Amide Group Coordination to the Pb2+ Ion

Article abstract of DOI:10.1021/ic950599h

The binary and ternary (2,2′-bipyridine) complexes of dipositive lead formed by N-carbonyl and N-sulfonyl amino acids, which are ligands containing the peptide and the sulfonamide group, respectively, were investigated in aqueous solution by NMR and differential pulse polarography, and some were also characterized crystallographically. N-Tosylglycine, N-tosyl-β-alanine, and N-benzoylglycine behave as simple carboxylate ligands at acid pH, while around neutrality they switch to dianionic N,O-bidentate chelating ligands due to the involvement of the deprotonated amide nitrogen as an additional donor site. The same coordination behavior is maintained in the presence of 2,2′-bipyridine. The binary and ternary species formed in solution, and their stability constants were determined and compared with those of the homologous complexes of Pd²⁼, Cu²⁺, Cd²⁺ and Zn²⁺. The Pb²⁺ ion is the only dipositive metal which is effective in promoting peptide nitrogen deprotonation in benzoylglycine. The molecular structures of [Pb(N-tosylglycinato-N,O)(H₂O)] (1), [Pb(N-benzoylglycinato-O)₂(H₂O) ₂]·2H₂O (2), and [Pb(N-tosylglycinato-O)₂(bpy)] (3) were determined by X-ray crystallography (O and N,O refer to the ligands binding as carboxylates and as N,O-chelating dianions, respectively). These compounds are all polymeric with six- to eight-coordinate metals showing distorted coordination geometries indicative of a stereochemically active metal lone pair. Polymerization is invariably determined by a bidentate chelate carboxylate group with one oxygen bridging between two metals, and in 2 and 3 it occurs through the formation of chains of Pb₂O₂ square-planar rings. The binding set in 1, involving a deprotonated amide nitrogen and a sulfonic oxygen, is unprecedented for the Pb²⁺ ion. This work provides new information on the solution and solid state chemistry of dipositive lead with ligands of biological interest, a research area that has received little attention in the past, although it is of great relevance for understanding the mechanisms of metal toxicity.

Full text of DOI:10.1021/ic950599h

Inorg. Chem. 1996, 35, 4239-4247
4239
Amide Group Coordination to the Pb2+ Ion
Gianantonio Battistuzzi, Marco Borsari, Ledi Menabue,* and Monica Saladini
Department of Chemistry, University of Modena, Via Campi 183, 41100 Modena, Italy
Marco Sola
Institute of Agricultural Chemistry, University of Bologna, Viale Berti Pichat 10, 40127 Bologna, Italy
ReceiVed May 16, 1995X
The binary and ternary (2,2′-bipyridine) complexes of dipositive lead formed by N-carbonyl and N-sulfonyl amino
acids, which are ligands containing the peptide and the sulfonamide group, respectively, were investigated in
aqueous solution by NMR and differential pulse polarography, and some were also characterized crystallographi-
cally. N-Tosylglycine, N-tosyl-â-alanine, and N-benzoylglycine behave as simple carboxylate ligands at acid
pH, while around neutrality they switch to dianionic N,O-bidentate chelating ligands due to the involvement of
the deprotonated amide nitrogen as an additional donor site. The same coordination behavior is maintained in
the presence of 2,2′-bipyridine. The binary and ternary species formed in solution, and their stability constants
were determined and compared with those of the homologous complexes of Pd2+, Cu2+, Cd2+, and Zn2+. The
Pb2+ ion is the only dipositive metal which is effective in promoting peptide nitrogen deprotonation in
benzoylglycine. The molecular structures of [Pb(N-tosylglycinato-N,O)(H2O)] (1), [Pb(N-benzoylglycinato-O)2-
(H2O)2]‚2H2O (2), and [Pb(N-tosylglycinato-O)2(bpy)] (3) were determined by X-ray crystallography (O and N,O
refer to the ligands binding as carboxylates and as N,O-chelating dianions, respectively). These compounds are
all polymeric with six- to eight-coordinate metals showing distorted coordination geometries indicative of a
stereochemically active metal lone pair. Polymerization is invariably determined by a bidentate chelate carboxylate
group with one oxygen bridging between two metals, and in 2 and 3 it occurs through the formation of chains of
Pb2O2 square-planar rings. The binding set in 1, involving a deprotonated amide nitrogen and a sulfonic oxygen,
is unprecedented for the Pb2+ ion. This work provides new information on the solution and solid state chemistry
of dipositive lead with ligands of biological interest, a research area that has received little attention in the past,
although it is of great relevance for understanding the mechanisms of metal toxicity.
Introduction
sites of several biomolecules and adapt to different coordination
geometries (with the large ionic radius as a limiting factor).
Toxic metal ions of environmental origin may specifically
bind to proteins, nucleic acids, and small metabolites in living
organisms, causing either alteration or loss of the biological
function, and may perturb the homeostatic control of essential
metals to which they are related in terms of ionic radius and
coordination chemistry.1-3 The Pb2+ ion, a large-scale envi-
ronmental pollutant, is known to obstruct heme biosynthesis,
to inhibit several zinc enzymes such as carbonic anhydrase,
acetylcholine esterase, and acid phosphatases, to interact with
nucleic acids and transfer RNA affecting protein synthesis,1,4-7
and to accumulate, also exchanging with Ca2+, in the apatite
structure of the bone. One of the determinants of Pb2+ toxicity
is likely the borderline “hard-soft”character.8 Lacking a marked
donor atom preference, this metal ion may fit in the binding
However, very little is known at present about specific interac-
tions at a molecular level. Moreover, although the coordination
chemistry of Pb2+ 1,9 is a key factor in determining the biological
effects of this metal, only a relatively few studies have appeared
concerning Pb2+ interactions with simple biomolecules. These
include studies of amino acids, small peptides, and nucleotides
for which stability constants of the complexes in aqueous
solution have been reported.5-7,10-14 Also, the X-ray structures
of Pb2+ complexes with ligands containing donor groups of
biological relevance have never been thoroughly investigated.
Metal coordination of the CONH group in small peptides may
occur through either the carbonyl oxygen or the deprotonated
amide nitrogen, provided the metal is initially coordinated by
the terminal amino group.15 Closure of the five-membered
* Corresponding author. Telefax: 39-59-373543.
X Abstract published in AdVance ACS Abstracts, June 1, 1996.
(1) Martin, R. B. In Encyclopedia of Inorganic Chemistry; King, R. B.,
Ed.; Wiley Interscience: Chichester, U.K., 1994; Vol. 4, p 2185.
(2) Martin, R. B. Met. Ions Biol. Syst. 1986, 20, 21.
(3) Martin, R. B. In Handbook on Toxicity of Inorganic Compounds;
Seiler, H. G., Sigel, H., Eds.; Marcel Dekker: New York, 1988;
Chapter 2, p 9.
(4) Harrison, P. G. In Encyclopedia of Inorganic Chemistry; King, R. B.,
Ed.; Wiley Interscience: Chichester, U.K., 1994; Vol. 4, p 1944.
(5) Kazakov, S. A.; Hecht, S. M. In Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; Wiley Interscience: Chichester, U.K., 1994; Vol. 5,
p 2697.
