Synthesis and characterization of a series of N3O-ligated mononuclear Mn(II) halide complexes

Article abstract of DOI:10.1016/j.ica.2005.09.008

Treatment of a N₃O-donor chelate ligand (mpppa = N-methyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylethyl)amine; bpppa = N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylmethyl)amine) with equimolar amounts of Mn(ClO₄)₂ ? 6H₂O and Me₄NX (X = Cl, Br, I) in methanol resulted in the production of a series of mononuclear Mn(II) halide complexes of the formula [(L)Mn-X(CH ₃OH)]ClO₄ (L = mpppa or bpppa). X-ray crystallographic studies of [(mpppa)Mn-Cl(CH₃OH)]ClO₄ ? CH ₃OH (2 ? CH₃OH), [(mpppa)Mn-Br(CH₃OH)] ClO₄ ? CH₃OH (4 ? CH₃OH), [(mpppa)Mn-I(CH₃OH)]ClO₄ ? CH₃OH (6 ? CH₃OH), and [(bpppa)Mn-I(CH₃OH)]ClO₄ ? O₂(CH₂CH₃)₂ (7 ? O(CH ₂CH₃)₂) revealed for each a mononuclear Mn(II) center having tetradentate coordination of the chelate ligand, one coordinated halide anion, and one molecule of coordinated methanol. An increase in the Mn-X distance through the halide series (Cl, Br, I) correlates linearly with the increase in the radius of the anion. The magnetic moment of each halide complex, measured via Evans method in methanol, is consistent with the presence of a high-spin distorted octahedral Mn(II) center. The EPR features of the halide complexes in methanol do not change as a function of the nature of the halide coordinated to the Mn(II) center.

Full text of DOI:10.1016/j.ica.2005.09.008

Inorganica Chimica Acta 359 (2006) 1282–1290
www.elsevier.com/locate/ica
Synthesis and characterization of a series of N3O-ligated
mononuclear Mn(II) halide complexes
a
a
b
a,
*
Amy L. Fuller , Rex W. Watkins , Atta M. Arif , Lisa M. Berreau
a
Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, UT 84322-0300, USA
b
Department of Chemistry, University of Utah, 315 S. 1400 E., Salt Lake City, UT 84112-0850, USA
Received 17 August 2005; received in revised form 12 September 2005; accepted 12 September 2005
Available online 18 October 2005
Abstract
Treatment of a N3O-donor chelate ligand (mpppa = N-methyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylethyl)amine;
bpppa = N-benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylmethyl)amine) with equimolar amounts of Mn(ClO4)2 Æ 6H2O
and Me4NX (X = Cl, Br, I) in methanol resulted in the production of a series of mononuclear Mn(II) halide complexes of the formula
[(L)Mn-X(CH3OH)]ClO4 (L = mpppa or bpppa). X-ray crystallographic studies of [(mpppa)Mn-Cl(CH3OH)]ClO4 Æ CH3OH
(2 Æ CH3OH), [(mpppa)Mn-Br(CH3OH)]ClO4 Æ CH3OH (4 Æ CH3OH), [(mpppa)Mn-I(CH3OH)]ClO4 Æ CH3OH (6 Æ CH3OH), and
[(bpppa)Mn-I(CH3OH)]ClO4 Æ O2(CH2CH3)2 (7 Æ O(CH2CH3)2) revealed for each a mononuclear Mn(II) center having tetradentate coor-
dination of the chelate ligand, one coordinated halide anion, and one molecule of coordinated methanol. An increase in the Mn-X dis-
tance through the halide series (Cl, Br, I) correlates linearly with the increase in the radius of the anion. The magnetic moment of each
halide complex, measured via Evans method in methanol, is consistent with the presence of a high-spin distorted octahedral Mn(II) cen-
ter. The EPR features of the halide complexes in methanol do not change as a function of the nature of the halide coordinated to the
Mn(II) center.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Manganese; Halides; X-ray crystal structures; EPR
1. Introduction
Fluoride and iodide are inhibitors for the enzyme, whereas
chloride and bromide do not significantly affect enzyme
activity.
Oxalate oxidase is a metalloenzyme found in a wide
variety of plants [1]. Within the active site of this enzyme
is a pseudo-octahedral Mn(II) center ligated by three histi-
dine residues, a monodentate glutamate, and two cis water
molecules [2]. Oxalate oxidase catalyzes the O2- and pro-
ton-dependent oxidative decomposition of oxalate to pro-
duce CO2 and hydrogen peroxide.
Recently, Whitaker et al. [3] investigated the EPR prop-
erties of the Mn(II) center of oxalate oxidase in the pres-
ence of oxalate and halides. Perturbation of the EPR
signal as a function of the nature of the anion present sug-
gests that the anions act as ligands to the Mn(II) center.
