Formate complexes of titanium(iv) supported by a triamido-amine ligand

Abstract

The terminal formate complex [(OCHO)Ti(N₃N)] (3) containing the trianionic triamido-amine ligand (Me₃SiNCH₂CH₂)₃N³⁻ (N₃N) was prepared via salt metathesis of [ClTi(N₃N)] (1) with sodium formate or alternatively by treatment of the alkyl complex [ⁿBuTi(N₃N)] (2) with ammonium formate [HNEt₃][OCHO]. Deprotonation of 3 with potassium hexamethyldisilazide gave a polymeric helical chain of the oxo complex {K[OTi(N₃N)]}_n (4). Reaction of 2 with the trityl salt [Ph₃C][B(3,5-Cl₂C₆H₃)₄] or the Brønsted acid [HNEt₃][B(C₆F₅)₄] gave [(Et₂O)Ti(N₃N)][BR₄] (6[BR₄]·Et₂O) with R = 3,5-Cl₂C₆H₃ or C₆F₅. The diethyl ether ligand was easily replaced by other L-type donor ligands such as tetrahydrofuran, pyridine, and 4-dimethylaminopyridine to give 6[BR₄]·L with L = thf, py, and dmap. Reaction of 6[BR₄]·Et₂O with a stoichiometric amount of CO₂ gave the dimeric, dicationic bis(carbamate)-bridged complexes [Ti{N(CH₂CH₂NSiMe₃)₂(CH₂CH₂NSiMe₃(μ-CO₂-ηO:ηO′))}]₂[BR₄]₂ (7[BR₄]₂) through insertion of one CO₂ into one of the titanium-amido bonds. Addition of pyridine to 7[B(C₆F₅)₄]₂ formed the monomeric carbamate complex [(py)Ti{((O₂C-κ²O,O′)NSiMe₃CH₂CH₂)N(CH₂CH₂NSiMe₃)₂}][B(C₆F₅)₄] (8[B(C₆F₅)₄]·py). The cationic formate-bridged species [(Ti(N₃N))₂(μ-OCHO-ηO:ηO′)][BR₄] (10[BR₄]) readily formed when the terminal formate complex 3 was reacted with the cationic 6[BR₄]. The reactivity of triamido-amine stabilized titanium(IV) complexes is shown to differ considerably from that of related titanium tris(anilide) complexes.