Heterocyclic cyclopentadienyl analogs with boron-nitrogen frameworks

The heterocyclic cyclopentadienyl analogs have provided a fertile area of research, owing to the interest in tuning the electronic properties of cyclopentadienyl (Cp) ligands for the development of more efficient catalysts and new materials. A variety of five-membered heterocyclic Cp analogs have been synthesized by the formal replacement of ring carbon atoms with isoelectronic main group fragments. Metal complexes containing these heterocyclic ligands have proven to be feasible ancillary ligands in catalysis.;The primary objective of this thesis is the development of a novel class of heterocyclic Cp ligands having ring carbon fragments (CR) formally substituted with isolobal boron (BR-) and nitrogen (NR'+) fragments. The synthesis and characterization of the 1,2-diaza-3,5-diborolyl ligands with cyclic CB2N2 frameworks is described and their coordination to a variety of metal fragments is reported. Just like cyclopentadienyl, the 1,2-diaza-3,5-diborolyl ligands are excellent pi ligands that exhibit interesting coordination properties. Alkali metal complexes of 1,2-diaza-3,5-diborolyls were synthesized and their investigation by single-crystal X-ray diffraction analysis revealed substantial coordinative similarities, but also remarkable differences in comparison to cyclopentadienyl ligands. Sandwich complexes containing these new heterocyclic pi ligands were synthesized by metathesis reaction of the corresponding alkali metal salts with various metal halides. The structural characterization of a series of electron-rich group 12 and 14 metallocenes, as well as metallocenes of the early and late transition metals, revealed some unique coordinative properties of the ligands: eta 1, eta3, eta4 and eta5 -coordination modes of the 1,2-diaza-3,5-diborolyl ligands were observed, depending on the electron properties of the coordinated metals. In addition, the synthesis of the heterobicyclic 1,5-diaza-2,4,6,8-tetraborolyl ligand featuring a C2B4N2 framework is described and the structural investigation of its dipotassium salt showed that this heterobicyclic ligand is a promising pi bridging linker for the construction of polydecker sandwich complexes. A triple-decker ruthenium sandwich complex featuring an unusual eight-membered C2B4N2 ring as the middle deck was synthesized through the insertion of two RuCp* fragments into the N-N bond of the heterobicyclic ligand. Electrochemical studies of the triple-decker ruthenocene are presented.;The coordination chemistry of the 1,2,4-triaza-3,5-diborolyl ligand, a carbon-free heterocyclic Cp analog, was also investigated. The alkali metal salts of this ligand were synthesized via selective deprotonation of the ring nitrogen utilizing appropriate metalating agents. In contrast to the former ligands, the solid-state structures of the alkali metal complexes featuring these ligands are dominated by sigma interactions of the ligand to the metal ions. A rhodium dimer containing 1,2,4-triaza-3,5-diborolyl ligands sigma-bridging two Rh(cod) fragments was synthesized and characterized. The synthesis and characterization of a tricyclic compound with a B8N4 framework consisting of fused five- and six-membered rings was discussed, along with a study of its electrochemical properties.