Transition Metal−Carboryne Complexes: Synthesis, Bonding, and Reactivity

Abstract

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...

Dicarba-closo-dodecarborane-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium: biological relevance and synthetic strategies

“…64-67 X-ray structure analyses show that the metallacycle MEC 2 is bent and that the pseudo-aromaticity is 10 partially or totally lost upon addition reactions. This decrease of the symmetry order (C 2v > C s ) was studied with complex 1 by addition of PMe 3 , the resulting complex being [Cp*Ir(Se 2 C 2 (B 10 H 10 ))(PMe 3 )] (7).…”

“…Clusters containing boron and carbon, termed carbaboranes, are a class of compounds that possess different sizes, architectures, charges and dipole moments. 10 Among them, icosahedral dicarba-closo-dodecarboranes (C 2 B 10 H 12 ) were 35 first reported in 1963.…”

“…This binary method consists of the nuclear reaction of nontoxic and nonradioactive 10 B atoms and low-energy thermal neutrons that produces high-energy 4 He 2+ α-particles and 7 Li 3+ ions. The dissipation of the high kinetic energy of 25 these particles is achieved in a small distance (less than one cell diameter), which allows accurate destruction of the targeted cells.…”

“…14 The degree of potential functionalization of dicarba-closo-dodecarboranes being much higher than that of 5 phenyl rings, dicarba-closo-dodecarboranes are considered as convincing bioisosteres for aryl, cycloalkyl or adamantyl units. For instance, this approach has been used by Valliant and co-workers to synthesise an analogue compound of tamoxifen in-which the A phenyl group has been substituted 10 by an o-dicarba-closo-dodecarborane unit. 37 On the other hand, the hydrophobic arene and cyclopentadienyl ligands and the hydrophilic metal centres give amphiphilic properties to the arene/cyclopentadienyl 15 metal units.…”

“…Only a binuclear complex was isolated during the reaction of 5 with 2,5-diethynylthiophene. With the other diynes (1,4-diethynylbenzene, 3',6-diethynyl-1,1'-binaphthyl-2,7'-diyl diacetate and 2-bromo-5-ethynylthiophene) the reactivity of 10 the second alkynyl group is inhibited. Electronic rather than steric effects play a key role in the generation of such binuclear complexes.…”