Alkyne scission in cobalt-molybdenum clusters: synthesis and structures of hexa- and heptanuclear alkylidyne clusters with unusual architectures

Heating [Mo-2 (mu-RC=CR)(CO)(4)Cp-2] (R = CO2Me; Cp = eta-C5H5) with an excess of dicobalt octacarbonyl affords, in addition to the expected product [Co2Mo2 (mu(4)-C2R2)(mu-CO)(4) (CO)(4)Cp-2], two clusters in which the alkyne ligand has been cleaved: hexanuclear [Co4Mo2 (mu(4)-CR)(2) (mu-CO)(2) (CO)(8)Cp-2] (2) and heptanuclear [Co5Mo2 (mu(4)-CR)(mu(5)-C) ( CO)(12) Cp-2] (3), both of which have been shown by X-ray diffraction to contain unusual edge-sharing bitetrahedral metal frameworks. An analogue of the latter, [Co5Mo2 (mu(4)-CH)(mu(5)-C)(CO)(12) Cp-2] (8), was obtained from a similar reaction with R = H, but no counterpart of 2 could be isolated. Treatment of 3 with CO afforded the trinuclear cluster [Co2Mo(mu(3)-CCH2CO2Me)(CO)(8)Cp] (11), presumably by carbido-alkylidyne coupling; the crystal structures of this cluster and of the related [Co2Mo(mu(3)-CMe)(CO)(8)Cp] are also reported.