Solution Characterization of the Iron(II) Bis(1,4,7-Triazacyclononane) Spin-Equilibrium Reaction

Iron(II) bis(1,4,7-triazacylononane), Fe(tacn)22+, exhibits 1A1g ⇌ 5T2g spin-crossover behavior in solution. Values of ΔH°se = 23 kJ mol-1 and ΔS°se = 67 J mol-1 K-1 are obtained for the reaction by variable-temperature UV−visible and 1H NMR spectroscopy. The thermodynamic parameters reflect intramolecular structural changes that accompany LS → HS conversion and depend only to a small extent on solvent, counterion, and the presence of excess electrolyte. A procedure is described for quantitative correction of Fe(tacn)23+/2+ thermodynamics for the accompanying Fe(II) spin equilibrium.