Cooperative anion recognition in copper(II) and zinc(II) complexes with a ditopic tripodal ligand containing a urea group

Article

pubs.acs.org/IC

Cooperative Anion Recognition in Copper(II) and Zinc(II) Complexes

with a Ditopic Tripodal Ligand Containing a Urea Group

Israel Carreira-Barral, Teresa Rodríguez-Blas, Carlos Platas-Iglesias, Andres de Blas,

́

and David Esteban-Gomez*

́

Departamento de Química Fundamental, Universidade da Coruna, Campus da Zapateira, Rua da Fraga 10, 15008 A Coruna, Spain

́

̃

S

* Supporting Information

ABSTRACT: The ability of Cu^IIand Zn^IIcomplexes of the ditopic receptor H₂L [1-(2-((bis(pyridin-2-ylmethyl)amino)-

methyl)phenyl)-3-(3-nitrophenyl)urea] for anion recognition is reported. In the presence of weakly coordinating anions such as

ClO₄, the urea group binds to the metal ion (Cu^IIor Zn^II) through one of its nitrogen atoms. The study of the interaction of the

−

2−

metal complexes with a variety of anions in DMSO shows that SO₄and Cl⁻bind to the complexes through a cooperative

binding involving simultaneous coordination to the metal ion and diﬀerent hydrogen-bonding interactions with the urea moiety,

depending on the shape and size of the anion. On the contrary, single crystal X-ray diﬀraction studies show that anions such as

NO₃⁻and PhCO₂⁻form 1:2 complexes (metal/anion) where one of the anions coordinates to the metal center and the second

one is involved in hydrogen-bonding interaction with the urea group, which is projected away from the metal ion.

Spectrophotometric titrations performed for the Cu^IIcomplex indicate that this system is able to bind a wide range of anions with

an aﬃnity sequence: MeCO₂⁻∼ Cl⁻(log K₁₁> 7) > NO₂⁻> H₂PO₄⁻∼ Br⁻> HSO₄⁻> NO₃⁻(log K₁₁< 2). In contrast to this,

the free ligand gives much weaker interactions with these anions. In the presence of basic anions such as MeCO₂or F⁻,

−

competitive processes associated with the deprotonation of the coordinated N−H group of the urea moiety take place. Thus, N-

coordination of the urea unit to the metal ion increases the acidity of one of its N−H groups. DFT calculations performed in

DMSO solution are in agreement with both an anion-hydrogen bonding interaction and an anion−metal ion coordination

collaborating in the stabilization of the metal salt complexes with tetrahedral anions.

exploits the anion binding properties of coordinatively

unsaturated coordination compounds.⁷The use of ditopic

receptors, which contain covalently linked sites for the

complexation of both a cation and an anion in a single

molecule, represents another attractive alternative.⁸The

binding of the cation increases the aﬃnity of the receptor for

the anion, thereby enhancing the aﬃnity toward diﬀerent

anions that normally coordinate poorly with monotopic

molecular receptors. In order to maximize the cooperative

eﬀect, the combined cation- and anion-recognition domains

have to be close enough into a preorganized platform.

INTRODUCTION

■

Selective anion recognition using organic receptors has been

the subject of intense research eﬀorts in the last 15 years due to

their fundamental role in a wide range of chemical, biological,

and environmental processes.¹The design of receptors with

improved anion recognition properties is not an easy task, and

diﬀerent strategies have been developed over the years for this

purpose. Factors such as the charge to radius ratio of the target

anion, as well as its geometry and its protonation state, must be

taken into account.²Traditionally, anion receptors comprised

organic frameworks containing suitable hydrogen-bonding

donor functions.^3,4More recently, organic anion receptors

taking advantage of halogen-bonding interactions have also

been developed.⁵A third approach to anion receptors is the use

of dynamically self-assembled hosts formed through reversible

bond formation.⁶An alternative to the use of organic scaﬀolds

Complexes of transition and post-transition metal ions with

polypyridyl ligands such as di(2-picolyl)amine (dpa) are still

Received: November 15, 2013

Published: February 17, 2014

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Article

commercial sources. Solvents were of reagent grade and used without

further puriﬁcation.

Caution: Although we have experienced no difficulties with the

perchlorate salts, these should be regarded as potentially explosive and

handled with care. To avoid the formation of peroxides due to contact with

air, reflux of ether solutions was carried out under Ar.

receiving much attention for the design of systems for a number

of biological, catalytic, and sensing applications.⁹For instance,

Cu^IIand Zn^IIcomplexes with dpa¹⁰and diﬀerent N-substituted

derivatives have been widely investigated.¹¹These complexes

are often coordinatively unsaturated, and therefore, they

possess anion recognition properties that have been exploited

for the design of luminescent sensors for diﬀerent anions.¹²

Urea-based receptors are known to be suitable for anion

recognition, as they can establish hydrogen-bonding inter-

actions with diﬀerent anions,¹³in particular Y-shape ones.¹⁴

Ditopic receptors containing urea- or thiourea units and metal

binding moieties have been developed for diﬀerent purposes

(i.e., metal controlled assembly of anion receptors,¹⁵extraction

of ion-pairs from aqueous media,¹⁶ion-pair sensing,¹⁷or anion

recognition via cooperative binding through anion coordination

to the metal ion and hydrogen-bonding interactions).¹⁸In view

of this, we envisaged that ditopic receptors containing a urea

moiety and a dpa unit might be useful for the recognition of

diﬀerent anions. Thus, in this paper we report the ability of the

Cu^IIor Zn^IIcomplexes of the ditopic receptor H₂L (Scheme 1)

General Methods. Elemental analyses were carried out on a

ThermoQuest Flash EA 1112 elemental analyzer. ESI-TOF mass

spectra were recorded from MeOH/H₂O/MeCN or MeOH solutions

using a LC-Q-q-TOF Applied Biosystems QSTAR Elite spectrometer

in the positive mode. IR spectra were recorded using a Bruker Vector

22 spectrophotometer equipped with a Golden Gate attenuated total

reﬂectance (ATR) accessory (Specac). ¹H and ¹³C NMR spectra were

recorded at 25 °C on Bruker Avance 300 and Bruker Avance 500

spectrometers, and spectral assignments were based in part on 2D

COSY, HSQC, and HMBC experiments. UV/vis spectra were

recorded with a Perkin-Elmer Lambda 900 spectrophotometer;

those performed in solution were recorded with quartz cells (path

length: 1 cm), and the cell holder was thermostatted at 25.0 °C,

through circulating water. The formation of the mononuclear Cu^II

complex was monitored by using spectrophotometric titrations at 25

°C on a 10⁻⁵M solution of ligand H₂L in DMSO. Typically, aliquots

−

of a fresh standard solution of Cu(TfO)₂(TfO⁻= CF₃SO₃) in the

same solvent (10⁻³M) were added and the UV/vis spectra of the

samples were recorded. Anion binding studies were performed by

monitoring the spectral changes of a 5 × 10⁻³M solution of ligand

H₂L in the presence of 1 equiv of Cu(TfO)₂upon addition of the

corresponding tetrabutylammonium salt (0.5 M in DMSO). Binding

constants were obtained by using a simultaneous ﬁt of the UV/vis

absorption spectral changes at 7−12 selected wavelengths in the range

500−1200 nm. A minimum of 26 absorbance data points at each of

these wavelengths was used, and all spectrophotometric titration

curves were ﬁtted with the HYPERQUAD program.¹⁹

Scheme 1. Synthesis of H₂L and Numbering Scheme used for

NMR Spectral Assignment

1-(2-(Chloromethyl)phenyl)-3-(3-nitrophenyl)urea (3). A solution

of 1-(chloromethyl)-2-isocyanatobenzene (1) (0.98 mL, 7.10 mmol)

and 3-nitroaniline (2) (1.00 g, 7.10 mmol) in diethyl ether (50 mL)

was heated to reﬂux under Ar atmosphere for 48 h. The precipitate

formed was isolated by ﬁltration and washed with diethyl ether (3 ×

10 mL) to give 1.584 g of 3 (73%) as a light yellow solid. δ_H(solvent

DMSO-d₆, 500 MHz, 298 K): 10.13 (s), 10.08 (s), 8.60 (s), 8.57 (t, ⁴J

= 2.1 Hz), 8.48 (s), 7.86 (m), 7.80 (m), 7.73 (m), 7.58−7.53 (m), 7.44

3

4

(dd, J = 7.7 Hz, J = 1.5 Hz), 7.35−7.31 (m), 7.24 (m), 7.09 (m),

7.03 (m), 4.88 (s), 4.55 (s). δ_C(solvent DMSO-d₆, 125.8 MHz, 298

K): 152.8, 152.7, 148.2, 148.2, 141.5, 141.2, 137.1, 136.9, 131.8, 130.7,

130.2, 130.1, 129.3, 128.3, 128.1, 127.5, 124.1, 124.0, 123.7, 122.9,

122.8, 121.6, 116.3, 116.1, 111.9, 61.0, 43.5. MS-ESI⁺, m/z (%BPI): [3

+ H]⁺, 306.1 (69.5%); [3 − Cl]⁺, 270.1 (100%). Elem. Anal. Calcd for

C₁₄H₁₂ClN₃O₃: C, 55.0; H, 4.0; N, 13.7%. Found: C, 54.9; H, 3.6; N,

13.4%. IR: 3291 ν(N−H), 3096 ν(C−H), 1651 ν(CO), 1594

ν(CC), 1561 δ(C−N−H), 1522 ν_a(NO₂), 1346 ν_s(NO₂), 687

ν(C−Cl) cm⁻¹.

1-(2-((Bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-

nitrophenyl)urea (H₂L). A solution of compound 3 (0.819 g, 2.678

mmol), bis(pyridin-2-ylmethyl)amine (dpa) (4) (0.452 mL, 2.434

mmol), N,N-diisopropylethylamine (dipea) (0.848 mL, 4.868 mmol),

and a catalytic amount of KI in acetonitrile (50 mL) was heated to

reﬂux with stirring for 16 h. The resulting solution was ﬁltered while

hot and allowed to cool down to room temperature and sonicated to

give a light yellow solid that was isolated by ﬁltration and washed with

acetonitrile (10 mL) and diethyl ether (10 mL) to give 0.561 g (48%)

of H₂L. δ_H(solvent DMSO-d₆, 500 MHz, 298 K): 10.30 (s, 1H, H14),

9.58 (s, 1H, H16), 8.61 (m, 1H, H22), 8.49 (m, 2H, H1), 7.89 (d, 1H,

for anion recognition. In particular, the aﬃnity of the

−

[Cu(H₂L)]²⁺complex toward diﬀerent anions (F⁻, NO₃,

−

HSO₄, H₂PO₄, NO₂, Cl⁻, Br⁻, MeCO₂, and PhCO₂) has

been investigated by using electronic absorption spectroscopy.

