Chemistry of Ruthenium(II) monohydride and dihydride complexes containing pyridyl donor ligands including catalytic ketone H2-hydrogenation

Article abstract of DOI:10.1021/ic0489387

In this study we determine the changes to the properties of dihydride catalysts for ketone H₂-hydrogenation by successively replacing the amine donors in the known dach complex RuH₂(PPh₃) ₂(dach) (2a), dach = 1,2-(R,R)-diaminocyclohexane, with one pyridyl group in the corresponding 2-(aminomethyl)pyridine (ampy) complexes RuH ₂(PPh₃)₂(ampy) (2b) and with two pyridyl groups in the complexes RuH₂(PPh₃)₂(bipy) (2c) and RuH₂(PPh₃)₂(phen) (2d). The ruthenium monohydride complex, (OC-6-54)-RuHCI(PPh₃)₂(ampy), (1b with CI trans to H) was prepared by the addition of 1 equiv of ampy to RuHCI(PPh₃)₃ in THF. Treatment of the monohydride complex with K[BH-(sec-Bu)₃] in THF or KO^tBu/H₂ in toluene resulted in the formation of a mixture of at least two isomers of the highly reactive, air-sensitive ruthenium dihydride complex 2b. One is the cis dihydride (OC-6-14)-2b or more simply c,t-2b with trans PPh₃ groups and another is the cis dihydride c,c-2b (OC-6-42) that has PPh₃ trans to H and PPh₃ trans to N(pyridyl). The isomer c,c-2b slowly converts to c,t-2b in solution. The reaction of 1b with KO^tBu under Ar results in the formation of a mixture that includes a complex with an imino ligand HN=CH-2-py while the same reaction under H₂ leads to c,c-2b and then c,t-2b. The dach complex c,t-2a, reacts with ampy, 2,2′- bipyridine (bipy), and 1,10-phenanthroline (phen) in refluxing THF to form the substituted cis-dihydride complexes c,t-2b, (OC-6-13)-RuH₂(PPh ₃)₂(bipy) (c,t-2c with trans PPh₃ groups) and (OC-6-13)-RuH₂(PPh₃)₂(phen), c,t-2d, respectively, The dihydrides containing amino groups and cis-PPh₃ groups, i.e., c,c-2a or c,c-2b, are active precatalysts for the H ₂-hydrogenation of acetophenone (neat or in benzene) under mild reaction conditions, whereas those with trans-PPh₃ groups, c,t-2a and c,t-2b are much less active. The combination of ampy complex 1b and KO ^tBu also provides a catalyst in benzene that is more active than the corresponding dach system. The complexes without amino groups c,t-2c and c,t-2d are air-stable and inactive as hydrogenation catalysts under comparable conditions. The mechanism of hydrogenation of ketones catalyzed by isomers of 2a,b is thought to be similar and to proceed via a trans-dihydride complex, t,c-2a or t,c-2b, and an amido complex, neither of which are directly observed for the ampy complexes. The dihydride complex c,t-2b reacts with formic acid to give (OC-6-45)-RuH(OCHO)(PPh₃)₂(ampy), 3b, with formate trans to hydride. The structures of 1b, c,t-2b, c,t-2c, and 3b have been determined by single-crystal X-ray diffraction.

Full text of DOI:10.1021/ic0489387

Inorg. Chem. 2005, 44, 2483−2492
Chemistry of Ruthenium(II) Monohydride and Dihydride Complexes
Containing Pyridyl Donor Ligands Including Catalytic Ketone
H2-Hydrogenation1
Kamaluddin Abdur-Rashid,† Robert Abbel, Alen Hadzovic, Alan J. Lough, and Robert H. Morris*
Department of Chemistry, UniVersity of Toronto, 80 St. George St.,
Toronto, Ontario M5S 3H6, Canada
Received August 3, 2004
In this study we determine the changes to the properties of dihydride catalysts for ketone H2-hydrogenation by
successively replacing the amine donors in the known dach complex RuH2(PPh3)2(dach) (2a), dach 1,2-(R,R)-
)
diaminocyclohexane, with one pyridyl group in the corresponding 2-(aminomethyl)pyridine (ampy) complexes RuH2-
(PPh3)2(ampy) (2b) and with two pyridyl groups in the complexes RuH2(PPh3)2(bipy) (2c) and RuH2(PPh3)2(phen)
(2d). The ruthenium monohydride complex, (OC-6-54)-RuHCl(PPh3)2(ampy), (1b with Cl trans to H) was prepared
by the addition of 1 equiv of ampy to RuHCl(PPh3)3 in THF. Treatment of the monohydride complex with K[BH-
(sec-Bu)3] in THF or KOtBu/H2 in toluene resulted in the formation of a mixture of at least two isomers of the highly
reactive, air-sensitive ruthenium dihydride complex 2b. One is the cis dihydride (OC-6-14)-2b or more simply c,t-2b
with trans PPh3 groups and another is the cis dihydride c,c-2b (OC-6-42) that has PPh3 trans to H and PPh3 trans
to N(pyridyl). The isomer c,c-2b slowly converts to c,t-2b in solution. The reaction of 1b with KOtBu under Ar
results in the formation of a mixture that includes a complex with an imino ligand HN
dCH-2-py while the same
reaction under H2 leads to c,c-2b and then c,t-2b. The dach complex c,t-2a, reacts with ampy, 2,2
′
-bipyridine
(bipy), and 1,10-phenanthroline (phen) in refluxing THF to form the substituted cis-dihydride complexes c,t-2b,
(OC-6-13)-RuH2(PPh3)2(bipy) (c,t-2c with trans PPh3 groups) and (OC-6-13)-RuH2(PPh3)2(phen), c,t-2d, respectively.
The dihydrides containing amino groups and cis-PPh3 groups, i.e., c,c-2a or c,c-2b, are active precatalysts for the
H2-hydrogenation of acetophenone (neat or in benzene) under mild reaction conditions, whereas those with trans-
PPh3 groups, c,t-2a and c,t-2b are much less active. The combination of ampy complex 1b and KOtBu also provides
a catalyst in benzene that is more active than the corresponding dach system. The complexes without amino
groups c,t-2c and c,t-2d are air-stable and inactive as hydrogenation catalysts under comparable conditions. The
mechanism of hydrogenation of ketones catalyzed by isomers of 2a,b is thought to be similar and to proceed via
a trans-dihydride complex, t,c-2a or t,c-2b, and an amido complex, neither of which are directly observed for the
ampy complexes. The dihydride complex c,t-2b reacts with formic acid to give (OC-6-45)-RuH(OCHO)(PPh3)2-
(ampy), 3b, with formate trans to hydride. The structures of 1b, c,t-2b, c,t-2c, and 3b have been determined by
single-crystal X-ray diffraction.
Introduction
diamines to RuCl2(PPh3)3 and base in 2-propanol as reported
by Noyori and co-workers.2,3 Our own interest in ruthenium
hydride and dihydrogen chemistry and, in particular, hy-
dridic-protonic M-H‚‚‚H-N bonds4-8 led us to investigate
The first demonstrations of the N-H effect on the
hydrogenation of ketones was the generation of very active
catalyst systems by the addition of ethylenediamine and other
(1) This work was presented at the Canadian Society for Chemistry
Conference, Vancouver, Cananda, 2002.
(2) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1995, 117, 2675-2676.
