The Stereoselective Self-Assembly of Chiral Metallo-Organic Cryptophanes

Cryptophanes are macropolycyclic cyclophanes constructed from two triply bridged concave cyclotriveratrylene analogues that encapsulate a large range of molecular and monoatomic substrates. Self-assembled metallo-organic cryptophanes based on M2+-carbonitrile (M = Pd or Pt) interactions have been obtained through the reactions of chiral nitrile-substituted cyclotribenzylenes (CTBs) 1 and 2 with [M(dppp)][OTf](2) (dppp = 1,3-bisdiphenylphosphinopropane, OTf = triflate) in 2:3 ratios in chlorinated solvents. The cryptophanes [Pd-3(dppp)(3)(1)(2)](6+) and [Pd-3(dppp)(3)(2)(2)](6+) were obtained exclusively in the chiral anti form in solution at room temperature whatever solvent was used; however the meso,syn form was identified as the major isomer of [Pd-3(dppp)(3)(1)(2)](6+) at low temperature in CD2Cl2, and - in the case of [Pt-3(dppp)(3)(1)(2)](6+) - it formed in a minor amount at room temperature. [Pd-3(dppp)(3)(1)(2)][OTf](6) and [Pt-3(dppp)(3)(1)(2)][OTf](6) crystallized in the anti forms with an encapsulated chloroform molecule, and gas-phase DFT calculations with the dispersion-corrected B97-D3 functional show that it is stabilized by 34.6 kJmol(-1). Interestingly, the heteroleptic cryptophane [Pd-3(dppp)(3)(1)(2)](6+) was not detected in mixtures of [Pd(dppp)](2+) and CTBs 1 and 2 in a 3:1:1 ratio or through the equilibration of a 1:1 mixture of the preformed homoleptic cryptophanes; this points to favored self-sorting processes.