The Carboxylate Twist: Hysteretic Bistability of a High‐Spin Diiron(II) Complex Identified by Mössbauer Spectroscopy

Abstract

Dedicated to Professor Dieter Fenske on the occasion of his 70th birthdayCarboxylates have always been among the most versatile and thus widely used ligands in coordination chemistry.[1] On account of their various possible modes of coordination, both terminal and bridging, they form a wide range of mono-and polynuclear complexes and are often found as ligands in metalloproteins. Facile interconversion between carboxylate binding modes, termed the carboxylate shift, is implicated as a critical step in many enzymatic reactions and is known to play a fundamental role in the catalytic cycles of, for example, soluble methane monooxygenase (sMMO) and the R2 subunit of ribonucleotide reductase (RNR R2).[2] Just recently, we reported a pyrazolate-based [3] diiron(II) complex in which the carboxylate shift could be observed in the solid state for the first time.[4] We reasoned that further exploration of this novel class of diiron(II) carboxylate cores may yield more detailed insight into carboxylate dynamics beyond the common carboxylate shift. Herein we describe a new bioinspired high-spin (m-pyrazolato)(m-carboxylato)diiron(II) complex 1 based on the bis(tetradentate) ligand HL ( Figure 1).[5] Complex 1 features pronounced conformational flexibility of the exogenous acetato ligand [6] which gives rise to an unprecedented thermal hysteresis, identified by Mçss-bauer spectroscopy and SQUID magnetometry (SQUID = superconducting quantum interference device). This hysteresis does not involve any spin transition or any configurational change and is hence a borderline case of the wellknown carboxylate shift. We will coin the underlying process the "carboxylate twist".Switchable molecule-based materials are indeed expected to have great potential for future data-storage and sensing applications. Hence there is much interest in molecular systems that show hysteretic bistability near room temperature, with the spin-crossover [7] and valence tautomeric compounds [8] probably being the most prominent examples. In the case of iron complexes, Mçssbauer spectroscopy has proven to be a valuable tool for investigating the effects associated with bistability.[7c, 9] However, hysteretic behavior distinct from spin crossover has not yet been observed for iron complexes, and hence has not yet been investigated by Mçssbauer spectroscopy.We previously reported a (m-azido)(m-pyrazolato)dinickel(II) complex which showed thermal magnetic bistability induced by a twist of the bridging azido coligand along the Ni-NNN-Ni dihedral angle, turning the antiferromagnetic exchange coupling on and off.[10] This behavior gives rise to an unprecedented hysteresis, clearly distinct from spin crossover and valence tautomerism scenarios. It is worth noting that there are only a handful of other examples reported in the literature where a subtle geometric distortion induces bistability and hysteresis without any spin or redox transitions.[11] Novel effects are now observed in the new pyrazolato-bridged diferrous complex 1 in which a flexible carboxylate is c...

A Heterobimetallic FeIIIMnII Complex of an Unsymmetrical Dinucleating Ligand: A Structural and Functional Model Complex for the Active Site of Purple Acid Phosphatase of Sweet Potato

“…The 57 Fe Mössbauer spectrum of 1 was analyzed using a Lorentzian line doublet and confirms unambiguously the high‐spin Fe III configuration (Figure S3 in the Supporting Information), which shows a single quadruple doublet with δ = 0.49 mm s –1 , Δ E Q = 0.71 mm s –1 , and linewidth Γ = 0.68 mm s –1 . The rather large linewidth might be rationalized in view of the presence of iron(III) ions in slightly different environments in the microcrystalline material (perhaps resulting from slightly different torsion angles of the bridging acetate39).…”

Modelling Binuclear Metallobiosites: Insights from Pyrazole‐Supported Biomimetic and Bioinspired Complexes

“…The acetato‐bridged structures described above represent snapshots of different static situations, whereas at more biologically relevant temperatures truly dynamic behavior is also feasible. In fact, the bioinspired di‐iron(II) complex 30 has recently been observed to show thermal hysteresis with respect to magnetization, induced by exactly such conformational “twisting” of the exogenous acetato co‐ligand (Scheme ) 74. Analogous plasticity with respect to carboxylate coordination is thought to be an intrinsic component of several zinc and iron metalloenzymes 7,75–77…”

Quenching and Restoration of Orbital Angular Momentum through a Dynamic Bond in a Cobalt(II) Complex

“…The change of magnetization observed here originates from the reversible quenching and restoration of the orbital angular momentum. This mechanism differs greatly from the preexisting ones, such as spin crossover between the high- and low-spin states of a transition-metal ion, − charge transfer, , spin-crossover-coupled charge transfer, − monomer–dimer transitions of the organic radicals, − spin-Peierls-type transitions, and modification of magnetic coupling induced by a structural phase transition. − …”

“…This mechanism differs greatly from the preexisting ones, such as spin crossover between the high-and lowspin states of a transition-metal ion, [3][4][5][6] charge transfer, 12,15 spincrossover-coupled charge transfer, [7][8][9][10][11] monomer-dimer transitions of the organic radicals, [16][17][18] spin-Peierls-type transitions, 35 and modification of magnetic coupling induced by a structural phase transition. [36][37][38]…”