Entropically driven self-assembly of a strained hexanuclear indium metal-organic macrocycle and its behavior in solution

The self-assembly of a polyprotic pentadentate ligand, N-cyclopentanoylaminobenzoylhydrazide (H4L4), and an In(III) nitrate hydrate in methanol led to a strained hexanuclear indium metal-organic macrocycle (In-MOM), [In(III)(6)(H2L4)(6)(NO3)(x)(solvent)(6-x)](NO3)(6-x) (where, the solvent is either methanol or a water molecule and x is the number of the nitrate anions ligated). The ligand in the doubly deprotonated state serves as an unsymmetric linear ditopic donor and the alternating indium ions in two different chelation modes serve as two different bent ditopic metal acceptors, which led to a D-3-symmetric hexanuclear In-MOM. Although the hexanuclear In-MOM is enthalpically unfavorable because of the ring strain, the combination of the soft coordination characteristic of the indium ion and the slight ligand deformation from the conjugated planar conformation allows the formation of the entropically favored hexanuclear In-MOM rather than the enthalpically favored octanuclear In-MOM. While the hexanuclear In-MOM is stable in acetonitrile, it partially dissociates into its components in dimethylsulfoxide, and then slowly reaches a new equilibrium state with several different indium species yet to be identified in addition to the free ligand.