Urease inhibitory activities of ZnBr2 and ZnI2 complexes of terpyridine derivatives: Systematic investigation of aryl substituents on urease inhibitory activities

Polyhedron 45 (2012) 9–14

Contents lists available at SciVerse ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Urease inhibitory activities of ZnBr2 and ZnI2 complexes of terpyridine derivatives: Systematic investigation of aryl substituents on urease inhibitory activities Ali Nemati Kharat a,⇑, Abolghasem Bakhoda b, Giuseppe Bruno c, Hadi Amiri Rudbari c a b c

School of Chemistry, University College of Science, University of Tehran, Tehran, Iran Department of Chemistry, Southern Illinois University, Edwardsville, IL 62026, United States Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy

a r t i c l e

i n f o

Article history: Received 17 April 2012 Accepted 1 July 2012 Available online 20 July 2012 Keywords: Urease inhibition Zinc bromide Zinc iodide Terpyridine X-ray crystallography

a b s t r a c t With the aim of discovering novel urease inhibitors, six new mononuclear complexes of five-coordinated Zn(II) bromide and iodide with terpyridine derivatives have been synthesized and their solid state structures were characterized by X-ray crystallography. Among these complexes, the zinc complexes bearing an electron withdrawing aryl group at the 40 -position show medium urease inhibitory activities, with IC50 values much lower than those of previously reported zinc species, at concentrations lower than 100 mM. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Urease (urea amidohydrolase; E.C.3.5.1.5) is a binuclear nickelcontaining metallo-enzyme which catalyzes the hydrolysis of urea to produce ammonia and carbamate [1,2]. The resulting carbamate subsequently converts to produce ammonia and carbon dioxide. It is now apparent that the high concentrations of NH3 generated from these reactions will result in pH elevation and have significant negative effects on human health, medicine and agriculture [3–5]. Restriction of the activity of urease using chemical inhibitors possibly will neutralize these negative side effects. Nowadays, urease inhibitors play a key role in the treatment of the infections caused by urease producing bacteria [6]. Urease inhibitors could be categorized into two types: (A) organic compounds, such as acetohydroxamic acid (AHA), humic acid and 1,4-benzoquinone [7–9]; (B) transition metal ion complexes, such as Cu2+, Zn2+, Pb2+ and Cd2+ complexes [10]. Metal complexes as urease inhibitors have been reported rarely. Research has shown that Sn4+, V5+, Bi3+, Mn3+, Zn2+ and Cu2+ complexes bear interesting urease inhibitory activities [11–16]. Zinc complexes with N-donor ligands are currently attracting attention for their interesting molecular topologies and crystal packing motifs, as well as the fact that they may be designed with specific applications in bioinorganic chemistry and material chem-

⇑ Corresponding author. Tel.: +98 21 61112499; fax: +98 21 66495291. E-mail address: [email protected] (A.N. Kharat). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.07.035

istry [17–20]. Studying the influence of the substituents on ligands may help us to understand and balance the physio-chemical and biochemical properties of synthetic compounds. In this paper, six new zinc(II) bromide and iodide complexes with terpyridine derivatives ([ZnBr2(40 -(4-chlorophenyl)-2,20 :60 ,200 -terpyridine)] (1), [ZnBr2(40 -phenyl-2,20 :60 ,200 -terpyridine] (2), [ZnBr2(40 -(pyridin-2yl)-2,20 :60 ,200 -terpyridine)] (3), [ZnI2(40 -phenyl-2,20 :60 ,200 -terpyridine] (4), [ZnI2(40 -2-thienyl-2,20 :60 ,200 -terpyridine)] (5) and [ZnI2 (40 -(4-methoxyphenyl)-2,20 :60 ,200 -terpyridine)] (6); Scheme 1) were synthesized to investigate the influence of 40 -position substitution on the urease inhibitory property. 2. Results and discussion 2.1. Synthesis The five terpyridine ligands in this study are similar tridentate ligands, which can coordinate to the metal atom through the pyridyl N-atoms. It is noteworthy that the terpyridine derivatives were prepared by simple one-pot reactions and their zinc complexes have not been reported previously. The complexes were synthesized according to the same method: a methanol solution (10 mL) of ZnX2 (1 mmol) was added with stirring to a chloroform solution (10 mL) of the corresponding terpyridine derivative (0.1 mmol). The mixture was stirred at room temperature for 10 min to give a white to yellow precipitate. Colorless to yellow block-shaped crystals of the complexes were formed by slow diffu-

