Site-Selective Ligand Bridging among Multiple Internal Coordination Sites of a Metallomacrocycle and Its Conformational Regulation

A hexanuclear palladium complex of hexapappossessing sixinternal coordination sites can take two different conformations, Alternate and Twisted, depending on theguests. Linear ditopic & alpha;,& omega;-diamines are captured in threedistinct cross-linking modes, and regulations between the two macrocyclicconformations have been realized by the lengths of the diamines. Furthermore,the heteroleptic site-selective bridging of two kinds of diamineshas been achieved. Metallomacrocycles with internal coordination sites havea highpotential to precisely control the positions of the guest ligandsand the overall shape of the assemblies by utilizing the directionalityand reversibility of the coordination bonds. However, when such coordinativehosts possess multiple coordination sites, it was difficult to controlto which coordination sites the incoming guest ligands bind, becausesuch systems often result in a random, uncontrolled mixture. The metallomacrocyclethat we now report, a hexanuclear palladium complex of hexapap possessingsix internal coordination sites, can take two different conformationsdepending on the guests. One is an Alternate conformation,in which six coordination sites of pap alternatively point to Up-Down-Up-Down-Up-Down.The other is a Twisted conformation, in which thecoordination sites direct Up-Middle-Down-Up-Middle-Down. Interestingly,linear ditopic & alpha;,& omega;-diamines are captured in three distinctcross-linking modes, and regulations between the two macrocyclic conformationshave been realized by the lengths of the diamines. Furthermore, theheteroleptic site-selective bridging of two kinds of diamines hasbeen achieved. It has been demonstrated that a slight difference inthe diamine lengths leads to a significant change in the structureand selection of the produced host-guest macrocyclic complexes.