Manganese(II) and copper(II) hexafluoroacetylacetonate 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine: Regiochemical parity analysis for exchange behavior of complexes between radicals and paramagnetic cations

Article abstract of DOI:10.1021/ja0343813

Mn(hfac)₂ and Cu(hfac)₂ form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)-pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)₂)₂ and (Cu[PyrimPh-NIT](hfac) ₂)₂, 1 and 2, re

Full text of DOI:10.1021/ja0343813

Published on Web 07/29/2003
Manganese(II) and Copper(II) Hexafluoroacetylacetonate 1:1
Complexes with
5-(4-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine:
Regiochemical Parity Analysis for Exchange Behavior of
Complexes between Radicals and Paramagnetic Cations
Lora M. Field,† Paul M. Lahti,*,† Fernando Palacio,‡ and Armando Paduan-Filho§
Contribution from the Department of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, Instituto de Ciencia de Materiales de Arago´n,
CSIC-UniVersidad de Zaragoza, 50009 Zaragoza, Spain, and
Instituto de Fisica, UniVersidade de Sa˜o Paulo, Sa˜o Paulo, Brazil
Received January 28, 2003; E-mail: lahti@chemistry.umass.edu
Abstract: Mn(hfac)2 and Cu(hfac)2 form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)-
pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)2)2 and (Cu[PyrimPh-NIT](hfac)2)2, 1 and 2, respectively,
are cyclic M2L2 dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication
(M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly
ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing
model with J/k ) -0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing
between S ) 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected
paramagnetic susceptibility ø(T) to an AFM spin pairing model at 200 Oe yields J/k ) (-)3.8 K, quite similar
to earlier measurements at 1000 Oe yielding J/k ) (-)5.0 K. At 1.40 K, the magnetization of 2 does not
approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization
curve shows steps characteristic of field-induced crossover between the S ) 0 ground state and excited
spin states. From the steps in the 0.55 K data, we estimate J/k ) (-)3.8-4.0 K for 2, in good agreement
with the analysis of ø(T).
Introduction
feature of this strategy is that even antiferromagnetic (AFM)
exchange coupling between transition metal and organic building
One very promising strategy for the design and synthesis of
molecular magnetic materials is to combine paramagnetic cations
with organic open-shell molecules to make hybrid systems.
Nitronylnitroxides, nitroxides, and verdazyls have all been
coordinated with paramagnetic ions to make hybrid materials
with varying types of magnetic behavior.1 Various qualitative
models for understanding spin density distribution in organic
radicals and polyradicals have been combined with overlap
models incorporating the magnetic spin-orbitals of transition
metals, as part of efforts to predict the magnetic behavior of
the hybrid molecular magnetic materials. Such materials com-
bine the increased magnetic moment of transition metals with
the structural control of organic chemistry. A particularly useful
block can lead to a material with a net magnetic moment, so
long as the transition metal has a higher spin moment than the
coordinated organic fragment.
In this article, we describe the crystallography and magnetic
behavior of two coordination complexes formed between
manganese(II) hexafluoroacetylacetonate/copper(II) hexa-
fluoroacetylacetonate and the conjugated radical 5-(4-[N-tert-
butyl-N-aminoxyl]phenyl)pyrimidine (PyrimPh-NIT): 1:1 cy-
clic, dimeric complexes[Mn(PyrimPh-NIT)(hfac)2] and [Cu-
(PyrimPh-NIT)(hfac)2], 1 and 2, respectively. We shall refer to
these complexes as 1:1 complexes because of the ion-to-radical
ratio, although they are structurally M2L2 type complexes. A
preliminary account of some of these results has been published
elsewhere.2
† University of Massachusetts.
‡ CSIC-Universidad de Zaragoza.
Results
§ Universidade de Sa˜o Paulo.
(1) For general references, see the following: (a) Kahn, O. Molecular
Magnetism; VCH: New York, 1993. (b) Iwamura, H.; Inoue, K.; Hayamizu,
T. Pure Appl. Chem. 1996, 68, 243. (c) Caneschi, A.; Gatteschi, D.; Sessoli,
R.; Rey, P. Acc. Chem. Res. 1989, 22, 392. (d) Caneschi, A.; Gatteschi,
D.; Sessoli, R. In Magnetic Molecular Materials; Gatteschi, D., Kahn, O.,
Miller, J. S., Palacio, F., Eds.; Kluwer: Dordrecht, The Netherlands, 1991;
p 215. (e) Gatteschi, D.; Rey, P. In Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999. (f) Caneschi
A.; Gatteschi, D.; Rey, P. In Progress in Inorganic Chemistry; Lippard, S.
J., Ed.; Wiley: New York, 1991; Vol 39, p 331.
Figure 1 summarizes the syntheses of complexes 1 and 2.
Silyl-protected hydroxylamine 3 was converted to boronic acid
4, subjected to palladium-catalyzed coupling with 5-bromopy-
rimidine to give 5, and deprotected to hydroxylamine 6. The
hydroxylamine was oxidized with lead dioxide to give large,
(2) Field, L. M.; Lahti, P. M.; Palacio, F. Chem. Commun. 2002, 636.
9
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J. AM. CHEM. SOC. 2003, 125, 10110-10118
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II
Mg /Cu Hexafluoroacetylacetonate Complexes
A R T I C L E S
Figure 1. (a) t-BuLi, Et2O, -78 °C; then B(OCHMe2)3; then NH4Cl. (b) Pd(CH3CO2)2, aq K2CO3, THF, 5-bromopyrimidine. (c) concd HCl, EtOH. (d)
PbO2, EtOAc. (e) Mn(hfac)2‚3H2O, acetone or Cu(hfac)2‚3H2O, EtOAc. (f) 0.5 equiv of Mn(hfac)2‚3H2O, acetone or 0.5 equiv Cu(hfac)2‚3H2O, EtOAc.
Figure 2. ORTEP diagram for complex 1. Fluorine atoms are hidden for
ease of viewing; multiple CF3 groups and one tert-butyl group are
rotationally disordered. Some atoms were not labeled for clarity of
presentation; see Supporting Information for more information.