(3) Noyori, R.; Koizumi, M.; Ishii, D.; Ohkuma, T. Pure Appl. Chem.
2001, 73, 227-232.
* Author to whom correspondence should be addressed. E-mail:
rmorris@chem.utoronto.ca.
† Current address: Kanata Chemical Technologies Inc., 2240 Speakman
Dr., Sheridan Science and Technology Park, Mississauga, Ontario L5K 1A9,
Canada.
10.1021/ic0489387 CCC: $30.25
Published on Web 03/08/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 7, 2005 2483

Abdur-Rashid et al.
Chart 1. Labels for the Complexes
Scheme 1. Succession of Dihydrides Produced by Reacting Complex
1a with Base under H2
the possible ruthenium monohydride and dihydride species
which are expected to be generated in these basic ruthenium-
(II) dichloride mixtures. We have demonstrated that trans-
dihydride species of the type RuH2(PPh3)2(diamine), diamine
) (R,R)-1,2-diaminocyclohexane (dach),9 ethylenediamine
(en),9 and 2,3-dimethyl-2,3-diaminobutane (tmen),10 or trans-
RuH2(diphosphine)(diamine)10,11 and the corresponding ami-
do complexes derived from these dihydrides are exceptionally
active catalysts for the hydrogenation of ketones and imines.
In this paper we determine the changes to the properties of
the catalysts by successively replacing the amine donors in
the dach complexes (these complexes are labeled with an a
below; see Chart 1) with one pyridyl group in corresponding
2-(aminomethyl)pyridine (ampy) complexes (b) and with two
pyridyl groups in 2,2′-bipyridine (c) and phenanthroline (d)
complexes. The dach and ampy ligands have an amino group
that is thought to be crucial for this catalytic mechanism
while the last two do not. Therefore, only the dach and ampy
complexes are expected to lead to active hydrogenation
catalysts, a finding that is verified here.
Scheme 2. Catalytic Cycle for the Hydrogenation of Acetophenone by
(R,R)-1,2-Diaminocyclohexane Complexes in Benzene
We have recently discovered a succession of hydrides
generated by reacting (OC-6-43)- RuHCl(dach)(PPh3)2 (1a)12,13
in benzene solution with a strong base under Ar (step i,
Scheme 1) and then under H2 (step ii, Scheme 1). The
unstable amido complex RuH(NHC6H10NH2)(PPh3)2, 4a, is
produced in step i. This reacts with 1 atm of H2 in step ii to
produce the trans-dihydride (OC-6-22)-RuH2(dach)(PPh3)2,
t,c-2a, that isomerizes quickly in step iii to the cis-dihydride
diastereomers ∆-c,c-2a and Λ-c,c-2a. The cis, cis isomers
slowly isomerize in step iv almost completely until reaching
equilibrium with the cis-dihydride with trans-phosphines, c,t-
2a.
(4) Lough, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. J. Am. Chem.
Soc. 1994, 116, 8356-8357.
(5) Morris, R. H. In Recent AdVances in Hydride Chemistry; Peruzzini,
M., Poli, R., Eds.; Elsevier: Amsterdam, 2001; pp 1-38.
(6) Abdur-Rashid, K.; Gusev, D.; Lough, A. J.; Morris, R. H. Organo-
metallics 2000, 19, 834-843.
(7) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J.; Morris,
R. H. J. Am. Chem. Soc. 1998, 120, 11826-11827.
(8) Landau, S. E.; Groh, K. E.; Lough, A. J.; Morris, R. H. Inorg. Chem.
2002, 41, 2995-3007.
Despite the instability of 4a and t,c-2a, we demonstrated
that these are the active catalysts in the ketone hydrogenation
mechanism (Scheme 2). The amido complex 4a is formed
in the catalytic cycle by donation of 1 equiv of H+/H- to
the ketone from a Ru-H‚‚‚H-N unit of the trans-dihydride.
The trans-dihydride is regenerated by reaction of 4a with
dihydrogen, presumably via an unstable η2-dihydrogen
ligand, which splits heterolytically into a hydride and a
proton. The proton adds to the amido nitrogen to regenerate
the N-H group.10,14 By contrast the more stable cis-dihydride
(9) Abbel, R.; Abdur-Rashid, K.; Faatz, M.; Hadzovic, A.; Lough, A. J.;
Morris, R. H. J. Am. Chem. Soc. 2005, 127, 1870-1882.
(10) 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.
(11) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am. Chem.
Soc. 2001, 123, 7473-7474.
(12) For the (OC-6-ij) stereochemical descriptor for octahedral complexes,
i is the Cahn-Ingold-Prelog priority of the atom trans to the highest
priority (1) atom and j is the priority of the atom trans to the next
highest priority atom along another axis of the octahedron. If there is
an ambiguity, then the axis with the maximum difference in numbers
of the trans atoms is chosen. E.g. for (OC-6-54)-1b the priorities are
Cl (1), P(PPh3) (2), N(pyridyl) (3), N(amino) (4), and H (hydride)
(5); see ref 13.
(13) Geoffroy, G. L. Inorganic and Organometallic Stereochemistry. In
Topics in Stereochemistry; Allinger, N. L., Eliel, E. L., Eds.; Wiley:
New York, 1981; Vol. 12.
(14) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2000,
19, 2655-2657.
2484 Inorganic Chemistry, Vol. 44, No. 7, 2005

Ruthenium(II) Monohydride and Dihydride Complexes
complex c,t-2a9,14 and monohydrides of the type RuHCl-
(PPh3)2(diamine) and RuHCl(diphosphine)(diamine) are in-
active and only function as precatalysts by reaction with base.
Therefore the dipolar proton-hydride motif, constituting a
hydride (Ru-H) and amine (N-H) coordinated cis on
ruthenium(II), plays a crucial role in the molecular recogni-
tion of the polar carbon-heteroatom double bond of a ketone
and facilitates its reduction by an outer-sphere hydrogenation
mechanism. This has been called metal-ligand bifunctional
catalysis by Noyori et al.15 or hydrogenation in the outer
sphere with ligand assistance (HOL) by some of us.16
Yamakawa et al.17 and our group10 have shown by use of
DFT calculations that this concerted transfer is a low-barrier
process. With the Shvo catalyst system, Casey and Johnson
have shown experimentally that this outer-sphere transfer
involves the concerted transfer of hydride and proton.18
In related work, Mizushima et al.19 have examined the use
of chiral ampy-type ligands 2-R1-6-R2-C5H3N where R1 )
H, Me, R2 ) CH2NHCHR3Me, and R3 ) phenyl, naphthyl
in the asymmetric transfer hydrogenation of aryl-alkyl
ketones catalyzed by ruthenium(II) complexes. These were
added to either RuCl2(PPh3)3 or RuHCl(PPh3)3 in THF to
generate uncharacterized solutions that were used to transfer
hydrogen from 2-propanol or HCOOH/NEt3 to the ketones
to produce alcohols with up to 86% ee. Baratta et al.20,21
prepared a 2-(aminomethyl)pyridine complex RuCl(CO){(2-
CH2-6-MeC6H3)PPh2}(ampy) that was an active precatalyst
for the transfer of hydrogen from 2-propanol to ketones.
Brunner’s group has examined the use of potentially triden-
tate ligands with 1-(pyridin-2-yl)ethylamine or Schiff base
ligands with amine and pyridyl donors as chiral units in
combination with RuCl2(PPh3)3 and KOtBu as catalyst
systems for the asymmetric transfer hydrogenation of
ketones.22,23
vacuum line, and glovebox techniques in dry, oxygen-free solvents.
