Tungsten(VI)-Copper(I)-Sulfur Cluster-Supported Metal-Organic Frameworks Bridged by in Situ Click-Formed Tetrazolate...

Six analogous two-dimensional (2D) [Tp*WS₃Cu₃]-based (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) networks, namely, {[(Tp*WS₃Cu₃)₂L₃](μ₃-N₃)}_n (2: L = 5-methyltetrazolate (Mtta); 3a: L = 5-ethyltetrazolate (Etta)) and {[(Tp*WS₃Cu₃)₂L₃]BF₄}_n (3b: L = Etta; 4: L = 5-propyltetrazolate (Ptta); 5: L = 5-butyltetrazolate (Btta); 6: L = 5-pentyltetrazolate (Petta)) were synthesized by reactions of [Et₄N][Tp*WS₃] (1), [Cu(CH₃CN)₄]BF₄, NaN₃, and NH₄BF₄ in different nitrile solvents (CH₃(CH₂)_nCN, n = 0, 1, 2, 3, and 4) under solvothermal conditions. In the structures of 2-6, each alkyl tetrazolate L as a bridging ligand was generated in situ from the "click" reaction between azide and nitrile. These 2D (6,3) networks support two types of voids wherein the pendant alkyl groups are accommodated. A tetrahedron cage-like cluster [Tp*W(μ₃-S)₃(μ₃-S')Cu₃]₄ (7) was also formed in some of the above reactions and can be readily separated by solvent extraction. The proportion of 7 increased with the elongation of the alkyl chains and finally became the exclusive product when heptylnitrile was employed. Further use of CuCN as a surrogate for [Cu(CH₃CN)₄]BF₄ with the aim of introducing additional CN bridges into the network led us to isolate a tetrazolate-free compound, {[Et₄N]{(Tp*WS₃Cu₃)[Cu₂(CN)_4.5]}₂·2PhCH₂CN}_n (8·2PhCH₂CN), a unique 2D network that features {(Tp*WS₃Cu₃)[Cu₂(CN)₅]}₂^2-, {(Tp*WS₃Cu₃)₃[Cu₃(CN)₇]₂[Cu(CN)₃]}^4-, and {(Tp*WS₃Cu₃)[Cu₄(CN)₉]}₂^6- ring subunits. Compounds 5-8 are soluble in DMF and exhibit a reverse saturable absorption and self-focusing third-order nonlinear optical (NLO) effect at 532 nm with hyperpolarizability γ values in the range of 4.43 × 10^-30 to 5.40 × 10^-30 esu, which are 400-500 times larger than that of their precursor 1. The results provide an interesting insight into the synergetic synthetic strategy related to the assembly of the [Tp*WS₃Cu₃]²⁺ cluster core, the "click" formation of the tetrazolate ligands, and the construction of the [Tp*WS₃Cu₃]²⁺ cluster-based 2D networks.