DaltonTransactions

COMMUNICATION

Cite this: Dalton Trans., 2014, 43,10690

Received 7th February 2014,Accepted 22nd May 2014

DOI: 10.1039/c4dt01469c

www.rsc.org/dalton

A truncated [MnIII12] tetrahedron from

oxime-based [MnIII3O] building blocks†

J. M. Frost,a S. Sanz,a T. Rajeshkumar,c M. B. Pitak,e S. J. Coles,e G. Rajaraman,*c

W. Wernsdorfer,d J. Schnack,*b P. J. Lusbya and E. K. Brechin*a

The use of the novel pro-ligand H4L combining the complimentary

phenolic oxime and diethanolamine moieties in one organic

framework, results in the formation of the first example of a

[MnIII12] truncated tetrahedron and an extremely rare example of a

Mn cage conforming to an Archimedean solid.

Synthetic coordination chemists have had enormous successin constructing large and aesthetically pleasing molecules con-taining paramagnetic metal ions, despite the many experi-mental difficulties in doing so.1 In small nuclearity clustersthe coordination sites of the metal ions can often be filled bydesign through the use of one polydentate ligand or throughsimple combinations of different ligand types, e.g. one brid-ging and one chelating ligand.2–6 However, as the nuclearity ofthe cluster increases the structural complexity increases dra-matically, the central cores often consist of numerous bridgingoxide and/or hydroxide ions whose presence, number andcoordination behaviour is difficult to predict, and the periph-eral skin of the cage is adorned by numerous (often different)organic bridging ligands coordinating in a variety of ways, withsolvent molecules filling what vacant metal sites remain.7–14

As such almost all large cages of paramagnetic metal ions areheteroleptic in nature and require the use of complementaryligands in [serendipitous] self-assembly. One way of achievingfuture success in this regard is to mine structural databases to

discover and then exploit the known coordination modes ofcertain ligand types. A particular ligand will often form thesame metal–organic moiety under a variety of reaction con-ditions, and it is how these building blocks assemble that dic-tates the nature of the final product. Different ligands will ofcourse form different building blocks and thus the combi-nation of different ligands that produce complementary build-ing blocks should produce beautiful new cages. Instead ofemploying these as two different reaction ingredients, we havebeen exploring a synthetic approach that combines theseligands into one structural framework, and two appealing can-didates in this regard are the phenolic oximes15 and thediethanolamines,16 both of which have long and distinguishedtrack records in cluster formation. Herein we report the syn-thesis, structure and magnetic behaviour of the truncatedtetrahedron [MnIII

12O4(H3L)8(H2L)4(TMA)4] (1) constructedusing the novel pro-ligand H4L (Fig. 1).

Reaction of MnBr2·4H2O, H4L and trimesic acid (H3TMA) ina basic MeOH solution results in the formation of black rod-like crystals after slow evaporation of the filtered motherliquor, after 5 days (see the ESI† for full details). The crystals

Fig. 1 (A) The pro-ligand H4L. (B) The molecular structure of 1. Colourcode: Mn = purple, O = red, N = blue, C = grey. H atoms have beenomitted for clarity.

†Electronic supplementary information (ESI) available: Synthetic procedures,crystallographic details and additional magnetic and theoretical data. CCDC949848. For ESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/c4dt01469c

aEaStCHEM School of Chemistry, The University of Edinburgh, West Mains Road,

Edinburgh, EH93JJ, UK. E-mail: ebrechin@staffmail.ed.ac.uk, Paul.Lusby@ed.ac.ukbUniversität Bielefeld, Fakultät für Physik, Postfach 100131, 33501 Bielefeld,

Germany. E-mail: jschnack@uni-bielefeld.decDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,

Mumbai – 400 076, IndiadInstitut Néel, 25 Avenue des Martyrs, BP 166, 38042 Grenoble, Cedex 9, France.

E-mail: wolfgang.wernsdorfer@grenoble.cnrs.freUK National Crystallographic Service, Faculty of Natural and Environmental

Sciences, University of Southampton, SO171BJ Southampton, UK.

