DaltonTransactions

Dynamic Article Links

Cite this: Dalton Trans., 2011, 40, 3295

www.rsc.org/dalton PAPER

Structure and magnetic properties of an unprecedented syn-antil-nitrito-1jO:2jO¢ bridged Mn(III)-salen complex and its isoelectronic andisostructural formate analogue†

Paramita Kar,a Rituparna Biswas,a Michael G. B. Drew,b Yumi Ida,c Takayuki Ishida*c and Ashutosh Ghosh*a

Received 3rd November 2010, Accepted 18th January 2011DOI: 10.1039/c0dt01521k

The preparation, crystal structures and magnetic properties of two new isoelectronic and isomorphousformate- and nitrite-bridged 1D chains of Mn(III)-salen complexes, [Mn(salen)(HCOO)]n (1) and[Mn(salen)(NO2)]n (2), where salen is the dianion of N,N¢-bis(salicylidene)-1,2-diaminoethane, arepresented. The structures show that the salen ligand coordinates to the four equatorial sites of the metalion and the formate or nitrite ions coordinate to the axial positions to bridge the Mn(III)-salen unitsthrough a syn-anti m-1kO:2kO¢ coordination mode. Such a bridging mode is unprecedented in Mn(III)for formate and in any transition metal ion for nitrite. Variable-temperature magnetic susceptibilitymeasurements of complexes 1 and 2 indicate the presence of ferromagnetic exchange interactions with Jvalues of 0.0607 cm-1 (for 1) and 0.0883 cm-1 (for 2). The ac measurements indicate negligible frequencydependence for 1 whereas compound 2 exhibits a decrease of cac¢ and a concomitant increase of cac¢¢ onelevating frequency around 2 K. This finding is an indication of slow magnetization reversalcharacteristic of single-chain magnets or spin-glasses. The m-nitrito-1kO:2kO¢ bridge seems to be apotentially superior magnetic coupler to the formate bridge for the construction ofsingle-molecule/-chain magnets as its coupling constant is greater and the cac¢ and cac¢¢ show frequencydependence.

Introduction

Magnetic molecular clusters have attracted much interest in thelast few years, due to the fundamental research of magnetic in-teractions and magneto-structural correlations.1 One of the majorchallenges to the researchers is the rational design of this typeof polymer. There are developments of a whole range of single-molecule magnets2 and single-chain magnets3 that bridge the gapbetween paramagnetism and long-range ordered magnetism andpotential applications of data storage and quantum calculations.1D magnetic chain systems are of special interest because oftheir importance in lower-dimensional magnetism.4 The single-chain magnets (SCMs) possess a large uniaxial anisotropy,strong ferromagnetic intrachain exchange interactions withoutspin compensation between the high-spin magnetic units, andgood isolation of the chains in order to avoid two- and three-

aDepartment of Chemistry, University College of Science, University ofCalcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: ghosh_59@yahoo.combSchool of Chemistry, The University of Reading, P.O. BOX 224,Whiteknights, Reading, RG6 6AD, U.K.cDepartment of Engineering Science, The University of Electro-Communications, Chofu, Tokyo, 182-8585, Japan. E-mail: ishi@pc.uec.ac.jp† Electronic supplementary information (ESI) available. CCDC referencenumbers 799078–799079. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c0dt01521k

dimensional ordering.5 However, it is still a big challenge todesign and prepare such ferromagnets due to the weakness ofthe ferromagnetic coupling compared to the antiferromagneticinteraction.6 Polynuclear transition-metal complexes are an inter-esting class in this regard and most of the new examples have beenfound in manganese(III) chemistry.7 Therefore, preparation of newpolynuclear manganese(III) complexes continues to receive a greatdeal of attention.

Employment of appropriate types of bridging ligands whichcan mediate the magnetic coupling between the local spin carriershas allowed access to a variety of high nuclearity products withinteresting structural and magnetic properties. The carboxylateligands play an important role in this field because they can assumea large palette of coordination modes such as syn-syn, anti-anti,syn-anti, and monatomic to form key building blocks for thesynthesis of polynuclear complexes.8 With extensive knowledgeof the coordination characteristics of the carboxylate group, itis clear that the formate ion, being the smallest carboxylate andconsequently having a small steric effect, is very effective in joiningtwo or more transition-metal ions as a three-atom connector toform a variety of zero-,9 one-,10 two-11 and three-dimensional12

complexes. All of the bridging modes of formate usually promoteeffective magnetic exchange pathways between moment carriersand consequently mediate ferro- or antiferromagnetic couplingbetween metal ions in different situations.13 The m1,3-azide bridge,

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3295

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online / Journal Homepage / Table of Contents for this issue

Scheme 1 The various bridging modes for m-HCOO- and m-NO2-.

which is known to produce analogous structures but usuallymediates a stronger coupling between metal ions than formate, hasbeen found to bring interesting variations in magnetic properties.3c

On the other hand the nitrite ion, which is isoelectronic withformate, is more versatile in bridging the metal ions because, inaddition to the two terminal oxygen atoms, the central nitrogenatom can also coordinate to the metal center (Scheme 1). Never-theless, it has been used only occasionally to construct polynuclearcomplexes and as a magnetic coupler. In fact, among its variousbridging modes, the m-nitrito-1kO:2kN bridging mode (Scheme 1f, g) is the most common one whereas the m-nitrito-1kO:2kO¢bridging modes (Scheme 1 a¢, b¢, c¢), which are analogous tocarboxylate, are very rare.14–16 Only recently, m-nitrito-1kO:2kO¢syn-syn (Scheme 1 a¢) and anti-anti (Scheme 1 c¢) nitrite bridgeshave been reported in some Cu(II)14 and Ni(II)15,16 systems.

Herein, we report the synthesis, single crystal X-ray structureand magnetic properties of two isoelectronic and isostructural one-dimensional chains of Mn(III)-salen complexes, one with a syn-anti formate bridge, [Mn(salen)(HCOO)]n (1) and the other witha syn-anti nitrite bridge [Mn(salen)(NO2)]n (2). To the best of ourknowledge, this syn-anti bridging mode of formate has not beenpreviously reported in Mn(III) and that of nitrite is unprecedentednot only in Mn(III) but also in any other transition metal ion.The frequency dependence of the ac susceptibility measurementsof these two ferromagnetically coupled compounds shows clearindications of SCM/spin glass behavior only in 2.

Experimental section

Salicylaldehyde, 1,2-ethanediamine, sodium nitrite (NaNO2) andsodium formate (HCOONa) were purchased from commercialsources and used as received. All other solvents were of reagentgrade and were used without further purification.

Caution! Perchlorate salts of metal complexes with organicligands are potentially explosive. Only a small amount of materialshould be prepared, and it should be handled with care.

