Manganese(II) chemistry of a new N3O-donor chelate ligand: Synthesis, X-ray structures, and magnetic properties of solvent- And oxalate-bound complexes

Article abstract of DOI:10.1039/b500534e

The synthesis and characterization of a new N₃O donor ligand N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylmethyl)amine (bpppa) is reported. Treatment of bpppa with Mn(II)(ClO₄)₂· 6H₂O in acetonitrile solution yielded the mononuclear [(bpppa)Mn(CH₃CN)(H₂O)](ClO₄)₂ (1) which was characterized by X-ray crystallography, elemental analysis, IR spectroscopy, mass spectrometry, and a solution magnetic moment measurement. Admixture of equimolar equivalents of bpppa and Mn(II)(ClO₄) ₂·6H₂O in methanol solution, followed by addition of 0.5 or 1 equivalents of sodium oxalate, yielded the binuclear complex [{(bpppa)Mn}₂(μ-C₂O₄)](ClO₄) ₂ (2), which was characterized by X-ray crystallography, elemental analysis, IR spectroscopy, mass spectrometry, and solid-state magnetic measurements. While 1 is mononuclear, the formation of the binuclear oxalate derivative 2 indicates that use of the bpppa ligand does not enable isolation of a complex that is structurally relevant to a proposed 1: 1 Mn(II)-oxalate adduct in the catalytic cycle of the oxalate degrading enzyme oxalate decarboxylase. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b500534e

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F U L L P A P E R
Manganese(II) chemistry of a new N3O-donor chelate ligand:
synthesis, X-ray structures, and magnetic properties of solvent- and
oxalate-bound complexes
Amy L. Fuller,a Rex W. Watkins,a Kim R. Dunbar,b Andrey V. Prosvirin,b Atta M. Arifc and
Lisa M. Berreau*a
a Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill,
Logan, UT, 84322-0300, USA. E-mail: berreau@cc.usu.edu; Fax: +1 435-797-3390;
Tel: +1 435-797-1625
b Department of Chemistry, Texas A & M University, PO Box 30012, College Station,
TX, 77842-3012, USA. E-mail: dunbar@mail.chem.tamu.edu
c Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City,
UT, 84112-0850, USA. E-mail: arif@chemistry.utah.edu
Received 12th January 2005, Accepted 12th April 2005
First published as an Advance Article on the web 28th April 2005
The synthesis and characterization of a new N3O donor ligand N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-
N-(2-pyridylmethyl)amine (bpppa) is reported. Treatment of bpppa with Mn(II)(ClO4)2·6H2O in acetonitrile solution
yielded the mononuclear [(bpppa)Mn(CH3CN)(H2O)](ClO4)2 (1) which was characterized by X-ray crystallography,
elemental analysis, IR spectroscopy, mass spectrometry, and a solution magnetic moment measurement. Admixture
of equimolar equivalents of bpppa and Mn(II)(ClO4)2·6H2O in methanol solution, followed by addition of 0.5 or 1
equivalents of sodium oxalate, yielded the binuclear complex [{(bpppa)Mn}2(l-C2O4)](ClO4)2 (2), which was
characterized by X-ray crystallography, elemental analysis, IR spectroscopy, mass spectrometry, and solid-state
magnetic measurements. While 1 is mononuclear, the formation of the binuclear oxalate derivative 2 indicates that use
of the bpppa ligand does not enable isolation of a complex that is structurally relevant to a proposed 1 : 1
Mn(II)–oxalate adduct in the catalytic cycle of the oxalate degrading enzyme oxalate decarboxylase.
Introduction
Oxalic acid is biosynthesized in plants and microbes via enzyme-
mediated reactions of oxaloacetate, glyoxylate or L-ascorbic
acid.1–3 To regulate oxalate levels, plants and microbes have
oxalate-degrading enzymes, an example of which is oxalate
decarboxylase (OxdC).4 This enzyme catalyzes a Mn(II)/O2-
dependent carbon–carbon bond cleavage reaction that results
in the conversion of oxalate to CO2 and formate.4–6 The oxalate
decarboxylase from Bacillus subtilis crystallizes as a hexamer,
with each monomer unit comprising two domains.7,8 Each
domain contains a binding site for a mononuclear Mn(II) ion.