Figure 3. ORTEP diagram for complex 2. Fluorine atoms are hidden for
ease of viewing; multiple CF3 groups are rotationally disordered. Some
atoms were not labeled for clarity of presentation; see Supporting Informa-
tion for more information.
deep-red sheets of needles of radical 7, which is stable to
ambient conditions and was readily characterized by both ESR
spectroscopy and elemental analysis.
Layered 1:1 mol:mol solutions of Mn(hfac)2‚3H2O with
radical 7 crystallized slowly in air to give deep red plates of 1.
The same procedure was carried out using Cu(hfac)2‚3H2O to
give red crystals of 2. Both 1 and 2 formed using various ratios
of M(hfac)2‚3H2O:7 g 1. When M(hfac)2‚3H2O:7 e 0.5, 1:2
complexes 8 and 9 are formed. Details about 8 and 9 will be
given elsewhere.3 Both 1 and 2 are stable and readily character-
ized by spectroscopy, elemental analysis, and magnetic suscep-
tibility. Figures 2 and 3 show crystallographic ORTEP repre-
sentations of the complexes, while Table 1 lists selected
structural parameters.
triplet and a variety of peaks attributable to Mn(II),4 while that
of 2 shows a more distorted nitroxide triplet and a set of peaks
attributable to Cu(II).4 The peaks are sharper in frozen solution
spectra (77 K) than at room temperature for both compounds
but otherwise do not change much upon cooling.
The solution spectra appear to show equilibrium mixtures of
metal cations, free or partially coordinated radicals, possibly
including 1 and 2 and/or 8 and 9. The spectra do not appear
sufficiently altered from individual component spectra to be due
mostly to discrete molecular 1 or 2, but there is enough spectral
complexity to support some level of coordination. We consider
it likely that dimers 1 and 2 are formed by reversible assembly
of ions and radicals, including possible dimerization of an initial
ML monomeric complex of metal cation and radical (Scheme
1).
Discussion
Synthesis of Complexes 1 and 2. Both dimeric complexes
are formed readily, so long as M(hfac)2:7 > 0.5. Upon mixing
of the cation with the radical, the solution at once turns very
dark (nearly opaque). UV-vis spectra of these solutions show
little difference by comparison to solutions of 7 or the
appropriate M(hfac)2, although UV-vis of mulls of solid 1 and
2 show extra, longer wavelength bands by comparison to solid
7. The ESR spectrum of 1 shows a somewhat distorted nitroxide
Structure and Spin Density Considerations. Both 1 and 2
are hexacoordinate with four oxygens from two hfac groups,
(4) For the effects of coordination on ESR spectra of a number of transition
metal cations, see (a) Rieger, P. H. Electron Spin Resonance Studies of
Organometallic Species. In Organometallic Processes; Trogler, W. C., Ed.;
Elsevier: Amsterdam, The Netherlands 1990; pp 270-305. (b) Bencini
A.; Gatteschi, D. EPR of Exchange Coupled Systems; Springer-Verlag: New
York, 1990.
(3) Field, L. M.; Lahti, P. M.; Palacio, F.; Moro´n, C. Unpublished.
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J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003 10111

A R T I C L E S
Field et al.
Table 1. Selected Molecular Structure and Close Contact
Parameters for 1:1 Complexes 1 and 2a
negative spin density on the nitrogen atoms of the pyrimidine
rings in 7. UB3LYP/6-31G* hybrid density functional5 model
computations using Gaussian 986 confirm this, showing Mul-
liken spin populations of 9.0% and (-)4.7% on the carbons
ortho and meta to the nitroxide group, but only about (-)0.5%
on the pyrimidine nitrogens. Further details are given in the
supporting material. These results are in excellent accord with
our estimates of spin populations of 9.6% and 4.3% on the
carbons ortho and meta to the nitroxide in 7, based on the
experimental aromatic proton hyperfine coupling constants aH
and using the equation aH ) QF, where F is the spin density on
the aromatic carbon and Q ) (-)22 G. There is no experimen-
tally resolvable hyperfine coupling from the pyrimidine ring.
Thus, computation and experiment both suggest that 7 will have
limited nitroxide to pyrimidine spin delocalization along the
pathway NIT-Ph-Pyrim‚‚‚M, where M ) paramagnetic ion.
However, we hoped that spin polarization (induced on the
pyrimidine unit by the coordinated paramagnetic ions) would
improve intramolecular exchange between the M-NIT spin units
directly along a M-NIT-Ph-Pyrim‚‚‚M-NIT pathway.
