The construction and transformation of metal-carbon (M-C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M-C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal-carbon (M-C(cage)) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M-C(cage) bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal-carboryne complexes. Our work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric problem and to activate the nontraditional M-C(cage) bonds, we prepared a series of group 4 and group 10 transition metal-carboryne complexes (where carboryne is 1,2-dehydro-o-carborane), because the formation of metallacyclopropane opens up the coordination sphere and creates ring strain, facilitating the reactions of M-C(cage) bonds with electrophiles. Structural and theoretical studies on metal-carboryne complexes suggest that the bonding interaction between the metal atom and the carboryne unit is best described as a resonance hybrid of the M-C σ and M-C π bonds, similar to that observed in metal-benzyne complexes. The nickel-carboryne complex (η(2)-C(2)B(10)H(10))Ni(PPh(3))(2) can (i) undergo regioselective [2 + 2 + 2] cycloaddition reactions with 2 equiv of alkyne to afford benzocarboranes, (ii) react with 1 equiv of alkene to generate alkenylcarborane coupling products, and (iii) also undergo a three-component [2 + 2 + 2] cyclotrimerization with 1 equiv of activated alkene and 1 equiv of alkyne to give dihydrobenzocarboranes. The reaction of carboryne with alkynes is also catalyzed by Ni species. Subsequently, a Pd/Ni co-catalyzed [2 + 2 + 2] cycloaddition reaction of 1,3-dehydro-o-carborane with 2 equiv of alkyne was developed, leading to the efficient formation of C,B-substituted benzocarboranes in a single process. In contrast, the zirconium-carboryne species, generated in situ from Cp(2)Zr(μ-Cl)(μ-C(2)B(10)H(10))Li(OEt(2))(2), reacts with only 1 equiv of alkyne or polar unsaturated organic substrates (such as carbodiimides, nitriles, and azides) to give monoinsertion metallacycles, even in the presence of excess substrates. The resultant five-membered zirconacyclopentenes, incorporating a carboranyl unit, are an important class of intermediates for the synthesis of a variety of functionalized carboranes. Transmetalation of zirconacyclopentenes with other metals, such as Ni and Cu, was also found to be a very useful tool for various chemical transformations. Studies of metal-carboryne complexes re...

Reaction of dilithiocarborane (1,2-Li2C2B10H10) with CuCl in toluene affords a single product, 1,1′:2,2′-[Cu(toluene)]2(C2B10H10)2, which gives 1,1′-bis(o-carborane) after hydrolysis. This serves as the most efficient method for the preparation of 1,1′-bis(o-carborane).

A zirconacyclopentene incorporating a carboranyl unit 1,2-[Cp2ZrC(Et)=C(Et)]-1,2-C2B10H10 (1), an analogue of well-known zirconacyclopentadienes, was prepared and fully characterized from the reaction of Cp2Zr(mu-Cl)(mu-C2B10H10)Li(OEt2)2 with EtC OCEt in reflux toluene. Complex 1 resembles zirconacyclopentadienes in some reactions, and on the other hand, it has unique properties of its own. Many carborane derivatives with interesting structural features such as 1-[CI(Et)=C(Et)]-1,2-C2B10H11, naphthalocarborane, and 1,2-[C(Et)=C(Et)]-1,2-C2B10H10 can be conveniently prepared from 1. This work shows that 1 provides a very valuable entry point to the synthesis of various kinds of carborane derivatives that can not be prepared by conventional methods.

A series of Zr-carboryne (carboryne = 1,2-dehydro-1,2-carborane) complexes were prepared and fully characterized. Both the electronic and steric factors of the ligands have significant effects on the formation of the resultant metal complexes. The reaction of organozirconium dichloride with 1 equiv of Li 2 C 2 B 10 H 10 gave a new class of zirconium-carboryne complexes, including (η 2 -C 2 B 10 H 10 )ZrCl 2 -(THF) 3 ( 6). Treatment of 6 with 2 equiv of amidinatolithium, guanidinatolithium, or t BuOK afforded the complex 9) was isolated from the reaction of 6 with ( t BuCOCHCO t Bu)Na. All complexes were fully characterized by 1 H, 13 C, and 11 B NMR techniques as well as elemental analyses. Their structures were further confirmed by singlecrystal X-ray analyses.

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