Inorganica Chimica Acta 362 (2009) 3860–3866

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Synthesis, crystal structures and spectroscopic properties of threeZn–cyclen–aminoacid complexes with new macrocyclic configurations

Z. Vargová a,*, R. Gyepes b, L. Arabuli a, K. Györyová a, P. Hermann b, I. Lukeš b

a Institute of Chemistry, P.J. Šafárik University, Košice, Moyzesova 11, 041 54 Košice 1, Slovak Republicb Department of Inorganic Chemistry, Universita Karlova (Charles University), Hlavova 2030, 128 40 Prague 2, Czech Republic

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 October 2008Received in revised form 17 March 2009Accepted 22 March 2009Available online 28 March 2009

Keywords:Cyclen complexesAminoacidsZinc complexesX-ray crystallography

0020-1693/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.ica.2009.03.027

* Corresponding author. Tel.: +421 552342343; faxE-mail address: zuzana.vargova@upjs.sk (Z. Vargov

Synthesis and characterization of a series of novel Zn–cyclen–aminoacid complexes are reported andnon-covalent interactions were used for the conformation changes description. In addition to theTRANS-I ++++ (same position of NH hydrogen atoms in both isomers) configuration we observed two dif-ferent isomers in the case of the [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1), [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2) and [Zn(D-cyclen)(S-HAla)](ClO4)2 (3) complexeswith pentacoordinated zinc atom. Moreover, work also deals with the correlation between spectraland structural properties of the prepared complexes and their thermal stability.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Aminoacids are the fundamental building blocks of a number ofimportant biomolecules such as peptides, proteins and molecularmessengers. Recognition of the common amino acids is thereforean important target for supramolecular chemists and many abioticreceptors and carriers have been reported. Such recognition sys-tems have relied heavily on the zwitterionic nature and side-chainfunctionality of amino acids and have either incorporated multiplebinding sites to accommodate a variety of host–guest interactions(enzymatic reactions) or have relied upon interaction with eitherthe charged ammonium or carboxylate moieties of the amino acid[1]. The conformational changes also perform significant role inenzymatic catalytic recognition of substrates or inhibitors. In Zn-LADH, the conformational change from the open to the closed formis included by coenzyme binding [2]. The presence of one or theother form has so far only been studied by crystallographic meth-ods. The open form gives rise to orthorhombic crystals whereas theclosed form gives monoclinic or triclinic crystals. From the com-paring studies (small substrate, large substrate) it emerges thatthe nature (steric role) of the substrate or inhibitor bound to thecatalytic zinc atom is an essential factor for proper positioning ofthe nicotinamide moiety of the coenzyme. Lot of experimentalwork has been done on mimicking enzyme catalysis with modelcoordination compounds with different multidentate organic li-gands (cyclodextrins, calix-arenes, macrocyclic polyamines) [3–5].

ll rights reserved.

: +421 556222124.á).

Kimura et al. postulated the formation of [Zn(cyclen)]2+ withOH�, HCO3

�, halide anions, acetate, sulfonamides, carboxamides,imidazole, alcohols with aim to provide explanation of someintrinsic chemical roles played by zinc in zinc enzymes [6]. It isclear from his findings that the role of molecular recognition re-mains a central issue in the enzymatic reaction.

In our previous work [7] we described that Zn–cyclen moleculeis suitable complex also for pyridinecarboxylic acids recognition.In addition, the found affinity of picolinic and dipicolinic acids indi-cates potential inhibition properties of their derivatives for theZn2+-enzymes. Generally, possibility of protonation of cyclen in itsZn2+-complexes should be taken into account while consideringthe [Zn(cyclen)]2+ fragment as a model for the enzymes above.Moreover, ternary species in solid state form moieties with differentconformation of five-member chelate rings around the zinc atom. Kconformation was observed in complex [Zn(cyclen)nad](ClO4)2. Inthe case of dinuclear solid complexes, where the two metal atomsare bridged through picolinic or nicotinic acid, additional conforma-tions were described.

