Do the chemists Do the chemists know howknow howto align the spins to align the spins of electronsof electronsin molecules,in molecules,parallel or antiparallel ?parallel or antiparallel ?

Understanding Understanding ……

why the spins of two neighbouring electrons why the spins of two neighbouring electrons (S = 1/2) become :(S = 1/2) become :

to get magnetic compounds to get magnetic compounds ……

antiparallel ?antiparallel ?

S=OS=O

or parallel ?or parallel ?

S=1S=1

Sa Sb1 2

A B

H = - J S1S2^ ^ ^

Interaction Models between Localised Electrons

Scalar Coupling : describes, does not explain

ET

ES

EJ

0

J<0

Interactionantiferro

J>0

ESE

0

ET J

Interactionferro

Energy levels

J = 2 k + 4ßS

if S = 0Orthogonality

if S≠0;|ßS|>>kOverlap

>0 <0

H2Aufbau

ES

ET JAntiferro

O2Hund

ESET

JFerro

J. Miró« Overlap » ?

Catalogue raisonné, N°1317

J. Miró, Pomme de terre, detail

H = - J S1S2^ ^ ^

An old game …

AF

FPalace Museum, TaiPei, Neolithic period, Yang-Shao Culture

Michelangelo, Sixtin Chapel, Rome

Exchange interactions can be very weak …

order of magnitude : cm-1 or Kelvins …

≈

order of magnitude : >> 150 kJ mol-1 …

« Chemical » bondsRobust !

Exchange interactions

Energy

≈ 5 ÅNegligible Interaction !

How to create the interaction … ?

Problem :

Cu(II) Cu(II)

Orbital Interaction …≈ 5 Å

Cu(II) Cu(II)

The ligand !Solution :

Ligand

A B

Examples with the ligand

• Cyanide

Ligand

Monet Claude, Charing Cross Bridge

Non linear and linear bridges

Monet Claude, Waterloo Bridge

Cyanide Ligand

Friendly ligand : small, dissymetric, forms stable complexesWarning : dangerous, in acid medium gives HCN, lethal

C≡N-

Dinuclear µ-cyano homometallic complexes

“Models” Compounds Cu(II)-CN-Cu(II)

Compounds exp

[Cu2(tren)2CN]3+

[Cu2(tmpa)2CN]3+

-160

-100

J/cm-1

Rodríguez-Fortea et al. Inorg. Chem. 2001, 40, 5868

Overlap : antiferromatic coupling …

Cr(III) Ni(II)

Dinuclear µ-cyano heterometallic complexes

NB : A dissymetric ligand helps to get stable heterometallic complexes …« Birds of the same feathers flock together » …

Polynuclear complex, synthetic strategy

Hexacyanometalate “Heart”Lewis Base

Mononuclear ComplexLewis Acid

Polynuclear Complex

3-

+ 6

2+ 9+

Hexacyanochromate complex

x

z t2g² oct

t2g

eg

Cr(III)

[CrIII(CN)6]3-

Electrons in

M-C≡N-M'

M C N

M'C N

S = 0, JF = 2k

π σ

Example :

Cr(III) (t2g)3 JF

Ni(II),(eg)2

Polynuclear complex, ferromagnetic strategy

Cr(III)Ni(II)6S = 6x1+3/2 =S = 15/2

Orthogonality :Ferromagnetism !

M-C≡N-M’

Cr(III)Mn(II)6S = 6x5/2 + 3/2 =

S = 27/2

Overlap = antiferromagnetism

Cr(III) (t2g)3

JAF

Mn(II) (t2g)3

Example

Polynuclear complex, ferrimagnetic strategy

π

Sab ° 0, JAF ∝ Sab√(²2-δ2)

