Cyclic M2(RL)2 coordination complexes of 5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine with paramagnetic transition metal dications

Article abstract of DOI:10.1021/ic050435t

5-(3-(N-tert-Butyl-N-aminoxyl)phenyl)pyrimidine (RL = 3NITPhPyrim) forms isostructural cyclic M₂(RL)₂ cyclic dimers with M(hfac)₂ (M = Mn, Co, Cu; hfac = hexafluoroacetylacetonate). Mn ₂(hfac)₄(RL)₂ exhibits strong antiferromagnetic Mn-RL exchange, with weak ferromagnetic exchange (0.7 cm^-1) between Mn-RL units that is consistent with a spin polarization exchange mechanism. The magnetic moment of Co₂(hfac)₄(RL)₂ at higher temperatures is consistent with strongly antiferromagnetic exchange within the Co-NIT units and tends toward zero below 50 K at lower magnetic fields. Cu ₂(hfac)₄(RL)₂ shows more complex behavior, with no high-temperature plateau in χT(T) up to 300 K but a monotonic decrease down to about 100 K. The Cu(II)-nitroxide bonds decrease by 0.2-0.3 A over the same temperature range, corresponding to a change of nitroxide coordination from axial to equatorial. This thermally reversible Jahn-Teller distortion leads to a thermally induced spin state conversion from a high-spin, paramagnetic state at higher temperature to a low-spin state at lower temperature. This spin state conversion is accompanied by a reversible solid-state thermochromic change between dull yellow-brown at room temperature and green at 77 K.

Full text of DOI:10.1021/ic050435t

Inorg. Chem. 2005, 44, 6725−6735
Cyclic M2(RL)2 Coordination Complexes of
5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine with Paramagnetic
Transition Metal Dications
Martha Baskett,† Paul M. Lahti,*,† Armando Paduan-Filho,†,‡ and Nei F. Oliveira, Jr.‡
Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003, and
Instituto de Fisica, UniVersidade de Sa˜o Paulo, S. P., Brazil
Received March 23, 2005
5-(3-(N-tert-Butyl-N-aminoxyl)phenyl)pyrimidine (RL
with M(hfac)2 (M Mn, Co, Cu; hfac hexafluoroacetylacetonate). Mn2(hfac)4(RL)2 exhibits strong antiferromagnetic
Mn RL units that is consistent with a
RL exchange, with weak ferromagnetic exchange (0.7 cm-1) between Mn
spin polarization exchange mechanism. The magnetic moment of Co2(hfac)4(RL)2 at higher temperatures is consistent
with strongly antiferromagnetic exchange within the Co NIT units and tends toward zero below 50 K at lower
magnetic fields. Cu2(hfac)4(RL)2 shows more complex behavior, with no high-temperature plateau in T(T) up to
300 K but a monotonic decrease down to about 100 K. The Cu(II) nitroxide bonds decrease by 0.2 0.3 Å over
the same temperature range, corresponding to a change of nitroxide coordination from axial to equatorial. This
thermally reversible Jahn Teller distortion leads to a thermally induced spin state conversion from a high-spin,
) 3NITPhPyrim) forms isostructural cyclic M2(RL)2 cyclic dimers
)
)
−
−
−
ø
−
−
−
paramagnetic state at higher temperature to a low-spin state at lower temperature. This spin state conversion is
accompanied by a reversible solid-state thermochromic change between dull yellow-brown at room temperature
and green at 77 K.
Introduction
structure of radical-ligands or in the coordination sphere of
such complexes can sometimes lead to large changes in
electronic behavior. As a result, a substantial amount of effort
in this area is ongoing, much of it aimed to identify predictive
structure-property trends.
Although a number of paramagnetic systems have been
reported with a general M2(L)2 dimer structure,1,2 we restrict
our attention to cyclic M2(RL)2 complexes where RL )
radical-ligand; hence, the organic fragment and the metal
ion bear unpaired spins. A variety of examples have been
reported where the RL spin-bearing unit is nitronyl
nitroxide,1a-c,f,h iminoyl nitroxide,1m-n verdazyl,1l-m or aryl
nitroxide.1c-f,h,j We have been most interested in systems
incorporating R ) aryl nitroxide, since these radical-ligands
exhibit the following desirable features for molecular mag-
The synthesis of coordination complexes between para-
magnetic cations and organic radical-ligands is a fruitful field
of study that has provided much information about structure-
property relationships for intramolecular exchange interac-
tions and molecular magnetism.1 Modest changes in the
* To whom correspondence should be addressed. E-mail: lahti@
chem.umass.edu.
† University of Massachusetts.
‡ Universidade de Sa˜o Paulo.
(1) (a) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res.
1989, 22, 392. (b) 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. (c)
Gatteschi, D.; Rey, P. In Magnetic Properties of Organic Materials;
Lahti, P. M., Ed.; Marcel Dekker: New York, 1999. (d) Iwamura,
H.; Inoue, K.; Hayamizu, T. Pure Appl. Chem. 1996, 68, 243. (e)
Iwamura, H.; Inoue, K.; Koga, N.; Hayamizu, T. NATO ASI Series,
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(2) Other M2L2 systems where L incorporates a radical include (not
exclusively): (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) Ishimaru, Y.;
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10.1021/ic050435t CCC: $30.25
Published on Web 08/05/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 19, 2005 6725

Baskett et al.
Scheme 1. Synthesis of Complexes 1-3
netic design: (1) they have high stability, (2) they possess
more delocalized unpaired spin distributions than nitronyl
nitroxides or iminoyl nitroxides, allowing more possibilities
for making structural connections with significant exchange
interactions.
We have previously reported the synthesis and magnetic
properties of M2(hfac)4(RL)2 type complexes, where RL )
5-(4-(N-tert-butyl-N-aminoxyl)phenyl)pyrimidine3a-b and 1-(4-
(N-tert-butyl-N-aminoxylphenyl))-1H-1,2,4-triazole3c (RL )
4NITPhPyrim and 4NITPhTriaz), and investigated their
differences in behavior due to variation of M2+ and incor-
poration of π-alternant vs π-nonalternant ligands. As part
of studies aimed at investigating differences in exchange
behavior due to changing connectivity, we now report the
synthesis, crystallography, and magnetic properties of the
M2(hfac)4(RL)2 complexes 1-3 of 5-(3-(N-tert-butyl-N-
aminoxyl)phenyl)pyrimidine (3NITPhPyrim) with M(hfac)2
(M ) Mn, Co, Cu; hfac ) hexafluoroacetylacetonate).
that was best used immediately after synthesis. The lower
stability of 3NITPhPyrim by comparison to 4NITPhPyrims
which can be isolated3b as a stable solidsmay be due to the
fact that it is not sterically blockaded in the position para to
the nitroxide unit, allowing increased reactivity at this
position. Despite the limited stability of 3NITPhPyrim, its
complexes made from carefully layered solutions with
M(hfac)2 (M ) Mn, Co, Cu) were highly stable and readily
characterized (Scheme 1). The Mn(II) and Co(II) coordina-
tions yielded 1 and 2, respectively, as the only isolated
products. We had more difficulty in obtaining pure 3 in a
reproducible fashion without other coordination side prod-
ucts. We have not carried out systematic study of the kinetics
of formation of 3 by comparison to alternate coordination
products, but its formation seems favored by very slow
evaporation of less polar solvent mixtures in larger volumes.
