Boron functionalization of hydridotris(pyrazolyl)borate ligand on osmium complexes

Abstract

[Work hypothesis and objective]: In most Tp-complexes (Tp=hydridotris(pyrazolyl)borate), the Tp ligand acts as an spectator that rarely undergoes transformations. As a part of our ongoing chemistry on Os-Tp complexes we report here the unexpected role of the B-H unit of the ligand, which acts as a claw-tail of the scorpionate to abstract an unsaturated group from the saturated metal center, allowing a subsequent reaction in another ligand. [Results]: The ethylene-pyridyl derivative OsTp{η1-C[NC5H3Me]}(η2-CH2=CH2)(PiPr3) reacts with two equivalents of phenylacetylene or 1,1-diphenyl-2-propyn-1-ol to give the corresponding alkynyl-vinylidene derivatives. Treatment of the hydroxyalkynyl-hydroxyvinylidene derivative with aluminium oxide promotes the dehydration of the hydroxyvinylidene to allenylidene and the subsequent isomerization of the hydroxyalkynyl ligand into an α,β-unsaturated acyl group. The B-H bond of the Tp ligand abstracts the allenylidene ligand from the metal center providing an allenyltris(pyrazolyl)borate ligand. As a result of the migration, the decarbonylation of the acyl group takes place to form the alkenyl-carbonyl derivative OsTpCH=C=CPh2(CH=CPh2)(CO)(PiPr3). The isomerization of the allenyl substituent of the borate ligand to alkynyl is promoted by a weak base as acetonitrile. These transformations represent very rare examples of boron functionalization of a Tp ligand.