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

Abstract

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 [( PtCl 2)( CH 3)6 LM ]( I )6, where L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion, M = Zn II , Mg II ( H 2 O ) 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 PdCl 2 or PtCl 2 unit at one of the external dipyridinopyrazine fragments only minimally disturbs the σ/π electronic distribution within the entire porphyrazine unit. In contrast, quaternization by CH 3 I of the six unligated pyridine N atoms of the species [( PtCl 2) LM ] leading to formation of the hexacations [( PtCl 2)( CH 3)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 σ/π electronic system towards the periphery of the macrocycle. Accordingly, there is a facilitated thermodynamic uptake of electrons upon going from [( PtCl 2)( CH 3)6 LM ]6+ to [( PtCl 2)( CH 3)6 LM ] n (n = 5+ → 2+). Noteworthy, the UV-visible spectra of the salt-like species [( PtCl 2)( CH 3)6 LM ]( I )6 in water at c = 5 × 10-5 M ) indicate the presence of a monomer-dimer equilibrium, persistent even at very low concentrations (ca. 5 × 10-7 M).