Copper(II) compounds with NNO tridentate Schiff base ligands: Effect of subtle variations in ligands on complex formation, structures and magnetic properties

Inorganica Chimica Acta 387 (2012) 373–382

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Copper(II) compounds with NNO tridentate Schiff base ligands: Effect ofsubtle variations in ligands on complex formation, structures and magneticproperties

Luca Rigamonti a,b,⇑, Alessandra Forni c, Roberta Pievo d,e, Jan Reedijk e,f, Alessandro Pasini a

a Università degli Studi di Milano, Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘Lamberto Malatesta’, via Venezian 21, 20133 Milano, Italyb Università degli Studi di Firenze, Dipartimento di Chimica ‘Ugo Schiff’, via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italyc CNR-ISTM, via Golgi 19, 20133 Milano, Italyd Max Planck Institute for Biophysical Chemistry, Am Faßberg 11, 37077 Göttingen, Germanye Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlandsf Department of Chemistry, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

a r t i c l e i n f o

Article history:Received 4 January 2012Received in revised form 13 February 2012Accepted 19 February 2012Available online 27 February 2012

Keywords:Copper(II) complexesTridentate Schiff basesCrystal structuresMagnetic susceptibilityEPR spectroscopy

0020-1693/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.ica.2012.02.030

⇑ Corresponding author at: Università degli StudiChimica ‘Ugo Schiff’, via della Lastruccia 3, 50019 Ses

E-mail address: [email protected] (L. Rig

a b s t r a c t

The formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and[Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1� = (E)-2-((3-aminoethylimi-no)methyl)phenolate or L2� = (E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monoconden-sation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) andpyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear deriva-tives, whose nature depends on the length of the diamine used and hence on the size of the correspond-ing metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) incompound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decompo-sition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres bysubstitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c),with exclusion of pyridine, or the trinuclear [Cu3(L2)3(l3-OH)](ClO4)2 (2d), where pyrazine acts as basegenerating OH�, and it does not appear in the product. Reaction of 1a with pyrazine yields only the tri-nuclear [Cu3(L1)3(l3-OH)](ClO4)2 (1d). Also with 2,20-bipyridine (2,20-bpy) dinuclear complexes are notformed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear com-pounds [Cu(L1)(2,20-bpy)](ClO4) (1e) and [Cu(L2)(2,20-bpy)](ClO4) (2e). The crystal structures of com-pounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities vM(T) of 1aand 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling(J = +0.96(3) cm�1) through the short dimeric Cu���O contacts is detected for 2a (tn, six-membered ring).The X-band EPR spectrum of 2a in a frozen methanol solution at 70 K shows the hyperfine coupling ofmononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Reaction of salicylaldehyde, salH, with primary diamines, suchas ethylenediamine, en, 1,3-propanediamine, tn, and 1,2-benzen-ediamine, o-phen, yields the well-known tetradentate Schiff baseproligands H2salen, H2saltn and H2salophen [1]. This kind of che-lating ligands forms very stable mono and oligonuclear complexeswith almost all the metals [2], with countless applications asmaterials [3], catalysts [4], etc.

ll rights reserved.

di Firenze, Dipartimento dito Fiorentino (FI), Italy.amonti).

If a monocondensation between the diamine and salH occurs,the so-called ‘half unit’ tridentate Schiff base proligands HL are ob-tained, with a NNO donor set [5]. However, as already observed,such monocondensation does not occur, especially with aliphaticdiamines [6], since a 1:1 M ratio of salH and diamines yields invari-ably the bicondensed products. The ligands L� can be obtained viatemplate syntheses as their metal complexes [7]. These compoundspossess a coordinated primary amino group that can be furtherfunctionalised and are therefore starting materials for new deriva-tives [8,9]. Moreover, complexes with L� are also interesting assuch, since they display useful features, such as antibacterialproperties, and may have catalytic applications [10].

In our previous study [6], the template syntheses of mono-, di-and trinuclear copper(II) complexes bearing tridentate Schiff bases

374 L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382

L�, derived from the monocondensation of the aliphatic diaminesethylenediamine, en, and 1,3-propylenediamine, tn, and 5-substi-tuted salicylaldehydes, 5-G-salH (with the groups NO2, H andOMe), have been reported.

The mononuclear copper(II) compounds [Cu(L1)(py)](ClO4) (1a)and [Cu(L2)(py)](ClO4) (2a), with the half units L1� = (E)-2-((3-aminoethylimino)methyl)phenolate and L2� = (E)-2-((3-amino-propylimino)methyl)phenolate (i.e. monocondensation productsof salH with en or tn, respectively; see Scheme 1) and a pyridineas the fourth ligand, can be synthesized in very high yields directlyfrom commercial starting materials and applying a room-temper-ature template method [6,8,11,12]. The reaction of 1a or 2a witha second differently substituted salicylaldehyde leads to the func-tionalisation of the coordinated primary amino group (imine for-mation), followed by the loss of the pyridine as fourth ligand,and formation of copper(II) compounds with unsymmetricallysubstituted tetradentate ‘salen-type’ Schiff base ligands [8,12],studied for their nonlinear optical features and applications [8].The reaction of compounds 1a and 2a with 4,40-bpy, instead, yieldsthe substitution of py and the formation of the dinuclear com-pounds [Cu2(L)2(l-4,40-bpy)](ClO4)2 [13].