Tetrahydrofuran (THF), diethyl ether (Et2O), and hexanes were
dried and distilled from sodium benzophenone ketyl. Deuterated
solvents were degassed and dried over activated molecular sieves.
NMR spectra were recorded on a Varian Unity-500 (500 MHz for
1
1H), a Varian Unity-400 (400 MHz for H), or a Varian Gemini
1
300 MHz spectrometer (300 MHz for H and 121.5 for 31P). All
31P chemical shifts were measured relative to 85% H3PO4 as an
1
external reference. H chemical shifts were measured relative to
partially deuterated solvent peaks but are reported relative to
1
tetramethylsilane. H NMR NOE experiments were done at 500
MHz. All infrared spectra were obtained on a Nicolet 550 Magna-
IR spectrometer. Microanalyses were performed by Guelph Chemi-
cal Laboratories Ltd. or in our Chemistry Department.
Synthesis. RuHCl(PPh3)2(ampy) (1b). A mixture of RuHCl-
(PPh3)3 (2.0 g, 2.16 mmol) and 2-(aminomethyl)pyridine (240 mg,
2.22 mmol) in THF (5 mL) was stirred for 30 min under an
atmosphere of nitrogen. The resulting solution was filtered, and
hexanes (10 mL) added to the filtrate, precipitating a yellow solid
that was filtered out, washed with hexanes, and dried under vacuum.
Yield ) 1.51 g of (OC-6-54)-1b, 70%. Once the product was
yellow-green due to the presence of a second isomer t-1b with trans
PPh3 groups. It is not clear what caused this variation. IR (Nujol):
1941, 2035 cm-1 (νRuH), 3140, 3214, 3345 cm-1 (νNH). Anal.
Calcd for C42H39ClN2P2Ru: C, 65.49; H, 5.10; N, 3.64. Found:
C, 64.94; H, 5.05; N, 3.56. NMR for 1b: 1H NMR (C6D6) δ 8.00-
8.20 ppm (m, pyridyl-H), 6.90-7.10 ppm (m, phenyl-H), 3.85, 3.15,
2
2.95, 2.05 ppm (m, CH2 and NH2), -16.70 ppm (dd, JHP ) 24
Hz, 30 Hz, RuH); 31P{1H} NMR (C6D6) δ 70.5 (d), 74.3 (d, 2JPP
)
37 Hz). NMR for t-1b: 1H NMR (C6D6) δ -11.81 (t, 2JHP ) 21.7
Hz, 1H, RuH); 31P{1H} NMR δ 50.8 (s).
RuH2(PPh3)2(ampy) (c,t-2b/c,c-2b). Method a. A solution of
potassium tri-sec-butylborohydride (350 mg of a 1.0 M solution in
THF, 0.39 mmol) was added to a solution of 1b (300 mg, 0.39
mmol) in THF (2 mL) under a nitrogen atmosphere. The mixture
became dark yellow immediately. It was stirred at room temperature
for 20 min and filtered using a fine-porosity sintered glass frit. The
filtrate was evaporated to dryness under vacuum, and diethyl ether
(1.0 mL) was added to the resulting solids. The slurry was stirred
vigorously for 10 min, and then hexanes (5 mL) were added. The
dark yellow solids were filtered out, washed with hexanes, and dried
under vacuum. Yield ) 278 mg, 97% as a mixture of isomers c,t-
2b (81%) c,c-2b (17.7%), and (OC-6-43)-c,c-2b (1.3%).
Experimental Section
General Methods. RuCl2(PPh3)3, RuHCl(PPh3)3,24 and RuH2-
(PPh3)2(dach), dach ) (R,R)-1,2-diaminocyclohexane,14 were syn-
thesized by following literature procedures. Potassium tri-sec-
butylborohydride,
potassium
tert-butoxide,
(R,R)-1,2-
Method b. Toluene (2 mL) was added to a mixture of RuHCl-
(PPh3)2(ampy) (300 mg, 0.39 mmol) and KOtBu (50 mg, 0.44
mmol) under a flow of hydrogen and the mixture stirred for 2 h
under H2 (1 atm). The resulting mixture was filtered, and evaporated
to dryness, and hexanes (10 mL) were added, precipitating a dark
yellow solid. This was filtered out, washed with hexanes, and dried
under vacuum. Yield ) 215 mg, 75% as isomers c,t-2b (73%) and
c,c-2b (27%). IR (Nujol): 1902, 1912, 1930 cm-1 (νRuH), 3294,
3300, 3353 cm-1 (νNH). Anal. Calcd for C42H40N2P2Ru: C, 68.56;
H, 5.48; N, 3.81. Found: C, 68.09; H, 5.66; N, 3.44. NMR (see
Table 1) results are as follows. c,t-2b: 1H NMR (C6D6) δ -18.2
diaminocyclohexane, 2-(aminomethyl)pyridine, 2,2′-bipyridine, and
1,10-phenanthroline were supplied by Aldrich Chemical Co. All
preparations and manipulations were carried out under hydrogen,
nitrogen, or argon atmospheres with the use of standard Schlenk,
(15) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66,
7931-7944.
(16) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201-2237.
(17) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
1466-1478.
(18) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001.
(19) Mizushima, E.; Ohi, H.; Yamaguchi, M.; Yamagishi, T. J. Mol. Catal.
1999, 149, 43-49.
2
2
2
(td, JHP ) 27 Hz, JHH ) 7 Hz, 1H, RuH), -16.3 (td, JHP ) 27
Hz, 2JHH ) 7 Hz, 1H, RuH), 1.7 (t, 2JHH ) 6 Hz, 2H, NH2), 2.7 (t,
2JHH ) 6 Hz, 2H, CH2), 5.8-8.1 (m, 34H); 31P{1H} NMR δ 67.7
(20) Baratta, W.; Ros, P. D.; Del Zotto, A.; Sechi, A.; Zangrando, E.; Rigo,
P. Angew. Chem., Int. Ed. 2004, 43, 3584-3588.
(21) We recently learned that some of this ruthenium (aminomethyl)pyridine
chemistry was done independently by another research group: Barrata,
W.; Rigo, P. Personal communication.
2
(s). c,c-2b: 1H NMR (C6D6) δ -5.2 (ddd, JHP ) 99 and 36 Hz,
2JHH ) 6 Hz, RuH), -14.9 (dt, 2JHP ) 26 Hz, 2JHH ) 6 Hz, RuH);
31P{1H} δ 83.1 (br), 57.8 (br). (OC-6-43)-c,c-2b: 1H NMR (C6D6)
(22) Brunner, H.; Niemetz, M. Monatsh. Chem. 2002, 133, 115-126.
(23) Brunner, H.; Henning, F.; Weber, M. Tetrahedron: Asymmetry 2002,
13, 37-42.
(24) Schunn, R. A.; Wonchoba, E. R.; Wilkinson, G. Inorg. Synth. 1971,
13, 131-134.
2
2
δ -4.97 (ddd, JHP ) 96 and 38 Hz, JHH ) 6 Hz), -16.3 (ddd,
2JHP ) 30 and 18.5 Hz, 2JHH ) 6 Hz). Upon standing overnight in
C6D6, all of the isomer c,c-2b completely isomerized to c,t-2b.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2485

Abdur-Rashid et al.
