Mn(II) complexes of monoanionic bidentate chelators: X-ray crystal structures of Mn(dha)2(CH3OH)2 (Hdha = dehydroacetic acid) and [Mn(ema)2(H2O)]2 · 2H2O (Hema = 2-ethyl-3-hydroxy-4-pyrone)

Inorganica Chimica Acta 359 (2006) 228–236 www.elsevier.com/locate/ica

Mn(II) complexes of monoanionic bidentate chelators: X-ray crystal structures of Mn(dha)2(CH3OH)2 (Hdha = dehydroacetic acid) and [Mn(ema)2(H2O)]2 Æ 2H2O (Hema = 2-ethyl-3-hydroxy-4-pyrone) Wen-Yuan Hsieh a, Curtis M. Zaleski b,1, Vincent L. Pecoraro Phillip E. Fanwick c, Shuang Liu a,* a

b,1

,

Department of Industrial and Physical Pharmacy, School of Pharmacy and Pharmaceutical Sciences, Purdue University, 575 Stadium Mall Drive, West Lafayette, IN 47907, United States b Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, MI 48109, United States c Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, United States Received 17 June 2005; received in revised form 15 September 2005; accepted 19 September 2005 Available online 25 October 2005

Abstract This report describes synthesis and characterization of bis-ligand Mn(II) complexes of bidentate chelators: maltol (3-hydroxy-2methyl-4-pyrone), ethylmaltol (2-ethyl-3-hydroxy-4-pyrone), 1,2-dimethyl-3-hydroxy-4-pyridinone (DMHP) and dehydroacetic acid. All four Mn(II) complexes were characterized by elemental analysis, IR, UV/Vis, EPR, cyclic voltammetry, and X-ray crystallography in cases of Mn(dha)2(CH3OH)2 and [Mn(ema)2(H2O)]2 Æ 2H2O. The bidentate chelator plays a significant role in the solid state structure of its Mn(II) complex. For example, dha forms the monomeric complex Mn(dha)2(CH3OH)2 while ethylmaltol forms the dimeric complex [Mn(ema)2(H2O)]2. Because of smaller size, maltol ligands in Mn(ma)2 are able to bridge adjacent Mn(II) centers to give a polymeric structure in solid state. Despite of the difference in their solid state structures, both Mn(ema)2 and Mn(ma)2 exist in solution as monomeric Mn(II) species, Mn(ema)2(H2O)2 and Mn(ma)2(H2O)2. This assumption is supported by the similarity in their UV/Vis spectra, EPR data and electrochemical properties. Replacing maltol with DMHP results in a decrease (by
100 mV) in the redox potential for the Mn(II)/Mn(III) couple, suggesting that DMHP stabilizes Mn(III) better than maltol. Since Mn(DMHP)2(H2O)2 is readily oxidized to form the more stable Mn(III) complex Mn(DMHP)3, DMHP has the potential as a chelator for removal of excess Mn(II) from patients with chronic Mn toxicity.
2005 Elsevier B.V. All rights reserved. Keywords: Maltol; DMHP; Mn(II) complexes; Mn toxicity

1. Introduction Maltol (3-hydroxy-2-methyl-4-pyrone, Hma), ethylmaltol (2-ethyl-3-hydroxy-4-pyrone, Hema), and 1,2-dimethyl-3-hydroxy-4-pyridinone (DMHP) are bidentate chelators (Fig. 1), and form neutral complexes with many biologically important metal ions [1,2]. DHMP is an orally active iron chelator approved (marketed by Apotex Inc., *

1

Corresponding author. Tel.: +1 765 494 0236; fax: +1 765 496 3367. E-mail address: [email protected] (S. Liu). Tel.: +1 734 643 1519; fax: +1 734 936 7628.

0020-1693/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.09.025

Toronto, Canada as Ferripoxe) for treatment of iron overload in b-thalassemia patients [1–5]. A number of 3-hydroxy-4-pyridinones (3,4-HPs) have been studied for their coordination chemistry with Al(III) [6,7], Cr(III) [8], Ga(III) [9], Fe(III) [10–13], In(III) [14,15], Zn(II) [16,17], and Sn(II) [17]. Recently, 3,4-HPs have been proposed as matrix metalloproteinase inhibitors for the treatment of cancer and other diseases [18]. The vanadyl(IV) complexes of 3,4-hydroxypyrones and 3,4-HPs have been reported to possess insulin enhancing activity [19–22]. Zn(II) and Sn(II) complexes of HPs are reported to be useful in dental care formulations [16,17]. The 67Ga, 111In and 99mTc

W.-Y. Hsieh et al. / Inorganica Chimica Acta 359 (2006) 228–236 O

O

O

O

O

Dehydroacetic Acid (Hdha)

O

O

O OH Me

Maltol (Hma)

OH

OH O

Et

Ethylmaltol (Hema)

N Me

Me

1,2-Dimethyl-3-Hydroxy4-Pyridinone (DMHP)

Fig. 1. Structures of bidentate chelators used in this study.

