Iridium(I) complexes of upper rim functionalized PTA derivatives. Synthesis, characterization, and use in catalytic hydrogenations (PTA = 1,3,5-Triaaza-7-phosphaadamantane)

Article abstract of DOI:10.1021/om1011177

Reaction of (1,3,5-triaza-7-phosphaadamantane-6-yl)lithium (1; PTA-Li), synthesized in high yield and purity, with p-(dimethylamino)benzaldehyde gave the novel water-soluble β-phosphino alcohol (4′-(dimethylamino) phenyl)(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol (2; PZA-NMe₂). The oxide O=PZA-NMe₂ (3) and sulfide S=PZA-NMe₂ (4) were also obtained. Ir(I) complexes of (SR,RS)-PZA-NMe₂ and the analogue phenyl(1,3,5-triaza-7- phosphatricyclo[3.3.1.1]dec-6-yl)methanol (PZA) were obtained and used as precatalysts for the hydrogenation of α,β-unsaturated aldehydes and ketones and acetophenone using different mild reduction protocols. The X-ray crystal structure of the SR,RS diastereoisomer of PZA-NMe₂ was also obtained and is described herein.

Full text of DOI:10.1021/om1011177

ARTICLE
pubs.acs.org/Organometallics
Iridium(I) Complexes of Upper Rim Functionalized PTA Derivatives.
Synthesis, Characterization, and Use in Catalytic Hydrogenations
(PTA = 1,3,5-Triaaza-7-phosphaadamantane)
Antonella Guerriero,† Mikael Erlandsson,† Andrea Ienco,† Donald A. Krogstad,‡ Maurizio Peruzzini,*,†
Gianna Reginato,† and Luca Gonsalvi*,†
†Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10,
50019 Sesto Fiorentino (Firenze), Italy
‡Concordia College at Moorhead, 334F Ivers, Moorhead, Minnesota 56562, United States
S Supporting Information
b
ABSTRACT: Reaction of (1,3,5-triaza-7-phosphaadamantane-
6-yl)lithium (1; PTA-Li), synthesized in high yield and purity,
with p-(dimethylamino)benzaldehyde gave the novel water-
soluble β-phosphino alcohol (40-(dimethylamino)phenyl)
(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol (2;
PZA-NMe2). The oxide OdPZA-NMe2 (3) and sulfide
SdPZA-NMe2 (4) were also obtained. Ir(I) complexes of (SR,RS)-PZA-NMe2 and the analogue phenyl(1,3,5-triaza-7-
phosphatricyclo[3.3.1.1]dec-6-yl)methanol (PZA) were obtained and used as precatalysts for the hydrogenation of R,β-unsaturated
aldehydes and ketones and acetophenone using different mild reduction protocols. The X-ray crystal structure of the SR,RS
diastereoisomer of PZA-NMe2 was also obtained and is described herein.
binding groups and/or chiral centers, has not yet been fully
identified and developed.
1. INTRODUCTION
The increasing need to replace old technologies with sustain-
able approaches,1 including minimization of large quantities of
organic solvents and waste, has prompteda renewed interest in the
search for alternative media for chemical transformations. In
particular, the use of water as an alternative solvent has witnessed
a true renaissance in the past few years, often in homogeneous
transition-metal-catalyzed processes.2 The first issue to be ad-
dressed in the quest for a water-soluble homogeneous catalyst is to
design a versatile structure able to convey the electronic and steric
properties of known active organometallic complexes into water.
The resulting precatalyst, and corresponding activated species,
must therefore be stable in water in order to bring about the
desired catalytic transformation with high activities and selectiv-
ities. This would allow for its recycling and reuse by simple phase
separation in the case of biphasic water/organic solvent media.
All these issues have been addressed by many research groups,
each proposing different ways to obtain water-soluble catalysts.3
The most common approach is to modify known ligands, such as
During the past few years, there has been a renewed interest in
the use of the neutral water-soluble cage monodentate phosphine
PTA (PTA = 1,3,5-triaza-7-phosphaadamantane)4 and its struc-
tural modifications.5 A large number of coordination compounds
have been synthesized, and their catalytic,6 medicinal,7 and
electrochemical properties8 have been investigated, together with
mechanistic aspects of Ru-PTA mediated hydrogen activation in
water.9 The easiest way tomodify PTA is toalkylate phosphorus or
nitrogen atoms10,11 or to open the cage, giving potentially
bidentate P,N12 or tridentate P,N,N13 derivatives. More recently,
the bidentate phosphine PTA-PPh2, containing a binding side arm
and a chiral center on the CR to P, was obtained by Frost and co-
n
workers by reaction of PTA with BuLi and ClPPh2.14 The key
intermediate, (1,3,5-triaza-7-phosphaadamantane-6-yl)lithium (1;
PTA-Li), was isolated as a highly pyrophoric compound, and the
subsequent reaction with the chlorophosphine was carried out in a
DME slurry at room temperature, resulting in only a 10% yield of
the final compound after workup (Scheme 1).
Later on, the Frost group15 and our laboratory16 indepen-
dently reported on the reaction of 1 with aryl aldehydes15,16 and
ketones,15 yielding a series of water-soluble β-phosphino alco-
hols (PTA-CRR0OH: R = C6H5, C6H4OCH3, ferrocenyl; R0 = H,
C6H5, C6H4OCH3). Some ligands were used to coordinate
(bidentate) phosphines and amines, amino alcohols, and their
-
combinations, by introducing polar charged groups, i.e. SO3
,
NR3þ, CO2-, and phosphonates.2c This in turn increasesthe water
solubility due to solvation and separate ion couple formation.
In spite of the great amount of literature data published, a class
of ligands having a molecular scaffold which is conceptually
simple, versatile in its coordination to transition metals, and easy
to handle and to functionalize, for example by introducing other
Received: November 29, 2010
Published: March 09, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis and Coordination Properties of PTA Upper Rim Derivatives
Scheme 2. Synthesis of PTA-Li (1), PZA, and PZA-NMe2 (2) and Their Ir(I) Complexes
Ru(II) arene moieties,15 whereas phenyl(1,3,5-triaza-7-phos-
phatricyclo[3.3.1.1]dec-6-yl)methanol (PZA) was used to bind
the Ir(III) pentamethylcyclopentadienyl core.16 In all cases a κ1P
η1 binding mode was observed, as shown in Scheme 1.
the reaction mixture as an analytically pure microcrystalline
powder after addition of BuLi (Scheme 2). The solid can be
therefore recovered by filtration under nitrogen and stored for
subsequent use. We noted that, although all efforts were made to
exclude moisture from the Schlenk tubes used for storage, the
batch should be used within 1 week to ensure the highest purity
of the reagent.
n
Herein, we describe an improved method for the synthesis of 1
and new derivatives of PTA upper rim functionalization, namely
the novel phosphino alcohol (40-(dimethylamino)phenyl)(1,3,5-
triaza-7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol, (2; PZA-
NMe2) and its oxide OdPZA-NMe2 (3) and sulfide SdPZA-
NMe2 (4) together with Ir(I) complexes of PZA and PZA-NMe2.
