Inorganica Chimica Acta 324 (2001) 117–122

www.elsevier.com/locate/ica

Crystal structure, spectroscopic characterisation and magneticproperties of [Cu(BIP)(N3)]·H2O

(BIP=3,3-bis(2-imidazolyl)propionate), a copper(II) polymericcompound with asymmetrical �-1,3-azido bridges

Hugo Nunez, Emilio Escriva, Juan Server-Carrio, A. Sancho, J. Garcıa-Lozano,Lucıa Soto *

Departament de Quımica Inorganica, Uni�ersitat de Valencia, c/ Vicent Andres Estelles, s/n, 46100 Burjassot (Valencia) Spain

Received 9 June 2001; accepted 4 July 2001

Abstract

The structure and the spectroscopic and magnetic properties of [3,3-bis(2-imidazolyl)propionato]azidocopper(II) monohydrateare described. The compound is built of [Cu(BIP)N3] entities which are connected through carboxylate groups from the BIPmolecules —which act as a tridentate ligand—and asymmetrical �-1,3-azido bridges, leading to a polymeric sheet-like structure.The copper atom is involved in a CuN3ON� chromophore and lies in a distorted square-pyramidal environment. Both electronicand EPR spectra are indicative of an essentially dx 2−y 2 ground state for the copper(II) ions. Magnetic susceptibility measurementsin the range 1.8–200 K show very weak antiferromagnetic exchange between the copper(II) ions (2J= −0.48 cm−1). This featureis discussed on the basis of the relative orientation of the coordination polyhedron around the metal atom and the bridgingnetwork. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Crystal structures; Copper(II) polymers; Azido bridges; Antiferromagnetic exchange; EPR spectroscopy

1. Introduction

In the field of magnetochemistry there is a continuingand growing interest in the design and characterisationof polymeric (from 1D to 3D) new materials. In thiscontext, the azido ligand is an excellent candidate to beemployed as bridging ligand due to its well knowversatility. So, in the last decade, a great number ofpublications have been devoted to the synthesis andcharacterisation of azido-containing polymeric coordi-nation compounds, many of them focussed on theanalysis of magneto-structural correlations [1–10].Mainly two coordination modes for azido bridgingligand have been observed: �-1,3 (end-to-end) and �-1,1(end-on), both in symmetrical and asymmetrical ways.In general, the end-to-end mode exhibits antiferromag-

netic interactions whereas the end-on bridge propagatesferromagnetic ones.

On the other hand, the anion 3,3-bis(2-imida-zolyl)propionate (BIP, see Scheme 1) is a versatil imida-zole-carboxylate polyfunctional system which can act asbidentate or tridentate ligand, thus playing an out-standing role in the building up of polymeric structures.Indeed, we have previously benefited from this chelat-ing ability to synthesize both dimeric and polymericBIP-containing copper(II) compounds [11,12]. Bearingin mind the above described coordinating behaviour ofthe BIP and N3

− anions and regarding the well provedplasticity of the copper(II) ions [13], it may be feasibleto obtain polymeric structures from [Cu(BIP)] units andazide anions.

In the present work we report the synthesis andcharacterization of [Cu(BIP)(N3)]·H2O, a copper(II)polymeric sheet-like structure with asymmetrical �-1,3-azido bridges and very weak antiferromagnetic interac-tions between the metal ions.

* Corresponding author. Tel.: +34-96-386-4532; fax: +34-96-386-4960.

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 -1693 (01 )00563 -1

H. Nunez et al. / Inorganica Chimica Acta 324 (2001) 117–122118

2. Experimental

2.1. Preparation of the complex

3,3-bis(2-imidazolyl)propionic acid (HBIP) was pre-pared according to Joseph et al. [14], and character-ized by NMR, IR spectroscopy and powder X-raydiffraction. The rest of the agents were purchasedfrom commercial sources and used as received. Anaqueous solution of copper(II) perchlorate (0.5 mmolin 5 ml) was added with stirring to an aqueous solu-tion of HBIP (0.5 mmol in 50 ml). Then, to theresulting blue solution was added a sodium azide solu-tion (0.5 mmol in 5 ml). The emerald green solutionwas stirred for a few minutes. Green prismatic crystalswere formed overnight and were filtered and storedin a dissicator over silica gel. Anal. Calc. forCuC9H11N7O3: Cu, 19.33; C, 32.88; H, 3.35; N, 29.83.Found: Cu, 19.10; C, 32.57; H, 3.43; N, 29.44%.

