Synthetic methodology allowing the interconversion of titanium-oxygen single bonds and double bonds: The self-assembly of bridging and terminal oxotitanium(IV) into oligomeric and polymeric linear titanoxanes

The controlled ionization of the linear [Cl-Ti-O-Ti-Cl] skeleton allowed the generation of the [Cl-Ti-O = Ti]+ dimer, which is nonsymmetrical as a consequence of extended Cl-Ti-O-π interactions. The [Ti = O] unit thus formed is a building block for a variety of titanoxane structures. This chemistry has been investigated from a theoretical point of view by ab initio MO analysis of the [Cl-Ti-O-Ti-Cl] and [Cl-Ti-O = Ti]+ fragments. These calculations lead to the conclusion that single ionization generates the [Ti = O] unit, whereas double ionization does not affect the μ-oxo bonding mode in [Ti-O-Ti]2+ or [S-Ti-O-Ti-S]2+ (where S is a pure σ-donor ligand or solvent). This observation has been confirmed experimentally by ionizing the following model complexes: [(Cl)(acacen)-Ti O Ti(acacen)(Cl)] (3) (acacen = N,N'-ethylenebisacetylacetoneiminato dianion) and [(Cl)(salen)Ti-O-Ti (salen)-(Cl)] (4) (salen = N,N'-ethylenebissalicylideneiminato dianion), where the linear Cl Ti O Ti Cl unit is assured by the square-planar bonding mode of the tetradentate Schiff base ligand. The double ionization of 3 with AgNO3 gave the conventional μ-oxo derivative [(acacen)(η1-ONO2)Ti O Ti(acacen)(η1-ONO2)] (5). In contrast, the stepwise ionization of 3 and 4 with NaBPh4 in THF led to the nonsymmetrical [Cl-Ti-O=Ti]+ intermediates, which are the parent compounds for a variety of linear titanoxanes. The following species containing a Ti=O unit have been isolated from the NaBPh4- assisted ionization of 3: [(acacen)Ti=O BPh3] (6) and [(L)(acacen)Ti=O-Ti (acacen) O(acacen)Ti-O = Ti(acacen)(L)]2+ 2 BPh4 (L = THF, 7; L = none, 8). The same reaction carried out on 4 led to [(THF)-(salen) Ti=O-Ti(salen) O (salen) Ti-(THF)]2+ 2 BPh4 (9) and [(L)(salen)-Ti=O Ti(salen) O (salen)Ti- O=Ti-(salen)(L)]2+ 2 BPh4 (L = THF, 10; L = Py, 11; L = none, 12, polymeric form). A scheme is proposed to explain the formation of the species derived from the single ionization of 3 and 4, where the origin and the binding properties of the [Ti=O] unit play a major role.