Synthesis and characterization of fluorinated tris(pyrazolyl)borate complexes. Observation of an (eta 5-pyrazole)-K+ interaction in the solid state.

Cyclopentadienyl (Cp) ligands have received considerable attention mainly because of their pi six-electron donation capability. Tris(pyrazolyl)borate ligands (Tp) are often compared with Cp because of their identical charge, number of donated electrons, and similar facial coordinating geometry. Their six-electron donation, however, is formally sigma-type. The X-ray structure of the [TpCF3,CH3CuK(mu 4-CO3)KTpCF3,CH3]2 aggregate, 1, reveals for the first time an unprecedented eta 5-TpCF3,CH3 potassium bonding interaction. 1 crystallizes in the triclinic space group P1 with a = 12.0411(2) A, b = 14.9791(2) A, c = 16.0567(3) A, alpha = 71.301(1) degrees, beta = 69.785(1) degrees, gamma = 66.539(1) degrees, and Z = 2. Both K-F and K-mu 4-CO3(2-) interactions stabilize the aggregate, as suggested by the lack of hexanuclear aggregation and K incorporation in the absence of fluorine groups or when O=CO2(2-) is replaced by CH3-CO2-. In the latter case we have isolated the complex [CuTpCF3,CH3(CH3CO2)], 2, which retains a Cu coordination sphere similar to that encountered in the TpCF3,CH3Cu subset of 1. The mononuclear complex 2 crystallizes in the monoclinic space group P2(1)/c with a = 14.1474(2) A, b = 14.1474(2) A, c = 19.0456(6) A, beta = 99.012(2) degrees, and Z = 4. The novel eta 5-coordination mode revealed in 1 suggests that Tp ligands might function not only as sigma donors but also as Cp-like pi donors. The eta 5-coordination mode might therefore constitute a new potential common denominator of these two important classes of ligands.

A comparison of an undecairon(III) complex with the ferritin iron core.

The iron core of ferritin is comprised of up to 4,500 Fe(III) atoms as Fe2O3.nH2O, which is maintained in solution by a surrounding, spherical coat of protein. Organisms as diverse as bacteria and man use the ferritin iron-protein complex as a reservoir of stored iron for other essential proteins. To extend studies of the steps in polynuclear iron core formation, a recently characterized undecairon(III) oxo-hydroxo aggregate [Fe11 complex] (Gorun et al., J. Am. Chem. Soc. 109, 3337 [1987]) was examined by x-ray absorption spectroscopy as a model for an intermediate. The results, which are comparable to the previous x-ray diffraction studies, show near neighbors (Fe-O) at 1.90 A that are distinct from those in ferritin and a longer distance of 2.02 A. However, contributions from neighbors (Fe-C) known to exist at ca. 2.7 A were obscured by a highly ordered Fe-Fe interaction and were not detectable in the Fe11 complex in contrast to a previously characterized Fe(III) cluster bound to the protein coat. Of the two Fe-Fe interactions detectable in the Fe11 complex, the shortest, at 3.0 A is particularly interesting, occurring at the same distance as a full shell (CN = 6) in ferritin, but having fewer Fe neighbors (CN = 2-3) characteristic of an intermediate in core formation. The incomplete Fe-Fe shell is much more ordered than in ferritin, suggesting that the disorder in ferritin cores may be associated with the later steps of the core growth. Differences between the Fe11 complex and the full core of ferritin indicate the possibility of intermediates in ferritin iron formation that might be like Fe11.

A new synthetic approach to the ferritin core uncovers the soluble iron(III) oxo-hydroxo aggregate [Fe11O6(OH)6(O2CPh)15].

Hydrolytic polymerization of iron(III) occurs in many reactions in vivo, for example, the formation of bacterial magnetite in magnetotactic organisms, biomineralization of iron and the synthesis of the metallic core of the iron-storage protein ferritin. The ferritin core contains aggregates of up to 4,500 oxygen-bridged, octahedrally coordinated, high-spin iron(III) centres and is attached to the protein shell through carboxylate groups of amino-acid side chains. The X-ray and electron-diffraction patterns of this core resemble those of the mineral ferrihydrite, a hydrated iron oxide formed in nature, inter alia, by iron-dependent bacteria. The preparation and structural characterization of such large poly-iron aggregates has been a challenge to inorganic chemists. We have recently shown that tri- and tetranuclear iron(III) oxo complexes of the type thought to be important in ferritin-core formation can be prepared by reacting mononuclear [FeCl4]- and binuclear [Fe2OCl6]2- components in aprotic solvents (ref. 9 and S.M.G., W. H. Armstrong and S.J.L., in preparation). Here we report the discovery of a remarkable new molecule, [Fe11O6(OH)6(O2CPh)15], obtained by hydrolysis of the [Fe2O]4+ unit in the presence of limited amounts of water and carboxylate salts. The synthesis and properties of this soluble iron(III) oxohydroxo aggregate should help to elucidate the mechanism of formation of poly-iron centres.