Influence of Tuned Linker Functionality on Modulation of Magnetic Properties and Relaxation Dynamics in a Family of Six Isotypic Ln2 (Ln = Dy and Gd) Complexes

Article

pubs.acs.org/IC

Inﬂuence of Tuned Linker Functionality on Modulation of Magnetic

Properties and Relaxation Dynamics in a Family of Six Isotypic Ln₂

(

Ln = Dy and Gd) Complexes

†

⊥,‡

⊥,§

†

,§

Soumya Mukherjee, Jingjing Lu, G_,_†unasekaran Velmurugan, Shweta Singh, Gopalan Rajaraman,*

,‡

Jinkui Tang,* and Sujit K. Ghosh*

†

Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune 411008, India

‡

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, P. R. China

§

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra 400076, India

S Supporting Information

*

ABSTRACT: A coordination complex family comprising of six

new dinuclear symmetric lanthanide complexes, namely,

[

Ln (L ) (L′) (CH OH) ]·yG (H L : three related yet distinct

2

x 2

2

3

2

2 x

Schiﬀ-base linkers; x = 1−3, according to the nomenclature of

the Schiﬀ-base linker employed herein. HL′: 2,6-dimethoxyphe-

nol. yG refers to crystallographically assigned guest solvent

species in the respective complexes; y = number of solvent

III

molecules; Ln = Dy/Gd) were isolated employing a mixed-

ligand strategy stemming out of a strategic variation of the

functionalities introduced among the constituent Schiﬀ-base

linkers. The purposeful introduction of three diverse auxiliary

groups with delicate diﬀerences in their electrostatic natures

III

aﬀects the local anisotropy and magnetic coupling of Ln ion-

environment in the ensuing Ln dinuclear complexes, con-

2

sequentially resulting into distinctly dynamical magnetic behaviors among the investigated new-fangled family of isotypic Ln₂

complexes. Among the entire family, subtle alterations in the chemical moieties render two of the Dy analogues to behave as

2

single molecule magnets, while the other Dy congener merely exhibits slow relaxation of the magnetization. The current

2

observation marks one of the rare paradigms, wherein magnetic behavior modulation was achieved by virtue of the omnipresent

inﬂuence of subtly tuned linker functionalities among the constituent motifs of the lanthanide nanomagnets. To rationalize the

observed diﬀerence in the magnetic coupling, density functional theory and ab initio calculations (CASSCF/RASSI-SO/

POLY_ANISO) were performed on all six complexes. Subtle diﬀerence in the bond angles leads to diﬀerence in the J values

observed for Gd complexes, while diﬀerence in the tunnel splitting associated with the structural alterations lead to variation in

2

the magnetization blockade in the Dy complexes.

2

INTRODUCTION

focus that seemingly converged on polynuclear 3d metal

■

(

25−42

aggregates, particularly large manganese clusters,

the

The breakthrough revelation of single-molecule magnet

SMM) feature in Mn₁₂acetate complex

1

−4

following years until now have witnessed stupendous

proved to be a

enthusiasm toward the development of mixed 3d−4f complexes

giant quantum leap, since it triggered a rapid development

behind exploring the diﬀerent intriguing facets of magnetic

properties for nanoscale magnetic coordination complex or

cluster-based materials of varied nuclearity, over the span of last

13,15,43−65

as well as 4f-based lanthanide clusters.

Particularly, the

attainment of maximum relaxation energy barrier and the

highest blocking temperature for Ln(III)-based multinuclear

23,66−68

5

−11

clusters deserves special attention.

The crucial, rather

two decades.

SMMs are molecular species typically

imperative involvement of Ln(III) ions is to take advantage of

the substantial magnetic anisotropy factor arising from the large

unquenched orbital angular momentum of the constituent 4f

metal ions, leading to an overall increase in the magnetic

anisotropy value for the concerned complex and therefore,

possessing the unmatched combination of high-spin (S) and

uniaxial Ising-like magnetic anisotropy (D), leading to an

anisotropy energy barrier (U) for the concomitant reversal of

2

12−15

magnetization vector S |D|.

Majority of these eﬀorts had

been primarily aimed at the development of novel molecular

magnets for miniaturizing devices in the nanoregime, involving

the exciting facets of high-density information storage, quantum

Received: August 1, 2016

1

6−24

computing, and molecule spintronics.

Barring the initial

©

XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.6b01863

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

a

Scheme 1. Schematic Illustration of the Various Synthetic Schemes Adopted to Obtain the Symmetric Ln Family (Compounds

2

1a, 1b, 2a, 2b, 3a, and 3b)

a

On the basis of mixed-ligand-based strategy.

resulting in superior anisotropy energy barriers. ^,⁶⁹⁻⁷¹Hence,

the well-adopted strategy to design and synthesize homo-

metallic Ln(III) SMMs stems out of this favorable criterion of

exploiting the high-anisotropy of 4f metal ions to utmost

5

related multidentate Schiﬀ-base bridging linkers, namely, H₂L_x

(x = 1−3; Scheme 1), in conjugation with a typical chelating

ligand, namely, 2,6-dimethoxyphenol (HL′), provides a unique

mixed linker-based strategy for the systematic study of the

magnetic modulation of a new dinuclear family of six symmetric

7

2−81

extent.

This is evident from the fact that hysteresis loop as

high as 30 K are witnessed for seven coordinated Dy(III)

SIMs.

Ln (Ln = Dy and Gd) complexes based on subtle electronic

2

8

2,83

changes of the constituent linkers. It is indeed mention-worthy

that such comparative introspection of the constituent linkers’

delicate electronic eﬀects onto magnetic properties of the

ensuing complexes, arising from strategic ligand-functionality

tuning, has been rarely scrutinized among the miscellaneous

Intriguingly from a chemist’s perspective, the intertwined

interplay of the coordination geometry and ligand ﬁeld eﬀect

coupled with the strength of magnetic interactions among the

adjacent lanthanide sites along with the rational design of

coordination chemistry assemblies’ play as vital decisive factors

behind the key route for synthesizing tailor-made functional

SMMs. Moreover, Ln(III)-based SMMs hold the potential to

come up with an exciting array of diverse quantum phenomena,

such as thermally activated two-phonon or multiphonon

109−112

spectra of SMM families.

Herein, we report six dysprosium and gadolinium-based

symmetric dinuclear compounds (Ln ), namely,

2

[Dy (L ) (L′) (CH OH) ] (1a), [Gd (L ) (L′) (CH OH) ]-

2

1 2

2

3

2

1 2

2

3

2

(EtOAc) (1b), [Dy (L ) (L′) (CH OH) ] (2a),

2

3

2

84−90

magnetization relaxation and/or even magnetic tunneling,

[Gd (L ) (L′) (CH OH) ] (2b), [Dy (L ) (L′) (CH OH) ]

2

2 2

2

3

2

3 2

2

3

2

which have already been comprehensively scrutinized in 3d

SMMs. Keeping the focus pretty streamlined of acquiring a

deeper insight into the structure−property correlations of

magnetic coordination clusters pertaining to diverse nuclearity,

a systematic synthetic approach is fundamentally essential to

explicate the genesis of slow magnetic relaxation aimed at

rational protocols of synthesizing better performing SMMs with

(3a), and [Gd (L ) (L′) (CH OH) ] (3b) (H L , H L , H L :

2

3 2

2

3

2

1

2

2 3

as illustrated in Scheme 1, and HL′ = 2,6-dimethoxyphenol) by

the designed variation principle of the participating Schiﬀ-base

linkers (Scheme 1), aimed at analyzing the outcomes of the

delicate disparity between the closely related ligands onto the

magnetic relaxation dynamics and characteristic properties for

this family of six analogous molecular nanomagnets. As a

distinct outcome of the aforesaid linker functionality-tuning

among the three dissimilar yet closely related Schiﬀ bases being

employed (deprotonated ligand forms: L , L , and L ,

8

8−94

high U and D values.

