Inorganic Chemistry Communications 12 (2009) 555–557

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Inorganic Chemistry Communications

journal homepage: www.elsevier .com/locate / inoche

A Michael-type reaction between acrylato ions and ethylenediamine coordinatedto Ni(II). Synthesis, crystal structure and magnetic properties of[Ni2(EDDP)2(H2O)2] � 2H2O (H2EDDP = ethylenediamine-N,N-dipropionic acid)

Mihaela Badea a,*, Rodica Olar a, Dana Marinescu a, Gina Vasile b, Bogdan Jurca a,Augustin M. Madalan a, Marius Andruh a

a Inorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucharest, Str. Dumbrava Rosie 23, 020464 Bucharest, Romaniab University of Agronomical Sciences and Veterinary Medicine, Department of Agrochemistry, Bd. Marasti 59, Bucharest, Romania

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 February 2009Accepted 15 April 2009Available online 24 April 2009

Keywords:Nickel complexesCarboxylato-bridged complexesCrystal structureMagnetic properties

1387-7003/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.inoche.2009.04.019

* Corresponding author.E-mail address: e_m_badea@yahoo.com (M. Badea

The reaction between NiCO3 � Ni(OH)2, acrylic acid and ethylenediamine in a 2:4:1 molar ratio affords thebinuclear complex, [Ni2(EDDP)2(H2O)2] � 2H2O 1. The organic ligand, EDDP2� (the dianion of the ethylene-diamine-N,N-dipropionic acid ligand), results from the addition of one amine group to the carbon–carbondouble bonds of two acrylato ions. The crystal structure of 1 consists of neutral centrosymmetric entities,with the nickel ions connected by two carboxylato groups, each one acting as a monoatomic bridge. Theintramolecular Ni� � �Ni distance is 3.212 Å. The metal ions exhibit an octahedral geometry. The cryomag-netic investigation of 1 reveals an antiferromagnetic coupling of the nickel(II) ions (J = �21.8 cm�1,H = �JSNi1SNi2).

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Ethylenediamine-polycarboxylic acids are well known andwidely used in industry and analytical chemistry as chelatingagents. By far, most of the references deal with the acetic acidderivatives: ethylenediamine-N,N,N0,N0-tetracetic acid [1], ethy-lenediamine-N,N,N0-triacetic acid [2], ethylenediamine-N,N0-diace-tic acid [3], ethylenediamine-N,N-diacetic acid [4]. The substitutionof the acetic acid moieties with the propionic acid ones leads alsoto good and selective chelators (e.g. ethylenediamine-N,N0-diace-ticacid, ethylenediamine-N,N0-dipropionicacid, ethylenediamine-N,N0,N0-tripropionicacid) [5,6]. Other biodegradable EDTA-likecompounds (e.g. ethylenediamine-N,N0-disuccinic acid, ethylenedi-amine-N,N0-dimalonic acid [7]) have been studied in order to de-crease the environmental impact of these compounds. Generally,these compounds contain the ethylenediamine moiety substitutedat both nitrogen atoms. However, there are only few referencesabout ethylenediamine-N,N-diacetic acid [4], and only one dealswith the ethylenediamine-N,N-dipropionic acid (H2EDDP) [8]. Thelast one has been prepared by the reaction of N-acetyl-ethylendi-amine with 3-chloropropionic acid followed by the hydrolysis. Itscomplex with 99mTc was obtained to be tested as a tumour-scan-ning agent.

Such an ethylenediamine-polycarboxylic acid was obtained byus in the attempt to synthesize complexes with mixed ligands(the acrylato ion and aliphatic amines). We observed an unex-

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).

pected reaction between ethylenediamine and the acrylato ionscoordinated to Ni(II). The resulted species (N,N-ethylenediamined-ipropionate ion, EDDP2�), acts as a tetradentate ligand, leading tothe binuclear complex, [Ni2(EDDP)2(H2O)2] � 2H2O 1, whose crystalstructure, spectroscopic and magnetic properties are reportedherein. The reactions between the organic ligands coordinated toa metal ion represent an important chapter in coordination chem-istry [9]. Various organic molecules can be obtained in higheryields than in the absence of the metal ions, or cannot be obtainedotherwise.

