A succession of isomers of ruthenium dihydride complexes. Which one is the ketone hydrogenation catalyst?

Article abstract of DOI:10.1021/ja039396f

Reaction of RuHCl(PPh₃)₂(diamine) (1a, diamine = (R,R)-1,2-diaminocyclohexane, (R,R)-dach; 1b, diamine = ethylenediamine, en) with KO^tBu in benzene quickly generates solutions of the amido-amine complexes RuH(PPh₃)₂(NHC₆H₁₀NH ₂), (2a′), and RuH(PPh₃)₂(NHCH ₂CH₂NH₂), (2b′), respectively. These solutions react with dihydrogen to first produce the trans-dihydrides (OC-6-22)-Ru(H)₂(PPh₃)₂(diamine) (t,c-3a, t,c-3b). Cold solutions (-20°C) containing trans-dihydride t,c-3a react with acetophenone under Ar to give (S)-1-phenylethanol (63% ee). Complexes t,c-3 have lifetimes of less than 10 min at 20° and then isomerize to the cis-dihydride, cis-bisphosphine isomers (OC-6-32)-Ru(H)₂(PPh ₃)₂(diamine) (Δ/Λ-c,c-3a, c,c-3b). A solution containing mainly Δ/Λ-c,c-3a reacts with acetophenone under Ar to give (S)-1-phenylethanol in 20% ee, whereas it is an active precatalyst for its hydrogenation under 5 atm H₂ to give 1-phenylethanol with an ee of 50-60%. Complexes c,c-3 isomerize to the cis-dihydride, trans-bisphosphine complexes (OC-6-13)-Ru(H)₂(PPh₃)₂(diamine) (c,t-3a, c,t-3b) with half-lives of 40 min and 1 h, respectively. A mixture of Δ/Λ-c,c-3a and c,f-3a can also be obtained by reaction of 1a with KBH(Bu^sec)₃. A solution of complex c,t-3a in benzene under Ar reacts very slowly with acetophenone. These results indicate that the trans-dihydrides t,c-3a or t,c-3b along with the corresponding amido-amine complexes 2a′ or 2b′ are the active hydrogenation catalysts in benzene, while the cis-dihydrides c,c-3a or c,c-3b serve as precatalysts. The complexes RuCl₂(PPh₃)₂((R,R)-dach) or 1a, when activated by KO^tBu, are also sources of the active catalysts. A study of the kinetics of the hydrogenation of acetophenone in benzene catalyzed by 3a indicates a rate law: rate = k[c,c-3a]_initial[H₂] with k = 7.5 M^-1 s^-1. The turnover-limiting step appears to be the reaction of 2a′ with dihydrogen as it is for RuH(NHCMe ₂CMe₂NH₂)(PPh₃)₂ (2c′). The catalysts are more active in 2-propanol, even without added base, and the kinetic behavior is complicated. The basic cis-dihydride c,t-3a reacts with [NEt₃H]BPh₄ to produce the dihydrogen complex (OC-14)-[Ru(η²-H₂)(H)(PPh₃) ₂-((R,R)-dach)]BPh₄ (4) and with diphenylphosphinic acid to give the complex RuH(O₂PPh₂)(PPh₃) ₂((R,R)-dach) (5). The structure of 5 models aspects of the transition state structure for the ketone hydrogenation step. Complex 2b′ decomposes rapidly under Ar to give dihydrides 3b along with a dinuclear complex (PPh₃)₂HRu(μ-η²;η⁴- NHCHCHNH)RuH(PPh₃)₂ (6) containing a rare, bridging 1,4-diazabutadiene group. The formation of an imine by β-hydride elimination from the amido-amine ligand of 2a′ under Ar might explain some loss of enantioselectivity of the catalyst. The structures of complexes 1a, 5, and 6 have been determined by single-crystal X-ray diffraction.

Full text of DOI:10.1021/ja039396f

Published on Web 01/21/2005
A Succession of Isomers of Ruthenium Dihydride Complexes.
Which One Is the Ketone Hydrogenation Catalyst?
Robert Abbel,† Kamaluddin Abdur-Rashid,†,‡ Michael Faatz,† Alen Hadzovic,†
Alan J. Lough,† and Robert H. Morris*,†
Contribution from the DaVenport Laboratory, Department of Chemistry, UniVersity of Toronto,
80 St. George Street, Toronto, Ontario M5S 3H6, Canada
Received November 3, 2003; Revised Manuscript Received November 18, 2004; E-mail: rmorris@chem.utoronto.ca
Abstract: Reaction of RuHCl(PPh3)2(diamine) (1a, diamine ) (R,R)-1,2-diaminocyclohexane, (R,R)-dach;
1b, diamine ) ethylenediamine, en) with KOtBu in benzene quickly generates solutions of the amido-
amine complexes RuH(PPh3)2(NHC6H10NH2), (2a′), and RuH(PPh3)2(NHCH2CH2NH2), (2b′), respectively.
These solutions react with dihydrogen to first produce the trans-dihydrides (OC-6-22)-Ru(H)2(PPh3)2(diamine)
(t,c-3a, t,c-3b). Cold solutions (-20 °C) containing trans-dihydride t,c-3a react with acetophenone under
Ar to give (S)-1-phenylethanol (63% ee). Complexes t,c-3 have lifetimes of less than 10 min at 20° and
then isomerize to the cis-dihydride, cis-bisphosphine isomers (OC-6-32)-Ru(H)2(PPh3)2(diamine) (∆/Λ-c,c-
3a, c,c-3b). A solution containing mainly ∆/Λ-c,c-3a reacts with acetophenone under Ar to give (S)-1-
phenylethanol in 20% ee, whereas it is an active precatalyst for its hydrogenation under 5 atm H2 to give
1-phenylethanol with an ee of 50-60%. Complexes c,c-3 isomerize to the cis-dihydride, trans-bisphosphine
complexes (OC-6-13)-Ru(H)2(PPh3)2(diamine) (c,t-3a, c,t-3b) with half-lives of 40 min and 1 h, respectively.
A mixture of ∆/Λ-c,c-3a and c,t-3a can also be obtained by reaction of 1a with KBH(Busec)3. A solution of
complex c,t-3a in benzene under Ar reacts very slowly with acetophenone. These results indicate that the
trans-dihydrides t,c-3a or t,c-3b along with the corresponding amido-amine complexes 2a′ or 2b′ are the
active hydrogenation catalysts in benzene, while the cis-dihydrides c,c-3a or c,c-3b serve as precatalysts.
The complexes RuCl2(PPh3)2((R,R)-dach) or 1a, when activated by KOtBu, are also sources of the active
catalysts. A study of the kinetics of the hydrogenation of acetophenone in benzene catalyzed by 3a indicates
a rate law: rate ) k[c,c-3a]initial[H2] with k ) 7.5 M-1 s-1. The turnover-limiting step appears to be the
reaction of 2a′ with dihydrogen as it is for RuH(NHCMe2CMe2NH2)(PPh3)2 (2c′). The catalysts are more
active in 2-propanol, even without added base, and the kinetic behavior is complicated. The basic cis-
dihydride c,t-3a reacts with [NEt3H]BPh4 to produce the dihydrogen complex (OC-14)-[Ru(η2-H2)(H)(PPh3)2-
((R,R)-dach)]BPh4 (4) and with diphenylphosphinic acid to give the complex RuH(O2PPh2)(PPh3)2((R,R)-
dach) (5). The structure of 5 models aspects of the transition state structure for the ketone hydrogenation
step. Complex 2b′ decomposes rapidly under Ar to give dihydrides 3b along with a dinuclear complex
(PPh3)2HRu(µ-η2;η4-NHCHCHNH)RuH(PPh3)2 (6) containing a rare, bridging 1,4-diazabutadiene group. The
formation of an imine by â-hydride elimination from the amido-amine ligand of 2a′ under Ar might explain
some loss of enantioselectivity of the catalyst. The structures of complexes 1a, 5, and 6 have been
determined by single-crystal X-ray diffraction.
Introduction
in the pharmaceutical, agrochemical, flavor, fragrance, and
material industries. A key discovery in catalyst development
Noyori and co-workers have developed extremely efficient
ruthenium catalysts for the hydrogenation of ketones to alcohols
and for the asymmetric hydrogenation of prochiral ketones to
valuable optically active alcohols.1-10 These alcohols find uses
was the beneficial effect of certain diamines with at least one
NH2 group, such as ethylenediamine (en) or 1,2-(R,R)-diami-
nocyclohexane ((R,R)-dach), on the catalytic activity of ruthe-
nium phosphine complexes toward ketone hydrogenation.7 Thus,
precatalysts are generated in situ or are isolated as complexes
† University of Toronto.
‡ Current address: Kanata Chemical Technologies, Inc., 2240 Speakman
Dr., Sheridan Science and Technology Park, Mississauga, Ontario L5K 1A9,
Canada.
(5) Ohkuma, T.; Koizumi, M.; Ikehira, H.; Yokozawa, T.; Noyori, R. Org.
Lett. 2000, 2, 659-662.
(6) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466-
1478.
(7) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40-73.
(8) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931-
7944.
(9) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008-2022.
(10) Ohkuma, T.; Koizumi, M.; Mun˜iz, K.; Hilt, G.; Kabuto, C.; Noyori, R. J.
Am. Chem. Soc. 2002, 124, 6508-6509.
(1) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1995, 117, 2675-2676.
(2) Ohkuma, T.; Ooka, H.; Yamakawa, M.; Ikariya, T.; Noyori, R. J. Org.
Chem. 1996, 61, 4872-4873.
(3) Ohkuma, T.; Koizumi, M.; Doucet, H.; Pham, T.; Kozawa, M.; Murata,
K.; Katayama, E.; Yokozawa, T.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1998, 120, 13529-13530.
(4) Noyori, R.; Ohkuma, T. Pure Appl. Chem. 1999, 71, 1493-1501.
9
1870
J. AM. CHEM. SOC. 2005, 127, 1870-1882
10.1021/ja039396f CCC: $30.25 © 2005 American Chemical Society

A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
Scheme 1. Proposed Roles of the Diamine and the Ruthenium
Hydride of a Ruthenium Complex in the Catalytic Hydrogenation of
Ketones: Metal-Ligand Bifunctional Catalysis (the [Ru] Notation
Represents the Phosphine and Other Ligands)
of the type RuCl2(PR3)2(diamine) or RuCl2(diphosphine)-
(diamine), including compounds containing enantiomerically
pure diamines and diphosphines. These complexes become
active when dissolved in basic 2-propanol solutions under H2-
(g). Attractive features of these catalysts include the use of low
pressures of hydrogen gas, high substrate-to-catalyst ratios, high
turnover numbers, functional group tolerance, high enantiose-
lectivity, and a high selectivity for the hydrogenation of CdO
over CdC bonds. Another more recently discovered feature is
their use in the hydrogenation and asymmetric hydrogenation
of CdN bonds.11-14
The source of the diamine effect was proposed to be the easy
transfer of a proton from the amine to the carbonyl oxygen and
a hydride from ruthenium to the carbonyl carbon of the ketone
in a unique second coordination sphere reaction.7,8,11,15-18
Experimental and theoretical studies of the mechanism of the
transfer hydrogenation catalysts of the type RuH(alkoxy-
amine)(η6-arene),6,19 RuH(Tosyl-amido-amine)(η6-arene),20-22
and RuH(η5-C5Ar4OH)(CO)2 23 have shown that this hydridic-
protonic transfer to the CdO group of the ketone is likely to
be a concerted process. Noyori and co-workers have called this
metal-ligand bifunctional catalysis.6-9 These catalysts are
regenerated by transfer of hydride to ruthenium from basic
2-propanol or ammonium formate but not from dihydrogen gas.
