Dihydridoamine and hydridoamido complexes of ruthenium(II) with a tetradentate P-N-N-P donor ligand

Article abstract of DOI:10.1021/om049565k

The preparation of tetradentate ligands and their use in making a new complex was discussed. The structure of 4 as isomer 4A showed that for the first time the presence of two N-H bonds on the same side as the Ru-H bond in such tetradentate ligand complexes. The formation of cis and trans dihydrides at ruthenium was an alternative explanation which could not be ruled out. Additional evidence for the mechanism of the hydrogenation of ketones catalyzed by tetradentate ligand systems.

Full text of DOI:10.1021/om049565k

Organometallics 2004, 23, 6239-6247
6239
Dihydridoamine and Hydridoamido Complexes of
Ruthenium(II) with a Tetradentate P-N-N-P Donor
Ligand
Tianshu Li, Raphae¨l Churlaud, Alan J. Lough, Kamaluddin Abdur-Rashid,† and
Robert H. Morris*
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario M5S 3H6, Canada
Received June 14, 2004
The new diphosphinediimine ligand PPh2C6H4CHdNCMe2CMe2NdCHC6H4PPh2 {tmeP2N2}
is prepared by reacting NH2CMe2CMe2NH2 with 2 equiv of PPh2(C6H4-2-CHO). The diamine
derivative PPh2C6H4CH2NHCMe2CMe2NHCH2C6H4PPh2 {tmeP2(NH)2} is prepared by reduc-
ing tmeP2N2 with LiAlH4. The reaction of RuHCl(PPh3)3 with tmeP2N2 in THF produces the
complex trans-RuHCl{tmeP2N2}, while a similar reaction of RuHCl(PPh3)3 with tmeP2(NH)2
gives the complex trans-RuHCl{tmeP2(NH)2}, as a mixture of two isomers. The isomer with
two N-H bonds syn to the Ru-H bond is converted on heating to an isomer thought to have
one N-H syn to the Ru-H. The reaction of the two isomers with KOtBu under Ar in toluene
produces the novel hydridoamido complex RuH{tmeP2NNH}, where the ligand is deproto-
nated at one nitrogen. Reaction of this amido complex with H2 gives exclusively the
dihydridoamine complex trans-Ru(H)2{tmeP2(NH)2}. The complexes have been characterized
by X-ray crystallography, NMR, IR, elemental analysis, and a deuterium exchange study.
Complex RuH{tmeP2NNH}, or a combination of RuHCl{tmeP2(NH)2} and KOtBu, slowly
catalyzes the hydrogenation of acetophenone (6 atm H2, 20 °C, benzene or 2-propanol). The
catalytic cycle is thought to be similar to that for the ketone hydrogenation precatalyst trans-
RuHCl{(S,S)-cyP2(NH)2)}. However in the present work both of the proposed catalysts trans-
RuH2{tmeP2(NH)2} and RuH{tmeP2NNH} have been completely characterized. The tmeP2-
(NH)2 system, with hindering methyl groups, is less active than the cyP2(NH)2 system with
a trans-1,2-substituted cyclohexyl backbone. The variable configuration of the amine
nitrogens that is observed for the tmeP2(NH)2 complexes might also occur in the cyP2(NH)2
systems and could explain the inconsistent selectivity of such catalysts. The diimine complex
RuHCl{tmeP2N2} reacts with KOtBu under H2 to produce the dihydridodiamine complex
trans-Ru(H)2{tmeP2(NH)2}, the catalyst for the hydrogenation of acetophenone.
Introduction
are also conceivable. The five-coordinate cationic com-
plexes of the type [RuCl(PPh2-NH-NH-PPh2)]+, pre-
pared by chloride abstraction from [RuCl2(PPh2-NH-
NH-PPh2)], catalyze the epoxidation of olefins with
hydrogen peroxide5 or air6 as oxidant and the cyclopro-
panation of olefins.7,8 The ruthenium hydridochloro
complexes trans-RuHCl(PPh2-NH-NH-PPh2) have
proven to be active precatalysts for transfer hydrogena-
tion and H2-hydrogenation9 of ketones to alcohols. The
relatively rigid, chiral, tetradentate ligand complexes
lead to enantioselective transfer hydrogenation reac-
tions,3 but less selective H2-hydrogenation.9 The kinetics
and mechanism of hydrogenation of acetophenone to
1-phenylethanol catalyzed by trans-RuHCl{(S,S)-cyP2-
(NH)2} (1) and alkoxide have recently been described.9
The proposed mechanism is shown in Scheme 1. The
reaction of precatalyst 1 with the base is thought to
Over the past several years, a variety of tetradentate
Ph2P-N-Z-N-PPh2 ligands with different amine back-
bone linkers Z (where PPh2 is a diphenylphosphino
group and N is an amine or an imine donor) have been
prepared by Gao et al. (Chart 1).1-4 These were used to
make ruthenium and rhodium precatalysts for the
asymmetric hydrogenation of ketones.1-4
These tetradentate ligands possess two “soft” phos-
phorus atoms and two “hard” nitrogen atoms that
restrict the coordination geometry around ruthenium,
usually to the trans isomer.2 However cis-â-dichloro
complexes have also been prepared and other isomers
* To whom correspondence should be addressed. E-mail:
rmorris@chem.utoronto.ca.
† Present address: Kanata Chemical Technologies, 2240 Speakman
Dr., Sheridan Science and Technology Park, Mississauga, Ontario,
L5K 1A9, Canada.
(5) Stoop, R. M.; Bachmann, S.; Valentini, M.; Mezzetti, A. Organo-
metallics 2000, 19, 4117-4126.
(6) Wong, W.-K.; Chen, X.-P.; Chik, T.-W.; Wong, W.-Y.; Guo, J.-P.;
Lee, F.-W. Eur. J. Inorg. Chem. 2003, 3539-3546.
(7) Bachmann, S.; Furler, M.; Mezzetti, A. Organometallics 2001,
20, 2102-2108.
(8) Bonaccorsi, C.; Bachmann, S.; Mezzetti, A. Tetrahedron Asym-
metry 2003, 14, 845-854.
(9) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid,
K.; Morris, R. H. Chem. Eur. J. 2003, 9, 4954-4967.
(1) Gao, J.-X.; Ikariya, T.; Noyori, R. Organometallics 1996, 15,
1087-1089.
(2) Gao, J.-X.; Wan, H.-L.; Wong, W.-K.; Tse, M.-C.; Wong, W.-T.
Polyhedron 1996, 15, 1241-1251.
