Direct access to pop-type osmium(II) and osmium(IV) complexes: Osmium a promising alternative to ruthenium for the synthesis of imines from alcohols and amines

Article abstract of DOI:10.1021/om200290u

An easy and direct access to POP-type osmium(II) and osmium(IV) complexes, including OsH₄{dbf(PⁱPr₂)₂} (dbf(PⁱPr₂)₂ = 4,6-bis(diisopropylphosphine) dibenzofuran), is reported. This tetrahydride derivative is an efficient catalyst for the selective formation of imines from alcohols and amines with liberation of H₂, proving that osmium is a promising alternative to ruthenium for catalysis.

Full text of DOI:10.1021/om200290u

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pubs.acs.org/Organometallics
Direct Access to POP-Type Osmium(II) and Osmium(IV) Complexes:
Osmium a Promising Alternative to Ruthenium for the Synthesis of
Imines from Alcohols and Amines
Miguel A. Esteruelas,* Nicole Honczek, Montserrat Olivꢀan, Enrique O~nate, and Marta Valencia
Departamento de Química Inorgꢀanica-Instituto de Ciencia de Materiales de Aragꢀon, Universidad de Zaragoza-CSIC, 50009 Zaragoza,
Spain
S Supporting Information
b
ABSTRACT: An easy and direct access to POP-type osmium(II) and
osmium(IV) complexes, including OsH4{dbf(PiPr2)2} (dbf(PiPr2)2 = 4,6-
bis(diisopropylphosphine)dibenzofuran), is reported. This tetrahydride
derivative is an efficient catalyst for the selective formation of imines from
alcohols and amines with liberation of H2, proving that osmium is a
promising alternative to ruthenium for catalysis.
mines are useful intermediates in organic synthesis, which act
as electrophilic reagents in many different transformations,
complexes capable of activating inert bonds.13 The most com-
monly encountered linker groups consist of either a metalated
aryl ring in anionic PCP ligands or uncharged PNP pyridines or
neutral POP ethers.14 In contrast to the remaining platinum-
group metals, osmium pincer chemistry has received little
attention,15 in particular that of POP ligands. In the search for
osmium pincer starting materials, we have recently synthesized
4,6-bis(diisopropylphosphino)dibenzofuran (dbf(PiPr2)2), which
has been used to prepare the osmium(III) derivative OsCl3{dbf-
I
including additions, condensations, and cycloadditions.1 Their
traditional preparation implies the reactions of ketones or
aldehydes with amines. Imines have been also synthesized by
self-condensation of amines upon oxidation,2 transition-metal-
promoted hydrogen transfer from secondary amines,3 direct
reaction of nitroarenes and primary alcohols in the presence of
an IrÀPd heterodimetallic catalyst,4 and cross-coupling between
amines and alcohols catalyzed by heterogeneous metallic systems
under an oxygen atmosphere.5
(PiPr2)2} from OsCl3 3H2O.16 We have now synthesized a
3
POP-type osmium tetrahydride pincer compound (Scheme 1),
which stops the alkylation of amines with alcohols at the imine,
liberating molecular hydrogen, and is more efficient than the
PNP-type ruthenium pincer compound previously reported.
Treatment of 2-propanol solutions of the tetrasolvento adduct
OsCl2(DMSO)4 (1) with 1.0 equiv of dbf(PiPr2)2 under reflux
for 18 h leads to OsCl2{dbf(PiPr2)2}(DMSO) (2), as a result of
the displacement of three DMSO molecules by the pincer ligand.
This complex, which is isolated as a yellow solid in 86% yield, has
been characterized by X-ray diffraction analysis. The coordina-
tion polyhedron around the osmium atom can be rationalized as
being derived from a distorted octahedron, with the POP group
occupying mer positions and the chloride ligands disposed trans
(Cl(1)ÀOsÀCl(2) = 171.61(5)°). The P(1)ÀOsÀP(2) bite
angle of 156.86(5)° compares well with that of OsCl3{dbf-
(PiPr2)2} (159.95(3)°). A singlet at 6.3 ppm in the 31P{1H}
NMR spectrum in dichloromethane-d2 is also characteristic of
this compound.
The alkylation of amines with alcohols is a ruthenium- and
iridium-promoted procedure, which takes place via imine
intermediates.6 However, the selective preparation of the imines
has been not achieved until very recently, when Milstein and co-
workers reported a homogeneous PNP-type ruthenium pincer
complex that, under argon, stops the reaction at the imine.7 This
environmentally benign method for the synthesis of imines, in
contrast to those using heterogeneous metallic systems under an
oxygen atmosphere, occurs with liberation of molecular hydro-
gen, which is the most efficient and clean energy carrier known.8
Osmium is more reducing than ruthenium, prefers to be
saturated by coordination, and forms redox isomers with more
metalÀcarbon bonds.9 These characteristics have been argued to
justify the versatility of the stoichiometric osmium chemistry and
its poorer catalytic activity in comparison with ruthenium.10 On
the other hand, we have recently shown that osmium complexes
