First Diastereomerically Controlled Aggregation ofL-Cysteinato Cobalt(III) Octahedra, Assisted by Silver(I) Ions

Abstract

During the past decade considerable progress has been made in the design and creation of supramolecular systems. [1] While the majority of supramolecular species have been constructed from organic ligands and metal ions by spontaneous self-assembly, our strategy is to use metal thiolate complexes as building blocks which can aggregate by forming S-bridged structures with a variety of metal ions. [2] In particular, we are interested in the aggregation of tris(chelate)-type octahedral complexes with simple bidentate N,S-thiolate ligands such as 2-aminoethanethiolate (aet NH 2 CH 2 CH 2 S À ) and l-cysteinate (l-cys NH 2 CH(COO À )CH 2 S À ), with the aim of creating chiral supramolecular architectures. [3] To date, a number of chiral aggregates based on the octahedral [M(aet) 3 ] unit (M Co III , Rh III , Ir III ) with three thiolate donors, such as trinuclear [Co III {M(aet) 3 } 2 ] 3 , [4] pentanuclear [Ag I 3 {M(aet) 3 } 2 ] 3 , [5] and octanuclear [Zn II 4 O{M(aet) 3 } 4 ] 6 , [6] as well as analogous aggregates based on the [M(l-cys-N,S) 3 ] 3À unit, have been prepared, and their unique stereochemical, spectroscopic, and redox properties have been extensively studied. In contrast, the aggregation of octahedral complex units with a single thiolate donor has been studied far less; only a few trinuclear and tetranuclear species such as [Co(aet)(en) 2 ] 2 or [Co(SCH 2 COO)(en) 2 ] (en NH 2 CH 2 CH 2 NH 2 ) have been reported. [7] To expand the range of chiral supramolecular architectures based on octahedral complex units with thiolate donors, it is desirable to find the key factors that control their aggregation by studying the fundamental [Co III (thiolato-S)(amine-N) 5 ] system. We therefore started by investigating the aggregation of [Co(aet)(en) 2 ] 2 and [Co(l-cys-N,S)(en) 2 ] assisted by metal ions. Recently, we found that the reaction of racemic [Co(aet)(en) 2 ] 2 with AgNO 3 in a 1:1 ratio gives an S-bridged Co III Ag I coordination polymer having a onedimensional infinite zigzag chain structure, namely, {[Ag{Co(aet)(en) 2 }](NO 3 ) 3 } I . [8] Interestingly, the chain structure discriminated the chiral configuration (L or D) of [Co(aet)(en) 2 ] 2 to give exclusively the enantiomeric (L-Co III Ag I ) I and (D-Co III Ag I ) I chains, which separate from one another as homochiral crystals. Given this result, we expected that similar (Co III Ag I ) I chiral chain structures would be formed on using the L L or D L diastereomer of [Co(l-cys-N,S)(en) 2 ] instead of racemic [Co(aet)(en) 2 ] 2 . Indeed, we found that the reactions of L L -or D L -[Co(l-cys-N,S)(en) 2 ] with AgNO 3 lead to the formation of fascinating chiral supramolecular structures, which are markedly dependent on whether the L L or D L diastereomer is used (Scheme 1).Treatment of a dark brown aqueous solution of L L -[Co-(l-cys-N,S)(en) 2 ](ClO 4 ) [9] with AgNO 3 in a 1:1 ratio at room temperature gave a dark red solution, from which red plate crystals of 1 were isolated by adding an aqueous solution of NaNO 3 . The electronic absorption s...