Reactions of 1,4-benzoquinones with s2 reducing centers

Article abstract of DOI:10.1039/b300055a

Aqueous solutions of Sn(II) and Ge(II) (in chloride media) and In(I) (in perchlorate media) react quantitatively with 1,4-benzoquinone and its 2,5-(OH)₂ and 2,5-Cl₂-3,6-(OH)₂ derivatives, reducing the oxo-functions to 1,4-(OH)₂. For Sn(II) and Ge(II), reaction is accelerated by incorporation of 2,5-(OH)₂ substituents and by chloroanation of the s² center. The most reactive reducing Sn(II) species are SnCl₃^- for benzoquinone and dihydroxyquinone but SnCl₂(aq)_x for the dichloroquinone. Reductions by Ge(II) proceed mainly through a species (probably GeCl ₄^2-) having one more chloride than the predominant form. The activated complex for the (OH)₂bzq-Ge(II) reaction features two germanium centers, only one of which is involved in the reduction act. Reductions of these quinones by In(I) proceed 10²-10³ times as rapidly as those by Sn(II) and Ge(II) and are not accelerated by hydroxylation of the quinone ring. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300055a

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Reactions of 1,4-benzoquinones with s2 reducing centers†
Zhiyong Yang and Edwin S. Gould*
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA.
E-mail: esgould@kent.edu
Received 3rd January 2003, Accepted 18th March 2003
First published as an Advance Article on the web 23rd April 2003
Aqueous solutions of Sn() and Ge() (in chloride media) and In() (in perchlorate media) react quantitatively
with 1,4-benzoquinone and its 2,5-(OH)2 and 2,5-Cl2-3,6-(OH)2 derivatives, reducing the oxo-functions to 1,4-(OH)2.
For Sn() and Ge(), reaction is accelerated by incorporation of 2,5-(OH)2 substituents and by chloroanation of
the s2 center. The most reactive reducing Sn() species are SnCl3Ϫ for benzoquinone and dihydroxyquinone but
2Ϫ
SnCl2(aq)x for the dichloroquinone. Reductions by Ge() proceed mainly through a species (probably GeCl4
)
having one more chloride than the predominant form. The activated complex for the (OH)2bzq–Ge() reaction
features two germanium centers, only one of which is involved in the reduction act. Reductions of these quinones
by In() proceed 102–103 times as rapidly as those by Sn() and Ge() and are not accelerated by hydroxylation
of the quinone ring.
Table 1 Stoichiometries of the reactions of 1,4-quinones with s2
Introduction
metal-ion reducing centersa
Quinones assume essential functions in a multitude of bio-
logical redox systems. They exhibit chemical and structural
versatility enabling them to act, depending on conditions,
as hydrogen atom acceptors, as single-, and as two-electron
acceptors. The ability of isopreinoid-substituted quinones to
transfer reducing equivalents across a membrane is reflected in
their central role in energy metabolism.2 Quinones, in addition,
are versatile oxidizing and dehydrogenating agents in the
organic laboratory,3a and, with suitable structural modification,
may feature significant herbicidal, antibiotic, or antineoplastic
activity.3b
The present study deals with the reactions of water-soluble
quinones [1,4-benzoquinone and its 2,5-(OH)2- and 2,5-Cl2-
3,6-(OH)2-substituted derivatives, I and II] with s2 metal-ion
reducing centers, two of which, indium()4 and germanium()5
have recently become available in readily workable concen-
trations. Since these reductants, like quinones, can enter into
both 1eϪ and 2eϪ transactions, several mechanistic possibilities
are conceivable. In examining eight separate quinone ϩ s2 redox
systems we find no fewer than seven different rate laws.
