Alkali metal directed assembly of heterometallic Vv/M (M = Na, K, Cs) coordination polymers: Structures, topological analysis, and oxidation catalytic properties

Article abstract of DOI:10.1021/ic400743h

The reactions of [VO(acac)₂] with bis(salicylaldehyde)- oxaloyldihydrazone (H₄L) and an alkali metal carbonate M ₂CO₃ (M = K, Na, Cs), in EtOH/H₂O medium upon reflux, resulted in the generation of three new heterometallic V^V/M materials, namely the 1D [(VO₂)₂(μ₄-L) {Na₂(μ-H₂O)₂(H₂O) ₂}]_n (1), 2D [{V(μ-O)₂}₂(μ ₄-L){K₂(μ-H₂O)₂(H ₂O)₂}]_n (2), and 3D [{V(μ-O) (μ₃-O)}₂(μ₈-L){Cs₂(μ-H ₂O)₂(H₂O)₂}]_n (3) coordination polymers. They were isolated as air-stable solids and fully characterized by IR, UV-vis, ¹H, and ⁵¹V NMR spectroscopy, ESI-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses, the latter showing that 1-3 are constructed from the resembling [(VO₂)₂(μ_4/8-L)]^2- blocks assembled by the differently bound aqua-metal [M₂(μ-H ₂O)₂(H₂O)₂]²⁺ moieties (M = Na, K, Cs). The main distinctive features of 1-3 arise from the different coordination numbers of Na (5), K (7), and Cs (9) atoms, thus increasing the complexity of the resulting networks from the ladder-like 1D chains in 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology. Compounds 1-3 also show solubility in water (S_{25 C} ≈ 4-7 mg mL^-1) and were applied as efficient precatalysts for the homogeneous oxidation of cyclohexane by aqueous H₂O₂, under mild conditions (50 C) in MeCN/H₂O medium and in the presence of an acid promoter. Total yields (based on substrate) of cyclohexanol and cyclohexanone up to 36% and turnover numbers (TONs) up to 5700 were achieved.

Full text of DOI:10.1021/ic400743h

Article
pubs.acs.org/IC
Alkali Metal Directed Assembly of Heterometallic Vv/M (M = Na, K,
Cs) Coordination Polymers: Structures, Topological Analysis, and
Oxidation Catalytic Properties
Samik Gupta,† Marina V. Kirillova,† M. Fatima C. Guedes da Silva,*,†,‡ Armando J. L. Pombeiro,*,†
́
and Alexander M. Kirillov*,†
†Centro de Química Estrutural, Complexo I, Instituto Superior Tec
Lisbon, Portugal
́
nico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001,
‡Universidade Lusof
́
ona de Humanidades e Tecnologias, ULHT, Lisbon, Av. do Campo Grande, 376, 1749-024, Lisbon, Portugal
S
* Supporting Information
ABSTRACT: The reactions of [VO(acac)2] with bis(salicylaldehyde)-oxaloyl-
dihydrazone (H4L) and an alkali metal carbonate M2CO3 (M = K, Na, Cs), in
EtOH/H2O medium upon reflux, resulted in the generation of three new
heterometallic VV/M materials, namely the 1D [(VO2)2(μ4-L){Na2(μ-
H2O)2(H2O)2}]n (1), 2D [{V(μ-O)2}2(μ4-L){K2(μ-H2O)2(H2O)2}]n (2), and
3D [{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-H2O)2(H2O)2}]n (3) coordination poly-
mers. They were isolated as air-stable solids and fully characterized by IR, UV−
1
vis, H, and 51V NMR spectroscopy, ESI−MS( ), elemental, thermal, and
single-crystal X-ray diffraction analyses, the latter showing that 1−3 are
constructed from the resembling [(VO2)2(μ4/8-L)]2− blocks assembled by the
differently bound aqua-metal [M2(μ-H2O)2(H2O)2]2+ moieties (M = Na, K, Cs).
The main distinctive features of 1−3 arise from the different coordination
numbers of Na (5), K (7), and Cs (9) atoms, thus increasing the complexity of
the resulting networks from the ladder-like 1D chains in 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The
topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/
kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology.
Compounds 1−3 also show solubility in water (S25 °C ≈ 4−7 mg mL−1) and were applied as efficient precatalysts for the
homogeneous oxidation of cyclohexane by aqueous H2O2, under mild conditions (50 °C) in MeCN/H2O medium and in the
presence of an acid promoter. Total yields (based on substrate) of cyclohexanol and cyclohexanone up to 36% and turnover
numbers (TONs) up to 5700 were achieved.
Scheme 1. Structural Formula of H4L
INTRODUCTION
■
Within the sweeping development of the research on
coordination polymers in recent years,1,2 the design of new
vanadium containing materials with attractive functional
properties3 is primarily associated with the unique redox,
catalytic, and coordinative versatility of the vanadium ions4 in
such metal−organic networks. In particular, the search for
novel organic building blocks and vanadium-based nodes to
generate coordination polymers with desired properties is a
promising research direction, which can lead to new materials
with unusual structural, topological, and functional character-
istics.
In this regard, we have focused our attention on the
oxaloyldihydrazone derivative [H4L, bis(salicylaldehyde)-oxa-
loyldihydrazone, Scheme 1], which, in spite of possessing up to
eight potential N,O-coordination sites and an ability to bind
vanadium5 and other metal centers,6,7 has not yet been applied
as a building block toward the design of coordination polymers,
as confirmed by a search of the Cambridge Structural
Database.7 From the other side, the common s-block metal
centers are known as versatile nodes8 to compose various V-
based secondary building units (SBUs) into coordination
networks.7 Bearing these points in mind, the first objective of
the present study was to probe the complexation of vanadium
ions with H4L, followed by assembling the resulting SBUs into
heterometallic V/M coordination polymers driven by different
alkali metals (M = Na, K, Cs). Moreover, given the recognized
application of various vanadium compounds with N,O-ligands
in oxidation catalysis9 and our interest toward the development
of efficient V-based catalytic systems for the mild oxidative
Received: March 26, 2013
© XXXX American Chemical Society
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a
Scheme 2. Simplified Representation of the Synthesis and Structural Formulae of 1−3
a
Numbers 1, 2, 3, and 4 correspond to extensions of polymeric motifs.
functionalization of alkanes,10,11 the second objective of the
current work consisted in exploring such a catalytic direction,
namely by testing the obtained coordination polymers as
precatalysts in the oxidation of cyclohexane as a model
substrate.
Hence, we describe herein the synthesis, full characterization,
structural features, and catalytic application of three new
heterometallic Vv/Na (1), Vv/K (2), and Vv/Cs (3)
coordination polymers. Apart from revealing an interesting
example of periodicity wherein the network complexity
increases from Na (1D) to K (2D) and Cs (3D), the obtained
compounds constitute the first coordination polymers of any
metal that derived from the oxaloyldihydrazone H4L building
block.7 Furthermore, the present study shows that 1−3 are rare
examples of aqua-soluble coordination polymers,12,13 which
also act as efficient precatalysts for the homogeneous oxidation
of cyclohexane, by H2O2 in MeCN/H2O medium and under
mild conditions, to give cyclohexanol and cyclohexanone.
related N,O-ligands.4,5 Although all the compounds 1−3 bear
resembling [(VO2)2(L)]2− units, the main distinctive features
arise from different coordination environments around Na, K,
and Cs atoms. In fact, the coordination number of M+ rises
from 5 (Na) to 7 (K) and 9 (Cs) on going from a lighter to a
heavier alkali metal ion. As a consequence, the complexity of
the resulting network increases from 1D chains in 1 to 2D
layers in 2, and 3D framework in 3. A key role in defining the
dimensionality of the obtained polymers is thus played by the
type of alkali metal. The new products 1−3 were isolated as
yellow air-stable solids in good yields (72−76% based on H4L)
1
and fully characterized by IR, H and 51V NMR, and UV−vis
spectroscopy, ESI-MS( ), elemental, thermal, and single-
crystal X-ray diffraction analysis.
