Synthesis, characterization, and reactivity of the first osmium β-diketiminato complexes and application in catalysis

Article abstract of DOI:10.1021/om400875r

The strongly chelating anionic β-diketiminate ligand has been employed to formulate complexes involving almost every metal of the periodic table; however, the heavier metals of the d block remain relatively unexplored. This paper describes the synthesis a

Full text of DOI:10.1021/om400875r

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivity of the First Osmium
β‑Diketiminato Complexes and Application in Catalysis
Dominique F. Schreiber,† Crystal O’Connor,†,‡ Christian Grave,† Helge Muller-Bunz,† Rosario Scopelliti,§
̈
Paul J. Dyson,§ and Andrew D. Phillips*,†
†School of Chemistry & Chemical Biology, University College Dublin (UCD), Belfield, Dublin 4, Ireland
‡SFI Strategic Research Cluster in Solar Energy Conversion, University College Dublin (UCD), Belfield, Dublin 4, Ireland
§Institut des Sciences et Ingen
́ ́ ́
ierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), 1014 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: The strongly chelating anionic β-diketiminate ligand has been
employed to formulate complexes involving almost every metal of the periodic
table; however, the heavier metals of the d block remain relatively unexplored.
This paper describes the synthesis and characterization of the first two osmium
β-diketiminato compounds, including a coordinatively unsaturated cationic
complex. In parallel to the analogous Ru(II) complexes, the cationic (η6-
arene)osmium(II) complex demonstrates bifunctional behavior through [4 + 2]
cycloaddition with ethylene, cleavage of dihydrogen under mild conditions, and protonation/chloride addition with [Et2OH]Cl.
Metal-centered activity in both the Ru(II) and Os(II) β-diketiminates has until now remained elusive, as the cationic Os complex
is shown to readily coordinate an aryl isonitrile. The applicability of Os(II) β-diketiminato complexes in catalytic olefin
hydrogenation demonstrates significantly greater activity in terms of conversion and TOF for a range of substrates, including
styrene, cyclohex-1-ene, and 1-methylcyclohex-1-ene. Moreover, selective hydrogenation of the exocyclic alkenyl group in
limonene was observed, whereas the corresponding isostructural Ru(II) complexes are inactive. In contrast, the cationic (η6-
arene)ruthenium(II) β-diketiminato complex proved more active for the catalytic dehydrogenation of N,N-dimethylamine
borane (Me2NBH3) than the equivalent Os(II) species. A detailed DFT study of the Ru(II) and Os(II) β-diketiminato species
using charge decomposition analysis (CDA) demonstrates differences in metal−ligand interactions, which in turn considerably
influences the extent of bifunctional reactivity.
recent increase in interest in Os chemistry, a number of reports
have focused on the synthesis of a wide variety of novel Os
complexes.17−23 Of particular relevance for the (η6-arene)-
osmium half-sandwich complexes discussed herein is the
synthesis of the dinuclear complex [{(η6-p-cymene)Os}2(μ-
η2:η2-L) (L = 2,5-bis[2-(methylthio)anilino]-1,4-benzoqui-
none) by Sommer et al.24
Previous work by some of us described the facile synthesis of
the coordinatively unsaturated (η6-arene)ruthenium β-diketi-
minato complex [(η6-C6H6)Ru{(2,6-Me2-C6H3NCMe)2CH}]-
OTf (2-Ru), via anion metathesis of (η6-C6H6)RuCl{(2,6-Me2-
C6H3NCMe)2CH} (1-Ru) (Figure 1). This bifunctional
complex demonstrated the ability to undergo [4 + 2]
cycloadditions with both ethylene and acetylene and,
importantly, heterolytically activate H2.25 Moreover, 2-Ru
performs efficient homogeneous catalytic hydrogenation of
styrene26 and dehydrogenation of amine−boranes.27 A key
property of the coordinatively unsaturated complex 2-Ru is the
thermal reversibility of the substrate addition, which is strongly
influenced by the substitution pattern of the flanking aryl
groups. Considering the propensity of Os compounds to
INTRODUCTION
■
Within the group 8 transition metals, osmium has received
comparatively less attention than iron and ruthenium, which is
generally due in part to slower ligand exchange kinetics and
hence lower catalytic activity. However, a handful of Os
compounds have been shown to have relevant catalytic activity
in their own right.1−3 In an extension of their work on Ru-
mediated catalytic hydrogenation, Vaska et al. pioneered the
first monometallic Os species active in homogeneous hydro-
genation, [OsH(Cl)(CO)(PPh3)3].4,5 Later on, Sanchez-
Delgado et al. published their findings on the rapid reduction
of 1-hexene, 1,3-cyclohexadiene, and phenylacetylene by
[OsH(Br)(CO)(PPh3)3], a brominated derivative of the
complex developed by Vaska.6 The same authors investigated
the reduction of L-carvone, featuring endo- and exocyclic CC
bonds.7,8 More recent work on osmium complexes concen-
trated mainly on the reduction of carbonyl compounds.9−15
A
particularly interesting example reported by Clapham and
Morris described the ability of an amido hydrido Os complex,
OsH(NHCMe2CMe2NH2)(PPh3)2, to reduce acetophenone.16
It was found that the corresponding Ru analogue showed
conversion similar to that of the Os complex; however, the
kinetic behavior indicated a continuously decreasing turnover
frequency (TOF) as the reaction progressed. As part of the
Received: August 31, 2013
Published: November 27, 2013
© 2013 American Chemical Society
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coordinatively saturated dichromatic purple-orange Os(II)
complex 1-Os was obtained in 73% yield. The characteristic
1H NMR signals in CD2Cl2 for the η6-C6H6 and β-CH protons
were observed at δ (ppm) 5.07 and 5.66, respectively (Table 1).
A comparison with the previously published solution NMR
data of 1-Ru, with δ (ppm) 4.60 and 5.54, respectively, reflects
a considerable downfield shift for the NMR signals in 1-Os.
This trend is paralleled in the corresponding 13C{1H} NMR
data (Table 1).25
Figure 1. β-Diketiminato metal complexes (η6-C6H6)MCl{(2,6-Me2-
C6H3NCMe)2CH} and [(η6-C6H6)M{(2,6-Me2-C6H3NCMe)2CH}]-
OTf (M = Ru/Os, OTf = CF3SO3 ).
