Homo-and heterometal complexes of oxido-metal ions with a triangular [V(V)O-MO-V(V)O] [M = V(IV) and Re(V)] core: Reporting mixed-oxidation oxido-vanadium(V/IV/V) compounds with valence trapped structures

Article abstract of DOI:10.1021/ic4013203

A new family of trinuclear homo-and heterometal complexes with a triangular [V(V)O-MO-V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido-metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, N,N′-bis(3-hydr

Full text of DOI:10.1021/ic4013203

Article
pubs.acs.org/IC
Homo- and Heterometal Complexes of Oxido−Metal Ions
with a Triangular [V(V)O−MO−V(V)O] [M = V(IV) and Re(V)]
Core: Reporting Mixed-Oxidation Oxido−Vanadium(V/IV/V)
Compounds with Valence Trapped Structures
Kisholoy Bhattacharya, Manoranjan Maity, Sk Md Towsif Abtab, Mithun Chandra Majee,
and Muktimoy Chaudhury*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
S
* Supporting Information
ABSTRACT: A new family of trinuclear homo- and
heterometal complexes with a triangular [V(V)O−MO−
V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido−metal
core have been synthesized following a single-pot protocol
using compartmental Schiff-base ligands, N,N′-bis(3-hydrox-
ysalicylidene)-diiminoalkanes/arene (H4L1−H4L3). The upper
compartment of these ligands with N2O2 donor combination
(Salen-type) contains either a V(IV) or a Re(V) center, while
the lower compartment with O4 donor set accommodates two
V(V) centers, stabilized by a terminal and a couple of bridging
methoxido ligands. The compounds have been characterized
by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in 1−3. Compound 1
crystallizes in a monoclinic space group P21/c, while both 2 and 3 have more symmetric structures with orthorhombic space
group Pnma that renders the vanadium(V) centers in these compounds exactly identical. In DMF solution, compound 1 displays
an 8-line EPR at room temperature with ⟨g⟩ and ⟨A⟩ values of 1.972 and 86.61 × 10−4 cm−1, respectively. High-resolution X-ray
photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p3/2 and
2p1/2 electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation,
confirm the trapped-valence nature of mixed-oxidation in compounds 1 and 2. Electrochemically, compound 1 undergoes two
one-electron oxidations at E1/2 = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V(IV/V)
oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers.
A larger cavity size in the upper compartment of the ligand H4L3 is spacious enough to accommodate Re(V) with larger size to
generate a rare type of all-oxido heterotrimetallic compound (3) as established by X-ray crystallography.
Herein, we report two trinuclear mixed-oxidation vanadium-
(V/IV/V) complexes 1 and 2 together with a trinuclear
heterometal complex 3 containing an oxido−rhenium(V) and a
couple of oxido−vanadium(V) centers using hexadentate
Schiff-base compartmental ligands having N2O4 donor
combinations. The compounds have been characterized by
single-crystal X-ray diffraction analyses, ESI−MS, EPR, and
X-ray photoelectron spectroscopy. Bond valence sum (BVS)
approach has been applied to confirm the oxidation states of
the vanadium centers in 1 and 2. Redox behavior of 1 has been
investigated using the technique of cyclic voltammetry.
INTRODUCTION
■
Coordination chemistry of vanadium1 deals with a large
number of vanadium oxidation states ranging from −3 to +5.
Among these, +4 and +5 are the most common oxidation states
and can be accessed easily under aerobic conditions when the
metal ion generates various types of oxido species. In these high
oxidation states, vanadium shows distinct preference for N/O
donor ligands in complex formation that leads to the
development of a large part of its coordination chemistry,
particularly in solution.2 Out of the various types of oxido−
vanadium compounds, mixed-oxidation (IV/V) complexes are
of special interest because of their intriguing spectroscopic
properties.3 Among these, dinuclear mixed-valence oxido−
vanadium complexes containing [V2O3]3+ core are the most
common in occurrence because of their favorable thermody-
namic stability and ease of formation.2c,d,4 Surprisingly, there
are only a few reported examples of mixed-oxidation vanadium-
(IV/V) compounds with vanadium nuclearity higher than
two.4c,5
EXPERIMENTAL SECTION
■
Materials. The precursor complexes [VO(acac)2]6 and [Re-
OCl3(PPh3)2]7 were prepared following literature methods. 2,3-
Dihydroxybenzaldehyde and 2,2-dimethyl-1,3-diaminopropane were
Received: May 27, 2013
© XXXX American Chemical Society
A
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Article
purchased from Aldrich. All other chemicals were commercially
available and used as received. Solvents were reagent grade, dried by
standard methods,8 and distilled under nitrogen prior to their use.
Preparation of the Ligands. N,N′-Bis(3-hydroxysalicylidene)-
1,2-diaminoethane (H4L1). To a stirred solution of 2,3-dihydroxy-
benzaldehyde (2.76 g, 20 mmol) in methanol (30 mL) was added
ethylenediamine (0.6 g, 10 mmol), and the solution was refluxed for
30 min. It was cooled to room temperature and filtered to obtain an
orange crystalline product. Yield: 2.2 g (73%). mp 170 °C. Anal. Calcd
for C17H20N2O5: C, 61.44; H, 6.07; N, 8.43. Found: C, 61.69; H, 6.06;
N, 8.15. 1H NMR (400 MHz, DMSO-d6, 298 K, δ/ppm): 13.88 (br s,
2H, phenolic−OH), 8.98 (br s, 2H, phenolic−OH), 8.48 (s, 2H,
azomethine), 6.85 (m, 4H, phenyl ring), 6.66 (t, J = 8.0 Hz, 2H,
phenyl ring), 3.48 (s, 4H, CH2), 0.99 (s, 3H CH3).
3.82; N, 3.41. FT-IR bands (KBr pellet, cm−1): 1641 vs, 1591 s, 1540 s,
1456 s, 1360 m, 1335 m, 1255 s, 1209 m, 945 s, 910 s, 760 m, 682 m,
575 m, 522 m.
