1006 | Chem. Commun., 2016, 52, 1006--1008 This journal is©The Royal Society of Chemistry 2016

Cite this:Chem. Commun., 2016,

52, 1006

An S = 12 semiquinoid radical-bridged Mn6 wheelcomplex assembled from an asymmetric redox-active bridging ligand†

Ie-Rang Jeon and T. David Harris*

The asymmetric redox-active ligand 4,5-bis(pyridine-2-carboxamido)-

1,2-catechol (N,OLH4) is prepared and metalated to afford the hexanuclear

complex [Mn6(N,OL)6]6�. Structural analysis and magnetic measurements

reveal this complex to feature MnII ions bridged by N,OL3�� radicals, which

are antiferromagnetically coupled to give an S = 12 ground state.

Molecules that possess a high-spin ground state can potentiallyretain their net magnetization upon removal of an applied dcmagnetic field, owing to the presence of a thermal spin relaxationbarrier.1 The molecules are known as single-molecule magnets,and they may find use in applications such as high-density spin-based information storage and processing.2 The realization ofsuch potential applications requires maximizing the spin relaxa-tion barrier, which is correlated to both spin ground state (S) andaxial zero-field splitting (D). In addition to these factors, thestrength of magnetic coupling between spin centres in single-molecule magnets is critical to ensure thermal isolation of thespin ground state from excited states and to shut down delete-rious fast relaxation pathways such as quantum tunneling.3

One synthetic strategy toward the engenderment of strongcoupling is to incorporate a paramagnetic radical ligand betweenhigh-spin metal centres.4 Here, the ligand magnetic orbital willoverlap with that of the metal, resulting in strong directexchange coupling. Indeed, a number of molecular systemscontaining semiquinoid radicals have been shown to involveexclusive population of the spin ground state up to even 300 K,stemming from strong metal-ligand radical magnetic inter-actions.5,6 Despite these advances with metal semiquinoidmolecules, to our knowledge no such complex of nuclearity higherthan two has been reported. Herein, we report the synthesis andcharacterization of a new asymmetric benzoquinoid bridgingligand, and its metalation to give a radical-bridged Mn6 wheel

complex. To our knowledge, this molecule represents the highestnuclearity and the highest spin ground state yet reported for ametal semiquinoid complex.

We targeted N,OLH4 based on previous reports of relatedligands that have been shown to support metal complexes withradical redox-isomers, albeit resulting exclusively in low-spincomplexes.7,8 The synthesis of N,OLH4 was effected through acoupling reaction between 4,5-diaminocatechol and 2-picolinicacid in pyridine in the presence of 1,10-carbonyldiimidazole,following a modified literature procedure (see Scheme 1).7

Reaction of N,OLH4 with Mn(CH3COO)3�2H2O in DMF, followedby treatment with a THF solution of Cp2Co, yielded a dark violetprecipitate. Subsequent diethyl ether vapour diffusion into aDMSO solution of the violet solid gave dark violet block-shapedcrystals of (Cp2Co)6[Mn6(N,OL)6]�8DMSO (1) in 38% yield.‡Crystals of 1 are stable at ambient temperature under a dinitro-gen atmosphere, but rapidly degrade upon exposure to air.

The structure of [Mn6(N,OL)6]6� comprises a wheel-shapedassembly formed by six each alternating Mn ions and depro-tonated N,OLn� bridging ligands (see Fig. 1). Each Mn centreresides in a distorted octahedral coordination environment,with two adjacent sites occupied by two oxygen donor atomsfrom the catechol unit of N,OLn� and the other four sites boundto nitrogen donor atoms from two pyridines and two carbox-amido groups of a second molecule of N,OLn�. The Mn centre isdisplaced by 0.814(2) Å out of the plane formed by four nitrogenatoms from the ligand. The average Mn–N and Mn–O distancesare 2.235(14) and 2.170(9) Å, which can be unambiguouslyassigned to a high-spin MnII ion configuration. The presenceof MnII ions, in conjunction with the overall 6+ complex charge,

Scheme 1 Synthesis of N,OLH4.

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,

IL 60208-3113, USA. E-mail: dharris@northwestern.edu

† Electronic supplementary information (ESI) available: Experimental details,crystallographic data, crystal structure of 1, additional magnetic data, andcrystallographic information files (CIF). CCDC 1430715. For ESI and crystal-lographic data in CIF or other electronic format see DOI: 10.1039/c5cc08482b.