complexes have been proposed as radiopharmaceuticals for scintigraphic imaging [23–25]. Recently, Co(II), Cu(II) and Cr(III) complexes of maltol have also been studied for their anti-hypoglycemic activity [26]. In contrast, there is very little information available about their Mn(II) and Mn(III) complexes. We are interested in the coordination chemistry of Mn(II) and Mn(III) with maltol and DMHP (Fig. 1) for their potential application as chelators in the treatment of chronic Mn toxicity. Recently, we reported two Mn(III) complexes: Mn(DMHP)3 and Mn(DMHP)2Cl [27]. A structural study of Mn(DMHP)2Cl Æ 0.5H2O revealed a five-coordinated Mn(III) complex containing two bidentate ligands with a square pyramidal coordination geometry. It was also found that the complex Mn(DMHP)3 Æ 12H2O is isostructural to complexes M(DMHP)3 Æ 12H2O (M = Al, Cr, Ga, Fe, and In) [6–15]. The Mn–O distance is almost identical to that of Fe–O [10]. The electrochemical data for Mn(DMHP)2Cl and Mn(DMHP)3 clearly demonstrated that the Mn(III) oxidation state is highly stabilized by DMHP [27]. Since Mn exists in biological systems mainly as Mn(II), it is necessary to study the coordination chemistry of Mn(II) with the bidentate chelators, such as maltol and DMHP. As a continuation of our previous studies [8,27], we prepared the bis-ligand Mn(II) complexes of maltol, ethylmaltol, and DMHP. The objective of this study is to demonstrate the ability of maltol and DMHP to form Mn(II) complexes. For comparison purpose, we also prepared the Mn(II) complex of dehydroacetic acid (Fig. 1: Hdha). We are particularly interested in the impact of chelators on structural and electrochemical properties of their Mn(II) complexes. In this report, we present the synthesis and characterization of Mn(II) complexes of bidentate chelators (Hma, Hema, DMHP, and Hdha), as well as X-ray crystal structures of [Mn2(ema)2(H2O)]2 Æ 2H2O and Mn(dha)2(CH3OH)2.

229

corded on a Perkin–Elmer spectrum One FT-IR spectrophotometer. The UV/Vis spectra were recorded on a Beckman DU-640 UV/Vis spectrophotometer. Variable-temperature magnetic susceptibility data were collected on a Quantum Design MPMS Controller SQUID susceptometer (Model 1822) equipped with a 5.5 T superconducting magnet at 5–300 K in a field of 2000 G. Powdered samples mulled in an eicosane matrix were used for data collection. Pascal
s constants were used to determine diamagnetic correction factors [28,29]. The X-band (9.4 GHz) Continuous-Wave (CW) perpendicular mode EPR spectra were collected on a Bruker EMX200E spectrometer, Department of Chemistry, University of Michigan. EPR spectra were obtained in the range 4–60 K using an Oxford Instruments ESR-900 Continuous flow cryostat, and 120 K spectra were collected using a Varian liquid Nitrogen continuous flow cryostat. The Mn(II) complexes were dissolved in a mixture of 10% glycerol and 90% H2O. Samples were frozen in liquid nitrogen before each measurement. Cyclic voltammograms of Mn(II) complexes were recorded on a Bioanalytical System BAS-100A electrochemical analyzer. A standard three-electrode cell was used with a polished glassy-carbon as the working electrode, a Pt wire as the auxiliary electrode, and an Ag/AgCl in 3 M NaCl solution as the reference electrode. The measurements were performed in aqueous solution containing 0.1 M sodium perchlorate at a scan rate of 100 mV/s. Before measurements, the sample solution was purged with extra pure nitrogen gas to remove the dissolved oxygen and a continuous nitrogen stream was blanketed over the solution during each measurement. 2.2. Mn(dha)2(CH3OH)2 Dehydroacetic acid (0.57 g, 3.4 mmol) and Mn(OAc)2 Æ 4H2O (0.42 g, 1.7 mmol) were mixed in 50 mL methanol. The resulting mixture was refluxed for 5 h. The yellow solid was filtered, washed with methanol, and dried under vacuum overnight. The yield was 0.72 g (93%). Slow evaporation of methanol gave microcrystals suitable for X-ray crystallographic analysis. IR (KBr, cm1): 1342, 1400, 1421, 1505, 1586, (s, mC–O and mring), 1660, 1697 (s, mC@O) and 3495 (bs, mO–H). Anal. Calc. for Mn(dha)2(CH3OH)2 Æ 0.25H2O: C, 47.22; H, 4.95. Found: C, 47.05; H, 4.90%. 2.3. Mn(ma)2

2. Experimental 2.1. Materials and methods Chemicals were purchased from Sigma–Aldrich (St. Louis, MO). Solvents were reagent grade from Mallinckrodt. Elemental analysis of Mn(II) complexes was performed by Dr. H. Daniel Lee using a Perkin–Elmer Series III analyzer, Department of Chemistry, Purdue University. Infrared (IR) spectra (4000–400 cm1) were re-

Maltol (3.79 g, 30 mmol) and sodium hydroxide (1.21 g, 30 mmol) were mixed in 120 mL water under nitrogen and the resulting mixture was heated until the solid was completely dissolved. MnCl2 (1.89 g, 15 mmol) dissolved in 20 mL degassed water was added to the solution under nitrogen while a yellow precipitate was formed in about 30 min. The reaction was continued for 2 h. The yellow solid was filtered, washed with water and dried under vacuum overnight. The yield was 4.22 g (85%). IR (KBr, cm1):