E-mail: M.Pitak@soton.ac.uk, s.j.coles@soton.ac.uk

10690 | Dalton Trans., 2014, 43, 10690–10694 This journal is © The Royal Society of Chemistry 2014

Publ

ishe

d on

22

May

201

4. D

ownl

oade

d by

IN

DIA

N I

NST

ITU

TE

OF

TE

CH

NO

LO

GY

BO

MB

AY

on

21/1

0/20

15 1

0:46

:29.

View Article OnlineView Journal | View Issue

were in a tetragonal crystal system and structure solution wasobtained in the I41/a space group.

The metallic skeleton of 1 describes a truncated tetrahe-dron (or simply a tetrahedron if one considers the [Mn3] tri-angles as nodes) in which [MnIII

3O]7+ triangles occupy the

vertices and the four µ6-TMA3− ligands panel the faces of thecage (Fig. 1 and 2). The µ3-O

2− ions lie at the centres ofthe [MnIII

3O]7+ triangles, slightly below the [Mn3] plane toward

the centre of the cage (0.216–0.233 Å). The H3L− and H2L

2−

ligands employ their –N–O– oxime moieties to each bridgeacross one edge of a [Mn3] triangle, with each Mn–N–O–Mnmoiety lying above the [Mn3] plane (Fig. 2E) and pointing awayfrom the centre of the cage. This in contrast to that observedin all phenolic-oxime based [Mn3] and [Mn6] molecules inwhich these moieties lie in the [Mn3] plane.15 Interestingly,however, the same [MnIII

3O(NO)3(O2CR)3] units have beenobserved in triangles stabilised with pyridyl oximes.17 The phe-nolic O-atoms are terminally bonded. There are two sets ofsymmetry equivalent triangles in 1 (Mn1–Mn3, Mn4–Mn6; seethe ESI† for crystallographic information) which, althoughappearing identical in nature are strictly scalene by definition.Although the Mn⋯Mn distances all fall within a very narrowrange (3.214–3.247 Å for Mn1–3 and 3.223–3.233 Å for Mn4–6)there are three rather different and distinct Mn–N–O–Mntorsion angles (11.59, 18.91 and 23.90° for Mn1–3; 8.04, 16.57and 23.43° for Mn4–6). The diethanolamine arms of theligand remain non-bonded, and are involved in a myriad ofH-bonding interactions O(diethanolamine)⋯O(diethanol-amine), ∼2.9 Å; O(diethanolamine)⋯O(phenol), ∼3.0 Å;O(diethanolamine)⋯O(oxime), ∼2.7 Å; O(diethanolamine)-

⋯O(TMA), ∼3.0 Å; O(diethanolamine)⋯O(MeOH), 2.894 Å.Closer inspection reveals eight O(diethanolamine)⋯O(diethanol-amine) H-bonding interactions per cage, with the N-atomsof the same moiety all H-bonded to the phenolic O-atoms ofthe same ligand (N⋯O, ∼2.65 Å). Both indicate the presence ofshared protons, suggesting assignment of eight H3L

− and fourH2L

2− ligands. The edges of the tetrahedron (the inter-triangleMn⋯Mn distances) are approximately 7.6–7.7 Å in length, withthe internal volume of the cage measuring ∼57 Å3 and occu-pied by highly disordered solvent (MeOH, H2O) molecules.The shortest inter-cluster contacts are of the order ∼3.2 Åbetween C-atoms of the diethanolamine arms and C-atoms ofthe Ph rings (Fig. S1†).

