Trigonal bipyramidal magnetic molecules based on [MoIII(CN)6]

3�w

Xin-Yi Wang, Matthew G. Hilfiger, Andrey Prosvirin and Kim R. Dunbar*

Received 15th March 2010, Accepted 15th April 2010

First published as an Advance Article on the web 13th May 2010

DOI: 10.1039/c0cc00399a

The trigonal bipyramidal molecules [M(tmphen)2]3[Mo(CN)6]2�(solvent) (M = Co, Ni; tmphen = 3,4,7,8-tetramethyl-1,10-

phenanthroline) based on the [MoIII(CN)6]3� unit were prepared

by loss of a cyanide ligand from [MoIII(CN)7]4� and found to

exhibit ferromagnetic interactions between the MII and MoIII

centers.

Research in molecular magnetism over the past decade has

witnessed dramatic growth owing to their diverse properties

and promise for practical applications for molecular materials.1

Molecular magnets that exhibit high ordering temperatures,

bistability (spin-crossover, single molecule magnetism and

single chain magnetism), multifunctionality (magnetism with

conductivity, magnetism with porosity) are among the major

achievements in this highly topical area.2

The syntheses of many of the aforementioned materials

involve the successful implementation of what is often referred

to as the building block approach. Among the materials

prepared by this method, those that involve cyanometallates

[M(CN)y]m� and substituted derivatives of general formula

[M(L)x(CN)y]m� (L = capping ligand) provide a myriad

of possibilities for engendering desired geometries, charges

and single ion as well as molecular anisotropy.3 Recently

cyanometallates of 4d and 5d metal centers have become the

focus of considerable attention owing to their diffuse orbitals,

high magnetic anisotropy due to strong spin–orbit coupling,

and their ability to exist in multiple oxidation states with

different coordination numbers.3e,f The cyanomolybdate

anions of general formula [Mo(CN)m]n� (m = 6, 7, and 8;

n = 3, 4) are a particularly fascinating class of homoleptic

cyanides. The molecules M9Mo6 (M = Mn, Co, Ni)

clusters4 with large ground state spin values several of

which exhibit SMM behaviour and the 4d–4f magnet

[Tb(pzam)3(H2O)Mo(CN)8]�H2O5 are two excellent examples

of molecular magnetic materials based on the dodecahedral

[MoV(CN)8]3� unit. Of additional fascination is the fact that

[MoIV(CN)8]4� has been demonstrated to participate in photo-

magnetic behaviour in several compounds.6

The pentagonal bipyramidal [MoIII(CN)7]4� building block

is also highly attractive because of its high anisotropy,

originating from the anisotropic MoIII center residing in a

D5h environment, and due to the possibility for engaging in

anisotropic exchange interactions as predicted by theory.7

Numerous two- and three-dimensional manganese containing

[Mo(CN)7]4� magnets have been reported but, suprisingly, no

molecular compounds based on this anion have appeared in

the literature.8

Only a few studies over the years have focused on the

chemistry of the octahedral [MoIII(CN)6]3� species, and,

indeed, it remained an elusive species until Beauvais and Long

reported the synthesis and structural characterization of this

anion in 2002.9 Several years later a theoretical study by Ruiz

et al.10 in 2005 led to the prediction that superexchange

between [Mo(CN)6]3� and the early 3d metal centers (VII, CrII)