(3) Gao, J.-X.; Zhang, H.; Yi, X.-D.; Xu, P.-P.; Tang, C.-L.; Wan, H.-
L.; Tsai, K.-R.; Ikariya, T. Chirality 2000, 12, 383-388.
(4) Gao, J.-X.; Xu, P.-P.; Yi, X.-D.; Yang, C.-B.; Zhang, H.; Cheng,
S.-H.; Wan, H.-L.; Tsai, K.-R.; Ikariya, T. J. Mol. Catal. A 1999, 147,
105-111.
10.1021/om049565k CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/20/2004

6240 Organometallics, Vol. 23, No. 26, 2004
Li et al.
Chart 1
Potassium tert-butoxide and 2-(diphenylphosphino)benzalde-
Scheme 1
hyde were supplied by the Aldrich Chemical Co. The diamine
11
12
NH2CMe2CMe2NH2 and the complex RuHCl(PPh3)3 were
prepared by the literature methods. NMR spectra were
1
recorded on a Varian Unity-500 (500 MHz for H), a Varian
Unity-400 (400 MHz for 1H), or a Varian Gemini 300 MHz
spectrometer (300 MHz for 1H and 121.5 for 31P). All 31P
chemical shifts were measured relative to 85% H3PO4 as an
external reference. 1H chemical shifts were measured relative
to partially deuterated solvent peaks but are reported relative
to tetramethylsilane. All infrared spectra were obtained on a
Nicolet 550 Magna-IR spectrometer. Microanalyses and mass
spectroscopy were performed at the University of Toronto.
Hydrogenation reactions were done at constant pressures of
H2 using a 50 mL Parr hydrogenation reactor. The Parr
hydrogenation reactor was flushed several times with hydro-
gen gas at the preset pressure prior to adding the reaction
mixture or its components. Aliquots of the reaction mixture
were quickly withdrawn with a syringe under a flow of
hydrogen at timed intervals by venting the Parr reactor.
Constant temperature was maintained using a constant-
temperature water bath. Samples were added to CHCl3 in air
to destroy the catalyst and terminate the reaction. Concentra-
tions of 1-phenylethanol and acetophenone were determined
by gas chromatography with a Chrompack capillary column
(Chirasil-Dex CB 25 m × 0.25 mm) and H2 as carrier gas at a
column pressure of 5 psi, an oven temperature of 130 °C, an
injector temperature of 250 °C, and an FID at 275 °C. The
retention times were acetophenone 5.0 min, (R)-1-phenyletha-
nol 8.5 min, and (S)-1-phenylethanol 9.1 min. The NMR
spectroscopic data of new ruthenium complexes are reported
in Table 1.
Synthesis of 1,6-Bis((2-diphenylphosphino)benzo)-
3,3,4,4-tetramethyl-2,5-diaza-1,5-hexadiene {tmeP2N2}.
The mixture of 2,3-diamine-2,3-dimethylbutane (0.210 g, 1.8
mmol) and 2-(diphenylphosphino)benzaldehyde (1.110 g, 3.75
mmol) in EtOH (10 mL) was refluxed under Ar for 24 h to
give a light yellow precipitate. The mixture was cooled to 0
°C, then filtered and washed with cold EtOH (20 mL), hexanes
(10 mL), and diethyl ether (10 mL) and dried under vacuum
to give a pale yellow solid (yield: 0.996 g, 80%). Anal. Calcd
for C44H42N2P2: C, 79.98; H, 6.41; N, 4.24. Found: C, 80.15;
H, 6.60; N, 4.11. 1H NMR (CDCl3): δ 8.88 (d, 2H, CHdN, 4JHP
) 5.2 Hz), 8.06 (dd, 2H, ArH), 7.50-7.30 (m, ArH, 24H), 6.90
(dd, 2H, ArH), 1.00 (s, 12H, CH3). 13C NMR: δ 155.0 (d, CHd
N), 140.3 (d, Ph), 137.4 (d, Ph), 136.4 (d, Ph), 134.3 (d, Ph),
132.8 (s, Ph), 129.8 (s, Ph), 128.9 (s, Ph), 128.7 (s, Ph), 128.6
(s, Ph), 127.2 (s, Ph), 65.6 (s, C(CH3)2), 22.8 (s, CH3). 31P{1H}
produce a reactive hydridoamido complex, RuH(PPh2-
C6H4CH2NC6H10NHCH2C6H4PPh2), abbreviated as RuH-
{cyP2NNH} (2), that could not be isolated. Dihydrogen
adds across the ruthenium-amido bond in the turnover
limiting step to give the dihydridoamine complex trans-
RuH2{cyP2(NH)2} (3). Experiments proved that trans
and cis dihydrides can be generated in solution.9 The
hydride complex in the catalytic cycle is thought to be
trans by analogy with the ketone hydrogenation catalyst
trans-RuH2(binap)(tmen), tmen ) NH2CMe2CMe2NH2,
prepared from RuH(binap)(NHCMe2CMe2NH2) with H2.
The use of the methylated diamine tmen, which lacks
hydrogen atoms alpha to the amine groups, allowed the
isolation and complete characterization of these hydro-
genation catalysts.10 The corresponding amido and
dihydride complexes prepared with the new tetraden-
tate ligand PPh2C6H4CH2NHCMe2CMe2NHCH2C6H4-
PPh2 {tmeP2(NH)2} have now been isolated and fully
characterized.
Experimental Section
General Procedures. All preparations and manipulations
were carried out under hydrogen, nitrogen, or argon atmo-
spheres with the use of standard Schlenk, vacuum line, and
glovebox techniques in dry, oxygen-free solvents. Tetrahydro-
furan (THF), diethyl ether (Et2O), and hexanes were dried and
distilled from sodium benzophenone ketyl. Deuterated solvents
were degassed and dried over activated molecular sieves.
(11) Hirel, C.; Vostrikova, K. E.; Pecaut, J.; Ovcharenko, V. I.; Rey,
P. Chem. Eur. J. 2001, 7, 2007-2014.
(12) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J. Chem. Soc.
A 1968, 3143-3150.
(10) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am.
Chem. Soc. 2001, 123, 7473-7474.