This culminated to the quite scarcely

9

5−100

adopted mixed-ligand-based strategy

targeted at the

development of quite a few related dysprosium-based SMMs,

which could come up with noteworthy magnetic property

variations based on subtle meteoric alterations in the electronic

1

2

3

respectively), this report summarizes the distinct magnetic

attributes of the new Ln family comprising of six symmetric

2

68,75,101−106

properties of the constituent linkers.

dinuclear clusters that are interrelated congeners, owing to the

Schiﬀ-base linkers, especially the ones based on o-vanillin, are

well-recognized to result into SMMs of varying nuclearity, since

they can eﬃciently act as bridging linkers with a number of

two sets of three Ln clusters. Further, CASSCF/RASSI-SO/

2

POLY_ANISO calculations were performed to gain insights

into the mechanism of magnetic relaxation and the diﬀerence in

the magnetic properties observed in this series.

57,104−108

ligand-coordination pockets.

To employ three of such

B

DOI: 10.1021/acs.inorgchem.6b01863

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Crystallographic Data and Structure Reﬁnement Details for the Ln Family of Compounds 1a, 2a, 3a, 1b, 2b, and 3b

2

compound

1a

2a

3a

1b

2b

3b

empirical formula

formula wt

crystal system

space group

a, Å

C H Dy N O

C H Gd N O

14

4

42

2

12

54 54

2

14

46 46

2

14

48 50

2

4

14

52 46

2

12

46 46

2

1117.79

1281.99

1177.85

1195.40

1207.41

1167.35

monoclinic

orthorhombic

Pbca

monoclinic

triclinic

orthorhombic

Pbca

P2 /n

1

P2 /n

1

P2 /c

1

P1

̅

8.6370(11)

14.4827(18)

17.300(2)

90.00

8.895(4)

12.546(6)

22.112(10)

90.00

8.5275(15)

20.048(3)

25.023(4)

90.00

18.745(6)

15.930(5)

18.388(6)

90.00

8.5153(9)

8.601(2)

20.178(4)

25.138(5)

90.00

b, Å

17.0101(19)

18.034(2)

74.521(2)

80.590(3)

80.393(3)

2462.6(5)

2

c, Å

α (deg)

β (deg)

101.278(2)

90.00

93.835(10)

90.00

119.316(7)

90.00

γ (deg)

V, Å³

90.00

2122.3(5)

2

2462.2(19)

2

4277.9(12)

4

4787(3)

4

4362.7(17)

4

Z

ρ_c_a_l_cg/cm³

μ, mm⁻

1.746

1.727

1.826

1.695

1.626

1.774

1

3.559

3.083

3.539

2.817

2.733

3.085

temperature (K)

θ_m_a_x(deg)

F(000)

100

28.41

28.202

1272

26.373

2320

28.629

2424

28.160

1192

28.301

2304

1096

reﬂ collected

independent reﬂ

GOF

20 924

5306

22 743

5988

27 474

4343

48 316

11 985

1.008

39 582

11 846

1.192

21 960

5410

1.708

0.896

1.051

0.794

ﬁnal R indices

R = 0.0309 wR =

R = 0.0363 wR =

R = 0.0556, wR =

R = 0.0346, wR =

R = 0.0458, wR =

R = 0.0397, wR =

1 2

0.1041

1

2

1

2

1

2

1

2

1

2

[

I > 2σ(I)]

0.0550

0.1021

0.1141

0.0651

0.1530

R indices (all data)

R = 0.0506, wR =

R = 0.0418 wR =

R = 0.0985, wR =

R = 0.0575, wR =

R = 0.0712, wR =

R = 0.0663, wR =

1

0

2

1

2

1

2

1

2

1

2

1

2

.0571

0.1154

0.1329

0.0738

0.1752

0.1238

0

.5 h, following which the reaction solution was cooled over an ice−

EXPERIMENTAL SECTION

■

salt bath, which yielded bright reddish-yellow crystals of H L . The

highly crystalline product, once washed well with hot EtOH, was dried

ﬁrst in open atmosphere and then under vacuum; subsequently, it was

used for further complexation-mediated syntheses, yield 17.89 g

2

1

Materials and Measurements. All the reagents and solvents were

commercially available and used without further puriﬁcation. Fourier

transform IR spectra were measured on NICOLET 6700 FT-IR

Spectro-photometer using KBr Pellets. Thermogravimetric analyses

were obtained in the temperature range of 30−800 °C on PerkinElmer

1

(

8

∼92%). H NMR (400 MHz, acetone-d ; Figure S4 ): δ 8.9 (s, 1H);

6

.6 (S, 1H), 7.6 (dd, J = 7.2, 1.2 Hz, 1H), 7.4 (m, 1H), 7.3 (dd, J = 8.0,

STA 6000 analyzer under a N atmosphere at a heating rate of 10 °C

13

2

1

.2 Hz, 1H), 7.15 (m, 1H), 6.9 (m, 4H); C NMR (100 MHz,

min.

acetone-d ; Figure S7): δ 206.3, 163.9, 162.1, 151.9, 137.0, 133.7,

1

(HRMS; electrospray ionization (ESI); Figure S1): Calcd for

C H NO [M + H] : 214.0868; Found: 214.0868. Elemental

Analysis: Anal. Calcd for C H NO : C, 73.23; H, 5.20; N, 6.57.

Found: C, 73.49; H, 5.14; N, 6.71%.

Synthesis of Linker H L . Similar procedure (as for H L ) was

also employed to synthesize linker 2-(((2-hydroxyphenyl)imino)-

methyl)naphthalen-1-ol (H L ); the only alteration being the use of

equimolar 2-hydroxy-1-naphthaldehyde in place of salicylaldehyde;

6

X-ray Structural Studies. Single-crystal X-ray data for all the six

Ln compounds (1a, 2a, 3a, 1b, 2b, and 3b, respectively; see Table 1)

28.9, 121.1, 120.7, 119.7, 117.7; high-resolution mass spectrometry

2

were collected at 100 K on a Bruker KAPPA APEX II CCD Duo

diﬀractometer (operated at 1500 W power: 50 kV, 30 mA) using

graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å),

mounting on nylon CryoLoops (Hampton Research) with mineral

oil (Paraton-N, Hampton Research). The data integration and

+

1

3

11

2

13

11

2

2 1

113

reduction were processed with SAINT

absorption correction was applied to the collected reﬂections. The

software. A multiscan

2

122

1

14

structures were solved by the direct method using SHELXTL and

1

yield 20.39 g (∼85%). H NMR (400 MHz, deuterated dimethyl

sulfoxide (DMSO-d ); Figure S5 ): δ 15.7 (d, J = 9.6 Hz, 1H), 10.3 (s,

2

were reﬁned on F by full-matrix least-squares technique using the

6

115

116

SHELXL-97 program package within the WINGX program. All

non-hydrogen atoms were reﬁned anisotropically. All hydrogen atoms

were located in successive diﬀerence Fourier maps, and they were

treated as riding atoms using SHELXL default parameters. The

1H), 8.4 (d, J = 8.0 Hz, 1H), 7.9 (dd, J = 8.0, 1.6 Hz, 1H), 7.8 (d, J =

9.2 Hz, 1H), 7.7 (dd, J = 7.6, 1.6 Hz, 1H), 7.5 (m, 1H), 7.3 (m, 1H),

7.1 (m, 1H), 7.0 (dd, J = 8.0, 1.2 Hz, 1H), 6.9 (m, 1H), 6.8 (d, J = 9.2

13

Hz, 1H); C NMR (100 MHz, DMSO-d ; Figure S8): δ 177.3, 149.5,

1

6

1

17

structures were examined using the Adsym subroutine of PLATON

48.4, 137.8, 133.8, 128.9, 128.6, 128.0, 126.7, 125.8, 125.0, 123.0,

to ensure that no additional symmetry could be applied to the models.

