Poly[(μ 3 -benzene-1,3-dicarboxylato-κ 4 O , O ′: O ′′: O ′′′)(μ 4 -benzene-1,3-dicarboxylato-κ 5 O , O ′: O ′: O ′′: O ′′′)(μ 2 -1,3-di-4-pyridylpropane-κ 2 N : N ′)dicobalt(II)]