Synthesis and characterization of new rhodium and iridium complexes with trianisylphosphine, PAn3, and evaluation of their catalytic behavior in the homogeneous hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1016/j.molcata.2008.11.013

A new family of rhodium and iridium compounds with the bulky tris(ortho-methoxyphenyl) phosphine (PAn₃) was synthesized and characterized by NMR methods. The X-ray crystal structures of RhCl(PAn₃)(COD) (1) and Ir[(PBz₃)(PA

Full text of DOI:10.1016/j.molcata.2008.11.013

Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
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Editor Choice Paper
Synthesis and characterization of new rhodium and iridium complexes with
trianisylphosphine, PAn3, and evaluation of their catalytic behavior in the
homogeneous hydrogenation of cinnamaldehyde
Vanessa R. Landaetaa,∗, Francisco López-Linaresb, Roberto Sánchez-Delgadoc,
Claudio Bianchinid, Fabrizio Zanobinid, Maurizio Peruzzinid,∗
a Universidad Simón Bolívar, Departamento de Química, Valle de Sartenejas, Baruta, Aptdo. 89000, Caracas 1080-A, Venezuela
b Schulich School of Engineering, University of Calgary, Alberta, Canada
c Chemistry Department, Brooklyn College and The Graduate Center of The City University of New York, Brooklyn, NY 11210, USA
d Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10, Polo Scientifico, 50019 Sesto Fiorentino (FI), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of rhodium and iridium compounds with the bulky tris(ortho-methoxyphenyl) phos-
phine (PAn3) was synthesized and characterized by NMR methods. The X-ray crystal structures of
RhCl(PAn3)(COD) (1) and Ir[(PBz3)(PAn3)(COD)]PF6 (4) have been determined. A stabilizing agostic inter-
action has been crystallographically observed in both compounds, due to the steric hindrance of the
ortho-substituted phosphine ligand and its presence has been associated to the fluxional behavior shown
by the complexes on the NMR timescale in solution. Iridium complexes containing PBz3 and/or PAn3
have been evaluated as catalyst precursors for the hydrogenation of trans-cinnamaldehyde (CNA), and
their activities have also been compared to those of other iridium complexes containing bulky phosphine
ligands, such as PTol3 (tris-ortho-tolyl-phosphine). The catalytic experiments show that irrespectively of
the phosphine combination, all of the evaluated catalysts prevalently hydrogenate the C C moiety. How-
ever, the product selectivity can be tuned by changing either the substrate/catalyst ratio or the phosphine
ligand at the metal center. The catalyst with PAn3 proved to be more efficient and also gave higher yields
of the enol product, indicating that stereoelectronic effects are responsible for the changes in selectivity.
© 2008 Elsevier B.V. All rights reserved.
Received 16 September 2008
Accepted 11 November 2008
Available online 21 November 2008
Keywords:
Hydrogenation
Trans-cinnamaldehyde
Iridium and rhodium complexes
Stereoelectronic properties
Phosphines
1. Introduction
occur not only through the phosphorus atom, but also through
at least one of the oxygen atoms from the methoxy groups [4,6].
The decoration of phenyl rings with different substituents in aro-
matic phosphine ligands has become one of the most effective ways
to tune the electronic and steric properties of these ligands and to
get the most out of catalytic systems [1,2]. Methoxy (OCH3) sub-
stituents have played a major role in these modifications [2–9],
specially when these are performed in ortho position, since they
can electronically tune the properties of the ligand as well as create
important steric effects.
Several reports [3–8] have appeared regarding the coordina-
tion chemistry of methoxyphenyl phosphines with more than
one substituent in the aromatic ring. As an example, it is worth
mentioning the in-depth studied family of ligands derived from
2,6-dimethoxyphenyl groups, MDMPP, BDMPP and TDMPP [3–6]
(Fig. 1), which show multiple hapticity since the coordination may
Also, low coordination numbers can be stabilized using this kind
of ligands as well as the more encumbering trisubstituted TMPP
[6].
Chiral methoxyphenyl phosphine ligands have been reported
[7,8] and among these, DIPAMP (Fig. 1) is the most success-
ful case [7]. Also, interesting synthetic methodologies have
been developed to obtain methoxy-substituted phosphine lig-
ands with polar groups [9]. However, regardless of the interesting
properties observed with these ligands, the simpler tris(ortho-
methoxyphenyl)phosphine, hereafter tris(ortho-anisyl)phosphine
(PAn3), has received only little attention [10–14]. Among miscella-
neous results, comparative studies on the basicity of PAn3 and other
phosphines have been carried out by evaluating the formation of
ionic pairs of gallium (III) with PR3 [10]. The reactivity of a number
of tertiary phosphine ligands, including PAn3, towards (Ph2Se2I2)2
has been reported [11] while some (ortho-anisyl)phosphides were
also described [12].
∗
Corresponding author. Tel.: +58 212 9063984; fax: +58 212 9063961.
E-mail addresses: vlandaeta@usb.ve (V.R. Landaeta), mperuzzini@iccom.cnr.it
The coordination chemistry of PAn3 has attracted scarce atten-
tion, essentially confined to M(CO)6 derivatives (M = Cr, Mo, W)
(M. Peruzzini).
1381-1169/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2008.11.013

2
V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
Fig. 1. Examples of methoxyphenylphosphines from the literature [4,6,7].
Table 1
[13] and gold complexes [14]. To the best of our knowledge, only
one rhodium(I) compound with PAn3, namely RhCl(PAn3)(COD),
COD = 1,5-cyclo-octadiene, has been briefly described [15] without
any crystallographic data. Also, one Rh(II) species containing PAn3
has been reported [16]. In contrast, no iridium/PAn3 complex has
been mentioned and no catalytic study involving PAn3 complexes
has been reported.
