PYRAZOLYLBORATE COMPLEXES OF Mn AND Co: SYNTHESIS, STRUCTURE AND REACTIVITY STUDIES

Abstract

The polypyrazolylborate ligand has more than 40 years old history as it was first time reported by S. Trofimenko in 1960's. In general, however, the polypyrazolylborate are tripodal ligands that include a diverse set of structure. The ligands can be tetradentate, bidentate or monodentate depending on the number of donor substituents on boron and the steric congestion around the metal. In addition, tripodal ligand can have central atoms other than boron such as carbon, phosphorus or gallium. The ligands also can bind metals through sulphur, phosphorus or other donor atoms. The pyrazolylborates / scorpionate chemistry signify a very large and growing aera. 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. The steric properties of the ligands can be altered by changing the substituents on the pyrazolyl rings. These ligands have been used extensively to prepare complexes of the transition metals. The polypyrazolylborate ligands have properties lying between those of parent [H4-nB(pz)n]" ligands (pz = pyrazolyl) and those of the recently reported derivatives [H4-nB(3-Rpz)n]", where R = tert-butyl and phenyl groups. The parents ligands, in particular the derivatives with n = 3, have been extensively used in inorganic and bioinorganic chemistry. However, the chemistry is severly limited because of the tendency of the first row transition metal ions to form the bisligand complex (i.e. M(HB(pz)3)2). On the other hand, the bulky group of the 3-R derivatives prevents formation of this type of complex. The 3-tert-butyl ligand allows no further access to the metal ion and is a "tetrahedral enforcer", whereas the 3- phenyl derivative accommodates five coordination with trigonal-bipyramidal geometry. Although bisligand complex formation is prevented, the metal ion is not readily accessible to other molecules because of the steric bulk of the 3-substituents. In contrast to cyclopentadienide ligands, with which they have often been compared, the tris(pyrazolyl)borate ligands offer much greater opportunities for modifying their coordinative features. Whereas only one substituent can beplaced on CsHstCsRs) to yield a derivative ligand with retention of the original ligand symmetry, there are 15 ways of placing from one to ten identical substituents on the parent HB(pz)3 ligand and still maintain their original C3v symmetry. It is well established in the literature that from a structural point of view the complex chemistry of the trispryrazolylborate anion frequently resembles that of cyclopentadienyl (or pentamethylcyclopentadienyl) complexes. However, as might be expected for a tridentate nitrogen donor ligand; the trispyrazolylborate complexes are kinetically less labile than their cyclopentadienyl analogues. In the hierarchy of increasing steric hindrance around the metal in 3- or 5- substituted tris(pyrazolyl)borates, the currently known series is H< CH3< C6H5 < 'Pr < 'Bu. In terms offorming octahedral L2M complexes, ligands with R=Hand CH3 do so rapidly whereas those with R = CeHs do so reluctantly, and those with R = 'Pr do not form L2M complexes. The ligands with R = 'Pr form octahedral complexes only with rearrangement of each L to HB(3-'Prpz)2(5-'Prpz); they also form mixed octahedral LML complexes, provided L is a relatively unhindered tris(pyrazolyl)borate ligand [19]. Finally, ligands with R = Bu' do not form octahedral complexes at all with first-row transition metals. Conversely, in terms offormation ofLMX (X= halide or pseudohalide) species, ligands with R= Bu1 form these stable complexes with ease.With R= 'Pr, LMX complexes are also readily formed. But they posses reactivity for solvation or for II displacement of X with anionic nucleophiles. The ligands with R = phenyl form LMX species, which are even more reactive towards solvation or substitution of X. When R = methyl, and even more so when R = H, the LMX species becomes progressively less stable and undergo transformation to the