(8) Basolo, F.; Pearson, R. G. In Mechanisms of Inorganic Reactions. A
Study of Metal Complexes in Solution, 2nd ed.; Wiley: New York,
1967.
(9) Harrison, P. G. In ComprehensiVe Coordination Chemistry; Wilkinson,
G., Grillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: Oxford,
U.K., 1987; Vol. 3, p 183.
(10) Khayat, Y.; Cromer-Morin, M.; Scharff, J. P. J. Inorg. Nucl. Chem.
1979, 41, 1496.
(11) Rabenstein, D. L.; Libich, S. Inorg. Chem. 1972, 11, 2960.
(12) Corrie, A. M.; Williams, D. R. J. Chem. Soc., Dalton Trans. 1976,
1068.
(13) Bizri, Y.; Cromer-Morin, M.; Scharff, J. P. J. Chem. Res., Synop. 1982,
192 and references therein.
(14) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum
Press: New York and London, 1974; Vol. 1.
(6) Eichhorn, G. L. In Inorganic Biochemistry; Eichhorn, G. L., Ed.;
Elsevier: Amsterdam, 1973; Vol. 2, p 1191.
(7) Izatt, R. M.; Christensen, J. J.; Rytting, J. H. Chem. ReV. 1971, 71,
439.
(15) Sigel, H.; Martin, R. B. Chem. ReV. 1982, 82, 385.
S0020-1669(95)00599-4 CCC: $12.00 © 1996 American Chemical Society

4240 Inorganic Chemistry, Vol. 35, No. 14, 1996
Battistuzzi et al.
pH was adjusted to 5 with NaOH. White platelets separated from the
mixture after a few hours at room temperature. Anal. Calcd for
C18H20N2O8PbS2: C, 32.56; H, 3.04; N, 4.22. Found: C, 32.47; H,
3.14; N, 4.17. Yield ) 70%.
Pb(bzgly-O)2‚4H2O (2). The compound was prepared as described
above for the corresponding species with tsgly. Separation of the
crystals was achieved in the pH range 4.5-6.3. The crystals were dried
on filter paper. Anal. Calcd for C18H24N2O10Pb: C, 33.99; H, 3.81;
N, 4.41. Found: C, 33.13; H, 3.95; N, 4.24. Yield ) 70%.
Pb(tsâala-O)2‚2H2O. The compound was prepared as described
above for the corresponding species with tsgly and bzgly. Separation
of the crystals was achieved in the pH range 4.5-6.7. At higher pH
values, metal hydroxide precipitation occurred. Anal. Calcd for
C20H28N2O10PbS2: C, 32.99; H, 3.88; N, 3.85. Found: C, 33.47; H,
3.62; N, 3.86. Yield ) 75%.
Pb(tsgly-O)2(bpy) (3). A 20 mL quantity of a methanolic solution
of bpy (0.025 M) was added to 30 mL of aqueous tsgly (0.017 M) at
pH 6. A 20 mL solution of aqueous Pb(NO3)2 was then added dropwise
to the resulting solution, adjusting the pH to 6 with NaOH. After a
few days at room temperature, poor crystals of the binary species 1
appeared together with well-formed crystals of formula Pb(tsgly-O)2-
(bpy). The solid was filtered off and washed with ethanol. The same
compound was also obtained using a Pb2+:tsgly molar ratio of 1:2.
Anal. Calcd for C28H28N4O8PbS2: C, 40.99; H, 3.43; N, 6.84.
Found: C, 41.05; H, 3.41; N, 6.82. Yield ) 40%.
chelate ring involving the deprotonated peptide nitrogen provides
a strong metal binding and occurs at acidic or slightly alkaline
pH values only with Pd2+, Cu2+, Co2+, and Ni2+ 15
N-Sulfonyl
.
amino acids are low molecular weight ligands which were found
to reproduce this coordination behavior and selectivity toward
dipositive metals. In fact, at pH values close to neutrality, these
compounds were shown to bind as dianions, i.e. through one
carboxylate oxygen (acting as the primary binding site) and the
deprotonated amide nitrogen, only to a few metals, namely Pd2+
and Cu2+ (like peptides) and also Cd2+ 16-19
Hence they were
.
used as models to probe for the factors determining metal
effectiveness in promoting amide nitrogen deprotonation. In
ternary 2,2′-bipyridine systems, the additional heteroaromatic
ligand was found to favor Cu2+ and Cd2+substitution for the
sulfonamide nitrogen-bound hydrogen and to induce the binding
of the these ligands as N,O-dianions also to the Zn2+ ion.20-23
In this paper, we describe the binding of the N-sulfonyl- and
N-carbonyl amino acids to the Pb2+ ion, focusing on the
structural characterization of the crystalline compounds and on
the determination of the nature and stability of the species
formed in aqueous solution in the pH range 4-10. Our goal is
to characterize the interaction of the Pb2+ ion with the amide
bond and to compare it with that of the other metals of biological
interest previously investigated.16-23 We found that the Pb2+
ion is effective in promoting peptide and sulfonamide nitrogen
deprotonation in these ligands. Polymeric compounds with
distorted coordination geometries indicative of a stereochemi-
cally active metal lone pair were invariably obtained in the solid
state. We believe that this study provides new information on
the metallobiochemistry of lead, which at present is still scarcely
explored.
Pb(tsâala-O)2(bpy). This compound was prepared with the pro-
cedure described above for the analogous species with tsgly. Separation
of the solid complex was achieved in the pH range 4.5-6.5. Anal.
Calcd for C30H32N4O8PbS2: C, 42.47; H, 3.81; N, 6.61. Found: C,
42.61; H, 3.78; N, 6.66. Yield ) 80%.
Polarography. Investigation of the binary systems was carried out
on 1 × 10-4 M Pb(NO3)2 solutions with ligand-to-metal molar ratios
in the range 2:1 to 20:1. The same metal ion concentration was used
for the ternary (bpy) systems, with metal:bpy:ligand molar ratios from
1:1:2 to 1:1:20. Sodium nitrate was used as the base electrolyte, and
the ionic strength was kept constant (I ) 0.1 M). The pH of the
solutions was adjusted by adding small amounts of concentrated
aqueous HNO3 or NaOH. Polarographic and differential pulse po-
larography (DPP) measurements were carried out with an Amel 472
Multipolarograph at 25 ( 0.1 °C using a scan rate of 2 mV s-1 and a
pulse height of 35 mV. A saturated calomel electrode (SCE) was used
as the reference and a platinum sheet as the counter electrode. All
E1/2 values were calculated according to the Parry-Osteryoung equa-
tion25 and referred to the SCE. Quasi-reversible reduction processes
were invariably observed for binary and ternary systems. The reversible
Experimental Section
Materials. N-Tosylglycine (tsgly), N-tosyl-â-alanine (tsâala), and
N-benzoylglycine (bzgly) (C. Erba) were twice recrystallized before
use. Nanopure water was used throughout. Stock solutions of Pb-
(NO3)2 were standardized with EDTA. N-Tosylglycine 13C-enriched
at the carboxylic carbon was obtained by reacting the 13C-enriched
amino acid with tosyl chloride.24
Preparation of the Complexes. Pb(tsgly-N,O)‚H2O (1). (In what
follows, O and N,O refer to the ligands binding as carboxylates and as
N,O-chelating dianions, respectively.) A 20 mL portion of an aqueous
Pb(NO3)2 solution (0.025 M) was added under continuous stirring to
30 mL of an aqueous tsgly solution (0.017 M) at pH 6.5. The pH was
kept constant by adding small amounts of concentrated aqueous NaOH.