Analogous to the case for 1-Ru, the protons of the o-CH3
−
substituents of the flanking aryl groups of 1-Os appear as a
1
single H NMR signal, indicating apparent C2v symmetry as in
the case for the cationic 2-Os. Importantly, this resonance
considerably broadens on changing to a more polar NMR
solvent, in this instance, THF-d8, where the signal for the o-
methyl groups is observed at 2.36 ppm. This suggests that in
solution Cl is only weakly associated with the Os(II) metal
center. In contrast, the solid-state structure of 1-Os reveals a
Os−Cl distance of 2.496(1) Å (Figure 2), which is significantly
shorter than the Ru−Cl bond of 2.521(1) Å in 1-Ru.25 Notably,
the Os−Cl bond length in 1-Os is the longest (η6-C6H6)Os−Cl
distance yet recorded in the Cambridge Crystallographic
Database (CCDC), with a calculated median distance of
2.402 Å for the entire data set.33 The metal−halogen charge
transfer (MXCT) resulting from the coordination of Os(II) to
the electron-donating β-diketiminate ligand, together with the
steric pressure exerted by the o-CH3 groups on the flanking
aryls, are believed to be the major contributions to this
homogeneously catalyze the hydrogenation of various olefin
and carbonyl substrates described above, we herein describe the
synthesis, characterization, and catalytic application of the novel
organoosmium β-diketiminato complexes (η6-C6H6)OsCl{(2,6-
Me2-C6H3NCMe)2CH} (1-Os) and [(η6-C6H6)Os{(2,6-Me2-
C6H3NCMe)2CH}]OTf (2-Os) (Figure 1).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Complexes. The
synthesis of the β-diketiminate ligand with o-methyl sub-
stituents (2,6-Me2-C6H3NCMe)2CH2 was carried out as
previously described via acid-catalyzed co-condensation of
2,6-dimethylaniline and 2,4-pentanedione.28−32 Analogous to
the synthesis of the chloro-substitued Ru(II) complex (η6-
n
C6H6)RuCl{(2,6-Me2-C6H3NCMe)2CH} (1-Ru), BuLi-medi-
ated deprotonation of the β-diketiminate ligand to Li{(2,6-Me2-
C6H3NCMe)2CH} was necessary to achieve transmetalation
with the dichloro(η6-C6H6)osmium(II) dimer.25 The resulting
unusually long Os−Cl bond. Moreover, the Os−Ccent (Ccent
Carene centroid) bond length is 1.682(2) Å, which is equivalent
=
1
Table 1. Selected Solution H and 13C{1H} NMR Chemical Shifts (ppm) for Compounds 1-Os−6-Os in CD2Cl2
a
compound
1-Os
2-Os
1H NMR
3-Os
2.14
4-Os
1.98
5-Os
2.05
6-Os
1.58
α-CH3
1.79
2.23
b
b
1.49
2.26
o-CH3
2.13
7.00
5.79
2.31/2.34
4.31/4.81
5.05
2.06/2.23
3.14/4.44
4.67
1.96/2.23
4.54
2.22/2.26
5.13
2.36 br
5.66
β-CH
b
4.86
η6-arene CH
aryl CH
5.07
4.67
4.85
b
4.60
7.04−7.08
6.92−7.13
7.18−7.22
13C{1H} NMR
7.10−7.14
7.05−7.08
7.10−7.13
7.06−7.34
b
α-CH3
o-CH3
23.3
23.2
24.8
23.8
22.7
22.4
b
23.2
19.3
19.1
18.7/19.0
79.9
18.0/18.2
79.0
18.0/18.9
80.1
18.5/19.7
86.2
b
19.6 br
η6-arene CH
β-CH
76.9
77.1
b
76.6
103.4
107.9
128.3
158.6
165.7
49.2
54.9
101.1
128.1
152.8
180.0
103.5
129.8
154.7
164.2
b
101.9
aryl p-CH
i-C
126.8
128.3
153.0
183.0
127.7
153.2
177.8
b
126.3
156.8
b
156.5
CN
162.9
b
161.3
a
b
Spectra were recorded in CD2Cl2, except for those of 3-Os, which were measured in CDCl3. Spectra recorded in THF-d8.
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Figure 2. ORTEP diagrams of the Os(II)-β-diketiminato complexes 1-Os and 2-Os. Thermal ellipsoids are drawn at the 50% probability level.
Solvates, anions, and internal disorder have been omitted for clarity. For 2-Os, the unit cell contains two crystallographically independent molecules,
of which only one is shown. The provided bond lengths and angles for 2-Os represent averaged values. Relevant bond lengths (Å) and bond angles
(deg): (1-Os)a) Os−Cl, 2.496(1); Os−N, 2.110(4); Os−Ccent, 1.682(2); N−Os−N, 87.3(1); Ccent−Os−Cl, 125.0(1); Ccent−Os−N,N′, 153.5(1);
Os−N,N′−Cβ, 158.3(2); N−Cα−Cβ, 124.5(4). (2-Os) Os−N, 2.000(8), 2.016(8); Os−Ccent, 1.711(5); N−Os−N, 89.3(3); Ccent−Os−N,N′,
178.3(3); Os−N,N′−Cβ, 176.2(5); N−Cα−Cβ, 122.9(9). Selected torsion angle (deg): Ru−N−Cα−Cβ (1-Os) 11.0, −11.12, (2-Os) 0.15, 2.52.
Closest contact between 2-Os and OTf counterion: η6-arene H22 to O2 2.416 Å. Ccent = Carene centroid, and N,N′ = Nligand midpoint.
Scheme 1. Reactivity of 2-Os with HCl, H2, C2H4, and CNXyl
to the distance of 1.687(3) Å in 1-Ru. The geometry at the
metal center in 1-Os has an Os−N,N′−Cβ angle of 158.3(2)°,
which is not as tripodal as in 1-Ru (N,N′ = Nligand midpoint)
and thus reflects the stronger interaction between Os and the β-
diketiminate ligand. The corresponding 16-valence-electron
mononuclear cationic complex [(η6-C6H6)Os{(2,6-Me2-
C6H3NCMe)2CH}]OTf (2-Os) was prepared in 88% yield
via a salt metathesis reaction of 1-Os and NaOTf, employing a
procedure slightly modified from that used for 2-Ru.25 This is
in contrast with the work presented by Carmona et al., where
the Ag(I)-mediated chloride abstraction of the osmium half-
sandwich complex [(η6-p-cymene)OsCl(L-α-aminocarboxy-
late)] led to trimerization and isolation of an Os cluster
compound linked through OC groups.34 As is evident from
the ORTEP representation, 2-Os features a vacant coordination
site at the metal center (Figure 2). The shortest contact
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Figure 3. ORTEP diagrams of 3-Os and 4-Os. Thermal ellipsoids are drawn at the 50% probability level. Solvates and the triflate anion have been
omitted for clarity. Relevant average bond lengths (Å) and bond angles (deg) are as follows. 3-Os: Os−Cl, 2.416(1); Os−N, 2.125(4); Os−Ccent
,
1.693(3); N−Os−N, 84.7(2); Ccent−Os−Cl, 123.7(1); Ccent−Os−N,N′, 152.5(1); Os−N,N′−Cβ, 145.0(2); N−Cα−Cβ, 122.2(5). 4-Os: Os−H,
1.55(2); Os−N, 2.106(1); Os−Ccent, 1.715(1); N−Os−N, 85.2(1); Ccent−Os−H, 126.4(1); Ccent−Os−N,N′, 155.2(3); Os−N,N′−Cβ, 138.6(1);
N−Cα−Cβ, 119.1(1). Ccent = Carene centroid, and N,N′ = Nligand midpoint.
́
between 2-Os and the counteranion OTf is 2.416 Å, measured
between O2 and H22 of the η6-coordinated benzene. Due to
the rigid, well-defined steric environment provided by the
ancillary β-diketiminate ligand, no dimerization in solution or in
the solid-state occurs. In comparison to 2-Ru, the metal center
in the cationic complex 2-Os is apparently more strongly
bound to the supporting ligands (on the basis of bond lengths),
mirroring a similar trend found between the chlorine-
substituted compounds 1-Ru and 1-Os.25 This is further
groups (Table 1). As expected, the β-CH2 position in 3-Os
shows distinctive sp3 hybridization, as evident by the 30.6° out-
of-plane folding with resspect to the planes defined by the “C
N” bonds.
In analogy with 2-Ru, 2-Os also mediates the heterolytic
bifunctional activation of H2 under mild conditions. The
synergic protonation of the nucleophilic β-carbon on the ligand
and the addition of the hydride to the Os center results in the
isolation of [(η6-C6H6)OsH{(2,6-Me2-C6H3NCMe)2CH2}]-
OTf (4-Os). Very few examples of this type of cooperative
Os(II)−ligand activation of H2 have been reported in the
literature. In particular, the Os complex OsH-
(NHCMe2CMe2NH2)(PPh3)2 of Clapham et al. showed
heterolytic cleavage of H2 to form the dihydride complex
trans-OsH2(NH2CMe2CMe2NH2)(PPh3)2, featuring a proto-
nated nitrogen at the diamine ligand.16 In parallel with complex
3-Os, the bifunctional H2-addition product 4-Os also features
an sp3-hybridized β-CH2 with strong diimine character (Figure
3). This observation is in stark contrast to the work of West et
al. on Pt(II) β-diketiminato complexes, where the β-
diketiminato β-CH group did not participate in the
dehydrogenation reaction, remaining sp2 hybridized through-
out.35 The Cs symmetry observed for 4-Os in solution is
evident from the corresponding 1H and 13C{1H} solution
NMR data (Table 1). Further evidence in favor of a strongly
evidenced by the shifted H NMR signals found for the η6-
1
C6H6 δ (ppm) 5.79, and β-CH δ (ppm) 7.00 groups within 1-
Ru (Table 1). A comparison of the Os−N and Os−Ccent bond
lengths in 2-Os with the equivalent metal−ligand distances
reported for 2-Ru features overall shortened Os(II)−ligand
distances in comparison to 2-Ru.25 Further discussion
regarding the solid-state structure of 2-Os is provided in the
Supporting Information.
