Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure

Article abstract of DOI:10.1021/jacs.8b10638

The size of the organic cation dictates both the composition and the extended 3-D structure for hybrid organic/inorganic Prussian blue analogues (PBAs) of A_aMn^II_b(CN)_a+2b (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC₆ and M′N₆ octahedral coordination sites and the alkali cation content depends on the M and M′ oxidation states. The reaction of Mn^II(O₂CCH₃)₂ and A⁺CN^- (A = NMe₄, NEtMe₃) forms a hydrated material of A₃Mn^II₅(CN)₁₃ composition. A₃Mn^II₅(CN)₁₃ forms a complex, 3-D extended structural motif with octahedral and rarely observed square pyramidal and trigonal bipyramidal Mn^II sites with a single layer motif of three pentagonal and one triangular fused rings. A complex pattern of Mn^IICN chains bridge the layers. (NMe₄)₃Mn^II₅(CN)₁₃ possesses one low-spin octahedral and four high-spin pentacoordinate Mn^II sites and orders as an antiferromagnet at 11 K due to the layers being bridged and antiferromagnetically coupled by the nonmagnetic cyanides. These are rare examples of intrinsic, chemically prepared and controlled artificial antiferromagnets and have the advantage of having controlled uniform spacing between the layers as they are not physically prepared via deposition methods. A₃Mn₅(CN)₁₃ (A = NMe₄, NEtMe₃) along with [NEt₄]₂Mn^II₃(CN)₈, [NEt₄]Mn^II₃(CN)₇, and Mn(CN)₂ form stoichiometrically related A_aMn^II_b(CN)_a+2b (a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing unprecedented stoichiometries and lattice motifs. These unusual structures and stoichiometries are attributed to the very ionic nature of the high-spin N-bonded Mn^II ion that enables the maximization of the attractive van der Waals interactions via minimization of void space via a reduced aMn^II_b(CN)_a+2b family of compounds are referred to as being cation adaptive in which size and shape dictate both the stoichiometry and structure.

Full text of DOI:10.1021/jacs.8b10638

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Article
Anomalous Stoichiometry, 3-D Bridged Triangular/pentagonal Layered
Structured Artificial Antiferromagnet for the Prussian Blue Analog
A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure
Saul H. Lapidus, Adora G. Graham, Christopher M Kareis,
Casey G. Hawkins, Peter W. Stephens, and Joel S. Miller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10638 • Publication Date (Web): 17 Dec 2018
Downloaded from http://pubs.acs.org on December 17, 2018
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Anomalous Stoichiometry, 3-D Bridged Triangular/pentagonal
Layered Structured Artificial Antiferromagnet for the Prussian
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Blue Analog A3MnII (CN)13 (A = NMe4, NEtMe3). A Cation Adaptive
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Structure
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Saul H. Lapidus,†‡ Adora G. Graham,§ Christopher M. Kareis,§ Casey G. Hawkins,§
Peter W. Stephens,*† Joel S. Miller*§
† Department of Physics & Astronomy State, University of New York. Stony Brook, NY 11794-3800 USA
§
Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850 USA
‡
Current Address: X-ray Science Division, Argonne National Laboratory, Lemont IL 60439
ABSTRACT: The shape of the organic cation dictates both the composition and the
extended 3-D structure for hybrid organic/inorganic Prussian blue analogs (PBAs) of
AaMnII (CN)a+2b (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC6
b
and
M'N6
octahedral
coordination sites, and the
alkali cation content depends
on the M and M' oxidation
states.
The reaction of
MnII(O2CCH3)2 and A+CN- (A
= NMe4, NEtMe3) forms a
hydrated
material
of
A3MnII (CN)13 composition.
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A3MnII (CN)13
forms
a
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complex,
3-D extended
structural motif with octahedral and rarely observed square pyramidal and trigonal
bipyramidal MnII sites with a single layer motif of three pentagonal and one triangular
fused rings. A complex pattern of MnIICN chains bridge the layers. (NMe4)3MnII (CN)13
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possesses one low-spin octahedral and four high-spin pentacoordinate MnII sites, and
orders as an antiferromagnet at 11 K due to the layers being bridged and
antiferromagnetically coupled by the non-magnetic cyanides. These are rare examples
of intrinsic, chemically prepared and controlled artificial antiferromagnets, and have the
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advantage of having controlled uniform spacing between the layers as they are not
physically prepared via deposition methods. A3Mn5(CN)13 (A = NMe4, NEtMe3) along with
[NEt4]2MnII (CN)8, [NEt4]MnII (CN)7, and Mn(CN)2 form stoichiometrically related
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AaMnII (CN)a+2b (a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing
b
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an unprecedented stoichiometries and lattice motifs. This These unusual structures and
stoichiometries are attributed to the very ionic nature of the high-spin N-bonded MnII ion
that enables the maximization of the attractive van der Waals interactions via minimization
of void space via a reduced ÐMnNC. This AaMnII (CN)a+2b family of compounds are
b
referred to as being cation adaptive which size and shape dictates both the stoichiometry
and structure.
