Shape-Complementary Dimers of Simple Mononuclear Cobalt(III) Complexes Formed Through Eightfold Hydrogen Bonding

Here we report the synthesis and dimerization behavior of a series of mononuclear cobalt(III) saloph complexes [LCoA₂](OTf) bearing primary amines (A) as axial ligands. Complexes (1-5) incorporating the 3-MeOsaloph ligand and the corresponding methoxy-free analogue (6) were prepared to clarify the roles of the axial amines (A) and the methoxy substituents in dimer formation. The ¹H NMR spectra exhibited clear concentration-dependent changes, indicating reversible dimerization in solution. Quantitative analysis showed that both the stability of the dimers (K_dim) and the monomer-dimer exchange kinetics (k_a, k_d) depend strongly on the nature of the coordinated primary amine ligands. Comparison of the methoxy-substituted complexes and the corresponding methoxy-free analogue further demonstrated an important contribution of the methoxy groups to dimer stability. X-ray crystallography revealed that, despite their different solution behaviors, all complexes adopt similar hydrogen-bonded dimeric structures in the crystalline state. In the dimers of the 3-MeOsaloph complexes (1-5), two mononuclear units are arranged in a slipped, parallel fashion and are held together by eight complementary N─H···O hydrogen bonds involving both phenoxo and methoxy oxygen atoms. The combination of the 3-MeOsaloph framework and axial primary amine ligands provide a modular platform for constructing shape-complementary hydrogen-bonded dimers of mononuclear metal complexes with tunable thermodynamic and kinetic properties.