r(Mn[1]-O[8]), hfac
r(Mn[1]-O[10])
r(Mn[1]-O[7])
r(Mn[1]-O[9])
r(Mn[2]-O[4])
r(Mn[2]-O[6])
r(Mn[2]-O[3])
r(Mn[2]-O[5])
2.119(8) Å r(Cu-O[3]), equatorial hfac 1.933(5) Å
2.133(8) Å r(Cu-O[5]), equatorial hfac 1.946(5) Å
2.159(9) Å r(Cu-O[2]), equatorial hfac 1.978(6) Å
2.179(8) Å
2.109(8) Å r(Cu-O[4]), axial hfac
2.145(8) Å
2.297(6) Å
2.167(8) Å
2.170(8) Å
r(Mn[2]-O[1]N), M-NIT
r(Mn[1]-O[2]N)
2.129(8) Å r(Cu-O[1]N), axial
2.142(9) Å
2.434(6) Å
2.062(6) Å
r(Mn[2]-N[5])
r(Mn[1]-N[3])
2.261(9) Å r(Cu-N[1]), equatorial
2.286(9) Å
r(Mn[1]‚‚‚Mn[2]) intra
r(Mn[1]‚‚‚Mn[2]′) inter
10.421(3) Å r(Cu-Cu) intra
6.068(3) Å r(Cu-Cu) inter
10.538(1) Å
8.191(1) Å
1.254(9) Å
r(N[1]-O[1])
r(N[2]-O[2])
1.306(12) Å r(N[1]-O[3])
1.275(12) Å
Ph-Pyrim, torsion
C[3]-C[4]-C[5]-C[10]
Ph-Pyrim, torsion
C[3]-C[4]-C[5]-C[6]
28.7(17)°
24.9(10)°
C[13]-C[14]-C[15]-C[20] 32.8(17)°
ON-Ph, torsion
O[1]-N[1]-C[8]-C[9]
ON-Ph, torsion
C[7]-C[8]-N[3]-O[1]
39.9(15)°
27.3(10)°
O[2]-N[2]-C[18]-C[19] 45.2(15)°
(Mn(PyrimPhNIT)hfac2)2, 1. We investigated the behavior
of paramagnetic susceptibility (ø) versus temperature for 1. A
plot of øT versus T gives a high-temperature limit corresponding
to about S ) 2.1 with fixed g ) 2 (Figure 4). Over the
temperature range of 1.8-300 K, 1/ø versus T showed linear
Curie-Weiss behavior, with slope C ) 3.311 emu‚K/Oe‚mol
corresponding to S ) 2 and g ) 2.12. These results show a
strong AFM exchange interaction between Mn(II) and the
nitroxide unit yielding an overall S ) (5/2) - (1/2) ) 2 at room
temperature. Axially coordinated nitroxide radicals tend to be
AFM exchange coupled to Mn(II) in a high-spin octahedral
environment, due to interactions between the radical π-SOMO
and the magnetic d-orbitals on Mn(II).1f,7
C[13]-N[6] (H-bond)
C[20]‚‚‚N[4] (H-bond)
3.144(19) Å C[2]-C[3]′ (π-stacking)
3.364(10) Å
3.649(18) Å
a Numbering schemes given in Figures 2 and 3 and Scheme 3.
Scheme 1
We carried out UB3LYP/6-31G* level state energy computa-
tions on a crystallographically derived half-dimer model for 1
to compare AFM versus FM Mn(II)-NIT exchange (Figure 5).
The comparison results for the model of 2 will be discussed
subsequently. We adjusted8 the computed gap for effects of spin
contamination in the S ) 2 versus the S ) 3 state and found
∆E ) -9 kJ/mol, that is, favoring the low spin S ) 2 state.
The effect of spin contamination in comparing computed to
experimental ∆E is not entirely clear here, but the qualitative
prediction by this procedure is correct for Mn-NIT.
The limiting value of øT for 1 at higher temperatures,
computed using the half-dimer molar weight, corresponds to
an effective magnetization of µeff/µB ) 2.828(øT)1/2 ) 7.4 for
one pyrimidine nitrogen, and one nitroxide (NIT) oxygen. The
CF3 groups in both are disordered, as is one tert-butyl group in
1. System 1 incorporates different conformations of 7, one with
a nitroxide syn to the coordinated pyrimidine nitrogen and one
anti. By comparison, system 2 is centrosymmetric. The Mn-O
bond length trends in 1 are not clearly differentiated, although
the coordination sphere about Mn(II) is roughly octahedral. The
Cu-O(NIT) bond lengths in 2 are notably longer than the other
Cu-O bond lengths (Table 1), consistent with coordination of
the nitroxide oxygen at a bond-elongated axial site of a distorted
octahedron. In both cases, the nitroxide oxygen is coordinated
anti to an hfac oxygen atom.
(5) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(6) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B.
G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V.
G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 98; Gaussian Inc.:
Pittsburgh, PA, 1998.
(7) Cf. the summary and citations in ref 1f, pages 355-356.
(8) Various schemes have been proposed for correcting exchange energy gaps
between high and low spin states formed by coupling of localized spin
sites, for example: (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (b)
Ruiz, E.; Cano, J.; Alvarez, S.; Alemany, P. J. Comput. Chem. 1999, 20,
1391. (c) Soda, T.; Kitagawa, Y.; Onishi, H.; Takano, Y.; Shigeta, Y.;
Nagao, H.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 2000, 319,
223. We used the method of ref 8c to make correction for the energy gaps
of 1 and 2, dividing the computed energy difference by the computed
difference in S2 between the two states.
An important part of the initial design strategy for 1 and 2
was to use the spin density distribution in 7 to link the
paramagnetic ions. Spin polarization considerations predict
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10112 J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003

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A R T I C L E S
Figure 4. For complex 1, (a) plot shows øT versus T (4) and 1/ø versus
T (O), all at 1000 Oe. Solid line for øT versus T uses eq 1; solid line for
1/ø versus T is a Curie-Weiss linear fit to all data. (b) Plot shows M versus
H data (M ) magnetization, H ) field) at 1.8 K.
the full dimer. Although slightly high, this is in reasonable
agreement with the value of µeff/µB ) 6.93 expected for a system
with two energetically degenerate S ) 2 units per mole. The
saturation magnetization M versus field/temperature (H/T)
extrapolates to about 4.16 µB/mol, consistent with an S ) 2
system having gavg ) 2.1 in the half-dimer.
The higher temperature 1/ø versus T data for 1 yields2 a Weiss
constant of θ ) -0.15 K, showing a weak generalized AFM
interaction between Mn-NIT spin units, despite the strong Mn-
NIT AFM exchange within a single unit. The downturn of the
øT versus T plot for 1 also shows AFM exchange coupling of
spin sites at low temperature. The øT versus T for 1 was fitted
to the model9 of eq 1 for spin pairing of S ) 2 spin units.
Figure 5. State energies for UB3LYP/6-31G* computations on half-dimer
models of 1 and 2, using the crystallographic geometries with replacement
of disconnection sites by C-H bonds (not shown). Energies are given
hartrees (au). Selected Mulliken spin densities are shown for the lowest
energy states.
mental data to the fitted model, which gives an exchange
coupling constant of J/k ) -0.25 K for g1 ) g2 ) 2.0.
