Novel scorpionate and pyrazole dioxovanadium complexes, catalysts for carboxylation and peroxidative oxidation of alkanes

Article abstract of DOI:10.1002/adsc.200900660

The dioxovanadium(V) complexes [VO₂(3,5-Me₂Hpz)₃] [BF₄] (1) (pz = pyrazolyl), [VO₂{SO₃C(PZ)₃}] (2), [VO₂{HB(3,5-Me₂pz)₃}] (3) and [VO₂(HC(pz)₃}j[BF₄] (4), bearing pyrazole or scorpionate ligands, were obtained by reaction of triethyl vanadate [VO(OEt)₃] with hydrotris(3,5-dimethyl-1-pyrazolyl)methane [HC(3,5-Me₂pz)₃] or 3,5-dimethylpyrazole (3,5-Me₂HpZ; 1), lithium tris(1-pyrazolyl)methanesulfonate {Li[SO₃C(pz)₃], 2}, potassium hydrotris(3,5-dimethyl-l-pyrazolyl)borate {K[HB (3,5-Me₂pz)₃], 3} and hydrotris(1-pyrazolyl)methane [HC(pz)₃, 4], respectively. Treatment of [VO(OEt)₃] with potassium hydrotris(1-pyrazolyl)borate (K[HB(pz)₃]) led to the mixed η³-tris(pyrazolyl)-borate and η₂-bis(pyrazolyl)borate oxovanadium(IV) complex [VO{HB(pz)₃}{H₂B(pz)₂}, 5]. The compounds were characterized by elemental analyses, IR, NMR and EPR spectroscopy, FAB and ESI mass spectrometry, cyclic voltammetry and, for 5, also by single crystal X-ray diffraction analysis. All complexes exhibit catalytic activity in the single-pot carboxylation [in trifluoroacetic acid/potassium peroxodisulfate (CF₃COOH/K₂S₂O₈)] of gaseous alkanes (methane and ethane) to carboxylic acids (yields up to 40%, TONs up to 157) and in the peroxidative oxidation [in water/acetonitrile (H₂O/NCMe)] of liquid alkanes (cyclohexane and cyclopentane) to the corresponding alcohols and ketones (yields up to 24%, TONs up to 117), under mild conditions.

Full text of DOI:10.1002/adsc.200900660

FULL PAPERS
DOI: 10.1002/adsc.200900660
Novel Scorpionate and Pyrazole Dioxovanadium Complexes,
Catalysts for Carboxylation and Peroxidative Oxidation of
Alkanes
Telma F. S. Silva,a Konstantin V. Luzyanin,a Marina V. Kirillova,a
M. Fꢀtima Guedes da Silva,a,b Luꢁsa M. D. R. S. Martins,a,c
and Armando J. L. Pombeiroa,*
a
Centro de Quꢁmica Estrutural, Complexo I, Instituto Superior Tꢂcnico, TU Lisbon, Av. Rovisco Pais, 1049-001 Lisboa,
Portugal
Fax : (+351)-21-846-4455; e-mail: pombeiro@ist.utl.pt
Universidade Lusꢃfona de Humanidades e Tecnologias, ULHT Lisbon, Campo Grande 376, 1749-024 Lisboa, Portugal
Departamento de Engenharia Quꢁmica, ISEL, R. Conselheiro Emꢁdio Navarro, 1959–007 Lisboa, Portugal
b
c
Received: September 22, 2009; Published online: December 30, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900660.
Abstract:
[VO2(3,5-Me2Hpz)3]
[VO2A{SO3C(pz)3}] (2), [VO2{HB(3,5-Me2pz)3}] (3) single crystal X-ray diffraction analysis. All com-
and [VO2{HC(pz)3}][BF4] (4), bearing pyrazole or plexes exhibit catalytic activity in the single-pot car-
scorpionate ligands, were obtained by reaction of boxylation [in trifluoroacetic acid/potassium peroxo-
triethyl vanadate [VO(OEt)3] with hydrotris(3,5-di- disulfate (CF3COOH/K2S2O8)] of gaseous alkanes
methyl-1-pyrazolyl)methane [HC(3,5-Me2pz)3] or (methane and ethane) to carboxylic acids (yields up
3,5-dimethylpyrazole (3,5-Me2Hpz; 1), lithium tris(1- to 40%, TONs up to 157) and in the peroxidative ox-
pyrazolyl)methanesulfonate {Li[SO3C(pz)3], 2}, idation [in water/acetonitrile (H2O/NCMe)] of liquid
The
dioxovanadium(V)
complexes IR, NMR and EPR spectroscopy, FAB and ESI mass
A
ACHTUNGRTEN[NUNG BF4] (1) (pz=pyrazolyl), spectrometry, cyclic voltammetry and, for 5, also by
A
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
potassium hydrotris(3,5-dimethyl-1-pyrazolyl)borate alkanes (cyclohexane and cyclopentane) to the corre-
{K[HB(3,5-Me2pz)3], 3} and hydrotris(1-pyrazolyl)- sponding alcohols and ketones (yields up to 24%,
methane [HC(pz)3, 4], respectively. Treatment of TONs up to 117), under mild conditions.
[VO
rate {K[HB(pz)3]} led to the mixed h3-tris(pyrazolyl)-
borate and h2-bis(pyrazolyl)borate oxovanadium(IV) Keywords: alkanes; carboxylation; C H bond activa-
complex [VO{HB(pz)3}{H2B(pz)2}, 5]. The com- tion; oxovanadium complexes; peroxidative oxida-
pounds were characterized by elemental analyses, tion; scorpionate ligands
ACHTUNGTRENNUNG(OEt)3] with potassium hydrotris(1-pyrazolyl)bo-
ꢀ
AHCTUNGTRENNUNG
Introduction
um, and the search for novel V complexes with phar-
macological and catalytic significance is a matter of a
The coordination chemistry of vanadium, in particular high current interest.[6,7]
with multidentate ligands, is receiving much attention
Poly(1-pyrazolyl)borates,[8] the parent compounds
on account of its involvement in various biological within the so-called scorpionate ligands (Figure 1),
processes: in the active site of metalloenzymes such and their coordination chemistry have been extensive-
as vanadium nitrogenase[1] and haloperoxidases,[2] as a ly developed[8,9] with particular significance for the
metabolic regulator,[3] as a mitogenic activator and es- control of the steric and electronic environment of
pecially as an insulin-mimicking agent;[4] vanadium the metal centre upon variation of the pyrazolyl
complexes can also affect the cardiac abnormality as- groups. The neutral hydrotris(1-pyrazolyl)methane
sociated with diabetes mellitus[5] and exhibit antican- HC(pz)3 (pz=pyrazolyl) and derivatives are formally
ACHTUNGTRENNUNG
cer activity.[6] Moreover, catalytic applications have related to the corresponding hydrotris(1-pyrazolyl)bo-
ꢀ
also stimulated the coordination chemistry of vanadi- rate HB(pz)3 species by replacing the central boron
Adv. Synth. Catal. 2010, 352, 171 – 187
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171

Telma F. S. Silva et al.
FULL PAPERS
Figure 1. Tris(1-pyrazolyl)scorpionates.
atom with a carbon,[10] but their coordination chemis-
Recently, we have observed that a few scorpionate
try, in particular at V centres and contrasting with chlorovanadium complexes exhibit a good catalytic
that of tris(pyrazolyl)borates, has so far been reported efficiency[13b,14] in the partial oxidation of cyclohexane
only rarely.[9a,11] This arises from the relatively small (to cyclohexanol and cyclohexanone), a reaction of in-
number of such ligands, the difficulties and the usual- dustrial importance. Hence, also taking into account
ly low yields associated with their syntheses. A partic- that high oxidation state dioxovanadium species con-
+
ular case is the sulfonate derivative tris(pyrazolyl)me- taining the VO2 moiety have been used as models of
thanesulfonate SO3C(pz)3 (Tpms)[12] which, in con- vanadium haloperoxidases,[6a–c] and with the aim of
ꢀ
trast to hydrotris(1-pyrazolyl)methane, is almost ex- extending the search for efficient scorpionate vanadi-
clusively soluble in water and moderately soluble in um compounds in the catalytic functionalisation of
methanol. Another distinct behaviour relative to light alkanes, we present here the application of the
HC(pz)3 and especially to hydrotris(1-pyrazolyl)bo- above dioxovanadium(V) complexes, bearing scorpio-
ꢀ
rate concerns the stability of SO3C(pz)3 over a wide nate ligands, as catalysts for the single-pot carboxyla-
range of pH in aqueous solution.
tion of methane and ethane and for the peroxidative
In view of the significance of vanadium chemistry oxidation of cyclopentane and cyclohexane, under
with poly(pyrazolyl)borate ligands towards mimicking mild conditions.
biocatalytic behaviour and also in pursuit of our inter-
est on transition metal complexes bearing scorpionate
ligands,[13] we have, in the current study, embarked Results and Discussion
upon the syntheses of oxovanadium complexes with
such ligands and with those of the related, but still Synthesis and Characterization of the Complexes
very little studied, tris(pyrazolyl)methane family.
Hence, the work has allowed us to extend the still Reaction of triethyl vanadate, a convenient starting
limited number of known complexes with the material in oxovanadium coordination chemistry,[15] in
ꢀ
HC(pz)3 and SO3C(pz)3 ligands, and in particular af- refluxing ethanol, with hydrotris(3,5-dimethyl-1-pyra-
forded the first dioxovanadium complex bearing the zolyl)methane, HC(3,5-Me2pz)3 (in 1:1 stoichiometric
latter ligand. We have also observed the conversion amounts), followed by addition of Na
ACHTUNGTRENN[UGN BF4], led to
of the disubstituted hydrotris(1-pyrazolyl)methane [VO2(3,5-Me2Hpz)3][BF4] 1 (reaction a, Scheme 1)
AHCTUNGTRENNUNG
HC(3,5-Me2pz)3 into the corresponding dimethylpyra- which was isolated as an air-stable brown solid (45%
zole, and of the related hydrotris(1-pyrazolyl)borate yield), soluble in DMSO, dichloromethane and
into the corresponding bis(1-pyrazolyl)borate, the chloroform. The formation of 1 involves the rupture
3
latter affording a mixed h3-tris(pyrazolyl)borate and of a C
G
ꢀ
h2-bis(pyrazolyl)borate oxovanadium(IV) complex. Other cases of metal-induced cleavage of such a type
On the other hand, by investigating the redox behav- of bond have been reported, namely the C N bond
iour of the obtained vanadium products, we have also rupture in HC(pz)3 or HC(3,5-Me2pz)3 by the ben-
gained an insight into the net electron-donor ability zoylhydrazido rhenium(V) chelate [ReCl2{h2-N,O-
of such ligands and in the redox V(V)V(IV) or N2C(O)Ph}
V(IV)/V
(III) interplay, essential for the versatility of (CH3)2C(pz)2 by Pt(II) complexes,[16] namely [PtCl2
vanadium as a catalyst. (RCN)2] (R=Me or Ph) or K2A[PtCl4]. Such a behav-
(PPh3)2][13f] or in bis(pyrazolyl)propane
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
E
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Novel Scorpionate and Pyrazole Dioxovanadium Complexes
Scheme 1.
iour, to the best of our knowledge, had not been pre- [VO2{HB(3,5-Me2pz)3}] (3) and [VO2{HC(pz)3}]ACHTUNGTRENNUNG[BF4]
viously shown for vanadium. In all the reported cases, (4), respectively (reactions c–e, Scheme 1). They were
the manner of the cleavage reaction occurs has not isolated (42–52% yields) as green (2 and 4) or brown
been established. Complex 1 is also obtained upon re- (3) air-stable solids and (as well as the other com-
action of [VO
yield, reaction b, Scheme 1).
Reactions of [VO
(OEt)3] with the anionic Li+ spectrometry and electrochemical methods.
salt, lithium tris(1-pyrazolyl)methanesulfonate Compounds 1 and 2 represent, to the best of our
Li[SO3C(pz)3], the related disubstituted potassium knowledge, the first examples of dioxovanadium com-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ꢀ
hydrotris(3,5-dimethyl-1-pyrazolyl)borate salt, i.e., plexes bearing the pyrazole or SO3C(pz)3 ligand; in 2
K[HB(3,5-Me2pz)3], or the neutral hydrotris(1-pyra- the ligated scorpionate shows the typical N,N,N-coor-
zolyl)methane HC(pz)3 (always in 1:1 stoichiometric dination mode. Moreover, [VO2{HB(3,5-Me2pz)3}] (3)
amounts), the latter followed by addition of Na
led to the formation of the scorpionate dioxovana- isolated and characterised. A related dinuclear vana-
dium(V) compounds [VO2A{SO3C(pz)3}] (2), dium complex [V2O4{HB(pz)3}2] was recently report-
ACHTUNGTREN[NNUG BF4], is the first mononuclear tris(1-pyrazolyl)borate to be
CTHUNGTRENNUNG
Adv. Synth. Catal. 2010, 352, 171 – 187
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173

