Reduction of terphenyl Co(II) halide derivatives in the presence of arenes: Insertion of Co(I) into a C-F bond

Article abstract of DOI:10.1021/ic202116s

Reduction of [(3,5-ⁱPr₂-Ar*)Co(μ-Cl)] ₂ (3,5-ⁱPr₂-Ar* = -C₆H-2,6- (C₆H₂-2,4,6-ⁱPr₃) ₂-3,5-ⁱPr₂) with KC₈

Full text of DOI:10.1021/ic202116s

Article
pubs.acs.org/IC
Reduction of Terphenyl Co(II) Halide Derivatives in the Presence of
Arenes: Insertion of Co(I) into a C−F Bond
Hao Lei, James C. Fettinger, and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: Reduction of [(3,5-iPr2-Ar*)Co(μ-Cl)]2
(3,5-iPr2-Ar* = -C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) with
KC8 in the presence of various arene molecules resulted in
the formation of a series of terphenyl stabilized Co(I) half-
sandwich complexes (3,5-iPr2-Ar*)Co(η6-arene) (arene =
toluene (1), benzene (2), C6H5F (3)). X-ray crystallographic studies revealed that the three compounds adopt similar bonding
schemes but that the fluorine-substituted derivative 3 shows the strongest cobalt-η6-arene interaction. In contrast, C−F bond
cleavage occurred when the analogous reduction was conducted in the presence of C6F6, affording the salt K[(3,5-iPr2-
Ar*)Co(F)(C6F5)] (4), in which there is a three-coordinate cobalt complexed by a fluorine atom, a C6F5 group, and the
terphenyl ligand Ar*-3,5-iPr2. This salt resulted from the formal insertion of a putative 3,5-iPr2-Ar*Co species as a neutral or
anionic moiety into one of the C−F bonds of C6F6. Reduction of [(3,5-iPr2-Ar*)Co(μ-Cl)]2 in the presence of bulkier
substituted benzene derivatives such as mesitylene, hexamethylbenzene, tert-butylbenzene, or 1,3,5-triisopropylbenzene did not
afford characterizable products.
reactivity in comparison to its iron counterpart (3,5-iPr2-
INTRODUCTION
Low-coordinate Co(I) complexes have received increasing
attention during the past decade owing to their exceptionally
high reactivity toward small molecules, as well as their potential
■
Ar*)Fe(η6-C6H6). For example, the iron compound reacts
readily with N2O at room temperature to afford a μ-oxo
bridged Fe(III) dimer.28 In contrast, 1 does not react with N2O
even at elevated temperature. This low reactivity may be
catalytic applications. Various low-coordinate Co(I) com-
attributed to the exceedingly strong Co-η6-arene interaction.
Thus we reasoned that the reactivity of 1 might be enhanced by
fine-tuning the Co-η6-arene interaction through modification of
the substituents on the arene ring to give weaker bonding to
the metal. We hoped to extend the previous work with the aim
of synthesizing a series of Co(I) compounds (3,5-iPr2-
Ar*)Co(η6-arene) and investigating the effects of the arene
substituents on the cobalt-arene interaction. While this work
was in progress Holland and his group showed that the unusual
Co(I) β-diketiminate [HC{C(But)N(C6H3-2,6-iPr2)}2]Co, that
is, CoLtBu, (HC{C(tBu)N(C6H3-2,6-iPr2)2 = LtBu), in which
the LtBu ligand has a masked Co(I) center in the unusual κN,η6-
arene bonding mode, reacted with fluorobenzene to afford
(LtBuCo(μ-F))2 and LtBuCoPh products.29
Herein we report the synthesis and characterization of
(3,5-iPr2-Ar*)Co(η6-arene) (arene = C6H6 (2), C6H5F (3)),
the formation of the latter being in contrast to the insertion
behavior of CoLtBu toward C6H5F. The solid-state structures of
2 and 3 were determined by X-ray crystallography and
compared to 1 to elucidate the strength of the cobalt-arene
interactions. We also show that the reduction of (3,5-iPr2-
Ar*)CoCl in the presence of C6F6 resulted in the formation of
the three-coordinate C−F inserted product K[3,5-iPr2-Ar*Co-
(F)C6F5], (4).
pounds bearing different ligand sets have been reported,1−15
and their synthesis and isolation usually require the use of
multidentate ligands to stabilize the unsaturated cobalt center.
