Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings 14: UV-visible spectral and electrochemical behavior of homo/heterobinuclear neutral and hexacationic macrocycles

Detailed UV-visible, electrochemical and spectroelectrochemical studies were carried out in DMSO on two known series of homo/heterobimetallic pyrazinoporphyrazine compounds, i.e. [(M'Cl-2) LM] and [(PtCl2)(CH3)(6)LM](I)(6), where L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion, M = Zn-II, Mg-II(H2O) or Pd-II and M' = Pd-II or Pt-II. The [(M'Cl-2)LM] compounds, as obtained from the mononuclear species [LM], undergo only subtle UV-visible spectral changes and exhibit practically unchanged half wave potentials for reduction; thus, peripheral coordination to the [LM] macrocycles of a single PdCl2 or PtCl2 unit at one of the external dipyridinopyrazine fragments only minimally disturbs the sigma/p electronic distribution within the entire porphyrazine unit. In contrast, quaternization by CH3I of the six unligated pyridine N atoms of the species [(PtCl2)LM] leading to formation of the hexacations [(PtCl2)(CH3)(6)LM](6+) results in a significant bathochromic shift (5-15 nm) of the Q-band positions, thus suggesting an enhanced electron-withdrawing effect determined by an incremented displacement of the sigma/p electronic system towards the periphery of the macrocycle. Accordingly, there is a facilitated thermodynamic uptake of electrons upon going from [(PtCl2)(CH3)(6) LM](6+) to [(PtCl2)(CH3)(6)LM](n) (n = 5+ -> 2+). Noteworthy, the UV-visible spectra of the salt-like species [(PtCl2)(CH3)(6)LM](I)(6) in water at c = 5 x 10(-5) M) indicate the presence of a monomer-dimer equilibrium, persistent even at very low concentrations (ca. 5 x 10(-7) M).