Chart 1. Isomers of Dihydride Complexes
In contrast, ruthenium diamine complexes such as RuCl2(en)-
4,7,24,25
(PPh3)2
are precatalysts that lead to much more active
catalysts for the hydrogenation of ketones than to catalysts for
the transfer hydrogenation of ketones.
The structures of the active ruthenium hydride catalysts in
the diamine systems are only now emerging. It was expected
that the dichloride precursors would be converted into hydride
species3,7,26,27 under the reducing catalytic conditions of hydro-
gen gas, 2-propanol, and strong base. Some of us found that
the monohydride complexes RuHCl(PPh3)2(diamine) and Ru-
HCl(diphosphine)(diamine) were inactive as ketone or imine
hydrogenation catalysts but could be converted into active
dihydride catalysts by reaction with a strong base and dihy-
drogen.11,12,17,18 Therefore, the catalytic species were pro-
posed7-9,11,12,15-18,28 to be the dihydride diamine and hydrido
amido-amine complexes as shown in Scheme 1. For example,
well-characterized ruthenium complexes with trans-dihydride
and cis-phosphine ligands, t,c-Ru(H)2(tmen)(PPh3)2, tmen )
NH2CMe2CMe2NH2, and trans-Ru(H)2(tmen)((R)-binap), are
active ketone hydrogenation catalysts without the need for
activation by a base.17,18 The dihydrides react directly with
ketones to produce alcohols and are regenerated by the reaction
of the hydrido amido-amine complexes RuH(NHCMe2CMe2-
NH2)(PPh3)2 and RuH(NHCMe2CMe2NH2)((R)-binap) with
hydrogen gas (Scheme 1). The heterolytic splitting across a
ruthenium-amido bond of an η2-dihydrogen ligand that is
weakly coordinated to ruthenium is the turnover limiting step
of the catalytic cycle. The complex trans-RuH(HBH3)((R)-
tolbinap)((R,R)-dpen), tolbinap ) 2,2′-bis(ditolylphosphino)-
1,1′-binaphthyl, dpen ) 1,2-diphenylethylenediamine, is also
an active catalyst without the need of added base;10 this
precatalyst was used in a kinetic study that supported the
mechanism of Scheme 1 for basic conditions in 2-propanol.28
Hartmann and Chen have proposed an alternate mechanism
where potassium is located on a hydridoamido intermediate and
serves to activate the ketone and promotes the heterolytic
splitting of dihydrogen by positioning an alkoxide ion for an
efficient deprotonation reaction.29,30 Similarly, a dihydride
complex RuH2(P-NH-NH-P) and amido-amine complex
RuH(P-N-NH-P) were proposed to be the active hydrogena-
(11) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2000, 19,
2655-2657.
(12) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2001, 20,
1047-1049.
(13) Cobley, C. J.; Henschke, J. P. AdV. Synth. Catal. 2003, 345, 195-201.
(14) Henschke, J. P.; Zanotti-Gerosa, A.; Moran, P.; Harrison, P.; Mullen, B.;
Casy, G.; Lennon, I. C. Tetrahedron Lett. 2003, 44, 4379-4383.
(15) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931-
7944.
(16) Noyori, R.; Koizumi, M.; Ishii, D.; Ohkuma, T. Pure Appl. Chem. 2001,
73, 227-232.
(17) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am. Chem.
Soc. 2001, 123, 7473-7474.
(18) 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.
(19) Alonso, D. A.; Brandt, P.; Nordin, S. J. M.; Andersson, P. G. J. Am. Chem.
Soc. 1999, 121, 9580-9588.
(20) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 285-288.
(21) Yamakawa, M.; Yamada, I.; Noyori, R. Angew. Chem., Int. Ed. 2001, 40,
2818-2821.
(22) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998-2001.
(23) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. J.
Am. Chem. Soc. 2001, 123, 1090-1100.
(24) Cenini, S.; Porta, F.; Pizzotti, M. J. Mol. Catal. 1982, 15, 297-308.
(25) Pamies, O.; Backvall, J. E. Chem.-Eur. J. 2001, 7, 5052-5058.
(26) Ikariya, T.; Ikehira, H.; Murata, K.; Kiyofuji, N.; Ooka, K.; Hashiguchi,
S.; Okuma, T.; Noyori, R. Jpn. Kokai Tokkyo Koho JP, 1999. CAN: 131:
87713.
(28) Sandoval, C. A.; Ohkuma, T.; Mun˜iz, K.; Noyori, R. J. Am. Chem. Soc.
2003, 125, 13490-13503.
(29) Hartmann, R.; Chen, P. AdV. Synth. Catal. 2003, 345, 1353-1359.
(30) Hartmann, R.; Chen, P. Angew. Chem., Int. Ed. 2001, 40, 3581-3585.
(27) Hartmann, R.; Chen, P. Angew. Chem., Int. Ed. 2001, 40, 3581-3585.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1871

A R T I C L E S
Abbel et al.
Table 1. Compound Numbering
diamines
amido−amine ligands
-
-
a
b
c
trans-(1R,2R)-diaminocyclohexane ((R,R)-dach)
1,2-diaminoethane (en)
2,3-diamino-2,3-dimethylbutane (tmen)
a′
b′
c′
(R,R)-NHC6H10NH2
NHCH2CH2NH2
NHCMe2CMe2NH2
-
complexes
1a, 1b, 1c
2a′, 2b′, 2c′
(OC-6-43)-RuHCl(PPh3)2(diamine) (trans-H‚‚‚Cl, cis-P‚‚‚P)
RuH(PPh3)2(amido-amine)
t,c-3a, 3b, 3c
∆-c,c-3a
Λ-c,c-3a
c,c-3b
(OC-6-22)-RuH2(PPh3)2(diamine) (with trans-H‚‚‚H, cis-P‚‚‚P)
∆-(OC-6-32)-RuH2(PPh3)2((R,R)-dach) (with cis-H‚‚‚H, cis-P‚‚‚P)
Λ-(OC-6-32)-RuH2(PPh3)2((R,R)-dach) (with cis-H‚‚‚H, cis-P‚‚‚P)
(OC-6-32)-RuH2(PPh3)2(en) (with cis-H‚‚‚H, cis-P‚‚‚P)
(OC-6-13)-Ru(PPh3)2(H)2(diamine) (with cis-H‚‚‚H, trans-P‚‚‚P)
(OC-6-14)-[Ru(η2-H2)H(PPh3)2((R,R)-dach)]BPh4
c,t-3a, 3b, 3c
4
5
6
(OC-6-.22)-RuH(O2PPh2)(PPh3)2((R,R)-dach)
Ru2H2(PPh3)4(µ-η2;η4-ΗΝdCHCHdNH)
tion catalysts for a tetradentate ligand system (R,R)-cyclo-C6H10-
{NHCH2C6H4PPh2}2.31
scheme). They are prepared here by the reaction of RuHCl-
(PPh3)3 with the appropriate diamine (eq 1).
Dihydrido diamine complexes of ruthenium with monodentate
phosphine ligands can potentially exist as four geometric
isomers, two of which are enantiomers (Chart 1). The fac-
stereochemistry of the hydride and diamine at ruthenium has
been proposed to be a requirement for catalytic activity.7 The
t,c and c,c isomers have this stereochemistry and potentially
have hydridic-protonic interactions32-34 between a ruthenium
hydride and an amino hydrogen that is axial with respect to the
diamine-Ru ring. However, the cis-dihydride complex c,t-Ru-
(H)2(PPh3)2((R,R)-dach) has a mer-diamine-hydride stereo-
chemistry and yet was also reported to be an active hydroge-
nation catalyst.11 Noyori and co-workers have provided evidence
for two dihydride isomers of the corresponding ethylenediamine
(en) system c,t-Ru(H)2(PPh3)2(en) and c,c-Ru(H)2(PPh3)2(en).26
In this article, we address the questions: which of these isomers
are competent catalysts or precatalysts for ketone hydrogenation
and what is the effect of the structure of each catalyst isomer
on the reactivity and obtained ee values? Although the ee of
the alcohol produced by the (R,R)-dach catalyst under study
here is lower than that observed for Noyori’s synthetically
valuable RuCl2(binap)(diamine) precatalysts, it is high enough
to answer some of these questions for the first time.
RuHCl(PPh3)3 + diamine f
(OC-6-43)-RuHCl(PPh3)2(diamine) + PPh3 (1)
Occasionally, a second, purple, insoluble diastereomer of 1a
is produced in small amount in eq 1. It is thought to be the
(OC-6-32) isomer with trans-PPh3 ligands. The structure of 1a
has been confirmed by X-ray crystallography (see Supporting
Information).
Observation of Amido-Amine Complexes. The reaction
of RuHCl(PPh3)2(tmen) 1c in THF under Ar with the strong
base potassium tert-butoxide produced the red amido-amine
complex RuH(PPh3)2(HNCMe2CMe2NH2)2 2c′, as described
elsewhere.18 This complex is stable and completely character-
ized. By contrast, the red solutions of complexes RuH(PPh3)2-
(HNC6H10NH2) 2a′ and RuH(PPh3)2(HNCH2CH2NH2) 2b′,
when prepared in a similar fashion (eq 2 shows the reaction for
2a′), are stable only for short periods of time. The neutral
phosphazene base tBuNP(NP(NMe2)3)336 (pKaTHF ≈ 4037,38) can
be used in place of KOtBu in eq 2, although an excess is required
to drive the reaction to completion. The low-temperature NMR
spectra of 2a′ in toluene-d8 are complex, probably because of
a monomer-dimer dynamic equilibrium (see Supporting In-
formation). NMR spectra of C6D6 solutions of 2a′ under Ar for
1 h start to show the presence of uncharacterized products of
its decomposition as well as the dihydride c,t-3a. The latter
compound likely forms from the abstraction by 2a′ of H2 from
other species in solution. The amido complex 2b′ has not been
produced in a pure state because decomposition starts im-
Results and Discussion
Starting Complexes. The starting complexes for this work
are the known yellow hydrido chloro complexes RuHCl(PPh3)2-
(diamine)11,35 that have hydride trans to chloride (OC-6-43) (1a,
diamine) (R,R)-dach, 1b, en; see Table 1 for the numbering
(31) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-Rashid, K.;
Morris, R. H. Chem.-Eur. J. 2003, 9, 4954-4967.
(32) Stevens, R. C.; Bau, R.; Milstein, D.; Blum, O.; Koetzle, T. F. J. Chem.
