Pb(II)-promoted amide cleavage: Mechanistic comparison to a Zn(II) analogue

Article abstract of DOI:10.1021/ic401782x

Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO₃)₂] (4-NO ₃) and [(epppa)Pb(ClO₄)₂] (4-ClO₄), were prepared and characterized. In the solid state, 4-NO₃ exhibits κ⁵-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO₃ is a 1:1 electrolyte with a coordinated NO₃^-, whereas 4-ClO₄ is a 1:2 electrolyte. Treatment of 4-ClO₄ with 1 equiv Me₄NOH·5H₂O in CH₃CN:CH ₃OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO ₄)₂ (3-ClO₄). ¹H NMR kinetic studies of the amide methanolysis reactions of 4-ClO₄ and 3-ClO₄ as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO ₄ and 3-ClO₄ differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO₄; second-order behavior for 3-ClO₄), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO₄, and the properties of an initial intermediate isolated from each reaction upon treatment with Me₄NOH·5H₂O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO₄ and 3-ClO₄, indicate that the Zn(II)-containing 3-ClO₄ initially undergoes amide deprotonation upon treatment with Me₄NOH·5H₂O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO₄ instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH₃ equilibrium mixture upon treatment with Me₄NOH·5H₂O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO₄ is nucleophilic attack of the Pb(II)-OCH₃ moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH₃ mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO₄.

Full text of DOI:10.1021/ic401782x

Article
pubs.acs.org/IC
Pb(II)-Promoted Amide Cleavage: Mechanistic Comparison to a Zn(II)
Analogue
Eric S. Elton,† Tingting Zhang,‡ Rajeev Prabhakar,‡ Atta M. Arif,§ and Lisa M. Berreau*,†
†Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, Utah 84322-0300, United States
‡Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, Florida 33146-0431, United States
§Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City, Utah 84112-0850, United States
S
* Supporting Information
ABSTRACT: Two new Pb(II) complexes of the amide-
appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-
((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)-
amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO3) and
[(epppa)Pb(ClO4)2] (4-ClO4), were prepared and charac-
terized. In the solid state, 4-NO3 exhibits κ5-epppa chelate
ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a
−
coordinated NO3 , whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH3OH
(3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue
[(epppa)Zn](ClO4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function
of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of
4-ClO4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order
behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-
containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·
5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4
and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·
5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and
methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide
protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-
containing, epppa-ligated Pb(II)−OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol.
The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on
the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable
direct formation of a Pb(II)−OH/Pb(II)-OCH3 mixture versus the initial amide deprotonation identified in the reaction of the
Zn(II)-containing 3-ClO4.
amide cleavage reactions, few detailed investigations of the
kinetics and thermodynamics of such reactions involving a
INTRODUCTION
■
Lead is a toxic environmental pollutant that has dramatic effects
in biological systems even in small amounts. Even after the
elimination of leaded gasoline, lead poisoning continues to be a
problem in the United States, with about 5% of children being
affected.1 While the exact process by which lead poisoning
occurs is not completely defined, it is known that lead binds to
sulfur rich sites in proteins, often displacing Zn(II).2 Recent
small molecule and peptide studies have focused on examining
the coordination chemistry of Pb(II) in Zn(II)-binding
environments.3−11 However, these studies do not include
comparisons of biologically relevant reactivity, which might
provide additional insight into the toxic properties of Pb(II).
Studies of zinc-promoted amide cleavage reactions continue
to be an active area of research. This is due to the important
role that mononuclear zinc centers play in catalyzing amide
cleavage reactions that are directly involved in human health
and disease,12−14 as well as in reagents for the selective cleavage
of peptides.15−17 Despite the importance of zinc-promoted
nonactivated amide substrate have been reported.18−20
A
strategy typically utilized in such systems has been the
attachment of the amide to a chelate ligand so as to position
the metal ion close to the scissile bond, such as in 1−3
(Scheme 1). In our laboratory, we have previously investigated
the amide methanolysis or hydrolysis reactions of 2 and 3-
ClO4.19,21−23
The interactions of Pb(II) with amide-containing ligands has
been previously investigated.2,24−26 However, to our knowl-
edge, no examples of Pb(II)-promoted hydrolysis or alcoholysis
of an amide linkage have been previously reported. In the
studies outlined herein, we have examined amide cleavage
reactivity in a system wherein the Zn(II) center can be replaced
by Pb(II). Using as a starting point our previously reported
Received: July 10, 2013
Published: September 25, 2013
© 2013 American Chemical Society
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Article
Scheme 1
Elemental analyses were performed by Atlantic Microlabs of Norcross,
GA, using a PE2400 automatic analyzer.
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only small amounts of material should be prepared,
and these should be handled with great care.31
amide cleavage studies of [(epppa)Zn](ClO4)2 (3-ClO4, epppa
= N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)-
methyl)-N-((2-pyridyl)methyl)amine; Scheme 1),22 we first
prepared and fully characterized two new Pb(II) complexes,
[(epppa)Pb(ClO4)2] (4-ClO4) and [(epppa)Pb(NO3)2] (4-
NO3), of the same chelate ligand. We then examined the
kinetics and thermodynamics of the amide methanolysis
reaction of 4-ClO4 and compared the results to those obtained
for 3-ClO4. Combined with investigations into the inter-
mediates in the amide methanolysis reaction pathways, as well
as computational studies of the amide methanolysis reaction
pathway as a function of metal ion,27,28 the research presented
herein provides the first detailed insight into the influence of
Pb(II) versus Zn(II) in a biologically relevant reaction. The
results of this study reveal how the inherent chemical properties
of Pb(II) influence an amide cleavage reaction.
[(epppa)Pb(ClO4)2] (4-ClO4). To a solution of Pb(ClO4)2·3H2O (54
mg, 0.11 mmol) in methanol (∼2 mL) was added a solution of epppa
(46 mg, 0.11 mmol) in methanol (∼1 mL). The resulting mixture was
stirred at ambient temperature for ∼20 min after which time excess
Et2O (∼15 mL) was added to induce precipitation of the product.
After being allowed to settle overnight at −20 °C, the excess solvent
was decanted and the deposited solid was dried under vacuum (91%).
1H NMR (CD3CN, 300 MHz) δ 9.06 (br, 1H, N-H), 8.92 (m, J = 5.4
Hz, 1H, α-H (py)), 8.08−7.98 (m, 2H, β-H (Py), γ-H (AmPy)),
7.68−7.62 (m, 2H, β-H (Py), γ-H (py)), 7.44 (d, J = 7.6 Hz, 1H, β-H
(AmPy)), 7.37 (d, J = 8.2 Hz, 1H, β-H (AmPy)), 4.71 (d, J = 15.0 Hz,
1H, benzylic), 4.69 (d, J = 15.0 Hz, 1H, benzylic), 4.52 (d, J = 15.0 Hz,
1H, benzylic), 4.34 (d, J = 15.0 Hz, 1H, benzylic), 3.69−3.58 (m, 2H,
CH2 (ethylene linker)), 2.99−2.90 (m, 1H, CH (ethylene linker)),
2.64−2.52 (m, 3H, CH (ethylene linker) and CH2 (ethyl)), 1.35 (s,
9H, C(CH3)3), 1.21 (t, J = 7.4 Hz, 3H, −CH3 (ethyl)); 13C{1H}
(CD3CN, 100 MHz) δ 182.4, 159.8, 154.9, 151.8, 149.6, 143.3, 141.8,
126.0, 125.4, 122.6, 117.3, 62.6, 62.5, 60.3, 41.4, 29.3, 27.2, 26.7, 14.5
(19 resonances expected and observed); FTIR (KBr, cm−1) 3397 (br,
νN−H), 1656 (νCO), 1609, 1523, 1456, 1417, 1366, 1163 (νClO4),
1096 (νClO4), 1018 (νClO4), 918 (νClO4), 622 (νClO4). Anal. Calcd for
C21H30Cl2N4O9PbS: C, 31.82; H, 3.81; N, 7.07. Found: C, 32.03; H,
3.89; N, 6.75.
