Self-exchange electron transfer in high oxidation state non-oxo metal complexes: Amavadin

Article abstract of DOI:10.1039/b610751f

The electron transfer self-exchange rate constant between the oxidized and reduced forms of amavadin equals ～1 × 10⁵ dm³ mol^-1 s^-1 at 25°C and represents the first unambiguous example for a vanadium(iv/v) couple. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610751f

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Self-exchange electron transfer in high oxidation state non-oxo metal
complexes: amavadin{
a
Jeremy Lenhardt, Bharat Baruah, Debbie C. Crans* and Michael D. Johnson*
b
b
a
Received (in Berkeley, CA, USA) 27th July 2006, Accepted 20th September 2006
First published as an Advance Article on the web 12th October 2006
DOI: 10.1039/b610751f
2
2
22
2
32
[
V(hidpa)
2
]
+ [IrCl
6
]
A [V(hidpa)
2
]
+ [IrCl
6
]
(2)
(3)
The electron transfer self-exchange rate constant between
the oxidized and reduced forms of amavadin equals y1 6
5
10
3
dm mol
21
21
s
at 25 uC and represents the first
2
22
+
2
[V(hidpa)2] + H Asc A 2[V(hidpa) ] + Asc + 2H
2
2
unambiguous example for a vanadium(IV/V) couple.
4
+
While the Ce oxidation rates were too fast to measure, the
oxidation stoichiometry is truly 1 : 1 which has been used by
The determination of self-exchange electron transfer rate constants
for vanadium(IV/V) couples remains a challenging problem that is
overcome in studies of the natural product, amavadin. Two major
impediments are: (1) coordination geometry changes between these
10
others to produce vanadium(V) containing amavadin. The
reaction kinetics for eqn 2 and 3 each showed a first order
dependence on both the vanadium complex and the oxidant (or
reductant). No hydrogen ion dependence was observed for the
1
two states and (2) a competing inner-sphere pathway through
2
their ‘‘yl’’ type oxygens. These have led to inconsistent estimates
2
2
23
[
IrCl
6
]
oxidation between 0.10 mol dm HCl (k = 3.2 ¡ 0.3 6
of self-exchange rate constants for hydroxyoxovanadate(IV/V) of
21 21 3,4
6
0 dm mol
6
0 dm mol
3
21 21
23
2
3
3
3
1
s ) and 0.0010 mol dm HCl (k = 3.3 ¡ 0.2 6
s ). This is expected since amavadin has no site
y10 and y10 dm mol
coordination geometry does not change when oxidized from the
s
.
In amavadin the metal
3
21 21
1
5
4 to the +5 oxidation states. Furthermore both amavadin and its
for protonation over this pH range. The following rate law can be
written, eqn 4.
+
5
oxidized form lack an ‘‘yl’’ type oxygen. As a result, amavadin is
an ideal system in which an unambiguous self-exchange rate
constant may be determined for a vanadium(IV/V) couple.
Amavadin possesses an eight-coordinate, non-oxo vanadium
center coordinated to two tetradentate ligands, 2,29-(hydroxyimi-
2
2
2
22
6
d[V(V)]/dt = kIr[V(hidpa)
][IrCl
]
(4)
In contrast, in the ascorbic acid reduction of oxidized amavadin,
+
a simple inverse dependence on H was observed, Fig. 1. A rate
5
no)dipropionate (hidpa). The unusual structure of this com-
law consistent with these observations can be written, eqn 5.
pound, its presence in mushrooms and its intriguing reactivity,
including oxygen activation, have maintained scientists’ interest in
6
this class of compounds for decades. Because vanadium forms
2
2
+ 21
Asc][H ]
2d[V(V)]/dt = kasc[V(hidpa)
][H
2
(5)
very strong high oxidation state oxo-metal complexes, outer-
sphere electron transfer studies of these systems are often
problematic, but can be accessed with this ligand. Since additional
The zero y-intercept suggests that reduction proceeds solely via
2
monohydrogen ascorbate, HAsc , since amavadin has no acidic
7
8
8
9
hydrogens in the pH range studied, Scheme 1.
metal ions such as Mo, Ti, Zr, and others yield similar
coordination complexes, analogous studies may also be carried
with other high oxidation state nonoxo-metal complexes with this
ligand.
