Studies on the relative stabilities of Mn(II) and Mn(III) in complexes with N4O2 donor environments: crystal structures of [Mn(pybzhz)2] and [Mn(Ophsal)(imzH)2] ClO4 (pybzhz=N-(benzoyl)-N′-(picolinylidene) hydrazine, Ophsal=N,N′-o-phenylenebis(salicylideneimine), imzH=imidazole)