Biphenyl derived Schiff-base vanadium(v) complexes with pendant OH-groups - Structure, characterization and hydrogen peroxide mediated sulfide oxygenation

Article abstract of DOI:10.1039/b403600j

A series of mononuclear oxovanadium(v) complexes of tridentate Schiff bases HL¹-HL⁴ and H₂L⁵-H ₂L⁸ derived from 6-phenylsalicylaldehyde and 6-(2-hydroxyphenyl)salicylaldehyde and four different amines was synthesized. The systematically selected ligands contain aliphatic or aromatic nitrogen, or alkoxy- and phenoxy-oxygen as third donor atom. The complexes were characterised by spectroscopic methods in solution and the solid state. Single-crystal X-ray analyses were performed with VO₂L¹ (1), VO ₂L³·1/2EtOH (3), VO₂L⁴ (4), VO(OiPr)L⁷·iPrOH (7), VO(OiPr)L⁸ (8) and H ₂L⁸. For all compounds the vanadium(v) cores contain distorted tetragonal pyramidal geometry around the dioxo- and oxovanadium site at which the N₂O- and NO₂-donor ligands bind equatorially. Complexes 1 and 3 display intramolecular hydrogen bonding of the pendant hydroxyphenyl group to a coordinated oxygen trans to a nitrogen atom and therefore serve as suitable models for the native site of vanadium dependent haloperoxidases. Variable-temperature ¹H NMR spectra revealed significant hydrogen bond interaction in acetonitrile solution. In situ prepared catalysts are active for hydrogen peroxide mediated oxygenation of ethyl phenyl sulfide and showed complete conversion of the substrate to ethyl phenyl sulfoxide, together with small amounts of the corresponding sulfone, as detected by GC/MS after 10 min. The complex of H₂L⁷ turned out to be most efficient while HL¹ and HL² were completely inactive. Catalysis is supported by the pendant OH group in the complex of HL³, the catalyst is twice as active as the complex of HL ⁴.

Full text of DOI:10.1039/b403600j

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F U L L P A P E R
Biphenyl derived Schiff-base vanadium(V) complexes with
pendant OH-groups—structure, characterization and hydrogen
peroxide mediated sulfide oxygenation
Patrick Plitt, Hans Pritzkow and Roland Krämer*
Anorganisch-Chemisches Institut der Ruprecht-Karls-Universität Heidelberg,
Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany.
E-mail: roland.kraemer@urz.uni-heidelberg.de; Fax: (+49) 6221-54-8599;
Tel: (+49) 6221-54-8438
Received 9th March 2004, Accepted 3rd June 2004
First published as an Advance Article on the web 25th June 2004
A series of mononuclear oxovanadium(V) complexes of tridentate Schiff bases HL1–HL4 and H2L5–H2L8 derived from
6-phenylsalicylaldehyde and 6-(2-hydroxyphenyl)salicylaldehyde and four different amines was synthesized. The
systematically selected ligands contain aliphatic or aromatic nitrogen, or alkoxy- and phenoxy-oxygen as third donor atom.
The complexes were characterised by spectroscopic methods in solution and the solid state. Single-crystal X-ray analyses
were performed with VO2L1 (1), VO2L3·ꢀEtOH (3), VO2L4 (4), VO(OiPr)L7·iPrOH (7), VO(OiPr)L8 (8) and H2L8. For all
compounds the vanadium(V) cores contain distorted tetragonal pyramidal geometry around the dioxo- and oxovanadium
site at which the N2O- and NO2-donor ligands bind equatorially. Complexes 1 and 3 display intramolecular hydrogen
bonding of the pendant hydroxyphenyl group to a coordinated oxygen trans to a nitrogen atom and therefore serve as
suitable models for the native site of vanadium dependent haloperoxidases. Variable-temperature 1H NMR spectra revealed
significant hydrogen bond interaction in acetonitrile solution. In situ prepared catalysts are active for hydrogen peroxide
mediated oxygenation of ethyl phenyl sulfide and showed complete conversion of the substrate to ethyl phenyl sulfoxide,
together with small amounts of the corresponding sulfone, as detected by GC/MS after 10 min. The complex of H2L7
turned out to be most efficient while HL1 and HL2 were completely inactive. Catalysis is supported by the pendant OH
group in the complex of HL3, the catalyst is twice as active as the complex of HL4.
studied several functional models in order to gain insight into the
Introduction
mechanism of sulfide oxygenation.20 Aryl alkyl sulfides were used
The interest in vanadium coordination chemistry has been
as substrates both in the bromoperoxidase and complex catalyzed
promoted by its presence in biological systems,1,2 its ability to
oxo transfer.21 Advantages of these substrates is their commercial
catalyze oxidations and oxo transfer reactions3 and its inhibitory
availability and the fast reaction with H2O2 and hydroperoxides.