Before a fundamental kinetic study of amavadin’s redox
reactions was undertaken, the reaction stoichiometries were
22
determined. Job plots for the oxidation of [V(hidpa) ] with
2
4
+
22
2
2
and the reduction of [V(hidpa) ] by ascorbic
Ce or [IrCl
6
]
acid (H Asc) in aqueous media all showed simple 1 : 1 (eqn 1 and
2
2) or 2 : 1 (eqn 3) stoichiometries.
22
4+
+ Ce A [V(hidpa)
2
3+
[V(hidpa)
2
]
2
]
+ Ce
(1)
a
Department of Chemistry and Biochemistry, New Mexico State
University, Las Cruces, New Mexico, USA. E-mail: johnson@nmsu.edu;
Fax: +1 505 646 2394; Tel: +1 505 646 3627
Department of Chemistry, Colorado State University, Fort Collins,
b
Colorado, USA. E-mail: crans@lamar.colostate.edu;
Fax: +1 970 491 1801; Tel: +1 970 491 7635
{ Electronic supplementary information (ESI) available: Job plots, kinetic
data. See DOI: 10.1039/b610751f
Fig. 1 Inverse hydrogen ion dependence of ascorbic acid reduction of
23
amavadin. Conditions: I = 1.0 mol dm LiCl, T = 25.0 uC.
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Chem. Commun., 2006, 4641–4643 | 4641

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complex, the three signals observed presumably represent three
5
1
different isomers of the oxidized form of amavadin. The V line
broadening experiment measures the line broadening induced by
vanadium(IV) of each signal. Fig. 2 (insert) shows that the amount
of broadening is directly proportional to the amavadin
(
vanadium(IV)) concentration. A value for k22 can be calculated
13,14
from the slope of these lines, Table 2.
Scheme 1
An average self-exchange
21 21
5
3
rate constant of 1 ¡ 0.5 6 10 dm mol
each isomer. Within experimental error, there was no difference in
s
was observed for
1
1
Based on Scheme 1, and using the pKa1 for ascorbic acid, 4.17,
a second order rate constant can be calculated since kasc = Ka1
The rate constant is given in Table 1.
a
k .
self-exchange rate constants for these three isomers.
The line broadening experiment was carried out at 0.10 and
2
3
23
Assuming that both these reactions are outer-sphere electron
transfer processes allows the use of Marcus theory to calculate a
self-exchange rate constant, k , for the V(IV/V) couple using the
10 mol dm HCl. As found for the iridium(IV) oxidation study
above, no difference was observed, thereby eliminating a proton
dependence in this acid range. This observation supports Scheme 1
where the observed proton dependence was ascribed to the
deprotonation of ascorbic acid and not deprotonation of
amavadin.
22
12
4
physical data provided in Table 1. Values for k22, of 1.6 6 10
4
3
21 21
and 2.8 6 10 dm mol
s , respectively, were calculated for the
iridium and ascorbate reactions. These two rate constants are self-
consistent and indicate the electron transfer reactions are adiabatic
in nature.
