PREPARATION AND PROPERTIES OF S-BRIDGED (COAG3COIII)-AG-III-CO-I PENTANUCLEAR COMPLEXES HAVING A TRIPLE-HELICAL CHIRALITY - CRYSTAL-STRUCTURE OF [AG-3(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 pre pared 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 A g+ in a ratio of 2:3. The crystal structure of 1(BF4)(3) was determine d by X-ray crystallography. 1(BF4)(3) . H2O, chemical formula C12H38B3 N6OF12S6Co2Ag3, 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) A ngstrom(3), Z = 4, and R = 0.066. In 1 the two thiolato S atoms from t wo 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 stru cture. I gave only racemic isomer, Delta Delta and Lambda Lambda, whic h were optically resolved by cation-exchange column chromatography, wh ile the Delta(LLL)Delta(LLL) isomer was selectively formed for 2 with a retention of the absolute configuration of the starting Delta(LLL)-f ac(S)-[Co(L-cys-N,S)(3)](3-). 1 was converted to the S-bridged tricoba lt(III) complex [Co{Co(aet)(3)}(2)](3+) by a reaction with I- in the p resence of Co2+, accompanied by an intermolecular exchange and racemiz ation of the Delta/Delta-fac(S)-[Co(aet)(3)] units. The electronic abs orption and CD spectral properties of 1 and 2 are also reported.