Synthetic routes to lead(II) derivatives of aromatic 1,2-diols and orthoquinones

Article abstract of DOI:10.1016/S0020-1693(03)00057-4

Complexes of lead(II) of the type Pb(O₂R) where R is a substituted aromatic group, can be prepared by the direct electrochemical oxidation of a lead anode in a non-aqueous solution of the appropriate substituted catechol, R(OH)₂. The products, which are colourless, air-stable, insoluble materials, and presumably cross-linked in the solid state, can be oxidised to the corresponding lead(IV) derivatives by iodine or tetrahalogeno-o-quinones. The direct reaction of elemental lead with an o-quinone in refluxing hexane or toluene yielded identifiable products in only two cases; with 2,5-tert-butyl-1,2-benzoquinone, the corresponding lead(II) catecholate was obtained, while with phenanthrenequinone the semiquinonate Pb(PSQ^?)₂ was isolated. These latter reactions are shown by electron spin resonance spectroscopy to proceed via free radical intermediates, and the spectra are analysed to give information on the species present in the reaction media.

Full text of DOI:10.1016/S0020-1693(03)00057-4

Inorganica Chimica Acta 349 (2003) 142ꢂ148
/
www.elsevier.com/locate/ica
Synthetic routes to lead(II) derivatives of aromatic 1,2-diols and
orthoquinones
Gareth M. Barnard, Martyn A. Brown, Hassan E. Mabrouk 1, Bruce R. McGarvey,
Dennis G. Tuck *
Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, Canada N9B 3P4
Received 16 October 2002; accepted 18 November 2002
Abstract
Complexes of lead(II) of the type Pb(O2R) where R is a substituted aromatic group, can be prepared by the direct electrochemical
oxidation of a lead anode in a non-aqueous solution of the appropriate substituted catechol, R(OH)2. The products, which are
colourless, air-stable, insoluble materials, and presumably cross-linked in the solid state, can be oxidised to the corresponding
lead(IV) derivatives by iodine or tetrahalogeno-o-quinones. The direct reaction of elemental lead with an o-quinone in refluxing
hexane or toluene yielded identifiable products in only two cases; with 2,5-tert-butyl-1,2-benzoquinone, the corresponding lead(II)
catecholate was obtained, while with phenanthrenequinone the semiquinonate Pb(PSQ+)2 was isolated. These latter reactions are
shown by electron spin resonance spectroscopy to proceed via free radical intermediates, and the spectra are analysed to give
information on the species present in the reaction media.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Lead complexes; Diol complexes; Orthoquinone complexes
1. Introduction
represents TBQ or one of its reduced forms) [3]. The
reaction is extremely slow, even in refluxing toluene,
requiring about 12 days to go to completion. When
tetrachloro- or tetrabromo-1,2-benzoquinone is substi-
tuted for TBQ, the reaction product is the completely
dehalogenated product Ge(O2C6H4)3; this unusual reac-
tion will be discussed elsewhere [4]. Tin reacts rapidly
with TBQ, with the reaction going to completion in
approximately 12 h, but it was not possible to isolate the
The chemistry of the complexes of lead has attracted
interest of late, in part because of the potential toxicity
of the compounds of this element, but also because this
is a relatively unexplored area of coordination chem-
istry. The latter topic has been reviewed by Parr [1], and
more recently the crystal structures of a number of
lead(II) complexes have been discussed by Foreman et
al. [2]. We now report some studies of preparative routes
to compounds in which lead(II) is coordinated by
catecholate or orthosemiquinone ligands.
The reactivity of the heavier Group 14 elements with
substituted orthoquinones demonstrates an interesting
range of behaviour. Germanium reacts with 3,5-di-tert-
butyl-1,2-benzoquinone (TBQ) to give GeQ2, GeQ3 or
GeQ4 depending on the initial mole ratio of reactants (Q
presumed product Sn(TBSQ+)2 (TBSQ+ꢀ ꢁ
3,5-di-tert-
/
butyl-1,2-benzo semiquinonate radical anion) in pure
form, although electron spin resonance (ESR) spectro-
scopy established the presence of a diradical species in
the reaction mixture [5]. Russian workers [6ꢂ8] studied
/
the reaction of tin amalgam with 3,6-di-tert-butyl-1,2-
benzoquinone; here again, no solid products were
isolated, but ESR spectroscopy identified diradical
species in solution. Tin reacts smoothly over a period
of 24 h with Cl4C6O2-o in refluxing toluene to give
tin(IV)-bis-catecholate derivatives which have been iso-
lated and characterised [9].