A structural analysis of anion recognition with the Zn^IIanalogue

in solution (NMR spectroscopy supported by DFT calcu-

lations) is also reported.

H12, J = 8.0 Hz), 7.86 (dd, 1H, H20, J = 8.2 Hz, ⁴J = 1.8 Hz), 7.80

(m, 1H, H18), 7.73 (m, 2H, H3), 7.61 (m, 1H, H19), 7.40 (m, 2H,

H4), 7.29−7.22 (m, 4H, H2, H9, and H11), 6.99 (m, 1H, H10), 3.83

(s, 4H, H6), 3.75 (s, 2H, H7). δ_C(solvent DMSO-d₆, 125.8 MHz, 298

K): 157.6 C5, 152.6 C15, 148.9 C1, 148.3 C21, 141.4 C17, 138.7 C13,

137.0 C3, 130.3 C19, 130.1 C9, 127.9 C11, 126.8 C8, 124.2 C18,

123.8 C4, 122.7 C2, 122.5 C10, 120.7 C12, 116.3 C20, 112.1 C22,

58.6 C6, 56.2 C7. MS-ESI⁺, m/z (%BPI): [H₂L + H]⁺, 469.2 (100%).

3

EXPERIMENTAL AND COMPUTATIONAL SECTION

Materials. 1-(Chloromethyl)-2-isocyanatobenzene (1), 3-nitroani-

line (2), and bis(pyridin-2-ylmethyl)amine (4) were obtained from

■

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Table 1. Crystal Data and Reﬁnement Details

Cu·L·NO₃

Cu·L·SO₄

Cu·L·Bz

Zn·L·Cl

formula

MW

C₂₇H₂₈CuN₈O₁₀

688.11

C₂₆H₃₀CuN₆O₁₀S

682.16

C₄₀H₃₆CuN₆O₈

792.29

C₂₇H₂₅Cl₅N₆O₃Zn

724.15

crystal system

space group

T/K

triclinic

monoclinic

P2₁/c

triclinic

P1

̅

100(2)

P1

̅

100(2)

̅

100(2)

9.4204(11)

26.459(3)

14.3146(18)

90

a/Å

9.2361(3)

12.3680(4)

13.5847(4)

99.696(2)

94.596(2)

106.471(2)

1453.38(8)

710

8.7439(5)

12.9521(7)

14.6569(8)

65.471(2)

89.183(3)

70.880(3)

1412.43(14)

706

9.0531(4)

12.3579(6)

15.7312(8)

91.820(3)

99.113(3)

106.040(3)

1664.80(14)

736

b/Å

c/Å

α/deg

β/deg

95.339(8)

90

γ/deg

V/Å³

3552.5(7)

1644

F(000)

Z

2

4

2

λ, Å (Mo Kα)

0.71073

1.572

0.71073

1.604

0.71073

1.481

0.71073

1.445

D

_calc/g cm⁻³

μ/mm⁻¹

0.824

0.916

0.680

1.176

θ range/deg

R_int

1.53−28.36

0.0447

2.52−28.39

0.0246

2.92−27.88

0.1146

2.70−28.39

0.0296

reﬂns measd

unique reﬂns

reﬂns obsd

GOF on F²

26362

41327

38143

22494

7137

7084

8447

8132

4975

6697

4805

5984

1.140

1.059

1.033

1.095

a

R1

0.0442

0.0247

0.0581

0.0579

b

wR2 (all data)

largest diﬀerences peak and hole/e Å⁻³

R1 = ∑||F₀| − |Fc||/∑|F₀|. wR2 = {∑[w(||F₀|²− |Fc|²|)²]/∑[w(F₀)]}^1/2

0.1194

0.0672

0.1396

0.1658

0.346 and −0.871

0.497 and −0.499

0.533 and −0.659

1.183 and −1.209

a

b

4

.

Elem. Anal. Calcd for C₂₆H₂₄N₆O₃0.5H₂O: C, 65.4; H, 5.3; N, 17.6%.

Found: C, 65.8; H, 5.1; N, 17.7%. IR: 3310−3190 ν(N−H), 3130−

2850 ν(C−H), 1703 ν(CO), 1519 ν_a(NO₂), 1345 ν_s(NO₂) cm⁻¹.

General Procedure for the Preparation of [M(H₂L)Cl₂], [M(H₂L)]-

(ClO₄)₂, [M(H₂L)(SO₄)], and [M(H₂L)(NO₃)](NO₃) (M = Cu or Zn). A

solution of MCl₂(M = Cu or Zn) or hydrated M(NO₃)₂or M(SO₄)

(0.105 mmol, M = Cu or Zn) in methanol (2 mL) was added to a

warm solution of H₂L (0.050 g, 0.105 mmol) in methanol (5 mL).

The mixture was stirred at room temperature for 16 or 1 h M(ClO₄)₂,

and the precipitate formed was isolated by ﬁltration, washed with

methanol (2 mL) and diethyl ether (2 mL), and dried under vacuum.

In some cases, an additional puriﬁcation step was used to remove

metal salt impurities: the precipitate was dissolved in chloroform (5

mL), the solution was ﬁltered, and the ﬁltrate was concentrated to

dryness. The solid was treated with diethyl ether (5 mL), ﬁltered, and

dried under vacuum.

Cu(H₂L)Cl₂·0.5MeOH (Cu·L·Cl). Light blue solid. Yield 0.043 g, 66%.

MS-ESI⁺, m/z (%BPI): [Cu(H₂L)Cl]⁺, 566.1 (100%); [Cu(HL)]⁺,

530.1 (2%). Elem. Anal. Calcd for C₂₆H₂₄Cl₂CuN₆O₃·0.5MeOH: C,

51.4; H, 4.2; N, 13.6%. Found: C, 51.0; H, 3.6; N, 13.2%. Λ_M

(methanol, 25 °C): 76 cm²Ω⁻¹mol⁻¹. IR: 3330−3190 ν(N−H), 1710

ν(CO), 1529 ν_a(NO₂), 1346 ν_s(NO₂) cm⁻¹. UV/vis diﬀuse

reﬂectance spectroscopy: 675 nm.

Cu(H₂L)(ClO₄)₂·2H₂O·MeOH (Cu·L·ClO₄). Dark green solid. Yield

0.075 g, 90%. MS-ESI⁺, m/z (%BPI): [Cu(H₂L)(ClO₄)]⁺, 630.1 (1%);

[Cu(HL)]⁺, 530.1 (61%); [Cu(H₂L)]²⁺, 265.6 (1%). Elem. Anal.

Calcd for C₂₆H₂₄Cl₂CuN₆O₁₁·2H₂O·MeOH: C, 40.8; H, 3.7; N,

10.0%. Found: C, 40.6; H, 4.0; N, 10.5%. Λ_M(methanol, 25 °C): 164

cm²Ω⁻¹mol⁻¹. IR: 3350 ν(N−H), 1695 ν(CO), 1525 ν_a(NO₂),

1349 ν_s(NO₂), 1053 ν_a(ClO₄), 619 δ_a(OClO) cm⁻¹. UV/vis diﬀuse

reﬂectance spectroscopy: 634 nm.

Cu(H₂L)(NO₃)₂·0.5MeOH·1.5H₂O (Cu·L·NO₃). Light blue solid. Yield

0.011 g, 16%. MS-ESI⁺, m/z (%BPI): [Cu(HL)]⁺, 530.1 (65%);

[Cu(H₂L)]²⁺, 265.6 (2%). Elem. Anal. Calcd for C₂₆H₂₄CuN₈O₉·

0.5MeOH·1.5H₂O: C, 45.5; H, 4.2; N, 16.0%. Found: C, 45.5; H, 3.5;

N, 15.5%. Λ_M(methanol, 25 °C): 110 cm²Ω⁻¹mol⁻¹(1:1

electrolyte). IR: 3323 ν(N−H), 1708 ν(CO), 1523 ν_a(NO₂),

1318, 1279 ν(NO₃) cm⁻¹. UV/vis diﬀuse reﬂectance spectroscopy:

652 nm [d_xz,d_yz→ d_{x −y}(²B₁→²E)]. Slow diﬀusion of diethyl ether

into a solution of the complex in an acetonitrile/methanol mixture

gave dark blue single crystals suitable for X-ray diﬀraction analysis.

Cu(H₂L)(SO₄)·1.5H₂O (Cu·L·SO₄). Light blue solid. Yield 0.049 g,

71%. MS-ESI⁺, m/z (%BPI): [Cu(HL)]⁺, 530.1 (91%); [Cu(H₂L)]²⁺,

265.6 (2%). Elem. Anal. Calcd for C₂₆H₂₄CuN₆O₇S·1.5H₂O: C, 47.7;

H, 4.2; N, 12.8%. Found: C, 47.5; H, 3.9; N, 12.8%. Λ_M(methanol, 25

°C): 4 cm²Ω⁻¹mol⁻¹(nonelectrolyte). IR: 3320−3180 ν(N−H),

1711 ν(CO), 1524 ν_a(NO₂), 1345 ν_s(NO₂), 1036, 1027 ν(SO₄)

2

cm⁻¹. UV/vis diﬀuse reﬂectance spectroscopy: 663 nm [d_xz,d_yz

→

d_{x −y}(²B₁→²E)]. Slow evaporation of a solution of the complex in

water provided dark blue single crystals suitable for X-ray diﬀraction

analysis.

