Eva Rentschler
Universität Mainz
The „metal-radical approach“
toward magnetic materials
O
+
N
O
N
O
N
M
e2
workshop synthetic strategies towards ....
Kaiserslautern, 23.-25.10.2005
“Toward Molecular Magnets: The metal-radical approach“
Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22, 392 (1989).
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.
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.
1901 Gomberg: triphenylmethyl radical
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
1901 Gomberg: triphenylmethyl radical
O
R
Cl
R
Cl
HOOC
R
ClCl
COOH
N
Cl
OCl
Cl
O
N
N
Cl
Cl
O
Cl
N
O
R
Cl
R
S
Cl
COOH
R
S
R
N
O
R
N
N
R
O
R
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
phenalenyl radical
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
phenalenyl radical
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
N
O
R
N
R
S
R
N
S
R
R
S
2-azaphenalenyl
radical
N
N
N
 2,5-diand
N
N
S
 2,5,8-triaza derivatives
R'
R
R
R
N
N
N
N
R'
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
O
R
R
R
OH
O
O
R
N
R
N
O
O
N
N
O
N
N
O
O
R
N
O
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
stable free radicals
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
Charge Transfer Salts
Fe
NC
CN
NC
CN
[FeCp*2]+.[TCNQ] -.
Fe
NC
CN
NC
CN
S
S
S
S
S
S
S
S
Bis(ethylenedithio)tetrathiafulvalene
[FeCp*2]+.[TCNE] -.
TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNE = tetracyanoethene
O
R
R
R
O
R
N
R
N
O
O
N
N
O
N
O
R
O
R
N
R
S
R
N
R
S
R
R
R
N
N
N
N
N
N
N
N
N
S
R
S
R'
R'
Nitronyl Nitroxide
O
O
N
N
R
R
N
N
O
O
NIT = 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-yloxyl-3-oxide
J.H. Osiecki, E.F. Ullman J. Am. Chem. Soc. 1968, 90, 1078-1079.
N1 – O1 1.285(1) Å
N4 – O4 1.287(1) Å
N – O 1.143 Å
N = O 1.316 Å
C5 – N 1.353(1) Å
C – N 1.438 Å
1N
C = N 1.260 Å
MS = +1/2
E
S=½
E = g H
MS = -1/2
H
2N
z
Spin
B
purely organic:
R
O

d
N
r
N
d1
O
including metal ions:
Cu(tfac)2NITMe
g = 2.0119(1), J = -143.1(1) cm-1,
TIP = 2.27 * 10-4 emu mol-1.
Typical Values of the Magnetic Coupling Constant J for
Metal-Nitroside Complexes
metal ion
type of coupling
copper(II)b
copper(II)c
nickel(II)
cobalt(II)
manganese(II)
a
AF
F
AF
AF
AF
J,a cm-1
 500
-10 to 70
 500
 300
150-300
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 orbitals

the metal-radical overlap is small, energy
separation between the two orbitals is large

molecular orbitals 1 and 2 mainly localized on
metal 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 squared
overlap between the magnetic orbitals.
J = 2k + 4S
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.
Structural and Magnetic Parameters for Diamagnetic Equatorially
Coordinated Copper(I1)-Nitroxide Complexes
compd
R



