A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: Synthesis, structure, and redox activity

Article abstract of DOI:10.1021/ic020259d

The concerned diols (general abbreviation, H₂L) are catechol (H₂L¹) and its 3,5-Bu₂^t derivative (H₂L²). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H₂L with VO(xsal)(H₂O) or VO(xsal)(OMe)(HOMe), where xsal^2- is the diionized salicylaldimine of glycine (x = g), L-alanine (x = a), or L-valine (x = v). The reaction of VO(acac)₂ with H₂L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO-(gsal)(HL¹), 2a, and VO(vsal)(HL²), 2f, the HL^- ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal^2- coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O···O, 2.60 A) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L²)·H₂O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O···O, 2.84 A). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single ⁵¹V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H₂L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O₂ to 2 is proposed to occur via hydrogen bonding with chelated HL^-.

Full text of DOI:10.1021/ic020259d

Inorg. Chem. 2002, 41, 4502−4508
A Family of Vanadate Esters of Monoionized and Diionized Aromatic
1,2-Diols: Synthesis, Structure, and Redox Activity
,†,‡
Bharat Baruah,† Samir Das,† and Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, and Jawaharlal Nehru Centre for AdVanced Scientific Research,
Bangalore 560 064, India
Received April 4, 2002
t
The concerned diols (general abbreviation, H L) are catechol (H L1) and its 3,5-Bu2 derivative (H L2). Esters of the
2
2
2
type VO(xsal)(HL), 2, are obtained by reacting H L with VO(xsal)(H O) or VO(xsal)(OMe)(HOMe), where xsal2- is
2
2
the diionized salicylaldimine of glycine (x ) g), L-alanine (x ) a), or L-valine (x ) v). The reaction of VO(acac)2
with H L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO-
2
(gsal)(HL1), 2a, and VO(vsal)(HL2), 2f, the HL- ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo
oxygen atom. The xsal2- coligand has a folded structure and the conformation of 2f is exclusively endo. In both
2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O‚‚‚O, 2.60 Å) to a carboxylic oxygen atom of
a neighboring molecule. In VO(psal)(L2)‚H O, 3b, the diionized diol is O,O-chelated to the metal and the water
2
molecule is hydrogen bonded to a phenoxidic oxygen atom (O‚‚‚O, 2.84 Å). The C−O and C−C distances in the
V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate−semiquinonate hybrid. In solution each
ester gives rise to a single 51V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease
in the ester LMCT band energy. The V(V)/V(IV) and catecholate−semiquinonate reduction potentials lie near −0.75
and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively
furnishing the corresponding o-quinone, and in the presence of H L the reaction becomes catalytic. In contrast,
2
type 3 esters are inert to oxygen. The initial binding of O to 2 is proposed to occur via hydrogen bonding with
2
chelated HL-.
Introduction
to trivalent,7,8 tetravalent7,9,10 and pentavalent10-12 vanadium.
Variants include semiquinonate and phenolato-bridged types.4,13
Monoionized aliphatic diols14,15 including modified car-
The binding of vanadium to aromatic 1,2-diols1 has been
of interest in contexts such as analytical color reactions,2
tunicate chemistry,3 and diol oxidation.4-6 Synthetic and
structural studies have demonstrated that the common diol
coordination mode is O,O-chelation in the diionized form
(5) (a) Galeffi, B.; Postel, M.; Grand, A.; Rey, P. Inorg. Chim. Acta 1989,
160, 87. (b) Galeffi, B.; Postel, M.; Grand, A.; Rey, P. Inorg. Chim.
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Garland, M.-T.; LeMarouille, J.-Y.; Giannotti, C. J. Organomet. Chem.
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(6) Preliminary communication based on hydrazone coligand: Rath, S.
P.; Rajak, K. K.; Chakravorty, A. Inorg. Chem. 1999, 38, 4376.
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104, 5092.
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Soc. 1990, 112, 2627.
* To whom correspondence should be addressed at the Indian Association
for the Cultivation of Science. E-mail: icac@mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
‡ Jawaharlal Nehru Centre for Advanced Scientific Research.
(1) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. ReV. 1981, 38,
45.
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(b) Lobinski, R.; Marczenko, Z. Anal. Chim. Acta 1989, 226, 281.
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Chasteen, N. D., Ed.; Kluwer Academic Publisher: Dordrecht, The
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4502 Inorganic Chemistry, Vol. 41, No. 17, 2002
10.1021/ic020259d CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/01/2002

Vanadate Esters of Ionized Aromatic 1,2-Diols
Table 1. Band Maxima in the Visible Regiona and 51V NMR Chemical
bohydrates16 have been shown to chelate VO3+ forming esters
of type 1 where the chelate ring is five- or six-membered
and the vacant sites are engaged by a tridentate coligand.
This has prompted us to search for 2, the aromatic ester
analogue of 1, and herein we report the isolation and
Shiftsb
band position
51V NMR
compd
number
(λmax, nm)
(δ, ppm)
VO(gsal)(HL1)
VO(asal)(HL1)
VO(vsal)(HL1)
VO(gsal)(HL2)
VO(asal)(HL2)
VO(vsal)(HL2)
VO(psal)(L1)
2a
2b
2c
2d
2e
2f
578
591
602
638
641
-460
-457
-455
-416
-414
-410
270
647
3a
3b
850, 525
855, 550
VO(psal)(L2)‚H2O
462
a The solvent is Me2CO. b The solvent is (CD3)2SO for 2a-2f and CDCl3
for 3a,b.
