Contiguous multiple π-coordination of π-conjugated polyenes: bonding nature and charge delocalization behaviour of polyene–(palladium chain) sandwich clusters

Abstract

The contiguous multiple π-coordination (CMπC) bonds are formed at the molecular interface between the extended π-conjugated sp²-carbon framework and metal–metal moieties. However, the static properties including the bonding nature of the CMπC bonds have not been well-understood. Here we report the charge delocalization behavior and bonding nature of the polyene–(palladium chain) sandwich clusters. We observed the chain-length dependent ¹³C NMR chemical shifts of the terminal phenyl groups in [Pd_m{Ph(CHCH)_m+1Ph}₂][B(Ar^F)₄]₂ (m = 3, 4, 5, 6), showing the efficient delocalization of positive charge over the polyene ligands. Furthermore, the detailed molecular orbital analysis of several model clusters provides insights into the bonding nature of the μ-η²:(η²:)_nη²- and μ-η³:(η²:)_nη³-poylene palladium chain sandwich clusters, where the former type is attributed to the corner-sharing Pd array and the latter to the edge-sharing/corner-sharing hybrid Pd array. The analogy of the μ-η³:(η²:)_nη³-type with the carbonyl-bridge coordination is discussed. The Pd string in each mode of sandwich chain clusters has a weak M–M bonding interaction.