The formation of 1 was serendipitous in nature – the alco-holic arms of the diethanolamine moieties being uninvolvedin bonding to the metal centres – and our initial aim was tolink the [Mn3oxime] units together via these alkoxide bridges.But it is clear that they must play an important structure-directing role, since the same reaction that produces complex1, but with H4L replaced with a simple phenolic oxime(R-saoH2; i.e. H4L minus the diethanolamine moiety) results inthe formation of a 2D framework of [Mn6oxime] Single-Mole-cule Magnets (SMMs).18 At this early juncture we can onlyspeculate that the extensive H-bonding interactions are thecause. This itself is an interesting observation however andsuggests that when synthetic chemists are designing bridgingligands they should think not only of the functional groups tobe employed in linking the metal centres together, but also inthe decoration of the organic framework that will eventuallyconstitute the peripheral skin of the final complex, directing,or at least influencing, intra- and inter-molecular interactionsand the packing and organisation of the molecules in thecrystal.

Molecules adopting Archimedean topologies have long heldfascination for chemists and physicists because of the unusualmagnetic properties they often display, a beautiful examplebeing the [Fe30] icosidodecahedron which has been the subjectof numerous papers.19 A search of the CSD reveals there to beno previously reported examples of any [Mn12] truncated tetra-hedra in the literature with the only other examples of Mncages adopting Archimedean geometries being a [Mn32] trun-cated cube built with tripodal alcohols20 and a [Mn12] cubocta-hedron constructed from phosphonates.21 The magneticsusceptibility (χ) of 1 was measured from 300 K to 5 K in anapplied field of 0.1 T and the results are plotted as the χTproduct versus T in Fig. 3. The χT value of ∼34.5 cm3 K mol−1

at 300 K is slightly below that expected for twelve non-interact-ing MnIII ions (36 cm3 K mol−1 for g = 2). This value increasessteadily as the temperature is decreased, reaching a maximumof 58.7 cm3 K mol−1 at 7.5 K, before dropping slightly to avalue of 58.3 cm3 K mol−1 at 5 K. This behaviour is suggestiveof the presence of competing ferro- and antiferromagneticexchange interactions. The low-temperature magnetisation(Fig. 3) rises to a value of ∼39.7µB at 7 T, lower than the satur-ation value (48µB for g = 2.0) expected for twelve MnIII ions.Modelling the experimental magnetic data of 1 via a standard

Fig. 2 (C) The molecular structure of 1 with the diethanolamine moi-eties, peripheral C and H atoms removed for clarity. (D) The truncatedtetrahedron highlighting the [Mn3O] triangles linked by the tricarboxy-lates. (E) The [Mn3O] triangular building block showing the bridgingmodes of the ligands. (F) The metallic skeleton highlighting both thetruncated tetrahedron and the position of the Jahn–Teller axes high-lighted in black. Colour code: Mn = purple, O = red, N = blue, C = grey.

Dalton Transactions Communication

This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 10690–10694 | 10691

Publ

ishe

d on

22

May

201

4. D

ownl

oade

d by

IN

DIA

N I

NST

ITU

TE

OF

TE

CH

NO

LO

GY

BO

MB

AY

on

21/1

0/20

15 1

0:46

:29.

View Article Online

spin Hamiltonian approach is computationally impossible,and thus we have turned to theory. DFT calculations (see theESI† for details; 5940 basis functions) were performed to esti-mate the intra-( J1–J3) and inter-( J4) triangle interactions, yield-ing J1 = +6.09 cm−1, J2 = +2.92 cm−1, J3 = −0.77 cm−1 and J4 =+0.02 cm−1. The latter is clearly very weak and near-negligible,as might be expected for the trimesate anion.22

Within the triangle the magnetic coupling is mediated via aμ3-O2− ion, an –N–O– oxime bridge and a carboxylate. The per-tinent structural factors dominating the sign and magnitudeof the exchange interactions, extracted from DFT, are listed inTable S2.† J1 is moderately ferromagnetic because both theMn–O–Mn angle and Mn–N–O–Mn torsion angle are large.J2 and J3 are much weaker in magnitude, the former beingferromagnetic and the latter antiferromagnetic. The switch insign correlates to the dz2–dz2 overlap which contributes to theAF part of the exchange, and is found to be significant for J3but negligible for J2 (Tables S3–4†). In the main, this is due tothe orientation of the dz2 orbitals and the variation in thestructural parameters associated with these orbitals (Table S2,Fig. S5†): here the dz2 orbitals are aligned along the carboxylatebridge (which are often responsible for counter-complementar-ity effects23) and the variation in the Mn–Ocarb distances playsa critical role in influencing the overlap integral and thus theobserved J values. The computed spin density plot for the high