Herein, we report the synthesis and characterization of
a family of rhodium and iridium complexes with tris(ortho-
anisyl)phosphine (PAn3) and describe the solid state struc-
ture of two exemplificative species like RhCl(PAn3)(COD) and
[Ir(PBz3)(PAn3)(COD)]PF6. The catalytic properties of some of these
species in the hydrogenation of trans-cinnamaldehyde (CNA) have
also been studied, since the production of allylic alcohols like cin-
namyl alcohol or cinnamol has importance due to their potential
use as building blocks in organic synthesis [17]. Finally, a compar-
ison of the catalytic properties of Ir/PAn3 precursors with respect
to other complexes bearing other bulky phosphine ligands, such as
PBz3 [18] or PTol3 is reported.
Summary of Crystal Data for RhCl(PAn3)(COD) (1) and [Ir(PBz3)(PAn3)(COD)]PF6 (4).
1
4
Formula
Mol. wt.
Cryst size (mm)
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
ˇ (degree)
V (Å3)
Z
RhC29H33PClO3
598.88
0.65 × 0.35 × 0.45
Monoclinic
P21/n
8.559 (3)
18.046 (2)
17.004 (3)
90.144 (19)
2626.4(11)
4
C50H54F6IrO3P3
1102.04
0.52 × 0.20 × 0.13
Triclinic
P
−1
10.1516 (19)
20.521 (4)
24.595 (3)
100.728 (14)
5034.1 (16)
4
1.454
2.808
2216
2.04–22.98
dcalc (mg/m3)
Abs. coeff. (mm−1
F(0 0 0)
1.515
0.842
1232
2.26–24.97
)
ꢀ range (degree)
Index ranges
−10 ≤ h ≤ 10
0 ≤ k ≤ 21
0 ≤ l ≤ 20
−11 ≤ h ≤ 10
0 ≤ k ≤ 22
0 ≤ l ≤ 27
Tot. number of data
4778
7152
2. Results and discussion
4617
[R(int) = 0.0196]
6966
Number of unique data,
I ≥ 2ꢁ(I)
S (goodness of fit on F2)
1.024
0.0310
0.0700
0.339, −0.334
0.851
0.0481
0.1317
1.290, −0.773
2.1. Synthesis and characterization of MCl(PAn3)(COD) (M = Rh,
Ir)
R1 (I ≥ 2ꢁ(I))
wR2 (all)
Largest diff. peak and hole (e/Å3)
The binuclear complexes [MCl(COD)]2 (M = Rh, Ir) react with
two equivalents of PAn3 under mild conditions (CH2Cl2, room tem-
perature), to give bright yellow crystals of the neutral complexes
MCl(PAn3)(COD) [M = Rh (1), Ir (2)] in high yield (>90%) (Scheme 1).
Compounds 1 and 2 are air-stable in the solid state and are soluble
in polar organic solvents.
Table 2
Selected bond distances (Å) and angles (◦) for RhCl(PAn3)(COD) (1) and
[Ir(PBz3)(PAn3)(COD)]PF6 (4).
1
4
An alternative lower yield synthesis of compound
already been reported by Tiburcio et al. [15]. Crystals of
1 has
Bond distances
Rh(1)–C(1)
Rh(1)–C(2)
Rh(1)–C(3)
Rh(1)–C(4)
Rh(1)–P(1)
Rh(1)–Cl(1)
1
2.108(3)
2.131(3)
2.156(4)
2.184(4)
2.3548(9)
2.3803(11)
Ir(1)–C(4)
Ir(1)–C (2)
Ir(1)–C(3)
Ir(1)–C(1)
Ir(1)–P(2)
Ir(1)–P(1)
2.189(10)
2.211(10)
2.193(11)
2.212(11)
2.349(3)
2.361(2)
suitable for an X-ray diffraction analysis were grown from a
dichloromethane/ethanol solution. An ORTEP drawing showing the
structure of compound 1 is given in Fig. 2. A summary of crystal data
and a selection of bond distances (Å) and angles (◦) are collected in
Tables 1 and 2.
Bond angles
Complex 1 crystallizes in a monoclinic system with a P21/n space
group. The molecular structure is a strongly distorted square pla-
nar one, reflecting the presence of the bulky phosphine ligand. The
P(1)–Rh(1)–Cl(1)
C(1)–Rh(1)–C(2)
C(1)–Rh(1)–C(3)
C(2)–Rh(1)–C(3)
C(1)–Rh(1)–C(4)
C(2)–Rh(1)–C(4)
C(3)–Rh(1)–C(4)
C(1)–Rh(1)–P(1)
C(2)–Rh(1)–P(1)
C(3)–Rh(1)–P(1)
C(4)–Rh(1)–P(1)
C(1)–Rh(1)–Cl(1)
C(2)–Rh(1)–Cl(1)
C(3)–Rh(1)–Cl(1)
C(4)–Rh(1)–Cl(1)
89.25(4)
38.11(15)
95.14(16)
80.75(15)
80.18(16)
89.17(15)
36.54(17)
95.87(10)
93.82(10)
154.18(15)
169.28(14)
155.69(12)
165.30(11)
90.25(12)
90.45(12)
C(4)–Ir(1)–C(2)
C(4)–Ir(1)–C(3)
C(2)–Ir(1)–C(3)
C(4)–Ir(1)–C(1)
C(2)–Ir(1)–C(1)
C(3)–Ir(1)–C(1)
C(4)–Ir(1)–P(2)
C(2)–Ir(1)–P(2)
C(3)–Ir(1)–P(2)
C(1)–Ir(1)–P(2)
C(4)–Ir(1)–P(1)
C(2)–Ir(1)–P(1)
C(3)–Ir(1)–P(1)
C(1)–Ir(1)–P(1)
P(2)–Ir(1)–P(1)
89.0(4)
35.5(4)
79.2(4)
78.7(5)
35.8(5)
90.5(5)
91.5(3)
159.8(4)
89.5(3)
162.8(4)
163.9(4)
90.3(3)
159.0(4)
91.4(3)
94.67(9)
Scheme 1.

V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
3
structure of the rhodium analogue in which one of the methoxy
groups and one of the ortho protons of different phenyl rings are
closer to the metal. Selective homonuclear irradiation experiments
also confirmed the assignment of each of the protons in species 1
and 2.