2

Zn(H₂L)Cl₂(Zn·L·Cl). Light yellow solid. Yield 0.048 g, 75%. MS-

ESI⁺, m/z (%BPI): [Zn(H₂L)Cl]⁺, 567.1 (100%); [Zn(HL)]⁺, 531.1

(42%); [Zn(H₂L)]²⁺, 266.1 (13%). Elem. Anal. Calcd for

C₂₆H₂₄Cl₂N₆O₃Zn: C, 51.6; H, 4.0; N, 13.9%. Found: C, 51.5; H,

3.7; N, 13.8%. Λ_M(methanol, 25 °C): 68 cm²Ω⁻¹mol⁻¹. IR: 3313

ν(N−H), 1707 ν(CO), 1523 ν_a(NO₂), 1348 ν_s(NO₂) cm⁻¹. δ_H

(solvent DMSO-d₆, 300 MHz, 298 K): 9.08 (s, 1H, HNCO), 8.84

(s, 2H), 8.52 (s, 1H), 7.83 (m, 3H), 7.61−7.43 (m, 7H), 7.32−7.25

(m, 2H), 7.09 (t, 1H, ³J = 7.0 Hz), 4.00 (s, 4H, H6), 3.81 (s, 2H, H7).

Slow evaporation of a solution of the complex in chloroform provided

colorless single crystals suitable for X-ray diﬀraction analysis.

Zn(H₂L)(ClO₄)₂·H₂O·MeOH (Zn·L·ClO₄). Yellow solid. Yield 0.070 g,

85%. MS-ESI⁺, m/z (%BPI): [Zn(H₂L)(ClO₄)]⁺, 631.1 (1%);

[Zn(HL)]⁺, 531.1 (100%); [Zn(H₂L)]²⁺, 266.1 (12%). Elem. Anal.

Calcd for C₂₆H₂₄Cl₂N₆O₁₁Zn·H₂O·MeOH: C, 41.2; H, 3.4; N, 10.4%.

Found: C, 41.4; H, 3.8; N, 10.7%. Λ_M(methanol, 25 °C): 184 cm²

Ω⁻¹mol⁻¹. IR: 3342 ν(N−H), 1663 ν(CO), 1524 ν_a(NO₂), 1350

ν_s(NO₂), 1054 ν_a(ClO₄), 620 δ_a(OClO) cm⁻¹. δ_H(solvent DMSO-d₆,

300 MHz, 298 K): 9.14 (s, 1H, HNCO), 8.59 (d, 2H, ³J = 5.9 Hz),

3

4

8.40 (s, 1H), 8.31 (s, 1H, HNCO), 7.99 (td, 3H, J = 7.6 Hz, J =

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2

0.9 Hz), 7.82 (m, 1H), 7.61−7.23 (m, 10H), 4.23 (d, 2H, H6A, J =

No symmetry constraints have been imposed during the optimizations.

The stationary points found on the potential energy surfaces as a result

of geometry optimizations were tested to represent energy minima

rather than saddle points via frequency analysis. The default values for

the integration grid (75 radial shells and 302 angular points) and the

SCF energy convergence criteria (10⁻⁸) were used in all calculations.

Since the calculations on the [Cu(H₂L)(OSMe₂)₂]²⁺system were

performed by using an unrestricted model, spin contamination²⁸was

assessed by a comparison of the expected diﬀerence between S(S + 1)

for the assigned spin state (S(S + 1) = 0.75) and the actual value of

⟨S²⟩. The results obtained indicate that spin contamination is

negligible [⟨S²⟩ − S(S + 1) = 0.002]. Throughout this work solvent

eﬀects (DMSO) were included by using the polarizable continuum

model (PCM), in which the solute cavity is built as an envelope of

spheres centered on atoms or atomic groups with appropriate radii. In

particular, we used the integral equation formalism (IEFPCM) variant

as implemented in Gaussian 09.²⁹

2

16.4 Hz), 3.80 (s, 2H, H7), 3.69 (d, 2H, H6B, J = 16.4 Hz).

Zn(H₂L)(NO₃)₂·1.5MeOH (Zn·L·NO₃). Light yellow solid. Yield 0.036

g, 48%. MS-ESI⁺, m/z (%BPI): [Zn(HL)]⁺, 531.1 (100%); [Zn-

(H₂L)]²⁺, 266.1 (3%). Elem. Anal. Calcd for C₂₆H₂₄N₈O₉Zn·

1.5MeOH: C, 46.8; H, 4.3; N, 15.9%; Found: C, 46.7; H, 3.7; N,

15.1%. Λ_M(methanol, 25 °C): 137 cm²Ω⁻¹mol⁻¹. IR: 3250−3160

ν(N−H), 1691 ν(CO), 1349 ν_s(NO₂), 1293 ν(NO₃) cm⁻¹. δ_H

(solvent DMSO-d₆, 300 MHz, 298 K): 9.28 (br, HNCO), 9.17 (br,

HNCO), aromatic protons appear at low ﬁeld as complicated

overlapping broad signals, 4.19 (br, −CH₂−), 3.75 (br, −CH₂−).

Zn(H₂L)(SO₄)·H₂O (Zn·L·SO₄). Light yellow solid. Yield 0.059 g,

86%. MS-ESI⁺, m/z (%BPI): [Zn(HL)]⁺, 531.1 (95%); [Zn(H₂L)]²⁺,

266.1 (100%). Elem. Anal. Calcd for C₂₆H₂₄N₆O₇SZn·H₂O: C, 48.2;

H, 4.0; N, 13.0%. Found: C, 48.2; H, 3.9; N, 12.8%. Λ_M(methanol, 25

°C): the low solubility of this complex in methanol has prevented us

from determining the conductivity value for this compound. IR:

3330−3200 ν(N−H), 1711 ν(CO), 1528 ν_a(NO₂), 1349 ν_s(NO₂),

1117 ν(SO₄) cm⁻¹. δ_H(solvent DMSO-d₆, 300 MHz, 298 K): 10.36

(br, 1H, H16), 9.76 (br, 1H, H14), 8.91 (d, 2H, ³J = 4.5 Hz), 8.44 (s,

1H), 7.88 (t, 2H, ³J = 7.7 Hz), 7.75 (m, 2H), 7.48−7.30 (m, 7H), 7.12

(t, 1H, ³J = 7.4 Hz), 6.97 (t, 1H, ³J = 7.3 Hz), 4.41 (d, 2H, H6A, ²J =

RESULTS AND DISCUSSION

■

Syntheses. The synthetic strategy used for the preparation

of H₂L is shown in Scheme 1. Compound 3 was prepared in

good yield (73%) by reaction of commercially available

2

16.0 Hz), 4.09 (s, 2H, H7), 3.92 (d, 2H, H6B, J = 16.0 Hz).

X-ray Crystal Structure Determinations. Single crystals were

obtained from solutions of the isolated compounds, as described

above. For the benzoate complex Cu·L·Bz, dark blue single crystals

were grown by slow evaporation of a solution in methanol containing

equimolar amounts of H₂L and copper(II) perchlorate in the presence

of an excess of tetrabutylammonium benzoate.

1

precursors 1 and 2. The H and ¹³C NMR spectra of 3

indicate the presence of two major isomers in solution with a

3:2 ratio, which are attributed to the (trans, trans) and (trans,

cis) conformations typically adopted by N,N′-diarylureas.³⁰N-

Alkylation of bis(pyridin-2-ylmethyl)amine (4) with 3 in

reﬂuxing acetonitrile in the presence of dipea and a catalytic

amount of KI gave H₂L in 48% yield. In contrast to compound

3, H₂L exists in solution as a single isomer, presumably the

(trans, trans) conformation.^30a

Three dimensional X-ray data were collected on a BRUKER-

NONIUS X8 APEX KAPPA diﬀractometer for Cu·L·NO₃, Cu·L·SO₄,

Cu·L·Bz, and Zn·L·Cl complexes. Data were corrected for Lorentz and

polarization eﬀects and for absorption by semiempirical methods²⁰

based on symmetry-equivalent reﬂections. Complex scattering factors

were taken from the program SHELX97²¹running under the WinGX

program system²²as implemented on a Pentium computer. The

structures were solved by Patterson methods with DIRDIF2008,²³

except that of Cu·L·SO₄, which was solved by direct methods with

SHELXS-97. All structures were reﬁned by full-matrix least-squares on

F². For the four compounds all hydrogen atoms were included in

calculated positions and reﬁned in riding mode, except those bonded

to the nitrogen atoms of the urea moiety, which were reﬁned either

freely (Cu·L·NO₃and Cu·L·SO₄) or in riding mode but with reﬁned

distances (Zn·L·Cl and Cu·L·Bz). Finally, the hydrogen atoms of the

water molecules in Cu·L·Bz were located in a diﬀerence electron-

density map and all the distances ﬁxed. The crystal structure of Cu·L·

NO₃shows positional disorder for the coordinated nitrate anion and

one pyridine arm of the ligand with an occupation factor of 0.57(3) for

the atoms labeled as A. For Zn·L·Cl we have cleaned up the data with

SQUEEZE²⁴in the last steps of the structure reﬁnement process to

avoid the problems generated by a disordered chloroform molecule

sitting in a special position. Finally, reﬁnement converged with

anisotropic displacement parameters for all non-hydrogen atoms for all

four crystals. Crystal data and details on data collection and reﬁnement

are summarized in Table 1.

Computational Details. All calculations presented in this work

were performed employing the Gaussian 09 package (Revision

B.01).²⁵Full geometry optimizations of Cu^IIand Zn^IIcomplexes

with the receptor H₂L were carried out in dimethyl sulfoxide solution

employing DFT within the hybrid meta-GGA approximation with the

TPSSh exchange-correlation functional.²⁶Calculations were per-

formed on the [M(H₂L)(OSMe₂)₂]²⁺(M = Cu, Zn) systems, which

explicitly include two DMSO ligands to satisfy the octahedral

coordination environment. Structure optimizations were also

performed on the hydrogen sulfate adduct [Zn(H₂L)(OSMe₂)(OS-

(O)₂OH)]⁺, which contains an explicit DMSO molecule and

[M(H₂L)Cl₂] complexes (M = Cu^IIor Zn^II). Input geometries were

generated from the crystallographic data of [Zn(H₂L)Cl₂]. For

geometry optimization purposes we used the standard Ahlrichs’

valence double-ξ basis set including polarization functions (SVP).²⁷

The coordination of H₂L to Cu^IIwas monitored by

spectrophotometric titrations. The UV/vis spectrum of H₂L

recorded in DMSO solution shows a band at 262 nm (ε =

45600 M⁻¹cm⁻¹) that is attributed to the so-called B-band

associated with a π → π* transition,³¹together with a weak

absorption at 350 nm (ε = 1870 M⁻¹cm⁻¹) due to π → π*

transitions centered on the nitro group. Upon addition of

Cu(TfO)₂, these absorption bands shift toward shorter

wavelengths as their intensity slightly decreases (Figure S1,

Supporting Information). These spectral changes indicate the

coordination of the pyridyl units to the metal ion. A 1:1

reaction stoichiometry was ascertained, as the data displayed a

single inﬂection point when the Cu^II/H₂L molar ratio is close

to one, and an isosbestic point at 290 nm. This is in agreement

with the formation of mononuclear complexes. Keeping in

mind the coordinating nature of DMSO, which is expected to

occupy the vacant positions of the coordination sphere of the

metal ions, these mononuclear complexes are denoted as

[M(H₂L)(OSMe₂)₂]²⁺(M = Cu, Zn), in agreement with their

optimized geometries obtained with DFT calculations. Notice

that addition of an excess of Cu^IIresults in the formation of a

band centered at 287 nm (ε = 3000 M⁻¹cm⁻¹) due to the

formation of [Cu(OSMe₂)₄]²⁺upon addition of an excess of

Cu(TfO)₂.³²

Reaction of H₂L with 1 equiv of MCl₂or hydrated M(NO₃)₂

or M(SO₄) (M = Cu or Zn) in methanol at room temperature

provided the desired complexes, which were isolated in 48−

86% yields, with the noticeable exception of Cu·L·NO₃, which

was obtained with a considerably lower yield (16%).