Dihydridoamine Complexes of Ruthenium(II)
Organometallics, Vol. 23, No. 26, 2004 6241
Table 1. NMR Data for the Ruthenium Hydride Complexes
1H NMR data (δ)
complex
solvent
hydride
CH3
CH2, CH, and NH
Ph
31P NMR data (δ)
4A
4B
RuHCl{tmeP2(NH)2} CD2Cl2
-17.54 (t), 2JHP
)
1.68 (s), 1.60 (s)
3.88 (d), 4.15 (d),
3JHH ) 10 Hz; 5.6
(br, NH)
7.65 (br), 7.3-7.0 65.9 (br, s)
22.7 Hz
(m), 6.40 (br)
CD2Cl2
at 200 K
-17.53 (dd), 2JHP
22.3, 21.6 Hz
)
1.24 (s), 1.34 (s),
1.54 (s), 1.72 (s)
3.96 (m, 2H, CH2),
4.31 (d, 1H, CH2),
4.63 (m, 1H, CH2),
5.81 (br, 2H, NH)
4.05 (m), 3.94 (m), 8.60 (br), 8.09 (t), 63.2 (d) 57.0 (d),
3
8.24-6.18 (m)
68.2 (d) 62.7 (d),
2Jpp ) 31 Hz
RuHCl{tmeP2(NH)2} CD2Cl2
-18.40 (dd), 2JHP
21.6, 29.2 Hz
)
)
1.37 (s), 1.42 (s),
1.44 (s), 1.62 (s)
4.97 (t, NH). 3JHH JHH ) 8.5 Hz,
2JPP ) 30.3 Hz
) 9 Hz
7.34-6.7 (m)
8.20 (br),
5
6
7
RuH{tmeP2NNH}
Ru(H)2{tmeP2(NH)2} C6D6
RuHCl{tmeP2N2} CD2Cl2
CD2Cl2
C6D6
-23.85 (dd), 2JHP
1.23 (s), 1.15 (s),
1.06 (s), 0.98 (s)
1.24 (s), 0.69 (s)
4.22 (m), 3.71 (s),
3.40 (br)
66.0 (d) 61.1 (d),
2JPP ) 17.7 Hz
77.5 (s)
25.5, 47.5 Hz
7.40-6.63 (m)
8.43 (br),
-5.28(t), 2JHP
)
4.10 (m), 3.49 (m)
17.7 Hz
7.59-6.81 (m)
7.66-6.98 (m)
-17.06 (t), 2JHP
)
1.66 (s), 1.58 (s)
8.91 (d), 2JHP
7.3 Hz
)
)
59.3 (br, s)
27.5 Hz
-16.90 (dd), 2JHP
)
0.88 (s), 1.24 (s),
1.40 (s), 1.80 (s)
9.06 (d), 2JHP
7.86-5.80 (m)
58.4 (d) 56.2 (d),
at 200 K
20.5, 30.5 Hz
6.9 Hz, 8.79 (d),
2Jpp ) 31 Hz
2JHP ) 7.1 Hz
5 + D2
C6D6
-5.15 (t), 2JHP
18 Hz, -5.29
(t), 2JHP ) 18 Hz
)
1.24 (s), 0.69 (s)
4.08 (m), 3.47 (m)
8.43 (br),
7.59-6.81 (m)
77.78 (s), 77.65 (s),
77.52 (s)
NMR: δ -12.51 (s). IR (Nujol): 1639 (s, CdN). MS for
C44H42N2P2: 660.1 (M+, 6%), 604.1 (76.1%), 330.1 (38%), 288.1
(100%), 183.0 (40.3%).
Calcd for C48H54N2OP2Ru (5‚THF): C, 68.80; H, 6.50; N, 3.34.
Found: C, 68.36; H, 6.98; N, 3.11. NMR data: see Table 1.
Synthesis of trans-RuH2{tmeP2(NH)2} (6). Method a.
A mixture of RuHCl{tmeP2(NH)2} (20 mg, 0.025 mmol) and
KOtBu (15 mg, 0.13 mmol) in C6D6 was sealed in an NMR tube
under H2 (1 atm). A yellow-brown solution was produced after
1 h. The dihydride is not stable under Ar. The brown crystal
of 6 was obtained by the vapor diffusion of hexane into the
C6D6 solution of 6 under H2. Method b. A dark red solution
of RuH{tmeP2NNH} (20 mg, 0.025 mmol) in C6D6 (0.6 mL)
was reacted with 1 atm H2 to give the yellow dihydride 6. NMR
data: see Table 1.
Synthesis of 1,6-Bis((2-diphenylphosphino)benzo)-
3,3,4,4-tetramethyl-2,5-diazahexane {tmeP2(NH)2}. The
tmeP2N2 compound (0.500 g, 0.758 mmol) in ether (10 mL) was
added to LiAlH4 (29 mg, 0.76 mmol) in ether (20 mL) under
Ar. The mixture was refluxed for 14 h. After the resulting
solution was cooled to room temperature, 1 mL of water was
added to destroy the excess LiAlH4. An aqueous solution of
NaOH (10%, 30 mL) was added. The ether layer was collected.
The aqueous layer was extracted with ether (10 mL × 3). The
combined ether layer was dried over MgSO4 and filtered, and
the solvent was removed by vacuum to give a white solid
(yield: 0.440 g, 87%). Anal. Calcd for C44H46N2P2: C, 79.49;
Synthesis of RuHCl{tmeP2N2} (7). A solution of tmeP2N2
(188 mg, 0.21 mmol) and RuHCl(PPh3)3 (143 mg, 0.21 mmol)
in THF (3 mL) was refluxed under Ar for 1 h to give a red
solution and red precipitate. Two-thirds of the solvent was
removed by vacuum. The mixture was filtered, and the solid
was washed with THF and ether and dried under vacuum to
give a pink powder (yield: 126 mg, 75%). A red crystal of 7
was obtained by the vapor diffusion of ether into the CH2Cl2
solution of 7 under Ar. Anal. Calcd for C44H43ClN2P2Ru: C,
66.20; H, 5.43; Cl, 4.44; N, 3.51. Found: C, 66.10; H, 5.22; N,
3.60. NMR data: see Table 1.
1
H, 6.99; N, 4.21. Found: C, 79.78; H, 6.60; N, 4.10. H NMR
3
(C6D6): δ 7.70-6.90 (m, 28H, ArH), 4.02 (d, 4H, CH2, JHH
)
6.3 Hz), 1.37 (brs, 2H, NH), 0.91 (s, 12H, CH3). 13C NMR: δ
147.0 (d, Ph), 137.9 (d, Ph), 135.9 (d, Ph), 134.2 (d, Ph), 133.8
(s, Ph), 129.6 (d, Ph), 129.2 (s, Ph), 128.8 (s, Ph), 128.7 (s, Ph),
127.1 (s, Ph), 59.2 (s, C(CH3)2), 45.9 (d, NHCH2), 20.9 (s, CH3).
31P{1H} NMR: δ -15.86 (s). IR (Nujol): 3177 (w, NH). MS for
C44H46N2P2: 663.3 (M+ - H, 7%), 332.1 (100%), 275.1 (44%).
