A new complex of manganese(II) with l -α-alanine: structure, spectroscopy and thermal study

Polyhedron 18 (1999) 2321–2326

A new complex of manganese(II) with L-a-alanine: structure, spectroscopy and thermal study ´ a , *, M. Sikorska-Iwan a , M. Jaroniec b , T. Gl«owiak c R. Mrozek a , Z. Rza¸czynska a

Faculty of Chemistry, Maria Curie-Skl«odowska University, M.C. Skl«odowska Sq. 2, Lublin, Poland b Department of Chemistry, Kent State University, Kent, OH 44242 -0001, USA c Faculty of Chemistry, University of Wrocl«aw, F. Joliot-Curie 14, Wrocl«aw, Poland Received 17 March 1999; accepted 27 April 1999

Abstract A polynuclear manganese(II) complex of L-a-alanine of the formula [Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O (where ala5L-a-alanine) has been synthesized and characterized. An X-ray crystallographic study shows that the polymer consists of double linear chains. One chain contains alternately located Mn(1) and Mn(2) ions, and the other chain is built of Mn(3) and Mn(4) ions. Each independent manganese(II) ion is in an octahedral arrangements. Mn(1), Mn(2) and Mn(3) atoms display the same coordination environment being surrounded by two carboxylate oxygen atoms of a-alanine molecules, three chloride ions and one aqua ligand. In the coordination octahedron of the Mn(4) atom one chloride ion is replaced by a water molecule. In chains Mn(II) ions are joined alternately by a chloride ion or by two carboxylate bridges from different a-alanine molecules and one Cl 2 ion. Three non-coordinated water molecules and one chloride ion are located between the chains. The crystal structure is stabilized by an extensive system of hydrogen bonds. Thermogravimetric analysis shows this complex is stable to 353 K. Upon heating the complex loses molecules of water and is transformed into the suitable oxides. The IR spectra of free ligand and complex have been discussed.  1999 Elsevier Science Ltd. All rights reserved. Keywords: a-Alanine complexes; Manganese(II); X-ray analyses

1. Introduction Manganese is a biologically important trace element as constituent of proteins and enzymes [1,2]. Plants and many bacteria require trace amounts of manganese for a healthy existence. Thus, the coordination chemistry of manganese is important and the studies of its reactivity are especially interesting. As a continuation of our study on the manganese(II) complexes with natural amino acids [3–6], the current work reports on structure, spectroscopy and thermogravimetric data for a new manganese(II) complex with L-aalanine. Based on the known crystal structures of a-alanine complexes, which are essential for life metal ions, several modes of binding can be clearly distinguished. The most common type of coordination is binding of metal ions through the nitrogen atom of an amino group and one carboxylate oxygen atom of a-alanine. In these cases, the *Corresponding author. Tel.: 148-81-537-5743; fax: 148-81-5333348. ´ E-mail address: [email protected] (Z. Rza¸czynska)

amino acid molecule acts as a chelating ligand. Such type of coordination is commonly observed in monomeric complexes of copper(II) with D,L and D,L-a-alanine [7–12], but also in complexes with other divalent metals such as cobalt(II) [13–20], nickel(II) [21–23], cadmium [24] as well as with trivalent metal ions, e.g. dimeric complex of chromium [19]. Another mode of coordination of M(II) ions by a-alanine assumes binding of these ions by a bidentate-bridging carboxylate group while the nitrogen atom is uncoordinated. In this group of complexes, the trimeric [25], tetrameric [26] and polymeric compounds [27–29] can be distinguished. In the polymeric complexes neighboring metal ions are linked only by one carboxylate group. The a-alanine molecule also acts as a monodentate ligand like in the molybdenum(IV) complex [30], or as a tridentate ligand in the copper(II) complex [31]. In this case the amino group and the carboxylate oxygen atom chelate metal ion and, additionally, the second carboxylate oxygen atom is linked to the neighbouring ion creating the polymeric-type structure. In spite of the fact that the silver(I) ion does not belong to biologically essential elements it is noteworthy to mention its complex with

0277-5387 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0277-5387( 99 )00140-0

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a-alanine [32]. In this compound the amino acid molecule bridges the silver ions in a polymeric chain via nitrogen and oxygen atoms. In the present work a new polymeric complex of manganese(II) with L-a-alanine is reported. It is shown that, in this complex, metal ions are linked by double carboxylate bridges and also by chloride ions.

