Tetraarylcyclobutadienecyclopentadienylcobalt complexes: Synthesis, electronic spectra, magnetic circular dichroism, linear dichroism, and TD DFT calculations

Article abstract of DOI:10.1021/om400403j

The known (tetraphenyl-η⁴-cyclobutadiene)- η⁵-cyclopentadienylcobalt (1) and a series of its new substituted derivatives have been prepared. The electronic states of a few representatives have been characterized by absorption and magnetic circular dichroism. Time-dependent density functional theory has been used to arrive at spectral assignments for several prominent low-energy bands. The absorption spectra of the radical ions of 1 have also been recorded.

Full text of DOI:10.1021/om400403j

Article
pubs.acs.org/Organometallics
Tetraarylcyclobutadienecyclopentadienylcobalt Complexes:
Synthesis, Electronic Spectra, Magnetic Circular Dichroism, Linear
Dichroism, and TD DFT Calculations
Gregg S. Kottas,† Thierry Brotin,† Peter F. H. Schwab,† Kamal Gala,† Zdenek Havlas,‡ James P. Kirby,§
̌
John R. Miller,§ and Josef Michl*,†,‡
†Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States
‡Institute of Chemistry and Biochemistry, Academy of Science of the Czech Republic, 16610 Prague 6, Czech Republic
§Chemistry Department, Brookhaven National Laboratory, Building 555A, Upton, New York 11973, United States
S
* Supporting Information
ABSTRACT: The known (tetraphenyl-η4-cyclobutadiene)-η5-cyclopentadienylcobalt (1) and a series of
its new substituted derivatives have been prepared. The electronic states of a few representatives have
been characterized by absorption and magnetic circular dichroism. Time-dependent density functional
theory has been used to arrive at spectral assignments for several prominent low-energy bands. The
absorption spectra of the radical ions of 1 have also been recorded.
The primary purpose of this paper is to report the synthesis
of additional symmetrical and unsymmetrical derivatives of 1.
In addition, we illustrate the excited states of a few of them by
electronic spectroscopy, magnetic circular dichroism, linear
dichroism, and DFT calculations. Since in molecular electronics
electron transfer to or from a building unit is often of interest,
we have also obtained the spectra of the radical ions of 1.
INTRODUCTION
■
For some time, we have been interested in the development of
a molecular Tinkertoy1 construction set permitting nanometer-
scale assembly of rods2,3 and connectors,4,5 especially for use in
the construction of molecular rotors.6−12 For a project in
molecular electronics, we sought rigid cross-shaped connectors
with tunable and reversible redox properties that could be easily
substituted at different positions for coupling into a grid.
(Tetraaryl-η4-cyclobutadiene)-η5-cyclopentadienylcobalt
(Ar4CbCoCp) complexes13,14 are an interesting class of
molecules for this purpose, due to their thermal and air
stability and resistance to a wide range of chemical reagents.
They are colored, but little is known about their excited states.
CbCoCp complexes have applications as building blocks in
oligomers15 and polymers,16−18 as catalysts,19−35 and as
structural units in other systems.36−43 Structural analyses of
such complexes have shown the Cb and Cp rings to be parallel
and the Cb ring to be square,44−47 making them interesting
connectors. In addition, the aryl rings on both rings can be
substituted, providing a broad synthetic scope. The parent
complex, Ph4CbCoCp (1), is readily available by the reaction of
two equivalents of diphenylacetylene (tolane) and cyclo-
pentadienylcobalt(I) dicarbonyl in xylenes.48−50 A variety of
4,4′-diphenylacetylenes have been found to give the corre-
sponding (p-RPh)4CbCoCp complexes47 (R = Br, I, COOEt, p-
CB10H10CH). More recently, several groups have reported the
RESULTS
■
Synthesis. Tolanes. Typically, tolanes (diphenylacety-
lenes) are synthesized by coupling an aryl halide with acetylene
or its derivative, using the procedures developed by
Sonogashira and Hagihara.58−62 4-Iodoaniline was converted
to 4-bromoiodobenzene on a 150 g scale by a Sandmeyer
reaction and into 4-iodo-N,N-dimethylaniline by phase-transfer-
catalyzed methylation, avoiding quaternization.63,64
Scheme 1 shows the preparation of the symmetrical tolanes
by the coupling of two aryl iodide derivatives with acetylene in
the presence of a palladium catalyst and copper cocatalyst and
triethylamine as a base, giving tolanes 3−6 in 31−83% yield.
Scheme 1
synthesis of such complexes using microwave heating.51,52
A
number of groups have also shown that substitution on the Cp
ring is possible.47,49,50,53−56 In general, most reports in the
literature concern the synthesis of symmetrically substituted
Ar4CbCoCp complexes, in which all four aryl groups are
equivalent. Unsymmetrical complexes have been studied much
less52,57 but promise potentially interesting electronic and
structural properties.
Received: May 7, 2013
© XXXX American Chemical Society
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The tolane 5 was also obtained in higher yield (86%) by
coupling with commercially available 4-ethynyltoluene.
Scheme 5
The low yield (31%) of 4,4′-N,N-dimethylaminodiphenyla-
cetylene (4) was observed even at high catalyst loadings and
long reaction times. We therefore abandoned the palladium
coupling in favor of the higher yielding and less time-
consuming conversion of 4,4′-bis(N,N-dimethylamino)-
benzophenone (Michler’s ketone) to 4 via a vinylidene
rearrangement, which proceeds in 85% yield (Scheme 2).65,66
Scheme 6
Scheme 2
The unsymmetrical tolanes were prepared by coupling the
corresponding aryl iodide to 4-ethynylbromobenzene as shown
in Scheme 3. Tolanes 7−9 were prepared in 84−93% yield.
as shown in Scheme 5. However, when tolanes 4 and 6 reacted
in the correct proportions, only the asymmetrical product 16b
was formed in an isolated yield of 48%. The complex 16a was
formed independently by reaction of two equivalents of 4 with
CpCo(CO)2, and 1 was synthesized according to the literature
procedure50 for use as a model system (Scheme 6).
Scheme 3
Finally, tolane 4 was cyclized with 4,4′-bipyridylacetylene to
form a mixture of the symmetrically substituted product 17a
(11%), the unsymmetrical cis complex 17c (17%), and the
carbonyl insertion product 17b (35%)47 (Scheme 7; complex
17d was not observed and has previously been shown to form
only in low yield47). Cobalt complex 17c was difficult to purify
but was oxidized with dimethyldioxirane to the N-oxide 18 in a
nearly quantitative yield, and the latter was obtained analytically
pure.
Electronic Spectra. We have chosen three symmetrically
(1, 17d, 16a) and two unsymmetrically (16b, 18) substituted
cobalt complexes to illustrate the UV−visible and magnetic
circular dichroism (MCD) spectra (Figures 1−5, respectively,
and Table 1) of these compounds. For the parent system 1 we
also measured linear dichroism (LD) of a sample aligned in
stretched polyethylene, and the absorption spectra of its radical
cation 1•+ and radical anion 1•− were obtained by pulse
radiolysis (Figure 6). A detailed spectroscopic and theoretical
study of all the new compounds lies beyond the scope of this
paper, which focuses on their synthesis.
All the complexes listed above show three or four major
absorption bands in the UV−vis region with high absorption
coefficients (εmax = 20 000−55 000 M−1 cm−1). For the parent
complex 1, the maximum occurs at 31 800 cm−1 (εmax = 17 600
M−1 cm−1). The presence of four dimethylamino groups (16a)
produces a small red shift to 31 500 cm−1 and triples the molar
absorptivity (53 400 M−1 cm−1). Replacing the four phenyl
units by pyridine rings (17d) shifts the principal peak to 33 500
cm−1 (εmax = 22 700 M−1 cm−1). The MCD spectra are
considerably more structured and leave little doubt that a fair
number of electronic transitions are present. The results of TD-
DFT calculations indeed indicate the presence of a large
number of distinct transitions. The transition moments
obtained from these calculations mostly fall into two categories:
those oriented approximately along an axis z that connects the
centers of the four-membered and the five-membered rings and
passes through or close to the cobalt atom (“z polarized”) and
When using the trimethylsilyl-protected phenol, in situ
hydrolysis gave 10 in 65% yield. The product 10 was
subsequently converted to the isopropoxy derivative 11 in
86% yield (Scheme 3).
Cobalt Complexes. The tolane derivatives were converted
into (tetraaryl-η4-cyclobutadiene)-η5-cyclopentadienylcobalt
(Ar4CbCoCp) complexes using the procedure developed by
Rausch and co-workers49,50 with two equivalents of the tolane
cyclizing to form the cyclobutadiene ring in the presence of
CpCo(CO)2 (Schemes 4−7). Complexes 12−15 were
prepared from the unsymmetrical tolanes 7−11, respectively,
permitting the formation of cis and trans isomers. For
complexes 14 and 15, both isomers were isolated, but in the
case of 12 and 13, only the trans isomer (12a and 13a52) was
isolated. Mixtures of two symmetrical tolanes can give three
products (two symmetrical and one asymmetrical, the cis form)
Scheme 4
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Scheme 7
Figure 2. Optical absorption and MCD spectra of 17d recorded in
DCM. Bottom: Absorption overlaid on TD-DFT(B3LYP/DZP)-
calculated oscillator strengths (blue: z polarized; orange: x,y polarized)
and simulated spectra (black dashed line: B3LYP/DZP, compared
with B3LYP/TZP, green line). The simulated spectra are scaled by a
factor of 0.4. Top: MCD spectrum overlaid on TD-DFT(B3LYP/
DZP)-calculated B terms (blue: z polarized; orange: x,y polarized) and
B3LYP/DZP-simulated MCD spectrum (black dashed line). The
simulated spectra are scaled by a factor of 0.75.
