New lead(II) catecholate and o-semiquinone complexes

Article abstract of DOI:10.1007/s11172-006-0391-z

Lead(II) catecholate complexes were prepared by reduction of 3,6-di-tert-butyl-o-benzoquinone and its derivatives with lead metal in THF. The molecular structure of the (CatPb)₄?(PbO)₂? 6C₃H₆O complex (Cat is the dianion of 3,6-di-tert- butylcatechol), which was synthesized by hydrolysis of lead 3,6-di-tert- butylcatecholate in acetone, was established by X-ray diffraction. A series of lead(II) o-semiquinone complexes, which were prepared by the addition of the phenoxyl radical to lead catecholates or by oxidation of the latter with mercury(II), copper(II), or silver(I) halides, were studied by the ESR method. Lead(II) mono-o-semiquinolate complexes undergo symmetrization to form stable bis-o-semiquinolates, which were isolated and characterized in individual state. Springer Science+Business Media, Inc. 2006.

Full text of DOI:10.1007/s11172-006-0391-z

Russian Chemical Bulletin, International Edition, Vol. 55, No. 7, pp. 1146—1154, July, 2006
1146
New lead(II) catecholate and oꢀsemiquinone complexes
G. A. Abakumov, V. K. Cherkasov, A. V. Piskunov,ꢀ A. V. Lado, G. K. Fukin, and L. G. Abakumova
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831 2) 12 7497. Eꢀmail: pial@iomc.ras.ru
Lead(II) catecholate complexes were prepared by reduction of 3,6ꢀdiꢀtertꢀbutylꢀoꢀbenzoꢀ
quinone and its derivatives with lead metal in THF. The molecular structure of the
(CatPb)4•(PbO)2•6C3H6O complex (Cat is the dianion of 3,6ꢀdiꢀtertꢀbutylcatechol), which
was synthesized by hydrolysis of lead 3,6ꢀdiꢀtertꢀbutylcatecholate in acetone, was established
by Xꢀray diffraction. A series of lead(II) oꢀsemiquinone complexes, which were prepared by the
addition of the phenoxyl radical to lead catecholates or by oxidation of the latter with
mercury(II), copper(II), or silver(I) halides, were studied by the ESR method. Lead(II) monoꢀ
oꢀsemiquinolate complexes undergo symmetrization to form stable bisꢀoꢀsemiquinolates, which
were isolated and characterized in individual state.
Key words: oꢀbenzoquinone, lead, oꢀsemiquinolate, catecholate, ESR, Xꢀray diffraction.
Among complexes of pꢀblock metals, lead(II) comꢀ
Results and Discussion
pounds are of particular interest for coordination chemꢀ
istry. Lead(II) equally readily form complexes with both
hard and soft bases, the coordination number of the metal
varying from 2 to 8.1 Due to considerable coordination
capabilities, the chemistry of lead(II) is very attractive but
poses additional problems for researchers. The coordinaꢀ
tion spheres of metals (true coordination number, the
geometry of substituents at the central atom, the equilibꢀ
rium, and the ligand exchange mechanism) are studied by
the spinꢀlabel method2 applied to both transition3—6 and
mainꢀgroup metals.7—8 The radical anions of sterically
hindered oꢀquinones are among the most widely used
spinꢀlabeled ligands. In this connection, the synthesis of
oꢀquinone lead(II) complexes and study of these comꢀ
plexes by the ESR method hold considerable promise.
Only a few oꢀquinone lead(II) complexes are known.
The formation of polymeric lead(II) 3,5ꢀdiꢀtertꢀbutylꢀ
catecholate in the course of oxidation of tetraethyllead
with 3,5ꢀdiꢀtertꢀbutylꢀoꢀbenzoquinone (1) was docuꢀ
mented.9 Lead metal reacts with compound 1 in various
aprotic solvents to give exclusively polymeric lead(II)
catecholate, which is poorly soluble in most of organic
solvents.10 Lead(II) bisꢀphenanthrenesemiquinolate was
synthesized by refluxing a lead wire with phenanthreneꢀ
quinone in hexane.11
One of the most widely used methods for the synthesis
of oꢀquinone metal complexes is based on metathesis reꢀ
actions of catecholate and oꢀsemiquinolate derivatives of
alkali metals or thallium with metal halides.12 However,
because of poor solubility of lead(II) halides, direct oxidaꢀ
tion of this metal with oꢀquinones is a method of choice.
