STUDIES ON SOME METAL COMPLEXES HAVING PYRAZOLYLBORATES AND HALOURACILS AS LIGANDS

Abstract

Poly(pyrazolyl)borate ligands, which include the tetrakis(pyrazolyl)borates, trischelating hydrotris(pyrazolyl)borates and the bis-chelating dihydrobis(pyrazolyl)- borates, are attractive ancillary species for the stabilization of lanthanide complexes. Since their first synthesis by S. Trofimenko, these ligands have found broad applications in coordination chemistry with majority of metals in periodic table. The poly(pyrazolyl)borates are often viewed as cyclopentadienyl analogues and are increasingly being used as sterically demanding ligands for the f-block elements. The most useful features of these ligands are their versatility in creating different steric environments around a metal by utilizing a modest degree of substitution. The addition of even small groups in the 3-positions of the pyrazolyl rings substantially changes the steric environment of the resulting complex, since these groups lie on three sides and in close proximity to the coordinating metal. Substituents in the 3- and 5-position of the pyrazolyl rings can also be used to control the stereochemical and electronic properties of the ligands, thus permitting the design of very individual or chiral environments for specific metals or functions. Thus the steric size of the TpR,R' (Tp = hydrotris(pyrazolyl)borate) ligands can be changed in a systematic and predictable way by introducing the appropriate substituents at the 3- and 5- position of the pyrazolyl rings which in turn allows the creation of unique coordination environments and the isolation of unusual species. Among the above poly(pyrazolyl)borates, the hydrotris(pyrazolyl)borate ligands ( Tp ' or HB(pz)3) have been extensively used in inorganic, organometallic and bioinorganic chemistry. These ligands are sterically bulky and can occupy three facial coordination sites ofa metal ion on complexation. Their uninegative and 6 electrons donative coordination mode bears comparison with that of the well known 7i-cyclopentadienyl system. Moreover, in view of the easy access to substituted pyrazoles, various substituents have been introduced on the pyrazolyl rings ofthis tripodal ligand system in the last two decades, with the hope offinetuning the electronic and steric properties ofreaction fields built up on the central metal ion in the complexes. On the other hand, it has been reported that lanthanide (III) ions have comparatively large ionic radii, show high and variable coordination numbers on complex formation. Accordingly, the poly(pyrazolyl)borate ligand system seems to be useful and promising in developing functionally designed and catalytically active coordination compounds of lanthanoids. The hydrotris(pyrazolyl)borate ligands are significantly larger than the cyclopentadienyl since their nonpolarity permits substituents to extend beyond the face capped by the ligand itself. Since steric saturation of the lanthanide coordination sphere is of critical importance in determining both structure and reactivity, TpR,R ligands therefore appear well suited to satisfying the coordination ii requirements of the trivalent lanthanide metal center. The tris(pyrazolyl)borate ligand is the focus of much research as a supporting ligand to control steric saturation and reactivity at a variety of metal centers. It displays many advantages as a supporting ligand in lanthanide complexes over analogous cyclopentadienyl ligands due to the relative ease oftuning the blocking substituent groups on the 3- position ofthe pyrazole rings. Alteration ofthese substituents has been shown to effect large changes in the reactivities of metal complexes, because of the alteration of steric saturation and electronic character at the metal center. In the present work, an attempt has been made for the synthesis and characterization of some zinc and lanthanide complexes by using hindered hydrotris(pyrazolyl)borate and plane hydrotris(pyrazolyl)borate as a tridentate nitrogen donar ligands. A potentiometric study has also been performed to investigate