(general abbreviation, H2xsal) and of 2-picolylamine, 5, are
employed as coligands. Upon reacting VIVO(xsal)(H2O)17 or
VVO(xsal)(OMe)(HOMe)18 with a slight excess of H2L in
methanol or acetonitrile at room temperature in air violet to
blue solutions are formed which afford VVO(xsal)(HL) as
dark crystalline solids in excellent yields. Violet VVO(psal)-
(L) was conveniently isolated by the stoichiometric reactions
19
of VO(acac)2 with Hpsal and H2L in methanol under
ambient conditions.
characterization of a family of this type incorporating
salicylaldimines of achiral and chiral R-amino acids as
coligands.6 For comparison, diionized diol chelates of type
3 also bearing a salicylaldimine coligand have also been
prepared. Structure determination has revealed that while the
diol ligand in 2 has model catechol character, it is signifi-
cantly semiquinonoid in 3. The distinctive spectral and
electrochemical features of 2 and 3 and the spontaneous
solution reactivity toward oxygen, a property exclusive to
2, are scrutinized.
Six esters of the type VO(xsal)(HL), 2a-f, and two of
the type VO(psal)(L), 3a,b (the latter occurring as a hydrate),
have been isolated (Table 1). Monoionized aromatic 1,2-
diol chelation as in 2 is rare in transition metal chemistry. A
notable example, relevant to catechol dioxygenase activity,20
occurs in iron(II) chemistry.21
Results and Discussion
(A) Diols, Coligands, and Vanadate Esters. (a) Syn-
thesis. The aromatic diol ligands (general abbreviation, H2L)
used are catechol, H2L1, and 3,5-di(tert-butyl)catechol, H2L2.
The salicylaldimines of glycine, L-alanine, and L-valine, 4a-c
(b) Spectral Characterization of the Esters. The type 2
species are characterized by a relatively broad OH stretch
(2450-2580 cm-1) suggesting the presence of strong hy-
1
drogen bonding. In H NMR the OH resonance (near 10
(10) Kabanos, T. A.; White, A. J. P.; William, D. J.; Woollians, J. D. J.
Chem. Soc., Chem. Commun. 1992, 17.
ppm) is very broad and is not observable in all cases. The
compounds display moderately strong (ꢀ 3000-5000 M-1
cm-1) absorption in the visible region: one band near 600
nm in 2 and two bands near 540 and 850 nm in 3 (Table 1).
In general the absorption shifts to lower energies in going
from (HL1)- to (HL2)- and (L1)2- to (L2)2- esters (Table 1)
consistent with pπ (HL-/L2-) f d (V) LMCT assignment.
The 51V chemical shifts of the esters are listed in Table 1.
Each ester gives rise to a single signal even for the type 2
(11) (a) Cass, M. E.; Gordon, N. R.; Pierpont, C. G. Inorg. Chem. 1986,
25, 3962. (b) Kabanos, T. A.; Slawin, A. M. Z.; Williams, D. J.;
Woollians, J. D. J. Chem. Soc., Chem. Commun. 1990, 193.
(12) (a) Cornman, C. R.; Colpas, G. J.; Hoeschele, J. D.; Kampf, J.;
Pecoraro, V. L. J. Am. Chem. Soc. 1992, 114, 9925. (b) Cornman, C.
R.; Kampf, J.; Pecoraro, V. L. Inorg. Chem. 1992, 31, 1981.
(13) (a) Mazzanti, M.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem.
Soc., Dalton Trans. 1989, 1793. (b) Manos, M. J.; Tasiopoulos, A. J.;
Raptopoulou, C.; Terzis, A.; Woollians, J. D.; Slawin, A. M. Z.;
Keramidas, A. D.; Kabanos, T. A. J. Chem. Soc., Dalton Trans. 2001,
1556.
(14) (a) Mondal, S.; Rath, S. P.; Dutta, S.; Chakravorty, A. J. Chem. Soc.,
Dalton. Trans. 1996, 99. (b) Mondal, S.; Ghosh, P.; Chakravorty, A.
Indian J. Chem. 1996, 35A, 171. (c) Rath, S. P.; Mondal, S.;
Chakravorty, A. Inorg. Chim. Acta 1997, 263, 247.
(15) (a) Mondal, S.; Rath, S. P.; Rajak, K. K.; Chakravorty, A. Inorg. Chem.
1998, 37, 1713. (b) Rath, S. P.; Rajak, K. K.; Mondal, S.; Chakravorty,
A. J. Chem. Soc., Dalton Trans. 1998, 2097.
(16) (a) Rajak, K. K.; Rath, S. P.; Mondal, S.; Chakravorty, A. Inorg. Chem.
1999, 38, 3283. (b) Rajak, K. K.; Rath, S. P.; Mondal, S.; Chakravorty,
A. Indian J. Chem. 1999, 38A, 405. (c) Rajak, K. K.; Rath, S. P.;
Mondal, S.; Chakravorty, A. J. Chem. Soc., Dalton Trans. 1999, 2537.
(d) Rajak, K. K.; Baruah, B.; Rath, S. P.; Chakravorty, A. Inorg. Chem.
2000, 39, 1598.
(17) Theriot, L. J.; Carlisle, G. O.; Hu, H. J. J. Inorg. Nucl. Chem. 1969,
31, 2841.
(18) (a) Dutta, S.; Mondal, S.; Chakravorty, A. Polyhedron 1995, 14, 1163.
(b) Nakajima, K.; Kojima, M.; Toriumi, K.; Saito, K.; Fujita, J. Bull.
Chem. Soc. Jpn. 1989, 62, 760.
(19) Rowe, R. A.; Jones, M. M. Inorg. Synth. 1957, 5, 133.
(20) Kruger, H.-J. In Biomemetic Oxidations Catalysed by Transition Metal
Complexes; Meunier, B., Ed.; Imperial College Press: London, UK,
2000; pp 363-413.
