Accepted Manuscript An unexpected Schiff base-type Ni(II) complex: synthesis, crystal structures, fluorescence, electrochemical property and SOD-like activities Lan-Qin Chai, Hong-Song Zhang, Jiao-Jiao Huang, Yu-Li Zhang PII: DOI: Reference:
S1386-1425(14)01275-X http://dx.doi.org/10.1016/j.saa.2014.08.084 SAA 12605
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 May 2014 7 August 2014 24 August 2014
Please cite this article as: L-Q. Chai, H-S. Zhang, J-J. Huang, Y-L. Zhang, An unexpected Schiff base-type Ni(II) complex: synthesis, crystal structures, fluorescence, electrochemical property and SOD-like activities, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.08.084
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An unexpected Schiff base-type Ni(II) complex： ：synthesis, crystal structures, fluorescence, electrochemical property and SOD-like activities Lan-Qin Chai a,*, Hong-Song Zhang b,*, Jiao-Jiao Huang a and Yu-Li Zhang a a b
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China Second Hospital of Lanzhou University, Lanzhou, Gansu 730030, China
ABSTRACT An unexpected Schiff base-type Ni(II) complex, [Ni(L2)2]·CH3OH (HL2 = 1-(2-{[(E)-3, 5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), has been synthesized via complexation
of
Ni(II)
acetate
tetrahydrate
with
HL1
(2-(3,
5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide) originally. HL1 and its corresponding Ni(II) complex were characterized by IR, 1H-NMR spectra, as well as by elemental analysis, UV-Vis and emission spectroscopy, respectively. Crystal structures of the ligand and complex have been determined by single-crystal X-ray diffraction. Each complex links two other molecules into an infinite 1-D chain via intermolecular hydrogen bonding interactions. Moreover, the electrochemical property of the nickle complex was studied by cyclic voltammetry. In addition, SOD-like activities of HL1 and Ni(II) complex were also investigated. Keywords: Quinazoline-type ligand; Schiff base-type complex; Crystal structures; Spectroscopic properties; Cyclic voltammetry; Superoxide dismutase activities Introduction Recently metal-organic frameworks (MOFs) have attracted considerable attention owing to their structural diversity and intriguing topologies as well as their potential applications in many fields such as catalysis, sensor, ion exchange, and magnetic materials [1-4]. However, it is still a great challenge to predict the exact structures and composition of the assembly products. As a result, so much effort has been devoted to modifying the building blocks and controlling the assembled motifs for required products via selecting different organic ligands [5-7]. The quinazoline ring system along with many alkaloids is a widely recognized moiety in inorganic syntheses and medicinal application [8], such as it was found in HIV reverse transcriptase *
Corresponding author. Tel.: +86 931 4938795; fax: +86 931 4938703. E-mail address: [email protected] (L.-Q. Chai), [email protected] (H.-S. Zhang) 1
inhibitors [9]. Nickel(II) complexes with quinazoline analogue ligands have been widely investigated in coordination chemistry and biological chemistry [10,11], but cleavage of two C–N bonds of Schiff base-type have not been observed when the ligands react with metal salts. Some of quinazoline-type ligands or their metal complexes are used as biological models in understanding the structure of biomolecules and biological processes [12-14]. We
have
recently
described
a
novel
cobalt(III)
2-(3,5-dichloro-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazolin
complex 3-oxide,
based with
on
which
coordination mode of cobalt(II) acetate tetrahydrate was explored as well [15]. Here, we report the extension of our investigations on the coordination chemistry of another quinazoline-type ligand 2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline
3-oxide (HL1)
with other different metal salts. We were able to obtain a new quinazoline-type ligand and the Ni(II)
complex,
[Ni(L2)2]·CH3OH,
(HL2
=
1-(2-{[(E)-3,5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), which is an unexpected mononuclear Ni(II) complex possessing a Schiff base-type instead of an anticipated quinazoline-type Ni–N4O2 complex. UV-Vis and fluorescence emission spectroscopy of HL1 and the Ni(II) complex are also investigated. In addition, the property of electrochemistry and superoxide dismutase (SOD) activity are also described. Experimental Materials and physical measurements 3, 5-Dibromo-2-hydroxybenzylidene from Alfa Aesar was used without further purification. The other reagents and solvents were analytical grade from Tianjin Chemical Reagent Factory and used without further purification. C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument. Elemental analysis for Ni was determined by an IRIS ER/S·WP–1 ICP atomic emission spectrometer. IR spectra were recorded on a Vertex 70 and Nicolet Tnstrument Corporation NEXUS 670 FT-IR spectrophotometer, with samples prepared as KBr (500-4000 cm-1) and CsI (100-500 cm-1) pellets. 1H NMR spectra were determined on a Mercury plus (300 MHz) instrument using (CD3)2CO as solvent and TMS as internal standard. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. The fluorescence spectrum was taken on a 970 CRT spectrofluorometer (Spectro, Shanghai). Cyclic voltammetry measurements were performed using Chi660 (The United States CHI) voltammetric analyzer, a three electrode arrangement 2
made up of a glassy carbon working electrode, a platinum wire auxiliary electrode and a Ag/AgCl reference electrode was used in DMF containing 0.05 mol L–1 tetrabutylammonium perchlorate at scan rates of 100 mV s–1. Single-crystal X-ray structure determination was carried out on a Bruker APEX-II CCD and Bruker Smart 1000 CCD area detector diffractometer. Melting points were obtained by use of a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and are uncorrected. Synthesis of HL1 The major reaction steps involved in the synthesis of HL1 are given in Scheme 1.
Scheme 1. Synthetic route to the quinazoline-type ligand HL1.