Synthesis of the Schiff-base ligandN ,N-bis(salicylidene)ethane-1,2-diamine (H2salen)

The tetradentate Schiff base ligand was prepared by the condensa-tion of salicylaldehyde (1.05 mL, 10 mmol) and 1,2-ethanediamine(0.31 mL, 5 mmol) in methanol (10 mL) as reported earlier.17

Synthesis of [Mn(salen)(HCOO)]n (1) and [Mn(salen)(NO2)]n (2)

A 10 mL methanol solution of the ligand H2salen (5 mmol)was added to a methanol solution of Mn(ClO4)2·6H2O (1.805 g,5 mmol) with constant stirring. After ca. 15 min, an aqueoussolution of HCOONa (6 mmol, 0.408 g) (for 1) was added tothe solution, followed by the dropwise addition of triethylamine(1.40 mL, 10 mmol). The color of the solution turned to darkbrown immediately. The solution was filtered and the filtrate wasleft to stand in air. Slow evaporation of the resulting brownsolution gave dark brown microcrystalline compounds for 1. Thesolid was filtered when the volume of the resulting solution wasreduced to ca. 10 mL and washed with diethyl ether. Complex2 was obtained by following a similar procedure to that of 1, butNaNO2 (6 mmol, 0.414 g) was used in place of HCOONa. Layeringof the brown acetonitrile solution of 1 and the methanol solutionof 2 with Et2O gave well-formed X-ray quality single crystals.

Complex 1: (Yield: 1.29 g; 71%). Anal. Calcd forC51H45Mn3N6O12: C, 55.75; H, 4.13; N, 7.65; Found: C, 55.73;H, 4.10; N, 7.63. IR (KBr pellet, cm-1): 1620 n(C N),1296 nas(C O), 1209 ns(C O). lmax/nm (emax/dm3 mol-1 cm-1)(CH3CN), 395 (930).

Complex 2: (Yield: 1.39 g; 76%). Anal. Calcd forC48H42Mn3N9O12: C, 52.33; H, 3.84; N, 11.44 Found: C, 52.30;H, 3.80; N, 11.30. IR (KBr pellet, cm-1): 1621 n(C N), 1543nas(NO2), 1438 ns(NO2). lmax/nm (emax/dm3 mol-1 cm-1) (CH3CN),398 (917).

Physical measurements

Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 series II CHN analyzer. IR spectra in KBr (4500–500 cm-1) were recorded using a Perkin-Elmer RXI FT-IR spec-trophotometer. The electronic absorption spectra (1000–200 nm)of the complexes were recorded in CH3CN with a Hitachi U-3501spectrophotometer. Dc magnetic susceptibilities on polycrystallinesamples of 1 and 2 were measured on a Quantum Design SQUIDmagnetometer (MPMS-7) at applied magnetic fields of 50, 500,and 5000 Oe in a temperature range of 1.8–300 K. Typical samplemasses were 20–50 mg. The magnetic response was corrected withdiamagnetic blank data of the sample holder obtained separately.The diamagnetic contribution of the sample itself was estimatedfrom Pascal’s constant. The magnetization curves were recorded

3296 | Dalton Trans., 2011, 40, 3295–3304 This journal is © The Royal Society of Chemistry 2011

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Table 1 Crystal data and structure refinement of complexes 1 and 2

1 2

Formula C51H45Mn3N6O12 C48H42Mn3N9O12

Formula weight 1098.75 1101.73Space group P1 P1Crystal system Triclinic Triclinica/A 13.8053(16) 13.767(4)b/A 13.8932(18) 14.3539(16)c/A 14.0281(16) 13.870(2)a (◦) 103.698(10) 108.718(13)b (◦) 106.301(10) 107.147(18)g (◦) 104.191(11) 103.931(15)V/A3 2364.7(6) 2303.9(10)Z 2 2Calculated density Dc/g cm-3 1.543 1.588Absorption coeff. (m) mm-1 (Mo-Ka) 0.861 (Mo-Ka) 0.886F(000) 1128 1128Crystal size/mm 0.05 ¥ 0.22 ¥ 0.23 0.03 ¥ 0.19 ¥ 0.22q range (deg) 2.5 to 30.0 2.4 to 30.0R(int) 0.051 0.035No. of data measured 16 595 16 382No. of unique data 13 129 12 876Data with I > 2s(I) 8315 6567R1, wR2 0.0796, 0.2593 0.0478, 0.0781

from -70 to +70 kOe. The ac measurements were obtained on aQuantum Design PPMS ac/dc magnetometer on polycrystallinesamples. The ac frequency was varied from 10 to 10 000 Hz with anamplitude of 5 Oe. Magnetization curves in ±90 kOe were recordedon the PPMS magnetometer.

Crystal data collection and refinement

Crystal data for the two crystals are given in Table 1. 13 129 and12 876 independent data were collected for 1 and 2 respectivelywith Mo-Ka radiation at 150(2) K using the Oxford DiffractionX-Calibur CCD System. The crystals were positioned 50 mmfrom the CCD. 321 frames were measured with a countingtime of 10 s. Data analyses were carried out with the CrysAlisprogram.18 The structures were solved using direct methods withthe SHELXS97 program.19 The non-hydrogen atoms were refinedwith anisotropic thermal parameters. The hydrogen atoms bondedto carbon were included in geometric positions and given thermalparameters equivalent to 1.2 times those of the atom to whichthey were attached. In 2, one of the nitrites was disorderedshowing two modes of bridging. The two sets of sites wererefined with population parameters x and 1 - x, x refiningto 0.57(1). Absorption corrections were carried out using theABSPACK program.20 The structures were refined on F 2 usingSHELXL9719 to R1 0.0796, 0.0482 wR2 0.2593, 0.0781 for 8315,6567 independent reflections above background respectively.

Results and discussion

Syntheses

Compounds 1 and 2 were obtained in high yield by the reaction ofMn(ClO4)2·6H2O, H2salen, HCOONa/NaNO2 and triethylaminein 1 : 1 : 1 : 2 molar ratios. The Mn(II) ion was oxidized by theoxygen in the atmosphere to Mn(III) under the reaction conditionsand the donor atoms of the deprotonated Schiff-base ligand (salen)coordinate to form the equatorial plane of the Mn(III) site, as isusual for the salen-type Schiff base complexes. The formate ornitrite ion, which is present in the reaction mixture, coordinatesthrough its m-1kO:2kO¢ bridging mode to the axial positions ofsuch [Mn(salen)]+ species to result in the one-dimensional chain(Scheme 2). When the molar ratio of formate or nitrite and Mn(III)is less than 1 : 1 the resulting solids are not pure 1 or 2. In the IRspectra of these impure samples a strong peak due to perchloratewas found in the region of 1100 cm-1 along with the characteristicpeaks of formate or nitrite, indicating that the composition ofthe impurities might be [Mn(salen)(H2O)]ClO4. Therefore, forpreparation of 1 and 2 a slight excess of the corresponding sodiumsalt over the stoichiometric requirement is necessary.