[(epppa)Pb(NO3)2] (4-NO3). This compound was prepared using
Pb(NO3)2 in a synthetic route analogous to that used for 4-ClO4
except with longer stirring time (∼40 min). Precipitation of the
product by Et2O addition resulted in the deposition of a white powder
(83%). Recrystallization of this powder by diethyl ether diffusion into a
iPrOH:CH3OH (1:1) solution yielded colorless crystals suitable for
single crystal X-ray diffraction analysis. 1H NMR (CD3CN, 300 MHz)
δ 9.15 (br, 1H), 8.82 (d, J = 4.8 Hz, 1H), 7.99−7.93 (m, 1H) 7.88 (t, J
= 8.2 Hz, 2H), 7.59−7.50 (m, 3H), 7.29 (d, J = 7.5 Hz, 1H), 4.52−
4.23 (m, 4H), 3.34−3.32 (m, 2H), 2.83−2.63 (m, 2H), 2.48−2.35 (m,
EXPERIMENTAL SECTION
■
General Methods. All reagents were purchased from commercial
sources and used as received unless otherwise noted. The chelate
ligand N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-
N-((2-pyridyl)methyl)amine (epppa) and the zinc complex [(epppa)-
Zn](ClO4)2 (3-ClO4) were prepared as previously described.22
Physical Methods. All NMR spectra for compound character-
ization were obtained on either a JEOL ECX-300 or Bruker ARX400
spectrometer at ambient temperature. Spectra are referenced to the
residual solvent peak(s) in CHD2CN (1H 1.94 ppm (quintet),
13C{1H} 1.39 ppm (heptet)). FTIR spectra were obtained on a
Shimadzu FTIR-8400 as KBr pellets. Conductance measurements
were made at 22(1) °C using a YSI model 31A conductivity bridge
with a cell having a cell constant of 1.0 cm−1 and using Me4NClO4 as a
1:1 electrolyte standard, and [(bnpapa)Ni](ClO4)229 as a 1:2 standard.
Solutions for conductance measurements were prepared as previously
described.30 Mass spectroscopy experiments for compound character-
ization were performed at the University of California, Riverside.
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Article
2H), 1.31 (s, 9H), 1.11 (t, J = 7.4 Hz, 3H); FTIR (KBr, cm−1) 3423,
1653 (νCO), 1606, 1523, 1456, 1383, 1292, 1242 (νNO3), 1155, 1030
(νNO3), 916, 819, 765, 632.
X-ray Crystallography. A crystal of 4-NO3 was mounted on a
glass fiber using a viscous oil and was transferred to a Nonius Kappa
CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) for data
collection at 150(1) K. Methods for the determination of cell
constants and unit cell refinement have been previously described.19
The structure was solved by a combination of direct methods and
heavy atom methods using SIR97.33 All of the non-hydrogen atoms
were refined with anisotropic displacement coefficients. Hydrogen
atoms were assigned isotropic displacement coefficients U(H) =
1.2U(C) or 1.5U(Cmethyl), and their coordinates were allowed to ride
on their respective carbons using SHELXL97.34
Computational Methods. All calculations were performed using
the Gaussian 09 program package.35 The geometries of reactants,
intermediates, transition states, and products were optimized in the gas
phase at the B3LYP/Lanl2dz level of theory without any symmetry
constraints using the corresponding Hay−Wadt effective core
potential (ECP) for zinc and lead.36−38 The final energies of the
optimized structures were further improved by performing single point
calculations including additional d and p polarization functions for O
(α = 0.96), N (α = 0.74), C (α = 0.59), and H (α = 0.36) atoms
(taken from the EMSL’s Gaussian basis set library), respectively, in the
basis set used for optimizations. Hessians were calculated at the same
level of theory as the optimizations to confirm the nature of the
stationary points along the reaction coordinate. The transition states
were confirmed to have only one negative eigenvalue corresponding to
the reaction coordinates. In this paper, the energies obtained at the
B3LYP/{Lanl2dz + d (O, N, and C) + p (H)}/ including thermal
corrections are discussed.
Amide Methanolysis Reactivity of 4-ClO4. Product Identi-
fication. To a methanol solution of 4-ClO4 (22 mg, 0.027 mmol) was
added a methanol solution of Me4NOH·5H2O (5.0 mg, 0.027 mmol).
The resulting cloudy mixture was stirred for 24 h at ambient
temperature, after which time a water solution containing excess
Me4NOH·5H2O (20 mg, 0.11 mmol) was added to the mixture and
the resulting solution was stirred for 20 min. The organic products
were extracted from this solution using CH2Cl2 (3 × 5 mL). The
combined organic fractions were then dried over Na2SO4, filtered, and
the solvent removed under vacuum. The resulting light yellow solid
1
exhibited H NMR features identical to those previously reported for
N-((ethylthio)ethyl)-N-((6-amino-2-pyridyl)methyl)-N-((2-pyridyl)-
methyl)amine (eappa: 8.1 mg, 96% yield).22 Me4NClO4 was isolated
from the reaction mixture as previously described.19 The methyl-
trimethylacetate produced in the amide methanolysis reaction of 4-
ClO4 was identified by 1H NMR (s, 1.13 ppm) and was quantified (1
equiv) via integration of this signal versus the internal standard
triphenylmethane.
Reaction of 4-ClO4 with Me4NOH·5H2O. Characterization of
4′-ClO4. To an acetonitrile solution of Me4NOH·5H2O (9.8 mg, 0.054
mmol) was added methanol (2 drops) to enhance dissolution of the
salt. A separate solution of 4-ClO4 (43 mg, 0.054 mmol) was prepared
in acetonitrile (∼2 mL). Admixture of these solutions produced a light
yellow solution containing a precipitate. Following removal of the
solvent under vacuum, the remaining solid was dissolved in CH2Cl2
and was filtered though a Celite/glass wool plug. The filtrate was
collected, and the solvent was removed under vacuum. 1H NMR
(CD3CN, 300 MHz) δ 8.80 (br, 1H), 8.68 (d, J = 4.9 Hz, 1H), 7.94−
7.87 (m, 2H), 7.61−7.56 (m, 2H), 7.45 (t, J = 6.0 Hz, 1H), 7.36 (d, J =
7.5 Hz, 1H), 4.31 (br, 2H), 4.27 (br, 2H), 3.15 (br, 2H), 2.65 (t, J =
6.5 Hz, 2H), 2.41(q, J = 7.4 Hz, 2H), 1.26 (s, 9H), 1.12 (t, J = 7.4 Hz,
3H); FT-IR (KBr, cm−1) 3354 (br), 1658 (νCO), 1606, 1528, 1521,
1455, 1406, 1300, 1159 (νClO4), 1092 (νClO4), 1002 (νClO4), 914
(νClO4), 803, 764, 620 (νClO4).
RESULTS
■
Preparation and Characterization of [(epppa)Pb(X)2]
−
−
(X = ClO4 or NO3 ). Synthesis. Treatment of epppa with an
equimolar amount Pb(ClO4)2·3H2O or Pb(NO3)2 in methanol
enabled the generation of [(epppa)Pb(X)2] (4-X). Precip-
itation of the perchlorate salt using Et2O resulted in the
isolation of analytically pure 4-ClO4, which was characterized
Kinetic Studies. 1H NMR kinetic studies of the amide
methanolysis reaction of 4-ClO4 were performed using a JEOL
ECX-300 NMR with a thermostatted probe. A temperature calibration
curve for the range of 220 to 295 K was obtained using ethylene glycol.