(a) Square-Planar or Square-Pyramidal Complexes
Cu(hfac)2NITPh
1.955
88.4
59.0
56.5
Cu(hfac)2TEMPO 1.920
84.7
56.2
63.7
CuCl2(NITPh)2
1.980
64.1
56.3
67.5
(b) Trigonal-Pyramidal Complexes
Cu(hfac)2NITPh
1.948
80.2
59.8 41.9
Cu(tcact)2TEMPO 1.942
81.5
56.5
7.0
Cu(tcact)2TEMPO 1.950
85.8
56.2
1.9
Cu(tcact)2PROXYL 1.970
79.4
47.4
75.8
Cu(tcact)2PROXYL 1.961
85.2
54.0
11.7
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.
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 19.7
Mn(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 Adduct
Mn(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
-1,3 bridging:
non-bridging:
O
N
N
O
M
M
O
N
R
O
N
M
-1,1 and -3,3 bridging:
M
M
O
O
M
R
O
R
-1,1 bridging:
N
N
N
N
M
M
O
M
R
Cambridge Structural Database
-1,3 bridging:
non-bridging:
O
N
N
O
M
M
O
N
R
N
40
-1,1 bridging:
O
M
R
180
N
O
N
-1,1 and -3,3 bridging:
M
M
O
O
M
R
N
M
N
O
M
M
R
3
-
non-bridging:
O
N
N
180
O
M
R
F3C
CF3
O
O
N
N
O
M
O
O
O
N
N
O
R
R
F3C
CF3
O
-1,3 bridging:
M
O
N
N
40
O
M
R
F3C
CF3
O
O
N
N
O
M
O
O
F3C
CF3
O
O
N
N
M
O
O
O
R
O
O
R
F3C
CF3
F3C
CF3
[Cu(hfac)2,(NITEt)
 vs. T follows the Curie law with C = 0.4639
S = 1/2 with g = 2.225.
J1
J2
J2
J1
Cu2------R------Cu1------R------Cu2
nitroxide occupies an
- equatorial position in the coordination environment of copper(II)
 strongly coupled 
- axial position
 a weak-to-moderate  coupling.
Spin transitions in non-classical systems
„head to tail“:
„Change in the Jahn-Teller axis of the Cu bipyramids“:
CF3
CF3
O
1D
O N
F3C
O
+
+
O N
N O
O
R
CF3
R
N
+
M
O
O
+
N
N
R
M
O
O
N
N
2D / 3D ?
X
M
N O
O
CF3
CF3
+
O N
O
CF3
N
O
O
N O
O
O
CF3
X
O
N
N
X
+
O
M
O
N
N
X
+
O
X
M
+
O
charge distribution
Ladung
-0.55
-0.70
-0.70
-0.55
-0.59
-0.58
-0.57
-0.72
-0.57
-0.68
Cu2(NIT-PhCOO)4(DMSO)2
N
N Cu O
N
O
O
O
-
O
N+
N
O
N
N+
O
O
N
N Cu O
N
O
O
O
Pyridin
2.0
-1
1.6
M·T / emu K mol
·T / emu K mol
-1
1.4
H = -2J SCu1 SCu2
gCu = 2.20
JCu1-Cu2 = -150.0 cm-1
Q==--0.85
0.85KK
1.2
0.8
1.2
 = + 0.50 K
0.4
1.0
0
100
200
T/K
300
0
100
200
T/K
300
NIT phenolates as ligands
O
N
N+
O-
O
+
N
N
X=N
X = C-CHO, C-COOH, C-P(O)Ph2
X
M
O
O
+
N
N
M
O
O
+
N
N
O
O
N
N+
O-
O-
O
N
O
N
O-
X
M
O
+
N
N
O
X
M
O
+
N
N O
X
M
ON
X
2D//3D
3Dnetwork
Netzwerk
2D
X
hohespin
Spindichte
high
density
N+
O-
O
4-hydroxo phenolates and their metal complexes
magnetic dilution of a nitronyl nitroxide
microcrystalline film
poly-vinylchloride matrix
• syntheses of molecular building blocks
• electronic structures
• magnetic dilution
• sign of the magnetic interaction
 construction of polynuclear compounds
Ni2+: d8
dx2-y2, dz2
dxy, dxz , dyz
octahedral coord. with axial CH3OH
Ni ( S = 1 ) + 2 NIT (S=1/2)
Planar quadratic coord.
Ni ( S = 0 ) + (S=1)
dx2-y2
dz 2
dxy
dxz , dyz
octahedral coord. with axial CH3OH
Ni ( S = 1 ) + 2 NIT (S=1/2)
Planar quadratic coord.
Ni ( S = 0 ) + (S=1)
next generation of nitronyl nitroxide ligands
Multifunctional Radicals
.
O
N
N
N
.
O
O
N
N
N
N
.
O
N
O
N
R
.
O
N
N
N
N
N
O
Chelating Radicals
R = Me 1
H 2
H. Oshio et a.l. Inorg. Chem. 36, 3014 (1997).
1 +[Cu(CH3CN)4]PF6
[Cu(1)2]+ (Td-symm.)
NIT-NIT: J = +55 cm–1.
S. Kaizaki. J. Chem. Soc., Dalton Trans. 1566 (2001).
MIICl2(2)2 (M = Mn, Co, Ni, Zn)
JM-rad = +95 (Ni) and +14.9 cm–1 (Co)
JMn-rad = –23.8 cm –1.
Jrad-rad = –9 cm –1.
Metal-nitronyl nitroxide homoleptic complexes
MII(ClO4)26H2O (MII = Ni, Mn, and Zn)