The IR spectra of 1−3 show comparable features due to the
presence of similar [(VO2)2(L)]2− units and hydrated alkali
metal ions. Hence, very strong bands in the 3700−3000 cm−1
region with maxima at 3430−3445 cm−1 are assigned to
ν(H2O) vibrations, their broad character being associated with
multiple hydrogen bonding interactions.14c Other characteristic
ν(CO) and ν(CN) vibrations appear as intense bands
with maxima at 1607−1608 and 1547−1548 cm−1, respec-
tively.5 As expected, these bands in 1−3 are shifted in
comparison with those of H4L [1692 ν(CO) and 1595
ν(CN) cm−1]. The detection of strong and broad ν(VO)
vibrations centered at 915 (1), 913 (2), and 921 (3) cm−1
supports the presence of the cis-VO2 groups in all compounds.5
In addition, the ν(VO) bands in 2 and 3 possess shoulders at
875−872 cm−1 that are presumably associated with the bridging
μ2- or μ3-O modes of oxido ligands, as a result of additional
binding to K or Cs atoms. The solution UV−vis spectra of 1−3
show broad absorptions with maxima at 324−325 and 409−414
nm that are assigned to ligand-to-metal charge transfer.15a The
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Aiming at the synthesis
of new heterometallic V/M (M = Na, K, Cs) coordination
polymers supported by multifunctional N,O-ligands, we
selected an oxaloyldihydrazone derivative H4L as a main
building block, since its application in coordination chemistry is
still very limited.5−7 Hence, treatment of H4L with [VO(acac)2]
in ethanol under reflux in air, followed by the addition of an
aqueous solution of alkali metal carbonate M2CO3, gave rise to
the series of coordination polymers [(VO2)2(μ4-L){Na2(μ-
H2 O)2 (H2 O)2 }]n (1), [{V(μ-O)2 }2 (μ4 -L){K2 (μ-
H2O)2(H2O)2}]n (2), and [{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-
H2O)2(H2O)2}]n (3) (Scheme 2). The observed herein
oxidation of the vanadium(IV) precursor [VO(acac)2] to give
the vanadium(V) products 1−3 is driven by the presence of
dioxygen, moist EtOH, and reflux conditions.4e,14a,b In fact,
[VO(acac)2] is known to undergo the oxidation to VV species
in the presence of dioxygen,14a opening up an entry to various
vanadium(V) derivatives that also include those with the
1
solution H NMR spectra of 1−3 are rather similar and reveal
expected signals due to aromatic C−H or C(H)N protons,
which occur as two multiplets at δ 6.86−6.98 and 7.44−7.62 or
singlets at δ 8.88−8.91, respectively.15b The absence of signals
corresponding to the NH and OH protons confirms the
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deprotonated L(4−) form of the ligand. The 51V NMR spectra
of 1−3 in aqueous CD3CN are also similar, showing one
resonance at δ −538 (Figure S1, Supporting Information).
A significant feature of 1−3 consists in their water solubility
(S25°C ≈ 4−7 mg mL−1) because hydrosoluble coordination
polymers are still rare,12,13 in spite of possessing rather high
potential for biological16 or catalytic12 applications in aqueous
media. Although the solubility of 1−3 in H2O is eventually
associated with their partial dissociation into ionic species via
decoordination of alkali metal ions, the ESI-MS(+) spectra of
1−3 also reveal the presence of moieties that resemble those of
molecular ions of the repeating units. Hence, the [(VO2)2(L)-
Na2 + Na]+ (m/z = 557) and [(VO2)2(L)K2(H2O)2 + H]+ (m/
z = 604) fragments are detected as the most intense peaks
(100% relative intensity) in the ESI-MS(+) spectra of 1 and 2.
The related [(VO2)2(L)Cs2 + Cs]+ (m/z = 886) and
[(VO2)2(L)Cs2 + H]+ (m/z = 755) fragments are also
observed in the respective spectrum of 3, although of low
intensity due to a poor fragmentation in the positive mode.
Other signals detected in ESI-MS(+) of 1 and 2 correspond to
[(VO2)2(L)Na2(H2O)4 + VO2]+ (m/z = 690), [(VO2)2(L)-
Na2(H2O)4 + H2O + H]+ (m/z = 625), [(VO2)2(L)K2(H2O)3
+ K]+ (m/z = 660), and [(VO2)(L)K2 + 2H]+ (m/z = 485)
fragments. The ESI−MS(−) plots further confirmed the
formulations of 1−3, revealing the characteristic [(VO2)2(L)-
M]− (m/z = 511, 527, and 621 in 1−3, respectively),
[(VO2)2(L) + H]− (m/z = 489), and [(VO2)2(L)]2− (m/z =
244) fragments with expected isotopic distribution patterns.
The differential thermal analyses of 1−3 show three to five
thermal effects. The endothermic effects in the 30−130 (1),
40−170 (2), or 40−160 (3) °C temperature range correspond
to removal of four (in 1) or two (in 2, 3) water ligands.
Interestingly, the elimination of the two remaining water
molecules in 2 and 3 only begins at 230 °C, supporting a
stronger binding of μ-H2O ligands in these compounds. The
subsequent exothermic effects in the 260−550 (1), 230−520
(2), or 230−400 (3) °C temperature intervals are mainly
associated to the multistep decomposition of L, resulting in
MVO3 (M = Na, K, Cs) as major decomposition products.
X-ray Crystal Structures. The crystal structures of 1−3 are
composed of one vanadium(V) atom, one alkali metal atom,
half of L(4−) moiety, two oxido ligands, and two aqua ligands
per asymmetric unit. These are arranged into resembling
[(VO2)2(L)]2− blocks that are interconnected through the
differently bound aqua-metal [M2(μ-H2O)2(H2O)2]2+ moieties
(M = Na, K, Cs), resulting in the formation of distinct
heterometallic 1D, 2D, and 3D metal−organic networks,
respectively.
Figure 1. Structural fragments of 1 showing (a) ellipsoid plot (50%
probability) with partial atom labeling scheme, (b) ladder-like 1D
chain (view along the a axis), (c) topological representation of the
underlying 1D network. Further details: (a, b) H atoms are omitted for
clarity, color codes: V (green), Na (magenta), N (blue), O (red), and
C (gray); (c) 3-connected Na nodes (magenta), centroids of the 2-
connected [(VO2)2(μ4-L)]2− (green) and μ-H2O (red) linkers.
Selected distances (Å): V1−N1 2.131(2), V1−O1 1.8821(16), V1−
O2 1.9993(16), V1−O3 1.6188(18), V1−O4 1.6434(17), Na1i−N2
2.419(2), Na1−O2 2.3529(17), Na1−O5 2.270(2), Na1−O6
2.3421(19), Na1ii−O6 2.3595(19), V1···Na1 3.856(1), Na1···Na1ii
3.355(1), Na1···Na1i 6.217(2), V1···V1i 7.0720(5). Symmetry codes:
(i) 1 − x, 1 − y, −z; (ii) 1 − x, 1 − y, 1 − z, (iii) x, 1 + y, z; (iv) 1 + x,
y, z.
