59th Annual Report on Research 2014

Our research program is aimed at the development of ligands that are inert towards undesirable intramolecular reactions with the metal center (cyclometalation) or reactive metal element bonds (insertions). Such intramolecular reactions often result in catalyst decomposition, and have inhibited the development of practical homogenous alkane functionalization catalysts. Our hypothesis is that by appending classical ancillary ligand frameworks with extraordinarily unreactive halogenated icosahedral carborane and or borane clusters, a new generation of “armor plated” ligands (APligands) will be born. In contrast to typical ligands that provide steric protection with hydrocarbon groups, which are susceptible to intramolecular attack by a reactive metal center, AP-Ligands will be immune to such decomposition pathways. This feature coupled with the cluster’s highly tunable steric and electronic properties, will render such AP-ligands ideally suited for a range of applications. A large part of our efforts to design AP-ligands requires the development of novel synthetic methods to functionalize the perhalogenated carborane anions. With the aid of ACS PRF funding, we have published four manuscripts (2 Inorganic Chemistry, full paper and communication; 1 Angewandte Chemie, communication; 1 Inorganic Chimica Acta, Communication) over the last two years, detailing the synthesis of novel molecules that are useful for ligand design. An outline of the manuscripts follows.

2012-2013 Funding Period

Publication #1, Inorg. Chem. 2013, 52, 6223: In the first, paper titled “Click-Like Reactions with the Inert HCB₁₁Cl₁₁^- Anion Leads to Carborane-Fused Heterocycles with Unusual Aromatic Character” we present the discovery of base induced cycloaddition reactions between the HCB₁₁Cl₁₁^-anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly-occupied p-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton.

Publication #2, Angew. Chem. Int. Ed.  2013, 52, 11560:Redox active ligands have found an important place in the design of catalysts for the functionalization of hydrocarbons. In the second paper, titled “Isolation of a Carborane-Fused Triazole Radical Anion” we present the discovery of a method to manipulate the electronic properties of carborane anions to render them amendable to facile chemical reduction. It was postulated that because carborane fused triazoles from publication #1, such as the N-phenyl substituted derivative, display some degree of exo-cluster delocalization, these species might possess interesting redox properties. Cyclic voltammetry (CV) experiments indicate that compound 1 undergoes no observable oxidations and only irreversible reductions at large negative potentials. We postulated that the large potential required to reduce the heterocyclic anion 1 was due to unfavorable electrostatic repulsion associated with producing a radical dianion. We reasoned that if the negative charge of 1 were masked, by forming a zwitterion, its electrochemical properties would be altered in such a way as to allow facile reduction. Hence, anion 1 was alkylated to afford an electronically neutral zwitterionic species. In marked contrast to the anionic carborane, the zwitterion undergoes a clearly reversible reduction at -0.87 V from Fc/Fc+. With these encouraging results in hand, we set out to chemically reduce the zwitterion an isolate the observed radical anion. Thus, a diethyl ether solution of cobaltocene was added to zwitterionic heterocycle after 12 hours a green/brown precipitate was isolated. A radical species could be detected by EPR (one broad signal at g = 2.003, the peak to peak width of the signal is 28.8 G, 28.2 G at 120 K), which is in very good agreement with the value calculated for the radical anion 3 (g = 2.004). The hyperfine coupling constants could also be calculated but were not observed experimentally because of the broad nature of the signal; a fact that may be attributed to the inherent asymmetry of the five-membered ring. The structure of the radical anion was unambiguously confirmed by a single crystal x-ray diffraction study, and is the first example of a stable triazoles radical anion. We are currently incorporating this structural motif into ligands for hydrocarbon functionalization chemistry.

2013-2014 Funding Period

Publication #3, Inorg. Chem. 2013, 52, 12308: The perchlorinated carba-closo-dodecaborate anion is typically inert toward B−Cl functionalization. We present here the observation of two competing reactions that occur with this anion at ambient temperature. When this molecule is treated with n-BuLi and subsequently reacted with tosyl azide, a cycloaddition occurs and results in chloride substitution at a B−Cl vertex. The competing and dominant pathway is a substitution reaction to form the azide N₃CB₁₁Cl₁₁⁻.This rare anionic carboranyl azide reacts with PPh₃ in FC₆H₅ to afford a stable anionic phosphazide. When dissolved in tetrahydrofuran, the phosphazide is in equilibrium with free PPh₃ and N₃CB₁₁Cl₁₁⁻. Both the triazole and phosphazide are characterized by single-crystal X-ray diffraction, NMR and IR spectroscopy, and high resolution mass spectrometry.

Publication #4, Inorg. Chim. Acta 2014, DOI: 10.1016/j.ica.2014.05.046: Here we describe the azide functionalization of polybrominated derivatives of the carba-closo-dodecaborate anion. Specifically, treatment of [HCB₁₁Br₁₁]^- and [HCB₁₁H₅Br₆]^- with n-BuLi, followed by subsequent reaction with tosyl azide, yields the novel carboranyl azides [N₃CB₁₁Br₁₁]^- and [N₃CB₁₁H₅Br₆]^-. In contrast to its perchlorinated congener, the [HCB₁₁Br₁₁]^- anion does not undergo competitive B–Br activation to produce a carborane-fused triazole; instead, it forms the azide in a perfectly selective manner. These polybrominated carboranyl azides have been characterized by IR, NMR, and HRMS. Additionally, we describe the first single-crystal X-ray diffraction studies of azide derivatives of the carba-closo-dodecaborate anion, as well as a neutral isoelectronic ortho-carborane azide (1-N₃-2-Ph-C₂B₁₀H₁₀). We are currently investigating the applications of these azides in the formation of metallo-nitrenes.