DOI: 10.1002/chem.201101372

A Trinuclear Copper(II) Cryptate and Its m3-CO3 Cascade Complex:Thermodynamics, Structural and Magnetic Properties

Pedro Mateus,[a] Rita Delgado,*[a, b] Francesc Lloret,*[c] Joan Cano,[c]

Paula Brand¼o,[d] and V�tor F�lix[e]

Introduction

Trinuclear copper clusters with trigonal symmetry play acentral role in the fundamental steps of biological catalysisby ubiquitous multicopper oxidases.[1] The study of modelcomplexes of these systems should not only provide betterunderstanding of the biological molecules but also assist inthe development of new low-molecular-weight catalysts[2]

and of new types of magnetic materials.[3]

Macrocyclic and macrobicyclic compounds with severaldonor atoms and cavities of suitable dimensions constitute aviable approach to coordinate two or more metal ions andbring them into close proximity so that the metal centresare able to interact magnetically or electronically, therebygiving rise to new properties or reactivities. Moreover, it iswell documented that dinuclear copper(II) complexes areuseful in the recognition and binding of anionic substrates.[4]

However, few complexes of macrocyclic ligands with threecopper(II) centres accommodated within the cyclic frame-work have been described[5] and only one involving a macro-bicycle was reported.[6]

Following our studies on anion recognition by macrocyclicand macrobicyclic receptors, a 2,4,6-triethylbenzene-cappedhexaamine macrobicycle with pyridyl spacers (pyr, seebelow) was studied and in its protonated forms proved to bean interesting anion receptor selective for dihydrogen phos-phate.[7] The structural motifs of this compound, the C3 sym-

Abstract: The 2,4,6-triethylbenzene-capped hexaamine macrobicycle withpyridyl spacers (pyr) was able to coor-dinate three copper(II) ions within itscavity. Potentiometric studies per-formed at 298.2 K in MeOH/H2O(50:50 v/v) and at ionic strength0.10 mol dm�3 in KNO3 revealed thattrinuclear species predominate in solu-tion from pH 5.0, the hydroxo com-plexes being the main species, whichstart forming at unusual very low pHvalues. The single-crystal X-ray deter-mination of the trinuclear complexshowed that the three copper centreshave square-planar geometry, arrangedin an almost equilateral triangle, and

have carbonate bridging the threemetal centres. The presence of carbon-ate resulted from fixation of atmos-pheric CO2. The present study repre-sents the first m3-CO3-bridged trinu-clear copper(II) complex located in theinterior of a macrobicyclic cavity. Themagnetic data of [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]·-ACHTUNGTRENNUNG(ClO4)4 showed ferromagnetic intramo-lecular interactions [J=3.80 cm�1,based on the Hamiltonian H=

�J(S1S2+S2S3 +S1S3)] yielding a spinquartet, S=3/2, ground state. Densityfunctional calculations on the experi-mental geometry of the trinuclear com-plex showed that the ferromagneticnature of the magnetic coupling can beattributed to the syn–anti conformationof the carbonato bridge, and a magne-to-structural correlation, based on thedifferent conformations (syn–anti, syn–syn and anti–anti), is presented. The in-teresting properties observed, namelythe lowering of the pKa of coordinatedwater molecules to unusual values andthe good fit of the carbonate anion be-tween the copper centres, derive fromthe special architecture of pyr.

Keywords: carbon dioxide fixation ·cryptands · density functional calcu-lations · magnetic properties · N li-gands

[a] P. Mateus, Prof. R. DelgadoInstituto de Tecnologia Qu�mica e Biol�gica, UNLAv. da Repfflblica—EAN, 2780-157 Oeiras (Portugal)Fax: (+351) 21-441-1277E-mail : delgado@itqb.unl.pt

[b] Prof. R. DelgadoInstituto Superior T�cnico, DEQBAv. Rovisco Pais, 1049-001 Lisboa (Portugal)

[c] Prof. F. Lloret, Dr. J. CanoDepartament de Qu�mica Inorg�nicaInstituto de Ciencia Molecular (ICMol)Universitat de Val�nciaC/Dr. Jos� Beltr�n 2, Paterna, Val�ncia 46980 (Spain)Fax: (+34) 963544441E-mail : francisco.lloret@uv.es

[d] Dr. P. Brand¼oDepartamento de Qu�mica, CICECOUniversidade de Aveiro3810-193 Aveiro (Portugal)

[e] Prof. V. F�lixDepartamento de Qu�micaCICECO and Secżo Aut�noma de CiÞncias da SaffldeUniversidade de Aveiro3810-193 Aveiro (Portugal)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201101372.

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FULL PAPER

metry and the three pyridyl spacers, prompted us to studyits copper(II) complexes in solution and in the solid state.Unexpectedly, the crystal X-ray diffraction characterisationof the trinuclear copper(II) complex revealed the presenceof a carbonate ion bridging the three copper(II) centres,formed by atmospheric CO2 fixation. Although a few exam-ples of m3-CO3-bridged trinuclear copper(II) complexesformed upon CO2 absorption have been reported, in allcases the trinuclear assembly is composed of three individu-al mononuclear complexes bound by carbonate,[8] whereasin the present study a single ligand coordinates the threecopper(II) ions which in turn are linked by carbonate, thusrepresenting the first m3-CO3-bridged trinuclear copper(II)assembly located in a macrobicyclic cavity.

Results and Discussion

Potentiometric studiesAcid–base behaviour : The protonation constants of pyr

were determined at ionic strength 0.10 mol dm�3 in KNO3

and at 298.2 K in H2O/MeOH (50:50 v/v), and the resultsare collected in Table 1. The mixed solvent system waschosen due to the precipitation of the compound at pH 7.22in pure water, which prevented the determination of all theprotonation constants in that medium. The present valuesagree well with those previously reported, obtained in tosy-late medium,[7] except K5

H and K6H, which are higher due to

the association of nitrate with the most protonated formsof pyr (log KH6pyrNO3

=2.54 in H2O/MeOH (50:50 v/v),T=298.2 K and I=0.10 mol dm�3 in KTsO).[7]

Complexation studies : The stability constants of pyr withcopper(II) were also determined at ionic strength0.10 mol dm�3 in KNO3 and at 298.2 K in H2O/MeOH (50:50v/v). The results are collected in Table 2.

When pyr was titrated in the presence of 0.5 equiv of cop-per(II) only mononuclear complexes were found (Fig-ure 1 a). At low pH copper coordinates to one of the pyrgroups, the two contiguous amines and one or more solventmolecules. As the pH increases, the successive deprotona-tion of the remaining ammonium groups takes place; how-

ever, due to the presence of themetal ion the basicity of theamines is decreased by an aver-age of 1.06 log units.