[Na1−O2 2.352(2), Na1−N2i 2.419(2), Na1−O5 2.270(2),
Na1−O6ii 2.358(2) Å], whereas an axial site is taken by the
second μ-O6 atom of μ-H2O ligand [Na1−O6 2.3595(19) Å].
Hence, each of the oxaloyldihydrazone ligands acts in a μ4-
bridging mode, simultaneously binding two V1 and two Na1
atoms with a {N4O4} donor set and resulting in the formation
of almost planar tetrametallic [V2Na2(μ4-L)] fragments with
the metal separations of 3.856(1) [V1···Na1], 6.217(2)
[Na1···Na1i], and 7.0720(5) Å [V1···V1i]. Together with the
additional oxido (O3, O4) and aqua (O5, O6) ligands, these
generate more complex [(VO2)2(μ4-L){Na2(μ-H2O)2(H2O)2}]
units that are mutually interconnected through μ-H2O (O6)
linkers into infinite ladder-like 1D metal−organic chains
(Figures 1b,c). Multiple interchain H-bonds [O5−H5B···O1iii
2.887(2), O6−H6B···O3iv 2.764(2) Å] provide further
extension of the 1D coordination network into a 3D
supramolecular assembly.
[{V(μ-O)2}2(μ4-L){K2(μ-H2O)2(H2O)2}]n (2). As in the case of 1,
the structure of 2 (Figure 2a) also bears the resembling
[(VO2)2(μ4-L)]2− blocks, which, therefore, are not discussed
further in detail. However, the main distinctive feature of 2
consists in different and more complex coordination behavior
of the K1 atoms, namely concerning their interactions with the
μ-oxido (O3, O4) and μ-H2O (O5) ligands. Hence, the
symmetry generated K1 atoms (two per formula unit) adopt
seven-coordinate {KNO6} environments, which are occupied
[(VO2)2(μ4-L){Na2(μ-H2O)2(H2O)2}]n (1). In 1, the
[(VO2)2(μ4-L)]2− blocks consist of the two symmetry related
V1 atoms, two pairs of terminal oxido O3 and O4 moieties, and
one tetradeprotonated μ4-L(4−) ligand in transoid conforma-
tion (Figure 1a). The five-coordinate V1 atoms adopt distorted
{VNO4} square-pyramidal environments (τ5 = 0.21), filled by
the N1, O1, and μ-O2 atoms of μ4-L and the O4 oxido ligand
in basal positions [V1−N1 2.131(2), V1−O1 1.8821(16), V1−
O2 1.9993(16), V1−O4 1.6434(17) Å], while the O3 oxido
ligand is located in the apical site [V1−O3 1.6188(18) Å].
Within the [Na2(μ-H2O)2(H2O)2]2+ moieties, the Na1 atoms
are five-coordinate and also possess distorted {NaNO4}
geometries that are better described as square-pyramidal (τ5
= 0.18). These are filled by the μ-O2 and N2 atoms of μ4-L,
and the O5 and μ-O6ii atoms of aqua ligands in equatorial sites
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and the O6 atom of the terminal H2O moiety [K1−O6
2.7380(16) Å]. As a result, the K1 and V1 centers are
assembled into helical 1D ribbons that are constructed from the
cyclic [V(μ-O)2(μ-OL)K2(μ-H2O)] subunits (Figure 2b).
These ribbons run along the b axis and have the pitch of
7.2879(7) Å, equal to the b unit cell dimension. The adjacent
ribbons are further interlinked via the μ4-L spacers generating
an infinite double-layered 2D metal−organic network. The
packing pattern of 2 along the c axis shows that the neighboring
2D layers are repeated every 9.0644(5) Å (shortest metal···-
metal distance), also being mutually interdigitated and held
together by means of interlayer H-bonds [O6−H6B···O1iv
2.968(2) Å] into a 3D supramolecular framework.
[{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-H2O)2(H2O)2}]n (3). In spite of
some similarities with 1 and 2, the cesium containing structure
of 3 is significantly more complex due to the presence of nine-
coordinate Cs1 atoms and their intricate binding with μ8-L, μ-
oxido (O4), μ3-oxido (O3), and μ-H2O (O5) ligands. In
contrast to 1 and 2, the N2 nitrogen of μ8-L is not bound to the
Cs1 atom, while its phenolate oxygen (O1) acts in a different
μ3-mode and bridges two Cs1 and one V1 atoms. Another
distinct feature of 3 concerns the overall μ8-L bridging mode of
the oxaloyldihydrazone ligand that is simultaneously linked to
six Cs1 and two V1 atoms with a {N2O4} donor set. Hence, the
{CsO9} coordination environment is occupied by a number of
symmetry equivalent oxygen atoms, namely two μ3-O1 and one
μ-O2 atoms of μ8-L [Cs1−O1 3.5111(18), Cs1−O1ii
3.2422(17), Cs1−O2iii 3.2571(16) Å], two μ3-O3 and one μ-
O4 oxido ligands [Cs1−O3ii 3.0334(16), Cs1−O3iii
3.0444(16), Cs1−O4 3.1524(16) Å], two μ-O5 atoms from
bridging water molecules [Cs1−O5 3.366(2), Cs1−O5iv
3.5272(19) Å], and the O6 atom of terminal H2O ligand
[Cs1−O6 3.1914(19) Å]. Besides, there are the Cs1···C1
[3.574(2) Å] and Cs1···C2 [3.487(2) Å] contacts that are
shorter than the sum of the van der Waals radii of the Cs and C
atoms [3.70 Å],17 and comparable to Cs···C distances in other
reported compounds.15a,18 The multiple binding of the Cs1
nodes with [(VO2)2(μ8-L)]2− blocks gives rise to the formation
of intricate 2D double layers [(VO)2(μ8-L)Cs2]n (Figure 3c),
which are further held together via μ-H2O pillars generating a
very complex layer-pillared 3D framework (Figure 3b).
Figure 2. Structural fragments of 2 showing (a) ellipsoid plot (50%
probability) with partial atom labeling scheme, (b) front (left) and side
(right) views of the metal−organic double 2D layer assembled from
helical 1D ribbons, (c) topological representation of the underlying
uninodal 4-connected 2D net with the kgm [Shubnikov plane net
(3.6.3.6)/kagome pattern] topology and the point symbol of
(32.62.72). Further details: (a, b) H atoms are omitted for clarity.
Color codes: V (green), K (magenta), N (blue), O (red), and C
(gray). (a, c) Views along the a (left) and b (right) axis; (c) 4-
connected K1 nodes (magenta), centroids of 4-connected μ4-L nodes
(gray), centroids of 2-connected [VO2]+ (green) and μ-H2O (red)
linkers. Selected distances (Å): V1−N1 2.1332(15), V1−O1
1.9108(12), V1−O2 1.9665(12), V1−O3 1.6211(13), V1−O4
1.6415(13), K1−N2 2.9035(15), K1i−O2 2.7858(13), K1ii −O3
2.9113(14), K1ii −O4 3.2262(15), K1−O5 3.2262(15), K1iii −O5
2.8036(15), K1−O6 2.7380(16), K1ii···V1 3.6962(5), K1···K1iii
4.9823(6). Symmetry codes: (i) −1 − x, −y, 1 − z; (ii) x, 0.5 − y,
−0.5 + z; (iii) −1 − x, −0.5 + y, 1.5 − z; (iv) −x, −y, 1 − z.
