Tuning the Magnetic Properties of Heterotrimetallic Co–M–Co (M = Ni and Pd) Chain Complexes via Redox Modulation and Metal Ion Replacement

Spin crossover (SCO) describes the reversible interconversion between low-spin and high-spin electronic configurations in transition metal complexes, arising from a delicate balance between ligand field splitting and electron pairing energy. Cobalt-based extended metal atom chains (EMACs) and their heterometallic analogues, HEMACs, offer a versatile platform for probing spin-state energetics and switchable magnetism through variations in metal–metal and metal–ligand interactions. Here, we report the synthesis, redox chemistry, and magnetic properties of the heterometallic chains [CoPdCo(dpa) 4 Cl 2 ] ( 1 ) and [CoNiCo(dpa) 4 Cl 2 ] ( 2, dpa = 2,2′-dipyridylamido), together with their one-electron oxidized derivatives [ 1 ][SbCl 6 ] and [ 2 ][PF 6 ]. Variable-temperature single-crystal X-ray diffraction, SQUID magnetometry, variable-temperature 1 H NMR spectroscopy, and density functional theory reveal that redox reaction and central metal substitution can modulate the spin-state equilibria of these Co–M–Co chains. The neutral complexes 1 and 2 feature antiferromagnetically coupled high-spin Co(II) termini, with 2 displaying structure-dependent spin crossover in the solid state. Upon oxidation, [ 1 ][SbCl 6 ] adopts a robust high-spin configuration over the entire temperature range studied, whereas [ 2 ][PF 6 ] undergoes an incomplete, temperature-driven spin crossover between low-spin and high-spin states, as evidenced by concerted structural, magnetic, and spectroscopic signatures. DFT calculations elucidate the delicate enthalpy–entropy balance governing these behaviors and highlight the role of central metal size and Co–N bond metrics in biasing the spin-state landscape. These results provide insight into the interplay between redox state, spin-state behavior, and heterometallic chain composition in cobalt-based HEMACs.