End-to-End Azido-Bridged Lanthanide Chain Complexes (Dy, Er, Gd, and Y) with a Pentadentate Schiff-Base [N3O2] Ligand: Synthesis, Structure, and Magnetism

Article

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

pubs.acs.org/IC

End-to-End Azido-Bridged Lanthanide Chain Complexes (Dy, Er, Gd,

and Y) with a Pentadentate Schiﬀ-Base [N O ] Ligand: Synthesis,

3

2

Structure, and Magnetism

†

,†,⊥

†,‡,§

Roman D. Svetogorov,^§

Tamara A. Bazhenova, Vladimir S. Mironov,*

Ilya A. Yakushev,

Olga V. Maximova,^†Yuriy V. Manakin, Alexey B. Kornev, Alexander N. Vasiliev,

,∥,#

†

∥,#,○

,

†

and Eduard B. Yagubskii*

†

Institute of Problems of Chemical Physics IPCP RAS, Chernogolovka 142432, Russia

Kurnakov Institute of General and Inorganic Chemistry IGIC RAS, Moscow 119991, Russia

National Research Center “Kurchatov Institute”, Moscow 123182, Russia

Lomonosov Moscow State University, Moscow 119991, Russia

‡

§

∥

⊥

Shubnikov Institute of Crystallography of Federal Scientiﬁc Research Centre “Crystallography and Photonics” RAS, Moscow

19333, Russia,

1

#

National University of Science and Technology “MISiS”, Moscow 119049, Russia

○

National Research South Ural State University, Chelyabinsk 454080, Russia

S Supporting Information

*

ABSTRACT: The syntheses, structure and magnetic proper-

ties are reported for ﬁve novel 1D polymeric azido-bridged

lanthanide complexes with the general formula {[Ln-

(

DAPMBH)(N )C H OH]C H OH} where H DAPMBH

2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone)a

3 2

3 2 5 2 5 n 2

=

new pentadentate pyridine-base [N O ] ligand and Ln = Dy

(

1), Y₀_.₉₃₀Dy₀_.₀₇₀(2), Er (3), Y₀_.₉₂₃Er₀_.₀₇₇(4), and Gd (5). X-

ray diﬀraction analysis of 1−5 show that the central

lanthanide atoms are eight-coordinated with the N O

5

3

donor set originating from the ligand DAPMBH, one

−

coordinated ethanol molecule and two end-to-end type N₃

bridges connecting the metal centers into inﬁnite chain. The [LnN O ] coordination polyhedron can be regarded as a distorted

5

3

dodecahedron (D ). AC magnetic measurements revealed that compounds 1−4 show ﬁeld-induced single-molecule magnet

2d

behavior, with estimated energy barriers U ≈ 47−17 K. The experimental study of magnetic properties was complemented by

eff

theoretical analysis based on crystal-ﬁeld calculations. Direct current magnetic susceptibility studies revealed marginally weak

intrachain exchange interaction between Ln³ions mediated by the end-to-end azide bridging groups (J ≈ −0.015 cm for 5).

Comparative analysis of static and dynamic magnetic properties of magnetically concentrated (1, 3) and diluted (2, 4) Dy and

Er compounds showed that, despite fascinating 1D azido-bridged chain structure, compounds 1 and 3 are not single-chain

+

−1

3

+

magnets; their magnetic behavior is largely due to single-ion magnetic anisotropy of individual Ln ions.

INTRODUCTION

are characterized by a spin reversal energy barrier, U , and

eff

■

blocking temperature, T , below which the magnetization of

Lanthanide coordination compounds have been the subject of

extensive research due to their remarkable, unique phys-

icochemical properties which are of great interest for designing

B

the magnetic molecule is blocked and retained for a long

period of time. Lanthanide SMMs have attracted increasing

attention since 2003 with the discovery of slow magnetic

1

2

3

4

new optical, electronic, catalytic, magnetic cooling, and

5

molecular magnetic materials. Over the past years, low-

9

relaxation in a bis(phthalocyaninato)lanthanide complex.

dimensional molecular lanthanide compounds featuring slow

relaxation of the magnetization have become highly relevant

owing to their applications in advanced new technologies such

Nowadays, lanthanide ions are recognized as unique spin

units for designing SMMs due to their large unquenched

orbital momentum, strong spin−orbit coupling and crystal-

6

as high-density information data storage, molecular spin-

6

b,7

6a,8

tronics,

and quantum computation;

they are known as

single-molecule magnets (SMMs, 0D compounds) and single-

Received: September 24, 2019

5

−8

chain magnets (SCMs, 1D compounds).

SMMs and SCMs

©

XXXX American Chemical Society

A

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Inorganic Chemistry

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1

12

ﬁeld (CF) eﬀect producing together very strong single-ion

magnetic anisotropy.

cm⁻was observed in, which seems to be close to the natural

limit of the CF splitting energy for 4f-electrons. Larger CF

splitting energies were debated for isolated small lanthanide

In the past decade, considerable eﬀort has been directed

toward synthesis and study of mononuclear lanthanide

complexes with a SMM behavior, which are commonly

13

molecules with short metal−ligand distances. This implies

that for mononuclear Ln complexes the barrier cannot be

scaled up far beyond a certain value (ca. 1500 cm ).

10

−1

referred to as single-ion magnets (SIMs). A large number

5

−12

of Ln-based SIMs have been reported,

many of which

Therefore, an alternative way to enhance SMM performance

of lanthanide complexes is to increase the strength of spin

coupling between Ln ions. Generally, this is a challenging

problem due to extremely localized nature of 4f electrons

leading to a weak magnetic coupling of Ln ions with other

spin carriers. More favorable situation occurs for radical

bridged lanthanide complexes providing more eﬃcient

magnetic coupling between Ln ions due to considerably

−

1 11

exhibit exceptionally high energy barriers (U = 1277 cm

) and blocking temperatures (T = 60 K and 80

K ), which are far superior to those of d-block SMMs. In Ln-

;

eff

−1 12

11

1

541 cm

B

1

2

3

+

SIMs, the barrier U_e_f_fis controlled by the CF splitting pattern

3+

of the ground J-multiplet of the Ln ion, which depends on

the local geometry of the ligand coordination. Maximum

barriers are provided by ligand coordinations with a distinct

1

0c,d,f

1

18

axial symmetry;

this is especially true for linear or quasi-

stronger metal−radical exchange interaction.

1,12

linear coordination,

degenerate ground state (such as m = ± / for Dy ) with the

maximum projection of total angular momentum J correspond-

which stabilizes the m = ±J doubly

The proper choice of bridging ligands is crucially important

to increase the spin coupling. In this respect, the azide ligand is

of special interest as it oﬀers rich possibilities for assembling

J

15

3+

J

2

19

ing to pure uniaxial magnetic anisotropy. Besides, linear

metal ions into polyatomic exchange-coupled clusters. Like

other pseudohalides (CN , NCS ), the azide anion is often

used as a bridging ligand in the design of polynuclear

3

+

−

coordination of Ln ion ensures the maximum energy spacing

between the ground CF state m = ± J and the upper CF state

J

3

+

20

m = ± 1/2 or 0 (for Kramers and non-Kramers Ln ions),

transition-metal complexes. Spin coupling mediated by

J

which correspond, respectively, to the bottom and top of the

double-well potential. This results in a perfect Ising-type

azide bridging ligands depends strongly on the coordination

mode: it is predominantly ferromagnetic for the end-on mode

1

9−22

single-ion magnetic anisotropy and high energy barrier U ; the

and antiferromagnetic for the end-to-end mode.

A

eff

latter considerably increases for short metal−ligand distan-

plethora of azide metal complexes was reported, mainly for

transition metals, including clusters, chains, and extended 2D

1

3

ces.

19−22

The pentagonal bipyramidal (PBP) coordination with D₅_h

and 3D magnetic structures.

By contrast, lanthanide azide

symmetry can provide similar uniaxial CF splitting pattern of

Ln ions and thus it is also attractive for obtaining high U_e_f_f

complexes are much less common; in fact, only a limited

number of Ln azide complexes are presently known.

3+

23

and T values; for instance, the PBP dysprosium(III) complex

Recently, tetranuclear lanthanide (Gd, Dy) azide complexes

have been characterized structurally and magnetically, in which

Ln ions are bridged by azide ligands in the end-to-end mode

B

+

[

(

Dy(OtBu) (py) ] (py = pyridyl) exhibits very high barrier

2

5

−

1

14

1262 cm ) and blocking temperature of T = 14 K. It is

B

24a

also essential that PBP lanthanide complexes are structurally

more robust than linear Ln complexes and thus they are more

resulting in a weak Ln-Ln exchange interaction.

