Imidazolyl-PTA derivatives as water-soluble (P,N) ligands for ruthenium-catalyzed hydrogenations

Article abstract of DOI:10.1021/om200896g

The first imidazolyl "upper-rim" derivatives of 1,3,5-triaza-7-phoshaadamantane (PTA), namely, 1-methylimidazolyl-(1,3,5-triaza- 7-phosphatricyclo[3.3.1.1^3,7]dec-6-yl)methanol (2, PTA-CH(1-MeIm)OH) and bis(1-methylimidazolyl)(1,3,5-triaza-7-phosphatricyclo[3.3.1.1 ^3,7]dec-6-yl)methanol (3, PTA-C(1-MeIm)₂OH), were synthesized in fair yields by reaction of PTA-Li with 1-methyl-2-imidazole carboxyaldehyde and bis(N-methylimidazole-2-yl) ketone, respectively. Compounds 2 and 3 exhibit higher water solubility than most upper-rim derivatives of PTA. The two ligands reacted cleanly with [Ru(η⁶-p-cymene)Cl ₂]₂ in refluxing CHCl₃ to form κ²-P,N-[(η⁶-p-cymene)Ru{PTA-CH(1-MeIm)OH}Cl]Cl (8) and κ²-P,N-[(η⁶-p-cymene)Ru{PTA-C(1-MeIm) ₂OH}Cl]Cl (9). Ligands phenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1. 1^3,7]dec-6-yl)methanol (PZA) and 4′-dimethylaminophenyl(1,3,5- triaza-7-phosphatricyclo[3.3.1.1^3,7]dec-6-yl)methanol (PZA-NMe ₂) were also used to coordinate to Ru, and the corresponding κ¹-P-[(η⁶-p-cymene)Ru(PZA)Cl₂] (6) and κ¹-P-[(η⁶-p-cymene)Ru(PZA-NMe ₂)Cl₂] (7) were isolated and characterized. The ability of the ligands to coordinate in κ¹-P vs κ²-P,E (E = O, N) modes was established by NMR experiments and complemented by DFT calculations. The X-ray crystal structure of the iodide analogue of 7 was obtained. Complexes 6-9 were tested as catalysts for acetophenone reduction using different hydrogen sources under mild conditions, and preliminary results are here described.

Full text of DOI:10.1021/om200896g

Article
pubs.acs.org/Organometallics
Imidazolyl-PTA Derivatives as Water-Soluble (P,N) Ligands for
Ruthenium-Catalyzed Hydrogenations
Donald A. Krogstad,*,† Antonella Guerriero,‡ Andrea Ienco,‡ Gabriele Manca,‡ Maurizio Peruzzini,*,‡
Gianna Reginato,‡ and Luca Gonsalvi*,‡
†Concordia College at Moorhead, 334F Ivers, Moorhead, Minnesota 56562, United States
‡Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10,
50019 Sesto Fiorentino (Firenze), Italy
S
* Supporting Information
ABSTRACT: The first imidazolyl “upper-rim” derivatives of
1,3,5-triaza-7-phoshaadamantane (PTA), namely, 1-methylimida-
zolyl-(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol
(2, PTA-CH(1-MeIm)OH) and bis(1-methylimidazolyl)(1,3,5-
triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (3, PTA-
C(1-MeIm)2OH), were synthesized in fair yields by reaction of
PTA-Li with 1-methyl-2-imidazole carboxyaldehyde and bis-
(N-methylimidazole-2-yl) ketone, respectively. Compounds 2
and 3 exhibit higher water solubility than most upper-rim
derivatives of PTA. The two ligands reacted cleanly with
[Ru(η6-p-cymene)Cl2]2 in refluxing CHCl3 to form κ2-P,N-
[(η6-p-cymene)Ru{PTA-CH(1-MeIm)OH}Cl]Cl (8) and κ2-
P,N-[(η6-p-cymene)Ru{PTA-C(1-MeIm)2OH}Cl]Cl (9). Ligands phenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)-
methanol (PZA) and 4′-dimethylaminophenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (PZA-NMe2) were
also used to coordinate to Ru, and the corresponding κ1-P-[(η6-p-cymene)Ru(PZA)Cl2] (6) and κ1-P-[(η6-p-cymene)Ru(PZA-
NMe2)Cl2] (7) were isolated and characterized. The ability of the ligands to coordinate in κ1-P vs κ2-P,E (E = O, N) modes was
established by NMR experiments and complemented by DFT calculations. The X-ray crystal structure of the iodide analogue of 7
was obtained. Complexes 6−9 were tested as catalysts for acetophenone reduction using different hydrogen sources under mild
conditions, and preliminary results are here described.
dmoPTA9 and PTN(R) (R = Me, Et, Ph)10 (Chart 1). Due to
the rigid structure and spatial arrangement of the heteroatoms,
INTRODUCTION
■
The need for more sustainable chemical processes, for example
by using water as a reaction medium instead of toxic organic
Chart 1
solvents, is attracting attention from both academia and
industry.1 From this point of view, the development of a
library of versatile water-soluble ligands with easily tunable
electronic, steric, and coordinating properties would be highly
desirable.2,3
Bifunctional ligands containing for example P and N donor
atoms are of great interest in the realms of coordination and
organometallic chemistry because of their possible hemilabile
behavior in their transition metal complexes.4 The bond
between a soft metal and a hard base (N) is often weak enough
to cause this moiety to reversibly dissociate and open a
coordination site thus available for the activation of reagents in
the coordination sphere of the metal.5,6 P,N ligands have thus
been employed in metal-catalyzed hydroxylations, allylic
substitutions, hydrogenations, hydrosilylations, hydroborations,
PTA8a,11 and dmoPTA9 have not been reported to behave as
carbonylations, hydroformylation, and many coupling reactions,
κ2-P,N chelating ligands but as κ1-P and bridging μ-P,N
usually run in organic solvents.7
To date, only a limited number of water-soluble amino-
phosphine ligands are known.2 Some of the few are 1,3,5-triaza-
7-phosphaadamantane (PTA)3a,8 and its derivatives including
Received: September 26, 2011
Published: November 3, 2011
© 2011 American Chemical Society
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Organometallics
Article
ligands. PTA has also been observed to bind κ1-N,12 and
dmoPTA to bind κ2-N,N.9 PTN(R), however, has been
reported to bind in both κ1-P and κ2-P,N modes.10,13 Rh com-
plexes with both binding motifs have recently been synthesized
by some of us and used as effective hydroformylation and
hydrogenation catalysts under aqueous and biphasic con-
ditions.13b Recently, κ2-P,N-[(η6-p-cymene)Ru{PTN(Me)}-
Cl]Cl (1) was proven to have antimetastatic activity toward
ovarian cancer cells.14 These studies clearly illustrated that
water-soluble P,N systems have great potential in both catalysis
and medicinal fields.
Frost et al. have derivatized PTA in the 6-position by reacting
it with n-BuLi to form the very useful synthon PTA-Li.15 PTA-
Li has then been used to prepare a library of PTA “upper-rim”
derivatives by reacting it with PPh2Cl, CO2, ketones, and
aldehydes.16−18 In particular, we have recently reported on the
synthesis of phenyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]-
dec-6-yl)methanol (PZA)17 and its p-dimethylamino analogue
(PZA-NMe2, Chart 1),18 whose Ir(I) complexes were recently
proven to be effective catalysts for the hydrogenation of
ketones and α,β-unsaturated carbonyl substrates.18 Following
on our interest in this field, we used PTA-Li to prepare two
new water-soluble phosphines that incorporate N-methyl
imidazole functionalities. The coordination chemistry of these
ligands toward [Ru(η6-p-cymene)Cl2]2, as well as that of PZA
and PZA-NMe2, has been studied. It was found by a
combination of NMR experiments and DFT calculations that
the imidazole systems preferentially bind as κ2-P,N chelates and
thereby may behave as bifunctional P,N ligands under catalytic
conditions. The abilities of the above-mentioned Ru(II)
complexes to catalyze the hydrogenation of acetophenone with
a variety of hydrogen sources and conditions were explored, and
the obtained results are here reported.
Aldrich and purified by recrystallization from n-pentane under N2.
1,3,5-Triaza-7-phosphaadamantane-6-yl lithium (PTA-Li),18 PZA,17
PZA-NMe2,18 [Ru(η6-p-cymene)Cl2]2,19 and bis(N-methylimidazole-
2-yl) ketone20 were prepared as described in the literature and the
latter further purified by recrystallization from THF under N2.
Compound water-solubilities were quantified by the addition of
water via a 100 μL Hamilton microsyringe to a 100 mg sample of the
phosphine or a 50 mg sample of the Ru complex in a Schlenk flask
under slow stirring, in a thermostated bath kept at 20 °C until
complete dissolution of the solids. For example, in a typical trial,
310 μL of water was required to dissolve 100 mg of 2 and a 160 μL
aliquot was required for a 50 mg sample of 8.
Caution: PTA-Li is a highly pyrophoric compound that readily
combusts when exposed to air.