INTRODUCTION
Several mixed-metal hexacyanometalates possessing the face centered cubic (fcc) (a ~
10.5 Å) Prussian blue structural motif magnetically order with critical temperatures, Tc, as
o
high as ~100 C.1-3 The relatively high Tc’s are attributed to the strong superexchange
via the conjugated -M'-NºC-M-CºN-M'- linkages.4 Albeit structurally related, Prussian
blues can have several compositions, e.g. M'III [MII(CN)6]3, A+M'II[MIII(CN)6],
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M'II [MIII(CN)6]2, A+M'III[MII(CN)6], A+ M'II[MII(CN)6], and M'III[MIII(CN)6] (A+ = alkali cation)
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that are typically solvated. The canonical unit cell contains significant void space that can
accommodate the A+ and/or water, and in some cases [M(CN)6]n- anions are missing.
A2MnII[MnII(CN)6] (A = Na,5 K,6 Rb6), however, has the same framework topology, but a
distorted lattice with nonlinear -M'-NºC- linkages with an average ÐM'-NºC << 180o. Use
of the larger NEt4 forms the layered (2-D) ferrimagnet [NEt4]2MnII (CN)8 and 3-D bridged-
+
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layered antiferromagnet [NEt4]MnII (CN)7 that can be thermally decomposed to Mn(CN)2
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with an interpenetrating diamondoid structure.8 To study the effect of cation size in
Prussian blue related magnets, and to identify new magnetic materials and structural
motifs, we investigated NMe4 and NEtMe3 to target A2MnII[MnII(CN)6] (A = NMe4,
NEtMe3). [NMe4]MnII[CrIII(CN)6]•4H2O9a and [NMe4]MnII[MIII(CN)6]•8H2O (M = Mn, Fe)9b
have been reported to have non-cubic orthorhombic and tetragonal structures,
respectively, but with ÐM'-NºC = 180o. Herein, we report the unexpected new structural
+
+
bridged layered motif of A3MnII (CN)13 (A = NMe4, NEtMe3) stoichiometry, and establish
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their antiferromagnetic ordering arising from antiferromagnetic coupling between adjacent
ferrimagnetic layers. Additionally, an overview of related family members reveals a
stoichiometric and structural relationship based on the concept of cation adaptive
structures and stoichiometry.
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EXPERIMENTAL SECTION
All syntheses were performed in an oxygen-free (<1.0 ppm O2) wet or dry box. Anhydrous
Mn(O2CMe)2 was used as purchased from Aldrich. H2O was purified through a Barnstead
“E-pure” water purification system and deoxygenated through distillation under N2. All
other solvents were distilled from the appropriate drying agents under nitrogen before
use.
[NMe4]CN was prepared from [NMe4][BF4] (1.30 g, 8.07 mmol) that was added a
10 mL acetonitrile solution containing [NEt4]CN (1.265 g, 8.10 mmol). After stirring for 24
hr the white solid was then collected and washed with cold acetonitrile (3 x 5 mL) and
then placed under vacuum for 5 hr. The white solid is then dissolved in a minimal amount
of methanol, filtered, and the methanol is removed under vacuum (Yield: 517 mg, 5.16
mmol, 64%). nCN 2050 cm-1.
[NEtMe3]Cl was made via Schlenk line techniques. To a stirring 13 mL CH2Cl2-
solution of N,N-ethyldimethylamine (1.46 g, 19.9 mmol) in a salt water ice bath was added
a 5 mL CH2Cl2 solution of methylchloroformate (1.39 g, 14.7 mmol) drop-wise over 10
min. After warming to room temperature for 8 hr a white precipitate formed, and the
solution was heated for 1 hr. After cooling the [NEtMe3]Cl precipitate was isolated via
filtration, and washed with 5 x 5 mL of cold dichloromethane before drying under reduced
pressure (Yield: 1.3393 g, 74%). IR (KBr): 3374 (br, s), 3009 (m), 1017 (s), 960 (s), 882
(s) cm-1; 1H NMR (300 MHz, D2O, 25 oC) δ:1.30-1.365 (tt, J1=7.29 Hz, J2=2.13 Hz, 2H),
3.06 (s, 9H), 3.33-3.40 (q, J=7.35 Hz, 3H).
[NEtMe3]CN was made from a 10 mL methanol solution of [NEtMe3]Cl (3.49 g,
28.3 mmol) that was added to a stirred 25 mL cloudy methanol solution containing NaCN
(2.4 g, 49.2 mmol). A white precipitate formed, and the reaction was stirred for 18 hr.
After which the slurry was filtered through Celite®, and the filtrate was taken to dryness
resulting in an off-white/pink solid. To the reaction flask containing the solid was added
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40 mL of dry MeCN and stirred for 4 hr, and then again filtered through Celite, and the
[NEtMe3]CN filtrate was taken to dryness to isolate product (Yield: 1.36 g, 42%). IR (KBr)
3426 (br, s), 3007 (m), νCN 2072 (m), 1019 (s), 960 (s), 880 (s) cm-1; 1H NMR (300 MHz,
D2O, 25 oC) δ:1.29 (tt, J1=7.29 Hz, J2= 2.01 Hz, 2H), 3.06 (s, 9H), 3.36 (q, 3H).
