New zirconium(IV) complexes were synthesized with bidentate ligands and characterized by elemental analysis, molar conductance measurements, molecular weight determinations, IR, electronic, NMR (1H and 13C), fluorescence and molecular modeling studies. All the complexes are 1 : 2 electrolytes in nature and may be formulated as [Zr(L)2Cl2] (where L is Schiff bases of amino acids and substituted isatin). The analytical data showed that the Schiff-base ligand acts as bidentate toward zirconium ion via the azomethine nitrogen and carboxylate oxygen. The conductivity values between 8.5–12.6 Ω−1, mol−1, cm2 in DMF imply the presence of nonelectrolyte species. On the basis of spectral and molecular modeling studies, the resulting complexes are proposed to have octahedral geometries.
The coordination chemistry of Schiff bases has been widely explored, though its use in supramolecular coordination chemistry remains largely unexplored. The Schiff-base moiety is potentially ambidentate and can coordinate through nitrogen with either oxygen or sulfur atoms. The vast literature on structural studies of Schiff-base complexes reveals some interesting features of their coordination behavior [
All chemicals used in the present work, 1
Solvents used were dried and purified by standard methods and moisture was excluded from the glass apparatus using CaCl2 drying tubes. The melting points of the compounds were determined on a capillary melting point apparatus and were not corrected. The purity of the compounds was confirmed by thin layer chromatography using silica gel-G glass plates as the stationary phase and benzene and ethanol (9 : 1) as the mobile phase. Zirconium was determined gravimetrically as its oxide, ZrO2. Nitrogen and sulfur were determined by Kjeldahl’s and Messenger’s methods, respectively. Molecular weight determinations were carried out by the Rast camphor method.
The IR spectra of samples in KBr pellets were recorded on an FTIR spectrophotometer model SP-2, Perkin Elmer in the range of 4000–400 cm−1. The electronic spectra of the ligands and their metal complexes were recorded in dry DMSO, on a thermo-, double-beam spectrophotometer UV 1, in the range of 800–200 nm. The fluorescence studies of Schiff base and its metal complexes were recorded on Shimadzu RF-5301PC spectrophotometer. The molar conductance of the complexes was measured on 10−3 M DMF solutions using Systronics conductivity bride model 305.
The molecular modeling of a representative compound is carried out on a CS Chem 3D ultramolecular modeling and analysis programme, interactive graphics programme that enables rapid structure building and geometry optimization with minimum energy and molecular display.
The complexes were prepared by treating oxozirconium(IV) chloride (1.61 mmol) in methanol with the corresponding Schiff bases (3.22 mmol) in the same solvent. The mixture was refluxed for three hours on a water bath after which the crystals of the complex separate out on cooling; the mixture was concentrated on a steam bath until about one-third of the solution remained. The concentrated solution was cooled after which the crystals were filtered, washed with methanol, and then dried in vacuum at
The reactions of zirconium(IV) dichloride with the monofunctional bidentate Schiff bases were carried out in dry methanol in 1 : 2 stoichiometric ratios and can be represented by the equation in Scheme
Representative equation illustrating the formation of Schiff bases and their zirconium(IV) complexes.
All these compounds are coloured solids, insoluble in common organic solvents and soluble in DMSO, DMF, CHCl3, MeOH, and so forth and insoluble in n-hexane and petroleum ether. The conductivity values for the Zr(L)2Cl2 complexes (in DMF, 10−3 moL, 25°C), ranging in the 8.5–12.6 Ω−1, moL−1, cm2 region, indicate that the nonelectrolytic nature of the compounds suggests that the anions are covalently bonded. The molecular weight determination by the Rast camphor method shows that the products are monomeric in nature.
