Reactions of tris(oxalato)cobaltate(in) with two-electron reductants

Article abstract of DOI:10.1039/b408758p

The tris(oxalato)cobaltate(in) complex [Co(C₂O₄) ₃]^3-, E^o_CoIII/II = +0.57 V) is readily reduced by the 2e^- reagents, Sn(II) and Ge(ii), in contrast to (NH₃)₅CoCl₂₊ and (NH₃) ₅CoBr₂₊, which are unreactive toward these donors. Rates for the oxalato oxidant are only 10^-3-10^-2 as great as those for vitamin B_12a (aquacob(III)alamin, E^o +0.35 V at pH 1), in accord with the suggestion that reductions of corrin-bound cobalt(III) by Sn(II) and Ge(II) occur predominantly through an additional path involving Co(I). Reductions of the oxalato complex by 2e^- donors are taken to proceed by initial formation of odd-electron intermediates (e.g., Sn ^III and Ge^III) which react rapidly with Co^III. Such a two-step sequence is in keeping with the observed behavior of the rare reductant, Ti^II, which is found to be oxidized by [Co(C ₂O₄)₃]^3- more slowly than (independently prepared) Ti(III) under comparable conditions.

Full text of DOI:10.1039/b408758p

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F U L L P A P E R
Reactions of tris(oxalato)cobaltate(III) with two-electron
reductants†‡
Zhiyong Yang and Edwin S. Gould*
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
Received 9th June 2004, Accepted 6th August 2004
First published as an Advance Article on the web 16th September 2004
The tris(oxalato)cobaltate(III) complex [Co(C2O4)3]3−, Eo III/II = +0.57 V) is readily reduced by the 2e− reagents, Sn(II)
Co
and Ge(II), in contrast to (NH3)5CoCl2+ and (NH3)5CoBr2+, which are unreactive toward these donors. Rates for
the oxalato oxidant are only 10−3–10−2 as great as those for vitamin B12a (aquacob(III)alamin, Eo +0.35 V at pH 1),
in accord with the suggestion that reductions of corrin-bound cobalt(III) by Sn(II) and Ge(II) occur predominantly
through an additional path involving Co(I). Reductions of the oxalato complex by 2e− donors are taken to proceed
by initial formation of odd-electron intermediates (e.g., SnIII and GeIII) which react rapidly with CoIII. Such a two-
step sequence is in keeping with the observed behavior of the rare reductant, TiII, which is found to be oxidized by
[Co(C2O4)3]3− more slowly than (independently prepared) Ti(III) under comparable conditions.
Stock solutions of indium(I) triflate in anhydrous CH3CN,2
Introduction
Sn(II) solutions in HCl–NaCl,8 Ge(II) solutions in HCl,3 and
Procedures for preparation of workable concentrations of the
Ga(I) solutions in H2O–CH3CN were prepared and analyzed
as described. Vanadyl triflate solutions were made by treatment
of VOSO4 with barium triflate, were reduced to V(II) using
hydrated p0s2-ions, indium(I),2 germanium(II),3 and gallium(I)4
have been recently described, allowing us to compare the
individual peculiarities of these strongly reducing species in
solution.
zinc amalgam in triflic acid, and the V(II) converted to V(III)
by mixing with a 10% excess of VO2+ in acid.9 Uranium(IV)
Striking differences are observed, for example, in reactions
solutions were prepared as described by Bose10 and potassium
with bound cobalt(III) . In contrast to In(I), which readily
tris(oxalato)cobaltate(III), K3Co(C2O4)3, was synthesized by the
reduces halo and oxalato derivatives of (NH3)5Co(III), both
Ge(II)aq and Ga(I)aq react immeasurably slowly with such
procedure of Bailar and Jones.11
Titanium(II) solutions were prepared under argon by a
oxidants, but cleanly reduce corrin-bound Co(III) (vitamin B12a
modification of the method of Kolle and Kolle.12 100 mg
related species).5
of titanium wire was dissolved in a mixture of 2 ml of neat
triflic acid, 8 ml of water, and 0.4 ml of 48% HF at room
temperature. The green solution was kept in a sealed container
The reductions of B12 have been attributed to intervention of
a Co(I) transient:
and used within 10 h of preparation. In our hands, such
(corrin)CoIII + GeII → (corrin)CoI + GeIV
(1)
solutions exhibited e660 = 3.5 ± 0.2 M−1 cm−1 and e430 = 6.0 ±
0.5 M−1 cm−1 rather than the much lower reported values, 0.5
and 0.8 M−1 cm−1.13 These preparations became discolored on
standing for 24 h.