Soc., Dalton Trans. 1990, 1429.
(36) Schwesinger, R.; Schlemper, H.; Hasenfratz, C.; Willaredt, J.; Dambacher,
T.; Breuer, T.; Ottaway, C.; Fletschinger, M.; Boele, J.; Fritz, H.; Putzas,
D.; Rotter, H. W.; Bordwell, F. G.; Satish, A. V.; Ji, G. Z.; Peters, E. M.;
Peters, K.; vonSchnering, H. G.; Walz, L. Liebigs Ann. 1996, 1055-1081.
(37) Abdur-Rashid, G.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.;
Landau, S. E.; Morris, R. H. J. Am. Chem. Soc. 2000, 122, 9155-9171.
(38) Hinman, J. G. MSc. Thesis, University of Toronto, Toronto, Canada, 2001.
(33) Crabtree, R. H.; Eisenstein, O.; Sini, G.; Peris, E. J. Organomet. Chem.
1998, 567, 7-11.
(34) Morris, R. H. In Recent AdVances in Hydride Chemistry; Peruzzini, M.,
Poli, R., Eds.; Elsevier: Amsterdam, 2001; pp 1-38.
(35) James, B. R.; Wang, D. K. W. Inorg. Chim. Acta 1976, 19, L17-L18.
9
1872 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
Table 2. NMR Properties of the RuH(PPh3)2(amido-amine)
Complexes 2 in C6D6
18
2a′
2b
′
2c′
δ
H (ppm)
-20.7 (br t)
33
72.5 (A of AB)
73.0 (B of AB)
25
-20.9 (t)
34
72.0 (m)
73.2 (br)
-21.1 (t)
36
71.1 (s)
2JHP (Hz)
δP (ppm)
2JPP (Hz)
The trans-dihydride complexes t,c-3a and t,c-3b are not easily
detectable because of their short lifetimes. However, t,c-3a can
be observed when a solution of 2a′ in toluene-d8 is prepared,
frozen in liquid N2, and then reacted with 1 atm of dihydrogen
at -60 to -20 °C. After 40 min at -20 °C, there is about 40%
conversion to t,c-3a; some ∆/Λ-c,c-3a also forms by isomer-
ization of t,c-3a. After 10 min at 20 °C, t,c-3a has almost
disappeared while c,c-3a continues to grow in and then it, in
turn, isomerizes to c,t-3a.
Figure 1. Plot of the fraction of the dihydride species t,c-3a (9), ∆/Λ-
c,c-3a (2), and c,t-3a (×) produced in the reaction of complex 2a′ ([)
(0.08 M) in toluene-d8 with dihydrogen (1 atm) as monitored by 1H NMR.
Isomerization of the Dihydrides. Two different diastereo-
mers (∆-c,c-3a and Λ-c,c-3a) are produced in the isomerization
of t,c-3a (Scheme 2) in a ratio of about 1:2. These result from
chirality at Ru (∆/Λ) and at the enantiopure (R,R)-dach ligand.
It is not clear which is the major diastereomer, although we
argue below that it may be the Λ isomer on the basis of the
results of the reaction the diastereomers with the prochiral
ketone, acetophenone. The ethylenediamine complex t,c-3b
isomerizes to c,c-3b (Scheme 3). Complex c,c-3b exists as a
racemate (chirality at Ru only).
mediately. An interesting dimeric product of the decomposition
of 2b′ is discussed below. The spectra and chemical properties
of 2a′ and 2b′ in solution are similar to those of 2c′ (Table 2).
The isomers c,c-3a and c,c-3b have half-lives of 40 min and
about 1 h, respectively, at 20 °C with respect to further
Reaction of the Amido-Amine Complexes with Dihydro-
gen To Produce trans-Dihydrides. Understanding the course
of the reaction of these amido-amine complexes with dihy-
drogen is important since this is thought to be a step in the
catalytic cycle in the hydrogenation of ketones (Scheme
1)7,9,11,17,18 and imines.11 When hydrogen is added to 2a′ or 2b′
in solution, the resulting dihydrides undergo a succession of
isomerization reactions, leading from the least stable trans-
dihydride, cis-PPh3 isomer t,c-3, via the intermediate cis-
dihydride, cis-PPh3 isomers ∆/Λ-c,c-3a, to the most stable cis-
dihydride, trans-PPh3 isomer c,t-3. In contrast, the reaction of
2c′ with dihydrogen stops at the isomer t,c-3c, presumably
because the methyl groups on the backbone of the diamine
hinder the formation of the other isomers.18 Figure 1 shows how
the concentrations of 2a′ and the dihydride isomers change with
time after the addition of hydrogen gas to a solution of 2a′ in
toluene-d8. As long as the amido and H2 are present there is
always a small amount of the trans-dihydride t,c-3a.
Table 3. NMR Properties of the trans-Dihydride Complexes, t,c-3,
in C6D6
trans-H, cis-PPh3
t,c-3a
t,c-3b
t,c-3c18
δ
H (ppm)
-5.45 (t)
18
86.5 (s)
-5.90 (t)
20
86.0 (s)
-5.50 (t)
18
87.8 (s)
2JHP (Hz)
δP (ppm)
cis-H, trans-PPh3,
c,t-3a
c,t-3b
δ
H (ppm)
-18.30 (t)
27
66.0 (s)
-18.35 (t)
27
66.0 (s)
2JHP (Hz)
δP (ppm)
Scheme 2. Isomerization of the Dihydride t,c-3a
The species that are formed first from the reaction of
dihydrogen at 1 atm pressure with 2a′ and 2b′ are the very
reactive trans-dihydride complexes t,c-3a or t,c-3b, respectively
(eqs 3 and 4). Because the formation of this isomer requires
the lowest degree of structural reorganization, it is reasonable
that it is produced fastest. This has also been observed for 2c′
and calculated for 2b′.18 Evidence for the structural assignment
of t,c-3a and t,c-3b comes from the NMR spectra (see Table
3).
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1873

A R T I C L E S
Abbel et al.
Scheme 3. Isomerization of the Dihydride t,c-3b
isomerization that allows an approach to equilibrium with a cis-
dihydride that has trans-phosphines (c,t-3a and c,t-3b; Schemes
2 and 3). The minor diastereomer of c,c-3a appears to isomerize
at a slightly greater rate than the major diastereomer on the basis
of changes in the intensities of the hydride resonances between
-5 and -6 ppm. At equilibrium there is about 8% c,c-3a and
92% c,t-3a. When a pure sample of crystalline c,t-3a is dissolved
in C6D6, the c,c-3a isomers can be observed to grow in over a
period of a day until equilibrium is established, although there
is also some decomposition. An equilibrium mixture of 3b is
richer in the cis,cis isomer than 3a and consists of about 30%
c,c-3b and 70% c,t-3b.
The NMR experiments showed that the rate of the isomer-
ization of c,c-3a to c,t-3a in C6D6 is independent of the starting
dihydride concentration and therefore appears to be a unimo-
lecular process. The addition of PPh3 (0.012 M PPh3 to 0.016
M of complex) did not cause a significant change either.
Therefore, the isomerization could involve a trigonal twist of
groups on the octahedral metal. However, this result does not
rule out a mechanism where isomerization takes place after the
rate-limiting dissociation of a phosphine. The rate of isomer-
ization from c,t-3a to c,c,3a is about 10 times slower than the
reverse process, making c,t-3a a less active ketone hydrogena-
tion precatalyst in benzene than c,c-3a (see below).
The isomerization between cis- and trans-dihydrides has been
reported in a few cases. Milstein and co-workers reported a case
where the cis-dihydride Ir{C6H3(CH2PiPr2)2}(H)2(CO) isomer-
izes to the more stable trans-dihydride.39 Theoretical calcula-
tions provide evidence for two consecutive trigonal twists as
the mechanism for this reaction; that is, there is no ligand
dissociation involved.40 The complex Ru(H)2(PPh2CH2PPh2)2
exists as a mixture of cis- and trans-isomers in a ratio of 1:4
and in dynamic equilibrium,41 while other complexes, M(H)2-
(diphosphine)2 42,43 and M(H)2(phosphine)4,44 M ) Fe, Ru, Os,
tend to have a more stable cis-isomer than a trans one.
Preparation of the cis-Dihydrides. Two effective procedures
to synthesize mixtures of c,c-3 and c,t-3 in high yield are given
by eqs 5 and 6.
Table 4. NMR Properties of the cis-Dihydride Complexes c,c-3 in
C6D6
Λ
-c,c-3aa
∆-c,c-3a
c,c-3b
δ
H (ppm)
-5.65 (ddd)
-5.35 (ddd)
- 6.00 (ddd)
99
33
6
2JHPtrans (Hz)
2JHPcis (Hz)
2JHH (Hz)
100
102
32
6
33
6
δ
H (ppm)
-15.4 (m)
-15.4 (m)
-15.75 (td)
2JHP (Hz)
2JHH (Hz)
δP (ppm)
δP (ppm)
2JPP (Hz)
not resolved
not resolved
24
6
6
6
55.5
84.0
13
57.5
84.0
13
56.5
83.5
14
a Arbitrarily assigned to the major isomer with ratios of intensities of
peaks of 2:1)Λ: ∆. These assignments may need to be reversed.
The KOtBu/H2 method (eq 5) produces small amounts of side
products in low concentrations that are difficult to remove. This
might be because this reaction proceeds via unstable amido-
amine complexes. The potassium selectride (KHBsBu3) method
(eq 6) might proceed via the initial formation of (OC-6-22)-
RuH(HBsBu3)(PPh3)2((R,R)-dach) (cf. trans-RuH(HBH3)((R)-
binap)(dpen) 10) followed by its decomposition, first to the trans-
dihydride t,c-3a, and then rapidly to the observed mixture of
cis-dihydrides that becomes enriched in c,t-3a over time.
RuHCl(PPh3)2(diamine) + KOtBu + H2 (1 atm) f
c,c-3 + c,t-3 + KCl + HOtBu (5)
Some spectroscopic properties of the cis-dihydrides c,c-3a
and c,c-3b are summarized in Table 4, while those of c,t-3a
and c,t-3b are listed in Table 3. The NMR properties and X-ray
crystal structure of c,t-3a have already been reported.11 The
trans-phosphine stereochemistry of c,t-3b was also verified by
NMR using a 15N labeled diamine in the complex Ru(H)2-
(PPh3)2(15NH2CH2CH215NH2) (see the Experimental Section).
RuHCl(PPh3)2((R,R)-dach) + KHBsBu3 f
c,c-3a + c,t-3a + KCl + BsBu3 (6)
In either case, the ratio of c,c-3 to c,t-3 depends on the
reaction time and the time until precipitation of the product.
Shorter reaction times produce the mixtures enriched up to 85%
in the c,c-3 isomers. The same 1:2 ratio of diastereomers of
∆/Λ-c,c-3a is produced by both methods. The ratio of the
isomers remains unchanged after storage in the solid state. The
cis-dihydrides c,c-3b and c,t-3b have been mentioned in a
patent.26
Reaction of the Dihydrides with Ketones and Alcohols.
