Synthesis, solution studies and structural characterisation of complexes of a mixed oxa–aza...

Synthesis, solution studies and structural characterisation ofcomplexes of a mixed oxa�/aza macrocycle bearing nitrile pendant

arms

Lorenzo Tei a, Alexander J. Blake a, Andrea Bencini b, Barbara Valtancoli b,Claire Wilson a, Martin Schroder a,*

a School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UKb Dipartimento di Chimica, Universita di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Florence, Italy

Received 19 February 2002; accepted 18 April 2002

Abstract

The mixed oxa�/aza macrocycle [15]aneN3O2 with cyanomethyl pendant arms on all three N-centres, l,4,7-tris(cyanomethyl)-l,4,7

triaza-10,13-dioxacyclopentadecane (L), and its complexes with Cu(II), Ni(II), Zn(II), Cd(II) and Pb(II) have been prepared. The

protonation constants of the ligand and the thermodynamic stabilities of the 1:1 metal:ligand complexes with Cu(II), Zn(II), Cd(II)

and Pb(II) have been investigated by means of potentiometric measurements in aqueous solutions. These metal ions form only

mononuclear complexes with unusually low stability constants, compared with those found for other cyclic or acyclic triamine

compounds due to the relatively poor s-donating properties of tertiary nitrogens and to the stiffening of the macrocyclic framework

imposed by the cyanomethyl side arms, which are unable to co-ordinate to the metal ion sitting within the macrocyclic cavity. The

single crystal X-ray structures of 1:1 complexes with Cu(II), Ni(II), Cd(II) and Pb(II) have been determined. The structures of

[Cu(L)(CH3CN)](ClO4)2 and [Ni(L)(CH3CN)](ClO4)2 show slightly distorted octahedral co-ordination geometries with the metal

cation bound to all the donor atoms of the macrocyclic ring and to a MeCN molecule. The structure of [Cd(L)(NO3)2] confirms

binding of all the donor atoms of the ring, but in this case the metal ion is also co-ordinated to two nitrate anions, one of them in a

bidentate fashion. In the case of Cu(II), Ni(II) and Cd(II) complexes, the nitrile arms do not take part in co-ordination to the metal

ion and are directed away from its co-ordination sphere. Interestingly, the structure of the Pb(II) complex [Pb(L)(ClO4)]22�

confirms the formation of a binuclear species with binding of a nitrile pendant arm from an adjacent molecule. Two ClO4� anions

bridge within the binuclear units in a bidentate fashion. NMR spectroscopic studies on the Zn(II), Cd(II) and Pb(II) complexes have

revealed a rigidity in the complexes which is lost on increasing temperature.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Crystal structure; Metal complexes; Azamacrocyclic complexes

1. Introduction

Although the co-ordination chemistry of functiona-

lised macrocyclic ligands has been extensively studied

over the last 20 years [1�/3], the presence of different

pendant arms and a mixed set of donor atoms makes the

co-ordination capability towards different metal ions

not entirely predictable. Many efforts have focused on

the development of new mixed oxa�/aza macrocycles

bearing different pendant arms attached to aza centres,

and on the study of their co-ordination chemistry with a

range of transition and post-transition metal ions. These

ligands often show specific complexation behaviour,

forming metal complexes with unusual structures and

stereochemistries, and exhibit remarkable metal ion

selectivity that is of interest in several areas [3�/8]. The

versatility of these ligands makes them useful for a range

of applications: while ligands with carboxylate or

phosphonate pendant arms form lanthanide complexes

which are effective contrast agents in magnetic reso-

nance imaging [4,5], those with pyridylmethyl, hydro-

* Corresponding author. Tel.: �/44-115-951 3490; fax: �/44-115-951

3563

E-mail address: [email protected] (M. Schroder).

Inorganica Chimica Acta 337 (2002) 59�/69

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0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 9 9 2 - 1

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xyalkyl or ethereal pendant groups exhibit good selec-

tivity and have been studied as ionophores for metal ion

recognition [3,6�/8].

We have previously reported the co-ordination prop-

erties of the ligand derived from the mixed oxa�/aza

macrocycle [15]aneN3O2 bearing primary aminoalkyl

pendant arms [9], and have also studied the co-ordina-

tive behaviour of a range of ligands bearing nitrile

pendant arms towards Ag(I) [10,11]. Nitrile functiona-

lised pendant arms prevent these polydentate ligands

from encapsulating tetrahedral metal centres such as

Ag(I), but promote the formation of polymeric com-

pounds. Thus, the Ag(I) complex of L is a linear

polymer in which one nitrile group belonging to an

adjacent molecule interacts with the metal centre and

forms a linear chain [11]. The inability of the nitrile

group to orient itself to co-ordinate linearly via s-

donation to a metal ion sited within the macrocyclic

cavity is further demonstrated by the paucity of reported

mononuclear complexes with cyanoalkyl pendant arm

derivatives of macrocyclic ligands [12]. We report herein

the co-ordinative behaviour of the ligand l,4,7-tris(cya-

nomethyl)-l,4,7-triaza-10,13-dioxacyclopentadecane (L)