through the cyanide ligand should be very strong and that

Prussian Blue phases based on the same combinations will

have critical temperatures higher than the well known CrIIIVII

derivatives11 (552 K for MoIIIVII and 308 K for MoIIICrII). In

spite of these intriguing predictions, progress in the chemistry

of [Mo(CN)6]3� has been hampered due to its instability and it

was only in 2009 that a cluster based on this cyanometallate

was prepared. The molecule contains a V4Mo core and

exhibits unusually strong antiferromagnetic coupling between

VII and MoIII ions (J = �61 cm�1)12 albeit not as strong as

what was predicted by the aforementioned theoretical calculations

for model dimers reported by Ruiz and Alvarez.10

In the course of our research in the area of cyanide

magnetism, our group has isolated numerous architectures

for heterobimetallic molecules including homologous series of

squares, cubes, and, the largest family, those with a trigonal

bipyramidal (TBP) geometry. The methods used to prepare

these compounds are based on specific combinations of ligand

protected metal ion building blocks that are designed to lead

to a predictable outcome.13,14 In recently published work

we have reported that stable TBP molecules with the 5d

cyanometallate [Os(CN)6]3� are also possible to isolate; in

the case of the Fe3IIOs2

III analog, unprecedented coupled

CTIST/SCO events were observed to occur.14

Herein we report two new 4d members of the TBP

family based on [Mo(CN)6]3�. The products were isolated

serendipitously from reactions involving the [Mo(CN)7]4�

precursor. The compounds [M(tmphen)2]3[Mo(CN)6]2�(solvent)(MII = Co (1), Ni (2)) which were fully characterized by X-ray

and magnetic studies constitute rare examples of molecular

clusters containing [Mo(CN)6]3� with only the aforementioned

example having been published to date.

Reactions of 18-Crown-6, K4Mo(CN)7�2H2O,15 M(NO3)2�6H2O and tmphen in a mixture of CH3OH and CH3CN lead

to single crystals of the pentanuclear complexes 1 and 2.zX-ray diffraction studiesy revealed that both of the compounds

adopt a TBP geometry in which the M2Mo3 unit is composed

of two [Mo(CN)6]3� axial groups and three [M(tmphen)2]

2+

fragments in the equatorial positions (Fig. 1). The local

Department of Chemistry, Texas A&M University, College Station,Texas 77840, USA. E-mail: dunbar@mail.chem.tamu.eduw Electronic supplementary information (ESI) available: CIF files ofcompounds 1 and 2; additional figures and tables. CCDC 723434 and724501. For ESI and crystallographic data in CIF or other electronicformat see DOI: 10.1039/c0cc00399a

4484 | Chem. Commun., 2010, 46, 4484–4486 This journal is �c The Royal Society of Chemistry 2010

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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environments of the MoIII centers are that of a slightly

distorted octahedron. The Mo–C bonds are in the range of

2.12–2.17 A for 1 and 2.12–2.19 A for 2, which are consistent

with the value of 2.181 A observed for mononuclear compound

Li3[Mo(CN)6]�6DMF.9 The bond angles are B1751 for trans-

C–Mo–C and 841 to 981 for cis-C–Mo–C. Within a single

cluster there are two p–p stacking interactions (3.5–3.6 A)

between tmphen ligands (Fig. 1) with additional weak inter-

molecular interactions involving p–p stacking (Fig. S1w) andinterstitial solvent molecules.

Given the fact that these compounds were synthesized from

the [Mo(CN)7]4� starting material, it is obvious that the anion

is labile under the reaction conditions, an event that is likely

triggered by formation of the highly stable TBP geometry.

This conclusion is based on the fact that, before reaction with

[M(tmphen)2]2+ precursors, the [Mo(CN)7]

4� anion is obviously

intact because diffusion of diethyl ether into a methanol

solution of 18-Crown-6 and K4Mo(CN)7�2H2O led to green

crystals of (18-Crown-6-K)K3[Mo(CN)7](MeOH) as determined

by single crystal X-ray analysis (Fig. S2w).The magnetic properties of 1 and 2 were investigated by

magnetic susceptibility measurements (1.8–300 K) and field

dependent magnetization measurements (0–70 kOe). Since the

samples readily lose solvent of crystallization it was imperative

to use the same set of samples for magnetic measurements

and elemental analyses. The crystals, which are needle-like in

morphology, were crushed into fine powders to avoid

preferential alignment of the crystals in the magnetic field.

The field dependence of the magnetization of compound 1

measured at 1.8 K and the temperature dependence of its

magnetic susceptibility measured at 1 kOe, displayed as wT(T)and w�1(T) respectively, are depicted in Fig. 2. The room

temperature value of 7.21 cm3 mol�1 K for wT (spin-only value

of 9.375 cm3 mol�1 K for Co3Mo2 with SCo = SMo = 3/2)