Recently we reported the synthesis and reactivity of a half-
sandwich Co(I) toluene complex (3,5-iPr2-Ar*)Co(η6-C7H8)
(1, 3,5-iPr2-Ar* = -C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2), which
was supported by a very bulky monodentate terphenyl ligand.16
In the solid-state structure of 1, the cobalt atom is η6-bound to
the aromatic ring of a toluene molecule. Such Co-η6-arene
interactions were not unprecedented and have been observed in
13−24
several other low-valent cobalt complexes.
The bonding
in 1 is composed of a 15-electron (arene)Co fragment which
contains three unpaired spins, one of which becomes paired
upon σ-bonding to the terphenyl ligand, to afford a
paramagnetic compound with two upaired electrons. Theoreti-
cal investigations of model compounds CH3M(C6H6) (M = Cr,
Fe, Co) revealed that the interaction energies between the
CH3M fragment and the benzene ring increase substantially
between Cr (−9.6 kcal/mol) and Co (−22.1 kcal/mol).25 This
is consistent with our observation that, the monomeric,
univalent chromium(I) compounds (3,5-iPr2-Ar*)Cr(L) (L =
THF or PMe3) decompose via rearrangement when reacted
with benzene or toluene in attempts to form (3,5-iPr2-
Ar*)Cr(η6-arene) complexes.26 In contrast, the analogous
Fe27 or Co16 derivatives form stable η6 complexes with
benzene or toluene (1), respectively. Furthermore, reactivity
studies of complex 1 showed that it had a relatively low
Received: September 29, 2011
Published: January 19, 2012
© 2012 American Chemical Society
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color change was observed, but no characterizable product was
isolated.
EXPERIMENTAL SECTION
■
General Methods. All manipulations were carried out by using
modified Schlenk techniques under an atmosphere of N2 or in a
Vacuum Atmospheres OMNI-Lab drybox. All solvents were dried over
an alumina column, followed by storage over 3 Å molecular sieves
overnight, and degassed three times (freeze−pump−thaw) prior to
use. The compound [(3,5-iPr2-Ar*)Co(μ-Cl)]2 was prepared accord-
ing to literature procedures.30 1H NMR data were obtained on a
Varian Mercury 300 MHz spectrometer and referenced to the residual
protio benzene in the C6D6 solvent. Melting points were measured in
glass capillaries sealed under N2 by using a Mel-Temp II apparatus and
are uncorrected. UV−vis data were recorded on a Hitachi-1200
spectrometer.
X-ray Crystallography. Crystals of appropriate quality were
removed from a Schlenk tube under a stream of nitrogen and
immediately covered with a thin layer of hydrocarbon oil. A suitable
crystal was selected, attached to a glass fiber on a copper pin, and
quickly placed in the cold N2 stream on the diffractometer.31 Data for
compound 3 were collected at 90 K on a Bruker APEX DUO
diffractometer with Cu Kα radiation (λ = 1.54178 Å). Data for
compounds 2 and 4 were collected at 90 K on a Bruker SMART APEX
II diffractometer with Mo Kα (λ = 0.71073 Å) radiation and a CCD
area detector. Absorption corrections were applied using SADABS
program.32 The crystal structures were solved by direct methods and
refined by full-matrix least-squares procedures in SHELXTL.33 All
non-H atoms were refined anistropically. All H atoms were placed at
calculated positions and included in the refinement using a riding
model.
SQUID Magnetic Studies. The samples for magnetic measure-
ments were sealed under vacuum in 4 mm quartz tubes. The magnetic
properties were measured on a Quantum Design MPMSXL7 SQUID
on 16.6 mg of 2, 15.9 mg of 3, and 14.8 mg of 4. The sample was
initially zero-field cooled to 5 K, and the magnetic susceptibility was
then measured from 5 to 300 K in a 0.010 T applied magnetic field.