spin state of complex 1 is shown in Fig. 4. Spin density analysisreveals a mixture of spin delocalisation and polarisation inoperation between neighbouring MnIII ions leading to weakF/AF coupling within the [Mn3] triangle. Between the triangleshowever the exchange is propagated predominantly via spinpolarisation (Fig. S3†), and the odd number of spacer atomspresent (the carboxylates are at positions 1, 3 and 5 on thephenyl ring) results in ferromagnetic exchange coupling, albeitvery weak.24

To examine whether the DFT-estimated exchange couplingconstants satisfactorily simulate the experimental data, wehave employed the Finite-Temperature Lanczos Method.25 Thisapproximate method can deal with very large Heisenbergsystems, therefore we could not include the single ion an-isotropy terms for the Jahn–Teller active MnIII ions at thispoint, see the solid curve in Fig. 3. Anisotropy is taken intoaccount later for fragment calculations in a simplified way byconsidering the dominant part of the local anisotropy tensors(d-term) with site-dependent directions, ei, but with the samestrength for all ions. The structure of this Hamiltonian is givenin eqn (1).26

H ¼ �2XN

i,j

Jij si �sj þXN

i

diðei �siÞ2 þ gμBBXN

i

si z ð1Þ

The spectroscopic splitting factor assumes a value of g =1.98. Due to the huge dimension of the Hilbert space(244,240,625) an exact eigenvalue determination including an-isotropy and thus a subsequent determination of the para-meters by fitting to the observables is impossible.27 Since J4can be assumed to be negligible in a first approximation wehave performed full diagonalisation studies, i.e. all eigenvaluesand vectors, assuming four uncoupled identical triangles withsingle-ion anisotropy. The orientations ei were chosen to bemutually perpendicular in each triangle as derived from theorientation of the Jahn–Teller axes from the single crystal X-raystructure as shown in Fig. 2F, and powder averaging wasapplied.28 The curves, depicting various scenarios for J1–J3,and di = D, in Fig. 3 and S7† do not represent fits, since this is

Fig. 3 (A) The magnetic susceptibility of 1 at B = 0.1 T (experimentaldata represented by crosses) and two simulations using g = 1.98. Thesolid curves are evaluated using the Heisenberg part of the Hamiltonianwith parameters from DFT; the dashed curves are produced with aHamiltonian that includes single-ion anisotropy, but only for a fragment.(B) The magnetisation at T = 2 K and T = 4 K; curves are for the modelsmentioned in (A).

Fig. 4 DFT computed spin density plot for the HS state of complex 1.Red = positive spin density, blue = negative spin density.

Communication Dalton Transactions

10692 | Dalton Trans., 2014, 43, 10690–10694 This journal is © The Royal Society of Chemistry 2014

Publ

ishe

d on

22

May

201

4. D

ownl

oade

d by

IN

DIA

N I

NST

ITU

TE

OF

TE

CH

NO

LO

GY

BO

MB

AY

on

21/1

0/20

15 1

0:46

:29.

View Article Online

impossible, but are intended to qualitatively estimate the orderof magnitude of exchange and anisotropy. It is clear that theparameters found by DFT lead to a reasonable χT dependence,but overestimate the low-temperature magnetisation due to thelack of anisotropic terms in the model employed. Our con-clusion therefore is that two (stronger) ferromagnetic and one(weaker) antiferromagnetic exchange interactions are presentin each triangle, and that the anisotropy assumes values thatare typical for MnIII ions.

