PULSE RADIOLYSIS OF AQUEOUS SOLUTIONS AS A TOOL
IN INORGANIC CHEMISTRY
D. Meyerstein
Department of Biological Chemistry, The College of Judea and
Samaria, Ariel, Israel
and Department of Chemistry, Ben-Gurion University of the Negev,
Beer-Sheva, Israel
The radiolysis of dilute aqueous solutions can be summed up by
the equation: [1-3]
 ; eH2O
e-aq (2.65); .OH (2.65); H. (0.60); H2 (0.45); H2O2 (0.75)
(1)
where the values in parentheses are the number of molecules of a given
product formed by the absorption of 100 eV in the medium. The radicals
thus formed are powerful redox reagents, i.e. e-aq (E° = -2.87 V); H.
(E° = ±2.31 V) and .OH (E° = +2.73 V). [4]
Thus a mixture of strong single electron oxidising- and
reducing-agents is formed. However the hydrated electrons can be
transformed into hydroxyl radicals via: [5]
H2O
.
e-aq + N2O
OH + N2 + OH
(2)
and into hydrogen atoms via: [5]
e-aq + H3O+
.
H + H2O
(3)
In pulse-radiolysis experiments these radicals are formed by a
short pulse, 10-12 – 10-6 s depending on the experimental set up, in
concentrations which enable their physical observation. The linear
electron accelerator of the Hebrew University of Jerusalem, which we
use, forms up to 2x10-5 M of radicals within < 1.5x10-6 s. The reactions
of the radicals thus formed, or their reactions products, can be followed
by a variety of physical techniques including spectrophotomety (this
technique is most commonly used), EPR, electrical conductivity,
polarography, resonance Raman, NMR etc. [1-3]
119
The primary radicals formed can be transformed via the addition
of different solutes into a variety of secondary inorganic (e.g. O2.-; HO2.;
O3.-; CO2.-; CO3.-; CN.; N3.; .NH2; .NO; .NO2; .NO3; NCO.; PO3.2-; PO4.2-;
HS.; RSSR.; SO2.-; SO3.-; SO4.-; SO5.-; (SCN)2.-; SeO3.-; HSeO4.2-;
(SeCN)2.-; Cl2.-; Br2.-; I2.-; ClO2.; BrO2.; IO2.; ) [6] and organic radicals
(e.g. .CH3; .CH2CH3; .CH2CH2CH3; .CH(CH3)2; .C(CH3)3; c-.C5H9;
.
CH2Cl; .CH2Br; .CF3; .CCl3; .CBr3; .CH2OH; .CH(CH3)OH; .C(CH3)2OH;
.
.
.
.
CH2CH2OH;
CH2C(CH3)2OH;
CH2OCH3;
CH(CH3)OC2H5;
.
.
.
.
CH(OH)CH2OH;
CH2CHO;
CH2CO2H;
CH(CH3)CO2H;
.
.
.
CH(OH)CO2H;
C(OH)(CH3)CO2H;
CH(CH2NH3+)CO2-;
.
CH(CH3)NH2; .CH2C(CH3)2NH3+; .CH2CN; .CH2C6H5; .SC2H5; CH3OO.:
CCl3OO.; NCCH2OO.; HO2CCH2OO.; etc.) [7-8] with desired redox
potentials. [4-9]
The reactions of the appropriate radicals with M(H2O)mn+ ions or
with MLmn+ complexes enables the formation of complexes with
unstable oxidation states and the study of their chemical properties. Thus
complexes of Ag°; Ag2+; AgII; Au°; AuII; BiII; BiIV; CdI; CoI; CrI; CrV;
Cu°; CuI; CuIII; FeIV; FeV; Hg°; HgI; Hg2+; InI; InII; MnIII; MnIV; MnV;
MoIV; MoV; NiI; NiIII; Pb°; PbI; PbIII; PdI; PdIII; PtI; PtIII; ReVI; Rh°; RhI;
RhII; RuI; SnIII; Tl°; TlII and ZnI were prepared [10,11] and their
chemical and physical properties studied. It should be pointed out that
the pulse-radiolysis technique is often also used to study fast reactions of
transition metal complexes with common oxidation states, e.g. ligand
exchange reactions and reactions with dioxygen.
