Iron(II) molecular framework materials with 4,4′-azopyridine

Article abstract of DOI:10.1071/CH04290

Three new iron(II) molecular-framework materials incorporating the bridging ligand 4,4′-azopyridine (azpy) have been synthesized and structurally characterized: Fe₂(azpy)₄(NCS)₄ ·(azpy) (A), Fe(azpy)(NCSe)₂(EtOH)₂ · (azpy) (B), and Fe(azpy)₂(NCSe)₂ · 2(MeCN) (C). A and C consist of non-interpenetrating (4,4) grids of iron(II) centres bridged by azpy ligands with non-coordinating azpy ligands or acetonitrile molecules occupying the spaces within and between the layers. For B, hydrogen-bonding interactions between coordinated ethanol molecules and non-coordinated azpy ligands link linear Fe-azpy chains to give a two-dimensional framework. CSIRO 2005.

Full text of DOI:10.1071/CH04290

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Aust. J. Chem. 2005, 58, 311–314
ꢀ
Iron(ii) Molecular Framework Materials with 4,4 -Azopyridine
A
A,B
Gregory J. Halder and Cameron J. Kepert
A
School of Chemistry, University of Sydney, Sydney NSW 2006, Australia.
Corresponding author. Email: c.kepert@chem.usyd.edu.au
B
ꢀ
Three new iron(ii) molecular-framework materials incorporating the bridging ligand 4,4 -azopyridine (azpy) have
been synthesized and structurally characterized: Fe2(azpy)4(NCS)4 · (azpy) (A), Fe(azpy)(NCSe)2(EtOH)2 · (azpy)
(
B), and Fe(azpy)2(NCSe)2 · 2(MeCN) (C). A and C consist of non-interpenetrating (4,4) grids of iron(ii) centres
bridged by azpy ligands with non-coordinating azpy ligands or acetonitrile molecules occupying the spaces within
and between the layers. For B, hydrogen-bonding interactions between coordinated ethanol molecules and non-
coordinated azpy ligands link linear Fe–azpy chains to give a two-dimensional framework.
Manuscript received: 21 December 2004.
Final version: 14 March 2005.
Interest in molecular-framework materials constructed from
metal ions and organic bridging ligands continues to
broaden as their diverse properties and subsequent poten-
grids constructed from Fe(ii) centres linked by four equatori-
ally coordinated azpy ligands, with axial thiocyanate ligands
completing the octahedral coordination sphere. Unlike in
Dguest, these grids do not interpenetrate but stack upon one
another parallel to the (103) plane, with an interlayer sepa-
ration of 4.48 Å (Fig. 1a). The asymmetric unit contains two
Fe(ii) centres located on special positions, each with one thio-
cyanate, one 1/2 azpy and one 1/4 azpy, defining crystallo-
graphically distinct alternate rhombic grids. The Fe1 and Fe2
octahedra are both compressed toward the thiocyanate lig-
ands and their corresponding bond lengths are characteristic
ofhigh-spinFe(ii)andagreetowithintwostandarddeviations
(Fe1–N(1/2 azpy) 2.267(6) Å, Fe2–N(1/2 azpy) 2.252(6) Å,
Fe1–N(1/4 azpy) 2.204(7) Å, Fe2–N(1/4 azpy) 2.207(7) Å,
Fe1–N(CS) 2.069(7) Å, Fe2–N(CS) 2.081(8) Å). The major-
[1,2]
tial applications continue to be realized.
Already there
are many examples of these materials that have been designed
with specific structures and interesting properties such as
porosity,[
3–6]
chirality, and magnetism.
[7]
[8,9]
With an interest in combining spin switching with
nanoporosity we have recently focussed synthetic efforts on
the incorporation of spin crossover centres into open molec-
ular frameworks.[
(
10,11]
The first such material, [Fe2(azpy)4-
[10]
NCS)4] · (guest) (Dguest),
two-dimensional grids of Fe(ii) centres linked by azpy ligands
Scheme 1). The coexistence of both nanoporosity and spin
consists of interpenetrating
(
crossover Fe(ii) centres in Dguest has resulted in a material
where the spin transition is ‘triggered’ by sorption of a guest
alcoholmolecule. Here, furtherinvestigationsofFe(ii)frame-
work materials with the azpy ligand have been undertaken,
including attempted preparations of selenocyanate analogues
of Dguest, with the expectation that the resulting increased
crystal-field splitting may yield materials with higher spin
crossover temperatures.[
◦
ity of the angles about the octahedra are ideal (90 ), with
only a slight distortion in the angle between the thiocyanate
◦
and the 1/2 azpy (87.1(2) and 89.4(3) for Fe1 and Fe2,
respectively). Both 1/4 azpy ligands of the asymmetric unit
define two-fold disordered azo bonds, where the four nitro-
gen positions are defined by a single atom (N4 or N14).
Non-coordinated azpy ligands occupy linear channels along
the crystallographic c-axis. The atom positions of the guest
azpy molecule were clearly identified from the difference
map but required several restraints for stable refinement. No
significant framework–guest interactions were evident in the
structure. Without sufficient product, the potentially interest-
ing magnetic and guest-exchange properties of A could not
be explored.
12]
[
Fe2(azpy)4(NCS)4] · (azpy) (A) was isolated as a minor
product from the preparation of D1-PrOH as brown platelet
crystals. Subsequent attempts to intentionally synthesize A
with appropriately modified stoichiometry were unsuccess-
ful, with repeated syntheses exclusively producing D1-PrOH.
The structure of A was solved and refined in the monoclinic
space group C2/m. The refinement revealed rhombic (4,4)
The first attempt to synthesize selenocyanate analogues
involved mimicking the preparation of DEtOH and sub-
stituting ammonium thiocyanate with potassium seleno-
cyanate. These diffusions yielded orange crystals that
have been structurally characterized as [Fe(azpy)(NCSe)2-
N
N
ꢀ
Scheme 1. Structure of 4,4 -azopyridine (azpy).
(EtOH) ] · (azpy) (B), revealing a structural motif very
2
©
CSIRO 2005
10.1071/CH04290
0004-9425/05/050311

