Synthesis, spectroscopic characterization and reactivity studies of oxovanadium(IV) complexes with bulky N,N′-polymethylenebis(3,5- tBu2salicylaldimine) ligands

Article abstract of DOI:10.1016/j.saa.2010.06.040

A series of new sterically hindered N,N′-polymethylenebis(3,5- ^tBu₂salicylaldimine) ligands (H₂L_x) VO(IV) complexes, [VO{(2-O-3,5-^tBu₂C₆H ₂)CH-N-R-N-CH-(3,5-^t

Full text of DOI:10.1016/j.saa.2010.06.040

Spectrochimica Acta Part A 77 (2010) 630–637
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic characterization and reactivity studies of
oxovanadium(IV) complexes with bulky
N,Nꢀ-polymethylenebis(3,5-tBu2salicylaldimine) ligands
Veli T. Kasumova,∗, F. Köksalb, M. Aslanoglua, Y. Yerlic
a Department of Chemistry, Harran University, Osmanbey, 63300 S¸ anlıurfa, Turkey
b Department of Physics, Ondokuz Mayıs University, 55139 Kurupelit, Samsun, Turkey
c Department of Physics, Gebze Institute of Technology, Gebze-Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
A series of new sterically hindered N,Nꢀ-polymethylenebis(3,5-tBu2salicylaldimine) ligands (H2Lx) VO(IV)
complexes, [VO{(2-O-3,5-tBu2C6H2)CH N-R-N CH-(3,5-tBu2-C6H2O-2)] (X), where R = –(CH2)3– (3),
–(CH2)4– (4), –(CH2)5– (5), –(CH2)6– (6) and –CH2C(CH3)2CH2– (7) and early reported –(CH2)2– (1) and
–CH2CH(CH3)– (2), has been synthesized and characterized by spectroscopic (IR, UV/vis, 1H NMR, EPR),
electrochemical and magnetic susceptibility measurements. Complexes 1–7 are described a trigonal dis-
torted pyramids. All seven compounds give nearly the same parallel hyperfine coupling constant (Az)
regardless that the geometry of VO(IV) changes from square pyramidal to trigonal distorted pyramids.
Chemical oxidation of 1–7 by one equiv Ce(IV) leads to the formation of stable [VO(V)Lx]+ complexes.
Cyclic voltammograms of 2–6 in DMSO along with a quasi-reversible VO(IV)/VO(V) redox couple also
showed irreversible phenolate/phenoxyl responses. Each 1 and 7 shows only one reversible VO(IV)
centered oxidation waves. Chemical oxidation of H2Lx forms the stable [H2Lx]+ radical species.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 26 March 2010
Received in revised form 11 June 2010
Accepted 24 June 2010
Keywords:
[N,Nꢀ-polymethylenebis(3,5-
tBu2salicylaldimine)]VO(IV)
EPR
UV/vis
Chemical oxidation
Electrochemistry
such as Mg2+, Ca2+, and Zn2+ has led to the increased use of
vanadyl ion (VO2+), as a spectroscopic probe of biological systems
1. Introduction
The vanadyl ion ranks among the most stable of diatomic
cations and dominates in the chemistry of vanadium. While the
bulk of the vast literature relating to it dwells on synthetic,
structural, spectroscopic, and magnetic aspects, the coordination
chemistry of vanadium has recently become of great interest due
to the presence of vanadium in enzymatic systems and catalytic
activity [1,2]. Structural and functional models for vanadium-
dependent enzymes, such as nitrogenases and haloperoxidases
[2–4], and other vanadium containing biological molecules like
amavadine [2e,g] have further stimulated research efforts on
coordination chemistry [5–9]. Another important biological activ-
ity of vanadium is its insulin-mimetic characteristic, which can
cause in vivo stimulation of the uptake and metabolism of glu-
cose [4e,g]. The distinct preference of this metal center for O-
and/or N-coordination environments has prompted the synthesis
of numerous model vanadium compounds containing O/N donor
ligands whose spectroscopic, magnetic, and redox properties have
been widely investigated [3–8]. In addition, the ability to sub-
stitute vanadyl ion for spectroscopically silent divalent cations,
using paramagnetic spectroscopic techniques, such as electron
paramagnetic resonance (EPR), electron spin echo envelope mod-
ulation (ESEEM) and ENDOR [2d,5c]. These techniques can provide
information such as the number, kinds and in some cases, ori-
entations of ligands around the metal binding site. Tetradentate
Schiff base–oxovanadium(IV) complexes along with a rich redox
chemistry also exhibit catalytic activity in asymmetric oxidation of
sulfides into the corresponding sulfoxides, selective epoxidation of
olefins and C–C coupling reactions of various organic compounds
[6d,8e].
This work is based upon our ongoing interest [9c,d] in the
coordination chemistry of the redox-active di-tert-butylated ster-
ically hindered phenol functionalized ligands transition metal
complexes. One of the unique properties of above complexes is that
they easily generate peripherally bonded stable M(II)-phenoxyl
radical complexes upon chemical oxidation [9]. The widespread
occurrence of tyrosine radicals [10] in metalloproteins involving
oxygen-dependent enzymatic radical catalysis has prompted inor-
ganic chemists to model tyrosine by using di-tert-butylphenolate
ligands [10,11]. In our previous reports [12,13], we have demon-
strated that some Cu(II), Co(II) and Pd(II) complexes with
bidentate N-aryl(alkyl)-3,5-tBu2salicylaldimines and tetraden-
tate N,Nꢀ-bis(3,5-tBu2-salicylidene)-polymethylenediamine type
∗
Corresponding author. Tel.: +90 414 3283461 3588; fax: +90 414 3440051.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.06.040

V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
631
cially available 2,4-di-tert-butylphenol according to the literature
[14].