2.2. Physical measurements

The IR spectrum (KBr pellets) was recorded on aPye Unicam SP 2000 spectrophotometer. Diffusereflectance spectrum was obtained using a Perkin–Elmer Lambda 9 UV–Vis/IR spectrophotometer.Polycrystalline powder EPR spectrum was recorded ona Bruker ESP-300 equipped with a standard Oxfordlow-temperature device working at X-band (9.45GHz). Magnetic susceptibility was measured by meansof a commercial SQUID magnetometer, Quantum De-sign model MPMS7 down to 1.8 K and using a mag-netic field of 0.5 T. Mercury(II) tetrakis(thiocyanato)-cobaltate(II) was used as a susceptibility standard. Theexperimental susceptibilities were corrected both forthe diamagnetic contributions using the Pascal con-stants [15] and for the temperature independent para-magnetism of the Cu(II) ion estimated to be 6×10−5

cm3 mol−1 per copper(II) ion.

2.3. X-ray structure determination

The selected green prismatic crystal, with approxi-mate size 0.10×0.15×0.20 mm, was mounted on anEnraf–Nonius CAD4 single-crystal diffractometer andintensity measurements were carried out at 293 K us-ing graphite-monochromated Mo K� radiation (�=0.71069 A� ). The unit cell dimensions were determinedfrom the angular settings of 25 reflections with 8���17°. The intensity data of 2750 reflections weremeasured between the limits 1���25° using �–2�

scan technique, variable speed, width (0.75+0.35 tan �)° in �, and maximum scan time of 60 s perreflection in the hkl range 0 to 9, 0 to 9, −23 to 23.Data reduction was done with the program XRAY-76[16]. The merging of a number of equivalent reflec-tions gave an Rint=0.011. From the 2315 independentreflections 1130 were considered observed with theFo�4�(Fo) criterion. An empirical absorption correc-tion, following the procedure DIFABS [17] with mini-mum and maximum correction coefficients of 0.76 and1.00 was applied.

The structure was solved by direct methods usingthe program SIR-92 [18]. All non-hydrogen atoms wereanisotropically refined by least-squares on F2 (181parameters) using SHELXL-97 [19]. The hydrogenatoms of the ligand molecules were geometrically cal-culated and the hydrogen atoms of the water moleculewere located by difference synthesis. Geometrical cal-culations were made with PARST [20]. Graphical ma-nipulations were produced by the ORTEP3 forWindows program [21]. Other relevant parametersof the crystal structure determination are listed inTable 1.

Scheme 1.

Table 1Crystallographic data and refinement parameters for the complex

C9H11N7O3CuEmpirical formulaCrystal system monoclinicSpace group P21/nUnit cell dimensions

8.033(1)a (A� )b (A� ) 7.725(1)c (A� ) 19.079(1)� (°) 93.60(1)

U (A� 3) 1181.7(2)Z 4Dcalc (g cm−3) 1.85

328.79M668F(000)

� (cm−1) 18.5Goodness-of-fit 0.855��max, ��min (e A� −3) 0.66, −0.48R1

a 0.043wR2

b 0.081

a R1=���Fo�−�Fc��/��Fo� for reflections with I�2�(I).b wR2={�[w(Fo

2–Fc2)2]/�[w(Fc

2)2]}1/2 for all reflections; w=1/[�2(Fo

2)+(aP)2+bP ], where P= [2Fc2+Fo

2]/3 and a and b are con-stants set by the program.