Clarkson et al. [8] investigated the influence of NH liability onthe aqueous solution chemistry of chelate species [Co(cyclen)-(O2C2O2)] and [Co(cyclen)-(O2CCH2CO2)] and reported crystalstructures of isomeric forms [9]. They used established conven-tions for the description of their actual configurations (positionof hydrogen atoms to the plane defined by nitrogen atoms of cy-clen). Similar aspects were used for [Zn(cyclen)(thiolate)]+ [10]and [Ni(cyclen)(H2O)2]2+ complexes [11].

In addition to the TRANS-I ++++ (same position of NH hydrogenatoms in both isomers) configuration we described two different

Z. Vargová et al. / Inorganica Chimica Acta 362 (2009) 3860–3866 3861

isomers (Scheme 1) in the case of unmodified Zn–cyclen–amino-acid (Gly, Ala, Phe) complexes. Similar approach is commonly usedfor the stereoisomers in modified M–cyclen complexes [12]. More-over, work also deals with the correlation between spectral andstructural properties of the prepared complexes and their thermalstability.

2. Experimental

2.1. Preparation of the zinc(II) complexes

2.1.1. [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1)Solution of zinc perchlorate (86 mg (Zn(ClO4)2 � 6H2O),

0.232 mmol) in water (10 ml) was slowly added to the aqueoussolution (10 ml) of cyclen (40 mg, 0.232 mmol). After 24 h, equi-molar amount of glycine (17 mg, 0.232 mmol) in water (10 ml)was added. The pH value of prepared solution was 5.2–5.6 andsolution was left under the normal atmospheric conditions to theisothermal crystallization. Anal. Calc. for C10H27Cl2N5O11Zn: C,22.68; H, 5.14; N, 13.22. Found: C, 21.65; H, 5.05; N, 12.46%.

2.1.2. [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2)Zn–cyclen solution was prepared by previous procedure. After

24 h, equimolar amount of (S)-Phenylalanine (38 mg, 0.232 mmol)in water (10 ml) was added. After next 2 h, the value of pH was ad-justed with 1 M NaOH to 5.5. After several days, colorless and pel-lucid crystals were obtained. Anal. Calc. for C17H31Cl2N5O10Zn: C,33.93; H, 5.19; N, 11.64. Found: C, 32.56; H, 4.87; N, 10.73%.

2.1.3. [Zn(D-cyclen)(S-HAla)](ClO4)2 (3)Aqueous solution (10 ml) of zinc perchlorate (86 mg

(Zn(ClO4)2 � 6H2O), 0.232 mmol) was slowly added to the aqueoussolution (10 ml) of cyclen (40 mg, 0.232 mmol). After 24 h, equi-molar amount of (S)-Alanine (21 mg, 0.232 mmol) in water(10 ml) was added. After next 2 h, the value of pH was adjustedwith 1 M NaOH to 5.5. After several days isothermal crystallization,colorless and pellucid crystals were obtained. Anal. Calc. forC11H25Cl2N5O10Zn: C, 25.06; H, 4.81; N, 13.37. Found: C, 24.14; H,5.03; N, 12.48%.

2.1.4. CrystallographyColorless prismatic crystals of the complexes selected from the

bulk before filtration were used for data collection. The cell deter-mination and data collection were carried out on a Nonius Kap-paCCD diffractometer using graphite monochromated Mo Karadiation (k = 0.71070 Å). The phase problem was solved bySIR-97 [13] and the structure refinement was carried out withfull-matrix least-squares on F2 using the SHELXL-97 [14] program.

N N

NN

HH

HH

M

N N

NN

H H

H

H

M

MNN

N N

+ +

+ +

λλλλ δδδδ

Scheme 1. Feasible isomers in pentacoordinated M–cyclen complexes.

All non-hydrogen atoms were refined anisotropically. All hydrogenatoms were included in idealized positions using the ‘‘riding mod-el” and refined isotropically. Experimental crystallographic param-eters of all complexes are listed in Table 1.

2.1.5. IR spectroscopy and elemental analysisInfrared spectra were recorded on Avatar Spectrophotometer in

the range 4000–400 cm�1. The content of C,H,N in prepared com-pounds was determined by means of Perkins–Elmer 2400 CHNanalyzer.