Physical Measurements. Elemental analyses (for C, H, and N)
were performed at IACS on a Perkin-Elmer model 2400 Series II
CHN Analyzer. The 1H NMR spectra were recorded on Bruker model
Avance DPX-500 and Avance DPX-400 spectrometers using SiMe4 as
internal reference. IR spectra of the samples prepared as KBr pellets
were recorded using a Shimadzu model 8400S FT-IR spectrometer.
The electrospray ionization mass spectra (ESI−MS) in positive ion
mode were measured on a Micromass QTOF model YA 263 mass
spectrometer. For UV−visible spectra in solution, a Perkin-Elmer
model Lambda 950 UV/vis/NIR spectrophotometer was employed.
EPR spectra in DMF solution were recorded on a JEOL model JES-FA
300 X-band spectrometer, equipped with a standard low temperature
(77 K) apparatus and data processing system ESPRIT 330. The
spectra at room temperature were recorded in a flat cell. X-ray
photoelectron spectroscopic (XPS) (Omicron) measurements were
done using an Mg Kα radiation source under 15 kV voltage and 5 mA
current conditions.
N,N′-Bis(3-hydroxysalicylidene)-1,2-diaminobenzene (H4L2). This
ligand was obtained following the same procedure as that for H4L1
using o-phenylenediamine (1.08 g, 10 mmol) as a replacement for
ethylenediamine. The product was collected as a dark red crystalline
solid. Yield: 74%. mp 192 °C. Anal. Calcd for C20H16N2O4: C, 68.96;
1
H, 4.63; N, 8.04. Found: C, 68.68; H, 4.71; N, 8.06. H NMR (500
Cyclic voltammetry (CV) in DMF was recorded on a EG&G PARC
electrochemical analysis system using a planar EG&G PARC G0229
glassy carbon milli electrode as the working electrode and a platinum
wire as counter electrode. Ag/AgCl was used for reference and Fc/Fc+
couple as the internal standard. Solutions were ∼1.0 mM in samples
and contained 0.1 M TBAP as supporting electrolyte.
MHz, CDCl3, 298 K, δ/ppm): 13.59 (br s, 2H, phenolic OH), 8.63 (s,
2H, azomethine), 7.31 (m, 4H phenyl ring), 7.02 (m, 4H, phenyl
ring), 6.83 (m, 2H, phenyl ring), 6.04 (br s, 2H, phenolic OH). UV−
vis (CH2Cl2) [λmax, nm (ε, L mol−1 cm−1)]: 226 (18 300), 281 (18
100), 333 (13 900).
N,N′-Bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-diaminopro-
pane (H4L3). This compound was synthesized following essentially the
same procedure as that for H4L1 using 2,2-dimethyl-1,3-diaminopro-
pane (1.02 g, 10 mmol) instead of ethylenediamine. The product was
collected as an orange crystalline solid. Yield: 79%. mp 169 °C. Anal.
Calcd for C19H22N2O4: C, 66.65; H, 6.48; N, 8.18. Found: C, 66.50;
X-ray Crystallography. Diffraction quality crystals of 1 (flakes,
blue, 0.16 × 0.10 × 0.07 mm3), 2 (needle, green, 0.17 × 0.13 × 0.09
mm3), and 3 (flakes, brown, 0.15 × 0.09 × 0.05 mm3) were collected
from their respective reaction pots. Crystals were mounted on glass
fibers coated with perfluoropolyether oil before mounting. Intensity
data for the compounds were measured employing a Bruker SMART
APEX II CCD diffractometer equipped with Mo Kα radiation (λ =
0.71073 Å) source using the ω/2θ scan technique at 150 K. No crystal
decay was observed during the data collections. The intensity data
were corrected for empirical absorptions. In all cases, absorption
corrections based on multiscans using the SADABS software9 were
applied.
1
H, 6.50; N, 8.20. H NMR (500 MHz, CDCl3, 298 K, δ/ppm): 8.24
(s, 2H, azomethine), 6.97 (d, J = 8.0 Hz, 2H, phenyl ring), 6.79
(d, J = 8.0 Hz, 2H, phenyl ring), 6.70 (t, 2H, phenyl ring), 3.51 (s, 4H,
methylene), 1.12 (s, 6H, methyl). UV−vis (CH2Cl2) [λmax, nm
(ε, L mol−1 cm−1)]: 225 (20 600), 263 (23 700), 302 (9800), 433 (2500).
Preparation of Complexes. [L1{(VIVO)CH3OH}{(VVO)(OCH3)-
(μ-OCH3)}2‑]·CH3OH 1. To a stirred solution of H4L1 (0.75 g,
0.25 mmol) in methanol (50 mL) was added [VO(acac)2] in solid
(0.195 g, 0.75 mmol), and the mixture was refluxed for 2 h. The
solution was filtered and kept overnight in the air to obtain dark blue
crystals. The product was collected by filtration and washed with
methanol. Some of these crystals were of diffraction grade and used
directly for X-ray crystallographic analysis. Yield: 0.080 g (53%). Anal.
Calcd for C22H32N2O13V3: C, 38.54; H, 4.71; N, 4.08. Found: C, 38.86,
H, 4.58, N, 4.14. FT-IR bands (KBr pellet, cm−1): 1639 vs, 1589 m,
1544 m, 1440 s, 1398 m, 1331 m, 1265 s, 1221 m, 1041 s, 976 vs, 775 m,
742 m, 661 m, 605 m, 561 m. ESI−MS (positive) in CH2Cl2: m/z, 644
[M − 2CH3OH+Na]+.