M C N

M'C N

² δ

π

Ferromagnetic …

ErosPaul KLEE

Nocturnal separation, 1922

Ferrimagnetic …

C rC u6S = 9/2

C rNi6S = 15/2

C rMn6S = 27/2

Hexagonal R -3a = b = 15,27 Å; c = 78,56 Åa = b= 90°; g = 120°; V = 4831 Å3

Hexagonal R -3a = b = 15,27 Å; c = 41,54 Åa = b= 90°; g = 120°; V = 8392 Å3

Hexagonal R -3a = b = 23,32 Å; c = 40,51 Åa = b= 90°; g = 120°; V = 19020 Å3

… High Spin Heptanuclear Complexes

Marvaud et al., Chemistry, 2003, 9, 1677 and 1692

Chemists have transformed good old Prussian blue, a blue pigment that Michael Faraday precipitated

in his time at Royal Institution, into a room temperature magnet

also useful in display devices.

Alfred Bader, « End of Mistery », Chemistry in Britain, 37, 7, July 2001, 99Greeting Card of RSC, Benevolent Fund, Thomas Graham House, Science Park, Milton Road, Registered Charity N° 207890.This painting does not depict neither W.T. Brande nor Michael Faraday making Prussian blue, Thomas Philips RA, ca 1816 (from Alfred Bader, Hon FRSC)

1704 …

Diesbach, draper in Berlin …

… prepares a blue pigment « Prussian blue »

… said to be the first coordination compound

2004 : 300th anniversary !

Anna Atkins, Cyanotypein Hart-Davis Adam Chain Reactions,pioneers of british science and technologyNational Portrait GalleryLondon, 2000

Classical coordination chemistry …

Fe2+aq+ 6CN-

aq [Fe(CN)6]4-aq

Complexes as Ligands, or « bricks »+ Lewis Acid-Base Interaction

FeH2O

H2O OH2

OH2

OH2

OH2

Fe+

[4-] [3+]

3[Fe(CN)6]4-aq + 4Fe3+aq {Fe4[Fe(CN)6]3}0•15H2O

since : 1936, modified 1972 …

an evergreen in inorganic chemistry …

a simple face-centered cubic structure …

Stoichiometry[AII]4[BII(CN)6]3or A4B3[AII]4 [BII] 3 1

= vacancy

J.F. Keggin, F.D. Miles, Nature 1936, 137, 577

A. Ludi, H.U. Güdel, Struct. Bonding (Berlin) 1973, 14, 1

Coming back to Prussian Blue

TC ∝ z |J|

z : number of magnetic neighbours|J| : coupling constant between nearest neighbours

Néel, Annales de Physique, 1948

Coming back to Prussian Blue

TC ∝ z |J|

z : number of magnetic neighbours|J| : coupling constant between nearest neighbours

Néel, Annales de Physique, 1948

TC = 5.6 K

Towards Prussian blue analogues …

TC ∝ z |J|

TC >> 5.6 K

JFerro > 0

Orthogonality

Towards Prussian blue analogues …

TC ∝ z |J|

TC >> 5.6 K

JAntiferro < 0

Overlap …

TC / K

Z

200

Ti V Cr Mn Fe Co Ni Cu Zn

300

100

• •• [Cr(CN)6]3-

(t2g)3 d3

• ••• •

6 F

9 AFMnII

d5

• ••

9 AF

(t2g)3

VIId3

• ••• 3 F

9 AF

CrII d4

• ••• •

• • •

6 F

NiIId8

V4[Cr(CN)6]8/3.nH2ORoom Temperature TCOn a rational basis !

Gadet et al., J.Am. Chem. Soc. 1992Mallah et al. Science 1993Ferlay et al. Nature, 1995

TC / K

Z

200

Ti V Cr Mn Fe Co Ni Cu Zn

300

100

• •• [Cr(CN)6]3-

(t2g)3 d3

• ••• •

6 F

9 AFMnII

d5

• ••

9 AF

(t2g)3

VIId3

• ••• 3 F

9 AF

CrII d4

• ••• •

•• ••

• ••

•• •••

•• •• •

6 F 6 F

3 F

6 AF 3 AF (eg)1

CuII

CoIIFeIId6 d7d9

• ••• •

• • •

6 F

NiIId8

V4[Cr(CN)6]8/3.nH2ORoom Temperature TCOn a rational basis !