Synthesis of RuHCl{tmeP2(NH)2} (4A, 4B). A mixture
of tmeP2(NH)2 (140 mg, 0.21 mmol) and RuHCl(PPh3)3 (192
mg, 0.21 mmol) in THF (5 mL) was refluxed under Ar for 1 h
to give a red solution and pink precipitate. Two-thirds of the
solvent was removed by vacuum. The mixture was filtered.
The solid was washed with ether (1 mL × 3) and dried under
vacuum to give a pink powder (121 mg, 73%). The pink powder
is composed of two isomers, 4A and 4B. The yellow isomer 4A
was isolated in 39% yield by washing the mixture with THF
to remove the red isomer 4B. Isomer 4A, when heated in CH2-
Cl2 for 16 h, converted into isomer 4B completely. A yellow
crystal of isomer 4A was obtained by the vapor diffusion of
ether into a CH2Cl2 solution of 4A under Ar. Anal. Calcd for
C44H47ClN2P2Ru: C, 65.87; H, 5.90; N, 3.49. Found: C, 65.62;
H, 6.02; N, 3.38. IR (Nujol): 3247 (w, NH), 3203 (w, NH), 2007
(m, RuH). MS for C44H47N2P2ClRu: 802 (M+, 5.9%), 764
(100%). NMR data: see Table 1.
Synthesis of RuH{tmeP2NNH} (5). A mixture of RuHCl-
{tmeP2(NH)2} (80 mg, 0.10 mmol) and KOtBu (17 mg, 0.15
mmol) in toluene (3 mL) was stirred under Ar for 1 h to give
a dark red solution. The mixture was filtered through Celite.
The solvent was removed by vacuum to give a red residue
(yield: 63 mg, 79%). A red crystal of 5 was obtained by the
vapor diffusion of hexane into a THF solution of 5 under Ar
after a week. IR (Nujol): 3233 (w, NH), 2015 (m, RuH). Anal.
Reaction of RuHCl{tmeP2N2} (7) with Base and H2. A
mixture of RuHCl{tmeP2N2} (7) (20 mg, 0.025 mmol) and KOt-
Bu (10 mg, 0.10 mmol) in C6D6 were sealed in an NMR tube
under H2 (1 atm). The mixture turned green in 10 min and
1
then yellow, to give a solution of the dihydride 6 in 1 h. H
NMR(C6D6) hydrides: δ -5.27 (t, 2J(HP) ) 18.3 Hz). 31P
NMR: δ 77.6 (s).
Reaction of [RuH{tmeP2(N)(NH)}] (5) with D2. A solu-
tion of RuH{tmeP2NNH} (5) (20 mg, 0.024 mmol) in C6D6 was
reacted with 1 atm of D2. The dark red solution turned dark
brown in 20 min and then dark red in 1 h.
X-ray Diffraction Structure Determination of RuHCl-
{tmeP2(NH)2} (4A), RuH{tmeP2(N)(NH)} (5), RuH2{tmeP2-
(NH)2} (6), and RuHCl{tmeP2N2} (7). Crystals suitable for
X-ray diffraction were obtained by vapor diffusion. Data were
collected on a Nonius Kappa-CCD diffractometer using Mo KR
radiation (λ ) 0.71073 Å). The CCD data were integrated and
scaled using the DENZO-SMN software package, and the
structure was solved and refined using SHELXTL V6.0. The
crystallographic data are listed in Table 2, and selected bond
distances and angles in Tables 3-6. The hydrides were
located and refined with isotropic thermal parameters (Figures
1-4).

6242 Organometallics, Vol. 23, No. 26, 2004
Li et al.
Table 2. Crystallographic Data for
RuHCl{tmeP2(NH)2} (4A), RuH{tmeP2NNH} (5),
RuH2{tmeP2(NH)2} (6), and RuHCl{tmeP2N2} (7)
4A(1/2CH2Cl2,
1/2THF)
5(THF)
6
7
formula
C
46.50H52Cl2- C48H54N2O- C44H48N2- C44H43Cl-
N2O0.50P2Ru
880.81
P2Ru
P2Ru
767.85
P1h
N2P2Ru
798.26
P2(1)/c
150 (1)
fw
837.94
P2(1)/n
150 (1)
9.3224 (1)
space group
T, K
P1h
150 (1)
150 (1)
A, Å
11.1340(3)
11.2830(3)
18.6220(5)
80.504(2)
75.354(2)
82.767(2)
2223.6(1)
2
11.2950(2) 11.3980(5)
B, Å
12.9914 (1) 12.0470(2) 9.0700(2)
33.9421(4) 14.9080(2) 35.610(1)
C, Å
R, deg
â, deg
γ, deg
V, Å3
Z
90
94.899(6)
90
75.7729(8) 90
80.7890(8) 94.718(1)
75.8090(6) 90
4095.74(7) 1895.53(5) 3668.9(2)
4
2
4
wavelength, Å 0.71073
0.71073
1.359
0.0743
0.0844
0.71073
1.345
0.0549
0.0912
0.71073
1.445
0.0990
0.1172
F
calc, mg/m3
1.316
0.0519
0.1176
R1 (all data)
wR2
Figure 1. Molecular structure of RuHCl{tmeP2(NH)2}
(4A).
Table 3. Selected Bond Distances and Angles for
RuHCl{tmeP2(NH)2} (4A)
Distances, Å
Ru(1)-H(1RU)
Ru(1)-N(2)
Ru(1)-P(2)
N(1)-C(17)
N(1)-H(1A)
N(2)-C(2)
1.56(3)
2.189(2)
2.2348(7)
1.487(3)
0.93
Ru(1)-N(1)
Ru(1)-P(1)
Ru(1)-Cl(1)
N(1)-C(1)
N(2)-C(47)
N(2)-H(2A)
2.179(2)
2.2519(6)
2.5970(6)
1.521(3)
1.485(3)
0.93
1.532(3)
Angles, deg
H(1RU)-Ru(1)-N(1)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-P(2)
H(1RU)-Ru(1)-P(1)
N(2)-Ru(1)-P(1)
85(1)
H(1RU)-Ru(1)-N(2)
87(1)
79.59(8) H(1RU)-Ru(1)-P(2)
166.81(6) N(2)-Ru(1)-P(2)
90(1)
87.86(6)
91.13(6)
100.81(2)
93.66(5)
91.20(2)
86(1)
N(1)-Ru(1)-P(1)
168.37(6) P(2)-Ru(1)-P(1)
H(1RU)-Ru(1)-Cl(1) 179(1)
N(1)-Ru(1)-Cl(1)
N(2)-Ru(1)-Cl(1)
P(1)-Ru(1)-Cl(1)
93.92(6) P(2)-Ru(1)-Cl(1)
93.64(2)
Table 4. Selected Bond Distances and Angles for
RuH{tmeP2NNH} (5)
Distances, Å
Ru(1)-H(1RU)
Ru(1)-N(2)
Ru(1)-P(2)
N(1)-C(7)
1.54(3)
Ru(1)-N(1)
Ru(1)-P(1)
2.164(2)
Figure 2. Molecular structure of RuHCl{tmeP2N2} (7).
