Electronic effects in PCP-pincer Ru(II)-based hydrogen transfer catalysis

Article abstract of DOI:10.1021/om060874f

The synthesis and characterization of novel cyclometalated ruthenium(II) complexes [RuCl(PCP^OMe)-(PPh₃)] and [RuCl(PCP ^CF3)(PPh₃)] containing monoanionic, tridentate coordinating PCP-pincer ligands [C₆H₃{CH ₂P(p-MeOC₆H₄)₂}₂-2,6] ^- (PCP^OMe) and [C₆H₃{CH ₂P(p-CF₃C₆H₄)₂} ₂-2,6]^- (PCP^CF3) are reported. These compounds have been tested as catalyst precursors in the hydrogen transfer reaction of cyclohexanone to cyclohexanol in 2-propanol using NaOH as a base. The initial rate of the hydrogen transfer reaction appeared to depend on the electronic character of the Ar₂P- groups of the PCP-pincer ligand. Among the catalyst precursors studied, the complex [RuCl(PCP^CF3)(PPh ₃)] was found to exhibit the highest activity and the initial TOF exceeded that observed for the Ph₂P analogue [RuCl(PCP ^Ph)-(PPh₃)]. Most importantly, catalysis performed with [RuCl(PCP^CF3)(PPh₃)] does not require pretreatment of the precursor in the absence of substrate. Conversely, a different catalytic profile and a low activity were observed when either the electron-poor [RuCl{C ₆H₃(CH₂P(C₆F₅) ₂)₂-2,6}(PPh₃)] ([RuCl(PCP^F20)- (PPh₃)]) complex or its triflate analogue was used as catalyst precursors. NMR studies and ESI-MS measurements provided information concerning the catalytically active species formed during the pretreatment of the precursor complexes. The results indicate that during the pretreatment period a monoanionic, monohydride ruthenium(II) species, Na[Ru(H)(PCP^ipr) (OiPr)(PPh₃)], is selectively formed. The latter hydride complex was also obtained via an independent synthetic route. On the basis of both the present results and those previously reported in literature, a mechanism is proposed.

Full text of DOI:10.1021/om060874f

Organometallics 2007, 26, 2219-2227
2219
Electronic Effects in PCP-Pincer Ru(II)-Based Hydrogen Transfer
Catalysis
Marcella Gagliardo, Preston A. Chase, Sander Brouwer, Gerard P. M. van Klink, and
Gerard van Koten*
Faculty of Science, Organic Chemistry and Catalysis, Utrecht UniVersity, Padualaan 8,
3584 CH Utrecht, The Netherlands
ReceiVed September 25, 2006
The synthesis and characterization of novel cyclometalated ruthenium(II) complexes [RuCl(PCPOMe)-
(PPh3)] and [RuCl(PCPCF3)(PPh3)] containing monoanionic, tridentate coordinating PCP-pincer ligands
[C6H3{CH2P(p-MeOC6H4)2}2-2,6]- (PCPOMe) and [C6H3{CH2P(p-CF3C6H4)2}2-2,6]- (PCPCF3) are reported.
These compounds have been tested as catalyst precursors in the hydrogen transfer reaction of
cyclohexanone to cyclohexanol in 2-propanol using NaOH as a base. The initial rate of the hydrogen
transfer reaction appeared to depend on the electronic character of the Ar2P- groups of the PCP-pincer
ligand. Among the catalyst precursors studied, the complex [RuCl(PCPCF3)(PPh3)] was found to exhibit
the highest activity and the initial TOF exceeded that observed for the Ph2P analogue [RuCl(PCPPh)-
(PPh3)]. Most importantly, catalysis performed with [RuCl(PCPCF3)(PPh3)] does not require pretreatment
of the precursor in the absence of substrate. Conversely, a different catalytic profile and a low activity
were observed when either the electron-poor [RuCl{C6H3(CH2P(C6F5)2)2-2,6}(PPh3)] ([RuCl(PCPF20)-
(PPh3)]) complex or its triflate analogue was used as catalyst precursors. NMR studies and ESI-MS
measurements provided information concerning the catalytically active species formed during the
pretreatment of the precursor complexes. The results indicate that during the pretreatment period a
monoanionic, monohydride ruthenium(II) species, Na[Ru(H)(PCPiPr)(OiPr)(PPh3)], is selectively formed.
The latter hydride complex was also obtained via an independent synthetic route. On the basis of both
the present results and those previously reported in literature, a mechanism is proposed.
corresponding metal complexes and the possibility to modulate
the reactivity of the metal center by fine-tuning and control of
the electronic and steric properties of the ligand framework (e.g.,
via introduction of an appropriate para-substituent in the pincer’s
aryl ring or by variation of the substituents at the E donor
atoms).6
Introduction
Hydrogen transfer catalysis is an attractive protocol for the
reduction of ketones to alcohols in both academic and industrial
research.1 The use of a hydrogen donor (e.g., 2-propanol) instead
of using molecular dihydrogen gas or hazardous reducing agents
(e.g., NaBH4 and LiAlH4) has a potential advantage in terms
of mild reaction conditions and excellent regioselectivity.1
Many metal complexes of Rh, Ir, and Ru have been found to
be active catalysts in (a)symmetric hydrogen transfer reactions
of polar groups (e.g., ketones and imines).1 In recent years a
number of studies appeared on the successful use of cyclo-
metalated ruthenium(II) complexes containing either tridentate,
cyclometalated PCP-,2 NCN-,2a and CNN-pincer,3 or bidentate
PC-4 and NC-ligands5 as catalyst precursors in hydrogen transfer
reactions. The great interest in the use of E,C,E-pincer ligands
(E ) N, P) arises from the remarkable stability of the
Van Koten and co-workers detailed the application of achiral2a
[RuX(PCPPh)(PPh3)] (X ) Cl (1a), OTf (1b), Scheme 1) and
chiral2c (Scheme 1) (S,S)-[RuCl(PCPtBu,Ph)(PPh3)] and (S,S)-
[RuCl(PCPiPr,Ph)(PPh3)] as catalyst precursors in the reduction
of aryl and diaryl ketones targeted as precursors to pharmaceuti-
cally desirable benzhydrols. Initial TOF up to 2.5 × 104 h-1
could be reached.2a Recent studies reported on the synthesis of
novel, cyclometalated PCP-pincer and PC-ligand Ru(II) com-
pounds, [RuCl(PCPCy)(PPh3)] (2) containing the bulky, electron-
rich, tridentate coordinating [C6H3(CH2PCy2)2-2,6]- pincer
ligand2b and [RuCl(CO)(PC)(ampy)] (PC ) [(2-CH2-6-MeC6H3)-
PPh2]-; ampy ) 2-(aminomethyl)pyridine).4 Results obtained
by van Koten,2a,c Fogg,2b and Baratta4 highlighted that the PCP-
Ru and PC-Ru structural units are preserved during the catalytic
cycle. Apparently, the presence of a unique, robust Ru-C
σ-bond between the ruthenium(II) center and the E,C,E-pincer
* Author to whom correspondence should be addressed. E-mail:
g.vankoten@chem.uu.nl. Fax: +31-30-252-3615. Tel: +31-30-253-3120.
(1) (a) Zassinovich, G.; Mestroni, G.; Gladiali, S. Chem. ReV. 1992, 92,
1051. (b) Noyori, N.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97. (c)
Ba¨ckvall, J. E. J. Organomet. Chem. 2002, 652, 105 (d) Zhang, X.; Tang,
W. Chem. ReV. 2003, 103, 3029.
(2) (a) Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G.
Angew. Chem., Int. Ed. 2000, 39, 743. (b) Amoroso, D.; Jabri, A.; Yap, G.