Telma F. S. Silva et al.
FULL PAPERS
ed[17] and considered to consist of two identical (951–912 cmꢀ1) and the antisymmetric VO2 stretch
+
VO{HB(pz)3} groups connected by two m-O atoms.
The mixed h3-tris(pyrazolyl)borate and h2-bis(pyra- The n
zolyl)borate complex [VO{HB(pz)3}{H2B(pz)2}] (5) is for several oxovanadium(IV) complexes bearing the
obtained upon reaction of [VO(OEt)3] with a two- tris(pyrazolyl)borate ligand, such as [VOCl{HB(3,5-
fold molar amount of hydrotris(1-pyrazolyl)borate Me2pz)3}
(Me2Hpz)].[21,23]
(888–839 cmꢀ1) of the cis-[VO2 ] structural unit.[2b,15,22]
AHCTUNGTRENNG(UN V=O) value in 5 is comparable to those quoted
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(reaction f, Scheme 1), in refluxing ethanol. In this
In 1, the equivalence of the pyrazole groups is indi-
1
case, fragmentation of the tris(pyrazolyl)borate oc- cated by both H and 13C{1H} NMR spectra which dis-
curred along the reaction course, to afford the di- play a single resonance for each type of protons or
ꢀ
hapto bis(pyrazolyl) ligand. The B N bond rupture carbons, i.e., for the 3, 4 or 5 position in the rings.
has been previously observed in K[HB(3,5-Me2pz)3] Moreover, the C resonances of the pyrazole rings are
2A
The molecular structure of 5 (Figure 2) was estab- complexes such as [ReCl
(PPh3)2]. The methine group was not detected for 1,
2ACHUTNGRNENUG{NNC(O)C6H5}ACHTUNGTRENNUNG(Hpz)-
AHCTUNGTRENNUNG
in accord with its formulation as a pyrazole com-
pound.
1
In the H NMR spectra, the methine hydrogen of 4
appears at d=8.96 in accord with the reported data,
e.g., for the tris(pyrazolyl)methane chloro-complexes
[MCln{HC(pz)3}] (M=V,[13b] Fe[13b] or Re,[13e] n=2 or
[BF4].[13f] The corre-
3) or [ReCl2{HC(pz)3}ACTHNUGTRNENUG(PPh3)]HCATUNGTRENNGUN
ꢀ
sponding resonance, i.e., B H, of 3 is observed at d=
8.50 and is comparable to that reported[24] for hydro-
tris(pyrazolylborate) complexes of other metals.
In the 13C{1H} NMR spectra of complexes 2–4, the
three carbon atoms from the pyrazolyl rings appear as
singlets, and the methine carbon resonances (in 2 and
4, at 80.1 and 82.3 ppm, respectively) are comparable
to those quoted, e.g., for our scorpionate complexes
[MCln{HC(pz)3}] (M=Fe[13b] or Re,[13e] n=2 or 3, re-
spectively), [ReCl2{HC(pz)3}
N
[BF4][13f] or [ReO3
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
MeCN, PTA (1,3,5-triaza-7-phosphaadamantane) or
HMT (hexamethylenetetramine)].[13a]
Compounds 1–4 exhibit strong 51 V NMR resonan-
ces in the ꢀ490 to ꢀ556 ppm range, relative to VOCl3,
as expected for dioxovanadium(V) complex-
es.[2b,15,21b,25]
Figure 2. ORTEP diagram of the neutral complex
[VO{HB(pz)3}{H2B(pz)2}] 5, indicating the atom numbering
scheme. Displacement ellipsoids are drawn at the 50% prob-
ACHTUNGTRENNUNG
ability level.
The FAB+ or ESI+-mass spectra of complexes 1–5
show the molecular ion [M]+. In FAB-MS, the frag-
lished by X-ray diffraction (see below). This com- mentation pathways occur upon cleavage of metal-
pound appears to have been obtained recently by ligand bonds and/or rupture of pyrazolyl rings. Cou-
others[19] using VOSO4 as the starting vanadium mate- pling of pyrazolyl (or pyrazole in 1) with metal frag-
rial, but the reported structure, in contrast to ours, ments and their oxygenation (by the NBA matrix) are
doubles the cell edge c and, consequently, the cell also observed.
volume.
The IR spectra of complexes 1–5 exhibit nACTHNUGRTNEUNG(C=C)
and nACHTUNGTRENNUNG(C=N) of the pyrazolyl or pyrazole rings at the Solid State Structure of Complex 5
usual[13,20] range of 1618–1503 cmꢀ1. n
G
E
ꢀ
ꢀ
ꢀ
H) (4) and n
N
{H2B(pz)2}] (5) displays a neutral mono-
acteristic[13,20,21]frequencies of 3141, 3127 and 2412– nuclear molecular structure that is depicted in
2365 cmꢀ1, respectively, also the n
C
(C S) vi- Figure 2, while the crystallographic details are given
brations of the methanesulfonate group (2) which are in Table 1, and selected bond distances and angles in
observed at the common[12,13a,b,e] ranges of 1046–1015 Table 2. This mononuclear V(IV) complex bears a tri-
and 647–638 cmꢀ1, respectively. Moreover, all the di- dentate tris(pyrazolyl)borate and a bidentate bis(pyr-
oxovanadium(V) complexes (1–4) exhibit two sharp azolyl)borate, apart from the oxo ligand. The coordi-
bands corresponding to the symmetrical VO2 stretch nation geometry around the vanadium atom is best
174
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ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 171 – 187