With all of this in mind, we have undertaken synthesis of

new lanthanide complexes with pentadentate chelating ligand

involving at the same time azide ligands. This work is

motivated by idea to combine strong uniaxial magnetic

anisotropy of pentagonal Ln complexes with advantages of

azide bridging ligands; besides, in contrast to numerous

reported low-dimensional lanthanide Schiﬀ-base complex-

15

suitable for designing polymetallic magnetic structures. This

especially concerns lanthanide complexes with pentadentate

chelating ligands supporting pentagonal coordination of the Ln

ion in the equatorial plane, which is supplemented by two

apical ligands to form a PBP coordination. Recently, several

mono- and binuclear dysprosium complexes with the H dapsc,

2

5

b−g

H dapbh, and H daps pentadentate ligands (Figure 1) were

es,

Ln complexes with pentadentate Schiﬀ-base ligands

2

4

16

synthesized and magnetically characterized; some of them

are still less common. Our eﬀorts have resulted in obtaining

Dy, Er, Gd, and Y chain complexes with end-to-end azide

bridging groups. In the present work, we report syntheses of

16

exhibit SMM behavior.

new pentadentate ligand H DAPMBH = 2,6-diacetylpyridine

2

bis(4-methoxybenzoylhydrazone) which is one more repre-

sentative of the family of the opened acyclic N O ligands on

3

2

the base of 2,6-diacethylpyridine, Figure 1, and studied its

reactions with DyCl and ErCl in ethanol. We found that the

3

addition of azide ion to the reaction solution led to the

formation of polymeric azido-bridged Ln(III) chain complexes

of general formula {[Ln(DAPMBH)(N )C H OH]C H OH}

n

^Fⁱ^g^u^r^e¹^.²^,⁶^-^Dⁱ^a^c^e^t^y^l^p^y^rⁱ^dⁱⁿ^e^-^b^a^s^e^d^a^c^y^c^lⁱ^c^lⁱ^g^aⁿ^d^s^.^R⁼^N^H2

(

H dapsc), C H (H dapbh), 2-OHC H (H daps), and 4-

2 6 5 2 6 4 4

3

2

5

2

5

OCH C H (L = H DAPMBH). The latter ligand is used in this

3

6

4

2

where H DAPMBH = 2,6-diacetylpyridine bis(4-methoxyben-

2

work.

zoylhydrazone) and Ln = Dy (1), Y₀_.₉₃₀Dy₀_.₀₇₀(2), Er (3),

Y₀_.₉₂₃Er₀_.₀₇₇(4), and Gd (5). Compounds 2 and 4 containing

3

+

3+

It is important to emphasize that, although mononuclear Ln

magnetic Dy and Er ions doped into the isostructural

diamagnetic yttrium(III) host compound {[Y(DAPMBH)-

(N )C H OH]C H OH} were synthesized with the aim to

complexes demonstrate record U_e_f_fand T values, there is a

B

fundamental upper limit for the barrier related to the fact that

maximum value of U_e_f_fis determined by the total CF splitting

energy of the ground J-multiplet, whose value is typically

within few hundreds of wavenumbers; only in few cases it

3

2

5

2

5

n

compare static and dynamic magnetic behavior of magnetically

concentrated (1, 3) and diluted (2, 4) lanthanide compounds

in order to examine the presence/absence of SCM behavior in

the end-to-end azido bridged lanthanide chains. Gadolinium

−1

17

exceeds 1000 cm . The maximum barrier of U_e_f_f= 1541

B

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C

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Table 2. Selected Bond Distances (Å) for Complexes 1−5

¹Dy

2_Y(Y−Dy)

2_D_y(Y−Dy)

3_E_r

4_Y(Y−Er)

4_E_r(Y−Er)

5_G_d

M(1)−O(3)

M(1)−O(2)

M(1)−O(1)

M(1)−N(3)

M(1)−N(1)

M(1)−N(5)

M(1)−N(6)

M(1)−N(7)

2.259(2)

2.294(2)

2.433(2)

2.441(3)

2.460(3)

2.474(2)

2.484(2)

2.492(2)

2.258(3)

2.280(3)

2.415(3)

2.427(3)

2.456(3)

2.463(3)

2.476(3)

2.482(3)

2.22(2)

2.29(2)

2.42(2)

2.45(2)

2.440(18)

2.46(2)

2.49(2)

2.244(5)

2.268(4)

2.402(5)

2.414(6)

2.423(6)

2.450(6)

2.450(5)

2.468(5)

2.250(4)

2.284(4)

2.418(5)

2.434(5)

2.447(5)

2.468(5)

2.474(4)

2.476(4)

2.27(2)

2.28(2)

2.40(2)

2.41(2)

2.46(2)

2.47(2)

2.49(2)

2.2852(18)

2.3173(18)

2.448(2)

2.453(3)

2.492(3)

2.504(2)

2.510(2)

2.521(2)

a

Symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z.

Figure 2. (a) Crystal structure of complexes 1−5, thermal ellipsoids are shown at 50% probability level. (b) Typical view of 1-D polymeric chain of

the complexes 1−5. All hydrogen atoms and solvated ethanol molecules (except coordinated on metal atoms) are omitted for clarity (Ln = Dy, Er,

Gd, Y).

compound 5 was prepared for a straightforward assessment of

the eﬃciency of the spin coupling between Ln³ions through

azide bridging group. We also report the structures and dc and

ac magnetic properties of these ﬁve compounds, which are

supplemented with a detailed theoretical analysis.

To the best of our knowledge, these complexes are the ﬁrst

example of azido-bridged lanthanide chain compounds with a

fascinating molecular structure.

of the reaction mixture, apparently due to the replacement of

chloride ligands by azide ion and the formation of a crystal

neutral polymeric azido-bridged chain.

+

In the solid state IR spectra of all complexes, there is, in

addition to a set of vibrations characteristic of a fully

deprotonated DAPMBH ligand coordinated to a metal, a

−

1

very intense band near 2100 cm , reﬂecting the presence of

azides that act as end-on bridges between Ln atoms in these

compounds. IR spectra of compounds 2 and 4 have two

distinctive maxima in the region. This conﬁrms the existence of

RESULTS AND DISCUSSION

■

two types of bridges, Y−N −Y and Y−N −Ln.

Synthesis. The all complexes described in this work were

obtained using the new representative of the pentadentate

ligand family with the [N O ] binding node, 1,1′- (pyridine-

3

Crystal Structure Descriptions. Single crystal X-ray

diﬀraction analyses revealed that these ﬁve compounds are

isostructural and crystallize in the orthorhombic space group

3

2

,6-diyl)bis(ethan-1-yl)-1-ylidene))di-4-methoxybenzohydra-

P2 2 2 . The phase purity of bulk samples 1−5 used for

zine ([H2DAPMBH]) that was synthesized by us via a simple

hydrazine condensation reaction between 1 equiv of 2,6-

diacetylpyridine and 2 equiv of 4-methoxybenzoylhydrazine in

1 1 1

magnetic measurements was con ﬁrmed by the X-ray powder

diﬀraction data (Figures S1 −S5, Supporting Information).

Detailed crystallographic parameters are given in Table 1 and

selected bond lengths of 1−5 are given in Table 2.

9

6% ethanol by analogy with previously published syntheses of

25

related ligands.

The composition of compound was

conﬁrmed by elemental analysis and IR spectra.

The asymmetric unit of these ﬁve compounds contains one

III

The complexes 1−5 were prepared via mixing of rare-earth

Ln core (Dy, Gd, Er or Y/Dy, Y/Er) ions, one anionic

metal chlorides (or bromidefor Gd) and a ligand

DAPMBH ligand, one bridging azide ligand, and two ethanol

molecules, one of them coordinated on the rare-earth metal

atom. Each eight-coordinated metal atom is surrounded by

three nitrogen atoms and two oxygen atoms of the negative

H DAPMBH in absolute ethanol followed by addition of

2

Et N to deprotonate the ligand and obtain most likely the

3

compound of the following composition [Ln(DAPMBH)-

+

16a

5

2−

Cl ]−(Et H)N

that is readily soluble in ethyl alcohol. The

charged almost planar η -DAPMBH , two nitrogen atom

2

3

2

−

addition of NaN to this solution leads to a change in the color

from azide ligands μ -N , and one oxygen atom from the

3

D

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Inorganic Chemistry

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nearest ethanol molecule (Figure 2a). The shape analysis of

these octacoordinate compounds has shown the most relevant

polyhedron is the triangular dodecahedron (TDD) (Table S1).