Reactivity. The coordinatively unsaturated complex 2-Os is
a Lewis acidic species and therefore should react to form stable
adducts with a variety of neutral σ-donors such as phosphines,
pyridines, and nitriles. However, no evidence for the reaction of
2-Os with PPh3, pyridine, or MeCN was observed by 1H NMR
spectroscopy. Nevertheless, 2-Os undergoes various other types
of reactions, as outlined in Scheme 1. Notably, 2-Os shows the
ability to activate suitable substrates through simultaneous
bifunctional metal and ligand interactions.
Upon treatment of 2-Os with an excess of [Et2OH]Cl in
Et2O, the saturated β-diimine complex [(η6-C6H6)OsCl{(2,6-
Me2-C6H3NCMe)2CH2}]OTf (3-Os) is rapidly and quantita-
tively formed as a bright orange solid. Presumably, protonation
of the β-C site of the β-diketiminate ligand affords a highly
electrophilic dicationic Os(II) species which rapidly combines
with Cl−. In contrast to 1-Os, the dominant CN character of
the β-diimine backbone in 3-Os is illustrated by a downfield
shift observed for the corresponding 13C{1H} NMR signal with
δ (ppm) 183.0 (Table 1). The neutral, weakly electron
donating β-diimine ligand mediates a highly electron deficient
Os(II) center, and consequently a shortened Os−Cl bond
length of 2.416(1) Å is observed (Figure 3). The increase in
Os−Cl bond strength is further underlined by the Cs-symmetric
1H NMR spectrum, which shows two different sets of signals
associated with the o-CH3 substituents on the flanking aryl
1
bound hydride in 4-Os is provided by a solution H NMR
inversion recovery experiment. Complex 4-Os shows a
diagnostic T1 value for the Os-bound hydride of 1.082 s,
which is significantly shorter than the 1.392 s measured
previously for the analogous Ru complex.25 A shorter T1
indicates more efficient relaxation due to a stronger Os−H
bond. Upon prolonged storage in solution and the solid state,
H2 is gradually lost, regenerating 2-Os. This process is
accelerated when 4-Os is exposed to reduced pressure, whereas
3-Os is indefinitely stable even when exposed to an O2- and
moisture-filled environment.25
As already observed in the case of 2-Ru,25 ethylene
undergoes a rapid [4 + 2] cycloaddition with 2-Os via a
synergic ligand−metal interaction, affording the light orange
adduct 5-Os. As a result, the β-diketiminato ligand in 5-Os is
transformed into a N,C,N-tridentate ligand with pronounced
imino bonding character, evidenced by the 13C{1H} NMR C
N shift of δ (ppm) 180.0 (Table 1). When it is coordinated to
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Figure 4. ORTEP representations of complexes 5-Os and 6-Os. Thermal ellipsoids are drawn at the 50% probability level. Solvates, the triflate anion,
and selected hydrogen atoms have been omitted for clarity. Relevant average bond lengths (Å) and bond angles (deg) are as follows. 5-Os: Os−C28,
2.164(2); Os−N, 2.108(2); Os−Ccent, 1.704(1); Cβ−C29, 1.569(3); C28−C29, 1.523(3); N−Os−N, 85.7(1); Ccent−Os−C28, 126.1(1); Ccent−Os−
N,N′, 154.5(1); Os−N,N′−Cβ, 134.1(1); N−Cα−Cβ, 118.4(2); C28−C29−Cβ, 113.6(2). 6-Os: Os−C28, 1.964(3); Os−N, 2.102(3); Os−Ccent
,
1.736(2); C28−N3, 1.164(5); N−Os−N, 86.8(1); Ccent−Os−C28, 127.8(1); Ccent−Os−N,N′, 150.5(1); Os−N,N′−Cβ, 153.8(2); N−Cα−Cβ,
123.9(3). Ccent = Carene centroid, and N,N′ = Nligand midpoint.
the ethylene group, the nucleophilic sp2-hybridized β-carbon of
spontaneously reacted with 2-Os in anhydrous CH2Cl2 to
afford the red-colored adduct 6-Os in 92% yield. Relevant
solution NMR data are given in Table 1, and the solid-state
structure is shown in Figure 4. The lengthened CN bond
distance of 1.164(5) Å indicates considerable π back-donation
from the metal to the isocyanide ligand. This is further
supported by the increase in double-bond character, as
evidenced by the IR ν(CN) stretch of 2144 cm−1, which is
at lower energy than that of the unbound 2,6-dimethylphenyl
isocyanide (i.e., 2160 cm−1).39 It is interesting to compare 6-Os
to similar compounds in the literature, such as [(η6-p-
c y m e n e ) O s ( C N C M e 3 ) ( M e ) C l ] , 4 0 [ ( η 6 - m e s ) -
OsCl2(CNXyl)],41 and [(η6-mes)OsH2(CNXyl)],42 where
ν(CN) is 2070, 2124, and 2055 cm−1, respectively. In
comparison, 6-Os features the least amount of metal−ligand π
back-bonding into the CN ligand. This result indicates that
both the CNXyl and the β-diketiminate ligand demonstrate
some degree of π-back-bonding with the metal center available
through energetically low-lying π*-type MOs. The latter
observation is well reflected in literature examples featuring a
second π-accepting ligand, such as [(η6-p-cymene)Os(η2-1,3-
PhNNNPh)(CNtBu)]BPh4,43 [η6-p-cymene)(Rpz)Os(HRpz)-
(CNtBu)]BPh4,44 and [(η6-p-cymene)OsCl(η2-CH2CH2)-
(CNtBu)]BPh4,45 with ν(CN) values of 2181, 2170, and
2191 cm−1, respectively.
the complex becomes sp3-hybridized, as evidenced by a more
1
shielded H NMR signal of δ (ppm) 4.54. The Os- and Cβ-
bound ethylene carbons resonate in 13C{1H} NMR at δ (ppm)
−7.5 and 16.9, respectively, and hence are more shielded than
the corresponding values of δ (ppm) 3.7 and 18.6 for 2-Ru.25
In particular, the negative 13C{1H} chemical shift associated
with the Os-bound ethylene carbon indicates strong metal−
ethylene bonding. Even a less shielded resonance of δ (ppm)
−23.0 is observed for the metal-bound methyl group in the
Os(II) complex Os(CO)2(Me)(PMe3)2I.36 The solid-state
structure of 5-Os is shown in Figure 4 and is isostructural
with that of the Ru analogue, whereby the only difference is a
greater bending of the flanking aryl groups. The 1.523(3) Å
distance measured for the eclipsing C(H2)−C(H2) unit within
5-Os is indicative of a single bond, a consequence of more
pronounced electron density at Os, as already suggested by the
solution NMR analysis. The C(H2)−C(H2) distance in 5-Os is
equivalent to that found in the structure of the dinuclear species
Os2(CO)8(μ2-η1:η1-C2H2) reported by Norton et al.37 Due to
the incorporation of the strongly electron releasing β-
diketiminate ligand, the Os−C(H2) distance of 2.164(2) Å is
longer than the mean distance of 2.030 Å compiled from the
Cambridge Crystallographic Database (CCDC).33 Of note is
the significantly long Cβ−C(H2) bond, 1.569(3) Å, as
compared to the average value of 1.530 Å for general
C(sp3)−C(sp3) bond connections. This suggests less than
ideal sp3 character at the Cβ position, providing an indication
that the ethylene addition to 2-Os is thermally reversible.
Similar to the case for 5-Os and to the previously reported
structurally related Ru(II) β-diketiminato acetylene complex,25
West et al. described a metal−ligand bifunctional cycloaddition
of terminal alkynes to the Pt(IV) β-diketiminato complex
SiPh3(H)2Pt−C2H2{(p-Cl-C6H4NCMe)2CH}.38
Computational Analysis. The reactivities of 2-Ru25 and 2-
Os with small molecules are similar, and therefore two reaction
pathways were examined by computational methods to allow
for a differentiation between the Os and Ru complexes.