2.001(2)
2.2303(7)
1.488(3)
1.482(3)
2.2495(7)
N(1)-C(1)
N(2)-C(8)
1.513(3)
1.475(3)
Table 6. Selected Bond Distances and Angles for
N(2)-C(2)
RuHCl{tmeP2N2} (7)
Angles, deg
90.5(9)
Distances, Å
H(1RU)-Ru(1)-N(1)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-P(2)
H(1RU)-Ru(1)-P(1)
N(2)-Ru(1)-P(1)
H(1RU)-Ru(1)-N(2) 116.9(9)
Ru(1)-H(1RU)
Ru(1)-N(2)
Ru(1)-P(2)
N(1)-C(17)
N(2)-C(2)
1.62(4)
Ru(1)-N(1)
Ru(1)-P(1)
Ru(1)-Cl(1)
N(1)-C(1)
2.099(3)
2.255(1)
2.591(1)
1.537(5)
1.278(5)
80.04(8) H(1RU)-Ru(1)-P(2)
170.44(6) N(2)-Ru(1)-P(2)
91.6(9)
2.112(3)
2.263(1)
1.286(5)
1.516(4)
90.68(6)
91.74(6)
97.82(2)
76.5(9)
N(1)-Ru(1)-P(1)
164.04(6) P(2)-Ru(1)-P(1)
N(2)-C(47)
Table 5. Selected Bond Distances and Angles for
Angles, deg
Ru(H)2{tmeP2(NH)2} (6)
H(1RU)-Ru(1)-N(1)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-P(2)
H(1RU)-Ru(1)-P(1)
N(2)-Ru(1)-P(1)
89(1)
H(1RU)-Ru(1)-N(2) 90(1)
H(1RU)-Ru(1)-P(2) 88(1)
79.1(1)
Distances, Å
166.86(9) N(2)-Ru(1)-P(2)
88.01(8)
94.00(9)
98.32(4)
91.78(8)
90.38(3)
Ru(1)-H(1RU)
Ru(1)-N(2)
Ru(1)-P(2)
N(1)-C(7)
N(1)-H(1N)
N(2)-C(2)
1.59(3)
Ru(1)-H(2RU)
Ru(1)-N(1)
Ru(1)-P(1)
N(2)-H(2N)
N(1)-C(1)
1.62(3)
82(1)
N(1)-Ru(1)-P(1)
2.196(2)
2.2265(6)
1.490(3)
0.88(3)
2.182(2)
2.2205(6)
0.88(3)
169.61(8) P(2)-Ru(1)-P(1)
H(1RU)-Ru(1)-Cl(1) 177(1)
N(1)-Ru(1)-Cl(1)
N(2)-Ru(1)-Cl(1)
P(1)-Ru(1)-Cl(1)
87.29(8) P(2)-Ru(1)-Cl(1)
1.517(3)
1.492(3)
100.83(3)
1.522(3)
N(2)-C(8)
Angles, deg
H(1RU)-Ru(1)-H(2RU) 175(1)
reaction of NH2CMe2CMe2NH2 with 2 equiv of PPh2-
(C6H4-2-CHO) (eq 1). The spectroscopic properties are
very similar to those of MeP2N2.4 The presence of the
imine groups is signaled by a CdN stretch at 1639 cm-1
H(1RU)-Ru(1)-N(1)
H(1RU)-Ru(1)-P(1)
H(2RU)-Ru(1)-N(2)
H(2RU)-Ru(1)-P(1)
N(1)-Ru(1)-P(2)
85(1)
87(1)
H(1RU)-Ru(1)-N(2) 90(1)
H(1RU)-Ru(1)-P(2) 93(1)
86.3(9) H(2RU)-Ru(1)-P(2) 89.5(9)
95.7(9) H(2RU)-Ru(1)-N(1) 91.2(9)
169.75(5) N(2)-Ru(1)-P(2)
170.77(5) N(1)-Ru(1)-P(1)
78.67(7) P(2)-Ru(1)-P(1)
91.17(5)
92.25(5)
97.85(2)
N(2)-Ru(1)-P(1)
in the IR spectrum, a doublet (4JPH ) 5.2 Hz) at 8.88
N(1)-Ru(1)-N(2)
1
ppm in the H NMR spectrum, and a doublet (3JPC
)
23.0 Hz) at 155.0 ppm in the 13C{1H} NMR spectrum.
The diamine, {tmeP2(NH)2}, is obtained by the reduction
of {tmeP2N2} with LiAlH4 in 87% yield (eq 2). The NH
Results and Discussion
Synthesis of the Ligands {tmeP2N2} and {tmeP2-
(NH)2}. The new tetradentate ligand {tmeP2N2} is
easily prepared13 in 80% yield by the condensation
1
protons appear at 1.37 ppm in the H NMR. NaBH4 is

Dihydridoamine Complexes of Ruthenium(II)
Organometallics, Vol. 23, No. 26, 2004 6243
stable in the solid state for days under air, but it is air
sensitive in solution. The isomers are present in variable
amounts. The yellow isomer 4A is only slightly soluble
in THF and can be isolated by washing the pink solid
mixture with THF to remove the red isomer 4B. Isomer
4A can be fully converted to isomer 4B by heating the
CH2Cl2 solution of 4A under Ar overnight (eq 4). This
shows that isomer 4B is the thermodynamically favored
product and isomer 4A is the kinetically favored prod-
uct. This also explains why the ratio of the two isomers
in the synthesis of the complex depends on the concen-
tration, temperature, and the reaction time.
Figure 3. Molecular structure of the hydridoamido com-
plex 5.
