Enhancing the Magnetic Anisotropy of Cyano-Ligated Chromium(II) and Chromium(III) Complexes via Heavy Halide Ligand Effects

Article abstract of DOI:10.1021/ic1002995

A method of increasing the axial zero-field splitting parameter for transition metal complexes of utility in the assembly of magnetic clusters is demonstrated through the use of heavy atoms as auxiliary ligands. The octahedral complexes [Cr(dmpe)₂(CN)X]⁺ (dmpe = 1,2- bis(dimethylphosphino)ethane, X = Cl, Br, I) and Cr(dmpe)₂(CN)X (X = Cl, I) are synthesized and structurally characterized. Variable-field magnetization measurements show the magnitude of D for these complexes to increase significantly as the halide ligand varies from chloride to iodide, ranging from 0.11 cm^-1 for [Cr(dmpe)₂(CN)Cl]⁺ to 6.26 cm^-1 for Cr(dmpe)₂(CN)I.

Full text of DOI:10.1021/ic1002995

4738 Inorg. Chem. 2010, 49, 4738–4740
DOI: 10.1021/ic1002995
Enhancing the Magnetic Anisotropy of Cyano-Ligated Chromium(II) and
Chromium(III) Complexes via Heavy Halide Ligand Effects
Hemamala I. Karunadasa, Kristine D. Arquero, Louise A. Berben, and Jeffrey R. Long*
Department of Chemistry, University of California, Berkeley, California 94720-1460
Received February 15, 2010
A method of increasing the axial zero-field splitting parameter for
transition metal complexes of utility in the assembly of magnetic
clusters is demonstrated through the use of heavy atoms as auxiliary
ligands. The octahedral complexes [Cr(dmpe)2(CN)X]þ (dmpe =
1,2-bis(dimethylphosphino)ethane, X = Cl, Br, I) and Cr(dmpe)2-
(CN)X (X = Cl, I) are synthesized and structurally characterized.
Variable-field magnetization measurements show the magnitude
of D for these complexes to increase significantly as the halide
ligand varies from chloride to iodide, ranging from 0.11 cm-1 for
[Cr(dmpe)2(CN)Cl]þ to 6.26 cm-1 for Cr(dmpe)2(CN)I.
cyano-bridged cluster is just Ueff = 33 cm-1 6
. A key to
enhancing these barriers may lie in the synthesis of new
high-anisotropy molecular building units for use in cluster
assembly. Ultimately, if a sufficiently large barrier can be
realized, single-molecule magnets could potentially find ap-
plications in areas such as high density information storage,
quantum computing, and molecular spintronics.2
In a number of cases, the incorporation of second- and
third-row transition metalcenters hasbeenshowntogenerate
a large magnetic anisotropy within cyano-bridged clusters
through spin-orbit coupling.5b,c,6,7 This approach can be
challenging, however, owing to their often sluggish ligand
substitution kinetics and difficulties with controlling the
coordination geometry to achieve high-anisotropy electron
configurations.8 An alternative strategy is to instill magnetic
anisotropy within first-row transition metal building units
through the spin-orbit coupling imparted by ligands with
heavy donor atoms. This would allow one to exploit the syn-
thetic and structural predictability of first-row transition metal
complexes while also delivering a large single-ion anisotropy.
