A Nonacoordinated Bridging Selenide in a Tricapped Trigonal Prismatic Geometry Identified in Undecanuclear Copper Clusters: Syntheses, Structures, and DFT Calculations

Undecanuclear copper clusters, [Cu11(μ9-Se)(μ3-Br)3{Se2P(OR)2}6] (R = Et, Pr, iPr) (1a−c), were isolated along
with closed-shell ion-centered cubes, [Cu8(μ8-Br){Se2P(OR)2}6] (PF6) (2a−c) and [Cu8(μ8-Se){Se2P(OR)2}6] (3a−c), from the reaction of [Cu(CH3CN)4](PF6), NH4[Se2P(OR)2], and Bu4NBr in a molar ratio of 2:3:2 in CH2Br2. The
molecular formulations of these clusters were confirmed by elemental analysis, positive FAB mass spectrometry,
and multinuclear NMR (1H, 31P, and 77Se). 77Se NMR spectra of Cu11 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 Cu11Se 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 μ8-Br ion. The shape of the molecule is a bromide-centered distorted Cu8
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−μ9-Se interactions for clusters [Cu11(μ9-Se)(μ3-X)3{Se2P(OR)2}6] (X = Br, I).
Calculations show that the formal bond order of each Cu-μ9-Se bond is slightly smaller than half of those calculated
for the terminal Cu−μ2-Se bonds.