a spectroscopic investigation of the complexing ability of catecholate or salicylate derivatives...

www.elsevier.com/locate/poly

Polyhedron 23 (2004) 2393–2404

A spectroscopic investigation of the complexing ability ofcatecholate or salicylate derivatives towards aluminium(III)

Sebastien Giroux a, Sabrina Aury a, Patrice Rubini a,*, Stephane Parant a,Jean-Roger Desmurs b, Michel Dury b

a Groupe de Chimie Physique Organique et Colloıdale, Laboratoire Structure et Reactivite des Systemes Moleculaires Complexes

(UMR CNRS-UHP 7565), Universite Henri Poincare – Nancy 1, BP 239, 54506, Vandœuvre-les-Nancy Cedex, Franceb Rhodia Recherches, Centre de Recherches de Lyon, 85 rue des Freres Perret, 69192 Saint Fons Cedex, France

Received 6 May 2004; accepted 27 July 2004

Available online 11 September 2004

Abstract

The coordination of the aluminium(III) cation to 2,3-dihydroxybenzoic acid (2,3-dhba), 5-nitrosalicylic acid (5-nsa), 4-nitrocate-

chol (4-ncat) and 3,5-dinitrocatechol (3,5-dncat) was studied by pH-potentiometry and by spectroscopic techniques. The stoichiom-

etry of the complexes and their formation constants were determined. NMR spectroscopy allowed us to obtain thermodynamical,

dynamical and structural information about the complexes formed in solution. The complexing power of these molecules was com-

pared to that of catechol and salicylic acid. Additionally, the proton exchange rate constant between the phenolic and the phenolate

forms of 5-nsa was determined by NMR line-shape analysis. This proton exchange that is usually very fast is considerably slowed

down by the formation of an intramolecular hydrogen bond between the carboxylate and the phenolic groups.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Aluminium(III); Catecholates; Salicylates; Complexation; Proton exchange; NMR

1. Introduction

Many trades are evolving in application domains

which resort to the complexation properties of metals

by organic molecules. For instance, the surface treat-

ment of metals in electronics, galvanoplasty and electro-

plating allows to improve the performances, to decorate

or to protect against oxidation, respectively. The treat-ment of water used in heat exchangers or in the deter-

gency domain, is also included in these areas; a large

variety (and amount) of complexing molecules is in-

volved in this activity.

There is an important class of compounds: the salicy-

late and catecholate derivatives, the complexing proper-

0277-5387/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2004.07.017

* Corresponding author. Tel.: +33 3 83 68 43 27; fax: +33 3 83 68 43

22.

E-mail address: [email protected] (P. Rubini).

ties of which are recognised. These compounds are

industrially produced in a great quantity. They are not

always employed for their complexing ability and could

be valorised in fields where this property is required (gal-

vanoplasty, electroplating for instance). Aluminium(III)

is concerned by this kind of use.

The study of the complexation of Al(III) by salicylate

and catecholate derivatives is also important to under-stand the phytotoxicity of this element when it is re-

leased from the mineral matter and solubilized under

acidic conditions [1,2]. The availability of Al in the soils

depends on its state: free or in organic complexes. Hu-

mic acids are able to complex the aluminium(III) cation

and have an influence on its speciation in soils. The com-

plexing parts of these organic macromolecules are con-

stituted by salicylate and catecholate functions. Theknowledge of the complexes that are formed in aqueous

solution with the Al(III) ion is crucial to evaluate the

mailto:[email protected]

2394 S. Giroux et al. / Polyhedron 23 (2004) 2393–2404

competition with the other organic ligands and to reach

the concentration of the different species.

This paper reports the study of the complexation of

the Al(III) ion with four ligands: 2,3-dihydroxybenzoic

acid (2,3-dhba), 5-nitrosalicylic acid (5-nsa), 4-nitrocate-

chol (4-ncat) and 3,5-dinitrocatechol (3,5-dncat) in aque-ous solutions as a function of pH. The first compound

was chosen because of the possible competition between

the salicylate and the catecholate coordination modes.

The three other compounds have one or two nitro substit-

uants on the aromatic ring that enhance the acidity of the

salicylic and catechol functions and consequently induce

a modulation of the complexing power of the molecules.

Besides the pH-potentiometric study that allowed usto propose a set of complexes for each Al(III)/ligand sys-

tem, an extensive spectroscopic investigation was made

(IR, UV–visible and especially 1H and 13C NMR) to

confirm the potentiometric results and to obtain infor-

mation on the structure and the dynamics of the com-

plexes. Additionally, the proton exchange kinetics

between the OH function and the phenolate form of 5-

nsa could be studied; the formation of an intramolecularhydrogen bond, inducing the slowing down of the pro-

ton exchange rate, leads to unusual line broadenings in

the NMR spectra.

2. Experimental

2.1. Materials

Aluminium(III) chloride (Fluka P99%) was used

with no further purification. The concentration of alu-

minium(III) solutions was determined by a standard

procedure using EDTA (Merck).

2,3-Dihydroxybenzoic acid (P99%), 2,4-dihydroxy-

benzoic acid (P97%), 5-nitrosalicylic acid (P99%) and

4-nitrocatechol (P97%) were purchased from Aldrichwithout further purification. 3,5-Dinitrocatechol was a

Rhodia product.