HL1: To an ethanol solution (5 mL) of 3, 5-dibromo-2-hydroxybenzaldehyde (279.9 mg, 1.0 mmol) was added an ethanol solution (3 mL) of (E)-1-(2-aminophenyl) ethanone oxime (150.2 mg, 1.0 mmol). After stirring at 55–60 for 10 h, the mixture was filtered, precipitates were collected on a suction filter to afford HL1 (316.12 mg, 73.5%) as pale yellow powder. m.p. 225–227℃, 1H NMR (300 MHz，(CD3)2CO) δ 13.39 (br，1H，OH)，δ 7.58 (d，J = 2.3Hz， 1H，CHarom)，δ 7.41 (m，3H，CHarom)，δ = 7.24-7.21 (d，J = 9.0Hz，1H，CHarom)，δ 6.97-6.92 (dd，J = 12.0 Hz，1H，CHarom)，δ 6.48 (br，1H，NH)，δ 5.62 (br，1H，CH)，δ 2.46 (s, 3H, CH3). Anal. Calcd for C15H12Br2N2O2 (Mw 412.08) (%): C, 43.72; H, 2.94; N, 6.80. Found (%): C, 43.76; H, 2.97; N, 6.84. A solution of HL1 (4.12 mg, 0.01 mmol) in acetonitrile (5 mL) was added dropwise to n-hexane (1 mL) at room temperature. The mixture was filtered and the filtrate was allowed to stand at room temperature for about one week. Then the solvent partially evaporated and several colorless prismatical single crystals suitable for X-ray crystallographic analysis were obtained. Synthesis of the Ni(II) complex A solution of Ni(II) acetate tetra-hydrate (2.83 mg, 0.005 mmol) in methanol (2 mL) was added dropwise to a solution of HL1 (8.78 mg, 0.01 mmol) in dichloromethane (4 mL). The 3
color of the mixing solution turned brown immediately from light green. The solvent was partially evaporated for two weeks at room temperature and several brown needle-like single crystals suitable for X-ray crystallographic analysis were obtained. Anal. Calcd. For C31H26Br4N4NiO5 [Ni(L2)2]·CH3OH): C, 40.79; H, 2.87; N, 6.14, Ni, 6.43. Found: C, 40.82; H, 2.91; N, 6.18; Ni, 6.47. Crystallography The single crystals of HL1 was placed on a Bruker APEX-II CCD and the Ni(II) complex was placed on a Bruker Smart 1000 CCD area detector. The reflections were collected using a graphite-monochromated Mo-Kα radition (λ = 0.71073 Å) at 298(2) K. The LP factor and Semi-empirical absorption corrections were applied to the intensity data. The structure was solved by direct methods (SHELXS-97) and subsequent difference Fourier map revealed positions of the remaining atoms [16]; all non-hydrogen atoms were refined anisotropically using full-matrix least-squares on F2 with SHELXL-97 [17,18]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were generated geometrically. Crystallographic data and structural refinements for HL1 and the Ni(II) complex are listed in Table 1. Table 1. Crystallographic data and data collection parameters for HL1 and the Ni(II) complex. Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å, º) a b c β Volume (Å3), Z Calculated density (Mg /m 3) Absorption coefficient (mm– 1) F(000) Crystal size (mm3) θ Range for data collection (°) Index ranges
981100 C15H12Br2N2O2
986641 C31H26Br4N4NiO5
412.09 298(2) 0.71073 Monoclinic P 2(1)/c
912.85 298(2) 0.71073 Monoclinic P 2(1)/c
10.826(7) 14.822(9) 9.992(6) 117.445(6) 1422.9(15), 4 1.924 5.702 808 0.22 × 0.21 × 0.20 2.52 – 27.58 -13≤ h ≤14; -18≤ k ≤18; -12≤ l ≤12
11.3726(13) 13.1258(16) 23.432(4) 92.8340 (10) 3493.5(8), 4 1.736 5.173 1792 0.40 × 0.38 × 0.17 2.33 – 25.02 -13≤ h ≤10; -15≤ k ≤15; -27≤ l ≤26
4
Reflections collected Independent reflections Completeness to θ = 25.02 (%) Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I >2σ(I)] R indices (all data) Largest difference peak and hole (e Å-3) a
15452 2851 [R(int) = 0.0233] 98.8 3224 / 0 / 192 1.014 R1 = 0.0220, wR2 = 0.0533a R1 = 0.0271, wR2 = 0.0553 0.634 and –0.483
15981 2660 [R(int) = 0.0832] 98.8 6158 / 1 / 413 1.027 R1 = 0.0524, wR2 = 0.1108b R1 = 0.0896, wR2 = 0.1254 1.409 and –1.325
w = 1 / [σ2 (Fo2) + (0.0253P)2 + 1.1933P], b w = 1 / [σ2 (Fo 2) + (0.0811P)2], Where P = (Fo2 + 2Fc2) / 3
Scavenger measurements of superoxide radicals The superoxide radicals were measured in the test system using NBT/VitB2/MET [19]. The methods are the same as reported early [20]. The antioxidant activity was described as the 50% inhibitory concentration (IC50). IC50 values were calculated from regression lines where x was the concentration of HL1 and Ni(II) complex in µM and y was the percent inhibition of HL1 and Ni(II) complex. Results and discussion Synthesis The anticipated quinazoline-type complex [Ni(L1 )2] was not formed, but an unexpected Schiff base-type complex [Ni(L2)2] was obtained, which was formed in the course of complexation of HL1 by Ni(II) acetate tetrahydrate. The ligand HL1 changed to be a new C=N–O ligand HL2 after forming Ni(II) complex. In the process of reaction between Ni(II) ion and HL1, unexpected cleavages of C–N bonds in HL1 took place forming HL2, which coordinated to Ni(II) ion forming a mononuclear Ni(II) complex with a six-membered ring Schiff base-type complex instead of an anticipated quinazoline-type Ni–N4O2 complex [21]. The result suggested that in the metal complex the Schiff base configuration is more stable than that of quinazoline configuration (Scheme 2).
Scheme 2. Complexation of HL1 with Ni(II) acetate. 5
The plausible mechanism for the formation of quinazoline-type ligand On the basis of experimental results, a plausible reaction mechanism for the formation of quinazoline-type ligand (HL1) is proposed in Scheme 3. Firstly, a condensation reaction between (E)-1-(2-aminophenyl) ethanone oxime (1) and 3,5-dibromo-2-hydroxybenzaldehyde (2) takes place to form imine intermediate (3), accompanied by eliminating one water molecule. In the imine (3), as the nitrogen atom is more electronegative than the carbon atom, the nitrogen atom of the C=N bond has more negative charge while the carbon atom has more positive charge. The lone pair electron on the oxime nitrogen attacks the positively charged C=N carbon, resulting in occur cyclization product (4) which has a stable six-membered ring system. In the six-membered ring configuration, the structure of oxime with hydroxyl group is similar to that of enol structure. Because the pKa of the alcoholic hydroxyl group (pKa = 10.66) is lower than that of the nitrogen atom in the secondary amine (pKa = 11.09), the molecule is stabilized as an inner salt (5) through an intramolecular 1,5-hydrogen proton transfer. Finally, we obtain the product 2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide (6), which is responsible for the observed experimental results.
Scheme 3. Proposed mechanism for the formation of quinazoline-type ligand.
X-ray crystal structures of HL1 and [Ni(L2)2]·CH3OH The molecular structures of HL1 and the Ni(II) complex are shown in Figure 1 and Figure 2. Selected bond lengths and angles for the Ni(II) complex are listed in Table 2.
6
Fig.1. Crystal structure and intramolecular hydrogen bonds of HL1 with atoms numbering. Hydrogen bonding is indicated by a dashed bond.
X-ray crystallographic analysis reveals the crystal structure of HL1, in which all bond lengths and angles are in normal ranges. The molecule crystallizes in the monoclinic system, space group P2(1)/c. HL1 adopts a V-shaped configuration, in which the benzene rings are approximately perpendicular, making a dihedral angle of 89.46(3)º, and dihedral angles of six-membered rings (N2–C15 –N1 –C13–C12–C7) with the corresponding benzene rings (C7–C8–C9 –C10–C11–C12) are 9.56(4)º.
Fig.2. Crystal structure of the Ni(II) complex with atoms numbering. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level. All hydrogen atoms are ignored for clarity. Table 2. Selected bond distances (Å) and angles (º) for the Ni(II) complex. Bond
Dist.
Bond
Dist.
