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Syntheses and characterization of two monomeric zinc complexes containing aqua ligands
Polyhedron 18 (1999) 1785–1790
Syntheses and characterization of two monomeric zinc complexes containing aqua ligands a, b a a b Bao-Hui Ye *, Feng Xue , Gen-Qiang Xue , Liang-Nian Ji , Thomas C.W. Mak a
b
Chemistry Department, Zhongshan University, Guangzhou 510275, People’ s Republic of China Chemistry Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 7 December 1998; accepted 19 February 1999
Abstract Two monomeric zinc complexes, [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?OHCH 2 CH 2 OH (H 2 biim52,29-biimidazole) (1) and [Zn(bipy) 2 (H 2 O)(CF 3 CO 2 )](ClO 4 ) (bipy52,29-bipyridine) (2), have been synthesized and characterized by NMR and single-crystal structural analysis. The zinc ions in both complexes are coordinated in a distorted N 4 O 2 octahedral geometry: the zinc ion in complex 1 is ˚ and two H 2 biim ligands (Zn–N 2.150 A), ˚ the acetate groups act as counter-anions and form surrounded by two aqua [Zn–O 2.150 (3) A] ˚ one aqua molecule [Zn–O 2.148(2) A] ˚ and hydrogen bonds with H 2 biim; the zinc ion in 2 is ligated by two bipy ligands (Zn–N 2.144 A), ˚ In both complexes, the aqua ligand forms strong donor hydrogen bonds with the one monodentate carboxylate [Zn–O 2.154(2) A]. hydroxyl or carboxylate groups, being analogous to those found for the active sites of several zinc enzymes. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Zinc complex; Aqua; 2,29-Biimidazole; 2,29-Bipyridine; Carboxylate
1. Introduction The functional unit at the active site of zinc-hydrolases is a zinc-bound water molecule, which is deprotonated near neutral pH to generate the strongly nucleophilic Zn–OH group [1–3]. The aqua ligand bound to zinc ion is further stabilized via the formation of a hydrogen bond with a carboxylate or a hydroxyl (phenolic) group from amino acid residues [3]; this H-bond has been postulated to play a role in the activation of the coordinated aqua ligand in the catalytic pathway [1,4,5]. Although a number of zinc model complexes have been synthesized and well studied [4,5], the weak or indirect interactions, such as the H-bond that is found to modulate the activity of metalloenzymes [6–8], have been largely neglected. Studies on low-molecular-weight model compounds may clarify the coordination geometries and properties of the active sites of metalloenzymes and facilitate an understanding of the mechanism of the hydrolytic reaction of enzymes. 2,29-Biimdazole (H 2 biim) can serve as an example for a *Corresponding author. Tel.: 186-20-8403-6461; fax: 186-20-84036737. E-mail address: [email protected] (B.-H. Ye)
biomimetic ligand. Interestingly, it occurs as the neutral bidentate molecule, H 2 biim, as the monoanion, Hbiim 2 , or as the dianion, Biim 22 . Furthermore, a variety of geometries and ligating schemes for H 2 biim-containing complexes have been observed, due to the ligand’s ability to accommodate a wide range of electronic and steric demands imposed by the bound metal. The complexation of H 2 biim to Cu(II) [9–12], Co(II) [9,11,13], Fe(II) [9,11,14], Fe(III) [11,15], Ni(II) [9,11,13,16] and Cd(II) [17] has been investigated. Compared with the coordination compounds of the other first transition metal ions, only two examples of zinc complexes containing H 2 biim, [Zn 2 (m-H 2 biim)(H 2 biim) 4 ](ClO 4 ) 4 ?3H 2 O and [Zn(H 2 biim) 2 (HCO 2 )](ClO 4 ), have been prepared, in which the zinc ions all display five-coordination [18]. In an ongoing effort to study the interaction of zinc ion with aqua ligand, we report here on the synthesis and characterization of two monomeric zinc complexes, [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?HOCH 2 CH 2 OH (1) and [Zn(bipy) 2 (CF 3 CO 2 )(H 2 O)](ClO 4 ) (where bipy is 2,29bipyridine) (2), in which the aqua ligand forms a donor hydrogen bond with an hydroxyl or carboxylate group, in an analogous manner to those found at the active sites of several zinc enzymes [1–3].
0277-5387 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0277-5387( 99 )00058-3
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2. Experimental Starting materials were purchased from commercial sources and were used without further purification. (CAUTION: No problems were encountered during the preparation of the perchlorate derivatives described above. However, suitable care and precautions must be taken when handling such potentially explosive compounds). Elemental analyses (C, H and N) were performed on a PerkinElmer 240Q elemental analyzer. FT–IR spectra were recorded on a Bruker IFS-66 spectrometer as KBr pellets (4000–400 cm 21 ). 1 H and 13 C NMR spectra were recorded on a Bruker ARX-300 NMR spectrometer in solution at room temperature; the solvent signal was used to lock the field, and all chemical shifts are given relative to Si(CH 3 ) 4 (TMS); 1 H NMR spectra were measured at 300.132 MHz and 13 C NMR spectra at 74.475 MHz using the solvent signal as an internal standard with wide band proton decoupling.
2.1. Synthesis H 2 biim was synthesized in accordance with a published procedure [19]. Yield, 32%. Found: C, 53.41; H, 4.24; N, 41.56%. Calc. for C 6 H 6 N 4 : C, 53.73; H, 4.48; N, 41.79%. FT–IR data (cm 21 ): 3143 m, 3074 m, 3001 s, 2896 s, 2806 s, 1546 s, 1436 m, 1405 vs, 1333 s, 1217 m, 1105 vs, 939 s, 888 m, 763 m, 748 s and 690 m. 1 H NMR (DMSO-d 6 , ppm): 7.06 (arom C–H), 3.34 (arom N–H).