10

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14

sion of methanol into the concentrated DMSO solution of the complexes. Yield: 87% (1), 89% (2), 92% (3), 85% (4), 89% (5) and 91% (6)1.

Ar

N

2.2. X-ray crystallography N

X-ray crystallography2 of complexes 1, 2, 4 and 5 reveals that the structures of the complexes are similar mononuclear five coordinated zinc complexes, except for the different aryl group on the 40 position of the terpyridine moiety. Complex 1 (Fig. 1), [ZnBr2(C21H14ClN3)], exists as a distorted trigonal bipyramid and crystallizes with one independent molecule in the asymmetric unit. The apical position of the square pyramid is occupied by one of the Br atoms, with the base of the pyramid consisting of the three N atoms of the terpyridine ligand, having para-chlorophenyl as a substituent in the 40 -position, and the other Br atom. In complex 2 (Fig. 2), [ZnBr2(C21H15N3)], the Zn2+ ion is five coordinated by the three N atoms from a 2,20 :60 ,200 terpyridine ligand, containing a phenyl group as a substituent in the 40 -position, and two bromide anions in a distorted trigonal bipyramidal configuration. It is noteworthy that complex 2 reported in this paper has a totally different structure in comparison with the ZnBr2 analogue of 2,20 :60 ,200 terpyridine (a-terpy) [21]. Fig. 3 presents the crystal structure of zinc iodide complex of the phenyl-substituted 2,20 :60 ,200 terpyridine (complex 4). In the complex 4, the Zn2+ cation is coordinated by the N atoms of the terpyridine ligand and by two 1 Anal. Calc. for 1 (C21H14Br2ClN3Zn): C, 44.33; H, 2.48; N, 7.38. Found: C, 44.9; H, 2.5; N, 7.7%. Selected IR data (Polyethylene, cm1): 3213w, 3091m (m C–H), 1619w, 1534m, 1450m, 1317m, 1199m, 1160w, 1064m, 1002w, 921w, 874w, 770m, 260m (m Zn–Br),252m (m Zn–N), 238 (m Zn–N). 1H NMR (DMSO-d6) d: 8.89 (m, 4H), 8.78 (d, 2H), 8.24 (dd, 2H), 7.92 (dd, 2H). ESI-MS of 1 in DMSO: m/z 569. Anal. Calc. for 2(C21H 15Br2N3Zn): C, 47.18; H, 2.83; N, 7.86. Found: C, 47.9; H, 2.9; N, 8.0%. Selected IR data (Polyethylene, cm1): 3250w, 3203w, 3128w, 3080 (m C–H), 1587m, 1533m, 1480s, 1435m, 1392m, 1223m, 1159m, 1112m, 1032w, 882m, 799m, 770s, 667m, 261m (m Zn–Br), 248m (m Zn–N), 236 (m Zn–N). 1H NMR (DMSO-d6) d: 8.78 (2H, d), 8.39 (2H, s), 8.02 (2H, dd), 7.96 (2H, dd), 7.88 (2H, d), 7.82 (2H, s,), 7.79 (1H, dd), 7.65 (2H, d). ESIMS of 2 in DMSO: m/z 533. Anal. Calc. for 3 (C20H 14Br2N 4Zn): C, 44.85; H, 2.63; N, 10.46. Found: C, 45.2; H, 2.7; N, 10.7%. Selected IR data (Polyethylene, cm1): 3208w, 3079m (m C–H), 1653w, 1511m, 1474m, 1306m, 1212m, 1155w, 1079m, 998w, 914w, 878w, 766m, 251m (m Zn–Br), 249m (m Zn–N), 231 (m Zn–N). 