Structure and magnetism of nickel (II) and manganese (II) complexes of a nitronyl nitroxide carboxylic acid

~

,-:-Er,v~-,vl~l

ELSEVIER

Inorganica Chimica Acta 248 (1996) 139-146

Structure and magnetism of nickel(II) and manganese(II) complexes of a nitronyl nitroxide carboxylic acid Niels Christian SchiCdt a, Fabrizia Fabrizi de Biani b, Andrea Caneschi b, Dante Gatteschi b.. a University of Copenhagen, Department of Chemistry, Fruebjergvej 3, DK-2100 Copenhagen, Denmark b University of Florence, Department of Chemistry, Via Maragliano 77, 1-50144 Florence, Italy Received 12 September 1995; revised 26 October 1995

Abstract

The organic anionic radical 2- (4-carboxy-phenyl)-4,4,5,5-tetramethyl-4,5-dihydro- 1H-imidazol-1-oxyl-3-oxide (NITpBA - ) has two different types of potentially metal-coordinating sites. One is the carboxylate group, the other consists of the two oxygen atoms belonging to the nitronyl nitroxide part. The nitronyl nitroxides have generally revealed a rather weak coordinating ability and they have been found only to coordinate metal ions with a low electron density as in the case of hexafluoro-acetylacetonates. However, in the simple carboxylate salt Mn(NITpBA)2(H20)2, each MnII ion is coordinated simultaneously by water, carboxylate oxygens and radical oxygens with the NITpBAligands bridging the Mnn ions such as to obtain a ladder-like structure. The NiH derivative has also been prepared and it was found to be isostructural with the Mn salt. Mn(NITpBA)2(H20)2 crystallizes in the monoclinic system, P2Jc (No. 14) space group, a--- 12.644(2), b ---9.203 (2), c = 13.598 (2),/3 = 115.21( 1)°, V= 1431.6(4) ,/k3, Z= 2, refinement with 2150 reflections, 196 parameters, yielded R -- 0.046 and Rw-- 0.051. The magnetic susceptibility as a function of temperature of these compounds is readily accounted for on the basis of their structure. A chain model was adopted to reproduce the observed magnetic susceptibility of the NITpBAH free radical. Keywords: Crystal structures; Magnetism; Nickel complexes; Manganese complexes; Nitronyl nitroxide complexes

1. Introduction

There is a wide interest in the study of the chemical and physical properties of the class of organic radical nitronyl nitroxides [ 1,2]. Accurate research about the possibility of obtaining cooperative magnetic phenomena using this type of radical both through dipolar interactions [ 3 ] and through a network of hydrogen bonds [4] has been developed and purely organic magnets operating in the region below 1 K have been synthesized. The scope of exploring the magnetic properties of compounds arising from the combination of transition metal or lanthanide ions with nitronyl nitroxides is steadily increasing. A great variety of addition compounds of nitronyl nitroxides with various Lewis acids have now been prepared displaying large variations in structure and magnetic properties and the subject has been reviewed [5]. Introduction of potential donor atoms into the substituent group of the nitronyl nitroxide opens up the possibility of achieving compounds with a high dimensionality and thus a high degree of ordering of the spins, as has previously been attempted with the tris-mono* Corresponding author. 0020-1693/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10020-1693 ( 9 5 ) 0 4 9 9 6 - 7

dentate ligands N1TpPy [6--9] and NITpCHO [ 10]. The possibility to play with anionic or cationic nitronyl nitroxides increases the chances for designing magnetic molecular materials. Investigations on the magnetic behavior of salts of cationic nitronyl nitroxides were recently performed [11] by various Japanese research groups and fascinating results have been obtained in the recent past [ 12] using a cation nitronyl nitroxide and first row transition metal ions. The use of anionic derivatives will probably offer more possibilities to synthesize extended systems. In fact, the tendency of metal carboxylates to form extended structures is well known, and it may hoped that the radical part of the new ligand can crosslink the polymeric moieties. We have prepared the ligand 2-(4-carboxy-phenyl)-4, 4,5,5-tetramethyl-4,5-dihydro-lH-imidazol-l-oxyl-3-oxide (N1TpBAH) in order to exploit the combination of the very extended transition metal--carboxylate chemistry with organic radicals, aiming for molecular ferro-/ferrimagnetic materials and high nuclearity spin clusters. We report here the magnetic properties of the pure ligand, the structure of Mn(NITpBA)2(H20)2 and the magnetic properties of both manganese and nickel derivatives.

