Toward a Better Understanding of the Reactivity of Titanium and Zirconium Complexes with an Aryl-Substituted Tripodal Triamido Ligand Derived from cis,cis-1,3,5-Triaminocyclohexane: A Density Functional Study

Tilley and co-workers have shown that titanium and zirconium hydride complexes containing the triamido [cis,cis-1,3,5-(3,5- t Bu2C6H3N)3C6H9]3− ligand (L) do not polymerize ethene. To get insight in this observation we have studied these systems and various modifications thereof using density functional theory. The reactivity toward ethene was investigated by computing the energy barriers for ethene insertion in the metal−hydride and metal−alkyl bonds of these compounds with the alkyl chain represented by a methyl or ethyl group. For titanium and zirconium complexes of L, the computational results agree with the experimental observation that insertion is possible in metal−hydride but not in metal−alkyl bonds. The energy barrier for insertion in the metal−alkyl bond can be lowered somewhat by reducing the steric bulk of the triamido ligand; however, even in the absence of any steric hindrance this barrier remains very high and no polymerization activity is expected. The ethene insertion barriers increase when reducing the rigidity of the triamido ligand by the introduction of methylene bridges between the coordinating nitrogen atoms and the cyclohexane backbone or replacing the cyclohexane-based ligand by three NH2 groups. The effect of changing the four-coordinate ligand environment of titanium to a five-coordinate one by adding an additional donor molecule (NH3) was found to be small. A significant lowering of the ethene insertion barriers in the metal−alkyl bonds was found only when creating a much more unsaturated metal center by removal of one of the −NR groups or replacement of one of these groups by a much weaker coordinating −OR group. Both the cationic d0 and neutral d1 variants of the latter compounds may be active polymerization catalysts. The calculated ease of ethene insertion is shown to be dominated by the electrophilicity of the metal center, quantified as the binding energy of the probe molecule ammonia, and roughly parallels the complexation energy of the ethene monomer.