2.2. Reactivity of the neutral complexes MCl(PAn3)(COD) (M = Rh,
Ir)
Removal of the chloride ligand from 1 or 2 can be readily
achieved either by reaction with silver salts in the presence of
appropriate ligands or by ligand substitution using an excess of a
strong ␴-donor such as pyridine (py) in the presence of NH4PF6.
Thus, treatment of 1 or 2 dissolved in acetone at room temper-
ature with silver triflate followed by addition of one equivalent
of PBz3, led to the formation of cationic mixed phosphine ligands
derivatives of formula [M(PAn3)(PBz3)(COD)]PF6 [M = Rh (3); Ir (4)]
(Scheme 2). Even simpler is the reaction of 1 or 2 with pyridine,
which straightforwardly results in the formation of the pyridine
adducts [M(PAn3)(py)(COD)]PF6 [M = Rh (5); Ir (6)] (Scheme 2).
The cationic mixed phosphine ligands 3 and 4 are air and mois-
ture stable solids. Upon reaction of the parent compounds 1 and
2 with Ag+ and addition of another equivalent of PAn3, we were
unable to isolate the bis-PAn3 complexes. It is clear that, in the coor-
dination sphere of rhodium or iridium, the presence of the sterically
bulky PAn3 together with the chelating ligand COD does not allow
two very large phoshine ligands to coordinate simultaneously at
the Rh(I) or Ir(I) metal centers in cis-disposition [21,22]. This con-
sideration may be safely expanded to PCy3 since, to the best of our
knowledge, no bis-PCy3 has been reported in the literature so far for
related square planar derivatives. However, a slightly less bulky lig-
and such as PBz3 (ꢀ = 165), is still able to coordinate to the rhodium
Fig. 2. ORTEP drawing of the complex RhCl(PAn3)(COD) (1).
space group is identical to the ones found for the analogous com-
plexes IrCl(PBz3)(COD) previously reported by us [18], and to other
analogous MCl(PAr3)(COD) species [19–21]. A variety of Rh(I) and
Ir(I) complexes with sterically demanding phosphine ligands (PCy3,
PPri3, PBz3) exhibit a s structure similar to that of compounds 1 and
2 [18–20].
The spectroscopic analysis of compound 1 is in keeping with
1
the data reported in the literature [15]. The 31P{ H} NMR of 2 in
CD2Cl2 shows a singlet at 13.34 ppm. The 1H NMR spectrum of 2
shows a set of 6 signals due to the magnetically inequivalent pro-
tons of the COD ligand, two pairs of CH protons and two sets of
diasterotopic CH2 protons. The aromatic protons of the phosphine
ligand appear, at room temperature, as one doublet of doublets,
two pseudo triplets and one broad signal centered at ı 7.75. As the
temperature decreases (Fig. 3), the broad aromatic resonance coa-
lesces while broadening of both the other aromatic resonances and
the methoxy groups signal also occurs. At 233 K decoalescence of
the aromatic signals occurs with new resonances rising at lower
fields and three new methoxy signals are distinguishable. At 183 K
the fluxional process is practically frozen and three methoxy sin-
glets have appeared while the originally broad aromatic resonance
is resolving into a doublet of doublets. This behavior is attributed
to a dynamic process, due to the demanding steric requirements of
PAn3 (cone angle, ꢀ = 194◦). The broad aromatic signal observed at
room temperature is likely ascribable to one of the ortho protons
of the phenyl rings in PAn3, which owing to the steric congestion
approaches the metal thus creating an agostic interaction. At room
temperature, fast ligand twisting around the M–P bond, forces one
of the three ortho protons of PAn3 to be close to the metal, and
results in a fluxional process on the NMR timescale. Lowering the
temperature slows down the exchange until at 183 K only one of the
three ortho protons is close to the metal thus appearing as a dou-
blet of doublets, due to the coupling with both the proton closer to
it and the phosphorus atom. This lack of equivalence at low temper-
atures is confirmed by the resolution of the methoxy signals into
three different singlets, one of them at lower fields than the other
two which are embedded in a more similar chemical environment.
A similar behavior is observed for compound 1 upon variation of
the temperature, and can also be confirmed from the solid state
Fig. 3. 1H NMR spectra of IrCl(PAn3)(COD) (2) at variable temperature (200.13 MHz,
CD2Cl2).

4
V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
The solution structure proposed for complexes 3 and 4 has been
confirmed in the case of 4, by X-ray crystallography, and the ORTEP
diagram of such species is shown in Fig. 4. A summary of crystal data
and a selection of bond distances (Å) and angles (◦) are collected in
Tables 1 and 2.
Complex 4 crystallizes in a triclinic system (space group P−1),
and exhibits a strongly distorted square planar geometry due to
the presence of two cis disposed bulky phosphine ligands. Ir–P
bond distances of ca. 2.35 Å and 2.36 Å were found for Ir–P(1) and
Ir–P(2), respectively, which are similar to each other and in the
range of distances found for this kind of complexes [20–22]. Also,
the P(1)–Ir–P(2) angle is higher than 90◦, due to steric repulsion
between the bulky phosphine ligands [P(1)–Ir–P(2) = 94.7◦].
Upon reaction with strong ␴-donors such as pyridine and
NH4PF6, which contains the bulk and weakly coordinating hex-
afluorophosphate counterion in polar solvents, e.g. methanol,
cationic complexes of formula [M(py)(PAn3)(COD)]+ (M = Rh, 5;
Ir, 6) could be isolated as air and moisture stable solids. Com-
plexes 5 and 6 are reminiscent of both the well-known Crabtree’s
catalyst [Ir(py)(PCy3)(COD)]PF6 [20] and the previously reported
PBz3 compounds [M(py)(PBz3)(COD)]PF6 (M = Rh; Ir) [18]. The two
complexes were characterized by NMR spectroscopy and exhibit a
fluxional behavior in solution at room temperature likely caused
by the steric crowding at the metal center due to the presence
of the bulky phosphine ligand PAn3. Rising the temperature to
50 ◦C narrows the 31P{ H} NMR signals for both complexes, par-
1
Scheme 2.
ticularly for compound 5 where the doublet due to 103Rh coupling
(JRhP = 153.9 Hz), typical of a Rh(I) compound is clearly discernable
[23].
or iridium centers even when a PAn3 ligand is already bonded to
the metal, thus allowing species 3 and 4 to be isolated as stable
solid derivatives. Several examples of bis-phosphine cationic Rh(I)
or Ir(I) complexes have been reported [18–22].
