Tetrakis[μ-2-(3,4-dimethoxyphenyl)acetato]-κ4O:O′;κ3O,O′:O;κ3O:O,O′-bis{[2-(3,4-dimethoxyphenyl)acetato-κ2O,O′](1,10-phenanthroline-κ2N,N′)samarium(III)}