Tetraazamacrocycle bearing quinoline pendant arms and its complexation properties

Inorganica Chimica Acta 356 (2003) 133�/141

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Tetraazamacrocycle bearing quinoline pendant arms and itscomplexation properties

Xiuling Cui a, M. Fatima Cabral b, Judite Costa a,b, Rita Delgado a,c,*a Instituto de Tecnologia Quımica e Biologica, UNL, Apartado 127, 2781-901 Oeiras, Portugal

b Faculdade de Farmacia de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisbon, Portugalc Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal

Received 5 March 2003; accepted 16 June 2003

In honor of Prof. J.J.R. Frausto da Silva

Abstract

A new tetraazamacrocycle containing two 5-chloro-8-hydroxyquinoline side arms, L1, was synthesized. The protonation

constants of L1 and the stability constants of its complexes with Cu2�, Zn2�, Cd2� and Pb2� were determined by potentiometric or

spectrophotometric methods, at 20 8C in methanol:water (5:1, v/v) and ionic strength 0.10 mol dm�3 in tetrabutylammonium nitrate

(TBAN). Compound L1 has three high protonation constants, the most basic centres being both phenolates of the arms. Mono- and

dinuclear complexes were found in solution for all the metal ions, the complexes of copper are thermodynamically very stable, and

the dinuclear complexes of lead(II) are the predominant species even in 1:1 mixtures. UV�/Vis spectra of some complexes and EPR

of the copper ones confirmed the presence of dinuclear complexes.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Tetraazamacrocycle; Stability constants; Dinuclear complexes; EPR copper complexes; 8-Hydroxyquinolinyl groups

1. Introduction

Macrocycles containing quinoline side arms have

found many applications as analytical reagents in

absorption spectrophotometry, fluorometry, solvent ex-

traction, and partition chromatography [1,2]. These

compounds are also used as pesticides [2].

The 8-hydroxyquinoline (HQ) forms a stable five-

membered chelate ring with metal centres and very

stable complexes with divalent transition and post-

transition metal ions of the type M(HQ)�, M(HQ)2 or

M(HQ)3� [3]. The attachment of two rigid 5-chloro-8-

hydroxyquinoline (CHQ) groups to diazamacrocycles

* Corresponding author. Tel.: �/351-214-46-97-37/38; fax: �/351-

214-41-12-77.

E-mail address: [email protected] (R. Delgado).

0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0020-1693(03)00465-1

results in an appropriate pre-organization of the mole-

cule increasing the selectivity and, in general, modifying

the co-ordination properties of the macrocycle to metal

ions [2,4�/7]. It is known that the phenol ring of CHQ is

electron-rich and the pyridine moiety electron-deficient

[4]. Therefore, if two CHQ rings approach to each other,

p�/p interactions will occur by overlapping of the phenol

moiety of one CHQ with the pyridine group of the

other, as already observed by Izatt and co-workers [4].

The p�/p interactions can be induced by co-ordination to

a metal ion so that a pseudo-second ring is formed,

resulting in a cryptate-like structure. In such a case, the

match between the cation size and the dimensions of the

pseudo-cavity formed is crucial [4]. In addition, the

CHQ molecule allows different positions to be appended

as N-substituents, by 2- or 7-positions, offering the

phenol or the pyridine as closer donor atom to the metal

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X. Cui et al. / Inorganica Chimica Acta 356 (2003) 133�/141134

centre with consequent different affinity of the resulting

molecule to metal ions. In most of the studies having

CHQ as substituents of macrocycles, this group is

appended by the 2-position [2,4�/7], and very few by

the 7-position [5,6]. In most cases, the CHQ groups were

attached to macrocycles containing oxygen donor

atoms, and consequently an special attention has been

given to the complexation of alkali and earth alkaline

metal ions, with few exceptions [2,7].

The extensive applications of quinoline derivatives

make them a very attractive field of research. We are

especially interested to obtain compounds, which could

form selective complexes with transition and post-

transition metal ions to be used as spectrophotometric

sensors for these metal ions. In the present work, we

explore CHQ groups appended by the 7-position to a

14-membered tetraazamacrocycle containing pyridine,

L1 (see Scheme 1). The 7-position CHQ derivative was

chosen because the resulting ligand has more affinity to

transition metal ions [5]. To understand the role of the

pyridine moiety of the CHQ groups, 5-bromophenol

groups were also appended to the same macrocycle, L3

[8]. The CHQ has a larger p electron area when

compared with the phenols and has the additional

nitrogen donor atom of the pyridine group then CHQ,

attached to macrocycles as side arms, can induce the

formation of di- or polynuclear complexes.

