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 iPr CoI 2 . 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 iPr CoAr 2 ” intermediate. The pivotal cobalt(I) Tp iPr Co (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 C 6 D 6 . 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 iPr CoAr 2 ” intermediate, which changes its coordination geometry from singlet spin state 18 electron OC ‐6 Tp iPr CoAr 2 ( thf ) to a triplet spin state 16 electron SPY ‐5 Tp iPr CoAr 2 where the Tp iPr ligand adopts a nearly κ 2 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.