Half-Sandwich Osmium(II) Complexes with Bidentate N,N-Chelating Ligands and Their Use in the Transfer Hydrogenation of Ketones

Article abstract of DOI:10.1002/ejic.201601249

The reaction of appropriate N,N-bidentate ligands with the [(η⁶-p-cymene)Os(μ-Cl)Cl]₂dimer (p-cymene = C₁₀H₁₄), followed by metathesis reaction with NH₄PF₆, gave the new osmium(II) arene complex salts [(η⁶-p-cymene)OsCl(C₅H₄N-2-CH=N–R)]PF₆, where [R = tert-butyl (1), isopropyl (2), 2,6-dimethylphenyl (3), or 2,6-diisopropylphenyl (4)]. The dimer was also reacted with the N,N′-bidentate ligands di-(2-pyridyl)amine (5), 4-phenyl-3,6-di(2-pyridyl)pyridazine (6), 4,4′-di-tert-butyl-2, 2′-bipyridine (7), and 5,5′-dimethyl-2,2′-bipyridine (8). In addition, the reaction of the precursor [(η⁶-C₆H₆)Os(μ-Cl)Cl]₂with the N,N′-bidentate ligands gave [(η⁶-C₆H₆)OsCl(N,N)]₂where N,N = 4,4′-di-tert-butyl-2,2′-bipyridine (9), 5,5′-dimethyl-2,2′-bipyridine (10), 3,6-bis(2-pyridyl)-4-phenyl pyridazine (11), or di-(2-pyridyl)amine (12). The compounds were characterized by using¹H and¹³C NMR, UV/Vis, FTIR spectroscopy and elemental analysis. The single-crystal X-ray structures for compounds 1, 4, 8, 10, 11, and 12 showed that the osmium(II) complexes adopted the classical three-legged piano stool geometry. These osmium(II) compounds were found to be effective catalysts for the transfer hydrogenation of ketones into alcohols with NaOH as base and 2-propanol as the solvent and hydrogen source. A range of cyclic, aromatic, and aliphatic ketones was studied and good turnover numbers achieved.

Full text of DOI:10.1002/ejic.201601249

A Journal of
Accepted Article
Title: A study of new half-sandwich Osmium(II) complexes with
bidentate N,N-chelating ligands and their use in the transfer
hydrogenation of ketones
Authors: Joel Gichumbi, Bernard Omondi, and Holger Bernhard
Friedrich
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601249
Link to VoR: http://dx.doi.org/10.1002/ejic.201601249

European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
A study of new half–sandwich Osmium(II) complexes with
bidentate N,N-chelating ligands and their use in the transfer
hydrogenation of ketones.
Joel M. Gichumbi, Bernard Omondi, and Holger B. Friedrich *[a, b]
[
a]
[a]
Dedication ((optional))
N
=4, 4’-ditert-butyl-2,2’-bipyridine (9); 5,5’-dimethyl-2,2’-bipyridine
Abstract: The reaction of appropriate N,N-bidentate ligands with
(10), 3,6-bis(2-pyridyl)-4-phenyl pyridazine (11) and di-(2-
6
the [(η -p-cymene)Os(μ-Cl)Cl]
2
dimer (p-cymene = C10
PF , gave the new osmium(II) arene
N-2-CH=N-R]PF , where (R =
H14), followed
1
pyridyl)amine (12). The compounds were characterized using H and
by metathesis reaction with NH
4
6
13
C NMR, UV-Vis, FT- IR and elemental analysis. The single crystal
6
complex salts [(η -p-cymene)OsCl(C
H
5 4
6
X-ray structures for compounds 1, 4, 8, 10 11 and 12 showed that the
osmium(II) complexes adopted the classical three legged piano stool
geometry. These osmium(II) compounds were found to be effective
catalysts for the transfer hydrogenation of ketones into alcohols with
NaOH as base and 2-propanol as the solvent and hydrogen source.
A range of cyclic, aromatic and aliphatic ketones was studied and
good turnover numbers achieved.
tert-butyl (1), iso-propyl (2), 2, 6-dimethylphenyl (3) and 2,6-diiso-
propylphenyl (4)). The dimer was also reacted with N,N’-bidentate
ligands di-(2-pyridyl)amine (5), 4-phenyl-3,6-di(2-pyridyl)pyridazine
(6), 4, 4’-ditert-butyl-2, 2’-bipyridine (7); 5, 5’-dimethyl-2, 2’-bipyridine
6
(
8). In addition, the reaction of the precursor [(η -C
6 6
H )Os(μ-Cl)Cl]
2
6
with the N,N’-bidentate ligands gave [(η -C
6 6
H )Os Cl(N-N)]
2
where N-
Introduction
with amino carboxylate ligands have been synthesized and
reported to have good activity in the TH of aromatic ketones [5]
.
Transfer hydrogenation (TH) involves the addition of hydrogen
from non- hydrogen (H ) sources to unsaturated molecules. It is a
Also, Baratta and coworkers prepared pincer complexes of Ru
and Os with benzo-quinolone ligands and reported the complexes
as efficient catalysts for TH of carbonyl compounds [6]. They also
synthesized CNN osmium pincer complexes and applied them in
the TH of ketones [7]. In addition, Vega and coworkers prepared
enantiopure osmium(II) pybox complexes, where pybox = 2,6-
bis(4’-(S)-isopropyl oxozolin 2’yl)pyridine, and applied then in TH
2
powerful and convenient route to a number of hydrogenated
compounds. The TH method has the advantage over direct
2
hydrogenation in that it doesn’t need pressurized H gas, which
can be hazardous nor complex experimental set-ups. In addition
the hydrogen donors are readily available and easy to handle, and
relatively cheap [1]
.
where they reported high conversions and enatioselectivities [8]
.
Compared to the other Group 8 transition metals, osmium has
been less studied than iron [1] and ruthenium [2] for catalytic
applications, because osmium is considered to have slower
ligand exchange kinetics which should cause lower catalytic
activity [3]. In particular, osmium-catalyzed TH has been rarely
studied, because osmium catalysts are generally regarded as
being less active than their ruthenium, rhodium and iridium
analogues [1, 3,4]. However, osmium(II) half sandwich complexes
Moreover, Putignano and co-workers synthesized osmium
complexes containing 1, 1’-bis (diphenyl phsosphino) ferrocene
(
dppf) and ampy (2-aminomethyl pyridine) moieties and they
reported that the complexes efficiently reduce various ketones
and aldehydes by means of TH [9]
.
Half-sandwich complexes are one of the most studied class of
after organometallic compounds because of their potential
applications in many areas of chemistry [10]. Research has shown
that the type of arene, the nature of the chelating ligands and
leaving groups in these compounds can significantly influence
their chemical and biological activity and they show structure
activity relationships [11]. In this study, in continuation of our study
[
[
a]
b]
School of Chemistry,
University of KwaZulu-Natal,
Private Bag X54001,
Durban 4000,
South Africa
on catalytic applications of half-sandwich compounds with
bidentate ligands [12], we describe the preparation of new half-
*Corresponding author. Tel.: +27312603107;
fax: +27 312603091.