2.3. Reaction of [IrCl(COD)]2 and PAn3 with H2
Species 3 and 4 were completely characterized by both multi-
nuclear NMR spectroscopy and FT-IR. Additionally, for the iridium
analogue, crystals suitable for an X-ray diffraction analysis were
grown from a diluted dichloromethane/ethanol solution.
In an attempt to determine whether the coordination to iridium
of two PAn3 species was possible, the iridium dimer [IrCl(COD)]2
was treated with four equivalents of PAn3 in a coordinating sol-
vent such as acetone-d6 (NMR-tube test). Only compound 2 was
As has already been described for the neutral complexes 1 and
2, a fluxional behavior is observed due to the steric hindrance
identified by 31P{ H} NMR spectroscopy. However, when a H2
1
around the metal center. The 31P{ H} NMR shows, at room temper-
1
ature, two broad singlets for 4 and two broad doublets for 3 due to
103Rh coupling with coupling constants typical of Rh(I) complexes
(JRhPAn3 = 136.9 Hz; JRhPBz3 = 155.3 Hz) [23]. Magnetic inequivalence
can also be observed from the 1H NMR spectra of both complexes,
which show the expected signals for the COD and the aromatic pro-
tons and broad signals for the benzyl CH2 and OMe anisyl proton
resonances.
Scheme 3.
Fig. 4. ORTEP drawing of the complex [Ir(PBz3)(PAn3)(COD)]PF6 (4).
Scheme 4.

V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
5
stream was passed through the solution cooled to ca. 0 ◦C, NMR sig-
nals ascribable to two new phosphine containing species could be
identified. The first of them could be assigned to the bis(hydride)-
bis(acetone)-bis(PAn3) Ir(III) product [Ir(H)2(acetone)2(PAn3)2]PF6
(7) (Scheme 3), analogous to other previously reported solvate dihy-
drides of tertiary phosphine ligands [18,24]. The other species (8)
1
shows two doublets in the 31P{ H} NMR (−32.5 and −40.1 ppm,
JPP = 43.2 Hz). Although the solution structure of complex 8 could
not be unequivocally assigned, the experimental evidence suggests
the presence of a chelating PAn3 ligand, similar to that found for
metal complexes of MDMPP, BDMPP or TDMPP ligands, in which the
phosphine coordinates through both the phosphorous and one or
more oxygen atoms [4]. Further evidence is required to substantiate
this putative structural hypothesis.
2.4. Homogeneous hydrogenation of trans-cinnamaldehyde.
Evaluation of iridium-bulky phosphine precursors
The selective hydrogenation of ␣,␤-unsaturated aldehydes is
an important process for the production of useful chemicals for
the fragrance and pharmaceutical industries [25]. Several reports
have appeared in which homogeneous [26], heterogeneous [27]
and biphasic [28] systems have been tested, probing the impor-
tance of this reaction. It has been shown [26] that the catalytic
synthesis of the saturated alcohol is favored, since it is easier to
hydrogenate C C double bonds compared to C O aldehyde moi-
eties [29]. A number of ruthenium and rhodium monophosphine
complexes have shown good activities and variable selectivity [29]
while their iridium counterparts generally show lower activities but
better selectivities [30].
The expected catalytic hydrogenation pathway for trans-
cinnamaldehyde (CNA) to the corresponding products, the
saturated aldehyde 3-phenyl propanal (PPAL), the allylic alcohol
cinnamol (CNOL), and the saturated alcohol 3-phenyl-1-propanol
(PPOL) is shown in Scheme 4.
Fig. 5. Hydrogenation of trans-cinnamaldehyde using [Ir(py)(PBz3)(COD)]PF6
as catalyst precursor. T = 80 ◦C; pH2 = 17.1 bar; catalyst = 1 mol%; toluene = 50 mL;
time = 6 h.
PPAL reaches values in the range of 25–50% in 6 h. However, at
100 ◦C, a maximum yield of PPAL is obtained in 4 h (60%), followed
by a decrease in the yield of this product, since the hydrogenation of
the C O moiety becomes more predominant. Therefore, a consec-
utive hydrogenation to yield PPAL followed by its transformation
into PPOL is proposed.
The screening of sterically hindered phosphine ligand com-
plexes was carried out initially, using the known complex
[Ir(py)(PBz3)(COD)]PF6 as catalyst precursor [18], which has been
previously evaluated by some of us for the hydrogenation of imines
[31]. The reaction conditions employed were: T = 80 ◦C; 17.1 bar of
H2 and a catalyst load of 1 mol%. The reaction profile obtained is
shown in Fig. 5. This catalyst precursor is able to convert CNA under
the studied conditions, in moderate yields, to the following product
distribution: 50.2% PPAL, 1.40% CNOL and 20.5% PPOL. No decar-
bonylation products were detected (e.g. allylbenzene), indicating
that the system works exclusively for hydrogenation in our exper-
imental conditions. This precursor reduces mainly the C C moiety
instead of the C O bond, which would indicate that the saturated
The low yields of CNOL (Fig. 8) are not notably affected by the
reaction temperature. For PPOL, the results displayed in Fig. 9 show
aldehyde (PPAL) is produced in a first stage followed by the C
O
reduction of the aldehyde, to produce the saturated alcohol, PPOL.
No decomposition of the catalyst precursor was observed and, in
fact, the homogeneity of the reaction was proven through the mer-
cury test [32].