2.1.6. Thermal analysisThe TG/DTG and DTA measurements were carried out using TA

instruments Netsch 409 PC. The mass of the used samples waswithin 10–15 mg range. The samples have been heated in nitrogenatmosphere in the temperature range from 25 to 900 �C.

3. Results and discussion

Simultaneously with aqueous solution studies [15] we at-tempted to prepare suitable crystals for X-ray analysis by isother-mal crystallization from aqueous solution with adjusted pH value.The equimolar ratio of required species at the concentration about0.02 M was applied to the crystal growth. We were successful inthe single crystal preparation and structures were determined forternary systems Zn–cyclen–aminoacid (Gly, Ala, Phe).

The similar complexes [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(H-Gly)}](ClO4)4 � 2H2O (1), [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2) and [Zn(D-cyclen)(S-HAla)](ClO4)2 (3) were pre-pared. In solid state complexes (1) and (2) form two independentmoieties (Figs. 1 and 2) created by two pentacoordinated Zn2+ nu-clei. The complex (3) consists of monomeric units with similarcoordination mode (Fig. 3). The central Zn2+ ions are surroundedby four nitrogen atoms of macrocycle and one oxygen atom of ami-noacid building the slightly distorted tetragonal pyramids (param-eter sZn1,Zn2 � 4 in complex 1, sZn1,Zn2 � 1 in complex 2 and s � 4 incomplex 3; this parameter should be 100 for an ideal trigonalbipyramid and 0 for an ideal tetragonal pyramid [16]). The zincatoms are placed above the plane of four nitrogen atoms of cyclenin distance 0.797 Å (complex 1), 0.779 Å (complex 2) and 0.794 Å(complex 3) which are slightly different to that found in the struc-tures with pyridinecarboxylic acids [7]. All four five-memberedchelate rings of cyclen have in the complexes (1) and (2) the samebut opposite conformation (D in one independent unit and K insecond independent unit) like was observed in similar complexes[Zn2(cyclen)2(lumizinato-N)2](ClO4)2 [17] and [Zn2(cyclen)2(30-azi-do-30-deoxythymidine-N3)2](ClO4)2 � 4H2O published in literature[18]. The same D conformation was observed for five-memberedchelate rings of cyclen in complex (3). The aminoacid moleculesare monodentate coordinated by oxygen atoms O1 and O21,respectively.

The bond distances and angles around the Zn2+ ions are listed inTable 2 and correspond to the expected Zn–O, Zn–N bond lengthsand N–Zn–O bond angles. All distances of Zn2+ ion from the N4

plane for each independent unit of all complexes is in the goodagreement with observed distances 0.75–0.80 Å found in the CCDC[30].

The monodentate structural motif is observed in the crystalstructure studies. Thus, if the aminoacid is monodentaly coordi-nated to the Zn–cyclen unit, the central Zn atom is surroundedby five donor atoms forming tetragonal pyramidal polyhedron.However, comparing our and published structures in the CCDCwe found the others structural motifs in the case of pentacoordi-nated M–cyclen–ligand complexes (M – Cu2+, Cd2+, Zn2+) [18–21].These three different coordination modes repeat:

Table 1Crystallographic parameters of the studied complexes.

Compound [{Zn(D-cyclen)(Hgly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1)

[{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2)

[Zn(D-cyclen)(S-HAla)](ClO4)2 (3)

Formula C10H27Cl2N5O11Zn C17H31Cl2N5O10Zn C11H25Cl2N5O10ZnMr 529.64 601.74 523.63Crystal color, habit colorless colorless colorlessCrystal dimensions (mm) 0.23 � 0.53 � 0.70 0.20 � 0.20 � 0.18 0.1 � 0.35 � 1.18Crystal system monoclinic triclinic orthorhombicSpace group P21/c (no. 14) P1 (no. 1) P212121 (no. 19)