towards transition and post-transition metal ions; we

wished to evaluate the behaviour of this ligand towards

cations such as Ni(II), Cu(II) or Zn(II) which generally

prefer N-donor binding and towards larger metal ions

that show a slight preference for O-donors such as

Cd(II) or Pb(II). We were interested in discovering

whether the nitrile pendant arms in this system would

dangle away from the metal centre or would participate

in some way to the co-ordination sphere. We were also

interested in comparing the ligand L with related

systems containing primary aminoalkyl pendant arms

[9]. Solution and structural studies have been published

previously describing the macrocyclic precursor [15]an-

eN3O2 [13,14]; the synthesis of its derivative with

hydroxyalkyl pendant arms and its stability constants

with Cu(II), Ni(II), Zn(II), Cd(II) and Pb(II) have also

been reported by Hancock and co-workers [13]. Re-

cently, the triacetic acid derivative of [15]aneN3O2 with

Cu(II), and an extensive study of its complexation

behaviour in solution have been reported by Delgado

and co-workers [15]. We report herein the crystal

structure determinations of the Cu(II), Ni(II), Cd(II)

and Pb(II) complexes of L, protonation studies of the

free ligand, stability constants for the Cu(II), Zn(II),

L. Tei et al. / Inorganica Chimica Acta 337 (2002) 59�/6960

Cd(II) and Pb(II) complexes, and NMR spectroscopic

studies on Zn(II), Cd(II) and Pb(II) complexes.

2. Results and discussion

The complexes [M(L)(CH3CN)](ClO4)2 [M�/Cu(II),

Ni(II)], [M(L)](ClO4)2 [M�/Zn(II) and Pb(II)] and

[Cd(L)(NO3)2] were prepared in good yields by reactingL with 1 equiv. of M(ClO4)2�H2O or Cd(NO3)2 �/6H2O in

MeCN at room temperature. Elemental analytical data

and mass spectra for all the complexes are consistent

with these formulations.

Single crystals suitable for X-ray diffraction studies

could be grown for all the complexes apart from

[Zn(L)](ClO4)2. [Ni(L)(CH3CN)](ClO4)2 and [Cu(L)-

(CH3CN)](ClO4)2 are very similar (Table 1), with thesix co-ordinate metal ion in a distorted octahedral

geometry (Figs. 1 and 2), and co-ordinated to the five

macrocyclic ring donors (three tertiary N- and two

ethereal O-donors) and one MeCN molecule. In both

structures the donor atoms in the equatorial plane are

N(1), N(7), O(10) and O(13), while the axial positions

are taken up by N(4) and N(3S) from the MeCN

molecule. The RMS deviation of the N- and O-atomsfrom the N2O2 equatorial plane is 0.036 A for the Ni(II)

structure and 0.077 A for the Cu(II) complex. The

difference between the two structures lies in the nature

of the distortion from octahedral geometry. The Ni(II)

structure presents a trigonal elongation with the trian-

gular face of the octahedron formed by N(1), N(4) and

N(7) elongated with respect to the face formed by O(10),

O(13) and N(3S) [bond lengths 2.110(7)�/2.157(8) A and

2.025�/2.055 A, respectively, Table 1]. The Cu(II)

structure presents a compression of the octahedron

with the two axial bonds shorter than the two equatorial

(Table 1). This Jahn�/Teller effect with the shortening of

the axial bonds is quite rare, elongation of the axis being

more common [16]. In the equatorial plane the two N-

donors of the tertiary amine centres [N(1) and N(7)] are

closer to the metal centre than the two O-donors [O(10)

and O(13)] (Table 1), giving rise to an additional

Table 1

Selected bond lengths (A) and angles (8) for [Ni(L)(CH3CN)](C1O4)2

and [Cu(L)(CH3CN)](C1O4)2

[Ni(L)(CH3CN)](ClO4)2 [Cu(L)(CH3CN)](ClO4)2

Bond lengths

M�N(l) 2.146(8) 2.161 (3)

M�N(4) 2.110(7) 2.029(3)

M�N(7) 2.157(8) 2.136(3)

M�O(10) 2.025(7) 2.332(3)

M�O(13) 2.055(7) 2.235(3)

M�N(3S) 2.047(9) 1.972(4)

Bond angles

N(1)�M�N(4) 83.4(3) 84.8(1)

N(1)�M�N(7) 123.3(3) 138.8(1)

N(4)�M�N(7) 83.6(3) 86.0(1)

N(1)�M�O(10) 158.7(3) 143.6(1)

N(1)�M�O(13) 81.2(3) 75.6(1)

N(1)�M�N(3S) 91.9(3) 91.8(1)

N(4)�M�O(10) 100.2(3) 94.0(1)

N(4)�M�O(13) 97.0(3) 99.8 (1)

N(4)�M�N(3S) 170.0(3) 174.7(1)

N(7)�M�O(10) 78.0(3) 77.0(1)

N(7)�M�O(13) 155.2(3) 145.5(1)

N(7)�M�N(3S) 91.8(4) 94.0(1)

O(10)�M�O(13) 77.5(3) 68.7(1)

O(10)�M�N(3S) 87.5(3) 91.1(1)

O(13)�M�N(3S) 90.9(4) 83.2(1)Fig. 2. Crystal structure of the [Cu(L)CH3CN)]2� cation with

numbering scheme adopted. Hydrogen atoms have been omitted for

clarity and displacement ellipsoids are drawn at 50% probability.

Fig. 1. Crystal structure of the [Ni(L)(CH3CN)]2� cation with


clarity. Displacement ellipsoids are drawn at 40% probability.