After 12 h at 40 °C, white crystals appeared. They were collected by
vacuum filtration and washed with ethanol. Anal. Calcd for
C9H11NO5PbS: C, 23.87; H, 2.45; N, 3.10. Found: C, 23.84; H, 2.45;
N, 3.00. Yield ) 20%.
E1/2 values were determined according to Matsuda and Ayabe.26
A
Jenway 3045 ion analyzer equipped with an Ingold HA 405-60-K1 pH
combination electrode was used for pH measurements.
Spectroscopy. 1H and 13C NMR spectra were obtained on a Bruker
AMX-400 spectrometer operating at 400.13 and 100.30 MHz, respec-
tively. Typical acquisition parameters were as follows. 1H NMR:
spectral bandwidth, 5 kHz; pulse width, 6.5 µs (90° pulse); pulse delay,
4 s; number of scans collected, 256-512. 13C NMR: spectral
bandwidth, 10 kHz; pulse width, 9 µs (90° pulse); pulse delay, 2 s;
number of scans collected, 2000. Spectra were run on aqueous (D2O)
millimolar solutions of the crystalline complexes at 27 ( 0.1 °C and
are referenced to tetramethylsilane. In the proton spectra the residual
HDO peak was suppressed by a presaturation pulse from the decoupler.
The pH was adjusted by adding small amounts of concentrated NaOH
or HClO4. pH-meter readings (pD values) are reported throughout the
paper. Infrared spectra were recorded with a Perkin-Elmer FT-IR 1600
Pb(tsgly-O)2. A 20 mL portion of aqueous lead acetate (0.02 M)
was added to 40 mL of ethanolic or aqueous tsgly (0.02 M), and the
(16) Antolini, L.; Battaglia, L. P.; Battistuzzi Gavioli, G.; Bonamartini
Corradi, A.; Grandi, G.; Marcotrigiano, G.; Menabue, L.; Pellacani,
G. C. J. Am. Chem. Soc. 1983, 105, 4327.
(17) Battistuzzi Gavioli, G.; Borsari, M.; Pellacani, G. C.; Menabue, L.;
Sola, M.; Bonamartini Corradi, A. Inorg. Chem. 1988, 27, 1587.
(18) Battistuzzi Gavioli, G.; Borsari, M.; Menabue, L.; Saladini, M.;
Pellacani, G. C.; Sola, M. J. Chem. Soc., Dalton Trans. 1990, 1585.
(19) Battistuzzi, G.; Gavioli, G.; Borsari, M.; Menabue, L.; Saladini, M.;
Sola, M. J. Chem. Soc., Dalton Trans. 1994, 279.
(20) Battistuzzi Gavioli, G.; Borsari, M.; Menabue, L.; Saladini, M.; Sola,
M.; Battaglia, L. P.; Bonamartini Corradi, A.; Pelosi, G. J. Chem.
Soc., Dalton Trans. 1990, 91.
(21) Battistuzzi Gavioli, G.; Borsari, M.; Menabue, L.; Saladini, M.; Sola,
M. J. Chem. Soc., Dalton Trans. 1991, 2961.
(22) Battistuzzi Gavioli, G.; Borsari, M.; Menabue, L.; Saladini, M.; Sola,
M. Inorg. Chem. 1991, 30, 498.
(23) Bonamartini Corradi, A.; Menabue, L.; Saladini, M.; Sola, M.;
Battaglia, L. P. J. Chem. Soc., Dalton Trans. 1992, 2623.
(24) Vogel’s Textbook of Practical Organic Chemistry Including QualitatiVe
Organic Analysis; Longman: Harlow, U.K., 1988; p 1173.
instrument as KBr pellets in the spectral range 4000-400 cm-1
.
X-ray Crystallography. Intensity data were collected on an Enraf-
Nonius CAD4 diffractometer at room temperature (293 K). Crystal
data and details of the data collection are listed in Table 1 and are
available as Supporting Information. The data were corrected for
Lorentz and polarization effects, and an empirical absorption correction
(25) Parry, E.; Osteryoung, R. Anal. Chem. 1965, 37, 1634.
(26) Matsuda, H.; Ayabe, Y. Z. Elektrochem. 1962, 66, 469.

Amide Group Coordination to the Pb2+ Ion
Inorganic Chemistry, Vol. 35, No. 14, 1996 4241
Table 1. Experimental Data for the X-ray Diffraction Studies on Crystalline Compounds
[Pb(tsgly-N,O)(H2O)]
[Pb(bzgly-O)2(H2O)2]‚2H2O
[Pb(tsgly-O)2(bpy)]
compd no.
empirical formula
M
crystal system
space group
a/Å
1
2
3
C9H11NO5PbS
452.45
monoclinic
P21/a
11.63(1)
7.672(5)
13.316(3)
90
98.16(4)
90
1176.1(8)
4
2.55, 2.40
146.3
0.72
0.035
0.033
C18H24N2O10Pb
C28H28N4O8PbS2
819.87
triclinic
P1h
6.920(6)
11.139(2)
19.756(4)
97.14(2)
96.77(4)
98.20(3)
1481(1)
2
1.84, 1.90
59.3
0.93
0.025
0.025
635.59
orthorhombic
Pccn
40.16(2)
6.847(4)
8.255(1)
90
90
90
2269(1)
4
1.86, 1.90
75.4
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
Dc, Dm/g cm-3
µ/cm-1
av transm factor
R ) ∑|∆F|/∑|Fo|
0.66
0.067
0.074
2
R′ ) [∑w(∆F2)/∑wFo ]1/2
Table 2. Logarithms of the Overall Stability Constants, â,a
Determined in 0.1 M NaNO3 at 298 K for Binary and Ternary (bpy)
Pb2+ Complexes with N-Sulfonyl and N-Carbonyl Amino Acids
based on a ψ scan27 was applied. The structures were solved by
conventional Patterson and Fourier techniques and refined with full-
matrix least-squares calculations with Σw (|Fo| - |Fc|)2 being mini-
mized. All non-hydrogen atoms were refined anisotropically, except
four carbon atoms of the phenyl ring in 1, which were found to be
statistically distributed over two positions (A, B) and which were
isotropically refined with site occupation factors of 0.5. For 2 and 3,
the hydrogen atoms, except those of water molecules, were located
from the difference Fourier maps. They were introduced in the
subsequent refinements as fixed atoms with isotropic thermal parameters
1.0 Å higher than those of the atoms to which they were bound. The
hydrogen atoms of complex 1 were ignored. Unit weight was adopted
in the refinement procedure of 1 and 3, while for 2 the weighting
L
species
tsgly
tsâala
bzgly
[PbHL]+
1.08
2.03
6.43
5.03
9.11
1.21
2.13
6.50
5.11
9.21
1.09
2.06
6.70c
5.06
9.51c
[Pb(HL)2]b
[PbL]
[Pb(bpy)(HL)2]b,d
[Pb(bpy)L]d
a The overall stability constants are relative to the equilibria Pb2+
+
nHL- (or L2-) ) [Pb(HL)n]2-n (or [PbLn]2-2n) for the binary complexes,
and to the analogous equilibria for the ternary bpy species. Estimated
standard deviation for log â is 0.07. b Observed only for ligand-to-
metal molar ratios greater than 8:1. c Value obtained by assuming a
pKNH of 15 for uncomplexed bzgly (see text). d Value obtained by using
2
scheme was w ) 3.5301/(σ2(Fo) + 0.01305Fo ). Complex neutral-
atom scattering factors28 were used throughout. All calculations were
carried out on an IBM 486/sx personal computer with the CRYS-
RULER29 crystallographic package. Final fractional coordinates are
available as Supporting Information.
a log â value of 2.9 for [Pb(bpy)]2+ 33
.