In particular, the presence of heavy halide ligands has been
shown to increase axial zero-field splitting for transition
metal complexes. For example, magnetic susceptibility mea-
surementsperformedon [ReX6]2- (X=Cl, Br) and high-field
Since the initial demonstration of magnetic bistability in
Mn12O12(CH3CO2)16(H2O)4,1 there has been considerable
interest in the possibility of generating molecular clusters
with higher blocking temperatures for a variety of potential
applications.2 Such molecules, known as single-molecule
magnets, exhibit a magnetic relaxation barrier, U, generated
by action of a negative axial zero-field splitting, D, on a high-
spin ground state, S. As U=S2|D| (for integer S), a variety of
approaches have been developed for attempting to gene-
rate clusters with a very high S value3 or a large magnetic
anisotropy |D|.4 Among these, the synthesis of high-nucle-
arity metal-cyanide coordination clusters presents a promis-
ing strategy, wherein the substitutions of the metal can
provide a means of manipulating S and D.5 To date, however,
the largest relaxation barrier that has been measured for a
(5) (a) Scuiller, A.; Mallah, T.; Verdaguer, M.; Nivorozkhin, A.; Tholence,
J. L.; Veillet, P. New J. Chem. 1996, 20, 1. (b) Shores, M. P.; Sokol, J. J.; Long, J. R.
J. Am. Chem. Soc. 2002, 124, 2279. (c) Sokol, J. J.; Hee, A. G.; Long, J. R. J. Am.
~
Chem. Soc. 2002, 124, 7656. (d) Berlinguette, C. P.; Vaughn, D.; Canada-Vilalta, C.;
*To whom correspondence should be addressed: E-mail: jrlong@
berkeley.edu.
(1) (a) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.;
Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem.
Soc. 1993, 115, 1804. (b) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A.
Nature 1993, 365, 141.
(2) (a) Garanin, D. A.; Chudnovsky, E. M. Phys. Rev. B 1997, 56, 11102.
(b) Leuenberger, M. N.; Loss, D. Nature 2001, 410, 789. (c) Stamp,
P. C. E.; Gaita-Arino, A. J. Mater. Chem. 2009, 19, 1718.
ꢀ
ꢀ
Galan-Mascaros, J. R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2003, 42, 1523.
(e) Marvaud, V.; Decroix, C.; Scuiller, A.; Guyard-Duhayon, C.; Vaissermann, J.;
Gonnet, F.;Verdaguer, M. Chem.;Eur. J. 2003, 9, 1677.(f)Song, Y; Zhang, P.;Ren,
X.-M.; Shen, X.-F.; Li, Y.-Z.; You, X.-Z. J. Am. Chem. Soc. 2005, 127, 3708. (g) Li,
D. F.; Parkin, S.; Wang, G. B.; Yee, G. T.; Prosvirin, A. V.; Holmes, S. M. Inorg.
Chem. 2005, 44, 4903. (h) Harris, T. D.; Long, J. R. Chem. Commun. 2007, 1360. (i)
Schelter, E. J.; Karadas, F.; Avendano, C.; Wernsdorfer, W.; Dunbar, K.
J. Am. Chem. Soc. 2007, 129, 8139. (j) Glaser, T.; Heidemeier, M.; Krickemeyer,
(3) (a) Zhong, Z. J.; Seino, H.; Mizobe, Y.; Hidai, M.; Fujishima, A.;
Ohkoshi, S.; Hashimoto, K. J. Am. Chem. Soc. 2000, 122, 2952–2953. (b)
Murugesu, M.; Habrych, M.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. J.
Am. Chem. Soc. 2004, 126, 4766. (c) Ako, A. M.; Hewitt, I. J.; Mereacre, V.;
€
€
E.; Bogge, H.; Stammler, A.; Frohlich, R.; Bill, E.; Schnack, J. Inorg. Chem. 2009,
48, 607.
(6) (a) Lim, J. H.; Yoon, J. H.; Kim, H. C.; Hong, C. S. Angew. Chem., Int.
Ed. 2006, 45, 7424. (b) Freedman, D. E.; Jenkins, D. M.; Iavarone, A. T.; Long,
J. R. J. Am. Chem. Soc. 2008, 130, 2884.
ꢀ
Clerac, R.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. Angew. Chem., Int. Ed.
2006, 45, 4926.
(7) (a) Schelter, E. J.; Prosvirin, A. V.; Dunbar, K. R. J. Am. Chem. Soc.
2004, 126, 15004. (b) Song, Y.; Zhang, P.; Ren, X.-M.; Shen, X.-F.; Li, Y.-Z.; You,
X.-Z. J. Am. Chem. Soc. 2005, 127, 3708. (c) Yoon, J. H.; Lim, J. H.; Kim, H. C.;
Hong, C. S. Inorg. Chem. 2006, 45, 9613. (d) Zhao, H.; Shatruk, M.; Prosvirin,
A. V.; Dunbar, K. R. Chem.;Eur. J. 2007, 13, 6573.
(4) (a) Gatteschi, D.; Sorace, L. J. Solid State Chem. 2001, 159, 253.
