Nano and dendritic structured carboranes and metallacarboranes: From materials to cancer therapy

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Abstract

An account of the current research carried out in our laboratories is presented. Included is the incorporation of several group 14 elements into charge-compensated carboranes. These species present a bonding pattern not found in other main group carboranes. In addition to our continuing studies of the syntheses and structures of organometallic compounds, the use of these compounds as catalysts and catalyst precursors has been investigated. The isotopic exchange reactions between 10B enriched boron hydrides with naturally abundant boranes catalyzed by Ru(0) nanoparticles has been studied. The Ru(0) nanoparticles were obtained by the reduction of [CpRuCpRuCp]PF6 (Cp = C5Me5) with hydrogen and stabilized by the ionic liquid trihexyltetradecylphosphonium dodecylbenzenesulfonate [THTdP][DBS]. This was found to be an excellent, long lived catalyst for the exchange reaction of B-10 enriched diborane and naturally abundant decaborane(14). Other approaches to the production and use of nano-metal catalysts have also been explored. The reduction of the iridium carborane, (PPh3)2IrH(7,8-C2B9H11) with hydrogen in the presence of trihexyltetradecylphosphonium methylsulfonate, [THTdP][MS], produced an Ir(0) nanoparticles that catalyzed the phenylborolation as did our Ir(sal = N-R = salicylaldiminato; COD = cyclooctadiene complex. Progress in the use of single wall carbon nanotubes (SWCNT) as boron delivery agents was also discussed.

Graphical abstract

Our research has dealt with two important aspects of organometallic chemistry: our continued interest in the structure and properties of the heterocarboranes, and, more recently in the use of metallacarboranes as precursors in the syntheses of nano-metal catalysts. This Account summarizes some of our more important results.

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Introduction

Our research has dealt with two important aspects of inorganic chemistry: our continued interest in the structure and properties of the heterocarboranes, and, more recently in the use of metallacarboranes as precursors in the syntheses of nano-metal catalysts. This Account summarizes some of our more important results.

One class of organometallic compounds that closely parallels the metallocenes are those in which the cyclopentadienide anion, [C5R5], is replaced by a heteroborane. Boranes are mixed hydrides of boron and hydrogen where the boron atoms are contained in electron deficient clusters. These clusters usually are associated with high negative charges. For example the general formula for an n vertex closed structure (closo) is [BnHn]2−, the more open borane structures have increasingly higher negative charges. As a result, these compounds are difficult to work with. However, if a three electron [B–H] vertex is replaced by a neutral three electron vertex, such as a [C–R] group, there is no change in structure, but the charge is decreased by one. Some of the most encountered heteroboranes are the dicarbaboranes (dicarborane) in which the cluster charge is decreased by two; the general formula for an n + 2 vertex closo-dicarborane is C2BnHn+2. If the primary bonding atom of the group replacing a [H–B] vertex is a metal, the cluster is referred to as a metallaborane, or more likely to be encountered, a metallacarborane. There are a simple, elegant set of rules, culminating in the Polyhedral Skeletal Electron Pair counting rules, for predicting the structures of the heteroboranes from the composition of the clusters [1], [2], [3], [4], [5]. Most of the metallacarboranes in this account are those with the general formula, MC2BnHn+2 (n = 4, 9) [5d], having pentagonal bipyramidal and icosahedral structures, respectively. These can be viewed as coordination complexes between the metal, M2+, and a [nido-C2BnHn+2]2− ligand, in which the metal group is bonded to the open C2B3 pentagonal face of the carborane, in much the same way as the cyclopentadienide, [C5R5], bonds to metals in the metallocenes. Indeed, the primary metal-bonding carborane orbitals are a set of three, filled, π-type orbitals delocalized about the pentagonal face of the carborane that are very similar to those of [C5R5]; this was initially recognized by Hawthorne in the syntheses of the first metallacarboranes [6]. There are two different arrangements of the atoms on the C2B3 face, one in which the carbons are next to one another (carbons adjacent) and another in which they are separated by a boron atom (carbons apart). They both seem to bond equally well with metals, but the latter is thermodynamically more stable [4], [5]. Analogous to that found for the metallocenes, metallacarboranes have been reported in which the metal group bonds to a single carborane cage, to give half-sandwich compounds, or that occupy vertices of two carboranes, to give full-sandwich (commo-) compounds. We have been exploring these compounds as potential catalysts, or catalyst precursors, as well as exploring their use as boron delivery agents in BNCT.

