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), But (4); hmbpH=6-hydroxymethyl-2,2'-bipyridine). The reaction of Fe(ClO4)3, hmbpH, and sodium acetate in a 1:1: approximately 4 ratio in EtOH gave 1, and the reaction between [Fe3O(O2CR)6(H2O)3](NO3) (R=Ph, Me, But) and hmbpH in a 1:1 ratio in MeCN gave 2-4, respectively. The core of 1 consists of a [Fe4(mu3-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(mu3-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/NmicroB 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.