The structures were solved by direct methods10 and refined on F2
by a full-matrix least-squares procedure10 based on all data minimizing
2
R = ∑∥Fo| − |Fc∥/∑|Fo|, wR = [∑[w(Fo2 − Fc2)2]/∑(Fo )2]1/2, and S =
2
[∑[w(Fo − Fc2)2]/(n − p)]1/2. SHELXL-97 was used for both
structure solutions and refinements.11 A summary of relevant
crystallographic data and final refinement details has been given in
Table 1. All non hydrogen atoms were refined anisotropically. The
hydrogen atoms were calculated and isotropically fixed in the final
refinement [d(C−H) = 0.95 Å, with the isotropic thermal parameter of
Uiso(H) = 1.2Uiso(C)]. The SMART and SAINT-plus software
packages12 were used for data collection and reduction, respectively.
Crystallographic diagrams were drawn using the DIAMOND software
package.13
[L2{(VIVO)CH3OH}{(VVO)(OCH3)(μ-OCH3)}2] 2. This compound was
obtained following the same procedure as that described for 1 using
H4L2 as a replacement for H4L1 as ligand to obtain dark green needle-
shaped crystals. Yield: 0.073 g (41%). Anal. Calcd for C25H28N2O12V3:
C, 42.81; H, 4.02, N, 3.99. Found: C, 42.67; H, 3.91; N, 4.03. FT-IR
bands (KBr pellet, cm−1): 1608 vs, 1579 s, 1531 s, 1446 m, 1382 m,
1332 m, 1265 m, 1199 s, 991 vs, 887 w, 787 s, 744 s, 650 m, 578 m,
544 m. ESI−MS (positive) in CH2Cl2: m/z, 692 [M − CH3OH+Na]+.
[L3{(ReVO)(OCH3)}{(VVO)(OCH3)(μ-OCH3)}2] 3. To a stirred solution
of H4L3 (0.068 g, 0.2 mmol) in methanol were added Et3N (0.4 g, 0.4
mmol) and [ReOCl3(PPh3)2] (0.166 g, 0.2 mmol), and the solution
was refluxed for 1 h during which time the color of the solution turned
to green. The solution was cooled to room temperature, and
[VO(acac)2] (0.104 g, 0.4 mmol) was added in solid to the reaction
mixture. The resulting solution was refluxed further for 2 h. The
solution was cooled to room temperature, filtered, and kept overnight
in the air to get dark brown crystals. The product was collected by
filtration, washed with methanol, and dried over CaCl2. Some of these
crystals were of diffraction grade and used directly for X-ray
crystallographic analysis. Yield: 0.096 g (51%). Anal. Calcd for
C24H33N2O12ReV2: C, 34.74; H, 3.98; N, 3.38. Found: C, 34.86; H,
RESULTS AND DISCUSSION
■
Syntheses. The compartmental Schiff-base ligands H4L1−
H4L3 have been synthesized in high yields (ca. 75%) following a
simple condensation reaction in methanol. These ligands have
two compartments as depicted in Scheme 1. The one with
N2O2 donor combination is referred to as the “upper
compartment”, while the other providing an O4 donor set is
cited as the “lower compartment” in the following part of this
discussion. The trinuclear mixed-oxidation oxido−vanadium
[V(V)−V(IV)−V(V)] complexes [L1{(VIVO)CH3OH}-
{(VVO)(OCH3)(μ-OCH3)}2]·CH3OH 1 and [L2{(VIVO)-
CH3OH}{(VVO)(OCH3)(μ-OCH3)}2] 2 are obtained in
moderate yields (ca. 45%) by a metathetical reaction (Scheme 1)
when stoichiometric amounts of the compartmental ligand
H4L1 (H4L2 for 2) and [VO(acac)2] (1:3 mol ratio) are reflux-
ed in methanol under aerobic environment. The obligatory steps
B
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Table 1. Summary of Relevant X-ray Crystallographic Data for Compounds 1−3
parameters
1
2
3
composition
formula wt
C22H32N2O13V3
685.32
C25H28N2O12V3
701.32
C24H33N2O12ReV2
829.62
crystal system
space group
a, Å
monoclinic
P21/c
orthorhombic
Pnma
orthorhombic
Pnma
15.094(4)
9.921(3)
18.717(4)
90.00
16.779(5)
14.807
24.191(5)
13.926(3)
8.5137(19)
90.00
b, Å
c, Å
11.232(3)
90.00
α, deg
β, deg
97.921(8)
90.00
90.00
90.00
γ, deg
V, Å3
ρcalc, g cm−3
90.00
90.00
2776.0(12)
1.640
2790.5(13)
1.691
2868.2(11)
4.385
temp, K
150
150
150
λ (Mo Kα), Å
Z
0.71073
4
0.71073
4
0.71073
4
F(000)/μ mm−1
2θmax [deg]
reflns collected/unique
Rint/GOF on F2
no. of parameters
1404/1.057
36.42
1464/1.052
46.24
3288/27.648
30.50
14 588/1962
0.1309/1.009
372
21 422/2061
0.1051/0.903
209
9692/649
0.1203/1.610
224
a
b
R1 (F0), wR2 (F0) (all data)
largest diff. peak, deepest hole, e Å−3
0.0421, 0.1117
0.0430, 0.1190
0.282, −0.249
0.0552, 0.1686
1.072, −0.668
0.277, −0.346
a
b
R = ∑∥Fo| − |Fc∥/∑|Fo|. wR = [∑[w(Fo − Fc2)2]/∑(Fo )2]1/2
.
2
2
The methodology has been extended to synthesize
heterometal compounds with V(V)−Re(V)−V(V) combina-
tion following the same protocol, albeit with a minor
modification, using the ligand, [ReOCl3(PPh3)2], and [VO-
(acac)2] in 1:1:2 mole ratio. Because Re(V) is much larger in
size as compared to V(IV), a new ligand H4L3 with upper
compartment of larger cavity size has been synthesized using
2,2-dimethyl-1,3-diaminopropane to provide enough flexibility
in the ligand backbone to accommodate all three metal centers
including Re(V). Compound [L3{(ReVO)(OCH3)}{(VVO)-
(OCH3)(μ-OCH3)}2] 3 has composition similar to that of 1
and 2 with V(IV) being replaced by Re(V) in the upper
compartment.
