The „metal-radical approach“

toward magnetic materials

N+

N

O

O

O

NMe 2

workshop synthetic strategies towards ....Kaiserslautern, 23.-25.10.2005

Eva Rentschler Universität Mainz

The strategy is simple:

- Strong direct metal-ligand magnetic exchange interactions are achieved from the coordination of stable free radicals to paramagnetic transition-metal ions,

- and if these interactions are extended in one, two, or three spatial directions, cooperative magnetic behavior is obtainable in these molecule-based systems.

Lemaire, Pure Appl. Chem., Vol. 76, No. 2, pp. 277–293, 2004.

“Toward Molecular Magnets: The metal-radical approach“

Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22, 392 (1989).

- Since (and prior to) 1989, literally hundreds of metal-radical complexes have been reported, including a number of magnetically ordered materials.

- A wealth of knowledge about the structure and magnetic properties of coordination complexes containing stable radical ligands has been unearthed, and as a result, the metal-radical approach is recognized as one of the more fruitful efforts toward molecular magnetic materials.

Lemaire, Pure Appl. Chem., Vol. 76, No. 2, pp. 277–293, 2004.

“Toward Molecular Magnets: The metal-radical approach“

Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22, 392 (1989).

The families of radicals to be discussed

are limited to stable, isolable free-radical species

i.e., radicals that can be prepared and stored under ambient conditions.

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

1901 Gomberg: triphenylmethyl radical

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

1901 Gomberg: triphenylmethyl radical

COOH

Cl

Cl

ClClCl

HOOC

ClCl

Cl

ClCl

Cl

COOH

Cl

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

phenalenyl radical

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

phenalenyl radical

N

2-azaphenalenyl radical 2,5-di- and 2,5,8-triaza derivatives

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

ON

O

O

N

O

OH

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

stable free radicals

FeNC

NC CN

CN

FeNC

NC CN

CN

S

S

S

S

S

SS

S

TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNE = tetracyanoethene

[FeCp*2]+.[TCNQ] -.

[FeCp*2]+.[TCNE] -.

Bis(ethylenedithio)tetrathiafulvalene

Charge Transfer Salts

O

R R

R

N

O

R R

N NOO

R

S

SN

N

R

S

N

SR

R

N

N N

NRR

R'

R

N

N N

NRR

R'

O

N

NO

O

N

N

R

O

O

N

NR

O

O

N IT = 4 ,4,5 ,5-te tram ethyl-4,5-d ihydro-1 -im idazole-1-yloxyl-3-oxideH

J.H . O siecki, E .F. U llm an , , 1078-1079.J. A m . C hem . S oc. 901968

Nitronyl Nitroxide

N – O 1.143 ÅN = O 1.316 Å

N1 – O1 1.285(1) ÅN4 – O4 1.287(1) Å

C5 – N 1.353(1) Å

C – N 1.438 Å C = N 1.260 Å

ES = ½

M = +1/2S

M = -1/2S

E = g H

1N 2N

H

BSpinz

purely organic: including metal ions:

NO

N O

d1d

r

R

g = 2.0119(1), J = -143.1(1) cm-1, TIP = 2.27 * 10-4 emu mol-1.Cu(tfac)2NITMe

Typical Values of the Magnetic Coupling Constant J for Metal-Nitroside Complexes

metal ion type of coupling J,a cm-1

copper(II)b AF 500copper(II)c F -10 to 70nickel(II) AF 500cobalt(II) AF 300manganese(II) AF 150-300

a Positive J means antiferromagnetic coupling. The energy separation between singlet and triplet is J. b The nitroxide is an equatorial site. c The nitroxide in an axial site.

Note: H = J S1.S2

rel. weak interaction between the magnetic orbitalsthe metal-radical overlap is small, energyseparation between the two orbitals is largemolecular orbitals 1 and 2 mainly localized onmetal and on the radical fragment,respectively.Since (2-2)1/2 S

JAF (2-2)1/2S

JAF is determined by the variation of the squaredoverlap between the magnetic orbitals.

