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

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 [(Mn2Mn4O2)-Mn-III-O-II(pyz)(0.61)/(MeOH)(0.39)(o-(NO2)C6H4COO)(10)center dot(H2O)center dot{(CH3)(2)CO}(2)]center dot(CH3)(2)CO (2) and [(Mn2Mn4O2)-Mn-III-O-II- pyz)(0.28)/(MeCN)(3.72)(o-(NO2)C6H4COO)(10)center dot(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 [(Mn2Mn4II)-Mn-III] 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 Mn-6 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.