increases to 10.2 cm3 mol�1 K at 26 K and then decreases to

9.38 cm3 mol�1 K upon further cooling down to 2 K. The w�1

data above 50 K obey the Curie–Weiss law with a Curie

constant C = 6.56 cm3 mol�1 K and a Weiss temperature

y = +28.2 K. The reason for the value of C being lower than

the spin-only value is not clear, but the results are consistent

among independent batches of crushed crystals that were

measured. The positive Weiss constant and the shape of the

wT curve are indications of ferromagnetic coupling between

the CoII and MoIII centers. The M–H curve at 1.8 K increases

to a value of of 8.5 mB without saturation at 70 kOe which

suggests a ground state of S = 9/2 for three CoII centers

ferromagnetically coupled to the two MoIII centers (assuming

S’Co = 1/2, SMo = 3/2, and gCo E gMo E 2.0). Although the

fact that a ferromagnetic interaction between the CoII and

MoIII ions is occurring can be gleaned from the magnetic data,

a fitting of the magnetic data was not possible because of the

complexity of the system owing to factors such as magnetic

anisotropy.

Compound 2 also exhibits a ferromagnetic interaction

between the NiII and MoIII centers (Fig. 3). The M versus H

data measured at 1.8 K saturates at Ms B 12.5 mB at 70 kOe

consistent with a ground state of S = 6 due to ferromagnetic

coupling between NiII and two MoIII ions (SNi = 1, SMo = 3/2,

and gNi E gMo E 2.0). The wT value increases from

8.46 cm3 mol�1 K at 300 K to 20.6 cm3 mol�1 K at 10 K,

and then decreases to 19.3 at 2 K. Fitting of the data above

100 K gave a Curie constant C = 7.33 cm3 mol�1 K and a

positive Weiss constant y = +43.0 K. The Curie constant is

consistent with the calculated value of 7.38 cm3 mol�1 K

(gNi = 2.2, gMo = 2.0, SNi = 1, SMo = 3/2). The decrease

in wT at low temperature is attributed to zero-field splitting

and/or weak antiferromagnetic interactions between the clusters.

Fig. 1 Molecular structure of the trigonal bipyramidal cluster of 1

and 2. Hydrogen atoms are omitted for the sake of clarity. The space

filling model of the tmphen ligands shows the p–p stacking within the

cluster.

Fig. 2 Field dependent magnetization for 1 at 1.8 K. Inset: temperature

dependence of wT and w�1 for 1. The line represents the Curie–Weiss fit.

Fig. 3 Field dependent magnetization data for 2 at 1.8 K. Inset:

Temperature dependence of wT and w�1 for 2. The red line represents

the Curie–Weiss fit and the blue line represents the MAGPACK

simulation.

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 4484–4486 | 4485

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The program MAGPACK16 was used to simulate the wT data

above 40 K using the following Hamiltonian:

H = �2JNi–Mo(SMo1 + SMo2)(SNi1 + SNi2 + SNi3)

+ bH[gavg(SMo1 + SMo2 + SNi1 + SNi2 + SNi3)]

(1)

where only one JNi–Mo and one gavg value were used for the

simulation to avoid overparameterization. The best simulation

of the data (Fig. 3) resulted in JNi–Mo = 8.2 cm�1 and gavg =

2.08. Simulations of the data at low temperature for the peak

of wT gave an unrealistically large value for DNi of B17 cm�1,

whereas the fitting of the reduced magnetization data (Fig. S3w)at low temperature using Anisofit2.017 gave a more reasonable

value of D = 0.145 cm�1 and a g of 2.04.

To check for SMM behavior of compounds 1 and 2,

zero-field-cooled and field-cooled magnetization (ZFC/FC)

and ac susceptibility data were measured at low temperatures

(Fig. S4–7w). No divergence in the ZFC/FC data or out-of-

phase signal in the ac data was observed, ruling out any SMM

behavior above 1.8 K for both compounds.

In summary, two magnetic TBP clusters containing the rare

[Mo(CN)6]3� anion were synthesized by in situ generation of

[Mo(CN)6]3� from [Mo(CN)7]

4�. Ferromagnetic interactions

were observed for both CoII and NiII through cyanide bridges

to MoIII centers; these data constitute the first time magnetic

interactions have been reported for these metal ions through

a cyanide bridge (or through any bridging group) to our

knowledge. Given the complexity of these systems, the

magnetic properties cannot be interpreted easily with simple

isotropic models. Detailed investigations, both experimental

and theoretical, on other TBP compounds containing the

[Mo(CN)6]3� unit, particularly the VII

3MoIII2 analog, are in

progress and will be reported in due course.