Table 1. NMR Properties of Isomers of Dihydrides 2a,b in C6D6
RuH2(PPh3)2(bipy) (c,t-2c). THF (5 mL) was added to a mixture
of RuH2(PPh3)2(dach) (300 mg, 0.40 mmol) and 2,2′-bipyridine (61
mg, 0.39 mmol), and the solution was refluxed for 3 h under an
argon atmosphere, resulting in a dark green solution. After the
solution was cooled to room temperature, hexanes (20 mL) were
added resulting in the precipitation of a deep violet solid, which
was filtered out, washed with hexanes, and dried under vacuum.
Yield ) 232 mg, 76%. IR (Nujol): 1877, 1921 cm-1 (νRuH). Anal.
dacha
ampy
trans-H, cis-PPh3
t,c-2b
t,c-2a
-5.45 (t)
18
86.5 (s)
Λ-c,c-2a
-5.6 (ddd)
100
δ
H (ppm)
2JHP (Hz)
δP (ppm)
cis-H, cis-PPh3
c,c-2b
-5.2 (ddd)
99
36
6
∆-c,c-2a
-5.5 (ddd)
103
34
6
δ
H (ppm)
2JHPtrans (Hz)
2JHPcis (Hz)
2JHH (Hz)
Calcd for C46H40N2P2Ru: C, 70.48; H, 5.14; N, 3.57. Found: C,
2
70.0; H, 5.0; N, 3.6. NMR: 1H NMR (C6D6) δ -13.71 (t, JHP
)
32
6
28 Hz, 2H, RuH), 6.10-8.52 (m, 38H); 31P{1H} NMR δ 63.7 (s).
RuH2(PPh3)2(phen) (c,t-2d). THF (5 mL) was added to a
mixture of RuH2(PPh3)2(dach) (300 mg, 0.40 mmol) and 1,10-
phenanthroline (72 mg, 0.40 mmol), and the solution was refluxed
for 3 h under an argon atmosphere, resulting in a violet solution.
After the solution was cooled to room temperature, hexanes (30
mL) were added resulting in the precipitation of a violet solid, which
was filtered out, washed with hexanes, and dried under vacuum.
δ
H (ppm)
-14.9 (td)
-15.4 (dt)
23
6
-15.4 (dt)
2JHP (Hz)
26
23
2JHH (Hz)
6
6
δP (ppm)
57.8 (d)
83.1 (d)
14
55.5
84.6
14
57.7
84.4
14
δP (ppm)
2JPP (Hz)
cis-H, trans-PPh3
c,t-2b
-18.2 (td)
27
c,t-2a
-18.3 (t)
27
δ
H (ppm)
HP (Hz)
HH (Hz)
J
J
Yield ) 265 mg, 82%. NMR: 1H NMR (C6D6) δ -13.3 (t, 2JHP
)
7
27.4 Hz, 2H, RuH), 5.85-8.68 (m, 38H); 31P{1H} NMR δ 64.5
(s). Anal. Calcd for C48H40N2P2Ru: C, 71.36; H, 4.99; N, 3.47.
Found: C, 71.2; H, 4.8; N, 3.4.
δ
H (ppm)
-16.3 (td)
27
7
J
J
HP (Hz)
HH (Hz)
δP (ppm)
67.6 (s)
66.0 (s)
RuH(HCO2)(PPh3)2(ampy) (3b). Formic acid (10 mg) was
added to a solution of RuH2(PPh3)2(ampy) (200 mg, 0.27 mmol)
in THF (1 mL) and the mixture stirred for 30 min. Hexanes (10
mL) were then added, resulting in the precipitation of a pale yellow
solid, which was filtered out, washed with hexanes, and dried under
a Reference 9.
Reaction of 1b with KOtBu. RuHCl(PPh3)2(ampy) (31 mg, 0.04
mmol) and KOtBu (10 mg, 0.08 mmol) were mixed under N2, C6D6
(0.5 mL) was added, and the mixture was stirred for 30 min. It
1
vacuum. Yield ) 132 mg, 65%. NMR: H NMR (C6D6) δ -18.8
1
2
turned blue-black. The H NMR spectrum contained evidence of
(t, JHP ) 26 Hz, 1H, RuH), 1.8 (br, 1H, NH), 3.5 (br, 1H, CH),
an imino-ampy hydride species along with several others: 1H NMR
3.8 (br, 1H, CH), 5.9-8.7 (m, 34H), 8.5 (br, 1H, NH), 9.3 (s, 1H,
(C6D6) 10.45 ppm (d, JHH ) 6 Hz, py-CHdNH), -11.65 ppm
CH); 31P{1H} δ 74.5 (d), 65.5 (d, JPP ) 36 Hz). The structure
3
2
(br, RuH); 31P{1H} NMR (C6D6) 62.0 ppm (s). There are several
was verified by single-crystal X-ray diffraction (see Supporting
Information).
1
other unidentified species with H resonances at -12.85 ppm (br
s), -15.45 ppm (br t, J ) 24 Hz), and -16.90 ppm (t, J ) 29 Hz)
and 31P{1H] resonances at 49.5 ppm (m), 56.0 ppm (s), 61.0 ppm
(m), 67.5 ppm (t, J ) 25 Hz), 71.0 ppm (t, J ) 25 Hz), and 77.0
ppm (s).
X-ray Structure Analysis. Crystals were obtained by the slow
diffusion of either diethyl ether or hexanes into THF or benzene
solutions of the desired compounds under a nitrogen atmosphere.
Data were collected on a Nonius Kappa-CCD diffractometer using
Table 2. Summary of X-ray Parameters for the Complexes
1b
c,t-2b
c,t-2c
3b
formula
Mr
C42H39ClN2P2Ru
770.21
0.25 × 0.24 × 0.20
triclinic
P-1
C42H40N2P2Ru
735.77
0.40 × 0.35 × 0.35
orthorhombic
P2(1) 2(1) 2(1)
150
9.885(1)
16.3564(2)
22.3784(4)
90
90
90
3618.3(4)
4
1.351
C46H40N2P2Ru
783.81
0.32 × 0.16 × 0.06
monoclinic
C2/c
C49H46N2O2P2Ru
857.89
0.24 × 0.20 × 0.18
monoclinic
P2(1)/c
150
27.0614(4)
17.0984 (4)
18.4155(7)
90
101.098(1)
90
8361.6(4)
8
1.363
0.493
3552
cryst size, mm
cryst class
space group
temp, K
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
150
150
10.8591(1)
18.4542(3)
22.6775(3)
81.380(1)
82.400(1)
79.001(1)
4385.4(1)
4
9.7052(2)
16.3191(7)
23.399(1)
90
90.205(3)
90
3706.0(2)
4
1.405
Z
D
calc, g cm-3
1.167
0.518
1584
µ(Mo KR), mm-1
F(000)
0.554
1520
0.545
1616
range θ, deg
no. of reflcns
2.55-27.51
46 671
19 453
0.0395
0.1333
1.067
890
0.694
3.00-27.48
32 449
8257
0.0290
0.0631
1.034
434
0.461
2.59-27.47
15 388
4230
0.0361
0.0779
1.028
2.55-27.50
46 851
18 815
0.0507
0.1192
1.015
1041
0.641
no. of indpdt reflcns
R1 [I > 2σ(I)]a
wR2 (all data)b
goodness of fit
no. of ref params
max peak, e Å-3
236
0.372
2
2
2
a R1 ) Σ(Fo - Fc)/Σ(Fo). b wR2 ) [Σ[w(Fo - Fc )2]/Σ[w(Fo )2]]1/2
.
