Amide group coordination to the Hg2+ ion. Potentiometric, 1H NMR and structural study on Hg2+-N-protected amino acid systems

Article abstract of DOI:10.1039/b100310k

The binary complexes of Hg²⁺ formed by N-carbonyl and N-sulfonyl amino acids, which are ligands containing peptide and sulfonamide groups respectively, are investigated in aqueous solution by ¹H NMR, UV spectroscopy and potentiometry. The corresponding ternary systems with 2,2′-bipyridine are studied in aqueous solution by potentiometry and in DMSO solutions by ¹H-NMR. All the amino acids behave as simple carboxylate ligands at acid pH, while, around neutrality, N-p-tolylsulfonylglycine (tsglyH₂), N-p-tolylsulfonyl-β-alanine (ts-β-alaH₂) and N-2-nitrophenylsulfonylglycine (NO₂psglyH₂) switch to dianionic N,O-bidentate chelating ligands due to the involvement of the deprotonated amide nitrogen as an additional donor site. The Hg²⁺ ion is ineffective in promoting peptide nitrogen deprotonation in N-benzoylglycine (bzglyH). The binary and ternary species formed in aqueous solution and their stability constants are determined and compared with those of the homologous complexes of Pd²⁺, Cu²⁺, Cd²⁺ and Pb²⁺. The molecular structure of [Hg(bpy)₂(NO₂-psgly-N,O)]·0.5H₂O is determined by X-ray crystallography. It represents a rare example of Hg²⁺ N,O coordination by an amino acid molecule. In the complex Hg²⁺ shows a distorted octahedral environment with a N₅O donor set. Four nitrogen atoms are derived from the two bpy ligands, while the oxygen and the fifth nitrogen are from the NO₂-psgly dianion. New information on the solution and solid state chemistry of Hg²⁺ with ligands of biological interest is provided which may be of great relevance in understanding the mechanism of metal toxicity.

Full text of DOI:10.1039/b100310k

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1
Amide group coordination to the Hg2؉ ion. Potentiometric, H
NMR and structural study on Hg2؉–N-protected amino acid
systems†
Monica Saladini,* Ledi Menabue, Erika Ferrari and Daniela Iacopino
Department of Chemistry, University of Modena and Reggio Emilia, Via Campi 183,
41100 Modena, Italy. E-mail: saladini@unimo.it
Received 2nd January 2001, Accepted 7th March 2001
First published as an Advance Article on the web 5th April 2001
The binary complexes of Hg2ϩ formed by N-carbonyl and N-sulfonyl amino acids, which are ligands containing
peptide and sulfonamide groups respectively, are investigated in aqueous solution by 1H NMR, UV spectroscopy
and potentiometry. The corresponding ternary systems with 2,2Ј-bipyridine are studied in aqueous solution by
potentiometry and in DMSO solutions by 1H-NMR. All the amino acids behave as simple carboxylate ligands at
acid pH, while, around neutrality, N-p-tolylsulfonylglycine (tsglyH2), N-p-tolylsulfonyl-β-alanine (ts-β-alaH2) and
N-2-nitrophenylsulfonylglycine (NO2psglyH2) switch to dianionic N,O-bidentate chelating ligands due to the
involvement of the deprotonated amide nitrogen as an additional donor site. The Hg2ϩ ion is ineffective in promoting
peptide nitrogen deprotonation in N-benzoylglycine (bzglyH). The binary and ternary species formed in aqueous
solution and their stability constants are determined and compared with those of the homologous complexes of
Pd2ϩ, Cu2ϩ, Cd2ϩ and Pb2ϩ. The molecular structure of [Hg(bpy)2(NO2-psgly-N,O)]ؒ0.5H2O is determined by X-ray
crystallography. It represents a rare example of Hg2ϩ N,O coordination by an amino acid molecule. In the complex
Hg2ϩ shows a distorted octahedral environment with a N5O donor set. Four nitrogen atoms are derived from the two
bpy ligands, while the oxygen and the fifth nitrogen are from the NO2-psgly dianion. New information on the solution
and solid state chemistry of Hg2ϩ with ligands of biological interest is provided which may be of great relevance in
understanding the mechanism of metal toxicity.
N-Carbonylamino acids such as N-benzoylglycine (bzglyH)
Introduction
were found to undergo nitrogen deprotonation only in the pres-
Mercury (Hg2ϩ) and methylmercury (CH3Hgϩ) cations are
ence of Pb2ϩ ion,15 while Pd2ϩ was unable to form N,O-chelate
known to react extensively with living organisms1 and the chain
complexes.16
targets for this type of complexation are thiol ligands2,3 such as
In ternary 2,2Ј-bipyridine systems the additional hetero-
the cysteine residues in proteins. Cysteine coordination to Hg2ϩ
,
atomic ligand favours Cu2ϩ and Cd2ϩ substitution for the sulfon-
amide nitrogen-bound hydrogen and induces binding of these
ligands as N,O-dianions also toward Zn2ϩ, Co2ϩ and Ni2ϩ
ions.17–19
in a mercury metalloregulatory protein (mercuric reductase),
has been investigated.4–7 Although Hg2ϩ coordination chem-
istry is a key factor in determining the biological effects of this
metal, relatively few studies, concerning Hg2ϩ interactions with
simple biomolecules, are available, while a great number of
studies on the CH3Hgϩ cation interacting with amino acids and
peptides are reported either in solid8 or in solution.9
In this work we investigate the binding ability of N-carbonyl-
and N-sulfonyl-amino acids toward Hg2ϩ in binary and 2,2Ј-
bipyridine (bpy) containing ternary systems. The presence of a
substituent in ortho position, such as a nitro group, on the
aromatic moiety of the ArSO2N amino acids was found to
enhance the coordination properties of such molecules both in
the solid and in the solution state.20–21 By means of potentio-
Potentiometric data on Hg2ϩ–dipeptide systems10 show the
formation of 1 : 1 complexes of [Hg(HL)]ϩ type and peptide
nitrogen deprotonation is not observed. Crystal structures of
Hg2ϩ complexes with ligands containing donor groups of bio-
logical relevance have never been throughly investigated and the
few reported data often concern HgCl2 adducts.11
N-Sulfonylamino acids are low molecular weight ligands that
were found to reproduce the coordination behaviour of
peptides and their selectivity towards metal ions. At pH values
close to neutrality these amino acids bind dipositive metal ions
as dianions, through one carboxylate oxygen (acting as primary
binding site) and the deprotonated sulfonamide nitrogen with
the formation of a five membered chelate ring. On increasing
pH, the deprotonation of the sulfonamide nitrogen in
N-p-tolylsulfonylglycine (tsglyH2) takes place with a markedly
lower pKNH than that of the free ligand and is promoted
to different extents according to the coordinated metal
(Pd2ϩ > Pb2ϩ > Cu2ϩ > Cd2ϩ).12–15
1
metric and H NMR data we find that Hg2ϩ ion is effective in
promoting sulfonamide nitrogen deprotonation in ArSO2-
N-protected amino acids in binary and in 2,2Ј-bipyridine con-
taining ternary systems. In addition the crystal and molecular
structure of the [Hg(bpy)2(NO2psgly-N,O)]ؒ0.5H2O complex
is reported (NO2psgly = N-2-nitrophenylsulfonylglycinate di-
anion). It represents a rare example of Hg2ϩ N,O-coordinated
by a amino acid molecule.