The observed molar magnetic susceptibilities were corrected for the
diamagnetic contribution of the constituents by subtracting
−0.000509, −0.000512, and −0.000546 emu/mol for 2, 3, and 4,
respectively, values that were obtained from tables of Pascal’s
constants.34
(3,5-iPr2-Ar*)Co(η6-C6H6) (2). A brown solution of [(3,5-iPr2-
Ar*)Co(μ-Cl)]2 (0.98 g, 0.74 mmol) in about 30 mL of benzene was
added dropwise to a suspension of KC8 (0.37 g, 2.76 mmol) at 0 °C in
about 20 mL of benzene. The dark mixture was stirred for about 24 h,
then the solvent was removed under reduced pressure, and the dark
residue was extracted with about 100 mL of hexane. The solution was
filtered, and the green filtrate was concentrated to about 20 mL, which
afforded X-ray quality green crystals of 2·C6H12 after storage at −18
°C for several days. Yield: 0.77 g (74%). M.p.: 182 °C. Anal. Calcd. for
2·C6H12: C, 82.4; H, 10.12. Found: C, 81.9: H, 9.81. 1H NMR (300.08
MHz, C6D6, 20.0 °C): δ 65.84 (br s), 16.17 (s), 14.28 (br s), 8.68 (s),
7.55 (s), 7.42 (s), 6.88 (s), 3.28 (s), 2.91 (s), 2.56 (s), 1.23 (d), 0.26
(d), −0.83 (s), −62.00 (s). UV−vis [hexanes; λmax, nm (ε,
L·mol−1·cm−1)]: 660 (100).
(3,5-iPr2-Ar*)Co(η6-C6H5F) (3). A mixture of [(3,5-iPr2-Ar*)Co(μ-
Cl)]2 (0.97 g, 0.73 mmol) and C6H5F (1.5 mL, 16.0 mmol) in about
30 mL of tetrahydrofuran (THF) was added dropwise to a suspension
of KC8 (0.47 g, 3.50 mmol) at 0 °C in about 20 mL of THF. The dark
mixture was stirred for about 24 h; then the solvent was removed
under reduced pressure and the dark residue was extracted with about
100 mL of hexane. The solution was filtered, and the green filtrate was
concentrated to about 20 mL, which afforded X-ray quality green
crystals of 3·C4H8O after storage at −18 °C for several days. Yield:
0.32 g (30%). M.p.: 165 °C. Anl. Calcd. for 3·C4H8O: C, 78.75; H,
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The reduction of
30
[(3,5-iPr2-Ar*)Co(μ-Cl)]2 with KC8 in benzene afforded
green crystals of the half-sandwich complex (3,5-iPr2-Ar*)Co-
(η6-C6H6) (2) in 74% yield (Scheme 1). A similar reduction
Scheme 1. Synthetic Routes of 1−4
1
9.41. Found C, 78.13; H, 9.19. H NMR (300.08 MHz, C6D6, 25.0
°C): δ 66.00 (br s), 16.17 (s), 14.30 (br s), 8.71 (br s), 7.55 (s), 7.24
(s), 6.88 (s), 3.30 (s), 2.91 (s), 2.56 (s), 1.30 (s), 1.18 (s), 0.31 (s),
−0.81 (s), −62.04 (s). UV−vis [hexanes; λmax, nm (ε, L·mol−1·cm−1)]:
386 (1000, sh), 664 (120).
K[(3,5-iPr2-Ar*)Co(F)(C6F5)] (4). A mixture of [(3,5-iPr2-Ar*)Co-
(μ-Cl)]2 (0.90 g, 0.68 mmol) and C6F6 (1.5 mL, 13.0 mmol) in about
30 mL of THF was added dropwise to a suspension of KC8 (0.55 g,
4.09 mmol) at 0 °C in about 20 mL of THF. The dark mixture was
stirred for about 24 h; then the solvent was removed under reduced
pressure and the dark residue was extracted with about 100 mL of
hexane. The solution was filtered, and the green filtrate was
concentrated to about 20 mL, which afforded X-ray quality green
crystals of 4·C6H14 after storage at −18 °C for several days. Yield: 0.28
g (25%). M.p.: 171 °C. Anal. Calcd for 4·C6H14: C, 69.28; H, 8.08.
