Regioselective Catenation-of Dinuclear Palladium and Platinum Metallocycles Promoted by π•••π Interactions

Dinuclear metallocycles were assembled from an L-shaped bidentate ligand based on 4,4'-bipyridine and palladium or platinum square-planar cis complexes. The palladium (4a,b) or platinum (5a,b) square metallocycles were obtained as 1:1 regioisomeric mixtures depending on which pair of coordinative nitrogen atoms binds to the metal center. The squares display a pi-deficient cavity suitable to incorporate two pi-donor aromatic systems. Therefore, the reaction with macrocyclic polyethers (BPP34C10 or DN38C10) resulted in the regioselective self-assembly of the [3]catenanes 4a(BPP34C10 or DN38C10)(2)center dot 6PF(6) and 5a(BPP34C10 or DN38C10)(2)center dot 6PF(6) because only macrocycles 4a and 5a present the correct disposition of the pi-deficient aromatic systems to maximize the;pi center dot center dot center dot pi stacking interactions. Single-crystal X-ray analyses of [3]catenanes revealed that the structures are additionally stabilized by [C-H center dot center dot center dot O], [N-H center dot center dot center dot O] bonds and [C-H center dot center dot center dot pi] interactions. The 1:2 inclusion complexes of metallocycles were prepared by self-assembly of three components: the ligand 1-(4-(pyridin-4-yl)benzyl)-4,4'-bipyridin-1-ium, a square planar complex M(en)(NO3)(2) (M = Pd or Pt, en = ethylenediamine), and a dioxoaromatic guest in a 2:2:2 ratio. The comparative study of the formation of 1:2 inclusion complexes has allowed us to conclude that the pi center dot center dot center dot pi interactions between the host and guests are responsible for the observed regioselectivity.