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This article was published in an Elsevier journal. The attached copyis furnished to the author for non-commercial research and
education use, including for instruction at the author’s institution,sharing with colleagues and providing to institution administration.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Copper(II) complexes with pyrazolyl-substituted nitronyland imino nitroxides
Eugene V. Tretyakov, Svyatoslav E. Tolstikov, Elena V. Gorelik, Matvey V. Fedin,Galina V. Romanenko, Artem S. Bogomyakov, Victor I. Ovcharenko *
International Tomography Center, Siberian Branch, Russian Academy of Sciences, Institutskaya Street, 3a, RU-630090 Novosibirsk, Russian Federation
Received 31 August 2007; accepted 2 November 2007
Abstract
It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide (HL1) and pyrazol-3-yl-substituted imino nitr-oxide (HL3) with Cu(II) acetate lead to self-assembly of the Cu4(OH)2(OAc)4(DMF)2(L1)2 tetranuclear and Cu2(OAc)2(H2O)2(L3)2
dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide(HL2) gave unexpected solid Cu2(H2O)2(L6)2 Æ 2DMF, in which L6 is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronylnitroxide, formed as a result of cleavage of an ester bond in the starting HL2. A similar transformation of the paramagnetic ligandwas observed in the reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted imino nitroxide (HL4). It led to theformation of Cu2(DMF)2(L7)2, where L7 is deprotonated 2-(5-carboxy-1H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole3-oxide. An X-ray diffraction study indicated that in Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(OAc)2(H2O)2(L3)2, the L1 and L3
paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu2(H2O)2(L6)2 Æ 2DMF and Cu2(DMF)2(L7)2, theparamagnetic L6 and diamagnetic L7 are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinatednitronyl nitroxide – Cu4(OH)2(OAc)4(DMF)2(L1)2 and Cu2(H2O)2(L6)2 Æ 2DMF is dictated by the dominant antiferromagneticexchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu2(OAc)2(H2O)2(L3)2 reflectsthe competition between the antiferromagnetic and ferromagnetic components, of which the latter is due to electron coupling inthe Cu(II) N=C–N ~ O exchange channels. EPR data confirm the results received from static magnetic measurements for multi-spin solids.� 2007 Elsevier Ltd. All rights reserved.
Pyrazolyl-substituted nitronyl (NN-Pz) and imino nitr-oxides (IN-Pz) are of particular interest as stable paramag-netic ligands useful in the design of magnetic structures,which stimulates the development of methods for theirselective synthesis. A wide range of spin-labeled pyrazolesare now accessible [1], with which mixed-ligand complexesof hexafluoroacetylacetonates [1a,2] and polynuclear metal
pivalates [3] have been obtained. At the same time, thereare few examples of chelate compounds with deprotonatedspin-labeled derivatives NN-Pz and IN-Pz and include onlytwo silver complexes: [Ag(L3)]6 and [Ag(L5)]n [1d] and twonickel complexes: [Ni3(L1)6] and [Ni3(L3)6] [4]. We haveattempted to expand this series of complexes by addingto them Cu(II) compounds with HL1, HL2, HL3, andHL4. Indeed, we have succeeded in isolating new complexeswith spin-labeled pyrazoles and defined their structure. Themagnetic properties of these compounds were analyzedusing DFT calculations. It has been found that the copperions are capable of promoting unusual transformations ofthe paramagnetic ligand.
0277-5387/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
IR spectra were recorded in the 400–4000 cm�1 regionon a VECTOR 22 Bruker spectrophotometer (KBr pellets).Microanalyses were performed on a Carlo Erba 1106instrument. The nitroxides HL1, HL2, HL3, and HL4 wereprepared by the procedures described in the literature [1e].All reagents and organic solvents were analytical qualityand used as purchased.