Scheme

2. Experimental

2.1. Reagents

The parent macrocycle 7-methyl-3,7,11,17-tetraazabi-

cyclo-[11.3.1]heptadeca-1(17),13,15-triene (L2) was

synthesized by previously reported procedures [9,10].

Compound 7,16-bis((5-chloro-8-hydroxy-7-quinoli-nyl)methyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctade-

cane (L4) was synthesized from the parent macrocycle

(Aldrich) following the described procedure [6]. All the

chemicals were of reagent grade and used as supplied

without further purification. Microanalyses were carried

out by the ITQB Microanalytical Service. IR spectra

were recorded from KBr pellets on a UNICAM

Mattson 7000 spectrometer.

2.1.1. Synthesis of 7-methyl-3,11-bis((5-chloro-8-

hydroxy-7-quinolinyl)methyl)-3,7,11,17-

tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene

(L1)

An anhydrous toluene solution (60 cm3) of L2 (0.5 g, 2

mmol), p -formaldehyde (0.12 g, 4 mmol), and CHQ

(0.72 g, 4 mmol) were refluxed for 2 days. After

evaporation of the solvent under reduced pressure, the

residue obtained was purified by silica column chroma-

tography using CHCl3/CH3OH (20:1, v/v) as eluent. The

1.

X. Cui et al. / Inorganica Chimica Acta 356 (2003) 133�/141 135

first fraction was CHQ and the second fraction eluted

gave a yellow powder after evaporation of the solvent,

which was verified to be the desired macrocycle (L1).

Yield: 0.3 g, 23.6%. m.p. 85�/86 8C. IR (KBr pellet):3413, 2945, 2810, 2360, 1622, 1591, 1496, 1456, 1385,

1263, 1153, 1111, 1045, 939, 789, 752 cm�1. 1H NMR

(CDCl3): d 1.41 (4H, q, NCH2CH2CH2N), 1.69 (3H, s,

NCH3), 2.16 (4H, bs, NCH2CH2), 2.77 (4H, t, J�/7.3

Hz, NCH2CH2), 3.48 (2H, s, OH ), 3.87 (4H, s, NCH2�/),

4.04 (4H, s, NCH2�/), 7.25 (2H, d, py), 7.41 (2H, Q), 7.49

(2H, m, Q), 7.50 (2H, t, py), 8.47 (2H, d, Q), 8.95 (2H, d,

Q) ppm. 13C NMR (CDCl3) and dept: d 23.7(NCH2CH2CH2N), 42.7 (CH3), 50.7, 54.1, 57.5, 59.2,

119.3 (C �/Cl), 119.9 (Q), 121.9 (py), 123.1 (Q), 126.1

(Q), 127.5 (Q), 132.7 (Q), 137.5 (py), 139.9 (C �/OH, Q),

149.3 (Q), 152.4 (Q), 156.9 (py) ppm. Found: C, 64.4; H,

5.0; N, 13.1. Calc. for C34H36Cl2N6O2: C, 64.66; H, 5.14;

N, 13.31%.

2.2. Potentiometric measurements

2.2.1. Reagents and solutions

Metal ion solutions were prepared at about 0.05 mol

dm�3 from the nitrate salts of the metals, and were

standardized as described [11]. Carbonate-free solution

of the titrant, KOH, was freshly prepared in methanol:-

water (5:1, v/v) solution, maintained in a closed bottle,

and was discarded when the percentage of carbonate

was about 0.5% of the total amount of base (tested byGran’s method) [12]. Perchloric acid solution (0.1 mol

dm�3) was prepared from a Fixanal ampoule in

methanol. The support electrolyte, tetrabutylammonium

nitrate (TBAN), was added to the solutions as a solid.

The ligand L1 was dissolved in methanol:water (5:1), in

concentrations of about 1.5�/10�3 mol dm�3. All the

solutions were prepared in a mixture of methanol:water

(5:1). This solvent was suggested from the solubility ofthe ligand. Methanol was purified by standard methods

[13] and water used was demineralized water obtained

by a Millipore/Milli-Q system.

2.2.2. Equipment and work conditions

The equipment used was described before [10,14]. The

glass electrode was pre-treated by soaking in solutions

of methanol:water of increasing rate of methanol over aperiod of 10 days, in order to prevent erratic responses.