(21) (a) Chiou, Y.-M.; Que, L., Jr. Inorg. Chem. 1995, 34, 3577. (b) Shu,
L.; Chiou, Y.-M.; Orville, A. M.; Miller, M. A.; Lipscomb, J. D.;
Que, L., Jr. Biochemistry 1995, 34, 6649.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4503

Baruah et al.
Table 2. Selected Bond Distance (Å) and Angles (deg) for
VO(gsal)(HL1), 2a, and VO(vsal)(HL2), 2f
2a
2f
distances
V-O(1)
1.576(3)
1.966(3)
1.855(3)
2.399(3)
1.827(3)
2.063(4)
1.366(5)
1.345(5)
1.372(7)
1.402(6)
1.386(7)
1.376(8)
1.377(8)
1.387(7)
1.592(3)
1.979(3)
1.865(3)
2.352(3)
1.843(3)
2.087(3)
1.366(4)
1.349(5)
1.370(6)
1.387(5)
1.404(5)
1.396(6)
1.396(6)
1.411(5)
V-O(2)
V-O(3)
V-O(4)
V-O(5)
V-N
O(4)-C(10)
O(5)-C(15)
C(10)-C(11)
C(10)-C(15)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
Figure 1. Perspective view and atom-labeling scheme of VO(gsal)(HL1),
2a. All non-hydrogen atoms are represented by their 30% probability
ellipsoids.
angles
O(1)-V-O(2)
O(1)-V-O(3)
O(1)-V-O(4)
O(1)-V-O(5)
O(2)-V-O(4)
O(3)-V-O(2)
O(3)-V-O(4)
O(5)-V-O(2)
O(5)-V-O(3)
O(5)-V-O(4)
O(1)-V-N
98.7(2)
99.5(2)
95.5(2)
99.7(2)
173.31(14)
100.9(2)
175.5(2)
100.3(2)
81.93(13)
156.86(14)
80.95(14)
88.50(14)
102.0(2)
75.32(13)
103.0(2)
76.73(14)
85.4(2)
80.63(12)
157.85(13)
85.58(12)
93.04(12)
99.76(13)
74.00(11)
106.1(2)
75.43(13)
84.88(13)
151.46(13)
78.35(11)
O(2)-V-N
Figure 2. Perspective view and atom-labeling scheme of VO(vsal)(HL2),
2f. All non-hydrogen atoms are represented by their 30% probability
ellipsoids.
O(3)-V-N
O(5)-V-N
154.0(2)
81.46(13)
N-V-O(4)
species bearing chiral amino acid residues, suggesting15a that
their solutions contain only one diastereoisomer as in the
solid state, vide infra. Similarly each proton type gives rise
to a single 1H NMR signal (see Experimental Section). The
51V signals generally shift12a downfield as the average LMCT
band energy decreases as between 2a and 2d (40 ppm),
between 3a and 3b (190 ppm), and between 2a and 3a (730
ppm). In the context of the very large shift between type 2
and type 3 species it is significant that in 2 the diol ligand
is catecholate in character while in 3 there is very significant
semiquinone contribution, vide infra.
In 2f the VdO bond lies endo to the C(2)-C(16) bond of
the chiral amino acid residue (Figure 2). The signs of the
atomic coordinates of 2f were chosen so as to conform the
S configuration of the L-amino acid residue. Viewed down
the VdO axis the equatorial atoms span clockwise when
ordered according to their priority sequence22 O(2) > O(5)
> O(3) > N. The absolute configuration of the endo form
is thus CS. The AS diastereoisomer (exo form) is not
observed either in the solid state or in solution (vide supra).
The chiral preference of the metal site is of steric origin and
can be related to the nonplanarity of the xsal2- ligand.15a
(b) (L)2- Esters. A molecular view of 3b is shown in
Figure 3 and selected bond parameters are listed in Table 3.
The metal atom is displaced by 0.28 Å toward the O(1) atom
from the equatorial plane of N(1), N(2), O(2), and O(4). The
psal- ligand is made up of two planar parts (mean deviation,
∼0.02 Å) C(1)-C(6), N(1) and C(7)-C(13), O(2), N(2), the
dihedral angle between them being 19.7°. The V(L2) frag-
ment (minus But groups) is a virtually perfect plane (mean
deviation <0.01 Å). The phenoxidic V-O(2) and V-O(4)
distances are ∼0.03 Å longer than the corresponding
distances (V-O(3) and V-O(5), respectively) in 2f. The
phenoxidic V-O(3) bond lying trans to V-O(1) is relatively
long but it is still shorter by 0.22 Å than the corresponding
phenolic bond V-O(4)H in 2f.
(B) Structures. (a) (HL)- Esters. The structures of VO-
(gsal)(HL1), 2a, and VO(vsal)(HL2), 2f, have been deter-
mined and molecular views are shown in Figures 1 and 2.
Selected bond parameters are listed in Table 2.
In the distorted octahedral VO5N coordination sphere the
metal atom is displaced toward the O(1) atom from the
equatorial plane of O(2), O(3), O(5), and N atoms by ∼0.35
Å in both 2a and 2f. The V-O bond lengths span the range
1.58-2.40 Å, the shortest and the longest being V-O(1)
and V-O(4), respectively, lying trans to each other. The
phenoxidic oxygen O(5) of HL- is more strongly coordinated
than the phenoxidic oxygen O(3) of xsal2-.
The meridionally disposed tridentate ligand is made up
of two planar segments (mean deviation, ∼0.02 Å) viz.
OC6H4CHN and CCO2. The dihedral angle between them is
37.3° in 2a and 49.9° in 2f. In VO(xsal) esters of monoion-
ized aliphatic diols the angle spans14-16 the range 26-40°.
The V(HL1) fragment in 2a and V(HL2) fragment (minus
But groups) in 2f are planar (mean deviation 0.04 Å).