The structure of 4A was established by the X-ray
diffraction study that revealed the octahedral configu-
ration shown in Figure 1. The hydride and chloride
ligands are trans to each other. Isomer 4A has two NH
hydrogen atoms syn to the hydride ligand, with H(1A)
axial and H(2A) equatorial with respect to the RuNCCN
five-membered ring. The stereogenic nitrogen atoms
have opposite configurations (R and S). At 200 K, 4A
exhibits two doublets at 68.2 and 62.7 ppm in the 31P-
{1H} NMR spectrum and four singlets for the methyl
groups and a doublet of doublets for the hydride at
1
-17.5 ppm in the H NMR spectrum. At room temper-
ature the spectra simplify to a broad peak at 65.9 ppm
for the 31P{1H} NMR and two singlets for the methyls
1
and a triplet for the hydride for the H NMR. A rapid
flipping of the RuNHCMe2CMe2NH five-membered
ring14 at room temperature would interchange axial and
equatorial NH and methyl groups and interchange the
environments of the PPh2 groups, thus averaging their
chemical shifts. Therefore this tetradentate ligand,
despite its hindered appearance, is quite flexible.
Isomer 4B exhibits two doublets in the 31P NMR
spectrum and four singlets for the methyl groups and a
doublet of doublets for the hydride in the 1H NMR. The
two phosphorus atoms and NH groups are not equiva-
lent because there is no molecular symmetry even with
a flipping of the backbone. The structure of 4B likely
has one NH hydrogen syn to the chloride and the other
NH syn to the hydride, both in axial positions with
respect to the RuNCCN ring, as shown in eq 4. Two
related X-ray crystal structures of RuCl2(enP2(NH)2)
and RuCl2(cyP2(NH)2) were reported by Gao et al.1 The
NH groups in these complexes are on opposite sides of
the tetradentate ligand plane as proposed for isomer 4B.
A small amount of a third isomer could be observed after
heating isomer 4B overnight in THF. The third isomer
was not isolated. The structure of the third isomer is
proposed to have two NH hydrogen atoms syn to
chloride.
Figure 4. Molecular structure of the dihydridoamine
complex 6.
not suitable forthis reduction. Its use led to a mixture
of incompletely reduced compounds.
Synthesis, Isomerization Reactions, and Struc-
tures of RuHCl{tmeP2(NH)2} 4A and 4B. The pink
complex trans-RuHCl{tmeP2(NH)2} is prepared as a
mixture of isomers 4A and 4B in 73% yield by the
reaction of RuHCl(PPh3)3 with 1 equiv of the ligand
{tmeP2(NH)2} in refluxing THF (eq 3). The mixture is
Synthesis and Structure of the Diimine Com-
plex RuHCl{tmeP2N2} (7). The complex trans-RuHCl-
(13) Jeffery, J. C.; Rauchfuss, T. B.; Tucker, P. A. Inorg. Chem. 1980,
19, 3306-3316.
(14) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104-15118.

6244 Organometallics, Vol. 23, No. 26, 2004
Li et al.
{tmeP2N2} (7) is prepared in 75% yield as a sparingly
soluble pink powder by heating a solution of RuHCl-
(PPh3)3 in THF with the ligand {tmeP2N2} (eq 5). In the
preparation of 7, only one isomer is formed. Therefore,
the occurrence of isomers 4A and 4B must be related
to the direction of the NH groups.
similar to that of the only other structurally character-
ized hydridoamido complex, RuH(NHCMe2CMe2NH2)-
(PPh3)2.14 The amido nitrogen-ruthenium bond length
of 2.001(2) Å (Ru(1)-N(2)) is similar to the ruthenium-
nitrogen double bond distance of 1.967(1) Å for the latter
complex and shorter than the amine nitrogen-ruthe-
nium distance, which is 2.164(2) Å (Ru(1)-N(1)). The
N-H bond is in an axial position with respect to the
RuNCCN ring and is aligned with the Ru-H bond. The
doublet of doublets pattern for the hydride at -23.8 ppm
in the 1H NMR spectrum and two doublets at 66.0 and
61.1 ppm with 2JPP ) 17.7 Hz in the 31P NMR spectrum
signal the presence of the two nonequivalent phosphorus
atoms, as observed in the X-ray structure. Even though
the P-Ru-P angles are similar (98-101°) in complexes
The X-ray crystal structure of 7 shows a trans
octahedral complex (Figure 2). The bond distances of
C(47)-N(2) and C(17)-N(1) are 1.278(5) and 1.286(5)
Å, as expected for imine groups. The Ru-P bond lengths
of 7 (2.255(1) and 2.263(1) Å) are almost identical to
those of 4A (2.252(1) and 2.235(1) Å, while the Ru-N
bond lengths are shorter (2.099(3) and 2.112(3) vs
2.179(2) and 2.189(2) Å, respectively). Those of 7 are
similar to the bond lengths in RuCl2(cyP2N2), with
Ru-P bond lengths of 2.288(2) and 2.295(2) Å and
Ru-N bond lengths of 2.091(5) and 2.100(5) Å.1
2
5, 4, and 7, the coupling constant JPP of the amido
complex 5 is noticeably smaller than those of the
octahedral complexes 4 and 7 (approximately 31 Hz).
1
No change was observed in the H and 31P{1H} NMR
spectra of 5 in toluene-d8 at 200 K.
Synthesis and Structure of the Dihydridoamine
Complex RuH2{tmeP2(NH)2} (6). The amido complex
RuH{tmeP2NNH} reacts with dihydrogen to produce
exclusively the dihydride complex trans-RuH2{tmeP2-
(NH)2}, 6 (eq 7). This complex is only stable under H2.
It loses H2 under Ar in solution and the solid state to
give complex 5. This complex can also be prepared
directly in about 40% yield from complex 4 by reaction
with KOtBu under H2 (1 atm) in C6D6, but other
hydride-containing species are also produced.
Evidence for the proposed structure is also provided
1
by the H NMR spectrum that reveals a triplet for the
hydride resonance at -17.1 ppm and a doublet for the
imine groups at 9.1 ppm. The triplet pattern of the
hydride and doublet of the imine hydrogen atoms are
due to coupling to the phosphorus nuclei. The 31P NMR
spectrum has a broad peak at 59 ppm. At 200 K, the
31P{1H} NMR spectrum of 7 in CD2Cl2 exhibits two
doublets at 58.4 and 56.2 ppm, while the 1H NMR
spectrum shows four singlets for the methyl groups, a
doublet of doublets for the hydride at -16.9 ppm, and
two doublets for the imine hydrogen atoms. These data
are similar to the spectroscopic properties of trans-
RuHCl{tmeP2(NH)2} (4A), again providing evidence for
the flipping of the RuNCMe2CMe2N ring. The similarity
of the NMR spectra and crystal structures of 4A and 7
suggests that the sp2- and sp3-hybridized nitrogen
donors place the Ru in a similar electronic state.