The latter were used as precatalysts for catalytic hydrogenations
of model substrates including linear R,β-unsaturated aldehydes
and ketones, namely cinnamaldehyde (CNA) and benzylidenea-
cetone (BZA), and cyclic species such as 2-cyclohexen-1-one and
simple aryl alkyl ketones (acetophenone). Different reduction
protocols were employed, specifically transfer hydrogenation
(TH) in the presence of either HCO2Na/H2O or KOH/iPrOH
or in a combined hydrogenation/transfer hydrogenation proto-
col (H2 30 bar/tBuOK/iPrOH) run at room temperature.17
Subsequent reaction of 1 with electrophiles such as benzalde-
hyde was carried out as previously described: i.e., by addition of
the aldehyde to a suspension of 1 in THF at -78 °C, followed by
quenching with ice-cold water. This afforded PZA as a mixture of
two diastereoisomers having comparable solubility properties in
water and other common organic solvents, as shown by solubility
tests and 31P{1H} spectra (D2O, two singlets at -103.4 and
-106.6 ppm, 1:1 ratio).16
We have previously shown16 in the case of the oxide OdPZA
that solubility can be pivotal to diastereoisomer separation by
simple crystallization from suitable solvent mixtures. For this
compound, the PdO oxygen atom forms a strong intermolecular
hydrogen bond with the hydroxyl group of a neighboring PZA-
(O) molecule (O OH distance 2.717(6) Å), giving a mono-
3 3 3
2. RESULTS AND DISCUSSION
dimensional infinite chain in the solid state. This could account
for the poor solubility of the SR,RS diastereoisomer. Thus, we
reasoned that subtle changes in the PZA framework by introduc-
tion of substituents on the phenyl ring could in principle modify
the hydrogen-bonding network and henceforth change the
solubility properties of the two diastereoisomers, without sig-
nificantly affecting the donor properties of the P atom of the cage.
Therefore, the new moderately water-soluble (S(H2O)20 °C
= 1.9 g/L) phosphino alcohol ligand (40-(dimethylamino)phenyl)
(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol (2;
PZA-NMe2) was synthesized by replacing benzaldehyde with
4-(dimethylamino)benzaldehyde. 31P{1H} NMR spectra of the
crude reaction mixture in CDCl3 showed the presence of two
singlets in a 3:1 ratio at -102.72 (major, 2a) and -106.27
(minor, 2b). As expected, the major diastereoisomer 2a was
isolated after fractional crystallization and characterized by
single-crystal X-ray diffraction, showing that 2a corresponds to
the SR,RS diastereoisomer (see section 2.2).
2.1. Synthesis and Characterization of PTA Derivatives.
Following our initial interest in PTA structural modifications, we
considered the use of (1,3,5-triaza-7-phosphaadamantane-6-
yl)lithium (1; PTA-Li) as a valuable intermediate to obtain
multidentate, water-soluble, enantiomerically enriched ligands
for transition-metal complexes. In order to improve on the yields
and purity of 1 and, in turn, of the ligands derived by its reaction
with electrophiles, we introduced slight but important changes in
the synthetic protocol14 originally comprising the reaction of
PTA with nBuLi in THF at -78 °C. Due to the limited solubility
of PTA under such conditions, the reaction suffered from the
main disadvantages of being carried out under heterogeneous
conditions and cumbersome workup being needed to separate
the products from omnipresent PTA after quenching with water.
We found that when the lithiation reaction was carried out in
warm benzene (ca. 30 °C), PTA was completely soluble under
such conditions, and the lithium derivative 1 precipitated out of
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Figure 1. ORTEP view of 2a. Ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å) and angles (deg) for 2a:
P1-C1 = 1.8695(16); P1-C2 = 1.859(2); P1-C3 = 1.8503(19); C1
-N1 = 1.476(2); C2-N2 = 1.476(3); C3-N3 = 1.470(2); C7-O1
= 1.420(2); C1-C7 = 1.531(2); C7-C1-P1 = 111.62(11); N1-C1
-C7 = 112.86(13).
Figure 2. (a) Intermolecular OH N hydrogen bonding in 2a, with a
view along the a axis. (b) Packing within the asymmetric unit, with a view
along the b axis parallel to hydrogen bonds.
In solution, the loss of symmetry of the PTA cage upon
functionalization of the 6-position was evident from the magnetic
inequivalence of the methylene protons in the 1H NMR
spectrum of 2a, with an AB multiplet centered at 4.85 ppm
(2JH H = 13.4 Hz) and a multiplet in the range 4.57-4.37
3 3 3
Figure 2a. The π π stacking of the (dimethylamino)phenyl
3 3 3
A
B
groups holds together two of the 1D chains. The latter supramo-
lecular units arrange themselves in the space, as seen from the
packing diagram (Figure 2b) along the b axis. The existence of a
hydrogen bond 1D chain was also found in OdPTA-CH-
(C6H4OCH3)OH15 and OdPTA-CH(C6H5)OH (OdPZA),16
while two OdPTA-C(C6H4OCH3)2OH and four PTA-C-
(C6H4OCH3)2OH units form an hydrogen bonded dimer and
tetramer, respectively. Interestingly, while the intermolecular
hydrogen-bonding network is established between the PdO
oxygen and the hydroxyl group of a neighboring OdPZA
molecule, for 2a this occurs by connecting the hydroxyl OH
group with a N atom of the adamantane cage.
ppm corresponding to the remaining four NCH2N protons. A
similar situation was observed for the upper rim protons appear-
ing as three overlapped groups of signals between 4.29 and 3.80
ppm. In keeping with the loss of the adamantane symmetry, the
13C{1H} NMR spectrum of 2a showed two signals at 74.20 and
67.78 ppm for the NCH2N carbon atoms and three doublets at
1
65.75 (PCHN) and 51.55 and 48.43 ppm (PCH2N) with JCP
values ranging from 19.9 to 24.0 Hz.
The main problem encountered in the syntheses of PZA and
PZA-NMe2 was the tedious workup, involving column chroma-
tography. This dramatically decreased the overall yields but was
needed to obtain analytically pure compounds. This procedure
was indeed mandatory for the following use as ligands toward
Ir(I) precursors, particularly in the case of PZA (see section 2.3).
The syntheses of OdPZA-NMe2 (3) and SdPZA-NMe2 (4)
were carried out straightforwardly by reacting 2a with either
aqueous H2O2 at room temperature or S8 under reflux condi-
tions. Selective protection (oxidation) of the P atom was
achieved in both cases, as evidenced by the presence of a singlet
in the 31P{1H} NMR spectra of the reaction mixtures, at 2.09 and
-14.07 ppm, respectively, at values expected from comparison
with the spectra for PTA4 and PZA.16
2.3. Synthesis and Characterization of Ir(I) Complexes of
PZA and PZA-NMe2. The coordination chemistry of PTA and its
derivatives with Ir has been far less investigated than for other late
transition metals relevant to homogeneous catalysis such as Ru,
Rh, and Pd.5 To date, only one Ir(0),18 four Ir(I),6f,19 and six
Ir(III) PTA16,20 complexes have been reported, and some of
these last six were used as catalysts for the hydrogenation of
bicarbonate to formate.21
On the basis of the limited amount of Ir complexes of PTA and
derivatives in the literature and in view of the possible applications
of such compounds as catalysts for reactions in water phase or
biphasic water/organic solvent,22 we thought it timely to explore
the coordination chemistry of PZA and (SR,RS)-PZA-NMe2 with
Ir(I). Both ligands were therefore reacted in a 2:1 ratio with the
dimeric precursor [Ir(cod)Cl]2 in either CH2Cl2 or CH2Cl2/
toluene at room temperature, to obtain the complexes
[Ir(cod)Cl(PZA)] (5) and [Ir(cod)Cl{(SR,RS)-(PZA-NMe2)}]
(6) in moderate to good yields after simple workup (Scheme 2).