H. Nunez et al. / Inorganica Chimica Acta 324 (2001) 117–122 119

Table 2Selected bond lengths (A� ) and bond angles (°) for the title complex

Azide ligandCopper coordination sphere

Bond lengthsN(5)–N(6) 1.210(7)Cu(1)–N(2) 1.984(5)N(6)–N(7)1.982(5) 1.144(7)Cu(1)–N(3)

1.986(5)Cu(1)–N(5)2.021(4)Cu(1)–O(1I)2.531(4)Cu(1)–N(7II)

Cu(1)–O(2I) 2.718(6)

Bond anglesN(2)–Cu(1)–N(3) N(7)–N(6)–N(5)90.4(2) 174.3(7)N(2)–Cu(1)–N(5) 178.9(2)

89.9(2)N(2)–Cu(1)–O(1I)N(2)–Cu(1)–N(7II) 97.5(2)

53.7(2)N(2)–Cu(1)–O(2I)N(3)–Cu(1)–N(5) 90.5(2)

173.7(2)N(3)–Cu(1)–O(1I)N(3)–Cu(1)–N(7II) 95.0(2)

120.1(2)N(3)–Cu(1)–O(2I)N(5)–Cu(1)–O(1I) 89.2(2)

83.1(2)N(5)–Cu(1)–N(7II)89.4(2)N(5)–Cu(1)–O(2I)53.6(2)O(1I)–Cu(1)–O(2I)

144.2(2)N(7)–Cu(1)–O(2I)

Symmetry operators: I x+1, y, z ; II −x+1/2, +y–1/2, −z+1/2.

cal �-1,3-azido bridges running along the [0,1,0] direc-tion, thus leading to a polymeric sheet-like structure.Through this extended array, a copper atom in a chainis connected to the other two copper atoms located inneighbouring chains, with copper–copper distances of5.66(1) A� . Finally, the formed sheets interact throughvan der Waals contacts and hydrogen bonds betweencarboxylate groups, nitrogen atoms (from the imidazoleand azide groups) and water molecules. Interatomicbond distances and angles are given in Table 2. Fig. 1shows a perspective view of the repeated unit with theatomic numbering scheme. Fig. 2 is a projection of thepolymeric structure perpendicular to the chains gener-ated by the BIP coordination showing the �-1,3-azidobridges.

The copper atom is involved in a CuN3ON� chro-mophore and lies in a distorted square-pyramidal envi-ronment. The equatorial plane comprises two nitrogenatoms belonging to two imidazole moieties, one nitro-gen atom from the azide anion and an oxygen atomfrom the carboxylate group. The apical site is occupiedby the nitrogen atoms: N(7)II from an asymmetrical�-1,3-azido bridging group. As usual, the copper atomis displaced (0.0064 A� ) out of the basal plane towardsthe water molecule (4+1 coordination mode). TheCu–Leq distances fall within the normal range andcompare well with the bond lengths found in otherCu(II)–BIP [11,22] and Cu(II)-�-1,3-azido complexes,respectively [10,23–27]. The distortion of the CuN3ON�

Fig. 1. Perspective view and atomic numbering of the [Cu-(BIB)(N3)]·H2O.

Fig. 2. Crystal packing of the title compound.

3. Results and discussion

3.1. Crystal structure description

The structure of the complex can be viewed as madeup of [Cu(BIP)N3] units and lattice water molecules.These [Cu(BIP)N3] entities are connected through car-boxylate groups from the BIP molecules—which act asa tridentate ligand— thus affording chains runningalong the [1,0,0] direction. The shortest separation be-tween two neighbouring copper atoms along thesechains is 8.033(1) A� . In addition, [Cu(BIP)N3] unitsbelonging to different chains are linked by asymmetri-

H. Nunez et al. / Inorganica Chimica Acta 324 (2001) 117–122120

Table 3Hydrogen bonds in the [Cu(C9H9N4O2)(N3)]·H2O crystal

X···Y (A� ) H···Y (A� ) �X–H···Y (°)X–H···Y (A� ) X–H (A� )

0.91 2.822(5)O(3)–H(3A) 1.93 168···O(1III)

O(3)–H(3B) 0.99 2.850(6) 1.95 149···N(5)

N(1)–H(1) 0.86 2.854(5) 2.05 156···O(3IV)

0.86 2.973(5) 2.22N(4)–H(4) 146···O(3III)

Symmetry operators: III −x–1/2, y+1/2, −z+1/2; IV x−1/2, −y+1/2, z−1/2.