Methodology and Results
The synthesis of 3NITPhPyrim is shown in Scheme 1.
Protected hydroxylamine 4 was made by a previously
reported route4 and then metalated and converted to the
boronic acid 5. While 5 and other boronic acids can form
cyclic boroxins (not shown), either form is useful for
carbon-carbon bond formation. Compound 5 was subjected
to Pd-catalyzed coupling with 5-bromopyrimidine to give
6, deprotected to precursor 7, and oxidized using Ag2O to
the desired nitroxide. The solution phase electron spin reso-
nance (ESR) spectrum of 3NITPhPyrim showed a nitroxide
1:1:1 triplet with aN ) 12.3 G and additional hyperfine
coupling (hfc) from the phenyl protons but no resolvable
hfc from the pyrimidine ring. The meta-connectivity of
3NITPhPyrim disallows direct conjugation of the nitroxide
spin onto the pyrimidine ring. UB3LYP/EPR-II computa-
tions5,6 show that the ligating nitrogen atoms in 3NITPhPyrim
have Mulliken spin populations of only ∼0.1%.
(6) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P.
Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J.
B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, B03 ed.;
Gaussian, Inc.: Pittsburgh, PA, 2003.
Although 7 was readily isolated with acceptable elemental
purity, 3NITPhPyrim itself was a somewhat unstable red oil
(3) (a) Field, L. M.; Lahti, P. M.; Palacio, F. Chem. Commun. 2002, 636.
(b) Field, L. M.; Lahti, P. M.; Palacio, F.; Paduan-Filho, A. J. Am.
Chem. Soc 2003, 125, 10110. (c) Field, L. M.; Lahti, P. M. Inorg.
Chem. 2003, 42, 7447.
(4) Liao, Y.; Xie, C.; Lahti, P. M.; Weber, R. T.; Jiang, J.; Barr, D. P. J.
Org. Chem. 1999, 64, 5176.
(5) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
6726 Inorganic Chemistry, Vol. 44, No. 19, 2005

Cyclic M2(RL)2 Coordination Complexes
Table 1. Crystallographic Analysis Parameters for 1-3 at Varying Temperaturesa
param
1
2
3
chem formula
fw
C48H36F24Mn2N6O10
1422.7
C48H36F24Co2N6O10
1430.69
C48H36F24Cu2N6O10
1439.91
cell setting, space group triclinic, P1h
triclinic, P1h
triclinic, P1h
a, b, c (Å)
9.4570(2), 10.6507(2), 16.4342(5)
9.4474(2), 10.6040(3), 16.4930(5)
[9.2310(2), 10.3568(2), 16.4551(4)]
9.6557(2), 16.0424(3), 19.6212(4)
[9.51840(10), 15.8656(2), 19.4793(3)]
{9.44230(10), 15.7864(2), 19.4298(4)}
76.6264(6), 81.6717(6), 82.5520(9)
[76.6884(5), 81.6336(5), 82.0372(7)]
{76.6127(6), 81.7446(6), 81.6369(8)}
2911.11(10) [2815.27(6)] {2769.31(7)}
2
{9.00800(10), 10.1214(2), 16.6591(3)}
R, â, γ (deg)
79.5093(8), 85.0207(8), 65.3736(17)
{79.5635(8), 86.6765(8), 69.2349(9)}
78.6172(9), 85.0945(10), 64.9927(14)
[79.1122(8), 85.5223(9), 66.7499(10)]
V (Å3)
Z
1479.49(6) {1396.68(4)}
1
1.618 {1.691}
Mo KR
712
0.561 {0.594}
yellow-brown plate
0.0236 {0.0169}
0.972 {0.989}
Refxyz
1.042{1.043}
0.872, -0.637 {0.849, -0.576)}
1467.92(7) [1419.38(5)]
1
1.6718 [1.674]
Mo KR
716
0.700 [0.724]
dark yellow needle
0.0257 [0.0185]
0.998 [0.985]
Refxyz
1.052[1.060]
0.867, -0.726 [0.646, -0.546]
Dx (Mg/m3)
radiatn type
F(000)
1.643 [1.699] {1.727}
Mo KR
1440
0.867 [0.896] {0.911}
yellow needle
0.0324 [0.0312] {0.0361}
0.989 [0.990] {0.986}
Refxyz
µ (mm-1
)
cryst color
Runt
completeness to 2θ
H-atom treatment
goodness of fit on F2
1.026[1.015] {1.029}
∆Fmax, ∆Fmin (e Å-3
R1, wR2 (I > 2σ(I))
R1, wR2 (all)
)
0.808, -0.739 [1.554, -0.798]
{1.053, -0.977}
0.0971, 0.2514
{0.0417, 0.1020}
0.1111, 0.2668
{0.0450, 0.1048}
293{100}
0.0882, 0.2212
[0.0526, 1297]
0.1085, 0.2392
[0.0568, 0.1332]
293 [173]
0.0793, 0.1932 [0.0667, 0.1579]
{0.0522, 0.1234}
0.1111, 0.2140 [0.0899, 0.1732]
{0.0702, 0.1336}
299 [173] {100}
temp (K)
a See final row in table.
Complexes 1-3 were analyzed by single-crystal X-ray
diffraction at various temperatures from 100 to 300 K. All
three complexes crystallize in the P1h triclinic space group.
Tables 1 and 2 show crystallographic and structural param-
eters from the analyses; further details are given in Support-
ing Information. Figure 1 shows structural diagrams7 for the
complexes at room temperature. Figure 2 summarizes
temperature-dependent geometry changes found for the
structure of 3, which are described in more detail in the
following section.
and the phenyl-pyrimidine torsions from 33 to 35°. The
coordination environments around the metal centers are
somewhat distorted from hexacoordinate octahedra, as shown
by the bond angles in Table 2. A number of anti L-M-L
angles range from 165 to 175° instead of 180°; other
L-M-L angles range from 84 to 103° instead of 90°. In all
of the structures, the pyrimidine unit is coordinated at an
83-86° angle relative to the nitroxide group, anti to an hfac
carbonyl unit as was observed3 in complexes of 4NITPh-
Pyrim and 4NITPhTriaz.
Polycrystalline samples of 1-3 were subjected to dc
magnetic susceptibility studies using a Quantum Design
MPMS-5 magnetometer. Samples were placed in gelatin
capsules and held in place with cotton, and were analyzed
from 1.8 to 300 K. Figures 3-5 show øT vs T plots obtained
for 1-3 at 1, 0.1, and 1 T, respectively. Figure 4 also shows
a 1/ø vs T Curie-Weiss plot of the same data for 2. Figure
5 also shows an inset of the same data for 3 as a ø vs T plot,
to emphasize the loss of overall paramagnetic susceptibility
as the temperature drops for this sample. The solid lines in
the figures represent fits to the data for models that are
discussed in detail below. Magnetization versus field mea-
surements for 1-3 were carried out from 0 to 17 T and are
compared for the three samples in Figure 6.
