Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 1981 www.rsc.org/dalton PAPER New iso-propoxides, tert-butoxides and neo-pentoxides of niobium(V): Synthesis, structure, characterization and stabilization by trifluoroheteroarylalkenolates and pyridine ligands† Linus Appel, Raquel Fiz, Wieland Tyrra and Sanjay Mathur* Received 3rd September 2011, Accepted 31st October 2011 DOI: 10.1039/c1dt11668a 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 b-alkenolate moiety. Introduction Niobium(V) oxide finds numerous applications in photoelectron- ics, as photocatalyst and capacitor material due to its interesting electrical and optical properties. 1 Recently, Nb 2 O 5 has attracted significant attention as a candidate for dye-sensitized and hybrid solar cells 2,3 and photocatalytic hydrogen production from water. 4 Niobium based oxides and heterometallic oxides are generally prepared from metal chlorides, metal alkoxides, metal alkylamides and heterometallic alkoxide complexes; however, the limited thermal stability and extreme sensitivity of these compounds towards moisture and air make their handling and storage rather difficult. 5–12 In terms of chemical design, the stability of group V metal alkylamides 10 or metal alkoxides 9,14,15 can be modulated by introducing ligands with a chelating function or possessing high steric profiles as we have shown for Sn(II) complexes. 13 Substituting University of Cologne, Institute of Inorganic and Materials Chemistry, Greinstrasse 6, 50939, Cologne, Germany. E-mail: sanjay.mathur@uni- koeln.de; Fax: +49 221 4704899; Tel: +49 221 4705627 †Electronic supplementary information (ESI) available: The TG/DTAs associated with this article and supplementary crystal data for compounds (3), (4) and (9). CCDC reference numbers 809950–809951 and 853078. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt11668a hydrogen atoms by fluorine atoms in the ligand shell generally increases the volatility as shown for b-diketonate complexes. 16,17 Since the most commonly commercially available niobium alkoxide is Nb 2 (OEt) 10 , of which no crystal structure is known, one focus of this work was to broaden the horizon of well characterized niobium alkoxides for the use as starting materials to produce niobium containing materials. Some of the reported homolep- tic niobium(V) alkoxides include Nb 2 (OMe) 10 , 18 Nb 2 (O i Pr) 10 , 19 Nb 8 O 10 (OEt) 20 , 20 and Nb 2 (OCH 2 SiMe 3 ) 10 . 21 Furthermore, some efforts have been undertaken to modify Nb 2 (OEt) 10 with different chelating ligands to improve their properties for chemical vapour deposition. 9,14,15 In this work, neo-pentoxide, tert-butoxide and iso-propoxide derivatives of niobium were synthesized in combination with N- aromatic donor ligands and stabilizing chelating ligands to obtain new precursors with higher thermal stability during vaporization, higher chemical stability due to a six coordinated metal center, a low tendency to form dimers (high N–M–p bonding contribution) and increased volatility caused by a highly electronegative outer periphery owing to the presence of fluorine atoms was envisaged (Scheme 1). Herein, we report the synthesis and characterization of nine new heteroleptic alkoxides of niobium and elucidate their thermal and structural properties. This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 1981–1990 | 1981 Downloaded by University of Cologne on 30 January 2012 Published on 16 December 2011 on http://pubs.rsc.org | doi:10.1039/C1DT11668A View Online / Journal Homepage / Table of Contents for this issue