The IR absorption frequencies for the ligands and their zirconium complexes were recorded in the range 4000–400 cm−1. The assignment of important infrared data for ligand and complexes are listed in experimental section. A strong band (due to the azomethine group) at 1620–1635 cm−1 in the spectra of the free ligands [
The infrared spectra of all the zirconium complexes do not show the strong band in the region 3105–2740 cm−1 due to
The electronic spectra of Schiff bases and its zirconium complexes have been recorded in methanol. In spectrum of the ligand, three bands are observed at 280, 300, and 380 nm. The bands at 280 and 300 nm are due to the
The characteristic resonance peaks for the synthesized compounds have been recorded in DMSO-d6 and data are given in experimental section. The expected resonances are assigned by their peak multiplicity, intensity patterns, and integration. The 1H NMR spectral data of the ligands show single resonance at
Schiff bases derived from glycine, alanine, valine, and methionine display four/three aromatic protons, as expected. In the spectrum of phenylalanine, the integral of the aromatic region corresponds to nine protons; five protons on the phenyl ring are recognizable at 7.4 ppm. Methylene (for glycine, alanine, valine, and methionine) protons on the
The
Though, it is also possible that the shifting of azomethine carbon is due to the change in hybridization of nitrogen attached to carboxylate group, but in the light of IR, UV, and
Metal-ligand coordination may lead to significant changes of the fluorescence properties of the ligand, including increase of the intensity, shift of the emission wavelength, appearance of new emissions, or quenching of the fluorescence. The fluorescence spectra of the Schiff base HL1 and HL3 and their metal complexes were recorded in DMF with excitation wavelength 380 nm at room temperature (298 K). The fluorescence emission spectrum of HL1 with its metal complexes is depicted in Figures
Emission spectra of the ligand (L3H).
Emission spectra of the Zr(L3)2Cl2 complex.
The ligand (L4H) has 50 filled molecular orbitals, and the orbitals of the HOMO and LUMO levels are shown in Figures
Various Bond Lengths of Compound [Zr(L4)2Cl2].
S. No. | Atoms | Actual (Å) | Optimal (Å) |
---|---|---|---|
1 | Zr(4)-N(3) | 2.1381 | |
2 | C(41)-H(66) | 1.1131 | 1.113 |
3 | C(41)-H(65) | 1.1126 | 1.113 |
4 | C(39)-H(64) | 1.1137 | 1.113 |
5 | C(39)-H(63) | 1.1153 | 1.113 |
6 | C(38)-H(62) | 1.1125 | 1.113 |
7 | C(38)-H(61) | 1.1135 | 1.113 |
8 | C(38)-H(60) | 1.1127 | 1.113 |
9 | C(36)-H(59) | 1.1132 | 1.113 |
10 | C(36)-H(58) | 1.1141 | 1.113 |
11 | C(35)-H(57) | 1.1142 | 1.113 |
12 | C(35)-H(56) | 1.1132 | 1.113 |
13 | C(34)-H(55) | 1.0939 | 1.1 |
14 | C(33)-H(54) | 1.1023 | 1.1 |
15 | C(32)-H(53) | 1.1023 | 1.1 |
16 | C(31)-H(52) | 1.1016 | 1.1 |
17 | N(27)-H(51) | 1.0444 | 1.05 |
18 | C(25)-H(50) | 1.1159 | 1.113 |
19 | C(25)-H(49) | 1.1146 | 1.113 |
