Conformational Prerequisites for the in vitro Inhibition of Cholate Uptake in Hepatocytes by Cyclic Analogues of Antamanide and Somatostatin
✍ Scribed by Prof. Dr. Horst Kessler; Mechtild Klein; Dr. Arndt Müller; Klaus Wagner; Jan Willem Bats; Dr. Kornelia Ziegler; Prof. Dr. Max Frimmer
- Publisher
- John Wiley and Sons
- Year
- 1986
- Tongue
- English
- Weight
- 481 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0044-8249
No coin nor oath required. For personal study only.
✦ Synopsis
Comparative bond length-bond strength calculations1"' for N b 0 2 and NaNb,05F show that the Nb(2) atoms are present in the t 4 oxidation state. Thus, a formal charge of -12 i s obtained for the [Nb,XxJ unit with Nb(l) in the oxidation state f 2 . In analogy to molecular species, e.g., Mo2(OR) (,,["I a &t4 triple bond comes into question for the Nb-Nb dumbbell.
However, Figure 2, right, indicates that each Nb2 pair in the structure of NaNb,O,F is "side-on" coordinated by four Nb" ions. The Nb4+ ions are displaced by about 15 pm from the centers of gravity of the (distorted) coordination octahedra in the direction of an edge X(S)-X(3) toward the Nb2 dumbbell [d(Nb(l)-Nb(2))=309 pm, distance to the center of gravity of the dumbbell 281 pm)]. The d ' ion Nb4+ apparently tends to engage in metal-metal bonding as observed in the structures of the tetrahalides NbX, (X=Cl, Br, For NaNb305F these bonds can be regarded as two-electron three-center bonds between Nb4' (dxz-,2) and n orbitals of the Nb, dumbbell; other configurations are also conceivable. The diamagnetism found for NaNb305F is in agreement with the assumption of a spin pairing of all ten valence electrons in the resulting [ 1.1.1. I]propelIane-Nb, cluster, but is not a proof for this idea of bonding, since NaNb02F1i31 also does not show the magnetism for a d ' configuration.
It is remarkable that the arrangement M2M4X8 (Fig. 2, right) is also found in the structure of Gd2Cl3,IZ3I although as part of an infinite chain. The electronic band structure calculated for the chain can also be interpreted in terms of the presence of 02n4-bonded Gd, pairs, which form Gd-Gd bonds in the direction of the chain via their TI sys-tern~.l'~'
Experimental Procedure
Dried NaF (p.A Merck) was ground into a mass with Nb powder (LAB, Merck) and NbZOs (Merck, Patinale) and placed in a corundum crucible. This was fused under argon in a quartz ampoule and annealed for 3 d at 800°C. In addition to NbO, NbOz, and oxide fluorides (e.g., NaNb02F), black. metallic appearing, parallelepiped-shaped single crystals of NaNblOiF were formed. By repeated grinding, compression, and annealing of a tablet containing NaF, Nb, and Nb205 (molar ratio 1.2 ; I : I ; each I d at 750"C), NaNb,O,F in ca. 80% yield was obtained. Such a product, washed with hydrochloric acid, contained 5.6% F. This value exceeds the calculated one for NaNbiOsF (4.7%) by only 0.9% and can be explained by the higher F content for the side products, e.g., NaNbOIF ( I 1.4% F).