The ionic strength was adjusted by potassium chlo-

ride (BDH AnalaR 99.5%).

pH-metric titration were performed with sodium

hydroxide from Prolabo (0.100 mol L�1 Titrinorm).

The pH of the solutions was adjusted with hydrochlo-

ric acid and sodium hydroxide from Prolabo (1.00 molL�1 Titrinorm).

2.2. pH-potentiometric measurements

The pH-potentiometric titrations were performed in

aqueous solutions to investigate the protonation and

coordination equilibria of the ligands at T = 298.0 ±

0.1 K and for an ionic strength I = 0.1 mol L�1 (KCl)under an argon atmosphere. They were recorded using

an automatic titration set, including a Metrohm Dosi-

mat 716 DMS Titrino titrator and an Orion 9103SC

type combined glass electrode.

The different equilibria were characterised by the fol-

lowing equation:

pMþ qLþ rH ¼ MpLqHr

and the corresponding constants are given by:

bpqr ¼½MpLqHr�

½M�p � ½L�q � ½H�r ;

where M denotes the aluminium(III) cation, L the non-

protonated ligand and H the proton. For simplicity,

charges are omitted in most cases. However, they can

be calculated knowing the fully protonated ligand is

LH2, leading to the LH� and L2� anions by successivedeprotonations. A detailed description of the experi-

mental procedure and data evaluation (from the PSE-

QUAD computer program [3]) was reported earlier

[4,5].

The protonation and complex formation constants

were determined from 6 to 8 independent titrations (be-

tween 30 and 70 data points per titration). The ligand to

metal ratios varied from 1/1 to 10/1. The ligand concen-tration was 0.01 mol L�1, except in the case of 5-nsa

(0.004 mol L�1) that is sparingly soluble. The pH-metric

data between pH 2 and 11.3 were used for the evalua-

tion, unless precipitation occured.

For the analysis of the titration curves, the formation

of the hydroxo complexes MH�1, M2H�2, M3H�4,

M13H�32 and MH�4 were taken in account, with the fol-

lowing formation constant values [6]: logb10–1 = �5.52,logb20–2 = �7.70, logb30–4 = �13.57, logb130–32 =�109.1 and logb10–4 = �23.46.

2.3. NMR spectroscopy

The 1H and a part of 13C NMR spectra were recorded

on a Bruker DRX 400 apparatus at 400 and 100.6 MHz,

respectively. The same apparatus was used for recordingHMQC 13C-1H correlation maps. Another part of the13C spectra were recorded on a Bruker AC 200 appara-

tus at 50.3 MHz. The solutions were mainly prepared in

D2O/H2O (20/80 v/v) mixtures. The chemical shifts were

referenced to dioxane and converted to the TMS scale,

knowing that ddioxane(13C) = 67.40 ppm and

ddioxane(1H) = 3.700 ppm. In the case of the exchange

rate measurements, an external capillary tube, filled upwith D2O and dioxane, was used.

The concentration of the ligand varied from 0.1 to

0.001 mol L�1.

2.4. IR spectroscopy

The IR spectra were recorded on a Perkin–Elmer

Spectrum One apparatus with an ATR accessory.

S. Giroux et al. / Polyhedron 23 (2004) 2393–2404 2395

The spectra were analysed after the subtraction of the

solvent (water) spectrum. The concentration of 2,3-dhba

was 0.05 mol L�1 and the ligand to metal ratio varied

from 1/1 to 5/1. According to ligand spectra, the 1390

and 1232 cm�1 vibration bands were attributed to

COO� and to COOH functions, respectively.

3. Results

3.1. pH-potentiometry

The values of the protonation and complexation con-

stants deduced from the pH-potentiometric studies areshown in Table 1.

The acidity constant values are in agreement with

those proposed in the literature for 2,3-dihydroxyben-

zoic acid (2,3-dhba) [7,8], 5-nitrosalicylic acid (5-nsa)

[9–11], 4-nitrocatechol (4-ncat) [12,13] and 3,5-dinitro-

catechol (3,5-dncat) [14,15] at 25 �C and for a 0.1 mol

L�1 ionic strength. All the ligands are noted LH2 with

two ionizable functions. The third potential acidity con-stant of 2,3-dhba could not be determined indicating a

pKa value higher than 12.

The set of complexes for the Al(III)–2,3-dhba system

determined in this work is less large than that found by

Kiss et al. [7] (25 �C and 0.2 mol L�1 KCl) or S. Desro-

ches et al. [8] (37 �C, 0.15 mol L�1 NaCl), but the species

ML, ML2, ML2H�1 and M2L2H�3 are common for all

the systems. The additional complexes are ML2H�2

and M2L2H�2 for the first reference and MLH, M2L

and M2L2H�2 for the second reference. The comparison

of the literature results [7] to our data, both obtained at

25 �C, indicates that the logbpqr values determined in

this work are slightly higher, probably because of the

presence of less species in our system. For this ligand,

the problem of the complexation mode: catecholate or

salicylate, arises and will be discussed in the discussionpart with the help of the NMR results.

For 5-nsa, the complexes formed with Al(III) are

ML, ML2, ML3. The formation constants of these com-

plexes are very close to those reported in the literature

[9–11]. The structure of these complexes and the exist-

ence of different isomers for the ML2 and ML3 species

have been analysed by NMR spectroscopy and dis-

cussed below.The 4-ncat ligand also leads to the formation of ML,

ML2 and ML3 complexes with the Al(III) cation. The

formation constant for the ML species is comparable

to that reported in the literature [12,13] and those for

ML2 and ML3 are slightly higher.