In the following sections some examples, from our studies, of
the use of pulse radiolysis to solve chemical problems are described.
I. Ligand exchange reactions.
The kinetics of the ligand loss of CoII(acac)3- and CrII(acac)3-,
where acac- = acetylacetonate, were studied [12] as a function of pH.
These complexes were prepared via the fast reaction of e-aq with the
corresponding trivalent complexes. The ligand exchange reactions were
followed using the conductivity detection technique. The results are in
accord with a mechanism in which the bidentate bound ligand is in
equilibrium with the ligand bound only as a monodentate, these
equilibria constants could be estimated. The ligand bound as a
monodentate is lost by an acid catalysed process with an acid
independent contribution. [12]
120
The kinetics of the ligation of fumarate and maleate to Cu+aq
were studied. [13,14] It was found that the kinetics of formation of d 
* complexes of this type approach the diffusion controlled limit. These
results were also the first evidence for the rates of ligand exchange of
Cu+aq. The results enabled also to measure the stability constants of these
complexes and the pKa’s of the bound ligands. [13,14]
The kinetics of SO42- axial ligation to NiIIIL(H2O)23+, where L =
5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane,
were
studied. [15] The results are in accord with a relatively fast ligand
exchange process as expected for a low spin d7 Jahn-Teller distorted
complex. The results also pint out that SO42- does not ligate to
NiIIIL(OH)2+, and enable the derivation of the pKa of NiIIIL(H2O)23+. [15]
The kinetics of ligation of NO to aminocarboxylato complexes
of Fe(II) were studied. [16] The results enable the determination of the
stability constants of these complexes and analysis of the effect of the
nature of the aminocarboxylate ligand on the kinetics. [16]
II. Reactions with dioxygen.
The kinetics of the reaction
Cu+aq + O2
Cu(O2)+
(1)
H+
+
+
Cu(O2) +Cu
aq
2Cu2+aq + H2O2
(2)
were studied. [17] From the results K1k2 = 2.3x108 M-2s-1 is derived.
The results indicate that fumarate as a ligand slows down considerably
the rate of this process. This observation is attributed to the formation of
the d * complex.
Surprisingly enough the stable NiIIIedta- complex reacts with
dioxygen via the reaction:
2NiIIIedta- + O2  2NiIIL- + 2CO2 + H2O2
k = 2.0x108 M-2s-1 (3)
where L = N,N,N’-ethylenediaminetriacetate. [18] The result points out
that the Ni(III) central cation has not the potential to oxidise the edta via
a single electron transfer to the carboxylate group and requires a second
121
oxidising agent. However the complex {NiIIIedta-;O2} seems also to be
stable, probably as its decomposition would form O 2.- which is
themodynamically an unfavourable process. Thus a coherent oxidation
of two edta ligands by two Ni(III) ions and by dioxygen occurs. It was
proposed that the transition state for this process is: [18]
O....O
2e
H2C
N
C
O
....
O
2e
1e C
1e
1e O
O 1e
Ni(III)
CH 2
N
Ni (III)
III. Intramolecular electron transfer reactions.
The rate of intramolecular electron transfer reactions of the type:
{(NH3)5CoIII--O2CC6H4NO2.--p} Co2+aq + 5NH4+ + p--O2CC6H4NO2(4)
3 -1
k = 2.6x10 s
were measured. [19] As these rates are slower, or equal to the rates of
the analogous intermolecular reactions: [20]
CoIII(NH3)5 X + p--O2CC6H4NO2.-  Co2+aq + 5NH4+ + X + p--O2CC6H4NO2- (5)
(X = NH3 or --O2CC6H5)
the results demonstrate that the carboxylate group has a poor electron
permeability. [20]
Analogous techniques were used to study the inramolecular
electron transfer processes in a variety of enzymes. [21-25]
IV. Reaction of aliphatic carbon-centered radicals with
transition metal complexes in aqueous solutions.