∆[Quinone]/
∆[reductant]
Quinone
Reductant
Benzoquinoneb
GeII
SnII
InI
0.94 0.01
0.94 0.02
0.99 0.01
0.96 0.02
1.00 0.06
0.94 0.03
0.95 0.01
1.04 0.03
0.94 0.02
2,5-Dihydroxybenzoquinonec
GeII
SnII
InI
2,5-Dichloro-3,6-dihydroxy-
GeII
SnII
InI
benzoquinoned (chloranilic acid)
a Reactions were carried out at 22 ЊC in 0.3 M HCl (GeII and SnII)
or 0.3 M HClO4 (InI). Reductant was taken in 30–80% deficiency.
b λ = 247 nm. c λ = 283 nm. d λ = 305 nm.
solutions 0.5 M in HCl and 1.0 M in NaCl, then centrifuging.
The clear supernatant was analyzed by titration with KI3 at 353
nm and kept under argon.
Stoichiometric studies
Stoichiometric determinations were carried out under argon
and monitored at λmax of the oxidizing quinone. Measured
deficient quantities of each reductant were added to a
known excess of the oxidant. Decreases in absorbance were
compared with those resulting from excess reductant. Results
are summarized in Table 1.
Kinetic studies
Experimental
Reactions were carried out under argon. Rates were evaluated
from absorbance decreases at λmax of the oxidant, using either a
Shimadzu 1601 instrument or a Durrum–Gibson stopped-flow
spectrophotometer interfaced with an OLIS computer system.
Temperatures were kept at 22.0 0.5 ЊC. Ionic strength was
maintained by addition of HCl–NaCl–NaClO4. Pseudo-first
order conditions were used, with the concentration of the
reagent in excess varying by less than 10% during the course
of a run. Rate constants were evaluated by nonlinear least-
squares fit to the relationship describing exponential decay. All
reactions were first order in quinone, and all but one (see
Results) were first order in reductant as well. No distortion of
kinetic curves indicative of the growth or decay of a reaction
intermediate on a time scale similar to that of the primary
reaction was observed.
Materials
All solutions were prepared using Millipore-Q-treated de-
ionized water that had been boiled for 2 h and then purged with
high purity argon for 2 h more to remove dissolved oxygen.
Germanium() oxide, tin() chloride, indium metal powder,
silver trifluoromethanesulfonate (silver triflate), and quinones
(Aldrich products) were used as received. Solutions of
indium() triflate were prepared by the method of Chandra,6
and solutions of germanium() were generated and analyzed
as described by Babich.5 Tin() solutions were prepared by
dissolving 0.60–1.0 g of SnCl2 in 6–8 ml of air-free aqueous
† Electron transfer, Part 155.1
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 3
D a l t o n T r a n s . , 2 0 0 3 , 2 2 1 9 – 2 2 2 3
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Table 2 Kinetic data for reduction of 1,4-quinones with tin()a
A. Benzoquinoneb B. 2,5-(OH)2bzqc
C. 2,5-Cl2-3,6-(OH)2bzqd
[Hϩ]/M
[ClϪ]/M
k2/MϪ1 sϪ1
[Hϩ]/M
[ClϪ]/M
10Ϫ2 k2/MϪ1 sϪ1
[Hϩ]/M
[ClϪ]/M
10Ϫ1 k2/MϪ1 sϪ1
0.10
0.10
0.10
0.10
0.10
0.17
0.20
0.30
0.40
0.50
0.10
0.20
0.30
0.40
0.50
0.50
0.50
0.50
0.50
0.50
0.34 (0.23)
1.15 (0.96)
1.67 (1.86)
2.5 (2.7)
3.5 (3.5)
3.8 (4.0)
4.3 (4.2)
4.6 (4.9)
5.6 (5.5)
6.2 (6.2)
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.50
1.00
0.20
0.30
0.50
0.70
0.80
0.90
1.00
1.00
1.00
0.45 (0.45)
0.63 (0.63)
0.97 (0.96)
1.28 (1.28)
1.40 (1.42)
1.49 (1.56)
1.68 (1.69)
1.74 (1.69)
1.65 (1.69)
0.050
0.10
0.25
0.30
0.40
0.50
0.20
0.20
0.20
0.20
0.20
0.50
0.50
0.50
0.50
0.50
0.50
0.11
0.21
0.31
0.41
0.50
5.9 (5.8)
5.4 (5.0)
3.8 (3.6)
3.7 (3.4)
3.2 (3.0)
2.8 (2.7)
1.41 (1.47)
2.3 (2.8)
2.9 (3.5)
3.4 (3.9)
3.5 (4.0)
a Reactions were carried out under pseudo-first order conditions at 22.0 0.5 ЊC; [quinone] = (2.0–5.0) × 10Ϫ5 M; [SnII] = (0.6–1.8) × 10Ϫ3 M. b µ = 0.5
M (HCl/NaCl/HClO4), 247 nm; parenthetical rates (right hand column) were calculated using eqn. (3) and parameters from Table 5. c µ = 1.0 M, 280
nm; parenthetical rates calculated using eqn. (7) and parameters from Table 5. d µ = 0.5 M, 305 nm; parenthetical rates calculated from eqn. (8) and
parameters from Table 5.