There is no previous study on the complexation of

Al(III) with 3,5-dncat to the best of our knowledge.

The pH-potentiometric study shows that ML2 andML3 complexes are formed with logb values equal to

21.80 and 31.68, respectively. Beyond pH = 5, another

species appears; it was not possible to determine its

stoichiometry.

The distribution curves deduced from the pH-

potentiometric measurements are plotted in Fig. 1.

3.2. NMR spectroscopy

The 1H NMR chemical shifts of all the systems de-

scribed below are summarised in Table 2.

3.2.1. 2,3-dhba1H NMR spectroscopy allowed us to follow the for-

mation of the four complexes detected by pH-potenti-

ometry. Besides the signal due to the free ligand(doublets for the H4 and H6, a doublet of doublets for

H5) the signals due to the different complexes succes-

sively appear when the pH is increased. For a ligand

to metal ratio L/M equal to 5/1, the proportion of ML

and ML2 can be easily quantified by integrating the cor-

responding NMR signals. The ML2H�1 complex exhib-

its many peaks that overlap the other signals; its

concentration cannot be determined.For L/M = 3/1 or 1.1/1, the different complexes can

be distinguished but the overlapping of the numerous

signals is very important preventing the determination

of their concentrations.

ML and ML2 present very similar spectra that are

slightly shifted to higher field relatively to the free

ligand.

M2L2H�3 appears in the NMR spectra for low L/Mratio such as 1.1/1. The spectrum presents two patterns

similar to that of the free ligand. They are more shifted

relatively to the free ligand than those of the ML and

ML2 complexes. The spectrum corresponding to

ML2H�1 is composed of many peaks due probably to

inequivalences in the complex and/or different isomeric

species.

A 13C NMR study was also performed in order todetermine the complexation mode with 2,3-dhba. The

chemical shifts of all the carbons (the carboxylate car-

bon is noted C7) for the LH2, LH and L forms are sum-

marised in Table 3. A shift amplitude from 4 to 9 ppm is

measured from the LH2 to the L species. When Al(III) is

added to the solution, new peaks appear in the spectra

corresponding to the formation of the complexes (Table

3). The chemical shifts of these signals do not depend onpH. For the spectra recorded between pH 2.0 and 6.5,

two series of ‘‘bound’’ peaks are successively observed.

3.2.2. 5-nsa

As for 2,3-dhba, the chemical shifts of the H3, H4 and

H6 aromatic protons depend on the pH. They were

measured as a function of pH and the chemical shifts

of the species L, LH and LH2 deduced from these meas-urements. As mentioned above, the peaks are broad for

a pH value around the pKa2 value.

Table 1

Protonation constants of the 2,3-dhba, 5-nsa, 4-ncat and 3,5-dncat molecules and complex formation constants with Al(III) at 298 K and for I = 0.1 mol L�1 compared to those of catechol and

salicylic acid ligands

2,3-dhba 5-nsa 4-ncat 3,5-dncat Catechola Salicylic acida

pKa1 2.76 (COOH) 2.07 (COOH) 6.68 (1-OH) 3.39 (2-OH) 9.28 2.79 (COOH)

pKa2 10.03 (3-OH) 9.90 (OH) 10.70 (2-OH) 9.69 (1-OH) 13.02 13.4 (OH)

logb110(AlL) 10.62 (10.32a, 10.50b) 10.65 (11.11c, 10.91d, 11.26e) 13.89 (13.74f, 13.75g) 16.20 13.22

logb120(AlL2) 19.20 (18.26a, 18.24b) 19.81 (19.73c, 19.67d) 26.33 (25.39f, 25.44g) 21.80 29.26 23.73

logb130(AlL3) 25.74 (25.86c, 25.03d) 37.08 (34.31f, 34.38g) 31.68 37.95 32.55

logb22�3(Al2L2H�3) 10.30 (8.87a, 9.74b) logb22�2 24.05 logb22�2 17.9

logb12�1(AlL2H�1) 13.50 (11.56a, 11.70b)

M + LH2 = ML + 2H+ �2.2 �1.3 �3.5 �6.1 �3.0

logK1 = logb110 � pKa1 � pKa2

M + 2LH2 = ML2 + 4H+ �6.4 �4.1 �8.4 �4.4 �15.3 �8.7

logK2 = logb120 � 2pKa1 � 2pKa2

M + 3LH2 = ML3 + 6H+ �10.2 �15.1 �7.6 �29.0 �16.0

logK3 = logb130 � 3pKa1 � 3pKa2

a Ref. [7], at 25 �C, I = 0.20 mol L�1 KCl.b Ref. [8], at 37 �C, I = 0.15 mol L�1 NaCl.c Ref. [9], I = 0.1 mol L�1 NaClO4.d Ref. [10], at 30 �C, I = 0.100 mol L�1 KNO3.e Ref. [11].f Ref. [16].g Ref. [13].

