Showing papers by "Georgina M. Rosair published in 2003"

Supraicosahedral (metalla)carboranes

Abstract: Although supraicosahedral (hetero)boranes have long been of interest to theoreti- cians, the area is under-developed from a synthetic viewpoint. The synthesis of supraicosa- hedral carboranes by reduction then capitation (RedCap) of C 2 B 10 species is attractive, but unsuccessful as long as the cage carbon atoms are free to separate in the reduction step. Studies on 4,1,6-MC 2 B 10 13-vertex metallacarboranes have shown that the partial degrada- tion of such species can be a facile process, in spite of the fact that the binding energy of the metal atom to the carborane framework can be at least as high as that of a {BH} fragment. These findings support the general concept of the kinetic instability of 1,6-C 2 B 11 species, ex- plaining why a supraicosahedral carborane could not be made from 1,2-C 2 B 10 H 12 . However, tethering together the two cage C atoms with a C 6 H 4 (CH 2 ) 2 strap ultimately allowed the syn- thesis of the first supraicosahedral carborane. This species has a henicosahedral geometry, and there is evidence that a facile rearrangement from kinetic to thermodynamic isomer has occurred. The RedCap synthesis of this unprecedented cluster has the potential to be applied successively, yielding 14-, 15-, 16-, etc. vertex carboranes, the larger of which may be suffi- ciently kinetically stable to exist without a C,C tether.

Nickelation of [3‐Et‐7,8‐Ph2‐7,8‐nido‐C2B9H8]2−: synthesis and characterization of 1,2 → 1,2 and 1,2 → 1,7 isomerized products

Abstract: The reaction between [3-Et-7,8-Ph2-7,8-nido-C 2 B 9 H 8 ] 2- and Ni(dppe)Cl 2 in tetrahydrofuran affords major and minor products, identified spectroscopically and crystallographically. The major species is 1,2-Ph2-4-dppe-6-Et-4,1,2-closo-NiC 2 B 9 H 8 , which arises from the presumed initial product 1,2-Ph 2 -3-dppe-6-Et-3,1,2-closo-NiC 2 B 9 H 8 by a sterically induced 1,2 - 1,2 C atom isomerization, typical of overcrowded nickelacarboranes. The minor species is 1,8-Ph2-2-dppe-4-Et-2,1,8-closo-NiC 2 B 9 H 8 , arising from an unexpected 1,2 → 1,7 carbon-atom isomerization of the initial product. The fact that one boron atom is labelled with an Et group, coupled with the fact that these isomerizations occur at low temperatures, allows comment on the isomerization mechanisms. The minor isomer cannot be explained by reference to Wales' sequential diamond-square-diamond mechanism for 1,2-closo-C 2 B 10 H 12 → 1,7-closo-C 2 B 10 H 12 . A simple rationalization of the formation of both major and minor isomers is available through triangle face rotation.

Crystal engineering with heteroboranes. III. 2-Carboxy-1-methoxymethyl-1,2-dicarba-closo-dodecaborane(12).

Abstract: The title compound, 1-CH(2)OCH(3)-2-COOH-1,2-closo-C(2)B(10)H(10) or C(5)H(16)B(10)O(3), forms a discrete centrosymmetric tetramer, via hydrogen bonding, involving two inner and two outer carborane molecules. One conventional eight-membered hydrogen-bonded ring [graph set R(2)(2)(8)] is formed between two carboxylic acid groups of the inner carboranes. This interaction is then supplemented by an open finite hydrogen bond (graph set D) between the ether O atom of the inner carborane and the carboxylic acid H atom of the outer carborane.