Novel Scorpionate and Pyrazole Dioxovanadium Complexes
ꢀ
Table 1. Crystallographic data for [VO{HB(pz)3}
(5).
ACHTUNGTRENNUNG
ꢀ1.965(6) ꢅ out of this plane. The V1 O1 bond
length of 1.607(4) ꢅ is similar to that reported for
other six-coordinate vanadyl complexes (1.62 ꢅ),[26,27]
although smaller than that found in a similar structure
Empirical formula
Formula weight
426.95
reported previously[28] [average 1.591(1) ꢅ]. The V N
Crystal system
Space group
Orthorhombic
Pca21
ꢀ
distances for both chelating ligands in 2 vary from
2.097(5) to 2.120(5) ꢅ (average 2.108 ꢅ); however,
the bond length involving the N31 atom in trans posi-
tion to the oxo ligand is considerably longer
[2.268(6) ꢅ]. This lengthening can be attributed to a
significant trans influence of the latter ligand. The V
N bond lengths in 5 are comparable to those observed
a (ꢀ)
b (ꢀ)
15.534(12)
8.484(6)
15.042(12)
1982(3)
4
c (ꢀ)
V (ꢅ3)
Z
ꢀ
Density (calculated) (Mg/m3)
1.431
Absorption coefficient (mmꢀ1) 0.530
for other V-pyrazolyl
ACHTUNGTRENNUNG
F
(000)
876
Reflections collected/unique
Goodness-of-fit on F2
Final R indices [I>2 s(I)]
R indices (all data)
GOF
11037/3566 [R
0.9810
R1 =0.0578, wR2 =0.1073
R1 =0.1058, wR2 =0.1264
0.981
ACHTUNGTREN(NUNG int)=0.1154]
[VOCl{HB(3,5-Me2pz)3}AHCTNUGTRENNNUG
Me2pz)3}] and [V{HB(pz)3}2]ACHTUNGTRNEUNG
[BPh4].[30]
Electrochemical Studies
Complexes 1, 2, 4 and 5 exhibit by cyclic voltammetry
(as illustrated by Figure 3 for compound 1), at a plati-
num disc electrode, 258C, in a 0.2M [n-Bu4N]ACHTUNGTRENNUNG[BF4]
Table 2. Selected bond lengths (ꢅ) and angles (8) for
[VO{HB(pz)3}ACHTUNGTRENNUNG{H2B(pz)2}] (5).
solution in dichloromethane or DMSO, a first irrever-
sible single-electron reduction (confirmed by con-
trolled potential electrolysis), assigned to the V(V)!
ꢀ
V1 O1
1.607(4)
2.120(5)
2.098(5)
2.268(6)
2.097(5)
2.118(5)
86.49(17)
80.30(17)
162.96(18)
90.64(17)
80.87(18)
91.82(18)
164.03(18)
82.69(17)
83.15(18)
86.34(17)
95.8(2)
ꢀ
V1 N11
I
red
V(IV) reduction (complexes 1, 2 and 4, Ep in the
ꢀ0.17 to ꢀ0.48 V vs. SCE range) or to the V(IV)!
ꢀ
V1 N21
ꢀ
V1 N31
VACHTUNGTERN(NNUG III) reduction (complex 5, as expected at the much
I
ꢀ
V1 N41
lower Epred value of ꢀ1.19 V vs. SCE) (Table 3).
ꢀ
V1 N51
ꢀ
ꢀ
For the V(V) complexes 1, 2 and 4 a second irrever-
N11 V1 N21
red
ꢀ
ꢀ
N11 V1 N31
sible reduction wave is also observed at IIEp ca.
ꢀ
ꢀ
N11 V1 N41
ꢀ1.25 to ꢀ1.82 V vs. SCE, conceivably involving the
ꢀ
ꢀ
N11 V1 N51
V(IV)!V
tion potential (IEpred =ꢀ1.19 V vs. SCE).
ACHTUNGTRENUN(NG III) reduction. Accordingly, the reduction
of the V(IV) complex 5 occurs at a comparable reduc-
ꢀ
ꢀ
N21 V1 N31
ꢀ
ꢀ
N21 V1 N41
ꢀ
ꢀ
N21 V1 N51
ꢀ
ꢀ
N31 V1 N41
ꢀ
N31 V1-N51
ꢀ
ꢀ
N41 V1 N51
ꢀ
ꢀ
O1 V1 N11
ꢀ
ꢀ
O1 V1 N21
96.41(19)
175.45(19)
101.2(2)
ꢀ
ꢀ
O1 V1 N31
ꢀ
ꢀ
O1 V1 N41
ꢀ
ꢀ
O1 V1 N51
99.49(18)
ꢀ
described as a distorted octahedron. The ligand N
V N angles are restrained by the chelate rings to
80.4–91.88.
ꢀ
ꢀ
Two nitrogen atoms from the tridentate HB(pz)3
ligand (N11 and N21) and two nitrogem atoms from
ꢀ
the bidentate H2B(pz)2 ligand (N41 and N51) form
the equatorial plane, while the axial positions are oc-
cupied by the remaining nitrogen (N31) from the tri-
dentate HB(pz)3 ligand and the oxygen atom (O1).
Figure 3. Cyclic voltammogram of [VO2(3,5-Me2Hpz)3]
(1), initiated by the cathodic sweep, at a Pt disc electrode, in
0.20M [n-Bu4N][BF4]/CH2Cl2 solution (u=200 mV/s).
ACHTUNGTRENNUNG[BF4]
ꢀ
The N11, N21, N41 and N51 atoms are on a plane
a
ACHTUNGTRENNUNG
and V1, O1 and N31 lay 0.302(3), 1.908(5) and Complex concentration: 1.5 mM.
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Telma F. S. Silva et al.
FULL PAPERS
Table 3. Cyclic voltammetric data[a] for [VO2(3,5-Me2Hpz)3]
Catalytic Studies
A
2A
ACHTUNGTERN[NUNG BF4] (4)
ACHTUNGTRENNUNG
Carboxylation of Gaseous Alkanes
red
IEp
red
IIEp
ox
Complex
Ep
The carboxylation reactions of methane and ethane
(reactions a and b, Scheme 2) were typically per-
formed under CO, in trifluoroacetic acid (TFA), at
808C for 20 h, by using a K2S2O8 as the oxidizing
agent, in the presence of a V-catalyst (0.020 mmol):
compounds 1–5 or the related chloro-complexes
G
E
N
[BF4] (1)[b]
ꢀ0.17
ꢀ0.46
ꢀ0.48
ꢀ1.19
ꢀ1.25
ꢀ1.82
ꢀ1.70
–
–
–
–
CHTUNGTRENNUNG
ACHTUNGTRENNUNG
[VO{HB(pz)3}
G
1.76
[a]
mVsꢀ1.
In CH2Cl2.
In DMSO.
[VCl3{HC(pz)3}] (6) and [VCl3ACTHNGUTERNNU{G SO3C(pz)3}] (7) pre-
[b]
[c]
pared elsewhere.[13b] The experimental conditions
(i.e., temperature, time and molar ratios of reactants)
were those found by some of us[32–37] and others[38] to
be optimal for the carboxylation reactions of alkanes
Although comparisons of the redox potentials have in TFA catalyzed by various transition metal coordi-
to be taken rather cautiously in view of the irreversi- nation compounds.
bility of the waves and the different solvents used, the
To the best of our knowledge, compounds 1–7 are
I
red
measured ꢀEp values suggest[31] that HC(pz)3 and the first scorpionate and related pyrazole vanadium
SO3C(pz)3 seem to display comparable electron- complexes to be used as catalysts for the carboxyla-
donor characters (in 4 and 2, respectively), while 3,5- tion of alkanes.
Me2Hpz (in 1) appears to behave as a weaker elec-
tron-donor than each coordinating arm of the former
The complexes bearing the tris(1-pyrazolyl)meth-
anesulfonate ligand [VO2A{SO3C(pz)3}] (2) and
[VCl3A{SO3C(pz)3}] (7) are the most active ones to-
wards methane carboxylation (yields up to ca. 40%),
N
E
CHTUNGTRENNUNG
ligands.
CHTUNGTRENNUNG
ox
Complex 5 displays one oxidation wave at Ep
=
1.76 V vs. SCE, assigned to V(IV)!V(V). This oxida- whereas the activity decreases on replacement of that
tion potential is markedly higher than that reported scorpionate by the analogous neutral hydrotris(1-pyr-
for the hydrotris(1-pyrazolyl)borate oxovanadium(IV) azolyl)methane (4 and 6, respectively) (Table 4 and
[VO{HB(pz)3}ACHTUNGTRENNUNG
that the h2-BH2(pz)2 ligand in 5 seems to behave as a of methane by our systems can proceed even without
weaker electron donor than h2-acetylacetonate (acac) CO gas as the TFA solvent can behave as a carbony-
in the latter complex. However, the above conclusions lating agent[32a,b,33] (Table S1, entry 2, Supporting In-
have to be confirmed by comparative studies of other formation). However, in this case the yield of acetic
complexes with those ligands which should exhibit re- acid is much lower. The effect of the pressure of CO
versible redox waves.
on the yield of the acetic acid product has been stud-
ied, showing an optimal value of 5 atm. Higher CO
pressures lead to an acetic acid yield drop (Figure 4,
Scheme 2.
176
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Novel Scorpionate and Pyrazole Dioxovanadium Complexes
Table 4. Carboxylation of alkanes catalysed by
[VO2{HB(3,5-Me2pz)3}] (3), [VO2{HC(pz)3}][BF4] (4), [VO{HB(pz)3}
ACHTUNGTRENNUNG
{SO3C(pz)3}] (7).[a]
V
complexes: [VO2(3,5-Me2Hpz)3]
ACHTUNTGRENNGU[BF4] (1), [VO2ACHTUGNTRNE{NUGN SO3C(pz)3}] (2),
N
ACHTUNGTRENNUNG
Carboxylation Reac-
tion:
Substrate: CH4
Substrate: C2H6
Product
Catalyst
MeCOOH TON
(Yield)
EtCOOH TON
(Yield)
MeCOOH TON
(Yield)
Total TON (Total
Yield)
1
2
14 (25)
22 (39)
9 (16)
traces
9 (16)
17 (31)
11 (20)
21 (37)
5 (6)
9 (11)
8 (10)
9 (11)
–
9 (11)
3 (3)
2 (3)
2 (2)
14 (17)
19 (23)
21 (25)
157 (0.5)
19 (23)
6 (7)
11 (13)
12 (14)
157 (0.5)
10 (12)
3 (4)
3
3[b]
4
5
6
7
2 (2)
5 (6)
4 (5)
7 (8)
[a]
Selected results (for the full results see Supporting Information, Table S1); turnover numbers (TON, moles of product/
mol of catalyst) determined by GC; values of product yields (moles of product/100 moles of alkane) are given in brackets.
Typical reaction conditions for carboxylation: pACHTUNGTNERN(UNG alkane)=5 atm (1.02 and 1.53 mmol for CH4 and C2H6, respectively),
p(CO)=5 atm, n(catalyst)=0.020 mmol, 808C, 20 h, K2S2O8 (4.00 mmol), CF3COOH (7.5 mL for CH4 and 5.5 mL for
AHCTUNGTRENNUNG
C2H6).
[b]
Conditions for carboxylation of CH4: pACTHNUGTRENUN(NG alkane)=5 atm, p(CO)=7.5 atm, nACTHUNGTRENNNUG
with K2S2O8 (4.00 mmol)), 808C, 20 h, CF3COOH (5.0 mL). Conditions for carboxylation of C2H6: p
p(CO)=25 atm, n
(5.0 mL).
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
The conversion of ethane into propionic (as a result
of carboxylation) and acetic (as a result of oxidation)
acids (reaction b, Scheme 2) proceeds under the
above mentioned conditions, in the presence of 1–7
(Table 4 and Table S2, Supporting Information). The
maximum activity was observed for complex 3 (over-
all turnover number TON of 157, although under con-
ditions that lead to a low yield), which, however, does
not reach the high levels we have observed for Ama-
vadin models.[34]
We found that our V complexes show some selec-
tivity in the ethane carboxylation: acetic acid is the
main product when using 1, whereas for 7 propionic
Figure 4. Effect of CO pressure on the yield of acetic acid acid is produced in a higher amount than that of
derived from methane (at 5 atm) by using catalyst
[VO2A{SO3C(pz)3}] (2) and K2S2O8 in CF3COOH. Reaction
conditions and point numbers are those of Table S1 (Sup-
acetic acid (Table 4 and Table S2, Supporting Infor-
mation).
G
CHTUNGTRENNUNG
porting Information).
Hydroxylation/Oxygenation of Liquid Alkanes
Table S1, entries 5 and 6, Supporting Information), All our vanadium complexes act as catalysts or catalyst
CO conceivably acting as a ligand with a resulting precursors for the oxidation, with aqueous H2O2 (30%
loss of the catalyst activity. The mechanism of carbox- aqueous solution), of cyclohexane and cyclopentane to
ylation of methane possibly involves the formation of the corresponding alcohols and ketones, in CH3CN
the methyl radical (upon hydrogen abstraction from and in the presence of HNO3, at room temperature, ac-
CH4) by the HSO4C radical (derived from thermolysis cording to Scheme 2 (reactions c and d). The reactions
2ꢀ
of S2O8 in acid medium), followed by carbonyla- were typically performed under dinitrogen and the
tion.[33]
final reactions solutions were analyzed by GC.
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Telma F. S. Silva et al.
FULL PAPERS
Table 5. Peroxidative oxidation of cycloalkanes catalyzed by V complexes: [VO2(3,5-Me2Hpz)3]
(2), [VO2{HB(3,5-Me2pz)3}] (3), [VO2{HC(pz)3}][BF4] (4), [VO{HB(pz)3}{H2B(pz)2}] (5), [VCl3{HC(pz)3}] (6) and [VCl3
ACHTUNGTRENNUNG
{SO3C(pz)3}] (7).[a]
ACHTUNTGRNENGU[BF4] (1), [VO2ACHTUNGTRNE{NUGN SO3C(pz)3}]
N
ACHTUNGTRENNUNG
Hydroxylation/Oxygenation Reac-
tion:
Substrate: C5H10
Substrate: C6H12
Product
Catalyst
C5H9OH
TON
C5H8O
TON
Total
TON
C6H11OH
TON
C6H10O
TON
Total
TON
1
2
3
4
5
6
7
12[c]
25[b]
7[c]
0[c]
12[c]
41[b]
11[c]
5[c]
46
47
33
38
67
70
53
61
113
117
86
16[b]
4[c]
3[c]
2[c]
99
15[c]
4[c]
14[c]
3[c]
29[c]
7[c]
32
43
75
74[d]
35[d]
93[d]
86[d]
167[d]
121[d]
18[b]
0[b]
18[b]
[a]
Selected results (for the full results see Supporting Information, Table S3 and Table S7): turnover numbers (TON, moles
of product/mol of catalyst) determined by GC; yields (%) can be estimed as: TONꢆ[n(catalyst)/n(cycloalkane)]ꢆ100.
Typical (unless otherwise stated) reaction conditions: NCMe (3.0 mL), H2O2 =10 mmol; n(HNO3)/n(catalyst)=100;
208C; 6 h. For cyclohexane oxidation: n(H2O2)/n(catalyst)=40000, n(catalyst)/n
(C6H12)=5ꢆ10ꢀ5.
(catalyst)=500; catalyst (0.010 mmol); n(catalyst)/n
(C5H10)=2ꢆ10ꢀ3.
(catalyst)=1500; catalyst (0.010 mmol); n(catalyst)/n
(C5H10)=2ꢆ10ꢀ3.
Included for comparative purposes.[13b]
AHCTUNGTRENNUNG
G
ACHTUNGTRENNUNG
E
E
G
ACHTUNGTRENNUNG
[b]
[c]
[d]
n
nE
G
N
G
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
Cyclohexane Oxidation
As for carboxylation reactions, the dioxovanadium
complex [VO2A{SO3C(pz)3}] (2) displays the highest
CHTUNGTRENNUNG
catalytic activity towards oxidation of cyclohexane,
within this work, leading to a maximum TON of 117
(Table 5) upon 6 h reaction time, in the presence of
HNO3. This TON is higher than those of some other
vanadium catalysts with N,O-ligands, such as Amava-
din (TONs of ca. 50),[39c] and of commercially avail-
able V oxides (TONs of 47).[33a] However, the activity
is slightly lower than those recently reported for the
half-sandwich vanadium compounds 6 and 7 (TONs
of 167 and 121, respectively).[13b]
Effect of the Catalyst Amount
The effect of catalyst amount was studied for com-
plexes 1–5. The obtained TON values are given in
Table S3 (Supporting Information), with typical
curves shown in Figure 5. In all cases the catalyst
amount has a relevant effect, since an increase of the
nACHTUNGTRENNUNG(H2O2)/nACHTUNGTRENNUNG(catalyst) molar ratio results in the rise of
Figure 5. Effect of the catalyst amount on the total (cyclo-
hexanone and cyclohexanol) TON, in the peroxidative oxi-
dation of cyclohexane catalysed by vanadium complexes:
the catalyst TON, e.g., the overall TON for complex 2
increases from ca. 10 to 117, upon changing that ratio
from 100 to 40,000 (entries 6 and 10, Table S3, Sup-
porting Information).
~
~
[VO2(3,5-Me2Hpz)3]
[VO2{HB(3,5-Me2pz)3}] (3) ( ), [VO2{HC(pz)3}]
^
N
2ACHTUNGTRENN{NUG SO3C(pz)3}] (2) ( ),
^
[BF4] (4)
( ) or [VO{HB(pz)3}
N
Reaction conditions and point numbers are those of
Table S3 (Supporting Information).
178
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Novel Scorpionate and Pyrazole Dioxovanadium Complexes
Figure 6. Effect of the solvent amount on the total (cyclo-
hexanone and cyclohexanol) TON in the reaction of peroxi-
dative oxidation of cyclohexane catalysed by vanadium com-
~
Figure 7. Effect of nitric acid on the total (cyclohexanone
and cyclohexanol) TON in the peroxidative oxidation of cy-
clohexane catalysed by vanadium complexes: [VO2(3,5-
plexes: [VO2(3,5-Me2Hpz)3]ACHTUNGRTNEN[NGU BF4] (1) ( ),ACHTUNTREG[GNNUN VO2ACHTNGUTRENUN{NG SO3C(pz)3}]
~
~
Me2Hpz)3]
N
2ACHTUNGTRENN{NUG SO3C(pz)3}] (2) ( ),
~
^
(2) ( ), [VO2{HB(3,5-Me2pz)3}] (3) ( ) or [VO2{HC(pz)3}]
^
[VO2{HB(3,5-Me2pz)3}] (3) ( ), [VO2{HC(pz)3}]
[BF4] (4)
^
N
^
( ) or [VO{HB(pz)3}
G
G
10ꢀ3 mmol. Reaction conditions and point numbers are
those of Table S5 (Supporting Information).
those of Table S4 (Supporting Information).
Effect of the Solvent Amount
The activity of the studied systems also depends on promoting effect of a slight amount of nitric acid up
the solvent, which determines the polarity of the to nACHTUNTRGENN(GU HNO3/nACHTUNGTNER(NUGN catalyst) of ca. 200 was observed for
medium.[13] The choice of acetonitrile, as the typical complexes 1, 2, 4 and 5. This fact can be related to a
solvent for our system, is due to its high resistance to decreased decomposition of hydrogen peroxide to
oxidising agents and also in view of the higher solubil- water and oxygen,[42] resulting in a faster formation of
ity of the alkane and organic products in this solvent possible intermediate peroxo complexes.[43,44] In con-
in contrast with other ones, e.g., methanol, ethanol or trast, for 3, a slight inhibiting effect is observed upon
acetone. Besides, it has also been used in many other nitric acid addition.
cases as the most appropriate solvent for such a type
However, interestingly, some of our catalytic sys-
tems operate even in the absence of acid (e.g., TONs
of alkane oxidations.[33a,36,37,39–41]
The formation of the products is observed even in of 39 for catalysts 3 and 5), which is of significance
the absence of acetonitrile (Table S4, entries, 1, 7, 13 for the design of a “green process”.
and 18, Supporting Information), what could be of
significance towards the establishment of an effective
“green” process. The increase of the acetonitrile Effect of the Reaction Time, Radical Initiator and
amount results in the rise of activity (Figure 6). The Radical Traps
maximum TON value was obtained for ca. 3.0 mL of
acetonitrile, for catalysts 1–3 (with 1 mL of 30% The effect of the reaction time on the activity of the
aqueous H2O2). Further increases of the CH3CN complexes 1, 2 and 4 was studied and an increase of
amount lead to lower TONs, most likely due to the TON with time up to 6 h (Figure 8) was observed. A
dilution effect of reagents and/or intermediates.
further increase of the reaction time results in a TON
drop, most likely due to subsequent reactions.
For complex 2, the effect of the presence of 3-chlo-
Effect of the Amount of Nitric Acid
ACHTUNGTRENNUNG
It has been shown that the presence of a small Figure 9. An accelerating effect is observed for the
amount of nitric acid increases the TONs in catalytic first 3 h, whereafter inhibition occurs. A similar be-
oxidation systems containing vanadium,[39] iron,[40] haviour was reported[37] for some of our Re scorpio-
copper[41a–f] or rhenium[33a,37,41g] complexes with N,O li- nate or pyrazole complexes, e.g., [ReClF
ACTHUNTRGNEUNG{N2C(O)Ph}-
gands as the catalysts.
N
2ACHTUNGTRNENUG(PPh3)] or [ReO3ACHTUNGTERN{NUGN SO3C(pz)3}].
The effects of the addition of nitric acid in the sys-
tems catalyzed by complexes 1–5 are shown in the reaction was performed in the presence of either
Figure 7 and Table S5 (Supporting Information). A a carbon or an oxygen radical trap (CBrCl3 or
Adv. Synth. Catal. 2010, 352, 171 – 187
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179