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2. Experimental 2.1. Preparations All solvents and reagents were obtained from Aldrich and used as received except for 2,3-dimethyl-2,3-bis(hydroxylamino)-butane which was prepared by a published method [ 13 ]. Although the carboxylic acid has been prepared recently [ 1 ], we used an alternative method that differs also from the classic one [ 14] ; details of the preparation are as follows.

2.1.1. 2-(4-Carboxy-phenyl )-4, 4,5,5-tetramethyl-4,5dihydro- l H-imidazol- l-oxyl-3-oxide (N1TpBAH) To a well stirred suspension of 5.5 g (36.7 mmol) of 4-carboxy benzoic acid in 150 ml methanol, 5.0 g (33.3 mmol) of 2,3-dimethyl-2,3-bis(hydroxylamino)-butane were added, which dissolved immediately. The temperature was raised to reflux and after a short time, a clear solution was obtained. This was cooled to room temperature and left for 24 h. To a solution of NaIO4 (2.6 g, 12 mmol) in 500 ml water were added 5 ml glacial acetic acid and 75 ml CHC13. While this was stirred vigorously, the methanolic solution was poured into it causing an immediate change in color from yellow to intense dark blue. The chloroform layer was separated and the water solution was extracted with a further two 75 ml portions of CHCI 3. The combined extracts were washed With a small amount of water and a white precipitate of unchanged starting material was separated at this time. The organic solution was extracted twice with 100 ml 1 M Na2CO3 solution and the combined aqueous extracts washed repeatedly with CHCI3 until the washings were colorless. To the aqueous solution of the sodium salt (violet), an excess of glacial acetic acid was added and the product was re-extracted into CHCI 3 as before (three times 75 ml). The combined extracts were washed three times with 10 ml portions of water, dried over anhydrous MgSO4, filtered and reduced in volume to 25 ml on the rotary evaporator. To this concentrate were added 400 ml hexane, the solution was cooled in ice for 1 h and the resulting dark blue microcrystals were filtered off. Yield 3.1 g (34%). Anal. Calc. for C14HITN204:C, 60.64; H, 6.18; N, 10.10. Found: C, 60.45; H, 6.16; N, 9.96%. Electronic absorption spectrum in CH2C12: A = 39 000 c m - t (shoulder, ~= 5000 M -1 c m - l ) ; 36500 (5700); 29 300 (sh, 2750); 27 800 (6000); 18 000 (sh, 285); 16 900 (340); 15 500 (sh, 265). The EPR spectrum of a powdered, crystalline sample recorded at room temperature showed a single absorption with g = 2.0078. When recorded in CH2C12 solution, a quintet spectrum was obtained with g = 2.0030 and a~ = 7.4 G.

2.2. Synthesis of the complexes 2.2.1. Mn(NITpBA)2(H20)2 The carboxylic acid NITpBAH (4 mmol, 1.11 g) was dissolved in 4 ml 1 M NaOH in methanol. A 0.5 M solution of Mn(C1On)2 in water (2 mmol, 4 ml) was added. The

solution was filtered through a cotton plug and left in a small beaker at room temperature for 24 h causing the methanol to evaporate slowly and the product to deposit as well shaped blue-violet crystals. These were filtered off, sucked as dry as possible and dried in air. One of the crystals obtained this way was picked for X-ray structure determination. Yield 0.65 g (50%). Anal. Calc. for C28H36/~rlN4Oto: C, 52.25; H, 5.64; N, 8.71. Found: C, 51.90; H, 5.73; N, 8.56%.

2.2.2. Ni(NITpBA)2(H20)2 This was prepared analogously. The crystals were smaller than those of the Mn compound but were found to be of a sufficiently good quality to establish the identity of the unit cell of this compound with that of Mn(NITpBA)2(H20)2, see Table 1. Anal. Calc. for C2sH36N4NiOlo: C, 51.96; H, 5.61; N, 8.66. Found: C, 50.98; H, 5.86; N, 8.55%. Caution! Perchlorate salts are potentially explosive. We recommend that the solution of Mn(CIO4)2 in water is prepared from treatment of an excess of MnCO3 with 1 M perchloric acid in water and used as obtained after filtration without isolation of the hydrated salt.

