Dr. Andrew Hughes - metallacarboranes

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Coordination chemistry of carborane anions

This project is investigating the reactions of the neutral nidocarborane fragment C₂B₉H₁₃ and its anions C₂B₉H₁₂^- and C₂B₉H₁₁^2-with metal amides, alkyls and chlorides. We are seeking to explore the synthesis and reactivity of metallacarboranes of the early transition metals in their highest oxidation states, with an emphasis on how the carborane ligand influences the chemistry of the metal fragment. As part of the work we have been exploring the reactions of carborane reagents with some other metal containing reagents, and have made some interesting observations, e.g., in zinc chemistry (see below). A feature of our work is that we study metallacarboranes obtained by removal of one B-vertex from all three isomers of C₂B₁₀H₁₂, whilst other research groups tend to focus mainly on ortho-carborane, and the meta-isomer to a lesser extent. The scheme to the right shows how the isomeric carboranes, C₂B₁₀H₁₂, can have one boron vertex (adjacent to a carbon) removed, to generate the three accesible isomers of the anions C₂B₉H₁₂^- and C₂B₉H₁₁^2-.

Structures of salts of C₂B₉H₁₂^-and C₂B₉H₁₁^2-

This work is being performed in collaboration with Dr. Mark Fox.
The 11 vertex carbon and boron hydrides C₂B₉H₁₃ and their mono- and di-anions C₂B₉H₁₂^- and C₂B₉H₁₁^2-obtained by deprotonation, were first described by Hawthorne and co-workers in the 1960s. These are the most common precursors used by our group and a great many others to synthesise metallacarboranes and a wide range of heterocarboranes. The structure of all three isomers of C₂B₉H₁₁^2-consists of an icosahedron with one vertex vacant, and with an exo-hydrogen atom on each of the 11 vertices; the isomers differ in the location of the carbon atoms. The mono-anions are a tiny bit more challenging structurally, and there is a facinating history of the structure proposed for the most widely used 7,8- C₂B₉H₁₂^- anion.
The location of the endo- or 12th hydrogen atom appears to depend on the identity of the counter-ion, and we find that the structure of carborane anion as the proton-sponge salt differs from that reported by other workers for other counter-ions.

The structure of (PSH⁺)(7,8-C₂B₉H₁₂^-) determined by neutron diffraction at natural isotopic abundance.
The ¹⁰B isotope of boron is a potent absorber of neutrons, and previous neutron diffraction studies of borane clusters have utilised expensive ¹⁰B-depleted materials, but the intense beam of the ISIS spallation source made this study possible.

The molecular structure of (PSH⁺)(7,8-C₂B₉H₁₂^-) determined by neutron diffraction (PS = proton sponge, 1,8-bis(dimethylamino)naphthalene); Inorganic Chemistry, 2001, 40, 173.

Zinc metallacarboranes

The reaction of ZnMe₂ with [nido-7,8-C₂B₉H₁₂][NMe₃H] in a 1:1 ratio, gives an unusual macropolyhedral metallacarborane, which is a novel dimer of the expected [closo-NMe₃-Zn-C₂B₉H₁₁] product. The structure of the product bears a passing resemblance to a pair of ear-muffs or headphones.

Unprecedented electron deficient bridging between zinc atoms by boron atoms of nido-carborane anions: preparation, crystal and molecular structure of the dimer [(nido-C₂B₉H₁₁)ZnNMe₃]₂; J. Chem. Soc., Chem. Commun., 1998, 1713.

Group 5 (niobium and tantalum) metallacarboranes

The carborane starting material nido-7,8-C₂B₉H₁₃ reacts with metal amides, e.g., M(NMe₂)₅, to liberate two equivalents of amine and coordinate the dicarbollide dianion to the resulting metal fragment, giving (C₂B₉H₁₁)M(NMe₂)₃ (M = Nb or Ta). Most of our exploratory work here has been carried out using exclusively tantalum, only a select few examples of niobium have been studied - the chemistry is essentially identical.

Transition metal dicarbollide complexes: Synthesis, molecular-, crystal- and electronic-structures of [M(nido-C₂B₉H₁₁)(NMe₂)₃] (M = Nb, Ta) and [Ta(nido-C₂B₉H₁₁)(O₂CNMe₂)₃]; J. Chem. Soc., Dalton Trans., 1999, 3867.
First structural characterisation of a 2,1,12-MC₂B₉ metallacarborane, [2,2,2-(NMe₂)₃-closo-2,1,12-TaC₂B₉H₁₁]. Trends in boron NMR shifts on replacing a {BH} vertex with a metal {ML_n} vertex in icosahedral carboranes, J. Chem. Soc., Dalton Trans., 2000, 3519
Insertion and cleavage reactions of closo-3,1,2-[Ta(NMe₂)₃(C₂B₉H₁₁)] with nitriles, phenols and thiols; structural characterisation of N,N-dimethyl amidinate ligands; J. Chem. Soc., Dalton Trans., 2000, 3526
m-oxo-bis{tantalum-bis-[N,N-(dimethyl)acetamidinate]-1,2-dicarba-undecaborate}; Acta Cryst. Section C, 2001, C57, 702.

Group 6 (tungsten) metallacarboranes

The metallacarborane chemistry of tungsten is in fact well established, and a large contribution to the chemical literature has already been made in this area. Most of the known group 6 metallacarboranes are from the group of Prof. F. G. A. Stone (at Bristol and then Baylor, Texas), and contain molybdenum or tungsten in low formal oxidation states, typically M(II), with p-acceptor spectator ligands, such as carbonyl. By contrast we have been exploring the synthesis and reactivity of tungsten metallacarboranes supported by imido (RN) ligands (with which (C₂B₉H₁₁ is related, both being s²,p⁴ ligands). Our work has involved exploring the balance between the p-donor abilities of the imido (RN), amido (RHN) and carborane (C₂B₉H₁₁) ligands.
In common with our work on zinc, niobium and tantaum metallacarboranes, our preferred means of coordinating a metal and carborane fragment is alkane or amine elimination between an acidic carborane precursor (usually Me₃NH⁺nido-7,8-C₂B₉H₁₂^-) and a metal alkyl or amide. A readily accessible tungsten precursor is the bis(imido) bis(amide), W(N^tBu)₂(NH^tBu)₂, prepared from WCl₆ and ^tBuNH₂ in high yield by a one-pot reaction, and this provides an entry into the chemistry described below.

Tungsten(VI) Metallacarborane Imido Complexes; Hydrogen Bonding to a Bent Imido Ligand in {W(N^tBu)₂[N(H)C(Me)NH^tBu](C₂B₉H₁₁)}, J. Chem. Soc., Dalton Trans., 2000, 1210

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This page last modified 16th March 2001, by Andrew Hughes