2396

S.Giro

uxet

al./Polyhedron23(2004)2393–2404

0%10%20%30%40%50%60%70%80%90%

100%

1.5 3.5 5.5 7.5 9.5 11.5

M MH-4

ML ML2

ML2H-1

M2L2H-3

pH0%

10%20%30%40%50%60%70%80%90%

100%

1.5 2.5 3.5 4.5 5.5

M MLML2

ML3

pH

0%10%20%30%40%50%60%70%80%90%

100%

1.5 3.5 5.5 7.5 9.5 11.5

M

MLML2

ML3

pH0%

10%20%30%40%50%60%70%80%90%

100%

1.5 2.5 3.5 4.5 5.5

M

ML2ML3

pH

(a) (b)

(d)(c)

Fig. 1. Distribution diagrams of the complexes formed at 25 �C between aluminium(III) and 2,3-dhba (a), 5-nsa (b), 4-ncat (c), 3,5-dncat (d) as a

function of the pH; [ligand] = 0.01 mol L�1, ligand to metal ratio 5/1.


When aluminium(III) is present in solution, new

peaks corresponding to the Al-bound ligand molecule

merge in the spectra. These different ‘‘bound’’ signals

are observed when pH is increased and correspond tothe formation of the ML, ML2, and ML3 complexes.

The chemical shift of the ML complex signals is only

shifted of a few hundredth ppm. The spectrum of ML2

is not well resolved at room temperature. At 5 �C, thereare two series of signals with similar intensities for this

complex.

The fine structure of ML3 is visible at room tempera-

ture and corresponds to four sets of signals with nearlyequal intensities (Fig. 2).

3.2.3. 4-ncat

The signals of the free ligand shift according to the

pH of the solutions and the knowledge of the chemical

shifts for the species LH2, LH and L are needed to dis-

criminate them from those of the bound ligand mole-

cules. The successive apparition of the complexes isseen when pH is increased. The structures of the pat-

terns corresponding to the bound ligand are similar to

that of the free ligand and are shifted to a higher field.

H5 is not very perturbed by the complexation of the lig-

and to the Al(III) cation. The shift of H3 is more pro-

nounced and almost corresponds to that of the

deprotonated ligand L. H6 also undergoes an important

shift and the shift of the complexes is near to that of thefree deprotonated ligand L. For ML2, the peaks are

broadened contrary to those of the ML3 complex for

which only one set of signal is observed.

3.2.4. 3,5-dncat

There are only two aromatic protons in the molecule.

H4 and H6 give two doublets in the NMR spectra

(J � 2.8 Hz). The chemical shift of these doublets de-pends on the pH. The pH effect is weak for H4 (LH2:

d = 8.56 ppm; LH: d = 8.56 ppm; L: d = 8.41 ppm) and

much stronger for H6 (LH2: d = 7.91 ppm; LH:

d = 7.53 ppm; L: d = 7.27 ppm).

For pH 6 5, two species are detected. The determina-

tion of their concentrations from NMR spectra and the

comparison to those obtained from the potentiometric

results allow us to assign the peaks to the ML2 andML3 complexes. The chemical shifts of ML2 and ML3

are similar for H4 whereas those for H6 are different:

7.42 and 7.58 ppm, respectively. The peaks correspond-

ing to ML3 are broadened when these two species are

present together whereas those of ML2 remain sharp.

Beyond pH 5, a new complex is detected in the NMR

spectra in agreement with the potentiometric results. It

gives rise to many peaks (two doublets and a tripletfor H6, for instance) but its structure could not be

determined.

3.3. Infrared spectroscopy

In order to study the complexation mode of the 2,3-

dhba ligand (salicylate or catecholate), IR spectra were

recorded. COO� and COOH bands can be easily distin-guished [17]. The peak at 1390 cm�1 corresponds to the

carboxylate in an aqueous phase. Another peak at 1232

cm�1 is characteristic of the COOH function. Their

Table 21H chemical shifts in ppm for 2,3-dhba, 5-nsa, 4-ncat and 3,5-dncat and for most of their Al(III) complexes

OH

OH

O OH

1

2

34

5

6

7

2,3-dhba

OH

O2N

O OH

1

2

3

4

5

6

7

5-nsa

OH

OH

NO2

1

2

3

45

6

4-ncat

OH

OH

NO2O2N

1

2

354

6

3,5-dncat

LH2 H3 7.039c 7.695c

H4 7.107d 8.284d 8.566c

H5 6.826e 7.740d

H6 7.406d 8.704c 6.930c 7.939c

LH H3 6.978c 7.563c

H4 6.997d 8.229d 8.564c

H5 6.756e 7.755d

H6 7.321d 8.642c 6.505c 7.552c

L H3 6.506c 7.417c

H4 6.686d 7.991d 8.327c

H5 6.632e 7.723d

H6 6.934d 8.148c 6.448c 7.091c

ML H3 6.796c 7.347c

H4 6.962d 8.155d

H5 6.604e 7.649d

H6 7.297d 8.702c 6.577c

ML2 H3 6.743c,a 6.649c,a 7.293c,a

H4 6.942d 8.147d,a 8.099d,a 8.360c

H5 6.586e 7.629d,a

H6 7.309d 8.724c,a 8.717c,a 6.502c,a 7.415c

ML3 H3 6.600c 6.565c 7.214c

6.556c 6.532c

H4 8.107d 8.084d 8.360c

8.048e 8.025e

H5 7.623d

H6 8.702c 8.697c 6.441c 7.385c

8.671c 8.656c

b H3

H4 6.884d 6.654d 8.340c 8.333c 8.323c

H5 6.793e 6.725e

H6 7.129d 7.082d 7.350c 7.313e 7.294c

The spectra were recorded at 400 MHz; coupling constant values: 3J = 9.0 Hz; 4J = 2.8 Hz; T = 298 K.a T = 278 K.b M2L2H�3 for 2,3-dhba and additional species for 3,5-dncat.c Doublet.d Doublet of doublets.e Triplet.


variations in intensity can be correlated to the propor-

tion of COO� group compared to that of COOH groups

in the solution. The intensity of the peak at 1390 cm�1

(COO�) is proportional to the sum of LH, ML and

ML2 concentrations in solution and not to the alone

LH concentration.