Data collection was performed using a method analogous to that used
to obtain rate data for the amide methanolysis reaction of
[(epppa)Zn](ClO4)2 (3-ClO4).22 At regular intervals, the integrated
intensity of the t-butyl methyl signal (1.27 ppm) was determined via
automatic integration available on the Delta NMR Processing and
Control Software (version 4.3.6) and normalized through comparison
against the integrated intensity of the internal standard CHPh3 (5.56
ppm). Pseudo-first order rate constants were determined from the
slopes of ln[4′-ClO4] versus time. Data in these plots represent more
than three half-lives and typical correlation coefficients for these plots
were ≥0.992. Second order rate constants were determined using the
known concentration of CD3OD in the mixture.32 Variable
concentration studies in terms of CD3OD (0.21 to 15.39 M) for the
amide methanolysis reaction of 4-ClO4 revealed saturation-type
kinetics with respect to the alcohol. Similar studies for the amide
methanolysis reaction of 3-ClO4 revealed a linear relationship. An
Eyring plot was constructed for the amide methanolysis reaction of 4-
ClO4 using rate constants over a range of 21 °C (274−295 K). All data
used in the Eyring plot was collected in triplicate with [CD3OD] =
15.39 M in CD3CN.
by elemental analysis, H and 13C NMR, IR, and conductivity
1
measurements. A similar isolation procedure yielded 4-NO3,
which was characterized by 1H NMR, IR, and X-ray
crystallography.
X-ray Crystallographic Characterization of 4-NO3. A
thermal ellipsoid view of 4-NO3 is shown in Figure 1(left).
Details of the X-ray data collection and refinement for 4-NO3
are given in Table 1. Selected bond angles for this complex are
given in Table 2. The overall coordination number of the
Pb(II) center in 4-NO3 is nine, with each nitrate anion
coordinated in a bidentate fashion to give an overall
holodirected-type structure.39 The epppa chelate ligand is κ5-
Solvent Kinetic Isotope Effect Studies. The influence of
solvent-derived protons was explored through the comparative use
of CD3OD and CD3OH in kinetic studies of the amide methanolysis
reactions of 3-ClO4 and 4-ClO4. The experimental procedure for
CD3OD is described above. An identical approach was used with
CD3OH with the exception of the presence of 4 equiv of CHPh3 as an
internal standard. All reactions were carried out at 295 K and
[CD3OH] = 15.39 M in CD3CN. All data used to examine the solvent
kinetic isotope effect were carried out in triplicate.
Figure 1. Thermal ellipsoid representations of 4-NO3 (left) and the
cationic portion of 3-ClO4 (right). Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms, except the amide proton, have been
omitted for clarity.
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12 analogues supported by either the epppa (3-ClO4: Zn−
O(amide), 1.998(7) Å)22 or the bmppa (N,N-bis(2-
methylthio)ethyl-N-((6-pivaloylamido-2-pyridyl)methyl)amine;
[(bmppa)Cd(ClO4)]ClO4: Cd−O(amide), 2.302(2) Å;21
[(bmppa)Hg(ClO4)]ClO4: Hg−O(amide), 2.271(5) Å46)
chelate ligands. The O(1)−M(1)−N(2)−C(6) torsion angle
(−21.62°) is similar to that found in the Hg(II) complex
[(bmppa)Hg(II)(ClO4)]ClO4 (−22.5°).46 Overall, the average
M-N distance, as well as the M-S(thioether) and M-O(amide)
distances associated with chelate ligand coordination in 4-NO3,
exceed those found in the zinc complex 3-ClO4 by 0.63−0.68
Å. This is a result of the larger ionic radius (Pb(II) (1.19 Å);
Zn(II) (0.74 Å))47 as well as the higher coordination number
of the Pb(II) complex. Finally, both nitrate ions are coordinated
to the Pb(II) center in 4-NO3 in a bidentate fashion, with the
nitrate coordinated via O(5) and O(6) being nearly symmetric
bidentate.48
Spectroscopic and Solution Conductivity Properties. The
solid-state IR spectrum of 4-NO3 contains vibrations at 1242
(ν1) and 1030 (ν2) cm−1, which are consistent with the
bidentate nitrate coordination identified via X-ray crystallog-
raphy.49 For the perchlorate analogue 4-ClO4, a series of four
vibrations at ∼1163, 1096, 1018, and 918 cm−1 suggest
bidentate coordination of the perchlorate anions.49
Table 1. Summary of X-ray Data Collection and
Refinement
a
4-NO3
empirical formula
formula weight
crystal system
space group
a (Å)
C21H30N6O7PbS
717.76
monoclinic
P 21/n
10.70150(1)
15.6611(2)
15.8963(2)
90
b (Å)
c (Å)
a (deg)
b (deg)
100.3770(8)
90
c (deg)
V (Å3)
2620.60(5)
4
Z
density (calcd), Mg m−3
1.819
temp (K)
150(1)
crystal size (mm)
diffractometer
abs. coeff. (mm−1
0.18 × 0.15 × 0.10
Nonius KappaCCD
6.569
)
2θ max (deg)
54.96
completeness to 2θ (%)
reflections collected
indep. reflections
99.9
11303
6011
When dissolved in acetonitrile, 4-ClO4 is a 1:2 electrolyte
(Figure 2) which is consistent with the dissociation of both
variable parameters
330
b
R1/wR2
0.0237/0.0455
1.061
goodness-of-fit (F2)
largest diff. (e Å−3
)
1.802/-0.671
a
b
Radiation used: Mo Kα (λ = 0.71073 Å). R1 = ∑||Fo| − |Fc||/∑|Fo|;
wR2 = [∑[w(Fo − Fc2)2]/[∑(Fo )2]]1/2 where w = 1/[σ2(Fo ) +
2
2
2
(aP)2 + bP].
Table 2. Selected Bond Distances (Å) and Angles (deg) for
a
4-NO3
Pb(1)−O(1)
Pb(1)−N(2)
Pb(1)−N(3)
Pb(1)−N(4)
Pb(1)−S(1)
Pb(1)−O(2)
Pb(1)−O(3)
Pb(1)−O(5)
Pb(1)−O(6)
2.645(2)
2.749(2)
2.720(2)
2.607(3)
3.0619(8)
2.592(2)
2.845(3)
2.912(2)
2.864(2)
O(1)−Pb(1)−N(4)
O(1)−Pb(1)−N(2)
N(4)−Pb(1)−N(2)
O(1)−Pb(1)−N(3)
N(4)−Pb(1)−N(3)
N(2)−Pb(1)−N(3)
N(3)−Pb(1)−S(1)
O(1)−Pb(1)−S(1)
N(4)−Pb(1)−S(1)
N(2)−Pb(1)−S(1)
167.56(7)
65.65(7)
125.89(7)
127.08(7)
64.90(8)
61.53(7)
69.04(6)
99.26(5)
81.48(6)
73.36(5)
Figure 2. Onsager plot of conductivity data for 4-NO3, 4-ClO4, the
1:1 standard Me4NClO4, and the 1:2 standard [(bnpapa)Ni](ClO4)2
in acetonitrile.
a
Estimated standard deviations in the last significant figure are given in
parentheses.
