Stepwise metal-ligand cooperation by a reversible aromatization/ deconjugation sequence in ruthenium complexes with a tetradentate phenanthroline-based ligand

Article abstract of DOI:10.1002/chem.201204003

The synthesis and reactivity of ruthenium complexes containing the tetradentate phenanthroline-based phosphine ligand 2,9-bis((di-tert- butylphosphino)methyl)-1,10-phenanthroline (PPhenP) is described. The hydrido chloro complex [RuHCl(PPhenP)] (2) undergoes facile dearomatization upon deprotonation of the benzylic position, to give [RuH(PPhenP-H)] (4). Addition of dihydrogen to 4 causes rearomatization of the phenanthroline moiety to trans-[Ru(H)₂(PPhenP)] (5), followed by hydrogenation of an aromatic heterocycle in the ligand backbone, to give a new dearomatized and deconjugated complex [RuH(PPhenP*-H)] (6). These aromatization/deconjugation steps of the coordinated ligand were demonstrated to be reversible and operative in the dehydrogenation of primary alcohols without the need for a hydrogen acceptor. This aromatization/deconjugation sequence constitutes an unprecedented mode of a stepwise cooperation between the metal center and the coordinated ligand. Obligation to cooperate: A dearomatization/deconjugation sequence, involving reversible hydrogenation of the ligand backbone, has been uncovered for the dihydrogen activation by ruthenium complexes with a tetradentate phenanthroline-based ligand. This unprecedented mode of stepwise cooperation between the metal center and the coordinated ligand is demonstrated to be reversible and operative in the dehydrogenation of primary alcohols without the need for a hydrogen acceptor (see scheme). Copyright

Full text of DOI:10.1002/chem.201204003

FULL PAPER
DOI: 10.1002/chem.201204003
Stepwise Metal–Ligand Cooperation by a Reversible Aromatization/
Deconjugation Sequence in Ruthenium Complexes with a Tetradentate
Phenanthroline-Based Ligand
Robert Langer,[a] Ido Fuchs,[a] Matthias Vogt,[a] Ekambaram Balaraman,[a]
Yael Diskin-Posner,[b] Linda J. W. Shimon,[b] Yehoshoa Ben-David,[a] and
David Milstein*[a]
Abstract: The synthesis and reactivity
of ruthenium complexes containing the
matization of the phenanthroline
moiety to trans-[Ru(H)2A(PPhenP)] (5),
followed by hydrogenation of an aro-
matic heterocycle in the ligand back-
bone, to give a new dearomatized and
deconjugated complex [RuH(PPhenP*-
H)] (6). These aromatization/deconju-
gation steps of the coordinated ligand
were demonstrated to be reversible
and operative in the dehydrogenation
of primary alcohols without the need
for a hydrogen acceptor. This aromati-
zation/deconjugation sequence consti-
tutes an unprecedented mode of a step-
wise cooperation between the metal
center and the coordinated ligand.
CHTUNGTRENNUNG
tetradentate
phenanthroline-based
phosphine ligand 2,9-bis((di-tert-butyl-
phosphino)methyl)-1,10-phenanthroline
(PPhenP) is described. The hydrido
chloro complex [RuHClACTHUNTGRNEUNG(PPhenP)] (2)
undergoes facile dearomatization upon
deprotonation of the benzylic position,
Keywords: cooperative effects · co-
ordination chemistry · dehydrogen-
ation
· homogeneous catalysis ·
to give [RuHACHTUNGTRENNUNG(PPhenP-H)] (4). Addi-
tion of dihydrogen to 4 causes rearo-
ruthenium
Introduction
cause the heterolytic cleavage of substrate bonds is often
rate determining for the catalytic conversion,[5a,7b,10b] the de-
velopment of new approaches for the activation of bonds is
important.
In recent years, the discovery of MLC by dearomatiza-
tion/rearomatization sequences in acridine- and pyridine-
based pincer complexes has led to unusual bond-activation
processes and new catalytic reactions.[12,13] For example,
dearomatized ruthenium PNP and PNN pincer-type com-
The activation of chemical bonds is a key step in reactions
catalyzed by transition-metal complexes.[1] In recent years,
bond activation by cooperation of the metal center with the
ligand has gained increasing attention and has been applied
in several catalytic transformations.[2–4] Reversible substrate
bond activation without a formal change of the oxidation
state of the metal center is characteristic of such metal–
ligand-cooperation (MLC) processes.