E-mail address: friedric@ukzn.ac.za (H.B. Friedrich);
catalysis.ukzn.ac.za
sandwich osmium(II) complexes with nitrogen based
bidentate ligands and then use of these complexes in the transfer
hydrogenation of ketones.
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European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
which showed two doublets at ca. 6.48- 5.44 ppm. For complexes
5-12 there was a downward shift of the CH=N pyridine proton
peak by ca. 0.3 ppm, which may be attributed to the CH=N group
donating electrons to the osmium metal. The 13C NMR spectra
also showed a downward shift of the corresponding carbon from
δ 149.4-155.5 ppm to δ 152.9-156.0 ppm, which may also be
attributed to the donation of electrons from the CH=N group to the
osmium. For complexes 9-12, complex formation was also
accompanied by a shift of the C H arene protons, from δ 6.15
Results and Discussion
Synthesis and characterization of the osmium(II)
complexes.
The osmium(II) dimers [(η - p-arene)Os(μ-Cl)Cl]
6
2
, where arene =
p-cymene (C10H14) or benzene (C
6
H
6
)
,
react at 40° C in methanol
with the N,N-donor ligands (Scheme 1) and, after metathesis with
6
a PF
6
salt, give the compounds [(η -p-cymene)Os (N-N)Cl]PF
6
in
good yields ranging from 73 to 85%. All the complexes were
soluble in acetonitrile, acetone, THF and DMSO, but were not
soluble in hexane, diethyl ether and toluene.
6
6
ppm in the precursor to δ 6.40 - 6.29 ppm for the complexes.
The formation of complexes 1-4 was also supported by observing
clear shifts of the proton CH=N and carbon NMR signals of the
imine between the complex and free ligands. Thus, the proton
signal of the imine in the complex shifted downfield relative to that
of the free ligand, due to the imines proton being deshileded
because by the nitrogen donates a lone pair of electrons to the
osmium [12, 14]. Also, the 13C NMR data supported complexes had
formed in that the carbon peak of the imine shifted ca. 6-10 ppm
downfield for the complexes 1- 4 relative to the shift of the free
ligand.
In complexes 1-4, 6 and 11, the osmium centre is a stereogenic
centre. Thus, these complexes are obtained as racemates and
this is supported by the x-ray crystal structures of 1,4 and 11
where both enantiomers are present.
The IR spectra of complexes 1-4 showed a strong absorption
band in the range of 1615-1617 cm-1 due to the symmetrical
vibration of the C=N bond, which had moved to lower wave
numbers than the equivalent band of the uncoordinated pyridine-
-1
imine ligands (1632 -1646 cm ), indicating complex formation.
The IR spectra of the complexes 5-12 exhibit bands due to the
chelated N,N-donor bidentate ligands in the 1626 to 1384 cm-1
region, corresponding to the different stretching frequencies of the
C=C and C=N bonds of these complexes. Also, the infrared
spectra of 1-12 showed a strong band between 836 and 847 cm-
6
Scheme 1. The reaction of [(η -arene)Os(μ-Cl)Cl]
2
with the N,N-donor ligands
in methanol
1,
due to the stretching frequency ν(P-F), of the PF
counter ion
6
The resonance patterns in the H and 13C NMR spectra for the p-
cymene in complexes 1-4 and 6 differed from that of the p-cymene
precursor. Thus, the aromatic protons of the p-cymene ligand in
these complexes give four doublets at ca. 6.61-4.84 ppm instead
of the two doublets found for p-cymene in the precursor. This new
pattern can be attributed to a loss of symmetry in the complex
caused by slow rotation of the cymene ligand [13]. This loss of
symmetry was, however, not observed for complexes 5, 7 and 8,
1
[12, 14-15]
of the complexes. The ESI-MS of complexes 1-12
revealed only one M/Z peak which matched the expected
+
molecular ions [M ]. Also, the characteristic osmium isotopic
pattern was seen, which further confirmed that the complexes had
formed.
UV-Vis spectra of the complexes 1-12, in acetonitrile, are shown
in Figure 1. They display bands at 387-420 nm of low intensity,
medium intensity at 294-350 nm and high intensity at 259-267 nm.
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European Journal of Inorganic Chemistry
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The bands at around 400 nm can be assigned to a metal-ligand
charge transfer transition (MLCT) (dπ-π*bands), due to the low
6
spin d configuration of the Os(II) complexes which provide filled
orbitals with correct symmetry that interact with the low lying π*
orbitals of the ligand. The bands of medium and high intensity are
assigned to the ligand centred π-π* and n-π* transitions,
respectively, in agreement with observations on complexes with
similar N, N’ bidentate ligands [12, 14]
.
Figure 2. TGA traces of 1-12
X-ray crystallography
The structures of complexes 1, 4, 8, 10 11 and 12 were
determined using single crystal X-ray diffraction and are shown in
Figures 3-8 while their selected bond distances and angles are
provided in Table 1 (Appendix). Compounds 4 and 8 crystallize
as red blocks while 1, 10, 11 and 12 all crystallize as red prisms.
All complexes have ligands that feature diversity in the
conformation and in the electronic and steric effects. Complexes
1
and 4 have one pyridinyl N atom and an amine N atom, the
difference between 1 and 4 being a tertiary butyl group and a
substituted phenyl ring attached to the amine groups in the two
respectively. The arene rings in the two complexes (p-cymene)
are the same. 8, 10 and 11 all have both coordinating N-atoms
constrained (pyridinyl) and while the arene rings in 8 and 10 are
different (p-cymene in 8 and benzene in 10 as in 11), the ligand is
the same. The ligand in 11 has extra heteroatom rings but rigid
while that in 12 has constrained pyridinyl N atoms even though it
has a flexible linker between the two pyridinyl rings. In the
arrangement described above, complexes 1, 4, 8, 10 and 11 all
have a planar 5-member metallacyle while 12 has a non-planar 6-
member metallacycle.
Fig 1. UV-Vis absorption spectra of the mononuclear complexes 1-12 in
acetonitrile at 298 K.
The TGA data were obtained from 30 °C to 700 ˚C (Figure 2). The
thermograms of the complexes show two key mass loss steps.
The first mass loss occurred between 180 and 350 ˚C, reflecting
an approximate mass loss of 30%. This is attributed to the loss of
the bidentate ligand. The second decomposition step which
occurs , of about 20%, is likely due to the loss of the arene ring.
The five-member metallacycle is co-planar with the pyridinyl rings
in 1 and 4. In 4 the 2,6-diisopropyl plane is twisted and forms a
dihedral angle of 84.83(13)º with the metallacycle pyridine ring
plane. In 8 the two pyridinyl rings are coplanar with a dihedral
angle of 3.17(8)º while in 10 the dihedral angle is 11.75(1)º. The
amine bridge in 12, makes the two pyridine rings non-coplanar.