Lattice parameters a = 28.3470(5) (Å) a = 10.3302(2) (Å) a = 8.6746(2) (Å)b = 8.8420(3) (Å) b = 11.3682(3) (Å) b = 8.8343(2) (Å)c = 17.3710(9) (Å) c = 11.3971(3) (Å) c = 27.6977(5) (Å)b = 107.8870(12) (�) a = 69.1580(12) (�)

b = 89.7025(14) (�)c = 86.8548(14) (�)

V (Å3) 4143.5(3) 1248.82(5) 2122.59(8)Z 8 2 4Dcalc (g cm�3) 1.698 1.600 1.639l (mm�1) 1.507 1.258 1.4662hmax (�) 55.02 54.86 55.02Number of measured reflections 26269 19063 27416Number of independent reflections 9049 10194 4858R1 (I > 2r(I)) 0.0417 0.039 0.056wR2 (all data) 0.1095 0.0972 0.1691

C6

2.384

O22.483

O22

C7

C23C10N22

C9

C5

C29

C30O200

C8

H3

O1

C22

O21

C21

N3

Zn1

N25

C24

N5

H23

N1

Zn2

N23

N21

C4

C1

N2

C25C3

C28

C2

N24

C26C27

O100

Fig. 1. DIAMOND drawing of two independent [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}]4+ units in complex cation at 30% ellipsoid probability found in thestructure [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1). Four per-chlorate anions and hydrogen atoms (expect H3 and H23) are omitted for clarity.Observed intramolecular hydrogen bond in complex cations are marked by dottedline.

C25

C2

C16

C26

N23

C1

N2

C15

C24

C17

O22C3

2.56

7

H1N24

N1

C30

C29

C14

C23

C4

O2

C12

C27

Zn2

C35

Zn1

O21

C8

C13

C10

N25

C11

O1

N22

N3

C28

C9

C7

C32C38

C22

N21

C5

N4

N5

C21

C33

C6

C37

C34C36

Fig. 2. DIAMOND drawing of two independent [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}]4+ units in the complex cation at 30% ellipsoid probability found inthe structure [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2). Fourperchlorate anions and some hydrogen atoms (expect H1) are omitted for clarity.Observed intramolecular hydrogen bond in complex cation is marked by dottedline.

C8 C1N1

C11

C7

C2

2.6

01

H4N4

O2

C9

C10

Zn1

N2

O1

N15

C6 C3

C5

N3

C4

Fig. 3. DIAMOND drawing of [Zn(D-cyclen)(S-HAla)]2+ cation at 30% ellipsoidprobability found in the structure [Zn(D-cyclen)(S-HAla)](ClO4)2 (3). Two perchlo-rate anions and some hydrogen atoms (expect H4) are omitted for clarity. Observedintramolecular hydrogen bond in complex cation is marked by dotted line.

3862 Z. Vargová et al. / Inorganica Chimica Acta 362 (2009) 3860–3866

– monodentate coordination of additional ligand through S, N or Odonor atom [10],

– bridge coordination of additional ligand, bridged with two ormore Zn–cyclen moieties [19–21],

– additional ligand is coordinated monodentately but forms twoindependent molecules [18].

The aminoacid coordination modes are given from their nativecoordination ability which results from their composition (N andO donor atoms in the suitable position). Aminoacid as additional li-gand creates monodentate coordination mode (through O or N do-nor atoms) [22], bidentate coordination mode (through twooxygen atoms of carboxylate [23–27] or through O and N atomsas bridge [28,29]) and like most common chelate coordinationmode (through both donor atoms) [30]. On the contrary, monoden-tate coordination of amino acid through carboxylate group withprotonated amino group was observed only in a few cases:[23,31–35]. The aminoacid configuration in complexes can be

Table 2Selected bond lengths (Å) and angles (�) found in the studied crystal structures.