À
plexes activate H X bonds with aromatization of the pyri-
Prominent examples are transition-metal complexes with
M=X multiple bonds, in which X can be, for example, an
imido,[5] amido,[6–9] or even a sulfido ligand.[10] Activation of
dine moiety (X=H, OR, OH, HNR). Eventually, the ability
of the corresponding trans-dihydride complexes to liberate
H2 with regeneration of the dearomatized complexes led to
the development of efficient catalytic processes, involving
dehydrogenative coupling of primary alcohols without the
need for a hydrogen acceptor to give esters and coupling of
alcohols with amines to give amides[12b] or imines.[12c] On the
other hand, the dearomatized PNP and PNN ruthenium
complexes readily react with H2 to form the trans-dihydride
species, which can be applied in the catalytic reduction of
challenging substrates, such as amides, carbamates, organic
carbonates, and urea derivatives.[13] Acridine-based rutheni-
um pincer complexes were reported to efficiently catalyze
the conversion of primary alcohols to acetals under neutral
conditions and to esters in the presence of a base,[12d] as well
as the reaction of alcohols with ammonia to give primary
amines selectively.[12g] A unique mode of MLC in the activa-
tion of H2 and NH3 that involves a “long-range” interaction
between the acridine C9 position and the metal center was
found for these complexes.[14] Long-range-type cooperation
À
R H bonds occurs across the M=X multiple bond, to give
1
À
À
À
À
the corresponding RM XH complex (R=H , Si , R O ,
R2). Highly active catalysts for the hydrogenation and dehy-
drogenation of organic substrates by using the MLC concept
for chemical bond activation have been reported.[2,11] Be-
[a] Dr. R. Langer, I. Fuchs, Dr. M. Vogt, Dr. E. Balaraman,
Y. Ben-David, Prof. Dr. D. Milstein
Department of Organic Chemistry
Weizmann Institute of Science, 76100 Rehovot (Israel)
Fax : (+972)8-934-4142
E-mail: david.milstein@weizmann.ac.il
[b] Dr. Y. Diskin-Posner, Dr. L. J. W. Shimon
Department of Chemical Research Support
Weizmann Institute of Science, 76100 Rehovot (Israel)
Supporting information for this article contains selected NMR spectra
for compounds 4 and 6 and is available from the author or on the
WWW under http://dx.doi.org/10.1002/chem.201204003.
Chem. Eur. J. 2013, 19, 3407 – 3414
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was observed also by Song and co-workers in ruthenium
complexes with diazafluorene ligands.[14b]
CS symmetry in complex 2 with the plane of symmetry per-
pendicular to the plane of the phenanthroline backbone.
Furthermore, a sharp triplet resonance at d=À19.90 ppm
Extending the concept of long-range cooperation to simi-
lar planar but tetradentate systems would give more stability
to the catalyst, whereas the typical meridional coordination
mode of pincer-type ligands can be retained. Besides, such
systems allow further functionalization of the ligand back-
bone and open up the possibility for additional reactive
carbon centers, which can cooperatively interact with the
metal center. Herein, we present a new class of aromatized,
dearomatized–deconjugated ruthenium complexes with the
tetradentate phenanthroline-based phosphine ligand. A new
mode of cooperation between the metal center and the phe-
nanthroline ligand has been discovered, including the rever-
sible hydrogenation of the conjugated ligand backbone. The
catalytic activity of these complexes has been examined in
the dehydrogenation of primary alcohols without the need
for a hydrogen acceptor.
1
was observed for the hydride ligand in the H NMR spec-
trum (2J
ACHTNUTRGNE(NUG H,P)=28.7 Hz). The molecular structure of 2, de-
termined by single-crystal X-ray diffraction analysis, indi-
cates a distorted octahedral coordination geometry around
the RuII center. The four donor atoms of the PPhenP ligand
reside in the central molecular plane with the hydride and
the chloride ligands located trans to each other and perpen-
dicular to the plane (Figure 1).
Results and Discussion
Reaction of [RuHClACHTNUGTRNEG(UN Ph3P)3] with the phenanthroline-based
diphosphine ligand 2,9-bis((di-tert-butylphosphino)methyl)-
1,10-phenanthroline (PPhenP, 1) takes place in THF at 608C
with displacement of PPh3, to give the hydrido chloride
complex [RuHClACHTUNGTRENNUNG(PPhenP)] (2) in good yield (Scheme 1).
Figure 1. ORTEP diagram of complex 2 with the thermal ellipsoids at
50% probability level. All hydrogen atoms, except for the hydride, were
The 31P{1H} NMR spectrum of 2 exhibits a singlet at d=
90.7 ppm, consistent with two chemically equivalent phos-
phorus nuclei. In the 1H NMR spectrum, the four tBu
À
omitted for clarity. Selected bond lengths [ꢁ] and angles [8]: Ru1 N2
À
À
À
À
2.044(4), Ru1 N1 2.052(4), Ru1 P2 2.340(1), Ru1 P1 2.343(1), Ru1 Cl1
groups give rise to two doublets at d=1.25 (3J
ACHTUNGTRENNUNG
À
2.610(1), Ru1 HRu1 1.485; N2-Ru1-N1 78.5(2), N2-Ru1-P2 159.2(1), N1-
Ru1-P1 157.8(1), P2-Ru1-P1 119.62(5), Cl1-Ru1-HRu1 166.1.
