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 (ΔG⧧393 = 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
(ΔG⧧393 = 13.4(3) kcal/mol), and exchange of diastereomeric methylene
protons occurs via inversion at Ta that interconverts antipodes (ΔG⧧393 ≈
14(1) kcal/mol).