Figure 1. Optical spectra of 1. Bottom: Absorption in THF overlaid
over TD-DFT(B3LYP/DZP)-calculated oscillator strengths (blue: z
polarized; orange: x,y polarized) and simulated spectra (black dashed
line: B3LYP/DZP, compared with B3LYP/TZP, green line). The
simulated spectra are scaled by a factor of 0.25. Center: MCD
spectrum in THF overlaid over TD-DFT(B3LYP/DZP)-calculated B
terms (blue: z polarized; orange: x,y polarized) and B3LYP/DZP-
simulated MCD spectrum (black dashed line), compared with
simulated B3LYP/TZP spectrum (green line) for limited number of
states. The simulated spectra are scaled by a factor of 0.5. Top:
Polarized absorption spectra in stretched polyethylene measured
parallel (EZ) and perpendicular (EY) to the stretching direction Z.
Linear dichroism of 1 in stretched polyethylene shows that
the transition moments of the three high-energy bands (Table
1, Figure 1) are preferentially aligned with the stretching
direction, while that of the band at 31 800 cm−1 is oriented
perpendicular to it.
Pulse Radiolysis. The radical cation and anion of 1 were
formed by standard pulse radiolysis techniques,67 and their
visible and near-infrared spectra are shown in Figure 6. The
cations were formed in 1,2-dichloroethane and the anions in
THF. The oxidized form shows transitions at ∼17 500 cm−1
(εmax = 10 600 M−1 cm−1) and 14 500 cm−1 (εmax = 7700 M−1
those oriented along a direction that lies approximately
perpendicular to z (“x,y polarized”). However, certain transition
moments lie at a direction intermediate between these two.
cm−1), with a weak transition just below 11 400 cm−1 (εmax
=
400 M−1 cm−1). The radical anion shows less intense
transitions with a shoulder barely in the observation window
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Figure 3. Optical absorption and MCD spectra of 16a recorded in
THF. Bottom: Absorption overlaid on TDDFT(B3LYP/DZP)-
calculated oscillator strengths (blue: z polarized; orange: x,y polarized,
green: mixed polarization) and simulated spectra (black dashed line:
B3LYP/DZP, compared with B3LYP/TZP, green line). The simulated
spectra are scaled by a factor of 0.4. Top: MCD spectrum overlaid on
TD-DFT(B3LYP/DZP)-calculated B terms (blue: z polarized; orange:
x,y polarized, green: mixed polarization) and B3LYP/DZP-simulated
MCD spectrum (black dashed line). The simulated spectra are scaled
by a factor of 1.3.
Figure 5. Optical absorption and MCD spectra of 18 recorded in
MeCN. Bottom: Absorption overlaid on TDDFT(B3LYP/DZP)-
calculated oscillator strengths (blue: z polarized; orange: x,y polarized,
green: mixed polarization) and simulated spectra (black dashed line:
B3LYP/DZP, compared with B3LYP/TZP, green line). The simulated
spectra are scaled by a factor of 0.4. Top: MCD spectrum overlaid on
TD-DFT(B3LYP/DZP)-calculated B terms (blue: z polarized; orange:
x,y polarized, green: mixed polarization) and B3LYP/DZP-simulated
MCD spectrum (black dashed line). The simulated spectra are scaled
by a factor of 0.5.
Figure 6. Optical absorption spectra of the radical ions of 1 recorded
∼100 ns after pulse radiolysis of 1 in THF to form the anion or in 1,2-
dichloroethane to form the cation. The lines represent interpolations
between the measured points. The spectrum of cobalticenium in
acetonitrile is shown for comparison.
that gave satisfactory results for the uncharged closed-shell
systems failed.
Figure 4. Optical absorption and MCD spectra of 16b recorded in
MeCN. Bottom: Absorption overlaid on TDDFT(B3LYP/DZP)-
calculated oscillator strengths (blue: z polarized; orange: x,y polarized,
green: mixed polarization) and simulated spectra (black dashed line:
B3LYP/DZP, compared with B3LYP/TZP, green line). The simulated
spectra are scaled by a factor of 0.5. Top: MCD spectrum overlaid on
TD-DFT(B3LYP/DZP)-calculated B terms (blue: z polarized; orange:
x,y polarized, green: mixed polarization) and B3LYP/DZP-simulated
MCD spectrum (black dashed line). The simulated spectra are scaled
by a factor of 0.9.
Molecular Orbitals. The frontier orbitals of 1, calculated at
the optimized geometry by the DFT-B3LYP/TZP method, and
their energies are given in Figure 7 and in the Supporting
Information. The orbitals shown are those that contribute to
configurations present with a weight of at least 10% in one of
the low-energy electronic transitions discussed below. Although
the molecule is not symmetric, one-half having a local 5-fold
axis and the other half a local 4-fold axis, and the phenyl
substituents on the cyclobutadienyl ring being twisted by about
40°, the molecular orbitals are close to symmetric and the metal
d orbitals form approximate symmetry sets in a way typical for
C5v transition metal complexes. The HOMO−4 is a mixed Cb/
metal d (z2) orbital of approximate “a” symmetry, the nearly
at 18 100 cm−1 (εmax = 7700 M−1 cm−1) and another broad
transition centered at ∼13 200 cm−1 (εmax = 4400 M−1 cm−1).
An attempt to reproduce the spectra at the same TDDFT level
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Table 1. UV−Vis and MCD Transitions of Cobalt Sandwich Complexes
a
b
c
d
R1−R4 are the functional groups in the para positions of the four phenyl substituents in 1, numbered in clockwise order. In THF. In CH2Cl2. In
CH3CN.
degenerate (almost e symmetry) pair HOMO−3 and HOMO−
2 are virtually pure metal d orbitals (xy and x2 − y2), and the
essentially exactly degenerate (nearly “e” symmetry) pair
HOMO−1 and HOMO are mixed Cp/Cb/d (xz and yz)
orbitals. The Cb contribution has the form of the Cb
nonbonding orbital. The almost exactly degenerate LUMO
and LUMO+1 pair is well described as the nonbonding orbital
pair of Cb, with very small contributions from Cp and metal d
(xz and yz) orbitals.
Electronic Transitions. Electronic absorption spectra were
calculated at ground-state DFT-B3LYP/TZP-optimized geo-
metries by the TDDFT method using DZP and TZP Slater-
type basis sets. The calculated vertical excitation energies and
oscillator strengths for 1, 16a, 16b, 17d, and 18 are depicted in
Figures 1−5.
transitions are well described as metal d to Cb (LUMO,
LUMO+1) transitions of very low intensity. Three of them are
composed of pairs of nearly exactly degenerate excitations,
polarized equatorially (in the Cb and Cp planes), and are
stronger than the remaining three. They correspond to
transitions from the xy and x2 − y2 orbitals and are calculated
at 19 930, 23 210, and 28 710 cm−1. The first really intense
transitions are calculated to lie just above 32 000 cm−1. One is
an equatorially polarized pair of excitations into the almost
exactly degenerate states produced by electron promotion from
the HOMO pair (HOMO, HOMO−1) to an orbital LUMO+2
that looks like the antibonding orbital of Cb but is also
delocalized onto the attached phenyl rings. The second intense
transition in this region is the nondegenerate combination of
approximately z (axially) polarized excitations from the HOMO
pair to the LUMO pair. Above these three intense transitions,
many weak transitions are calculated. The next intense
transitions are predicted near 38 000 cm−1 and represent
excitation from metal d orbitals to orbitals located on Cb and
The B3LYP/DZP results of calculations of the spectrum of 1
are presented in more detail in Table 2. Like the molecular
orbitals, the transitions, their oscillator strengths, and the
dominant configurations of the excited states reflect the
approximate symmetry of the molecule. The first nine
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oscillator strength of the x,y-polarized pair of excitations
predicted at about 27 000 cm−1. Inspection of the orbitals
involved in these transitions shows significant charge transfer
from amino groups and from d orbitals of Co into the nitro
groups. The calculated z-polarized transition of the parent 1 is
difficult to trace due to a large mixing caused by the asymmetric
substitution.
DISCUSSION
■
Synthesis. Diphenylacetylene and its derivatives (tolanes)
have been known for almost a hundred years, but it was not
until the development of Hagihara−Sonogashira coupling
conditions58−62 that a relatively cheap and functional group
tolerant approach for their synthesis became available. Since
aryl iodides undergo oxidative addition to palladium much
faster than aryl bromides, we exclusively used the iodides. While
many are commercially available, 1-bromo-4-iodobenzene and
4-iodo-N,N-dimethylaniline were prepared in high yield on
multigram scale from 4-iodoaniline (Scheme 1), the former via
a Sandmeyer reaction and the latter via methylation under
phase-transfer-catalysis conditions, which avoided quaternary
byproducts.63,64
Most tolanes were prepared in high yield using the palladium
coupling procedures developed by Hagihara and Sonogashira.
As shown in Scheme 1, tolanes 3, 5, and 6 formed in yields of
∼80% or higher by reaction of two equivalents of the
corresponding aryl iodide with acetylene gas in the presence
of PdII and CuI, with all reactions performed on multigram
scale. A slightly higher yield (86%) of 5 was obtained when
using commercially available 1-ethynyl-4-methylbenzene with
1-iodo-4-methylbenzene (Scheme 1). However, in the case of
dimethylaminotolane 4, the coupling of 4-iodo-N,N-dimethy-
laniline with acetylene gas gave poor yields and was abandoned.
