Reducing zirconium(iv) phthalocyanines and the structure of a Pc4−Zr complex

Abstract

The synthesis and characterization of the ring-unsubstituted zirconium phthalocyanine PcZrCl₂ (1; Pc^x− = phthalocyaninato^x−) and its reduction products are described. X-ray analysis of 1 (crystallized from hot 1-chloronaphthalene) reveals that 1 is a chloride-bridged dimer [PcZrCl]₂(μ-Cl)₂ in the solid-state; 1 was also characterized by UV-vis/MCD spectroscopy and cyclic voltammetry, which indicated reduction potentials at −0.55, −0.95 and −1.28 V. Although attempts to access these Pc-ring reduced species with KC₈ led to mixtures of reduced products due to the insolubility of both starting materials, one equivalent of the reducing agent KEt₃BH reacted with 1 to generate Pc³⁻-containing species, as indicated by visible Q-band spectral changes (from λ_max = 686 for 1 to 589/611 nm), a single ESR peak (g = 2.001) and paramagnetically shifted ¹H NMR resonances consistent with the presence of a Pc-radical anion. Addition of two equivalents of KEt₃BH to 1 generated Pc⁴⁻-containing species, confirmed by a shift in λ_max to 522 nm and upfield-shifted ¹H NMR peaks relative to 1. Reaction of 1 with one and two equivalents of LiCp* did not generate Cp*-substituted products but also effected reduction to analogous Pc³⁻ and Pc⁴⁻ species. This latter material, the air-sensitive ring di-reduced “ate”-complex Pc⁴⁻Zr(LiCl)_1.5(DME)₃, of the form [LiCl(DME)₄]_0.5[Pc⁴⁻ZrClLi(DME)] was structurally characterized, illustrating partial bond localization in the Pc⁴⁻ ring, which also adopts a saddle-shape vs. the more typical dome-configuration found in 1. This represents a rare example of an isolated and structurally characterized Pc⁴⁻ complex.