SYNTHESIS AND STRUCTURAL STUDIES OF Mn AND Ni COMPLEXES WITH HINDERED TRIS(PYRAZOLYL)BORATES

Abstract

The pyrazole nucleus is thermally and hydrolytically very stable. Although, it has generally been assumed that PzH coordinate exclusively through the unsubstituted N(2)( the "pyridine" N), in some instances data have been interpreted to imply coordination through the substituted N(1)(the "pyrole" N). When deprotonated, pyrazole becomes the pyrazolide ion, which can coordinate through both nitrogen atoms as an exobidentate ligand of C2v symmetry. The nucleophilicity of the nitrogens and their steric accessibility may be varied through appropriate ring substitution. Several pyrazoles with substitution at both 3- and 5-positions are reported in the literatures. Some of these substituted pyrazoles have been used for the synthesis of their borate salts i.e. dihydrobis-, hydrotris- and tetrakis(pyrazolyl)borate. Among these different types of pyrazolylborates, hydrotris(pyrazolyl)borate ligands have been used widely as supporting ligands for various inorganic and organometallic compounds for three decades. Its unique structural (i.e. facially capping tridentate ligand) and electronic characteristics (6e donating monomelic ligand) are similar to those of the cyclopentadienyl (CpR) ligands, which are the most commonly adopted ligands in organometallics chemistry. One of the advantages ofthe tris(pyrazolyl)borate ligands is the ease of controlling the properties of the (ii) 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 pyrazolyl rings. As facially capping tridentate nitrogen donors, tris(pyrazolyl)borate ligands have also been used by several workers in bioinorganic chemistry to mimic artificially an environment created by a set of three imidazole rings of histidine residue which is frequently found in the metal coordination site of various metalloproteins. One of the problems encounters in the use of tris(pyrazolyl)borates is their propensity to form bis-chelated octahedral ML2 complexes with first row transition metals. In order to prevent or retard the formation of such complexes, tris(pyrazolyl)borate ligands with the substitution at 3- and 5-position of the pyrazolyl ring have been synthesized for present study. In the present work an attempt has been made for the synthesis and characterization of manganese and nickel complexes as model for the various Mn and Ni-containing enzymes by using hindered hydrotris(pyrazolyl)borate as tridentate nitrogen donor ligand. For the sake of convenience the work embodied in the thesis is presented in the following chapter. The first chapter of thesis is general introduction and presents an up- to-date survey of literature related to the pyrazoles, substituted pyrazoles and (iii) their pyrazolylborate salt. The different type of metal complexes related to present research has been posed in the context of the cited work. The second chapter of the thesis deals with the synthesis and molecular structure of mononuclear and binuclear manganese complexes using hydrotris(3,5-diisopropylpyrazolyl)borate and benzoic acid. Single crystal X-ray measurements show the structure of synthesised compounds and also the different binding mode of benzoate groups. The reaction of hydrotris(3,5-diisopropyl-l-pyrazzolyl)borate (TplPr2) with MnCl2.4H20 resulted the formation of complex TplPr2MnCl 2c and Tp1 MnClPz 2d which have vacant coordination site on manganese for important reactivity iPr2 studies. Complex 2c and 2d are used to prepare the complex Tp Mn(n- OH^MnTp^2 2e, which upon reaction with one equivalent amount of benzoic acid in toluene gave complex TplPr2Mn([i-OH)(p.-OBz)MnTp 2f, as a possible model compound for the Mn-containing ribonucleotide reductase enzyme. Reaction of the toluene solution of 2e with solid benzoic acid yielded the complex TplPr2Mn(u-OBz)3MnTplPr2H 2g, contains a unique metallic core structure. The coordination environments (i.e. coordination number and geometry) of the two manganese centers are clearly different. One of the two Tp^2 ligands, which bound to the five-coordinated Mn center, was protonated by action of the third carboxylic acid and the (iv) resulting N-H moiety formed intramolecular hydrogen bond with the oxygen donor of a bridging carboxylate ligand. Steric congestion in the bimetallic core resulted in the large separation of the Mn centers bridged by the synanti carboxylate ligand. The above mentioned subjects (i.e. ligand exchange process involving proton transfer, bimetallic core bridged by carboxylates, variation of the coordination mode of the carboxylate) are occasionally observed events in the various metalloproteins and the present results may give some insight into the structural aspects of reaction intermediates. The reaction of 2g with 1equivalent solid NaOH gave mononuclear manganese complex Tp^MnfOBzXPzH) 2h. Complex 2h has distorted trigonal bipyramidal geometry. The synthesis and characterization of mononuclear manganese dioxygen complexes with hindered hydrotris(pyrazolyl)borate as the ligands are presented in the chapter three. Hie reaction of [Mn(Tp,,>r2)]2(0)2 with excess amount of H202 in the presence of 2-methylimidazole resulted the formation of Mn(02)(2-MeIM)(TplPT2) 3c. The complex 3c is first example of six coordinate mononuclear side-on peroxo complex of Mn(III), which is stabilized by intermolecular hydrogen bonding between peroxide and the imidazole proton. In comparison to Mn(02)('Tr2PzH)(TplM), complex 3c was unable to oxygenate triphenylphosphine and ethyl vinyl ether and its slow (v) decomposition at room temperature gave a bis(u.-oxo) complex [Mn(Tp,Pr2)]2(0)2. The similar reaction was also performed in the presence of 3,5-dimethylpyrazole, 3-phenyl-5-methylpyrazole and 1,2- dimethylimidazole. The isolated products were not stable at room temperature for detail characterization but IR spectral studies suggested the formation of peroxo compounds. The synthesis of mononuclear manganese peroxo complex was also tried by the reaction ofMn(Tp'~ u r)(OH) 3h with excess H202. The resultant compound Mn(Tp,BuiPr)(02) 3i is less stable than 3c but it was also unable to oxygenate triphenylphosphine. In chapter four of the thesis, the synthesis and characterization of a series of monomelic carboxylate nickel complexes with a tripodal N3 ligand TpiPl2 are described. The structure of Ni(OBz)(TplPr2) is determined by X-ray crystallography. The six coordinate Ni-benzoate complexes do not react with 1 arm. dioxygen neither at -78 °C nor at room temperature. Mononuclear and binuclear Ni(II)-hydroxo complexes were also prepared for their use as catalyst in ester hydrolysis. The esters were used in the reaction are pnitrophenylacetate, p-nitrophenylphosphate, bis(p-nitrophenyl)phosphate andtris(p-nitrophenyl)phosphate. The materials and reagents, synthetic procedures, experimental details and different type of spectroscopic measurements are described in chapter (vi) five of the thesis. Method for preparation of hindered hydrotris(pyrazolyl)borates and their complexes with Mn(II) and Ni(II) have been included.