[Pb(HL)n]
[Pb][HL]n
[PbLn]
[Pb][L]n
Results
âPb(HL)
)
and âPbL )
n
n
Polarography. For all binary systems, only one quasi-
reversible, two-electron, and diffusion-controlled reduction wave
(wave Ib) is observed in the pH range 2.5-5.8. In particular,
up to pH 4 the E1/2 value is independent of ligand concentration
and corresponds to that of the solvated Pb2+ ion. At higher pH
the E1/2 value moves toward more negative values with
increasing ligand concentration. Above pH 5.8 a second quasi-
reversible and bielectronic wave (wave IIb) appears, whose E1/2
depends upon ligand concentration and, above pH 10, is also
pH dependent. The two reduction waves coexist in the pH range
5.8-7.5, and the sum of their diffusion current is nearly
constant. The current of wave IIb decreases above pH 10 due
to metal hydroxide precipitation. N-Sulfonyl amino acids are
known to bind to a number of metal ions acting either as simple
carboxylate ligands at acid pH or as N,O-bidentate ligands
through one carboxylate oxygen and the deprotonated amide
nitrogen at higher pH values.16-19 Therefore, waves Ib and IIb
most probably correspond to the reduction of the above two
types of complexes, respectively. This is confirmed by X-ray
crystallography, which indicates that tsgly acts as a dianion in
the complex which precipitates above pH 6 (see below). The
overall stability constants (â) of the above low- and high-pH
complexes, defined as
respectively, where L is the fully deprotonated (dianionic) ligand
(charges are omitted for clarity), were obtained with the
DeFord-Hume method30 and are listed in Table 2. Selection
of the species to be included in the equilibrium calculations
was based on the Lingane plots31 which, for a given reduction
wave at a given pH, yield the number of ligands bound to the
metal in the prevailing species. The â values for the carboxylate
species were determined between pH 6.3 and 7.5 (at these pH
values the uncomplexed ligand in solution is the carboxylate
monoanion) by following the change in E1/2 (∆E1/2) of wave Ib
with ligand concentration. Those for the high-pH, nitrogen-
deprotonated, complexes were calculated from ∆E1/2 values vs
ligand concentration for wave IIb at pH values from 7.5 to 9,
by using pKNH values of 11.4 and 11.2 for free tsgly and tsâala.21
For bzgly, for which an experimental pKNH value is not
available, the â values listed in Table 2 were calculated using
a pKNH of 15, which is that determined for the peptide group
from equilibrium measurements and indirect kinetic methods
for closely related molecules and is the pKNH value assumed
for the prototypic amide group in small peptides.15 An
uncertainty of (1 unit in the above pKNH value causes an
uncertainty of (0.5 unit in the â values of the binary and ternary
complexes of the dianionic ligand. The species distribution
(27) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351.
(28) International Tables for X-ray Crystallography; Kynoch: Birmingham,
England, 1974; Vol. IV, pp 99-101, 149-150.
(29) Rizzoli, C.; Sangermano, V.; Calestani, G.; Andreetti, G. D. J. Appl.
Crystallogr. 1987, 20, 436.
(30) DeFord, D. D.; Hume, H. D. N. J. Am. Chem. Soc. 1951, 73, 5321.
(31) Crow, D. R. Polarography of Metal Complexes; Academic Press: New
York, 1969; p 56.

4242 Inorganic Chemistry, Vol. 35, No. 14, 1996
Battistuzzi et al.
Figure 1. Species distribution curves for (a) the binary Pb-tsgly
system and (b) the ternary Pb-tsgly-bpy system, with molar ratios of
1:20 and 1:20:1, respectively. [Pb2+]) 2 × 10-4 M.
Figure 2. (a) pD dependence of the chemical shift of the methylene
resonance in the H NMR spectrum of tsgly in D2O: (b) free ligand;
1
(1) saturated solution of [Pb(tsgly-N,O)(H2O)] (1); (9) saturated
solution of 1 plus bpy in a 1:1 molar ratio with the metal. T ) 300 K.
[Pb2+] ) 2 × 10-3 M. (b) pD dependence of the chemical shift of the
13C NMR resonance of the isotopically enriched carboxylic carbon of
tsgly in the presence of the Pb2+ ion in a 2:1 ligand-to-metal molar
ratio. T ) 300 K. [Pb2+] ) 1 × 10-3 M.
curves as a function of pH for tsgly, taken as a representative
system, are reported in Figure 1a.
The ternary 2,2′-bipyridine systems show a peculiar polaro-
graphic behavior. At acid pH values a single wave is still
observed (wave It), which shows E1/2 values and overall
electrochemical properties very similar to those of wave Ib. Two
new quasi-reversible and bielectronic waves, wave IIt and wave
IIIt, appear simultaneously above pH 6. The former is nearly
identical to wave IIb in the binary systems, while the latter falls
at more negative E1/2 values. We assign wave It to the reduction
of binary and ternary carboxylate species and waves IIt and IIIt
to the reduction of binary and ternary nitrogen-deprotonated
complexes, respectively. This equilibrium between binary and
ternary species over the entire pH range investigated provides
the best fit to the experimental data. The overall stability
constants for the ternary complexes, still selected on the basis
of Lingane plots, were calculated according to Shaap and
McMasters32 using a log â value of 2.9 for [Pb(bpy)]2+ 33 and
are collected in Table 2. The distribution curves as a function
of pH for the ternary Pb-tsgly-bpy system are shown in Figure
1b.
Figure 3. pD dependence of the chemical shift of the methylene
resonance in the 1H NMR spectrum of bzgly in D2O: (b) free ligand;
(1) saturated solution of [Pb(bzgly-O)2(H2O)2]‚2H2O (2); (9) saturated
solution of 2 plus bpy in a 1:1 molar ratio with the metal. T ) 300 K.
[Pb2+] ) 1 × 10-3 M.
NMR Spectroscopy. The pD dependence of the chemical
1
shift of the H NMR methylene peak(s) of tsgly, bzgly, and
tsâala for the free ligands and for binary and ternary (bpy) Pb2+
systems are reported in Figures 2a, 3, and 4, respectively. In
all cases, no additional resonances were observed in the presence
of the metal in the pH range investigated, a result of the fast
exchange of the amino acid between free and metal-bound states
on the NMR time scale. pKa values of 3.9, 3.8, and 4.7 for the
deprotonation of the carboxylic group in the free ligand were
determined for tsgly, bzgly, and tsâala, respectively. A further
(32) Shaap, W. B.; McMasters, D. L. J. Am. Chem. Soc. 1961, 83, 4699.