capabilities.4 Vanadium dependent chloroperoxidase isolated from
Here we describe a series of oxovanadium(V) complexes as
Curvularia inaequalis and vanadium bromoperoxidases from
catalysts for the oxygenation of sulfides with systematic variation
Corallina officinalis and Ascophyllum nodosum contain trigonal
of one donor arm in the equatorially coordinated, biphenyl-derived
bipyramidal coordination spheres including apical histidine and
ligand moiety and an additional pendant OH-group in the distant
a meridionally bound oxo group as revealed by X-ray crystallo-
aryl subunit. The first modification is important since electronic
graphic analyses.5–7 In the presence of hydrogen peroxide the
properties strongly depend on the nature of the ligand donor set
geometry changes towards a tetragonal pyramid involving an
and are crucial for catalytic activity.22 The latter modification is of
axially coordinated oxo function and equatorially bound peroxide
particular interest since hydrogen bonding towards the reaction site
and histidine.8 Multiple hydrogen bonding has been found between
seems to be essential for peroxide activation during the reaction
oxo or hydroxo ligands and the apoprotein both in the native and
with bromide and sulfide and hence for advanced enzyme and
in the active coordination environment. Structural models for
complex catalysis.23 The catalytic cooperation of metal ions and
the active site have been synthesized by Casny and Rehder9 and
H-bond donating functional groups is a common functional motif
Butler and coworkers.10 The capability of vanadium and hydrogen
in metalloenzyme active sites. It has been successfully mimicked
peroxide mediated activation of bromide towards bromination of
for phosphoryl and acyl group transfer enzymes24,25 but is less well
organic substrates was demonstrated.11 In the presence of molecular
explored by low molecular weight models of redox active enzymes.
oxygen, hydrogen peroxide or organic hydroperoxides, vanadium
The effects of the hydrogen bonding site and the electronic
complexes have also been found to accelerate the oxygenation
influence of the donor set on the biomimetic and catalytic artificial
of aromatic and aliphatic hydrocarbons12 and oxidize sulfides to
sulfoxidase activity is studied by spectroscopic and functional
form sulfoxides,13,14 respectively. The enantioselective synthesis
investigations on in situ prepared oxo peroxo vanadium(V) com-
of chiral sulfoxides was promoted by oxovanadium complexes
plexes. With the attached aryl moiety the complexes exclusively
of Schiff bases up to 78% ee15 and more recently by compounds
feature monomeric species.
derived from chiral diethanol amine ligands.16 It has recently
been demonstrated that dinuclear vanadium catalysts with chiral
binaphthyl units mediate the enantioselective oxidative coupling
Experimental
of 2-naphthols to binaphthols.17 Monomeric vanadium(V)
complexes are known as potent inhibitors for various enzymes.
Pentavalent species act as transition state analogues of the
pentagonal bipyramidal intermediate in phosphate hydrolysis.18
Tri- and tetradentate amine and imine ligands have shown con-
siderable abilities to regulate the properties of vanadium model
complexes, indicating that the oxo transfer reaction strongly
depends on the nature of the ligand set.19 Pecoraro and co-workers
The starting materials were purchased from commercial sources
(Acros, Aldrich, Fluka) and used without further purification. The
precursor aldehydes 2,2′-dihydroxybiphenyl-3-carbaldehyde26 and
2-hydroxybiphenyl-3-carbaldehyde27 were prepared according to
literature procedures. 1H, 13C and 51V NMR spectra were recorded
on a Bruker DRX 200 instrument, chemical shifts are given in
ppm referenced to the solvent peak or to external VOCl3 (for 51V
experiments). EI and FAB mass spectra were obtained on a JEOL
2 3 1 4
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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JMS-700 spectrometer; electrospray mass spectra were recorded on
a Waters ESI-Q-TOF Ultima API device, the sample concentration
was 0.1 mM in acetonitrile (mobile phase: acetonitrile; flow rate:
50 L min−1). Infrared spectra (KBr pellets) were recorded on
a Perkin Elmer 983 G spectrometer. Elemental analyses were
conducted by the Microanalytisches Laboratorium des Organisch-
Chemischen Instituts der Universität Heidelberg. GC/MS or GC
data were collected on an Agilent GC6890/ MS5973 equipped
with an HP-5MS capillary column (length: 30 m, inner diameter:
0.25 mm) with helium as carrier gas or on a Shimadzu GC-2100
equipped with an SGE BPX-5 capillary column (25 m, 0.22 mm)
with nitrogen as carrier gas. Yields of the components refer to
diphenyl ether as internal standard.
3-[(Pyridin-2-ylmethylimino)methyl]biphenyl-2,2′-diol
(HL3). 1H NMR (200 MHz, CDCl3):  4.96 (s, 2 H, CH2), 6.85–7.75
(m, 10 H, Har), 8.54 (s, 1 H, CHN), 8.60 (d, J = 4.4 Hz, 1 H, Hpy).
13C{1H} NMR (50 MHz, CDCl3):  61.29 (t, CH2), 117.33, 118.37,
118.69, 120.73, 122.12, 122.96, 126.42, 129.15, 129.71, 130.93,
131.95, 136.80, 137.22, 149.72, 154.99, 155.85, 163.34 (Car),
167.14 (d, CHN). HRMS (EI): m/z calc. for C19H16N2O2 (M+):
304.1212; found: 304.1218.