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European Journal of Inorganic Chemistry
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The choice of the arene ring seems not to affect the Os—AreneCg
distance much even though the ligands contribute somewhat this
distance. The distance is longer in 1 and 4 (1.6957(4) and
member metallacycle compound 12 while the N—Os—Cl bond
angles are pretty much the same with a smaller range between
75.92(10) Å to 86.66 (10) Å [16a, 17]
.
1
.7007(3) respectively) compared to 8, 10 and 11 (1.6791(3),
.6750(4) and 1.6823(4) respectively) and much shorter in 12
1
t
i
(
1.6572(4) Å). 1 and 4 have the Bu and PrPh groups which
contribute electronically and sterically, while 12 has a wider
metallacycle.
Apart from complex 10, all the other complexes crystallize with
6
+
one molecule of the cation [(η -p-cymene)ClOs(N-N))] and one
molecule of the anion (PF -
) in the asymmetric unit. Complex 10,
6
crystallizes in the highly symmetrical Cmca space group with one
whole 5,5'-dimethyl-2,2'-bipyridine ligand, the Cl atom, only a half
of the Os-arene moiety dissected at orthogonally (along the bc-
plane) to the arene ring as the cationic species, and only three
6
quarters of an also disordered PF as the anionic species. The
Figure 3. The molecular structure of 1 with the atomic numbering scheme. The
other half is generated by a mirror plane along the dissection
plane of the Os-arene moiety thus producing a reflection of the
displacement ellipsoids are drawn at a 50% probability level. The disordered
PF anion has been left out together with H atoms for clarity reasons.
6
5,5'-dimethyl-2,2'-bipyridine ligand and the Cl atom (see Fig 6).
This is in turn inverted through a center of inversion situated at
the mid-point of the crystallographic b axis. This is therefore a
perfect example of reflection positional disorder of a part of the
molecule accompanied further inversion positional disorder of
whole disordered molecule involving reflection and inversion
symmetry operations. The Os(II) centre is coordinated to the N,Nʹ-
bidentate ligands through the pyridinyl N atoms and the imine N
atoms in 1 and 4 and through two pyridinyl N-atoms in 8, 10, 11
and 12 in a bidentate manner. The two N atoms in each of the six
ligands together with a Cl atom occupy three coordination sites of
the Os(II) center while the C atoms of the p-cymene or the arene
rings occupy the remaining three sites of what is normally
described as a “pseudo-octahedral” geometry. In this kind of
arrangement, the Cl and two N atoms in each cationic species act
as the base and the arene ring forms the seat of a three-legged
piano stool” structure
Figure 4. The molecular structure of 4 showing the atomic numbering scheme.
The displacement ellipsoids are drawn at 50% probability level. The
disordered PF anion has been left out together with H atoms for clarity reasons.
a
The Os—Cl distance of the complexes range between 2.3871(8)
Å to 2.411(12) Å while the Os—N bond distances range between
6
2.053(3)Å and 2.108(2) Å and these values are comparable to
[
16]
reported values for similar compounds
and to osmium
complexes with N,N-bidentate ligands [16c, 17] respectively. The
N—Os—N bond angle seem not to be affected by the ligand or
the benzene ring and ranges between 75.93(11)° to 79.14(11)° in
compounds 1, 4, 8, 10 and 11 but is slightly higher in the six-
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European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
Figure 8. The molecular structure of 12 with the atomic numbering scheme. The
Figure 5. The molecular structure of 8 with the atomic numbering scheme. The
displacement ellipsoids are drawn at a 50% probability level. The disordered
displacement ellipsoids are drawn at a 50% probability level. The disordered
6
PF anion has been left out together with H atoms for clarity reasons..
6
PF anion has been left out together with H atoms for clarity reasons.
Figure 6. The molecular structure of 10 with the atomic numbering scheme. The
displacement ellipsoids are drawn at a 50% probability level. The disordered
6
PF anion has been left out together with H atoms for clarity reasons.
Figure 7. The molecular structure of 11 with the atomic numbering scheme. The
displacement ellipsoids are drawn at a 50% probability level. The disordered
6
PF anion and the disordered acetone molecule have been left out together with
H atoms for clarity reasons.
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European Journal of Inorganic Chemistry
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Table 1. Selected bond lengths (Å) and angles (º) for the complexes 1, 4, 8, 10, 11 and 12
1
4
8
10
11
12
Bond Lengths
Os(1) —N(1)
2.087 (2)
2.108 (2)
2.394 (7)
2.101 (3)
2.086 (4)
2.3917 (13
2.079(4)
2.084(4)
2.411(12)
2.089(3)
2.087(2)
2.402(8)
2.089(3)
2.053(3)
2.3871(8)
2.096(6)
2.101(6)
2.402(2)
Os (1) —N(2)
Os (1) —Cl(1)
Bond angles
N(1) —Os(1) —N(2)
76.27(7)
85.85 (5)
84.43 (5)
75.96 (15)
75.92 (10)
88.66 (10)
76.93(14)
82.61(11)
83.16(10)
76.55(10)
82.76(7)
84.34(7)
75.93 (11)
85.23 (7)
81.66 (8)
83.3 (2)
N(1) —Os(1) —Cl(1)
N(2) —Os(1) —Cl(1)
86.07 (19)
85.44 (16)
Catalytic results discussion
Optimization studies on transfer hydrogenation reactions using half-sandwich ruthenium complexes revealed that the best yields were
obtained at 82 °C using isopropanol as the solvent, NaOH as the base and 0.05 mol % of the catalyst [12]. Thus these conditions were
used as a basis to study the transfer hydrogenation of cyclohexanone using compounds 1-12. The results are presented in Table 2.
O
OH
1. base + catalyst
2. 2-propanol, 82°C
Table 2: TH of cyclohexanone catalyzed by catalysts 1-12 in 2-propanol (substrate/catalyst/base ratio is 10/0.005/4).
Catalyst
Conversion(%)
Yield
91
96
94
96
98
96
96
90
97
92
98
97
Time (h)
TONa
1916
1925
1884
1911
1914
1926
1912
1800
1932
1838
1920
1918
TOFb
639
481
471
478
638
642
637
600
483
460
640
639
1
2
3
4
5
6
7
8
9
97
97
99
97
99
97
98
98
99
99
99
99
3
4
4
4
3
3
3
3
4
4
3
3
10
11
12
a
=
Turnover number = mol of product/mol of catalyst,
b
=
Turnover frequency = mol of product/mol of catalyst/hour
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European Journal of Inorganic Chemistry
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When comparing the effects of the different arene groups and
bidentate ligands, on the transfer hydrogenation of
cyclohexanone, it is seen that the osmium cymene catalysts with
the ligands 2-methyl-N-(2-pyridinyl methylene)-2-propanamine
(Entries 1 and 2) likely due to steric reasons. A similar trend has
[
12,
18]. Indeed, the ketone
been seen for different systems
substituted by the methyl group in the 2-position (entry 2) showed
the lowest reactivity of the cyclic compounds examined, likely
caused by a combination of the steric influence of the substituent
α to the C=O group and the electronic contribution of the methyl
group [12, 18]. Entries 3-5 show that straight chain ketones (entries
3-5) took longer to react than their cyclic counterparts (entries 1
and 2). This is likely due to the C=O group being more accessible
in the more rigid cyclic compounds, in comparison to the reactive
site of the straight chain ketones, which is more sterically
hindered.
