Palladium(II) Weak-Link Approach Complexes Bearing Hemilabile N-Heterocyclic Carbene-Thioether Ligands

A new class of homoligated palladium(II) weak-link approach (WLA) complexes bearing hemilabile N-heterocyclic carbene (NHC)-thioether ligands is reported that, unlike previous tweezer-like WLA complexes, expand and contract in a linear fashion when switching between configurational states. These complexes can be chemically switched between a trans open state and a trans closed state via the addition or subsequent extraction of Cl-. These bis(NHC) complexes also display unusual isomerization behavior. For example, an NMR spectroscopic investigation into the solution state configuration of the open complex reveals the presence of interconverting syn,trans and anti,trans isomers, and a kinetic study shows that the barrier is large enough to isolate, store, and study the anti,trans isomer at room temperature. Notably, the linker length between the NHC and thioether moieties can be tailored with additional -CH2- groups, which affords considerable control over the geometric changes imposed by switching. Therefore, this class of complexes may be useful in the construction of allosterically regulated supramolecular assemblies and materials.