An eight-coordinate vanadium thiolate complex with charge delocalization between V(V)-thiolate and V(IV)-thiyl radical forms

Article abstract of DOI:10.1021/ja2004208

A six-coordinate oxovanadium(V) thiolate complex and an eight-coordinate non-oxovanadium thiolate complex, [PPh₄][VO(PS3′′) (OCH₃)] (1) and [NEt₄][V(PS3′′)₂] (2) (PS3′′ = P(C₆H₃-3-Me₃Si-2-S) ₃^3-), respectively, have been isolated and structurally characterized. The former belongs to a limited collection of oxovanadium(V) thiolate complexes. The latter has an unusual coordination number of eight. More importantly, its consensus electronic structure derived from its spectroscopic data should be considered as the resonance forms of V^V-thiolate and V^IV-thiyl radical species. This implies that V^IV-thiyl radical can maintain a stable presence in biological systems.

Full text of DOI:10.1021/ja2004208

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An Eight-Coordinate Vanadium Thiolate Complex with Charge
Delocalization between V(V)ꢀThiolate and V(IV)ꢀThiyl Radical Forms
Ya-Ho Chang,† Chia-Ling Su,† Ru-Rong Wu,† Ju-Hsiou Liao,‡ Yi-Hung Liu,|| and Hua-Fen Hsu*,†
†Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan
‡Department of Chemistry, National Chung Cheng University, Chia-Yi 621, Taiwan
Department of Chemistry, National Taiwan University, Taipei 116, Taiwan
S Supporting Information
b
VV with thiolate might lead to the formation of a VV thiolate
complex.17d,f Recently, studies have indicated that VVꢀthiol
ABSTRACT: A six-coordinate oxovanadium(V) thiolate
complex and an eight-coordinate non-oxovanadium thiolate
complex, [PPh4][VO(PS300)(OCH3)] (1) and [NEt4]-
[V(PS300)2] (2) (PS300 = P(C6H3-3-Me3Si-2-S)33ꢀ), re-
spectively, have been isolated and structurally characterized.
The former belongs to a limited collection of oxovanadium-
(V) thiolate complexes. The latter has an unusual coordina-
tion number of eight. More importantly, its consensus
electronic structure derived from its spectroscopic data
should be considered as the resonance forms of VVꢀthiolate
and VIVꢀthiyl radical species. This implies that VIVꢀthiyl
radical can maintain a stable presence in biological systems.
complexes indeed form. Redox chemistry was also evident during
the reaction of vanadate with 2-mercaptoethanol as well as
dithiothreitol under biological conditions.17c,18
In our previous studies, the redox conversion of VV/VIV
coupled with RSꢀ/RS• was demonstrated in the reactions of
VVO(O-i-Pr)3 with tris(benzenethiolato)phosphine derivatives,
P(C6H3-5-Me-2-S)3 (PS30) and P(C6H4-2-S)3 (PS3).17b
The pathway likely involves a thiyl radical-bound vanadium
complex as an intermediate. To observe or trap intermediates
of this transformation, PS300 ligand, which has the sterically
hindered substituent Si(CH3)3, in phenylthiolates was employed
to react with VVO(O-i-Pr)3. As a result, two species,
[PPh4][VO(PS300)(OCH3)] (1) and [NEt4][V(PS300)2] (2),
were isolated and characterized (Chart 1). Complex 1 is a VVdO
species, whereas complex 2 is a formal VV species. However, the
electronic structure of 2 is best described as resonance forms
between VVꢀthiolate and VIVꢀthiyl radical states. It is also
worth noting that 2 represents a new set of eight-coordinate non-
oxovanadium complexes (bare system) that has the same co-
ordination number as amavadin, a natural VIV product in
Amanita mushrooms. Reports on synthetic vanadium complexes
with a coordination number of eight are still scarce in the
literature.19
3ꢀ
3ꢀ
ysteineꢀthiyl radicals have been identified in several en-
C
zymes and have been suggested to play roles in their
functions,1 such as in ribonucleotide reductase (RNR).2 In
addition, a thiyl radical species is proposed as a key intermediate
in the biological radical sulfur insertion reaction.3 Furthermore, a
formal “Ni(III)Fe(II)” state containing cysteineꢀthiyl radical
character has been recognized in the active site of NiFe hydro-
genase according to spectroscopic and theoretical studies.4 Due
to this biological significance, there is an increasing interest in
studying (thiyl)metal complexes.5 Examples are [CoIIIL•]þ,6
(t-BuS•)[Cr(CO)5]2,7 and [RuIII(DPPBT)2(DPBT•)]þ8 gener-
ated from one-electron oxidation of their corresponding forms,
respectively, where L is a TACN-trithiolato ligand and DBPT is
2-(diphenylphosphino)thiaphenolate. A bis(μ-thiolato)dinickle-
(II) complex can be oxidized by one electron and yield a dinickle
species with the charge delocalization between the [NiIII(RSꢀ)]
and [NiIIꢀ(RS•)] redox extremes.9 A dithiosalicylidenediamine
NiII complex can be oxidized by one electron and generate a
complex with NiIIꢀthiyl radical character.10
An i-PrOH solution of VO(i-PrO)3 was added to a methanol
solution of Na3[PS300] to generate a deep green solution. By
adding different counterions, [PPh4]Br and [NEt4]Br, com-
plexes 1 and 2 were precipitated as green and brick red crystalline
solids, respectively. The structural data for 1 and 2 were obtained
by X-ray crystallography, and their ORTEP diagrams are shown
in Figures 1 and 2, respectively. Complex 1 belongs to a small
class of VVꢀthiolate complex.17d,f,k,20 The X-ray structure of 1
reveals a six-coordinate complex of V(V) through binding with
an oxo group, a methoxide ion, and a tetradentate PS300 ligand.