coordinated by the N3S-donor set along with the amide oxygen
atom. The Pb−N(chelate) distances found in 4-NO3
(2.607(2)−2.749(2) Å) are similar to the ranges found in
Pb(II) complexes of N4-donor chelate ligands ([(H2tpaa)Pb−
Cl] (2.646(6)−2.803(6) Å),40 [(TQA)Pb(NO3)2] (2.549(3)−
2.680(3) Å),41 and [(6-Me3TPA)Pb(NO3)]NO3 (2.546−2.677
Å)).42 The Pb(II)-S(thioether) distance of 3.0619(8) Å is
similar to the distances found in other Pb(II) thioether-
containing structures (e.g., [Pb([18]aneO4S2)(H2O)2(κ2-
BF4)]BF4, 3.0151(8), 3.0191(7) Å;43 [Pb([18]aneO4S2)-
(ClO4)2], 3.091(1) Å;44 [Pb([18]aneO4S2)(NO3)2], 3.114(6),
3.131(6) Å;44 [Pb([15]aneO3S2)2](BF4)2, 3.022(2)−3.116(2)
Å;43 [Pb([9]aneS3)2(ClO4)2], avg 3.076 Å45). In terms of the
amide coordination, the Pb(II)-O(amide) distance of 2.645(2)
Å is significantly longer than M(II)-O(amide) bonds in Group
perchlorate anions. These anions are likely replaced with
coordinated solvent ligands in acetonitrile-containing solutions
of the complex. The H NMR spectrum of 4-ClO4 in CD3CN
1
is shown in Figure 3a. The signals for the chelate ligand are
shifted downfield relative to the free ligand and are sharp, with
1
well-resolved H−1H coupling. These features are consistent
with chelate ligand coordination to the Pb(II) center in a
structure that is not fluxional on the NMR time scale. The
appearance of four distinct doublets for the benzylic protons, as
well as inequivalent protons for the methylene units of the
thioether appendage, is consistent with retention of the chelate
ligand coordination mode found in the solid state. Coupling
between 207Pb and the protons of the ligand could not be
1
clearly identified in the H NMR spectrum of 4-ClO4.
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Figure 3. 1H NMR spectral features of (a) 4-ClO4 and (b) 4-NO3 in CD3CN at ambient temperature. The asterisks denote trace diethyl ether that
1
was present in the H NMR sample.
Scheme 2
Notably, the 1H NMR spectrum of 4-NO3 differs in terms of
the chemical shifts of several of the resonances (Figure 3b).
Most notable is broadening and upfield shifts (relative to 4-
ClO4) for the proton resonances of the benzylic positions and
thioether appendage. Suspecting that this may indicate
interactions between the Pb(II) center and the nitrate
counterions,2,43 we investigated 4-NO3 via conductivity
measurements (Figure 2). Unlike 4-ClO4, the nitrate analogue
is not a 1:2 electrolyte, but instead exhibits 1:1 behavior,
indicating that one of the two nitrate anions remains
coordinated in acetonitrile solution. In the amide cleavage
reactivity studies outlined below, we have only employed 4-
ClO4.
with Me4NOH·5H2O in CD3CN:CD3OD (3:5) also results in
amide cleavage (Scheme 2). The products of this reaction,
including d3-methyltrimethylacetate, were identified and
quantified using approaches as previously described.19,22 We
have previously reported that exposure of free chelate ligands
containing the 6-pivaloylamido group to Me4NOH·5H2O in
CD3CN:CD3OD (3:5) does not result in amide cleavage in the
time required for completion of the amide methanolysis
reactions of the divalent metal complexes.21
Kinetic Studies. Using methods previously employed for
kinetic studies of the amide methanolysis reaction of 3-ClO4,22
we collected rate data for the amide methanolysis reaction that
occurs upon treatment of 4-ClO4 with Me4NOH·5H2O in
CD3CN:CD3OD (3:5). Pseudo first-order rate constants
(Table 3) were determined from slopes of plots of ln[4′-
ClO4] versus time. A typical correlation coefficient for these
first-order plots was ≥0.992. At a given temperature, the
Amide Cleavage Reactivity. Product Identification. We
have previously reported that upon treatment with Me4NOH·
5H2O in CD3CN:CD3OD (3:5), 3-ClO4 undergoes amide
methanolysis as shown in Scheme 1c.22 Treatment of 4-ClO4
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Table 3. Rate Constants for Amide Methanolysis Reaction of
4-ClO4 in CD3CN:CD3OD (3:5)
temp (K)
4-ClO4 (M)
[CD3OD] (M)
kobs (sec−1
)
295
288
281
274
295
295
295
0.020
0.020
0.020
0.020
0.020
0.020
0.020
15.39
15.39
15.39
15.39
9.23
8.07(43) × 10−4
4.98(34) × 10−4
2.47(21) × 10−4
1.20(6) × 10−4
7.42(43) × 10−4
5.27(35) × 10−4
1.50(4) × 10−4
0.92
0.21
observed rate of amide methanolysis was ∼10-fold faster for the
Pb(II) complex versus the Zn(II)-containing analogue.22
Examination of the rate of amide methanolysis as a function
of the concentration of CD3OD (0.21−15.39 M) in CD3CN
revealed additional differences between the reactions of 3-ClO4
and 4-ClO4. Specifically, the rate of amide methanolysis for the
zinc complex 3-ClO4 exhibits a linear correlation with
increasing methanol concentration (Figure 4), and therefore
Figure 5. Erying plots for the amide methanolysis reactions of 3-ClO4
and 4-ClO4.
Table 4. Thermodynamic Data for Amide Methanolysis
Reactions of 3-ClO4 and 4-ClO4 in CD3CN:CD3OD (3:5)
a
b
3-ClO4
4-ClO4
ΔH⧧ (kcal/mol)
17.3(2)
−24(1)
−7.2(3)
24.5(5)
d
16.0(6)
−17(2)
−5.2(7)
21.3(9)
ΔS⧧ (cal/(mol·K))
c
TΔS⧧ (kcal/mol)
d
ΔG⧧ (kcal/mol)
a
b
c
Reference 22. This work. 298 K. Calculated from ΔG = ΔH −
TΔS.
Solvent kinetic isotope effect (SKIE) experiments using
CD3OD and CD3OH revealed a SKIE (kH/kD) of 1.4 for the
amide methanolysis reaction of 3-ClO4. By contrast, the amide
methanolysis reaction of the Pb(II)-containing 4-ClO4 shows
no solvent kinetic isotope effect.
Evaluation of Possible Intermediates in the Amide
Methanolysis Reactions. As we have previously reported,
treatment of 3-ClO4 with Me4NOH·5H2O in acetonitrile or a
CD3CN:CD3OD (3:5) mixture initially results in the formation
of a deprotonated amide species (3′-ClO4, Scheme 3).22 This
1
complex has been isolated and characterized by H and 13C
NMR, IR, and elemental analysis.22 Unique H NMR spectral
1
Figure 4. Plot of [CD3OD] versus kobs for the amide alcoholysis
reactions of 3-ClO4 and 4-ClO4. The saturation curve for 4-ClO4 is fit
according to eq 1.
features of this compound relative to the starting material
include the absence of an N-H resonance and significant upfield
shifts of the resonances associated with the β-protons of the
amide-appended pyridyl ring (>0.45 ppm) and the t-butyl (0.22
ppm) protons. The upfield shifts of these resonances are
consistent with enhanced electron density within the
deprotonated amide unit, which is delocalized into the pyridyl
ring.19 Additionally, proton resonances associated with the
thioether appendage are shifted upfield relative to their position
in 3-ClO4. This change suggests a weakening of the Zn(II)-
S(thioether) interaction in 3′-ClO4, which is supported by
density functional theory (DFT) calculations reported herein
(vide infra). In terms of IR features, 3′-ClO4 does not exhibit a
1646 cm−1 vibration that is assigned to the coordinated amide
carbonyl unit in the starting amide complex. Combined, these
spectral features are consistent with the formation of a
deprotonated amide species. Upon introduction of methanol,
3′-ClO4 undergoes reaction to give amide methanolysis
products, providing evidence that it is a viable intermediate in
the amide methanolysis pathway of 3-ClO4.
overall second order behavior (rate = k[3-ClO4][CD3OD]).