P. A.; Gusev, D. G.; dos Santos, E. N.; Fogg, D. E. Organometallics 2004,
23, 4047. (c) Medici, S.; Gagliardo, M.; Chase, P. A.; Williams, S. B.;
Gladiali, S.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten, G.
HelV. Chim. Acta 2005, 88, 694.
(5) (a) Sortais, J. B.; Ritleng, V.; Voelklin, A.; Holuigue, A.; Smail, H.;
Barloy, L.; Sirlin, C.; Verzijl, G. K. M.; Boogers, J. A. F.; de Vries, A. H.
M.; de Vries, J. G.; Pfeffer, M. Org. Lett. 2005, 7, 1247. (b) Sortais, J. B.;
Barloy, L.; Sirlin, C.; de Vries, A. H. M.; de Vries, J. G.; Pfeffer, M. Pure
Appl. Chem. 2006, 78, 457.
(6) For review on pincer complexes see: (a) Albrecht, M.; van Koten,
G. Angew. Chem., Int. Ed. 2001, 40, 3750. (b) Dupont, J.; Pfeffer, M.;
Spencer, J. Eur. J. Inorg. Chem. 2001, 1971. (c) van der Boom, M. E.;
Milstein, D. Chem. ReV. 2003, 103, 1759. (d) Singleton, J. T. Tetrahedron
2003, 59, 1837.
(3) (a) Baratta, W. L.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti,
M.; Zanette, M.; Zangrando, E.; Rigo, P. Angew. Chem., Int. Ed. 2005, 44,
6214.
(4) Baratta, W.; Da Ros, P.; Del Zotto, A.; Sechi, A.; Zangrando, E.;
Rigo, P. Angew. Chem., Int. Ed. 2004, 43, 3584.
10.1021/om060874f CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/23/2007

2220 Organometallics, Vol. 26, No. 9, 2007
Gagliardo et al.
Scheme 1. PCP-Pincer Ru(II)-Catalyzed Hydrogen
Transfer Reaction of Cyclohexanone in a 2-Propanol/Base
Mixture
the [RuCl(PCP)(PPh3)] catalyst precursors 5, 6, 8a, 8b, and 9
([RuCl(PCPiPr)(PPh3)] are pretreated in 2-propanol in the
presence of an excess of base.
Results and Discussion
Synthesis. The p-MeO (3) and p-CF3 (4) ligands have been
synthesized according to a previously described procedure for
the preparation of the C6F5 ligand 710(Scheme 2). This procedure
involves the 1:2 molar reaction of the bis-Grignard reagent
[C6H4(ClMgCH2)2-1,3]11 with the phosphines (p-MeOC6H4)2-
PCl12 and (p-CF3C6H4)2PCl,13 respectively. In contrast to 7,
which is stable to air and moisture, 3 and 4 are sensitive to O2,
due to facile oxidation of the phosphorus centers.
The corresponding ruthenium(II) complexes 5, 6, and 8a9
have been prepared via the transcyclometalation (TCM) reaction,
an efficient method for the preparation of achiral14 and chiral2b,15
[RuCl(PCP)(PPh3)] complexes. Thus, reaction of the complex
[RuCl{C6H3(CH2NMe2)2-2,6}(PPh3)]16 ([RuCl(NCN)(PPh3)],
Scheme 2) with the PCP-pincer ligands 3, 4, and 7, respectively,
in 1:1 molar ratio at 80 °C, afforded the air-sensitive 5 and 6,
respectively, as deep green-colored solids and the remarkably
air/moisture-stable 8a as a red-colored powder.
or E,C-ligand allows for the formation of long-lived, catalytically
active species, which therefore may have great potential use in
future synthetic methods. Importantly, with PCP-pincer Ru(II)
complexes 1a, 1b, and 2 the duration of the pretreatment time
of the precursors with base in 2-propanol was critical to obtain
good activity.2
Ruthenium dihydride compounds are proposed to be the actual
catalytically active species in hydrogen transfer reactions.7
However, in the case of PCP-Ru(II) complexes the formation
of such species has not been unambiguously established.2b
Isolated monohydride derivatives of 2 have been applied as
catalyst precursors in the presence of base.2b A slightly higher
catalytic activity was attained. Nevertheless, the use of the more
robust chloride 2 was found to be preferred over the monohy-
drides, due to the sensitivity of the hydride species and its
subsequent deactivation in catalysis.2b
Despite extensive research, the mechanism and, in turn, the
steric and electronic effects of the PCP-pincer ligands on the
catalytic activity of the ruthenium metal centers have yet to be
determined systematically. In our previous investigations we
found that the catalytic activity drops with increasing bulk of
the phosphorus substituents.2c Therefore, we consider here the
possibility to investigate electronic effects in PCP-Ru(II)-based
hydrogen transfer by varying the donor properties of the P atoms
by substitution of the para phenyl hydrogens of the [C6H3-
{CH2P(C6H5)2}2-2,6]- pincer anionic ligand with MeO and CF3
groups. The position of the MeO and CF3 substituents in the
ligands [C6H3{CH2P(p-MeOC6H4)2}2-2,6]- (PCPOMe; 3, Scheme
2) and [C6H3{CH2P(p-CF3C6H4)2}2-2,6]- (PCPCF3; 4, Scheme
2) enables modification of electronic properties of the corre-
sponding P-sites with minimal steric perturbation.8 The catalytic
activity of the corresponding PCP-Ru(II) complexes [RuCl-
(PCPOMe)(PPh3)] (5, Scheme 2) and [RuCl(PCPCF3)(PPh3)] (6,
Scheme 2) has been investigated, as well as of the Ru(II)
complex [RuCl(PCPF20)(PPh3)]9 (8a, Scheme 2) containing the
electron-withdrawing [C6H4{CH2P(C6F5)2}2-2,6]10 (PCPF20) (7)
PCP-pincer ligand and its triflate analogue (8b, Scheme 2).
However, ligand 7 is expected to also exert considerable steric
effects on the ruthenium center because of its ortho-fluorine
substituents in all four P(C6F5) rings.
The spectroscopic properties of both 5 and 6 are similar to
those reported for 1a and 2, implying that the two compounds
have similar structures, i.e., square pyramidal structures with
the PPh3 ligand occupying the apical position.2,15 The 1H NMR
spectrum of both 5 and 6, recorded in CD2Cl2 at room
temperature, displayed two sets of virtual doublets of triplets
2
(2JHH ) 16 Hz and JPH ) 6 Hz) for the methylene protons of
the PCP-pincer ligand. This indicates that the two PR2 groups
are trans to each other and that the PCP-ligand is coordinated
to the ruthenium(II) center in a meridional fashion.2,15 In the
1H NMR spectrum of 5 and the 19F NMR spectrum of 6 two
resonance patterns are found for the MeOC6H4 and CF3C6H4
groups, respectively, indicating that in both complexes the (p-
MeOC6H4) and (p-CF3C6H4) groups are diastereotopic. The 31P-
{1H} NMR spectra exhibit one doublet for the equivalent PR2
groups of the respective PCP-pincer ligands and one triplet for
the apical PPh3 ligand, arising from 2JPP coupling with the cis-
positioned PR2 groups.2,15 Consistent with this geometry, the
13C NMR spectra for 5 and 6 show two characteristic reso-
nances: one virtual triplet resonance for the benzylic carbons
and a doublet Ru-Cipso resonance (2JCP ≈ 14 Hz).2,15
Since the low activity of 8a (Vide infra Table 1, entry 2) might
be ascribed to strong coordination of the chloride anion, also
the corresponding triflato ()trifluoromethanesulfonato; OTf )
[CF3(O)2SO-]) complex [Ru(PCPF20)(PPh3)](OTf) (8b) was
prepared by reacting 8a with an equimolar amount of Ag(OTf).