J = 2k + 4S

J 2

J S2

For = 0°, Cu-O-N angle = 180°, * and dx2-y2 orbitals , irrespective of and . afm-contribution = 0, a moderate ferromagnetic coupling can be developed.

( the shorter the copper-oxygen distance, the larger the coupling.)

0 and angles become important. When = 0, an increase in from 0 to 90°, causes an increase of the overlap. The effect is much more pronounced at = 90° than at smaller angles.

For = 0°, Cu-O-N angle = 180°, * and dx2-y2 orbitals , irrespective of and . afm-contribution = 0, a moderate ferromagnetic coupling can be developed.

( the shorter the copper-oxygen distance, the larger the coupling.)

0 and angles become important. When = 0, an increase in from 0 to 90°, causes an increase of the overlap. The effect is much more pronounced at = 90° than at smaller angles.

Structural and Magnetic Parameters for Diamagnetic Equatorially Coordinated Copper(I1)-Nitroxide Complexescompd R (a) Square-Planar or Square-Pyramidal ComplexesCu(hfac)2NITPh 1.955 88.4 59.0 56.5 Cu(hfac)2TEMPO 1.920 84.7 56.2 63.7CuCl2(NITPh)2 1.980 64.1 56.3 67.5(b) Trigonal-Pyramidal ComplexesCu(hfac)2NITPh 1.948 80.2 59.8 41.9Cu(tcact)2TEMPO 1.942 81.5 56.5 7.0Cu(tcact)2TEMPO 1.950 85.8 56.2 1.9Cu(tcact)2PROXYL 1.970 79.4 47.4 75.8 Cu(tcact)2PROXYL 1.961 85.2 54.0 11.7

Geometrical, Magnetic, and Molecular Orbital Parameters for Mn(hfac)2(radical)2 Complexes

r J S Trans Adducts

M(hfac)2(TEMPO) 2.127(4) 38.6 12.8 25.5 158 4.8 Mn(hfac)2(PROXYL) 2.150 (4) 79.6 34.7 14.0 210 19.7Mn(hfac)2(NITPh)2 2.144 (5) 77.2 49.5 29.8 180 9.8

2.154 (5) 81.4 47.1 27.9 Cis AdductMn(hfac)2 (NITMe)2 2.122 (5) 86.6 52.0 80.7 187 13.2

2.127 (5) 83.0 48.9 7.2

bridging Nitronyl Nitroxide radicals

N N

R

OOM

N N

R

OOM

N N

R

OOM

M

M

M

MN N

R

OOM

M

non-bridging: -1,3 bridging:

-1,1 and -3,3 bridging:-1,1 bridging:

N N

R

OOM

N N

R

OOM

N N

R

OOM

M

M

M

MN N

R

OOM

M

non-bridging: -1,3 bridging:

-1,1 and -3,3 bridging:-1,1 bridging:

180 40

3 -

Cambridge Structural Database

non-bridging: 180

N N

R

OOM

N N

R

OO MN N

R

OO

O O

OO

CF3F3C

F3C CF3

-1,3 bridging: 40

N N

R

OO MN N

R

OO

O O

OO

CF3F3C

F3C CF3

M

O O

OO

CF3F3C

F3C CF3

N N

R

OOMM

[Cu(hfac)2,(NITEt) vs. T follows the Curie law with C = 0.4639S = 1/2 with g = 2.225.

nitroxide occupies an - equatorial position in the coordination environment of copper(II) strongly coupled - axial position a weak-to-moderate coupling.

Cu2------R------Cu1------R------Cu2J1 J2 J2 J1

„head to tail“:

Spin transitions in non-classical systems

„Change in the Jahn-Teller axis of the Cu bipyramids“:

O

C F 3 CF 3

O

O

C F 3

O

C F 3 C F 3

O

F 3C

O

N+

NO ON+

NO O N+

NO O

O

CF 3

O

C F 3

RR R

N+

N

X

O O

N+

N

X

O O

M

N+

N

X

O O

M

M M N+

N

X

O O

MN+

N

X

O O

1 D

2D / 3D ?