The temperature was kept at 20.09/0.1 8C; atmospheric

CO2 was excluded from the cell during the titration by

passing purified argon across the top of the experi-

mental solution in the reaction cell. The ionic strength of

the solutions was kept at 0.10 mol dm�3 with TBAN.

2.2.3. Measurements

The [H�] of the solutions was determined by the

measurement of the electromotive force of the cell, E�/

E ?o�/Q log[H�]�/Ej . The term pH is defined as �/

log[H�]. E ?o, Q , Ej and K ?w were obtained as described

previously [10,14]. The value of K ?w; ionic product of

water in the methanol�/water mixture, used was found

equal to 10�14.40 mol2 dm�6 in our experimentalconditions, in agreement with that determined by

Rochester [15].

The potentiometric equilibrium measurements were

made on 20.00 cm3 of $/1.50�/10�3 mol dm�3 ligand

solutions diluted to a final volume of 30.00 cm3, in the

absence of metal ions and in the presence of each metal

ion for which the CM:CL ratios were 1:1 and 3:1. A

minimum of two replicates was performed. A correctionwas made for the small decrease in volume that occurs

on mixing methanol and water. Care has been taken to

maintain unaltered the methanol/water ratio in the

measured solution and the temperature was kept at

20 8C to prevent the evaporation of the solvent.

2.2.4. Calculation of equilibrium constants

Overall protonation constants bHi were calculated by

fitting the potentiometric data obtained for the free

ligand to the HYPERQUAD program [16].

bHi �

[HiL]

[L] � [H]i

Differences between two consecutive log bHi values

provide the stepwise protonation constants KHi :/

Stability constants of various species formed in

solution were obtained from the experimental data

corresponding to the titration of solutions of differentmetal:ligand ratios, also using the HYPERQUAD

program. The initial results were obtained in the form

of overall stability constants, bMm Hh Llvalues:

bMmHhLl�

[MmHhLl ]

[M]m � [H]h � [L]l

Mononuclear species, ML, MHiL (i�/1�/4) and also

dinuclear ones, M2L, M2HjL (j�/1�/2), M2H�1L and

M2(H�1)2L, were found for the metal ions studied,

being bM2(H�1)L�/bM2LOH�/KW and bM2(H�1)2L�/

bM2L(OH)2�/(KW)2. The species considered in a particu-

lar model were those that can be justified by the

principles of co-ordination chemistry. Differences, in

log units, between the values bMm (Hi L) [or bM2(H�1)j L]

and bMm (Hi�1L) [or bM2(H�1)j�1L] provide the stepwise

protonation reaction constants, as shown in Table 1.

The errors quoted are the standard deviations of the

overall stability constants given directly by the program

for the input data, which include all the experimental

points of all titration curves.

The protonation constants were obtained from 120

experimental points (two titration curves) and thestability constants for each metal ion were determined