Employing the Gaussian G09 package,46 high-level quantum
calculations using density functional theory were performed to
both probe all metal interactions and examine energy
differences in reaction pathways of dihydrogen cleavage and
ethylene addition. Where possible, previously established X-ray
structures were used as a starting point for the gas-phase
geometry optimizations, yielding model structures which are in
In contrast to the case for nitriles, strong π-accepting
isonitriles, such as 2,6-dimethylphenyl isocyanide (CNXyl),
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Figure 5. Gas-phase HOMO and LUMO electron densities associated with the cationic C2v complex 2-Os. The geometry was optimized on the basis
of the X-ray structure, using the M06 density functional with a 6-31G(d,p) and SDDAll (Os) basis set, as implemented in Gaussian G09.46 For
further details, see the Supporting Information.
Figure 6. Gas-phase energy diagrams for H2 (a) and C2H4 (b) adduct formation with 2-Os (black) and 2-Ru (gray). The relative energies are given
in kcal/mol, with the total energies for 2-Os/Ru and free substrate set to 0. The extent of the imaginary bond vibration associated with the transition
states of 4-Os (−961.51) and 5-Os (−332.53) is depicted graphically in the top part of the diagram. The calculated structural parameters for the
transition states are given in the Supporting Information.
good agreement with the experimentally measured structural
parameters (see the Supporting Information).
bonding observed for the β-diketiminate ligand in the case of 2-
Os. Moreover, the high electron density associated with the
metal in the metal β-diketiminate fragment leads to
considerable charge transfer to the η6-arene, which in turn
increases π back-donation, particularly in the case of 2-Os.
From the corresponding HOMO and LUMO orbitals of 2-
Os depicted in Figure 5, it is apparent that the strong π-
donating character of the β-diketiminate ligand significantly
reduces the potential for the binding by a third ligand.
However, the observed bifunctional reactivity of these
complexes with molecules such as H2 (4-Os/Ru) and C2H4
(5-Os/Ru) is the result of simultaneous weakening of the π-
donor ability of the β-diketiminate and σ-type bonding to the
metal center. Such interactions are dependent on symmetry-
matched interactions between the HOMO and LUMO of the
molecules involved. To estimate the energy differences involved
in the formation of the H2 (4-Os/Ru) and C2H4 (5-Os/Ru)
adducts, the coordinatively unsaturated 2-Os/Ru complexes
were considered as the respective starting points of the
reactions (Figure 6). In both sets of transition states, 4-Os/
Ru shows a simultaneous binding of C2H4 to metal and Cβ
center, while for 5-Os/Ru direct heterolytic cleavage of H2 by
the metal and Cβ site is calculated. A more detailed description
The coordinatively unsaturated cationic complexes 2-Os and
2-Ru show ΔEHOMO−LUMO gaps of 6.5 and 5.9 eV, respectively.
On this basis, the Os complex can be categorized as less
reactive in comparison to its Ru analogue. The shapes of the
probability distributions associated with the HOMO and
LUMO are very similar for the two complexes 2-Os and 2-
Ru (Figure 5).25 From a qualitative perspective, the HOMO is
β-diketiminate metallacycle centered, whereas the LUMO
spreads predominantly over the metal−arene fragment.
With respect to the differences in reactivity between 2-Os
and 2-Ru, it is informative to determine the degree by which
the two supporting ligands, η6-arene and β-diketiminate, engage
in synergic σ donation and π back-donation with the metal
center. Using the postanalytic electron partitioning package
AOMix,47,48 the metal−ligand bonding was investigated
through a charge decomposition analysis (CDA), as developed
by Frenking et al.49 CDA describes quantitative charge transfer
between the η6-arene/metal and β-diketiminate/metal frag-
ments. The results indicate that the more electron-rich Os ion
tends to accept less electron density from donating ligands in
comparison to Ru. This is supported by the slightly weaker σ
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Figure 7. Gas-phase electron densities associated with the dominant σ-donating and π-back-donating MOs, HOMO and HOMO-9, respectively, of
the isonitrile complex 6-Os. The geometry was optimized on the basis of the X-ray structure, using the M06 density functional with a 6-31G(d,p)
and SDDAll (Os) basis set, as implemented in Gaussian G09.46 For further details, see the Supporting Information.
of the transition states is provided in the Supporting
Information.
modeling indicate that the β-diketiminate folds along the N−N
axis and, to a lesser degree, along the M−Cβ axis. An inspection
of the MOs shows that distortion does not significantly disrupt
the π delocalization across the core atoms of the β-diketiminate
ligand but reduces the metal to ligand σ and π interactions.
Accordingly, the CDA indicates a decrease in charge transfer to
the metal center of 0.899 versus 1.046 in 2-Os.
The slightly higher reaction energies associated with the
adduct formation of H2 and C2H4 reflect the generally lower
reactivity associated with Os, in comparison to Ru. Examining
the structures of 4-Os and 4-Ru, interesting insights with
respect to the metal−hydride bond are possible. In general, the
strength of a metal−hydride bond is rather challenging to
assess. Although solution NMR and IR spectroscopy provide
useful diagnostics, a more complete understanding can be
gained in combination with DFT calculations. The Mayer bond
indices provide a normalized integration of the electron
population associated with each bond, therefore enabling a
direct comparison between different bond types.50 The
corresponding Mayer bond indices for 4-Os-H and 4-Ru-H
are 0.8288 and 0.8172, confirming the experimental solution
NMR analysis, attributing the stronger metal−hydride bond to
4-Os-H (see the Supporting Information for details). A CDA-
based comparison of the resulting neutral β-diimine in 4-Os
and the anionic β-diketiminate in 2-Os shows a reduction in the
charge transfer from ligand to metal with 0.711 versus 0.932.
Interestingly, the difference is more dramatic in the case of 4-
Ru and 2-Ru, with 0.723 versus 1.009. In both complexes,
Catalytic Studies. 1-Os and 2-Os were screened for the
ability to mediate the catalytic hydrogenation of styrene under
conditions similar to those employed for the corresponding
Ru(II) analogues (1-Ru and 2-Ru) and related chelating
bisphosphine Os complexes.25,26,51 All hydrogenation reactions
were performed with the careful exclusion of O2 to avoid
deactivation of the catalyst. In a separate experiment, a THF
solution containing 2-Os was opened to the atmosphere for
several hours. During this time, the solution darkened and a
dark brown insoluble powder was formed. Moreover, solution
1H NMR of 2-Os in nondegassed CD2Cl2 proved inconclusive
with respect to reaction with O2; however, no uncoordinated β-
diketiminate ligand was observed. Both 1-Os (TOF = 1842
h−1) and 2-Os (TOF = 1246 h−1) showed significantly higher
activity in the hydrogenation of styrene in comparison to the
Ru analogues (Table 2). With respect to the relative reactivity
of Ru and Os complexes, Bertoli et al. have observed a similarly
higher activity for the isostructural Os complexes over the
corresponding Ru analogues in the base-mediated transfer
hydrogenation of acetophenone.52 However, Baratta et al.
reported equal reactivity in the hydrogen transfer to ketones
catalyzed by the pincer complexes MCl(CNN)(Ph2P-
(CH2)4PPh2) (M = Ru, Os).13
Given that the OTf complexes 2-Os and 2-Ru feature a
vacant coordination site, the lower activity in comparison to the
chloro analogues 1-Os and 1-Ru was unexpected. It is
important to note that, for complexes 1-Os and 1-Ru, no
base cocatalyst was added to facilitate halogen abstraction, in
contrast with literature reports.14,15,53 However, on the basis of
the above solution NMR analysis of 1-Os (Table 1), the
chloride anion is only weakly interacting with the metal center,
leading to a cationic species present in solution. This weak Cl−
Os interaction therefore does not impede the catalytic activity.
In contrast, the weak interaction with the chloride anion could
2
minor back-donation from the metal HOMO dz orbital to the
π* NC bonds of the β-diimine ligand occurs, as indicated by
the CDA values of 0.056 for Ru and 0.038 for Os.
