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{Pc(2-)]0).MeIm (1) and [M(II)(MeIm)2{Pc(2-)}](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 +2 oxidation state in 1-4 and dianionic Pc(2-) 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 HOMO−LUMO gaps for MeIm solvated Cr(II)Pc and V(II)Pc. Essential diradicaloid character of the Pc macrocycles in [Cr(II)(MeIm)2{Pc(2-)}](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 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 linewidth (deltaH) of 81.8 mT for 1 (V(II), g = 1.8300 and linewidth deltaH = 161.0 mT for 2 (V(II), and g = 2.0534 and deltaH = 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 Pc(2-).