1
Found: C, 68.90; H, 7.83. H NMR (300.08 MHz, C6D6, 25.0 °C): δ
71.09 (s), 52.70 (br), 48.31 (br), 39.97 (s), 36.14 (s), 20.44 (br), 1.26
(m), 0.90 (s), 0.30 (s), −1.20 (br), −16.58 (s), −27.03 (s), −46.74
(s), −47.76 (s), −86.57 (br). UV−vis [hexanes; λmax, nm (ε,
L·mol−1·cm−1)]: 460 (32), 706 (42).
with KC8 in THF in the presence of C6H5F also afforded green
crystals of (3,5-iPr2-Ar*)Co(η6-C6H5F) (3), but in a lower 30%
yield (Scheme 1). In contrast, similar reduction in the presence
of bulkier arenes (mesitylene, hexamethylbenzene, tert-
butylbenzene or 1,3,5-triisopropylbenzene) did not yield any
characterizable products, possibly a result of instability because
of the steric repulsion between the substituents on the arenes
and the isopropyl groups on the flanking rings of the terphenyl
ligand, which may prevent complex formation. The results of all
the attempted reduction in the presence of various arene
molecules are summarized in Table 1.
Reduction of [(3,5-iPr2-Ar*)Co(μ-Cl)]2 in the Presence of
Bulkier Arenes. Representative procedure: a mixture of [(3,5-iPr2-
Ar*)Co(μ-Cl)]2 (0.58 g, 0.44 mmol) and C6Me6 (1.61 g, 9.9 mmol) in
about 30 mL of THF was added dropwise to a suspension of KC8
(0.36 g, 2.67 mmol) at 0 °C in about 20 mL of THF. The dark mixture
was stirred for about 24 h, and the solvent was removed under reduced
pressure. The dark residue was extracted with about 100 mL of hexane.
The solution was filtered, and the dark brown filtrate was concentrated
to about 10 mL and stored at −18 °C for a week. No characterizable
product could be isolated. Other reactions were implemented in a
similar fashion by replacing C6Me6 with the corresponding arenes
(mesitylene, tert-butylbenzene or 1,3,5-triisopropylbenzene). A similar
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Table 1. Summary of Results Obtained by Reduction in the
Presence of Various Arenes
arenes
products
toluene
(3,5-iPr2-Ar*)Co(η6-C7H8) (1)
benzene
(3,5-iPr2-Ar*)Co(η6-C6H6) (2)
fluorobenzene
(3,5-iPr2-Ar*)Co(η6-C6H5F) (3)
K[(3,5-iPr2-Ar*)Co(F)(C6F5)] (4)
N/A
N/A
N/A
N/A
hexafluorobenzene
mesitylene
hexamethylbenzene
tert-butylbenzene
1,3,5-triisopropylbenzene
1
The H NMR spectra of 2 and 3 showed paramagnetically
shifted resonances (see Supporting Information), suggesting
high-spin d8 electron configuration with two unpaired spins for
the cobalt atoms in both compounds. This was further
confirmed by the study of their magnetic properties in the
solid state. The magnetic susceptibilities of 2 and 3 were
measured from 5 to 300 K in a 0.010 T applied magnetic field.
A plot of the inverse molar magnetic susceptibility of 2 versus
temperature is linear and yields a Curie constant of 0.707(3)
mol emu−1 K−1, a Curie−Weiss temperature of −12.1(4) K, and
a corresponding effective magnetic moment of 2.35 μB. A plot
of the inverse molar magnetic susceptibility of 3 versus
temperature is almost linear and yields a Curie constant of
1.00(1) mol emu−1 K−1, a Curie−Weiss temperature of −18(1)
K, and a corresponding effective magnetic moment of 2.79 μB.
Both complexes displayed Curie paramagnetism, and the
resulting effective magnetic moment and inverse molar
susceptibility were plotted in Figures 1 and 2 for 2 and 3,
Figure 2. Effective magnetic moment of 3 between 5 and 300 K in a
0.010 T applied field. Inset: the corresponding inverse molar magnetic
susceptibility of 3.
the presence of two unpaired electrons in 1−3. In addition, all
three complexes were characterized by UV−vis spectroscopy,
and each compound shows a characteristic absorption in the
visible region (676 nm (1), 660 nm (2), 664 nm (3)).
