Synthesis and structural characterisation of the novel iron helicate [Fe2(µ-L)4(µ-Cl)2][FeCl4]2 and metalla-macrocycle [Fe2(µ-L)2(THF)4Cl2][FeCl4]2 {L = N,N′-di(n-butylcarbamoyl)pyridine-2,6-dicarboxamide}