20 | C(24)-H(48) | 1.1012 | 1.1 |
21 | C(23)-H(47) | 1.1061 | 1.1 |
22 | C(22)-H(46) | 1.1049 | 1.1 |
23 | C(21)-H(45) | 1.1012 | 1.1 |
24 | N(17)-H(44) | 1.0446 | 1.05 |
25 | C(8)-H(43) | 1.1138 | 1.113 |
26 | C(2)-H(42) | 1.1165 | 1.113 |
27 | S(40)-C(41) | 1.8167 | 1.815 |
28 | C(39)-S(40) | 1.8236 | 1.815 |
29 | C(25)-C(39) | 1.5388 | 1.523 |
30 | S(37)-C(38) | 1.8142 | 1.815 |
31 | C(36)-S(37) | 1.8255 | 1.815 |
32 | C(35)-C(36) | 1.5393 | 1.523 |
33 | N(9)-Zr(4) | 2.1345 | |
34 | Zr(4)-O(5) | 2.0916 | |
35 | C(2)-C(35) | 1.5486 | 1.523 |
36 | C(34)-C(29) | 1.4014 | 1.42 |
37 | C(33)-C(34) | 1.3978 | 1.42 |
38 | C(32)-C(33) | 1.3994 | 1.42 |
39 | C(31)-C(32) | 1.3929 | 1.42 |
40 | C(28)-C(31) | 1.3971 | 1.42 |
41 | C(26)-O(30) | 1.2293 | 1.208 |
42 | C(29)-C(15) | 1.4774 | 1.503 |
43 | C(28)-C(29) | 1.4077 | 1.42 |
44 | N(27)-C(28) | 1.3928 | 1.462 |
45 | C(26)-N(27) | 1.3678 | 1.462 |
46 | C(15)-C(26) | 1.5141 | 1.517 |
47 | C(8)-C(25) | 1.5398 | 1.523 |
48 | C(24)-C(19) | 1.4832 | 1.42 |
49 | C(23)-C(24) | 1.5134 | 1.42 |
50 | C(22)-C(23) | 1.5149 | 1.42 |
51 | C(21)-C(22) | 1.5092 | 1.42 |
52 | C(18)-C(21) | 1.4845 | 1.42 |
53 | C(16)-O(20) | 1.3098 | 1.208 |
54 | C(19)-C(14) | 1.5193 | 1.503 |
55 | C(18)-C(19) | 1.374 | 1.42 |
56 | N(17)-C(18) | 1.3476 | 1.462 |
57 | C(16)-N(17) | 1.3903 | 1.462 |
58 | C(14)-C(16) | 1.5503 | 1.517 |
59 | N(3)-C(15) | 1.2988 | 1.26 |
60 | N(9)-C(14) | 2.1882 | 1.26 |
61 | C(1)-O(13) | 1.2142 | 1.208 |
62 | C(7)-O(12) | 1.2117 | 1.208 |
63 | Zr(4)-Cl(11) | 2.4423 | |
64 | Zr(4)-Cl(10) | 2.4453 | |
65 | C(8)-N(9) | 1.4968 | 1.47 |
66 | C(7)-C(8) | 1.5279 | 1.509 |
67 | O(6)-C(7) | 1.3594 | 1.338 |
68 | C(41)-H(67) | 1.1129 | 1.113 |
69 | O(5)-C(1) | 1.3551 | 1.338 |
70 | Zr(4)-O(6) | 2.0914 | |
71 | C(2)-N(3) | 1.5032 | 1.47 |
72 | C(1)-C(2) | 1.5292 | 1.509 |
Various Bond Angles of Compound [Zr(L4)2Cl2].
S. No. | Atoms | Actual (°) | Optimal (°) |
---|---|---|---|
1 | H(67)-C(41)-H(66) | 109.418 | 109 |
2 | H(67)-C(41)-H(65) | 108.9614 | 109 |
3 | H(67)-C(41)-S(40) | 110.3566 | 109.3 |
4 | H(66)-C(41)-H(65) | 108.7912 | 109 |
5 | H(66)-C(41)-S(40) | 109.9408 | 109.3 |
6 | H(65)-C(41)-S(40) | 109.3437 | 109.3 |
7 | C(41)-S(40)-C(39) | 97.5564 | 96.3 |
8 | H(64)-C(39)-H(63) | 107.5686 | 109.4 |
9 | H(64)-C(39)-S(40) | 110.1883 | 112 |
10 | H(64)-C(39)-C(25) | 110.4229 | 109.41 |
11 | H(63)-C(39)-S(40) | 109.8119 | 112 |
12 | H(63)-C(39)-C(25) | 109.1207 | 109.41 |
13 | S(40)-C(39)-C(25) | 109.6938 | 106.5 |
14 | H(62)-C(38)-H(61) | 109.6623 | 109 |
15 | H(62)-C(38)-H(60) | 109.0937 | 109 |
16 | H(62)-C(38)-S(37) | 110.3571 | 109.3 |
17 | H(61)-C(38)-H(60) | 108.6879 | 109 |
18 | H(61)-C(38)-S(37) | 110.0128 | 109.3 |
19 | H(60)-C(38)-S(37) | 108.9946 | 109.3 |
20 | C(38)-S(37)-C(36) | 98.533 | 96.3 |
21 | H(59)-C(36)-H(58) | 107.1198 | 109.4 |
22 | H(59)-C(36)-S(37) | 110.417 | 112 |
23 | H(59)-C(36)-C(35) | 109.5515 | 109.41 |
24 | H(58)-C(36)-S(37) | 110.7645 | 112 |
25 | H(58)-C(36)-C(35) | 110.8347 | 109.41 |
