Trinuclear Terpyridine Frustrated Spin System with a Mn^IV₃O₄ Core: Synthesis, Physical Characterization, and Quantum Chemical Modeling of Its Magnetic Properties

The trinuclear oxo bridged manganese cluster, [MnIV3O4(terpy)(terpyO2)2(H2O)](S2O8)2 (5) (terpy = 2,2′:2′′,6′-terpyridine and terpyO2 = 2,2′:2′′,6′-terpyridine 1,1′′-dioxide), was isolated in an acidic aqueous medium from the reaction of MnSO4, terpy, and oxone as chemical oxidant. The terpyO2 ligands were generated in situ during the synthesis by partial oxidation of terpy. The complex crystallizes in the monoclinic space group P21/n with a = 14.251(5) Å, b = 15.245(5) Å, c = 24.672(5) Å, α = 90.000(5)°, β = 92.045(5)°, γ = 90.000(5)°, and Z = 4. The triangular {MnIV3O4}4+ core observed in this complex is built up of a basal Mn(μ-O)2Mn unit where each Mn ion is linked to an apical Mn ion via mono(μ-O) bridges. The facial coordination of the two tridentate terpyO2 ligands to the Mn(μ-O)2Mn unit allows the formation of the triangular core. 5 is also the first structurally characterized Mn complex with polypyridinyl N-oxide ligands. The variable-temperature magnetic susceptibility data for this complex, in the range of 10−300 K, are consistent with an S = 1/2 ground state and were fit using the spin Hamiltonian 2Heff=−2Ja(Ŝ1Ŝ2+Ŝ1Ŝ3)−2JbŜ2Ŝ3 with S1 = S2 = S3 = 3/2, Ja = −37 (±0.5) and Jb = −53 (±1) cm−1, where Ja and Jb are exchange constants through the mono-μ-oxo and the di-μ-oxo bridges, respectively. The doublet ground spin state of 5 is confirmed by EPR spectroscopic measurements. Density functional theory (DFT) calculations based on the broken symmetry approach reproduce the magnetic properties of 5 very well (calculated values: Ja = −39.4 and Jb = −55.9 cm−1), thus confirming the capability of this quantum chemical method for predicting the magnetic behavior of clusters involving more than two metal ions. The nature of the ground spin state of the magnetic {MnIV3O4}4+ core and the role of ancillary ligands on the magnitude of J are also discussed.