Chameleonic capacity of [3,3′-Co(1,2-C2B9H11)2]- in coordination.: Generation of the highly uncommon s(thioether)-Na bond

The multiple binding modes of [3,3'-Co(1,2-C2B9H11)(2)](-) allow it to adjust to different coordinating or geometric demands. [3,3'-Co(1,2-C2B9H11)(2)](-) can behave as a platform on which two types of coordinating moieties exist: the BH's and boron- or carbon-bonded elements with available lone pairs. Until now, no compounds containing Na+ bonded to thioether ligands were known other than Na[7,8-mu-{S(CH2CH2O)(3)CH2CH2S}-7,8-C2B9H10]. The dithioether moiety in the latter compound is preorganized to favor chelation and produces a five-membered ring. This definitely assists in coordination to sodium. To avoid this preorganization and assess the independent strength of the C-S(thioether)-Na bond, the anionic nonpreorganized dithioether [1,1'-mu-{S(CH2CH2O)(3)CH2CH2S}-3,3'-Co(1,2-C2B9H10)(2)](-) ligand was synthesized. The two cluster moieties have sufficient accessible rotamers to allow adequate metal coordination, but they do not preorganize. The synthetic procedure leading to [1,1'-mu-{S(CH2CH2O)(3)CH2CH2S}-3,3'-Co(1,2-C2B9H10)(2)](-) implied the formation of idealized meso and racemic forms. These forms behave differently toward Na+. In the meso form coordination to sodium is achieved via the two sulfur (C-S) and three oxygen elements in the exo cluster chain. In the racemic form coordination is achieved via the three oxygen elements in the exo cluster chain, with no thioether participation. The remaining Na+ coordination sites are filled by oxygen atoms from ancillary ligands. These coordination environments were determined by the crystal structures of Na[1,1'-mu-{S(CH2CH2O)(3)CH2-CH2S}-3,3'-Co(1,2-C2B9H10)(2)].(CH3)(2)CO.CHCl3.CH2Cl2 (meso form) and Na[1,1'-mu-{S(CH2-CH2O)(3)CH2CH2S}-3,3'-Co(1,2-C2B9H10)(2)].2(CH3)(2)CO (racemic form). Therefore, C-S(thioether)Na coordination has been demonstrated for the first time with nonpreorganized ligands, although the cluster participation has been found necessary. A situation similar to that obtained with the racemic form of [1,1'-mu-{S(CH2CH2O)(3)CH2CH2S}-3,3'-Co(1,2-C2B9H10)(2)](-), having only three coordinating oxygen atoms, is also possible for [3,3'-Co(8-{O(CH2CH2O)(2)-CH3}-1,2-C2B9H10)(1',2'-C2B9H11)](-). This ligand does not have thioethers, and it was interesting to discover how the [3,3'-Co(1,2-C2B9H11)(2)](-) moiety would behave with a low supply of coordination sites in a more flexible -O-CH2CH2-O-CH2CH2-O-CH2CH3 arrangement. The structure of Na[3,3'-Co(8-{O(CH2CH2O)(2)CH2CH3}-1,2-C2B9H10)(1',2'-C2B9H11)] shows that the chain contributes three oxygen atoms for coordination to Na+ and, interestingly, the [3,3'-Co(1,2-C2B9H11)(2)](-) moiety provides three extra B-H coordination sites.