0.05 g, 0.164
mmol) was added. The liberated CO was removed by
purging the solution with dinitrogen for 5 min. Upon
slow evaporation of the solvent at about 0°C, orange
crystals were formed in 85% yield. C28H21NO5PRh
requires C, 57.45; H, 3.62; N, 2.39. Found: C, 57.2;
H, 3.4; N, 2.8%. NMR (acetone-d6, 20°C): lP 126.3
(
M=481.28, triclinic, space group P1 (No 2), a=
8.777(3), b=10.424(4), c=12.904(5) A, h=73.25(2),
,
3
,
i=70.63(2), k=82.74(2)°, V=1065.9(7) A , T=
295(2) K, Z=2, v=0.897 mm−1, 15 200 reflections
(q530°), 6016 unique reflections (Rint=0.019) which
were used in all calculations. The final wR(F2) was
0.065, R1=0.032. Rh(oxinate)(CO)(P(OPh)3) 5.
ꢀC28H21NO5PRh, M=585.34, monoclinic, space group
1
(d, JRhꢀP=281.1 Hz). wmax/cm−1 1983 (CO).
P21/n (No 14), a=14.271(4), b=10.966(3), c=
3
,
,
16.936(5) A, i=99.24(2)°, V=2530.1(12) A , T=
295(2) K, Z=4, v=0.777 mm−1, 35 628 reflections
(q530°), 7267 unique reflections (Rint=0.026) which
were used in all calculations. The final wR(F2) was
0.075, R1=0.045.
3.1.2. Synthesis of Rh(oxinate)(P(OPh)3)2 (6)
To a solution of Rh(oxinate)(CO)2 (0.05 g, 0.165
mmol) in benzene (0.3 ml) P(OPh)3 (0.12 g, 0.387
mmol) was added and the solution was purged with
dinitrogen for 5 min. The solvent was then removed
under vaccum and the residue was washed with n-hex-
ane and ethanol and dried in vacuo. Yield: 75%.
C45H36NO7P2Rh requires C, 62.30; H, 4.18; N, 1.61.
Found: C, 62.0; H, 3.92; N, 1.83%. NMR (acetone-d6,
4. Supplementary material
Listings of atomic coordinates, anisotropic tempera-
ture factors, complete bond lengths and angles, and
least-squares planes of complexes 2a, 2c, and 5 can be
obtained from the authors on request.
1
20°C): lP 127.4 (dd, JRhꢀP=280.0 Hz, JPP=101 Hz,
1
P trans to N), 125.3 (dd, JRhꢀP=293.0 Hz, JPP=101
Hz, P trans to O).
3.2. Kinetics
Acknowledgements
8
The stopped-flow kinetic data were obtained with a
Hi-Tech SF-61 instrument. The optical signal at 320
was monitored for the reactions of Rh(acac)(CO)2 and/
or Rh(acac)(CO)(PR3) with PR3. For the reaction of
Rh(oxinate)(CO)2 with P(OPh)3 the optical signal at
340 nm was monitored. The data were collected as sets
of 512 points. They all involved pseudo-first order
conditions with the phosphines and phosphites (PR3) in
large excess over the rhodium complexes. The calcula-
tion of rate constants was based on a numerical integra-
tion of the family of simultaneous differential equations
from Eqs. 1 and 2 (3 and 4) and nonlinear least squares
regression to the experimental curves (see Fig. 1) which
was carried out with the Scientist data analysis
package.
Financial support by the ‘Osterreichischer Akademis-
cher Austauschdienst’ is gratefully acknowledged (Pro-
ject 6/99). The authors also thank Marcin Drag, MSc,
for the preparation of samples for the kinetic measure-
ments and Drs Ryszard Grzybek and Grzegorz Ma*lecki
for the synthesis of pyrrolylphosphines.
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