* Corresponding author. Tel.: ꢃ
7098.
/
1-519-253 3526; fax: ꢃ1-519-973
/
The present work describes studies of the reaction of
elemental lead with various substituted orthoquinones.
E-mail address: dgtuck@uwindsor.ca (D.G. Tuck).
1
Deceased.
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00057-4

G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ
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143
There are marked differences between the behaviour of
germanium and tin, both in the reactivities and the
native of the derivatives. No identifiable product was
The volume of the residual green filtrate was reduced
by evaporation, and on cooling gave a green solid, in
which both n(CÄ
/
O) and n(CÃ/O) vibrations were
obtained in the Pbꢃ
/
X4C6O2-o systems (Xꢁ/Cl, Br), but
detected. No satisfactory chemical analysis could be
obtained for this material; the ESR spectrum of the
solution phase is discussed below. Addition of picoline
to the filtrate from a Pb:2TBQ reaction produced a
brown solution which yielded a colourless solid analys-
we were able to obtain the corresponding lead*
/
tetrahalogenocatecholate species by the method of direct
electrochemical synthesis, which had been used earlier to
prepare Sn(O2R) compounds [10,11].
A useful comparison can also be made between the
present work and an earlier study in which thallium was
found to react with TBQ to give Tl(TBSQ+), which
slowly decomposes to the corresponding bis-thallium
catecholate [12].
ing as Pb(TBCAT)×
showed only n(CÃO) vibrations in the infrared spec-
trum.
/
0.5 picoline. This is ESR-silent, and
/
2.2.2. Pbꢃphenanthrenequinone (PQ)
/
The procedure here was essentially identical to that
described for TBQ, except that a reaction time of 18 h
was required for complete consumption of the lead. A
dark green solution, essentially free of solid, was
2. Experimental
obtained over the range Pb:PQꢁ1:2, 1:3 and 1:4. This
/
2.1. General
solution is ESR active (see below), and on cooling
deposited a green solid which was identified as the bis-
semi-quinonate derivative Pb(PSQ+)2.
Lead shot was 99.99% pure (Alfa). Solvents were
distilled and dried before use; all other reagents were
used as supplied.
On exposure to air, this solid became grey, and the
ESR spectrum lost its diradical character. The chemical
analysis is essentially unchanged (C, 52.4; H, 2.66%; c.f.
Table 1, line 3), and it appears that Pb(PC) is being
formed, presumably with elimination of PQ, by pro-
cesses analogous to the decomposition of the thallium(I)
Metal analysis was by atomic absorption spectro-
photometry, using an IL-250 instrument. Iodine analysis
was by the Volhard method, involving back-titration
with potassium thiocyanate and silver nitrate. Micro-
analysis was by Canadian Microanalytical Services Ltd.
Infrared spectra were run on a Nicolet 5DX instrument,
with samples pressed as KBr discs. Electron spin
resonance (ESR) spectra were recorded on a Bruker
ESP-300E instrument, using the calibration and other
methods described earlier [13].
ꢀ
derivative of TBSQ+ [10].
2.2.3. Pbꢃ1,2-naphthoquinone (NQ)
/
Refluxing a 1:2 Pb:Q mixture in tetrahydrofuran over
48 h resulted in only approximately 50% of the metal
being consumed. Addition of hexane to the cooled
In this work, as elsewhere, we use the abbreviations
solution gave a dark brownꢂ
/
black solid, for which no
QX
/
SQ+ꢀ X
the triad of oxidised/reduced forms of the ligand.
/
CAT2ꢀ, with suitable prefixes, to identify
satisfactory analytical results could be obtained.
2.2.4. Pbꢃ
/
X4C6O2-o (XꢁCl, Br)
/
2.2. Preparative
In the case of Cl4C6O2-o, only 25% of the metal was
consumed under similar conditions. The resultant red
solution showed a weak ESR activity, but it proved
impossible to obtain crystalline material, and the addi-
tion of picoline gave a only red syrup. Analysis was
unsatisfactory, and attempts to obtain pure products by
fractional crystallisation, or chromatography (TLC),
failed. Similar results were obtained with Br4C6O2-o,
except that approximately 15% of the metal was
consumed over 48 h.