26 | S(37)-C(36)-C(35) | 108.1586 | 106.5 |
27 | H(57)-C(35)-H(56) | 104.9845 | 109.4 |
28 | H(57)-C(35)-C(36) | 108.6578 | 109.41 |
29 | H(57)-C(35)-C(2) | 110.8409 | 109.41 |
30 | H(56)-C(35)-C(36) | 108.9072 | 109.41 |
31 | H(56)-C(35)-C(2) | 109.5018 | 109.41 |
32 | C(36)-C(35)-C(2) | 113.5779 | 109.5 |
33 | H(55)-C(34)-C(29) | 122.3033 | 120 |
34 | H(55)-C(34)-C(33) | 117.5306 | 120 |
35 | C(29)-C(34)-C(33) | 120.1661 | |
36 | H(54)-C(33)-C(34) | 119.6447 | 120 |
37 | H(54)-C(33)-C(32) | 119.4129 | 120 |
38 | C(34)-C(33)-C(32) | 120.9424 | |
39 | H(53)-C(32)-C(33) | 119.7786 | 120 |
40 | H(53)-C(32)-C(31) | 120.1954 | 120 |
41 | C(33)-C(32)-C(31) | 119.9868 | |
42 | H(52)-C(31)-C(32) | 121.1904 | 120 |
43 | H(52)-C(31)-C(28) | 120.678 | 120 |
44 | C(32)-C(31)-C(28) | 118.1313 | |
45 | C(34)-C(29)-C(15) | 134.3781 | 120 |
46 | C(34)-C(29)-C(28) | 117.1629 | 120 |
47 | C(15)-C(29)-C(28) | 108.4549 | 120 |
48 | C(31)-C(28)-C(29) | 123.0239 | 120 |
49 | C(31)-C(28)-N(27) | 126.8775 | 120 |
50 | C(29)-C(28)-N(27) | 110.0936 | 120 |
51 | H(51)-N(27)-C(28) | 126.0741 | 118 |
52 | H(51)-N(27)-C(26) | 125.3088 | 118 |
53 | C(28)-N(27)-C(26) | 108.5425 | |
54 | O(30)-C(26)-N(27) | 120.8581 | 122.6 |
55 | O(30)-C(26)-C(15) | 129.2063 | 123 |
56 | N(27)-C(26)-C(15) | 109.9356 | 122 |
57 | H(50)-C(25)-H(49) | 106.678 | 109.4 |
58 | H(50)-C(25)-C(39) | 108.7135 | 109.41 |
59 | H(50)-C(25)-C(8) | 108.393 | 109.41 |
60 | H(49)-C(25)-C(39) | 111.2794 | 109.41 |
61 | H(49)-C(25)-C(8) | 109.454 | 109.41 |
62 | C(39)-C(25)-C(8) | 112.1296 | 109.5 |
63 | H(48)-C(24)-C(19) | 117.3341 | 120 |
64 | H(48)-C(24)-C(23) | 117.009 | 120 |
65 | C(19)-C(24)-C(23) | 119.9311 | |
66 | H(47)-C(23)-C(24) | 113.9434 | 120 |
67 | H(47)-C(23)-C(22) | 114.0848 | 120 |
68 | C(24)-C(23)-C(22) | 118.3693 | |
69 | H(46)-C(22)-C(23) | 115.8207 | 120 |
70 | H(46)-C(22)-C(21) | 115.0649 | 120 |
71 | C(23)-C(22)-C(21) | 118.1479 | |
72 | H(45)-C(21)-C(22) | 119.4891 | 120 |
73 | H(45)-C(21)-C(18) | 118.9569 | 120 |
74 | C(22)-C(21)-C(18) | 118.0658 | |
75 | C(24)-C(19)-C(14) | 124.1732 | 120 |
76 | C(24)-C(19)-C(18) | 120.434 | 120 |
77 | C(14)-C(19)-C(18) | 114.4563 | 120 |
78 | C(21)-C(18)-C(19) | 123.9331 | 120 |
79 | C(21)-C(18)-N(17) | 126.1831 | 120 |
80 | C(19)-C(18)-N(17) | 109.7946 | 120 |
81 | H(44)-N(17)-C(18) | 126.029 | 118 |
82 | H(44)-N(17)-C(16) | 126.0518 | 118 |
83 | C(18)-N(17)-C(16) | 107.8705 | |
84 | O(20)-C(16)-N(17) | 122.4042 | 122.6 |
85 | O(20)-C(16)-C(14) | 123.6108 | 123 |
86 | N(17)-C(16)-C(14) | 113.7879 | 122 |
87 | C(29)-C(15)-C(26) | 101.2183 | 117.6 |
88 | C(29)-C(15)-N(3) | 129.5068 | 120 |
89 | C(26)-C(15)-N(3) | 129.128 | 120 |
90 | C(19)-C(14)-C(16) | 93.7144 | 117.6 |
91 | C(19)-C(14)-N(9) | 101.1826 | 120 |
92 | C(16)-C(14)-N(9) | 103.5499 | 120 |
93 | Zr(4)-N(9)-C(14) | 104.5285 | |
94 | Zr(4)-N(9)-C(8) | 111.1129 | |
95 | C(14)-N(9)-C(8) | 111.1476 | |
96 | H(43)-C(8)-C(25) | 108.1727 | 109.39 |
