Chelate ring opening rate constants

Rate constants for chelate ring-opening in four-coordinated square-planar cf-platinum complexes [ 1,2-bis(phenylsulfinyl)ethane]dichloroplatinum(II) by a series of pyridines show from a negative deviation in a Bronsted plot (a = 0.53) an estimated rate decrease of steric origin of 200 for the 2-picoline (81IC71) (Scheme 33). 

Fig. 12. Rate constant for the lanthanide-chelate ring-opening step as a function of the stability constant for several car-boxylate ligands.

The conclusion is that chelate ring opening, k y is an unexpectedly slow process, and this accounts for the high formation constants of chelate complexes. It should be noted that this analysis applies for dissociative type substitutions where the rate is insensitive to the entering group. 

Figure 3.5. Some chelates of (H20) Ni(amino-pyridine) and their rate constants for chelate ring opening.

Chelate ring-opening processes also have been studied in these systems. Graham and Angelici studied the reaction of W(CO)4(bpy) with phosphites and found that the pseudo-flrst-order rate constant is given by... 

The trans-cis isomerization of K[Cr(ox)2(OH2)2] ion has been examined in aqueous/organic solvents (EtOH, dioxane, Mc2CO). The first-order rate constant decreases with increasing concentration of organic solvent. Both chelate-ring-opening and twist mechanisms are involved. This reaction is also catalyzed by ion pairing with Mg " ions. ... 

Similarly, for the BNPP complex (10) hydrolysis and generation of the chelate occurs with a rate constant of 4 x 10s" at pH 8 and 25 °C. Intermolecular attack of hydroxide ion on the uncoordinated ester NPP occurs with a rate constant of 2 x 10" s" in 0.5M NaOH at 25 °C. If the ring opening of the product chelate NPP occurs by intermolecular attack of OH" then to achieve a rate of 4 x lO"" s" at pH 8 a second-order rate constant of 4 x 10 L mol" s is required. This value represents a rate increase of 10"-fold for P-O bond rupture over that for the uncoordinated ester. 

The major product of the hydrolysis of cis-[(en)2lr(OH)(BNPP)] (10) the corresponding monoester, is also hydrolyzed by attack of the cis coordinated hydroxide ion with a rate constant of 8 x 10 s at 40 C, or 2 X 10 S at 25 °C in the pH independent region. The product of this reaction is the ring opened monodentate phosphato complex cis-[(en)2lr(0H)(0P03)j. Thus, unlike the corresponding Co(III) complex, the chelate is not thermodynamically stable even at pH 9. The rate of attack of the cis coordinated hydroxide ion on the P center is also —500-fold slower than that reported for the analogous Co(III) reaction. 

The rate constants for the reaction were ki = 2.4 x 10 L-mol s and k2 = 2.9 X 10 L mol" s Both paths produced two phosphorus containing metal complexes in the same relative yields. The minor species (19% yield) was readily identified as the ethylphosphate complex [(NH3)5lrEP]. The major species, however, was not the chelate phos-phoramidate ester, but the ring opened species, N-coordinated monoden-tate ethylphosphoramidate, 14. Both products were indefinitely stable in hydroxide ion solution. The reaction proceeds some 10 -fold slower than the reaction of the analogous Co(IIl) complex, which is rather significant and will be discussed in detail later. The slower rate of production of the... 

Some evidence for this is found for amino-pyridine systems, as shown in Figure 3.5. The rate constant for ring opening of the six-membered chelate is about 10 times larger than that for either of the five-membered rings. A methyl group on the coordinated amine has a more modest effect that could be ascribed to steric acceleration of a dissociative process. 

The phosphodiester complexes (ethyl 4-nitrophenyl phosphate )pentaam-mineiridium(III) and bis(4-nitrophenyl phosphate )pentaammineiridium(III) have also been prepared and their reactivities under basic conditions studied. Both complexes react predominantly via intramolecular attack of a deprotonated coordinated ammonia to liberate 4-nitrophenolate ion. The four-membered phosphoramidate chelate ring thus formed rapidly ring-opens, probably via P—O and P—N rupture, to yield N—bonded phosphoramidate monoester and O—bonded phosphate ester complexes. The rate constant... 

---