This method is commonly used for the synthesis of
oꢀquinone complexes of mainꢀgroup metals.13
Reduction of oꢀquinones 2—4 with excess lead metal
in a THF solution occurs stepwise. In the first step, the
initial red color of oꢀquinone disappears and the reaction
mixture turns green. These changes are accompanied by
the appearance of the ESR spectrum. The isotropic ESR
spectra of the reaction mixtures are complex superposiꢀ
tions of different signals. In the initial step, the ESR specꢀ
tra are observed regardless of the nature of oꢀquinone
involved in the reaction with metal. These spectra corꢀ
respond to the free oꢀsemiquinone radical anions
(gi = 2.0041, ai(2H) = 0.35 mT; gi = 2.0045, ai(3H) =
0.05 mT, ai(H) = 0.39 mT; gi = 2.0055, ai(35,37Cl) =
0.05 mT, ai(H) = 0.33 mT for oꢀquinones 2, 3, and 4,
respectively). In addition, the ESR spectra show signals
belonging to monoꢀoꢀsemiquinone derivatives of lead, as
evidenced by satellite splitting on the 207Pb magnetic isoꢀ
topes (22.1%, I = 1/2, µN = 0.5926).14 The ESR paramꢀ
eters of the paramagnetic compounds are as follows:
gi = 2.0018, ai(2H) = 0.35 mT, ai(207Pb) = 7.95 mT;
gi = 1.9998, ai(2H) = 0.35 mT, ai(207Pb) = 6.03 mT;
gi = 1.9990, ai(2H) = 0.35 mT, ai(207Pb) = 5.01 mT for
oꢀquinone 2; gi = 2.0015, ai(3H) = 0.05 mT, ai(H) =
0.39 mT, ai(207Pb) = 7.14 mT; gi = 1.9997, ai(3H) =
In the present study, we synthesized lead(II) cateꢀ
cholate complexes by reduction of 3,6ꢀdiꢀtertꢀbutylꢀ
oꢀbenzoquinone (2) and its derivatives, viz., 3,6ꢀdiꢀtertꢀ
butylꢀ4ꢀmethoxyꢀoꢀbenzoquinone (3) and 3,6ꢀdiꢀtertꢀ
butylꢀ4ꢀchloroꢀoꢀbenzoquinone (4), with lead metal in
THF and investigated the redox reactions with the resultꢀ
ing compounds.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1103—1111, July, 2006.
1066ꢀ5285/06/5507ꢀ1146 © 2006 Springer Science+Business Media, Inc.

Lead semiquinolate and catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
1147
0.05 mT, ai(H) = 0.39 mT, ai(207Pb) = 5.46 mT; gi =
1.9989, ai(3H) = 0.05 mT, ai(H) = 0.39 mT, ai(207Pb) =
4.41 mT for oꢀquinone 3; gi = 2.0004, ai(35,37Cl) =
0.05 mT, ai(H) = 0.33 mT, ai(207Pb) = 5.04 mT;
gi = 1.9999, ai(35,37Cl) = 0.05 mT, ai(H) = 0.33 mT,
ai(207Pb) = 4.52 mT for oꢀquinone 4. The g factors
(1.9990—2.0018) and the hyperfine coupling constants
with the lead magnetic isotope ai(207Pb) (4.41—7.95 mT)
unambiguously indicate that the resulting compounds are
paramagnetic lead(II) complexes. The ESR parameters
observed in the course of the reaction substantially differ
from those obtained earlier for oꢀsemiquinone (SQ) deꢀ
rivatives of lead(IV). The latter are characterized by higher
g factors (2.0022—2.0044)9 and substantially lower hyperꢀ
fine coupling constants ai(207Pb) (0.90—0.93 mT).9 The
observed differences are also characteristic of thallium(I)15
and thallium(III)16 oꢀsemiquinolates and have been disꢀ
cussed in detail in the studies.9,16
×0.1
5 mT
Fig. 1. Anisotropic spectrum of the reaction mixture prepared by
reduction of oꢀquinone 4 with lead metal (2ꢀmethyltetraꢀ
hydrofuran, T = 130 K).
Table 1. Zeroꢀfield splitting parameters and the average disꢀ
tances between the radical centers in biradical lead derivaꢀ
tives 5—7
Complex
|D|||
|E |
r/Å
The presence of several signals belonging to monoꢀ
oꢀsemiquinone derivatives of lead(II) in the ESR spectra
measured in the early step of the reaction is apparently
attributed to dissolution of an oxide film from the surꢀ
face of the metal involved in the reaction. This should
lead to the appearance of SQPb—OH, SQPb—O—Pb,
[SQPb]+•THF, etc. compounds in solution. The same
conclusion has been drawn by researchers, who studied
reduction of oꢀquinone 1 with tin,17 magnesium,18,19
zinc,20 and cadmium19,20 by ESR spectroscopy and obꢀ
served the formation of monoꢀoꢀsemiquinone derivatives
of metals. The presence of noncoordinated semiquinone
radical anions in the reaction mixture is indicative of the
presence of solventꢀseparated ion pairs in solution.21 These
data are consistent with the results of ESR studies of the
reaction of lead metal with oꢀquinone 1.10,11
The reaction is accompanied by the appearance of the
intense blue color, and the components of the isotropic
ESR spectrum become strongly broadened. In this step,
the anisotropic ESR spectra of the reaction mixtures in a
2ꢀmethyltetrahydrofuran matrix are superpositions of a
singlet and signals characteristic of biradical species
(Fig. 1). The ESR spectra have also a halfꢀfield singlet
(H ≈ 1700 G) corresponding to the transition ∆ms = 2.