(b) Oshio, H.; Nakano, M. Chem.;Eur. J. 2005, 11, 5178. (c) Milios, C. J.;
Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou,
G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 2754. (d) Cirera, J.; Ruiz, E.;
Alvarez, S.; Neese, F.; Kortus, J. Chem.;Eur. J. 2009, 15, 4078.
(8) Karunadasa, H. I.; Long, J. R. Angew. Chem., Int. Ed. 2009, 48, 738.
r
pubs.acs.org/IC
Published on Web 05/05/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 11, 2010 4739
Scheme 1. Synthesis of [Cr(dmpe)2(CN)X]1þ/0 Complexes
EPR studies of Tp*NiX (Tp*=hydrotris(3,5-dimethylpyra-
zole)borate; X = Cl, Br, I) and (tpa)MnX2 (tpa = tris-
2-picolylamine; X=Cl, Br, I) have shown the complexes with
the heavier halide ligands to exhibit D values of significantly
greater magnitude.9 Here, we demonstrate that this effect can
extend to octahedral CrII and CrIII complexes rendered sui-
table for use as cluster building units through the appendage
of a terminal cyanide ligand.
Octahedral chromium complexes featuring a halide ligand
placed trans to a cyanide ligand were synthesized with the use
of two 1,2-bis(dimethylphosphino)ethane (dmpe) auxiliary
ligands. Complexes of the form Cr(dmpe)2(CN)X (X=Cl, I)
and [Cr(dmpe)2(CN)X]þ (X=Cl, Br, I) were generated, star-
ting from Cr(dmpe)2Cl2,10 as outlined in Scheme 1. Although
some of these complexes have been reported previously,
here we employ different synthetic routes with improved
yields and structurally characterize compounds 1-8 for the
first time. The complex Cr(dmpe)2(CN)Cl (1) was isolated
from the reaction of Cr(dmpe)2Cl2 with (nBu4N)CN and was
subsequently converted into [Cr(dmpe)2(CN)Cl](PF6) (2) via
chemical oxidation with [Cp2Fe](PF6).11 Repeating the reac-
tion with [Cp2Fe](BPh4) afforded the analogous compound
[Cr(dmpe)2(CN)Cl](BPh4) (20). In an attempt to access the
CrII iodo and bromo complexes, Cr(dmpe)2Cl2 was reduced
to Cr(dmpe)2(N2)2 using 2 equiv of n-butyllithium.12 Oxida-
tion of Cr(dmpe)2(N2)2 with 2 equiv of iodobenzene then
afforded Cr(dmpe)2I2 (5),13 while analogous reactions em-
ploying bromobenzene or t-butylbromide were found to yield
[Cr(dmpe)2Br2]Br (3).11 Further oxidation of Cr(dmpe)2-
(N2)2 to [Cr(dmpe)2I2]I (7) can be accomplished using 1.5
equiv of I2.11 Reaction of (nBu4N)CN with Cr(dmpe)2I2 or
NaCN with [Cr(dmpe)2X2]X (X=Br,11 I) affords the mixed
Figure 1. Structures and reduced magnetization data for the octahedral
complexes [Cr(dmpe)2(CN)X]þ (X = Cl, Br, I), as observed in 2, 4, and 8.
Green, purple, orange, yellow, light blue, blue, and gray spheres represent
Cr, I, Br, Cl, P, N, and C atoms, respectively; H atoms are omitted for
clarity. Solid lines represent fits to the data (see text for details).
cyanide-halide compounds Cr(dmpe)2(CN)I (6) and
[Cr(dmpe)2(CN)X](X) (4 and 8) in high yield.
Compounds 1, 20, and 3-8 were characterized by single
crystal X-ray analysis. In each of the mixed cyanide-halide
structures, the cyanide and halide ligands were equally dis-
ordered between the two trans positions. In most of the struc-
tures, disorder was also evident in the dmpe ligand. For the
complexes [Cr(dmpe)2X2]þ, the mean Cr-X distance in-
14
˚
˚
creases from 2.293(4) A for X = Cl to 2.457(2) A for X =
˚
Br to 2.662(5) A for X = I. This trend is conserved in the
structures of the [Cr(dmpe)2(CN)X]þ complexes, which dis-
˚
play a mean Cr-X distance of 2.452(2) A for X = Cl and
(9) (a) Busey, R. H.; Sonder, E. J. Chem. Phys. 1962, 36, 93. (b) Desrochers,
P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J.; Vicic, D. A. Inorg.
