Synthesis of a highly strained uranacycle: molecular structures of organometallic products arising from reduction, oxidation and protonolysis

The reaction of [U(NN3')I] [NN3' = N(CH2CH2NSiMe2Bu')(3)] with Group 1 and 2 metal alkyls gives, via metalation of a methylsilyl group, a highly strained metallacycle [U(bit-NN3')] in high yield. The molecular structure of this compound shows that it has an unusually long U-C bond (ca. 2.75 Angstrom) and additional C-H ... M agostic interactions. On exposure of the complex in solution to H-2(2) the methylsilyl groups and the metallacyclic UCH2Si group are deuterated, leaving the tert-butyl and methylene groups intact. The analogous thorium metallacycle is prepared from the reactions of [Th(NN3')Cl], or by attempted reduction of [Th(NN3')I] with potassium metal in pentane. Attempted reduction of [U(NN3')I] in a similar manner in toluene gave a high yield of a fascinating dimeric metallacyclic anion {[K(eta(6)-C6H5Me)][U(bit -NN3')]}(2), which was characterised by X-ray crystallography. The authenticity of this compound as U(III) is established by near-IR spectroscopy, and also by its production from the reaction of the neutral uranacycle [U(bit-NN3')] with potassium. A mechanism for the formation of the anion is postulated. Its reaction with benzyl chloride led to oxidation to the parent uranacycle. Oxidation of [U(bit-NN3')] with air or oxide sources leads to formation of an ore-bridged dimer with intermonomer metalated methylsilyl groups and pentavalent uranium centres. Reactions of [U(bit-NN3')] with a range of acids HX occur rapidly, reprotonating the previously metalated methylsilyl group and forming complexes [U(NN3')X]. For example, lutidinium hydrochloride, diethylamine and tert-butanol gave [(NN3')UCl], [(NN3')U(NEt2)] and [(NN3')U(OBut)], respectively. Reactions with carbon acids were also successful. For example, beta-hydropyridines gave eta(2)-dehydropyridyl complexes. One of these, [U(NN3')(eta(2)-NC5H4)], is shown in the first X-ray crystallographic study of such an actinide complex to contain an essentially planar U(eta(2)-NC5H4) unit. Monosubstituted alkynes react with the uranacycle in a similar manner to give eta(1)-alkynyls. The molecular structure of one example shows that the U-C=C unit is bent (156 degrees) in contrast to other structurally characterised non-bridging alkynyls of the f-elements. (C) 1999 Elsevier Science S.A. All rights reserved