Whereas the racemic mixture containing 2a as a single diaster-
eoisomer was used to synthesize 6, complex 5 was obtained as a
racemic mixture of four enantiomers containing both diastereoi-
somers present in PZA. Since all our attempts to grow suitable
crystals to determine the solid-state structure of 5 and 6 had failed
thus far, the proposed formulas were demonstrated by elemental
2.2. X-ray Crystal Structure of (SR,RS)-(40-(Dimethyla-
mino)phenyl)(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]dec-6-
yl)methanol (2a). The X-ray structure of 2a contains both SR and
RS enantiomers in the unit cell. Figure 1 give an ORTEP drawing
with selected distances and angles in the caption. As previously
noted for other upper rim PTA compounds,15,16 the P1-C1 bond
(1.8695(16) Å) is slightly longer than the other two P-C bonds
(P1-C2 = 1.859(2) Å, P1-C3 = 1.8503(19) Å), while the C-N
bonds of the adamantane cage do not differ significantly.
The hydroxyl group forms a strong intramolecular hydrogen
bond with a nitrogen atom of a neighboring molecule
(O1 N3#1 = 2.7821(19) Å, where the symmetry transforma-
3 3 3
tion used to generate the equivalent atom is x, -1 þ y, z). A 1D
infinite chain can be recognized along the b axis, as shown in
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Scheme 3. Proposed Mechanism for the Switch from a κ1P to a κ2P,O Coordination Mode in 6
analysis and solution NMR and ESI-MS data. Complex 6 exhibited
a singlet at -64.09 ppm in the 31P{1H} NMR spectrum (one
diastereoisomer), while the 1H NMR signals corresponding to the
η4-coordinated cyclooctadiene were partially overlapping the
PCH2N methylene signals and were partially identifiable in the
range 2.25-1.68. The 13C{1H} NMR data are more diagnostic for
this ligand, in particular the CH signals at 98.07 (d, 2JCP = 12.8 Hz)
and 97.13 (d, 2JCP = 13.7 Hz), again reflecting the lack of symmetry
in the molecule. The NMR characterization is slightly more
complicated for 5, due to the presence of two diastereoisomers
giving 31P{1H} NMR singlets at -64.97 and -58.50 ppm, but is
substantially similar to the case for 6. The C, H, N elemental
analysis and ESI-MS data (m/z 564.2 [Ir(cod)(PZA)]þ, 605.3
[Ir(cod)(PZA-NMe2)]þ) confirm the presence of (labile) chlor-
ide ligand and support the proposed formulas. Conductivity data
were obtained for 6 in various solvents. As expected, in CH2Cl2 the
complex was not substantially conductive in nature, whereas partial
chloride dissociation may be possible in polar solvents such as
MeOH and H2O, however, giving low conductivity values (0.032
and 0.064 mS/cm, respectively).
protonation-deprotonation intermolecular equilibrium could
be possible in this case (Scheme 3).
DFT theoretical calculations were used to verify the possible
replacement of Cl with an OH group on the ligand for 5 and 6
and the energies associated with a protonation-deprotonation
intermolecular equilibrium. The effects of the substituent of the
phenyl group on the geometrical parameters was found to be
relatively small, and for sake of conciseness only the results
relevant to 6 are discussed here. Several models were considered.
In particular the structures related to 6a, a model of 6 with κ1P
coordination mode of PZA-NMe2 and 6b, having a κ2P,O
bonding mode of the ligand, were optimized in the gas phase.
Regarding the intermolecular equilibrium, there are in principle
four nitrogen atoms (N1-N4, Scheme 4) that could act as
proton acceptors for the hydroxyl deprotonation and conse-
quently four models (6c-f) should be considered. On a closer
look (Scheme 4), two of them (6d,e), namely with protonation at
the N2 and N3 nitrogen atoms, are equivalent; thus, only 6d was
optimized. Moreover, in the discussion of the structures only 6c
is considered, as the main geometrical features of the optimized
species 6c,d,f were found to be quite similar.
The lability of the chloride ligand in polar solvents and the
presence of phosphorus and oxygen donor atoms on PZA and
PZA-NMe2 could, in principle, favor the formation of κ2P,O
chelate analogues of 5 and 6 in solution. In order to verify this
possibility, complex 6 was reacted with a chloride scavenger
(TlPF6) in CH2Cl2 at room temperature. The initial yellow
solution immediately turned red. After 1 h of stirring, TlCl was
removed by filtration and an aliquot of the solution was analyzed
by 31P{1H} NMR, showing a singlet at -17.04 ppm and a septet
at ca. -146 ppm (PF6). The low-field NMR shift from -64.1ppm
due to 6 to -17.04 ppm can be attributed to both the formation
of a cationic species such as [Ir(cod)(PZA-NMe2)]þ and the
Δ-ring contribution upon κ2P,O coordination.23 To further
validate this hypothesis, 2a was reacted in a 1:1 ratio with the
Ir(I) precursor [Ir(cod)2]PF6 in CH2Cl2 at room temperature.
After 2 h of stirring, the 31P{1H} NMR spectrum confirmed
the presence of the singlet at -17.04 ppm. Interestingly, when
the solution was left standing for 24 h, the singlet at -17.04 ppm
decreased in intensity and a new signal at -53.4 ppm appeared,
for a final 1:5 ratio between the two species on the basis of NMR
data.24 In the absence of coordinating solvents which could
stabilize κ1P cationic derivatives, equilibria involving different
κ2P,O species may be active. In particular, due to the relative
acidity of the OH proton and the basicity of the N atoms in the
triazacyclohexane bottom rim on the PTA skeleton, a
In the conformers 6a-c, reported in Figure 3, the Ir atoms
have a square-planar coordination environment. The Ir-C
distances trans to the phosphine ligand are slightly longer than
those trans to chlorine or oxygen atoms (average 2.21 Å vs 2.13 Å
and 2.26 Å vs 2.14 Å for 6a,b, respectively) while the CdC bonds
trans to P (1.40 Å) are shorter than those trans to Cl and O atoms
(1.43 Å) in all the optimized models. The metal back-donation is
more effective for the CdC bonds trans to O and Cl atoms, but
the substitution of the oxygen atom with chlorine did not have an
influence on CdC distances. The Ir-P distance is calculated at
2.31 Å in 6b (2.33 Å for 6c), while it was 2.35 Å in 6a. The
shortening of the M-P distance is a consequence of the ring-
closing strain. The latter effect can also be measured by compar-
ing the Cl-Ir-P (90.1°) and O-Ir-P (78.4 and 81.5° for 6b,c)
angles and the Ir-P-C1 angles (114.5, 102.7, and 98.6° for 6a-
c, respectively).
Compounds 6b-f were examined in an effort to identify the
most likely protonated species of the intermolecular equilibrium.
