This journal is © The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 7621--7624 | 7621
Cite this: Chem. Commun., 2014,
50, 7621
A layered erbium phosphonate in pseudo-D
5h
symmetry exhibiting field-tunable magnetic
relaxation and optical correlation†
Min Ren,
a
Song-Song Bao,
a
Rute A. S. Ferreira,
b
Li-Min Zheng*
a
and
Luis D. Carlos*
b
A layered erbium(III) phosphonate compound, [Er(notpH
4
)(H
2
O)]ClO
4
3H
2
O (1), in which the Er
III
ion has a pseudo-D
5h
symmetry exhibits
field tunable multiple magnetic relaxation. The near-IR emission
spectrum of 1, excited at 1064 nm (Nd:YAG laser), provides a direct
probe of the crystal field splitting correlated to the magnetic data.
Lanthanide based single-molecule magnets (SMMs) have attracted
significant current interest due to the large magnetic moment and
inherent magnetic anisotropy of the 4f elements.
1
An increasing
number of Ln-SMMs have been reported in recent years, among
which some display rather high anisotropy energy barriers and
blocking temperatures.
2,3a–c
The majority of Ln-SMMs reported so
far are Dy
III
complexes. In contrast, SMMs based on Er
III
are rare,
3
although the Er
III
ion also possesses a Kramers ground state of
4
I
15/2
. The crystal field (CF) around the Er
III
ion, including the
symmetry and ligand donation, is a key factor in dictating the M
J
ground state and hence the magnetic anisotropy barrier of these
materials. Unlike the oblate Dy
III
ion, an equatorial ligand field
favors stabilization of the most prolate M
J
= 15/2 state of the Er
III
ion. As far as we are aware, Er-SMMs with CF symmetries of D
4d
,
3d
C
1
,
3a
and D
8h
3b
have been described. The exploration of new
Er-SMMs with other CF symmetries and the understanding of
their relaxation mechanisms remain to be a goal to pursue.
In this communication, we report a layered erbium phosphonate,
[Er(notpH
4
)(H
2
O)]ClO
4
3H
2
O [notpH
6
= 1,4,7-triazacyclononane-
1,4,7-triyl-tris(methylenephosphonic acid)] ( 1) and its Y-analogue
[Y(notpH
4
)(H
2
O)]ClO
4
3H
2
O( 2), in which the lanthanide ion has a
pseudo- D
5h
symmetry. Field-induced slow magnetization relaxation
is observed at low temperature. Interestingly, compound 1 also
shows field-tunable dual magnetic relaxation processes, both of
which are thermally activated. Furthermore, the near-IR emission
spectrum of 1 is also recorded, giving a direct probe to correlate the
CF splitting derived from the magnetic data.
Compound 1 crystallizes in the triclinic space group P % 1.‡
It is isostructural to compounds [Ln(notpH
4
)(H
2
O)]ClO
4
3H
2
O
(Ln = Gd, Tb),
4
showing a layered structure in which the
equivalent Er
III
ions are cross-linked by O–P–O units from the
notpH
4
2
ligands (Fig. 1). The Er atom is seven-coordinated and has
a distorted pentagonal bipyramidal geometry with D
5h
symmetry,
determined by Continuous Shape Measure (CShM) analysis.
5
The
five equatorial positions are occupied by four phosphonate oxygen
atoms (O7, O4, O2A and O6B) from three equivalent notpH
4
2
ligands and one oxygen (O1W) from the water molecule [Er–O
eq
:
2.241(3)–2.473(3) Å]. Two axial positions are filled with two phos-
phonate oxygen atoms (O1 and O8C) from two equivalent notpH
4
2
ligands [Er–O
ax
: 2.201(3)–2.243(3) Å]. The average length of the axial
bonds (2.222 Å) is much shorter than that of the equatorial bonds
(2.331 Å), indicating that the pentagonal bipyramidal geometry is
compressed in 1.
Compound 2 is isostructural to 1.‡ The Y–O
eq
and Y–O
ax
bond
lengths are 2.246(4)–2.475(5) Å and 2.191(5)–2.238(5) Å, respectively.
Compared with 1, the pentagonal-bipyramidal coordination geo-
metry around the Y
III
ion in 2 is more distorted with a larger CShM
value (1.79 vs. 1.61 for 1). The axial O1–Y1–O8C angle [172.6(2) 1] is
smaller than that in 1 [174.0(1)1].
Fig. 1 (left) The molecular structure of complex 1; (right) packing diagram
of structure 1 along the c-axis. The ClO
4
counterions, the lattice water
molecules and all H atoms are omitted for clarity. Symmetry codes: (A) x + 1,
y, z + 1; (B) x, y, z + 1; (C) x, y + 1, z + 1.
a
State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China.
E-mail: lmzheng@nju.edu.cn
b
Physics Department, CICECO, University of Aveiro, 3810-193, Aveiro, Portugal.
E-mail: lcarlos@ua.pt
† Electronic Supplementary Information (ESI) available: Synthetic procedure;
experimental setup; powder XRD; TG; additional ac susceptibility data. CCDC
992503 and 992504. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc02085e
Received 20th March 2014,
Accepted 27th May 2014
DOI: 10.1039/c4cc02085e
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