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 S = 0 to S = 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.

Magneto-structural correlations in a family of di-alkoxo bridged chromium dimers

“…27 The largest homometallic Cr(III) complex reported to date, [Cr 14 (bta) 6 O 6 (OMe) 18 Cl 6 ] (btaH = benzotriazole), whose metallic skeleton describes a hexacapped hexagonal bipyramid also contains no carboxylates. 28 These latter two species hint that structurally novel clusters may be obtained by employing different ligand sets than those traditionally used.…”

Cryogenic magneto-caloric effect and magneto-structural correlations in carboxylate-bridged Gd(iii) compounds

“…1,2 As for conventional cryogenic magnetic refrigerants, e.g., Gadolinium Gallium Garnet (GGG), 3 these so-called molecular coolers usually contain the isotropic Gd(III) ion because it has zero orbital angular momentum and provides the largest entropy per single ion. Although both homo-and heterometallic high nuclearity complexes have been reported to have high MCE, 4,5,6 small lightweight Gd(III) compounds represent the best compromise in terms of weak magnetic interactions, magnetic/non-magnetic elements ratio and spin multiplicity. 7,8 Enhancement of the latter through intramolecular ferromagnetic exchange is of course welcome, as it helps reaching large magnetic entropy at comparatively lower applied magnetic fields.…”

{Fe6O2}-Based Assembly of a Tetradecanuclear Iron Nanocluster

“…To our knowledge, only few tetradecanuclear Fe III clusters are known. These include triazole-bridged [Fe 14 O 6 (L) 6 (OMe) 18 Cl 6 ] clusters (where L = 1,2,3-triazole, benzotriazole, 5-methylbenzotriazole, 5,6-dimethylbenzotriazole, 5-chlorobenzotriazole) [22,27] with a {Fe 14 O 6 } core, an alkoxide-based [Bu 4 N] 2 [Fe 14 O 8 (OCH 2 Me) 20 Cl 8 ] [28] cluster and a oxo/peroxo pivalate-type [Fe 14 O 4 (O 2 ) 2 (Piv) 12 (L) 8 (H 2 O) 12 ](NO 3 ) 2 cage (where L = phosphonate ligand) [29] possessing a {Fe 14 O 8 } core, and finally a nitride (Et 4 N) 4 [Fe 14 N 8 (L)Cl 12 ] (where L = (Me 3 Sn) 3 N) cluster [30] with a {Fe 14 N 8 } core, yet no tetradecanuclear cluster with a {Fe 14 O 14 } core has been reported.…”