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1426, 1460, 1519, 1575 and 1607 (s, mC@O and mring), and 3420 (bs, mO–H). ESI-MS (positive mode): m/z = 306.0 for [C12H11MnO6 ]+. Anal. Calc. for Mn(ma)2 Æ 0.25H2O: C, 46.55; H, 3.42. Found: C, 46.40; H, 3.43%. 2.4. [Mn(ema)2(H2O)]2 Ethylmaltol (2.81 g, 20 mmol) in 40 mL water was heated under nitrogen until it was completely dissolved. To the solution was added Mn(OAc)2 Æ 4H2O (2.45 g, 10 mmol) dissolved in 20 mL degassed water. The yellow solution was refluxed for 2 h while a yellow precipitate was formed. The yellow solid was filtered while the solution was hot, washed with water, and dried under vacuum overnight. The solid was recrystallized from hot water to give yellow crystals suitable for X-ray crystallographic analysis. The yield was 1.95 g, (92%). The X-ray quality crystals were obtained from slow evaporation of aqueous solution. IR (KBr, cm1): 1451, 1467, 1509, 1575 and 1600 (s, mC@O and mring), and 3420 (bs, mO–H). ESI-MS (positive mode): m/z = 334.0 for [C14H15MnO6]+. Anal. Calc. for [Mn(ema)2 (H2O)]2: C, 47.88; H, 4.59. Found: C, 47.67; H, 4.54%. 2.5. Mn(DMHP)2(H2O)2 To a slurry of MnCl2 (0.126 g, 1 mmol) in acetonitrile (40 mL) was added a solution of DMHP (0.28 g, 2 mmol) and triethylamine (280 lL, 2 mmol) in 40 mL acetonitrile under nitrogen. The reaction mixture was refluxed overnight, during which time MnCl2 was completely dissolved and the solution color turned yellow. The resulting solution was cooled to room temperature, and the volume was reduced to
10 mL by stream of nitrogen gas. Cold diethyl ether (80 mL) was added to form a beige precipitate, which was filtered under nitrogen and washed with acetonitrile and diethyl ether. The yield was 0.305 g (83%). The sample was dried under vacuum overnight before being submitted for elemental analysis. IR (KBr, cm1): 1464, 1503, 1554 and 1625 (s, mC@O and mring), and 3423 (bs, mO–H). ESI-MS (positive mode): m/z = 331.13 for [C14H16MnN2O4 ]+. Anal. Calc. for Mn(DMHP)2 Æ 2.5H2O: C, 44.68; H, 5.59; N, 7.44; Found: C, 44.95; H, 5.50; N, 7.36%. 2.6. X-ray crystallographic analysis Crystallographic data for [Mn(ema)2(H2O)]2 Æ 2H2O and [Mn(dha)2(CH3OH)2] were collected on a Nonius Kappa CCD diffractometer. The selected crystallographic data are listed in Table 1. The selected bond distances and bond angles for [Mn(ema)2(H2O)]2 Æ 2H2O and [Mn(dha)2 (CH3OH)2] were are listed in Tables 2 and 3. Crystals were mounted on a glass fiber in a random orientation. Preliminary examination and data collection were performed using graphite monochromated Mo Ka radiation (k = ˚ ). Cell constants and an orientation matrix for 0.71073 A data collection were obtained from least-squares refine-

Table 1 Selected crystallographic Mn(dha)2(CH3OH)2

data

[Mn(ema)2(H2O)]2 Æ 2H2O

[Mn(ema)2(H2O)]2 Æ 2H2O Formula C28H36Mn2 O16 Fw 738.47 Space group P 1 (No. 2) ˚) a (A 7.7557 (10) ˚) b (A 10.535 (2) ˚) c (A 10.7859 (12) a () 109.91 (7) b () 97.75 (6) c () 102.77 (8) ˚ 3) V (A 786.6 (2) Z 1 dcalc (g/cm3) 1.559 Temperature (K) 150 Crystal dimensions 0.39 · 0.30 · 0.30 (mm3) ˚) Radiation (k, A Mo Ka (0.71073) Linear absorption 0.842 coefficient (mm1) Transmission factors 0.74, 0.78 R(Fo) 0.040a Rw ðF 2o Þ 0.106b P P a R ¼
jjF o j  jF c jj= jF o j for F 2o > 2rðF 2o Þ. 1=2 P P b 2 Rw ¼ wðjF o j  jF 2c jÞ2 = wjF 2o j2 .

Table 2 Selected bond distances Mn(dha)2(CH3OH)2

of

Mn(dha)2(CH3OH)2 C18H22MnO10 453.31 P21/c (No. 14) 7.7588 (7) 15.8440 (17) 8.8965 (8) 90.00 114.012 (4) 90.00 999.01 (17) 2 1.507 150 0.44 · 0.38 · 0.35 Mo Ka (0.71073) 0.684 0.40, 0.79 0.073a 0.183b

[Mn(ema)2(H2O)]2 Æ 2H2O

[Mn(ema)2(H2O)]2 Æ 2H2O

Mn(dha)2(CH3OH)2

Mn–O(1) Mn–O(2) Mn–O(4) Mn–O(4) Mn–O(5) Mn–O(7) O(1)–C(1) O(2)–C(2) O(4)–C(8) O(5)–C(9) C(1)–C(2) C(8)–C(9)

Mn–O(1) Mn–O(1) Mn–O(3) Mn–O(3) Mn–O(2) Mn–O(2) O(1)–C(1) O(2)–C(21) O(3)–C(11)