As shown in Fig. 1, the N-terminal domain Mn(II) center,
which is believed to be the dominant or sole site of catalytic
activity,8 is ligated in the resting state by a facial array of
three histidine residues (His95, His97 His140), one glutamate
Fig. 2 Possible coordination modes of oxalate anion in OxdC.
(Glu101), and two water molecules, to yield an overall distorted
octahedral geometry. It has been proposed that oxalate binds as
a monoanion in a unidentate fashion to the Mn(II) center via
However, a recent X-ray crystallographic study of a struc-
turally similar putative oxalate decarboxylase from Thermotoga
displacement of a metal-bound water molecule (Fig. 2(a)).9 Re-
action with O2, coupled with displacement of the second Mn(II)-
coordinated water, is proposed to yield a Mn(III) superoxide
maritim revealed bidentate oxalate coordination to the active
site manganese ion (Fig. 2(b)).10 In the chemical literature,
species. Deprotonation of the metal-bound oxalate is suggested
1 : 1 manganese–oxalate species have been proposed to exist
to initiate decarboxylation via a radical pathway.9
in aqueous solution.11 A search of the Cambridge Crystallo-
graphic Database (v. 5.25, July 2004) revealed that structurally
characterized examples of Mn(II) oxalate complexes, wherein
To date, an X-ray crystal structure of the OxdC from Bacillus
subtilis containing the oxalate substrate has not been reported.
four coordination positions of the Mn(II) ion are occupied by
biomimetic donors akin to the amino acid ligands found in
OxdC enzymes, are limited to two binuclear complexes where
the oxalate dianion is coordinated in a bridging bis-bidentate
type mode.12 For Co(III) and Cr(III), use of supporting chelate
amine-type ligands has enabled the isolation of mononuclear
2−
metal oxalate complexes having bidentate C2O4 coordination
to a single metal center,13,14 as was found in the putative OxdC
from T. maritim.
Fig. 1 Coordination environment of the active site Mn(II) center in
oxalate decarboxylase (OxdC).
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 8 9 1 – 1 8 9 6
1 8 9 1
©

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In this contribution, we outline studies of the man-
ganese(II) coordination chemistry of a new N3O-donor chelate
ligand, N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-
pyridylmethyl)amine (bpppa). Using this ligand, a mononuclear
six-coordinate Mn(II) complex, [(bpppa)Mn(CH3CN)(H2O)]-
(ClO4)2 (1) having two cis coordination sites occupied by
solvent molecules, has been prepared and structurally charac-
terized. Treatment of the bpppa ligand with Mn(ClO4)2·6H2O
in methanol followed by addition of sodium oxalate results in
the isolation of [{(bpppa)Mn}2(l-C2O4)](ClO4)2 (2), a binuclear
manganese(II) complex having bis-bidentate coordination of the
oxalate dianion.
Results and discussion
Synthesis of a new N3O-donor ligand
Chelate ligands containing a pyridyl donor having a 2-amido
substituent have been shown to coordinate to first-row metal
centers via the amide oxygen.15 As a first attempt toward
modeling the protein-derived N3O coordination environment of
the active site manganese center in OxdC, we have constructed
a new N3O-donor ligand containing a 2-pivaloylamido moiety
as the oxygen donor. Specifically, as shown in Scheme 1, treat-
ment of 2-(pivaloylamido)-6-(bromomethyl)pyridine16 with N-
benzyl-N-2-methylpyridine,17 followed by work-up and purifica-
tion using silica gel column chromatography, yielded the N3O-
donor ligand N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-
N-(2-pyridylmethyl)amine (bpppa) in 90% isolated yield. The
bpppa ligand is a pale yellow oil that is best stored under vacuum
in the freezer.
Scheme 2 Synthetic routes for preparation of 1 and 2.
been characterized by X-ray crystallography, elemental analysis,
IR spectroscopy, mass spectrometry, and a solution magnetic
moment measurement.
Stirring of the bpppa ligand with equimolar amounts of
Mn(ClO4)2·6H2O and sodium oxalate in methanol solution for
several days, followed by filtration and recrystallization from
CH3CN–diethyl ether, resulted in the crystallization of only
one complex, the bridging oxalate derivative [{(bpppa)Mn}2(l-
C2O4)](ClO4)2 (2) in 78% yield (Scheme 2). Thus, despite provid-
ing one equivalent of oxalate dianion per manganese ion in the
reaction mixture, the major product possesses a 2 : 1 manganese–
oxalate stoichiometry. For reporting purposes, we have repeated
the reaction using 0.5 equivalents of sodium oxalate and have
obtained 2 in 80% yield. Complex 2 has been characterized by
X-ray crystallography, elemental analysis, IR spectroscopy, mass
spectrometry, and solid-state magnetic measurements. While
complex 2 crystallizes with two non-coordinated molecules of
acetonitrile (2·2CH3CN), these solvates are lost upon drying of
the bulk sample.