2.1.1. [ Zn( H2 biim)2 ( H2 O)2 ]( OAc)2?( HOCH2 CH2 OH) (1) Zn(OAc) 2 ?2H 2 O (0.219 g, 1 mmol), dissolved in methanol (10 ml), was added to an ethylene glycol solution (10 ml) containing H 2 biim (0.134 g, 1 mmol), with stirring. The solution was stirred at 508C for 5 h. After cooling to room temperature, the resulting solution was filtered off. The filtrate was left in air to evaporate the solvent and crystals were obtained after 2 days. The product was collected by filtration, washed with acetone and dried in air. Yield, 52.0%. Found: C, 39.52; H, 5.32; N, 20.54%. Calc. for C 18 H 28 N 8 O 8 Zn: C, 39.30; H, 5.09; N, 20.37%. 21 FT–IR data (cm ): 3300–2552 br, 1562 vs, 1534 s, 1442 s, 1414s, 1346 m, 1133 s, 1038 s, 997 m, 927 m, 784 s, 695 s, 658m, 471 w, 429 w. 1 H NMR data (DMSO-d 6 , ppm): 7.17 (s, 8H, arom C–H), 3.48 (br, 4H, N–H), 2.50 (s, 4H, CH 2 ), 1.90 (s, 6H, CH 3 ). 13 C NMR data (DMSOd 6 , ppm): 173.70, 171.95, 139.24, 62.68, 20.97. A single crystal suitable for X-ray analysis was obtained by evaporating a methanol and ethylene glycol solution of compound 1. 2.1.2. [ Zn( bipy)2 ( CF3 CO2 )( H2 O)]( ClO4 ) To a methanol solution (15 Zn(CF 3 CO 2 ) 2 ?2H 2 O (0.29 g, 1 mmol) (0.178 g, 1 mmol). The mixture was
(2) ml) containing was added bipy stirred at room
temperature for 1 h. The addition of NaClO 4 (0.2 g) to the resulting solution and cooling in a refrigerator lead to the formation of a colorless crystalline product within 3 days. Yield, 51%. Found: C, 43.61; H, 3.13; N, 9.43%. Calc. for C 32 H 18 ClF 3 N 4 O 7 Zn: C, 43.41; H, 2.96; N, 9.21%. FT–IR data (cm 21 ): 3433 br, 1691 vs, 1598 vs, 1576 m, 1566 m, 1492 m, 1475 s, 1442 vs, 1418 m, 1317 m, 1250 m, 1207 s, 1193 s, 1120 vs, 1076 vs, 1018 s, 778 m, 765 vs, 737 m, 718 m, 629 m, 415 m. A single crystal suitable for X-ray analysis was obtained by evaporating a methanol solution of compound 2.
2.2. Crystallographic measurements and structure resolution The selected single crystals of complexes 1 and 2 were mounted on a glass fiber and placed on a Siemens P3 / V diffractometer (1) or RIGAKU AFC7R (2) (graphite˚ monochromated Mo–Ka, l50.71073 A), respectively. The determination of the crystal class, orientation matrix and unit-cell dimensions was performed according to established procedures, and parameters were calculated from least-squares fitting of 2u angles for 25 reflections. Three standard reflections were monitored after every 100 data measurements, showing only small random variations. The crystal structures were solved by direct methods. Data processing, solution, and full-matrix least-squares refinement were performed with the SHELXL-PC program package [20]. All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated geometri˚ and were assigned isotropic thermal cally (C–H50.96 A), parameters. In 2, the CF 3 group exhibits 1 / 3 orientational disorder. Crystal data as well as details of data collection and refinement for complexes are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.
3. Results and discussion
3.1. Syntheses We have previously reported that the treatment of Zn(OAc) 2 ?2H 2 O with imidazole (Im) in a ratio of 1:2 afforded a mononuclear zinc complex, [Zn(Im) 2 (OAc) 2 ], in which the zinc ion is four coordinated and the carboxylate–histidine–zinc triad system found in metalloenzymes was observed [21]. When bidentate ligand H 2 biim was used in place of the monodentate ligand Im to investigate the carboxylate–histidine–zinc triad system, complex 1 was obtained. The zinc ion in complex 1 is six coordinated; the acetate anions do not bind to zinc but act as counter-anions and form hydrogen bonds to H 2 biim ligands. These arrangements are similar to those found in [Fe(H 2 biim) 2 (MeOH) 2 ](OAc) 2 [14] and [Ni(H 2 biim) 2 (H 2 O) 2 ](NO 3 ) 2 [16], but different from that found in the
B.-H. Ye et al. / Polyhedron 18 (1999) 1785 – 1790
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Table 1 Summary of crystallographic and experimental data for 1 and 2 Complex
1
2
Formula Mol. wt. T (K) ˚ l (A) Crystal system Space group ˚ a(A) ˚ b(A) ˚ c(A) b (8) ˚ 3) V(A Z Dcalc (g cm 23 ) m (mm 21 ) F(000) Crystal size (mm) 2u range (8) Reflections collected Independent reflections Observed reflections [F.4s (F )] Parameters Goodness-of-fit on F R, Rw [I.2s(I)] a
C 18 H 28 N 8 O 8 Zn 549.9 294 MoKa (0.71073) Monoclinic P2 1 / n 8.369(1) 9.551(1) 15.579(1) 102.81(1) 1214.2(5) 2 1.504 1.071 572 0.4030.4430.50 3.0–52.0 2559 2392 1387 161 1.30 0.0543, 0.0416
C 22 H 18 ClF 3 N 4 O 7 Zn 608.2 294 MoKa (0.71073) Monoclinic P2 1 / c 18.955(4) 7.299(1) 18.267(4) 104.22(3) 2449.9(12) 4 1.649 1.186 1232 0.1530.2030.40 3.0–52.0 5079 4802 3149 400 1.39 0.0379, 0.0428
a
R5o([Fo ]2[Fc ]) / o[Fc ], R w 5[ow(Fo 2Fc )2 / ow(Fo )2 ] 1 / 2 .