The kinetics and mechanisms of reaction of many carboncentered radicals with a large variety of transition metal complexes were
studied. The specific rates of most of these reactions are summed up in
reference [7]. In principle these reactions might proceed via one of the
following mechanisms:
122
Mn±1Lm + R-/+
LmMn+1-R or Lm-1Mn+1-R + L
MnLm + R.Mn-1Lm-1 + L-R or L± + R-/+
Lm-1Mn-LR. Mn±1Lm-1 + L-R-/+ or + L± + R-/+
Lm-1Mn(L±) + R-/+Mn±1Lm + R-/+
(6)
(7)
(8)
(9)
(10)
Reaction (6) describes outer-sphere redox processes. As the self
exchange rates for the R./- and R+/. couples are usually slow these
reactions are not abundant. However outer-sphere reductions of
transition metal complexes by .CR1R2OH or .CR1R2O- radicals [26-34]
were observed. This is reasonable as these radicals are powerful
reducing agents and as the oxidation of these radicals does not require
major bond rearrangements. Thus the self exchange rate for the couple
C(CH3)2OH+/0 has been estimated to be ~ 103 M-1s-1 [32]. It should be
noted that CO2.- is also a powerful reducing agent [4,35] but as it is bent
and CO2 is linear most of its reactions proceed via the inner-sphere
mechanism.
Processes following the mechanism described in equation (8) are
mainly observed for halide, and analogous complexes as the oxidising
species, e.g. [26,30,34,36]:
M(NH3)5X2+ + R.
RX + M2+aq + 5NH4+
(11)
In principle also the reactions [see for example ref. 37,38]:
LmMn+1-R + R’.
MnLm + RR’ or RH + R-H
(12)
which are discussed below in detail follow an analogous mechanism.
Processes following the mechanism described in equation (9) are
observed for the addition of alkyl [39] and substituted alkyl [40] radicals
to aromatic ligands followed by the reduction of the central cation and
proton loss from the aromatic ring, e.g. [39,40]. In principle the central
cation could also be oxidised by its radical-ligand. However, such
reactions are not known.
Processes following the mechanism described in equation (10)
are abundant, though only processes in which the ligand is reduced are
123
known [20,41-50] an example to this is the formation of {(NH3)5CoIII-O2CC6H4NO2.--p} followed by reaction (4).
Processes following the mechanism described in equation (7) are
the common mechanism of reaction for “low valent” complexes with
ligand exchange rates, and/or steric structures, which enable a bond
formation between the central metal cation and the attacking radical. The
products of these reactions are transient or stable complexes with metalcarbon  bonds. The question whether the mechanism involves ligand
exchange, or is accompanied by an increase in the coordination number
is often not clear. In some cases the results indeed suggest that an
increase in the coordination number occurs at least initially, [51-53]
whereas in other cases the reaction seems to require a loss of one of the
ligands L [54]. Recently measurements of the volumes of activation of
several of these reactions pointed out that the rate determining step in
most of these reactions is the ligand interchange step, i.e. that the
number of ligands is not increased in these processes [55]. The specific
rates of reactions proceeding via this mechanism were measured for first
row transition metal complexes with different ligands for which M n =
Ti(III) [30,56,57] (though the transient complexes with a TiIV-C bond
were not observed); V(II) [9,58] (though the transient complexes with a
VIII-C bond were not observed); Cr(II) [59-69]; Cr(III) [69]; Mn(II)
[70,71]; Mn(III) [71] (though the transient complexes with a MnIV-C
bond were not observed); Fe(II) [70,72-78]; Fe(III) [76,78,79]; Co(II)
[37,38,65,70,80-91]; Ni(I) [72,92-94]; Ni(II) [72,94-97]; Cu(I) [98-107]
and Cu(II) [100,107-115].
V. Mechanisms of decomposition of the transient complexes
LmMn+1-R.
The mechanisms and kinetics of decomposition of the transient
complexes LmMn+1-R in aqueous solutions depend on the nature of the
central cation, of the ligands, L, of the substituents on the aliphatic
residue, R, on the pH and on the presence and nature of various
substrates, S, in the medium, e.g. O2. In the following paragraphs the
major mechanisms observed are discussed:
1. The major mechanism observed is the heterolysis of the
metal-carbon  bond:
124
LmM
n+1
-R + H2O 
Mn+1Lm + RH + OH-
(13)
Mn-1Lm + ROH/R-H + H3O+
(14)
Reaction (13) is observed mainly for complexes MnLm which are
relatively strong reducing agents and which are not expected to be
reduced, e.g. for complexes of Cr(II) [59,61,67-69]; Mn(II) [71,72];
Fe(II) [56,73], Co(II) [39]; Ni(I) [72,92,93,116] and Cu(I) [98,99,105].