are more efficient catalysts than tungsten, ruthenium, and
rhodium systems for the regioselective 7-endo heterocyclization
of aromatic alkynols into benzoxepines.11
Complex 2 reacts with molecular hydrogen under 3 atm, in the
presence of a Brønsted base. The products of the reactions
Pincer ligands are having a tremendous impact in homoge-
neous catalysis.12 As a consequence of the disposition of their
donor atoms, they develop marked abilities to stabilize metal
Received: April 4, 2011
Published: April 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om200290u Organometallics 2011, 30, 2468–2471
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Organometallics
COMMUNICATION
Scheme 1
Scheme 2
H(03)ÀH(04) separations are 1.816, 1.651, and 1.849 Å,
respectively. As expected for two inequivalent hydride positions,
two broad resonances centered at À3.3 and À14.3 ppm are
1
observed at 193 K in the high-field region of the H NMR
spectrum in toluene-d8. At about 223 K, coalescence between
them takes place. This behavior indicates that the hydride ligands
H(01) and H(02) (or H(04) and H(03)) undergo a thermally
223
can be estimated for the process. The 31P{1H} NMR spectrum
contains a singlet at 64.9 ppm.
activated position exchange. A ΔGq value of 9.8 kcal molÀ1
depend upon the base and the experimental conditions. Triethy-
lamine produces the abstraction of a chloride ligand. Thus, the
trihydride OsH3Cl{dbf(PiPr2)2} (3) is formed after 60 h, in
toluene, at 90 °C, and in the presence of the amine. On the other
hand, sodium hydride removes both chloride ligands. As a result
of this, the tetrahydride OsH4{dbf(PiPr2)2} (4) is obtained after
60 h, in tetrahydrofuran, at 50 °C, and in the presence of the salt.
Complexes 3 and 4 are isolated as pale yellow solids in 50% and
65% yields, respectively, and have been characterized by X-ray
diffraction analysis.
The geometry around the osmium atom of 3 can be rationa-
lized as a distorted pentagonal bipyramid, with the phosphorus
atoms of the POP group occupying axial positions (P(1)ÀOsÀ
P(2) = 158.99(5)°) and the chloride situated between the
oxygen atom and the hydride H(03). The B3W91 optimized
structure confirms the trihydride character of the OsH3 unit. The
H(01)ÀH(02) and H(02)ÀH(03) separations are 1.611 and
1.555 Å, respectively. At room temperature, the 1H NMR
spectrum in dichloromethane-d2 shows a hydride resonance
at À11.86 (t, JHÀP = 9.4 Hz) ppm, which is consistent with the
operation of two thermally activated site exchange processes
between the hydrides. At about 243 K, decoalescence occurs and,
between 240 and 187 K, two signals are observed at À12.2
at À14.0 ppm in a 2:1 intensity ratio. In agreement with the
trihydride character of the complex, at 223 K, T1(min) values
of 68 ( 1 and 70 ( 1 ms were found for these resonances. The
31P{1H} NMR spectrum contains a singlet at 54.2 ppm.
The tetrahydride complex 4 reacts with 1.0 equiv of benzyl
alcohol in toluene under reflux to afford after 7 h the hydride aryl
carbonyl derivative OsH(Ph)(CO){dbf(PiPr2)2} (5), which is
isolated as a yellow solid in 53% yield, according to eq 1. The
behavior of 4 is in agreement with the tendency shown by
osmium polyhydrides to dehydrogenate alcohols17 and with our
previous observation that the tetrahydride species OsH4(PiPr3)2,
generated from OsH6(PiPr3)2 by thermal activation, reacts with
benzaldehyde to give OsH(Ph)(CO)2(PiPr3)2 via a hydride acyl
intermediate.10c The presence of a hydride in 5 is revealed by a
triplet (JHÀP = 19.5 Hz) at À8.07 ppm in the 1H NMR spectrum,
which agrees well with that of OsH(Ph)(CO)2(PiPr3)2
(δ À6.59), where the hydride and carbonyl are also disposed
trans. In the 13C{1H} NMR spectrum the carbonyl and meta-
lated aryl resonances appear at 189.9 (t, JPÀC = 7 Hz) and 157.6
(t, JPÀC = 8.5 Hz) ppm, respectively. The 31P{1H} NMR
spectrum contains a singlet at 56.8 ppm.
The geometry around the osmium atom of 4 is similar to that
of 3 and can be rationalized as a distorted pentagonal bipyramid
with the phosphorus atoms of the POP group occupying axial
positions (P(1)ÀOsÀP(2) = 158.84(2)°) and the hydride
H(04) in the position of the chloride ligand of 3. The B3W91
optimized structure confirms the tetrahydride character of
the OsH4 unit. The H(01)ÀH(02), H(02)ÀH(03), and
The formation of 5 can be rationalized through the inter-
mediates AÀD shown in Scheme 2. The deprotonation of the
alcohol by the basic tetrahydride 4 could afford the trihydride
alkoxy intermediate A, which should give the unsaturated
species B by dissociation of molecular hydrogen. A β-elimina-
tion reaction in the alcoholate group could lead to the dihydride
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COMMUNICATION
Table 1. Direct Synthesis of Imines from Alcohols and
Amines Catalyzed by POP-Type Osmium Complex 4a
98% yield after 3 h, by treatment of equimolecular amounts of