Scheme 1. Schematic Representation of the Protocol
Followed for the Synthesis of Compounds 1−3
Compounds 1 and 2 are electrically neutral, and their charge
neutralization requires that two of the vanadium centers are in
+5 oxidation state while the third one is in +4 oxidation state.
Because the alkoxido groups are known to stabilize vanadium in
the +5 oxidation state,4h we believe the vanadium centers in the
lower compartment are in the +5 state. In compound 3, the
upper compartment is occupied by Re(V) with an extra unit of
positive charge as compared to 1 and 2, neutralized by a methoxido
group coordinated trans to its terminal oxido group. Of particular
interest is the innocent behavior of the catecholate part of these
ligands toward vanadium(v) in 1 and 2. Because of their non-
innocent nature, the [VV-(catecholate2−)] compounds15 are also
known to exist in their valence tautomeric [VIV-(semiquinone)1−]
form.16
All three compounds reported above have poor solubility
characteristics. Both 1 and 2 are only sparingly soluble in
dichloromethane, while 1 has a reasonable solubility in DMF.
Compound 3, on the other hand, is practically insoluble in all
common organic solvents.
Selected IR data for the compounds (1−3) have been listed
in the Experimental Section. Compound 1 displays two strong
bands at 1639 cm−1 (corresponding band appears at 1608
and 1641 cm−1 in 2 and 3, respectively) and 1589 cm−1
in this preparative procedure are the use of methanol as solvent and
the exposures of the reaction media to atmospheric oxygen as
confirmed by several control experiments. The upper compartment
of these ligands has a Salen2− type moiety that accommodates and
stabilizes a VO2+ ion.14 The remaining two vanadium centers are
oxidized to VO3+ when exposed to atmospheric oxygen in
methanol medium4h and are accommodated in the more flexible
and spacious lower compartment of the ligands, thus providing
trinuclear mixed-oxidation compounds with a rare type of V(V)−
V(IV)−V(V) triangular core.5c The solvent CH3OH plays a crucial
role here as it supplies the −OCH3 donor groups, both bridging
and monodentate, to stabilize vanadium in the +5 oxidation
state. No such compound has been isolated in our hand when
the reactions were carried out with other alcohols like ethanol,
1-propanol, or 2-propanol as solvent.
C
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(1579 and 1591 cm−1) due to the ν(CN) and ν(CC)
stretching modes of the coordinated Schiff-base and phenyl ring
vibrations, respectively.17 In addition, a sharp strong band is
also observed at 1265 cm−1 (1265 and 1255 cm−1) due to
ν(C−O/phenolate) vibration.17 The spectra of these com-
pounds in the 1050−900 cm−1 range also reveal interesting
information about their metal−oxygen vibrational modes. For
compound 1, a pair of strong bands at 1041 and 976 cm−1 are
observed for the terminal ν(VOt) stretches, while for 2 only
a single such mode in the form of a very strong band with
distorted shape is observed at 991 cm−1, apparently due to the
overlap of two closely spaced ν(VOt) signals. This is as
expected due to two types of vanadium centers [V(V) and
V(IV)] present in 1, while in 2 that difference is somewhat
much less pronounced. Interestingly for compound 3, two
strong ν(MOt) bands are observed at 945 and 910 cm−1 as
per our expectation.18 The latter band is most likely due to
ν(ReOt) vibrations due to the involvement of the heavier Re
atom.19
Figure 3. Molecular structure and the crystallographic numbering
scheme for compound 1. H atoms and solvent molecule have been
omitted for clarity.
Mass Spectrometry. ESI−MS spectra (in positive ion
mode) for complexes 1 and 2 have been recorded in
dichloromethane solution, and the data are listed in the
Experimental Section. Both compounds demonstrate their
respective molecular ion peaks due to the [M − 2CH3OH+Na]+
(for 1) and [M − CH3OH + Na]+ (for 2) ionic species.
Unfortunately, the mass spectrum of 3 could not be recorded
because of its lack of solubility in common organic solvent. In
Figures 1 and 2 are displayed the isotope distribution patterns
Figure 4. Molecular structure and atom labeling scheme for
compound 2. H atoms have been omitted for clarity.
Figure 1. Molecular ion peak in the ESI mass spectrum (positive) for
complex 1 in dichloromethane with (a) simulated and (b) observed
isotopic distributions.
Figure 2. Molecular ion peak in the ESI mass spectrum (positive) for
complex 2 in dichloromethane with (a) simulated and (b) observed
isotopic distributions.
Figure 5. Molecular structure and atom labeling scheme for
compound 3. H atoms have been omitted for clarity.
for the molecular ion peaks of 1 and 2, respectively, along with
their simulation patterns, thus providing support in favor of
their proposed trinuclear compositions, which retain their
integrity in CH2Cl2 solution.
