A stable hydrido amido ruthenium complex bearing a tridentate iminophosphorane-phosphine-amine ligand

Article abstract of DOI:10.1021/om050738v

A heterotrideniate (P,N,N′) ligand featuring phosphine, iminophosphorane, and amine groups has been prepared in high yield. The corresponding dichlororuthenium(II) complex reacts with base in 2-propanol to give a stable hydrido amido complex. The lack of hydrogens that are a to the amine allowed for the isolation and structural characterization of this hydrido amido species by X-ray diffraction.

Full text of DOI:10.1021/om050738v

Organometallics 2006, 25, 315-317
315
A Stable Hydrido Amido Ruthenium Complex Bearing a Tridentate
Iminophosphorane-Phosphine-Amine Ligand
Le¨ıla Boubekeur, Simon Ulmer, Louis Ricard, Nicolas Me´zailles, and Pascal Le Floch*
Laboratoire “He´te´roe´le´ments et Coordination”, UMR CNRS 7653 (DCPH), De´partement de Chimie,
Ecole Polytechnique, 91128 Palaiseau Ce´dex, France
ReceiVed August 26, 2005
Scheme 1
Summary: A heterotridentate (P,N,N′) ligand featuring phos-
phine, iminophosphorane, and amine groups has been prepared
in high yield. The corresponding dichlororuthenium(II) complex
reacts with base in 2-propanol to giVe a stable hydrido amido
complex. The lack of hydrogens that are R to the amine allowed
for the isolation and structural characterization of this hydrido
amido species by X-ray diffraction.
Among the most studied catalytic processes, the reduction
of polar double bonds is one of the most fundamental trans-
formations in organic synthesis.1,2 In the catalytic hydrogenation
of ketones, Noyori and co-workers have shown the crucial role
played by the ligand in enhancing the activity of the catalyst
(“NH effect”).3 Following these pioneering works, numerous
groups developed ligands bearing both soft P-donor and hard
N-donor moieties. A multitude of highly chemo- and enantio-
selective ruthenium systems have thus been reported. However,
few examples of nonsymmetrical heterotridentate or tetradentate
ligands have been designed.4
In this communication the synthesis of a new mixed (P,N,N′)
tridentate ligand bearing phosphine, iminophosphorane, and
amine functions and its coordination to the [RuCl2(PPh3)]
fragment are reported. Additionally, we also show that the use
of an aromatic amine is the key point in the isolation of a rare
hydrido amido ruthenium complex postulated to be involved
as an intermediate in the Ru-catalyzed transfer hydrogenation
of ketones.
We recently reported an efficient synthesis of mixed bidentate
phosphine-iminophosphorane ligands from the commercially
available diphosphine dppm (1,2-bis(diphenylphosphino)-
methane).5 This method, which was previously applied to
aliphatic, benzylic, and aromatic primary amines (chiral or not),
could be extended here to a primary diamine. The synthetic
approach employed is depicted in Scheme 1. The selective
monobromination of the symmetrical diphosphine dppm using
1 equiv of bromine, followed by reaction with 2 equiv of
o-diaminobenzene, yielded the aminophosphonium salt 2 in an
excellent yield (92% after isolation). This salt was characterized
by conventional NMR techniques, elemental analysis, and X-ray
diffraction (see Supporting Information). As expected, the 31P
NMR spectrum exhibits two doublets (AX spin system) at 38.8
and -30.2 ppm (2JPP ) 74 Hz), which were respectively
assigned to the aminophosphonium moiety and the phosphine.
The new unsymmetrical tridentate salt 2 was then deprotonated
quantitatively to yield the desired heterotridentate (P,N,N′)
ligand 3 (phosphine-iminophosphorane-amine). As previously
noted, the formation of the iminophosphorane ligand results in
a significant upfield shift and the 31P NMR spectrum of 3
showed two doublets at 4.1 and -27.1 ppm (2JPP ) 48 Hz,
∆δ = -35 ppm for the iminophosphorane moiety).
The coordination behavior of this potentially tridentate ligand
was then studied. Reaction of 3 with [Ru(PPh3)4Cl2] readily
led to the formation of complex 4 through the displacement of
three PPh3 ligands (eq 1).