3
12
G. J. Halder and C. J. Kepert
similar to that reported for the material [Fe(azpy)(NCS)2-
to be Fe(azpy)2(NCSe)2 · 2(MeCN) (C). The structure was
solved and refined in the monoclinic space group C2/m. The
structure consists of rhombic (4,4) grids of Fe(ii) centres
linked by the equatorial coordination of four azpy ligands,
with two axial selenocyanate ligands completing the
compressed octahedral coordination sphere (Fe1–N11(NCS)
2.115(2) Å, Fe1–N1(azpy) 2.212(7) Å, Fe1–N3(azpy)
2.237(7) Å; N–Fe(ii)–N angles range from 86.1(4) to
[13,14]
(
MeOH)2] · (azpy).
A similar supramolecular pattern
ꢀ
has also been observed in several 4,4 -bipyridine-containing
coordination frameworks.[
15–18]
For B, linear chains of
Fe(ii) centres are linked by azpy ligands, with pairs of
trans-coordinated selenocyanate anions and oxygen bound
ethanol molecules completing the octahedral coordination
sphere. All non-hydrogen atoms were modelled anisotropi-
cally and no disorder was observed in the structural refine-
ment. The Fe(ii) octahedra are elongated toward the azpy
ligands (Fe–N(azpy) 2.462(19) Å, Fe–N(CSe) 2.117(2) Å,
Fe–O(EtOH) 2.1148(17) Å), and are mildly distorted with
◦
93.79(8) ).Thestructurallyequivalenttwo-dimensionalgrids
stack in a similar fashion to that observed forA (Fig. 1c), with
acetonitrile guest molecules zig-zagging through the layers
(Fig. 1d). Subsequent layers are spaced 4.42 Å apart, slightly
closerthanthe4.48 ÅspacingobservedforA.Theacetonitrile
guest molecules exhibit two-fold symmetry related disor-
der. A head-to-tail hydrogen-bonding interaction was located
◦
N/O–Fe–N/O angles ranging from 86.91(7) to 93.09(7) .
Hydrogen-bonding interactions between the coordinated
ethanol molecules and unbound azpy ligands (O12 · · · N3
ꢀ
2
.698 Å) link parallel chains to form a pseudo two-
between neighbouring acetonitrile molecules (C21 · · · N21
dimensional grid structure (Fig. 1b). These grids stack paral-
lel to one another in the (101) direction. The coordination of
ethanol deems the material to be of insufficient interest for
both magnetic and guest-exchange studies.
Attempts to synthesize selenocyanate analogues of Dguest
were also carried out with several other solvents (methanol,
acetone, acetonitrile, 1-propanol); however, only diffusions
with acetonitrile were successful in producing single crystals.
These brown platelet crystals differed greatly in morphology
from the dark-blue rod-shaped crystals observed for Dguest
and were structurally characterized by X-ray diffraction
3.445(9) Å). Calculations of the channel volume and com-
[
19]
position within PLATON
suggest a solvent-accessible
volume of 29.7% and an electron population of 19 electrons
per cavity, in good agreement with the expected value of 22
electrons per acetonitrile molecule.
Variable temperature magnetic susceptibility measure-
ments were employed to investigate the spin state of C. The
magneticmomentremainedapproximatelyconstantat5.4 µB
on cooling from 225 to 15 K.These values are consistent with
the presence of only high-spin Fe(ii) sites (S = 2), with no
indication of a spin-transition.
(
a)
(b)
(
c)
(d )
Fig. 1. Stick representations of the three crystal structures. (a) Stacking of four layers in A and the uncoordinated azpy ligands, the latter shown
in a lighter shade. (b) Two-dimensional coordination and hydrogen-bonded layer in the structure of B (hydrogen bonds are shown as dashed lines).
(
c) Stacking of the layers in C with subsequent layers coloured differently. (d) Linear channels in C occupied by acetonitrile guest molecules (space
filling representation). All hydrogen atoms, except those of the ethanol OH in B, have been omitted for clarity.