Mononuclear manganese(II) complexes having a coordi-
nation environment akin to that found in oxalate oxidase,
as well as one or more halide ligands, are rare. For exam-
ple, a Cambridge Crystallographic Database search (version
5.26, updated May 2005) for mononuclear N3O-ligated
manganese(II) complexes with coordinated halide ligands
produced only four chloride derivatives and two bromide
complexes (Fig. 1) [4–8]. Thus, no systematic study of the
coordination properties of halides with a N3O-ligated
Mn(II) center has been reported to date. A series of mono-
nuclear terpyridine-ligated Mn(II)X2 (X = Clꢀ, Brꢀ, Iꢀ)
complexes has been systematically examined (Fig. 2) [9].
However, the pentacoordinate nature of the Mn(II) center
in these complexes, coupled with the N3-donor environ-
ment of the supporting chelate ligand, limits the relevance
*
Corresponding author. Tel.: +1 435 797 1625; fax: +1 435 797 3390.
E-mail addresses: arif@chemistry.utah.edu (A.M. Arif), berreau@
cc.usu.edu (L.M. Berreau).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.008

A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
1283
2.2. Physical
FTIR spectra were recorded on a Shimadzu FTIR-8400
spectrometer as KBr pellets. 1H NMR spectra were re-
corded at 25(1) ꢁC on a Bruker ARX-400 or JEOL-270
spectrometer. Chemical shifts are referenced to the residual
solvent peak in CD3CN (1H: 1.94 (quintet) ppm). Solution
magnetic moments were determined by the Evans method
[12]. Elemental analyses were performed by Atlantic
Microlabs of Norcross, GA. Fast atom bombardment
(FAB) mass spectra were collected at the Mass Spectrome-
try Facility at the University of California, Riverside. EPR
spectra were recorded at 70(5) K on a Bruker ESP-300 X-
band EPR spectrometer fitted with a liquid helium cooled
probe.
2.3. Synthesis of ligand and complexes
Fig. 1. Active site Mn(II) center in oxalate oxidase [1] and known
synthetic Mn(II) halide complexes having N3O or N4 coordination of a
chelate ligand: (a) [Mn(Hmimppa)Cl2] Æ CH3OH [4], (b) [Mn(HL3)Cl2] Æ
CH3OH (R = H), [Mn(HL4)Br2] (R = Br) [5], (c) [MnBr2(mebpa)] [6], (d)
[MnCl(H2O)(phen)2]Cl [7], and (e) [Mn(bpy)2Cl(H2O)]ClO4 [8].
N-Benzyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-(2-
pyridylmethyl)amine (bpppa) was prepared according to
the literature procedure [10].
2.3.1. N-Methyl-N-((6-pivaloylamido-2-pyridyl)methyl)-N-
(2-pyridylethyl)amine (mpppa)
To a CH3CN solution (80 mL) of 2-(2-methylamino-
ethyl)pyridine (1.25 g, 9.18 mmol) was added 2-bromom-
ethyl-6-pivaloylamidopyridine [13] (2.49 g, 9.19 mmol),
sodium carbonate (1.95 g, 18.4 mmol) and tetrabutylam-
monium bromide (ꢁ3 mg). This solution was heated at re-
flux for 16 h under a N2 atmosphere. After cooling the
yellow-brown solution to room temperature, an equal vol-
ume of 1 M NaOH was added. Extraction of this organic/
aqueous solution with CH2Cl2 (3 · 80 mL), followed by
drying of the combined organic fractions with Na2SO4, fil-
tration, and removal of solvent under reduced pressure,
yielded a thick yellow-brown oil. Purification of this oil
via column chromatography on silica gel (200–400 mesh,
CH3OH, Rf ꢁ 0.7 (trailing band)) provided an analytically
pure yellow oil (1.6 g, 53%). (C19H26N4O requires C,
69.90; H, 8.03; N, 17.17. Found: C, 68.65; H, 8.21; N,
16.82%); 1H NMR (CD3CN, 270 MHz) d 8.44 (d,
J = 4.6 Hz, 1H), 8.20 (br, 1H, N–H), 7.97 (d, J = 8.0 Hz,
1H), 7.68–7.58 (m, 2H), 7.20 (d, J = 7.9 Hz, 1H), 7.17–
7.10 (m, 1H), 6.99 (d, J = 7.6 Hz, 1H), 3.57 (s, 2H), 3.00–
2.90 (m, 2H), 2.84–2.75 (m, 2H), 2.28 (s, 3H), 1.26 (s, 9H)
ppm; 13C{1H} NMR (CD3CN, 67.9 MHz) d 177.9, 161.6,
159.0, 152.2, 150.1, 139.5, 137.2, 124.2, 122.2, 119.5,
112.6, 63.9, 58.1, 42.7, 40.5, 36.6, 27.6 ppm (17 signals ex-
pected and observed); FTIR (neat): mmax/cmꢀ1 (N–H)
3164, (C@O) 1688 (neat); LREI-MS, m/z (relative intensity)
327 ([M + H]+, 100%); HREI-MS, m/z: Calc. for
[C19H26N4O + H]+, 327.2185. Found: 327.2184.
