A nonacoordinated bridging selenide in a tricapped trigonal prismatic geometry identified in undecanuclear copper clusters: syntheses, structures, and DFT calculations

Undecanuclear copper clusters, Cu(11)(micro(9)-Se)(micro(3)-Br)(3)Se(2)P(OR)(2)](6)] (R = Et, Pr, (i)Pr) (1a-c), were isolated along with closed-shell ion-centered cubes, Cu(8)(micro(8)-Br)Se(2)P(OR)(2)](6)] (PF(6)) (2a-c) and Cu(8)(micro(8)-Se)Se(2)P(OR)(2)](6)] (3a-c), from the reaction of Cu(CH(3)CN)(4)](PF(6)), NH(4)Se(2)P(OR)(2)], and Bu(4)NBr in a molar ratio of 2:3:2 in CH(2)Br(2). The molecular formulations of these clusters were confirmed by elemental analysis, positive FAB mass spectrometry, and multinuclear NMR ((1)H, (31)P, and (77)Se). (77)Se NMR spectra of Cu(11) clusters (1a-c) are of special interest as two inequivalent selenium nuclei of the diselenophosphate (dsep) ligand exhibit different scalar coupling patterns with the adjacent phosphorus nuclei. X-ray analysis of 1c reveals a Cu(11)Se core stabilized by three bromide and six dsep ligands. The central core adopts the geometry of a 3,3,4,4,4-pentacapped trigonal prism with a selenium atom in the center. The coordination geometry for the nonacoordinate selenium atom is tricapped trigonal prismatic. The X-ray structure 2a or 2c consists of a cationic cluster in which eight copper ions are linked by six diselenophosphate ligands with a central micro(8)-Br ion. The shape of the molecule is a bromide-centered distorted Cu(8) cube. Each diselenophosphate ligand occupies square faces of the cube and adopts a tetrametallic tetraconnective coordination pattern. Each copper atom of the cube is coordinated by three selenium atoms with a strong interaction with the central bromide ion. Molecular orbital calculations at the B3LYP level of the density functional theory have been carried out to study the Cu-micro(9)-Se interactions for clusters Cu(11)(micro(9)-Se)(micro(3)-X)(3)Se(2)P(OR)(2)](6)] (X = Br, I). Calculations show that the formal bond order of each Cu-micro(9)-Se bond is slightly smaller than half of those calculated for the terminal Cu-micro(2)-Se bonds.