Spin-state crossover with structural changes in a cobalt(II) organometallic species: Low-coordinate, first row, heteroleptic amido transition metal aryls. synthesis and characterization of Ar'MN(H)Ar# (M = Mn, Fe, Co) (Ar' = C6H

Article abstract of DOI:10.1021/ic801660a

The synthesis and characterization of the monomeric aryl transition metal amido complexes Ar'MN(H)Ar^# (Ar' = C₆H3-2,6-(C ₆H3-2,6-'Pr₂)2, Ar^# = C₆H3-2,6- (C₆H₂-2,4,6-Me

Full text of DOI:10.1021/ic801660a

Inorg. Chem. 2009, 48, 2443-2448
Spin-State Crossover with Structural Changes in a Cobalt(II)
Organometallic Species: Low-Coordinate, First Row, Heteroleptic Amido
Transition Metal Aryls. Synthesis and Characterization of Ar′MN(H)Ar#
(M ) Mn, Fe, Co) (Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2,
Ar# ) C6H3-2,6-(C6H2-2,4,6-Me3)2)
Chengbao Ni,† James C. Fettinger,† Gary J. Long,‡ and Philip P. Power*,†
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, California 95616, and
Department of Chemistry, Missouri UniVersity of Science and Technology, Rolla, Missouri 65409
Received September 3, 2008
The synthesis and characterization of the monomeric aryl transition metal amido complexes Ar′MN(H)Ar# (Ar′ )
C6H3-2,6-(C6H3-2,6-iPr2)2, Ar# ) C6H3-2,6-(C6H2-2,4,6-Me3)2, M ) Mn (1), Fe (2), Co(3a, b)) are reported. The
compounds were characterized by X-ray crystallography, electronic and infrared spectroscopy, and magnetic
measurements. At about 90 K the complexes 1 and 2 possess quasi-two coordinate geometry with a weak, secondary,
M---C interaction involving a flanking aryl ring from an amido group. In contrast, at the same temperature, their
cobalt analogue 3a features a strong Co-η6-flanking ring interaction to give an effectively higher coordination geometry.
Magnetic studies of 1-3a showed that 1 and 2 have high spin configurations, whereas the cobalt species 3a has
a low-spin configuration (S ) 1/2). However, 3a undergoes a spin crossover to a high spin (S ) 3/2) state 3b near
229 K. An X-ray structural determination above the crossover temperature at 240 K showed that the low temperature
structure of 3a had changed to 3b which involves a weak secondary M---C interaction analogous to those in 1 and
2. The complexes 1-3 are very rare examples of heteroleptic quasi-two coordinate open shell transition metal
complexes.
Introduction
of inherent interest because of the ability of the amido ligand
(-NR2) to combine steric flexibility, good metal-ligand bond
strength, and relatively straightforward synthetic accessibility.
This enables a wide variety of complexes with unusual
bonding modes and coordination numbers to be isolated.1
These attributes led to their pioneering use in the synthesis
of low (2 or 3) coordinate open shell (d1-d9) complexes.23-27
Amido complexes of transition metals1 are attracting
increased attention as supporting ligands for molecular
catalysts2-14 and for their involvement as important inter-
mediates in many catalytic processes.8,15-22 They are also
* To whom correspondence should be addressed. E-mail: pppower@
ucdavis.edu. Fax: (+01) 530-752-8995.
†
University of California, Davis.
Missouri University of Science and Technology.
‡
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10.1021/ic801660a CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 6, 2009 2443
Published on Web 02/12/2009

Ni et al.
in glass capillaries sealed under N2 and are uncorrected. UV-vis
data were recorded on a Hitachi-1200 spectrometer.
