Synthesis, Structure, and Characterization of the New [L(OH)Fe(μ-O)Fe(OH₂)L]³⁺ Complex (L = N,N‘-Dimethyl-N,N‘-bis(2-pyridylmethyl)ethane-1,2-diamine). Detection of an Equilibrium with the Protonated Diamond Form [LFe(μ-O)(μ-OH)FeL]³⁺ in Acetonitrile

Abstract

The complex [L(H2O)Fe(μ-O)Fe(OH)L](ClO4)3·H2O, where L = N,N‘-dimethyl-N,N‘-bis(2-pyridylmethyl)ethane-1,2-diamine, was synthesized. It crystallizes in the orthorhombic space group P212121 with a = 13.283(5) Å, b = 16.050(9) Å, c = 20.050(9) Å, V = 4476(6) Å3, and Z = 4. It presents the hydrogen-bonded [(H2O)Fe(μ-O)Fe(OH)]3+ core unit characterized by an Fe−O−Fe angle of 137.5(2)° and an Fe−Fe distance of 3.396(1) Å. The measurement of the magnetic susceptibility as a function of the temperature indicated an antiferromagnetic coupling between the two high-spin Fe(III) ions J = −184 cm-1 (H = −J S 1 ·S 2 ). In the solid state the symmetric stretching vibration was observed at 438 cm-1. Upon dissolution in dry acetonitrile, this vibration was no longer detected and an intense band was observed at 600 cm-1. This frequency can be correlated with an Fe−O−Fe angle of 111°. This value suggests that the species which exists in these conditions is the protonated diamond core [Fe(μ-O)(μ-OH)Fe]3+, analogous to that identified by Zang et al. (J. Am. Chem. Soc. 1994, 116, 3653) and by Hazell et al. (J. Chem. Soc., Dalton Trans. 1995, 707). Upon addition of water, the original aquated species is observed in equilibrium with the protonated diamond unit. By analysis of the changes in UV−vis spectra as a function of the amount of water added, the equilibrium constant of the formation of the aquated species was found to be 5.4 M-1.

Synthesis, Structure and Characterization of the New Complex [L1(H2O)Fe(μ-O)Fe(OH2)L1]4+ [L1 =N,N′-Bis(1-methylimidazolyl-2-methyl)-N,N′-Bismethyl-1,2-ethanediamine] − Formation of the (μ-O)(μ-H3O2) Complex upon Deprotonation

“…For compound 2 it is possible to evaluate from the J value an average FeϪ(µ-O) distance of 1.84 Å , a value slightly greater than the ones measured by X-ray diffraction studies on the three [(H 2 O)Fe(µ-O)Fe(OH)] 3ϩ core structures for which the P parameter ranges from 1.80 to 1.82 Å . [14,15,24] One can argue that the exchange coupling constant calculated here for compound 2 is closer to the values determined for the double-bridged [LFe(µ-O)(µ-OH)FeL] 3ϩ systems {Ϫ113 cm Ϫ1 for L ϭ 6-Me 3 -TPA, [19] Ϫ106 cm Ϫ1 for L ϭ bispicEt 2 en, [19] Ϫ84 cm Ϫ1 for L ϭ bispicMe 2 en [16] (see also remark 42 in ref. [19] )}, a formulation that cannot be rejected on the basis of the elementary analysis alone.…”

“…The J value calculated for 1 is of the same magnitude as the one measured for the other di-aquo-di-iron-µ-oxo complexes: [24] Ϫ220 cm Ϫ1 in [(phen) 2 [22] and [27] However, the value determined for 2 is significantly less negative than the ones measured for [(H 2 O)Fe(µ-O)Fe(OH)] 3ϩ core complexes: Ϫ194 cm Ϫ1 with TPA, [24] Ϫ196 cm Ϫ1 with 5-Et 3 -TPA [15] and Ϫ184 cm Ϫ1 with bispicMe 2 en. [14] This weaker coupling interaction reveals a less efficient superexchange pathway. As previously mentioned in the literature, the latter is essentially governed by the FeϪ(µ-O) distances for bridging angles ranging from 120 to 180°.…”

“…[14Ϫ18] Starting with a [(H 2 O)Fe(µ-O)Fe(OH)] 3ϩ core unit, we have shown recently the possibility to obtain the double-bridged structure [Fe(µ-O)(µ-OH)Fe] 3ϩ . [14] The protonated form of the diamond core structure is thus formed in the same way as would be the di-iron-di-µ-oxo unit of compound Q, an oxygenated group converting from a η 1 -to a µ-type coordination. This behavior seems to be common to several systems.…”

“…This behavior seems to be common to several systems. [14,16,19] In our effort to model the active site of MMOH, the synthesis of two new iron-oxo complexes with the tetradentate ligand containing imidazole groups N,NЈ-bis(1-methylimidazolyl-2-methyl)-N,NЈ-bismethyl-1,2-ethanediamine (L 1 , see Scheme 1) are presented. (2).…”

Synthesis, Structure and Characterizations in Solid State and Solution of Dinuclear Pentacoordinated FeII and MnII Complexes and of a Linear Tetranuclear FeIII Complex Obtained with the Ligand N,N,N′,N′‐Tetrakis[(6‐methyl‐2‐pyridyl)methyl]propane‐1,3‐diamine

“…In this 320Ϫ350 nm area, intense Cl Ϫ ǞFe III charge-transfer bands are expected, as reported previously for dinuclear complexes with ϪOϪFeCl 3 fragments, [23,25] and possibly amine-to-metal charge transfer as well. [33,34] The other intense band (381 nm, ε ϭ 20000  buted to an oxoǞFe III charge-transfer transition. [35,36] The spectral features reported for µ-oxodiiron() complexes with aminopyridine and chloro ligands, which present an intense band around 320 nm and another at about 380 nm, [37,38] are also consistent with these assignments.…”