Complex 4, which was isolated as a brown solid in 84% yield,
was characterized by the usual NMR techniques and elemental
analyses.6 The 31P NMR spectrum of 4 displays three doublets
of doublets at 32.4, 41.5, and 52.3 ppm that were respectively
assigned, on the basis of 2D (31P, 1H) correlation, to the
iminophosphorane, phosphine PPh2, and phosphine PPh3 moi-
* To whom correspondence should be addressed. Tel: +33 1 69 33 45
70. Fax: +33 1 69 33 39 90. E-mail: lefloch@poly.polytechnique.fr.
(1) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201.
(2) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40.
(3) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97.
(4) Barbaro, P.; Bianchini, C.; Giambastiani, G.; Togni, A. Eur. J. Inorg.
Chem. 2003, 4166. Bhattacharyya, P.; Loza, M. L.; Parr, J.; Slawin, A. M.
Z., J. Chem. Soc., Dalton Trans. 1999, 2917. Das, C.; Ghosh, A. K.; Hung,
C. H.; Lee, G. H.; Peng, S. M.; Goswami, S. Inorg. Chem. 2002, 41, 7125.
Flores-Lopez, C. Z.; Flores-Lopez, L. Z.; Aguirre, G.; Hellberg, L. H.; Parra-
Hake, M.; Somanathan, R. J. Mol. Catal. A: Chem. 2004, 215, 73. Song,
D. T.; Morris, R. H. Organometallics 2004, 23, 4406. Zhang, J.; Gandelman,
M.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. Organometallics 2004,
23, 4026.
1
eties. In the H NMR spectrum, the resonances corresponding
to methylenic protons and the NH2 hydrogen atoms were found
at 4.22 and 5.10 ppm, respectively (∆δ ) 0.88 ppm with respect
to free ligand 3). Altogether, these data suggest the coordination
of the ligand to the ruthenium center in a tridentate fashion.
The pseudo-octahedral geometry was confirmed by an X-ray
diffraction study (Figure 1). It is noteworthy that the Ru(1)-
P(2) and Ru(1)-P(3) bond distances at 2.269(1) and 2.288(1)
(5) Boubekeur, L.; Ricard, L.; Mezailles, N.; Le Floch, P. Organo-
metallics 2005, 24, 1065.
10.1021/om050738v CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/16/2005

316 Organometallics, Vol. 25, No. 2, 2006
Communications
in favor of the hydrido amido species.9 The presence of an
aromatic diamine in the design of our ligand allowed us to
expect the possible formation of a hydrido amido species under
typical reaction conditions.
As expected, reaction of the dichloro complex 4 with 2 equiv
or more of tBuOK in 2-propanol led to the quantitative
formation of complex 5 (by 31P NMR), which precipitated from
the reaction medium as a red solid (60% isolated yield) (eq 2).
Figure 1. Molecular structure of complex 4. Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and bond angles (deg):
Ru(1)-P(2), 2.269(1); Ru(1)-P(3), 2.288(1); Ru(1)-N(1), 2.144(3);
Ru(1)-N(2), 2.181(4); Ru(1)-Cl(1), 2.421(1); Ru(1)-Cl(2), 2.425(1);
C(1)-P(1), 1.803(4); C(1)-P(2), 1.859(4); N(1)-Ru(1)-N(2),
77.5(1); N(1)-Ru(1)-P(2), 87.2(1); P(3)-Ru(1)-N(2), 96.0(1);
N(2)-Ru(1)-P(2), 164.1(1); N(1)-Ru(1)-Cl(2), 84.1(1); N(1)-
Ru(1)-Cl(1), 87.0(1); P(2)-Ru(1)-Cl(1), 101.89(4); P(2)-Ru(1)-
Cl(2), 92.00(4); Cl(1)-Ru(1)-Cl(2), 163.10(4).
This complex, which proved to be very reactive toward dioxygen
in solution as well as in the solid state, was fully characterized
by NMR techniques and X-ray diffraction.10 The 31P NMR
spectrum consists of three sets of signals at 29.7, 57.9, and 74.5
ppm assigned to the iminophosphorane, PPh3, and PPh2 frag-
2
ments, respectively. The coupling constant JPP ) 26 Hz
indicates the two phosphorus atoms to be in cis positions.11 On
1
the other hand, the H NMR spectrum reveals the presence of
a doublet of doublets (2JHP ) 31.2 and 41.5 Hz) at -22.9 ppm
(1H), which is a typical chemical shift for a terminal hydride.12
Moreover, the signal for the NH proton appears at 5.56 ppm
(1H) as a broad singlet. Together these data pointed to the
formation of the desired hydrido amido species. Due to a
different electronic environment, the two methylenic protons
Å, respectively, reflect the higher trans effect of the imino-
phosphorane moiety compared to the amine moiety.