For example, the use of sterically encumbering amido ligands
has permitted the synthesis of several stable two-coordinate
derivatives of Cr, Mn, Fe, Co, and Ni.28,29 Analogous
complexes involving other bulky ligands such as alkyls or
aryls are rare. The first stable, open-shell, two coordinate
organometallic species in the solid state was the dialkyl
Mn(C(SiMe3)3)2.30 The corresponding iron compound Fe(C-
Ar′MnN(H)Ar# ·0.5 n-hexane (1 ·0.5 n-hexane). About 30 mL
of hexanes was added to a mixture of [Li(THF)Ar′MnI2]2 (0.393
g, 0.25 mmol) and LiN(H)Ar# (0.171 g, 0.51 mmol) at room
temperature. The mixture was stirred for 1 day by which time the
solution had become light yellow and some white precipitate had
formed. The solution was filtered, and the light yellow filtrate was
concentrated to about 5 mL, which afforded X-ray quality pale
yellow crystals of 1 after storage for 4 days at -18 °C. Yield 0.248
g (63.4%). mp 163-165 °C. UV-vis (hexane, nm [ε, cm-1 M-1]):
382(700).
31
(SiMe3)3)2 has also been reported. The use of the bulky
aryl ligand Mes* (Mes* ) C6H2-2,4,6-tBu3) permitted this
class of compounds to be extended to include MMes*2 (M
) Mn,32 or Fe32,33) and more recently the terphenyl ligand
Ar#(C6H3-2,6-Mes2) has been used to stabilize the complexes
MAr#2 (M ) Mn - Co).34 A feature of these amido, alkyl or
aryl derivatives is that they are homoleptic. Currently there
are very few examples of heteroleptic two coordinate
complexes and they are limited to ((Me3Si)2N)Fe(SAr#)35 and
the recently reported Fe(SAr#){SC6H3-2,6-(SiMe3)2}36 and
Cr(1) complex (L)Cr(C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) (L
) THF or PMe3).37 A major obstacle to the synthesis of the
heteroleptic complexes is the scarcity of suitable starting
materials that would afford two-coordinate complexes upon
derivatization with amido or organo groups. However, the
recent isolation of well characterized monoaryl transition
metal halides of the formula (Ar′MX)2 (M ) Cr, X ) Cl,
Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2) or ((Et2O)Li)2Mn2Ar′2I4 (M
) Mn, Fe, Co)38 has provided suitable building blocks for
low-coordinate heteroleptic species. Here we report the
synthesis and characterization of a series of heteroleptic
complexes Ar′MN(H)Ar# (M ) Mn (1), Fe(2), Co(3))
stabilized by bulky terphenyl and primary arylamido ligands.
Ar′FeN(H)Ar# (2). About 30 mL of hexanes was added to a
mixture of [Ar′Fe(µ-Br)]2 (0.267 g, 0.25mmol) and LiN(H)Ar#
(0.171 g, 0.51mmol) at room temperature. The amber solution had
become red in 2 h. The mixture was stirred for 1 day by which
time the solution had become bright red and some white precipitate
had formed. The solution was filtered and the bright red filtrate
was concentrated to about 5 mL, which afforded X-ray quality
bright red crystals of 2 after storage for overnight at -18 °C. Yield
0.285 g (73.0%). mp 110 °C(dec). Anal. Calcd. For C54H63FeN: C,
82.94; H, 8.12; N, 1.79; Found: C, 81.8, H, 7.89; N, 1.754.UV-vis
(hexane, nm [ε, cm-1 M-1]): 450(2500).
Ar′CoN(H)Ar# (3a and 3b). About 30 mL of hexanes was added
to a mixture of [Ar′Co(µ-Cl)]2 (0.246 g, 0.25mmol) and LiN(H)Ar#
(0.171 g, 0.51mmol) at room temperature. The orange brown
solution became deep purple after about 30 min. The mixture was
stirred for 1 day by which time the solution had become deep purple
and a white precipitate had formed. The solution was filtered and
the deep purple filtrate was concentrated to ca. 15 mL, which
afforded X-ray quality dark purple crystals of 3 after storage for
overnight at 7 °C. Yield 0.289 g (68.5%). mp 174-177 °C. Anal.
