Microwave heating was used to prepare eta5-rhenium carborane complexes in aqueous reaction media. For carboranes bearing sterically demanding substituents, isomerization of the cage from 3,1,2 to 2,1,8 derivatives occurred concomitantly with complexation. Microwave heating was equally effective at the tracer level using technetium-99m, affording access to a new class of synthons for designing novel molecular imaging agents.

A series of metallocarboranes of the types rac-[M(CO)3(eta(5)-7-R-7,8-C2B9H11)]-, rac-[M(CO)3(eta(5)-7-R-8-R'-7,8-C2B9H11)]-, and rac-[M(CO)3(eta(5)-7-R-7,9-C2B9H11)]- (M=Re) were prepared by reacting [NEt4]2[Re(CO)3Br3] or [Re(CO)3(OH2)3]Br with the corresponding carboranes in the presence of aqueous solutions of either alkali metal or tetraalkylammonium fluoride salts. Carborane derivatives that were investigated included those containing pyridine, amino, carboxylic acid, carbohydrate, and aryl substituents. During the course of the research, it was discovered that Re metallocarboranes can be prepared directly from the respective closo-clusters under similar reaction conditions used with nido-carboranes. Reaction yields ranged from modest to excellent depending on the carborane isomer and the nature of the cage substituent(s). A crystal structure of an amine-substituted Re metallocarborane was obtained where the complex crystallized in the orthorhombic space group P2(1)2(1)2(1) with a=8.982(2) A, b=11.563(3) A, c=16.811(4) A, alpha=beta=gamma=90 degrees, V=1746.1(7) A3, Z=4, and R1=0.0684.

A series of carborane-carbohydrate derivatives were prepared as model platforms for developing targeted radiopharmaceuticals in which a single ligand can be tagged with two different classes of radionuclides. A nido-carborane linked to glucose was synthesized and the cage labelled with a radiometal (99mTc) and a radiohalogen (125I) in good radiochemical yield. The 99mTc-metallocarborane derivative was prepared via microwave-assisted reactions, while the iodinated derivative was prepared using oxidative labelling methods in mere minutes. Fully characterized reference standards were prepared using nonradioactive rhenium and iodine and used to demonstrate that the isolated radioactive compounds were in fact the desired targets. The work reported, which is one of the rare examples of a single ligand system that can be labelled with both radiohalogens and radiometals, forms the basis of a new paradigm for developing targeted radiopharmaceuticals.Key words: carboranes, medical isotopes, technetium-99m, iodine-125, glucose.

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