Synthesis, structure and functions of discrete titanium-based multinuclear architectures

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(OiPr)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.

A porous and photoactive Ti-MOF based on a novel tetranuclear [Ti2Tb2] cluster

A robust and porous titanium metal-organic framework (Ti-MOF; LCU-505) has been solvothermally synthesized based on an unprecedented tetranuclear Ti2(μ3-O)2Tb2(μ2-CH3COO)2(H2O)4(OOC-)8 cluster (abbreviated as [Ti2Tb2]) and tritopic 4,4',4''-s-triazine-2,4,6-triyl-tribenzoic acid ligand (H3TATB). LCU-505 shows remarkable water stability and permanent porosity for N2 and CO2 gas adsorption. Moreover, LCU-505 demonstrates n-type semiconductor behavior and good photocatalytic activity in the degradation of organic dyes.

Going Up the Ladder: Stacking Four 4,4'-Bipyridine Moieties within a Ti(IV)-Based Tetranuclear Architecture

Biphenol-based ligands have proven their ability to bind titanium(IV) centers and generate sophisticated self-assembled structures in which auxiliary nitrogen ligands often complete the coordination sphere of the metal and improve stability. Here, a central 4,4'-bipyridine, which acts as both a spacer and a source of monodentate nitrogen to complete the coordination sphere of the Ti(IV) complex, was incorporated within two bis-2,2'-biphenol strands, 3H₄ and 4H₄. Both proligands possess structural features that are well adapted to form self-assembled structures built from titanium-oxygen-nitrogen units; however, their different degrees of torsional freedom strongly influenced the nuclearity of the complexes formed. The presence of a phenyl spacer between the bipyridine and the biphenol moieties of 3H₄ provided enough flexibility for the ligand to wrap around one titanium(IV) center to form a mononuclear complex Ti(3)(DMF)₂ in the presence of dimethylformamide (DMF). Assembly of the more rigid ligand 4H₄ with Ti(OiPr)₄ afforded a tetranuclear complex Ti₄(4)₂(4H)₂(OEt)₂ containing four stacked 4,4'-bipyridine units as shown by the X-ray structure of the complex. Density functional theory studies suggested that the assembly of this tetrametallic complex involves a dimetallic intermediate with TiO₆ nodes that is converted to the thermodynamically stable tetranuclear complex with two TiO₆ nodes and two TiO₅N units with enhanced covalent character.