The zeroꢀfield splitting constants and the average disꢀ
G
5
6
7
368
316
334
9.0
6.0
7.5
5.3
5.6
5.5
tances between the radical centers calculated in the
dot—dipole approximation2 for the resulting biradicals
are given in Table 1.
These ESR spectra provide evidence for the formation
of the corresponding bisꢀoꢀsemiquinone metal complexes
5—7 in the reactions of oꢀquinones 2—4 with lead metal
(Scheme 1).
The further contact of the reaction mixture with exꢀ
cess lead leads to a change in the color to yellow and the
disappearance of the ESR signal. After completion of the
reaction, one mole of the metal was consumed per mole
of oꢀquinone involved in the reaction. The resulting lead
complexes can be isolated from a THF solution as
yellow finely crystalline precipitates by the addition of
toluene or hexane. The characteristic sets of bands in the
700—1600 cm–1 region of the IR spectra and elemental
analysis data for the reaction products indicate that the
reactions afford the corresponding catecholate derivatives
of lead(II) 8—10 (see Scheme 1).
Scheme 1
R = H (2, 5, 8), MeO (3, 6, 9), Cl (4, 7, 10)

1148
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
Abakumov et al.
O(1s)
O(3)
The resulting compounds are insoluble in acyclic
ethers, hydrocarbons, their chloro derivatives, and niꢀ
trogenꢀcontaining solvents (pyridine, acetonitrile, and
amines) but are readily soluble in THF, dioxane, and
dimethoxyethane, as well as in acetone on heating. The
complexes isolated from THF solutions (IR spectroscopic
data) contain no coordinated solvent molecules. In the
crystalline state, compounds 8—10 exist apparently as
coordination polymers. The latter can be formed through
additional bridging Pb—O—Pb bonds involving the oxyꢀ
gen atoms of the catecholate ligands. This behavior is
characteristic of oxygenꢀcontaining divalent lead comꢀ
plexes.1 The same conclusion was drawn in the studies,
where lead(II) catecholate was synthesized starting from
oꢀquinone 1.10,11 The presence of sterically nonshielded
oxygen atom at position 1 of oꢀquinone 1 results in even
lower solubility of the resulting complex compared to
compounds 8—10.
C(5)
C(4)
C(16)
O(2s)
C(17)
O(2)
C(15)
Pb(3)
C(6)
C(18)
O(1A)
Pb(2)
O(5)
Pb(1)
C(1)
O(1)
C(20)
C(19)
O(5A)
C(3)
O(4)
C(2)
O(4A)
Pb(1A)
Pb(3A)
Pb(2A)
O(2A)
O(3A)
Fig. 2. Molecular structure of complex 11. The atoms are repreꢀ
sented by anisotropic displacement ellipsoids drawn at the
50% probability level. The H atoms, the tertꢀbutyl groups, and
the acetone molecules (except for the O(1s) and O(2s) atoms
involved in coordination with the lead atoms) are omitted.
In the study of lead(II) carboxylates, it was demonꢀ
strated23 that such coordination polymers can be transꢀ
formed into lead(II) oxideꢀcontaining clusters by partial
hydrolysis. We found that hydrolysis of compound 8 by
refluxing in acetone containing ~1% of water afforded the
new lead complex, tetrakis(3,6ꢀdiꢀtertꢀbutylcatechoꢀ
lato)bis(µ4ꢀoxo)hexalead(II) hexakis(acetone) solvate (11)
(Scheme 2, the acetone molecules in the structural forꢀ
mula of complex 11 are omitted).
acceptor bonds and consists of the PbO dimer, in which
the Pb(3)O(5) and Pb(3A)O(5A) fragments are additionꢀ
ally coordinated by two lead(II) catecholate molecules
through the oxygen atoms (Fig. 2). Selected bond lengths
and bond angles are given in Table 2. The catecholate
fragments are differently arranged relative to the PbO
dimer (Fig. 3). There is only one donorꢀacceptor bond,
a
Scheme 2
C(2)
O(2)
Pb(3)
O(5)
C(1)
O(1)
Pb(1)
O(5A)
Pb(3A)
b
O(4)
Pb(2)
Pb(3A)
C(21)
C(15)
O(5A)
O(3)
O(5)
Pb(3)
This compound is an example of hybrid "metalꢀorꢀ
ganic framework" complexes. The chemistry of these comꢀ
plexes has been extensively developed in recent years beꢀ
cause they are promising reagents in chemistry and techꢀ
nology.24
Fig. 3. Fragments A (a) and B (b) of the structure of complex 11.
The atoms are represented by anisotropic displacement ellipꢀ
soids drawn at the 50% probability level. The H atoms and the
acetone molecules are omitted.