(
1), di-(2-pyridyl)amine (5), 4-phenyl-3,6-di(2-pyridyl)pyrazine (6),
, 4’-ditert-butyl-2, 2’-bipyridine (7) and 5, 5’-dimethyl-2, 2’-
4
bipyridine (8) gave highest conversions within 3 hours and thus
the highest TONs and TOFs. For those complexes with N-(2-
pyridinyl methylene)-2-propanamine (2) and 2,6-(dimethyl
phenyl)-pyridin-2-propanamine (3) and 2,6-diisopropyl-N-(2-
pyridylmethylene)-aniline (4) substituents on the ligand the
reaction took 4 hours to reach completion. The high TONs and
TOFS shown by the osmium cymene complexes 1,5, 6, 7, 8 and
osmium benzene complexes 11 and 12 can be attributed to the
higher electron density on the osmium, which weakens the
osmium chloride bond and hence favors the formation of the
postulated osmium hydride intermediate. On comparing the
osmium cymene and benzene moieties with the same bidentate
ligands, 7& 9 and 8 & 10 the cymene complexes perform much
better. This can be attributed to the electron donating methyl
groups donating electrons to the osmium metal which might
weaken the Os-Cl bond making it easier for the formation of the
Os-hydride intermediate species. This was supported by the X -
ray crystal structure for compounds 8 and 10 with the same
bidentate ligand, where in the compound 8 with the longer and
hence weaker Os-Cl bond the reaction occurred faster and had
higher TON and TOF. The increase in electron density would also
make the hydride more reactive and hence the reaction would
reach completion faster. Therefore, the loss of the chlorine may
be rate determining step in the proposed mechanism. However,
when the complexes with the same bidentate ligand 6 &11 and 5
When comparing the linear ketones, 3-pentanone and 2-
pentanone (entries 3 and 4), the steric hindrance in 3-pentanone
resulted in a longer reaction time to reach completion (20 h over
6
and 21 h over 11) as compared 2-pentanone (16 h for 6 and 17
h for 11). Also looking at entries 4 and 5, where both compounds
have equal steric hindrance on one side of the ketone group, 2-
methyl-3-butanone which has a stronger electron donating group,
(
entry 5), took longer to react to completion than the linear 2-
pentanone. This is likely due to the electron donating methyl
groups which donate electrons to the C=O group which make it
harder to reduce.
The electronic effect of substituting acetophenone at the 4-
position with an electron withdrawing chloro group (entry 7) and
an electron donating methoxy group (entry 8) was also
investigated (Table 3). 4-Chloro acetophenone required
significantly less time as compared to acetophenone to react to
completion, which is likely due to the electron withdrawing chloro
group withdrawing electrons from the C=O moiety, making it more
susceptible to attack. The reaction of the 4-methoxy substituted
acetophenone, in contrast, took ten hours to reach completion
and this is due to the the methoxy group donating electrons to the
C=O group, making its reduction more difficult, as also noted for
entry 2 discussed previously. As noted, for the reactions of
cyclohexanone, the catalyst with the cymene group, 6, showed
higher TOFs for all the reactions than the catalyst where the arene
group is benzene, likely due to the increased electron density on
the metal.
&
12 with very electron pushing ligands, the effect of the arene is
less dominant.
The transfer hydrogenation reactions was then studied over a
range of ketones with catalysts 6 and 11, because they gave the
highest TON and TOF for the osmium cymene and benzene
complexes respectively, in order to gain insight into the generality
and scope of this catalytic system (Table 3). These ketones were
selected to allow a study of potential effects that steric and
electronic variations on the substrate backbone may have.
Moving the substituent from the
2 to the 4 position of
cyclohexanone caused a substantial increase in catalytic activity
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European Journal of Inorganic Chemistry
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FULL PAPER
Table 3. Transfer hydrogenation of ketones catalyzed complex 6 and 11
O
OH
1.Base + Catalyst
R1
R2
R1
R2
2. 2-propanol, 82°C
Entry
Substrate
Conversion (%)
Yield (%)
Time (h)
TONb
6
TOFc
6
11
6
11
6
11
11
6
11
1
2
99
98
90
86
3
3
1783
1718
594
572
O
55
54
52
52
12
13
1045
1047
87
81
O
3
4
99
99
91
91
81
80
76
89
20
16
21
17
1600
1669
1592
1721
80
76
O
104
101
O
5
6
7
8
97
93
96
80
96
93
95
83
83
90
87
74
80
87
85
77
22
24
1596
1724
1732
1486
1590
1737
1670
1544
75
66
O
6
7
287
866
149
248
560
128
O
2
3
O
Cl
10
12
O
O
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
As previously mentioned, the mechanism of the transition metal
catalyzed hydrogenation of ketones has been proposed to involve
the formation of a metal hydride species from the catalyst
precursor [1]. In our efforts to confirm this mechanism of the
reaction, attempts to establish the formation of an Os-H species
reported here [5]. However, the complexes were more active when
compared to the enantiopure osmium(II) pybox complexes
reported by Vega and coworkers who required higher catalyst
loadings of 0.20- 0.6 mol% [8]. Furthermore, the complexes
performed better than the six-coordinate chloro(hydrido) and
dihydrido metal derivatives reported by Schlunken and
coworkers, who used 0.1 mmol of catalyst and the reactions took
from 16 h-7 days to reach completion and where some were found
to be inactive in the transfer hydrogenation of ketones [21]. When
the complexes 2-4, 9 and 10 are compared to equivalent
ruthenium compounds [12], the complexes are less active, giving
lower TOF values, the likely due to the slower ligand exchange
kinetics [3]. However, for complexes 1,5-8 and 11 and 12
complexes are as active as the equivalent ruthenium compounds
1
by H NMR spectroscopy were not successful and no signal in the
region of -10.00 ppm was found [19]. This we attribute to the likely
instability or very high reactivity of the intermediate hydride
species. Catalyst 6 was recovered at the end of the reaction and
subjected to an ESI-MS study. The ESI-MS studies showed one
1
major species with a peak at 311 (M/Z). The H NMR spectrum of
the recovered catalyst 6 showed a mixture of compounds. This
can be attributed the fragmentation of the intermediate osmium
hydride species due to its instability. The peaks for the cymene
and the ligand were not seen.
with N,N-ligands [12]
.