Another procedure, which relies on a vinylidene to alkyne
rearrangement, was developed by Savignac and co-workers
(Scheme 2).65,66 Using the Horner modification of the Wittig
reaction,68−70 benzophenone derivatives can be converted to
1,1-dichloro-2,2-diarylalkenes by treatment with a phospho-
nate.71 Reaction of the gem-dichloride with a lithium reagent
generates the vinylidene, which rearranges to give the tolane.
Unsymmetrical tolanes have been studied less7,72,73 and can
be prepared by coupling an appropriate arylacetylene with an
aryl iodide as shown in Scheme 3. Here we employed 4-
bromophenylacetylene74 to prepare a number of unsymmetrical
tolanes (7−10) in good to excellent yields. Coupling of 4-
bromophenylacetylene with the protected 4-trimethylsilylox-
yiodobenzene led to in situ deprotection to give tolane 10,
which in this case was desirable, as 10 was alkylated with
isopropyl bromide to give 11.
As mentioned above, cyclization of the unsymmetrical
tolanes in the presence of CpCo(CO)2 gives two isomers of
the corresponding sandwich complexes, cis and trans with
respect to the disposition of the substituent groups (Scheme 4).
In the case of unsymmetrical tolanes 7 and 8, only the trans
forms 12a and 13a were isolated (we were able to isolate but
not fully purify small quantities of 12b and 13b). When tolanes
9 and 11 were cyclized, a slight excess of the cis (14a and 15a)
over the trans (14b and 15b) form was formed. The lower
yields were due to difficulties in separating the isomers with
flash chromatography due to their often nearly identical
retention times and the resulting need for multiple chromato-
graphic separations (see Experimental Section).
Figure 7. B3LYP/DZP frontier orbitals of 1.
its phenyl substituents. Most are nearly degenerate and
approximately x,y (equatorially) polarized.
Dimethylamino substitution in the para positions of the four
phenyl groups (16a) does not affect the excitation energy of the
first observed absorption band noticeably, but its intensity is
almost tripled. This increase is well reproduced by the
calculations. In the parent 1 the first intense calculated band
consists of three excitations of almost identical energy, one
polarized along z and two along x,y. In 16a, the z-polarized
component has a similar excited-state composition and
oscillator strength as in 1 and is red-shifted by ∼1500 cm−1.
The oscillator strength of the pair of almost degenerate x,y-
polarized transitions is tripled relative to the parent 1, and the
excited state contains one additional important configuration.
This corresponds to an electron promotion from the orbital
HOMO−2 of 16a, with a large amplitude on the amino
nitrogen atoms (Supprting Information, Figure S2), to the
LUMO pair and transfers electron density from the amino
groups to Cb, Co, and Cp, roughly equally.
The substitution of parent 1 by donor and acceptor groups,
dimethylamino and nitro (16b), destroys significantly its
original approximate symmetry. This is reflected in the mixed
polarization of most calculated transitions. The observed
intensity of the band near 30 000 cm−1 is 1.5 times higher,
and this is reproduced by the calculated increase of the
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Table 2. TDDFT (B3LYP/DZP)-Calculated Vertical Transitions in 1
dominant
configurations
dominant
b
a
b
a
ν
̃
/cm−1
f
B term
polarization
ν
̃
/cm−1
f
B term
configurations
polarization
1
2
3
19 932
19 936
23 205
0.0029
0.0029
0.0058
1.5972 3→−1, 4→−2
1.5993 3→−2, 4→−1
x,y
28
29
39 236
39 720
0.0008
0.0004
2.7667 3→−4
z
z
2.6604 5→−3, 12→−1,
13→−2
−2.0614 4→−1, 3→−2,
x,y
10→−1
30
31
40 294
40 334
0.0000
0.0001
−0.3506 4→−4
z
z
4
23 216
0.0058
−2.0567 4→−2, 3→−1,
10→−2
−0.8042 12→−2, 13→−1,
8→−1, 7→−2
5
6
7
8
24 184
25 615
26 957
28 709
0.0001
0.0000
0.0001
0.0059
0.7686 1→−2, 2→−1
0.9628 2→−1, 1→−2
1.1239 1→−1, 2→−2
z
32
33
34
40 715
40 722
40 781
0.0196
0.0199
0.0005
−1.0367 6→−1, 1→−8
−1.1156 6→−2, 2→−8
−0.5867 5→−3, 12→−1,
13→−2, 3→−5,
4→−4
x,y
z
z
z
−0.8210 5→−1, 10→−1,
x,y
4→−1
35
36
41 100
41 210
0.0003
0.0163
−0.7315 3→−5
z
z
9
28 717
32 142
0.0058
0.1868
−0.8045 5→−2, 10→−2,
4→−2
−0.6299 8→−2, 7→−1,
1→−9
10
−5.8507 2→−2, 1→−1,
z
4→−3
37
38
41 415
41 444
0.0030
0.0037
−2.8706 7→−2, 8→−1
z
11
12
13
14
15
16
32 271
32 283
33 874
35 217
35 233
36 822
0.1848
0.1832
0.0084
0.0073
0.0073
0.0237
10.2793 1→−3
10.2508 2→−3
1.1501 3→−3, 4→−3
−2.0264 1→−4
−2.0034 2→−4
x,y
−2.2830 3→−7, 3→−6,
x,y
3→−5
39
40
41
42
41 453
41 581
41 772
41 838
0.0042
0.0001
0.0029
0.0275
−0.6026 3→−6, 3→−7
0.9520 8→−1, 7→−2
1.4403 1→−9, 7→−1
z
z
z
z
x,y
1.3164 2→−10, 1→−9,
18.3868 4→−3, 1→−7,
z
8→−2
2→−6
43
41 993
0.0282
1.6490 2→−10, 11→−1,
7→−1, 8→−2,
1→−9
6.1265 11→−1
7.2710 11→−2
0.4604 4→−5
z
17
18
37 142
37 482
0.0002
0.1422
0.3158 1→−7, 2→−6,
1→−6, 2→−7
z
−10.8875 4→−3, 1→−7,
z
44
45
46
47
42 016
42 019
42 253
42 286
0.1100
0.1328
0.0035
0.0168
x,y
2→−6
19
20
21
37 589
37 604
37 636
0.0739
0.0867
0.0401
−8.4849 1→−5
−9.7680 2→−5, 2→−6
x,y
z
2.4497 9→−1, 4→−7,
x,y
−3.9135 1→−6, 2→−7,
z
10→−1, 4→−6
1→−7
48
49
50
42 294
42 448
42 833
0.0154
0.0005
0.0150
2.2386 9→−2, 4→−6,
4→−7,10→−2
22
23
37 680
38 396
0.0064
0.0026
−0.7898 2→−7, 2→−6,
1→−7 1→−6
z
−0.6079 2→−9, 1→−10,
z
0.6140 13→−1, 12→−2,
z
4→−5
12→−1, 13→−2
0.9520 4→−7, 4→−6,
x,y
24
25
26
38 891
38 903
39 131
0.0784
0.0769
0.1087
−14.4975 1→−8, 5→−1
−15.1171 2→−8, 5→−2
x,y
9→−2, 4→−5
a
b
In units of 10−5 D2βe/cm−1. Dominant configurations with weight
more than 10% (bold: weight more than 25%). 1 = HOMO, −1 =
LUMO, 2 = HOMO−1, −2 = LUMO+1, etc.
7.7446 1→−8, 5→−2,
x,y
10→−1
27
39 145
0.1027
9.8763 2→−8, 5→−2,
10→−2
For the symmetrical tolanes, no isomers are possible, but
three different products can be formed: the two complexes
obtained from cyclization of the same tolane and the cross-
product formed by the coupling of different tolanes (Scheme
5). For these cyclizations with CpCo(CO)2, the reactivity of
the tolane must be taken into consideration, as those with
electron-withdrawing groups tend to be less reactive. Therefore,
the reactant ratio must be changed to optimize the conditions
under which the mixed cobalt complex forms in highest yield.
In the case of 16b, it was found that three equivalents of the
dinitrotolane 6 for every equivalent of the dimethylaminotolane
4 were necessary to optimize the yield. As shown in Scheme 5,
the coupling reaction proceeds in a 48% yield, without
observation of either symmetrically substituted cobalt complex.
It should be noted that tetrakis(N,N-dimethylaminophenyl)-
CbCoCp (16a) is observed when the ratio of the tolanes
deviates from three-to-one. However, the tetrakis(nitrophenyl)-
CpCoCp complex (16c) is never observed under the reaction
conditions, and unreacted dinitrotolane 6 could be recovered
after the reaction. Compound 16a was also synthesized
independently by the reaction of two equivalents of 4 with
CpCo(CO)2 in 56% yield, as shown in Scheme 6.
The products from the reaction of two equivalents of 4,4′-
dipyridylacetylene with one equivalent of dimethyltolane 5 and
one equivalent of CpCo(CO)2 were mostly the cyclo-
pentadienone complex 17b (35%) and little of the desired
compound 17c (17%). Brotin et al.47 have previously shown
that treating two equivalents of 4,4′-dipyridylacetylene with
CpCo(CO)2 in refluxing xylenes gives a low yield (15%) of the
tetrapyridyl complex 17d along with a significant amount
(50%) of the tetrapyridylcyclobutadienone complex 17b.