This reaction is usually acid [9,61,64,68,105] and general base catalysed
[64,117,118]. Measurements of the volumes of activation of some of
these reactions [119] suggest that they proceed via a transition state of
the type:
LmMn+ 1
C
O
H
H
Reaction (14) is mainly observed for complexes MnLm which
can be reduced and are difficult to oxidise, e.g. Fe(III) [79]; Cu(II)
[110] and with proper ligands for Co(II) [81,90]. Measurements of the
volumes of activation of some of these reactions [119] suggest that they
proceed via a transition state of the type:
(H2O)5M
n+1
1 2 3
CR R R
OH2
It is of interest to note the mechanisms of decomposition of
several complexes of the type LCuIII-CH3:
CuIII-CH32+aq  Cu+aq + CH3OH [120]
II
(H2NCH2CO2-)2CuIII-CH3
(H2NCH2CO2-)2+ + CH4 [113]
125
(15)
(16)
(glycylglycylglycine)CuIII-CH3+  CuI(glycylglycylglycine) + CH3OH [121] (17)
The shift in mechanism caused by the glycine ligands was
attributed [113] to their effect on the redox potential of the Cu III/II
couple. However the glycylglycylglycine ligand stabilises the CuIII
oxidation state even more than the glycines. Thus it seems that the
choice of the mechanism of decomposition of the transient complexes
depends on the activation energies of the different plausible mechanisms
and not on the total free energy gain. This is plausible as the .CH3
radicals are both powerful oxidising and reducing agents [4,80], and
therefore both reaction mechanisms are highly exothermic.
2. In many systems studied the results point out that reaction (7) is an
equilibrium process, and the mechanism of decomposition of the
transient complex LmMn+1-R involves radical processes:
LmMn+1-R + L 
M nL m + R
followed by:
2R.  R2/ RH + R-H
or by:
R. + S 
products
.
(-7)
(18)
(19)
This mechanism of decomposition has been observed for a large
variety of complexes of Cr(III) [61,68]; Mn(III) [70]; Fe(III) [70,72];
Fe(IV) [79]; Co(III) [37,84,85]; Ni(III) [95-97] and Cu(II) [100,122125]. It should be noted that as reaction (7) proceeds usually via a
mechanism involving a ligand interchange [37,55,85] the measurement
of H# of reaction (-7) does not yield the M-C bond strength as was
proposed [126-129], though the determination of the bond strengths by
this method were performed mainly in aprotic solvents which do not
behave as ligands.
The homolytic decomposition is often followed by the reaction:
L
n+1
.
Lm-1M -R + R  MnLm + R2/(R+ + R-)
(20)
126
This reaction leads in many systems to unexpected products.
The detailed mechanism of this reaction was studied only in few
systems. [37,38]
Another important reaction which follows homolytic
decomposition is: [53,85]
R. + O2

RO2.
(21)
Which is usually followed by: [53,85]
RO2. + MnLm  Lm-1Mn+1-OOR + L
(22)
The properties of the transient complexes Lm-1Mn+1-OOR thus
formed is discussed below.
3. The  elimination mechanism of decomposition of transient
complexes of the type LmMn+1-R occurs when a good leaving group, X,
e.g. X = -OR, -NR2, -NHC(O)R and halides, is bound to the  carbon:
LmMn+1-CR1R2CR3R4X  Mn+1Lm + R1R2C=CR3R4 + X-
(23)
These reactions, which are often acid catalysed, were
observed for a large variety of such complexes, see for example for X
= OH [59,61,66,68,74,81,83,89,98,101,130-132]; X =OR [133]; X =
NR2 [104,115,134-136]; OP(O)(OH)2 [137] and X = NHC(O)R [138].