4. Discussion

4.1. 2,3-dhba

The 1H NMR study allowed us to confirm the

potentiometric results. Four different complexes were

Table 313C chemical shifts in ppm for 2,3-dhba and for ML and ML2 complexes

13C LH2 LH L ML ML2

C1 114.11 119.47 118.48 a a

C2 150.18 150.17 155.99 153.37 154.10

C3 145.04 144.88 153.72 148.00 148.27

C4 122.17 120.15 122.23 117.91 117.81

C5 120.27 119.20 119.57 116.58 116.02

C6 122.64 122.52 115.87 122.90 122.88

C7 173.44 176.44 177.89 a 174.61

The spectra were recorded at 400 MHz; T = 298 K; the peaks were assigned according to ChemDraw Ultra� Softwareb and to HMQC 1H–13C

correlation maps.a Peak not observed.b ChemDraw Ultra� v 7.0.1 (2002), Chemical Structure Drawing Standard, Cambridge Soft Corporation.

Fig. 2. Simulation (dark lines) of H4 NMR pattern for Al(5-nsa)3 complex; spectrum (light lines) recorded at 400 MHz: [5-nsa] = 4 · 10�3 mol L�1,

[Al(III)] = 1.33 · 10�3 mol L�1, pH 9.7; T = 278 K; a: simulation in the absence of coupling (chemical shifts of the four species: 8.103, 8.087, 8.041

and 8.034 ppm); b: simulation with coupling (2.8 Hz with H6 and 9.2 Hz with H3).


detected. For L/M = 5/1, we could determine the con-

centrations of two of them. The values correspond to

those deduced from potentiometric results for the ML

and ML2 complexes. The ML2H�1 species gives rise to

many peaks that overlap the other NMR signals. For

this reason, it was not possible to quantify its concen-

tration. For L/M = 1/1, M2L2H�3 is the major com-

plex. From the integrals of the NMR spectra, thelogb values could be evaluated: 10.96 and 10.97 at

pH 3.0 and 4.0, respectively, for ML; 19.55 and

19.39 at pH 3.0 and 4.0, respectively, for ML2 and

10.49 at pH 4.0 for M2L2H�3. These values are

similar to those found by pH-potentiometry: 10.62,

19.20 and 10.30 for ML, ML2 and M2L2H�3,

respectively.

Once the stoichiometry of the species known, the

question of the structure and of the coordination modeof the ligand arises. It is well recognised that catecholate


leads to stronger complexes than salicylate. 2,3-dhba can

act as both of these ligands. The salicylate complexation

mode implies that the less acidic OH function must be

deprotonated. The catecholate complexation implies

that the carboxylic function be not ionised in the ML

and ML2 complexes in which only two (OH) deprotona-tions occur. A 13C NMR study was performed to solve

this question. Up to pH 3.5, the C7 carbon gives one sig-

nal. From pH 4.5, two signals are observed: one at 176.4

ppm for the free carboxylate and another one at 174.6

ppm for the complex ML2. This latter value approxi-

mately corresponds to the chemical shift of the free

COOH ligand. The shifts of the C2 and C3 carbons are

more important: nearly 3.0 ppm and 4.0 ppm at lowfieldfor C2 in ML and ML2, respectively, and nearly 3.1 ppm

and 3.4 ppm at lowfield for C3 in ML and in ML2,

respectively. These raw results are in favour of a catech-

olate complexation for which the carboxylic function re-

mains protonated. Nevertheless this observation is

inconsistent with the fact that the logb values for 2,3-

dhba are similar to the corresponding one for 5-nsa (Ta-

ble 1) which is a salicylate-type ligand and very differentfrom the values found for 4-ncat which is a catecholate-

type ligand.

This difficulty was solved by an IR study that showed

that the intensity of the COO� band corresponds to the

sum of the concentrations of the free ligand, LH, and of

the ML and ML2 complexes, and not to the concentra-

tion of LH. Thus, the carboxylate function is deproto-

nated in the complex: this result proves a salicylatecomplexation.

This conclusion was reinforced by the 13C NMR

study of 2,4-dhba for which the complexation can only

be of the salicylate type. The shift of the C7 carbon of

the free 2,4-dhba and that of the C7 carbon in the com-

plex ML2 at pH 5.2 are identical to the corresponding

ones for 2,3-dhba.

For all these reasons it can be concluded that thecomplexation of 2,3-dhba with Al(III) occurs through

the carboxylate and the deprotonated a-OH function

in the ML and ML2 complexes as for the salicylate

ligand.

The stoichiometry of the M2L2H�3 species supposes

that at less one hydroxide ion participates to the

coordination of Al(III). The 1H NMR spectrum of this

species shows two sets of peaks with an equal intensityindicating that there is either one complex with two

inequivalent ligands or two isomeric forms in which ra-

pid intramolecular ligand exchange occurs.