2.3. Synthesis of ligands
The synthesis and spectroscopic (1H NMR, FTIR, UV–vis) char-
acterization of the H2Lx ligands except H2L7 were described early
[8g,9c,10,13]. The ligand H2L7 was prepared by the condensation
reaction between 1,3-diamino-2,2ꢀ-dimethylpropane and 3,5-di-
tert-butyl-salicylaldehyde. Yield 88%, M.p. 181–183 ◦C, Anal. Calc.
for C35H54N2O2 C, 78.60; H, 10.18; N, 5.23%. Found: C, 77.63; H,
9.92; N, 4.98%. IR (KBr pellet, ꢀ [cm−1]): 1632 (CH N), 2860–2970
Scheme 1. Formula of the complexes.
ligands upon chemical oxidation affords directly coordinated
phenoxyl radical complexes. Herein, we describe the synthesis,
spectroscopic characterization, and chemical and electrochemical
redox behaviors of a series of VO(IV) chelates (VOLx, abbreviated
as 1–7) with tetradentate N,Nꢀ-bis(3,5-tBu2salicylidene)-poly-
methylenediamine ligands (H2Lx) (Scheme 1).
t
(C–H of –CH2– and Bu). 1H NMR (300 MHz; solvent CDCl3; stan-
dard SiMe4) ı: 13.91 (s, 2H, OH), 8.41 (s, 2H, HC N), 7.43–7.44 (d.
2H, meta-coupled H in salicylic ring, J = 2.67 Hz), 7.15–7.16 (d, 2H,
meta-coupled H in salicylic ring, J = 2.67 Hz), 1.51 [s, 18H, C(CH3)3],
1.35 [(s, 18H, C(CH3)3, 1.14 (s, 6H, CH3))]. UV/vis (C2H5OH: ꢁ
(log ε M−1 cm−1)): 225(4.8), 231(sh), (4.1), 263(4.9), 331 (4.5),
414(2.36).
2. Experimental
2.1. Measurements
2.4. Synthesis of complexes
All the methods of analysis [(C, H, N), FTIR, EPR, 1H NMR]
were carried out as described previously [12,13]. The ESR spectra
were recorded on a Varian E-109 C model X-band spectrom-
eter with 100 kHz frequency modulation. The g-values were
determined by comparison with a g = 2.0036 of DPPH sample.
The errors for g and A parameters are 0.001 and 0.005 G,
respectively. The program SIMFONIA version 2.1 by Bruker was
used for numerical simulation of the EPR spectra for an S = 1/2
electron spin coupled to the I = 7/2 nuclear spin from the 51V
nucleus. IR spectra obtained on a Perkin-Elmer FTIR spectrome-
ter using KBr pellet. Electronic spectra were recorded by using a
Perkin-Elmer Lambda 25 spectrophotometer in the 200–1100 nm
region in CHCl3, CH3CN and DMF. The room temperature (r.t.)
The complexes 1, 2 and 7, were prepared by the following
procedures. The hot methanol (30 ml) solution of VOSO4·5H2O
(0.5 mmol) was added to a deoxygenated hot mixture of Et3N
(1.0 mmol) and the ligand (0.5 mmol) dissolved in 50 ml of
CHCl3/MeOH (1:10 by volume). The mixture was heated to reflux
with stirring for about 30–45 min under nitrogen. After reducing
the volume to ca. 15 ml under nitrogen, reaction mixture allowed
to stand at room temperature. The green precipitate was filtered
and washed several times with water and methanol, and dried
in the oven at ca. 50 ◦C. The complexes were recrystallized from
chloroform-methanol mixture. Yield 85–92%. In our all attempts
to prepare the 3–6 complexes by above procedure, a green pre-
cipitate that was collected by filtration and recrystallized from
methanol/CHCl3 mixture yield compounds which contain slightly
amounts of insoluble part. Therefore, the 3–6 complexes were pre-
pared as follows. To a hot deoxygenated methanol-acetone solution
of ligand (0.005 mol) and sodium acetate trihydrate (0.01 mol) the
oxovanadium(IV) sulfate pentahydrate (0.005 mol) in 1 ml water
was added. The reaction mixture was heated under a dinitro-
gen atmosphere for 40–50 min. After reducing the volume to ca.
15–20 ml, the obtained olive green precipitate, collected by filtra-
tion, washed with water and methanol, and dried in the oven at ca.
70 ◦C. Yields: 71–77%. The 3–6 complexes prepared by this pro-
cedure are good soluble in CHCl3, CH3CN, DMSO and DMF. The
complexes were characterized by IR, UV–vis and elemental anal-
ysis (C, H, N). Complexes: 3: yield: 79%. M.p. 266–268 ◦C, elemental
Anal.: calcd. for C33H48N2O3V (571.69): C, 69.33; H, 8.46; N, 4.89%.
Found: C, 69.62; H, 9.43; N, 4.47%. IR (KBr pellet, ꢀ [cm−1]): 1620
(CH N), 2867–2957 (C–H of –CH2– and tBu), new bands at 1536
and 1553 ((Ph-)C–C N) bond, 978 (V O). 4: yield: 71%. M.p. dec.