As precursors for further compounds, in this paper we presentin more detail other aspects of the conversion of the compounds1a and 2a into new mono-, di- and trinuclear derivatives (Scheme2). The nature of the products depends, among other factors, on thelength of the diamine, en or tn. All these transformations are sup-ported by X-ray crystal structure determinations. Moreover, themagnetic behaviour of 1a and 2a will be presented, showing thatthe size of the chelating ring due to the diamine used determinesalso the magnetic properties of these compounds.

2. Experimental

2.1. Materials and methods

All chemicals were reagent grade. Solvents were used as re-ceived. Elemental analyses were performed at the MicroanalyticalLaboratory at the Università degli Studi di Milano. Infrared spectrawere recorded as KBr disks using a JASCO FT-IR 410 spectropho-tometer with a 2 cm�1 resolution. Compounds [Cu(L1)(py)](ClO4)(1a) [6,8] and [Cu(L2)(py)](ClO4) (2a) [6,8,11] were prepared by lit-erature procedures.

2.2. Synthetic procedures

2.2.1. Synthesis of [Cu(L2)(dmf)](ClO4) (2b)2a (154.6 mg, 0.37 mmol) was dissolved and heated at 80 �C in

DMF (10 mL) for 3 h. Slow diffusion of diisopropyl ether into thereaction mixture gave the product as blue crystals. Yield:148.4 mg (97%). Anal. Calc. for C13H20ClCuN3O6 (413.32): C, 37.78;H, 4.88; N, 10.17. Found: C, 37.41; H, 5.02; N, 9.93%. IR (KBr):m(NH2) 3316, 3266, m(C@Odmf) 1655, m(C@NL) 1627, m(ClO4)1093 cm�1.

O-

N NH2

O-

N NH2

L1- L2-

Scheme 1. Schematic representation of the tridentate Schiff base ligands L1� andL2�.

2.2.2. Synthesis of [Cu2(L2)2(ClO4)2] (2c)Pyrazine (12.4 mg, 0.15 mmol) was added to a suspension of 2a

(115.0 mg, 0.27 mmol) in methanol (10 mL) and the reaction mix-ture was left under stirring at room temperature for 3 h. The prod-uct was recovered as dark green crystalline solid. More crystals,suitable for X-ray diffraction, were obtained from slow diffusionof diisopropyl ether into the reaction mixture. Yield: 30.8 mg(34%). Anal. Calc. for C20H26Cl2Cu2N4O10 (680.44): C, 35.30; H,3.85; N, 8.23. Found: C, 35.45; H, 3.95; N, 7.81%. IR (KBr): m(NH2)3323, 3290, 3272, 3250, m(C@N) 1627, m(ClO4) 1080 cm�1.

2.2.3. Synthesis of [Cu(L1)(2,20-bpy)](ClO4) (1e) [11]2,20-bipyridine (46.3 mg, 0.30 mmol) was added to a suspension

of 1a (119.1 mg, 0.28 mmol) in methanol (10 mL) and the reactionmixture was left under stirring at room temperature for 1 day, till agreen solution was obtained. Slow diffusion of diisopropyl etheryielded the product as deep green crystals, suitable for X-ray dif-fraction. Yield: 35.1 mg (26%). Anal. Calc. for C19H19ClCuN4O5

(482.38): C, 47.31; H, 3.97; N, 11.61. Found: C, 47.25; H, 3.62; N,11.44%. IR (KBr): m(NH2) 3337, 3249, m(C@N) 1645, m(ClO4) 1118,1078 cm�1.

2.2.4. Synthesis of [Cu(L2)(2,20-bpy)](ClO4) (2e)2,20-Bipyridine (68.2 mg, 0.44 mmol) was added to a suspension

of 2a (152.1 mg, 0.36 mmol) in methanol (10 mL) and the reactionmixture was left under stirring at room temperature for 4 h, thenthe mixture was filtered and the meagre light green solid was dis-charged. The deep green solution was left at slow evaporation for1 day, yielding the compound as deep green crystalline needles.Yield: 82.3 mg (46%). Anal. Calc. for C20H21ClCuN4O5 (496.24): C,48.41; H, 4.27; N, 11.26. Found: C, 48.22; H, 4.02; N, 11.29%. IR(KBr): m(NH2) 3319, 3273, m(C@N) 1619, m(ClO4) 1091 cm�1.

2.3. X-ray crystal structure determinations

Crystals of 2b, 2c and 1e suitable for X-ray diffraction were ob-tained from slow diffusion of diisopropyl ether into the DMF reac-tion mixture (2b) or into the methanolic reaction mixture (2c, 1e).Crystal data and details of data collection and refinement are givenin Table 1. Intensity data were collected with a Bruker Apex CCDarea detector by using graphite monochromated MoKa radiation.Data reduction was performed with SAINT, and absorption correc-tions based on multiscan were obtained with SADABS [14]. All thestructures were solved by direct methods with SHELXS-97 [15] andrefined by SHELXL-97 [15]. The program ORTEPIII was used for graphics[16]. Anisotropic thermal parameters were used for all non-hydro-gen atoms. The isotropic thermal parameters of H atoms were fixedat 1.2 (1.5 for methyl groups) times those of the atom to whichthey were attached. All H atoms were placed in calculated posi-tions and refined by a riding model, except for the methyl hydro-gens of a DMF carbon atom (C3D) in compound 2b. Coordinatesof the latter H atoms required to be refined because the standard‘AFIX 137’ instruction gives a misoriented N-methyl moiety. In 2cand 1e, some oxygen atoms of one perchlorate anion present largedisorder, which was partially resolved by splitting such atoms intwo positions, labelled by adding the letters A and B to the num-bering scheme. In 2c, the Cl–O distances involving such splitoxygen atoms required to be restrained during the refinement inorder to give a satisfactory geometry (SADI instruction).