2.3. X-ray structure determination X-ray data were collected on an Enraf-Nonius CAD-4 fourcircle diffractometer equipped with Mo K a radiation. Accurate unit cell parameters for Mn(NITpBA)2(H20)2 were derived from least-squares refinement of the setting angles of 25 reflections ( 1 6 < 2 0 < 2 4 °) and are reported in Table 1 with other experimental parameters. Data were corrected for Lorentz and polarization effects but not for absorption and extinction. The systematic absences showed that the compound belongs to the monoclinic system, space group P2 t/c (No. 14). The crystal structure was solved by direct methods with the program sir92 [ 15] which gave the positions of all nonhydrogen atoms. Structure refinements were carried out by using the SHELX-76 program package [ 16] which utilizes least-squares methods, with anisotropic parameters for all non-hydrogen atoms. The final refinement model included all hydrogen atoms in fixed and idealized positions. The unit cell of Ni(NITpBA)2(H20)2 was found (using the equipment and conditions described above) from 25 reflections (18 < 20< 28 °) and in this way proved to be isostructural with the Mn compound. The unit cell parameters for NITpBAH were obtained from 25 reflections ( 16 < 20-~ 28°). The structure was refined with the same procedure described above and the final result was found consistent with the one previously reported [ 1]. Crystal data: space group C2/c (No. 15); a = 25.622(3), b = 9.424(1), c = 12.776(1) /~,, /3 = 107.99(1) °, V = 2934.3(5)/~3, Z = 8; R = 0.047, Rw = 0.048, 1724 reflections with I > 3 o ' ( I ) , 213 variables, weight=3.0795/

[cr2(Fo)+ 0.00007Fo2].

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N.C. SchiCdt et al. / lnorganica Chimica Acta 248 (1996) 139-146 Table 1 Crystal data and experimental parameters for Mn(NITpBA)2(H20)2 and Ni(NITpBA)2(H20)2 Formula Molecular weight (g mol- t ) Space group a (/~,) b (/~) c (/k) /3 (°) •~ (/~3) 2 D~alc (g cm -3) Crystal size (mm) A (/~) (cm- t ) Data limits (°) Octant Unique reflections Observed data ( F > 3o.F) Parameters /¢ Rw(w = 1 / [ o'2(Fo) + 0.00005Fo 2] )

Cz~HsoN4OIoNi 647.31

C2sH36N,,OtoMn 643.55 P2~/c, monoclinic, No. 14 12.644(2) 9.203(2) 13.598(2) 115.21(1) 1431.6(4) 2 1.49 0.2 × 0.2 × 0.3 0.71069 34.71 5 < 2 0 < 50 -t- h, - k, + l 2765 2150 196 0.042 0.043

12.492(4) 9.192(2) 13.480(4) 115.17(3) 1400.9(8)

0.15×0.1×0.15 0.71069

2.4. Magnetic susceptibility measurements Magnetic susceptibilities were measured by using a Metronique Ingenierie MS02 super conducting SQUID susceptometer. The data were collected for free acid NITpBAH in the temperature range 2-70 K at a field strength of 1 T. For the manganese compound data were collected at a field strength of 0.1 T in the temperature range 32-2 K and at a field strength of 1.0 T in the range 28-265 K. For the Ni compound data were collected in the temperature range 4-280 K at a field strength of 1 T. Data were corrected for the magnetization of the sample holder and for diamagnetic contributions, which were estimated from the Pascal constants.

3. Results 3.1.

Crystal structure

The crystal structure of NITpBAH was previously reported [ 1 ] but not completely described. In particular the packing arrangement was not shown, while it could provide the key for the interpretation of the magnetic behavior. The radicals are connected in a one-dimensional chain structure, as shown in Fig. 1, given by the hydrogen bonds between the carboxylic acid group and the nitronyl nitroxide oxygen atom of the radical reported by the glide plane, with a bond distance of 2.611(3) /~. The shortest O-O distance between the NO groups of radicals related by an inversion center is 3.885 (3) A,, and in this sense we could see the structure as a dimeric one. Actually the structure could be described as couples of chains parallel to the c axis. There are no relevant contacts between the couples of chains. An ORTEP drawing of the asymmetric unit of Mn(NITpBA)z(HzO)2 is shown in Fig. 2. Atomic coordi-

Fig. 1. Picture of NITpBAH chain structure given by intermolecular short contacts.