However, the Pb(II) complex 4-ClO4 instead exhibits
saturation behavior (Figure 4), indicating a reaction that is
more complicated than a single step. The saturation behavior is
consistent with a reaction pathway involving an equilibrium
prior to an irreversible step having a rate constant k2 as shown
in eq 1. The calculated values of K1 and k2 from the fitting of
this equation to the variable [CD3OD] concentration data for
4-ClO4 gave K1 = 1.5(4) and k2 = 8.3(5) × 10−4 M−1 s−1.
k2k1[CD3OD]
1 + K1[CD3OD]
kobs
=
(1)
Eyring plots were constructed for the amide methanolysis
reactions of 3-ClO4 and 4-ClO4 (Figure 5). Activation
parameters derived from these plots are given in Table 4.
This data reveals that the slightly faster rate of methanolysis for
the Pb(II) complex 4-ClO4 is associated with lower values of
enthalpy and entropy of activation. Both complexes exhibit ΔS⧧
values that are consistent with an intramolecular rate-
determining step.20
Treatment of 4-ClO4 with 1 equiv of Me4NOH·5H2O in
acetonitrile or CD3CN:CD3OD (3:5) results in the initial
formation of a new species, 4′-ClO4. This complex has a t-butyl
1H NMR resonance that is shifted slightly upfield (0.09 ppm)
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Scheme 3
Figure 6. 1H NMR spectral features of (a) 4-ClO4 and (b) 4′-ClO4 in CD3CN at ambient temperature. The asterisks denote trace diethyl ether that
1
was present in the H NMR sample. Water and CH2Cl2 impurities are also denoted in the spectra.
Scheme 4
relative to the chemical shift of the same resonance in the
starting material, but does not exhibit significant upfield shifts
for any other protons of the amide-appended pyridyl ring
(Figure 6). A broad N−H resonance is present in the 1H NMR
spectrum of 4′-ClO4 at ∼8.8 ppm. Combined, these results
suggest that 4′-ClO4 does not contain a deprotonated amide
moiety. This conclusion is supported by the solid state IR
spectral features of 4′-ClO4 wherein an amide carbonyl
stretching vibration is still present at ∼1658 cm−1. A notable
difference in the H NMR spectrum of 4′-ClO4 versus that of
4-ClO4 is that the former shows significant changes in the
1
multiplicity of signals for the methylene protons of the
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Figure 7. DFT-optimized structures of reactant, intermediates, transition states, and product in the amide methanolysis reaction pathway of 3-ClO4.
thioether appendage, which suggests a loss of diastereotopic
character for these protons (Figure 6), perhaps due weakening
or loss of the Pb(II)-S(thioether) interaction upon anion
coordination. This change in the Pb(II)-S(thioether) inter-
action is supported by DFT calculations described herein (vide
infra).
A possible structure for 4′-ClO4 is an epppa-ligated Pb(II)-
OH or Pb(II)-OCD3 species (Scheme 4). Evidence for a
mixture of such species in the amide methanolysis reaction of
4-ClO4 is the formation of a small amount of amide hydrolysis
product (identified via a trimethylacetate t-butyl resonance at
0.96 ppm; verified via treatment of 4-ClO4 with CD3CN:D2O
(3:5) which results in exclusively this hydrolysis product).
Importantly, treatment of isolated 4′-ClO4 with CD3OD results
in amide methanolysis, providing evidence that this compound
is a viable intermediate in the amide methanolysis reaction
pathway.
Computational Studies. The amide methanolysis path-
ways of 3-ClO4 and 4-ClO4 were modeled using density
functional theory (DFT) calculations to correlate computed
energetics with the results of the kinetic studies described
above, as well as to provide insight into the mechanisms.
Structures of geometry optimized reactants, transition states,
and intermediates in the respective amide cleavage pathways of
3-ClO4 and 4-ClO4 are provided in Figures 7 and 8,
respectively. Table 5 lists the M(II)/chelate ligand bond
distances in each of these structures. The potential energy
surface (PES) diagrams of these pathways are shown in Figure
9. The starting reactant complex for the calculated pathway of
each metal ion is a deprotonated amide species containing a
coordinated methanol, [(epppa−)Zn(CH3OH)]+ (RZn) and
[(epppa−)Pb(CH3CN)2(CH3OH)]+ (RPb). RZn was developed
from the X-ray structural data of [(epppa)Zn](ClO4)2 (3-
ClO4),22 followed by geometry optimization for a structure that
exhibits amide deprotonation and methanol coordination. RPb
was developed from the X-ray crystallographic data of 4-NO3.
Two molecules of coordinated acetonitrile were found to
optimally model the solvated form of the [(epppa)Pb]2+ cation
based on comparisons to the X-ray crystallographically
determined bond distances involving the epppa chelate ligand
i n 4 - N O 3 . T h e s t r u c t u r e o f [ ( e p p p a − ) P b -
(CH3CN)2(CH3OH)]+ (RPb, Figure 8), wherein the amide
has been deprotonated and a coordinated methanol has been
added, was then geometry optimized.
In the optimized structure of the deprotonated amide Zn(II)
reactant complex (RZn), the coordinated methanol molecule
interacts with coordinated amide oxygen via a hydrogen bond
(Figure 7). This structure contains a six-coordinate Zn(II)
center. The coordination number of the Zn(II) ion is 5 or 6
throughout the reaction pathway. The long Zn−S(thioether)
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Figure 8. DFT-optimized structures of reactant, intermediates, transition states, and product in the reaction pathway leading to amide methanolysis
of 4-ClO4.
distance (3.01 Å) in RZn suggests a weak interaction as, while it
is within the sum of the van der Waals radii (3.19 Å) of the two
atoms, the observed distance is >0.7 Å longer than that found
in 3-ClO4. This elongation of the Zn(II)-S(thioether)
Comparison of the amide C−N bond distances in In2Zn (1.49
Å) and In1Zn (1.38 Å) reveals elongation that is favorable for
the subsequent amide cleavage step. A second molecule of
methanol that is required for amide cleavage was then added to
In2Zn to form In3Zn. In the optimized structure of In3Zn, the
additional methanol interacts with the amide N−H via a
hydrogen bond (Figure 7). From In3Zn, which is endergonic by
3.7 kcal/mol relative to RZn, proton transfer from methanol to
the amide nitrogen leads to cleavage of the amide C−N bond.
This transformation happens with a barrier of 17.6 kcal/mol
from In3Zn and an overall barrier of 21.3 kcal/mol from RZn. In
the final product (PZn), the Zn(II) center coordinates a
methoxide anion and is supported by the primary amine-
containing chelate ligand (eappa). One equivalent of a
noncoordinated carboxyl ester (methyltrimethylacetate) is
also generated.
1
interaction is consistent with the H NMR features of the
thioether appendage of 3′-ClO4 as discussed above. The other
zinc-ligand distances in RZn are generally elongated to a lesser
extent (<0.3 Å) upon amide deprotonation and introduction of
the anionic charge into the metal coordination environment.