2.4. Magnetic measurements

Variable temperature magnetic susceptibility measurements(2–300 K, 1000 G) were carried out using a Quantum DesignMPMS-XL SQUID magnetometer. Data were corrected for the mag-netisation of the sample holder and for diamagnetic contributions

O

N NH2

Cu

O

NH2N

Cu

(ClO4)2

2+

2c

1/2

O

N

Cu

NH2

OH

Cu

Cu

O N

NH2

N

OH2N

2+

(ClO4)2

(CH2)n

(CH2)n

n(H2C)

1/3

1d (n = 2), 2d (n = 3)[6]

1/2 NN MeOH, RT

only with tn

NN MeOH, RT

O

N NH2

Cu

O

(ClO4)

+

N

H

2b

O

N

(CH2)n

NH2

Cu

N

+

(ClO4)

1a (n = 2), 2a (n = 3)

DMF, 80 °C

only with tnwith en decomposition

O

N NH2

Cu

N

(ClO4)

+

N

MeOH, RT

1e (n = 2), 2e (n = 3)

N

N

(CH2)n

Scheme 2. Overview of the different reactions of 1a and 2a (legend: 1 ? L1�, 2 ? L2�; a ? py, b ? DMF, c ? dinuclear, d ? trinuclear, e ? 2,20-bpy).

Table 1Crystal data and structure refinement for compounds 2b, 2c and 1e.

2b 2c 1e

Crystal dataEmpirical formula C13H20ClCuN3O6 C20H26Cl2Cu2N4O10 C19H19ClCuN4O5

Moiety formula [C13H20CuN3O2](ClO4) [C20H26Cu2N4O2(ClO4)2] [C19H19CuN4O](ClO4)M 413.31 680.43 482.37Crystal system monoclinic monoclinic triclinicSpace group P21/n P21/n P�1Unit cell dimensionsa (Å) 17.3933(12) 13.4213(7) 13.1133(9)b (Å) 11.9929(8) 12.3554(6) 13.4199(9)c (Å) 18.0655(12) 15.5984(8) 13.9503(17)a (�) 90 90 99.058(1)b (�) 114.428(1) 94.567(1) 99.958(1)c (�) 90 90 115.242(1)V (Å3), Z, Z0a 3431.1(4), 8, 2 2578.4(2), 4, 1 2112.2(3), 4, 2Dx (Mg m�3) 1.600 1.753 1.517Reflections for cell determination 7574 9941 60192h (�) for cell determination 5.0–53.9 4.5–52.0 4.7–51.9l (mm�1) 1.463 1.918 1.199T (K) 293(2) 293(2) 293(2)Colour, habit blue, prism green, rhombic prism green, prismDimensions (mm) 0.42 � 0.29 � 0.17 0.28 � 0.20 � 0.10 0.34 � 0.28 � 0.14

Data collectionRadiation, k (Å) MoKa, 0.71073 MoKa, 0.71073 MoKa, 0.71073Scan type u and x u and x u and x2hmax (�) 64.8 55.0 55.0h range �25 ? 26 �17 ? 17 �17 ? 17k range �17 ? 18 �16 ? 16 �17 ? 17l range �26 ? 27 �20 ? 20 �18 ? 18Intensity decay (%) none none noneMeasured reflections 71943 41811 30373Independent reflections 11834 5925 9488Reflections with I > 2r(I) 7631 4516 7473Rint 0.032 0.033 0.021

Refinement on F2

R[F2 > 2r(F2)], wR[F2 > 2r(F2)] 0.0415, 0.1184 0.0300, 0.0786 0.0426, 0.0550S 1.018 1.008 1.034Parameters, restraints 445, 0 370, 21 550, 0(D/r)max 0.002 0.001 0.001Dqmax, Dqmin (e�3) 0.764, �0.509 0.632, �0.268 0.663, �0.484

a Z0 is the number of molecules per asymmetric unit.

L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382 375

Fig. 1. ORTEP plot of the asymmetric unit of compound 2b with the atom numbering scheme and short intermolecular contacts (dashes: Cu���O interaction; points: NAH���Ohydrogen bonds). Displacement ellipsoids are drawn at the 20% probability level.

Table 2Selected bond lengths (Å) and angles (�) for [Cu(L2)(dmf)](ClO4) (2b).

Molecule 1 Molecule 2

Cu1–N1 1.992(2) Cu2–N3 1.996(2)Cu1–N2 1.9632(18) Cu2–N4 1.9675(18)Cu1–O1 1.9059(16) Cu2–O2 1.9184(15)Cu1–O1D 1.9752(16) Cu2–O2D 1.9860(15)N1–Cu1–N2 94.36(9) N3–Cu2–N4 94.29(9)N2–Cu1–O1 93.15(7) N4–Cu2–O2 93.08(7)O1–Cu1–O1D 90.03(7) O2–Cu2–O2D 90.27(6)O1D–Cu1–N1 82.10(9) O2D–Cu2–N3 81.43(8)

Fig. 2. ORTEP plot of the centrosymmetric pairs of compound 2b with the atomnumbering scheme and short intermolecular contacts (dashes: Cu���O interaction;points: NAH���O hydrogen bonding and metal–ligand interaction). Displacementellipsoids are drawn at the 20% probability level.