(11) (a) Lappert, M. F.; Martin, T. R.; Atwood, J. L.; Hunter, W. E. J.
Chem. Soc., Chem. Commun. 1980, 476. (b) Lappert, M. F.; Martin, T. R.;
Raston, C. L.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans.
1982, 1959.
(12) Ogasawara, M.; Takizawa, K.; Hayashi, T. Organometallics 2002,
21, 4853.
(13) Unruh, J. H.; Christenson, J. R. J. Mol. Catal. 1982, 14, 19.
(14) Dani, P.; Karlen, T.; Gossage, R. A.; Smeets, W. J. J.; Spek, A. L.;
van Koten, G. J. Am. Chem Soc. 1997, 119, 11317. (b) Dani, P.; Albrecht,
M.; van Klink, G. P. M.; van Koten, G. Organometallics 2000, 19, 4468.
(15) (a) Karlen, T.; Dani, P.; Grove, D. M.; Steenwinkel, P.; van Koten,
G. Organometallics 1996, 15, 5687. (b) Jia, G.; Lee, H. M.; Xia, H. P.;
Williams, I. D. Organometallics 1996, 15, 5453. (c) van der Boom, M. E.;
Kraatz, H.-B.; Hassner, L.; Ben-David, Y.; Milstein, D. Organometallics
1999, 18, 3873. (d) Dani, P.; van Klink, G. P. M.; van Koten, G. Eur. J.
Inorg. Chem. 2000, 7, 1465.
NMR studies and ESI-MS measurements were performed in
order to shed light on the PCP-pincer Ru species formed when
(7) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004,
248, 2201.
(8) Tolman, C. A. Chem. ReV. 1977, 77, 315.
(9) Gagliardo, M.; Chase, P. A.; Lutz, M.; Spek, A. L.; Hartl, F.;
Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G. Organometallics
2005, 24, 4553.
(16) Sutter, J.-P.; James, S. L.; Steenwinkel, P.; Karlen, T.; Grove, D.
M.; Veldman, N.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organo-
metallics 1996, 15, 941.
(10) Chase, P. A.; Gagliardo, M.; van Klink, G. P. M.; Lutz, M.; Spek,
A. L.; van Koten, G. Organometallics 2005, 24, 2016.

PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis
Organometallics, Vol. 26, No. 9, 2007 2221
Scheme 2. Synthesis of [RuCl(PCPR)(PPh3)] Complexes via the Transcyclometalation (TCM) Reaction
Table 1. Hydrogen Transfer Reaction Catalyzed by
PCP-Pincer Ru(II) Complexesa
The catalytic experiments have been performed following
pretreatment of the catalyst precursor for 1 h in refluxing
2-propanol (82 °C) in the presence of a 20-fold excess of base.
For all the complexes the elevated temperature as well as the
presence of an excess of base were essential for a high catalytic
activity. Addition of the base to solutions of either 5 or 6 caused
a color change of the solution from green to yellow. The results,
visualized in Figure 1, show that after 1 h of pretreatment the
reaction starts immediately upon addition of the substrate (t )
0), and no further induction period was observed. Moreover,
no deactivation was observed and full conversion could be
attained in 10-20 min.
catalyst precursor
[RuCl(L)(PPh3)]
b
entry
TOF (h-1
)
conversion (%)/t (min)
1
2
3
4
5
PCPPh (1a)
PCPF20 (8a)
PCPOMe (5)
PCPCF3 (6)
PCPiPr (9)
33 600
41
8000
35 700
980
98/10
98/780
96/30
98/10
98/300
a Reaction conditions: 2.0 mmol of cyclohexanone; [Ru] ) 0.1 mol %;
2 mL of 2-propanol; [NaOH]:[Ru] ) 20:1; T ) 82 °C; N2 atmosphere.
b
Pretreatment time ) 1 h. Turnover frequency (TOF) calculated at 20%
conversion.
The results reported in Table 1 highlight that PCP-pincer Ru-
(II) complexes 5 and 6 are highly active precursors and initial
TOFs (at 20% conversion) of 8000 and 38 000 h-1 were
obtained. It must be noted that the initial TOFs for the majority
of ruthenium(II) complexes do not exceed a few thousands per
hour.7 Comparison of the entries in Table 1 reveals that the
precursors 1a, 5, and 6 containing bisarylphosphine PCP-pincer
ligands (entries 1, 3, and 4, respectively) are more active
precatalysts than the complex containing the bulky, electron-
rich bis(isopropyl)phosphine PCP-pincer ligand 9 (entry 5).
Complexes 1a and 6 showed similar activities and are the most
active of the series.
Complex 8b was isolated in good yield as a purple-colored solid.
The 1H NMR spectrum showed that the benzylic hydrogens are
2
diastereotopic. Moreover, the small JCP coupling constant of
the doublet for the Cipso in the 13C NMR spectra indicates the
mutual cis arrangement of Cipso and PPh3. However, analogously
as found for 1b,15a 31P{1H} NMR spectroscopy shows that 8b
exhibits temperature-dependent behavior. The 31P{1H} NMR
spectrum contains a broad resonance for the PPh3 ligand (55.6
ppm) and a sharp doublet resonance for the PCP-pincer P-donor
2
atoms (25.8 ppm, JPP ) 35 Hz). The position and the
multiplicity of the resonances point to a meridional coordination
of the PCP-pincer ligand as well as the cis arrangement between
the P(C6F5) donor atoms and the PPh3 ligand. The broad signal
for the PPh3 ligand may be ascribed to the occurrence of an
association/dissociation process that involves the triflate ion.
In fact, the 19F NMR spectrum of 8b recorded in CH2Cl2 at
room temperature displayed a singlet at -80.47 ppm consistent
with the presence of a noncoordinating triflato group.17 Mo-
lecular modeling confirms that the bulkiness of the P(C6F5)2
groups would exert considerable pressure to a η1-O monodentate
coordinated triflate group. Moreover, the structure of 8a in the
solid state has revealed that the ortho-F substituents are ideally
positioned for intramolecular coordination to a vacant site at
the ruthenium center. Indeed, analysis of the IR spectrum of
solid 8b in the region 1000-1300 cm-1 shows broad absorptions
So far, for all the PCP-Ru(II) systems reported, the duration
of the pretreatment time appeared to be critical to activity.2 The
fact that a pretreatment time is required indicates that from the
starting complexes new intermediates are formed. Interestingly,
in the case of 6, the observed activity is independent of any
pretreatment. The similar activity profiles with 0 and 1 h
pretreatment time (Figure 2) suggest that the catalytically active
for the asymmetric S-O stretching at 1293 and at 1261 cm-1
,
which are characteristic of an uncoordinated OTf- ion.18
Hydrogen Transfer Catalysis. The catalytic activity of the
novel complexes 5 (entry 3) and 6 (entry 4) as hydrogen transfer
catalyst precursors using cyclohexanone as a model substrate
was studied and compared to that of 1a (entry 1).
Figure 1. Electronic effect on the hydrogen transfer reaction of
cyclohexanone catalyzed by 6 (9), 1a ([), and 5 (2) ([Ru] ) 0.1%
mol) in 2-propanol in the presence of base (NaOH:[Ru] ) 20:1) at
T ) 82 °C. Pretreatment time ) 1 h.
(17) van Stein, G. C.; van Koten, G.; Vrieze, K.; Brevard, C.; Spek, A.
L. J. Am. Chem. Soc. 1984, 106, 4486.
(18) Dedert, P. L.; Thompson, J. S.; Ibers, J. A.; Marks. T. J. Inorg.
Chem. 1982, 21, 969.

2222 Organometallics, Vol. 26, No. 9, 2007
Gagliardo et al.