Topological Analysis. To get an insight into the intricate
2D and 3D metal−organic networks of 1 and 2, we performed
their topological analysis19 following the concept of the
simplified underlying net.20 Thus, after omitting terminal
water ligands and reducing the μ4-L, [VO2]+, and μ-H2O
moieties to their centroids, the structure of 2 can be described
as an underlying 2D net built from the 4-connected K1 and μ4-
L nodes (topologically equivalent), and the 2-connected
[VO2]+ and μ-H2O linkers (Figure 2c). The topological
analysis of this network discloses a uninodal 4-connected net
with the kgm [Shubnikov plane net (3.6.3.6)/kagome pattern]
topology and the point symbol of (32.62.72).19,21 In spite of
being not very common, a few examples of coordination
polymers with the kgm topology have been reported.22
Following the above-mentioned strategy,19,20 we also
obtained an underlying network of 3 (Figure 3d) by contracting
the μ8-L, [VO2]+, and μ-H2O ligands to their centroids and
eliminating terminal water molecules. The resulting trinodal
4,7,8-connected net is composed of the 4-connected [VO2]+, 7-
connected Cs1, and 8-connected μ8-L nodes, as well as 2-
connected μ-H2O linkers. Its topological analysis19 reveals a
n o v e l t o p o l o g y w i t h t h e p o i n t s y m b o l o f
by the N2 and μ-O2 atoms of μ4-L [K1−N2 2.9035(15), K1i−
O2 2.7858(13) Å], the μ-O3 and μ-O4 oxido ligands [K1−O3ii
2.9113(14), K1−O4ii 3.2262(15) Å], two μ-O5 atoms of μ-
H2O molecules [K1−O5 2.7273(15), K1−O5iii 2.8036(15) Å],
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Figure 3. Structural fragments of 3 showing (a) ellipsoid plot (50% probability) with partial atom labeling scheme, (b) intricate layer-pillared 3D
framework (view along the a axis) assembled from three [(VO)2(μ8-L)Cs2]n double layers (side view) and μ-H2O linkers, (c) front view of one 2D
double layer, (d) topological representation of the underlying trinodal 4,7,8-connected 3D net with a new topology, which is composed of (e)
topologically unique 2D metal−organic layers (front view of one layer). Further details: (a−c) all H atoms and terminal H2O ligands (in b, c) are
omitted for clarity. Color codes: V (green), Cs (magenta), N (blue), O (red), and C (gray); (d, e) 7-connected Cs nodes (magenta), centroids of 8-
connected μ8-L nodes (gray), centroids of 4-connected [VO2]+ nodes (green), centroids of 2-connected μ-H2O linkers (red). Selected distances (Å):
V1−N1 2.1360(19), V1−O1 1.9130(16), V1−O2 1.9955(16), V1−O3 1.6321(17), V1−O4 1.6326(17), Cs1−O1 3.5111(18), Cs1ii−O1
3.2422(17), Cs1iii −O23.2571(16), Cs1ii −O3 3.0334(16), Cs1iii −O3 3.0444(16), Cs1−O4 3.1524(16), Cs1−O5 3.366(2), Cs1iv−O5 3.5272(19),
Cs1−O6 3.1914(19), Cs1···C1 3.574(2), Cs1···C2 3.487(2). Symmetry codes: (i) −x, −y, 2 − z; (ii) 1 − x, 1 − y, 1 − z; (iii) −1 + x, y, z; (iv) 2 − x,
−y, 1 − z.
(33.43)2(33.49.52.66.7)2(36.48.54.67.72.8), wherein the notations
Scheme 3. Mild Oxidation of Cyclohexane
(33.43), (33.49.52.66.7), and (36.48.54.67.72.8) correspond to the
[VO2]+, Cs1, and μ8-L nodes, respectively. The undocumented
character of the present topology has been confirmed by the
search in different databases.19,21 Interestingly, the 2D layers
[(VO)2(μ8-L)Cs2]n of 3 (Figure 3c) after simplification result
in an underlying trinodal 4,6,8-connected net (Figure 3e) that
also features a unique topology described by the point symbol
of (33.43)2(33.49.52.6)2(36.48.54.67.72.8). Hence, the present
study contributes to the identification and classification of
coordination polymers with new topologies.
cyclohexanol and cyclohexanone as final products. The activity
values discussed below concern the total yields, that is, sum of
the molar % yields of the two final products (cyclohexanol and
cyclohexanone) based on cyclohexane.
Mild Oxidation of Cyclohexane. The compounds 1−3
were tested as precatalysts in the oxidation of cyclohexane by
H2O2 at 50 °C in MeCN/H2O medium (Scheme 3). The
reactions were monitored by control sampling, typically after
treatment by PPh3 to reduce cyclohexyl hydroperoxide
(primary intermediate product) to cyclohexanol, and then
analyzed by gas chromatography determining the formation of
Although 1−3 are almost inactive as such (less than 0.5%
total product yield), their catalytic performance is dramatically
increased upon addition of a small amount of an acid promoter
(cocatalyst). Thus, the total yield of cyclohexanol and
cyclohexanone rose up to 19, 22, and 15% (Figure 4a) in the
reactions catalyzed by 1−3, respectively, in the presence of
trifluoroacetic acid (TFA). Such a promoting behavior of an
23
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Figure 4. (a) Effect of the precatalyst type on the total yield of cyclohexanol and cyclohexanone in the oxidation of C6H12 by H2O2 catalyzed by 1−3
(5 × 10−4 M) in the presence of TFA (5 × 10−3 M). (b) Effect of the acid promoter on the total yield of cyclohexanol and cyclohexanone in the
oxidation of C6H12 catalyzed by 2 (5 × 10−4 M) in the presence (5 × 10−3 M) of PCA (curve A), TFA (curve B), HCl (curve C), and H2SO4 (curve
D). (a, b) General conditions: C6H12 (0.46 M), H2O2 (2.2 M), 50 °C in MeCN.
acid cocatalyst is well-known for other V-catalyzed systems in
oxidative transformation of alkanes.11,23
exhibited by 3 resulting in the 32% total yield after 2 h with the
corresponding TON of 2990, followed by 2 with the 24% total
yield after 3 h and TON of 2220, and 1 with the 15% total yield
after 4 h and TON of 1400 (Table 1, entries 1−3). Although
the total yields drop off upon further decreasing the precatalyst
concentration to 5 × 10−6 M, very high TONs of 5700, 5220,
and 4400 can be achieved after 5 h in the cyclohexane oxidation
catalyzed by 3, 2, and 1, respectively (Table 1, entries 4−6).
The analysis of the kinetic curves of the products accumulation
in the oxidation of cyclohexane catalyzed by 2 taken in different
concentrations (5 × 10−4 to 5 × 10−6 M) and in the presence of
a constant PCA amount (Figure 6a) reveals a linear
dependence of the maximum reaction rate (Wmax) on the
precatalyst concentration (Figure 6b).