3-[(Pyridin-2-ylmethylimino)methyl]biphenyl-2-ol (HL4). 1H
NMR (200 MHz, CDCl3):  4.96 (s, 2 H, CH2), 6.98 (dd, J = 7.6 Hz,
1 H, Har), 7.17–7.70 (m, 10 H, Har), 8.56 (d, J = 4.2 Hz, 1 H, Hpy),
8.62 (s, 1 H, CHN), 13.93 (br s, 1 H, OH). 13C{1H} NMR (50 MHz,
CDCl3):  64.97 (t, CH2), 118.73, 118.93, 122.05, 122.39, 127.13,
128.12, 129.32, 129.84, 131.09, 136.91, 137.68, 149.38, 157.86,
158.44 (Car), 166.97 (d, CHN). HRMS (EI): m/z calc. for C19H16N2O
(M+): 288.1263; found: 288.1259.
Preparations
General procedure for the preparation28 of the ligands HL1–HL4 and
H2L5–H2L8 (Fig. 1): In a typical Schiff-base condensation 1.0 mmol
of the appropriate amine were added to a stirred solution of an
equimolar amount of 2,2′-dihydroxybiphenyl-3-carbaldehyde or
2-hydroxybiphenyl-3-carbaldehyde in 10 mLof either dry dichloro-
methane or methanol. The resulting yellow solution was refluxed
for 1 h and the solvent was removed under reduced pressure. For
HL1–HL4 and H2L5–H2L6 the products were dried under vacuum to
yield yellow oils quantitatively. For complex syntheses the ligands
were used without further purification. For H2L8 the synthesis and
melting point have been reported previously.29
3-[(2-Hydroxy-1,1-dimethylethylimino)methyl]biphenyl-2,2′-
diol (H2L5). 1H NMR (200 MHz, CDCl3):  1.27 (s, 6 H, CH3), 3.37
(s, 2 H, CH2), 6.69 (dd, J = 7.4 Hz, 1 H, Har), 6.96–7.31 (m, 6 H, Har),
7.55 (d, J = 7.2 Hz, 1 H, Har), 8.07 (s, 1 H, CHN). 13C{1H} NMR
(50 MHz, CDCl3):  23.21, 59.68, 69.13, 115.41, 115.95, 118.99,
120.24, 127.44, 128.83, 130.72, 132.64 133.43, 138.47, 156.00,
163.25, 171.09. HRMS (EI): m/z calc. for C17H19NO3 (M+):
285.1365; found: 285.1382.
3-[(2-Hydroxy-1,1-dimethyl-ethylimino)-methyl]-biphenyl-
1
2-ol (H2L6). H NMR (200 MHz, CDCl3):  1.32 (s, 6 H, CH3),
3.57 (s, 2 H, CH2), 6.94 (dd, J = 7.6 Hz, 1 H, Har), 7.31–7.63
(m, 7 H, Har), 8.43 (s, 1 H, CHN). 13C{1H} NMR (50 MHz, CDCl3):
 23.53 (q, CH3), 60.96 (s, C(CH3)2), 70.97 (t, CH2), 118.19, 118.71,
127.06, 128.15, 129.28, 130.21, 131.15, 133.39, 137.85, 159.69
(s, Car), 162.46 (s, CHN). HRMS (FAB+): m/z calc. for C17H20NO2
([M + H]+): 270.1494; found: 270.1445.
3-[(2-Hydroxyphenylimino)methyl]biphenyl-2,2′-diol (H2L7).
The orange precipitate was isolated by filtration and recrystallized
from a 5:1 mixture of toluene and methanol yielding 208 mg (68%)
of an orange microcrystalline powder. 1H NMR (200 MHz, DMSO-
d6):  6.82–7.00 (m, 5 H, Har), 7.12–7.23 (m, 3 H, Har), 7.39 (d,
J = 7.4 Hz, 1 H, Har), 7.47–7.59 (m, 2 H, Har), 9.10 (s, 1 H, CHN),
9.56 (br s, 1 H, OH), 9.95 (br s, 1 H, OH). 13C{1H} NMR (50 MHz,
DMSO-d6):  116.01, 116.44, 117.40, 118.49, 118.70, 119.14,
119.59, 125.20, 128.10, 128.19, 128.29, 131.11, 132.10, 132.60,
135.40, 150.65, 155.01, 161.36, 161.54. MS (FAB+) m/z: 306.1
([M + H]+). Elemental analysis (%) calc. for C19H15NO3: C 74.74,
H 4.95, N 4.59; found: C 74.21, H 4.94, N 4.57.
3-[(2-Hydroxyphenylimino)methyl]biphenyl-2-ol
(H2L8).
Fig. 1 Structures of the Schiff-base ligands HL1–H2L4 and H2L5–H2L8.
The orange precipitate was isolated by filtration and dried in
vacuum yielding 243 mg (84%). Orange prisms suitable for an
X-ray crystallographic analysis were grown from a 5:1 mixture of
toluene and methanol by slow evaporation of the solvent at room
temperature. 1H NMR (200 MHz, CDCl3):  5.79 (br s, 1 H, O′H),
6.94–7.66 (m, 12 H, Har), 8.76 (s, 1 H, CHN), 12.92 (s, 1 H, OH).
13C{1H} NMR (50 MHz, CDCl3):  115.99, 118.20, 119.44, 119.61,
121.03, 127.45, 128.27, 128.88, 129.33, 130.21, 132.18, 134.72,
135.36, 137.21, 149.96, 157.90, 163.84. MS (FAB+) m/z: 290.2
([M + H]+). Elemental analysis (%) calc. for C19H15NO2: C 78.87,
H 5.23, N 4.84; found: C 78.71, H 5.25, N 4.95.
