A novel metallaborane compound, [(PPh3)3(PPh2)2Pd4B20H16], prepared from [PdCl2(PPh3)2] and [B10H10]2- in EtOH, exhibits a globular cluster architecture consisting of two twelve-vertex closo {PdB11} cluster units fused to generate a condensed globular {PdB20} assembly that has a tetrahedral {B4} borons-only core. There is dynamic bonding between this borane-based assembly and an unusual exopolyhedral palladium-based domain consisting of a {(PPh3)Pd(PPh2)Pd(PPh2)Pd(PPh3)} string. The central tetrahedral borons-only core suggests a new type of globular "megaloborane" architectural principle for higher condensed boron hydrides, which is assessed by ab initio studies at the STO-3G level on protonated [B28H18]2- and related structures: results suggest energetic minimisation at neutral [B27H21].

Electronic interactions and metal-metal communication in a wide range of cobaltacarborane-hydrocarbon complexes containing one to six metal centers, and exhibiting a variety of modes of inter-cage connectivity and molecular architectures, have been investigated via cyclic voltammetry, controlled potential coulometry, and UV-visible spectroelectrochemistry. The properties of mixed-valent Co(III)/Co(IV) and Co(II)/Co(III) species that are generated on oxidation or reduction of dinuclear and polynuclear Co(III) complexes were examined and classified as Robin-Day Class I (localized), Class II (partially delocalized), or Class III (fully delocalized) systems. The extent of metal-metal communication between metallacarborane cage units is strongly influenced by the type of intercage connection (e.g., cage B-B or Cp-Cp); the vertexes involved (equatorial vs apical); the nature of the linking unit, if any; and the presence of substituents on the carborane cages. In multi-tripledecker complexes where three CpCo(C(2)B(3)H(4))CoCp units are linked through a central triethynyl benzene connector, the data suggest that Co-Co electronic communication is extensive (Class III) within individual sandwich units while intersandwich delocalization is weak or absent. An extended Hückel study of CpCoC(2)B(4)H(6) double-decker and CpCo(C(2)B(3)H(5))CoCp triple-decker sandwich model complexes shows significant differences in the orbital contributions involved in the HOMO and LUMO of the former vs the latter type. The calculations afford additional insight into the electronic structures and properties of these systems as elucidated by the experimental studies.

A series of B(5)- and B(7)-alkynyl-substituted derivatives of CoC2B3, CoC2B4, and Co2C2B3 cobaltacarboranes has been prepared and linked via metal-promoted cross-coupling reactions to generate several types of polynuclear molecular complexes, which were structurally characterized and studied via electrochemical techniques. The target compounds prepared include linear dicobalt species containing BC⋮CC⋮CB linkages, e.g., [Cp*Co(2,3-Et2C2B4H3-7-C⋮C)]2 (8) and [Cp*Co(2,3-Et2C2B4H3-5-C⋮C)]2 (17), trigonally symmetrical 1,3,5-triethynylbenzene-anchored tricobalt and hexacobalt complexes, e.g., {Cp*Co(2,3-Et2C2B4H3-5-C⋮C)}3C6H3 (19), {Cp*Co(2,3-Et2C2B4H3-7-C⋮C)}3C6H3 (22), and 1,3,5-[Cp*2Co2(2,3-Et2C2B3H2-5-C⋮C)]3C6H3 (21), and novel cobaltacarborane−Co2C2 mixed-cluster systems, e.g., Cp*Co(2,3-Et2C2B4H3)-7-C2HCo2(CO)6 (5) and {Cp*Co[2,3-Et2C2B4H3-7-(C2Co2CO)6]}3C6H3 (23), a nonacobalt complex. The new compounds were obtained in all cases as air-stable crystalline solids and were characterized by multinuclear NMR and mass spectra, supplemented by X-ray diffraction data for 8, 19, and 21. Electrochemical data on the alkynyl-linked compounds indicate that metal−metal communication in oxidized and reduced species occurs to a limited extent in most cases and to a considerable degree (Robin−Day class III) in the C2B3-bridged triple-decker systems.

The study of dendrimers 2 is well advanced since their first appearance 25 years ago. 3 Dendritic materials have attracted wide-ranging interest because of their actual or potential application in a broad spectrum of areas, including electrode coatings, nonlinear optical (NLO) materials, controlled artificial energy antennas (light harvesters), electrochemical biosensors, exoreceptors for molecular recognition, catalysis, biomedical applications, and organic electrical conductors. While most characterized dendrimers are purely organic in composition, attention has been increasingly directed to metalcontaining systems, 4 particularly those containing electroactive metal centers. 4a Transition metals with partially filled d shells may function as electrophores, chromophores, or active catalytic sites, with the metal center residing at the core (rarely), the interior branches, and/ or the periphery of the molecule. The choice of transition-metal-containing units that are suitable for inclusion in dendritic systems is rapidly growing: known metallodendrimers incorporate ferrocenyl, 5 cobaltocenium, 5h,6 chromium carbonyl, 7 Co 2 C 2 , 8 and organoruthenium groups 9 and others. 4 Polyhedral metal-boron clusters afford an as yet unexplored potential for creating novel families of electronically tailorable metallodendrimers. Metallacarboranes seem especially promising for this role, given their well-established thermodynamic and redox stability, ease of modification via organosubstitution, and synthetic versatility. 10 Although dendrimers containing nonmetalated C 2 B 10 carborane cages have been prepared, 8b,9e,11 we know of none having metallaborane or metallacarborane cluster units. Here we report the synthesis and characterization of the first metallacarborane dendrimers.As the metal-containing units of choice for attachment to poly(propyleneimine) dendrimers, 6-and 7-vertex CoC 2 B n (n ) 3, 4) cobaltacarborane clusters were selected because of their relatively small, metallocene-like steric requirements and their well-established redox properties and synthetic tailorability. 12 Scheme 1 shows the conversion of CpCo(2,3-Et 2 C 2 B 4 H 4 ) (1) to its Cpsubstituted carboxylic acid and acyl derivatives [η 5 -C 5 H 4 C(O)R]Co(2,3-Et 2 C 2 B 4 H 4 ) (2, R ) OH; 3, R ) Cl), For some very recent reviews of metallodendrimers, see: (a) Juris, A.; Venturi, M.; Ceroni, P.; Balzani, V.; Campagna, S.; Serroni, S. Casado, C. M.; Alonso, B.; Moran, M.; Losada, J.; Belsky, V. Moran, M.; Casado, C. M.; Alonso, B.; Lobete, F.; Garcia, B.; Ibisate, M.; Losada, J. Casado, C. M.; Alonso, B.; Cuadrado, I.; Moran, M.; Wang, Y.; Kaifer, A. E. Chem. Commun. 1998, 2569. (b) Valerio, C.; Ruiz, J.; Fillaut, J.-L.; Astruc, D. C. R. Acad. Sci., Ser. II 1999, 79. (c) Gonzá les, B.; Cuadrado, I.; Casado, C. M.; Alonzo, B.; Pastor, C. J. Organometallics 2000, 19, 5518. (7) Lobete, F.; Cuadrado, I.; Casado, C. M.; Alonso, B.; Moran, M.; Losada, J.