The closest distance between metal ions in 1-3 is
intradimeric, 7-7.5 Å; the closest intermolecular M‚‚‚M
distances are about 9-10 Å. With no close interdimer
contacts between sites of significant spin density in the crystal
structures of any of these complexes, the magnetic effects
discussed in the following section are attributed to intramo-
lecular exchange mechanisms. The crystallographic nitroxide
and interannular torsions in the 3NITPhPyrim units seem
not so large to preclude conjugation of the nitroxide into
the π-system, which could contribute to intradimer spin
polarization and exchange between the M-NIT spin units.
But, the ESR spectrum of 3NITPhPyrim and the spin density
computations described above show little spin density on
the pyrimidine ring in the isolated radical. The coordinating
metal ion could directly spin-polarize the pyrimidine ring,
but previous studies suggest3b such polarization to be small.
There is only one molecule in the unit cells of 1 and 2.
Complex 3 has two similar but crystallographically distinct
structures. The M-ON bond lengths in 1 and 2 are not
greatly differentiated from the other M-O bond lengths, so
axial versus equatorial distinctions for the nitroxide coordina-
tion is not strong. But, at room temperature, one molecule
of 3 in the unit cell (form 3A) shows strong Jahn-Teller
Discussion
Crystallography. Figure 1 shows that the complexes 1-3
all have similar room-temperature centrosymmetric geom-
etries that incorporate 3NITPhPyrim ) RL with the nitroxide
group syn to the pyrimidine group. There is a limited torsion
in the RL units: the Ph-NO torsions range from 9 to 24°,
(7) Generated using ORTEP-III for Windows: Farrugia, L. J. J. Appl.
Crystallogr. 1997, 30, 565.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6727

Baskett et al.
Table 2. Selected Structural Parameters for 1-3 at Various Temperaturesa
param
1 (293 K)
1 (100 K)
2 (299 K)
2 (173 K)
3 (299 K)
3 (173 K)
3 (100 K)
M-ON
M-O(5)
2.135(4)
2.139(2)
2.056(4)
2.058(2)
2.292(4) (A)
2.166(4) (B)
2.111(3) (A)
2.002(3) (B)
2.031(3) (A)
1.984(3) (B)
M-O Bond Lengths (hfac)
M-O(1)
M-O(2)
M-O(3)
M-O(4)
2.114(4)
2.172(5)
2.143(4)
2.154(4)
2.116(2)
2.179(2)
2.148(2)
2.144(2)
2.031(4)
2.089(5)
2.037(4)
2.089(4)
2.031(2)
2.200(6)
2.068(8)
2.033(4)
2.124(5)
2.052(4)
2.131(5)
1.945(4)
1.962(4)
2.062(5)
1.991(4)
2.165(4)
2.254(4)
2.212(4)
2.293(4)
1.921(3)
1.949(3)
2.028(3)
1.990(3)
2.238(3)
2.262(3)
2.287(3)
2.302(3)
1.912(3)
1.947(3)
2.093(2)
2.037(2)
2.083(2)
M-N Bond Length
M(1)-N(pyri midine)
O(5)-N(1)
2.274(4)
1.308(6)
7.491
2.253(2)
1.302(2)
7.446
2.158(4)
2.148(2)
2.013(4)
2.026(4)
2.001(4)
2.005(4)
1.996(3)
2.008(3)
N-O Bond Length
1.315(6)
1.303(3)
1.296(5)
1.292(6)
1.295(5)
1.298(5)
1.300(4)
1.301(4)
M···M Intradimer Distance
M(1)···M(1)
7.371
7.327
7.111
7.570
6.986
7.376
6.952
7.339
Phen-NO Torsion
O(5)-N(1)-C(17)-C(16)
C(16)-C(15)-C(14)-C(11)
19.79
19.57
20.11
19.79
12.20
23.76
10.09
21.75
9.15
20.68
Phen-Pyrim Torsion
36.52
32.84
34.79
34.39
34.72
32.91
35.65
33.47
34.84
32.63
Angles
O(1)-M-O(3)
O(4)-M-N(2)
O(5)-M-O(2)
O(5)-M-O(3)
O(5)-M-N(2)
166.21(18)
163.23(16)
167.28(15)
103.71(16)
85.10(15)
163.00(6)
161.92(7)
163.00(6)
106.10(6)
84.13(6)
170.41(18)
170.83(17)
173.28(16)
100.05(16)
84.46(17)
169.22(9)
170.80(9)
171.77(8)
101.22(9)
84.48(9)
170.16(17)
172.2(2)
173.20(18)
173.2(2)
174.40(16)
178.00(19)
100.57(16)
94.59(18)
85.50(16)
83.15(17)
166.70(14)
171.93(14)
174.15(16)
173.22(15)
175.01(15)
177.47(14)
102.19(13)
94.92(14)
86.43(14)
83.79(14)
164.99(10)
172.16(10)
173.82(12)
173.14(12)
175.63(11)
177.66(11)
102.80(10)
95.18(10)
86.40(11)
83.54(11)
a See Figure 1 for atom numbering. All distances in angstroms, all angles in degrees.
elongation of the coordinated nitroxide Cu(1)-O(5)N bond
(2.29 Å) and the anti-related Cu(1)-O(1) hfac bond (2.2 Å);
the other, equatorial Cu(II)-O(hfac) bond lengths range from
1.99 to 2.03 Å. Form 3B is less distorted, with the Cu(2)-
O(5a)N bond being longest by 0.03 Å among four M-O
bond lengths ranging from 2.07 to 2.17 Å. Save for the
different bond lengths, the coordination spheres and confor-
mational geometries of the two molecules of 3 are analogous.
The variable-temperature crystallographic analyses were
important to show the differences in behavior among these
compounds. Mn(II) complex 1 showed no significant mo-
lecular structural changes upon cooling, although the unit
cell undergoes an approximately 0.5 Å contraction of the a-
and b-axes with slight elongation of the c-axis and a unit
cell volume contraction of only 6.5%. Co(II) complex 2
similarly showed no significant molecular structural changes
upon cooling, with only a 3.4% contraction of the unit cell
volume down to 173 K, at which temperature all crystals
studied broke, presumably as a result of some sort of phase
change.
(see Tables 1 and 2). Figure 2 summarizes the coordination
sphere changes around the Cu(II) ions. The Cu-ON bonds
in both forms of 3, which are elongated at room temperature,
contract markedly by 100 K, as do the anti-related Cu-
O(hfac) bonds; there is also a small contraction of the Cu-N
bonds and their anti-related Cu-O(hfac) bonds. Simulta-
neously, the remaining anti-related pair of Cu-O(hfac) bonds
elongates significantly. These geometric changes are revers-
ible upon rewarming the sample. This corresponds to a
change from an axial environment at room temperature to
an equatorial environment at 100 K for the Cu-ON bond
and its anti-related Cu-O(hfac) bond. This is very important
for explaining the magnetic behavior of 3 that is described
below.