>270 ◦C. Calcd. for C34H50N2O3V (585.71): C, 69.72; H, 8.60; N,
4.78%. Found: C, 69.52; H, 8.73; N, 5.57%. IR (KBr pellet, ꢀ [cm−1]):
1624 (CH N), 2868-2955 (C–H of –CH2– and tBu), new bands at
1542 and 1554 due to vibration of the (Ph–)C–C N bond, 978 (V O).
5: yield: 77%. M.p. dec. >250 ◦C. Calcd. for C35H52N2O3V (599.74):
C, 70.08; H, 8.74; N, 4.67%. Found: C, 69.87; H, 8.63; N, 4.57%. IR
(KBr pellet, ꢀ [cm−1]): 1626 (CH N), 2866-2955 (C–H of –CH2–
magnetic susceptibility was measured by using
a Sherwood
Scientific magnetic susceptibility balance. The diamagnetic cor-
rections for ligand susceptibility were evaluated using Pascal’s
constants.
Chemical oxidation of 1–7 complexes (5 × 10−3–3 × 10−2 M)
was carried out by the addition of one or two molar equivalent
(NH4)2Ce(NO3)6 [Ce(IV)] to their CHCl3, CH3CN, DMF or DMSO
solutions at r.t. and under air conditions by using in situ UV–vis
and EPR spectral measurements. UV–vis experiments were car-
ried out in DMF or CH3CN solutions of X by using equivalent
molar Ce(IV). The observed spectral changes were monitored with
a Perkin-Elmer Lambda 25 spectrophotometer. Electrochemical
measurements were recorded at r.t. under a dry nitrogen using a
PC-controlled Eco Chemie-Autolab-12 potentiostat in DMSO solu-
tions of ca. 10−3 to 10−4 M of X containing 0.05 M n-Bu4NClO4 as
the supporting electrolyte.
A conventional three-electrode configuration was used con-
sisting of a Pt disc working and a Pt wire auxiliary electrodes
and a Ag/AgCl reference electrode. Prior to each voltammetric
experiment, oxygen free nitrogen was bubbled through the elec-
trochemical cell to remove the oxygen for a few min. The potentials
are referenced to Ag/AgCl in the +1.5 to −1.5 V potential range.
2.2. Materials
t
and Bu), new bands at 1542 and 1554, 976 (V O). 6: yield: 74%.
All chemicals and solvents were of reagent grade and were
used without further purification. VO(SO4)·5H2O, (NH4)2Ce(NO3)6
(ACN), 2,4-di-tert-butylphenol, all diamines, NH2–(CH2)n–NH2,
where n = 2–6, 2-methyl-1,2-diaminoethane and 1,3-diamino-2,2-
dimethyl-propane were obtained from Aldrich Chemical Co. The
reagent 3,5-di-t-butylsalicylaldehyde was prepared from commer-
M.p. dec. >270 ◦C. Calcd. for C36H54N2O3V (613.77): C, 70.44; H,
8.87; N, 4.56%. Found: C, 68.89; H, 6.81; N, 3.37%. IR (KBr pellet, ꢀ
[cm−1]): 1625 (CH N), 2868–2956 (C–H of –CH2– and tBu), new
bands at 1541 and 1554; 979 (V O). 7: yield: 92%. M.p. >295 ◦C.
Calcd. for C35H52N2O3V (599.74): C, 70.08; H, 8.74; N, 4.67%. Found:
C, 70.27; H, 8.53; N, 4.27%. IR (KBr pellet, ꢀ [cm−1]): 1614 (CH N),

632
V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
Table 1
Electronic spectral data for 1–7 complexes.
Complex
ꢂeff ꢂB
Solvent
Electronic spectra ꢁmax (nm) (ε M−1 cm−1
)
1
2
3
4
5
6
7
1.74
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
CHCl3
DMF
388(12600), 500s(100), 625(207), 850s(31)
377(9630), 400s(4406), 490s(64), 619(155), 660s(136), 820s(30)
387(24200), 420s (11200), 5004(59), 630(186)
1.70
1.67
1.68
1.71
1.73
1.85
377(9889), 410s(4730), 490s(102), 622(144)
329(8862), 382(7560), 530s(46), 630s(27), 865s(18)
358(1873), 541(52), 686s(25), 846(15)
381(6290), 400s(2312), 540s(71), 630s(44), 900(10)
329(7677), 317(5279), 420s(2263), 530s(41), 634(23), 852(11)
360(3870), 400s(1310), 535s(69), 635s(36), 914(8)
339(7260), 350s(7106), 540s(62), 640s(34), 856(22)
356(6346), 400s(2268), 540s(74), 640s(45), 864(15)
340(6585), 353s(2964), 532s(48), 634(27), 852(30)
385(11640), 430s(2490), 490s(140), 620s(112), 63(124),
378(6292), 420s(1780), 490s(113), 588(93), 660s(86), 890s(36)
sShoulder.
2869–2959 (C–H of –CH2– and tBu), new bands at 1542 and 1555,
984 (V O).