Experimental
Materials
N-p-Tolylsulfonylglycine (tsglyH2), N-p-tolylsulfonyl-β-alanine
(ts-β-alaH2) and N-(2-nitrophenylsulfonyl)glycine (NO2psgly-
H2) were synthesized as in ref. 20, while N-benzoylglycine
(bzglyH) was from Carlo Erba. Stock solutions of HgCl2 were
standardized by means of a SPECTROFLAME D ICP plasma
spectrometer; samples contained 1% of HNO3 (BDH-Aristar).
† Electronic supplementary information (ESI) available: IR bands,
least-squares planes. See http://www.rsc.org/suppdata/dt/b1/b100310k/
DOI: 10.1039/b100310k
J. Chem. Soc., Dalton Trans., 2001, 1513–1519
This journal is © The Royal Society of Chemistry 2001
1513

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Preparation of the complexes
range of 10Ϫ4 to 10Ϫ1 M acid. All the spectra were performed at
30 0.1 ЊC and referenced to tetramethylsilane.
Hg(L-N,O). (In what follows O and N,O refer to ligands
binding as carboxylate monoanions and as N,O-chelating
dianions respectively; L = tsgly, ts-β-ala or NO2psgly). A 10 ml
portion of an Hg(CH3CO2)2 aqueous solution (0.05 M) was
added under continuous stirring to 20 ml of an aqueous
ethanolic (2 : 1 v/v) solution of the appropriate amino acid
solution (0.05 M) at pH 8. The pH was kept constant by adding
small amounts of concentrated aqueous NaOH. On standing
for a few days white powders separated. They were collected by
vacuum filtration and washed with ethanol. Hg(tsgly-N,O):
Calc. for C9H9HgNO4S C, 25.16; H, 2.11; N, 3.26; S, 7.46%;
Found C, 25.12; H, 2.27; N, 3.23; S, 7.30%; yield = 50%.
Hg(ts-β-ala-N,O): Calc. for C10H11HgNO4S C, 27.16; H, 2.51;
N, 3.17; S, 7.26%; Found C, 27.32; H, 2.58; N, 3.21; S, 7.10%;
yield 60%. Hg(NO2psgly-N,O): Calc. for C8H6HgN2O6S C,
20.93; H, 1.32; N, 6.11; S, 6.99%; Found C, 21.03; H, 1.50; N,
5.98; S, 7.10%; yield = 40%.
Spectrophotometric titration were performed using a Perkin-
Elmer Lambda 19 spectrophotometer at 25 0.1 ЊC in the
200–500 nm spectral range employing a 1 cm cell length. The
solutions contained Hg2ϩ/L in 1 : 1 molar ratio, with
[Hg2ϩ] = 1 × 10Ϫ4 M. The pH of the solutions was adjusted by
adding small amounts of concentrated aqueous NaOH
solutions.
Infrared spectra of solid compounds were recorded with a
Perkin-Elmer FT-IR 1600 instrument as KBr pellets in the
spectral range 4000–400 cmϪ1; a table reporting the more rele-
vant IR bands, with their tentative assignment, is available as
ESI supplementary material.
Potentiometry
Potentiometric measurements were performed in aqueous
solutions at 25 0.1 ЊC using a fully automated ORION 960
Autochemistry system and following the general procedures
previously reported.23 All experiments were carried out in a
nitrogen atmosphere at ionic strength 0.1 M (adjusted with
solid NaNO3); the equivalence point was determined by the first
derivative technique with constant volume increments. The
stability constants (βpqrs), which are defined by eqns. (1) and (2),
Hg(bzgly-O)2. The compound was prepared as described
above. Separation of solid compounds was achieved in
the pH range 4.5–6. Calc. for C18H16HgN2O6: C, 38.80; H,
2.90; N, 5.03%. Found: C, 38.68; H, 3.02; N, 5.00%. Yield =
30%.
pM ϩ qA ϩ rL ϩ sH
MpAqLrHs
(1)
(2)
Hg(bpy)(L-N,O) (L ؍ tsgly, ts-ꢀ-ala or NO2psgly). A 10 ml
portion of an aqueous solution of Hg(CH3CO2)2 (0.05 M) was
added under continuous stirring to an aqueous ethanolic (2 : 1
v/v) solution of the appropriate amino acid and bpy, both 0.05
M. After a few days of evaporation, white powders separated
from the solutions. They were collected under vacuum filtration
and washed with ethanol. [Hg(bpy)(tsgly-N,O)]: Calc. for
C19H17HgN3O4S C, 39.10; H, 2.93; N, 7.20; S, 5.49%; Found C,
40.58; H, 3.29; N, 6.69; S, 6.32%; yield = 40%. [Hg(bpy)(ts-β-
ala-N,O)]: Calc. for C20H19HgN3O4S C, 40.19; H, 3.20; N, 7.03;
S, 5.36%; Found: C, 38.65; H, 3.36; N, 6.53; S, 5.10%; yield =
50%. [Hg(bpy)(NO2psgly-N,O)]: Calc. for C18H14HgN4O6S: C,
35.17; H, 2.29; N, 9.11; S, 5.22%. Found: C, 34.92; H, 2.10; N,
8.90; S, 4.75%; yield = 40%.