Ni(1)–O(2) Ni(1)–O(4) Ni(1)–N(1)
2.018(3) 2.040(4) 2.076(5)
Ni(1)–N(4) Ni(1)–N(2) Ni(1)–N(3)
2.021(5) 2.046(4) 2.117(5)
7
Bond
Angles
Bond
Angles
O(2)–Ni(1)–N(4) N(4)–Ni(1)–O(4) N(4)–Ni(1)–N(2) O(2)–Ni(1)–N(1) O(4)–Ni(1)–N(1) O(2)–Ni(1)–N(3) N(2)–Ni(1)– O(4) O(4)–Ni(1)–N(3) N(1)–Ni(1)–N(3) O(3)–N(1)–Ni(1) C(19)–N(2)–Ni(1) O(1)–N(3)–Ni(1) C(4)–N(4)–Ni(1) C(26)–O(4)–Ni(1)
88.53(16) 89.06(16) 103.27(18) 86.06(17) 91.29(17) 99.51(17) 84.91(16) 166.83(17) 99.84(18) 114.9(3) 115.2(3) 121.3(3) 114.0(3) 118.8(3)
O(2)–Ni(1)–O(4) O(2)–Ni(1)–N(2) O(4)–Ni(1)–N(2) N(4)–Ni(1)–N(1) N(2)–Ni(1)–N(1) N(4)–Ni(1)–N(3) N(1)–Ni(1)–O(4) N(2)–Ni(1)–N(3) C(17)–N(1)–Ni(1) C(24)–N(2)–Ni(1) C(2)–N(3)–Ni(1) C(9)–N(4)–Ni(1) C(11)–O(2)–Ni(1)
88.18(14) 166.19(17) 84.91(16) 174.57(18) 82.16(18) 80.52(18) 91.29(17) 89.63(18) 129.6(4) 124.9(4) 124.8(4) 126.4(4) 122.7(3)
Table 3. Hydrogen bonding distances (Å) and angles (º) for HL1 and the Ni(II) complex. D–H···A HL1 O2–H2··· Br1a N2–H2A··· O1b C4–H4·· ·N2 C14–H14C···O1 C11–H11···Cg(2)c C5–Br2···Cg(3)d Ni(II) complex O3–H3··· O2 C16–H16C···O3 O1–H1··· O5e O5–H5A··· O2f C9–H9·· ·O4g C14–Br2···Cg(7)h C27–Br3···Cg(6)
d(D–H)
d(H···A)
d(D···A)
∠DHA
0.82 0.86 0.93 0.96 0.93 1.90
2.92 2.32 2.46 2.33 2.77 3.824(3)
3.533(3) 3.065(3) 2.816(3) 2.778(3) 3.601(4) 5.160(4)
133 145 103 108 150 125.41(6)
0.82 0.96 0.82 0.82 0.93 1.899(6) 1.909(11)
2.15 2.19 1.86 1.96 2.56 3.589(3) 3.816(3)
2.777(6) 2.645(8) 2.676(6) 2.736(6) 3.334(7) 4.896(6) 4.793(6)
133 108 172 157 141 123.29(19) 109.10(16)
Symmetry code: (a) -x, -y, 2-z; (b) x, 1/2-y, -1/2+z; (c) 1-x, -y, 2-z; (d)1-x, -1/2+y, 3/2-z; (e) -1+x, y, z; (f) 1+x, y, z; (g) -x, -1/2+y, 1/2-z; (h) -x,-1/2+y, 1/2-z. Cg(2) is the centroids for benzene ring C1–C6, Cg(3) is the centroids for benzene ring C7–C12, Cg(6) is the centroids for benzene ring C10–C15, Cg(7) is the centroids for benzene ring C18–C23 respectively.
Molecules of HL1 are connected by intramolecular C−H···O and C−H···N hydrogen bonding, intermolecular N−H···O and O−H···X hydrogen bonding, C−H···π and C−X···π (Ph) interactions (Table 3), which play a role in stabilizing the structure of the crystal. Two pairs of intramolecular C14−H14C···O1 and C4−H4···N2 hydrogen bonds are formed two five-membered rings. One intermolecular N2−H2A···O1 hydrogen bond is between quinazoline 8
nitrogen and phenolic oxygen, anther intermolecular O2−H2···Br1 hydrogen bond is between quinazoline nitrogen and bromine atom. Furthermore, three pairs of C11−H11···Cg(2), C5−Br2···Cg(3) and Cg(2)···Cg(2) (the distance between aromatic ring centroid for C1−C6 is 3.631Å) weak interactions are between aromatic ring. Above-mentioned hydrogen bonds contain intermolecular C−H···π and C–X···π (Ph) play an import role in stabilizing the crystal structure. In conclusion, with the help of hydrogen bonds and other weak interactions, adjacent molecular units are linked together by interactions to give an infinite 2-D chain supramolecular structure along the a-axis, as shown in Figure 3.
Fig.3. View of the 2-D chain motif of the HL1 along the a-axis (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
X-ray crystallographic analysis of complex reveals the formation of a mononuclear structure. Single crystal X-ray analysis reveals that the metal complex has 2:1 ligand to metal stoichiometry, crystallizes in the monoclinic system with P 2(1)/c space group and consists of one Ni(II), two (L2)−, and one non-coordinated methanol molecule. Ni(II) is bonded to oxygen and nitrogen of the two (L2)− molecules in a cis arrangement in which Ni(II) is six-coordinated. As shown in Figure 4, the coordination environment around Ni(II) is octahedral with some distortion, by one imine nitrogen (N1), one Schiff base nitrogen (N4), and two deprotonated phenolic oxygens (O2 and O4) defining the cis-N2O2 basal plane, plus one imine nitrogen (N3) atom and one Schiff base nitrogen (N2) occupying apical positions from two deprotonated (L2)− units [22,23]. The coordination environment around Ni(II) is best regarded as a slightly distorted octahedral geometry with the distance of the apical nitrogen N1 and N4 to the N2O2 basal plane 9
being 2.585(4) Å and 1.431(4) Å. The center Ni is in the N2O2 basal plane. The bond lengths of Ni–O are longer than Ni–N. These structural features, from steric hindrance of the L1− fragments, account for the distortion of the ligand in the Ni(II) complex.
Fig.4. Coordination configuration of Ni(II) complex.
Molecules of complex are connected by intramolecular C−H···O and O−H···O hydrogen bonds, intermolecular hydrogen bonds and C−X···π (Ph) interactions (Table 3), which play a role in stabilizing the structure of the crystal. Two pairs of intramolecular O3−H3···O2 and C16−H16C···O3 hydrogen bonds form two five-membered rings. Three intermolecular O1–H1···O5, O5–H5A···O2 and C9–H9···O4 hydrogen bonds between aromatic carbon, phenolic oxygen and non-coordinated methanol form a seven-membered ring. Two pairs of C14–Br2···Cg(7) and C27–Br3···Cg(6) hydrogen bonds exist between aromatic rings [24]. To sum up, with the help of interactions hydrogen bonds, the crystal packing shows a self-assembling 1-D supramolecular structure (Figure 5).
Fig.5. View of the 1-D chain motif of the Ni(II) complex along the b-axis. Hydrogen bonding is indicated by a dashed bond (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
IR spectra The main FT-IR absorptions of HL1 and the Ni(II) complex in the 400-4000 cm-1 region 10
are given in Table 4. The free ligand exhibits a characteristic C=N stretching band at 1611 cm-1, while C=N of the Ni(II) complex is observed at 1623 cm-1. The C=N stretch is shifted to higher frequency by ca. 12 cm-1 upon complexation, indicates a decrease in C=N bond order due to the coordination of Ni(II) with oxime nitrogen [25]. The Ar–O stretch is a strong band, occurring at 1265 cm-1 for HL1 and at 1294 cm-1 for the Ni(II) complex. The Ar–O stretching frequency is shifted to higher frequency, indicating that Ni–O bond was formed between Ni(II) and oxygen of the phenol [26]. The IR spectrum of HL1 shows unexpected strong absorption due to ν(N→O) at 1286 cm-1, which disappears in the complex, indicating the C–N bond in the Ni(II) complex has broken and at the same time oxime N participated in coordination. The N–H in HL1 at 3070 cm−1, which disappears in the complex, indicating the nitrogen atom coordinated to Ni(II). The O–H stretch of free HL1 appears at 3253 cm-1, while the infrared spectrum of the Ni(II) complex shows the expected strong absorption due to ν(O–H) at 3205 cm−1, which is the evidence of O–H group from oxime. The far-infrared spectrum of the Ni(II) complex is also obtained in the region 100–500 cm-1 in order to identify frequencies due to Ni–O and Ni–N bonds. The FT-IR spectrum of the complex shows ν(Ni–N) and ν(Ni–O) at 517 and 450 cm-1, respectively. Table 4. Selected FT-IR bands for the ligand and its Ni(II) complex (cm-1). Compound
ν(O–H)
ν(N–H)
ν(C=N)
HL1 Complex
3253 3205
3070 ─
1611 1623
νN→O 1286 ─
νAr–O 1265 1294
ν(Ni–N)
ν(Ni–O)
─ 517
─ 450
UV-vis absorption spectra UV-vis aborption spectra of HL1 and the Ni(II) complex were determined in 5 × 10 -5 mol L-1 CH2Cl2 solution. Absorptions of the Ni(II) complex were obviously different from those of HL1 as shown in Figure 6. Compared with the Ni(II) complex, an important feature of the absorption spectrum of HL1 was that three absorptions were observed at 240, 305 and 363 nm, respectively. The latter absorptions were absent in the spectrum of the Ni(II) complex. The absorption at 252 nm in the Ni(II) complex could be due to the d–d transition of Ni(II). The Ni(II) complex with 3d8 electronic configuration could be assigned to 3A2g → 3T1g transition of Ni(II) in an octahedral environment [27]. These observations were in line with common spectral features of d-block metal complexes. The intense band near 439nm was assigned for 11
ligand-to-metal charge transfer (LMCT) transitions [28]. The splitting of the 3d levels in the crystal field of the coordination compounds causes d–d electronic transitions.
