Towards hydroperoxovanadium complexes: The X-ray crystal structure of a peroxovanadium(v) complex containing a V(O2)(RCO2H)(H2O)2 cluster with hydrogen bond inter-linkages

Article abstract of DOI:10.1039/b101010g

The molecular structure of the oxo-peroxovanadium complex [VO(O₂)(bpaH)]ClO₄·2H20, containing the new ligand N,N-bis(2-pyridylmethyl)-β-alanine (bpaH) reveals tight binding of the carboxylic acid function to the vanadium centre through its doubly bonded oxygen; the carboxylic acid proton mediates hydrogen bonding interactions comprising the peroxo group and the two waters of crystallisation, thus providing the basis for the potential formation of a hydroperoxo species.

Full text of DOI:10.1039/b101010g

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Towards hydroperoxovanadium complexes: the X-ray crystal
structure of a peroxovanadium( ) complex containing a
V
V(O2)(RCO2H)(H2O)2 cluster with hydrogen bond inter-linkages†
ˆ
Marian Casny´ and Dieter Rehder*
Chemistry Department, University of Hamburg, D-20416 Hamburg, Germany.
E-mail: rehder@xray.chemie.uni-hamburg.de
Received (in Cambridge, UK) 30th January 2001, Accepted 22nd March 2001
First published as an Advance Article on the web 3rd May 2001
The molecular structure of the oxo-peroxovanadium com-
plex [VO(O2)(bpaH)]ClO4·2H2O, containing the new ligand
N,N-bis(2-pyridylmethyl)-b-alanine (bpaH) reveals tight
binding of the carboxylic acid function to the vanadium
centre through its doubly bonded oxygen; the carboxylic acid
proton mediates hydrogen bonding interactions comprising
the peroxo group and the two waters of crystallisation, thus
providing the basis for the potential formation of a
hydroperoxo species.
side-on coordination of RO22 has previously been reported for
[VO(O2)(HO2)(bipy)] (bipy = 2,2A-bipyridyl12a), [VO(H2O)-
tBuO2)(dipic)]
(dipic
=
dipicolinate(22))12b
and
1
2
[{VO(O2)2}(m-h ,h -O2){VO(O2)H2O}]22 13
.
Based on kinetic
investigations, hydroperoxo intermediates have also been made
plausible in oxo transfer reactions to metal-centred thiolates
using [VO(O2)2(pic)]22 14
, and peroxidative halogenations with
[VO(O2)(bpg)] (bpgH = N,N-bis(2-pyridylmethyl)glycine).5,9
An additional point of considerable interest in the context of
peroxovanadium complexes is their ability to inhibit phospho-
tyrosine phosphatases and thus act as insulin mimetics.15
In the present work we report on a peroxovanadium complex
giving rise to a tight hydrogen bonding network in which three
different functionalities participate, viz. a coordinated and
protonated carboxylate, two inter-lattice water molecules and
the peroxo ligand. Hence a complex which may represent a
preformed hydroperoxo species, or a compound which is able to
rapidly and reversibly provide a proton for a hydroperoxo
intermediate in catalytic turnover. Our complex thus models a
situation which, in the haloperoxidases, is represented by
aspartate or the distal histidine plus water molecules at the
active site of the enzyme (Fig. 1, right). Aspartate as a mediator
for proton-transfer has also been reported for vanadate-
incorporated phytase,16 a semi-synthetic vanadium perox-
idase.17
Authentic peroxovanadium complexes as well as peroxo-
vanadium complexes generated in situ have been employed as
catalysts and in stoichiometrically conducted reactions as oxo
transfer reagents for alcohols, arenes, alkenes and thioethers.1,2
Likewise, the bromination of functionalised arenes is carried
out by peroxovanadium complexes, or by vanadium compounds
in the presence of H2O2.3,4 Peroxovanadium intermediates have
also been proposed for the enzymatic conversion of halide to
hypohalous acid (or another Hal+ species) by vanadate-
dependent algal and fungal haloperoxidases in the presence of
peroxide,4–7 an assumption which has been fortified by the
structural characterisation of the peroxo form of chloroperox-
idase from the fungus Curvularia inaequalis.8 Both the in vitro
and in vivo oxidation of halide such as bromide is facilitated by
an increase of the electrophilicity at the reaction site, i.e. by the
intermediate formation of a hydroperoxo complex (Fig. 1, left)
in the case of the peroxo ligand representing the reaction
site,4,5,9 a suggestion which finds support in the fact that (i)
protons are consumed in these reactions,9 (ii) alkylperoxo
complexes are often more effective than the parent peroxides,1
(iii) the cationic hydroperoxo–oxovanadium moiety is on a
lower energy level (more stable) than the corresponding
peroxo–oxovanadium species,10 and (iv) a hydroperoxo form of
the active site vanadium in bromoperoxidase from the marine
alga Ascophyllum nodosum is quite within the bounds of
probability as based on 17O NMR evidence.10 The asymmetric
The new ligand N,N-bis(2-pyridylmethyl)-b-alanine (bpaH)
was synthesised† (Scheme 1) by modifying the procedure
described for N,N-bis(2-pyridylmethyl)-glycine (bpgH).19 The
complex [VO(O2)(bpaH)]ClO4·2H2O was obtained† by adding
the ligand to an aqueous solution of K[VO3], followed by
treatment with H2O2 and finally with dilute HClO4 to adjust the
solution to pH 1.7. Orange-coloured crystals were obtained
from the ethanolic solution in the cold.