L. Tei et al. / Inorganica Chimica Acta 337 (2002) 59�/69 61

distortion from the octahedral geometry. The distortion

from the equatorial co-ordination geometry in both the

structures is also due to the presence of different chelate

rings. While the five-membered chelate rings are verysimilar both in the angle at the metal centre [78.0(3)�/

81.2(3)8 for [Ni(L)(CH3CN)]2� and 68.7(1)�/77.0(1)8 for

[Cu(L)(CH3CN)]2�] and in the distance between the

donor atoms [2.555(10)�/2.735(9) A for Ni(II) and

2.577(4)�/2.786(5) A for Cu(II)], the angles N(1)�/M�/

N(7) within an eight-membered chelate ring are larger

than the expected 908 [123.3(3)8 in [Ni(L)(CH3CN)]-

(ClO4)2 and 138.8(1)8 in [Cu(L)(CH3CN)](ClO4)2]. Themacrocyclic framework adopts a very similar conforma-

tion in both complexes with eight out of 15 torsion

angles less than 908. In both structures, the macrocycle

is folded along the direction N(1)� � �N(7) and the

dihedral angles between the two mean planes defined

by the seven-membered N(1)� � �N(7) and the ten-mem-

bered N(7)� � �N(l) chains of the macrocycle are 90.38 in

the Ni(II) complex and 85.38 for the Cu(II) complex.Interestingly, comparison of the structure of

[Cu(L)(CH3CN)]2� with other Cu(II) complexes of the

macrocyclic precursor [15]aneN3O2 and its derivatives

L2 and L3 shows that in the Cu(II) complex of L all the

atoms from the macrocycle participate in co-ordination

to the metal ion, while in all the others at least one O-

donor does not bind [9,14,15]. In [Cu(L2)], the metal ion

is five co-ordinate and sits outside the macrocyclic cavitywith only two out of five donors of the ring interacting

with the Cu(II) centre [9]. The Cu(II) complexes with L1

and L3 are also five co-ordinate. With the macrocyclic

precursor [I5]aneN3O2 (L1) a peculiar trinuclear com-

plex with a m-CO32� bridging three Cu(II) centres has

been reported with each Cu(II) adopting a distorted

square-pyramidal geometry with one O-donor of the

macrocycle not bound [14]. In [Cu(HL3)] �/0.5H2O, theco-ordination geometry around Cu(II) is a distorted

compressed trigonal bipyramid with one oxygen of the

macrocycle interacting only very weakly with the metal

centre at a distance of 2.727(4) A [15]. The preference for

N- over O-donors, typical of Cu(II), requires a folded

arrangement of the macrocycle, but even in this

conformation the macrocyclic cavity is not too large to

prevent both the O-donors of the macrocyclic ring frombinding to the metal ion, as demonstrated by the

structure of [Cu(L)(CH3CN)]2�.

The structure of the complex [Cd(L)(NO3)2] is shown

in Fig. 3 and selected bond lengths and angles are

reported in Table 2. The Cd(II) centre is eight co-

ordinate with all of the donor atoms of the macrocyclic

ring co-ordinated, with one bidentate and one mono-

dentate nitrate anion. While the Cd�/N bond lengths liein the range 2.436 (1)�/2.535 (2) A, the Cd�/O distances

span in the range 2.307(1)�/2.672(1) A, both falling in

the range reported for similar bonds in other Cd(II)

complexes [7,17]. The bond length Cd�/O(6) from the

Table 2

Selected bond lengths (A) and angles (8) for [Cd(L)(NO3)2]

Bond lengths

Cd�N(1) 2.535(2)

Cd�N(4) 2.436(1)

Cd�N(7) 2.474(2)

Cd�O(1) 2.307(1)

Cd�O(3) 2.672(1)

Cd�O(6) 2.361(1)

Cd�O(10) 2.486(1)

Cd�O(13) 2.390(1)

Bond angles

N(4)�Cd�N(1) 72.76(5)

N(7)�Cd�N(1l) 146.80(5)

O(10)�Cd�N(1) 134.12(5)

O(13)�Cd�N(1) 70.54(5)

O(1)�Cd�N(1) 78.62(5)

O(3)�Cd�N(1) 120.48(5)

O(6)�M�N(1) 81.99(5)

N(4)�Cd�N(7) 74.80(5)

O(13)�Cd�N(4) 88.07(5)

O(l)�Cd�N(4) 151.31(5)

N(4)�Cd�O(10) 123.73(5)

O(3)�Cd�N(4) 147.72(5)

O(6)�Cd�N(4) 80.08(5)

N(7)�Cd�O(10) 71.78(5)

O(13)�Cd�N(7) 115.39(5)

O(1)�Cd�N(7) 133.17(5)

O(3)�Cd�N(7) 84.75(5)

O(6)�Cd�N(7) 85.69(5)

O(13)�Cd�6(10) 68.03(4)

O(1)�Cd�O(10) 78.53(5)

O(3)�Cd�O(10) 70.50(4)

O(6)�Cd�O(10) 138.86(5)

O(1)�Cd�O(13) 84.28(5)

O(3)�Cd�O(13) 123.60(5)

O(6)�Cd�O(13) 152.28(4)

O(3)�C�O(1) 51.02(5)

O(1)�Cd�O(6) 94.17(5)

O(3)�Cd�O(6) 73.62(5)

Fig. 3. View of the [Cd(L)(NO3)2] complex with numbering scheme

adopted. Hydrogen atoms have been omitted for clarity and displace-

ment ellipsoids are drawn at 50% probability.


monodentate nitrate group [2.361(1) A] falls within the

range (2.31�/2.44 A) previously observed in this kind of

Cd(II) complex [18,19]. Cd�/O bond lengths in bidentate

nitrato complexes (which are usually unsymmetrical)

span a wider range (2.29�/2.83 A) [19]; therefore, the

bond lengths Cd�/O(1) and Cd�/O(3) of 2.307(1) and

2.672(1) A, respectively, lie within this range. The

macrocyclic framework is folded differently to that

observed for [Ni(L)(CH3CN)](ClO4)2 and [Cu(L)(CH3-

CN)](ClO4)2: in [Cd(L)(NO3)2l the folding is along the

direction N(1)� � �C(9) with a dihedral angle of 59.08

between the two mean planes defined by the nine-

membered N(1)� � �C(9) and the eight-membered

C(9)� � �N(1) chains of the macrocycle.