The energy differences calculated in CH2Cl2 solution show that
6c is more stable (3.7 kcal mol-1) relative to 6f, which is expected
considering amines substituted by aromatics have basicities lower
than those substituted by alkyl groups. It is also noteworthy that the
energy difference between 6c and 6d is 3.8 kcal mol-1. The latter
value has to be compared with the difference (ΔG = 0.1 kcal mol-1
)
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Scheme 4. Possible Structures 6a-f Deriving from Protonation and κ1P to κ2P,O Coordination Switch in 6
between the N1 and N2 protonated species. Finally, the hydroxyl 6b
complex is more stable than 6c (2.9 kcal mol-1). These results
support the hypothesis of the possible presence of an N-
protonated zwitterionic complex (6c) and also discriminate
between the nitrogen atoms of the ligand as the preferred
protonation site.
2.4. Catalytic Hydrogenations of Aldehydes and Ketones
under Mild Conditions. Few examples of catalytic applications
of Ir complexes bearing PTA or derivatives as ancillary ligands
can be found in the literature. The iridium compounds
[Ir(cod)(PTA)3]Cl, [IrCl(CO)(PTA)3], and [Ir(CO)(PTA)4]
Cl were found to be active for the cyclization of 4-pentyn-1-amine
to 2-methylpyrroline in water at 50 °C.6f [Cp*IrCl(PTA)2]Cl was
tested as a catalyst for CO2/HCO3- reduction to formate, in the
absence of base additive and under mild conditions, inthepH range
5.3-10.5 and in the temperature range 30-100 °C. The complex
Figure 3. Optimized structures for the isomers κ1P (6a), κ2P,OH (6b)
and κ2P,O (6c). Color code: oxygen, red; nitrogen, blue; phosphorus,
pink; chlorine, green; iridium, purple (large sphere); carbon, black
(small sphere); hydrogen, white.
showed good activity at elevated temperatures and in slightly basic
aqueous solution (pH 9), forming the corresponding cationic
[Cp*IrH(PTA)2]þ complex as the catalytically active species.21
Ir-catalyzed chemo- and enantioselective hydrogenations25 and
transfer hydrogenations26 of compounds of industrial interest such
as alkenes, ketones, aldehydes, and R,β-unsaturated carbonyl
compounds in water or biphasic systems are known, using
bidentate ligands such as 4,40-dihydroxy-2,20-bipyridine,27
Noyori’s (R,R)-TsDPEN and derivatives, ((R,R)-TsDPEN
= (1R,2R)-N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine)),28
(1S,2R)-(þ)-norephedrine and (1R,2S)-(-)-ephedrine,29 and
other systems such as [Ir(cod)Cl]2 in combination with HSA
(HSA = human serum albumin),30 to name just a few. No Ir-PTA
complexes have so far been reported to perform these transforma-
tions. More attention has been paid to Ru and Rh compounds such
as cis-RuCl2(PTA)4,31 trans-[RuI4(mPTA)2], [RuI2(mPTA)3
(H2O)]I3, and [RhI4(mPTA)2]I (mPTA = N-methyl-PTA),32
and half-sandwich Ru-PTA complexes bearing ancillary ligands such
as Cp, Cp*,33 Dp, and Ind (Cp = η5-cyclopentadienyl; Cp* = η5-
pentamethylcyclopentadienyl; Dp = η4-dihydrocyclopentadie-
nyl; Ind = η5-indenyl),6a,34 which were observed to promote
catalytic hydrogenations of alkenes, unsaturated aldehydes,
ketones and other substrates under either transfer hydrogenation
or hydrogen pressure in water or biphasic systems.5 In all cases,
although promising, these water-phase systems rarely match the
performance obtained by the most efficient Ru-based catalysts
showing very high turnover frequencies (TOFs).35 The need is
indeed for mild protocols which can be applied using water as
solvent and the tolerance to different functional groups, resulting
in high chemoselectivities.2
We tested complexes 5 and 6 as catalysts for the chemoselective
hydrogenation of model substrates including linear R,β-unsatu-
rated aldehydes and ketones (CNA, BZA), cyclic species (2-
cyclohexen-1-one), and simple aryl alkyl ketones (acetophenone)
using different reduction protocols (Scheme 5).36
Sodium formate is one of the mildest reducing agents and
affords a wide compatibility between functional groups and water
solubility. In order to ensure complete solubility of all reagents
and formation of a homogeneous phase at moderate tempera-
tures, it is often required to add a variable percentage of an organic
solvent, usually an alcohol. Thus, transfer hydrogenation (TH) of
CNA using HCO2Na was carried out in water/methanol at
different temperatures and variable catalyst/CNA ratios.
A black precipitate, likely bulk metal, was observed upon
running the tests at 80 and 60 °C, suggesting that the complexes
were not stable under the conditions applied. This assumption
was supported by the fact that phosphine oxide 3 was the only
P-containing species observed by 31P NMR analysis of an aliquot
of the reaction mixture. The conversions observed at 6 h, 80 °C
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Scheme 5. Hydrogenation Test Reactions Involving R,β-Unsaturated Aldehydes, Ketones, and Acetophenone
Table 1. Transfer Hydrogenation of CNA with 5 and 6 using HCO2Na/H2O/MeOH
yield (%)
solvent ratio
MeOH/H2O
conversn (%)
(time (h))
entry
precat.
Ab
Bb
Cb
TON (av)
1
2
3
4
5
6
5a
5a,c
6a
6a
6a,c
6a,c
1/2
1/2
1/1
1/1
1/1
1/1
99.4 (8)
68.5 (8)
96.6 (6)
99.3 (24)
57.4 (6)
78.7 (24)
0.7
4.5
3.1
0.6
9.7
7.3
77.6
47.2
78.0
79.0
43.2
61.9
21.1
16.8
15.5
19.7
4.5
99
69
97
99
57
79
9.5
a Conditions: CNA, 0.75 mmol; catalyst, 7.5 ꢀ 10-3 mmol; HCO2Na, 7.5 mmol; total volume of solvents, 6 mL; 40 °C. b GC values based on pure
samples: A = hydrocinnamaldehyde; B = cinnamol; C = 3-phenyl-1-propanol. c One drop of Hg(0) added.
(99.1%, 5; 83.3%, 6), yielding a ca. 2:1 mixture of cinnamol and
3-phenyl-1-propanol, could be therefore due to the formation of
unperceived phosphine oxide stabilized metal nanoparticles or
soluble metal colloids.37 The runs were repeated at 40 °C in order
to test the effect of temperature on the stability of the catalyst and
its performance under milder conditions. In parallel, Hg(0)
poisoning tests were carried out under the same conditions to
establish whether a significant amount of heterogeneously cata-
lyzed reactions were responsible for the results observed.38
The results are summarized in Table 1. It can be seen that,
although cinnamol is always the favored product, a significant
decrease in activity and selectivity is observed in the Hg-poisoning
tests for both catalysts, thus confirming that a mixed heteroge-
neous/homogeneous system is active under these conditions. The
tests were also run at higher catalyst/substrate ratios (1/500 and 1/
1000, respectively), but in this case the competing base-catalyzed
aldol condensation reaction dominates, yielding significant amounts
of dimerization side products, identified by GC-MS analysis.