Iwamura and co-workers studied a set of complexes related
to 1 and 2.10 The Mn(hfac)2 1:1 cyclic complex with N-(4-tert-
butylnitroxylphenyl)imidazole (4-NITPh-Im), 10,10a shows strong
Mn-NIT AFM exchange interaction, with an onset below 100
K of an FM interaction of J/k ) +0.59 K between the S ) 2
Mn-NIT spin sites. The connectivity isomer with 3-(4-tert-
N0g1g2µ2B
ø )
‚
(
)
kT
84 + 6 exp(-10x) + 30 exp(-6x) + 180 exp(8x)
7 + exp(-12x) + 3 exp(-10x) + 5 exp(-6x) + 9 exp(8x)
(x ) J/kT)
(1)
(10) (a) Ishimura, Y.; Inoue, K.; Koga, N.; Iwamura, H. Chem. Lett. 1994, 1693.
(b) Kitano, M.; Ishimaru, Y.; Inoue, K.; Koga, N.; Iwamura, H. Inorg.
Chem. 1994, 33, 6012. (c) Iwamura, H.; Koga, N. Mol. Cryst. Liq. Cryst.
1999, 334, 437. (d) See Koga, N.; Iwamura, H. In Magnetic Properties of
Organic Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999; p
641. (e) Rabu, P.; Drillon, M.; Iwamura, H.; Gorlitz, G.; Itoh, T.; Matsuda,
K.; Koga, N.; Inoue, K. Eur. J. Inorg. Chem. 2000, 211.
Here, g1 and g2 are Lande´ constants for component spin units,
k is the Boltzmann constant, µB is the Bohr magneton constant,
and N0 is Avogadro’s number. Figure 4 compares the experi-
(9) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, Germany, 1986.
9
J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003 10113

A R T I C L E S
Field et al.
Chart 1
Scheme 2
Scheme 3
butylnitroxylphenyl)imidazole (3-NITPh-Im), 11, shows AFM
exchange behavior between the S ) 2 units with J/k ) -0.22
K. The difference was attributed to different regiochemical
connectivities of the complexes, although it was not clear
whether the effects in 10 and 11 were purely intramolecular.
Complex 12, a connectivity analogue of 1 with 3-pyrimidyl-
tert-butylnitroxide (3-NIT-Py), has not been described to our
knowledge. But, the known para complex 1310b,e with 4-pyrim-
idyl-tert-butylnitroxide (4-NIT-Py) exhibits strong AFM ex-
change within an Mn-NIT spin unit (S ) 2) and moderate FM
exchange10e of J/k ) +8.7 K between units. The stronger
exchange in 13 by comparison to 10 and 11 is probably due to
a smaller number of bonds in the intramolecular exchange
pathway (Chart 1).
In the absence of 12, complexes 10 and 13 seem to be the
best benchmarks for the behavior of 1. Kitano et al.10b described
the spin polarization pathways expected in π-conjugated ni-
troxides coordinated to Mn(II). Their model is consistent with
the FM exchange observed10e between Mn-NIT spin sites in
13. Extrapolating, the Mn-NIT spin sites should be intramo-
lecularly AFM exchange coupled in 1, as shown in Scheme 2
and consistent with the observed behavior.
The comparison of 1 to 10 (or 11) is less straightforward.
The imidazole rings in 10 and 11 introduce nonalternant effects
into the spin-density distribution. Although 10 is structurally
similar to 1 (Scheme 2), 10 exhibits FM exchange between Mn-
NIT spin units, the qualitative reverse of 1. UB3LYP/6-31G*
model spin density computations on the organic radical portion
of 10 show a small positive spin density on the coordinating
imidazole nitrogen, +0.35% (see Supporting Information for
further details). Interannular delocalization in 10 is thus quite
limited, as in radical 7. But, the positive spin polarization on
the coordinating imidazole nitrogen, opposite to the negative
sign of the spin density on the coordinating nitrogen of radical
7, is consistent with the reversal of qualitative exchange behavior
between 10 and 1.
We considered the possibility that through-space interaction
between Mn-NIT spin sites could account for the small exchange
couplings observed for 1 and 10. In 1, the intramolecular and
closest intermolecular Mn‚‚‚Mn distances are 10.42 and 6.07
Å, respectively. The corresponding distances in 10 are 10.65
and 8.21 Å. If intermolecular Mn‚‚‚Mn distances alone con-
trolled exchange in 10, even stronger effects would seem likely
in 1 due to the closer contacts. So simplistic an argument would
not, of course, account for differences in orbital overlap for
different geometries.
We also considered possible intermolecular exchange by spin
polarization through close atomic contacts. The closest inter-
dimer contacts in 1 are hydrogen-bond type interactions formed
by complementary C-H‚‚‚N interactions between pyrimidine
rings (Scheme 3, C20-H‚‚‚N4). The very small spin densities
on the pyrimidine ring suggest this to be a very weak exchange
mechanism. Although the observed exchange between Mn-NIT
spin sites is quite weak, the model of Scheme 3 also suggests
a qualitatively FM exchange linkage, opposite to the observed
behavior. No other intermolecular close-contact pathways link-
ing Mn-NIT spin sites in 1 seem likely to cause AFM exchange.
Thus, the AFM exchange in 1 is consistent with spin
polarization models10 for through-bond coupling of the Mn-
NIT spin units in 10 and 11 and 13. Because of the weakness
of the exchange, it is difficult to ascribe it definitively to an
intramolecular spin polarization mechanism. Nonetheless, there
are no clear intermolecular pathways that clearly fit the
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Mg /Cu Hexafluoroacetylacetonate Complexes
A R T I C L E S
Figure 7. Magnetization versus field, H, isotherms at 0.55 K (red) and
1.4 K (black). Arrows show positions of spin state level crossing steps
described in the text.
Figure 6. For complex 2, the plot shows ø versus T (4) and 1/ø versus T
(O), at 200 Oe. Solid lines for ø versus T uses eq 3; solid line for 1/ø
versus T is a Curie-Weiss linear fit to the data for T > 150 K.
Accordingly, the ground state of 2 will be ST ) 0, while ST )
1 and ST ) 2 will be excited state spin levels at energetic
spacings of 2J and 6J, respectively.
The ø versus T data was fitted to the above spin-pairing
model,9 with incorporation of an additional mean field interac-
tion term. Equation 5 gives the expression for the temperature
dependence of the susceptibility: the constants have the same
meanings as given for eq 1 earlier, with the addition of θ as the
mean-field term.
qualitative exchange behavior. Given the consistency of our
results with models for the behavior of previously studied 10,
11, and 13, we feel that use of the spin polarization mechanism
is justified as a working model for all of these systems.