the stability ofthe complexes ofEu(III), Gd(III) and Tb(III) by using benzoic acid, p-nitrobenzoic acid, p-fluorobenzoic acid, p-chlorobenzoic acid, pbromobenzoic acid as primary ligands and uracil, 5-fluorouracil, 5-chlorouracil, 5- bromouracil as secondary ligands. For the sake of the convenience, the work embodied in the thesis is presented in the following chapter: The first chapter of the thesis is the general introduction and presents an up-todate survey of literature related to the various pyrazolylborate ligands. The different types ofmetal complexes related to the present research have been posed 111 in the context of the cited work. The synthesis of mononuclear zinc hydroxo, Zn(OH)(HB(3-But-5-Pripz)3) by using sterically hindered pyrazolylborate ligand i.e. hydrotris(3-tert-butyl-5- isopropyl-l-pyrazolyl)borate ligand is described in chapter 2. The structure of Zn(OH)(HB(3-But-5-Pr,pz)3) is very similar to the active site of the enzyme carbonic anhydrase. The complex Zn(OH)(HB(3-But-5-Pripz)3) was found to stabilize zinc complexes of uracil, its halo derivatives, adenine and thymine where uracil, its halo derivatives and thymine are bound as monodentate ligand via its deprotonated Nl and adenine via N9. The halogenated uracil complexes were injected against Dalton's Lymphoma tumor system in mice and was found that Zn(HB(3-But-5-Pr1pz)3)(5-fluorouracilate) exhibits significant antitumor activity. The complex Zn(OH)(HB(3-But-5-Pripz)3) is very reactive species and was used as catalyst in ester hydrolysis. Zn(OH)(HB(3-But-5-Pripz)3) was found to promote the hydrolysis of various esters and the maximum rate of hydrolysis was observed with p-nitrophenylacetate. The synthesis and molecular structure and emission properties of benzoatobridged lanthanide complexes with hydrotris(pyrazolyl)borate as ligands are presented in chapter 3. Reaction of EuCl3, GdCl3 and TbCl3 with one equivalent of the potassium salt of the tridentate ligand hydrotris(pyrazolyl)borate [K{HB(pz)3}] and two equivalents of sodium p-X-benzoate (X =-H, -CI, -Br, and -N02) yielded the complexes of composition [{(HB(pz)3)Ln(u-p-X-OBz)2}2] (Ln IV = Eu, Gd, Tb) whereas the reaction of these lanthanum chloride with 2 equivalents of [K{HB(pz)3}] and one equivalent of sodium azide, gave the tetranuclear complex [{(HB(pz)3)2Ln(u-N3)}4], The crystal structures of the benzoate and azido complexes have been determined. The luminescence properties of the unsubstituted and para-substituted Gd- and Tb-benzoate complexes have been studied in solution at room temperature. The solution studies of some binary (1:1) and ternary (1:1:1) complexes of Eu(III), Gd(III) and Tb(III) with benzoic acid, its derivatives, uracil and its halo derivatives in dioxane-water (30:70 V/V) medium (ionic strength I = 0.1 M NaN03) using Bjerrum-Calvin's pH-titration technique as adopted by Irving and Rossotti at 25 °C are presented in chapter 4. The experimental pH titration data were analysed with the help of the BEST computer programme in order to evaluate the formation constants of various intermediate species formed. The stability constant of the binary (1:1) complexes of Eu(III), Gd(III) and Tb(III) with benzoic acid, p-nitrobenzoic acid, p-fluorobenzoic acid, p-chlorobenzoic acid, pbromobenzoic acid, uracil, 5-fluorouracil, 5-chlorouracil and 5-bromouracil were determined and decreases in the order of Tb(III) > Eu(III) > Gd(III). The formation of 1:1:1 mixed ligand complexes by Eu(III), Gd(III) and Tb(III) of benzoic acid, p-nitrobenzoic acid, p-fluorobenzoic acid, p-chlorobenzoic acid, pbromobenzoic acid as primary ligands and uracil, 5-fluorouracil, 5-chlorouracil, 5- bromouracil as secondary ligands have also been studied. The ternary metal complexes of Eu(lll), Gd(lll) and Tb(lll) are more stable than binary complexes. The materials, reagents, theoretical treatment, synthetic procedures, experimental details and different types of spectroscopic measurements are described in chapter five of the thesis. Methods for the preparation of ligands and their complexes have also been included.