(c) Nature of the Chelated Diol. The C(10)-O(4) and
C(15)-O(5) distances in the V(HL2) fragment of 2f are
1.366(4) and 1.349(5) Å, respectively. The aromatic C-C
(22) Nomenclature of Inorganic Chemistry; Leigh, G. J., Ed.; Blackwell
Scientific Publications: Oxford, UK, 1990; pp 186-187.
4504 Inorganic Chemistry, Vol. 41, No. 17, 2002

Vanadate Esters of Ionized Aromatic 1,2-Diols
Figure 3. Perspective view and atom-labeling scheme of VO(psal)(L2)‚
H2O, 3b. All non-hydrogen atoms are represented by their 30% probability
ellipsoids.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
VO(psal)(L2)‚H2O, 3b
Figure 4. Hydrogen bonding in crystalline (a) VO(gsal)(HL1), 2a, and
(b) VO(vsal)(HL2), 2f, represented in ball-and-stick form (selected atoms
only).
distance
angles
O(1)-V-O(4)
V-O(1)
V-O(2)
V-O(3)
V-O(4)
V-N(1)
V-N(2)
1.572(3)
1.898(3)
2.134(3)
1.861(3)
2.099(4)
2.044(3)
1.275(5)
1.319(4)
1.385(6)
1.364(6)
1.392(6)
1.366(6)
1.398(5)
1.403(6)
97.32(14)
99.4(2)
98.76(13)
101.5(2)
159.38(13)
86.38(14)
94.2(2)
93.40(13)
160.36(14)
77.1(2)
172.29(14)
76.93(12)
86.63(13)
83.50(13)
81.14(13)
O(1)-V-O(2)
O(4)-V-O(2)
O(1)-V-N(2)
O(4)-V-N(2)
O(2)-V-N(2)
O(1)-V-N(1)
O(4)-V-N(1)
O(2)-V-N(1)
N(2)-V-N(1)
O(1)-V-O(3)
O(4)-V-O(3)
O(2)-V-O(3)
N(2)-V-O(3)
N(1)-V-O(3)
Table 4. Electrochemical Data in MeCN
esters
E
1/2, Va (∆Ep, mV)b
Epa, Vc
VO(gsal)(HL1), 2a
VO(asal)(HL1), 2b
VO(vsal)(HL1), 2c
VO(gsal)(HL2), 2d
VO(asal)(HL2), 2e
VO(vsal)(HL2), 2f
VO(psal)(L1), 3a
-0.60(80)
1.02
1.10
1.12
0.98
1.08
1.04
0.76
0.66
-0.77(300)
-0.78(260)
-0.73(180)
-0.84(320)
-0.81(180)
-0.29(100)
-0.40(160)
O(3)-C(14)
O(4)-C(19)
C(14)-C(15)
C(15)- C(16)
C(16)-C(17)
C(17)-C(18)
C(18)-C(19)
C(14)-C(19)
VO(psal)(L2)‚H2O, 3b
a Calculated as the average of anodic and cathodic peak potentials. b Peak-
to-peak separation. c Anodic peak potential.
lengths (Table 2) are normal. The ligand is thus chelated in
the catecholate form. The expected C-O lengths in catecho-
late and semiquinonate forms are 1.35 and 1.29 Å, respec-
tively.1,6,23,24 The V(HL1) fragment in 2a is very similar
(Table 2).
(d) Hydrogen Bonding. Many of the hydrogen atoms of
2a and 2f were directly observed in difference Fourier maps,
and those include the crucial phenolic hydrogen atoms which
are explicity shown in Figures 1 and 2. The O(4) atom is
strongly hydrogen bonded to the uncoordinated carboxyl
oxygen atom, O(6), of a neighboring molecule (O(4)‚‚‚O(6),
2.596(5) Å in 2a and 2.582(5) Å in 2f), thus generating
infinite chains (Figure 4).
In contrast, the V(L2) fragment in 3b is a resonance hybrid
2
of the catecholate (specified as VV(LC )) and semiquinonate
2
(VIV(LS )) canonical forms. The C(14)-O(3) and C(19)-
O(4) distances are 1.275(5) and 1.314(4) Å, respectively.
The C(15)-C(16) and C(17)-C(18) bonds (average length
1.36 Å) are significantly shorter than the other four C-C
bonds (average, length 1.39 Å), Table 3. Semiquinone
contribution has earlier been documented in two other VO-
(L) salicylaldimine chelates.12a
In summary, the coligand controls the protic and electronic
nature of the metal-diol fragment. In 2 the diionized
salicylaldimine (xsal2-) stabilizes the monoionized catecho-
late form. In 3 the monoionized salicylaldimine (psal-)
promotes diol diionization attended with internal fractional
electron transfer expressed in the form of a VIV(LS) contribu-
tion.
In 3b hydrogen bonding occurs between the phenoxidic
O(3) atom and the aqua O(5) atom forming discrete hydrated
molecules, Figure 3. The bonding (O(3)‚‚‚O(5), 2.836(4) Å)
is significantly weaker than those in 2a and 2f.
(C) Redox Activity. (a) Cyclic Voltammetry. Both 2 and
3 display a quasireversible reductive one-electron response
vanadium(V)/vanadium(IV) at negative potentials and a
virtually irreversible oxidative response catecholate/semi-
quinonate at positive potentials. Reduction potential data are
listed in Table 4 and a pair of representative voltammograms
are displayed in Figure 5. The two responses occur respec-
tively at more negative and more positive potentials in 2
compared to those in 3. The processes are thus thermody-
namically less favorable in 2 than in 3, consistent with both
the higher level of ligand protonation in 2 and the contribu-
tion of the VIV(LS) form to the ground state in 3.
(23) Carngo, O.; Castellani, C. B.; Djinovic, K.; Rizzi, M. J. Chem. Soc.,
Dalton Trans. 1992, 837.