In order to get some additional mechanistic information, we
applied compounds 1−3 in combination with PCA for the
oxidation of linear and branched cyclic alkanes, and the
obtained results were compared with some related systems
(Table 2).11d,25 The oxidation of n-heptane proceeds with
rather low regioselectivity [C(1):C(2):C(3):C(4) ≈ 1:(7−8):
(7−9):(6−8)] and the secondary carbon atoms are oxidized
preferably to the primary ones, although without particular
selectivity to C(2), C(3), or C(4) position. In the oxidation of
methylcyclohexane (MCH), the bond selectivity 1°:2°:3°
parameters of 1:(6−7):(17−21) are very close for 1−3 and
also comparable to those observed in other V-catalyzed
oxidation reactions.11d,25 The oxidation of cis- and trans-
dimethylcyclohexane undergoes with some stereoselectivity and
a partial inversion of the configuration, as attested by the trans/
cis ratios in the 0.7−0.4 range between the formed tertiary
alcohol isomers with the mutual trans and cis orientation of the
methyl groups.
To understand whether the type of acid promoter has an
influence on the oxidation of cyclohexane, we studied the effect
of various acids using 2 as a precatalyst (Figure 4b). Hence, in
the presence of HCl and H2SO4 the kinetic curves of products
accumulation are very similar. Although the oxidation reactions
proceed faster than those in the presence of TFA, the maximum
product yields are the same (∼22%) for all the three acids
(Figure 4b, curves B−D). However, a different pattern is
observed in the presence of 2-pyrazinecarboxylic acid (PCA),
resulting in a significantly faster and more efficient reaction:
36% total yield and TON 340 after 1 h (Figure 4b, curve A).
This behavior is consistent with the recognized application of
PCA as a highly efficient cocatalyst in various V-catalyzed
oxidative transformations.9a,11a,b,d,23−25
We also compared the activity of 1−3 (used in the 5 × 10−5
M concentration) in the presence of PCA (Figure 5). The
kinetic curves of the product accumulation in cyclohexane
oxidation are rather different for the three compounds, both in
terms of the reaction rate and efficiency. The highest activity is
By analogy with other catalytic systems,11d,25 the observed
selectivity features indicate the involvement of hydroxyl radicals
as oxidizing species in the present alkane oxidations by the 1−
3/PCA/H2O2 systems. Most likely [(VO2)2(L)]2− cores react
with PCA to form active oxido-peroxido V-intermediates that
participate in the generation of the HO• radical from H2O2.
The hydroxyl radical then abstracts an H atom from the alkane
generating the alkyl radical R•, which further reacts with O2
(e.g., from air) affording ROO• and leading to the formation of
alkyl hydroperoxide ROOH as a primary intermediate
Figure 5. Effect of the precatalyst type on the total yield of
cyclohexanol and cyclohexanone in the oxidation of C6H12 (0.46 M)
by H2O2 (2.2 M) catalyzed by 1−3 (5 × 10−5 M) in the presence of
PCA (5 × 10−3 M), at 50 °C in MeCN.
F
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a
Table 1. Effect of the Precatalyst Type and its Concentration in the Oxidation of Cyclohexane by H2O2 in the Presence of PCA
product yield [%]
b
c
entry
precatalyst
precatalyst concentration [M]
reaction time [min]
cyclohexanol
cyclohexanone
total
TON
1
2
3
4
5
6
1
2
3
1
2
3
5 × 10−5
5 × 10−5
5 × 10−5
5 × 10−6
5 × 10−6
5 × 10−6
240
180
120
300
300
300
9.0
16.6
20.9
2.2
6.2
7.6
15.2
24.2
32.5
4.8
1400
2220
2990
4400
5220
5700
11.6
2.6
1.5
4.2
5.7
2.6
3.6
6.2
a
b
Reaction conditions: C6H12 (0.46 M), PCA (5 × 10−3 M), H2O2 (50% aq., 2.2 M), MeCN (up to 5 mL total volume), 50 °C. Moles of products
c
(cyclohexanol + cyclohexanone)/100 mol of C6H12, determined by GC after the treatment with PPh3. Moles of products (cyclohexanol and
cyclohexanone) per mol of precatalyst.
Figure 6. (a) Effect of the precatalyst amount. Accumulation of oxygenates (total concentration of cyclohexanol and cyclohexanone, mM) with time
in the oxidation of C6H12 (0.46 M) by H2O2 (2.2 M) in the presence of PCA (5 × 10−3 M) at 50 °C in MeCN catalyzed by 2: 5 × 10−4 M (curve A),
5 × 10−5 M (curve B), 5 × 10−6 M (curve C). (b) Dependence of the maximum reaction rate Wmax (M s−1) on the concentration of 2 (data derived
from part a).
a
Table 2. Selectivity Parameters in the Oxidation of Linear and Branched Cyclic Alkanes
stereoselectivity trans/cis
catalytic system
1/PCA/H2O2
regioselectivity (n-C7H16) C(1):C(2):C(3):C(4)
bond selectivity (MCH) 1°:2°:3°
cis-DMCH
trans-DMCH
1:7:7:6
1:8:8:7
1:8:9:8
1:9:7:7
1:6:6:5
1:7:6:7
1:7:21
1:6:18
1:6:17
1:6:18
1:4:10
0.70
0.56
0.55
0.75
0.70
0.90
0.41
0.41
0.50
0.80
0.75
2/PCA/H2O2
3/PCA/H2O2
VO3−/PCA/H2O2
b
b
[VO{N(C2H4O)3}]/PCA/H2O2
b
hν/H2O2
a
Abbreviations: MCH, methylcyclohexane; cis- and trans-DMCH, isomers of 1,2-dimethylcyclohexane. Reaction conditions: precatalyst 1−3 (5 ×
10−4 M), [PCA]0 = 0.005 M, [substrate]0 = 0.46 M, [H2O2]0 = 2.2 M (50% aq.), MeCN up to 5 mL total volume, 50 °C. All parameters were
measured after reduction of the reaction mixtures with PPh3 before GC analysis and calculated based on the ratios of isomeric alcohols. The
calculated parameters were normalized taking into account the number of H atoms at each carbon atom. Parameters C(1):C(2):C(3):C(4) are
relative reactivities of H atoms at C(1), C(2), C(3), and C(4) atoms of n-heptane. Parameters 1°:2°:3° are the relative normalized reactivities of the
H atoms at primary, secondary, and tertiary carbon atoms of methylcyclohexane. Parameter trans/cis is determined as the ratio of the formed tertiary
b
alcohol isomers with mutual trans and cis orientation of methyl groups. Taken from reference 11d.
product.9a,11,23−25 This rapidly decomposes (conceivably by V-
catalyzed processes) to give alcohol and ketone as final
products.23−25 Alkyl hydroperoxides are common primary
products in vanadium catalyzed alkane oxidation.9a,11,25 Their
generation in the present study was confirmed by performing
the GC analyses of the reaction mixtures before and after the
treatment with PPh3, following a method developed by
Shul’pin.23 The involvement of HO• and R• radicals was
further corroborated by a strong inhibiting effect on the
formation of oxidation products, observed in the C6H12
oxidation in the presence of O- or C-centered radical
quenchers, either diphenylamine (Ph2NH) or bromotrichloro-
methane (CBrCl3), respectively.12a Hence, upon addition of
Ph2NH to the 2/PCA/H2O2 system, the total yield of
oxygenates dropped from 35 to 6% (conditions are those of
Figure 6a, 2: 5 × 10−4 M), while the introduction of CBrCl3
resulted in the formation of cyclohexyl bromide as a main
product (37% yield) instead of cyclohexanol and cyclo-
hexanone.
