Aqueous solutions of hypovalent gallium; reductions using gallium( )†
I
Shawn Swavey* and Edwin S. Gould*
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA
Received (in Cambridge, UK) 19th June 2000, Accepted 25th September 2000
First published as an Advance Article on the web
Solutions 0.2 mol dm23 in GaI, prepared by dissolving
Ga2Cl4 in dry acetonitrile, are stable for more than seven
days and may be diluted 300- to 1000-fold with O2-free water
le2 oxidants IrCl6 and Fe(bipy)33+, reflecting the expected
conversion to GaIII with oxidants of either type [eqn. (1) and
(2)]:
22
to give GaI preparations that may be handled by conven-
GaI + Br2 ? GaIII + 2Br2
GaI + 2 Fe(bipy)3 ? GaIII + 2 Fe(bipy)3
Oxidation by HCrO42 utilizes 0.64 ± 0.01 mol of CrVI in 0.01
M HClO4 but 1.1 mol of oxidant when carried out in 2-ethyl-
2-hydroxybutanoic acid buffer (EHBA/EHB2, pH 3.3), indicat-
ing predominant conversion to CrIII in the absence of this
chelating ligand [eqn. (3)] but formation of CrIV in its presence
[eqn. (4)]:
(1)
tional techniques; these GaI(aq) solutions readily reduce I3
,
2
3+
2+
(2)
Br2(aq), IrCl622, Fe(bipy)3 and aquacob(III)alamin (B12a
)
3+
but are inert to Co(NH3)5Cl2+ and Co(NH3)5Br2+; reduction
of HCrO42 in 2-ethyl-2-hydroxybutanoate buffers yields the
CrIV chelate of the buffering anion.
Accounts of the generation of gallium( ) species in aqueous
I
solution are exceedingly scarce,2,3 and no redox studies of this
unusual state appear to have been described. A standard
3GaI + 2 CrVI ? 3 GaIII + 2 CrIII (pH 1–2)
(3)
potential for Ga(III
, ), 20.755 V (25 °C) has been docu-
I
GaI + CrVI æEæHBæAÆGaIII + CrVI (EHB– complex)
mented.2
(4)
buffer
The crystalline compound ‘gallium dichloride’ (Ga2Cl4) is
known to feature equal numbers of Ga(
) and Ga(III) centers (GaI
GaIIICl42).4 Employing this as a source of hypovalent gallium,
we have prepared aqueous Ga( ) solutions which have allowed
The pink product of eqn. (4) showed a strong peak at 510 nm,
typical of EHB-chelated CrIV.5
I
The reactions listed in Table 1 are first order in each of the
I
3+
redox partners. Although oxidations by IrCl622 and Fe(bipy)3
us, utilizing conventional methods, to compare rates at which
almost certainly pass through the intermediate state GaII, kinetic
profiles for these oxidants exhibit no discontinuity attributable
to the accumulation or decay of this odd-electron species,
implying a two-step sequence (5), in which
this s2-center reacts with a variety of oxidants.
Manipulations of Ga2Cl4 (Aldrich) were carried out under
high purity N2 or Ar. Solutions were prepared by dissolving
1.25 g of this halide in 8.0 ml of anhydrous MeCN under a
constant flow of protective gas. After 5 min of stirring, a silver-
colored precipitate separates. The clear yellow supernatant
solution, which was obtained by centrifuging, was found to be
0.20 mol dm23 in GaI (spectrophotometric redox titration vs.
KI3 at 353 nm) and remained unchanged on standing for seven
days. Aqueous solutions for kinetic experiments, prepared by
300- to 1000-fold dilutions of the MeCN solutions with O2-free
water, were stable for 10–15 min in the absence of electrolyte.
For slow reactions (e.g. reduction of vitamin B12a) fresh
IrIV
IrIV
GaI ææÆGaII ææÆGaIII
(5)
slow
rapid
the initial step is rate-determining and the more rapid follow-up
step is kinetically silent. The relative rates of the two steps
suggest that GaII, the s1 intermediate, is much more strongly
reducing than the parent s2 cation, a difference applying also to
8
the related p-block triads, Tl(III
,
II,
I
),7 In(III
,
II,
I)
and
Ge(IV
, ,
II).9
III
2
I
I
In contrast, we find the cobalt(III) corrin derivative, aqua-
cob(III)alamin (B12a) to be reduced smoothly to its Co(II) analog.
We suspect that this reaction is initiated by the two-unit
reduction (very likely by oxo-transfer) to the known CoI
complex, B12s [cob(I)alamin], a hypovalent species which has
been shown11 to undergo very rapid comproportionation with
a
Table 1 Reductions with aqueous gallium(I)
Oxidant
Product
k/dm3 mol21 s21
2
I3
I2
(1.47 ± 0.09) 3 104
(2.05 ± 0.05) 3 104
(7.3 ± 0.05) 3 102
(8.9 ± 0.2) 3 104
(2.7 ± 0.1) 3 103
7.1 ± 0.3
Br2 (aq)
IrCl6
Br2
22
32
B
12a [eqn. (6)]:
CoIII ææÆCoI æææÆ2 CoII
We are grateful to the National Science Foundation for
IrCl6
3+
2+
GaI
CoIII
Fe(bipy)3
Fe(bipy)3
(6)
HCrO42 (pH 2.0)
CrIII
B12a (CoIII
)
B12r (CoII)b
[Co(NH3)5Br]2+
[Co(NH3)5Cl]2+
< 0.02
< 0.01
support of this work and to Ms Arla McPherson and Ms Carol
Haven for technical assistance.
a Reactions at 25 °C; m = 0.5 M (NaClO4, LiCl or KI); [H+] = 0.01–0.05
M; [GaI] = (1.0–12.0) 3 1024 M; [oxidant] = (4.0–12.0) 3 1025 M.
b Spectrum of product corresponded to that reported by Pratt.6
Notes and references
1 O. A. Babich and E. S. Gould, Inorg. Chem., 2000, 39, 4119.
2 L. Ph. Kozin, N. M. Openko and T. A. Tishura, Ukr. Khim. Zh. (Eng.
Ed.), 1993, 59, 227.
† Electron Transfer, part 145. For part 144, see ref. 1.
DOI: 10.1039/b004869k
Chem. Commun., 2000, 2159–2160
2159
This journal is © The Royal Society of Chemistry 2000