Description of Crystal Structures. The molecular
structures and the atom labeling schemes for compounds
1−3 are shown in Figures 3, 4, and 5, respectively, providing
confirmatory evidence in support of their trinuclear compositions
with metal centers forming triangular cores. Their selected
metrical parameters are listed in Tables 2 and 3. Compound
1 crystallizes in a monoclinic space group P21/c with four
molecular mass units accommodated per unit cell. Compounds
2 and 3, on the other hand, crystallize in a more symmetric
D
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Table 2. Selected Bond Lengths and Angles in 1 and 2
parameters
1
parameters
2
parameters
1
parameters
2
Bond Distances (Å)
Bond Angles (deg)
V1−O3
1.593(6)
V1−O3
V1−O1#
V1−O1
V1−N1#
V1−N1
V1−O4
V2−O5
V2−O6
V2−O2
V2−O8
1.587(5)
2.001(3)
2.001(3)
2.077(4)
2.077(4)
2.379(6)
1.595(3)
1.804(3)
1.855(3)
1.977(3)
2.008(3)
2.312(3)
1.595(3)
1.804(3)
1.855(3)
1.977(3)
2.008(3)
2.312(3)
N2−V1−O4
N1−V1−O4
O7−V2−O9
O7−V2−O5
O9−V2−O5
O7−V2−O12
O9−V2−O12
O5−V2−O12
O7−V2−O11
O9−V2−O11
O5−V2−O11
O12−V2−O11
O7−V2−O1
O9−V2−O1
O5−V2−O1
O12−V2−O1
O11−V2−O1
O8−V3−O10
O8−V3−O6
O10−V3−O6
O8−V3−O11
O10−V3−O11
O6−V3−O11
O8−V3−O12
O10−V3−O12
O6−V3−O12
O11−V3−O12
O8−V3−O2
O10−V3−O2
O6−V3−O2
O11−V3−O2
O12−V3−O2
82.0(3)
N1#−V1−O4
N1−V1−O4
O5−V2−O6
O5−V2−O2
O6−V2−O2
O5−V2−O8
O6−V2−O8
O2−V2−O8
O5−V2−O7
O6−V2−O7
O2−V2−O7
O8−V2−O7
O5−V2−O1
O6−V2−O1
O2−V2−O1
O8−V2−O1
O7−V2−O1
O5#−V2#−O6#
O5#−V2#−O2#
O6#−V2#−O2#
O5#−V2#−O8
O6#−V2#−O8
O2#−V2#−O8
O5#−V2#−O7
O6#−V2#−O7
O2#−V2#−O7
O8−V2#−O7
O5#−V2#−O1#
O6#−V2#−O1#
O2#−V2#−O1#
O8−V2#−O1#
O7−V2#−O1#
78.58(18)
78.58(18)
100.40(18)
97.89(17)
98.28(16)
102.31(19)
89.41(16)
156.72(16)
95.34(18)
158.03(17)
94.61(15)
72.22(15)
173.32(17)
83.86(13)
76.31(12)
82.77(15)
81.99(15)
100.40(18)
97.89(17)
98.28(16)
102.31(19)
89.41(16)
156.72(16)
95.34(18)
158.03(17)
94.61(15)
72.22(15)
173.32(17)
83.86(13)
76.31(12)
82.77(15)
81.99(15)
V1−O2
V1−O1
V1−N2
V1−N1
V1−O4
V2−O7
V2−O9
V2−O5
V2−O12
V2−O11
V2−O1
V3−O8
V3−O10
V3−O6
V3−O11
V3−O12
V3−O2
2.010(7)
2.025(7)
2.060(9)
2.068(9)
2.273(10)
1.582(7)
1.779(7)
1.863(8)
1.976(7)
2.013(7)
2.337(7)
1.588(7)
1.809(8)
1.827(7)
1.953(7)
1.993(6)
2.345(6)
80.2(4)
101.4(3)
94.2(4)
98.2(3)
98.2(3)
92.3(3)
161.8(3)
100.8(3)
154.1(3)
93.3(3)
71.4(3)
169.2(3)
84.1(3)
75.7(3)
90.8(3)
76.4(2)
98.3(4)
94.8(4)
100.2(3)
101.5(3)
91.8(3)
158.1(3)
98.0(3)
159.1(3)
91.3(3)
72.2(3)
168.9(3)
88.8(3)
75.5(3)
86.7(3)
77.2(2)
V2−O7
V2−O1
V2#−O5#
V2#−O6#
V2#−O2#
V2#−O8
V2#−O7
V2#−O1#
Bond Angles (deg)
O3−V1−O2
O3−V1−O1
O2−V1−O1
O3−V1−N2
O2−V1−N2
O1−V1−N2
O3−V1−N1
O2−V1−N1
O1−V1−N1
N2−V1−N1
O3−V1−O4
O2−V1−O4
O1−V1−O4
98.2(3)
99.4(3)
101.6(3)
98.3(3)
88.5(4)
158.1(3)
96.0(3)
162.0(4)
87.0(3)
78.4(4)
176.0(4)
85.8(4)
79.5(3)
O3−V1−O1#
O3−V1−O1
O1#−V1−O1
O3−V1−N1#
O1#−V1−N1#
O1−V1−N1#
O3−V1−N1
O1#−V1−N1
O1−V1−N1
N1#−V1−N1
O3−V1−O4
O1#−V1−O4
O1−V1−O4
102.56(14)
102.56(14)
96.47(18)
97.70(17)
89.24(13)
157.16(14)
97.70(17)
157.16(14)
89.24(13)
77.6(2)
175.2(3)
80.57(15)
80.57(15)
orthorhombic space group Pnma with as many molecular mass
units accommodated per cell. Thus, unlike in 1, compound 2
has a 2-fold symmetry that renders the two vanadium centers
V(2) and V(2)# in the lower ligand compartment exactly
identical. The V(1) centers in the upper compartment of both
compounds have distorted octahedral environments. Four
donor atoms O(1), N(1), N(2), O(2) [O(1), N(1), N(1)#,
O(1)# in 2] from the Salen-type moiety of this compartment
occupy the basal plane. The apical positions are taken up by the
terminal oxido atom O(3) and the oxygen atom of the
coordinated methanol molecule O(4). The trans angles O(1)−
V(1)−N(2) 158.1(3)° [O(1)−V(1)−N(1)# 157.16(14)°] and
O(2)−V(1)−N(1) 162.0(4)° [N(1)−V(1)−O(1)#
157.16(14)°] are somewhat compressed, forcing the V(1)
atom to move out of the basal plane by 0.286 Å [0.365 Å]
toward the apical oxido atom O(3). In the lower compartment,
both the vanadium centers V(2) and V(3) [V(2) and V(2)#]
also have octahedral environments, each made up of an O6
core, which differ marginally in 1 and are identical in 2. The
basal positions around V(2) in 1 are completed by O(5), O(9),
O(12), O(11) donor atoms [O(6), O(2), O(7), O(8) in 2],
three of which (two bridging and a monodentate) are
contributed by the methoxido donor atoms, which stabilize
the metal center in the +5 oxidation state.4h The apical
positions, on the other hand, are occupied by the terminal
oxido atom O(7) [O(5)] and a bridging phenoxido atom O(1).
