Synthesis and properties of N-methylimidazole solvates of vanadium(ii), chromium(ii) and iron(ii) phthalocyanines. Strong NIR absorption in VII(MeIm)2(Pc2−)

Abstract

N-Methylimidazole (MeIm) solvates of vanadium(II), chromium(II) and iron(II) phthalocyanines: [V^II(MeIm)₂(Pc²⁻)]⁰·MeIm (1) and [M^II(MeIm)₂(Pc²⁻)]⁰·2C₆H₄Cl₂ (M = V (2), Cr (3), and Fe (4)) have been obtained and studied in a crystalline form. It has been shown that central metal atoms have the +2 oxidation state in 1–4 and dianionic Pc²⁻ macrocycles are formed. Optical spectra of 1–3 (V^II, Cr^II) are different from that of 4 (Fe^II) due to the appearance of intense absorption bands in the NIR range at 1187 nm for 1 and 2 and 1178 nm for 3 and manifestation of multiple bands in the visible range. Absorption in the NIR range is explained by unusually small gaps between the molecular orbitals at around HOMO and LUMO for MeIm solvated Cr^IIPc and V^IIPc. The essential diradicaloid character of the Pc macrocycles in [Cr^II(MeIm)₂(Pc²⁻)]⁰, in which α- and β-orbitals are distributed in different regions of the macrocycle, has also been shown. In this case, the diradicaloid character is comparable to that of aromatic hydrocarbon heptacene. Magnetic properties of 1–3 are defined by metal atoms with a S = 3/2 spin state for V^II and S = 1 for Cr^II. Compounds 1–3 manifest broad isotropic EPR signals at 4.2 K with g = 2.0188 and a linewidth (ΔH) of 81.8 mT for 1 (V^II), g = 1.8300 and a linewidth ΔH = 161.0 mT for 2 (V^II), and g = 2.0534 and ΔH = 60.27 mT for 3 (Cr^II). These signals shifted to smaller g-factors with the temperature increase. Integral intensity of narrow EPR signals from Pc˙³⁻ does not exceed 0.1% from those of broad signals in 1–3. Complex 4 is EPR silent due to the presence of diamagnetic Fe^II and Pc²⁻.