3-[(2-Dimethylaminoethylimino)methyl]biphenyl-2,2′-diol
(HL1). 1H NMR (200 MHz, CDCl3):  2.33 (s, 6 H, CH3), 2.69 (t, 2 H,
CH2, J = 6.2 Hz), 3.74 (t, 2 H, CH2, J = 6.2 Hz), 6.95 (dd, J = 7.7 Hz,
1 H, Har), 7.02 (m, 2 H, Har), 7.28 (m, 4 H, Har), 7.57 (dd, 1 H, Har),
8.27 (s, 1 H, CHN), 13.95 (br s, 1 H, OH). 13C{1H} NMR (50 MHz,
CDCl3):  45.39 (q, CH3), 51.77 (t, NCH2), 58.70 (t, CH2N), 107.57,
116.34, 117.09, 118.83, 120.42, 126.85, 128.97, 130.80, 132.19,
137.52, 155.56 (Car), 166.62 (d, CHN). HRMS (FAB+): m/z calc.
for C17H21N2O2 ([M + H]+): 285.1603; found: 285.1619.
VO2L1 (1) and VO2L2 (2). To a 2-propanol solution of the
ligand (0.5 mmol in 10 mL) was added VO(OiPr)3 (122 mg, 0.5
mmol) and the mixture was stirred at room temperature overnight.
The mixture was allowed to stand at room temperature for slow
evaporation of the solvent and pale yellow needles separated which
were filtered off.
3-[(2-Dimethylaminoethylimino)methyl]biphenyl-2-ol
1
(HL2). H NMR (200 MHz, CDCl3):  2.29 (s, 6 H, CH3), 2.64
(t, 2 H, CH2N, J = 6.7 Hz), 3.73 (t, 2 H, NCH2, J = 6.7 Hz), 6.95
(dd, J = 7.8 Hz, 1 H, Har), 7.23–7.66 (m, 7 H, Har), 8.42 (s, 1 H,
CHN), 14.15 (br s, 1 H, OH). 13C{1H} NMR (50 MHz, CDCl3): 
45.99 (q, CH3), 57.72 (t, NCH2), 60.11 (t, CH2N), 118.61, 119.16,
127.26, 128.31, 129.55, 130.06, 130.95, 133.40, 138.10, 159.03
(Car), 165.95 (d, CHN). HRMS (FAB+): m/z calc. for C17H21N2O
([M + H]+): 269.1654; found: 269.1624.
1. Yield 73 mg (39%). Crystals suitable for X-ray analysis were
grown from a dichloromethane solution by slow evaporation at room
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0
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temperature. 1H NMR (200 MHz, CD3CN):  2.80 (s, 6 H, CH3), 2.99
(t, J = 6.3Hz, 2H, NCH2), 4.19(t, J = 6.3 Hz, 2H, CH2N), 6.93–7.11
(sh, 3 H, Har), 7.25–7.33 (sh, 2 H, Har), 7.64 (d, J = 7.2 Hz, 2 H,
Har), 7.77 (s, 1 H, OH), 8.92 (s, 1 H, CHN). 51V NMR (52.58 MHz,
CD3CN):  −509. IR (KBr):  = 3434, 2357, 1624, 1562, 929, 869,
762 cm−1. Elemental analysis (%) calc. for: C17H19N2O4V: C 55.74,
H 5.23, N 7.65; found: C 53.81, H 5.24, N 7.31.
7. Yield 44 mg (51%). 51V NMR (52.58 MHz, CD3CN):  −547.
IR (KBr):  = 3426, 2372, 1601, 1401, 991, 748 cm−1. HRMS
−
(ESI−): m/z calc. for C19H13NO5V (L7VO2 ): 386.0233; found:
386.0222.
8. Yield 51 mg (62%). 51V NMR (52.58 MHz, CD3CN):  −550.
IR (KBr):  = 3425, 1610, 1469, 988, 744 cm−1. Elemental analysis
(%) calc. for C22H20NO4V: C 63.93, H 4.88, N 3.39; found: C 63.77,
H 4.80, N 3.51.
2. Yield 79 mg (45%). 1H NMR (200 MHz, CD3CN):  2.78 (s,
6 H, CH3), 2.94 (t, J = 6.2 Hz, 2 H, NCH2), 4.13 (t, J = 6.2 Hz, 2 H,
CH2N), 6.96 (dd, J = 7.5 Hz, 1 H, Har), 7.31–7.61 (sh, 8 H, Har),
8.85 (s, 1 H, CHN). 51V NMR (52.58 MHz, CD3CN):  −512. IR
(KBr):  = 3437, 1624, 1557, 1416, 1230, 930, 864, 759, 698 cm−1.
Elemental analysis (%) calc. for C17H19N2O3V: C 58.29, H 5.47, N
8.00; found: C 58.03, H 5.41, N 8.00.
Single-crystal X-ray diffraction
Details of the crystal data and refinement are given in Table 1.
Intensities were collected using a Bruker AXS CCD Smart 1000
diffractometer with graphite-monochromated Mo-K radiation
( = 0.71073 Å). Correction for Lorentz and polarization effects
were applied.Absorption corrections were performed based on mul-
tiple scans of equivalent reflections using the SADABS routine.30
The structures were solved by direct methods (SHELXS-97)31
and refined by full-matrix least squares (SHELXTL NT V.5.1.)32
anisotropically for all non-hydrogen atoms. Complex 3 contains
one disordered ethanol molecule of crystallization per two complex
molecules and complex 7 contains non-metal-coordinated dis-
ordered 2-propanol which is hydrogen bound to the ligand hydroxy
group.