βpqrs = [MpAqLrHs]/[M]p[A]q[L]r[H]s
where M is the metal, A 2,2Ј-bipyridine, L the amino acid in the
deprotonated form and H is proton, were refined by least-
squares calculation using computer program SUPERQUAD,24
taking into account the presence of [Hg(OH)]ϩ and [Hg(OH)2]
species.25
The concentration of the starting solution of the amino acid
together with the protonation constants was determined by at
least four titrations of 5 × 10Ϫ3 M solutions.
In the binary system the starting solutions for each titration
were prepared by addition of known volumes of HgCl2 (0.1 M)
and amino acid (0.01 M) solutions in 1 : 1, 1 : 2 and 1 : 4 metal-
to-ligand molar ratios. Hg2ϩ concentration ranged from
5 × 10Ϫ3 to 5 × 10Ϫ4 M, for all systems. Aqueous NaOH (0.05
M) was used as titrant. In the ternary systems the starting solu-
tions were prepared as in the binary with the addition of known
volumes of aqueous bpy solution (0.01 M) to have 1 : 1 : 1,
1 : 1 : 2, 1 : 1 : 4 M : bpy : L molar ratios. Hg2ϩ was 1.25 × 10Ϫ4
M for all systems and the titrant was NaOH (5 × 10Ϫ3 M). Ten
measurements at least were performed for each system with 40
data points in each titration in the pH range 3–10.
[Hg(bpy)2(NO2psgly-N,O)]ؒ0.5H2O. By slow evaporation of
mother liquor, after separation of the [Hg(bpy)(NO2psgly-
N,O)] complex, white crystals separated, useful for X-ray
analysis. Calc. for C28H23HgN6O6.5S: C, 43.08; H, 2.97; N,
10.78; S, 4.11%. Found: C, 44.12; H, 2.80; N, 10.20; S, 3.98%.
Yield = 10%.
[Hg(bpy)(bzgly-O)2]. This was prepared as were the [Hg(bpy)-
(L-N,O)] complexes. Calc. for C28H24HgN4O6: C, 47.17; H,
3.39; N, 7.86%. Found: C, 46.95; H, 3.50; N, 7.48%.
X-Ray crystallography
Spectroscopy
A single crystal of [Hg(bpy)2(NO2psgly-N,O)]ؒ0.5H2O was
mounted on a glass fiber and data were collected on an Enraf
Nonius CAD4 diffractometer. Crystallographic data are sum-
marized in Table 1. All data were corrected for Lorentz-
polarization effects, while an absorption correction was not
applied due to the absence of appropriate reflections. The struc-
ture was solved by conventional Patterson and Fourier tech-
niques. The structure was refined by full-matrix least-squares
calculations with anisotropic thermal parameters for all non-
hydrogen atoms. The phenyl ring of the amino acid molecule
was refined as a rigid group. Hydrogen atoms were calculated
and used as fixed contributors. All calculations were carried out
on a personal computer with SHELX 76,26 SHELXL 9327 and
ORTEP28 programs.
1H NMR spectra were obtained on a Bruker Avance DPX-200
spectrometer at 200.13 MHz with a Broad Band 5 mm probe
(inverse detection). The typical acquisition parameters are as
follows: spectral bandwidth 2 KHz, pulse width 7.6 µs (90Њ
pulse), pulse delay 1 s, number of scans 216–512. The ligands
and binary complexes spectra were run on aqueous (D2O)
millimolar solutions. Small amounts of concentrated NaOH or
HNO3 were added in order to adjust the pD values. pD Values
are reported throughout the paper. The ternary complexes,
because of their low solubility in aqueous media, and binary
complexes were run in deuteriated DMSO millimolar solutions.
A combined electrode was standardized by titration of a known
volume of pure dimethyl sulfoxide with a standard solution of
salicylic acid in DMSO [(pKHA
)
DMSO = 6.8; (pKHA)w = 3.0].22
CCDC reference number 156235.
An excellent adherence to the Nernst equation of E(mV) vs.
paH was observed, with maximum deviations of 1 mV over a
See http://www.rsc.org/suppdata/dt/b1/b100310k/ for crystal-
lographic data in CIF or other electronic format.