X-Ray crystal structures of 1 and 2·2CH3CN
An ORTEP representation of the cationic portion of 1 is shown
in Fig. 3. Details of the data collection and refinement are given
Scheme 1 Synthetic route for preparation of the bpppa ligand.
Synthesis of Mn(II) solvent- and oxalate-bound complexes
Treatment of bpppa with an equimolar amount of
Mn(ClO4)2·6H2O in acetonitrile, followed by recrystallization
from acetonitrile–diethyl ether solution, yielded crystalline
[(bpppa)Mn(CH3CN)(H2O)](ClO4)2 (1) in 34% isolated yield
(Scheme 2). The low overall yield of crystalline complex is due
to the high solubility of 1 in acetonitrile solution. Complex 1 has
Fig. 3 ORTEP drawing of the cationic portion of 1. Ellipsoids are
plotted at the 50% probability level. All hydrogen atoms, except those
on the amide and manganese-coordinated water molecule, have been
omitted for clarity.
1 8 9 2
D a l t o n T r a n s . , 2 0 0 5 , 1 8 9 1 – 1 8 9 6

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Table 1 Summary of X-ray data collection and refinement
1
2·2CH3CN
Empirical formula
Mr
C26H33N5Cl2O10Mn
701.41
Orthorhombic
Pna21
32.1996(8)
9.2019(2)
10.5751(2)
90
C50H56N8Cl2O14Mn2·(CH3CN)2
1255.92
Triclinic
Crystal system
Space group
¯
P1
˚
a/A
14.5730(2)
14.6144(2)
16.9418(2)
71.2141(9)
66.7123(9)
63.7564(8)
2924.77(7)
2
1.426
150(1)
Colorless
Prism
0.30 × 0.23 × 0.20
Nonius KappaCCD
0.595
54.96
99.6
23355
13322
0.0206
1004
0.0518/0.1216
1.017
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
90
90
3
˚
V/A
3133.38(12)
4
1.487
150(1)
Colorless
Prism
Z
Dc/Mg m−3
T/K
Color
Crystal habit
Crystal size/mm3
Diffractometer
l/mm◦−1
0.30 × 0.25 × 0.23
Nonius KappaCCD
0.653
54.96
99.4
6699
6699
—
424
2hmax
/
Completeness to h = 27.48◦ (%)
Reflections collected
Independent reflections
Rint
Variable parameters
R1/wR2b
0.0425/0.0984
1.020
Goodness-of-fit (F2)
−3
˚
Dqmax/min/e A
0.514/−0.418
1.048/−0.929
ꢀ
ꢀ
ꢀ
ꢀ
Radiation used: Mo-Ka (k = 0.71073 A). b R1 = ||Fo| − |Fc| |/ |Fo|; wR2 = [ [w(Fo − Fc2)2]/[ (Fo2)2]]1/2 where w = 1/[r2(Fo2) + (aP)2 +
a
2
˚
bP].
in Table 1. Selected bond distances and angles are given in
Table 2. The overall geometry of the Mn(II) ion in 1 is distorted
octahedral, with a notable feature being the O(1)–Mn(1)–N(3)
angle (154.03(9)◦), which is significantly below 180◦ due to the
chelate nature of the bpppa ligand. This angle is acute relative
to the largest NHis–Mn–OGlu angle in the active site Mn(II) center
of OxdC (173◦).8 However, similar to the enzyme active site
Mn(II) center in OxdC, 1 exhibits N3O-coordination of the
chelate ligand and two cis solvent-occupied coordination sites.