11.4 Hz) and 1.51 ppm (3J
ACHTUNGTRENNUNG
zylic protons two doublets of doublets (d=3.74 and
3.99 ppm) are observed showing a geminal coupling constant
2
of J
A
G
The reaction of [RuCl
608C gives the dichloro complex [RuCl CHUTNGTRENNUNG
(Ph3P)3] with ligand 1 in THF at
2A(PPhenP)] (3). A
singlet at d=70.6 ppm is observed for the two equal phos-
ACHTUNGTRENNUNG
phorus atoms in the 31P{1H} NMR spectrum. C2 symmetry in
1
7.79 ppm for the aromatic protons suggest the presence of
3 is indicated in the H NMR spectrum by the observation
of only one doublet at d=1.45 ppm (3J
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
four benzylic protons, and three signals for all aromatic pro-
tons. The crystal structure of complex 3 reveals a distorted
octahedral coordination sphere around the RuII center with
the two chloride ligands located trans to each other
(Figure 2).
Treatment of 2 with one equivalent of LiNACHTUNTGRNEUNG(SiMe3)2 result-
ed in selective deprotonation at the benzylic position and
the formation of the dearomatized hydride complex 4
(Scheme 1). The 31P{1H} NMR spectrum of 4 reveals an AB
spin system with two doublets at chemical shifts at d=83.0
and 95.7 ppm, respectively (2J
ACTHNUTRGNE(NUG P,P)=11.5 Hz), suggesting the
1
inequivalence of the two phosphorus atoms. The H NMR
spectrum displays a doublet of doublets at d=À33.11 ppm
(2J(H,P)=29.4, 2J
ACTHNUTRGENNUG CAHTUNGTREN(NUGN H,P)=25.3 Hz) for the hydride ligand;
Scheme 1. Synthesis of ruthenium complexes 2–4.
the low-frequency chemical shift indicates a square-pyrami-
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Metal–Ligand Cooperation through Aromatization/Deconjugation
FULL PAPER
with complex 4. The remaining two spin systems in the
1H NMR spectrum, consisting of two sets of doublet reso-
nances, can be assigned (by 1H–1H-COSY, 1H–13C-HSQC,
1H–13C-HMBC NMR analyses) to the protons of one aro-
matic heterocycle and the aromatic protons in the backbone
of the phenanthroline moiety, respectively. In addition, two
multiplet resonances centered at d=2.72 and 2.83 ppm, as
well as one doublet of triplets at d = 2.98 ppm resonance
(2J(H,H)=14.3, 3J
ACHTNUTRGENNUG HCATUNGTRENN(NGU H,H)=5.4 Hz) were detected in the
region of the benzylic protons and identified as one spin
system by 1H–1H COSY NMR corresponding to four pro-
tons of two neighboring CH2 groups. Based on
1H–1H COSY,
1H–31P HMQC,
1H–13C HSQC,
1H–13C HMBC, 13C{1H} and 13C DEPTQ NMR analyses, this
spin system corresponds to the third ring of the phenanthro-
line moiety, including two CH2 groups and one amido group
that binds to the ruthenium center. The benzylic CH2 group
gives rise to two doublet of doublets resonances at d=2.84
Figure 2. ORTEP diagram of complex 3 with the thermal ellipsoids set at
50% probability. All hydrogen atoms were omitted for clarity. Selected
À
À
bond lengths [ꢁ] and angles [8]: Ru1 N1 2.034(3), Ru1 N2 2.038(2),
1
À
À
À
À
Ru1 P2 2.3810(9), Ru1 P1 2.3842(9), Ru1 Cl1 2.4529(9), Ru1 Cl2
2.4652(9); N1-Ru1-N2 79.9(1), N1-Ru1-P2 159.24(8), N2-Ru1-P1
159.42(8), P2-Ru1-P1 119.73(3), Cl1 Ru1 Cl2 167.51(3).
and 3.26 ppm in the H NMR spectrum, with a geminal cou-
pling constant of 2J
N
2
stant values of J(H,P)=7.3 and 8.9 Hz, respectively. Based
G
on 1D and 2D NMR spectroscopy, we conclude that this
CH2 group is adjacent to the aromatic heterocycle of the
phenanthroline scaffold. A singlet resonance at d=4.62 ppm
corresponding to an integration value of one proton in the
1H NMR spectrum is assigned to the second benzylic proton
of a CH group, which forms a double bond to the aliphatic
heterocycle of the phenanthroline moiety.[15] Full assignment
of all atoms reveals the structure shown in Scheme 2 for
complex 6. Unexpectedly, under these conditions, a dearom-
atized and deconjugated complex was formed as a result of
dihydrogen addition to the endocyclic double bond, whereas
the expected trans-dihydride complex was not observed.