The ruthenium-catalyzed transfer hydrogenation of ketones
has been thoroughly studied, both experimentally and mecha-
nistically. In terms of activity, Noyori and co-workers have
developed remarkably efficient catalysts. These complexes bear
an amine moiety which is able to assist the hydrogenation (outer-
sphere mechanism, “NH effect”). In this mechanism, the in situ
formation of a hydrido amido complex has been proposed.1,7
Such types of complexes have been proven to be very difficult
to isolate and characterize because the formation of a dihydrido
imine complex is usually favored when a â-hydrogen is
accessible. Only two complexes have been fully characterized
by Morris and co-workers. One could be isolated because of
the lack of such a hydrogen atom,8 while for the other, the
equilibrium (hydrido amido T dihydrido imine) could be shifted
1
are split in the H NMR spectrum into two multiplets at 3.87
and 4.19 ppm, which appear as two doublets (2JHH ) 14.3 Hz)
1
in the H{31P} NMR spectrum. Finally, the structure of the
hydrido amido complex was confirmed by an X-ray diffraction
study. A view of one molecule of 5 is presented in Figure 2.
The P(3)-Ru(1)-N(1) (170.54(4)°) and N(2)-Ru(1)-P(2)
(157.91(5)°) angles suggest that the geometry of 5 is better
described as a distorted square-based pyramid. The quality of
the X-ray data allowed the refinement of the hydrogen atoms
(9) Li, T. S.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R.
H. Organometallics 2004, 23, 6239.
(10) 2-Propanol (5 mL) was added to a mixture of complex 4 (100 mg,
0.11 mmol) and tBuOK (25 mg, 0.22 mmol) under argon in the Schlenk.
The cloudy brown solution was heated at 80 °C for 5 min. The solution
turned to red immediately, and a red precipitate formed. The latter was
isolated by filtration and washed with 2-propanol and then hexanes to obtain
5 as a very air-sensitive red solid. Yield: 60% (57 mg). 31P{1H} NMR
(THF-d8): 29.7 (d, 2JPP ) 0 Hz, 3JPP ) 26 Hz, PdN), 57.9 (dd, 2JPP ) 3JPP
(6) [Ru(PPh3)4Cl2] (1.40 g, 1.15 mmol) was added to a solution of 3
(565 mg, 1.15 mmol) in 10 mL of THF. The solution turned from yellow
to brown immediately. After 30 min the solvent was removed under vacuum
and the residue dissolved in toluene. After 15 min a brown solid precipitated.
The latter was isolated by filtration on a frit, washed with toluene and then
hexanes, and dried under vacuum. Yield: 84% (894 mg). Anal. Calcd for
C49H43Cl2N2P3Ru: C, 63.64; H, 4.69. Found: C, 63.25; H, 4.70. 31P{1H}
2
1
) 26 Hz, PPh3), 74.5 (apparent d, JPP ) 36 Hz, PPh2). H NMR (THF-
d8): 22.91 (dd, 2JHP ) 41.5 Hz, 2JHP ) 31.2 Hz, 1H, Ru-H hydrido), 3.87
(m, ABXY, 1H, JHH ) 14.3 Hz, PCH2P), 4.19 (m, ABXY, 1H, JHH )
2
3
2
2
NMR (THF-d8): 32.4 (dd, JPP ) 22 Hz, JPP ) 6 Hz, PdN), 41.5 (dd,
2JPP ) 22 Hz, JPP ) 36 Hz, PPh2), 52.3 (dd, JPP ) 36 Hz, JPP ) 6 Hz,
14.3 Hz, PCH2P), 5.56 (br s, 1H, NH), 5.71-5.82 (m, 1H, CH5), 6.14 (vd,
JHH ) 7.7 Hz, C(H2) or C(H3)), 6.24-6.33 (m, 1H, C(H2) or C(H3)), 6.37-
6.45 (m, 1H, C(H4)), 6.75-7.56 (m, 35H). 13C{1H} NMR (THF-d8): 48.4
(b, AXY, PCH2P), 111.3 (s, C5), 115.2 (s, C4), 117.6 (d, JCP ) 9 Hz, C2 or
2
2
3
PPh3). 1H NMR (THF-d8): 4.22 (dd, A2XY, 2H, 2JP H ) 2JP H ) 10.0 Hz,
PCH2P), 5.10 (s, 2H, NH2), 6.08-6.17 (m, 1H, CH2), 6.40-6.45 (m, 2H,
C(H3) and C(H4)), 6.76-6.95 (m, 5H), 6.97-7.24 (m, 10H), 7.25-7.39
(m, 5H), 7.40-7.62 (m, 12H), 7.85-8.06 (m, 4H, Ph2P). 13C{1H} ΝΜR
