Monomeric, trimeric, and tetrameric transition metal complexes (Mn, Fe, Co) containing N,N-bis(2-pyridylmethyl)-2-aminoethanol/-ate: preparation, crystal structure, molecular magnetism and oxidation catalysis

Abstract

The reaction of N,N–bis(2-pyridylmethyl)-2-aminoethanol (bpaeOH), NaSCN/NaN₃, and metal (M) ions [M = Mn(II), Fe(II/III), Co(II)] in MeOH, leads to the isolation of a series of monomeric, trimeric, and tetrameric metal complexes, namely [Mn(bpaeOH)(NCS)₂] (1), [Mn(bpaeO)(N₃)₂] (2), [Fe(bpaeOH)(NCS)₂] (3), [Fe₄(bpaeO)₂(CH₃O)₂(N₃)₈] (4), [Co(bpaeOH)(NCS)₂] (5), and [Co₃(bpaeO)₂(NO₃)(N₃)₄](NO₃) (6). These compounds have been investigated by single crystal X-ray diffractometry and magnetochemistry. In complex 1 the Mn(II) is bonded to one bpaeOH and two thiocyanate ions, while in complex 2 it is coordinated to a deprotonated bpaeO⁻ and two azide ions. The oxidation states of manganese ions are 2+ for 1 and 3+ for 2, respectively, indicating that the different oxidation states depend on the type of binding anions. The structures of monomeric iron(II) and cobalt(II) complexes 3 and 5 with two thiocyanate ions are isomorphous to that of 1. Compounds 1, 2, 3, and 5 exhibit high-spin states in the temperature range 5 to 300 K. 4 contains two different iron(III) ions in an asymmetric unit, one is coordinated to a deprotonated bpaeO⁻, an azide ion, and a methoxy group, and the other is bonded to three azide ions and two oxygens from bpaeO⁻ and a methoxy group. Two independent iron(III) ions in 4 form a tetranuclear complex by symmetry. 4 displays both ferromagnetic and antiferromagnetic couplings (J = 9.8 and −14.3 cm⁻¹) between the iron(III) ions. 6 is a mixed-valence trinuclear cobalt complex, which is formulated as Co^III(S = 0)–Co^II(S = 3/2)–Co^III(S = 0). The effective magnetic moment at room temperature corresponds to the high-spin cobalt(II) ion (∼4.27 μ_B). Interestingly, 6 showed efficient catalytic activities toward various olefins and alcohols with modest to excellent yields, and it has been proposed that a high-valent Co^V–oxo species might be responsible for oxygen atom transfer in the olefin epoxidation and alcohol oxidation reactions.