Alkynyl-linked poly(cobaltacarboranes). Directed synthesis, structures, and electrochemistry of linear complexes and benzene-centered pinwheels

A series of B(5)- and B(7)-alkynyl-substituted derivatives of CoC2B3, CoC2B4, and Co2C2B3 cobaltacarboranes has been prepared and linked via metal-promoted cross-coupling reactions to generate several types of polynuclear molecular complexes, which were structurally characterized and studied via electrochemical techniques. The target compounds prepared include linear dicobalt species containing BCCCCB linkages, e.g., [Cp*Co(2,3-Et2C2B4H3-7-CC)]2 (8) and [Cp*Co(2,3-Et2C2B4H3-5-CC)]2 (17), trigonally symmetrical 1,3,5-triethynylbenzene-anchored tricobalt and hexacobalt complexes, e.g., {Cp*Co(2,3-Et2C2B4H3-5-CC)}3C6H3 (19), {Cp*Co(2,3-Et2C2B4H3-7-CC)}3C6H3 (22), and 1,3,5-[Cp*2Co2(2,3-Et2C2B3H2-5-CC)]3C6H3 (21), and novel cobaltacarborane−Co2C2 mixed-cluster systems, e.g., Cp*Co(2,3-Et2C2B4H3)-7-C2HCo2(CO)6 (5) and {Cp*Co[2,3-Et2C2B4H3-7-(C2Co2CO)6]}3C6H3 (23), a nonacobalt complex. The new compounds were obtained in all cases as air-stable crystalline solids and were characterized by multinuclear NMR and mass spectra, supplemented by X-ray diffraction data for 8, 19, and 21. Electrochemical data on the alkynyl-linked compounds indicate that metal−metal communication in oxidized and reduced species occurs to a limited extent in most cases and to a considerable degree (Robin−Day class III) in the C2B3-bridged triple-decker systems.