Efficient resolution of a dinuclear triple helicate by asymmetric extraction/precipitation with TRISPHAT anions as resolving agents

Tetradentate 1,2-bis[4-(4'-methyl-2,2'-bipyridyl)]ethane ligand (3) and Fe( NH4)(2)(SO4)(2). 6H(2)O combine in a 3:2 ratio to form the racemic helicate [Fe2L3](4+) (4), as reported by Elliott et al. We now show that the enanti omeric purity of 4 can be efficiently measured by H-1 NMR by the use of the TRISPHAT (1) salt as a chiral shift reagent. Large differences in chemical shifts (Delta Delta delta of up to 0.3 ppm, 20% [D-6]DMSO in CD3CN) are ob served between the enantiomers of 4 upon addition of [nBu(4)N][Delta -1]. T he resolution of 4 by asymmetric extraction was attempted: addition of an o rganic solution of [cinchonidinium][Delta -1] salt (2 equiv) to an aqueous solution of helicate 4-(SO4)(2) led, after vigorous stirring, to the extrac tion of the homochiral diastereomer [P-4][Delta -1](4), into the organic la yer along with the precipitation of the heterochiral diastereomer [M-4][Del ta -1](4) at the interface (diastereomeric ratio > 49:1 for both processes) . An enantioenriched fraction of [P-4][SO4](2) remained in the aqueous laye r. To obtain only two fractions of resolved helicate and develop this proce dure into an efficient resolution protocol, four equivalents of [cinchonidi nium][Delta -1] salt were used as the resolving agent. Chemically and diast er-eomerically pure [P-4][Delta -1](4) and [M-4][Delta -1](4) helicate salt s were then obtained in excellent yields.