Reduction of Dioxygen by a Dimanganese Unit Bonded Inside a Cavity Provided by a Pyrrole-based Dinucleating Ligand

A novel class of dinucleating ligands has been introduced into manganese chemistry to study the reactivity of this metal towards dioxygen under strictly controlled conditions. Such N4 ligands combine some of the major peculiarities of tetradentate Schiff bases and the porphyrin skeleton. They are derived from the condensation between 2-pyrrolaldehyde and ethylenediamine or o-phenylenediamine, leading to pyrenH2 (LH2, 1), pyrophenH2 (L'H2, 2) and Me2pyrophenH2, (L"H2, 3), respectively. Their metallation with [Mn3-(Mes)6] (Mes = 2,4,6-trimethylphenyl) led to [Mn2L2] (4), [MnL'(thf)2] (5) and [MnL"(thf)2] (6). Complex 4 displays a double-stranded helical structure, while 5 and 6 are mononuclear complexes containing hexacoordinated metals. Regardless of their structure, complexes 5 and 6 behave in a similar manner to 4 in their reaction with dioxygen, namely, as a dimetallic unit inside a cavity defined by two dinucleating ligands. These reactions led to dinuclear MnIII/MnIV oxo-hydroxo derivatives, [Mn2L2(mu-O)(mu-OH)] (7), [Mn2L'2(mu-O)(mu-OH)] (8) and [Mn2L"2(mu-O)(mu-OH)] (9), in which the two Mn ions are strongly antiferromagnetically coupled [J = -53 (7), J = -64 (8), J = -60 cm(-1) (9)]. The crystal structure of 7 could only be solved with synchrotron radiation as the crystals diffracted very poorly and suffered from twisting and disorder. The formation of 7-9 has been proposed to occur through the formation of an intermediate dinuclear hydroperoxo species.