2.1725 (15) 2.1933 (16) 2.1531 (15) 2.1929 (15) 2.2322 (17) 2.2299 (16) 1.322 (3) 1.267 (3) 1.323 (3) 1.267 (3) 1.448 (3) 1.448 (3)

and

and

2.103 2.103 2.212 2.212 2.145 2.145 1.260 1.244 1.430

(3) (3) (4) (4) (3) (3) (5) (6) (7)

ment, using the setting angles in the range of and 20 < h < 27 for [Mn(ema)2(H2O)]2 Æ 2H2O Mn(dha)2(CH3OH)2. For [Mn(ema)2(H2O)]2 Æ 2H2O, a total of 10 404 reflections were collected and 3689 reflections were unique. For Mn(dha)2(CH3OH)2, a total of 6128 reflections were collected and 1973 reflections were unique. Lorentz and polarization corrections were applied to the data. A linear absorption coefficient is 8.4/cm in [Mn(ema)2(H2O)]2 Æ 2H2O and 6.8/cm in Mn(dha)2(CH3OH)2. An empirical correction was applied using the program SCALEPACK [30]. The structure of [Mn(ema)2(H2O)]2 Æ 2H2O was solved using the program PATTY in DIRDIF-99 [31], and the structure of Mn(dha)2(CH3OH)2 was solved by direct method using SIR-2002 [32]. Both structures were refined on an AlphaServer 2100 using SHELXL-97 [33]. Crystallographic drawings were produced using the program ORTEP.

W.-Y. Hsieh et al. / Inorganica Chimica Acta 359 (2006) 228–236 Table 3 Selected bond angles Mn(dha)2(CH3OH)2

of

[Mn(ema)2(H2O)]2 Æ 2H2O O(1)–Mn–O(2) O(1)–Mn–O(4) O(1)–Mn–O(4) O(1)–Mn–O(5) O(1)–Mn–O(7) O(2)–Mn–O(4) O(2)–Mn–O(4) O(2)–Mn–O(5) O(2)–Mn–O(7) O(4)–Mn–O(4) O(4)–Mn–O(5) O(4)–Mn–O(5) O(4)–Mn–O(7) O(4)–Mn–O(7) O(5)–Mn–O(7) C(1)–O(1)–Mn C(2)–O(2)–Mn C(8)–O(4)–Mn C(8)–O(4)–Mn C(9)–O(5)–Mn

75.75 159.55 122.82 86.25 82.48 91.24 102.36 100.63 154.12 77.33 74.00 150.68 88.88 102.91 91.64 113.21 113.95 115.23 141.88 114.76

[Mn(ema)2(H2O)]2 Æ 2H2O

and

Mn(dha)2(CH3OH)2 (6) (6) (6) (6) (6) (6) (6) (6) (6) (7) (6) (6) (6) (6) (6) (13) (14) (16) (14) (14)

O(1)–Mn–O(1) O(1)–Mn–O(3) O(1)–Mn–O(3) O(1)–Mn–O(2) O(1)–Mn–O(2) O(2)–Mn–O(3) O(2)–Mn–O(3) O(2)–Mn–O(2) O(21)–Mn–O(3) C(1)–O(1)–Mn C(11)–O(3)–Mn C(21)–O(2)–Mn

180.00 (8) 88.01 (15) 91.99 (15) 80.33 (13) 99.67 (13) 88.11 (14) 91.89 (14) 180.00 (14) 180.00 (14) 134.4 (3) 126.7 (4) 135.7(3)

3. Results and discussion In a normal individual, the Mn level is under tight control by proteins, such as transferrin. However, Mn overload becomes significant among workers in the mining, welding and battery manufacturing industries [34–41]. Thus, it would be a considerable medicinal interest to develop effective chelators for the removal of excess body Mn in patients. Mn resembles Fe in several ways: having similar ionic radii, carrying both +2 and +3 oxidation states under physiological conditions, and processing a similar affinity for the carrier protein, transferrin [42,43]. As demonstrated in our previous report [27], the complex Mn(DMHP)3 Æ 12H2O is almost identical to complexes M(DMHP)3 Æ 12H2O (M = Cr, Ga, and Fe) in solid state [8–11]. The electrochemical data also showed that Mn(III) is highly stabilized by DMHP. A direct comparison of M–O bond distances suggests that Mn(DMHP)3 should have a stability constant close to that of Fe(DMHP)3 (log b3 = 35.9) [12,13]. This estimation is supported by the fact that Fe(III)–EDTA and Mn(III)–EDTA have almost identical stability (log bFe(III)–EDTA = 25.1, log bMn(III)–EDTA = 25.2) [44]. DMHP has the potential as a chelator for the removal of excess intracellular Mn(III) and Fe(III). 3.1. Synthesis of bis-ligand Mn(II) complexes To demonstrate the ability of bidentate chelators to form Mn(II) complexes, we prepared four bis-ligand Mn(II) complexes by reacting a Mn(II) salt with two equivalents of a respective bidentate chelator in the presence of a base, such as sodium hydroxide, under nitrogen atmosphere. Mn(dha)2(CH3OH)2 could also be prepared from