Tables S1−S6 contain the complete crystallographic data set for these

compounds. CCDC Nos. 1420355, 1420356, 1420357, 1420358,

119.8, 119.6, 117.6, 115.9, 107.7; HRMS (ESI; Figure S2): Calcd for

+

C H NO [M + H] : 264.1024; Found: 264.1027. Elemental

1

7

13

2

Analysis: Anal. Calcd for C H NO : C, 77.55; H, 4.98; N, 5.32.

17

13

2

1420359, and 1420360 (1a, 1b, 2a, 2b, 3a, and 3b, respectively)

Found: C, 77.84; H, 5.82; N, 5.26%.

Synthesis of Linker H L . Similar synthetic protocol (as described

contain the supplementary crystallographic data for these compounds.

Additional crystallographic information is available following the

Supporting Information. X-ray powder patterns were recorded on

Bruker D8 Advanced X-ray diﬀractometer at room temperature using

Cu Kα radiation (λ= 1.5406 Å).

2

3

for H L ) was followed for linker 2-((2-hydroxybenzylidene)amino)-6-

2

1

methoxyphenol (H L ); the single essential change being the

2

3

123,124

replacement of salicylaldehyde by equimolar o-vanillin;

yield

1

20.94 g (∼94%). H NMR (400 MHz, acetone-d ; Figure S6 ): δ 8.1 (s,

6

Synthesis of Linker H L . The Schiﬀ-base ligand 2-((2-

1H); 7.8 (s, 1H), 6.5 (dd, J = 8.4, 2.8 Hz, 1H), 6.4 (m, 1H), 6.34 (m,

1H), 6.29 (dd, J = 7.6, 1.2 Hz, 1H), 6.2 (dd, J = 8.0, 1.2 Hz, 1H), 6.1

(dd, J = 7.6, 1.2 Hz, 1H), 6.0 (m, 1H), 3.1 (s, 1H); ¹C NMR (100

2

1

hydroxybenzylidene)amino)phenol (L H ) was synthesized via a

1

2

3

slightly modiﬁed synthetic protocol from the ones reported in the

118−121

literature.

under reﬂux conditions with salicylaldehyde (12.2 g, 100 mmol) in a

20 mL binary solvent mixture of toluene and ethanol (7:5, v/v) for

o-Aminophenol (10.9 g, 100 mmol) was heated

MHz, acetone-d ; Figure S9): δ 206.3, 163.7, 152.7, 151.9, 149.5,

6

136.9, 128.9, 124.9, 121.2, 120.7, 119.2, 117.4, 116.45, 56.6; HRMS

+

1

(ESI; Figure S3): Calcd for C H NO [M + H] : 244.0973; Found:

14

13

3

C

DOI: 10.1021/acs.inorgchem.6b01863

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

1

25

2

44.0977. Elemental Analysis: Anal. Calcd for C H NO : C, 69.12;

constants,

and magnetic data were corrected for diamagnetic

14

13

3

H, 5.39; N, 5.76. Found: C, 68.93; H, 5.37; N, 5.91%.

Synthesis of [Dy (L ) (L′) (CH OH) ] (1a). A methanolic solution

contributions of the sample holder.

Computational Details. To compute the g-tensors and the

energies of Kramers doublet the ab initio calculations were performed

on the complexes using MOLCAS 7.8 quantum chemistry pack-

2

1 2

2

3

2

of H L (21.3 mg, 0.1 mmol in 5 mL) and HL′ (15.4 mg, 0.1 mmol)

2

1

was deprotonated with triethylamine (41.5 μL, 0.3 mmol), to which

solid dysprosium nitrate hydrate (45.7 mg, 0.1 mmol) was added,

along with 5 mL of ethyl acetate (EtOAc). This reaction mixture was

kept in undisturbed conditions at room temperature. Yellow single

crystals of compound 1a suitable for single-crystal X-ray analysis were

obtained after slow evaporation of the binary solvent mixture of

MeOH/EtOAc over a span of just 10 h (overnight). ∼62% yield

1

26−129

age.

In this multiconﬁgurational approach, relativistic approach

was treated based on Douglas−Kroll Hamiltonian. We employed

atomic natural (ANO-RCC) basis set for the calculations of g-tensor.

The following contraction schemes were employed [8s7p5d3f2g1h]

for Dy, [4s3p2d1f] for N, [4s3p2d1f] for O, 3s2p] for C, and [2s] for

III

6

H. The ground-state atomic multiplicity of Dy is H15/2^,^w^hⁱ^c^h^r^e^s^u^l^t

in eight low-lying Kramers doublets. The CASSCF calculation

comprises an active space of nine active electrons in the seven active

orbitals (CAS (9,7)). In CASSCF calculation active space (9,7) is

adopted, and hence, 21 sextets were considered. In the last step we

−1

(

based on metal). IR (KBr, cm ): 1894 (vw), 1597 (m), 1473 (s),

1308 (s), 1169 (w), 1088 (m), 1007 (w), 918 (w), 837 (m), 737 (s),

5

63 (vw). Anal. Calcd (found) for compound 1a (C H Dy N O ):

44

42

2

12

C, 47.36 (47.64); N, 2.51 (2.69); H, 3.79 (3.38)%.

1

30

used SINGLE_ANISO code

implemented in the MOLCAS to

Synthesis of [Gd (L ) (L′) (CH OH) ](EtOAc) (1b). Pale brown

2

1 2

2

3

2

III

compute the g-tensors of Dy ions.

single crystals of compound 1b, suitable for X-ray diﬀraction analysis,

were obtained; the synthetic protocol being similar (identical molar

ratios for the respective reactants involved) to the reaction for

To further account for the exchange interaction between the metal

sites using the above acquired lowest spin−orbit states, Lines model

was used. The Lines model has been proved to be useful owing to its

intrinsic single parameter (J) consideration, which corresponds to an

eﬀective isotropic magnetic exchange interaction.

exchange coupling followed by the estimation of magnetic properties

1

31

−1

synthesizing 1a. ∼59% yield. IR (KBr, cm ): 1778 (vw), 1593 (m),

470 (w), 1304 (s), 1169 (vw), 1091 (m), 1007 (w), 930 (vw), 837

m), 741 (s), 563 (w). Anal. Calcd (found) for compound 1b (C₄₈H₅₀

Gd N O ): C, 47.20 (46.98); N, 4.59 (4.88); H, 4.13 (4.07)%.