The first transition state investigated was for proton transfer
from the coordinated methanol to the deprotonated amide of
the chelate ligand. This step proceeds with an activation barrier
of 22.4 kcal/mol to give In1Zn, a structure containing a neutral
amide and a coordinated methoxide anion. This intermediate is
endergonic by 8.1 kcal/mol relative to RZn. The Zn(II)-chelate
ligand distances in In1Zn are slightly elongated relative to RZn
(Table 5). Nucleophilic attack of the methoxide ligand on the
amide carbonyl carbon results in the formation of the
tetrahedral intermediate In2Zn. Notably, this transformation
results in a decrease in the Zn−S(thioether) distance to 2.69 Å.
Similar to the X-ray structure of 4-NO3, the geometry
optimized structure of the deprotonated amide Pb(II) reactant
complex (RPb) exhibits coordination of the chelate ligand with
coordination space available for the binding of two solvent
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Table 5. Structural Data for DFT-Calculated Structures (Å)
complex
M-S
M-N4
M-N2
M-N3
M-O1
a
[(epppa)Zn](ClO4)2
2.399(3)
3.01
1.983(8)
2.14
2.0470(16)
2.09
2.169(8)
2.29
1.998(7)
2.01
RZn
TS1Zn
In1Zn
TS2Zn
In2Zn
In3Zn
TS3Zn
PZn
3.19
2.11
2.10
2.36
2.36
3.12
2.15
2.24
2.27
2.17
2.86
2.13
2.16
2.36
2.10
2.69
2.12
2.12
2.33
1.97
2.67
2.10
2.07
2.34
1.93
2.83
2.18
2.17
2.39
2.05
2.74
2.11
2.11
2.32
1.93
b
[(epppa)Pb(NO3)2]
3.0619(8)
3.11
2.607(2)
2.91
2.749(2)
2.48
2.720(2)
2.74
2.645(2)
2.31
RPb
TS1Pb
In1Pb
TS2Pb
In2Pb
In3Pb
TS3Pb
PPb
3.30
2.90
2.56
2.95
2.81
3.59
2.60
2.78
2.62
2.71
3.26
3.06
2.57
2.96
2.29
3.05
3.00
2.54
2.86
2.19
3.07
2.93
2.46
2.74
2.14
3.36
3.08
2.62
3.02
2.23
3.38
2.86
2.56
2.86
2.19
a
b
Ingle, G. K.; Makowska-Grzyksa, M. M.; Szajna-Fuller, E.; Sen, I.; Price, J. C.; Arif, A. M.; Berreau, L. M. Inorg. Chem. 2007, 46, 1471−1480. This
work.
endergonic relative to RPb. As was seen in the Zn(II) system,
the Pb−S(thioether) interaction is stronger in In2Pb (3.05 Å)
and the C−N bond is elongated in In2Pb (1.48 Å) relative to
In1Pb (1.39 Å). Once again, a second molecule of methanol
required for amide cleavage was added to In2Pb to form In3Pb.
In the optimized structure of In3Pb, the additional methanol
interacts with the amide N−H via a hydrogen bond (Figure 8).
From In3Pb, which is endergonic by 6.7 kcal/mol relative to
RPb, proton transfer from methanol to the amide nitrogen leads
to cleavage of the amide C−N bond. This transformation
happens with a barrier of 4.7 kcal/mol from In3Pb and an
overall barrier of 11.4 kcal/mol from RPb. In the final product
(PPb), the Pb(II) center coordinates a methoxide anion and is
supported by the primary amine-containing chelate ligand
(eappa). One equivalent of a noncoordinated carboxyl ester
(methyltrimethylacetate) is also generated.
Based on the observation of small amounts of amide
hydrolysis product in the amide methanolysis reaction of 4-
ClO4, we also used DFT calculations to examine the relative
energies of the Pb(II)-OCH3 species (In1Pb) and a Pb(II)−OH
analogue. We found that the hydroxide compound is only 1.04
kcal/mol lower in energy in the gas phase, suggesting that these
two compounds can form an acid/base equilibrium (Figure 10).
Notably, the calculated free energy difference between these
two compounds matches well with that derived for the
equilibrium component in the saturation kinetics of the
amide methanolysis of 4-ClO4 (0.2 kcal/mol).
Figure 9. Free energy profiles for the amide methanolysis reactions for
3-ClO4 and 4-ClO4.
molecules. The coordination number of the Pb(II) ion is 7 or 8
throughout the reaction pathway. Proton transfer to the
deprotonated amide from a coordinated methanol occurs in
this case with an activation barrier of 18.9 kcal/mol to give
In1Pb. This intermediate is 7.2 kcal/mol exergonic relative to
RPb. The structure of In1Pb contains a neutral, coordinated
amide as well as a coordinated methoxide anion. The Pb(II)-
S(thioether) distance in In1Pb is 3.59 Å which, while within the
sum of the van der Waals radii of the two atoms, represents a
significant elongation relative to the same interaction in RPb
(3.11 Å). Notably, the elongation of the Pb(II)-S(thioether)
interaction in In1Pb is consistent with the observed changes in
the 1H NMR spectral features of the thioether appendage of 4′-
ClO4 versus the starting 4-ClO4 complex. Nucleophilic attack
of the methoxide at the amide carbonyl carbon occurs with a
barrier of 21.9 kcal/mol from In1Pb and 14.7 kcal/mol relative
to RPb. The intermediate formed, In2Pb, is 10.5 kcal/mol
DISCUSSION AND CONCLUSIONS
■
Amide cleavage reactions catalyzed by enzymes containing a
mononuclear zinc center are well-known in biological
systems.50 Though Pb(II) substitution for the active site zinc
center has not been reported for an amide hydrolase to date,
the general properties introduced by heavy metal substitution
into zinc proteins is of current interest.11 The amide hydrolysis
reactivity of Zn(II) in biological systems is often proposed to
involve the formation of a Zn(II)−OH nucleophile.20 The
involvement of Pb(II)−OH as a general base catalyst in tRNA
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Figure 10. DFT optimized structures of In1Pb (right) and a Pb−OH analogue and the equilibrium that relates these compounds.
hydrolysis has been proposed,51,52 as well as a Pb(II)−OH
acting as a nucleophile in the hydrolysis of uridine 2′,3′-cyclic
monophosphate.53 In the experiments described herein, we
have examined in detail the coordination chemistry and amide
methanolysis reaction pathways of mononuclear Pb(II) and
Zn(II) complexes of an amide-appended chelate ligand.
Pb(II) and Zn(II) differ in a number of important ways.
First, the ionic radius for the former is considerably larger
(Pb(II) 1.19 Å; Zn(II) 0.74 Å).47 This is reflected in the higher
coordination numbers that are typically found for Pb(II)
complexes versus Zn(II) analogues, including compound 4-
NO3 versus 3-ClO4, which exhibit overall coordination
numbers in the solid state of nine and five respectively.22 In
both of these compounds, the epppa chelate ligand is
coordinated in a κ5-fashion. However, whereas the coordination
of the chelate ligand in 4-NO3 leaves available positions for
anion or solvent coordination, in 3-ClO4 the chelate ligand fully
encapsulates the metal center. This is also the case for the
geometry-optimized structures of the deprotonated amide
species RPb and RZn. As outlined below, we propose that it is
this difference in accessibility of coordination positions at the
Pb(II) center in 4-ClO4 that enables a reaction pathway for
amide cleavage in this complex that initially differs from that of
the Zn(II) analogue 3-ClO4.
analogue 3-ClO4. However, differences in the influence of
methanol concentration, saturation kinetics for 4-ClO4 versus
second-order behavior for 3-ClO4, and the identification of a
small solvent deuterium kinetic isotope effect identified only for
the reaction involving 3-ClO4, suggest differences in the
reaction pathway. This led us to carefully investigate
spectroscopically identifiable intermediates in the reaction
pathways, as well as to examine the reaction as a function of
metal ion via DFT calculations.