Telma F. S. Silva et al.
FULL PAPERS
served upon addition of PPh3, indicating that either
CyOOH had already decomposed (further reacted) in
the presence of this catalyst or the reaction did not
proceed via that hydroperoxide.
Regioselectivity
In order to gain a further mechanistic insight[46,47c,48]
into the cycloalkane oxidation, we have also studied
the oxidation of methylcyclohexane with hydrogen
peroxide, catalysed by 2. The reaction was run for 6 h,
and the sample was analysed by GC after treatment
with PPh3 (in order to reduce the alkyl hydroperox-
ides to the corresponding alcohols). The reaction
products show the following 18:28:38 selectivity:
1:7:17.5, respectively. The molar ratios of the obtained
alcohols were normalised taking into account the
equivalent hydrogen atoms at each position. This lim-
ited selectivity is indicative of the involvement of the
highly reactive hydroxyl (HOC) radical, as report-
Figure 8. Effect of reaction time on the total (cyclohexanone
and cyclohexanol) TON, in the peroxidative oxidation of cy-
clohexane catalysed by vanadium complexes: [VO2(3,5-
~
~
Me2Hpz)3]
[VO2{HC(pz)3}]
N
2ACHTUNGTREN{UNGN SO3C(pz)3}] (2) ( ) and
[BF4] (4) ( ). Reaction conditions and point
^
N
numbers are those of Table S6 (Supporting Information).
AHCTUNGTRENNUNG
ed[33a,39b,40b,41e,43,46,49,54] for other systems, e.g., H2O2-
[47c]
Ph2NH, respectively), thus supporting a radical mech- Fe
(ClO4)3
and H2O2-[NBu4]ACTHGNUTRENNU[G VO3]-PCA (PCA=
anism for the peroxidative oxidation of the alkane.
pyrazine-2-carboxylic acid).[46,47c]
Effect of the Addition of PPh3
Cyclopentane Oxidation
An increase in the amount of alcohol, ca. 10% for Effect of the Oxidant Amount
complex 4, with a concomitant decrease in the
amount of ketone is observed (Table S3, entry 21, Complexes 1–7 also catalyse the peroxidative oxida-
Supporting Information) when the final reaction solu- tion of cyclopentane to the corresponding cyclopenta-
tion is treated with an excess of PPh3, prior to the GC none and cyclopentanol, but their catalytic activity
analysis, according to a method reported by Shulꢇ- (Table 5) is lower than that for the oxidation of cyclo-
pin.[46,47] This shows that at the end of the reaction hexane (e.g., overall TONs of 41 and 117, respective-
there is a significant amount of cyclohexyl hydroper- ly, for 2). In contrast, a higher selectivity is observed
oxide (CyOOH) which is reduced to the alcohol by for the formation of the alcohol, i.e., cyclopentanol,
PPh3. However, in the case of 2, no appreciable which can be the only oxidation product detected in
change in the relative amounts of products was ob- the presence of 1 or 7. The total TONs increase
Figure 9. Effect of radical initiator (3-ClC6H4COOOH) on the total (cyclohexanone and cyclohexanol) TON, in the peroxi-
dative oxidation of cyclohexane catalysed by [VO
2ACHTUNGTREN{NUNG SO3C(pz)3}] (2). Reaction conditions and point numbers are those of
Table S6 (Supporting Information).
180
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Adv. Synth. Catal. 2010, 352, 171 – 187

Novel Scorpionate and Pyrazole Dioxovanadium Complexes
Figure 10. Effect of the catalyst amount on the total (cyclo-
pentanone and cyclopentanol) TON, in the peroxidative oxi-
dation of cyclopentane catalysed by vanadium complexes:
~
~
[VO2(3,5-Me2Hpz)3]
N
2ACHTUNGTRENN{NUG SO3C(pz)3}] (2) ( ),
^
[VO2{HB(3,5-Me2pz)3}] (3) ( ), [VO2{HC(pz)3}]
N
^
( ), [VO{HB(pz)3}
{H2B(pz)2}] (5) (x), [VCl3{HC(pz)3}] (6)
Scheme 3.
&
(–) and [VCl3ACHTUNGTRENNUNG{SO3C(pz)3}] (7) ( ). H2O2 =10 mmol. Reac-
tion conditions and point numbers are those of Table S7
(Supporting Information).
since the reactions are essentially suppressed when
they are carried out in the presence of either a C- or
(Figure 10) with the H2O2/catalyst molar ratio (i.e., an O-centred radical trap (such as CBrCl3 and
with an increase of the H2O2 amount or a decrease of Ph2NH).[14b,33a,34,37,41b]
that of the catalyst), but up to maximum values for 2
and 7. The further TON decrease for the high be derived from oxo-metal-promoted decomposition
(H2O2)/n
(catalyst) ratios conceivably results from reactions of H2O2,[7,33a,43,46,47] followed by H-abstraction
Peroxo-metal V-OOC and hydroxyl HOC radicals can
n
N
ACHTUNGTRENNUNG
overoxidation.
from the alkane (RH) to form the alkyl radical RC
(Scheme 3, reactions 1–4). The process could then
proceed via formation of the organo-peroxyl radical
ROOC (upon reaction of RC with O2) and the organo-
hydroperoxide ROOH[55] (reactions 5 and 6) which
Mechanistic Considerations
The carboxylations of methane and ethane (RH) are could undergo metal-assisted homolytic decomposi-
expected to proceed via free radical mecha- tion to the organooxyl ROC and organoperoxyl ROOC
nisms,[33,34,50] and theoretical calculations[33b,34] per- radicals,[56] the latter thus being regenerated (reac-
formed for other oxo-vanadium systems with TFA/ tions 7 and 8). H-abstraction from the alkane by ROC
K2S2O8 disclose a particularly favourable process in- would form the alcohol ROH and RC (reaction 9),
volving the sequential formation of RC, RCOC and whereas ROOC could either decompose to the alcohol
RCOOC which, upon H-abstraction (from TFA or and the ketone (reaction 10) or regenerate ROOH
RH),[33b,34] form the RCOOH acid.
and RC upon H-abstraction from RH (reaction 11), as
RC can be formed by H-atom abstraction from the proposed[42,47] for some metal-catalysed alkane oxida-
ꢀ
alkane by the sulfate radical SO4 C (or its protonated tions with O2. The alcohol can also be formed from RC
HSO4C form)[33b,34,51a] derived from thermolytic decom- and a metal hydroperoxide (reaction 12),[33b] the latter
[50,51b]
position of K2S2O8.
Carbonylation of RC would being derived, e.g., upon H-abstraction from RH by
form the acyl radical RCOC that upon oxygenation by V(IV)-OOC.
a peroxo-metal complex would lead to RCOOC from
The involvement of the highly reactive HOC radical
which RCOOH would be formed (see above).[33b] Al- is suggested by the restricted regioselectivity of the
ternatively, RCOC could be oxidized to RCO+ which, oxidation of methylcyclohexane, whereas the partici-
on reaction with TFA, would lead to the mixed anhy- pation of the hydroperoxide ROOH is corroborated
dride CF3COOCOR. Further reaction of the latter for the catalyst 4 by the increase of the alcohol
with TFA would yield the acid RCOOH and amount with a concomitant decrease of that of the
CF3COOCOCF3 (trifluoroacetic anhydride), as pro- ketone when the final reaction solution, prior to GC
posed[33b,50] in other cases.
analysis, is treated with an excess of PPh3 (method re-
The mechanisms of the peroxidative oxidations pos- ported by Shulꢇpin.[46]). For the metal catalyst 2, no
sibly involve both C-centred and O-centred radicals, ROOH was detected at the end of the experiments
Adv. Synth. Catal. 2010, 352, 171 – 187
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181

