Ligand-to-metal ratio controlled assembly of tetra- and hexanuclear clusters towards single-molecule magnets

A simple template-mediated route, starting from triethalolamine 1, sodium h ydride or caesium carbonate, and iron(III) chloride led to the six- and eig ht-membered iron coronates [Na subset of {Fe-6[N(CH2CH2O)(3)](6)}](+) (2) a nd [Cs subset of {Fe-8[N(CH2CH2O)(3)](8)}](+) (3). In the reaction of N-met hyldiethanolamine 4 (H2L1) or N-(2,5-dimethylbenzyl)iminodiethanol 6 (H2L2) with calcium hydride followed by addition of a solution of iron(III) chlor ide, the neutral unoccupied coronands [Fe6Cl6(L-1)(6)] (5) and [Fe6Cl6(L-2) (6)] (7) were formed. Subsequent exchange of the chloride ions of 7 by brom ide orthiocyanate ions afforded the ferric wheels [Fe6Br6(L-2)(6)] (8) or [ Fe-6(NCS)(6)(L-2)(6)] (9), respectively. Titration experiments of solutions of dianion (L-1)(2)- with iron(III) chloride in THF revealed interesting m echanistic details about the self-assembling process leading to 5. At an ir on/ligand ratio of 1:1.5 star-shaped tetranuclear [Fe{Fe(L-1)(2)}(3)] (11) was isolated. However, at an iron/ ligand ratio of 1:2, complex 11 was tran sformed into the ferric wheel 5. It was shown, that the interconversion of 5 and 11 is reversible. Based on the mechanistic studies, a procedure was d eveloped which works for both the synthesis of homonuclear 11 and the star- shaped heteronuclear clusters [Cr{Fe(L-1)(2)}(3)] (12) and [Al(Fe(L-1)(2)}( 3)] (13). The structures of all new compounds were determined unequivocally by single-crystal X-ray analyses.