(33) Anderegg, G. HelV. Chim. Acta 1963, 46, 2397.

Amide Group Coordination to the Pb2+ Ion
Inorganic Chemistry, Vol. 35, No. 14, 1996 4243
Figure 5. ORTEP view of the Pb atom environment in [Pb(tsgly-
N,O)(H2O)] (1) with the atom-numbering scheme.
precipitation, or by increasing the ligand-to-metal molar ratio,
which reduces the spectral changes due to amide nitrogen
deprotonation of the ligand. The pD titration of the 13C NMR
resonance of the isotopically enriched carboxylic carbon of the
ligand in the same binary Pb-tsgly system (Figure 2b) closely
reproduces the above 1H NMR titration. The increase in
frequency of the first step with a pKa value of 3.9 corresponds
to the deprotonation of the carboxylic group of the ligand, while
the additional frequency increase can, as above, be attributed
to the formation of the nitrogen-deprotonated complexes.
Carboxylic groups are known to undergo shielding effects on
the 13C NMR resonance upon metal binding.34-38
1
The pD profiles of the H NMR methylene resonances of
the amino acid ligands in the ternary systems containing a Pb2+
:
bpy:ligand molar ratio of 1:1:2 (Figures 2a, 3, and 4), are
invariably very similar to that of the binary Pb-tsgly system.
Hence, the titration step around pH 4 corresponds to the
deprotonation of the carboxylic group, whose pKa is almost
unmodified as compared to those of the free ligands, followed
by a frequency increase due to deprotonation and metal binding
of the amide nitrogen of the amino acid ligand. Consistently,
for tsâala such an equilibrium is clearly detected only for the
â-CH2 group which is adjacent to the amide nitrogen (Figure
4b).
Description of the Structures. Drawings of the structures
and selected interatomic distances and angles for 1-3 are shown
in Figures 5-7 and reported in Tables 3-5, respectively.
[Pb(tsgly-N,O)(H2O)] (1). The Pb atom is coordinated at
short distances by N, O1, and Ow atoms (in the range 2.314-
(8)-2.423(9) Å) and at longer distances by the chelating
carboxylate oxygens O1′ and O2′ and the sulfonic O4′′ oxygen
(in the range 2.683(8)-2.827(5) Å). Each tsgly dianion
coordinates three Pb2+ atoms giving rise to a three-dimensional
polymeric arrangement. The shortest Pb‚‚‚Pb separation is
4.748(2) Å. The Pb-N bond within the glycine-like chelate
ring (2.38(1) Å) is, as expected, remarkably shorter than Pb-N
bonds involving amino, pyridyl, and imino nitrogens (2.470-
2.668 Å)39-42 and longer than Pb-N bonds in Pb2+ complexes
with dialkylamides (2.22-2.26 Å).43 The Pb-O1 bond (2.314-
Figure 4. pD dependence of the chemical shift of the central peak of
the two methylene triplets in the 1H NMR spectrum of tsâala in D2O:
(a) R-CH2; (b) â-CH2; (b) free ligand; (1) saturated solution of the
crystalline compound [Pb(tsâala-O)2]‚H2O; (9) saturated solution of
the crystalline compound [Pb(tsâala-O)2]‚H2O plus bpy in a 1:1 molar
ratio with the metal. T ) 300 K. [Pb2+] ) 1 × 10-3 M.
equilibrium, due to the deprotonation of the sulfonamide
nitrogen, is observed with pKa values in the range 10.7-11 (not
shown). The same pH titrations performed on millimolar
aqueous (D2O) solution of the crystalline binary complexes for
all of the above ligands yielded apparent pKa values for the
carboxylic group very similar to the pKa of the free ligands (4.0,
3.3, and 4.8 for tsgly, bzgly, and tsâala, respectively). At pD
values above 5.5 the chemical shift of the methylene peak of
tsgly clearly increases up to pH ∼7. Metal hydroxide precipita-
tion occurs thereafter. For the other ligands, Pb(OH)2 precipi-
tates slightly above pH 6. Comparison with the polarographic
data indicates that the above chemical shift change observed
for tsgly is most probably due to the Pb2+-promoted amide
nitrogen deprotonation of the ligand (the increase in frequency
of the methylene peak, opposite to the frequency decrease caused
by deprotonation of the amide nitrogen in the free ligand,
suggests that the metal itself affects the chemical shift of the
methylene group). For all ligands no apparent pKa value for
the equilibrium Pb2+ + HL- ) PbL + H+ can be obtained,
since metal hydroxide precipitation occurs at a pH value much
lower than the theoretical high-pH limit of the titration (i.e.,
the pH above which all the metal is present as PbL), which
turns out to be about 9, as indicated by the species distribution
curves calculated for the concentration of the reactants employed
in the NMR spectra. This problem cannot be avoided either
by using more diluted solutions, since the end of the titration
moves at higher pH and is still preceded by metal hydroxide
(34) Hammel, J. C.; Smith, J. A. S. J. Chem. Soc. A 1969, 2883.
(35) Howarth, O. W.; Moore, P.; Winterton, N. J. Chem. Soc., Dalton Trans.
1975, 360.
(36) Meiske, L. A.; Angelici, R. J. Inorg. Chem. 1982, 21, 738.
(37) Sherry, A. D.; Yang, P. P.; Morgan, L. O. J. Am. Chem. Soc. 1980,
102, 5755.
(38) Juranic, N.; Celap, M. B.; Vucelic, D.; Malinar, M. J.; Radivojsa, P.
N. Inorg. Chem. 1980, 19, 802.
(39) Zoziol, A. E.; Palenik, R. C.; Palenik, G. J. Inorg. Chim. Acta 1986,
116, L51.
(40) Bailey, N. A.; Fenton, D. E.; Jackson, I. T.; Moody, R.; Rodriguez de
Barbarin, C. J. Chem. Soc., Chem. Commun. 1983, 1463.
(41) Battaglia, L. P.; Belicchi Ferrari, M.; Boggia, R. Inorg. Chim. Acta
1994, 215, 85.
(42) Reger, D. L.; Huff, M. F.; Rheingold, A. L.; Haggerty, B. S. J. Am.
Chem. Soc. 1992, 114, 579.
(43) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A. J.
J. Chem. Soc., Chem. Commun. 1983, 639.

4244 Inorganic Chemistry, Vol. 35, No. 14, 1996
Battistuzzi et al.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Complex 2a
Pb-O1
Pb-O2′
Pb-O2′′
Pb-Ow1
C2-N
2.75(1)
2.46(1)
2.75(1)
2.62(1)
1.45(2)
O1-C1
O2-C1
C3-O3
N-C3
1.22(2)
1.27(2)
1.26(3)
1.32(2)
O1-Pb-O1′
O2′′′-Pb-Ow1′
O2′′′-Pb-Ow1
O2′′-Pb-O2′′′
O2-Pb-Ow1′
O2-Pb-Ow1
O1-Pb-O1′
O1-Pb-O2′′
O1-Pb-O2′
O1-Pb-O2
141.3(4)
140.3(3)
76.8(3)
70.5(3)
73.8(3)
76.8(3)
177.6(3)
105.9(3)
128.7(3)
48.9(3)
O2-Pb-O2′′′
O2-Pb-O2′′
O2-Pb-O2′
O1-Pb-Ow1′
O1-Pb-Ow1
O1-Pb-O2′′′
O1-C1-O2
C2-N-C3
119.3(3)
139.5(3)
80.1(3)
92.2(3)
87.0(4)
76.2(3)
122(1)
123(2)
1
1
1
1
1
a Key: ′ ) /2 - x, /2 - y, z; ′′ ) /2 - x, y, /2 + z; ′′′ ) x, /2 -
1
y, /2 + z.