The three nitrogen donors of the bpppa ligand form a facial
array on the metal center akin to the three histidine donors
found in OxdC, albeit the average of the N–Mn–N bond angles
in 1 (84◦) is smaller than the average of the NHis–Mn–NHis
bond angles in OxdC (94◦), again due to the chelate nature
of the bpppa ligand.8 The average Mn–N distance for the bpppa
˚
nitrogen donor atoms in 1 (2.24 A) is similar to the average
8
˚
Mn–NHis distance (2.27 A) in the enzyme. In addition, the
˚
Mn(1)–O(1) interaction in 1 (2.089(2) A) is generally similar
to the N-terminal Mn(II)–OGlu interaction (2.04 A). However,
the metal–water interaction in 1 (Mn(1)–O(2) 2.136(2) A) is
˚
˚
◦
a
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1 and 2·2CH3CN
[(bpppa)Mn(H2O)(CH3CN)](ClO4)2 (1)
[{(bpppa)Mn}2(l-C2O4)](ClO4)2·2CH3CN (2·2CH3CN)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–N(5)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–N(4)
2.089(2)
2.136(2)
2.290(3)
2.229(3)
2.270(2)
2.215(3)
Mn(1)–O(2)
Mn(1)–O(5)
Mn(1)–O(6)
Mn(1)–N(6)
Mn(1)–N(7)
Mn(1)–N(8)
2.1150(18)
2.1689(17)
2.1611(17)
2.272(2)
2.282(2)
2.240(2)
Mn(2)–O(1)
Mn(2)–O(3)
Mn(2)–O(4)
Mn(2)–N(2)
Mn(2)–N(3)
Mn(2)–N(4)
2.116(2)
2.1378(17)
2.1967(18)
2.249(2)
2.298(2)
2.254(2)
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(3)
O(1)–Mn(1)–N(4)
O(1)–Mn(1)–N(5)
O(2)–Mn(1)–N(2)
O(2)–Mn(1)–N(3)
O(2)–Mn(1)–N(4)
O(2)–Mn(1)–N(5)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(4)
N(2)–Mn(1)–N(5)
N(3)–Mn(1)–N(4)
N(3)–Mn(1)–N(5)
N(4)–Mn(1)–N(5)
102.17(11)
81.25(9)
154.03(9)
93.46(9)
O(2)–Mn(1)–O(5)
O(2)–Mn(1)–O(6)
O(2)–Mn(1)–N(6)
O(2)–Mn(1)–N(7)
O(2)–Mn(1)–N(8)
O(5)–Mn(1)–O(6)
O(5)–Mn(1)–N(6)
O(5)–Mn(1)–N(7)
O(5)–Mn(1)–N(8)
O(6)–Mn(1)–N(6)
O(6)–Mn(1)–N(7)
O(6)–Mn(1)–N(8)
N(6)–Mn(1)–N(7)
N(6)–Mn(1)–N(8)
N(7)–Mn(1)–N(8)
97.16(7)
108.95(7)
79.96(7)
104.96(7)
89.27(8)
76.60(6)
164.47(7)
115.29(7)
83.48(7)
89.83(7)
99.65(7)
154.34(8)
73.99(7)
111.60(8)
74.34(7)
O(1)–Mn(2)–O(3)
O(1)–Mn(2)–O(4)
O(1)–Mn(2)–N(2)
O(1)–Mn(2)–N(3)
O(1)–Mn(2)–N(4)
O(3)–Mn(2)–O(4)
O(3)–Mn(2)–N(2)
O(3)–Mn(2)–N(3)
O(3)–Mn(2)–N(4)
O(4)–Mn(2)–N(2)
O(4)–Mn(2)–N(3)
O(4)–Mn(2)–N(4)
N(2)–Mn(2)–N(3)
N(2)–Mn(2)–N(4)
N(3)–Mn(2)–N(4)
90.85(7)
128.17(8)
80.24(8)
139.95(8)
86.56(10)
76.38(6)
156.93(7)
123.26(8)
91.83(8)
92.41(7)
83.84(7)
142.68(9)
74.35(8)
108.66(8)
73.25(9)
89.47(9)
166.99(11)
101.85(10)
93.18(10)
83.18(10)
77.53(8)
99.17(10)
84.31(10)
75.49(10)
103.04(10)
175.75(10)
a Estimated standard deviations indicated in parentheses.