In general the environment of each metal ion is N O . M−N

5

3

and M−O distances are in the range 2.41(2)−2.521(2) and

.22(2)−2.3173(18) Å, respectively, except for extra coordi-

2

nated ethanol with a longer M−O distance (2.40(2)−2.448(2)

Å). Metal atoms and azide ligands forms inﬁnite 1-D polymeric

chains along the a-axes with metal−metal distances ranging

from 7.0000(6) to 7.07180(10) Å, which prevent eﬃcient

interaction even between closest metal atoms in M−N −M

3

chains (Figure 2b). Inspections of the hydrogen bonds in

complexes 1−5 shows that the interlayer ethanol molecules as

hydrogen-bonding connectors join molecular chains via strong

O(1)−H(1)···O(6) and O(6)−H(6A)···N(4)#3 (symmetry

Figure 4. Temperature dependence of χT for

2

(Y₀_.₉₃Dy₀_.₀₇(DAPMBH)) measured at H = 0.1 T (black open circles).

−

4

χ_T_I_P= 3.61 × 10 emu/mol. The solid red curve corresponds to the

experimental χT product of 1 (Figure 3) scaled by a factor of 0.072

1

code #3 −x + 1, y + / , −z + / ; additional information is

2

(

with some extra TIP correction). In the inset: magnetization vs ﬁeld

given in the Supporting Information ) hydrogen bonds, Tables

S2 −S6 . The structural studies also revealed the same distances

and coordination environment for positional disordered metal

atoms in 2 and 4 with interatomic distances less than 0.05 Å

for 2 measured at T = 2.2, 3, and 4 K (open circles).

(

Y(1)−Dy(1) 0.04(2); Y(1)−Er(1) 0.03(3)).

Static Magnetic Properties. The temperature depend-

ence of magnetic susceptibility for complexes 1−5 were

measured under dc applied ﬁeld of 1000 Oe in the temperature

range of 2−300 K in ﬁeld cooled (FC) mode, as shown in

Figures 3−7. The χT products at 300 K for complexes 1, 3, 5

Figure 5. Temperature dependence of χT for 3 (Er(DAPMBH))

−

4

measured at H = 0.1 T (black open circles). χ_T_I_P= 3.67 × 10 emu/

mol. Simulated temperature dependence of χT is shown in the solid

red line (see text for detail, eqs 1−6 and Table 3). In the inset:

magnetization vs ﬁeld for 3 measured at T = 2, 3, and 4 K (open

circles).

Figure 3. Experimental temperature dependence of the χT product

for 1 (Dy(DAPMBH)) measured at H = 0.1 T (black open circles);

Temperature-independent correction of χ_T_I_P= 3.68 × 10⁻emu/mol

is applied. Simulated temperature dependence of χT is shown in the

solid red line (see text for detail, eqs 1-6 and Table 3). In the inset:

magnetization vs ﬁeld for 1 measured at T = 2, 3, and 4 K (open

circles).

4

3

−1

(

13.8, 10.9, and 8.1 cm K mol , respectively) are close to the

3+

3+ 6

expected values for free Ln ions, 14.17 (Dy , H₁₅_/₂), 11.48

Er , I ) and 7.87 cm K mol (Gd , S₇_/₂). The χT

3

+

4

3

−1

3+ 8

(

Figure 6. Temperature dependence of χT for

4

1

5/2

product for compounds 1−4 gradually decreases with

temperature lowering down to 100 K and then rapidly drops

below 100 K being characteristic of many lanthanide

complexes (Figures 3−6). Closer inspection of the dc data

for magnetically diluted compounds 2 and 4 (Figures 4 and 6)

reveals that their magnetic susceptibilities per one magnetic

Ln ion are virtually identical to those of the corresponding

concentrated compounds 1 and 3 (Figures 3 and 5). In fact,

the experimental χT curves of 2 and 4 are well reproduced by

scaling the respective experimental χT data of 1 and 3 with the

concentration factors of 0.072 (for 2) and 0.061 (for 4), which

are fairly close to the corresponding nominal concentrations

(Y₀_.₉₂₃Er₀_.₀₇₇(DAPMBH)) measured at H = 0.1 T (black open

−

4

circles). χTIP = 3.61 × 10 emu/mol. The solid red curve corresponds

to the experimental χT product of 3 (Figure 5) scaled by a factor of

0

.061 (with some extra TIP correction). In the inset: magnetization vs

ﬁeld for 4 measured at T = 2.2, 3, and 4 K (open circles).

3+

that the crystal-ﬁeld (CF) splitting patterns of Dy³⁺and Er³⁺

ions vary insigniﬁcantly upon dilution in the diamagnetic

yttrium host compound. The value of χT for the Gd complex 5

is constant in temperature range of 300−10 K that is typical of

3

+

isolated magnetically isotropic Gd ions (Figure 7). Subtle

decrease in χT at low temperatures (T < 6 K) may indicate

very weak intrachain antiferromagnetic spin coupling in 5, J ≈

(

0.07 and 0.077, respectively), Figures 4 and 6. This suggests

E

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this point is examined in more detail by means of comparison

of ac magnetic properties of concentrated Dy and Er chain

compounds 1 and 3 and their magnetically diluted counter-

parts 2 and 4.

The ﬁeld dependencies of the magnetization (M/μ vs H/T)

B

for all complexes have been measured at 2, 3, and 4 K in the

ﬁeld range of 0−7 T as shown in the insets in Figures 3−7.

The magnetization of 1−4 does not saturate and reaches the

values of 5.39 μ , 7.46 μ , 5.46 μ , and 4.1 μ , respectively, at 7

B

T and 2 K; such behavior was observed in many Ln complexes.

The magnetic ﬁeld dependences of magnetization for

complexes 1−4 plotted in M vs H/T axes at diﬀerent

temperatures do not superimpose, indicating the presence of

considerable magnetic anisotropy in the complexes. In

contrast, the magnetization of isostructural Gd complex 5

Figure 7. Temperature dependence of χT for 5 (Gd(DAPMBH))

−

4

measured at H = 0.1 T (open black circles). χ_T_I_P= 3.61 × 10 emu/

reaches the saturation value of 7 μ at 7 T and 2 K and M vs

mol. The solid red line represents the χT curve simulated in terms of

B

7

the isotropic spin Hamiltonian − JΣ (S S ) (S = / ) for a ﬁnite ﬁve-

H/T curves plotted at diﬀerent temperatures coincide (Figure

7).

i

i+1

2

membered Gd chain with the AF exchange parameter of J = −0.015

cm . In the inset: magnetization vs ﬁeld for 5 measured at T = 2, 3,

−

1

Analysis of Dc−Magnetic Data. To understand the

magnetic behavior of magnetically concentrated compounds 1

and 3, we relate their dc magnetic characteristics with the

and 4 K (open circles).

3

+

₀_.₀₁₅_c_m₋1 or less, as estimated from the modeling of

electronic structure of Ln ions in terms of the CF theory for

f-electrons, which is based on the model Hamiltonian H

composed of the free-ion part H and the CF term H

−

7

magnetic susceptibility for ﬁnite Gd chains (S = / ) in terms

2

of the conventional isotropic spin Hamiltonian −JΣ (S S )

0

CF

i

i i+1

(

Figure 7, the solid red line); this decrease can also be partially

H = H + H

(1)

0

CF

8

due to the zero-ﬁeld splitting (ZFS) of the S ground state of

7/2

3+

Gd ions. Thus, dc magnetic data for gadolinium compound 5

provide an important insight into the character of magnetic

interactions in 1−5 as they evidence marginally weak exchange

The free-ion Hamiltonian H

0

is written in the parametric form

l S + αL(L + 1) + βG(R )

k

H =

∑

f F + ξ

∑

i

0

4f

i i

7

k

_i_n_t_e_r_a_c_t_i_o_n_b_e_t_w_e_e_n_L_n3+ ions mediated by the end-to-end

k=2,4,6

azide bridging groups in the chain, which is comparable in

magnitude with direct magnetic dipole−dipole interaction.

The value of the calculated exchange parameter (J ≈ −

+

γG(G₂)

(2)

k

where f and F are the angular and radial Slater parameters,

respectively, the second term is the spin orbit coupling (SOC)

k

−

1

0

.015 cm ) is quite consistent with the recent data on

tetranuclear Ln (Ln = Gd, Dy) complexes revealing weak

for 4f electrons, and α, β, and γ are Trees parameters

4

−

1

26−28

ferromagnetic spin coupling (J = +0.016 cm , in the − J

describing two-electron correlations.

The H_C_Fterm

notation of the spin Hamiltonian) between lanthanide ions

incorporates metal−ligand interactions represented in the

frame of the Wybourne formalism,

24a

bridged by azide groups in the end-to-end mode. The origin

of the diﬀerence between ferromagnetic (F) behavior of Gd

complex and antiferromagnetic (AF) behavior of Gd chain 5

4

k

2

4a

^HCF

=

∑

B C

kq

q

featuring the same end-to-end azido-bridged groups Gd-(N )-

k,q

(3)

3

Gd may be due to a complicated interplay of F and AF

contributions related to several competing mechanisms.