One of the most startling developments in the last few decades, is in the syntheses and catalytic applications of nano-scaled particles prepared from groups 8–10 elements [7]. We have studied the use of these nanoparticle catalysts, derived from the corresponding metallacarborane precursors.

Our results led us to conclude that there remains a wealth of unexplored research that still to be done on the application of metallacarboranes to problems in catalysis and medicine. The most significant results of this research are presented in this account.

Section snippets

Chemistry of main group metallacarboranes

Main group metallacarboranes in which a silicon or germanium occupies an apical position above a [nido-(RC)2BnHn]2− cage are among the earliest reported [5](a), [5](b). These compounds can exist as either half-sandwich or distorted full-sandwich species with the group 14 element, in both +2 and +4 oxidation states, respectively. In the corresponding metallocenes, only the +2 is found. The dinegative charge on the carborane ligands might play a key role in determining the oxidation state of the

Nanoparticles-catalyzed isotopic exchange reactions

Nanoscale metal particles have been attracting much attention and are widely explored for their intriguing chemical and physical properties, as well as potential applications [9]. The extremely high surface areas and the subsequent high density of active sites of these nanoparticles make them more attractive catalysts than the bulk metals [10], [11]. Transition metals such as Ru, Os, Ir, and Rh, were found to activate B–H bonds in boron clusters to form metal–boron (M–B) bonds [12]. Also,

Metallacarboranes of constrained-geometry and their catalyric activities

In a continuing investigation of the catalytic behavior of constrained-geometry metallacatboranes, we have synthesized K[nido-7-Me-8-(2′-hydroxycyclohexyl)-7,8-C2B9H10, which was then reacted with BuLi in a 1:2 carborane:base molar ratio, to give an intermediate, identified as the trianionic carborane, that reacted with either ZrCl4 · 2THF or TiCl4 · 2THF to give the constrained geometry metallacarboranes, closo-1-M(Cl)-2-Me-3-(2′-σ-O-cyclohexyl)-η5-2,3-C2B9H9 (M = Zr (3), Ti (4)). The polymer

Catalytic arylborylation

We have developed an interest in the catalytic arylborolation reactions using iridium based catalysts. As part of this investigation, several iridium(I) salicylaldiminato–cyclooctadiene complexes of the form, Ir(sal = N-R)(cod) (sal = salicyladehyde; R = CH2Ph (1), Ph (2); cod = 1,5-cyclooctadiene) (see Fig. 5, Fig. 6 for their crystal structures), have been synthesized, thoroughly characterized, and used as catalysts for arylborylation via C–H activation [19]. The highest isolated yield of 91%) was

Carborane-appended star-shaped clusters

Symmetrical star shaped molecules with carborane clusters on the periphery have been synthesized in good yields via silicon tetrachloride mediated cyclotrimerization reactions of the 9-benzyl derivatives of carboranes with an acetyl group substitution on the benzene ring (see Scheme 7 and Fig. 7) [22]. Functionalization of these symmetrical core structures with either 1-iodoheptane and trivinylchlorosilane produced compounds which could be used as precursors for synthesis of higher order

Nanotechnology in cancer therapy

Boron neutron capture therapy (BNCT) is a binary cancer treatment in which compounds containing 10B are selectively introduced into tumor cells and then irradiated with thermal neutrons. The 10B nucleus then absorbs a neutron forming an excited 11B nucleus that undergoes a rapid fission reaction, producing a high energy α-particle (1.47 MeV) and 7Li ion (0.84 MeV), in addition to a low energy gamma γ-ray (478 keV). The linear energy transfer (LET) of these heavily charged particles have a range of

Acknowledgements

We gratefully acknowledge the financial support through grants by the National Science Foundation (CHE-0601023 to N.S.H.), the Institute of Chemical and Engineering Sciences (ICES) in Singapore, the Robert A. Welch Foundation (N-1322 to J.A.M.), and the second time research prize from the Alexander von Humboldt Foundation (to N.S.H.).

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