Aromatic Imines in the Titanocene Coordination SphereTitanaaziridine vs 1‑Aza-2-titanacyclopent-4-ene Structures

Two independent synthetic routes to η²-imine titanocene complexes were developed. On one hand side, ligand exchange reactions of bis­(trimethylsilyl)­acetylene by (<i>p</i>-Tolyl)­HCNPh (<b>3</b>) employing the <i>Rosenthal</i> reagent Cp₂Ti­{η²-C₂(SiMe₃)₂} (<b>1</b>) lead to Cp₂Ti­{η²-(<i>p</i>-Tolyl)­CHNPh} (<b>5</b>), exhibiting a titanaaziridine structure. On the other hand, the direct reductive complexation of <b>3</b> by using Cp₂TiCl₂ (<b>2</b>) and Mg as reducing agent leads also to <b>5</b>, one of the rare known titanoceneaziridines without additional ligands. By using the ketimine (<i>p</i>-Tolyl)₂CNPh (<b>4</b>) instead of the aldimine <b>3</b>, an unexpected coordination mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene structure involving one tolyl fragment. In such a way, via the reductive complexation of <b>4</b>, employing <b>2</b> or Cp*TiCl₃ (<b>12</b>), the 1-aza-2-titanacyclopent-4-ene complexes <b>6</b> and <b>13</b> are formed. Density functional calculations at the M06-2X level identify these new complexes <b>6</b> and <b>13</b> as 1-aza-2-titanacyclopent-4-enes, in agreement with an analysis based on the experimental structural parameters. A theoretical study of the bonding between the titanocene fragment and the imine ligand reveals that steric factors are more pronounced for titanaaziridines and disfavor their formation compared to azatitanacyclopentenes. This provides a rationalization for the preferred formation of titanoceneaziridines in the case of aldimine ligands and azatitanacyclopentenes when ketimines are applied. Whereas titanoceneaziridine <b>5</b> undergoes insertion reactions into the Ti–C carbon σ-bond with aldehydes, ketones, or carbodiimides to the five-membered titanacycles <b>20</b> and <b>21</b>, complex <b>6</b> appears to be inert in comparable reactions