Calcd. for C54H63CoN: C, 82.62; H, 8.09; N, 1.78. Found: C, 81.91;
H, 8.0; N, 1.76. UV-vis (hexane, nm [ε, cm-1 M-1]): 548(3400)
and 696(1400).
Experimental Section
X-ray Crystallographic Studies. Suitable crystals of 1-3a and
3b were selected and covered with a layer of hydrocarbon oil under
a rapid flow of argon. They were mounted on a glass fiber attached
to a copper pin and placed in the cold N2 stream on the
diffractometer. For 1 and 2, X-ray data were collected on a Bruker
SMART 1000 diffractometer at 90(2) K using Mo KR radiation (λ
) 0.71073 Å), while for 3, X-ray data were collected on a Bruker
SMART Apex II diffractometer at both 90(2) and 240(2) K using
Mo K radiation (λ ) 0.71073 Å). Absorption corrections were
applied using SADABS.40 The structures were solved using direct
methods and refined by the full-matrix least-squares procedure in
SHELX.41 All of the non-hydrogen atoms were refined anisotro-
pically. The hydrogen atom on the nitrogen in all three compounds
was located by use of a Fourier difference map, and the other
hydrogens in each structure were placed at calculated positions and
included in the refinement using a riding model.
Magnetic Studies. The samples for magnetization measurements
were sealed under vacuum in 3 mm quartz tubing. The sample
magnetization was measured using a Quantum Designs MPMSXL7
superconducting quantum interference device (SQUID) magnetom-
eter. For each measurement, the sample was zero-field cooled to 2
K, and the magnetization was measured in 0.5 K increments to 4
K, 1 K increments to 10 K, 1.5 K increments to 24 K, 5 K
increments to 70 K, and 10 K increments to 320 K under constant
field of either 0.001 T or 0.01 T and with a long delay time.
General Procedures. All manipulations were carried out using
modified Schlenk techniques under an argon atmosphere or in a
Vacuum Atmospheres HE-43 drybox. All of the solvents were dried
over an aluminum column, stored over 3 Å molecular sieves
overnight, and degassed three times (freeze-thaw) prior to use.
The metal halide precursors38 and Ar#N(H)Li39 were prepared
according to literature procedures. Melting points were recorded
(23) Bradley, D. C. Chem. Br. 1975, 11, 393.
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Chem. ReV. 1977, 24, 1.
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(28) Power, P. P. Comments Inorg. Chem. 1989, 8, 177.
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A. C. Chem. Commun. 1985, 1380. (b) Viefhaus, T.; Schwarz, W.;
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(31) LaPointe, A. M. Inorg. Chim. Acta 2003, 345, 359.
(32) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264.
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34, 325.
(34) Kays, D. L.; Cowley, A. R. Chem. Commun. 2007, 1053.
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Ed. Engl. 1994, 33, 1178.
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Chem. 2007, 692, 4792.
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Soc. 2007, 129, 6076.
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G. J.; Power, P. P. Inorg. Chem. 2007, 46, 4809.
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(40) SADABS, Version 5.0 package; an empirical absorption correction
program from the SAINTPlus NT; Bruker AXS: Madison, WI, 1998.
(41) SHELXL, Version 5.1; Bruker AXS: Madison, WI, 1998.
