SYNTHETIC, STRUCTURAL AND CATALYTIC STUDIES ON Co AND Cu COMPLEXES WITH PYRAZOLYLBORATES

Abstract

A family of compounds that can act as bidentate as well as tridentate ligands are poly(pyrazolyl)borates. Among the different types of pyrazolylborate, hydrotris(pyrazolyl)borate, TpR, ligands have been widely used as supporting ligand for various inorganic and organometallic compounds for more than three decades. It's unique structural (i.e., facially capping tridentate ligand) and electronic characteristics (6e" donating mono anionic ligand) are similar to those of the family of cyclopentadienyl (CpR) ligands, which are the most commonly adopted ligands in organometallic chemistry. One of advantages of the tris(pyrazolyl)borate ligands is the ease of controlling the properties of the resulting metal complexes (coordination environment and reactivity of metal centers, solubility in organic solvents, facility of crystallization, etc.) by introduction of various substituents into the pyrazole rings. It is notable that highly sterically demanding hydrotris(pyrazolyl)borate ligands containing bulky substituents at the 3-position of the pyrazole rings can stabilize a coordinatively unsaturated metal center and provide a vacant site for substrate binding. In addition, a hydrophobic shading pocket formed by the bulky substituents surrounding the metal center may kinetically stabilize unstable functional groups such as 0-0 and metal-carbon bond moieties. From a bioinorganic point of view, the coordination environment provided by tris(pyrazolyl)borate ligand is fascinating, since the geometry and the ligand donor set reasonably mimic the coordination of three histidyl nitrogen atoms in a pyramidal array often found at the metal sites in proteins. Previously, with HB(pz)3" or HB(3,5-Me2pz)3' as a ligand, model complexes for blue copper proteins, hemerythrin and hemocyanin have been successfully synthesized. However, their marked peculiarity in forming bis-chelated complexes ML2 causes the utility of these ligands to be very limited. As stated above, poly(pyrazolyl)borate ligands have been widely used in organometallic and bioinorganic chemistry, their use in stereochemistry chemistry and asymmetric catalysis is very limited. The great number of existing pyrazoles with a wide range of electronic and steric properties permit the construction of many unsymmetrical bis(pyrazolyl)borates, chiral tripod bis(pyrazolyl)borates and chiral tris(pyrazolyl)borates with different electronic and steric properties. The possibility of tailoring the pyrazolylborate, in addition to the great stability of its complexes, led us to attempt the synthesis of optically active poly(pyrazolyl)borates. In the present work, an attempt has been made for the synthesis and characterization ofsome optically active pyrazolylborate as possible ligands for enantioselective cyclopropanation reactions. The synthesis and characterization of cobalt and copper complexes as model for the various cobaltand copper-containing proteins by using hindered hydrotris(pyrazolyl)borate as a tridentate nitrogen donar ligand have also been attempted. For the sake of the convenience, the work embodied in the thesis is presented in the following chapter: n The first chapter of the thesis is the general introduction and presents an up-todate survey of literature related to the various substituted pyrazoles, optically active pyrazoles and their borate salts. The different types of metal complexes related to the present research have been posed in the context of the cited work. The second chapter of the thesis deals with the synthesis of optically active pyrazolylborate ligands and their uses in various enantioselective cyclopropanation reactions. Optically active 4,5,6,7-tetrahydro-7,8,8-trimethyl- 2H-4,7-methanoindazole (camphpz) was prepared by the reaction of (+)-3- hydroxymethylenecamphor and hydrazine sulphate whereas (-)-3(5)-methyl-lphenylethylaminomethylpyrazole was prepared by the reduction of (-)-N-methyl- N-(l-phenylethyI)-3(5)pyrazolecarboxylic-amide with lithium aluminium hydride. Optically active hydrotris[NaHB(camphpz)3] and tetrakis[NaB(camphpz)4] salts were prepared in a manner analogous to that for KHB(pz)3 and KB(pz)4. NaBH4 was heated in the presence of 3/4 equivalents camphpz at 220-240 °C until 3/4 equivalents amount of H2 gas was evolved. Sodium salt of hydrotris((-)-3(5)- methyl-l-phenylethylaminomethylpyrazolyl)borate, [NaHB(Pl)3] was prepared by the reaction of (-)-3(5)-methyl-l-phenylethylaminomethylpyrazole with NaBH4. These ligands were characterized by elemental analysis, IR and 'H-NMR techniques. The hydrotris- and tetrakis(pyrazolyl)borate ligands were used in various metal catalyzed enantioselective cyclopropanation reactions of styrene with several diazoacetates. The highest enantiomeric excess obtained was 62% for cis isomer and 42% for trans isomer. The chemical yield was upto 91%. With the iii use of menthyl diazoacetate, the drastic changes were observed from cis to trans and enantiomeric excess decreases. The synthesis and molecular structural studies of some mononuclear and binuclear cobalt complexes with hindered hydrotris(pyrazolyl)borate as ligands are presented in chapter three. The reaction of hydrotris(3,5-diisopropyl-lpyrazolyl) borate TplPr2 with Co(N03)2.6H20 resulted the formation of complex [Tp Co(N03)] which has vacant coordination sites on cobalt for important reactivity studies. The complex [Tp'Pr2Co(N03)] has been used for the preparation ofcomplex [TplPr2Co]2(OH)2 which upon reaction with one equivalent ofbenzoic acid in toluene gave complex [TplPr2Co]2(u-OH)(u-OBz). The complex [Tp1 r Co]2(u-OH)(|i-OBz) has both hydroxo and carboxylato bridging groups which is very similar to the bridging groups present in iron- and manganesecontaining ribonucleotide reductase enzymes. The reaction of [TplPr2Co]2(OH)2 with excess amount of benzoic acid in toluene yielded the complex [Tp1 r Co(OBz)], which has mononuclear six-coordinated structure where the benzoate group is coordinated in bidentate fashion. The same compound has also been prepared by reacting [TplPr2Co(N03)] with sodium benzoate i.e., [Tp1 r Co(OBz)]. Other mononuclear benzoate complexes such as [TplPr2Co(FOBz)] is prepared by the reaction of [TplPr2Co(N03)] with fluoro substituted sodium benzoate. The single crystal X-ray measurement suggested the isostructural nature of complex [TpiPr2Co(F-OBz)] with [TpiPr2Co(OBz)]. In other