D a l t o n T r a n s . , 2 0 0 5 , 1 8 9 1 – 1 8 9 6
1 8 9 3

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shorter than that found at the active site Mn(II) center in
OxdC (av. Mn(II)–O(H2O) 2.35 A). This is likely due to a
˚
ically, the Mn(2)–O(3) bond is ∼0.06 A shorter than the
Mn(2)–O(4) interaction. This contrasts with identical Mn–O
bond lengths for the Mn(1) ion of 2·2CH3CN (Mn(1)–O(5)
8
˚
reduced Lewis acidity for the enzyme N3O(glutamate)-ligated
Mn(II) center. Thus, while the bpppa ligand provides a N3O
donor environment for the Mn(II) ion in which the chelate
ligand–Mn(II) bond distances are similar to those found for
Mn(II)–protein interactions in OxdC, the bpppa ligand does
not accurately reproduce the primary ligand environment of
the enzyme Mn(II) center in terms of bond angles and charge.
Finally, both protons of the Mn(II)-bound water molecule
of 1 are involved in moderate hydrogen-bonding interactions
with a perchlorate anion in the solid state (O(2) · · · O(7)/O(8)
˚
2.1689(17), Mn(1)–O(6) 2.1611(17) A) and both manganese cen-
ters in [(bispicMe2en)Mn(C2O4)Mn(bispicMe2en)](ClO4)2 (Mn–
O 2.160(3) and 2.168(3) A). Finally, the Mn–N and Mn–O
distances involving the bpppa chelate ligands in 2·2CH3CN are
slightly longer than those found in 1, consistent with reduced
Lewis acidity of the Mn(II) centers in the binuclear complex due
to oxalate dianion coordination.
12
˚
Magnetic properties of 1 and 2
◦
18
˚
2.75/2.73 A; O(2)–H · · · O(7)/O(8) 168/165 ). A moderate
The solution magnetic moment of 1 was determined at 298 K
using the Evans method.19 The mononuclear Mn(II) complex
1 exhibits leff = 5.8 lB, consistent with the expected spin-only
value (5.9 lB) for a mononuclear high-spin Mn(II) center (S =
5/2). Solid-state magnetic susceptibility measurements of 2 were
performed on a polycrystalline sample of 2 at 1000 Oe over the
temperature range 2–300 K. A plot of the vT vs. T susceptibility
data for 2 is shown in Fig. 5. The value of vT at 300 K is 8.32
emu mol−1 K, which is slightly lower than the expected spin-only
value for two Mn(II) ions (2v(S = 5/2)T = 8.75 emu mol−1 K). As
the temperature is lowered, the vT value decreases gradually to
0.36 emu mol−1 K at 2 K. The temperature dependence of 1/v
in the temperature range 7–300 K approximates Curie–Weiss
behavior with C = 9 emu mol−1 K and h = −20 K. The negative
Curie–Weiss constant indicates that the coupling between the
Mn(II) centers is antiferromagnetic.20 The weak antiferromag-
netic coupling observed for 2 is similar to that found for
binuclear Mn(II) oxalate complexes of the tetradentate N4-
donor ligands N,Nꢀ-bis(2-pyridylmethyl)-1,2-ethanediamine,
N,Nꢀ-bis(2-pyridylmethyl)-1,3-propanediamine, and N,N-bis(2-
pyridylmethyl)-N,Nꢀ-dimethyl-1,2-ethanediamine. In this set of
complexes, the effective magnetic moment at ambient tempera-
ture is 8.3–8.4 lB.12
hydrogen bonding interaction is defined as having a heteroatom
distance (A · · · B) of 2.5–3.2 A, a A–H · · · B angle of 130–180 ,
◦
˚
and energy of 4–15 kcal mol−1.18
ORTEP representations of the (a) binuclear cationic portion,
and (b) Mn2(l-C2O4) core of 2·2CH3CN are shown in Fig. 4.
As in 1, the bpppa ligand is bound to each Mn(II) center
of the binuclear cation in a tetradentate fashion with each
metal center having a distorted octahedral geometry. The
oxalate dianion is bound between the two Mn(II) ions of
2·2CH3CN in a bis-bidentate fashion. This structural mo-
tif has been previously identified in two Mn(II) complexes
having N4-donor tetradentate supporting chelate ligands.12
˚
The Mn(1) · · · Mn(2) separation (5.62 A) and oxalate C–O
and C–C bond lengths in 2·2CH3CN are identical to those
found in [(bispicMe2en)Mn(C2O4)Mn(bispicMe2en)](ClO4)2.12
However, one distinct feature of 2·2CH3CN is a slight
asymmetry for the oxalate anion binding to Mn(2). Specif-
Fig. 5 Temperature dependence of the vT product (᭺) and the inverse
susceptibility (᭛) for 2. The solid line corresponds to the best fit
obtained with eqn. (2), and the dashed line corresponds to the best fit
to the Curie–Weiss law. The inset shows the deviation from Curie–Weiss
behavior at low temperature.
