Expanding the Family of Pyrazole‐Bridged Mixed‐Spin and Mixed‐Valence Tetranuclear [2 × 2] Iron Grid Complexes

Abstract

The new bis(tridentate) compartmental proligand 3,5-bis[6-(1-methyl-1H-imidazole-2-yl)pyrid-2-yl]pyrazole (HL im ) was synthesized and used for the self-assembly of a rugged tetranuclear [2 × 2] Fe 4 grid complex [Fe II 4 L im 4 ](ClO 4 ) 4 (1), which readily forms upon reaction of HL im with Fe(ClO 4 ) 2 ·6H 2 O in the presence of a base (NaOtBu). According to cyclic voltammetry, all-ferrous 1 can be sequentially oxidized in four steps, with pronounced stability of the twice-oxidized dimixed-4333 Scheme 2. Synthesis of the new ligand HL im . 4335 Figure 2. SWVs in the cathodic (A) and anodic regimes (B) and anodic regime of the CV (scan rate 1000 mV/s; C) of [Fe II 4 L im 4 ](ClO 4 ) 4 (1) in CH 3 CN/0.1 M NBu 4 PF 6 as an electrolyte; potentials vs. SCE. The dashed line in C shows a CV in the range from 0.2 to 1.1 V (at different current scale).

Bis(pyrazolato) Bridged Diiron Complexes: Ferromagnetic Coupling in a Mixed‐Valent HS‐FeII/LS‐FeIII Dinuclear Complex

“…[5,11] As in the mixed-spin systems, in these di-mixedvalence complexes metal ions of the same oxidation state are located at diagonally opposed vertices of the grid, resulting in a [Fe II -Fe III -Fe II -Fe III ] configuration; [5] a similar situation has also been observed in di-mixed-valence tetrairon grids and squares with other types of ligand strands. [16][17][18][19] For complexes based on pyrazolate-bridging ligand scaffolds [L R ] -, magnetic signatures evidenced ferromagnetic coupling between adjacent HS-Fe II and LS-Fe III ions. [5,11,19] More generally, mixed-valent Fe II /Fe III iron complexes have been of interest in a variety of fields ranging from moleculebased magnetic materials to models of metalloprotein active sites.…”

“…[16][17][18][19] For complexes based on pyrazolate-bridging ligand scaffolds [L R ] -, magnetic signatures evidenced ferromagnetic coupling between adjacent HS-Fe II and LS-Fe III ions. [5,11,19] More generally, mixed-valent Fe II /Fe III iron complexes have been of interest in a variety of fields ranging from moleculebased magnetic materials to models of metalloprotein active sites. [20][21][22] The magnetic coupling as well as the extent of delocalization according to the Robin and Day classification [23] may vary widely and depend on, inter alia, the individual coordination environments and the nature of the bridging ligands.…”

“…The Fe•••Fe distance in 2 (4.14 Å) is much larger than in mixed-valent [Fe 2 (OH) 3 (tmtacn) 2 ] 2+ (2.51 Å), so that any direct interaction can be safely excluded, as in the case of 1. Ferro-magnetic coupling has previously been observed in mixedvalent [2 × 2] grids of ligands [L R ]with similar Fe•••Fe separations in which adjacent metal ions are bridged by a single pyrazolate: [5,19] X-ray crystallography and Mössbauer spectroscopy of the mixed-valent grid [L H 4 Fe II 2 Fe III 2 ] 6+ showed the presence of HS-Fe II and LS-Fe III ions situated at opposite corners, and magnetic measurements revealed ferromagnetic coupling between the HS-Fe II and LS-Fe III ions (J = +7.9 cm -1 ). [5] Similarly, a related tetranuclear mixed valent grid [L im 4 Fe II 2 Fe III 2 ] 6+ with peripheral imidazole units instead of pyridine moieties in the ligand strand also displayed ferromagnetic coupling via two intramolecular pathways with J 1 = +5.0 cm -1 and J 2 = +3.0 cm -1 ; these values are comparable to the coupling observed in the mixed valent dinuclear complex 2.…”

“…[5] Similarly, a related tetranuclear mixed valent grid [L im 4 Fe II 2 Fe III 2 ] 6+ with peripheral imidazole units instead of pyridine moieties in the ligand strand also displayed ferromagnetic coupling via two intramolecular pathways with J 1 = +5.0 cm -1 and J 2 = +3.0 cm -1 ; these values are comparable to the coupling observed in the mixed valent dinuclear complex 2. [19] Partial orthogonality of the magnetic orbitals at the neighbouring HS-Fe II and LS-Fe III sites was proposed to give rise to these magnetic signatures of the mixed-valent Fe II 2 Fe III 2 grid complexes. Substantial ferromagnetic coupling due to orthogonality of magnetic orbitals has further been observed in bis(pyrazolato) or pyrazolato/peroxido bridged dicopper(II) complexes with Cu-N-N-Cu or Cu-O-O-Cu torsion angles close to 90°, respectively.…”

“…[ 16–19 ] For complexes based on pyrazolate‐bridging ligand scaffolds [L R ] – , magnetic signatures evidenced ferromagnetic coupling between adjacent HS‐Fe II and LS‐Fe III ions. [ 5,11,19 ]…”