Reactivity Profile of a Formally Dicobalt(0) Complex Bound by a Redox-Active Macrocycle

In this article, we discuss the synthesis, characterization, and reactivity of a neutral dicobalt complex, Co 2 (PMe 3 ) 2 0 , supported by a highly reduced, redox-active macrocyclic ligand. The reducing equivalents stored within the ligand framework proved capable of activating a wide scope of small molecule substrates. The addition of a weak acid, lutidinium triflate, led to the clean formation of a bridging hydride that was found to undergo fluxional geometric changes in solution, while exposure of Co 2 (PMe 3 ) 2 0 to H2 resulted in a rare example of net binuclear oxidative addition to give two bridging hydrides. The addition of diphenylacetylene to Co 2 (PMe 3 ) 2 0 gave an unusual metallacyclobutane configuration for the coordinated alkyne, a geometry that was mirrored by the product obtained on activation of CO2 by Co 2 (PMe 3 ) 2 0 . Dinitrogen was not observed to bind to Co 2 (PMe 3 ) 2 0 , but the use of catalytic quantities of Co 2 (PMe 3 ) 2 0 in mixtures of KC8 and Me3SiCl under 1 atm of N2 yielded N(SiMe3)3, which was quantified by conversion into NH4Cl. A screen of related dicobalt complexes revealed a dependence on the identity of the pre-catalyst, with yields of fixed nitrogen ranging up to 50(6) %.