five coordination complex [Zn(H 2 biim) 2 (HCO 2 )](ClO 4 ) [18]. It is difficult to trace the main factor that determines the nature of the complex, including the coordinational preference of the metal ion, steric requirements of the ligands, packing considerations in the crystal and the
influence of the external charge of the complex. We have also reported that the treatment of Zn(OAc) 2 ?2H 2 O with bipy in a ratio of 1:1 afforded a dinuclear zinc complex, [Zn 2 (bipy) 2 (m-OAc) 3 ] 1 , in which the acetate groups act as monoatomic and triatomic bridge ligands [22]. When a
Table 2 ˚ and bond angles (8) of complexes 1 and 2 Selected bond lengths (A) Complex 1 Zn(1)–O(1w) 2.150(3) C(1)–N(1) 1.334(6) C(4)–N(4) 1.351(6) O(1w)???O(3) 2.746 O(1w)–Zn(1)–N(1) 93.1(1) N(1)–Zn(1)–N(3) 79.9(2) N(1)–Zn(1)–O(1wa) 86.9(1) N(1)–Zn(1)–N(1a) 180.0(1) N(3)–Zn(1)–N(3a) 180.0(1) N(4)–H(4)???O(1) 172.2 O(1w)–H(1wa)???O(3) 161.6 Symmetry code: (a) 2x, 2y, 12z; (b) 2x, 212y, 12z;
Zn(1)–N(1) 2.171(4) C(1)–N(2) 1.346(6) N(2)???O(2) 2.756 O(2)???O(3c) 2.680 O(1w)–Zn(1)–N(3) 92.4(1) O(1w)–Zn(1)–O(1wa) 180.0(1) N(3)–Zn(1)–O(1wa) 87.6(1) N(3)–Zn(1)–N(1a) 100.1(2) N(2)–H(2)???O(2) 165.5 O(1wd)–H(1wb)???O(1) 148.4 O(3c)–H(3c)???O(2) 170.6 (c) 0.51x, 20.5 2y, 0.51z; (d) 12x, 2y, 12z.
Zn(1)–N(3) C(4)–N(3) N(4)???O(1) O(1)???O(1wd)
2.129(4) 1.319(6) 2.678 2.681
Complex 2 Zn(1)–N(3) Zn(1)–N(4) C(21)–O(1) N(1)–Zn(1)–N(2) N(2)–Zn(1)–N(3) N(2)–Zn(1)–N(4) N(1)–Zn(1)–O(1w) N(3)–Zn(1)–O(1w) N(1)–Zn(1)–O(1) N(3)–Zn(1)–O(1) O(1w)–Zn(1)–O(1)
Zn(1)–N(2) Zn(1)–O(1w) C(21)–O(2) N(1)–Zn(1)–N(3) N(1)–Zn(1)–N(4) N(3)–Zn(1)–N(4) N(2)–Zn(1)–O(1w) N(4)–Zn(1)–O(1w) N(2)–Zn(1)–O(1) N(4)–Zn(1)–O(1) O(1)–C(21)–O(2)
Zn(1)–N(3) Zn(1)–O(1) O(1w)???O(2)
2.149(2) 2.154(2) 2.672
2.149(2) 2.145(2) 1.251(3) 76.9(1) 95.0(1) 96.7(1) 90.3(1) 88.3(1) 93.9(1) 166.1(1) 88.2(1)
2.131(2) 2.148(2) 1.227(3) 99.6(1) 172.2(1) 76.5(1) 167.2(1) 96.2(1) 91.4(1) 90.5(1) 130.8(2)
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bulky ligand CF 3 CO 2 2 was used in place of the acetate ligand to investigate the above reaction, complex 2 was obtained.
3.2. Crystal structure of complex 1 The crystal structure consists of [Zn(H 2 biim) 2 (H 2 O) 2 ] 21 complex cations, acetate anions and ethylene glycol solvent molecules, which are self-assembled via hydrogen bonds (Fig. 1). The asymmetric unit, therefore, comprises half a zinc ion, one biimidazole molecule, one aqua molecule, one acetate anion and one ethylene glycol molecule. The zinc ion, situated on a crystallographic inversion center in the unit cell, is coordinated to the N(1) and N(3) of two H 2 biim ligands that are trans to each other and O(1w) of two aqua molecules that are also trans to each other. The main distortion of the resulting octahedral coordination sphere originates in the small N(1)???N(3) bite angle of the chelating ligand [N(1)–Zn(1)–N(3)579.9 ˚ (2)8]. Furthermore, the average Zn–N distance (2.150 A) ˚ is slightly shorter than the apical distance (2.220 A), but is ˚ in a significantly longer than the basal distance (2.025 A) trigonal bipyramidal sphere [18]. The Zn–O distance ˚ is markedly longer than that observed in four [2.150(3) A] ˚ [23]. coordination (ca. 2.0 A) The [Zn(H 2 biim) 2 (H 2 O) 2 ] 21 complex cations are linked together into infinite chains along the b axis through an O(1w)–H(1wa)???O(3) hydrogen bond involving the coordinated aqua and the solvent molecule ethylene glycol (Fig. 1). Each acetate anion is connected to one H 2 biim through two strong N–H???O hydrogen bonds, with 2.678 ˚ N???O distances, and further accepts two and 2.756 A hydrogen bonds from the coordinated aqua and the solvent ethylene glycol molecule, with distances of 2.681 and ˚ respectively (Fig. 1). A hydrogen bond between 2.680 A, the carboxylate and bound water has been postulated to play a role in the activation of the coordinated aqua ligand in the catalytic pathway [1,4]. In carboxypeptidase A and
Fig. 1. ORTEP view of [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?HOCH 2 CH 2 OH with hydrogen bonds, showing 35% probability thermal ellipsoids and atoms labels.