It was noted that transfer hydrogenation does not occur without
the base being present, which implies that this catalytic
transformation follows the classical mechanistic pathway, which
is shown in Scheme 2 [20]. Here, the active species is an iso-
propoxide complex (A), which forms in the presence of the base
which abstracts a proton from the hydrogen donor giving an
isopropoxide species which displaces the chloride from the
complex. The hydride complex is formed by β-elimination (B).
Cyclohexanone then coordinates to B giving the catalytic hydride
intermediate (C). Hydride transfer from osmium to the carbonyl
group gives (D). The active species A then is regenerated by an
alkoxide exchange reaction.
On comparing these complexes with other reported osmium
complexes, these complexes are less active than the osmium(II)
CNN pincer complexes reported by Baratta and coworkers. They
reported lower catalyst loadings of 0.001 mol% and TOFs of 1.4
x 10 (h ) [7]. In addition half-sandwich osmium(II) complexes with
amino carboxylate ligands have been reported by Carmona and
coworkers. Their experiments used a lower catalyst loading of
5
-1
Scheme 2: The proposed mechanism for the catalyzed transfer hydrogenation
by complexes 1-12
0.04 mmol and required a shorter reaction time of between 1-3 h
but they showed lower TONs and TOFs than the catalysts
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
Conclusions
6
2
.2 Synthesis and characterization of the complexes, [(η -arene)OsCl(N-
N] PF
6
6
The [(η -arene)OsCl(N-N]PF
6
complexes 1-12 were successfully synthesized
dimers (where arene = p-cymene
or benzene) with the N,N-bidentate ligands. The structures of 1, 4, 8, 10, 11 and
2 were solved X-ray diffraction and were shown to have adopted the classical
Compounds 1-12 were prepared using the following general procedure. A
from the reactions of [(η6-arene)Os(μ-Cl)Cl]
2
6
mixture of [(η - arene)Os(μ-Cl)Cl]
2
(100 mg, 0.20 mmol), the ligand in slight
PF (82 mg, 0.5 mmol) in dry
excess (0.42 mmol) and 2.5 equivalents of NH
4
6
1
methanol (20 ml) was stirred at 40° C for 3 h. The solution was then kept at 0°C
overnight. The formed precipitate was filtered off, then washed with diethyl ether
and dried under vacuum.
pseudo-octahedral three legged piano stool geometry. These complexes 1-12
were found to be effective catalysts for the transfer hydrogenation of
cyclohexanone with NaOH and gave high conversions and yields to
cyclohexanone. When comparing the effect of different arene groups, the
cymene complexes performed better than their benzene complexes, likely due
to higher electron density on the metal. When comparing the effects of changing
the bidentate ligands, on the transfer hydrogenation of cyclohexanone, the
ligands with electron donating substituents showed highest conversions, TONs
and TOFs, showing that the complexes could be tuned by substitution on the
ligand. Furthermore, catalysts 6 and 11 showed a wide applicability. They
converted cyclic, aliphatic and aromatic ketones to their corresponding alcohols
in 2-propanol as both the solvent and hydrogen source. Furthermore, the
proposed mechanism suggests that an osmium hydride species forms and
evidence suggests that cleavage of the Os-Cl bond may be a rate limiting step.
In addition, evidence suggests that the compounds with a weaker Os-Cl bond
are better catalysts. Furthermore, increased electron density on the metal likely
also activates the postulated hydride intermediate.
1.
Red powder, yield 80 %, m.p. 170.0 °C (decomp
.
)
.
1H NMR (400 MHz, DMSO-
6
): δ 9.42 (d, JHH = 5.6 Hz, 1H, Py); 9.24 (s, 1H, CH=N); 8.34 (d, JHH = 7.3 Hz,
d
1
1
1
H, Py); 8.26 (d, JHH = 7.3 Hz, 1H, Py); 7.67 (m, 1H, Py); 6.6 (d, JHH = 5.4 Hz,
H, p-cymene); 6.31 (d, JHH = 5.9 Hz, 1H, Ar, p-cymene); 6.24 (d, JHH = 5.6 Hz,
H, Ar, p-cymene), 6.03 (d, JHH = 5.3 Hz, 1H, Ar, p-cymene), 2.34 (m, 1H,
CH(CH
HH = 6.8 Hz, 3H, CH-Me
NMR (400 MHz, DMSO-d
3 3
(Py); 129.47 (Py); 129.40 (Py); 67.6 (Ar, p-cymene); 31.8 C(CH )
3
)
2
, cymene); 2.26 (s, 3H, cymene); 1.6 (s, 9H, (CH
, cymene); 0. 79 (d, JHH = 6.7 Hz, 3H, cymene). 13C
). δ 168.46 (CH=N), 156.38 (Ar), 155.12 (Py); 139.90
, t-butyl);
, p-cymene); 18.18
3
)3, t-butyl), 0.91 (d,
J
2
6
30.70 (CH
CH
3
3 3
, p-cymene); 22.53 (CH , p-cymene); 21.53 (CH
-1
(
3
, p-cymene). IR (KBr, cm ): 1615.0 υ(CH=N), 828.0 υ(P-F). Anal. Calcd for
[
C
20
H
28ClN
2
Os]PF
6
: C, 36.01; H, 4.23; N, 4.20. Found: C, 35.71; H, 4.10; N,
28ClN
Os] +
4
.69. MS (ESI, m/z): 523.0 for [C20
H
2
2
Experimental Section
Red powder, yield 73%, m.p. 190.0 °C (decomp.). 1H NMR (400 MHz, DMSO-
): δ 9.44 (d, JHH = 5.6 Hz, 1H, Py); 9.26 (s, 1H, CH=N); 8.27 (m, 2H, Py); 7.76
m, 1H, Py); 6.53 (d, JHH = 5.9 Hz, 1H, p-cymene ); 6.4 (d, JHH = 5.72 Hz, 1H, p-
cymene); 6.12 (d, JHH = 5.7 Hz, 1H, Ar, p-cymene); 6.03 (d, JHH = 5.9 Hz, 1H, Ar,
p-cymene), 4.9 (m, 1H, CH(CH iso-propyl); 2.23 (s, 3H, CH(CH iso-
propyl); 1.57 (d, JHH = 6.4 Hz, 3H, cymene); 1.40 (d, JHH = 6.6 Hz, 3H, cymene);
. 97 (d, JHH = 6.9 Hz, 3H, cymene); 0. 88 (d, JHH = 6.8 Hz, 3H, cymene). 13C
NMR (400 MHz, DMSO-d ). δ 166.25 (CH=N), 156.41 (Ar), 155.24 (Py); 139.79
Py); 128.64 (Py); 96.72 (Ar); 79.84 (Ar, p-cymene); 76.65 (Ar, p-cymene); 75.31
Ar, p-cymene); 74.11 (Ar, p-cymene); 66.86 (CH(CH , iso-propyl); 30.69
General
d
6
Experiments were carried out under nitrogen using standard Schlenk
(
2
techniques Acetonitrile and acetone were distilled over anhydrous CaCl and
stored over molecular 4 Å sieves. Diethyl ether and hexane were distilled from
sodium/benzophenone and used immediately. All reagents were used as
received (Aldrich or Merck). Low and high resolution mass spectra were
obtained using a Waters Micromass LCT Premier TOF-MS, while elemental
)
3 2
,
3 2 ,
)
0
6
analyses were obtained using
a ThermoScientific Flash 2000 elemental
(
(
(
(
analyzer. Electronic spectra were obtained in acetonitrile with a Perkin-Elmer
LAMBDA 35 UV-visible spectrophotometer and infrared spectra were recorded
with an ATR Perkin-Elmer Spectrum 100 spectrometer between 4000-400 cm-
3 2
)
CH(CH
3
)2, iso-propyl); 23.47 (CH
3
, p-cymene); 22.5 (CH
3
, p-cymene); 22.23
-1
CH , p-cymene); 21.72 (CH
3
3
, p-cymene); 18.23 (CH
3
). IR (KBr, cm ): 1616
1. 1
The chemical shifts are reported in ppm. The deuterated solvent DMSO-d
Aldrich) were used as obtained. Thermogravimetric analyses (TGA) was
H and 13C NMR spectra were obtained using a Bruker 400 MHz spectrometer.
