NCLN blocking peptide

Dynamic (column) studies on the ion exchange of triaquatricarbonyltechnetium(I) cation, fac-[99mTc(CO)3(H2O)3]+ (1), in the system: amphoteric ion exchange resin (Purolite S-950) - aqueous HNO3 solutions (0.05-2M) corroborate the +1 charge of 1. Stability constants of fac-[99mTc(CO)3(H2O)3-nCln]1-n complexes, determined from the distribution coefficients in the system anion exchanger (Dowex 1X4) - aqueous HCl solutions (0.1-12M) differ from the literature values determined at a constant ionic strength. This difference, resulting from the decrease in the activity of water with increasing HCl concentration, may be a measure of the effect of non-constant ionic strength on ion-exchange equilibria. Weak acidic properties of 1 in aqueous solution at n.c.a. (no carrier added) level (pKa 9) have been demonstrated by paper electrophoresis.

We outline in this combined experimental and theoretical NMR study that sign and magnitude of J(Si,H) coupling constants provide reliable indicators to evaluate the extent of the oxidative addition of Si-H bonds in hydrosilane complexes. In combination with experimental electron density studies and MO analyses a simple structure-property relationship emerges: positive J(Si,H) coupling constants are observed in cases where M -> L pi-back-donation (M = transition metal; L = hydrosilane ligand) dominates. The corresponding complexes are located close to the terminus of the respective oxidative addition trajectory. In contrast negative J(Si,H) values signal the predominance of significant covalent Si-H interactions and the according complexes reside at an earlier stage of the oxidative addition reaction pathway. Hence, in nonclassical hydrosilane complexes such as Cp2Ti(PMe3)(HSiMe3-nCln) (with n = 1-3) the sign of J(Si,H) changes from minus to plus with increasing number of chloro substituents n and maps the rising degree of oxidative addition. Accordingly, the sign and magnitude of J(Si,H) coupling constants can be employed to identify and characterize nonclassical hydrosilane species also in solution. These NMR studies might therefore help to reveal the salient control parameters of the Si-H bond activation process in transition-metal hydrosilane complexes which represent key intermediates for numerous metal-catalyzed Si-H bond activation processes. Furthermore, experimental high-resolution and high-pressure X-ray diffraction studies were undertaken to explore the close relationship between the topology of the electron density displayed by the eta(2)(Si-H)M units and their respective J(Si,H) couplings.