Ac susceptibility studies carried out on crystalline samplesof 1 in the 1.8–10.0 K range in a 3.5 G field oscillating at fre-quencies up to 1000 Hz (Fig. S6†) display the tails of fre-quency-dependent out-of-phase (χ″) signals suggestive of SMMbehaviour but no peaks. Hysteresis loop measurements weretherefore carried out on single crystals using a micro-SQUIDassembly with the field applied along the easy axis of magneti-sation.29 These show temperature and sweep rate dependenthysteresis loops confirming SMM behaviour (Fig. 5). The loopsare indicative of an SMM with a modest anisotropy barrier, nosignificant intercluster interactions and one in which manyexcited states are mixed with the ground state. Low tempera-ture dc relaxation data recorded between 0.03–1.00 K revealnon-exponential behaviour consistent with [multiple] excitedstate involvement in the relaxation process, renderingany meaningful assignment of an anisotropy barrier heightimpossible.

Conclusions

In conclusion, the use of the novel pro-ligand H4L combiningthe complimentary phenolic oxime and diethanolamine moi-eties in one organic framework, has resulted in the formationof the first example of a [MnIII

12] truncated tetrahedron and anextremely rare example of a Mn cage conforming to an Archi-medean solid. The formation of such an aesthetically beautifulcage, despite the diethanolamine moieties being non-bonded,suggests synthetic variation should lead to a plethora of beau-tiful and novel polymetallic clusters possessing fascinating

physical properties. Replacement of the trimesate ion withredox-active congeners, if possible, will enable both strongerinter-triangle magnetic interactions and open the door to thepossibility of switching the interaction on and off via chemicaland electrochemical stimuli.

Notes and references

1 G. Aromí and E. K. Brechin, Struct. Bonding, 2006, 122,1–69.

2 G. Aromí, D. Aguilà, P. Gamez, F. Luis and O. Roubeau,Chem. Soc. Rev., 2012, 41, 537–546.

3 M. Andruh, Chem. Commun., 2011, 47, 3025–3042.4 V. Marvaud, C. Decroix, A. Scuiller, C. Guyard-Duhayon,

J. Vaissermann, F. Gonnet and M. Verdaguer, Chem. – Eur.J., 2003, 9, 1677–1691.

5 F. Habib and M. Murugesu, Chem. Soc. Rev., 2013, 42,3278–3288.

6 L. N. Dawe, K. V. Shuvaev and L. K. Thompson, Chem. Soc.Rev., 2009, 38, 2334–2359.

7 T. Glaser, Chem. Commun., 2011, 47, 116–130.8 C. J. Milios, T. C. Stamatatos and S. P. Perlepes, Polyhedron,

2006, 25, 134–194.9 (a) G. A. Timco, E. J. L. McInnes and R. E. P. Winpenny,

Chem. Soc. Rev., 2013, 42, 1796–1806; (b) G. A. Timco,T. B. Faust, F. Tuna and R. E. P. Winpenny, Chem. Soc. Rev.,2011, 40, 3067–3075.

10 R. Bagai and G. Christou, Chem. Soc. Rev., 2009, 38, 1011–1026.

11 R. Sessoli and A. K. Powell, Coord. Chem. Rev., 2009, 253,2328–2341.

12 M. Murrie, Chem. Soc. Rev., 2010, 39, 1986–1995.13 X.-Y. Wang, C. Avendaño and K. R. Dunbar, Chem. Soc.

Rev., 2011, 40, 3213–3238.14 K. S. Murray, Aust. J. Chem., 2009, 62, 1081–1101.15 (a) R. Inglis, C. J. Milios, L. F. Jones, S. Piligkos and

E. K. Brechin, Dalton Trans., 2012, 48, 181–190;(b) R. Inglis, L. F. Jones, C. J. Milios, S. Datta, A. Collins,S. Parsons, W. Wernsdorfer, S. Hill, S. P. Perlepes,S. Piligkos and E. K. Brechin, Dalton Trans., 2009, 3403–3412; (c) C. J. Milios, S. Piligkos and E. K. Brechin, DaltonTrans., 2008, 1809–1817.