from 70 to 120 experimental points (two or three

titration curves). First, each titration curve of a given

Table 1

Protonation constants of L1 and stability constants of its metal complexes

Metal ion Species m h l log bMm

Hh

Ll

L1 Equilibrium quotient log KMm

Hh

Ll

L1 log KMm

Hh

Ll

L4 a

H� 0 1 1 12.42(1) [HL]/[H]�/[L] 12.42 12.1

0 2 1 22.77(3) [H2L]/[HL]�/[H] 10.35 8.15

0 3 1 31.71(4) [H3L]/[H2L]�/[H] 8.94

0 4 1 37.80(6) [H4L]/[H3L]�/[H] 6.09

0 5 1 41.89(6) [H5L]/[H4L]�/[H] 4.09

0 6 1 44.36(9) [H6L]/[H5L]�/[H] 2.47

Cu2� 1 0 1 23.51(9) [ML]/[M]�/[L] 23.51 10.1

1 1 1 32.07(4) [MHL]/[ML]�/[H] 8.56

1 2 1 37.71(1) [MH2L]/[MHL]�/[H] 5.64

1 3 1 40.48(2) [MH3L]/[MH2L]�/[H] 2.77

2 0 1 32.79(4) [M2L]/[ML]�/[M] 9.28

2 1 1 36.86(3) [M2HL]/[MHL]�/[M] 4.07

2 �/1 1 26.14(5) [M2L]/[M2L(OH)]�/[H] �/6.65

2 �/2 1 19.10(4) [M2L(OH)]/[M2L(OH)2]�/[H] �/7.04

Zn2� 1 0 1 19.43(5) [ML]/[M]�/[L] 19.43 5.12

1 1 1 28.01(5) [MHL]/[ML]�/[H] 8.58

1 2 1 33.51(4) [MH2L]/[MHL]�/[H] 5.50

1 3 1 38.03(3) [MH3L]/[MH2L]�/[H] 4.52

1 4 1 41.98(4) [MH4L]/[MH3L]�/[H] 3.95

2 0 1 26.63(5) [M2L]/[ML]�/[M] 7.20

2 1 1 32.87(3) [M2HL]/[M2L]�/[H] 6.24

2 2 1 37.57(4) [M2H2L]/[M2HL]�/[H] 4.70

Cd2� 1 0 1 18.57(5) [ML]/[M]�/[L] 18.57

1 1 1 27.74(5) [MHL]/[ML]�/[H] 9.17

1 2 1 34.64(4) [MH2L]/[MHL]�/[H] 6.90

1 3 1 39.38(1) [MH3L]/[MH2L]�/[H] 4.74

2 0 1 26.25(4) [M2L]/[ML]�/[M] 7.68

2 1 1 33.09(4) [M2HL]/[MHL]�/[M] 6.84

Pb2� 1 0 1 16.23(3) [ML]/[M]�/[L] 15.23

1 1 1 25.75(4) [MHL]/[ML]�/[H] 9.52

1 2 1 33.49(3) [MH2L]/[MHL]�/[H] 7.74

1 3 1 38.62(2) [MH3L]/[MH2L]�/[H] 5.13

1 4 1 42.77(2) [MH4L]/[MH3L]�/[H] 4.15

2 0 1 27.38(4) [M2L]/[ML]�/[M] 11.15

2 1 1 33.22(3) [M2HL]/[M2L]�/[H] 5.84

2 2 1 38.00(3) [M2H2L]/[M2HL]�/[M] 4.78

Values in parentheses are standard deviations in the last significant figures. T�/20.0 8C; I�/0.10 M in TBAN.a Methanol at 25 8C [5].


system was treated separately and finally the data of all

the titration curves were merged and treated simulta-

neously to give the values of stability constants.

2.2.5. Spectroscopic studies1H NMR spectra were recorded with a Bruker AMX-

300 spectrometer at probe temperature. The electronic

spectra of the complexes prepared by the addition of

increasing amounts of the metal ion (in the form of

nitrate or chloride salts) to the ligands at the appropriate

pH value were recorded with a UNICAM UV�/Visspectrophotometer model UV-4 or a Shimadzu model

UV-3100 spectrophotometer for UV�/Vis�/near IR, in

methanol:water (5:1, v/v) or in DMSO. One titration of

a solution of L1 with KOH was also followed by UV�/

Vis spectroscopy from pH 7 to 14, to determine the two

higher protonation constant values of this ligand. The

protonation constants were determined using pHab [17]

or HYPERQUAD2000 programs [16].

EPR spectroscopy measurements were recorded with

a Bruker ESP 380 spectrometer equipped with contin-

uous-flow cryostats for liquid helium or liquid nitrogen,

operating at X-band. The spectra of the copper(II)

complexes, 1.0�/10�3 and 2.2�/10�2 mol dm�3, were

recorded in the ranges 102�/130 and 7.5�/32 K, in

DMSO/water mixtures and in 1.0 mol dm�3 TBAN.

Computer simulations of the spectra were carried out

with a program for a microcomputer [18].


3. Results and discussion

3.1. Synthesis of macrocycles

Compound L1 was prepared by the Mannich reaction,

reacting the parent macrocyclic amine L2 with p -

formaldehyde and the corresponding substituted CHQ

in a particular solvent, as described to append the samegroup to other macrocycles [6,19,20]. The reaction is

quite sensitive to the solvent used [19]. Although the

reaction ortho - to the OH of phenol is preferred to that

of the para -position, the latter was blocked with Cl, the

commercial available starting material, to avoid parallel

reactions. From the two possible ways of synthesis, the

previous preparation of 3,11-bis(methoxymethyl) deri-

vative of L2 and the one-pot reaction [6,8], the latter oneled to better yields, although quite low and impossible to

improve. Izatt and co-workers [7] attempted also to

prepare CHQ derivatives of tetraazamacrocycles using

the same technique but could not obtain the desired

product.

3.2. Acid�/base behaviour of L1

The acid�/base reactions of L1 have been studied by

potentiometric and/or spectrophotometric methods at

20.0 8C and in a mixture 5:1 methanol:water (v/v). Theresults are collected in Table 1. The temperature was

chosen to minimize the evaporation of the solvent.

