Do the chemists know how to align the spins of electrons...

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



… 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


Ground State Energy Levels

H = 0 M=±10

M=±9

M=±8


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


H=0 M=S M=-S

Tunneling effect : new !Towards equilibrium :


M

H

Mn12 is a Hard Magnet

+ Steps in the magnetisation curve

Msaturation

Hcoercive

Mremnant


Resonnant Tunneling Effect for H = nD/gµB

H = nD/gµB

M=-S

M=S


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”


M

HTunneling Effect


No resonnant Tunneling Effectwith a magnetic field parallel to z

H ≠ nD/gµB

M = -S

M = S


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"

Do the chemists know how to align the spins of electrons...

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