Topographic Steric Maps and Noncovalent Interactions of Dinuclear Silicon-Bridged Bis(indenyl)zirconocene Cations: Insights from DFT.

Dinuclear zirconocene cations such as [(Cp2ZrMe)2(μ-Me)]+ are adducts of a cationic monomethyl zirconocene and a neutral dimethyl zirconocene joined by a bridging methyl (μ-Me) group and act as dormant species in equilibrium with the active mononuclear catalyst. Using density functional theory at the M06-2X/def2-TZVP level with a polarizable continuum (PCM) toluene solvent model, we examined four configurational isomers of the silicon-bridged bis-(indenyl) complex [(Ind2ZrMeSiMe2)2(μ-Me)]+ (Ind = indenyl): cis-RS, cis-RR, trans-RS, and trans-RR. The relative stability follows the order cis-RS > trans-RS > trans-RR > cis-RR, with the heterochiral (RS) isomers more stable than the homochiral (RR) isomers in both the cis and trans series. The stability ordering tracks with a larger buried volume around the μ-Me bridge, a more negative charge on the bridging methyl, and a larger HOMO-LUMO gap, although these are reported as exploratory trends given the small set of isomers. The most stable isomers adopt near-perpendicular torsion angles between the metal centers and Zr-Zr distances of about 4.8 Å. Topographic steric maps show pronounced hindrance around the methyl bridge, and a noncovalent interaction analysis identifies attractive, repulsive, and van der Waals regions whose balance accompanies the stability ordering. Because olefin insertion requires dissociation of the adduct, these results bear on catalyst behavior through the stability of the dormant species and the ease of its dissociation, clarifying how steric and electronic factors shape the energy landscape of these μ-Me-bridged bimetallic complexes.