Synthesis, Characterization and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal
pendadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3,
CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These
diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also
remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic
techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic
cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-
cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-
hexenyl group to the methylcyclopentyl derivative over the course of several hours.
Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a
very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can
accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent,
leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for
Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to
divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand
as opposed to protonation of the methyl group. The produce of protonation at nitrogen is
the dimeric species 4 which was prepared via separate synthesis and characterized by Xray crystallography.