Carbaboranes as Pharmacophores: Properties, Synthesis, and Application Strategies

Intramolecular electronic interactions were investigated in five series of 12-, 10-, and 6-vertex closo -boranes and hydrocarbons (benzene, acetylene, bicyclo[2.2.2]octane, and cubane) substituted with triple-bonded groups Y≡Z (C≡CR, C≡N, C≡O, and N≡N). Structural data (single crystal X-ray crystallography), spectroscopic properties (UV, IR, and NMR), chemical behavior (dediazoniation reactions), and electronic structure calculations (hybridization and π bond order) are all in agreement that the degree of electronic conjugation between the cluster and the Y≡Z substituent is lowest for the 12-vertex closo -borane and highest for the 6-vertex analog.

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Investigations of electronic interactions between closo-boranes and triple-bonded substituents

2-Thienylpyridyl iridium(III) complexes containing an o-, m-, or p-carboranylvinyl-2,2'-bipyridine ligand and various counteranions (denoted o-PF6, m-BF4, m-PF6, m-SbF6, m-ClO4, m-OTf, m-NO3, m-BPh4, m-F, m-Cl, and p-PF6) were synthesized by using C-formyl carboranes as starting materials. The solid-state structures of o-PF6, m-PF6, m-ClO4, and m-BF4 showed that the cations form twisted cavities in which the anions are fixed by multiple hydrogen bonds. Anion-hydrogen interactions were investigated for nine m-carborane-based complexes with different counteranions. All carborane-based iridium(III) complexes show similar phosphorescence yields in solution but significantly different emission in the solid state. Anion-exchange titration and theoretical calculations revealed the relationships between structures and optical properties. The size of the anion and C-H⋅⋅⋅X anion-hydrogen bonds strongly influence the phosphorescence quantum yield in the solid state. In particular, the Ccar-H⋅⋅⋅X hydrogen bonds between the carboranyl unit and the anion play an important role in solid-state phosphorescence. Complex p-PF6 was successfully applied in phosphorescence-lifetime bioimaging owing to its low toxicity and near-infrared emission.

Tuning the optical properties of 2-thienylpyridyl iridium complexes through carboranes and anions.

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me3SiCC-1,12-C2B10H11, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrCCSiMe3 is reported. The low-yield preparation of 1,12-(Me3SiCC)2-1,12-C2B10H10 from the C,C′-dicopper para-carborane derivative with 1-bromo-2-(trimethylsilyl)ethyne, BrCCSiMe3, has been re-investigated and other products were identified including the C-monoethynyl-carborane 1-Me3SiCC-1,12-C2B10H11 and two-cage assemblies generated from cage–cage couplings. The contrast in the yields of the monoethynyl and diethynyl products is due to the highly unfavourable coupling process between 1-RCC-12-Cu-1,12-C2B10H10 and the bromoalkyne. The ethynyl group at the cage carbon C(1) strongly influences the chemical reactivity of the cage carbon at C(12)—the first example of the ‘antipodal effect’ affecting the syntheses of para-carborane derivatives. New two-step preparations of 1-ethynyl- and 1,12-bis(ethynyl)-para-carboranes have been developed using a more readily prepared bromoethyne, 1-bromo-3-methyl-1-butyn-3-ol, BrCCCMe2OH. The molecular structures of the two C-monoethynyl-carboranes, 1-RCC-1,12-C2B10H11 (R = H and Me3Si), were experimentally determined using gas-phase electron diffraction (GED). For R = H (RG = 0.053) a model with C5v symmetry refined to give a CC bond distance of 1.233(5) A. For R = Me3Si (RG = 0.048) a model with Cs symmetry refined to give a CC bond distance of 1.227(5) A. Molecular structures of 1,12-Br2-1,12-C2B10H10, 1-HCC-12-Br-1,12-C2B10H10 and 1,12-(Me3SiCC)2-1,12-C2B10H10 were determined by X-ray crystallography. Substituents at the cage carbon atoms on the C2B10 cage skeleton in 1-X-12-Y-1,12-C2B10H10 derivatives invariably lengthen the cage C–B bonds. However, the subtle substituent effects on the tropical B–B bond lengths in these compounds are more complex. The molecular structures of the ethynyl-ortho-carborane, 1-HCC-1,2-C2B10H11 and the ethene, trans-Me3SiBrCCSiMe3Br are also reported.

Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes.

Synthesis and structure of 1,12-diethynyl-para-carborane

Two methods have been developed for the preparation of 1-ethynyl-o-, m-, and p-carboranes: reaction of 1-o-, m-, and p-carboranylcopper derivatives with 1-bromo-2-trimethylsilylacetylene and subsequent decomposition of the C-Si bond in the trimethylsilylethynyl-carboranes with either ethanolic base or KF in a two-phase system, and, secondly, reaction of 1-o-, m-, and p-carboranylcopper with trans-1-halo-2-iodoethylenes (halogen=Cl or I), followed by dehydrohalogenation of the resulting 1-(trans-β-halovinyl)carboranes with sodium amide in liquid ammonia.

A. I. Kovderov

Papers

Preparation of 1-ethynyl-o, m − , and p-carboranes from carboranylcopper compounds