Addressing the question whether the initial step of the di-
hydrogen activation is formation of a trans-dihydride inter-
mediate, NMR studies of the reaction of complex 4 with H2
were performed. During the reaction of 4 with H2, a singlet
resonance of low intensity at d=118.91 ppm appears in the
31P{1H} NMR spectrum, which correlates to a triplet reso-
dal coordination geometry with no ligand trans to the hy-
dride ligand. The protons of the phenanthroline moiety give
rise to three spin systems, consisting of simple first-order
doublets (confirmed by 1H–1H COSY NMR analysis). The
latter finding and the observation of four distinct doublet
resonances at d=1.12, 1.16, 1.35, and 1.48 ppm in the
1H NMR spectrum for the tBu groups indicate the absence
of C2 or CS symmetry in complex 4. A singlet resonance at
1
4.35 ppm in the H NMR spectrum of 4, which is assigned to
the benzylic proton that corresponds to a methine carbon
atom in the 13C distortionless enhancement by polarization
transfer including quaternary carbons (DEPTQ) NMR spec-
1
3
trum (d=82.2 ppm, dd, JACTHNUTRGENN(UG C,P)=45.2, JAHCTUNGTERNN(UGN C,P)=3.2 Hz), sug-
gests the formation of an anionic ligand system, with one of
the heterocycles being dearomatized (confirmed by
1H–13C HSQC NMR analysis). Thus, the phenanthroline
backbone undergoes dearomatization upon deprotonation
of the benzylic position.
Complex 4 was reacted with H2 under various conditions.
Surprisingly, the expected trans-dihydride complex [Ru(H)2-
ACHTUNGTRENNUNG(PPhenP)] (5) was not formed at ambient temperature and
1
nance at À8.65 ppm in H NMR for the hydride ligands (2J-
ACHTUNGTREN(NGNU H,P)=28.8 Hz). The appearance of one singlet for the
phosphorus atoms and one triplet for the hydrides is consis-
tent with a rearomatized C2- or Cs-symmetric species.[13a,16]
In addition, the chemical shift of À8.65 ppm for the hydride
ligand suggests the coordination of a ligand of a strong trans
influence (such as a hydride) in the trans position. There-
fore, it is very likely that the trans-dihydride complex 5 is an
intermediate in the formation of complex 6.
Additional evidence for the intermediacy of the dihydride
complex 5 was provided by the reaction of the dichloride
complex 3 with two equivalents of NaHBEt3 at À408C in
[D8]toluene. Initially, the assumed dihydride complex 5 was
formed in low concentrations, followed by the generation of
complexes 4 and 6. After warming to ambient temperature,
NMR spectroscopy showed a mixture of two products, con-
sisting of 68% complex 4 and 32% complex 6. This observa-
tion is consistent with two possible reaction pathways, in the
hydrogen pressures between one and five bars. Instead,
upon reaction of 4 with H2, the 31P{1H} NMR spectrum
2
showed a new AB spin system (d=86.2 and 95.5 ppm, J-
ACHTUNGTRENNUNG(P,P)=10.83 Hz). The hydride ligand in the newly formed
complex (6) gives rise to a slightly shifted doublet of dou-
1
blets resonance in the H NMR spectrum at d=À31.52 ppm
(2J(H,P)=32.7 and 2J
ACHTUNGTRENNUNG ACHTUNGTREN(NUNG H,P)=25.2 Hz), which integrates to
the value of one proton, indicating that still no ligand re-
sides trans to this hydride. The tBu groups in 6 exhibit a
1
slightly shifted set of four doublets in the H NMR spectrum
with respect to complex 4, consistent with no change in sym-
metry during the reaction with H2. A detailed analysis of
1
the signals for the aromatic protons in the H NMR spec-
trum revealed that one spin system is absent in comparison
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3409

D. Milstein et al.
labeling experiments using D2
and CD3OD partial deutera-
tion of the hydride, the benzyl-
ic positions and the deconju-
gated ring of the phenanthro-
line moiety was observed by
NMR spectroscopy. This reac-
tivity of complex 4 towards H2
and primary alcohols repre-
sents a unique and unprece-
dented mode of stepwise MLC
by an aromatization/deconju-
gation sequence.
Based on these findings,
complexes 4 and 6 are expect-
ed to be competent catalysts
for the dehydrogenation of pri-
mary alcohols without the
need for a hydrogen acceptor.
Exploring the possibility of
catalysis, a solution of n-hexa-
Scheme 2. Reversible formation of complex 6 via different reaction pathways.
nol and 1 mol% of
4 in
absence of dihydrogen, for the intermediate 5: liberation of
H2 to form complex 4 and hydrogenation of the phenanthro-
line backbone forming complex 6. However, under hydro-
gen pressure >1 atm, complex 6 is formed exclusively, and
no further reaction of complex 6 with H2 was observed. No-
tably, heating complex 6 at reflux in toluene in an open
system under argon for six hours gives complex 4, as well as
unidentified decomposition products.
A similar observation was made upon treatment of com-
plex 4 in [D8]toluene with an excess of MeOH at À408C, re-
sulting in an immediate color change from dark purple to
brown. Complex 5 was initially formed, but disappeared
after a short time, and a mixture of 4 and 6 was detected,
whereas the corresponding alkoxide complex could not be
observed over the whole course of the reaction. These re-
sults demonstrate that complex 6 does not directly activate
dihydrogen or alcohols.