To get further information on the vanadium speciation in the
catalytic reaction mixtures, we have performed the 51V NMR
studies on the model aqueous (11 M H2O) CD3CN solutions
containing 1 (5 mM). The 51V NMR spectrum of 1 (δ −538)
remains unchangeable upon addition of up to 10 equivalents of
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H2O2 (50 mM), while at high H2O2 concentrations (>0.5 M) a
new signal at δ −719 (Figure S2, Supporting Information) is
observed that corresponds to a di(peroxido) species such as
[VO(O2)2(CD3CN)]−.11d,26 The addition of PCA (20 mM, 4
equiv.) to the solution of 1 results in a negligible shift of the
parent resonance from δ −538 to −536, along with some
broadening of the signal (Figure S3, Supporting Information).
CONCLUSIONS
■
In the present study, we synthesized and fully characterized
three novel heterometallic vanadium(V)-alkali metal coordina-
tion polymers 1−3 composed of the [(VO2)2(μ4/8-L)]2− cores
and [M2(μ-H2O)2(H2O)2]2+ moieties. The type of the alkali
metal ion plays a key role in defining the complexity of the
obtained coordination networks, which span from the Na-
driven ladder-like 1D chains in 1 to K-driven double 2D layers
in 2, and Cs-driven layer-pillared 3D framework in 3. This set
of compounds represents an example of periodicity when the
coordination number of the alkali metal [Na (5) < K (7) < Cs
(9)] increases with its ionic radius, also with a concomitant
growth of the network dimensionality [Na (1D) < K (2D) < Cs
(3D)]. The work also contributes to the identification and
classification of metal−organic materials with new topologies.20
In particular, the structure of 3 features a trinodal 4,7,8-
connected underlying net with a hitherto undocumented
topology, whereas the structure of 2 shows a uninodal 4-
connected underlying net with a rare kgm topology.
In addition, the compounds 1−3 represent rare examples of
aqua-soluble coordination polymers that can be used as
precatalysts in oxidation catalysis. In fact, 1−3 act as highly
active catalyst precursors for the mild oxidation of cyclohexane,
by aqueous H2O2 in MeCN/H2O medium, to cyclohexanol and
cyclohexanone. This transformation also requires the presence
of an acid promoter that dramatically increases the rate of
cyclohexane oxidation. The effect of different acid promoters
(HCl, H2SO4, TFA, or PCA) has been studied, revealing the
highest activity of 2-pyrazinecarboxylic acid. In spite of the
presence of the resembling [(VO2)2(μ4/8-L)]2− cores in 1−3,
their relative efficiencies depend on the reaction parameters,
such as the type of acid promoter and the precatalyst
concentration. Interestingly, under optimized conditions, the
activity of 1−3 in the presence of PCA follows the trend 1 < 2
< 3 that can be explained by the different degrees of
stabilization that the alkali metal ions provide to intermediate
V-species. In fact, alkali metal aqua moieties are known to
stabilize various oxido-peroxido complexes of vanadium.7,27
The best total yields (based on substrate) of cyclohexanol and
cyclohexanone up to 36% and turnover numbers (TONs) up to
5700 were shown by the 3/PCA catalytic system.
1
The H NMR spectrum of this solution reveals a shift of the
PCA aromatic protons in comparison with “free” PCA,
indicating that a considerable amount of PCA has been
coordinated to vanadium, forming most likely the
[VO2(pca)2]− or other di(oxido) derivatives containing both
L(4−) and pca(1−) ligands. In fact, it was reported earlier11d
that the [VO2(pca)2]− species in MeCN solution shows a 51V
resonance at δ −536. In the presence of 10 equiv. of PCA (50
mM), we have observed some widening of the resonance at δ
−536 in the 51V NMR spectrum, whereas two sets of proton
signals corresponding to the coordinated and “free” PCA
1
(∼2:1) were found in the aromatic region of the H NMR
spectrum.
The addition of H2O2 (10 mM, 2 equiv.) to the solution of
complex 1 (5 mM) containing PCA (20 mM, 4 equiv.) alters
the 51V spectrum (Figure S4, Supporting Information). Apart
from the signal at δ −536, two new resonances at δ −552 and
−566 appeared, which correspond to the two isomers (trans/
cis) of the mono(peroxido) vanadium complex [VO(O2)-
(pca)2]−.25a,11d The presence of these PCA containing species
1
was also confirmed by H MNR. Besides, there are two very
weak 51V resonances at δ −664 and −719 that correspond to
the previously identified di(peroxido) species [VO-
(O2)2(H2O)]− and [VO(O2)2(CD3CN)]−.11d,25a A higher
excess of both PCA (50 mM, 10 equiv.) and H2O2 (0.5 M)
(Figure S5) led to the disappearance of the [VO2(pca)2]−
signal (δ −536). However, the intensity of other signals
increased, namely those assigned to the cis and trans isomers of
the PCA containing (mono)peroxido vanadium species, as well
as the above di(peroxido) species bearing H2O and MeCN
ligands.
The ESI-MS(−) study of a model MeCN/H2O solution
containing 1, H2O2 (10 equiv.), and PCA (4 equiv.) showed
the generation of various oxido-peroxido species, in accord with
the detection of the following fragments: [{VO(O2)}2L-
(Na)2(pca)2 + H]− (m/z = 814, 25% relative intensity, pca =
anionic form of PCA), [{VO(O2)}2L(Na)3(pca) − H]− (m/z =
712, 55%), and [VO(O2)(pca)2]− (m/z = 345, 100%), the
latter being a recognized intermediate in other vanadium
catalyzed oxidations promoted by PCA.9a,11b,d,24,25 The
formation of resembling oxido-peroxido species was also
observed in the 2/PCA/H2O2 and 3/PCA/H2O2 model
solutions. Interestingly, the oxido-peroxido derivatives do not
appear to form upon treatment of 1−3 with H2O2 (10 equiv.)
in the absence of PCA, as attested by the detection of [(VO2)L
+ 2H+]− (m/z = 407, 100% relative intensity) as major
fragments in the corresponding ESI-MS(−) spectra. This fact
apparently explains the inactivity of 1−3 in the absence of the
PCA promoter. All these observations also testify that 1−3 are
not intact in the course of alkane oxidations and act as
homogeneous precatalysts, which furnish various oxido-
peroxido V/M intermediates responsible for the generation of
HO• radicals as active oxidizing species.
In summary, the present work provides a contribution
toward (i) widening the growing family of heterometallic
vanadium-containing coordination polymers driven by alkali
metals, (ii) identifying new structural and topological types of
such materials, and (iii) applying them in oxidation catalysis.
We believe future research should focus on the exploration of
both synthetic and catalytic directions, namely by employing
other types of vanadium building blocks and by broadening the
substrate versatility of oxidative transformations.
EXPERIMENTAL SECTION
■
Materials and General Methods. All synthetic work was
performed in air. All chemicals were obtained from commercial
sources and used as received. Bis(salicylaldehyde)-oxaloyldihydrazone
(H4L) has been prepared by modification of a previously reported
method.6b C, H, and N elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Tec
spectra (4000−400 cm−1) were recorded on a BIO-RAD FTS 3000
MX instrument in KBr pellets (wavenumbers are in cm−1
Abbreviations: vs, very strong; s, strong; m, medium; w, weak; br,
broad; sh, shoulder). 1H NMR spectra were recorded at ambient
temperature on a Bruker Avance II 300 (UltraShield Magnet)
spectrometer operating at 300.130 MHz. The chemical shifts (δ) are
́
nico. Infrared
.