IR and UV/VIS spectra of complexes

The attributions of the IR spectra in the 1530–1625 cm-1 regionare difficult due to the appearance of several absorption bandsfrom both the Schiff base and the carboxylate ligands. However,by comparing the IR spectra of the Mn(III) complexes of the sameligand but with other anions (e.g. azide, halides), the moderatelystrong and sharp band at 1621 cm-1 for 1 is assigned to theazomethine n(C N) group. The strong band at 1578 cm-1 islikely to be due to the antisymmetric and the band at 1441 cm-1

to the symmetric stretching modes for the formate. Complex 2shows a sharp peak at 1620 cm-1 corresponding to the azomethinen(C N) group. The strong band at 1296 cm-1 is attributed to theantisymmetric stretching of nitrite and the band at 1209 cm-1 isdue to the symmetric stretching mode for the nitrite group. Theabsorption spectra of all these complexes show a broad band atca. 395 nm, as is usual for salen-based Mn(III) complexes.21

Description of structure of complexes 1 and 2

Complex 1 features syn-anti m-formato-1kO:2kO¢ bridged one-dimensional helical chains of Mn(salen) units, as shown inFig. 1 together with the atomic numbering scheme. The structurecontains three metal atoms in the asymmetric unit.

All three independent manganese atoms are six-coordinateoctahedral being bonded to four atoms of salen ligand in anequatorial plane and two formate oxygen atoms in axial positions.Each formate ion links the Mn(salen) moieties through a syn-

Scheme 2 Formation of the complexes (the minor m-nitrito-1kO:2kN bridging configuration between two Mn atoms in 2 is omitted).

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3297

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Fig. 1 ORTEP view of 1 with ellipsoids at 30% probability (left) and helical polymeric structure (right).

Fig. 2 Formation of supramolecular network by C–H ◊ ◊ ◊ p interactions in complex 1.

anti m-formato-1kO:2kO¢ coordination mode to result in the one-dimensional polymeric structure. The four donor atoms from salenand the metal atom are nearly co-planar with r.m.s. deviations of0.023, 0.073, 0.008 A for the five atoms in the three equatorialplanes based on Mn(1), Mn(2) and Mn(3) respectively. The bondlengths in the equatorial plane fall in the range of 1.874(4)–1.902(4)A for Mn–O, 1.975(4)–2.010(4) A for Mn–N, while axial Mn–Odistances are in the range of 2.212(4)–2.257(4) A. Selected bondlengths and angles are summarized in Table 2. The axial bondsare significantly longer than the equatorial bonds, as expected forthe Jahn–Teller distortion of Mn ions with a +3 oxidation state.The intrachain Mn ◊ ◊ ◊ Mn distances are in the range of 5.482(2)–5.990(2) A (Table 2).

In the complex there is no considerable p-stacking interactionbetween the aromatic rings. However, there are several C–H ◊ ◊ ◊ p

(phenyl) interactions between the neighboring chains to generatea two-dimensional supramolecular network as shown in Fig. 2.The distance H20(B) ◊ ◊ ◊ Cg(4) is 2.71 A, H(55) ◊ ◊ ◊ Cg(1) is 2.58A, H(46) ◊ ◊ ◊ Cg(3) is 2.88 A and H(60B) ◊ ◊ ◊ Cg(2) is 2.68 A, theC20–H20(B) ◊ ◊ ◊ Cg (4) angle is 119◦, C(55)–H(55) ◊ ◊ ◊ Cg(1) is 162◦,C(46)–H(46) ◊ ◊ ◊ Cg(3) is 148◦ and C(60)–H(60B) ◊ ◊ ◊ Cg(2) is 149◦

[Cg = centroid of the phenyl ring].Complex 2 is isomorphous with complex 1. The only difference

between the two structures is that in 2 Mn atoms are bridgedthrough syn-anti 1kO:2kO¢ nitrite rather than formate. Thestructure of 2 is shown in Fig. 3 with the atomic numbering scheme.

As in 1 the structure contains three metal atoms in theasymmetric unit, all of which are six-coordinate with octahedralenvironments, being bonded to four atoms of salen ligand inthe equatorial plane and two bridging nitrites in axial positions.

3298 | Dalton Trans., 2011, 40, 3295–3304 This journal is © The Royal Society of Chemistry 2011

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Table 2 Bond distances (A) and angles (◦) in the metal coordinationspheres of complexes 1 and 2a

1 2

Mn(1)–O(30) 1.894(4) 1.877(2)Mn(1)–O(11) 1.902(4) 1.866(2)Mn(1)–N(22) 1.975(4) 1.976(2)Mn(1)–N(19) 2.010(4) 1.984(2)Mn(1)–O(73) 2.212(4) 2.246(3)Mn(1)–O(77) 2.257(4) 2.335(2)Mn(2)–O(50) 1.899(4) 1.876(2)Mn(2)–O(31) 1.900(4) 1.880(2)Mn(2)–N(39) 2.001(5) 1.983(2)Mn(2)–N(42) 2.005(4) 1.985(2)Mn(2)–O(71) 2.228(4) 2.370(7)Mn(2)–O(74) 2.230(4) 2.312(2)Mn(3)–O(70) 1.874(4) 1.870(2)Mn(3)–O(51) 1.901(4) 1.867(2)Mn(3)–N(59) 1.987(4) 1.981(2)Mn(3)–N(62) 2.000(4) 1.971(2)Mn(3)–O(76)b 2.242(4) 2.302(2)Mn(3)–O(79) 2.252(4) 2.272(2)Mn(1)–Mn(2) 5.482(2) 5.228(2)Mn(1)–Mn(3) 5.990(2) 5.498(2)Mn(3)–Mn(2)a 5.916(2) 5.851(2)O(30)–Mn(1)–O(11) 95.04(16) 93.59(8)O(30)–Mn(1)–N(22) 90.61(17) 92.02(9)O(11)–Mn(1)–N(22) 173.99(17) 174.38(9)O(30)–Mn(1)–N(19) 172.53(18) 173.64(9)O(11)–Mn(1)–N(19) 92.41(17) 92.17(9)N(22)–Mn(1)–N(19) 81.96(18) 82.22(10)O(30)–Mn(1)–O(73) 92.37(18) 87.94(10)O(11)–Mn(1)–O(73) 90.68(17) 93.92(10)N(22)–Mn(1)–O(73) 87.02(17) 86.15(9)N(19)–Mn(1)–O(73) 88.02(17) 88.99(9)O(30)–Mn(1)–O(77) 88.84(16) 80.90(9)O(11)–Mn(1)–O(77) 91.37(16) 88.40(9)N(22)–Mn(1)–O(77) 90.80(17) 92.62(9)N(19)–Mn(1)–O(77) 90.49(15) 101.96(8)O(73)–Mn(1)–O(77) 177.52(16) 168.73(8)O(50)–Mn(2)–O(31) 94.71(16) 94.90(8)O(50)–Mn(2)–N(39) 172.27(18) 172.22(9)O(31)–Mn(2)–N(39) 92.30(18) 92.23(9)O(50)–Mn(2)–N(42) 91.72(18) 92.10(9)O(31)–Mn(2)–N(42) 171.93(17) 170.96(9)N(39)–Mn(2)–N(42) 81.59(19) 81.14(10)O(50)–Mn(2)–O(71) 91.14(17) 86.54(16)O(31)–Mn(2)–O(71) 85.22(15) 77.18(17)N(39)–Mn(2)–O(71) 86.22(17) 92.01(16)N(42)–Mn(2)–O(71) 99.55(16) 109.02(18)O(50)–Mn(2)–O(74) 95.94(16) 94.56((7)O(31)–Mn(2)–O(74) 86.75(16) 86.30(7)N(39)–Mn(2)–O(74) 87.68(16) 89.01(8)N(42)–Mn(2)–O(74) 87.74(17) 87.44(8)O(71)–Mn(2)–O(74) 169.70(15) 163.47(17)O(70)–Mn(3)–O(51) 95.45(16) 93.27(8)O(70)–Mn(3)–N(59) 174.21(17) 173.90(9)O(51)–Mn(3)–N(59) 90.33(17) 92.68(8)O(70)–Mn(3)–N(62) 92.51(16) 91.13(8)O(51)–Mn(3)–N(62) 172.02(17) 175.49(8)N(59)–Mn(3)–N(62) 81.71(18) 82.90(9)O(70)–Mn(3)–O(76)b 86.63(15) 85.84(7)O(51)–Mn(3)–O(76)b 92.28(17) 91.86(8)N(59)–Mn(3)–O(76)b 92.98(16) 95.31(8)N(62)–Mn(3)–O(76)b 87.54(16) 89.45(8)O(70)–Mn(3)–O(79) 87.16(16) 89.22(8)O(51)–Mn(3)–O(79) 93.48(15) 94.33(8)N(59)–Mn(3)–O(79) 92.68(16) 88.99(9)