Aside from the bifunctional reactivity of 2-Os/Ru,
coordination of a third ligand to the metal center is possible
only when moderate to strong π-accepting molecules are
introduced. Here, the prototypical example is an aryl isonitrile
bound to a cationic complex (6-Os). The CDA of the
interaction between 2-Os and CNXyl shows only a moderate
charge transfer of 0.492 from the C lone pair of the isonitrile to
the Os dxy orbital, formerly the LUMO of 2-Os (Figure 7). The
2
corresponding back-donation from the tilted Os dz orbital to
the π* CN orbital amounts to a charge transfer of 0.129, which
justifies the observed moderate decrease in the CN stretching
frequency to 2144 cm−1, in comparison to unbound CNXyl
(2160 cm−1).39 It is possible to probe the electronic changes
associated with the β-diketiminate ligand upon the addition of a
third ligand. Both solid-state structures and computational
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activity in comparison to the case for styrene. For the reduction
of cyclohexene, the Os complexes are more active than the
corresponding Ru analogues. In contrast to the reactions
involving styrene, the cationic complexes 2-Os/Ru show higher
activity for the conversion of cyclohexene. The propensity of a
homogeneous Os complex to activate cyclohexene type
substrates was investigated by Harman et al., who reported
an X-ray structure of [Os(NH3)5(η2-3-methoxycyclohex-
ene)]2+.60 Only very few Os complexes have been tested in
the hydrogenation of cyclohexene. Examples include OsHCl-
(CO)(PPh3)3 (TOF = 191 h− 1),61 [OsH(CO)-
(NCMe)2(PPh3)2][BF4] (TOF = 167 h−1),61 and [{Pd-
(cod)}2(μ-biim)2Os(OPPh3)2](NO3)3.62 The heterotrinu-
clear Pd2−Os complex only showed marginal activity, and
when the {Pd(cod)} moiety was omitted, no activity was
observed. In comparison to the examples cited, 2-Os is clearly
the most active homogeneous Os hydrogenation catalyst for the
reduction of cyclohexene to cyclohexane. However, as
evidenced by a literature comparison, Os is not the metal of
choice for this type of transformation, as for example Crabtree’s
catalyst [Ir(cod)(PCy3)(py)]PF6 significantly exceeds the
activity of 2-Os.63,64
Table 2. Hydrogenation of Styrene to Ethylbenzene
Catalyzed by Complexes 1-Os/Ru and 2-Os/Ru
a
b
c
complex
time (h)
conversion (%)
TOF (h‑1)
1-Os
2-Os
1-Ru
0.5
0.5
0.5
1.0
92
62
66
84
1842
1246
1326
838
d
2-Ru
a
Conditions: 1 g (9.6 mmol) of styrene and 0.1 mol % of catalyst were
dissolved in 12 mL of anhydrous and N2-saturated THF under inert
conditions and transferred to an individually stirred multicell
autoclave. The sealed reactor was purged and pressurized to 40 bar
b
of H2 and heated to 80 °C. After the appropriate time the conversion
c
was measured by gas chromatography. Turnover frequency (TOF)
values were calculated on the basis of the listed conversion and
d
reaction time. Results taken from earlier reports.26
facilitate the adoption of a tripodal geometry at the metal
center, leading to increased activity of 1-Os over 2-Os. Only
very few Os complexes have been reported to show catalytic
activity in the hydrogenation of styrene and closely related
substrates.54−58 More recently, Albertin et al. showed that the
p-cymene Os triazenide complex [Os(η2-1,3-ArNNNAr)(η6-p-
cymene)L]BPh4 (Ar = Ph; L = P(OEt)3) hydrogenates styrene
to ethylbenzene with a TOF of approximately 22 h−1 at 100
°C.43 In comparison to the previously reported mono- and
tetranuclear osmium complexes [OsHCl(CO)(PiPr3)2],59
OsH2Cl2(PiPr3),56 and H3Os4(CO)12(I),54 1-Os shows consid-
erably higher activity.
The reduction of 1-methylcyclohex-1-ene to methylcyclohex-
ane proceeds significantly more slowly than for cyclohexene, as
a consequence of the increased steric bulk of the substrate.
Only 2-Ru shows acceptable activity with a TOF of 406 h−1
(Table 3). While the [RuH(CO)(MeCN)(tppms)3]BF4
complex reported by Baricelli et al. is inactive in the reduction
of 1-methylcyclohex-1-ene,65 [(η4-Ph4C4CO)(CO)2Ru]2 is by 1
order of magnitude more active than 2-Ru.66 2-Ru has similar
67
reactivity to that of [(cod)Ir(PCy3)(py)]PF6 but shows
higher TOFs than [Ir(PPh3)2(H)2(ClCH2CH2Cl)]BArF68 and
(MeCNC)Fe(N2)2 (MeCNC = (2,6-Me2-C6H3-imidazol-2-yli-
dene)2-C5H3N).69 However, the most active catalyst for this
type of transformation involves a Rh species.70
The complexes were further tested in the ability to mediate
the catalytic hydrogenation of cyclohexene and 1-methylcyclo-
hex-1-ene (Table 3). The increase in steric bulk of these
substrates is clearly reflected in a decrease of the catalytic
Limonene is a prominent member of the terpene class,
featuring endo- and exocyclic alkenyl bond. The naturally
occurring (+)-limonene is used in the flavor and fragrance
industry. Importantly, due to its high availability, it can also
serve as a cheap building block in asymmetric synthesis.
Moreover, work by Grzybek et al. and Shephard et al. showed
the preference for the exocyclic reduction of limonene by Rh
and Ru/Cu transition-metal complexes.71,72 Complexes 1-Os/
Ru and 2-Os/Ru mediated exclusive exocyclic hydrogenation
of limonene, with the osmium complexes being considerably
more active than the Ru counterparts (Table 4). In the case of
1-Os, quantitative conversion to p-menthene was achieved with
a TOF of 647 h−1. To the best of our knowledge, this is the first
report of an Os-catalyzed hydrogenation of limonene. However,
considering the similarity of L-carvone, the work by Sanchez-
Delgado et al. needs to be mentioned; they found selective
reduction of the exocyclic CC bond in L-carvone by the
complex [OsH(Br)(CO)(PPh3)3] in the presence of H2 (1
atm).6 Moreover, the study also revealed that, under an
increased H2 pressure of 5 atm, both CC bonds of L-carvone
were hydrogenated. The TOF for the hydrogenation of
limonene catalyzed by 1-Os is one of the highest reported in
the literature for a monometallic homogeneous complex. Even
in comparison to certain heterogenized systems, such as Pd/
Al2O3,73 Pd-AEAPSi/SiO2,74 NH2-SBA-15-[RuHCl(CO)-
(PPh3)2],75 and ionic liquid suspended [Ru(cod)(cot)] derived
nanoparticles,76 1-Os shows higher reactivity. In their initial
report on the olefin hydrogenation of limonene to p-menthene
Table 3. Catalytic Hydrogenation of Cyclohexene (R = H)
and 1-Methylcyclohex-1-ene (R = Me) by Complexes 1-Os/
Ru and 2-Os/Ru
a
b
c
complex
R
time (h)
conversion (%)
TOF (h‑1)
1-Os
1-Ru
2-Os
2-Ru
1-Os
1-Ru
2-Os
2-Ru
H
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
55
42
75
64
7
1090
840
1492
1276
66
H
H
H
Me
Me
Me
Me
7
69
6
60
41
406
a
Conditions: 1 g of substrate (cyclohexene, 12.2 mmol; 1-
methylcyclohex-1-ene, 10.4 mmol) and 0.1 mol % of catalyst were
dissolved in 12 mL of anhydrous and N2-saturated THF under inert
conditions and transferred to an individually stirred multicell
autoclave. The sealed reactor was pressurized with H2 (cyclohexene,
b
40 bar; 1-methylcyclohex-1-ene, 50 bar) and heated to 80 °C. After
the appropriate time, the conversion was measured by gas
c
chromatography. Turnover frequency (TOF) values were calculated
on the basis of the listed conversions and reaction times.