In the solid state, 2 contains two S = 5/2 Mn(II) centers
bridged by an oxalate ligand which is expected to lead to
dominant intramolecular exchange interactions. Accordingly,
the variable-temperature magnetic susceptibility data for this
complex were fit to the modified Van Vleck equation (eqn. (1))
using the isotropic exchange Hamiltonian (H = −JS1·S2
+
glBSzH − zJꢀꢁSzꢂSz) for two interacting S = 5/2 centers with x =
J/kT.20 In an effort to include intermolecular interactions be-
tween individual binuclear molecules, the expression in eqn. (1)
was corrected using the molecular field approximation (eqn. (2))
where v is the exchange coupled magnetic susceptibility that was
measured, vdim is the magnetic susceptibility of the Mn dimer, zJꢀ
Fig. 4 ORTEP representations of the cationic portion (top) and
Mn(II) coordination environments (bottom) in 2·2CH3CN. Ellipsoids
are plotted at the 50% probability level. All hydrogen atoms, except the
proton on the amide moiety, have been omitted for clarity.
1 8 9 4
D a l t o n T r a n s . , 2 0 0 5 , 1 8 9 1 – 1 8 9 6

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is the exchange parameter and the other symbols have their usual
meanings. The magnetic data can be fit very well to eqn. (2) in
the temperature range of 2–300 K (Fig. 6) with J = −2.95 cm−1,
g = 2.0 and zJꢀ ≈ 0 cm−1. The fact that zJꢀ is essentially zero
means that intermolecular interactions are negligible and can be
ignored.
distilled under N2 prior to use.21 1H and 13C NMR were obtained
in CD3CN solution on a Bruker ARX-400 spectrometer at
25(1) ◦C. Chemical shifts are reported relative to the residual
1
solvent signals (1H NMR: CHD2CN, d 1.94 (quintet); 13C{ H}
NMR: d 1.39 (heptet)). FTIR spectra were recorded on a
Shimadzu FTIR-8400 spectrometer as KBr pellets. Room-
temperature magnetic susceptibilities were determined by the
Evans method.19 Mass spectrometry data was obtained at
the Mass Spectrometry Facility, Department of Chemistry,
University of California, Riverside. Elemental analyses were
performed by Atlantic Microlabs, Inc., Norcross, Georgia.
Variable temperature magnetic susceptibility data for 2 were
collected using a solid microcrystalline sample on a Quantum
Design MPMS-2 SQUID magnetometer in the range of 2–
300 K.
2Ng2b2 ex + 5e3x + 14e6x + 30e10x + 55e15x
vdim
=
(1)
(2)
kT 1 + 3ex + 5e3x + 7e6x + 9e10x + 11e15x
vdim
v =
ꢀ
2
1 − (zJ /Ng b2) vdim
The ligand precursors 2-(pivaloylamido)-6-(bromomethyl)-
pyridine and N-benzyl-N-2-methylpyridine were prepared
according to literature procedures.16,17
Synthesis of ligand and complexes
N-Benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridyl-
methyl)amine (bpppa). To a 500 mL round bottom flask
containing 2-(pivaloylamido)-6-(bromomethyl)pyridine (2.1 g,
7.5 mmol, 50 mL) in dry acetonitrile (70 mL) was added
N-benzyl-N-2-methylpyridine (1.5 g, 7.5 mmol, 20 mL),
Na2CO3 (1.6 g, 15 mmol), and a catalytic amount of
tetrabutylammonium bromide (∼5 mg). The reaction mixture
was purged with nitrogen and refluxed for 22 h. After cooling
the solution to room temperature, 1 M sodium hydroxide was
added (90 mL). The resulting biphasic solution was extracted
with dichloromethane (3 × 150 mL) and the organic fractions
were combined and dried over anhydrous sodium sulfate.
After filtration, the organic solution was brought to dryness
under reduced pressure. Column chromatography on silica gel
(230–400 mesh; ethyl acetate, Rf = 0.55), followed by removal
of the solvent under reduced pressure, yielded a yellow oil
(2.63 g, 90%) (Found: C, 74.03; H, 7.25; N, 14.32. C24H28N4O
requires C, 74.18; H, 7.27; N, 14.43%); mmax/cm−1: (N–H) 3350
Fig. 6 Temperature dependence of the vT product (᭺) and the molar
susceptibility (᭛). The solid and dashed lines correspond to the best fit
obtained with eqn. (2).