2.2. Syntheses of the complexes
2.2.1. Cu4(OH)2(OAc)4(DMF)2(L1)2 (1)
A solution of Cu(OAc)2 Æ H2O (180 mg, 0.9 mmol) inwater (1.5 ml) was added to a solution of nitronyl nitrox-ides HL1 (100 mg, 0.45 mmol) in a mixture of water(1.5 ml) and MeOH (1.5 ml). The resulting solution waskept for 12 h, whereupon the solvent was distilled off invacuum at room temperature. The residue was dissolvedin a minimal amount of DMF (�4 ml). The solution wasfiltered into a conical flask, which was then connected viaa U-tube with a flask containing Et2O (20 ml). Single crys-tals of good quality were grown due to gradual dilution ofthe DMF solution of the complex with diethyl ether thattook place under these conditions. After 7 days, the green-ish blue crystals were separated by filtration and washedwith ether. Yield 200 mg (80%). IR: ~mmax ¼ 415; 485;546; 614; 672; 699; 790; 872; 972; 1020; 1080; 1103; 1136;1171; 1211; 1252; 1328; 1382; 1428; 1477; 1576; 1613; 1667;2928; 2998; 3129; 3436 cm�1. Elemental Anal. Calc. forC34H56Cu4N10O16: C, 36.6; H, 5.1; N, 12.6. Found: C,36.8; H, 5.3; N, 12.4%. Compound 1 also formed in�80% yield when the reaction of Cu(OAc)2 Æ H2O(180 mg, 0.9 mmol) with HL1 (100 mg, 0.45 mmol) wasconducted in DMF.
2.2.2. Cu2(H2O)2(L6)2 Æ 2DMF (2)
A solution of Cu(OAc)2 Æ H2O (130 mg, 0.66 mmol) inwater (2 ml) was added to a solution of nitronyl nitroxideHL2 (100 mg, 0.33 mmol) in MeOH (3 ml). The resultingsolution was stored for 12 h; then the solvent was distilledoff in vacuum at room temperature. The residue was crys-
tallized as described for 1. Compound 2 was obtained asgreenish blue crystals suitable for an X-ray analysis. Yield110 mg (77%). IR: ~mmax ¼ 481; 548; 607; 662; 798; 848; 875;1026; 1074; 1101; 1138; 1170; 1298; 1324; 1351; 1379; 1403;1467; 1510; 1627; 1666; 2993; 3393 cm�1. Elemental Anal.
Calc. for C28H44Cu2N10O12: C, 40.1; H, 5.3; N, 16.7.Found: C, 39.9; H, 5.3; N, 16.7%. When conductedin DMF, the reaction also gave compound 2 in �85%yield.
N N–N+
NO
L6
O–
O
O–
2.2.3. Cu2(OAc)2(H2O)2(L3)2 (3)
A solution of Cu(OAc)2 Æ H2O (80 mg, 0.39 mmol) inwater (1.5 ml) was added to a solution of imino nitroxidesHL3 (80 mg, 0.39 mmol) in a mixture of water (1.5 ml),MeOH (1 ml), and EtOH (1.5 ml). The resulting solutionwas stored for 12 h; then the solvent was distilled off in vac-uum at room temperature. The residue was processed asdescribed for 1. Complex 3 was obtained as dark greencrystals suitable for an X-ray analysis. Yield 40 mg(30%). IR: ~mmax ¼ 422; 468; 567; 611; 633; 684; 723; 795; 891;935; 968; 1020; 1077; 1137; 1188; 1260; 1340; 1369; 1400; 1482;1593; 1676; 2928; 2972; 3248; 3454 cm�1. Elemental Anal.
Calc. for C24H40Cu2N8O8: C, 41.4; H, 5.8; N, 16.1. Found:C, 41.6; H, 5.7; N, 16.1%.
2.2.4. Cu2(DMF)2(L7)2 (4)
A solution of Cu(OAc)2 Æ H2O (131 mg, 0.72 mmol) inwater (2 ml) was added to a solution of imino nitroxidesHL4 (100 mg, 0.36 mmol) in MeOH (1 ml); this immedi-ately resulted in the formation of a brown fine precipitate.Then MeOH (2 ml) was added until the precipitate dis-solved. The resulting solution was kept for 15 min; thenthe solvent was distilled off in vacuum at room tempera-ture. The residue was treated as described for 1. After 2days, the solution became greenish blue, and green needlesgradually crystallized. After 5 days, the crystals were sepa-rated by filtration and washed with ether. Yield 50 mg(36%). IR: ~m max ¼ 480; 570; 607; 714; 774; 800; 858; 898;1026; 1074; 1113; 1142; 1202; 1246; 1331; 1348; 1391; 1447;1544; 1625; 2812; 2885; 2990; 3199; 3454 cm�1. ElementalAnal. Calc. for C28H42Cu2N10O8: C, 43.5; H, 5.5; N,18.1. Found: C, 42.5; H, 5.6; N, 16.8%.