Table 2 (Contd.)

1 2

N(62)–Mn(3)–O(79) 87.55(15) 84.72(8)O(79)–Mn(3)–O(76)b 171.90(15) 172.29(7)

a Dimensions (A, ◦) involving the minor nitrite component in 2 areMn(2)–N(72B) 2.311(10) A, O(50)–Mn(2)–N(72B) 89.1(2), O(31)–Mn(2)–N(72B) 94.4(3), N(39)–Mn(2)–N(72B) 87.2(2), N(42)–Mn(2)–N(72B)91.4(3), O(74)–Mn(2)–N(72B) 176.2(2). b Symmetry element 1 + x, 1 + y,1 + z.

However in one of the nitrites, namely O(71), N(72), O(73)which bridges Mn(1) and Mn(2), while the position of O(73)is ordered, those of O(71) and N(72) are disordered over twosites A and B, with occupation parameters for A of 0.57(1)and B of 0.43(1). In A this results in Mn(2)–O(71A)–N(72A)–O(73)–Mn(1) bridging (syn-anti 1kO:2kO¢) while in configurationB the bridging pattern is Mn(2)–N(72B)–O(73)–Mn(1) withO(71B) unbonded, thus can be described as bridging type g(m-nitrito-1kO:2kN) in Scheme 1. Presumably as a result of thisdisorder and the possibility of type g bridging involving just twobridging atoms, the Mn(1) ◊ ◊ ◊ Mn(2) distance is the shortest metalmetal distance at 5.228(2) A with Mn(1) ◊ ◊ ◊ Mn(3) at 5.498(2) Aand Mn(3) ◊ ◊ ◊ Mn(2)a at 5.851(2) A.

The Mn–O bridge distances fall in the range of 2.246(3)–2.370(7) A with Mn(2)–N(72B) 2.311(10) A. The distances inthe equatorial planes are similar to those in 1 with Mn–Oand Mn–N in the ranges of 1.866(2)–1.880(2) and 1.971(2)–1.985(2) A. The four donor atoms and the metal atom in theequatorial plane show r.m.s. deviations of 0.022, 0.071 and 0.008A for Mn(1), Mn(2) and Mn(3) respectively. Here also, the 1Dchains form a supramolecular 2D network through C–H ◊ ◊ ◊ p(phenyl) interactions as shown in Fig. S1, ESI.† The distancesH(55) ◊ ◊ ◊ Cg(5) and H(60B) ◊ ◊ ◊ Cg(6) are 2.71 A and 2.69 A and theangles between C(55)–H(55) ◊ ◊ ◊ Cg(5) and C(60)–H(60B) ◊ ◊ ◊ Cg(6)are 134 and 121◦, respectively.

It is to be noted that although formate bridging is welldocumented in several metal ions including Mn(II), it is very rare inMn(III) complexes. A CSD search reveals that a formate bridge ispresent only in manganese(III) formate22 and in two manganese(III)tetraphenylporphyrin complexes.23 However, in these compoundsthe bridging mode of formate is anti-anti. Therefore, complex1 is the first example of a Mn(III) system in which a m1,3 syn-anti formate bridge is present. Interestingly, the structure of theacetate analogue of complex 1, [Mn(salen)(CH3COO)]n is also aone-dimensional chain but the bridging mode of acetate is anti-anti.24 On the other hand, to our knowledge, nitrite bridged Mn(III)complexes have not previously been observed. Therefore, complex2 is the first example of nitrite-bridged Mn(III). Moreover, the m-nitrito-1kO:2kO¢ modes which are analogous to carboxylate arevery rarely found in transition metal complexes. Only recentlyhave these bridges been found in [Ni2(L)(l-O2N)(NO2)(OH2)] (L =cyclam based ligand),16 [M(pyrazine)2(NO2)]ClO4 (M = Co, Cu),14

[Ni(NO2)2]n and [Ni(4,4¢-bipy)(NO2)2]n.15 In these complexes thebridging mode of nitrite is syn-syn in [M(pyrazine)2(NO2)]ClO4

(M = Co, Cu) and [Ni2(L)(l-O2N)(NO2)(OH2)], whereas it is anti-anti in Ni(NO2)2 and [Ni(4,4¢-bipy)(NO2)2]n. Thus the syn-antibridging mode of nitrite which is observed in complex 2 is notonly unprecedented in Mn(III) but also in any other transition

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3299

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Fig. 3 ORTEP view of complex 2 with ellipsoids at 30% probability (left). The two configurations of the bridging nitrite O(71), N(72), O(73) are shownA with 57%, B (dotted lines) with 43% occupation, O(73) is common. Helical polymeric structure with minor nitrite configuration omitted (right).