(Cu(PyrimPhNO)hfac2)2, 2. A 1/ø versus T plot at 200 Oe
for 2 yields a Curie constant of C ) 0.909 emu K/Oe mol and
a Weiss constant of θ ) -0.2 K (Figure 6). The Curie constant
corresponds to S ) 0.94 if g ) 2. Thus, 2 exhibits strong FM
interaction between the Cu(II) ions and the nitroxide units, with
effective S ) 1. High temperature values of øT versus T
approach an equivalent magnetization value of µeff/µB = 4.0,
in good agreement with the value of 4.00 expected for a
molecule composed with two S ) 1 Cu-NIT units.
We carried out UB3LYP/6-31G* computations on a crystal-
lographically derived half-dimer model of 2. Figure 5 shows
computed energies for the S ) 0 and S ) 1 states of the model,
as well as the Mulliken spin density populations in the high
spin state. The high spin state is favored by ∆E ) 1.7 kJ/mol
when the computed energies are adjusted using eq 1. The
favoring of a high spin state for Cu(II)-NIT in axial coordination
been explained elsewhere in terms of orthogonality between the
SOMO’s of Cu(II) and the nitroxide.1f,11-12 The results of Figure
5 are best considered a qualitative comparison to experiment,
but the computed high spin state is correctly favored in 2.
The ø versus T plot for 2 maximizes at 8 K and then decreases
rapidly down to the minimum measurement temperature. This
is indicative of antiferromagnetic spin pairing between S ) 1
units, for which the Hamiltonian is given by eq 2,
2
N0g1g2µB
exp(2x) + 5 exp(6x)
ø )
(5)
(
)
k(T - θ) 1 + 3 exp(2x) + 5 exp(6x)
(x ) J/kT)
The behavior is slightly field dependent. We previously
described2 a preliminary study of 2 at 1000 Oe, from which we
found J/k ) -5.0 K, θ ) -1.1 K, using fixed values of g1 )
2.00 and g2 ) 2.20. By comparison, fitting of the data at 200
Oe gives J/k ) -3.79 K, using fixed values of g1 ) g2 ) 2.00
and θ ) -0.2 K (Figure 6). Overall, 2 exhibits stronger AFM
exchange coupling between M-NIT spin units than 1 under all
conditions tested.
The magnetization versus field plot does not saturate up to
50 kOe; instead, it increases smoothly and exhibits a shape that
strongly differs from a Brillouin function. However, magnetiza-
tion experiments made at higher fields show an S-shaped
behavior, as shown for measurements made at 1.4 and 0.55 K
in Figure 7. The shape is very smooth in the 1.4 K isotherm,
whereas at 0.55 K a clear “ripple” is observed. In the latter
plot, over 0-40 kOe, the magnetization is quite small and
increases very slowly, but at higher fields, it rapidly increases
to form a shoulder at approximately 2µB corresponding to the
S ) 1 state. Increasing the magnetic field further gives another
rapid increase in magnetization and a second shoulder at
approximately 4µB corresponding to the S ) 2 spin state. The
width of these transitions is independent of the applied magnetic
field.
H ) -2J S1‚S2 + gµBH (S1z + S2z)
(2)
This Hamiltonian is diagonal in the magnitude ST of the total
spin of the pair and in the projection m of the total spin ST
along H. The energy levels can be obtained by noting that ST
) S1 + S2 obeys the relation given by eq 3,
ST‚ST ) S1‚S1 + S2‚S2 + 2S1‚S2
(3)
Since S1 ) S2 ) S, this leads to a set of energies given by eq
4,
This magnetization behavior strongly supports the spin pairing
model used to model the temperature dependence of the
susceptibility. The shoulders observed in the 0.55 K magnetiza-
tion isotherm originate from crossovers of higher spin multiplets
at larger applied fields. At zero-field, the |ST ) 0, M ) 0 state
E ) STm|H|STm )
-J[ST(ST + 1) - 2S(S + 1)] + gµBmH (4)
9
J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003 10115

A R T I C L E S
Field et al.
seem not to have been described. Complex 1413 is a parity
relative of 2 but has such long Cu-O(NIT) bonds (2.79 Å,
versus 2.43 Å in 2) that Cu(II) occupies a square-pyramidal,
pentacoordinate environment, quite different from the Cu(II)
environment in 2. Iwamura and co-workers studied 1:2 com-
plexes 15-17 related to 8 and 9,10b-d,13 but these do not
incorporate direct Cu-NIT bonding. The following trends were
described in the latter studies: (1) exchange constants J/k in
the Cu(II) complexes were stronger than in the analogous Mn-
(II) complexes, (2) exchange behavior seemed to be of opposite
sign to that in the Mn(II) structural analogues, and (3) the spin
density of the coordinated pyridine nitrogen atoms in 15 and
17 was proposed to be polarized to the same sign as the Cu(II).
We tried to compare these trends to the behavior of 2.
Scheme 2 postulates spin polarization exchange pathways for
1 and 2. A positive spin polarization on the Cu(II)-coordinated
pyrimidine nitrogen is important to yield a qualitative result,
wherein both 1 and 2 are AFM coupled. The computations
summarized in Figure 5 are consistent with the proposed Cu-
(II)-pyrimdine spin polarization pattern. The same parity was
also computed for Mn(II)-pyrimidine polarization in 1, against
the Scheme 2 proposal, but the significant S ) 3 state
contamination in the S ) 2 computation for 1 may effect this
result. The high spin S ) 1 state of the model for 2 is not
significantly spin contaminated, so we deem the spin density
results for S ) 1 Cu-NIT to be more reliable for comparison to
experiment.
Figure 8. (a) Energy level diagram for the pair model (see text) and Zeeman
splitting of these levels in a magnetic field H. Note that the crossing of
levels at H1 and H2 changes the ground state. (b) Schematic magnetization
curve at very low temperature showing the steps in the magnetization due
to the successive increases of |m| in the ground state before reaching real
saturation.