(THF-d8): 44.0 (dd, AXY, 1JCP ) 90 Hz, 1JCP ) 10 Hz, PCH2P), 117.6 (s,
C3), 120.7 (d, JCP ) 10 Hz, C2), 126.3 (s, C4), 127.0 (CIV-P), 127.5 (d,
JCP ) 9 Hz), 127.6 (d, JCP ) 10 Hz, C5), 128.8, 129.0, 129.4, 129.5, 133.1
(d, JCP ) 3 Hz), 134.4 (d, JCP ) 10 Hz), 134.6 (d, JCP ) 10 Hz), 135.6 (d,
JCP ) 10 Hz), 137.8 (d, JCP ) 39 Hz, CIV-P), 138.9 (s, C1), 139.2 (dd, JCP
) 36 Hz, JCP ) 3 Hz, CIV-P), 150.5 (br s, C6).
V
III
C3), 118.9 (s, C2 or C3), 126.0, 127.6, 128.0, 128.5, 128.7, 129.4 (d, JCP
)
12 Hz), 129.6 (d, JCP ) 12 Hz), 132.8, 132.9, 133.1 (d, JCP ) 10 Hz),
133.5 (d, JCP ) 10 Hz), 134.6 (d, JCP ) 13 Hz), 134.9 (d, JCP ) 11 Hz),
139.0 (b, C6), 141.3 (d, JCP ) 37 Hz, CIV-P), 148.8 (b, CIV-P), 161.0 (s,
C1).
(11) Baratta, W.; Herdtweck, E.; Siega, K.; Toniutti, M.; Rigo, P.
Organometallics 2005, 24, 1660.
(12) Hampton, C.; Cullen, W. R.; James, B. R.; Charland, J. P. J. Am.
Chem. Soc. 1988, 110, 6918. Chaudret, B.; Chung, G.; Eisenstein, O.;
Jackson, S. A.; Lahoz, F. J.; Lopez, J. A. J. Am. Chem. Soc. 1991, 113,
2314.
(7) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid,
K.; Morris, R. H. Chem. Eur. J. 2003, 9, 4954.
(8) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104.

Communications
Organometallics, Vol. 25, No. 2, 2006 317
In this complex, as in 4, the Ru(1)-P(2) and Ru(1)-P(3) bond
distances of 2.2125(5) and 2.2298(5) Å respectively reflect a
slightly higher trans effect of the iminophosphorane moiety
compared to the amido moiety. In terms of thermal and kinetic
stabilities, solutions of complex 5 proved to be very stable under
argon or nitrogen for extended periods or after heating at 100
°C for 1 week. As postulated, this remarkable stability can be
traced back to the lack of an H atom on the carbon atom R to
the amino group. The usual decomposition pathway involving
â-hydride elimination from a CH group, which can lead to the
corresponding imine moiety,15,16 is simply not possible.
Preliminary reactivity tests in the transfer hydrogenation of
ketones under the “standard conditions” (80 °C in 2-propanol
with excess of base vs ruthenium catalyst: acetophenone/Ru/
Base ) 200/1/20) with either complex 4 or complex 5 (without
added base) showed a very moderate activity (35% and 30%,
respectively after 4 h). The fast formation (and precipitation)
of complex 5 from complex 4 under the reaction conditions
explains their similar activities. The low activity of our
complexes 4 or 5 can then be ascribed to the high stability of
complex 5 and its reluctance to give a dihydride species, which
are the postulated hydrogen-transferring species.1
In conclusion, we have devised a straightforward and high-
yielding access to a new heterotridentate (P,N,N′) phosphine-
iminophosphorane-amine ligand from the commercially avail-
able bis-phosphine dppm and studied its coordination toward
the [RuCl2(PPh3)] fragment. Taking advantage of the amine sub-
structure of the ligand, we isolated and fully characterized a
rare ruthenium hydrido amido complex. Such complexes are
postulated to act as intermediates in the “outer sphere” mech-
anism of the ruthenium-catalyzed transfer hydrogenation of
ketones. The reactivity of this key intermediate toward dihy-
drogen and DFT calculations aimed at rationalizing its stability
are currently under investigation, and these results will be
presented in due course.
