Controlling the nuclearity of low-spin iron(II) phthalocyanine complexes through long-chain alkyldiamine coordination

New iron(II) phthalocyanine complexes featuring long-chain alkyldiamine axial ligands have been synthesized, isolated, and comprehensively characterized by single-crystal X-ray diffraction and multinuclear NMR spectroscopy ( 1 H, 13 C, and 15 N), among other techniques. In tetrahydrofuran solution, these complexes exist in a dynamic equilibrium between mononuclear ( M ) and variable amounts of linear ( D ) and cyclic ( Cy ) polynuclear assemblies, depending on concentration and ligand stoichiometry, while maintaining a six-coordinate Fe(II) environment across all assemblies. Quantum-chemical calculations reveal the energetic minima associated with each assembly, providing a coherent picture of the structure–stability relationships that govern nuclearity and aggregation in solution. Long-chain alkyldiamines act as programmable axial linkers that control the nuclearity of low-spin Fe(II) phthalocyanines, establishing a dynamic equilibrium between monomeric, dimeric, and cyclic assemblies in THF. Multinuclear NMR and PGSE diffusion measurements, complemented by DFT calculations, rationalize the structure–stability relationships governing aggregation. • Controlled nuclearity switching in Fe(II) phthalocyanines via axial diamines. • Stoichiometry- and concentration-dependent equilibria between monomeric, dimeric and cyclic species in solution. • PGSE diffusion NMR identifies mono- and polynuclear FePc assemblies. • DFT calculations rationalize stability and aggregation of FePc complexes.