Figure 4. Hydrogen transfer reaction of cyclohexanone catalyzed
by 8a (9) and 8b ([) ([Ru] ) 0.1% mol) in 2-propanol in the
presence of base (NaOH:[Ru] ) 20:1) at T ) 82 °C. Pretreatment
time ) 0 h.
Figure 2. Effect of pretreatment time on the hydrogen transfer
reaction of cyclohexanone catalyzed by 6 ([Ru] ) 0.1% mol) in
2-propanol in the presence of base (NaOH:[Ru] ) 20:1) at T ) 82
°C. Pretreatment time ) 1 h (9) and 0 h ([).
8a does not immediately cause change of the color of the
solution. Only after prolonged heating under catalysis conditions
does the color of the solution turn slowly from red to yellow,
which seemed to indicate that initially 8a remained unchanged.
1
Analysis of the H, 31P{1H}, and 19F NMR spectra recorded
during the pretreatment period revealed that eventually also 8a
underwent a transformation in the presence of base. A series of
well-resolved 19F NMR signals was observed indicating the
presence of at least three different species, which however could
not be identified. We are aware that metal-mediated carbon-
fluorine bond activation in P(C6F5)2 groups has been observed
in the presence of an excess of alkoxide.20 However, from the
obtained NMR data it can be excluded that under the applied
experimental conditions substitution of the para-fluorine atoms
by the isopropoxide anion occurred.20
Figure 3. Effect of pretreatment time on the hydrogen transfer
reaction of cyclohexanone catalyzed by 8a ([Ru] ) 0.1% mol) in
2-propanol in the presence of base (NaOH:[Ru] ) 20:1) at T ) 82
°C. Pretreatment time ) 0 h (9), 1 h ([), and 2 h (2).
In order to study whether the low activity of 8a was due to
the strong coordination of the chloride ligand, the cationic
complex 8b with the triflate ion was used as catalyst precursor
(Figure 4). The results indicated that 8b had an even lower
activity than that found for 8a. It is of interest to note that this
compound has a remarkable stability to air and moisture,
whereas its protio analogue is extremely reactive and difficult
to handle.15a The low reactivity of the Lewis acidic complexes
8a and 8b can be ascribed to a pronounced deactivation (e.g.,
strong coordination of substrate, 2-propanol, acetone) or de-
composition of the catalytically active species formed during
the pretreatment period. However, in analogy with what was
observed with the chiral complexes (S,S)-[RuCl(PCPtBu,Ph)-
(PPh3)] and (S,S)-[RuCl(PCPiPr,Ph)(PPh3)], containing bulky
PCP-pincer ligands, the reactivity of the active species formed
during the pretreatment period may be considerably altered by
the steric effects exerted by the bulky P(C6F5) groups.21
Fogg and co-workers reported the formation of the neutral
complex [Ru(H)(PCPCy)(N2)(PPh3)] when [RuCl(PCPCy)(PPh3)]
(2) is refluxed in a 2-propanol/KOtBu mixture.2b We have not
attempted to isolate analogous species when the catalyst
precursors are heated to reflux in a NaOH/NaOiPr mixture.
However, to rule out whether the activity of the catalytically
active species is influenced by nitrogen coordination, we carried
out the catalysis under an Ar atmosphere. With the highly active
complexes 1a, 5, 6, and 9 the results were essentially identical
to those obtained under N2. When the less active complexes 8a
and 8b were used, the initial rate was slightly higher. This is
probably due to the fact that coordination of N2 to the ruthenium
stabilizes, to a certain extent, catalytically active species formed
species is formed rapidly when 6 reacts with 2-propanol in the
presence of base and that the resulting species is stable in the
absence of substrate. Regrettably, the intrinsic higher catalytic
activity of 6 did not allow the observation of deactivation
pathways at the applied substrate to catalyst ratio. Replacement
of phenyl rings in the R2P groups by strongly electron-
withdrawing C6F5 rings, which, however, are also ortho-
hindering through the ortho-fluorine atoms (Vide supra) (entry
2), resulted in a dramatic decrease of the catalytic activity, cf.
8a with 1a.
In contrast to the findings for 5 and 6, the reaction profile of
the reaction catalyzed by 8a (Figure 3) is strongly dependent
on the length of the pretreatment time. As compared with 1a
and 5, which reach the highest activity after 1 h of pretreatment,
8a suffered from a long induction time (Figure 3). Moreover,
after the induction period, a low initial activity was observed
while the catalytic solution undergoes deactivation with time.
After 400 min conversion ceased at 20%. Application of a longer
pretreatment time (2 h) led to analogous results. However, in
this case, a longer induction period but a slightly higher activity
are attained (Figure 3). Interestingly, when substrate was directly
added to a fresh solution of 8a, i.e., without applying any
pretreatment, no induction period was observed and a higher
activity was found. Full conversion was reached in 13 h,
accompanied by the formation of traces of aldol condensation
products.19
These observations suggest that during pretreatment several
species are formed in different concentrations and with intrinsi-
cally different activities, which all suffer from deactivation under
pretreatment conditions in the absence of substrate. In contrast
to what is observed with 1a, 5, and 6, addition of the base to
(20) Mohr, W.; Stark, G. A.; Jiao, H.; Gladysz, J. A. Eur. J. Inorg. Chem.
2001, 925.
(19) Analysis of the aldol products (GS-MS) in the reaction mixture
revealed that they were the result of reaction of acetone with the substrate.
(21) Gagliardo, M.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten,
G. J. Organomet. Chem. 2006, 691, 4411.

PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis
Organometallics, Vol. 26, No. 9, 2007 2223
additional excess of cyclohexanone resulted in an increase of
the latter signals. A rationalization for these observations is the
presence of an equilibrium in which the coordinated isopro-
poxide ion in 10 is replaced by the substrate.
This binding of the isopropoxide anion to the ruthenium center
in 10 in solution is supported by high-resolution ESI-MS
measurements. In the ESI-MS(+) spectrum the highest mass
peak at m/z ) 958.7 (70%) corresponds to the adduct {[Ru-
(H)(PCPiPr)(OiPr)]Na[Ru(PCPiPr)]}+, showing the correct iso-
topic pattern for this formulation. The peak of the oxygen-
containing adduct is also present ([Madduct + O]+; m/z ) 974.7;
50%). Apart from these peaks, no other peaks were observed
that could be assigned to a Ru-containing adduct or a ligand
degradation product. The signals in the 31P{1H} NMR spectra
confirmed the presence of resonances typical for the oxidized
P(iPr)2 group. The presence of [Madduct + O]+ in the early stages
of the pretreatment time shows that oxidation of the phosphorus
atoms of the PCP ligand may constitute an important deactiva-
tion route. Our ESI-MS studies of reaction mixtures during the
pretreatment time indicate that dimeric Ru-alkoxide species are
accessible under the applied conditions. However, we are aware
that a detailed study is required in order to establish unambigu-
ously whether the formation of these species is due to the
presence of an excess of NaOiPr.
Figure 5. Effect of pretreatment on the hydrogen transfer reaction
of cyclohexanone catalyzed by 9 ([Ru] ) 0.1% mol) in 2-propanol
in the presence of base (NaOH:[Ru] ) 20:1) at T ) 82 °C.
Pretreatment time ) 1 h (9) and 0 h ([).
during the pretreatment, decreasing their susceptibility toward
decomposition.
Mechanistic Aspects. In agreement with previous studies,
our results show that the use of a strong base such as NaOH as
promotor in the hydrogen transfer is essential to effect conver-
sion.2,7 As already reported for analogous systems, no reaction
occurs in the absence of base or if its concentration is too low,
whereas in the presence of a large excess (30-100 equiv to the
catalyst precursor) the catalyst precursors decompose quite fast.
Since for most of the analyzed systems the pretreatment of the
catalyst precursor with an excess of a basic promotor is
absolutely necessary, attention had been directed toward the
identification of the species formed during the pretreatment time.