Namely, the 4f−4f superexchange interaction between two

k

where B_k_qare CF parameters (k = 2,4,6; q ≤ k) and C are

q

N

spherical tensor operators for the 4f electronic conﬁguration;

details of CF calculations for Ln ions are well documented in

3

+

7

magnetic centers with the half-ﬁlled 4f shell favors for AF spin

26−28

the literature.

The set of B parameters speciﬁes the CF

kq

coupling, while F spin coupling is mainly due to electron

3+

potential of 4f-electrons; Bkq are usually treated as adjustable

parameters, which are ﬁtted to the optical or magnetic data for

lanthanide compounds.

transfers into the empty 5d states of Gd ions (so-called

24b

Goodenough mechanism ). These mechanisms are both

sensitive not only to the bridging groups, but also to the local

3+

Magnetic properties of lanthanide complexes can be treated

atomic environment of two exchange-coupled Gd ions, which

3

+

24a

in terms of the electronic structure of Ln ions obtained from

CF calculations, eqs 1-3). The magnetization M and applied

magnetic ﬁeld H are related by M = χH, where χ is the tensor

of magnetic susceptibility, which is represented by a 3 × 3

matrix χ_α_β,

is diﬀerent in 5 and in Gd complex; as a result, the net

4

balance may switch between F and AF spin coupling. However,

3+

detailed analysis of the mechanism of the Gd −(N )−Gd

3

spin coupling in 5 is far beyond its scope of the article. Small

value of the intrachain exchange parameter implies that the

decrease in the χT product of 1−4 at low temperature is

caused by thermal depopulation of excited CF energy levels of

lanthanide ions rather than by the antiferromagnetic spin

coupling. Besides, although 1 and 3 show a distinct 1D crystal

structure with well-isolated metal chains inherent to single-

chain magnets, their magnetic behavior would likely have a

dominant single-ion (0D) character typical of mononuclear

lanthanide complexes rather than SCM (1D) character. Below,

M = ∑ χ H

α

αβ

β

(4)

where α, β = x, y or z; note that M and H are generally

noncollinear. The matrix elements χ_α_βof the χ tensor are

expressed via the eigenvectors |i > and eigenenergies E of the

CF Hamiltonian (1) using the Gerloch−McMeeking equa-

i

29

tion,

F

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

l

states of both the lowest J-multiplet and several excited

o

⟨i|μ |j⟩⟨j|μ |i⟩

α β

^Na

o

6

3+

4

χ_α_β

=

× m∑ ∑

−

multiplets ( H15/2, H13/2, H11/2, H9/2 for Dy and I15/2,

o

^∑i exp(−E /kT)

o

kT

4

3+

i

j

I₁₃_/₂, I , I , for Er ) were involved in the calculations of

11/2

9/2

n

the χ_α_βtensor in (5). To attain the best agreement with the

experimental dc magnetic data, especially at low temperatures,

we apply reﬁned CF calculations, in which the rank two (k = 2)

|

⟨

i|μ |j⟩⟨j|μ |i⟩ + ⟨i|μ |j⟩⟨j|μ |i⟩ o

o

α

β

α

o

∑

} exp(−E /kT)

o

i

E − E

o

j≠i

i

j

~

(5)

B_k_qparameters are varied instead of the b “intrinsic” CF

2

parameter (Figures 3 and 5). The meaning of this technique is

that the second rank CF parameters are sensitive to long-range

interactions whose source lies outside the position of the

where N is the Avogadro number, E is the energy of the CF

state |i⟩, k is the Boltzmann constant, T is the absolute

temperature, and μ , μ are the components of the operator of

a

i

α

β

3+

nearest atomic environment of the Ln ion; therefore, these

the magnetic momentum

interactions cannot generally be well quantiﬁed in the frame of

the superposition CF model. Numerical calculations are

carried out with routines outlined in ref 35.

μ = μ (L + 2S)

(6)

B

where L and S are, respectively, the operators of the total

orbital momentum and spin and μ is the Bohr magneton. The

eigenvalues of the 3 × 3 matrix χ_α_β(7) correspond to the

principal components of the magnetic susceptibility (χ , χ and

The best agreement for 1 is obtained at b = 280, b = 420

4

6

−1

B

cm (for O atoms) and b = 200, b = 180 cm (for N

atoms); for 3, calculations resulted in b = 300, b = 250 cm

4 6

−1

4

6

x

y

(O, N); calculated rank two B parameters are listed in Table

2q

^χz⁾^aⁿ^d^t^h^e^p^o^w^d^e^r^m^a^gⁿ^e^tⁱ^c^s^u^s^c^e^p^tⁱ^bⁱ^lⁱ^t^yⁱ^s^w^rⁱ^t^t^eⁿ^a^s^χ⁼⁽^χ

x

S7 . The simulated χT curves for 1 and 3 are in close agreement

with the experimental data in the whole temperature range

+

χ + χ )/3.

y z

Using eqs 1-6), we simulate dc magnetic susceptibility of 1

and 3 with the aim to establish the CF splitting pattern of Dy

and Er ions in the low-symmetry N O coordination (Figure

a). The CF parameters B can be obtained from the ﬁtting of

the simulated χT curve to the experimental dc magnetic data

Figures 3 and 5). However, for low-symmetry lanthanide

complexes such calculations should be performed with caution

because ﬁtting to the χT curve can rapidly lead to an

overparametrization problem due to a large number of B_k_q

parameters (up to 27 B_k_qparameters for the C₁point

symmetry, as is the case for 1−5). In this situation, it is

preferable to take advantage of the superposition CF

that relates the B_k_qparameters with the geometry

of the metal site in terms of intrinsic CF parameters b (R )

describing local metal−ligand interactions,

(

Figures 3 and 5). Some uncertainty in the lanthanide

3

+

concentration in the powdered samples of 1 and 3 was

covered with a scaling factor for the magnetic susceptibility

3

+

5

3

2

kq

(

+1.2% for 1 and +2.6% for 3, respectively). These data

3+

indicate that the N O 8-fold coordination of Ln ions in 1

5

3

(

and 3 produces in a rather low CF splitting energy, about 300

−1

cm or less (Table 3); more speciﬁcally, the strength of the

36

CF potential can be quantiﬁed by the CF strength criterion,

which is S = 388 cm for 1 and 439 cm for 3.

−1

Table 3. Calculated CF Splitting Energies (cm ) of the

3+

Ground J-Multiplets of Dy and Er in 1 and 3 and the

Ground-State g-Tensors

3

0−32

model

k

0

3+

6

3+ 4

Dy , H15/2

Er , I15/2

0

10.8

63.5

tk

k

B_k_q= ∑ b (R )(R /R ) C (ϑ , φ)

k

0

n

q

n

2

3

4.7

8.2

(7)

n

1

00.9

47.3

85.4

99.7

where index n numerates metal−ligand pairs involved in the

coordination polyhedron of the Ln³ion, b (R ) are the three

+

220.4

258.2

295.2

324.8

k

0

(

k = 2, 4, 6) intrinsic CF parameters, (R , θ , φ ) are polar

n n n

k

237.8

62.1

coordinates of the nth ligand atom, t are power-law exponents

2

and R is the reference distance (which is normally set to the

0

^gx ⁼⁰^.³⁷⁶

^gy ⁼²^.¹⁰¹

^gz ⁼¹⁶^.⁹⁰³

^gx ⁼⁰^.³⁶⁰

^gy ⁼²^.¹³⁹

^gz ⁼¹²^.⁵²⁸

average metal−ligand distance). The superposition CF model

30−32

and its applications were discussed elsewhere.

It is also of

special note that the intrinsic CF parameters b (R ) are

k

0

transferable between diﬀerent Ln³ions in the similar ligand

surrounding; this allows estimates of CF parameters for Ln

sites with known coordination geometry.

+

CF analysis shows that the entering the extra ligand (EtOH)

in the apical position (in addition to azide ligand) of the

coordination polyhedron [LnN O ] signiﬁcantly reduces the

Following this approach, the B parameters are determined

kq

5

3

from the ﬁtting to the experimental χT curves of complexes 1

and 3 (Figures 3 and 5). Intrinsic CF parameters b (R ) vary

eﬀective axial symmetry of the CF potential for 4f-electrons.

More speciﬁcally, the perfect PBP geometry (D ) of the

k

0

5h

3

+

independently for the O and N coordinating atoms; the

reference distances corresponds to average metal−ligand

distances (R (O) = 2.34 Å and R (N) = 2.45 Å) and the

coordination polyhedron of Ln ions results in the strict axial

symmetry of the CF potential, which is represented by only

three nonzero axial (q = 0) CF parameters (i.e., B , B and

0

20

40

2

4

6

30−32

power-law indexes are ﬁxed at t = 5, t = 8, and t = 11.

