Enantiomerization of Chiral Uranyl-Salophen Complexes via Unprecedented Ligand Hemilability: Toward Configurationally Stable Derivatives.

[摘要]：In the search for configurationally stable inherently chiral uranyl-salophen complexes, the newly synthesized compd. UO2(L1) (3, H2L1 = I: R1 = R2 = R3 = H, R4-R5 = -(CH2)12-) featuring a dodecamethylene chain was expected to be a promising candidate. Unexpectedly, dynamic HPLC on a enantioselective column showed that it still undergoes enantiomerization at high temp. By comparison with the dynamic behavior of compds. UO2(L2) (4, H2L2 = I: R1 = Bu-t; R2 = R3 = H, R4-R5 = -(CH2)12-) and UO2(L3) (5, H2L3 = I: R1 = H; R2-R3 = -CH:CH-CH:CH-, R4-R5 = -(CH2)12-) the enantiomerization rate is independent of the size of the ligand. This finding definitely rules out a jump rope-type mechanism for the enantiomerization process and points to reaction pathways involving preliminary rupture of one of the O鬃譛 coordinative bonds. This provides unprecedented evidence of the occurrence of ligand hemilability in metal-sal(oph)en complexes. Such findings inspired the synthesis of compd. UO2(L4) (6, H2L4 = I: R1 = R2 = endowed with a more rigid spacer, i.e., that derived from 4,4'-(1,4-phenylenediisopropylidene)bisphenol. DHPLC studies showed that the new structural motif imparts a higher configurational stability, thus raising the half-life for the enantiomerization to >2 mo at room temp. This compd. represents the 1st member of a new class of inherently chiral receptors, whose potential in chiral recognition and catalysis now can be feasibly explored.

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