Transfer hydrogenation of BZA using HCO2Na in water/
methanol at 60 °C in the presence of 6 (1/100 catalyst to
substrate ratio) showed that the reaction gave a maximum
conversion of ca. 55% at 24 h and a ca. 4:1 ratio between saturated
ketone and unsaturated alcohol (Table 2, entries 1 and 2).
For the reduction of 2-cyclohexen-1-one, both the HCO2Na/
H2O/MeOH protocol (Table 2, entries 3 and 4) and the more
commonly used KOH/iPrOH method (Table 2, entries 7 and 8)
were applied in the presence of 6, using catalyst/substrate ratios
of 1/100 and 1/500, respectively. In the first case, hydrogenation
was selective for cyclohexanone, with a maximum conversion of
67% after 24 h. In contrast, after 24 h at 80 °C the stronger
reducing agent KOH brings about the reduction to cyclohexanol
(71% yield) with TON = 440. Acetophenone was also reduced to
1-phenylethanol using KOH/iPrOH under the same conditions
with a maximum TON = 472 (Table 2, entry 12). Interestingly,
in these runs the decrease in activity associated with Hg(0)
poisoning is less important than in the case of CNA reduction
(Table 1).
Finally, the hydrogenation protocol already applied to
(substituted) acetophenones in our laboratories,17 namely tBuO-
K/iPrOH under hydrogen pressure (30 bar) at room tempera-
ture, was tested for the reduction of acetophenone and
2-cyclohexen-1-one in the presence of 6 (Table 3).
Blank runs (no catalyst, entry 4; no base, entry 5) showed that
acetophenone was not reduced without the presence of both a
catalyst and a base. [Ir(cod)Cl]2 was chosen for control runs and
performed poorly (ca. 20% conversion at 4 h) in comparison to 6
(78% conversion, 4 h, TON = 195), which gave almost complete
conversion (94%) after 6 h. Reduction of 2-cyclohexen-1-one in
the presence of 6 reached 60% conversion, yielding hydrogena-
tion products with a ketone/alcohol ratio of 2.5.
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Table 2. Transfer Hydrogenation of BZA, 2-Cyclohexen-1-
one, and Acetophenone with 6 using either the HCO2Na/
H2O/MeOH or the KOH/iPrOH Protocol
the well-known phosphine PTA, were obtained. A combination of
experimental and theoretical data suggest that in polar solvents
κ1P-[IrCl(cod)(L)] may easily displace the chloride ligand to
form the cationic κ2P,O-[Ir(cod)(L)]þ, which in turn may be in
equilibrium with an isomer containing the ligand L in a zwitter-
yield (%)
ionic NHþ O- form. The precatalysts were tested for the
3 3 3
conversn (%)
hydrogenation of R,β-unsaturated aldehydes and ketones, includ-
ing acetophenone, using different mildreductionprotocolssuchas
transfer hydrogenation with sodium formate, the classic iPrOH/
KOH, and a combined hydrogenation/transfer hydrogenation
protocol, run at room temperature under a pressure of 30 bar of
H2 and in the presence of tBuOK as base in iPrOH. When aqueous
conditions were used, Hg(0) poisoning experiments showed that
a mixed homogeneous/heterogeneous system may be active
under the conditions applied for the catalytic tests, especially for
cinnamaldehyde reduction. Current studies are in progress to
obtain more stable water-soluble transition-metal complexes
based on different “upper rim” functionalizations of PTA.
entry
substrate
(time (h))
Ac
Bc Cc TON (av)
1
BZAa
BZAa
25.5 (5)
55.1 (24)
31.0 (5)
67.4 (24)
25.6 (5)
64.4 (24)
83.3 (5)
88.1 (24)
79.6 (5)
87.8 (24)
84.5 (5)
94.5 (24)
83.8 (5)
94.2 (24)
21.1 4.4 0.0
43.7 11.5 0.0
24.6 0.1 0.9
57.9 0.2 6.3
29.7 0.1 1.2
60.0 0.2 7.2
24.9 0.6 57.8
16.3 0.5 71.3
29.5 0.5 49.6
22.0 0.5 65.3
84.5
26
55
2
3
2-cyclohexen-1-onea
2-cyclohexen-1-onea
2-cyclohexen-1-onea,d
2-cyclohexen-1-onea,d
2-cyclohexen-1-oneb
2-cyclohexen-1-oneb
2-cyclohexen-1-oneb,d
2-cyclohexen-1-oneb,d
acetophenoneb
31
4
67
5
26
6
64
7
416
440
398
439
422
472
419
471
8
9
10
11
12
13
14
4. EXPERIMENTAL SECTION
acetophenoneb
acetophenoneb,d
acetophenoneb,d
94.5
83.8
4.1. General Methods and Materials. All synthetic procedures
were carried out using standard Schlenk techniques under an inert atmo-
sphere of dry nitrogen. All glassware was dried overnight in the oven. PTA,4
PZA,16 and [Ir(cod)Cl]239 were prepared as described in the literature. All
solvents were distilled and degassed prior to use according to standard
procedures.40 Doubly distilled water was used. Deuterated solvents and
other reagents were bought from commercial suppliers and used without
further purification. Column chromatographic purification was performed
using glass columns (10-50 mm wide) and silica gel (230-400 mesh
particle size). The 1H, 13C{1H}, and 31P{1H} NMR were recorded on a
Bruker Avance II 300 spectrometer (operating at 300.13, 75.47, and 121.50
MHz, respectively) and a Bruker Avance II 400 spectrometer (operating at
400.13, 100.61, and 161.98 MHz, respectively). Peak positions are relative
to tetramethylsilane and were calibrated against the residual solvent
resonance (1H) or the deuterated solvent multiplet (13C). 31P{1H}
NMR was referenced to 85% H3PO4, with the downfield shift taken as
positive. All of the NMR spectra were recorded at room temperature
(20 °C). IR spectra (KBr pellets) were recorded on a Perkin-Elmer
Spectrum BX II series FT-IR spectrophotometer in the range 4000-
400 cm-1. Elemental analyses were performed on a Perkin-Elmer 2400
series II elemental analyzer. ESI-MS spectra were measured on a LCQ
Orbitrap mass spectrometer (ThermoFischer, San Jose, CA) equipped with
a conventional ESI source by direct injection of the sample solution and are
reported in the form m/z (intensity relative to base 100). Conductivities
were measured with an Orion Model 990101 conductance cell connected
to a Model 101 conductivity meter. GC analyses were performed on a
Shimadzu 2010 gas chromatograph (with polar column) equipped with a
flame ionization detector and a VF-WAXms capillary column (30 m, 0.25
mm i.d., 0.25 μm film thickness), on a Shimadzu GC-14A gas chromato-
graph (with apolar column) equipped with a flame ionization detector and
an SPB-1 Supelco fused silica capillary column (30 m, 0.25 mm i.d., 0.25 μm
film thickness), and on a Shimadzu GC-17A gas chromatograph (with a
chiral column Lipodex-E Macherey-Nagel) equipped with a flame ioniza-
tion detector (50 m, 0.25 mm i.d., 0.40 mm o.d.).
