Novel hetero-bimetallic metalla-macrocycles based on the bis-1-pyridyl ferrocene [Fe(eta(5)-C5H4-1-C5H4N)(2)] ligand. Design, synthesis and structural characterization of the complexes [Fe(eta(5)-C5H4-1-C5H4N)(2)](Ag-I)(2)(2+)/(Cu-II)(2)(4+)/(ZnII)(2)(4+)