[{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1) [{Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)(S-HPhe)}](ClO4)4 (2) [Zn(D-cyclen)(S-HAla)](ClO4)2 (3)

1st independent moiety 2nd independent moiety 1st independent moiety 2nd independent moiety

Coordination sphereZn1–O1 1.950(3) Zn2–O21 1.938(3) Zn1–O1 1.951(2) Zn2–O21 1.944(3) Zn1–O1 1.924(3)Zn1–N1 2.153(4) Zn2–N21 2.153(3) Zn1–N1 2.126(3) Zn2–N21 2.146(3) Zn1–N1 2.140(5)Zn1–N2 2.130(4) Zn2–N22 2.146(3) Zn1–N2 2.123(3) Zn2–N22 2.134(3) Zn1–N2 2.117(5)Zn1–N3 2.146(4) Zn2–N23 2.154(4) Zn1–N3 2.148(3) Zn2–N23 2.148(4) Zn1–N3 2.137(5)Zn1–N4 2.120(3) Zn2–N24 2.136(3) Zn1–N4 2.154(4) Zn2–N24 2.115(3) Zn1–N4 2.124(5)

AnglesN1–Zn1–O1 115.9(1) N21–Zn2–O21 113.3(1) N1–Zn1–O1 114.9(1) N21–Zn2–O21 113.9(1) N1–Zn1–O1 112.6(2)N2–Zn1–O1 111.8(1) N22–Zn2–O21 112.9(1) N2–Zn1–O1 118.9(1) N22–Zn2–O21 104.5(1) N2–Zn1–O1 109.0(2)N3–Zn1–O1 107.3(1) N23–Zn2–O21 109.2(1) N3–Zn1–O1 107.5(1) N23–Zn2–O21 108.3(1) N3–Zn1–O1 110.2(2)N4–Zn1–O1 113.4(1) N24–Zn2–O21 112.1(1) N4–Zn1–O1 104.6(1) N24–Zn2–O21 118.6(1) N4–Zn1–O1 115.5(2)N1–Zn1–N3 136.9(1) N21–Zn2–N23 137.5(1) N1–Zn1–N3 137.3(1) N21–Zn2–N23 137.4(1) N1–Zn1–N3 137.0(2)N4–Zn1–N2 134.7(1) N24–Zn2–N22 135.0(1) N4–Zn1–N2 136.4(1) N24–Zn2–N22 136.9(1) N4–Zn1–N2 135.4(2)N2–Zn1–N1 81.7(1) N22–Zn2–N21 82.6(1) N2–Zn1–N1 82.6(1) N22–Zn2–N21 82.2(1) N2–Zn1–N1 81.6(2)N1–Zn1–N4 82.0(2) N21–Zn2–N24 82.0(2) N1–Zn1–N4 82.0(1) N24–Zn2–N21 82.4(1) N1–Zn1–N4 82.1(2)N2–Zn1-N3 81.5(1) N22–Zn2–N23 81.6(1) N2–Zn1–N3 82.0(1) N22–Zn2–N23 81.7(1) N2–Zn1–N3 82.0(2)N3–Zn1–N4 82.3(1) N23–Zn2–N24 81.9(1) N3–Zn1–N4 82.4(1) N24–Zn2–N23 83.0(1) N3–Zn1–N4 82.3(2)

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influenced by original configuration of free ligands. The configura-tion of (S)-Ala and (S)-Phe in our ternary complexes is remained.However, the description of the Zn–cyclen configuration by estab-lished conventions [11] lead us to the observations that the posi-tion of hydrogen atoms to the plane defined by nitrogen atomsof cyclen is TRANS-I (++++) in all crystallographic units of our com-plexes. The independent units in complexes (1,2) differ from eachother by conformation of chelate rings around the zinc atom. Firstis the ‘clock-wise D’ conformation, second is opposite ‘anticlock-wise K’ conformation. We observed only ‘clock-wise D’ conforma-tion in complex (3). We found similar differences in structurespublished in CCDS, therefore we were interested in the reason ofdifferent cyclen conformations around the zinc atom. The follow-ing can play significant role:

1. the conformation (S,R) of additional ligand,2. the steric configuration of additional ligand,3. crystal packing decides about gauche conformation of M–cyclen

complexes.

We have found similar pentacoordinated Zn–cyclen complexeswith two independent moieties with different gauche conforma-tion of chelate rings in the two cases [17,18]. In the first case theadditional ligand is achiral molecule. In the second case complex((+,+,+,+)-1,4,7,10-tetra-azacyclododecane)-(phenylmethanethiola-to)-zinc(II) perchlorate and complex ((+,+,+,+)-1,4,7,10-tetra-aza-

Table 3Selected hydrogen bonds (Å) found in the studied crystal structures.

