Triplet State-Promoted Kumada-Corriu Coupling Catalyzed by Hydrotris(3,5-diisopropylpyrazolyl)boratocobalt(III)diiodide

The Kumada-Corriu hetero-coupling between an halogeno-arene and an arylmagnesiumbromide can be catalyzed with yields >80 % by the new hydrotris(3,5-diisopropylpyrazolyl)boratocobalt(III)diiodide, i. e. Tp^iPrCoI₂. The catalysis, which is significantly improved upon exposure to light as compared to darkness, is determined by the coexistence of low and high spin states in a respective ratio of ~3 : 2 for the crucial “Tp^iPrCoAr₂” intermediate. The pivotal cobalt(I) Tp^iPrCo^(I)(thf)_n intermediate is shown to be exclusively a triplet state in the ground state with a measured μ_eff value of 3.08 μ_B at 293 K in C₆D₆. DFT investigations confirm the key role of triplet states for the bisaryl-cobalt(III) intermediates in that they provide a reaction pathway with much lower activation barriers as compared to the singlet state. The low-to-high spin state transition in THF enhances the reactivity of the “Tp^iPrCoAr₂” intermediate, which changes its coordination geometry from singlet spin state 18 electron OC-6 Tp^iPrCoAr₂(thf) to a triplet spin state 16 electron SPY-5 Tp^iPrCoAr₂ where the Tp^iPr ligand adopts a nearly κ² bonding mode. The quantitative Independent Gradient Model analysis of the noncovalent interactions that prefigure the C−C covalent bond in the key Co(Ar)₂ intermediates informs of the peculiar importance of the singlet to triplet spin state crossover in catalysis.