Synthesis, Characterization and Reactivity of Electrophilic Organometallic Compounds of Ruthenium, Tantalum and Silicon
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The work presented herein will discuss the synthesis, characterization and reactivity of electrophilic organometallic compounds of ruthenium, tantalum, and silicon in order to explore unusual properties, reactivity, or structure. Several of the chemical species described are cationic, electronically and coordinatively unsaturated, and require the use of weakly coordinating anions in order to retain their high electrophilic character. The first study consisted in the devolpment of a method to synthesize C?alkylated carborane anions of the type [RCB11Cl11]^? (R = Me, Et, Pr, Bu, Hex) was developed, in order increase the solubility of its salts in weakly coordinating organic solvents. Salts of these C?alkylated anions form crystalline compounds. On the basis of this work, the possibility of the synthesis of a silylium?like zwitterionic compound was investigated. The compound was designed around the anionic [CB11Cl11]^? fragment, attached covalently via the C?vertex to a silylium?type cation through a methylene linker. Its synthesis was succesfuly achieved, and an X?ray diffraction study of this compound revealed that the cationic silicon center is stabilized intramolecularly by weak coordination to a chlorine atom on the ortho B?Cl ring of the anionic fragment. This compound retains the fundamental structural and chemical features of its two?component counterparts, but it is extraordinarily insoluble in weakly coordinating organic solvents. This feature can be traced back to the inherent molecular symmetry and charge distribution. The synthesis of a family of d^6 ML5 and d^6 ML6 ruthenium triflate complexes of the pincer (P2C=)Ru(X) (X = Cl, H, OAc, acac) architecture by ligand exchange using Me3SiOTf will be presented. It was speculated that metathesis of chloride with the more weakly coordinating triflate would be a convenient way to generate compounds that could be potential Lewis?acidic precatalysts. These compounds can be regarded as synthetic equivalents of truly cationic complexes, with the added advantage that no need for the isolation of highly reactive species is necessary. Finally, the reactivity of (?^5?C5Me5)Ta(?CPh)(PMe3)2Cl towards internal and terminal alkynes was investigated. In addition to this, chloride abstraction from (?^5?C5Me5)Ta(?CPh)(PMe3)2Cl was effected, in order to potentially enhance any reactivity towards internal and terminal alkynes. It was found that, upon chloride abstraction from neutral (?^5?C5Me5)Ta(?CPh)(PMe3)2Cl, a reversible C?H bond activation process at PMe3 takes place, concomitant with protonation of the carbyne to form two cationic, isomeric alkylidenes of the type [(?^5?C5Me5)Ta(=CHPh)(CH2PMe2)(PMe3)]^+. In terms of reactivity towards alkynes, it was found that both the neutral and cationic systems form stable tantalacyclobutadienes, one of which was characterized structurally through an X?ray diffraction study.