X-ray Crystal Structures. The solid state structures of Cu·

L·NO₃, Cu·L·SO₄, Cu·L·Bz, and Zn·L·Cl were determined by

using single crystal X-ray diﬀraction analyses. Bond distances

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Table 2. Bond Distances (Å) and Angles (deg) of the Metal Coordination Environments Determined by X-ray Diﬀraction

Measurements

Cu·L·NO₃

Cu·L·SO₄

Cu·L·Bz

Zn·L·Cl

M(1)−N(3)

M(1)−N(4)

M(1)−N(5)

M(1)−N(6)

M(1)−O(4)

M(1)−O(5)

2.987(1)

2.039(1)

1.996(1)

1.986(1)

2.354(1)

1.9501(9)

2.861(3)

2.010(3)

1.961(3)

1.955(3)

2.637(3)

1.918(2)

2.038(2)

1.978(2)

1.93(1)

2.370(3)

2.067(3)

2.055(3)

2.616(9)

1.91(1)

M(1)−O(10)

2.221(2)

M(1)−Cl(1)

M(1)−Cl(2)

2.327(1)

2.303(1)

75.8(1)

N(4)−M(1)−N(5)

N(4)−M(1)−N(6)

N(4)−M(1)−O(4)

N(4)−M(1)−O(5)

N(4)−M(1)−Cl(1)

N(4)−M(1)−Cl(2)

N(5)−M(1)−N(6)

N(5)−M(1)−O(4)

N(5)−M(1)−O(5)

N(5)−M(1)−Cl(1)

N(5)−M(1)−Cl(2)

N(6)−M(1)−O(4)

N(6)−M(1)−O(5)

N(6)−M(1)−Cl(1)

N(6)−M(1)−Cl(2)

O(4)−M(1)−O(5)

Cl(1)−M(1)−Cl(2)

O(10)−M(1)−N(4)

O(10)−M(1)−N(5)

O(10)−M(1)−N(6)

O(10)−M(1)−O(5)

82.88(9)

153.9(3)

82.29(4)

83.27(4)

97.02(4)

175.25(4)

84.0(1)

84.2(1)

77.2(1)

84.61(9)

175.3(1)

173.41(8)

87.97(8)

114.4(1)

158.0(3)

95.1(3)

165.56(4)

91.91(4)

96.81(4)

167.2(1)

89.8(1)

94.5(1)

98.49(9)

129.8(1)

89.68(4)

97.59(4)

84.4(1)

97.7(1)

98.9(4)

102.59(9)

107.4(1)

87.66(4)

99.85(9)

98.31(5)

107.08(8)

93.52(7)

100.9(3)

99.0(3)

a

Chart 1. Possible Conformations of [M(H₂L)]²⁺Complexes

a

Only one of the two possible enantiomers for each conformation is represented. Note that the tertiary amine nitrogen atom is placed behind the

coordinated metal ion. Possible intramolecular π−π stacking interactions are shown in green.

and angles of the metal coordination environments are given in

Table 2.

would imply that the two pyridyl groups are twisted like in the

“propeller” form, with the urea group bending in the opposite

direction “bent propeller”. The third and fourth conformations

may be denoted as “open wing butterﬂy” and “closed wing

butterﬂy” to emphasize the relative arrangement of the two

pyridyl groups.

Crystals of Zn·L·Cl (Figure 1) contain the [Zn(H₂L)Cl₂]

complex and a chloroform molecule. The metal coordination

environment in the [Zn(H₂L)Cl₂] complex can be described as

distorted trigonal bipyramidal. The equatorial plane of the

trigonal bipyramid is deﬁned by the two nitrogen atoms of the

Tridentate ligand H₂L may fold around the metal ion tilting

the pyridyl and phenylurea groups with four diﬀerent spatial

arrangements with respect to the metal−amine axis. This gives

rise to eight possible stereoisomers, existing as four

enantiomeric pairs that diﬀer in the layout of the pyridyl and

phenylurea groups (Chart 1). One of these conformations

resembles a propeller, with the three groups attached to the

central amine atom twisting either clockwise or anticlockwise in

the corresponding enantiomeric form. A second conformation

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and long (2.95−3.15 Å).³⁷The N−H···Cl contacts observed

can be therefore regarded as intermediate [N(3)−H(3)···

Cl(2)] or short [N(2)−H(2)···Cl(2)]. It was suggested that

hydrogen bonds involving N−H donors and Cl−M acceptors

(M = transition metal) are favored for M−Cl···H angles in the

range 100−110°.³⁸The values observed for the Zn^IIcomplex

reported here (∼80°, Table 3) are somewhat smaller, but they

point to an angular preference for angles close to the

perpendicular approach probably because of the higher basicity

of the halogen p-type lone pairs relative to the sp lone pair.³⁸

The crystal packing is determined by intermolecular halogen-

bonding interactions between the oxygen atoms of the nitro

group and a chloroform molecule that is bridging two complex

units, assisted by weak C−H···Cl hydrogen bonds with the

coordinated chloride anions. The two C−Cl···O distances

[Cl(3)···O(2) 3.12 Å, C−Cl(3)···O(2) 161.47° and Cl(5)···

O(1) 3.09 Å, C−Cl(5)···O(1) 147.37°] are shorter than the

sum of the van der Waals radii of Cl and O (1.75 and 1.52 Å,

respectively).³⁹However, the C−Cl···O angles deviate

considerably from the ideal value of 180°, which points to

rather weak interactions.

Figure 1. X-ray molecular structure of the Zn·L·Cl complex. A

chloroform molecule and hydrogen atoms, except those of the urea

group, are omitted for clarity. The ORTEP plot is at the 30%

probability level.

pyridyl groups [N(5) and N(6)] and one of the coordinated

chloride anions [Cl(2)], while the apical positions are occupied

by the amine nitrogen atom of the ligand [N(4)] and the

second chloride anion [Cl(1)]. The metal ion is placed 0.36 Å

above the plane deﬁned by the three donor atoms of the

equatorial plane. The N−M−N and Cl−M−N angles of the

equatorial plane (107−130°) are relatively close to those

expected for a regular trigonal bipyramidal coordination, while

the N(4)−M−Cl(1) angle (ca. 173.4°) deviates by less than 7°

from the ideal one (180°). This is in line with the value of the

index of trigonality τ, which amounts to 0.73 (τ = 0 for a

perfect square-pyramidal geometry and τ = 1 for an ideal

trigonal bipyramidal geometry).³³The Zn(1)−N(5) and

Zn(1)−N(6) distances are similar to those observed for

diﬀerent ﬁve-coordinated Zn^IIcomplexes with tripodal ligands

containing pyridyl units (2.02−2.09 Å), while the Zn(1)−N(4)

distance (2.370 Å) is somewhat longer than those observed for

related complexes with tripodal ligands.³⁴

Crystals of Cu·L·NO₃contain the [Cu(H₂L)(ONO₂)-

(OHMe)]⁺cation and a nitrate anion interacting with the

urea group of the ligand through a bifurcated hydrogen bond

(Figure 2). The ligand wraps around the metal ion, giving an

The ‘bent propeller’ conformation adopted by the ligand in

the [Zn(H₂L)Cl₂] complex is imposed by the presence of

intramolecular slipped π−π stacking interactions involving the

pyridyl ring containing N(5) and the benzylurea fragment.³⁵

The 1-(3-nitrophenyl)-3-benzylurea fragment is nearly planar,

with mean deviation from planarity of 0.06 Å. The mean-square

planes deﬁned by the 1-(3-nitrophenyl)-3-benzylurea and

pyridyl fragments intersect at 18.1°, and the shortest contacts

between them correspond to the N3···N5 (3.193 Å) and C13···

C16 (2.964 Å) distances. This conformation appears to favor

Figure 2. X-ray molecular structure of Cu·L·NO₃. Hydrogen atoms,

except those involved in hydrogen-bonding interactions, are omitted

for clarity. The ORTEP plot is at the 30% probability level.

N_pyridyl-M-N_pyridylangles of about 120°, and therefore a trigonal

bipyramidal coordination environment.