Upon introduction of 1 equiv of Me4NOH·5H2O to
acetonitrile solutions of 4-ClO4 and 3-ClO4 different isolable
1
products are generated, as determined by H NMR and IR.
Whereas the Zn(II) complex 3-ClO4 undergoes amide
deprotonation to give 3′-ClO4, no evidence was identified for
amide deprotonation in the Pb(II) analogue. Instead, the
spectroscopic properties of the intermediate suggest the
formation of a mixture of epppa-ligated Pb(II)-OH and
Pb(II)-OCH3 species containing a neutral amide. This
difference in reactivity with Me4NOH·5H2O is a consequence
of the available coordination sites for anion binding in 4-ClO4,
whereas anion coordination to 3-ClO4 is considerably more
difficult because of the more encapsulating chelate ligand
coordination motif. This is reflected in the ∼15 kcal/mol
difference in energy between In1Pb and In1Zn.
Pb(II) and Zn(II) are also similar in a number of important
ways. According to Pearson’s classification of hard and soft
acids and bases, both are considered intermediate acids and
therefore bind a variety of donor atoms.54 In the X-ray crystal
structures of 4-NO3 and 3-ClO4, each metal ion binds a
mixture of nitrogen, oxygen, and sulfur donors from the chelate
ligand, with the Pb(II) complex exhibiting longer bond
distances because of the larger size of the metal ion. The
multidentate chelate nature of the epppa ligand ensures that the
metal ion in 4-ClO4 and 3-ClO4 is not released during the
transformations involved in amide cleavage.
Treatment of 4-ClO4 or 3-ClO4 with 1 equiv of Me4NOH·
5H2O in CD3CN:CD3OD (3:5) results in amide methanolysis
and the formation of similar products. Thus, regardless of the
divalent metal ion present, the same amide methanolysis
products are obtained. Variable temperature kinetic studies of
the reactions revealed only small differences in the activation
parameters, with the ΔG⧧ for the Pb(II)-containing 4-ClO4
being ∼3 kcal/mol lower than that found for the Zn(II)
DFT calculations indicate that both 4-ClO4 and 3-ClO4 may
feasibly undergo amide methanolysis starting from a deproto-
nated amide species, with activation barriers for the rate-
determining step that are generally consistent with exper-
imentally determined free energies of activation. For example,
for the Zn(II)-containing 3-ClO4, the rate-determining step
would involve either protonation of the deprotonated amide, or
protonation of the amine leaving group, as both steps have
similar activation barriers (22.4 and 21.3 kcal/mol, respec-
tively), which are similar to the experimentally determined free
energy of activation (ΔG⧧ = 24.5 kcal/mol). We note that these
activation barriers are also similar to the ΔG⧧ values recently
reported for the methanolysis of an internal amide substrate in
a Cu(II) complex.55 The small observed normal SKIE identified
for the reaction of 3-ClO4 (1.4) may be consistent with
intramolecular proton transfer from a coordinated methanol, or
general acid assistance by a solvent molecule for departure of
the leaving group.55 General base assistance by a Zn(II)-OCH3
moiety in accepting a proton from an exogenous solvent
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Scheme 5
molecule that would nucleophilically attack the carbonyl would
be expected to exhibit a larger SKIE.56
departure, relative to the Zn(II) analogue (Figure 9). These
effects likely relate to a more flexible metal coordination
environment for Pb(II), wherein a greater range of metal−
ligand distances may be accessed during the amide cleavage
reaction.59
Overall, the purpose of this study was to evaluate how the
presence of Pb(II) versus Zn(II) would influence an amide
cleavage reaction. Our results indicate that Pb(II) may serve as
a Lewis acid to promote amide cleavage and lower the energy
barrier for key steps in the reaction pathway. The differences in
the reaction pathway as compared to a Zn(II) analogue relate
to the inherent chemical properties of this heavy metal ion.
The DFT studies indicate that the reaction of Pb(II)-
containing 4-ClO4 starting from a deprotonated amide species
(RPb) would involve rate-limiting amide protonation from a
coordinated methanol, a reaction with a calculated activation
barrier of 18.9 kcal/mol. This value is comparable to the
experimentally determined free energy of activation (21.3 kcal/
mol). However, based on the lack of evidence for the initial
formation of a deprotonated amide species in the reaction of 4-
ClO4 upon treatment with Me4NOH·5H2O, we propose that
this reaction proceeds via the direct formation of a Pb(II)−OH
species [(epppa)Pb−OH(sol)n]+ (sol = CH3CN or CH3OH; n
= 1−2), that equilibrates with a corresponding Pb(II)-OCH3
(In1Pb) species. The equilibration of these two compounds is
consistent with the observed saturation kinetic data wherein the
Pb(II)−OH/Pb(II)-OCH3 equilibrium precedes nucleophilic
attack of the methoxide on the coordinated amide. The
calculated activation barrier associated with this nucleophilic
attack (In1Pb → TS2Pb, 21.9 kcal/mol) is a very good match
with the experimentally determined activation barrier (21.3
kcal/mol). The lack of an observed SKIE is consistent with this
step being rate-determining, as no proton is in flight.56
Therefore, we propose that the amide methanolysis reactions
of 3-ClO4 and 4-ClO4, while giving the same products, proceed
via different initial reactants, 3′-ClO4 (modeled as RZn) and 4′-
ClO4 (modeled as In1Pb) that are the result of the differences
in the size and coordination preferences of Zn(II) versus Pb(II)
(Scheme 5). That being said, in both cases the metal center is
likely playing multiple roles in the amide cleavage reaction.
These include electrophilic activation of the carbonyl via
coordination of the oxygen atom and stabilization of the
nucleophile.57,58 In terms of Pb(II), our calculations indicate
that the larger Pb(II) ion more effectively stabilizes the double
Lewis activated structure having both amide and nucleophile
coordination (In1Pb). Notably, the Pb(II) center also
substantially lowers the activation barrier for leaving group
ASSOCIATED CONTENT
■
S
* Supporting Information
Analysis of solid-state IR features of 4-ClO4 versus 3-ClO4; rate
constant data for the amide methanolysis of 3-ClO4; CIF data
for 4-NO3; complete reference 35; atomic coordinates for all
reactants, transition states, intermediates and products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lisa.berreau@usu.edu. Phone: (435) 797-3509. Fax:
(435) 797-3390.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Utah State University Office of Research and
Graduate Studies for a SURCO grant (to E.S.E.) as well as the
Utah State University Department of Chemistry and
Biochemistry for financial support. R.P. thanks the U.S.
National Science Foundation (CHE-1152846) for funding.
11491
dx.doi.org/10.1021/ic401782x | Inorg. Chem. 2013, 52, 11480−11492

Inorganic Chemistry
Article
(33) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119.
REFERENCES
■
(1) CDC, Morbidity and Mortality Weekly Report; Center for Disease
Control and Prevention: Atlanta, GA, 2005; pp 513−516.
(2) Claudio, E. S.; Godwin, H. A.; Magyar, J. S. Prog. Inorg. Chem.
2003, 51, 1−144.
(3) Bridgewater, B. M.; Parkin, G. J. Am. Chem. Soc. 2000, 122,
7140−7141.
(4) Magyar, J. S.; Weng, T.-C.; Stern, C. M.; Dye, D. F.; Rous, B. W.;
Payne, J. C.; Bridgewater, B. M.; Mijovilovich, A.; Parkin, G.; Zaleski, J.
M.; Penner-Hahn, J. E.; Godwin, H. A. J. Am. Chem. Soc. 2005, 127,
9495−9505.