VO2L3 (3) and VO2L4 (4). To a stirred solution of the ligand
(0.2 mmol) in 10 mL of 2-propanol VO(OiPr)3 (47 mg, 0.2 mmol)
was added and the red–brown mixture was kept at room tempera-
ture for 2 h. The solvent was removed in vacuo and the solid was
extracted with a mixture of dichloromethane–EtOH (v/v = 5:1).
The solution was allowed to stand at room temperature for slow
evaporation of the solvent to yield yellow crystals suitable for X-ray
crystallographic analysis.
CCDC reference numbers 233275–233280.
See http://www.rsc.org/suppdata/dt/b4/b403600j/ for crystallo-
graphic data in CIF or other electronic format.
3. Yield: 17 mg (22%). 51V NMR (52.58 MHz, acetone-d6):
 −536. IR (KBr):  = 3445, 2363, 2338, 1624, 1402, 949, 757 cm−1.
HRMS (ESI−): m/z calc. for C19H14N2O4V ([L3VO2 − H]−):
385.0393; found: 385.0350.
Catalytic oxygenation of ethyl phenyl sulfide
4. Yield: 25 mg (34%). 51V NMR (52.58 MHz, acetone-d6):
 −538. IR (KBr):  = 3438, 1616, 1405, 936, 756 cm−1. HRMS
(ESI+): m/z calc. for C19H16N2O3V ([L4VO2 + H]+): 371.0601;
found: 371.0590.
A solution containing equimolar amounts of the ligand and
vanadyl acetylacetonate (1 mol) in 0.2 mL of acetonitrile was
stirred in a glass tube at room temperature for 2 h. Ethyl phenyl
sulfide (200 mol in MeCN) and diphenyl ether (50 mol in
MeCN, as internal standard) were added. The reaction was initi-
ated by addition of 220 mol of H2O2 (1 M in MeCN, prepared
from 30% aqueous hydrogen peroxide) (final concentration of the
catalyst: 0.5 mM; final volume: 2 mL; ratio of metal:ligand:sul-
fide:H2O2 = 1:1:200:220). After defined intervals of time, 50 L
portions of the mixture were diluted in an excess of dichloro-
methane. Quantification and characterization of the corresponding
sulfoxide and sulfone were achieved with GC/MS.
VO(OiPr)L5 (5) and VO(OiPr)L6 (6). VO(OiPr)3 (0.5 mmol)
was slowly added to a solution of an equimolar amount of the
ligand in 15 mL of dichloromethane. The solution immediately
turned dark brown and was stirred at room temperature for
2 h. The solvent was removed in vacuo to yield the product
quantitatively.
1
5. H NMR (200 MHz, CD3OD):  1.17 (d, J = 6.2 Hz, 6 H,
OCHMe2), 1.42 (s, 3 H, NCMeMe), 1.67 (s, 3 H, NCMeMe),
3.94 (sept, J = 6.2 Hz, 1 H, OCHMe2), 4.54 (br d, J = 9.4 Hz, 1 H,
CMe2CHHO), 4.80 (d, J = 9.4 Hz, 1 H, CMe2CHHO), 6.93–7.09
(m, 3 H, Har), 7.20–7.37 (m, 2 H, Har), 7.60–7.65 (m, 2 H, Har), 8.75
(s, 1 H, CHN). 51V NMR (52.58 MHz, CD3OD):  −551, −525. IR
(KBr):  = 3423, 2965, 1627, 1423, 1262, 1061, 984, 802, 753 cm−1.
Results and discussion
Syntheses
The synthesized ligands are phenylated or 2-hydroxyphenylated
derivatives of salicylaldimines. They selectively leave cis
oriented coordination sites at the oxovanadium(V) center
unoccupied. A systematic variation (Fig. 1) of one ligand arm
was achieved with N,N-dimethylethane-1,2-diamine (HL1, HL2),
2-(aminomethyl)pyridine (HL3, HL4), 2-amino-2-methylpropanol
(H2L5, H2L6) and 2-aminophenol (H2L7, H2L8). The ligands HL1–HL4
and H2L5–H2L8 provide N2O- and NO2-donor sets, respectively. The
latter donor set mimics in addition to oxo and hydroxo donors the
metal coordination sphere in vanadate dependent haloperoxidases.
The tridentate ligands were prepared by Schiff-base
condensation of the appropriate primary amine with 2-
hydroxybiphenyl-3-carbaldehyde or 2,2′-dihydroxybiphenyl-
3-carbaldehyde. Single crystals of H2L8 suitable for X-ray
crystallography were obtained upon slow cooling and standing
of the reaction media. The oxovanadium(V) complexes 1–8
were synthesized (Scheme 1) by a similar synthetic route from
oxo-tris(isopropoxy)vanadium(V) and the corresponding ligand
in 2-propanol or dichloromethane. Single crystals of 1, 3, 4, 7
and 8 suitable for X-ray diffraction analyses were obtained upon
slow evaporation at room temperature of ethanol, 2-propanol or
dichloromethane solutions of the products. The structures of the
complexes are shown in Figs. 3–7. Table 2 shows a comparison of
selected bond lengths and angles for the five structures.