2.2.1. PbꢃTBQ
/
Lead (0.21 g, 1 mmol) was refluxed with TBQ (0.22 g,
0.44 g, 0.66 g; 1,2 or 3 mmol) in n-hexane (50 ml) for 12
h. In each case, the reaction product was a white
powder, which was collected by filtration, washed with
toluene and dried in vacuo. This diamagnetic solid,
which is insoluble in all common organic solvents, was
identified as Pb(TBCAT) (TBCAT2ꢀ ꢁ
3,5-di-tert-bu-
/
tylcatecholate) (see Table 1 for analytical results). The
yields of Pb(TBCAT) were 65, 61 and 69% (based on
initial quantity of lead) for Pb:TBQ ratios of 1, 2 or 3,
respectively, so that the stoichiometry has no effect on
the nature of the product. The infrared spectra of these
2.3. Electrochemical synthesis
The method followed the techniques developed in this
laboratory, with a cell consisting of a 100-ml tall-form
beaker containing a solution of the appropriate ligand in
freshly distilled MeCN [11,12]. In each case, the anode
was a lead billet previously cleaned in conc. HNO3,
suspended from a platinum metal wire, and the cathode
solids confirmed the absence of n(CÄ
/O), found at 1665
cmꢀ1 in TBQ; n(CÃ
/
O) was identified at 1459 and 1434
cmꢀ1. Substitution of toluene as the reaction medium
did not significantly change any of these findings.

144
G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ
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Table 1
Analytical results for lead derivatives of substituted o-benzoquinones
Compound
Colour
Found (%)
Calcd. (%)
Pb
Pb
C
H
C
H
Pb(C14H20O2)
colourless
colourless
green
40.0
44.0
53.3
21.7
12.0
4.78
39.3
43.1
53.9
22.9
12.7
4.68
4.96
2.56
1.33
0.71
Pb(C14H20O2)×
Pb(C14H8O2)2
Pb(C6H4O2)
/
0.5C6H7N
5.02
2.52
1.33
0.94
colourless
green
66.1
36.0
41.7
37.2
27.8
56.7
38.5
40.3
32.9
26.0
65.7
36.4
41.8
36.9
28.0
56.7
38.0
39.7
32.8
26.3
a
Pb(C6H4O2)I2
Pb(C6H4O2)phen b
Pb(C6H4O2)(Cl4C6O2)
Pb(C6H4O2)(Br4C6O2)
Pb(C10H6O2)
goldꢂ/yellow
blue
dark blue
colourless
yellow
Pb(C10H6O2)phen
Pb(C10H6O2)bpy
Pb(Br4C6O2)
yellow
yellow
Pb(Br4C6O2)bpy
goldꢂyellow
/
a
I analysis; found 44.9, calcd. 44.6.
b
phenꢁ
/
1,10-phenanthroline; bpyꢁ2,2?-bipyridyl.
/
was a platinum wire, 0.5 mm diameter. The cell solution
was deoxygenated with a stream of dry nitrogen prior to
electrolysis, and was blanketed with dry nitrogen to
exclude oxygen and moisture during the electrochemical
synthesis. The electrolyte added to the solution in order
to give a reasonable current was generally tetraethylam-
monium perchlorate, but tetra n-butylammonium tri-
fluoromethanesulfonate was also used satisfactorily. In
all cases, subsequent analysis of the prepared com-
pounds confirmed that there had been no incorporation
of the electrolyte into the products.
Details of solution composition and other experimen-
tal details are given in Table 2. The applied voltage was
that required to give a steady current of 40 mA.
Hydrogen gas evolved at the cathode, and the product
deposited around the anode; this material was collected
by filtration at the end of the experiment, washed with
MeCN and then Et2O, and dried in vacuo.
In the cases of catechol, tetrabromocatechol and 2,3-
dihydroxynaphthalene (ꢁR(OH)2), the product was the
lead(II) derivative Pb(O2R), (see Table 1 for analytical
results); when 2,2-bipyridine or 1,10-phenanthroline (ꢁ
L) was also present, the 1:1 adducts Pb(O2R)L were
obtained.