The vanadium center adopts a highly distorted octahedral
geometry with the oxo group trans to the thiolate and the
methoxide trans to the phosphine donor. The three trans angles
are 151.0° (S1ꢀVꢀS2), 160.0° (O1ꢀVꢀP1), and 168.6°
(O2ꢀVꢀS3) away from 180° for an ideal octahedron. The
distance of the VdO bond is 1.603 Å, comparable to those in
reported oxovanadium(V) thiolate complexes (1.57ꢀ1.60 Å).
The VꢀS bond distance (2.662 Å) trans to the oxo group is much
The broad relevance of vanadium thiolate chemistry to
biological systems has garnered increasing attention from
researchers.11 The chemistry of high-valent vanadium ions
interacting with cysteine and glutathione has several biomedical
aspects, such as the protein tyrosine phosphatase (PTP) inhibi-
tion by vanadate,12 the anti-diabetic behavior of vanadium
compounds,13 the redox conversion of vanadium in ascidians,14
the function of amavadin isolated from Amanita mushrooms,15
and the toxicity of vanadium in biological systems.16 Based on
these backgrounds, we and others have made efforts to investi-
gate the reactions of high-valent vanadium ions with thiol-
containing ligands.17 Examples have shown that the reaction of
Received: January 16, 2011
Published: March 25, 2011
r
2011 American Chemical Society
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Chart 1
Figure 2. Molecular structure of 2 shown with 35% thermal ellipsoids.
The countercation and H atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°]: VꢀS1 2.4486(13), VꢀS2 2.4065(13), VꢀS3
2.6108(12), VꢀS4 2.6564(15), VꢀS5 2.4545(14), VꢀS6 2.4410(14),
VꢀP1 2.4828(14), VꢀP2 2.4561(13), S1ꢀVꢀS2 93.88(5), S1ꢀVꢀS3
142.01(5), S1ꢀVꢀS4 82.74(5), S1ꢀVꢀS5 85.05(5), S1ꢀVꢀS6
148.57(5), S2ꢀVꢀS3 82.79(4), S2ꢀVꢀS4 65.16(4), S2ꢀVꢀS5
151.07(5), S2ꢀVꢀS6 103.75(5), S3ꢀVꢀS4 127.96(5), S3ꢀVꢀS5
80.56(4), S3ꢀVꢀS6 67.12(4), S4ꢀVꢀS5 142.68(5), S4ꢀVꢀS6
81.45(5), S5ꢀVꢀS6 91.32(5), P1ꢀVꢀS1 76.44(4), P1ꢀVꢀS2
76.45(5), P1ꢀVꢀS3 65.96(4), P1ꢀVꢀS4 134.62(5), P1ꢀVꢀS5
75.21(4), P1ꢀVꢀS6 132.65(5), P2ꢀVꢀS1 72.89(4), P2ꢀVꢀS2
129.75(5), P2ꢀVꢀS3 136.12(4), P2ꢀVꢀS4 65.17(4), P2ꢀVꢀS5
77.54(4), P2-V-S6 75.84(4), P1ꢀVꢀP2 140.27(4).
Figure 1. Molecular structure of 1 shown with 35% thermal ellipsoids.
The countercation and H atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°]: VꢀS1 2.3957(10), VꢀS2 2.4276(10), VꢀS3
2.6615(9), VꢀO1 1.790(2), VꢀO2 1.603(2), VꢀP1 2.3880(9),
S1ꢀVꢀS2 151.04(4), S1ꢀVꢀS3 89.18(4), S2ꢀVꢀS3 82.59(3),
P1ꢀVꢀS1 79.15(3), P1ꢀVꢀS2 71.93(3), P1ꢀVꢀS3 76.19(3),
O1ꢀVꢀS1 93.70(7), O1ꢀVꢀS2 113.01(7), O1ꢀVꢀS3 84.87(7),
O1ꢀVꢀP1 159.76(7), O2ꢀVꢀS1 94.83(9), O2ꢀVꢀS2 88.79(9),
O2ꢀVꢀS3 168.55(9), O2ꢀVꢀP1 94.00(8), O1ꢀVꢀO2 105.52(11).
longer than the other two (2.428 and 2.396 Å); the latter are
much longer than those in the reported oxovanadium(V) thiolate
complexes (2.24ꢀ2.31 Å) due to the difference in their coordi-
nation numbers.