thermolysin, attack of water at the peptide carbonyl may be assisted through general base catalysis by the carboxylate groups of Glu-270 and Glu-143, which form strong hydrogen bonds with the bound water [1]. Each imidazole ring is planar, with no atom deviating by ˚ from the best five-atom least-squares more than 0.010 A plane. The two rings of H 2 biim are nearly coplanar, making an angle of 1.358 with each other. The bond ˚ and C(1)–N(2) distances of C(1)–N(1) [1.334 (6) A] ˚ are almost equivalent, within experimental [1.346 (6) A] error, indicating that the effects of metal coordination and hydrogen-bonded protonation at these imidazole nitrogen atoms have equivalent effects on the ring’s electronic structure [15,16]. However, the bond length of C(4)–N(3) ˚ is significantly shorter than that of C(4)– [1.319 (6) A] ˚ This fact does not agree with the N(4) [1.351 (6) A]. above observation and prompts us to further examine the data carefully. We noticed that the bond distance of Zn(1)– ˚ is markedly shorter than that of N(3) [2.129 (4) A] ˚ and the hydrogen bond of Zn(1)–N(1) [2.171 (4) A], ˚ N(4)???O(1) (2.678 A, 172.28) is stronger than that of ˚ 165.58). It seems that the bond N(2)???O(2) (2.756 A, distance of C–N not only depends on the strength of metal coordination but also on hydrogen-bonded protonation. ˚ of the The C–O distances [1.250 (7) and 1.246 (7) A] acetate anion indicate the delocalization of the p bond between the two C–O bonds.
3.3. [ Zn( bipy)2 ( CF3 CO2 )( H2 O)]( ClO4 ) (2) The crystal structure of complex 2 consists of discrete [Zn(bipy) 2 (CF 3 CO 2 )(H 2 O)] 1 cations and perchlorate anions. As shown in Fig. 2, the zinc(II) ion is coordinated by
Fig. 2. ORTEP view of [Zn(bipy) 2 (H 2 O)(CF 3 CO 2 ](ClO 4 ), showing 35% probability thermal ellipsoids and atoms labels.
B.-H. Ye et al. / Polyhedron 18 (1999) 1785 – 1790
a unidentate CF 3 CO 2 2 group, an aqua ligand and two chelating bipy ligands in a distorted octahedral N 4 O 2 environment. The main distortion of the resulting octahedral coordination sphere originates in the small N(3)– Zn(1)–N(4)576.5 (1)8 bite angle of the chelating ligand. The Zn–N bond distances, in the range of 2.131(2) to ˚ and of Zn(1)–O(1) [2.154(2) A], ˚ are normal 2.149(2) A, [23]. The aqua ligand coordinates to zinc ion with a ˚ distance, which is comparable Zn(1)–O(1w)52.148(2) A ˚ and is further to that observed in complex 1 [2.150(3) A], stabilized through the formation of two hydrogen bonds, one with the non-coordinated oxygen atom O(2) of the ˚ CF 3 CO 2 2 group (2.67 A) and other with the oxygen atom ˚ O(3) of the perchlorate anion (3.06 A).
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In conclusion, complexes 1 and 2 were synthesized and further confirmed by single crystal structural analysis and spectroscopic studies. In both complexes, the aqua ligand forms strong donor hydrogen bonds with an hydroxyl or carboxylate group, analogous to those found for the active sites of several zinc enzymes. Therefore, the complexes can be used as models to observe how the water molecule is fixed and further activated in metallohydrolases.
Supplementary data Supplementary data are available from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, on request, quoting the deposition numbers 112075 and 112076.
3.4. Spectroscopic properties The IR spectra show broad absorption bands at 3300– 2552 and 3433 cm 21 for complexes 1 and 2, respectively, which are assigned to the n (N–H)1n (O–H) stretching vibrations. The shift of these absorption bands to lower wavenumbers, in comparison with the free ligands, can be attributed to the forming of intermolecular and / or intramolecular hydrogen bonds. The asymmetric and symmetric absorption observed for the uncoordinated acetates in complex 1 display at 1562 and 1414 cm 21 , respectively. The D[(D5nas (COO 2)2ns (COO 2)] value of 148 cm 21 is lower than that observed for an ionic acetate (164 cm 21 ) [24], indicating a lowered difference in the stretching ability of the C–O 2 and C=O bonds probably arising from the involvement of both acetate oxygen atoms in hydrogen bonds. In complex 2, D5249 cm 21 is lower than those values observed in unidentate geometry (D.300 cm 21 ), but similar to those in tri-atomic bridged fashion [24]. This can also be attributed to the formation of intramolecular hydrogen bonds. Complex 1 was also characterized by NMR spectroscopy in solution. The resonance peak at 7.17 ppm was assigned to H(2) and H(3), which was different from the observation in the [Zn(H 2 biim) 2 (HCO 2 )] 1 complex [18]. In the latter, this peak was split into two peaks, at 7.49 and 7.31 ppm. These may be interpreted by the formation of strong hydrogen bonds between the oxygen atoms of acetate anion and non-coordinated N–H groups from H 2 biim ligand, with the result that the effects of metal coordination and hydrogen-bonded protonation at the imidazole nitrogen atoms have equivalent effects on the ring’s electronic structure [15,16]. This was further confirmed by 13 C NMR spectra. Only a peak at 139.24 ppm was observed in the range of 170–70 ppm, which was assigned to the resonance of C(2) and C(3). The ethylene glycol molecule was also identified by the NMR technique; the signals at 2.50 and 62.68 ppm were consistent with the 1 H and 13 C resonances of the CH 2 group, respectively [25].