1514
J. Chem. Soc., Dalton Trans., 2001, 1513–1519

View Article Online
Table 1 Crystallographic data for the complex [Hg(bpy)2(NO2psgly-
N,O)]ؒ0.5H2O
Table 2 Selected bond distances (Å) and angles (deg) for [Hg(bpy)2-
(NO2psgly-N,O)]ؒ0.5H2O
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
C28H23HgN6O6.5
780.17
Monoclinic
C2/c
20.459(3)
18.653(3)
17.972(3)
109.97(2)
6446(2)
8
S
Hg–N(5)
Hg–N(3)
Hg–O(1)
2.14(2)
Hg–N(4)
Hg–N(1)
Hg–N(2)
N(1)–C(1)
N(2)–C(10)
N(3)–C(11)
N(4)–C(16)
O(2)–C(21)
C(22)–N(5)
S–O(3)
2.33(2)
2.42(2)
2.68(2)
1.35(2)
1.33(2)
1.34(2)
1.33(2)
1.23(2)
1.48(3)
1.42(2)
1.79(1)
1.18(3)
2.42(2)
2.60(2)
1.33(2)
1.36(2)
1.31(2)
1.31(2)
1.18(3)
1.63(4)
1.56(2)
1.45(2)
1.49(3)
1.22(3)
N(1)–C(5)
N(2)–C(6)
N(3)–C(15)
N(4)–C(20)
O(1)–C(21)
C(21)–C(22)
N(5)–S
S–O(4)
C(28)–N(6)
N(6)–O(6)
β/deg
V/Å3
Z
T/K
293
0.71069
48.89
S–C(23)
N(6)–O(5)
λ(Mo-Kα)/Å
µ/cmϪ1
Reflections collected
Reflections used in the refinement
(I > 2σI)
Unique reflections
wR(all)
R [I > 2σ(I)]
wR(abs)
6633
2666
N(5)–Hg–N(4)
N(4)–Hg–N(3)
N(4)–Hg–N(1)
N(5)–Hg–O(1)
N(3)–Hg–O(1)
N(5)–Hg–N(2)
N(3)–Hg–N(2)
O(1)–Hg–N(2)
C(5)–N(1)–Hg
C(10)–N(2)–Hg
C(11)–N(3)–Hg
C(20)–N(4)–Hg
O(1)–C(21)–O(2)
C(22)–N(5)–S
S–N(5)–Hg
152.8(6)
71.4(6)
92.0(6)
69.7(6)
110.2(6)
89.6(6)
155.3(6)
85.5(5)
121.0(12)
123.2(14)
124(2)
119(2)
127(3)
117(2)
119.7(9)
112.6(9)
103.8(9)
107.5(9)
122(2)
126(3)
112(3)
N(5)–Hg–N(3)
N(5)–Hg–N(1)
N(3)–Hg–N(1)
N(4)–Hg–O(1)
N(1)–Hg–O(1)
N(4)–Hg–N(2)
N(1)–Hg–N(2)
C(5)–N(1)–C(1)
C(1)–N(1)–Hg
C(10)–N(2)–C(6)
C(6)–N(2)–Hg
C(11)–N(3)–C(15)
C(15)–N(3)–Hg
C(20)–N(4)–C(16)
C(16)–N(4)–Hg
C(21)–O(1)–Hg
C(22)–N(5)–Hg
O(3)–S–O(4)
113.4(6)
113.3(7)
96.4(6)
83.4(6)
149.8(5)
92.5(6)
64.8(5)
124(2)
115.4(12)
123(2)
109.1(11)
121(2)
114.5(14)
122(2)
118(2)
114(2)
123(2)
5694
0.202
0.086
0.195
O(3)–S–N(5)
O(3)–S–C(23)
N(5)–S–C(23)
O(5)–N(6)–C(28)
O(5)–N(6)–O(6)
O(6)–N(6)–C(28)
118.7(10)
106.5(9)
107.1(8)
O(4)–S–N(5)
O(4)–S–C(23)
2.54–3.75 Å. Hg2ϩ often prefers sulfur coordination as in the
structure of methioninediperchloratomercury()30 where
Hg2ϩ ion is octahedrally coordinated by carboxylic oxygen
atoms and sulfur atoms of methionine in a near collinear
S–Hg–S bond. On the contrary CH3Hgϩ has a strong tendency
to form complexes with amino acids or small peptides, but it
prefers linear or trigonal coordination geometry.31
The N–Hg–N angles within the chelate rings range from
64.8(5) to 71.4(6)Њ and are similar to those found in other Hg2ϩ
–
bpy complexes.32,33 The Hg–Nbpy bond distances are in the
range 2.33–2.68 Å, of the 2.22–2.56 Å interval of other Hg–bpy
complexes.32–34 The Hg–N(5) sulfonamide distance [2.14(2) Å]
is the shortest, because of the great ligand strength of the
deprotonated sulfonamide nitrogen, and is similar to those
reported for complexes with amino acids.29 The Hg–O(1) bond
distance falls in the range [2.2–2.9 Å] found in carboxylate
complexes.35 In addition longer contacts with the S atom
(3.22(2) Å) and sulfonic O4 atom (3.07(2) Å) are observed,
within the sum of van der Waals radii. The glycine-like ring
forms dihedral angles of 64.7(2) and 55.4(2)Њ with the chelate
rings involving the bipyridine molecules; the angle between the
chelate rings of bipyridine is 76.4(3)Њ. Bond distances and
angles in the amino acid moiety are similar to those observed in
the free ligand and in other NO2psgly containing metal com-
plexes.20 The main difference is in the dihedral angle formed by
the benzene ring and the NO2 group, which is 59.9(2)Њ in this
complex and 31.5(1)Њ in free NO2psglyH2. This may be attrib-
uted to the presence of intramolecular interactions between
NO2psgly and bpy(1) (range 3.7–3.9 Å), which causes also a
distortion in bpy; in fact the internal rotation angle about the
2,2Ј bond is 27.4Њ in bpy(1) and 2.4Њ in bpy(2). Ring stacking
interactions involving bpy(1) and symmetry related bpy(2)
(range 3.5–3.8 Å) are also present, while the water molecule is
involved in a possible hydrogen bond with sulfonic oxygen
O3.
Fig. 1 An ORTEP view of the [Hg(bpy)2(NO2psgly-N,O)] moiety
showing atom numbering ellipsoids (40%) for non-hydrogen atoms.