charge distribution

-0.57

-0.72 -0.68

-0.57

Ladung

-0.70 -0.70 -0.59 -0.58

-0.55 -0.55

Cu2(NIT-PhCOO)4(DMSO)2

C u

O

N O

N

N

O

O

C u

O

N O

N

N

O

O

N

N+

O

-

O

N

N+

O

-

O

Pyridin

0 100 200 3000.4

0.8

1.2

1.6

2.0

∙T /

emu

K m

ol-1

T / K

0 100 200 3001.0

1.2

1.4

M∙T

/ em

u K

mol

-1

T / K

H J S S = -2 Cu1 Cu2

= -0.85 KQ

gCu = 2.20JCu1-Cu2 = -150.0 cm-1

= + 0.50 K = - 0.85 K

N+

N

X

O O

N+

N

X

O O

M

N+

N

X

O O

M

M M N+

N

X

O O

MN+

N

X

O O

hohe Spindichte2D / 3D Netzwerk

X = NX = C-CHO, C-COOH, C-P(O)Ph2

O-

N

N+

O

O-

O

N

N

O

O-

O

N

N+

O-

O-

XN

N+

O

O-

2D / 3D network high spin density

NIT phenolates as ligands

4-hydroxo phenolates and their metal complexes

microcrystalline film poly-vinylchloride matrix

magnetic dilution of a nitronyl nitroxide

• syntheses of molecular building blocks

• electronic structures

• magnetic dilution

• sign of the magnetic interaction

construction of polynuclear compounds

Ni2+: d8

dx2-y

2, dz2

dxy, dxz , dyz

dxy

dxz , dyz

dz2

dx2-y

2octahedral coord. with axial CH3OHNi ( S = 1 ) + 2 NIT (S=1/2)

Planar quadratic coord.Ni ( S = 0 ) + (S=1)

octahedral coord. with axial CH3OHNi ( S = 1 ) + 2 NIT (S=1/2)

Planar quadratic coord.Ni ( S = 0 ) + (S=1)

next generation of nitronyl nitroxide ligands

N NOO

.

N

N NOO

N N

N NO.

N

N NOO

N N

R

. .

Multifunctional Radicals

Chelating Radicals

1 +[Cu(CH3CN)4]PF6

[Cu(1)2]+ (Td-symm.)

NIT-NIT: J = +55 cm–1.

H. Oshio et a.l. Inorg. Chem. 36, 3014 (1997).

MIICl2(2)2 (M = Mn, Co, Ni, Zn)

S. Kaizaki. J. Chem. Soc., Dalton Trans. 1566 (2001).

JM-rad = +95 (Ni) and +14.9 cm–1 (Co)JMn-rad = –23.8 cm –1. Jrad-rad = –9 cm –1.

R = Me 1 H 2

MII(ClO4)26H2O (MII = Ni, Mn, and Zn)

[M(NITim)3](ClO4)2, [M(NITbzim)3](ClO4)2.

exhibiting strongly antiferromagnetic metal-radical interactions (-111 < J < -53 cm–1).

Metal-nitronyl nitroxide homoleptic complexes

coordination polymers

[Mn(NITIm)(NITImH)]ClO4

Ziessel

[Mn2(NITIm)3]ClO4

Rey and Luneau

Ferromagnetic ordering temperatures

1.4 K [Mn2(NITIm)3]ClO4

40 K [Mn2(NITBzIm)3]ClO4

Chelating Radicals

R. Ziessel et al. Inorg. Chem., 37 (20), 5078 -5087, 1998

R. Ziessel et al. Inorg. Chem., 37 (20), 5078 -5087, 1998

R. Ziessel et al. Inorg. Chem., 37 (20), 5078 -5087, 1998

For the Ln(III) with 4f1 to 4f5 electronic config. the {Ln-organic radical} interaction

Conversely, for the configurations 4f7 to 4f10

Rare earth coordination compounds

4f7-ion CuII (S=1/2): NIT (S=1/2):

J small usually not direct available from experimentdue to L.S. coupling

Rare earth coordination compounds

For the Ln(III) with 4f1 to 4f5 electronic config. the {Ln-organic radical} interaction