ꢀ
Iron(ii) Molecular Framework Materials with 4,4 -Azopyridine
313
Despite a similar iron(ii) coordination geometry to that
present in Dguest, no spin crossover was observed for C. Geo-
metric structural parameters that may influence the occur-
rence of spin crossover include the average iron(ii)–nitrogen
bond length (dFe−N), the global octahedral distortion (Σ: the
sum of the deviation of each of the 12 cis angles) and the
torsion angle of the pyridyl groups with respect to the equato-
Three new molecular-framework materials incorporating
iron(ii) and azpy have been synthesized and structurally
characterized; A and C consist of non-interpenetrating (4,4)
grids of iron(ii) centres bridged by azpy ligands, while in
B, hydrogen-bonding interactions between bound ethanol
molecules and unbound azpy link linear Fe–azpy chains to
give an overall two-dimensional structure.These results high-
light the sensitivity of the self-assembly process that the
synthesis of systems of this type rely on, with subtle changes
in the chemical reagents or diffusion conditions resulting in
an array of different structures. However, the synthesis of new
and different examples of framework materials continues to
clarifyourunderstandingoftherangeandrelativeimportance
of the interactions between metals, ligands, and solvents.
[
20]
rial plane of the octahedral coordination sphere (ψ). Short
◦
dFe−N distances, minimal Σ and ψ angles approaching 45
(
allowing maximum π back-bonding) strengthen the crystal
field and stabilize the low-spin configuration. These struc-
[
10]
tural parameters for C, the high-spin structure of DEtOH
and the desolvated material D
[10]
are included in Table 1.
There is no strong relationship between these parameters
and the existence of spin crossover; however, those of the
spin crossover Fe2 centre of DEtOH are collectively more
favourable. For C, the parameters more closely resemble
those of the spin crossover inactive Fe1 centre of DEtOH. In a
recent study of an extensive series of discrete [FeLn(NCS)2]
Experimental
Ligand Synthesis
The synthesis of azpy was adapted from published methods.[22,23]
An aqueous solution (200 mL) of 4-aminopyridine (16.9 g, 92 mmol)
was added over 40 min to an aqueous sodium hypochlorite solution
(
L = N-donor ligand, n = 2 (bidentate) or 4 (unidentate))
complexes, a similar range of inter- and intra-molecular
structural properties were tabulated and compared to spin
crossover features.[ Again, no single structural reason was
found to explain why spin crossover occurs in some com-
plexes and not others, highlighting the subtle nature of the
phenomenon.
◦
◦
(
12.5% w/v, 1200 mL) at 0 C. The reaction was stirred at 0 C for a
further 20 min. The orange product was extracted with diethyl ether
(1500 mL) and evaporated to dryness on a rotary evaporator. The