Complex 17d was not observed in our reaction mixture,
while the tetratolylcyclobutadienylcyclopentadienecobalt com-
plex 17a was formed in an 11% yield (Scheme 7). Attempts to
optimize the formation of 17c over the formation of 17b failed
to increase the yields significantly. The use of CpCo-
(cyclooctadiene), devoid of a carbonyl group that could insert
into the cyclobutadiene ring, did not produce any reaction.
Likewise, attempts to purge CO from the reaction mixture with
bubbling argon did not significantly decrease the amount of
17b formed. Therefore, the carbonyl insertion product was
deemed to be a more stable electronic structure unique in the
presence of the pyridine groups. Rausch and Genetti50 had
previously reported the formation of the tetraphenylcyclopen-
G
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tadienone CoCp complex in the cyclization of diphenylacety-
lene, but in much lower yields (<10%). Due to difficulties in
our attempts to purify 17b, it was converted to the N-oxide
with dimethyldioxirane in nearly quantitative yield (Scheme 7)
to give compound 18.
linear dichroism if the molecules orient in the expected fashion.
We thus conclude that they apparently do and that in the lower
energy band near 32 000 cm−1 the axially polarized transition is
actually stronger than the overlapping equatorially polarized
ones. In the 40 000 cm−1 region, substituents modify the
spectra more strongly, and it is difficult to draw simple
conclusions.
In summary, it appears that in the lower energy region the
TDDFT calculations provide a quite reasonable description of
the energies and relative intensities of electronic excitations in
1, although the finer details of substituent-induced shifts are not
reproduced well or at all. Given the large number of transitions
present, the MCD spectra that reflect their mixing by the
magnetic field are obviously quite challenging to reproduce
computationally. The DFT procedure used, which is the best
available at the moment for molecules of this size, is not up to
the task, although in certain parts of some of the spectra some
agreement can be detected.
The visible absorption spectra of the radical ions of 1 (Figure
6) are relatively featureless. Both the cation and anion have
absorptions with extinction coefficients ε of several thousand in
the visible region, in contrast to the case of cobalticenium,
which has almost no significant absorption in the visible. We
expect the bands of the radical ions of 1 to be useful for the
detection of charge transfer events if 1 finds use as a connector
or as an active redox element in studies directed at molecular
electronics.
Electronic Spectra. The absorption spectra shown in
Figures 1−5 show remarkable similarity, and only that of 16b
differs significantly from the others. We focus primarily on the
absorption and MCD spectra of the parent complex 1.
In the low-energy region, they agree remarkably well with
expectations from the TDDFT calculations. Below 30 000
cm−1, where three moderately weak equatorially polarized
nearly degenerate transitions and three extremely weak axially
polarized transitions are calculated and the absorption curve
shows only an indistinct shoulder, the MCD curve is very
helpful. It shows a negative B term and two positive B terms at
the expected locations, easily assignable to the three predicted
transitions. A nearly degenerate transition could also exhibit an
apparent A term, but those seem to be weak since there is no
indication of any in the spectral curve. The agreement with the
DFT-calculated B terms is poor.
The linear dichroism of these three transitions could not be
measured reliably, and we were unable to obtain an
independent confirmation of their equatorial (x,y) polarization.
Nevertheless, it seems that these electron promotions from
metal d orbitals (xy and x2 − y2) to the nonbonding orbitals of
the Cb ligand are correctly described by the TDDFT
calculation. The same three transitions appear in the MCD
spectra of the substituted derivatives, except for 18, where both
calculations and spectra indicate a more complicated situation,
with a larger number of transitions present.
SUMMARY
■
Efficient synthetic procedures have been described for
derivatives of 1 substituted symmetrically or unsymmetrically
in the phenyl rings, and an initial examination of their
electronically excited states has been performed. Some
information on the low-energy excited states of the parent 1
and their response to substitution has been obtained by a
combination of time-dependent density functional theory with
absorption, linear dichroism, and magnetic circular dichroism
spectroscopy on the parent and four of its derivatives. An
attempt to reproduce the absorption and MCD spectra by DFT
computations has met with only partial success, particularly in
the case of MCD. The absorption spectra of the radical ions of
1 have been recorded.
The first intense band at 32 000 cm−1 looks deceptively
simple, and only the spectra of some of the derivatives,
especially 18, suggest that it contains more than one transition.
According to the calculations, it should contain one nearly
degenerate pair of equatorially polarized transitions plus
another transition at nearly the same spectral location,
polarized axially, all of comparable intensity. Its weakly negative
linear dichroism (EZ < EY) suggests that either the molecule of
1 orients in an unexpected way in stretched polyethylene, given
its shape,75 or the axially polarized transition is actually stronger
than the equatorially polarized ones. In any event, the weakness
of the LD signal agrees with expectations. Even if the molecule
oriented well, the presence of overlapping transitions of both
polarizations would cause LD to be weak. The shape of the
MCD curve is compatible with a negative A term overlapped at
the high-energy end with a negative B term, and this would be
in perfect accord with the calculations, but the actually
predicted spectral shape does not agree with the observed
spectrum. The pseudo-A term of the nearly degenerate
transition should be negative76 if the pair of transitions can
indeed be described as a promotion of an electron from the
nearly degenerate (HOMO, HOMO−1) orbital pair, the
nonbonding orbital on Cb, into the antibonding orbital on
Cb, as the TDDFT calculation suggests.
EXPERIMENTAL SECTION
■
Measurements. Most measurements were conducted with
standard commercial instruments as detailed in the Supporting
Information. Linear dichroism spectra were measured in stretched
polyethylene sheets with a JASCO J-720 spectropolarimeter in the
linear dichroism mode. Polyethylene sheets were cut from commercial
polyethylene bags and cleaned by soaking in chloroform for several
hours before washing with clean chloroform and drying. The clean
sheets were stretched in a homemade stretcher to approximately five
times their original length. The sheet was doped by placing a drop of a
saturated sample solution in chloroform on the stretched sheet in a
closed, chloroform-saturated environment. After 1 h, the sheet was
removed and the chloroform was allowed to evaporate. Excess sample
that was not absorbed into the polymer was washed from the sheet
with methanol, and the sheet was dried. The doped, stretched sheet
was placed in the spectrometer in an orientation such that the
photoelastic modulator produced light that alternated between
polarizations parallel and perpendicular to the stretching direction.
As a baseline, an identical stretched sheet without incorporation of the
sample, but otherwise handled in the identical manner, was measured
before measurement of the doped sheet.
The next intense band, located near 40 000 cm−1, shows
evidence of its complex nature in both the absorption and the
MCD spectra. Calculations suggest that the lower energy peak
is due to the overlap of three intense and several other
transitions, and the higher energy shoulder is due primarily to
one transition, with many others contributing weakly. The
calculated polarization of all the intense transitions in this
region is equatorial, and this agrees with the observed positive
H
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Pulse Radiolysis. These experiments were done at the Brookhaven
National Laboratory. Solutions of 0.7 μM of cations in 1,2-
dichloroethane or anions in THF were produced by pulse radiolysis.
About 0.7 mM solutions of 1 in 1,2-dichloroethane (cations) or THF
(anions) were exposed to <50 ps pulses of 9 MeV electrons at the
LEAF accelerator, and the spectra were recorded 100 ns after the
pulse. The uncertainty in the scale factors used to obtain molar
extinction coefficients from the observed absorbances is 30%.
Computations. Calculations were performed by the DFT-B3LYP
method using the program ADF, version 2013.77 The internally stored
Slater-type basis sets78 DZP (excitation spectra, MCD) and TZP
(geometry optimization, in selected calculations of excitation spectra
and MCD for comparison) were used. Description of the B3LYP
funtional79−81 is available. Geometry optimizations were performed
without constraints. Electronic excitation was calculated using the
time-dependent DFT method.82 MCD B terms83 were calculated for
the lowest 50 states by the iterative conjugate gradient procedure.
Excitation and MCD spectra were simulated by a home-developed
code, using Gaussian shape functions with square-root band
broadening (wi = w√(vi/30 000), w = 3000 cm−1). The orbitals
were plotted using ADF-GUI.
showed complete conversion of the starting material. The reaction
mixture was then filtered, and the solid residue thoroughly rinsed with
toluene. The filtrate was washed with dilute NaOH (2 M, 3 × 50 mL).
The combined organic layers were dried over sodium sulfate, and the
solvent was evaporated. The solid residue was dissolved in 2 M
hydrochloric acid (150 mL) and extracted with hexanes (3 × 50 mL).
Then, concentrated NaOH was added to the aqueous layer until the
solution was strongly basic and a white precipitate appeared. After
extraction with hexanes (3 × 100 mL), the combined organic layers
were dried over sodium sulfate and the solvent was evaporated to yield
18.5 g (75 mmol, 82%) of 2 as a white solid. Mp: 64 °C (lit.88 66−67
°C). 1H NMR (CDCl3, 300 MHz): δ 2.90 (s, 6 H, CH3), 6.47 (dt, J =
9.0 Hz, J = 2.1 Hz, 2 H, Ar), 7.54 (dt, J = 9.0 Hz, J = 2.1 Hz, 2 H, Ar).
4,4′-Dibromodiphenylacetylene (3). 1-Bromo-4-iodobenzene (125
g, 0.44 mol), CuI (265 mg, 1.1 mmol), and Pd(PPh3)2Cl2 (1.05 g, 1.1
mmol) were charged into a dry 1 L three-necked flask under an argon
atmosphere. Freshly distilled triethylamine (600 mL) was added from
a syringe, and the solution was cooled to 10 °C with an ice/water bath.