The attempted synthesis of (3,5-iPr2-Ar*)Co(C6F6), to
investigate the strong electron-withdrawing effect of the six
fluorine atoms on metal-arene bonding, was performed in a
similar fashion to that of 2 and 3, by reduction of [(3,5-iPr2-
30
Ar*)Co(μ-Cl)]2 with KC8 in THF in the presence of C6F6
(Scheme 1). The reaction gave, after workup and crystal-
lization, green crystals of the unexpected C−F activated
product K[(3,5-iPr2-Ar*)Co(F)(C6F5)] (4) in 25% yield
which is a very rare example of a three coordinate transition
metal fluoride complex.35 Although C−F bond activation by
Ni(0) complexes is well documented,36 for Co(I) complexes it
is a rarity.29,37,38 For example, Li and co-workers demonstrated
that a cyclometalation reaction involving C−F activation occurs
when N,N′-bis-(2,6-difluorobenzylidene)hydrazone is reacted
with MeCo(PMe3)4, to afford an mono-ortho-metalated
cobalt(III) fluoride.37 Holland and co-workers described an
unusual, masked two-coordinate cobalt(I) complex, LtBuCo
(LtBu = HC{C(tBu)N(C6H3-2,6-iPr2)}2 in which the cobalt
atom is bound to one of the ligand nitrogens and η6 to one of
the C6H3-2,6-Pri2 rings, and showed that it cleaves to the C−F
bond in fluorobenzene to give [LtBuCo(μ-F)]2 and LtBuCoPh
products.29 In this binuclear oxidative addition reaction, the
hemilabile β-diketiminate ligand can rearrange from the
“masked” κN,η6-arene binding mode to the two-coordinate
κ2N,N′ ligation mode during reductive cleavage of the C−F
bond in fluorobenzene. The mechanism of the formation of 4
remains unknown. The cleavage of the C−F bond in C6F6
could be a result of direct reaction of C6F6 with KC8, to
produce KC6F5, which could then react with the 3,5-iPr2-
Ar*CoCl to generate 3,5-iPr2-Ar*CoC6F5 which then forms a
complex with KF to generate 4. The reaction could also
proceed via the generation of a neutral, masked intermediate,
Co(I) species of formula 3,5-iPr2-Ar*Co which has a structure
that is related to the κ,N,η6-arene structure of LtBuCo29 that is
generated in the reduction process. The neutral 3,5-iPr2Ar*Co
may then insert into a C−F bond in C6F6 to give the Co(III)
species 3,5-iPr2Ar*Co(C6F5)F which can be reduced to KC8 to
give 4. Alternatively, the formation of 4 could result from the
Figure 1. Effective magnetic moment of 2 between 5 and 300 K in a
0.010 T applied field. Inset: the corresponding inverse molar magnetic
susceptibility of 2.
respectively. The magnetic behavior of 2 and 3 conform to the
Curie−Weiss law; however, they display somewhat different
room temperature effective magnetic moments of 2.35 (2) and
2.79 (3) μB which correspond roughly to the spin-only value of
2.83 μB for an S = 1 ground state. Thus, the magnetic results are
consistent with a high-spin Co(I) electron configuration in each
case. The bonding in the Co(I) half sandwich complex and
related Fe(I) species and the effects of geometrical distortion
on their magnetic properties have been studied experimentally
and computationally.15,25 For Co(I) they are consistent with
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generation of an [3,5-iPr2-Ar*Co]− anion from the reduction of
{3,5-iPr2Ar*Co(μ-Cl)}2 with K8 which then inserts into a C−F
bond of C6F6 to afford the anion of 4 which can crystallizes
with the K+ cation. However, we note that over reduction of σ-
bonded aryl Co(I) to give an anionic Co(0) species has not
been described in the literature and that the generation of a
neutral 3,5-iPr2Ar* moiety is consistent with the formation of
1−3 upon its η6-complexation with an arene ring.