In contrast to pyridine-based trans-dihydride ruthenium
pincer complexes,[12] the trans-dihydride 5 is unstable even
under an atmosphere of hydrogen. There are two major re-
action pathways for complex 5: 1) the elimination of dihy-
drogen to re-form the dearomatized complex 4, with all phe-
nanthroline-derived carbon atoms being conjugated; and
2) the hydrogenation of an aromatic double bond to give
complex 6, resulting in deconjugation of the phenanthroline
moiety. It seems likely that the latter is formed via a dear-
omatized dihydrogen complex, which could react in a bi- or
unimolecular reaction to form complex 6. To develop a
deeper understanding of this unusual aromatization/decon-
jugation sequence, we performed the reaction of complex 4
with H2 in the presence of similar polyaromatic compounds,
such as phenanthrene or phenanthroline, which might get
hydrogenated in the case of a bimolecular reaction. Re-
markably, the reaction proceeds completely without partici-
pation of the added polyaromatic compounds. In deuterium-
[D8]toluene was stirred for three days at ambient tempera-
ture and subsequently analyzed by NMR spectroscopy and
GC-MS. Complex 6 was identified by 31P{1H} NMR spectro-
scopy as the only complex present in the reaction mixture,
whereas GC-MS confirmed the formation of small amounts
of n-hexyl hexanoate together with traces of n-hexanal. By
heating at reflux the same reaction mixture, quantitative for-
mation of hexyl hexanoate after six hours was observed, in-
dicating that complex 6 liberates dihydrogen via intermedi-
ate 5 back to 4 upon heating.
Additional catalytic experiments are outlined in Table 1.
By heating at reflux a mixture of 5 mmol benzyl alcohol in
2 mL of toluene in the presence of 0.2 mol% of complex 4
for 72 h, 67% conversion of benzyl alcohol was determined
by GC analysis to give benzyl benzoate in 66% yield
(Table 1, entry 2). A similar yield of benzyl benzoate was
obtained when 0.2 mol% of complex 6 was used instead of
4 (Table 1, entry 3). Complex 4 was also investigated as a
catalyst for the dehydrogenative coupling of primary alco-
hols with amines. The formation of imines with elimination
of H2 and H2O, rather than amide formation, took place, as
was previously observed for ruthenium PNP pincer com-
plexes[12c] and unlike for ruthenium PNN complexes.[12b] The
81% yield (TON=405) of N-benzylidene-1-phenylmethana-
mine is quite similar to the yield obtained with the pincer
complex [RuH(CO)ACHTNUGRTENUNG(tBu-PNP*)] (87%, TON=435, tBu-
PNP=bis(2,6-di-tert-butylphosphinomethyl)pyridine; [*] de-
notes a dearomatized ligand).[12c]
In contrast, no catalytic activity was observed when com-
plexes 4 and 6 were tested in the hydrogenation of hexyl
hexanoate under conditions comparable to those, under
which the corresponding pyridine-based ruthenium com-
plexes were active (THF, 1208C, 5.3 bar H2).[13a,b] This find-
ing is consistent with the observation that complex 6, which
is preferably generated under a hydrogen atmosphere, is not
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Metal–Ligand Cooperation through Aromatization/Deconjugation
Table 1. Dehydrogenative coupling of primary alcohols.[a]
FULL PAPER
Entry
1[b]
Complex
Substrate
Product
t [h]
Conversion [%][c]
Yield [%][c]
TON
99
4
6
>99
99
66
2
4
6
4
72
72
72
67
65
94
330
3
65
325
4[d]
81 imine, 6 ester
405+30
[a] Reaction conditions: complex (0.01 mmol), benzyl alcohol (5.00 mmol), m-xylene (1.00 mmol, internal standard), and toluene (2 mL) were heated at
reflux. [b] Complex (0.01 mmol), n-hexanol (1.00 mmol), m-xylene (1.00 mmol, internal standard), and [D8]toluene (1 mL) were heated at reflux. [c] De-
termined by GC analysis with m-xylene as internal standard. [d] Benzylamine (5.05 mmol) was added.
able to add dihydrogen. Although in principle it can isomer-
ize back to the intermediate trans-dihydride complex 5, the
concentration of the latter may be too low to promote cata-
lytic activity.
Conclusion
The concept of MLC by dearomatization/aromatization se-
quences has been extended to non-pincer-type complexes.
Similar to the pyridine-based pincer complexes, dearomat-
ized RuII complexes with a tetradentate phenanthroline-
based bisphosphine ligand were synthesized. An unprece-
dented mode of stepwise MLC by an aromatization/deconju-
gation sequence was disclosed. Detailed NMR studies re-
vealed the reversible addition of hydrogen to one of the C=
C double bonds of the ligand backbone. Complexes 4 and 6
efficiently catalyze the dehydrogenative coupling of primary
alcohols to form esters, although they do not catalyzed the
reverse reaction, namely, the hydrogenation of esters. Cata-
lytic imine formation by dehydrogenative coupling of an
amine and alcohol was also observed. Mechanistic studies
and extension of this new mode of MLC to other metal sys-
tems are planned.