The coordination environment around V(3) [V(2)#] is almost
[exactly] identical to that of V(2). The terminal VOt
distances V(1)−O(3) 1.593(6), V(2)−O(7) 1.582(7), and
V(3)−O(8) 1.588(7) Å [1.587(5), 1.595(3), and 1.595(3) Å,
respectively] are all in the expected range. The V(1)−O(4)
distance 2.273(10) Å [2.379(6) Å] is much elongated due to
trans labilization influence of the terminal oxido group. The
V(2)−O(9) 1.779(7), V(2)−O(11) 2.013(7), and V(2)−
O(12) 1.976(7) Å distances [corresponding distances in 2
are V(2)−O(6) 1.804(3), V(2)−O(7) 2.008(3), and V(2)−
O(8) 1.977(3) Å] are in the expected ranges for terminal and
bridging methoxido groups, respectively.5e,20 The trans angles
in the basal plane O(5)−V(2)−O(12) 161.8(3)° [158.03(17)°]
and O(9)−V(2)−O(11) 154.1(3)° [156.72(16)°] are much
short of linearity, forcing the V(2) atom to move out of the basal
plane by 0.261 Å [0.296 Å] toward the terminal oxido atom
O(7) [O(5)] and generate distorted octahedral geometry around
the metal center. For V(3) atom, the trans angles O(10)−V(3)−
O(12) and O(6)−V(3)−O(11) are 159.1(3)° and 158.1(3)°,
respectively, and the V(3) atom is displaced from the mean basal
plane by 0.280 Å [0.296 Å]. The dihedral angle between the two
basal planes containing V(1) and V(2) is 87.32° and V(1) and
V(3) is 87.77° [both are 80.72° in 2]. The distances between the
V(1)···V(2) and V(1)····V(3) centers are 3.880 and 3.915 Å,
E
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coordination environments around the vanadium centers are
very much the same as observed in 2. The Re(1)−O(3)
1.716(19) Å and Re(1)−O(4) 1.95(3) Å bonds are as expected
for ReVOt and ReV−alkoxide distances, respectively.22 The
V(1)−O(5) distance 1.54(2) Å is as expected for the terminal
VOt bond. Also, the V(1)−O(6), V(1)−O(7), and V(1)−
O(8) distances of 1.78(3), 1.95(2), and 1.98(2) Å, respectively,
are in the expected ranges for terminal and bridging −OCH3
distances in a vanadium(V) compound.5e,20 The trans angles
O(1)−Re(1)−N(1)# and O(1)#-Re(1)−N(1) 170.6(12)° are
slightly short of linearity, which forces the Re(1) atom to move
out of the basal plane by 0.164 Å toward the apical oxido atom
O(3). The V(1) atom, on the other hand, is displaced by 0.306 Å
[same for V(1)# atom] from its basal plane containing O(2),
O(6), O(7), and O(8) atoms. The Re····V distance is 3.995 Å,
and that between the two V centers is 3.163 Å. The dihedral
angle between the basal planes containing the Re and V atom is
76.02°.
Table 3. Relevant Bond Distances and Angles in 3
Bond Distances (Å)
Re1−O3
Re1−O4
Re1−O1
Re1−O1#
Re1−N1
Re1−N1#
V1−O5
1.72(2)
1.97(2)
2.071(15)
2.071(15)
2.09(2)
2.09(2)
1.52(2)
1.80(2)
V1−O2
V1−O7
V1−O8
V1#−O5#
V1#−O6#
V1#−O2#
V1#−O7
V1#−O8
1.869(17)
1.974(16)
1.999(16)
1.52(2)
1.80(2)
1.869(17)
1.974(16)
1.999(16)
V1−O6
Bond Angles (deg)
O3−Re1−O4
O3−Re1−O1
O4−Re1−O1
O3−Re1−O1#
O4−Re1−O1#
O1−Re1−O1#
O3−Re1−N1
O4−Re1−N1
O1−Re1−N1
O1#−Re1−N1
O3−Re1−N1#
O4−Re1−N1#
O1−Re1−N1#
O1#−Re1−N1#
N1−Re1−N1#
O5−V1−O6
178.3(9)
95.6(6)
85.6(5)
95.6(6)
85.6(5)
93.9(8)
92.3(7)
86.5(6)
87.1(8)
172.0(7)
92.3(7)
86.5(6)
172.0(7)
87.1(8)
90.7(12)
101.7(10)
97.4(9)
100.2(8)
101.9(9)
90.3(8)
155.7(9)
96.6(10)
O6−V1−O8
O2−V1−O8
O7−V1−O8
O5−V1−O1
O2−V1−O1
O7−V1−O1
O8−V1−O1
V1−O7−V1#
O5#−V1#−O6#
O5#−V1#−O2#
O6#−V1#−O2#
O5#−V1#−O7
O6#−V1#−O7
O2#−V1#−O7
O5#−V1#−O8
O6#−V1#−O8
O2#−V1#−O8
O7−V1#−O8
O5#−V1#−O1#
O2#−V1#−O1#
O7−V1#−O1#
O8−V1#−O1#
156.9(10)
91.2(8)
72.1(9)
172.2(8)
76.3(7)
83.2(8)
79.2(7)
106.9(12)
101.7(10)
97.4(9)
100.2(8)
101.9(9)
91.(1)
90.3(8)
96.6(10)
156.9(10)
91.2(8)
72.1(9)
172.2(8)
76.3(7)
83.2(8)
79.2(7)
O5−V1−O2
O6−V1−O2
O5−V1−O7
O6−V1−O7
O2−V1−O7
O5−V1−O8
Figure 6. X-band EPR spectrum of compound 1 at room temperature
in DMF.
respectively [3.883 Å]. The V(2)····V(3) distance is 3.191 Å
[3.196 Å].