Fig. 4 The cmT vs. T plot for a randomly oriented polycrystalline specimen of 1 (a) and 2 (b). The solid lines show curves calculated with the Fisherequation. For the equation and parameters, see the text.

metal ion. The syn-anti bridging mode of the carboxylate/nitritehas important implications in the magnetic properties of thecompounds which are discussed in subsequent sections. In thiscontext it is worth comparing the structure of these two complexeswith the azide bridged Mn(III)-salen complex.17 The comparisonshows that the syn-anti bridging mode of nitrite or formate isanalogous to m-1,3 N3 to result in helical 1D chains in thesesystems.

Magnetic study for compounds 1 and 2

We measured the magnetic properties of 1 and 2 on a SQUIDmagnetometer at an applied field of 0.05 T. As Fig. 4 shows, thecmT values increased with a decrease of temperature down to ca.5 K. The data obeyed the Curie–Weiss law, cm = C/(T - q), and theparameters were optimized above 5 K to give C = 3.044(5) cm3 K

mol-1 and q = +1.04(3) K for 1 and C = 2.838(4) cm3 K mol-1 andq = +0.70(3) K for 2. The C values are close to the spin-only valueof 3.0 cm3 K mol-1 expected from two high-spin d4 (S = 2) species.The g values of 1 and 2 were 2.01 and 1.95, respectively, and arecompatible with those of the high-spin Mn3+ compounds.25,26 Thepositive q values suggest the presence of dominant ferromagneticcoupling. On further cooling below 5 K, the cmT value decreased,which is partly due to the saturation effect, as clarified by the M-Hmeasurements.

The nearest-neighbor interaction is ferromagnetic as shownby positive q values, which can be attributed to the through-bond Mn3+–Mn3+ coupling across the bridges. The carboxylatebridges are well-known to serve as ferro- and antiferromagneticsuperexchange couplers, depending on the geometry and themutual lobe direction of the magnetic orbitals.8–13,27 Several Mn3+

carboxylate complexes showed ferromagnetic couplings,28 and its

3300 | Dalton Trans., 2011, 40, 3295–3304 This journal is © The Royal Society of Chemistry 2011

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Fig. 5 (a) The M-H curves on polycrystalline 1 (a) and 2 (b) measured at 1.8 and 10 K. Solid lines are drawn for a guide to the eye.

mechanism has been proposed as follows. The parameter J canbe related to the individual J ij interactions between orbitals i ofion A and orbitals j of ion B; namely, J = R J ij/nAnB, wherenX is the number of unpaired electrons on ion X. There areferro- and antiferromagnetic contributions in J. The magneticorbital that can make a significant contribution to the couplingin manganese(III) ions having Jahn–Teller elongation is the singly-occupied 3dz2 . Thus the axial ligands will provide a superexchangepathway. However, due to the weakened axial interatomic orbitaloverlap, this can only produce a weak interaction and underpossible conditions the ferromagnetic contributions surpass theantiferromagnetic one. The nitrite bridge is somewhat rare,29 butit is isoelectronic to the formate bridge, and accordingly it isreasonable to assume similar magnetic behavior.

Analysis in a high-temperature region of the cmT vs. T plot issupposed to give information of the largest exchange parameter(J). Although there are three crystallographically independent Mnions, we assume that the Mn chain would consist of equally-spacedS = 2 spins for simplifying the analysis. The exchange parameterscan be estimated from numerical fitting (Fig. 4), according to ananalytical expression for the magnetic susceptibility of an infiniteclassical-spin chain derived by Fisher (eqn (1) and 2),30,31 where Jis defined by the spin Hamiltonian H = -2J R Si·Si + 1.

c mm

A B

B

=+ +

−N g S S

k T

u

u

2 2 1

3

1

1

( )(1)

and

uJS S

k T

k T

JS S=

+⎡

⎣⎢⎢

⎤

⎦⎥⎥ − +

⎡

⎣⎢⎢

⎤

⎦⎥⎥coth

( )

( )

2 1

2 1B

B (2)

The parameters were optimized by using the data above 5 K, togive J/kB = 0.088(5) K with g = 2.026(4) for 1 and J/kB = 0.128(3)K with g = 1.936(2) for 2. The calculated curve almost reproducedthe experimental data (the solid lines in Fig. 4). The Mn ◊ ◊ ◊ Mndistances of 2 are slightly shorter (5.228–5.851 A) than those of 1(5.482–5.990 A). This geometrical difference is responsible for thelarger exchange coupling in 2.

The saturation effect was actually observed by the final cmTdrop, when the applied field was varied. The cmT plot was recordedin as small as 0.05 T, but the cmT drop still remained, suggestingthe presence of interchain interaction. As the crystallographicanalysis revealed, inter-chain atomic contacts occurs aroundthe C–H ◊ ◊ ◊ p interactions, and magnetic exchange coupling can

hardly be expected there. The coupling may be attributed toclassical dipolar interaction, which is sometimes found in low-dimensional magnets.32,33

Fig. 5 shows the magnetizations for 1 and 2. They showed astepwise increase of magnetization at 1.8 K whereas a monotonicincrease was observed at 10 K. At 1.8 K, the magnetization oncereached a plateau, which seems to be related to the cmT drop below5 K. The second magnetization increase was very gradual andcharacterized by means of differentiation analysis (i.e., dM/dHvs. H plots), giving HC = 6.5 and 8.3 T for 1 and 2, respectively.The magnetizations at 9 T (3.6 and 3.2 NAmB, respectively) weresomewhat lower than the theoretical saturation magnetization (4NAmB), because of the imperfect saturation even at 9 T. The slopeswere particularly steep below 104 Oe, being consistent with thenotion that the specimens are dominantly ferromagnetic as seenin Fig. 4.

The present magnetic properties of both compounds can beregarded as canted ferromagnetic systems. Some manganese(II)and cobalt(II) coordination compounds showed similar anomaliesin magnetization, which have been characterized as spin-cantedsystems,35 and several three-atom bridging ligands are well knownto afford canted-spin structures.36 This situation loses centrosym-metry between the neighboring spins, favoring antisymmetric in-teraction, known as the Dzjaloshinsky–Moriya (DM) interaction,H = -DDM·(Si¥Sj).37,38 However, the magnetization plateau valuesfor 1 and 2 are 2.9 and 2.5 NAmB, suggesting the cant angles of 43and 50◦, respectively; the relatively large cant angles here may notbe explained solely from the DM interaction.6,36

The conventional canted spin systems show a linear fielddependence on further applied field, once the ground cantedspin structure is formed. In general, manganese(III) compoundshave a uniaxial anisotropy owing to Jahn–Teller distortion,and such an anisotropy is usually defined by the zero-field-splitting parameters with D < 0. Step-like behavior in the M-Hcurves may be explained in terms of the competing interactionsbetween the zero-field splittings and the magnetic couplings withdifferent amplitudes.26,34 The formato and nitrito bridges forma zigzag chain with neighboring single-ion magnetic anisotropydirections canted each other, and such non-colinear arrangementof anisotropy tensors of adjacent spins may be responsible forthe large spin-canting.6,36 Since there are too many parametersto characterize them quantitatively, we can at least say thatthese auxiliary effects are smaller than the dominant ferro-magnetic J.

