New iso-propoxides, tert-butoxides and neo-pentoxides of niobium(V): synthesis, structure, characterization and stabilization by trifluoroheteroarylalkenolates and pyridine ligands

The synthesis and characterization of nine new heteroleptic alkoxides of niobium is described. Metathesis reactions of Nb(2)Cl(10) with (t)BuCH(2)OH and pyridine (py) or 4-dimethylaminopyridine (DMAP) affords monomeric octahedral complexes Nb(OCH(2)(t)Bu)(5)py (1) and Nb(OCH(2)(t)Bu)(5)DMAP (2), respectively, in high yields (>60%). The same reaction with (t)BuOH resulted in a chloro functionalized alkoxide Nb(O(t)Bu)(4)pyCl (3) and could not be pushed to complete removal of remaining Cl(-) ligand. The introduction of a chelating bidental ligand 3,3,3-trifluoro-1-(pyridine-2-yl)propen-2-ol (2-PyCHCOHCF(3)) (4') in the dimeric framework of Nb(2)(O(i)Pr)(10) (4') produced a heteroleptic, monomeric niobium complex Nb(O(i)Pr)(4)(2-PyCHCOCF(3)) (4) with significantly enhanced stability and volatility. As a comparison to (4), five different heteroaryl systems (5-9) with the same side chain have been synthesized and examined in order to understand the influence upon physio-chemical properties. All the new compounds (1-9) have been characterized by microanalysis, variable temperature multinuclear NMR spectroscopy, mass spectrometry, thermal analysis and single crystal X-ray diffraction studies ((3), (4) and (9)). The molecular structure of (3) revealed mononuclear species with Nb atoms present in the distorted octahedral environment of four (t)BuO, one chloride and one pyridine ligand. Compounds (4) and (9) consisting of four (i)PrO and a trifluoroheteroarylenolate exhibited a stronger distortion in the molecular geometry due to the rigidity of chelating β-alkenolate moiety.