Synthesis of 1-Methylimidazolyl(1,3,5-triaza-7-
phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (2, PTA-CH(1-
MeIm)OH). A 50 mL Schlenk flask was charged with PTA-Li (2.10 g,
12.88 mmol) and THF (20 mL) and cooled to −70 °C. A separate
50 mL Schlenk flask was charged with 1-methyl-2-imidazolecarbox-
aldehyde (1.71 g, 15.40 mmol) and THF (15 mL) at −70 °C. The
aldehyde solution was then added to the stirred PTA-Li slurry under
N2 over a 3 min period. After stirring for 30 min at −70 °C, the
reaction was allowed to warm to room temperature. After 12 h, the
reaction mixture was quenched by the dropwise addition of H2O (0.65 mL),
resulting in an orange suspension. After evaporation of the solvents
and addition of CHCl3, the orange slurry was dried with Na2SO4, then
filtered through Celite. Evaporation of the solvent gave an orange
residue, which was purified by column chromatography (Rf = 0.17,
eluent: methanol), to give a pale yellow solid, which was recrystallized
from THF (10 mL)/benzene (20 mL) as an off-white powder. Yield:
1.40 g (40.7% based on PTA-Li). S(H2O)20 °C = 320 g/L. The product
is formed as a mixture of two diastereoisomers (1:1 ratio), which may
be enriched in one of the two diastereoisomers as follows: 300 mg
of the crude product was combined with acetone (35 mL) under N2
and filtered hot. The resulting solution was then put into the freezer at
−20 °C for 12 h. After this time, the mother liquor was decanted off,
leaving 122 mg of a white powder containing the two diastereoisomers
in 15:1 ratio. Spectroscopic data for 2 (major diastereoisomer, values
for minor diastereoisomer are reported in parentheses when
distinguishable): 1H NMR (400.13 MHz, CDCl3): δ (ppm) 6.84
EXPERIMENTAL SECTION
■
General Methods. All manipulations were carried out under a
purified N2 atmosphere with use of standard Schlenk techniques unless
otherwise noted. Elemental analyses were carried out at the University
of Florence, Italy, by using a Perkin-Elmer 2400 series II elemental
analyzer. Electrospray mass spectrometry (ESI-MS) analyses were
carried out at the University of Florence, Italy, on a LCQ Orbitrap
mass spectrometer (ThermoFischer, San Jose, CA, USA) 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). Infrared spectra were measured in CH2Cl2 solution or in KBr
pellets on a Perkin-Elmer FT-IR Spectrum BX II instrument. GC
analyses were performed on a Shimadzu GC-17A gas chromatograph
(with a chiral column Lipodex-E Macherey-Nagel) equipped with a
flame ionization detector (50 m, 0.25 mm i.d., 0.40 mm o.d.).
Hydrogenation reactions were performed either with a magnetically
stirred home-built stainless steel autoclave with sample carousel (100 mL)
at room temperature or with a Parr autoclave (100 mL) with mechanical
stirring and external digital controller.
3
3
(7.00) (d, JHH = 1.1 Hz, 1H, H4-Im); 6.81 (6.89) (d, JHH = 1.1 Hz,
1H, Im, H5-Im); 5.29 (5.31) (pst, 3JHH = 7.7 Hz, 1H, PCHCHImOH);
2
4.75 (AB system, JHAHB = 13.7 Hz, 1H, NCH2N); 4.62 (AB system,
2JHAHB = 13.7 Hz, 1H, NCH2N); 4.50−4.42 (m, 3H, NCH2N +
PCHN); 4.26−4.20 (m, 2H, NCH2N); 4.15−4.00 (m, 2H, PCH2N);
3.90−3.80 (m, 2H, PCH2N); 3.74 (3.81) (s, 3H, Me-Im). 13C{1H}
NMR (100.61 MHz, CDCl3): δ 148.24 (146.04) (d, 3JCP = 4.1 Hz, C2-
Im); 126.56 (127.47) (s, C4-Im); 122.22 (122.10) (s, C5-Im); 76.33
3
3
(76.01) (d, JCP = 1.1 Hz, NCH2N); 74.02 (74.00) (d, JCP = 2.6 Hz,
NCH2N); 69.05 (61.11) (d, 2JCP = 9.3 Hz, CHOH); 67.52 (66.44) (d,
3JCP = 2.2 Hz, NCH2N); 63.32 (63.72) (d, JCP = 21.0 Hz, PCHN);
1
1
1
51.25 (50.71) (d, JCP = 20.1 Hz, PCH2N); 47.63 (47.39) (d, JCP
=
25.1 Hz, PCH2N); 33.21 (33.04) (s, Me-Im). 31P{1H} NMR (161.98
MHz, CDCl3): δ −103.39 (−104.65) (s); Anal. Found (calcd) for
C11H18N5OP (267.27 g mol−1): C, 49.41 (49.43); H, 6.85 (6.79); N,
26.26 (26.21). IR (KBr, cm−1): ν(O−H) 3397 (sh); ν (CN) 1654.
ESI-MS observed: m/z 268.19 (100) [M+ + 1].
1H, 13C{1H}, and 31P{1H} NMR spectra 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) at
room temperature. The 13C and 31P spectra were run with proton
decoupling, and 31P spectra are reported in ppm relative to an external
85% H3PO4 standard, with positive shifts downfield. 13C and 1H NMR
spectra are reported in ppm relative to residual solvent resonances
with positive shifts downfield.
Solvents were distilled and dried prior to use. Doubly distilled water
was used. Column chromatography purification was performed using
glass columns (10−50 mm wide) and silica gel (230−400 mesh
particle size). 1-Methyl-2-imidazolecarboxaldehyde was obtained from
Synthesis of Bis(1-methylimidazolyl)(1,3,5-triaza-7-
phosphatricyclo[3.3.1.13,7]dec-6-yl)methanol (3, PTA-C(1-MeI-
m)2OH). A 100 mL Schlenk flask was charged with PTA-Li (0.49 g,
3.00 mmol) and THF (15 mL) and put into an alcohol/liquid nitrogen
bath at −70 °C. A 100 mL Schlenk flask was charged with bis(N-
methylimidazole-2-yl) ketone (0.65 g, 3.42 mmol) and THF (15 mL)
kept at −70 °C. The ketone slurry was added to the PTA-Li under N2
over 1 min. Over the next 30 min, the reaction changed from a brown
slurry to a slightly cloudy, gold-colored solution. The reaction was
allowed to warm to room temperature. After 12 h, the clear, orange
solution was quenched by the dropwise addition of H2O (0.3 mL),
resulting in an orange suspension. The solvents were removed
in vacuo, and the product was combined with CHCl3. The orange
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Organometallics
Article
slurry was dried with Na2SO4 and filtered through Celite, and the
resulting orange solution was evaporated to dryness. The orange
residue was extracted with Et2O under N2 (5 × 50 mL), and the
combined solutions were evaporated to dryness, leaving a pale yellow
residue. Yield: 0.62 g (59.7% based on PTA-Li). S(H2O)20 °C = 78 g/L.
The residue was recrystallized from hot Et2O under N2 to produce
white microcrystals after sitting overnight at −20 °C. Data for 3 are as
follows. 1H NMR (400.13 MHz, CD2Cl2): δ (ppm) 6.94 (d, 3JHH = 1.1 Hz,
Found (calcd) for C15H22N7O2P (363.35 g mol−1): C, 49.85 (49.58);
H, 6.22 (6.10); N, 26.05 (26.98). IR (KBr, cm−1): ν(O−H) 3283 sh;
ν(CN) 1597; ν(PO) 1146. ESI-MS (MeOH): observed m/z
364.17 (100) [M+ + 1].
Synthesis of κ1-P-[(η6-p-cymene)RuCl2(PZA)] (6). [(η6-p-
cymene)RuCl2]2 (0.22 g, 0.36 mmol) was dissolved under nitrogen
in dry, degassed CH2Cl2 (13 mL). The solution of PZA (0.19 g,
0.72 mmol, two diastereomers, ratio 1:1) in CH2Cl2 (7 mL) was added
dropwise, and the reaction was left under stirring at room temperature
for 1 h. The volume was then reduced to approximately 10 mL, and
diethyl ether (15 mL) added to precipitate the product. The solid was
isolated and washed with cold ether to afford a reddish-orange powder
(0.26 g, 63% yield based on the PZA ligand), containing the two
diastereoisomers in 1:2 ratio. S(H2O)20 °C = 2.7 g/L. Data for 6 are as
follows (major diastereoisomer, values for minor isomer are reported
in parentheses). 1H NMR (400.13 MHz, CDCl3): δ (ppm) 7.41−7.30
(m, 5H, Ar); 5.65−5.51 (m, 3H, p-cymene); 5.47−5.40 (m, 1H,
CHOH); 5.22−5.19 (m, 1H, p-cymene); 4.87−4.14 (m, 11H, 3 ×
NCH2N, 2 × PCH2N, PCHN); 2.82−2.77 (m, 1H, CHMe2); 2.20
3
3
1H, H4-Im); 6.90 (d, JHH = 1.1 Hz, 1H, H4-Im′); 6.87 (d, JHH
=
3
1.1 Hz, 1H, H5-Im); 6.83 (d, JHH = 1.1 Hz, 1H, H5-Im′); 6.24 (br s,
3
1
1H, OH); 5.55 (br d, JHP= 1.3 Hz, 1H, NCH2N); 4.77 (br d, JHP
=
2
13.6 Hz, 1H, PCHN); 4.66 (AB system, JHAHB= 13.2 Hz, 1H,
2
NCH2N); 4.60 (AB system, JHAHB= 13.2 Hz, 1H, NCH2N); 4.4−4.5
(m, 3H, NCH2N); 4.0−4.15 (m, 3H, PCH2N); 3.83 (m, 1H,
PCH2N); 3.49 (s, 3H, Me-Im); 3.35 (s, 3H, Me-Im′). 13C{1H} NMR
(100.61 MHz, CD2Cl2): δ 147.48 (d, 3JCP = 4.1 Hz, C2-Im); 145.26 (s,
C2-Im′); 125.44 (s, C4-Im); 124.55 (s, C4-Im′); 124.20 (s, C5-Im);
124.11 (s, C5-Im′); 78.49 (d, 2JCP= 8.1 Hz, COH); 77.77 (s, NCH2N);
3
3
74.24 (d, JCP = 1.5 Hz, NCH2N); 67.49 (d, JCP= 3.3 Hz, NCH2N);
62.30 (d, 1JCP = 24.4 Hz, PCHN); 51.94 (d, 1JCP = 19.6 Hz, PCH2N);
1
(2.11) (s, 3H, p-cymene CH3); 1.30 (1.25) (d, JHH = 4.9 (2.3) Hz,
1
1
49.13 (d, JCP = 23.7 Hz, PCH2N); 34.40 (s, Me-Im); 33.18 (s, Me-
3H, CH-Me2); 1.28 (1.23) (d, JHH = 4.9 (2.3) Hz, 3H, CH-Me2).
