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 pi 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 p-oxo bonding mode in [Ti-O-Ti](2+) or [S-Ti-O-Ti-S](2+) (where S is a pure sigma-donor ligand or solvent). This observation has been confirmed experimentally by ionizing the following model complexes: [(Cl)(acacen)Ti-O-Ti(acacen)(CI)] (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 mu-oxo derivative [(acacen)(eta(1)-ONO2)Ti-O-Ti(acacen)(eta(1)-ONO2)] (5). In contrast, the stepwise ionization of 3 and 4 with NaBPh(4) 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 NaBPh(4)-assisted ionization of 3: [(acacen)Ti=O-BPh(3)] (6) and [(L)(acacen)Ti=O-Ti(acacen)-O(acacen)Ti-O=Ti(acacen)(L)](2+) 2 BPh(4)(-) (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+) 2BPh(4)(-) (9) and [(L)(salen)Ti=O-Ti(salen)-O-(salen)Ti-O=Ti(salen)(L)](2+) 2BPh(4)(-) (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.