97 | H(43)-C(8)-N(9) | 109.7641 | 107.5 |
98 | H(43)-C(8)-C(7) | 106.4941 | 107.9 |
99 | C(25)-C(8)-N(9) | 111.9987 | |
100 | C(25)-C(8)-C(7) | 113.8847 | 109.9 |
101 | N(9)-C(8)-C(7) | 106.3523 | |
102 | O(12)-C(7)-C(8) | 125.0648 | 122.5 |
103 | O(12)-C(7)-O(6) | 124.6366 | 122 |
104 | C(8)-C(7)-O(6) | 110.2827 | 107.1 |
105 | Zr(4)-O(6)-C(7) | 111.3282 | |
106 | Zr(4)-O(5)-C(1) | 114.0377 | |
107 | N(3)-Zr(4)-O(5) | 80.1135 | |
108 | N(3)-Zr(4)-O(6) | 128.6218 | |
109 | N(3)-Zr(4)-N(9) | 149.0865 | |
110 | N(3)-Zr(4)-Cl(11) | 88.0663 | |
111 | N(3)-Zr(4)-Cl(10) | 84.2757 | |
112 | O(5)-Zr(4)-O(6) | 148.2304 | |
113 | O(5)-Zr(4)-N(9) | 78.2701 | |
114 | O(5)-Zr(4)-Cl(11) | 85.1672 | |
115 | O(5)-Zr(4)-Cl(10) | 116.7647 | |
116 | O(6)-Zr(4)-N(9) | 78.9572 | |
117 | O(6)-Zr(4)-Cl(11) | 82.9908 | |
118 | O(6)-Zr(4)-Cl(10) | 83.2655 | |
119 | N(9)-Zr(4)-Cl(11) | 111.6987 | |
120 | N(9)-Zr(4)-Cl(10) | 86.0092 | |
121 | Cl(11)-Zr(4)-Cl(10) | 154.9219 | |
122 | Zr(4)-N(3)-C(15) | 121.0647 | |
123 | Zr(4)-N(3)-C(2) | 107.937 | |
124 | C(15)-N(3)-C(2) | 120.234 | |
125 | H(42)-C(2)-C(35) | 110.432 | 109.39 |
126 | H(42)-C(2)-N(3) | 106.5807 | 107.5 |
127 | H(42)-C(2)-C(1) | 102.9891 | 107.9 |
128 | C(35)-C(2)-N(3) | 118.1743 | |
129 | C(35)-C(2)-C(1) | 109.8863 | 109.9 |
130 | N(3)-C(2)-C(1) | 107.6576 | |
131 | O(13)-C(1)-O(5) | 124.3628 | 122 |
132 | O(13)-C(1)-C(2) | 123.8243 | 122.5 |
133 | O(5)-C(1)-C(2) | 111.734 | 107.1 |
(a) HOMO orbitals of the MM2 geometry-optimized structure of the ligand (L4H). (b) LUMO orbitals of the MM2 geometry-optimized structure of the ligand (L4H).
3D molecular structure of Zr(L4)2Cl2.
In all, 205 measurements of the bond lengths (72 in numbers) plus the bond angles (133 in numbers) are listed. Except for a few cases, optimum values of both the bond lengths and the bond angles are given in the tables, with the actual values. The actual bond lengths/bond angles given in Tables
New Schiff bases and their zirconium complexes have been successfully synthesized. Elemental analysis data obtained are in good agreement with the predicted formula. Distorted octahedral geometries have been proposed for Zr(IV) complexes with the help of various physicochemical studies. The infrared spectra of these complexes show the presence of monofunctional and bidentate ligands. The Schiff base exhibits a strong fluorescence emission; in contrast with this partial fluorescence quenching phenomena is observed in its metal complexes. The proposed structures of metal complexes are presented in Figure
The authors are thankful to the Dean, Faculty of Engineering & Technology, Mody Institute of Technology and Science, Deemed University, Lakshmangarh, Sikar, for providing necessary facilities and financial support. They are also thankful to the Head, SAIF, Panjab University, Chandigarh, for providing elemental analysis and NMR facilities. The authors are also grateful to Dr. Sangeeta Jhajharia for linguistic corrections.