The coordinated chloride anion Cl(2) is involved in a

bifurcated hydrogen-bonding interaction with the NH groups

of the urea fragment (Table 3).³⁶The N(2)···Cl(2) distance is

somewhat shorter than the N(3)···Cl(2) one, while the N(2)−

H(2)···Cl(2) angle is closer to linearity in comparison with the

N(3)−H(3)···Cl(2) one. This indicates a stronger interaction

of the chloride anion with N(2). The D-H···Cl interactions

were categorized in three groups depending on the observed

H···Cl distances: short (≤2.52 Å), intermediate (2.52−2.95 Å),

“open wing butterﬂy” conformation. The 3-nitrophenyl group

and the N(2)−C(7)−O(3) fragment of the urea group are

nearly coplanar, with the corresponding least-squares plane

(mean deviation from planarity 0.045 Å) intersecting with the

least-squares plane deﬁned by the benzyl group and N(3) at

47.5°. This value clearly shows that the 1-(3-nitrophenyl)-3-

benzylurea unit is not planar, in contrast to the situation

observed for the [Zn(H₂L)Cl₂] complex. Most likely this is to

Table 3. Intramolecular Hydrogen Bonds Involving the Urea Group in Zn·L·Cl

d(D−H)

d(H···A)

d(D···A)

∠(DHA)

∠(ZnClH)

N(2)−H(2)···Cl(2) Å/deg

N(3)−H(3)···Cl(2) Å/deg

Zn−Cl(2)···H(2)/deg

0.85

2.42

2.58

3.240(3)

3.352(3)

163.6

152.2

80.9

78.7

Zn−Cl(2)···H(3)/deg

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Table 4. Hydrogen Bonds in Compounds Involving the Urea Group in Cu^IIComplexes

d(D−H)

d(H···A)

Cu·L·SO₄

d(D···A)

∠(DHA)

∠(XOH)

N(2)−H(2)···O(6) Å/deg

N(3)−H(3)···O(8) Å/deg

S(1)−O(6)···H(2)/deg

S(1)−O(8)···H(3)/deg

0.78(2)

0.80(2)

2.06(2)

2.03(2)

2.834(1)

2.819(1)

169.2(2)

169.8(2)

112.3

107.2

Cu·L·NO₃

N(2)−H(2)···O(9) Å/deg

N(3)−H(3)···O(9) Å/deg

N(8)−O(9)···H(2)/deg

N(8)−O(9)···H(3)/deg

0.80(3)

0.75(3)

2.10(3)

2.26(3)

2.857(3)

2.928(3)

160(3)

149(3)

116.3

140.3

Cu·L·Bz

N(2)−H(2)···O(7) Å/deg

N(3)−H(3)···O(8) Å/deg

C(34)−O(7)···H(2)/deg

C(34)−O(8)···H(3)/deg

0.79

0.75

2.21

2.12

2.976(3)

2.771(3)

161.5

145.5

99.3

126.4

maximize the hydrogen-bonding interaction of the non-

coordinated nitrate anion with the urea group of the ligand.

Ligand H₂L binds to the Cu^IIion in a distorted octahedral

coordination where the equatorial plane of the octahedron is

deﬁned by the three nitrogen atoms of the dpa unit [N(4),

N(5), and N(6)] and an oxygen atom of a coordinated nitrate

anion [O(5)]. The apical positions are occupied by the oxygen

atom of a methanol molecule [O(10)] and one of the oxygen

atoms of the nitrate anion [O(4)], both placed along the ideal

C₄perpendicular axis. The Cu(1)−O(10) and Cu(1)−O(4)

distances are very long, as expected for a tetragonal elongation

due to a strong Jahn−Teller distortion.⁴⁰The mean deviation

from planarity of the donor atoms of the equatorial plane is

0.11 Å, with the Cu^IIion being placed 0.335 Å above that plane.

The Cu(1)−O(10) distance [2.221(2) Å] is ca. 0.2−0.3 Å

longer than the distances to the donor atoms of the equatorial

plane. The cis angles of that plane range between 77.1 and 98.9°

and, therefore, are reasonably close to the expected values of a

regular geometry (90°). The angles deﬁned by the Cu(1)−

O(10) vector and the donor atoms of the equatorial plane

(93.5−107.1°) are also relatively close to the expected value

(90°).

[O(10)···O(7) 2.740(2) Å; O(10)−H(10)···O(7) 2.00(4) Å;

O(10)−H(10)−O(7) 161(4)°].

Crystals of Cu·L·SO₄(Figure 3) contain the [Cu(H₂L)-

(OH₂)(OS(O)₂O)] complex and two water molecules of

The geometrical data characterizing the asymmetrical

bifurcated hydrogen bond involving the urea group and the

uncoordinated nitrate anion are shown in Table 4. Both the

N···O(9) distances and N−H···O(9) angles point to a stronger

interaction of the anion with the hydrogen-bonding donor

N(2) compared with N(3). The N(8)−O(9)···H(2) angle

(116.3°) is close to the ideal value of 115 10°,⁴¹while the

larger value of the angle N(8)−O(9)···H(3) (140.3°) also

reﬂects a weaker interaction. Furthermore, the dihedral angles

O(8)−N(8)−O(9)-H(2) and O(7)−N(8)−O(9)-H(2) (4.2

and 174.7°, respectively) are close to the values providing

strong interactions (0° and 180°), while the corresponding

values involving H(3) deviate considerably from the ideal

angles [O(8)−N(8)−O(9)−H(3) = 71.1° and O(7)−N(8)−

O(9)−H(3) = 107.7°]. The interaction of the nitrate anion

with the urea receptor appears to also be reinforced by weak

C−H···O interactions involving oxygen atoms of the nitrate

anion and C−H groups of the 1-(3-nitrophenyl)-3-benzylurea

unit. Finally, a strong hydrogen-bonding interaction also exists

between the O−H group of the coordinated methanol

molecule and one of the oxygen atoms of the nitrate anion

Figure 3. X-ray molecular structures of Cu·L·SO₄(top) and Cu·L·Bz

(bottom). Hydrogen atoms, except those involved in hydrogen-

bonding interactions, are omitted for clarity. Two water molecules

present in the crystal lattice of Cu·L·SO₄are also omitted. The

ORTEP plots are at the 30% probability level.

crystallization that form a hydrogen-bonding network with

O(7) and O(8), while those of Cu·L·Bz contain the

[Cu(H₂L)(O₂CPh)(OH₂)]···O₂CPh entity (O₂CPh = Bz⁻).

Again, the Cu^IIions also present a distorted octahedral

coordination with a strong Jahn−Teller distortion.⁴⁰The

equatorial plane of the octahedron is deﬁned by the three

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nitrogen atoms of the dpa unit [N(4), N(5), and N(6)] and an

oxygen atom of a coordinated sulfate or benzoate anion

[O(5)], while the apical positions are occupied by one of the

nitrogen atoms of the urea group [N(3)] and the oxygen atom

of the water molecule [O(4)] placed along the ideal C₄

perpendicular axis. The cis angles among the donor atoms of

the equatorial plane range between 82.3 and 97.7°, while the

corresponding trans angles fall in the range 165.6−175.3°,

pointing to relatively small distortions of the metal coordina-

tion environments. Both complexes show an “open wing

butterﬂy” conformation, but the conformation of the 1-(3-

nitrophenyl)-3-benzylurea unit changes considerably depending

on the particular anion. Indeed, the least-squares plane deﬁned

by the 3-nitrophenyl group and the N(2)−C(7)−O(3)

fragment forms an angle of 55.8° with the plane deﬁned by

the benzyl group and N(3) in the sulfate complex, while this

angle is much smaller (47.5°) in the case of the nitrate anion.

These very diﬀerent values are probably related to the diﬀerent

shapes of the two anions. In the case of the sulfate complex,

cooperative binding involving simultaneous anion coordination

to the urea H-donor group and Cu^IIis observed. However, in

the benzoate analogue the urea moiety and the metal ion

interact each with a diﬀerent benzoate anion. The coordinated

water molecule in [Cu(H₂L)(O₂CPh)(OH₂)]···O₂CPh shows

hydrogen-bonding with an oxygen atom of the coordinated

benzoate anion [O(4)···O(6) 2.709(3) Å; O(4)−H(2W)···

O(6) 1.89(4) Å; O(4)−H(2W)−O(6) 159(4)°].

Figure 4. ¹H NMR spectrum (300 MHz, DMSO-d₆) of H₂L and

spectra recorded after addition of [Bu₄N]·Cl, [Bu₄N]·MeCO₂, or

diﬀerent Zn^IIsalts [H₂L/Zn^II(1:1) ensemble (40 mM)].

as Cl⁻causes the deshielding of H16 (9.95 ppm) and a

concomitant shielding of H14 (10.14 ppm), which indicates an

asymmetrical H-bond interaction where only H16 is interacting

with the anion. Anion binding polarizes the N−H16 bond, and

Both sulfate and benzoate anions provide a Y-shape

directional hydrogen-bonding interaction with the urea frag-

ment. The geometrical data characterizing the H-bonds (Table

4) point to a rather symmetrical interaction of the sulfate anion

with the urea receptor, with the two N−H···O contacts

showing very similar distances and angles. In the case of the

benzoate analogue, this interaction is less symmetrical, and the

N(urea)−O(benzoate) distances observed (2.770 and 2.975 Å)

situate them within the group of “moderate” hydrogen bonds

according to Jeﬀrey’s classiﬁcation.⁴²Furthermore, while the

C(34)−O(7)···H(2) and C(34)−O(8)···H(3) angles are

relatively close to the ideal value for a trigonal planar anion

1

therefore, the H NMR signal of this proton is deshielded

(through-space eﬀect). The increased partial negative charge on

the nitrogen atom of the N−H14 group is then delocalized over

the urea moiety (through-bond eﬀect), thereby inducing the

shielding of H14.^14b,16d,44

The presence of Zn^IIin solution drives to a diﬀerent

situation. Upon coordination of the metal ion, the signals of the

protons corresponding to the aromatic region shift with respect

to their positions in the free ligand and overlap as complicated

multiplets, which prevented their unequivocal assignment.

However, the proton signals of the NH urea fragment and

those of protons H6 and H7 can still be used to monitor the

interaction of the anion with the metal-based receptor. Thus,

addition of Zn(ClO₄)₂provokes an important shielding of the

N−H resonances, in particular that corresponding to H14,

which experiences an upﬁeld shift of ca. 2 ppm from 10.30 to

8.31 ppm while the signal due to H16 shifts upﬁeld by 0.44

ppm from 9.58 to 9.14 ppm. These unprecedented upﬁeld

shifts of the signals due to urea N−H protons can only be

explained by the coordination of the urea group to the Zn^IIion.

Indeed, the signal due to protons H6, which is observed as a

(115

12°),⁴¹the O(8)−C(34)−O(7)−H(2) and O(7)−

C(34)−O(8)−H(3) dihedrals (18.7 and 45.8°, respectively)

deviate considerably from the ideal values (0 and 180°). In the

case of the sulfate anion, both the S(1)−O(6)−H(2) and

S(1)−O(8)−H(3) angles are close to the ideal value (122

12°).⁴³Taken together, the data given in Table 4 indicate a

stronger interaction of the sulfate anion with the urea unit

compared with nitrate and benzoate anions.