Chem. 2006, 45, 8930. (c) Duboc, C.; Phoeung, T.; Zein, S.; Pecaut, J.; Collomb,
M.-N.; Neese, F. Inorg. Chem. 2007, 46, 4905.
(10) Girolami, G. S.; Wilkinson, G.; Galas, A. M. R.; Thornton-Pett, M.;
Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1985, 1339.
˚
˚
Cr-X distances of 2.552(3) A for X=Br and 2.663(2) A for
X=I. We note, however, that too much emphasis cannot be
placed on the bond distances which are influenced by the
disorder in the structure. Analogous trends are apparent for
the structures of the Cr(dmpe)2(CN)X complexes, with a
mean Cr-Cl distance of 2.436(9) and a Cr-I distance of
(11) Suzuki, T.; Kashiwabara, K.; Usami, T.; Imamura, T.; Kiki, M.;
Fujita, J.; Kaizaki, S. Bull. Chem. Soc. Jpn. 2001, 74, 1055.
(12) Berben, L. A.; Kozimor, S. A. Inorg. Chem. 2008, 47, 4639.
(13) The reaction between Cr(dmpe)3 and excess MeI was reported to give
Cr(dmpe)2I2; however, the product was not crystallographically characterized,
and the green color of the solid suggests that it is actually [Cr(dmpe)2I2]I. As
isolated here, Cr(dmpe)2I2 is a brown compound. (a) Cloke, F. G. N.; Fyne,
P. J.; Gibson, V. C.; Green, M.L. H.; Ledoux, M. J.; Perutz, R. N.; Dix, A.;
Gourdon, A.; Prout, K. J. Organomet. Chem. 1984, 277, 61. (b) Halepoto, D. M.;
Holt, D. G. L.;Larkworthy, L. F.; Povey, D. C.; Smith, G. W.; Leigh,G. J.Polyhedron
1989, 8, 1821.
˚
2.733(1) A.
Magnetic susceptibility measurements were performed on
finely ground polycrystalline powders of compounds 1, 2, 4,
6, and 8 suspended in eicosane. At room temperature, the
(14) Salt, J. E.; Girolami, G. S.; Wilkinson, G.; Motevalli, M.; Thornton-
Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1985, 685.

4740 Inorganic Chemistry, Vol. 49, No. 11, 2010
Karunadasa et al.
Table 1. Magnetic Parameters for Compounds Containing the Octahedral
Complexes [Cr(dmpe)2(CN)X]þ (X=Cl, Br, I) and Cr(dmpe)2(CN)X (X=Cl, I)
compound
μeff (μB)a
g
|D| (cm-1
)
[Cr(dmpe)2(CN)Cl](PF6) (2)
[Cr(dmpe)2(CN)Br]Br (4)
[Cr(dmpe)2(CN)I]I (8)
Cr(dmpe)2(CN)Cl (1)
Cr(dmpe)2(CN)I (6)
3.64
3.69
3.87
2.79
2.67
1.82
1.84
1.82
1.66
1.61
0.11
1.28
2.30
3.97
6.26
a At 298 K.
Figure 3. Structure of the cyano-bridged cluster [(H2O)(salen(OMe)2)-
Mn(μ-NC)Cr(dmpe)2I]2þ, as observed in 9. Purple, brown, green, light blue,
red, blue, and gray spheres represent I, Mn, Cr, P, O, N, and C atoms,
respectively; H atoms are omitted for clarity.
Figure 2. Structures and reduced magnetization data for the octahedral
complexes Cr(dmpe)2(CN)X (X=Cl, I), as observed in 1 and 6. Purple,
green, yellow, light blue, blue, and gray spheres represent I, Cr, Cl, P, N,
and C atoms, respectively; H atoms are omitted for clarity. Solid lines
represent fits to the data (see text for details).
complex Cr(dmpe)2(CN)Cl exhibits a large zero-field split-
ting, with |D| = 3.97 cm-1. Nevertheless, a substantial gain
in anisotropy is again achieved through the substitution
of chloride with iodide, resulting in |D| = 6.26 cm-1 for
Cr(dmpe)2(CN)I.
compounds exhibit effective magnetic moments close to the
expected spin-only moments of 3.87 and 2.83 μB for isolated
CrIII (S=3/2) and low-spin CrII (S=1) centers, respectively,
as given in Table 1. Magnetization data collected at applied
fields between 1 and 7 T and temperatures below 5 K were fit
using ANISOFIT 2.05b to extract the axial zero-field splitting
parameters and g values, as listed in Table 1. Since, in general,
the sign of D is not reliably obtained from such measure-
ments, we report here only its magnitude.