Fig.6. UV-vis absorption spectra: HL1 and the Ni(II) complex in CH2Cl2 (5×10-5 mol L-1).
Fluorescence spectra Fluorescent properties of HL1 and its corresponding Ni(II) complex were investigated at room temperature (Figure 7). The ligand exhibits two intense emissions at 423 and 463 nm upon excitation at 363 nm, which should be assigned to the intraligand π-π* transition [29]. The Ni(II) complex shows an intense broad photoluminescence with maximum emission at ca. 495 nm upon excitation at 436 nm, which is red-shifted to that of HL1. This red shift might be related to the auxochrome group (–Br), which makes the conjugated system larger. The compounds emit strong fluorescence in the green light area and may be potential functional fluorescent material which can emit green light.
Fig.7. Emission spectra of HL1 and the Ni(II) complex in DMF at room temperature (c = 5 × 10-5 mol L-1). 12
Electrochemistry Studies The electrochemical property of Ni(II) complex has been studied in DMF containing 0.05 mol L–1 tetrabutylammonium perchlorate over the potential range from –2 to 0.7 V (vs. Ag/AgCl) using CV techniques, with scanning rate of 100 mV s–1. As it observed in Figure 8, the Ni(II) complex exhibits two reversible redox waves, which are assigned to consecutive Ni(II)/Ni(I) and Ni(III)/Ni(II) redox processes [30]. The first pair of oxidation-reduction peaks correspond to the oxidatio-reduction couples Ni(III)/Ni(II), Epa1 = 0.196 V, Epc1 = –0.895 V, with the average formal potential [E1/2 = (Epa1+Epc1)/2] is –0.347 V. The peak-to-peak separation between the anodic and cathodic (△Ep1) is 1.093 V and the proportion of the peak current (ipc1/ipa1) is 1.523. The second pair of oxidation–reduction peaks correspond to Ni(II)/Ni(I), Epa2 = –0.723 V, Epc2 = –1.561 V, with the average formal potential [E1/2 = (Epa2 + Epc2) /2] is 1.142 V, the peak-to-peak separation between the anodic and cathodic processes (△Ep 2) of 0.836V and the proportion of the peak current (ipc2/ipa2) of 0.771. These features are indicative of a quasi-reversible electrode process [31].
Fig.8. Cyclic voltammogram of Ni(II) complex in DMF solution. Scan rate at 100 mV s–1.
SOD-like Activity The SOD-like activities of HL1 and Ni(II) complex were investigated and shown in Figure 9. The average suppression ratio against O2˙– increases with increasing concentration in the range of the tested concentration. The observed IC50 values of the compounds were compared with earlier reported values for nickel(II) complexes [20,32,33]. The catalytic activity of NiSOD [34], however, is on the same high level as that of Cu–ZnSOD at about 109 (mol L-1)-1s-1 per 13
metal center. The IC50 data of the SOD activity assay along with kinetic catalytic constants of compounds [35,36] are presented in Table 5. The results indicate that the prepared compounds are more efficient antioxidants than Vitamin C, which is standard superoxide dismutase [37,38]. The brown color of the octahedral Ni(II) complex exhibits more active scavenging effects against O2˙– than HL1 and other nickel complexes under the same conditions. The scavenging effect of Ni(II) complex may be ascribed to the chelating function of ligand with metal ion to achieve significant selectivity of radical scavenging activity in biological system [39].
Fig.9. Scavenging effects of the ligand HL1 and the Ni(II) complex on O2˙– Table 5. IC50 values and kinetic constant of compounds IC50 (µmol)
kMcCF ((mol L-1)-1s-1) × 104
852
─
[37]
51.58
─
This work
8.24 8.26 31 34
33.14 33.08 3.06 1.76
This work
Ni(HL)(bipy)(H2O)(NO3 )(ClO4 )(H2O)
38
2.5
Ni(HL)(dien)(ClO4 )(H2O)
Compound Vc 1
HL
2
[Ni(L )2] ·CH3OH [Ni(L)2] ·CH3OH [Ni(L)2] [Ni(L)(HL)](ClO4)(H2O)
95
1.0
1
35
2.71
2
55
1.73
3
60
1.58
[Ni(L )2] ·2H2O [Ni(L )2](ClO4)2 [Ni(L )(bipy)](ClO4)2
Reference
[20] [32] [32] [32] [32] [33] [33] [33]
kMcCF were calculated by K = kNBT × [NBT]/IC50, kNBT (pH 7.8) = 5.94×104 (mol L–1 ) –1s–1 [36].
Conclusions We have successfully reported the synthesis and structural characterization of an 14
unexpected mononuclear Ni(II) complex based on analytical and spectral data. Ni(II) caused an unexpected cleavage of C–N bonds in HL1, giving a new C=N–O ligand HL2, which coordinates to Ni(II) forming Ni–N4O2 complex involving a Schiff base-type instead of an anticipated quinazoline complex. Moreover, the quasi-reversible one-electron Ni(III)/Ni(II) and Ni(II)/Ni(I) redox waves were determined by cyclic voltammetry. In addition, the Ni(II) complex exhibits more active scavenging effects against O2˙– than of HL1 and other nickel complexes under the same conditions. Analytic, spectroscopic, electrochemical, and crystal structure determinations show that a new nickel(II) complex has octahedral geometry. Supplementary material CCDC 981100 and 986641 contain the supplementary crystallographic data of the ligand and its complex for this article. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif., or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK. Telephone: (44) 01223 762910 Facsimile: (44) 01223 336033; or e-mail: [email protected].
Acknowledgements We are thankful for the financial support by the Fundamental Research Funds for the Universities (214086) and the Natural Science Foundation of Gansu Province (No: 1107RJZA165). References [1] H. Furukawa, K.E. Nirdova, M. O′Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science. 341 (2013) 974-986. [2] Q.X.Yang, L.F. Huang, M.D. Zhang, An unprecedented homochiral metal-organic framework based on achiral nanosized pyridine and v-shaped polycarboxylate acid ligand, Cryst. Growth Des. 13 (2013) 440-445. [3] S. Saha, R.K. Kottalanka, P. Bhowmik, S. Jana, K. Harms, T.K. Panda, S. Chattopadhyay, H.P.
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19
GRAPHICAL ABSTRACT An unexpected Schiff base-type Ni(II) complex instead of an anticipated quinazoline-type complex has been synthesized and structural characterized by spectroscopic methods. Crystal structures of the ligand and complex have been determined by single-crystal X-ray diffraction. Moreover, the electrochemical property of the nickle complex was studied by cyclic voltammetry. In addition, SOD-like activities of HL1 and Ni(II) complex were also investigated.
31
HIGHLIGHTS •
An unexpected
Schiff base-type
Ni(II)
complex instead
of an anticipated
quinazoline-type complex has been synthesized firstly. •
A new ligand and complex have been characterized structurally by spectroscopic methods and determined by single-crystal X-ray diffraction.