With the increase of theamount of copper to 1.0 equiv(Figure 1 b), dinuclear speciesstart to form and they are thepredominant species at Cu2+

/pyr 2:1 ratio (Figure 1 c). Thestability constant corresponding

Table 1. Overall (biH) and stepwise (Ki

H) protonation constants of pyr in H2O/MeOH (50:50 v/v).[a]

Equilibrium log biH[b] Equilibrium log Ki

H[c]

pyr+H+ÐHpyr+ 8.76(1) pyr+H+ÐHpyr+ 8.76 [8.72]pyr+2 H+ÐH2pyr2+ 16.70(1) Hpyr+ +H+ÐH2pyr2+ 7.94 [7.89]pyr+3 H+ÐH3pyr3+ 23.86(1) H2pyr2+ +H+ÐH3pyr3+ 7.16 [7.15]pyr+4 H+ÐH4pyr4+ 30.44(1) H3pyr3+ +H+ÐH4pyr4+ 6.59 [6.40]pyr+5 H+ÐH5pyr5+ 36.44(1) H4pyr4+ +H+ÐH5pyr5+ 5.99 [5.70]pyr+6 H+ÐH6pyr6+ 41.76(1) H5pyr5+ +H+ÐH6pyr6+ 5.32 [4.54]

[a] T= (298.2�0.1) K; I= (0.10�0.01) mol dm�3 in KNO3. [b] Values in parentheses are standard deviations inthe last significant figure. [c] Values in brackets were determined in H2O/MeOH (50:50 v/v), I= 0.10 mol dm�3

in KTsO (Ts= tosyl) and T =298.2 K.[7]

Table 2. Overall (log bMm Hh Ll) and stepwise (log KMm HhLl

) stability constants of the copper(II) complexes of pyr in H2O/MeOH (50:50 v/v).[a]

Equilibrium log bMm HhLl

[b] Equilibrium log KMm Hh Ll

Cu2+ +5 H+ +pyrÐ ACHTUNGTRENNUNG[CuH5 ACHTUNGTRENNUNG(pyr)]7+ 41.06(5) ACHTUNGTRENNUNG[CuH4ACHTUNGTRENNUNG(pyr)]6+ +H+Ð ACHTUNGTRENNUNG[CuH5ACHTUNGTRENNUNG(pyr)]7+ 2.56Cu2+ +4 H+ +pyrÐ ACHTUNGTRENNUNG[CuH4 ACHTUNGTRENNUNG(pyr)]6+ 38.50(1) ACHTUNGTRENNUNG[CuH3ACHTUNGTRENNUNG(pyr)]5+ +H+Ð ACHTUNGTRENNUNG[CuH4ACHTUNGTRENNUNG(pyr)]6+ 5.37Cu2+ +3 H+ +pyrÐ ACHTUNGTRENNUNG[CuH3 ACHTUNGTRENNUNG(pyr)]5+ 33.13(3) ACHTUNGTRENNUNG[CuH2ACHTUNGTRENNUNG(pyr)]4+ +H+Ð ACHTUNGTRENNUNG[CuH3ACHTUNGTRENNUNG(pyr)]5+ 6.28Cu2+ +2 H+ +pyrÐ ACHTUNGTRENNUNG[CuH2 ACHTUNGTRENNUNG(pyr)]4+ 26.86(3) ACHTUNGTRENNUNG[CuH ACHTUNGTRENNUNG(pyr)]3+ +H+Ð ACHTUNGTRENNUNG[CuH2 ACHTUNGTRENNUNG(pyr)]4+ 6.82Cu2+ +H+ +pyrÐ ACHTUNGTRENNUNG[CuH ACHTUNGTRENNUNG(pyr)]3+ 20.04(3) [Cu ACHTUNGTRENNUNG(pyr)]2+ +H+Ð ACHTUNGTRENNUNG[CuH ACHTUNGTRENNUNG(pyr)]3+ 7.67Cu2+ +pyrÐ[CuACHTUNGTRENNUNG(pyr)]2+ 12.37(3) pyr+Cu2+Ð[CuACHTUNGTRENNUNG(pyr)]2+ 12.37Cu2+ +pyrÐ[CuACHTUNGTRENNUNG(pyr)OH]+ +H+ 3.50(5) [Cu ACHTUNGTRENNUNG(pyr)OH]+ +H+Ð[Cu ACHTUNGTRENNUNG(pyr)]2+ 8.872Cu2+ + 2H+ + pyrÐ ACHTUNGTRENNUNG[Cu2H2ACHTUNGTRENNUNG(pyr)]6+ 33.69(2) ACHTUNGTRENNUNG[Cu2H ACHTUNGTRENNUNG(pyr)]5+ +H+Ð ACHTUNGTRENNUNG[Cu2H2 ACHTUNGTRENNUNG(pyr)]6+ 5.122Cu2+ + H+ +pyrÐ ACHTUNGTRENNUNG[Cu2H ACHTUNGTRENNUNG(pyr)]5+ 28.57(2) ACHTUNGTRENNUNG[Cu2ACHTUNGTRENNUNG(pyr)]4+ +H+Ð ACHTUNGTRENNUNG[Cu2H ACHTUNGTRENNUNG(pyr)]5+ 6.572Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu2 ACHTUNGTRENNUNG(pyr)]4+ 22.00(3) [Cu ACHTUNGTRENNUNG(pyr)]2+ +Cu2+Ð ACHTUNGTRENNUNG[Cu2 ACHTUNGTRENNUNG(pyr)]4+ 9.632Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu2 ACHTUNGTRENNUNG(pyr)OH]3+ +H+ 14.52(4) ACHTUNGTRENNUNG[Cu2ACHTUNGTRENNUNG(pyr)OH]3+ +H+Ð ACHTUNGTRENNUNG[Cu2ACHTUNGTRENNUNG(pyr)]4+ 7.483Cu2+ + H+ +pyrÐ ACHTUNGTRENNUNG[Cu3H ACHTUNGTRENNUNG(pyr)]7+ 32.01(3) ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(pyr)]6+ +H+Ð ACHTUNGTRENNUNG[Cu3H ACHTUNGTRENNUNG(pyr)]7+ 5.73Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)]6+ 26.3(1) ACHTUNGTRENNUNG[Cu2ACHTUNGTRENNUNG(pyr)]4+ +Cu2+Ð ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)]6+ 4.33Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)OH]5+ +H+ 21.84(2) ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(pyr)OH]5+ +H+Ð ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(pyr)]6+ 4.43Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)(OH)2]

4+ + 2H+ 15.14(3) ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(pyr)(OH)2]4+ +H+Ð ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)OH]5+ 6.70

3Cu2+ + pyrÐ ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)(OH)3]3+ + 3H+ 6.33(5) ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(pyr)(OH)3]

3+ +H+Ð ACHTUNGTRENNUNG[Cu3 ACHTUNGTRENNUNG(pyr)(OH)2]4+ 8.81

[a] T= (298.2�0.1) K and I= (0.10�0.01) mol dm�3 in KNO3. [b] Values in parentheses are standard deviations in the last significant figure.

