Reactions of Mn(OAc)2, Co(OAc)2, and Bromide Salts
Inorganic Chemistry, Vol. 40, No. 13, 2001 3229
Chart 1
controlled cell holder. Faster reactions were studied by the stopped-
flow method with a Bio-Sequential DX-17MW instrument from Applied
ψ
Photophysics. The pseudo-first-order rate constant, k , was evaluated
by the nonlinear least-squares fitting to the absorbance-time values
to the equation
-
kψt
Abs ) Abs + (Abs - Abs ) × e
t
∞
0
∞
Results
Bromide-Co(II) Complexation Equilibria. Equilibrium
constants are known for the stepwise formation of bromocobalt-
8
(
II) complexes in glacial acetic acid at 25.0 °C. Spectropho-
tometric and electrochemical determinations were carried out
in solutions containing 48.5 mmol/L H2O (0.1 wt %) in acetic
acid over a range of LiBr concentrations (3.15-100 mmol/L)
at temperatures of 25.0-60.0 °C and a cobalt(II) concentration
employs an in situ oxidation procedure based upon the oxidation of
8
•
14
of 12.3 mmol/L as described by Sawada and Tanaka. Spectral
Co(II) by ArCH
Co(OAc) , p-xylene, and sodium bromide; the cobalt(III) concentration
builds up until the oxygen has been depleted, whereupon [Co(III)]
2
OO . In it, oxygen is introduced into a solution of
scans were recorded over the range 400-800 nm at intervals
2
of 1.0 nm for at least 20 bromide concentrations at each selected
II
25
diminishes as it oxidizes Co Br
n
complexes. Further addition of oxygen
temperature. Figure S-1 shows representative results. The data
can be repeated, so that the rise and fall of the cobalt(III) absorbance
can be made to recur. The falling stage of this transformation,
representing the oxidation of HBr by Co(III) in the presence of Co(II),
was the portion of interest to us in this study.
at each temperature were analyzed simultaneously at all
wavelengths and bromide concentrations using the program
PSEQUAD. In addition, these solutions were analyzed for
21
free bromide by the electrochemical method described by
Equilibrium mixtures of bromocobalt(II) and bromomanganese(II)
8
Sawada and Tanaka. The equilibrium constants so determined,
complexes were prepared from Co(OAc)
2 2
or Mn(OAc) in glacial acetic
presented in Table 1, indicate the diminution of the binding
constants caused by even this low water concentration. Table 1
also includes the values of ∆H° and ∆S° from the fits to the
acid with a stoichiometric deficiency of NaBr or LiBr. The distribution
II
8
n
of Co Br was calculated from the stepwise formation constants. Under
the conditions chosen, the monobromocobalt(II) complex is the
predominant species, but as we shall see it is the concentration of
dibromocobalt(II) that is most important in the reaction chemistry. Water
is known to affect the extent to which given species of bromide-cobalt-
2
5
van’t Hoff equation, Figure S-2. One form in which the
temperature-dependent equilibria will be used in this research
rests on the quotient K2/K1, which represents KD, a ligand
distribution equilibrium; it shows but a small temperature effect.
Bromide-Mn(II) Complexation Equilibria. None of the
bromomanganese(II) species exhibit an absorption band of useful
intensity. Three bromomanganese(II) species were identified in
ternary mixtures of Mn(OAc)2, Co(OAc)2, and LiBr. Addition
of the manganese salt lowers the absorbance of the solution by
competitively lowering the concentrations of the Co(II)Brn
species responsible for light absorption. Using the PSEQUAD
(II) are formed, and so the stability constants were redetermined at the
water concentrations used here using the electrochemical method
8
described by Sawada and Tanaka.
Acetic acid is not an ionizing medium; hydrobromic acid is a weak
acid with pK
a
6.7. Cobalt(II) acetate can be crystallized from water as
‚(H O) with four waters coor-
Slow crystallization of 0.12 M Co(OAc)
from acetic acid afforded a different structure of hydrated
cobalt(II) acetate with an infinite chain structure, catena-poly-
the six-coordinate complex Co(OAc)
2
2
4
2
2,23
dinated to Co(OAc)
O)
2
.
2
-
(H
2
4
2
1
II
program, three bromomanganese(II) species were identified.
[
H
monoaquadiacetatocobalt(II) monohydrate], [Co (CH
3
COO)
Cobalt(II) acetate forms a pink solution in wet acetic acid,
(H O) (OAc) ; in chemical
equations it will be written simply as Co(OAc) . The monobromocobalt-
II) complex is also pink, whereas the dibromo and higher complexes
are deep blue, thus tetrahedral complexes. Similarly, the bromo
complexes will be designated CoBrOAc and CoBr
2 2
(H O)‚
II
2
4
The stability constants of Mn Br were determined by the
2
O]
n
.
n
8
consistent with six-coordinate Co(HOAc)
n
2
m
2
electrochemical method described by Sawada and Tanaka.
These experiments used 2.20 mmol L Mn(OAc) and 0-21.8
mmol L LiBr in glacial acetic acid at 25 °C. These formation
-1
2
2
(
-1
constants were calculated for MnBrn: K1 ) 701 ( 8, K2 ) 91
2
.
(
10, and K3 ) 18 ( 8. When expressed as overall formation
Trivalent Mn(III) and Co(III) exist in di- and trinuclear forms in
acetic acid, as well as transient mononuclear species. This point is
particularly pertinent for Co(III), the kinetic inertness of which ensures
that such forms will not likely equilibrate with one another in the
reaction time. We have recently commented upon the Mn(III) and Co-
constants, the values become log â1 ) 2.84, log â2 ) 4.80, and
log â3 ) 6.06. At 50 °C, K1 ) 415 ( 13, K2 ) 19 ( 3, and K3
∼
0. No association between Co(II) and Mn(II) could be
detected spectrophotometrically.
1
,16-20
The formation constants K for both cobalt and manganese
(
III) species.
Except when speciation itself becomes an issue, the
will be used.
n
notation M(OAc)
3
decrease markedly with the water content. This becomes an
Kinetics. Most of the reactions were examined with UV-visible
important point in the practical autoxidation chemistry, as water
is a byproduct. For that reason, autoxidation becomes ineffective
once water reaches a certain level. This depends on the
hydrocarbon used, but it is about 10% water in the case of
terephthalic acid.
methods, for which Shimadzu 2101 and 3101 and Shimadzu MultiSpec
1
501 instruments were used. The reaction cuvettes were maintained at
constant temperature (5-80 °C, as desired) by an electronically
(
21) Z e´ k a´ ny, L.; Nagyp a´ l, I. In Computational methods for the determi-
nation of formation constants; Leggett, D., Ed.; Plenum Press: New
York, 1985. We are grateful to G a´ bor Lente for assistance in its
implementation.
Manganese(III) Oxidation of Bromide in the Presence of
Mn(II). We have reported the oxidation of HBr and MBr (M
1
II
) alkali metal) by Mn(III). When Mn Brn complexes are also
(
(
22) Kaduk, J. A.; Partenheimer, W. Powder Diffr. 1997, 12, 27-39.
present in solution, a new pathway opens that arises from direct
oxidation of Mn Brn. The net reaction remains as before:
23) Niekerk, J. N. V.; Schoening, F. R. Acta Crystallogr. 1953, 6, 609-
II
612.
(
24) Jiao, X.-D.; Guzei, I. A.; Espenson, J. H. Z. Kristallogr.sNew Cryst.
Struct. 2000, 215, 173.
(25) See the Supporting Information.