Tetrakis[μ-2-(3,4-dimethoxyphenyl)acetato]-κ3O1,O1′:O1;κ3O1:O1,O1′;κ4O1:O1′-bis{[2-(3,4-dimethoxyphenyl)acetato-κ2O1,O1′](1,10-phenanthroline-κ2N,N′)erbium(III)}