The effect of the reaction temperature was also studied, with
temperatures ranging between 40 ◦C and 100 ◦C. These results are
summarized in Fig. 6. As can be observed, a rise in the reaction
temperature promotes the conversion of the substrate, reaching a
maximum yield of 96% at 100 ◦C. This linearity is confirmed since,
at lower temperatures, the conversion decreases considerably, and
at 40 ◦C, only 44% of the substrate is transformed.
However, it was also observed that rising the reaction temper-
ature not only increases the amount of converted CNA, but also
affects the product distribution. Figs. 7–9 show the effect of chang-
ing the reaction temperature on the yields and product distribution.
From Fig. 7, we may conclude that the yield of PPAL increases pro-
gressively with the temperature. Below 100 ◦C, the production of
Fig. 6. Effect of the reaction temperature on the hydrogenation of trans-
cinnamaldehyde using [Ir(py)(PBz3)(COD)]PF6 as catalyst precursor. T = 80 ◦C;
pH2 = 17.1 bar; catalyst = 1 mol%; toluene = 50 mL; time = 6 h.

6
V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
Fig. 9. Yield of 3-phenyl-1-propanol (PPOL) as a function of the reaction temperature
in the hydrogenation of trans-cinnamaldehyde using [Ir(py)(PBz3)(COD)]PF6 as cata-
lyst precursor. T = 80 ◦C; pH2 = 17.1 bar; catalyst = 1 mol%; toluene = 50 mL; time = 6 h.
Fig. 7. Yield of 3-phenyl-propanal (PPAL) as a function of the reaction temperature in
the hydrogenation of trans-cinnamaldehyde using [Ir(py)(PBz3)(COD)]PF6 as catalyst
precursor. T = 80 ◦C; pH2 = 17.1 bar; catalyst = 1 mol%; toluene = 50 mL; time = 6 h.
hydrogen pressure does not affect the selectivity or the rate of the
conversion.
that the effect of the reaction temperature is more predominant at
100 ◦C, achieving 52% conversion after 6 h of reaction. Below this
temperature, there is practically no conversion to the saturated
alcohol.
The effect of the hydrogen pressure was also studied. The conver-
sion of CNA does not suffer practically any modification by varying
the H2 pressure. At 9.5 bar, 44% conversion is achieved, in compari-
son with 72.1% at 17.1 bar, and at 30.6 bar, only 52% of the substrate
is transformed. In any case, C C bond hydrogenation prevails over
the reduction of the C O moiety, indicating that modification of the
The effect of the substrate/catalyst ratio (S/C) on the conversion
of CNA was also studied, and the results are summarized in Table 3.
As can be observed, the conversion decreases from 87.2% to 28.0%
as the substrate:catalyst ratio increases. The product distribution
is also affected, showing that at low S/C ratios, the yield of PPAL
is about 50.0%, while the production of the saturated alcohol PPOL
reaches its maximum value of 36.4%. Furthermore, when the S/C
ratio changes from 50 to 200 the major product is the saturated
aldehyde PPAL and the production of PPOL decreases considerably.
Finally, at the highest S/C ratio studied, the conversion decreases
considerably, but the product distribution still favors the produc-
tion of PPAL. Some changes in selectivity also occur, showing that
the S/C ratio orients the C C and C O hydrogenation. At S/C ratios
of 50 and 100, the hydrogenation of C C double bond prevails over
Table 3
Effect of the catalyst load on the hydrogenation of trans-cinnamaldehyde using
[Ir(py)(PBz3)(COD)]PF6 as catalyst precursor.
Entry CNA:cat.
ratio
Conversion (%) Product distribution (%)
Selectivity
PPAL/CNOL
PPAL CNOL PPOL
1
2
3
4
50
100
200
300
87.2
72.1
47.3
28.0
47.6
50.2
33.6
18.5
3.30
1.40
4.50
3.50
36.4
20.5
9.10
6.00
14.6
36.0
7.50
5.30
T = 80 ◦C; pH2 = 17.1 bar; toluene = 50 mL; time = 6 h.
Table 4
Effect of the solvent on the hydrogenation of trans-cinnamaldehyde using
[Ir(py)(PBz3)(COD)]PF6 as catalyst precursor.
Entry
Solvent
Product distribution (%)
PPAL
CNOL
PPOL
1
2
3
Toluene
THF
50.2
2.10
6.40
1.40
0.30
0.00
20.5
0.00
0.00
MeOHa
Fig. 8. Yield of cinnamol (CNOL) as a function of the reaction temperature in the
hydrogenation of trans-cinnamaldehyde using [Ir(py)(PBz3)(COD)]PF6 as catalyst
precursor. T = 80 ◦C; pH2 = 17.1 bar; catalyst = 1 mol%; toluene = 50 mL; time = 6 h.
T = 80 ◦C; pH2 = 17.1 bar; catalyst = 1 mol%; time = 6 h.
a
Slight decomposition observed.

V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
7
Table 5
Effect of the variation of the phosphine ligand in the hydrogenation of trans-cinnamaldehyde using [Ir(py)(PAr3)(COD)]PF6 as catalyst precursors.
Entry
Ar
Cone angle (ꢀ)
Conversion (%)
Product distribution (%)
C
C/C O ratio
PPAL
CNOL
PPOL
a
1
2
3
o-OCH3C6H4
–CH2C6H5
o-CH3C6H4
194
165
195
99.5
72.1
67.8
57.5
50.2
54.0
1.40
1.40
5.00
41.1
20.5
8.80
10.3
36.0
41.3
T = 80 ◦C; pH2 = 17.1 bar; toluene = 50 mL; catalyst = 1 mol%; time = 6 h.
a
Slight decomposition observed.
the C O bond (entries 1 and 2). When this ratio increases to values
of S/C of 200 or 300, C O hydrogenation becomes more important.
This suggests that the modification of the catalyst concentration
can be used to modify the product selectivity in this reaction.
