1,2,3-Triazolate-bridged tetradecametallic transition metal clusters [M14(L)6O6(OMe)18X6] (M = FeIII, CrIII and VIII/IV) and related compounds:: Ground-state spins ranging from S=0 to S=25 and spin-enhanced magnetocaloric effect

We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M-14(L)(6)O-6(OMe)(18)Cl-6] (M = Fe-III, Cr-III) and [V-14(L)(6)O-6(OMe)(18)Cl6-xOx] (L = anion of 1,2,3-triazole or derivative). Crystal structure data are reported for the {M-14} complexes [Fe-14(C2H2N3)(6)O-6(OMe)(18)Cl-6], [Cr-14(bta)(6)O-6(OMe)(18)Cl-6] (btaH = benzotriazole), [V14O6(Me(2)bta)(6)(OMe)(18)Cl6-xOx] [Me(2)btaH = 5,6-Me-2-benzotriazole; eight metal sites are V-III, the remainder are disordered between {V-III-Cl}(2+) and {(VO)-O-IV}(2+)] and for the distorted [(Fe14O9)-O-III(OH)(OMe)(8)(bta)(7)(MeOH)(5)(H2O)Cl-8] structure that results from non-solvothermal synthetic methods, highlighting the importance of temperature regime in cluster synthesis. Magnetic studies reveal the {Fe-14} complexes to have ground state electronic spins of S <= 25, among the highest known, while in contrast the {Cr-14} complex has an S = 0 ground state despite having a very similar structure and all complexes being dominated by intramolecular antiferromagnetic exchange interactions. The {Fe-14} 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 (T-N = 1.8 and 3.4 K for L = bta(-) and H2C2N3-, respectively). The huge spins of the {Fe-14} complexes lead to very large magnetocaloric effects (MCE)the largest known for any material below 10 Kwhich is further enhanced by spin frustration within the molecules due to the competing antiferromagnetic interactions. The largest MCE is found for [Fe-14(C2H2N3)(6)O-6(OMe)(18)Cl-6] with an isothermal magnetic entropy change -Delta S-m of 20.3 J kg(-1) K-1 at 6 K for an applied magnetic field change of 0-7 T.