Rate law (3) indicates that this reduction occurs via competing
Results
protonated and non-protonated (k3) paths, both involving the
The close approach to 1 : 1 stoichiometry observed for all
reactions examined (Table 1), in conjunction with the absence
of spectral features attributable to a semiquinone, points to a
relatively clean reduction to a hydroquinone in each case:
trichloro reductant, SnCl3Ϫ. Refinement of data in terms of (3)
yields rate constants 8.3 0.6 MϪ1 sϪ1 for the k3 path and 19
3
MϪ2 sϪ1 for the protonated component. Alternate sequences
involving the lower chloro complexes of SnII as well did not
improve the fit. Calculated and observed rates are compared on
the right-hand column of the benzoquinone entries.
(1)
The 2,5-dihydroxy oxidant reacts much more rapidly than
its parent quinone and exhibits a substantially altered kinetic
profile. Rates vary according to eqn. (7).
(7)
(2)
The “speciation denominator”, D is again there, but no [Hϩ]-
dependent contribution is noted. Moreover, there are minor
but perceptible contributions involving SnClϩ and SnCl2.
Analogous reductions of quinone I to tetrol III, using tin metal
have been reported.7
Refinement yields: k1 = 39 8 MϪ1 sϪ1, k2 = (5 1) × 10 MϪ1 sϪ1
,
and k3 = (5.7 0.3) × 102 MϪ1 sϪ1
.
Although the net reaction is the same in all instances, the
kinetic picture features remarkable variation. At one extreme,
the very rapid In() reductions of benzoquinone (k = 8.9 × 104
Unlike the parent quinone, the (OH)2Cl2-substituted oxidant
is reduced with Sn() in a reaction retarded by increase in
acidity. The redox burden in this instance is carried by SnCl2,
but partition into two paths at different protonation levels
requires inclusion of an [Hϩ]-dependent “partition ratio” in the
rate law (8)
MϪ1 Ϫ1) and its 2,5-(OH)2 derivative (3.2 × 104 MϪ1 sϪ1
s )
exhibit straightforward second order behavior with no acidity
dependence in the range 0.01–0.50 M Hϩ.
Reductions with Sn(II)
The Sn() and Ge() reaction mixtures contain chloride since
both of these reductants are prepared in strong HCl.5 Reduc-
tions with these centers may exhibit [ClϪ]- or [Hϩ]-dependencies
or both. Data for Sn() are assembled in Table 2.
Rates for the Sn()-benzoquinone reaction conform to
expression (3)
(8)
H
where k2 and k2 pertain to the deprotonated and protonated
paths and KHA is the acidity constant governing the conversion.
Refinement of data based on (8) yields k2 = (2.2 0.1) × 102
M
Ϫ1 sϪ1, k2H = 35 9 MϪ1 sϪ1, and KHA = 0.18 M. As expected,
the rate constant for the deprotonated contribution exceeds that
for the protonated.
Reductions with Ge(II)
(3)
Data for Ge() reductions of benzoquinone and its dichloro-
dihydroxy derivative appear in Table 3. Rate laws for both
oxidants, (9) and (10), feature the denominator (1 ϩ K [ClϪ]),
which may be taken to reflect the partition of reductant
between a principal and a subsidiary complex, the latter with
one additional chloride.