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to the addition of Cu2+ to [CuACHTUNGTRENNUNG(pyr)]2+ is lower than the sta-bility constant of [Cu ACHTUNGTRENNUNG(pyr)]2+ by 2.74 log units due to thecharge repulsion between the two metal ions, which are inclose proximity, besides the statistical factor. Insertion of athird copper ion is even more disfavoured due to the build-up of positive charge, which results in a formation constantof only 4.3 log units. Nonetheless, at 3:1 Cu/pyr ratio (Fig-ure 1 d), trinuclear species predominate starting from pH 5.0but exist mainly in the form of hydroxo complexes. Thesehydroxo complexes start to form at a very low pH, mostlikely to lower the amount of positive charge within thecryptand cavity to minimise electrostatic repulsions. A 1:1ratio of hydroxide to copper ions was found, which suggestsa four-coordinate environment for each metal centre. How-ever, based only on the potentiometric measurements it isnot possible to establish whether the hydroxo groups bridgetwo copper centres.

Crystallographic studies : Crystals of the trinuclear coppercomplex were grown from H2O/MeOH at pH�6, by mixingpyr with 3 equiv of CuACHTUNGTRENNUNG(NO3)2 [or CuACHTUNGTRENNUNG(ClO4)2] followed byslow evaporation. The production of [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+

under different crystallisation conditions, that is, in the pres-

ence of NO3� or ClO4

� counter-ions, indicates that the car-bonate bridging ligand formed by absorption of CO2 fromthe atmosphere is preferred to nitrate in spite of the samegeometry of both anions and the higher concentration of ni-trate in solution. Interestingly, in another example of CO2

fixation by a dinuclear copper(II) cryptate,[9] the authors re-ported that in the presence of acetate and nitrate, theseanions bridged two copper centres to form dinuclear com-plexes instead of the carbonate-bridged complex.

The two single-crystal X-ray structure determinations ofthe copper complex revealed unit cell contents consistentwith molecular formulae [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·ACHTUNGTRENNUNG(MeOH)·2H2O (1) and [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(NO3)4·9H2O (2).In the nitrate salt 2, the copper complex displays a twofoldcrystallographic axis perpendicular to the CO3

2� bridgingligand plane intercepting its carbon atom and the 2,4,6-trie-thylbenzene ring caps. Consequently, in the crystal structure,the CO3

2� was found to be disordered over two positionswith the same probability. However, in both salts, the [Cu3-ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ has comparable distances and angles sub-tend at the copper centres, as can be seen from their dimen-sions given in Table 3. The molecular structure of [Cu3ACHTUNGTRENNUNG(pyr)-ACHTUNGTRENNUNG(m3-CO3)]4+ in 1 is shown in Figure 2 with the labellingscheme adopted, and its structure in 2 is presented in Fig-

Figure 1. Species distribution diagram calculated for copper(II)/pyr solutions: a) at 0.5:1 ratio, CCu = 1=2Cpyr =0.5 10�3 mol dm�3 ; b) at 1:1 ratio, CCu =

Cpyr = 1.0 10�3 mol dm�3 ; c) at 2:1 ratio, CCu =2Cpyr =2.0 10�3 mol dm�3 ; and d) at 3:1 ratio, CCu =3Cpyr =3.0 10�3 mol dm�3. Charges are omitted forclarity. This Figure is presented in colour in the Supporting Information.

Chem. Eur. J. 2011, 17, 11193 – 11203 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 11195

FULL PAPERA Trinuclear Copper(II) Cryptate

ure S1 in the Supporting Information. The two structuresare equivalent regarding the dimensions of the most rele-vant structural parameters associated with metal coordina-tion spheres, and henceforth subsequent discussion of thecomplex [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ is focused on the crystalstructure of 2. Each copper centre exhibits a distortedsquare-planar coordination environment composed of threenitrogen donors from the pyr macrobicycle and one oxygenatom of the m3-CO3 ligand. The Cu–O distances are compa-rable to those found previously for m3-CO3-bridged trinu-clear copper(II) complexes (see Table 4). The Cu–N distan-

ces are also in agreement withthose found for macrocycliccomplexes containing 2,6-bis(aminomethyl)pyridine clefts,and the distances to the pyri-dine rings are slightly shorterthan those to the aliphaticamines, as would be expectedfor metal complexes incorporat-ing this structural unit.[10] TheCO3

2� is encapsulated into themacrobicyclic cavity almostequidistant from two 2,4,6-trie-thylbenzene caps with thecarbon atom located at 2.923(4)and 3.060(4) � from benzenering least-square planes. Fur-

thermore, the three CuN3O coordination planes interceptthe carbonate plane at angles ranging from 46.27(3) to54.49(4)8. The intramolecular distances between the threecopper centres (4.9263(8), 4.8492(7) and 4.8381(7) �) indi-cate the absence of metal–metal bonds. The top view of thecomplex presented in Figure 2 shows that the 2,4,6-triethyl-benzene caps adopt a roughly eclipsed arrangement with anaverage rotation angle around the centroids of the benzenering of 17.08.

Obviously, in both nitrate and perchlorate salts, the [Cu3-ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ complex cation participates with theanions (NO3

� or ClO4�) and mother liquor solvent mole-

cules in extensive hydrogen bonds, which are not describedhere. Indeed, the detailed analysis of these bonding interac-tions is not significant to understanding the [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ chemistry reported.

CO2 fixation : CO2 fixation should derive from the nucleo-philic attack of the hydroxo group of the [Cu3 ACHTUNGTRENNUNG(pyr)OH]5+

complex (the main species present at pH�6) on the electro-philic carbon atom of CO2 (see Scheme 1), as observed inmono-[8,11] and dinuclear complexes[9,12] of copper(II) andother metal centres. In every m3-CO3-bridged trinuclear com-plex reported, CO2 fixation occurs in alkaline media (pH>

7.5). Curiously, the only exception to this was observed witha dinuclear copper(II) complex of a macrobicyclic com-pound,[9] which is also able to form hydroxo complexes at apH lower than usual. Apparently, the ability of macrobicy-clic compounds to bring into close proximity several metalions plays a crucial role in lowering the pKa of coordinatedwater molecules, which allows hydroxo complexes to beformed in slightly acid media that in turn permits CO2 fixa-tion to occur at a lower pH. This is similar, although with adifferent metal ion, to what happens in the hydration ofcarbon dioxide to yield bicarbonate by carbonic anhydrase,the catalytic activity of which requires a zinc-bound watermolecule with a pKa near 7, a lower than usual value whichresults not only from zinc coordination but also from assis-tance of nearby amino acid residues.[13] However, unlike car-

Table 3. Selected bond lengths [�] and angles [8] for [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ in salt complexes 1 and 2.