Conversely, for the configurations 4f7 to 4f10

But:

K. E. Vostrikova, D. Luneau, W. Wernsdorfer, P. Rey, and M. Verdaguer, J. Am. Chem. Soc., 122, 718-719 (2000)

A S = 7 Ground Spin-State Cluster Built from Three Shells of Different Spin Carriers Ferromagnetically Coupled, Transition-Metal Ions and Nitroxide Free Radicals

[Fe2(CN)12Ni3(IM-2Py)6]:

2 [Fe(CN) 6] + 3 CN-Ni(IM-2Py)2-NC

Ni(ClO4)2.4H2O + IM-2Py [Fe2(CN)12Ni3(IM-2Py)6]

K3[Fe(CN)6]

H2OMeOH

2 (S = ½) + 3 (S = 2)

K. E. Vostrikova, D. Luneau, W. Wernsdorfer, P. Rey, and M. Verdaguer, J. Am. Chem. Soc., 122, 718-719 (2000)

A S = 7 Ground Spin-State Cluster Built from Three Shells of Different Spin Carriers Ferromagnetically Coupled, Transition-Metal Ions and Nitroxide Free Radicals

1.2 K

R. J. Glauber. J. Math. Phys. 4, 294 (1963). 1D Ising ferri- or ferromagnetic materials

could exhibit slow relaxation of their magnetization.

molecular magnetic nanowires

slow relaxation in chains

favoured by spin correlation along the chains

reorientation of the magnetization becomes more difficult

The height of the barrier to magnetization reversal should scale

with the nearest-neighbor exchange coupling.

R. Sessoli, et al. Angew. Chem. Int. Ed. 40, 1760 (2001):

zig-zag / helical

molecular magnetic nanowires

slow relaxation in chains

Co(hfac)2(NITPhOMe)

1 D- helix (trigonal crystallographic symmetry).

is highly anisotropic (gCo = 7.4) below 50 K

and slow magnetization relaxation

as well as hysteresis effects are observed.

magnetization barrier 154(2) K, (J = 220 K).

R. Sessoli, et al. Chem. Eur. J. 8, 286 (2002).

“molecular magnetic nanowires” for information storage on the molecular level

Incorporation of an asymmetric center

into the structure of the radical ligand.

Luneau and Veciana

Inoue

Solutions exhibit optical activity, and thelow-temperature solid-state magnetic properties suggest a field-induced transitionto a ferromagnetic state (metamagnetic behavior) below 5.4 K.

chirality and magnetism in helical 1 D metal-nitroxide complexes

Quest for new magneto-chiral materials

that could exhibit novel properties that result

from the interaction of chirality and magnetism

M. Minguet, D. Luneau, E. Lhotel, V. Villar, C. Paulsen, D. B. Amabilino, J. Veciana,Angew. Chem., Int. Ed. 41, 586 2002

An Enantiopure Molecular Ferromagnet

chiral ligand chiral molecule

Chirality induced by – atomic stereogenic centersor atropoisomeric conformations

chiral spacegroupP212121

M. Minguet, D. Luneau, E. Lhotel, V. Villar, C. Paulsen, D. B. Amabilino, J. Veciana,Angew. Chem., Int. Ed. 41, 586 2002

An Enantiopure Molecular Ferromagnet

unusual dynamic behavior

at Tc domains with long-laminar form along easy axis

near Tc domain wall are soft and easily displaced

at higher Temp. domains become more rigid

Verdazyl radicals

structure SOMO

Lemaire, Hicks

The first transition-metal complex of a

Verdazyl radical 1997 by Fox et al.

J intra –271 cm-1, X = IJ intra –190 cm-1, X = ClJ intra –200 cm-1, X = Br

J inter negligable

Robin Hicks, Martin T. Lemaire; Pure Appl. Chem. 76, 277 (2004)

Lemaire, Hicks

Nickel-verdazyl exchange strongly ferromagnetic ()J Ni-vd ≥ +240 cm–1

Manganes-verdazyl exchange antferromagnetic ()J Mn-vd = -45 cm –1

Robin Hicks, Martin T. Lemaire; Pure Appl. Chem. 76, 277 (2004)

A series of 4,5-diazafluorene derivatives of Koelsch’s free radical

Reaction with CuCl2 has reportedly generated analytically pure metal-radical complexes, but which have not yet been structurally or magnetically characterized.