1

(

5

3,132−134

The Lines

2

4

14

of the dinuclear complex were undertaken using the POLY_ANISO

Synthesis of [Dy (L ) (L′) (CH OH) ] (2a). A methanolic solution

2

2 2

2

3

2

135−137

program,

interfaced with the SINGLE_ANISO module. The

of H L (26.3 mg, 0.1 mmol in 3 mL) and HL′ (15.4 mg, 0.1 mmol)

2

resultant exchange spectrum and correlated wave functions of the

dinuclear complexes were used to determine temperature and ﬁeld-

dependent magnetic properties of the dinuclear complexes.

was deprotonated with triethylamine (41.5 μL, 0.3 mmol), to which

solid dysprosium nitrate hydrate (45.7 mg, 0.1 mmol) was added,

along with 7 mL of EtOAc. The homogeneous solution was kept

uninterrupted at room temperature, as yellow platelike crystals of

compound 2a suitable for single-crystal X-ray analysis emerged on

slow evaporation of the binary solvent mixture of MeOH/EtOAc just

Density Functional Theory Calculations. Density functional

calculations were performed for complexes 1b−3b using the B3LYP

138

139

functional,

with the G09 code.

We used the double-ζ quality

140

basis set employing Cundari-Stevens (CSDZ) relativistic eﬀective

core potential on Gd atom, TZV basis set for the rest of the atoms.

−1

within 4 h. ∼71% yield. IR (KBr, cm ): 1724 (w), 1597 (s), 1470

141

(

5

m), 1381 (m), 1277 (s), 1092 (m), 1011 (vw), 837 (s), 741 (s), 552

The density functional theory (DFT) calculations combined with the

broken symmetry (BS) approach were employed to estimate exchange

m). Anal. Calcd (found) for compound 2a (C H Dy N O ): C,

54

2

14

0.67 (51.10); N, 2.19 (2.31); H, 4.25 (4.89)%.

III

constant between the Gd −Gd ions. This computational approach is

Synthesis of [Gd (L ) (L′) (CH OH) ] (2b). Dark red single

2

2 2

2

3

2

proven to be reliable for computing exchange coupling constants in

crystals of compound 2b, suitable for X-ray diﬀraction analysis, were

formed from an analogous reaction as the one for synthesizing 2a.

142,143

Gd(III) complexes.

The simulation of magnetic susceptibility

144

data (1b−3b) is obtained using PHI software. DFT calculations on

−

1

∼

64% yield. IR (KBr, cm ): 1905 (vw), 1724 (w), 1601 (m), 1485

m), 1385 (s), 1281 (m), 1169 (w), 1092 (m), 1007 (vw), 837 (m),

25 (s), 552 (w). Anal. Calcd (found) for compound 2b

C H Gd N O ): C, 51.81 (51.03); N, 2.32 (2.45); H, 3.85

145

complexes 1a−3a were performed using ORCA software.

In

(

146

ORCA, BS-DFT

calculations, we employed B3LYP functional

1

47−149

7

(

conjunction with SARC−DKH

rest of the elements.

basis set for Dy and TZVP for

52

46

2

12

3.51)%.

Synthesis of [Dy (L ) (L′) (CH OH) ] (3a). A methanolic solution

2

3 2

2

3

2

RESULTS AND DISCUSSION

of H L (24.3 mg, 0.1 mmol in 3 mL) and HL′ (15.4 mg, 0.1 mmol)

■

2

3

was deprotonated with triethylamine (41.5 μL, 0.3 mmol), to which

solid dysprosium nitrate hydrate (45.7 mg, 0.1 mmol) was added,

along with 7 mL of EtOAc. This reaction mixture was kept in

undisturbed conditions at room temperature, as yellow single crystals

of compound 3a, suitable for X-ray diﬀraction analysis, were obtained

after slow evaporation of the binary solvent mixture of MeOH/EtOAc

All the aforementioned six compounds’ family (1a, 1b, 1c, 2a,

b, and 2c) were prepared at room temperature by slow

evaporation of the respective low-boiling reaction solvent

mixtures like MeOH or EtOAc. Single-crystal X-ray analysis

technique was employed to indisputably determine the

structures for all of them. With the salicylaldehyde-based

−1

over a span of just 6 h. ∼66% yield. IR (KBr, cm ): 1878 (vw), 1732

Schiﬀ-base linker L in unison with the new-fangled bridging

1

(

m), 1612 (m), 1473 (m), 1385 (vw), 1311 (w), 1173 (vw), 1092 (w),

ligand L′, the compounds 1a and 1b got crystallized in

968 (w), 849 (s), 725 (s), 555 (w). Anal. Calcd (found) for compound

monolinic space groups P2 /n and P2 /c, with Z = 2 and Z = 4,

3

a (C₄₆H₄₆Dy N O ): C, 46.99 (46.68); N, 2.38 (2.44); H, 3.94

1

2

14

(

3.78)%.

respectively; the more conjugated naphthalene-based linker L

derived analogous compounds 2a and 2b crystallized in

monoclinic and triclinic space groups P2 /n (Z = 2) and P1

-

2

Synthesis of [Gd (L ) (L′) (CH OH) ] (3b). Bright red single

2

3 2

2

3

2

crystals of compound 3b, suitable for X-ray diﬀraction analysis, were

derived from an analogous reaction as the one for synthesizing 3a.

1

̅

(

Z = 2), respectively. As a correlated ﬁnding, the ubiquitously

−

1

∼

69% yield. IR (KBr, cm ): 1855 (vw), 1728 (m), 1601 (s), 1385

used o-vanillin-based ligand L₃led to the formation of

compounds 3a and 3b, both endowed with the orthorhombic

crystallographic space group Pbca.

(

4

m), 1470 (m), 1308 (m), 1092 (s), 952 (vw), 849 (m), 725 (s), 552

w). Anal. Calcd (found) for compound 3b (C₄₆H₄₆Gd N O ): C,

7.41 (47.09); N, 2.40 (2.46); H, 3.9 (3.99)%.

Magnetic Measurement (Experimental) Details. All magnet-

2

14

The molecular structures of compounds 1a and 1b (with

partial labeling), presenting the symmetric dinuclear core, is

shown in Figure 1, while the similar ones for compounds 2a−

ization data were recorded on a Quantum Design MPMS-XL7 SQUID

magnetometer. The variable-temperature magnetization was measured

with an external magnetic ﬁeld of 1000 Oe in the temperature range of

2

b and 3a−3b are illustrated in Figures 2 and 3, respectively.

1

.9−300 K. Diamagnetic corrections for the bulk phase-pure

The compound 1a is composed of two crystallographically

polycrystalline compounds were estimated from the Pascal’s

unique Dy1 atoms bridged by two μ -oxo (O4) bridges from

2

D

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Inorganic Chemistry

Article

Figure 3. Partially labeled molecular structures of complexes (a) 3a

and (b) 3b, developed by virtue of the mixed-ligand strategy of

employing H₂L₃and HL′ both. Hydrogen atoms and solvent

molecules were omitted for clarity.

Figure 1. Partially labeled molecular structures of complexes (a) 1a

and (b) 1b, harnessed from the mixed-ligand strategy approach

adopted herein. Hydrogen atoms and solvent molecules were omitted

for clarity.

center, coordinate to each of the two Dy-centers. On the one

hand, two methanol molecules coordinating via O6 satisfy the

octacoordination environment for each of the Dy(III) centers.