Synthesis and Structure of the Hydridoamido
Complex RuH{tmeP2NNH} (5). The reaction of a
mixture of 4A and 4B with KOtBu under Ar in THF
produces the red amido complex RuH{tmeP2NNH} (5)
in 79% yield (eq 6). This complex is air sensitive in both
the solid state and in solution.
Complex 6 was crystallized under H2, and the struc-
ture was determined by X-ray diffraction (Figure 4).
Complex 6 has an octahedral geometry with approxi-
mately C2 symmetry. The Ru(1)-N(1) and Ru(1)-N(2)
bond lengths of 2.196(2) and 2.182(2) Å are comparable
to those of RuH2(tmen)(R-binap), 2.202(2) and 2.193(2)
Å.10 The two hydrides are trans (H(1Ru)-Ru(1)-H(2Ru)
175(1)°). Each of the two N-H bonds is aligned with
one of the two Ru-H bonds. The X-ray crystal structure
also reveals that the RuH‚‚‚HaxN distances of about 2.4
Å are at the outer limit of hydridic-protonic bonding.15,16
The C2 symmetry of the complex in solution is revealed
by the triplet pattern at -5.28 ppm (2JPH ) 18 Hz) of
the hydride in the 1H NMR spectrum and a sharp
singlet at 77.5 ppm in the 31P{1H} NMR spectrum. The
dihydride trans-RuH2(tmen)(R-binap) shows similar
features, notably a hydride triplet at -4.8 ppm with a
small coupling of 17 Hz.10
Complex 5 was crystallized under Ar, and the struc-
ture was determined by X-ray diffraction (Figure 3). The
amido complex 5 is best viewed as a distorted trigonal
bipyramidal with the N(1) and P(2) in pseudoaxial posi-
tions, forming the angle N(1)-Ru(1)-P(2) ) 170.44(6)°.
This places the amido nitrogen in an equatorial position
for favorable dative π-bonding to Ru. This interaction
causes the hydride and P(1) to squeeze together with
H(1RU)-Ru(1)-P(1) ) 76.5(9)°. This is a geometry very
The stereochemistry of 6 also can be established by
reacting RuH{tmeP2NNH} with D2(g) to form the trans
isotopomer RuHD{tmeP2(ND)(NH)} (shown in eq 8)
along with other isotopomers resulting from further H/D
exchange at RuH, NH, and CH2. The isotopomers with
(15) Lough, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. J. Am.
Chem. Soc. 1994, 116, 8356-8357.
(16) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold,
A. L.; Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348-354.

Dihydridoamine Complexes of Ruthenium(II)
Organometallics, Vol. 23, No. 26, 2004 6245
Scheme 2. Mechanism for H/D Exchange between
Hydrogenation of Acetophenone Catalyzed by
Complex 5. Complex 5 in benzene, toluene, or 2-pro-
panol is a well-defined catalyst for the hydrogenation
of acetophenone. Some rate determination measure-
ments have served to define the kinetic properties
(Table 7).9,10,14
a
D2 (g) and Complex 6-d2
In benzene solution, complex 5 is 24 times less active
than RuH2(tmen)(R-binap) and 141 times less active
than the analogous catalyst system RuHCl(cyP2(NH)2)/
KOtBu (entries 1, 2, and 3 in Table 7). It is significant
that replacing a cyclohexyl backbone between the
amines in the last complex with a tetramethylethylene
backbone in 5 results in such a reduction in rate.
Presumably this is a combination of steric and electronic
interference of the addition of dihydrogen to the amido
complex, the rate-determining step of the catalytic
reaction, at least for the catalyst system 1/KOtBu/
iPrOH. Complex 5 is 4 times more active in 2-propanol
than in benzene (entry 4) and does not require the
addition of base for activity. The increase in activity of
this catalyst as well as that of 4A/4B/KOtBu (entry 5)
and that of 1/KOtBu (entry 6) in 2-propanol compared
to similar systems in benzene can be attributed to the
more polar solvent stabilizing the transition state of
what is believed to be the turnover limiting step, the
heterolytic splitting of dihydrogen. Complex 5 in iPrOH
shows activity similar to that of the catalyst system 4A/
4B/KOtBu/iPrOH, where a mixture of the isomers of the
precatalyst is used. Therefore the isomers of 4 react with
base quickly to form the amido complex 5, which then
serves as the catalyst in the hydrogenation of acetophen-
one (as shown in Scheme 1 for precatalyst 1). This also
indicates that potassium ions do not play a role in
accelerating the catalysis for this system in 2-propanol,
although such an effect has been reported for a RuCl2-
(binap)(dpen) system.18 No significant potassium effect
was observed in the use of complex 1 either.9 However
the tmeP2(NH)2 systems (entries 4 and 5) are more than
1000 times less active than the corresponding catalyst
system 1/KOtBu/iPrOH (entry 6).
a Only atoms directly involved are shown for clarity.
a trans H-Ru-D motif produce overlapping triplets for
1
the hydrides in the H NMR spectrum at -5.15 ppm.
This large downfield shift of 0.13 ppm from the hydride
resonance of undeuterated 6 is caused by the large trans
influence of deuterium. This proves that the hydrides
are trans in the dihydride since a deuterium cis to a
hydride would not have such a large influence on the
chemical shift.
Hydrogenation of Acetophenone Catalyzed by
Complex 7 and Base. RuHCl{tmeP2N2} reacts with
KOtBu in 2-propanol under H2 (6 atm) to produce a
catalytic system for the hydrogenation of acetophenone.
The initial rate of hydrogenation catalyzed by this
system (entry 7) is listed relative to the other catalysts
in Table 7.9
For comparison the diimine complex RuHCl{(R,R)-
cyP2N2} (8) was prepared by a method similar to that
for complex 7. This complex was found to produce a
catalyst that is 50 times more active than the RuHCl-
{tmeP2N2} precatalyst (entry 8 vs entry 7 in Table 7)
and gives R-1-phenylethanol with an ee of ca. 37%. The
diamine system is also more active than the diimine
system under these conditions (entries 6 vs 8). However,
the reason the RuHCl{tmeP2N2}/base system is about
3 times more active than the amidoamine complex
(entries 4 and 5 in Table 7) is not clear at this stage.