L7
N N–N+
NH
O–
O
O–
N NHX
NO
HL1: R = H, X = N+–O–
HL2: R = CO2Et, X = N+–O–
HL3: R = H, X = NHL4: R = CO2Et, X = N
R
N NHN
NO
N
NO
HL5
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2.3. X-ray measurement and structure determination
Diffraction reflections were obtained on a SMARTAPEX CCD (Bruker AXS) diffractometer (Mo Ka,k = 0.71073 A, T = 295 K; an absorption correction wasapplied using the Bruker SADABS program, version2.10). The structures were solved by direct methodsand refined by the full-matrix least-squares method inan anisotropic approximation for all non-hydrogenatoms. Some of the H atoms were located in a differenceelectron density map, while others were located at calcu-lated positions. The methyl H atoms were refined as arigid group in an isotropic approximation. All calcula-tions on structure solution and refinement were carriedout with a Bruker SHELXTL Version 6.14 program com-
plex. The crystal data for the complexes, details of exper-iment, and selected bond lengths and angles are given inTables 1 and 2.
2.4. Magnetic measurements
The magnetic susceptibility of the polycrystalline com-plexes was measured with a Quantum Design MPMS-5SSQUID magnetometer in the temperature range 2–300 Kwith magnetic fields of up to 5 kOe. None of the complexesexhibited any field dependence of molar susceptibility atlow temperatures. Diamagnetic corrections were madeusing the Pascal constants. The effective magnetic momentwas calculated as leff(T) = [(3k/NAlB
2) v0MT]1/2, where v0Mis corrected molar susceptibility.
Table 1Crystal data and details of experiment for complexes 1–4
Compound 1 2 3 4
Formula Cu4(OH)2(OAc)4(DMF)2(L1)2 Cu2(H2O)2(L6)2 Æ 2DMF Cu2(OAc)2(H2O)2(L3)2 Cu2(DMF)2(L7)2
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2.5. EPR measurements
The experiments have been carried out at commercial X-band (9 GHz) continuous wave EPR spectrometer (BrukerEMX) equipped with the homemade cryogenic system. Forall studied compounds (1–4) the powder CW EPR spectrahave been recorded within a temperature range 20–300 Kin a standard way and then analyzed.
2.6. Computational details
All DFT calculations reported in this paper were per-formed with the ORCA program package [5]. The exchangecorrelation was treated in Perdew–Burke–Ernzerhof’s(PBE) generalized gradient approximation [6]. An accurateall-electron Gaussian basis set of triple-zeta quality withtwo sets of polarization functions Wachters + f =(14s11p6d3f)/ [8s6p4d2f] [7] was used for the copper atom.All other (carbon, nitrogen, and hydrogen) atoms weredescribed using the polarized 6-31G(d) basis set [8] of dou-ble-zeta quality.
Since our quantum-chemical study aimed at the inter-pretation of the experimentally observed magnetic proper-ties that were obtained for polycrystalline samples, itdictated the use of X-ray data for electronic structure inves-tigations of the complexes without further geometryoptimization.
Molecular orbitals and spin density distributions wereplotted using the MOLEKEL program [9]. The broken sym-metry (BS) approach [10] was employed to elucidate themagnetic properties of multispin systems under study.
The exchange coupling constants were calculated by thegeneralized spin projection method suggested by Yamagu-chi et al. [11].
3. Results and discussion
3.1. Syntheses and structures of the complexes
Interaction of Cu(II) acetate with HL1 forms a hetero-spin complex, which crystallizes into perfect single crystalsafter the DMF solution of the complex has been graduallysaturated with diethyl ether. The compound is formed atany ratio of the starting reagents. The optimal ratio,Cu2(OAc)4(H2O)2/HL1 = 2/1, leads to at least 80% yieldof the product. The yield is drastically reduced when theinitial reagent ratio is decreased under the same conditions;for example, the product yield is less than 8% when theratio is 1/1. An X-ray diffraction study showed that thecompound is a tetranuclear complex Cu4(OH)2(OAc)4
(DMF)2(L1)2 (1).The complex has a centrosymmetric molecule (Fig. 1).
The square environment of Cu1, formed by the N3 atomof the pyrazole fragment and by the O atoms of the >N–O and OH groups and bridging bidentate acetate anion,is completed to the elongated bipyramid by the OOAc
and ODMF atoms. The OOH atom lies at the apex ofthe Cu2 square pyramid, while the N4 and OOH atomsand two OOAc atoms lie at its base. The paramagneticligand performs the cyclic tridentate bridging function,connecting the two Cu atoms. The planes of the pyrazolering and the CN2 fragment of the imidazole ring are
Fig. 1. Structure of molecule 1.
