Synthesis, Structure, and Magnetic Studies of Manganese–Oxygen Clusters of Reduced Coordination Number, Featuring an Unchelated, 5‑Coordinate Octanuclear Manganese Cluster with Water-Derived Oxo Ligands

The synthesis of reduced coordination (less than 6),
unchelated
manganese oxygen cluster systems is described. Addition of phenols
to Mn­(NR2)2 (R = SiMe3) results in
protolytic amide ligand replacement, and represents the primary entry
into the described chemistry. Addition of PhOH to Mn­(NR2)2 results in the formation of the heteroleptic dimer
Mn2(μ-OPh)2(NR2)2(THF)2 (1). Usage of the sterically larger
2,6-diphenylphenol (Ph2C6H3OH) as
the ligand source results in the formation of a 3-coordinate heteroleptic
dimer without THF coordination, Mn2(μ-OC6H3Ph2)2(NR2)2 (2). Attempts to generate 2 in the presence
of THF or Et2O resulted in isolation of monomeric Mn­(OC6H3Ph2)2L2 (3, L = THF, Et2O). Use of the sterically intermediate
2,4,6-trimethylphenol (MesOH) resulted in formation of the linear
trinuclear cluster Mn3(μ-OMes)4(NR2)2(THF)2 (4). Reaction
of Mn­(NR2)2 with PhOH in the presence of water,
or reaction of 1 with water, results in the formation
of a 5-coordinate, unchelated Mn–O cluster, Mn8(μ5-O)2(μ-OPh)12(THF)6 (5). Preparation, structures, steric properties, and
magnetic properties are presented. Notably, complex 5 exhibits a temperature-dependent phase transition between a 4-spin
paramagnetic system at low temperature, and an 8-spin paramagnetic
system at room temperature.