KRD gratefully acknowledges the support of the Department

of Energy (DE-FG02-02ER45999) for this work. Funds for

the SQUID magnetometer were obtained from the National

Science Foundation.

Notes and references

z All experiments were performed under a N2 atmosphere. Synthesisof 1: 59 mg of Co(NO3)2�6H2O (0.2 mmol) and 106 mg of tmphen(0.45 mmol) were dissolved in 5 mL of MeOH–MeCN (v : v = 1 : 2). Amethanol solution of (18-Crown-6-K)4Mo(CN)7 was diffused into theabove solution using the same MeOH–MeCN ratio as a buffer layer inan H-tube. Dark green crystals suitable for X-ray diffraction formedafter one week. The crystals were washed with MeCN and dried in aninert atmosphere glove box (yield: 60 mg, 36%). Elemental analysis for[Co(tmphen)2]3[Mo(CN)6]2�(MeCN)2�(H2O)3: calcd. (found) C, 60.19(59.01); N, 16.29 (16.72); H, 4.87 (4.92); O, 2.15 (2.20)%. IR(KBr, cm�1): 2115 cm�1. Synthesis of 2: A 58 mg sample ofNi(NO3)2�6H2O (0.2 mmol) and 106 mg of tmphen (0.45 mmol) weredissolved in 10 mL of MeCN and 1 mL of MeOH was added. To thisclear pink solution was added 1 mL of a methanol solutionof (18-Crown-6-K)4Mo(CN)7 (0.1 mmol) which resulted in theinstantaneous formation of a brown precipitate. The solution was leftto stand undisturbed for two weeks during which time red crystalshad formed in the vial. The crystals were washed with MeCNand dried (yield: 50 mg, 30%). Elemental analysis for[Ni(tmphen)2]3[Mo(CN)6]2�(MeCN)2�(H2O)6: calcd. (found) C,58.79 (58.40); N, 15.91 (15.91); H, 5.02 (4.76); O, 4.20 (4.23)%. IR(KBr, cm�1): 2127 and 2094 cm�1.

y Crystal data for 1: C119H136N26O10Co3Mo2, monoclinic, P21/c,a = 19.387(2), b = 25.463(3), c = 24.634(3) A, b = 98.205(2)1,V=12036(4) A3,Z=4, T=110K,Mo-Ka radiation (l=0.71073 A).R1 = 0.0663 for 1461 parameters and 11451 unique reflections with(I > 2s(I)) and wR2 = 0.2132 for all 15 460 reflections. CCDC: 723434.Crystal data for 2: C115H139Mo2N25Ni3O15, monoclinic, P21/c,a = 19.394(2), b = 25.236(3), c = 24.423(3) A, b = 98.144(2)1, V =11833(2) A3, Z = 4, T = 110 K, Mo-Ka radiation (l = 0.71073 A).R1 = 0.0772 for 1442 parameters and 9756 unique reflections with(I > 2s(I)) and wR2 = 0.2758 for all 20 830 reflections. CCDC: 724501.

1 (a) Magnetism: Molecules to Materials, ed. J. S. Miller andM. Drillon, Wiley-VCH, 2002–2005, vol. I–V; (b) H.-J. Kruger,Ed., Special issue on Deutsche Forschungsgemeinschaft MolecularMagnetism Research Report, Coord. Chem. Rev., 2009, 253,2241–2460.

2 For examples: (a) S. Ferlay, T. Mallah, R. Ouahes, P. Veillet andM. Verdaguer, Nature, 1995, 378, 701; (b) P. Gutlich and H. A.Goodwin, ed., Top. Curr. Chem., 2004, 233–255; (c) D. Gatteschiand R. Sessoli, Angew. Chem., Int. Ed., 2003, 42, 268;(d) C. Coulon, H. Miyasaka and R. Clerac, Struct. Bonding,2006, 122, 163; (e) O. Sato, J. Tao and Y. Z. Zhang, Angew.Chem., Int. Ed., 2007, 46, 2152; (f) E. Coronado and P. Day, Chem.Rev., 2004, 104, 5419.