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non-coplanar (\CN2-Pz, Table 2). The OH group iscoordinated simultaneously by three Cu atoms. TheOOH atom lies in the {HCu3} trigonal pyramid. One ofthe two AcO� anions is coordinated as a monodentateligand, while the other, as (l2-O,O 0), which affects the
C–O bond lengths. In the first ligand, these bond lengthsare appreciably different (1.272(5) and 1.221(5) A), whilein the second, they are almost the same (1.255(4) and1.260(4) A). The N–O distances in the paramagnetic L1
ligand are non-equivalent: 1.301(4) for the coordinated
Fig. 2. Structure of the molecule (a) and layer (b, Me groups and H atoms omitted for clarity) in 2.
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NO group versus 1.267(4) A for the uncoordinatedgroup.
Structure analysis of complex 2, formed by interactionof Cu(II) acetate with HL3 (an imine analog of HL1),showed that this is a dinuclear complex (Fig. 2a). The rad-ical retains its cyclic tridentate bridging function on passingfrom nitronyl nitroxides to imino nitroxide. In molecule 2,each of the two Cu atoms is surrounded by a square pyra-mid with the O atom of the water molecule at the apex. Thebase of the pyramid is formed by three N atoms of theparamagnetic ligands and the O atom of the acetate anion.The uncoordinated O atoms of the acetate anions form H-bonds with the coordinated water molecules of the neigh-boring chelates, which leads to the formation of layers instructure 2 (Fig. 2b).
The coordination mode of L3 was quite predictable;according to the data of the Cambridge Structural Data-base, this is just the way 3-(2-pyridyl)pyrazole is coordi-
nated to Cu(II) [12,13] to give various products includingdinuclear complexes [14]. For L1, however, the coordina-tion mode cannot be compared with the available databecause the database has no structural information forCu(II) complexes with ligands containing a combinationof donor functional groups similar to L1 [15]. Therefore,one can only assume that the size of the chelate ring thatincreased from 2 to 1, as well as the lower donor abilityof the O atom of the nitroxyl fragment compared to theimine N atom, enhance dissociation of coordinated water,provoking the condensation of two dinuclear fragmentsinto a tetranuclear fragment in 1 via two l3-OH groups.
Based on the data obtained it might be assumed thatsimilar complexes would also be produced from the esterderivatives HL2 and HL4. However, the reaction of Cu(II)acetate with HL2 under conditions similar to the conditionsof synthesis of 1 gave the [CuL6(H2O)]2 Æ 2DMF complex(3) as a solid product in a high yield, in which L6 is the
Fig. 3. Structure of the molecule (a) and ribbon (b) in 3.
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product of transformation of the starting nitronyl nitrox-ide. The complex is the major reaction product that maybe reproduced at any starting ratio of reagents. As indi-cated by the results of X-ray diffraction investigation of3, transformation of L2 into L6 occurs without affectingthe free valence. The structure of the dinuclear compound3 (Fig. 3a) suggests that the reaction is a cascade processinvolving the splitting of the O–Et bond in the ester groupof HL2 and further formation of a five-membered metallo-cycle due to coordination of the O atom of the deproto-nated carboxyl group and the N atom of the pyrazolefragment. The structure of the six-membered chelate frag-
ment in 3 is similar to that in 1. As a result, the L6 dianionin the dinuclear compound 3 performs the bridging bicyclictetradentate function.
In the centrosymmetric molecule 3, the square environ-ment of each Cu atom is completed to pyramidal by the Oatoms of the water molecule A (Fig. 3a), flanking the sidesof the chelate plane. The water molecules of the two che-lates, in turn, form a couple of contradirectional intermo-lecular H-bonds with the carboxyl O atoms. This leads tothe formation of polymer ribbons in the solid state withshort contacts (3.426(4) A) between the Cu atoms and theN–O groups of the neighboring chelates arising within
Fig. 4. Structure of the molecule (a) and ribbon (b) in 4.
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the ribbons. The DMF molecules H-bonded with the watermolecules lie on the ‘‘periphery’’ of the ribbons (Fig. 3b).
