Role Played by Isopropyl Substituents in Stabilizing the Putative Triple Bond in Ar′EEAr′ [E = Si, Ge, Sn; Ar′ = C6H3‑2,6-(C6H3‑2,6-Pri2)2] and Ar*PbPbAr* [Ar* = C6H3‑2,6-(C6H2‑2,4,6-Pri3)2]
journal contributionposted on 05.08.2013 by Issaka Seidu, Michael Seth, Tom Ziegler
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A theoretical study of the bonding in ArEEAr (where E = Si, Ge, Sn, Pb; Ar = terphenyl ligand) revealed for the first time why bulky isopropyl substituents electronically are required in order to isolate stable ArEEAr species. This was accomplished by combining the natural orbitals for chemical valence (NOCV) method with the extended transition state (ETS) scheme. The NOCV–ETS analysis was based on two ArE fragments in their doublet ground state with the configuration σ2π1. For E = Si, Ge, and Sn, it revealed one π-bond perpendicular to the CEEC plane and two σ/π-type bonds in the plane, whereas the ArPbPbAr system was found to have a single σ bond with a C–Pb–Pb trans-bent angle close to 90°. While similar bonding pictures have been obtained in previous model studies with Ar = H and CH3, the NOCV–ETS scheme was able to obtain quantitative estimates for the strength of various σ/π components without artificial truncations or twisting of the system. More importantly, NOCV–ETS analysis was able to show that the electronic influence of the isopropyl substituents on the σ/π components differs little from that found in a system where they are replaced by hydrogen. Instead, the favorable role of the isopropyl substituents is due to dispersive van der Waals attractions between Pri groups on aryl rings attached to different E atoms as well as hyperconjugation involving donation into σ* orbitals on Pri. Dispersive interaction amounts to −27.5 kcal/mol (Si), −29.1 kcal/mol (Ge), −26.2 kcal/mol (Sn), and −44.0 kcal/mol (Pb). The larger dispersive stabilization for Pb reflects the fact that the longer Pb–Pb and Pb–C bonds sterically allow for more isopropyl groups with Ar = C6H3-2,6-(C6H2-2,4,6-Pri3)2. This is compared to the other elements where Ar = C6H3-2,6-(C6H3-2,6-Pri2)2. It is finally concluded from the analysis that real ArEEAr systems reveal little character of the EE bond in contrast to the findings of previous studies on model systems.