υ(CH=N), 831 υ(P-F). Anal. Calcd for [C19
H
26ClN
2 6
Os]PF : C, 34.94; H, 4.01; N,
6
4
.29. Found: C, 34.94; H, 4.02; N, 4.58. MS (ESI, m/z): 509.0 for [C19
H
26ClN Os]
2
(
+
carried out using a SDTQ600 TGA-DSC under nitrogen. Melting points were
obtained using an Ernest Leitz Wetzlar hot-stage microscope. The transfer
hydrogenation reactions were followed using gas chromatography (GC) (Agilent
Capillary GC 6820 equipped with a DB wax polyethylene column (0.25 mm
3
Red powder, yield 78%, m.p. 200.0 °C (decomp
): δ 9.6 (d, JHH = 5.5 Hz, 1H, Py); 9.5 (s, 1H, CH=N); 8.44 (m, 1H, Py); 8.32
). 1H NMR (400 MHz, DMSO-
.
d
6
6
diameter, 30 m length) and a flame ionization detector). The dimers [(η -p-
(m, 1H, Py); 7.9 (m, 1H, Py); 7.49 (m, 2H, Ar); 7.42 (m, 1H, Ar); 6.37 (d, JHH
=
=
arene)Os(μ-Cl)Cl]
according to published procedures [22], as were the pyridine imine ligands [23]
The ligand 3,6-bis(2-pyridyl)-4-phenyl pyridazine was also synthesized
according to a literature procedure [24]
2 6 6
, where arene = C H or p-cymene, were synthesized
6
5
1
1
.0 Hz, 1H, p-cymene ); 5.78 (d, JHH = 6.00 Hz, 1H, p-cymene); 5.68 (d, JHH
.
.8 Hz, 1H, Ar, p-cymene); 5.57 (d, JHH = 5.8 Hz, 1H, Ar, p-cymene), 3.63 (d,
H, JHH = 5.8 Hz, CH(CH ); 2.34 (s, 3H, p-cymene); 2.17 (s, 3H, CH(CH );
.40 (d, JHH = 6.7 Hz, 3H, CH(CH ); 1.23 (d, JHH = 6.7 Hz, 3H, CH(CH ); 1.14
1.03 (m, 6H, cymene); 0.88 (d, JHH = 6.6 Hz, 3H,
cymene). 13C NMR (400 MHz, DMSO-d
). δ 174.4 (CH=N), 155.55 (Ar), 157.69
)
3 2
3 2
)
.
)
3 2
3 2
)
3 2 ;
(d, JHH = 6.6 Hz, 3H, CH(CH ) )
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201601249
FULL PAPER
1
(Py); 141.29 (Py); 140.70 (Py); 139.85 (Ar); 130.24 (Ar); 129.63 (Ar); 129.29
Brown powder, yield 73%, m.p. 205.0 °C (decomp
.
). H NMR (400 MHz, DMSO-
(
Ar); 124.31 (Ar); 96.44 (Ar, p-cymene); 93.13 (Ar, p-cymene); 79.15 (Ar, p-
d
6
): δ 9.34 (d, JHH = 6.1 Hz, 2H, Py); δ 8.72 (d, JHH = 1.6 Hz, 2H, Py); 7.68 (m,
2H, Py); 6.38 (d, JHH = 5.9 Hz, 2H, p-cymene ); 6.11 (d, JHH = 5.8 Hz, 2H, p-
cymene); 2.2 (s, 3H, CH ); 1.44 (s, 18H, C(CH , tert-butyl); 1.57 (d, JHH = 6.4
Hz, 3H, CH-Me , cymene); 1.40 (d, JHH = 6.6 Hz, 3H CH-Me , cymene); 0. 92
(d, JHH = 6.9 Hz, 6H, (CH ). δ 155.09 (Py),
). 13C NMR (400 MHz, DMSO-d
155.39 (Py), 125.01 (Py); 121.19 (Py); 95.25 (Py); 94.46 (Py); 74.36 (Ar, p-
cymene); 74.36 (Ar, p-cymene); 35.6 (CH ); 30.51 (CH ); 30.02 (CH ); 21.91
). IR (KBr, cm ): 825.0 υ(P-F). Anal. Calcd for [C28 38ClN Os]
C, 43.49; H, 4.95; N, 3.62. Found: C, 42.98; H, 5.19; N, 3.60. MS (ESI, m/z):
29 for [C28 38ClN
Os] +
cymene); 74.11 (Ar, p-cymene); 30.47 (CH
CH 22.40 (CH ); 21.61 (CH , p-cymene); 21.52 (CH
p-cymene). IR (KBr, cm ): 1616 υ(CH=N), 831 υ(P-F). Anal. Calcd for
36ClN Os]PF : C, 43.60; H, 4.70; N, 3.63. Found: C, 44.10; H, 5.00; N, 4.01.
3
); 26.57 (CH
3 3
); 25.82 (CH ); 23.1
(
)
3 ;
3
3
3
, p-cymene); 17.74 (CH
3
,
3
3 3
)
-
1
2
2
[C
28
H
2
6
3
)
2
6
MS (ESI, m/z): 627.0 for [C28H36ClN
2
Os] +
3
3
3
4
-
1
(CH
3
); 18.10 (CH
3
H
2
Red powder, yield 73%, m. p. 210.0 °C (decomp.) 1
PF
6
.