Fig. 2. [Mn(terpy)(X)2] X = Clꢀ, Brꢀ, Iꢀ, and SCNꢀ [9].
of this series to the chemistry of oxalate oxidase. As a first
approximation toward modeling the active site coordina-
tion environment of oxalate oxidase, we recently reported
the use of an N3O(amide)-donor ligand to prepare mono-
nuclear Mn(II) complexes [10]. In the results described
herein, we have used the same type of chelate ligand to pre-
pare and characterize the first series of mononuclear N3O-
ligated Mn(II) complexes having a single halide ligand.
These complexes have been characterized by X-ray crystal-
lography, elemental analysis, mass spectrometry, a solution
magnetic moment measurement, and infrared and EPR
spectroscopy.
2. Experimental
2.1. General
All reagents and solvents were obtained from commer-
cial solvents and used as received unless otherwise noted.
Solvents were dried according to published procedures
and were distilled under N2 prior to use [11]. Air-sensitive
reactions were performed in a MBraun Unilab glovebox
under an atmosphere of purified N2.
Caution. Perchlorate salts of metal complexes with or-
ganic ligands are potentially explosive. Only small amounts
of material should be prepared and these should be han-
dled with great care [14].

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A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
2.3.2. [(mpppa)Mn](ClO4)2 (1)
fiber with traces of viscous oil and then transferred to a Non-
To a MeOH (ꢁ3 mL) solution of mpppa (67 mg,
0.21 mmol) was added a MeOH (ꢁ3 mL) solution of
Mn(ClO4)2 Æ 6H2O (74 mg, 0.21 mmol). This yellow solution
was stirred overnight. The solvent was removed under reduced
pressure. Recrystallization from 1:1 CH2Cl2:ACN/Et2O diffu-
sion at ꢀ20 ꢁC yielded crystalline material (102 mg, 86%).
ius 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 1ꢁ/frame and an exposure time of 20 s/frame [15]. Indexing
and unit cell refinement based on all observed reflections
from those 10 frames indicated monoclinic P lattices for
2 Æ CH3OH, 4 Æ CH3OH, and 6 Æ CH3OH and a orthorhombic
P lattice for 7 Æ O(CH2CH3)2. Final cell constants for each
complex were determined from a set of strong reflections
from the actual data collection. For each data set, these
reflections were indexed, integrated, and corrected for Lor-
entz, polarization, and absorption effects using DENZO/
SMN and SCALEPAC [16]. The structures were solved by
a combination of direct methods and heavy atom using SIR
97 [17]. All of the non-hydrogen atoms were refined with
anisotropic displacement coefficients. Unless otherwise sta-
ted hydrogen atoms were assigned isotropic displacement
coefficients U(H) = 1.2U(C) or 1.5U(Cmethyl), and their coor-
dinates were allowed to ride on their respective carbons using
SHELLXL 97 [18]. Scattering factors were taken from the Inter-
national Tables for Crystallography, Vol. C [19,20].
2.3.3. General procedure for the preparation of metal halide
complexes
To a MeOH (ꢁ3 mL) solution of ligand (ꢁ0.12 mmol)
was added a MeOH (ꢁ3 mL) solution of Mn(ClO4)2 Æ 6-
H2O (ꢁ0.12 mmol). This yellow solution was stirred for
ꢁ15 min and was then added to a MeOH (ꢁ3 mL) slurry
of Me4NX (ꢁ0.12 mmol; X = Cl, Br, I). The resulting mix-
ture was stirred overnight which produced a yellow solu-
tion containing a white precipitate. The solvent was
removed under reduced pressure and the solid was redis-
solved in a minimal amount of MeOH (ꢁ2 mL). The pre-
cipitate (Me4NClO4) was removed via gravity filtration
through a glass wool/Celite plug. The solvent was removed
under reduced pressure. Each complex was recrystallized
from MeOH via Et2O diffusion.
In 2 Æ CH3OH and 4 Æ CH3OH, all of the hydrogen atoms
were located and refined independently. In 6 Æ CH3OH,
hydrogen atoms H1N (N1), H2 (O2), and H7A (O7) were
located, whereas for 7 Æ O(CH2CH3)2 H2 (O2) was the only
hydrogen atom to be located and refined independently. A
solvate molecule of methanol present in the lattice of
2 Æ CH3OH, 4 Æ CH3OH, and 6 Æ CH3OH forms a hydrogen
bonding interaction (average distance of Mn(II)–
2.3.4. [(mpppa)Mn(Cl)(HOCH3)]ClO4 (2)
Clear, colorless plate-type crystals suitable for X-ray
crystallography (44 mg, 65%). FAB-MS DCM:NBA, m/z
(relative intensity), 416 ([M ꢀ ClO4 ꢀ CH3OH]+, 34%).
2.3.5. [(bpppa)Mn(Cl)(HOCH3)]ClO4 (3)
Colorless, polycrystalline material (86 mg, 84%). FAB-
MS DCM:NBA, m/z (relative intensity), 478 ([M ꢀ ClO4 ꢀ
CH3OH]+, 100%).