H–bonds D–H d(D–H) [Å] d(D� � �A) [Å]

ZnCnGly N23–H23 0.910 3.122d-isomer N25–H25C 0.890 2.775

O200–H200 0.645 2.835

ZnCnGly N3–H3 0.910 3.058k-isomer N5–H5E 0.890 2.769

O100–H101 0.767 2.813

ZnCnPhe N1–H1 0.930 3.172d-isomer N5–H5D 0.910 2.759

ZnCnPhe N25–H25D 0.910 2.766k-isomer

ZnCnAla N4–H4 0.910 3.171d-isomer N15–H15B 0.890 2.818

Cn = cyclen.

cyclododecane)-(4-methylbenzenethiolato)-zinc(II) perchlorate[10] form opposite isomers although involve large hydrophobicaromatic ligands. Therefore, we can suppose that the (S,R) confor-mation and the steric configuration of additional ligand is probablynot factor controlling the macrocyclic configurations in solidZn(II)–cyclen–additional ligand complexes.

The electrostatic nature of the X–H� � �Y (X,Y = N or O) hydrogenbond determines its role in influencing crystal packing. We ob-served some differences in our crystal structures. The similarhydrogen bonds were observed in both independent units (see Ta-ble 3) in structure with glycin. The intramolecular hydrogen bondswere detected in both isomers (see Fig. 1). Other hydrogen atoms ofcyclen amino group are connected to the intermolecular hydrogenbonds by means of perchlorates and water molecules. Moreover,cyclen molecules interact through hydrophobic interaction amongthemselves. We identified some interesting features in the struc-tures with chiral aminoacids (S)-Ala and (S)-Phe. The intramolecu-lar hydrogen bond in dddd-isomer of complex (2) and (3) was foundbetween Ocabox� � �HNcyclen (2.57 Å, 123�), (2.60 Å, 126�), respectively(Figs. 2 and 3). On the contrary, non-coordinated oxygen carboxyl-ate atom of kkkk-fragments does not create intramolecular hydro-gen bond. Further, two independent units in complexes (2) areconnected by hydrogen bond between oxygen carboxylate atomand hydrogen atom of amino group of opposite isomer(O22� � �HNamine(k), 1.87 Å, 165�; O2� � �HNamine(d), 1.88 Å, 164�). Thecomparable intermolecular hydrogen bonds were observed be-

A� � �HD d(H� � �A) [Å] h(DHA)i (�)

O22� � �HNcyclen 2.48 127O200� � �HNamine 1.89 176O22� � �HOwater 2.20 170

O2� � �HNcyclen 2.39 131O100� � �HNamine 1.88 176O2� � �HOwater 2.10 158

O2� � �HNcyclen 2.57 123O22� � �HNamine(k) 1.87 165

O2� � �HNamine(d) 1.88 164

O2� � �HNcyclen 2.60 126O2� � �HNamine 1.98 155

Fig. 4. MERCURY drawing of significant hydrogen bonds in complex cations considerably influencing the D(mas � ms) value in IR spectra. Perchlorate anions are omitted forclarity.

3864 Z. Vargová et al. / Inorganica Chimica Acta 362 (2009) 3860–3866

tween identical monomeric units in chiral complex (3)(O2� � �HNamine, 2.00 Å, 155�). Moreover, hydrogen atoms ofcyclen molecule are bonded to perchlorate oxygen atom byhydrogen bonds forming 3D crystal structure. We foundsimilar intramolecular hydrogen bonds (Nlumazinato� � �HNcyclen(d),2.58 Å, 111�; OC=O(lumazinato)� � �HNcyclen(d), 2.20 Å, 121�;OC@O(lumazinato)� � �HNcyclen(k), 2.29 Å, 125�) between two indepen-dent isomers with lumazinato ligand coordinated to the Zn atomin the published work [17]. Opposite isomers are connectedthrough hydrogen atom of cyclen (d isomer) and oxygen atom ofcarbonyl lumazinato group (k isomer). The same isomers interactby intermolecular bonds of OC@O(lumazinato)(d)� � �HNlumazinato(k),OC@O(lumazinato)(k)� � � HNcyclen(d).