Xꢀray diffraction study demonstrated that 11 is a cenꢀ
trosymmetric complex formed through the Pb—O donorꢀ

Lead semiquinolate and catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
1149
Table 2. Selected bond lengths (d) and bond angles (ω) in complex 11
Bond
d/Å
Bond
d/Å
Angle
ω/deg
O(1)—C(1)
O(2)—C(2)
O(3)—C(15)
O(4)—C(16)
C(1)—C(2)
C(1)—C(6)
C(2)—C(3)
C(3)—C(4)
C(4)—C(5)
C(5)—C(6)
C(15)—C(16)
C(15)—C(20)
C(16)—C(17)
C(17)—C(18)
C(18)—C(19)
C(19)—C(20)
Pb(1)—O(1)
Pb(1)—O(2)
Pb(1)—O(5)
Pb(2)—O(3)
Pb(2)—O(4)
1.345(5)
1.354(5)
1.376(5)
1.365(5)
1.429(6)
1.412(6)
1.409(6)
1.386(7)
1.386(7)
1.394(6)
1.418(6)
1.401(6)
1.417(6)
1.396(6)
1.378(7)
1.402(6)
2.212(3)
2.228(3)
2.291(3)
2.278(3)
2.284(3)
Pb(2)—O(5)
Pb(3)—O(3)
Pb(3)—O(4A)
Pb(3)—O(5)
Pb(3)—O(5A)
Pb(1)—Pb(2)
Pb(2)—Pb(3)
Pb(2)—Pb(3A)
Pb(3)—Pb(3A)
Pb(1)—O(2S)
Pb(1)—C(15)
Pb(1)—C(16)
Pb(1)—C(17)
Pb(1)—C(18)
Pb(1)—C(19)
Pb(1)—C(20)
Pb(2)—O(2)
Pb(2)—O(1S)
Pb(2)—O(2S)
Pb(3)—O(2S)
2.349(3)
2.554(3)
2.548(3)
2.256(3)
2.291(3)
3.667(3)
3.591(2)
3.616(3)
3.527(3)
3.221(3)
3.441(3)
3.412(3)
3.505(3)
3.526(3)
3.525(3)
3.526(3)
3.045(3)
3.022(3)
2.819(3)
2.877(3)
O(1)—Pb(1)—O(2)
O(1)—Pb(1)—O(5)
O(2)—Pb(1)—O(5)
O(3)—Pb(2)—O(4)
O(3)—Pb(2)—O(5)
O(4)—Pb(2)—O(5)
O(5)—Pb(3)—O(5A)
O(5)—Pb(3)—O(4A)
O(5A)—Pb(3)—O(4A)
O(5)—Pb(3)—O(3)
O(5A)—Pb(3)—O(3)
O(4A)—Pb(3)—O(3)
Pb(2)—O(3)—Pb(3)
Pb(2)—O(4)—Pb(3A)
Pb(3)—O(5)—Pb(1)
Pb(3)—O(5)—Pb(3A)
Pb(1)—O(5)—Pb(3A)
Pb(3)—O(5)—Pb(2)
Pb(1)—O(5)—Pb(2)
Pb(3A)—O(5)—Pb(2)
74.20(10)
83.50(11)
93.99(10)
71.69(10)
81.27(10)
80.18(10)
78.28(11)
86.71(10)
75.94(10)
77.33(10)
89.92(10)
160.60(9)
95.85(10)
96.74(10)
123.04(13)
101.72(11)
119.77(12)
102.46(11)
104.42(11)
102.38(11)
Pb(1)—O(5), between the catecholate metallocycle and
the PbO dimer in the fragment A (see Fig. 3, a). In the
fragment B (see Fig. 3, b), there are three such interacꢀ
tions, Pb(2)—O(5), Pb(3)—O(3), and Pb(3A)—O(4).
The dihedral angle between the Pb(2)O(3)O(4) and
Pb(3)O(5)Pb(3A)O(5A) planes is 8.5°.
repulsions would occur between all components of comꢀ
plex 11.
In the molecule of complex 11, there is an interaction
between the Pb(1) atom and the aromatic system of
the catecholate ligand C(15)—C(20). It should be
noted that the average Pb(1)—C(15—20) distance in
molecule 11 (3.49 Å) is substantially longer than the
average Pb—C(arom.) distance (3.20 Å) in lead(II)
bis((η6ꢀoꢀxylene)tetrachloroaluminate),25 which is the
only known compound with η6 coordination of the aroꢀ
matic system to lead(II).