Received 12th October 2015,Accepted 11th November 2015

DOI: 10.1039/c5cc08482b

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implies the presence of six N,OL3�� units. The Mn6 complexfeatures an intramolecular Mn� � �Mn distance of 15.493(4) Åalong the diameter of the wheel. This distance is slightly longerthan that of 14.734(8) Å previously reported for a structurallysimilar Mn6 wheel complex containing nitronyl nitroxide bridgingligands.9

Within 1, the charge of each [Mn6(N,OL)6]6� anionic complexis compensated by six [Cp2Co]+ cations (see Fig. S1 in the ESI†).Three [Cp2Co]+ cations are situated above and three below theMn6 wheel, as viewed normal to the wheel diameter along thecrystallographic c axis, with a closest Mn� � �Co distance of6.243(4) Å. In addition, the individual Mn6 complexes areclosely packed in the solid state, as evidenced by shortestintermolecular Mn� � �Mn and C� � �C distances of 8.846(3) and3.36(3) Å, respectively.

To probe and confirm the presence of a trianionic radicalconfiguration for the bridging ligand, bond distances involvingligand-based atoms were carefully investigated. The mean C–Cdistance of the quinoid ring of 1.41(2) Å falls exactly intermediateof 1.405(4) and 1.425(3) Å previously reported for a related N,NL4�

ligand and its two-electron oxidized benzoquinone isomer, N,NL2�,respectively.8 In addition, the mean C–Namide distance in 1 of1.37(2) Å follows similar trend, being shorter and longer, respec-tively, than the C–N distances of 1.407(3) and 1.350(3) Å for N,NL4�

and N,NL2�. Finally, the C–O bond distance in 1 of 1.29(2) Å isclose to that of 1.293(3) Å reported for molecular complexescontaining trianionic tetraoxolene radicals.10 Taken together,these structural observations confirm that the bridging ligandin 1 is best described as N,OL3��, giving an overall anionic complexformulation of [MnII

6(N,OL3��)]6�. To the best of our knowledge,1 represents the first structurally-characterized complex containinga radical isomer of (pyridine-2-carboxamido)benzene derivatives,and the highest-nuclearity metal semiquinoid complex reportedto date.

The presence of MnII and N,OL3�� in 1 likely results from acombination of ligand-to-metal electron-transfer and reductionby Cp2Co. Here, a 1 : 1 electron-transfer from MnIII to N,OL4�,followed by one-electron reduction of the ensuing product byCp2Co, would give rise to a [MnII

6(N,OL4�)6]12� complex ratherthan the observed [MnII

6(N,OL3��)6]6� species. If such a fully-reduced species is indeed formed transiently, it may then beoxidized by excess MnIII or solvent. Alternatively, the reactionpossibly proceeds via two-electron oxidation of the startingN,OL4� ligand, assisted by excess MnIII, and then reduction byCp2Co. Nevertheless, the exact mechanism associated with theformation of 1 is beyond the scope of this report. Note,however, that carrying out the reaction in the absence of Cp2Cogives rise to a different product whose study is ongoing.

To probe the magnetic interactions in 1, variable-temperature dc magnetic susceptibility data were collected ona solid sample under an applied field of 1000 Oe, and theresulting plot of wMT vs. T is shown in Fig. 2. At 300 K, wMT =25.5 cm3 K mol�1, which is slightly lower than the valueof 28.5 cm3 K mol�1 expected for magnetically isolated sixS = 5/2 MnII centres and six S = 1/2 radical ligands with g = 2,suggesting the presence of antiferromagnetic interactionsbetween the metal ions and the ligand radicals. Upon loweringthe temperature, the wMT data undergo a gradual decrease,reaching a minimum of wMT = 24.0 cm3 K mol�1 at 165 K. Uponfurther lowering the temperature, the wMT data undergo anabrupt increase to a maximum of wMT = 64.9 cm3 K mol�1 andthen a steep downturn below 4 K. The observed maximum ofwMT is slightly lower than the value of 78 cm3 K mol�1 expectedfor an S = 12 ground state, most likely due to the effectsof intermolecular antiferromagnetic interactions and/or thepopulation of low-lying excited states.

