Bifunctional ruthenium(II) PCP pincer complexes and their catalytic activity in acceptorless dehydrogenative reactions

Article abstract of DOI:10.1021/om400285r

New air-stable ruthenium complex 1, bearing the dibenzobarrelene-based cooperating ligand, was synthesized in 96% yield and fully characterized. The examination of its coordination chemistry revealed that the ruthenium center in 1 is slightly distorted from octahedral geometry, and ³¹P NMR is consistent with the formation of trans-spanned pincer complexes bearing strongly bent dibenzobarrelene-based ligands. The catalytic activity of the new complex was examined in acceptorless dehydrogenation of alcohols and acceptorless dehydrogenative coupling of alcohols with amines.

Full text of DOI:10.1021/om400285r

Article
pubs.acs.org/Organometallics
Bifunctional Ruthenium(II) PCP Pincer Complexes and Their Catalytic
Activity in Acceptorless Dehydrogenative Reactions
Sanaa Musa,† Sveta Fronton,† Luigi Vaccaro,‡ and Dmitri Gelman*,†
†Institute of Chemistry, The Hebrew University, Edmond Safra Campus, Givat Ram, 91904 Jerusalem, Israel
‡Dipartimento di Chimica, Universita di Perugia, Via Elce di Sotto, 8, Perugia, Italy
S
* Supporting Information
ABSTRACT: New air-stable ruthenium complex 1, bearing the dibenzobarrelene-
based cooperating ligand, was synthesized in 96% yield and fully characterized. The
examination of its coordination chemistry revealed that the ruthenium center in 1 is
slightly distorted from octahedral geometry, and 31P NMR is consistent with the
formation of trans-spanned pincer complexes bearing strongly bent dibenzobarre-
lene-based ligands. The catalytic activity of the new complex was examined in
acceptorless dehydrogenation of alcohols and acceptorless dehydrogenative
coupling of alcohols with amines.
intramolecular interaction between the metal center and the
ligand’s side arm (Scheme 1, bottom).
INTRODUCTION
■
Cooperating ligands play an active role in reversible structural
transformations of transition metal complexes, thus modifying
their reactivity over the course of a catalytic cycle. Initially, the
ligand−metal interplay was observed in enzyme-catalyzed
processes;1 however, nowadays, the design of noninnocent
ligands for transition metal-catalyzed processes attracts much
attention.2 For example, it was found that metal-amide/metal-
amine interconversion facilitates heterolytic cleavage of non-
polar bonds.3 This discovery led to the development of a
powerful family of bifunctional catalysts for hydrogenation of
carbonyl and carbonyl-like compounds by Noyori.4 A similar
metal-amide/metal-amine interplay5 in unique aromatic PNP
pincer systems was employed by Milstein for the efficient and
green catalytic dehydrogenative synthesis of esters,6,7 acetals,8
imines,9,10 and amides11,12 from alcohols and vice versa, as well
as for hydrogenation of carbonic acid derivatives.13 Aliphatic
PNP pincer complexes catalyze transfer hydrogenation of
ketones and imines,14 acceptorless dehydrogenative trans-
formations of alcohols to esters and amines,15,16 hydrogenation
of esters,17,18 and dehydrogenation of ammonia-borane adducts
via a ligand−metal cooperation mechanism.19,20 Other unique
processes have been also described.21 Shvo20,22 and Yamaguchi
and Fujita23 exploited a different ligand−metal cooperation
mechanism for acceptorless dehydrogenation of alcohols and
related processes under very mild conditions.
Scheme 1. Activation and Formation of Polar and Nonpolar
Bonds by Ligand−Metal Cooperation in PC(sp3)P-Based
Pincer Complexes
In this article, we extended our studies to ruthenium
complexes, and we wish to report on the synthesis, character-
ization, and properties of a ruthenium complex bearing the
dibenzobarrelene-based cooperating ligand and its catalytic
activity in selective dehydrogenative coupling of primary and
secondary alcohols to form imines (Scheme 2, top).