2444 Inorganic Chemistry, Vol. 48, No. 6, 2009

First Row, Heteroleptic Amido Transition Metal Aryls
Scheme 1. Synthesis of Aryl Metal Amido Complexes 1-3
Table 1. Selected Crystallographic Data and Collection Parameters for 1-3a
1·0.5 n-hexane
2
3a
3b
formula
fw
C57H70MnN
824.08
C54H63FeN
781.90
C64H63CoN
784.98
C64H63CoN
784.98
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
pale yellow, plate
triclinic
P1
red, block
monoclinic
P21/c
19.6531(13)
11.4831(7)
19.8523(13)
90
99.605(1)
90
4417.4(5)
4
deep purple, block
monoclinic
P21/c
19.962(5)
11.288(3)
19.485(5)
90
103.042(4)
90
4277.5(18)
4
deep purple, block
monoclinic
P21/c
19.9318(9)
11.5995(5)
19.6369(9)
90
99.932(1)
90
4472.0(3)
4
j
10.6554(7)
12.5210(8)
20.2032(13)
75.825(1)
82.460(1)
68.015(1)
2421.0(3)
2
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
dcalcd, Mg/m3
θ range, deg
µ, mm-1
obs data, I > 2σ(I)
R1(obs data)
wR2(all data)
1.13
1.04-27.5
0.309
9745
0.0405
0.1218
1.176
1.05-27.55
0.378
8870
0.0338
0.0988
1.219
2.76-25.25
0.439
6657
0.0576
0.1479
1.166
2.72-27.18
0.419
7689
0.0382
0.1108
a 3a was collected at 90 K, 3b was collected at 240 K.
Table 2. Selected Bond Lengths (Å) and Angles (deg)
Results and Discussion
1 · 0.5n-hexane
2
3a
3b
Synthesis and Spectroscopy. Reaction of Ar#N(H)Li with
half an equivalent of the terphenyl metal halides
[Li(THF)Ar′MnI2]2, [Ar′Fe(µ-Br)]2, or [Ar′Co(µ-Cl)]2 in
hexanes at room temperature yielded the aryl metal amido
complexes 1-3 according to Scheme 1. The complexes 1-3
could be isolated from hexanes as very air and moisture
sensitive pale yellow, bright red, or deep purple crystals,
respectively. They can be stored at room temperature under
an inert atmosphere for several months.
The UV-visible spectra of 1-3 are characterized by
intense absorptions below 250 nm corresponding to π-π*
transitions within the terphenyl ligand. In addition, 1 features
a low-intensity absorption centered at 382 nm (ε ) 700 cm-1
M-1), and has a featureless spectrum in the visible region,
which is as expected for a high-spin d5 system. Compound
2 displays a relatively intense absorption centered at 450 nm
(ε ) 2500 cm-1 M-1) and 3 shows very broad absorptions
at 548 nm (ε ) 3400 cm-1 M-1) and 696 nm (ε ) 1400
cm-1 M-1). In their infrared spectra, the N-H stretching
absorptions are very similar for all 3 compounds and are
observed near 3390 cm-1.
M-C(ipso)
M-N
M-C(37)
Co-arene
2.0953(13)
1.9807(12)
2.5945(13)
2.0455(13) 1.977(1)
1.9324(12) 1.875(3)
2.4567(12) 2.077(3)
2.150(3),
1.992(2)
1.880(2)
2.393(2)
2.519(2),
2.692(2)
2.920(2),
3.049(2)
3.190(2)
1.374(2)
1.488(2)
1.493(2)
2.157(3)
2.211(3),
2.220(3)
2.143(3)
N-C31
1.3733(17)
1.5009(18)
1.4964(19)
132.58(5)
152.62(5)
126.35(9)
119.75(12)
1.3768(16) 1.385(4)
1.4889(18) 1.490(4)
1.4966(17) 1.493(4)
134.97(5)
149.25(4)
125.82(9)
C32-C37
C36-C46
C1-M-N1
C1-M-C37
M-N1-C31
C31-C32-C37
101.75(11) 133.90(6)
142.22(6)
118.0(2)
124.57(11)
116.24(13)
115.24(11) 112.2(3)
Structures. The structures of compounds 1-3 were
determined by X-ray crystallography. Important data col-
lection and refinement parameters for 1-3 are provided in
Table 1, and selected structural data are given in Table 2.