Telma F. S. Silva et al.
FULL PAPERS
probably due to its possible[41a,b,48] metal-promoted de- Experimental Section
composition to the alcohol and ketone.
Materials and Instrumentation
Pyrazole, 3,5-dimethylpyrazole and V2O5 were used as re-
ceived from the supplier (Aldrich). Triethyl vanadate,
Conclusions
[VO
dure immediately before use, and employed as a solution in
ethanol (1.00 mmol of [VO(OEt)3] corresponds to ca. 20 mL
(OEt)3] was synthesized in situ via a known[15a] proce-
ACHTUNGTRENNUNG
We have found a simple and convenient route for the
formation of dioxovanadium complexes with scorpio-
AHCTUNGTRENNUNG
nate or pyrazole ligands based on the use of the easily
of ethanol solution) without further isolation. Potassium hy-
+
drotris(l-pyrazolyl)borate,[57] potassium hydrotris(3,5-di-
generated in situ [VO
N
methyl-1-pyrazolyl)borate,[57]
hydrotris(l-pyrazolyl)me-
source, thus contributing to expand the still relatively
little explored coordination chemistry of HC(pz)3 and
thane,[10] hydrotris(3,5-dimethyl-1-pyrazolyl)methane[10] and
lithium tris(l-pyrazolyl)methanesulfonate[12a] were prepared
in accord with the published procedures. Complexes
ꢀ
SO3C(pz)3 types of scorpionate ligands. That route
has allowed the synthesis of the first dioxovanadium
complexes bearing tris(1-pyrazolyl)-methanesulfonate
or borate, or pyrazole ligands. The work also shows
that the oxovanadium centre can promote the rupture
3ACTHNUTRGNEUNG
[VCl3{HC(pz)3}][13b] (6) and [VCl {SO3C(pz)3}][13b] (7) were
prepared from VCl3, also according to procedures described
earlier.
Solvents were purified by standard procedures and freshly
distilled immediately prior to use. All manipulations and re-
actions were performed under an atmosphere of dinitrogen
using standard vacuum and inert-gas flow techniques. Cyclo-
hexane (Merck), methylcyclohexane (Merck), cyclopentane
(Merck), acetonitrile (Riedel-de-Haꢈn), hydrogen peroxide
(30%) (Fluka), nitric acid (65%) (Riedel-de-Haꢈn), 3-chlo-
3
ꢀ
of a C
(sp ) N(pyrazolyl) bond in HC(3,5-Me2pz)3 and
ꢀ
ꢀ
of a B N bond in HB(pz)3 to afford pyrazole and
ꢀ
H2B(pz)2 complexes, what has be taken into account
when studying coordination reactions of those scorpi-
onates.
The vanadiumACHTUNGTRENNUNG(V and IV) complexes are redox
A
acid
(Aldrich),
triphenylphosphine
active, what allows us to gain an insight into the rela-
tive electron-donor abilities of the scorpionate and
pyrazole ligands, based on the redox potential values,
although a firm comparison is precluded by the irre-
versibility of the redox waves.
The work also shows that oxovanadium complexes
with scorpionate ligands, such as those reported
herein, can act as catalysts or catalyst precursors for
alkane functionalisation reactions under mild condi-
tions, including their one-pot carboxylation and per-
oxidative hydroxylation or oxygenation. Complex
(Merck), potassium and ammonium peroxodisulfates
(Fluka), trifluoroacetic acid (Aldrich), bromotrichlorome-
thane (Fluka), diphenylamine (Fluka), n-butyric acid (Al-
drich), cycloheptanone (Aldrich), cyclopentanone (Aldrich),
diethyl ether (Riedel-de-Haꢈn), ethane (AlphaGaz), carbon
monoxide (Air Products), dioxygen (Air Liquid Portugal)
and dinitrogen (Air Liquid Portugal) were used as pur-
chased.
Infrared spectra (4000–400 cmꢀl) were recorded on a
Nicolet Impact 400D or a BIO-RAD FTS 3000 MX spectro-
photometer instrument in KBr pellets; wavenumbers are in
cmꢀ1; abbreviations: vs, very strong; s, strong; m, medium.
1H, 13C, 13C{1H}, 19F and 51 V NMR spectra were recorded on
Bruker Avance II+ 300 and 400 MHz (UltraShieldTM
Magnet) or a Varian Unity 300 spectrometer at ambient
temperature. d values are in ppm relative to Me4Si (1H and
13C), CCl3F (19F) or [VOCl3] (51 V). Coupling constants are
in Hz; abbreviations: s, singlet; d, doublet; t, triplet; m,
complex multiplet; br, broad. In the 13C NMR data, assign-
ments and coupling constants common to the 13C{1H} NMR
spectra are not repeated. Atom labelling for the Hpz ligand:
HN(1)N(2)C(3)HC(4)HC(5)H; the numbering of the coor-
dinated pyrazolyl ring is VN(1)N(2)C(3)HC(4)HC(5)H.
EPR spectra were recorded on a Bruker ESP 300E X-band
spectrophotometer equipped with an ER 4111 VT variable-
temperature unit.
[VO2ACHTUNGTRENNUNG{SO3C(pz)3}] (2) exhibits the highest catalytic ac-
tivity for the carboxylation of methane to acetic acid
and for the cycloalkane oxidations, whereas, for
ethane carboxylation, the most active catalyst is
[VO2{HB(3,5-Me2pz)3}] (3). The dioxo-scorpionate-
V(V) complexes 1–4 are usually more active than the
monooxo-discorpionate-V(IV) 5 and than the tri-
chloro-V(IV) compounds 6 and 7. However, for the
oxidation of cyclohexane, their activities are compara-
ble or lower than those of the latter compounds. Ner-
vertheless, the generality of these observations should
be taken cautiously and tested further for other types
of alkane reactions and vanadium catalysts. The
search for greener systems should proceed, and the
favourable features of the oxo-scorpionato-vanadium
catalysts reported here (including their operation
under mild conditions and, in some cases, even in
acid-free systems) should encourage further investiga-
tion within this challenging field of research.
FAB+ and FABꢀ mass spectra were obtained on a Trio
2000 spectrometer by bombarding 3-nitrobenzyl alcohol
(NBA) matrices of the samples with 8 keV (ca. 1.28ꢆ
1015 J) Xe atoms. Mass calibration for data system acquisi-
tion was achieved using CsI. ESI+/ESIꢀ mass spectra were
obtained on a Varian 500-MS LC ion trap mass spectrome-
ter [solvent: dimethyl sulfoxide; flow: 20 mLminꢀ1; needle
spray voltage: ꢁ5 kV, capillarity voltage: ꢁ100 V; nebulizer
gas (N2): 35 psi; drying gas (N2): 10 psi; drying gas tempera-
ture (N2): 3508C]. For the mass spectra description, M de-
182
asc.wiley-vch.de
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 171 – 187

Novel Scorpionate and Pyrazole Dioxovanadium Complexes
notes the complex part of compounds 1–5. The C, H, N, S
elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Tꢂcnico.
The electrochemical experiments were performed on an
EG&G PAR 273 A potentiostat/galvanostat connected to
personal computer through a GPIB interface. Cyclic voltam-
ture was filtered, and the brown-yellow filtrate was concen-
tred under vacuum„ followed by slow addition of Et2O
(5 mL), resulting in the precipitation of 1 as a dark brown
solid. This was filtered off, washed with Et2O and dried
under vacuum; yield: 206 mg (45% yield).
Complex 1 can also be obtained by an alternative way: a
mixture of 3,5-dimethylpyrazole (288 mg, 3.00 mmol) and
mograms (CV) were obtained in 0.2M [n-Bu4N]ACTHNUTRGENUG[N BF4]/
CH2Cl2 or DMSO, at a platinum disc working electrode (d=
1 mm). Controlled-potential electrolyses (CPE) were carried
out in electrolyte solutions with the above-mentioned com-
position, in a three-electrode H-type cell. The compartments
were separated by a sintered glass frit and equipped with
platinum gauze working and counter electrodes. For both
CV and CPE experiments, a Luggin capil1ary connected to
a silver wire pseudo-reference electrode was used to control
the working electrode potential. The CPE experiments were
monitored regularly by cyclic voltammetry, thus assuring
that no significant potential drift occurred along the elec-
trolyses. Ferrocene was used as an internal standard for the
measurement of the oxidation potentials of the complexes;
the redox potential values are quoted relative to the SCE by
using as internal reference[58] the ferrocene/ferricinium
couple (E1/2ox =0.525 V vs. SCE in CH2Cl2 or 0.44 V vs. SCE
in DMSO).
[VO
during ca. 24 h, resulting in the formation of an orange solu-
tion. Then Na[BF4] (326 mg, 3.00 mmol) in THF (5 mL) was
ACHTUNGRTENUN(NG OEt)3] (1.00 mmol) in ethanol (20 mL) was refluxed
AHCTUNGTRENNUNG
added, and the solution was refluxed for further ca. 2 h. The
filtered yellow solution was concentred under vacuum to ca.
one half followed by slow addition of Et2O, resulting in the
precipitation of the brown compound 1; yield: 308 mg
ꢀ
(50%).IR (KBr pellet): n=3206 [s, nACTHNUTRGNEUNG(N H)], 2022, 1638 and
ꢀ
1298 [s, n
(N=N), n
E
N
ꢀ
n
N
[m, n
(V N)]; 1H NMR: (DMSO-d6), d=5.75 [s, br, 3H,
ꢀ
H(4) 3,5-Me2Hpz], 2.12 [s, br, 18H, CH3, 3,5-Me2Hpz];
13C{1H} NMR (DMSO-d6): d=143.2 [s, C(3) and C(5), 3,5-
Me2Hpz], 103.9 [s, C(4), 3,5-Me2Hpz], 12.3 [s, CH3, 3,5-
Me2Hpz];13C NMR (DMSO-d6): d=103.3 [d, JCH =132 Hz,
C(4), 3,5-Me2Hpz], 11.8 [q,
JCH =92.3 Hz, CH3, 3,5-
Me2Hpz]; 51V NMR (DMSO-d6): d=ꢀ472; 19F NMR
(DMSO-d6): d=ꢀ148.3; FAB+-MS: m/z=553 [M+(3,5-
Me2Hpz)]+, 387 [M+O]+, 371 [M]+, 276 [Mꢀ(3,5-
Me2Hpz)]+(MI); ESI+-MS: m/z=371 [M]+; FABꢀ-MS:
m/z=87 [BF4]ꢀ; anal. calcd. for C15H24N6BF4O2V: C 39.3, H
5.3, N 18.3%; found: C 39.7, H 5.9, N 18.4%.
Gas chromatographic (GC) measurements were carried
out using a Fisons Instruments GC 8000 series gas chroma-
tograph with an FID detector and a capillary column (DB-
WAX, column length: 30 m; internal diameter: 0.32 mm)
and the Jasco-Borwin v.1.50 software. The temperature of
injection was 2408C. Helium was used as the carrier gas.
This complex is soluble in DMSO, dichloromethane and
chloroform, but insoluble in diethyl ether, acetone, acetoni-
trile, toluene, hexane and methanol.
X-Ray Structure Determinations
A
2ACHTUNGTRNEN{UNG SO3C(pz)3}] (2): A solution of lithium tris(pyrazo-
Intensity data were collected on a Bruker AXS-KAPPA
APEX II diffractometer using graphite monochromated
Mo-Ka radiation. Data were collected at 150 K using omega
scans of 0.58 per frame and a full sphere of data was ob-
tained. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT on all
the observed reflections. Absorption corrections were ap-
plied using SADABS. The structure was solved by direct
methods by using the SHELXS-97 package[59] and refined
with SHELXL-97[60] with the WinGX graphical user inter-
face.[61] All hydrogens were inserted in calculated positions.
Least square refinement with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic for
the remaining atoms gave R1 =0. 0578 [I>2s(I); R1 =0.
1058 (all data)]. The maximum and minimum peaks in the
final difference electron density map are of 0.32 and
ꢀ0.43 eꢅꢀ3.
ACHTUNGTRENNUNG
(202 mg, 1.00 mmol) at 20–258C. The reaction mixture was
then refluxed for ca. 24 h, resulting in a green solution. Con-
centration under vacuum of the filtered green solution fol-
lowed by slow addition of Et2O (ca. 5 mL) resulted in the
precipitation of 2 as a green solid. This was filtered off,
washed with Et2O (two portions of 10 mL) and dried under
vacuum; yield: 188 mg (50%); IR (KBr pellet): n=1639 and
1524 [s, n
O)], 986 [m, nACHTNUTRGNEUNG(V=O)], 921, 860 [s (sym. and antisym. VO2)],
647 [m, n
ꢀ
(N=C) and n
E
N
1
ꢀ
ꢀ
(S C)] and 424 [m, n
G
ꢀ
d6): d=8.11 [s, bꢀr, 3H, H(3), SO3C(pz)3 ], 7.39 [s, br, 3ꢀH,
H(5), SO3C(pz)3 ], 6.30 [s, br, 3H, H(4), SO3C(pz)3ꢀ];
13C{1H} NMR (DMSO-d6): d=138.3 [s, C(3), SO3Cꢀ(pz)3 ],
ꢀ
132.2 [s, C(5), SO3C(pz)3 ], 105.8 [s, C(4), SO3C(pz)3 ], 80.1
[s, SO3C(pz)3]; 13C NMR ꢀ(DMSO-d6): d=138.2 [d, JCH
=
CCDC 697173 contains the supplementary crystallograph-
ic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
187.9 Hz, C(3), SO3C(pz)3 ], 132.1 [d, JCH =192.8 Hz, C(ꢀ5),
ꢀ
SO3C(pz)3 ], 105.7 [d, JCH =168.1 Hz, C(4), SO3C(pz)3 ];
51V NMR (DMSO-d6): d=ꢀ556; FAB+-MS: m/z=376 [M]+,
309 [Mꢀpz]+; ESI+-MS: m/z=376 [M]+; anal. calcd. for
C10H9N6O5SV: C 31.9, H 2.4, N 22.3, S 8.5%; found: C 31.5,
H 2.5, N 22.4, S 8.5%.
Synthesis and Characterization of the Complexes
This complex is soluble in DMSO, but insoluble in diethyl
ether, acetone, acetonitrile, toluene, hexane, dichlorome-
thane, chloroform and methanol and water.
G
ACHTUNGTRENN[UGN BF4] (1): A mixture of hydrotris(3,5-di-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(329 mg, 3.00 mmol) in THF (5 mL) was then added, and
the mixture was refluxed for further 2 h. The reaction mix-
AHCTUNGTRENNUNG
Adv. Synth. Catal. 2010, 352, 171 – 187
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
183