The polar coordinates (R , θ , φ ) in (7) describe atomic

B ) with other B parameters being zero. This case is most

60 kq

favorable for obtaining high energy barrier because the CF

n

positions of the N and O atoms of the LnN O coordination

potential splits the lower J-multiplet to produce pure ± m_J

5

3

+

2

4

15

polyhedrons in 1−5; atomic parameters of Ln ions (F , F ,

ground state (such as m = ± / in the record PBP dysprosium

J

2

6

14

F , ζ , α, β, and γ) involved in the free-ion Hamiltonian H (3)

single-ion magnets with U > 1000 K ). Departures from the

4

f

0

eff

are taken from ref 33 and 34. For a more accurate assessment

of the second-order contributions from excited states to the

magnetic susceptibility (the second term in eq 5), the CF

PBP geometry (due to distortions and the presence of two

ligands (EtOH and N ) in the same axial direction) results in

considerable nonaxial (q ≠ 0) B_k_qCF parameters (Table S7)

3−

G

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 8. (a) Frequency dependence of the in-phase χ′ and (b) out-of-phase ac susceptibility χ″ at diﬀerent temperatures between 2.15 and 2.9 K

and H = 915 Oe for 1 (Dy(DAPMBH). Solid lines correspond to ﬁtted data obtained within the generalized Debye model with parameters listed

DC

in Table S10 .

Figure 9. (a) Cole−Cole plot of 1 (Dy(DAPMBH))under 915 Oe dc ﬁeld in the temperature range from 2.15 to 6.15 K. Open circles are the

experimental data, the solid lines correspond to the simulated data ﬁtted within the generalized Debye model with parameters listed in Table S10 .

1

(

b) ln(τ) vs T⁻plot of 1 under 915 Oe dc ﬁeld. Black circles are the experimental data, and the solid red line corresponds to the linear ﬁt

−

8

accounting only thermal activation process, by ln(τ) = ln(τ ) + U /kT, with parameters τ = 1.42 × 10 s and U_e_f_f= 47 K.

0

eff

0

that cause strong mixing of the |J,m ⟩ states. Thus, the wave

interaction between the magnetic ions may promote the

J

functions of the ground and lower excited Kramers doublets of

quantum tunneling of magnetization (QTM) thus accelerating

3

+

3+

38,39

Dy and Er ions in 1 and 3 reveal a strong mixture of the |

the process of magnetic relaxation;

on the other hand,

J,m ⟩ states with only a small percent of states with high m_J

strong magnetic exchange coupling can increase the blocking

J

18c

projection (Tables S8 and S9 ); this reduces considerably the

eﬀective energy barrier U_e_f_fdue to fast QTM processes typical

of low-symmetry Ln complexes. In addition, the degree of the

departure from the axial symmetry of the CF potential is

measured by the anisotropy of the ground-state g-tensor. In

temperature of SMM. Whether the slow magnetic relaxation

originates from the single-ion anisotropy or it comes from intra

and/or intermolecular interactions of magnetic ions is

important for unravelling the nature of the dynamic magnetic

behavior of 4f-based complexes. The magnetic dilution is an

eﬀective approach to study the relaxation behavior of a 4f

SMM. Indeed, the lanthanide ions in diluted sample retain

their coordination environments while the magnetic coupling

between them is substantially weakened, so that the magnetic

behavior of diluted sample reﬂects the properties of the single

ion and allow to get insight into magnetic behavior of the

15

high-performance Dy SMMs with the m = ± / ground state,

J

2

the anisotropic g-tensor approaches to a pure Ising limit, g , g

x

y

1

1,12,14

≈

0, g = 20;

in distorted complexes, the g-tensor is less

z

anisotropic, with larger g , g components and a smaller g

x

y

z

component. Thus, in chain compounds 1 and 3 based on Dy

and Er azido complexes with low-symmetry [LnN O ]

5

3

40

coordination, the ground-state g-tensor (g = 0.376, g =

undiluted sample. This method was successfully used for

studying the SMM properties of 0D systems such as

x

y

2

.101, g = 16.903 in 1 and g = 0.360, g = 2.139, g = 12.528

z x y z

4

1,42

39

in 3, Table 3), albeit retaining overall Ising-type anisotropy (g_z

mononuclear,

dinuclear, and tetranuclear lanthanide

3

8

≫

g , g ), diﬀers considerably from the pure Izing pattern due

complexes. Recent studies report that the magnetic exchange

in lanthanide compound may realize through the end-to-end

x

y

to the presence of pronounced transversal components g and

x

2

3c,24

g_y, which favors for fast quantum tunneling of magnetization.

In this regard, a more favorable situation occurs for the PBP

dysprosium complex (Et NH)[(H L)−DyCl ] featuring only

azido bridges.

Herein, to study the SMM properties of

novel polymeric azido-bridged dysprosium and erbium 1D

chain systems we thoroughly investigated the dynamic

magnetic properties of undiluted 1 (Dy) and 3 (Er) and

isostructural diluted 2 (Y_Dy) and 4 (Y_Er) complexes.

The alternating current susceptibilities as a function of

frequency were measured at diﬀerent temperatures for each

compound in zero and nonzero dc ﬁeld. None of the

compounds exhibit the slow relaxation of magnetization

3

2

16a

one Cl atom in each apical position; this corresponds to a

more distinct PBP geometry, which is characterized by a less

mixed ground state and higher energy barrier (70 K).

Dynamic Magnetic Properties. Magnetic interactions

have a dual eﬀect on the SMM properties of 4f ions-based

3

7

complexes. On one hand, the weak intramolecular magnetic

H

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 10. (a) Frequency dependence of the in-phase χ′ and (b) out-of-phase ac susceptibility χ″ recorded at diﬀerent temperatures between 2.15

and 2.9 K and H = 715 Oe for 2 (Y Dy (DAPMBH)). Solid lines correspond to the ﬁtted data obtained within the generalized Debye model

DC

0.93

0.97

with parameters listed in Table S11.

Figure 11. (a) Cole−Cole plot of 2 (Y₀_.₉₃Dy₀_.₀₇(DAPMBH)) under 715 Oe dc ﬁeld in the temperature range from 2.15 to 3.05 K. Open circles are

the experimental data, solid lines correspond to the simulated data ﬁtted within the generalized Debye model with parameters listed in Table S11 .

−

1

(

×

b) ln(τ(s)) vs 1/T (K ) plot at H = 750 Oe for 2 (black circles) and its ﬁt (solid red line) by ln(τ) = ln(τ ) + U /kT with parameters τ = 2.4

D

C

0

e

f

0

−

10

10 s and U_e_f_f= 36 K.

behavior in the absence of dc ﬁeld in 10−10000 Hz frequency

range (Figure S6 −S9).

The pronounced maxima in ﬁeld dependence of the out-of-

phase magnetic susceptibility, χ″ marks the optimal magnetic

ﬁ eld at 915 and 715 Oe for 1 and 2, respectively (Figures S6

and S7 ). The ac magnetic susceptibility for 1 and 2 complexes

recorded under optimum magnetic ﬁeld are shown in Figures 8

and 10. In the temperature range of 2.15−2.9 K the position of

the χ″ maximum of complex 1 varies insigniﬁcantly with

temperature and steadily moves to higher frequencies with

further temperature increase above 3 K, while the peak of χ″ of

diluted complex 2 shifts toward higher frequencies in the

whole temperature range. The magnetic dynamic of 2 is more

rapid compared to 1, and the χ″ maximum of 2 passes the

At 2.15 K in a zero dc ﬁeld the out-of-phase component of

magnetic susceptibility χ″ of dysprosium complexes 1 (Dy)

and 2 (Y_Dy) increases in frequency range of 10- 10000 Hz

and tentatively reaches maximum near 10000 Hz, that is

beyond the capability limits of our experimental setup (Figures

S6 and S7). Noteworthy, the magnitude of χ″ of a magnetically

diluted sample 2 exhibits more appreciable growth comparing

to undiluted sample (Figure S7). Given that weak magnetic

−

1

interactions between the Ln ions (such as J ≈ − 0.015 cm in

our compounds) usually tend to activate QTM processes, the

diﬀerence in the ac properties of complexes 1 and 2 is likely

due to reduced magnetic interactions between more distant

10000 Hz threshold already at 3.05 K. The diﬀerence in the

magnetic relaxation of 1 and 2 complexes is likely due a

combined eﬀect of exchange driven magnetic relaxation

processes and single-ion magnetic anisotropy that may act in

opposite ways upon magnetic dilution. Larger spacing between

40,44

dysprosium ions in 2, that suppresses the QTM.

contrast to the dysprosium complexes, the 3 (Er) and 4

Y_Er) complexes do not show any frequency dependence of

In

(

3+

magnetic Ln ions in diluted compound 2 weakens magnetic

the out-of-phase (χ″) ac susceptibility at 2.15 K in a zero dc

ﬁeld.

coupling and thus suppresses exchange driven magnetic

relaxation, while changes in the CF splitting pattern of

To diminish the impact of the quantum tunneling eﬀects,

the ac measurements were performed under external dc

magnetic ﬁeld. The optimal magnetic ﬁeld was determined

from the magnetic ﬁeld dependencies of ac susceptibility (χ′

3+

magnetic Ln ions doped in the diamagnetic yttrium host

compound may both accelerate or suppresses magnetic

relaxation process, especially due to their high sensitivity to

even tiny variations in the local coordination geometry of

lanthanide ions. The net balance between these two factors

may be opposite, as can be seen from the comparison of

changes in dynamic magnetic properties of concentrated and

diluted Dy and Er compounds (1 and 2 vs 3 and 4, see below).

