SYNTHESIS, CHARAaERIZATION AND CATALYTIC ACTIVITY OF MANGANESE AND ZINC COMPLEXES

Abstract

In general, the poly(pyrazolyl)borates are tripodal ligands and have been used for providing steric shielding to the metal center in complexes. These ligands can be tetradentate, tridentate, bidentate or monodentate depending on the number of donor substituents on boron atom and the steric congestion around the metal. Nitrogen heterocycle other than pyrazole can be used such as pyrrole, imidazole and indole. In addition, tripodal ligand can have central atoms other than boron such as carbon, phosphorus or gallium. The ligands can also bind metals through sulphur, phosphorus or other donor atoms. The pyrazole nucleus both thermally and hydrolytically is very stable and occupies a position similar to that of pyridine or ammonia in spectrochemical series. As a ligand, it coordinates to metals and metalloids through the 2-N but after deprotonation, the formed pyrazolate anions can coordinate through both nitrogen atoms as an exobidentate ligand of C2V symmetry. The nucleophilicity of the nitrogen and their steric accessibility may be varied through appropriate ring substitution. Due to these attractive features, the coordination chemistry of pyrazole and its derivatives has attracted much attention. The pyrazolylborates / scorpionate chemistry signify a very large and growing area. These ligands are extremely versatile because the number of donor atoms in each ligand can be varied from two to three while going from the bidentate ligands to the potentially tridentate ligands. Among the different types of pyrazolylborates, hydrotris(pyrazolyl)borate (TpR) ligand has been widely used as supporting ligand for various inorganic and organometallic compounds. For the sake of 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 to date survey of literature related to the various pyrazole and their borate salts. The different types of metal complexes related to the present research have been posed in the context ofthe cited work. The second chapter of the thesis deals with the synthesis of some mononuclear and binuclear manganese complexes having TpPhMe (hydrotris(3-phenyl-5-methylpyrazolyl) borate), TplPr2 (hydrotris(3,5-diisopropyl-pyrazolyl)borate) and 2,4,6-tri(2- pyridyl)-l,3,5-triazine as supporting ligand. IR and X-ray structure suggested that the complexes [Tpph'MeMn(u-N3)(u-acac)MnTpph'Me] and [TplPr2Mn(u-N3)(upic) Mn(PzlPr2)Tp,Pr2] are binuclear with azido and bidentate ligands (acac/pic) as bridging whereas the complexes [Tpph'Me Mn(acac)] and [TpiPr2Mn(pic)] are mononuclear with 5- coordinated metal center. In [TpiPr2Mn(u-N3)(n-pic)Mn(PziPr2)TpiPr2] complex, the picolinate is coordinated as tridentate in n3-fashion but in complex [TpPh,Me Mn(u-N3)(uacac) Mn Tp ' e], the acac behaves as bidentate whereas azide is coordinated in bridging bidentate u-l,3-manner in both [TpPh'MeMn(u-N3)(u-acac)MnTpph'Me] and [TpiPr2Mn(u- N3)(^-pic)Mn(PziPr2)TpiPr2]. The [TpiPr2Mn(^-N3)(u-pic)Mn(PziPr2)TpiPr2] complex was tested for the catalytic activity towards the disproportionation of hydrogen peroxide and was suggested as suitable structural as well as functional model for the pseudocatalase enzyme. [Mn2(tptz)2Cl4].CH3CN, [Mn(tptz)(ac)(N3)(H20)].H20 (where tptz = 2,4,6- tri(2-pyridyl)-l,3,5-triazine, ac = acetate anion). The complex [Mn2(tptz)2Cl4].CH3CN is binuclear with a pair of chlorine atoms bridging the manganese ions at the Mn-Mn of 11 (3.777(27)) A. [Mn(tptz)(ac)(N3)(H20)].H20 has one-dimensional layered structure where three independent molecules are linked by uncoordinated water molecule present in lattice. The magnetic susceptibility measurement in the temperature range 5-300 K for both binuclear [Mn2(tptz)2Cl4].CH3CN complex show the presence of magnetic interaction between the local high-spin manganese (II) ions which is antiferromagnetic in