(corrin)CoI + (corrin)CoIII → 2(corrin)CoII (rapid) (2)
but may reflect, at least in part; the more positive potential
associated with the corrin-derived system.6,7 Since, however,
Hin-Fat and Higginson7 have noted that replacement of ligating
Stoichiometric studies. Stoichiometric determinations were
carried out under argon, most often with the oxidant in excess,
and were monitored at the absorbance maximum of that
oxidant. Measured deficient quantities of the reductant were
added, and the decreases in absorbance were compared to those
resulting from addition of an excess of the reductant.
3+
ammonias in Co(NH3)6 by three chelating oxalato groups
boosts its potential markedly (by approximately 500 mV), we
anticipated that examination of the reactions of the readily
prepared Co(C2O4)33− complex with Ge(II), Ga(I) and other 2e−
reductants might be instructive.
This report includes, we believe for the first time, kinetic data
pertaining to electron transfer reactions of the unusual state,
titanium(II) in aqueous media.
Kinetic studies. Reactions were performed under argon in
a darkened room since Co(C2O4)3 is light-sensitive. Freshly
3−
prepared Co(III) was used for each series of runs. Rates were
obtained from measurements of absorbance decreases of
the oxidant, using a Durrum–Gibson stopped-flow spectro-
photometer interfaced with an OLIS computer system or a
Shimadzu 1601 UV-visible spectrophotometer. Temperatures
were kept at 22.0 ± 0.5 °C. Ionic strength was maintained with
NaCl–HCl or HCl–HClO4. Concentrations were most often
adjusted so that no more than 10% of the reagent in excess was
consumed in a given run (pseudo-first order conditions). The
resulting exponential curves were followed for at least five half-
life periods, and rate constants were evaluated by nonlinear
least-squares fittings to the relationship corresponding to
first-order decay. The very rapid reaction of Ga(I) could not be
monitored via stopped-flow, due to interference by H2 bubble
formation; when the reaction was examined using conventional
mixing, only a lower limit, k2 > 2 × 102 M−1 s−1 (22 °C) could
be obtained.
Experimental
Materials
All solutions were prepared from Millipore-Q system deionized
water that had been boiled for 2 h and then sparged with pure
argon for a further 2 h to remove dissolved oxygen. Inorganic
reagents, including indium metal, germanium dioxide, “gallium
dichloride” (GaIGaIIICl4), titanium trichloride (in HCl), silver
trifluoromethanesulfonate (silver triflate), barium triflate, and
ammonium iron(II) sulfate hexahydrate (all Aldrich products)
were used as received.
† Electronic supplementary information (ESI) available: Tables
S1–S4; detailed kinetic data for redox reactions. See http://www.rsc.org/
suppdata/dt/b4/b408758p/
‡ Electron Transfer. Part 159. For Part 158, see ref. 1.
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
D a l t o n T r a n s . , 2 0 0 4 , 3 6 0 1 – 3 6 0 3
3 6 0 1

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Table 1 Stoichiometry of the reactions of [Co(C2O4)3]3− with single-
Sn(II)–Co(III) rates (Table S3A, ESI†) conform to expression
(8)
electron and two-electron reductantsa
[CoIII ][SnII ](k3K3[Cl− ]3 )
1+ K1[Cl− ] + K2 [Cl− ]2 + K3[Cl− ]3
D[CoIII]b/D[red]
−d[CoIII ]/dt =
Reductant (red)
1e− reagents
(8)
where the denominator indicates the partition of Sn(II) into
three chloro complexes. The numerator points to a preponderant
path, k3K3[Cl−]3, involving SnCl3−. Refinement of data in terms
of eqn. (8) yields rate constants in Table 2. Alternate sequences,
involving the lower chloro complexes as well, did not improve
the fit. Calculated and observed rates are compared at the right
of this table.
Reductions by Ge(II) (Table S3B, ESI†) are less sensitive
to changes in the medium than those by Sn(II) eqn. (8). The
resulting rate law is given by eqn. (9); refinement of data yields
k1 = (2.01 ± 0.03) M−1 s−1.