Complex 1[a]

Cu(1)�O ACHTUNGTRENNUNG(101) 1.902(3) Cu(2)�O ACHTUNGTRENNUNG(103) 1.907(3) Cu(3)�O ACHTUNGTRENNUNG(102) 1.907(3)Cu(1)�N(8) 1.914(4) Cu(2)�N(44) 1.930(4) Cu(3)�N(26) 1.917(4)Cu(1)�N(1) 2.072(4) Cu(2)�N(37) 2.027(4) Cu(3)�N(28) 2.099(4)Cu(1)�N(10) 2.077(4) Cu(2)�N(46) 2.056(4) Cu(3)�N(19) 2.070(4)N(1)-Cu(1)-N(10) 164.8(2) N(8)-Cu(1)-O ACHTUNGTRENNUNG(101) 173.3(2) N(37)-Cu(2)-N(46) 163.0(1)N(44)-Cu(2)-O ACHTUNGTRENNUNG(103) 168.3(2) N(37)-Cu(3)-N(46) 163.0(1) N(26)-Cu(3)-O ACHTUNGTRENNUNG(102) 173.1(1)Complex 2[a]

Cu(1)�O ACHTUNGTRENNUNG(103) 1.978(4) Cu(1)�N(10) 2.098(3) Cu(2)�N(24) 1.903(4)Cu(1)�N(8) 1.925(3) Cu(1)�O ACHTUNGTRENNUNG(101) 1.987(5) Cu(2)�N(19) 2.061(3)Cu(1)�N(1) 2.088(3) Cu(2)�O ACHTUNGTRENNUNG(102) 1.984(5)N(1)-Cu(1)-N(10) 165.7(1) N(19)-Cu(2)-N(19)[b] 164.4(2) N(8)-Cu(1)-O ACHTUNGTRENNUNG(101) 154.5(1)N(24)-Cu(2)-O ACHTUNGTRENNUNG(102) 154.8(1) N(8)-Cu(1)-O ACHTUNGTRENNUNG(103) 158.7(1)

[a] Nitrogen atoms labelled N(8), N(26) and N(44) in 1 and N(8) and N(24) in 2 are pyridine nitrogen atoms.The remaining nitrogen atoms belong to the amine groups. [b] Denotes the symmetry operation: 1�x, y,1.5�z.

Figure 2. Molecular structure of [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ in complex salt 1with the Cu3 ACHTUNGTRENNUNG(m3-CO3) central core drawn in ball-and-stick style. The per-spective view (top) shows the overall structure of the complex with theCO3

2� trapped in the macrobicyclic cavity, whereas the top view (down)emphasises the pseudo C3 symmetry of the complex cation. This Figure ispresented in colour in the Supporting Information.

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R. Delgado, F. Lloret et al.

bonic anhydrase that has low affinity for the bicarbonateanion and thus readily liberates it, the [Cu3ACHTUNGTRENNUNG(pyr)]6+ keepsCO3

2� tightly bound between the three copper(II) ions.Dinuclear metal complexes able to fix atmospheric CO2

can also further promote the reaction of the bridged carbon-ate with primary alcohols to yield monoalkyl carbona-tes.[9,12b,c] However, in the case of the trinuclear copper(II)cryptate, the esterification reaction did not take place inspite of the crystals being grown in methanol solution, dueto the fact that the carbonate is held tightly by the threecopper ions and kept inside the cryptand cavity.

IR and ESI mass spectra : The bands corresponding to thering-stretching vibrations of the pyridine n ACHTUNGTRENNUNG(C=C) andn ACHTUNGTRENNUNG(C=N) observed at 1594 and 1577 cm�1, respectively, forthe free ligand (Figure S2 in the Supporting Information)are shifted to higher frequencies in the complex (1613,1587 cm�1, see Figure S3 in the Supporting Information),which is characteristic of pyr nitrogen coordination.[14] Thestrong broad absorption band centred at 1100 cm�1, the veryweak absorption at 941 cm�1 and the strong sharp band at626 cm�1, along with the absence of a second band near650 cm�1, are consistent with the presence of non-coordinateperchlorate ions,[15] as observed in the crystal structure.

The carbonate species, as the free ion, exhibits a D3h

space group and displays three bands in the infrared spec-trum corresponding to the anti-symmetrical stretching, theout-of-plane bending, and the in-plane bending modes.However, changes will occur due to coordination, namelythe lowering of symmetry to C2v or Cs and consequent lossof degeneracy of the bands.[14] The carbonate-related bandsare somewhat difficult to assign unambiguously, due to over-

lap of ligand-related bands. Nonetheless, the strong band ob-served at 1456 cm�1, along with two shoulders at 1479 and1497 cm�1, should correspond to the C–O anti-symmetricalstretching of carbonate, although the CH2 in-plane bendingof the ligand also occurs in this region (1452 cm�1). The out-of-plane bending mode appears as a weak band at 814 cm�1

with a shoulder at 820 cm�1 and the in-plane bending as twoweak bands at 793 and 772 cm�1.

The ESI mass spectrum acquired in positive polaritymode for a solution of the crystals of [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]·ACHTUNGTRENNUNG(ClO4)4·H2O dissolved in H2O/MeOH (50:50 v/v) at pH 6.0(Figure S4 in the Supporting Information) showed only onepeak at m/z 1357.0 corresponding to the {[Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)3}

+ ion, which indicates that the structure ofthe complex is retained in solution.

Magnetic properties : The magnetic properties of [Cu3ACHTUNGTRENNUNG(pyr)-ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O, in the form of the cMT versus T plot,are shown in Figure 3 (cM being the magnetic susceptibilityper three copper(II) ions).

At room temperature cMT is 1.21 cm3 mol�1 K, a valuewhich is expected for three magnetically isolated spin dou-blets. Upon cooling, cMT increases monotonically to attain avalue of 1.90 cm3 mol�1 K at 2.0 K. The shape of this plotclearly shows the occurrence of intramolecular ferromagnet-ic interactions, and so a spin quartet (S=3/2) as groundstate is expected. Having in mind the triangular topology ofthe three crystallographically independent copper(II) ions in[Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ (see Figure 2), three different ex-change pathways through a syn–anti carbonato bridge couldbe expected (J12, J13 and J23). However, because of the simi-larity of the structural parameters for all these three inde-pendent CuII ions, we can assume J12 = J13 =J23 = J. Takinginto account this coupling scheme and considering that the

Scheme 1. Proposed mechanism for CO2 fixation by [Cu3ACHTUNGTRENNUNG(pyr)]6+ . Coor-dinated water molecules and charges are omitted for simplicity.

Figure 3. Temperature dependence of the product of the magnetic sus-ceptibility and temperature for [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ . The solid line is thetheoretical curve based on Equation (1). The inset shows the Q-bandEPR spectrum of a powder sample of [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ at 5 K.