Conclusion
Recent X-ray crystallographic and mechanistic studies of
Mn(II)-containing oxalate decarboxylase (OxdC) suggest that
the catalytic pathway for this enzyme involves reaction of a
N3O-ligated Mn(II)–oxalate adduct with O2 to yield carbon
dioxide and formate.7–9 While the structural features of the
Mn(II)–oxalate species in OxdC remain unknown, both mono-
and bidentate coordination modes have been observed in
first-row transition-metal complexes and may be possible in
the enzyme active site.8,10 In the work presented herein, we
have evaluated whether use of the new N3O-donor bpppa
chelate ligand would enable the isolation of synthetic complexes
with relevance to species proposed in the catalytic cycle of
OxdC. While 1 is mononuclear and supported by a N3O-donor
ligand environment, isolation of the binuclear oxalate derivative
2, which has structural and magnetic properties similar to
previously reported complexes supported by N4-donor chelate
ligands,12 indicates that the bpppa ligand is not suitable for the
isolation of a mononuclear Mn(II) oxalate derivative relevant
to an enzyme/substrate adduct. On the basis of these data,
efforts are underway to incorporate steric hindrance and/or
hydrogen-bond donors into a supporting N3O-donor chelate
ligand, with the goal of preventing the formation of binuclear
oxalate derivatives. In this regard, it is worth noting that a
proposed structure for the enzyme/substrate adduct in OxdC
includes hydrogen-bonding interactions between the Mn(II)-
bound oxalate and surrounding amino acid resides.8
=
(br), (C O) 1687 (neat); dH (400 MHz, solvent CD3CN): 1.26
(9H, s), 3.65 (2H, s), 3.66 (2H, s), 3.75 (2H, s), 7.15–7.27 (2H,
m), 7.30–7.36 (3H, m), 7.42 (2H, d, J = 7.4 Hz), 7.57 (1H, d,
J = 7.8 Hz), 7.69 (2H, t, J = 7.8 Hz), 7.98 (1H, d, J = 8.2 Hz),
8.14 (1H, br s, N–H), 8.46 (1H, d, J = 4.5 Hz); dC (100 MHz,
CD3CN): 27.7, 40.5, 59.1, 60.3, 60.7, 112.7, 119.4, 123.1, 123.9,
128.1, 129.3, 130.0, 137.4, 139.6, 140.4, 149.9, 152.3, 159.5,
160.8, 178.0 (20 signals expected and observed); FAB-MS: m/z
389 (MH+, 100%).
CAUTION! Perchlorate complexes of organic ligands are
potentially explosive. They should be handled in small quantities
with great care.22
[(bpppa)Mn(CH3CN)(H2O)](ClO4)2 (1). To a acetonitrile
(3 mL) solution of bpppa (0.054 g, 0.14 mmol) was added
a acetonitrile (3 mL) solution of Mn(ClO4)2·6H2O (0.050 g,
0.14 mmol). The resulting yellow solution was stirred for ∼3 h
at ambient temperature. The solvent was then removed under
reduced pressure. The remaining solid was dissolved in ∼4 mL
of acetonitrile and diethyl ether (∼10 mL) was added until the
solution became cloudy. From this solution, block-type colorless
crystals were obtained (0.033 mg, 34%) (Found C, 44.55 H,
4.78 N, 9.88. C26H33N5O10ClMn requires C, 44.57; H, 4.75; N,
10.00%); mmax/cm−1: 3386s, 3319s, 1654s, 1619s, 1528m, 1459s,
1438m, 1420m, 1162m, 1112s, 1066s, 623s (KBr); FAB-MS
(DCM–NBA), m/z (relative intensity): 542 ([M + ClO4−]+, 32%);
leff(CH3CN, 25(1) ◦C) = 5.8 lB.
Experimental
General
All chemicals were purchased from commercial sources and
used as received unless otherwise noted. All reactions involving
Mn(II) salts were performed in a MBraun Unilab glovebox under
an atmosphere of purified dinitrogen. Solvents for glovebox
procedures were dried according to published methods and
[{(bpppa)Mn}2(l-C2O4)](ClO4)2 (2). To a methanol solution
(3 mL) of bpppa (0.052 g, 0.13 mmol) was added a methanol
solution (3 mL) of Mn(ClO4)2·6H2O (0.048 g, 0.13 mmol).