This transformation is of great importance, as imines are
versatile synthetic intermediates that are used in both
laboratory synthesis and industrial production.26 They are
typically prepared from carbonyl compounds and amines by
employing acid catalysts or by means of one-pot oxidation/
condensation cascade. However, the traditional methods either
Some time ago, our group reported on a new family of
PC(sp3)P pincer cooperating catalysts24,25 used for efficient
acceptorless dehydrogenation of alcohols to carbonyl and
carboxyl compounds (TON = 3600) and transfer hydro-
genation of ketones (TON = 100 000). These catalysts take
advantage of their unique three-dimensional shape and can
activate/form chemical bonds either via dissociation/associa-
tion of a carbon−metal bond (Scheme 1, top) or via
Received: April 6, 2013
© XXXX American Chemical Society
A
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Scheme 2. Dehydrogenative Coupling of Primary Alcohols
with Amines
RESULTS AND DISCUSSION
■
Complex 1 was prepared by the reaction of the previously
reported bifunctional ligand with Ru2Cl4(CO)6 in chloroform
at room temperature (Scheme 3). Analysis of the reaction
mixture by 31P NMR indicated the presence of three isomers of
the target compound in 1:0.3:0.14 ratio, apparently different in
the position of the chloride ligand (the major doublets were
located at ca. 51 ppm, JP−P = 281 Hz).
Observation of the phosphine donor signals as a set of
doublets in the 31P NMR spectra in this complex confirmed the
nonequivalence of the two phosphorus nuclei, while the
magnitude of the coupling constant (J = 281 Hz) supported
their mutually trans disposition.32
X-ray crystallographic analysis of the species was undertaken
(Scheme 3). As one would expect for the complexes bearing
strongly bent dibenzobarrelene-based ligands, the ruthenium
center in 1 is slightly distorted from the octahedral geometry.
For example, the observed interplanar angle between the
C(1)−Ru(1)−P(1) and C(1)−Pd(1)−P(2) planes is 157.2°,
while the C(1)−Ru(1)−C(20) angle is 173.34(2)°.
The catalytic activity of the new species in acceptorless
dehydrogenation of alcohols and related reactions has been
checked and compared to that of the previously published
systems. In particular, the dehydrogenative homocoupling of
primary alcohols to form esters and dehydrogenative coupling
of primary alcohols with amines were targeted.
In general, the reactivity of 1 was found to be moderate in
comparison with the Ir-based catalysts of this class (e.g., as in
Scheme 1). Thus, oxidation of benzyl alcohol to benzyl
benzoate in the presence of 5 mol % of Cs2CO3 as a base in p-
xylene with heating under reflux and in a N2 atmosphere led to
the formation of the desired product in 90% yield after 24 h.
Similar results were obtained with p-methylbenzyl alcohol and
1-hexanol (Table 1). However, a relatively high catalyst loading
require harsh conditions or generate oxidation waste.27 Thus,
the development of efficient, environmentally benign, and
atom-economical synthesis of imines is welcome.
Recently Milstein and co-workers suggested selective imine
synthesis directly from primary alcohols and amines using a
pincer Ru(II) complex bearing 2,6-bis(di-tert-
butylphosphanylmethyl)pyridine (2) as a catalyst (Scheme 2,
bottom).9,28 Interestingly, substitution of the ligand with 2-(di-
tert-butylphosphanylmethyl)-6-(diethylaminomethyl)pyridine
(3) reversed the selectivity of the reaction toward the amide
formation.12 Hanson and co-workers reported a versatile cobalt
PNP-based catalyst capable of catalyzing the imine formation
under relatively mild conditions.29 Reports from other groups
on similar reactions are also available.30,31
Thus, in this article we wish to compare the catalytic activity
of the new Ru catalyst with the previously reported
compounds.