2486 Inorganic Chemistry, Vol. 44, No. 7, 2005

Ruthenium(II) Monohydride and Dihydride Complexes
Scheme 3. Preparation of the RuHCl and RuH2 Complexes
Table 3. Selected Bond Distances and Angles for the Known Complex
c,t-2a and the New Complexes c,t-2b and c,t-2c
c,t-2a14
c,t-2b
c,t-2ca
Ru(1)-H(1Ru)
Ru(1)-H(2Ru)
Ru(1)-N(1)
1.53(2)
1.62(3)
1.56(2)
1.56(3)
2.235(2)
2.2504(6)
2.2661(6)
2.141(2)
1.59(2)b
2.225(3)
2.2497(8)
2.2512(7)
2.284(2)
0.82(3)
0.87(3)
0.99(3)
0.84(3)
2.126(2)b
Ru(1)-P(2)
Ru(1)-P(1)
Ru(1)-N(2)
N(1)-H(1NB)
N(1)-H(1NA)
N(2)-H(2NA)
N(2)-H(2NB)
2.2843(6)b
H(1Ru)-Ru(1)-H(2Ru)
H(1Ru)-Ru(1)-N(1)
H(2Ru)-Ru(1)-N(1)
H(1Ru)-Ru(1)-P(2)
H(2Ru)-Ru(1)-P(2)
N(1)-Ru(1)-P(2)
83(1)
96(1)
177(1)
83(1)
80(1)
97.34(8)
82(1)
83(1)
99.14(9)
158.21(3)
172(1)
105(1)
76.6(1)
99.72(8)
97.89(7)
92(1)
86(2)
97.2(9)
170.6(9)
80(1)
85(1)
98.62(6)
79(1)
99.0(9)b
174.3(9)b
99.99(5)b
81.3(9)b
H(1Ru)-Ru(1)-P(1)
H(2Ru)-Ru(1)-P(1)
N(1)-Ru(1)-P(1)
81(1)
83.1(9)b
98.46(6)
154.96(2)
174.1(9)
93.6(9)
77.14(7)
102.06(5)
77.14(7)
96.87(5)b
158.57(3)b
P(2)-Ru(1)-P(1)
H(1Ru)-Ru(1)-N(2)
H(2Ru)-Ru(1)-N(2)
N(1)-Ru(1)-N(2)
P(2)-Ru(1)-N(2)
The solution of the ketone, followed by the solution of the
ruthenium complex and then the base (if required), was injected
into the already-thermostated reactor at 5 atm H2 pressure to give
a final working volume of 5 mL with concentrations of 0.17 M
acetophenone and 0.00024 M Ru with 15 equiv of base per Ru
when required. The reaction time was measured from the time of
the injection of the precatalyst solution. The amount of acetophe-
none remaining and 1-phenylethanol produced after 1 h and 20
min was determined by gas chromatography (see Table 4).
Deuteration Studies. A weighed sample of the desired com-
pound was dissolved in the required deuterated solvent in a NMR
tube under an argon atmosphere. The tube was then fitted with an
adapter, by which it was connected to a vacuum line. The sample
was cooled to liquid nitrogen temperature, degassed, and closed.
After being warmed to room temperature, the NMR tube was filled
with D2 gas at 1 atm pressure and then flame-sealed.
75.9(1)
N(1)-Ru(1)-N(2)
a There is a second bond distance or angle related by symmetry with the
same distance or angle (H(1Ru)#1 ) H(1Ru)#1, N(1)#1 ) N(1)#1, P(1)#1
) P(1)#1). b There is a second, equivalent bond distance or angle because
of the symmetry.
Mo KR radiation (λ ) 0.710 73 Å). The CCD data were integrated
and scaled using the DENZO-SMN software package, and the
structures were solved and refined using SHELXTL V5.0 (see
Tables 2 and 3). The hydrides were located and refined with
isotropic thermal parameters.
Catalytic Hydrogenation of Neat Acetophenone. Acetophe-
none (1.0 g) was added under a flow of hydrogen gas to 5 mg of
1b and 5 mg of KOiPr in a 500 mL Schlenk flask. The flask was
then cooled to liquid nitrogen temperature, filled with H2 gas,
closed, and allowed to warm to room temperature. The mixture
Results
1
was then vigorously stirred for 1 h. A H NMR spectrum of the
Synthesis of the Ruthenium(II) Monohydride and
Dihydride Complexes. The yellow monohydride complex
RuHCl(PPh3)2(ampy), 1b, is prepared by reaction of RuHCl-
(PPh3)3 with 2-(aminomethyl)pyridine (Scheme 3). It contains
mutually cis-phosphines with the hydride trans to the chloride
ligand, as deduced by its solution 1H and 31P NMR spectra.
The X-ray structure of 1b is shown in Figure 1. Selected
bond distances and angles are listed in Table 2. The structure
is that of a distorted octahedron, with the hydride trans to
the chloride ligand. The ruthenium-nitrogen bond distances
mixture indicated complete conversion of the ketone to phenyl-
ethanol. Similar results were obtained using the isomer mixture
c,t-2b/c,c-2b as a catalyst without added KOiPr.
Catalytic Hydrogenation of Acetophenone in Benzene. Reac-
tions were done at 5 atm of H2 using a 50 mL Parr hydrogenator
reactor. A constant temperature (20 °C) was maintained by use of
a Fisher Scientific Isotemp 1016D water bath. All solutions were
handled under a nitrogen atmosphere. Standard solutions of
acetophenone and catalyst precursors were prepared in benzene
while that of potassium tert-butoxide was made up in 2-propanol.
Table 4. Hydrogenation of Acetophenone Catalyzed by Ruthenium Complexes
time/conversion
to CMePh(H)OH
entry
precatalysta
ketone/solvent
MePhCO (4 g)
MePhCO (1 g)
MePhCO (1 g)
other reagents
ketone:Ru:base
1
2
3
4
5
6
7
8
9
c,c-RuH2(dach)L2/c,t-RuH2(dach)L2 ) 30/70
c,c-RuH2(ampy)L2/c,t-RuH2(ampy)L2 ) 18/82
t-RuHCl(ampy)L2
3 atm H2
5000:1
2000:1
2000:1:8
100:1
<8 h/100%14
1 h/100%
1 h/100%
24 h/0%
3 atm H2
3 atm H2, KOiPr
3 atm H2
c,t-RuH2(bipy)L2
MePhCO (1 g)
c,t-RuH2(phen)L2
c,c-RuH2(ampy)L2/c,t-RuH2(ampy)L2 ) 7/83
t-RuHCl(dach)L2
t-RuHCl(ampy)L2
t-RuHCl(bipy)L2
MePhCO (1 g)
3 atm H2
5 atm H2
100:1
709:1
709:1:17
709:1:17
709:1:17
24 h/0%
MePhCO (0.17 M)/C6H6
MePhCO (0.17 M)/C6H6
MePhCO (0.17 M)/C6H6
MePhCO (0.17 M)/C6H6
1.3 h/12%
1.3 h/55%
1.3 h/98%
3 h/0%
5 atm H2, KOtBu
5 atm H2, KOtBu
5 atm H2, KOtBu
a L ) PPh3.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2487

Abdur-Rashid et al.