Figure 6. ORTEP view of the Pb atom environment in [Pb(bzgly-
O)2(H2O)2]‚2H2O (2) with the atom-numbering scheme.
Table 5. Selected Bond Distances (Å) and Angles (deg) for
Complex 3a
Pb-O1
Pb-O2
Pb-O2′
Pb-O5′′
Pb-O5
Pb-N3
Pb-N4
S1-N1
S1-O3
S1-O4
2.627(4)
2.827(5)
2.745(4)
2.649(3)
2.523(5)
2.612(4)
2.609(5)
1.617(5)
1.426(5)
1.428(5)
S1-C3
S2-O7
S2-O8
S2-N2
S2-C2
O1-C1
O2-C1
O5-C10
O6-C10
1.769(6)
1.438(5)
1.428(5)
1.599(6)
1.775(5)
1.260(8)
1.249(8)
1.274(7)
1.236(7)
N3-Pb-N4
O5-Pb-N4
O5-Pb-N3
O5′′-Pb-N4
O5′′-Pb-N3
O5′′-Pb-O5
O2′-Pb-N4
O2′-Pb-N3
O2′-Pb-O5
O2′-Pb-O5′′
O2-Pb-N4
O2-Pb-N3
O5-C10-O6
62.6(1)
80.4(1)
79.8(1)
101.0(1)
146.4(2)
68.1(1)
O2-Pb-O5
O2-Pb-O5′′
O2-Pb-O2′
O1-Pb-N4
O1-Pb-N3
O1-Pb-O5
O1-Pb-O5′′
O1-Pb-O2′
O1-Pb-O2
S1-N1-C2
S2-N2-C11
O1-C1-O2
121.2(1)
97.3(1)
89.1(1)
143.4(1)
85.4(1)
76.6(2)
96.3(1)
134.1(1)
47.6(1)
117.3(4)
123.5(5)
123.5(6)
82.3(1)
129.3(1)
131.3(1)
71.2(1)
156.0(2)
108.0(1)
125.3(5)
Figure 7. ORTEP view of the Pb atom environment in [Pb(tsgly-O)2-
(bpy)] (3) with the atom-numbering scheme.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complex 1a
Pb-O1
Pb-N
Pb-O2′
S-O3
S-N
O1-C1
N-C2
2.314(8)
2.38(1)
2.78(1)
1.450(9)
1.56(1)
1.30(1)
1.46(1)
Pb-O4′′
Pb-Ow
Pb-O1′
S-O4
2.683(8)
2.423(9)
2.827(9)
1.436(9)
1.77(1)
a Key: ′ ) -x + 1, -y, -z; ′′ ) -x, -y, -z.
distorted bicapped square-pyramidal with O4′′, N, Ow, and O1′
atoms acting as equatorial ligands (with deviations from the
mean plane of (0.3 Å) and O1 and O2′ atoms as apical ligands.
The vacancy of a coordination position in trans to O1 and O2′
suggests that the metal lone pair is stereochemically active. The
five atoms of the glycine-like ring are almost planar, with
deviations from the mean plane from -0.077 to 0.156 Å. Bond
distances and angles within the ligand fall in the typical range
for metal complexes of tsgly dianions.47 Crystal packing is
mainly due to contacts involving the oxygen atoms (in the range
2.66(1)-2.98(1) Å).
[Pb(bzgly-O)2(H2O)2]‚2H2O (2). The Pb atom lies on the
binary axis, and only one bzgly anion is crystallographically
independent. The ligand coordinates one Pb atom through the
chelate carboxylate group and a second Pb atom through the
carboxylate O2 atom, which thus acts as a monoatomic bridge.
A further bond is formed by the Ow1 atom of one water
molecule. Each Pb-O bond is duplicated by symmetry
transformation, thus originating a polymeric chain of Pb2O2
square-planar rings with Pb‚‚‚Pb distances of 4.127(1) Å. The
S-C3
O2-C1
1.28(1)
N-Pb-Ow
O4′′-PbN
82.7(3)
89.2(3)
138.3(3)
69.8(3)
119.2(3)
79.9(3)
76.3(3)
112.5(3)
119.2(7)
119.6(5)
120.4(8)
118(1)
O4′′-Pb-Ow
O2′-Pb-Ow
O2′-Pb-O4′′
O1′-Pb-N
O1′-Pb-O2′
O1-Pb-N
O1-Pb-O2′
Pb-O1′-Pb′
N-C2-C1
156.1(3)
81.9(3)
89.7(3)
151.4(3)
46.7(3)
68.3(3)
71.0(3)
134.7(4)
107(1)
O2′-Pb-N
O1′-Pb-Ow
O1′-Pb-O4′′
O1-Pb-Ow
O1-Pb-O4′′
O1-Pb-O1′
Pb-O1-C1
Pb-N-S
S-N-C2
O1-C1-O2
O1-C1-C2
120.0(9)
120(1)
123(1)
Pb-N-C2
O2-C1-C2
1
1
1
1
a Key: ′ ) -x + /2, y - /2, -z + 1; ′′ ) x + /2, -y + /2, z.
(8) Å) is much shorter than Pb-Ocarboxylate bonds (2.40-3.11
Å).44-46 The coordination geometry can be described as
(44) Harrison, P. G.; Steel, A. T. J. Organomet. Chem. 1982, 239, 105.
(45) Chandler, C. D.; Hampden-Smith, M. J.; Duesler, E. N. Inorg. Chem.
1992, 31, 4891.
(46) Bryant, R. G.; Chacko, V. P.; Etter, M. C. Inorg. Chem. 1984, 23,
3580.
(47) Bonamartini Corradi, A. Coord. Chem. ReV. 1992, 117, 45 and
references therein.

Amide Group Coordination to the Pb2+ Ion
Inorganic Chemistry, Vol. 35, No. 14, 1996 4245
Discussion
Binary Systems. The pH dependence of the H and 13C
chemical shifts of the ligand in the Pb-tsgly system unequivo-
cally indicate that above pH 6 a change in the coordination mode
of the ligand occurs. This is confirmed by the X-ray structure
of the complex separated in the solid state from solutions at
these pH values, which shows that the ligand behaves as an
N,O-bidentate dianion. As a consequence, the polarographic
waves Ib and IIb can be confidently attributed to the reduction
of the complexes in which tsgly acts as simple carboxylate and
as an N,O-bidentate ligand, respectively. The same pH-
dependent coordination behavior can be proposed for the other
two ligands, which show identical electrochemistries and very
similar pH dependences of the NMR spectra. Since the E1/2
values of the polarographic wave IIb are independent of pH in
all binary systems, the chemical shift change above pH 6 cannot
be due to the formation of a mixed hydroxy-carboxylate
complex. Overall, these observations indicate that the Pb2+ ion
is effective in promoting amide nitrogen deprotonation in these
ligands.