Telma F. S. Silva et al.
FULL PAPERS
ꢀ
ꢀ
ꢀ
was refluxed for ca. 24 h, resulting in a yellow solution. The
solution was filtered and concentred under vacuum to ca.
one half, followed by slow addition of Et2O (ca. 5 mL), re-
sulting in the precipitation of a brown solid of 3. The latter
was filtered off, washed with Et2O (two portions of 10 mL)
and dried under vacuum; yield: 198 mg (52%); IR (KBr
ꢀ
nACHTUNGRTNENG(U C=N), nACHUTGTNREN(UNG B N), nACHTNUGTREG(NNUN C N), HB(pz)3 ], 973 [s, nACHTNUGRTEN(NUGN V=O)], 839
ꢀ
ꢀ
[m, nACTHNUTRGENN(GU V O)], 460 [m, nACHTUNGTREN(NUGN V N)]. EPR (r.t., CH2Cl2): A=
101.9 G, g=1.9989; FAB+-MS: m/z=510 [M+O+pz]+, 443
[M+O]+, 427 [M]+, 414 [MꢀH2B(pz)2 +2pz]+, 360 [Mꢀpz]+,
347
[MꢀH2B(pz)2 +pz]+,
280
[MꢀHB(pz)2]+,
229
[MꢀH2B(pz)2 +Oꢀpz]+, 213 [MꢀH2B(pz)2ꢀpz]+; anal.
calcd. for C15H18N10B2OV: C 42.2, H 4.3, N 32.8%; found: C
42.3, H 4.3, N 32.8%.
This complex is soluble in dichloromethane, chloroform,
DMSO and water.
pellet): n=2450, 2365, 1638, 1421, ꢀ1296 [s, n
ACHUTGTNREN(UNG H B), nCAHTUNGTRENN(UGN N=C),
ACHTUNGTRENNUNG(C N), HB(3,5-Me2pz)3 ], 947, 886 [s (sym. and
ꢀ
ꢀ
n
G
1
ꢀ
(V N)]; HꢀNMR (DMSO-d6):
G
d=5.75 [s, br, 3H, H(4), HB(3,5-Me2pz)3 ], 2.08 [s, br, 18H,
ꢀ
CH3, HB(3,5-Me2pz)3 ]; 13C{1H} NMR (DMSO-d6): d=142.1
ꢀ
[s, C(3) ꢀand C(5), HB(3,5-Me2pz)3 ], 103.2 [s, ꢀC(4), HB(3,5-
Me2pz)3 ], 11.8 [s, CH3, HB(3,5-Me2pz)3 ]; 13C NMR
(DMSOꢀ-d6): d=103.5 [d, JCH =128.9 Hz, C(4), HB(3ꢀ,5-
Me2pz)3 ], 11.8 [q, JCH =92.9 Hz, CH3, HB(3,5-Me2pz)3 ];
Catalytic Activity Studies
Carboxylation of gaseous alkanes: In a typical experiment
the reaction mixtures were prepared as follows. To a mixture
of 20.0 mmol of the vanadium catalyst with 4.00 mmol
(1.08 g) of K2S2O8, contained in a 13.0-mL stainless steel au-
toclave, were added 7.5 mL or 5.0 mL of CF3COOH (TFA)
for methane or ethane carboxylation, respectively. Then the
autoclave was closed and flushed with dinitrogen three
times for replacing the air inside and finally pressurized with
CH4 (5 atm, 1.02 mmol) or C2H6 (5 atm, 1.53 mmol) and CO
(0–15 atm, 0.00–4.59 mmol). The reaction mixture was vigo-
rously stirred for 20 h at 808C using a magnetic stirrer and
an oil bath, whereupon it was cooled in an ice bath, de-
gassed and opened. The product analysis was undertaken as
follows. To 1.0 mL of the reaction mixture were added
5.0 mL of diethyl ether and 90 mL of internal standard n-bu-
tyric acid. The obtained mixture was stirred, then filtered
and analysed by gas chromatography (GC). Blank experi-
ments were performed for all tested alkanes in the presence
of CO and K2S2O8, and confirmed that no carboxylic acid
formation was detected unless the catalyst was used.
Hydroxylation/oxygenation of liquid alkanes: In typical
conditions, the reaction mixtures were prepared as follows:
0.010 mmol of V catalyst were dissolved in 2.50 mL of
MeCN. The required amount of this solution for the desired
catalyst amount was transferred to a second flask, where-
after MeCN was added until a total acetonitrile volume of
3.00 mL, 5.00 mmol of H2O2 (30% H2O solution, 0.50 mL)
and 5.00 mmol of cycloalkane were then added (in this
order) and the reaction solution was stirred for 6 h, at room
temperature and under N2 atmosphere. In the experiments
with HNO3, this acid (0–11 mmol) was added before the ad-
dition of the substrate. For the products analysis, 90 mL of
cyclopentanone or cycloheptanone (internal standards) for
cyclohexane or cyclopentane oxidations, respectively, and
6.5 mL of diethyl ether (to extract the substrate and the or-
ganic products from the reaction mixture) were added. The
obtained mixture was stirred during 10 min and then a
sample (1 mL) was taken from the organic phase and ana-
lysed by GC by the internal standard method. Blank experi-
ments were performed for both cycloalkanes with H2O2 and
confirmed that no product of alkane oxidation was obtained
unless the vanadium catalyst was used. The amount of alkyl
hydroperoxide (if formed) was estimated from the variations
in the alcohol and ketone yields, determined by GC analy-
ses, upon addition of PPh3 to the final reaction solution, ac-
cording to a method reported by Shulꢇpin[46,47,62] The carbon
radical trap[63] CBrCl3 was used in a stoichiometric amount
relative to substract (cyclohexane) and the oxygen radical
trap Ph2NH was added in a stoichiometric amount relative
51V NMR (DMSO-d6): d=ꢀ501; FAB+-MS: m/z=381 [M]+
,
286 [Mꢀ(3,5-Me2pz)]+; ESI+-MS: m/z=381 [M]+; anal.
calcd. for C15H23N6BO2V: C 47.3, H 6.1, N 22.1%; found: C
47.1, H 6.2, N 22.5%.
This complex is soluble in DMSO, dichloromethane,
chloroform, acetone and acetonitrile, but insoluble in diethyl
ether and methanol.
A
ACHTUNGTRENNUNG[BF4] (4): A mixture of hydrotris(1-pyra-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
added, and the solution was further refluxed for 2 h. The re-
sulting yellow-green solution was concentred under vacuum
to ca. one half, followed by slow addition of Et2O (ca. 5 mL)
to yield a green solid of 4 which was filtered off, washed
with Et2O (two portions of 10 mL) and dried under vacuum;
ꢀ
yield: 161 mg (42%); IR (KBr pellet): n=3127 [m, nACTHNUTRGNEUNG(C
ꢀ
H)], 1618, 1524, 1295 [s,
nACHTUNGTREG(NNUN N=C), nACHTUNREGTG(NNUN C=C), nACHUTGTNRENNUG(C N),
ꢀ
HC(pz)3], 1068 [vs, n
sym. VO2)], 431 [m, n
G
(V N)]; 1H NMR (DMSO-d6): d=
8.96 [s, 1H, HC(pz)3], 7.88 [m, br, 3H, JHH =2.4 Hz, H(3)
or H(5), HC(pz)3], 7.66 [m, br, 3H, JHH =2.2 Hz, H(5) or
ꢀ
3
3
3
H(3), HC(pz)3], 6.42 [m, br, JHH =2.0 Hz, H(4), HC(pz)3];
ACHTUNGTRENNUNG
13C{1H NMR} (DMSO-d6): d=140.9 [s, C(3), HC(pz)3],
130.1 [s, C(5), HC(pz)3], 106.8 [s, C(4), HC(pz)3], 82.3 [s,
HC(pz)3]; 13C NMR (DMSO-d6): d=140.9 [d, JCH
139.1 Hz, C(3), HC(pz)3], 130.1 [d, JCH =144.3 Hz, C(5),
=
HC(pz)3], 106.9 [d,
JCH =137.5 Hz, C(4), HC(pz)3];
51V NMR (DMSO-d6): d=ꢀ538; 19F NMR (DMSO-d6): d=
ꢀ148.3; FAB+-MS: m/z=297 [M]+, 230 [Mꢀpz]+; FABꢀ-
MS, m/z=87 [BF4]ꢀ; ESI+-MS: m/z=297 [M]+; anal. calcd.
for C10H10N6BF4O2V: C 31.3, H 2.6, N 21.9; found: C 31.5,
H 2.3, N 21.5.
This complex is soluble in DMSO, but insoluble in diethyl
ether, dichloromethane, chloroform, acetone, acetonitrile,
toluene, hexane and methanol.
[VO{HB(pz)3}
drotris(1-pyrazolyl)borate (504 mg, 2.00 mmol) and [VO-
ACHTUNGTRENNUNG(OEt)3] (202 mg, 1.00 mmol) in ethanol (20 mL) was re-
ACHTUNGTRENNUNG{H2B(pz)2}] (5): A mixture of potassium hy-
fluxed for ca. 24 h, yielding a blue solution. The latter was
concentrated under vacuum, followed by slow addition of
Et2O (5 mL), resulting in the precipitation of blue crystals
of 2, which were separated by filtration and dried in air;
yield: 158 mg (37%); IR (KBr pellet): n=2412, 2384 [s,
ꢀ
ꢀ
ꢀ
n
n
G
(B H), H2B(pz)2 ], 1740, 1637, 1407, 1153 [s, n
184
asc.wiley-vch.de
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 171 – 187

