COMPLEXES OF Sm(III), Nd(III) AND Pr(III): SYNTHESIS, STRUCTURAL AND THERMAL STUDIES

Abstract

Lanthanides are f-block elements, with atomic numbers from 57 to 71. In nature, they never exist as pure elements, but only as sparsely distributed minerals. Although lanthanides are termed as rare-earth elements, they are not rare in nature because their levels in the earth's crust are often equal to or higher than those of some physiologically significant elements. The lanthanide series is characterized by a gradual filling of the 4f shell. Since, in most cases, the 4f electrons are chemically inert and atomic like, all lanthanides behave in a very similar manner and basic properties such as the atomic volume, bulk modulus and melting temperature vary in a more or less regular manner across the series [1]. Their unique chemical similarity is due to the shielding of 4f valence electrons by the completely filled 5s and 5p orbitals. Although the members are very similar from a chemical point of view, each of them has its own very specific physical properties-including color, luminescent behavior and nuclear magnetic properties. The ionic radii of lanthanides range from 0.1034 nm (Ce) to 0.0848 nm (Lu), the values being relatively higher than those in other elements with the same oxidation number. Despite the high charge, the large size of the Ln(III) ions results in low charge densities and their compounds are predominately ionic in character. The lanthanide (III) ions are typical 'A' type cations in the Ahrlansd-Chatt-Davies sense or hard acid in the Pearson sense, so they tend to bound to ligands containing pure oxygen donors (e.g. oxalate ion, /?-diketonate ions), mixed oxygen-nitrogen donors (e.g. polyaminepolycarboxylate ions) and nitrogen donors (e.g. 1,10 phenanthroline, bipyridine, terpyridine). The past several years have witnessed the rapid growth of interest in the lanthanides and their coordination chemistry. Studies based on lanthanide ions are special challenge due to their specific electronic, magnetic or spectroscopic properties result from aprecise description of coordination sphere around the metal ions. Coordination compounds of lanthanides have found avariety of applications in material science including superconductors, magnetic materials, and catalyst [2-5]. In all these applications two main points must be consider for optimization of the success of a lanthanide complex for achieving aspecific objective. First, thermodynamic stability is crucial to assure that the complex can reach the target point (an organ, a cell, etc.) unchanged. Second, the structure of the complex is very important to determine the in vivo behavior of the agent. For example, the coordination number, the presence of coordinated water and the formation of polynuclear species can dramatically change the usefulness of the compound. Due to interesting luminescence properties of lanthanide complexes, several complexes with samarium (III), neodymium (III), praseodymium (III) with different ligands have been prepared and their structural, thermal as well as photo physical properties have been studied in the present thesis. For the sake of the 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 to the photo physical properties of different lanthanide complexes. The different kind of metal complexes related to the present research have been posed in the context of the cited work. The synthesis, structural and thermal studies of some samarium (III) complexes are presented in chapter two. One samarium coordination polymer (chain like) with composition [{Sm(OBz)3(MeOH)2}2]n has been isolated by the reaction of SmCl3 and 11 sodium benzoate in 1:3 ratio. Four binuclear samarium complexes with chemical composition [{(tp)Sm(u-/?-X-OBz)2}2] have been prepared by the reaction of SmCl3, potassium hydrotris(pyrazol-l-yl)borate [K(tp)] and sodium/?-X-benzoate (where X = H, F, CI,N02) in 1:1:2 ratio,whereas the reaction of SmCl3 with 2 equivalents of [K(tp)] and one equivalent of sodium azide, gave the tetranuclear complex [{(tp)2Sm(u-N3)}4]. Several other complexes like [Sm(bipy)2Cl3.CH3OH], [Sm(terpy)Cl2(CH3OH)3].Cl, [Sm2(u-Cl)2(phen)4Cl4] and [Sm(Phen)2(SCN)3(OCH3)].phen have also been prepared. These complexes have been characterized by elemental analysis, IR spectroscopy, thermogravimetry, X-ray and magnetic measurements studies. The single crystal X-ray studies revealed that the complex [{Sm(OBz)3(MeOH)2}2]„ is polymer in nature with eight coordination number around the metal center, where as four binuclear samarium complexes with chemical composition [{(tp)Sm(u-p-X-OBz)2}2] are isostructural with coordination number seven around the metal center completed by three nitrogen atoms of pyrazolylborate ligand and four oxygen atoms from benzoate groups. The molecular structure of [{(tp)2Sm(u-N3)}4] consists of a tetrameric cycle of [(tp)2SmN3] units owing a crystallographic center of inversion. The samarium ions in the tetramer are linked by one end-to-end bridging azide ion. Each samarium ion is eight coordinated with six nitrogen atoms from two tp ligands and two nitrogen atoms from bridging azide ions. In mononuclear complexes [Sm(bipy)2Cl3.CH3OH] and [Sm(terpy)Cl2(CH3OH)3].Cl, the metal center is in eight coordination environment. In host guest complex [Sm(Phen)2(SCN)3(OCH3)].phen, one free phenanthroline molecule in lattice is playing a role of guest molecule while the metal complex is playing a role of host molecule. The photophysical properties of samarium benzoate bridged polymer 111 [{Sm(OBz)3(MeOH)2}2]„and dimmers [{(tp)Sm(u-/?