231

the reaction of Mn(III) acetate with two equivalents of Hdha in the presence of air, suggesting that the Mn(II) oxidation state is stabilized by dha ligands. Since Hdha was not in large excess, methanol would most likely be the reducing agent. Mn(dha)2(CH3OH)2 was isolated from methanol as yellow crystals suitable for X-ray crystallography. Maltol and ethylmaltol formed complexes Mn(ma)2 and [Mn(ema)2(H2O)]2, respectively, in the presence of air; but there was always a small amount of Mn(III) complex formed. Crystals suitable for X-ray crystallographic analysis of [Mn(ema)2(H2O)]2 Æ 2H2O were obtained by recrystallization from hot water. Mn(DMHP)2(H2O)2 had to be prepared under nitrogen. In the air, Mn(DMHP)2Cl or Mn(DMHP)3 was the only product isolated from the reaction of Mn(II) chloride with DMHP depending on the Mn:DMHP ratio. Characterization of Mn(DMHP)2Cl and Mn(DMHP)3 has been described in our previous report [27]. The solubility of [Mn(ema)2(H2O)]2 in water was
35 mg/mL (50 mM). The water solubility of Mn(ma)2 was 5 mg/mL (16 mM). The water solubility of Mn(dha)2(CH3OH)2 in water was only 0.3 mg/mL (
0.75 mM). The water solubility of Mn(DMHP)2(H2O)2 was > 75 mg/mL ( > 100 mM); but it was not stable due to rapid oxidation of Mn(II) as evidenced by the presence of the green color for the Mn(III) complex. 3.2. IR and UV/Vis spectroscopy The IR spectra of [Mn(ema)2(H2O)]2 and Mn(ma)2 show four strong bands in the frequency region of 1600– 1400 cm1, which are characteristic of the coordinated 3hydroxy-4-pyrones. The IR spectra of [Mn(ema)2(H2O)]2 and Mn(ma)2 also show a broad band at
3420 cm1 due to the coordinated or crystallization water. The IR spectrum of Mn(dha)2(CH3OH)2 show several strong bands between 1600 and 1400 cm1 due to the mC–O and mC@C from the two coordinated dha ligands. A broad band is observed at
3450 cm1 is due to mO–H from the coordinated methanol molecules. The UV/Vis spectra of Mn(ema)2 and Mn(ma)2 in water are almost identical, and display an intense metal-to-ligand charge transfer (MLCT) band at the wavelength of 330–340 nm with e = 24 000 for Mn(ema)2 and e = 18 400 for Mn(ma)2. The UV/Vis spectrum of Mn(dha)2(CH3OH)2 was not obtained due to its very low water solubility. Attempts to obtain the UV/Vis spectrum of Mn(DMHP)2(H2O)2 were unsuccessful due to its rapid oxidation in air. The ES-MS spectral and elemental analysis data were consistent with their proposed formula. 3.3. X-ray crystal structure of Mn(dha)2(CH3OH)2 Fig. 2 shows the ORTEP drawing of Mn(dha)2(CH3OH)2. The selected crystallographic data are listed in Table 1. There are two Mn(dha)2(CH3OH)2 molecules in each unit cell. The Mn(II) was bonded to four oxygen

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Fig. 2. ORTEP diagram of Mn(dha)2(CH3OH)2 (ellipsoids are at 50% probability). Hydrogen atoms are omitted for the sake of clarity.

atoms from two dha ligands and two oxygen atoms from the coordinated methanol molecules. The coordination geometry of Mn(dha)2(CH3OH)2 is best described as octahedral with four oxygen atoms of dha ligands occupying the four equatorial positions and the two methanol molecules filling the two axial sites. The dha ligand and Mn(II) form a planar six-membered chelate ring with the maxi˚ from the calculated least mum deviation being 0.045(5) A square plane. The dihedral angle between the two dha ligands is 0.00(15). The Mn(II) is at the center of the square plane defined by four oxygen atoms from the two dha ligands, which are in a trans-configuration without a significant distortion from the equatorial plane. The significant distortion occurs at the angles between Mn(II) and the donor atoms of the two dha ligands, ranging from 80.33(13) to 99.67(13) (Table 3). The structure of Mn(dha)2(CH3OH)2 is very similar to that of Co(dha)2(DMF)2 [45], but is different from that of Mn(dha)2(H2O)2, in which one water molecule is at the axial position and the other at the equatorial position [46]. The coordinated methanol molecules are involved in the intermolecular hydrogen bonding with the non-coordinating carbonyl oxygen atom from the adjacent Mn(dha)2(CH3OH)2 molecule. These hydrogen bonds strengthen the crystal packing between Mn(dha)2(CH3OH)2 molecules and form a infinite chain (Figure SI), which may contribute to its low water solubility. 3.4. X-ray crystal structure of [Mn(ema)2(H2O)]2 Æ 2H2O Fig. 3 shows the ORTEP drawing of [Mn(ema)2(H2O)]2. Crystallization water molecules and hydrogen atoms are omitted for the sake of clarity. The selected crystallographic data are listed in Table 1. There is only one [Mn(ema)2(H2O)]2 molecule along with two water molecules of crystallization in each unit cell. The structure of [Mn(ema)2(H2O)]2 is dimeric with each Mn(II) bonding to four oxygen atoms from two ethylmaltol ligands, a water molecule and a bridging hydroxyl oxygen atom from one of the two ethylmaltol ligands. The coordination geometry at each Mn(II) is best described as distorted octahedron. The two oxygen atoms (O1, O2) from the terminal ethyl-

Fig. 3. ORTEP diagram of [Mn(ema)2(H2O)]2 (ellipsoids are at 50% probability). Crystallization water and hydrogen atoms are omitted for the sake of clarity.