On the other hand, compound 1b (adopted space group: P2₁/

c) composed of exactly same set of units; coordinating ligands

assembles with Gd(III) to crystallize in two crystallographically

unique, but structurally similar, Gd ; consequently (see Table

2

S18 ), only one is considered in the ensuing discussion. The O4,

N1, and O6 atoms of the tridentate Schiﬀ-base L coordinate to

1

each of the crystallographically identical Gd1 centers; O5 and

O1 centers of the L′ ligand act as the ones from an apposite

capping ligand, while simultaneously bridging the Gd1 centers

via O2 center. For the other crystallographically distinct core

comprising of Gd2 pair, centers O8, O9, and N2 serve as the

chelating centers from the Schiﬀ base L , alongside O7, O11

1

centers capping the two Gd2 centers’ coordination spheres held

by a bridging μ -oxo center (O13). The intermetallic distances

2

among the two Ln centers (two pairs of Gd1 and Gd2) in the

2

symmetric complex 1b are nearly similar: 3.899 and 3.887 Å,

respectively, being comparable to that in the single Dy core of

2

1

a, 3.833 Å. The Dy−O−Dy bond angles in compounds 1a and

b are 111.12° (Dy1−O4−Dy1) and 111.87° (Gd2−O12−

Gd2), 112.97° (Gd2−O12−Gd2), respectively.

Figure 2. Partially labeled molecular structures of complexes (a) 2a

and (b) 2b, stemming out of the mixed-ligand strategy approach

followed herein. Hydrogen atoms and solvent molecules were omitted

for clarity.

The molecular architectures of Ln compounds 2a and 2b

2

(with requisite labeling) is shown in Figure 2, while the similar

ones for compounds 3a and 3b are exhibited in Figure 3. The

compound 2a is constituted from two crystallographically

distinctive Dy1 atoms bridged by two μ -oxo (O5) bridges

2

the two L′ linkers, which also coordinates in bidentate fashion

stemming out of the two L′ linkers, which also involves

bidentate coordination via O6 and O4 centers. The analogous

ligating centers for compound 2b composed of two crystallo-

via O5 and O3 centers. Considering deprotonated form of the

3

κ Schiﬀ-base L , two O-centers O2 and O1, along with N1

1

E

DOI: 10.1021/acs.inorgchem.6b01863

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Inorganic Chemistry

Article

graphically distinguishable Gd units are μ -oxo (O11), O8,

interlanthanide distances and the metal−ligand−metal nodal

angles is indeed observed, owing to the strategically introduced

manipulation in the Schiﬀ-linker environment of the tuned

linker functionality. This might play as one of the crucial

contributing factors behind the ensuing exhibition of

considerable alterations in their magnetic properties.

2

O10 (for Gd2 center) and μ -oxo (O3), O4, O5 (for Gd1

2

center). While O2, N1, and O1 atoms of the two coordination

pocket-containing imine ligand L coordinate to the Dy(III)

2

centers, the pairs of O9, N2, O7 (Gd2) and O1, N1, O6 (Gd1)

enjoy favorable ligation to the Gd(III) centers in 2b. Terminal

methanol molecules coordinating via O3 (Dy1 in 2a), O12 (for

Gd2 in 2b), and O2 (via Gd1 in 2b) fulﬁll the eightfold square

antiprismatic coordination geometry adopted by each of the

Ln(III) centers in case of this related pair. The interlanthanide

distance between the two metal centers for the Dy(III)

complex 2a being 3.831 Å, is evidently quite close to the ones

observed for the double core-complex 2b: 3.851 Å (Gd2) and

The systematic analysis on the coordination geometry was

150

performed using SHAPE software

on the octacoordinated

lanthanide ions of all the complexes. The results reveal that all

the complexes are found to be intermediate geometry between

square antiprism (D ) and dodecahedron (D ; Tables S17 and

4d

2d

S18 ). The slight deviation was observed within Gd(III) and

Dy(III) analogues due to the various substituents. Within a

complex both the Gd(III) and Dy(III) centers show similar

values, which clearly indicates that both the centers are highly

symmetry. All the Dy(III) complexes (1a−3a) show similar

values due to its isotypic environment.

3

1

.837 Å (Gd1), respectively. The Dy1−O5−Dy1 bond angle of

11.52° observed in compound 2a (Dy1−O4−Dy1) analo-

gously has a proximity to the 110.79° (Gd2−O11−Gd2) and

1

2

10.43° (Gd1−O3−Gd1), respectively, observed for complex

b.

Direct-current (dc) magnetic susceptibility studies of 1a−3b

were performed in an applied magnetic ﬁeld of 1 kOe over the

The isotypic compounds 3a and 3b constituted from two

sets of crystallographically unique Ln1 (Ln = Dy and Gd for 3a

and 3b, respectively) atoms, which are similarly bridged by two

μ₂-oxo (O6 and O1, respectively) bridges arising from the

respective L′ linkers (see Table S18). This pair also engages

bidentate coordination of L′ via the combination of O5, O7 and

O6, O2 paired centers (for compounds 3a and 3b,

respectively). Interestingly enough, tridentate ligand L₃

possesses three coordination pockets, unlike the double-

containing ones realized in its previously discussed congeners

L and L , owing to the presence of chelating methoxy moiety

temperature range of 2−300 K. The plots of χ

where χ is the molar magnetic susceptibility, are shown in

Figure 4. The observed χ T products at 300 K for all

T versus T,

M

1

2

(O2 and O5 for 3a and 3b, respectively) in ortho position to

the chelating hydroxyl group therein. Unlike literature reports

of this linker utilizing its all three coordination pockets for

92

complexation with 4f ions, this compound’s architecture

presents only tridentate coordination of L , merely exploiting

3

its two imine bond-adjacent pockets. The ligating centers of

Schiﬀ-base L for compounds 3a and 3b are O1, N1, O3 and

3

O4, N1, O7, respectively; leading to two symmetric Ln₂

structures with Dy−Dy and Gd−Gd distances of 3.837 and

3.859 Å alongside Dy1−O6−Dy1 angle assuming 111.60° and

a proximally anticipated Gd1−O1−Gd1 angle of 110.81°. The

square antiprismatic geometry for all the Dy and Gd centers are

requisitely completed by the linked methanol molecules

donating via O4 to Dy1 (in 3a) and O3 to Gd2 (in 3b).

Figure 4. Temperature dependence of χ T products at 1 kOe for 1a−

M

3b. The solid red lines are best ﬁts as described in the text.

2

Considering all the six complexes, apart from the μ -η :η

compounds are almost in good agreement with the expected

value for two uncoupled lanthanide ions. When cooled,

bridged L′ spacer, the coordination modes adopted by the

constituent analogous Schiﬀ-base ligands are precisely matching

however, the χ T values show diﬀerent tendency, which is

M

3

(

Figure S20). In addition to the anticipated η binding mode

dependent on lanthanide ion. Herein, we classify the

compounds based on lanthanide ions to describe the magnetic

properties.

assumed by L and L , ligand L in spite of possessing one

1

2

3

additional chelating pocket prefers to bind via the same η

coordination mode to the lone Ln(III) centers on each of the

For complexes 1a and 2a, the χ T values at 300 K are 29.15

M

3

−1

vertices of the Ln architecture leading to the exact resemblance

and 28.57 cm K·mol , respectively, which is in agreement

3

2

−1

of the linker binding modes. Thermogravimetric analysis data

recorded for all these six isotypic compounds (Supporting

Information, Figures S16 and S17) show signiﬁcant thermal

stability up to nearly 155 °C with almost no weight loss,

corresponding to the absence of any guest molecule in all the

air-dried phases of this dinuclear family. Powder X-ray

diﬀraction studies for all the six compounds (1a, 1b, 2a, 2b,

with the expected value of 28.36 cm K·mol for two

III 6

uncoupled Dy ions ( H₁₅_/₂, J = 15/2, g = 4/3). But the

χ T value of 26.62 for 3a at 300 K is slightly smaller than the

M

6

theoretical value due to the splitting of the H

state.

ground

1

5/2

1

51−154

The χ T products gradually decrease with

M

lowering temperature, reaching a minimum value of 26.29,

3

−1

28.59, and 25.93 cm K·mol at ∼40 K, which is mainly

3a, and 3b) reveal highly crystalline characteristic proﬁles

ascribed to the depopulation of the Stark sublevels and/or

signiﬁcant magnetic anisotropy present in Dy systems. The

155

signifying phase purity for each of the discussed series of

coordination complexes. As discussed in the aforementioned

discussion highlighting the coordination environment features

for the Ln₂family, the anticipated variation of the

χ T value then increases sharply to a maximum of 32.85, 36.75,

M

3

−1

and 29.91 cm K·mol at 2 K for 1a, 2a, and 3a, respectively,

indicating the presence of weak intramolecular ferromagnetic

F

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Inorganic Chemistry

Article

III

interactions between Dy spin carriers as observed in other

104,156

dysprosium systems.