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bonding impedes the rapid release of H2, as evidenced through
the above T1 NMR analysis of 4-Os.
Table 4. Hydrogenation of Limonene to p-Menthene
Catalyzed by Complexes 1-Os/Ru and 2-Os/Ru
a
CONCLUSIONS
■
Employing standard synthetic methodology for the trans-
metalation of aryl-substituted β-diketiminate ligands, the
preparation and characterization of the first β-diketiminato
osmium complexes, 1-Os and 2-Os, has been achieved. The Ru
and Os arene β-diketiminate complexes demonstrate a strong
propensity to engage in bifunctional activity involving the metal
and β-carbon site. This work demonstrates, however, that the
metal-based coordination and activation of a third ligand is only
possible if the latter has π-accepting capabilities. Both
spectroscopic evidence and DFT calculations indicate that the
η6-arene Os(II) β-diketiminate differ from the isostructural
Ru(II) counterparts in σ- and π-based interactions between the
metal and supporting ligands. This differentiation is further
evidenced in the hydrogenation of various types of olefins,
where Os(II) proved to be more active. In contrast, Ru seems
to be the superior catalyst in dehydrogenation reactions, as
evidenced by the results obtained with N,N-dimethylamine
borane. Interestingly, using 2-Os, it is possible to hydrogenate
the exo-olefinic bond of limonene, which is normally limited to
heterogeneous-based catalysts.
b
c
complex
time (h)
conversion (%)
TOF (h‑1)
1-Os
1-Ru
2-Os
2-Ru
1.5
1.0
1.5
1.0
97
8
647
84
50
20
334
195
a
Conditions: 1 g (9.6 mmol) of limonene and 0.1 mol % of catalyst
were dissolved in 12 mL of anhydrous and N2-saturated THF under
inert conditions and transferred to an individually stirred multicell
autoclave. The sealed reactor was pressurized to 50 bar of H2 and
heated to 80 °C. After the appropriate time, the conversion was
measured by gas chromatography. Turnover frequency (TOF) values
b
c
were calculated on the basis of the listed conversion and reaction time.
by (iPrPDI)Fe(N2)2 (iPrPDI = {(2,6-CHMe2)2C6H3N
CMe}2C5H3N), Bart et al. stated a TOF of 164 h−1 in
toluene77 and a TOF of 1085 h−1 in n-pentane.78 Very recently,
the most active systems for the selective hydrogenation of
limonene have been reported, both based on Rh nanoparticles,
79
EXPERIMENTAL SECTION
suspended either in ethanol
or in the ionic liquid
■
[bimim]PF6.80
The complexes were synthesized using standard Schlenk techniques,
whereas subsequent syntheses and manipulations of all products and
reagents were performed in a glovebox with a N2 atmosphere
containing less than 1 ppm of O2 and H2O. All glassware was dried at
130 °C, and the flasks underwent three N2 purge/refill cycles prior to
the introduction of solvents or reagents. All solvents were dried
according to literature procedures involving distillation over the
appropriate drying agents and storage in Schlenk flasks equipped with
a Teflon stopcock.86 Extra-dry THF was purchased from Acros
Organics (Fisher Scientific). Benzene-d6, n-pentane, and toluene were
dried and distilled over sodium. THF-d8 was purchased dry from
Aldrich and used as received. CH2Cl2 and CD2Cl2 were distilled over
CaH2 and stored under inert conditions. As a filtration aid, Celite was
kept in an oven at 130 °C and flushed with N2 prior to use. The
precursor dichloro(η6-C6H6)osmium(II) dimer was prepared accord-
ing to literature procedures.25,87 Other reagents were purchased from
commercial sources and used as received if not specified otherwise.
NMR spectra were recorded using Varian VNMRS 300 and 400 and a
In view of the ongoing interest in hydrogen storage related
catalytic dehydrogenation of amine borane substrates,81−85 2-
Os was tested for its capability to catalytically dehydrocouple
N,N-dimethylamine borane (DMAB). As depicted in Figure 8,
1
Varian INOVA 500 instruments. Chemical shifts for H and 13C{1H}
NMR spectra were referenced to the relevant solvent peaks, observed
as residual signals. 19F NMR spectra were referenced to the relevant
residual solvent peak and CCl3F. Infrared spectra were recorded on a
Varian 3100FT-IR Excalibur spectrometer. Samples were prepared as
Nujol mulls on KBr disks. Elemental microanalyses were obtained
using an Exeter Analytical EA-1110 elemental analyzer. Mass spectra
were recorded on a Waters alliance HT Micromass Quattro LCT
(MeOH/H2O, 60/40) TOF instrument with a cone voltage of 35 V
and a capillary voltage of 2800 V (+) and 2500 V (−). Gas
chromatography measurements were performed on a Varian CP-3380
Gas Chromatographic Analyzer.
Figure 8. H2 gas evolution (DMAB equivalents) as a function of time
(h) for the dehydrogenation of DMAB (3.2 M in THF) at 42 °C,
catalyzed by 0.5 mol % of 2-Os ( ) and 2-Ru ( ), respectively. The
background reaction without catalyst added is shown as a solid line
(). Further details are provided in the Experimental Section.
□
○
Synthesis of (η6-C6H6)OsCl{(2,6-Me2-C6H3NCMe)2CH} (1-Os).
Dichloro(η6-C6H6)osmium(II) dimer (300 mg, 0.44 mmol) was
weighed into a 50 mL Schlenk tube under N2. In a second 50 mL
Schlenk tube, Li{(2,6-Me2-C6H3NCMe)2CH} (304 mg, 0.97 mmol)
was dissolved in dried and N2-saturated CH2Cl2 (15 mL) and added to
the dichloro(η6-C6H6)osmium(II) dimer by cannula. The reaction
mixture was stirred for 24 h under N2. The mixture was filtered over
Celite under N2, and the solvent was reduced to 1 mL. Dried and N2-
saturated n-pentane was added to precipitate the crude product. After
filtration under N2 and three washings with n-pentane, 1-Os (392 mg,
73%) was obtained as a light brown solid. Anal. Found (calcd): C,
0.5 mol % of 2-Os affords nearly quantitative dehydrogenation
of DMAB after 30 h at 42 °C. The observed activity is
significantly lower than that previously reported for 2-Ru,
which showed quantitative dehydrogenation of DMAB after
only 0.5 h.27 The modest reactivity of 2-Os for amine borane
dehydrogenation is in strong contrast with the superior activity
of Os observed in the above olefin hydrogenation reactions. A
possible explanation might be that the strong Os−hydride
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47.23 (46.84); H, 4.54 (4.74), N, 3.85 (3.77). 1H NMR (400 MHz, 25
°C, CD2Cl2): δ (ppm) 1.79 (s, 6H, α-CH3), 2.26 (s, 12H, o-CH3),
5.07 (s, 6H, η6-C6H6), 5.66 (s, 1H, β-CH), 7.04−7.08 (m, 2H, Ar p-
CH), 7.21−7.23 (m, 4H, m-CH). 13C{1H} NMR (100 MHz, 25 °C,
CD2Cl2): δ (ppm) 19.3 (s, o-CH3), 23.3 (s, α-CH3), 76.9 (s, η6-C6H6),
103.4 (s, β-CH), 126.8 (s, Ar p-CH), 128.9 (s, Ar m-CH), 132.4 (s, Ar
o-C), 156.8 (s, Ar i-C), 162.9 (s, CN). ESI MS+ (25 °C, CH2Cl2)
(m/z): 575.240 [parent M − Cl−, 100%, calcd 575.201]. FT-IR (25
°C, ATR): ν (cm−1) 3391 (m, br), 3054 (w), 2956 (w), 2919 (m),
1641 (w), 1622 (w), 1590 (w), 1553 (w), 1528 (s), 1465 (w), 1450
(m), 1428 (m), 1392 (vs), 1334 (w), 1264 (w), 1242 (w), 1186 (s),
1172 (w), 1098 (w), 1090 (w), 1024 (w), 978 (w), 907 (w), 852 (s),
843 (m), 817 (w), 798 (w), 773 (m), 767 (vs), 760 (s), 756 (s), 711
(w).
