Heptacoordinate Transition-Metal-decorated Metallo-Borospherenes and Multiple-Helix Metallo-Boronanotubes.

The recent discovery of lanthanide-metal-decorated metallo-borospherenes LM3B18- (LM = La, Tb) marks the onset of a new class of boron-metal binary nanomaterials. Using the experimentally observed or theoretically predicted borospherenes as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel chiral metallo-borospherenes C2 Ni6 ∈ B39- (1), C1 Ni6 ∈ B41+ (3), C2 Ni6 ∈ B422+ (4), C2 Ni6 ∈ B42 (5), and C2 Ni8 ∈ B56 (6) as the global minima of the systems decorated with quasi-planar heptacoordinate Ni (phNi) centers in η7-B7 heptagons on the cage surfaces, which are found to be obviously better favoured in coordination energies than hexacoordinate Ni centers in previously reported D2d Ni6 ∈ B40 (2). Detailed bonding analyses indicate that these phNi-decorated metallo-borospherenes follow the σ + π double delocalization bonding pattern, with two effective (d-p)σ coordination bonds formed between each phNi and its η7-B7 ligand, rendering spherical aromaticity and extra stability to the systems. The structural motif in elongated axially chiral Ni6 ∈ B422+ (4), Ni6 ∈ B42 (5), and Ni8 ∈ B56 (6) can be extended to form the metallic phNi-decorated boron double chain (BDC) double-helix Ni4 ∈ B28 (2, 0) (P4̄m2) (8), triple-helix Ni6 ∈ B42 (3, 0) (P3̄m1) (9), and quadruple-helix Ni8 ∈ B56 (4, 0) (P4mm) (10) metallo-boronanotubes, which can be viewed as quasi-multiple-helix DNAs composed of interconnected BDCs decorated with phNi centers in η7-B7 heptagons on the tube surfaces in the atomic ratio of Ni : B = 1 : 7.