2. Experimental

gravimetric analyzer (TA Instruments, Inc. New Castle, DE, USA). The instrument was equipped with an open platinum pan and an automatically programmed temperature controller. The TG curves were recorded at a heating rate of b 558 min 21 . in the temperature range 293–1273 K. IR spectra of L-a-alanine and its complex were recorded in the 4000–100 cm 21 spectral range using Nujol mulls between KBr windows with a Bruker 113 V FT-IR spectrophotometer.

2.1. Synthesis

2.3. X-ray crystallography

Crystals of [Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O were grown by slow evaporation of an aqueous solution (0.05 mol) of L-a-alanine and manganese(II) chloride (0.05 mol) of 1:1 molar ratio. Light pink crystals were formed after a few days. Crystals were isolated by washing with ethanol and dried in air at room temperature. Analytical data (%): found: C, 14.41; H, 4.35; N, 5.52. Calc.: C, 14.34; H, 4.38; N, 5.58.

Data were collected at 293 K temperature on a KM4 diffractometer, in the v / 2u scan mode with the v scan width51.210.35 tan u, using graphite monochromated Mo Ka radiation. Crystallographic data and other pertinent information are given in Table 1. The crystal structure was solved by heavy-atom methods using SHELXS-86 [33] and refined by full-matrix least squares method using SHELXL93 program [34]. H-atoms of water molecules were located from difference Fourier map. Other hydrogen atoms were placed in the geometrically calculated positions with isotropic temperature factors taken as 1.2 Ueq of neighbouring heavier atoms. Afterwards, positional parameters were calculated in riding mode with Uiso fixed. The discrepancy factors indicates R 5 S(uFo u 2 uFc u) / SuFo u and 2 2 2 2 2 1/2 R w 5 h(S[w(F o 2 F c ) ] / S[w(F 0 ) ]j and are listed in Table 1.

2.2. Instrumentation CHN microanalysis was carried out on a Perkin-Elmer model 2400 elemental analyzer. Thermogravimetric measurements of the complex were carried out in static air atmosphere and flowing nitrogen atmosphere using a TGA 2950 high-resolution thermo-

Table 1 Summary of data collection and crystal parameters Formula Mr Crystal system Space group T (K) ˚ Wavelength (A) ˚ a (A) ˚ b (A) ˚ c (A) a (8) b (8) g (8) ˚ 3) V (A Z F(000) Dcalc (mg m 23 ) Absorption coefficient (cm 21 ) Crystal dimensions (mm) Reflections measured 2u range / deg. Index ranges Criterion for observed reflections No. of observed reflections GOF R1 wR 2 ˚ 23 ) Largest diff. peak and hole (e A

C 12 H 44 Cl 8 Mn 4 N 4 O 16 1003.88 Triclinic P1 293 0.71069 7.544(2) 10.869(3) 11.207(3) 85.55(3) 87.02(3) 82.84(3) 908.2(3) 1 508 1.836 20.11 0.3530.3530.40 3202 5.0–50.1 0#h# 228, 24#k # 20, 219#l # 218 I .3d (I) 3143 1.082 0.0292 0.0749 0.651, 20.795

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3. Results and discussion

3.1. Structural description The new complex of L-a-alanine and manganese(II) has the formula [Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O. a-Alanine molecules appear in zwitterionic form. The compound is built up from two linear chains as shown in Fig. 1. There are four crystallographically independent Mn(II) ions having octahedral coordination, which is the most common for Mn(II). The Mn(1), Mn(2) and Mn(3) atoms are bonded with two carboxylate oxygen atoms of two different L-a-alanine molecules, one oxygen atom from water molecule and three chloride ions. The Mn(4) atom displays different coordination arrangement being surrounded by two carboxylate oxygen atoms, two water molecules and two chloride ions. Additionally, one chloride ion appears in the second coordination sphere of Mn(II) ion being hydrogen bonded with the coordinated water molecule and amino group of a-alanine. The remaining three water molecules fill the voids between the polymeric chains and hold the chains together through hydrogen bonds of type O–H? ? ?Cl. In the crystal structure, the adjacent manganese(II) ions are connected alternately through one and three bridges. When the chloride ion bridges the metal ions