94.2
a Conditions: substrate, 0.75 mmol; catalyst, 7.5 ꢀ 10-3 mmol; HCO2-
Na, 7.5 mmol; MeOH/H2O (1:1), 6 mL. b Conditions: substrate, 4.40
mmol; catalyst, 8.8 ꢀ 10-3 mmol; KOH, 0.88 mmol; iPrOH, 8.8 mL.
c GC values based on pure samples. For BZA: A = 4-phenyl-2-butanone;
B = 4-phenyl-3-buten-2-ol; C = 4-phenyl-2-butanol, T = 60 °C. For
2-cyclohexen-1-one: A = cyclohexanone; B = 2-cyclohexen-1-ol;
C = cyclohexanol, T = 80 °C. For acetophenone: A = 1-phenylethanol,
T = 80 °C. d One drop of Hg(0) added.
Table 3. Hydrogenation of 2-Cyclohexen-1-one and Aceto-
phenone with 6 in iPrOH/tBuOK under a Pressure of H2
yield (%)
conversn (%)
entry
substrate
(time (h))
Ab Bb Cb TON (av)
1
2
3
4
5
6
7
8
9
10
acetophenonea
acetophenonea
acetophenonec
acetophenoned
78.3 (4) 78.3
94.1 (6) 94.1
19.6 (4) 19.6
195
240
47
<1 (4)
<1 (4)
<1
<1
0
acetophenonee
0
acetophenonea,f
2-cyclohexen-1-onea
2-cyclohexen-1-oned
2-cyclohexen-1-onee
2-cyclohexen-1-onea,f
76.2 (4) 76.2
191
150
50
60.1 (4) 42.5 0.9 16.7
19.8 (4) 17.1 0.0 2.7
17.1 (6) 17.1 0.0 0.0
59.5 (4) 36.0 0.4 23.1
43
149
a Conditions: substrate, 1.75 mmol; catalyst, 7.0 ꢀ 10-3 mmol; tBuOK,
0.35 mmol; iPrOH, 2 mL. p(H2), 30 bar; 25 °C. b GC values based on
pure samples. For 2-cyclohexen-1-one: A = cyclohexanone; B = 2-cy-
clohexen-1-ol; C = cyclohexanol. For acetophenone: A = 1-phenyletha-
nol. For cinnamaldehyde: A = hydrocinnamaldehyde; B = cinnamol;
C = 3-phenyl-1-propanol. c As above, catalyst [Ir(cod)Cl]2. d As above,
no catalyst. e As above, no base. f One drop of Hg(0) added.
4.2. Improved Synthesis of (1,3,5-Triaza-7-phosphaada-
mantane-6-yl)lithium (1; PTA-Li). The synthetic protocol described
in the literature14 has been modified. PTA (1.57 g, 10.0 mmol) was
completely dissolved in dry and degassed benzene (160 mL) at 50 °C
with strong stirring. The solution was then cooled to approximately
30 °C, and n-butyllithium solution (1.6 M solution in hexanes, 10 mL, 16
mmol) was slowly added. After a few minutes a white solid was formed
and the reaction mixture was stirred at room temperature for 2 h. The
3. CONCLUSIONS
In summary, examples of coordination to Ir(I) of water-soluble
β-phosphino alcohols such as (SR,RS)-PZA-NMe2 and PZA,
members of the class of “upper rim” derivatized analogues of
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solid was filtered on a frit under nitrogen and washed with dry n-pentane
(3 ꢀ 30 mL). PTA-Li was obtained in quantitative yield (1.63 g). Due to
its highly pyrophoric nature and reactivity with atmospheric moisture, the
solid must be stored under nitrogen and the batch consumed within a few
days. CAUTION! The compound ignites spontaneously if exposed to air and
is very sensitive to moisture. A protective nitrogen atmosphere and gloves must
be used at all times for storage and handling.
4.5. Synthesis of (SR,RS)-(40-(Dimethylamino)phenyl)-
(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]]dec-6-yl)methanol
Sulfide (4; SdPZA-NMe2). In a three-necked round bottomed flask
equipped with a reflux condenser, 2a (0.50 g, 1.6 mmol) was dissolved in
degassed water (150 mL) and heated to 90 °C to completely dissolve the
compound. An excess of elemental sulfur (0.13 g, 4.1 mmol of S) was
added and the suspension refluxed for 90 min with strong stirring. The
reaction mixture was cooled to room temperature and the precipitate
filtered on a B€uchner funnel. The solid was washed with hot CCl4 (2 ꢀ
3 mL) and dried under vacuum, yielding a white solid (0.35 g, 65%).
S(H2O)20 °C = 0.5 g/L. 1H NMR (400.13 MHz, DMSO-d6): δ 7.30 (d,
3JHH = 8.5 Hz, 2H, arom); 6.66 (d, 3JHH = 8.5 Hz, 2H, arom); 5.32 (dd,
4.3. Synthesis of (40-(Dimethylamino)phenyl)(1,3,5-triaza-
7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol,
(2;
PZA-
NMe2). In a 100 mL Schlenk flask, 1 (1.80 g, 11.0 mmol) was suspended
in 30 mL of dry THF and maintained in a liquid nitrogen/acetone bath at
-78 °C under an inert atmosphere of nitrogen. 4-(Dimethylamino)
benzaldehyde (1.80 g, 12.1 mmol) was dissolved in 10 mL of dry THF,
cooled to 0 °C, and transferred via cannula to the slurry. The resulting
yellow mixture was left at -78 °C for 15 min and then warmed to reach
room temperature and stirred for an additional 2 h. A 0.5 mL portion of
doubly distilled water was added to quench the reaction, and the solvent
was removed under reduced pressure. The resulting solid was washed with
acetone (10 mL) to give a pale yellow powder (2.10 g, 62% yield)
containing the two diastereoisomers. Silica gel flash column chromatogra-
phy gave pure product 2 (1.40 g, 41% yield) (Rf = 0.23, eluent MeOH).
The major diastereoisomer 2a was obtained pure after fractional crystal-
lization from CH2Cl2 (0.77 g, white powder, 23% yield) and fully
characterized. Crystals suitable for X-ray diffraction were obtained by the
slow evaporation under nitrogen of a solution of 2a in CH2Cl2 and EtOH.
S(H2O)20 °C = 1.9 g/L. Data for the major diastereoisomer (SR,RS)-2a are
as follows. 1H NMR (CDCl3, 400.13 MHz): δ 7.35 (d, 3JHH = 8.6 Hz, 2H,
Ar);6.74 (d, 3JHH =8.6Hz, 2H, Ar);5.16(dd, 3JHH =3.7 Hz,3JHP =8.4Hz,
3
3JHP = 10.1 Hz, JHH = 3.6 Hz, 1H, CHOH); 4.39-4.18 (m, 7H,
NCH2N þ PCH2N); 4.00-3.84 (m, 4H, PCH2N þ PCHN); 2.87 (s,
6H, N(CH3)2). 31P{1H} NMR (161.98 MHz, DMSO-d6): δ -14.07
(s). 13C{1H} NMR (100.61 MHz, DMSO-d6): δ 150.26 (s, Ar); 130.43
(d, 3JCP = 6.8 Hz, Ar); 128.48 (s, Ar); 112.27 (s, Ar); 74.85 (s, CHOH);
73.82 (d, 3JCP = 5.8 Hz, NCH2N); 71.71 (d, 3JCP = 9.2 Hz, NCH2N);
70.17 (d, 1JCP = 31.4 Hz, PCHN); 65.92 (d, 3JCP = 12.6 Hz, NCH2N);
58.11 (d, 1JCP = 35.4 Hz, PCH2N); 55.36 (d, 1JCP = 34.5 Hz; PCH2N);
40.73 (s, N(CH3)2). Anal. Found (calcd for C15H23N4OPS) (338.41gmol-1):
C, 53.71 (53.24); H, 6.93 (6.85); N, 16.71 (16.56). IR (KBr, cm-1):
νOH 3360 (vs); νarom 1615 (s), 1529 (s); νPdS 646 (m).