05

¢~O2

C3

O4

1 ~C 1 ¢ ~ C 4

Ol,

Ol

L2

~,C13

N

03

C11

C12 Fig. 2. ORTEP view of the molecular arrangment of Mn (NITpBA) 2(H20) 2.

N. C Schi¢dt et al. / lnorganica Chimica Acta 248 (1996) 139-146

142

Table 2 Atomic coordinates for Mn (NITpBA) 2(H20) 2

Mn O1 02 03 04 05 N1 N2 CI C2 C3 C4 C5 C6 C7 C8 C9 C10 CII C12 C13 C14

Table 3 Selected bond lengths (A) and angles (°) for Mn(NITpBA)2(H20)2 Lb

x/a

y/b

z/c

U~

1.0000 1.0562(2) 1.0415(2) 1.165l(2) 1.4210(2) 1.0529(2) 1.2528(2) 1.3693(2) 1.0684(2) 1.1221(2) 1.1606(3) 1.2113(3) 1.2231(2) 1.1838(2) 1.1349(3) 1.2790(2) !.3453(2) 1.3994(3) 1.3047(3) 1.4147(3) 1.3479(3) 1.5216(3)

-5.0000 -0.2119(2) -0.2468(2) -0.9446(2) -0.8104(2) 0.0477(2) -0.9424(3) -0.8825(3) -0.2854(3) -0.4321(3) -0.5063(3) -0.6393(3) -0.7000(3) -0.6265(3) -0.4929(3) -0.8396(3) -1.0478(3) - 1.0357(3) - 1.1954(3) -0.9857(4) - 1.1319(4) - 1.0541(4)

1.0000 1.0526(2) 1.2196(2) 1.0078(2) 1.3733(2) 1.1872(2) 1.1045(2) 1.2799(2) 1.1407(3) 1.1517(2) 1.2564(3) 1.2683(3) 1.1747(2) 1.0684(3) 1.0579(3) 1.1867(2) 1.1304(3) 1.2651(3) 1.0812(3) 1.0724(3) 1.3254(3) 1.3204(3)

0.020(1) 0.031(1) 0.040(1) 0.027(1) 0.041(1) 0.031(1) 0.024(1) 0.027(1) 0.027(2) 0.024(1) 0.028(2) 0.028(2) 0.025(2) 0.027(2) 0.028(2) 0.024(1) 0.027(1) 0.029(2) 0.039(2) 0.041(2) 0.044(2) 0.048(2)

nates and selected bond distances and bond angles are given in Tables 2 and 3, respectively. The crystal structure of Mn(NITpBA)e(H:O)2 reveals two ladder-like one-dimensional chains of repeating Mn(NITpBA)2(H20)2 units, related by a glide plane. In Fig. 3, for the sake of clarity, only one of the two arrays is shown. The Mn n ion lies on an inversion center and is coordinated by two pairs of NITpBAions, the ligands in each pair being therefore trans to each other. One pair binds through one oxygen atom of the carboxylic acid group, while the other coordinates through one of the nitronyl nitroxide oxygens. Each ligand coordinates the next Mn H ion in the opposite fashion thus forming the chains. The Mn n ions are coordinatively saturated by two water molecules in the apical positions, thus completing the MnO6 core. The bond angles within the coordination octahedron are slightly distorted. The Mn-O distances involving both types of radical oxygen atoms are almost equal and shorter than those involving the water molecules. Interchain contacts are found between the water molecule oxygen 0 5 and the non-coordinated carboxylic oxygen 0 2 separated by 2.809(3) /~ indicative of hydrogen bonding. The NITpBA fragment has the usual geometry of substituted phenyl nitronyl nitroxides [ 17-20] and shows a little distortion with respect to the free acid with an angle of 31.14(11) ° between the five-membered heterocyclic ring and the phenyl ring, just a little larger than that found in the radical (27.99(8)°). The N--O distances are slightly different from each other and in the Mn compound they are longer than those found in NITpBAH ( 1.331 (3) and 1.310(2)/~, versus 1.298(2) and 1.273(2)/~,) but comparable to those previously reported for the coordinated NITR radical [5,21]. Both in the free and