[NMe4]3Mn5(CN)13•zH2O (1•H2O). [NMe4]CN (517 mg, 5.16 mmol) was dissolved
in a 5 mL of H2O, and the solution was dropwise added to a 5 mL aqueous solution
containing Mn(O2CMe)2 (255 mg, 1.47 mmol). A yellow precipitate formed that turned
light green, and the mixture was stir for ~12 hr. The light green solid was then filtered,
using a medium frit, washed with H2O (2 x 5 mL), MeOH (2 x 5 mL), and Et2O (2 x 5 mL).
The solid was then placed under vacuum, on the frit, for 2 hr and then isolated (Yield 250
mg; 98%). The solvent content z = 2 was determined from thermogravimetric analyses.
The solvent is evident by the uOH at 3658 and 3633 cm-1. Calculated for
(NMe4)3Mn5(CN)13•3.5H2O: C25H43Mn5N16O3.5 %(calc'd): C: 33.67 (33.42); H: 4.23 (4.82);
N: 24.51 (24.94); Mn: 30.16 (30.57). IR (KBr) nCN; 2137 (w), 2110 (w), 2073 (s), 2047 (s).
[NMe4]3Mn5(CN)13•zMeOH (1•MeOH). [NMe4]CN (510 mg, 5.09 mmol) was
dissolved with 5 mL MeOH, and this solution was dropwise added to a vigorously stirred
MeOH solution (10 mL) containing Mn(O2CCH3)2 (250 mg, 1.44 mmol). A yellow
precipitate formed that turned green. The mixture was stirred overnight (~12 hr). The
green solid was filtered using a medium frit and washed with MeOH (2 x 5 mL) and Et2O
(2 x 5mL). The product was dried in vacuo for 2 hr (Yield 245 mg, 97%). The solvent
content z = 1.25 (S = MeOH) was determined from thermogravimetric analyses. IR (KBr)
nCN; 2137 (w), 2110 (w), 2074 (s), 2048 (s). The peaks and positions in the nCN (2134,
2108, 2075, and 2050 cm-1) region are the same, but with differing in peak intensities.
[NEtMe3]3Mn5(CN)13•H2O (2•H2O). Using [NEtMe3]CN (107.1 mg; 0.938 mmol)
and Mn(O2CCH3)2 (54.5 mg, 0.315 mmol) the aforementioned reaction was repeated and
the grey precipitate turned yellow within minutes. The reaction mixture was left to stir for
24 hr. The precipitate was isolated via filtration and washed with methanol (4 x 3 mL)
and ether (3 x 3 mL), and dried under vacuum at room temperature for 4 hr (Yield: 39.80
mg, 72%). IR (KBr) nOH 3629 (w, br), nCN 2141(w), 2098 (m), 2075 (s), 2049 (s), and 1017
(m), 964 (w), 881 (w) cm-1.
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Thermogravimetric analysis (TGA) was performed on a TA Instrument Q500 series
equipped with an evolved gas furnace and attached to a Pfeiffer Thermostar GSD 301
mass spectrometer. This instrument was located in a Vacuum Atmospheres DriLab box
under nitrogen to protect air and moisture sensitive samples. Samples were placed in an
aluminum pan and heated at 2 °C/min under a continuous 10-mL/min N2 flow. Infrared
spectra (400 to 4000 cm-1; ±1 cm-1 resolution) were obtained on a Bruker Tensor 37 FT
IR spectrophotometer as KBr pellets.
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Magnetic susceptibilities were measured in 1000 Oe applied fields between 5 and
300 K on a Quantum Design MPMS superconducting quantum interference device
(SQUID) equipped with a reciprocating sample measurement system, low field option,
and continuous low temperature control with enhanced thermometry features, as
previously described.10 Powder samples for magnetic measurements were loaded in
gelatin capsules. The temperature dependence of the magnetization was obtained by
cooling in zero-field and collecting the data on warming. The remnant magnetization was
taken in zero applied field upon warming after cooling in a 5 Oe field. AC susceptibilities
were measured at 33, 100, and 1000 Hz. In addition to correcting for the diamagnetic
contribution from the sample holder, the Pascal core diamagnetic corrections were used.
Powder X-ray diffraction (PXRD) measurements for Rietveld structure analyses
were performed at Beam Line X16C of the National Synchrotron Light Source at
Brookhaven National Laboratory. The powdered samples were held in an 1.0-mm
diameter thin-wall quartz capillary. Monochromatic X-rays were selected by a Si(111)
channel-cut monochromator; diffracted X-rays were selected by a Ge(111) analyzer and
detected by a scintillation counter. The incident intensity was monitored by an ion
chamber and used to normalize the measured signal. The TOPAS-Academic program
was used to index, solve, and refine the crystal structures.11,12 Additional powder X-ray
diffraction patterns were collected on all samples with a Bruker D8 Diffractometer (Cu Ka)
using Mica (NIST Standard Reference Material 675) as an internal standard. Images of
structures generated using CrystalMaker®; CrystalMaker Software Ltd, Oxford, UK.