The mixed cyanide-halide complexes characterized here
should be of utility in the assembly of cyano-bridged coordi-
nation clusters. In particular, reactions with metal complexes of
thetype[LxcapM(solvent)y]nþ (where Lcap represents a capping
ligand) can be expected to give star-like clusters of formula
{LxcapM[(μ-NC)Cr(dmpe)2X]y}nþ/(nþy)þ. Of greatest interest
for this purpose will be the high-anisotropy iodo complexes
[Cr(dmpe)2(CN)I]þ (S=3/2) and Cr(dmpe)2(CN)I (S=1).
As an initial example demonstrating cluster formation, the
former species was found to react with [(salen(OMe)2)-
Mn(H2O)](ClO4) (H2salen(OMe)2 =N,N0-bis(3-methoxysali-
cylidene)ethyl-enediamine) to afford [(H2O)(salen(OMe)2)-
Mn(μ-NC)Cr(dmpe)2I](ClO4)2 (9). This compound features
the cyano-bridged cluster depicted in Figure 3. The formation
of a cyanide bridge precludes the halide/cyanide disorder seen in
the crystal structures of the monomeric complexes and is
evidence that the crystallographic disorder is not a manifestation
of the cocrystallization of [Cr(dmpe)2X2]0/1þ and [Cr(dmpe)2-
(CN)2]0/1þ. Although this molecule is expected to have a ground
state of just S = 1/2 owing to antiferromagnetic coupling,
related reactions can be anticipated to yield high-spin, high-
anisotropy clusters and, potentially, single-molecule magnets
with high relaxation barriers.
The magnetization data for the CrIII complexes are depic-
ted in Figure 1. Typically, an octahedrally coordinated metal
3
center with a t2g configuration has an essentially spherical
electron distribution, leading to a D value near zero. This situ-
ation is borne out in the data for [Cr(dmpe)2(CN)Cl]þ,
wherein the isofield lines show little deviation from the
Brillouin function for an S=3/2 ground state, leading to |D|=
0.11 cm-1. Importantly, replacement of the chloride ligand
with bromide or iodide results in an increasing separation
between isofield lines, corresponding to anisotropy para-
meters of |D| = 1.28 and 2.30 cm-1, respectively. Thus, the
simple substitution of a single halide ligand can lead to in-
creases in the axial zero-field splitting by more than an order
of magnitude. To the best of our knowledge, the value obta-
ined for [Cr(dmpe)2(CN)I]þ is the largest yet observed for a
pseudooctahedral CrIII complex.15
As shown in Figure 2, the magnetization data for the CrII
complexes indicate even greater anisotropy. Here, the elec-
Acknowledgment. This research was funded by NSF
Grant No. CHE-0617063. We thank Tyco Electronics for
partial support of H.I.K.; Drs. F. J. Hollander, A. G.
Oliver, and S. J. Teat for experimental assistance; and
Dr. S. A. Kozimor for helpful discussions.
4
tronic structure stems from the t2g configuration of a low-
spin octahedral complex, which carries with it first-order
orbital angular momentum. Accordingly, even the chloride
Supporting Information Available: Complete experimental
details (PDF) and X-ray crystallographic files for 1-9 (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(15) (a) Solano-Peralta, A.; Sosa-Torres, M. E.; Flores-Alamo, M.; El-
Mkami, H.; Smith, G. M.; Toscano, R. A.; Nakamura, T. Dalton Trans.
2004, 2444. (b) Rebilly, J.-N.; Catala, L.; Charron, G.; Rogez, G.; Riviere, E.;
Guillot, R.; Thuery, P.; Barra, A.-L.; Mallah, T. Dalton Trans. 2006, 2818.