maltol, a water molecule (O7), and the hydroxyl oxygen atom (O4) from the bridging ethylmaltol occupy the four equatorial positions while the hydroxyl oxygen atom (O4a) and the carbonyl oxygen atom (O5) from two bridging ethylmaltol ligands occupy two axial positions. The Mn–O1 and Mn–O4 bonds are in the trans position while the Mn–O2 and Mn–O5 bonds are in the cis-position. Each ethylmaltol and Mn(II) forms a planar five-membered chelate ring. The dihedral angle between these two planes is 79.91(8). The O1–Mn–O2 and O4–Mn–O5 angles are 75.75(6) and 74.00(6), respectively, due to the small ‘‘bite distance’’ of the coordinated ethylmaltol (Table 3). The O1–Mn–O4 and O5–Mn–O4a angles are 159.55(6) and 150.68(6), respectively. Apparently, these angles are deviated from an ideal octahedral coordination geometry (90 for O1–Mn–O2 and O4–Mn–O5 angles; and 180 for O1–Mn–O4 and O5–Mn–O4a angles). The arrangement of ethylmaltol ligand in [Mn(ema)2(H2O)]2 is very similar to that in Mn(dha)2(H2O)2 [46]. The two coordinated water molecules are involved in both intermolecular and intramolecular hydrogen bonding (Figure SII). Each coordinated water molecule serves as a hydrogen bond donor to O1 and O5 of the adjacent [Mn(ema)2(H2O)]2 molecule, and a hydrogen bond acceptor to the crystallization water, which also forms a hydrogen bond with the coordinated carbonyl-oxygen (Figure SII), which is important for the stabilization of the dimeric structure of [Mn(ema)2(H2O)]2 Æ 2H2O. Attempts to grow crystals of Mn(ma)2 were not successful due to its low water solubility. However, the elemental analysis data indicates that there are no water molecules bonded to the Mn(II). Since its preferred coordination number is 6, it is reasonable to believe that Mn(ma)2 may exist as a polymer in the solid state (Figure SIII). The two maltol ligands in each Mn(ma)2 can be in transconfiguration to form square plane with one maltol ligand bridging one neighboring Mn(II) and the second maltol bridging the other Mn(II) center in the opposite direction.

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The two maltol ligands in Mn(ma)2 can also be in the cisconfiguration with both bridging the Mn(II) center in the same direction. The polymeric structure may contribute to its very low water solubility. The difference between ethylmaltol and maltol is the 2-substituent of the pyrone ring. It seems that the ethyl group in [Mn(ema)2(H2O)]2 is probably bulky enough to prevent the formation of a polymeric chain. As a result, the water molecule is bonded to the Mn(II) to form a dimeric complex [Mn(ema)2(H2O)]2. We attempted to grow crystals for X-ray crystallographic analysis of Mn(DMHP)2(H2O)2 but were not successful for due to its air-sensitivity. Mn(DMHP)2(H2O)2 remains air-stable in the solid state. As soon as it is dissolved in water, methanol, or a combination thereof, it will be rapidly oxidized to form the corresponding Mn(III) complexes in the presence of air. Based on elemental analysis data and water solubility, we believe that Mn(DMHP)2(H2O)2 most likely has a structure similar to that of Mn(dha)2(CH3OH)2 or Mn(dha)2(H2O)2 [46]. The main difference between the coordinated maltol and DMHP is their electron density distribution over two oxygen donors. In the coordinated maltol, the delocalization of electrons between the carbonyl and hydroxyl group is minimal, and the charge is largely localized at the hydroxy-O atom [8,11], which makes it easier for the hydroxyO to bridge the two neighboring Mn(II) centers. In the coordinated DMHP, there is much more delocalization of electrons between the carbonyl and hydroxyl groups and the negative charge is more evenly distributed over the two oxygen donors and the pyridinone ring [6–11]. As a result, it becomes more difficult for the hydroxy-O to bridge the two neighboring Mn(II) centers. Thus, the DMHP ligands coordinate to the Mn(II) to form the monomeric complex Mn(DMHP)2(H2O)2 while ethylmaltol is able to form the dimeric complex [Mn(ema)2(H2O)]2. 3.5. Magnetic properties To explore magnetic interactions between the two neighboring Mn(II) centers in [Mn(ema)2(H2O)]2, we determined its variable-temperature magnetic susceptibility at 5–300 K in an applied magnetic field of 2000 G. The magnetic susceptibility data of all four Mn(II) complexes are summarized in Table 4. The observed vMT products at 300 K for Mn(dha)2(CH3OH)2 (4.53 cm3 K mol1), Mn (DMHP)2 Æ 2.5H2O (4.20 cm3 K mol1), and Mn(ma)2 Æ

233

0.25H2O (4.39 cm3 K mol1) were close to the spin-only value for a high-spin Mn(II) (4.37 cm3 K mol1). The vMT product was 8.43 cm3 K mol1 at 300 K for [Mn(ema)2(H2O)]2, which is close to the value expected for the two non-coupled Mn(II) ions (8.75 cm3 K mol1 at g = 2). The vMT product reaches a maximum value of 9.20 cm3 K mol1 at 40 K. The increase in vMT product indicates that the two Mn(II) centers in [Mn(ema)2(H2O)]2 are ferromagnetically coupled. The small increase in the vMT product suggests that the coupling between two Mn(II) centers in [Mn(ema)2(H2O)]2 is very weak. The magnetic susceptibility data for [Mn(ema)2(H2O)]2 was fit to a model incorporating the exchange interaction, J, and the primary components of the g-tensor. The Hamiltonian used in the data analysis is given in H ¼ gbH  ðS 1 þ S 2 Þ  2J ðS 1  S 2 Þ.