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in polymeric chains, the Mn? ? ?Mn separations are ˚ The shorter Mn? ? ?Mn distances 4.405(1) and 4.607(1) A. ˚ and appear when are equal to 3.834(1) and 3.887(1) A neighbouring Mn(II) ions are simultaneously bridged through two carboxylate groups and one chloride ion. The known polymeric manganese complex with D,L-alanine [27] contains only single carboxylate bridges and the ˚ Note distance between Mn(II) ions is longer, i.e. 4.790 A. that three bridges were observed in polymeric complexes of Cd(II) and Co(II) with L-a-alanine [28,29], but in this case only one bridge originated from the carboxylate ligand group. On the other hand, in polymeric complexes of manganese with a-amino acid the single carboxylate bridge is typical [4,6,27]. The carboxylate groups in the complex studied have the bidentate-bridging character and form syn–syn bonds with manganese(II) ions. The C–O– Mn angles are within the range of 129.7(3)–139.9(3)8 and the O–C–O angles of carboxylate bridging groups are within the range of 126.3(4)–127.6(4)8. They are slightly wider than the O–C–O angle in L-a-alanine which is equal to 125.7(8)8 [35]. The O–C bond lengths being within the ˚ agree well with the range from 1.232(6) to 1.263(6) A corresponding values found for other polymeric a-alanine complexes [27–29]. The deviations of manganese atoms from planes defined

Fig. 1. The crystal structure of [Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O. Symmetry code: (i) x 1 1, y, z.

R. Mrozek et al. / Polyhedron 18 (1999) 2321 – 2326

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by carboxylate groups range from 21.223(7) to 1.107(8) ˚ The torsion angles in a-alanine molecules are not the A. same. The nitrogen atoms deviate from the planes of the carboxylate group within a wide range from 224.1(6) to 213.7(6)8. The selected bond lengths and angles are given in Table 2. All manganese atoms are in a distorted octahedral arrangement. The Mn–O carboxyl bond lengths range from ˚ The Mn–O water distances have 2.136(3) to 2.204(3) A. very similar values being within the range of 2.134(4)– ˚ The chloride ions act as monodente- as well as 2.216(4) A. bidentate-bridging ligands. The Mn–Cl bond lengths are ˚ These signifiwithin the range of 2.4720(14)–2.674(2) A. cant differences in bond lengths force a large distortion of co-ordination polyhedrons. The valence angles in the octahedron differ from 908 by a maximum of 17.208 while the angles which should be 1808 differ by a maximum of 17.208. In the crystal structure of the complex studied is a complicated system of hydrogen bonds (Table 3). Three water molecules of outer coordination sphere (O6, O7, O8) are embedded between the chains connecting them by means of nearly linear hydrogen bonds of different types. The O6 and O7 water molecules participate in two hydrogen bonds as proton donors to the chloride and water oxygen atoms. Additionally, O6 and O8 molecules act as proton acceptors in the N–H? ? ?O hydrogen bonds. The O8 molecule does not take part in hydrogen bond formation as proton donor. Water molecules of the inner sphere (O1, O2, O3, O4, O5) also act as proton donors: the O3 and O5 molecules form only single hydrogen bond, the O1, O2, O4 molecules are the proton donors to chloride and carboxylic oxygen atoms and the O1 oxygen atom acts as proton acceptor in the O–H? ? ?O hydrogen bond. All NH 1 3 Table 2 ˚ and angles (8) with e.s.d.’s in parentheses Selected bond lengths (A) Mn(1)–Mn(2) Mn(1)–Mn(2)a Mn(1)–O(11) Mn(1)–O(21) Mn(1)–O(1) Mn(1)–Cl(1) Mn(1)–Cl(3) Mn(1)–Cl(2) Mn(2)–O(22) Mn(2)–O(2) Mn(2)–O(12) Mn(2)–Cl(4) Mn(2)–Cl(1) Mn(2)–Cl(2)b O(11)–C(11) O(12)–C(11) O(21)–C(21) O(22)–C(21) O(11)–C(11)–O(12) O(21)–C(21)–O(22) a b

3.834(1) 4.607(1) 2.136(3) 2.181(3) 2.189(4) 2.4964(14) 2.5866(14) 2.5896(14) 2.157(3) 2.163(4) 2.199(3) 2.4720(14) 2.562(2) 2.674(2) 1.232(6) 1.253(6) 1.263(6) 1.236(6) 127.3(4) 126.9(4)

Symmetry code: x 2 1, y, z. Symmetry code: x 1 1, y, z.

