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

Abstract

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