In the crystal structure of the centrosymmetric dimer

[Pb(L)]2(ClO4)4 disorder was identified in part of the

macrocyclic backbone and in one nitrile arm and only

the major component is described herein. Bond lengths

and angles are reported in Table 3. In this structure, one

nitrile arm and a perchlorate anion bridge two metal

ions to form a binuclear species as shown in Fig. 4. The

Pb(II) is co-ordinated to nine donor atoms, but some of

these bonds are longer than others (Table 4). Four of the

five macrocyclic donors are more strongly co-ordinated

to the metal centre with bond lengths in the range

2.548(8)�/2.643(15) A, while the remaining N-donor

from the macrocycle [N(l)] and O(5) from a perchlorate

anion are at distances of 2.797(8) and 2.803(8) A,

Table 3

Selected bond lengths (A) and angles (8) for [Pb(L)]2(ClO4)4

Bond lengths

Pb(1)�N(1) 2.797(8)

Pb(1)�N(4) 2.575(10)

Pb(1)�N(7) 2.643(15)

Pb(1)�O(10) 2.576(14)

Pb(1)�O(13) 2.548(8)

Pb(1)�O(5) 2.803(8)

Pb(l)�O(8) 3.068(10)

Pb(1)�N(3A)? 2.977(12)

Pb(1)�O(8)? 2.993(13)

Bond angles

N(4)�Pb(1)�N(1) 69.0(3)

N(7)�Pb(1)�N(1) 138.2(3)

O(10)�Pb(1)�N(1) 120.3(3)

O(13)�Pb(1)�N(1) 63.0(3)

N(1)�Pb(1)�O(5) 76.1(3)

N(1)�Pb(1)�O(8) 100.5(3)

N(1)�Pb(1)�O(8)? 74.5(3)

N(1)�Pb(1)�N(3A)? 146.1(3)

N(4)�Pb(1)�N(7) 69.8(3)

N(4)�Pb(1)�O(10) 91.0(4)

O(13)�Pb(1)�N(4) 85.5(3)

N(4)�Pb(1)�O(5) 78.0(3)

N(4)�Pb(1)�O(8) 122.9(3)

N(4)�Pb(1)�O(8)’ 143.4(3)

N(4)�Pb(1)�N(3A)’ 142.7(4)

O(10)�Pb(1)�N(7) 67.0(4)

O(13)�Pb(1)�N(7) 120.3(3)

N(7)�Pb(1)�O(5) 88.8(3)

N(7)�Pb(1)�O(8) 95.8(4)

N(7)�Pb(1)�O(8)? 146.7(3)

N(7)�Pb(l)�N(3A)? 73.2(4)

O(13)�Pb(1)�O(10) 59.7(3)

O(10)�Pb(1)�O(5) 155.7(4)

O(10)�Pb(1)�O(8) 135.1(3)

O(10)�Pb(1)�O(8)? 105.6(4)

O(10)�Pb(1)�N(3A)’ 79.3(4)

O(13)�Pb(1)�O(5) 139.0(3)

O(13)�Pb(1)�O(8) 141.3(3)

O(13)�Pb(1)�O(5)? 75.7(3)

O(13)�Pb(1)�N(3A)? 118.3(3)

O(5)�Pb(1)�O(8) 45.8(2)

O(5)�Pb(1)�O(8)? 95.9(2)

O(5)�Pb(1)�N(3A)? 96.4(3)

N(3A)?�Pb(1)�O(8)? 73.5(4)

N(3A)?�Pb(1)�O(8) 55.9(3)

O(8)?�Pb(1)�O(8) 65.9(4)

Symmetry operation: �x�1, �y�2, �z�1.

Fig. 4. Crystal structure of the dimeric unit in [Pb(L)(ClO4)]22� with


clarity. Displacement ellipsoids are drawn at 40% probability, [i�/

�/x�/1, �/y�/2, �/z�/1].

Table 4

Stability constants (log K ) of the metal complexes with L at 298.1 K,

I�0.1 M

Reaction log K

L�Cu(II)� [Cu(L)]2� 8.8(1)

[Cu(L)2��H�� [Cu(LH)]3� 4.7(1)

[Cu(L)]2��OH�� [Cu(L)OH)]� 6.6(1)

[Cu(L)(OH)]��OH�� [Cu(L)(OH)2] 3.0(1)

L�Zn(II)� [Zn(L)]2� 4.6(1)

L�Cd(II)� [Cd(L)]2� 5.6(1)

L�Pb(II)� [Pb(L)]2� 6.4(1)


respectively. The Pb(II) centre is further co-ordinated by

two perchlorate oxygens, [O(8) and its symmetry

equivalent O(8)?], and a nitrile group of a symmetry

related molecule [N(3A)?], all at about 3 A from the

metal centre. Pb�/N and Pb�/O bond lengths are similar

to those found in other N,O-macrocyclic complexes of

Pb(II) [20,21]. The macrocyclic framework assumes a

folded conformation along the N(1)� � �N(7) direction,

with a dihedral angle of 64.28 between the two mean

planes defined by the seven-membered N(1)� � �N(7) and

the ten-membered N(7)� � �N(1) chains of the macrocycle.