The ﬁeld-dependent magnetizations

for all compounds are shown in Figures S21−S29. For 1a, 2a,

and 3a, the magnetization rises abruptly at low ﬁelds and

reaches 11.60, 15.64, and 10.10 μ at 7 T, without saturation

B

(

Figure S21 −S23 ). This suggests the presence of magnetic

157

anisotropy and/or the considerable crystal-ﬁeld eﬀects. The

non-superposition of the M versus H/T data (Figure S27−S29)

also suggests the presence of signiﬁcant magnetic anisotropy

and/or low-lying excited states.

III

With the replacement of Dy ions with Gd ions, analogous

compounds 1b, 2b, and 3b are isolated. The χ T products at

M

room temperature are close to the expected value for two

III

uncoupled Gd ions. As the temperature is lowered, the χ_MT

^p^r^o^d^u^c^t^r^e^m^aⁱⁿ^s^c^oⁿ^s^t^aⁿ^t^uⁿ^tⁱ^l³⁰^K^,^a^t^w^hⁱ^c^h^p^oⁱⁿ^t^χM^T

Figure 6. (a) Temperature-dependent ac susceptibility for (a) 1a and

(b) 2a in the absence of a static ﬁeld.

monotonically decreases, ultimately reaching 11.70, 10.87, and

3

−1

1

0.81 cm K·mol at 2 K. To evaluate the coupling parameters,

153

the expression

Similar SMM behavior can be observed in 2a, which exhibits

a peak at 12 K and rapid increase below 5 K (Figure 6b). The

rapid increase in the low-temperature region could be

attributed to quantum tunneling eﬀect at zero dc ﬁeld. The

χ″ peaks show a frequency dependence in the high-temperature

region, signaling the relaxation of the spins through the

anisotropy barrier. When decreasing the temperature, the

maxima of the out-of-phase χ″ peaks shift toward lower

frequency until 3 K, then maintain in the same frequency,

conﬁrming the classic quantum regime. Meanwhile, an increase

in the χ″ component observed again in the low-temperature

̂

Z

H = −J·S ·S + β(S ·g + S ·g )·H

A

B

A

B

A

B

is applied on compounds 1b−3b with S = 7/2. The ﬁts for

compounds 1b−3b give J = −0.07(9), −0.09(2), and −0.09(7)

−

1

cm , respectively. This negative J conﬁrms the anti-

III

ferromagnetic interaction is very weak between the Gd

centers. The ﬁeld-dependent magnetization approximate to

III

the expected values for two isolated Gd ions without

saturation (Figures S24−S26) suggests the presence of the

157

considerable crystal-ﬁeld eﬀects.

71

region is also typical of the onset of tunneling relaxation.

In view of the SMM behavior, alternating-current (ac)

susceptibility measurements were also performed under zero dc

ﬁelds for 1a, 2a, and 3a (Figures 5, 6, and S30 ). Temperature-

From the one set of peaks χ″(υ) curves, we can see that the

thermally activated spin reversal is gradually replaced at low

temperature by a tunneling relaxation mechanism (Figure 5b).

To study the magnetic process, Cole−Cole plots of χ″ versus

χ′ (Figure 7a,b for 1a and 2a, respectively) were constructed

Figure 7. (a) Cole−Cole plots for complex 1a between 8 and 14 K;

the solid lines are the best ﬁts to the experimental data, obtained with

the generalized Debye model with α parameters in the range of 0.10−

0

.16. (b) Cole−Cole plots for complex 2a between 6 and 22 K; the

Figure 5. Frequency-dependent ac susceptibility for 1a (left) and 2a

solid lines are the best ﬁts to the experimental data, obtained with the

generalized Debye model with α parameters in the range of 0.09−0.24.

(

right) in the absence of a static ﬁeld.

dependent in-phase (χ′) and out-of-phase (χ″) magnetic

susceptibility signals for 1a exhibit a peak at 14 K; when

cooled, a new peak tail of χ′ and χ″ is observed below 5 K

and ﬁtted to a generalized Debye model to determine α values

and relaxation times τ in the temperature ranges of 8−14 and

6−22 K for 1a and 2a, respectively. The plots reveal relatively

symmetrical semicircles, indicating a single relaxation process,

with α values in the ranges of 0.10−0.16 and 0.08−0.24 for 1a

and 2a, respectively, indicating a broad distribution of

relaxation times. On the basis of the ac susceptibility analysis,

a higher energy can be expected for 1a compared to 2a because

of the relatively weak tunneling of magnetization in 1a. To

further investigate this prediction, the energy barriers were

evaluated with ﬁtting to the Arrhenius law (Figure 8). The

magnetization relaxation time τ was extracted from the maxima

(

Figure 6a). Frequency-dependent susceptibility data were

collected in the range of 1−1500 Hz under zero applied dc

ﬁeld. As shown in Figure 5a, the maximum values of χ″ were

temperature-dependent between 4.5 and 13 K over the entire

frequency range. This indicates that the relaxation does not

cross over to a pure mechanism of quantum tunneling

mechanism (QTM) at temperatures above 4.5 K, contrary to

III

what is often observed for the majority of Ln -based SMMs,

99,158

where the QTM is fast.

G

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Inorganic Chemistry

Article

the complexes 1b−3b. The DFT-computed J values are found

−1

−

1

−1

to be −0.034 cm (1b), −0.054 cm (2b ), and −0.049 cm

(

3b; see Table S8 in Supporting Information). All the

complexes found to exhibit weak anti-ferromagnetic exchange

interactions due to weak overlap between the 4f orbitals. The

computed J values are in agreement with the experimental

estimate, and it reproduces not only the sign of J but also their

magnitude and the trends. The magnitude of J shows the

presence of weak exchange interactions as evidenced in other

III

142

[

Gd −Gd ] complexes. The comparison of experimental

Figure 8. Magnetization relaxation time constant τ vs T⁻¹for 1a (left)

and 2a (right) in a zero static ﬁeld from best ﬁt to the Arrhenius law of

the thermally activated regime (solid line).

and calculated magnetic susceptibility (simulated based on the

DFT J values) for complexes 1b−3b are shown in Figure 10. It

of the out-of-phase peaks. The best ﬁts yield eﬀective energy

barriers of U = 69 K with pre-exponential factor τ = 1.37 ×

eff

0

−6

1

0

s, and U_e_f_f= 51 K with τ = 4.54 × 10 s for 1a and 2a,

0

respectively. The barrier of compound 2a seems to be smaller

compared with that of 1a as a result of the faster quantum

tunneling.