(ppm) −6.65 (s, 1H, Os-H), 1.98 (s, 6H, o-CH3), 2.06 (s, 6H, o-
CH3′), 2.23 (s, 6H, α-CH3), 3.14 (d, 2JHH = 14,8 Hz, 1H, β-CH), 4.44
(d, 2JHH = 14,8 Hz, 1H, β-CH), 4.67 (s, 6H, η6-C6H6), 7.05−7.08 (m,
2H, Ar p-CH), 7.12−7.14 (m, 2H, Ar m-CH), 7.21−7.22 (m, 2H, Ar
m-CH′). 13C{1H} NMR (100 MHz, 25 °C, CD2Cl2): δ (ppm) 18.0 (s,
o-CH3), 18.2 (s, o-CH3′), 23.8 (s, α-CH3), 54.9 (s, β-CH), 79.0 (s, η6-
−
C6H6), 121.2 (q, 1JCF = 321.5 Hz, CF3SO3 ), 127.7 (s, Ar p-CH), 127.9
(s, Ar o-C), 129.0 (s, Ar m-CH), 129.2 (s, Ar m-CH′), 129.5 (s, Ar o-
C′), 153.2 (s, Ar i-C), 177.8 (s, CN). Note: T1 of Os−H is 1.082 s
(as measured by the inversion recovery method). FT-IR (25 °C,
ATR): ν (cm−1) 3067 (vw), 2976 (w), 2922 (w), 2867 (w), 1981 (w,
Os−H), 1633 (vw), 1589 (vw), 1562 (w), 1511 (w), 1469 (w), 1431
(w), 1383 (w), 1469 (w), 1431 (w), 1383 (w), 1346 (w), 1305 (w),
1259 (vs), 1223 (m), 1149 (s), 1095 (w), 1067 (vw), 1030 (s), 910
(w), 859 (w), 825 (m), 771 (m), 753 (w), 715 (w), 637 (s).
Synthesis of [(η6-C6H6)Os{(2,6-Me2-C6H3NCMe)2CH}]OTf (2-
Os). 1-Os (300 mg, 0.49 mmol) and NaOTf (93 mg, 0.54 mmol) were
added to a 50 mL Schlenk tube under a N2 atmosphere. The solids
were dissolved in dried and N2-saturated CH2Cl2 (10 mL) and stirred
overnight. The solution was filtered over Celite under inert conditions
to remove NaCl. The solvent was evaporated, and the crude product
was washed with dried and N2-saturated n-pentane, decanted, and
dried under vacuum to afford 2-Os (312 mg, 88%) as a dark brown
solid. Anal. Found (calcd): C, 46.40 (46.35); H, 4.31 (4.36); N, 3.87
(3.83). ESI MS+ (25 °C, CH2Cl2) (m/z): 575.267 [parent M+, 100%,
Synthesis of [(η6-C6H6)Os(C2H4){(2,6-Me2-C6H3NCMe)2CH}]-
OTf (5-Os). The title compound was isolated as a light orange solid
(43 mg, 41%), following a procedure identical with that reported for
the corresponding Ru(II) complex.25 Anal. Found (calcd): C, 46.95
(47.99); H, 4.50 (4.70); N, 3.75 (3.73). Elemental analysis was
affected by the partial loss of ethylene. ESI MS+ (25 °C, CH2Cl2) (m/
z): 577.6035 [parent (M − C2H4 + H+)+, 100%, 577.2163]. 1H NMR
(25 °C, 400 MHz, CD2Cl2): δ (ppm): 1.35 (m, 2H, Os−CH2), 1.96
(s, 6H, o-CH3), 2.05 (s, 6H, α-CH3), 2.23 (s, 6H, o-CH3′), 2.35 (m,
2H, Os−CβH2), 4.54 (s, 1H, β-H), 4.67 (s, 6H, η6-C6H6), 7.10−7.13
(m, 2H, Ar p-CH), 7.16−7.18 (m, 2H, Ar m-CH), 7.22−7.24 (m, m-
CH′). 13C{1H} NMR (25 °C, 100 MHz, CD2Cl2): δ (ppm) −7.5 (s,
Os-CαH2), 16.9 (s, Os-CβH2), 18.0 (s, o-CH3), 18.9 (s, o-CH3′), 22.7
1
575.201]. H NMR (400 MHz, 25 °C CD2Cl2): δ (ppm) 2.13 (s, 12
H, o-CH3), 2.23 (s, 6H, α-CH3), 5.79 (s, 6H, η6-C6H6), 7.00 (s, 1H, β-
CH), 7.18−7.22 (m, 2H, Ar p-CH), 7.33−7.35 (m, 4H, m-CH).
13C{1H} NMR (101 MHz, 25 °C, CD2Cl2): δ (ppm) 19.1 (s, Ar o-
CH3), 23.2 (s, α-CH3), 77.1 (s, η6-C6H6), 107.9 (s, β-CH), 121.6 (q,
1
(s, α-CH3), 61.4 (s, β-CH), 80.1 (s, η6-C6H6), 121.6 (q, JCF = 321.5
−
−
1JCF = 320.9 Hz, CF3SO3 ), 128.3 (s, Ar p-CH), 129.2 (s, Ar m-CH),
Hz, CF3SO3 ), 128.1 (s, Ar p-CH), 128.9 (s, Ar o-C), 129.4 (s, Ar m-
130.0 (s, Ar o-C), 158.6 (s, Ar i-C), 165.7 (s, CN). 19F NMR (376
CH), 129.6 (s, Ar o-C′), 129.7 (s, Ar m-CH′), 152.8 (s, Ar i-C), 180.0
(s, CN). FT-IR (25 °C, ATR): ν (cm−1) 2955 (vw), 2917 (vw),
1626 (vw), 1612 (w), 1590 (vw), 1471 (w), 1420 (w), 1398 (w), 1381
(w), 1333 (vw), 1260 (vs), 1223 (m), 1183 (w), 1150 (s), 1132 (m),
1098 (w), 1028 (vs), 998 (vw), 883 (vw), 832 (m), 804 (w), 791 (w),
782 (w), 774 (m), 766 (m), 753 (w), 718 (w), 711 (vw).
−
MHz, 25 °C, CD2Cl2): δ (ppm) −78.89 (s, CF3SO3 ). FT-IR (25 °C,
ATR): ν (cm−1) 3068 (vw), 2919 (vw), 1592 (w), 1560 (w), 1512
(w), 1470 (w), 1430 (w), 1382 (w), 1345 (w), 1271 (vs, S−O), 1221
(m), 1180 (w), 1152 (s, C−F), 1098 (w, C−F), 1088 (w), 1029 (s),
979 (w), 876 (w), 863 (m), 778 (m), 752 (w), 716 (w).