The four term denominator in (3) (abbreviated D) often occurs
in rate laws associated with reactions of Sn() in chloride
media8 and reflects partition of this reductant into four ligation
levels9
SnII ϩ ClϪ
SnClϩ (K1 = 11.2)
(4)
(5)
(6)
SnII ϩ 2ClϪ
SnCl2 (K2 = 32)
(9)
Ϫ
SnII ϩ 3ClϪ
SnCl3 (K3 = 14, µ = 1.0 M)8b
D a l t o n T r a n s . , 2 0 0 3 , 2 2 1 9 – 2 2 2 3
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Table 3 Kinetic data for reduction of 1,4-quinones with Ge()a
A. Benzoquinoneb
B. 2, 5-Cl2-3,6-(OH)2bzqc
[Hϩ]/M
[ClϪ]/M
10Ϫ2 k2/MϪ1 sϪ1
[Hϩ]/M
[ClϪ]/M
10Ϫ2 k2/MϪ1 sϪ1
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.20
0.40
0.50
0.10
0.15
0.20
0.25
0.30
0.40
0.45
0.50
0.50
0.50
0.50
0.37 (0.36)
0.50 (0.48)
0.58 (0.59)
0.62 (0.71)
0.81 (0.82)
1.05 (1.03)
1.17 (1.14)
1.34 (1.24)
1.33 (1.24)
1.33 (1.24)
1.30 (1.24)
0.10
0.20
0.30
0.40
0.50
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.50
0.50
0.50
0.50
0.50
0.033
0.10
0.15
0.20
0.30
0.40
0.45
0.58 (0.53)
1.07 (1.07)
1.63 (1.60)
2.10 (2.14)
2.6 (2.7)
0.47 (0.48)
0.57 (0.57)
0.67 (0.64)
0.70 (0.70)
0.79 (0.83)
0.93 (0.95)
1.08 (1.01)
a For the 2,5-(OH)2bzq–Ge() systems, which exhibits second order reductant dependency see Table 4. The reactions above were monitored under
pseudo-first order conditions at 22.0 0.5 ЊC; [quinone] = (2.0–3.0) × 10Ϫ5 M; [GeII] = (0.5–3.2) × 10Ϫ3 M. b µ = 0.5 M (HCl/NaCl/HClO4), λ = 247
nm; parenthetical rates were calculated using eqn. (9) and parameters from Table 5. c λ = 305 nm; parenthetical rates were calculated using eqn. (10)
and parameters from Table 5.
Table 4 Reduction of 2,5-dihydroxybenzoquinone with germanium()a
from a hydroxy group of the oxidant) Refinement (keeping K1
as 0.2) yields k = (9.9 0.1) × 106 MϪ1 sϪ1 11
Rate laws and
kinetic parameters are summarized in Table 5.
.
[Hϩ]/M
[ClϪ]/M
104[GeII]/M
kobs/sϪ1
0.30
0.50
0.80
1.00
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
1.00
1.00
1.00
1.00
0.50
0.60
0.70
1.00
1.00
1.00
1.00
1.00
1.00
1.00
7.00
7.00
7.00
7.00
8.00
8.00
8.00
8.00
2.20
6.80
9.00
13.5
18.0
30.0
2.6 (2.7)
1.74 (1.62)
1.14 (1.02)
0.92 (0.81)
1.20 (1.21)
1.49 (1.43)
1.60 (1.62)
1.92 (2.12)
0.13 (0.16)
1.21 (1.53)
2.69 (2.69)
6.2 (6.0)
Discussion
Kinetic profiles of these quinone reductions exhibit no
irregularities pointing to the formation or decay of a semi-
quinone intermediate on a time scale comparable to that of
the principal conversions. We may then infer either that two
electrons are transferred from the donor center in a single
transaction, or that if a pair of single electron transfers are
involved, rates are determined by the initial act, followed by a
rapid (and kinetically silent) follow-up step.