˚
CH3OH. . .OHCH3 2.66 A; average bond angle O–H. . .O
168ꢁ) with the Mn(II)-coordinated methanol molecule. In
7 Æ O(CH2CH3)2, one solvent molecule of diethylether is
present in the asymmetric unit and the perchlorate anion
is disordered.
2.3.6. [(mpppa)Mn(Br)(HOCH3)]ClO4 (4)
Colorless block crystals suitable for X-ray crystallogra-
phy (99 mg, 82%). FAB-MS DCM:NBA, m/z (relative
intensity), 460 ([M ꢀ ClO4 ꢀ CH3OH]+, 100%).
3. Results and discussion
2.3.7. [(bpppa)Mn(Br)(HOCH3)]ClO4 (5)
Colorless polycrystalline material (61 mg, 59%). FAB-
MS DCM:NBA, m/z (relative intensity), 522 ([M ꢀ ClO4 ꢀ
CH3OH]+, 89%).
3.1. Synthesis
The N3O-donor ligand N-methyl-N-((6-pivaloylamido-
2-pyridyl)methyl)-N-(2-pyridylethyl)amine (mpppa, Fig. 3
(top)) was prepared in a single step by treatment of 2-bro-
momethyl-6-pivaloylamidopyridine [13] with 2-(2-methyla-
minoethyl)pyridine in acetonitrile in the presence of
sodium carbonate. The analog ligand, N-benzyl-N-((6-
pivaloylamido-2-pyridyl)methyl)-N-(2-pyridylmethyl)amine
(bpppa, Fig. 3(bottom)) was prepared as previously de-
scribed [10]. Admixture of molar equivalent amounts of
mpppa and Mn(ClO4)2 Æ 6H2O in methanol, followed by
recrystallization from 1:1 acetonitrile:methylene chloride/
diethyl ether yielded [(mpppa)Mn(ClO4)2] (1). Complex 1
has been characterized by infrared spectroscopy, electron
paramagnetic resonance, and elemental analysis (Table 1).
Treatment of either mpppa or bpppa with equimolar
amounts of Mn(ClO4)2 Æ 6H2O and Me4NX (X = Cl, Br, I;
2.3.8. [(mpppa)Mn(I)(HOCH3)]ClO4 (6)
Colorless block crystals suitable for X-ray crystallogra-
phy (79 mg, 65%). FAB-MS DCM:NBA, m/z (relative
intensity), 508 ([M ꢀ ClO4 ꢀ CH3OH]+, 7%).
2.3.9. [(bpppa)Mn(I)(HOCH3)]ClO4 (7)
Colorless plate-type crystals suitable for X-ray crystal-
lography (68 mg, 66%). FAB-MS DCM:NBA, m/z (rela-
tive intensity), 570 ([M ꢀ ClO4 ꢀ CH3OH]+, 84%).
2.4. X-ray crystallography
A
crystal of compound 2 Æ CH3OH, 4 Æ CH3OH,
6 Æ CH3OH, and 7 Æ O(CH2CH3)2 was mounted on a glass

A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
1285
Fig. 3. Ligands used in preparation of Mn(II)-halide complexes.
Scheme 1) in methanol, followed by recrystallization from
methanol/diethyl ether, yielded two series of N3O-ligated
mononuclear Mn(II) halide complexes (2–7). These halide
derivatives were characterized by multiple methods, includ-
ing X-ray crystallography.
3.2. X-ray crystallography
Single crystals suitable for X-ray crystallographic exper-
iments were obtained for 2 Æ CH3OH, 4 Æ CH3OH,
6 Æ CH3OH, and 7 Æ O(CH2CH3)2. Details of the data
collection and refinement of these structures are given in
Scheme 1. Synthesis of Mn(II) halide complexes.