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3301

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

Fig. 6 The ac magnetic susceptibilities, cac¢ (in-phase) and cac¢¢ (out-of-phase), for 1 (a) and 2 (b). The ac field was applied with an amplitude of 5 Oeand frequencies of 10–10 000 Hz. Solid lines are drawn for a guide to the eye. The inset in (b) shows the Cole–Cole plot at 1.8 K for 2. A solid line standsfor the calculated curve with a = 0.18.

We have no evidence for long-range order down to 1.8 K in1 or 2 from the static magnetic studies (field-cooled, zero-field-cooled, or remnant magnetizations). The alternating-current (ac)magnetometry is a versatile tool to investigate single-moleculemagnet (SMM) and SCM behavior showing slow magnetizationreversal.39 The ac magnetic susceptibilities of 1 and 2 weremeasured (Fig. 6). Practically identical plots were obtained witha dc bias field of 0.1 T. Negligible frequency dependence wasobserved for 1. Compound 2 exhibited a decrease of cac¢ anda concomitant increase of cac¢¢ on elevating frequency around2 K. This finding seems to be an indication of slow magnetizationreversal. No cac¢¢ peak was observed down to 1.8 K in anycase, suggesting that the meaningful retardation of magnetizationreversal would take place much lower than 2 K.

The Cole–Cole analysis40 was performed on the data of 2 at1.8 K (Fig. 6b, inset). Only a small portion of a semicircle wasdrawn, but the a value was successfully optimized according tothe theoretical equation,41 giving a = 0.18 ± 0.05. When a dcbias field (0.1 T) was applied, a similar experiment and analysisafforded a = 0.045 ± 0.009. These small a values imply the singlerelaxation pathway for the magnetization reorientation. Althoughthe presence of relaxation process with an activation energy isconfirmed, the possibility of spin-glass is not completely rejected,depending on the long or short intrachain correlation length.Coexistence of canted-spin properties and SCM-like behavior hasbeen reported and discussed recently.42

Considering of the possibility of the SCMs for the presentcompounds, we have to recall several dinuclear Mn3+ buildingblocks involving salen-based bridges those are known to bepotential SMMs.43–46 Very recently, a homometallic Mn3+

3-basedone-dimensional compound has been reported to behave asa SCM.46 In the context of the Glauber theory,47,48 the acti-vation energy of the magnetization reversal is related to theintrachain magnetic exchange interaction (eqn (3)), where t is1/(2pf ).

t = t 0 exp(DGlauber/kBT) = t 0 exp(4 S2J/kBT) (3)

The present compounds showed very small magnetic exchangecoupling (J/kB = 0.088(5) and 0.128(3) K for 1 and 2, respectively).Consequently, the activation energy is considerably small (DGlauber

£ ca. 2 K), leading to the low blocking temperature. Though it islow, the frequency dependence of the ac susceptibility would bean indication of a SCM or spin-glass.

In complexes 1 and 2, though the bridging mode of nitriteand formate is the same, the nitrite bridge is a stronger magneticcoupler than the formate bridge, as is evidenced from the largermagnetic coupling (Fig. 4), and accordingly the activation energyfor the magnetization reversal appreciably higher (Fig. 6). Thereason seems to reside in the shorter Mn ◊ ◊ ◊ Mn distances in thenitrite system.

Conclusions

We have prepared two isoelectronic and isostructural syn-anti m-1kO:2kO¢ formate and nitrite bridged one-dimensional chains ofsalen-based Mn(III) complexes. Formate bridged Mn(III) systemsare very rare and compound 1 is the first example of syn-anti formate bridging in Mn(III) compounds. The nitrite bridgedMn(III) system is even rarer and the syn-anti m-nitrito-1kO:2kO¢bridge is unprecedented not only in Mn(III) but also in anyother transition metal ion. Both the chains are ferromagneticallycoupled. The nitrite bridged complex (2) shows clear indicationof slow magnetization reversal characteristic of a SCM or spin-glass. The structural similarity of the present two complexes tothat of m1,3-azide bridged Mn(III)-salen reveals that they can besuitable analogues of azide - one of the most popular choices inrecent years as a bridging group for construction of polynuclearcomplexes. More importantly, these bridging modes with formateand nitrite (syn-anti 1kO:2kO¢) as the ferromagnetic couplerovercome the disadvantages of usual antiferromagnetic couplingof the m1,3-azide bridge. In this regard, it is worth mentioningthat analogous structural and magnetic properties of formate,azide and other short bridging ligands (OH-, CN-, C2O4

2- etc)have been investigated and reviewed.36a However, this hitherto

3302 | Dalton Trans., 2011, 40, 3295–3304 This journal is © The Royal Society of Chemistry 2011

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

unexplored syn-anti m-nitrito-1kO:2kO¢ bridge seems to be apotentially superior magnetic coupler to the formate bridgefor the construction of single-molecule/-chain magnets, as itscoupling constant is greater and the cac¢ and cac¢¢ show frequencydependence. We will pay attention to exploit this m-nitrite bridgefor future work on SCM and SMM.

Acknowledgements

P.K and R.B are thankful to CSIR, India, for research fellowship[Sanction no. 09/028(0733)/2008-EMR-I] and [Sanction no.09/028(0746)/2009-EMR-I]. We thank EPSRC and the Univer-sity of Reading for funds for the X-Calibur system. We alsoare thankful for financial support to Scientific Research (Nos.21110513 and 22350059) from the Ministry of Education, Culture,Sports, Science and Technology, Japan.

References

1 (a) T. Taguchi, M. R. Daniels, K. A. Abboud and G. Christou, Inorg.Chem., 2009, 48, 9325; (b) O. Kahn and C. J. Martinez, Science, 1998,279, 44; (c) P. Gutlich, Y. Garcia and T. Woike, Coord. Chem. Rev.,2001, 219, 839; (d) M. Morimoto, H. Miyasaka, M. Yamashita and M.Irie, J. Am. Chem. Soc., 2009, 131, 9823; (e) H-L. Sun, Z-M. Wang andS. Gao, Coord. Chem. Rev., 2010, 254, 1081.

2 (a) H. Hiraga, H. Miyasaka, R. Clerac, M. Fourmigue and M.Yamashita, Inorg. Chem., 2009, 48, 2887; (b) D. Wu, D. Guo, Y. Song,W. Huang, D. Chunying, Q. Meng and O. Sato, Inorg. Chem., 2009,48, 854; (c) S. J. Shah, C. M. Ramsey, K. J. Heroux, A. G. DiPasquale,N. S. Dalal, A. L. Rheingold, E. Del Barco and D. N. Hendrickson,Inorg. Chem., 2008, 47, 9569.