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RESULTS
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The reaction of [NMe4]CN and Mn(O2CMe)2 formed a dark green precipitate as both the
hydrate, 1•H2O, and methanol solvated 1•MeOH depending on these solvents. Use of
[NEtMe3]CNforms light-green 2•H2Ousing aqueous media.13 The compositions of 1•H2O,
1•MeOH, and 2•H2O were unknown prior to structure determination by real space
simulated annealing, model building, and Rietveld refinement from the powder samples
(Figure 1). The powder diffraction patterns of 1•H2O and 1•MeOH are sufficiently similar
to conclude that they are isostructural, but the former is much better resolved; thus, the
analysis is restricted to the hydrate. At ambient temperature, 1•H2O has hexagonal lattice
parameters a = 13.6227(4) Å, c = 5.7794(2) Å (extinction symbol P---, eventually
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determined space group P 2m, Z = 1) and 2•H2O has a = 13.5280(6) Å, c = 17.8231(8)
Å (extinction symbol P31--, space group P3121, Z = 3). By inspection of the powder
diffraction patterns, they do not have the face centered cubic (fcc) structure that is typical
of Prussian blue compounds, or any simple distortion thereof.14,15 The Rietveld
refinements yield water contents z ~ 2.0(1) for 1•H2O and z ~ 0.33(5) for 2•H2O, consistent
with the elemental and thermogravimetric analyses.
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Figure 1.
High-resolution synchrotron powder diffraction data (dots) and Rietveld fit
of the data for 1•H2O (top) and 2•H2O (bottom). The lower trace is the differences,
measured – calculated, plotted to the same vertical scale.
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Structure
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The close similarity of their a lattice parameters, and that the c lattice parameters of 1•H2O
and 2•H2O are close to a ratio of 1:3, and overall similarity of their diffraction patterns
imply that 1•H2O and 2•H2O have very similar structures, which is borne out in the
eventual structural analysis. 1•H2O turns out to be a more disordered version of 2•H2O,
and to clarify that the disorder is intrinsic, data on the same sample of 1•H2O was also
collected at 22 K. It is more straightforward and convincing to describe disordered 1•H2O
in the context of ordered 2•H2O; for that reason, the ordered structure of 2•H2O is initially
described. Crystallographic details are given in Table 1.
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Table 1. Summary of crystallographic results for 1•H2O and 2•H2O.
1•H2O (1•2H2O)
1•H2O (1•2H2O)
2•H2O (2•0.33H2O)
Formula
(N(CH3)4)3Mn5(CN)13 (N(CH3)4)3Mn5(CN)13 (N(C2H5)(CH3)3)3Mn5(CN)13
•2H2O
•2H2O
•0.33H2O
C25H40N16Mn5O2
Formula mass 869.35
C25H40N16Mn5O2
869.35
C29H42.67N16Mn5O0.33
882.79
T, K
295
22
295
Space Group
P3121 (#152)
6
6
P 2m (#189)
P 2m (#189)
a, Å
c, Å
Z
13.6227(4)
5.7794(2)
1
928.82
1.55
5.89 %
3.88 %
13.5940(12)
5.7260(6)
1
916.4(2)
1.57
6.63 %
3.11 %
13.5280(6)
17.8231(8)
3
2824.77
1.56
4.94 %
1.68 %
3
V, Å
r
Rwp
calcd, g/cm3
a
b
Rexp
RBragg
2.28 %
2.54 %
2.53 %
Pawley Rwp
4.95 %
5.77 %
3.13 %
0.69977(2)
0.69997(6)
0.69983(3)
l, Å
w (y calc − yi obs
)
2
∑
i
i
, where yicalc and yiobs are the calculated and observed intensities
a
i
Rwp
=
w (y obs
)
2
∑
i
i
i
at the ith point in the profile, including background, normalized to monitor intensity. The
weight wi is 1/s2 from counting statistics, with the same normalization factor.
b
2
(ycalc − yiobs
)
∑
i
i
Rp =
2
(yobs
)
∑
i
i
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The a-b planes of 2•H2O has a network of MnII connected by CN groups, forming
three- and five-membered rings, Figure 2a. Each layer of 2•H2O has three chemically
distinct MnII ions (Figure 2b). Mn1 is a 5-coordinate square pyramid with two locations in
each plane; Mn2 is a 5-coordinate trigonal bipyramid with two locations in each plane,
and Mn3 is octahedrally coordinated, with one location in each plane. It should be noted
both 5-coordinate square pyramidal and trigonal bipyramidal MnII sites are unusual, with
Mn(NO)(CO)4 being a rare example,16 although these geometries are observed for
[CrII(CN)5]3-.17
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Perpendicular to the plane, Mn1 and Mn3 form Mn1-N-C-Mn3-C-N-Mn1 chains,
Figure 3a, with the 5.25 Å Mn1-Mn3 distance being typical of Prussian blue structured
compounds. The triangular Mn1-N-C-Mn3-C-N-Mn1 component possesses each of the
three Mn's noted in Figure 3a such that a layer is connected to the layer above and below
it, Figure 3b (and movie S1).