The ﬁt of χ′ and χ″ plots by the generalized Debye

and χ″) recorded at ﬁxed frequency f = 100 Hz and

ac

temperature T = 2.15 K as shown in Figures S10 −S13. The in-

phase χ′ and out-of-phase χ″ frequency dependence of ac

susceptibility studied in optimal dc ﬁelds evidence the slow

relaxation of magnetization behavior under applied dc

magnetic ﬁeld indicating the ﬁeld-induced SMM property of

studied complexes (Figures 8−11).

43,44

model

gives the values of parameter α (Table S10) for 1

(Dy) within the range 0.27−0.07, signifying the one

I

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 12. (a) Frequency dependence of the in-phase χ′ and (b) out-of-phase ac susceptibility χ″ at diﬀerent temperatures and H = 915 Oe for 3

DC

(

Er(DAPMBH)).

Figure 13. (a) Cole−Cole) plot of 3 (Er(DAPMBH) under 915 Oe dc ﬁeld in the temperature range from 2.0 to 2.6 K. Empty circles are the

experimental data, solid lines correspond to the simulated data ﬁtted within the generalized Debye model with parameters listed in Table S14 ; (b)

−

1

ln(τ(s)) vs 1/T (K ) plot at H = 915 Oe for 3 (Er(DAPMBH)) (black circles) and its ﬁt (solid red line) by ln(τ) = ln(τ ) + U /kT with

DC

0

eff

−

8

parameters τ = 2.25 × 10 and U_e_f_f= 17 K.

0

Figure 14. Frequency dependence of the in-phase χ′ and (b) out-of-phase ac susceptibility χ″ at diﬀerent temperatures and H = 1500 Oe for 4

DC

(

Y

0

Er (DAPMBH)).

.923 0.077

dominating mechanism of magnetic relaxation at higher

temperature range. The values of parameter α obtained from

ﬁtting the data of 2 does not change much in the temperature

range of 2.15 K −3.05 K and take values 0.17−0.21

corresponding to a relatively narrow distribution of the

relaxation time Table S11 . Fitting the temperature dependence

of relaxation time for the data for 1 (Dy) and 2 (Y_Dy) with

Arrhenius law, τ = τ exp(U /k T), results in U = 47 K and

counting for the direct relaxation, QTM and phonon assistant

processes. For the 1 the ﬁt quite well captures the experimental

−

2

−1

data with the set of parameters: A = 164 Oe

s

K , n =

−

5

−13

6.95, C = 0.083 K /s, Ueff = 116 K and τ = 8.5 × 10 s and

0

^p^a^r^a^m^e^t^e^r^τQTM ﬁxed at zero (see Table S12). The value of the

^eⁿ^e^r^g^y^b^a^r^rⁱ^e^r^Ueff is higher than that obtained from the

Arrhenius ﬁt, probably due to some overparameterization in

the frame of the four-term ﬁtting model, in which one ﬁtting

^p^a^r^a^m^e^t^e^r^s^m^a^y^a^b^s^o^r^b^o^t^h^e^r^s^.^T^h^e^v^a^l^u^e^o^f^Ueff = 47 K is

preferable since it is consistent with the barrier of 36 K

obtained for the diluted Y−Dy compound 2 and it also better

agrees with the energy of the ﬁrst excited CF state of 35 K

0

eff

B

eff

−

8

τ = 1.42 × 10 s for 1 (Figure 9b) and U = 36 K, τ = 2.4 ×

0

eff

0

−

10

1

0

s for 2 (Figure 11b).

As shown in Figure 9b, the plot of ln(τ) versus 1/T of 1

deviates from the linear ﬁt below 3.0 K, that is suggestive of the

coexisting multiple relaxation processes. To elucidate the

impact of diﬀerent mechanisms on magnetic dynamic, the

temperature dependence of relaxation time was ﬁtted with

^Ueff

−

1

3+

(

24.7 cm ) of Dy ions in 1. Similar problems were reported

for the related PBP Dy complex (Et NH)[(H L)−DyCl ], for

3

2

which inclusion of contributions of Orbach, Raman and direct

16a

processes has led to poor ﬁtting for the relaxation time. The

best ﬁt of the temperature dependence of relaxation time for 2

expression τ⁻= τ_Q_T_M⁻¹+ AT + CT + τ exp −

1

n

−1

ac-

0

( )

kT

J

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 15. (a) Cole−Cole plot of 4 (Y₀_.₉₂₃Er₀_.₀₇₇(DAPMBH)) under 1500 Oe dc ﬁeld in the temperature range from 2.15 to 3.05 K. Color circles

are the experimental data, solid lines correspond to the simulated data ﬁtted within the generalized Debye model with parameters listed in Table

−

1

S15 . (b) Plot of ln(τ(s)) vs 1/T (K ) at H = 1500 Oe for 4 (black circles) and its Arrhenius ﬁt (solid red line) by ln(τ) = ln(τ ) + U /kT with

DC

0

eff

−

9

parameters τ = 3.31 × 10 and U_e_f_f= 24 K.

0

is achieved with parameters A = 217 Oe s⁻¹K , U = 40 K

−

2

−1

temperatures for 3 and 4 are listed in Tables S14 and S15,

respectively The parameter α does not show appreciable

temperature dependence and varies in range of 0.26−0.15 for 3

and 0.22−0.14 for 4. The temperature dependence of

relaxation time for 3 and 4 can be ﬁtted by Arrhenius law

eff

−

11

and τ = 7.5 × 10

s, which implies the coexistence of

thermal activation and direct relaxation of magnetization

processes in studied temperature range (Table S13 ).

0

The energy barriers in 1 and 2 are comparable with the U_e_f_f

values of related Dy complexes with 2,6-diacetylpyridine-based

pentadentate ligands producing similar pentagonal N O

with the values of the eﬀective energy barrier U = 17 K and

eff

−

8

parameter τ = 2.25 × 10 s for 3 (Figure 13b) and U = 24

K and parameter τ = 3.31 × 10 s for 4 (Figure 15b). The

3

2

0

eff

1

6a−e

−9

coordination in the equatorial plane.

the barrier ranges from 18 to 36 K being fairly consistent with

7 K in 1 and 36 K in 2; a signiﬁcantly larger barrier (70 K)

was observed in ref 16a for compound (Et NH)[(H L)-

In these compounds,

0

erbium compounds 3 and 4 exhibit lower eﬀective energy

barriers as compared to dysprosium compounds 1 and 2. This

diﬀerence is qualitatively consistent with an approximate PBP

4

3

2

3

+

DyCl ] (where H L = 2,6-diacetylpyridine bis-

salicylhydrazone)) with apical Cl atoms, whose coordination

is more close to the perfect PBP symmetry (D ). The

importance of tuning the coordination geometry of the

equatorial donor atoms in PBP Dy complexes to enhance

single-molecule magnet properties has also been discussed in a

coordination geometry of Ln ions in 1−5, which shows axial

2

4

3+

(

negative charge distribution being preferable for Dy ion.

More quantitatively, lower barrier in Er compounds agrees

with the fact that U_e_f_fcorrelates with energy separation

between the ground state and ﬁrst excited CF energy level,

5h

−

1

3+

−1

which is 10.8 cm for Er ions in 3 vs 24.7 cm in Dy

compound 1, as obtained from the CF analysis (Table 3). The

close values of the energy barrier obtained at 500 and 1500 Oe

in 4 are indicative of the absence of contribution from

magnetic ﬁeld-driven relaxation process.

1

6f

recent paper.

In order to get more insight into the mechanism of slow

magnetic relaxation, it is of interest to compare SMM

characteristics of Ln ions with oblate (Dy ) and prolate

3+

3

+

(

Er ) 4f electron density shape in the same ligand

In contrast to dysprosium compounds 1 and 2, where slow

magnetic relaxation in Dy complexes is partially suppressed by

magnetic dilution, the concentrated 3 and diluted 4 Er

complexes exhibit similar ac properties and comparable values

3+

coordination geometry, especially because Dy ions reveal

better SMM performance in compounds with negative charge

distribution on axial direction, while Er³ions are preferable in

compounds with negative charge distribution in the equatorial

+

of the energy barrier U , suggesting diﬀerent eﬃcacy of

eff

1

0c,45

plane.