Fe(II)c
Ti(III)c
1.01
1.02
2e− reagents
In(I)d
1.93
1.94
1.84
1.91
Ge(II)c
Sn(II)c
Ti(II)e
a Reactions at 22 °C in a darkened room. b Determined spectro-
photometrically at 603 nm. c In 0.50 M HCl. d In 0.20 M HAc–Ac− buffer
(pH 4.7). e In 0.4 M NaCl–0.2 M HCl.
[CoIII ][GeII ](k1K[Cl− ])
−d[CoIII ]/dt =
(9)
1+ K[Cl− ]
Results
This reaction then utilizes a path requiring just one extra Cl−.
The small anation constant, KCl = 0.20 M−1, associated with
attachment of an additional Cl− to the principal Ge(II) center, is
consistent with that reported earlier.5
Somewhat unexpectedly, reductions by Ti(II) (Table S4, ESI†)
are slower than those by Ti(III) under comparable conditions.
Slight but perceptible inhibition, both by H+ and by Cl−, is
observed. The rates are correlated by expression (10):
In addition to the 2e− reductants (SnII, GeII, InI and TiII) some
experiments with the single-electron reagents FeII, TiIII, and VIII
are included for comparison. The le− donors are seen (Table 1) to
exhibit very nearly 1:1 stoichiometry, whereas the 2e− reagents
(including TiII) approach the expected 1:2. Slight deviations
with the slower reductants probably reflect loss from traces
of O2 during the longer waiting periods. Monophasic kinetic
profiles obtained in all instances rule out the intervention of
transients formed or consumed on a time scale comparable to
that of the principal redox transformation.
rate = [CoIII][TiII](k0 + a/[H+] + b/[Cl−])
(10)
Refinement yields parameters k0 = 62 ± 2 M−1 s−1, a = 6.1 ±
0.5 s−1 and b = 3.3 ± 0.4 s−1. It is most unlikely that the a
term in (10) is mechanistically meaningful. If it is attributed
to reaction by the deprotonated form of the reductant,
[TiII(OH)]+, a bimolecular rate constant, obtained by division
by Ka for TiII(aq) (pKA near 12.0),17 greatly exceeds the diffusion
controlled limit. We therefore ascribe this term to a medium
effect. Interpretation of the b term in eqn. (10) requires (as yet
unavailable) knowledge of the anation quotients applicable to
chloro complexes of Ti(II) in this medium.
One-electron reductions
Reductions of [Co(C2O4)3]3− by our le− reagents are
relatively straightforward. Data for the FeII–CoIII reaction
generate the simple monomial rate law: rate = k[Ox][Red]
with k = 35 ± 1 M−1 s−1, under our conditions. Reductions by
Ti(III) and V(III) (Tables S1 and S2, ESI†) exhibit inverse acid
terms, a type of dependence which has long been recognized for
these reductants.9,14 The relatively high Fe(II) rate (Eo = 0.77 V
for FeIII/II), in contrast to that for the more strongly reducing
V(III) (Eo = 0.36 V for V3+/VO2+) points to an important inner-
sphere component for the d6 center, but Grossman and Haim15
have pointed out that the very rapid consumption of the same
oxidant by VII (k25 °C = 2 × 104 M−1 s−1) must be predominantly
outer sphere since it proceeds much more quickly than ligand
attachment to the V(II) center.
Rate laws and kinetic parameters are summarized in Table 2.
Discussion
In our view, the most noteworthy aspects of this study are: (a) the
facile reduction of the Co(III) oxidant by Sn(II) and Ge(II), and
(b) the relatively sluggish redox action of the Ti(II) center.
The usual reluctance of SnII and GeII to utilize ligand bridging
in reductions of Co(III)5 implies that reductions by these s2 centers
use non-bridged paths, as has been demonstrated with V(II).15
If, as we suspect, these reductions are initiated by single
electron transfers, our observations confirm the suggestion
by Higginson17 that this oxidant, converts Sn(II) to short-lived
SnIII and we may extend this inference to the conversion of GeII
to GeIII:
Two-electron reductions
The facile reduction (k = 10.4 M−1 s−1) by In(I), nominally a
2e− donor, brings to mind the versatility of its action toward
(NH3)5CoIII oxidants.2 Initiation of reaction in such cases, is
taken to proceed through short-lived In(II):
CoIII + InI → CoII + InII (rate determining)
CoIII + InII → CoII + InIII (rapid)
(3)
(4)
CoIII + GeII → CoII + GeIII
(11)
(12)
Both Sn(II) and Ge(II) are likewise found to react readily.