From the eight basic centres of the molecule, four

amines of the macrocycle, two phenolate and two

pyridine centres of the arms, it was possible to determine

six protonation constants, the other two were very low

to be determined by the techniques used. The value of

the first protonation constant was determined byspectrophotometric titration and the second one con-

firmed by the same technique (see Fig. 1). The bands at

Fig. 1. Spectrophotometric titration curves of L1 in methanol:water

(5:1, v/v), at 20.0 8C in 0.1 mol dm�3 in TBAN, between pH 7.11 and

13.55: 1*/pH 7.11; 2*/pH 7.86; 3*/pH 8.67; 4*/pH 9.70; 5*/pH

10.26; 6*/pH 10.72; 7*/pH 11.45; 8*/pH 12.01; 9*/pH 12.77; 10*/

pH 13.10; 11*/pH 13.55.

250 and 335 nm at pH 8 disappear at pH about 10.5 to

give rise to the bands at 260, 345 and 380 nm which

increase with the pH. The same happens with the CHQ

itself, which exhibits, in the same experimental condi-tions, a band at 335 nm that is converted to the bands at

345 and 380 nm with the increase of the pH.

The most basic centres of the molecule are the

phenolate groups, for which the values of 12.42 and

10.35 can be ascribed. The third and fourth values, 8.94

and 6.09, correspond to the protonation of amine

centres of the macrocycle. The third one would corre-

spond to the protonation of the amine opposed to thepyridine of the macrocycle, and the protonation of the

second amine probably implies a rearrangement of

charges into the macrocyclic cavity with both proto-

nated amines trans to each other, as found by 1H NMR

titrations for compounds containing the same macro-

cycle but different side arms [21]. The next one, 4.09, can

be attributed to the protonation of the pyridine of one

quinoline group or the third amine of the macrocycle.The sixth and the others correspond to the protonation

of the remaining centres, which are very acidic due to

the presence of positive charges at short distance of

already protonated amines.

The protonation constants of 8-hydroxyquinoline are

9.63 and 4.95 in water (in log units) [3b] and, 11.30 and

4.33 in 75% ethanol [3b,22]. For CHQ, they are slightly

lower: 9.29 and 3.79 (in water) [3a]. On the other hand,the parent macrocycle, L2, presents the following values:

9.74, 8.67, 4.67 and B/1 in water [10], and slightly lower

in a mixture methanol:water (1:1, v/v): 9.60, 8.12, 3.95

and B/2 [23]. Then, the presence of the CHQ groups

decreases the basicity of the amine centres of the

macrocycle and, on the other hand, the presence of the

amines of the macrocycle at short distance increases the

basicity of the phenolate groups of CHQ. These effectsare well established for linear amines having N-o -

hydroxybenzyl substituents [24,25], and the same effects

were found for cyclic amines with the same type of

substituents [5,26]. It is assumed to be due to the

formation of intramolecular hydrogen bonding interac-

tions between the phenolic hydrogen and the amine

nitrogen. These intramolecular hydrogen bonding inter-

actions were found in the crystal X-ray diffractionstructures of L4 or L5, involving the hydroxy groups

of CHQ of the first compound or the phenol groups of

the second one and the nitrogen atoms of the macro-

cycle, resulting in the elongation of the macrocycle along

the N� � �N direction. On the other hand, the same

macrocycle with appended CHQ groups bound by the

2-position, where the pyridine moiety is closer to the

nitrogen of the ring, does not exhibit intramolecularhydrogen bonding, but intermolecular ones between the

hydroxyl groups that point away from the macrocycle

and a similar group of other molecule [5]. The intramo-

lecular hydrogen bonds stabilize the deprotonated form


of the amine contributing to decrease its basic character,

and, at the same time, they stabilize the phenol form

conferring a more basic character to the phenolate. It

can be inferred that the same effects should occur withL1 taking into account the relative position of the

phenol group in CHQ substituents to the nitrogen

atom of the macrocycle (see Table 1).

Compound L3 was synthesized as a model for the

study of L1, but it was impossible to find a common

medium for the study of both. However, we believe that

they should present rather similar behaviour in solution,

although the basicity of the phenolate groups of L3

would be slightly higher and its metal complexes would

be thermodynamically less stable [5].

Fig. 3. Species distribution curves calculated for the Cu(II):L1

complexes in methanol:water (5:1, v/v): (a) 1:1 ratio, CL�/CM�/

8.333�/10�4 mol dm�3; (b) 2:1 ratio (M:L), CL�/8.333�/10�4 mol

dm�3 and C �/1.666�/10�3 mol dm�3.