Magnetism and Spectroscopy. Neither of complexes 1
and 2 showed an X-band TE102 mode ESR spectrum in the
solid state at either room temperature or 77 K. Any ESR
spectral transitions are apparently either too weak or too
broad to observe under these conditions. Complex 3 shows
a weak and broadened peak at 77 K at g ) 2.06 versus DPPH
standard.
Cu(II) complex 3 differs from 1 and 2 by having two
crystallographically distinct forms that exhibit coordination
sphere bond length changes of up to 0.26 Å upon cooling to
100 K, during which its unit cell volume decreases by 4.9%
The magnetic measurements for the high-temperature
region of manganese complex 1 yield a Curie constant of C
) 5.8 emu-K/(Oe-mol), in good accord with the value of
6728 Inorganic Chemistry, Vol. 44, No. 19, 2005

Cyclic M2(RL)2 Coordination Complexes
Figure 1. ORTEP style structures of 1-3 showing 50% probability ellipsoids, with atom numbering used in the main text. Hydrogen and fluorine atoms
are omitted for clarity of viewing. Structure 3 has two similar but crystallographically distinct forms, A and B.
6.0 emu-K/(Oe-mol) expected for a dimer of S ) 4/2 units
that arise from antiferromagnetic (AFM) Mn(II) to radical
exchange. The magnetization versus field plot for 1 at 1.4
K (Figure 6) saturates at M(H) ) 8.0 µB/mol, also appropriate
for a dimer of S ) 4/2 spin units. Since all of the d-orbitals
of octahedral Mn(II) are singly occupied, overlap with the
nitroxide SOMO ensures AFM exchange.1a,b,j The slow initial
rise of the øT versus T plot and the rapid decrease below 20
K (Figure 3) suggest multiple exchange mechanisms.
A rectangular four-spin fit to the magnetic data was carried
out using the program MAGMUN,8 varying the effective
Lande factor geff, and using two exchange constants J1/k and
J2/k plus a temperature-independent paramagnetic contribu-
tion TIP. No zfs or spin-orbit effects were included in the es-
timation procedure. A similar procedure has been described
for a Mn2(RL)2 type dimer by Rabu et al.2e Figure 3 shows a
good fit to the data for 1 with geff ) 1.97, J1/k ) (-)790 K
) (-)550 cm-1, J2/k ) (+)1.0 K ) (+)0.7 cm-1, and TIP
) (-)0.0019 emu/(Oe-mol). The strongly AFM exchange
occurs within the Mn(II)-ON bond (based on the Curie
constant described above), with the weak ferromagnetic (FM)
exchange assigned to interaction between the Mn(II)-ON
units, presumably in an intramolecular fashion due to a lack
of close crystallographic contacts between Mn(II)-ON units.
A spin polarization mechanism analogous to those applied
to the complexes of 4NITPhPyrim and 4NITPhTriaz3b-c is
consistent with the FM exchange between the Mn(II)-ON
spin units (Scheme 2). But, the results show that the
intradimer FM exchange mechanism between Mn(II)-ON
units in 1 is quite weak, probably due to the difficulty of
(8) MAGMUN is an integrated Windows-based software package for the
solution of magnetic exchange problems. It was written by Dr.
Zhiqiang Xu in collaboration with Prof. L. K. Thompson and Dr. O.
Waldmann. A copy may be obtained from http://www.ucs.mun.ca/
∼lthomp/magmun.html.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6729

Baskett et al.
Figure 2. Geometric deformations observed for the two forms of 3 upon cooling. Bond lengths given in Å. Axial bonds in the schematic are correlated with
longer bond lengths; the arrows in the high-temperature picture show the elongation/contraction of bonds in the 3A form with cooling.
Figure 4. øT versus T (O) and 1/ø versus T plots (b) for 2 at 0.1 T. The
solid line shows a Curie-Weiss fit to 1/ø versus T over 1.8-300 K.
Figure 3. øT versus T plot for 1 at 1 T. The solid line shows a rectangular
four-spin fit described in the text.
emu-K/(Oe-mol), much higher than observed. But, using the
transmitting spin polarization from the phenyl nitroxide unit
through the pyrimidine unit in the 3NITPhPyrim ligands.
Interestingly, we found no evidence of a discontinuity in
relationship C ) 0.125(geff)2S(S + 1), a dimer of S ) 2/2
spin units with geff ) 2.45 would give C ) 3.0 emu-K/
(Oe-mol), in good agreement with the observed value. This
the øT versus T plot for polycrystalline cobalt complex 2 at
shows strong AFM exchange in the Co(II)-NIT units to give
about 170 K (Figure 4), the temperature below which single
crystals of 2 fractured, as mentioned earlier. The lack of
effective S ) (3/2)-(1/2). The deviation from the free
electron value of the Lande´ constant is not atypical for
magnetic sensitivity to the presumed phase change that
octahedral Co(II).9
causes the fracture supports the assumption that intramo-
lecular effects dominate the magnetism of 2. At 100 K and
Below 100 K, øT(T) for 2 drops steadily to 0.13 emu-K/
(Oe-mol) at 1.8 K. AFM spin pairing in the Co-ON unit
higher, the øT(T) data plateau at C ∼ 3 emu-K/(Oe-mol). A
makes them effectively S ∼ 0 with loss of paramagnetic
Curie-Weiss fit to 1/ø(T) above 120 K yields C ) 2.96 emu-
moment at low temperature. However, paramagnetic states
K/(Oe-mol) and θ ) (-)1.7 K (Figure 4). Typically, each
are still accessible at high field. M(H) at 1.4 K rises
monotonically to the maximum measured field of 17 T,
without giving a plateau (Figure 6). The maximum effective
magnetization is about 2 µB, about half the value expected
octahedral Co(II) contributes9 ∼3.2 emu-K/(Oe-mol) to øT,
so two Co(II) ions plus two nitroxides should give øT ∼ 7
(9) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986; pp
65-67.
for two S ) 2/2 units in the dimer. The overall results show
6730 Inorganic Chemistry, Vol. 44, No. 19, 2005

Cyclic M2(RL)2 Coordination Complexes
Scheme 3. Qualitative Spin-Orbital Overlap Diagram Showing
Essentially Zero Overlap of Nitroxide SOMO of 3NITPhPyrim with
Cu(II) SOMO in Axial Coordination at Higher Temperature but
Significant Overlap in Equatorial Coordination at Lower Temperature
same at both 0.01 and 1.0 T. The magnetization versus field
plot of 3 at 1.25 K shows only a small saturation moment
above 4 T.
Figure 5. øT versus T plot for 3 at 1.0 T. (The inset shows the same data,
ø versus T.)
On the basis of the low value of øT at room temperature,
there must be significant AFM exchange between spins in
the sample of 3. The AFM coupling is presumably strongest
in the molecules of 3 in the unit cell having the shorter
Cu(II)-ON bonds. The decrease in øT with temperature
correlates with the shortening of all the Cu(II)-ON bonds
upon cooling, during which the magnetic moment disappears
save for a remnant (Figure 6) that is likely due to crystal
defect paramagnetic sites or other small paramagnetic
impurities that are below the limits detectable by elemental
analysis. The lower temperature øT rise is not due to spin
canting, since the saturation magnetization of the sample
quickly plateaus at only a few percent of the total possible
spins in the sample for fields up to 17 T.