A precise quantitative determination of the magnetic inter-action between MnII ions and radicals in 1, considering a

Fig. 1 X-ray crystal structure of [Mn6(N,OL)6]6�, as viewed along thecrystallographic c axis. Purple, red, blue, and gray spheres representMn, O, N, and C atoms, respectively; H atoms are omitted for clarity.

Fig. 2 Variable-temperature dc magnetic susceptibility data for 1, collectedunder an applied field of 1000 Oe. The red line corresponds to a fit ofthe data. Inset: low-temperature magnetization data for 1 at selectedtemperatures.

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1008 | Chem. Commun., 2016, 52, 1006--1008 This journal is©The Royal Society of Chemistry 2016

model based on a 12-member ring, was hindered due to thelarge number of spins. Nevertheless, a fit to the data using achain model for alternating classical spins (S = 5/2) andquantum spins (S = 1/2) provided an estimated couplingconstant of J = �15.6(4) cm�1 with g = 2.0(1).11 Note that, inthe case of alternating S = 5/2 and S = 1/2 spins, the theoreticalestimation of magnetic susceptibility for a ring comprisingthree pairs of spins and that for an infinite chain are nearlyidentical above the temperature at which wMT reaches theminimum, while the increase of wMT at low temperature issteeper for a chain owing to longer-range magnetic correlation.11

This divergence between the two models at low temperaturelikely explains the observed discrepancy between the fit and theexperimental data for 1.

In addition, according to this alternating chain analysis, thetemperature at which wMT reaches a minimum value shouldprovide an estimate of the strength of magnetic exchangebetween spins following the relationship T = 7.194|J|.11,12

As such, this expression gives an estimated coupling constantof | J| = 23 cm�1 for 1. In view of these two components of thechain model analysis, the magnetic coupling between MnII andN,OL3�� in 1 most likely falls within the range of 15–23 cm�1.Finally, the observed coupling strength is smaller than whathas been estimated for the FeIII complexes containing NL�� andN,NL3��,7,8 which likely stems from the high-spin MnII configu-ration and the significant displacement of the MnII centre fromthe N4 ligand plane within the wheel structure in 1.

To confirm the spin ground state of 1, variable-field magne-tization data were collected at 1.8, 3, 5, and 8 K (see Fig. 2,inset). The magnetization curve at 1.8 K follows an abruptincrease at low field and reaches a maximum of M = 24.4 mB

at 4 T. This magnetization value confirms the presence ofstrong antiferromagnetic interaction between MnII ions andN,OL3��, leading to a spin configuration of S = 12 even under anapplied dc field of 7 T. Additionally, such a rapid increase ofmagnetic moment at low field is reflective of the high-spinground state of this compound. Indeed, magnetization datawere compared with the simulated Brillouin function for anS = 12 ground state, revealing moderate agreement betweenexperimental and simulated data (see Fig. S2, ESI†). The observedmismatch at lower field may stem from significant intermolecularantiferromagnetic interactions, as was discussed above. Note that 1is a rare example of a compound that features a high-nuclearityradical-bridged molecule with a large spin ground state, exhibitingsimilarities to a wheel complex containing nitronyl nitroxidederivatives.9 Moreover, to our knowledge, 1 represents the highestspin ground state yet reported for a metal semiquinoid complex.Finally, as depicted in the plot of M vs. H/T, the magnetizationcurves at different temperatures are superimposed, suggesting theabsence of any significant magnetic anisotropy in 1 (see Fig. S2,ESI†). In line with the observation, the variable-frequency acsusceptibility data collected on a solid sample of 1 indicate theabsence of slow magnetic relaxation down to 1.8 K at frequenciesup to 1500 Hz (see Fig. S3, ESI†).

The foregoing results demonstrate the utility of the newasymmetric redox-active ligand, 4,5-bis(pyridine-2-carboxamido)-1,2-catechol, in generating a semiquinoid radical-bridgedhexanuclear Mn6 wheel complex with an S = 12 ground state.To the best of our knowledge, this complex represents the firststructurally-characterized example of a molecule incorporatingradical isomers of (pyridine-2-carboxamido)benzene derivativesand the highest nuclearity and spin of any metal semiquinoidcomplex. Future efforts will focus on investigating the redoxchemistry of 1 and synthesizing related wheel complexes withanisotropic metal ions.

This research was supported by the National Science FoundationGrant DMR-1351959, the Institute for Sustainability and Energy atNorthwestern, and Northwestern University. Purchase of theSQUID magnetometer was supported in part by the InternationalInstitute of Nanotechnology.