3 (a) C-M. Liu, D-Q. Zhang and D-B. Zhu, Inorg. Chem., 2009, 48, 4980;(b) A. J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboud andG. Christou, Angew. Chem., Int. Ed., 2004, 43, 2117; (c) H.-B. Xu, B.-W.Wang, F. Pan, Z.-M. Wang and S. Gao, Angew. Chem., Int. Ed., 2007,46, 7388.

4 (a) E. J. Schelter, A. V. Prosvirin and K. R. Dunbar, J. Am. Chem. Soc.,2004, 126, 15004; (b) M. Ferbinteanu, H. Miyasaka, W. Wernsdorf, K.Nakata, K.-I. Sugiura, M. Yamashita, C. Coulon and R. Clerac, J. Am.Chem. Soc., 2005, 127, 3090.

5 (a) L. M. Toma, R. Lescouezec, F. Lloret, M. Julve, J. Vaissermannand M. Verdaguer, Chem. Commun., 2003, 1850; (b) T. Kajiwara, M.Nakano, Y. Kaneko, S. Takaishi, T. Ito, M. Yamashita, A. Igashira-Kamiyama, H. Nojiri, Y. Ono and N. Kojima, J. Am. Chem. Soc., 2005,127, 10150.

6 X.-Y. Wang, Z.-M. Wang and S. Gao, Inorg. Chem., 2008, 47, 5720.7 (a) C.-M. Liu, D.-Q. Zhang and D.-B. Zhu, Inorg. Chem., 2009, 48,

4980; (b) J. T. Brockman, T. C. Stamatatos, W. Wernsdorfer, K. Abboudand G. Christou, Inorg. Chem., 2007, 46, 9160; (c) M. Yuan, F. Zhao,W. Zhang, Z-M. Wang and S. Gao, Inorg. Chem., 2007, 46, 11235.

8 (a) C. Biswas, P. Mukherjee, M. G. B. Drew, C. J. Gomez-Garcıa, J. M.Clemente-Juan and A. Ghosh, Inorg. Chem., 2007, 46, 10771; (b) A. K.Ghosh, D. Ghoshal, E. Zangrando, J. Ribas and N. R. Chaudhuri,Inorg. Chem., 2007, 46, 3057; (c) P. Mukherjee, M. G. B. Drew, C. J.Gomez-Garcia and A. Ghosh, Inorg. Chem., 2009, 48, 4817; (d) M. S.Ray, A. Ghosh, A. Das, M. G. B Drew, J. R- Arnio, J. Novoa and J.Ribas, Chem. Commun., 2004, 1102; (e) C. Biswas, M. G. B. Drew, E.Ruiz, M. Estrader, C. Diaz and A. Ghosh, Dalton Trans., 2010, 39,7474.

9 C. Cadiou, R. A. Coxall, A. Graham, A. Harrison, M. Helliwell, S.Parsons and R. E. P. Winpenny, Chem. Commun., 2002, 1106.

10 M. J. Sanchis, P. Gomez-Romero, J. V. Folgado, R. Sapinaz, A. Beltran,J. Garcia and D. Beltran, Inorg. Chem., 1992, 31, 2915.

11 (a) Y. F. Zeng, X. Hu, J. P. Zhao, B. W. Hu, E. C. Sanudo, F. C. Liuand X. H. Bu, Chem.–Eur. J., 2008, 14, 7127; (b) H. Kubo, K. Zenmyo,M. Matsumura, K. Takeda, K. Alhara and K. Yamagata, J. Phys. Soc.Jpn., 1999, 68, 253.

12 (a) Z.-M. Wang, B. Zhang, H. Fujiwara, H. Kobayashi and M.Kurmoo, Chem. Commun., 2004, 416; (b) M. Viertelhaus, H. Henke,C. E. Anson and A. K. Powell, Eur. J. Inorg. Chem., 2003, 2283.

13 (a) V. Paredes-Garcia, A. Vega, M. A. Novak, M. G. F. Vaz, D. A.Souza, D. Venegas-Yazigi and E. Spodine, Inorg. Chem., 2009, 48,4737; (b) X-Y. Wang, L. Gan, S-W. Zhang and S. Gao, Inorg. Chem.,2004, 43, 4615.

14 T. Liu, Y-H. Chen, Y-J. Zhang, Z-M. Wang and S. Gao, Inorg. Chem.,2006, 45, 9148.

15 V. Vo, Y. Kim, N. V. Minh, C. S. Hong and S-J. Kim, Polyhedron, 2009,28, 1150.

16 (a) J. C. Byun, W. H. Lee and C. H. Han, Inorg. Chem. Commun., 2006,9, 563; (b) S. Naiya, H-S. Wang, M. G. B. Drew, Y. Song and A. Ghosh,Dalton Trans., DOI: 10.1039/C0DT00978D.

17 A. Panja, N. Shaikh, P. Vojtisek, S. Gao and P. Banerjee, New J. Chem.,2002, 26, 1025.

18 CrysAlis, (2006) Oxford Diffraction Ltd., Abingdon, U.K..19 G. M. Sheldrick, Shelxs97 and Shelxl97, Programs for Crystallographic

solution and refinement, Acta Crystallogr., Sect. A: Found. Crystallogr.,2008, A64, 112.

20 ABSPACK (2005) Oxford Diffraction Ltd, Oxford, U.K..21 (a) L. Lecren, W. Wernsdorfer, Y.-G. Li, A. Vindigni, H. Miyasaka

and R. Clerac, J. Am. Chem. Soc., 2007, 129, 5045; (b) Z. Lu, M.Yuan, F. Pan, S. Gao, D. Zhang and D. Zhu, Inorg. Chem., 2006, 45,3538.

22 A. Cornia, A. Caneschi, P. Dapporto, A. C. Fabretti, D. Gatteschi, W.Malavasi, C. Sangregorio and R. Sessoli, Angew. Chem., Int. Ed., 1999,38, 1780.

23 P. Turner, M. J. Gunter, T. W. Hambley, A. H. White and B. W. Skelton,Inorg. Chem., 1992, 31, 2297.

24 J. E. Davies, B. M. Gatehouse and K. S. Murray,, J. Chem. Soc., DaltonTrans., 1973, 2523.

25 P. Kar, P. Mukherjee, M. G. B. Drew, T. Ishida and A. Ghosh,Eur. J. Inorg. Chem. in press (2011).

26 (a) Q. Scheifele, C. Riplinger, F. Neese, H. Weihe, A.-L. Barra, F.Juranyi, A. Podlesnyak and P. L. W. Tregenna-Piggott, Inorg. Chem.,2008, 47, 439; (b) H. A. Goodwin and R. S. Sylva, Aust. J. Chem., 1967,20, 629; (c) V. V. Zelentsov, J. Struct. Chem., 1967, 8, 581.