3.2. Magnetic moments
The magnetic moment data of the X complexes are presented in
Table 1. The oxovanadium(IV) complexes belong to S = 1/2 system
and the magnetic moments for magnetically dilute mononuclear
VO(IV) complexes would be very close to the spin-only mag-
netic moment of 1.73 BM, since the spin-orbit coupling constant
(ꢁ) is positive and the orbital contribution is almost completely
quenched in VO(IV) complexes [5a,b]. The r.t. effective magnetic
moments of 1–7 complexes are in the range 1.67–1.85 BM indi-
cating the magnetically dilute nature of the complexes [5a,b]. The
observed magnetic moment data for 1–6 complexes (Table 1),
except 7 (1.84 BM) are close to the expected 1.73 BM and do
not indicate any antiferromagnetic or ferromagnetic coupling of
spins at room temperature [5]. These results suggest that there
are no direct V–V interactions in the solid complexes. It is of
interest that the ꢂeff values of these complexes estimated from
solid state and frozen glass EPR spectra were in the range of
1.71–1.75 BM and indicate the absence of magnetic exchange in
solution.
3. Results and discussion
The complexes 1 and 2, reported before [8d,13a], also were pre-
pared for comparative studies of their oxidative behaviors. All 1–7
complexes are scarcely soluble in organic solvents except for CHCl3,
DMSO and DMF. Compounds 3–6 are soluble in all above solvents
on a slight heating. Complexes 3–6 upon recrystallization from
CHCl3 particularly undergoes to the oxidation giving corresponding
diamagnetic VO(V) compounds. All complexes are green color and
their IR spectra exhibit strong band in the region 975–984 cm−1 due
to ꢀ(V O) stretching suggesting their mononuclear nature [5,6].
3.1. IR
The spectroscopic characteristics and elemental analysis results
of the H2L1–H2L6 ligands were reported in literature [13c,d].
The selected IR spectral data of H2L7 ligand and 3–7 complexes,
along with their assignments, are given in Section 2. The lig-
ands exhibit characteristic bands in regions 2860–2970, 2600–2700
and 1629–1634 cm−1 due to C–H of tBu and (CH2)x groups, OH
(intramolecular H-bands) and CH N stretches, respectively. The
bands of ꢀ(CH N) stretching (1629–1634 cm−1) of H2L3–H2L7
undergoes shifts to lower frequencies by about of 6–16 cm−1 in
the spectra of 3–7 (1614–1625 cm−1), indicating coordination of
the imine nitrogen. The IR spectra of HLx free ligands exhibit a
broad absorption band at ca. 2700 cm−1 due to ꢀ(OH) involved
in intramolecularly H-bonding that disappears upon complexa-
tion. These changes suggest that the ligands are coordinated via
imine nitrogen and deprotonated OH group oxygen atoms to the
VO(IV) center. The spectra of X unlike the free ligands also exhibit
two new bands at ca. 1540 and 1550 cm−1 which may origi-
nate from the coupling between the C C/C N bonds and the
aromatic ring [15]. The ꢀ(V O) stretching frequency is relatively
insensitive to the square pyramidal-to-trigonal bipyramidal dis-
tortion. For complexes 3–7 the V O frequencies are 978, 978,
976, 979 and 984 cm−1, respectively, consistent with the terminal,
non-bridged, oxo V O bond [5,9]. In general, IR spectra of the sal-
icylaldimine VO(IV) chelates show the ꢀ(V O) stretching band at
about 980 cm−1 for a monomeric form and nearly at 860–880 cm−1
for a linear chin (V O· · ·V O· · ·) polymeric form [5c,d]. These find-
ings indicate that all 1–7 complexes unlike their non-tert-butylated
analogous do not exhibit polymeric structure with the formation
of V O· · ·V O· · · chains [5c,d]. The intermolecular steric repul-
sions of di-t-butyl-substituents seem to prevent the formation of
3.3. Electronic spectral study
The electronic absorption spectral data obtained from spectra
of 1–7 which recorded in CHCl3 and DMF solutions are listed in
Table 1. Representative visible spectra of 1–7 MF are given in Fig. 1.
As can be seen from Table 1 and Fig. 1, the spectra of 3–7 com-
plexes in non-coordinated CHCl3 and coordinated DMF solutions
exhibit three broad bands of low intensity (ε = 13–199 M−1cm−1
)
in 525–905 nm region. The spectra of 1 and 2 are nearly identi-
cal, except that the spectrum of 2 exhibits poorly distinguishable
inflection around 830 nm (ε = 25 M−1 cm−1) absorption, and that
their spectra practically do not exhibit solvent dependent behav-
ior. It is interesting that the visible spectra of 3–7 compounds are
significantly different from those for 1 and 2 complexes. The vis-
ible spectra of 3–7 show three low intensity absorption bands in
the 500–910 nm region at 525–564, 630–680 and 780–905 nm in
CHCl3 and at 490–550, 600–650, and 613–850 nm in DMF solutions
(Table 1). These absorptions due to their low extinction coeffi-
cients (13–230 M−1 cm−1) can be assigned (from lowest to highest
2
energy) as the 2B → E *(dxy → dxz,dyz) (I), 2B2
→
2B1 ∗ (dxy
→
␲
2
dx
2 ) (II), and 2B2
→
2A1 ∗ (dxy → d z2 ) (III) transitions, according
2
− y
to Ballhausen and Gray molecular orbital scheme [17]. Comparison
of the visible absorptions of 3–7 complexes revealed that the low
energy bands appeared in coordinating DMF solvent display a red
shifts comparing to those in non-coordinating (or poorly coordi-
nated) CHCl3 solvent. The spectra of all 1–7 display higher intensity
absorptions with ε > 3 × 103 M−1 cm−1 in the 350–410 nm region,
V
O· · ·V O linkages.