A possible reaction mechanism for the dehydrogenative
coupling of primary alcohols catalyzed by the phenanthro-
line based ruthenium complexes might involve the addition
of alcohol to complex 4 to give the rearomatized hydrido–
alkoxide complex A (Scheme 3). b-Hydride elimination of
the alkoxide complex would result in the trans-dihydride
complex 5. It is not clear if the generation of a vacant coor-
dination side is involved in this step, because no evidence
for hemilability was found. Moreover, the b-hydride elimi-
nation without generation of a vacant coordination side has
been reported for some transition-metal alkoxide com-
plexes.[17] The trans-dihydride complex 5 can formally rear-
range to give complex 6 or eliminate dihydrogen and regen-
erate complex 4. It seems likely that both pathways proceed
via the dearomatized dihydrogen complex B (Scheme 3).
Experimental Section
Materials and methods: All experi-
ments were carried out under an at-
mosphere of purified nitrogen in a
vacuum-atmosphere glove box or by
using standard Schlenk techniques.
THF, toluene, benzene, and n-pen-
tane were heated at reflux over
sodium/benzophenone,
distilled
under argon atmosphere, and stored
over molecular sieves. Deuterated
solvents (except CD2Cl2) were de-
gassed with argon and kept in the
glove box over molecular sieves. The
substrates used in catalytic reactions
were purified by vacuum distillation.
The complexes [RuCl
[RuHCl
(Ph3P)3][19] were prepared ac-
(Ph3P)3][18] and
2ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
cording to literature procedures.
1H, 13C, and 31P NMR spectra were
recorded on a Bruker AvanceIII-300,
AvanceIII-400, and
a AvanceII-500
Scheme 3. Possible mechanism for the dehydrogenative coupling of alcohols catalyzed by 4 or 6.
NMR spectrometers. 1H and 13C{1H}
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3411

D. Milstein et al.
and 13C DEPTQ NMR chemical shifts are reported in ppm downfield
from tetramethylsilane. 31P NMR chemical shifts are reported in ppm
downfield from H3PO4 and referenced to an external 85% solution of
phosphoric acid in D2O. Assignment of the signals is based on
1H–1H COSY, 1H–1H NOESY, 1H–31P HMQC, 1H–13C HSQC, and
1H–13C HMBC NMR analyses.
Synthesis of [RuCl
2ACHTUNGTRENUN(NG PPhenP)] (3): PPhenP (1) (67 mg, 0.14 mmol) of
and [RuCl2A(Ph3P)3] (130 mg, 0.14 mmol) were dissolved in THF (20 mL),
CTHUNGTRENNUNG
the mixture was heated at 608C for 16 h to give a deep blue violet solu-
tion. Prior to removal of all volatiles in vacuo, the solution was passed
through a syringe filter (25 mm porosity). Subsequently, the crude solid
was dissolved in a minimal volume of THF (ca. 1 mL) and precipitated
with n-pentane (15 mL). The superjacent solution was decanted and the
purple solid washed with diethyl ether/pentane (15:2 mL). Drying at high
vacuum gave pure 3. Single crystals suitable for X-ray diffraction analysis
were grown by layering concentrated THF solution with n-pentane or
ether. Yield: 40 mg (42%); 1H NMR (300 MHz, CD2Cl2, 238C): d=1.45
Synthesis of 2,9-bis(di-tert-butylphosphino)-methyl)-1,10 phenanthroline
(1): Neocuprine (2.50 g, 12.0 mmol) was dissolved in a toluene/ether 10:1
mixture (50 mL), and nBuLi solution in n-hexane (24 mL, 1.6m,
38.4 mmol, 3.2 equiv) was added slowly at 08C. After two hours stirring
at ambient temperature, the solution was cooled to À788C, and tBu2PCl
(4.32 g, 24.0 mmol, 2 equiv) was added dropwise to the solution. After
4 h at À788C, the reaction mixture was slowly warmed to ambient tem-
perature and stirred for 16 h. Degassed H2O (50 mL) was added to the
suspension, the organic layer was separated, and the aqueous phase was
extracted two times with toluene (10 mL) under N2 atmosphere. The
combined organic phases were dried over anhydrous Na2SO4. All vola-
tiles were removed in vacuo, and the residue was recrystallized from
(d, 36H, 3J
CH2PtBu2), 7.89 (s, 2H, phen-H5,6), 7.92 (d, 2H, 3J
H3,8), 8.19 ppm (d, 2H, 3J(H,P)=8.3 Hz, phen-H4,7); 31P{1H} (121 MHz,
CD2Cl2, 238C): d=70.61 ppm (s); 13C{1H} NMR (126 MHz, CD2Cl2,
238C): d=30.72 (vt, 6C, 2J (CH3)3), 37.73 (vt, 4C, 1J-
(C,P)=2.0 Hz, PC
(C,P)=3.8 Hz, PC(CH3)3) AA’XX’, 40.52 (m, 2C, J(C,P)=12.4, 5.5 Hz,