experiments, the reaction of [TpiPr2Co(N03)] with one equivalent of KSCN/NaN3 were performed in order to know the binding nature of these biologically important molecules with metal ions. In the chapter four of the thesis, the synthesis, molecular structure and catalytic properties of mononuclear cobalt- and copper-hydroxo complexes with hydrotris(3-tert-butyl-5-isopropyl-l-pyrazolyl)borate are described. By using the sterically more hindered hydrotris(pyrazolyl)borate ligand (TptBu'lPr), mononuclear [TptBu,iPrCo(N03)] and [TptBu'iPrCu(Cl)] complexes have been prepared. The reaction of [TptBu'iPrCo(N03)] and [TptBu'iPrCu(Cl)] with IN NaOH resulted the formation of mononuclear [TptBu'iPrCo(OH)] and [TptBu'iPrCu(OH)] complexes. These complexes were characterized by different spectroscopic techniques including X-ray. The single crystal X-ray study of [TptBu'iPrCo(OH)] established that the complex [TptBu'iPrCo(OH)] is mononuclear with four-coordinated metal center (N30). The complexes [TptBu'iPrCo(OH)] and [TptBu'iPrCu(OH)] were found to react with carbon dioxide, even with atmospheric carbon dioxide, afford ucarbonato dinuclear complexes of cobalt [TptBu'iPrCo]2(C03) and copper [TptBu'iPrCu]2(C03). The carbonate group in [TptBu'iPrCo]2(C03) is coordinated to one cobalt ion bidentately and to other cobalt ion monodentately. [TpiPr2Co]2(OH)2, [TptBuiPrCo(OH)] and [TptBu'iPrCu(OH)] were examined for their catalytic activity in the hydrolysis of 4-nitrophenyl acetate, mono(4- nitrophenyl)phosphate, bis(4-nitrophenyl)phosphate and tris(4- nitrophenyl)phosphate and it was found that these complexes can catalyze the hydrolysis of above esters efficiently with different pseudo-first order rate constants. The materials and reagents, synthetic procedures, experimental details and different types of spectroscopic measurements are described in chapter five of the thesis. Methods for the preparation of different types of ligands and their complexes with Co(II) and Cu(II) have been included.