1H NMR (DMSO-d6) d: 8.88 (2H, d), 8.76 (1H, d), 8.13 (1H, dd), 8.02 (2H, dd), 7.93 (2H, dd), 7.89 (1H, dd), 7.86 (2H, dd), 7.78 (2H, s,), 7.64 (1H, dd). ESI-MS of 3 in DMSO: m/z 536. Anal. Calc. for 4 (C21H15I2N3Zn): C, 40.13; H, 2.41; N, 6.68. Found: C, 40.8; H, 2.5; N, 6.9%. Selected IR data (Polyethylene, cm1): 3198w, 3115w, 3076w (m C–H), 1594m, 1565m, 1494s, 1451m, 1396m, 1231m, 1179m, 1146m, 1049w, 898m, 768m, 748s, 692m, 171m (m Zn–I), 245m (m Zn–N), 234 (m Zn–N). 1H NMR (DMSO-d6) d: 8.88 (2H, d), 8.34 (2H, dd), 8.05 (2H, dd), 7.93 (1H, d), 7.76 (1H, dd), 7.71 (2H, dd), 7.66 (2H, s), 7.61 (1H, d). ESIMS of 4 in DMSO: m/z 627. Anal. Calc. for 5(C19H13I2N3SZn): C, 35.96; H, 2.06; N, 6.62. Found: C, 36.5; H, 2.2; N, 6.7%. Selected IR data (Polyethylene, cm1): 3239w, 3128w, 3069w (m C–H), 1560m, 1524m, 1476s, 1421m, 1339m, 1249m, 1195m, 1144m, 1104m, 1088w, 957m, 879m, 844 (m C–S), 638m, 178m (m Zn–I), 245m (m Zn–N), 239 (m Zn–N). 1H NMR (DMSO-d6) d: 8.86 (2H, d), 8.81 (2H, dd), 8.11 (2H, dd), 7.84 (1H, d), 7.73 (1H, dd), 7.68 (2H, dd), 7.61 (2H, s), 7.53 (1H, d). ESI-MS of 5 in DMSO: m/z 633. Anal. Calc. for 6 (C22H17I2N3OZn): C, 40.12; H, 2.60; N, 6.38. Found: C, 40.5; H, 2.7; N, 6.5%. Selected IR data (Polyethylene, cm1): 3235w, 3196w, 3114w, 3068w (m C–H), 2853(m C–H, Me) 1601m, 1541m, 1484s, 1440m, 1374m, 1236m, 1167m, 1128m, 1036w, 874m, 771m, 763s, 649m, 161m (m Zn–I), 242m (m Zn–N), 237 (m Zn–N). 1H NMR (DMSO-d6) d: 8.63 (2H, d), 8.62 (2H, dd), 8.58 (2H, dd), 7.87 (2H, d), 7.77 (2H, dd), 7.31 (2H, dd), 7.12 (2H, s), 3.84 (3H, s). ESI-MS of 6 in DMSO: m/z 657. 2 Crystal data for 1 (C21H14Br2ClN3Zn): Mw = 568.99, Monoclinic, space group P21/ n, a = 9.0278(4), b = 15.3710(6), c = 14.6727(5) Å, V = 1981.87(14) Å 3 , Z = 4, qcalc = 1.907 Mg/m3, l (Mo Ka) = 5.419 mm1, R1 = 0.0465, wR2 = 0.0938 (all data), T = 296(2) K. Crystal data for 2 (C21H15 Br2ClN3Zn): Mw = 534.55, Monoclinic, space group C2/c, a = 13.6700(2), b = 14.9655(2), c = 18.7708(3) Å, V = 3839.73(10) Å3, Z = 8, qcalc = 1.849 Mg/m3, l (Mo Ka) = 5.452 mm1, R1 = 0.0435, wR2 = 0.0881 (all data), T = 296(2) K. Crystal data for 4 (C21H15I2N3Zn): Mw = 628.559, Monoclinic, space group C2/c, a = 13.7769(4), b = 15.2209(4), c = 19.4317(6) Å, V = 4074.3(2) Å3, Z = 8, qcalc = 2.049 Mg/m3, l (Mo Ka) = 4.247 mm1, R1 = 0.0459, wR2 = 0. 1162 (all data), T = 296(2) K. Crystal data for 5 (C19H13I2N3SZn): Mw = 634.55, Monoclinic, space group P21/n, a = 10.1344(2), b = 17.9080(4), c = 11.2606(2) Å, V = 2025.58(7) Å3, Z = 4, qcalc = 2.081 Mg/m3, l (Mo Ka) = 4.372 mm1, R1 = 0.0308, wR2 = 0.0913 (all data), T = 296(2) K.