2[CrIII(CN)6]3-+3Vaq

2+ [V3[CrIII(CN6)]2]

0

A blue, transparent, low density magnet at room temperature

MagnetMolecular

Holder (1)(2)PermanentMagnet

Light, Sun …

(2)

Image

Len

Screen(3)

Oscillating Magnet : Experiment

A thermodynamical machine transformingLight

inMechanical

Energy

QuickTime™ et un décompresseurDV - PAL sont requis pour visualiser

cette image.

Permanent Magnet

Hot

CoupleSample ( MM)

Magnetic Switch…Or thermal probe …

Other device

Up to 2004 … magnetic analogues used as …

QuickTime™ et un décompresseurDV - PAL sont requis pour visualiser

cette image.

… devices and demonstrators

Chemists have managed to transform

isolated single molecules into magnets

molecule-based magnets ? Why ?

Low densityTransparentNanosized, identical moleculesOften biocompatible and biodegradableVery flexible chemistryMild chemistry : Room T, Room P, Solution Chemistry

Fragile AgingDiluted

Specific properties

To overcome

To improve

Top down

Nanosystems• Nice Chemistry• Single molecule magnets

• Applications (far …)• Recording• Quantum computing

Fragments Threads

Dots

• New Physics• Quantum / Classical• Quantum tunneling

Bottom up

Giant Molecular Clusters

0D, Molecules

3DMetalsOxydes

Synthesis

Properties

Theory

Idea

NewMaterials

NewFunctions

NewConcepts

… Single molecule magnets Giant Molecular Clusters

Mn4and many others

High Spin + Anisotropy∆E = DSz

2 Mn12Fe8

See GatteschiHendricksonChristouWinpenny …

=

What is named Single Molecule Magnet ?

High SpinAnisotropic

High Spin Paramagnetic Molecule

H

Single Molecule Magnet

Towards information storage at the molecular level ?

Below T < TBlocking

H

Single Molecule Magnet

Towards information storage at the molecular level ?

Below T < TBlocking

Single molecule magnetsz

yx

E

- Sz

Sz+Sz

0

DSz2

0-2-4 +2 +4

Anisotropy Barrier

Tunneling

ThermalActivation

DSz2 = 400K ? |D| = 1K

S = 20(D < 0)

K. Vostrikova, P. Rey et al., JACS 2000, 122, 718

2nd generation

NCM

NC CN

CN

C

CN

N

NCM

NC CN

CN

C

CN

N

NCM

NC CN

CN

C

CN

N

Spin = 2 = Ni(II)(Rad°)2

= Ni(II)(tetren)Spin = 1

Rad°

1rst generation

Complex

Decurtins, Angewandte, 2000Hashimoto, JACS, 2000

Marvaud, Chemistry, 2003, 9, 1677 y 1692

Rey, JACS 2000, 122, 718

Some examples …

S = 27/2

S = 39/2 (AF)

S = 14/2

CoNi5

CoNi3

CoCo3

CoCu3CrNi 5/2

CrNi2

CoCo2 CoNi2CoCu2

7/2

Marvaud et al., Chemistry, 2003, 9, 1677 and 1692 Ariane Scuiller, Caroline Decroix, Martine Cantuel, Fabien Tuyèras …

CrNi2CoCo2

CoCu6

CrNi

CoNi5CrNi3

CoNi2

CoNi3CrNi3

CoCo6

CoCu2

CoCo3CrCo3

CoCu3

5/2

7/2

9/2

27/2

Anisotropy

High spin

V. Marvaud

CrMn6

CoMn6

CrNi6

15/2

CrCu6

[Mn[Mn1212OO1212(CH(CH33COO)COO)1616(H(H22O)O)44].2CH].2CH33COOH.4HCOOH.4H2200

Mn(IV)

