Organic compounds -- part of the class 532-570 series – Organic compounds – Heavy metal containing
Reexamination Certificate
2000-04-07
2002-10-01
Nazario-Gonzalez, Porfirio (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heavy metal containing
C568S014000, C568S707000, C562S014000, C562S487000, C562S488000, C556S032000, C556S034000, C556S045000
Reexamination Certificate
active
06458980
ABSTRACT:
The present invention relates to chelating agents, in particular to chelating agents exhibiting selectivity for tri-valent (manganic) manganese ions, to the corresponding manganic chelates, and to their use for the treatment of manganese deficiency in plant cultivation.
Manganese deficiency is a common problem in agriculture, in field crops as well as in fruit orchards, gardening, and other forms of plant cultivation. As a remedy some soils may be treated with manganese salts, usually the sulphate, but the manganese soon forms insoluble oxides which are no longer available to the plants. “Banded” application is therefore recommended, whereas general or broad-cast application is ineffective. In most cases manganese deficiency instead is treated or prevented by foliar application of manganese sulphate.
In the case of other microelements, e.g. iron, deficiencies often are more conveniently treated by general soil application, such as broadcasting or drip irrigation. The metal is kept soluble in the form of a suitable chelate. It has long been desired to find a manganese chelate of such utility. Attempts to use ordinary manganese (Mn
2+
) chelates of known chelating agents like EDTA and DTPA have proved counter-productive, the problem being that the chelating agent is taken over by ferric iron ions from the soil and the manganese set free is soon oxidised to insoluble oxides.
An object of the present invention is to provide a manganic chelate with a high stability with regard to ferric iron ions and other metal ions present in the soil and with regard to decomposition by hydrolysis and, more particularly, to provide a manganic chelate which remains unchanged for an effective period and can therefore be used for the treatment of manganese deficiency by general application to soil and other growing substrates.
The chelating agents (chelants) according to the present invention have the general structure:
wherein:
n=2-4
X=—CR
1
R
2
—, or
whereby
at least two of X are of formula III
Y=COOH, —PO
3
H
2
or o-hydroxyphenyl
R
1
, R
2
and R
3
are independently H or C
1
-C
8
alkyls
m=0-3, preferably m=1, 2 or 3, and most preferably m=1
Z and Z′ are unsubstituted or substituted C
C*=C or CH or N
Z
1
, Z
2
, Z′
1
and Z′
2
are independently selected from H (or nothing for Z
2
and Z′
2
if C* is N) and C
1
-C
10
groups that optionally contain one or more N atoms (optionally attached directly to C*), whereby one of the pairs Z/Z
1
, Z/Z
2
and Z
1
/Z
2
and one of the pairs Z′/Z′
1
, Z′/Z′
2
and Z′/Z′
2
may be connected to form substituted or unsubstituted (hetero)(poly)cyclic structures of less than 20 atoms.
C* is either part of an aromatic (hetero)(poly)cyclic structure or linked by a double bond to Z or Z
2
and/or Z′ or Z′
2
.
These aromatic cyclic structures or double bonds are preferably conjugated with the respective C═N bonds to allow each N atom to share the negative charge resulting from dissociation of the OH group.
The inventors have surprisingly found that the hexadentate chelating agents according to the present invention are highly selective chelants for manganic ions in the presence of ferric iron ions and that the manganic chelates also exhibit a remarkable stability with regard to hydrolysis, as well as a good ability to remain soluble in the presence of an iron-containing soil.
More specifically, the manganic chelates according to the present invention have been found to form stable solutions up to a pH of 10.5-11, indicating a pKa for hydrolysis of at least about 11. The ferric chelates of the same chelating agents exhibit stability up to a pH of about 8-9.
Alkaline, neutral or weakly acidic soils are usually more or less aerated. When manganese is applied to such a soil, it will slowly, but inevitably be oxidised to insoluble manganese dioxide. However, if the manganese is very strongly chelated, it can, in principle, be kept soluble and available to the plants for a sufficient time, e.g. weeks or months.
To uphold such strong chelation of manganese ions requires high stability of the chelate with regard to hydrolysis, especially at a high soil pH. At least as important is a high stability with regard to other metal ions that may compete for the chelant and thereby set free manganese ions. This problem is especially critical and well-known with ferric iron ions, which tends to be abundant in soils. Due to its tri-valent positive charge, ferric iron ions is known to form very stable chelates with most chelating agents, e.g. EDTA or DTPA. For manganese, the normal state is the divalent manganese cation, which forms chelates of much lower stability than ferric iron ions. Consequently, it is well-known that manganous chelates when applied to soil are rapidly decomposed, and made useless, by ferric iron ions in the soil.
An interesting option would be to use a chelate of tri-valent (manganic) manganese, which is known to form some chelates of the same order of stability as ferric iron ions. The phenolic chelating agents, e.g. EDDHA (EHPG), long used in the form of their ferric chelates on alkaline soils, would be candidates for forming manganic chelates of fair hydrolytic stability, but the inventors have found that in the presence of an iron-containing soil these manganic chelates will decompose rapidly.
This is in accordance with the findings reported by Ahrland, Dahlgren, and Persson (
Acta Agric. Scand,
4:101-111, 1990). These authors report that manganic chelates generally are more prone to hydrolysis than ferric chelates. For manganic EDDHA (EHPG) a pKa value of 9.3 is reported for hydrolysis, whereas results according to the present invention indicate a corresponding pKa for the new chelates of at least 11. For ferric iron ions the situation is the reverse. A pKa value for hydrolysis of 12.7 is reported by the same authors, whereas the chelate according to the present invention has a pKa for ferric iron ions of about 9-10.
A manganic chelate with a stability of approximately the same order as that of the ferric chelate will allow a considerable proportion of manganic ions to be set free in the soil. A chelant is sought with a high selectivity for manganic ion over ferric ion. A very high stability of the manganic chelate is important, since the formation of insoluble manganese oxides will be accelerated by a so-called dismutation of two manganic ions to form one manganous ion and one mole of manganese dioxide. Certain soil bacteria also promote the oxidation of soil manganese to the dioxide.
Not wishing to be bound by any theory, the inventors believe the selectivity of the chelating agents according to the present invention to be found in the distorted configuration of the manganic ion in many of its compounds. While the ferric ion always prefers to co-ordinate in a regular octahedral (hexadentate) fashion, the manganic ion is known to display so-called Jahn-Teller distortion. The effect is that two opposite (“axial”) bonds tend to be elongated in comparison with the remaining four (“equatorial”) bonds. It is thought, in order to explain the selectivity of the chelating agents according to the present invention, that the two positions where the axial coordinating groups branch out are more or less locked by a rigid structure including the four equatorial bonds. With such restrictions on the two axial coordinating groups, the importance of the elongated manganic valences is understandable.
In one of the preferred manganic chelates of the invention the rigid, equatorial structure is thought to be formed by the phenolic and iminic groups in the well-known salen structure (short for salicylaldehyde-ethylenediamine adduct), which is known to prefer a stable, planar structure in its metal chelates, as shown in FIG.
1
.
The negative charges are partly delocalised from the phenolic oxygen to the imine nitrogen. This resonance serves to favour the rigid, planar structure. The complete chelate has two carboxymethyl groups on the N-N bridge
Hakanson Christer L.
Heus Martin
Akzo Nobel NV
Fennelly Richard P.
Nazario-Gonzalez Porfirio
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