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.