CH2PtBu2, AA’XX’), 122.15 (vt, 2C, 3J
(C,P)=5.0 Hz, phen-C3,8), 125.63
(s, 2C, phen-C5,6), 128.77 (s, 2C, phen-C4a,6a), 133.20 (s, 2C, phen-C4,7),
150.22 (s, 2C, phen-C10a,10b), 166.85 ppm (vt, 2C, 2J
(C,P)=1.9 Hz, phen-
C2,9, AA’XX’); MS (ESI): m/z (%): 633.19 (100% [Ru(Cl)
(PPhenP)]+]);
A
A
ACHTUNGTREN(NUNG H,P)=8.5 Hz,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
1
methanol. Yield: 3.70 g (62%). H NMR (500 MHz, C6D6, 238C): d=1.14
AHCTUNGTRENNUNG
(d, 36H, 3J
N
(CH3)3), 3.42 (d, 4H, 2J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
IR (thin film): n˜ =2988 (w), 2945 (m), 2917 (m), 2863 (m), 1571 (w),
1500 (w), 1474 (w), 1437 (w), 1393 (m), 1385 (m), 1221 (w), 1179 (w),
1164 (w), 1112 (w), 1021 (m), 933 (m), 859 (s), 809 (m), 777 (w), 749 (w),
691 (w), 666 cmÀ1 (m); elemental analysis calcd (%) for C30H46Cl2N2P2Ru
(668.62 gmolÀ1): C 53.89, H 6.93, N 4.19; found: C 54.44, H 6.71, N 4.25.
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Synthesis of [(PPhenP-H)RuH] (4): LiHMDS (13 mg, 0.08 mmol,
AHCTUNGTRENNUNG
1.2 equiv) was added to a stirred suspension of [RuHClACTHNUTRGNE(UNG PPhenP)] (2)
AHCTUNGTRENNUNG
(42 mg, 0.07 mmol) in toluene (2 mL). After two hours, the dark purple
reaction mixture was filtered, and all volatiles were removed in vacuo.
The residue was washed with pentane, and the remaining solid was dried
under high vacuum. All attempts to obtain single crystals suitable for X-
ray diffraction resulted either in precipitation or decomposition. Yield:
26 mg (66%); 1H NMR (500 MHz, C6D6, 238C): d=À33.11 (dd, 1H,2J-
15.0 Hz, phen-C2,9); MS (ESI): m/z (%): 497.17 (80%, [PPhenÀH]+); IR
(thin film): n˜ =2978 (w), 2946 (m), 2891 (w), 2859 m, 1615 (w), 1604 (w),
1588 (s), 1545 (m), 1504 (m), 1487 (s), 1469 (m), 1463 (m), 1443 (s),
1420 (m), 1398 (m), 1386 (m), 1364 (s), 1360 (s), 1317 (w), 1303 (w),
1280 (w), 1241 (w), 1211 (w), 1200 (m), 1171 (w), 1169 (m), 1144 (m),
1136 (w), 1119 (w), 1015 (w), 991 (w), 938 (w), 934 (w), 852 (s), 845 (m),
811 (m), 797 (m), 747 (w), 701 cmÀ1 (w); elemental analysis calcd (%) for
C30H46N2P2 (496.65 gmolÀ1): C 72.55, H 9.34, N 5.64; found: C 71.08, H
9.58, N 5.31.
A
(H,P)=25.3 Hz, Ru-H), 1.12 (d, 9H,3J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
A
ACHUTGTNREN(UNG H,P)=12.1 Hz, PCACTHUNGTRENNNUG
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Synthesis of [RuHCl
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
0.18 mmol) and [RuHClAHCTUNGTRENNUNG
U
ACHTUNGTRENNUNG
THF (20 mL), and the mixture was heated at 608C for 16 h, to give a
deep blue violet solution. Prior to removal of all volatiles in vacuo, the
solution was passed through a syringe filter (20 mm porosity). The residue
was washed with pentane (15 mL). Subsequently, the crude solid was dis-
solved in a minimal volume of THF (ca. 1 mL) and precipitated with
pentane (30 mL). The purple solid was filtered off and dried at high
vacuum to give pure 2. Single crystals suitable for X-ray diffraction anal-
ysis were grown by layering concentrated THF solution with n-pentane
or ether. Yield: 91 mg (79%); 1H NMR (500 MHz, CD2Cl2, 238C): d=
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
E
ACHTUNGTRENNUNG
R
U
A
ACHTUNGTRENNUNG
G
E
ACHTUNGTRENNUNG
G
N
ACHTUNGTRENNUNG
R
A
ACHTUNGTRENNUNG
E
G
ACHTUNGTRENNUNG
2
3
À19.90 (t, 1H, J
G
ACHTUNGTREN(NUNG H,P)=11.4 Hz,
U
ACHTUNGTRENNUNG
PC
(CH3)3), 1.51 (d, 18H, 3J
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
G
U
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
125.49 (s, 1C, phen-C5), 128.98 (s, 1C, phen-C6a), 129.04 (s, 1C, phen-C4),
131.34 (s, 1C, phen-C7), 147.18 (br, 1C, phen-C10a), 154.43 (d, 1C, 4J-
ACHUTNGRENNUG CAHTUNGTRENNUNG
(C,P)=4.0 Hz, phen-C10b), 159.02 (d, 1C, 2J
AHCTUNGTRENNUNG
165.88 ppm (d, 1C, 2J
ACHTUNGTRENNUNG
G
G
ACHTUNGTRENNUNG
1
MeOH solvent): calcd for [M+H+]: 599.2258; found: 599.2266 (D=
1.3 ppm); IR (thin film): n˜ =3048 (w), 2949 (s), 2918 (s), 2865 (s),
1612 (m), 1579 (w), 1518 (w), 1479 (w), 1479 (s), 1436 (m), 1393 (m),
1364 (w), 1259 (w), 1091 (w), 1020 (w), 912 (m), 838 (s), 845 (m), 814 (m),
728 (w), 695 (s), 671 cmÀ1 (s); elemental analysis was precluded due to
the extraordinary sensitivity towards moisture and air.