benzyl alcohol and aniline in toluene under reflux and in the
presence of 0.2 mol % of 4. Because osmium has a tendency
higher than that of ruthenium to be saturated by coordination
and to activate CÀH bonds, in contrast to the PNP-type
ruthenium pincer system, 1 mol % of KOH is also necessary
in order to increase the nucleophilicity of the medium. This
facilitates the amine attack, preventing the decarbonylation of
the aldehyde, which produces the catalytic deactivation of the
system. The catalytic cycle is closed with the addition of the
OÀH bond of the alcohol to E. Complexes 2 and 3 also catalyze
the reaction but are significantly less active than 4 (49% and
54% yield, respectively, after 3 h).
Complex 4catalyzes the selective formation of a variety of imines
from alcohols and amines with liberation of molecular hydrogen,
under an argon atmosphere (eq 2). The reactions include the
formation of aliphatic imines, which are inherently more challen-
ging because of their instability. High yields are obtained after 24 h
(Table 1), while in the presence of the PNP-type ruthenium pincer
system, the reactions need between 48 and 56 h.
Benzyl alcohol reacts with anilines and aliphatic amines,
including cyclohexylamine. The position of the substituent of
the anilines has a marked influence on the reaction rates
(Table 1, runs 2 and 3). While N-benzylidene-p-methylaniline
is formed after 3 h in 92% yield, along with a small amount of
secondary amine (5%), N-benzylidene-o-methylaniline is ob-
tained in 87% yield after 24 h. The reactions with the aliphatic
amines are slower. After 24 h, cyclohexylamine (run 4),
benzylamine (run 5), and dodecylamine (run 6) lead to the
corresponding imines in yields between 61% and 82%. Cyclo-
hexylmethanol reacts more slowly than benzyl alcohol. After
24 h, N-cyclohexylmethylene-p-methylaniline (run 7), N-cyclo-
hexylmethylenecyclohexylamine (run 8), and N-cyclohexyl-
methylenedodecylamine (run 9) are formed in 55%, 65% and
37% yields, respectively. The reactions between linear alcohols
and aliphatic amines are also very efficient (runs 10À13). Both
butan-1-ol and octan-1-ol react with cyclohexylamine and
dodecylamine to afford selectively the corresponding aliphatic
imine in high yields (80À94%). The secondary cyclohexanol
and dodecylamine give N-cyclohexylidenedododecylamine in
30% yield, after 24 h (run 14). Small amounts of secondary
amine are also formed from the reactions of dodecylamine with
cyclohexylmethanol (11%, run 9) and octan-1-ol (7%, run 13).
The current lack of development of the osmium chemistry in
comparison with that of ruthenium is certainly in part a
consequence of the scarce effort to find starting materials.
These results show an easy and direct access to POP-type
osmium(II) and osmium(IV) pincer complexes and prove that
osmium is a promising alternative to ruthenium for the direct
synthesis of imines, including aliphatic imines, from alcohols
and amines with liberation of molecular hydrogen.
a Complex 4 (0.02 mmol), alcohol (10 mmol), amine (10 mmol), KOH
(0.1 mmol), and toluene (1 mL) were heated at 150 °C in a Schlenk tube
under an argon atmosphere. b Yields of products were determined by 1H
NMR spectroscopy.
benzaldehyde C, which should generate the unsaturated hy-
dride acyl intermediate D. The formation of the latter could
take place by heterolytic CÀH bond activation of the aldehyde,
promoted by one of the hydride ligands, or alternatively by
reductive elimination of molecular hydrogen and subsequent
homolytic CÀH bond activation of the aldehyde. Finally, the
deinsertion of the phenyl group should give 5. Intermediate D
is similar to that proposed for the formation of OsH(Ph)
(CO)2(PiPr3)2.10c
Intermediate C can dissociate the aldehyde to afford an
equilibrium mixture with the unsaturated dihydride E. Thus,
when the reaction of 4 and benzyl alcohol is carried out in the
presence of aniline, the generation of N-benzylideneaniline is
observed. According to this, the latter is catalytically formed in
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, tables, and CIF
b
files giving experimental procedures regarding the synthesis,
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dx.doi.org/10.1021/om200290u |Organometallics 2011, 30, 2468–2471

Organometallics
COMMUNICATION
characterization data for the complexes and the imines, X-ray
data for complexes 2À4, and computational details and Carte-
sian coordinates for the calculated structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Corresponding Author
*E-mail: maester@unizar.es.
’ ACKNOWLEDGMENT
Financial support from the MICINN of Spain (CTQ2008-
00810 and Consolider Ingenio 2010 CSD2007-00006), the
Diputaciꢀon General de Aragꢀon (E35), and the European Social
Fund is acknowledged.
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