Lanthanide(III) and Group IV metal triflate catalysed electrophilic nitration: ‘nitrate capture’ and the rôle of the metal centre†

Abstract

The lanthanide(III) triflate, [Ln(OH₂)₉](OTf)₃ (Ln = La–Lu), catalysed nitration of a representative arene, viz. bromobenzene, is reported. The extent of nitration is found to be dependent on the charge-to-size ratio of the tripositive metal centre. A mechanistic scenario involving ‘nitrate capture’ in the auto-ionisation of nitric acid is presented where triflic acid plays a key role in the generation of the de facto nitration agent: the nitronium ion. Preparation of the putative intermediates, [(H₂O)_xLn(NO₃)](OTf)₂ (Ln = La–Lu), and characterisation by IR spectroscopy shows the nitrate anion is inner sphere and the triflate anions are outer sphere. Additionally, these salts show a steady increase in nitrate stretching IR frequencies as the charge-to-size ratio of the tripositive lanthanide increases, providing strong evidence for the nitrate capture model. Extrapolation to higher charge-to-size ratio predicts [Hf(OH₂)_x](OTf)₄ to be a superior nitration catalyst. Experimental confirmation of theory is obtained by the application of [Hf(OH₂)_x](OTf)₄ to the successful nitration of the strongly electron deficient arene o-nitrotoluene.