The Mn2 sites, illustrated in Figure 3c, are separated by 5.94 Å, which is
unprecedentedly long for an Mn-C-N-Mn linkage as they typically are ~5.25 Å.1,15 The
refinement presented here has a disordered CN between two Mn2 sites along the c axis,
but the distances are unusual: Mn2-(C/N) = 2.34 Å, and C-N = 1.27 Å. Models with split
sites for Mn2 or the bridging CN give marginally better R-factors, but none of them is
sufficiently compelling, either from a chemical standpoint or from improvement of the
+
structural refinement, to be regarded as a definitive structure. The NEtMe3 cations reside
in two distinct sites located within the cavities between the layers in the five-membered
rings. A small amount of solvent water appears in the void between adjacent Mn1 sites.
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Figure 2. Structure a single layer of the 2•H2O looking normal to the ab plane (top), and
the complete unit cell viewed along the c axis (bottom) (C is black, N is light blue, and
orientationally disordered C/N is dark blue, Mn is magenta (H atoms are not shown).
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(a)
(b)
(c)
Figure 3. a) Structure of the Mn1-NC-Mn3-CN-Mn1 in 2•H2O, showing coordination by
five, six, and five cyanides, respectively (N light blue, C black, disordered N/C dark blue).
b) Triangular column illustrating that every layer (gray) has two square pyramidal Mn1
sites connected to an octahedral Mn3 site in a different layer. The blue, brown and red
coloring illustrates the registration of the Mn1-NC-Mn3-CN-Mn1 sequences between
layers. Movie S1 depicts this unusual arrangement more clearly. c) Structure of infinite
-Mn2-(C/N)-Mn2- chains in 2•H2O.
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The results of X-ray diffraction do not convincingly distinguish the C from the N
atoms on the basis of scattering amplitude, but the bond distances to the MnII sites
generally clarify the assignments of CN orientations. Some of the cyanides are
disordered because they consist of two atoms equivalent by symmetry. Further disorder
of the cyanide orientations, not required by symmetry, cannot be excluded; the simplest
model consistent with the powder diffraction data is described. The octahedral Mn3 site
is coordinated by six C-bound cyanides, Figure 3a, consistent with the propensity of
cyanide to bond to an octahedral M site via carbon,1,15 The equatorial Mn3-C distances
are 1.91(1) and 2.11(1) Å, while the axial Mn3-C distance is 1.91(1) Å. This is not a
Jahn-Teller distortion (as observed for Na4[Mn(CN)6]•10H2O,18a,b and K2Mn[Mn(CN)6]6)
because the two long distances are adjacent, not trans. The square-pyramidal Mn1 site
is N-bonded to three equatorial CN (Mn1-N = 2.11(1) Å, 2.26(1) Å, and 2.26(1) Å) and
one symmetry-disordered C/N at 2.38(1) Å [as occurs for MnII(CN)2],8 and N-bonded
(Mn1-N = 2.17 Å) in the axial direction. Due to reduced crystal field stabilization energies,
the 5-coordinated Mn1 and Mn2 are assumed to be high spin, as occurs for [CrII(CN)5]3-
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.
+
1•H2O, with the slightly smaller NMe4 cation, has essentially the same in-plane
structure as 2•H2O (Figure 4a), but the Mn1 and Mn3 sites of 2•H2O become equivalent.
Various models of disorder were explored and the Mn1 site, which has 4-fold equatorial
coordination in the ab plane is significantly extended along the c axis, while the Mn2 site
(3-fold equatorial coordination) is not. The straightforward interpretation is that 1•H2O
contains the same Mn-cyanide trimers, two square-pyramidal sites joined in the axial
direction to an octahedral site, as seen in 2•H2O, but they are disordered along the c axis
relative to one another. Alternatively, it may be that the square-pyramidal and octahedral
Mn sites are randomly distributed along the channels in a ratio of 2:1. Either model is
consistent with the diffraction data and with the necessity to balance charge as
[NMe4]3MnII (CN)13. The model used for refinement, shown in Figure 4b, has three sites
5
for Mn1, each with occupancy of 1/3. Likewise, the refinement favors a split site for the
disordered CN that spans the in-plane Mn1-Mn1 connection. The axial Mn1-Mn1 linkages
along the c axis are 2/3 occupied by a disordered CN, and 1/3 occupied by solvent water.
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There is also solvent water between the three-membered rings. The infinite -Mn2-(CN)-
Mn2- chains in 1•H2O are essentially identical to those in 2•H2O, except that all of the
Mn2 sites, with trigonal bipyramidal coordination, appear to be C-bonded to all three of
their equatorial cyanides (Figure 4c).
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(a)
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(c)
Figure 4. a) View of 1•H2O along the c axis. b) Average configuration of the disordered
chains of Mn1-(CN)-Mn1-(NC)-Mn1 in 1•H2O. One third of the axial (C/N) disordered
sites are occupied by solvent water (red). c) Structure of infinite -Mn2-(CN)-Mn2- chains
in 1•H2O. The color scheme is the same as Figures 2 and 3, plus red for oxygen in solvent
water.