3 (a) M. Shatruk, C. Avendano and K. R. Dunbar, Prog. Inorg.Chem., 2009, 56, 155; (b) L. M. C. Beltran and J. R. Long, Acc.Chem. Res., 2005, 38, 325; (c) J. N. Rebilly and T. Mallah, Struct.Bonding, 2006, 122, 103; (d) S. Tanase and J. Reedijk, Coord.Chem. Rev., 2006, 250, 2501; (e) P. Przychodzen, T. Korzeniak,R. Podgajny and B. Sieklucka, Coord. Chem. Rev., 2006, 250, 2234;(f) M. Atanasov, P. Comba, S. Hausberg and B. Martin, Coord.Chem. Rev., 2009, 253, 2306 and references therein.

4 (a) J. Larionova, M. Gross, M. Pilkington, H. Andres, H. Stoeckli-Evans, H. N. Gudel and S. Decurtins, Angew. Chem., Int. Ed.,2000, 39, 1605; (b) Y. Song, P. Zhang, X. M. Ren, X. F. Shen,Y. Z. Li and X. Z. You, J. Am. Chem. Soc., 2005, 127, 3708;(c) J. H. Lim, H. S. Yoo, J. H. Yoon, E. K. Koh, H. C. Kim andC. S. Hong, Polyhedron, 2008, 27, 299.

5 F. Prins, E. Pasca, L. J. de Jongh, H. Kooijman, A. L. Spek andS. Tanase, Angew. Chem., Int. Ed., 2007, 46, 6081.

6 A. Bleuzen, V. Marvaud, C. Mathoniere, B. Sieklucka andM. Verdaguer, Inorg. Chem., 2009, 48, 3453.

7 V. S. Mironov, L. F. Chibotaru and A. Ceulemans, J. Am. Chem.Soc., 2003, 125, 9750.

8 (a) J. Milon, M. C. Daniel, A. Kaiba, P. Guionneau, S. Brandesand J. P. Sutter, J. Am. Chem. Soc., 2007, 129, 13872;(b) K. Tomono, Y. Tsunobuchi, K. Nakabayashi andS. Ohkoshi, Inorg. Chem., 2010, 49, 1298; (c) O. Kahn,J. Larionova and L. Ouahab, Chem. Commun., 1999, 945 andreferences therein.

9 L. G. Beauvais and J. R. Long, J. Am. Chem. Soc., 2002, 124, 2110.10 E. Ruiz, A. R. Fortea, S. Alvarez and M. Verdaguer, Chem.–Eur.

J., 2005, 11, 2135.11 (a) E. Dujardin, S. Ferlay, X. Phan, C. Desplanches, C. C. Dit

Moulin, P. Sainctavit, F. Baudelet, E. Dartyge, P. Veillet andM. Vergaguer, J. Am. Chem. Soc., 1998, 120, 11347;(b) Ø. Hatlevik, W. E. Buschmann, J. Zhang, J. L. Manson andJ. S. Miller, Adv. Mater., 1999, 11, 914.

12 D. E. Freedman, D. M. Jenkins and J. R. Long, Chem. Commun.,2009, 4829.

13 K. E. Funck, M. G. Hilfiger, C. P. Berlinguette, M. Shatruk,W. Wernsdorfer and K. R. Dunbar, Inorg. Chem., 2009, 48,3438.

14 (a) M. G. Hilfiger, M. Shatruk, A. Prosvirin and K. R. Dunbar,Chem. Commun., 2008, 5752; (b) M. G. Hilfiger, M. Chen,T. V. Brinzari, T. M. Nocera, M. Shatruk, D. T. Petasis,J. L. Musfeldt, C. Achim and K. R. Dunbar, Angew. Chem., Int.Ed., 2010, 49, 1410; (c) R. Sessoli, Nat. Chem., 2010, 2, 346.

15 R. C. Young, J. Am. Chem. Soc., 1932, 54, 1402.16 J. J. Borras-Almenar, J. M. Clemente-Juan, E. Coronado and

B. S. Tsukerblat, J. Comput. Chem., 2001, 22, 985.17 M. P. Shores, J. J. Sokol and J. R. Long, J. Am. Chem. Soc., 2002,

124, 2279.

4486 | Chem. Commun., 2010, 46, 4484–4486 This journal is �c The Royal Society of Chemistry 2010

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