a
Scheme 3. ORTEP Drawing (50% probability ellipsoids) of the Structures 1
a
Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1−C1 (2.235(3)), Ru1−P1
(2.3588(9)), Ru1−P2 (2.3380(9)), Ru1−Cl1 (2.4541(9)), Ru1−C19 (1.851(4)) and Ru1−C20 (1.929(4)). P1−Ru1−P2 (162.93(3)), C1−Ru1−
C20 (173.78(15)), C1−Ru1−C19 (93.22(16)), C19−Ru1−C20 (92.96(18)), Cl1−Ru1−C19 (175.40(13)), C1−Ru1−Cl1 (83.48(9)), Cl1−Ru1−
P2 (84.20(3)), Cl1−Ru1−P2 (84.74(3)).
B
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a
Table 1. Representative Dehydrogenative Coupling Results
a
Alcohol (1 mmol), amine (1,2 mmol), base (0.05 mmol), and 1 (0.02 mmol) in refluxing p-xylene. Yield was determined by NMR (using
diphenylmethane as an internal standard).
Scheme 4. Suggested Mechanism of the Metal−Ligand Cooperation in 1
Scheme 5. Catalytic Activity of 1
C
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(2 mol %) was required in order to drive the reaction to
completion in a reasonable time. To compare, the analogous Ir-
based catalyst published by us in the past25 promotes this
reaction in 6 h at only 0.1 mol % loading. The state-of-the-art
alcohol dehydrogenation catalysts also demonstrate signifi-
cantly higher TONs.7,18,33 The highest TON demonstrated by
1 was ca. 2100 (0.04 mol % of the catalyst).
Mechanistically, the transformation is likely to proceed via
(a) a ligand exchange step, leading to the arm-opened
ruthenium alkoxide species; (b) generation of the Ru−H
species via β-hydride elimination with subsequent formation of
the oxidized product; and (c) a H2-forming step, leading to the
formation of the catalytically active arm-closed ruthenium
species (Scheme 4).25 Indeed, hydrogen formation was
observed upon stoichiometric interaction of 1 with 1-phenyl-
ethanol.
Noteworthy, the moderate catalytic activity of the catalyst 1
can be explained by the slow dissociation of the CO ligand
required to create a vacant coordination site.
Dehydrogenation of benzyl alcohols in the presence of 1.2
equivalents of benzylamine under the same conditions led to
the 1:1:1 mixture of N-benzyl benzamide, N,N-dibenzyl amine,
and N-benzylidene benzylamine (Scheme 5).
The distribution of these products can be rationalized as
follows: benzaldehyde formed via dehydrogenation of the
starting benzyl alcohol forms hemiaminal upon being attacked
by benzylamine. The hemiaminal can be either dehydrogenated
to produce amide or dehydrated to imine. Dibenzyl amine,
apparently, originates from hydrogenation of the corresponding
imine.
Although our attempts to shift the chemoselectivity of the
transformation toward the formation of amide failed, we were
able to establish the reaction conditions for the selective
synthesis of imines. We found that the nature of the base is
crucial for the selectivity of the reaction. Thus, we found that
replacement of Cs2CO3 with amidine bases, such as DBU,34
results in the complete conversion of the benzyl alcohol/
benzylamine mixture to N-benzylidene benzylamine in the
presence of 2 mol % of 1 in refluxing xylene. A maximal TON
of ca. 200 (under 0.05 mol % catalyst loading) was observed.
Here again, the performance of 1 is less impressive than of the
state-of-the-art catalysts.9,16,31
EXPERIMENTAL SECTION
■
All manipulations were performed using standard Schlenk techniques
under an atmosphere of dry N2. Anhydrous solvents, ruthenium salts,
and complexes as well as all chemicals used in the catalytic experiments
were purchased from Aldrich and used without further purification.
Ligands were prepared following published procedures. NMR spectra
were recorded on a Bruker instrument operating at 400 MHz for
protons, 100 MHz for carbons, and 121 MHz for phosphorus.