value of 3.87 μB for high-spin Co(II) S = 3/2 ground state,
possibly as a result of spin orbit coupling.39
X-ray Crystal Structures. The formulation and solid state
structures of 2 and 3 were established by X-ray crystallography
(Figure 4 and Table 2). The structures of both compounds are
Table 2. Selected Crystallographic Data and Collection
Parameters for 2−4
1
Compound 4 was also characterized by UV−vis and H
2·C6H12
3·C4H8O
4·C6H14
NMR spectroscopy. It shows broad absorption bands at 460
and 706 nm in its UV−vis spectrum, which may be due to the
4F → 4P transitions of Co(II) ion in a distorted trigonal ligand
field. The 1H NMR spectrum of 4 features shifted and
broadened resonances in the region between 71 and −86 ppm,
consistent with the paramagnetic nature of the cobalt center in
4. The magnetic properties of 4 were measured from 5 to 300
K in a 0.010 T applied magnetic field. It shows nearly ideal
Curie paramagnetism, and the temperature effective magnetic
moment and inverse molar susceptibility are plotted in Figure
formula
fw
C54H79Co
787.10
green
C52H74CoFO
793.03
green
C54H75CoF6K
936.17
color
light green
block
habit
block
block
space group
Pnn2
Pnn2
P21/c
́
a, Å
14.4108(7)
14.8230(7)
10.9922(5)
90
14.4101(4)
14.8119(3)
10.9705(2)
90
16.0295(9)
10.4041(6)
30.7218(18)
90
́
b, Å
́
c, Å
α, deg
β, deg
γ, deg
90
90
99.3690(10)
90
90
90
3
́
V, Å
2348.06(19)
2
2341.55(9)
2
5055.2(5)
4
Z
d
calcd, Mg/m3
1.113
1.126
1.230
θ range, deg
2.31−27.50
0.399
4.28−71.57
3.154
2.59−25.25
0.478
−1
μ, mm
no. of obsd data, I >
2σ(I)
4876
2088
5933
R1 (obsd data)
wR2 (all data)
0.0467
0.1211
0.0740
0.2246
0.0592
0.1771
broadly similar to the structure of the toluene analogue
(3,5-iPr2-Ar*)Co(η6-C7H8) (1) reported earlier.16 A compar-
ison of the key structural parameters of compounds 1−3 is
illustrated in Table 3. In the solid state structure, the Co atom is
Table 3. Selected Bond Distances (Å) and Angles (deg) for
1−3
Figure 3. Effective magnetic moment of 4 between 5 and 300 K in a
0.010 T applied field. Inset: the corresponding inverse molar magnetic
susceptibility of 4.
1
2
3
Co(1)−C(1)
Co(1)−centroid
C(1)−Co(1)−centroid
C(2)−Co(1)−C(6)
2.021(2)
1.659(1)
167.6(2)
117.23(18)
1.985(3)
1.634(2)
180.0
2.069(8)
1.557(5)
180.0
3. The resulting room temperature effective magnetic moment
117.0(3)
117.0(7)
of 4 is 4.47 μB, which is higher than the expected spin-only
Figure 4. Solid-state molecular structure of 2 (left) and 3 (right) (H atoms and solvent molecules are not shown; thermal ellipsoids are shown at
30% probability; F(23) in 3 is distorted over six positions around the ring, only one position is shown). Selected bond distances (Å) and angles
(deg) for 2: Co(1)−C(1) 1.985(3), Co(1)−centroid 1.634(2), C(1)−Co(1)−centroid 180.0; For 3: Co(1)−C(1) 2.069(8), Co(1)−centroid
1.557(5), C(1)−Co(1)−centroid 180.0.
