Synthesis of mixed-valence hexanuclear Mn(II/III) clusters from its Mn(II) precursor: variations of catecholase-like activity and magnetic coupling

Abstract

One Mn(II) coordination polymer, [Mn(o-(NO₂)C₆H₄COO)₂(pyz)(H₂O)]_n (1), has been synthesized and oxidized with n-Bu₄NMnO₄ in non-aqueous media to two mixed-valence hexanuclear Mn(II/III) complexes [Mn^III₂Mn^II₄O₂(pyz)_0.61/(MeOH)_0.39(o-(NO₂)C₆H₄COO)₁₀•(H₂O)•{(CH₃)₂CO}₂]•(CH₃)₂CO (2) and [Mn^III₂Mn^II₄O₂(pyz)_0.28/(MeCN)_3.72(o-(NO₂)C₆H₄COO)₁₀•(H₂O)] (3) (where pyz = pyrazine). All three complexes were characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. The structural analyses reveal that complex 1 is comprised of linear chains of pyz bridged Mn(II), which are further linked to one another by syn–anti carboxylate bridges, giving rise to a two-dimensional (2D) net. Complexes 2 and 3 feature mixed valence [Mn^III₂Mn^II₄] units in which each of the six manganese centres reside in an octahedral environment. Apart from the variations in terminal ligands (acetone for 2 and acetonitrile for 3), the complexes are very similar. Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, the catecholase-like activity of the complexes has been studied and it is found that the mixed valent Mn6 complexes (2 and 3) are much more active towards aerial oxidation of catechol compared to the Mn(II) complex (1). Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed the presence of antiferromagnetic coupling in all three complexes. The magnetic data have been fitted with a 2D quadratic model derived by Lines, giving the exchange constant J/k_B = −0.0788(5) K for 1. For 2 and 3, antiferromagnetic interactions within the Mn₆ cluster have been fitted with models containing three exchange constants: J_A/k_B = −70 K, J_B/k_B = −0.5 K, J_C/k_B = −2.9 K for 2 and J_A/k_B = −60 K, J_B/k_B = −0.3 K, J_C/k_B = −2.8 K for 3.