Acknowledgements We gratefully acknowledge financial support from the Natural Science Foundation of Guangdong Province and the Education Ministry of China.
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Syntheses and characterization of two monomeric zinc complexes containing aqua ligands a, b a a b Bao-Hui Ye *, Feng Xue , Gen-Qiang Xue , Liang-Nian Ji , Thomas C.W. Mak a
b
Chemistry Department, Zhongshan University, Guangzhou 510275, People’ s Republic of China Chemistry Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 7 December 1998; accepted 19 February 1999
Abstract Two monomeric zinc complexes, [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?OHCH 2 CH 2 OH (H 2 biim52,29-biimidazole) (1) and [Zn(bipy) 2 (H 2 O)(CF 3 CO 2 )](ClO 4 ) (bipy52,29-bipyridine) (2), have been synthesized and characterized by NMR and single-crystal structural analysis. The zinc ions in both complexes are coordinated in a distorted N 4 O 2 octahedral geometry: the zinc ion in complex 1 is ˚ and two H 2 biim ligands (Zn–N 2.150 A), ˚ the acetate groups act as counter-anions and form surrounded by two aqua [Zn–O 2.150 (3) A] ˚ one aqua molecule [Zn–O 2.148(2) A] ˚ and hydrogen bonds with H 2 biim; the zinc ion in 2 is ligated by two bipy ligands (Zn–N 2.144 A), ˚ In both complexes, the aqua ligand forms strong donor hydrogen bonds with the one monodentate carboxylate [Zn–O 2.154(2) A]. hydroxyl or carboxylate groups, being analogous to those found for the active sites of several zinc enzymes. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Zinc complex; Aqua; 2,29-Biimidazole; 2,29-Bipyridine; Carboxylate
1. Introduction The functional unit at the active site of zinc-hydrolases is a zinc-bound water molecule, which is deprotonated near neutral pH to generate the strongly nucleophilic Zn–OH group [1–3]. The aqua ligand bound to zinc ion is further stabilized via the formation of a hydrogen bond with a carboxylate or a hydroxyl (phenolic) group from amino acid residues [3]; this H-bond has been postulated to play a role in the activation of the coordinated aqua ligand in the catalytic pathway [1,4,5]. Although a number of zinc model complexes have been synthesized and well studied [4,5], the weak or indirect interactions, such as the H-bond that is found to modulate the activity of metalloenzymes [6–8], have been largely neglected. Studies on low-molecular-weight model compounds may clarify the coordination geometries and properties of the active sites of metalloenzymes and facilitate an understanding of the mechanism of the hydrolytic reaction of enzymes. 2,29-Biimdazole (H 2 biim) can serve as an example for a *Corresponding author. Tel.: 186-20-8403-6461; fax: 186-20-84036737. E-mail address: [email protected] (B.-H. Ye)
biomimetic ligand. Interestingly, it occurs as the neutral bidentate molecule, H 2 biim, as the monoanion, Hbiim 2 , or as the dianion, Biim 22 . Furthermore, a variety of geometries and ligating schemes for H 2 biim-containing complexes have been observed, due to the ligand’s ability to accommodate a wide range of electronic and steric demands imposed by the bound metal. The complexation of H 2 biim to Cu(II) [9–12], Co(II) [9,11,13], Fe(II) [9,11,14], Fe(III) [11,15], Ni(II) [9,11,13,16] and Cd(II) [17] has been investigated. Compared with the coordination compounds of the other first transition metal ions, only two examples of zinc complexes containing H 2 biim, [Zn 2 (m-H 2 biim)(H 2 biim) 4 ](ClO 4 ) 4 ?3H 2 O and [Zn(H 2 biim) 2 (HCO 2 )](ClO 4 ), have been prepared, in which the zinc ions all display five-coordination [18]. In an ongoing effort to study the interaction of zinc ion with aqua ligand, we report here on the synthesis and characterization of two monomeric zinc complexes, [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?HOCH 2 CH 2 OH (1) and [Zn(bipy) 2 (CF 3 CO 2 )(H 2 O)](ClO 4 ) (where bipy is 2,29bipyridine) (2), in which the aqua ligand forms a donor hydrogen bond with an hydroxyl or carboxylate group, in an analogous manner to those found at the active sites of several zinc enzymes [1–3].
0277-5387 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0277-5387( 99 )00058-3
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2. Experimental Starting materials were purchased from commercial sources and were used without further purification. (CAUTION: No problems were encountered during the preparation of the perchlorate derivatives described above. However, suitable care and precautions must be taken when handling such potentially explosive compounds). Elemental analyses (C, H and N) were performed on a PerkinElmer 240Q elemental analyzer. FT–IR spectra were recorded on a Bruker IFS-66 spectrometer as KBr pellets (4000–400 cm 21 ). 1 H and 13 C NMR spectra were recorded on a Bruker ARX-300 NMR spectrometer in solution at room temperature; the solvent signal was used to lock the field, and all chemical shifts are given relative to Si(CH 3 ) 4 (TMS); 1 H NMR spectra were measured at 300.132 MHz and 13 C NMR spectra at 74.475 MHz using the solvent signal as an internal standard with wide band proton decoupling.
2.1. Synthesis H 2 biim was synthesized in accordance with a published procedure [19]. Yield, 32%. Found: C, 53.41; H, 4.24; N, 41.56%. Calc. for C 6 H 6 N 4 : C, 53.73; H, 4.48; N, 41.79%. FT–IR data (cm 21 ): 3143 m, 3074 m, 3001 s, 2896 s, 2806 s, 1546 s, 1436 m, 1405 vs, 1333 s, 1217 m, 1105 vs, 939 s, 888 m, 763 m, 748 s and 690 m. 1 H NMR (DMSO-d 6 , ppm): 7.06 (arom C–H), 3.34 (arom N–H).