/
/
2.4. Oxidation of Pb(O2R) compounds
2.4.1. Pb(O2C6H4) and I2
A suspension of freshly prepared Pb(O2C6H4) (0.47 g,
1.50 mmol) in 10 ml MeCN was prepared under N2.
When a solution of iodine (0.38 g, 1.50 mmol) in the
same solvent (10 ml) was added, the mixture became
green, and a finely divided olive-green solid was detected
after 1 h. After 6 h, the product was collected by
filtration under vacuum, and washed with MeCN (3ꢄ
/
25 ml) and then Et2O (3ꢄ25 ml). The solid was finally
/
Table 2
Experimental conditions for the direct electrochemical synthesis of Pb(O2R) compounds and adducts
a
Diol
Solution composition b Time of electrolysis c (h) Metal dissolved c (mg) Mass of product (g) Yield d (%) EF (mol Fꢀ1
)
Diol (g)
Ligand (g)
phen, 0.33
C6H4(OH)2
C6H4(OH)2
0.25
0.20
3.0
492
348
355
387
305
455
315
0.749
0.832
0.551
0.967
0.702
1.30
98
89
88
95
92
94
91
0.49
0.50
0.50
0.50
0.51
0.51
0.51
2.25
2.25
2.5
C10H6(OH)2 0.25
10H6(OH)2 0.25
C
phen, 0.28
bpy, 0.24
C10H6(OH)2 0.25
C6Br4(OH)2 0.55
C6Br4(OH)2 0.55
2.0
3.0
bpy, 0.20
2.0
1.09
a
C6H4(OH)2ꢁ
/
1,2-dihydroxybenzene; C6Br4(OH)2ꢁ
In 50 ml acetonitrile containing 100 mg electrolyte (see text).
At 40 mA, with applied voltages in the range 20ꢂ40 V.
/
1,2-dihydroxytetrabromobenzene; C10H6(OH)2ꢁ2,3-dihydroxynaphthalene.
/
b
c
/
d
Based on mass of metal dissolved.

G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ
/148
145
dried overnight under vacuum, and identified as Pb(O2-
C6H4)I2 (yield; 0.69 g, 81%), olive-green in colour.
3.2. Reaction of lead with orthoquinones
The preparative results (Sections 2.2.1, 2.2.2, 2.2.3
and 2.2.4) show that there are surprising differences in
reactivity between the five substituted o-quinones used.
With TBQ, the product obtained under all the condi-
tions investigated is the highly insoluble lead(II) com-
pound Pb(TBCAT). Similarly, each residual reaction
2.4.2. Pb(O2C6H4) and Br4C6O2-o
A
grey/black suspension of freshly prepared
Pb(O2C6H4) (0.47 g, 1.50 mmol) in MeCN (10 ml)
under nitrogen was treated with a solution of o-O2C6Br
(0.64 g, 1.50 mmol) in the same solvent (10 ml). The
mixture was stirred for 6 h under an inert atmosphere,
during which time it became dark blue, with a finely
divided deep blue solid product being visible after some
hours. The product was filtered under vacuum and
ꢀ
mixture apparently contains species in which TBSQ+
is interacting with a lead centre (see below). The
addition of picoline to this solution produces a diamag-
netic 2:1 adduct of Pb(TBCAT) (see Table 1).
When phenanthrenequinone (PQ) was used, the
product obtained on cooling the green reaction mixture
was Pb(PSQ+)2. A solution prepared by dissolving this
compound in toluene showed the characteristic ESR
spectra of a diradical (see below). Attempted reactions
of lead with naphthoquinone or tetrahalogeno-o-qui-
nones showed that only a small fraction of the metal was
consumed, even over long reaction times, and in the case
of X4C6O2-o, there is a strong similarity to the
behaviour of germanium [3,4], with both elements
differing markedly from tin [5,9] in this respect.
washed with MeCN (3ꢄ
/
25 ml) and then Et2O (3ꢄ25
/
ml). The product was dried overnight under vacuum,
and identified as Pb(O2C6H4)(O2C6Br4) (yield: 0.95 g,
86%), dark blue in colour. A similar reaction using
Cl4C6O2-o gave the oxidative addition product Pb(O2-
C6H4)(O2C6Cl4).