V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
633
Fig. 2. EPR spectrum of 2 in CHCl3 at r.t.
unpaired electron to the 51V (I = 7/2, 99,75%; S = 1/2) nucleus. The
EPR spectra of 1–7 recorded in CHCl3/toluene solution at 160 K
show well-resolved eight line axial anisotropy with two sets of
eight line pattern with g < g and A⊥ ꢁ A relationships, charac-
||
⊥
||
teristic of an axially compressed d1 configuration [17–19]. The
xy
dipolar term |A⊥ − A | remains approximately the same for all
||
the complexes. This is consistent with the non-bonding nature of
the electron-containing dxy level [18a]. The possible superhyper-
fine couplings from ligand nitrogen or hydrogen nucleus on the
vanadium lines are not observed. This indicates that the unpaired
Fig. 1. (A) Electronic spectra of 1–7 complexes in DMF; (B) Electronic spectra of 3–6
complexes in CHCl3 and 4 in DMF (7)
electron to be in a non-bonding b2g (dxy,
2B2 ground state) orbital
which could be assigned as ligand to metal charge transfer (LMCT),
pointing away from the ligands in the equatorial (xy) plane and thus
excluding the possibility of its direct interaction with the ligand
[18,19]. The values of spin-Hamiltonian parameters extracted from
simulated experimental frozen glass (160 K) EPR spectra of 1–7 are
presented in Table 2. The simulation of the spectra was carried out
by using the Bruker WINEPR Symfonia program. Representative
experimental and simulated spectra of some complexes are shown
in Fig. 3. The values of isotropic g-factors and hyperfine coupling
constant, Aiso, were estimated as an average of the anisotropic g and
A values calculated from simulated spectra [Aiso = 1/3(Az + Ax + Ay)
and giso = 1/3(gz + gx + gy)]. The spin-Hamiltonian parameters A and
g are found to be in good agreement with those for analogous
vanadyl complexes with square-pyramidal geometry [2d,7, and 8].
The r.t. and frozen glass spectra of 1–7 in CHCl3 do not exhibit
superhyperfine structure from ligands nitrogen or hydrogen atoms
nucleus on the vanadium hyperfine line components. This indi-
p(␲) → d(␲*) transitions.
3.4. EPR spectra
Solid state EPR spectra. While the powder EPR spectra of 1 and 2
at r.t. exhibit a single broad isotropic signals with giso = 1.983 and
1.981, respectively, the solid state spectra of 3–7 display a broad
structured features. Powdersamples of 3 show structured EPRspec-
trum with gav = 1.985. The spectra of solid 4 and 5 consist of five
broad shoulders centered at ca. g = 1.946 and are more complicated
for interpretation. At the same time, powder spectrum of 6 clearly
displays structured symmetric feature centered at gav = 1.971. How-
ever, the EPR spectra of all these solid samples recorded at 160 K
do not exhibit any hyperfine structure and triplet state signal at
∼1500 G due to the coupling of interacting 51V nucleus. The appear-
ance of the structured spectra for some VO(IV) complexes with
longer (CH2)x linkages ensure that in the polycrystalline state the
VO(IV) centers are well spaced in the lattice diminishing mutual
line broadening interactions [16a,17]. On the other hand, the pos-
sibility of the existence of various conformers in powder samples
of these complexes [16b] also could complicate the solid state EPR
spectra.
cates that the unpaired electron to be in b2g (3dxy
state) orbital localized on metal, thus excluding the possibility of
,
2B2 ground
its direct interaction with the ligand atoms [15,18]. The A hyper-
||
fine coupling constant values (158–164 × 10−4 cm−1) (Table 2) for
X, are in the range expected for complexes with the similar salicy-
laldimine ligands donor atoms [2,4–6]. As can be seen from Table 2
the difference |Axx − Ayy| generally increases with the number of
(–CH2–) groups in X. For all complexes |Axx| < |Ayy| and gyy > gzz were
observed. Comparison of the in plain anisotropy indicates that both
gxx and gyy are sensitive to changes in the structure. In addition,
Solution EPR spectra. The r.t. isotropic EPR spectra of 1–7
complexes recorded in chloroform/toluene (V/V = 2:1) solution
exhibit similar well resolved eight-line signals (Fig. 2) with
giso ∼ 1.975–1.992 0.005 and average hyperfine splitting Aiso
of 86–92 × 10−4 cm−1 (Table 2) resulting from coupling of the
the obtained A values are similar to the values calculated from
||
additivity relationship described by Chasteen [1a] for VO(IV) com-
Table 2
The values of isotropic and anisotropic g and A components obtained from simulated frozen glass EPR spectra and calculated molecular orbital coefficients ˇ22 and ˛2 for 1–7
compounds.
ˇ22
˛
a
b
2
Complex X
giso
Aiso
gxx
gyy
gzz
Axx
Ayy
Az
1
2
3
4
5
6
7
1.981
1.972
1.964
1.964
1.990
1.968
1.977
−88.3
−89.8
−86.9
−91.6
−86.1
−88.4
−86.9
1.989
1.983
1.978
1.979
1.997
1.990
1.990
1.985
1.980
1.965
1.974
1.973
1.971
1.985
1.970
1.955
1.949
1.940
1.945
1.945
1.955
−52.3
−53.4
−49.5
−54.2
−44.8
−51.4
−49.5
−54.2
−55.6
−52.8
−57.0
−54.2
−55.1
−52.4
−157.9
−160.6
−158.8
−163.5
−159.8
−158.8
−159.8
0.92
0.92
0.93
0.92
0.91
0.92
0.93
0.41
0.60
0.67
0.47
0.73
0.71
0.60
a
Estimated errors are 0.001 for g-factors.
b
Values for A are given in units of 10−4 cm−1 with estimated errors of 0.5 × 10−4 cm−1
.

