Circular Heterochiral Titanium-Based Self-Assembled Architectures

Circular trinuclear helicates have been synthesized from a bis-biphenol strand (LH₄), titanium isopropoxide, and various diimine ligands. These self-assembled architectures constructed around three TiO₄N₂ nodes have a heterochiral structure (C₁ symmetry) when 2,2'-bipyridine (A), 4,4'-dimethyl-2,2'-bipyridine (B), 4,4'-bromo-2,2'-bipyridine (C), or 4,4'-dimethyl-2,2'-bipyrimidine (D) is employed. Within these complexes, one nitrogen ligand is endo-positioned inside the metallo-macrocycle, whereas the other two diimine ligands point outside the helicate framework. This investigation highlights that the nitrogen ligand which does not participate in the helicate framework of the complex controls the overall symmetry of the helicate since the 2,2'-bipyrimidine chelate (F) ends in the formation of a homochiral aggregate (C₃ symmetry). The lack of symmetry found in the solid state for the trinuclear species ([Ti₃L₃(B)₃], [Ti₃L₃(C)₃], and [Ti₃L₃(D)₃]) is observed for these complexes in solution (dichloromethane or chloroform). Remarkably, the 2,2'-bipyrazine ligand (ligand E) ends in the formation of a hexameric aggregate formulated as [Ti₆L₆(E)₆], whereas the use of 4,4'-dimethyl-2,2'-bipyrimidine (ligand D) permits to generate the dinuclear complexes ([Ti₂L(D)₂(OPr)₄] and [Ti₂L₂(D)₂]) in addition to the trimeric structure [Ti₃L₃(D)₃]. The behavior of [Ti₃L₃(A)₃] in solution, on the other hand, is unique since an equilibrium between the homochiral and the heterochiral form is reached within 17 days after the complex has been dissolved in dichloromethane (C₃-[Ti₃L₃(A)₃]/C₁-[Ti₃L₃(A)₃] ratio = 0.3). In chloroform, the heterochiral form of [Ti₃L₃(A)₃] is stable for the same period of time, evidencing the dependence of this stereochemical transformation toward the solvent medium. The thermodynamic and kinetic parameters linked to this stereochemical equilibrium have been obtained and point to the fact that the transformation is intramolecular and not induced by the presence of external ligands. The thermodynamic constant of the C₁-[Ti₃L₃(A)₃]/C₃-[Ti₃L₃(A)₃] equilibrium is found to be K = 0.34 ± 10%. Further evidence to rationalize this solvent-induced symmetry switch is obtained via a DFT calculation and classical molecular dynamics. In particular, this computational investigation elucidates the reason why the stereochemical transformation of a heterochiral architecture into a homochiral structure is possible only for a trinuclear assembly containing ligand A.