These are, to our knowledge, the first reported reductions
of non-corrin-bound Co(III) by these 2e− donors. Sequences
analogous to eqns. (3) and (4) may be involved here.
However, consideration of the Sn(II) reduction in Cl-
containing media (Table S3, ESI†) must take into account the
partial conversion of this reaction into four ligation levels:16
CoIII + GeIII → CoII + GeIV (rapid)
Since the increased reactivity of [Co(C2O4)3]3− reflects, in
large part, its favorable potential (Eo = 0.57 V),4 one may ask
whether the remarkable reactivity of Sn(II) and Ge(II) toward
5
corrin-bound Co(III) in B12a is simply potential-related. We
think not. Rates for Sn(II)–B12a are about 3 × 102 as great as
those for reduction of our tris-oxalato complex despite a 0.20 V
potential gap favoring the latter. Similarly, rates for Ge(II)–B12a
are about 103 as large as those for Ge(II) –Co(ox)33−. Even when
potential differences are taken into account, evidence persists for
reduction of B12a via its Co(I) counterpart (sequence (1), (2)).
Sn2+ + Cl− → SnCl+ (K1 = 11.2)
Sn2+ + 2Cl → SnCl2 (K2 = 32)
(5)
(6)
(7)
Sn2+ + 3Cl− → SnCl3 (K3 = 14)
−
3 6 0 2
D a l t o n T r a n s . , 2 0 0 4 , 3 6 0 1 – 3 6 0 3

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Table 2 Rate laws and kinetic parameters for reductions of Co(C2O4)33−a
Red
Rate law
Parameters
Fe(II)
Ti(III)
k[CoIII][FeII]
k[CoIII][TiIII][(k0 + k/[H+])
k = 35 ± 1 M−1 s−1
k0 = (1.5 ± 0.2) × 102 M−1 s−1
k = (56 ± 3) s−1
V(III)
k[CoIII][VIII](k0 + k/[H+])
k0 = (1.3 ± 0.1) × 10−2 M−1 s−1
k = (4.3 ± 0.2) × 10−3 s−1
In(I)c
V(II)
k[CoIII][InI]
k[CoIII][VII]
k = 10.4 ± 1.3 M−1 s−1b
k = 2 × 104 M−1 s−1b
[CoIII ][SnII ](k3K3[Cl− ]3 )
1+ K1[Cl− ] + k2[Cl− ]2 + K3[Cl− ]3
Sn(II)
k3 = (0.41 ± 0.01) M−1 s−1
K1 = 11.2; K2 = 32; K3 = 14
[CoIII ][GeII ]kK[Cl− ]
1+ K[Cl− ]
Ge(II)
k = (2.01 ± 0.03) M−1 s−1; K = 0.20
Ti(II)d
Ga(I)
[CoIII][TiII](k0 + a/[H+] + b/[Cl−])
k[CoIII][GaI]
k0 = (62 ± 2) M−1 s−1
a = (6.1 ± 0.5) s−1; b = (3.3 ± 0.4) s−1
k > 200 M−1 s−1
a Reactions were carried out at 22.0 ± 0.5 °C; l = 0.50 M (HCl/NaCl) unless otherwise indicated; k = 603 nm. b Ref. 15. c l = 0.050 M. d l = 1.0–1.2 M.
Here and elsewhere2–4 those reductions of FeIII, CoIII, or IrIV
References
1 Z. Yang and E. S. Gould, Dalton Trans, 2004, 1858.
by 2e− donors which exhibit exponential profiles have been taken
to proceed via two-step sequences such as (11) + (12), in which
2 S. K. Chandra and E. S. Gould, Inorg. Chem., 1996, 35, 3881.
the rate constant associated with the odd electron transient
3 O. A. Babich and E. S. Gould, Inorg. Chem., 2000, 39, 4119.
(e.g., GeIII) is much greater than that for the parent reductant.