Solution Structure. Before studying the ability of the Cu^II-

based ditopic receptor for anion recognition, we investigated

the structure of the Zn^IIanalogue in solution in the presence of

diﬀerent coordinating anions by using NMR spectroscopy. The

experiments were carried out in DMSO-d₆due to the low

solubility of these systems in water. Addition of 1 equiv of

1

singlet in the H NMR spectrum of the free ligand, is now

observed as an AB spin system (²J = 16 Hz) upon addition of

Zn(ClO₄)₂. It has been shown that Zn^IIcomplexes with

symmetrical tripodal ligands with “propeller-like” structures

provide single-line spectra due to a fast enantiomerization

process that does not require the cleavage of the Zn-donor

bonds.⁴⁵Thus, the diastereotopic nature of the signals due to

H6 is most likely an indication of an “open wing butterﬂy’

structure. Zn^IIcomplexes prepared with other weakly

coordinating anions such as TfO⁻provide identical spectra,

indicating a similar behavior of the metal-based receptor with

this anion. We notice that the molar conductivity value

obtained for this complex in methanol (184 cm²Ω⁻¹mol⁻¹) is

in agreement with a 2:1 electrolyte behavior, which indicates

−

MeCO₂, HSO₄, or H₂PO₄(as their tetrabutylammonium

salts) to a solution of the free receptor causes important shifts

of the two N−H signals that reﬂect the establishment of

directional H-bond interactions with these oxoanions. In the

−

case of MeCO₂, both proton signals experience a downﬁeld

shift to 11.93 and 11.16 ppm, respectively (Figure 4). A similar

−

spectral pattern is also observed for H₂PO₄(11.16 and 10.52

ppm) whereas addition of HSO₄⁻provokes negligible chemical

shift changes (10.32 and 9.57 ppm), pointing to a rather weak

interaction. On the contrary, addition of a spherical anion such

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that the perchlorate anion does not interact with the cation

complex in solution.

1

The H NMR spectrum of [Zn(H₂L)(SO₄)] (Figure 4)

shows that sulfate binding provokes an important deshielding

and broadening of the urea NH proton signals, which are

observed at 9.76 and 10.36 ppm. These chemical shift changes

are attributed to the formation of H-bonds between the anion

and the NH groups of the urea moiety, with concomitant

decoordination of the nitrogen atom of the urea unit upon

anion addition. On the other hand, the presence of 2 equiv of a

more basic anion such as acetate causes the deprotonation of

the urea unit, as the N−H14 signal disappears, while the

resonance due to N−H16 experiences a slight deshielding. The

addition of D₂O to a solution of the Zn²⁺complex in the

presence of 2 equiv of acetate causes the attenuation of the N−

H16 signal, while the 9.0−6.5 region remains unaﬀected, which

supports the deprotonation process (Figure S8, Supporting

Information). We also notice that the signals due to H6 protons

become a singlet upon MeCO₂⁻addition, which is indicative of

a “propeller-like” structure of the complex in solution. A similar

conformation is probably adopted in the presence of 2 equiv of

Cl⁻, where the H6 protons are also shown as a singlet.

However, in this case the signals due to the NH urea protons

are quite broad, reﬂecting dynamic exchange processes, with

their unequivocal assignment being impossible. This broad-

ening may be related to the presence of a dynamic exchange

process between a 1:1 electrolyte and a nonelectrolyte species

in solution, as suggested by the conductivity value obtained for

this complex in methanol (68 cm²Ω⁻¹mol⁻¹). Nonetheless, in

Figure 5. Geometries of the [M(H₂L)(OSMe₂)₂]²⁺complexes (M =

Cu or Zn) obtained from DFT calculations (TPSSh/SVP level) in

DMSO solution. Cu(1)−N(3) 2.680 Å, Cu(1)−N(4) 2.080 Å,

Cu(1)−N(5) 2.041 Å, Cu(1)−N(6) 2.037 Å, Cu(1)−O(4) 2.228 Å,

Cu(1)−O(5) 2.029 Å, Zn(1)−N(3) 2.621 Å, Zn(1)−N(4) 2.289 Å,

Zn(1)−N(5) 2.107 Å, Zn(1)−N(6) 2.118 Å, Zn(1)−O(4) 2.079 Å,

Zn(1)−O(5) 2.096 Å.

1

(Figure 5), as suggested by the shift of H(14) in the H NMR

spectrum in the presence of Zn(ClO₄)₂(vide supra). Further

support for the coordination of this urea nitrogen atom comes

from the ¹³C NMR spectra, where the signal due to the carbon

nucleus of the urea group is observed at 152.6 ppm for H₂L but

experiences a deshielding to 153.3 ppm in the perchlorate salt

of the Zn^IIcomplex. Complexation of substituted ureas to Cu^II

or Zn^IIis very rare, and to the best of our knowledge there is

only one example of a Cu^IIcomplex where an aromatic urea

group coordinates via the nitrogen atom.⁴⁶

DFT calculations performed on the [Zn(H₂L)(OSMe₂)(OS-

(O)₂OH)]⁺complex give a Zn(1)−N(3) distance of 2.441 Å

(Figure 6), which represents a slight shortening of ca. 0.18 Å

1

the H NMR spectrum of the Zn^IIcomplex recorded in the

presence of 10 equiv of Cl⁻, the urea NH proton signals appear

at 10.88 (H16) and 9.66 ppm (H14). These shifts indicate an

important deshielding of the NH signals upon Cl⁻addition,

which is in line with the decoordination of the urea group and

the establishment of a hydrogen-bonding interaction between

urea and the coordinated Cl⁻anions. This situation has also

been observed in the crystal molecular structure of Zn·L·Cl

described above.

As seen in the molecular crystal structures of Cu·L·SO₄and

Cu·L·Bz (vide supra), a weak interaction between the nitrogen

atom of the urea group N(3) and the Cu^IIion is observed. The

metal coordination environment in Cu^IIcomplexes appears to

be conditioned by a strong Jahn−Teller distortion that results

in very long Cu(1)−N(3) and Cu(1)−O(4) distances. One

could expect that these distances were shorter in the complexes

of the d¹⁰metal ion Zn^II. Unfortunately, we have not obtained

single crystals of the Zn(II) perchlorate complex, but the

minimized geometries obtained by DFT calculations (TPSSh/

SVP) on the [M(H₂L)(OSMe₂)₂]²⁺systems (M = Cu, Zn) in

DMSO solution are in agreement with this hypothesis. The

optimized geometry of [Cu(H₂L)(OSMe₂)₂]²⁺presents

Cu(1)−N(3) and Cu(1)−O(4) distances of 2.680 Å and

2.228 Å, respectively, while the remaining four donor atoms of

the ligand provide considerably shorter Cu-donor bonds

(2.03−2.08 Å, Figure 5). This is in line with an octahedral

coordination around Cu^IIwith strong Jahn−Teller distortion.

Analogous calculations performed on the [Zn(H₂L)-

(OSMe₂)₂]²⁺complex indicate octahedral coordination around

the metal ion with a Zn(1)−N(3) distance of 2.621 Å.

Although longer than the remaining bond distances of the

metal coordination environment, which fall within the range

2.08−2.29 Å, these data clearly indicate a weak coordination of

the nitrogen atom of the urea group N(3) to the metal ion

Figure 6. Geometries of the [Zn(H₂L)(OS(O)₂OH)(OSMe₂)]⁺(left)

and [Zn(H₂L)Cl₂] (right) complexes obtained from DFT calculations

(TPSSh/SVP level) in DMSO solution. Hydrogen atoms, except those

involved in hydrogen-bonding interactions, are omitted for the sake of

simplicity.

with respect to the distance obtained for [Zn(H₂L)-

(OSMe₂)₂]²⁺(Figure 5). The anion binds to the urea group

through directional H-bonds, in line with the deshielding of the

¹H NMR signals of the NH groups observed upon sulfate

addition. Furthermore, the AB spin system (²J = 16 Hz)

observed for protons H6 pointed to an “open wing butterﬂy”

structure of this complex in solution, in agreement with the

DFT optimized geometry obtained for HSO₄⁻adduct. A similar

structure was observed for the Cu^IIanalogue in the X-ray

molecular structure of Cu·L·SO₄described above. Taken

together, these results indicate that tetrahedral anions such as

−

HSO₄and SO₄are recognized in solution through a

cooperative binding involving H-bonding interaction with the

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Figure 7. Family of UV/vis spectra taken during the course of the titration of [Cu(H₂L)(OSMe₂)₂]²⁺(5 × 10⁻³M in DMSO) with a standard

solution (0.5 M in DMSO, 25 °C): (a) [Bu₄N]·HSO₄; (b) [Bu₄N]·NO₂; (c) [Bu₄N]·MeCO₂; and (d) [Bu₄N]·Cl. Inset: titration proﬁle at a

selected wavelength vs equivalents of anion and species distribution diagram (coordinated DMSO molecules were omitted for the sake of simplicity).

urea unit of the ligand and coordination to the metal ion (Cu^II

or Zn^II).

titration proﬁle is characteristic of a very weak binding, which

prevents an accurate calculation of the association constant.

Solutions of the Cu^IIcomplex (5 × 10⁻³M) were titrated

with standard solutions of the diﬀerent oxoanions (as their

tetrabutylammonium salts) up to a 10-fold excess. The

absorption spectrum of [Cu(H₂L)(OSMe₂)₂]²⁺shows a

broad band in the range 500−1200 nm with a maximum at

656 nm (ε = 100 M⁻¹cm⁻¹) characteristic of d−d transitions

centered on the Cu^IImetal ion. These spectral data are very

similar to those reported for the Jahn−Teller distorted

octahedral perchlorate derivative [Cu(dpa)(OHMe)](ClO₄)₂

(dpa = di(2-picolyl)amine) and related complexes,⁴⁷which

match the predictions of our DFT calculations. Thus, this band

The optimized geometry obtained for the [Zn(H₂L)Cl₂]

complex in solution (Figure 6) shows a cooperative binding on

anion recognition where the nitrogen atom of the urea

fragment N(3) is not coordinated to the metal ion. This

situation has been observed in the X-ray molecular structure of

Zn·L·Cl and is also in line with the shifts of the NH proton

1

signals of the urea group seen in the H NMR spectra. DFT

calculations provide a very similar structure for the Cu^II

analogue.

