Mismatched Interactions in Bis(phenolato) Metal Complexes: Insights from Experimental Charge Density

Bis(phenolate) complexes of the early transition metals, such as titanium, are precatalysts for olefin polymerization. The ligand structure has a strong influence on the stereochemistry of the resulting polymer. For this reason, the bis(phenolate) ligands have been tuned by different substituents on the aromatic rings (R1 and R2) as well as by modifying the bridging unit. S atoms as part of the bridge can improve the catalytic activity, obviously as a result of hemilabile interactions. However, little is known about the bonding situation between the central metal and the ligands' sulfur atoms in these complexes. To clarify the bonding situation between these, according to the HSAB principle, mismatched atoms, bis(phenolato)titanium complexes as model compounds for active catalysts in the homogeneous olefin polymerization were synthesized and examined with high resolution Xray single crystal analysis.With the cyclohexyl bridged ligand a system was found which renders the base frame of the active catalysts. These model compounds do not comprise sterically demanding or disordering groups, which prevent a good resolution in single crystal X-ray analysis. It was possible to get high quality crystals of two different bis(phenolato)titanium complexes and one of the corresponding ligands.Furthermore a simplified cyclohexyl bridged salen ligand was used for complexation with titanium tetrachloride. The first crystal structure for dichlorido-N,N'-bis(salicylidene)-1,2cyclohexanediamino-titanium(IV) is reported here.Based on high-resolution data sets from the single crystal X-ray analysis, a multipole refinement based on the Hansen-Coppens multipole formalism was performed. The multipole refinement leads to an aspherical electron density, which enables the inspection of the bonding conditions in the molecule. With the aspherical electron density, a topological analysis of the bonding states was conducted.