Figure 6. Magnetization versus field measurements for 1 (1.4 K), 2 (1.4
K), and 3 (1.2 K).
Axial elongation of the Cu(II)-ON bonds yields FM
Cu(II)-ON exchange due to the orthogonality of the Cu(II)
axial SOMO with the nitroxide SOMO. Shortening the
Cu(II)-ON bond leads to SOMO-SOMO overlap and AFM
exchange, as the nitroxide changes to equatorial coordination
(Scheme 3).1a-c,10 The molecule of 3 in the unit cell that has
less Jahn-Teller distortion at room temperature is part way
along the axial to equatorial Cu(II)-ON changeover and
presumably is responsible for the reduced moment of 3 at
300 K relative to the value expected. As a result 3 exhibits
a strongly temperature-modulated magnetic moment, as two
different structural contributors to the moment separately
undergo changes in Cu(II)-ON exchange strength.
The magnetic and structural changes in copper complex
3 are accompanied by a strong thermochromic change. At
room temperature, 3 is a dull yellowish-brown solid. Upon
cooling, it becomes progressively more greenish, yielding a
bright lime green at 77 K. This thermochromism is reversible
over as many cycles as we have tested. It seems clear that
the green color is associated with the low-spin state and a
strongly AFM exchange coupled Cu-ON bond, while the
yellow-brown color is associated with the high-spin or poorly
exchange coupled Cu(II)-ON state.
Scheme 2. Qualitative Spin Polarization Mechanism for
Intramolecular FM Exchange in 1
a strong AFM exchange within the Co(II)-NIT units but
do not readily allow extraction of the intradimer exchange
energy between the Co(II)-NIT units due to the complexity
of Co(II) magnetic behavior by comparison to 1.
For copper complex 3, a noninteracting set of four S )
1/2 spins should give a high-temperature øT plateau corre-
sponding to a Curie constant of C ∼ 1.5 emu-K/(Oe-mol).
Two strongly FM coupled S ) 1 Cu(II)-ON units in the
dimer should give C ∼ 2.0 emu-K/(Oe-mol). However,
øT(300 K) ∼ 0.7 emu-K/(Oe-mol) and decreases steadily
from 300 to 100 K. There is no higher temperature plateau
(Figure 5). At 100 K, øT reaches a minimum value of 0.10
emu-K/(Oe-mol) and then weakly increases to 0.13 emu-K/
(Oe-mol) at 1.8 K. The paramagnetic susceptibility ø also
decreases from 300 to 100 K. This behavior is virtually the
(10) (a) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. Chem. 1991, 39,
331. (b) Luneau, D.; Rey, P.; Laugier, J.; Fries, P.; Caneschi, A.;
Gatteschi, D.; Sessoli, R. J. Am. Chem. Soc. 1991, 113, 124. (c)
Ressouche, E.; Boucherle, J.-X.; Gillon, B.; Rey, P.; Schweizer, J. J.
J. Am. Chem. Soc. 1993, 115, 3610. (d) Boucherle, J.-X.; Ressouche,
E.; Schweizer, J.; Gillon, B.; Rey, P. Z. Naturforsch. 1993, 48, 120.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6731

Baskett et al.
Table 3. Comparison of Selected Structural Parameters for 3 (100 K) and 8 (293 K)a
a Letters in table refer to accompanying figures.
We were also able to test the effect of forming a short,
strong Cu(II)-ON bond in this type of Cu2(RL)2 system by
making model complex 8. Radical 3NITPhPyr was synthe-
geometric parameters (Table 3). Details are given in the
Supporting Information and the Experimental Section.
Notably, at room temperature the Cu(II)-ON bonds and
the anti-related Cu-O(hfac) bonds in 8 are already shortened
by comparison to the room-temperature structures of 3. The
room-temperature structure of 8 is thus analogous to the low-
temperature form of 3, as shown in Table 3. At temperatures
down to 1.8 K, 8 showed no appreciable paramagnetic
susceptibility, showing strongly AFM Cu(II)-ON exchange.
The correlation of short Cu-ON bond lengths with strong
AFM exchange in 8 is consistent with the interpretation given
above for the magnetostructural variation of 3 as a function
of temperature. Interestingly, low-spin complex 8 is green
even at room temperature, consistent with the above-
described green color of the low-temperature, low-spin state
of complex 3.
In work closely related to ours, Lanfranc de Panthou et
al.11 described a study of complex 9, a cyclic dimer end-
capped with Cu(II) ions. Upon cooling, the complex exhib-
ited Cu(II)-ON bond shortening of ∼0.2 Å in the dimeric
portion with a drop in øT from 2.5 to 0.8 emu-K/(Oe-mol)
over the 150-100 K temperature range, corresponding to a
sized by replacing 5-bromopyrimidine with 3-bromopyridine
in the route of Scheme 1. Cyclic 8 was then obtained after
some effort by crystallization of 3NITPhPyr with Cu(hfac)2
using procedures similar to those used to make 3. There was
only one structure of 8 in the crystal lattice. It has
conformation and torsions of the radical ligand portion
similar to those for the structures of 3, as well as other
(11) Lanfranc de Panthou, F.; Belorizky, E.; Calemczuk, R.; Luneau, D.;
Marcenat, C.; Ressouche, E.; Turek, P.; Rey, P. J. Am. Chem. Soc.
1995, 117, 11247.
6732 Inorganic Chemistry, Vol. 44, No. 19, 2005

Cyclic M2(RL)2 Coordination Complexes
was separated and washed with water. The aqueous layer was
extracted with diethyl ether, and the combined organic layers were
dried over magnesium sulfate. Evaporation of the solvent yielded
pale yellow crystals of 3-bromo-N-tert-butyl-N-hydroxylaminoben-
zene that were dried under vacuum and used immediately in the
next step (see ref 4).
change from six to two isolated S ) 1/2 units. These workers
pointed out that shortening the Cu(II)-ON bonds changed
the nitroxide coordination from axial to equatorial, giving a
loss of magnetic moment in the dimeric portion of the
structure for the reasons exemplified in Scheme 3. They
described this behavior as a thermally induced spin conver-
sion11 (as opposed to a spin crossover, which is mechanisti-
cally distinct12). Ovcharenko13 has also recently described a
series of coordination complexes derived from Cu(hfac)2 and
one nitronyl nitroxide radical, some of which show temper-
ature-dependent bond length changes that modulate magnetic
moment.
The crystals from the previous step were dissolved in 20 mL of
dry DMF and then added to a mixture of tert-butyldimethylchlo-
rosilane (5.11 g, 33.9 mmol) and imidazole (3.46 g, 50.8 mmol) in
a predried flask. The resulting solution was stirred under argon for
24 h at 50 °C. Distilled water (20 mL) was added, and the solution
was extracted with hexanes. The combined organic layers were dried
over magnesium sulfate and evaporated under reduced pressure.