634
V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
Fig. 4. Electronic spectra of 1 (a–c) and 1 + 1 equiv Ce(IV) (d) in DMF at r.t.; (e)
spectrum of diluted (d).
orbit coupling constant (ꢁ) of 170 cm−1, ˛2 is calculated from Eq.
(4). While the spectra yield only the magnitudes of A and A , a
||
⊥
negative sign of the hyperfine tensor components is required to
obtain positive values for ˇ22 from Eq. (3).
The values of ˛2 and ˇ22 calculated from Eqs. (4) and (3) for all
1–7 complexes are given in Table 2. The obtained values for ˇ22
(0.91–0.93) are close to the largest value of 1, which show that
the in-plane ␲-bonding between dxy orbital of VO(IV) and coor-
dinating atoms is negligible. The quantity C = 1 − ˇ22(0.07 − 0.09),
which corresponds to the fraction of unpaired d-electrons that is
delocalized over ligand orbitals, indicate that the unpaired electron
in 1–7 practically localized in a 3dxy orbital. The values for in plane
␴-bonding parameter, ˛2 (0.41–0.73) indicate that there is consid-
erable ␴-bonding covalency between VO(IV) and coordinating N/O
atoms of the ligands in the 1–7 complexes.
Fig. 3. Experimental and simulated EPR spectra of 2 and 4 complexes in CHCl3 at
160 K.
3.5. Chemical oxidation of 1–7
plexes with ligands bearing two azomethine N and two phenolate O
atoms in the coordination site [5–8]. Thus, the solution EPR spectra
of 1–7 in CHCl3 solution at 160 K are typical for square-pyramidal
or distorted square-pyramidal geometries.
Our recent studies on the chemical oxidation of bis[N-
alkyl(aryl)-3,5-But2-salicylaldi-minato]M(II) [M = Cu(II), Co(II),
Pd(II)] complexes by (NH4)2[Ce(NO3)6] [Ce(IV)] in CHCl3 have
revealed that the oxidation process was accompanied by the
decrease of intensity of their EPR spectra [in the case of Cu(II)
complexes] and appearance of the radical signals [12,13]. In the
chemical oxidation of Co(II) complexes with di-tert-butylated
bidentate, tridentate and tetradentate ligands unlike their
non-di-tert-butylated analogues the generation of the stable
Co(III)-phenoxyl radical complexes were detected in their EPR
(octet of duplets at ca. g = 2.00).
Molecular orbital theory is used to relate the measured prin-
cipal components of g and A tensors to changes in the geometry
and in the electronic structure of the vanadyl species. The relation-
ship between the g and A hyperfine coupling constant and bond
covalency has been studied by Kivelson Lee and McGarvey [18].
The principal components of the hyperfine tensor are given by
the approximate relationships of (1)–(4). The squared MO coeffi-
cient ˇ22 can be determined from a suitable linear combination of
(1) and (2) according to Eq. (3). Using the obtained values of ˇ22,
The investigation of oxidative behavior of 1–7 complexes and
H2Lx ligands were carried out by using in situ UV-visible spectral
measurements in CH3CN and DMF. It has been found that upon
addition of one molar equiv Ce(IV) to 1–7 in DMF at r.t., a simultane-
ous change of the light green to dark green blue, the disappearance
of low intense d-d bands in the 600–850 nm region and appear-
ance of new intense broad maximum band in the 592–780 nm were
observed. In the oxidation of 1–7 complexes with one equiv molar
Ce(IV) in DMF solution, along with instantly disappearance of orig-
inal visible bands, the appearance of new absorptions with ꢁmax(ε,
ꢁ = 170 cm−1 and ꢃEx
was estimated from Eq. (4)
= Ex
− Edxy, in-plane ␴-bonding ˛2
−y
2
2
2
2
−y
ꢀꢁ
ꢂ
ꢁ ꢂ
ꢃ
4
7
3
7
A
= −P
K +
ˇ22 + ꢃg
+
ꢃg
⊥
(1)
(2)
(3)
||
||
d
ꢀꢁ
ꢂ
ꢁ
ꢂ
ꢃ
4
11
14
A
⊥
= −P
K −
ˇ22
+
ꢃg
⊥
d
7
7
6
5
7 A|| − A
⊥
ˇ22
=
ꢃg
−
ꢃg
−
⊥
6
||
12
Pd
M
−1 cm−1) of 613 (1738), 612 (1265), 664(1918), 682 (780), 760
8ꢁ˛2ˇ22
(213) and 780 (167) and 660 nm (1785), respectively, were detected
(Figs. 4–6). As can be seen from Fig. 5 upon one-electron oxida-
tion of 2, along with the disappearance weak shoulder bands at
ca. 800, 480 nm and UV bands 280 and 377 nm, the appearance
of a new intense maximum band at 612 nm was detected. Similar
spectral changes were also observed in CH3CN. When the oxidation
was carried out in CH3CN the visible spectra of oxidized 1+, 2+, 3+
and 6+ species showed absorptions with ꢁmax (ε, M−1 cm−1) of 615
(1300), 668 (1790), 663 (3090), and 660 (2090) nm, respectively,
ꢃg
=
(4)
||
2
2
ꢃE( B2→ B1)
where Pd = 128 × 10−4 cm−1 for VO(IV), ꢃg = ge − g ; ꢃg = ge − g ,
||
||
⊥
⊥
K is Fermi contact term and related to the unpaired spin density at
the nucleus [18b], ˛ and ˇ2 are the coefficient of the vanadium
dx
and dxy orbitals in the molecular orbital, respectively. Using
2
2
− y
the values of ˇ22, along with the observed energies for the electronic
transitions 2B2 → B1 appeared at around 600–650 nm and a spin-
2

V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
635
Fig. 7. Electronic spectra of some H2Lx ligands and their one electron oxidation
species generated by one equiv molar Ce(IV) in CH3CN: (1) and (2) initial and dilute
spectra of H2L1; (3)–(7) spectra of phenoxyl radical species scanned with 1 min
intervals after oxidation of H2L1: (8) and (9) initial and dilute spectra of H2L2; (10)
spectrum of phenoxyl generated from H2L2; (11) spectrum of phenoxyl radicals
generated from H2L7; (12) spectrum of diluted (11).
