ORGANOTRANSITION-METAL METALLACARBORANES .29. SYNTHESIS OF SELECTIVELY C-SUBSTITUTED AND B-SUBSTITUTED DOUBLE-DECKER AND TRIPLE-DECKER SANDWICHES - (ETA-5-C5ME5)COIII(ETA-5-R2C2B3R'3H2) COBALTOCENIUM ANALOGS

In earlier publications from our laboratory, methods were described for the regiospecific introduction of organic substituents onto cage boron and carbon atoms in small metallacarborane clusters. Here we report the selective application of these approaches in the targeted synthesis of several double-decker, triple-decker, and tetradecker sandwich compounds including the parent species (C5Me5)CO(C2B3H7) and (C5Me5)Co(C2B3H5)Co(C5Mes). The peralkyl species (CsMe5)CoIII[(Me2C2B3Me3(mu-H)2] (5), an isostructural and isoelectronic analogue of the decamethylcobaltocenium ion [(C5Me5)2CoIII]+, was prepared and structurally characterized via X-ray diffraction. The C2B3 and C5 rings in 5 are staggered and nearly parallel, and the metal is approximately centered over each ring. The cobalt-C5Me5 centroid distance (1.68 angstrom) in 5 is virtually identical to that reported in the literature for (C5Me5)2Co+, but the cobalt-C2B3 centroid vector is much shorter (1.54 angstrom), indicating greater covalent character in the Co-carborane vs the Co-C5Me interaction. In 5 the C5Me5 methyl groups are bent out of the C5 plane away from the metal, in accord with theoretical predictions, but the methyls on the carborane ligand are bent toward the metal atom, an observation that probably reflects differences in orbital hybridization in the two rings. The permethyl triple-decker complex (C5Me5)Co(Me2C2B3Me3)Co(C5Me5) was synthesized via deprotonation and metal-capping of (C5Me5)Co(Me2C2B3Me3H2). The unsubstituted triple-decker complex (C5Me5)Co(C2B3H5)Co(C5Me5) was prepared via two routes: (i) desilylation of the CC'-bis(trimethylsilyl) derivative and (ii) deprotonation and metal capping of the parent double-decker (C5Me5)Co(C2B3H7). Attempted tetradecker stacking of (C5Me5)Co(C2B3H5-5-Cl)- ion with NiBr2 led to the fusion product (C5Me5)2Co2C4B6H8Cl2, but (C5Me5)Co(Me2C2B3H3-5-Me)-and NiBr2 gave the desired tetradecker [(C5Me5)Co(Me2C2B3MeH2)]2Ni in low yield. New compounds were isolated as air-stable solids and characterized via H-1 and B-11 NMR, IR, and mass spectra.