The interlayer Mn•••Mn separation, c, is 5.78 Å for 1•H2O and 5.94 for 2•H2O, and
are >0.4 Å longer than reported for Cs2Mn[Mn(CN)6] (5.303 Å),6 [NEt4]2MnII (CN)8 (5.162
3
Å), and [NEt4]MnII (CN)7 (5.115 Å).7 This is attributed to the long axial Mn-(C/N) bond in
3
the trigonal bipyramidal sites in both materials, and/or the stacking of the square
pyramidal sites, whether ordered in 2•H2O (Figure 3a), or disordered in 1•H2O (Figure
4b). The overall 3-D topology is complicated and cannot be reduced to any simple 2-
nodal topology.
Magnetic properties
The magnetic susceptibility, c, of [NMe4]3Mn5(CN)13•S [S = H2O (1•H2O), MeOH
(1•MeOH)] and [NEtMe3]3Mn5(CN)13•H2O (2•H2O) were measured from 5 to 300 K, and
are plotted as c(T) (Figure 5), cT(T), and c-1(T) (Figure 6). The c(T) for 1•H2O, 1•MeOH,
and 2•H2O increase with decreasing temperature until they reach sharp maxima at 12.6,
10.1, and 17.1 K, respectively, before decreasing upon further cooling. The cusp-like
peaks in c(T) are suggestive of antiferromagnetic ordering. c-1(T) is linear above 60 K
with fitting the data to the Curie-Weiss expression, c µ (T - q)-1, yields a Weiss constant,
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q ~ -35 K for 1•H2O and 1•MeOH, and -40.5 K for 2•H2O. The significant negative q-
values are further evidence for antiferromagnetic coupling. 1•H2O and 1•MeOH have
room temperature cT-values of 16.18 ± 0.01 emuK/mol. 2•H2O, however, has a 10%
larger room temperature cT-value of 17.55 emuK/mol.19 Upon cooling cT(T) decreases
with increasingly greater rates at lower temperatures for each compound.
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Figure 5. c(T) for [NMe4]3Mn5(CN)13•S (S = H2O (p), MeOH) (q), and
[NEtMe3]3Mn5(CN)13•H2O (•) in a 1000 Oe applied field.
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Figure 6. a) cT(T) and b) c-1(T) for [NMe4]3Mn5(CN)13•S (S = H2O (p), MeOH) (q) and
[NEtMe3]3Mn5(CN)13•H2O (•) in a 1000 Oe applied field. Inset in (a) is d(cT)/dT for
[NMe4]3Mn5(CN)13•S (S = H2O (p), MeOH) (q), and [NEtMe3]3Mn5(CN)13•H2O (•). The
solid lines are fits to the Curie-Weiss expression (see the text).
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These room temperature cT-values for 1•H2O and 1•MeOH are substantially lower
than the spin-only, i.e. c µ T-1, expectation of i) 21.875 emuK/mol for five high-spin (hs)
MnII sites, and ii) 17.875 emuK/mol for one low-spin (ls) and four hs MnII sites, due to the
aforementioned antiferromagnetic coupling. However, above ~25 K cT(T) is well fit by
the Curie-Weiss expression, c µ (T - q)-1, with q = -35 K for 1•H2O and 1•MeOH based on
one ls and four hs MnII sites, Figure 5. The fit is much poorer when all five MnII sites are
assumed to be hs or more than one site is ls. Higher values of cT(T) were obtained for
2•H2O, Figure 6 top, which cannot be fit to one ls and four hs MnII sites.20 A good fit was
obtained for 0.5 ls and 4.5 hs MnII sites, Figure 6 top. The fractional number of high- and
low-spin sites are attributed to all five coordinate sites being hs,17 and the octahedral sites
being a mix of hs and ls, as they will be low spin when all cyanides are C-bonded, but
with several N-bonded cyanides, the site should be high spin. This suggests that more
of the cyanides are disordered than the simple structural model described above, and
and this disorder is more prevalent for 2•H2O than for either 1•H2O or 1•MeOH. The lower
symmetry of the NEtMe3 cation in 2•H2O may be a factor in this disorder.
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The cusp in the c(T) data is additional evidence for antiferromagnetic ordering.21-
24
The temperature at which the maximum in c(T) occurs lies above Tc,25,26 and Tc can
be determined from the temperature at which the maximum in d(cT)/dT occurs, i.e.,
Fisher's specific heat.27,28 The d(cT)/dT data for 1•H2O, 1•MeOH, and 2•H2O have peaks
at 11, 8.8, and 14 K, respectively, Figure 6 top inset, which are their antiferromagnetic
ordering temperatures, Tc, respectively. The higher Tc for 2•H2O is in accord with 10%
more high spin S = 5/2 Mn(II), and less low spin S = 1/2 Mn(II) being present with respect
to 1•H2O, as Tc µ S(S + 1).1,29,30
The field dependent magnetization, M(H), at 5 K from ±90 kOe is linear and is still
increasing with applied field, and is 27,000, 28,750, 38,700 emuOe/mol at 90 kOe for
1•H2O, 1•MeOH, and 2•H2O respectively, with no evidence of a saturation or a remnant
magnetization, or a coercive field, Figure 7, as is expected for an antiferromagnet.