376 L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382

of the ligand systems, which were estimated from Pascal constantsas �4.23 and �4.48 � 10�4 cm3 mol�1 for a dimer of 1a and 2a,respectively. The fitting of the experimental curves was achievedusing Origin�. X-band EPR spectra were obtained on a Bruker EMX-plus electron spin resonance spectrometer at RT, 70, 20 and 5 K(field calibrated with DPPH (g = 2.0036)), and the simulation ofthe spectrum reported in Fig. 8 was done with Easyspin [17].

3. Results and discussion

3.1. Complex formation

The compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4)(2a) have been previously used as precursors for the synthesis ofunsymmetrically substituted salen-type copper(II) complexes [8],by reaction with a second salicylaldehyde under reflux in methanol,for about half an hour. With this in mind, the coordinated primaryamino groups in 1a and 2a appear reactive towards carbonyl deriv-atives, and therefore they could be functionalised, yielding differentcopper complexes with higher nuclearities. Unfortunately, our at-tempts to functionalise the NH2 group by reaction with numberof carbonyl compounds different from salH so far failed. For exam-ple, upon refluxing 2a with 4-hydroxybenzaldehyde in N,N-dimeth-ylformamide (DMF) for 1 day, the only detectable change was thesubstitution of the coordinated pyridine with a molecule of solvent,yielding [Cu(L2)(dmf)](ClO4) (2b) in very small amounts, plusmainly decomposition products. This observation suggests thatthe driving force in the activation of the NH2 group may be theformation of the final N2O2 salen-type tetradentate ligand, coordi-nated to copper. Compound 2b was obtained quantitatively as themain product upon heating 2a in DMF at 80 �C for 3 h, and crystall-ising the reaction mixture with diisopropyl ether. With 1a, instead,only oily decomposition products were observed.

Therefore, a different approach was applied to transform 1a and2a into new compounds, and this appeared possible by the substi-tution of the coordinated pyridine with bridging linkers, since theCu–N(py) bond has been found quite labile in solution [6].

Reactions of 1a and 2a with pyrazine were then performed inthe hope of obtaining dinuclear complexes with bridging pyrazinebetween two CuII centres. Despite the examples reported in theliterature with other copper systems [18–20], pyrazine does not

Table 3Selected bond lengths (Å) and angles (�) for [Cu2(L2)2(ClO4)2] (2c).a

Cu1–N1 1.9582(19) Cu2–N3 1.960(2)Cu1–N2 1.9807(19) Cu2–N4 1.959(2)Cu1–O1 1.9155(16) Cu2–O2 1.9219(15)Cu1–O2 2.0173(15) Cu2–O1 1.9881(16)Cu1���O6A/O6Bb 2.525(7)/2.697(11) Cu2���O1P 2.511(2)Cu1���O3Pi 2.758(2) Cu2���O3Pi 2.854(2)Cu1���Cu2 2.9406(4) – –N1–Cu1–N2 98.27(8) N3–Cu2–N4 98.00(10)N2–Cu1–O1 90.93(8) N4–Cu2–O2 93.18(8)O1–Cu1–O2 75.19(7) O2–Cu2–O1 75.74(7)O2–Cu1–N1 95.17(7) O1–Cu2–N3 92.06(9)

a i = 1/2 � x, 1/2 + y, 1/2 � z.b Suffixes A and B refer, respectively to the two sets of the partially disordered

perchlorate ion.

Fig. 3. ORTEP plot of the asymmetric unit of compound 2c with the atomnumbering scheme and Cu���O intermolecular contacts. The longest axial Cu���Odistances have been left out for clarity. Displacement ellipsoids are drawn at the20% probability level.

L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382 377

give rise to an adduct, but if used in a 1:2 stoichiometric amount(or in a very low excess) in the reaction with 2a, the dinuclear com-pound [Cu2(L2)2(ClO4)2] (2c), with bridging coordination of thephenoxido oxygen atoms, can be isolated in medium–low yield.If pyrazine is used in a larger excess, in an attempt to increasethe yield, the trinuclear species [Cu3(L2)3(l3-OH)](ClO4)2 (2d) is in-stead obtained, suggesting that in this case the higher concentra-tion of pyrazine in solution raises the basicity to a level to whichthe trinuclear derivative is the most stable, in accordance withwhat already observed with stronger bases, such as NaOH [6,11].The trinuclear species [Cu3(L1)3(l3-OH)](ClO4)2 (1d) is always themain product when reacting 1a with pyrazine, in any stoichiome-tric ratio, suggesting that 1d is the most stable complex even in aslightly basic medium (Scheme 2). By the way, the two nitrogenson the same aromatic ring of the pyrazine probably do not have en-ough donor power to bridge two copper ions by coordination inthis kind of systems.