D a l t o n T r a n s . , 2 0 0 5 , 1 8 9 1 – 1 8 9 6
1 8 9 5

View Article Online
The resulting pale yellow solution was stirred for ∼20 min
at ambient temperature. This solution was added to Na2C2O4
(0.009 g, 0.07 mmol) and the reaction mixture was stirred for
six days at which point the solvent was removed under reduced
pressure. The solid was re-dissolved in methylene chloride and
filtered through a glass wool/Celite plug. The CH2Cl2 solvent
was removed from the filtrate under reduced pressure leaving a
white solid. Recrystallization of the solid from CH3CN–diethyl
ether at −20(1) ◦C yielded colorless crystalline blocks (0.062 g,
80%) (Found: C, 50.84; H, 4.85; N, 9.54. C50H56N8 O14Cl2Mn2
requires C, 51.19; H, 4.81; N, 9.56%); mmax/cm−1 1664s, 1641s,
1614s, 1524s, 1483m, 1458s, 1310m, 1163m, 1096s, 1016m, 793m,
770m, 752m, 706m, 623s; FAB-MS (DCM–NBA), m/z (relative
intensity): 1073 ([M − ClO4−]+, 15%).
CCDC reference numbers 259639 and 259640.
See http://www.rsc.org/suppdata/dt/b5/b500534e/ for cry-
stallographic data in CIF or other electronic format.
Acknowledgements
L. M. B. gratefully acknowledges the Herman Frasch Foun-
dation (501-HF02) for financial support. K. R. D. gratefully
acknowledges the National Science Foundation (DMR0103455,
NSF-9974899) and the Telecommunication and Informatics
Task Force (TITF) at Texas A & M University. The SQUID
magnetometer was purchased with funds provided by the
National Science Foundation.
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X-Ray crystallography
A crystal of each manganese complex was mounted on a
glass fiber using a viscous oil and then transferred to a
Nonius KappaCCD diffractometer with Mo-Ka radiation (k =
˚
0.71073 A) for data collection at 150(1) K. For each compound,
an initial set of cell constants was obtained from 10 frames
of data that were collected with an oscillation range of 1◦
frame−1 and an exposure time of 20 s frame−1. Indexing and unit
cell refinement based on all observed reflections from those
10 frames indicated an orthorhombic P lattice for 1 and a
triclinic P lattice for 2·2CH3CN. Final cell constants for each
complex were determined from a set of strong reflections from
the actual data collection. For each data set, reflections were
indexed, integrated, and corrected for Lorentz, polarization, and
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of direct methods and heavy atom methods using SIR 97.24
For 1, all of the non-hydrogen atoms except for the oxygen
atoms of one perchlorate anion were refined with anisotropic
displacement coefficients. The hydrogen atoms on the coordi-
nated water molecule and the amide proton in 1 were located
and refined independently. All other hydrogen atoms for 1 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 SHELXL97.25 There is one disordered
perchlorate anion in 1. The O(7), O(8) and O(9) oxygen atoms,
bonded to Cl(2), were each split into two fragments (O(7)/O(7ꢀ),
O(8)/O(8ꢀ), O(9)/O(9ꢀ)) and were refined. This refinement led
to a 0.87 : 0.13 ratio in occupancy over two positions for
O(7) and O(8) and a 0.60 : 0.40 ratio in occupancy over
two positions for O(9). The O(10) atom was split into three
fragments (O(10)/O(10ꢀ)/O(10ꢀꢀ)), with refinement leading to a
0.55 : 0.27 : 0.18 ratio in occupancy over three positions.
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For 2·2CH3CN, all of the non-hydrogen atoms, except
select oxygen atoms of the perchlorate anions, were re-
fined with anisotropic displacement coefficients. All hydrogen
atoms, except those of two non-coordinated acetonitrile sol-
vate molecules, were located and refined independently. The
solvate hydrogen atoms of 2·2CH3CN 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 SHELXL97.25 The position of the methyl carbon
of one acetonitrile solvate molecule is disordered over two
positions. Splitting of C(54) into two fragments (C(54)/C(54ꢀ))
and refinement led to a 0.54 : 0.46 ratio in occupancy. Oxygen
atoms of both perchlorate anions in 2·2CH3CN exhibit disorder.
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