From pyrazole- to imidazole-based N-donor Ligands: Coordination Chemistry and Applications of New Late Transition Metals Complexes with Scorpionates, Poly(pyrazolyl)alkanes and NHCs

The nature of ligands coordinated to the metal ion mostly determines the properties of the inorganic, organometallic or bioinorganic model compounds. The pyrazole is one of the most studied heterocyclic aromatic compounds in coordination chemistry as single ligand or incorporated in polidentate ligands, such as in poly(pyrazolyl)borates (‘scorpionates'), poly(pyrazolyl)alkanes, etc. The great versatility of the pyrazole allows the formation of several ligands with different functionalizations, which mainly influence the geometry and nuclearity of the diverse coordination complexes afforded by the reaction of the pyrazole-based chelating ligands and a metal ion. Another of the most studied heterocyclic aromatic compounds in coordination chemistry is an analogue of pyrazole with two non-adjacent nitrogen atoms, the imidazole, which represents the main structure of the most common and studied N-Heterocyclic carbenes (NHCs), in particular the imidazol‐2‐ylidenes. These general informations arouse curiosity in main features and reactivity, synthesis and coordination modes, potential and ensured applications of various N-donor ligands, such as poly(pyrazolyl)borates or scorpionates, their isoelectronic and isosteric neutral analogues poly(pyrazolyl)alkanes and N-heterocyclic carbenes, presented, thus, in detail, in Chapter 1. The focus on the electronic and steric properties, the coordinating behaviour and several applications of scorpionates metal complexes, the state-of-art on coordination chemistry and reactivity of the poly(pyrazolyl)alkanes derivatives, the advantages of NHCs over phosphines, including ease of handling, minimal toxicity and powerful electron-donating properties, as enhanced stability of their transition metal complexes, which lead to the synthesis of highly active derivatives in homogeneous catalysis, anticipate the research work of this thesis on the nitrogen pyrazole- and imidazole-based ligands and their organometallic compounds. Beginning from Chapter 2, which describes the aims of the research work, all the chapters are divided in four sub-chapters. In the first sub-chapter differently halogen substituted bis- and tris-(pyrazolyl)borate ligands (BpBr3, TpBr3 and TpiPr,4Br) are compared in terms of electron-donor character through their interactions with various ruthenium(II) arene dimers, as also through electrochemical studies. Interestingly, an unexpected dinuclear ionic compound accounted for by its low thermodynamic stability, confirmed by theoretical calculations at the DFT level, due to steric reasons. The second sub-chapter is centered on the synthesis, characterization and crystal structure of the complexes of the second-lowest steric hindrance scorpionate ligand, TpTn, towards CuI, CuII, ZnII and CdII acceptors. A remarkable coordinative aspect of this ligand is pointed out, as well as the thermal and air stability of the relative complexes, confirmed by thermogravimetric and differential thermal analysis (TGA-DTA). By contrast, the interaction between TpTn and PtI2 proceeded through ligand decomposition, affording a monomeric complex. In the third sub-chapter, the varying, different stoichiometries and the N,N-chelating coordination modes of the bis(pyrazolyl)methanes, in particular of the two indazolyl-based regio-isomeric ligands, L1, bis(1H-indazol-1-yl)methane, and L2, bis(2H-indazol-2-yl)methane, toward XI group metal centers are studied. The synthesis and full characterization of analogous adducts of both regio-isomers allow a systematic comparison of their structural and spectral features, depending on the regio-isomeric ligand and counter-ion used, and the reaction conditions employed. The synthesis and characterization of a new series of stable, cationic gold(I/III) NHC complexes, as well as their catalytic activity in five well-established organic gold-mediated transformations, performed at the University of Strasbourg, under guidance of Director of Research Pierre Braunstein and coworkers (Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universita'© de Strasbourg, France) is described in the fourth sub-chapter. Interestingly, various attempts to synthesize gold(III) NHC bis- or tris-pyridine complexes, as also the comparative behaviour of NHCs respect to phosphine ligands, are presented. Going further on the thesis structure, all the general procedures and experimental data are reported in Chapter 3, while the discussion and elaboration of the results are described in Chapter 4. Concluding remarks and future perspectives are discussed in Chapter 5, whereas all the references and some notes are given in Chapter 6. In Annexes section, the Curriculum Vitae, the List of Pubblications and the Contributions to Congresses are presented.