An attempt to explain the reactivity towards pyrazine can bemade taking into account the different crystal structures of 1a[21] and 2a [6]. In 1a, each copper can be considered as five-coordinated, in which the fifth position is occupied by a perchlorateanion, while the monomeric units interact each other mainlythrough N–H���O hydrogen bond (2.905(6) Å) [21]. In 2a, instead,the copper atom is further involved in a relatively strong interactionwith the phenoxido oxygen of a centrosymmetric related molecule,forming a head-to-tail dimer, in which copper is practically hexaco-ordinated [6] (see Fig. 7). In methanol solution, the pyridine is mostprobably lost in both compounds, as suggested by mass spectros-copy analyses [6]; the copper–oxygen interactions in 2a betweentwo monomeric units (Cu���O1�x,2�y,�z = 2.4008(9) Å [6]) may thenbecome stronger, taking the role of the fourth copper coordinationbond after exclusion of pyridine (Cu1–O2 = 2.0193(17), Cu2–O1 = 1.9891(19) Å) and yielding the dinuclear compound 2c. Thelack of the Cu���O interactions in 1a [21] leads to the isolated[Cu(L1)(MeOH)]+ solvated species in solution [13], that even in amild basic medium prefers to undergo aggregation and form thetrinuclear compound 1d with triply bridging hydroxido ions.

The solid state structure of 2a can also be taken into account toexplain the different behaviour of 1a and 2a in reaction with DMF:as the crystal structure of [Cu(L2)(dmf)](ClO4) (2b) confirms (seebelow), weaker Cu���O interactions between two monomeric units

are still present (Cu1���O1�x,�y,1�z = 2.580(2), Cu2���O22�x,2�y,2�z =2.436(2) Å), and then only substitution of pyridine in 2a withDMF occurs when the temperature is kept below 80 �C. At higherT, instead, the Cu���O intermolecular interactions do not surviveleading to decomposition, as in the case of 1a.

Reaction of 1a and 2a with 2,20-bipyridine has then been testedin the hope to form new di- or oligonuclear complexes withtwisted 2,20-bpy molecules coordinating two copper ions. How-ever, the chelating properties of 2,20-bpy prevails and the twomononuclear complexes [Cu(L1)(2,20-bpy)](ClO4) (1e) and[Cu(L2)(2,20-bpy)](ClO4) (2e) can be instead isolated, in which thebidentate ligand 2,20-bpy coordinates the same copper centre inan asymmetric fashion (see below in the crystal structure of 1e).As we already reported [13], the use of 4,40-bipyridine, instead ofpyrazine and 2,20-bpy, in the reaction with 1a and 2a leads tothe formation of the dinuclear compounds [Cu2(L1)2(l-4,40-bpy)](ClO4)2 and [Cu2(L2)2(l-4,40-bpy)](ClO4)2 with bridging 4,40-bpymolecules between two copper ions.

3.2. Crystal structure of [Cu(L2)(dmf)](ClO4) (2b)

Crystals of 2b suitable for X-ray diffraction were obtained fromslow diffusion of diisopropyl ether into the reaction mixture of 2awith DMF. The asymmetric unit includes two independent mole-cules of the complex, named molecules 1 and 2, respectively (seeFig. 1 and Table 2 for selected bond lengths and angles). Each cop-per ion is coordinated, in the basal plane, by the NNO set of donorsfrom the L2� ligand, and by the oxygen atom of a dmf molecule.The two independent molecules are very similar in geometry, withmolecule 2 showing slightly longer Cu–O distances than molecule1. Comparison with the analogous mononuclear complex 2a,where the fourth equatorial site is occupied by the nitrogen atomof pyridine [6], indicates a lengthening of 0.02–0.03 Å, on goingfrom dmf to pyridine, in all the distances between copper andthe atoms of the NNO set, suggesting a larger electron-donorstrength for the nitrogen of pyridine with respect to the oxygenof dmf. The Cu–Odmf distances are comparable with those observedin other copper complexes with dmf in the basal plane [20,22].When the dmf is instead coordinated in the apical position as fifthligand, the Cu–Odmf distances are longer [23].

Both independent molecules of the complex interact with anadjacent molecule, related by a centre of symmetry, to formhead-to-tail dimeric pairs, where each copper ion interacts withthe centrosymmetric phenolato oxygen (see Fig. 2). The Cu���Ointermolecular distances are consistent with a square-pyramidalcoordination geometry of the copper ion, where the centrosym-metric phenolato oxygen occupies the apical position(Cu1���O1�x,�y,1�z = 2.581(2) Å, Cu1���Cu1�x,�y,1�z = 3.3772(7) Å,Cu2���O2�x,�y,�z = 2.436(2) Å, Cu2���Cu2�x,�y,�z = 3.2412(7) Å). The

Fig. 4. ORTEP view of the zig-zag chain connecting molecules of compound 2c through perchlorate anions (dashes: apical Cu���O interactions are in the range 2.75–2.85 Å).Displacement ellipsoids are drawn at the 20% probability level.

Fig. 5. ORTEP plot of the asymmetric unit of 1e with atom-numbering scheme and short intermolecular contacts (dashes: C���C contact; points: H���O and H���C contacts).Displacement ellipsoids are drawn at 20% probability.

Table 4Selected bond lengths (Å) and angles (�) for [Cu(L1)(2,20-bpy)](ClO4) (1e).

