Four [Tp*W(μ3-S)3Cu3(μ3-Br)]-based clusters: synthesis, structural characterization and third-order NLO properties

Abstract

Treatment of [Et₄N][Tp*W(μ₃-S)₃(CuBr)₃] (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) (1) with an excess of α-methylpyridine (α-MePy) and NH₄PF₆ in CH₂Cl₂ afforded a cationic cluster [Tp*W(μ₃-S)₃Cu₃(α-MePy)₃(μ₃-Br)](PF₆) (2) while the reaction of 1 with an excess of 1,4-pyrazine (1,4-pyz) and NH₄PF₆ in MeCN–CH₂Cl₂ at 65 °C produced a polymeric cluster [Tp*W(μ₃-S)₃Cu₃(1,4-pyz)((1,4-pyz)_0.5)₂(μ₃-Br)][Tp*W(μ₃-S)₃(CuBr)₃] (3). Reactions of 1 with melamine (MA) in 1 : 1 or 1 : 2 gave rise to another polymeric cluster [{Tp*W(μ₃-S)₃Cu₃Br(μ₃-Br)}₂(MA)₂] (4) and a neutral cluster [Tp*W(μ₃-S)₃Cu₃Br(μ₃-Br)(MA)₂] (5), respectively. Compounds 2–5 were characterized by elemental analysis, IR spectra, UV-vis spectra, ¹H NMR, electrospray ionization (ESI) mass spectra and X-ray crystallography. The cation of 2 has a cubane-like [Tp*W(μ₃-S)₃Cu₃(μ₃-Br)] structure with each α-MePy ligand coordinated at one Cu(I) center. For 3, each [Tp*W(μ₃-S)₃Cu₃(μ₃-Br)] core is interconnected by 1,4-pyz bridges to form a 1D cationic zigzag chain with the [Tp*W(μ₃-S)₃(CuBr)₃]⁻ anions arranged along its two sides. For 4, each [Tp*W(μ₃-S)₃Cu₃(μ₃-Br)] core is interlinked by MA bridges to afford a 1D spiral chain. 5 adopts a cubane-like [Tp*W(μ₃-S)₃Cu₃(μ₃-Br)] structure in which one terminal Br and two MA ligands are coordinated at three Cu centers. The third-order nonlinear optical (NLO) properties of 1–5 in DMF were investigated by femtosecond degenerate four-wave mixing (DFWM) technique with a 80 fs pulse width at 800 nm. Compounds 1–5 exhibit good NLO responses, and 3 and 4 possess the largest second-order hyperpolarizability γ values among the known W/Cu/S clusters bearing the [Tp*WS₃] unit.