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 (C
1 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 (C
3 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(O
i
Pr)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 (C
3-[Ti3L3(A)3]/C
1-[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 C
1-[Ti3L3(A)3]/C
3-[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.