2.1.1. [ Zn( H2 biim)2 ( H2 O)2 ]( OAc)2?( HOCH2 CH2 OH) (1) Zn(OAc) 2 ?2H 2 O (0.219 g, 1 mmol), dissolved in methanol (10 ml), was added to an ethylene glycol solution (10 ml) containing H 2 biim (0.134 g, 1 mmol), with stirring. The solution was stirred at 508C for 5 h. After cooling to room temperature, the resulting solution was filtered off. The filtrate was left in air to evaporate the solvent and crystals were obtained after 2 days. The product was collected by filtration, washed with acetone and dried in air. Yield, 52.0%. Found: C, 39.52; H, 5.32; N, 20.54%. Calc. for C 18 H 28 N 8 O 8 Zn: C, 39.30; H, 5.09; N, 20.37%. 21 FT–IR data (cm ): 3300–2552 br, 1562 vs, 1534 s, 1442 s, 1414s, 1346 m, 1133 s, 1038 s, 997 m, 927 m, 784 s, 695 s, 658m, 471 w, 429 w. 1 H NMR data (DMSO-d 6 , ppm): 7.17 (s, 8H, arom C–H), 3.48 (br, 4H, N–H), 2.50 (s, 4H, CH 2 ), 1.90 (s, 6H, CH 3 ). 13 C NMR data (DMSOd 6 , ppm): 173.70, 171.95, 139.24, 62.68, 20.97. A single crystal suitable for X-ray analysis was obtained by evaporating a methanol and ethylene glycol solution of compound 1. 2.1.2. [ Zn( bipy)2 ( CF3 CO2 )( H2 O)]( ClO4 ) To a methanol solution (15 Zn(CF 3 CO 2 ) 2 ?2H 2 O (0.29 g, 1 mmol) (0.178 g, 1 mmol). The mixture was
(2) ml) containing was added bipy stirred at room
temperature for 1 h. The addition of NaClO 4 (0.2 g) to the resulting solution and cooling in a refrigerator lead to the formation of a colorless crystalline product within 3 days. Yield, 51%. Found: C, 43.61; H, 3.13; N, 9.43%. Calc. for C 32 H 18 ClF 3 N 4 O 7 Zn: C, 43.41; H, 2.96; N, 9.21%. FT–IR data (cm 21 ): 3433 br, 1691 vs, 1598 vs, 1576 m, 1566 m, 1492 m, 1475 s, 1442 vs, 1418 m, 1317 m, 1250 m, 1207 s, 1193 s, 1120 vs, 1076 vs, 1018 s, 778 m, 765 vs, 737 m, 718 m, 629 m, 415 m. A single crystal suitable for X-ray analysis was obtained by evaporating a methanol solution of compound 2.
2.2. Crystallographic measurements and structure resolution The selected single crystals of complexes 1 and 2 were mounted on a glass fiber and placed on a Siemens P3 / V diffractometer (1) or RIGAKU AFC7R (2) (graphite˚ monochromated Mo–Ka, l50.71073 A), respectively. The determination of the crystal class, orientation matrix and unit-cell dimensions was performed according to established procedures, and parameters were calculated from least-squares fitting of 2u angles for 25 reflections. Three standard reflections were monitored after every 100 data measurements, showing only small random variations. The crystal structures were solved by direct methods. Data processing, solution, and full-matrix least-squares refinement were performed with the SHELXL-PC program package [20]. All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated geometri˚ and were assigned isotropic thermal cally (C–H50.96 A), parameters. In 2, the CF 3 group exhibits 1 / 3 orientational disorder. Crystal data as well as details of data collection and refinement for complexes are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.
3. Results and discussion
3.1. Syntheses We have previously reported that the treatment of Zn(OAc) 2 ?2H 2 O with imidazole (Im) in a ratio of 1:2 afforded a mononuclear zinc complex, [Zn(Im) 2 (OAc) 2 ], in which the zinc ion is four coordinated and the carboxylate–histidine–zinc triad system found in metalloenzymes was observed [21]. When bidentate ligand H 2 biim was used in place of the monodentate ligand Im to investigate the carboxylate–histidine–zinc triad system, complex 1 was obtained. The zinc ion in complex 1 is six coordinated; the acetate anions do not bind to zinc but act as counter-anions and form hydrogen bonds to H 2 biim ligands. These arrangements are similar to those found in [Fe(H 2 biim) 2 (MeOH) 2 ](OAc) 2 [14] and [Ni(H 2 biim) 2 (H 2 O) 2 ](NO 3 ) 2 [16], but different from that found in the
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Table 1 Summary of crystallographic and experimental data for 1 and 2 Complex
1
2
Formula Mol. wt. T (K) ˚ l (A) Crystal system Space group ˚ a(A) ˚ b(A) ˚ c(A) b (8) ˚ 3) V(A Z Dcalc (g cm 23 ) m (mm 21 ) F(000) Crystal size (mm) 2u range (8) Reflections collected Independent reflections Observed reflections [F.4s (F )] Parameters Goodness-of-fit on F R, Rw [I.2s(I)] a
C 18 H 28 N 8 O 8 Zn 549.9 294 MoKa (0.71073) Monoclinic P2 1 / n 8.369(1) 9.551(1) 15.579(1) 102.81(1) 1214.2(5) 2 1.504 1.071 572 0.4030.4430.50 3.0–52.0 2559 2392 1387 161 1.30 0.0543, 0.0416
C 22 H 18 ClF 3 N 4 O 7 Zn 608.2 294 MoKa (0.71073) Monoclinic P2 1 / c 18.955(4) 7.299(1) 18.267(4) 104.22(3) 2449.9(12) 4 1.649 1.186 1232 0.1530.2030.40 3.0–52.0 5079 4802 3149 400 1.39 0.0379, 0.0428
a
R5o([Fo ]2[Fc ]) / o[Fc ], R w 5[ow(Fo 2Fc )2 / ow(Fo )2 ] 1 / 2 .