Purification by column chromatography on silica gel with hexanes
yielded 5.35 g (89%) of 4 as a clear colorless oil that is
spectroscopically identical to that previously reported in ref 4. This
Conclusions
Dimers 1 and 2 show quite strong AFM metal ion to
nitroxide exchange, which is consistent with the strong
overlap between spin-bearing orbitals. A weak FM exchange
observed in 1 between Mn-NIT spin sites is in accord with
expectations of a spin polarization model. The metal ion to
nitroxide exchange in copper complex 3 is variable due to a
thermally reversible Jahn-Teller distortion of the Cu(II)
coordination sphere in two crystallographically different
molecules, which leads to conversion from ferromagnetic
(high-temperature) to antiferromagnetic (low-temperature)
Cu(II)-NIT coupling. Complex 3 also shows a strong
thermochromic and magnetochromic effect that correlates
qualitatively with changes in its magnetic behavior as a
function of temperature. This thermally induced spin state
change allows possibilities for modulating the magnetic
moments and optical properties of compounds that contain
3 by modulating the temperature.
1
material can be used directly in the next synthetic step. H NMR
(200 MHz, CDCl3): δ -0.12 (broad s, 6 H), 0.90 (s, 9 H), 1.09 (s,
9 H), 7.04-7.24 (m, 3 H), 7.43 (s, 1 H).
(N-tert-Butyl-N-(tert-butyldimethylsiloxy)amino)phenyl-3-bo-
ronic Acid (5). A solution of 1.54 M n-butyllithium in hexanes
(3.9 mL, 6.0 mmol) was added dropwise to a solution of 4 (2.17 g,
6.05 mmol) in 40 mL of THF under argon at -78 °C. The reaction
was allowed to stir for 1.5 h. Next, triisopropyl borate (1.75 mL,
7.57 mmol, Avocado Research Chemicals) was added via syringe,
and the reaction was allowed to warm to room temperature
overnight. Aqueous ammonium chloride (5 mL) was added, and
the reaction was allowed to stir for 1 h. The organic layer was
separated and washed with brine. The combined aqueous portions
were extracted with diethyl ether. The combined organic portions
were dried over anhydrous magnesium sulfate and evaporated under
reduced pressure. Purification by column chromatography (silica
gel, 7:3 ethyl acetate-hexanes) yielded 1.57 g (80%) of 5 as a
colorless crystalline powder, mp 49-50 °C. This product can be
used as is in the subsequent step. 1H NMR (300 MHz, CDCl3): δ
-0.10 (broad s, 6 H), 0.96 (s, 9 H), 1.15 (s, 9 H), 7.37 (d, 1 H, J
) 7.22 Hz), 7.46 (broad s, 1 H), 7.94 (d, 1 H, J ) 7.22 Hz), 8.09
(broad s, 1 H).
5-[3-(N-tert-Butyl-N-(tert-butyldimethylsiloxyl)amino)phenyl]-
pyrimidine (6). A flask charged with 5-bromopyrimidine (0.477
g, 3.00 mmol) and palladium(II) acetate (0.0350 g, 0.156 mmol)
was evacuated and backfilled with argon three times. Tetrahydro-
furan (10 mL) was added, and the suspension was allowed to stir
for 10 min. Deaerated solutions of boronic acid 5 (1.26 g, 3.89
mmol) in THF (5 mL) and of potassium carbonate (1.03 g, 7.48
mmol) in distilled water (4.5 mL) were added via cannula. The
stirring reaction was heated to reflux for 24 h under argon. The
organic layer was separated. The aqueous layer was extracted with
diethyl ether. The combined organic portions were dried over
magnesium sulfate and evaporated under reduced pressure. Purifica-
tion by column chromatography (silica gel, 8:2 hexanes-ethyl
acetate) yielded 0.83 g (78%) of 6 as a white crystalline powder.
Recrystallization in hexanes yielded clear colorless needle crystals,
mp 119-120 °C. 1H NMR (300 MHz, CDCl3): δ -0.10 (broad s,
6 H), 0.93 (s, 9 H), 1.14 (s, 9 H), 7.30-7.41 (m, 3 H), 7.51 (broad
s, 1 H), 8.93 (s, 2 H), 9.20 (s, 1 H). Anal. Calcd for C20H31ON3Si:
C, 67.18; H, 8.74; N, 11.75. Found: C, 67.24; H, 8.76; N, 11.81.
5-[3-(N-tert-Butyl-N-hydroxylamino)phenyl]pyrimidine (7).
Concentrated HCl (1 mL) was added to a solution of 6 (0.367 g,
1.03 mmol) in ethanol (5 mL). The solution was allowed to stir
under argon overnight, concentrated under vacuum, and then diluted
with distilled water (5 mL). The resulting solution was brought to
pH 5 with 1 M sodium hydroxide and extracted with dichlo-
Experimental Section
General Procedures. Reactions were carried out in oven-dried
glassware, under an argon atmosphere. 2-Methyl-2-nitrosopropane
was prepared according to the procedure of Stowell.14 Diethyl ether
and tetrahydrofuran (THF) were distilled under argon after drying
over sodium. Dri-Solv N,N-dimethylformamide (DMF) was ob-
tained from EM Science and used without further purification. Other
chemicals were purchased from Acros, Avocado, and Aldrich
Chemical Cos. Diethyl ether was distilled under argon after drying
over sodium. Hexanes were distilled under argon after drying over
calcium hydride.
3-Bromo-N-tert-butylamino-N-tert-butyldimethylsiloxylben-
zene (4). A solution of 1.5 M n-butyllithium in hexanes (11.3 mL,
17.0 mmol) was added to a solution of 1,3-dibromobenzene (4.00
g, 17.0 mmol) in 80 mL of diethyl ether under argon at -78 °C.
The suspension was allowed to stir for 30 min, warmed to room
temperature at which time it went into solution, and then cooled to
-78 °C. A solution of 2-methyl-2-nitrosopropane14 (2.22 g, 25.4
mmol) in diethyl ether (15 mL) was added, and the reaction was
left to stir overnight at ambient temperature. Aqueous saturated
ammonium chloride (25 mL) was added, and then the organic layer
(12) For a recent example, see: Sunatsuki, Y.; Ohta, H.; Kojima, M.; Ikuta,
Y.; Goto, Y.; Matsumoto, N.; Iijimi, S.; Akashi, H.; Kaizaki, S.; Dahan,
F.; Tuchagues, J.-P. Inorg. Chem. 2004, 43, 4154. See also cited work
in this reference.
(13) Fokin, S.; Ovcharenko, V.; Romanenko, G.; Ikorski, V. Inorg. Chem.
2004, 43, 969.
(14) Stowell, J. C. J. Org. Chem. 1971, 36, 3055.