The lead atoms in molecule 11 form a distorted octaꢀ
hedron with the Pb(1)Pb(2)Pb(1A)Pb(2A) base and the
vertices occupied by the Pb(3) and Pb(3A) atoms. This
cluster can alternatively be described by two edgeꢀsharing
tetrahedra. The µ4ꢀoxo ligands O(5) and O(5A) are loꢀ
cated in the centers of the tetrahedra. An analogous core,
Pb6O2, was found in the compound produced by the hyꢀ
drothermal synthesis of lead(II) benzoylbenzoate.23
Complex 11 contains also six acetone molecules. These
molecules are arranged in three equivalent pairs relative
to the lead atoms. It should be noted that one pair does
not form donorꢀacceptor bonds with the metal atom. In
two other pairs, the Pb—O distances are in the range of
2.819(3)—3.221(3) Å, which is indicative of coordination
interactions. The O(1s) and O(2s) atoms of two nonꢀ
equivalent coordinated acetone molecules are shown in
Fig. 2. One acetone molecule (O(1s)) is coordinated to
the Pb(2) atom, and the other molecule (O(2s)) forms
donorꢀacceptor bonds with all three lead atoms,
The Pb(1)—O(5) distance (2.291(3) Å) in the fragꢀ
ment A is substantially larger than the endocyclic
Pb(1)—O(1) and Pb(1)—O(2) bond lengths (2.212(3) and
2.228(3) Å, respectively) in the fiveꢀmembered metalloꢀ
cycle but is comparable with the analogous distances
in lead(II) oxide (2.256(3) and 2.291(3) Å). In the
fragment B, the Pb(2)—O(5), Pb(3)—O(3), and
Pb(3A)—O(4) distances (2.349(3), 2.554(3), and
2.548(3) Å, respectively) are substantially longer than the
analogous distance in A. In addition, the Pb(2)—O(3)
and Pb(2)—O(4) distances (2.278(3) and 2.284(3) Å) in
the catecholate moiety of the fragment B are also substanꢀ
tially longer than the analogous distances in A and are
comparable with the Pb(1)—O(5) bond length. The O—C
distances in the catecholate metallocycles of the fragꢀ
ments A and B of complex 11 are equal within experimenꢀ
tal error (1.345(5)—1.376(5) Å) and are in the range charꢀ
acteristic of this type of ligands.12
Unlike the fragment A, in which the catecholate
metallocycle is virtually planar, the fragment B is folded
along the O(3)...O(4) line (28.0°) away from the Pb(1)
and Pb(1A) atoms. Apparently, the folding along the
O(3)...O(4) line is attributed to steric effects in the comꢀ
plex. If the fragment B were planar, substantial nonbonded

1150
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
Abakumov et al.
Scheme 3
R = H (8, 12, 15, 16, 19, 22, 25), MeO (9, 13, 17, 20, 23, 26), Cl (10, 14, 18, 21, 24, 27)
Pb(1)—Pb(3). Therefore, taking into account the lone
electron pairs of the metal atoms occupying the vacancies
in the coordination environment, the lead atoms have the
following coordination numbers (coordination polyhedra):
Pb(1), 4 (distorted tetragonal pyramid); Pb(2), 6 (disꢀ
torted monocapped trigonal prism); Pb(3), 5 (distorted
octahedron).
Complex 11, unlike compounds 8—10, is much more
readily soluble and can be recrystallized, for example,
from hot toluene.
Compounds 8—10 in a THF solution are very sensiꢀ
tive to oxidizing agents, due to which they can be used for
the preparation of various oꢀsemiquinone derivatives of
lead(II) (Scheme 3).
Oxidation of lead catecholates 8—10 with oꢀquinones
2—4 affords the corresponding bisꢀoꢀsemiquinone derivaꢀ
tives 5—7. The parameters of the anisotropic ESR spectra
of the latter are identical to those observed in the course
of reduction of oꢀquinones with lead (see Table 1). Comꢀ
plexes 5 and 6 were isolated in individual state by recrysꢀ
tallization from hexane. Compound 7 gradually decomꢀ
poses in solution to form lead(II) chloride.
zoyl peroxide in THF solutions generate new threeꢀcoorꢀ
dinate monoꢀoꢀsemiquinone lead(II) complexes. The ESR
spectra of the resulting compounds reveal interactions of
the unpaired electron with the magnetic nuclei of the
oꢀsemiquinone ligand and the 207Pb magnetic isotope.
For derivatives 12—18, the hyperfine coupling constants
with the magnetic isotopes of the halogen atoms are also
observed (Fig. 4). The ESR parameters of the resulting
complexes are given in Table 3.
It was reported9 that lead(IV) diethylcatecholate can
add the phenoxyl radical to give the corresponding oꢀsemiꢀ
quinone derivative. The addition of stable 3,6ꢀdiꢀtertꢀbuꢀ
tylꢀ2ꢀethoxyphenoxyl to solutions of compounds 8—10
leads to the disappearance of the ESR signal of the startꢀ
ing aroxyl radical and the appearance of new spectra
(Fig. 5), which are indicative of the formation of oꢀsemiꢀ
quinone lead(II) complexes 19—21.