The ligand can be coordinated to the Al3+ in a catech-

olate manner, so the complex can be more properly writ-

ten M2(LH�1)2(OH) in this case.

Mixed catecholate/salicylate coordination modes

with two OH� ions, as already reported in the literature[7], or only salicylate coordination with three OH� ion

can also be evoked.

4.2. 5-nsa (see NMR spectra on Fig. 3)

4.2.1. Kinetics study of the phenolic proton exchange

The NMR study of the ligand leads to the exceptional

direct observation of the slow exchange of the OH pro-

ton in the LH species between the ligand and the bulk.Indeed, the 1H and 13C NMR signals are broadened

around pH 9.90 (Fig. 3), whereas they are sharp for

pH 6 pKa2 � 1 and for pH P pKa2 + 1. This broaden-

ing is due to the observation of an averaged signal be-

tween the LH and the L species: the full coalescence is

not achieved because of the slow exchange of the OH

proton relatively to the NMR time scale. It is effectively

known that the intermolecular exchange of this protonbetween the protonated form of the ligand and a base

present in the solution can be slowed down because of

the intramolecular hydrogen bond between the carboxy-

late group and the OH function [18–22]. The NMR

study allows the determination of the rate constants

for the exchange of the phenolic OH proton between

LH� and L2� or OH� [23]

LH� þ L2�¢k

kL2� þ LH�

LH� þOH�¢kþ

k�L2� þH2O

The exchange rate is the sum of the rates corresponding

to these two mechanisms, leading to:

kNMR ¼ k½L2�� þ kþ½OH��For pH = pKa, the concentration of L2� and LH� spe-

cies are equal and it can be written that:

k½L2�� þ kþ½OH�� ¼ k½LH�� þ k�½H2O�:In this case: k+[OH�] = k�[H2O] = k 0, since the water

concentration is constant. k, k+ and k� can be calculated

from the measurement of the NMR linewidths for differ-

ent ligand concentrations.13C NMR was preferred to 1H NMR because the

chemical shift differences Dd between the signals corre-

sponding to the acidic and basic forms of the ligand

are higher. The chemical shifts for the different carbon

atoms in the LH� and L2� species are summarised in

Table 4. Dd values are particularly important for C1,

C2 and C5 carbons (10.7, 7.8 and 6.4 ppm, respectively).

Beyond the coalescence of the LH� and the L2� sig-nals, at pH = pKa = 9.90, and consequently for

[LH�] = [L2�], the exchange rate kNMR is given by the

formula [23]:

Dm1=2� �

obs� Dm1=2� �

0¼ p mLH� � mL2�ð Þ2

2kNMR

where (Dm1/2)obs is the observed linewidth at half height,

(Dm1/2)0, that of the signals not submitted to the ex-change and m, the chemical shift in Hz.

Fig. 3. 1H NMR spectra of aluminium(III)–5-nsa (4 · 10�3 mol L�1) solutions for a ligand to metal ratio 3/1, recorded at 400 MHz in D2O/H2O (20/

80 v/v); a: T = 298 K, pH 3.92, b: T = 278 K, pH 4.08 and c: T = 298 K, pH 9.80, the peaks corresponding to the LH/L species are broad; this is due

to the relatively slow exchange of the phenolic proton.

Table 413C chemical shifts (d) in ppm for 5-nsa, linewidth at half-height of 13C peaks ((Dm1/2)0) in Hz for the LH species; linewidth at half-height of 13C peaks

((Dm1/2)obs) in Hz for LH/L mixtures (50/50 mol/mol) at pH = pKa = 9.9 and for different concentrations

d (Dm1/2)0 (Dm1/2)obs

LH L 0.05 mol L�1 0.1 mol L�1 0.05 mol L�1 0.035 mol L�1

C1 118.66 129.56 1.3 13.9 a a

C2 167.86 175.68 1.7 7.1 14.3 17.0

C3 118.31 121.45 0.8 1.9 2.8 4.0

C4 129.83 128.62 0.6 0.8 1.0 1.2

C5 139.69 133.29 3.3 10.6 12.4 12.3

C6 127.68 127.30 0.5 0.6 0.6 0.7

C7 174.12 177.21 0.5 1.6 2.0 1.8

The spectra were recorded at 50 MHz; T = 298 K.a Peak too broad to be detected.


kNMR was found to be dependent on [LH�] or [L2�].

For these different concentration values, a straight line

was obtained when kNMR is plotted versus [L2�] (Fig.

4). The slope of this line gave a k value equal to

5.7 · 105 L mol�1 s�1 and k 0 = 4.5 · 103 s�1

(k+ = 5.7 · 107 L mol�1 s�1 and k� = 80 L mol�1 s�1).

The k value found in this work with 5-nsa is in the

same order of magnitude as the value reported by

10000

15000

20000

25000

30000

35000

0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055

kex (L.mol-1.s-1)

[L] (mol.L-1)

Fig. 4. Exchange constant (kex) of the phenolic proton in 5-nsa

measured by 13C NMR as a function of ligand concentration; T = 298

K; pH 9.9.


Diebler et al. [19] for the exchange rate constant corre-

sponding to the exchange of the dinitrosalicylic anion

with the NH3 base. (k = 1.9 · 106 L mol�1 s�1). The

k+ value is higher than the value obtained for alizarine

yellow S [22] (k+ = 1.4 · 107 L mol�1 s�1) and compara-ble to that obtained for the exchange of thiosalicylic acid

with OH� (4.4 · 108 L mol�1 s�1 [19] or 5.3 · 107 L

mol�1 s�1 at 12 �C [21]). The literature values were ob-

tained from T-jump experiments.