−
HRMS (ESI−): m/z calc. for C17H17NO5V (L5VO2 ): 366.0546;
found: 366.0529.
1
6. H NMR (200 MHz, CD3OD):  1.17 (d, J = 6.2 Hz, 6 H,
OCHMe2), 1.39 (s, 3 H, NCMeMe), 1.66 (s, 3 H, NCMeMe),
3.94 (sept, J = 6.2 Hz, 1 H, OCHMe2), 4.52 (br d, J = 9.4 Hz,
1 H, CMe2CHHO), 4.75 (d, J = 9.4 Hz, 1 H, CMe2CHHO), 7.05
(t, J = 7.6 Hz, 1 H, Har), 7.29–7.47 (m, 3 H, Har), 7.59–7.75 (m,
4 H, Har), 8.75 (s, 1 H, CHN). 51V NMR (52.58 MHz, CD3OD):
 −549, −528. IR (KBr):  = 3402, 2970, 1627, 1423, 1065, 984,
−
761 cm−1. HRMS (ESI−): m/z calc. for C17H17NO4V (L6VO2 ):
350.0597; found: 350.0598.
VO(OiPr)L7·iPrOH (7) and VO(OiPr)L8 (8). To a stirred
solution of the ligand (0.2 mmol) in 10 mLof 2-propanol VO(OiPr)3
(47 mg, 0.2 mmol) was added. The resulting brown reaction mix-
ture was stirred at room temperature for 1 h. The precipitate was
filtered off, extracted with 10 mL of dichloromethane and filtered
again. Brown needles, suitable for X-ray diffraction analysis were
obtained by storing the filtrate at room temperature overnight. The
solvent was removed by filtration.
2 3 1 6
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0

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D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0
2 3 1 7

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For comparison of this selected coordination environment with
that of native haloperoxidase see Fig. 8. However, the similar
complex 7 does not feature intramolecular hydrogen bonding but
an inter-linkage is observed between the oxygen atom O3 of the
hydroxyphenyl group and one 2-propanol molecule of crystallisa-
tion (d[O3O6)] = 2.655(1) Å).
Scheme 1 Preparation of the complexes used in this study (R = OH, H;
R1 = “third donor arm”; X = O, OiPr).
Crystal structure of the ligand H2L8
The X-ray diffraction analysis shows nearly coplanar aromatic
rings within the salicylaldimine subunit having a maximum twist
at the N1–C1 bond (C7–N1–C1–C6 = 10.6(2)°). The phenyl group
attachedtoC12isat −56.3(1)°tothisplane.Intramolecularhydrogen
bonding of the type OHN was found (d[O1N1] = 2.602(1) Å).
Bond lengths and angles of the same range have been reported
for 5-chloro-N-(2-hydroxy-5-methylphenyl)salicylaldimine33 and
N-(2-hydroxyethyl)-2-oxy-1-naphthaldimine.34 Two neighbouring
molecules form a dimer by a double O1O2 hydrogen linkage
between their phenolic oxygen atoms (d[O1O2] = 2.662(1) Å,
Fig. 2).
Fig. 3 Molecular structure of 1 with thermal ellipsoids (30% probability);
hydrogen atoms omitted for clarity.
Fig. 4 Molecular structure of 3 with thermal ellipsoids (50% probability);
hydrogen atoms and solvent molecules omitted for clarity.
Fig. 2 Molecular structure of the ligand H2L8 showing a hydrogen bonded
dimer. Selected bond lengths (Å) and angles (°): C7–C8 1.413(1), C7–N1
1.312(1), O1N1 2.602(1), O2O1 2.662(1), C13–C8–C7–N1 −1.4(2),
C7–N1–C1–C6 10.6(2), C13–C12–C14–C15 −56.3(1).
Structural description of complexes 1, 3, 4, 7 and 8
The structure analyses of the five mononuclear complexes
show similar features (Figs. 3–7). The vanadium(V) cores are in
tetragonal-pyramidal coordination spheres involving the N2O3-
and the NO4-donor sets, respectively, of the Schiff-base ligands
together with the cis dioxo function or the oxo-isopropoxo site. For
complexes 1, 3 and 4 the four basal coordination sites are occupied
by the Schiff-base ligand NNO-donor set and another oxo donor
opposite to the imino nitrogen. For 7 and 8 the opposite position to
the imino-nitrogen of the ONO-donor set is occupied by a mono-
dentate isopropoxy ligand. The apical position is occupied by an
oxo function. The bond lengths d[V–O(oxo)] are slightly shorter
for 7 and 8 with NO4-environment (1.596(2) and 1.595(2) Å) than
for 1, 3 and 4 bearing a N2O3 donor set (1.615(2), 1.623(1) and
1.624(3) Å). The d[V–N(imine)] distances are 2.173(2) Å (7),
2.159(2) Å (8), 2.138(2) Å (1), 2.130(3) Å (4) and shortest for
compound 3 with 2.128(1) Å. All complexes feature a fused ring
system of one five- and one six-membered ring, with the imino-
nitrogen atom common to both chelate rings. The complexes are
distorted towards a trigonal-bipyramidal environment. Values of
the angular parameter  increase from 0.27 (for 8), 0.32 (for 7) and
0.35 (for 1) to 0.40 and 0.42 for the pyridin-2-ylmethyl modified
complexes 3 and 4 ( values are 0 and 1 for an ideal square
pyramid and a trigonal bipyramid, respectively35). In complexes
1 and 3 intramolecular hydrogen bonding is found between the
oxygen atom of the hydroxyphenyl moiety and the vanadium
coordinated phenoxidic O1 atom. The O1O distances are 2.609
and 2.597 Å and are within the typical range for OHO-bonding.
