A New Tetranuclear Oxohydroxoiron(III) Cluster: Crystal Structure, Magnetic Properties, and Exafs Investigation of [L4Fe4(μ-O)2(μ-OH)4]I4.3H2O (L = 1,4,7-Triazacyclononane)

Hydrolysis of the cation [L₂Fe₂(acac)₂(μ-O)]²⁺in aqueous solution yields the new tetranuclear species [L₄Fe₄(μ-O)₂μ-OH)₄]⁴⁺, where L represents the cyclic amine 1,4,7-triazacyclononane (L; C₆H_l5N₃) and acac is acetylacetonate(l-). The green tetraiodide trihydrate and the tetraperchlorate trihydrate have been isolated as solid materials. The crystal structure of [L₄Fe₄(μ-O)₂(μ-OH)₄]I₄.3H₂O has been determined by X-ray crystallography. Crystal data: monoclinic; P2₁/a (C2h); a = 14.653 (6) Å; b = 19.092 (7) Å; c = 17.309 (8) Å; = 91.80 (3)°; V = 4839.9 A³; Z = 4. Four ferric ions are connected by two μ₂-oxo and four μ2-hydroxo bridges, forming the distorted adamantane-like core [Fe₄(μ-O)₂(μ-OH)₄]⁴⁺; each Fe^IIIcenter is capped by the tridentate amine L (fac-FeN₃O₃donor set at each Fe^III), The complex may be envisaged as a dimer of the dimer containing a [Fe-0-Fe]⁴⁺core. From temperature-dependent susceptibility measurements strong intramolecular antiferromagnetic coupling between all four high-spin ferric ions is established (S = 0 ground state; J = -106.3 (2) cm^-1and J’= -15.1 (2) cm^-1). EXAFS spectra of the tetraiodide and of the tetraperchlorate are in excellent agreement with the results obtained by crystallography: in particular, the two different intramolecular Fe.Fe distances at 3.33 and 3.53 Å and the differing Fe-O_oxo and Fe-O_hydroxo distances at 1,82 and 2.00 A, respectively, are clearly detected in both salts. In alkaline solution the tetranuclear complex is shown to dissociate, generating most probably two of the binuclear species [LFe(OH)₂(μ-O)Fe(OH)₂L]. The process is reversible; reacidification regenerates the tetranuclear complex.