Overall, therefore, the intramolecular spin polarization model
of Scheme 2 is plausible to explain comparative magnetic
behaviors of 1 and 2 (with a caveat described in the subsequent
paragraph). If we accept that this model governs their exchange
natures, our experimental results clarify a structure-property
deficit left by the absence of systems such as 12 and analogues
summarized in Table 2.
is lowest in energy. Under application of a magnetic field, the
upper spin states undergo Zeeman splitting, as shown in Figure
8a. Only the ST ) 0 zero-field ground state level is not split.
Due to the Zeeman splitting, the ground state of the overall
dimer 2 changes with increasing H. Thus, as the magnetic field
is increased, the |ST ) 1, m ) -1 energy level from the ST )
1 manifold drops in energy according to -gµBH, becoming the
ground state at H1. Because of the jump in |m| at this point, the
low temperature magnetization exhibits a step at H1. Further
increase of the field results in a second crossover at H2 between
the |1, -1 and the |2, -2 spin states; the latter becomes the
ground state, and so the magnetization |m| again increases by
one unit. Since this is the highest spin value of the pair, the
magnetization will saturate at sufficient high field. These steps
in the magnetization are sketched qualitatively in Figure 8b. If
the temperature is sufficiently low compared to |J/k|, the steps
and subsequent saturation plateaus in the magnetization can be
clearly observed. However, as T approaches the value of |J/k|,
thermal population obscures the steps, and the magnetization
appears to increase smoothly up to the saturation limit. Just such
behaviors are observed for the magnetization plots of Figure 7
at 0.55 and 1.4 K, respectively.
One ambiguity in a connectivity-based analysis for 2 is the
strong intermolecular overlap between π-stacked pyrimidine
rings in adjacent molecules (Figure 9). No similar contact is
present in 1. Pyrimidine rings in adjacent molecules of 2 are
stacked in a parallel, offset manner, only 3.364 Å apart. At this
distance, the π-clouds are in direct contact. The strong overlap
between sites of opposite spin density could provide an AFM
exchange pathway.14,15 Although DFT computations on 7 and
on the model for 2 (Figure 5) show <2% spin densities on the
pyrimidine ring, the net through-space effect might occur due
to the close stacking despite the small spin overlap involved.
Qualitatively, spin-pairing interaction in 2 thus might occur by
(11) For example, see: (a) Laugier, J.; Ramasseul, R.; Rey, P.; Espie, J. C.;
Rassat, A. NouV. J. Chim. 1980, 7, 11. (b) Anderson, O. P.; Kuechler, T.
C. Inorg. Chem. 1980, 19, 1417. (c) Grand, A.; Rey, P.; Subra, R. Inorg.
Chem. 1983, 22, 391.
(12) For some cases of AFM CuNIT exchange, see: (a) Lim, Y. Y.; Drago, R.
S. Inorg. Chem. 1972, 11, 1134. (b) Porder, L. C.; Dickman, M. H.;
Doedens, R. J. Inorg. Chem. 1985, 24, 1006. (c) Luneau, D.; Rey, P.;
Laugier, J.; Fries, P.; Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Am. Chem.
Soc. 1991, 113, 124.
According to this model the magnetic fields at the centers of
the magnetization steps are given by eq 5,
gµBHn ) 2|J|n
(5)
(13) Ishimaru, Y.; Kitano, M.; Kumada, H.; Koga, N.; Iwamura, H. Inorg. Chem.
1998, 37, 2273.
(14) McConnell, H. M. J. Chem. Phys. 1963, 39, 1910.
where n ) 1, 2. Based on the data of Figure 7, we used H1 )
57 kOe and H2 ) 120 kOe. Assuming g ) 2.0, one obtains
|J/k| values for the two steps of 3.8 and 4.0 K, respectively, in
excellent agreement with the fit of the susceptibility at low field
in Figure 6.
There is a dearth of structural analogues to which to compare
the behavior of 2. Cu(II)-based versions Mn(II)-based 10-12
(15) (a) Kawakami, T.; Yamanaka, S.; Mori, W.; Yamaguchi, K.; Kajiwara,
A.; Kamachi, M. Chem. Phys. Lett. 1995, 235, 257. (b) Yamaguchi, K.;
Kawakami, T.; Oda, A.; Yoshioka, Y. In Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999 and
references therein. (c) Yoshiozawa, K.; Hoffmann, R. J. Am. Chem. Soc.
1995, 117, 6921. (d) Yoshiozawa, K. In Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999 and
references therein.
9
10116 J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003

II
II
Mg /Cu Hexafluoroacetylacetonate Complexes
A R T I C L E S
Table 2. Comparison of Exchange Behavior as a Function of Structure in Metal-Radical Complexesa
complex
J/k (M-NIT)
J/k (S−S)
reference
[Mn(3-NITPh-Im)(hfac)2]2, 11
[Mn(4-NITPh-Im)(hfac)2]2, 10
[Mn(PyrimPhNIT)(hfac)2]2, 1
[Mn(4-NIT-Py)(hfac)2]2, 13
[Cu(PyrimPhNIT)(hfac)2]2, 2
Cu(4-NIT-Py)(hfac)2, 14
strong, AFM at 300 K
strong, AFM at 300 K
strong, AFM at 300 K
strong, AFM at 300 K
strong, FM at 300 K
AFM -0.22 K (S ) 2)
FM +0.59 K (S ) 2)
AFM -0.25 K (S ) 2)
FM +8.7 K (S ) 2)
AFM -3.8 K (S ) 1)
FM +58.6 K
10a
10a
This work
10e
This work
13
(2 × S ) 1/2)
a J(M-NIT) shows exchange from metal cation to nitroxide (M-ON), while J/k (S-S) shows exchange between M-NIT spin units, except for 14, where
the exchange model used treats the Cu(II) and NIT spins separately.