Molecule 1 Molecule 2

Cu1–O1 1.9269(18) Cu2–O2 1.9272(19)Cu1–N1 2.049(2) Cu2–N5 2.050(2)Cu1–N2 1.942(2) Cu2–N6 1.936(2)Cu1–N3 2.235(2) Cu2–N7 2.207(2)Cu1–N4 2.014(2) Cu2–N8 2.006(2)N1–Cu1–N2 83.93(10) N5–Cu2–N6 83.89(11)N2–Cu1–O1 92.47(9) N6–Cu2–O2 92.49(9)O1–Cu1–N4 91.05(8) O2–Cu2–N8 92.74(9)N1–Cu1–N4 93.63(9) N5–Cu2–N8 94.71(10)N3–Cu1–N4 77.07(8) N7–Cu2–N8 77.76(10)N1–Cu1–O1 154.55(10) N5–Cu2–O2 148.67(10)N2–Cu1–N4 176.07(9) N6–Cu2–N8 172.03(10)

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shorter Cu���O intermolecular contact observed in molecule 2, com-pared with that of molecule 1, is consistent with its longer equato-rial Cu–O bond lengths (see above) and with the larger distance ofcopper from the N2O2 least-squares (l.s.) plane (0.133(1) Å inmolecule 2 versus 0.087(1) Å in molecule 1). Dimeric units areconnected to each other through very weak metal–ligand interac-tions, where Cu2 points towards the middle of the C7–C8 bond (seeFig. 2 for the Cu2���C7 contact, 3.356(3) Å). Only in the case ofmolecule 1, showing the weaker head-to-tail dimeric interaction,the copper atom weakly interacts also with the oxygen atom ofan apical perchlorate anion (Cu1���O1P = 2.962(5) Å). Both complexmolecules are involved in hydrogen bonds with the perchlorateanions through the amine groups (N1–H1A���O5P, rH1A���O5P =

Fig. 6. Experimental (s) and theoretical (continuous lines) temperature depen-dences of vM and vMT for compound 1a (data are reported for two copper ions forcomparison with data in Fig. 7).

L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382 379

2.50(1) Å, rN1���O5P = 3.308(3) Å, \ = 150(1)�; N3–H3A���O3P,rH3A���O3P = 2.43(1) Å, rN3���O3P = 3.148(4) Å, \ = 137(1)�, see Fig. 1).

3.3. Crystal structure of [Cu2(L2)2(ClO4)2] (2c)

Crystals suitable for X-ray diffraction were obtained from slowdiffusion of diisopropyl ether into the methanolic reaction mixtureof 2a with pyrazine. The asymmetric unit of the dinuclear complex2c contains two half units in which two square-planar copper cen-tres are bridged equatorially by two phenolate oxygen atoms of thetridentate L2� ligand, with the remaining equatorial sites occupiedby two nitrogen atoms of the ligand (see Fig. 3 and Table 3 for se-lected bond lengths and angles). The complex is therefore charac-terised by a pair of ‘‘long’’ Cu–O bonds (Cu1–O2 and Cu2–O1,2.0173(15) and 1.9881(16) Å, respectively, see Table 2), bridgingthe two mononuclear half units, as distinguished from the otherpair of Cu–O coordination bonds (Cu1–O1 and Cu2–O2,1.9155(16) and 1.9219(15) Å, respectively).

Fig. 7. Experimental (s) and theoretical (continuous lines) temperature dependences ostructure from Ref. [6].

The molecular structure of the dinuclear cation is then coinci-dent with that previously reported for [Cu2(L2)2(l-MeOH)](ClO4)2

by Ray et al. [24], who, however, found a different coordinationenvironment around the copper ions with respect to the presentstructure (see above). In that work, in fact, the copper centres areaxially bridged by an oxygen atom of co-crystallized methanolmolecule, yielding a distorted square pyramidal geometry to themetal ions. The perchlorate anions are only involved in H-bondingwith the amine and methanol hydrogen atoms. In the structure of2c, on the other hand, both copper ions show a pseudo-octahedralcoordination, where the apical positions are occupied on eitherside by an oxygen atom of the perchlorate anions. The strongestCu���O interactions, which are observed on the same side of themolecule, are shown in Fig. 3. Such different coordination environ-ment does not imply significant changes in the geometrical param-eters. The most relevant one concerns the Cu1���Cu2 distance,which as expected is found longer by 0.038 Šin the present struc-ture with respect to that reported for [Cu2(L2)2(l-MeOH)](ClO4)2

[24]. The deviation of the structure with respect to a hypotheticalplanar conformation, as given for example by the dihedral anglebetween the two N2O2 basal planes, is also very similar in thetwo compounds (45.88(4)� in 2c versus 42.5(2)� from Ref. [24]).The structure of 2c is also very close to that of the analogous dinu-clear compounds carrying a methoxy group in position 5 of the sal-icylaldehyde residues (see Ref. [6]), indicating no significantinfluence of such donor group on the coordination geometry ofthe copper ions. The six-membered chelate rings Cu1–N1–C1–C2–C3–N2 and Cu2–N3–C11–C12–C13–N4 assume, respectivelyan envelope and half-chair conformation, with puckering parame-ters u = 175.0(5)�, 146.4(5)� and m = 37.4(2)�, 37.1(2)� [25,26].

The coordination of perchlorate ions to the copper atoms gener-ates infinite zig-zag chains in the crystal structure of 2c, wheremolecules of complex are linked together by the same perchlorateion through atoms O1P and O3P (see Fig. 4).