The synthesis of [Ni2(EDDP)2(H2O)2] � 2H2O 1 was achieved bythe reaction of NiCO3�Ni(OH)2, acrylic acid and ethylenediaminein 2:4:1 ratio, according to the experimental conditions [10]. Anunexpected reaction between the acrylato ions and the ethylenedi-amine coordinated to the nickel ion was observed (Scheme 1). Theaddition of an amine group to the electron deficient alkenes(Michael-type additions) is well known in organic chemistry [11],but it was never reported as a synthetic method for ethylenedia-mine-N,N-dipropionic acid.

The infrared spectrum of 1 shows the characteristic vibrationbands of the different moieties of the organic ligand. The asymmet-rical stretching mode of the carboxylato group, masym, is found at1566 cm�1 and the symmetrical one, msym, at 1399 and 1341cm�1. The splitting of this last band suggests that the carboxylatogroups adopt different coordination modes [12,13].

The electronic spectrum of 1 is typical for an octahedral nicke-l(II) ion [14]. The three absorption bands at 369, 599 nm and

H2N CH2 CH2 NH2 +CH2 CH COO-

CH2 CH COO-H2N CH2 CH2 N

COO-CH2CH2

COO-CH2CH2

Scheme 1.

556 M. Badea et al. / Inorganic Chemistry Communications 12 (2009) 555–557

943 nm are assigned to the spin-allowed d–d transitions: 3A2g ?3T1g(P), 3A2g ?

3T1g(F) and 3A2g ?3T2g(F), respectively. A weak

absorption due to the 3A2g ?1Eg spin forbidden transition is ob-

served at 756 nm. These data allow the calculation of the ligandfiled parameters, 10Dq and Racah (B): 10Dq = 10,600 cm�1;B = 798 cm�1. The value of the nephelauxetic parameter, b, is 0.768.

The crystallographic investigation [15] reveals neutralcentrosymmetric binuclear entities and crystallization watermolecules (Fig. 1). The nickel ions are connected by two carboxy-lato groups arising from two EDDP2� ligands (monoatomicbridging mode). The EDDP2� anion acts as a tris-chelating ligand.Each nickel ion is coordinated by the primary and tertiary aminenitrogen atoms [Ni1–N1 = 2.0832(16); Ni1–N2 = 2.069(5) Å], twocarboxylato bridging oxygens [Ni1–O3 = 2.0720(13), Ni1–O3i =2.1060(13) Å; i = 1 � x, 2 � y, �z], one oxygen from the othercarboxylato group [Ni1–O1 = 2.028(4) Å], and one aqua ligand[Ni1–O1W = 2.0969(15) Å]. The coordination geometry of the nick-

Fig. 1. Perspective view of the binuclear complex 1, along with the atom numberingscheme.

Fig. 2. vMT vs. T curve for compound 1 (the solid line represents the best fit curve).

el ions is octahedral. The intramolecular distance between thenickel atoms is 3.212 Å.

Magnetic susceptibility data for 1 have been collected in thetemperature range 1.9–300 K (Fig. 2). The value of the vMT productat room temperature is 2.24 cm3 mol�1 K, that corresponds to theexpected one for two uncoupled nickel(II) ions. Below 100 K thevMT product decreases abruptly, indicating an antiferromagneticcoupling of the nickel(II) ions.

The magnetic data are analysed by using the isotropic spinHamiltonian:

H ¼ �JSNi1SNi2

(SNi1 and SNi2 are the local spins of the nickel ions).The temperature dependence of the magnetic susceptibility is

given by the equation:

vMT ¼ Nb2

3kg2 6ex þ 30e3x

1þ 3ex þ 5e3x

where x ¼ JkT. Least-squares fit to the data leads to J = �21.8 cm�1,

g = 2.1 (a TIP contribution of 100 � 10�6 cm3 mol�1 was taken intoaccount for each nickel ion).