4.2.2. Complexation study1H NMR spectroscopy confirms the formation of

three different complexes when pH is varied. Separatesignals from those of the free ligand are observed for

each complex. The integration of these signals allows

the determination of the logb values for the ML and

ML2 complexes. Because of the broadness of the free lig-

and peaks around pH 10.0 (see above) the determination

of the logb values for ML3 is not possible. The logb val-

ues calculated for ML and ML2 from NMR spectra at 3

different pH values are close to the values deduced fromthe potentiometry experiments (11.3 against 10.65 for

ML and 20.35 against 19.61 for ML2).

The NMR spectrum of the ML species is similar to

that of the free ligand, but slightly shifted.

The signals of ML2 are broad at room temperature.

By cooling down the solution to 5 �C, two sets of signals

with equal intensities are observed for this complex (Fig.

3). This can be due to the non-equivalence of the two lig-and molecules in the complex or to two different iso-

meric forms of equivalent stability. If we consider a

cis-complexation of the two salicylate ligands with the

hexa-coordinated Al3+ cation with two axial water mol-

ecules, there are two possible isomers in which the lig-

ands are equivalent (probabilities 1/2–1/2).

=

-O

O

-ONO2

H2O

H2O

Al3+

H2O

H2O

Al3+

isomers of the cis-ML2 species

An intramolecular exchange can convert an isomer

into another and leads to the partial coalescence of the

signals at room temperature.

The signals of the ML3 complex are sharp at roomtemperature and present four different patterns of equal

intensities (Fig. 2).

The hexa-coordination of Al(III) with the three

unsymmetrical chelating ligands leads to the formation

of two isomers. In the fully symmetrical isomer, all the

three ligands are equivalent. In the other isomer, the

hydrogen atoms of each ligand are inequivalent. It can

be considered that the stability of the two isomers areclose together. Moreover, if the steric effects are weak,

the population for the symmetrical and for the other iso-

mer must be 1/4 and 3/4. This leads to four patterns of

equal intensities as observed.

Al3+ Al3+

isomers of the ML3 species

4.3. 4-ncat

1H NMR allows to follow the formation of three suc-

cessive complexes. The signals of the first and third spe-cies are sharp whereas those of the second complex are

broad. When the second species is present, the peaks

of the free ligand are also broadened. At 5 �C, these sig-nals become sharper. The integration of the free and

bound signals allows the determination of the percent-

ages of all the species present in solution. On the basis

of the formation of the ML, ML2 and ML3 complexes,

the proportions determined by NMR are fully consistentwith the results of the potentiometric study. It can be


concluded that the three species which are detected in

the 1H NMR spectra are effectively ML, ML2 and ML3.

The logb110 and logb120 for the ML and ML2 com-

plexes were determined from the 1H NMR spectra. The

values found at pH 2.0 and 3.0 (13.96 for ML and 25.62

for ML2) are very close to that determined from thepotentiometric study (13.89 for ML and 26.33 for

ML2). The logb values for ML3 could not be evaluated

because of the quantitative complexation which occurs

for the pH range where this species is formed.

It must be mentioned that, from pH 8, additional

peaks due to the formation of other species are detected

in the 1H NMR spectra but their total concentration do

not exceed 4% of the total complex concentration.The broadening of the ligand and ML2 signals when

the ML2 complex is formed can be due to the exchange

of the LH2 molecules with the bound L2� ligands:

ML�2 þ L�H2 ¼ MLL�� þ LH2

where the asterisk is only a typographical distinction.

This kind of exchange is not observed for the ML+

and ML33� species at the NMR time scale level.

4.4. 3,5-dncat

The NMR results are consistent with the potentio-

metric results. For pH 6 5.0, the ML2 and ML3 com-

plexes are formed in solution. The determination of

the formation constants from the NMR experiments isdifficult because the poor solubility of the ligand leads

to the use of low concentrations. Nevertheless, the aver-

age values obtained from six different experiments (two

different ligand to metal ratios at three different pH val-

ues for each) are 21.2 and 31.7 for logb120 and logb130,respectively. The values are close to those obtained from

the potentiometric experiments: logb120 = 21.80 and

logb130 = 31.68.It must be mentioned that these logb values are lower

than those obtained for 4-ncat: logb120 = 26.33 and

logb130 = 37.08. It might be due to the presence of one

0

2

4

6

8

10

12

1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

logα

pH

3,5-dncat 4-ncat

5-nsa

2,3-dhba

sal

cat

Fig. 5. loga as a function of pH for 2,3-dhba, 4-ncat, 5-nsa, 3,5-dncat,

catechol (cat) and salicylic acid (sal); a is the ratio of the total

aluminium(III) concentration to that of free aluminium(III); [lig-

and] = 0.01 mol L�1, ligand to metal ratio 5/1, I = 0.1 mol L�1,

T = 298 K.

nitro-substituent in an ortho-position relatively to one

OH function in the 3,5-dncat ligand leading to a de-

crease of the negative charge on the phenolate oxygen

atom owing to mesomeric effects.

