Role of the Acceptor in Tuning the Properties of Metal [M(II) = Ni, Pd, Pt] Dithiolato/Dithione (Donor/Acceptor) Second-Order Nonlinear Chromophores: Combined Experimental and Theoretical Studies

Abstract

The mixed-ligand complexes [M(II)(Et2dazdt)(mnt)] (M = Ni, 1; Pd, 2; Pt, 3) [Et2dazdt = N,N'-diethyl-perhydrodiazepine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate] have been prepared and fully characterized. X-ray diffractometric studies on 1-3 (the structure of 1 was already known) show that the crystals are isostructural (triclinic, P-1), and two independent molecular entities are present in the unit cell. These entities differ in the orientation of the ethyl substituents with respect to the epta-atomic ring. In the C2S2MS2C2 dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the M-S bond distances involving the two ligands are similar, while the C-S bond distances in the C2S2 units exhibit a significant difference in Et2dazdt (dithione) and mnt (dithiolato) ligands. 1-3 show in the visible region one or two moderately strong absorption peaks, having ligand-to-ligand charge-transfer (CT) character with some contribution of the metal, and show negative solvatochromism and molecular quadratic optical nonlinearity, which was determined by the EFISH (electric-field-induced second-harmonic generation) technique. These complexes are redox active and show two reversible reduction waves and one irreversible oxidation wave. Theoretical calculations based on DFT and TD-DFT calculations on complexes 1-3 as well as on [Pt(Bz2pipdt)(mnt)] (4) and [Pt(Bz2pipdt)(dmit)] (5) highlight the factors which affect the optical properties of these second-order redox-active NLO chromophores. Actually, the torsion angle of the dithione system (δ2) inversely correlates either with the oscillator strengths of the main transition of the complexes or with their beta values. The high beta value of 5 can be attributed both to its lowest torsion angles and to the extent of the π system of its dithiolate ligand, dmit.

Structural changes in MII dithione/dithiolato complexes (M = Ni, Pd, Pt) on varying the dithione functionalization

“…The shoulder at higher energy is the relatable HOMO-1 → LUMO transition, where the HOMO-1 is formed by a mixture of metal, dithiolate and dithione orbitals, but the very low value of the oscillator strength of this transition can make this peak not observable. 3c, 4 EFISH experiments, as described in the Experimental section, allowed determination of the scalar product μβ λ ( μ = ground state dipole moment; β λ = projection of the vectorial component of the quadratic hyperpolarizability tensor along the dipole moment axis). The μβ λ values determined at 1907 Table 3, and can be extrapolated to zero frequency by applying the equation: β 0 = βλ[1 − IJ2λ max /λ) 2 ]ij1 − IJλ max /λ) 2 ] giving for μβ 0 (10 −48 esu) −1247 (−1089) (1), −1332 (−938) (2), and −1896 (−1735) (3).…”

Proton‐Reduction Reaction Catalyzed by Homoleptic Nickel–bis‐1,2‐dithiolate Complexes: Experimental and Theoretical Mechanistic Investigations

“…More specifically, complexes of 1,2‐dithiolenes have been the subject of both experimental and theoretical work since the early reports of their synthesis and properties . This is because they exhibit nonlinear optical properties, are of bioinorganic interest owing to their similarity with molybdenum enzymes, and have found application in dye‐sensitized solar cells as chromophores, as catalysts in hydrogen evolution reactions, and as metallodrugs …”

A mixed valence zinc dithiolene system with spectator metal and reactor ligands

“…This orbital partitioning suggests that ligand-to-ligand charge-transfer proceeds from dithiolate ligand to dithione ligand, as has been previously documented in redox-active metal systems. [11] In 4 , however, the dithiolate and dithione ligands make almost equal contribution to the HOMO and LUMO frontier orbitals. This disparity in MO composition between 3 and 4 would explain the electronic spectrum observations.…”