Figure 1. Structure and atomic numbering of RuHCl(PPh3)2(ampy), 1b.
Figure 2. Structure and atomic numbering of c,t-RuH2(PPh3)2(ampy), c,t-
2b.
Ru(1A)-N(1A) and Ru(1A)-N(2A) are similar at 2.163(2)
and 2.171(2) Å, respectively. This is unlike what is observed
for the dihydride complexes (below), in which the ruthenium-
nitrogen distances involving the pyridyl groups are typically
much shorter than those involving the amine moieties.
irradiation of the NH2 signal produces a large increase in
the intensity of the Hb signal but no change in the signal of
Ha.
1
The hydride region of the H NMR spectrum of c,c-2b
shows a doublet of doublet of doublet pattern at -5.2 ppm
for the hydride Hc trans to phosphorus (2JHPtrans 99 Hz). The
second hydride (Hd) shows a doublet of triplet pattern at
Isomers of the dihydride RuH2(ampy)(PPh3)2 are obtained
by the addition of KHBsBu3 to 1b in THF or by the reaction
of 1b with H2 (1 atm) and KOtBu in toluene (Scheme 3). At
first the 1H and 31P{1H} NMR spectra of the reaction
mixtures in C6D6 show the presence of two or three isomers,
with the isomer c,t-2b, containing cis-hydrides and trans-
phosphines, predominating relative to those containing cis-
hydrides and cis-phosphines, namely the isomers (OC-6-42)-
c,c-2b (unless otherwise indicated this isomer will simply
be called isomer c,c-2b) and (OC-6-43)-c,c-2b (<2%). With
warming or over the course of 20 h at room temperature,
the minor dihydride isomers in solution are converted to c,t-
2b. No trans-dihydride species t,c-2b was detected in solution
at room temperature.
2
cis
-14.9 ppm with JHP couplings. The structure of c,c-2b
1
was assigned on the basis of a H NOE experiment. Upon
irradiation of the hydride with the chemical shift correspond-
ing to Hc there was an increase in the intensity of the hydride
resonance corresponding to Hd and the NH2 resonance at
4.50 ppm. Upon irradiation of Hd there was a corresponding
NOE enhancement of Hc; however, there was no increase in
the intensity of the NH2 resonance. These observations are
consistent with the structure of c,c-2b as shown in Scheme
3.
The solid-state (Nujol) infrared spectrum of 2b shows three
νRu-H peaks at 1902, 1912, and 1930 cm-1 and three νN-H
bands at 3294 (w), 3300 (w), and 3353 cm-1. Only c,t-2b
formed X-ray-quality crystals upon layering a benzene
solution of the mixture of isomers with hexanes. The
structure is shown in Figure 2, and the bond lengths are listed
in Table 3. The Ru-H bond lengths are both 1.56(3) Å. The
Ru(1)-N(1)(pyridyl) bond length of 2.141(2) Å is much
shorter than that of Ru(1)-N(2)(amino) with a bond length
of 2.284(2) Å.
1
The hydride region of the H NMR spectrum of c,t-2b
shows two sets of triplet of doublet patterns at -18.2 and
-16.3 ppm in a 1:1 ratio (Table 1). The 31P{1H} NMR
spectrum shows a sharp singlet at 67.7 ppm, indicative of
equivalent phosphorus nuclei, while the hydride-coupled 31
P
spectrum shows a doublet of doublet pattern. A comparison
of the proton NMR spectrum of c,t-2b with those of c,t-
RuH2(PPh3)2(dach) and c,t-RuH2(PPh3)2(bipy) (below) indi-
cates that the hydride chemical shifts at -18.2 and -16.3
ppm (labeled Ha and Hb, respectively, in Scheme 3) are likely
due to their locations trans to the amino and pyridyl moieties
of the bidentate ligand, respectively. The assignment of the
Observation of a Diimine Species. The reaction of the
monohydride complex 1b with KOtBu in THF under Ar
results in a dark blue solution due to the formation of a
hydride diimine species (Scheme 4) as well as several other
products. The diimine may be produced by â-hydride
migration from the methylene group of the undetected
intermediate hydrido-amido complex 4b. A distinctive
1
hydride chemical shifts was verified by a H NMR NOE
experiment. Irradiation of the hydride with a resonance at
-18.2 ppm (Ha) shows a large increase in the intensity of
the other hydride resonance at -16.3 ppm (Hb). On the other
hand, irradiation at -16.3 ppm shows a correspondingly large
increase in the hydride intensity at -18.2 ppm, as well as
an increase in the NH2 signal at 1.7 ppm. Likewise,
1
singlet at 10.4 and triplet at -11.7 ppm in the H NMR
spectrum and a singlet in the 31P{1H} spectrum are also
consistent with the dihydride structure shown in Scheme 4.
Other diimine complexes such as the bipyridine complex c,t-
2488 Inorganic Chemistry, Vol. 44, No. 7, 2005

Ruthenium(II) Monohydride and Dihydride Complexes
Scheme 4. Reaction of Complex 1b with Base Producing Imine Complexes, Probably via an Amido Complex 4b
Scheme 5. Reaction of c,t-RuH2(PPh3)(dach) with Pyridyl-Donor
Ligands To Give the Dihydrides 2b-d
The single-crystal X-ray structure of c,t-2c is shown in
Figure 3. The structure is similar to those of c,t-2a and c,t-
2b14 with mutually trans phosphine ligands and cis hydrides.
The Ru-H bond lengths of 1.59(2) Å are similar to the 1.56-
(3) and 1.57(3) Å (average) distances observed in c,t-2b and
c,t-2a, respectively. However, the Ru-N distances of 2.129-
(2) Å are much shorter than the 2.255(3) Å average distance
observed in the diamine complex c,t-2a but similar to 2.141-
(2) Å observed for the Ru-N(2)(pyridyl) bond in c,t-2b.
Reactions of the Dihydrides with Deuterium Gas. Upon
exposure of a solution of 2b to an atmosphere of D2 gas in
C6D6, the 1H NMR spectrum of the solution shows the slow
incorporation of deuterium into the hydrides and NH2
positions (Scheme 6). For the major isomer, c,t-2b, two
isotopomers were observed to have resonances at -16.36
(triplet of doublets) and -16.33 ppm (triplet). These cor-
respond to the resonance of Ha of the starting RuH2(PPh3)2-
(ampy) complex and the isotopomer RuHD(PPh3)2(ampy)
resulting from the replacement of the hydride located trans
to the NH2 group with deuteride. On the other hand, a series
of six triplets between -18.16 and -18.32 ppm was observed
for Hb, the hydride located trans to the pyridyl group. These
include the resonances for RuH2(PPh3)2(pyCH2NH2) and the
series of isotopomers RuH2(PPh3)2(pyCH2NHD) and RuH2-
(PPh3)2(pyCH2ND2), as well as the corresponding HD
isotopomers RuHD(PPh3)2(pyCH2NH2), RuHD(PPh3)2(pyCH2-
NHD), and RuHD(PPh3)2(pyCH2ND2).
2c and the phenanthroline complex c,t-2d (see below) also
have this deep color, probably because of metal to diimine
charge-transfer transitions.
When this solution containing the diimine species is
reacted with 1 atm of H2(g), the dihydride isomers, RuH2-
(PPh3)2(ampy), shown in Scheme 3, are generated. This might
be evidence that the conversion of 4b to the diimine is
reversible as discussed below.