Results and discussion
Description of the structure of [Hg(bpy)2(NO2psgly-N,O)]ؒ
0.5H2O
Main bond distances and angles are reported in Table 2 with
the atom numbering as in Fig. 1. In the complex molecule the
Hg2ϩ environment can be described as distorted octahedral
with a N5O donor set. Four nitrogen atoms are derived from the
two bpy ligands, while the oxygen and the fifth nitrogen are
from the NO2psgly dianion, so three five-membered chelate
rings are formed. N(1), N(2), N(3) and O(1) act as equatorial
ligands with deviation from the main plane of about 0.1 Å;
N(4) and N(5), at shorter distances, are the apical ligands with a
trans angle of 152.8(6)Њ. Six-coordination is rather common
for Hg2ϩ ion although this structure represents a rare example
of Hg2ϩ coordinated by an amino acid molecule acting as chel-
ating agent via N,O donor atoms. The structure of bis[(S-
methyl--cysteinato)mercury()]29 is known where the amino
acid is N,O bonded with two almost collinear Hg–N bonds of
2.16(3) and 2.13(3) Å, and eight Hg ؒ ؒ ؒ O distances in the range
J. Chem. Soc., Dalton Trans., 2001, 1513–1519
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Fig. 3 pD dependence of the chemical shift of the methylene
resonance in the 1H NMR spectra of a saturated D2O solution of
[Hg(ts-β-ala)]: (᭹) α-CH2; (᭢) β-CH2.
Fig. 2 pD Dependence of the chemical shift of the methylene reson-
ances in the 1H NMR spectra in D2O: (᭿) NO2psglyH2 free ligand; (᭹)
saturated solution of [Hg(NO2psgly-N,O)]; (᭢) saturated solution of
[Hg(tsgly-N,O)].
Infrared spectra
A common feature for all the complexes is the position of
νasym(OCO) (ca. 1600 cmϪ1) as a consequence of the dissoci-
ation and coordination to the metal ion of the carboxylic
group. In N-sulfonylamino acid containing complexes the dis-
appearance of ν(N–H), which is present for bzgly containing
complexes, is in line with the presence of a deprotonated and
metal coordinated nitrogen atom.
As a general behaviour, the deprotonation and metal
coordination of sulfonamide nitrogen increases the bond order
of S–N and slightly decreases the S–O bond order.36 This leads
to ν(S–N) at higher frequency whereas asymmetric and
symmetric ν(SO2) are shifted to lower frequencies.
1H NMR spectroscopy
D2O solutions. The pD dependence of methylene peak(s)’s
chemical shift are investigated for the free ligand NO2psglyH2
(tsglyH2, bzglyH and ts-β-alaH2 are reported in a previous
work 15) and for binary and ternary bpy containing Hg2ϩ
systems.
Fig. 4 paH dependence of the chemical shift of the methylene
resonance in 1H NMR spectra of saturated solutions of binary solid
complexes in deuteriated DMSO: (᭹) [Hg(NO2psgly-N,O)]; (᭢)
[Hg(tsgly-N,O)]; (᭜) [Hg(ts-β-ala-N,O)] (β-CH2).
The titration of NO2psglyH2 (Fig. 2) gives two pKa values
(correction applied pH = pD Ϫ 0.4;37 all the calculated pKa
values have estimated error of 0.2): 2.9 and 10.6 correspond-
ing respectively to the equilibrium of carboxylic group dissoci-
ation and sulfonamide nitrogen deprotonation. These values
are consistent with those found through potentiometric and
spectrophotometric analysis.21
amide nitrogen deprotonation is observed. The calculated pKNH
values are 6.5 for the Hg2ϩ/NO2psglyH2, 6.7 for Hg2ϩ/ts-β-alaH2
and 7.1 for Hg2ϩ/tsglyH2 systems.
DMSO solutions. The 1H NMR study of the ligands
in this solvent was prevented by the insolubility of their alkal-
metal salts, while binary and ternary systems were investigated
on varying pH. The titolative trend observed by adding NaOH
and HNO3 to a deuteriated DMSO solution of the solid binary
complexes (Fig. 4) parallels the behaviour in D2O. The pKa
values for the carboxylic group were 6.9, 7.2, 7.4 and 7.0 for
The titrations of binary systems (Figs. 2 and 3), performed
on millimolar solutions, yield apparent pKa values for the carb-
oxylic group of 3.2, 3.3, 3.9 and 3.6 respectively for
Hg2ϩ/NO2psglyH2, Hg2ϩ/tsglyH2, Hg2ϩ/ts-β-alaH2 and Hg2ϩ
/
bzglyH; these values are almost unchanged with respect to
those of the free ligands.
Hg2ϩ/NO2psglyH2, Hg2ϩ/tsglyH, Hg2ϩ/ts-β-alaH2 and Hg2ϩ
/
bzglyH respectively, while the pKNH were 9.8 for Hg2ϩ/NO2-
psglyH2 and 10.3 for Hg2ϩ/tsglyH2 and Hg2ϩ/ts-β-alaH2. In the
binary Hg2ϩ/bzglyH system no further variation of the methyl-
ene group chemical shift is observed, after the carboxylic group
dissociation.
In all binary systems (except bzglyH) at pD values greater
than 6 the chemical shift (δ) of the methylene peak(s) increases
to reach a maximum around pD 9. The Hg2ϩ/ts-β-alaH2 spec-
trum shows a β-CH2 peak broadening with loss of structure
consistent with a quadrupolar interaction with a nitrogen atom.
On increasing pD value the spectra reveal a triplet structure
deformation of α-CH2 due to the inequivalency of β-CH2
protons in the chelate ring. Binary systems are stable and no
precipitation of HgO is observed, even at high pD values.
In the Hg2ϩ/bzglyH binary system no hint of Hg2ϩ inducing
The 1H NMR spectra run on deuteriated DMSO solutions of
the solid ternary complexes show only one equivalence point
with pKa of 6.8 for Hg2ϩ/NO2psglyH2 and Hg2ϩ/tsglyH2, 8.6 for
Hg2ϩ/ts-β-alaH2 (Fig. 5). This behaviour suggests the simul-
taneous deprotonation of both carboxylic group and sulfon-
amide nitrogen, as was previously observed in the Pd2ϩ
-
1516
J. Chem. Soc., Dalton Trans., 2001, 1513–1519

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Fig. 5 paH dependence of the chemical shift of the methylene reson-
ance in 1H NMR spectra of saturated solutions of ternary solid
complexes in deuteriated DMSO: (᭹) [Hg(bpy)(NO2psgly-N,O)];
(᭢) [Hg(bpy)(tsgly-N,O)]; (᭜) Hg(bpy)(ts-β-ala-N,O)].