Plater et al. J.Chem.Soc., Perkin Trans. 1, 971 (2000)

COOH

Cl

Cl

ClClCl

HOOC

ClCl

Cl

ClCl

Cl

COOH

Cl

Daniel Maspoch, thesis, Valencia, 2004

Triphenylmethyl-radical

S=1/2 S=1/2S=0

Zn2+

S=1/2 S=1/2S=1/2

Cu2+

S=1/2 S=1/2S=0

Ni2+

Chemical Communications, 2002, (24), 2958 - 2959

D. Maspoch, D. Ruiz-Molina, K. Wurst, C. Rovira, J. Veciana

D. Maspoch, J. Gómez-Segura, N. Domingo, . Ruiz-Molina, K. Wurst, C. Rovira, J. Tejada, J. Veciana ; Inorg. Chem. 44, 6936 (2005)

An Unusually Stable Trinuclear Manganese(II) Complex Bearing Bulk Carboxylic Radical Ligands:

FeNC

NC CN

CN

FeNC

NC CN

CN

S

S

S

S

S

SS

S

TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNE = tetracyanoethene

[FeCp*2]+.[TCNQ] -.

[FeCp*2]+.[TCNE] -.

Bis(ethylenedithio)tetrathiafulvalene

Charge Transfer Salts

A.H. Reis, Jr., L.D. Preston, J.M. Williams, S.W. Peterson, G.A. Candela, L.J. Swartzendruber,J.S. Miller, J. Am. Chem. Soc. 101 (1979) 2756.

Tc 4.8 K [FeIII(C5Me5)2]2[TCNE] magnet: with(i) spins residing in a p-orbital(ii) exhibiting magnetic hysteresis(iii) lacks an extended one-, two,- or three-dimensional network structure;(iv) is soluble in conventional organic solvents(v) does not require metallurgical processing.

Variation of Tc with increasing spin number per metalin [MIII(C5Me5)2]+[TCNE]-.

1 D- ferrimagnet

large remanent magnetizationand coercice fields27 500 Oe at 2K !!!

Tc 16 K

V(TCNE)x · y(CH2Cl2) (x2; y 1/2) is the first example of an organic-basedmaterial with a critical temperature exceeding room temperature.

J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, J.S. Miller, Science 252 (1991) 1415.

J. Zhang, P. Zhou, W.B. Brinckerhoff, A.J. Epstein, C. Vazquez, R.S. McLean, J.S. Miller, Am.Chem. Soc. Symp. Ser. 644 (1996) 311.

V(TCNE)x · y(CH2Cl2)

by reaction of V0(CO)6 with TCNE

spin diverse ligands,

containing two or more different kinds of spin carriers per molecule,

Nitronyl nitroxide/semiquinone hybrid biradicals very strong intraligand ferromagnetic exchange couplingJNN = > +300 and +100 cm–1

The semiquinone is stabilized by coordination to ZnTpCum,Me, where TpCum,Me = hydro-tris(3-cumenyl-5-methylpyrazolyl)-borate]:

The coupling is so strong, in fact, that these systems may be treated as singleS = 1 units.

Shultz et al.J. Am. Chem. Soc. 123, 3133 (2001)J. Am. Chem. Soc. 125, 1607 (2003)

Y. Takano, et. al. J. Am. Chem. Soc., 124, 450 (2002)

M(II)(hfac)2(di-(4-pyridyl)phenylcarbene)

Y. Takano, et. al. J. Am. Chem. Soc., 124, 450 (2002)

Y. Takano, et. al. J. Am. Chem. Soc., 124, 450 (2002)

Mn(II):

d-pd-pd-p

Cu(II):

d-pd-p

exchange pathways:

Y. Takano, et. al. J. Am. Chem. Soc., 124, 450 (2002)

Di- and triradicals

Di- and triradicals

Di-radicals and Pressure effects