Table 1
Summary of X-ray data collection and refinementa
2 Æ CH3OH
4 Æ CH3OH
6 Æ CH3OH
7 Æ O(CH2CH3)c
Empirical formula
Formula weight
Crystal system
Space group
C21H34N4Cl2O7Mn
580.36
monoclinic
P21/n
17.2112(4)
8.0366(2)
20.5002(4)
90
111.2040(10)
90
C21H34N4BrClO7Mn
624.82
monoclinic
P21/n
17.3012(2)
8.0510(2)
20.4912(4)
90
111.1361(10)
90
C21H34N4IClO7Mn
671.81
monoclinic
P21/n
17.3627(3)
8.1189(2)
20.6408(5)
90
110.9714(13)
90
C29H42N4IClO7Mn
775.96
orthorhombic
P212121
10.9095(3)
12.3635(3)
25.1066(7)
90
˚
A (A)
˚
B (A)
˚
C (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90
3
˚
V (A )
2643.60(10)
4
2662.24(9)
4
2716.91(10)
4
3386.37(16)
4
Z
Density (calc.) (Mg mꢀ3
Temperature (K)
Crystal size (mm)
Diffractometer
)
1.458
150(1)
0.45 · 0.25 · 0.25
Nonius KappaCCD
0.748
1.559
150(1)
0.35 · 0.25 · 0.20
Nonius KappaCCD
2.144
1.642
150(1)
0.38 · 0.33 · 0.20
Nonius KappaCCD
1.766
1.522
150(1)
0.28 · 0.25 · 0.15
Nonius KappaCCD
1.429
Absorption coefficient (mmꢀ1
)
2h max (ꢁ)
54.98
99.4
9737
6038
449
0.0567/0.1139
1.091
1.121/ꢀ0.495
54.92
99.3
10435
6056
452
0.0399/0.0687
1.031
0.479/ꢀ0.428
54.94
99.3
10455
6189
334
0.0520/0.1120
1.013
0.952/ꢀ1.241
54.98
99.2
7581
7581
400
0.0674/0.1080
1.066
1.108/ꢀ0.647
Completeness to 2h (%)
Reflections collected
Independent reflections
Variable parameters
b
R1/wR2
Goodness-of-fit (F2)
ꢀ3
˚
Largest differential peak and hole (e A
)
a
˚
Radiation used: Mo Ka (k = 0.71073 A).
P
P
P
P
2
2
1=2
2
b
c
R1 ¼ jjF o ꢀ F cjj= jF oj; wR2 ¼ ½ ½wðF 2o ꢀ F c2Þ ꢂ=½ ðF o2Þ ꢂꢂ ; where w ¼ 1=½r2ðF 2oÞ þ ðaPÞ þ bPꢂ.
Flack parameter: 0.40705.

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A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
Table 1. Important bond distances and angles are given in
Table 2. ORTEP drawings of the cationic components of
these complexes are shown in Fig. 4.
For example, for 7 Æ O(CH2CH3)2, the average Mn–N dis-
˚
tance is only slightly shorter (ꢁ2.28 A) than that found
˚
for the mpppa-ligand complexes (ꢁ2.30–2.31 A). However,
˚
Complexes 2 Æ CH3OH, 4 Æ CH3OH, and 6 Æ CH3OH
crystallize in the monoclinic crystal system in space group
P21/n with four cations, four perchlorate counteranions,
and four molecules of non-coordinated methanol in the
unit cell. Complex 7 Æ O(CH2CH3)2 crystallizes in the
orthorhombic crystal system in space group P212121. In
the unit cell of 7 Æ O(CH2CH3)2 there are four cations, four
perchlorate anions, and four molecules of diethyl ether.
in 7 Æ O(CH2CH3)2, the Mn(1)–N(3) distance (2.307(4) A)
involving the tertiary amine donor is longer than the
˚
Mn–N(pyridyl) bonds (Mn(1)–N(2) (2.283(5) A) and
˚
Mn(1)–N(4) (2.243(5) A)). Also notable is the fact that
˚
˚
˚
the Mn(1)–O(1) distance (2.088(4) A) is ꢁ0.05 A shorter,
and the Mn(1)–O(2) bond is ꢁ0.05 A longer, than the anal-
ogous bonds in the mpppa-ligated complexes. Thus,
whereas the Mn(1)–O(1) and Mn(1)–O(2) bond lengths in
˚
the mpppa-supported complexes differ by ꢁ0.08 A, this dif-
˚
3.2.1. Comparison of chelate ligand environments
ference is ꢁ0.18 A in 7 Æ O(CH2CH3)2.
For each of the mpppa-ligated complexes (2 Æ CH3OH,
4 Æ CH3OH, and 6 Æ CH3OH), the mononuclear Mn(II) cen-
ter exhibits a distorted octahedral geometry. The Mn–N
In terms of bond angles, comparison of the mpppa-
and bpppa-ligated complexes revealed that the most sig-
nificant difference is found in the O(1)–Mn(1)–N(3) angle.
˚
bond lengths average ꢁ2.30–2.31 A, with the shortest of
For
the
mpppa-ligated
complexes
(2 Æ CH3OH,
these interactions always involving the tertiary nitrogen
of the supporting chelate ligand. The Mn–O bonds are
shorter than the Mn–N bonds, and are notably different
from one another. Specifically, for each complex, the
Mn(1)–O(1) interaction involving the chelating amide is
4 Æ CH3OH, and 6 Æ CH3OH), regardless of the nature of
the coordinated anion, this angle is found in the range
153–154ꢁ. For 7 Æ O(CH2CH3)2, the O(1)–Mn(1)–N(3) an-
gle is more acute (149.2(2)ꢁ). However, it should be noted
that this is not a consequence of the difference in sup-
porting chelate ligands, as the bpppa-ligated complex
[(bpppa)Mn(CH3CN)(H2O)](ClO4)2 has a O(1)–Mn(1)–
N(3) angle of 154.03(9)ꢁ [10]. Finally, the mpppa and
bpppa ligands differ in the chelate ring size produced
upon coordination of the tertiary nitrogen and unsubsti-
tuted pyridyl donor to the Mn(II) center. Specifically,
whereas coordination of mpppa results in the formation
of a six-membered chelate ring, with a N(3)–Mn(1)–
N(4) bond angle of approximately 88–89ꢁ, the bpppa li-
gand produces a five-membered ring and angle of
75.3(2)ꢁ.