N

Zn X

X

Cl

OMe

Ar = N

S

X=Br, I Scheme 1. Structures of the terpyridine derivatives used in the present study.

iodide anions, forming a distorted trigonal bipyramidal environment again. For complex 5, Zn2+ is similarly five coordinated with three nitrogen atoms from the terpyridine skeleton and two iodide atoms, with the Zn atom being located at the distorted trigonal bipyramidal center (shown in Fig. 4). In this complex the terpyridine ligand bears a 2-thienyl ring as a substituent. Further investigation of the structures reveals that the Zn–N and Zn–Br bond lengths in 1 and 2 are comparable to each other. The Zn–N bond length averages are in the range of 2.12–2.18 Å. All of the coordinate bond lengths can be considered as normal values by comparison with those reported in the literature [21– 25]. The packing of the complexes (See Figs. S1–S4) are stabilized by medium to weak hydrogen bonds as well as p. . .p stacking (for complexes 1, 2 and 4). There are p–p stacking interactions in the lattice of 1, where the chlorophenyl ring (Cg1; C16–C21) and one flanking pyridyl ring (Cg2; N3, C8–C11) of the terpyridine backbone are stacked with a centroid–centroid distance of 3.924 Å. (Fig. S1). In addition, in the crystal packing of 2, the pyridyl rings are parallel (Cg1 and Cg 2 where Cg1 is N1,C1–C5 and Cg2 is N3, C8–C12) and p–p stacking interactions are found between each side ring of a pyridyl ring and another contiguous counterpart, with a centroid–centroid separation of 3.930 Å. Thus, a 3-D framework sustained by weak p–p stacking interactions is formed (Fig. S2). Also, in the packing of 4, the pyridyl rings (Cg1 and Cg 2 where Cg1 is N1,C1–C5 and Cg2 is N3, C8–C12) are parallel and p–p stacking interactions are found between neighboring pyridyl rings with a centroid–centroid distance of 3.955 Å. These interactions have been shown in Fig. S3. Table 1 summarizes the most significant bond lengths and angles for complexes 1, 2, 4 and 5. 2.3. Urease inhibitory activity The measurement of jack bean urease inhibitory activity was carried out in triplicate according to the phenol-red literature method [26]. The assay mixture, containing 25 lL of jack bean urease (10 kUL1) and 25 lL of the test materials of various concentrations (dissolved in DMSO:H2O solution 1:1 (V/V)), was pre-incubated for 1 h at 37 °C in a 96-well assay plate. Aliquots of 0.2 mL of 100 mM Hepes (N-[2-hydroxyethyl]piperazine-N0 -[2-ethanesulfonic acid]) buffer at pH 6.8 containing 500 mM urea and 0.002% phenol red were added and incubated at 37 °C. The reaction time, which was required to produce enough ammonium carbonate to raise the pH of the Hepes buffer from 6.8 to 7.7, was measured by a micro-plate reader (at 570 nm) with the end-point determined by

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14

11

Fig. 1. Molecular structure of 1 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.

