1,2,3-Triazolate-Bridged Tetradecametallic Transition Metal Clusters [M₁₄(L)₆O₆(OMe)₁₈X₆] (M = Fe^III, Cr^III and V^III/IV) and Related Compounds: Ground-State Spins Ranging from <i>S</i> = 0 to <i>S</i> = 25 and Spin-Enhanced Magnetocaloric Effect

Abstract

We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M14(L)6O6(OMe)18Cl6] (M=FeIII, CrIII) and [V14(L)6O6(OMe)18Cl6-xOx] (L=anion of 1,2,3-triazole or derivative). Crystal structure data are reported for the {M14} complexes [Fe14(C2H2N3)6O6(OMe)18Cl6], [Cr14(bta)6O6(OMe)18Cl6] (btaH=benzotriazole), [V14O6(Me2bta)6(OMe)18Cl6-xOx] [Me2btaH=5,6-Me2-benzotriazole; eight metal sites are VIII, the remainder are disordered between {VIII-Cl}2+ and {VIV=O}2+] and for the distorted [FeIII14O9(OH)(OMe)8(bta)7(MeOH)5(H2O)Cl8] structure that results from non-solvothermal synthetic methods, highlighting the importance of temperature regime in cluster synthesis. Magnetic studies reveal the {Fe14} complexes to have ground state electronic spins of S<or=25, among the highest known, while in contrast the {Cr14} complex has an S=0 ground state despite having a very similar structure and all complexes being dominated by intramolecular antiferromagnetic exchange interactions. The {Fe14} complexes undergo a magnetic phase transition to long-range ordering at relatively high temperatures for molecular species, which are governed by the steric bulk of the triazole (TN=1.8 and 3.4 K for L=bta- and H2C2N3-, respectively). The huge spins of the {Fe14} complexes lead to very large magnetocaloric effects (MCE)-the largest known for any material below 10 K-which is further enhanced by spin frustration within the molecules due to the competing antiferromagnetic interactions. The largest MCE is found for [Fe14(C2H2N3)6O6(OMe)18Cl6] with an isothermal magnetic entropy change -DeltaSm of 20.3 J kg-1 K-1 at 6 K for an applied magnetic field change of 0-7 T.