˚
ꢁ0.08 A shorter than the Mn(1)–O(2) bond involving the
coordinated methanol molecule. This difference is a reflec-
tion of the fact that the amide moiety can have partial an-
ionic character at the oxygen atom via resonance. The
contribution of the anionic resonance form is enhanced
in the metal complex, versus that present in the free ligand,
and is reflected in the lower energy position of the mCO
vibration (vida infra).
The bpppa-ligated complex 7 Æ O(CH2CH3)2 also exhib-
its a distorted octahedral geometry and reveals interesting
structural differences relative to the mpppa-ligated systems.
Table 2
a
˚
Selected bond lengths (A) and angles (ꢁ)
2 Æ CH3OH
4 Æ CH3OH
6 Æ CH3OH
7 Æ O(CH2CH3)2
Mn(1)–X(1)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–N(4)
2.441(1)
2.140(2)
2.221(2)
2.319(2)
2.293(2)
2.320(2)
2.564(1)
2.135(1)
2.218(2)
2.313(2)
2.290(2)
2.312(2)
2.771(1)
2.135(2)
2.215(3)
2.303(3)
2.284(3)
2.304(3)
2.776(1)
2.088(4)
2.267(4)
2.283(5)
2.307(4)
2.243(5)
X(1)–Mn(1)–O(1)
X(1)–Mn(1)–O(2)
X(1)–Mn(1)–N(2)
X(1)–Mn(1)–N(3)
X(1)–Mn(1)–N(4)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(3)
O(1)–Mn(1)–N(4)
O(1)–Mn(1)–O(2)
O(2)–Mn(1)–N(2)
O(2)–Mn(1)–N(3)
O(2)–Mn(1)–N(4)
N(3)–Mn(1)–N(2)
N(3)–Mn(1)–N(4)
N(4)–Mn(1)–N(2)
a
102.32(5)
92.29(6)
175.57(5)
104.32(5)
94.54(5)
77.53(7)
153.22(7)
92.34(7)
87.51(7)
83.28(7)
88.79(8)
173.03(8)
75.70(7)
88.22(7)
89.88(7)
101.24(4)
92.28(4)
175.73(4)
104.88(4)
93.99(4)
77.79(5)
153.71(5)
92.59(6)
87.71(6)
83.54(6)
88.37(6)
173.53(6)
75.94(6)
88.52(6)
90.21(6)
100.27(7)
91.85(8)
175.72(8)
105.09(7)
93.57(7)
78.15(10)
154.40(10)
93.01(10)
88.09(10)
84.14(11)
87.60(11)
174.19(11)
76.31(10)
88.95(10)
90.50(10)
99.64(12)
90.57(12)
169.49(12)
108.72(11)
95.96(12)
79.72(16)
149.16(16)
90.23(18)
96.04(19)
79.11(16)
95.55(17)
170.08(17)
74.57(16)
75.30(15)
94.53(17)
Estimated standard deviations indicated in parentheses.

A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
1287
Fig. 5. Mn(II) halide distance vs. ionic radius [21].
chelate ligand, increases linearly with the ionic radius of
the halide anion (Fig. 5) [21]. All other metal/ligand dis-
tances are similar through the halide series, with the
Mn(1)–X(1) distance being the longest in each complex.
No significant perturbations in bond angles are found upon
change in the halide ligand.
The Mn(1)–X(1) (X = Cl, Br) distances in these com-
plexes are similar to those found in the mononuclear Mn(II)
halide derivatives shown in Fig. 1. For example, the chlo-
ride derivative 2 Æ CH3OH exhibits a Mn(1)–Cl(1) bond dis-
˚
tance of 2.441(1) A, which is generally similar to the
Mn(II)–Cl linkages found in [Mn(Hmimppa)Cl2] Æ CH3OH
˚
(Fig. 1(a), 2.398(1) and 2.449(1) A), [Mn(HL3)Cl2] Æ
˚
CH3OH (Fig. 1(b), 2.514(1) and 2.382(2) A), and
˚
[Mn(bpy)2Cl(H2O)]ClO4 (Fig. 1(e), 2.447(1) A) [4,5,8].
˚
The Mn(II)–Br distance of 4 Æ CH3OH (2.564(1) A) is in
the range of Mn–Br distances found for [Mn(HL4)Br2]
˚
˚
(Fig. 1(b), 2.624(2) and 2.550(2) A) and [MnBr2(mebpa)]
(Fig. 1(c), 2.6224(7) and 2.5048(7) A), respectively [5,6].