Fig. 5. Electronic spectra of 2 (0.016 M) and its oxidized species in DMF generated by
the oxidation of one equiv molar Ce(IV) (0.016 M): (1), (2) and (3) spectra of 2 before
the oxidation; spectra (4) just after oxidation and (5) after diluted 4 (3.2 × 10−3); (6)
spectrum of diluted 5.
but oxidized 4+ and 5+ exhibited absorptions with ꢁmax 592 (440)
and 750 (sh, 303) nm, 662 (1496) and 780 nm(sh) in MeCN solu-
tion, respectively. It is of interest to note that upon addition of one
equiv Ce(IV) to heterogeneous mixture of complexes 3–5 in MeCN,
the insoluble part instantly dissolves and the reaction mixture
becomes homogenous. The deep green blue colors of the oxidized
1+–7+ species are presumably due to phenolate to oxovanadium(V)
charge transfer transitions, and are comparable to other oxovana-
dium complexes of phenolate derived ligands [8]. When the course
of oxidation process was monitored by EPR in CH3CN, without
detection of radical signals, the disappearance of VO(IV) EPR signal
was observed. In the presented complexes one-electron oxida-
tion process may be either metal-centered [VO(IV)Lx → VO(V)Lx]
or ligand-centered [VO(IV)Lx → VO(IV)Lx•+] and in both cases the
generated oxidized species should be diamagnetic (if antiferromag-
netic coupling between d1 and phenoxyl spins takes place for the
latter one). While in both cases the generated 1+–7+ species exhibit
absorption bands in the 550–900 nm region, the visible absorptions
in phenoxyl radicals originate from forbidden n → ␲* transitions
and appeared as low intensity bands (ε = 100–500 M−1 cm−1). In
addition, phenoxyl radicals generally are unstable (under aero-
bic conditions they are rapidly decay) and also display typical
strong absorption maxima around 350–400 nm and low intensity
absorption at around 700–800 nm and ꢁ > 1100 nm [9,10]. At the
same time, the visible bands of VO(V) complexes originate from
allowed LMCT transitions and exhibit high intensity absorptions
(ε = 1000–5000 M−1 cm−1) and generally are stable under aerobic
conditions. Since all of the spectra of oxidized 1–7 species did not
exhibit absorption bands around 400 nm and display strong visi-
ble absorption bands in the 615–780 nm (ε = 1300–3090 M−1 cm−1
)
region and the oxidized species are stable under aerobic conditions
it can be concluded that all these oxidation process is metal-
centered and the oxidized species can be described as [VO(V)Lx]+
cations. Thus, while the presented 1–7 complexes have two easily
oxidizable metal centered VO(IV) and ligand centered 3,5-di-tert-
butylphenolate group, the chemical oxidation takes place at the
metal center.
Spectral change from light yellow to dark green or brown-
ish orange also takes place upon oxidation of H2Lx ligands under
above conditions (Fig. 7). The oxidation of H2L2, H2L2 and H2L6
by one molar equiv Ce(IV) in CH3CN, along with the simulta-
neous disappearance of original absorption at ca 410 nm and
appearance of a new broad bands at 590(270), 600(186) and
704 nm, respectively, as well as absorption at ꢁ > 1100 nm for
H2L5•+, were observed. The appeared spectral patterns are typ-
•+
ical for phenoxyl radical cations, H2Lx [10,21]. The generation
of radical species was also supported by (the) detection of a
single isotropic signal with g = 2.005 in the EPR spectra of the oxi-
dized species in CHCl3. In the oxidation of other H2Lx ligands,
the appeared green color instantly (for 4–5 s) converts to brown-
ish orange and the detection of the absorbance of the generated
species under above conditions were not possible. Thus, the pre-
sented experimental findings indicate that unlike Cu(II) and Co(II)
complexes chemical oxidation of N,Nꢀ-polymethylenebis(3,5-
tBu2salicylaldimine)VO(IV) leads to the formation of [N, Nꢀ-
polymethylenebis(3,5-tBu2salicylaldimine)VO(V)]+ complexes.
Table 3
Voltammetric data for 1–7 complexes.