Reaction of RuHCl{tmeP2N2} (7) with Base and
H2. The diimine precatalyst trans-RuHCl{tmeP2N2} can
be converted to the diaminedihydride catalyst trans-Ru-
(H)2{tmeP2(NH)2} given a sufficiently long reaction time
The three overlapped singlets at 77.78, 77.65, and
77.52 ppm in the 31P{1H} NMR spectrum suggest that
there are at least three different isotopomers in the
exchange process. Surprisingly, the analysis by 1H NMR
shows the slow decrease of the peaks due to CH2 groups
caused by a hydrogen-deuterium exchange process. The
exchange of methylene protons with solvent deuteriums
was reported in a ruthenium(II) hydrido complex of 2,6-
(diphenylphosphinomethyl)pyridine.17 We propose that
the C-D bonds form by a â-hydride elimination (Scheme
2) from the amido complex 5 and its isotopomers to
produce imine complexes of the type 9-d1.6 This step
might proceed via an intermediate where the imine Cd
N is π-bonded to ruthenium. The addition of the
deuteride to the imine creates a new C-D bond. In the
presence of extra D2, this process eventually leads to
almost complete deuteration of the methylene positions.
There is no deuteration of the phenyl C-H bonds.
(17) Rahmouni, N.; Osborn, J. A.; De Cian, A.; Fischer, J.; Ezzama-
rty, A. Organometallics 1998, 17, 2470-2476.
(18) Hartmann, R.; Chen, P. Angew. Chem., Int. Ed. 2001, 40, 3581-
3585.

6246 Organometallics, Vol. 23, No. 26, 2004
Li et al.
Table 7. Comparison of Initial Rates of Reaction of Catalyst Systems for the Hydrogenation of
Acetophenone (0.167 M) under 6 atm H2 at 293 K
entry
catalyst
solvent
[Ru]
rate (M s-1
)
rate/[Ru]
relative rates
ref
1
2
3
4
5
6
7
8
5
C6H6
2.0 × 10-4
2.3 × 10-4
2.0 × 10-4
2.0 × 10-4
2.0 × 10-4
2.0 × 10-4
2.0 × 10-4
2.0 × 10-4
9 × 10-7
4.5 × 10-3
1
24
141
4
4
5555
14
667
this work
14
RuH2(tmen)(R-binap)
C6H6
3 × 10-5
0.12
1/KOtBu
C6H6
1.2 × 10-4
3.7 × 10-6
4.6 × 10-6
5.0 × 10-3
1.3 × 10-5
6.0 × 10-4
0.60
9
5
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
0.02
this work
this work
9
this work
this work
4A/4B/KOtBu
1/KOtBu
0.02
25
7/KOtBu
0.065
3
8a/KOtBu
a RuHCl{(R,R)-cyP2N2}.
to the ketone in the outer coordination sphere21-23 as
shown in Figure 5 results in the ionic hydrogenation24
Scheme 3. Catalytic Cycle Showing the
Structures of the Catalytic Species
or higher temperatures (eq 9). The mechanism must
involve â-hydride addition as in Scheme 2. Therefore
the imine functions of the ligand are hydrogenated to
the amine functions in the course of the reaction. A
similar diimine dichloride complex has been reduced to
the corresponding diamine dichloride complex by use
of NaBH4.6
Figure 5. Proposed transition state for the ionic hydro-
genation of the ketone in the outersphere of the dihydride
complex 6.
of the ketone and regeneration of the hydridoamido
complex.
Conclusions
The new tetradentate ligands PPh2C6H4CHdNCMe2-
CMe2NdCHC6H4PPh2 {tmeP2N2} and PPh2C6H4CH2-
NHCMe2CMe2NHCH2C6H4PPh2 {tmeP2(NH)2} were pre-
pared and used to make the new complexes RuHCl-
{tmeP2(NH)2} (4), RuH{tmeP2NNH} (5), trans-RuH2-
{tmeP2(NH)2} (6), and RuHCl{tmeP2N2} (7). The struc-
ture of 4 as isomer 4A shows for the first time the
presence of two N-H bonds on the same side as the
Ru-H bond in such tetradentate ligand complexes. The
ease of formation of the isomers derived from the
different configurations at nitrogen might explain why
the ee of the alcohol product produced by catalysts
containing the (S,S)-cyP2(NH)2 ligand can be high for
transfer hydrogenation and low for H2-hydrogenation.9
The formation of cis and trans dihydrides at ruthenium
is an alternative explanation that cannot be ruled out
at this stage. The Ru hydridoamido complex 5 quickly
reacts with H2 to give the dihydridoamine complex 6
as proposed in the catalytic cycle for the analogous RuH-
{cyP2NNH}/RuH2{cyP2(NH)2} system. This provides
additional evidence for the mechanism of the hydroge-
nation of ketones catalyzed by these tetradentate ligand
systems.9 Alternative, more classical mechanisms that
involve the coordination of the substrate to the metal
Proposed Mechanism for the Hydrogenation of
Acetophenone. The structural studies and experimen-
tal observations of these complexes derived from the
{tmeP2(NH)2} ligand provide evidence for the mecha-
nism of hydrogenation proposed earlier for the related
{cyP2(NH)2} system (Scheme 1). Specifically the two
catalytic species, the trans-dihydride 6 and the hydri-
doamido complex 5, have been completely characterized
and react in a catalytic fashion as shown in Scheme 3.
For the {cyP2(NH)2} system the dihydride was observed
only in solution and the amido complex was too unstable
to be characterized. The hydridoamido complex 5 is
proposed to coordinate dihydrogen and split it hetero-
lytically19,20 into a hydride on ruthenium and proton on
nitrogen to produce the trans-dihydride 6.14 As men-
tioned, the observed increase in the rate of hydrogena-
tion on going from the less polar solvent benzene to the
more polar solvent 2-propanol lends support to this
proposal. The concerted transfer of hydride and proton
(21) Clapham, S.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.
2004, in press.
(22) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40-
(19) Morris, R. H. In Recent Advances in Hydride Chemistry;
Peruzzini, M., Poli, R., Eds.; Elsevier: Amsterdam, 2001; pp 1-38.
(20) Kubas, G. J. Metal Dihydrogen and Sigma-Bond Complexes;
Kluwer Academic/Plenum: New York, 2001.
73.
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Dihydridoamine Complexes of Ruthenium(II)
Organometallics, Vol. 23, No. 26, 2004 6247
can be ruled out because they would have to proceed
via decoordination of a donor group of the tetradentate
ligand, a very improbable event under the mild condi-
tions of catalysis described here. The correct steric and
electronic properties of the ligand are clearly very
important for the activation of dihydrogen, and therefore
the catalyst activity. The exchange of deuterium for
hydrogen on the CH2 groups in the dihydride complex
suggests that the reversible formation of an imine ligand
is occurring by â-hydride elimination.
Acknowledgment. This work was supported in
part by a discovery grant to R.H.M. from NSERC
Canada and a PRF grant, as administered by the
American Chemical Society.
Supporting Information Available: X-ray crystallo-
graphic file (CIF) containing X-ray structural information for
complexes 4A, 5, 6, and 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM049565K