ð1Þ

This Hamiltonian has been used in the data analysis of other Mn(II)–Mn(II) dimers [47–50]. The Zeeman part of the Hamiltonian assumes that the g values for the two Mn(II) are isotropic and equivalent. Eq. (2) shows the Van Vleck formula used to fit the magnetic data of the dimer   Ng2 b2 X S i ðS i þ 1Þð2S i þ 1Þ expðEi =kT Þ vm ¼ . ð2Þ ð2S i þ 1Þ expðEi =kT Þ 3kT i The zero-field intradimer exchange energies (Ei) are given in Ei ¼ J ½S i ðS i þ 1Þ  S 1 ðS 1 þ 1Þ  S 2 ðS 2 þ 1Þ;

ð3Þ

where S1 and S2 are individual spins associated with two Mn(II) centers in [Mn(ema)2(H2O)]2 and Si assumes values of Si = |S1 + S2| +    + |S1  S2|. Fig. 4 shows the plot of v and vMT versus temperature for [Mn(ema)2(H2O)]2. Solid lines represent the best fit calculation using Eq. (2). The best fits between the observed magnetic susceptibility data and those calculated from Eq. (2) was marginal, but for the purpose of this paper this fit is adequate to give a J value of 0.5 ± 0.1 cm1 at g = 2.02 for [Mn(ema)2(H2O)]2. The small decrease in the vMT product at temperatures <40 K is probably caused by Zeeman effects or by intermolecular antiferromagnetic interactions between the two Mn(II) centers at the very low temperatures. [Mn(ema)2(H2O)]2 represents one of a few structurally characterized Mn(II) complexes in which the two Mn(II) centers are ferromagnetically coupled [47–52].

Table 4 Summary of the spectroscopic and magnetic data for Mn(II) complexes

[Mn(ma)2 Æ 0.25H2O] [Mn(ema)2(H2O)]2 [Mn(DMHP)2 Æ 2.5H2O] [Mn(dha)2(CH3OH)2] a b

Extinction coefficient e (cm1 Æ M1)

Magnetic susceptibility vMT (cm3 K mol1)

Mn(III)/Mn(II) redox potential DE1/2 (mV)

18 400 24 000 NAa NAb

4.39 8.43 4.20 4.53

405 410 295 NAb

Data not obtained due to fast oxidation. Data not obtained due to low solubility.

(300 K) (300 K); 9.23 (40 K) (300 K) (300 K)

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Fig. 4. Plot of vM and vMT vs. temperature for [Mn(ema)2(H2O)]2 Æ 2H2O. Solid lines represent the best fit calculation using Eq. (2).

3.6. EPR spectroscopy The EPR spectra are very sensitive to multiplicity of the metal complexes in solution. For the Mn(II) with I = 5/2 nuclear spin and S = 5/2 as electron spin ground state, free ions or ions in a weak ligand field, a well-defined six-line signal is observed in perpendicular-mode EPR spectrum. This six-line signal is centered at g = 2 with an averaging hyperfine splitting (A) of 90 G. EPR samples were prepared by dissolving the Mn(II) complex in 10% glycerol aqueous solution, and were frozen in liquid nitrogen before each measurement. The perpendicular-mode EPR spectra of Mn(ema)2 and Mn(ma)2 were collected at 5 and 120 K. Fig. 5 shows the X-band perpendicular-mode EPR spectra of Mn(ema)2 and Mn(ma)2 at 120 K. Both Mn(ema)2 and Mn(ma)2 show a six-line signal centered at 3250 G (g = 2.03) with an average A value of 86 G. At 5 K, Mn (ema)2 and Mn(ma)2 show a six-line signal centered at 3300 G (g = 2.03) with a average A value of 86 G. The shape of the six-line signal and A value suggest that the coordinated ethylmaltol and maltol ligands remain bonded to the Mn(II) in solution. The six-line signals are almost identical at 120 K indicate that both Mn(ema)2 and Mn(ma)2 exist in solution as the monomeric Mn(II) species, Mn(ema)2(H2O)2 and Mn(ma)2(H2O)2. This assumption is also supported by the similarity of their UV/Vis spectral and electrochemical properties, and is completely consistent with their ESI-MS spectral data of Mn(ema)2 and Mn(ma)2. The X-band perpendicular-mode EPR spectrum of Mn(DMHP)2(H2O)2 is almost identical to those of Mn(ema)2 and Mn(ma)2. 3.7. Electrochemistry We examined the electrochemical properties of Mn(ma)2, Mn(ema)2 and Mn(DMHP)2 by cyclic voltam-

Fig. 5. Perpendicular-mode EPR spectra of Mn(ema)2 (top) and Mn(ma)2 (bottom) at 120 K in 10% glycerol/H2O. EPR parameters: microwave frequency, 9.303 GHz; Power, 20 mW; modulation amplitude, 1 G; modulation frequency, 100 kHz.