Novel Scorpionate and Pyrazole Dioxovanadium Complexes
to H2O2. The radical initiator (3-ClC6H4COOOH)[45] was
used in a stoichiometric amount relative to the substrate.
For studying the selectivity with methylcyclohexane, the re-
action was run for 6 h, whereafter 90 mL of cyclopentanone
(internal standard) and 6.5 mL of diethyl ether (to extract
the substrate and the organic products from the reaction
mixture) were added, and the sample was then analysed by
GC.
Chemistry, 2nd edn., (Ed.: R. B. King), Wiley, 2005, Vol.
IX, pp 5767–5803; d) T. Hirao, Chem. Rev. 1997, 97,
2707; e) A. S. Tracey, G. R. Willsky, E. S. Takeuchi, Va-
nadium – Chemistry, Biochemistry Pharmacology and
Practical Applications, CRC Press, 2007.
[7] a) M. V. Kirillova, M. L. Kuznetsov, V. B. Romakh,
L. S. Shulꢇpina, J. J. R. Frafflsto da Silva, A. J. L. Pom-
beiro, G. B. Shulꢇpin, J. Catal. 2009, 267, 140; b) L. S.
Shulꢇpina, M. V. Kirillova, A. J. L. Pombeiro, G. B.
Shulꢇpin, Tetrahedron 2009, 65, 2424.
[8] a) S. Trofimenko, J. Am. Chem. Soc. 1967, 89, 3170;
b) S. Trofimenko, Scorpionates – The Coordination
Chemistry of Polypyrazolylborate Ligands, Imperial
College Press, London, 1999.
[9] a) C. Pettinari, Scorpionates II: Chelating Borate Li-
gands, Imperial College Press, World Scientific Pub.,
2008; b) C. Pettinari, C. Santini, in: Comprehensive Co-
ordination Chemistry II, Vol. 1, (Eds.: J. A. McCleverty,
T. J. Meyer), Elsevier, Oxford, 2004, Chap. 10, p 159;
c) S. Trofimenko, Chem. Rev. 1993, 93, 943; d) R.
Garcia, A. Paulo, I. Santos, Inorg. Chim. Acta 2009,
362, 4315.
Acknowledgements
This work has been partially supported by the Fundażo
para a CiÞncia e a Tecnologia (FCT), Portugal, its PPCDT
(FEDER funded) programme. TFSS and MVK are grateful
to FCT for fellowships (grants: SFRH/BD/48087/2008 and
SFRH/BPD/34926/2007). The authors also thank Dr. Maria
Cꢁndida Vaz (IST) for the direction of the elemental analysis
service, Mr. Indalꢂcio Marques (IST) and Dr. M. Conceiżo
Oliveira (IST) for running the FAB-MS and ESI-MS, respec-
tively, Dr. Jo¼o Paulo Telo (IST) for the EPR spectrum and
Portuguese NMR Network (IST-UTL Center) for providing
access to the NMR facility.
[10] D. L. Reger, T. C. Grattan, K. J. Brown, C. A. Little,
J. J. S. Lamba, A. L. Rheingold, R. D. Sommer, J. Orga-
nomet. Chem. 2000, 607,120.
[11] a) C. Pettinari, R. Pettinari, Coord. Chem. Rev. 2005,
249, 525; b) H. R. Bigmore, S. C. Lawrence, P. Mount-
ford, C. S. Tredget, Dalton Trans. 2005, 635.
[12] a) W. Klaui, M. Berghahn, G. Rheinwald, H. Lang,
Angew. Chem. 2000, 112, 2590; Angew. Chem. Int. Ed.
2000, 39, 2464; b) W. Klaui, D. Schramm, W. Peters, G.
Rheinwald, H. Land, Eur. J. Inorg. Chem. 2001, 1415;
c) W. Klaui, M. Berghahn, W. Frank, G. J. Reiß, T.
Schonherr, G. Rheinwald, H. Lang, Eur. J. Inorg.
Chem. 2003, 2059.
[13] a) R. Wanke, P. Smolenski, M. F. C. Guedes da Silva,
L. M. D. R. S. Martins, A. J. L. Pombeiro, Inorg. Chem.
2008, 47, 10158; b) T. F. S. Silva, L. M. D. R. S. Martins,
E. C. B. A. Alegria, A. J. L. Pombeiro, Adv. Synth.
Catal. 2008, 350, 706; c) G. S. Mishra, E. C. B. Alegria,
L. M. D. R. S. Martins, J. J. R. Frafflsto da Silva, A. J. L.
Pombeiro, J. Mol. Catal. A: Chemical, 2008, 285, 92;
d) T. F. S. Silva, L. M. D. R. S. Martins, M. F. C. G.
Silva, A. J. L. Pombeiro, Acta Crystallogr. E 2007, 63,
m1979; e) E. C. B. Alegria, L. M. D. R. S. Martins, M.
Haukka, A. J. L. Pombeiro, Dalton Trans. 2006, 4954;
f) E. C. B. Alegria, L. M. D. R. S. Martins, M. F. C. G.
Silva, A. J. L. Pombeiro, J. Organomet. Chem. 2005,
690, 1947.
[14] G. S. Mishra, T. F. S. Silva, L. M. D. R. S. Martins,
A. J. L. Pombeiro, Pure Appl. Chem. 2009, 81, 1217.
[15] a) C. A. Root, J. D. Hoeschele, C. R. Cornman, J. W.
Kampf, V. L. Pecoraro, Inorg. Chem. 1993, 32, 3855;
b) M. R. Maurya, Coord. Chem. Rev. 2003, 237, 163.
[16] M. A. Cinellu, S. Stoccorro, G. Minghetti, A. L. Bandi-
ni, G. Banditelli, B. Bovio, J. Organomet. Chem. 1989,
372, 311.
References
[1] a) R. R. Eady, Coord. Chem. Rev. 2003, 237, 23;
b) J. J. R. Frafflsto da Silva, R. J. P. Williams, The Bio-
logical Chemistry of the Elements - The Inorganic
Chemistry of Life, 2nd edn., Oxford University Press,
Oxford, 2001; c) W. Kaim, B. Schwederski, Bioinor-
ganic Chemistry: Inorganic Elements in the Chemistry
of Life, John Wiley & Sons, Chichester, 1994; d) D.
Rehder, Coord. Chem. Rev. 1999, 182, 297.
[2] a) D. Rehder, J. Inorg. Biochem. 2008, 102, 1152;
b) M. R. Maurya, S. Agarwal, C. Bader, D. Rehder,
Eur. J. Inorg. Chem. 2005, 147; c) D. Rehder, G. Santo-
nin, G. M. Licini, C. Schulzke, B. Meier, Coord. Chem.
Rev. 2003, 237, 53; d) W. Plass, Coord. Chem. Rev.
2003, 237, 205; e) K. H. Thompson, C. Orvig, Coord.
Chem. Rev. 2001, 219, 1033; f) A. Butler, Coord. Chem.
Rev. 1999, 187, 17.
[3] a) I. Osinska-Krolicka, H. Podsiadly, A. Bukietynska, J.
Inorg. Biochem., 2004, 98, 2087; b) K. H. Thompson,
J. H. McNeil, C. Orvig, Chem. Rev. 1999, 99, 2561.
[4] a) D. C. Crans, Pure Appl. Chem. 2005, 77, 1497; b) H.
Sakurai, Y. Kojima, Y. Yoshikawa, K. Kawabe, H.
Yasuri, Coord. Chem. Rev. 2002, 226, 187; c) Y. Shecht-
er, I. Goldwaser, M. Mironchik, M. Fridkin, D. Gefel,
Coord. Chem. Rev. 2003, 237, 3; d) H. Sakurai, Y.
Kojima, Y. Yoshikawa, K. Kawabe, H. Yasui, Coord.
Chem. Rev. 2002, 226, 187; e) K. H. Thompson, C.
Orvig, Chem. Rev. 2001, 101, 1033.
[5] a) Y. Shechter, S. J. D. Karlish, Nature 1980, 286, 556;
b) Y. Shechter, Diabetes 1990, 39, 1.
[6] a) D. Rehder, Bioinorganic Vanadium Chemistry, John
Wiley & Sons, Chichester, 2008; b) K. Kustin, J. C.
Pessoa, D. C. Crans, (Eds.), Vanadium: The Versatile
Metal, American Chemical Society, Washinton, DC,
2007; c) R. L. Richards, in: Encyclopedia of Inorganic
[17] Y. H. Xing, K. Aoki, F. Y. Bai, Y. H. Zhang, B. L.
Zhang, J. Chem. Crystallogr., 2008, 38, 327.
[18] C. J. Jones, J. A. McCleverty, A. S. Rothin, J. Chem.
Soc. Dalton Trans. 1986, 109.
Adv. Synth. Catal. 2010, 352, 171 – 187
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
185

