Effect of reduction on the molecular structure and optical and magnetic properties of fluorinated copper(ii) phthalocyanines

Abstract

The reduction of copper(II) octafluoro- {Cu^II(F₈Pc)} and hexadecafluorophthalocyanines {Cu^II(F₁₆Pc)} by NaCpCo(CO)₂ in the presence of cryptand[2.2.2] yields new crystalline {cryptand(Na⁺)}[Cu^II(F₈Pc)˙³⁻]⁻·2C₆H₄Cl₂ (1) and {cryptand(Na⁺)}₂[Cu^II(F₁₆Pc)⁴⁻]²⁻·C₆H₁₄ (2) salts. Together with two previously characterized salts of Cu^II(F_xPc) (x = 8 and 16), this allows the study of the molecular structure and optical and magnetic properties of fluorinated copper phthalocyanines in different reduction states (−1 and −2). The blue shift of the Q-band increases together with the negative charge on the macrocycle, and new weak bands of the anions appear at 820–1013 nm. Alternation of the N_meso–C bonds manifests itself in reduced macrocycles due to a partial disruption of macrocycle aromaticity. In the case of Cu^II(F₈Pc) this effect is nearly two times stronger for dianions than for monoanions. The alternation of the bonds is less pronounced for perfluorinated Cu^II(F₁₆Pc)ⁿ⁻ (n = 1, 2) anions most probably due to a partial delocalization of the negative charge on fluoro-substituents. The reduction also noticeably elongates the average C–F bonds in Cu^II(F₈Pc). The first reduction centered on the macrocycle leads to the formation of [Cu^II(F₈Pc)˙³⁻]⁻ in 1 with two S = 1/2 spins positioned on Cu^II and the radical trianion (F₈Pc)˙³⁻ macrocycle. As a result, a broad EPR signal is observed with g = 2.1652 at RT attributable to both paramagnetic species having exchange interactions. The formation of dimerized stacks from [Cu^II(F₈Pc)˙³⁻]⁻ in 1 results in strong enough magnetic coupling of the (F₈Pc)˙³⁻ spins within the dimers (J/k_B = −21.8 cm⁻¹), and weaker intramolecular coupling is observed between Cu^II and (F₈Pc)˙³⁻ (J/k_B = −10.8 cm⁻¹). Coupling between (F₈Pc)˙³⁻ spins from the neighboring dimers is nearly 1.5 times weaker (−14.6 cm⁻¹). Under reduction conditions, a second electron also comes to the macrocycle forming diamagnetic F₁₆Pc⁴⁻ tetraanions. In this case S = 1/2 spin is preserved on Cu^II. Magnetic coupling between these centers is weak due to the long distances between them in the [Cu^II(F₈Pc)⁴⁻]²⁻ chains of 2. Salt 2 shows an EPR signal with a HF splitting characteristic of Cu^II with g_∥ = 2.1806 (A_∥ = 20.11 mT), and g_⊥ = 1.9597 at RT.