Study of the Interaction with Oxoanions. The

interaction of [Cu(H₂L)(OSMe₂)₂]²⁺(prepared in situ by

mixing stoichiometric amounts of H₂L and Cu(TfO)₂) with

is attributed to the d_xz,d_yz→ d_{x −y}(²B₁→²E) transition in a

tetragonal ligand ﬁeld.⁴⁸The molar conductivity value obtained

for the perchlorate analogue in methanol (164 cm²Ω⁻¹mol⁻¹)

is in agreement with a 2:1 electrolyte behavior, conﬁrming that

2

−

diﬀerent oxoanions such as NO₃, NO₂, HSO₄, H₂PO₄,

−

MeCO₂, and PhCO₂was followed by spectrophotometric

titrations in DMSO solution. For comparative purposes, we

have also investigated the interaction of the free ligand with the

corresponding tetrabutylammonium salts of these anions. The

results show that only acetate establishes weak H-bond

interactions with the free ligand (Figure S2, Supporting

Information). The titration proﬁle and the presence of only

one isosbestic point at 360 nm indicates the presence of a single

equilibrium in solution, in agreement with the formation of a

−

weakly coordinating anions such as ClO₄or TfO⁻do not

interact with the cation complex in solution.

Addition of NO₃⁻up to a 20-fold excess only provokes slight

changes in the absorption spectra, indicating a very weak

binding of this anion to the metal complex. Indeed, the

intermediate value obtained for the conductivity of this

complex in methanol (137 cm²Ω⁻¹mol⁻¹) is indicative of the

presence of an equilibrium in solution between 2:1 and 1:1

−

1:1 (MeCO₂/H₂L) adduct. The smooth curvature of the

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Table 5. Spectroscopic Data and Association Constants (log K Values) Obtained from Spectrophotometric Titrations in DMSO

Solutions (25 °C)

[Cu(H₂L)(X)]⁺

λ_max, nm (ε, M⁻¹·cm⁻¹

[Cu(H₂L)(X)₂]

λ_max, nm (ε, M⁻¹·cm⁻¹

)

X

log K₁₁

)

log K₁₂

−

MeCO₂

Cl⁻

>7

653 (96)

678 (109)

629 (109)

684 (86)

679 (141)

679 (89)

>7

1.48(1)

2.11(1)

2.29(1)

1.14(1)

744 (173)

664 (140)

720 (83)

−

NO₂

5.46(9)

3.81(4)

3.81(6)

3.37(9)

−

H₂PO₄

Br⁻

737 (197)

−

HSO₄

−

electrolyte species. However, addition of HSO₄to a solution

of [Cu(H₂L)(OSMe₂)₂]²⁺causes a red shift of the d−d band as

its intensity decreases, pointing to the coordination of the anion

to the metal center (Figure 7a), which retains a distorted

octahedral geometry. The titration proﬁle and the formation

during the course of the titration of two simultaneous isosbestic

points at 540 and 705 nm are indicative of the presence of a

single equilibrium in solution, which is in agreement with the

formation of a 1:1 complex. The analysis of the titration data

provided an association constant of log K₁₁= 3.37(9) (Table

5).

monodentate nitrite anion giving the expected trigonal

bipyramidal geometry, with the second nitrite anion being

involved in a hydrogen-bonding interaction with the N-

coordinated urea group.

Addition of acetate to a solution of [Cu(H₂L)(OSMe₂)₂]²⁺

gives rise to a more intricate behavior with very important

changes in the d−d absorption band (Figure 7c). The intensity

of the band at 656 nm decreases while experiencing a slight

blue shift to 653 nm, reaching the lowest absorption after

addition of 1 equiv of anion. After that, the intensity of the d−d

transition band suddenly increases as its maximum shifts to 699

nm. At the same time, the tail of a ligand-centered charge

transfer band from the −NH fragment to the −NO₂group,

characteristic of the deprotonation of the receptor, is also

observed in the high energy side of the spectrum. This CT

band is clearly observed when the titration is performed at low

concentration (10⁻⁴M) (see also Figure S6, Supporting

Information).^14b,cThe titration proﬁle shows a second

−

In contrast to the situation observed for HSO₄, two

diﬀerent processes could be identiﬁed during the course of the

titration of Cu^IIcomplex with NO₂(Figure 7b). Addition of

−

up to 1 equiv of NO₂⁻results in the coordination of the anion

to the metal ion to form a 1:1 ([Cu(H₂L)]²⁺/NO₂) complex

−

(eq 1). Addition of a second equivalent of NO₂provokes a

drastic structural change evidenced by the important variations

observed in the absorption spectra (eq 2) (solvent molecules

were omitted for the sake of clarity):

inﬂection point at a 1:2 ([Cu(H₂L)]²⁺/MeCO₂) stoichiom-

−

etry. Further addition of an excess of anion results in subtle

changes in the absorption spectrum.

[Cu(H₂L)(OSMe₂)₂]²⁺+ X⁻

These data are rationalized in terms of the formation of three

diﬀerent species in solution upon acetate addition. The ﬁrst

species formed during the course of the titration is

characterized by a very high association constant (log K >

7.0) and is attributed to the coordination of the anion to the

metal center according to the following:

⇆ [Cu(H₂L)(X)(OSMe₂)]⁺+ OSMe₂

(1)

[Cu(H₂L)(X)(OSMe₂)]⁺+ X⁻

⇆ [Cu(H₂L)(X)(OSMe₂)]⁺···X⁻

(2)

−

[Cu(H₂L)(OSMe₂)₂]²⁺+ MeCO₂

[Cu(H₂L)(X)(OSMe₂)]⁺+ X⁻

⇆ [Cu(H₂L)(O₂CMe)(OSMe₂)]⁺+ OSMe₂

(4)

⇆ [Cu(H₂L)(X)₂] + OSMe₂

(3)

The spectral changes recorded upon addition of H₂PO₄⁻to a

The second process represents deprotonation of the urea

moiety according to equilibrium 5, which involves the

formation of a hydrogen-bond complex between acetate and

its conjugated acid:^14c

solution of [Cu(H₂L)(OSMe₂)₂]²⁺in DMSO are very similar

−

to those observed for NO₂(Figure S3, Supporting

Information), indicating that both anions provide a very similar

behavior described by eqs 1 and 2. The equilibrium constants

determined for these processes amount to log K₁₁= 5.46(9)

and log K₁₂= 2.11(1) for NO₂⁻and log K₁₁= 3.81(4) and log

[Cu(H₂L)(O₂CMe)(OSMe₂)]⁺+ MeCO₂

−

⇆ [Cu(HL)(OSMe₂)]⁺+ [(MeCO₂)₂H]⁻

(5)

−

K₁₂= 2.29(1) for H₂PO₄, respectively.

−

In the case of NO₂anion, the spectral changes for the

The analysis of the titration data provides a pK_avalue of

10.12(3). This value is 17 orders of magnitude lower than that

of urea (pK_a= 26.9 in DMSO)⁵⁰and ∼9 units lower than the

value reported for N,N′-diphenylurea (pK_a= 18.7 in DMSO).⁵¹

The urea group in [Cu(H₂L)(O₂CMe)]⁺is even considerably

more acidic than that of 1-(3,5-bis(triﬂuoromethyl)phenyl)-3-

phenylurea (pK_a= 16.1 in DMSO),⁵¹which contains two −CF₃

substituents with strong electron-withdrawing ability. Thus, the

deprotonation of the urea moiety in [Cu(H₂L)(O₂CMe)]⁺can

only be explained by its N-coordination to the metal ion. The

deprotonation of the Zn^IIanalogue after addition of 2 equiv of

acetate has also been observed in the ¹H NMR spectrum where

second step indicate the formation of a complex species with

trigonal bipyramidal coordination around the Cu^IIion. Indeed,

trigonal bipyramidal Cu^IIcomplexes usually exhibit a broad

band extending from 500 to 1000 nm with two components.⁴⁹

Although the low energy band is often more intense than the

high-energy counterpart, in the present case this situation

appears to be reversed, with the band envelope showing a

shoulder on the low energy side. In this case, the three nitrogen

atoms of the dpa unit and one nitrogen atom of the urea group

also bind to the metal center as seen for Cu·L·Bz. The

coordination polyhedron might be completed with a

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one of the NH signals of the urea group disappears, while the

second NH signal experiences a slight deshielding (vide supra).

The red shift observed in the d−d transitions of the Cu^II

complex is also indicative of a structural change from the Jahn−

Teller distorted octahedral coordination to a trigonal

bipyramidal geometry upon complex deprotonation (see

Fluoride displays a very intricate behavior. Addition of an

excess of F⁻to a solution of the free receptor H₂L results in the

formation at 325 nm of the typical charge-transfer transition

from the −NH fragment to the −NO₂group. This is indicative

of the double deprotonation of the urea group and formation of

the biﬂuoride anion [HF₂]⁻(Figure S2, Supporting Informa-

tion).¹⁴Further support for the deprotonation of the receptor

comes from the observation of the ¹⁹F NMR signal of [HF₂]⁻

at δ = −144 ppm,⁵²and the ¹H NMR signal of the same species

at δ = 16.0 ppm.

−

above). Addition of an excess of MeCO₂results in the

formation of an additional species, presumably due to the

coordination of the anion to the metal ion according to the

following:

In the presence of Cu^IIor Zn^II, the metal-based receptor

undergoes stepwise deprotonation of the two NH fragments of

the urea subunit, events that are signaled by the development of

vivid colors. The typical CT band due to the double

deprotonation of the urea moiety is clearly observed when

the titration is performed at low concentration (10⁻⁴M)

(Figure S6, Supporting Information). The titration proﬁles

obtained upon F⁻addition to [Cu(H₂L)]²⁺in DMSO solution

show four inﬂection points at Cu²⁺/F⁻molar ratios of 1:1, 1:2,

1:3, and 1:4 (Figure S7, Supporting Information). The ﬁrst of

these processes involves the interaction of a F⁻anion with the

urea group via hydrogen-bonding. Addition of a second

equivalent of anion provokes the monodeprotonation of the

urea group. The third equivalent of added anion interacts with

the urea NH group via hydrogen-bonding, while addition of a

fourth equivalent promotes the deprotonation of the second

urea NH group. The double deprotonation of ureas by F⁻with

concomitant formation of [HF₂]⁻has been observed

previously.^14eDeprotonation of the receptor is again conﬁrmed

by the observation of the ¹⁹F NMR signal of [HF₂]⁻at δ =

−

[Cu(HL)(OSMe₂)]⁺+ MeCO₂

⇆ [Cu(HL)(O₂CMe)] + OSMe₂

(6)

The equilibrium constant determined for this process is low

and amounts to log K = 1.06(3).