The molecular structure‡ of the compound is shown in Fig. 2;
hydrogen bonding interactions have been indicated by dashed
lines. The basic structure of the cation [VO(O2)(bpaH)]+ is a
pentagonal bipyramid, with the peroxo ligand (O1 and O2) and
the three nitrogens (N1, N2 and N3) of bpaH in the plane, and
the oxo group O3 and carboxylic acid oxygen O4 in the apical
positions. The ligand bpaH coordinates to the vanadium
forming one six-membered and two five-membered chelate
rings. The two pyridines are in the equatorial plane. The five
atoms in the equatorial plane are nearly coplanar; the vanadium
Fig. 1 Left: proposed mechanism for bromide oxidation (su = supporting
group). Right: active site of the peroxo form of vanadate-dependent
bromoperoxidase from A. nodosum, based on the structure of the native A.
nodosum enzyme18 and the peroxo form of the C. inaequalis peroxidase.8
Potential mediators for proton transfer are His418, His411 and Asp278.
† Electronic supplementary information (ESI) available: synthesis of bpaH
and [VO(O2)(bpaH)ClO4·nH2O. See http://www.rsc.org/suppdata/cc/b1/
b101010g/
Scheme 1
DOI: 10.1039/b101010g
Chem. Commun., 2001, 921–922
This journal is © The Royal Society of Chemistry 2001
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which the electrophilicity of this site can be adapted to an
appropriate substrate to be oxidised, such as bromide; cf. Fig.
1.
Notes and references
‡ Crystal data: C15H21ClN3O11V, M = 505.74, monoclinic, space group
C2/c, a = 31.612(7), b = 7.1835(17), c = 21.290(16) Å, b = 123.274(4)°,
V = 4042.1(16) Å3, T = 293(2) K, Z = 8, m(Mo-Ka) = 0.69 mm21
.
Independent reflections 4382 (Rint = 0.0265), final R values [I > 2s(I0)] R1
=
0.0408, wR2 0.1005. CCDC 157361. See http://www.rsc.org/
=
suppdata/cc/b1/b101010g/for crystallographic data in .cif or other elec-
tronic format.
§ Additional hydrogen bonding contacts exist between the peroxo group
and the neighbouring pyridines of the ligand bpaH, viz. O1 and H12 (2.749
Å), and O2 and H6 (2.771 Å).
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Fig. 2 Molecular structure of [VO(O2)bpaH]ClO4·2H2O at the 50%
probability level, including the hydrogen bonds (dashed lines). Selected
bond lengths (Å) and angles (°): V–O1 1.8734(16), V–O2 1.8827(16), V–
O3 1.5877(15), V–O4 2.2095(16), V–N1 2.132(2), V–N2 2.192(2), V–N3
2.133(2), O1–O2 1.422(2), C1–O4 1.222(3), C1–O5 1.302(3), O5–H5
0.8201(10); O1–V–N1 81.68(8), O1–V–O2 44.48(7), O2–V–N3 81.83(8),
N1–V–N2 75.53(7), N2–V–N 374.13(7), O3–V–O4 172.25(7). Hydrogen
bonds (Å): O1…H10C–O10 3.250, O2…H10C–O10 2.995, O2…H10D–
O10 2.806, O5–H5···O11 2.550, O11–H11B…O10 2.711, O11–H11A…O6
(perchlorate) 2.993.