Fig. 5 Molecular structure of 4 with thermal ellipsoids (50% probability);
details as in Fig. 3.
Fig. 6 Molecular structure of 7 with thermal ellipsoids (50% probability);
details as in Fig. 3.
2 3 1 8
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0

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Table 3 Selected spectroscopic data for 1–8
Complex
(51V)/ppm
(VO)/cm−1
(CN)/cm−1
1
2
3
4
5
6
7
8
−509a
929
930
949
936
984
984
991
988
1624
1624
1624
1616
1627
1627
1601
1610
−512a
−536b
−538b
−525, −551c
−528, −549d
−547a
−550a
a Recorded in CD3CN. b In (CD3)2CO. c In CD3OD, peak ratio 10:1. d In
CD3OD, peak ratio 7:1.
Table 4 Hydrogen peroxide mediated oxygenation of ethyl phenyl sulfide
in the presence of catalysts 1a–8a
Fig. 7 Molecular structure of 8 with thermal ellipsoids (50% probability);
details as in Fig. 3.
Catalyst
% PhSOEta
% PhSO2Eta
SO:SO2
TONb
after 5/10 min
after 5/10 min
after 10 min.
1a
2a
3a
4a
5a
6a
7a
8a
0/1
0/0
0/0
0/0
0/5
0/2
8/22
4/7
—
—
—
—
110
60
196
174
196
196
11/50
6/28
80/76
60/80
78/78
75/79
91:9
93:7
78:22
92:8
80:20
80:20
16/20
19/19
a Yield related to the educt. b Turnover number of all oxygenated products
related to vanadium after 10 min (TON = {[SO] + [SO2]}/[V]).
Fig. 8 Comparison of the coordination environment of complex 3 (left)
to that of V-bromoperoxidase based on the structure of native A. nodosum37
(right).
51V NMR, IR and MS spectral data
The complexes were studied in acetonitrile or methanol solution
by 51V NMR (see Table 3 for details). The signals are observed
in the typical range for complexes with similar ligand donor sets.
For complexes 1 and 2 the chemical shifts are −509 and −512, for
3 and 4 they are −536 and −538, while for 5–8 the signals range
from −547 to −550 ppm. This can be explained by higher negative
charge on the ligand moiety and the smaller number of oxo ligands.
Complexes 5 and 6 show two signals in methanol solution with a
peak ratio of 10:1 and 7:1, respectively. Its origin remains unclear.
The IR spectra show frequencies at 929 to 991 cm−1 for (VO)
which is in the standard range seen for V-Schiff-base complexes.
The value increases with an increasing number of oxygen atoms
around the metal center and correlates well with the VO-distances
described by X-ray analyses. The CN-group shows typical
resonances around 1620 cm−1 with no significant difference to an
unbound imino function.
Fig. 9 (a) Temperature dependent 1H NMR spectra of 1 in CD3CN show-
ing the aromatic region from 233 to 298 K (bottom to top). (b) Room temper-
ature spectrum after addition of CD3OD. The signal at 7.8 ppm disappears.
The formation of peroxo complexes on addition of hydrogen
peroxide to an acetonitrile solution of 2 was qualitatively proven by
electrospray mass spectrometry. The complex ion [VO(O2)L2Na]+
(m/z = 389.0663, calc. 389.0682) was found in the catalytic aceto-
nitrile solution. A structurally related complex with side-on coordi-
nated peroxide was characterised by X-ray diffraction analysis by
Meicheng et al.36
Catalytic oxygenation of sulfides
Vanadium Schiff-base complexes bearing both biphenyl moieties
and various donor sets have not so far been employed as catalysts
in the oxidation of sulfides. In a preliminary investigation of the
sulfide oxygenation activity, the catalysts were prepared in situ by
mixing the appropriate ligand with vanadyl acetylacetonate.15 The
reactions have been conducted with 100 mM solutions of ethyl
phenyl sulfide, hydrogen peroxide and 0.5 mol% catalyst at room
temperature in acetonitrile. The results are summarized in Table 4
for in situ prepared complexes 1a–8a. For the overall reaction see
Scheme 2.
Variable-temperature 1H NMR spectra
1
To investigate hydrogen bonding of complex 1 H NMR spectra
were recorded at different temperatures in CD3CN. Upon cooling
the OH-proton of the hydroxyphenyl subunit significantly shifts
from 7.80 ppm at room temperature to 8.05 ppm at 233 K (Fig. 9).
This is typical for enhanced H-bonding which possibly results from
a slower (or frozen) rotation of the hydroxyphenyl group around
the C4–C12 bond at low temperature. The chemical shift of the OH
proton in 1 is not concentration dependent in the range 1–10 mM.