Only the isomers of RuH2(PPh3)2(diamine) with cis-phosphine
ligands react with acetophenone (PhMeCdO) and benzophenone
(Ph2CdO). Cold solutions containing a mixture of the trans-
dihydride t,c-3a and the amido complex 2a′ react with ac-
etophenone to produce predominately (S)-phenylethanol (see
below). This parallels the high activity of complex t,c-3c in
rapidly transferring dihydrogen to acetophenone to produce
1-phenylethanol.18
(39) Rybtchinski, B.; BenDavid, Y.; Milstein, D. Organometallics 1997, 16,
3786-3793.
(40) Li, S.; Hall, M. B. Organometallics 1999, 18, 5682-5687.
(41) Ayllon, J. A.; Gervaux, C.; Sabo-Etienne, S.; Chaudret, B. Organometallics
1997, 16, 2000-2002.
The cis,cis-isomers ∆/Λ-c,c-3a and c,c-3b in solution at 20
°C are consumed completely within 3 min after the addition of
acetophenone or benzophenone, while the cis,trans-isomers c,t-
3a and c,t-3b remain unchanged. When dihydrogen is added,
the cis,cis-dihydrides are regenerated, presumably via the trans-
dihydrides t,c-3, while the amount of the dihydrides c,t-3 still
(42) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc. 1991,
113, 3027-3039.
(43) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.; Schweitzer,
C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991, 113, 4876-4887.
(44) Jesson, J. P. In Transition Metal Hydrides; Muetterties, E. L., Ed.; Marcel
Dekker: New York, 1971; Ch. 4.
9
1874 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
Scheme 4. Stoichiometric Hydrogenation of Acetophenone by
t,c-3a
Scheme 5. Stoichiometric Hydrogenation of Acetophenone by the
Diastereomers of c,c-3a To Produce 1-Phenylethanol
remains unchanged. This demonstrates that the cis,cis-isomers
are potential catalysts for the hydrogenation of these ketones.
In contrast, the cis,trans-isomers need at least several days to
fully react and can even be detected after the addition of a large
excess of PhCOMe. The cis,trans-isomers may have to isomer-
ize to the cis,cis-isomer to react since the time scale of the
reaction with the c,t-3 compounds is similar to the time scale
for the c,t-3 to c,c-3 isomerization. This is the explanation for
the report of a reaction between c,t-3a and acetophenone in
C6D6.11
by the hydride. A concerted transfer of H- and H+ equivalents
gives the (S)-alcohol and the amido-amine complex 2a′. A
similar mechanism has been established for trans-dihydrides
3c and RuH2((R)-binap)(tmen).18 The existence of rotamers of
the PPh3 ligands in t,c-3a could explain why the enantioselec-
tivity is poor.
When a mixture containing predominantly the cis,cis-dia-
stereomers ∆-c,c-3a and Λ-c,c-3a is reacted with acetophenone
at 20 °C, an ee of 16-18% (S) is obtained. If it is assumed that
the most important reason for stereoselectivity is the avoidance
of the bulky syn-PPh3 ligand by the large phenyl group of the
acetophenone docked by hydrogen bonding in the second
coordination sphere, then the following model provides a
possible explanation for the reduction in enantioselectivity. The
proposed transition state for Λ-c,c-3a should favor (S)-1-
phenylethanol, whereas that for ∆-c,c-3a should favor the (R)-
enantiomer (Scheme 5) with the assumption that the larger Ph
group of the ketone is placed over the smaller hydride ligand.
The result will be a low overall ee.
The very low reactivity of the dihydride c,t-3a with the ke-
tone may be due to poor positioning of the amine protons and
hydrides for the dihydrogen transfer reaction. The X-ray crystal
structure of c,t-3a11 revealed that these are not aligned by
RuH‚‚‚HN hydridic-protonic interactions unlike the trans-
dihydride RuH2((R)-binap)(tmen) that does react readily with
ketones.17,18
Hydrogenation of Acetophenone Catalyzed by Complexes
3a in Benzene. The dihydride Ru(H)2(PPh3)2((R,R)-dach) 3a
was reported to be an active catalyst for the hydrogenation of
neat acetophenone to 1-phenylethanol with an enantiomeric
excess of 60% (S).11 However, the role of the different isomers
of 3a in catalysis was not studied.
The metal-containing products of the stoichiometric reaction
of benzophenone and acetophenone with c,c-3a or c,c-3b in
benzene have not been completely characterized. These could
be alkoxide complexes or the adducts of amido species and
alcohols (see the Supporting Information). Only the c,c-3a
isomer in a mixture of c,c-3a/c,t-3a reacts quickly with added
rac-1-phenylethanol in C6D6 under Ar, probably to give H2 and
the alcohol adduct of 2a′, RuH(OHCHMePh)(PPh3)2((R,R)-
1
dach), with a broad H NMR resonance at -20.4 ppm. This
behavior was observed for the reaction of the amido-amine
complex 2c′ with this alcohol.18
Enantioselective Stoichiometric Hydrogenation of Ac-
etophenone by the Dihydride Isomers of 3a. Solutions
containing increasing amounts of the trans-dihydride t,c-3a (and
smaller amounts of the cis,cis-dihydrides, c,c-3a) were prepared
by reacting the amido-amine complex 2a′ in toluene at -20
°C with dihydrogen for time intervals increasing from 10 to 40
min. When these were reacted with PhCOMe for approximately
1 min at -20 °C, the conversion to 1-phenylethanol that was
isolated after workup increased from 18 to 42% in accordance
with the amount of trans-dihydride present, while the ee stayed
at 63 ( 2% (S). The fact that the dihydrogen transfer to the
ketone is very fast, even at -20 °C, supports the mechanism
proposed below for the catalytic ketone hydrogenation. If the
ketone is left in contact with the hydrides for longer periods of
time (5-30 min at -20 °C), the cis,cis-dihydride isomers begin
to react to give a higher conversion to the alcohol, but with a
lower ee (50-30% (S)).
By examining the postulated structure of the transition state
(Scheme 4), we can explain the production of the excess of the
S enantiomer of 1-phenylethanol by hydrogen transfer to
PhCOMe from the trans-dihydride t,c-3a that contains the
optically active (R,R)-dach ligand. There is one axial NH aligned
with the Ru-H bond at each side of this C2-symmetrical
molecule. Hydridic-protonic interactions are postulated to favor
this alignment with a RuH‚‚‚HN distance of 2.2 Å as is observed
for trans-RuH2(tmen)((R)-binap).17 The ketone approaches either
side of the dihydride so that the oxygen forms a hydrogen bond
with an axial NH. Then the smaller Me group fits between the
bulky cis-PPh3 ligands, while the carbonyl carbon is attacked
Here we find that a mixture of about 85% ∆/Λ-c,c-3a and
15% c,t-3a45 in benzene that is prepared according to eq 5 serves
as an active catalyst precursor to hydrogenate PhCOMe to (S)-
PhCH(OH)Me with an ee of about 50%. When a sample of
this mixture of 3a is left to isomerize to an equilibrium mixture
of about 8% ∆/Λ-c,c-3a/92% c,t-3a in benzene, almost no
catalytic activity is observed using exactly the same procedure.
In general, we observe that the catalytic activity of 3a decreases
(45) During catalyst preparation and loading, this mixture should evolve into a
mixture of approx 75% c,c-3a and 25% c,t-3a.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1875

A R T I C L E S
Abbel et al.
Scheme 6. Proposed Catalytic Cycle and Routes into the Cycle
Kinetic Study of the Hydrogenation of Acetophenone
Catalyzed by Dihydrides 3a in Benzene. The rates of
hydrogenation as a function of concentrations of the dihydrides
3a, ketone and dihydrogen in benzene, benzene-d6, or 2-propanol
were studied by withdrawing samples from a thermostated
reactor at set time intervals and analyzing them by NMR and
gas chromatography over a chiral column. The conditions for
obtaining reproducible results are exacting (see the Experimental
Section). It is necessary to determine by NMR the starting ratio
of c,c-3a/c,t-3a isomers since only the c,c-3a isomers are
effective catalyst precursors in benzene as shown in Scheme 6.
Hence, fresh catalyst solutions have to be made up before each
run. The activity of a precatalyst benzene solution enriched in
c,c-3a decreases with time, decreasing as c,c-3a isomerizes to
c,t-3a. When a solution of 3a is added to the ketone solution
under H2 at the start of the hydrogenation reaction, the c,c-3a
present at that instant rapidly reacts with the ketone to start the
reaction.
The catalytic reaction in benzene was found to be first-order
with respect to the initial concentration of c,c-3a in the catalyst
mixture, first-order in dihydrogen concentration, and zero-order
in ketone concentration (eq 7, k7 ) 7.5 ( 0.4 M-1 s-1) at ketone
concentrations of less than 0.02 M (Table 5). The presence of
0.01 M PPh3 or 0.05 M racemic 1-phenylethanol (Table 5,
entries 4 and 5) did not significantly alter the reaction rate as
expected on the basis of Scheme 6. The first-order dependence
on the initial c,c-3a concentration is illustrated in Figure 2, while
the first-order dependence on hydrogen concentration is dem-
onstrated in Figures 3 and 4 (entries 2, 7, and 8 of Table 5).
The approximately linear dependence of concentration with time
throughout the reaction indicates that the catalytic reaction is
independent of the ketone and alcohol concentrations.
as the ratio of c,c-3a/c,t-3a decreases. Therefore, we can rule
out c,t-3a as being the active catalyst or catalyst precursor in
benzene.
Samples of 3a that contain the c,c-3a isomer prepared
according to eq 6 provide the alcohol in 55-61% ee (S) in
catalytic hydrogenation reactions. Samples prepared according
to eq 5 with similar amounts of c,c-3a give a product of lower
ee (48-51% ee (S)). We suspect that a small amount of
racemization of the (R,R)-dach ligand occurs by reversible
â-hydride elimination from the amido-amine intermediate of
reaction 5 to give an imine intermediate as in the formation of
6 discussed below. This would result in some loss of enan-
tiopurity.
The 48-61% ee (S) of the product in the catalytic hydroge-
nation approaches the 63% ee of the alcohol observed in the
stoichiometric reaction of predominantly the trans-dihydride t,c-
3a with acetophenone at low temperature and not the ee of 16-
18% (S) of the stoichiometric reaction of mainly ∆/Λ-c,c-3a
(see above). Therefore, the trans-dihydride t,c-3a is the active
catalyst along with the amido-amine complex 2a′, whereas the
isomers ∆/Λ-c,c-3a are only catalyst precursors to these active
catalysts.
Scheme 6 shows the proposed catalytic cycle for the
hydrogenation of acetophenone conducted in benzene solutions
by analogy to that proposed for 2c′/t,c-3c. The amido complex
2a′ reacts with H2 to produce t,c-3a that rapidly transfers the
hydride and proton to the ketone to produce 1-phenylethanol
in 50-60% ee (S) (cf. Scheme 4). This transfer is apparently
more rapid than the isomerization to ∆/Λ-c,c-3a because the
ee obtained is 50-60% (S), not the 16-18% (S) resulting from
the reaction of the cis,cis-isomers with acetophenone (cf.