The Franck–Condon barrier to outer sphere 2eϪ transfers is
generally taken to be prohibitively high,12 but the structure of
our oxidants allows initiation via metal–carbonyl coordination
(12), after which internal electron transfer within the IIM(bzq)
complex (13) yields the dihydroxy product.
11.9 (10.7)
29.8 (29.8)
a Reactions were carried out under pseudo-first order conditions at 22.0
0.5 ЊC; [quinone] = (2.0–5.0) × 10Ϫ5 M. b µ = 1.0 M (HCl/NaCl/
HClO4; λ = 280 nm; parenthetical rates calculated using eqn. (11) and
parameters from Table 5.
MII ϩ bzq
MII:bzq
(12)
(10)
(13)
Refinements yield k1 = (1.25 0.07) × 102 MϪ1 sϪ1 and ko = 12
3 MϪ1 sϪ1 for benzoquinone and k1 = (3.7 0.1) × 103 MϪ2 sϪ1
and ko = (2.2 0.1) × 102 MϪ2 sϪ1 for the dichloro oxidant.
Unlike its unsubstituted parent, the dichloroquinone is
reduced predominantly through protonated forms. The proton-
ations required for activation must involve an acid center with
a pK value well below zero since there is no hint of kinetic
saturation involving [Hϩ]. If the pKA of the protonated quinone
is assumed to lie near the documented value, Ϫ6,10 bimolecular
rate constants for these highly acidic forms (featuring the
Although the general sequence (12)–(13) may apply to all
redox systems here examined, differences in the detailed
behavior of Sn() and Ge() in solution can elaborate the
resulting rate laws. The denominators of each of the three Sn()
reactions mirror the known speciation9 of Sn() in aqueous
chloride. In addition, the Sn()–bzq reaction [eqn. (3)] receives
a modest kinetic boost from Hϩ and a much more dramatic
push from tris-anation of the reductant (the [ClϪ]3 dependence).
function >C᎐OHϩ) fall in the range 108–109 MϪ1 sϪ1
.
᎐
Data for the Ge() reduction of 2,5-(OH)2bzq are summar-
ized in Table 4. Observed accelerations with added reductant
are clearly more steep than that corresponding to a propor-
tional relationship. Rate law (11), which correlates these data,
H
The K3 [Hϩ] contribution doubtless arises from partial
protonation of the oxidant, which would be expected to ease
internal electron transfer within the MII(bzq) complex. The
[ClϪ]3 proportionality brings to mind similar dependances
observed for SnII reactions which are presumed to proceed
through SnIII,8b,13 and leads us to wonder whether the internal
transfer (13) takes place in a single act or in a pair of 1eϪ steps.
The Ge()–bzq reaction, involving a much stronger reductant
(11)
again includes a denominator indicating partition of Ge() and
points to preponderant activated complex having two units of
Ge() and a single ClϪ with one proton removed (presumably
(E 0 = Ϫ0.22 V at pH 0), requires only partial help from a
,
[ClϪ]1-assisted path [eqn. (9)] and no protonation. The two-term
denominator indicates that Ge() in the systems at hand exists
D a l t o n T r a n s . , 2 0 0 3 , 2 2 1 9 – 2 2 2 3
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Table 5 Rate laws and kinetic parameters for reductions of 1,4-benzoquinones with s2 metal reducing centersa
Reductant
Oxidant
Rate law
Eqn. no.