(1.144(7) and 1.210(7) A� ) is in agreement with theasymmetric coordination behaviour exhibed by theazido groups. Moreover, the angles N–N–Cu are133.5(5)° for the short Cu–N bond distance and113.6(5)° for the longer one.

The hydrogen bonds listed in Table 3 determinemainly the crystal packing shown in Fig. 2. Latticewater molecules are four coordinated: as H-donors withthe azido and carboxylate equatorial copper-coordi-nated atoms, N(5) and O(1), from different complexunits of the same polymeric sheet; and as H-acceptorswith the imidazole nitrogen atoms, N(1) and N(4), fromdifferent polymeric sheets. As expected for a nearlytetrahedral arrangement, all six H–O(3)–H� angles(mean value of 108.5°) are close to 109.5°. The resultinghydrogen bond system leads to an infinite three-dimen-sional array. The crystal packing is completed by �–�stacking interactions of parallel imidazole rings (alongthe b direction) with distances between the imidazolenucleus of approximately 3.7–4.0 A� [31].

3.2. Vibrational and electronic spectra

The IR spectrum of the complex shows a strongsharp band in the 3600–3400 cm−1 range assignable to�(OH) stretching vibrations.The N–H vibration fromthe imidazole rings appears in the 3220–3120 cm−1

region. The most characteristic feature is a strong sharpband at 2059 cm−1, due to the azide group, consistentwith an asymmetrical end-to-end coordination mode[26,32]. In the 1650–800 cm−1 region, there are severalbands from the BIP ligand, assignables to the imidazolemoieties and carboxylate groups. The two strong ab-sorption bands observed at 1616 and 1390 cm−1 areassigned to antisymmetric (�as) and symmetric (�s) O–C–O stretching modes, respectively, from the coordi-nated carboxylate groups of the BIP ligand.

The diffuse reflectance spectrum of the complexshows a broad absorption in the visible region with amaximum centered at ca 15800 cm−1, wich is consistentwith a five-coordinated copper(II) with square-pyrami-dal geometry [13].

3.3. Electron spin resonance spectroscopy and magneticproperties

The room temperature polycrystalline X-band EPRspectrum of the title compound shows a wide axialsignal giving g��=2.23 and g�=2.06 (gav=2.12). Low-ering the temperature to 100 K produces no effect onthe shape and the position of the signal which is devoidof any hyperfine structure. The g-values are in agree-ment with those expected for CuN3ON� chromophoreswith the copper atom exhibiting a very elongated octa-hedral environment, and indicate a basically dx 2−y 2

ground state for the Cu(II) ion.

polyhedron can be quantified through the parameter

as defined by Addison et al. [28]. The calculated value=0.08 (relative to 0 for a regular square pyramid(SPY) and 1 for a regular trigonal bypiramid (TBPY))clearly indicates the predominance of the SPY form. Onthe other hand, if the very weak Cu–O(2)I is consideredas an extension of the fivefold coordination, the stereo-chemistry about the copper atom could be described asan elongated and strongly distorted octahedron. In thiscase the carboxylate group is contemplated as a chelat-ing bidentate ligand in an off-the-z-axis coordinationwith a N(7)II–Cu–O(2)I angle of 144.2(1)°. The coordi-nation geometry about the copper atom could thus beviewed as 4+1+1* (CuN3ON�O* chromophores), in-termediate between five- and six-coordination. For theoff-the-z-axis bonding, the Cu–Oax distance exceeds theCu–Oeq one (see Table 2) by approximately 0.70 A� ,and according to Hathaway’s criterion [29] the axialoxygen atoms must be considered as semicoordinated.In that respect the Cu–O(2)I interaction must be re-garded as very weak, according to the criterion estab-lished for the off-the-z-axis bonding in anisobidentatetriangular and tetrahedral oxoanions [30].

The geometry of BIP molecules (interatomic dis-tances and angles) is similar to that previously found inHBIP/BIP-containing copper(II) compounds [11,12,24].The imidazole rings of ligands are planar as expected,with deviations from the mean planes not greater than0.08(5) A� . The dihedral angle between the two imida-zole rings is 23.3(2)°, and the rings form dihedral angleswith the basal coordination planes of 10.9(2) and16.2(2)°. The carboxylate group is nearly perpendicularto the basal least-square plane (dihedral angle of89.5(3)°). On the other hand, C–O bond distancessatisfy the trend C–Ocoord�C–Osemicoord, as expectedfrom the polarization of the charge density toward themetal-bonded oxygen atoms.

