A Remarkable Solvent Effect on the Nuclearity of Neutral Titanium(IV)-Based Helicate Assemblies

The spontaneous self-assembly of a neutral circular trinuclear Ti-IV-based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n-pentane into a solution with dichloromethane. The circular helicate has been characterized by using single-crystal X-ray diffraction study, C-13 CP-MAS NMR and (HNMR)-H-1 DOSY solution spectroscopic, and positive electrospray ionization mass-spectrometric analysis. These analytical data were compared with those obtained from a previously reported double-stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double-stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double-stranded helicates was characterized by H(toluene)=-30kJmol(-1) and S(toluene)=+357JK(-1)mol(-1), whereas these values were H(CH2Cl2)=-75kJmol(-1) and S(CH2Cl2)=-37JK(-1)mol(-1) for the ring helicate. The transformation of the ring helicate into the double-stranded helicate was a strongly endothermic process characterized by H(CH2Cl2)=+127kJmol(-1) and H(n-pentane)=+644kJmol(-1) associated with a large positive entropy change S=+1115JK(-1)mol(-1). Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n-pentane. Suggestions for increasing the stability of the ring helicate are given.