In [Pb(L)](ClO4)2 the overall N4O5 donation at the

Pb(II) centre gives a holo -directed [22] co-ordination

geometry around the metal ion with the bonds to the

donor atoms evenly distributed about the co-ordination

sphere and the 6s2 lone-pair of Pb(II) apparently

stereochemically inactive. Presumably, the separation

of the two metal ions within the dimer [Pb� � �Pb 5.087(4)

A] allows the ClO4� anions to act as bidendate moieties

and to bridge the metal centres by permitting a

favourable Cl�/O�/Pb angle. In this way, the inert lone-

pair of electrons is prevented from expanding the co-

ordination sphere of the Pb(II) ions. The co-ordination

sphere at Pb(II) centre can be described as a very

distorted tricapped trigonal prism (Fig. 5). The two

triangular faces are formed by N(1), N(4) and O(5) and

by O(10), O(8)? and N(3A)? and are inclined with respect

to each other by 22.08. The first triangular face of the

prism is essentially equilateral (angle range 59.9�/64.68),but the second is more distorted, having one side and

one angle larger than the other two, with N(7), O(8) and

O(13) capping the three rectangular faces of the prism.

The two triangular faces are twisted with respect to one

another; the twist between the two triangular faces can

be quantified by considering the torsion angles at the

four atoms of every rectangular face of the prism. The

average of the three torsion angles in this Pb(II)

structure (one for every triangular side) is 26.08.It should be noted that the crystal structures of the

Ni(II), Cd(II) and Pb(II) complexes generally show M�/

N bond lengths comparable with the M�/O distances. In

contrast, the Cu(II) complex shows M�/O bond lengths

longer than the M�/N distances [2.332(3) and 2.235(3) A]

against an average of 2.075 A for M�/N bonds.

2.1. Metal binding properties in aqueous solution

The formation of the Cu(II), Zn(II), Cd(II) and Pb(II)

complexes with L has been studied by means of

potentiometric measurements (298.1 K, 0.1 M

Me4NCl)1. The stability constants of the complexes are

reported in Table 4, while the distribution diagram forthe Cu(II) complexes is shown in Fig. 6. In the case of

Ni(II), the low solubility of the complex does not allow

the study of its complexation equilibria in aqueous

solution.

The most interesting feature in Table 4 is the far lower

stability of complexes with the present ligand in

comparison with other cyclic or acyclic triamine com-

pounds. For example, the stability constants of the samemetal complexes with the macrocyclic precursor [15]ane-

N3O2 are much higher as can be seen in Table 5. The

large decrease in stability observed for L, can in part be

attributed to the presence of three tertiary amine groups,

which usually show a lower binding ability than primary

and secondary ones. In fact, tertiary N-centres can be

poorer s-donors than primary or secondary amines,

since N-functionalisation prevents the formation of

Fig. 5. Coordination geometry about the Pb(II) centre in complex

[Pb(L)](ClO4). Displacement ellipsoids are drawn at 40%.

1 Protonation constant of L, determined by means of

potentiometric measurements: log K1�/9.10(3), log K2�/2.94(2).

K1�/[LH�]/[L][H�], K2�/[LH22�]/[LH�][H�]. These values are

much lower than those found for [15]aneN3O2, due to the lower

basicity generally observed for tertiary amine groups compared with

secondary centres [23].

Fig. 6. Distribution diagram for the system Cu(II)/L ([Cu(II)]�/[L]�/

1�/10�3 M, 298.1 K, I�/0.1 M).


hydrogen bonds between water and amine groups,

which contribute, via the H2O� � �H�/N interaction, to

the s-donating ability of amine groups in aqueous

solution [24]. On the other hand, the stability constants

of the present complexes are much lower even than

those found for the complexes with the ligand 2,5,8-

trimethyl-2,5,8-triazanonane (e.g. log K�/12.16 for the

Cu(II) complex) [25], which contains three tertiary

amine groups separated by ethylene chains. This ob-

servation indicates that the low complex stability is not

only due to the nature of tertiary N-donors. Most likely,

the insertion onto the macrocyclic backbone of three

CH2CN side arms, which are unable to bind the metal

ion sited within the macrocyclic cavity, leads to greater

rigidity in the molecule, which does not allow the donor

atoms to achieve an optimal arrangement around the

metal ion. In fact, the complex [Cu(L)]2� forms a

protonated species [Cu(HL)]� at slightly acidic pH,

indicating that a co-ordinated amine group may easily

protonate and detach from the metal. The complex

[Cu(L)]2� also shows a marked tendency to form

hydroxylated species at alkaline pH. Such behaviour is

usually attributed to an unsaturated co-ordination

sphere for the metal and may suggest that some donors

in [Cu(L)]2� are only weakly bound to the metal ion. In

the solid state we have observed that all the donor atoms

of the macrocyclic ring co-ordinate the Cu(II). Two

amine groups, however, are bound at longer distance

[Cu�/N(1) 2.161(3), Cu�/N(7) 2.136(3) A] than normally

found in Cu(II) complexes with oxa�/aza macrocycles

such as [15]aneN3O2, where the Cu�/N bond lengths

range between 1.99 and 2.05 A [14]. Furthermore, as

previously noted, in the present complex the O-donors

are also bound at longer distance than the N-centres.

These structural data may account for the reduced

thermodynamic stability of the Cu(II) complex in

aqueous solutions.