The effect of the solvent was also studied, using toluene,
methanol and THF. The results are presented in Table 4. As can
be observed, the solvent has a strong influence on the activity
as well as in the product selectivity. In toluene 72.1% of conver-
sion is achieved and the main product is the saturated aldehyde
PPAL (entry 1). In THF (entry 2) the reaction is practically sup-
pressed. Probably a THF ligand coordinates to the iridium center,
blocking the vacant site required to coordinate the substrate dur-
ing the hydrogenation step. In fact, mechanistic studies previously
performed by some of us [18,31] indicate that under a H2 stream
a THF-d8 solution of [Ir(COD)(py)(PBz3)]PF6 generates a dihydrido-
solvento complex [Ir(H)2(THF)2(py)(PBz3)]PF6. On the other hand,
when MeOH is used, the conversion of CNA is close to 80.0% but the
product analysis reveals that only 6.40% of the PPAL is produced
and a new product, 1,1-dimethoxy-benzenepropane, is formed in
74.5%. This indicates that an acetalyzation of the aldehyde is pro-
moted [33]. To figure out whether the metal complex leads to the
formation of this new product, a blank experiment was carried
out, showing that under these conditions, trans-cinnamaldehyde
is the only product obtained. When the metal complex is present,
it promotes C C double bond hydrogenation, originating CNA. The
absence of 1,1-dimethoxy-benzenepropane led us to the conclusion
that the substrate is hydrogenated to CNA, which in a successive
step reacts with MeOH to give the corresponding acetal.
a stronger base (pKb = 4.47 [10b]) due to the presence of OCH3 donor
groups, has a different electronic character as compared to the other
ligands used, explaining the lower C C/C O ratio obtained.
3. Conclusions
A new family of rhodium and iridium complexes with PAn3 has
been obtained. These complexes are straightforwardly prepared
using simple procedures, and are air and moisture stable. A fluxional
behavior in solution has been observed for all complexes due to the
high steric hindrance at the metal centers, which was confirmed in
the solid state by X-ray diffraction studies.
A screening of the hydrogenation reaction of cinnamaldehyde
(CNA) was performed, using a series of cationic [Ir(py)(PAr3)(COD)]+
(PAr3 = PAn3, PBz3, PTol3) complexes indicating that all of the cat-
alyst precursors preferentially hydrogenate the C C double bonds
instead of the C O bond. Steric and electronic properties of the
phosphine ligands in the iridium catalyst precursor have been found
to influence both the activity and selectivity with the PAn3 complex
showing superior activity and C C/C O selectivity.
4. Experimental
4.1. General information
All reactions and manipulations were routinely performed
under dry nitrogen or argon atmosphere using standard Schlenk
1
techniques. 1H and 13C{ H} NMR spectra were recorded either on a
Bruker ACP 200 (200.13 and 50.32 MHz), a Bruker AM 300 (300.13
and 75.47 MHz) or a Bruker Avance 500 (500.13 and 125.80 MHz)
spectrometers. Peak positions are relative to tetramethylsilane and
were calibrated against the residual solvent resonance (1H) or
A
comparative study within the series of complexes
[Ir(py)(PAr3)(COD)]PF6 (PAr3 = PAn3 (6); PBz3 [18], PTol3 [34]) has
also been undertaken with the aim of studying the effect of phos-
phine ligands with variable steric and electronic characteristics
[35]. These results are summarized in Table 5.
1
the deuterated solvent multiplet (13C). 31P{ H} NMR spectra were
Replacing PBz3 (cone angle 165◦) with PAn3, makes the conver-
sion of CNA almost quantitative (entry 2 vs. entry 1), even though
some catalyst decomposition was observed at the end of the reac-
tion. Using PTol3 (ꢀ = 194◦), slightly decreases the activity compared
to the PBz3 complex (entry 3 vs. entry 2). A change in selectivity
is also observed. The PBz3 compound hydrogenates the C C bond
36 times more selectively than the C O bond. With PTol3, even
if the steric hindrance at the metal center is high and compara-
ble to that of PAn3, the hydrogenation of CNA occurs, converting
mainly the C C double bond (C C/C O = 41.3) and practically all of
the PPAL generated is transformed into the saturated alcohol PPOL.
Confirmative evidence for assessing whether the steric hindrance is
responsible for the selectivity towards the olefin can be addressed
by using the PAn3 complex. This system is selective also towards the
recorded on the same instruments operating at 81.01, 121.49, and
202.53 MHz, respectively. Chemical shifts were measured relative
to external 85% H3PO4, with downfield shifts considered positive.
All the NMR spectra were recorded at room temperature (25 ◦C)
unless otherwise stated. Homonuclear decoupling experiments
were carried out on a Bruker ACP 200 (200.13 MHz). Infrared spec-
tra were recorded either on a PerkinElmer 1600 series or a Nicolet
Magna IR 560 FT-IR spectrometers, using samples mulled in Nujol
between KBr plates or in KBr disk. Elemental analyses (C, H, N) were
performed using a Carlo Erba model 1106 elemental analyzer by the
Microanalytical Service of the Department of Chemistry at the Uni-
versity of Florence (I). Reactions under controlled gas pressure were
performed on Parr reactors.
C
C moiety, but the ratio C C/C O changes from 36 to 10.3, with a
4.2. Materials
low production of the saturated alcohol PPOL, but a slightly higher
yield of CNOL as compared to the PBz3 and PTol3 systems. These
results suggest that a combination of electronic and steric proper-
ties of the phosphine ligands might be responsible for the changes
in the activities and selectivities observed. Although sterically dif-
ferent, PBz3 and PTol3 are electronically very similar and this is
probably associated with the C C/C O ratios obtained. PAn3 being
Unless otherwise stated, all solvents were distilled just prior
to use from appropriate drying agents. Methanol was dis-
tilled from Mg(OMe)2, dichloromethane and acetonitrile from
CaH2, and tetrahydrofuran (THF) from sodium/benzophenone.