H NMR (400 MHz, DMSO-
6
H
2
d
6
): δ 9.57 (d, JHH = 5.5 Hz, 1H, Py); 9.35 (s, 1H, CH=N); 8.37 (d, JHH = 7.6 Hz,
1H, Py); 8.30 (m, 1H, Py); 7.91 (m, 3H, Ar); 7.08 (s, 1H, Ar); 6.22 (d, JHH = 6.0
8
Hz, 1H, p-cymene ); 5.71 (m, 2H, p-cymene); 5.54 (d, JHH = 5.7 Hz, 1H, Ar, p-
cymene); 2.29 (s, 3H, CH ); 2.15 (s, 1H, CH(CH , p-cymene); 2.10 (s, 3H,
CH ); 1.03 (m, 6H, CH(CH ). δ
2, p-cymene)). 13C NMR (400 MHz, DMSO-d
1
3
3
)
2
Brown powder, yield 83%, m.p. 200.0 °C (decomp
): δ 9.28 (s, 2H, Py); 8.55 (d, JHH = 8.4 Hz, 2H, Py); 8.09 (t, JHHH = 5.9 Hz, 2H,
Py); 6.47 (d, JHH = 5.9 Hz, 2H, p-cymene); 6.13 (d, JHH = 5.9 Hz, 2H, p-cymene);
2.43(m, 1H, CH(CH ); 2.36 (s, 3H, (CH )); 0.85 (d, JHH = 6.9 Hz, 6H, (CH ).
13C NMR (400 MHz, DMSO-d
). δ 155.09 (Py); 152.74 (Py); 140.31 (Py); 138.61
(Py); 122.78 (Py); 97.71 (Py); 93.67 (Py); 78.14 (Ar, p-cymene); 73.18 (Ar, p-
cymene); 30.56 (CH ); 21.95 (CH ); 18.32 (CH ); 17.73 (CH
). IR (KBr, cm-1):
822.1 υ(P-F). Anal. Calcd for [C22 26ClN Os] PF C, 38.34; H, 3.80 ; N, 4.07.
.
). H NMR (400 MHz, DMSO-
3
3
)
6
d
6
1
74.01 (CH=N), 155.77 (Ar), 150.36 (Py); 140.35 (Py); 131.2 (Py); 130.07 (Ar);
29.7 (Ar); 129.21 (Ar); 128.87 (Ar); 128.59 (Ar); 128.37 (Ar); 97.35 (Ar); 92.55
1
3
)
2
3
3 2
)
(
Ar,); 79.06 (Ar, p-cymene); 78.79 (Ar, p-cymene); 78.65 (Ar, p-cymene); 77.4
CH(CH ); 30.64 (CH ) ; 22.2 (CH ); 21.85 (CH , p-cymene); 19.39 (CH , p-
, p-cymene). IR (KBr, cm ): 1617 υ(CH=N), 832 υ(P-F).
28ClN Os]PF : C, 40.31; H, 3.95; N, 3.92. Found: C, 40.81;
H 4.00, N, 4.01. MS (ESI, m/z): 571.0 for [C24 28ClN
Os] +
6
(
3
)
2
3
3
3
3
-
1
cymene); 17.80 (CH
Anal. Calcd for [C24
3
3
3
3
3
H
2
6
H
2
6
H
2
Found: C, 38.46; H, 3.50; N, 4.00. MS (ESI, m/z): 545 for [C22H26ClN
2
Os] +
5.
9
. .
)
1H NMR (400 MHz, DMSO-
Red powder, yield 73%, m.p. 195.0 °C (decomp
): δ 8.48 (d, JHH = 5.3 Hz, 2H, Ar); 7.93 (m, 2H, Ar); 7.22 (d, JHH = 8.2 Hz, 2H,
Py); 7.15 (m, 2H, Ar); 5.99 (d, JHH = 5.8 Hz, 2H, p-cymene ); 5.89 (d, JHH = 5.8
1
Brown powder, yield 75 %, m.p. 210.0 °C (decomp.). H NMR (400 MHz, DMSO-
d
6
d
6
): δ 9.43 (d, JHH = 6.0 Hz, 2H, Py); 9.72 (s, 2H, Py); 7.68 (d, JHH = 5.2 Hz, 2H,
Py); 6.29 (s, 6H, C ).
); 1.44 (s, 18H, tert-butyl). 13C NMR (400 MHz, DMSO-d
δ 164.17 (Py); 155.35 (Py); 155.08 (Py); 124.86 (Py); 121.18 (Py); 77.62 (Ar, p-
6
H
6
6
Hz, 2H, p-cymene); 2.43 (s, 1H, CH(CH
cymene); 1.13 (d, JHH = 6.9 Hz, 6H, (CH
DMSO-d
5.76 (Ar, p-cymene); 91.53 (Ar, p-cymene); 75.50 (Ar, p-cymene); 75.34 (Ar,
p-cymene); 30.54 (CH , p-cymene); 22.0 (CH3, p-cymene); 17.69 (CH , p-
cymene); IR (KBr, cm ): 1627 υ(C=N), 831 υ(P-F). Anal. Calcd for
23ClN Os]PF : C, 35.53; H, 3.43; N, 6.22. Found: C, 36.00; H, 3.80; N, 6.52.
)
3 2
, p-cymene); 2.06 (s, 3H, CH3, p-
)
3 2
, p-cymene). 13C NMR (400 MHz,
-
1
cymene); 35.60 (CH
3
); 30.00 (CH
3
). IR (KBr, cm ): 826.8 υ(P-F). Anal. Calcd
6
). δ 154.93 (Ar), 151.42 (Ar), 140.69 (Py); 119.55 (Py); 113.45 (Py);
for [C24 30ClN Os] PF
H
2
6
C, 40.19; H, 4.22 ; N, 3.91. Found: C, 40.57; H, 4.01; N,
9
3
.78 MS (ESI, m/z): 573 for [C24H30ClN
2
Os] +
3
3
-
1
10
[C
20
H
3
6
1
MS (ESI, m/z): 532.0 for [C20
H
23ClN
3
Os] +
Brown powder, yield 80 %, m.p. 190.0 °C (decomp
): δ 9.40 (s, 2H, Py); 8.50 (d, JHH = 8.3Hz, 2H, Py); 8.09 (d, JHH = 8.2 Hz, 2H,
Py); 6.33 (s, 6H, C ); 2.50 (s, 6H, 2CH ). δ
). 13C NMR (400 MHz, DMSO-d
);
C,
.
). H NMR (400 MHz, DMSO-
d
6
6
6
H
6
3
6
. .