This supports the idea of intramolecular rather than intermolecular
H-bonding. In a vanadium haloperoxidase model complex, a similar
temperature dependent shift of NH hydrogen bonded to V-coordi-
nated oxygen was observed, with a value of 6.4 kcal mol−1 for the
H-bond strength estimated.10
The reaction proceeded with a maximum turnover of 196 (98%,
total amount of oxygenated substrate related to the catalyst). Along
with the sulfoxide as the main product, some sulfone is also formed
due to a slight excess (1.1 eq. related to sulfide) of hydrogen
Scheme 2 Catalytic oxygenation of ethyl phenyl sulfide.
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0
2 3 1 9

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4 A. Messerschmidt and R. Wever, Proc. Natl. Acad. Sci., 1996, 93, 392.
peroxide. Systems 5a–8a compared to 1a–4a show better substrate
conversion probably due to the presence of a different ligand donor
set (NO4 vs. N2O3). A reason for the inactivity of catalysts 1a and 2a
might be attributed to the easily protonated tertiary amine donor arm
forming a bidentate ligand which allows oligomerism. Of particular
interest is the about two times higher reactivity of 3a compared with
4a, suggesting a catalytic cooperation of the vanadium centre and
the pendant OH-group of 3a.
Density functional studies on the mechanism of vanadium
catalysed oxidation of disulfides favour a direct oxo-transfer
from side-on coordinated hydroperoxide to the sulfur atom of the
substrate.38 It is evident from molecular models that the pendant
OH-group in 3a could form an H-bond with side-on bound peroxide
in the L3V(V)-coordination plane. Such an interaction could
activate the peroxide ligand for electrophilic attack, or it could
simply promote the formation of the peroxo complex. The pendant
OH-group appears to a reaction step in the catalytic cycle which
is rate limiting with catalysts 3a/4a but not with 5a–8a where no
significant effect on rate is detectable.
5 M. N. Isupov, A. R. Dalby, A. A. Brindley, I. Yoshikazu, T. Tanabe,
G. N. Murshudov and J. A. Littlechild, J. Mol. Biol., 2000, 299, 1035.
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7 A. A. Brindley, A. R. Dalby, M. N. Isupov and J. A. Littlechild, Acta
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8 A. Messerschmidt, L. Prade and R. Wever, Biol. Chem., 1997, 378,
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9 M. Casny and D. Rehder, Chem. Commun., 2001, 921.
10 C. Kimblin, X. Bu and A. Butler, Inorg. Chem., 2002, 41, 161.
11 M. J. Clague, N. L. Keder and A. Butler, Inorg. Chem, 1993, 32, 4754;
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12 K. B. Sharpless and R. C. Michaelson, J. Am. Chem. Soc., 1973, 95,
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14 C. Bolm, Coord. Chem. Rev., 2003, 237, 245–256.
15 A. Vetter and A. Berkessel, Tetrahedron Lett., 1998, 39, 1741.
16 G. Santoni, G. Licini and D. Rehder, Chem. Eur. J., 2003, 9, 4700.
17 Z. Luo, Q. Liu, L. Gong, X. Cui, A. Mi and Y. Jiang, Chem. Commun.,
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Conclusions
18 D. Rehder, Biometals, 1992, 5, 3–12.
Eight five-coordinate oxovanadium(V) complexes containing
distorted tetragonal pyramidal N2O3 or NO4 donor sets consisting
of a sterically hindered tridentate salicylaldimine derivative
together with a dioxo or oxo-alkoxy functionality have been
characterized. The compounds are essentially monomeric in solid
and solution phase proved by X-ray crystallography, IR and 51V
NMR spectroscopy. Complexes 1 and 3 serve as an appropriate
model for the environment of the native center of vanadate
dependent haloperoxidases. Like other vanadium(V) species the
used complexes enhance the oxygenation of sulfides to sulfoxides.
The highest turnover numbers were obtained with a system contain-
ing a ligand with NO2 donor set and hydrogen peroxide as oxidant.
A pendant OH-group in 3 accelerates the catalytic rate about two-
fold compared with unfunctionalized complex 4.
19 A. Vetter and A. Berkessel, Tetrahedron Lett., 1998, 39, 1741;
G. Santoni, G. Licini and D. Rehder, Chem. Eur. J., 2003, 9, 4700.
20 T. S. Smith, II and V. L. Pecoraro, Inorg. Chem., 2002, 41, 6754.
21 J. V. Walker and A. Butler, Inorg. Chem. Acta, 1996, 243, 201.
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23 T. S. Smith, II and V. L. Pecoraro, Inorg. Chem., 2002, 41, 6754.
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25 R. Krämer, Coord. Chem. Rev., 1999, 182, 243.
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28 A. D. Westland and M. T. H. Tarafder, Inorg. Chem., 1981, 20,
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29 R. J. Argauer and C. E. White, Anal. Chem., 1964, 36, 2141.
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31 G. M. Sheldrick, SHELXS-97, Program for Crystal Structure Solution,
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft for financial
support (KR1218/6-1). P. P. would like to thank C. Dienemann and
K. Gabel for the measurement of ESI mass spectra and B. Termin
for conducting NMR experiments.
32 G. M. Sheldrick, SHELXTL, Version NT V. 5.1., BrukerAXS, Madison,
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2 3 2 0
D a l t o n T r a n s . , 2 0 0 4 , 2 3 1 4 – 2 3 2 0