21]
crude product was recrystallized from boiling H2O (200 mL). Yield
2
1
0.7 g (64%). δH (200 MHz, CDCl3, TMS): 8.87 (d, 4H, J 6), 7.75
2
(
d, 4H, J 6).
Framework Syntheses
Table 1. Selected structural parameters for the high-spin struc-
tures of C, DEtOH,[10] and the desolvated material D. [10] Of the two
crystallographically distinct iron(ii) sites in DEtOH only the Fe2 site
exhibits spin crossover
A: A diffusion of azpy (90 mg, 0.50 mmol) and a mixture of iron(ii)
perchlorate (42 mg, 0.17 mmol) and ammonium thiocyanate (25 mg,
0.33 mmol) in 1-propanol gave brown platelet crystals of A as a minor
product (major product: D1-PrOH). Yield approx. 20 mg (19%).
B: Diffusions of azpy (40 mg, 0.22 mmol) and a mixture of iron(ii)
perchlorate (28 mg, 0.11 mmol) and potassium selenocyanate (31 mg,
◦
◦
Structure
SCO
dFe−N [Å]
Σ [ ]
ψ [ ]
C
No
No
Yes
No
2.188
2.170
2.167
2.252
16
21
10
26
60
56
52
47
0
5
.22 mmol) in ethanol gave dark yellow crystals of B. Yield approx.
5 mg (70%).
DEtOH (Fe1)
DEtOH (Fe2)
D
C: Diffusions of azpy (40 mg, 0.22 mmol) and a mixture of iron(ii)
perchlorate (28 mg, 0.11 mmol) and potassium selenocyanate (31 mg,
Table 2. Crystallographic data for A, B, and C
B
Parameter
Formula
A
C
Fe2(C10N4H8)4(NCS)4 · (C10N4H8)
Fe(C10N4H8)(NCSe)2(C2H6O)2 · (C10N4H8)
Fe(C10N4H8)2(NCSe)2 · 2(CH3CN)
M [g mol 1]
−
1265.04
150(2)
726.35
150(2)
Monoclinic
P21/c (no. 14)
7.2556(19)
17.061(4)
12.298(3)
96.843(4)
1511.5(7)
2
716.33
150(2)
T [K]
Crystal system
Space group
a [Å]
Monoclinic
C2/m (no. 12)
22.514(5)
13.402(3)
10.922(2)
101.613(4)
3228.0(12)
2
Monoclinic
C2/m (no. 12)
9.2518(16)
20.446(4)
8.5790(15)
103.215(3)
1579.9(5)
2
b [Å]
c [Å]
◦
β [ ]
3
V [Å ]
Z
ρcalc [Mg m 3]
−
1.301
0.633
1.596
2.949
1.506
2.818
−1
µ [mm
]
Data
4002
3573
1940
Restraints
54
0
0
Parameters
231
191
108
R(F) {I > 2σI}
R(F) {all data}
0.0798
0.1535
0.2225
0.3119
1.051
0.0344
0.0426
0.0857
0.0915
1.098
0.0344
0.0435
0.0775
0.0847
1.092
2
Rw(F ) {I > 2σI}
2
Rw(F ) {all data}
GoF

3
14
G. J. Halder and C. J. Kepert
0
.22 mmol) in acetonitrile gave a small number of orange platelet
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with MoKα (λ 0.71073 Å) radiation and nitrogen (100–375 K) and
helium (25–375 K) cryostreams (Oxford Instruments). Crystals were
mounted on a mohair fibre in a thin film of perfluoropolyether oil and
quench cooled to 150 K. Full spheres of data were collected over a range
of incident angles, up to 2000 frames, with a 30 s exposure time per
frame. Empirical absorption corrections were applied to all data using
[
[
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22, 5158. doi:10.1021/JA993814S
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1
075948
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[
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