The azido groups are almost linear within experimen-tal error (angle N(5)–N(6)–N(7) of 174.3(7)°). Besidethis, the observation of two different N–N distances

H. Nunez et al. / Inorganica Chimica Acta 324 (2001) 117–122 121

The title compound does not show a maximum insusceptibility in the studied temperature range.Notwithstanding, a plot of MT versus the temperature(Fig. 3) exhibits a significant decrease at T�15 K,indicating weak antiferromagnetic interactions betweenthe copper(II) ions. As it has been discussed previously,the title compound exhibits a polymeric sheet-like struc-ture. Nevertheless, taking into account the topology ofthe framework bridges, it may be assumed that theinterchain interaction—via the �-1,3-azido brigde—must be the only one significant compared with thatpropagated through the propionate bridges (intra-chains). In fact, this very extended (ca. 11 A� ) exchangepathways involves two C–C � bonds, that offer a verypoor support for the interactions [33]. Moreover, therelated compound [Cu(HBIP)(BIP)](C4O4)1/2·2H2O [12],where the [Cu(HBIP)(BIP)] units are connected in athrough the propionate arms affording a 1D polymericstructure exhibits a quasi-negligible antiferromagneticbehaviour (�J ��0.1 cm−1).Thus, from a magneticpoint of view, the title compound might be consideredas an assembly of ‘quasi’ isolated chains and, accord-ingly, its magnetic behavior could be analysed in termsof a regular S=1/2 Heisenberg chain using the expres-sion of Brown et al. [34] derived from the numericalresults of Bonner and Fisher [35]:

M=N�2g2

kT0.25+0.14995x+0.30094x2

1+1.9862x+0.68854x2+6.0626x3

where x= �J �/kT. The best fit is obtained with J=−0.48(1) and g=2.15, with an agreement factor ofR=4×10−4 (R is defined as �[(M)obsd−(M)calcd]2/�[(M)obsd]2).

This behaviour may be understood in terms of thenature of the orbitals involved in the exchange interac-tions together with structural considerations on thebridging network. As discussed above, in the presentcompound the unpaired electron of the copper(II) ion isessentially described by a magnetic orbital built fromthe dx 2−y 2 metallic orbital with little contribution ofthe dz 2 orbital, and being localized basically in the basal

planes, which are nearly normal to the direction of theexchange propagation. Hence, as it is usual for axial-equatorial (parallel–planar) polymeric copper(II) sys-tems, the observed weak coupling must be attributed tothe small z2-type character acquired by the magneticorbitals.

Acknowledgements

We are grateful to the DGICYT (PB98-1453) forfinancial support. We thank Drs C. Gomez and A.Marie for their assistance with susceptibility measure-ments and EPR experiences, respectively.

References

[1] L. Casella, M. Gulloti, G. Pallanza, M. Buga, Inorg. Chem. 30(1991) 221.

[2] J. Ribas, M. Monfort, B.K. Ghosh, X. Solans, Angew. Chem.,Int. Ed. Engl. 33 (1994) 2087.

[3] L.K. Thompson, S.S. Tandon, M.E. Manuel, Inorg. Chem. 34(1995) 2356 and references therein.

[4] G. De Munno, M. Julve, G. Viau, F. Lloret, J. Faus, D. Viterbo,Angew. Chem., Int. Ed. Engl. 35 (1996) 1807.

[5] J. Ribas, M. Monfort, I. Resino, X. Solans, P. Rabu, F. Main-got, M. Drillon, Angew. Chem., Int. Ed. Engl. 35 (1996) 2420.

[6] A. Escuer, R. Vicente, F.A. Mautner, M.A.S. Goher, Inorg.Chem. 36 (1997) 1233.

[7] L.K. Thompson, S.S. Tandon, F. Lloret, J. Cano, M. Julve,Inorg. Chem. 36 (1997) 3301.

[8] M.A.S Goher, F.A. Mautner, J. Chem. Soc., Dalton Trans.(1999) 1535.