Mn(3)–Mn(4) Mn(3)–Mn(4)a Mn(3)–O(3) Mn(3)–O(31) Mn(3)–O(41) Mn(3)–Cl(7) Mn(3)–Cl(5) Mn(3)–Cl(6) Mn(4)–O(4) Mn(4)–O(32) Mn(4)–O(42) Mn(4)–O(5) Mn(4)–Cl(5) Mn(4)–Cl(6)b O(31)–C(31) O(32)–C(31) O(41)–C(41) O(42)–C(41) O(31)–C(31)–O(32) O(41)–C(41)–O(42)

3.887(1) 4.405(1) 2.162(3) 2.171(4) 2.204(3) 2.5364(14) 2.5570(14) 2.580(2) 2.134(4) 2.149(3) 2.193(3) 2.216(4) 2.4810(14) 2.5881(14) 1.231(6) 1.263(6) 1.251(6) 1.250(6) 126.3(4) 127.6(4)

Table 3 ˚ and angles (8) with e.s.d.’s in parentheses Hydrogen bond lengths (A) D–H? ? ?A

D–H

H? ? ?A

D? ? ?A

,D–H? ? ?A

O(1)–H(1)? ? ?Cl(8) O(1)–H(2)? ? ?O(12)a O(2)–H(3)? ? ?O(21)b O(2)–H(4)? ? ?Cl(7)c O(3)–H(5)? ? ?O(32)a O(4)–H(7)? ? ?Cl(3)d O(4)–H(8)? ? ?O(41)b O(5)–H(9)? ? ?Cl(8)b O(6)–H(11)? ? ?Cl(3) O(6)–H(12)? ? ?Cl(7) O(7)–H(13)? ? ?O(1) O(7)–H(14)? ? ?Cl(5)a N(11)–H(11A)? ? ?O(8) N(11)–H(11B)? ? ?Cl(8) N(11)–H(11C)? ? ?O(6) N(21)–H(21A)? ? ?O(7)c N(21)–H(21B)? ? ?Cl(7)c N(31)–H(31A)? ? ?Cl(4)e N(31)–H(31B)? ? ?Cl(8)f N(41)–H(41A)? ? ?Cl(2)d N(41)–H(41B)? ? ?O(6)g N(41)–H(41C)? ? ?Cl(3)d

0.93 0.91 0.89 0.93 0.84 0.98 0.73 0.94 0.93 1.10 1.11 1.03 0.89 0.89 0.89 0.89 0.89 0.89 0.89 0.89 0.89 0.89

2.16 1.84 1.89 2.29 1.96 2.20 2.00 2.23 2.37 2.20 2.02 2.12 1.95 2.33 2.27 1.94 2.31 2.37 2.26 2.46 2.21 2.51

3.222(5) 2.748(6) 2.763(6) 3.216(5) 2.776(6) 3.101(5) 2.723(6) 3.113(5) 3.282(6) 3.292(6) 3.108(6) 3.114(6) 2.821(7) 3.204(6) 3.010(7) 2.776(7) 2.659(6) 3.175(5) 3.191(6) 3.094(5) 3.021(7) 3.308(5)

178 174 168 176 165 153 167 156 166 171 166 172 166 166 140 156 165 154 155 145 151 150

a

Symmetry code: 2 1 1 x, y, z. Symmetry code: 1 1 x, y, z. c Symmetry code: x,1 1 y, z. d Symmetry code: 1 1 x, y, 2 1 1 z. e Symmetry code: x, 2 1 1 y, 2 1 1 z. f Symmetry code: 1 1 x, 2 1 1 y, z. g Symmetry code: 1 1 x, y, 2 1 1 z. b

groups occur as proton donors to chlorine atoms and water molecules in the hydrogen bonds: N–H? ? ?Cl and N–H? ? ? O.