Figure 9. π-Stacking interaction between pyrimidine rings in different molecules of 2. Ball-and-stick atoms show the interaction paths between Cu-NIT
spin sites on the left-hand side of the figure. The right-hand side of the figure shows qualitative, relative spin density, based on a qualitative polarization
model for AFM exchange.
a [Cu-NIT]-Pyrim‚‚‚‚‚Pyrim-[Cu-NIT] through-space interdimer
overlap mechanism rather than by a spin polarization pathway
between Cu-NIT sites within the cyclic dimer.
intradimer exchange pathway is dominant; there even may be
some contribution from both, whichever dominates. Despite this
complexity, we must note that the exchange coupling of and
between M-NIT spin sites in 1 and 2 is quite consistent with
the qualitative, parity-based intramolecular spin parity models.
These models remain convenient paradigms to design hybrid
inorganic/organic magnetic materials. Unless clearly contra-
dicted, the models are the most consistent means by which to
try to predict and explain the behavior of 1 and 2 and related
systems. By correlating the relative strength of exchange
interactions as a function of structure, researchers will hopefully
be better able to design hybrid magnetic materials from first
principles.
Conclusions
Mn(hfac)2 and Cu(hfac)2 both form cyclic, dimeric 1:1
complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimi-
dine that incorporate both metal-nitroxide and metal-pyrimidine
coordination sites. Both have the same head-to-head cyclic dimer
connectivity, despite the geometric differences that break the
symmetry in 1 by comparison to the centrosymmetry in 2. These
systems provide information about the little-known 1:1 com-
plexes of paramagnetic dications with meta connectivity organic
radicals, models for hybrid inorganic-organic exchange-linked
systems. The Mn-NIT unit is strongly and antiferromagnetically
coupled (S ) 2), while the axial Cu-NIT unit is strongly and
ferromagnetically coupled (S ) 1); for both, this behavior is
observed even at room temperature.
Experimental Section
General. Diethyl ether and tetrahydrofuran (THF) were distilled
under argon from sodium. N,N-Dimethylformamide (DMF) was dried
over anhydrous magnesium sulfate. tert-Butyllithium in pentane and
n-butyllithium in hexanes were obtained from Acros. tert-Butyldimethyl
chlorosilane was obtained from Alfa Aesar. All other chemicals were
obtained from Aldrich and used as received. All melting points are
uncorrected.
The AFM interaction in 1 is consistent with an intramolecular
spin parity analysis, although the weakness of the exchange
makes it hard to rule out an intermolecular contribution
conclusively. The AFM exchange in 2 is also consistent with
intramolecular spin parity analysis, although this situation is
complicated by a favorable interdimer π-overlap. The close
intermolecular π-stacking in 2 could give AFM exchange and
confuse the intramolecular spin parity analysis. It is therefore
difficult to decide conclusively whether the interdimer or the
X-ray crystallographic analyses were carried out by Dr. A. Chan-
drasekaran at the X-ray Structural Characterization Facility at the
University of Massachusetts-Amherst. Elemental analysis was carried
out by Dr. G. Dabkowski of the University of Massachusetts-Amherst
Microanalysis Laboratory. NMR spectra were obtained on Bruker EMX-
200 FT-NMR spectrometers. UV-vis spectra were obtained on a
Shimadzu UV-260 spectrometer.
9
J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003 10117

A R T I C L E S
Field et al.
tert-Butyl(4-bromophenyl)-tert-butyldimethylsiloxylamine (3). This
compound was synthesized in 74% yield by the method of Inoue and
Iwamura.16 See Supporting Information for details.
4-(N-tert-Butyl-N-[tert-butyldimethylsiloxyl]amino)phenyl Bo-
ronic Acid (4). This compound was synthesized following the
procedures of Inoue and Iwamura and of Lahti et al.17 See Supporting
Information for details.
(2) Å3, Z ) 4, Dcalc ) 1.585 g/cm3; 6164 unique reflections with I >
2σ(I) were modeled with 511 variables to yield the structure with R )
0.1341, wR ) 0.3377. The large R-value is due to rotational disorder
in half of the CF3 groups and one tert-butyl group.
[Cu(hfac)2(PhPyrim-NIT)]2 (2). Atop a solution of 0.053 g (0.22
mmol) of 7 in 8 mL of ethyl acetate was layered a second solution of
0.11 g (0.22 mmol) of Cu(hfac)2‚3H2O in 0.1 mL of acetone and 8 mL
of hexane. The resulting solution was allowed to stand for 1 day and
then to evaporate slowly for another day to give a crude solid. This
solid was then recrystallized from dichloromethane and hexane to give
0.084 g (54%) of red, square platelike crystals with mp 132-134 °C.
Anal. Calcd for C24H18N3O5F12Cu: C, 40.52; H, 2.55; N, 5.91. Found:
C, 40.65; H, 2.48; N, 5.88. UV-vis (λmax, tetrahydrofuran; nm[M-1
cm-1]): 314 [62 000], 500 [12 000]. Monoclinic space group C2/c
(#15), with a ) 13.500(2) Å, b ) 14.53(7) Å, c ) 33.696(2) Å, â )
101.02(9)°, V ) 6485.91(16) Å3, Z ) 4, Dcalc ) 1.475 g/cm3; 5710
unique reflections with I > 2σ(I) were modeled using 382 variables to
yield the structure with R ) 0.1280, wR ) 0.3634. The large R-value
is due in part to rotational disorder in multiple CF3 groups.
5-[4-(N-tert-Butyl-N-[tert-butyldimethylsiloxyl]amino)phenyl]-
pyrimidine (5). A flask containing 0.38 g (2.38 mmol) of 5-bromopy-
rimidine and 0.027 g (0.12 mmol) of palladium acetate was evacuated
and recharged with argon twice. Then, 10 mL of THF was added, and
the solution was allowed to stir for 10 min. Deaerated solutions of 1.0
g (3.09 mmol) of 4 in 5 mL of THF and 0.82 g (5.95 mmol) of
potassium carbonate in 4.5 mL of water were then added. The reaction
was then heated to reflux and allowed to stir under argon for 1 day.
The aqueous layer was then separated and extracted with diethyl ether.