Fig. 7 Species distribution curves for the binary systems in HgHg2ϩ/ :
H2L 1 : 2 molar ratio, [Hg2ϩ] = 10Ϫ3 M: 1, 2, 3 [Hg(HL)]ϩ; 4, 5, 6
[Hg(HL)2]; 7, 8, 9 [Hg(HL)(OH)]; 10, 11, 12 [HgL2]2Ϫ; 13, 14, 15
[HgL(OH)]Ϫ; ——, NO2psglyH2; ······, tsglyH2; – – – – ts-β-alaH2.
systems (L = tsgly, ts-β-ala or NO2psgly in the dianionic form)
in 1 : 1 and 1 : 2 molar ratio show two equivalence points in
accordance to eqns. (3) and (4) where m stands for number of
mNaOH = mL ϩ mHg
mNaOH = mL ϩ 2mHg
(3)
(4)
moles and L is the amino acid. The first equivalence point
(pH ≈ 5) is consistent with the formation of [HgL] or
[Hg(HL)(OH)] species, the second (pH ≈ 8) with the species
1
[HgL(OH)]Ϫ and [HgL2]2Ϫ. Analysis of the electronic and H
NMR spectra suggests that the deprotonated [HgL] species is
formed beyond pH 6.5, so the prevailing species at pH ≈ 5.5
may reasonably be [Hg(HL)(OH)]; the presence of mixed-
Fig. 6 Electronic spectra of the system Hg2ϩ/tsglyH2 in the molar
ratio 1 : 1, [Hg2ϩ] = 10Ϫ4 M, at pH varying from 4.27, 5.04, 6.28, 6.69,
7.20, 7.70, 8.80, 10.20 to 11.16 in order of decreasing absorbance. The
inset shows the overall pH dependence of the molar absorption
coefficient at 228 nm.
ligand hydroxo complexes has previously been found in Hg2ϩ
-
containing systems with carbohydrate α-amino acids,38 while in
Cu2ϩ/tsglyH2 and Cd2ϩ/tsglyH2 systems such species were
suggested to be intermediates in the reaction mechanism of
sulfonamide nitrogen deprotonation.
12
13
containing systems with ArSO2N-protected amino acids.14,16 In
the Hg2ϩ/bzglyH ternary system the pKa value observed is quite
similar to that found for the carboxylic group in the binary
system; in addition a broad peak assigned to the sulfonamide
nitrogen proton is observed in all spectra at different paH,
confirming that the only complex species formed is of the
carboxylate type.
The potentiometric titration of Hg2ϩ/bzglyH shows only one
equivalence point corresponding to neutralisation of the carb-
oxylic group of the ligand followed by precipitation of the
metal hydroxide. The calculated formation constants of the
complexes are reported in Table 3, while in Fig. 7 the species
distribution curves are shown. In the Hg2ϩ/NO2psglyH2, Hg2ϩ
/
tsglyH2 and Hg2ϩ/ts-β-alaH2 systems the same prevailing
species are observed. Owing to the electron-withdrawing effect
of the sulfonyl group, log β values of [Hg(HL)]ϩ and [Hg(HL)2]
complexes are lower than those found for Hg2ϩ complexes with
unsubstituted monocarboxylic acids (in the ranges 3.66–4.33
and 7.10–8.80 respectively),40 while similar to those found for
chloroacetate complexes (2.95 and 5.61 respectively).40
UV Spectroscopy
The spectrophotometric titrations of Hg2ϩ/H2L systems at 500–
200 nm reveal the presence of an isosbestic point (Fig. 6); plot-
ting absorbance vs. pH near 230 nm for tsglyH2 and ts-β-alaH2
and 270 nm for NO2psglyH2, a titration pattern is observed.
The estimated values of apparent pKNH observed are 6.9(2),
6.5(2) and 6.4(2) respectively, near to those found by 1H NMR
data. The trend is consistent with the order of basicity of the
ligands confirming also for Hg2ϩ that the amount of metal
induced decrease in pKNH is independent of the nature of the
Regarding the deprotonated species [HgL2]2Ϫ and the
[HgL(OH)]2Ϫ, the lower stability of the ts-β-ala complex with
respect to the tsgly one is attributed to the effect of the greater
strain of a six-membered chelate ring as compared to a five-
membered one,41 while the lower stability of NO2psgly com-
plexes is due to the steric hindrance of the NO2 group which
disfavours coordination of a second ligand molecule.
amino acid, as was previously found for Cu2ϩ/and Cd2ϩ
/
N-sulfonylamino acid systems.21 In the Hg2ϩ/bzglyH system no
spectral changes are observed on increasing pH, therefore
excluding any amide nitrogen deprotonation, also in the
presence of Hg2ϩ, confirming 1H NMR data.
The log β values of [HgL2]2Ϫ species are remarkably greater
with respect to those of Hg2ϩ complexes with dipeptides (mean
value 5.8 0.8)10 and amino acids such as 2-(benzylamino)-2-
deoxy--glycero--guloheptonic acid (log β = 9.78)42 owing to
the greater basicity of the deprotonated sulfonamide nitrogen
Potentiometry
Binary systems. The pH-metric titration curves of Hg2ϩ/H2L
J. Chem. Soc., Dalton Trans., 2001, 1513–1519
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Table 3 Logarithm of protonation constants of ligands and complex formation constants at T = 298 K, I = 0.1 M (NaNO3)
Species
NO2psgly
tsgly
ts-β-ala
bzgly
HLϪ
H2L
log β0011
log β0012
log Kal
10.62(2)
13.84(1)
3.22
13.56(8)
2.94
26.08(5)
4.84
11.35(3)
14.56(2)
3.21
14.32(5)
2.97
27.98(3)
5.28
11.19(2)
15.49(1)
4.30
14.39(4)
3.20
28.38(4)
5.99
HLb
a
log β0011
[HgL]ϩ b
log β1010
[HgL2]b
log β1020
3.81(1)
3.01(6)
5.73(4)
[Hg(HL)]ϩ
[Hg(HL)2]
log β1011
c
log K1
log β1022
d
log K2
[Hg(HL)(OH)]
[HgL2]2Ϫ
log β1010
log β1020
log β101-1
log β1110
log β111-1
log β1120
log β112-1
8.87(2)
15.72(3)
1.69(4)
18.58(2)
10.95(5)
9.40(3)
16.91(4)
2.69(3)
19.08(6)
11.97(4)
9.20(3)
16.42(4)
2.60(3)
18.88(6)
11.76(5)
[HgL(OH)]2Ϫ
[Hg(bpy)L]e
[Hg(bpy)L(OH)]Ϫ
[Hg(bpy)L2]b
17.59(2)
11.66(7)
[Hg(bpy)L2(OH)]Ϫ b
a log Ka1 = log β0012 Ϫ log β0011
.