Fig. 2. Molecular structure of 2 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.

the color of the phenol red indicator. The complexes remained intact under these conditions (in the presence of DMSO and Hepes buffer) as we checked their solutions by 1H NMR.1 The results are summarized in Table 2. Complexes 1 and 3 show medium urease inhibitory activity with IC50 values being marginally higher than those of acetohydroxamic acid (AHA), co-assayed as a standard urease inhibitory agent, while complexes 2, 4, 5 and 6 show poor activity. The results in this paper are not in accordance

with those reported formerly, in which it was stated that zinc(II) complexes have no urease inhibitory activity [27,28]. As is clearly shown in Table 2, the electron withdrawing aryl substituted terpyridine ligands (40 -(4-chlorophenyl)-2,20 :60 ,200 -terpyridine and 60 -(pyridin-2-yl)-2,20 :40 ,200 -terpyridine) could enhance the inhibition property of the complexes, while electron donating substituted aryl rings have a minimal effect on the urease inhibitory activity. Control experiments were carried out and the

12

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14

Fig. 3. Molecular structure of 4 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.

Fig. 4. Molecular structure of 5 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.

results are presented in Table 2, as well. Both zinc(II) halides as well as the free ligands displayed negligible activities. The results of the control experiments are in accordance with those reported in the literature [28]. The mechanism of this inhibition has not been elucidated. Considering the effect of Zn compounds on urease [10], the following

suggestion is probable. The active site of urease contains two Ni ions, which is essential to the enzymatic activity. If those Ni ions are substituted by Zn ions, urease is significantly deactivated. We propose that the exchange of Ni by Zn happens considering the comparable complex-forming ability of these two transition metal ions and the deactivation of urease. Hence, the electron withdraw-

13

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14 Table 1 Bond lengths (Å) and angles (°) for complexes 1, 2, 4 and 5. Complex (1)

Complex (2)

Complex (4)

Complex (5)

Zn1–Br1 Zn1–Br2 Zn1–N1 Zn1–N2 Zn1–N3

2.3934(5) 2.3899(4) 2.212(3) 2.109(2) 2.214(3)

Zn1–Br1 Zn1–Br2 Zn1–N1 Zn1–N2 Zn1–N3

2.3826(4) 2.4050(4) 2.198(2) 2.098(2) 2.224(2)

Zn1–I1 Zn1–I2 Zn1–N1 Zn1–N2 Zn1–N3

2.6030(6) 2.5714(6) 2.205(4) 2.090(4) 2.229(4)

Zn1-I1 Zn1–I2 Zn1–N1 Zn1–N2 Zn1–N3

2.6067(5) 2.5957(5) 2.192(3) 2.079(3) 2.200(3)

Br1–Zn1–Br2 Br1–Zn1–N1 Br1–Zn1–N2 Br1–Zn1–N3 Br2–Zn1–N1 Br2–Zn1–N2 Br2–Zn1–N3 N1–Zn1–N2 N1–Zn1–N3 N2–Zn1–N3

112.467(18) 101.71(7) 110.79(6) 100.01(7) 96.62(6) 136.74(6) 98.96(6) 73.91(9) 145.91(9) 73.95(9)

Br1–Zn1–Br2 Br1–Zn1–N1 Br1–Zn1–N2 Br1–Zn1–N3 Br2–Zn1–N1 Br2–Zn1–N2 Br2–Zn1–N3 N1–Zn1–N2 N1–Zn1–N3 N2–Zn1–N3

114.091(17) 97.96(6) 138.99(6) 96.33(6) 98.44(6) 106.90(6) 104.66(6) 74.20(8) 144.87(8) 73.96(8)