3.3. Metal complexes studies

The stability constants of L1 with Cu2�, Zn2�, Cd2�

and Pb2� have also been studied by potentiometric and/

or spectrophotometric methods in the conditions used

for the determination of the protonation constants. The

results are also listed in Table 1.Compound L1 forms in solution mono- and dinuclear

complexes, each of them presenting several protonated

species, with all the metal ions studied. For the Cu2�

complexes, two hydroxocomplexes, M2L(OH)1 or 2 were

also found at pH�/5.5 in the 2:1 ratio (metal:ligand).

Fig. 2. Species distribution curves calculated for the Pb(II):L1 com-

plexes in methanol:water (5:1, v/v): (a) 1:1 ratio, CL�/CM�/8.333�/

10�4 mol dm�3; (b) 2:1 ratio (M:L), CL�/8.333�/10�4 mol dm�3

and CM�/1.666�/10�3 mol dm�3.

M

With the other metal ions studied, the hydroxocom-

plexes could not be found due to precipitation of the

ML complex, which is a neutral species, and conse-

quently less soluble in the polar solvent mixture used.

The ML1 complexes (1:1 ratio) follow the usual trend of

thermodynamic stability: CuL1�/ZnL1�/CdL1�/PbL1,

but an unexpected order of stability constants was found

for the dinuclear complexes: [Pb2L1]2��/[Cu2L1]2��/

[Cd2L1]2��/[Zn2L1], revealing a clear tendency of Pb2�

to form dinuclear species. The [Pb2L1]2� species not

only appear in the 2:1 ratio, but already in the 1:1 ratio

starting to form at pH 5 and having a maximum

percentage at pH:/7.5 (see the speciation diagram of

Fig. 2). For the complexes of the other metal ions with

the same ligand, L1, the dinuclear species only appear in

the 2:1 ratio, as can be seen in Fig. 3 for the copper(II)

complexes, which exhibits a behaviour similar to that

found for the other metal ions.

It is interesting to emphasize that in the 1:1 ratio, the

completely deprotonated complexes only form at high

pH values (pH�/7 for the copper and �/8 for the lead

complexes), implying a very basic centre on the ligand,

the protonation constant being 8.56 for Cu2�, 8.58 for

Zn2�, 9.17 for Cd2� and 9.52 for Pb2� complexes (in

log units) (see Table 1). However, an excess of metal ion

(2:1 ratio) leads to the complete deprotonation of the


complex at much lower pH due to the formation of

dinuclear complexes (see Figs. 2 and 3).

It is reasonable to think that the remaining proton on

the MHL species is bound to the oxygen of one

phenolate, which is the most basic centre of the ligand,

and supported by the crystal X-ray structures of the

monoprotonated nickel and zinc complexes of the model

compound L3, [M(HL3)]� with M�/Cu2� and Zn2�

[8]. More difficult to rationalize is the relative position

of the two metal ions in the dinuclear complexes,

although the formation of these species at low pH

values implies the involvement of the CHQ pendent

arms.

The formation of the dinuclear species was also

confirmed by UV�/Vis spectroscopy of the copper and

lead complexes and by EPR spectroscopy of the copper

ones.

The ligand L1 exhibits an UV�/Vis spectrum with

bands at 380, 345 and 260 nm (Fig. 1). Upon addition of

metal ions, the peaks shift or change their absorptivity,

the largest shift was verified for the copper complex.

Upon addition of increasing amounts of a Cu2�

solution, the bands of the ligand decrease and those of

the CuL1 complex at 628 (170), 415 (1515), 345 (1890)

and 270 (11870) nm appear. Further addition of the

Cu2� solution leading to [Cu2L1]2� shift two of the

previously formed bands to 410 (2214) and 280 (11 720)

nm [the values given correspond to lmax (nm) (omolar

(dm3 mol�1 cm�1))]. The electronic spectra of the Zn2�

and Pb2� complexes of L1, and the Cu2� and Zn2�

complexes of L4 were also carried out in methanol and

in all cases the maximum of absorption bands of the ML

complexes displayed shifts similar to those described for

the Cu2� complexes of L1 (5�/35 nm) when the dinuclear

complexes were formed.