3. Results and discussion
A schematic summary of these preparative results is
shown in Scheme 1. As noted earlier, the decomposition
of Pb(SQ+ꢀ)2 to Pb(CAT) is reminiscent of the inherent
instability of Tl(TBSQ+), which decomposes sponta-
neously to Tl2(TBCAT) [12].
3.1. Electrochemical synthesis
The electrochemical oxidation of lead in the presence
of aromatic 1,2-diols is a direct route to Pb(O2R)
compounds. The yields are high, the apparatus is simple,
and it is relatively easy to scale up the experiment to
produce larger quantities of product than in the present
work. Equally, the same procedure can be used to
prepare adducts with neutral bidentate donor ligands,
and it is reasonable to assume that other diols, and other
neutral donors, could be utilized in the same way.
The electrochemical efficiency, EF, defined as quan-
tity of lead dissolved from the anode per Faraday of
Scheme 1.
˚
charge passed through the cell, is found to be 0.50 A
0.01 mol Fꢀ1 for all the systems studied in this work.
The simplest interpretation of this result is in terms of
the reactions
3.3. ESR results
As noted above, the reaction of lead with 3,5-di-tert-
butyl-1,2-quinone was carried with initial mole ratios
(Pb:TBQ) of 1:1, 1:2 and 1:3. In each, the reaction
product is the insoluble Pb(TBCAT). The residual green
solutions were all ESR active, and the spectrum for
cathode R(OH)2 ꢃ2e 0 H2(g)ꢃRO22ꢀ
anode RO22ꢀ ꢃPb 0 Pb(O2R)ꢃ2e
(1)
(2)
in which the RO22ꢀ dianion is the current carrier. Other
explanations are also possible (see Ref. [10]), but there is
no experimental evidence on which to resolve this matter
at present. The same EF value applies to the synthesis of
both Pb(O2R) and Pb(O2R)L, so that the formation of
the adduct occurs, not surprisingly, subsequent to the
oxidation of Pb to Pb(O2R). The mechanism in Eqs. (1)
and (2) supports the formulation of these products as
lead(II) species.
TBQ:Pbꢁ3 is shown in Fig. 1; for starting ratios of 2:1
/
and 4:1, the central portion of the spectrum remains the
same but the 207Pb hyperfine peaks are changed. In the
central portion of the spectrum there are two doublets,
the more intense of which has gꢁ
/
1.9995 and A(H)ꢁ
/
3.20 G. This must be TBSQ+ coordinated to Pb2ꢃ, since
the 207Pb hyperfine peaks are centered about this peak.
The smaller doublet has gꢁ
/
2.0039 and A(H)ꢁ3.30 G
/

146
G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ148
/
Fig. 1. ESR spectrum, at room temperature, of the residual solution from the reaction of Pb with 3,5-di-tert-butyl-1,2-quinone (initial Pb: quinone
mole ratioꢁ1:3).
/
and is identified as uncoordinated TBSQ+. The existence
of multiple 207Pb hyperfine peaks indicates the presence
of several conformers of Pb(TBSQ+) species in the
solution. When the solutions are frozen to 100 K, the
above). The major difference between the two systems
lies in the relative solubilities of the Pb(SQ+)2 species,
but the overall mechanism is presumably the same in
each system.
spectra of both Sꢁ
/
1/2 and Sꢁ
/
1 species appear,
1 state
The presence of multiple conformers of these lead(II)
complexes in solution may be related to the known
tendency of such species to form multiply bridged
suggesting the presence of Pb(TBSQ+)2; the Sꢁ
/
is confirmed by the presence of a half-field transition. In
previous work on other metal-TBQ systems, addition of
a nitrogen base produced an ESR spectrum of a single
species, but in the present case addition of picoline
structures in the solid state (see below), and the
ꢀ
formation of oligomers of Pb(SQ+
Pb(O2R) species in solution would then account for
the multiple ESR spectra observed.
)
or related
2
produced a precipitate of Pb(TBCAT)×
/0.5 picoline and
the residual filtrate showed only weak ESR activity.