On contrary to the complexes (2) and (3), complex (1) has twounits connected through hydrogen bonds by means of water mol-ecule {O(200,100)� � �HNamine and O(22,2)carboxylate� � �HOwater} (Fig. 4,Table 3). Similarly, intermolecular hydrogen bonds were observedbetween two isomers through water molecules in complex cyclen-30-azido-30-deoxythymidine-N3)-zinc(II) perchlorate dihydrate[18]. Moreover, the crystal packing of our complexes is influencedby hydrophobic interactions among cyclen molecules (complex 1),cyclen molecules to methyl group (complex 3) and cyclen mole-cules to phenylalanine group (complex 2). The hydrophobic func-tionalities of methyl group (Ala) and phenyl group (Phe) arrangethe orientation of hydrophilic parts (carboxylate, perchlorate andamino groups) to intramolecular or intermolecular hydrogenbonds.

Table 4Characteristic absorption bands (m/cm�1) in prepared complexes, D(mas � ms) values, calcu

Assignment [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}](ClO4)4 � 2H2O (1)

[{(S

m(C–H)cyclen 2913 2m(N–H)cyclen 3178 3m(N–H)Phe 3mas(COO�)amino acid 1590 1d(NH2)Phe 1ms(COO�)amino acid 1334 1D(mas � ms) 256 1dr(d isomer) 0.04 0dr(k isomer) 0.039 0hOCO � 120

(d isomer)7.19 6

hOCO � 120(k isomer)

6.34 5

D (calculated)(d isomer)

258 2

D (calculated)(k isomer)

242 2

From the previous comparison we can predict, that in the pen-tacoordinated Zn–cyclen-additional ligand complexes in solid statethe gauche conformation of five-membered chelate rings is proba-bly influenced by intra or intermolecular hydrogen and hydropho-bic weak interactions.

3.1. Correlation between spectral and structural properties

Before determination of the crystal structure we used the IRspectroscopic technique for the organic groups identification. Thecharacteristic absorption bands for prepared complexes are givenin Table 4. The cyclen molecule is characterized by the presenceof stretching vibrations of the methylene and amino group. Theamino acid presence signifies by carboxylate and amino stretchingvibrations in IR spectra (see Table 4). Perchlorate anion is shown atits typical position of stretching vibration (�1090 cm�1). Generally,the difference between the asymmetric and symmetric carboxylatestretches (D = mas(COO�) � ms(COO�)) is often used for the correla-tion of the infrared spectra with the structures of metal carboxyl-ates. These criteria are applied to assign the type of carboxylatecoordination. According to the D values, the following order wasproposed for carboxylate complexes of divalent metal cations [36]:

DðmonodentateÞ > DðionicÞ > Dðbridging bidentateÞ> Dðchelating bidentateÞ:

lated parameters in Eq. (1) and D (calculated) values for individual isomers.

Zn(D-cyclen)(S-HPhe)}{Zn(K-cyclen)-HPhe)}](ClO4)4 (2)

[Zn(D-cyclen)(S-HAla)](ClO4)2

(3)

916 2918175 3175304589 1589614 1618405 136484 225.034 0.048.038.07 6.15

.39

29 255

25

0

2

4

6

8

10

DTA

TG /%

exo

DTA[μV/mg]

DTG [%/min]

Temperature / oC200 400 600 800

0

20

40

60

80

100

120

TG

-6

-4

-2

0

2

DTG

Fig. 5. Thermoanalytical curves of [{Zn(D-cyclen)(HGly)}{Zn(K-cyclen)(HGly)}]-(ClO4)4 � 2H2O (1).

Z. Vargová et al. / Inorganica Chimica Acta 362 (2009) 3860–3866 3865

From the D(mas � ms) values assigning (256 for complex 1, 184 forcomplex 2 and 225 for complex 3) we predicted the monodentatecoordination for all complexes.

