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Ligand isomerism in coordination cages: to interlock or not to interlock
Moumita Sarkar1, Ayan Ghosh1, Shobhana Krishnaswamy1
1IoE Center of Molecular Architecture, Department of Chemistry, Indian Institute of Technology Madras Chennai 600036 India dillip@zmail.iitm.ac.in.
None:
The combination of Pd(ii) with a set of three regioisomeric neutral bis-monodentate ligands in DMSO medium, in separate pots, yielded Pd2L4-type cages, depicting ligand isomerism in coordination cages (isomer-1, -2 and -3). These tetra-cationic cages, having spacious cavities and wide windows, exhibited differential supramolecular behaviours in the solution state upon aging in terms of remaining intact as a Pd2L4-type cage or undergoing the interlocking of a pair to form a (Pd2L4)2-type interlocked cage. One of the isomeric cages (isomer-3) could form the corresponding (Pd2L4)2-type interlocked cage when parameters like counter anion, concentration, and time were optimized. However, isomer-1 and -2 remained intact as Pd2L4-type cages irrespective of parameter variation. For isomer-3, the counter anion of choice either exclusively favoured (for NO3 -) or largely favoured (for BF4 - or ClO4 -) interlocking or arrested the cage (for OTf- or OTs-) with no interlocking. The compatibility of selected anions with the newly created pair of peripheral cavities of the interlocked cage and other supramolecular interactions could overcome the entropy penalty of interlocking. Additional special features of the interlocked cage include triply interlocked architecture instead of quadruply; a relatively longer central cavity (1.8 times that of the peripheral cavities) instead of being equal sized with the peripheral cavities; and the presence of solvent molecules in the central cavity of the interlocked cage instead of accommodating counter anions.
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