First-Known High-Nuclearity Copper-Nickel Carbonyl Cluster: [Cu~xNi~3~5~-~x(CO)~4~0]^5^- (with x = 3 or 5) Containing an Unprecedented 35-Atom Three-Layer hcp Triangular Stacking Metal-Core Geometry

Abstract

Reactions of [Ni6(CO)12]2- (1) with CuBr2 have given rise in small yields (∼10%) to the first example of a close-packed copper-nickel carbonyl cluster; its formulation as [CuxNi35-x(CO)40]5- (with x=3 or 5) is based upon a low-temperature CCD X-ray crystallographic determination coupled with an elemental analysis and X-ray fluorescence measurements. This air-unstable black pentaanion (2) together with one co-crystallized bromide anion, six [NMe4]+ counterions, and one solvated acetone molecule comprise the crystallographic independent part in a monoclinic unit cell of P21/n symmetry (with Z=4). The geometrically unprecedented 35-atom hcp M(A)2M(B)3Ni30 polyhedron of pseudo-D3h symmetry consists of a central 15-atom equilateral ν4M(B)3Ni12 triangle that is capped on both sides by two symmetry-related 10-atom equilateral ν3M(A)Ni9 triangles. The 35-atom M(A)2M(B)3Ni30 core is encapsulated by 40 COs, whose connectivities, due to an “extra” CO ligand on one of the three triangular sides, reduce the pseudo D3h symmetry of the metal core to Cs. An elemental analysis via AA and X-ray fluorescence measurements resulted in Cu/Ni ratios of 3.2/31.8 and 3.7/31.3, respectively, that are consistent with the metal core being either Cu5Ni30 (i.e., M(A)=M(B)=Cu) or Cu3Ni32 (i.e., M(A)=Ni; M(B)=Cu). Several attempts to determine the actual stoichiometry of the metal core by use of electrospray FT/MS/ICR measurements were unsuccessful. The maximum metal-core diameter of 2 is ca. 0.41 nm parallel and 0.85 nm perpendicular to the principal 3-fold axis.