Complexes 12—27 are the first lead derivatives in the
series of threeꢀcoordinate complexes of divalent 14 Group
elements containing paramagnetic ligands. Analogous deꢀ
rivatives of silicon,26 germanium,27—29 and tin8,30 were
documented known. The (BIAN)GeCl complex (BIAN is
the N,Nꢀsubstituted acenaphthenediimine radical anion)
was studied by Xꢀray diffraction27 and it was demonꢀ
Oxidation of catecholates 8—10 with copper(II),
mercury(II), and silver(I) salts, molecular iodine, and benꢀ

Lead semiquinolate and catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
1151
Table 3. ESR parameters of lead oꢀsemiquinone derivatives 12—27
Comꢀ
plex
gi
ai(Н) (1 H)
ai(R)
ai(207Pb)
ai(Hal)
mT
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2.0002
2.0003
2.0012
2.0032
2.0011
2.0009
2.0024
2.0015
2.0011
2.0008
1.9960
1.9961
1.9971
1.9998
1.9994
2.0006
0.345
0.385
0.335
0.325
0.350
0.380
0.330
0.347
0.370
0.330
0.355
0.380
0.350
0.350
0.380
0.345
0.345 (1 H)
0.055 (3 H)
6.240
5.380
5.720
6.850
6.080
5.390
5.610
7.540
6.800
5.350
2.130
1.760
2.120
5.820
5.180
5.230
0.150 (35Cl), 0.125 (37Cl)
0.147 (35Cl), 0.128 (37Cl)
0.145 (35Cl), 0.120 (37Cl)
0.890(127I)
0.690 (79Br), 0.750 (81Br)
0.670 (79Br), 0.720 (81Br)
0.670 (79Br), 0.720 (81Br)
0.060 (35Cl), 0.050 (37Cl)
0.325 (1 H)
0.350 (1 H)
0.060 (3 H)
0.060 (35Cl), 0.050 (37Cl)
0.347 (1 H)
0.050 (3 H)
0.045 (35Cl), 0.037 (37Cl)
0.355 (1 H)
0.050 (3 H)
0.055 (35Cl), 0.045 (37Cl)
0.350 (1 H)
0.055 (3 H)
0.055 (35Cl), 0.045 (37Cl)
in the studies.8,28,30 Analogous structures were also typiꢀ
cal of the known threeꢀcoordinate lead(II) complexes.31,32
Similar trigonal structures would be expected for comꢀ
plexes 12—27.
a
×5
×5
1 mT
b
This is also evidenced by the ESR parameters of halide
complexes 12—18. The high hyperfine coupling conꢀ
stants with the magnetic isotopes of the halogen atoms
indicate that the σ(σ∗) orbitals of PbꢀHal make a subꢀ
stantial contribution to πꢀMO occupied by the unpaired
electron. This interaction is favored by the geometry of
complexes in which the Pb—Hal bonds are orthogonal
to the plane of the metallocycle. The hyperfine coupling
constants ai(35,37Cl) for the paramagnetic threeꢀcoorꢀ
dinate complexes decrease in the Ge—Sn—Pb series
(0.9—1.1 mT 27,28 > 0.5—0.7 mT 8,30 > 0.14—0.15 mT)
with the simultaneous increase in the ionic radius of the
divalent elements (0.90 Å > 0.93 Å > 1.32 Å),14 which is
also consistent with the proposed geometry.
The hyperfine coupling constants ai(207Pb) and the
g factors for the ESR spectra of complexes 12—27 vary in
a wide range, whereas the hyperfine coupling constants
with the magnetic nuclei of the oꢀsemiquinone ligand are
virtually equal within experimental error (see Table 3).
Taking into account the proposed geometry of the lead
Fig. 4. Experimental (a) and calculated (b) ESR spectra of comꢀ
plex 12 in THF (T = 290 K).
×5
×5
1 mT
Fig. 5. ESR spectrum of complex 19 in THF (T = 290 K).
strated that the structure of this complex can be described
as a distorted trigonal pyramid, whose base is formed by
two nitrogen atoms and the chlorine atom and the apex is
occupied by the lone electron pair of the germanium atom.
Based on the ESR parameters of the derivatives LSnCl
and LGeCl (L is the radical anions of the diazabutadiene
series), the trigonalꢀpyramidal geometry was proposed also

1152
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
Abakumov et al.
oꢀsemiquinone complexes, it can be stated that the forꢀ
mation of oꢀsemiquinone complexes is accompanied by
hybridization of the occupied 6s orbital and the unoccuꢀ
pied 6p orbital of lead to form a distorted tetrahedral
coordination sphere. The vertices are occupied by the
oxygen atoms, the substituent X, and the lone electron
pair of lead. In this case, the s orbital, which has a nonꢀ
zero density on the metal nucleus, makes a substantial
contribution to the orbitals, which can directly interact
with the orbital of the unpaired electron. This results in an
additional large contribution to the hyperfine coupling
constants without changes in the spin density of the ligand.