Then a steady stream of acetylene was bubbled through the solution
1
until H NMR showed complete conversion of the starting material
after 3 h. The reaction mixture was poured into 4 M hydrochloric acid
(500 mL) and extracted with chloroform (4 × 200 mL). After drying
over sodium sulfate, the solvents were evaporated and the crude
product was flash chromatographed (silica gel, chloroform/hexane,
1:3). 4,4′-Dibromodiphenylacetylene (3) was recrystallized from
chloroform/hexanes: 59.8 g (0.18 mol, 81%). Mp: 181 °C (lit.89
182−184 °C). 1H NMR (CDCl3, 300 MHz): δ 7.35 (dt, J = 8.7 Hz, J
= 1.8 Hz, 4 H, Ar), 7.47 (dt, J = 8.7 Hz, J = 1.8 Hz, 4 H, Ar).
4,4′-N,N-Dimethylaminodiphenylacetylene (4). 4-Iodo-N,N-dime-
thylaniline (2) (2.1 g, 8.5 mmol), CuI (26 mg, 0.13 mmol), and
Pd(PPh3)2Cl2 (100 mg, 0.13 mmol) were charged to a dry 100 mL
three-necked flask under an argon atmosphere. Freshly distilled dry
triethylamine (80 mL) was added from a syringe, and a steady stream
of acetylene was bubbled through the solution under vigorous stirring
in the dark. After 12 h, a second portion of Pd(PPh3)2Cl2 (100 mg,
0.13 mmol) was added, and the gas flow was continued until 1H NMR
showed the complete conversion of starting material after 48 h. The
reaction mixture then was poured into 4 M NaOH (100 mL) and
extracted with toluene (3 × 50 mL). The organic layers were
combined and dried over sodium sulfate, and the solvent was
evaporated under reduced pressure. The crude product (525 mg, 47%)
was chromatographed (silica gel: toluene) and recrystallized from
THF to yield 348 mg (1.3 mmol, 31%) of 4 as large pale red cubes.
Mp: 242 °C (lit.90 240−243 °C). 1H NMR (CDCl3, 300 MHz): δ 2.96
(s, 12 H, CH3), 6.64 (dt, J = 9.0 Hz, J = 2.1 Hz, 4 H, Ar), 7.37 (dt, J =
9.0 Hz, J = 2.1 Hz, 4 H, Ar).
Synthetic Procedures. Compound 4,65,66 dimethyldioxirane,84 4-
iodophenyltrimethylsilane,74 4-bromophenylacetylene,74 4,4′-bipyridy-
lacetylene,85 1,48−50 and 17d47 were prepared according to the
literature. Cyclopentadienylcobalt(I) dicarbonyl and tetrakis-
(triphenylphosphine)palladium(0) were from Strem Chemicals, and
the former was distilled prior to use. Dichlorobis(triphenylphosphine)-
palladium(II) (Aldrich) was used as received. All other reagents were
purchased and used as received. All reactions were carried out under
an argon atmosphere with dry solvent, freshly distilled under
anhydrous conditions,86 unless otherwise noted. Xylenes, THF, and
diethyl ether were dried over and distilled from sodium benzophenone
ketyl. Triethylamine was dried over and distilled from calcium hydride.
Standard Schlenk- and vacuum-line techniques were employed for all
manipulations with air- or moisture-sensitive compounds. Yields refer
to isolated, chromatographically and spectroscopically homogeneous
materials, unless otherwise stated.
1-Bromo-4-iodobenzene. A three-neck flask equipped with a
mechanical stirrer and containing NaNO2 (54.5 g, 0.79 mol) was
evacuated, placed under argon, and cooled to 0 °C. Under mechanical
stirring, H2SO4 (concd, 300 mL) was added slowly to minimize the
formation of nitrogen oxides. Acetic acid (concd, 500 mL) was then
added from an addition funnel, and the mixture was stirred for 30 min.
In the meantime, 4-bromoaniline (100 g, 0.58 mol) was dissolved in
acetic acid. Under stirring at 0 °C, H2SO4 (concd, 45 mL) was added
slowly. The resulting white precipitate was dissolved by addition of a
minimum of ice. This solution was transferred to a cooled addition
funnel and added dropwise very slowly to the NaNO2/H2SO4 solution
at 0 °C. After 2 h of continued stirring, the reaction mixture was
transferred to an ice-cooled addition funnel and added dropwise but
rapidly under vigorous stirring to a solution of KI (230 g, 1.36 mol), I2
(202 g, 0.79 mol), and urea (30.5 g) in 1000 mL of water and 450 mL
of chloroform at 0 °C. The reaction mixture was stirred overnight
while warming to room temperature. The two layers were separated in
a separation funnel, and the aqueous layer was extracted with
chloroform (4 × 250 mL). The organic phases were combined, dried
over Na2SO4, and evaporated under reduced pressure. The crude
product was flash chromatographed (silica gel, hexanes/chloroform,
4:1) to yield 137.2 g (0.48 mol, 84%) of 1-bromo-4-iodobenzene (1)
as a white solid. Mp: 89 °C (lit.87 92 °C).
4,4′-Dimethyldiphenylacetylene (5). Method A: 1-Iodo-4-methyl-
benzene (5.0 g, 23 mmol) and CuI (108 mg, 0.57 mmol) were charged
into a dry 100 mL three-neck flask under an argon atmosphere.
Freshly distilled 1-ethynyl-4-methylbenzene (2.9 mL, 23 mmol) and
freshly distilled, dry triethylamine (80 mL) were added from a syringe,
and the solution was degassed by three freeze−pump−thaw cycles.
Subsequently, Pd(PPh3)4 (670 mg, 0.57 mmol) was added from a tip
tube. The reaction mixture was warmed to 50 °C and stirred for 5 h in
1
the dark. After H NMR showed a full conversion of the starting
material, the solution was poured into dilute hydrochloric acid (2 M,
250 mL) and extracted with toluene (3 × 100 mL). The organic layers
were combined and dried over sodium sulfate. After evaporation of the
solvent, the crude reaction mixture was chromatographed (silica gel,
chloroform/hexane, 1:5). The desired product 5 was recrystallized
from hexane to yield 3.9 g (20 mmol, 86%) of 5 as white plates. Mp:
4-Iodo-N,N-dimethylaniline. 4-Iodoaniline (20.0 g, 91 mmol) was
charged into a dry two-necked flask, and toluene (200 mL) was added
under an argon atmosphere. NaOH (7.3 g, 182 mmol) was finely
pulverized and added to the solution at once. Tris[2-(2-
methoxyethoxy)ethyl]amine (TDA-1, 1.4 mL, 4.3 mmol) was then
added from a syringe. Under vigorous stirring, iodomethane (11.4 mL,
182 mmol) was added dropwise to the solution, and the reaction
mixture was warmed to 50 °C for 6 h while a dry ice condenser was
attached. More NaOH (7.3 g, 182 mmol) and more TDA-1 (1.4 mL,
135 °C (lit.91 136 °C). H NMR (CDCl3, 300 MHz): δ 2.27 (s, 6 H,
1
CH3), 6.91 (dt, J = 8.1 Hz, J = 1.8 Hz, 4 H, Ar), 7.54 (dt, J = 8.1 Hz, J
= 1.8 Hz, 4 H, Ar). Method B: 1-Iodo-4-methylbenzene (5.0 g, 23
mmol), CuI (29 mg, 0.15 mmol), and Pd(PPh3)2Cl2 (110 mg, 0.15
mmol) were charged into a dry 100 mL three-necked flask under an
argon atmosphere. Freshly distilled triethylamine (80 mL) was added
from a syringe. A steady stream of acetylene was then bubbled through
the solution until 1H NMR showed complete conversion of the
starting material after 12 h. The reaction mixture was poured into
1
4.3 mmol) were added, and the solution was stirred until H NMR
I
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dilute hydrochloric acid (2 M, 250 mL) and extracted with chloroform
(3 × 100 mL). After drying over sodium sulfate, the solvents were
evaporated and the crude product was chromatographed (silica gel,
chloroform/hexane, 1:5), yielding 5 as a white solid: 3.6 g (17 mmol,
74%).
1-(4-Bromophenyl)-2-(4-hydroxyphenyl)acetylene (10). 4-Trime-
thylsilyliodobenzene (4.23 g, 14.5 mmol), 4-bromophenylacetylene
(2.75 g, 15.0 mmol), CuI (20.0 mg, 0.11 mmol), PdCl2(PPh3)2 (0.105
g, 0.15 mmol), and triethylamine (100 mL) were charged to a 250 mL
three-neck flask and stirred overnight at room temperature. The
solvent was removed under reduced pressure, and the residue
dissolved in dichloromethane and washed once with 1 M HCl and
twice with water. The organic layer was dried over sodium sulfate, and
the solvent was removed under reduced pressure. The crude product
was chromatographed on silica gel (eluent: dichloromethane) to give
10 as a gray-white solid (2.6 g, 9.5 mmol, 65%), which was then
recrystallized from dichloromethane and hexanes. Mp: 149−150 °C
(change of color) (lit.95 148−149 °C; lit.96 182 °C). 1H NMR (C6D6,
300 MHz): δ 7.34 (m, 2 H, Ar), 7.08 (m, 4 H, Ar), 6.32 (m, 2 H, Ar),
3.9 (s, 1 H, OH). 13C{1H} NMR (DMSO, 300 MHz): δ 86.30, 91.25.
112.07, 115.75, 121.45, 122.14, 131.67, 132.93, 133.06, 158.25. MS (EI
+): m/z (rel int) 272 ([M]+, 100), 165 (30), 139 (10).