Scheme 5). Scheme 6 shows two possible entries into the cycle.
The catalyst precursors can be c,c-3a, by the reactions of Scheme
5 to produce 2a′ and other ruthenium species, or 1a, by reaction
with base to produce mainly 2a′. The reaction of c,c-3a with
the ketone in benzene is very fast, and thus there is no induction
period for catalyst formation while there is an induction period
of several seconds starting with 1a and a suspension of KOtBu.
The second entry, the addition of complex 1a to a mixture of
ketone and base under H2, provides the most active system in
benzene and gives the alcohol in 50-62% ee (S) depending on
the conditions. The isomer c,t-3a is apparently not a good
precatalyst in benzene but will, over the course of several hours,
isomerize to c,c-3a that is a precatalyst.
d[alcohol] -d[ketone]
rate )
)
) k7[c,c-3a]initial[H2],
dt
dt
k7 ) 7.5 ( 0.4 M-1 s-1 (7)
The second entry route into the catalytic cycle of Scheme 6
is the addition of 1a to the KOtBu/ketone/H2/benzene mixture
to form 2a′. This results in a catalyst mixture that is about twice
as active as one from 3a on the basis of total ruthenium
concentration. It seems that this route provides a better yield of
2a′, but possibly at the expense of partial racemization of the
(R,R)-dach ligand, as mentioned above.
If the addition of dihydrogen to 2a′ is the turnover limiting
step of Scheme 6 as it is for 2c′ (k ) 110 M-1 s-1 at 20 °C),18
then a rate law like that of eq 7 would be expected except that
the concentration of the amido complex 2a′ is used instead of
[c,c-3a]initial (eq 8).
d[alcohol]
rate )
) k8[2a′][H2]
(8)
dt
We conclude that k8 is 8 M-1 s-1 or slightly greater depending
on the yield of catalyst 2a′ from the precatalyst c,c-3a or 1a.
The rate constant increases with temperature, but the interpreta-
tion of the temperature dependence will be complicated by the
presence of the various dihydride and alkoxide equilibria.
Figure 5 shows three runs at higher catalyst and ketone
concentrations. As mentioned previously, when mainly c,t-3a
is present, there is almost no catalytic activity. When 75% c,c-
9
1876 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
Table 5. Dependence of the Zero Order Rate of Production of Alcohol on the Concentrations of the Precatalyst c,c-3a and Hydrogen in
Benzenea
[Ru]total
,
% c,c-3a
5%)
[c,c-3a]init.
M ( 5%)
,
[H2],
M
v0, initial rate,
k
)
v0/([c,c-3a][H2]),
1
1
run
M
(
±
±
M-1 s-
(±
5%)
M-1 s-
%conversion/% ee
1
2
3
4
5
6
7
8
9
1.1 × 10-4
2.2 × 10-4
3.3 × 10-4
2.2 × 10-4
2.2 × 10-4
2.2 × 10-4
2.2 × 10-4
2.2 × 10-4
1.1 × 10-3
35
35
40
40
40
10
40
30
75
3.6 × 10-5
7.7 × 10-5
1.3 × 10-4
8.8 × 10-5b
8.8 × 10-5c
2.2 × 10-5d
8.8 × 10-5
6.6 × 10-5
8 × 10-4e
0.013
0.013
0.013
0.013
0.013
0.013
0.021
0.029
0.013
3.5 × 10-6
7.8 × 10-6
1.3 × 10-5
9.5 × 10-6
9.4 × 10-6
2.8 × 10-6
1.37 × 10-5
1.40 × 10-5
6.9 ( 0.5
7.7 ( 0.5
7.5 ( 0.5
8.2 ( 0.5
8.1 ( 0.5
9 ( 1
7.6 ( 0.5
7.3 ( 0.5
4 to 12
7/61; 31/55
16/60; 30/59; 73/55
49/62; 96/61
15/63; 90/55
14/60; 24/58
20/59;77/58
32/58; 96/56
8/47; 30/50
4 × 10-5 to 1.2 × 10-4e
a [Acetophenone]0 ) 0.0167 except for run 9 where it is 0.167 M; constant [H2] ) 0.013 M (5 atm), 0.021 M (8 atm), or 0.029 M (11 atm); 293 K; [Ru]tot
) [active c,c-3a] + [inactive c,t-3a]. b [PPh3] ) 0.01 M. c [rac-1-phenylethanol]0 ) 0.05 M. d A solution of 35% c,c-3a after sitting for 2.5 h. e [acetophenone]0
) 0.167 M (Figure 5).
Figure 4. Plot of rate of alcohol formation divided by the starting
concentration of the isomer c,c-3a as determined by NMR versus the
Figure 2. Plots of production of 1-phenylethanol as a function of time in
the hydrogenation of acetophenone (0.0167 M) by precatalyst 3a in benzene
hydrogen concentration. The slope of the line is the rate constant, k7 ) 7.5
M-1 s-1
.
at 20 °C, 5 atm H2, with total initial concentrations of 3a of 3.3 × 10-4
M
(40% c,c-3a, 9), 2.2 × 10-4 M (35% c,c-3a, 2), and 1.1 × 10-4 M (35%
c,c-3a, [). The solid lines represent the numerical integration of eq 7 with
k7 ) 7.5 M-1 s-1 with the initial conditions as indicated above. The ee
values decrease from 60 to 55% (S) over the course of the reaction.
Figure 5. Plot of 1-phenylethanol concentration versus time during the
hydrogenation (5 atm) of acetophenone (0.167 M) in benzene at 20 °C
catalyzed by 0.0011 M 3a as 75% c,c-3a/25% c,t-3a ([) or >95% c,t-3a
(2) or >95% c,t-3a that was preactivated by reaction with 0.05 M racemic
1-phenylethanol for 15 min under H2 (9). The ee range from 49 to 51%
(S).
tion precatalyst RuHCl{(R,R)-(PPh2C6H4CH2NH)2C6H10}.31 The
latter acceleration might be due to the high concentration of
the alcohol product that assists in the activation of dihydrogen
as observed in 2-propanol.
The upper curve in Figure 5 demonstrates that the trans-
phosphine isomer c,t-3a can be activated by reaction with excess
racemic 1-phenylethanol (0.05 M) for 15 min under H2 before
adding the ketone. Again there is also increasing activity with
time. Lower concentrations of the dihydride c,t-3a and alcohol
do not display this activation phenomenon. The higher concen-
tration of this acidic alcohol may allow the protonation of this
dihydride to give a transient dihydrogen complex that loses H2
to produce an alkoxide complex RuH(OCHMePh)(PPh3)2-
((R,R)-dach). This alkoxide complex might then isomerize to
Figure 3. Plots of 1-phenylethanol concentration as a function of time in
the hydrogenation of acetophenone (0.0167 M) by precatalyst 3a in benzene
at 20 °C, with total initial concentrations of 3a of 2.2 × 10-4 M and constant
hydrogen concentrations of 0.0132 M (5 atm, 35% c,c-3a, [), 0.0212 M
(8 atm, 40% c,c-3a, ×), and 0.0292 M (11 atm, 30% c,c-3a, 9). The solid
lines (dotted line for the 8 atm run) represent the numerical integration of
eq 7 with k7 ) 7.5 M-1 s-1 and the hydrogen concentrations as indicated.
The ee values decrease from 60 to 57% (S) over the course of the reaction.
3a is present, there is good activity and the plot of conversion
versus time curves upward. The initial rate is lower than that
expected on the basis of eq 8, while the maximum rate is slightly
higher than expected (Table 5, entry 7). A possible explanation
for the former low rate might be an inhibition by ketone; this
has been observed previously for the related ketone hydrogena-
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1877

A R T I C L E S
Abbel et al.
the trans-isomer and eliminate alcohol to give the ketone
hydrogenation catalyst 2a′. Thus, catalytically inactive c,t-3a
is converted into active 2a′. Chaudret and co-workers have
reported observing the first steps of such a reaction sequence.41
The cis-dihydride cis-RuH2(dppm)2, dppm ) PPh2CH2PPh2,
reacts with phenol to produce first the dihydrogen-bonded
complex trans-Ru(H‚‚‚HOPh)(H)(dppm)2 along with the dihy-
drogen complex trans-[Ru(H2)(H)(dppm)2][OPh], and then the
alkoxide complex trans-RuH(OPh)(dppm)2.
Figure 6. Structure i, analogous to the transition state structure ii.
Scheme 7. Preparation of the Phosphinate Complex 5
While no dihydrogen-bonded or dihydrogen complexes have
been detected by reaction of complex 3a with alcohols under
H2 thus far, the related dihydrogen complex (OC-14)-[Ru(η2-
Η2)(H)(PPh3)2((R,R)-dach)]BPh4 (4) with trans-phosphines has
been observed in solution by reacting c,t-3a with [HNEt3][BPh4]
under 1 atm H2 (eq 9). Equation 9 shows that the dihydride
c,t-3a is more basic than triethylamine in THF. Therefore, it is
reasonable that the acidic alcohol 1-phenylethanol could pro-
tonate this dihydride when in high enough concentration. Noyori
and co-workers have reported evidence for the formation of
trans-[Ru(H2)(H)((S)-tolbinap)((S,S)-dpen)]+ in alcohol solu-
tion.28
to 60% (S) in 2-propanol, while they decrease in benzene. The
complexities of this system would require further extensive
investigation.