five coordination complex [Zn(H 2 biim) 2 (HCO 2 )](ClO 4 ) [18]. It is difficult to trace the main factor that determines the nature of the complex, including the coordinational preference of the metal ion, steric requirements of the ligands, packing considerations in the crystal and the
influence of the external charge of the complex. We have also reported that the treatment of Zn(OAc) 2 ?2H 2 O with bipy in a ratio of 1:1 afforded a dinuclear zinc complex, [Zn 2 (bipy) 2 (m-OAc) 3 ] 1 , in which the acetate groups act as monoatomic and triatomic bridge ligands [22]. When a
Table 2 ˚ and bond angles (8) of complexes 1 and 2 Selected bond lengths (A) Complex 1 Zn(1)–O(1w) 2.150(3) C(1)–N(1) 1.334(6) C(4)–N(4) 1.351(6) O(1w)???O(3) 2.746 O(1w)–Zn(1)–N(1) 93.1(1) N(1)–Zn(1)–N(3) 79.9(2) N(1)–Zn(1)–O(1wa) 86.9(1) N(1)–Zn(1)–N(1a) 180.0(1) N(3)–Zn(1)–N(3a) 180.0(1) N(4)–H(4)???O(1) 172.2 O(1w)–H(1wa)???O(3) 161.6 Symmetry code: (a) 2x, 2y, 12z; (b) 2x, 212y, 12z;
Zn(1)–N(1) 2.171(4) C(1)–N(2) 1.346(6) N(2)???O(2) 2.756 O(2)???O(3c) 2.680 O(1w)–Zn(1)–N(3) 92.4(1) O(1w)–Zn(1)–O(1wa) 180.0(1) N(3)–Zn(1)–O(1wa) 87.6(1) N(3)–Zn(1)–N(1a) 100.1(2) N(2)–H(2)???O(2) 165.5 O(1wd)–H(1wb)???O(1) 148.4 O(3c)–H(3c)???O(2) 170.6 (c) 0.51x, 20.5 2y, 0.51z; (d) 12x, 2y, 12z.
Zn(1)–N(3) C(4)–N(3) N(4)???O(1) O(1)???O(1wd)
2.129(4) 1.319(6) 2.678 2.681
Complex 2 Zn(1)–N(3) Zn(1)–N(4) C(21)–O(1) N(1)–Zn(1)–N(2) N(2)–Zn(1)–N(3) N(2)–Zn(1)–N(4) N(1)–Zn(1)–O(1w) N(3)–Zn(1)–O(1w) N(1)–Zn(1)–O(1) N(3)–Zn(1)–O(1) O(1w)–Zn(1)–O(1)
Zn(1)–N(2) Zn(1)–O(1w) C(21)–O(2) N(1)–Zn(1)–N(3) N(1)–Zn(1)–N(4) N(3)–Zn(1)–N(4) N(2)–Zn(1)–O(1w) N(4)–Zn(1)–O(1w) N(2)–Zn(1)–O(1) N(4)–Zn(1)–O(1) O(1)–C(21)–O(2)
Zn(1)–N(3) Zn(1)–O(1) O(1w)???O(2)
2.149(2) 2.154(2) 2.672
2.149(2) 2.145(2) 1.251(3) 76.9(1) 95.0(1) 96.7(1) 90.3(1) 88.3(1) 93.9(1) 166.1(1) 88.2(1)
2.131(2) 2.148(2) 1.227(3) 99.6(1) 172.2(1) 76.5(1) 167.2(1) 96.2(1) 91.4(1) 90.5(1) 130.8(2)
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bulky ligand CF 3 CO 2 2 was used in place of the acetate ligand to investigate the above reaction, complex 2 was obtained.
3.2. Crystal structure of complex 1 The crystal structure consists of [Zn(H 2 biim) 2 (H 2 O) 2 ] 21 complex cations, acetate anions and ethylene glycol solvent molecules, which are self-assembled via hydrogen bonds (Fig. 1). The asymmetric unit, therefore, comprises half a zinc ion, one biimidazole molecule, one aqua molecule, one acetate anion and one ethylene glycol molecule. The zinc ion, situated on a crystallographic inversion center in the unit cell, is coordinated to the N(1) and N(3) of two H 2 biim ligands that are trans to each other and O(1w) of two aqua molecules that are also trans to each other. The main distortion of the resulting octahedral coordination sphere originates in the small N(1)???N(3) bite angle of the chelating ligand [N(1)–Zn(1)–N(3)579.9 ˚ (2)8]. Furthermore, the average Zn–N distance (2.150 A) ˚ is slightly shorter than the apical distance (2.220 A), but is ˚ in a significantly longer than the basal distance (2.025 A) trigonal bipyramidal sphere [18]. The Zn–O distance ˚ is markedly longer than that observed in four [2.150(3) A] ˚ [23]. coordination (ca. 2.0 A) The [Zn(H 2 biim) 2 (H 2 O) 2 ] 21 complex cations are linked together into infinite chains along the b axis through an O(1w)–H(1wa)???O(3) hydrogen bond involving the coordinated aqua and the solvent molecule ethylene glycol (Fig. 1). Each acetate anion is connected to one H 2 biim through two strong N–H???O hydrogen bonds, with 2.678 ˚ N???O distances, and further accepts two and 2.756 A hydrogen bonds from the coordinated aqua and the solvent ethylene glycol molecule, with distances of 2.681 and ˚ respectively (Fig. 1). A hydrogen bond between 2.680 A, the carboxylate and bound water has been postulated to play a role in the activation of the coordinated aqua ligand in the catalytic pathway [1,4]. In carboxypeptidase A and
Fig. 1. ORTEP view of [Zn(H 2 biim) 2 (H 2 O) 2 ](OAc) 2 ?HOCH 2 CH 2 OH with hydrogen bonds, showing 35% probability thermal ellipsoids and atoms labels.