4,4′-Dinitrodiphenylacetylene (6). 4-Iodo-1-nitrobenzene (5.0 g,
20 mmol), CuI (26 mg, 0.13 mmol), and Pd(PPh3)2Cl2 (100 mg, 0.13
mmol) were charged into a dry 100 mL three-necked flask under an
argon atmosphere. Freshly distilled triethylamine (80 mL) was added
from a syringe. A steady stream of acetylene was then bubbled through
the solution until 1H NMR showed complete conversion of the
starting material after 68 h. The reaction mixture was poured into 2 M
hydrochloric acid (250 mL) and extracted with chloroform (3 × 100
mL). After drying over sodium sulfate, the solvents were evaporated
and the crude product was chromatographed (silica gel, chloroform/
hexane, 1:3). The product was recrystallized from hexanes to yield 4.5
g (17 mmol, 83%) of 6 as pale yellow needles. Mp: 208 °C (lit.92 205−
207 °C); 1H NMR (CDCl3, 300 MHz) δ 7.68 (dt, J = 8.7 Hz, J = 1.8
Hz, 4 H, Ar), 8.23 (dt, J = 8.7 Hz, J = 1.8 Hz, 4 H, Ar).
1-(4-Bromophenyl)-2-(4-trimethylsilylphenyl)acetylene (7). 4-Io-
dophenyltrimethylsilane (9.93 g, 36.0 mmol), 4-bromophenylacetylene
(7.15 g, 39.5 mmol), CuI (110 mg, 6 mmol), PdCl2(PPh3)2 (840 mg,
1.2 mmol), and diethylamine (185 mL) were charged to a 500 mL
three-neck flask and stirred overnight at room temperature. The
solvent was removed under reduced pressure, and the residue
dissolved in diethyl ether and washed twice with water. The organic
layer was dried over sodium sulfate, and the solvent was removed
under reduced pressure. The residue was triturated with hexanes and
filtered on sintered glass, which removed a white solid. The filtrate was
concentrated under reduced pressure, giving an orange residue, which
was chromatographed on silica gel (eluent: hexanes) to give 7 as a
white solid (10 g, 30 mmol, 84%), which was recrystallized from cold
pentane. Mp: 82 °C. 1H NMR (C6D6, 300 MHz): δ 7.58 (m, 2 H, Ar),
7.34 (m, 2 H, Ar), 7.08 (m, 4 H, Ar), 0.14 (s, 9 H, Si(CH3)3). 13C{1H}
NMR (C6D6, 300 MHz): δ −1.34 (Si−C), 89.42. 91.33, 122.58,
122.81, 123.86, 131.08, 131.87, 133.29, 133.72, 141.44. MS (EI+): m/z
(rel int) 328 ([M]+, 40), 272 ([M − CH3]+, 100). Anal. Calcd for
C17H17SiBr: C, 62.00; H, 5.20. Found: C, 61.84; H, 5.35.
1-(4-Bromophenyl)-2-(4-methoxyphenyl)acetylene (8). 4-Bromo-
phenylacetylene (5 g, 27.6 mol), 4-iodoanisole (6.15 g, 26.2 mmol),
PdCl2(PPh3)2 (0.646 g, 0.92 mmol), CuI (90 mg, 0.46 mmol), and
triethylamine (120 mL) were charged to a 500 mL three-neck flask.
The mixture was stirred overnight at room temperature. The solvent
was removed under reduced pressure, and the solid residue was
dissolved in CH2Cl2, washed twice with water, and dried over sodium
sulfate. The solvent was removed under reduced pressure, and the
crystalline product was chromatographed on silica gel (dichloro-
methane/hexanes, starting at 25:75 and gradually increasing the
proportion of dichloromethane) to give 8 as white crystals (6.4 g, 22
mmol, 85%). It was recrystallized from dichloromethane and hexanes.
Mp: 154−155 °C (lit.93 150−152 °C; lit.94 155 °C).
1-(4-Acetoxyphenyl)-2-(4-bromophenyl)acetylene (9). 4-Acetox-
yiodobenzene (0.5 g, 1.9 mmol), 4-bromophenylacetylene (0.36 g, 2.0
mmol), CuI (6 mg, 0.032 mmol), PdCl2(PPh3)2 (45 mg, 0.064 mmol),
and triethylamine (20 mL) were charged to a 100 mL three-neck flask
and stirred for 3 h at room temperature. The solvent was removed
under reduced pressure. The residue was dissolved in dichlormethane
and washed twice with water. The organic layer was dried over sodium
sulfate, and the solvent was removed under reduced pressure. The
crude product was chromatographed on silica gel (eluent: acetone/
hexanes, 1:1) to give 9 as a gray-white solid (0.56 g, 1.8 mmol, 93%). It
was then recrystallized from acetone and hexanes. Mp: 136−137 °C.
1H NMR (C6D6, 300 MHz): δ 7.37 (m, 2 H, Ar), 7.08 (s, 4 H, Ar),
6.88 (m, 2 H, Ar), 1.68 (s, 3 H, CH3). 13C{1H} NMR (C6D6, 300
MHz): δ 20.42 (CH3), 88.89. 90.33, 120.75, 122.14, 122.46, 122.83,
131.90, 132.92, 133.22, 151.37, 167.97 (CO). MS (EI+): m/z (rel
int) 314 ([M]+, 10), 272 ([M − COCH3]+, 100), 165 (18), 43
(OCCH3, 27). Anal. Calcd for C16H11O2Br: C, 60.98; H, 3.52. Found:
C, 60.86; H, 3.23.
1-(4-Bromophenyl)-2-(4-isopropoxyphenyl)acetylene (11). Com-
pound 10 (2.11 g, 7.73 mmol), 2-bromopropane (1.14 g, 9.3 mmol),
potassium carbonate (2.7 g, 19.5 mmol), and dry DMF (75 mL) were
charged to a 250 mL three-neck flask. The mixture was heated to 120
°C for 3 h. Then 2-bromopropane (1.14 g, 9.3 mmol) was added, and
the mixture was stirred overnight at 120−130 °C under an argon
atmosphere. The mixture was poured into a 1 M HCl solution (100
mL) and extracted with chloroform (3 × 30 mL). The combined
organic layers were washed with water (3 × 30 mL) and dried over
sodium sulfate. The solvent was removed under reduced pressure, and
the residue was chromatographed on silica gel (eluent: chloroform/
hexanes, 60:40) to give 11 as a white solid (2.1 g, 6.7 mmol, 86%).
1
Mp: 117−118 °C. H NMR (C6D6, 300 MHz): δ 7.46 (m, 2 H, Ar),
7.10 (m, 4 H, Ar), 6.66 (m, 2 H, Ar), 4.04 (hept, J = 6.0 Hz, 1 H, CH),
1.03 (d, J = 6.0 Hz, 6 H, CH3). 13C{1H} NMR (C6D6, 300 MHz): δ
21.84 (CH3), 69.62 (CH), 87.59, 91.49, 115.13, 115.98, 122.30,
123.11, 131.84, 133.12, 133.48, 158.70. MS (EI+): m/z (rel int) 314−
316 ([M]+, 25), 272 (100), 163 (22), 43 (12). Anal. Calcd for
C17H15OBr: C, 64.77; H, 4.80. Found: C, 64.64; H, 4.63.
[1,3-Bis(4-bromophenyl)-2,4-bis(4-trimethylsilylphenyl)cyclo-
butadiene]cyclopentadienylcobalt (12a). Compound 7 (1.62 g, 4.9
mmol), freshly distilled CpCo(CO)2 (0.44 g, 2.47 mmol), and
degassed xylenes (100 mL) were charged to a 200 mL three-neck flask
and refluxed overnight in the dark. After cooling, the dark solution was
poured onto a dry column of silica gel and eluted with hexanes. An
orange band was collected, and the solvent was removed under
reduced pressure. The solid was collected on a frit and washed with
copious amounts of hexanes and a few milliliters of acetone.
Compound 12a was recrystallized from a mixture of dichloromethane
and acetonitrile (0.5 g, 0.64 mmol, 26%). Single crystals were obtained
from a mixture of benzene and acetonitrile. Mp: 285−295 °C (dec).
1H NMR (CD2Cl2, 300 MHz): δ 7.38 (m, 8 H, Ar), 7.33 (m, 8 H, Ar),
4.62 (s, 5 H, Cp), 0.28 (s, 18 H, Si(CH3)3). 13C{1H} NMR (C6D6,
300 MHz): δ −1.12 (Si−C), 74.24 (Cb), 75.42 (Cb), 83.62 (Cp),
120.04 (Ar), 128.32 (Ar), 133.70 (Ar), 131.45 (Ar), 133.48 (Ar),
136.06 (Ar), 136.59 (Ar), 139.06 (Ar). MS (FAB+, 3-NOBA): m/z
780 (maximum at 782). HRMS calcd for C39H39Si2Br2Co: 780.0289.
Found: 780.0269. Anal. Calcd for C39H39Si2Br2Co·1/2CH2Cl2: C,
57.50; H, 4.89. Found: C, 57.26; H, 4.81.
[1,3-Bis(4-bromophenyl)-2,4-bis(4-methoxyphenyl)cyclobuta-
diene ]cyclopentadienylcobalt (13a). The procedure described for
12a was followed, using 8 (2.48 g, 8.7 mmol), CpCo(CO)2 (0.78 g,
4.3 mmol), and degassed xylenes (100 mL) in a 250 mL three-neck
flask and refluxing overnight in the absence of light. After cooling, the
dark solution was poured onto a dry column of silica gel, and an
orange band eluted with dichloromethane. The first fraction (150 mL)
contains mainly the desired isomer. The solvent was removed under
reduced pressure, and the brown residue was collected on a frit and
washed with diethyl ether. The product was then purified by flash
chromatography on silica gel (eluent: chloroform/pentane, 60:40).