(3)
Parameters
Sn()
Benzoquinone
k3H = 19 3 MϪ2 sϪ1
k3 = 8.3 0.6 MϪ1 sϪ1
K1 = 11.2, K2 = 26, K3 = 55
(OH)2bzqb
(7)
k1 = 39 8 MϪ1 sϪ1
k2 = (5 1) × 10 MϪ1 sϪ1
k3 = (5.7 0.3) × 102 MϪ1 sϪ1
Cl2(OH)2bzq
Benzoquinone
(OH)2bzqb
(8)
k2 = (2.2 0.1) × 102 MϪ1 sϪ1
k2H = 35 9 MϪ1 sϪ1
kHA = 0.18
Ge()
(9)
ko = 12 3 MϪ1 sϪ1
k1 = (1.25 0.07) × 102 MϪ1 sϪ1
K = 0.20
(11)
(10)
k = (9.9 0.1) × 106 MϪ1 sϪ1
K = 0.20
Cl2(OH)2bzq
ko = (2.2 0.1) × 102 MϪ2 sϪ1
k1 = (3.7 0.1) × 103 MϪ2 sϪ1
In()
Benzoquinone
(OH)2bzq
k[ox][red]
k[ox][red]
k = (9.4 0.2) × 104 MϪ1 sϪ1
k = (3.1 0.1) × 104 MϪ1 sϪ1
a Reactions at 22.0 0.5 ЊC, µ = 0.5 M unless otherwise indicated. Supporting electrolyte was (HCl/NaClO4/HClO4) for Sn() and Ge() reactions,
(HClO4/NaClO4) for In() reactions. b µ = 1.0 M.
primarily as two species differing by a single chloride. Quanti-
tative data concerning speciation of Ge() at high [ClϪ] are
scarce, but Poshkozim14 has presented evidence that the
predominant form in 0.1–0.5 M HCl is GeCl3Ϫ. Data are then
consistent with equilibrium (14)
more thinly populated. Hence in this case, reaction is favored at
lower acidities. Why a similar partition of paths is not observed
in Ge() reduction of the same quinone [eqn. (10)] remains
a puzzling point.
The most unusual kinetic behavior in this series is
encountered with the (OH)2bzq–Ge() system. The reaction
is very nearly second order in reductant (in the range 0.2–3.0
mM) although the observed stoichiometry lies close to 1 : 1
(Table 1). In addition, there is a [ClϪ]-dependence and a 1/[Hϩ]
proportionality. The implied activated complex, consisting of
a unit of quinone (from which one Hϩ has been removed), two
germaniums, and one chloride, is consistent with Scheme 1.
Here, we suggest that the role of the second (non-oxidized)
GeII is simply as a positive center [analogous to Hϩ in the Sn()
reductions] which may facilitate electron flow from the active
Ge()–Cl through the ring system.
GeCl3Ϫ ϩ ClϪ
GeCl4
(14)
2Ϫ
for which the modest equilibrium constant 0.20 0.02 has been
recorded.8a
The proverbial electron-donating capabilities of aryl-bound
hydroxy substituents would be expected to disfavor acceptance
of external electrons by the quinone. For the systems at hand,
however, incorporation of hydroxy groups in the unit greatly
facilitates reduction. We suspect that chelation has entered the
picture here, with formation of a cyclic intermediate (e.g. IV),
thus increasing the concentration of this precursor complex. It
is likely that the predominant k3 path for the (OH)2bzq–Sn()
reaction [eqn. (7)] utilizes chelation, but this is less certain for
the marginal k1 and k2 paths.
The enhanced acidity of the Cl2(OH)2-substituted quinone
comes into play in its reduction by Sn() [eqn. (8)]. Here,
virtually every act of reduction involves the dichloro reductant
but the familiar kinetic acidity dependence corresponds to
partition into a deprotonated path (k2) and a protonated com-
ponent (k2H). Of the two precursors, the protonated is evidently
Scheme 1
D a l t o n T r a n s . , 2 0 0 3 , 2 2 1 9 – 2 2 2 3
2222

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The very high rates (104–105 MϪ1 Ϫ1) and simple monomial
s
References and notes
rate laws for reductions by In() hint at a change of mechanism
for this donor. Moreover, incorporation of 2,5-(OH)2 sub-
stituents is seen to retard, rather than accelerate reaction,
indicating that chelation plays no appreciable part with this uni-
positive center. The difference brings to mind the recognized
redox versatility of In(). Although normally a 2eϪ reagent, it
can reduce single electron oxidants, utilizing both bridged6 and
outer-sphere15 routes. In this case, the lack of kinetic response
to ring substituents leads us tentatively to favor initiation by an
outer-sphere single electron transfer to the quinone, yielding
a semiquinone radical and In2ϩ. Subsequent reaction of these
two unstable species must be very rapid (possibly geminate
recombination) since we see no distortions of kinetic profiles
attributable to their intervention.