In order to confirm our prediction we compared the structuraland IR data by the following equation after structural datacollection:

D ¼ 1818:1dr þ 16:47ðhOCO � 120Þ þ 66:8; ð1Þ

where dr is difference between the two CO bond lengths (Å) andhOCO is the OCO angle (Å) [37]. The comparison of experimental dataD(mas � ms) and calculated ones is given in Table 4. The D value cal-culated by Eq. (1) from the structural data is 250 cm�1 (as averageof values for d and k isomer of complex 1), 227 cm�1 (as average ofvalues for d and k isomer of complex 2) and 255 cm�1 (value for disomer of complex 3). Experimental {D = mas � ms = 256 (1), 184(2), 225 (3) cm�1} and calculated data indicate the approximatelymonodentate coordination mode for all cases, which is in an agree-ment with structural results. In addition to intramolecular hydro-gen bond, the oxygen atom of carboxylate (complex 1) is in theslight interaction with hydrogen atom of water (1.88 Å) (Fig. 4A),therefore the separation value calculated from structural data cor-relates to the monodentate coordination mode. Intramolecularand intermolecular hydrogen bond among units (see Fig. 4B) causethe shift of separation to the lower value in complex 3. Shift to thebidentate separation value (complex 2) is probably caused by com-bination of different vibration mode belonging to diverse intra(Ocabox� � �HNcyclen) or intermolecular (Ocabox� � �HNamine(k), Ocabox� � �H-Namine(d)) interactions (see Fig. 4C and Table 3).

3.2. Thermal stability of prepared complexes

Thermal behavior of prepared complexes was observed in inertatmosphere of nitrogen. Process of decomposition is similar for allcomplexes. Therefore the TG/DTG and DTA curves of complex 1 isonly depicted in Fig. 5. Thermal stability of prepared complexes de-creases in the following order:

2ð240 �CÞ ! 1ð220 �CÞ ! 3ð200 �CÞ:

The thermal stability is in the good correlation with non-covalentinteraction presence.

Thermal decomposition of all complexes can be described asprocess with two significant steps. The first step of thermal decom-

position belongs to the release of organic part of molecules and thisprocess is accompanied by slight exothermic effect with peak from290 to 270 �C (complex 3 ? 1 ? 2). Thermal decomposition con-tinues by decomposition of the rest of the molecule. The solidresidual part of thermal decomposition depicts approximately20% in the case of complexes (1) and (3) and 7% in complex (2), thatapproximately respond to ZnO and Zn, respectively.

4. Conclusion

Aminoacids (Gly, Ala, Phe) are monodentately coordinated tothe Zn–cyclen fragment by oxygen atom of carboxylate group.Amino group of aminoacids is protonated in all cases. Two inde-pendent units Zn1N4O and Zn2N4O were observed in complex 1and 2. The opposite gauche conformation is characteristic for inde-pendent fragments. Monomeric unit Zn1N4O of complex 3 createsN (from Zn–cyclen fragment) and C (from alanine fragment) chiralcenters. The combination of hydrophobic and intra- or intermolec-ular interactions determines the coordination modes of Zn–cyclen–aminoacid units in pentacoordinated complexes.

The IR results indicate that D(mas � ms) value is also influencedby non-covalent interactions (mainly by intra and intermolecularbonds close to carboxylate group) and thermal stability order de-creases from complex 2, through 1 to 3.

In conclusion, to the absolute configuration description ofunmodified M–cyclen-additional ligand complexes is appropriateto insert also the conformation of five-membered chelate rings.

5. Supplementary material

CCDC 725314, 725315 and 725316 contain the supplementarycrystallographic data for 1, 2 and 3. These data can be obtained freeof charge from The Cambridge Crystallographic Data Centre via<http://www.ccdc.cam.ac.uk/data_request/cif>.

Acknowledgments

Financial support was provided by grants of the Ministry of theEducation of the Slovak Republic (VEGA 1/0122/08, MVTS CR/SR/UPJŠ/08), APVV agency (bilateral contract between the CzechRepublic and the Slovak Republic SK-77/CZ-81) and SAIA agency(National Scholarship Programme of the Slovak Republic).

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