Mn-Ol Mn-O3" Mn-O5 O1-CI O2-C1 O3-N1 O4-N2 NI-C8 N1--C9 N2-C8 C1--C2

2.094(2) 2.107(2) 2.381 (2) 1.326(4) 1.305(5) 1.310(2) 1.331 (3) 1.393(4) 1.442(4) 1.353(3) 1.490(4)

O1-Mn-Ol' O1-Mn--O3" O1-Mn-OY" O1-Mn-O5 O 1-Mn-O5' O3"--Mn-O3" OY'-Mn-O5 OY'Mn--O5' O5-Mn--O5' Mn-O1-CI O3-N1-C8 C8-NI-C9 O4-N2-C8 C8-N2--C10 O1--C1-O2 O2--C1--C2 O1-C1--C2 N2--C8-C5 N1--C8--C5 N 1--C8-N2 N1-C9--C12

180.00(9) 91.35(9) 88.65(9) 84.65(9) 95.35 ( 9 ) 180.00 (9) 94.62 ( 8 ) 85.38(8) 180.00(9) 132.48(19) 128.74(24) 111.69(24) 128.57(25) 108.07(24) 129.27(27) 115.54(26) 115.18(26) 122.87(26) 125.27(26) 111.77(25) 101.97(25)

a Standarddeviations in the last significant digits are reported in parentheses. b 03" translatedby x, y + 1, z.

coordinated radical there are two shorter bond distances in the phenyl ring, suggesting a contribution of the quinoid structure. The phenyl ring and the carboxylic group are nearly coplanar, the dihedral angle being 3.4(2) ° in NITpBAH and a little larger in Mn(NITpBA)2(H20)2 where it becomes 12.0 ( 2 ) °. These structural features suggest a conjugation path consistent with a non-zero spin density on the carboxylic group.

3.2. Magnetic data The temperature dependence of the magnetic susceptibility of NITpBAH is shown in Fig. 4. Near 3 K the X versus Tplot shows a maximum (0.044 emu tool - l ) as a signature of antiferromagnetic interaction. In the light of the crystal structure we should expect a rather weak coupling between the paramagnetic centers as confirmed by the low value of Trr~. The temperature dependence of the magnetic susceptibility of Mn(NITpBA ) 2(H20) 2 is shown in Fig. 5. At 265 K a xT value of 2.2 emu K mol- 1 is observed, much lower than the one expected for uncorrelated spins (5.125 emu mol-1 K). On cooling, the value decreases until a plateau is reached at

N. C. Schi#dt et al. /lnorganica Chimica Acta 248 (1996) 139-146

143

b

C Fig. 3. Picture of the Mn(NITpBA)2(H20)2 chain developed along the ab plane.

2.5

0.05

0.04 2.0 E

2 v ~t

0.03

0.02

1.5

0.01

0

20

40

60

80

T CI<) Fig. 4. Temperature dependence of the X value of a polycrystalline sample of NITpBA. The circles are experimental values. Full line represents the fit

1.0 0

100

200

300

( see text).

T(K) Fig. 5. Temperaturedependenceof the xT valueof a polyerystallinesample of Mn(NITpBA)2(H20)2.The triangles are experimentalvalues.Full line representsthe fit (see text).

temperatures between 80 and 40 K with a value of 1.89 emu K m o l - l , very close to the spin-only value for an S = 3/2 system (X T= 1.875 emu K m o l - t ) . This is in agreement with what should be expected from the structure; a strong antiferromagnetic interaction between each Mn n ion with S = 5 / 2 and the two NO-bonded radicals each having S = 1/2. Below 40 K, xT tends to decrease further, eventually reaching a sharp drop around 20 K, presumably arising from antiferromagnetic coupling of the S = 3/2 units. A CurieWeiss law behavior is observed at low temperature and no

maximum is observed in the susceptibility curve. The value of x T a t 4.2 K is 1.5 emu Kmo1-1. In Fig. 6 is shown the XT versus T curve for Ni(NITpBA)2(H20)2. A steady decrease from 0.38 emu K m o l - l at 280 K down to 0.077 at about 85 K is observed, then the xT value remains almost constant. The expected value at low temperature is zero, since the spin of the Ni II ion exactly cancels that of the two radicals. The small residual susceptibility is probably due to a paramagnetic impurity in the sample (see Section 4).