Synthesis of [(η6-C6H6)-Os(CNXyl){(2,6-Me2-C6H3NCMe)2CH}]-
OTf (6-Os). 2-Os (100 mg, 0.138 mmol) and 2,6-dimethylphenyl
isocyanide (19 mg, 0.145 mmol) were weighed into a 50 mL Schlenk
tube under inert conditions and dissolved in dried and N2-saturated
CH2Cl2 (10 mL). The reaction mixture was stirred overnight under N2
and the solvent removed under vacuum to afford 6-Os as a red solid
(108 mg, 92%). Crystals suitable for X-ray analysis were grown from a
concentrated CH2Cl2 solution by slow vapor diffusion of n-pentane at
−10 °C. Anal. Found (calcd): C, 51.43 (51.92); H, 5.13 (4.71); N,
4.47 (4.91). ESI MS+ (25 °C, CH2Cl2) (m/z): 707.6621 [parent (M +
H+)+, 100%, 707.3000]. 1H NMR (400 MHz, 25 °C, CD2Cl2): δ
(ppm) 1.58 (s, 6H, α-CH3), 2.22 (s, 6H, Ar o-CH3), 2.26 (s, 6H, Ar o′-
CH3), 2.56 (s, 6H, Xyl Ar o-CH3), 4.85 (s, 6H, η6-C6H6), 5.13 (s, 1H,
β-CH), 7.06−7.34 (m, 9H, Ar CH). 13C{1H} NMR (100 MHz, 25 °C,
CD2Cl2): δ (ppm) 18.5 (s, Ar o-CH3), 19.6 (s, Xyl o-CH3), 19.7 (s, Ar
o′-CH3), 22.4 (s, α-CH3), 86.2 (s, η6-C6H6), 103.5 (s, β-CH), 121.6 (q,
Synthesis of [(η6-C6H6)OsCl{(2,6-Me2-C6H3NCMe)2CH2}]OTf
(3-Os). 2-Os (100 mg, 0.138 mmol) was suspended in dried and
N2-saturated Et2O (10 mL) in a 50 mL Schlenk tube under inert
conditions and cooled to −20 °C in a N2(l)/EtOH slurry. A 1 N
solution of Et2O/HCl (0.15 mL, 0.15 mmol) was added dropwise by
syringe with stirring. The reaction was warmed to room temperature
and stirred for 2 h under N2. The crude product was filtered under
inert conditions and washed with Et2O to afford 3-Os (97.6 mg, 93%)
as a light green solid. Crystals suitable for X-ray analysis were grown by
diffusion of n-pentane into a saturated solution of the title compound
in CH2Cl2. Anal. Found (calcd): C, 44.72 (44.29); H, 4.64 (4.25); N,
3.26 (3.69). ESI MS+ (25 °C, MeCN) (m/z): 575.2101 [parent (M −
Cl)+, 100%, 575.2102]. 1H NMR (400 MHz, 25 °C, CDCl3): δ (ppm)
2.14 (s, 6H, α-CH3), 2.31 (s, 6H, o-CH3), 2.34 (s, 6H, o-CH3′), 4.31
2
(d, 1H, JHH = 20.2 Hz, β-CH), 4.81 (d, 2JHH = 20.2 Hz, 1H, β-CH′),
5.05 (s, 6H, η6-C6H6), 7.10−7.14 (m, 2H, Ar p-CH), 7.19−7.22 (m,
4H, Ar m-CH). 13C{1H} NMR (101 MHz, 25 °C, CDCl3): δ (ppm)
18.7 (s, o-CH3), 19.0 (s, o-CH3′), 24.8 (s, α-CH3), 49.2 (s, β-CH2),
79.9 (s, η6-C6H6), 128.3 (s, Ar p-CH), 129.2(6) (s, Ar m-CH),
129.2(9) (s, Ar m-CH′), 129.8 (s, Ar o-CH), 130.2 (Ar o-CH′), 153.0
(Ar i-C), 183.0 (CN). 19F NMR (376 MHz, 25 °C, CDCl3): δ
−
1JCF = 321.5 Hz, CF3SO3 ), 127.4 (s, Xyl p-CH), 129.1 (s, Xyl Ar m-
CH), 129.7 (s, Ar m-CH), 129.8 (s, Ar p-CH), 130.4 (s, Xyl Ar o-CH),
131.6 (s, Ar o′-CH), 132.8 (s, Ar o-C), 136.6 (s, Xyl Ar i-CH), 154.7 (s,
Ar i-C), 164.2 (s, NCCH3). FT-IR (25 °C, ATR): ν (cm−1) 3062
(w), 2953 (w), 2920 (w), 2144 (s, CN), 1552 (w), 1523 (w), 1453
(m), 1435 (m), 1387 (s), 1263 (vs), 1225 (m), 1187 (m), 1168 (w),
1148 (m), 1134 (m), 1101 (vw), 1027 (vs), 975 (vw), 858 (vw), 839
(m), 808 (vw), 770 (s), 754 (w), 713 (w), 669 (w).
Hydrogenation Catalysis. The alkene substrates were distilled
from CaH2 and degassed with three freeze−thaw cycles prior to use.
The experiments were conducted in a custom-built multicell (10 mL
each) steel autoclave reactor. A THF stock solution (2 mL) containing
a magnetic Teflon stirrer and 0.1 mol % of 1-Os/Ru or 2-Os/Ru along
with the appropriate alkene (1.0 g) and dried (CaH2) and degassed n-
octane (100 mg) as a GC standard were added to the autoclave under
inert conditions. After the autoclave was sealed, the cells were purged
with high-purity H2 gas (99.8%, 5 bar). The autoclave was heated to
the required temperature (80 °C) under H2 pressure (10 bar). After
−
(ppm) −78.25 (s, CF3SO3 ). FT-IR (25 °C, Nujol mull, KBr disks), ν
(cm−1) 2953 (s), 2898 (s), 2861 (s), 1641 (w), 1463 (m), 1375 (m),
1267 (vs), 1229 (w), 1155 (s), 1099 (vw), 1030 (m), 910 (vw), 853
(w), 776 (w), 720 (vw), 637 (m).
Synthesis of [(η6-C6H6)OsH{(2,6-Me2-C6H3NCMe)2CH2}]OTf (4-
Os). 2-Os (120 mg, 0.17 mmol) was weighed into a narrow 20 mL
Schlenk tube under inert conditions and dissolved in dried and N2-
saturated THF (10 mL). The solution was degassed under slight
vacuum, and H2(g) was run over the solution for 2 min and allowed to
diffuse into the THF solution at −10 °C for 48 h to afford orange
crystals suitable for X-ray analysis. Elemental analysis was affected by
partial loss of hydrogen. Anal. Found (calcd): C, 45.98 (46.40); H,
1
4.33 (4.59); N, 3.78 (3.87). H NMR (400 MHz, 25 °C, CD2Cl2): δ
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Organometallics
Article
the desired temperature was reached, the reactor was fully pressurized
to 40 or 50 bar with stirring. The reaction temperature (80 °C) was
maintained for the duration of the experiment. Afterward, all catalytic
reactions were quenched by depressurizing and cooling the reactor to
ambient conditions. The autoclave was opened under air, and small
aliquots were taken and diluted with toluene. The conversion and
selectivity of the individual hydrogenation reactions were analyzed by
GC employing the following conditions. A Varian CP-3380 Gas
Chromatographic Analyzer was used with a CP-Sil 5CB normal phase
(low polarity) 25 m × 0.25 mm column. The reported TOF values are
averages from three separate runs.
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prior to each run. Unless otherwise stated, the reactions were carried
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ASSOCIATED CONTENT
* Supporting Information
■
S
(20) Esteruelas, M. A.; Fernandez, I.; Gomez-Gallego, M.; Martin-
Ortiz, M.; Molina, P.; Olivan, M.; Oton, F.; Sierra, M. A.; Valencia, M.
Dalton Trans. 2013, 42, 3597−3608.
Text, tables, figures, and CIF files giving crystallographic details
for all relevant complexes, computational methodology, Mayer
bond indices, and a detailed comparison of the charge
decomposition analysis for selected model complexes. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) Mandal, S.; Mandal, S.; Seth, D. K.; Mukhopadhyay, B.; Gupta,
P. Inorg. Chim. Acta 2013, 398, 83−88.
(22) Jantke, D.; Cokoja, M.; Poethig, A.; Herrmann, W. A.; Kuehn, F.
E. Organometallics 2013, 32, 741−744.
(23) Albertin, G.; Antoniutti, S.; Bonaldo, L.; Botter, A.; Castro, J.
Inorg. Chem. 2013, 52, 2870−2879.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.D.P.: andrew.phillips@ucd.ie.
■
(24) Sommer, M. G.; Schweinfurth, D.; Weisser, F.; Hohloch, S.;
Sarkar, B. Organometallics 2013, 32, 2069−2078.
(25) Phillips, A. D.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J.
Organometallics 2007, 26, 1120−1122.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The initial part of this research was funded through the
European Commission Marie Curie Action EIF (ADP, MEIF-
CT-2005-025287) and the Swiss National Science Foundation.
This research was further supported through a commercializa-
tion grant from Enterprise Ireland, project no. CF20111039,
and by Science Foundation Ireland (SFI) through a grant
supporting the Solar Energy Conversion Cluster (07/SRC/
B1160). Finally, A.D.P. thanks Science Foundation Ireland
(SFI) for a Stokes Lectureship at UCD.
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