Late transition metal complexes supported by an 18-atom benzimidazolylidene tetracarbene macrocycle

The synthesis and characterization of an 18-atom tetra-benzimidazolylidene macrocycle, ^Me,EtTC^B (1), and the corresponding tetranuclear silver transmetallation reagent, [(^Me,EtTC^B)₂Ag₄(NCCH₃)₂](PF₆)₄ (2), are reported. Transmetallation reactions between the silver complex and first-row transition-metal iodide salts provide Fe(II), Co(II), and Ni(II) complexes. Single-crystal X-ray diffraction reveals that the tetracarbene ligand enforces distinct coordination geometries across the metal series, giving octahedral Fe(II) (3), square pyramidal Co(II) (4), and square planar Ni(II) (5) complexes. Downfield shifts in carbene ¹³C NMR spectra relative to analogous tetra-imidazolylidene systems indicate diminished σ-donor strength for benzimidazolylidene donor groups. Cyclic voltammetry experiments show substantial anodic shifts in the M(II/III) redox couples (ΔE_1/2 = 0.36-0.38 V) relative to ^Me,EtTC^Ph analogues, consistent with stabilization of the metal d-orbital manifold. Density functional theory calculations support these experimental observations and indicate that benzimidazolylidene tetracarbene 1 generates a strong equatorial ligand field while providing reduced σ-donation compared to related tetra-imidazolylidene macrocycles. Chemical oxidation reactions afforded {FeNO}⁶ (6), Fe(III) (7), and Co(III) (8) macrocyclic tetracarbene complexes. These findings demonstrate that modification of macrocyclic tetra-NHC donor frameworks can significantly influence the electronic structures and redox properties of late first-row transition metal complexes.