Synthesis of a Bis(thiophenolate)pyridine Ligand and Its Titanium, Zirconium, and Tantalum Complexes

A precursor to a new tridentate LX2 type ligand, bis­(thiophenol)­pyridine ((SNS)­H2 = (2-C6H4SH)2-2,6-C5H3N), was prepared. Bis­(thiophenolate)­pyridine complexes of Ti, Zr, and Ta having dialkylamido coligands were synthesized and structurally characterized. The zirconium complex (SNS)­Zr­(NMe2)2 (4) displays C2 symmetry in the solid state, unlike a related bis­(phenolate)­pyridine compound, Cs-symmetric (ONO)­Ti­(NMe2)2. This change is likely the result of strain about the sulfur atom in the six-membered chelate with longer metal–sulfur and carbon–sulfur bonds. Solid-state structures of tantalum complexes (SNS)­Ta­(NMe2)3 (5) and (SNS)­TaCl­(NEt2)2 (6) also display pronounced C2 twisting of the SNS ligand. 1D and 2D NMR experiments show that 5 is fluxional, with rotation about the Ta–N­(amide) bonds occurring on the NMR time scale that interchange the equatorial amide methyl groups (ΔG393 = 25.0(3) kcal/mol). The fluxional behavior of 6 in solution was also studied by variable-temperature 1H NMR. Observation of separate signals for the diastereotopic protons of the methylene unit of the diethylamide indicates that the complex remains locked on the NMR time scale in one diastereomeric conformation at temperatures below −50 °C, fast rotation about the equatorial amide Ta–N bonds occurs at higher temperature (ΔG393 = 13.4(3) kcal/mol), and exchange of diastereomeric methylene protons occurs via inversion at Ta that interconverts antipodes (ΔG393 ≈ 14(1) kcal/mol).