Formate complexes of tri- and tetravalent titanium supported by a tris(phenolato)amine ligand

Abstract

Titanium(III) and titanium(IV) formate complexes supported by the sterically encumbering tris(phenolato)amine ligand (H₃(O₃N) = tris(4,6-di-tert-butyl-2-hydroxybenzyl)amine) are described. Salt metathesis of the chlorido precursor [(O₃N)TiCl] (1-Cl) with sodium formate in a 2 : 1 ratio in THF gave a dimer of sodium dititanium triformate units with 12-membered ring [Na{(O₃N)Ti}₂(μ-OCHO-ηO:ηO′)₃]₂ (3-Na) when crystallized from acetonitrile. Complex 3-Na was also prepared by reacting the previously reported terminal formate complex [(O₃N)Ti(OCHO)] (2) with excess sodium formate. Salt metathesis of 1-Cl with potassium formate gave a tetratitanium cluster of the composition [K₃{(O₃N)Ti}₄(OCHO)₇] (3-K) which can be also obtained by treating 2 with potassium formate. In 3-K both Ti and K centers are six-coordinate. The titanium(III) complexes [(O₃N)Ti(L)] (4-L, L = THF, THP, Et₂O) and solvent free dimeric [(O₃N)Ti]₂ (5) were synthesized by reduction of 1-Cl with sodium sand or magnesium in THF, THP, Et₂O, and n-pentane, respectively. The tert-butyl formate adduct of titanium(III)-[(O₃N)Ti(^tBuOCHO)] (6) was isolated by reacting 4-L or 5 with tert-butyl formate. Complex 6 is thermolabile and slowly decomposed in solution to produce a formate-bridged mixed-valence titanium(III)/titanium(IV) complex [{(O₃N)Ti}₂(μ-OCHO-ηO:ηO′)] (7) which further decomposed to a mixture containing 2, [(O₃N)Ti(OH)] and [(O₃N)Ti–O–Ti(O₃N)]. All new complexes were isolated in moderate to good yields and fully characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, and single crystal X-ray diffraction analysis. For the titanium(III) complexes solution magnetic moments were measured by the Evans method and EPR spectra recorded as toluene glass at 77 K.