Hexacoordinated MIV (M=Ti, Zr, Hf) tetrachlorido complexes with chelating dithienylethane based 1,2 diketone ligand : π-conjugation as decisive factor for axial chirality mode

Abstract

1,2-bis(2,5-dimethylthiophen-3-yl)ethane-1,2-dione (1, DTEthane) reacts with MCl4 metal precursors of group four (M=Ti, Zr, Hf) via coordination of the carbonyl groups. The molecular structure of complex 2–4 were determined in scXRD studies in the solid state and characterized by means of multi-nuclear and multi-dimensional NMR spectroscopy in solution. While the resulting titanium complex [TiCl4(DTEthane)] 2 shows a monomeric structure, where 1 binds in a bidentate fashion, complexes with a Zr (3) and Hf (4) center have dimeric scaffolds in which the ligands adopt a bridging mode. Quantum chemical calculations using density functional theory (G16, B97D3/def2-TZVP) were used to evaluate the general trend of dimer formation (Ti<Zr<Hf). The molecular structures derived from both scXRD and the DFT optimized structures reveal the carbonyl groups in conjugation with the adjacent thiophene substituent. As a result, they are coplanar and rotation about the two C−C axes (C1−C7; C8−C9) is restricted allowing for only one chiral axis along C7−C8. This gains special importance with respect to previously described complexes carrying the closely related 1,2-endiolato ligand (1,2-bis(2,5-dimethylthiophen-3-yl)ethene-1,2-diolate), in which no coplanarity of the thiophene rings to their neighboring metallacycle was observed allowing for two chiral axes. Noteworthy, further DFT calculations addressing the pathway of racemization found transition states, which are characterized by contrary rotations of both thiophene rings and a loss of conjugation rather than a direct rotation around the axis C7−C8.