Reactions of RuH2(PPh3)2(dach) with Pyridyl-Donor
Ligands. The diamine ligand in the complex c,t-RuH2(PPh3)2-
(dach) (c,t-2a)14 can be displaced by a range of bidentate
ligands containing nitrogen donor groups upon heating or
under reflux in THF solutions (Scheme 5). Thus, the addition
of 1 equiv of ampy to a THF solution of RuH2(PPh3)2(dach)
under a nitrogen or argon atmosphere and warming to 50
°C resulted in the gradual formation of the cis-dihydride
complex, RuH2(PPh3)2(ampy), c,t-2b.
The observation of these isotopomers also confirms that
the chemical shift at -18.23 ppm in c,t-2b is due to the
hydride ligand (Ha) trans to the NH2 moiety, since deuteration
of the amine hydrogens located trans to the hydride would
When c,t-RuH2(PPh3)2(dach) is refluxed in THF with 1
equiv of 2,2′-bipyridine or 1,10-phenanthroline under an inert
atmosphere, the substituted dihydride complexes c,t-RuH2-
(PPh3)2(bipy) (c,t-2c) or c,t-RuH2(PPh3)2(phen) (c,t-2d) are
formed quantitatively, as dark green and violet solutions,
respectively, which both yield deep violet crystals upon
addition of hexanes. Unlike complexes c,t-2a and c,t-2b
which are very air-sensitive, complexes c,t-2c and c,t-2d are
air-stable as solids, while their solutions are only moderately
air-sensitive, requiring at least several hours for decomposi-
1
tion at room temperature. The hydride regions of the H
NMR spectra of c,t-2c and c,t-2d show triplets at -13.7 ppm
(2JHP ) 28 Hz) and -13.3 ppm (2JHP ) 27 Hz), respectively,
while the 31P{1H} NMR spectra show sharp singlets at 63.7
and 64.5 ppm, respectively. The deshielding of the hydride
chemical shifts in these two complexes relative to that
observed for the diamine complex RuH2(PPh3)2(dach) with
trans-PPh3 (-18.2 ppm) is apparently characteristic of the
imine ligands in the former two complexes. Solid-state
infrared spectra of c,t-2c and c,t-2d show two sharp νRu-H
peaks as expected for a cis-dihydride stereochemistry.
Figure 3. Structure and atomic numbering of RuH2(PPh3)2(bipy), c,t-2c.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2489

Abdur-Rashid et al.
Scheme 6. Isomers of Dihydride 2b Undergoing H/D Exchange with
D2 Gas While Complexes 1b, c,t-2c, and c,t-2d Do Not
Scheme 7. Formation of the Formate Complex 3b
mixture of isomers c,c-2b and c,t-2b mixture (Table 4, entry
6) is less than that of a 1b/KOtBu mixture in benzene (entry
8). The ampy complex 1b is an even more active precatalyst
than the dach complex 1a (entry 8 vs entry 7). The complex
RuHCl(bipy)(PPh3)225 that has cis phosphines but no amino
group is not a precatalyst for the hydrogenation of acetophe-
none in benzene in the presence of an excess of KOtBu (entry
9).
Discussion
In past work we have identified amido complexes10,11,26
(e.g. RuH(NHC6H10NH2)(PPh3)2, 4a,9 in Scheme 1) as the
key reactants in the formation of dihydrides and in the cycle
for the catalytic hydrogenation of ketones (Scheme 2). Our
attempts to generate the corresponding amido complex 4b
as in Scheme 4 resulted in the formation of a mixture of
species including a diimine complex that is presumably
formed by â-hydride elimination from the CH2 group of the
coordinated ampy ligand. The ethylenediamine ligand in
RuHCl(en)(PPh3)2, upon treatment with base, also undergoes
this reaction to form dimeric complexes containing a bridging
diimine NHCHCHNH ligand.9 Conversely, the complex
RuHCl(NH2CMe2CMe2NH2)(PPh3)2, with a diamine ligand
that lacks hydrogens â to the metal, reacts with KOtBu to
give the stable amido complex RuH(NHCMe2CMe2NH2)-
(PPh3)2.10 Nevertheless, the mixture that included this diimine
reacted with dihydrogen to give a succession of dihydride
isomers (Scheme 8) similar to that produced by the reaction
of amido 4a with dihydrogen (Scheme 1). Therefore, the
diimine species could serve as a reservoir of the amido
complex 4b if both are in equilibrium as in Scheme 8. By
analogy to the reactions shown in Scheme 1, this amido
complex 4b would react with H2 to first produce the, as yet,
unobserved trans-dihydride RuH2(ampy)(PPh3)2, t,c-2b. As
in the case of the dach trans-dihydride complex t,c-2a
(Scheme 1), t,c-2b would quickly isomerize to the cis-
dihydride c,c-2b (Scheme 8). This isomer, in turn, converts
within less than 20 h into pure c,t-2b that is stable in benzene
under N2 for at least several days. However, at this stage
we cannot rule out the direct reaction of 4b with H2 to give
c,c-2b, a reaction that would bypass the formation of a trans-
dihydride.
result in a large change in the hydride chemical shift as
observed; the change would be small if it were located in
the cis position. The incorporation of deuterium into the
hydrides of c,c-2b resulted in a broadening of the hydride
resonances and a decrease in their intensities.
There was no incorporation of deuterium into the mono-
hydride complex 1b or into dihydride complexes c,t-2c and
c,t-2d upon exposure of their solutions to deuterium gas for
several days.
Reaction of RuH2(PPh3)2(ampy) with Formic Acid. The
dihydrides are very reactive toward acids. The compound
RuH(HCO2)(PPh3)2(ampy), 3b, was prepared by the addition
of formic acid to a THF solution of c,t-2b (Scheme 7). The
1
hydride region of the H NMR spectrum shows a triplet at
-18.8 ppm while the formate hydrogen presents a sharp
singlet at 9.34 ppm. Two NH hydrogens are observed at 1.8
and 8.5 ppm. The extensive deshielding of the latter proton
signal is clearly indicative of a strong hydrogen bond between
one of the amine hydrogens and the noncoordinated formato
oxygen that is also observed in the X-ray crystal structure
(Supporting Information). The complex RuH(HCO2)(PPh3)2-
(NH2CMe2CMe2NH2) has a very similar structure.10 The trans
configuration of the complex and the hydrogen-bonded
carbonyl groups are important features of the transition state
structure for ketone hydrogenation catalyzed by the corre-
sponding trans-dihydride complex t,c-2b as in Scheme 9 (see
below).
Catalytic Hydrogenation of Acetophenone. Like the
dach dihydride complexes c,c/c,t-2a, the mixture of ampy
dihydride isomers c,c/c,t-2b is a very active catalyst for the
hydrogenation of neat acetophenone to 1-phenylethanol at
room temperature and 3 atm H2 (Table 4, entries 1 and 2).
An active catalyst system is also obtained starting from
complex 1b and KOiPr (entry 3). On the other hand, there
was no detection of phenylethanol when a mixture of
acetophenone and the bipyridine or phenanthroline com-
plexes (c,t-2c or c,t-2d) was stirred under hydrogen gas (3
atm) for 24 h (Table 4, entries 4 and 5). The activity of a
If the steps of Scheme 8 are reversible, then the mechanism
of the slow reaction of D2 with c,c-2b and c,t-2b can be
explained. The prior elimination of H2 to form the amido
complex 4b followed by reaction with D2, would produce
(25) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J. Chem. Soc. A 1968,
3143-3150.