Im′). 31P{1H} NMR (161.98 MHz, CD2Cl2): δ −97.55 (s). Anal.
Found (calcd) for C15H22N7OP (347.37 g mol−1): C, 51.04 (51.87);
H, 6.31 (6.38); N, 28.39 (28.23). IR (KBr, cm−1): ν(O−H) 3250 (sh);
ν(CN) 1634. ESI-MS (MeOH): observed m/z 348.14 (100) [M+ + 1].
Synthesis of OPTA-CH(1-MeIm)OH (4). PTA-CH(1-MeIm)-
OH (2) (40 mg, 0.15 mmol) was dissolved in D2O (1 mL) in the air.
The clear solution was then combined with 35% H2O2 (20 μL),
transferred to a 5 mm NMR tube, and analyzed by NMR spectroscopy.
According to the 31P spectrum, the phosphine was quantitatively
converted into its oxide. The solvent was then removed under vacuum
at 30 °C. The resulting white residue was washed with acetone (3 × 5
mL) and dried in vacuo. Yield: 24 mg (55.9% based on PTA-CH(1-
MeIm)OH). S(H2O)20 °C = 80 g/L. Spectroscopic data for 4 (major
diastereoisomer, values for minor diastereoisomer are reported in
3
13C{1H} NMR (100.61 MHz, CDCl3): δ 141.09 (141.52) (d, JCP
=
9.1 Hz, Ar); 128.53 (127.99) (s, Ar); 128.16 (127.12) (s, Ar); 126.69
(126.28) (s, Ar); 108.53 (106.83) (s, CCH-Me2); 99.86 (96.48) (s,
C-CH3); 89.69 (88.66) (s, CH p-cymene); 87.52 (88.42) (s, CH
p-cymene); 83.64 (85.70) (s, CH p-cymene); 83.49 (85.07) (s, CH
p-cymene); 75.88 (75.47) (s, NCH2N); 73.54 (73.97) (s, NCH2N);
1
72.12 (s, CHOH); 69.72 (68.71) (d, JCP = 9.4 Hz, PCHN); 67.51
(65.70) (d, 3JCP = 4.6 Hz, NCH2N); 54.72 (51.25) (d, 2JCP = 12.8 Hz,
1
PCH2N); 45.50 (d, JCP = 18.8 Hz, PCH2N); 31.10 (30.80) (s, CCH-
Me2); 22.54 (22.36) (s, CCH-Me2); 22.49 (22.03) (s, CCH-Me2);
19.03 (18.50) (s, C-CH3). 31P{1H} NMR (161.97 MHz, CDCl3): δ
−40.32 (−28.71) (s). Anal. Calcd for C23H32Cl2N3OPRu (569.47 g
mol−1). Anal. Found (calcd): C 48.31 (48.51); H 5.45 (5.66); N 7.40
(7.38). IR (KBr, cm−1): ν(OH) 3372 (br, s); ν(arom) 1446.
1
Synthesis of κ1-P-[(η6-p-cymene)RuCl2{(SR,RS)(PZA-NMe2)}]
(7). Dry, degassed CH2Cl2 (25 mL) was added under nitrogen to a
Schlenk tube charged with ligand (SR,RS)(PZA-NMe2) (0.20 g,
0.65 mmol) and [(η6-p-cymene)RuCl2]2 (0.20 g, 0.33 mmol). The
resulting clear red solution was stirred for 2 h at room temperature.
The solvent was reduced to about half of the initial volume, and upon
addition of 15 mL of cold n-pentane, a reddish-pink precipitate was
formed. This was filtered under nitrogen and washed with few drops of
cold n-pentane, yielding a reddish-pink powder (0.22 g, 56% yield
parentheses when distinguishable). H NMR (400.13 MHz, D2O): δ
(ppm) 6.94 (6.92) (d, 3JHH = 1.1 Hz, 1H, H4-Im); 6.81 (d, 3JHH = 1.2
Hz, 1H, H5-Im); 5.57 (vt, 3JHH ≈ 3JHP = 9.0 Hz, 1H, CHOH); 4.62 (m,
2
1H, PCHN); 4.1−4.3 (m, 6H, NCH2N); 3.95 (br d, JHP = 14.2 Hz,
1H, PCH2N); 3.88−3.81 (m, 3H, PCH2N); 3.64 (3.61) (s, 3H, Me-
Im). 13C{1H} NMR (100.61 MHz, D2O): δ 145.42 (d, 3JCP = 10.0 Hz,
C2-Im); 126.28 (126.54) (s, C4-Im); 123.62 (123.04) (s, C5-Im);
3
3
72.44 (d, JCP = 5.6 Hz, NCH2N); 70.31 (d, JCP = 8.9 Hz, NCH2N);
1
2
66.10 (66.52) (d, JCP = 52.0 Hz, PCHN); 65.03 (d, JCP = 11.5 Hz,
CHOH); 64.65 (60.98) (s, NCH2N); 53.19 (d, JCP = 54.0 Hz,
1
1
based on the PZA-NMe2 ligand). S(H2O)20 °C = 1.4 g/L. H NMR
PCH2N); 50.41 (d, 1JCP = 52 Hz, PCH2N); 32.74 (32.53) (s, Me-Im).
31P{1H} NMR (161.98 MHz, D2O): δ 0.59 (−0.48) (s), Anal. Found
(calcd) for C11H18N5O2P (283.27 g mol−1): C, 46.35 (46.64); H, 6.37
(6.40); N, 24.45 (24.72). IR (KBr, cm−1): ν(O−H) 3317(sh); ν(P
O) 1152. ESI-MS (MeOH): observed m/z 284.25 (100) [M+ + 1].
Synthesis of OPTA-C(1-MeIm)2OH (5). By following the
same procedure as described for 4, PTA-C(1-MeIm)2OH (3) (32 mg,
0.09 mmol) was quantitatively converted into its oxide according to
31P NMR. After evaporation and an absolute ethanol wash, a white
powder was collected. Yield: 26 mg (76.7% based on PTA-C(1-
(300.13 MHz, CD2Cl2): δ (ppm) 7.26 (d, 1JHH = 8.7 Hz, 2H, Ar); 6.75
(d, 1JHH = 8.7 Hz, 2H, Ar); 5.65−5.54 (m, 3H, p-cymene); 5.38−5.30
1
(m, 1H, CHOH); 5.23 (d, JHH = 5.9 Hz, 1H, p-cymene); 4.65−4.35
(m, 9H, 3 × NCH2N + 1 × PCH2N + PCHN); 4.25−4 0.15(m, 2H,
1
PCH2N); 2.97 (s, 6H, N(CH3)2); 2.79 (sept, JHH = 6.9 Hz, 1H, CH-
1
Me2); 2.19 (s, 3H, p-cymene CH3); 1.32 (d, JHH = 6.9 Hz, 3H, CH-
1
Me2); 1.30 (d, JHH = 6.9 Hz, 3H, CH-Me2). 13C{1H} NMR (75.47
MHz, CD2Cl2): δ 150.38 (s, Ar); 128.90 (d, 3JCP = 8.2 Hz, Ar); 127.51
(s, Ar); 112.17 (s, Ar); 108.10 (d, JCP = 2.7 Hz, CCH-Me2); 99.41 (s,
C-CH3); 89.66 (d, JCP = 4.5 Hz, CH p-cymene); 87.55 (d, JCP = 5.6
Hz, CH p-cymene); 83.76 (s, CH p-cymene); 83.48 (d, JCP = 3.7 Hz,
1
MeIm)2OH). S(H2O)20 °C = 66.7 g/L. Data for 5 are as follows. H
NMR (400.13 MHz, D2O): δ (ppm) 6.92 (s, 1H, H4-Im); 6.90 (s, 1H,
3
CH p-cymene); 75.84 (br s, NCH2N); 73.60 (d, JCP = 6.6 Hz,
H4-Im′); 6.84 (s, 1H, H5-Im); 6.79 (s, 1H, H5-Im′); 5.57 (br d, 1JHH
=
3
1
NCH2N); 71.63 (d, JCP = 2.6 Hz, CHOH); 69.57 (d, JCP = 9.5 Hz,
1
3
1
14.0 Hz, 1H, NCH2N); 4.47 (br d, JHP= 14.0 Hz, 1H, PCHN); 4.34
PCHN); 67.43 (d, JCP = 7.4 Hz, NCH2N); 54.78 (d, JCP = 13.5 Hz,
(vt, 3JHH ≈ 3JHP = 12.2 Hz, 1H, NCH2N); 4.21−4.14 (m, 2H, NCH2N
1
PCH2N); 45.25 (d, JCP = 18.8 Hz, PCH2N); 40.34 (s, N(CH3)2);
1
+ 1H AB system NCH2N); 4.08 (AB system, JHAHB= 13.6 Hz, 1H,
31.02 (s, CCH-Me2); 22.23 (s, CCH-Me2); 22.15 (s, CCH-Me2);
18.66 (s, C-CH3). 31P{1H} NMR (121.50 MHz, CD2Cl2): δ −40.35
(s). Anal. Found (calcd) for C25H37Cl2N4OPRu (612.54 g mol−1): C
48.96 (49.02); H 5.98 (6.09); N 9.02 (9.15). IR (KBr, cm−1): ν(OH)
3383 (br, s); ν(arom) 1611 (m), 1521 (m).
