η2-Arene Binding at High-Spin Fe(I) Enabled by a Sterically Accommodating Tris(pyrazolyl)hydroborate Ligand

Abstract

Transition-metal arene complexes are important species in catalysis and arene functionalization. Certain electron-rich metal fragments are capable of binding arenes across two adjacent carbon atoms, inducing significant dearomatization through strong back-bonding interactions. Such complexes generally adopt low-spin, closed-shell configurations with electronic and coordinative saturation. Herein, we report an asymmetric tris­(pyrazolyl)­hydroborate (Tp) ligand and its Fe­(I) fragment, which forms η2 complexes with a range of arenes and heteroarenes, including benzene, trifluoromethylbenzene, naphthalene, anthracene, and furan. X-band EPR and solution magnetometry definitively establish these complexes as high spin (S = 3/2), owing to the relatively weak ligand field provided by Tp. These compounds expand a small but growing family of complexes that feature strongly back-bonding metal fragments in high-spin configurations.