To this end, we measured the dynamic magnetic

properties of the isostructural erbium compounds 3 and 4

dilution on the SMM properties of isostructural azido-bridged

erbium and dysprosium chain compounds. These ﬁndings can

be rationalized in terms of the interplay between ionic radii of

(Figures 12−15).

The in-phase χ′ and out-of-phase χ″ frequency dependence

3+

Dy , Er and Y ions: since smaller ionic radius of Er is

of the ac susceptibility for Er complex 3 under optimal

magnetic ﬁeld of 915 Oe are shown in Figure 12; the Cole−

Cole plot for 3 is shown in Figure 13. The ﬁeld dependence of

χ″ recorded at 100 Hz for diluted erbium complex 4 does not

show maxima up to 5000 Oe and the values of χ″ remained at

the noisy level as in zero dc ﬁeld (Figure S13 ). However, the

in-phase χ′ and out-of-phase χ″ frequency dependences for 4

examined in the presence of the static dc ﬁelds of 500 and

3+

closer to that of Y as compared to that of Dy , entering

erbium ions into the yttrium host matrix would result in

smaller structural changes in the local geometry of the

[

LnN O ] coordination polyhedron and thus results in smaller

5 3

changes in the CF splitting pattern of the ground J-multiplet in

passing from concentrated to diluted Ln compound (Table 3).

CONCLUSION

1

500 Oe indicate the ﬁeld-induced SMM properties of 4

■

(Figure 14).

In summary, we report synthesis, structure, and magnetic

properties of new lanthanide 1D coordination polymers of

general formula {[Ln(DAPMBH)(N )C H OH]C H OH} ,

The dynamic magnetic properties of erbium complexes were

studied under applied magnetic ﬁeld in temperature range of

3

2

5

2

5

n

2

.15−2.6 K for 3 (Er) and 2.15−3.05 K for 4 (Y_Er) since at

1−5. Their crystal structure reveal a fascinating end-to-end

3+

higher temperatures the maxima of the frequency dependence

azido bridged lanthanide chains, in which Ln ions adopt a

pentagonal coordination in the equatorial plane produced by

the N O chelating ring of the newly synthesized pentadentate

of out-of phase magnetic susceptibility χ″ are located above 10

0

00 Hz (Figures 12 and 14). The parameters of ﬁtting the χ′

3

2

43,44

and χ″ plots with the generalized Debye model

at diﬀerent

ligand H DAPMBH (2,6-diacetylpyridine bis(4-methoxyben-

2

K

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

zoylhydrazone). To the best of our knowledge, these

complexes represent the ﬁrst example of azido-bridged

lanthanide chain compounds. The underlying idea behind

this study was to combine strong uniaxial magnetic anisotropy

of pentagonal Ln complexes with rich possibilities of the azide

anion as a bridging ligand with the aim to obtain advanced

lanthanide chain magnets. A series of chain compounds 1−5

was synthesized to provide better insight into their magnetic

behavior. dc magnetic data indicate marginally weak spin

coupling between Ln ions mediated by bridging azido groups, J

Syntheses of all the complexes were performed under anaerobic

conditions following practically the same procedure. An exception is

the synthesis of the Gd complex (5), for which GdBr ·6H O was used

3

2

as the starting compound instead of the anhydrous chloride (DyCl₃,

ErCl , YCl ) that were used in all other cases. Mixtures of

3

corresponding chlorides in molar ratios noted in chemical formulas

were used for the preparation of magnetically diluted complexes 2 and

4

.

{[Dy(DAPMBH)(N )C H OH]C H OH} (1). To a suspension of

3

2

5

2

5

n

H DAPMBH (0.48 mmol, 220 mg) in absolute ethanol, a solution of

2

DyCl

in absolute ethanol (0.48 mmol, 128 mg in 15 mL of

3

−

1

C H OH) was added using a dropping funnel at room temperature.

≈

0

− 0.015 cm for 5 (Ln = Gd). Compounds 1−4 (Ln = Dy,

2

5

The white suspension turned to lemon-yellow immediately, and the

white precipitate started gradually dissolved. The reaction mixture was

stirring at 60 °C for additional 1 h. During this time, the white

precipitate of the ligand was completely dissolved, and the reaction

mixture turned into a yellow homogeneous solution. After that, the

solution was cooled to room temperature and triethylamine (0.13 mL,

0.98 mmol) was added to it with stirring. As a result, the color of the

solution changed from lemon yellow to bright yellow. Then the solid

Y

Dy , Er, Y₀_.₉₂₃Er , respectively) exhibit ﬁeld-induced

.93

0.07

0.077

single-molecule magnet behavior with the eﬀective energy

barriers U ranging from 47 K in 1 to 17 K in 3. Comparative

eff

analysis of dc and ac magnetic properties of concentrated (1,

3) and magnetically diluted (2, 4) Dy and Er compounds

indicated that, despite distinct 1D structure with well isolated

lanthanide chains characteristic to single-chain magnets, their

dynamic magnetic behavior is largely governed by the single-

NaN (200 mg, 3.08 mmol) was added to the reaction mixture under

3

_i_o_n_m_a_g_n_e_t_i_c_a_n_i_s_o_t_r_o_p_y_o_f_i_n_d_i_v_i_d_u_a_l_L_n3+ ions rather than by

stirring and the solution was stirring at 60 °C for about 15 min.

During this time, a small amount of light yellow precipitate appeared.

After cooling the reaction mixture to room temperature, the

precipitate was ﬁltered. The ﬁltrate was left standing undisturbed at

room temperature. Large bright yellow crystals of 1 suitable for X-ray

diﬀraction precipitated in a course of several days. The mother liquor

was decanted from the crystals that washed with diethyl ether (2 × 3

mL) and dried in vacuo aﬀording 0.2 g of product 1 of analytical

purity. Yield: 55%. Anal. Calcd for DyC H N O : C,46.19; H, 4.68;

the SCM regime. CF analysis showed that the presence of two

−

ligands (EtOH and N ) in the same axial direction results in

3

considerable lowering the axial symmetry of the CF potential

of 4f-electrons, which reduces the energy barrier. This study

evidence poor eﬃciency of end-to-end bridging azide ligands

as mediators of the spin coupling between lanthanide ions.

2

9

35

8

6

‑1

N, 14.86. Found: C, 46.15; H, 4.61; N, 14.90. FT-IR (solid, in cm ):

3467 (m), 3186 (m), 2984 (m), 2838 (m), 2097 (vs, νazide), 1608

(s), 1585 (s), 1552 (m), 1504 (s), 1485 (s), 1361 (vs), 1300 (m),

1260 (s), 1165 (vs), 1034 (s), 882 (m), 842 (m), 765 (m).

EXPERIMENTAL SECTION

■

Materials and Methods. 2,6-Diacetylpyridine, Et N, DyCl

3

anhydrous, GdBr ·6H O, YCl anhydrous, ErCl anhydrous, NaN ,

3

2

3

ethanol anhydrous, 4-methoxybenzoic acid, thionyl chloride, and

{[Y₀_.₉₃Dy₀_.₀₇(DAPMBH)(N )C H OH]C H OH} (2). Yield: 60%.

3

2

5

2

5

n

hydrazine hydrate solution (50−60% N H ) were purchased from

Anal. Calcd. for Y₀_.₉₃Dy₀_.₀₇C H N O : C, 50.82; H, 5.15; N, 16.35.

2

4

29 35 8 6

−

1

commercial sources and used without further puriﬁcation.

Found: C, 49.97; H, 4.90; N, 15.87. FT-IR (solid, in cm ): 3269

(m), 2976 (m), 2839 (m), 2150 (s, ν_a_z_i_d_e), 2102 (vs, ν_a_z_i_d_e), 1607 (s),

1585 (s), 1552 (s), 1501 (s), 1481 (s), 1361 (vs), 1259 (s), 1164

(vs), 1033 (s), 844 (m), 764 (m), 638 (m).

The infrared spectra were measured on solid samples using a

PerkinElmer Spectrum 100 Fourier Transform infrared spectrometer

−

1

in the range of 4000−500 cm .

Elemental analyses were carried out by the Analytical Department

service at the Institute of Problems of Chemical Physics RAS using a

Vario Micro cube (Elementar GmbH) equipment.

{[Er(DAPMBH)(N )C H OH]C H OH} (3). Yield: 25%. Anal.

3

2

5

2

5

n

Calcd. for ErC H N O : C,45.90; H, 4.65; N, 14.77. Found: C,

2

9

35

8

6

−

1

46.12; H, 4.55; N, 14.85. FT-IR (solid, in cm ): 2838 (m), 2101 (vs,

ν_a_z_i_d_e), 1606 (s), 1585 (s), 1553 (m), 1490 (s), 1361 (vs), 1247 (vs),

1164 (vs), 1036 (s), 808 (m), 619 (m).