I1–Zn1–I2 I1–Zn1–N1 I1–Zn1–N2 I1–Zn1–N3 I2–Zn1–N1 I2–Zn1–N2 I2–Zn1–N3 N1–Zn1–N2 N1–Zn1–N3 N2–Zn1–N3

112.62(2) 97.68(10) 106.34(10) 105.45(10) 97.83(10) 140.95(10) 97.22(10) 73.99(13) 144.86(14) 74.32(14)

I1–Zn1–I2 I1–Zn1–N1 I1–Zn1–N2 I1–Zn1–N3 I2–Zn1–N1 I2–Zn1–N2 I2–Zn1–N3 N1–Zn1–N2 N1–Zn1–N3 N2–Zn1–N3

114.320(17) 98.24(9) 122.24(8) 98.64(8) 96.40(9) 123.41(8) 99.35(8) 74.92(11) 149.65(11) 74.75(11)

Table 2 Inhibition of urease by the tested materials. Tested materials

IC50a (lM)

Tested materials

IC50a (lM)

>100

72.96 ± 0.12

>100

92.84 ± 0.50

>100

56.12 ± 0.72

>100

91.72 ± 0.48

>100

92.36 ± 0.24

96.42 ± 0.36

45.32 ± 0.27 (continued on next page)

14

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14

Table 2 (continued) Tested materials

IC50a (lM)

Tested materials

IC50a (lM)

Acetohydroxamic Acid (AHA)

ZnBr2 a

>100

ZnI2

>100

All IC50 values were expressed as mean ± S.D. values of the triplicate tests.

ing substituents on the terpyridyl rings can help the metal center to be separated and coordinate the Ni centers of the urease. This phenomenon is reported by another research group elsewhere. [29]. 3. Conclusion The present paper reports the syntheses, structures and urease inhibitory activities of a series of structurally similar zinc(II) bromide and iodide complexes with a-terpyridine derivatives. The structure–activity relationship specifies that electron withdrawing aryl groups on the 2,20 :60 ,200 -terpyridine ligands could improve the urease inhibitory activities of the zinc(II) complexes. Bearing in mind that zinc complexes have interesting biological activities and have been used in medicine, the complexes reported in this paper may possibly be used in the treatment of toxicities caused by urease producing bacteria. 4. Experimental The ligand was synthesized according to the Wang and Hanan procedure, with a slight modification [30]. 2-Acetylpyridine and the corresponding carboxaldehyde were mixed together all at once. Addition of 28–30% ammonia solution resulted in the synthesis of crude 40 -(aryl)- 2, 20 ; 60 , 200 -terpyridine (see Scheme 1 of the paper). After crystallization of the impure ligand from a CHCl3:Et2O (1:1) mixture, the analytically pure 40 -(aryl)- 2, 20 ; 60 , 200 -terpyridine was obtained in 40–86% yield. 4.1. Measurement of urease inhibition

Appendix A. Supplementary data CCDC Nos. 846606–846610 contains the supplementary crystallographic data for complexes 1, 2, 4 and 5, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223– 336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.035. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

The assay mixture, containing 25lL (4U) of jack bean urease and 25 lL (100 lg) of the test compound, was pre-incubated for 30 or 180 min at room temperature in a 96-well assay plate. After preincubation, 0.2 mL of 100 mM phosphate buffer pH 6.8 containing 500 mM urea and 0.002% phenol red indicator were added and incubated at room temperature. The reaction time was measured by a micro plate reader at 570 nm, which was required for enough ammonium carbonate to form to raise the pH of a phosphate buffer from 6.8 to 7.7.

[18]

Acknowledgments

[26] [27] [28] [29] [30]

This work was financially supported by the University of Tehran. The authors are also grateful to Prof. Sadegh Khazaeli for his useful comments.

