Preparation, crystal structure and single-crystal polarized spectra of bis(1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinido)zinc(II)–tetrahydrofuran (1/4)

Abstract

The complex bis(1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinido)zinc(II), as its tetrahydrofuran (thf) adduct, Zn(C₁₁N₇H₂)₂·4thf, has been synthesized and its crystal structure determined. The zinc atom, which lies on a crystallographic symmetry centre, co-ordinates four imino groups in a square-planar arrangement [Zn–N_imino 2.070(8)Å] and two thf units in the apical positions [Zn–O_thf 2.249(7)Å]; the other two thf molecules are equatorially linked to the imino hydrogens. The electronic absorption spectra have been derived through a Kramers–Krönig transformation of the single crystal polarized specular reflectance spectra and then resolved via Gaussian deconvolution. They show an extensive mixed polarization, since their prominent components are distributed along different molecular directions. The low-energy band (Q) is polarized mainly along the in-plane molecular short axis (y), the next band (B) along the long axis (x) and the third band (N) along z. The interpretation is based on the only available theoretical model, that is INDO/S molecular orbital calculations, carried out on the analogous Ni^II complex (D_2h). The agreement between the observed and predicted transitions is qualitatively quite good, in the sense that several spectroscopic states have been calculated within the energy range investigated, of symmetry mainly B_2u(y) at low energy, B_3u(x) at intermediate energy and B_1u(z) at high energy. While crystal optics may contribute slightly to polarization mixing, there is evidence for vibronic coupling between states of different symmetry; the proposed assignment (D_2h) is thus based on the extrapolation of the above calculations by using the vibronic coupling, with the frequent occurrence of the type B_2u× b_1g× B_3u.