Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

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Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imid- azol-5-ylidene) mesoionic carbene ligand, a scaffold which is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, have been synthesized and characterized. This new ligand’s strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order pertur- bative energy analysis, are on par with those of the well-studied alkyl substituted tris(imidazol-2-ylidene)borates which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron withdrawing trifluorome- thyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups towards oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition metal species.