The EPR spectra of the copper complexes of L1 and

L3 were performed in water and in a mixture of

water:DMSO (1:1) in the 1:1 and 2:1 M:L ratios. For

L4 only the 2:1 spectrum was recorded, in the same

conditions. The spectra in both solvents are identical,

but better defined in the mixture of solvents. In the case

of 1:1 metal to ligand ratio, the spectra of the L1 and L3

complexes are similar, and already described for L3 [8]

Table 2

Spectroscopic EPR data for the Cu2� complexes of L1, and other similar co

Complex lmax (nm) Species EPR Ai (�/104 cm

gx gy

CuL1 628 A 2.049 2.110

B 2.037 2.057

CuL3 614 A 2.049 2.068

B 2.050 2.060

[CuL2]2� 560 2.034 2.060

[CuL6] 630 2.027 2.084

(see Table 2). The spectra indicate the presence of two

species (A and B), as easily observed by the splitting of

the bands at low field. Taking into account the pH of

the solutions, the speciation diagram suggests that the

signals can be attributed to [Cu(HL1)]� and [CuL1]

species. Each species exhibits three well-resolved lines at

low field without superhyperfine splitting, and the

strong and not resolved band of the high field part of

the spectra overlaps the fourth line. The hyperfine

coupling constants (A) and g values of glassy solutions

of these complexes, obtained by simulation of the

spectra [18], are collected in Table 2. The parameters

of both species are typical of copper(II) complexes in

rhombic symmetry axially elongated and a dx2�y2

ground state, consistent with elongated rhombic-octa-

hedral or distorted square-based pyramidal stereoche-

mistries [27,28]. These parameters, together with the

position of the d�/d absorption band, of both species of

CuL1 indicate penta- or hexaco-ordination environ-

ments for the complexes. Complex B exhibits para-

meters and also the visible band maximum value similar

to those of [CuL6] (see Table 2), which adopts an

octahedral structure where the four nitrogen atoms of

the macrocycle determine the equatorial plane and the

six-co-ordination is completed via the two oxygen atoms

of the carboxylate arms [30]. Therefore, a similar

structure should be expected for B, suggesting that this

is the species [CuL1]. However, the gz value for the A

species is higher than suggested for a square-pyramidal

geometry and the Az value higher than expected for an

octahedral arrangement, then important distortions of

these geometries can be ascribed to this copper(II)

complex, which should be ascribed to the monoproto-

nated complex [Cu(HL1)]� [8].

The increase of the amount of copper(II) to 2:1 ratio

(metal to ligand) in solutions of CuL1 leads to an EPR

spectrum which can be interpreted as that of a triplet

state of a coupled dinuclear copper centre with an S�/1

state and zero-field splitting, having two types of signals

in the DMs�/1 and DMs�/2 regions (see Fig. 4a). The

hyperfine structure corresponding to the first transition

is only observed at temperatures of �/100 K and diluted

solutions (of about 1.1�/10�3 mol dm�3), while the

mplexes

�1) Reference

gz Ax Ay Az

2.236 13.5 20.3 173.5

2.210 19.8 31.8 166.9 This work

2.229 13.6 13.5 178.1

2.194 3.0 11.3 163.8 [8]

2.188 0.5 3.4 192.9 [29]

2.221 14.9 21.3 165.4 [30]

Fig. 4. EPR X-band spectra of the Cu2� complexes of L1 (a), of L3 (b)

and of L4 (c), in the 2:1 M:L ratio in H2O:DMSO (1:1). The high-field

part of the spectra were recorded at 114 K, frequency (n ) 9.408 GHz

for (a), at 102 K, frequency (n ) 9.643 GHz for (b) and at 102 K,

frequency (n ) 9.404 GHz for (c). The low-field part of the spectra were

recorded at 9 K, frequency (n ) 9.643 GHz for (a) and (b), and at 8 K,

frequency (n ) 9.644 GHz for (c). Microwave power of 2.4 mW and

modulation amplitude of 0.9 mT for all complexes. Concentration of

the complexes: 2.00�/10�3 mol dm�3.


signal at DMs�/2 transition is only observed at tem-

peratures lower than 30 K and concentrations of about

2�/10�2 mol dm�3. Similar signals, but with hyperfine

structures better defined, were observed for the 2:1

solutions of copper complexes of L4 (see Fig. 4c).

However, the spectrum of corresponding solutions of

Cu2�:L3 (2:1 ratio) recorded in the same conditions

only exhibits a signal of very low intensity in the DMs�/

2 transition at 9 K which disappear at 19 K (see Fig. 4b).