Diphenylphosphinate Complex with Similar Structure to
the Proposed Transition State for Ketone Hydrogenation
in Benzene. The diphenylphosphinate group has been used as
an analogue (Figure 6, i) of the transition state for the transfer
of hydride and proton from a ruthenium transfer hydrogenation
catalyst Ru(η6-arene)(RC6H4SO2NCH2CH2NH2)(H), arene )
cymene, R ) CH2dCH-, to benzophenone (Figure 6, ii).49
In a similar approach, the reaction of diphenylphosphinic acid
with complex 3a (Scheme 7) resulted in the slow evolution of
H2 gas and the gradual formation of the sparingly soluble, air-
stable diphenylphosphinato complex, 5, which was isolated as
pale yellow air-stable crystals. This compound was also prepared
by adding solid diphenylphosphinic acid to a freshly prepared
solution of 2a′, whereupon 5 was formed without the evolution
of H2 gas. Two diastereomers are in the unit cell of a crystal of
5. The structure of each consists of a distorted octahedron, with
cis-phosphines and with the hydride ligand trans to the
coordinated phosphinato oxygen. In one isomer (Figure 7), there
is a PO‚‚‚HN hydrogen bond between the noncoordinated
phosphinato oxygen (PdO) and the amine hydrogen (H1BB)
of the dach ligand that is axial with respect to the Ru-N-C-
C-N- ring. The O(2B)‚‚‚N(1B) distance is 2.944(5) Å. A
ketone oxygen is proposed to form a similar hydrogen bond to
an axial NH in the concerted hydrogen transfer transition state
(Scheme 4), and thus this diastereomer of 5 is an interesting
transition state analogue. The other isomer in the crystal has a
PO‚‚‚HN hydrogen bond with a longer O(2A)‚‚‚N(2A) distance
of 3.016(5) Å involving an amine hydrogen that is bisectional50
with respect to the five-membered ring. The latter hydrogen
bond is similar to the one observed in Ru(η6-arene)(RC6H4-
The dihydrogen compound 4 is only stable in solution under
an atmosphere of H2 gas. The 1H NMR spectrum of the salt in
THF-d8 shows a single triplet at -10.65 ppm (JHP )15.0 Hz)
for the hydride and dihydrogen ligands in rapid exchange, while
a singlet and a quartet at 52.0 ppm in the 31P{1H} and 31P NMR
spectra, respectively, are consistent with the presence of three
coordinated hydrogens and mutually trans-PPh3 ligands. When
a solution of the salt in THF-d8 is sealed under deuterium gas,
the 1H NMR spectrum shows the presence of the three
observable isotopomers, 4-H3, 4-H2D, and 4-HD2. After equili-
bration under a large excess of deuterium gas, only 4-HD2 was
detected. The hydride region of the 1H{31P} NMR spectrum of
this species shows a nonbinomial quintet with a averaged HD
coupling constant of 4.5 Hz, which was used to calculate a 1JHD
coupling constant of 13.5 Hz in the HD ligand. This value is
consistent with the presence of an elongated dihydrogen ligand
with an H-H distance of 1.2 Å.46,47 The dihydrogen complexes
[RuH(H2)(PCy3)2(L)]+, L ) bipy or phen, have very similar
properties to complex 4.48
Study of the Hydrogenation of Acetophenone Catalyzed
by Dihydrides 3a in 2-Propanol. Solutions of 3a in 2-propanol
in the absence of added base change over time and have variable
catalytic activity in the hydrogenation of acetophenone. The
plots of 1-phenylethanol production versus time in the catalytic
hydrogenation reactions are curved with decreasing rate with
increasing time (see the Supporting Information). The initial
rates are higher than those for corresponding conditions with
benzene as a solvent. The ee values increase with time from 56
(46) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.;
Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996,
118, 5396-5407.
(47) Luther, T. A.; Heinekey, D. M. J. Am. Chem. Soc. 1997, 119, 6688-6689.
(48) Heinekey, D. M.; Mellows, H.; Pratum, T. J. Am. Chem. Soc. 2000, 122,
6498-6499.
(49) Polborn, K.; Severin, K. Chem.-Eur. J. 2000, 6, 4604-4611.
(50) Bisectional: between axial and equatorial. See Luger; P.; Bu¨low, R. J.
Appl. Crystallogr. 1983, 16, 431-432 for ring descriptor classifications.
9
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A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
Figure 8. Structure and atomic numbering of the diazabutadiene complex
5.
σ-fashion by the nitrogens to Ru(1) and in an η2-CdN, η2-C′d
N′ π-fashion to Ru(2). The planar Ru(1)-diimine grouping acts
like an η5-cyclopentadienyl analogue in its bonding to the second
ruthenium. This type of η2;η4-bridging diimine interaction has
been observed previously in the structures of derivatives of the
compounds M2(RNdCHCHdNR)(CO)4(X2), M ) Fe, Ru, Os,
R ) alkyl, aryl,52-54 where X2 represents a bridging carbonyl,
alkyne, or Ru(CO)4 unit. Therefore, the diimine bridges between
two “MX(CO)2” groups in this case. Such complexes have imine
CH resonances in the 1H NMR spectrum that range between 5
and 8 ppm (cf. 5.1 ppm for 6). The Ru-Ru distances fall in
the range from 2.7 to 2.9 Å, similar to that of 6 (Ru(1)-Ru(2)
2.9735(3)).
The 1H and 31P{1H} NMR signals of 6 in C6D6 are concordant
with a molecular mirror plane containing the two inequivalent
Ru-H groups. The CHdN 1H NMR resonance is a doublet at
5.1 ppm, and pairs of PPh3 ligands produce singlets in the 31P-
{1H} NMR spectrum at 70.0 and 70.5 ppm. The hydride
resonances are triplets at -7.15 and -19.85 ppm in the 1H NMR
spectrum.
Figure 7. Molecular structure of isomer B of RuH(O2PPh2)(PPh3)2((R,R)-
dach) 5.
SO2NCH2CH2NH2)(O2PPh2) with O‚‚‚N 2.972(3) Å.49 One
difference is the much shorter Ru-O distance in the latter
complex of 2.155(3) Å where the oxygen is trans to a Ru-C
bond than in complex 5 (Ru-O 2.295(3) or 2.298(3)) where
oxygen is trans to a Ru-H bond. In solution, there must be
rapid isomerization between the hydrogen-bonded structures of
5 because only one hydride resonance (broad triplet) and three
inequivalent phosphorus resonances are observed by 1H and 31
P
NMR, respectively.
1
Decomposition of 2b′. The H and 31P {1H} NMR spectra
of the amido-amine complex 2b′ indicate that this compound
decomposes in C6D6 under Ar via a dehydrogenation of the
ethylenediamine ligand (eq 10) to produce two dihydride species,
c,c-3b and c,t-3b, and at least two other complexes, one of which
is likely to be the dinuclear complex 6. After 60 min, all of the
amido complex 2b′ is gone. Therefore, hydrogen is transferred
from one molecule of 2b′ to another to generate the dihydrides
and the diimine species. This process may be initiated by
â-hydride elimination from the amido-amine ligand in 2b′, a
process that is shut down by the methyl groups in the stable
complex RuH(NHCMe2CMe2NH2)(PPh3)2 (2c′).
The decomposition of the amido-amine complex 2a′ under
Ar may also produce a similar dinuclear complex (a disordered
crystal structure with some of the features of 6 was obtained in
one case) along with a multitude of other compounds.
Conclusions
The reactive amido-amine complexes RuH(NH-R-NH2)-
(PPh3)2 2 can be prepared by dehydrohalogenation of the
complexes RuHCl(diamine)(PPh3)2 1. The stability of the amido
complexes decreases as the diamine changes in the order NH2-
CMe2CMe2NH2 > (R,R)-dach > en. The last complex decom-
poses to dihydrides and an interesting dinuclear complex 6 with
a bridging η2;η4-diazabutadiene ligand.
The amido-amine complexes react with dihydrogen to
produce a succession of dihydride isomers RuH2(diamine)-
(PPh3)2 3 when the diamine is (R,R)-dach or en: first, the trans-
To reduce the complexity of the reaction, acetophenone was
added to act as the dihydrogen acceptor in place of equivalents
of 2b′. Indeed, the main product of the reaction of 1b with KOt-
Bu in the presence of PhCOMe under Ar in C6D6 is the dinuclear
complex Ru2H2(PPh3)4(HNdCHCHdNH) 6, which can be
isolated from this reaction in a yield of about 30% but still
contaminated by at least one other compound that also appears
(51) Lane, B. C.; Lester, J. E.; Basolo, F. J. Chem. Soc., Chem. Commun. 1971,
1618-1619.
1
to have an imine ligand on the basis of H NMR.
(52) Keijsper, J.; Polm, L.; Koten, G. V.; Vrieze, K. Inorg. Chem. 1984, 23,
2142-2148.
The determination of the crystal structure of complex 6
reveals that it is composed of the rare diimine ligand, 1,4-
diazabutadiene, HNdCHCHdNH,51 that bridges between two
RuH(PPh3)2 fragments (Figure 8). The diimine is bonded in a
(53) Keijsper, J.; Polm, L. H.; Koten, G. V.; Vrieze, K.; Seignette, P. F. A. B.;
Stam, C. H. Inorg. Chem. 1985, 24, 518-525.
(54) Muller, F.; Koten, G. v.; Vrieze, K.; Heijdenrijk, D. Inorg. Chim. Acta
1989, 158, 69-79.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1879

A R T I C L E S
Abbel et al.
(10 mL) in toluene (150 mL) was degassed with hydrogen and RuCl2-
(PPh3)3 (20.0 g, 20.9 mmol) added under a flow of hydrogen gas. The
resulting mixture was refluxed for 2 h under hydrogen with vigorous
stirring. The mixture was then cooled to room temperature and then in
ice (1 h), and the purple solid was filtered under argon, washed with
ethanol (3 × 20 mL), then ether (2 × 20 mL), and dried under vacuum
(17.2 g, 89%). 1H NMR (C6D6, δ): -17.9 (qrt, JHP ) 26 Hz, 1H, RuH),
7.5-7.0 ppm (m, 45H, Ph). 31P{1H} NMR: 58.8 ppm (s, br).
RuHCl(PPh3)2((R,R)-dach) (1a).11 RuHCl(PPh3)3 (600 mg, 0.65
mmol) and (R,R)-dach (80 mg, 0.70 mmol) were mixed in an
atmosphere of N2, 4 mL of THF was added, and the reaction mixture
was stirred for 4 h, producing a dark yellow solution that was filtered.
In a few instances, a small amount of an insoluble deep blue compound,
thought to be (OC-6-32)-RuHCl(PPh3)2((R,R)-dach), was recovered. The
filtrate was concentrated to 1.5 mL in vacuo, and 5 mL of hexanes
was added to give a yellow precipitate. This was filtered, washed with
5 mL of hexanes, and dried in vacuo to give 1a as a pale yellow powder
(470 mg, 93%). Yellow crystals were obtained by layering a C6D6
solution with hexanes. 1a: E. A. Calcd for C42H45ClN2P2Ru: C, 64.98;
dihydride isomers, then the cis,cis-isomers, and finally an
equilibrium mixture of the cis,cis- and cis,trans-isomers. This
isomerization possibly proceeds via two trigonal twists of the
octahedral coordination geometry. The fast, stoichiometric,
enantioselective reduction of acetophenone by the cis,cis- and
trans,cis-dihydride isomers of RuH2((R,R)-dach)(PPh3)2 is a
significant mechanistic result. The low enantiomeric excess in
the alcohol produced in the reactions of the c,c-3a diastereomers
indicates that these isomers are only precatalysts, while the 63%
ee from reaction with t,c-3a strongly suggests that this trans-
dihydride is the catalyst in benzene and in 2-propanol. The
original report11 that the dihydride c,t-3a is likely to be within
the catalytic cycle is incorrect, although this isomer can serve
as a precatalyst when reacted with an alcohol. This is yet another
instance where an isolated, stable “catalyst” turns out to be only
a precatalyst. Thus, the cis-phosphine isomers are more reactive
than the trans-phosphine one. The very active and selective
Noyori-type catalysts for the asymmetric H2-hydrogenation of
ketones contain a chelating, enantiopure diphosphine ligand and
thus necessarily have phosphorus donors that are mutually cis.
This appears to be an important structural feature of the catalysts.
The turnover-limiting step in the hydrogenation of acetophe-
none in benzene catalyzed by the dihydrides appears to be the
activation of dihydrogen at the amido-amine complex, as it is
for the tmen system 2c′/t,c-3c. The complex RuH(O2PPh2)-
(PPh3)2((R,R)-dach) is a good model of the transition state for
the concerted transfer of hydride and proton to the ketone. The
mechanism when the solvent is phenylethanol/benzene or neat
2-propanol is more complex and would require extensive
additional study. The catalyst is more active in the presence of
alcohol. 2-Propanol is the solvent of choice in Noyori’s
enantioselective catalysts for ketone hydrogenation.