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FULL PAPERA Trinuclear Copper(II) Cryptate

zero-field splitting of the ground spin quartet is axial (2D isthe energy gap between the �3/2 and �1/2 Kramers dou-blets), the derived expression for the magnetic susceptibilitycould be given by Equation (1)[5g]

cM ¼Nb2g2

4kðT � qÞð3þ 2=dÞ þ ð3� 2=dÞ expð�2dÞ� þ 2 expð�jÞ

1þ expð�2dÞ þ 2 expð�jÞ

� �

ð1Þ

in which q is the Weiss parameter for the intermolecular in-teractions, d=D/kT and j=3J/2kT (for jD j < jJ j). We haveassumed the same value of g for all three copper(II) ionswith gjj ¼ g? ¼ g. Least-squares fitting of the magnetic dataof [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ through Equation (1) leads to J= ++

3.80(2) cm�1 and g=2.08(1). The parameters D and q arestrongly correlated and can be varied in the range jD j = 0–0.1 cm�1 and �q=0.1–0.14 K, thereby indicating that boththe intermolecular interactions (q) and the zero-field split-ting effects (D) of the low-lying spin quartet (S=3/2) arevery small. In this respect, the Q-band (hn�35 GHz�1.2 cm�1) EPR spectrum on a polycrystalline sample of[Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ at 5 K (see inset of Figure 3) shows aquasi-isotropic signal at g =2.089, which indicates that D�0.[16]

Figure 4 shows the experimental isothermal magnetisationcurves at different temperatures (2–15 K). The saturationmagnetisation tends to 3 mB, as expected for the S= 3/2ground state. The plot of M versus the H/T ratio shows thatthe different isothermal curves do not collapse on a uniquemaster curve. This fact is not due to the existence of a zero-field splitting of the S= 3/2 ground state but to the differentpopulation of the excited spin doublets (S=1/2), which areseparated from the ground state by 3J/2�5.7 cm�1 (seeFigure 5).

In fact, we can fit these experimental magnetisationcurves by using the isotropic Hamiltonian. The best-fit pa-rameters obtained from a numerical matrix diagonalisationmethod through the Hamiltonian of Equation (2) areJ=4.20(3) cm�1 and g= 2.07(1).

H ¼ �JðS1S2 þ S2S3 þ S1S3Þ þ gbHX3

i¼1Si ð2Þ

A fit taking into account both the experimental magnetisa-tion and cMT curves simultaneously through Equation (2)yields J= 4.0 (1) cm�1 and g=2.07(1) with D and q equal tozero.

Density functional calculations and magneto-structural cor-relations : Density functional (DF) calculations on the exper-imental geometry of the trinuclear complex were carried outto estimate the three magnetic coupling constants present.The values obtained (J= ++ 5.2, + 5.6 and +6.5 cm�1) areclose to those found from the experimental data (J= ++

4.0 cm�1) and they are included in Table 4 with some oftheir more relevant structural parameters.

It deserves to be noted that these values are also close toeach other because there are small geometrical differences.The ferromagnetic nature of the interactions leads to a spinquartet (S=3/2) ground state. However, the magnetic cou-plings are weak enough to allow the nearby presence of theexcited doublet states (see Figure 5) accounting for the ob-served thermal dependence of the magnetisation curves(Figure 4). The ferromagnetic nature of the magnetic cou-pling can be attributed to the syn–anti conformation ofthe carbonato bridge (Figure 6 c), as known for othercopper(II) complexes in which a carboxylate[17] or carbona-te[8b,d–h,j,k,m,o,p, 18] group adopts the syn–anti conformation.

To study the influence of the different carbonato-bridgeconformations (syn–anti, syn–syn or anti–anti) on the natureand magnitude of the magnetic coupling (J), we considereda copper(II) dinuclear model (see Experimental Section).The CuOO angles (see Figure 6 a) were chosen to visualisethe way that syn–syn is transformed into syn–anti and anti–anti conformations. In this model, the carbonate group and

Figure 5. Variation of the spin-state energies (E/J versus the x= j/J ratio)for an isosceles triangle with local spins S1 = S2 =S3 =1/2, with thecommon notation jS,S*> , in which S is the total spin (S=S1 +S2 +S3)and S*= S1 +S3.

Figure 4. Magnetisation curves for [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ at different tem-peratures. The solid lines are the theoretical curves using the best-fit pa-rameters (see text).

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R. Delgado, F. Lloret et al.

copper atoms were kept in the same plane. Thus, an idealsyn–syn conformation is found when the CuOO angle is 908for both metal ions (see Figure 6 b). The magnetic couplingparameters (J) calculated through this model are displayedin Figure 7 as a contour map for each value of J as a func-tion of the CuOO angle of both copper(II) ions.

These results show that antiferromagnetic couplings pre-vail in a syn–syn conformation because the overlapping be-tween the magnetic orbitals is too far to be cancelled (Fig-ure 6 b). This overlap increases for an anti–anti conformation(1808 for both CuOO angles; Figure 6 d), thereby yielding astronger antiferromagnetic interaction.

As shown by Figure 7, ferromagnetic coupling can be ob-served for the syn–anti conformation (about 908 for oneCuOO angle and about 1808 for the other), which can beexplained by an accidental orthogonality between the mag-netic orbitals (Figure 6 c). Table 4 shows the most relevantmagnetic (J) and structural (CuOO angles) parameters forthe m3-carbonato tricopper(II) complexes, the magnetismand structure of which have been studied as far as we know.

Table 4. Magneto-structural data for the compounds in which a carbonate group acts as a bridge between three copper(II) ions through only the OCOpathway.

Label Compound[a] J[b] Cu�O[c] Cu1OO[d] Cu2OO[d] q[e] Ref. code[f] Ref.

1 ACHTUNGTRENNUNG[Cu3(L4)6ACHTUNGTRENNUNG(m3-CO3)] (BF4)4 29.3 1.9877.0 204.5

1.8 MOGPUT [8j]1’ 79.5 222.01’’ 96.3 220.92 ACHTUNGTRENNUNG[Cu3(L5)3ACHTUNGTRENNUNG(m3-CO3) ACHTUNGTRENNUNG(ClO4)3] (ClO4) 18.0 1.97 81.0 219.0 2.1 RAMVIL [8m]

3 ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(L10)3 ACHTUNGTRENNUNG(m3-CO3)] (ClO4)4 17.2 2.00 82.1 217.9 0.0 NOJZOB [8f]

4 ACHTUNGTRENNUNG[Cu3(L2)3ACHTUNGTRENNUNG(m3-CO3) ACHTUNGTRENNUNG(ClO4)3] (ClO4) 12.6 1.97 88.7 211.7 5.5 ZOKDIM [8d]

5 ACHTUNGTRENNUNG[Cu3(L6)3ACHTUNGTRENNUNG(m3-CO3) ACHTUNGTRENNUNG(ClO4)2ACHTUNGTRENNUNG(H2O)] (ClO4) 12.6 1.9989.1 213.3

OGEHIR [8k]5’ 110.4 211.8 2.35’’ 87.0 199.06 ACHTUNGTRENNUNG[Cu3(L1)3ACHTUNGTRENNUNG(m3-CO3)] (ClO4)4 11.3 1.98 87.8 212.2 1.2 IFIDOR [8p]