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40000
35000
30000
25000
20000
15000
10000
5000
2•H2O
1•MeOH
1•H2O
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5 K
0
0
20000
40000
60000
80000
100000
Magnetic Field, H, Oe
Figure 7. 5 K M(H) for [NMe4]3Mn5(CN)13•S (S = H2O (p), MeOH (q)), and
[NEtMe3]3Mn5(CN)13•H2O (•).
The in-phase, real, c'(T), and out-of-phase, complex, c"(T), components of the AC
susceptibility for 1•H2O, 1•MeOH, and 2•H2O are also characteristic of antiferromagnetic
ordering, Figure 8, as c"(T) is not evident. The c'(T) data are frequency independent.
The maximum in the 33-Hz c'(T) data occur at 13.5, 10.4, and 19 K for 1•H2O, 1•MeOH,
and 2•H2O, respectively, and are a little higher, but in accord with Tc. The broader peak
for 2•H2O is attributed to Et group disorder present in the lower symmetry cation with
+
respect to NMe4 .
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Figure 8. Real, c'(T) (filled symbols), and imaginary, c"(T) (hollow symbols), AC
susceptibilities for [NMe4]3Mn5(CN)13•S [S = H2O, MeOH)] and [NEtMe3]3Mn5(CN)13•H2O
(green) at 33 (red), and 100 (green), and 1000 (blue) Hz.
The small magnetic irreversibility observed in the c"(T) data is not intrinsic. It is
attributed to the unusual nature of this antiferromagnet arising from antiferromagnetic
coupling of ferromagnetically ordered layers, and not opposing equivalent paramagnetic
spins, and is presumably due to defects sites or impurities that results in incomplete
compensation via antiferromagnetic coupling. Similarly, a small magnetic irreversibility is
observed in the M(T) data, and while unexpected for antiferromagnetic ordering materials,
it has been reported.27,31 Furthermore, this behavior has also been observed for
7
[NEt4]MnII (CN)7 and M(TCNE)[C4(CN)8]1/2 (M = Mn, Fe)32 antiferromagnets, but is not
3
observed for antiferromagnets arising from antiferromagnetic coupling of the identical spin
site, e.g. Mn(CN)2,8] or the more classical antiferromagnets.
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Triangular and pentagonal arrangements of magnetic spin sites, as occurs for
1•H2O, 1•MeOH, and 2•H2O should lead to magnetically frustrated materials.33,34
Magnetic frustration can be assessed from the figure-of-merit, f {= |q|/Tc}.33 For f < 5
frustration is minimal, but for f > 10 it is significant, as occurs for many compounds.26,33-35
The f values are <3.6 for these compounds (Table 2), and indicate that 1•H2O, 1•MeOH,
and 2•H2O are only minimally magnetically frustrated. As every triangle and pentagon
contains only one low-spin MnII site, the frustration should not be as pronounced as it
would be if all spins were identically high spin. Furthermore, significant, non-frustrated
antiferromagnetic coupling occurs between these layers via cyanide bridges. Spin glass
behavior can contribute magnetic frustration, but, as c'(T) is frequency independent, this
can be ruled out.36
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Table 2. Summary of Magnetic Properties for 1•H2O, 1•MeOH, and 2•H2O.
A
[NMe4]+
•2H2O
16.17a
21.875
17.875
19.59
16.01
16.86
-35
[NMe4]+
[NEtMe3]+
•S
•1.25MeOH •0.41H2O
16.19a
21.875
17.875
19.59
16.01
16.86
-35
17.55a
21.875
17.875
19.27
15.75
17.13
-40.5
17.1
cobsT, emuK/mol (300 K)
c
c
c
c
c
calcT, emuK/mol (spin-only): 5MnIIhs
calcT, emuK/mol (spin-only): 4MnIIhs+MnIIls
calcT, emuK/mol (CW; qb): 5MnIIhs
calcT, emuK/mol (CW; qb): 4MnIIhs+MnIIls
calcT, emuK/mol (CW; qb): 4.5MnIIhs+0.5MnIIls
q,b K (Curie-Weiss)
12.6
10.1
Tcusp [Cusp or maximum in c(T)]
Tc [Tmax for d(cT)/dT]
11
8.8
14
13.5
10.4
19
Tc, K (AC) from c'(T) (33 Hz)
33
3.6
3.4
2.9
f = |q|/Tc
M, emuOe/mol (5 K, 90 kOe), emuOe/mol
27,000a
28,750a
38,700a
a Still increasing and not saturated. b c µ (T - q)-1
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The antiferromagnetic ground states arise from antiferromagnetic coupling via the
layer of nonmagnetic cyanides that bridge between adjacent ferrimagnetic layers. Each
layer is ferrimagnetic due to the antiferromagnetic coupling between adjacent high- (S =
9
5/2) and low-spin (S = 1/2) MnII sites, as occurs for antiferromagnet [NEt4]MnII (CN)7 and
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3
ferrimagnet [NEt4]2MnII (CN)8.7 Hence, A3Mn5(CN)13 (A = NMe4, NEtMe3) are rare
3
examples of intrinsic artificial antiferromagnets (AAF),37 which are increasingly important
for the development of spin valves (albeit using extrinsic, deposition-prepared AAFs).38
These materials along with M(TCNE)[C4(CN)8]1/2 (M = Mn, Fe32 are the first intrinsic,
chemically prepared and controlled AAFs, and have the advantage of having controlled
uniform spacing between the layers as they are not physically prepared via deposition
methods.