Addition of PhCO₂⁻to a solution of [Cu(H₂L)]²⁺in DMSO

provokes spectral changes very similar to those observed upon

−

MeCO₂addition (Figure S4, Supporting Information),

indicating that reactions 3−5 are also valid to account for the

speciation in solution in the presence of PhCO₂. The pK_a

value of the urea group obtained from the titration with

PhCO₂(pK_a= 9.30) is very similar to that obtained for

MeCO₂, while the association constant for the third step [log

−

K = 1.09(3), eq 6] is identical within experimental error.

Study of the Interaction with Halides. In the absence of

a bound metal ion, halide anions (except F⁻) establish very

weak H-bond interactions with the ligand, which prevented the

determination of reliable association constants for the

formation of 1:1 adducts. As previously observed for the

oxoanions, the intramolecular hydrogen-bonding interactions

between the NH fragment of the urea moiety and the pivotal

nitrogen of the dpa unit block the anion binding site, which

results in the observed weak interaction with Cl⁻(Figure 4).

Addition of Cl⁻or Br⁻to a solution of [Cu(H₂L)-

(OSMe₂)₂]²⁺in DMSO provokes a slight red shift of the d−

d absorption band from 656 to 678 nm (Cl⁻) and 679 nm

(Br⁻) up to 1 equiv of added halide anion (Figure 7d; see also

Figure S5, Supporting Information). Addition of an excess of

anion results in the formation of a very broad d−d absorption

band with a maximum at 744 nm (Cl⁻) and 737 nm (Br⁻),

characteristic of a trigonal bipyramidal coordination around the

metal ion.

The spectral changes can be rationalized in terms of the

equilibria (eqs 1 and 3) where the ﬁrst step involves the

coordination of an X⁻anion to the metal ion by replacing a

coordinated DMSO molecule. The resulting [Cu(H₂L)-

(OSMe₂)(X)]⁺cation retains the Jahn−Teller distorted

tetragonal geometry of the [Cu(H₂L)(OSMe₂)₂]²⁺species, as

indicated by the corresponding absorption proﬁles. The second

step implies the coordination of a second X⁻anion to the metal

ion with a concomitant change of the metal coordination

environment to trigonal bipyramidal [Cu(H₂L)(X)₂], where

the Cu^IIcomplex presents a “bent propeller” conformation with

cooperative binding mode observed in the X-ray molecular

structure of [Zn(H₂L)Cl₂] (Figure 1). The structural analysis

1

−144 ppm and the H NMR signal of the same species at δ =

16.0 ppm in the Zn^IIanalogue. The [HF₂]⁻species is not

interacting with the deprotonated metal-receptor, as demon-

1

strated by H DOSY measurements, which were recorded in

the presence of 5 equiv of F⁻(Figure S9, Supporting

Information). These studies provided a diﬀusion coeﬃcient D

= 7.96 × 10⁻¹⁰m²·s⁻¹for [HF₂]⁻, which gives a hydrodynamic

radius r_H= 1.29 Å using the Stokes−Einstein equation. As

expected due to its larger hydrodynamic radius, the [Zn(HL)-

(OSMe₂)]⁺gives a considerably lower diﬀusion coeﬃcient (D

= 2.17 × 10⁻¹⁰m²·s⁻¹).

Association Constants. The analysis of the spectrophoto-

metric titrations described above allowed us to calculate the

association constants for the interaction of the diﬀerent anions

with the Cu^IIcomplex (Table 5). The log K₁₁values obtained

−

for the diﬀerent anions follow the expected order: MeCO₂

∼

Cl⁻> NO₂⁻> H₂PO₄⁻∼ Br⁻> HSO₄⁻> NO₃. In the case of

−

MeCO₂and Cl⁻, the steep curvature of the titration proﬁles

−

indicates especially high equilibrium constants (log K₁₁> 7). In

particular, the p parameter was found to be close to 1.0 in both

cases (p = [complex concentration]/[maximum possible

complex concentration]), a condition that does not permit

the determination of a reliable equilibrium constant.⁵³On the

−

opposite side of this series of ligands is NO₃, which provides a

weak binding to [Cu(H₂L)]²⁺; a value of logK₁₁< 2 can be

estimated from the titration data.

1

It is clear that the observed sequence of anion aﬃnity does

not parallel the sequence in solvation terms. For instance, small

anions such as Cl⁻, with a relatively high charge to radius ratio,

should be highly solvated, and the endothermic desolvation

term should disfavor binding to the metal ion.⁵⁴The

association constant log K₁₁determined for Cl⁻is higher

than that with Br⁻, a fact that is in line with the thermodynamic

performed by H NMR spectroscopy for the related system

[Zn(H₂L)Cl₂] in DMSO solution (vide supra) suggested the

presence of dynamic exchange processes, presumably involving

the [Zn(H₂L)Cl₂] and [Zn(H₂L)Cl(OSMe₂)]⁺species. Addi-

tion of an excess of anion shifts the equilibrium to the

[Zn(H₂L)Cl₂] form, causing an important deshielding of the

NH proton signals.

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stability often observed for metal-halide complexes derived

from the direct charge−charge interaction with the metal ion,⁵⁵

even if this is not a general rule. In both cases, a cooperative

eﬀect between the coordinatively unsaturated metal ion and the

urea fragment, involved in a bifurcated hydrogen-bonding

interaction, reinforces the binding of halides to the metal ion.

However, the particularly high association constant observed

for Cl⁻is probably related to the better hydrogen-bonding

acceptor properties of this anion that could further stabilize

chloride compared to bromide.^8e

only one anion may interact with the urea group through

hydrogen bonds. The X-ray structure of the complex with

copper sulfate and DFT calculations performed in DMSO

solution point to an unexpected N-coordination mode of the

urea group to the metal ion in the absence of coordinating

anions. The N-coordination of the urea unit enhances its acidity

with respect to the free receptor, so that addition of basic

−

anions such as MeCO₂, PhCO₂, and F⁻promotes the

deprotonation of this group. The binding aﬃnity sequence of

the diﬀerent anions to [Cu(H₂L)(OSMe₂)₂]²⁺follows the

−

The observed binding aﬃnity toward the diﬀerent anions is

probably primarily related to the strength of the metal−anion

interaction, but steric eﬀects derived from the cooperative

hydrogen-bonding interactions probably also play an important

following trend: MeCO₂∼ Cl⁻> NO₂> H₂PO₄∼ Br⁻>

− −

HSO₄> NO₃, where a cooperative bifurcated hydrogen-

bonding interaction with halides seems to reinforce their

binding to the metal ion. This complex has a certain degree of

preorganization for the recognition of tetrahedral oxoanions

over trigonal planar ones. Indeed, the orientation of the

coordinated urea unit allows tetrahedral anions to interact

simultaneously with the coordinatively unsaturated metal

center and the urea group through directional hydrogen

bonds. Current eﬀorts are focused on the design of more

sophisticated receptors capable of enhancing the selectivity

toward tetrahedral anions in aqueous media for selective metal

salt extraction.

−

role. The especially high aﬃnity constants of NO₂and

−

MeCO₂can also be related to their relatively low steric

requirements and their ability to act as bidentate chelate

ligands. The lower log K₁₁value of the adduct with NO₂⁻is in

line with the lower partial charge of its oxygen atoms in

comparison to those of acetate (the atomic polar tensor (APT)

−

charges of the oxygen atoms of NO₂and MeCO₂calculated

at the TPSSh/6-311+G(d,p) level in DMSO solution are −1.00

and −1.27, respectively). The highest aﬃnity toward the

tetrahedral anions H₂PO₄⁻and HSO₄⁻compared with NO₃⁻is

also probably in line with the APT charges on the oxygen atoms

ASSOCIATED CONTENT

* Supporting Information

■

−

of these oxoanions (H₂PO₄, −1.25; HSO₄, −1.19; NO₃,

−0.99 at the TPSSh/6-311+G(d,p) level in DMSO solution)

and the cooperative eﬀect evidenced in the solid state for the

adduct with SO₄²⁻, where a N-coordination type of the urea

fragment is observed. Such a cooperative eﬀect likely prevents

S

X-ray crystallographic ﬁles in CIF format for Cu·L·NO₃, Cu·L·

SO₄, Cu·L·Bz, and Zn·L·Cl and optimized Cartesian

coordinates (Å) of the complexes investigated in this work,

main geometrical parameters of the minimum-energy con-

formations calculated at the (TPSSh/SVP) level, and UV/vis

spectra showing the formation of the Cu^IIcomplexes. This

material is available free of charge via the Internet at http://

pubs.acs.org.

−

the coordination of a second HSO₄anion, and as a result the

formation of a ﬁve-coordinate species with a “bent propeller”

conformation is not observed.

CONCLUSIONS

■

A series of metal complexes with the tripodal ditopic ligand

H₂L containing a dpa unit for cation binding and a urea motif

for anion recognition have been prepared and characterized.

The analysis of the X-ray molecular structures of Cu^IIand Zn^II

complexes derived from H₂L indicates a diﬀerent behavior of

the urea moiety when the receptor interacts with diﬀerent

anions. The solid state structure of the Cu^IIcomplex with

AUTHOR INFORMATION

Corresponding Author

*E-mail: david.esteban@udc.es.

■

Notes

The authors declare no competing ﬁnancial interest.

2−

ACKNOWLEDGMENTS

The authors thank Xunta de Galicia (EM 2012/088) and (CN

2012/011) for generous ﬁnancial support and Centro de

SO₄shows a cooperative binding involving simultaneous

■

coordination of the anion to the metal ion and to the urea

subunit through two directional hydrogen−bonding interac-

tions. A similar situation is observed in the Zn^IIcomplex with

Cl⁻, but only one of the coordinated anions interacts with the

urea fragment in a bifurcated mode. On the contrary, for anions

Supercomputacion

computer facilities. I.C.-B. thanks Ministerio de Educacion

Ciencia (FPU program) for a predoctoral fellowship.

́

de Galicia (CESGA) for providing the

́

y

−

such as NO₃and PhCO₂, two independent and non-

cooperative interactions are observed. On one side, the ﬁrst

anion interacts only with the coordinatively unsaturated metal

ion, while the second anion is involved in a directional

hydrogen-bonding interaction with the urea group, which is

projected away from the metal center.

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Cooperative anion recognition in copper(II) and zinc(II) complexes with a ditopic tripodal ligand containing a urea group

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