7 ACHTUNGTRENNUNG[Cu3(L8)6ACHTUNGTRENNUNG(m3-CO3)] (BF4)4 10.6 2.0288.4 223.5

WOJSOD [8h]7’ 81.0 212.7 12.47’’ 78.5 218.78 ACHTUNGTRENNUNG[Cu3(L9)3ACHTUNGTRENNUNG(m3-CO3) ACHTUNGTRENNUNG(H2O)3] (NO3)4 9.6 1.96 –[g] –[g] –[g] COPYCU [8b]

9 ACHTUNGTRENNUNG[Cu3(L3)3ACHTUNGTRENNUNG(m3-CO3)] (ClO4)4 8.2 1.93 97.2 204.3 8.7 ZINXAV [8e]

10 ACHTUNGTRENNUNG[Cu3(L7)6ACHTUNGTRENNUNG(m3-CO3)2 ACHTUNGTRENNUNG(ClO4)2 ACHTUNGTRENNUNG(CH3OH)0.5]n 6.2 1.95 77.6 222.4 0.2 WANLED [8o]

11 ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(L11)ACHTUNGTRENNUNG(m3-CO3)] (ClO4)4·H2O 4.0 1.9092.1 205.1

– this work11’ 91.3 208.6 5.011’’ 94.712 ACHTUNGTRENNUNG[Cu3ACHTUNGTRENNUNG(L12)3 ACHTUNGTRENNUNG(m3-CO3)] (ClO4)4 �1.2 1.95 –[g] –[g] –[g] MUWWOQ [8g]

[a] L1 =N-(2-thiophenoethyl)-N,N-bis(3-aminopropyl)amine; L2 =bis(3-aminopropyl)methylamine; L3 =1,4-dioxa-7,10,13-triazacyclopentadecane;L4 =4,4’-dimethyl-2,2’-bipyridine; L5 =methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine; L6 =bis(3-aminopropyl)amine; L7 =4-aminopyridine; L8 =2,2’-bi ACHTUNGTRENNUNGpyridine; L9 =2-[2-(2-pyridyl)ethyliminomethyl]pyridine; L10 =1,4,7,10-tetraazabicycloACHTUNGTRENNUNG[5.5.3]pentadecane; L11 =pyr; L12 = tris-(pyridylmethyl)amine.[b] In cm�1. [c] Average distance in angstroms. [d] In degrees. [e] Angle (in degrees) that measures the planarity of the fragment Cu carbonate, whichtakes a null value when all atoms in the fragment are placed in the same plane. [f] Code from Cambridge Database. [g] No data available from Cam-bridge Database.

Figure 6. a) Definition of the CuOO angles. Schematic views of the over-lapping between magnetic orbitals in the b) syn–syn, c) syn–anti andd) anti–anti conformations.

Figure 7. Contour map showing the dependence of the magnetic couplingconstant J (in cm�1) on the CuOO angles (in degrees). The syn–syn, syn–anti and anti–anti conformations are shown in the corresponding cornersof the plot. Antiferro- and ferromagnetic couplings are represented byAF and F, respectively.

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A label has been assigned to each complex according to themagnitude of the magnetic coupling (a stronger magneticcoupling receives a lower label) and they are placed in thecontour map of Figure 8 as a function of their CuOO angles.Figure 8 is similar to Figure 7 but only for the narrowdomain of CuOO known angles of the complexes (Table 4).

However, it is not easy to establish a good magneto-struc-tural correlation for the following reasons: 1) the magneticcoupling depends at least on two CuOO angles, 2) in manycases the copper(II) ions are structurally independent (threedifferent CuOO angles) but only a magnetic coupling hasbeen considered in the analysis of the experimental magnet-ic data and 3) the number of compounds reported in the lit-erature is scarce (only ten complexes are fully character-ised). In any case, when Figure 8 is carefully examined, agood agreement is evidenced between the experimental andtheoretical results. Nevertheless, the experimental couplingsfor compounds labelled 7 and 10 are much weaker thanthose expected from the theoretical study. Although gener-ally the copper(II) ions are placed in the same plane of thecarbonate group (see q angle in Table 4), it is not the casefor complex [Cu3(L8)6 ACHTUNGTRENNUNG(m3-CO3)] (BF4)4 (7), which presentsthe larger values of q (12.48) and Cu�O bond length in thisfamily. Clearly, both factors induce a decrease of the antifer-romagnetic coupling because the spin delocalisation and theoverlapping between magnetic orbitals are reduced. On theother hand, the structure of the compound [Cu3(L7)6ACHTUNGTRENNUNG(m3-CO3)2 ACHTUNGTRENNUNG(ClO4)2ACHTUNGTRENNUNG(CH3OH)0.5]n (10) is a complicated 2D net-work, for which the law to simulate the magnetic behaviouris unknown at present, so the authors have fitted the experi-mental data using a law developed for a simpler hexagonalnetwork.

Conclusion

Solution studies of the copper(II) complexes of a macrobicy-clic ligand showed that the trinuclear species predominatefrom pH 5.0, the main species being the hydroxo complexes.The stepwise stability constants of the complexes formed be-tween copper(II) and pyr decrease in the order [Cu ACHTUNGTRENNUNG(pyr)]2+

> [Cu2ACHTUNGTRENNUNG(pyr)]4+> [Cu3ACHTUNGTRENNUNG(pyr)]6+ mainly due to the build-up ofpositive charge in the macrobicyclic cavity. To lower thispositive charge and minimise the electrostatic repulsions,the hydroxo complexes start forming at very unusual lowpH values.

Crystals of the trinuclear copper complex grown at pH�6revealed the presence of carbonate (formed by spontaneousCO2 uptake from air) bridging the three copper centres.CO2 fixation is likely to derive from the nucleophilic attackof the hydroxo group of the [Cu3ACHTUNGTRENNUNG(pyr)OH]5+ complex onthe electrophilic carbon atom of CO2. Apparently, the abili-ty of pyr to bring into close proximity three metal ions playsa crucial role in lowering the pKa of coordinated water mol-ecules, which allows hydroxo complexes to be formed inslightly acid media that in turn permits CO2 fixation tooccur without the need for high pH. The Cu–Cu distances ofthe [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ complex are comparable to thoseof the known examples of m3-CO3-bridged trinuclear cop-per(II) complexes formed by assembly of three individualmononuclear complexes[8] for which no special constraintsexist.