For binuclear compounds featuring a Ni2O2 core, the magnitudeof the exchange interaction is influenced by several factors, one ofthem being the Ni–O–Ni angle [16]. It has been noticed that theantiferromagnetic interaction dramatically increases for Ni–O–Niangles higher than 102�, although a simple relationship relatingthe J value and the bridging angle is not applicable. Other factors(e.g. the angle between the Ni–O–Ni plane and the remaining coor-dination plane, the apical distances, etc.) play also a non-negligiblerole [16b]. For compound 1 the value of the Ni–O–Ni angle is100.5�, the interaction between the nickel ions being moderatelyantiferromagnetic.

In conclusion, we have described the addition reaction of oneamine group of the ethylenediamine to the carbon–carbon doublebonds from two acrylato ions coordinated to nickel(II) ions. A newcarboxylato-bridged binuclear complex was characterized. Thisreaction deserves to be investigated by using other organic amines,as well as other transition metal ions. Moreover, the resultedcomplexes can be employed as ligands toward other metal ions,in order to obtain heterometallic complexes. Further work is inprogress.

Acknowledgement

We are grateful to the Romanian Ministry of Education andResearch for the financial support to purchase the SQUIDmagnetometer.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.inoche.2009.04.019.

References

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[9] E.C. Constable, Metals and Ligand Reactivity, Wiley VCH, Weinheim, 1996.[10] Synthesis of [Ni2(EDDP)2(H2O)2] � 2H2O 1: An aqueous mixture (20 mL) of

NiCO3 � Ni(OH)2 (0.952 g, 8 mmol), acrylic acid (1.1 ml, 16 mmol) andethylenediamine (0.27 mL, 4 mmol) was stirred at room temperature for 1 h,and the resulted pale green solution was filtered off (ethylenediamine was

added after the formation of nickel(II) acrylate from the reaction ofNiCO3 � Ni(OH)2 with acrylic acid). The filtrate was allowed to slowlyevaporate at room temperature for about two weeks and then 15 ml DMSOwas added. After other 2 weeks the resulted dark blue crystals were filteredoff, washed with small amounts of cold ethanol and air-dried (yield: 40%).Single crystals suitable for X-ray diffraction were obtained by recrystallisationfrom a DMSO/ethanol (1:1, v/v). Elemental chemical analyses: 32.61 C; 5.86 H;9.7 N, 20.04% Ni, (found); 32.36 C; 6.11 H; 9.43 N; 19.77% Ni (calcd). SelectedIR data (KBr, cm�1): 3581s; 3472m; 3337s; 1645m; 1566vs; 1399s, 1341m.Diffuse reflectance spectrum: 369, 599, 756 and 943 nm.

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New York, 1986. pp. 448, 507, 544.[15] X-ray data for crystal 1 were collected at room temperature on a STOE IPDS II

diffractometer. The structure was solved by direct methods and refined byfull-matrix least squares techniques based on F2. The non-H atoms wererefined with anisotropic displacement parameters. Calculations wereperformed using SHELX-97 crystallographic software package. Chemicalformula: C16H36N4Ni2O12; M (g mol�1): 593.88; temperature (K): 298;wavelength (Å): 0.71073, crystal system: monoclinic; space group: P21/n;unit cell dimensions: a (Å), 10.4103(8), b (Å), 9.7661(5), c (Å), 11.8373(9), b (�),101.65(1), V (Å3), 1178.69(14); Z = 2; Dc (g cm�3): 1.662; l (mm�1): 1.665;F(0 0 0) = 616; goodness-of-fit on F2: 1.131; final R1, wR2 [I > 2r(I)] indices:0.0429, 0.0826; R1, wR2 (all data): 0.0538, 0.0858, largest diff. peak and hole(eÅ�3), 0.584; �0.753.

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