SYNTHESIS AND STRUCTURAL STUDIES OF IRON, COBALT AND NICKEL COMPLEXES

Abstract

The poly(pyrazolyl)borate ligands have more than 40 years old history as they were first time reported by S. Trofimenko in 1966. 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 also be used such as pyrrole, imidazole and indole. 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 have attracted much attention. Among the different types of pyrazolylborates, hydrotris(pyrazolyl)borate (TpR) ligand has been widely used as supporting ligand for various inorganic and organometallic compounds. Thioimidazole (N,N,S donor) can also be used in place of pyrazole as soft donor ligand and can bindto metal center through N or/and S atoms. In recent years, N-alkyl derivatives thioimidazoles have been directly coordinated to a variety of transition and main group metals and used as building blocks for the assembly of poly(mercaptoimidazolyl)borates. These soft analogues of Trofimenko's versatile poly(pyrazolyl)borates ligands, have been developed by Reglinski and co-workers in 1996and also found interesting applications in the preparation of model compounds for sulfur-rich metalloenzymes such as liver alcohol dehydrogenase (LADH) and [NiFe] hydrogenases. An important property of these ligands is the delocalized anionic charge on the thioimidazole group, which brings about the thiolate-like character of the thioimidazole sulfurs. After Reglinski's discovery of the tris(thioimidazolyl)borates (S3 donor), Parkin and Vahrenkamp introduced the pyrazolylbis(thioimidazolyl)borates (NS2 donor), and bis(pyrazolyl)(thioimidazolyl)borates (N2S donor) ligands to give these coordination environment around metal centers. Further they have suggested that varying the substituents on the pyrazole and thioimidazole groups of these ligands, an appropriate electronic and geometrical environment can also be generated on metal centre. 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, thioimidazole 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 chapter two of the thesis deals with azido complexes of Iron (III) having hydrotris(l-pyrazolyl)borate [Tp] or hydrotris(3,5-dimethyl-pyrazolyl)borate [TpMe2] and some bidentate ligands (acetylacetonate/picolinate) as supporting ligands. To check these complexes as azide-selective, one of the iron complex [Tp e Fe(acac)Cl] was used as an ionophore for the preparation of a polyvinyl chloride) (PVC) membrane sensor for azide anion. After confirming the selectivity of azide ion, the reaction of [TpMe2Fe(acac)Cl] with the methanolic solution of sodium azide was performed to get single azide bonded compound [TpMe2Fe(acac)N3]. The complex [TpMe2Fe(PzMe2H)(N3)2] was also synthesized by the reaction of FeCl3 and KTpMe2 with two equivalent sodium azide in presence of 3,5-dimethylpyrazole [Pz e H]. All of these compounds were 11 characterized by different spectroscopic methods including single crystal X-ray crystallography. The one electron electrochemically reduced product of [TpMe2Fe(PzMe2H)(N3)2] was crystallized in ionic form [(TpMe2)2Fe][FeCl4]. Further attempts have been made to prepare more ion-pair complexes by co-crystallization of nickel (II) phenanthroline complex cations and different complex anions like [MnCU] ", [CotmOcy", [NiCtnOCh,]" and [ZnCtm^Cls]' (un1"®11 = N-terf-butyl-2- thioimidazole). Chapter three deals with synthesis of some cobalt complexes using 3,5- diisopropylpyrazole [PzlPr2H], hydrotris(3,5-diisopropyl-l-pyrazolyl)borate [TplPr2], substituted benzoates and N-tert-butyl-2-thioimidazole [tm'"Bu]. The reaction of [TplPr2Co(N03)] with sodium para-fluorobenzoate [Na(p-F-OBz)] led to the formation of five coordinate complex of the type [TplPr2Co(/?-F-OBz)]. The oxidation of this compound with H202 in the presence of PzlPr2H resulted the formation of a unique compound [{HBC«-3-OCMe2-5-PziPr)(PzlPr2)2}Co(PziPr2H)(p-F-OBz)] where only one methine group of diisopropyl pyrazole ring of TplPr2 ligand was oxidized and coordinated to cobalt center. Pyrazolato bridged binuclear cobalt (II) complex [(PziPr2H)2Co2(u-PziPr2)2(p-F-OBz)2] was formed when the reaction of cobalt (II) chloride with PzlPr2H in presence of sodium p-fluorobenzoate was performed. The same reaction in presence of 50 equivalent of H202 resulted in the formation of host-guest complexes ofthe type [(Co(3-OCMe2,5-PziPrH)3].2(p-X-OBz) (where X= F, CI, CH3, N02, CN, CHO) with different packing. Some cobalt complexes with N-tert-butyl-2- thioimidazole [tml"Bu] were also synthesized and characterized. The chapter four of the thesis deals with mononuclear nickel (II) complexes having hydrotris(3-phenyl-5-methyl-l-pyrazolyl)borate [Tpph,Me], 3-phenyl-5-methyl- 111 pyrazole [PzphMeH] and N-tert-butyl-2-thioimidazole [tm1"8"] as supporting ligands. [(TpPhMe)2Ni] were synthesized by the reaction of TpphMe and nickel (II) chloride whereas the same reaction in presence of PzPhMeH resulted in the formation of [Tpph'MeNi(Cl)PzPhMeH]. The complex [TpphMeNi(Cl)PzPhMeH] has been used as an ionophore to prepare its polymeric membrane as a sensor for benzoate anion. The different benzoate complexes ofthe type [TpPhMeNi(p-X-OBz)PzPhMeH] (X = H, F, CI, N02, Me, OMe, OH, CHO, CN, NH2) were also prepared by the reaction of [Tpph,MeNi(Cl)Pzph,MeH] and different sodium ^-X-benzoate for structural studies in solid state using single crystal X-ray method. In all these complexes except aminobenzoate complex, the benzoate groups are coordinated as monodentate and the uncoordinated oxygen atom of the benzoate groups form intramolecular hydrogen bonds with NH group present on pyrazole ring. The complex [Tpph,MeNi(Cl)PzphMeH] has also been used as ionophores for azide and thiocyanate anions. Some nickel (II) complexes with N-tert-butyl-2-thioimidazole [tm'"Bu] were also synthesized and characterized. 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 metal complexes have also been included.