Those results suggest that in the presence of excess
TBQ, Pb(TBCAT) reacts by
3.4. Properties of Pb(O2R) compounds
Pb(CAT)ꢃTBQX[Pb(TBSQ+)]ꢃ ꢃTBSQ+
(3)
ꢀ
The Pb(O2R) compounds prepared are air-stable
powders of high melting point; long exposure to
moisture appears to cause decomposition, since the
solids blacken, probably due to the formation of
metallic lead. The solids are insoluble in non-donor
solvents, and in acetonitrile and diethyl ether. Lead(II)
catecholate is insoluble in cold dimethylsulphoxide but
slightly soluble in the hot. These properties suggest a
homo-polymeric solid state structure for Pb(O2R)
and that on cooling the diradical complex Pb(TBSQ+)2
is formed. Eq. (3) is a modified form of the well-known
equilibrium
CAT2ꢀ ꢃQX2SQ+
(4)
ꢀ
and of the analogous intramolecular electron transfer
which has been shown to occur in a number of Main
Group complexes of these ligands.
Similar results were obtained from the residual filtrate
resulting from the reaction of Pb with phenanthrene-
quinone. The ESR spectrum indicates the presence of a
mixture of conformers of Pb(PSQ+ꢀ), but in this case
addition of a nitrogen base produced a clean spectrum
(Fig. 2) of a single species, well simulated with the
compounds, involving cross-linking through PbÃ
/
OÃ
/
Pb
bonding. A recent paper [2] has reviewed the earlier
literature on the solid state structures of complexes of
lead(II) with mono- and polybasic carboxylates, and has
shown that OÃ
/
PbÃ
/
O cross-linking frequently leads to
polymeric or oligomeric structures in which the metal
can display a variety of coordination numbers. We were
unable to obtain crystals of any of the Pb(O2R)
compounds prepared, but the properties support the
proposed cross-linked solid state structures.
parameters gꢁ
A(207Pb)ꢁ
53.45 G. The ESR spectra of the frozen
solution at 100 K showed the presence of both Sꢁ1/2
and Sꢁ1 species, as was the case for TBQ and Pb (see
/
1.9998, A(6H)ꢁ
/
1.66, A(2H)ꢁ
/0.6,
/
/
/

G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ
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147
Fig. 2. ESR spectrum, at room temperature, of the residual solution from the reaction of Pb with phenanthrenequinone.
The formation of adducts of Pb(O2R) with the
bidentate donors 2,2-bipyridine and 1,10-phenanthro-
line is an interesting feature of the coordination
chemistry of the compounds. In view of the homo-
polymerization discussed above, it is not surprising that
the lead atom of Pb(O2R) should also expand its
coordination shell by ligation.
stances confirmed that the characteristic n(CÄ
/
O) of the
o-quinones at 1445 and 1270 cmꢀ1 was absent, and the
n(CÃ
/
O) modes of both RO22ꢀ ligands appear as a
complex set of bands in the 1450 cmꢀ1 region. The
intense colour of these products (see Table 2) suggests
inter-ligand charge-transfer processes.
3.6. Synthetic routes to Pb(O2R)
3.5. Oxidation of Pb(O2R)
The direct electrochemical synthesis of lead(II) cate-
cholate, and two typical substituted catecholates (Table
2) leads to stable insoluble compounds whose stability
and insolubility is presumably related to the cross-
linking discussed above. This pattern of preparative
behaviour is very different from that seen in the
reaction, or attempted reaction, of lead with o-quinones.