(5) Andersen, R. J.; diTargiani, R. C.; Hancock, R. D.; Stern, C. L.;
Goldberg, D. P.; Godwin, H. A. Inorg. Chem. 2006, 45, 6574−6576.
(6) Farrer, B. T.; Pecoraro, V. L. Curr. Opin. Drug Discovery Dev.
2002, 5, 937−943.
(34) Sheldrick, G. M. SHELXL-97; University of Gottingen:
̈
Gottingen, Germany, 1997.
̈
(35) Frisch, M. J., et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT,
2009.
(36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(37) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(39) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem.
1998, 37, 1853−1867.
̀ ̀
(40) Pellissier, A.; Bretonniere, Y.; Chatterton, N.; Pecaut, J.;
Delangle, P.; Mazzanti, M. Inorg. Chem. 2007, 46, 3714−3725.
(41) Williams, N. J.; Gan, W.; Reibenspies, J. H.; Hancock, R. D.
Inorg. Chem. 2009, 48, 1407−1415.
(7) Matzapetakis, M.; Ghosh, D.; Weng, T. C.; Penner-Hahn, J. E.;
Pecoraro, V. L. J. Biol. Inorg. Chem. 2006, 11, 876−890.
(8) Neupane, K. P.; Pecoraro, V. L. Angew. Chem., Int. Ed. 2010, 49,
8177−8180.
(42) Zhang, Z.; Xinahe, B.; Zhu, Z.; Chen, R. Chin. J. Inorg. Chem.
1997, 13, 58.
(43) Farina, P.; Latter, T.; Levason, W.; Reid, G. Dalton Trans. 2013,
42, 4714−4724.
(9) Neupane, K. P.; Pecoraro, V. L. J. Inorg. Biochem. 2011, 105,
1030−1034.
(44) Park, I.-H.; Park, K.-M.; Lee, S. S. Dalton Trans. 2010, 39,
9696−9704.
(10) Chakraborty, S.; Kravitz, J. Y.; Thulstrup, P. W.; Hemminsen, L.;
DeGrado, W. F.; Pecoraro, V. L. Angew. Chem., Int. Ed. 2011, 50,
2049−2053.
(45) Kuppers, H.-J.; Weighardt, K.; Nuber, B.; Weiss, J. Z. Anorg. Allg.
Chem. 1989, 577, 155−164.
(46) Makowska-Grzyska, M. M.; Doyle, K.; Allred, R. A.; Arif, A. M.;
Bebout, D. C.; Berreau, L. M. Eur. J. Inorg. Chem. 2005, 822−827.
(47) Shannon, R. D. Acta Crystallogr. 1976, A32, 751−767.
(48) Parkin, G. Adv. Inorg. Chem. 1995, 42, 291−393.
(49) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 5th ed.; John Wiley & Sons, Inc.: New York,
1997; Vol. Part B.
(11) Zampella, G.; Neupane, K. P.; De Gioia, L.; Pecoraro, V. L.
Chem.Eur. J. 2012, 18, 2040−2050.
(12) Zitka, O.; Kukacka, J.; Krizkova, S.; Huska, D.; Adam, V.;
Masarik, M.; Prusa, R.; Kizek, R. Curr. Med. Chem. 2010, 17, 3751−
3768.
(13) Gantt, S. L.; Gattis, S. G.; Fierke, C. A. Biochemistry 2006, 45,
6170−6178.
(50) Lipscomb, W. N.; Strater, N. Chem. Rev. 1996, 96, 2375−2434.
̈
(14) Dowling, D. P.; Gattis, S. G.; Fierke, C. A.; Christianson, D. W.
Biochemistry 2010, 49, 5048−5056.
(51) Brown, R. S.; Hingerty, B. E.; Dewan, J. C.; Klug, A. Nature
1983, 303, 543−546.
(15) Kita, Y.; Nishii, Y.; Higuchi, T.; Mashima, K. Angew. Chem., Int.
Ed. 2012, 51, 5723−5726.
(52) Brown, R. S.; Dewan, J. C.; Klug, A. Biochemistry 1985, 24,
4785−4801.
(16) Yashiro, M.; Kawakami, Y.; Taya, J.; Arai, S.; Fujii, Y. Chem.
Commun. 2009, 1544−1546.
(53) Kuusela, S.; Lonnberg, H. J. Phys. Org. Chem. 1992, 5, 803−811.
(54) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533−3539.
(55) Raycroft, M. A.; Maxwell, C. I.; Oldham, R. A.; Andrea, A. S.;
Neverov, A. A.; Brown, R. S. Inorg. Chem. 2012, 51, 10325−10333.
(56) Maxwell, E.; Neverov, A. A.; Brown, R. S. Org. Biomol. Chem.
2005, 3, 4329−4336.
(17) Yashiro, M.; Sonobe, Y.; Yamamura, A.; Takarada, T.;
Komiyama, M.; Fujii, Y. Org. Biomol. Chem. 2003, 1, 629−632.
(18) Groves, J. T.; Chambers, R. R. J. Am. Chem. Soc. 1984, 106,
630−638.
(19) Szajna, E.; Makowska-Grzyska, M. M.; Wasden, C. C.; Arif, A.
M.; Berreau, L. M. Inorg. Chem. 2005, 44, 7595−7605.
(20) Berreau, L. M. Adv. Phys. Org. Chem. 2006, 41, 79−181.
(21) Berreau, L. M.; Makowska-Grzyska, M. M.; Arif, A. M. Inorg.
Chem. 2000, 39, 4390−4391.
(22) Ingle, G. K.; Makowska-Grzyska, M. M.; Szajna-Fuller, E.; Sen,
I.; Price, J. C.; Arif, A. M.; Berreau, L. M. Inorg. Chem. 2007, 46, 1471−
1480.
(57) Sayre, L. M. J. Am. Chem. Soc. 1986, 108, 1632−1635.
(58) Hegg, E. L.; Burstyn, J. N. Coord. Chem. Rev. 1998, 173, 133−
165.
(59) Harrowfield, J. Helv. Chim. Acta 2005, 88, 2430−2433.
(23) Szajna-Fuller, E.; Ingle, G. K.; Watkins, R. W.; Arif, A. M.;
Berreau, L. M. Inorg. Chem. 2007, 46, 2353−2355.
(24) Battistuzzi, G.; Borsari, M.; Menabue, L.; Saladini, M.; Sola, M.
Inorg. Chem. 1996, 35, 4239−4247.
(25) Claudio, E. S.; ter Horst, M. A.; Forde, C. E.; Stern, C. L.; Zart,
M. K.; Godwin, H. A. Inorg. Chem. 2000, 39, 1391−1397.
(26) Clapp, L. A.; Siddons, C. J.; Whitehead, J. R.; VanDerveer, D.
G.; Rogers, R. D.; Griffin, S. T.; Jones, S. B.; Hancock, R. D. Inorg.
Chem. 2005, 44, 8495−8502.
(27) Bora, R. P.; Ozbil, M.; Prabhakar, R. J. Biol. Inorg. Chem. 2010,
15, 485−495.
(28) Kumar, A.; Zhu, X.; Walsh, K.; Prabhakar, R. Inorg. Chem. 2010,
49, 38−46.
(29) Grubel, K.; Fuller, A. L.; Chambers, B. M.; Arif, A. M.; Berreau,
L. M. Inorg. Chem. 2010, 49, 1071−1081.
(30) Allred, R. A.; McAlexander, L. H.; Arif, A. M.; Berreau, L. M.
Inorg. Chem. 2002, 41, 6790−6801.
(31) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335−A337.
(32) Wilkins, R. G., Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH Publishers: New York, 1991.
11492
dx.doi.org/10.1021/ic401782x | Inorg. Chem. 2013, 52, 11480−11492