Ligands protonation

Polymeric compounds are formed from 1,3,5-benzene tricarboxylate and zinc with 2,2 -bipyridyl, phenanthroline, or a pyridyl-2-(l-methyl-lH-pyrazol-3-yl) derivative. A number of compounds were characterized with varying ligand protonation and some, such as [ZnL(pyridine-2-(l-methyl-4-nitro-lH-pyrazol-3-yl))(H20)], containing single-stranded coordinative chains.383... 

The complexes formed from salicylhydroxamic acid and acetylsalicylhydroxamic acid (104) with zinc show loss of one ligand proton and loss of two ligand protons respectively. The zinc complex of the acetyl derivative appears to show coordination via the deprotonated amide and phenolate oxygen of the ligand. IR studies are presented.849... 

A.R. Raitsimring and F.A. Walker, Porphyrin and ligand protons as internal labels for determination of ligand orientations in ESEEMS of low-spin d5 complexes in glassy media ESEEM studies of the orientation of the g tensor with respect to the planes of axial ligands and porphyrin nitrogens of low-spin ferriheme systems, J. Am. Chem. Soc., 1998, 120, 991. 

Effect of ligand protonation. For strongly basic amine ligands, protonation will occur spontaneously in water and electrostatic effects between the resulting positively charged ligand and the metal ion may lead to rate decreases relative to the non-protonated cases (Leugger, Hertli Kaden, 1978). 

We carried out some simulations using two hypothetical models consisting of five ligand protons and five protein protons with the configurations shown in Fig. 2a (the symmetric model) and Fig. 2b (the asymmetric model). 

Even if a given set of protein proton signals P2 and P2 protons in Scheme 2) are instantaneously saturated, the saturation will take a finite time to spread to other protein protons PI and PI sets of protons) and the bound ligand protons ( ) through dipolar networks, and through chemical exchange from the bound to the free ligand (L) protons. This is illustrated in Fig. 3, which... 

It may be noted that the STD intensities are changing even at 3 s of saturation, and generally reach a plateau in intensities at times greater than 4 s. The time-dependent intensity changes in the initial regions (< 0.1 s in Fig. 3) are more reflective of the spatial proximity of the ligand protons to the saturated protein proton in the bound state. 

In Fig. 3b, L2 and L4 have substantially different STD values, with L4 showing significantly smaller effect, even though these two protons are equidistant from the P3 proton. This is a simple consequence of the differences in the relaxation rates for these L2 and L4 protons due to differences in their local environments (e.g., in the asymmetric model, the L4-L5 distance is shorter than the L3-L2 distance, and thus L4-L5 protons experience a faster longitudinal relaxation rate than the L4-L5 protons). These observations suggest that caution is needed in quahtative attempts to relate the magnitudes of steady state STDs to spatial proximity of ligand protons to the protein protons. 

Since the protein signals at 0 ppm were irradiated for the STD experiment, we made the reasonable assumption that the Leu, Val and He methyl protons were instantaneously saturated. From the intensity matrix I(t), the fractional intensity changes for different ligand protons k were calculated, and compared to the experimental STD values using an NOE R-factor Eq. 6. The value of Wk was assumed to be 1. 

The splitting patterns due to ligand protons and nuclei differ in the free ligand and the complexes. There is rapid intramolecular exchange of between two possible sites (shown in 13) and this leads to line broadening which can be used to measure the exchange rate constants (5 x 10 s to 2 x 10 s ). See Table 7.4. 

FIG. 5. Schematic representation of the orbital overlap between the chromium d, v orbital and the ligand protons Ha and Hb for oxo-Cr(V) complexes of 6 (left) and 7 (right). 

Ligand (protonated form) Complex K logK Temp. Medium Refs. 

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