NCH2N); 3.75−3.95 (m, 2H, PCH2N); 3.65−3.75 (m, 2H, PCH2N);
3.10 (s, 3H, Me-Im); 2.90 (s, 3H, Me-Im′). 13C{1H} NMR (100.61
3
MHz, D2O): δ 144.64 (d, JCP = 6.7 Hz, C2-Im); 143.99 (s, C2-Im′);
125.43 (s, C4-Im); 125.17 (s, C4-Im′); 125.14 (s, C5-Im); 125.04 (s,
3
3
C5-Im′); 75.78 (d, JCP = 2.2 Hz, NCH2N); 74.03 (d, JCP = 4.4 Hz,
Synthesis of κ1-P-[(η6-p-cymene)RuI2{(SR,RS)(PZA-NMe2)}]
(7-I). Following the synthetic procedure described above for 7 and
using [(η6-p-cymene)RuI2]2 (70 mg, 0.09 mmol) in place of the
chloride analogue, the corresponding κ1-P complex 7-I was obtained
in ca. 60% yield (based on the PZA-NMe2 ligand). S(H2O)20 °C = 0.5 g/L.
NCH2N); 70.40 (d, 3JCP = 8.1 Hz, NCH2N); 68.34 (d, 1JCP = 53.0 Hz,
2
1
PCHN); 65.17 (d, JCP = 13.7 Hz, COH); 53.92 (d, JCP = 52.0 Hz,
PCH2N); 52.74 (d, 1JCP = 56.0 Hz, PCH2N); 33.17 (s, Me-Im); 32.67
(s, Me-Im′). 31P{1H} NMR (161.98 MHz, D2O): δ −0.20 (s). Anal.
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X-ray-quality crystals were obtained as red needles by addition of
EtOH to a CH2Cl2 solution of the complex.
PCH2N); 4.23 (s, 3H, Me-Im); 4.24−4.14 (m, 2H, PCH2N); 4.02 (AB
system, JHAHB= 14.5 Hz, 1H, PCH2N); 3.01 (s, 3H, Me-Im′); 2.80
(sept, J = 6.9 Hz, 1H, CH-Me2); 2.05 (s, 3H, p-cymene CH3; 1.36 (d,
1
1H NMR (300.13 MHz, CDCl3): δ (ppm) 7.25 (d, JHH = 8.7 Hz,
1
1
3
3JHH = 6.8 Hz, 3H, CH-Me2); 1.27 (d, JHH = 6.8 Hz, 3H, CH-Me2).
2H, Ar); 6.73 (d, JHH = 8.7 Hz, 2H, Ar); 5.67 (d, JHH= 5.8, 1H,
1
13C{1H} NMR (100.61 MHz, CD2Cl2): δ 150.39 (s, C2-Im); 144.64
p-cymene); 5.63−5.55 (m, 2H, p-cymene); 5.45 (dd, JHH = 7.3 Hz,
3JHP = 9.6 Hz, 1H, CHOH); 5.36 (d, JHH= 5.8 Hz, 1H, p-cymene);
1
3
(d, JCP = 7.0 Hz, C2-Im′); 134.73 (s, C4-Im); 127.31 (s, C4-Im′);
123.96 (s, C5-Im); 122.82 (s, C5-Im′); 107.63 (s, C-CH3); 105.67 (s,
CCH-Me2); 96.86 (d, JCP = 5.9 Hz, CH p-cymene); 95.69 (d, JCP = 7.4
Hz, CH p-cymene); 82.14 (s, CH p-cymene); 81.53 (s, CH p-cymene);
4.82−4.42 (m, 9H, 3 × NCH2N + 1 × PCH2N + PCHN); 4.16−4.04
(m, 2H, PCH2N); 3.64 (d, 1JHH= 5.6 Hz, 1H, OH); 3.19 (sept, 1JHH
=
5.0 Hz, 1H, CH-Me2); 2.96 (s, 6H, N(CH3)2); 2.45 (s, 3H, p-cymene
1
1
78.48 (s, NCH2N); 76.85 (d, 3JCP = 3.3 Hz, NCH2N); 74.07 (d, 3JCP
6.7 Hz, NCH2N); 68.23 (d, 2JCP = 9.3 Hz, COH-Im2); 64.47 (d, 1JCP
14.1 Hz, PCHN); 54.14 (d, 1JCP = 22.9 Hz, PCH2N); 52.85 (d, 1JCP
=
=
=
CH3); 1.31 (d, JHH = 6.9 Hz, 3H, CH-Me2); 1.30 (d, JHH = 6.9 Hz,
3H, CH-Me2). 13C{1H} NMR (100.61 MHz, CDCl3): δ 150.35 (s,
Ar); 128.11 (d, JCP = 6.1 Hz, Ar); 127.68 (s, Ar); 112.32 (s, Ar);
3
112.01 (d, JCP= 3.1 Hz, CCHMe2); 101.28 (s, C-CH3); 89.27 (d, JCP
=
19.3 Hz, PCH2N); 36.78 (s, Me-Im); 36.65 (s, Me-Im′); 31.20 (s,
CCH-Me2); 22.48 (s, CCH-Me2); 22.08 (s, CCH-Me2); 19.28 (s,
C-CH3). 31P{1H} NMR (161.98 MHz, CD2Cl2): δ −19.77 (s). Anal.
Found (calcd) for C25H36N7Cl2OPRu (653.6 g mol−1): C, 45.88
(45.94); H, 5.65 (5.55); N, 15.08 (15.01). IR (CH2Cl2, cm−1): ν(O−
H) 3284 (sh); ν (CN) 1606.
3.1 Hz, CH p-cymene); 87.60 (d, JCP = 3.3 Hz, CH p-cymene); 83.93
(s, CH p-cymene); 84.80 (d, JCP = 2.8 Hz, CH p-cymene); 76.20 (d,
3
3JCP = 2.5 Hz, NCH2N); 73.32 (d, JCP = 5.0 Hz, NCH2N); 71.50 (d,
1
3JCP = 2.2 Hz, CHOH); 70.22 (d, JCP = 7.5 Hz, PCHN); 67.72 (d,
3JCP = 5.6 Hz, NCH2N); 59.62 (d, 1JCP = 11.7 Hz, PCH2N); 54.20 (d,
1JCP = 17.0 Hz, PCH2N); 40.53 (s, N(CH3)2); 31.95 (s, CCH-Me2);
X-ray Diffraction Data for κ1-P-[(η6-p-cymene)RuI2{(SR,RS)-
(PZA-NMe2)}] (7-I). X-ray data collections for compound 7-I were
carried out at room temperature by using a CCD diffractometer
equipped with Mo Kα radiation (0.71073 Å). The program CrysAlis
CCD21 and CrysAlis RED22 were used for data collection and
reduction, respectively. Absorption correction was applied through the
program ABSPACK. The structure was solved by using the direct
methods in Sir9723 and refined with SHELXL24 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 the atom to which they are bound. The hydrogen
atom of the OH group was located in the Fourier difference map, and
its coordinate was freely refined. All calculations were performed under
the WINGX package.25 The molecular drawings were made by using
ORTEP-III for Windows.26
23.71 (s, CCH-Me2); 22.15 (s, CCH-Me2); 20.71 (s, C-CH3). 31P{1H}
NMR (121.50 MHz, CDCl3): δ −51.91 (s). Anal. Found (calcd) for
C25H37I2N4OPRu (795.44 g mol−1): C 37.86 (37.75); H 4.86 (4.69);
N 7.32 (7.04). IR (KBr, cm−1): ν(OH) 3411 (br, s); ν(arom) 1615
(m), 1522 (m).
