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

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]}22–, {(Tp*WS3Cu3)3[Cu3(CN)7]2[Cu(CN)3]}4–, and {(Tp*WS3Cu3)[Cu4(CN)9]}26– 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.