3.4. Crystal structure of [Cu(L1)(2,20-bpy)](ClO4) (1e)

Crystals suitable for X-ray diffraction were obtained from slowdiffusion of diisopropyl ether into the methanolic reaction mixtureof 1a with 2,20-bpy. The crystal structure of [Cu(L1)(2,20-bpy)]+ ionshows the presence of two discrete mononuclear complexes in theasymmetric unit, named molecules 1 and 2, respectively (see Fig. 5and Table 4 for selected bond lengths and angles). The Cu atoms

f vM and vMT for compound 2a (treated as a dimer), and redrawing of its crystal

Fig. 8. (a) Experimental and (b) simulated first derivative X-band EPR spectra of afreshly prepared methanolic solution of 2a at 70 K. Experimental parameters:microwave frequency: 9.398 GHz; 100 kHz field modulation amplitude, 1 mT; timeconstant: 0.010 ms; conversion time: 30 ms; scan time: 30 ms. Simulation param-eters: g1,2,3 = 2.049, 2.052, 2.240, A1,2,3 (63Cu) = [55, 62, 555] MHz, A1,2,3 (14N) = [31,44, 44] MHz, linewidth = [42.5, 7.1] MHz.

380 L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382

assume a distorted square-pyramidal coordination with the threeNNO atoms from the L1� Schiff ligand in the equatorial position,and the bipyridine N atoms in an equatorial–axial binding mode.The two independent molecules are very similar in geometry. Bonddistances and angles are comparable with those found in othersquare-pyramidal Schiff base Cu(II) compounds, including 2,20-bipyridine [27]. The donor atoms of the N3O basal plane show asignificant tetrahedral distortion from planarity, with maximumdeviations of 0.259(2) and 0.360(2) Å for molecules 1 and 2,respectively, from their l.s. plane. Copper ions are significantlypushed away from their coordinating basal planes towards thenitrogen atoms N3, N7 in apical position (0.2021(14) and0.2125(16) Å for molecules 1 and 2, respectively). The values ofthe distortion index s [28] (s = 0.0 for ideal square-pyramidaland 1.0 for ideal trigonal–bipyramidal geometries), amounting to0.36 and 0.39 for the two molecules of the asymmetric unit, indi-cate a geometry somewhat intermediate between the two idea-lised structures. Distortion from the square-pyramidalcoordination can be also quantified through the N3–Cu1–N4 andN7–Cu2–N8 angles (77.07(8)� and 77.76(10)� in molecules 1 and2, respectively), which greatly deviate from the ideal value. Thedihedral angles between the normal to the l.s. basal planes of mol-ecules 1 and 2, and the line through Cu1,N3 and Cu2,N7 atomsmeasure 13.5(1)� and 9.9(1)�, respectively. The bipyridine ligandis almost planar, with dihedral angles of 2.5(1)� and 3.0(1)� be-tween the two pyridyl rings in molecules 1 and 2, respectively,and it is approximately perpendicular to the N3O basal plane (dihe-dral angles of 84.7(1)� and 86.23(1)� in molecules 1 and 2,respectively).

In the crystal structure of 1e, molecules stack on each otherforming infinite columns with p���p interactions involving atomsof bipyridyl (C10���C37 = 3.354(6) Å) and C–H���p interactionswhere a bipyridyl hydrogen atom (H37) points perpendicular tobond C4–C9 of the salicylaldehyde moiety in a ‘T configuration’(H37���C4 = 2.63(1), H37���C9 = 2.68(1) Å) (see Fig. 5 for the stron-gest intermolecular interactions). Perchlorate anions are primarilyinvolved in moderate NAH���O hydrogen bonds with the aminogroup of the L1� ligand.

3.5. Magnetic susceptibility

The temperature dependences of the molar susceptibility vM forcompounds 1a and 2a in the 2–300 K range are shown in Figs. 6 and7, respectively, as vMTversus T and vM versus T graphics. The mono-nuclear compound 1a shows a flat vMT(T) experimental curve,meaning that there are no interactions between the mononuclearunits in the solid state, and that even at very low temperaturesthe copper centres behave as uncoupled paramagnetic 1/2 spins,with an almost constant value of vMT around 0.80 cm3 K mol�1

per two copper ions (leff � 1.80 bM per Cu ion). In fact, the crystalstructure of 1a [21] shows only very weak p–p interaction betweenthe salicylaldehyde aryl rings (shortest p���p interaction 3.649(8) Å)and an N–H���O hydrogen bond (2.905(6) Å), which have probably anegligible magnetic communication path.

