Synthesis and properties of a heteroleptic µ-nitrido tetrapyrrolic complex

Abstract

Multi-component organic chromophores having intense visible absorption characteristics covering large part of solar radiation are currently an object of much attention in view of their exploitation as artificial light-harvesting antenna. In particular those containing the phthalocyanine (Pc) and porphyrin (P) macrocyclic ligands have been recently studied being interesting for the peculiar photoinduced energy transfer processes[1] and being also attractive for their potential use in the field of molecular photovoltaics, artificial photosynthesis and molecular electronic applications.[2-4] Several strategies can be used to link each other the two different subunits to obtain a hetero phthalocyanine-porphyrin dyads, resulting in different geometries and properties. It is possible for example to link the different moieties through a conjugated or not spacer between the peripheral positions of one macrocycle[5] toward either the periphery or the central metal of the other macrocycle.[1] Moreover single atom (O, N, C) bridged systems as μ-oxo, μ-nitrido and μ-carbido can also allow a combination, via metal centres, leading to homo/heterometallic and homo/heteroleptic systems.[6] To the aim recalled above, it should be important that the orbital interactions could promote an electronic communication along the stack thereby optical absorption will change and, in the case of complementary chromophores, extend. As other analogues mixed-ligand porphyrin/phthalocyanine bridged systems are expected to mutate the electronic configuration and to extend the visible absorption to the near IR wavelength. Among the μ-nitrido species, the mixed-ligand tetraphenylporphyrin-phthalocyanine μ-nitrido complex of iron was synthesised and fully characterised as solid material by Ercolani et al.;[7,8] it showed a good thermal stability (> 300°C) and an interesting electronic and magnetic exchange between the iron centres trough the linear Fe-N-Fe bond system. Unfortunately its optical (UV-vis) spectrum was not reported likely because of its very scarce solubility in non-donor solvent. Being this feature indispensable for the processability of these material in view of technological applications, we attempted to increase the solubility of such species and we present here preliminary investigations on the substituted analogous (μ-nitrido)[((tetraphenylporphyrinato) iron)-(tetra-tert-butylphthalocyaninato)iron], I.§