Near-infrared pigments based on ion-pair charge transfer salts of dicationic and dianionic metal–dithiolene [M(ii) = Pd, Pt] complexes

Abstract

Mixing [M(Et₂dazdt)₂](BF₄)₂ [M = Ni(II), Pd(II), Pt(II); Et₂dazdt = N,N′-diethyl-perhydrodiazepine-2,3-dithione] with (Bu₄N)₂[M(mnt)₂] (mnt = maleonitrile-2,3-dithiolate) in CH₃CN produces the known mixed ligand dithiolene complex [Ni(Et₂dazdt)(mnt)] in the nickel case and ion-pair salts [M(Et₂dazdt)₂][M(mnt)₂] in the palladium (1) and platinum (2) cases. Structural characterization of 2 shows that the anionic (donor) and cationic (acceptor) complexes form an irregular stack that lies in the bc crystallographic plane. The shortest contacts exchanged by the anion and cation molecules within each stack are those occurring through the hydrogen atoms of the CH₂ groups of Et₂dazdt and the Pt(2)–S(22) segment (d(H(81a)–S(22) = 2.981(3) Å) and the nitrogen atom of the cyano group of mnt and the carbon atom of one of the thione moieties (d(N(12)–C(11) = 3.179(3) Å). The Pt atom of [Pt(mnt)₂]²⁻ is surrounded by two hydrogen atoms of the Et₂dazdt ligand, whereas the Pt atom of [Pt(Et₂dazdt)₂]²⁺ is surrounded by two carbon atoms of the dithiolate moiety of mnt. Intramolecular interactions are due to contacts exchanged mainly through H-atoms, which are suitable to mediate charge-transfer (CT) interactions. In fact, these salts are characterized by a long wavelength CT peak [λ_max = 905 nm (1), 937 nm (2)], which makes them candidates as near-infrared pigments, whose properties are tunable with the redox features of the components, the energy of the NIR absorption being relatable to the driving force of electron transfer from the donor (dianion) to the acceptor (dication). A thorough description of interactions occurring between the complex anions and complex cations has been achieved by investigating the Hirshfeld surface (HS) properties. Computational methods are in agreement with experimental findings and allow us to highlight the electronic features of the components of these CT salts, providing a structure–property relationship, useful in designing new candidates to optimize the desired properties.