Ligand-Controlled Magnetic Interactions in Mn₄ Clusters

Abstract

A method is presented to design magnetic molecules in which the exchange interaction between adjacent metal ions is controlled by electron density withdrawal through their bridging ligands. We synthesized a novel Mn(4) cluster in which the choice of the bridging carboxylate ligands (acetate, benzoate, or trifluoroacetate) determines the type and strength of the three magnetic exchange couplings (J(1), J(2), and J(3)) present between the metal ions. Experimentally measured magnetic moments in high magnetic fields show that, upon electron density withdrawal, the main antiferromagnetic exchange constant J(1) decreases from -2.2 K for the [Mn(4)(OAc)(4)] cluster to -1.9 K for the [Mn(4)(H(5)C(6)COO)(4)] cluster and -0.6 K for the [Mn(4)(F(3)CCOO)(4)] cluster, while J(2) decreases from -1.1 K to nearly 0 K and J(3) changes to a small ferromagnetic coupling. These experimental results are further supported with density-functional theory calculations based on the obtained crystallographic structures of the [Mn(4)(OAc)(4)] and [Mn(4)(F(3)CCOO)(4)] clusters.

A Computational Framework for Magnetic Polyoxometalates and Molecular Spin Structures: CONDON 2.0

“…A family of tetranuclear Mn II complexes with Mn 4 O 4 cubane cores illustrates this capability. The neutral complexes [Mn 4 L 2 (RCOO) 4 ] (L represents the two‐fold deprotonated pentadentate pyridine‐diimine‐type ligand 2,6‐bis(1‐(2‐hydroxyphenyl)iminoethyl)pyridine) have recently been isolated for R = CH 3 ( 8 ), C 6 H 5 ( 9 ), and CF 3 ( 10 ) 43. The carboxylate ligands (acetate, benzoate, and trifluoroacetate) differ significantly in the electron density withdrawal caused by the R groups their bridging ligands.…”

“…As expected for such a spin‐only system, fitting the data with CONDON 2.0 resulted in very similar parameters for J 1–3 , though the multiparameter fitting implementation in CONDON allows for a further refinement leading to a slight improvement in the quality of the fit, measured as ${SQ = \sqrt {\sum\nolimits_{i = 1}^n {([\chi _{obs} (i) - \chi _{calc} (i)]/\chi _{obs} (i))^2 } } }$ . Using standard values for the spin‐orbit coupling constant and the Racah parameters (ζ=347 cm −1 , B =960 cm −1 , C =3325 cm −1 ) for all three compounds, fitting to all data sets (Figure 9) yields for 8 : J 1 = J 2 =−1.17 cm −1 , J 3 =+0.13 cm −1 (SQ decreases from 2.2 % for the published fitted data43 to 1.7 %); for 9 : J 1 =−1.39 cm −1 , J 2 =−0.066 cm −1 , J 3 =+0.19 cm −1 (SQ decreases from 2.1 % to 0.8 %); for 10 : J 1 = J 2 =−0.32 cm −1 , J 3 =+0.045 cm −1 (SQ decreases from 1.5 % to 1.2 %). Similar measurements on Co II analogues of 8–10 are currently underway.…”

Covalent functionalization of carbon nanotubes with tetramanganese complexes

“…[Mn II 4 L 2 (CH 3 COO) 4 ], in the following abbreviated as {Mn 4 }, was synthesized according to Ref. 9. We used two different types of CNT material.…”

“…Neither the geometrical structure nor the overall magnetic character of the complex is affected by the choice of the carboxylate ligands. However, some degree of tuning of the strength of the magnetic interaction between the manganese centers has been achieved when the acetates were replaced by more electron withdrawing carboxylates such as benzoate and trifluoroacetate 9.…”

Quantum transport in carbon nanotubes covalently functionalized with magnetic molecules

“…For this purpose, we functionalize a CNT with charge‐neutral {Mn 4 } complexes, specifically [Mn II 4 L 2 (OAc) 4 ] (H 2 L = 2,6‐bis(1‐(2‐hydroxyphenyl) iminoethyl) pyridine). The choice of this particular {Mn 4 } complex, featuring a cubane‐type Mn 4 O 4 core, is primarily motivated by its high stability and the fact that their magnetic properties do not strongly change upon ligand exchange .…”