5. Conclusion

The spectroscopic investigation of some salicylate or

catecholate/aluminium(III) systems allowed us to obtain

dynamical, thermodynamical and structural informa-

tion on the ligands and the complexes formed in aque-

ous solutions. These results prove that the studied

ligands are good complexing agents for the Al(III) cat-ion. This complexing ability can be easily shown by con-

sidering the plot of loga versus pH, where a is the ratio

of the total cation concentration to that of free cation. A

high a value is characteristic of a strong complexation. It

can be seen on Fig. 5 that 3,5-dncat is the best ligand

from pH � 2.5 up to pH � 5. 5-nsa and 2,3-dhba have

a similar complexing ability. The complexing power of

4-ncat which is the lowest up to pH 3.5, strongly in-creases from pH 3.5 and becomes comparable to that

of 3,5-dncat at pH � 5.0. The logb1n0 values for the for-mation of the MLn complexes are higher for catecholate

derivatives than for salicylate compounds but, at low

pH values, the complexing ability of salicylate ligand

is nevertheless good, owing to low pKa1 values. The

logb1n0 values are lower for 3,5-dncat and 4-ncat, but

they are offset by a low pKa1 value leading to a verygood complexing power.

For systems that do not lead to precipitate formation

(4-ncat and 2,3-dhba) beyond pH � 5.0, the complexa-

tion is challenged by the formation of the aluminate spe-

cies AlðOHÞ4� from pH � 6.0.

The logb1n0 formation constants for 5-nsa are lower

than those for salicylic acid (logb110 = 13.2,

logb120 = 23.7 for salicylic acid [7]). Similarly, these val-ues for 4-ncat are lower than those for catechol

(logb110 = 16.2, logb120 = 29.3 and logb130 = 37.95 [7]).

The presence of one nitro-substituant on the aromatic

ring decreases the negative charge borne by the pheno-

late oxygen atoms owing to mesomeric effects. When a

second NO2-substituent is present in an ortho-position

relatively to the OH function, this effect is enhanced,

leading to lower values for the formation constant asnoted for 3,5-dncat and 4-ncat. This is true if the inter-

actions between the Al3+ cation and the ligand are essen-

tially electrostatic. Theoretical calculations on these

systems might allow to precise these points.

Acknowledgements

The authors are grateful to Professor J.-J. Delpuech

for helpful discussions about the proton exchange

study.


References

[1] G. Guibaud, C. Gauthier, J. Inorg. Biochem. 97 (2003) 16.

[2] S. Hiradate, N.U. Yamaguchi, J. Inorg. Biochem. 97 (2003) 26.

[3] L. Zekany, L. Nagypal, in: D. Leggett (Ed.), Computational

Methods for the Determination of Stability Constants, Plenum,

New York, 1985 (Chapter 8).

[4] T. Gajda, B. Henry, A. Aubry, J.-J. Delpuech, Inorg. Chem. 35

(1996) 386.

[5] P. Surdy, P. Rubini, N. Buzas, B. Henry, L. Perrito, T. Gajda,

Inorg. Chem. 38 (1999) 346.

[6] L. Ohman, L. Sjoberg, Acta. Chem. Scand. A 36 (1982) 47.

[7] T. Kiss, K. Atkari, M. Jezowska-Bojczuk, P. Decock, J. Coord.

Chem. 29 (1993) 81.

[8] S. Desroches, F. Biron, G. Berthon, J. Inorg. Biochem. 75 (1999)

27.

[9] L. Lajunen, M. Tikanmaki, Acta Univ. Oul. Chem. A 74 (1978) 8.

[10] A.L.R. Merce, A.S. Mangrich, B. Szpoganicz, N.M. Levy, J.

Felcman, J. Braz. Chem. Soc. 7 (2) (1996) 97.

[11] B. Perlmutter-Hayman, E. Tapuhi, Inorg. Chem. 18 (3) (1979) 875.

[12] P. Hakkinen, Finn. Chem. Lett. (1984) 9.

[13] A.J. Downard, R.J. Lenihan, S.L. Simpson, B. O�Sullivan, K.J.

Powell, Anal. Chim. Acta 345 (1997) 5.

[14] V.A. Nazarenko, N.V. Lebedeva, L.I. Vinarova, Zhur. Neorgan.

Khimii 15 (3) (1970) 643.

[15] P. Hakkinen, Finn. Chem. Lett. 13 (4) (1986) 85.

[16] P. Hakkinen, Finn. Chem. Lett. (1984) 151.

[17] L.J. Bellamy, The Infra-Red Spectra of Complex Molecules, vol.

1, 3rd ed., Chapman and Hall, London, 1975 (Chapter 10).

[18] W.L. Mock, L.A. Morsch, Tetrahedron 57 (2001) 2957.

[19] H. Diebler, F. Secco, M. Venturini, J. Phys. Chem. 88 (1984)

4229.

[20] M. Eigen, Angew. Chem. Int. Edit. 3 (1) (1964) 1.

[21] M. Eigen, W. Kruse, G. Maass, L. De Maeyer, Prog. React.

Kinet. 2 (1964) 286.

[22] M. Eigen, W. Kruse, Zeit. Naturforsch. B 18 (1963) 857.

[23] J.-J. Delpuech, Dynamics of Solutions and Fluid Mixtures by

NMR, Wiley, New York, 1995.

a spectroscopic investigation of the complexing ability of catecholate or salicylate derivatives...

Documents