EPR Spectroscopy. The EPR spectrum of compound 1 has
been recorded in DMF (Figure 6) at ambient temperature.
Unfortunately, the data are not available for compound 2
because of its poor solubility in common organic solvents. The
spectrum shows an 8-line profile (for 51V, I = 7/2) with ⟨g⟩ =
1.972 and ⟨A⟩ = 86.61 × 10−4 cm−1, providing indication of a
valence-trapped situation for the odd electron of this V(V)−
V(IV)−V(V) core on the time-scale of EPR spectroscopy. Had
the unpaired electron been delocalized on all three vanadium
centers, one would have expected a 22-line spectrum or a 15-
line one if the delocalization covered two of the vanadium
centers.4a,c,h,i Indirect support in favor of such a trapped-valence
character also comes from the results of X-ray crystallography,
which reveals the V(1) center to be in the +4 oxidation state
and V(2) and V(3) in the +5 state. Similar trapped-valence
character has been also reported for the only other triangular
V(V)−V(IV)−V(V) mixed-oxidation compound reported thus
far in the literature.5c Compound 1 thus retains its structural
integrity in DMF solution.
Interestingly, both compounds 1 and 2 are trapped valence
(class I)21 mixed-oxidation compounds with the V(1) center
being in the +4 oxidation state with larger ionic radius as
compared to V(2) and V(3) [V(2#)] sites, which are in the +5
oxidation state as reflected from the bond length data. Thus, in
1, the V(1)−N(1) distance 2.068(9) Å is much larger as
compared to the V(2)−O(5) distance 1.863(8) Å. On that
basis, the bridging V(1)−O(1) distance 2.025(7) Å should have
been larger than the V(2)−O(1) distance. Interestingly,
contrary to that expectation, the later distance 2.337(7) Å is
much larger due to the opposing effect of the strong trans
labilization influence of the terminal V(2)−O(7) bond. Because
the bridging V(2)−O(1)−V(1) angle 127.5(4)° is much short
of linearity, it restricts the symmetry constrained dxy orbital of
V(1) center that accommodates the unpaired electron, to
interact effectively with the remaining vanadium centers, thus
generating a trapped valence system in this case.3b
The molecular structure of the remaining compound
[L3{(ReVO)(OCH3)}{(VVO)(OCH3)(μ-OCH3)}2] 3 is quite
similar to those of 1 and 2, the only difference being in the
upper compartment where V(IV) in the latter two compounds
is replaced by a Re(V) center. The necessary charge balance
requires a concomitant replacement of the coordinated
methanol O(4) in 2 by a methoxy group in 3 as evident
from the short Re(1)−O(4) bond length of 1.95(3) Å as
compared to the larger V(1)−O(4) distance of 2.379(6) Å. The
X-ray Photoelectron Spectroscopy. The XPS analysis of
compound 1 also provides convincing evidence in support of a
trapped-valence mixed-oxidation V(V)−V(IV)−V(V) compo-
sition. The spectral profile depicted in Figure 7 contains two
well-resolved peaks at 515.14 and 522.14 eV, which correspond
to the binding energy of the vanadium 2p3/2 and 2p1/2 electrons
in the oxidation states +5 and +4, respectively.23 A similar result
from XPS study has been reported earlier for a mixed-oxidation
divanadium [V(IV/V)] compound.4m
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couple of redox processes in the positive potential range, which
includes a reversible electron transfer at E1/2 = 0.52 V (ΔEp =
80 mV), tentatively arising from a metal-based [V(IV/V)]
oxidation as represented by eq 1. The remaining oxidation
observed at a higher anodic potential E1/2 = 0.83 V (ΔEp = 100 mV)
is most certainly originating from a ligand-based oxidation
involving one of the catecholate moieties. The cathodic part of
the later process reveals a decrease in current height, thus
confirming the onset of a ligand decomposition process after
the initial oxidation. Under identical experimental conditions,
the ferrocenium/ferrocene couple appears at 0.60 V (ΔEp = 90 mV).
Coulometric confirmations of electron stoichiometry for these
processes have been vitiated by the instability of this compound
in the longer time-scale of CPC experiment.
Figure 7. High-resolution XPS spectrum of vanadium (2p) in
compound 1.
1
[L(VIVO)(VVO)2(OCH3)4]
−e−
Electronic Spectroscopy. The electronic spectrum of 1 in
DMF in the visible−NIR range (200−1200 nm) is displayed in
Figure 8, which features no band in the NIR region, thus
HoooooooooooI [L(VVO)3(OCH3)4]+
1
E1/2=0.52V
(1)
The voltammetry experiments with compounds 2 and 3
remain unreported due to their poor solubility in common
organic solvents.
Bond Valence Sum Calculation. The use of bond valence
sum (BVS) calculation26 provides further support in favor of a
V(V)−V(IV)−V(V) valence distribution in the trinuclear
compounds 1 and 2. In short, the bond valence (sij) and the
bond length (rij) between two atoms (i, j) are related by eq 2
where r0 and B are two empirically determined parameters, the
former being the characteristic of the bond in question.26 For a
polyatomic system containing a central metal atom, the bond
valence sij of a particular bond may be contemplated as the
amount of electrons by which the metal center (i) is depleted in
forming that bond with ligand (j). Consequently, the BVS (Vi) of
the metal atom is a measure of the total number of electrons it
loses (i.e., oxidation state) in forming that compound and is
calculated using eq 3. It appears from eq 2 that the sij contribution
of a particular bond decreases as the length (rij) of that bond
increases and vice versa. Recently, BVS model has been
successfully applied to estimate the vanadium oxidation states in
native as well as reduced bromoperoxidase.27 This method worked
really well when applied also to many model compounds with
vanadium in IV and V oxidation states.27 In our calculation, we
have used the required r0 values available from the literature,27,28
and B, the empirical constant, is taken as 0.37. The rij values are
taken as the bond lengths of the individual bonds determined by
X-ray crystallography. In Table 4 are summarized the BVS values
calculated for the individual vanadium centers in 2 taken as a
representative compound. Similar results are also obtained for
compound 1 (Table S1 in the Supporting Information). In this
calculation, all of the vanadium centers have been considered
either in +4 or in +5 oxidation states. The results have
convincingly established the V(1) center to be in the +4 oxidation
state, and the rest of the vanadium centers are in the +5 state.