H, 5.84; N, 3.61. Found: C, 62.97; H, 5.63; N, 3.87. IR (Nujol), cm-1
:
1987 (s), 1957 (s) (νRuH), 3332 (s), 3270 (m), 3229 (m) (νNH). 1H NMR
2
(C6D6, δ): -17.99 (t, JHP ) 26 Hz, 1H, RuH), 0.12-0.31 (m, 2H,
CH), 0.45 (br, 2H, NH), 0.8-0.98 (m, 4H, CH), 1.75-2.04 (m, 3H,
CH), 2.38 (br, 2H, NH), 2.83 (t, 1H, CH), 6.96-7.20, 8.00 (m, 30H,
Ph). 31P{1H}: 70.6 ppm (s). (OC-6-32)-RuHCl(PPh3)2((R,R)-dach): 1H
NMR (DMF-d7, δ): -12.07 (t, 2JHP ) 21.75 Hz, 1H, RuH), 0.4-4.48
(m, 14H), 7.63-7.91 (m, 30H, Ph). 31P{1H}: 48.06 ppm (s).
RuHCl(PPh3)2(en) (1b).35 RuHCl(PPh3)3 (600 mg, 0.65 mmol) and
en (40 mg, 0.67 mmol) were mixed in an atmosphere of N2, 3 mL of
THF was added, and the resulting mixture was stirred for 4 h under
N2. It was filtered and reduced in vacuo to about 1 mL, and 10 mL of
hexanes was added to precipitate a yellow solid after being stirred for
10 min. The precipitate was collected by filtration, washed with 2 mL
of hexanes, and dried in vacuo for 1 h. This was recrystallized twice
to remove free PPh3 by adding hexanes to a concentrated solution of
1b in CH2Cl2. Yield of 1b as a pale yellow powder: 460 mg (>95%).
The dehydrogenation of the amine ligand during amido-
amine formation, as observed for the ethylenediamine complex,
could explain why the dihydrides prepared by the RuHCl((R,R)-
dach)(PPh3)2/KOtBu/H2 route provide 1-phenylethanol of lower
ee when used as hydrogenation catalysts than dihydrides
prepared by the RuHCl((R,R)-dach)(PPh3)2/KsecBu3BH route.
The dehydrogenation and rehydrogenation of the dach ligand
could result in some racemization of this ligand and cause the
formation of other diastereomers of RuH2(dach)(PPh3)2.
2
1H NMR (C6D6, δ): -18.15 (t, JHP ) 26 Hz, 1H, RuH) 1.60, 1.85,
2.05, 2.40, (br s, 8H, CH2 and NH2), 6.9-7.2, 7.85, (m, 30H, Ph).
31P{1H} NMR (C6D6): 70.0 ppm (s).
Preparation of a Solution of RuH(PPh3)2((R,R)-HNC6H10NH2)
(2a′). The monohydride 1a (62 mg, 0.08 mmol) and KOtBu (20 mg,
0.18 mmol) were mixed in an atmosphere of N2, C6D6 (1.0 mL) was
added, and the reaction mixture was stirred for 15 min, resulting in the
formation of 2a′. The NMR data were quickly collected without
isolation, using the filtered, dark red reaction mixture. 1H NMR (hydride
2
region, C6D6, δ): -20.70 (br t, JHP ) 33 Hz). 31P{1H} NMR (C6D6):
Experimental Section
2
2
72.5 (d, JPP ) 24.5 Hz), 73.0 ppm (d, JPP ) 24.5 Hz).
General. All preparations and manipulations were carried out under
hydrogen, nitrogen, or argon atmospheres with the use of standard
Schlenk, vacuum line, and glovebox techniques in dry, oxygen-free
solvents. All of the chemicals were purchased from Aldrich Chemical
Co. unless otherwise noted. The 99.5% 15N-labeled ethylenediamine
was obtained from CDN Isotopes. Instruments and the methods of
solvent purification have been described previously.18 The acetophenone
was distilled and deoxygenated before use.
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 Mo KR
radiation (λ ) 0.71073 Å; see the Supporting Information). The CCD
data were integrated and scaled using the DENZO-SMN software
package, and the structures were solved and refined using SHELXTL
V5.0. Wherever reported, hydrides were located and refined with
isotropic thermal parameters.
Observation of RuH(PPh3)2(NHCH2CH2NH2) (2b′). The mono-
hydride 1b (60 mg, 0.08 mmol) and KOtBu (20 mg, 0.18 mmol) were
mixed in an atmosphere of N2, C6D6 (0.6 mL) was added, and the
reaction mixture was stirred for 3 min, resulting in the formation of a
red solution. This was frozen by cooling with liquid N2, H2 gas was
added, and the mixture was warmed to room temperature. The 1H NMR
spectrum, obtained 3 min after melting, showed the presence of an
amido species 2b′ (37%) and the three dihydride isomers t,c-3b (2%),
c,c-3b (54%), c,t-3b (7%), and a small amount of the dinuclear
compound 6. 1H NMR (hydride region, C6D6, δ) 2b′: -20.9 (br t, 2JHP
2
) 33 Hz). t,c-3b: -5.9 (t, JHP 20). c,c-3b, c,t-3b, and 6 (see below).
31P{1H} NMR (C6D6): 2b′: 72.0 (br), 73.1 ppm (br); t,c-3b: 86.0 (s);
c,c-3b, c,t-3b and 6 (see below).
Observation of the Successive Formation of the Dihydrides t,c-
3a, ∆/Λ-c,c-3a and c,t-3a. (a) Complex 1a (31 mg, 0.04 mmol) and
KOtBu (10 mg, 0.09 mmol) were mixed in an atmosphere of N2,
Synthesis. RuHCl(PPh3)3. A modification of the reported proce-
dure55 was used to prepare this complex. A solution of triethylamine
(55) Schunn, R. A.; Wonchoba, E. R.; Wilkinson, G. Inorg. Synth. 1971, 13,
131-134.
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A Succession of Isomers of Ruthenium Dihydride Complexes
A R T I C L E S
toluene-d8 (0.6 mL) at -20 °C was added, and the resulting mixture
was stirred for 10 min while it warmed to room temperature and was
filtered and frozen in liquid N2. Hydrogen gas was added, and the
mixture was allowed to warm to -78 °C and stored at that temperature
for 45 min. 1H and 31P spectra were taken at -60, -20, and 25 °C. At
low temperatures, they clearly showed the conversion of 2a′ to the
dihydride species t,c-3a that then started isomerizing to ∆-c,c-3a and
Λ-c,c-3a at -20 °C and totally disappearing at room temperature after
2a′ was consumed (Figure 1). t,c-3a: 1H NMR (hydride region, toluene-
d8, δ): -5.40 (t, 2JHP ) 18 Hz). 31P{1H} NMR (toluene-d8): 86.5 ppm
(s). Similar experiments were conducted using C6D6 to monitor the
conversion of the c,c-3a isomers to c,t-3a.
Observation of (OC-14)-[Ru(H2)(H)(PPh3)2((R,R)-dach)]BPh4 (4).
Cold THF-d8 (-30 °C, 0.6 mL) was added to a mixture of c,t-3a (30
mg, 0.04 mmol) and [HNEt3]BPh4 (17 mg, 0.04 mmol) in an NMR
tube. The mixture was then sealed under hydrogen gas. The NMR
spectra showed the clean formation of the dihydrogen salt [RuH(H2)-
(PPh3)2(dach)]BPh4. 1H NMR (THF-d8) δ: -10.65 (t, 2JHP ) 15.0 Hz,
3H, RuH), 0.37-2.67 (m, 14H), 6.69-7.71 (m, 50H). 31P{1H} δ: 51.9
ppm (s).
RuH(PPh3)2(Ph2PO2)((R,R)-dach) (5). A suspension of diphen-
ylphosphinic acid (87 mg, 0.40 mmol) in THF (5 mL) was added with
stirring to a solution of the dihydride c,t-3a (300 mg, 0.40 mmol) in
THF (5 mL). Evolution of H2 gas was accompanied with the formation
of a yellow crystalline solid over a period of 4 h. The solid was collected
by filtration, washed with ether and then hexanes, and dried under
vacuum (353 mg, 92%). A crystal from this preparation was chosen
for the X-ray diffraction study. 1H NMR (C6D6, δ): -21.93 (br t or td,
2JHP ) 20.2, 3JHP 4.2 Hz, 1H, RuH), 0.21-3.25 (m, 12H), 2.47 (b, 1H,
NH), 5.55 (b, 1H, NH), 6.90-8.20 (m, 40H). 31P{1H} NMR: 71.1 (d,
2JPP 38.6 Hz), 69.0 (d), 25.0 (s). IR (Nujol): 2000 cm-1 (νRuH), 3207
(br), 3251, 3279, 3319, 3324, 3333, 3340 cm-1 (νNH). E. A.
C54H55N2O2P3Ru, Calcd: C, 67.70; H, 5.79; N, 2.92. Found: C, 67.27;
H, 5.98; N, 2.76.
Ru(H)2(PPh3)2((R,R)-dach) (3a, Enriched in Isomer c,t-3a). THF
(2 mL) was added to the monohydride 1a (1.0 g, 1.28 mmol) and
potassium tri-sec-butylborohydride (1.5 g of a 1.0 M solution, 1.7
mmol), and the mixture was stirred for 2 h under nitrogen. The resulting
mixture was filtered and evaporated to dryness. Hexanes (10 mL) and
a few drops of 2-propanol were added, precipitating a bright yellow
solid. This was collected by filtration, washed with hexanes, and dried
under vacuum to give the yellow solid (782 mg, 82%) as a mixture of
c,t-3a (72%), ∆-c,c-3a (8%), and Λ-c,c-3a (20%).56 Crystals of c,t-3a
were obtained by diffusion of hexanes into an ether solution as described
previously.11 E. A. Calcd for C42H46N2P2Ru: C, 68.0; H, 6.2; N, 3.9.