Synthesis of κ2-P,N-[(η6-p-cymene)Ru{PTA-CH(1-MeIm)OH}-
Cl]Cl (8). A flame-dried 100 mL Schlenk flask was charged with [(η 6-
p-cymene)RuCl2]2 (76 mg, 0.12 mmol), the diastereomerically
enriched ligand 2 (77 mg, 0.29 mmol), and CHCl3 (25 mL). The
clear red solution was heated to 60 °C for 5 h under N2. The resulting
dark brown solution was filtered through Celite, and the volume was
reduced to approximately 10 mL under vacuum. Et2O was added
dropwise to produce a tan powder, which was collected under N2,
washed with Et2O, and dried in vacuo. Yield: 122 mg (85.9% based on
Ru). S(H2O)20
= 320 g/L. Spectroscopic data for 8 (major
°C
diastereoisomer, values for minor diastereoisomer are reported in
parentheses when distinguishable): 1H NMR (400.13 MHz, CDCl3): δ
(ppm) 7.76 (br s, 1H, H4-Im); 6.93 (7.03) (br s, 1H, H5-Im); 6.05
(6.13) (AB system, JHAHB = 5.5 Hz, 1H, p-cymene); 5.98 (AB system,
Catalytic Hydrogenation Tests. Two different setups were used,
depending on whether the tests were run at room temperature or
higher. In the first case, up to seven 3 mL vials were charged under a
protective nitrogen atmosphere with each precatalyst (7.0 × 10−3
mmol), base, substrate, 2 mL of solvent, and a micro stir bar. The vials
were then transferred to the autoclave, which was flushed with H2 for
10 min, then pressurized to 30 bar. After stirring at room temperature
for the chosen time, the autoclave was depressurized and an aliquot of
the reaction mixture (0.1 mL) was diluted with methanol (0.4 mL)
and analyzed by GC; in the case of water as solvent, the products were
extracted from the aqueous phase with dichloromethane, and the
organic phase was analyzed by GC. In the second case, when heating
was required, a 100 mL Parr autoclave was charged under nitrogen
with precatalyst (2.6 × 10−2 mmol), base, substrate, and 40 mL of
solvent, flushed with H2 for 10 min, then pressurized to 30 bar, stirred
by mechanical stirring, and heated to the desired temperature by an
external temperature controller (thermocouple). At the end of the test,
the pressure was released and an aliquot of the reaction mixture (2 mL)
was diluted in methanol (6 mL) and analyzed by GC; in the case of
water, the aqueous phase was extracted with ethylacetate and analyzed
by GC. Each test was repeated twice to check for reproducibility.
J
HAHB = 5.5 Hz, 1H, p-cymene); 5.80 (5.72) (br s, 1H, p-cymene); 5.55
(br s, 1H, p-cymene); 5.54−5.43 (m, 2H, NCH2N); 5.22 (br d, 1H,
CHOH); 4.78 (br d, 1H, PCHN); 4.60 (m, 3H, NCH2N); 4.51 (AB
system, JHAHB = 14.0 Hz, 1H, PCH2N); 4.40 (ad, 1H, PCH2N); 4.28
(AB system, JHAHB = 14.0 Hz, 1H, PCH2N); 4.10 (br d, 3JHP = 15.0 Hz,
1H, PCH2N); 4.02 (s, 3H, Me-Im); 3.47 (br s, 1H, PCH2N); 2.68
(m, 1H, CH-Me2); 1.81 (1.93) (s, 3H, p-cymene CH3); 1.29 (d, 3JHH
=
6.7 Hz, 3H, CH-Me2); 1.19 (d, 3JHH = 6.7 Hz, 3H, CH-Me2). 13C{1H}
NMR (100.61 MHz, CDCl3): δ 149.53 (s, C2-Im); 134.64 (132.90) (s,
C4-Im); 122.22 (s, C5-Im); 105.47 (s, C-CH3); 100.17 (s, CCH-Me2);
98.05 (d, JCP = 6.7 Hz, CH p-cymene); 95.60 (d, JCP = 7.0 Hz, CH
p-cymene); 82.79 (s, CH p-cymene); 81.75 (s, CH p-cymene); 76.51
3
3
(d, JCP = 2.6 Hz, NCH2N); 74.17 (d, JCP = 6.3 Hz, NCH2N); 68.18
2
1
(d, JCP = 9.2 Hz, CHOH); 66.72 (s, NCH2N); 59.22 (d, JCP= 13.0
Hz, PCHN); 54.50 (d, 1JCP = 14.0 Hz, PCH2N); 52.56 (d, 1JCP = 19.0
Hz, PCH2N); 35.23 (s, Me-Im); 30.63 (s, CCH-Me2); 23.81 (s, CCH-
Me2); 20.89 (s, CCH-Me2); 18.50 (s, CCH3). 31P{1H} NMR (161.98
MHz, CDCl3): δ −26.37 (−22.88) (s). Anal. Found (calcd) for
C21H32N5Cl2OPRu (573.4 g mol−1): C, 43.50 (43.98); H, 5.44 (5.62); N,
12.17 (12.21). IR (CH2Cl2, cm−1): ν(O−H) 3370 (sh); ν(CN) 1604.
Synthesis of κ2-P,N-[(η6-p-cymene)Ru{PTA-C(1-MeIm)2OH}-
Cl]Cl (9). Following the same procedure as described for 8, the
reaction of [(η6-p-cymene)RuCl2]2 (75 mg, 0.12 mmol) and PTA-
C(1-MeIm)2OH (3) (102 mg, 0.29 mmol) produced a brown powder.
Yield: 114 mg (71.4% based on Ru). S(H2O)20 °C = 170 g/L. Data for
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Water-Soluble
Imidazolyl-Aminophosphines and Their Oxides. The
monoimidazolyl “upper-rim”-functionalized PTA, 1′-
methylimidazolyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)-
methanol (2, PTA-CH(1-MeIm)OH), and the bis-imidazolyl
analogue bis-1′-methylimidazolyl(1,3,5-triaza-7-phosphatricyclo-
[3.3.1.13,7]dec-6-yl)methanol (3, PTA-C(1-MeIm)2OH) were
respectively produced by combining a THF slurry of PTA-Li,
obtained as recently described,18 with a solution of 1-methyl-2-
imidazolecarboxaldehyde or bis(N-methylimidazole-2-yl) ketone at
1
9: H NMR (400.13 MHz, CD2Cl2): δ (ppm) 7.41 (s, 1H, H4-Im);
6.97 (s, 1H, H5-Im); 6.86 (s, 1H, H4-Im′); 6.81 (s, 1H, H5-Im′); 6.08
(AB system, JHAHB= 6.3 Hz, 1H, p-cym); 6.02 (AB system, JHAHB
=
6.3 Hz, 1H, p-cymene); 5.57 (br s, 1H, p-cymene); 5.56 (br s,
2
1H, p-cymene); 5.29 (AB system, JHAHB = 13.5 Hz, 1H, NCH2N);
4.65−4.55 (m, 4H, NCH2N + 1H AB system NCH2N); 4.48 (br d,
2JHP = 13.3 Hz, 1H, PCHN); 4.30 (AB system, JHAHB = 14.5 Hz, 1H,
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Scheme 1. Synthesis of Ligands 2 and 3 and Their Oxides 4 and 5
−70 °C (Scheme 1). The resulting deep orange solutions were
slowly warmed to room temperature, stirred overnight, and
quenched with H2O. Upon removal of the solvent, 2 was purified
by column chromatography, while 3 was purified via extraction and
recrystallization with Et2O. Once isolated, compounds 2 and 3
were found to be air-stable solids that slowly oxidized in solution
after several days.
the C atoms of the two imidazole rings were observed as six
separate signals at 147.48 (d, 3JCP = 4.1 Hz, NCNMe), 145.26 (s,
NCNMe), 125.44 (s, NCCNMe), 124.55 (s, NCCNMe),
124.20 (s, NCCNMe), and 124.11 (s, NCCNMe) ppm.
Although PTA is quite water-soluble (S(H2O)20 °C = 235 g/L),8
most of its upper-rim derivatives possess limited water solubility
(S(H2O)20 °C = 3−14 g/L).16,17 It was hypothesized that the
imidazole rings of compounds 2 and 3 could alleviate this
problem, as the additional N atoms would enhance hydrogen
bonding with water. As expected, both systems readily dissolved
in water. Phosphine 2 had a solubility of 320 g/L, while 3 had a
solubility of 78 g/L, the difference likely due to a different
extension of the hydrogen bonding with the solvent.28
Solutions of 2 and 3 oxidized into OPTA-CH(1-MeIm)OH
(4) and OPTA-C(1-MeIm)2OH (5), respectively, over several
days when not under an inert atmosphere. This transformation
was also readily accomplished by employing 35% H2O2 (Scheme
1). The oxidation was clearly observed in the IR spectra of 4 and
5 via PO stretches at 1152 and 1146 cm−1, respectively. The
31P{1H} NMR spectrum of 4 exhibited two singlets at +0.59 and
−0.48 ppm, as expected for its diastereomeric nature, while that
of 5 displayed a single resonance at −0.20 ppm. A diastereomeri-
cally enriched sample (ca. 13:1) of 4 was produced by reacting
H2O2 with an enhanced sample of 2. The 1H spectra of 4 and 5
largely mirrored those of their parent compounds. In terms of 5,
this implied that the imidazole fragments were inequivalent.
