[BETA-NA2FE18S30]8- AND [NA9FE20SE38]9- - HIGH-NUCLEARITY CLUSTERS BY MONOCYCLIZATION OR BICYCLIZATION OF UNIDIMENSIONAL POLYMERIC FRAGMENTS AND THE EXISTENCE OF ISOMERIC MONOCYCLIC CLUSTERS

Alteration of conditions in the assembly system that has previously afforded the cyclic cluster [alpha-Na2Fe18S30]8-has led to the preparation of two new high-nuclearity clusters of unprecedented structure. The system FeCl3:3Na-(PhNCOMe):nLi2Q/EtOH (n = 1.8 for Q = S; n = 2 for Q = Se), when treated with BU4NBr after 10-12 h and allowed to react and crystallize over 5 days, affords the black Compounds (BU4N)6Na4Fe18S30 (1) or (Bu4N)4.5Na13.5Fe20Se38. 2PhNHCOMe.12EtOH (2.12EtOH) in good yield. Compound 1-8EtOH (2.15EtOH) crystallizes in monoclinic (orthorhombic) space group P2(1)/n (Pbcn) with a = 17.862 (5) (34.551 (11)) angstrom, b = 27.383 (11) (26.169 (5)) angstrom, c = 18.534 (6) (24.244 (8)) angstrom, beta = 91.12 (2)-degrees, and Z = 2 (4). The crystal structures include discrete BU4N+ ions, solvate molecules (EtOH, PhNHCOMe). and cluster anions. Compound 1.8EtOH contains the monocyclic cluster [beta-Na2Fe18S30]8-, which has a centrosymmetric toroidal shape of lateral dimensions 12.4 X 16.3 angstrom, a maximum thickness of 3.3 angstrom, and no terminal ligands. It is constructed by the fusion f 22 nonplanar Fe2S2 rhombs in edge- and vertex-sharing modes such that there are 18-mu-2-S and 12-mu-3-S atoms binding 18 nearly coplanar iron atoms in tetrahedral FeS4 sites. Two sodium ions are bound to interior sulfur atoms of the cluster whose framework can be considered to arise from the sequential connection of two Fe2S2 and four cuboidal Fe3S4 fragments and two tetrahedral FeS4 sites. This cluster is a structural isomer of [a-Na2Fe18S30]8- whose atom connectivity is different (20-mu-2-S, 8-mu-3-S, 2-mu-4-S, 2 Fe3S4 + 2 Fe6S9 fragments). A structural comparison of the two isomers is presented. Compound 2.12EtOH contains the bicyclic cluster [Na9Fe20Se38]9- of prolate ellipsoidal shape with the dimensions 11.2 x 17.4 angstrom, imposed C2 symmetry, and no terminal ligands. The structure is built up from 21 nonplanar Fe2Se2 rhombs, 15 of which share vertices to form three separate but equivalent Fe6(mu-2-Se)10 chains. These chains, positioned roughly parallel to an idealized C3 axis that contains the additional two nonchain Fe atoms and two Se atoms, are anchored at each end of the molecule by attachment to three edge-shared bridgehead rhombs. Nine sodium ions are encapsulated within the selenium-rich interior cavity, distinguishing the molecule as inorganic thiacryptand. [Na9Fe20Se38]9 is the only known bicyclic cluster. Both clusters have singlet ground states, are electronically delocalized and antiferromagnetically coupled, and are of dimensions that place them in the nanometer size range. Magnetic and Mossbauer spectroscopic data are reported. Reaction of [Na9Fe20Se38]9- with p-toluenethiol causes breakdown to the major product [Fe4Se4(S-p-tol)4]2- and minor amounts of [Fe6Se9(S-p-tol)2]4- and [Fe2Se2(S-p-tol)4]2-. The electronic structures of the two clusters as analyzed at the extended Huckel level have quasiband structures in which the highly delocalized MO's divide into five distinct blocks. Leading results from this analysis are described. It is suggested that clusters [alpha-Na2Fe18S30]8-, [beta-Na2Fe18S30]8-, and [Na9Fe20Se38]9- can be assembled from a common cyclic precursor [FeIII14Q28]14-and that the alpha- and beta-isomers of the cyclic cluster can be interconverted by relatively minor bond rearrangements. The existence of these isomers is predicted from molecular topological arguments, which are briefly summarized. The general concept demonstrated by these clusters is that, under promotion by an appropriate reagent, high-nuclearity clusters can be formed by termination of unidimensional polymeric fragments via mono- or bicyclization.