The dynamics of magnetization for 3a were also investigated

from ac susceptibility measurements, in a zero static ﬁeld at the

indicated frequencies given in Figure 9a. The χ″ component of

Figure 10. A comparison of the experimental and calculated (solid

line) magnetic susceptibility for complexes 1a−3b.

clearly demonstrates that the simulation of the magnetic

properties with the computed exchange constants reproduces

the magnetic susceptibility exactly. Among the three complexes,

complex 1b is found to less anti-ferromagnetic than complexes

2b and 3b, and this is correlated to relatively large Gd−O−Gd

angles found in complex 1b compared to complexes 2b and 3b

(average Gd−O−Gd angles are 112.4°, 110.6°, and 110.8° for

complexes 1b, 2b, and 3b, respectively). This is consistent with

Figure 9. (a) Temperature dependence of the out-of phase ac

susceptibility for 3a in the absence of a static ﬁeld. (b) Natural

logarithm of the ratio of χ″ over χ′ vs 1/T of the data for 3a given in

Figure 9a. Slope corresponding to energy barrier U = 1.0 K.

eff

the susceptibility has a strong frequency dependence below 8 K

to the lowest measured temperature of 1.9 K, indicating the

onset of the slow magnetization (M) relaxation expected for

SMM behavior.

142

the Gd−O−Gd magneto−structural correlation proposed.

To understand the mechanism of magnetic coupling, the

computed spin densities are analyzed. The net J value has

contributions from ferromagnetic part and anti-ferromagnetic

parts, and the sign of exchange is decided by the dominating

factor. As per our earlier mechanism of magnetic exchange for

{Gd−Gd} pair, the anti-ferromagnetic contributions arise from

the overlap between the 4f orbitals, while the polarization by

the 4f orbitals to the empty 5d/6s/6p shells contribute to

Although the expected maximum due to blocking could not

159

be observed down to this temperature, a method assuming

that there is only one characteristic relaxation process of the

Debye type with one energy barrier and one time constant.

With this assumption, one can evaluate roughly the energy

barrier and τ based on the following relationship (eq):

0

ln(χ″/χ′) = ln(ωτ ) + U /kT

eff

142

°

ferromagnetic coupling. Relatively larger Gd−O−Gd angles

This methodology had been applied earlier in the

determination of τ in Mn₁₂acetate

estimated for complexes 1b−3b, and the low symmetric nature

leads to signiﬁcant overlap between the 4f orbital pairs. This

contribute signiﬁcantly for the anti-ferromagnetic part of the

exchange leading to negative Js in all three complexes. The

computed spin densities for complex 1b is shown in Figure 11

(S = 0 state; for complexes 2b and 3b see Figure S32 of

160

and U_e_f_fand τ in

0

Fe Ln. As shown in Figure 9b, by ﬁtting the experimental χ″/χ″

3

−4

data to eq, the parameter values U ≈ 1.0 K and τ ≈ 1 × 10

eff

0

s were obtained. A more precise result must wait for very low-

temperature measurements (T < 1 K) by using a micro-

SQUID.

III

Supporting Information). The spin densities of Gd ion

Theoretical Studies. To gain further insight into the

mechanism of magnetic relaxation and to rationalize the

diﬀerence in the observed magnetic behavior we performed ab

initio and DFT calculations for the complexes 1a−3a and 1b−

(∼7.02 for both) show the presence of spin polarization, as the

4f orbitals are deeply buried. It is noteworthy that spin

delocalization is poor is due to the contracted nature of 4f

orbitals, whereas spin polarization is exceeded leading to

opposite spin densities on the atoms connected to Gd.

Eventually the bridging oxygen atoms are having more spin

densities (due to polarization of the both Gd atoms) than other

3b, respectively.

To evaluate exchange parameter and their relationship

between the structures, the DFT studies were performed on

H

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Figure 12. (a−c) Ab initio SINGLE_ANISO computed magnetization

blocking barrier for 1a−3a. The thick black lines imply the Kramers

doublet as a function of magnetic moment. The green lines indicate

the possible pathway of the Orbach (Raman) contribution of magnetic

relaxation. The blue lines show the most suitable relaxation pathway

for magnetization reorientation. The red lines correspond to the

QTM/TA-QTM of relaxation contribution between the connecting

pairs. The black texts near the black lines reveal the nature of the KDs

in terms of major wave function contribution. (d, e) Ab initio

POLY_ANISO computed low-lying exchange spectrum and position

of the magnetization blocking barrier (yellow dashed line) for 1a−3a.

Here, the thick black lines represent exchange-coupled energy levels,

while the solid lines signify similar description as for SINGLE_ANISO

results. Here the red lines indicate tunnel splitting within the

exchange-coupled states of the exchange spectrum.

Figure 11. Spin density plot of the complex 1b (a) high spin and (b)

broken symmetry shown with isosurface values of 0.001 Å .

−

3

atoms, and this favors the weak anti-ferromagnetic coupling as

142

reported in our earlier studies.

To understand the mechanism of magnetic relaxation in

Dy(III) complexes, ab initio CASSCF calculations are

performed. The qualitative mechanism of magnetic relaxation

of the uncoupled Dy(III) single ion corresponding to

complexes 1a−3a are shown in Figures 12a−c. Here, the x-

axis indicates the magnetic moment of each state along main

magnetic axis, while y-axis denotes the energy of the respective

states. The numbers shown in Figure are the mean absolute

values of the matrix elements of the magnetic moment. The

energy of each Kramers doublet (KD) on each individual

Dy(III) ion is given in Tables S12 −S14 in Supporting

Information. The large value of g (g = 20) reveals the

than Orbach process. In addition, the overestimated excitation

energies of the local Dy(III) multiplets, as obtained in CASSCF

163

approximation, might also be one of the reasons.

The crystal-ﬁeld parameters were investigated for better

understanding of the relaxation mechanism (Table S15 ).

Interestingly, the major part of the ligand-ﬁeld eﬀect comes

from both axial and nonaxial parameters. The two Dy(IIII) ions

experience symmetric ligand ﬁeld, which is evident in the local

anisotropy axes. The nearly equal strength of the axial and

nonaxial terms suggests that it will not relax via ground-state

KD but via the ﬁrst excited KD.

zz

presence of large magnetic moment, approaching that expected

161

for a pure m = ±15/2 state. It was observed that QTM is

J

suppressed in all the cases due to the pure Ising nature of the

ground-state anisotropy. As shown in Table S10, the energy

spectrum of the states arising from the ground-state free ion

diﬀers in 1a−3a. The large energy separation between the

ground and excited doublet is observed in all the cases.

To further understand the mechanism of magnetic relaxation,

dinuclear Dy(III) relaxation mechanism diagrams are con-

structed using POLY_ANISO program employing Lines

model. Within the dimer, dipolar interaction between the

Dy(III) centers were calculated and given in the Table 2. With

the Lines parameter, the simulated susceptibility (J_D_y₋_D_y) for

Signiﬁcant tunneling at the excited KDs suggest TA-QTM

via ﬁrst excited state to be operational for magnetic relaxation,

and this suggests the barrier heights of 159, 159, and 147 cm⁻¹

for complexes 1a−3a, respectively. These values are much

−

1

complex 1a−3a are −0.090, 0.13, and −0.097 cm ,

respectively. This yielded a reasonable ﬁt to the experimental

data for complexes 1a−3a as shown in Figure 10 (total

higher than the estimated U values. This may be due to the

coupling parameter J ). Interestingly, for complex 2b

eff

tot

computational limitation of not considering intermolecular

calculations yield ferromagnetic coupling, while 1a and 3a are

estimated to be anti-ferromagnetically coupled. As the

experimental susceptibility obtained is slightly higher than the

calculated one, a scaling factor of 6−8% for the calculated

162

interactions and QTM between the ground-state KDs. The

same was observed in the literature that the energy diﬀerence

observed is clear indication of several relaxation pathways other

I

DOI: 10.1021/acs.inorgchem.6b01863

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 2. Exchange and Dipolar Interactions and Computed Anisotropic g-Tensor Values (g , g , and g ) Obtained from