Complex [RuCl{C6H3(CH2P(iPr)2)2-2,6}(PPh3)]15c (9), which
has a reaction profile similar to those of 1a and 6 (Figure 5),
was chosen to identify and investigate such species in more
detail.
In a separate experiment, the selective synthesis of 10 was
attempted by reaction of 9 with exactly 2 equiv of NaOiPr in
1
refluxing toluene. H and 31P{1H} NMR spectroscopy clearly
demonstrated that at a 1:2 molar ratio of 9 and NaiOPr, hydride
10 is present in a low amount. Addition of NaOiPr to a 1:5 or
1:10 molar ratio resulted in the steady increase of the signals
related to 10 at the expense of the other signals. An almost
quantitative formation of 10 was observed at 1:20 molar ratio
(20-fold excess). These results indicate that a large excess of
base is required in order to shift the equilibria depicted in
Scheme 3 toward 10 and that several hydride species can coexist.
Several mechanisms have been proposed, in which mono-
and dihydride ruthenium complexes are key intermediates.7
Little is known about the mechanism of the PCP-Ru(II)-
catalyzed hydrogen transfer reaction. During the pretreatment
time, no loss of PCP-pincer ligand or PPh3 ligand is observed.
This suggests that the catalytically active species indeed contains
[(cis-PPh3)(mer-PCP)Ru-H]) as reaction platform. It seems
likely that the reactivity of the Ru-hydride PCP-pincer species
(Scheme 3) toward carbonyl substrates is high. In fact, it has
been shown that the presence of a ligand such as PPh3 or Cipso
with a high trans-influence trans to a hydride enhances the
reactivity of this Ru-H bond.7
On the basis of these observations and on the fact that loss
of PPh3 has not been observed (see below),2b the catalytic cycle
depicted in Scheme 3, based on the mechanisms proposed in
the literature,7can be envisaged. The cycle involves a hydrogen
transfer reaction of coordinated ketones via an inner-sphere
hydritic route.7
This cycle starts from the monohydride complex A, which
is formed by coordination of the ketone substrate to the Ru-
hydride intermediate 12.22 This species could also be formed
prior to the formation of 12 by loss of NaOiPr from 10. Complex
A is subsequently converted into B via intramolecular addition
of the hydride to the R-C atom of the ketone. Recently, Baratta
and co-workers obtained solid evidence for the formation of a
ruthenium species by insertion of benzophenone into the Ru-H
bond of the complex [Ru(H)(CNN)(dppb)] (CNN ) 6-(4′-
Reaction of 9 in refluxing 2-propanol in the presence of an
excess of NaOH was monitored by taking aliquots of the
1
reaction mixture and subsequently analyzing them by H and
31P{1H} NMR spectroscopy in C6D6. Within a few minutes, 9
reacted completely to several hydride species, including the
complex that was identified as Na[Ru(H)(PCPiPr)(OiPr)(PPh3)]
(10, Scheme 3). It turned out that after 1 h 10 is quantitatively
formed and is the only species present in solution. The 1H NMR
spectrum of 10 in C6D6 displays two doublet of triplets at -9.17
2
ppm (2JHP ) 20.7 Hz and JHP ) 82.2 Hz), indicating that the
hydride ligand is trans to the PPh3 ligand and cis to the
phosphorus atoms of the PCP-pincer ligand. The proton signal
for the Ru-OCH moiety appears as a broad resonance at 3.83
ppm, while the 13C NMR peak for the OCH moiety appears at
59.0 ppm. The 31P{1H} NMR spectrum shows a doublet at 72.1
ppm and a triplet at 35.8 ppm (2JPP ) 15.5 Hz) for the two
equivalent phosphorus atoms of the PCP-pincer ligand and the
1
PPh3 ligand, respectively. No additional peaks in the H and
31P{1H} NMR spectra are observed upon prolonged heating of
the reaction mixture. The quantitative formation of 10 under
pretreatment is in agreement with the results communicated by
Dani et al.2a and with the formation of Ru-hydride-isopropoxide
species obtained by refluxing analogous Ru(II) catalyst precur-
sors in 2-propanol in the presence of an excess of NaOH or
NaOiPr.4-7
In the presence of an excess of substrate, under reflux
conditions, 10 undergoes transformation toward a new hydride
1
species. The hydride region of the H NMR spectra shows the
2
appearance of an additional quartet (-6.16 ppm, JHP ) 21.3
Hz), which may be attributed to a hydride cis to the PPh3 ligand.
The 31P{1H} NMR spectrum shows an additional doublet at 71.4
ppm (2JPP ) 16.0 Hz) for the PCP-pincer ligand and a triplet at
48.5 ppm (2JPP ) 16.0 Hz) for the PPh3 ligand. Addition of an
(22) Jia, G.; H. M.; Lee, I. D.; Wiiliams J. Organomet. Chem. 1997,
534, 173.

2224 Organometallics, Vol. 26, No. 9, 2007
Gagliardo et al.
Scheme 3. Mechanistic Proposal for the Hydrogen Transfer Reaction of Ketones Catalyzed by PCP-Pincer Ru(II) Complexesa
a
In the catalytic cycle the drawings are not meant to show a particular stereochemistry.
methylphenyl)-2-pyridylmethylamine; dppb ) Ph2P(CH2)4-
PPh2).3 The hydrogen transfer agent (2-propanol) releases the
coordinated alkoxide by protonation, thereby forming C and
the new alcohol. At this stage, the PCP-pincer ligand may play
a crucial role by stabilizing the cationic charge developed at
the metal center that is produced during the hydride transfer
and prior to coordination of the isopropoxide anion to give C.
Subsequently, C delivers a hydride to the ruthenium center by
â-H elimination giving D. Exchange of the oxidized hydrogen
donor (acetone) for the substrate results in the formation of the
monohydride A, which is the start of the cycle.
The remarkable difference in activity between mono- and
dihydride species made Ba¨ckvall23 and Fogg2b propose that
instead of A a ruthenium-dihydride complex is formed as the
catalytically active species in hydrogen transfer of ketones via
related Ru systems. In this respect, the anionic complex 10 could
be envisaged as a potential reservoir of such dihydride species.
Formation of a dihydride species from 10 would require a loss
of the PPh3 ligand in order to create a vacant coordination site
available for an additional hydride. In the present reactions
catalyzed by PCP-pincer ruthenium species the formation of
Ru(H)2 and free PPh3 ligand has not been observed, in
agreement with the results of Fogg and co-workers.2b Addition
of an excess of PPh3 (5-10 equiv to Ru) results in a lower
catalytic activity, in agreement with the results reported in the
literature.2b Unfortunately, these experiments do not prove, nor
disprove, the loss of PPh3 during the catalytic cycle. Excess
PPh3 could simply hamper catalysis by coordination to a vacant
site. In this respect no definite conclusions may be drawn, even
though the most active precursor, 6, contains the most electro-
philic Ru(II) center, from which decomplexation of PPh3 is less
likely to occur.
Using the mechanism proposed in Scheme 3, the beneficial
effect of the employment of the PCPCF3-pincer ligand containing
CF3 electron-withdrawing groups on the formation of active
species from 6 during the pretreatment time or in the catalytic
cycle can be explained: a higher Lewis acidic character is
expected to make the ruthenium center a better hydrogen
abstractor. Thus, during the pretreatment time, the highly
reactive monohydride 12 is expected to be formed more
readily,22 which reacts in the absence of substrate with the excess
of isopropoxide to give 10. In the catalytic cycle the formation
of the monohydride D is faster as well when 6 is used as catalyst
precursor.