4 S. Swavey, V. Manivannan and E. S. Gould, Inorg. Chem., 2001,
However, direct observation of such intermediates has proved
elusive. In the present study, comparisons of reductions by
Ti(II) with those by Ti(III) (prepared independently) confirm
the reasonableness of this path. Once again, the odd-electron
reductant is found to be the more rapid reactant.
40, 1312.
5 O. A. Babich and E. S. Gould, Inorg. Chim. Acta, 2002, 336, 80.
6 D. Lexa and J.-M. Saveant, Acc. Chem. Res., 1983, 16, 235, report a
formal potential near +0.35 V for (corrin)CoII/IIII at pH 1. Although
values for the Co(NH3)5 series have not been recorded, they are
expected to lie near +0.06 V listed for (NH3)6CoII/III by Hin-Fat and
Higginson7.
Why is reduction by Ti(II) so slow? This cannot be attributed
to an inadequate potential, but if considered on the basis of the
model of Marcus for outer-sphere reactions,18 may instead imply
a very low value for the Ti(III,II) self-exchange rate in our media.
Both TiII (3d2) and TiIII (3d1) are, in principle, subject to minor
Jahn–Teller distortions, but the degree of such distortion and
the occupancy of the non-degenerate orbitals [(one short, two
long) vs. (two short, one long)] would not necessarily match. The
two states might then feature substantially different, geometries,
and this mismatch would presumably be reflected in a higher
Franck–Condon barrier to exchange. Alternatively, partial
olation of TiII centers to unreactive dimeric or higher associated
species cannot be ruled out, although it would not be expected
in the strongly acidic solutions used.
7 L. Hin-Fat and W. C. E. Higginson, J. Chem. Soc. A, 1967, 298.
8 Z. Yang and E. S. Gould, J. Chem. Soc., Dalton Trans., 2002,
3840.
9 M. C. Ghosh and E. S. Gould, J. Am. Chem. Soc., 1993, 115, 3167.
10 R. N. Bose and E. S. Gould, Inorg. Chem., 1986, 25, 94.
11 J. C. Bailar, Jr. and E. M. Jones, Inorg. Synth., 1939, 1, 7.
12 U. Kolle and P. Kolle, Angew. Chem., Int. Ed., 2003, 115, 3167.
13 Values of emax in the range 4–15 M−1 cm−1 may be considered
typical of 3d1 and 3d2 aqua-substituted cations of this type (VIII
7.0, 11.0; TiIII 3.9).
;
14 (a) A. H. Martin and E. S. Gould, Inorg. Chem., 1975, 14, 873;
(b) M. S. Ram, A. H. Martin and E. S. Gould, Inorg. Chem., 1983,
22, 1103.
15 B. Grossman and A. Haim, J. Am. Chem.. Soc., 1971, 93, 6490.
16 R. M. Smith and A. E. Martell, Critical Stability Constants, Plenum,
New York, 1976, vol. 4, pp. 1–6, 109. The pKA value for TiII(aq)
is taken to fall between those for Ca2+(aq) (12.7) and Mn2+ (11.1).
17 W. C. E. Higginson, R. T. Leigh and R. Nightingale, J. Chem. Soc.,
1962, 435.
18 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155.
19 For a detailed treatment, see: Standard Potentials in Aqueous
Solution, ed. A. J. Bard, R. Parsons and J. Jordan, Dekker, New York,
1985, pp. 539–546. Substantial variation of this potential by partial
conversion of one or both states to fluoro complexes would not be
expected since [F−] would be very small in the highly acidic media
used.
The ease with which Ti(II) can be prepared12 bears upon the
current uncertainty in the value of Eo(TiIII/TiII).19 Of the two most
frequently cited potentials, we favor −0.37 V forwarded by Forbes
and Hall20 over the much more negative figure −2.1 V suggested
by Oliver and Ross.21 The latter is difficult to reconcile with the
lack of redox activity of Ti(II) with H+ and Co2+ in our systems.
Acknowledgements
We are grateful to the National Science Foundation for support
of this work and to Mrs Arla Dee McPherson for technical
assistance.
20 G. S. Forbes and I. F. Hall, J. Am. Chem. Soc., 1924, 46, 583.
21 J. W. Oliver and J. W. Ross, Jr., J. Am. Chem.. Soc., 1963, 85, 2565.
D a l t o n T r a n s . , 2 0 0 4 , 3 6 0 1 – 3 6 0 3
3 6 0 3