Process for preparing a complex of a ligand, metal and base

Details Process for preparing a complex of a ligand, metal and base Process for preparing a complex of a ligand, metal and base

2001-05-07

2002-10-29

Padmanabhan, Sreeni (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

Reexamination Certificate

active

06472548

ABSTRACT:

This invention relates to a process for the preparation of a complex comprising a ligand, a metal and a base.
As discussed in U.S. Pat. No. 5,717,123 chelants or chelating agents are compounds which form coordinate covalent bonds with a metal ion to form chelates. Chelates are coordination compounds in which a central metal atom is bonded to two or more other atoms in at least one other molecule (ligand) such that at least one heterocyclic ring is formed with the metal atom as part of each ring.
Chelants are used in a variety of applications including food processing, soaps, detergents, cleaning products, personal care products, pharmaceuticals, pulp and paper processing, water treatment, metalworking and metal plating solutions, textile processing solutions, fertilizers, animal feeds, herbicides, rubber and polymer chemistry, photofinishing, and oil field chemistry.
In the bleaching stage of photographic materials, a particularly important class of bleaching agents are the aminopolycarboxylic acid bleaching agents, such as an ammonium or alkali metal salt of a ferric complex of ethylenediaminetetraacetic acid (EDTA) or of ethylenediamine disuccinic acid. The production of iron ammonium salts of EDTA is taught in U.S. Pat. No. 4,364,871 and U.S. Pat. No. 4,438,040. Ferric complex salts of propylenediaminetetraacetic acid (PDTA) having a higher bleaching power than EDTA have also been widely used as bleaching agents.
The preparation of complexes of EDDS with metals ions and bases, such as ammonia, are known in the art. For example EP-A-0641168 discloses (S,S)-ethylenediamine-N,N′-disuccinic acid iron (III) ammonium salt. This compound is represented by the formula:
EP-A-0641168 teaches that the above compound can be synthesised by simple contact of (S,S) EDDS with ammonia and iron or an iron compound. It has been recognised in the art that such prior processes for the production of chelates comprising ligands comprising optically active amino acids, such as ethylenediamine-N,N′-disuccinic acid, may result in undesirable precipitation of insoluble oxides and/or hydroxides. This is in contrast to the production of chelates of products such as EDTA which does not result in problems of solid production.
U.S. Pat. No. 5,717,123 teaches that a concentrated stable iron chelate solution may be obtained by addition of a water soluble alkali metal salt of a polyamino disuccinic acid, for example EDDS, to an aqueous ferric salt solution.
U.S. Pat. No. 5,763,634 teaches that ferric chelate of a polyamino disuccinic acid may be prepared without insoluble oxides/hydroxides precipitating out by the simultaneous addition to a vessel of a ferric salt and an alkali metal salt of the polyamino disuccinic acid.
The present invention alleviates the problems of the prior art.
In a first aspect the present invention provides a process for the preparation of a complex of the formula
L·B·M
wherein
(a) L is of the formula (R
1
)(R
2
)N—(CH
2
)
n
—N(R
3
)(R
4
), wherein each of (R
1
)(R
2
)N and N(R
3
)(R
4
) is an optically active amino acid, which may be the same or different; and wherein n is an integer from 1 to 10,
(b) B is a base and
(c) M is a metal,
the process comprising the steps of
(i) providing a reaction mixture comprising L and B and having a pH of less than 7.5;
(ii) contacting the reaction mixture with M to provide an first complex mixture; and
(iii) contacting the first complex mixture with a further amount of B to provide a second complex mixture.
In the present specification by the term “base” it is meant a substance that can accept a proton.
It will be appreciated by one skilled in the art that L of formula L·B·M is in the form of a salt and that L used in the process of the invention may in the form of an acid of a salt thereof.
It will be appreciated that in the process of the present invention B and M can be utilised in the process in ionic, elemental and/or molecular form.
We have surprisingly found that by splitting the contact of the base B with the metal M and ligand L, such that the pH of the reaction mixture of step (i) is kept below 7.5, a process for preparing the complex is provided in which formation of solids is substantially reduced. The present process provides at least some base in the reaction mixture of step (i) and subsequently contacts further base with the mixture of metal M, base B and ligand L. The split addition ensures that the pH is retained at a level below that which results in the undesirable solid formation.
Process
Preferably the amount of B contacted with the first complex mixture in (iii), is such that the pH of the first complex mixture and/or second complex mixture has a value of less than 7.5. Preferably the amount of B provided in (i), and/or the amount of B contacted with the first complex mixture in (iii), is such that the pH of the reaction mixture, first complex mixture and/or second complex mixture has a value of from 4 to 6. In one or more steps of the invention, or in one aspect in all steps (i), (ii) and (iii) of the claimed invention, the pH of the reaction mixture has a value of less than 7.5, preferably from 4 to 6.
During addition of M, such as ferric ions in the form of ferric nitrate, and until B such as ammonia has added in step (iii) thereby raising the pH, the reaction mixture typically reaches a pH between 1 and 2. Low pH may result in decomposition of ligand L, for example of the decomposition of EDDS to lactam and ethylenediamine monosuccinate. Thus in a preferred aspect from the commencement of step (ii) until the commencement of step (iii) the reaction mixture is maintained at a temperature of no greater than 30° C.
In some aspects the present process generates as by-product oxides of M, for example oxides of iron, which are insoluble under the reaction conditions. Insoluble by-products may be removed, for example by filtration. In a preferred aspect to minimise the amount of solids produced and/or to ease their filtration, the process provides a stoichiometric excess of L, such as for example ethylenediamine disuccinic acid, over M, for example ferric ion. In a preferred aspect the ratio of M to L is 0.95 to 0.85.
In one aspect the process further comprises the step of heating the second complex mixture. In this preferred aspect the second complex mixture may be heated to a temperature of 60-100° C., preferably 70-90° C., more preferably approximately 80° C.
When the second complex mixture is heated, in one aspect it may be heated for a period of 30 to 90 minutes, typically for 45 to 75 minutes, or typically approximately 60 minutes.
After the constituents of the complex forming mixture have been contacted and the mixture has been optionally heated as discussed above, the process of the present invention preferably comprises the further step of adjusting the pH of second complex mixture to a value of from 6 to 7.5 or 6 to 7.
The reaction medium (reaction mixture/first complex mixture/second complex mixture) of the present invention is normally wholly aqueous but the presence of other solvents such as ethanol is not excluded.
The process of the present invention may comprise further steps of sequentially contacting the second complex mixture with a further amount of M and a further amount of B. In this way further control of the pH and solid formation may be exercised.
Ligand L
L is of the formula (R
1
)(R
2
)N—(CH
2
)
n
—N(R
3
)(R
4
) wherein each of (R
1
)(R
2
)N and N(R
3
)(R
4
) is an optically active amino acid, which may be the same or different. Preferably n is 2, 3 or 4.
The amino acids will normally be one of the 25 or so naturally occurring optically active amino acids listed in standard textbooks viz. alanine, valine, leucine, norleucine, phenylalanine, tyrosine, serine, cystine, threonine, methionine, di-iodotyrosine, thyroxine, dibromotyrosine, tryptophan, proline and hydroxyproline (which are all “neutral”), aspartic acid, glutamic acid and &bgr;-hydroxyglutamic acid (which are all “acidic”) and omithine, arginine, lysine and histidine (which are all “basic” and less preferred for the reasons stat

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