(26) Li, T.; Churlaud, R.; Lough, A. J.; Abdur-Rashid, K.; Morris, R. H.
Organometallics 2004, 23, 6239-6247.
2490 Inorganic Chemistry, Vol. 44, No. 7, 2005

Ruthenium(II) Monohydride and Dihydride Complexes
Scheme 8. Succession of Dihydrides Produced by Reacting the Diimine Mixture of Scheme 4 with Dihydrogen
Scheme 9. Proposed Steps in Catalyst Activation and Function
the isotopomers RuHD(NHDCH2py)(PPh3)2. The elimination
of HD or H2 from these compounds and reaction with 4b
would eventually result in the formation of the other
of these compounds appear to reflect this increase in stability
(Table 3). The Ru-pyridyl distances (2.12-2.14 Å in c,t-
2b and c,t-2c) are shorter than those to amino nitrogens
(2.25-2.23 Å in c,t-2a and c,t-2b).
1
isotopomers that are observed by H NMR. The lack of
reactivity of D2 with complexes 2c,d can be explained by
the fact that they do not have N-H bonds and cannot form
amido complexes. Therefore, they do not react with D2 as
observed. These complexes clearly do not eliminate H2 to
produce a ruthenium(0) intermediate such as Ru(N-N)-
(PPh3)2 since this would also be expected to react with D2.
This also argues against Ru(0) species as intermediates in
the catalytic hydrogenation mechanism involving complexes
2a and b.
The mechanism of the hydrogenation reaction is thought
to involve two very reactive species, the trans-dihydride
isomer t,c-RuH2(ampy)(PPh3)2, 2b, and the amido complex
RuH(NHCH2py)(PPh3)2, 4b (Scheme 9). In the ketone
hydrogenation step, the dihydride transfers a hydride from
ruthenium and a proton from the amino group to the ketone
to generate the alcohol and the amido complex 4b. The fact
that the bipyridine or phenanthroline complexes (c,t-2c and
c,t-2d) are catalytically inactive provides evidence for the
importance of the amino group in the hydrogenation of
ketones as first discovered by Noyori and co-workers.2 The
structure of the formate complex 3b (Scheme 7) illustrates
the possible geometry of the transition state for H+/H-
transfer where the carbonyl oxygen is hydrogen-bonded to
the amino NH and an oxygen instead of a hydrogen is
attached to the carbonyl carbon.
The isomeric dihydride complexes RuH2(ampy)(PPh3)2
(c,t-2b/c,c-2b) have properties quite similar to those of dach
1
(c,t-2a/c,c-2a). Their H (hydride) and 31P chemical shifts
and coupling constants are very similar (Table 1). A
difference is that the ampy complex isomerizes completely
to the c,t-isomer over time while the dach complex remains
as a 92:8 c,t:c,c mixture.9 The pyridyl-donor ligands ampy,
bipy, and phen form more stable complexes than the dach
ligand since the dach ligand in c,t-2a is substituted quanti-
tatively by these ligands to produce the corresponding
dihydrides c,t-2b, c,t-2c, and c,t-2d. The Ru-N distances
In the dihydrogen activation step, the amido complex 4b
is proposed to split heterolytically an η2-dihydrogen ligand
into a hydride on ruthenium and a proton on nitrogen to
regenerate the trans-dihydride complex as shown in Scheme
Inorganic Chemistry, Vol. 44, No. 7, 2005 2491

Abdur-Rashid et al.
8. The higher reactivity of the ampy amido complex 4b with
dihydrogen compared to that of the dach amido complex 4a
might explain why the ampy catalysts are more active than
the dach catalysts in the hydrogenation of acetophenone
(Table 4, entry 8 vs entry 7). At least for 4a, this is the
turnover-limiting step in the catalytic cycle. As shown in
Scheme 9, we cannot rule out a second catalytic cycle where
4b reacts with H2 to give c,c-2b, which in turn reacts with
ketone to give the alcohol product and regenerate the amido
catalyst.
The routes into the catalytic cycle are also similar for the
dach (Scheme 2) and ampy (Scheme 9) complexes. The cis-
dihydride with cis-PPh3 groups, c,c-2b, reacts quickly with
acetophenone to enter the catalytic cycle via 4b while the
cis-dihydride with trans-PPh3 groups, c,t-2b, needs an
activation step, probably isomerization to c,c-2b to enter the
cycle. This is consistent with the result shown in Table 4,
entry 4, where pure c,t-2b is a catalytically inactive complex.
Therefore, active catalysts of this type should have a cis-
phosphine configuration. However, this is not the only
requirement since the use of the complex RuHCl(bipy)-
(PPh3)2 with cis-PPh3 ligands does not lead to a catalyst
(entry 9, Table 4). The activity of a mixture of isomers c,c-
2b and c,t-2b (Table 4, entry 6) is less than that of a 1b/
KOtBu mixture in benzene (entry 7). Presumably this latter
mixture under H2 leads to a larger fraction of dihydride
isomers with cis-PPh3 groups (Scheme 8).
Conclusions
There are similarities and differences between the com-
plexes of 2-(aminomethyl)pyridine and (R,R)-trans-diami-
nocyclohexane. The structures and properties of the hydrido-
chloro and cis-dihydride complexes are similar. For both
nitrogen-donor ligands, the dihydride isomer with trans-PPh3
groups is more stable than those with cis-PPh3. However it
is the later that are the active hydrogenation catalysts. The
ampy-amido complex 4b that is proposed in Schemes 4, 8,
and 9 is very unstable compared to the dach-amido complex
4a and decomposes to an imine-containing complex as well
as to other species. The high reactivity of such an amido
complex 4b might translate into a faster reaction with
dihydrogen compared to the dach amido complex 4a. This
would explain the higher catalytic activity of the ketone
hydrogenation catalysts containing the ampy ligand compared
to those containing the dach ligand. The use of bipyridine
or phenanthroline ligands that lack an amino N-H group
produces catalytically inactive compounds as expected on
the basis of a key step in the proposed mechanism for ketone
hydrogenationsthe transfer of the proton from nitrogen and
hydride from ruthenium to the ketone carbonyl in the outer
coordination sphere.
Acknowledgment. This work was supported by grants
to R.H.M. from the NSERC Canada and PRF, as adminis-
tered by the American Chemical Society, and by a loan of
ruthenium salts from Johnson-Matthey. Dr. Timothy Burrow
provided valuable assistance with the high-field NMR
studies.
Another difference between the ampy and dach complexes
is the qualitatively faster rate of dehydrochlorination of the
hydrido chloro complex 1b versus that of 1a. The formation
of isomers of the dihydride RuH2(ampy)(PPh3)2 (c,t-2a/
c,c-2a) by the addition of KHBsBu3 to 1a (Scheme 3) or
KOtBu/H2 to 1a (Scheme 3, Scheme 9) is much faster than
similar reactions for the formation of RuH2(PPh3)2(dach), 2b,
from RuHCl(PPh3)2(dach), 1b. The ampy ligand clearly
facilitates this dehydrochlorination reaction.
Supporting Information Available: Crystallographic files in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0489387
2492 Inorganic Chemistry, Vol. 44, No. 7, 2005