2C, J
G
N
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3052 (w), 2945 (m), 2917 (m), 2863 (m), 1585 (w), 1478 (s), 1433 (s),
1393 (w), 1362 (w), 1180 (w), 1164 (w), 1089 (m), 1020 (m), 932 (w),
853 (m), 813 (w), 743 (s), 695 (s), 681 (w), 543 (m), 468 cmÀ1 (w); elemen-
tal analysis calcd (%) for C30H47ClN2P2Ru (634.18 gmolÀ1): C 56.82, H
7.47, N 4.42; found: C 56.93, H 7.43, N 4.36.
Synthesis of [RuH(PPhenP*-H)] (6): [RuHACTHNUTRGNEUNG(PPhenP-H)] (4; 25 mg,
0.04 mmol) was dissolved in toluene (5 mL) and was placed in Fisher–
Porter tube, pressurized with H2 (6 atm) and stirred for 16 h at RT. After
this period, all volatiles were removed in vacuo to give the product as a
brown powder. Complex 4 reacts already under low pressure (1 atm)
3412
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Chem. Eur. J. 2013, 19, 3407 – 3414

Metal–Ligand Cooperation through Aromatization/Deconjugation
FULL PAPER
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unreacted these conditions. Quantitative yield; 1H NMR (500 MHz,
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C6D6, 258C): d=À31.52 (dd, 1H,2J
A
E
3
3
Ru-H), 1.09 (d, 9H, J
(H,P)=11.3 Hz, PC
E
11.8 Hz, PC
(CH3)3), 1.40 (d, 9H,3J
E
ACHTUNGTRENNUNG
9H, 3J
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
G
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
A
E
U
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
G
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
phen-C5), 128 (superimposed, 1C, phen-C6a), 131.65 (s, 1C, phen-C7),
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144.77 (dd, 1C, 4J
A
E
4
2
1C, J
N
163.77 ppm (d, 1C, 2J
ACHTUNGTRENNUNG
calcd for [M+ÀH]: 599.2258; found: 599.2281 (D=3.8 ppm); IR (KBr
pellet): n˜ =3053 (w), 2957 (s), 2923 (m), 2901 (m), 2867 (s), 1623 (brs),
1480 (m), 1479 (s), 1436 (m), 1395 (m), 1363 (w), 1260 (s), 1181 (w),
1160 (w), 1092 (s), 1020 (s), 914 (m), 853 (m), 814 (s), 747 (w), 722 (w),
695 (m), 667 (s), 542 cmÀ1 (m); elemental analysis was precluded due to
the extraordinary sensitivity towards moisture and air.
General procedure for catalytic dehydrogenation: The catalyst (4 or 6,
0.01 mmol), toluene (2 mL), primary alcohol (5.05 mmol), amine (if
specified, 5.00 mmol), and m-xylene (1 mmol) were placed in a Schlenk
tube. The tube was equipped with a condenser, and the mixture was
heated at reflux under argon flow for the specified time. The extent of
conversion to the corresponding esters or imines was determined by GC
analysis with m-xylene as internal standard, by using a Carboxen 1000
column on a HP 690 series GC system.
CCDC-909143 (2) and CCDC-909144 (3) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This research was supported by the European Research Council under
the FP7 framework (ERC No. 246837) and by the MINERVA Founda-
tion. R.L. received a postdoctoral fellowship from the Deutsche For-
schungsgemeinschaft (DFG). M.V. received a postdoctoral fellowship
from the Schweizer Freunde des Weizmann Instituts. D.M. holds the
Israel Matz Professorial Chair of Organic Chemistry.
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Received: November 7, 2012
Published online: January 29, 2013
3414
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Chem. Eur. J. 2013, 19, 3407 – 3414