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centre is displaced from this plane by 0.2163(8) Å towards the
oxo group. The unusual coordination mode of the carboxylic
acid function – through the doubly bonded oxygen O4 – has
been described previously for a number of dipicolinato
complexes of copper,20 iron21 and zinc22 with bond lengths
ranging from 2.3 to 2.5 Å (as compared to ca. 2.1 Å for the
corresponding bond lengths of deprotonated carboxylic acid
functions). The angle O3–V–O4, 172.25°, only slightly deviates
from linearity. The bond V–O4 [d = 2.2095(16) Å] is thus
surprisingly strong, even more as it is subject to the trans effect
exerted by the oxo group. It compares to the V–O (carboxylate)
distances of 2.144 and 2.045 Å for the carboxylato group trans
and cis to VNO, respectively, in [VO(L)H2O(O2)]2 (H2L =
carboxymethylhistidine),23 d[V–O (carboxylate)] = 2.138 Å
for the trans-standing carboxylate in [VO(O2)(bpg)],9 and d[V-
O(carboxylate)] between 2.01 and 2.04 Å in other peroxo-
vanadium complexes containing supporting ligands with cis-
standing carboxylato functions.24,25 The complex is the first
example of a cationic monoperoxo–oxovanadium complex with
an N3O donor set. Other cationic vanadium complexes which
have been described so far contain two NO donor sets
([VO(O2)(picolinamide)2]+),26 or one or two N2 sets ([VO(O2)-
(phen)(H2O)2]+;27 [VO(O2)L2]+, L = phen, bipy28).
ˆ
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Biochem., 2000, 80, 157.
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Fischer and R. Weiss, Nouv. J. Chim., 1983, 7, 467.
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2837.
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Zhang-Sun, I. George Fantus, J. B. Ng, D. A. Hall, B. Soo Lum and A.
Shaver, J. Biol. Chem., 1994, 269, 4596.
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Chem. Commun., 1998, 189.
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Schomburg, J. Mol. Biol., 1999, 293, 595.
19 D. D. Cox, S. J. Benkovic, L. M. Bloom, F. C. Bradley, M. J. Nelson,
L. Que, Jr. and D. E. Wallick, J. Am. Chem. Soc., 1988, 110, 2026.
20 M. Biagini-Cingi, A. Chiesi-Villa, C. Guastini and M. Nardelli, Gazz.
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21 P. Laine, A. Gourdon and J.-P. Launay, Inorg. Chem., 1995, 34, 5138;
P. Laine, A. Gourdon, J.-P. Launay and J.-P. Tuchagues, Inorg. Chem.,
1995, 34, 5150.
The uncoordinated OH of the carboxylic acid group of
[VO(O2)(bpaH)]+ links a water of crystallisation (O11) via a
strong hydrogen bond of 2.550 Å, which in turn hydrogen binds
to the perchlorate anion and a second water of crystallisation,
O10. Two hydrogen bonds of weak to medium strength keep
this water in the proximity of the coordinated peroxo group.§
Although only O2 of the peroxide is involved in this H-bonding
network to a sizable extent, there are no significant differences
in the V–O (peroxide) bond lengths. Bonding parameters [d(V–
O1) 1.8734(16), d(V–O2) 1.8827(16) Å; angle O1–V–O2 =
44.48(7)°] are comparable to those in other peroxovanadium
complexes;9,13,23–25,29 the O–O bond length, 1.421(3), is in-
between the margins noted for [V2O2(O2)4H2O]22 13 and
22 K. Hakansson, M. Lindahl, G. Svensson and J. Albertsson, Acta Chem.
Scand., 1993, 47, 449.
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1998, 1267.
24 M. Kaliva, T. Giannadaki, C. P. Raptopoulos, A. Tzerzis and A.
Salifoglou, Inorg. Chem., in the press; M. Tsaramyrsi, D. Kavousanaki,
C . P. Raptopoulou, A. Terzis and T. Salifoglou, Inorg. Chim. Acta, in
press.
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L. Kuchta, M. Sivák, J. Marek, F. Pavelcˆik and M. Casny´, New J. Chem.,
1999, 43.
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[VO(O2)(tp)(pz)] (tp
= tris(pyrazolyl)borate, pz = pyr-
azole).30
This special arrangement of carboxylic acid, water and
peroxide provides a basis for a mechanism for rapid – and
reversible – transfer of a proton to the activated (by coordination
to vanadium) peroxo function, and hence a mechanism by
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Chem. Commun., 2001, 921–922