Bis[μ2-2,2′-dimethyl-N,N′-bis-(2-oxido-benz-yl)propane-1, 3-diamine]-14 O,N,N′,O′:22 O,O′;2 2 O,O′:34 O,N,N′,O′-bis-(N,N′- dimethyl-formamide)-1O,3O-di-k2-formato-1:22 O:O′;2:3 2 O:O′-trinickel(II)