It is also of interest that in the solid state the co-

ordination sphere of the metal ion is effectively satu-

rated by a solvent molecule, thereby adopting an

octahedral geometry. Although conclusions regarding

the co-ordination properties of ligands in solution

derived from solid state data may be misleading, these

observations can reasonably explain the tendency to

form hydroxylated species at alkaline pH, considering

that in aqueous solution one co-ordination site of the

Cu(II) would be occupied by a water molecule.Unfortunately, the low stability of the Zn(II), Cd(II)

and Pb(II) complexes prevents any investigation of their

completion at alkaline pH due to metal hydroxide

precipitation even at slightly alkaline pH. However,

the Cd(II) and Pb(II) complexes are somewhat more

stable than the Zn(II) complex, suggesting the involve-

ment of the oxygen donors in co-ordination to Cd(II)

and Pb(II). These structural features may indicate abetter fit between the large macrocyclic cavity and the

larger Cd(II) and Pb(II) ions, leading to the observed

higher stability of the Cd(II) and Pb(II) complexes with

respect to the smaller Zn(II) ion; this behaviour is the

same for almost all the other ligands described in this

paper (Tables 5 and 6) [9,13�/15], and is best ascribed to

the dimension of the cavity rather than to the different

pendant arms attached to the N-centres. Although fromthese potentiometric studies we cannot conclude

whether or not the nitrile arms interact in some way

with the metal ions in solution, we can nevertheless say

that their effect is to decrease quite drastically the

stability of all the metal complexes studied.

2.2. NMR spectroscopic studies

In order to investigate further the structural featuresof the complexes in solution,1H NMR spectra in

CD3CN were recorded at various temperatures. The1H NMR spectra of all the complexes recorded at 300

MHz at room temperature show a complicated splitting

pattern. In the 1H NMR spectra of [Zn(L)]2� and

[Cd(L)]2� it is not possible to assign clearly any of the

signals because of overlap. We can observe two series of

multiplets, one due to the macrocyclic protons adjacentto the amines (in the region between 2.7�/3.1 ppm for

[Zn(L)]2� and 2.8�/3.2 ppm for [Cd(L)]2�), and the

other due to the protons of the CH2CN arms and to the

CH2 adjacent to the oxygens on the ring (in the region

between 3.5�/4 ppm for [Zn(L)]2� and 3.6�/4 ppm for

[Cd(L)]2�). However, for [Pb(L)]2�a clearer splitting

pattern can be observed in the 1H NMR spectrum, A

2D-COSY experiment combined with a 1H�/13C cou-

pling 2D experiment allowed a partial assignment of the

resonances. Two triplets of doublets have been assigned

to two axial protons of the ethylene moiety

[OCH2CH2N] of the macrocycle. This splitting pattern

is the result of the coupling between axial and equatorial

protons of the two CH2 groups of this moiety. These

two triplets of doublets are at 4.23 and 3.30 ppm due to

the axial proton adjacent to the oxygen and to thenitrogen, respectively. The coupling constants are 11.4

Hz for Jgem for both axial protons, while Jtrans, due to

the coupling between equatorial and axial protons of

Table 5

Stability constants (log K ) of the metal complexes with L1, L2, L3 and

L4

Reaction L1 a L2 b H3L3 c L4 a

L�Cu2�� [Cu(L)]2� 15.27(1) 17.38(2) 17.25(4) 12.68(2)

L�Zn2�� [Zn(L)]2� 8.85(1) 12.15(1) 16.82(2) 7.21(1)

L�Cd2�� [Cd(L)]2� 10.05(1) 13.02(2) 17.83(3) 9.15(2)

L�Pb2�� [Pb(L)]2� 10.07(1) 10.45(4) 16.92(2) 9.09(2)

a In 0.1 M NaNO3 at 298 K (ref. [12]).b In 0.1 M Me4NCl, 298.1 K (ref. [9]).c In 0.1 M (Me4N)NO3, 298 K (ref. [14]).


different GHz, are 3.7 and 5.5 Hz, respectively. All other

peaks overlap each other, and it has not been possible to

assign them; between 3.8 and 4.1 ppm we can observe a

complicated multiplet which includes the resonances ofthe protons of the arms, the protons of the other

methylene group adjacent to the oxygen and the

equatorial proton of OCH2CH2N. Another multiplet

due to the protons adjacent to the amines appears in the

region between 3.0 and 3.2 ppm. The assignment of the13C NMR spectrum of [Pb(L)]2�, is reported in Section

4.

The presence of a splitting pattern where axial andequatorial protons can be detected and the peaks

assigned (at least for [Pb(L)]2�) means that the com-

plexes are quite rigid at room temperature. Variable-

temperature experiments have been carried out to

determine fluxionality and differences between the two

complexes. Upon increasing the temperature, all the

peaks broaden and those due to axial and equatorial

protons tend to collapse into a single broad peak. Thiscan be explained by a loss of rigidity of the complexes

and by enhanced fluxionality of the macrocycle. All the

complexes lose their rigidity by 40 8C perhaps due to

their low stability. In contrast, analogous experiments

performed with the complexes of the ligand L2 showed

that the Pb(II) complex remained rigid up to 70 8C [9].

3. Conclusion

The structural characterisation of the Ni(II) Cu(II),

Cd(II) and Pb(II) complexes of l,4,7-tris(cyanomethyl)-

l,4,7-triaza-l0,13-dioxacyclopentadecane (L) has con-

firmed that these metal ions are bound to the donor

atoms of the macrocyclic ligand, with solvent molecules

or anions completing their co-ordination spheres. Only

in the case of the Pb(II) complex was the formation of abinuclear species with binding of a nitrile pendant arm

from an adjacent molecule observed. The markedly low

stability constants determined for complexes of Cu(II),

Zn(II), Cd(II) and Pb(II) with L are due mainly to the

inability of the nitrile groups to bind the metal ion co-

ordinated within the macrocyclic cavity, with the

pendant arms oriented away from the metal co-ordina-

tion sphere and increasing the rigidity of the cycle.

