Chemistry of the WSe₄²⁻ Anion: Synthesis, NMR Spectroscopy, and Crystal Structure of the Selenium Insertion Product [NEt₄]₂[μ-WSe₄][(SC₄H₃)SeCu]₂

Reaction of a CH₃CN solution of [NEt₄]₂[WSe₄] with 1 equiv of lithium (2-thienyl)cyanocuprate, Li[(CN)Cu-(C₄H₃S)], affords [NEt₄]₂[μ-WSe₄] [(SC₄H₃)SeCu]₂ (1), in which a Se atom from the decomposition of the WSe₄²⁻ anion formally has been inserted into the 2-thienylcopper bond of Cu(C₄H₃S) to form the 2-selenothiophene ligand. [NEt₄]₂[μ-WSe₄] [(SC₄H₃)SeCu]₂ crystallizes with four formula units in the tetragonal space group D_2d⁷ − P4̄b2 in a cell of dimensions a = 15.732(2) Å and c = 14.992(3) Å (T = 113 K). Full anisotropic refinement (4934 unique data, 173 variables) of the structure on F_o² led to a value of R(F) = 0.055 for those 4282 data having F_o² ≥ 2σ(F_o²). The molecule has a crystallographically imposed 2-fold axis. Four Se atoms are arranged in an almost perfect tetrahedron about the W center; the Se atoms bridge to two Cu atoms, each of which in turn is bonded to a third Se atom in a distorted trigonal planar array. Each of these latter Se atoms is bonded to a thiophene group at the 2-position. The ⁷⁷Se NMR spectrum of a DMF solution of 1 exhibits two resonances at δ 905.8 and −18.1 ppm. The ¹³C NMR spectrum displays six peaks at δ 15.5, 52.7, 65.9, 124.5, 127.9, and 131.6 ppm, consistent with the solid-state structure.