•
The brown color of the octahedral Ni(II) complex exhibits more active scavenging effects against O2˙– than of HL1 and other nickel complexes under the same conditions.
32
S1386-1425(14)01275-X http://dx.doi.org/10.1016/j.saa.2014.08.084 SAA 12605
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 May 2014 7 August 2014 24 August 2014
Please cite this article as: L-Q. Chai, H-S. Zhang, J-J. Huang, Y-L. Zhang, An unexpected Schiff base-type Ni(II) complex: synthesis, crystal structures, fluorescence, electrochemical property and SOD-like activities, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.08.084
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An unexpected Schiff base-type Ni(II) complex： ：synthesis, crystal structures, fluorescence, electrochemical property and SOD-like activities Lan-Qin Chai a,*, Hong-Song Zhang b,*, Jiao-Jiao Huang a and Yu-Li Zhang a a b
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China Second Hospital of Lanzhou University, Lanzhou, Gansu 730030, China
ABSTRACT An unexpected Schiff base-type Ni(II) complex, [Ni(L2)2]·CH3OH (HL2 = 1-(2-{[(E)-3, 5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), has been synthesized via complexation
of
Ni(II)
acetate
tetrahydrate
with
HL1
(2-(3,
5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide) originally. HL1 and its corresponding Ni(II) complex were characterized by IR, 1H-NMR spectra, as well as by elemental analysis, UV-Vis and emission spectroscopy, respectively. Crystal structures of the ligand and complex have been determined by single-crystal X-ray diffraction. Each complex links two other molecules into an infinite 1-D chain via intermolecular hydrogen bonding interactions. Moreover, the electrochemical property of the nickle complex was studied by cyclic voltammetry. In addition, SOD-like activities of HL1 and Ni(II) complex were also investigated. Keywords: Quinazoline-type ligand; Schiff base-type complex; Crystal structures; Spectroscopic properties; Cyclic voltammetry; Superoxide dismutase activities Introduction Recently metal-organic frameworks (MOFs) have attracted considerable attention owing to their structural diversity and intriguing topologies as well as their potential applications in many fields such as catalysis, sensor, ion exchange, and magnetic materials [1-4]. However, it is still a great challenge to predict the exact structures and composition of the assembly products. As a result, so much effort has been devoted to modifying the building blocks and controlling the assembled motifs for required products via selecting different organic ligands [5-7]. The quinazoline ring system along with many alkaloids is a widely recognized moiety in inorganic syntheses and medicinal application [8], such as it was found in HIV reverse transcriptase *
Corresponding author. Tel.: +86 931 4938795; fax: +86 931 4938703. E-mail address: [email protected] (L.-Q. Chai), [email protected] (H.-S. Zhang) 1
inhibitors [9]. Nickel(II) complexes with quinazoline analogue ligands have been widely investigated in coordination chemistry and biological chemistry [10,11], but cleavage of two C–N bonds of Schiff base-type have not been observed when the ligands react with metal salts. Some of quinazoline-type ligands or their metal complexes are used as biological models in understanding the structure of biomolecules and biological processes [12-14]. We
have
recently
described
a
novel
cobalt(III)
2-(3,5-dichloro-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazolin
complex 3-oxide,
based with
on
which
coordination mode of cobalt(II) acetate tetrahydrate was explored as well [15]. Here, we report the extension of our investigations on the coordination chemistry of another quinazoline-type ligand 2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline
3-oxide (HL1)
with other different metal salts. We were able to obtain a new quinazoline-type ligand and the Ni(II)
complex,
[Ni(L2)2]·CH3OH,
(HL2
=
1-(2-{[(E)-3,5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), which is an unexpected mononuclear Ni(II) complex possessing a Schiff base-type instead of an anticipated quinazoline-type Ni–N4O2 complex. UV-Vis and fluorescence emission spectroscopy of HL1 and the Ni(II) complex are also investigated. In addition, the property of electrochemistry and superoxide dismutase (SOD) activity are also described. Experimental Materials and physical measurements 3, 5-Dibromo-2-hydroxybenzylidene from Alfa Aesar was used without further purification. The other reagents and solvents were analytical grade from Tianjin Chemical Reagent Factory and used without further purification. C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument. Elemental analysis for Ni was determined by an IRIS ER/S·WP–1 ICP atomic emission spectrometer. IR spectra were recorded on a Vertex 70 and Nicolet Tnstrument Corporation NEXUS 670 FT-IR spectrophotometer, with samples prepared as KBr (500-4000 cm-1) and CsI (100-500 cm-1) pellets. 1H NMR spectra were determined on a Mercury plus (300 MHz) instrument using (CD3)2CO as solvent and TMS as internal standard. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. The fluorescence spectrum was taken on a 970 CRT spectrofluorometer (Spectro, Shanghai). Cyclic voltammetry measurements were performed using Chi660 (The United States CHI) voltammetric analyzer, a three electrode arrangement 2
made up of a glassy carbon working electrode, a platinum wire auxiliary electrode and a Ag/AgCl reference electrode was used in DMF containing 0.05 mol L–1 tetrabutylammonium perchlorate at scan rates of 100 mV s–1. Single-crystal X-ray structure determination was carried out on a Bruker APEX-II CCD and Bruker Smart 1000 CCD area detector diffractometer. Melting points were obtained by use of a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and are uncorrected. Synthesis of HL1 The major reaction steps involved in the synthesis of HL1 are given in Scheme 1.
Scheme 1. Synthetic route to the quinazoline-type ligand HL1.
HL1: To an ethanol solution (5 mL) of 3, 5-dibromo-2-hydroxybenzaldehyde (279.9 mg, 1.0 mmol) was added an ethanol solution (3 mL) of (E)-1-(2-aminophenyl) ethanone oxime (150.2 mg, 1.0 mmol). After stirring at 55–60 for 10 h, the mixture was filtered, precipitates were collected on a suction filter to afford HL1 (316.12 mg, 73.5%) as pale yellow powder. m.p. 225–227℃, 1H NMR (300 MHz，(CD3)2CO) δ 13.39 (br，1H，OH)，δ 7.58 (d，J = 2.3Hz， 1H，CHarom)，δ 7.41 (m，3H，CHarom)，δ = 7.24-7.21 (d，J = 9.0Hz，1H，CHarom)，δ 6.97-6.92 (dd，J = 12.0 Hz，1H，CHarom)，δ 6.48 (br，1H，NH)，δ 5.62 (br，1H，CH)，δ 2.46 (s, 3H, CH3). Anal. Calcd for C15H12Br2N2O2 (Mw 412.08) (%): C, 43.72; H, 2.94; N, 6.80. Found (%): C, 43.76; H, 2.97; N, 6.84. A solution of HL1 (4.12 mg, 0.01 mmol) in acetonitrile (5 mL) was added dropwise to n-hexane (1 mL) at room temperature. The mixture was filtered and the filtrate was allowed to stand at room temperature for about one week. Then the solvent partially evaporated and several colorless prismatical single crystals suitable for X-ray crystallographic analysis were obtained. Synthesis of the Ni(II) complex A solution of Ni(II) acetate tetra-hydrate (2.83 mg, 0.005 mmol) in methanol (2 mL) was added dropwise to a solution of HL1 (8.78 mg, 0.01 mmol) in dichloromethane (4 mL). The 3