Mn(III)

Ion Oxyde

Carbone

S=10

or Mnor Mn1212

From D.Gatteschi and R. Sessoli

S=2

S=3/2

S =8x2 -4x3/2 =

Mn12 is a hard magnet

-3 -2 -1 0 1 2 3

-20

-10

0

10

20T=2.1K

MA

GN

ETI

ZATI

ON

(µB)

MAGNETIC FIELD (T)

Bistability : in zero field the magnetisation can be positive or negative depending of the story of the sampleCoercive Field

Remnant Magnetisation

From D.Gatteschi and R. Sessoli

Ground State Energy Levels

H = 0 M=±10

M=±9

M=±8

From D.Gatteschi and R. Sessoli

At low temperature, a magnetic field populates only the M = -S state

M=-S

M=S

Energy Levels in a Magnetic Field

H≠0

S

-S

Going back to equilibrium :Thermal activation :trivial

H=0∆E=DS2

M=-SM=S

τ = τ0 exp(∆E/kBT)

Axial symmetry

E(M) = DM2

From D.Gatteschi and R. Sessoli

H=0 M=S M=-S

Tunneling effect : new !Towards equilibrium :

From D.Gatteschi and R. Sessoli

M

H

Mn12 is a Hard Magnet

+ Steps in the magnetisation curve

Msaturation

Hcoercive

Mremnant

From D.Gatteschi and R. Sessoli

Resonnant Tunneling Effect for H = nD/gµB

H = nD/gµB

M=-S

M=S

From D.Gatteschi and R. Sessoli

Conditions to observe tunnelling effect

• Degenerated wave functions must superpose• A transversal field must couple the two wave

functions • Coupling splits the two levels : “tunnel splitting”• Tunnelling Effect Probability increases with

tunnel splitting”

From D.Gatteschi and R. Sessoli

M

HTunneling Effect

From D.Gatteschi and R. Sessoli

No resonnant Tunneling Effectwith a magnetic field parallel to z

H ≠ nD/gµB

M = -S

M = S

From D.Gatteschi and R. Sessoli

M

H No tunneling effectFrom D.Gatteschi and R. Sessoli

Anisotropic precursor

[Fe(III)(bipy)(CN)4]-

R. Lescouëzec, M. Julve, Valencia, Spain D. Gatteschi, W. WernsdorferAngewandte Chem. 2003, 142, 1483-6

Feasibility of « Molecular nanowires » (or SCM) ?

[{FeIII(bpy)(CN)4}2MII(H2O)] .CH3CN . 1/2H2O [M = Mn, Co, Cu]

[{FeIII(L)(CN)4}2CoII(H2O)4] .4H2O

[{FeIII(L)(CN)4}2MII(H2O)4].4H2O [L = 2,2’-bipy and 1, 10-phen), M = Mn, Co, Cu, Zn]

Trinuclear speciesDouble

zig-zag chainsBis double

zig-zag chains

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10 100 1000M

/Ms

t (s)

1.5 K

1.6 K

1.7 K

1.8 K

1.9 K

2.8 K

2.7 K

2.6 K2.5 K 2.3 K

2.2 K

2.1 K

2.4 K

2.0 K

M vs. t plots along the b axis.

Magnetization as a function of time

Thermally activated relaxation of the magnetization

ac: Ea= 142 K, τ0 = 6.10-11 s

Slow relaxation of the magnetisation …

W. Wernsdorfer, Grenoble

The dream …

Surface

Magnetic Tip H

≈ 10 nm

High Spin"down"

High Spin"down"

Surface

Magnetic Tip H

≈ 10 nm

High Spin"down"

High Spin"down"

… information storage at the molecular level !

Surface

Magnetic Tip H

≈ 10 nm

HSM «up»High Spin"down"

The dream …

Nanosciences …

… a challenge for chemists and friends …

Surface

H

≈ 10 nm

HSM «up»High Spin"down"