Bis(pyrazolato) Bridged Diiron Complexes: Ferromagnetic Coupling in a Mixed‐Valent HS‐Fe^II/LS‐Fe^III Dinuclear Complex

Abstract

Using a new bis(tridentate) compartmental pyrazolate‐centered ligand HL, the bis(pyrazolato)‐bridged diiron complex [L2FeII2][OTf]2 (1) and its singly oxidized mixed‐valent congener [L2FeIIFeIII][OTf]3 (2) have been synthesized and structurally characterized. While 1 features two HS‐FeII ions coordinated to two cis‐axial pyridine moieties in a highly distorted octahedral environment, the metal ions in 2 are coordinated by the ligand strand in a trans‐axial configuration. Very different Fe–N bond lengths and distinctly different coordination polyhedra are associated with pronounced valence localization in the case of 2. The electronic structures and magnetic properties of 1 and 2 have been further investigated by Mössbauer spectroscopy and variable temperature magnetic susceptibility measurements. In the case of 1, weak antiferromagnetic coupling is observed between the two HS‐FeII ions (J = –0.6 cm–1), while the HS‐FeII and LS‐FeIII ions in 2 are ferromagnetically coupled (J = +5.2 cm–1) to give an ST = 5/2 ground state with significant zero‐field splitting (DFe(II) = 2.3 cm–1). The findings are rationalized with the help of DFT computations.

Effects on the magnetic interaction caused by molecular recognition in complexes of 1,2-azole-based oxamate and [Cu(bpca)]+ units

“…Focusing on 1H-pyrazole-based ligands, their coordination chemistry is very rich due to the possibility of coordinating to metal atoms in their neutral or deprotonated form since its NH group can be easily deprotonated in the presence of transition metal ions and a base (apparent pK a = 19,8 for 1Hpyrazole in DMSO). The pyrazolate anion acts as a bismonodentate kN:kN′ ligand, forming di-, [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] tri-, [22][23][24][37][38][39][40][41][42][43][44][45][46][47] tetra- 22,[48][49][50][51][52][53][54] and polymetallic synthons [55][56][57][58][59][60] or even coordination polymers. [61][62]…”

“…Focusing on 1 H -pyrazole-based ligands, their coordination chemistry is very rich due to the possibility of coordinating to metal atoms in their neutral or deprotonated form since its NH group can be easily deprotonated in the presence of transition metal ions and a base (apparent p K a = 19,8 for 1 H -pyrazole in DMSO). The pyrazolate anion acts as a bis-monodentate kN:kN′ ligand, forming di-, 22–38 tri-, 22–24,37–47 tetra- 22,48–54 and polymetallic synthons 55–60 or even coordination polymers. 61–64 Relatively strong magnetic interactions between paramagnetic centers such as Cu II , Co II , and Fe II can be mediated through this coordination mode.…”