4.6. Synthesis of [Ir(cod)Cl(PZA)] (5). In a Schlenk tube under
nitrogen, a solution of [Ir(cod)Cl]2 (0.26 g, 0.4 mmol) in toluene (5 mL)
was added by cannula to a solution of PZA (0.20 g, 0.8 mmol) in CH2Cl2
(10 mL). The solution was stirred at room temperature for 1 h, after
which the volume was reduced to approximately a third, resulting in the
formation of an orange precipitate. The suspension was filtered and the
product washedonce with ice-cold tolueneandthen twice with n-pentane
and dried under vacuum (0.33 g, 74% yield). S(H2O)20 °C = 3.4 g/L.
Spectroscopic data for compound 5(twodiastereoisomers, 1:2ratio,values
for minor isomer in parentheses) are as follows. 1H NMR (400.13 MHz,
CD2Cl2): δ 7.13-7.42 (m, 5H, Ar); 5.32-5.26 (5.68-5.63) (m, 2H,
NCH2N); 5.18-5.09 (m, 2H, cod þ CHOH); 5.08-4.84 (m, 3H, cod);
4.62-4.43 (m, 2H, NCH2N); 4.38-4.19 (m, 3H, NCH2N þ PCHN);
4.12-3.84 (m, 4H, PCH2N); 2.32-2.08 (m, 4H, cod); 1.94-1.64 (m,
4H, cod). 31P{1H} NMR (161.98 MHz, CD2Cl2): δ -64.97
(-58.50) (s). 13C{1H} NMR (100.61 MHz, CD2Cl2): δ 140.88
(140.97) (s, Ar); 128.26 (128.02) (s, Ar); 127.89 (127.36) (s, Ar);
1H, CHOH); 4.85 (AB system, 2JH H = 13.4 Hz, 2H, NCH2N); 4.57-
A
4.37 (m, 4H, NCH2N); 4.29-4.24 (mB, 1H, PCH2N); 4.15-4.08 (m, 1H,
PCH2N); 3.90-3.80 (m, 3H, PCH2N þ PCHN); 2.97 (s, 6H, N(CH3)2).
31P{1H} NMR (CDCl3, 161.98 MHz): δ -102.72 (s). 31P{1H}NMR
(D2O, 121.50 MHz): δ -99.07 (s). 13C{1H} NMR (100.61 MHz,
CDCl3): δ 150.42 (s, Ar); 130.51 (s, Ar); 127.24 (s, Ar); 112.30 (s,
2
Ar); 77.64 (d, JCP = 6.5 Hz, CHOH); 74.20 (s, NCH2N); 67.78 (s,
NCH2N); 65.75 (d, 1JCP = 23.1 Hz, PCHN); 51.55 (d, 1JCP = 19.9 Hz,
PCH2N); 48.43 (d, 1JCP = 24.0 Hz, PCH2N); 40.54 (s, N(CH3)2). Anal.
Found (calcd for C15H23N4OP) (306.34 g mol-1): C, 58.67 (58.81); H,
7.55 (7.57); N, 18.58 (18.29). ESI-MS (m/z (%)): 307.06 (7) [Mþ],
246.02 (100) [C13H17N3Pþ]. IR (KBr, cm-1): νOH 3401 (br, s); νarom
1612 (s), 1525 (s).
127.15 (126.91) (s, Ar); 97.93 (d, 2JCP = 16.9 Hz, cod); 97.20 (d, 2JCP
=
4.4. Synthesis of (SR,RS)-(40-(Dimethylamino)phenyl)-
(1,3,5-triaza-7-phosphatricyclo[3.3.1.1]dec-6-yl)methanol
Oxide (3; OdPZA-NMe2). In a one-necked round-bottomed flask
containing ligand 2a (0.55g, 1.8mmol), methanol(140mL) wasaddedand
the mixture was slightly heated to dissolve the solid. The solution was cooled
to room temperature, and an excess of aqueous H2O2 (35%, 260 μL)
diluted in ethanol (8 mL) was slowly added. The reaction mixture was
stirred at room temperature for 1 h, and a small amount of a white
precipitate appeared. The solvent was then removed by rotary evaporation,
yielding 0.52 g (90%) of pure product. S(H2O)20 °C = 12.5 g/L. 1H NMR
(400.13 MHz, D2O): δ 7.33 (d, 3JHH = 8.2 Hz, 2H, Ar); 6.94 (d, 3JHH = 8.2
Hz, 2H, Ar); 5.34 (dd, 3JHP = 10.9 Hz, 3JHH = 3.4 Hz, 1H, CHOH); 4.19
17.5 Hz, cod); 75.65 (s, NCH2N); 73.83 (d, 3JCP = 7.7 Hz, NCH2N);
72.83 (s, NCH2N); 67.65 (d, 1JCP = 41.9 Hz, PCHN); 67.48 (d, 2JCP
=
1
28.8 Hz, CHOH); 49.59 (49.98) (d, JCP = 24.4 Hz, PCH2N); 44.82
(46.0) (d,1 JCP = 23.0 Hz, PCH2N); 34.60 (34.26) (s, cod); 33.11 (33.66)
(s, cod); 29.58 (28.95) (s, cod); 28.59 (27.98) (s, cod). Anal. Found
(calcd for C21H30ClIrN3OP) (599.13 g mol-1): C, 41.86 (42.10); H,
4.94 (5.05): N, 7.06 (7.01). MS (nESIþ; m/z): 564.2 [Ir(cod)(PZA)]þ.
IR (KBr, cm-1): νOH 3433 (br, s); νarom 1610 (s), 1524 (s).
4.7. Synthesis of [Ir(cod)Cl{(SR,RS)-(PZA-NMe2)}] (6). A
Schlenk tube was charged under nitrogen with ligand 2a (0.20 g,
0.6 mmol) and [Ir(cod)Cl]2 (0.22 g, 0.3 mmol). Dry degassed CH2Cl2
(15 mL) was added, and the resulting yellow solution was stirred at room
temperature for 1 h 30 min. The solution was concentrated under
vacuum to half of the volume, and dry n-pentane (10 mL) was added.
The yellow precipitate was filtered on a frit, washed with cold n-pentane,
and dried under vacuum, yielding 0.26 g of the complex (67% yield).