The DMs�/2 transition signal reveals a spin exchange

interaction between the two copper atoms. If assumed

that two unpaired electrons, each located on a Cu2�, are

exchanged with an energy larger than the hyperfine

coupling energy, each unpaired electron (/S� 12) couples

with two copper nuclei (/ICu�32) in an equivalent

manner, the hyperfine structure consists of seven lines.

Indeed, seven hyperfine lines can be observed in the

DMs�/2 region of the L4 complexes, with an average

hyperfine spacing of 95.1 cm�1. The hyperfine constant

is approximately half the value found in mononuclear

copper(II) complexes having the same donor atoms and

geometry.

The triplet state (S�/1) involving the coupling of two

copper(II) ions can be represented by the spin Hamilto-

nian, assuming that the system is axially symmetric:

H�gIbHzSz�g�b(HxSx�HySy)�D[S2z �

13S(S�1)]

where D describes the zero-field splitting constant

within the triplet state and can be considered to be built

up from a dipolar Ddip and an exchange, Dex contribu-

tions:

D�Dex�Ddip

where

Dex��1

8J

�1

4(gI�2)2�(g��2)2

�and

Ddip��(g2

I � 12g2�)b2

r3

where �/J is the energy separation between the singlet

ground state and the first excited triplet state, and r is

the Cu�/Cu distance, which appears in the dipole�/dipolecontribution [31�/33].

The J values were determined by the signal intensity

of the EPR spectra at different temperatures by the

expression: I�/c{T [3�/exp(�/J /kT )]}, where I is the

spectral intensity of the DMs�/2 signal, which is

proportional to the paramagnetic susceptibility of the

dinuclear system, c being the proportionality constant

and k the Boltzmann constant. As the linewidth did notdepend on the temperature, the relative intensity of the

triplet spectra is equal to the relative peak height of a

certain line in the first derivative spectra of the DMs�/2

transition [34]. The determined values were 5.03 cm�1

for the L1 complex and �/7.76 cm�1 for that of L4.

The 2D value was calculated by the difference

between the Hz2and Hz1

transitions, respectively

[32,35], yielding to 0.0185 and 0.0262 cm�1 for L1 andL4 complexes, respectively. Taking gI�2:240 and g��/

2.082 for both complexes, obtained by simulation of the

DMs�/2 transitions signal of the L4 complex, and

considering the same values for the L1 complex, due to

the complete superimposition of the signals of both

complexes (see Fig. 4), the values of 6.04 and 6.12 A

were determined for the copper�/copper distance from

the above expression for the L1 and L4 complexes,respectively.

On the other hand, as the intensity of the DMs�/2

signal of the EPR spectrum of the L3 copper(II) complex

is very low, even at 9 K, then a different kind of

dinuclear species is probably formed.

4. Conclusions

Compound L1, which has a very high overall basicity,

forms in solution stable mono- and dinuclear metal

complexes. Unfortunately, it was not possible to obtain


crystals with adequate size for X-ray diffraction deter-

mination of metal complexes of L1. However, the crystal

structures of metal complexes of the model compound

L3, [Ni(HL3)]� and [Zn(HL3]� [8], indicate that in bothcases the nitrogen donors of the macrocycle determine a

square-planar arrangement around the metal centre. But

while the nickel complex can be described as a distorted

octahedron with the axial positions occupied by the

oxygen atoms of the phenolic pendant arms, in

[Zn(HL3]� the oxygen atom of the protonated pheno-

late is also directed towards to zinc but at a long

distance from the metal suggesting a square-pyramidalgeometry rather than an octahedral one. Furthermore,

DFT theoretical calculations suggested that the cop-

per(II) complex adopts a geometry which can be

considered as between distorted octahedral and

square-pyramidal [8]. Our EPR results indicate that

the monoprotonated copper(II) complex of L1 also

adopts a comparable distorted arrangement, although

the deprotonated complex should adopt an octahedralgeometry.

Our potentiometric measurements for the complexes

of all the metal ions studied and the EPR spectroscopy

of the copper(II) complexes clearly indicate the forma-

tion of dinuclear complexes. However, on the basis of

these results it is impossible to predict the co-ordination

sphere and the relative position of the second metal ion,

but they suggest that the quinoline arms are necessaryinvolved.

Acknowledgements

The authors acknowledge the financial support fromFundacao Para a Ciencia e Tecnologia (FCT) and

POCTI, with coparticipation of the European Commu-

nity fund FEDER (Project No. POCTI/1999/QUIM/

35396). X.C. also acknowledges FCT for the grant

(SFRH/BPD/1502/2000).

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Tetraazamacrocycle bearing quinoline pendant arms and its complexation properties

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