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

Abstract

Six analogous two-dimensional (2D) [Tp*WS 3 Cu 3 ]-based (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate) networks, namely, {[(Tp*WS 3 Cu 3 ) 2 L 3 ](μ 3 -N 3 )} n ( 2: L = 5-methyltetrazolate (Mtta); 3a: L = 5-ethyltetrazolate (Etta)) and {[(Tp*WS 3 Cu 3 ) 2 L 3 ]BF 4 } n ( 3b: L = Etta; 4: L = 5-propyltetrazolate (Ptta); 5: L = 5-butyltetrazolate (Btta); 6: L = 5-pentyltetrazolate (Petta)) were synthesized by reactions of [Et 4 N][Tp*WS 3 ] ( 1 ), [Cu(CH 3 CN) 4 ]BF 4, NaN 3, and NH 4 BF 4 in different nitrile solvents (CH 3 (CH 2 ) n CN, 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(μ 3 -S) 3 (μ 3 -S′)Cu 3 ] 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(CH 3 CN) 4 ]BF 4 with the aim of introducing additional CN bridges into the network led us to isolate a tetrazolate-free compound, {[Et 4 N]{(Tp*WS 3 Cu 3 )[Cu 2 (CN) 4.5 ]} 2 ·2PhCH 2 CN} n ( 8 ·2PhCH 2 CN), a unique 2D network that features {(Tp*WS 3 Cu 3 )[Cu 2 (CN) 5 ]} 2 2–, {(Tp*WS 3 Cu 3 ) 3 [Cu 3 (CN) 7 ] 2 [Cu(CN) 3 ]} 4–, and {(Tp*WS 3 Cu 3 )[Cu 4 (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*WS 3 Cu 3 ] 2+ cluster core, the “click” formation of the tetrazolate ligands, and the construction of the [Tp*WS 3 Cu 3 ] 2+ cluster-based 2D networks.