3.2. Infrared spectroscopy The wavenumbers of the bands and their assignments are listed in Table 4 [36–38]. Both compounds are characterized by a broad band with submaxima within the region 3100–2000 cm 21 resulting from stretching vibrations of the N–H bond of NH 1 3 group. The band overlaps with symmetric and asymmetric stretching bands of CH 3 group. The free a-amino acid shows no absorption in the region of 3650–3200 cm 21 , while on the spectrum of the complex a wide band with several peaks is observed because of the presence of water molecules as well as hydrogen bonds. As expected from the zwitterionic structure of the amino acid, the carboxylate stretching vibrations of the carboxylate group appear alike in the free ligand in the complex. For a-alanine the asymmetric nas and symmetric nsym stretching vibrations of COO group appear at 1588 and 1413 cm 21 , respectively. For the a-alanine complex the asymmetric stretching modes of COO group are obscured by deformation vibration of water molecules. We assume that the nas vibration appears at 1594 cm 21 . The coordination of the carboxylate group is reflected in the spectrum of the complex through an

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Table 4 The wavenumbers and assignment of the bands observed in the IR spectra of L-a-alanine and its manganese complex a L-a-alanine

[Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O

Vibration modes

1620

1626

1588 1520, 1506 1456 1413 1362 1307 1237 1152, 1115

1594 – 1460 1377 1352 1293 1202 –

– – – 1014 919, 850

1138 1109 998, 973 – 930, 850

772 649 540 487 – 411, 322 293, 278, 259 214 169

764 656 553 – 356 – 277, 256 215 –

NH 1 3 def, NH 2 bend, OH def COO asym str NH 1 3 sym def CH 3 asym def COO sym str CH 3 sym def C–H bend NH 1 3 rock, CH 3 –C–N asym str NH 1 3 rock, CH 3 rock, CH 3 –C–N asym str CH 3 rock, CO str C–N str OH out of plane CH 3 rock, CH 3 –C–N sym str CH 3 –C–N asym str, COO scissor COO scissor, CC str COO rock COO wag NH 1 3 tors Mn–O str NCCO def CCO def in-plane skeletal tors CH 3 tors

a

Abbreviations: asym, asymmetric; sym, symmetric; str, stretch; def, deformation; tors, torsion; wag, wagging; rock, rocking; bend, bending.

insignificant increase in the asymmetric frequencies of COO and a decrease (1377 cm 21 ) of the frequency of symmetric vibrational mode in comparison to the corresponding bands of the free ligand. The coordination mode of the carboxylate ligand is often proposed on the grounds of the frequency difference between the asymmetric and symmetric COO vibrational modes (Dn 5 nas 2 ns ) [39]. The Dn splitting value in the complex studied being 214 cm 21 is comparable to that of 211 cm 21 observed in the case of manganese(II) triphenylacetate [40]. In that case COO 2 also acts as a bidentate-bridging group. The asymmetric bending bands of NH 31 group are observed in the complex and free ligand at 1620 and 1626 cm 21 , respectively. The complex spectrum shows a combination band at 998 cm 21 which could result from the O–H? ? ?O bridges [41]. Below 600 cm 21 most of the bands occurring in the spectrum of free a-alanine exhibit reduced intensities or disappear in the spectrum of the complex. A band at 356 cm 21 with respect to free ligand is assigned to stretching vibrations of the Mn–O bond.

3.3. Thermal analysis The [Mn 4 (ala) 4 Cl 7 (H 2 O) 5 ]Cl?3H 2 O complex is stable at room temperature. Upon heating, dehydration process begins at about 353 K. A calculated weight loss due to removal of the all water molecules is 14.34%, while the measured weight losses are 13.44% (air atmosphere) and

15.52% (nitrogen atmosphere). In spite of the use of a high-resolution technique it was impossible to distinguish two stages of dehydration related to the loss of outer and inner sphere water molecules. The anhydrous compound is stable up to 503 K and then it decomposes in several steps to suitable oxides. Under both conditions, the first stage of the decomposition process (to about 623 K) occurs in the same way. Above this temperature the shapes of the TG curves differ entirely. In air atmosphere the formation of Mn 2 O 3 is observed at 873 K. During heating to 1173 K the Mn 2 O 3 transforms into Mn 3 O 4 which is the final product of decomposition (found and calculated weight loss is 69.65%). Under a nitrogen atmosphere MnO is found as a final product (formation temperature, 5008C; found weight loss, 71.71%; calculated, 71.85%).

Supplementary data Supplementary data are available from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK on request, quoting the deposition number 116382.

Acknowledgements This work was supported by the Polish State Committee Scientific Research, Grant 3T09A 109 15.

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