color of the mixing solution turned brown immediately from light green. The solvent was partially evaporated for two weeks at room temperature and several brown needle-like single crystals suitable for X-ray crystallographic analysis were obtained. Anal. Calcd. For C31H26Br4N4NiO5 [Ni(L2)2]·CH3OH): C, 40.79; H, 2.87; N, 6.14, Ni, 6.43. Found: C, 40.82; H, 2.91; N, 6.18; Ni, 6.47. Crystallography The single crystals of HL1 was placed on a Bruker APEX-II CCD and the Ni(II) complex was placed on a Bruker Smart 1000 CCD area detector. The reflections were collected using a graphite-monochromated Mo-Kα radition (λ = 0.71073 Å) at 298(2) K. The LP factor and Semi-empirical absorption corrections were applied to the intensity data. The structure was solved by direct methods (SHELXS-97) and subsequent difference Fourier map revealed positions of the remaining atoms [16]; all non-hydrogen atoms were refined anisotropically using full-matrix least-squares on F2 with SHELXL-97 [17,18]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were generated geometrically. Crystallographic data and structural refinements for HL1 and the Ni(II) complex are listed in Table 1. Table 1. Crystallographic data and data collection parameters for HL1 and the Ni(II) complex. Identification code Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å, º) a b c β Volume (Å3), Z Calculated density (Mg /m 3) Absorption coefficient (mm– 1) F(000) Crystal size (mm3) θ Range for data collection (°) Index ranges
981100 C15H12Br2N2O2
986641 C31H26Br4N4NiO5
412.09 298(2) 0.71073 Monoclinic P 2(1)/c
912.85 298(2) 0.71073 Monoclinic P 2(1)/c
10.826(7) 14.822(9) 9.992(6) 117.445(6) 1422.9(15), 4 1.924 5.702 808 0.22 × 0.21 × 0.20 2.52 – 27.58 -13≤ h ≤14; -18≤ k ≤18; -12≤ l ≤12
11.3726(13) 13.1258(16) 23.432(4) 92.8340 (10) 3493.5(8), 4 1.736 5.173 1792 0.40 × 0.38 × 0.17 2.33 – 25.02 -13≤ h ≤10; -15≤ k ≤15; -27≤ l ≤26
4
Reflections collected Independent reflections Completeness to θ = 25.02 (%) Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I >2σ(I)] R indices (all data) Largest difference peak and hole (e Å-3) a
15452 2851 [R(int) = 0.0233] 98.8 3224 / 0 / 192 1.014 R1 = 0.0220, wR2 = 0.0533a R1 = 0.0271, wR2 = 0.0553 0.634 and –0.483
15981 2660 [R(int) = 0.0832] 98.8 6158 / 1 / 413 1.027 R1 = 0.0524, wR2 = 0.1108b R1 = 0.0896, wR2 = 0.1254 1.409 and –1.325
w = 1 / [σ2 (Fo2) + (0.0253P)2 + 1.1933P], b w = 1 / [σ2 (Fo 2) + (0.0811P)2], Where P = (Fo2 + 2Fc2) / 3
Scavenger measurements of superoxide radicals The superoxide radicals were measured in the test system using NBT/VitB2/MET [19]. The methods are the same as reported early [20]. The antioxidant activity was described as the 50% inhibitory concentration (IC50). IC50 values were calculated from regression lines where x was the concentration of HL1 and Ni(II) complex in µM and y was the percent inhibition of HL1 and Ni(II) complex. Results and discussion Synthesis The anticipated quinazoline-type complex [Ni(L1 )2] was not formed, but an unexpected Schiff base-type complex [Ni(L2)2] was obtained, which was formed in the course of complexation of HL1 by Ni(II) acetate tetrahydrate. The ligand HL1 changed to be a new C=N–O ligand HL2 after forming Ni(II) complex. In the process of reaction between Ni(II) ion and HL1, unexpected cleavages of C–N bonds in HL1 took place forming HL2, which coordinated to Ni(II) ion forming a mononuclear Ni(II) complex with a six-membered ring Schiff base-type complex instead of an anticipated quinazoline-type Ni–N4O2 complex [21]. The result suggested that in the metal complex the Schiff base configuration is more stable than that of quinazoline configuration (Scheme 2).
Scheme 2. Complexation of HL1 with Ni(II) acetate. 5
The plausible mechanism for the formation of quinazoline-type ligand On the basis of experimental results, a plausible reaction mechanism for the formation of quinazoline-type ligand (HL1) is proposed in Scheme 3. Firstly, a condensation reaction between (E)-1-(2-aminophenyl) ethanone oxime (1) and 3,5-dibromo-2-hydroxybenzaldehyde (2) takes place to form imine intermediate (3), accompanied by eliminating one water molecule. In the imine (3), as the nitrogen atom is more electronegative than the carbon atom, the nitrogen atom of the C=N bond has more negative charge while the carbon atom has more positive charge. The lone pair electron on the oxime nitrogen attacks the positively charged C=N carbon, resulting in occur cyclization product (4) which has a stable six-membered ring system. In the six-membered ring configuration, the structure of oxime with hydroxyl group is similar to that of enol structure. Because the pKa of the alcoholic hydroxyl group (pKa = 10.66) is lower than that of the nitrogen atom in the secondary amine (pKa = 11.09), the molecule is stabilized as an inner salt (5) through an intramolecular 1,5-hydrogen proton transfer. Finally, we obtain the product 2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline 3-oxide (6), which is responsible for the observed experimental results.
Scheme 3. Proposed mechanism for the formation of quinazoline-type ligand.
X-ray crystal structures of HL1 and [Ni(L2)2]·CH3OH The molecular structures of HL1 and the Ni(II) complex are shown in Figure 1 and Figure 2. Selected bond lengths and angles for the Ni(II) complex are listed in Table 2.
6
Fig.1. Crystal structure and intramolecular hydrogen bonds of HL1 with atoms numbering. Hydrogen bonding is indicated by a dashed bond.
X-ray crystallographic analysis reveals the crystal structure of HL1, in which all bond lengths and angles are in normal ranges. The molecule crystallizes in the monoclinic system, space group P2(1)/c. HL1 adopts a V-shaped configuration, in which the benzene rings are approximately perpendicular, making a dihedral angle of 89.46(3)º, and dihedral angles of six-membered rings (N2–C15 –N1 –C13–C12–C7) with the corresponding benzene rings (C7–C8–C9 –C10–C11–C12) are 9.56(4)º.
Fig.2. Crystal structure of the Ni(II) complex with atoms numbering. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level. All hydrogen atoms are ignored for clarity. Table 2. Selected bond distances (Å) and angles (º) for the Ni(II) complex. Bond
Dist.
Bond
Dist.
