Self-Assembled E2L3 Cryptands (E = P, As, Sb, Bi): Transmetalation, Homo- and Heterometallic Assemblies, and Conformational Isomerism

A series of Group 15-containing homometallic (E2L3, E = P, As, Sb, Bi) and heterometallic (AsSbL3, AsBiL3, PSbL3) supramolecular cryptands were prepared by the self-assembly of pnictogen halides with dithiolate ligand or by direct transmetalation from a heavier congener. Structural characterization by single crystal X-ray diffraction shows that the E−S bond distances and S−E−S bond angles are significantly affected by the identity of the pnictogen. 1H NMR spectroscopy reveals that the homometallic cryptands are dynamic in solution: surprisingly one ligand “flips”, perturbing the C3 symmetry of the complex and giving a new asymmetric conformer. Density functional theory calculations were carried out on both the symmetric and the asymmetric conformations of the cryptands, and the energies were compared to those observed by NMR spectroscopy. It was found that the relative stability of the asymmetric cryptand to its symmetric conformer increases with increasing size of the Group 15 element. Finally, it is reported that if two metals are present during the self-assembly process, heterometallic cryptands form. These supramolecular cryptands are reminiscent of their organic analogues, but result from a self-assembly process rather than a stepwise synthesis. Surprisingly, they possess conformational isomerism and exhibit dynamic transmetalation in their reactivity which provides access to otherwise unattainable assemblies.