The desired isomer (top spot on TLC) was collected, and the same
purification procedure was applied to the other fractions. Compound
13a was then washed with diethyl ether (0.33 g, 0.43 mmol, 6%) and
J
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Organometallics
Article
recrystallized from DMSO as dark red crystals. Mp: 308−310 °C. MS
(ESI+): m/z 696 (maximum at 698). Anal. Calcd for C35H27O2Br2Co:
C, 60.20; H, 3.90. Found: C, 60.31; H, 3.71. The NMR data agree with
those reported.52
on silica gel (eluent: chloroform/hexanes, 50:50) to give 15b as a
yellow solid (0.17 g, 0.23 mmol, 9%). Mp: 170−180 °C (dec). H
1
NMR (C6D6, 300 MHz): δ 7.46 (m, 4H, Ar), 7.26 (m, 4 H, Ar), 7.17
(m, 4 H, Ar), 6.73 (m, 4 H, Ar), 4.45 (s, 5 H, Cp), 4.19 (hept, J = 6.0
Hz, 2 H, CH), 1.11 (d, J = 6.0 Hz, 6 H, CH3). 13C{1H} NMR
(CD2Cl2, 300 MHz): δ 22.26 (CH3), 70.15 (CH), 72.93 (Cb). 76.06
(Cb), 83.37 (Cp), 115.62 (Ar), 119.81 (Ar), 127.52 (Ar), 130.33 (Ar),
130.58 (Ar), 131.48 (Ar), 136.51 (Ar), 157.08 (2C, Ar). MS (ESI+):
m/z 751 (maximum at 753). Anal. Calcd for C39H35O2Br2Co: C,
62.09; H, 4.68. Found: C, 62.49; H, 4.46.
[1,3-Bis(4-acetoxyphenyl)-2,4-bis(4-bromophenyl)cyclobuta-
diene]cyclopentadienylcobalt (14a). The procedure described for
12a was followed, using 9 (2.87 g, 9.1 mmol), CpCo(CO)2 (0.82 g,
4.5 mmol), and degassed xylenes (150 mL) in a 500 mL three-neck
flask and refluxing for 20 h in the absence of light. After cooling, the
dark solution was poured onto a dry column of silica gel (h = 30 cm),
and an orange band eluted with chloroform. The first fraction (200
mL) contains mainly the desired isomer. The solvent was removed
under reduced pressure, the product was rechromatographed on silica
gel (eluent: chloroform), and the fractions containing the desired
isomer (top spot on TLC) were collected. The second fraction (200
mL) contains a mixture of the two isomers. The same purification
procedure was applied to isolate both isomers. After removing the
solvent, all the fractions corresponding to the expected isomer were
combined and rechromatographed on silica gel (eluent: chloroform)
to give 14a as a yellow-orange solid (0.52 g, 0.69 mmol, 15%). The
product was then recrystallized from a mixture of dichloromethane and
hexanes. Single crystals were obtained from a mixture of THF and
[1,2-Bis(4-nitrophenyl)-3,4-bis(4-N,N-dimethylaminophenyl)-
cyclobutadiene]cyclopentadienylcobalt (16b). Compound 4 (500
mg, 1.9 mmol) and compound 6 (1.5 g, 5.7 mmol) were suspended in
100 mL of degassed xylenes in a 250 mL three-neck flask with stir bar
and condenser. The mixture was subsequently heated to 80 °C, at
which time both components dissolved, and 1.0 g (5.7 mmol) of
freshly distilled CpCo(CO)2 was added via syringe. The reaction was
heated to reflux overnight in the dark. After cooling, the reaction
mixture was poured onto a plug of silica gel on a glass frit and eluted
with ethyl acetate. The solvent was removed under reduced pressure
to give a black oil. The oil was chromatographed on silica gel
[gradient: hexanes (100−80)/ethyl acetate (0−20)], first eluting a
yellow band containing unreacted 6 (214 mg). Compound 16b eluted
in a second, red band, which gave an indigo-colored solid upon
removal of the solvent. The product was crystallized by slow
evaporation of CH2Cl2 from a ∼3:1 MeOH/CH2Cl2 solution to give
1
ethanol. Mp: 244−246 °C. H NMR (DMSO, 300 MHz): δ 7.45 (m,
4 H, Ar), 7.37 (m, 4 H, Ar), 7.27 (m, 4 H, Ar), 7.04 (m, 4 H, Ar), 4.70
(s, 5 H, Cp), 2.27 (s, 6 H, CH3). 13C{1H} NMR (CD2Cl2, 300 MHz):
δ 21.31 (CH3), 74.13 (Cb), 74.59 (Cb), 83.67 (Cp), 120.26 (Ar),
121.86 (Ar), 129.95 (Ar), 130.66 (Ar), 131.64 (Ar), 133.62 (Ar),
135.61 (Ar), 149.66 (2C, Ar), 169.59 (CO). MS (FAB+, 3-NOBA):
m/z 752 (maximum at 754). Anal. Calcd for C37H27O4Br2Co: C,
58.91; H, 3.61. Found: C, 58.50; H, 3.37.
1
532 mg (0.91 mmol, 48%) of pure 16b. Mp: 268−270 °C. H NMR
(CD2Cl2, 500 MHz): δ 2.97 (s, 12 H, N−CH3), 4.59 (s, 5 H, Cp),
6.59 (d, J = 8 Hz, 4 H, Ar), 7.27 (d, J = 8 Hz, 4 H, Ar), 7.55 (d, J = 8
Hz, 4 H, Ar), 8.06 (d, J = 8 Hz, 4 H, Ar). 13C{1H} NMR (CD2Cl2, 500
MHz): δ 40.52 (N−CH3), 83.39 (Cb), 112.10 (Cp), 123.65 (Ar),
128.85 (Ar), 130.08 (Ar), 145.63 (Ar), 146.76 (Ar). MS (EI+) m/z
(rel int): 956 ([M]+, 100), 388 (15), 250 (10), 220 (15), 69 (30). IR
(KBr, cm−1): 554, 729, 736, 782, 808, 827, 854, 945, 1067, 1107, 1168,
1194, 1224, 1340, 1511, 1444, 1511, 1589, 1609, 2800, 1442. UV−vis
(solvent): λ (ε) 200 (79 000), 260 (35 000), 320 (34 000), 352 (sh,
30 000), 450 (12 000) nm (L mol−1 cm−1). Anal. Calcd for
C37H33N4O4Co: C, 67.68; H 5.07; N, 8.53. Found: C, 67.78; H,
5.26; N, 8.21. HRMS (ESI+): calcd for C37H33N4O4Co: 656.1828.
Found: 656.1841.
[1,2-Bis(4-acetoxyphenyl)-3,4-bis(4-bromophenyl)cyclobuta-
diene]cyclopentadienylcobalt (14b). The third fraction (300 mL)
contains mainly the second isomer (second spot by TLC). The same
purification procedure was applied. After removing the solvent, all the
fractions corresponding to this isomer were combined and flash
chromatographed on silica gel (eluent: chloroform) to give 14b as a
yellow-orange solid (0.33 g, 0.43 mmol, 10%). The product was then
recrystallized from a mixture of chloroform and hexanes. Mp: 258−
260 °C. 1H NMR (DMSO, 300 MHz): δ 7.46 (m, 4 H, Ar), 7.36 (m, 4
H, Ar), 7.28 (m, 4 H, Ar), 7.04 (m, 4 H, Ar), 4.70 (s, 5 H, Cp), 2.26 (s,
6 H, CH3). 13C{1H} NMR (CD2Cl2, 300 MHz): δ 21.31 (CH3), 73.94
(Cb), 74.81 (Cb), 83.68 (Cp), 120.27 (Ar), 121.85 (Ar), 129.95 (Ar),
130.68 (Ar), 131.65 (Ar), 133.64 (Ar), 135.62 (Ar), 149.66 (2C, Ar),
169.61 (CO). MS (ESI+): m/z 752 (maximum at 754). Anal. Calcd
for C37H27O4Br2Co: C, 58.91; H, 3.61. Found: C, 58.90; H, 3.42.
[1,3-Bis(4-bromophenyl)-2,4-bis(4-isopropoxyphenyl)cyclobuta-
diene]cyclopentadienylcobalt (15a). The procedure described for
12a was followed, using 11 (1.65 g, 5.2 mmol), CpCo(CO)2 (0.47 g,
2.6 mmol), and degassed xylenes (100 mL) in a 250 mL three-neck
flask and refluxing for 20 h in the dark. After cooling, the dark solution
was poured onto a dry column of silica gel, and an orange band eluted
with chloroform. The first fraction (200 mL) contains mainly the
desired isomer. The solvent was removed under reduced pressure, the
product was rechromatographed on silica gel (eluent: chloroform/
hexanes, 50:50), and the fractions containing the desired isomer (top
spot on TLC) were collected. The second fraction contains a mixture
of the two isomers. The same purification procedure was repeated
twice to isolate 15a as a yellow-orange solid (0.32 g, 0.42 mmol, 16%).
Single crystals have been obtained from a mixture of THF and ethanol.
Tetrakis(4-N,N-dimethylaminophenyl)cyclobutadienecyclopenta-
dienylcobalt (16a). To 80 mg (0.30 mmol) of 4 dissolved in 20 mL of
dry, degassed xylenes in a 50 mL three-neck flask with stir bar and
condenser was added 27 mg (0.152 mmol) of CpCo(CO)2. The
reaction mixture was brought to reflux for 3 h, at which point TLC
(4:1 hexanes/ethyl acetate) showed the absence of starting material.