The magnetic data of [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]4+ showed theexistence of ferromagnetic intramolecular interactionsthrough the carbonato bridge in anti–syn conformation. TheQ-band EPR spectrum at 5 K clearly shows that the spinquartet, S=3/2, ground state is basically degenerated (D�0). DF calculations indicated that the ferromagneticnature of the magnetic coupling could be attributed to thesyn–anti conformation of the carbonato bridge.

In conclusion, the architecture of pyr is responsible forthe interesting properties observed, namely lowering of thepKa of coordinated water molecules to unusual values andthe good fit of the carbonate anion between the copper cen-tres that allows its encapsulation.

Experimental Section

General considerations : The pyr compound was synthesised as describedpreviously from 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene and 2,6-pyridinedicarbaldehyde.[7] All solvents and chemicals were reagent-gradequality, commercially purchased and used as supplied, except for 1,3,5-tri-s(aminomethyl)-2,4,6-triethylbenzene and 2,6-pyridinedicarbaldehydewhich where prepared according to literature methods.[19] KNO3, Cu-ACHTUNGTRENNUNG(NO3)2·2.5H2O and CuACHTUNGTRENNUNG(ClO4)2·6H2O were analytical-grade quality, com-mercially purchased and used as supplied. All the solutions were pre-pared with demineralised water (obtained by a Millipore/Milli-Q system)and methanol purified by standard methods.[20] IR spectra were recordedin the 4000–450 cm�1 region at room temperature by using KBr pelletsand a UNICAM Mattson 7000 spectrometer. Microanalyses were carriedout by the ITQB Microanalytical Service.

Figure 8. Contour map showing the dependence of the magnetic couplingconstant J (in cm�1) on the CuOO angles (in degrees). The experimentalpoints are marked with a dot and a label identifying the compound. Com-pounds 1, 5, 7 and 11 (see Table 4) have a low symmetry and eachcopper atom presents two angles; therefore, these compounds are repre-sented by three dots but the same label with different primes. Antiferro-and ferromagnetic couplings are represented by AF and F, respectively.

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Potentiometric measurements

Reagents and solutions : All solutions were prepared in water/methanol(50:50 v/v) solvent. A stock solution of the receptor was prepared at ap-proximately 2.0 10�3 mol dm�3. Carbonate-free solutions of the KOH ti-trant were prepared as described earlier.[21] These solutions were discard-ed every time the carbonate concentration was about 0.5% of the totalamount of base. The titrant solutions were standardised (tested by Gran�smethod).[22]

Equipment and working conditions : The equipment used has been de-scribed before.[21] The ionic strength of the experimental solutions waskept at (0.10�0.01) mol dm�3 with KNO3; the temperature was main-tained at (298.2�0.1) K. Atmospheric CO2 was excluded from the titra-tion cell during experiments by passing purified nitrogen across the topof the experimental solution. The glass electrode was pre-treated bysoaking it in the water/methanol (50:50 v/v) solution over a period of2 days, to prevent erratic responses.

Measurements : The [H+] of the solutions was determined by the mea-surement of the electromotive force of the cell, E= E8’+Q log [H+]+Ej,where E8’ is the formal cell potential, Q =2.303RT/F and Ej is the liquid-junction potential. The term pH is defined as �log [H+]. E8’, Q, Ej andKw (the ionic product of water) were determined by titration of a solu-tion of known hydrogen-ion concentration at the same ionic strength, byusing the acid pH range of the titration. Ej was found to be negligibleunder the experimental conditions used. The value of Kw was determinedfrom data obtained in the alkaline range of the titration, by consideringE8’ and Q valid for the entire pH range, and was found to be equal to10�13.91 under our experimental conditions. Before and after each set of ti-trations the glass electrode was calibrated as a [H+] probe by titration of1.0 10�3 mol dm�3 standard HCl solution with standard KOH. Measure-ments were carried out with 0.0475 mmol of ligand in a total volume of40 cm3, in the absence and in the presence of Cu ACHTUNGTRENNUNG(NO3)2 in CCu/Cpyr ratiosof 0.5, 1.0, 2.0 and 3.0. Each titration curve consisted typically of 60–90points in the 2.5–9.0 pH range and a minimum of two replicate titrationswere performed for each CCu/Cpyr ratio. Back-titrations with standardHNO3 solution were performed to confirm the values of the final E8’readings. Care was taken to maintain unaltered the methanol/water ratioin the measured solution. The exact amount of ligand was obtained bydetermination of the excess of acid present in a mixture of pyr and stan-dard HNO3 (1.3 10�2 mol dm�3) by titration with standard KOH solu-tion.

Calculation of equilibrium constants : Overall protonation constants, biH,

of the free ligand were calculated by fitting the potentiometric data ob-tained for all the performed titrations under the same experimental con-ditions with the HYPERQUAD program.[23] Stability constants of thevarious species formed in solution were obtained from the experimentaldata corresponding to the potentiometric titration of solutions of the dif-ferent metal ions, each of them with different metal-to-ligand ratios, alsousing the HYPERQUAD program. The initial computations were ob-tained in the form of overall stability constants, bMm HhLl

= [MmHhLl]/[M]m[H]h[L]L. The errors quoted are the standard deviations of the over-all stability constants given directly by the program for the input data,which include all the experimental points of all titration curves. TheHYSS program[24] was used to calculate the concentration of equilibriumspecies from the calculated constants from which distribution diagramswere plotted. The species considered in a particular model were thosethat could be justified by the principles of coordination chemistry.

Synthesis of [Cu3 ACHTUNGTRENNUNG(pyr)ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(NO3)4·9H2O crystals : The pyr com-pound (2 mg, 2.5 mmol) was dissolved in MeOH (0.05 cm3) and Cu ACHTUNGTRENNUNG(NO3)2

(0.15 cm3, 0.05 mol dm�3) was added. Immediately a blue precipitate wasformed. MeOH (0.5 cm3) was added and the solution slightly heated untilit was clear, then the mixture was left exposed to the atmosphere for3 days, after which blue single crystals suitable for X-ray crystallographywere obtained.

Synthesis of [Cu3 ACHTUNGTRENNUNG(pyr)ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O crystals : The pyr compound(82 mg, 0.1 mmol) was dissolved in MeOH (2 cm3) and Cu ACHTUNGTRENNUNG(ClO4)2 (6 cm3,0.05 mol dm�3) was added. Immediately a blue precipitate was formed.MeOH (20 cm3) was added and the solution slightly heated until it wasclear, then the mixture was placed inside a box in a CO2 atmosphere (dry

ice) where it was allowed to stand for 3 d. Blue single crystals suitablefor X-ray crystallography were isolated by filtration and washed withcold water. Yield: 105 mg, 70%; IR (KBr pellet): n = 3450br n ACHTUNGTRENNUNG(N�H),2966m, 2931m, 2912m, 2870m n ACHTUNGTRENNUNG(CH2), 1613m n ACHTUNGTRENNUNG(C=C, py), 1587m n ACHTUNGTRENNUNG(C=N,py), 1497sh, 1479sh, 1455s nd ACHTUNGTRENNUNG(C�O); 1100br na ACHTUNGTRENNUNG(Cl�O), 941w ns ACHTUNGTRENNUNG(Cl�O),820sh, 814w p ACHTUNGTRENNUNG(CO3), 793w, 772w dd ACHTUNGTRENNUNG(OCO), 626 d ACHTUNGTRENNUNG(Cl�O) cm�1; ESI-MS(H2O/MeOH): m/z (%): 1357.0 {[Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]·3ClO4}

+ ; elementalanalysis calcd (%) for [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]·4ClO4·H2O: calcd for: C 42.35,H 4.85, N 8.55; found: C 42.40, H 4.84, N 8.44.