-X-OBz)2}2] have been studied with ultraviolet absorption, excitation and emission spectral studies. Thermogravimetric analyses showed that after complete decomposition, all complexes resulted the formation of thermally stable samarium oxide (Sm203). The chapter three deals with the structural and thermal properties studies of some neodymium (III) complexes. Reaction of NdCl3 with one equivalent of potassium salt of potassium hydrotris(pyrazol-l-yl) borate [K(tp)] and two equivalents of sodium p-Xbenzoate (where X = H, F, CI, N02) in 1:1:2 ratio yielded the binuclear complexes of composition [(tp)Nd(u-/?-X-OBz)4Nd(tp)], whereas as the same reaction in presence of one equivalent of free pyrazole (pz) yielded the complex with composition [(tp)(pz)Nd(up- F-OBz)4Nd(pz)(tp)]. In other set of experiments some mononuclear neodymium complexes of the type [Nd(bipy)2Cl3.CH3OH], [Nd(phen)2Cl3.OH2] and [Nd(terpy)Cl3(CH3OH)3] have been prepared by the reaction of NdCl3 with bipy (2,2' bipyridine), phen (1,10 phenanthroline) and terpy (2,2':6',2" terpyridine). The treatment of [Nd(phen)2Cl3.OH2] with three equivalents sodium benzoate gave benzoate bridged complex of composition [Nd(OBz)3(Phen)]2. These complexes have been characterized by elemental analysis, IR, thermal, magnetic measurements and X-ray crystallographically. The X-ray studies demonstrated that complexes [(tp)Nd(u- OBz)4Nd(tp)], [(tp)(pz)Nd(u-/?-F-OBz)4Nd(pz)(tp)] and [Nd(bipy)2Cl3.CH3OH] crystallize in triclinic space group P\ with cell dimensions a = 11.745(3), b = 12.566(3), c = 17.654(5) Afor [(tp)Nd(u-OBz)4Nd(tp)]; a = 11.561(5), b = 12.020(5), c = 12.286(5) A for [(tp)(pz)Nd(u-/?-F-OBz)4Nd(pz)(tp)]; a = 8.474(3), b = 8.832(3), c = 15.788(5) A for [Nd(bipy)2Cl3.CH3OH]. Complexes [Nd(phen)2Cl3.OH2] and [Nd(OBz)3(Phen)]2 iv crystallize in the monoclinic space group P2(l)/c with cell dimensions a = 18.396(12), b = 10.764(6), c = 13.112(9) A for [Nd(phen)2Cl3.OH2]; a = 12.587(4), b = 10.629 (3), c = 22.696 (8) A for complex [Nd(OBz)3(Phen)]2. In all the complexes except [(tp)Nd(u- OBz)4Nd(tp)], the coordination number of each neodymium is eight whereas in complex [(tp)Nd(u-OBz)4Nd(tp)], it is seven. The IR studies suggested only bidentate bridging binding mode of carboxylate groups in complexes [(tp)Nd((^-OBz)4Nd(tp)] and [(tp)(pz)Nd(u-p-F-OBz)4Nd(pz)(tp)] whereas in complex [Nd(OBz)3(Phen)]2, the carboxylate groups are coordinated in both bridging as well as in bidentate chelating manner. Thermogravimetric analyses showed that all the complexes under go complete decomposition with the formation of thermally stable neodymium oxide (Nd203). The chapter four deals with the synthesis, structural and thermal properties studies of some praseodymium (III) complexes. Reaction of PrCl3 with one equivalent of potassium salt of hydrotris(pyrazol-l-yl) borate [K(tp)] and two equivalents of sodium /?-X-benzoate (where X = H, F, CI, N02) in 1:1:2 ratio yielded the binuclear complexes of composition [(tp)Pr(u-/?-X-OBz)4Pr(tp)]. The reaction of PrCl3 with bipy (2,2' bipyridine) and terpy (2,2':6',2" terpyridine) yielded the chloride bonded mononuclear complexes but with phen (1,10 phenanthroline) ligand, the formation of chloride bridged binuclear complex of composition [Pr2(u-Cl)2(phen)4Cl4] has been observed. These complexes have been characterized by elemental analysis, IR spectroscopy, thermal as well as X-ray and magnetic measurements. Single crystal X-ray diffraction studies demonstrated that complexes [(tp)Pr(u-OBz)4Pr(tp)], [(tp)Pr(u-p-Cl-OBz)4Pr(tp)] and [Pr(bipy)2Cl3.CH3 OH] crystallize in triclinic space group P\ with cell dimensions a = 11.761(13) A, b = 12.536(13) A, c = 17.726(19) A for [(tp)Pr(u-OBz)4Pr(tp)]; a = 9.309(8) A, b = 12.667(11) A, c = 14.421(12) A for [(tp)Pr(u-p-Cl-OBz)4Pr(tp)]; a = 8.473(6) A, b = 8.839(7) A, c = 15.818(15) A for [Pr(bipy)2Cl3.CH3OH]. The complex [Pr2(u- Cl^tphen^CU] crystallizes in the monoclinic space group P2(l)/c with the cell dimensions a = 9.541(2) A, b = 17.321(4) A, c = 14.596(3) A. In complexes [(tp)Pr(p.-p- X-OBz)4Pr(tp)] (where X = H, F, CI, N02), one tp ligand and four benzoate groups are coordinated to one praseodymium metal center and accounted seven coordination numbers. In mononuclear complex [Pr(bipy)2Cl3.CH30H], each praseodymium center is coordinated by four nitrogen atoms from bipyridine ligands and three terminal chlorine atoms, whereas in chloride bridged binuclear complex [Pr2(|i-Cl)2(phen)4Cl4], each praseodymium center is coordinated by four nitrogen atoms from two phenanthroline ligands, two terminal chlorine atoms and two bridging chlorine atoms having coordination number eight. Thermogravimetric analysis showed that all complexes after complete decomposition converted to thermally stable praseodymium oxide (Pr60n). 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 samarium (III), neodymium (III) and praseodymium (III) have been included.