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N.C. Schi#dt et aL / lnorganica Chimica Acta 248 (1996) 139-146

0.4

0.3

v

0.2

0.1

0

100

200

300

T (K)

Fig. 6. Temperature dependence of the x T value of a polycrystalline sample of Ni(NITpBA)2(H20)2. The circles are experimental values. Full line represents the fit (see text).

4. Discussion The magnetic behavior of NITpBAH shows the features of an antiferromagnetic interaction. Actually there are two possible paths for the exchange interaction: through the O O contact between the nitronyl nitroxide groups and through the hydrogen bonds between the carboxyl and the nitronyl nitroxide groups. In all cases we should expect a weak interaction due to the long distance between the nitronyl nitroxide groups or to the low spin density on the carboxylic group. We tried to fit the magnetic susceptibility both by a dimeric model [22] and a chain model and found that the best fit could be obtained by Bonner and Fisher's [23] expression for an antiferromagnetic chain of S = 1/2 spins:

Ng2fl 2 0.25 + 0.074975x + 0.075235x 2 kT 1.0+0.9931x +O.172135x2 +O.757825x 3 with x = IJI/kT, the effective Heisenberg Hamiltonian ( 1 ) being: H = JY'.iSi • S i + 1

(

1)

The best fit parameters are g = 1.94 and J = 3.29(4) c m with a value of R = 8 . 0 × 10 -5 ( R = [E(xTcalc--XTobs)2/ E(XTobs) 2] ~/2). The low value o f g can be ascribed to experimental artefacts. Actually we could not obtain any accurate fit of the data by using the dimeric model, the best value of R being 9.6× 10 -4 obtained for g = 1.90 and J = 4 . 3 0 ( 1 5 ) cm

--1

The crystal structure of Mn(NITpBA)2(H20)2 reveals a new feature of nitronyl nitroxide ligands. Hitherto, the nitronyl nitroxide NO groups have been found to coordinate only fairly strong Lewis acids such as hexafluoro-acetylacetonates and metal ions in salts of per-fluorinated carboxylic acids and usually only under anhydrous conditions. Furthermore, very few other cases of simultaneous coordination of

a nitronyl nitroxide ligand and water to the same metal ion are known [24]. Of course, the nitronyl nitroxide NO groups are still rather weakly coordinating in N I T p B A - , but the crystallization of Mn(NITpBA)z(H20)2 shows that it is possible to overcome the low degree of coordinating ability. The magnetic data of Mn(NITpBA)2(H20)2 show a strong antiferromagnetic coupling between the manganese ions and the NO-bound radicals as would be expected from the data reported in the literature for compounds with similar Mn ~ nitronyl nitroxide bonds. This is apparently also the case for Ni(NITpBA)2(H20)2. In both cases a fit of the magnetic susceptibility in the high temperature region was made by approaching the M(NITpBA)2 units to an isosceles triangle and using the fitting parameters: JMR and JRR (M and R refer to metal and radicals, respectively). The effective Heisenberg Hamiltonian (2) is H = 2JMRSMS R -~-JRRSRSR,

(2)

