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Eva Rentschler Universität Mainz
The metal-radical approach
toward magnetic materials
workshop synthetic strategies towards
.... Kaiserslautern, 23.-25.10.2005
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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.
277293, 2004.
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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.
277293, 2004.
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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.
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1901 Gomberg triphenylmethyl radical
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1901 Gomberg triphenylmethyl radical
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phenalenyl radical
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phenalenyl radical

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

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stable free radicals
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Charge Transfer Salts
FeCp2.TCNQ -.
Bis(ethylenedithio)tetrathiafulvalene
FeCp2.TCNE -.
TCNQ 7,7,8,8-tetracyano-p-quinodimethane, TCNE
tetracyanoethene
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Nitronyl Nitroxide
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purely organic
including metal ions
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g 2.0119(1), J -143.1(1) cm-1, TIP 2.27
10-4 emu mol-1.
Cu(tfac)2NITMe
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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 ? 500 cobalt(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
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rel. weak interaction between the magnetic
orbitals ? the metal-radical overlap is small,
energyseparation between the two orbitals is
large ? molecular 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 4?S J ? ?2 J ? S2
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• 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.

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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)2TEMP
O 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.

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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
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bridging Nitronyl Nitroxide radicals
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non-bridging
?-1,3 bridging
?-1,1 and ?-3,3 bridging
?-1,1 bridging
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Cambridge Structural Database
non-bridging
?-1,3 bridging
180
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?-1,1 and ?-3,3 bridging
?-1,1 bridging
-
3
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non-bridging
180
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?-1,3 bridging
40
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• 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.

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Spin transitions in non-classical systems
head to tail
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Change in the Jahn-Teller axis of the Cu
bipyramids
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1 D
2D / 3D ?
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charge distribution
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Cu2(NIT-PhCOO)4(DMSO)2
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? 0.50 K
? - 0.85 K
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NIT phenolates as ligands
2D / 3D network
high spin density
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4-hydroxo phenolates and their metal complexes
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magnetic dilution of a nitronyl nitroxide
poly-vinylchloride matrix
microcrystalline film
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• syntheses of molecular building blocks
• electronic structures
• magnetic dilution
• sign of the magnetic interaction
• ? construction of polynuclear compounds

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Ni2 d8
dx2-y2, dz2
dxy, dxz , dyz
dx2-y2
octahedral coord. with axial CH3OH Ni ( S 1 )
2 NIT (S1/2) Planar quadratic coord. Ni ( S
0 ) (S1)
dz2
dxy dxz , dyz
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octahedral coord. with axial CH3OH Ni ( S 1 )
2 NIT (S1/2) Planar quadratic coord. Ni ( S
0 ) (S1)
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next generation of nitronyl nitroxide ligands
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Multifunctional Radicals
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Chelating Radicals
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R Me 1 H 2
H. Oshio et a.l. Inorg. Chem. 36, 3014 (1997).

• 1 Cu(CH3CN)4PF6
• Cu(1)2 (Td-symm.)
• NIT-NIT J 55 cm1.