Several other factors than electronic effects may influence
the rate of the fundamental steps as well as the structure of the
intermediates proposed to be involved in the catalytic cycle. In
the case of the slowest catalysts, the 31P{1H} NMR spectrum
of the reaction mixture after consumption of the substrate
showed the presence of oxidized free ligand, indicating that in
(23) Aranyos, A.; Csjernyik, G.; Szabo´, K.; Ba¨ckwall, J.-E. J. Chem.
Soc., Chem. Commun. 1999, 351.

PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis
Organometallics, Vol. 26, No. 9, 2007 2225
or in a nitrogen-filled MBraun 150 G1 glovebox. Pentane and
toluene were dried over Na sand, THF and diethyl ether were dried
over Na/benzophenone, and CH3CN was dried over CaH2. All
solvents were freshly distilled under nitrogen prior to use. CD2Cl2
was purchased from Cambridge Isotope Laboratories, dried over
CaH2, stored in a Schlenk flask over 4 Å molecular sieves under
nitrogen, and distilled prior to use. 1D NMR spectra [1H (300.1
MHz), 13C (75.5 MHz), 19F (282.4 MHz), and 31P{1H} (121.4
MHz)] were recorded on a Varian Inova 300 MHz spectrometer.
Chemical shift values are reported in ppm (δ) and referenced
internally to residual solvent signals (1H, 13C) or externally (19F,
C6F6, δ ) -164.9; 31P, H3PO4 in D2O, δ ) 0)). All reagents were
purchased from Acros Chemicals and used as received. Complexes
[RuCl{C6H3(CH2NMe2)2-2,6}(PPh3)],16 8a,9and 915cand the ligands
(p-MeOC6H4)PCl12, (p-CF3C6H4)PCl,13and 710 were synthesized via
literature procedures. Elemental analyses were performed by Dornis
und Kolbe, Mikroanalytisches Laboratorium, Mu¨llheim a.d. Ruhr,
Germany. High-resolution electrospray ionization (ESI) mass
spectra were recorded with a Micromass LC-TOF mass spectrom-
eter of the Department of Chemistry, Biomolecular Mass Spec-
trometry, Utrecht University. The electrospray ionization mass
spectrometry (ES-MS) experiments were conducted on an API III
(PE SCIEX) triple quadrupole MS system with an IonSpray
(pneumatically assisted electrospray) source equipped with a gas
curtain, which are contained in a closed chamber that can be
evacuated, flushed, and maintained under nitrogen. Typical sample
analysis: in a glovebox, a sample of the catalyst, 2-propanol, and
NaOH were taken up into a 500 µL syringe (Model 1750 RNR,
Hamilton) and electrosprayed via a syringe pump operating at 10
µL/min. The capillary voltage was 3.5 kV. Mass spectra were
recorded from m/z 50 to 900 at 10 s per scan using a step size of
0.1 Da. The sampling orifice (nozzle) was at +70 V. The skimmer
located behind the sampling orifice was at +25 V in all experiments.
the course of the reaction catalyst decomposition occurred. In
addition, after each catalytic run, poorly soluble, light brown
ruthenium-containing deposits were recovered from the reaction
mixture. Infrared and 13C NMR spectra of these residues pointed
to the presence of carbonyl complexes, which apparently were
formed during the reaction. Ozerov reported that reaction of
[RuCl2(PNP)] with NaOH in 2-propanol resulted in the forma-
tion of [RuH(PNP)(CO)], which is inactive in hydrogen transfer
reactions.24 In addition, the 31P{1H} NMR spectra of the deposits
revealed that oxidized ligands are also present, while ESI-MS
experiments also showed that during the pretreatment time the
P atoms of the PCP-pincer ligand are oxidized. The high
catalytic activity obtained with 9, together with ESI-MS results,
indicates that the introduction of electron-withdrawing substit-
uents at the phosphorus atoms of the Ru-PCP-diphenylphos-
phine manifold may decrease their susceptibility toward oxi-
dative degradation by a stronger coordination of the PCP-pincer
ligand to the metal center.
Conclusions
In this work, novel PCP-pincer Ru(II) complexes were
synthesized, and their catalytic activity in the hydrogen transfer
of cyclohexanone in a 2-propanol/NaOH mixture was studied.
It was shown that the complexes [RuCl{C6H3(CH2P(p-XC6H4)2)2-
2,6}(PPh3)] (X ) H, OMe, CF3) can be efficiently employed
as catalyst precursors and that higher reaction rates are obtained
in the presence of an excess of base at reflux temperature. Our
experimental data showed that the initial rate of the reaction
increases when electron-withdrawing CF3 groups replace the
para phenyl hydrogens in the pincer ligand [C6H3(CH2P(p-
XC6H4)2)2-1,3]-. More importantly, in this case the duration of
the pretreatment time (refluxing in 2-propanol with base) was
not critical to activity. Thus, the origin of the higher initial rate
may be related to the faster formation of catalytically active
species.
An induction period followed by a significantly lower activity
have been obtained applying the precursor [RuCl(PCPF20)-
(PPh3)], which possesses different structural features due in part
to the considerable steric effects exerted by the bulky P(C6F5)
rings on the ruthenium center. These findings clearly show that
the steric effects can be at least as important as electronic effects
and, as observed in many cases, can dominate the formation of
catalytically active species.
The results of stoichiometric reactions, NMR experiments,
and ESI-MS measurements provide support to the formation
of the Ru-hydride-alkoxide complex Na[Ru(H)(PCPiPr)(OiPr)-
(PPh3)] during the pretreatment time. The concentration of this
species is strongly dependent on the amount of base and
temperature. Therefore, upon changing the experimental condi-
tions, different PCP-pincer Ru(II) species may be present
simultaneously. The fact that a higher rate is obtained when
the Na[Ru(H)(PCPiPr)(OiPr)(PPh3)] species is quantitatively
formed is indicative that the latter may be regarded as the active
species or as a stabilizing reservoir for the active species.
The results of this work are a worthwhile subject for further
studies devoted to the isolation of robust PCP-Ru hydride
species and their application in homogeneous (a)symmetric
hydrogen transfer reaction of ketones.
[C6H4{CH2P(p-MeOC6H4)2}2-1,3] (PCPOMe) (3). An excess of
Mg turnings (0.48 g, 20 mmol) was stirred in dry THF (50 mL) in
a round-bottom flask equipped with a pressure-equalizing dropping
funnel, a sintered glass frit, and a nitrogen inlet for 1 h. To this
was added BrCH2CH2Br (0.2 mL, 2.0 mmol), and the flask was
heated to reflux for 1 min. After cooling for 5 min, the THF was
removed in Vacuo and fresh THF (100 mL) was added. In a separate
Schlenk flask, R,R′-dichloro-meta-xylene (1.21 g, 7.0 mmol) was
dissolved in dry THF (200 mL), transferred via cannula to the
dropping funnel, and added dropwise at room temperature over 1
h to the Mg turnings. On addition, the solution became light green.
After the addition was complete, the reaction was stirred overnight
at room temperature. The green solution was filtered to remove
the excess Mg and subsequently cooled to -78 °C. (p-MeOC6H4)2-
PCl (3.9 g, 14.0 mmol) in dry THF (100 mL) was added slowly
via cannula. The yellow-green color of the bis-Grignard reagent
dissipated at the end of the addition. The reaction was stirred at
-78 °C for 5 min and warmed to room temperature overnight. All
volatiles were removed in Vacuo, and fresh pentane (30 mL) was
added. The mixture was stirred for 30 min, and all volatiles were
removed in Vacuo. A fresh portion of pentane (200 mL) was added,
and the reaction mixture was filtered. The white magnesium salts
were washed with dry pentane (3 × 50 mL), and the filtrate was
pumped dry to obtain a white, thick oil. Yield: 7.3 g, 88%.