Compounds 4 and 5 were soluble in H2O (S(H2O)20 °C = 80 and
66.7 g/L, respectively), alcohols, and CH2Cl2. It was found,
however, that 4 was insoluble in CHCl3. Hence, this solvent may
be used to separate phosphine 2 from its oxide.
ESI-MS and elemental analysis data confirm the empirical
formulas of compounds 2 and 3 as C11H18N5OP and
C15H22N7OP, respectively. The presence of the OH moiety
in the phosphines was identified via IR as each exhibited a
broad stretching band near 3370 cm−1. The presence of two
diastereomers in 2 was confirmed by 31P{1H} NMR, showing,
as expected,16−18 two singlets at −103.39 and −104.65 ppm,
respectively. Compound 3 consists of a pair of enantiomers;
hence its spectrum contained only one singlet at −97.55 ppm.
The diastereomeric mixture of 2 made the interpretation of its
1H and 13C{1H} NMR spectra quite complicated. This mixture
could be diastereomerically enriched by extraction and
recrystallization from hot acetone, to a final 15:1 ratio based
1
on integration of the 31P{1H} NMR singlets. The H spectrum
of the enhanced system was still complicated by second-order
effects. The presence of the imidazole fragment was clearly
evident from the two doublets (3JHH = 1.1 Hz, 1H) at 6.84 and
6.81 ppm and the singlet with an integration of 3H at 3.74 ppm.
These originated from the CHCH and NCH3 groups,
respectively.27 The imidazole moiety was also observed in the
13C{1H} spectrum, as the C atoms of the ring resonated at
3
148.24 (d, JCP = 4.1 Hz, NCNMe), 126.56 (s, NCCNMe),
and 122.22 (s, NCCNMe) ppm. Additionally, the N−CH3
carbon was observed as a singlet at 33.21 ppm. As for 2, the
1H NMR spectrum of 3 was not first order in the region 3.8−
4.8 ppm, and the signals had significant overlaps. The imidazole
protons, however, were quite informative. Two singlets (3H)
were observed at 3.49 and 3.35 ppm for the NCH3 groups,
while four doublets (JHH = 1.1 Hz, 1H) resonated at 6.94, 6.90,
6.87, and 6.83 ppm for the CHCH protons. These two
distinct sets indicated that the two imidazolyl moieties were
magnetically inequivalent due to the presence of the adjacent
stereocenter. The presence of two inequivalent imidazole
groups was further confirmed by the 13C{1H} NMR spectrum.
Two singlets were observed at 34.40 and 33.18 ppm,
corresponding to the C atoms of the NCH3 moieties. Additionally,
Synthesis and Characterization of Ruthenium Com-
plexes. Phosphines 2 and 3 have the potential to bind to
transition metals in both monodentate κ1-P (or κ1-N) and
chelating κ2-P,N modes. Following on our interest in the
coordination chemistry of PTA derivatives to Ru and the
application of the resulting complexes in catalysis29 and as
anticancer agents,29d,30 PZA, PZA-NMe2, 2, and 3 were allowed
to react with [(η6-p-cymene)RuCl2]2 (Scheme 2).
PZA and PZA-NMe2 reacted cleanly with [(η6-p-cymene)-
RuCl2]2 in CH2Cl2 at room temperature to form exclusively the
neutral, three-legged piano stool compounds κ1-P-[(η6-p-
cymene)Ru(PZA)Cl2] (6) and κ1-P-[(η6-p-cymene)Ru(PZA-
NMe2)Cl2] (7) (Scheme 2).
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on the phosphine ligands.33 Similar considerations are valid for
13C{1H} spectra of 8 and 9, both containing four different
resonances for the carbon atoms of the arene ring in the range
81−98 ppm.34,35
Scheme 2. Synthetic Route to Ru Complexes 6−9
The 1H and 13C data of the imidazole rings were important, as
they indicated that the ligands were binding in the κ2-P,N mode
in 8 and 9 (see Tables 1 and 2). In the diastereomerically
Table 1. 1H and 13C{1H} Chemical Shifts (ppm) and
Coupling Constants (Hz) for the Imidazole Rings of 2 and 8
a
atom code
1H, 2
13C{1H}, 2
1H, 8
13C{1H}, 8
2
148.24 d (4.1)
126.56 s
149.53 s
134.64 s
122.22 s
35.23 s
4
6.84 d (1.1)
6.81 d (1.1)
3.74 s
7.76 br s
6.93 br s
4.02 s
5
122.22 s
NCH3
33.21 s
a
For atom code see Chart 2.
13
Table 2. 1H and
C{1H} Chemical Shifts (ppm) and
Coupling Constants (Hz) for the Imidazole Rings of 3 and 9
atom
a
code
1H, 3
13C{1H}, 3
1H, 9
13C{1H}, 9
2
147.48 d (4.1)
145.26 s
125.44 s
124.55 s
124.20 s
124.11 s
34.40 s
150.39 s
144.64 d (7.0)
134.73 s
127.31 s
123.96 s
122.82 s
36.78 s
When the imidazolyl ligands 2 and 3 were reacted with the
Ru precursor in CH2Cl2 at room temperature, the 31P{1H}
NMR spectrum showed signals at −39.42, −35.21, −26.37, and
−22.88 ppm in the case of 2 and singlets at −38.11 and −19.77
ppm for 3. As the upfield signals were in the region observed
with compounds 6 and 7 (vide infra), we can assume that the
signals at −39.42 and −35.21 ppm belong to the complexes
obtained with the diastereomeric mixture of ligand 2 bound in
the κ1-P mode to Ru. Analogously, the signal at −38.11 ppm is
likely due to a similar behavior of ligand 3. The 31P{1H} signal
of PTN(R) has previously been reported to shift downfield
upon changing from the κ1-P and κ2-P,N mode.13 Therefore, it
was reasonable that the resonances −26.37 and −22.88 ppm
were due to the κ2-P,N product obtained with ligand 2 and the
singlet at −19.77 ppm to that of ligand 3. All attempts to
separate the mixture or to prepare the pure κ1-P species by
altering the reaction conditions have failed to date. As κ2-P,N-
[(η6-arene)Ru{PTN(Me)}Cl]Cl could be formed by refluxing
the corresponding Ru arene dimer precursors with the P,N
ligand in CHCl3,14 we reasoned that a similar behavior could be
shown by our imidazolyl ligands. As expected, when 2 and 3
were refluxed with chloroform solutions of [(η 6-p-cymene)-
RuCl2]2, only complexes κ2-P,N-[(η6-p-cymene)Ru{PTA-CH-
(1-MeIm)OH}Cl]Cl (8) and κ2-P,N-[(η6-p-cymene)Ru{PTA-
C(1-MeIm)2OH}Cl]Cl (9) were obtained in good yields
(Scheme 2).31
4
6.94 d (1.1)
6.90 d (1.1)
6.87 d (1.1)
6.83 d (1.1)
3.49 s
7.41 s
6.86 s
6.97 s
6.81 s
4.23 s
3.01 s
5
NCH3
3.35 s
33.18 s
36.65 s
a
For atom code see Chart 2.
enriched free ligand 2, the H4 proton resonance of the imidazole
ring (Chart 2) was observed at 6.84 (d, JHH = 1.1 Hz) ppm.
3
Chart 2
In 8, however, the corresponding signal was dramatically
shifted downfield to 7.76 ppm (br s). This large shift was likely
due to the close proximity of the proton to the N donor, as this
H would encounter the greatest σ-effect.36 These data thus
indicated that N3 was binding to the Ru center. This hypothesis
was supported by the fact that only a slight shift was observed
for H5 (Table 1). Similar results were seen in the 13C NMR.
The C5 singlet was observed at the same resonance (122.22
ppm) for both the free ligand and the complex. However, a
downfield shift was observed with C4. In the free ligand, C4
resonated at 126.56 ppm, while in the complex, its singlet was
observed at 134.64 ppm. Similar dramatic chemical shift effects
have previously been observed with other P-imidazole ligands
and their κ2-P,N complexes.33,36
The 31P{1H} NMR spectrum of 8 contained two singlets at
−26.37 and −22.88 ppm (CDCl3) in the same diastereomeric
ratio of the P,N ligand, while in the case of 9 only one
resonance at −19.77 ppm (CD2Cl2) was observed, as expected.
The 1H NMR spectra of complex 8 obtained using a
diastereomeric enriched sample of the ligand (15:1) and that
of 9 both contained four resonances for the p-cymene ring
protons in the range 5.5−6.1 ppm and two separate doublets
due to CH3 groups at 1.29 (3JHH = 6.8 Hz) and 1.19 (3JHH = 6.6
Hz) ppm and 1.36 (3JHH = 6.9 Hz) and 1.27 (3JHH = 6.8 Hz)
ppm, respectively.32,33 The same behavior was found also for
κ1-P complexes 6 and 7 due to the presence of two stereocenters
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Similar spectroscopic changes were observed with the bis-
imidazole ligand and its Ru complex (Table 2). Since
phosphine 3 contained two inequivalent imidazoles, two H4
doublets (3JHH = 1.1 Hz) were observed at 6.94 and 6.90 ppm.