Ni(1)–O(2) Ni(1)–O(4) Ni(1)–N(1)
2.018(3) 2.040(4) 2.076(5)
Ni(1)–N(4) Ni(1)–N(2) Ni(1)–N(3)
2.021(5) 2.046(4) 2.117(5)
7
Bond
Angles
Bond
Angles
O(2)–Ni(1)–N(4) N(4)–Ni(1)–O(4) N(4)–Ni(1)–N(2) O(2)–Ni(1)–N(1) O(4)–Ni(1)–N(1) O(2)–Ni(1)–N(3) N(2)–Ni(1)– O(4) O(4)–Ni(1)–N(3) N(1)–Ni(1)–N(3) O(3)–N(1)–Ni(1) C(19)–N(2)–Ni(1) O(1)–N(3)–Ni(1) C(4)–N(4)–Ni(1) C(26)–O(4)–Ni(1)
88.53(16) 89.06(16) 103.27(18) 86.06(17) 91.29(17) 99.51(17) 84.91(16) 166.83(17) 99.84(18) 114.9(3) 115.2(3) 121.3(3) 114.0(3) 118.8(3)
O(2)–Ni(1)–O(4) O(2)–Ni(1)–N(2) O(4)–Ni(1)–N(2) N(4)–Ni(1)–N(1) N(2)–Ni(1)–N(1) N(4)–Ni(1)–N(3) N(1)–Ni(1)–O(4) N(2)–Ni(1)–N(3) C(17)–N(1)–Ni(1) C(24)–N(2)–Ni(1) C(2)–N(3)–Ni(1) C(9)–N(4)–Ni(1) C(11)–O(2)–Ni(1)
88.18(14) 166.19(17) 84.91(16) 174.57(18) 82.16(18) 80.52(18) 91.29(17) 89.63(18) 129.6(4) 124.9(4) 124.8(4) 126.4(4) 122.7(3)
Table 3. Hydrogen bonding distances (Å) and angles (º) for HL1 and the Ni(II) complex. D–H···A HL1 O2–H2··· Br1a N2–H2A··· O1b C4–H4·· ·N2 C14–H14C···O1 C11–H11···Cg(2)c C5–Br2···Cg(3)d Ni(II) complex O3–H3··· O2 C16–H16C···O3 O1–H1··· O5e O5–H5A··· O2f C9–H9·· ·O4g C14–Br2···Cg(7)h C27–Br3···Cg(6)
d(D–H)
d(H···A)
d(D···A)
∠DHA
0.82 0.86 0.93 0.96 0.93 1.90
2.92 2.32 2.46 2.33 2.77 3.824(3)
3.533(3) 3.065(3) 2.816(3) 2.778(3) 3.601(4) 5.160(4)
133 145 103 108 150 125.41(6)
0.82 0.96 0.82 0.82 0.93 1.899(6) 1.909(11)
2.15 2.19 1.86 1.96 2.56 3.589(3) 3.816(3)
2.777(6) 2.645(8) 2.676(6) 2.736(6) 3.334(7) 4.896(6) 4.793(6)
133 108 172 157 141 123.29(19) 109.10(16)
Symmetry code: (a) -x, -y, 2-z; (b) x, 1/2-y, -1/2+z; (c) 1-x, -y, 2-z; (d)1-x, -1/2+y, 3/2-z; (e) -1+x, y, z; (f) 1+x, y, z; (g) -x, -1/2+y, 1/2-z; (h) -x,-1/2+y, 1/2-z. Cg(2) is the centroids for benzene ring C1–C6, Cg(3) is the centroids for benzene ring C7–C12, Cg(6) is the centroids for benzene ring C10–C15, Cg(7) is the centroids for benzene ring C18–C23 respectively.
Molecules of HL1 are connected by intramolecular C−H···O and C−H···N hydrogen bonding, intermolecular N−H···O and O−H···X hydrogen bonding, C−H···π and C−X···π (Ph) interactions (Table 3), which play a role in stabilizing the structure of the crystal. Two pairs of intramolecular C14−H14C···O1 and C4−H4···N2 hydrogen bonds are formed two five-membered rings. One intermolecular N2−H2A···O1 hydrogen bond is between quinazoline 8
nitrogen and phenolic oxygen, anther intermolecular O2−H2···Br1 hydrogen bond is between quinazoline nitrogen and bromine atom. Furthermore, three pairs of C11−H11···Cg(2), C5−Br2···Cg(3) and Cg(2)···Cg(2) (the distance between aromatic ring centroid for C1−C6 is 3.631Å) weak interactions are between aromatic ring. Above-mentioned hydrogen bonds contain intermolecular C−H···π and C–X···π (Ph) play an import role in stabilizing the crystal structure. In conclusion, with the help of hydrogen bonds and other weak interactions, adjacent molecular units are linked together by interactions to give an infinite 2-D chain supramolecular structure along the a-axis, as shown in Figure 3.
Fig.3. View of the 2-D chain motif of the HL1 along the a-axis (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
X-ray crystallographic analysis of complex reveals the formation of a mononuclear structure. Single crystal X-ray analysis reveals that the metal complex has 2:1 ligand to metal stoichiometry, crystallizes in the monoclinic system with P 2(1)/c space group and consists of one Ni(II), two (L2)−, and one non-coordinated methanol molecule. Ni(II) is bonded to oxygen and nitrogen of the two (L2)− molecules in a cis arrangement in which Ni(II) is six-coordinated. As shown in Figure 4, the coordination environment around Ni(II) is octahedral with some distortion, by one imine nitrogen (N1), one Schiff base nitrogen (N4), and two deprotonated phenolic oxygens (O2 and O4) defining the cis-N2O2 basal plane, plus one imine nitrogen (N3) atom and one Schiff base nitrogen (N2) occupying apical positions from two deprotonated (L2)− units [22,23]. The coordination environment around Ni(II) is best regarded as a slightly distorted octahedral geometry with the distance of the apical nitrogen N1 and N4 to the N2O2 basal plane 9
being 2.585(4) Å and 1.431(4) Å. The center Ni is in the N2O2 basal plane. The bond lengths of Ni–O are longer than Ni–N. These structural features, from steric hindrance of the L1− fragments, account for the distortion of the ligand in the Ni(II) complex.
Fig.4. Coordination configuration of Ni(II) complex.
Molecules of complex are connected by intramolecular C−H···O and O−H···O hydrogen bonds, intermolecular hydrogen bonds and C−X···π (Ph) interactions (Table 3), which play a role in stabilizing the structure of the crystal. Two pairs of intramolecular O3−H3···O2 and C16−H16C···O3 hydrogen bonds form two five-membered rings. Three intermolecular O1–H1···O5, O5–H5A···O2 and C9–H9···O4 hydrogen bonds between aromatic carbon, phenolic oxygen and non-coordinated methanol form a seven-membered ring. Two pairs of C14–Br2···Cg(7) and C27–Br3···Cg(6) hydrogen bonds exist between aromatic rings [24]. To sum up, with the help of interactions hydrogen bonds, the crystal packing shows a self-assembling 1-D supramolecular structure (Figure 5).
Fig.5. View of the 1-D chain motif of the Ni(II) complex along the b-axis. Hydrogen bonding is indicated by a dashed bond (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).
IR spectra The main FT-IR absorptions of HL1 and the Ni(II) complex in the 400-4000 cm-1 region 10
are given in Table 4. The free ligand exhibits a characteristic C=N stretching band at 1611 cm-1, while C=N of the Ni(II) complex is observed at 1623 cm-1. The C=N stretch is shifted to higher frequency by ca. 12 cm-1 upon complexation, indicates a decrease in C=N bond order due to the coordination of Ni(II) with oxime nitrogen [25]. The Ar–O stretch is a strong band, occurring at 1265 cm-1 for HL1 and at 1294 cm-1 for the Ni(II) complex. The Ar–O stretching frequency is shifted to higher frequency, indicating that Ni–O bond was formed between Ni(II) and oxygen of the phenol [26]. The IR spectrum of HL1 shows unexpected strong absorption due to ν(N→O) at 1286 cm-1, which disappears in the complex, indicating the C–N bond in the Ni(II) complex has broken and at the same time oxime N participated in coordination. The N–H in HL1 at 3070 cm−1, which disappears in the complex, indicating the nitrogen atom coordinated to Ni(II). The O–H stretch of free HL1 appears at 3253 cm-1, while the infrared spectrum of the Ni(II) complex shows the expected strong absorption due to ν(O–H) at 3205 cm−1, which is the evidence of O–H group from oxime. The far-infrared spectrum of the Ni(II) complex is also obtained in the region 100–500 cm-1 in order to identify frequencies due to Ni–O and Ni–N bonds. The FT-IR spectrum of the complex shows ν(Ni–N) and ν(Ni–O) at 517 and 450 cm-1, respectively. Table 4. Selected FT-IR bands for the ligand and its Ni(II) complex (cm-1). Compound
ν(O–H)
ν(N–H)
ν(C=N)
HL1 Complex
3253 3205
3070 ─
1611 1623
νN→O 1286 ─
νAr–O 1265 1294
ν(Ni–N)
ν(Ni–O)
─ 517
─ 450
UV-vis absorption spectra UV-vis aborption spectra of HL1 and the Ni(II) complex were determined in 5 × 10 -5 mol L-1 CH2Cl2 solution. Absorptions of the Ni(II) complex were obviously different from those of HL1 as shown in Figure 6. Compared with the Ni(II) complex, an important feature of the absorption spectrum of HL1 was that three absorptions were observed at 240, 305 and 363 nm, respectively. The latter absorptions were absent in the spectrum of the Ni(II) complex. The absorption at 252 nm in the Ni(II) complex could be due to the d–d transition of Ni(II). The Ni(II) complex with 3d8 electronic configuration could be assigned to 3A2g → 3T1g transition of Ni(II) in an octahedral environment [27]. These observations were in line with common spectral features of d-block metal complexes. The intense band near 439nm was assigned for 11
ligand-to-metal charge transfer (LMCT) transitions [28]. The splitting of the 3d levels in the crystal field of the coordination compounds causes d–d electronic transitions.