The combined organic layers were washed with brine, dried over
magnesium sulfate, filtered, and concentrated in vacuo. The crude
material was purified by chromatography on silica gel with a 2:8
mixture of ethyl acetate/hexanes to give a yellow solid product: 0.57
Magnetic Measurements. DC magnetic susceptibility experiments
were carried out on two Quantum Design MPMS SQUID magnetom-
eters in Zaragoza and in Sa˜o Paulo. The samples were packed into
gelatin capsules with cotton, subjected to at least 3-fold helium purge.
Molar paramagnetic susceptibilities were determined after correction
for diamagnetic and temperature independent contributions. Diamag-
netic contributions to total susceptibility were computed using Pascal’s
tables. Magnetization versus field strength measurements between 0
and 50 kOe were also carried out in a Quantum Design MPMS
magnetometer at 1.8 and 3 K. A vibrating sample magnetometer (VSM)
was used for high-field magnetization measurements up to 17 T, which
were carried out at 1.4 and 0.55 K. (In ref 2, a preliminary description
of the magnetic analysis for 2 was done using eq 1 listed in this article,
but the equation was incorrectly shown in that reference. The exchange
constant J/k of 2 in ref 2 thus is given correctly.)
1
g, 66%, mp 73-74 °C. H NMR (CDCl3): δ -0.10 (broad s, 6 H),
0.93 (s, 9H), 1.13 (s, 9 H), 7.38 (d, 2 H, J ) 8 Hz), 7.47 (d, 2 H, J )
8 Hz), 8.96 (s, 2 H), 9.17 (s, 1 H). Anal. Calcd for C20H31N3OSi: C,
67.18; H, 8.74; N, 11.75. Found: C, 67.65; H, 9.03; N, 11.72.
5-[4-(N-tert-Butyl-N-hydroxylamino)phenyl]pyrimidine (6). To a
solution of 0.182 g (0.509 mmol) of 5 in 5 mL of ethanol was added
1 mL of concentrated HCl. The resulting solution was allowed to stir
overnight under argon. The solution was concentrated, then diluted with
5 mL water, neutralized to pH 5 with 1 M sodium hydroxide, and then
extracted with dichloromethane repeatedly. The organic layers were
combined, washed with water and then brine, dried over magnesium
sulfate, filtered, and concentrated in vacuo to give an oil. Pale yellow,
square crystals of the product formed upon recrystallization from ethyl
1
acetate/hexane: 0.99 g, 80%, mp 142-145 °C. H NMR (CDCl3): δ
-1.52 (s, 9 H), 7.66 (d, 2 H, J ) 8 Hz), 8.02 (d, 2 H, J ) 8 Hz), 8.97
(s, 2 H), 9.26 (s, 1 H). Anal. Calcd for C14H17N3O: C, 69.11; H, 7.04;
N, 17.27. Found: C, 68.95; H, 7.24; N, 17.21.
Acknowledgment. The authors wish to dedicate this work
to Prof. Domingo Gonzalez, of the University of Zaragoza, on
the occasion of his retirement. This work was supported by the
National Science Foundation (NSF CHE-9809548 and CHE-
0109094), the Comision Interministerial de Ciencia y Tecnologia
(CICYT-MAT2000-1388-C03-03), the Fullbright Espan˜a com-
mission, and the Fundac¸a˜o de Amparoa Pesquisa do Estado de
Sa˜o Paulo (A.P.-F.). The authors thank Dr. A. Chandrasekaran
of the University of Massachusetts-Amherst X-ray Structural
Characterization Center (NSF CHE-9974648) for crystallo-
graphic analyses and Dr. G. Dabkowski of the University of
Massachusetts-Amherst microanalytical laboratory for elemental
analyses. The University of Massachusetts-Amherst authors
acknowledge support for magnetic measurements from the
National Science Foundation (Nanomagnetics Characterization
Facility, NSF CTS-0116498).
5-(4-N-tert-Butyl-N-aminoxylphenyl)pyrimidine (7). To a solution
of 0.84 g (0.37 mmol) of 6 in 5 mL of ethyl acetate was added 0.090
g (0.37 mmol) of lead dioxide. The resulting suspension was allowed
to stir for 2 h under argon. The suspension was then filtered through
Celite, and the filtrate was allowed to slowly evaporate under air to
give red crystals 0.080 g (89%) with mp 103-105 °C. Anal. Calcd for
C14H16N3O: C, 69.40; H, 6.65; N, 17.34. Found: C, 69.50; H, 6.78;
N, 17.45. ESR (benzene): g ) 2.0066, aN ) 11.70 (nitroxide N), aH
) 2.11 and 0.94 G. UV-vis (λmax, tetrahydrofuran; nm[M-1 cm-1]):
319 [37 000], 500 [5400].
[Mn(hfac)2(PhPyrim-NIT)]2 (1). Over a solution of 0.053 g (0.22
mmol) of 7 in 8 mL of ethyl acetate, a solution of 0.10 g (0.22 mmol)
of Mn(hfac)2‚3H2O in 0.1 mL of acetone and 8 mL of hexane was
layered on top. The resulting layered solution was allowed to stand for
1 day and then allowed to evaporate for another day to give a crude
solid. This solid was recrystallized from ether and hexane to give 0.064
g (41%) of dark red plates with mp 142-143 °C. UV-vis (λmax
,
Supporting Information Available: Crystallographic sum-
maries for 1 and 2, summaries of DFT computations for models
of 1, 2, and 7, ESR spectra for 1 and 2 (fluid and frozen
solutions) and 7 (fluid solution). This material is available free
of charge via the Internet at http://pubs.acs.org.
tetrahydrofuran; nm[M-1 cm-1]): 313 [73 000], 500 [6800]. Anal. Calcd
for C24H18N3O5F12Mn: C, 40.04; H, 2.52; N, 5.84. Found: C, 40.28;
H, 2.63; N, 5.64. Monoclinic space group P21/c (#14), with a ) 9.888-
(9) Å, b ) 28.922(5) Å, c ) 21.273(1) Å, â ) 78.339(7)°, V ) 5958.8-
(16) Inoue, K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 927.
(17) Lahti, P. M.; Liao, Y.; Julier, M.; Palacio, F. Synth. Met. 2001, 122, 485.
JA0343813
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10118 J. AM. CHEM. SOC. VOL. 125, NO. 33, 2003