Structural Chemistry of Titanium (IV) Oxo Clusters, Part 2: Clusters without Carboxylate or Phosphonate Ligands

Homometallic titanium oxo clusters (TOC) are one of the most important groups of metal oxo clusters. In a previous article, TOC structures with carboxylato and phosphonato ligands were reviewed and categorized. This work is now extended to clusters with other ligands. Comparison of the different cluster types shows how the interplay between condensation of the titanium polyhedra by means of bridging oxygen atoms and the coordination characteristics of the ligands influences the cluster structures and allows working out basic construction principles of the cluster core.

How Many O-Donor Groups in Enterobactin Does It Take to Bind a Metal Cation?

The E. coli siderophore enterobactin, the strongest Fe^III chelator known to date, forms hexacoordinate complexes with Si^IV , Ge^IV , and Ti^IV . Synthetic protocols have been developed to prepare non-symmetric enterobactin analogues with varying denticities. Various benzoic acid residues were coupled to the macrocyclic lactone to afford a diverse library of ligands. These enterobactin analogues were bound to Si^IV , Ge^IV , and Ti^IV , and the complexes were investigated through experimental and computational techniques. The binding behavior of the synthesized chelators enabled assessment of the contribution of each of the phenolic hydroxy groups in enterobactin to metal-ion complexation. It was found that at least four O-donors are needed for enterobactin derivatives to act as metal binders. Density functional theory calculations indicate that the strong binding behavior of enterobactin can be ascribed to a diminished translational entropy penalty, a common feature of the chelate effect, coupled with the structural arrangement of the three catechol moieties, which allows the triseryl base to be installed without distorting the preferred local metal-binding geometry of the catecholate ligands.

Ortho-phenylene-bridged aryloxy phosphine ligands and their coordination chemistry with tantalum(V)

The lithium complexes RP(3,5-tBu2C6H2OLi)2(THF)4, where R = Ph or iPr, (R[OPO]Li2)2(THF)4, can be converted quantitatively into the protonated forms, R[OPO]H2, by reaction with Et3NHCl. Reaction of the protonated compounds with KH yields the potassium complexes (Ph[OPO]K2)2(THF)6 and (iPr[OPO]K2)3(THF)3. The reaction of R[OPO]H2 with TaCl5 yields the sparingly soluble tantalum trichloride complexes R[OPO]TaCl3 via elimination of HCl. Reaction of Ph[OPO]H2 with TaMe3Cl2 leads to the monomethyl complex Ph[OPO]TaMeCl2, or the dimethyl halide complex, Ph[OPO]TaMe2Cl, via protonolysis, and the potassium complex (Ph[OPO]K2)2(THF)6 yields the trimethyl complex, Ph[OPO]TaMe3 by a metathesis process.

Biphenolate Phosphine Complexes of Group 4 Metals

The preparation and structural characterization of a series of group 4 complexes supported by 2,2'-phenylphosphinobis(4,6-di-tert-butylphenolate) ([OPO]2-) are described. The reaction of either H2[OPO] with Ti(OR)4 (R = Et, iPr) or Li2[OPO] with TiCl4(THF)2 produced yellowish-orange crystals of Ti[OPO]2, regardless of the stoichiometry of the starting materials employed. Comproportionation of the bis-ligand complex Ti[OPO]2 with 1 equiv of TiCl4(THF)2 led to the formation of [OPO]TiCl2(THF) as brownish-red crystals. Surprisingly, treatment of H2[OPO] with [(Me3Si)2N]2MCl2 (M = Zr, Hf), irrespective of the molar ratio, generated colorless crystals of the corresponding bis-ligand complex [OPO]2M(OH2) as an aqua adduct. The solution and solid-state structures of these group 4 complexes were all characterized by multinuclear NMR spectroscopy and X-ray crystallography, respectively.