CQBCentro

Química e Bioquímica

A Mn(III) single ion magnet with tridentate Schiff-base ligandsS. Realista,a A. J. Fitzpatrick,b G. Santos,a L. P. Ferreira,c,d S. Barroso,e L. C. J. Pereira,f N. A. G. Bandeira,a,g P. Neugebauer,h J. Hrubý,h G. G.

Morgan,b J. van Slageren,h M. J. Calhordaa and P. N. Martinhoa

aCentro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal. Email: pnmartinho@ciencias.ulisboa.pt

bSchool of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland.

cBioISI, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal.

dDepartment of Physics, University of Coimbra, 3004-516 Coimbra, Portugal.

eCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.

fC2TN, Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, ao Km 139,7, 2695-066 Bobadela LRS, Portugal.

gInstitute of Chemical Research of Catalonia (ICIQ). Avda. Països Catalans, 16- 43007 Tarragona, Spain.

hInstitut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany

Single molecule magnets (SMMs) potential application as high-density magnetic memories and quantum-computing devices.1

First observation of slow magnetic relaxation by Sessoli and co-workers2 (1993) complex [Mn12O12(CH3CO2)(H2O)4]·4H2O·2CH3CO2H in the absence of a

magnetic field.

Upon magnetisation the unpaired spins align in a preferential direction and at low temperatures, the magnetisation is retained for some time after removal of the

magnetic field.2

The barrier for the loss of magnetisation (Ueff) depends on the uniaxial magnetic anisotropy (D) and the total ground state spin (ST) Ueff = |D|ST2.1

To improve SMM behaviour: manipulating the total spin by increasing the number of spin active nuclei (higher ST) and/or searching for a high D value.

In recent years emerged as a class of single ion magnets (SIMs) with high barriers.3

INTRODUCTION

RESULTS AND DISCUSSION

Synthetic method and crystallographic details

Magnetisation measurements High field - EPR

Computational studies

Figure 2 Field dependence of the magnetisation of

complex 1 at 2.5, 3 and 4 K. Red lines represent fit to the

data.

Variable temperature magnetisation measurements -

between 10 K and 300 K.

χMT = 3.02 cm3 mol-1 K S=2 high-spin Mn(III)

complex.

Sharp decrease in the χMT value non-negligible

zero field splitting. 𝑯 = 𝑫 𝑺𝒛

2+ 𝑬 𝑺𝒙

2− 𝑺𝒚

2+ 𝒈𝝁B 𝒊𝑩 ∙

𝑺 Eq.1

Zero Dc field no slow relaxation on the temperature

dependence of the AC susceptibility typical SIM

behaviour of Mn(III) complexes Quantum tunneling of

the magnetisation (QTM).

Application DC field split ms levels inhibition of QTM.Figure 3. Frequency dependence of the out

of phase susceptibility, χm’’, for complex 1,

under a static DC field of 2000 Oe with a 2

Oe oscillating field in the range 1.6-4K.

Figure 5. Cole-Cole plots for complex 1

under a DC field of 2000 Oe. The lines

represent the least-square fits with a

generalised Debye model to a

distribution of single relaxation modes.

Cole-Cole plots

values.

Arrhenius plot of

ln() vs. 1/T

• Ueff = 10.19 K

• 0 = 1.476 x 10-6 s

𝑙𝑛𝜏 = 𝑙𝑛𝜏0 + Ueff/𝑘𝐵𝑇 Eq,2

Figure 6. Arrhenius plot of ln τ vs. 1/T for

complex 1 under a DC field of 2000 Oe. The

red line is the best fit curve.

Better estimate of g, D and E.

Absorption-shaped parallel features at low fields

and derivative-shaped perpendicular features are

located at high fields a negative D.

Zero field crossing at 300 GHz D ≈ 3.3 cm-1.

Figure 3. High Frequency EPR spectra (in black) of

compressed powder recorded at 5 K at various

frequencies indicated in the plot, together with simulation

(in red) obtained using parameters indicated in Table 1.

Crystallographic unit A

D/cm-1 -4.60 ±0.05

E/cm-1 1.5 ±0.05

giso 1.987 ±0.015

Crystallographic unit B

D/cm-1 -4.18 ±0.05

E/cm-1 1.5 ±0.10

giso 1.987 ±0.015

Table 2. Spin Hamiltonian parameters extracted from HF-EPR spectra

Simulations of spectra (Eq. 1)

Two species in a 1:1 ratio in

accordance with the crystallographic data

units A and B.