Figure 8. Electronic absorption spectrum of 1 in DMF (concentration:
1 × 10−3 M).
confirming its status as a typical trapped-valence compound21
in solution as established already by EPR spectroscopy.
A strong band at λmax = 583 nm (ε = 3600 M−1 cm−1),
possibly originating from a PhO−→V charge transfer,4e,24 is the
only feature observed in the visible region that masks the d−d
bands expected to be seen in this region due to a vanadyl
center.4k,25 Unfortunately, because of solubility restrictions, the
spectra of 2 and 3 remain unreported.
Electrochemistry. The cyclic voltammogram of 1 has been
recorded at a glassy carbon working electrode under an
atmosphere of purified dinitrogen in DMF solution (0.1 M
TBAP) at 25 °C in the potential range from −0.1 to +1.1 V vs
Ag/AgCl reference. The voltammogram (Figure 9) displays a
sij = exp[(r0 − r )/B]
ij
(2)
(3)
V = ∑ sij
i
j
bond r0, Å
V4+−O 1.784
V5+−O 1.803
V4+−N 1.874
V5+−N 1.894
Figure 9. Cyclic voltammogram of compound 1 in DMF; potentials vs
Ag/AgCl, 0.1 M TBAP at a glassy carbon working electrode, scan rate
of 100 mV s−1.
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Table 4. Bond Valence Sum Calculated for the Vanadium Centers in 2
bond
r0, Å (considering r0, Å (considering
sij (considering sij (considering
Vij = ∑j sij
Vij = ∑j sij
vanadium description
V(IV))
V(V))
rij, Å
V(IV))
V(V))
(considering V(IV))
(considering V(V))
4.36
V(1)
V(2)
V(2)#
V1−O3
V1−O1#
V1−O1
V1−O4
V1−N1#
V1−N1
V2−O5
V2−O6
V2−O2
V2−O8
1.784
1.803
1.587
2.001
2.001
2.379
2.077
2.077
1.595
1.804
1.855
1.977
2.008
2.312
1.595
1.804
1.855
1.977
2.008
2.312
1.70
0.55
0.55
0.20
0.57
0.57
1.66
0.94
0.82
0.59
0.54
0.24
1.66
0.94
0.82
0.59
0.54
0.24
1.79
0.58
0.58
0.21
0.60
0.60
1.75
0.99
0.86
0.62
0.56
0.25
1.78
0.98
0.93
0.66
0.59
0.23
4.14
4.79
4.79
1.874
1.784
1.894
1.803
5.03
5.03
V2−O7
V2−O1
V2#−O5#
V2#−O6#
V2#−O2#
V2#−O8
V2#−O7
V2#−O1#
1.784
1.803
Series 974; American Chemical Society: Washington, DC, 2007.
(c) Tracey, A. S.; Willsky, G. R.; Takeuchi, E. S. Vanadium Chemistry,
Biochemistry, Pharmacology and Practical Applications; CRC Press: Boca
Raton, FL, 2007. (d) Crans, D. C.; Smee, J. J.; Gaidamauskas, E.; Yang,
CONCLUSIONS
■
Mixed-oxidation trinuclear vanadium compounds 1 and 2 in
V(V)−V(IV)−V(V) valence distribution have been synthesized
and characterized in the solid state by single-crystal X-ray
diffraction analysis. For 1, results of both EPR and XPS
spectroscopy have confirmed this mixed-valence compound to
be of trapped-valence type. Bond valence sum calculation also
supported this view. Replacement of the V(IV) center in these
compounds by Re(V) has led us to the successful isolation of a
heterotrinuclear compound (3) with a V(V)−Re(V)−V(V)
core. Result of X-ray structure analysis has confirmed the details
of the molecular structure of this interesting heterotrinuclear
compound with all oxido−metal ions forming a triangular core.
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J. P. The Biological Chemistry of the Element; Oxford University Press:
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98, 849. (h) Sigel, H., Sigel, A., Eds. Metal Ions in Biological Systems;
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(2) See, for example: (a) Hamstra, B. J.; Houseman, A. L. P.; Colpas,
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Pinkerton, A. A.; Chaudhury, M. Inorg. Chem. 1999, 38, 1982.
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ASSOCIATED CONTENT
* Supporting Information
BVS data for compound 1 (Table S1), and X-ray crystallo-
graphic files in CIF format for compounds 1−3. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(4) (a) Nishizawa, M.; Hirotsu, K.; Ooi, S.; Saito, K. J. Chem. Soc.,
Chem. Commun. 1979, 707. (b) Kojima, A.; Okazaki, K.; Ooi, S.; Saito,
K. Inorg. Chem. 1983, 22, 1168. (c) Launay, J.-P.; Jeannin, Y.; Daoudi,
M. Inorg. Chem. 1985, 24, 1052. (d) Costa-Pessoa, J.; Silva, J. A. L.;
Vieira, A. L.; Vilas-Boas, L.; OBrien, P. O.; Thornton, P. J. Chem. Soc.,
AUTHOR INFORMATION
Corresponding Author
*E-mail: icmc@iacs.res.in.
Notes
■
Dalton Trans. 1992, 1745. (e) Schulz, D.; Weyhermuller, T.;
̈
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Council of Scientific and
Industrial Research (CSIR), New Delhi. K.B., M.M., S.M.T.A.,
and M.C.M. also thank the CSIR for the award of Research
Fellowships. The single-crystal X-ray diffraction data were
recorded on an instrument supported by DST, New Delhi as a
National facility at IACS under the IRHPA program. XPS facility
was also provided by DST through the Unit of Nanoscience at
IACS.
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