thermolysin, attack of water at the peptide carbonyl may be assisted through general base catalysis by the carboxylate groups of Glu-270 and Glu-143, which form strong hydrogen bonds with the bound water [1]. Each imidazole ring is planar, with no atom deviating by ˚ from the best five-atom least-squares more than 0.010 A plane. The two rings of H 2 biim are nearly coplanar, making an angle of 1.358 with each other. The bond ˚ and C(1)–N(2) distances of C(1)–N(1) [1.334 (6) A] ˚ are almost equivalent, within experimental [1.346 (6) A] error, indicating that the effects of metal coordination and hydrogen-bonded protonation at these imidazole nitrogen atoms have equivalent effects on the ring’s electronic structure [15,16]. However, the bond length of C(4)–N(3) ˚ is significantly shorter than that of C(4)– [1.319 (6) A] ˚ This fact does not agree with the N(4) [1.351 (6) A]. above observation and prompts us to further examine the data carefully. We noticed that the bond distance of Zn(1)– ˚ is markedly shorter than that of N(3) [2.129 (4) A] ˚ and the hydrogen bond of Zn(1)–N(1) [2.171 (4) A], ˚ N(4)???O(1) (2.678 A, 172.28) is stronger than that of ˚ 165.58). It seems that the bond N(2)???O(2) (2.756 A, distance of C–N not only depends on the strength of metal coordination but also on hydrogen-bonded protonation. ˚ of the The C–O distances [1.250 (7) and 1.246 (7) A] acetate anion indicate the delocalization of the p bond between the two C–O bonds.
3.3. [ Zn( bipy)2 ( CF3 CO2 )( H2 O)]( ClO4 ) (2) The crystal structure of complex 2 consists of discrete [Zn(bipy) 2 (CF 3 CO 2 )(H 2 O)] 1 cations and perchlorate anions. As shown in Fig. 2, the zinc(II) ion is coordinated by
Fig. 2. ORTEP view of [Zn(bipy) 2 (H 2 O)(CF 3 CO 2 ](ClO 4 ), showing 35% probability thermal ellipsoids and atoms labels.
B.-H. Ye et al. / Polyhedron 18 (1999) 1785 – 1790
a unidentate CF 3 CO 2 2 group, an aqua ligand and two chelating bipy ligands in a distorted octahedral N 4 O 2 environment. The main distortion of the resulting octahedral coordination sphere originates in the small N(3)– Zn(1)–N(4)576.5 (1)8 bite angle of the chelating ligand. The Zn–N bond distances, in the range of 2.131(2) to ˚ and of Zn(1)–O(1) [2.154(2) A], ˚ are normal 2.149(2) A, [23]. The aqua ligand coordinates to zinc ion with a ˚ distance, which is comparable Zn(1)–O(1w)52.148(2) A ˚ and is further to that observed in complex 1 [2.150(3) A], stabilized through the formation of two hydrogen bonds, one with the non-coordinated oxygen atom O(2) of the ˚ CF 3 CO 2 2 group (2.67 A) and other with the oxygen atom ˚ O(3) of the perchlorate anion (3.06 A).
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In conclusion, complexes 1 and 2 were synthesized and further confirmed by single crystal structural analysis and spectroscopic studies. In both complexes, the aqua ligand forms strong donor hydrogen bonds with an hydroxyl or carboxylate group, analogous to those found for the active sites of several zinc enzymes. Therefore, the complexes can be used as models to observe how the water molecule is fixed and further activated in metallohydrolases.
Supplementary data Supplementary data are available from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, on request, quoting the deposition numbers 112075 and 112076.
3.4. Spectroscopic properties The IR spectra show broad absorption bands at 3300– 2552 and 3433 cm 21 for complexes 1 and 2, respectively, which are assigned to the n (N–H)1n (O–H) stretching vibrations. The shift of these absorption bands to lower wavenumbers, in comparison with the free ligands, can be attributed to the forming of intermolecular and / or intramolecular hydrogen bonds. The asymmetric and symmetric absorption observed for the uncoordinated acetates in complex 1 display at 1562 and 1414 cm 21 , respectively. The D[(D5nas (COO 2)2ns (COO 2)] value of 148 cm 21 is lower than that observed for an ionic acetate (164 cm 21 ) [24], indicating a lowered difference in the stretching ability of the C–O 2 and C=O bonds probably arising from the involvement of both acetate oxygen atoms in hydrogen bonds. In complex 2, D5249 cm 21 is lower than those values observed in unidentate geometry (D.300 cm 21 ), but similar to those in tri-atomic bridged fashion [24]. This can also be attributed to the formation of intramolecular hydrogen bonds. Complex 1 was also characterized by NMR spectroscopy in solution. The resonance peak at 7.17 ppm was assigned to H(2) and H(3), which was different from the observation in the [Zn(H 2 biim) 2 (HCO 2 )] 1 complex [18]. In the latter, this peak was split into two peaks, at 7.49 and 7.31 ppm. These may be interpreted by the formation of strong hydrogen bonds between the oxygen atoms of acetate anion and non-coordinated N–H groups from H 2 biim ligand, with the result that the effects of metal coordination and hydrogen-bonded protonation at the imidazole nitrogen atoms have equivalent effects on the ring’s electronic structure [15,16]. This was further confirmed by 13 C NMR spectra. Only a peak at 139.24 ppm was observed in the range of 170–70 ppm, which was assigned to the resonance of C(2) and C(3). The ethylene glycol molecule was also identified by the NMR technique; the signals at 2.50 and 62.68 ppm were consistent with the 1 H and 13 C resonances of the CH 2 group, respectively [25].
Acknowledgements We gratefully acknowledge financial support from the Natural Science Foundation of Guangdong Province and the Education Ministry of China.
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