Due to the aforementioned problems in determining self-
exchange rate constants for vanadium(IV/V) couples, there is a
In addition to the Marcus estimations, a direct measurement
paucity of data for comparison. The self-exchange electron transfer
0/2
51
2
was also carried out using V NMR line broadening. The
vanadium(V) form of amavadin was produced by addition of one
equivalent of Ce(IV) to a solution of amavadin. Fig. 2 shows the
rate constant for aqueous alkaline vanadium(IV/V), [VO(OH)
3
]
,
has been reported using the Marcus cross relation. Two very
21 21
2
3
3
different values were reported, (k22 y 10 dm mol
(k22 y 10–10 dm mol
s
) and
51
3
3
21 21 3,4
V spectrum of oxidized amavadin in D O at acidic pH. Since a
2
s ). The inconsistency was attributed
3
racemic mixture of the hidpa ligand was used to synthesize the
to non-adiabaticity of the cross reactions. In contrast, the self-
exchange rate constants in the present study, obtained by both
Marcus calculation and direct measurement, vary by less than a
Table 1 Kinetic and thermodynamic data for amavadin redox
reactions
0/2
factor of four. Despite the ambiguity in the values for [VO(OH)3]
,
a
3
3
b
3
the self-exchange rate constant in the amavadin system is at least
2 orders of magnitude higher than the corresponding aqueous
oxohydroxide couple. This reflects the smaller structural differences
between the oxidized and reduced forms of amavadin than those in
k
12 /dm
K
mol
11/dm
Eu/V,
NHE
k
22 /dm
2
mol
1
21
s
21 21
21 21
mol
Reactant
s
s
?
2
5
5 c
5 e
d
4
HAsc /HAsc
[
1.4 6 106
3.2 6 10
1.0 6 10
2.0 6 10
0.70
0.96
2.8 6 10
1.6 6 10
2
2
f
4
IrCl
6
]
2
a
b
the aqueous vanadium(IV/V) system. These smaller changes reduce
k
12 = k
a
or kIr in text, T = 25.0 uC, I = 1.0 M (LiCl). Calculated
V/IV c d
) = 0.75 V, NHE, ref. 3. See ref. 18. See ref. 19
the inner sphere reorganization energy barrier and thereby increase
the self-exchange rate constant for amavadin.
using Eu (V
for the HAsc /HAsc . See ref. 20. See ref. 21.
?
2
e
f
Iron containing enzymes have been reported to enhance
biological electron transfer processes compared to aqueous iron.
For example, the observed self-exchange rate constant for the
2
+/3+
3
21 21 15
[
Fe(OH ) ]
2
couple is 33 dm mol
s
whereas for the
6
4
cytochrome c couple, rate constants above 10 dm mol
16,17
reported.
3
21 21
s
are
These rate enhancements have been attributed
primarily to reductions in reorganization energies within the
1
7
protein matrix. Similarly, the natural product, amavadin, shows
significantly enhanced electron transfer processes compared to the
aqueous vanadium(IV/V) system.
In summary, this work provides a self-consistent vanadium(IV/
5
3
21 21
s
V) self-exchange rate constant, k = 1.0 ¡ 0.2 6 10 dm mol
using Marcus theory and line broadening experiments. As the first
example of a non-oxo vanadium(IV/V) redox system in aqueous
Table 2 Self-exchange rate constants for amavadin enantiomers
a
b
3
21 21
s
Isomer (resonance frequency, ppm)
k
22 /dm mol
5
2
217
1.0 ¡ 0.5 6 10
1.2 ¡ 0.5 6 10
1.3 ¡ 0.5 6 10
5
5
5
1
23
Fig. 2 The V NMR spectrum of 0.050 mol dm oxidized amavadin,
2234
2252
a
23
3
VOCl used as reference. Conditions: 0.10 mol dm HCl, I = 1.0 mol
2
3
Chemical shifts reported in reference to VOCl3, 50 mM
3
amavadin used for kinetic study. Conditions: 0.10 mol dm HCl,
dm LiCl, T = 25.0 uC. Insert: line width broadening as a function of
vanadium(IV) concentration. Inset linear fits correspond to: 2252 ppm m,
2
2
3
b
I = 1.0 mol dm LiCl, T = 25.0 uC. Calculated as in ref. 13.
2234 ppm $, 2217 ppm &.
4
642 | Chem. Commun., 2006, 4641–4643
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8
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electron transfer enhancement observed for amavadin should be
considered in elucidation of its biological function.
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Chem. Commun., 2006, 4641–4643 | 4643