Measurements of the volumes of activation of some of these reactions
[136] suggest that their transitions states of the type:
R
1
R
2
C
LmM
n+1
R
3
C
R
127
4
X
4. Another mechanism of decomposition which was observed up
to date only for Mn = Cu(II) [114,115] is the  elimination of
carboxylates:
LmMn+1-(CR1R2CR3R4CO2-)  Mn-1Lm + R1R2C=CR3R4 + CO2
(24)
5.  hydride shift reactions were also observed, though up to
now only in a few systems, e.g.:
L(H2O)CoIII-CH(CH3)OH2+  L(H2O)CoIII-H2+ + CH3CHO [81]
(25)
where L is a tetraaza-macrocyclic ligand. And in [139]:
III
(H2 O)5 Cr -C(CH 3 )2 OH
(H2 O)4 Cr
2+
CH 2
III
C
H
CH 3 OH
(26)
III
(H2 O)5 Cr -CH2 CH(OH)(CH 3 )
H3 O
Cr(H2 O)6
3+
2+
+
+ H 2 O + CH 2=CHCH 3
6. CO insertion, or methyl migration was observed till now only
in one system: [106]
9
-1 -1
.
1.1x10 M s
+
Cu(CO)n + CH3
II
(CO)n Cu -CH3
+
3 -1
.
+ C(O)CH3
1x10 s
3.5x10 9 M -1s-1
II
(CO)n-1 Cu -C(O)CH3
H2 O
2+
aq
Cu
4 -1
1.1x10 s
+ CH3 CHO + (n-1)CO + OH
128
(27)
+
-
However analogous studies clearly can be carried out for other
systems.
7. In principle one can expect that the formation of transient
complexes of the type LmMn+1-R might result in the rearrangement of the
carbon-skeleton of R in analogy to B-12 catalysed processes [140].
However, only one such reaction was observed till now, [102] probably
due to the fact that very simple alkyl radicals are used in most studies.
This rearrangement is discussed in the section “elucidation of the
mechanisms of catalytic processes” below.
Reduction of alkenes.
Recently it was observed that {NiIL1}+ (where L1 = 1,4,8,11tetraazacyclotetradecane) reduces alkenes, two such reactions were
studied up to date:[141]
a. The reduction of ethylene yields mainly ethane with butane as
a side product (straces of hexane are also obtained). The results suggest
the following mechanism:

{NiIIL1}2+ + .C2H5

{L1(NiIII-C2H5}2+
{L1(NiIII-C2H5}2+ + {NiIL1}+  {L1(NiII-C2H5}+ + {NiIIL1}2+
{NiIL1}+ + .C2H5 
{L1(NiII-C2H5}+
{L1(NiII-C2H5}+ + H+  {NiIIL1}2+ + C2H6
{NiIL1}+ + C2H4
(28)
(29)
(30)
(31)
b. The reduction of maleic acid proceeds via a significantly
different mechanism, i.e.:
{NiIL1}+ + (-O2C)HC=CH(CO2-)
[{NiIL1}+;(-O2C)HC=CH(CO2-)]
(32)
i.e. the formation of a d -> 
H+
[{NiIL1}+;(-O2C)HC=CH(CO2-)]  {NiIIL1}2+ + .CH(CO2-)CH2CO2{NiIL1}+ + .CH(CO2-)CH2CO2 -
{L1NiII-CH(CO2-)CH2CO2-}
129
(33)
-
(34)
{L1NiII-CH(CO2-)CH2CO2-}- + .CH(CO2-)CH2CO2-{NiIL1}+ + (-CH(CO2-)CH2CO2-)2 (35)
{NiIIL1}2+ + .CH(CO2-)CH2CO2-  {L1NiIII-CH(CO2-)CH2CO2-}
(34)
I 1 +
1 III
1 II
- II 1 2+
{Ni L } + {L Ni -CH(CO2 )CH2CO2 }  {L Ni -CH(CO2 )CH2CO2 } + {Ni L } (35)
Properties of transient complexes of the type Lm-1Mn+1-OOR
Transient complexes of the type Lm-1Mn+1-OOR are formed as
transients in all the transition metal catalysed oxidations by peroxides,
[143-145] in many transition metal catalysed oxidations by dioxygen
[146] and via reaction (22) which was proposed to be of importance in
radical induced deleterious biological processes. [147] However usually
in these systems the steady state concentration of these transient
complexes is too low to be followed directly. Pulse-radiolysis turned out
to be the optimal technique for the elucidation of the chemical properties
of these transient complexes. The following mechanisms of
decomposition of these transient complexes were observed:
a.