Crystallography : The X-ray data of [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·-ACHTUNGTRENNUNG(MeOH)·2H2O (1) and [Cu3ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(NO3)4·9H2O (2) were collect-ed on a CCD Bruker APEX II instrument at 150(2) K with graphite-monochromatised MoKa radiation (l= 0.71073 �). The selected crystalwas positioned 35 mm from the CCD and the spots were measured witha counting time of 100 s for 1 and 60 s for 2. Data reduction including amulti-scan absorption correction was carried out using the SAINT-NTsoftware package from Bruker AXS. The structures were solved by acombination of direct methods with subsequent difference Fourier syn-theses and refined by full matrix least squares on F2 by using theSHELX-97 suite.[25] Anisotropic thermal displacements were used for allnon-hydrogen atoms. The hydrogen atoms of the C�H and N�H bondswere placed at geometrical positions and refined with Uiso =1.2Ueq of theatom to which they are attached. The O�H hydrogen atom of the metha-nol molecule present in salt 1 was also refined in an ideal position calcu-lated by a suitable AFIX instruction. The atomic positions of the hydro-gen atoms of the water of crystallisation molecules of both complexeswere discernible from difference Fourier maps and they were included inthe structure refinement with individual isotropic thermal parameters. Inaddition, in complex 1, one perchlorate counter-ion was found to be dis-ordered over two tetrahedral positions, which were included in the re-finement with refined occupancies of 1�x and x, x being 0.55(1). Molecu-lar diagrams were drawn with PYMOL software.[26] The crystal data andrefinement details are summarised in Table S1 in the Supporting Infor-mation.

CCDC-823349 and 823350 contain the supplementary crystallographicdata for this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ESI-MS : A solution of [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O (1 10�4 mol dm�3) in water/methanol (50:50 v/v) at pH 6.0 was prepared andthe mass spectrum was acquired in the positive polarity mode, afterdirect injection with a syringe pump into a Bruker Daltonics Esquire3000plus mass spectrometer equipped with an ESI source. The followingtuning conditions were used: ion-spray voltage, 50–300 V ramp range; ca-pillary exit and temperature of the heated capillary, 150 8C. Nitrogen wasused as drying gas at a flow rate of 5 dm3 min�1 and at a constant pressureof 15 psi.

Magnetic measurements : Magnetic susceptibility measurements of poly-crystalline samples were obtained over the temperature range 2–300 Kwith a superconducting quantum interference device (SQUID) magneto-meter in the temperature range 2.0–300 K and under an applied magneticfield of 0.02 T for T<20 K, to avoid saturation effects, and 1 T for T>20 K. Diamagnetic corrections of the constituents atoms were estimatedfrom Pascal�s constants and a value of 60 10�6 cm3 mol�1 was used forthe temperature-independent paramagnetism of each copper(II) ion. Q-band EPR spectroscopy on a polycrystalline sample of [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O was performed with a Bruker ER 200 spectrometerequipped with a helium continuous-flow cryostat.

Computational details : Calculations were performed through the Gaussi-an 09 package by using the B3LYP functional and the quadratic conver-gence approach.[27] Triple-z and double-z all-electron basis sets proposedby Ahlrichs et al. were used for the metal and for the rest of the atoms,respectively.[28, 29] The broken symmetry approach was employed to de-scribe the unrestricted solutions of the antiferromagnetic spin states,[30]

which were obtained from the guess functions generated with the frag-ment tool implemented in the Gaussian code. The full geometry wasused for [Cu3 ACHTUNGTRENNUNG(pyr)ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O. DF calculations were also per-formed on a dinuclear model in which the copper(II) ions were perfectly

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FULL PAPERA Trinuclear Copper(II) Cryptate

placed in the carbonate plane and, to allow close positions for the metalatoms, the peripheral ligands were placed in a plane perpendicular tothat built from the carbonate group. To prevent contacts between theseperipheral groups from different metal ions (strong hydrogen bonds),small fluoride groups were chosen rather than bulkier ammonia groups.In this model, the metal–ligand distances were optimised in the singletspin state. The metal fragments were shifted to reach anti–anti conforma-tions from the syn–syn and syn–anti configurations.

On the other hand, the broken-symmetry methodology allows all the ex-change coupling constants that are present in polynuclear transition-metal complexes to be obtained.[30] To calculate the n Ji exchange cou-pling constants of a polynuclear complex, we must at least perform n +1energy calculations of different spin configurations that correspond tosingle-determinant Kohn–Sham solutions. Such spin configurations mustbe selected in such a way that it is possible to solve a system of n equa-tions with n unknowns, the Ji values. In this way, four calculations werenecessary on the complex [Cu3 ACHTUNGTRENNUNG(pyr) ACHTUNGTRENNUNG(m3-CO3)]· ACHTUNGTRENNUNG(ClO4)4·H2O to obtain thethree possible exchange coupling constants (see Table 3). In this case,one of them is that for which the local spin moment is parallel alignedleading to a quartet spin state (S=3/2), which can be chosen as energyreference. The other spin configurations correspond to the doublet spinstates, for which one of the local spin moments associated with the metalions is contrarily aligned to the rest.

Acknowledgements

The authors acknowledge FCT, with co-participation of the EuropeanCommunity funds FEDER, POCI, QREN and COMPETE for the finan-cial support under project PTDC/QUI/67175/2006. The authors alsothank the MICINN (Spain) (Projects CTQ2010-15364 and the ConsoliderIngenio in Molecular Nanoscience CSD2007-00010) and the GeneralitatValenciana (Spain) (Project PROMETEO/2009/108). The NMR spec-trometers are part of The National NMR Network (REDE/1517/RMN/2005), supported by “Programa Operacional CiÞncia e Inovażo(POCTI) 2010” and Fundażo para a CiÞncia e a Tecnologia (FCT). Wealso acknowledge M. C. Almeida for providing elemental analysis andESI-MS data from the Elemental Analysis and Mass Spectrometry Ser-vice at the ITQB. P.M. thanks FCT for the grant (SFRH/BD/36159/2007).

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Received: May 4, 2011Published online: August 23, 2011

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FULL PAPERA Trinuclear Copper(II) Cryptate