Found: C, 67.3; H, 6.2; N, 3.3. IR (Nujol): 1833 (s), 1944 (s) cm-1
(νRuH), 3266 (w), 3282 (w), 3327 (m), 3337 (w) cm-1 (νNH). 1H NMR
Ru2H2(PPh3)4(HNdCH-CHdNH) (6). Complex 1b (100 mg, 0.14
mmol), KOtBu (31 mg, 0.28 mmol), and acetophenone (18 mg, 0.15
mmol) were mixed, 1 mL of THF was added, and the resulting mixture
was stirred for 4 h under N2, giving a red-brown suspension that was
filtered to leave a white solid (KCl) and a dark red-brown filtrate. About
10 mL of hexanes was added to the filtrate, and the mixture was stirred
for 1 h to produce a yellow-brown suspension. A yellow solid was
collected by filtration and dried in vacuo (32%). Recrystallization from
THF/hexanes did not result in the separation of 6 from the other species
present. A recrystallization from benzene/hexanes gave crystals of 6
in one instance. 6: 1H NMR (C6D6, δ): 6.9-7.8 (m, Ar), 5.10 ppm
2
(C6D6, 300 MHz, δ) c,t-3a: -18.26 (t, JHP 26.7 Hz, 2H), 0.17 (m,
2H), 0.37 (t, 2H), 0.96 (d, 4H), 1.20 (m, 4H, CH2 + NH2), 2.04 (d,
J
HH 6.3 Hz, 2H, NH2), 7.02 (t, 3JHH 7.2 Hz, 6Hpara), 7.15 (dd, 3JHH 7.2,
6.6 Hz, 12Hmeta), 8.18 (dt, 3JHH 6.6, 3JPH + 5JPH 3.9 Hz, 12Hortho). ∆-c,c-
3a: -15.5 (dt, 2JHP 23, 2JHH 6 Hz, RuH), -5.5 (ddd, 2JHP 103, 34, 2JHH
6 Hz, RuH), 0.1-2.1 (m, 14H, CH, NH), 6.1 (s br, 2Hortho), 6.9-8.0
(m, 28 H). Λ-c,c-3a: -15.4 (dt, 2JHP 23, 2JHH 6 Hz, RuH), -5.6 (ddd,
2
2JHP 100, 32, JHH 6 Hz, RuH), 0.0 (br, 2H), 0.1 (br, 2H), 0.3-2.1
(8H), 2.3 (m, 2H), 2.6 (m, 2H), 6.9-8.0 (m, 30 H). 31P{1H} (C6D6)
2
(d, J ) 6 Hz, CHdNH), -7.15 ppm (t, JHP 23 Hz, RuH), -19.85
2
2
ppm (t, J 30 Hz, RuH). 31P{1H} NMR (C6D6): 70.0 (s), 70.5 ppm (s).
Other species: 1H NMR (C6D6, δ): 5.35 (d, J ) 6 Hz, CHdNH),
-9.55 (m, RuH), -13.30 (t, J ) 32 Hz, RuH). 31P{1H} NMR (C6D6):
67.0 ppm (s).
c,t-3a: 67.2 ppm (s); ∆-c,c-3a: 84.4 (d, JPP 13.7 Hz), 57.7 (d, JPP
2
2
13.7 Hz); Λ-c,c-3a: 84.6 (d, JPP 13.7 Hz), 55.5 (d, JPP 13.7 Hz).
Ru(H)2(PPh3)2((R,R)-dach) (3a, Enriched in Isomers c,c-3a).
Complex 1a (200 mg, 0.26 mmol) and KOtBu were mixed in an
atmosphere of N2, toluene (1.5 mL) was added, and the mixture was
stirred for 3 min, yielding a dark red solution, which was filtered. H2
was bubbled through the filtrate for 2 min while being stirred, and
then 15 mL of hexanes was added. After being stirred for 1 h, the light
beige precipitate was collected by filtration and dried in vacuo to give
a yellow powder (150 mg, 0.20 mmol; 78%) as a mixture of ∆-c,c-3a
(30%), Λ-c,c-3a (55%), and c,t-3a (15%).56 NMR properties as above.
Kinetic Measurements. (a) General. Kinetic runs were carried out
at constant pressures 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
an Ar or N2 atmosphere in a glovebox. It is important that the glovebox
is free of traces of vapors such as CH2Cl2 and HCl that react with the
dilute solutions of the dihydrides. The benzophenone should be
deoxygenated and distilled before use and the solvent dried, distilled,
and deoxygenated carefully. For runs in benzene, standard solutions
of acetophenone (0.083 M), 3a (0.0055 M), and 1a (0.0026 M) were
prepared by dissolving the required amounts of acetophenone and
ruthenium complexes in 10 mL (for ketone) or 5 mL (for catalyst
precursors) of C6H6. In parallel, a solution of 3a in C6D6 was prepared
to determine by use of NMR the c,c-3a(active)/c,t-3a(inacative) ratio
at the time of the addition of the solution of 3a to the reactor. For runs
in 2-propanol, standard solutions of acetophenone (0.083 M) and 3a
(0.0013 M) were prepared by dissolving the required amounts in 10
mL of 2-propanol. These standard solutions were further diluted to the
required concentrations. In all of the runs (except for the run described
in section f), the solution of the ketone, followed by the solution of a
ruthenium complex, was injected into the already-thermostated reactor
under the required H2 pressure to give a final working volume of 5
mL. The reaction time was measured from the time of the injection of
the precatalyst solution. Anisole (phenylmethyl ether) was added as
internal standard when needed (see below). In the cases when base
was needed, a suspension of potassium t-butoxide (10 mg, 0.089 mmol)
was added. Solutions containing 3a were always prepared and handled
last to keep the time from making the stock solution to injection at
Ru(H)2(PPh3)2(en) (3b as a Mixture of Isomers c,c-3b and c,t-
3b). Complex 1b (360 mg, 0.50 mmol) and KOtBu (60 mg, 0.52 mmol)
were mixed in an atmosphere of N2, THF (2 mL) was added, and the
mixture was stirred for 1 min, yielding a dark red solution, which was
filtered. H2 was bubbled through the filtrate for 2 min while being
stirred, and then 20 mL of hexanes was added. After being stirred for
30 min, the brown precipitate was collected by filtration and dried in
vacuo to give an orange solid (154 mg. 0.24 mmol, 45%) as a mixture
1
of c,c-3b (85%) and c,t-3b (15%). H NMR (C6D6, 300 MHz, δ) c,c-
3b: -15.7 (td, 2JHP 24, 2JHH 6 Hz, RuH), -6.0 (ddd, 2JHP 99, 33, 2JHH
6 Hz, RuH), 0.8-1.6 (m, 5H, CH, NH), 1.8 (s br, 1H), 2.0 (s br, 1H),
2.2 (s br, 1H), 5.8 (s br, 1Hortho), 6.9-7.1 (m, 17H), 7.7 (td, 6H), 7.8
(m, 6H). c,t-3b: -18.35 (t, 2JHP 27 Hz, 2H), 1.2 (m, 4H), 1.4 (m, 4H),
7.0 (m, 6H), 7.15 (m, 12H), 8.10 (dt, 3JHH 7, 3JPH + 5JPH 4 Hz, 12Hortho).
31P{1H} (C6D6, δ) c,c-3b: 83.5 (d, 2JPP 14 Hz), 56.5 (d, 2JPP 14 Hz, P
trans to hydride); c,t-3b: 66.0 ppm (s). cis,trans-Ru(H)2(PPh3)2(H215N-
(CH2)215NH2): 1H NMR (C6D6, δ): -18.26 (AA′ part of AA′MM′X2
where A,A′ ) 1H, M,M′ ) 15N, X ) 31P, 2JHP 26.9, 2JHNtrans 7.5, 2JHNcis
2
2
-0.8, JHH 6.5, JNN < 1 Hz, RuH).
(56) The assignment of the major isomer to the Λ configuration is not certain.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1881

A R T I C L E S
Abbel et al.
approximately 10 min and hence to ensure a known ratio of isomers in
the precatalyst. The largest errors in the kinetic study are the
approximately 5% error in the estimate of the percentage of the isomer
c,c-3a and 5% error in the determination of the initial rates.
(e) Kinetics with RuHCl(PPh3)2((R,R)-dach) as Catalyst Precur-
sor. For this run, a suspension of potassium t-butoxide (10 mg, 0.089
mmol) in C6H6 (0.2 mL) was added after the solution of 1a was injected
into the reactor, giving the final concentration of base of 0.011 M.
Timing was started after the base was added. Reaction conditions: [1a]
) 2.1 × 10-4 and 1.0 × 10-4 M, [acetophenone] ) 0.0167 M, H2
pressure 5 atm, temperature, 273 K
(f) Activation of c,t-3a by Reaction with rac-MeHC(OH)Ph. A
solution (4 mL) containing c,t-3a and rac-MeHC(OH)Ph in benzene
was stirred under 5 atm H2 at 20 °C. After 15 min, a solution containing
anisole and acetophenone was injected, giving a total volume of the
reaction mixture of 5 mL and the following concentrations: [c,t-3a] )
0.0011 M, [rac-MeCH(OH)Ph] ) 0.05 M, [acetophenone] ) 0.167
M, and [anisole] ) 0.17 M. The reaction time was measured from the
addition of the ketone. Samples were taken as described above.
(g) Numerical Analysis. The differential equations were ap-
proximated and integrated using dynamic models on a spreadsheet.57
(b) Sampling and Sample Analysis. In all of the kinetic runs,
sampling was done at regular time intervals of 5 min for a total reaction
time of 25 min. The samples were withdrawn from the reactor with a
syringe against a flow of hydrogen. The pressure in the reactor dropped
to nearly atmospheric during the sampling time of about 10 s. These
samples were analyzed for conversion and ee using a Perkin-Elmer
Autosystem XL gas chromatograph with Chrompack capillary column
ChirasilDex CB 25 m × 0.25 mm). The carrier gas was H2 at column
pressure of 5 psi, with an oven temperature of 130 °C, injector
temperature of 250 °C, and FID temperature of 275 °C. The retention
times were: anisole 3.0 min, acetophenone 5.0 min, (R)-1-phenyle-
thanol 8.5 min and (S)-1-phenylethanol 9.1 min. The injected sample
volume was 1 µL.
(c) Effect of Added PPh3. The hydrogenation of a solution
containing acetophenone (0.0167 M) and 3a (2.2 × 10-4 M) in benzene
was conducted for 10 min, and during this time, two samples were
withdrawn to establish a baseline rate. Then a benzene solution (0.2
mL) containing PPh3 (14 mg, 0.053 mmol) was injected, giving the
final PPh3 concentration of 0.01 M in the reaction mixture. After the
addition of PPh3 solution, sampling was continued as described in
section b.
Acknowledgment. We thank NSERC and the Petroleum
Research Fund, as administered by the American Chemical
Society, for grants to R.H.M. and Johnson-Matthey for a loan
of RuCl3. Deutscher Akademischer Austauschdienst provided
funding to R.A. and M.F. We thank Prof. Jeremy Harvey for
valuable suggestions.
Supporting Information Available: Further details of the
synthetic and kinetic studies (PDF). Crystallographic data in
electronic form (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(d) Effect of Added 1-Phenylethanol. A standard solution of rac-
1-phenylethanol (0.5 M) in C6H6 (5 mL) was prepared under Ar. After
a baseline rate was verified by withdrawing one sample at 5 min, 0.3
mL of alcohol stock solution was injected into the reactor to give the
starting alcohol concentration of 0.03 M. Other reaction conditions:
[3a] ) 2.2 × 10-4 M, [acetophenone] ) 0.0167 M, [anisole internal
standard] ) 0.0167 M, H2 pressure 5 atm, temperature 273 K.
JA039396F
(57) Atkinson, D. E.; Brower, D. C.; WcClard, R. W.; Barkley, D. S. Dynamic
Models in Chemistry; N. Simonson: Marina del Rey, CA, 1990.
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1882 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005