4. Experimental

Spectra were recorded on a Bruker DPX 300 (1H and13C NMR) and on a Perkin�/Elmer 1600 spectrometer

(FTIR, KBr discs). Elemental analytical data were

obtained by the Microanalytical Service (Perkin�/Elmer240B analyser) at the University of Nottingham. Fast

atom bombardment (FAB) mass spectra were obtained

by the EPSRC National Mass Spectrometry Service at

the University of Swansea. 1,4,7-Tris (2-cyanomethyl)-

l,4,7-triaza-10,13-dioxacyclopentadecane (L) was pre-

pared as described in the literature [9]. All starting

materials were obtained from Aldrich Chemical Co.Ltd. and were used without further purification.

4.1. Synthesis of L �/HCl �/H2O

The hydrochloride salt was obtained in quantitativeyield by adding a 1:1 solution of HCl in EtOH to an

ethanolic solution containing the free amine until

precipitation of a white solid, which was filtered off

and washed with EtOH. Elemental analysis: Found:

(Calc. for C16ClH27N6O2) C, 51.67 (51.82); H, 7.31

(7.34); N, 22.53 (22.66)%.

4.2. Synthesis of the complexes

The appropriate metal salt MX2 [M�/Ni(II), Cu(II),

Zn(II), Cd(II) and Pb(II) and X�/ClO4� or NO3

�]

(0.06 mmol) was dissolved in CH3CN (10 cm3) andadded to a solution of L (0.06 mmol) in CH3CN (20

cm3). The mixture was stirred for 3 h at room

temperature (r.t.). The solution was partially concen-

trated under vacuum and addition of Et2O yielded a

solid. The products were filtered off and dried. Yields

65�/80%. Single crystals suitable for X-ray analysis were

obtained by diffusion of Et2O vapour into a CH3CN

solution of the complex at r.t.

4.2.1. [Ni(L)(CH3CN)](ClO4)2

FAB mass spectrum (3-NOBA matrix) m /z: 493 for

[C16H26N6O2Ni �/ClO4]�; IR (KBr disc): 2969w, 2252m[n(C�/N)], 1471m, 1326w, 1091s [n(ClO4

�)], 628m.

Elemental analysis: Found: (Calc. for C16H26Cl2N6-

O10Ni �/2CH3CN): C, 35.46 (35.63); H, 6.15 (6.29); N,

13.71 (13.80)%.

4.2.2. [Cu(L)(CH3CN)](ClO4)2

FAB mass spectrum (3-NOBA matrix) m /z : 498

[C16H26N6O2Cu �/ClO4]�; IR (KBr disc): 2910w, 2260w

[n(C�/N)], 1469m, 1338m, 1091s [n(ClO4�), 628m.

Elemental analysis: Found: (Calc. for Cl6H26Cl2N6O10-

Cu �/CH3CN): C, 33.92 (33.89); H, 4.81 (4.58); N,

15.53(15.37)%.

4.2.3. [Zn(L)](ClO4)2

FAB mass spectrum (3-NOBA matrix) m /z : 500 for

[C16H26N6O2Zn �/ClO4]�; IR (KBr disc): 2907w, 2249w

[n(C�/N)], 1484m, 1382m, 1091s [n(ClO4�)] 940m,

628m. Elemental analysis: Found: (Calc. for

C16H26Cl2N6O10Zn �/CH3CN): C, 33.57 (33.79); H, 4.46

(4.57); N, 15.29 (15.33)%.


4.2.4. [Cd(L)(NO3)2]


[C16H26N6O2Cd �/NO3 �/H2O]�; IR (KBr disc): 2918w,

2263 [n (C�/N)], 1471w, 1384s [n (NO3�)], 1294s, 1100w.

Elemental analysis: Found: (Calc. for C16H26N8O8Cd �/2H2O): C, 31.51 (31.67); H, 4.94 (4.98); N, 18.55

(18.46)%.

4.2.5. [Pb(L)](ClO4)2


[C16H26N6O2Pb �/ClO4]�; IR (KBr disc): 2901w, 2264w

[n(C�/N)]. 1464m, 1362m, 1093s [n (ClO4�)], 929m,

628m. Elemental analysis: Found (Calc. for C16H26Cl2-N6O10Pb): C, 26.20 (25.95); H, 3.35 (3.54); N, 11.19

(11.35)%. 13C NMR: d (CDCl3) 70.3 (OCH2), 69.9

(OC¯

H2CH2N), 54.1 (OCH2C¯

H2N), 55.7 (NC¯

H2CH2N),

46.5 (NCH2C¯

H2N). 44.8 (NC¯

H2CN), 39.6 (NC¯

H2CN),

115.8 (C¯�/N) ppm.

CAUTION!! Transition metal perchlorates are poten-

tially explosive. These materials should be made in small

quantities and handled with extreme caution.

5. Crystal structure determinations

Crystal data, data collection and refinement para-

meters for compounds [M(L)(CH3CN)](ClO4)2 [M�/

Cu(II), Ni(II)], [Cd(L)(NO3)2] and [Pb(L)](ClO4)2 are

given in Table 6 and selected bond lengths and angles in

Tables 1�/3. Data for the Ni(II), Cu(II) and Pb(II)structures were collected on a Stoe Stadi-4 four-circle

diffractometer equipped with an Oxford Cryosystems

open-flow cryostat using graphite-monochromated Mo

Ka radiation (l�/0.71073 A). Numerical absorption

corrections based on face indexing were applied to the

data for all compounds. Data for the Cd(II) structure

were collected on a Bruker SMART CCD area detector

diffractometer with a similar cryostat, using graphite-

monochromated Mo Ka radiation (l�/0.71073 A).