Both ac and dc magnetic properties were measured using Physical

Properties Measurements System PPMS-9 (Quantum Design) in the

temperature range of T = 2−300 K under magnetic ﬁeld up to B = 7

T. The ﬁeld dependences of the magnetization were measured at

several temperatures between 2 and 7 K with dc magnetic ﬁeld up to 5

T. The samples in polycrystalline form were loaded into an insulating

capsule. The experimental data were corrected for the sample holder.

The diamagnetic contribution from the ligand was calculated using

Pascal’s constants.

{[Y₀_.₉₂₃Er₀_.₀₇₇(DAPMBH)(N )C H OH]C H OH} (4). Yield: 55%.

3

2

5

2

5

n

Anal. Calcd. for Y₀_.₉₂₃Er₀_.₀₇₇C H N O : C, 50.73; H, 5.14; N, 16.32.

29

35

8

6

−

1

Found: C, 49.58; H, 4.87; N, 15.65. FT-IR (solid, in cm ): 3290

(m), 3191 (m), 2974 (m), 2838 (m), 2146 (s, νazide), 2102 (vs,

νazide), 1608 (s), 1585 (vs), 1552 (s), 1486 (s), 1448 (s), 1406 (s),

1362 (vs), 1324 (m), 1301 (m), 1260 (s), 1165 (vs), 1034 (vs), 842

(m), 807 (m), 638 (m).

Synthetic Procedures. 4-Methoxybenzoic acid hydrazide was

prepared in 75% yield from methyl 4-methoxybenzoate (prepared in

{[Gd(DAPMBH)(N )C H OH]C H OH} (5). Yield: 60%. Anal.

3

2

5

2

5

n

Calcd for GdC H N O : C, 46.51; H, 4.71; N, 14.96. Found: C,

2

9

35

8

6

8

5% yield from 4-methoxybenzoic acid, methanol and thionyl

46.63; H, 4.57; N, 14.69. FT-IR (solid, in cm-1): 3131 (m), 3079

(m), 2974 (m), 2838 (m), 2095 (vs, ν_a_z_i_d_e), 1608 (s), 1585 (s), 1550

(m), 1501 (s), 1481 (s), 1358 (vs), 1260 (vs), 1164 (vs), 1027 (s),

807 (m), 619 (m).

4

6

chloride ) and hydrazine hydrate in methanol according to

previously published procedure. Anal. Calcd for C H O N : C,

47

8

10

2

5

7.83; H, 6.02; N, 16.87. Found: C, 58.03; H, 6.05; N, 16.86. FT-IR

−1

(

solid, in cm ): 3323 (s), 1618 (vs), 1606 (vs), 1495 (vs), 1343 (vs),

X-ray Data Collection, Structure Solution, and Reﬁnement.

X-ray diﬀraction data for 1, 2, 4, and 5 were obtained on a Bruker

SMART APEX II diﬀractometer (CCD detector, Mo Kα, λ = 0.71073

Å, graphite monochromator) using the w-scan mode at the Center of

shared equipment IGIC RAS. X-ray diﬀraction data for 3 were

collected on the “Belok” beamline (λ = 0.79272 Å) of the Kurchatov

Synchrotron Radiation Source (National Research Center “Kurchatov

Institute”, Moscow, Russian Federation) in the j-scan mode using a

Rayonix SX165 CCD detector at 150 and 100 K, respectively. The

data were indexed, integrated and scaled using the Bruker SAINT

program [SAINT; Bruker AXS Inc.: Madison, WI, 2016] and the XDS

1

7

328 (s), 1256 (vs), 1188 (s), 1174 (s), 1038 (s), 928 (s), 844 (vs),

65 (s).

H DAPMBH ligand was prepared in a simple hydrazine

2

condensation reaction between 1 equiv of 2,6-diacetylpyridine and

equiv of 4-methoxybenzoylhydrazine in 96% ethanol similarly to

2

48

previously reported procedure. The yields were more than 90%.

Anal. Calcd for C H N O : C, 65.35; H, 5.48; N, 15.24. Found: C,

25

5

4

−

1

6

(

1

(

5.62; H, 5.71; N, 15.35. FT-IR (solid, in cm ): 3411 (m), 3218

m), 2296 (m) 1646 (s), 1606 (s), 1580 (m), 1547 (s), 1502 (vs),

455 (m), 1365 (m), 1285 (s), 1250 (vs), 1176 (vs), 1145 (m), 1120

m), 1025 (s), 919 (m), 834 (vs), 758 (m).

4

9

program suite. The reﬂection intensities were corrected for

L

DOI: 10.1021/acs.inorgchem.9b02825

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

absorption using the SADABS software (SADABS; Bruker AXS Inc.:

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

49

Madison, WI, 2016) and the XDS program The structures were

2

solved by direct methods and reﬁned by full-matrix least-squares on F

with anisotropic displacement parameters for all non-hydrogen atoms.

The acidic hydrogen atoms of the coordinated ethanol molecules

were found from the electron density map and reﬁned isotropically.

All other hydrogen atoms were placed in calculated positions and

reﬁned using the riding model with U (H) = 1.5 × U (C, O) for the

AUTHOR INFORMATION

Corresponding Authors

E-mail (V.S.M.): mirsa@list.ru .

*E-mail (E.B.Y.): yagubski@icp.ac.ru.

■

*

iso

eq

methyl and hydroxyl groups and 1.2 × U (C) for hydrogen atoms of

eq

ORCID

the ligand and methylene groups. In 1, 3, and 5, atomic coordinates

and thermal parameters were reﬁned independently without usage of

any constraints or restraints. As for 2 and 4, the EADP instruction

were used for both occupied metal positions, and additionally, the

TWIN instruction for 4 was applied. The chemical occupancies for

positional disordered yttrium and dysprosium or yttrium and erbium

were derived from the synthetic protocol (see below) and set in

reﬁnement as a rigid metal:metal ratio, 0.930:0.070 and 0.923:0.077

for Y/Dy and Y/Er respectively. Calculations were carried out using

the SHELXTL program suite. Crystallographic data for 1−5 have

been deposited with the Cambridge Crystallographic Data Center,

No. 1937344 −1937348 (deposit@ccdc.cam.ac.uk , http://www.ccdc.

cam.ac.uk/data_request/cif ).

Vladimir S. Mironov: 0000-0001-7313-8279

Ilya A. Yakushev: 0000-0001-6784-947X

Roman D. Svetogorov: 0000-0003-0360-1023

Yuriy V. Manakin: 0000-0001-8619-8991

Alexey B. Kornev: 0000-0002-4217-4063

Alexander N. Vasiliev: 0000-0003-3558-6761

Author Contributions

50

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

Notes

X-ray powder diﬀraction study of the samples was performed at X-

ray structural analysis beamline (XSA) of Kurchatov Synchrotron

The authors declare no competing ﬁnancial interest.

51

Radiation Source. Monochromatic radiation with a wavelength of

.8 Å (photon energy 15498 eV) was used. The sample was placed in

0

ACKNOWLEDGMENTS

This work was supported by the Russian Science Foundation

Project No. 18-13-00264). A.N.V. acknowledges support by

■

(

a cryoloop of 300 μm in size and rotated around the horizontal axis

during the measurement, which made it possible to average the

diﬀraction patterns according to the orientations of the sample.

Diﬀraction patterns were collected by the 2D Rayonix SX165

detector, which was located perpendicular to the SR beam at a

distance of 120 mm, Debye−Scherrer (transmissional) geometry was

used with a 400 μm beam size. The exposure time was 5 min. To

calibrate the sample−detector distance we need a polycrystalline

standard with a known position of the diﬀraction peaks; in this series

NUST “MISiS” Grant No. K2-2017-084 through Act 211 of

the Government of Russian Federation, Contracts No.

Dynamic

0

2.A03.21.0004 and No. 02.A03.21.0011, in the section

Magnetic Properties,

. The structural researches were performed

using the equipment of the JRC PMR IGIC RAS. This work

was partially supported by the Ministry of Science and Higher

Education within the State assignment FSRC “Crystallography

and Photonics” RAS in part of designing computer codes for

of measurements LaB (NIST SRM 660a) powder was used. The two-

6

dimensional diﬀraction patterns obtained on the detector were further

integrated to the standard form of the dependence of the intensity on

52

calculations of magnetic characteristics of highly anisotropic

the scattering angle I(2θ) using Dionis software. Whole powder

proﬁle modeling was carried out by the Rietveld method using

Analysis of Dc−Magnetic Data,

spin systems in the section “

(V.S.M.).

53

Jana2006 software.

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Article Doi

End-to-End Azido-Bridged Lanthanide Chain Complexes (Dy, Er, Gd, and Y) with a Pentadentate Schiff-Base [N3O2] Ligand: Synthesis, Structure, and Magnetism

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