Mechanism of Pyridine−Ligand Exchanges at the Different Labile Sites of 3d Heterometallic and Mixed Valence μ₃-oxo Trinuclear Clusters

The syntheses and single crystal X-ray structural analysis of five novel hetero- and homometallic μ3-oxo trinuclear cluster with the formula [FeIII2MII(μ3-O)(μ-O2CCH3)6(4-Rpy)3]·x(4-Rpy)·y(CH3CN) where R = Ph for 1(Fe2Mn), 2(Fe2Fe), 3(Fe2Co), 4(Fe2Ni) and R = CF3 for 5(Fe2Co), are reported. The persistence of the structure for compounds 2−5 in dichloromethane solution in the temperature range 190−320 K is demonstrated by 1H and 19F NMR spectroscopy. Even at the lowest temperature, the electron exchange in the homometallic mixed-valence compound 2(Fe2Fe) is in the fast regime at the NMR time scale. Variable temperature and pressure NMR line broadening allowed quantifying the fast coordinated/free 4-Rpy exchanges at the two labile metal centers in these clusters: 2: FeIII(k298/103 s−1 = 16.6; ΔH‡ = 60.32 kJ mol−1; ΔS‡ = + 34.8 J K−1 mol−1; ΔV‡ = + 12.5 cm3 mol−1); 3: Fe (11.9; 58.92; +30.7; +10.6) and Co (2.8; 68.24; +49.8; +13.9); 4: Fe(12.2; 67.91; +61.0; −) and Ni (0.37; 78.62; +67.8; +12.3); 5: Fe (46; 58.21; +39.3; +14.2) and Co (4.7; 55.37; +11.2; +10.9). A limiting D mechanism is assigned to these exchange reactions. This assignment is based on a first-order rate law, the detection of intermediates, the positive and large entropies and volumes of activation. The order of reactivity kCo > kNi is expected for a D mechanism at these metal centers: their low exchange rates are due to their strong binding with the 4-Rpy donor. Surrounded by oxygen donors the d5 iron(III) usually reacts associatively; however, here due to low affinity of this ion for nitrogen the mechanism is D and the rate of exchange is very fast, even faster than on the divalent ions. There is no significant effect of the divalent ion in cluster 2, 3, and 5 on the exchange rates of 4-Phpy at the iron center, which seems to indicate that the specific electronic interactions between the three ions making the clusters do not influence the FeIII−N bond strength.