Organometallic chemistry of the f elements: towards new developments: cyanide complexes of the f elements.

Summary If the chemistry of cyanide complexes of d metals has been a highly studied discipline for many years, it is however little developed with f metals and in particular with actinides. The cyanide ion, which is an extremely coordinating ligand, bidentate and capable of stabilizing both the high and low oxidation states of the metal centers, seems particularly well suited to uranium. Its use with the f-elements offers, in addition to attractive synthetic perspectives that have already overturned some commonly accepted ideas, a definite interest in obtaining compounds with interesting physicochemical properties. This PhD concerns the development of the chemistry of cyanide complexes of the f elements (Ce, U, Th) in the two series of precursors [Mf(N*)₃]ʲ˖ (j = 0, 1) and [An(Cot)₂] (N* = -N(SiMe₃)₂ . Cot = C₈H₈²-). The first chapter deals with the reactivity of trivalent [Mf(N*)₃] complexes (Mf = Ce, U) with cyanide ion. The syntheses and crystal structures of the mono- and polycyanide complexes [Mf(N*)₃(CN)][M] [Mf(N*)₃(CN)₂][M]₂ and [Mf(N*)₂(CN)₃][M]₂, and of the bimetallic complexes [{Mf(N*)₃}₂(µ-CN)][M] (M = NR₄, K(18-C-6)) are presented. Depending on the ion considered, Ce³˖ or U³˖, the structural characterizations show a different coordination mode of the cyanide ligand. For example, bis-cyanide complexes reveal coordination of cerium by the nitrogen atom (Ce-NC) and uranium by the carbon atom (U-CN). The isocyanide coordination mode is extremely rare with the transition metals d and f. Cyanide complexes of U(IV) presented in Chapter 2 were also obtained from the new tris-amide precursor [U(N*)₃][BPh₄]. The bimetallic species [{U(N*)₃}₂(µ-CN)][BPh₄] and especially the neutral [U(N*)₃(CN)] and anionic [U(N*)₃(CN)₂][M] terminal cyanide derivatives [M] (M = NEt₄, K) were isolated. These monometallic species exhibit the U-NC isocyanide ligation mode, in contrast to other previously known uranium cyanide complexes. Theoretical DFT studies explain the origin of these different coordination modes of the cyanide ligand towards Ce³˖ and U³˖ ions by the involvement of the 5f orbitals and the distinct hardnesses (according to Pearson) of the metal centers. These calculations are in agreement with experiment and highlight the slightly greater energy stability of the Ce³˖-NC , U³˖-CN and U⁴˖-NC bonded compounds. The contribution of cyanide to the chemistry of actinocenes [An(Cot)₂] (An = U, Th), mythical sandwich compounds considered until recently as species without any coordination chemistry, is evident with the preparation of a new class of compounds with bent geometries. While the mono-cyanide complex [U(Cot)₂(CN)][NEt₄] is the only bent compound characterized with uranium, [Th(Cot)₂] exhibits a much more varied chemistry that is the subject of the third chapter. Thorocene thus reacts with CN-, N₃-, and H- ions to give the anionic complexes [Th(Cot)₂(X)][M] (X = CN-, N₃-, and M = Na(18-C-6) and NBu₄), dianionic [Th(Cot)₂(CN)₂][NBu₄]₂ and bimetallic [{Th(Cot)₂}₂(μ-X)][M] (X = CN-, H- and M = Na(18-C-6) and NBu₄]. The structures of the [An(Cot)₂(CN)][NR₄]₂ analogues are notably different (monomer for U and polymer for Th). Cyanide-terminated complexes are interesting molecular building blocks for the elaboration of polynuclear systems and materials that, due to the particular physicochemical properties of the f-elements, are particularly attractive for applications in the field of molecular magnetism and/or luminescence.