Notes and references‡ X-ray analysis for 1 (C184H168Co6Mn6N24O32S8, FW = 4161.07) atT = 100 K: space group R%3, a = b = 23.1209(12) Å, c = 27.281(2) Å,V = 12629.7(16) Å3, Z = 3. d = 1.641 g cm�1, R1 = 0.0905, wR2 = 0.2923.

1 D. Gatteschi, R. Sessoli and J. Villain, Molecular Nanomagnets, 2006;R. E. P. Winpenny, Single Molecule Magnets and Related Phenomena,Springer, Berlin, 2006, vol. 122.

2 M. N. Leuenberger and D. Loss, Nature, 2001, 410, 789; A. R. Rocha,V. M. Garcıa-suaez, S. W. Bailey, C. J. Lambert, J. Ferrer and S. Sanvito,Nat. Mater., 2005, 4, 335; L. Bogani and W. Wernsdorfer, Nat. Mater.,2008, 7, 179; P. C. E. Stamp and A. Gaita-Arino, J. Mater. Chem., 2009,19, 1718; A. Ardavan and S. J. Blundell, J. Mater. Chem., 2009, 19, 1754.

3 O. Kahn, Molecular Magnetism, VCH, New York, 1993; S. Demir,I.-R. Jeon, J. R. Long and T. D. Harris, Coord. Chem. Rev., 2015,289, 149.

4 A. Caneschi, D. Gatteschi, R. Sessoli and P. Rey, Acc. Chem. Res.,1989, 22, 392; K. E. Vostrikova, Coord. Chem. Rev., 2008, 242, 1409;R. G. Hicks, Stable Radicals, John Wiley and Sons, Ltd., UnitedKingdom, 2010; M. T. Lemaire, Pure Appl. Chem., 2011, 83, 141;H. Iwamura, Polyhedron, 2013, 66, 3.

5 R. M. Buchanan, S. L. Kessel, H. H. Downs, C. G. Pierpont andD. N. Hendrickson, J. Am. Chem. Soc., 1978, 100, 7894; C. Benelli,A. Dei, D. Gatteschi, H. U. Gudel and D. I. Pardi, Inorg. Chem., 1989,28, 3089; D. E. Wheeler and J. K. McCusker, Inorg. Chem., 1998,37, 2296; H.-C. Chang, T. Ishii, M. Kondo and S. Kitagawa, J. Chem.Soc., Dalton Trans., 1999, 2467; K. Chłopek, E. Nill, T. Weyhermullerand K. Wieghardt, Inorg. Chem., 2005, 44, 7087; K. Chłopek, E. Bothe,F. Neese, T. Weyhermuller and K. Wieghardt, Inorg. Chem., 2006,45, 6298; D. Guo and J. K. McCusker, Inorg. Chem., 2007, 46, 3257.

6 I.-R. Jeon, J. G. Park, D. J. Xiao and T. D. Harris, J. Am. Chem. Soc.,2013, 135, 16845; J. A. DeGayner, I.-R. Jeon and T. D. Harris, Chem.Sci., 2015, 6, 6639.

7 A. K. Patra, M. Ray and R. Mukherjee, Inorg. Chem., 2000, 39, 652;S. K. Dutta, U. Beckmann, E. Bill, T. Weyhermuller and K. Wieghardt,Inorg. Chem., 2000, 39, 3355.

8 U. Beckmann, E. Bill, T. Weyhermuller and K. Wieghardt, Inorg.Chem., 2003, 42, 3355.

9 A. Caneschi, D. Gatteschi, J. Laugier, P. Rey, R. Sessoli and C. Zanchini,J. Am. Chem. Soc., 1998, 110, 2795.

10 K. S. Min, A. L. Rheingold, A. G. DiPasquale and J. S. Miller, Inorg.Chem., 2006, 45, 6135; K. S. Min, A. G. DiPasquale, J. A. Golen,A. L. Rheingold and J. S. Miller, J. Am. Chem. Soc., 2007, 129, 2360.

11 J. Seiden, J. Phys., 1983, 44, L947. The Hamiltonian used for the fit isgiven as H = �2J

PSLi�(SMni

+ SMni+1).

12 A. Gleizes and M. Verdaguer, J. Am. Chem. Soc., 1984, 106, 3727.The relation is modified according to the Hamiltonian and the unitof J used in this paper.

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