Diethylether and petroleum ether were dried with sodium. Hydro-
gen was purified passing it through two columns in series

8
V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
1
31P{ H} NMR (CD2Cl2, 25 ◦C, 202.5 MHz): ı 13.6 (br s); ı −10.8 (br
containing CuO/Al2O3 and CaSO4, respectively. Deuterated sol-
vents were dried over activated 4 Å molecular sieves prior to
use. Trans-cinnamaldehyde (CNA) was purified by distillation
under reduced pressure. All other chemicals were commer-
cial products and used as received without further purification.
Literature methods were employed for the synthesis of tris(ortho-
anisyl)phosphine (PAn3) [36], tribenzylphosphine (PBz3) [18],
[RhCl(COD)]2 [37], [IrCl(COD)]2 [38], [Ir(py)(PBz3)(COD)]PF6 [18],
and [Ir(py)(PTol3)(COD)]PF6 [34]. A modification of the method
reported in the literature [15], using dichloromethane as solvent
and crystallizing from ethanol/dichloromethane, was followed for
the synthesis of RhCl(PAn3)(COD) (1), which led to an improved
yield (90%), and the possibility to grow suitable crystals to perform
X-ray diffraction. The solid complexes were collected on a sintered
glass-frit and washed with ethanol and light petroleum ether (bp
40–60 ◦C) or n-pentane before being dried in a stream of nitrogen.
GC–MS analyses were performed using a HP 5890 SERIES II PLUS
with FI detector and a Megabore type capillary column, 15 m (DB-5
phase; 1.5 u FT, J and W Scientific) with the following temperature
program: 100 ◦C/3 min–10 ◦C/min–150 ◦C/6 min.
s); ı −143.0 (sept, JPF = 713).
4.6. [Rh(py)(PAn3)(COD)]PF6 (5)
A suspension of 1 (250 mg; 0.42 mmol) in methanol (20 mL),
under stirring, was treated with an excess of pyridine (0.20 mL,
2.53 mmol) for 60 min. NH4PF6 (150 mg; 0.92 mmol) was added.
Concentration under vacuum separated a yellow solid which after
filtration was washed with water (5 mL), methanol (5 mL) and
diethyl ether (5 mL). The solid was vacuum dried. Yield: 285 mg;
87%. 1H NMR (CDCl3, 50 ◦C, 300.13 MHz): ı 2.06 (d, 4H, CH2 COD);
ı 2.54 (br s, 4H, CH2 COD); ı 3.58 (s, 9H, OCH3 PAn3); ı 4.03 (br s,
2H, CH COD); ı 4.54 (br s, 2H, CH COD); ı 6.86 (dd, 3H, aromatic
PAn3); ı 6.94 (t, 3H, aromatic PAn3); ı 7.06 (t, 3H, aromatic PAn3);
ı 7.43 (m, 7H, aromatics PAn3 and py); ı 8.30 (d, 2H, aromatic py).
1
31P{ H} NMR (CDCl3, 50 ◦C, 121.5 MHz): ı 12.25 (d, JRhP = 153.9); ı
−142.9 (sept, JPF = 713).
4.7. [Ir(py)(PAn3)(COD)]PF6 (6)
4.3. IrCl(PAn3)(COD) (2)
A suspension of 2 (250 mg; 0.73 mmol) in methanol (20 mL),
under stirring, was treated with an excess of pyridine (0.18 mL,
2.28 mmol) for 60 min. NH4PF6 (150 mg; 0.92 mmol) was added.
Concentration under vacuum separated a shiny orange solid which
after filtration was washed with water (5 mL), methanol (5 mL) and
diethyl ether (5 mL). The solid was vacuum dried. Yield: 280 mg;
88%. 1H NMR (CDCl3, 25 ◦C, 300.13 MHz): ı 1.84 (br d, 4H, CH2 COD);
ı 2.34 (m, 4H, CH2 COD); ı 3.44 (br s, 11H, 2H COD y 9H–OCH3);
ı 4.08 (br s, 2H, CH COD); ı 6.78 (br s, 3H, aromatic PAn3); ı 6.99
(br s, 3H, aromatic PAn3); ı 7.17 (t, 3H, aromatic PAn3); ı 7.40 (t,
4H, aromatic PAn3 and py); ı 7.54 (td, 2H, aromatic py); ı 8.27 (d,
[IrCl(COD)]2 (500 mg; 0.74 mmol) and PAn3 (540 mg;
1.50 mmol) were dissolved in dichloromethane (15 mL) and
vigorously stirred for 30 min. Upon addition of ethanol and
concentration, a yellow-orange solid precipitated. The solid was
filtered off and washed with ethanol and n-pentane. Yield: 0.91 g;
89%. 1H NMR (CDCl3, 25 ◦C, 500.13 MHz): ı 1.57 (m, 4H, CH2 COD);
ı 1.69 (m, 4H, CH2 COD); ı 2.18 (m, 2H, CH COD); ı 4.82 (br s, 2H,
CH COD); ı 3.50 (s, 9H, –OCH3); ı 6.86 (dd, 3H, aromatic); ı 6.97
(t, 3H, aromatic); ı 7.37 (t, 3H, aromatic); ı 7.72 (t, 3H, aromatic).
1
31P{ H} NMR (CDCl3, 25 ◦C, 121.5 MHz): ı 13.34 (s). IR (KBr):
1
2H, aromatic py). 13C{ H} NMR (CDCl3, 25 ◦C, 75.5 MHz): ı 29.4
ꢂ(Ir–Cl) 506 cm−1 (m). Anal. Calc. for C29H33PO3ClIr: C, 50.61; H,
(CH2 COD); ı 31.7 (CH2 COD); ı 54.8 (–OCH3); ı 66.9 (CH COD
trans to py); ı 85.3 (d, CH COD trans to P); ı 111.0 (CH aromatic);
ı 116.0 (C aromatic); ı 120.4 (d, CH, aromatic ipso, JCP = 10.5 Hz); ı
124.9 (CH aromatics); ı 132.4 (CH aromatic); ı 135.6 (C aromatic);
ı 137.1 (CH para py); ı 150.0 (CH meta py); ı 159.7 (CH ortho py).