The compound 2a, instead, shows a ferromagnetic behavioursince, lowering the temperature, there is an increase of the vMT va-lue from 0.90 cm3 K mol�1 in the range 50–300 K (two uncoupled1/2 spins) to 1.02 cm3 K mol�1 at 2 K (Fig. 7). Therefore, couplingbetween two copper ions in two different monomers is present.Taking into account the crystal structure of 2a [6], it is clear thatin this case the magnetic path is given by the relatively strong cop-per–oxygen dimeric interactions between two monomers(Cu���O1�x,2�y,�z = 2.4008(9) Å, see Fig. 7), responsible for the ferro-magnetic exchange [29]. Therefore, the experimental data can bedescribed by the dimer HDVV (Heisenberg, Dirac, Van Vleck) model(Eq. (1)):

Hex ¼ �2JS1 � S2 ð1Þ

(J is the exchange coupling constant, J < 0 refers to antiferromagnet-ism and J > 0 to ferromagnetism; S1 and S2 are the spin operatorsfor the two coppers) which, with use of the Van Vleck formalism[30], leads to the Eq. (2) (Bleaney–Bowers formula [31]):

vMT ¼ ð2Nb2g2=kÞ½3þ expð�2J=kTÞ��1ð1� PÞ þ ðNb2g2=2kÞPþ 2Na ð2Þ

where N is the Avogadro’s number, b is the Bohr magneton, k is theBoltzmann constant, g is the g-factor, P represents the fraction ofparamagnetic species, and Na is the temperature independent para-magnetism (TIP), assumed to be 60 � 10�6 cm3 mol�1 per Cu.

The best fit of the data for compound 2a leads to the followingvalues: J = + 0.96(3) cm�1, g = 2.21(1), without paramagneticuncoupled fraction (P < 0.001, within the fitting error) with anagreement factor R = 2 � 10�5 (defined as

Pi[(vM)exp � (vM)calc]2/

Pi[(vM)obs]2). The singlet–triplet state energy separation of

+1.92(3) cm�1 is small, but in line with other literature data of sim-ilar compounds [29]. The experimental data for compound 1a arewell described by the second term of Eq. (2), and the best fittingled to g = 2.05(1) with R = 3 � 10�5.

3.6. Electron paramagnetic resonance

The X-band EPR spectra at 70, 20 and 5 K (with no significantvariations with the temperature), recorded on powder of com-pound 1a, show classical resonance lines of axial symmetry, withg|| = 2.22 and g\ = 2.06. These values are in agreement with thedata available in the literature for similar molecules [13,29b,32].For compound 2a, instead, only a giso = 2.09 is present. Hyperfinesplittings remain unresolved, which is not uncommon for Cu(II)compounds where the copper ions are relatively close in thelattice.

The X-band EPR spectra at room temperature for both com-pounds 1a and 2a in methanol show a very similar series of signals(four lines with giso = 2.14 and Aiso = 27.6 MHz), due to a mononu-clear isotropic copper species. This signal is likely to be due to thesolvated cations [Cu(L1)(MeOH)]+ and [Cu(L2)(MeOH)]+, consider-ing the lability of pyridine in solution [6,13]. The X-band EPR spec-trum of a freshly prepared methanol solution of 2a quickly frozenat 70 K shows again the classical four lines for a mononuclear Cu(II)species, but also the hyperfine coupling due to the three 14N atoms

L. Rigamonti et al. / Inorganica Chimica Acta 387 (2012) 373–382 381

(nuclear spin I = 1) [33], highlighted by the splitting into seven nar-rower lines of the higher-field copper resonance (Fig. 8). Thismeans that the rapid freezing of the methanol solution of 2a doesprevent the loss of pyridine. The same hyperfine splitting cannot beobserved in the spectrum of 1a under the same conditions.

4. Conclusions

The results presented above underline the usefulness of theNNO tridentate Schiff base ligands L� derived from the monocon-densation of salH and aliphatic diamines, en and tn. In fact, whencoordinated to copper, they leave the forth equatorial coordinationposition available for new monodentate ligands (pyridine, dmf,OH�, etc.). If weak donors, they can be removed or exchanged,leading to the conversion into different mono-, di-, oligonuclearcompounds. A fifth donor atom can also interact with copper, witha longer bond distance (a weaker interaction, e.g. 2,20-bpy, com-pound 1e), but it results in a distorted coordination environment,midway between a square pyramidal and a trigonal bipyramidalfashion.

Another important point here presented is how the change ofethylenediamine, en, with 1,3-propylenediamine, tn, on going from1a to 2a, gives rise to very different reactivity, molecular structuresand magnetic behaviour. When the diamine has only two carbonatoms, the chelate ring formed is five-membered, and hence thecoordination environment around copper is close to square planar.The propylene chain, instead, due to its larger steric-demandingconformation and the consequent flexibility of the six-memberedmetallacycle formed, distorts the copper coordination sphere, giv-ing rise to larger N–Cu–N angles and, more important, to a relevantdisplacement of Cu from the coordination plane. Therefore, morelikely copper seeks a fifth interaction, that can be given by dimerformation (case of 2a and 2b), perchlorate ions (2c) or solvent mol-ecules ([Cu2(L2)2(l-MeOH)](ClO4)2 [24]). In addition to the com-plex formation, the different magnetic properties between 1a and2a fully reflect how a simple aliphatic carbon atom, added in thechelate ring, can determine the features of this class of coppercompounds, as a change from non-interacting copper ions (1a) toa ferromagnetic intermolecular Cu–Cu exchange (2a) is observed.

Acknowledgements

This work has been supported by the Italian Ministero dell’Ist-ruzione, Università e Ricerca. Assistance with the magnetic andEPR measurements by Mr. Gerard A. van Albada (Leiden Univer-sity) is kindly acknowledged.

Appendix A. Supplementary material

Supplementary material CCDC 827358–827360 contains thesupplementary crystallographic data for the compounds 2b, 2cand 1e. These data can be obtained free of charge from The Cam-bridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with thisarticle can be found, in the online version, at doi:10.1016/j.ica.2012.02.030.

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