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η1 bonded to the terphenyl ligand and η6 bonded to the arene
molecule, affording a linear coordination C(ipso)−Co−centroid
arrangement that has 180.0° angles in both 2 and 3. The
Co(1)−centroid distance in 2 is 1.634(2) Å, which is slightly
shorter than the corresponding distance in 1 (1.659(1) Å). In
comparison, 3 has a considerably shorter Co(1)−centroid
distance (1.557(5) Å), indicating a much stronger Co-η6-arene
interaction in this case. In contrast, the related dimeric complex
Ar′CoCoAr′ (Ar′ = -C6H3-2,6-(C6H3-2,6-iPr2)2) in which Ar′
behaves as an η1:η6-μ2 bridging ligand, contains a much longer
Co−centroid distance ( 1.7638(16) Å),15 possibly a result of
the steric repulsion between the two Ar′Co fragments. In view
of the lower electron density on C6H5F ring because of the
electron-withdrawing effect of the fluorine atom, the observed
trend of the Co(1)−centroid distances in 1−3 implies that the
major contribution of the Co-η6-arene interaction involves
backbonding from the cobalt atom to the aromatic ring, which
is strengthened by electron withdrawing ring substituents. This
is consistent with our previous observation in Ar′CoCoAr′,
where the average C−C bond length within the metal-
coordinated flanking arene rings is approximately 0.02 Å
longer than that in the uncomplexed rings as a result of electron
density transferred from the metal into the ring π* orbitals.15
The Co(1)−C(1) σ-bond length in 2 (1.985(3) Å) is 0.036 Å
shorter than that in 1. However, the Co(1)−C(1) distance in 3
(2.069(8) Å) is considerably longer. This is in agreement with
the fact that 3 has the strongest Co-η6-arene interaction among
compounds 1−3, as a result of which its Co(1)−C(1) σ-bond
is weakened. Both the Co−C σ-bond and Co−centroid
distances in the toluene complex 1 are slightly larger than
those in the benzene complex 2 probably for steric reasons.
The Co(1)-C(ipso) bond lengths in 1−3 are similar to those
Figure 5. Solid-state molecular structure of 4 (H atoms and solvent
molecules are not shown; thermal ellipsoids are shown at 30%
probability) shown the K--F interactions with one of its neighbors.
Selected bond distances (Å) and angles (deg.): Co(1)−F(1) 1.896(2),
Co(1)−C(1) 2.007(3), Co(1)−C(43) 2.070(4), C(1)−Co(1)−F(1)
117.77(12), C(1)−Co(1)−C(43) 142.71(14), F(1)−Co(1)−C(43)
99.53(12).
Ar′CoI2]2 (1.998(5) Å),29 Ar′CoCoAr′ (2.008(3), 2.019(3)
Å),15 CoAr′2 (2.014(2) Å),43 and Ar′CoN(SiMe3)2 (1.9732(16)
Å).43 The Co(1)−F(1) bond length (1.896(2) Å) is shorter
than those in the fluoride salts [Co(Me)(F){C6H3-2-
(C6H5CO)-3-F}(PMe3)2] (1.9316(14) Å),54 [Co(Me)(F)-
{C6H3-2-(CHNNCH-2,6-F2C6H3)-3-F}(PMe3)2] (1.941(1)
Å),54 and {η3-HB-(3-tBu-pyrazolyl)3}CoF (2.060(3) Å),55
which is probably due to the lower coordination number at
cobalt in 4. The potassium ion weakly interacts with two carbon
atoms (C(16A) and C(17A)) from one flanking aryl ring of the
terphenyl ligand, as well as four fluorine atoms (F(5), F(1A),
F(1B), F(2A)). The K(1)···C(16A) (3.225(4) Å) and
K(1)···C(17A) (3.080(4) Å) distances are within the range of
K···C(arene) distances (3.028−3.303 Å) in the salt KAr′Ge-
GeAr′.56 The K···F distances are in the range of 2.501−3.060 Å.
With such K···F and K···C secondary interactions, the
K[(3,5-iPr2-Ar*)Co(F)(C6F5)] unit is repeated in three
dimensions in the solid state, thus forms an infinite
coordination polymer framework structure.
found for low-coordinate Co(II) complexes such as CoAr#
(
2
2.001(3) Å, Ar# = -C6H3-2,6-(C6H2-2,4,6-Me3)2),40 Ar′CoN-
(H)Ar# (1.977(1) Å),41 [BrCoAr#(THF)]2 (2.053(8) Å),42
[Li(OEt2)Ar′CoI2]2 (1.998(5) Å),29 CoAr′2 (2.014(2) Å),43
and Ar′CoN(SiMe3)2 (1.9732(16) Å).43 A feature of the
structures is that the difference in oxidation state between
Co(I) and Co(II) has only a minor effect on the Co−C( ipso)
bond distances. The almost linear C(ipso)−Co−centroid
arrangement in 1−3 may be contrasted with the bent geometry,
C(Ipso)−Co−centroid angle = 143.7(3)°, in Ar′CoCoAr′15 and
the similarly bent 140° angle for N−Co−centroid in κN,η6-
arene bonded LtBuCo.29
The formulation and solid-state structure of 4 was
determined by X-ray crystallography (Figure 5 and Table 2).