)
1H NMR (400 MHz, DMSO-
Red powder, yield 78%, m.p. 200.0 °C (decomp
): δ 9.56 (d, JHH = 5.4 Hz, 1H, Py); δ 8.80 (d, JHH = 7.9 Hz, 1H, Py); δ 8.74 (d,
HH = 7.9 Hz, 1H, Py); 8.56 (s, 1H, Py); 8.15 (m, 1H, Py); 7.91 (d, 1H, Py); 7.67
m, 5H, Ar); 7.50 (d, JHH = 6.8 Hz, 1H, p-cymene); 6.36 (d, JHH = 5.9 Hz, 1H, p-
cymene); 6.21 (d, JHH = 5.9 HZ, 1H, p-cymene); 6.11 (d, JHH = 5.9 Hz, 1H, p-
cymene); 2.65 (s, 1H, CH(CH , p-cymene); 2.21 (s, 3H, CH3, p-cymene); 1.03
m, 6H, (CH ). δ 158.36 (Py),
, p-cymene). 13C NMR (400 MHz, DMSO-d
56.75 (Py), 156.01 (Py); 153.22 (Py); 150.30 (Py); 149.71(Py); 138.35 (Py);
1
1
3
6 6
55.17 (Py); 152.88 (Py); 140.35 (Py); 138.45 (Py); 122.75 (Py); 72.70 (C H
d
6
-1
7.65 (CH
3
). IR (KBr, cm ): 821.7 υ(P-F). Anal. Calcd for [C16
H
14ClN
2
Os] PF
6
J
4.15; H, 2.87; N, 4.43. Found: C, 34.55; H, 2.69; N, 4.22. MS (ESI, m/z): 573
(
Os] +
for [C16H14ClN
2
3
)
2
11
(
)
3 2
6
1
Brown powder, yield 78 %, m.p. 210.0 °C (decomp
.
). H NMR (400 MHz, DMSO-
): δ 9.72 (d, JHH = 5.2 Hz, 1H, Py); 8.85 (m, 2H, Py); 8.62 (s, 1H, Py); 8.20 (m,
H, Py); 7.97 (m, 1H, Py); 7.74 (m, 5H, Ar); 7.60 (m, 2H, Py); 7.23 (d, JHH = 8.2
). 13C NMR (400 MHz, DMSO-d
). δ 158.20 (Py);
56.88 (Py); 150.25 (Py); 149.73 (Py); 144.95 (Py); 138.93 (Py); 138.25 (Py);
34.68 (Py); 130.63 (Py); 128.52 (Ar); 128.28 (Ar); 128.07 (Ar); 127.42 (Ar);
1
1
1
d
6
34.6 (Py); 130.68 (Py); 129.97 (Py); 128.77 (Py); 128.12 (Py); 127.54 (Py);
1
26.76 (Py); 122.63 (Py), 96.60 (Py); 95.95 (Py); 81.42 (Ar, p-cymene); 79.82
Hz, 1H, Py); 6.40 (s, 6H, C
6
H
6
6
(
Ar, p-cymene); 77.30 (Ar, p-cymene); 77.07 (Ar, p-cymene); 30.60 (CH(CH
3
)
2
);
, p-cymene). IR (KBr,
: C, 44.20; H, 3.46; N,
.87. Found: C, 44.60; H, 3.68; N, 6.50. MS (ESI, m/z): 671 for [C30 28ClN
Os]+
1
1
1
2
2.31 (CH
3
, p-cymene); 21.61 (CH
3
p-cymene); 18.01 (CH
28ClN Os]PF
3
-
1
cm ): 831.0 υ(P-F). Anal. Calcd for [C30
H
4
6
-
1
22.76 (Ar); 79.77 (C
20ClN Os]PF C, 41.40; H, 2.66; N, 7.38. Found: C, 41.60; H, 2.86; N, 7.39.
MS (ESI, m/z): 615 for [C26 20ClN
Os] +
6 6
H ). IR (KBr, cm ): 826.5 υ(P-F). Anal. Calcd for
6
H
4
[C
26
H
4
6
7
H
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201601249
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with a reflux condenser. The solution was heated to 82 °C under nitrogen
atmosphere using an oil bath. Biphenyl was used as an internal standard. The
reaction was monitored using a GC, by taking aliquots, which were first passed
through silica pads, and subsequently injected (0.1 μL) into the gas
chromatography. The alcohols were confirmed by comparing their retention with
pure commercially obtained samples (Aldrich) and by GC-MS analysis.
1
2
1
Brown powder, yield 73 %, m.p. 200.0 °C (decomp
.
). H NMR (400 MHz, DMSO-
d
2
1
6
): δ 8.60 (d, JHH = 4.8 Hz, 2H, Py); 7.93 (m, 2H, Py); 7.22 (d, JHH = 8.04 Hz,
). 13C NMR (400 MHz, DMSO-d
H, Py); 7.12 (s, 2H, Py); 6.11 (s, 6H, C
6
H
6
6
). δ
).
C, 31.00; H,
.44; N, 6.78. Found: C, 30.40; H, 2.17; N, 6.58. MS (ESI, m/z): 476 for
15ClN
Os] +
6 6
55.0 (Py); 151.69 (Py); 140.71 (Py); 119.45 (Py); 113.36 (Py); 73.36 (C H
Supplementary material
-
1
3 6
IR (KBr, cm ): 824.54 υ(P-F). Anal. Calcd for [C16H15ClN Os] PF
CCDC numbers 1505440 – 1505445 contain the supplementary
crystallographic 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 or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
2
[C
16
H
3
2.3 X-ray crystallography
Suitable crystals of compounds 1, 4, 8, 10 11 and 12 for X-ray diffraction studies
were grown by the solvent diffusion method. Thus solutions of the compounds
in dry acetone were layered with a fourfold volume of hexane and were left to
stand in the dark at room temperature for 2 days. Details of the data collection
method are given in the supplementary material. The data was collected using
a Bruker Smart APEXII diffractometer. The program SHELXS [25] and SHELXL
1
EZ, UK; fax: +44 1223 336033.
Acknowledgement
We thank the NRF, THRIP (Grant no. TP 1208035643) and UKZN
(URF) for financial support. Joel M. Gichumbi further thanks Prof.
E. N. Njoka for his support.
[
26]
were used to solve the structure. Structure refinement and crystal data
information for compound 1, 4, 8, 10 11 and 12 are summarized in Supporting
information Table 1. Disorder was found in all PF molecules and were treated
Keywords: Osmium(II) arene complexes; transfer
6
hydrogenation; N,N-bidentate ligands; crystal structure.
using PART instructions. Compound 11 also has a molecule of acetone also
disordered over two positions as resolved using PART instructions in the
refinement.
General procedure for the transfer hydrogenation reactions
The osmium(II) complex (0.00425 mmol), the ketone (8.5 mmol) and the base
NaOH (3.4 mmol), were added 2-propanol(10 ml) in a Schlenk tube equipped
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European Journal of Inorganic Chemistry
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4
2
023-4024; c). J. W. Faller, A. R. Lavoie, Organometallics
001, 20, 5245-5247.
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Osmium(II) arene complexes; transfer hydrogenation;
6
We report on the synthesis of new [(η -arene)OsCl(N-N)]PF
6
complexes with bidentate ligands. The
single crystal X-ray structures for five of these compounds showed the classical three legged piano stool
geometry. The compounds were found to be effective catalysts for the transfer hydrogenation of ketones
into alcohols with NaOH as base and 2-propanol as the solvent and hydrogen source.
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