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

AbstractThe spontaneous self‐assembly of a neutral circular trinuclear TiIV‐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, 13C CP‐MAS NMR and 1H NMR 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 ΔH(toluene)=−30 kJ mol−1 and ΔS(toluene)=+357 J K−1 mol−1, whereas these values were ΔH(CH2Cl2)=−75 kJ mol−1 and ΔS(CH2Cl2)=−37 J K−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 ΔH(CH2Cl2)=+127 kJ mol−1 and ΔH(n‐pentane)=+644 kJ mol−1 associated with a large positive entropy change ΔS=+1115 J K−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.