The Co(1) atom is in a three-coordinate planar coordination
environment (sum of interligand angles is 360.01°), and is σ-
bonded to a terphenyl ligand, a C6F5 group, and a fluorine
atom. Three-coordinate cobalt complexes are relatively rare,
and their synthesis and isolation requires the employment of
bidentate,13,14,44−48 tridentate,4,6 or bulky monodentate ligands.
Our group has reported several three-coordinate cobalt
compounds using monodentate ligands, such as bulky
silylamido or alkyloxo groups.49−53 Compound 4 differs in
that it contains three different monodentate ligands attached to
the cobalt atom. The wide C(1)−Co(1)−C(43) angle
(142.71(14)o) is probably due to the strong steric repulsion
between the very bulky terphenyl ligand and the C6F5 group.
The Co(1)−C(1) (2.007(3) Å) and Co(1)−C(43) (2.070(4)
Å) distances are similar to reported Co−C lengths in other aryl
CONCLUSIONS
■
In summary, compounds (3,5-iPr2-Ar*)Co(η6-arene) (arene =
C6H6 (2), C6H5F (3)) were synthesized by reduction of
[(3,5-iPr2-Ar*)Co(μ-Cl)]2 with KC8 in the presence of the
corresponding arene molecules. Both complexes were charac-
1
terized by UV−vis and H NMR spectroscopy, as well as
SQUID magnetic study, which confirm the high-spin d8
electron configuration for the cobalt centers. The solid-state
structures of these two compounds in comparison to that of
(3,5-iPr2-Ar*)Co(η6-C7H8) (1) indicate that 3 has the strongest
Co-η6-arene interaction among the three compounds. Fur-
thermore, attempts to reduce [(3,5-iPr2-Ar*)Co(μ-Cl)]2 with
KC8 under the same conditions in the presence of C6F6
afforded the unexpected three-coordinate Co(II) complex
cobalt species, such as CoAr# (2.001(3) Å),40 Ar′CoN(H)Ar#
2
(1.977(1) Å),41 [BrCoAr#(THF)]2 (2.053(8) Å),42 [Li(OEt2)-
1825
dx.doi.org/10.1021/ic202116s | Inorg. Chem. 2012, 51, 1821−1826

Inorganic Chemistry
Article
K[(3,5-iPr2-Ar*)Co(F)(C6F5)] (4), which arose from the
formal insertion of a 3,5iPr2-Ar*Co moiety into a C−F bond
of C6F6.
(24) Kleigrewe, N.; Steffen, W.; Blomker, T.; Kehr, G.; Frohlich, R.;
̈
̈
Wibbeling, B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. C. J. Am.
Chem. Soc. 2005, 127, 13955−13968.
(25) La Macchia, G.; Gagliardi, L.; Power, P. P.; Brynda, M. J. Am.
Chem. Soc. 2008, 130, 5104−5114.
ASSOCIATED CONTENT
■
(26) Wolf, R.; Brynda, M.; Ni, C.; Long, G. J.; Power, P. P. J. Am.
Chem. Soc. 2007, 129, 6076−6077.
S
* Supporting Information
1H NMR spectra and crystallographic information files (CIFs)
for 2−4. This material is available free of charge via the Internet
at http://pubs.acs.org.
(27) Ni, C.; Ellis, B. D.; Fettinger, J. C.; Long, G. J.; Power, P. P.
Chem. Commun. 2008, 1014−1016.
(28) Lei, H.; Ni, C.; Fettinger, J. C.; Long, G. J.; Power, P. P.
unpublished results.
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Cundari, T. R.; Holland, P. L. J. Am. Chem. Soc. 2011, 133, 12418−
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: pppower@ucdavis.edu.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-0948417)
for support of this work.
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