A search of the Cambridge Crystallographic Database
(version 5.26, updated May 2005) did not reveal any re-
ported examples of mononuclear N3O-ligated Mn(II) iodide
derivatives. The Mn(1)–I(1) distances in 6 Æ CH3OH and
˚
7 Æ O(CH2CH3)2 are 2.771(1) and 2.776(1) A, respectively.
3.2.3. Secondary hydrogen bonding interactions
In the solid-state X-ray structures of 2 Æ CH3OH,
4 Æ CH3OH, and 6 Æ CH3OH, the Mn(II) coordinated meth-
anol molecule acts as a hydrogen bond donor to a second
molecule of methanol present in the lattice (average dis-
˚
tance of Mn(II)–CH3OH. . .OHCH3 2.66 A; average bond
angle O–H. . .O 168ꢁ). In each of these complexes, the
non-coordinated methanol molecule also acts as a hydro-
gen bond donor to a perchlorate anion. These secondary
interactions are characterized by a slightly longer average
Fig. 4. ORTEP representations of the cationic portions of 2 Æ CH3OH,
4 Æ CH3OH, 6 Æ CH3OH, and 7 Æ O(CH2CH3)2.
˚
distance (average O(7). . .O(ClO3) 2.8 A). The amide N–H
3.2.2. Effect of differing halide
The Mn(1)–X(1) distance in 2 Æ CH3OH, 4 Æ CH3OH,
and 6 Æ CH3OH, all of which are supported by the mpppa
also participates in a weak hydrogen bonding interaction
involving a perchlorate anion, with a heteroatom distance
of ꢁ3.1 A. In 7 Æ O(CH2CH3)2, the Mn(II)-coordinated
˚

1288
A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
methanol molecule donates a hydrogen bond to the oxygen
˚
atom of a perchlorate anion (O(2). . .O(5)#1 2.828(8) A).
Similar to the mpppa-ligated complexes, the amide N–H
in 7 Æ O(CH2CH3)2 donates a hydrogen bond to a perchlo-
rate oxygen atom. The heteroatom distance of this interac-
˚
tion is ꢁ3.1 A.
3.3. Characterization of 2–7 by elemental analysis
Extensive drying of crystalline samples of 2–6 resulted in
loss of both the coordinated and non-coordinated metha-
nol molecules, as determined by elemental analysis (C, H,
N) measurements (Table 3). Specifically, for each complex,
the elemental analysis data was best fit using an empirical
formula of [(L)Mn(X)]ClO4 (L = mpppa or bpppa; X =
Cl, Br, I). Interestingly, the bpppa-ligated iodide derivative
7 exhibited a different behavior. Drying of a crystalline
sample of this complex under the same conditions used
for 2–6 resulted only in the loss of the non-coordinated
diethyl ether. In Table 3, we have presented all of the com-
plexes as having one coordinated methanol molecule, as
magnetic moment measurements shown in Table 3 (Evans
method), and performed in methanol solution, are consis-
Fig. 6. FTIR fingerprint region of 1.
and 1687 cmꢀ1, respectively. In solid-state infrared spectra
of the Mn(II) halide complexes 2–7, obtained as KBr pel-
lets, this vibration is found in the range of 1648–
1655 cmꢀ1. The shift to lower energy indicates weakening
of the C@O interaction which occurs upon coordination
of the amide carbonyl oxygen atom to the Lewis acidic
Mn(II) center. As noted above, this is consistent with a lar-
ger contribution from the amide resonance form having an-
ionic character at the oxygen atom.
The infrared spectral properties of 1 differ from those of
the halide complexes 2–7. Most notably, there is evidence
in the infrared spectrum of 1 for bidentate metal coordina-
tion of a perchlorate anion (Fig. 6). Specifically, the m3
tent with the presence of
a
high-spin six-
coordinate Mn(II) center, as is found in the solid-state
structures of 2 Æ CH3OH, 4 Æ CH3OH, 6 Æ CH3OH, and
7 Æ O(CH2CH3)2. However, it should be noted that the so-
lid-state infrared measurements described below were per-
formed using the dry, powdered samples sent for
elemental analysis.
3.4. Solid state infrared spectroscopic analysis
ꢀ
vibration, which is an asymmetric stretch of the ClO4 an-
The mpppa and bpppa ligands, when analyzed as neat
oils between salt plates, exhibit mCO vibrations at 1688
ion, splits into three bands at 1146, 1117, and 1089 cmꢀ1
.