The homolytic pathway. i.e.: [148]
Lm-1Mn+1-OOR + L
RO2. + MnLm
(-22)
Surprisingly enough the equilibria constants of these reactions
are not directly related to the oxidation potentials of the radicals. It was
argued that this observation stems from the stabilisation of the
complexes Lm-1Mn+1 by the ligand ROO-. It seems that substituents on R
which raise the redox potential of the radical decrease its stabilising
effect on the Lm-1Mn+1 complex. At least for Lm-1Mn+1 =Fe(H2O)53+ these
effects seem to nearly cancel each other. [148]
b.
Heterolysis of the M-O bond: [149]
Lm-1Mn+1-OOR + H3O+  Mn+1L(H2O)n+1 + HOOR
130
(36)
c.
“Fenton Like” reactions, i.e.: [148,149]
L; 2H+
Lm-1M
n+1
-OOR + M Lm
2Mn+1Lm + H2O + .OR
(37)
n
L; 3H+
 Mn+1Lm + 2Mn+2Lm + ROH + H2O
(38)
It should be noted that though reaction (37) is the classical way
in which the “Fenton” reaction is written, at least the reaction
(H2O)5FeIII-OOR2+ + Fe(H2O)62+ follows the mechanism described by
reaction (38). [148] It is also of interest to note that the latter reaction is
four orders of magnitude faster than the reaction H2O2 + Fe(H2O)62+,
[148] i.e. the metal cation activates the peroxide towards this reaction.
d.
The transient complexes Lm-1Mn+1-OOR are strong
oxidising agents. Thus they might oxidise the ligands L, i.e.: [150]
{L1(H2O)NiIII--OOSO3-}+  {NiIIIL’(H2O)2}3+ + SO42-
(39)
where L’ is the product of oxidation of L1 in which one N=C double
bond was formed.
e. In principle the heterolysis of the O-O bond:
Lm-1Mn+1-OOR + H+  Lm-1Mn+2=O + ROH
(40)
is expected to occur in analogy to enzymatic processes, i.e. to
cytochrome P-450. However, we have still not observed a reaction
following this mechanism in aqueous solutions.
f. As the transient complexes Lm-1Mn+1-OOR are powerful oxidising
agents they are capable of oxidising reducing substrates, S, present in the
solution:
H+; L
Lm-1Mn+1-OOR + S
 Mn+1Lm + S=O + ROH
131
(41)
Thus for example ascorbate is oxidised by such transient
complexes. [151,152]
g.
Surprisingly enough it was recently observed that the reaction
Lm-1Mn+1-(-OOR) + RO2.  Lm-1Mn+1-(.OOR) + RO2-
(42)
is fast, its rate often approaches the diffusion controlled limit. [149] The
mechanism of decomposition of the transient complex L m-1Mn+1-(.OOR)
depends on its constituents. This reaction is expected to be of
importance in many radical processes.
The mechanism of decomposition of peroxonitrite
Measurement of the volume of activation of the isomerisation
reaction

ONOOH

NO3- + H+
(43)
Point out that this reaction proceeds via the homolysis of the O-O bond,
[153] thus solving a long debated question.
Elucidation of the mechanisms of catalytic processes
Finally it should be pointed out that the pulse radiolysis
technique can be applied to the elucidation of the detailed mechanisms
of a variety of catalytic processes. Here three examples for this
application will be presented.
a.
The
autocatalytic
reduction
of
(NH3)5CoIII
3+
2+
(isonicotineamide) by Eu aq.
It
was
reported
that
the
reduction
of
(NH3)5CoIII(isonicotineamide)3+, (NH3)5CoIII(INA)3+,
by Eu2+aq is
autocatalytic
whereas
the
analogous
reduction
of
(NH3)5CoIII(nicotineamide)3+, (NH3)5CoIII(NA)3+, is not. [154] In order to
elucidate the source of this difference the kinetics of reduction of several
cobalt(III) complexes by the HINA. and HNA. radicals were studied.