After cooling, the reaction was poured over a plug of silica gel on a
glass frit and eluted with ethyl acetate. The solvent from the eluent was
removed under reduced pressure, and the resulting dark solid was
chromatographed on silica gel (4:1 hexanes/ethyl acetate) to yield 64
mg (0.10 mmol, 66%) of 16a as a gold solid. The product was
crystallized by slow evaporation of CH2Cl2 for a ∼3:1 MeOH/CH2Cl2
solution. Mp: >285 °C. 1H NMR (CDCl3, 500 MHz): δ 2.93 (s, 24 H,
N(CH3)2), 4.55 (s, 5 H, Cp), 6.57 (d, J = 8 Hz, 8 H, Ar), 7.36 (d, J = 8
Hz, 8 H, Ar). 13C{1H} NMR (CDCl3, 400 MHz): δ 40.65 (N(CH3)2),
74.63 (Cb), 82.61 (Cp), 112.05 (Ar), 125.24 (Ar), 129.80 (Ar),
148.46 (Ar). MS (EI+) m/z (rel int): 652 ([M]+, 20), 388 ([M − 4]+,
10), 264 ([4]+,100), 205 (25), 129 (30). IR (KBr, cm−1): 664, 800,
1029, 1103, 1262, 1348, 1511, 1600, 2792, 2850, 2928, 2963, 3449.
UV−vis (solvent): λ (ε) 229 (27 500), 262 (sh, 23 000), 318 (53 500),
410 (br, 6000) nm (L mol−1 cm−1). Anal. Calcd for C41H45N4Co: C,
75.45; H, 6.95; N, 8.58. Found: C, 75.65; H, 7.02; N, 8.42.
1
Mp: 252−256 °C. H NMR (C6D6, 300 MHz): δ 7.40 (m, 4 H, Ar),
7.33 (m, 4 H, Ar), 7.17 (m, 4 H, Ar), 6.73 (m, 4 H, Ar), 4.45 (s, 5 H,
Cp), 4.21 (hept, J = 6.0 Hz, 2 H, CH), 1.12 (d, J = 6.0 Hz, 6 H, CH3).
13C{1H} NMR (CD2Cl2, 300 MHz): δ 22.26 (CH3), 70.18 (CH),
[1,2-Bis(4-pyridyl)-3,4-bis(4-methylphenyl)cyclobutadiene]cyclo-
pentadienylcobalt (17c). 4,4′-Dipyridylacetylene (1.75 g, 9.7 mmol)
and compound 5 (1.0 g, 4.85 mmol) were suspended in 50 mL of
degassed xylenes in a 100 mL three-neck flask. The mixture was heated
to 80 °C, at which time both components went into solution, and 1.75
g (9.71 mmol) of CpCo(CO)2 was added from a syringe. The reaction
mixture was heated to reflux overnight in the dark. After cooling, the
solvent was removed under reduced pressure, and the dark red solid
73.25 (Cb), 75.81 (Cb), 83.38 (Cp), 115.66 (Ar), 119.68 (Ar), 127.34
(Ar), 130.26 (Ar), 130.64 (Ar), 131.42 (Ar), 136.65 (Ar), 157.17 (2C,
Ar). MS (ESI+): m/z 752 (maximum at 754). Anal. Calcd for
C39H35O2Br2Co: C, 62.09; H, 4.68. Found: C, 62.06; H, 4.60.
[1,2-Bis(4-bromophenyl)-3,4-bis(4-isopropoxyphenyl)cyclobuta-
diene]cyclopentadienylcobalt (15b). All fractions corresponding to
the second isomer were combined and flash chromatographed twice
K
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Organometallics
Article
was dissolved in methylene chloride and poured over silica gel on a
glass frit. Elution with methylene chloride brought off 17a as a yellow
band. The silica gel was then eluted with methanol to give a mixture of
17c and 17b as a dark red solid after removal of the methanol. The
solid was triturated with methanol, and an orange solid was collected
on a glass frit. The dark red filtrate contained only compound 17b
(800 mg, 35% crude yield). The orange solid, which contained mostly
17c, was crystallized from CHCl3/MeOH to give 420 mg (0.82 mmol,
17%) of 17c as a yellow-orange powder. The product was oxidized
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under Grant No. CHE-1265922. We thank
Drs. Benjamin T. King, Evgueni Pinkhassik, and Darren K.
MacFarland for initial synthetic studies. Work in Prague was
supported by the Institute of Organic Chemistry and
Biochemistry (RVO:61388963) and the Czech Science
Foundation (P208/12/G016). The authors gratefully acknowl-
edge support of the Division of Chemical Sciences, Geo-
sciences, and Biosciences, Office of Basic Energy Sciences of
the U.S. Department of Energy, through Grant DE-AC02-98-
CH10886, and for use of the LEAF Facility of the BNL
Accelerator Center for Energy Research.
1
without further purification. Mp > 200 °C. H NMR (CD2Cl2, 300
MHz): δ 2.32 (s, 6 H, CH3), 4.65 (s, 5 H, Cp), 7.07 (d, J = 7.2 Hz, 4
H, Ar), 7.30 (d, J = 7.8 Hz, 4 H, Ar), 7.31 (m, 4 H, Ar), 8.41 (m, 4 H,
Ar). 13C{1H} NMR (CDCl3, 400 MHz): δ 21.61 (CH3), 70.53 (Cb),
83.71 (Cp), 123.10 (Ar), 128.98 (Ar), 129.19 (Ar), 131.62 (Ar),
137.09 (Ar), 146.24 (Ar), 149.70 (Ar). MS (EI+) m/z (rel int): 510
(M, 95), 317 (22), 206 (10), 124 (52), 84 (48), 69 (20), 57 (28), 31
(100). IR (KBr, cm−1): 484, 526, 531, 596, 618, 629, 642, 663, 724,
818, 832, 991, 1001, 1061, 1106, 1180, 1215, 1412, 1507, 1515, 1541,
1590, 1597, 3021, 3442.
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(17a). This material was formed during the synthesis of 17c. After
elution on silica gel, the solvent was removed under reduced pressure,
and the resulting yellow solid triturated with methanol. The resulting
solid was recrystallized from hexanes to give pure 17a (280 mg, 0.52
mmol, 11%). Mp: 308−310 °C. 1H NMR (CDCl3, 500 MHz): δ 2.28
(s, 12 H, CH3), 4.57 (s, 5 H, Cp), 6.99 (d, J = 7.6 Hz, 8 H, Ar), 7.32
(d, J = 7.6 Hz, 8 H, Ar). 13C{1H} NMR (CDCl3, 400 MHz): δ 21.58
(CH3), 74.86 (Cb), 83.27 (Cp), 128.82 (Ar), 128.95 (Ar), 133.80
(Ar), 135.84 (Ar). MS (EI+) m/z (rel int): 536 ([M]+, 100), 330 ([M
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553, 592, 632, 703, 722, 735, 807, 828, 841, 93, 1001, 1018, 1110,
1179, 1516, 1608, 1714, 1902, 2857, 2914, 3017, 3053. UV−vis
(CH2Cl2): λ (ε) 312 sh (21 000), 269 (39 000), 243 (33 000) nm (L
mol−1 cm −1). Anal. Calcd for C37H33Co: C, 82.82; H, 6.20. Found: C,
82.83; H, 6.12.
[1,2-Bis(4-pyridine-N-oxide)-3,4-bis(4-methylphenyl)cyclobuta-
diene]cyclopentadienylcobalt (18). Compound 17c (1.0 g, 2.0
mmol) was dissolved in 40 mL of dichloromethane and cooled to 0
°C in an ice water bath. To the solution was added dropwise an excess
of dimethyldioxirane in acetone.97 After warming to room temper-
ature, 40 mL of water was added and the layers were separated. The
aqueous layer was washed twice more with dichloromethane (20 mL).
After drying over sodium sulfate the solvent was removed under
reduced pressure to yield 1.0 g (1.9 mmol, 95%) of compound 18.
Mp: >280 °C (dec). 1H NMR (CD2Cl2, 500 MHz): δ 2.29 (s, CH3, 6
H, CH3), 4.62 (s, 5 H, Cp), 7.08 (d, J = 7.9 Hz, 4 H, Ar), 7.21 (d, J =
8.1 Hz, 4 H, Ar), 7.23 (d, J = 7.0 Hz, 4 H, Ar), 7.96 (d, J = 7.3 Hz, 4 H,
Ar). 13C{1H} NMR (CD2Cl2, 400 MHz): δ 21.52 (CH3), 69.15 (Cb),
77.88 (Cb), 83.55 (Cp), 124.21 (Ar), 128.75 (Ar), 129.38 (Ar),
130.69 (Ar), 136.39 (Ar), 137.56 (Ar), 139.08 (Ar). IR (KBr, cm−1):
743, 821, 846, 995, 1023, 1087, 1106, 1163, 1244, 1452, 1485, 1505,
1614, 2862. UV−vis (solvent): λ (ε) 227 (33 000), 229 (41 500), 345
(22 000), 400 (6500) nm (L mol−1 cm−1). Anal. Calcd for
C33H27N2O2Co: C, 73.06; H, 5.02; N, 5.16. Found: C, 72.95; H,
4.99; N, 5.01.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Computational methods and optimized geometries. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: michl@eefus.colorado.edu.
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Notes
The authors declare no competing financial interest.
L
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