The crystals of 1, 2, and 3a (at 240(2) K) are isomorphous,
and their structures are very similar as illustrated in Figures
1, 2, and 3b. They feature quasi two-coordinate metal centers
bonded to the ipso-carbon of the aryl and the nitrogen of
the primary arylamido ligand, respectively. In addition, the
Inorganic Chemistry, Vol. 48, No. 6, 2009 2445

Ni et al.
Figure 1. Molecular structure of 1 at 90 Kwith thermal ellipsoids presented
at a 30% probability level. Hydrogen (except N-H) atoms are not shown
for clarity.
Figure 3. (a) Molecular structure of 3a at 90 K with thermal ellipsoids
presented at a 30% probability level. Hydrogen (except N-H) atoms are
not shown for clarity. (b) Molecular structure of 3b at 240 K with thermal
ellipsoids presented at a 30% probability level. Hydrogen (except N-H)
atoms are not shown for clarity.
Figure 2. Molecular structure of 2 at 90 K with thermal ellipsoids presented
at a 30% probability level. Hydrogen (except N-H) atoms are not shown
for clarity.
2.0455(13) Å is comparable to those in FeAr# (2.040(3) Å)34
2
and FeMes*2 (2.058(6)Å).32,33 The Fe-N distance (1.9324(12)
Å) is similar to those amido complexes, such as Fe(N-
(Mes)B(Mes)2)2 (1.938(2) Å)45 and Fe(N(SiMePh2)2)2
(1.917(2) Å).46 The C(1)-Fe-N angle is 134.97(5)°, which
is more than 30° narrower than those of 166.1(1) and
169.0(1)° in Fe(N(Mes)B(Mes)2)245 and Fe[N(SiMePh2)2]2.45
The Fe---C(37) distance (2.4567(12) Å) is about 0.5 Å longer
than the Fe-C(1) bond length and is similar to that in
(Me3Si)2NFeSAr# (2.457 Å),35 but is somewhat shorter than
the interactions observed in Fe(N(Mes)B(Mes)2)2 (2.521 Å)45
and Fe(N(SiMePh2)2)2 (2.695(4) Å).46 This might be due to
the fact that in these examples, each Fe(II) center interacts
with two aromatic rings, while in 2, the Fe(II) center only
interacts with one ring.
metal centers interact with the ipso carbon (C(37)) of a
flanking -C6H2-2,4,6-Me3 ring from the arylamido ligand.
In 1, the Mn-C(1) σ-bonded distance (2.0953(13) Å) is
essentially the same as those in the aryl Mn(II) complexes
MnAr# (2.095(3) Å)34 and MnMes*2 (2.108(2) Å).32 The
2
Mn-N distance (1.9807(12) Å) is similar to those in the
amido complex Mn(N(SiMePh2)2)2 (1.989(3) Å),42 but
somewhat shorter than those in Mn(N(Mes)B(Mes)2)2 (2.046(4)
Å),43 possibly as a result a weakened Mn-N bond because
of multiple bonding between the boron and nitrogen atoms.
The C(1)-Mn-N angle of 132.58(5)° deviates from linearity
because of the interaction between Mn(II) and the ipso
carbon(C(37)) of the flanking -C6H2-2,4,6-Me3 ring. The
Mn---C(37) distance is 2.5945(13) Å, is about 0.5 Å longer
than the Mn-C(1) bond length, and is similar to the 2.536(5)
Å observed in Mn(N(Mes)B(Mes)2)2.43 Both of the Mn-C
interactions in 1 may be compared to 2.37 Å predicted from
the sum of their covalent radii.44
(42) Chen, H.; Bartlett, R. A.; Dias, H. V. R.; Olmstead, M. M.; Power,
P. P. J. Am. Chem. Soc. 1989, 111, 4338.
(43) Bartlett, R. A.; Feng, X.; Olmstead, M. M.; Power, P. P.; Weese, K. J.