H
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reported in ppm using tetramethylsilane as the internal reference
(abbreviations: s, singlet; m, multiplet). 51V NMR spectra were
measured on a Bruker 400 (UltraShield Magnet) spectrometer at
ambient temperature in aqueous CD3CN. The vanadium chemical
shifts are quoted relative to the external VOCl3. A typical 51V spectrum
for a 5 mM solution of 1−3 was obtained from the accumulation of
10000 transients with a 900 ppm spectral window, at about 10 scans
per second. An exponential line broadening up to 200 Hz was applied
before Fourier transformation. UV−vis spectra of 10−4 M solutions of
1−3 in DMF were recorded on a Perkin-Elmer Lambda 35 instrument.
ESI−MS( ) spectra were run on a 500-MS LC Ion Trap instrument
(Varian Inc.) equipped with an electrospray (ESI) ion source, using ca.
10−3 M solutions of 1−3 in water or acetonitrile. Thermal analyses
were performed with a Perkin-Elmer STA 6000 instrument. The
crystalline samples were heated under N2 atmosphere at the rate of 10
°C min−1 in the 30−950 °C temperature range. Gas chromatography
(GC) analyses were performed on a Fisons Instruments GC 8000
series gas chromatograph with FID (He as carrier gas) and a capillary
column BP20 (SGE) 30 m × 0.22 mm × 25 μm, using the Jasco-
Borwin v. 1.50 software.
General Synthetic Procedure for 1−3. In a round-bottom flask
(50 mL), H4L (41 mg, 0.125 mmol) was mixed with ethanol (20 mL)
under constant stirring. To that stirred suspension [VO(acac)2] (66
mg, 0.25 mmol) was added. The obtained mixture was refluxed for 2 h
resulting, after cooling to room temperature (∼25 °C), in a brown
clear solution. To this solution an aqueous solution (10 mL) of alkali
metal carbonate [Na2CO3 (27 mg, 0.25 mmol) for 1; K2CO3 (35 mg,
0.25 mmol) for 2; or Cs2CO3 (81 mg, 0.25 mmol) for 3] was added,
causing the immediate formation of a yellow precipitate. It was allowed
to settle for about 1 h, then filtered, washed with ethanol, and dried
over fused CaCl2 to furnish compounds 1, 2, and 3 as yellow solids in
72, 70, and 76% yields based on H4L, respectively. The X-ray quality
single crystals of 1−3 were grown by a slow evaporation of their H2O/
DMF solutions (3:2, v/v), in air at room temperature.
(w), 1369 (m), 1291 (s), 1268 (s), 1211 (w), 1154 (w), 1129 (w),
1029 (w), 921 (vs br) ν(VO), 872 (sh), 810 (w), 754 (s), 678 (w),
1
622 (w), 575 (m), and 459 (m) cm−1. H NMR (300 MHz, D2O,
Me4Si), δ: 6.92−6.97 (m, 4H, Ar−H), 7.51−7.62 (m, 4H, Ar−H),
8.91 (s, 2H, C(H)N). 51V NMR (105 MHz, CD3CN, VOCl3), δ:
−538. UV−vis (DMF) λmax, nm (ε, L mol−1 cm−1): 414 (15500), 325
(21300). ESI−MS( ) (H2O), selected fragments with relative
abundance >1%, MS(+): 886 (1%) [(VO2)2(L)Cs2 + Cs]+, 755
(1%) [(VO2)2(L)Cs2 + H]+, 133 (100%) [Cs]+. MS(−) m/z: 621
(100%) [(VO2)2(L)Cs]−, 244 (100%) [(VO2)2(L)]2−.
X-ray Crystal Structure Determinations. The X-ray diffraction
data of 1−3 were collected using a Bruker AXS-KAPPA APEX II
diffractometer with graphite monochromated Mo Kα radiation. Data
were collected using omega scans of 0.5° per frame, and a full sphere
of data was obtained. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT on all the observed
reflections.28 Absorption corrections were applied using SADABS.28
Structures were solved by direct methods using the SHELXS-9729 or
SIR9730 programs and refined with SHELXL-97.29 Calculations were
performed with the WinGX System-Version 1.80.03.31 All hydrogen
atoms were inserted in calculated positions except those of
coordinated water molecules; they were located from the final Fourier
difference map, their coordinates were blocked during the refinement
process, and the isotropic thermal parameters were set at 1.5 times the
average thermal parameter of the belonging oxygen atoms. Crystal data
and details of data collection for 1−3 are reported in Table 3.
Table 3. Crystal Data and Structure Refinement Details for
Compounds 1−3
1
2
3
empirical formula
Fw
C8H9N2NaO6V
303.10
C8H9KN2O6V
319.21
C8H9CsN2O6V
412.90
T (K)
150(2)
150(2)
150(2)
[(VO2)2(μ4-L){Na2(μ-H2O)2(H2O)2}]n (1). Soluble in H2O (S25°C ≈ 7
mg mL−1) and DMSO, slightly soluble in DMF and MeCN. Found: C,
31.35; H, 3.07; N, 8.93. C16H18N4Na2O12V2 (MW 606.2) requires C,
31.70; H, 2.99; N, 9.24. IR (KBr): 3435 (vs br) ν(H2O), 1607 (vs)
ν(CO), 1547 (s) ν(CN), 1472 (w), 1441 (w), 1372 (m), 1296
(s), 1272 (s), 1212 (w), 1149 (w), 915 (vs br) ν(VO), 813 (w), 749
(s), 618 (w), 572 (w), and 462 (m) cm−1. 1H NMR (300 MHz, D2O,
Me4Si), δ: 6.86−6.98 (m, 4H, Ar−H), 7.44−7.60 (m, 4H, Ar−H),
8.88 (s, 2H, C(H)N). 51V NMR (105 MHz, CD3CN, VOCl3), δ:
−538. UV−vis (DMF): λmax, nm (ε, L mol−1 cm−1): 412 (12500), 324
(20500). ESI−MS( ) (H2O), selected fragments with relative
abundance >10%; MS(+) m/z: 690 (10%) [(VO2)2(L)Na2(H2O)4 +
VO2]+, 625 (30%) [(VO2)2(L)Na2(H2O)4 + H2O + H]+, 557 (100%)
[(VO2)2(L)Na2 + Na]+. MS(−) m/z: 511 (90%) [(VO2)2(L)Na]−,
489 (60%) [(VO2)2(L) + H]−, 244 (100%) [(VO2)2(L)]2−.
