Structural Diversity and Spectral Properties of the Crystals of Heterometallic Complexes Derived from TM(acacen) and Pb(diketonate)2, TM = Cu, Ni, Pd

Six stable heterometallic complexes with metal ratios 1:1 and 2:1 were prepared by crystallization of TM­(acacen) and Pb­(hfa)2 from organic solvents (TM = Cu­(II), Ni­(II), Pd­(II); acacen = N,N′-ethylenebis­(acetylacetoniminate); hfa = hexafluoroacetylacetonate). The prepared complexes show unique molecular structures. Tetranuclear heterocomplexes [TM­(acacen)­Pb­(hfa)2]2 with a special arrangement of metal atoms are basic units of crystals of 1:1 heterometallic complexes. At the components ratio of 2:1, trinuclear complexes [(TM­(acacen))2Pb­(hfa)2] are formed whose shape and crystal packing are TM-specific. The chemical interactions between monometallic subunits are shown to differ depending on the TM used and to alter crystal structure features and optical properties of the prepared heterometallic complexes. Among the compounds obtained, only crystals of [Ni­(acacen)­Pb­(hfa)2]2 exhibit high-contrast dichroism which is observed as the Alexandrite effect for visible light and by optical microscopy under irradiation with polarized light. TD-DFT study of the electronic excited states in the range 350–750 nm confirmed the charge transfer between the monometallic subunits in the complex alters the position of absorption bands to favor the observed optical properties. All prepared compounds are volatile and can sublimate under heating in vacuum with partial decomposition.