[19] [20] [21] [22] [23] [24] [25]

P.A. Karplus, M.A. Pearson, R.P. Hausinger, Acc. Chem. Res. 30 (1997) 330. J.B. Sumner, J. Biol. Chem. 69 (1926) 435. C. Montecucco, R. Rappuoli, Nat. Rev. Mol. Cell Biol. 2 (2001) 457. R.A. Burne, Y.Y.M. Chen, Microb. Infect. 2 (2000) 533. D. Foster-Hartnett, P.J. Cullen, K.K. Gabbert, R.G. Kranz, Mol. Microbiol. 9 (1993) 907. B. Krajewska, J. Mol. Catal. B: Enzym. 59 (2009) 9–21. W. Zaborska, M. Kot, K. Superata, J. Enzym. Inhib. Med. Chem. 17 (2002) 247. Z. Amtul, A. Rahman, R.A. Siddiqui, M.I. Choudhary, Curr. Med. Chem. 9 (2002) 1323. M.A. Pearson, L.O. Michel, R.P. Hausinger, P.A. Karplus, Biochemistry 36 (1997) 8164. G.H. Perez, C.B. Gower, M. Blazer, J. Infect. Immun. 62 (1994) 299. Z.L. You, X. Han, G.N. Zhang, Z. Anorg. Allg. Chem. 634 (2008) 142. C.Y. Wang, Z.P. Han, J.Y. Ye, X. Wu, Synth. React. Inorg. Met. Org. Chem. 40 (2010) 831. M.I. Khan, M.K. Baloch, M. Ashfaq, J. Enzym. Inhib. Med. Chem. 22 (2007) 343. N. Zhang, C.Y. Huang, D.H. Shi, Z.L. You, Inorg. Chem. Commun. 14 (2011) 1636. C.Y. Wang, J. Coord. Chem. 62 (2009) 2860. E. Farkas, D. Bátka, E. Csapó, P. Buglyó, W. Haase, D. Sanna, Polyhedron. 26 (2007) 543. E. Gao, T. Sun, S. Liu, S. Ma, Z. Wen, Y. Wang, M. Zhu, L. Wang, X. Gao, F. Guan, M. Guo, F. Liu, Eur. J. Med. Chem. 45 (2010) 4531–4538. E.C. Constable, C.E. Housecroft, M.K. Kocik, J.A. Zampese, Polyhedron 30 (2011) 2704. A. Tarushi, G. Psomas, C.P. Raptopoulou, V. Psycharis, D.P. Kessissoglou, Polyhedron 28 (2009) 3272. E. Katsoulakou, N. Lalioti, C.P. Raptopoulou, A. Terzis, E. Manessi-Zoupa, S.P. Perlepes, Inorg. Chem. Commun. 5 (2002) 719. Q.L. Zhao, G.P. Li, Acta Cryst. Sec. E: Struct. Report. E65 (2009) m693. B. Whittle, E.L. Horwood, L.H. Rees, S.R. Batten, J.C. Jeffery, M.D. Ward, Polyhedron 17 (1998) 373. W. You, W. Huang, Y. Fan, C. Yao, J. Coord. Chem. 62 (2009) 2125. M. Wriedt, I. Jess, C. Näther, Acta Cryst. Sec. E: Struct. Report. E64 (2008) m11. S.H. Lee, S.H. Kim, P.G. Kim, C. Kima, Y. Kim, Acta Cryst. Sec. E: Struct. Report. E64 (2008) m511. Z.L. You, P. Zhou, Inorg. Chem. Commun. 10 (2007) 1273. K. Cheng, Z.L. You, H.L. Zhu, Aust. J. Chem. 60 (2007) 375. Z.L. You, P. Hou, L.L. Ni, S. Chen, Inorg. Chem. Commun. 12 (2009) 444. T. Matsukura, H. Tanaka, Biochemistry (Moscow) 65 (2000) 817. J. Wang, G.S. Hanan, Synlett 8 (2005) 1251.