According to the proposed mechanism, excitation of the
lone electron pair to the πSQ* molecular orbital should
cause an increase in the g factor with a simultaneous
increase in the hyperfine coupling constant on the lead
magnetic isotope, ai(207Pb). This dependence is clearly
seen among related (closest environment of the lead atom
consists of three oxygen atoms) lead oꢀsemiquinolate comꢀ
plexes 19—27, including paramagnetic lead derivatives
observed in the course of reduction of oꢀquinones with
metal (Fig. 6).
Fig. 7. Molecular orbital diagram for lead oꢀsemiquinone comꢀ
plexes 19 and 22.
Due to the ability of catecholate derivatives of lead(II)
to add Oꢀcentered radicals, compounds 8—10 can be used
as spin traps for this type of radicals. The strong depenꢀ
dence of the ESR parameters for lead(II) oꢀsemiquinone
complexes on the nature of the substituent X can be used
for the identification of the radical being attached.
Complexes 12—27 are stable in THF solutions in the
absence of atmospheric oxygen and moisture for several
days. The replacement of THF by other solvents causes
symmetrization of lead monoꢀoꢀsemiquinolates to form
biradical complexes 5—7 (Scheme 4). The signals of startꢀ
ing complexes 12—27 in the isotropic ESR spectra disapꢀ
pear, and the anisotropic spectra show signals characterꢀ
istic of compounds 5—7.
The g factor increases according to the equation
∆g = 2λ/∆E,
where λ is the spinꢀorbital coupling constant and ∆E is the
energy difference between the πSQ* molecular orbital ocꢀ
cupied by the unpaired electron and MO occupied by the
lone electron pair of the metal atom (Fig. 7).
Scheme 4
The effective positive charge of the lead atom in comꢀ
plexes containing more electronegative substituents X is
higher compared to that in compounds containing less
electronegative substituents, which, in turn, results in an
increase in the MO level occupied by the lone electron
pair and an increase in the g factor. As an example, the
relative changes in the energy of MO of the triflate (22)
and phenolate (19) lead oꢀsemiquinone complexes are
presented in Fig. 7.
Experimental
gi
All experiments associated with the synthesis and study of
the properties of catecholate and oꢀsemiquinolate lead(II) comꢀ
plexes were carried out under reduced pressure in the absence of
traces of oxygen and water.
The solvents were purified and dried according to recomꢀ
mendations.33 oꢀQuinones 2 34, 3 35, and 4 36 and 3,6ꢀdiꢀtertꢀ
butylꢀ2ꢀethoxyphenoxyl37 were prepared according to known
procedures. Copper, mercury, and silver halides, silver triflate,
bezoyl peroxide, and iodine (Aldrich) were used without addiꢀ
tional purification.
The IR spectra were measured on a Specord Mꢀ80 instruꢀ
ment in Nujol mulls. The ESR spectra were recorded on a Bruker
ER 200 DꢀSRC spectrometer equipped with an ER 4105 DR
double resonator and an ER 4111 VT temperature controller.
The g factors were measured with the use of diphenylpicrylꢀ
hydrazyl as the standard (g = 2.0037).
2.001
2.000
1.999
1.998
1.997
1.996
2
1
3
2
3
4
5
6
ai(207Pb)/mT
Fig. 6. Plots gi—ai(207Pb) for lead(II) oꢀsemiquinone complexes
derived from oꢀquinones 2 (1), 3 (2), and 4 (3).

Lead semiquinolate and catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 7, July, 2006
1153
Synthesis of complexes 8—10 (general procedure). A soluꢀ
tion of oꢀquinone 2 (1.10 g, 5.0 mmol) in THF (30 mL) was
added with continuous stirring to an excess of lead metal (3.11 g,
15.0 mmol), after which the color of the reaction mixture
changed from redꢀgreen through intense greenꢀblue to yellow.
Then the reaction mixture was separated from the unconsumed
metal by decantation. After the addition of toluene or hexane to
the resulting solution, a finely crystalline precipitate of comꢀ
plex 8 was obtained (2.05 g, 4.8 mmol) in 96.0% yield. Comꢀ
plex 8 is soluble in THF, dioxane, and dimethoxyethane, as well
as in acetone on heating. Complex 8 is sensitive to atmospheric
oxygen both in solution and the crystalline state.
Complexes 9 (91.9% yield) and 10 (89.3% yield) were preꢀ
pared analogously starting from oꢀquinones 3 and 4, respecꢀ
tively.
Lead(II) 3,6ꢀdiꢀtertꢀbutylcatecholate (8), yellow finely crysꢀ
talline compound; decomposes without melting at a temperaꢀ
ture higher than 260 °C. Found (%): C, 32.50; H, 3.98; Pb, 54.64.
than 40 °C, compound 11 loses acetone. At a temperature higher
150 °C, 11 decomposes without melting. Found (%): C, 35.24;
H, 4.17; Pb, 49.86. C74H116O16Pb6. Calculated (%): C, 35.49;
H, 4.64; Pb, 49.64. IR, ν/cm–1: 1688, 1413, 1376, 1240, 1150,
966, 945, 931.