Characterization was performed with the borane-protected form of
3, [C6H4{CH2P(p-MeOC6H4)(BH3)}2-2,6] (3b), prepared by ap-
plying the following synthetic procedure: to a solution of 3 (0.21
g, 0.35 mmol) in CH2Cl2 (30 mL) was added BH3‚SMe2 (0.054 g,
0.71 mmol) at room temperature. The mixture was stirred at room
temperature for 10 h. All volatiles were evaporated and the white
sticky solid washed with hot EtOH and dried in Vacuo to give 3b
as a white air-stable powder. Yield: 0.10 g, 47%. 1H NMR (300.1
Experimental Section
General Procedures. All reactions were performed under an
atmosphere of dry, oxygen-free nitrogen using Schlenk techniques
2
MHz, CD2Cl2): δ 7.51 (t, JHH ) 9 Hz, 8H, ortho-H ArOCH3),
(24) C¸ elenligil-C¸ etin, R.; Watson, L. A.; Guo, C.; Foxman, B. M.;
Ozerov, O. V. Organometallics 2005, 24, 186.
6.95 (d, 2JHH ) 9 Hz, 8H, meta-H ArOCH3), 6.87 (d, 1H, H-C(5)),

2226 Organometallics, Vol. 26, No. 9, 2007
Gagliardo et al.
6.77 (m, 2H, H-C(4,6)), 6.67 (s, 1H, H-C(2)), 3.84 (s, 12H,
MS: m/z 1143.9 ([M + H]+, 60%), 1108.8 ([M - Cl]+, 100%),
881.6 ([M - PPh3]+, 100%), 845.7 ([M - Cl - PPh3]+, 100%).
2
CH3O), 3.43 (d, JHH ) 12 Hz, 4H, CH2P), 1.7-0.5 (br q, 6H,
BH3). 13C NMR (75.5 MHz, CD2Cl2): δ 162.3 (s, para-C ArOCH3),
[Ru(PCPF20)(PPh3)](OTf) (8b). AgOTf (0.028 g, 0.1 mmol) in
CH2Cl2 (5 mL) was added to a solution of 8a (0.14 g, 0.1 mmol)
in CH2Cl2 (10 mL). A color change of the reaction mixture from
red to purple was observed. After stirring at room temperature for
5 h, the purple solution was separated by filtration over Celite from
the AgCl formed. Removal of the solvent in Vacuo gave 8b as an
1
134.4-114.4 (Ar), 57.0 (s, CH3O), 34.5 (d, JCP ) 33 Hz, CH2).
31P{1H} NMR (121.4 MHz, CD2Cl2): δ 16.0 (br s). Anal. Calcd
for C36H42B2O4P2‚2CH2Cl2: C, 57.62; H, 5.85; P, 7.92. Found: C,
57.62; H, 5.43; P, 7.72.
[C6H4{CH2P(p-CF3C6H4)2}2-1,3] (PCPCF3) (4). Ligand 4 was
prepared by applying the synthetic procedure described for ligand
4 employing 0.33 g (0.14 mmol) of Mg, 0.87 g of R,R′-dichloro-
meta-xylene (5.0 mmol), and 4.0 g of (p-CF3C6H4)2PCl (10.0
mmol). Yield: 3.5 g, 95%. Characterization was performed with
the borane-protected form of 4 ([C6H4{CH2P(p-CF3C6H4)(BH3)}2-
2,6] (4b)) prepared by adding BH3‚SMe2 (0.10 g, 1.3 mmol) to a
solution of 4 (0.5 g; 0.67 mmol) in CH2Cl2 (30 mL) at room
temperature. The mixture was stirred for 10 h and, subsequently,
all volatiles were evaporated, yielding 4b as a white, air-stable solid.
The solid was further purified by column chromatography on silica
gel (hexane/CH2Cl2, 3:1 v/v), yielding a white solid. Yield: 0.29
1
air/moisture-stable, purple solid. Yield: 0.12 g, 89%. H NMR
(300.1 MHz, CD2Cl2): δ 7.34 (m, 3H, para-H PPh3), 7.16 (m, 12H,
ortho- and meta-H PPh3), 7.06 (d, 3JHH ) 7.5 Hz, 2H, Ar), 6.98 (t,
3JHH ) 7.0 Hz, 1H, Ar), 4.14 (m, 2H, CH2), 3.07 (bd, 2JHH ) 18.6
Hz, 2H, CH2). 13C NMR (74.4 MHz, CD2Cl2): δ 156.1 (Cipso),
1
149.9 (t, Ar), 147.7 (doublet of multiplet, JCF ) 211.5 Hz, ArF),
143.9 (doublet of multiplet, 1JCF ) 283.5 Hz, ArF), 137.7 (doublet
1
of multiplet, JCF ) 258.9 Hz, ArF), 134.8-104.72 (C Ar), 40.33
(m, CH2). 31P{1H} NMR (121.4 MHz, CD2Cl2): δ 55.6 (br m,
PPh3), 25.8 (d, 2JPP ) 35.0 Hz, P-ArF). 19F{1H} NMR (282.4 MHz,
CD2Cl2, RT): δ -80.47 (s, OTf), -125.23 (br s, 4F, ortho-F ArF),
-137.03 (br s, 4F, ortho-F ArF), -148.83 (br s, 2F, para-F ArF),
-149.77 (br s, 2F, para-F ArF), -161.72 (br s, 4F, meta-F ArF),
-162.15 (br s, 4F, meta-F ArF). IR: νas ) 1293.53 and 1261.06
cm-1; νas ) 1231.39 and 1211.23 cm-1 (CF3); νas ) 1168.06 and
1088.86 cm-1 (CF3); νas ) 1022.49 cm-1 (SO3). Anal. Calcd for
C51H22F23P3RuO4S: C, 45.52; H, 1.65; P, 6.90. Found: C, 45.33;
H, 1.74; P, 6.73. ESI-MS: m/z 1196.9 ([M]+, 100).
1
g, 56%. H NMR (300.1 MHz, CD2Cl2): δ 7.92-7.62 (m, 16H,
ArH), 6.99 (s, 1H, H-C(2)), 6.79 (m, 3H, H-C(5) and H-C(6)),
3.63 (d, 2JHH ) 18 Hz, 4H, CH2P), 1.28-0.2 (br q, 6H, BH3). 13
C
)
1
NMR (75.5 MHz, CD2Cl2): δ 133.8-122.0 (Ar), 34.5 (d, JCP
31 Hz, CH2). 31P{1H} NMR (121.4 MHz, CD2Cl2): δ 20.9 (br s).
19F{1H} NMR (282.4 MHz, CD2Cl2): δ -64.97 (s, CF3). Anal.
Calcd for C36H30B2F12P2: C, 55.85; H, 3.91; P, 8.00. Found: C,
56.05; H, 3.98; P, 7.96.
[RuH(PCPiPr)(OiPr)(PPh3)]Na (10). Complex 9 (0.11 g, 0.16
mmol) was mixed with an excess of NaOiPr (0.13 g, 1.6 mmol) in
toluene (15 mL). The reaction mixture, the color of which turned
instantaneously from green to yellow upon heating, was stirred at
reflux temperature for 1 h. The solvent was concentrated in Vacuo.
Slow diffusion of pentane caused the precipitation of complex 10
as a pale yellow solid. Unfortunately, due to the concomitant
precipitation of NaOiPr, which could not be separated, elemental
analysis of 10 did not provide correct values for the complete
characterization. However, the structure of 10 in solution can be
readily assigned on the basis of NMR techniques. 1H NMR (300.1
MHz, C6D6): δ 7.45 (m, 6H, ortho-H PPh3), 7.10 (m, 3H, para-H,
[RuCl(PCPOMe)(PPh3)] (5). Ligand 3 (0.27 g, 0.4 mmol) was
dissolved in dry benzene (10 mL) and transferred, at room
temperature, into a Schlenk flask containing complex [RuCl-
{C6H3(CH2NMe2)2-2,6}(PPh3)] (0.27 g, 0.4 mmol) in 10 mL of
dry benzene. The purple reaction mixture turned to deep green after
5 h of heating. After 48 h of heating under reflux, the solvent was
removed in Vacuo. The obtained green residue was washed with
cold hexane and dissolved in ether. The ether layer was filtered
through a sintered-glass filter, and subsequently, the solvent was
dried in Vacuo, yielding 5 as a green, air/moisture-sensitive solid.