octahedral L2M complexes. Among the different types of pyrazolylborates, hydrotris(pyrazolyl)borate (Tp ) ligand has been widely used as supporting ligand for various inorganic and organometallic compounds. It has 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. The TpR ligand is also able to tune the electronic and steric environment of the central metal atoms by introducing various substituents to the pyrazolyl ring. In addition to this feature, a coordination environment created by the Tp ligand can mimic an environment created by a set of three imidazole rings of histidine residue, which is frequently found in the metal coordination site of metalloproteins. For the sake of the convenience, the work embodied in the thesis is presented in the following chapter: III The First chapter of the thesis is the general introduction and present an up to date survey of literature related to the various pyrazole and their borate salts. The different kind ofmetal 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 the mononuclear and binuclear of manganese complexes with 3,5-diisopropylpyazole (3,5-iPr2pzH) and its borate salt [KHB(3,5-Pr'2pz)3] as ligand. The reaction of 3,5-diisopropylpyazole with MnCl2.4H20 and sodium fluorobenzoate (NaOBz-F)/sodium nitrobenzoate (NaOBz- N02) resulted the formation of [Mn(F-OBz)2(3,5-iPr2pzH)4] and [Mn(N02-OBz)2(3,5- iPr2pzH)4]. Mn(OBzF)2(iPr2H)4 and Mn(N02-OBz)2(3,5-iPr2pzH)4 have six coordinated structure with intramolecular hydrozen bonding between the uncoordinated oxygen of mono-coordinated fluorobenzoate/nitrobenzoate and the free hydrogen of coordinated 3,5-iPr2pzH. The single crystal X-ray structure also shows the CH3-P7t interaction between the two molecules of above complexes. A dimanganese(H) complex [Mn(HB(3,5-Pri2pz)3)(u-FOBz)3Mn(3)5-iPr2pzH)2] was prepared from a mixture of [Mn(Cl)(HB(3,5-Pri2pz)3)], 3,5-iPr2pzH, MnCl2.4H20 and (NaOBz-F). The X-ray studies established the unsymmetric coordination environment of each manganese ion with a structurally unique bridging unit consisting of three carboxylate groups. Some other mononuclear complexes was also prepared for finding out the coordination mode of fluorobenzoate, azide, thiocyanate and cyanide group. The synthesis and structural studies of binuclear cobalt complexes having different bridging ligands are presented in chapter three. The reaction of Co(N03)2.6H20 with 3,5-diisopropylpyazole (3,5-iPr2pzH) and sodium nitrobenzoate resulted the rv formation [Co2(3,5-iPr2pzH)2(u-3,5-iPr2pz)2(OBz-N02)2] complex. In this complex both cobalt ions are four coordinated, bridged by two 3,5-diisopropylpyrazole. The other two molecules 3,5-diisopyropylprazole are unidentately coordinated to the metal ions and also form the hydrogen bonds with un-coordinated oxygen of nitrobenzoate group. The complex [CoN03(HB(3,5-Pr'2pz)3)] was synthesized by the reaction of Co(N03)2.6H20 with [KHB(3,5-Pr'2pz)3] which upon reaction with NaOH resulted the formation of binuclear hydroxo complex [HB(3,5-Pr'2pz)3Co)2(OH)2]. This complex reacted with two equivalent of bis(4-nitrophenyl)phosphate and resulted the complex [(HB(3,5- Pr'2pz)3)2Co2bis(u-phosphodiester)]. The reaction of [HB(3,5-Pri2pz)3Co)2(OH)2] with two equivalent KSCN / NaN3 / KCN were performed in order to know the binding mode of these molecules in resultant complexes. The chapter four of the thesis deals with the synthesis and characterization of some mononuclear hydroxo of the type [(HB(3-Bu'-5-Pr'pz)3)MOH] (M = Mn, Ni, Zn, Cd). These complexes were characterized by different available techniques and were used as a catalyst in the hydrolysis of different types of phosphate, carboxylate and sulphonate esters. The hydrolysis of different ester was monitored spectrophotometrically. The material and reagents, synthetic procedures, experimental details and different type of spectroscopic measurement are described in chapter five of the thesis. Methods for the preparation of different types of ligands and their complexes with Mn(II) and Co(II) have been included.