Inorganic Chemistry, Vol. 44, No. 19, 2005 6733

Baskett et al.
romethane. The combined organic portions were dried over
magnesium sulfate and evaporated under reduced pressure and then
dried under vacuum and ground up to yield 0.225 g (90%) of pale
yellow crystalline powder 7, mp 126-128 °C. 1H NMR (200 MHz,
CDCl3): δ 1.18 (s, 9 H), 7.32-7.45 (m, 4 H), 8.91 (s, 2 H), 9.19
(s, 1 H). IR (KBr disk, cm-1): 3233 (N-OH). HRMS EI: calcd
for C14H17ON3, [M + H]+ 243.1372; found, 243.1344. The structure
was confirmed by crystallographic analysis (see Supporting Infor-
mation). The crystal and structural parameters in CIF format were
deposited at the Cambridge Crystallographic Databank Center,
CCDC Deposition No. 234245.
least squares in SHELXTL9715 to yield the structure with R1 )
0.0882 and wR2 ) 0.2212; for all data, R1 ) 0.1085 and wR2 )
0.2392. The crystal and structural parameters in CIF format were
deposited at the Cambridge Crystallographic Databank Center,
CCDC No. 272167.
Cu2(hfac)4(3NITPhPyrim)2 (3). 3NITPhPyrim (0.223 g, 0.917
mmol) was dissolved in 6.5 mL of dichloromethane, and then a
solution of Cu(hfac)2 (0.454 g, 0.917 mmol) in 6.0 mL of
dichloromethane and 14.0 mL of hexanes was layered on top. After
1 week, a mixture of brown-green and light green crystals formed
that was separated manually. The light green crystals were dis-
solved in toluene and filtered into a small vial. After 2 weeks fine
needle crystals formed, mp 104-106 °C. Anal. Calcd for
C48H36O10N6F24Cu2: C, 40.03; H, 2.52; N, 5.84. Found: C, 40.06;
H, 2.28; N, 5.87. ESR (solid, 77 K): g ) 2.06. Crystallographic
analysis was carried out on a Bruker P4 at ambient temper-
ature with Mo KR radiation: λ ) 0.7107 Å, µ(Mo KR) ) 0.561
mm-1; yellow plate crystals 1.00 × 0.75 × 0.10 mm; formula
C48H36Cu2F24N6O10 (fw 1439.91); triclinic space group P1h, with a
) 9.6557(2) Å, b ) 16.0424(3) Å, c ) 19.6212(4) Å, R )
76.6264(6)°, â ) 81.6717(6)°, γ ) 82.5520(9)°, V ) 2911.11(10)
Å3, Z ) 2; Dcalc ) 1.643 g/cm3. N ) 10 202 unique reflections
were measured using ω scans for 4.08 < θ < 25.05. The structure
was solved by direct methods and refined on F2 for all reflections
with I > 2σ(I) using 799 parameters and full-matrix least squares
in SHELXTL9715 to yield the structure with R1 ) 0.0793 and wR2
) 0.1932; for all data, R1 ) 0.1111 and wR2 ) 0.2140. See Table
1 for parameters at lower temperatures, for which R1 and wR2
improve. The crystal and structural parameters in CIF format were
deposited at the Cambridge Crystallographic Databank Center,
CCDC No. 272168.
5-[3-(N-tert-Butyl-N-aminoxyl)phenyl]pyrimidine (3NITPh-
Pyrim). To a stirred solution of 5-[3-(N-tert-butyl-N-hydroxylami-
no)phenyl]pyrimidine, 7 (0.223 g, 0.917 mmol), in benzene (20
mL) under argon was added Ag2O (0.531 g, 2.29 mmol). After 2
h the reaction was filtered to yield a clear red/orange solution. This
was used directly for spectroscopic studies and as a reactant in
subsequent reactions. The radical is not stable to prolonged storage
and so is best used as soon as possible after preparation. ESR
(benzene, room temperature, 9.649 GHz): aN ) 12.28 G; aH
)
2.03, 1.92, 1.82, 0.83 G; correlation 0.996 for fitting with Duling’s
WINSIM program.15
Mn2(hfac)4(3NITPhPyrim)2 (1). 3NITPhPyrim (0.010 g, 0.041
mmol) was dissolved in 0.1 mL of dichloromethane, and then a
solution of Mn(hfac)2 (0.019 g, 0.041 mmol) dissolved in 0.2 mL
of diethyl ether plus 1.0 mL of hexanes was layered on top. Yellow-
brown plates formed after several days, mp 168-169.5 °C. Anal.
Calcd for C48H36O10N6F24Mn2: C, 40.52; H, 2.55; N, 5.91. Found:
C, 40.72; H, 2.69; N, 5.78. Crystallographic analysis was carried
out on a Bruker P4 at ambient temperature with Mo KR radiation:
λ ) 0.7107 Å, µ(Mo KR) ) 0.561 mm-1; yellow-brown plate
crystals 0.75 × 0.50 × 0.15 mm; formula C48H36F24Mn2N6O10 (fw
1422.7); triclinic space group P1h; cell parameters a ) 9.4570(2)
Å, b ) 10.6507(2) Å, c )16.4342(5) Å, R ) 79.5093(8)°, â )
85.0207(8)°, γ ) 65.3736(17)°, V ) 1479.49(6) Å3, Z ) 2; Dcalc
) 1.597 g/cm3. N ) 5086 unique reflections were measured using
ω scans for 4.25 < θ < 25.05. The structure was solved by direct
methods and refined on F2 for all reflections with I > 2σ(I) using
406 parameters and full-matrix least squares in SHELXTL9715 to
yield the structure with R1 ) 0.0971 and wR2 ) 0.2514; for all
data, R1 ) 0.1111 and wR2 ) 0.2668. The crystal and structural
parameters in CIF format were deposited at the Cambridge
Crystallographic Databank Center, CCDC No. 272166.
3-[3-(N-tert-Butyl-N-(tert-butyldimethylsiloxyl)amino)phenyl]-
pyridine. A flask charged with 0.279 g (1.77 mmol) of 3-bro-
mopyridine and 0.021 g (0.092 mmol) of palladium(II) acetate was
evacuated and backfilled with argon three times. Dry tetrahydro-
furan (10 mL) was added, and the suspension allowed to stir for
10 min. Deaerated solutions of 0.742 g (2.29 mmol) of boronic
acid 5 in THF (5 mL) and 0.609 g (4.41 mmol) of potassium
carbonate in distilled water (4.5 mL) were added via cannula. The
stirred reaction was heated to reflux for 24 h under argon and then
allowed to cool. The organic layer was separated. The aqueous layer
was extracted with diethyl ether. The combined organic portions
were dried over magnesium sulfate and evaporated under reduced
pressure. Purification of the residue by column chromatography
(silica gel, 8:2 hexanes-ethyl acetate) yielded 0.28 g (42%) of
product as a clear colorless oil, which was used as is in the next
synthetic step. Anal. Calcd for C21H32ON2Si: C, 70.73; H, 9.05;
Co2(hfac)4(3NITPhPyrim)2 (2). 3NITPhPyrim (0.0600 g, 0.247
mmol) was dissolved in 1.5 mL benzene, and a solution of 0.117
g (0.247 mmol) of Co(hfac)2 in 2 mL of distilled diethyl ether and
5 mL of hexanes was layered on top. This was then layered again
with 5 mL of hexanes. After 4 days dark yellow needles formed,
mp 165-167 °C. Anal. Calcd for C48H36O10N6F24Co2: C, 40.29;
H, 2.54; N, 5.87. Found: C, 40.27; H, 2.57; N, 5.92. Crystal-
lographic analysis was carried out on a Bruker P4 at ambient
temperature with Mo KR radiation: λ ) 0.7107 Å, µ(Mo KR) )
0.561 mm-1; dark yellow needle crystals 0.75 × 0.20 × 0.02 mm;
formula C48H36Co2F24N6O10 (fw 1430.69); triclinic space group P1h,
cell parameters a ) 9.4474(2) Å, b ) 10.6040(3) Å, c ) 16.4930(5)
Å, R ) 78.6172(9)°, â ) 85.0945(10)°, γ ) 64.9927(14)°, V )
1467.92(7) Å3, Z ) 1; Dcalc ) 1.645 g/cm3. N ) 5122 unique
reflections were measured using ω scans for 4.14 < θ < 25.01.