(24) Bag, N.; Lahiri, G. K.; Basu, P.; Chakravorty, A. J. Chem. Soc., Dalton
Trans. 1992, 113.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4505

Baruah et al.
level and the reaction of eq 1 can start over again. A catalytic
cycle for the oxidation of H2L2 into Lq2 by oxygen, eq 3, is
readily realized by combining the two reactions. It is an
unusual catecholase reaction in which the active
V2O3(vsal)2 + 2H2L2 f 2VO(vsal)(HL2) + H2O (2)
2H2L2 + O2 f 2 Lq2 + 2H2O
(3)
intermediate, VO(vsal)(HL2), and the catalyst, V2O3(vsal)2,
are vanadium complexes. In biological catecholase reactions
the active sites are generally copper based.25 Vanadium(III,
IV, V) species have been documented to mediate the
oxidation of 1,2-diphenols giving mixtures of products
including semiquinone, quinone, muconic acid anhydride,
and 2-pyrone.4,5 Vanadium-promoted clean catecholase reac-
tions such as those reported here are rare.6
Figure 5. Cyclic voltammograms of (- - -) VO(gsal)(HL2), 2d, and (-)
VO(psal)(L2)‚H2O, 3b, in MeCN.
The VO(psal)(L) species are inert to oxygen in solution.
This is somewhat surprising since electrochemically the diol
ligand in 3 is more easily oxidizable than that in 2. It is
believed that the hydrogen atom of the coordinated HL-
ligand plays a crucial role in the initial binding of the oxygen
molecule. Oxygen binding via hydrogen bridges has been
documented among metalloenzymes.26 The propensity of the
coordinated HL- hydrogen to form a strong hydrogen bond
was cogently revealed in the structures of the VO(xsal)(HL)
species (Figure 4). The intimate mechanism of the subsequent
events in the oxidation process, eq 1, is unclear at present.
Concluding Remarks
The main findings of the present work will now be
recapitulated. Esters of the type VO(xsal)(HL), 2, incorporat-
ing the rarely observed chelation of monoionized aromatic
1,2-diols have been isolated and characterized along with
the diionized diol esters VO(psal)(L), 3. The coligand
influences the protic and electronic nature of the diol
fragment, which is bonded as a catechol in 2 and as a
catechol-semiquinone hybrid in 3.
The type 2 species with chiral amino acid residues occur
exclusively in the endo form, which is selectively stabilized
by the folded structure of the salicylaldimine coligand. The
phenolic OH group in 2 is strongly hydrogen bonded to a
carboxylic oxygen atom of a neighboring molecule, thus
forming a chain in the lattice.
The type 2 species are electrochemically more difficult to
reduce (VV f VIV) as well as to oxidize (catecholate f
semiquinonate) than the type 3 species. Yet only 2 is oxidized
spontaneously and quantitatively by O2 in solution furnishing
the corresponding quinone, Lq, and V2O3(xsal)2, which is
reconverted to 2 upon addition of H2L. These reactions
provide a method for catalytic oxidation (catecholase reac-
tion) of H2L to Lq. It is proposed that the O2 molecule is
Figure 6. (a) Time evolution spectra (visible region) of VO(vsal)(HL2),
2f, in O2-saturated acetone solution at 295 K (At is absorbance). (b) Time
evolution of tert-butyl signals in 2f in O2-saturated acetone-d6. The signals
at δ 1.25 and 1.26 due to free Lq2 and those at δ 1.19 and 1.51 due to
coordinated (HL2)- respectively grow and decay with time.
Finally, the vanadium(V)/vanadium(IV) response in ali-
phatic species of type 1 incorporating xsal2- coligands lies
near -0.25 V.14,15 The aromatic diol in 2 thus promotes
superior redox stability to vanadium(V).
(b) Reaction with Oxygen. The VO(xsal)(HL) esters are
stable in the solid state under ambient conditions. Their dark
solutions, which are otherwise stable, get progressively
bleached in the presence of oxygen. The only products
isolated from the yellowish brown solutions are the dimeric
vanadyl chelate V2O3(xsal)218b and the o-quinone correspond-
ing to the (HL)- ligand. The reaction has been quantitated
in the case of VO(vsal)(HL2) in acetone solution. The
experimentally determined mass balance corresponds to eq
1. The progress of the reaction is observable in both
1
electronic and H NMR spectra (Figure 6). In oxygen-
2VO(vsal)(HL2) + O2 f V2O3(vsal)2 + 2Lq2 + H2O (1)
(25) (a) Bioinorganic Chemistry of Copper; Karlin, K. D., Tyeklar, Z., Eds.;
Chapman and Hall: New York, 1993. (b) Solomon, E. I.; Baldwin,
M. J.; Lowery, M. D. Chem. ReV. 1992, 92, 521. (c) Kitajima, N.;
More-Oka, Y. Chem. ReV. 1994, 94, 734.
(26) (a) Perutz, M. F.; Fermi, G.; Luisi, B.; Shaanan, B.; Liddington, R.
C. Acc. Chem. Res. 1987, 20, 309. (b) Shaanan, B. J. Mol. Biol. 1983,
171, 31. (c) Stenkamp, R. E. Chem. ReV. 1994, 94, 715.
saturated acetone the pseudo-first-order rate constant is found
to be 5.64 × 10-4 s-1 at 295 K.
Upon adding free H2L2 to the solution during the reaction
or after its completion the ester is regenerated to the original
4506 Inorganic Chemistry, Vol. 41, No. 17, 2002

Vanadate Esters of Ionized Aromatic 1,2-Diols
Found: C, 59.48; H, 5.97; N, 3.09. UV-vis (λmax, nm (ꢀ, M-1 cm-1
)
initially attached to 2 via hydrogen bonding with the
coordinated HL- ligand. Further work is in progress.