Fig.6. UV-vis absorption spectra: HL1 and the Ni(II) complex in CH2Cl2 (5×10-5 mol L-1).
Fluorescence spectra Fluorescent properties of HL1 and its corresponding Ni(II) complex were investigated at room temperature (Figure 7). The ligand exhibits two intense emissions at 423 and 463 nm upon excitation at 363 nm, which should be assigned to the intraligand π-π* transition [29]. The Ni(II) complex shows an intense broad photoluminescence with maximum emission at ca. 495 nm upon excitation at 436 nm, which is red-shifted to that of HL1. This red shift might be related to the auxochrome group (–Br), which makes the conjugated system larger. The compounds emit strong fluorescence in the green light area and may be potential functional fluorescent material which can emit green light.
Fig.7. Emission spectra of HL1 and the Ni(II) complex in DMF at room temperature (c = 5 × 10-5 mol L-1). 12
Electrochemistry Studies The electrochemical property of Ni(II) complex has been studied in DMF containing 0.05 mol L–1 tetrabutylammonium perchlorate over the potential range from –2 to 0.7 V (vs. Ag/AgCl) using CV techniques, with scanning rate of 100 mV s–1. As it observed in Figure 8, the Ni(II) complex exhibits two reversible redox waves, which are assigned to consecutive Ni(II)/Ni(I) and Ni(III)/Ni(II) redox processes [30]. The first pair of oxidation-reduction peaks correspond to the oxidatio-reduction couples Ni(III)/Ni(II), Epa1 = 0.196 V, Epc1 = –0.895 V, with the average formal potential [E1/2 = (Epa1+Epc1)/2] is –0.347 V. The peak-to-peak separation between the anodic and cathodic (△Ep1) is 1.093 V and the proportion of the peak current (ipc1/ipa1) is 1.523. The second pair of oxidation–reduction peaks correspond to Ni(II)/Ni(I), Epa2 = –0.723 V, Epc2 = –1.561 V, with the average formal potential [E1/2 = (Epa2 + Epc2) /2] is 1.142 V, the peak-to-peak separation between the anodic and cathodic processes (△Ep 2) of 0.836V and the proportion of the peak current (ipc2/ipa2) of 0.771. These features are indicative of a quasi-reversible electrode process [31].
Fig.8. Cyclic voltammogram of Ni(II) complex in DMF solution. Scan rate at 100 mV s–1.
SOD-like Activity The SOD-like activities of HL1 and Ni(II) complex were investigated and shown in Figure 9. The average suppression ratio against O2˙– increases with increasing concentration in the range of the tested concentration. The observed IC50 values of the compounds were compared with earlier reported values for nickel(II) complexes [20,32,33]. The catalytic activity of NiSOD [34], however, is on the same high level as that of Cu–ZnSOD at about 109 (mol L-1)-1s-1 per 13
metal center. The IC50 data of the SOD activity assay along with kinetic catalytic constants of compounds [35,36] are presented in Table 5. The results indicate that the prepared compounds are more efficient antioxidants than Vitamin C, which is standard superoxide dismutase [37,38]. The brown color of the octahedral Ni(II) complex exhibits more active scavenging effects against O2˙– than HL1 and other nickel complexes under the same conditions. The scavenging effect of Ni(II) complex may be ascribed to the chelating function of ligand with metal ion to achieve significant selectivity of radical scavenging activity in biological system [39].
Fig.9. Scavenging effects of the ligand HL1 and the Ni(II) complex on O2˙– Table 5. IC50 values and kinetic constant of compounds IC50 (µmol)
kMcCF ((mol L-1)-1s-1) × 104
852
─
[37]
51.58
─
This work
8.24 8.26 31 34
33.14 33.08 3.06 1.76
This work
Ni(HL)(bipy)(H2O)(NO3 )(ClO4 )(H2O)
38
2.5
Ni(HL)(dien)(ClO4 )(H2O)
Compound Vc 1
HL
2
[Ni(L )2] ·CH3OH [Ni(L)2] ·CH3OH [Ni(L)2] [Ni(L)(HL)](ClO4)(H2O)
95
1.0
1
35
2.71
2
55
1.73
3
60
1.58
[Ni(L )2] ·2H2O [Ni(L )2](ClO4)2 [Ni(L )(bipy)](ClO4)2
Reference
[20] [32] [32] [32] [32] [33] [33] [33]
kMcCF were calculated by K = kNBT × [NBT]/IC50, kNBT (pH 7.8) = 5.94×104 (mol L–1 ) –1s–1 [36].
Conclusions We have successfully reported the synthesis and structural characterization of an 14
unexpected mononuclear Ni(II) complex based on analytical and spectral data. Ni(II) caused an unexpected cleavage of C–N bonds in HL1, giving a new C=N–O ligand HL2, which coordinates to Ni(II) forming Ni–N4O2 complex involving a Schiff base-type instead of an anticipated quinazoline complex. Moreover, the quasi-reversible one-electron Ni(III)/Ni(II) and Ni(II)/Ni(I) redox waves were determined by cyclic voltammetry. In addition, the Ni(II) complex exhibits more active scavenging effects against O2˙– than of HL1 and other nickel complexes under the same conditions. Analytic, spectroscopic, electrochemical, and crystal structure determinations show that a new nickel(II) complex has octahedral geometry. Supplementary material CCDC 981100 and 986641 contain the supplementary crystallographic data of the ligand and its complex for this article. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif., or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK. Telephone: (44) 01223 762910 Facsimile: (44) 01223 336033; or e-mail: [email protected].
Acknowledgements We are thankful for the financial support by the Fundamental Research Funds for the Universities (214086) and the Natural Science Foundation of Gansu Province (No: 1107RJZA165). References [1] H. Furukawa, K.E. Nirdova, M. O′Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science. 341 (2013) 974-986. [2] Q.X.Yang, L.F. Huang, M.D. Zhang, An unprecedented homochiral metal-organic framework based on achiral nanosized pyridine and v-shaped polycarboxylate acid ligand, Cryst. Growth Des. 13 (2013) 440-445. [3] S. Saha, R.K. Kottalanka, P. Bhowmik, S. Jana, K. Harms, T.K. Panda, S. Chattopadhyay, H.P.
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19
GRAPHICAL ABSTRACT An unexpected Schiff base-type Ni(II) complex instead of an anticipated quinazoline-type complex has been synthesized and structural characterized by spectroscopic methods. Crystal structures of the ligand and complex have been determined by single-crystal X-ray diffraction. Moreover, the electrochemical property of the nickle complex was studied by cyclic voltammetry. In addition, SOD-like activities of HL1 and Ni(II) complex were also investigated.
31
HIGHLIGHTS •
An unexpected
Schiff base-type
Ni(II)
complex instead
of an anticipated
quinazoline-type complex has been synthesized firstly. •
A new ligand and complex have been characterized structurally by spectroscopic methods and determined by single-crystal X-ray diffraction.
•
The brown color of the octahedral Ni(II) complex exhibits more active scavenging effects against O2˙– than of HL1 and other nickel complexes under the same conditions.
32