[M(NITim)3](ClO4)2, [M(NITbzim)3](ClO4)2.
exhibiting strongly antiferromagnetic metalradical interactions (-111 < J < -53 cm–1).
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
Rare earth coordination compounds
4f7-ion
CuII (S=1/2): 
NIT (S=1/2): 
J small
usually not direct available from experiment
due to L.S. coupling
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
But:
For the Ln(III) with 4f1 to 4f5 electronic config. the {Ln-organic radical} interaction 
Conversely,  for the configurations 4f7 to 4f10
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
2 (S = ½) + 3 (S = 2)
Ni(ClO4)2.4H2O
K3[Fe(CN)6]
+ IM-2Py 

MeOH H2O
[Fe2(CN)12Ni3(IM-2Py)6]
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
K. E. Vostrikova, D. Luneau, W. Wernsdorfer, P. Rey, and M. Verdaguer, J. Am. Chem. Soc., 122, 718-719 (2000)
molecular magnetic nanowires
slow relaxation in chains
R. J. Glauber. J. Math. Phys. 4, 294 (1963). 1D Ising ferri- or ferromagnetic materials
could exhibit slow relaxation of their magnetization.
 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
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
Incorporation of an asymmetric center
Inoue
into the structure of the radical ligand.
Solutions exhibit optical activity, and the
low-temperature solid-state magnetic
properties suggest a field-induced transition
to a ferromagnetic state
(metamagnetic behavior) below 5.4 K.
Luneau and Veciana
An Enantiopure Molecular Ferromagnet
chiral ligand
chiral molecule
Chirality induced by –
atomic stereogenic centers
or
atropoisomeric conformations
chiral spacegroup
P212121
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
M. Minguet, D. Luneau, E. Lhotel, V. Villar, C. Paulsen, D. B. Amabilino, J. Veciana,Angew. Chem., Int. Ed. 41, 586 2002
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 = I
J intra –190 cm-1, X = Cl
J 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)
Plater et al. J.Chem.Soc., Perkin Trans. 1, 971 (2000)
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.
Triphenylmethyl-radical
Cl
HOOC
Cl
ClCl
COOH
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
COOH
Daniel Maspoch, thesis, Valencia, 2004
Zn2+
S=1/2
S=0
S=1/2
Cu2+
S=1/2
S=1/2 S=1/2
Ni2+
S=1/2
D. Maspoch, D. Ruiz-Molina, K. Wurst, C. Rovira, J. Veciana
Chemical Communications, 2002, (24), 2958 - 2959
S=0
S=1/2
An Unusually Stable Trinuclear Manganese(II) Complex Bearing Bulk Carboxylic Radical Ligands:
D. Maspoch, J. Gómez-Segura, N. Domingo, . Ruiz-Molina, K. Wurst, C. Rovira, J. Tejada, J. Veciana ; Inorg. Chem. 44, 6936 (2005)
Charge Transfer Salts
Fe
NC
CN
NC
CN
[FeCp*2]+.[TCNQ] -.
Fe
NC
CN
NC
CN
S
S
S
S
S
S
S
S
Bis(ethylenedithio)tetrathiafulvalene
[FeCp*2]+.[TCNE] -.
TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, TCNE = tetracyanoethene
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 metal
in [MIII(C5Me5)2]+[TCNE]-.
Tc 16 K
1 D- ferrimagnet
large remanent magnetization
and coercice fields
27 500 Oe at 2K !!!
J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, J.S. Miller, Science 252 (1991) 1415.
V(TCNE)x · y(CH2Cl2)
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.
by reaction of V0(CO)6 with TCNE
V(TCNE)x · y(CH2Cl2) (x2; y 1/2) is the first example of an organic-based
material with a critical temperature exceeding room temperature.
spin diverse ligands,
containing two or more different kinds of spin carriers per molecule,
Nitronyl nitroxide/semiquinone
hybrid biradicals
very strong intraligand ferromagnetic
exchange coupling
JNN = > +300 and +100 cm–1
Shultz et al.
J. Am. Chem. Soc. 123, 3133 (2001)
J. Am. Chem. Soc. 125, 1607 (2003)
The semiquinone is stabilized by
coordination to ZnTpCum,Me, where
TpCum,Me = hydro-tris(3-cumenyl-5methylpyrazolyl)-borate]:
The coupling is so strong, in fact, that
these systems may be treated as single
S = 1 units.
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)
Y. Takano, et. al. J. Am. Chem. Soc., 124, 450 (2002)
exchange pathways:
Mn(II):
d-p
d-p
d-p
Cu(II):
d-p
d-p
Di- and triradicals
Di- and triradicals
Di-radicals and Pressure effects