27 (a) H. Kageyama, D. I. Khomskii, R. Z. Levitin and A. N. Vasilev,Phys. Rev. B: Condens. Matter, 2003, 67, 224422; (b) S. J. Rettig, R. C.Thompson, J. Trotter and S. H. Xia, Inorg. Chem., 1999, 38, 1360; (c) F.Sapina, M. Burgos, E. Escriva, J. V. Folgado, D. Marcos, A. Beltran andD. Beltarn, Inorg. Chem., 1993, 32, 4337; (d) M. Viertelhous, H. Henke,C. E. Anson and A. K. Powell, Eur. J. Inorg. Chem., 2003, 2283; (e) V.Paredes-Garcia, A. Vega, M. A. Novak, M. G. F. V. Vaz, D. A. Souza,D. Venegas-Yazigi and E. Spodine, Inorg. Chem., 2009, 48, 4737; (f) C.Ruiz-Perez, J. Sanchiz, M. H. Molina, F. Lloret and M. Julve, Inorg.Chem., 2000, 39, 1363; (g) S. M. Saadeh, K. L. Trojan, J. W. Kampf,W. E. Hatfield and V. L. Pecararo, Inorg. Chem., 1993, 32, 3034.

28 (a) K. Mitra, D. Mishra, S. Biswas, C. R. Lucas and B. Adhikary,Polyhedron, 2006, 25, 1681; (b) K. Wieghardt, U. Bossek, B. Nuber, J.Weiss, J. Bonvoisin, M. Corbella, S. E. Vitols and J.-J. Girerd, J. Am.Chem. Soc., 1988, 110, 7398; (c) C. Canada-Vilalta, W. E. Streib, J. C.Huffman, T. A. O’Brien, E. R. Davidson and G. Christou, Inorg. Chem.,2004, 43, 101; (d) M. Corbella, R. Costa, J. Ribas, P. H. Fries, J. M.Latour, L. Ohrstrom, X. Solans and V. Rodriguez, Inorg. Chem., 1996,35, 1857.

29 (a) B. Sarkar, S. Konar, C. J. Gomez-Garcia and A. Ghosh, Inorg.Chem., 2008, 47, 11611; (b) J. Hausmann, M. H. Klingele, V. Lozan, G.Steinfeld, D. Siebert, Y. Journaux, J. J. Girerd and B. Kersting, Chem.–Eur. J., 2004, 10, 1716; (c) C. Diaz, J. Ribas, R. Costa, J. Tercero, F.El, S. Mohamed, X. Solans and M. Font-Bardia, Eur. J. Inorg. Chem.,2000, 675.

30 30. O. Kahn, “Molecular Magnetism” Chapt. 11.2, pp. 257–261, VCHPublishers,Inc., New York, Weinheim and Cambridge (1993).

31 M. E. Fisher, Am. J. Phys., 1964, 32, 343.32 C. M. Wynn, M. A. Girtu, W. B. Brinckerhoff, K.-i. Sugiura, J. S. Miller

and A. J. Epstein, Chem. Mater., 1997, 9, 2156.33 (a) N. Ishii, Y. Okamura, S. Chiba, T. Nogami and T. Ishida, J. Am.

Chem. Soc., 2008, 130, 24; (b) Y. Okamura, T. Nogami and T. Ishida,Chem. Lett., 2009, 38, 740; (c) Y. Okamura, N. Ishii, T. Nogami and T.Ishida, Bull. Chem. Soc. Jpn., 2010, 83, 716.

34 W.-X. Zhang, W. Xue, Y.-Z. Zheng and X.-M. Chen, Chem. Commun.,2009, 3804.

35 (a) H.-P. Jia, W. Li, Z.-F. Ju and J. Zhang, Chem. Commun., 2008,371; (b) O. Sengupta and P. S. Mukherjee, Dalton Trans., 2009, 7599;(c) K. C. Mondal, O. Sengupta, M. Nethaji and P. S. Mukherjee, DaltonTrans., 2008, 767.

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3303

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online

36 (a) X.-Y. Wang, Z.-M. Wang and S. Gao, Chem. Commun., 2008, 281;(b) N. Takagami, T. Ishida and T. Nogami, Bull. Chem. Soc. Jpn., 2004,77, 1125; (c) S. Mossin, H. Weihe, H. O. Sorensen, N. Lima and R.Sessoli, Dalton Trans., 2004, 632.

37 (a) I. Dzjaloshinsky, J. Phys. Chem. Solids, 1958, 4, 241; (b) I.Dzjaloshinski, Soviet Phys. JETP., 1958, 6, 1120.

38 (a) T. Moriya, Phys. Rev., 1960, 117, 635; (b) T. Moriya, Phys. Rev.,1960, 120, 91.

39 (a) D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, OxfordUniversity Press: New York, 2006; (b) D. Gatteschi and R. Sessoli,Angew. Chem., Int. Ed., 2003, 42, 268.

40 K. S. Cole and H. R. Cole, J. Chem. Phys., 1941, 9, 341.41 R Sessoli and A. K. Powell, Coord. Chem. Rev., 2009, 253, 2328.42 (a) K. Bernot, J. Luzon, R. Sessoli, A. Vindigni, J. Thion, S. Richeter,

D. Leclercq, J. Larionova and A. Van Der Lee, J. Am. Chem. Soc.,2008, 130, 1619; (b) Y.-Z. Zheng, W. Xue, M.-L. Tong, X.-M. Chen,F. Grandjean and G. J. Long, Inorg. Chem., 2008, 47, 4077; (c) J. H.

Yoon, D. W. Ryu, H. C. Kim, S. W. Yoon, B. J. Suh and C. S. Hong,Chem.–Eur. J., 2009, 15, 3661.

43 (a) Z. Lu, M. Yuan, F. Pan, S. Gao, D. Zhang and D. Zhu, Inorg.Chem., 2006, 45, 3548; (b) H. Miyasaka, R. Clerac, T. Ishii, H. Chang,S. Kitagawa and M. Yamashita, J. Chem. Soc., Dalton Trans., 2002,1528.

44 H. Miyasaka, A. Saitoh and S. Abe, Coord. Chem. Rev., 2007, 251,2622.

45 R. Clerac, H. Miyasaka, M. Yamashita and C. Coulon, J. Am. Chem.Soc., 2002, 124, 12837.

46 X. Song, P. Yang, X. Mei, L. Li and D. Liao, Eur. J. Inorg. Chem., 2010,1689.

47 R. J. Glauber, J. Math. Phys., 1963, 4, 294.48 (a) C. Coulon, R. Clerac, L. Lecren, W. Wernsdorfer and H. Miyasaka,

Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 132408; (b) T.-F.Liu, D. Fu, S. Gao, Y.-Z. Zhang, H.-L. Sun, G. Su and Y.-J. Liu, J. Am.Chem. Soc., 2003, 125, 13976.

3304 | Dalton Trans., 2011, 40, 3295–3304 This journal is © The Royal Society of Chemistry 2011

Publ

ishe

d on

23

Febr

uary

201

1. D

ownl

oade

d by

Tuf

ts U

nive

rsity

on

27/1

0/20

14 1

7:15

:18.

View Article Online