1
Figure 8. Arrangement of the polymeric square-planar Pb2O2 rings in
3.
The same prevailing species are observed for the three ligands
(Table 2). Due to the electron-withdrawing effect of the sulfonyl
and carbonyl groups, the log â values of the [Pb(HL)]+ and
[Pb(HL)2] complexes are slightly lower than those for Pb2+
complexes with monocarboxylate ligands (in the ranges 1.1-
2.68 and 2.0-3.5, respectively).48,49 The stability constants for
bzgly are in good agreement with values previously determined
electrochemically for the same ligand50 and for acetylglycine.48,51
As expected, the log â values for the nitrogen-deprotonated PbL
species are greater than those for Pb2+ complexes with amino
acids with noncoordinating side chains (from 4.0 to 5.5),14 due
to the greater donor strength of the deprotonated amide nitrogen
as compared to the amino nitrogen. Comparison with other
bond distances involving the metal are in the range 2.46(1)-
2.75(1) Å. The benzoylglycinate anion acts as an asymmetric
bidentate chelate and a bridging ligand through the carboxylate
group. This group behaves analogously to that observed in 1.
The metal coordination polyhedron in this compound cannot
be unambiguously defined, as well as the direction of lone-pair
stereochemical activity. Crystal packing is mainly due to O‚‚‚O
contacts involving a lattice water molecule. Bond distances and
angles within the ligand fall in the range of literature data.47
[Pb(tsgly-O)2(bpy)] (3). The Pb atom is coordinated by the
N3 and N4 atoms of the bpy molecule, two symmetry-related
carboxylate O5 oxygens of two bridging tsgly ligands, the O1
and O2 atoms of an anisobidentate chelating carboxylate group,
and the carboxylate oxygen O2′. Overall, the metal is seven-
coordinated with bond distances in the range 2.523(5)-
2.827(5) Å. Each tsgly monoanion bridges two Pb atoms
forming a polymeric chain running along the x axis. The chain
is formed by exactly planar Pb2O2 rings, nearly orthogonal to
one another (Figure 8). The Pb‚‚‚Pb′ distance is 4.285(3) Å,
and Pb‚‚‚Pb′′ is 3.970(2) Å. The coordination polyhedron can
be described as a distorted octahedron where N3, N4, O5′′ and
the midpoint O1, O2 are coplanar and the normal to this plane
forms angles of 14.9 and 34.6° with the Pb-O5 and Pb-O2′
bonds, respectively. The value of the latter angle suggests that
the lone pair is stereochemically active in the direction opposite
to that of the Pb-O5 bond. Bond distances and angles within
the ligand and the bpy molecule fall in the range of literature
data.47 Crystal packing is mainly due to interchain ring stacking
interactions (in the range 3.39-3.88 Å) involving bpy mol-
ecules.
binary metal-N-substituted amino acidate systems reveals the
MHL
ML
following: (i) The magnitude of the step constants K
relative to the formation of the nitrogen-deprotonated species
from the precursor carboxylate complex, which follows the order
Pd2+ . Pb2+ > Cu2+ g Cd2+ 18,19
indicates that metal
,
effectiveness in substituting for the sulfonamide nitrogen-bound
hydrogen upon closure of the five-membered ring closely
follows the order of “softness” of the acidic character, namely
Pd2+ . Pb2+ > Cu2+ > Cd2+ 8
. This correlation can hardly be
considered accidental and may indicate that metal affinity for
nitrogen as the donor atom plays a role in determining the
binding selectivity of these ligands and, by analogy, that of
peptides toward dipositive metals.15 Of course, the overall metal
ability to promote amide nitrogen deprotonation in these ligands
also depends on the stability of the precursor carboxylate
complex (and of the complex formed by the primary ligating
amino group in peptides), as well as on the solubility of the
metal hydroxide. However, the relevance of the above factor
emerges clearly if we consider that Co2+, Ni2+, and Zn2+, which
are “harder” acids than the above metal ions, are indeed
ineffective in promoting nitrogen deprotonation in the present
sulfonamide ligands and less effective than Pd2+ and Cu2+ in
peptides. (ii) The Pb2+ ion is the only metal found so far to
substitute for the proton of the NHCO group in binary
complexes with N-carbonyl amino acids. The greater stability
of the corresponding [PbL] species as compared to the N-
sulfonyl amino acids (Table 2) must be ascribed to the greater
Infrared Spectroscopy. For all compounds, the asymmetric
and symmetric stretching modes of the carboxylate group fall
in the ranges 1574-1546 and 1408-1384 cm-1, respectively,
with ∆ν values (∆ν ) ν(COO)as - ν(COO)s) between 170 and
146 cm-1. This is in line with the small difference in C-O
bond lengths within the carboxylate group among 1-3 and
suggests that the carboxylate group binds in the same way also
in the complexes not structurally characterized. In 1, the
deprotonation of the sulfonamide nitrogen causes the expected
high energy shift of the stretching vibrations of the SO2 and
S-N bonds as compared to the case of the free ligand.16
(48) Bunting, J. W.; Thong, K. M. Can. J. Chem. 1970, 48, 1654.
(49) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum
Press: New York and London, 1977; Vol. 3.
(50) Gupta, J. K.; Gupta, C. M. J. Indian Chem. Soc. 1969, 46, 491.
(51) Rabenstein, D. L. Can. J. Chem. 1972, 50, 1036.

4246 Inorganic Chemistry, Vol. 35, No. 14, 1996
Battistuzzi et al.
basicity of the peptide nitrogen as compared to a sulfonamide
nitrogen. In the light of the observations in the previous point,
the existence of a stable [PbL] complex for bzgly is somewhat
surprising, given that the Pd2+ ion was found unable to form
the same complex.18,19 One may speculate that the Pb2+ ion,
possessing a greater affinity for oxygen donors than the Pd2+
ion, forms more stable carboxylate precursors and, hence, sets
up a stronger interaction with the primary binding group (or
“anchoring group”), which is essential for successive peptide
nitrogen deprotonation. (iii) Comparison between the stability
constants for the [PbL] complexes of tsgly and tsâala indicates
that stability is not affected by the chelate ring size. Most
probably the remarkable donor strength of the deprotonated
amide nitrogen strongly reduces the effects of the greater strain52
of a six-membered ring as compared to a five-membered ring
in Pb2+ chelate complexes. Analogous behavior was observed
previously for the same complexes of the Cd2+ ion.53 The
greater basicity of the carboxylate group in tsâala as compared
to the N-substituted R-amino acids justifies the increase in
stability of the corresponding carboxylate complexes.
carboxylate O-donor to N,O-bidentate dianion appears unfavored
in the presence of a metal-coordinated bpy. The molecular bases
for this effect are not obvious. It can be proposed that the likely
distorted coordination geometry around the Pb2+ ion in these
nitrogen-deprotonated ternary species (see below) would prevent
the above π-conjugation. It may also be relevant to note
that the Pd2+ ion, which shows negative ∆ log K values for
homologous systems,55 shares with the Pb2+ ion a “softer” acidic
nature than Cu2+, Cd2+, Zn2+, Co2+, and Ni2+, for which the
favorable effect of bpy on nitrogen deprotonation of this class
of ligands has been observed.22,56 It is apparent that, with Pb2+
and Pd2+, which are remarkably active in nitrogen deprotonation
in the binary systems, this effect is overcome possibly by
structural factors related to coordination geometry and mecha-
nistic factors,55 respectively.