S. Kaizaki. J. Chem. Soc., Dalton Trans. 1566
(2001).
MIICl2(2)2 (M Mn, Co, Ni, Zn)
JM-rad 95 (Ni) and 14.9 cm1 (Co) JMn-rad
23.8 cm 1. Jrad-rad 9 cm 1.
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Metal-nitronyl nitroxide homoleptic complexes
MII(ClO4)26H2O (MII Ni, Mn, and Zn) ?
M(NITim)3(ClO4)2, M(NITbzim)3(ClO4)2. exhibit
ing strongly antiferromagnetic metal-radical
interactions (-111 lt J lt -53 cm1).
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coordination polymers
Mn(NITIm)(NITImH)ClO4 Ziessel
Mn2(NITIm)3ClO4
Rey and Luneau
Ferromagnetic ordering temperatures 1.4 K
Mn2(NITIm)3ClO4 40 K Mn2(NITBzIm)3ClO4
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Chelating Radicals
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R. Ziessel et al. Inorg. Chem., 37 (20), 5078
-5087, 1998
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R. Ziessel et al. Inorg. Chem., 37 (20), 5078
-5087, 1998
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R. Ziessel et al. Inorg. Chem., 37 (20), 5078
-5087, 1998
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Rare earth coordination compounds
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
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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
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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)
K3Fe(CN)6
Ni(ClO4)2.4H2O IM-2Py ? ?
Fe2(CN)12Ni3(IM-2Py)6
H2O
MeOH
K. E. Vostrikova, D. Luneau, W. Wernsdorfer, P.
Rey, and M. Verdaguer, J. Am. Chem. Soc., 122,
718-719 (2000)
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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)
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molecular magnetic nanowires slow relaxation in
chains
R. J. Glauber. J. Math. Phys. 4, 294 (1963). 1D
Ising ferri- or ferromagnetic materialscould
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
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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
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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 into the
structure of the radical ligand.
Inoue
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
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An Enantiopure Molecular Ferromagnet

chiral molecule
chiral ligand
chiral spacegroup P212121
Chirality induced by atomic stereogenic
centers or atropoisomeric conformations
M. Minguet, D. Luneau, E. Lhotel, V. Villar, C.
Paulsen, D. B. Amabilino, J. Veciana,Angew.
Chem., Int. Ed. 41, 586 2002
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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
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Verdazyl radicals
structure
SOMO
Lemaire, Hicks
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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
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Robin Hicks, Martin T. Lemaire Pure Appl. Chem.
76, 277 (2004)
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Lemaire, Hicks
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Nickel-verdazyl exchange strongly ferromagnetic
(??) J Ni-vd 240 cm1 Manganes-verdazyl
exchange antferromagnetic (??) J Mn-vd -45 cm
1
Robin Hicks, Martin T. Lemaire Pure Appl. Chem.
76, 277 (2004)
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Plater et al. J.Chem.Soc., Perkin Trans. 1, 971
(2000)
A series of 4,5-diazafluorene derivatives of
Koelschs free radical Reaction with CuCl2 has
reportedly generated analytically pure
metal-radical complexes, but which have not yet
been structurally or magnetically characterized.
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Triphenylmethyl-radical
Daniel Maspoch, thesis, Valencia, 2004
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Zn2
S1/2
S1/2
S0
Cu2
S1/2
S1/2
S1/2
Ni2
S1/2
S1/2
S0
D. Maspoch, D. Ruiz-Molina, K. Wurst, C. Rovira,
J. Veciana
Chemical Communications, 2002, (24), 2958 - 2959
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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)
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Charge Transfer Salts
FeCp2.TCNQ -.
Bis(ethylenedithio)tetrathiafulvalene
FeCp2.TCNE -.
TCNQ 7,7,8,8-tetracyano-p-quinodimethane, TCNE
tetracyanoethene
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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)22TCNE magnet with
• spins residing in a p-orbital
• 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.

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Variation of Tc with increasing spin number per
metal in MIII(C5Me5)2TCNE-.
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Tc 16 K
1 D- ferrimagnet
large remanent magnetization and coercice
fields 27 500 Oe at 2K !!!
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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.
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spin diverse ligands,
containing two or more different kinds of spin
carriers per molecule,
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 single S 1 units.
Nitronyl nitroxide/semiquinone hybrid biradicals
very strong intraligand ferromagnetic exchange
coupling JNN gt 300 and 100 cm1
Shultz et al. J. Am. Chem. Soc. 123, 3133
(2001) J. Am. Chem. Soc. 125, 1607 (2003)
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Y. Takano, et. al. J. Am. Chem. Soc., 124, 450
(2002)
M(II)(hfac)2(di-(4-pyridyl)phenylcarbene)
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Y. Takano, et. al. J. Am. Chem. Soc., 124, 450
(2002)
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Y. Takano, et. al. J. Am. Chem. Soc., 124, 450
(2002)
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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?
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Di- and triradicals
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Di- and triradicals
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Di-radicals and Pressure effects