4.83. Found: C, 49.61; H, 4.73%.
4.4. [Rh(PAn3)(PBz3)(COD)]PF6 (3)
An acetone solution of 1 (500 mg; 0.83 mmol) was treated
with AgOSO2CF3 (290 mg; 1.78 mmol), under vigorous stirring for
30 min. The solution was filtered out and PBz3 (280 mg; 0.92 mmol)
was added, and the solution turned orange. NH4PF6 (275 mg;
1.69 mmol) and ethanol (20 mL) were added, and concentration
under a brisk current of nitrogen precipitated a yellow solid,
which after filtration was washed with ethanol and n-pentane,
and vacuum dried. Yield: 750 mg; 89%. 1H NMR (CD2Cl2, 25 ◦C,
300.13 MHz): ı 1.87 (m, 4H, CH2 COD); ı 2.40 (m, 4H, CH2 COD);
ı 2.96 (br s, 3H, OCH3 PAn3); ı 3.50 (br s, 3H, OCH3 PAn3); ı 3.87
(d, 6H, CH2 PBz3, JHP = 9.5 Hz); ı 3.94 (br s, 3H, OCH3 PAn3); ı 5.49
1
31P{ H} NMR (CDCl3, 25 ◦C, 121.5 MHz): 9.63 (s); −143.03 (sept,
JPF = 712).
4.8. In situ reaction of [IrCl(COD)]2 with PAn3 and H2 in
acetone-d6
[IrCl(COD)]2 (30 mg; 0.045 mmol) and PAn3 (33 mg;
0.094 mmol) were dissolved in acetone-d6 (1 mL) and the solution
was introduced into a 5 mm NMR tube under an inert atmosphere.
After the tube was cooled to 0 ◦C, H2 was gently bubbled into
1
the solution for 5 min. 1H and 31P{ H} NMR spectra showed the
(br s, 4H, CH COD); ı 7.00–7.63 (m, 27H, aromatic). 31P{ H} NMR
1
formation of [Ir(H)2(acetone)2(PAn3)2]+ (7). All our attempts to
isolate 7 by scaling up the reaction were unsuccessful. In all cases,
products without hydride ligands or a mixture of unidentified
products were obtained.
(CD2Cl2, 25 ◦C, 202.5 MHz): 23.9 (br d, JRhP = 155.3); ı −2.1 (br d,
JRhP = 136.9); ı −143.0 (sept, JPF = 713).
4.5. [Ir(PAn3)(PBz3)(COD)]PF6 (4)
4.8.1. Selected NMR data for 7
An acetone solution of 2 (500 mg; 0.73 mmol) was treated
with AgOSO2CF3 (200 mg; 0.77 mmol), under vigorous stirring for
30 min. The solution was filtered out and PBz3 (240 mg; 0.79 mmol)
was added. NH4PF6 (250 mg; 1.53 mmol) and ethanol (20 mL) were
added, and concentration under a brisk current of nitrogen pre-
cipitated a shiny red solid, which after filtration was washed with
1H NMR (acetone-d6, 0 ◦C, 200.13 MHz): ı −31.6 (t, hydrides,
1
JHP = 16.0 Hz). 31P{ H} NMR (acetone-d6, 0 ◦C, 81.01 MHz): ı 17.93
(s).
4.9. X-ray diffraction studies
ethanol and n-pentane, and vacuum dried. Yield: 650 mg; 81%. 1
H
NMR (CD2Cl2, 25 ◦C, 300.13 MHz): ı 1.26 (m, 4H, CH2 COD); ı 1.84
(br s, 3H, OCH3 PAn3); ı 2.20 (br s, 4H, CH2 COD); ı 2.58 (br s, 3H,
OCH3 PAn3); ı 2.84 (br s, 3H, OCH3 PAn3); ı 3.30 (br s, 6H, CH2
PBz3); ı 4.26 (br s, 4H, CH COD); ı 6.97–7.64 (m, 27H, aromatic).
Summary of crystal data and structure refinement parame-
ters for RhCl(PAn3)(COD) (1) and [Ir(PBz3)(PAn3)(COD)]PF6 (4) are
reported in Table 1. Selected distances and angles for both com-
pounds are summarized in Table 2.

V.R. Landaeta et al. / Journal of Molecular Catalysis A: Chemical 301 (2009) 1–10
9
4.9.1. RhCl(PAn3)(COD) (1)
Acknowledgements
Crystallographic data for
1 were collected on a CAD4
diffractometer using graphite monochromated Mo K␣ radiation
(ꢃ = 0.7107 Å) at room temperature. A set of 25 carefully centered
reflections in the range 7◦ < ꢀ < 9◦ was used for determining the
lattice constants. As a general procedure, the intensity of three
standard reflections were measured periodically every 200 reflec-
tions for orientation and intensity control. This procedure revealed
an 8% decay of intensities during the data collection period, for
which the intensities were corrected. The data were corrected for
Lorentz and polarization effects. Atomic scattering factors were
those tabulated by Cromer and Waber [39] with anomalous disper-
sion corrections taken from reference [40]. An empirical absorption
correction was applied via scan with correction factors in the
range 0.8401–0.8977. The computational work was carried out
using the program SHELX97 [41]. All non-hydrogen atoms were
refined with anisotropic displacement parameters (adps). Hydro-
gens atoms were geometrically placed and allowed to ride on their
parent C atom with Uiso(H) = 1.2Ueq(C). Idealized C–H distances
were fixed at 0.93 Å (for C–H in phenyl groups), 0.97 Å (for C–H
in CH2 groups) and 0.98 Å (C–H in the 1,5-cyclo-octadiene group).
The authors thank FONACIT (Venezuela) for financial support to
V.L. (Ph.D. Fellowship and Project S3), FONACIT and CNR (Italy) for
an International Cooperation Grant. We also thank Mr. Dante Masi
(CNR) for collecting the X-ray diffraction data.
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