A New N,N,O Chelate for Transition Metal Chemistry:  Fe₅ and Fe₆ Clusters from the Use of 6-Hydroxymethyl-2,2‘-bipyridine (hmbpH)

The initial use of the anion of 6-hydroxymethyl-2,2‘-bipyridine (hmbpH) as a chelate in coordination chemistry is described. The syntheses, crystal structures, and magnetochemical characterization are reported of four new iron(III) clusters [Fe₅O₂(OH)(O₂CMe)₅(hmbp)₃](ClO₄)₂ (1) and [Fe₆O₂(OH)₂(O₂CR)₆(hmbp)₄](NO₃)₂ (R = Ph (2), Me (3), Bu^t (4); hmbpH = 6-hydroxymethyl-2,2‘-bipyridine). The reaction of Fe(ClO₄)₃, hmbpH, and sodium acetate in a 1:1:∼4 ratio in EtOH gave 1, and the reaction between [Fe₃O(O₂CR)₆(H₂O)₃](NO₃) (R = Ph, Me, Bu^t) and hmbpH in a 1:1 ratio in MeCN gave 2−4, respectively. The core of 1 consists of a [Fe₄(μ₃-O)₂]⁸⁺ butterfly unit to which is attached a fifth Fe atom by bridging O atoms. The core of 2−4 also consists of a [Fe₄(μ₃-O)₂]⁸⁺ butterfly unit to which are attached an Fe atom on either side by bridging O atoms. Variable-temperature (T) and -field (H) solid-state DC and AC magnetization (M) studies were carried out on complexes 1−4 in the 5.0−300 K range. Fitting of the data revealed that 1 has an S = 5/2 ground state spin whereas 2−4 possess an S = 5 ground state. Fitting of the M/Nμ_B vs H/T data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave values of the axial ZFS parameter |D| of 0.75, 0.36, 0.46, and 0.36 cm^-1 for 1−4, respectively.