Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 = MeC(CH2NPh-o-NPh)3; L* = tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S = 6 or S = 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low-temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger σ-donating pyridine or tertiary phosphine ligands reduces the ground state to S = 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally-induced variation in molecular spin state between S = 2 and S = 4. Fits to these data provide thermodynamic parameters of ΔH = 406 cm−1 and Tc = 187 K for 2 and ΔH = 604 cm−1 and Tc = 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the σ orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.
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