Synthesis of complexes 12—27 (general procedure). Equimoꢀ
lar amounts of complexes 8 (9 or 10) and different oxidizing
agents (see Scheme 3) in THF were mixed in an ESR tube.
Complexes 12—27 were studied in solution without isolation.
Xꢀray diffraction study. Single crystals of compound 11 were
grown by slow cooling of an acetone solution. Xꢀray diffracꢀ
tion data were collected on a Smart Apex diffractometer
(MoꢀKα, graphite monochromator). The crystal dimensions are
0.25×0.20×0.10 mm. A total of 31929 reflections were meaꢀ
sured, of which 7225 reflections (Rint = 0.0341) were indepenꢀ
dent with I > 2σ(I). The unit cell parameters for C37H58O8Pb3
at 100(2) K: a = 15.7862(8) Å, b = 16.7512(9) Å, c =
15.9984(8) Å, β = 103.557°, space group P2(1)/n, Z = 4, V =
C
14H20O2Pb. Calculated (%): C, 32.76; H, 4.05; Pb, 54.74. IR,
4112.7(4) Å3, dcalc = 2.023 g cm–3, µ = 12.296 mm–1
,
ν/cm–1: 1245, 1150, 970, 920, 805, 730, 660.
F(000) = 2360, 1.80° < θ < 25.00°, R1 = 0.0210 and wR2 = 0.0485
(I > 2σ(I )), R1 = 0.0248 and wR2 = 0.0495 (based on all
data)), S(F 2) = 1.129, the residual electron density (max/min)
Lead(II) 3,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethoxycatecholate (9), orange
finely crystalline compound; decomposes without melting at a
temperature higher than 315 °C. Found (%): C, 39.30; H, 4.67;
Pb, 45.56. C15H22O3Pb. Calculated (%): C, 39.38; H, 4.81;
Pb, 45.29. IR, ν/cm–1: 1235, 1100, 970, 870.
Lead(II) 3,6ꢀdiꢀtertꢀbutylꢀ4ꢀchlorocatecholate (10), orange
finely crystalline compound; decomposes without melting at a
temperature higher than 230 °C. Found (%): C, 36.50; H, 4.09;
Pb, 44.30; Cl, 7.70. C14H19O2PbCl. Calculated (%): C, 36.39;
H, 4.12; Pb, 44.88; Cl, 7.68. IR, ν/cm–1: 1290, 1235, 1155,
1055, 975, 945, 845.
0.919/–1.240 e Å–3
.
The structure was solved by direct methods and refined by
the fullꢀmatrix leastꢀsquares method against F 2 with the use of
the SHELXTL program package.38 The semiempirical absorpꢀ
tion correction was applied based on equivalent reflections using
the SADABS program.39 All nonhydrogen atoms were refined
anisotropically. The hydrogen atoms were located in difference
electron density maps and refined isotropically, except for the
H atoms of the solvent molecules.
Synthesis of complexes 5 and 6 (general procedure). Soluꢀ
tions of complex 8 (2.14 g, 5.0 mmol) and oꢀquinone 2 (1.1 g,
5.0 mmol) in THF (30 mL) were mixed in a 100ꢀmL tube, after
which the reaction mixture turned intense blueꢀgreen. The solꢀ
vent was removed under reduced pressure, and the residue was
dissolved in hot hexane. After cooling of the solution, needleꢀ
like crystals of complex 5 (3.00 g, 4.6 mmol, 92.8% yield) preꢀ
cipitated. Complex 5 is soluble in most of organic solvents. In
the crystalline state, the complex is resistant to atmospheric
oxygen. In solution, complex 5 slowly decomposes to form the
corresponding oꢀquinone.
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 04ꢀ03ꢀ
32413), the Council on Grants of the President of
the Russian Federation (Program for State Support of
Leading Scientific Schools of the Russian Federation,
Grant NShꢀ4947.2006.3, and Young Doctors, Grant
MKꢀ8752.2006.3), and the Russian Science Support
Foundation.
References
Complex 6 was prepared analogously starting from cateꢀ
cholate 9 and oꢀquinone 3 in 85.6% yield.
Lead(II) bis(3,6ꢀdiꢀtertꢀbutylꢀoꢀsemiquinolate) (5), thin blueꢀ
green needleꢀlike crystals, m.p. 173 °C. Found (%): C, 51.83;
H, 6.24; Pb, 32.01. C28H40O4Pb. Calculated (%): C, 51.93;
H, 6.18; Pb, 31.99. IR, ν/cm–1: 1470, 1360, 1280, 1200, 950, 830.
Lead(II) bis(3,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethoxyꢀoꢀsemiquinolate)
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1250, 1190, 1100, 990, 880, 830.
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Received February 8, 2006;
in revised form May 23, 2006