Complex 2 consists of a dodecahedral vanadium center bound
to two phosphorus and six sulfur atoms of two PS300 ligands. S1,
S3, S6, and P1 atoms form the same plane, with a mean plane
deviation of 0.0083 Å. This plane is nearly perpendicular to the
plane generated by S2, S4, S5, and P2 atoms, with a mean plane
deviation of 0.0425 Å. The dihedral angle of 89.8° between these
two planes is close to the ideal value of 90° found in a regular
dodecahedron.21 The vanadium center has a dodecahedral
geometry which could be considered as one elongated tetrahe-
dron formed with P1, P2, S3, and S4 atoms and a collapsed
tetrahedron arising from S1, S2, S5, and S6 atoms.21a In the
elongated tetrahedron, the average length of the VꢀS bonds is
2.634 Å, 0.184 Å longer than in the collapsed one (2.450 Å).
Complex 1 has a VdO stretching band of 941 cmꢀ1. The
electronic spectrum of 1 displays an intense shoulder around
390 nm which is associated with a ligand-to-metal charge-transfer
band (Figure 3). The 51V NMR spectrum exhibits a single peak
at ꢀ276 ppm (relative to VOCl3), indicating a single VV species
in solution. The value is closer to the only reported six-coordi-
nate oxovanadium(V) thiolate complex (ꢀ186 ppm) but more
upfield shifted compared to other five-coordinate oxovanadium-
(V) thiolate complexes (338, 557, and 191 ppm).17d,f,20
Figure 3. Electronic spectra of 1 (0.10 mM, solid line) and 2 (0.15 mM,
dotted line) in CH3CN.
Interestingly, the electronic spectrum of 2 exhibits very different
absorption bands compared to 1, one at 533 nm (ε = 4.61 ꢁ 103
M
ꢀ1 cmꢀ1) and the other at 908 nm (ε = 4.96 ꢁ 103 Mꢀ1 cmꢀ1
)
with a shoulder at 740 nm (Figure 3). The pattern is relatively
similar to that for an electrochemically generated (thiyl)CoIII
complex, [CoIIIL•]þ, where L is a TACN-trithiolato ligand.6 This
implies that 2 might consist of a thiyl radical-bound vanadium
species. The 51V and H NMR spectra, as shown in Figure 4,
1
further suggest that the electronic structure of 2 can be better
understood as resonance forms [VV(PS300)2]— T [VIV(PS300)-
•
(PS300 )]—. The 51V NMR spectrum shows a single peak at
δ = 483 ppm, indicating a þ5 oxidation state for the vanadium
1
center. However, the H NMR spectrum displays some char-
acteristic peaks at 20.24, 19.02, 11.70, 9.61, and ꢀ3.20 ppm,
suggesting a paramagnetic character of the metal center. Notably,
the reported 51V NMR data for eight-coordinate non-oxovana-
dium complexes are very few. Compound 2 has a chemical shift at
δ = 483 ppm is more downfield shifted compared to oxidized
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vanadiumꢀsulfur bond requires a more sophisticated examination;
one that considers the presence of different resonance forms. This
implies that VIVꢀthiyl radical can maintain a stable presence in
biological systems. Furthermore, the reactivity of a vanadium-bound
thiyl radical is also likely responsible for inhibiting PTP as well as
being involved in the mechanism of vanadium-containing bio-
medical systems. The details of this chemistry and theoretical work
are currently under investigation in our laboratories.
’ ASSOCIATED CONTENT
Figure 4. 1H (top) and 51V (bottom) NMR spectra of 2 in CD2Cl2.
S
Supporting Information. Synthetic details and addi-
b
tional characterization and crystallographic data. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
konopka@mail.ncku.edu.tw
’ ACKNOWLEDGMENT
This work was supported by the National Science Council in
Taiwan (NSC 96-2113-M-006-011). The XAS data were col-
lected at the Synchrotron Radiation Research Center (SRRC) in
Taiwan. The authors are grateful to SRRC for providing beam-
time and thank Dr. Jyh-Fu Lee for support during the beamtime.
Prof. Kui-Feng Hsu at National Cheng Kung University is
thanked for discussion.
Figure 5. Vanadium K-edge X-ray absorption spectra of 1 (solid line)
and 2 (dotted line).
amavadin and its related analogues (approximately ꢀ200 to
ꢀ300 ppm),19c,22 likely attributed to S ligands for the former
versus N/O ligands for the latter cases.23
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