Diffraction data were collected with a Bruker APEX CCD instrument
(Mo Kα radiation (λ = 0.71073 Å)). Crystals were mounted onto glass
fibers using epoxy. Single-crystal reflection data were collected on a
Bruker APEX CCD X-ray diffraction system controlled by a Pentium-
based PC running the SMART software package. The integration of
data frames and refinement of cell structure were done by the SAINT+
program package. Refinement of the structure on F2 was carried out by
the SHELXTL software package. Further information may be found
within CIF files provided as Supporting Information.
Synthesis of 1. A solution of Ru2Cl4(CO)6 (40 mg, 0.078 mmol)
and the ligand (100 mg, 0.157 mmol) in ca. 3 mL of chloroform was
stirred at room temperature overnight under inert gas. Evaporation of
the solvent afforded 125 mg (96%) of a yellow solid (three isomers in
a 1:0.3:0.14 ratio). Single crystals of the major isomer suitable for X-
ray analysis were grown by slow evaporation of the saturated CH2Cl2
solution. 1H NMR (CDCl3) δ: 1.75 (t, 2H, J = 10 Hz), 2.61 (t, 2H, J =
10 Hz), 2.90 (d, 1H, J = 10 Hz), 3.36 (m, 1H), 4.35 (s, 1H), 7.3−7.5
(m, 18H), 7.75 (m, 6H), 8.02 (t, 2H, J = 7 Hz). 31P NMR (CDCl3) δ:
51.5 (dd, J = 281 Hz). Anal. Calcd for C44H37ClO4P2Ru: C 63.81; H,
4.50. Found: C 63.33; H 4.28. IR (KBr): 3395.2 (bs, OH), 2122.3,
2036.4 (CO).
General Procedure for Acceptorless Dehydrogenation of
Alcohols. A 10 cm3 round-bottomed flask equipped with a double
surface condenser was charged with 1 (1.1 × 10−2 mmol, 2 mol %)
and Cs2CO3 (89.3 mg, 0.27 mmol, 5 mol %) in p-xylene (1 mL) under
Ar. Alcohol (5.5 mmol) was injected, and the mixture was heated
under reflux with stirring for the specified time. The solvent was
removed under reduced pressure, and the product was isolated by
filtration through a pad of silica.
General Procedure for Acceptorless Dehydrogenative
Coupling of Alcohols and Amines. A 10 cm3 round-bottomed
flask equipped with a double surface condenser was charged with 1
(1.1 × 10−2 mmol, 2 mol %) and DBU (0.27 mmol, 5 mol %) in p-
xylene (1 mL) under Ar. Alcohol (5.5 mmol) and amine (6.5 mmol)
were injected, and the mixture was heated under reflux with stirring for
the specified time. The solvent was removed under reduced pressure,
and the product was isolated by filtration through a pad of silica and
analyzed.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic files in CIF format for all structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
The method showed satisfactory generality, and we
demonstrated that a variety of amine/alcohol combinations
result in the formation of the corresponding imines in 60−98%
yields (Table 1). Lower yields (25−40%) were obtained in the
runs with aliphatic and, especially, bulky amines. Noteworthy as
well, no racemization was observed when chiral amines were
employed.
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: dgelman@chem.ch.huji.ac.il. Fax: (+) 972-2-6585279.
■
CONCLUSIONS
Author Contributions
Equal contribution by Sanaa Musa and Sveta Fronton.
■
We synthesized and characterized new ruthenium complexes
bearing a bifunctional PC(sp3)P ligand. The new compounds
demonstrated acceptable catalytic activity in acceptorless
dehydrogenation of alcohols to form esters and acceptorless
dehydrogenative coupling of alcohols with amines to form
imines. The activity of the new compounds is currently lower
than that of other recently developed catalysts of this type;
however, the modularity of our ligand gives hope to develop
more active catalysts for this and other transformations.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was partially supported by the Israeli Science
Foundation (ISF) and the Israeli Ministry of Science,
Technology, Israel−Italy. We thank Dr. Shmuel Cohen and
Mr. Benny Boguslavsky for solving the X-ray structures.
D
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