1 Part 154: Z. Yang and E. S. Gould, Res. Chem. Intermed., 2003, 29,
in press.
2 Reviews: (a) P. R. Rich, Biochim. Biophys. Acta, 1984, 768, 53; (b)
S. de Vries, Chem. Scr., 1987, 27, 155; (c) H. Inouye and E. Leistner,
in The Chemistry of Quinonoid Compounds, S. Patai and
Z. Rappoport, eds., vol. 2, Wiley, New York, 1988, ch. 22.
3 (a) H.-D. Becker and A. B. Turner, in The Chemistry of Quinonid
Compounds, S. Patai and Z. Rappoport, eds. vol. 2, Wiley, New
York, 1988, ch. 23; (b) H. Nohl, W. Jordan and R. J. Youngman, Adv.
Free Rad. Biol. Med., 1986, 2, 211.
4 S. Swavey and E. S. Gould, Inorg. Chem., 2000, 39, 1200.
5 O. A. Babich and E. S. Gould, Inorg. Chem., 2000, 39, 4119.
6 S. K. Chandra and E. S. Gould, Inorg. Chem., 1996, 35, 3881.
7 (a) P. R. Weider, L. S. Hegedus, H. Asada and S. V. D’Andreq,
J. Org. Chem., 1985, 50, 4276; (b) S. Aldridge, R. J. Calder,
M. H. Cunningham, K. M. Malik and J. W. Steed, J. Organomet.
Chem., 2000, 614, 188.
8 See, for example: (a) O. A. Babich and E. S. Gould, Inorg. Chim.
Acta, 2002, 336, 80; (b) Z. Yang and E. S. Gould, J. Chem. Soc.,
Dalton Trans., 2002, 3840.
9 R. M. Smith and A. E. Martell, Critical Stability Constants, vol. 4,
Plenum, New York, 1976, p. 109.
Conclusions
The bewildering array of rate laws describing the Sn() and
Ge() reductions of the quinones examined, (reflecting, in part,
differing speciation behavior of the donors) should not obscure
similarities pertaining to the systems. Our evidence is consistent
with reactions occurring by two-unit inner-sphere transactions,
with conversions particularly with Sn(), by polychloro anation
of the reductant.
The picture with regard to In() reductions is less clear, but
the marked differences in observed patterns serve to remind us
that for this donor, the possibility of reaction via successive
single electron steps should not be ignored.
10 See, for example, E. M. Arnett, Prog. Phys. Org. Chem., 1963, 1, 223
(Table XXVI).
11 Preliminary examination of the reduction of chloranilic acid with
In() points to operation of a path which is also second order in
reductant. However, this component competes with a first order
contributor, and the system is further complicated by kinetic acidity
dependence which pertains to both components.
12 See, for example, A. G. Lappin, Redox Mechanisms in Inorganic
Chemistry, Ellis Horwood, New York, 1994, ch. 5.
13 E. A. M. Wetton and W. C. E. Higginson, J. Chem. Soc., 1965,
5890. This effect has been attributed to the structural similarity
of intermediate SnCl3 and SnIIICl3(H2O)3, a resemblance that
Ϫ
Acknowledgements
should lower the Franck–Condon barrier to the Sn()–Sn()
conversion.
We are grateful to the National Science Foundation for support
of this work (grant CHE-0090856) and to Mrs Arla McPherson
for technical assistance.
14 P. S. Poshkozim and A. L. Stone, J. Inorg. Nucl. Chem., 1970, 32,
1391.
15 S. K. Chandra and E. S. Gould, Inorg. Chem., 1997, 36, 3485.
D a l t o n T r a n s . , 2 0 0 3 , 2 2 1 9 – 2 2 2 3
2223