The structure was solved by direct methods and refined on F2 for
all reflections with I > 2σ(I) using 406 parameters and full-matrix
1
N, 7.86. Found: C, 70.97; H, 9.24; N, 7.64. H NMR (400 MHz,
CDCl3): δ -0.09 (broad s, 6 H), 0.93 (s, 9 H), 1.13 (s, 9 H), 7.30-
7.36 (overlap m plus broad s, 4 H), 7.50 (broad s, 1 H), 7.83-7.86
(ddd, 1 H, J ) 7.83, 2.28, 1.52 Hz), 8.57-8.58 (dd, 1 H, J ) 4.80,
1.77 Hz), 8.83-8.84 (dd, 1 H, J ) 2.28, 0.76 Hz).
3-[3-(N-tert-Butyl-N-hydroxylamino)phenyl]pyridine. Con-
centrated HCl (1 mL) was added to a solution of 0.10 g (0.28 mmol)
of 3-[3-(N-tert-butyl-N-(tert-butyldimethylsiloxyl)amino)phenyl]-
pyridine in ethanol (5 mL). The solution was allowed to stir under
argon overnight, concentrated under vacuum, and then diluted with
distilled water (5 mL). The resulting solution was adjusted to pH
5 with 1 M sodium hydroxide and extracted with dichloromethane.
The combined organic portions were dried over magnesium sulfate
and evaporated under reduced pressure to yield 0.04 g (61%) of
pale yellow crystalline powder, mp 144-147 °C. Anal. Calcd for
C15H18ON2: C, 74.35; H, 7.49; N, 11.56. Found: C, 74.18; H, 7.30;
(15) Sheldrick, G. M. SHELXTL97 Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
6734 Inorganic Chemistry, Vol. 44, No. 19, 2005

Cyclic M2(RL)2 Coordination Complexes
1
N, 11.58. H NMR (400 MHz, CDCl3): δ 1.18 (s, 9 H), 7.25-
) 14.8870(2) Å, R ) 91.8560(10)°, â ) 94.6500(1)°, γ )
113.86500(10)°, V ) 1458.73(3) Å3, Z ) 2; Dcalc ) 1.637 g/cm3.
N ) 6940 unique reflections were measured using ω scans for 4.1
< θ < 27.9. The structure was solved by direct methods and refined
on F2 for all reflections with I > 2σ(I) using 594 parameters and
full matrix least squares in SHELXTL9715 to yield the structure
with R1 ) 0.0326 and wR2 ) 0.0831; for all data, R1 ) 0.0425
and wR2 ) 0.0874. The crystal and structural parameters in CIF
format were deposited at the Cambridge Crystallographic Databank
Center, CCDC No. 272171.
7.30 (m, 4 H), 7.43 (s, 1 H), 7.79 (d, 1 H, J ) 7.6 Hz), 8.56 (d, 1
H, J ) 3.2 Hz), 8.78 (s, 1 H). IR (KBr disk, cm-1): 3250 (broad,
OH str).
3-[3-(N-tert-Butyl-N-aminoxyl)phenyl]pyridine (3NITPhPyr).
3-[3-(N-tert-Butyl-N-hydroxylamino)phenyl]pyridine (0.032 g, 0.13
mmol) was dissolved in 3.0 mL of dichloromethane under argon.
Freshly prepared silver oxide was added and the suspension stirred
for 1 h. Thin-layer chromatography in ethyl acetate showed
appearance of a radical as an orange spot in the same location as
the starting material. The suspension was centrifuged and the bright
red radical solution decanted and filtered through a cotton plug.
This was used directly for spectroscopic studies and as a reactant
in subsequent reactions. The radical is not stable to prolonged
storage and so is best used as soon as possible after preparation.
ESR (benzene, room temperature, 9.645 GHz): aN ) 12.31 G; aH
) 2.00, 1.89, 1.82, 0.83 G; correlation 0.997 for fitting with
Duling’s WINSIM program.16
Cu2(hfac)4(3NITPhPyr)2 (8). The radical solution from above
was placed in a clean vial and layered with 0.066 g (0.13 mmol)
of Cu(hfac)2 dissolved in 0.5 mL of diethyl ether and 3.0 mL of
dichloromethane. This solution was layered with 6 mL of hex-
anes. Dark green block crystals formed after 1 week, sinters at 66-
68 °C and chars and melts 109-110 °C. Anal. Calcd for
C50H38O10N4F24Cu2: C, 41.66; H, 2.66; N, 3.88. Found: C, 41.53;
H, 2.72; N, 3.62. Crystallographic analysis was carried out on a
Bruker P4 at ambient temperature with Mo KR radiation: λ )
0.7107 Å, µ(Mo KR) ) 0.864 mm-1; yellow plate crystals 1.00 ×
0.75 × 0.10 mm; formula C50H38Cu2F24N4O10 (fw 1438); triclinic
space group P1h, with a ) 10.35900(10) Å, b ) 10.40400(1) Å, c
Acknowledgment. This work was supported by the U.S.
National Science Foundation (NSF Grants CHE-0109094 and
CHE-0415716) and the Brazilian agencies CNPq and Fun-
dac¸a˜o de Amparoa Pesquisado Estado de Sa˜o Paulo (Grant
99/10359-7). Support for the UMasssAmherst X-ray Struc-
tural Characterization Facility (NSF Grant CHE-9974648)
and Nanomagnetics Characterization Facility (NSF Grant
CTS-0116498) was also provided by the U.S. National
Science Foundation. Crystallographic analyses were provided
by Dr. A. Chandrasekaran and Dr. P. Khalifah. We also thank
the reviewers of this paper for numerous helpful, improving
recommendations.
Supporting Information Available: Crystallographic data for
1-3, 7, and 8 (including CIF format files), computational modeling
of hfc data for 3NITPhPyrim, and an ESR spectrum and hfc
assignments for 3NITPhPyrim. This material is available free of
charge via the Internet at http://pubs.acs.org. The CIF files have
been also submitted to the Cambridge Crystallographic Data Centre.
(16) Duling, D. R. J. Magn. Reson. 1994, B104, 105-110.
IC050435T
Inorganic Chemistry, Vol. 44, No. 19, 2005 6735