In the Ru complex 9, the corresponding signals were at 7.41
and 6.86 ppm. The substantial chemical shift change of only
one signal indicated that one of the imidazole rings had coordinated
via N3, while the other ring was not attached. Therefore, in this
particular case, ligand 3 acted in a κ2-P,N mode and not a
tridentate κ3-P,N,N fashion. This has previously been observed for
other P,N,N ligands.37
[{Ru(η6-C10H14)}2(Cl)3]+. The phosphorus atom binds to
ruthenium in the κ1-P mode with a Ru−P distance of
2.3416(16) Å. Regarding the adamantane cage, the P1−C1
distance is slightly longer than the other P−C distances, while
there are no significant differences in the C−N distances. This
feature was previously observed for other PZA cage
structures.16−18 An intramolecular hydrogen bond between
the OH group and one iodine atom (O1···I1 = 3.4698(45) Å)
was present, similar to what was observed for [Cp*IrCl2(PZA)].17
Interestingly, in the crystal structure of the free PZA molecule, the
OH group forms a strong intermolecular hydrogen bond with one
nitrogen atom of the adamantane cage, forming a 1D infinite
chain.18
The conclusions supported by NMR data indicating
preferential κ1-P binding of PZA and PZA-NMe2 and κ2-P,N
binding of 2 and 3 to Ru, respectively, have been substantiated
by DFT calculations (vide infra).
Computational DFT Studies on κ1-P vs κ2-P,E (E = O,
N) Coordination Behavior of Ligands to Ru. The lability
of the chloride ligand in the polar solvents and the donor
abilities of the oxygen and/or nitrogen atoms could, in
principle, favor the formation of a κ 2-P,O or κ 2-P,N
coordination mode. In order to gain more details on the
geometric and energetic factors that govern the coordina-
tion mode around the Ru center, a theoretical investigation
at the DFT/B3LYP level of theory was carried out. On the
basis of a previous study,18 all the possible coordination
modes of PTA derivatives were investigated. No structural
approximations were introduced, and the calculations were
performed in chloroform solution, the same solvent used
during the synthesis. As pointed out before,18 for compound
7, four nitrogen atoms could in principle act as proton
acceptors following hydroxyl group deprotonation, so six
models were investigated: the first one with the coordina-
tion κ 1-P (7a), four related with the κ 2-P,O coordination
with a variable proton acceptor (7b+: H on N1; 7c+: H on
N2; 7d+: H on N3; 7e+: H on N4), and the last one with κ 2-
P,OH coordination (7f+). All of the structures were
optimized and confirmed as minima by the frequencies
calculations (see Supporting Information for details). In all
conformers 7a−f+ the Ru atom displays an octahedral
coordination environment with the Ru−cymene centroid
distances in the range 2.21−2.34 Å and the Ru−Cl of 2.47 Å. In
7a, the proton of the OH group is involved in hydrogen bonding
with one of the two coordinated chloride atoms [d(O−H‑‑‑Cl) ca.
2.2 Å]. The changes in the coordination modes of PTA
derivatives did not strongly affect the remaining coordination
environment of the Ru center, with the average Ru−O bond
length of 2.1 Å. Only in 7f+ was an elongation of the Ru−O
bond distance (ca. 0.13 Å) observed. This was likely due to a
residual interaction of the bonded proton with the adjacent
chloride (2.98 Å). In order to identify the most stable species in
solution and the most probable protonation center, the relative
energy of each conformer was evaluated together with the energy
of the free chloride in chloroform solution. Among all the κ2-P,O
coordination isomers, the most stable free energies were
associated with the conformers 7b+ and 7f+, with the latter
more stable by ca. 8.7 kcal mol−1. Nevertheless, the most stable
conformer was 7a, with an energy stabilization of 18.7 kcal mol−1
with respect to the system 7f+ + Cl−. The optimized structures of
the conformers 7a, 7b+, and 7f+ are shown in Figure 2. The
optimized structures of the remaining conformers are reported in
the Supporting Information.
The Ru compounds 6−9 were found to be soluble in most
halogenated hydrocarbons, alcohols, and water. In fact, the
water solubilities of 6−9 were determined to be 2.7, 1.4, 320,
and 170 g/L, respectively. The solubilities of 8 and 9 are
substantially greater than those of related Ru(arene)
complexes that contained upper-rim-derivatized PTA ligands,
usually falling in the range 4−11 g/L.16 This remarkable
difference is likely due to the noncoordinating imidazole
nitrogen atoms, which may undergo hydrogen bonding with
H2O.
X-ray Crystal Structure of κ 1-P-[(η 6-p-cymene)-
RuI2{(SR,RS)(PZA-NMe2)}] (7-I). Red needles of 7-I were
obtained from the slow diffusion of dry ethanol in a CH2Cl2
solution of the complex. These were suitable for X-ray crystal
diffraction. The X-ray crystal structure shows that the complex
crystallizes in a centrosymmetric space group, and both the SR
and RS isomers are present in the cell. Figure 1 shows an
Figure 1. ORTEP plot (ellipsoids drawn at 50% probability level) of
complex 7-I; O−H···I hydrogen bonding shown as empty lines.
Selected bond distances (Å) and angles (deg): Ru1−P1, 2.3416(16);
Ru1−I1, 2.7375(8); Ru1−I2, 2.7398(8); Ru1−C17, 2.243(6); Ru1−
C18, 2.196(6); Ru1−C19, 2.203(7); Ru1−C20, 2.288(6); Ru1−C21,
2.250(5); Ru1−C22, 2.192(6); P1−C1, 1.885(7); P1−C2, 1.845(7);
P1−C3, 1.845(6); P1−Ru1−I1, 85.95(5); P1−Ru1−I2, 86.68(5); I1−
Ru1−I2, 89.84(2).
ORTEP drawing with selected distances and angles in the
caption. The geometry around the ruthenium atom in 7-I is an
octahedron, where p-cymene occupies three coordination
positions, while two iodine atoms (in cis position) and one
PZA-NMe2 ligand occupy the other three sites. The Ru−C
distances are between 2.192(6) and 2.288(6) Å, thus slightly
longer than the corresponding distances38 in the cation
The high energy difference between κ1-P (7a) and the κ2-
P,OH coordination (7f+) suggests that at room temperature
only the former conformer is reasonably present in solution. In
order to investigate all the possible products, the neutral κ2-P,O
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Figure 2. Optimized structures and relative energies (kcal mol−1) for κ1-P (7a), κ2-P,O (7b+), and κ2-P,OH (7f+) coordination modes. Color code:
oxygen, red; nitrogen, blue; phosphorus, pink; chlorine, green; ruthenium, gray; carbon, black; hydrogen, white. The methyl substituents on the p-
cymene ring are omitted for clarity.
coordination (7g), formed by the loss of a HCl molecule, was
also considered, and the optimized structure is reported in
Figure 3. The compound shares with the aforesaid complexes
all the structural features with no significant differences.
proton acceptor (8b+), κ2-P,N (8c+), κ2-P,OH (8d+), and κ2-
P,O coordination without N protonation (8e). The optimized
structures are reported in Figure 4.
Figure 3. Optimized structure and relative energy (kcal mol−1) for
isomer κ2-P,O (7g).
The comparison of 7g + HCl with 7f+ + Cl− shows a slight
stabilization (less than 0.5 kcal mol−1) of the latter. Thus, at
room temperature the κ1-P coordination mode is confirmed as
the only one available. When the temperature is increased, for
example at reflux in chloroform (around 70 °C), both
coordination modes could be active.
When an imidazole unit was introduced in the “upper rim”,
such as in compound 8, κ1-P and two chelate coordination
modes, i.e., κ2-P,O and κ2-P,N, can be expected. Calculations
on compound 8 lead to the optimization of five different
structures, namely, κ1-P (8a), κ2-P,O coordination with N1 as
Figure 4. Optimized structures and relative energies (kcal mol−1) for
isomers κ1-P (8a), κ2-P,O (8b+), κ2-P,N (8c+), κ2-P,OH (8d+), and
κ2-P,O (8e).
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As for compound 7, in all the investigated conformers the
octahedral coordination around Ru was maintained. In
conformers 8a and 8c+ a hydrogen bond between the
coordinated chloride and the OH was found (2.18 Å);
= 1.9 Å]. Noteworthy is the fact that all efforts to optimize the
geometry of κ2-P,OH (9d+) failed. Thus, the metal center
remained with a free coordination position, and in turn the
metal−C(p-cymene) distances were found to decrease. The
corresponding coordinative unsaturation around the Ru metal
resulted in the particularly high energy of conformer 9d+
compared to 9c+ (ca. 17.5 kcal mol−1), leading to an isomer
that is unlikely to form experimentally. Finally, the structure of
a neutral κ2-P,O isomer, 9e, resulting from formal elimination
of HCl from 9a without N-protonation was also optimized. 9e
was ca. −4.0 kcal mol−1 more stable than 9d+ and −2.9 kcal
mol−1 than 9b+, but less stable by 13.4 kcal mol−1 than the 9c+/
Cl− couple. This suggested that, also in this case, OH group
deprotonation must be followed by protonation at the N atom
of the PTA moiety.