The interaction of the ester derivative of imino nitroxideHL4 with Cu(II) acetate confirmed that a cascade process isthe general tendency for the given compounds. The reac-tion occurred with deprotonation of HL4, cleavage of theC–O ester bond, and reduction of the imino nitroxide frag-ment to the oxoamidine fragment. The reaction ultimatelyformed the Cu2(DMF)2(L7)2 complex (4) whose structure issimilar to that of Cu2(H2O)2(L6)2 Æ 2DMF except that theaxial position in the square pyramidal environment of eachcopper atom is occupied by the O atoms of the DMF mol-ecule (Fig. 4a). In structure 4, as well as in structure 3,polymer ribbons are formed, but here they are formed ina different way – via the O� � �H–N bonds between the unco-ordinated O atoms of the carboxyl groups and the NHgroups of the neighboring molecules.
3.2. Magnetic properties of the compounds
The temperature dependences of the effective magneticmoment (leff) for complexes 1–4 are presented in Fig. 5.
It is reasonable to start this section by discussing themagnetic behavior of compound 4. This will enable us toevaluate the efficiency of the pyrazolate bridges in exchangeinteractions between the odd electrons of the Cu(II) ions,which are also present in more complex exchange clustersin 1–3. For complex 4, the high-temperature value of themagnetic moment is about 2.50 b, which is close to thevalue expected for the non-interacting spins of two cop-per(II) atoms (spin-only value is 2.45 b). The reduced mag-netic moment at lower temperatures points to theantiferromagnetic exchange interactions. To evaluate theoptimal parameters for 4 (JCuCu, gCu) we employed theexchange-coupled dimer model [16]. The resulting parame-ters are listed in Table 3. Quantum-chemical calculationof the parameter of the exchange interaction gave
JCuCu = �186 cm�1 (the calculated values are given inparentheses in Table 3). The calculated value of J coincides(in the order of magnitude and sign) with the experimentalvalue. The slightly overestimated value of JCuCu isexplained by the well-known tendency for DFT methodsto overstabilize the low-spin states, which generally leadsto the parameters of antiferromagnetic exchange exagger-ated by a factor of 1.5–2 [17].
For complex 2, the leff increases at elevated tempera-tures and approaches 3.46 b, which is the theoretical valuefor four practically uncoupled spins with S = 1/2. From X-ray data for 2 it follows that interaction between the oddelectrons of the Cu(II) ion and the N ~ O group in themolecule occurs via the Cu(II) N=C–N ~ O channel,due to which the interaction is ferromagnetic [18]. As in4, the intramolecular interaction between the odd electronsof the two Cu(II) ions occurs via the pyrazolate bridges,owing to which this is a strong antiferromagnetic interac-tion. Since the intermolecular distances between the para-magnetic centers exceed 4.379 A in solid 2, otherinteractions may be neglected in analysis of the experimen-tal dependence leff(T). Therefore, the exchange structure 2
may be regarded as a system of isolated four-centerclusters:
Cu2+ Cu2+
JCuCu
JRCuN
NN O
NO
Approximation of the experimental dependence leff(T)by the isotropic spin-Hamiltonian of the cluster [19] allowedus to obtain the optimal values of the parameters gCu andexchange coupling energies JCuCu and JRCu (Table 3). Theyshow that in solid 2 the antiferromagnetic exchange prevailsover the ferromagnetic exchange, which was also confirmedby our quantum-chemical data (Table 3).
The high-temperature asymptotics of leff for 3, as well asfor 2, tends to reach the value 3.46 b, which is the theoreticalvalue for a system of four spins with S = 1/2 provided thatJ� kT. Spin coupling gradually takes place at lower tem-peratures. In molecule 3, the exchange interaction of theodd electrons of the Cu(II) ions via the pyrazolate bridgesshould be antiferromagnetic, as in the case of 2 and 4.Moreover, each of the Cu(II) ions coordinates the O atom
0 50 100 150 200 250 3000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
T(K)
eff( )
Fig. 5. Dependences leff(T) for 1 (.), 2 (h), 3 (j), and 4 (D). Solid lines –theoretical curves.