Me2CO solution): 638 (3530); 371 (1370). IR (KBr, cm-1): ν(VdO)
1
993; ν(CHdN) 1630; ν(OH) 2533. H NMR (δ (J, Hz) (CD3)2CO):
4.70 (H(2A), br); 5.22 (H(2B), br); 8.97(H(3), s); 7.72 (H(5), d,
7.8); 7.02 (H(6), t, 7.4); 7.62 (H(7), t, 7.8); 6.87 (H(8), d, 8.4);
7.01 (H(11), s); 6.74 (H(13), s); 1.24 (C(16)Me3, s); 1.53 (C(20)-
Me3, s); 10.28 (OH, br).
Experimental Section
Materials. VIVO(xsal)(H2O),17 VOV(xsal)(OMe)(HOMe),18 VO-
(acac)2,19 and tetraethylammonium perchlorate (TEAP)27 were
prepared by reported methods. 2-Picolylamine, salicylaldehyde, and
catechols were obtained from Fluka and Aldrich. All other
chemicals and solvents were of analytical grade and were used as
received.
VO(vsal)(HL1), 2c. The procedure is similar to that for 2d but
the initial washing of the product was done with water (to remove
excess H2L1) and then with a minimum volume of cold methanol.
Yield: 75%. Anal. Calcd For C18H18NO6V: C, 54.69; H, 4.59; N,
3.54. Found: C, 54.78; H, 4.41; N, 3.67. UV-vis (λmax, nm, Me2-
CO solution): 602; 378. IR (KBr, cm-1): ν(VdO) 997; ν(CHdN) 1612;
Physical Measurements. UV-vis spectral measurements were
carried out with a Shimadzu UVPC 1601 spectrometer fitted with
thermostated cell compartments. IR spectra were measured with a
1
ν(OH) 2533. H NMR (δ (J, Hz) (CD3)2SO): 4.23 (H(2), br); 8.94
Nicolet Magna IR750 Series II spectrometer. Proton NMR and 51
V
(H(3), s); 7.69 (H(5), d, 7.3); 6.96 (H(6), t, 6.0); 7.51 (H(7), t, 8.3);
6.79 (H(8), d, 8.4); 6.58 (H(11), H(12), m); 6.70 (H(13), H(14),
m); 3.14 (H(16), m); 1.13 (C(16)Me(17), d, 6.6); 1.02 (C(16)Me-
(18), d, 6.7).
VO(asal)(HL1), 2b. The procedure is the same as for 2c. Yield:
80%. Anal. Calcd for C16H14NO6V: C, 52.33; H, 3.84; N, 3.81.
Found: C, 52.47; H, 3.95; N, 3.91. UV-vis (λmax, nm, Me2 CO
solution): 591; 373. IR (KBr, cm-1): ν(VdO) 998; ν(CHdN) 1619; ν(OH)
2535. 1H NMR (δ (J, Hz) (CD3)2SO): 4.70 (H(2), br); 9.02 (H(3),
s); 7.70 (H(5), d, 6.8); 6.99 (H(6), t, 6.0); 7.57 (H(7), t, 8.3); 6.83
(H(8), d, 8.2); 6.59 (H(11), H(12), m); 6.71 (H(13), H(14), m);
1.68 (C(2)Me, d, 5.3).
NMR spectra were respectively recorded on a Bruker FT 300 MHz
spectrometer and a Varian spectrometer at 78.8 MHz (VOCl3
external reference). The atom numbering scheme used for 1H NMR
is the same as that used in crystallography. Spin-spin structures
are abbreviated as follows: s, singlet; d, doublet; t, triplet; m,
multiplet; br, broad. Cyclic voltammetry was performed (scan rate,
50 mVs-1) in MeCN solution (0.1 M TEAP) under nitrogen
atmosphere on a PAR 370-4 electrochemical system,28 using a
platinum working electrode with reference to a saturated calomel
electrode (SCE). A Perkin-Elmer 2400 elemental analyzer was used
for microanalysis (C, H, N).
Preparation of Esters. The VO(xsal)(HL) and VO(psal)(L)
esters were prepared by general methods starting from VO(xsal)-
(H2O)/VO(xsal)(OMe)(HOMe) and VO(acac)2, respectively. Details
are given below for representative cases.
VO(gsal)(HL1), 2a. The procedure is the same as for 2c. Yield:
92%. Anal. Calcd for C15H12NO6V: C, 51.01; H, 3.42; N, 3.97.
Found: C, 50.87; H, 3.36; N, 3.82. UV-vis (λmax, nm, Me2CO
solution): 578; 376. IR (KBr, cm-1): ν(VdO) 992; ν(CHdN) 1627;
VO(vsal)(HL2), 2f. To a stirred methanolic solution (10 mL) of
VO(vsal)(H2O) (0.20 g, 0.66 mmol) was added a slight excess of
H2L2 (0.219 g, 0.99 mmol). The deep blue solution thus obtained
was kept undisturbed at room temperature to evaporate slowly under
nitrogen atmosphere. After 12 h a dark crystalline solid was
obtained, which was washed with hexane (to remove excess H2L2)
and then with a minimum volume of cold methanol. It was finally
dried over fused CaCl2 in vacuo. Yield: 0.240 g (72%). Anal. Calcd
for C26H34NO6V: C, 61.53; H, 6.75; N, 2.76. Found: C, 61.57; H,
6.68; N, 2.72. UV-vis (λmax, nm (ꢀ, M-1 cm-1) Me2CO solution):
647 (3650); 376 (1430). IR (KBr, cm1): ν(VdO) 996, ν(CHdN) 1644;
ν
(OH) 2492. 1H NMR (δ (J, Hz) (CD3)2SO): 4.71 (H(2A), d, 18.9);
5.16 (H(2B), d, 15.9); 8.98 (H(3), s); 7.33 (H(5), d, 6.0); 7.02 (H(6),
t, 6.0); 7.61 (H(7), t, 6.6); 6.91 (H(8), d, 7.8); 6.67 (H(11), H(12),
m); 6.78 (H(13), H(14), m).