b In the case of bzgly L stands for the carboxylate anion. c log K1 = log β1011 Ϫ log β0011
.
d log K2 = log β1022 Ϫ 2log β0011
.
e log β value for [Hg(bpy)]2ϩ is 9.6.39
Table 4 Logarithm of stability constants of M2ϩ complexes, T = 298 K, I = 0.1 M (NaNO3)
System
[M(HL)]ϩ
[M(HL)2]
[ML]
[ML2]2Ϫ
[ML(OH)]Ϫ
Ref.
Hg/tsglyH2
14.32(5)
14.39(4)
13.56(8)
12.48(8)
12.40(7)
12.20(2)
27.98(3)
28.38(4)
26.08(5)
24.83(7)
24.52(7)
25.73(5)
28.0(1)
16.91(4)
16.42(4)
15.72(3)
2.69(3)
2.60(3)
1.69(4)
Hg/ts-β-alaH2
Hg/NO2psglyH2
Pb/tsglyH2
Pb/ts-β-alaH2
Pb/NO2psglyH2
Cu/tsglyH2
6.43(7)
6.50(7)
6.90(2)
7.6(1)
15
15
21
12
17
21
13
43
21
14
14
11.3(1)
Cu/ts-β-alaH2
Cu/NO2psglyH2
Cd/tsglyH2
Cd/ts-β-alaH2
Cd/NO2psglyH2
Pd/tsglyH2
27.9(1)
12.36(6)
8.59(1)
4.90(1)
5.34(4)
6.24(3)
17.8(1)
16.8(1)
14.22(3)
6.00(1)
9.90(8)
10.45(9)
23.4(1)
20.5(1)
0.62(7)
15.00(1)a
30.10(1)a
28.14(7)
12.5(3)
Ϫ2.95(5)
Pd/ts-β-alaH2
a Values in water–CH3OH solutions.
compared to that of the terminal amine nitrogen in Hg2ϩ
coordinated amino acids or peptides.
-
oxylic oxygen coordination to the metal ion. The stabilising
effect of 2,2Ј-bipyridine is reflected by a marked diminishing of
pKNH, whose value becomes independent of the nature of the
amino acid ligand, being around 4.5 for all the three ligands as
shown from the distribution curves. Regarding the stability of
the complex species we may observe the same trend found in the
corresponding binary systems; in fact tsglyH2 is the ligand that
forms more stable complexes. In particular the log X value47
Comparing the behaviour of these ligands with various
metal() ions (Table 4) it is seen that Hg2ϩ forms more stable
complexes with N-sulfonylamino acids, apart from Pd2ϩ. In the
case of Pd2ϩ complexes the sulfonamide nitrogen deprotonation
doesn’t pass through the preliminary formation of carboxylate
species, as is found for other investigated metal ions,16,44
although other authors excluded the formation of carboxylate
complexes of Cu2ϩ with N-p-amino-(or nitro)-phenylsulfonyl
amino acid derivatives.45,46 For Pb2ϩ, Cd2ϩ, Cu2ϩ and Pd2ϩ the
effectiveness in substituting for the sulfonamide nitrogen-
bound hydrogen and the stability of the complexes formed
parallel well the behaviour toward oligopeptides;47 even though
the complex formation of Hg2ϩ with dipeptides has not been
investigated frequently, it was found that Hg2ϩ was unable to
undergo amide nitrogen deprotonation in dipeptides.10 There-
fore the behaviour of Hg2ϩ with N-sulfonylamino acids is rather
surprising and may be attributed to its affinity for the sulfur
atom, as evidenced by the bond interaction found in the crystal
structure of the solid ternary complex, which may favour the
sulfonamide nitrogen deprotonation reaction.
Hg
Hg
Hg
2Ϫ
2ϩ
{log X = 2logβ
Ϫ [log β
ϩ log β[Hg(bpy) ] ]} is 5.74
[Hg(bpy)L]
[HgL2]
2
for tsgly, 4.64 for ts-β-ala and 4.74 for NO2psgly. These differ-
ences may be attributed to the different ability of bpy to give
rise to π conjugation with the aromatic moiety of the amino
acid molecule. In fact the formation of a non-planar six-
membered chelate ring in the ts-β-ala complex and the presence
of the NO2 group in the NO2psgly one handicap the π
conjugation with the aromatic system of the bpy molecule,
diminishing the value of log X.
The overall stability constants for these ternary complexes
are similar to those found for Pb2ϩ complexes15 and, in particu-
lar for the tsglyH2 system, follow the order of stability Pd2ϩ
ӷ
Hg2ϩ > Pb2ϩ > Cu2ϩ > Ni2ϩ > Zn2ϩ > Co2ϩ > Cd2ϩ
; indicating
that the metal affinity for N,O-donor ligands is the major factor
determining the stability of the complexes.
Ternary system. The complex species observed in the bpy
containing systems are [Hg(bpy)L] and [Hg(bpy)L(OH)]Ϫ and
their evaluated stability constants are reported in Table 3. Fig. 8
shows the species distribution curves for the ternary systems.