Crystal engineering with heteroboranes. II. 1,2-Dicarboxy-1,2-dicarba-closo-dodecaborane(12) ethanol hemisolvate

Abstract: The title compound, 1,2-(COOH)(2)-1,2-closo-C(2)B(10)H(10).0.5C(2)H(6)O or C(4)H(12)B(10)O(4).0.5C(2)H(6)O, forms a tetramer by incorporating ethanol (solvent) molecules through hydrogen bonding. Two eight-membered rings [graph set R(2)(2)(8)] are formed by hydrogen bonding between two carboxylic acid groups, whereas two ten-membered rings [R(3)(3)(10)] are formed by hydrogen bonding between two carboxylic acid groups and the OH group of an ethanol molecule (solvent). Two crystallographically independent tetramers are present in the crystal structure.

Synthesis and structures of monocyclopentadienylniobium complexes with bi- and tri-dentate dithiolate ligands

Abstract: Reactions of cyclopentadienyl and methylcyclopentadienyl-tetrachloroniobium with thallium(I) 1,3-propanedithiolate yield compounds [NbCpx(1,3-S2C3H6)2] [1, Cpx = η-C5H5 (Cp); 2, Cpx = η-C5H4Me (Cp′)]; similar reactions of the thallium(I) derivative of 2,2′-thiodiethanethiol afford [NbCpxCl2{(SC2H4)2S}] (3, Cpx = Cp; 4, Cpx = Cp′). Reaction of [NbCp′Cl4] with thallium(I) 2,2′-oxydiethanethiolate produces both [NbCp′Cl2{(SC2H4)2O}] 5 and [NbCp′O{(SC2H4)2O}] 6. Products have been characterised analytically and by NMR spectroscopy; the solid-state structures of 1, 3, 5 and 6 have been established by XRD. Compound 1 is five-coordinate, with a distorted four-legged piano-stool geometry, compound 3 is six-coordinate with tridentate thiolate and cis-equatorial Cl ligands, compound 5 is also six-coordinate but with trans-equatorial Cl ligands, and compound 6 is five-coordinate, approximating to a trigonal bipyramidal geometry with an equatorial oxo ligand. Compounds 1 and 2 are fluxional in solution due to non-rigidity of the twist conformations of the six-membered chelate rings, as shown by VT 1H NMR spectroscopic studies with ΔG‡ = 42.7 ± 1.4 kJ mol−1 (at 231 K) and ΔG‡ = 43.1 ± 1.3 kJ mol−1 (at 233 K), respectively, for ring inversion in toluene solutions. Cyclic voltammetric studies on 1 and 2 showed two consecutive, quasi-reversible, reduction processes, attributable to the sequential formation of mono- and di-anionic derivatives; reductions of the other compounds showed less evidence for reversibility. ESR spectra of the initial products of chemical reduction of 1 and 2 in thf, using either a sodium film or cobaltocene as reductant, are typical of those from Nb(IV) species and assigned to the respective mono-anions.

Synthesis, characterisation and structural studies of [Zn(phen)3][Fe(CN)5(NO)].2H2O.0.25MeOH and [(bipy)2(H2O)Zn(μ-NC)Fe(CN)4(NO)].0.5H2O

Abstract: Two new bimetallic complexes [Zn(phen) 3 [Fe(CN) 5 (NO)].2 H 2 O.0.25 MeOH, (1) and [(bipy) 2 (H 2 O)Zn(μ-NC)Fe(CN) 4 (NO).0.5 H 2 O, (2), have been isolated (where phen = 1,10-phenanthroline and bipy = bipyridyl) and characterised by X-ray crystallography [as the 2 H 2 O.0.25 CH 3 OH solvate for (1) and hemihydrate for (2)] infrared spectroscopy and thermogravimetric analysis. Substitution of phenanthroline for bipyridyl resulted in a cyano-bridged bimetallic species rather than two discrete mononuclear metal complexes. The bond angles of Fe-N-O were shown to be practically linear for both 1 [179.2(7)°] and 2 [178.3(3)°], and the Zn atoms have distorted octahedral geometry. The solvent molecules in both crystal lattices take part in forming hydrogen-bonded networks.