Reactivity of the 16-electron CpCo half-sandwich complex containing a B(3,6)-disubstituted o-carborane-1,2-dithiolate ligand

Abstract

The 16-electron (16e) half-sandwich complex CpCo(S2C2B10H8)(CHCHCO2Me)2 (1; Cp = cyclopentadienyl) was considered as inert owing to hindered di-substitution at B sites. However in methanol complex 1 reacts with ethynylferrocene (FcCCH) and dimethyl acetylenedicarboxylate (DMAD, MeO2CCCCO2Me) to lead to two-fold alkyne insertion at Co–S bond to give rise to unusual stable 18e adduct CpCo(S2C2B10H8)(CHCHCO2Me)2(HCCFc)(MeO2CCCCO2Me) (2) containing mixed alkynes. Also in methanol complex 1 undergoes intramolecular hydrogen transfer of one CHCHCO2Me unit to adjacent carbon to form a CH2CCO2Me unit, thus producing the 18e complex CpCo(S2C2B10H8)[CH2CCO2Me][CHCHCO2Me] (3). Further investigation indicates that complex 1 can catalyze [2 + 2 + 2] cycloaddition of alkynes. The catalytic efficiency is determined by temperature and nature of alkynes. In THF in the presence of [N(n-Bu)4]Br complex 1 is transformed to ionic complex [N(n-Bu)4]+[Co(S2C2B10H8)2(CHCHCO2Me)4]− (4), in which metal-dithiolene is planar. Analogous [N(n-Bu)4]+[Co(S2C2B10H9)2(CHCHCO2Me)2]− (5) was also synthesized from mono-substituted precursor CpCo(S2C2B10H9)(CHCHCO2Me) (1′). The electrochemical properties of both 4 and 5 show significant positive shift of redox peaks versus the analogous bis(1,2-benzenedithiolato)cobaltate (III) (Co(bdt)2−). Reactivity of both neutral 16e CpCo complexes and ionic metal-dithiolene derivatives declines with the increase of substituents on carborane cage. All the new compounds have been characterized by NMR, IR, mass spectrometry, elemental analysis, and 2, 3, and 4 have been structurally characterized by X-ray diffraction.