nature with J = - 0.17 cm"1 . The synthesis, structural and hydrolytic activity of mono-, bi- and tri-nuclear zinc (II) hydroxo complexes having different pyrazolylborate and thioimidazolylborate ligands are presented in chapter three. The zinc hydroxo complexes -[TpphMeZn-OH] (Tpph,Me = hydrotris(3-phenyl-5-methylpyrazolyl)borate), [TpCumMeZn-OH] (TpCum-Me = hydrotris(3-cumyl-5-methyl pyrazolyl)borate), [Tti2'6xylylZn(u-OH)ZnTti2'6-xylyl]C104 (Tti2,6-xyiyi = nydrotris(2,6-xylyl-2-thioimidazol-l-yl)borate), [Tpph,MeZn(^OH)(^ PzphMe)Zn(^OH)(u-Pzph'Me) TpphMeH] and [TpCum'MeZn(u-OH)(u-PzCumMe)Zn(u-OH)(u- PzCumMe)TpCum,MeH] have been synthesized as nucleophile for ester hydrolysis. The kinetic study were performed on hydrolytic cleavage of carboxylate esters and found that [Tti2,6-xylylZn(^_OH)ZnTti2,6-xylyl]cl04 and [JpC^Me^^ effect stoichiometric hydrolysis of different carboxylate esters. The resulted suggested that the hydrolysis is faster with [TpCum'MeZn-OH] in comparison to [Tti2'6-xylylZn(u-OH)ZnTti2'6"xylyl]C104 and no hydrolysis was observed with [TpphMeZn(^OH)(u-PzPhMe)Zn(^-OH)(^Pzph-Me) Tpph'MeH] and [TpCumMeZn(^OH)(^-PzCumMe)Zn(^OH)(n-PzCum'Me)TpCum'MeH]. Among various esters taken, the increasing rate of hydrolysis was observed from 4- nitrophenylacetate to 4-nitrophenyllaurate. The kobs is also greater for 4- nitrophenyllaurate and lowest for 4-nitrophenyltrifluoroacetate with both [TpCum'MeZniii OH] and [Tti2'6-xylylZn(^-OH)ZnTti2'6xylyl]C104 because the trifluoroacetate is behaving as electron withdrawing group whereas the acetate, propionate, caprylate and laurate are electron donating group. The chapter four of the thesis deals with effect of steric hindrance on the structure of zinc complexes as well asthe size ofthe formed cavity. The reaction of hydrated zinc (II) nitrate with four or more equivalents of different pyrazole (pyrazole = PzH, 3,5- dimethylpyrazole = PzMe2H, 3-phenyl-5-methylpyrazole = PzphMeH, 3,5- diisopropylpyrazole =PzlPr2H and 3-tert-butyl-5-isopropylpyrazole =PztBlUPrH) yielded different type of compounds. Thenitrate complex contains C2-symmetric four-coordinate zinc (II) centers with a slightly flattened tetrahedral geometry. The crystals of [Zn(PziPr2H)4][N03]2 and [Zn(PztBu-iPrH)4][N03]2 are composed ofsupramolecules that are formed from N-H--0 hydrogen bonding between zinc bound pyrazole rings and nitrate ions. Both [Zn(PztBu'iPrH)4]2+ and [Zn(PzlPr2H)4]2+ moieties are linked into 30- sheet by hydrogen bonding to the nitrate anions lying in the lattice. In both these, the N03" ions are encapsulated in near-spherical cavities of approximate dimensions 5.2 + 2 Awhich are formed by interlocking of different number of supramolecular dications. This nitrate anion was exchanged by other anions like C104", OAc", CI", S042" etc. Complex [Zn(PzPhMeH)3(N03)](N03) also crystallizes as discrete ions with N30 coordination. The complex [(PzH)2Zn(u-Pz)2Zn(PzH)2](N03)2 and [(PzMe2H)(fx- PzMe2)(N03)Zn)]2 are dinuclear where each centre is four coordinate and pyrazole is behaving both as neutral as well as pyrazolato ion. The di positive charge on metal centers is balanced by both pyrazolato as well as nitrate anions present in lattice. Both these complexes seem to be new example of zinc complexes where the pyrazole is IV showing dual behavior although with other metal ions, this type of behavior is known in literature. In [(PzH)2Zn(|a-Pz)2Zn(PzH)2](N03)2, both complex as well as nitrate ion act as acceptor for each other whereas in [(PzMe2H)(u-PzMe2)(N03)Zn)]2, only nitrate ion act as acceptor for complex as well as acetonitrile. The material and reagents, synthetic procedures, experimental details and different type of spectroscopic measurements are described in chapter five of the thesis. Methods for the preparation of different type of ligands and their complexes with Mn(II) and Zn(II) have been included.