16 (a) A. J. Tasiopoulos and S. P. Perlepes, Dalton Trans., 2008,5537–5555; (b) R. W. Saalfrank, A. Scheurer, R. Prakash,F. W. Heinemann, T. Nakajima, F. Hampel, R. Leppin,B. Pilawa, H. Rupp and P. Müller, Inorg. Chem., 2007, 46,1586–1592.

17 T. C. Stamatatos, D. Foguet-Albiol, S.-C. Lee, C. C.Stoumpos, C. P. Raptopoulou, A. Terzis, W. Wernsdorfer,S. O. Hill, S. P. Perlepes and G. Christou, J. Am. Chem. Soc.,2007, 129, 9484–9499.

18 A. D. Katsenis, R. Inglis, A. Prescimone, E. K. Brechin andG. S. Papaefstathiou, CrystEngComm, 2012, 14, 1216–1218.

19 U. Kortz, A. Müller, J. van Slageren, J. Schnack, N. S. Dalaland M. Dressel, Coord. Chem. Rev., 2009, 253, 2315–2327.

Fig. 5 Magnetisation (M) versus field hysteresis loops for a single crystalof 1 at the indicated temperatures (top) and field sweep rates (bottom).M is normalised to its saturation value.

Dalton Transactions Communication

This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 10690–10694 | 10693

Publ

ishe

d on

22

May

201

4. D

ownl

oade

d by

IN

DIA

N I

NST

ITU

TE

OF

TE

CH

NO

LO

GY

BO

MB

AY

on

21/1

0/20

15 1

0:46

:29.

View Article Online

20 R. T. W. Scott, S. Parsons, M. Murugesu, W. Wernsdorfer,G. Christou and E. K. Brechin, Angew. Chem., Int. Ed., 2005,44, 6540–6543.

21 L. Zhang, R. Clérac, P. Heijboer and W. Schmitt, Angew.Chem., Int. Ed., 2012, 51, 3007–3011.

22 P. Christian, G. Rajaraman, A. Harrison, M. Helliwell,J. J. W. McDouall, J. Raftery and R. E. P. Winpenny, DaltonTrans., 2004, 2550–2555.

23 M. Atanasov, B. Delley, F. Neese, P. L. Tregenna-Piggott andM. Sigrist, Inorg. Chem., 2011, 50, 2112–2124.

24 J. Cano, E. Ruiz, S. Alvarez and M. Verdaguer, CommentsInorg. Chem., 1998, 20, 27–56.

25 (a) J. Jaklič and P. Prelovsěk, Phys. Rev. B: Condens. Matter,1994, 49, 5065; (b) J. Schnack and O. Wendland, Eur.

Phys. J. B, 2010, 78, 535–541; (c) J. Schnack and C. Heesing,Eur. Phys. J. B, 2013, 86, 46.

26 (a) T. Glaser, M. Heidemeier, E. Krickemeyer, H. Bögge,A. Stammler, R. Fröhlich, E. Bill and J. Schnack, Inorg.Chem., 2009, 48, 607–620; (b) J. Schnack, Condens. MatterPhys., 2009, 12, 323–330.

27 V. Hoeke, M. Heidemeier, E. Krickemeyer, A. Stammler,H. Bögge, J. Schnack, A. Postnikov and T. Glaser, Inorg.Chem., 2012, 51, 10929–10954.

28 V. I. Lebedev and D. N. Laikov, Dokl. Akad. Nauk, 1999, 366,741–745.

29 (a) W. Wernsdorfer, Adv. Chem. Phys., 2001, 118, 99–190;(b) W. Wernsdorfer, Supercond. Sci. Technol., 2009, 22,064013.

Communication Dalton Transactions

10694 | Dalton Trans., 2014, 43, 10690–10694 This journal is © The Royal Society of Chemistry 2014

Publ

ishe

d on

22

May

201

4. D

ownl

oade

d by

IN

DIA

N I

NST

ITU

TE

OF

TE

CH

NO

LO

GY

BO

MB

AY

on

21/1

0/20

15 1

0:46

:29.

View Article Online