VO(psal)(L1), 3a. To an oxygen-saturated methanolic solution
(20 mL) of VO(acac)2 (0.10 g, 0.38 mmol) and Hpsal (0.08 g, 0.38
mmol) was added H2L1 (0.084 g, 0.38 mmol). The violet solution
was stirred for 2 h and then kept undisturbed to evaporate at room
temperature in air. The dark violet crystalline product that deposited
within 1 day was filtered off, washed with cold methanol, and dried
over fused CaCl2 in vacuo. Yield: 0.128 g (88%) Anal. Calcd for
C19H15N2O4V: C, 59.08; H, 3.91; N, 7.25. Found: C, 59.17; H, 3.95;
N, 7.32. UV-vis (λmax, nm (ꢀ, M-1 cm-1) Me2CO solution): 850
(4620); 525 (3600); 375 (2950). IR (KBr, cm-1): ν(VdO) 950;
1
ν(OH) 2580. H NMR (δ (J, Hz) (CD3)2CO): 4.23 (H(2), br); 8.91
(H(3), s); 7.71 (H(5), d, 7.6); 7.01 (H(6), t, 6.7); 7.59 (H(7), t, 7.4);
6.83 (H(8), d, 8.4); 6.97 (H(11), s); 6.62 (H(13), s); 2.79 (H(16),
br); 1.11 (C(16)Me2, d, 6.7); 1.19 (C(19)Me3, s); 1.51 (C(23)Me3,
s); 10.03 (OH, br).
ν
(CHdN)1622. 1H NMR (δ (J, Hz) CDCl3): 8.46 (H(1), d, 5.1); 7.54
(H(2), H(4), H(9), H(11), m); 7.91 (H(3), t, 6.6); 5.59 (H(6A), d,
19.2); 5.44 (H(6B), d, 18.9); 8.54 (H(7), s); 6.83 (H(10), t, 7.2);
7.06 (H(12), d, 8.7); 6.72 (H(15), H(18), m); 6.30 (H(16), H(17),
m).
VO(asal)(HL2), 2e. Here VO(asal)(OMe)(HOMe) was used as
the starting material in place of VO(asal)(H2O) and the reaction
medium was acetonitrile instead of methanol. The rest of the
procedure remains the same as above. Yield: 82%. Anal. Calcd
for C24H30NO6V: C, 60.12; H, 6.31; N, 2.92. Found: C, 60.23; H,
6.39; N, 3.10. UV-vis (λmax, nm (ꢀ, M-1 cm-1) Me2CO solution):
641 (3380); 373 (1290). IR (KBr, cm-1): ν(VdO) 991; ν(CHdN) 1619;
VO(psal)(L2)‚H2O. 3b. The procedure is the same as for 3a.
Yield: 89%. Anal. Calcd for C27H33N2O5V: C, 62.79; H, 6.44; N,
5.42. Found: C, 62.86; H, 6.47; N, 5.51. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) Me2CO solution): 855 (4760); 550 (5150); 375 (2900).
1
ν(OH) 2475. H NMR (δ (J, Hz) (CD3)2CO): 4.69 (H(2), br); 8.96
1
IR (KBr, cm-1): ν(VdO) 960; ν(CHdN) 1625. H NMR (δ (J, Hz)
(H(3), s); 7.68 (H(5), d, 7.4); 7.01 (H(6), t, 7.6); 7.58 (H(7), t, 7.2);
6.85 (H(8), d, 8.2); 6.98 (H(11), s); 6.67 (H(13), s); 1.79 (C(2)Me,
br); 1.21 (C(17)Me3, s); 1.51 (C(21)Me3, s); 10.15 (OH, br).
VO(gsal)(HL2), 2d. The method is the same as for 2f. Yield:
76%. Anal. Calcd for C23H28NO6V: C, 59.36; H, 6.06; N, 3.01.
CDCl3): 8.46 (H(l), H(7), br); 7.44(H(2), H(4), H(11), m); 7.88
(H(3), t, 7.5); 5.59 (H(6A), d, 18.0); 5.38 (H(6B), d, 18.0); 7.34
(H(9), d, 6.0); 6.75 (H(10), t, 7.5); 7.04 (H(12), d, 9.0); 6.23 (H15),
H(17), br); 1.52 (C(24)Me3, br), 1.20 (C(20)Me3, br).
Reaction of VO(vsal)(HL2) with Oxygen: Isolation of Prod-
ucts. A solution of 2f (0.10 g, 0.2 mmol) in O2-saturated acetone
solution (50 mL) was taken in a two-necked flask fitted with a
balloon filled with O2 and stirred for 10 h. At the end of the reaction
(reaction solution yellowish brown) H2L2 (0.044 g, 0.2 mmol) was
(27) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry for
Chemists; Wiley: New York, 1974; p 212.
(28) Lahiri, G. K.; Bhattacharya, S.; Ghosh, B. K.; Chakravorty, A. Inorg.
Chem. 1987, 26, 4324.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4507

Baruah et al.
Table 5. Crystallographic Data for VO(gsal)(HL1), 2a, VO(vsal)(HL2),
2f, and VO(psal)(L2)‚H2O, 3b
added and the original deep blue color was reestablished. This
solution was again bleached with O2 as above and two more
instalments (each 0.044 g) of H2L2 were added repeating the events.
The solvent was then removed and the solid thus obtained was dried
in a vacuum over fused CaCl2. The dry mass was extracted with
petroleum ether (60-80 °C) leaving pure V2O3(vsal)2 as the residue.