Table 3Experimental and calculated (in parentheses) effective values of exchangeinteractions (in cm�1) for 2–4
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of the nitronyl nitroxyl fragment in the equatorial position,which is known [18] to predetermine the antiferromagneticcharacter of exchange in the Cu(II)–O ~ N< cluster.Another possible channel of exchange interactions may bedictated by the short distances (3.426(4) A) between the Oatoms of the >N ~ O groups of the neighboring molecules.To substantiate the model of dominant exchange interac-tions, we performed quantum-chemical calculations ofJCuCu and JRCu based on the real geometry of molecule 3.These calculations showed that the intramolecularexchange interactions are antiferromagnetic (Table 3).The efficiency of intermolecular interaction was evaluatedbased on the calculated data for the cluster that includestwo neighboring molecules 3. According to this estimation,|JRR| is of the order of 1 cm�1. Therefore, exchange struc-ture 3 may be regarded as a system of intramolecularfour-center clusters weakly interacting with one another:
N+N O
Cu2+ Cu2+N+
NOJCuCu
JRCu
O–O–
n
JRR
The optimal values of the parameters gCu, JCuCu, andJRCu obtained by approximation of the experimentaldependence leff(T) within the framework of the isotropicspin-Hamiltonian are listed in Table 3.
For complex 1, leff decreases at reduced temperatures,and around 100 K the dependence leff(T) becomes a pla-teau with leff � 2.41 b, which is close to the theoreticalvalue 2.45 b for two weakly interacting spins S = 1/2 withg = 2. This behavior of the dependence leff(T) suggests thatat T 6 100 K the four spins are completely coupled in 1.Since the X-ray study showed that in solid compound 1
the intermolecular distances between the paramagnetic cen-ters exceed 6 A, the strong antiferromagnetic interactionsare evidently concentrated in the tetranuclear moleculesof the complex. In the centrosymmetric molecule 1, fourCu(II) ions are linked with one another by pairs of variouschannels, forming an exchange triangle. Moreover, each ofthe two Cu(II) ions lying at the opposite vertices of animaginary exchange triangle interacts with the paramag-netic ligand coordinated to it. Thus, exchange structure 1
may be viewed as a system of isolated six-centered clusters:
N+N O
Cu2+ Cu2+
Cu2+ Cu2+N+
NOJCuCu
J'CuCu
JRCu
–O
O–
The optimal values of the parameters gCu, JCuCu, J 0CuCu,and JRCu could not be determined because of the characterof the experimental dependence leff(T) for 1. Our quantum-chemical calculation indicated that the exchange interac-
tion channels JCuCu via the pyrazolate complex and thecoordinated OH group and J 0CuCu via the acetate bridge dif-fer considerably in intensity. Interaction via the acetatebridge is weak antiferromagnetic; J 0CuCu � �11 cm�1. Atthe same time, the exchange interaction channel betweenthe Cu(II) ions linked by the pyrazolate bridge and theOH group is characterized as having an unusually highexchange coupling parameter JCuCu = �938 cm�1. Thismay be assumed to be exactly the channel that is responsi-ble for the effective spin coupling in the exchange rectangle.An antiferromagnetic interaction as strong as this is a con-sequence of direct overlapping of the copper-centered mag-netic orbitals on the oxygen atom of the coordinated OHgroup (Fig. 6). As would be expected for this type ofcoordination of the nitronyl nitroxyl fragment, thecopper–radical interaction is antiferromagnetic withJRCu = �60 cm�1.
3.3. EPR characterization of the compounds
The EPR spectrum of complex 4 and its temperaturedependence are typical for the two-spin system coupledantiferromagnetically (Fig. 7a). At high temperatures thespectrum consists of the strong absorption band atg � 2.15 and weaker line at the half-field region of the spec-trum (g � 4.2), characteristic for spin dimers [20,21]. Onlowering the temperature, an absolute intensity of the spec-trum decreases and spectrum completely disappears belowca. 30 K. At T = 20 K only a weak and narrow signal withg � 2.013 is observed, which is most likely to be assigned tothe small admixture of nitroxide radical. Double integra-tion of spectra at T = 296 and 20 K allows for the estimatethat the concentration of admixture does not exceed 0.4%and thus its influence on the magnetic properties of 4 canbe safely neglected.
The room temperature (296 K) EPR spectrum of thecompound 1 consists of the single broad line (peak-to-peakwidth ca. 90 mT) at g � 2.054 (Fig. 7b). The line shape isclose to Lorentzian, implying an assignment to thestrongly-coupled exchange cluster. On lowering the tem-perature, this line gradually transforms into a narrow linewith g � 2.008 observed at T = 20 K. The value of the g-
Fig. 6. Spin density in complex 1 for the minimal-energy BS state(H atoms omitted for clarity).