J. Am. Chem. Soc. 1987, 109, 4851.
The structure of 2 is similar to that of 1. The iron is
similarly coordinated to the terphenyl ligand, the arylamido
ligand, and also interacts with the ipso carbon(C(37)) of the
flanking -C6H2-2,4,6-Me3 ring. The Fe-C(1) distance of
(44) Cardeno, B.; Go´mez, V.; Platero-Prats, A. E.; Rves, M.; Echeveria,
J.; Cremades, E.; Barraga´n, F.; Alvarez, S. Dalton Trans. 2008, 2832.
(45) Chen, H.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner,
S. C. J. Am. Chem. Soc. 1990, 112, 1048.
(46) Bartlett, R. A.; Power, P. P. J. Am. Chem. Soc. 1987, 109, 7563.
2446 Inorganic Chemistry, Vol. 48, No. 6, 2009

First Row, Heteroleptic Amido Transition Metal Aryls
Table 3. Magnetic Properties from Curie-Weiss Law Fits
field temperature
range, K
C, Emu
K/mol
cpd.
Τ
ΝR
θ, K
µeff, µΒ
S
1
2
3
0.001 2-320
0.001 2-320
0.001 2-150
0
0.77(5)
4.17(10) 5.78(15) 5/2
0.000090 -2.20(6) 3.58(10) 5.36(15)
2
1.346
-82
1.416
-58
0.387
3.456
0.390
3.298
1.76
1/2
3/2
1/2
3/2
250-320
2-150
250-320
4.65a
1.77
0.01
4.70a
a Calculated at 300 K from µeff ) 2.828(xMT)1/2
.
Figure 5. Temperature dependence of the effective magnetic moment of
3 obtained upon warming. Upper inset: the inverse molar magnetic
susceptibility of 3 obtained upon warming and its corresponding Curie-Weiss
law fit. Lower inset: The hysteresis in the effective magnetic moment of 3
observed upon warming, red, and cooling, blue.
Figure 4. Temperature dependence of the effective magnetic moment of
1, red, and 2, green. The parameters for the fit of 2 shown between 2 and
100 K are S ) 2, g ) 2.15, and D ) 6 cm-1. Inset: the inverse molar
magnetic susceptibility of 1 and 2 and their corresponding Curie-Weiss
law fits.
distance is 1.636(3) Å, which is shorter than that in
Ar′CoCoAr′ (1.7638(16) Å),47 but is in the previously
observed range of 1.561 to 1.766 Å.48 The strong interaction
causes the C(1)-Co-N(1) bending angle 101.75(11)° to be
much narrower than the 133.90(6)° observed at 240(2) K.
The strong Co-η6-arene interaction is consistent with calcula-
tions for first row transition metal-arene interactions, which
show that they are often stronger for cobalt.49 The Co(1)-C(1)
(1.977(1) Å) and Co(1)-N91) (1.875(3) Å) σ-bonded
distances are marginally shorter than those in 3b consistent
with the low spin of the cobalt center. However, all the
σ-bond lengths involving the transition metals (M-C(ipso
carbon of the aryl) and the M-N distance) decrease in the
sequence Mn > Fe > Co, and this is in agreement with the
decrease in the size of these metals.44,50 The C(32)-C(37)
distances in 3a and 3b, as well as in 1 and 2, are essentially
the same and show no significant change from that of
C(36)-C(46), which is the corresponding C-C distance of
the non-interacting flanking -C6H2-2,4,6-Me3 ring. Although
the secondary M---C interactions in the four structures induce
The structure of 3 was determined by X-ray crystal-
lography at both 90(2) K (3a) and 240(2) K (3b), and the
structures are illustrated in panels a and b of Figure 3,
respectively. The structural data for both forms are given in
Table 2. It is noteworthy that the incipient structural change
from the high spin, high temperature form (3b) to the low
spin, low temperature form (3a) is discernible even at 240
K. At this temperature, the cobalt center is disordered over
two positions. The main component is structure 3b, which
has 95% occupancy (the other 5% is structure 3a), and is
similar to those of 1 and 2 at 90 K. The cobalt center is
coordinated to the aryl and arylamido ligands in a manner
similar to that in 1 and 2, and it interacts with the ipso carbon
(C(37) of the flanking -C6H2-2,4,6-Me3 ring at a distance of
2.393(2) Å which is considerably longer than the Co(1)-C(37)
distance (2.077(3) Å) in the low spin form 3a (see below).