X-ray Structures. The large ionic radius, the remarkable
ionic character of metal-ligand bonding, and the presence of a
nonbonding pair of electrons most often possessing stereochem-
ical activity cause the coordination geometry of the Pb2+ ion to
be dominated by high coordination numbers and distorted
geometries, the latter strongly affected by the nature and steric
requirements of the ligands.4,9 In all of the present compounds,
the carboxylate group chelates to one Pb atom, with one oxygen
bridging between two metals. This simultaneous chelation and
bridging, leading to polymerization, is typical of lead(II)
carboxylates, as well as the formation of chains of Pb2O2 square
planar rings.44-46 In particular, lead(II) formate is a three-
dimensional polymer and the metal shows a distorted dodeca-
hedral eight-coordination.44 In the monodimensional polymers
lead(II) bis(pentafluorobenzoate)-bis(methanol),44 lead(II) ac-
etate trihydrate,44,46 and the pyridine adduct of lead(II) benzoate4
the metal shows a square-antiprismatic geometry, being eight-
coordinate in the first two cases and showing a seven-
coordination plus one unoccupied site in the last complex. The
lower metal coordination number in 1 might be related to the
remarkable binding strength of the deprotonated nitrogen. In
all cases, the distorted geometries indicate a stereochemically
active nonbonding metal electron pair.
Ternary Systems. Both ligand coordination modes, namely
as carboxylate O-donors and chelating N,O-dianions, are
maintained in the presence of 2,2′-bipyridine as an additional
ligand (Table 2). The binding of the dianionic ligands to the
[Pb(bpy)]2+ species is to some extent disfavored as compared
to the aquo ion, as indicated by negative values of ∆ log K
M(bpy)
M(bpy)L
()log K
- log KMML) for all ligands (∆ log K ) -0.22,
-0.19, and -0.09 for tsgly, tsâala, and bzgly, respectively).
Thus, the heteroaromatic base appears not to favor amide
nitrogen deprotonation in this class of ligands, at variance with
previous observations for the same ternary systems with Cu2+
,
Cd2+, and Zn2+ ions, for which pKNH values 0.4-3 units lower
than those for the binary systems and positive ∆ log K values
were determined.20-22,54 This effect was attributed to a
preferential binding of the carboxylate group, acting as primary
ligating group, to the [M(bpy)]2+ species as compared to the
solvated metal ion and possibly to a favorable influence of
π-conjugation between the aromatic system of bpy and that of
the aromatic moiety of the N-protecting group of the amino
acid ligand in the nitrogen-deprotonated species. Due to the
two-step mechanism of the formation of the nitrogen-deproto-
nated species, with the involvement of the carboxylate species
as precursors, ∆ log K can be factored as follows:
Compound 1 is the first example of a structurally character-
ized Pb2+ N,O-chelate complex. The Pb-N bond (2.38(1) Å)
is 0.4 Å longer than the average Cu-N bond in analogous
complexes of the five-coordinate Cu2+ ion.47 Since the ionic
radius of a six-coordinate Pb2+ ion is 0.53 Å greater than that
of a five-coordinate Cu2+ ion,57 it turns out that the deprotonated
amide nitrogen has a stronger affinity for dipositive lead than
for the copper ion, consistent with the greater step constant for
the formation of the nitrogen-deprotonated species from the
precursor carboxylate complex. The N-Pb-O1 angle in the
five-membered N,O-chelate ring of 1 (68.3(3)°) is, as expected,
much smaller than the corresponding angle in analogous Cu2+
and Pd2+ complexes (average value of 82.9°). This determines
a remarkable opening of the two adjacent angles, M-O1-C1
and M-N-C2, as compared to the cases of the analogous
copper and palladium complexes. The metal binding of a
sulfonic oxygen in 1, which creates a three-dimensional
polymeric structure, is a novel feature in the coordination
chemistry of dipositive lead. Compound 3 is one of the few
ternary carboxylate Pb2+ complexes with a heteroaromatic base
reported to date, in addition to Pb(phen)(O2CH3)2‚2H2O4 and
Pb(O2CPh)2‚py.4 The phenanthroline complex shows the short-
M(bpy)
M(bpy)(HL)2
∆ log K ) (log K
- log KMM(HL) ) +
2
M(bpy)(HL)2
M(bpy)L
(log K
- log KMM(LHL)2
)
The above two effects determine an increase in the first and
the second term of this equation, respectively. Average values
of 0.1 and -0.26 for the first and second terms are obtained
for the present Pb2+ systems, against corresponding values of
0.1 and 0.4 determined for the same ternary (bpy) complexes
of dipositive copper.19,54 Hence, it is apparent that the car-
boxylate ligands bind more strongly to both metals in the ternary
systems, but while for the Cu2+ ion the stabilizing effect of a
π-delocalization on the ternary nitrogen-deprotonated species
is also operative, this is not the case for the Pb2+ ion, for which
the change in the coordination mode of the ligand from
(52) Martell, A. E.; Hancock, R. D.; Motekaitis, R. J. Coord. Chem. ReV.
1994, 133, 39.
(53) Battistuzzi Gavioli, G.; Menabue, L.; Saladini, M.; Sola, M.; Bonama-
rtini Corradi, A.; Battaglia, L. P. J. Chem. Soc., Dalton Trans. 1989,
1345.
(55) Battistuzzi, G.; Gozzoli, E.; Borsari, M.; Menabue, L.; Saladini, M.;
Sola, M. J. Chem. Soc., Dalton Trans. 1994, 285.
(56) Unpublished results from our laboratory.
(57) Huheey, J. E. Inorganic Chemistry: Principles of Structure and
ReactiVity; Harper: Cambridge, U.K., 1983; p 73.
(54) Borsari, M. Trends Inorg. Chem. 1993, 3, 355.

Amide Group Coordination to the Pb2+ Ion
Inorganic Chemistry, Vol. 35, No. 14, 1996 4247
est Pb-Pb distances detected in lead(II) carboxylates (4.05 and
3.73 Å). One of the two Pb‚‚‚Pb contacts in 3 (4.285 and 3.97
Å) is very short as well. This common feature may well be
accidental, but the heteroaromatic base may also play a role,
possibly through ring stacking interactions, which in 3 occur
between the aromatic rings of adjacent chains.
logica of Italy is acknowledged for financial support (MURST
40%).
Supporting Information Available: Listings of IR data and
elemental analyses, tables of full crystallographic data collection and
reduction and structure refinement parameters, fractional atomic
coordinates, anisotropic thermal parameters for non-hydrogen atoms,
complete bond lengths and angles, and mean least-squares planes for
1-3, and tables of hydrogen atom parameters for 2 and 3 (19 pages).
Ordering information is given on any current masthead page.
Acknowledgment. NMR and FT-IR measurements and
X-ray data collection were carried out at the Centro Interdipar-
timentale Grandi Strumenti of the University of Modena. The
Ministero dell’Universita` e della Ricerca Scientifica e Tecno-
IC950599H