Figure 2. Molecular structure of hydrido amido complex 5.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond distances (Å) and bond
angles (deg): Ru(1)-P(2), 2.2125(5); Ru(1)-P(3), 2.2298(5);
Ru(1)-N(1), 2.148(2); Ru(1)-N(2), 2.031(2); Ru(1)-H(1), 1.53(2);
C(1)-P(1), 1.794(2); C(1)-P(2), 1.865(2); N(2)-H(2), 0.93(2);
P(1)-N(1), 1.612(2); N(1)-Ru(1)-N(2), 77.09(6); N(1)-Ru(1)-
P(2), 85.66(4); P(3)-Ru(1)-N(2), 94.59(5); N(1)-Ru(1)-P(3),
170.54(4); N(2)-Ru(1)-P(2), 157.91(5); N(1)-Ru(1)-H(1),
100.2(2); N(2)-Ru(1)-H(1), 113.8(8); P(2)-Ru(1)-H(1), 82.5(7);
P(3)-Ru(1)-H(1), 87.3(8).
on both the nitrogen atom and the Ru center, H(2) and H(1),
respectively. The sp2 character of nitrogen is corroborated by
the planarity at the nitrogen atom N(2), as shown by the sum
of the angles, 358.7°. Moreover, the amido nitrogen-ruthenium
bond length Ru(1)-N(2) of 2.031(2) Å is shorter than the amine
nitrogen-ruthenium distance in complex 4 (2.181(4) Å). These
data are consistent with a dative π donation from the amido
nitrogen to the ruthenium center.13 A deviation of phosphorus
P(2) from the mean plane (defined by the plane containing P(3),
Ru(1), N(1), and N(2)) could be related to the significant
deshielding (∆δ = 30 ppm vs 4) observed in the 31P NMR
spectrum for this phosphine, while other phosphorus centers
are less affected by the transformation to the hydrido complex.14
As expected, the strongest σ-donor ligand hydride is located in
the apical position trans to the vacant site and the Ru-H(1)
distance (1.53(2) Å) is typical of a ruthenium-hydride bond.12
Acknowledgment. This work was supported by the CNRS
and the Ecole Polytechnique.
Supporting Information Available: Text giving experimental
details and CIF files and tables giving crystallographic data for 2,
4, and 5, including atomic coordinates, bond lengths and angles,
and anisotropic displacement parameters. This material is available
free of charge via the Internet at http://pubs.acs.org. CCDC ref-
erence numbers 279051-279053 also contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB21EZ, U.K.; fax, (+44) 1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk).
(13) Yi, C. S.; He, Z. J.; Guzei, I. A. Organometallics 2001, 20, 3641.
Yi, C. S.; Yun, S. Y.; Guzei, I. A. Organometallics 2004, 23, 5392.
Celenligil-Cetin, R.; Watson, L. A.; Guo, C. Y.; Foxman, B. M.; Ozerov,
O. V. Organometallics 2005, 24, 186. Hsu, G. C.; Kosar, W. P.; Jones, W.
D. Organometallics 1994, 13, 385.
OM050738V
(14) Pregosin, P. S.; Kunz, R. W. 31P and 13C NMR of Transition Metal
Phosphine Complexes. In Basic Principles and Progress; Springer-Verlag:
Berlin, 1979. Hoffman, P. R.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97,
4221.
(15) Abbel, R.; Abdur-Rashid, K.; Faatz, M.; Hadzovic, A.; Lough, A.
J.; Morris, R. H. J. Am. Chem. Soc. 2005, 127, 1870.
(16) Abdur-Rashid, K.; Abbel, R.; Hadzovic, A.; Lough, A. J.; Morris,
R. H. Inorg. Chem. 2005, 44, 2483.