Telma F. S. Silva et al.
FULL PAPERS
[19] Y. Xing, B. Zhang, Y. Zhang, H. Yuan, Z. Sun, M. Ge,
Polyhedron 2007, 26, 3037.
[20] a) M. Mohan, S. M. Holmes, R. J. Butcher, J. P. Jasinski,
C. J. Carrano, Inorg. Chem. 1992, 31, 2029; b) D. V. Fo-
mitchev, C. C. McLauchlan, R. H. Holms, Inorg. Chem.
2002, 41, 958.
[21] a) Z-P. Li, Y-H. Xing, Y-Z. Cao, X-Q. Zeng, M-F. Ge,
S-Y. Niu, Polyhedron 2009, 28, 865; b) Z. Li, Y. Xing,
Y. Zhang, G. Zhou, C. Wang, J. Li, X. Zeng, M. Ge, S.
Niu, J. Coord. Chem. 2009, 28, 865.
[33] a) M. V. Kirillova, A. M. Kirillov, P. M. Reis, J. A. L.
Silva, J. J. R. Frafflsto da Silva, A. J. L. Pombeiro, J.
Catal., 2007, 248, 130; b) M. V. Kirillova, M. L. Kuznet-
sov, P. M. Reis, J. A. L. Silva, J. J. R. Frafflsto da Silva,
A. J. L. Pombeiro, J. Am. Chem. Soc. 2007, 129, 10531.
[34] M. V. Kirillova, M. L. Kuznetsov, J. A. L. Silva, M. F. C.
Guedes da Silva, J. J. R. Frafflsto da Silva, A. J. L. Pom-
beiro, Chem. Eur. J. 2008, 14, 1828.
[35] P. M. Reis, J. A. L. Silva, A. F. Palavra, J. J. R. Frafflsto
da Silva, A. J. L. Pombeiro, J. Catal. 2005, 235, 333.
[36] M. V. Kirillova, A. M. Kirillov, M. L. Kuznetsov,
J. A. L. Silva, J. J. R. Frafflsto da Silva, A. J. L. Pom-
beiro, Chem. Commun. 2009, 2353.
[37] E. C. B. Alegria, M. V. Kirillova, L. M. D. R. S. Martins,
A. J. L. Pombeiro, J. Appl. Catal. A: Gen. 2007, 217, 43.
[38] a) C. Jia, T. Kitamura, Y. Fujiwara, Acc. Chem. Res.
2001, 34, 633; b) Y. Fujiwara, K. Takaki, Y. Taniguchi,
Synlett 1996, 591; c) Y. Taniguchi, T. Hayashida, H. Shi-
basaki, D. Piao, T. Kitamura, T. Yamaji, Y. Fujiwara,
Org. Lett. 1999, 1, 557; d) D. Piao, K. Inoue, H. Shiba-
saki, Y. Taniguchi, T. Kitamura, Y. Fujiwara, J. Organo-
met. Chem. 1999, 574, 116.
[39] a) P. M. Reis, J. A. L. Silva, J. J. R.Frafflsto da Silva,
A. J. L. Pombeiro, J. Mol. Cat. A: Chem. 2004, 224,
189; b) L. S. Shulꢇpina, M. V. Kirillova, A. J. L. Pom-
beiro, G. B. Shulꢇpin, Tetrahedron 2009, 65, 2424;
c) P. M. Reis, J. A. L. Silva, J. J. R.Frafflsto da Silva,
A. J. L. Pombeiro, Chem. Commun. 2000, 1845.
[40] a) M. N. Kopylovich, A. M. Kirillov, A. K. Baev,
A. J. L. Pombeiro, J. Mol. Catal. A: Chem. 2003, 206,
163; b) R. R. Fernandes, M. V. Kirillova, J. A. L. da
Silva, J. J. R. Frafflsto da Silva, A. J. L. Pombeiro, Appl.
Catal. A 2009, 353, 107; c) D. S. Nesterov, V. N. Koko-
zay, V. V. Dyakonenko, O. V. Shishkin, J. Jezierska, A.
Ozarowski, A. M. Kirillov, M. N. Kopylovich, A. J. L.
Pombeiro, Chem. Commun. 2006, 4605.
[41] a) A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova, M.
Haukka, M. F. C. G. da Silva, A. J. L. Pombeiro, Angew.
Chem. 2005, 117, 4419; Angew. Chem. Int. Ed. 2005, 44,
4345; b) A. M. Kirillov, M. N. Kopylovich, M. V. Kiril-
lova, E. Y. Karabach, M. Haukka, M. F. C. G. da Silva,
A. J. L. Pombeiro, Adv. Synth. Catal. 2006, 348, 159;
c) C. Di Nicola, Y. Y. Karabach, A. M. Kirillov, M.
Monari, L. Pandolfo, C. Pettinari, A. J. L. Pombeiro,
Inorg. Chem. 2007, 46, 221; d) Y. Y. Karabach, A. M.
Kirillov, M. Haukka, M. N. Kopylovich, A. J. L. Pom-
beiro, J. Inorg. Biochem. 2008, 102, 1190; e) M. V. Kiril-
lova, A. M. Kirillov, M. F. C. Guedes da Silva, A. J. L.
Pombeiro, Eur. J. Inorg. Chem. 2008, 3423; f) K. R.
Gruenwald, A. M. Kirillov, M. Haukka, J. Sanchiz,
A. J. L. Pombeiro, Dalton Trans. 2009, 2109; g) A. M.
Kirillov, M. Haukka, M. F. C. G. da Silva, A. J. L. Pom-
beiro, Eur. J. Inorg. Chem. 2005, 2071.
[22] a) K. Nakamoto, Infrared and Raman Spectra of Inor-
ganic and Coordination Compounds – Part B, John
Wiley & Sons, Chicester, 1997, p 169; b) G. Asgedom,
A. Sreedhara, J. Kivikoski, J. Valkonen, J. Chem. Soc.
ˇ
´
Dalton Trans. 1995, 2459; c) M. Casny, D. Rehder,
Dalton Trans. 2004, 839.
[23] a) C. J. Carrano, M. Mohan, S. M. Holmes, R. Rosa, A.
Butler, J. Charnock, C. D. Garner, Inorg. Chem. 1994,
33, 646; b) D. Collison, D. Eardley, F. Mabbs, A.
Powell, S. Turner, Inorg. Chem. 1993, 32, 664; c) R. C.
Maurya, S. Rajput, J. Mol. Struct. 2007, 833, 133.
[24] a) E. Rba, C. Gemel, C. Slugovc, K. Mereiter, R.
Schmid, K. Kirchner, Organometallics 1999, 18, 2275;
b) C. Gemel, G. Trimmel, C. Slugovc, S. Kremel, K.
Mereiter, R. Schmid, K. Kirchner, Organometallics
1996, 15, 3998.
[25] a) C. R. Cornman, G. J. Colpas, J. D. Hoeschele, J.
Kampf, V. L. Pecoraro, J. Am. Chem. Soc. 1992, 114,
9925; b) A. S. Tracey, Coord. Chem. Rev. 2003, 237,
113; c) V. Conte, F. Furia, S. Moro, J. Mol.Catal A:
Chemical, 1995, 104, 159; d) A. Butler, C. J. Carrano,
Coord. Chem. Rev. 1991, 109, 61; e) D. Rehder, C. Wei-
demann, A. Duch, W. Priebsch, Inorg. Chem. 1988, 27,
584; f) D. Rehder, Transition Metal Nuclear Magnetic
Resonance, (Ed.: P. S. Pregosin), Elsevier, New York,
1991, pp 1; g) O. W. Howarth, Progr. Magn. Reson. Spe-
trosc. 1990, 22, 453; h) I. Andersson, S. Angus-Dunne,
O. Howarth, L. Pettersson, J. Inorg. Biochem. 2000, 80,
51.
[26] M. Koppen, G. Fresen, K. Weighardt, R. Llusar, B.
Nuber, J. Weiss, Inorg. Chem. 1988, 27, 721.
[27] M. Herberhold, G. Frohmader, T. Hofmann, W. Milius,
J. Darkwa, Inorg. Chim. Acta, 1998, 267, 17.
[28] Y. Xing, B. Zhang, Y. Zhang, H. Yuan, Z. Sun, M. Ge,
Polyhedron 2007, 26, 3037.
[29] E. Kime-Hunt, K. Spartalian, M. DeRusha, C. Nunn,
C. J. Carrano, Inorg. Chem. 1989, 28, 4392.
[30] F. Mani, Inorg. Chim. Acta 1980, 38, 97.
[31] A. J. L. Pombeiro, Eur. J. Inorg. Chem. 2007, 1473.
[32] a) P. M. Reis, J. A. L. Silva, A. F. Palavra, J. J. R. Fraffls-
to da Silva, T. Kitamura, Y. Fujiwara, A. J. L. Pom-
beiro, Angew. Chem. Int. Ed. Angew Chem. Int. Ed.
2003, 42, 821; b) A. J. L. Pombeiro, J. J. R. Frafflsto da
Silva, Y. Fujiwara, J. A. L. Silva, P. M. Reis, A. F. Palav-
ra, World Patent WO2004/037416A3, 2004; c) A. M.
Kirillov, M. Haukka, M. V. Kirillova, A. J. L. Pombeiro,
Adv. Synth. Catal. 2005, 347, 1435; d) M. V. Kirillova,
J. A. L. da Silva, J. J. R. Frafflsto da Silva, A. J. L. Pom-
beiro, Appl. Cat. A, 2007, 332, 159; e) M. V. Kirillova,
J. A. L. da Silva, J. J. R. Frafflsto da Silva, A. F. Palavra,
A. J. L. Pombeiro, Adv. Synth. Catal. 2007, 349, 1765.
[42] G. B. Shulꢇpin, G. Süss-Fink, J. R. L. Smith, Tetrahedron
1999, 55, 5345.
[43] A. E. Shilov, G. B. Shulꢇpin, Chem. Rev. 1997, 97, 2879.
[44] I. I. Moiseev, D. I. Shishkin, A. E. Gekhaman, New J.
Chem. 1989, 13, 682.
[45] T. Hogan, A. Sen, J. Am. Chem. Soc. 1997, 119, 2642.
[46] G. B. Shulꢇpin, J. Mol. Cat. A: Chem. 2002, 189, 39.
[47] a) G. B. Shulꢇpin, Mini-rev. Org. Chem. 2009, 6, 95;
b) G. B. Shulꢇpin, in: Transition Metals for Organic Syn-
186
asc.wiley-vch.de
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 171 – 187

Novel Scorpionate and Pyrazole Dioxovanadium Complexes
thesis, (Eds.: M. Beller, C. Bolm), Wiley-VCH: Wein-
heim/New York, 2nd edn., 2004, Vol. 2, pp 215–242;
c) G. B. Shulꢇpin, G. V. Nizova, Y. N. Kozlov, L. G.
Cuervo, G. Süss-Fink, Adv. Synth. Catal. 2004, 346, 317.
[48] a) A. A. Fokin, P. R. Schreiner, Adv. Synth. Catal. 2003,
345, 1035; b) A. A. Fokin, P. R. Schreiner, Chem. Rev.
2002, 102, 1551.
[49] G. Süss-Fink, L. G. Cuervo, B. Therrien, H. Stoeckli-
Evans, G. B. Shulꢇpin, Inorg. Chim. Acta 2004, 357, 475.
[50] M. Asadullah, T. Kitamura, Y. Fujiwara, Angew. Chem.
2000, 112, 2609; Angew. Chem. Int. Ed. 2000, 39, 2475.
[51] a) R. E. Huie, C. L. Clifton, Int. J. Chem. Kinet. 1989,
21, 611; b) F. Minisci, A. Citterio, G. Giordano, Acc.
Chem. Res. 1983, 16, 27.
[54] G. B. Shulꢇpin, Y. N. Kozlov, G. V. Nizova, G. Süss-
Fink, S. Stanislas, A. Kitaygorodskiy, V. S. Kulikova, J.
Chem. Soc. Perkin Trans. 1 2001, 2, 1351.
[55] M. Hartmann, S. Ernst, Angew. Chem. 2000, 112, 916;
Angew. Chem. Int. Ed. 2000, 39, 888.
[56] A. E. Shilov, G. B. Shulꢇpin, Activation and Catalytic
Reactions of Saturated Hydrocarbons in the Presence of
Metal Complexes, Kluwer Academic Publishers, Dor-
drecht, The Netherlands, 2000.
[57] S. Trofimenko, J. Am. Chem. Soc. 1966, 88, 1842.
[58] A. J. L. Pombeiro, M. F. C. G. da Silva, M. A. N. D. A.
Lemos, Coord. Chem. Rev. 2001, 219, 53.
[59] G. M. Sheldrick, Acta Crystallogr. Sect. A 1990, 46, 467.
[60] G. M. Sheldrick, SHELXL-97, University of Gottingen,
Germany, 1997.
[61] L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837.
[62] G. B. Shulꢇpin, C. R. Chim. 2003, 6, 163.
[52] U. Schuchardt, D. Mandelli, G. B. Shulꢇpin, Tetrahedron
Lett. 1996, 37, 6487.
[53] G. Fu, X. Xu, X. Lu, H. Wan, J. Am. Chem. Soc. 2005,
127, 3989.
[63] L. M. Slaughter, J. P. Collman, T. A. Eberspacher, J. I.
Brauman, Inorg. Chem. 2004, 43, 5198.
Adv. Synth. Catal. 2010, 352, 171 – 187
ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
187