Removal of solvent from the filtrate afforded the quinone, Lq2.
The observed yields of Lq2 and V2O3(vsal)2 are 0.169 and 0.056 g,
respectively. Theoretical yields for complete conversion are 0.174
and 0.058 g, respectively. The complex V2O3(vsal)2 has been
characterized. Anal. Calcd for C24H26N2O9V2: C, 48.99; H, 4.45;
N, 4.76. Found: C, 49,03; H, 4.52; N, 4.71. IR (KBr, cm-1): ν(VdO)
2a
2f
3b
chemical formula
fw
space group
a, Å
b, Å
C15H12NO6V
353.20
C26H34NO6V
507.48
C27H33N2O5V
516.49
P21/n (no. 14) R3 (no.146)a
P21/n (no. 14)
9.382(7)
11.617(5)
23.979(11)
97.31(5)
2592(2)
4
10.436(4)
9.757(2)
14.410(5)
101.70(3)
1436.8(8)
4
23.754(5)
23.754(5)
12.836(3)
c, Å
â, deg
V, Å3
Z
6272(2)
9
T, °C
λ, Å
20
0.71073
1.633
20
20
0.71073
1.323
0.71073
1.209
3.92
1
992; ν(CHdN) 1686. H NMR (δ (J, Hz) CDCl3): 4.02 (H(2A), br);
F
calcd, mg m-3
µ(Mo KR), cm-1
7.22
4.22
4.25 (H(2B), br); 8.44 (H(3), s); 7.68 (H(5), d, 6.0); 7.11 (H(6), t,
7.5); 7.64 (H(7), t, 9.0); 7.09 (H(8), d, 9.0); 2.32, 2.54 (H(10), m);
0.97, 1.04 (C(10)Me2, d, 6.0). The quinone, Lq2, was characterized
R1,b wR2c [I > 2σ(I)] 0.0577, 0.1447 0.0515, 0.1344 0.0589, 0.1402
goodness of fit on F2 1.081
1.089
1.079
1
a Hexagonal axes. b R1 ) ∑|Fo| - |Fc|/∑|Fo|. c wR2 ) [∑w(Fo2 - Fc2)2/
by H NMR.
Synthesis of VO(vsal)(HL2) from V2O3(vsal)2. To a stirred
acetone solution (30 mL) of V2O3(vsal)2 (0.10 g, 0.17 mmol) was
added H2L2 (0.076 g, 0.34 mmol) under N2 atmosphere. After 0.5
h the reaction mixture was concentrated to 10 mL and then kept at
4 °C. The precipitate thus obtained was filtered off and washed
with hexane and dried in vacuo affording 0.079 g (92%) of VO-
(vsal)(HL2).
∑w(Fo )2]1/2
.
2
and empirical absorption correction was performed on each set of
data on the basis of azimuthal scans29 of six reflections.
All calculations of data reduction, structure solution, and
refinement were done with the programs of SHELXTL, Version
5.03.30 The three structures were solved by direct methods and were
refined by full-matrix least-squares on F2. All non-hydrogen atoms
were refined anisotropically. The number of hydrogen atoms located
directly in difference Fourier maps for compounds VO(gsal)(HL1)
and VO(vsal)(HL2) was 8 and 23, respectively. The remaining
hydrogen atoms were included in calculated positions. Significant
crystal data are listed in Table 5.
Rate Measurements. The initial O2 concentration in O2-saturated
acetone determined by using an oxygen-sensitive electrode was
found to be 0.90 × 10-3 M. The concentration of the complex was
varied in the range (1-4) × 10-5 M. The spectrum of a solution
of VO(vsal)(HL2) in O2-saturated acetone (295 K) was followed
spectrophotometrically (quartz cell path length, 1 cm) by measuring
the absorbance (At) at 647 nm as a function of time, t. The
absorbance AR at the end of 24 h of reaction was also measured.
The values of kobs were obtained from the slope of the linear plot
of -ln(AR - At) versus t. At substrate concentrations of 3.90 ×
10-5, 2.95 × 10-5, and 1.65 × 10-5 M the kobs values were 5.64 ×
10-4, 5.70 × 10-4, and 5.58 × 10-4 s-1 (average 5.64 × 10-4 s-1),
respectively.
X-ray Structure Determination. Crystals of VO(gsal)(HL1),
VO(vsal)(HL2), and VO(psal)(L2)‚H2O were grown by slow
evaporation of their solutions in methanol. Cell parameters were
determined by the least-squares fit of 30 machine-centered reflec-
tions (14 e 2θ e 28°). Data were collected by the ω-scan technique
in the 2θ range 3-50° for all three complexes on a Siemens R3m/V
four-circle diffractometer with graphite-monochromated Mo KR (λ
) 0.71073 Å) radiation at 293 K. Two check reflections after each
198 reflections showed no significant intensity reduction for any
of the crystals. Data were corrected for Lorentz-polarization effects
Acknowledgment. We thank the Department of Science
and Technology and Council of Scientific and Industrial
Research, New Delhi, for financial support. Thanks are due
to Drs. S. P. Rath and K. K. Rajak for performing certain
preliminary experiments.
Supporting Information Available: For VO(gsal)(HL1), VO-
(vsal)(HL2) and VO(psal)(L2)‚H2O crystallographic data, atomic
coordinates and equivalent isotopic coefficients, complete bond
distances and angles, anisotropic thermal parameters, hydrogen atom
positional parameters. The crystallographic files, in CIF format are
available free of charge via the Internet at http://www.pubs.acs.org.
IC020259D
(29) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351.
(30) Sheldrick, G. M. SHELXTL, Version 5.03; Siemens Analytical
Instruments, Inc: Madison, WI, 1994.
4508 Inorganic Chemistry, Vol. 41, No. 17, 2002