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

Abstract

The synthesis and characterization of nine new heteroleptic alkoxides of niobium is described. Metathesis reactions of Nb₂Cl₁₀ with ^tBuCH₂OH and pyridine (py) or 4-dimethylaminopyridine (DMAP) affords monomeric octahedral complexes Nb(OCH₂^tBu)₅py (1) and Nb(OCH₂^tBu)₅DMAP (2), respectively, in high yields (>60%). The same reaction with ^tBuOH resulted in a chloro functionalized alkoxide Nb(O^tBu)₄pyCl (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-PyCHCOHCF₃) (4′′) in the dimeric framework of Nb₂(OⁱPr)₁₀ (4′) produced a heteroleptic, monomeric niobium complex Nb(OⁱPr)₄(2-PyCHCOCF₃) (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 ^tBuO, one chloride and one pyridine ligand. Compounds (4) and (9) consisting of four ⁱPrO and a trifluoroheteroarylenolate exhibited a stronger distortion in the molecular geometry due to the rigidity of chelating β-alkenolate moiety.