Characterisation of electronic

structure and reproduce the zero field

splitting.

Significant contributor to the axial

anisotropy (D) ground state triplet

• 3𝑑𝑥𝑦1 3𝑑𝑦𝑧

1 3𝑑𝑥𝑧2 3𝑑𝑧2

0 3𝑑𝑥2−𝑦20 dominant

configuration (87.9%) doubly

filling the d orbital bisecting the plane

of the nitrogen donor ligands.

3𝑑𝑥𝑦1 3𝑑𝑦𝑧

1 3𝑑𝑥𝑧1 3𝑑𝑧2

1 3𝑑𝑥2−𝑦20

configuration

• Ground state quintet Mn-N anti-

bonding interaction between the

occupied 3𝑑𝑧2 orbital with the

nitrogen donor ligands structural

distortion.

MOLCAS ORCA

Method CASSCF+SOC NEVPT2+SOC

Crystallographic unit A

D/cm-1 -3.225 -3.054 -3.809

E/cm-1 +0.645 +0.583 +1.046

giso 1.987 1.987 1.988Crystallographic unit B

D/cm-1 -3.224 -3.050 -3.747

E/cm-1 +0.625 +0.565 +0.983

giso 1.987 1.987 1.988

Table 3. Calculated ZFS parameters with several approaches.

CONCLUSIONSFirst example of SIM in an octahedral Mn(III) complex with tridentate Schiff-base

ligands.

Both magnetic measurements and HF-EPR spectroscopy size of the axial

anisotropy is the highest reported to date.

Ueff (10.19 K) and τ0 (1.476x10-6 s) at 2000 Oe are in agreement with other Mn(III)

SIMs.

Computational studies easy (D < 0) axis is orthogonal to the Jahn-Teller axis.

The largest source of anisotropy t2g4 eg*

0 triplet state admixed with the quintet

ground state.

1 D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, 2007.

2 R. Sessoli, D. Gatteschi, A. Caneschi, M. A. Novak, Nature, 1993, 365, 141–143.

3 H. L. C. Feltham, S. Brooker, Coord. Chem. Rev., 2014, 276, 1–33.

This work: S. Realista, A. J. Fitzpatrick, G. Santos, L. P. Ferreira, S. Barroso, L. C. J. Pereira, N. A. G. Bandeira, P. Neugebauer, J. Hrubý, G. G. Morgan, J. van Slageren, M. J.

Calhorda and P. N. Martinho, in review

REFERENCESThis work was supported by Fundação para a Ciência e a Tecnologia (FCT), Portugal (Projects UID/MULTI/00612/2013, UID/MULTI/04046/2013, UID/Multi/04349/2013 and

PTDC/QEQ¬QIN/3414/2014). FCT is gratefully acknowledged by S.R. for a PhD grant (PD/BD/52368/2013) and by P.N.M. for a postdoctoral grant (SFRH/BPD/73345/2010).

N.A.G.B. gratefully acknowledges the COFUND 291787-ICIQ-IPMP grant. A Science Foundation Ireland Investigator Project Award 12/IP/1703 is gratefully acknowledged by

G.G.M. and funding from the National University of Ireland, University College Dublin and the Cultural Service of the French Embassy in Ireland for scholarships is gratefully

acknowledged by A.J.F. The COST action CM1305 is acknowledged by P.N.M. and G.G.M.

ACKNOWLEGEDMENTS

Scheme 1 Synthesis of [Mn(3-OEt-salme)2]BPh4, 1.

Figure 1 ORTEP-3 diagram of complex 1, using 30% probability level ellipsoids. Equivalent atoms

labelled with #

Tetragonal elongation N2−Mn−N2# axis:

Mn1-N1 = 2.0286(10)

2.0342(10)

Mn1-N2 = 2.355(1) Å) 2.343(1)

Å

Mn1-O2 = 1.8663(8) 1.8745(8)

Jahn–Teller distorted d4 Mn(III) ion.

Two crystallographically distinct Mn(III) half-occupancy

cations in the asymmetric unit.

Octahedral coordination with two tridentate ligands in a

fac arrangement with:

two trans-phenolate (O2-Mn1-O2#),

trans-amine (N2-Mn1-N2#),

trans-imine (N1-Mn1-N1#) donors.