Preparation and properties of S-bridged CoIIIAgI3CoIII pentanuclear complexes having a triple helical chirality.: Crystal structure of [Ag3{Co(aet)3}2](BF4)3 (aet = 2-aminoethanethiolate)

Novel S-bridged Co(III)Ag(I)3Co(III) pentanuclear complexes, [Ag-3{Co(aet)(3)}(2)](3+) (1) and [Ag-3{Co(L-cys-N,S)(3)}(2)](3-) (2), were prepared by reacting fac(S)-[Co(aet)(3)] (aet = 2-aminoethanethiolate) or Delta(LLL)-fac(S)-[CO(L-cys-N,S)(3)](3-) (L-cys= L-cysteinate) with Ag+ in a ratio of 2:3. The crystal structure of 1(BF4)(3) was determined by X-ray crystallography. 1(BF4)(3) . H2O, chemical formula C12H38B3N6OF12S6Co2Ag3, crystallizes in the orthorhombic space group Pnma with a = 16.397(2), b = 13.239(2), c = 15.803(4) Angstrom, V = 3430.5(9) Angstrom(3), Z = 4, and R = 0.066. In 1 the two thiolato S atoms from two octahedral fac(S)-[Co(aet)(3)] units coordinate to each of three Ag atoms, such that the three S-Ag-S linkages form a triple helical structure. I gave only racemic isomer, Delta Delta and Lambda Lambda, which were optically resolved by cation-exchange column chromatography, while the Delta(LLL)Delta(LLL) isomer was selectively formed for 2 with a retention of the absolute configuration of the starting Delta(LLL)-fac(S)-[Co(L-cys-N,S)(3)](3-). 1 was converted to the S-bridged tricobalt(III) complex [Co{Co(aet)(3)}(2)](3+) by a reaction with I- in the presence of Co2+, accompanied by an intermolecular exchange and racemization of the Delta/Delta-fac(S)-[Co(aet)(3)] units. The electronic absorption and CD spectral properties of 1 and 2 are also reported.