Optically-active tris(O,O′-dialkyl dithiophosphato-S,S′)chromium(III) complexes. Isolation of diastereoisomers and isomerization reactions

Two optically-active O,O′-dialkyl dithiophosphate ligands, (Bor)2PS2 − and (Men)2PS2 −, were prepared from P2S5 and the monoterpene alcohols, d-borneol and l-menthol. The chromium(III) complexes, [Cr{(Bor)2PS2}3] and [Cr{(Men)2PS2}3], were prepared by reaction with Cr(NO3)3·9H2O in acetone. The diastereoisomers (Δ and Λ) were separated by fractional crystallization of the complexes from toluene ([Cr{(Bor)2PS2}3]) or acetone ([Cr{(Men)2PS2}3]). The (−)680 CD-Δ-[Cr{(Bor)2PS2}3] isomer was obtained from the less soluble part. This isomer shows extremely strong CD, Δε (680 nm, toluene)=−15.0 M−1 cm−1. The Δ-[Cr{(Men)2PS2}3] isomer could not be obtained in the optically-pure state, since the complex inverts appreciably during the recrystallization procedure. Activation parameters for isomerization (epimerization), Δ→Λ and Λ→Δ were determined for both complexes, [Cr{(Bor)2PS2}3] and [Cr{(Men)2PS2}3]. [Cr{(Men)2PS2}3] isomerizes much faster than [Cr{(Bor)2PS2}3].