When the unusual structure of A3Mn5(CN)13 (A = NMe4, NEtMe3) is considered
along with layered [NEt4]2MnII (CN)8, bridged-layered [NEt4]MnII (CN)7,6 and
3
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interpenetrating diamondoid Mn(CN)2,7 they can be grouped as a stoichiometrically
related AaMnII (CN)a+2b (a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and the present a = 3, b =
b
5) family each possessing an unprecedented stoichiometry and lattice motif. The
extended structure forms around the cation, and its shape and charge lead to unusual
stoichiometries and lattices. The driving force is ascribed to the very ionic nature of the
high-spin MnII ion that enables a reduced ÐMnNC to minimize the void space in the lattice.
Hence, this AaMnII (CN)a+2b family of compounds are referred to as being cation adaptive
b
which size and shape dictates both the stoichiometry and structure.
Related aspects of the differing compositions and structures have been observed
for some oxides as infinitely adaptive structures,39 in addition to dimensional reduction.40
Oxides examples include: TixO2x-1 (x < 37) [= TiOn (2 > n > 1.75)], and M2xO(16x-2)/3 (M =
Mo, Ta) [= y M10O26 + z M22O58]. This is attributed to different and shifting crystallographic
shear planes, or ordered stacking for the latter family.
Alternatively, the structures of [NMe4]3Mn5(CN)13 and [NEtMe3]3Mn5(CN)13 may be
viewed as "dimensional reduction" albeit with an increase in connectivity that is well
known for monoatomic anionic oxides/sulfides and halides.40 Hence, it is extended to
more complex anions as well as polyatomic cations. Zero dimensional (no extended
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structure) is observed for both [MnII(CN)6]4- and [MnII(CN)4]2- with Mn:CN ratios of 1:6 and
1:4, respectively, while 2-D [NEt4]2MnII (CN)8 has Mn:CN of 1:2.67, and 3-D
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K2MnII[MnII(CN)6], [NEt4]MnII (CN)7, and [NMe4]3MnII (CN)13 have Mn:CN of 1:3.0, 1:2.33,
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and 1:2.60, respectively. Such complex, but variable stoichiometries are observed for
several transition metal oxides.40
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CONCLUSIONS
The structure of A3MnII (CN)13 (A = NMe4, NEtMe3) has a complex (and in the case
5
+
of NMe4 a very disordered) structure with octahedral, and rare square pyramidal, and
trigonal bipyramidal MnII sites. It has a layer motif based on three pentagons and one
triangle perpendicularly interconnected by orientationally disordered cyanides.
A3MnII (CN)13 is key member of AaMnII (CN)a+2b (a = 0, b = 1; a = 1, b = 3; a = 2, b = 3;
5
b
and a = 3, b = 5) that form cation adaptive structures. These unusual stoichiometries and
extended lattices are attributed to the highly ionic nature of the high-spin N-bonded MnII
ion that enables the maximization of the attractive van der Waals interactions via a
reduced ÐMnNC. A3MnII (CN)13 magnetically order as antiferromagnets, and are
5
minimally magnetically frustrated. The intrinsic antiferromagnetic ordering arises from the
non-magnetic cyanide layer providing antiferromagnetic coupling between the
ferrimagnetic layers that result, as occurs for artificial antiferromagnets used in spin
valves.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/jacs.XXXXXX.
The powder X-ray crystallographic CIF files for 1•H2O (295 and 22 K) and 2•H2O
(CCDC1543750 -1543752).
A movie showing the interlayer connectivity.
AUTHOR INFORMATION
Corresponding Authors
* peter.stephens@stonybrook.edu
* jsmiller@chem.utah.edu
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DEDICATION
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Dedicated to the late Hugo Rietveld, on the fiftieth anniversary of the invention of the
technique that bears his name, which has revolutionized the applicability of powder
diffraction to crystallography
9
ORCID
Peter W. Stephens 0000-0002-8311-7305
Joel S Miller 0000-0001-5743-8327
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We appreciate the insightful discussion on network structures with Prof. Lars Öhrström (Chalmers
University of Technology), and spin frustration with Prof. Arthur P. Ramirez (University of
California, Santa Cruz) and the initial support by the Department of Energy Division of Material
Science (Grant No. DE-FG03-93ER45504) for the chemical synthesis and magnetic studies
(C.M.K., C.G.H., and J.S.M.). Use of the National Synchrotron Light Source, Brookhaven
National Laboratory for powder X-ray diffraction studies (S.H.L. and P.W.S.) was supported by
the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-
98CH10886. All authors contributed to writing of the manuscript. C.M.K. was also supported by
the NSF MRSEC Grant # DMR-1121252 CFDA NO. 47.049.
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