Takele Seda

Amorphous intergranular phases control the properties of rodent tooth enamel

Key trace minerals greatly strengthen teeth The outer layers of teeth are made up of nanowires of enamel that are prone to decay. Gordon et al. analyzed the composition of tooth enamel from a variety of rodents at the nanometer scale (see the Perspective by Politi). In regular and pigmented enamel, which contain different trace elements at varying boundary regions, two intergranular phases—magnesium amorphous calcium phosphate or a mixed-phase iron oxide—control the rates of enamel demineralization. This suggests that there may be alternative options to fluoridation for strengthening teeth against decay. Science, this issue p. 746; see also p. 712 Differences in strength and stability of various tooth enamels may be due to trace minerals at boundary regions. [Also see Perspective by Politi] Dental enamel, a hierarchical material composed primarily of hydroxylapatite nanowires, is susceptible to degradation by plaque biofilm–derived acids. The solubility of enamel strongly depends on the presence of Mg2+, F−, and CO32–. However, determining the distribution of these minor ions is challenging. We show—using atom probe tomography, x-ray absorption spectroscopy, and correlative techniques—that in unpigmented rodent enamel, Mg2+ is predominantly present at grain boundaries as an intergranular phase of Mg-substituted amorphous calcium phosphate (Mg-ACP). In the pigmented enamel, a mixture of ferrihydrite and amorphous iron-calcium phosphate replaces the more soluble Mg-ACP, rendering it both harder and more resistant to acid attack. These results demonstrate the presence of enduring amorphous phases with a dramatic influence on the physical and chemical properties of the mature mineralized tissue.

Preparation and Magnetic Properties of Iron(3+) Spin-Crossover Complexes Bearing a Thiophene Substituent: Toward Multifunctional Metallopolymers

The synthesis of a new 3-ethynylthienyl-substituted QsalH ligand (QsalH is the short form for N-(8-quinolyl)salicylaldimine) (ThEQsalH 3), and the preparation, electronic, and magnetic properties of three homoleptic and cationic iron(3+) complexes containing this ligand with PF(6)(-) 4, SCN(-) 5, and ClO(4)(-) 6 counteranions are reported. In all three complexes a spin-crossover is observed in the solid state by variable temperature magnetic susceptibility measurements and Mossbauer spectroscopy, indicating that the synthetic modification of the QsalH ligand has not significantly altered the electronics at the metal center. This includes the observation of a very rare S = 5/2 to 3/2 spin-crossover in a non-porphyrin iron(3+) complex 5. The molecular structure and magnetic properties of an unusual iron(2+) complex 7 generated by reduction of complex 6 serendipitously during a recrystallization attempt in aerobic acetone solution is also reported. Complexes 4-6 feature iron(3+) reduction and oxidation of the thiophene ring at potentials of approximately -0.7 and +1.2 V (vs Fc), respectively.

Pressure induced Fe2++Ti4+ → Fe3++Ti3+ intervalence charge transfer and the Fe3+/Fe2+ ratio in natural ilmenite (FeTiO3) minerals

The Fe3+/Fe2+ ratioin natural ilmenite (FeTiO3) samples shows rapid increase under pressure from ambient up to GPa, as deduced from 57Fe Mossbauer pressure studies. The ratio, which is initially 0.2 at ambient pressure, saturates at beyond 2–4 GPa to the highest pressure of 14 GPa in this study. Dramatic changes occur at low pressure where the compressibility of the unit cell is appreciably anisotropic, the c-axis being more compressible than the a-axis, and coincides with the rapid decrease observed in the electrical resistance under pressure. Pressure induced intervalence charge transfer away from the ferrous sites, conceivably across regions of octahedra, to the empty 3d manifold of the Ti4+ cation in an adjacent face-sharing layer along the c-axis may account for the change in the Fe3+/Fe2+ ratio.

Ligand-based reduction of CO2 and release of CO on iron(II).

A synthetic cycle for the CO(2)-to-CO conversion (with subsequent release of CO) based on iron(II), a redox-active pydridinediimine ligand (PDI), and an O-atom acceptor is reported. This conversion is a passive-type ligand-based reduction, where the electrons for the CO(2) conversion are supplied by the reduced PDI ligand and the ferrous state of the iron is conserved.

π-Extended and six-coordinate iron(II) complexes: structures, magnetic properties, and the electrochemical synthesis of a conducting iron(II) metallopolymer.

Herein, we describe the preparation of three new bidentate π-extended derivatives of the ligand N-phenyl-2-pyridinalimine (ppi) containing a 3-thienyl (4) substituent at position 4 of the aniline ring or 2-thienyl (6) or phenyl (2) substituents at each of the 2,5 positions of the aniline rings. Three iron(2+) complexes (7-9) containing these ligands were prepared by combining two equivalents each of 2, 4, or 6 with Fe(NCS)(2), and the resulting neutral, six-coordinate complexes were fully characterized, including with single crystal X-ray diffraction experiments in the case of complexes 7 and 9. Variable temperature magnetic susceptibility and Mössbauer experiments confirm the presence of spin-crossover in complexes 7 and 8, and the unusual solid state variable temperature magnetic properties of complex 9 likely result from crystal packing forces. Electropolymerization of the 2,5-dithienyl-substituted complex (9) produces a conducting and electrochromic metallopolymer film (poly-9).

Pyridinediimine Iron Complexes with Pendant Redox-Inactive Metals Located in the Secondary Coordination Sphere.

A series of pyridinediimine (PDI) iron complexes that contain a pendant 15-crown-5 located in the secondary coordination sphere were synthesized and characterized. The complex Fe((15c5)PDI)(CO)2 (2) was shown in both the solid state and solution to encapsulate redox-inactive metal ions. Modest shifts in the reduction potential of the metal-ligand scaffold were observed upon encapsulation of either Na(+) or Li(+).

Probing the Protonation State and the Redox-Active Sites of Pendant Base Iron(II) and Zinc(II) Pyridinediimine Complexes

Utilizing the pyridinediimine ligand [(2,6-(i)PrC6H3)N═CMe)(N((i)Pr)2C2H4)N═CMe)C5H3N] (didpa), the zinc(II) and iron(II) complexes Zn(didpa)Cl2 (1), Fe(didpa)Cl2 (2), [Zn(Hdidpa)Cl2][PF6] (3), [Fe(Hdidpa)Cl2][PF6] (4), Zn(didpa)Br2 (5), and [Zn(Hdidpa)Br2][PF6] (6), Fe(didpa)(CO)2 (7), and [Fe(Hdidpa)(CO)2][PF6] (8) were synthesized and characterized. These complexes allowed for the study of the secondary coordination sphere pendant base and the redox-activity of the didpa ligand scaffold. The protonated didpa ligand is capable of forming metal halogen hydrogen bonds (MHHBs) in complexes 3, 4, and 6. The solution behavior of the MHHBs was probed via pKa measurements and (1)H NMR titrations of 3 and 6 with solvents of varying H-bond accepting strength. The H-bond strength in 3 and 6 was calculated in silico to be 5.9 and 4.9 kcal/mol, respectively. The relationship between the protonation state and the ligand-based redox activity was probed utilizing 7 and 8, where the reduction potential of the didpa scaffold was found to shift by 105 mV upon protonation of the reduced ligand in Fe(didpa)(CO)2.

Uncoupled Redox-Inactive Lewis Acids in the Secondary Coordination Sphere Entice Ligand-Based Nitrite Reduction

Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(15bz5PDI)(CO)2 was shown to encapsulate the redox-inactive alkali ion, Na+, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na+, can entice the corresponding counterion, NO2-, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.