In both cases it was clear, due to the relevant distance between the radicals, that JRR' was extremely small (less than 0.01 c m - ~) and it was therefore fixed at zero. In the case of Mn(NITpBA)z(H20)2, JMR = 193(3) c m - l and gM = 2.01 ( R = 2 . 2 X 10 -5 ). For Ni(NITpBA)2(H:O)2 the fitting of the data assuming the presence of a paramagnetic impurity with a xT value of 0.077 emu K mol-1 gives JMR = 466(4) cm -~ and gM=2.47 ( R = 2 . 3 × 10-5). The values of the coupling constants compare well with those reported for the compounds M(hfac)2(NITPh)2 ( M = M n , Ni; hfac= hexafluoro-acetylacetonate; NITPh = 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro- 1H-imidazolyl- 1-oxyl-3-oxide) which are monomeric, trans-octahedral bis-adducts of nitronyl nitroxides with Mn u and Ni u, with the metal ions being coordinated only by oxygen atoms. In fact, for Mn(hfac)2(NITPh)2 JMR=180 cm -1 [25] and for Ni(hfac)E(NITPh)2 JMR "~ 4 0 0 c m - 1 [26]. The decrease of X T of the manganese compound below 20 K might be due either to zero-field splitting of the S = 3 /2 state or to additional antiferromagnetic interactions along the chains transmitted through the carboxylic groups of the nitronyl nitroxide ligands. The latter hypothesis seems to be the more probable; in fact we should not expect a relevant ZFS effect for a compound of this nature, while on the other side the results obtained for NITpBAH show that the interaction through the carboxylic group should be active. Therefore we fitted the low-temperature data with the expression derived by Fisher for an antiferromagnetic chain [27] of spins 3/2, making the assumption that the S = 3/2 is the only populated spin state below 20 K. The parameters found for Mn(NITpBA)2(H20)2 are: ,/3/2=0.30(4) cm -1 and gs=3/2 = 2.02 (R = 1.75 × 10-5). The fit is shown as a solid line in Fig. 6 together with the experimental data. ,13/2 is an effective coupling constant. If we assume that the true interaction is through two neighboring radicals it can be shown [ 28 ] that:

J3/2 --" - 2 X 7/25 × J

(3)

N.C. Schicdt et al. / Inorganica Chimica Acta 248 (1996) 139-146

where J is the coupling constant relative to the interaction between the Mn n of a triad with a radical of another triad. The factor 2 originates from the fact that for a pair of triads there are two identical exchange pathways. The factor - 7 / 2 5 originates from the fact that in the ground 3/2 state of a triad the projection of the spin of the radical on the total spin is - 1/5, and that of the manganese(II) is 7/5. The negative sign is very important: it indicates that in order to have an effective antiferromagnetic coupling constant between the Mn(NITpBA)2(H20)2 moieties is necessary that the true coupling constant is ferromagnetic. In fact from Eq. (3) we calculated J = - 0 . 5 4 cm -~. The observation of a weak ferromagnetic coupling between Mn n and a radical for such a relative orientation is not new. In fact values o f J = - 1.06 (10) and J = - 0.95 (8) cm- l were reported [8] for an Mn II NITpPy interaction, respectively, in Mn(hfac) 2(NITpPy)2 and MnCI2(NITpPy) 2, and J = - 2.7 crn-I in [Mn(hfac)2(NITpPy) ]2 where NITpPy=2-(4pyridyl) -4,4,5,5-tetramethyl-4,5-dihydro- 1 H-imidazolyl- 1oxyl-3-oxide [9].

5. Conclusions The crystal structure of NITpBAH has been considered from a more accurate point of view and its magnetic behavior has been interpreted in the light of the chain structure given by the hydrogen bonds between the carboxylic acid group and the nitronyl nitroxide oxygen atom. Even if only a small fraction of the spin density is expected to be localized on the carboxylic substituent of the phenyl ring it seems to be responsible for the low temperature magnetic behavior of the ladder-like chain compound obtained by reacting NITpBAwith Mn(C104)2. The magnetic interaction between the S = 1/2 of the radical and the S = 5/2 of the manganese ion through the carboxylic group has been found to be ferromagnetic and about 0.5 c m - ~. Also interesting is the kind of coordination encountered in the manganese and nickel derivatives, where the radical is bound through the weakly coordinating NO group and the carboxylic group in the presence of coordinated water molecules. It seems therefore possible to combine the rich chemistry of metal clusters based on oxo, hydroxo and carboxylate bridges with that of nitronyl nitroxide radicals, with the aim t~f obtaining high nuclearity clusters with a large spin and interesting magnetic properties.

Acknowledgements This work is supported by the Human Capital and Mobility Program through contract ERBCHRX-CT920080 and by Ministero della Ricerca Scientifica e Tecnologica (MURST) of Italy.

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