Ruthenium-Catalyzed Hydrogenation Reactions. Ru
complexes have a rich history as homogeneous catalysts for a
variety of reactions, including the hydrogenation of carbonyl
groups to alcohols.39−41 This reaction can be run either under
transfer hydrogenation conditions,42 typically in i-PrOH in the
presence of a strong base, or under a pressure of hydrogen.
Mixed systems using transfer hydrogenation conditions under a
pressure of hydrogen are also known.43 We tested compounds
6−9 as catalysts for CO bond reduction, mainly using
acetophenone as a model substrate. The most relevant results
are summarized in Table 3.
however, in 8d+ hydrogen bonding seems unlikely [d(Cl‑‑‑H)
=
2.8 Å]. When the energy differences were calculated, the κ2-P,N
coordination mode of 8c+ was ca. 14.3 kcal mol−1 more stable
than the κ2-P,O mode of 8d+. It is possible that the Ru
preferentially binds to the imidazole N donor atom, due to the
softer nature if compared to the OH group and adamantane N
atoms. A slight further stabilization of compound 8c+ could be
due to the presence of an intramolecular Cl---H−O hydrogen
bond. Even though the most stable compound is 8c+, the
comparison among the three κ2-P,O coordination modes
reveals that 8e is ca. 4.0 kcal mol−1 more stable than the
parent 8d+. However, the energy comparison between κ1-P and
κ2-P,N reveals only a very small stabilization of the former by
ca. 6.7 kcal mol−1 with respect to the latter, suggesting that in
solution both conformers can be present.
The energy difference becomes even smaller when two
imidazole substituents are introduced, as in complex 9. In fact,
in such a compound, the stabilization energy difference
between the κ1-P (9a) and κ2-P,N (9c+) coordination is only
4.0 kcal mol−1 (see Supporting Information for details). This is
in good agreement with the experimental data, which show the
presence of both coordination modes at room temperature.
Furthermore, due to the small energy difference, it can be
predicted that when the temperature is increased, the entropic
contribution can be fundamental and the κ2-P,N isomer (9c+)
becomes predominant, as found experimentally in chloroform
under reflux conditions. An intramolecular hydrogen bonding is
present for 9c+, where the uncoordinated OH is involved in an
interaction with one nitrogen atom of the PTA unit [d(O−H‑‑‑N)
From Table 3, it can be observed that all catalysts were
active, although not rivaling the state-of-the-art bifunctional Ru
catalysts,44 using the protocol involving H2 (30 bar), BuOK,
t
i
and PrOH as solvent, giving conversions ranging from
moderate to excellent depending on the precatalyst and
reaction temperature. In the presence of the most active
precatalyst, namely, complex 8 used as the 1:1 diastereomeric
Table 3. Hydrogenation of Acetophenone Catalyzed by 6−9 under a Pressure of H2 in the Presence of Added Base
b
run
catalyst
solvent
cat/sub/base
base
temperature (°C)
time (h)
conversion (%)
a
1
6
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
H2O
iPrOH
iPrOH
H2O
1/250/5
1/250/50
1/250/5
1/250/5
1/250/50
1/250/5
1/250/10
1/250/5
1/250/5
1/100/50
1/250/5
1/250/5
1/250/5
1/500/5
1/1000/5
1/250/5
1/250/5
1/500/5
1/250/5
1/250/5
1/100/50
1/250/5
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
KOH
tBuOK
tBuOK
KOH
25
25
60
80
25
60
60
80
80
25
25
25
40
40
40
60
60
60
60
25
25
60
80
4
4
4
4
4
4
4
4
4
4
4
4
1
4
4
1
4
4
4
4
4
4
15.85
26.06
45.98
95.31
2.01
a
2
6
c
3
6
c
4
6
a
5
7
c
6
7
48.49
25.41
92.94
95.89
4.88
c
7
7
c
8
7
cd
,
9
7
8
8
8
8
8
8
8
8
8
8
9
9
9
9
a
a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
H2O
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
KOH
84.14
82.04
94.82
97.91
89.71
100
ad
,
c
c
c
c
cd
,
98.4
c
100
c
38.19
3.48
a
a
c
iPrOH
tBuOK
H2O
KOH
0
iPrOH
iPrOH
tBuOK
tBuOK
53.02
c
1/250/5
4
96.91
a
b
c
Solvent, 2 mL; pH2, 30 bar; catalyst, 7.0 × 10−3 mmol. Conversions determined by GC based on pure samples. Solvent, 40 mL; pH2, 30 bar;
d
catalyst, 2.6 × 10−2 mmol. Addition of one drop of Hg(0).
6300
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mixture, the reactions could be run with high efficiency at room
temperature (run 11) or under very mild (40 °C) temperature
conditions (runs 13−15). The catalyst load could be decreased
from the standard 0.4% to as low as 0.1% without significant
loss of activity. The homogeneous nature of the catalysis was
proven by a standard Hg(0) poisoning test,45 showing very low
decrease of activity under these conditions (runs 9, 12, 17).
Disappointingly, only a maximum 38% conversion (run 19) was
obtained using H2O/KOH/H2 in the presence of precatalyst 8
(1:250 catalyst to substrate ratio) at 60 °C.
conversions with catalyst loadings as low as 0.1%. More detailed
structure−activity relationship and mechanistic studies are in
progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Cartesian coordinates for complexes 6−9; cif files, crystal data,
and structure refinement for 7-I. This material is available free
of charge via the Internet at http://pubs.acs.org.
The catalysts were also tested for the reduction of 2-
cyclohexenone under the experimental conditions described in
Table 3 (catalyst/substrate/base = 1:250:5). Only catalyst 8
gave good conversion (64.7%, 4 h, 25 °C), although the
selectivity of the reaction was not excellent (cyclohexanone,
22.1%; 2-cyclohexenol, 1.0%; cyclohexanol, 41.6%).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: krogstad@cord.edu; maurizio.peruzzini@iccom.cnr.it;
l.gonsalvi@iccom.cnr.it.
Acetophenone hydrogenation was also tested with catalysts
6−9 under standard transfer hydrogenation conditions, i.e.,
iPrOH/KOH at reflux (80 °C). Using catalyst/substrate ratios
of 1:500, the κ1-P compounds 6 and 7 gave conversions of
3.8% and 6.8% after 3 h. As these results compare well with
conversions obtained in our hands using the ligand-free
complex [(p-cymene)RuCl2]2 under the same conditions (4.2%
conversion after 3 h), we can suggest that ligand decoordina-
tion may occur, leaving unknown Ru species to catalyze the
hydrogenation reactions. On the other hand, compound 8 was
quite effective, reaching ca. 73% conversion at 0.2% mol and 95%
conversion at 1.0% mol catalyst loading after 3 h, respectively.
Poor stability of complex 9 under the same conditions was
deduced from the immediate darkening of the solution upon
mixing the reagents, and no conversion was observed. Poor
conversions (less than 5%) were observed using H2O/HCOONa
as mild reducing protocol at 80 °C, accompanied by phosphine
decoordination and oxidation as deduced by 31P{1H} NMR
experiments run on aliquots of the reaction mixtures after the
catalytic runs, giving signals at the same chemical shift values of
isolated phosphine oxides (vide infra).
ACKNOWLEDGMENTS
■
The authors thank MATTM and DPM-CNR for financial sup-
port through PIRODE and EFOR projects. MIUR is thanked
for support through PRIN 2009. GDRE project “Catalyse
̀ ́
Homogene pour le Developpement Durable (CH2D)” and
COST Action CM0802 “PhoSciNet” are also thanked for
funding mobility. L.G. thanks MAE for a JRP grant within the
Executive Programme of Cooperation Italy-USA (2008−2010).
D.A.K. thanks American Chemical Society Petroleum Research
Fund (grant 49372-UFS) and Fulbright Research Scholarships
(2009−2010) for partially supporting his stay at ICCOM
CNR. A.I. and G.M. thank the ISCRA-CINECA HP grant
“HP10BNL89W” for the computational time.
DEDICATION
■
Dedicated to our friend Prof. Christian Bruneau on the
occasion of his 60th birthday.
REFERENCES
■
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CONCLUSIONS
■
In this study, the first imidazole derivatives of PTA, 1-methy-
limidazolyl(1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]dec-6-yl)-
methanol (2, PTA-CH(1-MeIm)OH) and bis(1-methylimidazolyl)-
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upper-rim analogues, likely due to extensive hydrogen bonding
in solution. It was shown that reaction of these ligands with
[Ru(η6-p-cymene)Cl2]2 in refluxing CHCl3 gave the chelate
compounds κ 2-P,N-[(η 6-p-cymene)Ru(PTA-CH(1-MeIm)-
OH)Cl]Cl (8) and κ 2-P,N-[(η 6-p-cymene)Ru(PTA-C(1-
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potential (P,O) ligands such as PZA and PZA-NMe2, which
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Cl2] (6) and [(η6-p-cymene)Ru(PZA-NMe2)Cl2] (7). This
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were tested in base-assisted catalytic hydrogenation of
acetophenone, with compound 8 outperforming the others
both at room temperature and at 40 °C, giving complete
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