[9] F.F. DE Biani, E. Ruiz, J. Cano, J.J. Novoa, S. Alvarez, Inorg.Chem. 39 (2000) 3221 and references therein.

[10] P.S. Mukherjee, T.K. Maji, G. Mostafa, T. Mallah, N.R.Chaudhuri, Inorg. Chem. 39 (2000) 5147.

[11] Y. Akhriff, J. Server-Carrio, A. Sancho, J. Garcıa-Lozano, E.Escriva, J.V. Folgado, L. Soto, Inorg. Chem. 38 (1999) 1174.

[12] Y. Akhriff, J. Server-Carrio, A. Sancho, J. Garcıa-Lozano, E.Escriva, L. Soto, Inorg. Chem., in press.

[13] B.J. Hathaway, Struct. Bonding 57 (1984) 55 and referencestherein.

[14] M. Joseph, T. Leigt, M.L. Swain, Synthesis (1977) 459.[15] O. Kahn, Molecular Magnetism, VCH Publisher, New York,

1993, p. 3.[16] J.M. Stewart, P.A. Machin, C.W. Dickinson, H.L. Ammon, H.

Heck H. Flack, The X-RAY 76 System, Technical report TR-446,Computer Science Center, University of Maryland, College Park,MD, 1976.

[17] N. Walker, D. Stuart, Acta Crystallogr., Sect. A 39 (1983) 158.[18] A. Altomare, M.C. Burla, M. Camalli, G. Cascarano, G. Giaco-

vazzo, A. Guagliardi, G. Polidori, J. Appl. Crystallogr. 27 (1994)435.

[19] G.M. Sheldrick, SHELXL-97, Program for the Refinement ofCrystal Structures, University of Gottingen, Germany, 1997.

[20] M. Nardelli, Comput. Chem. 7 (1983) 95.[21] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.[22] A. Sancho, B. Gimeno, J.M. Amigo, L.E. Ochando, T. De-

baerdemaeker, J.V. Folgado, L. Soto, Inorg. Chim. Acta 248(1996) 153.

[23] I. Brouche-Waksman, M.-L. Boillot, O. Kahn, S. Sikorav, Inorg.Chem. 23 (1984) 4454.Fig. 3. Magnetic behaviour of [Cu(BIB)(N3)]·H2O.

H. Nunez et al. / Inorganica Chimica Acta 324 (2001) 117–122122

[24] P. Chaudhuri, K. Order, K. Wieghardt, B. Nuber, J. Weiss,Inorg. Chem. 23 (1986) 2818.

[25] F.A. Mautnier, M.A.S. Goher, Polyhedron 11 (1992) 2537.[26] G. De Munno, M.G. Lombardi, M. Julve, F. Lloret, J. Faus,

Inorg. Chim. Acta 282 (1998) 82 and references therein.[27] A. Escuer, M. Font-Bardia, E. Penalba, X. Solans, R. Vicente,

Inorg. Chim. Acta 298 (2000) 195.[28] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Ver-

schoor, J. Chem. Soc., Dalton Trans. (1984) 1349.[29] B.J. Hathaway, in: G. Wilkinson (Ed.), Comprehensive

Coordination Chemistry, vol. 5, Pergamon, Oxford, 1985, p.604.

[30] C.C. Addison, N. Logan, S.C. Wallwork, C.D. Garner, Q. Rev.Chem. Soc. 25 (1971) 289.

[31] E.J. Dirks, J.G. Haasnoot, A.J. Kinneging, J. Reedijk, Inorg.Chem. 26 (1987) 1902.

[32] K. Nakamoto (Ed.), Infrared and Raman spectra of Inorganicand Coordination Compounds, 4th ed., Wiley, New York, 1986,p. 290.

[33] P.J. Hay, J.C. Thibeault, R. Hoffman, J. Am. Chem. Soc. 97(1975) 4884.

[34] D.B. Brown, J.A. Donner, J.W. Hall, S.R. Eilson, R.B. Wilson,D.J. Hodgson, W.E. Hatfield, Inorg. Chem. 18 (1979) 2635.

[35] J.C. Bonner, M.E. Fischer, Phys. Rev. 165 (1968) 647.