The Co(1)-C9(1) (1.992(2) Å) and Co(1)-N(1) (1.880(2)
Å) distances are similar to those previously observed in the
diaryl CoAr2 (2.001(3) Å)37 and in the amides Co(N-
#
(Mes)BMes2)2 (1.910(3) Å)45 and Co(N(SiMe3Ph2)2)2 (1.901(3)
Å).46 The C(1)-Co-N angle is 133.90(6)°, which lies
between 127.1(2)° in Co(N(Ph)B(Mes)2)245 and 140.0(1)° in
Co(N(SiMePh2)2)2.45 The geometry of the minor 5% com-
ponent has a structure that is the same as that observed for
3a at 90(2) K. The major structural difference is that at the
low temperature there is a strong cobalt-η6 interaction with
one of the flanking -C6H2-2,4,6-Me3 rings instead of the
weaker M---C interaction observed in (3b). The Co-centroid
(47) Nguyen, T.; Merrill, A.; Ni, C.; Lei, H.; Fettinger, J. C.; Ellis, B. D.;
Long, G. J.; Brynda, M.; Power, P. P. Angew. Chem., Int. Ed. 2008,
47, 9115.
(48) Co-centroid distance of 31 Co-arene (benzene or toluene) complexes
from Cambridge database (version 5.29, Feb, 2008).
(49) (a) Pandey, R.; Rao, B. K.; Jena, P.; Blanco, M. A. J. Am. Chem.
Soc. 2001, 123, 3799. (b) Jaeger, T. D.; van Heijnsbergen, D.;
Klippenstein, S. J.; von Helden, G.; Meijer, G.; Duncan, M. A. J. Am.
Chem. Soc. 2004, 126, 10981. (c) La Macchia, G.; Gagliardi, L.;
Power, P. P.; Brynda, M. J. Am. Chem. Soc. 2008, 130, 5105.
(50) Emsley, J. The Elements; Oxford University Press: Oxford, 1998.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2447

Ni et al.
considerable bending in their geometries, there are only
relatively small effects on other structural parameters. Also,
the M---C interaction with the flanking rings produce little
structural change within the interacting rings. The N-C(31)
distances in all four complexes remain essentially the same
and marginally shorter than the C-N (1.396(3) Å) distance
in the corresponding terphenyl aniline.51
values that are typical for high spin iron(II) in a distorted
coordination environment.
The magnetic properties of 3 are especially interesting,
see Figure 5, because they indicate that the cobalt(II) ion in
this complex undergoes an electronic spin-state transition
from the low-spin, S ) 1/2, state below 229 K (3a) to the
high-spin, S ) 3/2 state (3b) above this temperature; as
expected the results obtained at a 0.01 and 0.001 T applied
field are virtually identical, which is in agreement with the
structural change of 3 at 240 K. As is often the case for an
electronic spin-state crossover, the transition exhibits an about
8 K hysteresis upon warming and cooling, which is shown
in the lower inset to Figure 5. The presence of this hysteresis
indicates that the transition is probably first-order. The
magnitude and the small decrease in the observed µeff of 3
upon cooling from 320 to 230 K results, no doubt, from either
the influence of spin-orbit coupling or the imminent onset
of further cooling of the spin state crossover; because of this
spin-state crossover it is not possible to evaluate the relative
importance of these influences. Spin-state crossover in
cobalt(II) systems has been well studied; however, the
majority of those examined concern multidentate ligand
complexes with six-coordination, or less commonly five-
coordination, at cobalt.55 In this case it is associated with a
change in the effective coordination number from quasi three-
coordinate in 3a to quasi five-coordinate in 3b.55
Magnetism. Magnetic susceptibility studies were carried
out on crystalline samples of 1-3 using a SQUID magne-
tometer.Adiamagneticcorrection52 of-0.000571,-0.000533,
and -0.000532, emu/mol, obtained from tables of Pascal’s
constants, have been applied to the measured susceptibility
of 1-3. As would be expected from the structures of 1-3,
these complexes are magnetically dilute and exhibit para-
magnetic behavior, except perhaps rather low temperatures.