Yield: 0.27 g, 60%. No further purification by crystallization of
the compound was possible due to its high solubility in all common
PPh3), 7.04 (m, 3H, Ar), 6.92 (m, 6H, meta-H, PPh3), 4.23 (br sep,
1
2
organic solvents. H NMR (300.1 MHz, CD2Cl2): δ 7.81-6.46
CH-O, exc. NaOiPr), 3.83 (br s, 1H, CH-O), 2.91 (dvt, JHH
)
2
2
(m, 34H, ArH), 3.85 (s, 6H, CH3O), 3.65 (s, 6H, CH3O), 3.42 (dvt,
16 Hz, JHP ) 5 Hz, 2H, CH2P), 2.31 (dvt, JHH ) 16 Hz, 2H,
2
2
2
2
JHH ) 16 Hz, JHP ) 6 Hz, 2H, CH2P), 2.31 (bd, JHH ) 16 Hz,
CH2P), 1.96 (m, 4H, CH), 1.30 (d, JHH ) 5.4 Hz, CH3, exc.
2H, CH2P). 13C NMR (75.5 MHz, CD2Cl2): δ 173.0 (d, 2JCP ) 14
Hz, Cipso), 151.2 (t, 2JCP ) 8 Hz, C-2,6), 136.7 (dvt, 1JCC ) 50 Hz,
1JCP ) 2-3 Hz, Cquat. PPh3), 134.3-126.0 (C Ar), 122.9 (vt, C-3,5),
NaOiPr), 0.83 (dd, JHH ) 6.8 Hz, JHP ) 13.7 Hz, 12H, CH3),
3
3
3
3
0.72 (dd, JHH ) 6.0 Hz, JHP ) 11.1 Hz, 12H, CH3), -9.17 (dt,
2JHH ) 20.0 and JHP ) 82.2 Hz, 1H, Ru-H). 13C NMR (75.5
2
1
2
2
121.7 (s, C-4), 55.5 (s, CH3O), 39.3 (vt, JCP ) 16 Hz, CH2P).
MHz, C6D6): δ 177.9 (t, JCP ) 5 Hz, Cipso), 149.2 (t, JCP ) 9
Hz, C-2,6), 140.8 (d, JCP ) 25 Hz, Cquat. PPh3), 134.17-127.43
31P{1H} NMR (121.4 MHz, CD2Cl2): δ 82.7 (t, JPP ) 31.5 Hz),
2
2
35.2 (d, JPP ) 31.5 Hz). ESI-MS: m/z 957 ([M - Cl]+, 100%).
(ortho, meta, and para-C PPh3), 122.7 (s, C-4), 120.5 (vt, C-3,5),
62.6 (br s, CH-O, exc. NaOiPr), 59.0 (br m O-CH), 44.4 (vt,
1JCP ) 13 Hz, CH2P), 29.8 (br s, CH3, exc. NaOiPr), 29.0 (vt, 4JCP
) 8.8 Hz, CH3), 26.0 (br s, CH), 22.6, 20.9, 18.6, 16.7 (s, all CH3).
2
[RuCl(PCPCF3)(PPh3)] (6). The reaction mixture obtained by
adding ligand 4 (0.8 g, 1.1 mmol) in dry benzene (10 mL) and
complex [RuCl{C6H3(CH2NMe2)2-2,6}(PPh3)] (0.7 g, 1.1 mmol)
in dry benzene (20 mL) was stirred at reflux temperature for 48 h.
The color of the mixture turned to deep green upon heating. The
solvent was evaporated, and the green residue obtained was washed
with cold hexane. 6 was then extracted with ether. The ether layer
was filtered through a glass filter and the solvent removed in Vacuo
to give 6 as an air/moisture-sensitive, green solid. No further
purification of the compound by crystallization was possible due
to its high solubility in all common organic solvents. Yield: 0.63
2
31P{1H} NMR (121.4 MHz, C6D6): δ 72.1 (d, JPP ) 15.5 Hz),
2
35.8 (t, JPP ) 15.5 Hz).
Procedure for the Hydrogen Transfer Reaction. In a typical
experiment performed under a N2 atmosphere, the ruthenium
complex (0.02 mmol) was mixed with NaOH (0.4 mmol) in
2-propanol (5 mL) into a two-neck round-bottom flask equipped
with a reflux condenser and a septum. Next, the flask was lowered
into a preheated oil bath of 82 °C, and the resulting mixture was
heated to reflux for 1 h. A color change was observed from green
to yellow for complexes 5 and 6 and to red for 9 within a few
minutes. Subsequently, cyclohexanone (20 mmol) in 2-propanol
(10 mL) was added by a syringe. Samples of the reaction mixture
were taken by an airtight syringe. The reaction was monitored in
time by gas chromatography until total conversion of the substrate
was reached. The experiments performed under an Ar atmosphere
were performed employing solvents and reagents degassed under
1
g, 46%. H NMR (300.1 MHz, CD2Cl2): δ 8.04-7.04 (m, 26H,
2
2
Ar), 6.90 (m, 6H, meta-H PPh3), 3.58 (dvt, JHH ) 16 Hz, JHP
)
6 Hz, 2H, CH2P), 2.50 (bd, JHH ) 16 Hz, 2H, CH2P). 13C NMR
(75.5 MHz, CD2Cl2): δ 170. 3 (d, 2JCP ) 14 Hz, Cipso), 150.2 (vt,
C-2,6), 138.9-124.9 (C Ar), 123.7 (m, C-3,5), 122.7 (s, C-4), 38.7
(vt, 1JCP ) 16 Hz, CH2P). 31P{1H} NMR (121.4 MHz, CD2Cl2): δ
2
76.1 (t, JPP ) 31.5 Hz), 36.4 (d, JPP ) 31.5 Hz). 19F{1H} NMR
2
2
(282.4 MHz, CD2Cl2): δ -65.09 (s, CF3), -65.53 (s, CF3). ESI-

PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis
Organometallics, Vol. 26, No. 9, 2007 2227
Ar, by applying the procedure described above for the experiments
carried out under N2.
Acknowledgment. The authors kindly acknowledge Dr. S.
Araujo Bambirra for his assistance during ESI measurements.
This work was supported by the Council for Chemical Sciences
from the Netherlands Organization for Scientific Research (CW-
NWO) and Natural Sciences and Engineering Research Council
(NSERC) of Canada (P.A.C.).
Monitoring the Effect of Pretreatment Time on the Activity
of [RuCl(PCPiPr)(PPh3)] (9). To a solution of 9 (0.02 mmol) in
2-propanol (5 mL) was added NaOH (0.4 mmol) in 2-propanol (10
mL). The resultant mixture was heated to reflux under an N2
atmosphere. Aliquots of the solution were withdrawn by a syringe
and poured into a Schlenk flask. All the volatiles were dried in
Vacuo, and the yellow-brown residue obtained was dissolved in
dry benzene. Addition of pentane caused the precipitation of a light
yellow solid, which was collected by centrifugation and dried in
Vacuo. Then, 1H and 31P{1H} NMR spectra were recorded at room
temperature in C6D6.
Supporting Information Available: 1H, 13C, and 31P{1H} NMR
spectra of complexes 5, 6, and 10. ESI-MS spectra of complexes
5 and 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM060874F