Only with 3,5-di-tert-butyl-1,2-benzoquinone is the
corresponding lead(II) catecholate obtained, and there
Oxidative addition reactions of low oxidation state
Main Group compounds have been the subject of many
investigations, but few of these have included lead(II)
species, in part because for most systems involving
common inorganic ligands, lead(II) compounds are
more stable than the lead(IV) analogues. We have now
shown that Pb(O2C6H4) prepared in this work is readily
oxidized by iodine to give the corresponding lead(IV)
species Pb(O2R)I2, which is air-stable, showing no sign
of loss of iodine in vacuo. This reaction parallels the
behaviour of Sn(O2R) and In[O(HO)R] compounds
[10,14]. The correspondence is further extended by the
oxidative reaction with the tetrahalogeno-o-quinones,
is evidence that the residual solution contains Pbꢂ
/
TBSQ+ derivatives which are presumably intermediates
in the formation of the catecholate (see Scheme 1). The
bis-semiquinonate Pb(PSQ+ꢀ)2 has been identified with
phenanthrenequinone, but none of the other PbꢃQ
/
o-O2C6X4 (Xꢁ
/
Cl, Br), which have been shown to
reactions showed signs of significant consumption of
lead. The contrast between this result for Br4C6O2-o,
and the simple electrochemical synthesis of Pb(O2C6Br4)
from C6Br4(OH)2-1,2, is very marked, and shows that
the lack of a facile reaction between Pb and the
tetrabromoquinone is not the result of the absence of
a stable product. Rather, it indicates that here, as in the
case of germanium [4], the strongly oxidising tetrahalo-
genoquinones do not react with lead in the expected
oxidize indium(I) [15] and tin(II) [16] halides, as well
as In[O(HO)R] [14], In[S(HS)R] [17] and Sn(O2R) [10]
complexes, in each case yielding a tetrahalogenocate-
cholate derivative of indium(III) or tin(IV). In the
present work, the reaction of o-O2C6Ã
Pb(O2R) causes discharge of the characteristic redꢂ
brown colour of the tetrahalogenoquinone, and analysis
of the products identified the formation of
Pb(O2R)(O2C6X4). The infrared spectra of these sub-
/
X4 with
/
ꢀ
fashion to give Pb/SQ+ or Pb/CAT2ꢀ products. We

148
G.M. Barnard et al. / Inorganica Chimica Acta 349 (2003) 142ꢂ148
/
[3] A.A. El-Hadad, B.R. McGarvey, B. Merzougui, R.G.W. Sung,
A.R. Trikha, D.G. Tuck, J. Chem. Soc., Dalton Trans. (2001)
1046.
did not identify the products of whatever slow reactions
do occur in the Pb/X4C6O2-o system, but if the
germanium case offers any parallel, it may be that
[4] B.R. McGarvey, R.G.W. Sung, B. Merzougui, A. Trikha, D.G.
Tuck, Unpublished results.
reactions between the metal and the CÃ
/
X sites of the
quinone
[5] (a) B.R. McGarvey, Z. Tian, D.G. Tuck, Unpublished results; (b)
Z. Tian, Ph.D. Thesis, University of Windsor, 1993.
[6] A.I. Prokof’ev, T.I. Prokof’eva, N.N. Bubnov, S.P. Solodovnikov,
I.S. Belostotskaya, V.V. Ershov, M.I. Kabachnik, Dokl. Chem.,
Engl. Edn. 245 (1979) 135.
quinone occur instead of the expected metalꢂ
/
electron transfer processes.
[7] A.I. Prokof’ev, N.A. Malysheva, N.N. Bubnor, S.P. Solodovni-
kov, M.I. Kabachnik, Dokl. Chem., Engl. Edn. 252 (1980) 236.
[8] R.R. Rakhimov, A.I. Prokof’ev, T.S. Lebedev, Him. Fiz. 8 (1989)
1265.
4. Supplementary material
The material is available from the authors on request.
[9] T.A. Annan, D.G. Tuck, Can. J. Chem. 67 (1989) 1807.
[10] H.E. Mabrouk, D.G. Tuck, J. Chem. Soc., Dalton Trans. (1988)
2539.
Acknowledgements
[11] D.G. Tuck, in: A.J.L. Pombeiro, J.A. McCleverty (Eds.),
Molecular Electrochemistry of Inorganic, Bioinorganic and
Organometallic Compounds, Kluwer, Dordrecht, 1992, p. 15.
[12] A.A. El-Hadad, J.E. Kickham, S.J. Loeb, L. Taricani, D.G. Tuck,
Inorg. Chem. 34 (1995) 120.
This work was supported in part by Research Grants
(to BRM and DGT) from the Natural Sciences and
Engineering Research Council of Canada.
[13] M.A. Brown, B.R. McGarvey, D.G. Tuck, J. Chem. Soc., Dalton
Trans. (1998) 1371.
[14] H.E. Mabrouk, D.G. Tuck, Can. J. Chem. 67 (1989) 7466.
[15] T.A. Annan, D.G. Tuck, Can. J. Chem. 66 (1988) 2935.
[16] T.A. Annan, R.K. Chadha, D.G. Tuck, K.D. Watson, Can. J.
Chem. 65 (1987) 2670.
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