The results53 of a Curie-Weiss law fit of the inverse molar
susceptibility of compounds 1-3 are given in Table 3.
The temperature dependence of the effective magnetic
moments, µeff, and the inverse molar magnetic susceptibilities
of 1 and 2 are shown in Figure 4; for each complex the
magnetic properties are fully consistent with the presence
of the expected high-spin manganese(II) and high-spin
iron(II) ions. The effective magnetic moment of 1 is less
than the expected 5.92 µΒ (S ) 5/2) probably as a result of
the presence of the diamagnetic impurities in the sample.
The very small decrease in the µeff of 1 below about 5 K
may be due to the onset of very weak long-range antiferro-
magnetic ordering; no attempt has been made to model this
small decrease because of the presence of probably impuri-
ties. The µeff for 2 is 5.36 µΒ, which is significantly more
than the spin only value 4.90 µΒ (S ) 2), indicates a
considerable orbital contribution to the effective magnetic
moment and is consistent with incomplete quenching of the
high orbital moment observed in a strictly linear, two
coordinate Fe(II) species such as Fe(C(SiMe3)3)2.54 The
decrease in µeff of 2 below about 50 K, see Figure 4, is
predominantly due to zero-field splitting; a fit between 2 and
100 K with S ) 2 yields g ) 2.1(52) and D ) 6(1) cm-1
Conclusions
In summary, complexes 1-3 represent rare examples of
monomeric, heteroleptic, open-shell Mn(II), Fe(II), and
Co(II) amido complexes. Their synthesis is made possible
by the availability of terphenyl stabilized aryl metal halide
starting materials. The low spin configuration of 3 at low
temperature (< ca. 229 K, i.e., 3a) is consistent with the
strong Co-η6-arene interaction, and the high spin configu-
ration of (3b) at high temperature is consistent with the lower
effective coordination number of the metal as demonstrated
by X-ray crystallography.
Acknowledgment. We thank the National Science Foun-
dation (Grant CHE-0641020) and Peter Klavins for assistance
in magnetization measurements.
(51) Wright, R. J.; Steiner, J.; Beaini, S.; Power, P. P. Inorg. Chim. Acta
2006, 359, 1939.
(52) The diamagnetic corrections are believed to be accurate to at least
(10% of the value given and are probably more accurate. See Bain,
G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532.
(53) The rather higher than expected errors reported in Table 3 arise
predominately from the difficulty in accurately determining the exact
mass of the sample placed in a sealed quartz tube under dry nitrogen
inside a Vacuum Atmospheres glove box. It is estimated that this error
should be no more than 10% of the mass and is probably less.
(54) Reiff, W. M.; LaPointe, A. M.; Whitten, E. H. J. Am. Chem. Soc.
2004, 126, 10206.
Supporting Information Available: The crystallographic data
for 1-3. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC801660A
(55) Goodwin, H. A. Top. Curr. Chem. 2004, 234, 23.
(56) Alvarez, S.; Cirena, J. Angew. Chem., Int. Ed. 2006, 45, 3012.
2448 Inorganic Chemistry, Vol. 48, No. 6, 2009