Semi-empirical absorption corrections based on equiva-

lent reflections were applied.

All the structures were solved by direct methods [26]

and completed by iterative cycles of full-matrix least

squares refinement and DF syntheses. All non-H atoms,

except for those in disordered groups, were refined

anisotropically. All the H atoms were placed in calcu-

lated positions and refined using a riding model [27],

except those on the acetonitrile molecules which were

located from difference maps and refined as a rigid

body.

The perchlorate anions were found to exhibit disorder

in all three compounds, and this disorder was modelled

using partial occupancy models over either two or three

sites. In [Cu(L)(CH3CN)](ClO4)2 the occupancy factor is

0.50 for O(1) and 0.33 or O(4) from a perchlorate anion

found disordered over three sites [0.25 for O(1)? and

O(1)ƒ, 0.33 for O(4)? and O(4)ƒ]. The oxygens of the

other perchlorate are disordered over two sites. In

[Ni(L)(CH3CN)](ClO4)2 are each two oxygens from a

perchlorate anion disordered over three sites; the

occupancies are 0.50, 0.25 and 0.25 for O(1), O(1)?,and O(1)ƒ, and 0.33 for each of O(4), O(4)? and O(4)ƒ. In

Table 6

Crystal data and X-ray experimental details for [Ni(L)(CH3CN)](ClO4)2 �CH3CN, [Cu(L)(CH3CN)](ClO4)2, [Cd(L)](NO3)2(CH3CN) and

[Pb(L)](ClO4)2

Formula C16H26N6O2Ni �CH3CN �2ClO4 C16H26O2Cu�CH3CN �2ClO4 C16H26N6O2Cd �CH3CN �2NO3 C32H52O4Pb2 �4ClO4

M 671.12 637.92 611.90 740.52

Crystal system monoclinic monoclinic monoclinic triclinic

Space group P21/n P21/n P21/n P/1/

a (A) 9.571 (4) 9.359 (4) 9.5360 (8) 8.908 (6)

b (A) 19.475 (9) 17.351 (13) 19.221 (2) 10.118 (5)

c (A) 15,663 (6) 15.817 (4) 13.596 (1) 14.049 (8)

a (8) 105.86(4)

b (8) 90.71 (4) 90.59 (4) 93.545 (1) 100.72 (6)

g (8) 92.15(5)

V (A3) 2919.2 (15) 2568.5 (13) 2487.3 (6) 1191.6 (12)

T (K) 150 (2) 150 (2) 150 (2) 150 (2)

Z 4 4 4 1

Dcalc (g cm�3) 1.527 1.650 1.634 2.064

Crystal size (mm) 0.29�0.16�0.15 0.35�0.34�0.10 0.28�0.17�0.12 0.30�0.26�0.08

m (mm�1) 0.912 1.125 0.939 7.368

Unique data, Rint 3797, 0.00 4513, 0.0668 6036, 0.045 4203, 0.0386

Observed data 2644 3597 4889 3737

Parameters 371 350 326 304

Maximum D/s ratio 0.08 0.004 0.001 0.002

R1, wR2a 0.0846, 0.2252 0.0502, 0.1316 0.0247, 0.0637 0.0573, 0.1556

Goodness-of-fit 1.054 1.019 1.033 1.056

aSHELXL-97 R1 based on observed data with I �2s (I ), wR2 on all unique data.


[Pb(L)](ClO4)2 disorder way identified in part of the

macrocyclic backbone, in parts of the nitrile arms and in

one perchlorate. It was modelled using partial occu-

pancy models over two sites for each disordered atom.The occupancy factor is 0.60 for all the disordered

atoms [from N(7) to C(12), from C(1C) to N(3C) and

from O(1) to O(4)] and 0.40 of the major for the minor

component. Appropriate restraints were applied to all

bond distances involving disordered atoms.

5.1. Potentiometric measurements

Equilibrium constants for protonation and complexa-

tion reactions with L were determined by pH measure-

ments in 0.1 mol dm�3 Me4NCl at 298.19/0.1 K, by

using potentiometric equipment that has been already

described [28]. The combined glass electrode was

calibrated as a hydrogen concentration probe by titrat-

ing known amounts of HCl with CO2-free NaOHsolutions and determining the equivalent point by the

Gran’s method [29] which allows determination of the

standard potential E8, and the ionic product of water

(pKW�/13.83(1) at 298.1 K in 0.1 mol dm�3 Me4NCl).

Concentrations of 0.5�/10�3�/1�/10�3 mol dm�1 of

ligand and metal ions were employed in the potentio-

metric measurements and three titration experiments

were performed in the pH range 2�/11, each giving about100 data points. The computer program HYPERQUAD

[30] was used to calculate equilibrium constants from

e.m.f. data. For each system, all titrations were treated

either as single sets or as separated entities without

significant variation in the values of the constants

determined.

Acknowledgements

We thank the EPSRC and the University of Notting-

ham for support, and the EPSRC National Service for

Mass Spectrometry at the University of Swansea.

Financial support from the Italian Ministero dell’Uni-

versita e della Ricerca Scientifica e Tecnologica (COFIN2000) is gratefully acknowledged.

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