Octanuclear Copper(I) Clusters Inscribed in a Se12 Icosahedron: Anion-induced Modulation of the Core Size and Symmetry

Synthesis and structural characterization of an octanuclear Cu(I) cluster [Cu-8{Se2P((OPr)-Pr-i)(2)}(6)](PF6)(2) (1) with an empty Cu-8 cubic core involving diisopropyl diselenophosphate (dsep) ligand has been demonstrated despite its high tendency to abstract anions even from the traces of impurities in the solvent. Reaction of 1 with anion sources (BU4NF for F-; NaBH4 for W, and NaSH for S2-) in a 1:1 ratio produced anion-centered Cu-8 clusters with a formula [Cu-8(X){Se2P((OPr)-Pr-i)(2)}(6)](PF6) (X = F, 2a; H, 3a; D, 3a') and [Cu-8(S){Se2P((OPr)-Pr-i)(2)}(6)] (4) in high yields, In addition, fluoride- and hydride-centered Cu-8(1) clusters [Cu-8(X){Se2P(OEt)(2)}(6)](PF6) (X = F, 2b; H, 3b) could be generated in similar to 80% yield by direct reaction of [Cu(CH3CN)(4)](PF6), NH4Se2P(OEt)(2), and the anion sources (Bu4NF for F-; NaBH4 for H-) in 8:61 ratio. Whereas the structural elucidation of complexes 2 and 4 revealed an anion-centered cubic Cu-8 core surrounded by six dsep ligands, it was a tetracapped tetrahedral copper framework with a hydride in the center in compounds 3. All Cu center dot center dot center dot Cu distances along either the edge of the cube in 2 and 4 or the tetracapped tetrahedron in 3 are shorter than those identified in 1. Although the cubic (or spherical) contraction of the copper framework that was identified in a series of closed-shell anion-centered (except a hydride) Cu-8 cube having T-h symmetry could be explained by the existence of strong anion-cation attractions, it was definitely a surprise that the hydride, which is the smallest closed-shell anion and spherical too, induced a tetrahedral contraction of four out of the eight Cu atoms in the empty cube 1, resulting in a tetracapped-tetrahedral geometry and reducing the symmetry to T from T-h, Furthermore the fact that the encapsulated anion induced modulation of the copper core size and symmetry was fully reproduced by DFT calculations on model compounds. To the best of our knowledge, this demonstrated the first example of the reduction of molecular symmetry (from T-h to T) simply by changing the encapsulated species without altering the general bonding pattern of the surrounding ligands. We also demonstrated that the hydride can easily replace other anions (Cl-, Br-, F-, S2-, Se2-) in a very facile manner to produce hydride-centered species. Eventually, compounds 3 were stable in the presence of other anions, and hydride/deuteride exchange could not be achieved.