S(H2O)20 °C = 1.7 g/L. 1H NMR (300.13 MHz, CDCl3): δ 7.33 (d, 3JHH
= 8.6 Hz, 2H, Ar); 6.72 (d, 3JHH = 8.6 Hz, 2H, Ar); 5.30-5.16 (br m, 2H,
NCH2N þ cod); 5.04-5.01 (m, 1H, cod); 4.67-4.44 (m, 6H, NCH2N
þ PCHN þ CHOH); 4.35-3.87 (m, 7H, cod þ PCH2N); 2.95 (s, 6H,
N(CH3)2); 2.25-1.68 (m, 8H, CH2, cod). 31P{1H} NMR (121.50
MHz, CDCl3): δ -64.09 (s). 31P{1H} NMR (121.50 MHz, D2O): δ
-18.12 (s). 13C{1H} NMR (75.47 MHz, CDCl3): δ 150.36 (s, Ar);
(AB system, 2JH H = 14.0 Hz, 2H, NCH2N); 4.31-4.10 (m, 6H, NCH2N
B
þ PCH2N); 3A.89-3.77 (m, 3H, PCH2N þ PCHN); 2.79 (s, 6H,
N(CH3)2). 31P{1H} NMR (161.98 MHz, D2O): δ 2.09 (s). 13C{1H}
NMR (100.61 MHz, D2O): δ 151.44 (s, Ar); 131.44 (d, 3JCP = 7.1 Hz, Ar);
127.57 (s, Ar); 115.52 (s, Ar); 72.97 (d, 2JCP = 2.3 Hz, CHOH); 72.44 (d,
3JCP =6.1Hz, NCH2N); 70.45 (d, 3JCP =8.4 Hz, NCH2N); 68.50 (d, 1JCP
=
50.7 Hz; PCHN); 64.89 (d, 3JCP = 11.6 Hz, NCH2N); 53.31 (d, 1JCP = 52.7
1
Hz, PCH2N); 50.79 (d, JCP = 52.0 Hz, PCH2N); 40.96 (s, N(CH3)2).
Anal. Found (calcd for C15H23N4O2P) (322.34 g mol-1): C, 56.24
(55.89); H, 7.88 (7.19); N, 17.67 (17.38). IR (KBr, cm-1): νOH 3227
(br, m); νarom 1613 (s), 1525 (s); νPdO 1155 (m).
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127.91 (s, Ar); 127.56 (d, 3JCP = 6.9 Hz, Ar); 112.26 (s, Ar); 98.07 (d,
2JCP = 12.8 Hz, cod); 97.13 (d, 2JCP = 13.7 Hz, cod); 75.74 (d, 3JCP = 4.2
Hz, NCH2N); 73.98 (d, 3JCP = 6.0 Hz, NCH2N); 72.97 (d, 3JCP = 3.9 Hz,
NCH2N); 67.52 (d, 1JCP = 17.3 Hz, PCHN); 67.32 (d, 2JCP = 8.3 Hz,
CHOH); 49.76 (d, 1JCP = 18.2 Hz, PCH2N); 44.89 (d,1 JCP = 17.3 Hz,
PCH2N); 40.49 (s, N(CH3)2); 34.62 (d, 3JCP = 3.6 Hz, cod); 33.09 (d,
3JCP = 3.0 Hz, cod); 29.65 (d, 3JCP = 2.1 Hz, cod); 27.98 (d, 3JCP = 4.2
Hz, cod). Anal. Found (calcd for C23H35ClIrN4OP) (642.19 g mol-1):
C, 43.48 (43.02); H, 5.67 (5.49); N, 8.58 (8.72). MS (nESIþ; m/z):
605.3 [Ir(cod)(PZA-NMe2)]þ. IR (KBr, cm-1): νOH 3423 (br, s);
νarom 1613 (m), 1524 (m).
4.8. X-ray Diffraction Data for (SR,RS)-PZA-NMe2 (2a). X-ray
data collection for compound 2a was carried out at room temperature by
using a CCD diffractometer equipped with Mo KR radiation (0.710 73
Å). The program CrysAlis CCD41 and CrysAlis RED42 were used for
data collection and reduction, respectively. Absorption correction was
applied through the program ABSPACK.42 The structure solution was
obtained by using the direct methods in Sir97.43 Structure refinement
was performed with SHELXL44 by using the full-matrix least-squares
method for all the available F2 data. All of the non-hydrogen atoms were
refined anisotropically, and the H atoms bonded to the carbon atoms
were fixed in calculated positions and refined isotropically with thermal
factors 20% larger (50% for the H of the methyl groups) than those of
the atoms to which they are bound. The hydrogen atom of the OH was
located in the Fourier difference map, and its coordinate was freely
refined. All calculations were performed with the WINGX package.45
The molecular drawings were made by using both ORTEP-III for
Windows46 and SCHAKAL97.47
4.9. Transfer Hydrogenation Tests. The reactions were carried
out in Schlenk tubes under an inert atmosphere using degassed solvents.
For transfer hydrogenation of cinnamaldehyde (CNA) and 2-cyclohex-
en-1-one, the catalyst and HCO2Na were placed in two different Schlenk
tubes, to which 3 mL of a H2O/MeOH mixture was added (1/1 or 1/2
as specified in Table 1) after three freeze-thaw-pump cycles. The
formate solution was added by cannula to the solution of catalyst, and
the temperature was set. The liquid substrate was directly added by
syringe to the reaction mixture. In the case of benzylideneacetone
(BZA), HCO2Na was dissolved in 2 mL of water and added via cannula
to the solution of catalyst in MeOH (2 mL). The temperature was set,
and the solution of BZA in MeOH (2 mL) was added to the mixture. At
the end of the catalytic runs, an aliquot of the reaction mixture (0.1 mL)
was taken by syringe and diluted with methanol (0.4 mL) or extracted
with dichloromethane and analyzed by GC. For the hydrogenation of
2-cyclohexen-1-one and acetophenone with KOH/iPrOH, the catalyst
and the base were placed in a Schlenk tube and dissolved in dry isopropyl
alcohol (ca. 9 mL). When the reaction temperature was reached, the
substrate was slowly added by syringe. At the end of the reaction, an
aliquot of the mixture was taken, diluted with MeOH, and analyzed by
GC. The aliquots were usually passed through a plug of silica before the
injection to remove traces of metal. Each test was repeated twice to
check for reproducibility.
’ ASSOCIATED CONTENT
S
Supporting Information. Tables, figures, and a CIF file
b
giving Cartesian coordinates for all the considered complexes
6a-f and crystal data and structure refinement details for 2a.
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: l.gonsalvi@iccom.cnr.it (L.G.); mperuzzini@iccom.cnr.it
(M.P.).
’ ACKNOWLEDGMENT
We thank the Italian Ministries MIUR and MATTM and the
Ente Cassa di Risparmio di Firenze (ECRF) for financial support
through the PRIN 2007, PIRODE, and FIRENZE HYDROLAB
projects, respectively. Thanks are expressed to the EC for promot-
ing this scientific activity through the FP6 Research Training
Network AQUACHEM (contract MCRTN-2003-503864). The
GDRE project “Catalyse Homogꢀene pour le Dꢁeveloppement Durable
(CH2D)” and COST Action CM0802 “PhoSciNet” are also
thanked for funding mobility. L.G. thanks the MAE for a JRP
grant within the Executive Programme of Cooperation Italy-USA
(2008-2010). M.E. thanks the foundation Bengt Lundqvist minne
(Sweden) for extra financial support. D.A.K. thanks the donors of
the Petroleum Research Fund, administred by the American
Chemical Society, and Fulbright Research Scholarships (2009
-2010) for partial supportof thisresearch. A.I. thanks the ISCRA-
CINECA HP grant “HP10BNL89W” for computational time.
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