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factor, as well as the width of the line at T = 20 K, indicatethat at low temperatures the spectrum is dominated by nitr-oxides and the contribution of copper spins is negligible.This is consistent with the magnetic behavior and the aboveconclusions, that 4 spins of coppers are strongly-coupledantiferromagnetically and thus virtually do not influencethe magnetism below ca. 100 K.
The EPR behavior of the compounds 2 and 3 is rathersimilar to each other. The spectrum consists of the singlebroad Lorentzian line (peak-to-peak width ca. 150 mT atroom temperature) with no any additional signals. Forboth compounds, the positions of these lines are tempera-ture dependent. The value g = 1.969 is measured for com-plex 2 at room temperature (296 K), whereas at T = 20 Kmuch higher value g = 2.025 is observed. For complex 3the value g = 1.963 is found at T = 296 K, the valueg = 2.037 at T = 120 K, and at T = 20 K the spectrumintensity is too small to be safely detected. The unusualg-values <2 at room temperatures, as well as their temper-ature dependence, might result from the antiferromagneticcoupling within a four-spin system. Similar effects havebeen recently observed by us and interpreted for the caseof spin triads nitroxide–copper(II)–nitroxide [22–24].Therefore, these peculiarities are very interesting and willbe the future subject of separate investigation beyond thiswork.
4. Conclusions
Thus our study has shown that the reactions of HL1 andHL3 with Cu(II) acetate lead to self-assembly of the Cu4-(OH)2(OAc)4(DMF)2(L1)2 tetranuclear and Cu2(OAc)2-(H2O)2(L3)2 dinuclear complexes, respectively, in which
L1 and L3 perform the bridging cyclic tridentate functionby linking the two Cu atoms. The greatest differencesbetween these complexes are due to the character of theintramolecular exchange interactions. The magnetic behav-ior of Cu4(OH)2(OAc)4(DMF)2(L1)2 is dictated by thedominant antiferromagnetic exchange interactions, as con-firmed by the results of quantum-chemical analysis of theexchange coupling channels. The magnetic properties ofCu2(OAc)2(H2O)2(L3)2 reflect the competition betweenthe antiferromagnetic and ferromagnetic contributions, ofwhich the latter is due to the interaction of electrons inthe Cu(II) N=C–N ~ O exchange channels.
It has been found that the reaction of Cu(II) acetatewith HL2 or HL4 occurs with cleavage of the ester group,resulting in the formation of Cu2(H2O)2(L6)2 Æ 2DMF andCu2(DMF)2(L7)2 solid dinuclear complexes, where L6 andL7 are the products of transformation of the starting nitr-oxides, which in the case of HL2 occurs without affectingthe free valence. This possibility should be taken intoaccount in designing heterospin systems based on transi-tion metal complexes with polyfunctional nitroxides.
Acknowledgements
This work was supported by the Russian Foundationfor Basic Research (Grant Nos. 05-03-32305, 06-03-32157, 06-03-32742, 06-03-04000, 05-03-32264-a), FASI(State Contract No. 02.513.11.3044), CRDF (RUE1-2839-NO-06), President of the RF (Grant NSh-4821.2006.3, MK-4362.2006.3, MK-6673.2006.3), the programsof the Presidium and OKhNM, Russian Academy of Sci-ences, and the programs of the Siberian (Lavrentiev
100 200 300 400
0 100 200
20 K
50 K
110 K
170 K
296 K
Magnetic field / mT
100 200 300 400
Magnetic field / mT
20 K
296 K
Fig. 7. EPR spectra of 4 at T = 20-296 K. The arrow marks the signal of nitroxide admixture. Inset: magnified signal of Dms = 2 transition at T = 296 K(a). EPR spectra of 1 at T = 20 and 296 K. The intensities are normalized (b).
748 E.V. Tretyakov et al. / Polyhedron 27 (2008) 739–749
Author's personal copy
Grant SB RAS No. 79) and Ural Branches of the RussianAcademy of Sciences.
Appendix A. Supplementary material
CCDC 645063, 645062, 645064 and 645065 contain thesupplementary crystallographic data for this paper 1, 2, 3
and 4. These data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, or fromthe Cambridge Crystallographic Data Centre, 12 UnionRoad, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementarydata associated with this article can be found, in the onlineversion, at doi:10.1016/j.poly.2007.11.003.
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