metry. Fig. 6 shows cyclic voltammograms for Mn(ema)2, Mn(ma)2 and Mn(DMHP)2. Redox potentials are given as those versus NHE (Table 4). Both Mn(ma)2 and Mn (ema)2 exhibited an oxidation wave at
600 mV and a reduction wave at
200 mV versus NHE (Fig. 6: left). The E1/2 values of the Mn(II)/Mn(III) couple in Mn(ma)2 and Mn(ema)2 are 400 mV versus NHE. The anodic/cathodic potential difference (DEp = Epc  Epa) was
400 mV, which is much larger than the theoretical value (DEp = 59 mV) for the reversible one-electron Nerstian transfer. Thus, these waves are best described as irreversible. Since no redox waves are observed for maltol or ethylmaltol, these redox waves are due to oxidation from Mn(II) to Mn(III). The low oxidation potential indicates that Mn(III) is stabilized in Mn(ma)2 and Mn(ema)2. The irreversibility suggests that the oxidation of Mn(II) to Mn(III) probably involves significant conformational changes. Addition of free ligand (maltol or ethylmaltol) did not have a significant impact on their redox potentials. The similarity in electrochemical properties of Mn (ma)2 and Mn(ema)2 indicates that the 2-substituent of 3-hydroxy-4pyrones has little impact on electrochemical properties of their bis-ligand Mn(II) complexes. Mn(DMHP)2 shows a quasi-reversible redox wave at E1/2 = 0.30 V (Fig. 6, right). The anodic/cathodic potential difference (DEp = Epc  Epa) was
100 mV. Addition of DMHP causes a significant decrease in the redox potential (by
120 mV) for the Mn(II)/Mn(III) couple; but the

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235

Fig. 6. Typical cyclic voltammograms of Mn(ma)2, Mn(ema)2, Mn(DMHP)2, and Mn(DMHP)2 + DMHP. The concentration was
1 mM. Potentials are given as those vs. NHE since the Ag/AgCl electrode has a potential of +0.2 V vs. NHE. The scan rate was 100 mV/s.

reversibility remains relatively unchanged. A direct comparison between Mn(ma)2 and Mn(DMHP)3 shows a significant decrease (by
100 mV) in the redox potential (E1/2) for the Mn(II)/Mn(III) couple when maltol is replace by DMHP. This suggests that DMHP stabilizes the Mn(III) oxidation state better than maltol and ethylmaltol. From this point of view, DHMP has the advantage over both maltol and ethylmaltol. 4. Conclusions In this study, we prepared the bis-ligand Mn(II) complexes of four bidentate chelators (maltol, ethylmaltol, DMHP, and Hdha). These Mn(II) complexes have been characterized by elemental analysis, IR, UV/Vis, EPR, electrochemical method, magnetic susceptibility measurements, and in cases of Mn(dha)2(CH3OH)2 and [Mn (ema)2(H2O)]2 Æ 2H2O by the X-ray crystallography. It was found that the bidentate chelator plays a significant role in the solid state structure of its Mn(II) complex. For example, Mn(dha)2(CH3OH)2 is monomeric with two dha and two methanol molecules bonding to the Mn(II) in a distorted octahedral coordination geometry. [Mn (ema)2(H2O)]2 has a dimeric structure with the two Mn(II) centers bridged by the hydroxyl-O atoms from ethylmaltol ligands. Because of smaller size, maltol ligands in Mn(ma)2 are able to bridge the neighboring Mn(II) centers to give a polymeric structure in solid state. In the coordinated DMHP, the negative charge is more evenly distributed over two oxygen donors. As a result, it forms the monomeric complex Mn(DMHP)2(H2O)2. Despite the difference in their solid state structures, Mn(ema)2 and Mn(ma)2 exist in solution as the monomeric Mn(II) species, Mn(ema)2 (H2O)2 and Mn(ma)2(H2O)2. Changing the 2-substituents in 3-hydroxy-4-pyrones has little impact on electrochemical properties of their Mn(II) complexes. However, replacing maltol with DMHP results in a significant decrease (by
100 mV) in the redox potential of the Mn(II)/Mn(III) couple, suggesting that DMHP stabilizes Mn(III) better than maltol.

For a chelator to remove the excess intracellular Mn(II) and Mn(III) under physiological conditions, it should have the ability to form Mn(II) and/or Mn(III) complexes with high thermodynamic stability. Hdha is able to form the neutral Mn(II) complex Mn(dha)2, which has an accumulative stability constant of log b2 = 6.34 [53]. Thus, Hdha will not be an efficient chelator to remove the excess intracellular Mn(II) due to the low stability of its Mn(II) complex. Both maltol and DMHP are able to form the neutral Mn(II) complexes Mn(ma)2 and Mn(DMHP)2. However, attempts to determine their stability constants using the potentiometric and spectrophotometric titration techniques were not successful due to their high air-instability in aqueous solution. As soon as Mn(II) salt and DMHP were mixed in the air, the corresponding Mn(III) complex would form regardless the solvent (water, methanol or combination thereof) and relative Mn(II)/ligand ratio most likely due to its high affinity for Mn(III) as demonstrated by the low oxidation potentials of complexes Mn(DMHP)2 Cl and Mn(DMHP)3 [27]. That is why the bis-ligand complex Mn(DMHP)2 had to be prepared under nitrogen. Since Mn(DMHP)2 can be readily oxidized to form the more stable Mn(III) complex Mn(DMHP)3, DMHP may have the potential as a chelator to remove excess body Mn in patients with chronic Mn toxicity. Acknowledgment This work is supported by Purdue University and Bristol-Myers Squibb Medical Imaging Inc. Appendix A. Supplementary data X-ray crystallographic files are in CIF format for the reported structures of Mn(dha)2(CH3OH)2 and [Mn(ema)2(H2O)]2 Æ 2H2O. These materials are available free of charge at http://pubs.acs.org. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2005.09.025.

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