xx

yy

zz

−1

POLY_ANISO Calculation for Complexes 1a−3a [cm ]

interaction

1a

2a

3a

dipolar

exchange

total

0.180

−0.090

0.090

0.200

0.130

0.330

0.194

−0.097

0.097

doublet

g_x_x

Δ_t_u_n

g_x_x

Δ_t_u_n

g_x_x

Δ_t_u_n

^gyy

g_y_y

^gzz

g_z_z

7.7 × 10⁻

1.1 × 10⁻

2.0 × 10⁻

0.000 20

0.000 17

0.000 11

06

05

0.00

39.39

0.00

0.040

0.00

35.60

0.00

35.60

0.00

0.900

0.00

0.889

4.1 × 10⁻⁰⁷

2.1 × 10⁻⁰⁶

0.000 32

0.000 34

0.000 36

0.000 29

0.00

39.26

0.00

0.026

0.00

35.75

0.00

35.74

0.00

0.002

0.00

0.003

2.0 × 10⁻⁰⁵

3.1 × 10⁻⁰⁵

0.000 11

0.000 60

0.000 57

0.000 33

1

2

3

4

5

6

0.00

0

3

.00

9.31

0.00

0

.00

.023

0.00

0

3

.00

6.11

0.00

0

3

.00

6.10

0.00

0

.00

.013

0.00

0

.00

.028

curves are required. This may be attributed to the loss of

solvation, as possible evaporation of coordinate/uncoordinated

solvent molecules are not often accounted for corrections. The

estimated exchange interaction is smaller than that of the

dipolar magnetic coupling; as the result, exchange and dipolar

parts are not stabilizing the nonmagnetic state. The Dy−Dy

axes slightly favor the strong dipolar magnetic interaction and

thus the parallel arrangement of the local magnetic axes arises.

To validate the computed exchange parameter, we computed

the Dy −Dy exchange using broken symmetry DFT

approach implemented in Orca software. The result reveals

that the calculated Js values are in agreement with the J

exchange obtained from POLY_ANISO program. The

established mechanism of magnetic relaxation for the dinuclear

systems is in agreement with the dynamic magnetic

susceptibility data measured. Besides the diﬀerence in the

observed barrier heights are found to be correlated to the

structural alterations.

The calculated local magnetic moments in the ground

exchange doublet state and the main magnetic anisotropy axis

are shown in Figures 12 and Figure 13, respectively. Because of

the symmetric nature the orientation of the local magnetic

anisotropy axes and magnetic moments are parallel to each

other. The orientation of the principle anisotropy axes of the

Dy(III) ions are perpendicular to the two bridging ligands. And

it lies close to the Dy−O bonds as expected. The g -tensor for all

the Dy(III) ions are mostly Ising type (Tables S12 −S14 ). For

complexes 1a−3a, six exchange Ising doublets are obtained

from six lowest-lying KDs on the two Dy(III) sites (Table S16,

Figure 12d,e). The exchange states in Figure 12d,e are arranged

in compliance with their corresponding maximal magnetic

moments. The lowest exchange levels were grouped into

components are found to be nearly zero (g = g = 0 < 1 ×

xx yy

9

−

10 ), and orientation of the magnetic anisotropy of the ground

state resembles that of the Dy(III) single-ion behavior.

However, the tunnel splitting (Δtu) for the ground state is

−

6

estimated to be diﬀerent with values larger than 7.7 × 10

−1

cm estimated for complexes 1a and 2a, while relatively larger

−5

−1

tunnelling splitting observed for complex 3a (2 × 10 cm ),

suggesting possible ground-state tunnelling for this com-

164,165

plex.

This suggests relaxation of magnetization for

III

complexes 1a and 2a via the third excited KD placing the

^e^s^tⁱ^m^a^t^e^o^f^Ucal as 159 and 160 cm⁻for complexes 1a and 2a,

respectively.

1

Relatively larger tunnelling at the ground state for complex

3

a suggests meager barrier height, and this is consistent with

the experimental estimates. The estimated barrier heights for

complexes 1a and 2a are however much larger than the

experimental estimate, and this may be attributed to the fact

^t^h^a^tⁱⁿ^t^h^e^e^s^tⁱ^m^a^tⁱ^oⁿ^o^f^Ucal value other eﬀects such as

intermolecular interaction/hyperﬁne interaction/non-Orbach

53,137,162,163,166,167

mechanisms are not taken into consideration.

167 Although the diﬀerences are marginal, slightly larger

,

tunnelling gap for complex 3b observed is due to small

diﬀerence in the structural parameters leading to enhanced

transverse anisotropy and larger tunnelling gap at the coupled

Dy(III) states. This suggests the importance of ﬁne-tuning the

barrier height by employing diﬀerent ligand architecture.

CONCLUSION

■

On a closing note to this report, a new family of dinuclear and

symmetric lanthanide complexes, comprising of six congeners,

which have been assembled by adopting a mixed-ligand strategy

based on three minutely altered linkers with small yet distinct

changes in one particular constituent ligand functionality’s

electronic nature. DFT was employed to rationalize the

doublets, and these doublets are split by tunnel splitting (Δ )

tun

as indicated. For all the exchange-coupled states, the transverse

J

DOI: 10.1021/acs.inorgchem.6b01863

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

reﬁnement parameters, plots of measured and calculated

magnetic susceptibility with temperature, spin density

plots, computed spin density values, temperature-

dependent ac susceptibility, reduced magnetization

versus H/T, illustrated coordination modes of ligands,

calculated J values, tabulated energies of spin-free and

spin-orbit states, tabulated results of ab initio calcu-

lations, computed crystal ﬁeld parameters, crystal ﬁeld

Hamiltonian parameter, computed anisotropic g-tensor

values, SHAPE measures. (PDF )

X-ray crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Authors

E-mail: rajaraman@chem.iitb.ac.in. (G.R.)

E-mail: tang@ciac.ac.cn . (J.T.)

E-mail: sghosh@iiserpune.ac.in. Fax: +91 20 2590 8186.

■

*

(

S.K.G.)

Author Contributions

The manuscript was written through contributions from all

authors. J.L. and G.V. contributed equally as second authors.

Notes

The authors declare no competing ﬁnancial interest.

⊥

J.L. and G.V. contributed equally as second authors.

Additional crystallographic information is available. These data

can be obtained free of charge from Cambridge Crystallo-

graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ACKNOWLEDGMENTS

■

S.M. is thankful to IISER Pune for research fellowship, while

the research facilities provided by the same are also sincerely

acknowledged. DST (Project No. GAP/DST/CHE-12-0083) is

acknowledged for the ﬁnancial support. DST-FIST (SR/FST/

CSII-023/2012) is acknowledged for microfocus SC-XRD

facility. G.R. would like to thank the DST (No. EMR/2014/

0

00247) for funding. G.V. would like to thank Indian Institute

Figure 13. Orientation of the main local magnetic moment in the

ground exchange doublet state of the Dy(III) ion in the complexes (a)

of Technology Bombay for Post-Doctoral Fellowship.

1

a, (b) 2a, and (c) 3a, where H atoms were omitted for clarity (color

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Influence of Tuned Linker Functionality on Modulation of Magnetic Properties and Relaxation Dynamics in a Family of Six Isotypic Ln2 (Ln = Dy and Gd) Complexes

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