Table 3
Characterization data for 1–7
c
Complex
EA
C
H
N
IR ðmC@O; mClO
Þ
lB
4
[(mpppa)Mn](ClO4)2 (1)
Calculateda
Found
39.38
39.48
4.53
4.59
9.67
9.63
1663, 1117, 626
1655, 1108, 623
1653, 1102, 623
1654, 1108, 623
1654, 1100, 623
1648, 1091, 624
1655, 1101, 623
5.8
5.9
6.1
5.8
6.0
5.7
5.9
[(mpppa)Mn(Cl)(HOCH3)]ClO4 (2)
[(bpppa)Mn(Cl)(HOCH3)]ClO4 (3)
[(mpppa)Mn(Br)(HOCH3)]ClO4 (4)
[(bpppa)Mn(Br)(HOCH3)]ClO4 (5)
[(mpppa)Mn(I)(HOCH3)]ClO4 (6)
[(bpppa)Mn(I)(HOCH3)]ClO4 (7)
Calculatedb
Found
44.27
43.32
5.09
5.10
10.87
10.54
Calculatedb
Found
49.91
49.57
4.89
4.94
9.71
9.52
Calculatedb
Found
40.79
40.65
4.69
4.71
10.02
9.92
Calculatedb
Found
46.37
46.27
4.54
4.52
9.02
8.92
Calculatedb
Found
37.56
37.58
4.32
4.45
9.23
9.24
Calculated
Found
42.79
42.66
4.60
4.80
7.99
8.02
a
Calculated as [(mpppa)Mn](ClO4)2.
b
Calculated as [(L)Mn(X)]ClO4 indicating loss of coordinated and non-coordinated methanol from the crystalline sample after drying for several hours
under vacuum at ambient temperature.
c
Measured by Evans method [12].

A.L. Fuller et al. / Inorganica Chimica Acta 359 (2006) 1282–1290
1289
This is consistent with a reduction from Td to C2v symme-
try and bidentate coordination of the perchlorate anion.
For Mn(ClO4)2 Æ 2H2O, these vibrations are found at
1210, 1138, and 945 cmꢀ1, respectively [22].
For 2–7, a broad, unresolved band is found in the region
of ꢁ3600–3000 cmꢀ1. The energy and shape of this feature
is consistent with the involvement of the amide N–H pro-
ton in a hydrogen bonding interaction with a perchlorate
anion, as previously discussed.
6 [6]. Spectra obtained for 1 and 4 are shown in Fig. 7.
The spectrum of 4 is characteristic of that collected for each
of the halide complexes. The similarity of the spectra for 2–
6 indicates that for this series of complexes, the EPR signal
is not perturbed as a function of the nature of the anion
present. This is further emphasized by the fact that the per-
chlorate derivative 1 exhibits a similar spectrum to the ha-
lide complexes.
4. Summary and conclusions
3.5. Solution magnetic properties
In this work, we have prepared and characterized a series
of mononuclear Mn(II) halide complexes. To our knowl-
edge, this is the first such series to be reported that has struc-
tural relevance to possible halide-bound species in the active
site of oxalate oxidase. In 2–7, a distorted octahedral
Mn(II) center is supported by a tetradentate N3O-donor li-
gand and coordinates one halide anion and one molecule of
methanol. X-ray crystallographic studies demonstrate that
the Mn–X (X = Cl, Br, I) distance increases in a linear fash-
ion with the anionic radius of the halide. The coordinated
molecule of methanol participates in a secondary hydrogen
bonding interaction in the solid state, but for the majority of
the complexes (2–6), is lost upon extensive drying of the
crystalline solid. Analysis of the solution magnetic proper-
ties of 2–7, via Evans method in methanol, suggests that sol-
vent is likely present in the Mn(II) coordination sphere in
solution, as the observed magnetic moment is consistent
with the presence of a high-spin distorted octahedral Mn(II)
center. Notably, change in the coordinated anion does not
produce any perturbation in the observed EPR signal.
Based on these results, we suggest that within the active site
of oxalate oxidase, the presence of differing halides may also
be accompanied by additional perturbations, for example
changes in secondary interactions, that can influence the
coordination and/or EPR properties of the mononuclear
Mn(II) center.
Analysis of the solution magnetic properties of 1–7 in
acetonitrile by Evans method indicated the presence of a
mononuclear high-spin Mn(II) center for each complex
[21]. Specifically, the magnetic moment (leff) of each com-
plex was found in the range of 5.7–6.1 lB, which is near
the spin-only mangnetic moment expected for a high-spin
mononuclear Mn(II) center (5.92 lB).
3.6. Electron paramagnetic resonance
The EPR properties of oxalate oxidase change as a func-
tion of the nature of the halide anion present, which may
indicate a change in the coordination number of the active
site Mn(II) center [3]. For example, the presence of iodide
anion perturbed the EPR signal in a way that suggested
formation of a five-coordinate Mn(II) center [3].
The EPR spectra of 1–6 were recorded in methanol at
70(5) K. Each Mn(II) complex of this series exhibits an ax-
ial EPR signal at g ꢁ 2.0 with a six-line pattern (average
Ai ꢁ 90 G) that is due to coupling of the electron spin with
the 55Mn nucleus (I = 1/2). Unlike the EPR properties of
[MnBr2(mebpa)] (Fig. 1(c)) and analogs, no broad signals
above or below g = 2 are found in the EPR spectra of 2–
5. Supplementary material
CCDC 281018–281021 contain the supplementary crys-
tallographic data for this paper. This data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
We gratefully acknowledge the Herman Frasch Founda-
tion (501-HF02 to L.M.B.) for financial support.
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