λ (Å)
0.71069
Triclinic
P-1
0.71069
0.71069
Triclinic
P-1
crystal system
space group
a (Å)
Monoclinic
P21/c
7.8121(4)
8.5243(4)
9.0869(6)
84.285(2)
70.705(2)
80.472(1)
562.60(5)
2
8.9859(4)
7.2879(4)
17.9664(7)
7.4312(2)
9.2421(3)
10.1449(3)
64.3800(10)
83.907(2)
77.9490(10)
614.30(3)
2
b (Å)
c (Å)
α (deg)
β (deg)
91.315(2)
γ (deg)
V (Å3)
1176.28(9)
4
Z
ρcalcd (g/cm3)
μ(Mo Kα) (mm−1
1.789
1.803
1.218
9258
2.233
)
0.941
3.748
[{V(μ-O)2}2(μ4-L){K2(μ-H2O)2(H2O)2}]n (2). Soluble in H2O (S25 °C
≈
no. of collected
reflns
5752
11110
5 mg mL−1) and DMSO, slightly soluble in DMF and MeCN. Found:
C, 30.16; H, 2.55; N, 8.43. C16H18K2N4O12V2 (MW 638.4) requires C,
30.10; H, 2.84; N, 8.78. IR (KBr): 3442 (vs br) ν(H2O), 1608 (vs)
ν(CO), 1548 (s) ν(CN), 1475 (m), 1442 (w), 1371 (m), 1293
(m), 1270 (s), 1213 (m), 1154 (m), 1127 (w), 1030 (w), 913 (vs br)
ν(VO), 875 (sh), 814 (w), 749 (s), 621 (w), 575 (m), 518 (w), and
no. of unique reflns 2545
Rint 0.0336
2593
2229
0.0322
0.0261
a
b
Final R1 , wR2 (I
≥ 2σ)
0.0350, 0.0943
0.0263, 0.0689
0.0162, 0.0418
GOF on F2
1.145
1.049
0.977
1
459 (m) cm−1. H NMR (300 MHz, D2O, Me4Si), δ: 6.86−6.98 (m,
a
b
R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. wR2 = [Σ[w(Fo2 − Fc2)2]/ Σ[w(Fo )2]]1/2
2
4H, Ar−H), 7.44−7.61 (m, 4H, Ar−H), 8.89 (s, 2H, C(H)N). 51V
NMR (105 MHz, CD3CN, VOCl3), δ −538. UV−vis (DMF), λmax, nm
(ε, L mol−1 cm−1): 409 (3600), 324 (6400). ESI−MS( ) (H2O),
selected fragments with relative abundance >10%, MS(+) m/z: 686
(30%) [(VO2)2(L)K2(H2O)2 + VO2]+, 660 (20%) [(VO2)2(L)-
K2(H2O)3 + K]+, 604 (100%) [(VO2)2(L)K2(H2O)2 + H]+, 485
(20%) [(VO2)(L)K2 + 2H]+. MS(−) m/z: 527 (85%) [(VO2)2(L)-
K]−, 489 (30%) [(VO2)2(L) + H]−, 244 (100%) [(VO2)2(L)]2−.
[{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-H2O)2(H2O)2}]n (3). Soluble in H2O
(S25°C ≈ 4 mg mL−1) and DMSO, slightly soluble in DMF and MeCN.
Found: C, 22.90; H, 2.00; N, 6.46. C16H18Cs2N4O12V2 (MW 826.0)
requires C, 23.26; H, 2.20; N, 6.78. IR (KBr): 3433 (vs br) ν(H2O),
1608 (vs) ν(CO), 1547 (s) ν(CN), 1523 (m), 1474 (m), 1443
Alkane Oxidation Studies. The alkane oxidations were typically
carried out in air in thermostatted (50 °C) Pyrex cylindrical vessels
with vigorous stirring and using MeCN as solvent (up to 5.0 mL total
volume). Typically, precatalyst 1−3 was introduced into the reaction
mixture as a stock solution in acetonitrile (1.25 × 10−3 M). Then, an
acid promoter (0.025 mmol, optional) was added either as solid
(PCA) or stock solution in MeCN (TFA: 0.44 M. H2SO4: 0.355 M.
HCl: 1.2 M). The substrate, typically cyclohexane (0.25 mL, 2.3
mmol), was introduced and the reaction started upon addition of
hydrogen peroxide (50% in H2O, 0.68 mL, 11 mmol) in one portion.
The final concentrations of the reactants in the reaction mixture were
I
dx.doi.org/10.1021/ic400743h | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
as follows: precatalyst 1−3 (5 × 10−4 − 5 × 10−6 M), acid promoter
(5 × 10−3 M), substrate (0.46 M), and H2O2 (2.2 M). The oxidation
reactions were monitored by withdrawing small aliquots after different
periods of time, which were then cooled and treated with PPh3 (for
reduction of remaining H2O2 and alkyl hydroperoxides23 that are
formed as major primary products in alkane oxidations) followed by
GC analysis. Attribution of peaks was made by comparison with the
chromatograms of authentic samples. Nitromethane (0.05 mL) was
used as GC internal standard.
J. C.; Crans, D. C. (Eds.), Vanadium: The Versatile Metal, ACS Symp.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1−S5 with selected 51V NMR spectra, crystallographic
files in CIF format for compounds 1−3 (CCDC 927412-
927414). This material is available free of charge via the
Internet at http://pubs.acs.org.
Frausto da Silva, J. J. R.; Pombeiro, A. J. L. Coord. Chem. Rev. 2011,
́
255, 2232. (b) Maurya, M. R.; Kumar, A.; Pessoa, J. C. Coord. Chem.
Rev. 2011, 255, 2315. (c) Mizuno, N.; Kamata, K. Coord. Chem. Rev.
2011, 255, 2358. (d) Conte, V.; Coletti, A.; Floris, B.; Licini, G.;
Zonta, C. Coord. Chem. Rev. 2011, 255, 2165.
AUTHOR INFORMATION
Corresponding Author
*Fax: +351 218464455. E-mail: kirillov@ist.utl.pt, pombeiro@
ist.utl.pt, fatima.guedes@ist.utl.pt.
■
(10) (a) Kirillova, M. V.; Kuznetsov, M. L.; Reis, P. M.; da Silva, J. A.
L.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L. J. Am. Chem. Soc. 2007,
́
Notes
129, 10531. (b) Kirillova, M. V.; Kuznetsov, M. L.; da Silva, J. A. L.;
The authors declare no competing financial interest.
Guedes da Silva, M. F. C.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L.
́
Chem.Eur. J. 2008, 14, 1828. (c) Kirillova, M. V.; da Silva, J. A. L.;
ACKNOWLEDGMENTS
Frausto da Silva, J. J. R.; Palavra, A. F.; Pombeiro, A. J. L. Adv. Synth.
́
■
Catal. 2007, 349, 1765. (d) Kirillova, M. V.; Kirillov, A. M.; Reis, P.
This work was supported by the Foundation for Science and
Technology (FCT) (projects PTDC/QUI-QUI/121526/2010,
PTDC/QUI-QUI/102150/2008, and PEst-OE/QUI/UI0100/
2013), Portugal. We also thank Dr J. Lasri for assistance with
M.; Silva, J. A. L.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L. J. Catal.
́
2007, 248, 130. (e) Reis, P. M.; Silva, J. A. L.; Palavra, A. F.; Frausto da
́
Silva, J. J. R.; Pombeiro, A. J. L. J. Catal. 2005, 235, 333. (f) Reis, P. M.;
Silva, J. A. L.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L. J. Chem. Soc.,
́
1
some H NMR measurements and Dr M. C. Oliveira for ESI-
Chem. Commun. 2000, 1845. (g) Reis, P. M.; Silva, J. A. L.; Palavra, A.
MS (IST-node of the RNEM/FCT).
F.; Frausto da Silva, J. J. R.; Kitamura, T.; Fujiwara, Y.; Pombeiro, A. J.
́
L. Angew. Chem., Int. Ed. 2003, 42, 821.
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