Lanthanide(III) and group IV metal triflate catalysed electrophilic nitration: 'nitrate capture' and the role of the metal centre

The lanthanide(III) triflate, [Ln(OH2)(9)](OTf)(3), (Ln = La-Lu), catalysed nitration of a representative arene, viz. bromobenzene, is reported. The e xtent of nitration is found to be dependent on the charge-to-size ratio of the tripositive metal centre. A mechanistic scenario involving 'nitrate cap ture' in the auto-ionisation of nitric acid is presented where triflic acid plays a key role in the generation of the defacto nitration agent: the nit ronium ion. Preparation of the putative intermediates, [(H2O)(x)Ln(NO3)](OT f)(2) (Ln = La-Lu), and characterisation by IP spectroscopy shows the nitra te anion is inner sphere and the triflate anions are outer sphere. Addition ally, these salts show a steady increase in nitrate stretching IR frequenci es as the charge-to-size ratio of the tripositive lanthanide increases, pro viding strong evidence for the nitrate capture model. Extrapolation to high er charge-to-size ratio predicts [Hf(OH2)(x)](OTf)(4) to be superior nitrat ion catalyst. Experimental confirmation of theory is obtained by the applic ation of [Hf(OH2)(x)](OTf)(4) the successful nitration of the strongly elec tron deficient arene 0-nitrotoluene.