Interaction and Energy Decomposition Analyses to Predict Stability of Tetraaryl Square Planar Cobalt Complexes

The sodium-mediated cobaltation of pentafluorobenzene using the bimetallic base [NaCo(HMDS)3] (HMDS=N(SiMe3)2) has been reported to afford a novel tetraaryl Co(II) square planar complex. Yet, the preparation of analogue structures with 1,2,3,4-tetrafluorobenzene, 1,3,5-trichlorobenzene, and 1,4-dibromo-2,5-difluorobenzene remains elusive. While the metalation step proceeds leading to stable [NaCo(HMDS)2Ar] species, the ligand redistribution process to afford the tetraaryl Co(II) square planar complexes does not take place. Herein we report a density functional theory study in combination with electronic structure and energy decomposition analyses to shed light on the electronic and steric requirements to afford such complexes. Our findings show that the formation of the Co(II) square planar complexes depends on the right balance between intramolecular X⋅⋅⋅X and Na⋅⋅⋅X (X=H, F, Cl, Br) interactions. The latter further induces a ‘seesaw effect’, whereby the aryl ligand acts as a ‘seesaw’ allowing two X atoms in ortho positions to interdependently interact with Na. Only by considering both attractive and repulsive Na(X)⋅⋅⋅X interactions, the correct stability of the square planar complexes observed in experiments can be predicted computationally. We envision these insights to guide the rational design of novel square planar metal complexes for C−C coupling, a field that is still dominated by scarce and expensive precious metals.