1
1
2
2
Complex
Epa
Epc
ꢃEp
E1/2
Epa
Epc
1
2
3
4
5
6
7
0.290
0.298
0.376
0.356
0.373
0.425
0.319
0.220
0.235
0.247
0.271
0.196
0.209
0.250
0.070
0.063
0.129
0.085
0.177
0.216
0.069
0.255
0.266
0.312
0.313
0.280
–
–
1.05
0.95
1.01
1.13
1.01
–
−0.442
−0.490
–
–
0,317
0.285
−0.459
–
Supporting electrolyte = 0.05 M n-Bu4ClO4, scan rate = 50 mV/s.
Potentials are presented in volts.
Fig. 6. Electronic spectra: (A) Visible spectra of 3 (a and b) and 3 + 1 equiv Ce(IV) (c
and d) in DMF. (B) Spectra of 4 (a) and 4 + 1 equiv Ce(IV) (b) in DMF; spectra of 6 (c)
and 6 + 1 equiv Ce(IV) in DMF recorded at r.t.
E1 and E2 represent the metal centered and ligand centered redox process, respec-
tively.

636
V.T. Kasumov et al. / Spectrochimica Acta Part A 77 (2010) 630–637
Fig. 8. (A) A cyclic voltammogram of complex 3 in DMSO and 0.05 M n-Bu4NClO4 as supporting electrolyte. Scan rate 50 mV/s. (B) Cyclic voltammograms of complex 7 in
DMSO at increasing scan rates. Scan rates: 50, 100, 150, 200 and 250 mV/s.
3.6. Electrochemistry
similar to each other and exhibit only one irreversible wave in the
range from +0.98 to 1.17 V versus Ag/AgCl [13c], indicating that the
process accompanies O–H bond dissociation [22].
The electrochemical behavior of 1–7 complexes were stud-
ied by cyclic voltammetry (CV) in the range +1.5 to −1.5 V in
DMSO at Pt working electrode and Ag/AgCl reference electrode
using n-Bu4NClO4 (0.05 M) as the supporting electrolyte. The
results of cyclic voltammetry (CV) measurements are given in
Table 3. Representative CV scans are shown in Fig. 8. The CV
of 1–7 exhibit a quasi-reversible waves at E1/2 of 0.266–0.317
and ꢃEp = 0.064–0.216 V versus Ag/AgCl which can be ascribed
to VO2+/VO3+ oxidation couples (Table 3). The observed E1/2 data
for VO2+/VO3+ waves are shifted to negative potentials com-
paring to those found for non-tert-butylated salen type VO(IV)
complexes in DMF [6a] as expected for the greater ␴ elec-
tron donation of the tert-butyl groups attached to salicylic
ring. The 1, 2, 4, 7 and 3, 5, 6 complexes exhibit approx-
imately reversible (ꢃEp = 0.063–0.085 mV) and quasi-reversible
(ꢃEp = 0.129–0.216 V) processes, respectively, in DMSO at ca.
0.255–0.317 V versus Ag/AgCl due to VO3+/VO2+ couple
4. Conclusions
In conclusion, a series of sterically hindered salen complexes of
VO(IV) with N,Nꢀ-polymethylenebis(3,5-tBu2salicylaldimine) lig-
ands have been prepared and characterized spectroscopically,
magnetochemically and electrochemically. The bonding coeffi-
cients ˛2 and ˇ22 were evaluated from the available MO theory
and experimental UV/vis and EPR data. The presented com-
plexes unlike their non-di-tert-butylated analogues did not form
polymeric structure in solid state and adopted particularly trig-
onal distorted square-pyramidal geometry in CHCl3 solution. It
was also shown that the chemical oxidation of all complexes
without generation of phenoxyl radical species readily forms cor-
responding stable [VO(V)Lx]+ complexes. Cyclic voltammograms
of 1–7 along with a quasi-reversible VO(IV)/VO(V) redox couples
also showed irreversible oxidation waves assigned to pheno-
late/phenoxyl responses.
VO(IV) − e = VO(V)
(5)
The CV of 2–6 complexes also exhibit
VO(V) = VO(IV) + e
(6)
Acknowledgment
The CV of 2–6 complexes also exhibit an irreversible anodic peak
(Fig. 8a) in the potential range of +0.95 to +1.13 V (Table 3), which
might be attributed to the one-electron oxidation of coordinat-
ing phenolates into phenoxyl radical species. The irreversibility of
these curves suggests the instability of the generated radical species
under electrochemical time scale. The CV of 2, 3 and 6 display
an irreversible peaks at −0.44, −0.49 and −0.46 V versus Ag/AgCl,
respectively, which are absent in the CV of the free ligands, proba-
bly originate from products of the oxidized species. These reduction
waves cannot be assigned to VO(IV)/VO(III), since the reduction
potential of this couple is lower than −1.5 V in DMF [19,20]. The
CV of 1 and 7 display only one reversible redox couple with E1/2
at 0.255 and 0.285 V (Table 3), respectively. The quasi-reversibility
of the complex 7 was also assured by the increase in peak cur-
rent versus the square root of the scan rate as given in Fig. 8b. The
Ip/v1/2 value is almost constant for all scan rates. This establishes
the electrode process as diffusion controlled [21]. It is interesting
to note that the CV curves of all oxidized H2Lx ligands in DMF, are
Financial support by the Harran University Research Grants
Council is gratefully acknowledged.
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