The most interesting feature of these systems is that no carb-
oxylate type complexes are detected, confirming the hypothesis
In the bzglyH ternary system the complex species found are
[Hg(bpy)L2] (where L, in this case, is the monoanionic form of
the amino acid), isolated also in the solid state at pH ≈ 5, and
[Hg(bpy)L(OH)], where the amino acid acts invariably as a
carboxylate ligand; their stability constants are reported in
Table 3.
1
suggested by H NMR spectra that the sulfonamide nitrogen
deprotonation takes place almost contemporarily with carb-
In conclusion we have proved the ability of Hg2ϩ ion to
coordinate N-protected amino acid molecules forming N,O-
1518
J. Chem. Soc., Dalton Trans., 2001, 1513–1519

View Article Online
11 S. Menzer, E. C. Hillgeris and B. Lippert, Inorg. Chim. Acta, 1993,
211, 221 and ref. therein.
12 L. Antolini, L. P. Battaglia, G. Battistuzzi Gavioli, A. Bonamartini
Corradi, G. Grandi, G. Marcotrigiano, L. Menabue and G. C.
Pellacani, J. Am. Chem. Soc., 1983, 105, 4333.
13 G. Battistuzzi Gavioli, M. Borsari, G. C. Pellacani, L. Menabue,
M. Sola and A. Bonamartini Corradi, Inorg. Chem., 1988, 27, 1587.
14 G. Battistuzzi Gavioli, M. Borsari, L. Menabue, M. Saladini,
G. C. Pellacani and M. Sola, J. Chem. Soc., Dalton Trans., 1990,
1585.
15 G. Battistuzzi, M. Borsari, L. Menabue, M. Saladini and M. Sola,
Inorg. Chem., 1996, 35, 4239.
16 G. Battistuzzi, G. Gavioli, M. Borsari, L. Menabue, M. Saladini and
M. Sola, J. Chem. Soc., Dalton Trans., 1994, 279.
17 G. Battistuzzi Gavioli, M. Borsari, L. Menabue, M. Saladini and
M. Sola, J. Chem. Soc., Dalton Trans., 1991, 2961.
18 G. Battistuzzi Gavioli, M. Borsari, L. Menabue, M. Saladini and
M. Sola, Inorg. Chem., 1991, 30, 498.
19 M. Borsari, L. Menabue and M. Saladini, J. Chem. Soc., Dalton
Trans., 1996, 4201.
20 D. Iacopino, L. Menabue and M. Saladini, Aust. J. Chem., 1999, 52,
741.
21 M. Saladini, D. Iacopino and L. Menabue, Inorg. Biochem., 2000,
78, 355.
22 I. M. Kolthoff, M. K. Chantooni Jr. and S. Bhowmik, J. Am.
Chem. Soc., 1968, 3, 23.
23 M. Borsari, L. Menabue and M. Saladini, Polyhedron, 1999, 18,
1983.
24 P. Gans, A. Sabatini and A. Vacca, J. Chem. Soc., Dalton Trans.,
1985, 1185.
Fig. 8 Species distribution curves for ternary systems in Hg : bpy : H2L
1 : 1 : 2 molar ratio, [Hg2ϩ] = 1.25 × 10Ϫ4 M: 1 [Hg(bpy)]2ϩ; 2, 3, 4
[Hg(bpy)L]; 5, 6,
7
[Hg(bpy)L(OH)]Ϫ; ——, NO2psglyH2; ······,
tsglyH2; – – – –, ts-β-alaH2.
25 NIST WebBook Standard Reference Database 46, Critically
Selected Stability Constants, Version 5, webbook.nist.gov.
26 G. M. Sheldrick, SHELX 76, Program for Crystal Structure
Determination, University of Cambridge, 1976.
27 G. M. Sheldrick, SHELXL 93, Program for the Refinement of
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28 C. K. Johnson, ORTEP, Report ORNL 3794, Oak Ridge National
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29 L. Book, A. J. Carty and C. Chich, Can. J. Chem., 1981, 59, 144.
30 A. J. Carty and N. J. Taylor, J. Chem. Soc., Chem. Comm., 1976, 214.
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chelate complexes, either in the solid or solution state, in a
wide range of pH. The stability of these complexes is greater
than that found for Hg2ϩ complexes with methionine and
-cysteine48 and is comparable to that of methylmercury()–
thiol complexes.49 The presence, in the ligand molecule, of a
sulfur atom interacting with the metal ion, seems the driving
force for the NH deprotonation and the formation of stable
complexes.
These data confirm the great ability of Hg2ϩ to coordinate
sulfur-containing biological ligands, when the sulfur atom is
not considered a coordination site, a feature that can be the
basis of its toxicity.
33 D. C. Craig, Y. Farhangi, D. P. Graddon and N. C. Stephenson,
Cryst. Struct. Commun., 1974, 3, 155.
34 D. C. Bebout, D. E. DeLanoy, D. E. Ehmann, M. E. Kastner,
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35 E. C. Alyea, S. A. Dias, G. Ferguson, M. A. Khan and P. J. Roberts,
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36 L. Antolini, L. P. Battaglia, A. Bonamartini Corradi, G.
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37 D. D. Perrin and B. Dempsey, Buffers for pH and Metal Ion Control,
Chapman and Hall, London, 1979.
Acknowledgements
We are grateful to the Centro Interdipartimentale Grandi
Strumenti (CIGS) of the University of Modena and Reggio
Emilia which supplied the diffractometer and the NMR spec-
trometer, and also thankful to the Ministero dell’Università e
della Ricerca Scientifica e Tecnologica of Italy for financial
support.
38 M. A. Diaz Diez, F. J. Garcia Barros, A. Bernalte Garcia and C.
Valenzuela Calahorro, J. Inorg. Biochem., 1994, 54, 141.
39 R. G. Anderegg, Helv. Chim. Acta, 1963, 46, 2397.
40 F. J. C. Rossotti and R. J. Wheweel, J. Chem. Soc., Dalton Trans.,
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41 A. E. Martell, R. D. Hancock and R. J. Motekaitis, Coord. Chem.
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