The results demonstrated that the HNA. radical is a more potent reducing
agent than the HINA. radical.[155] Thus the source of the different
132
activity is that the rate of reduction of HNA+ by Eu2+aq is considerably
lower than that of HINA+.
b.
The catalytic oxidation of cyclohexene by S2O82- in the
presence of copper ions.
The oxidation of cyclohexene by S2O82- is catalysed by copper ions.
[156] It was proposed that the mechanism of this process involves the
following steps: [156]
S2 O8

2-
2SO4
+ SO4
.-
(44)
+
.-
. + SO4
(45)
2-
OH
+
.
+ 2H2 O
.
+ H3O
OH
2+
Cu aq
(47)
III
Cu aq
2+
OH
H
O
+ Cu
Cu
Cu
+
aq
(46)
2+
OH
+
.
+
III
+
aq
+ H3 O
(48)
+
aq
+ S2 O 8
2-
Cu
2+
aq
+ SO4
2-
+ SO4
.-
(49)
It seemed of interest to study the detailed mechanism of reaction
(48). For this purpose it was decided to start with the study of reaction
(47). Surprisingly it was found that this reaction is very slow and instead
the reaction observed is: [102]
OH
OH
+
aq
Cu
+
+
9
-1 -1
k = 2.6x10 M s
.
II
Cu aq
133
(50)
which is followed by:
OH
+
+
-H
HC
Cu
II
Cu aq
HC
II
aq
+
HC
-1
(51)
+
Cu2+aq
k = 4.5 s
-1
(52)
+
OH
HC
H
3
k = 3.5x10 s
II
aq
OH
OH
Cu
OH
Cu2+aq
fast
.
CHOH
(53)
+
aq
+ Cu
.
CHOH
+
H
2+
Cu aq
fast
O
+
aq
+ Cu
+ H
+
(54)
c.
The “Fenton Like” reactions.
There is a major debate concerning the question whether free
hydroxyl radicals are formed in “Fenton Like” reactions.[147] In order
of solving this question it was decided to use reactions (23) in the
following way: [157-159] Solutions containing a mixture of alcohols,
e.g. (CH3)2CHOH and (CH3)3COH, in the presence of a low valent
complex MnLm are first irradiated and the relative yields of the alkenes
formed in the reactions:
OH + (CH3)2CHOH/(CH3)3COH .CH2CH(OH)CH3/.CH2C(CH3)2OH + H2O (55)
CH2CH(OH)CH3/.CH2C(CH3)2OH + MnLm  Lm-1Mn+1-CH2CH(OH)CH3/
Lm-1Mn+1-CH2C(CH3)2OH + L
(56)
L
LmMn+1-CH2CH(OH)CH3/LmMn+1-CH2C(CH3)2OH 
.
.
Mn+1Lm + H2C=CCH3/ H2C=C(CH3)2 + OH (57)
134
are measured. Then to identical solutions hydrogen-peroxide is added
and the relative yields of the same alkenes are measured. If the results
are identical then hydroxyl radicals are formed in the system and if not,
not. The results point out that the first reaction is always the formation
of a complex between the peroxide and the low valent metal complex:
[160,161]
MnLm + H2O2
Lm-1Mn.H2O2
(58)
This reaction might be followed by one of the following reactions:
L
 Mn+1Lm + .OH + OHLm-1Mn.H2O2
 Lm-1Mn+2=O + H2O
(59)
(60)
RH; L
 Mn+1Lm + .R + H2O
(61)
The mechanism occurring depends on the nature of Mn; L; pH and the
concentration and nature of RH.
Concluding remark
It is hoped that the examples given in this manuscript testify that
the pulse-radiolysis technique is indeed a powerful tool for solving of a
large variety of important problems concerning the inorganic chemist.
Acknowledgements
I am indebted to my co-workers whose work is cited on this
manuscript. The financial help via grants from the Israel Science
Foundation, The planning and Granting Committee and the Israel
Atomic Energy Committee, The German Israel Binational Science
Foundation and The American Israel Binational Science Foundation and
The Alexander von Humboldt Foundation enabled these studies. Finally
I appreciate the continuous interest and support by Mrs. Irene Evens.
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