Haptotropic rearrangement in tricarbonylchromium complexes of 2-aminobiphenyl and 4-aminobiphenyl

Article abstract of DOI:10.1039/c1dt10994d

The para-aminobiphenyl compound [(η⁶-C₆H ₅)(C₆H₄-4-NH₂)]Cr(CO)₃ (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34

Full text of DOI:10.1039/c1dt10994d

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PAPER
Haptotropic rearrangement in tricarbonylchromium complexes of
2-aminobiphenyl and 4-aminobiphenyl†
Curtis J. Czerwinski,*a Ilia A. Guzei,b Kevin M. Riggle,a Jason R. Schroedera and Lara C. Spencerb
Received 26th May 2011, Accepted 7th July 2011
DOI: 10.1039/c1dt10994d
6
The para-aminobiphenyl compound [(h -C6H5)(C6H4-4-NH2)]Cr(CO)3 (1) has an arene–phenyl
dihedral angle of 38.01(6)◦, as determined by single-crystal X-ray crystallography, and 34.7(11)◦, as
determined by DFT calculations. It undergoes haptotropic rearrangement at 140 ◦C in solution to form
6
[(h -C6H4-4-NH2)(C6H5)]Cr(CO)3 (2), even though previous reports have suggested that such
rearrangements should not be observed in compounds with arene–phenyl dihedral angles greater than
22◦. NMR analysis gave a rate constant of k = 5.0 ¥ 10-5 s-1 for the rearrangement of 1 to 2. The
6
ortho-substituted analog [(h -C6H5)(C6H4-2-NH2)]Cr(CO)3 (3) has an arene–phenyl dihedral angle of
67.70(7)◦, as determined by single-crystal X-ray crystallography, and 51.9(10)◦, as determined by DFT
calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to
◦
6
[(h -C6H4-2-NH2)(C6H5)]Cr(CO)3 (4) at 140 C in solution with a rate constant of k = 2.6 ¥ 10-4 s-1.
This approximately five-fold rate enhancement likely results from the ortho-amino group providing
intramolecular stabilization for intermediates formed during the rearrangement.
and indicated that a necessary condition for rearrangement is an
Introduction
arene–arene dihedral angle less than 22◦, as determined either
by calculation or single crystal X-ray crystallography.5 Inter-
ring steric hindrance effected by a non-hydrogen substituent in
the ortho position was shown to twist the arene rings out of
coplanarity to a great enough degree so as to prevent the metal
from migrating between rings. We found it somewhat surprising
that solid-state or gas-phase structure determination could predict
this inhibition, especially considering the expected low rotational
barrier about the arene–arene sigma bond in ortho-substituted
Organometallic compounds with p-coordinated polycyclic aro-
matic ligands may undergo haptotropic rearrangements where a
coordinated metal fragment migrates between aromatic rings. The
most commonly studied examples involve movement of metals
between the rings of naphthalene, fluorene, indenyl, biphenyl,
or other related ligands.1,2 Recent examples have suggested
6
important potential applications of this process, such as (h -
polyarene)Cr(CO)3 compounds that could be used as molecular
switches when their haptotropic rearrangements can be directed
by changes in electronic or steric factors.3
6
compounds of the type [(h -C6H5)C6H4X]Cr(CO)3.6 Noting that
this series of compounds had not included cases where biphenyl
6
In most examples that specifically involve substituted (h -
ligands had strongly electron-donating substituents, we wished
6
biphenyl)Cr(CO)3 complexes, the Cr(CO)3 fragment migrates from
the less electron-rich ring to the more electron-rich ring, but
migration has been shown to be inhibited in some cases by
conformational effects, especially when substituents force the
arene rings out of coplanarity.4 In fact, a recent study describing a
to examine amino-substituted biphenyl compounds such as [(h -
C6H5)C6H4NH2]Cr(CO)3. It seemed plausible that the strong
electron donating character of the amino group could facilitate
haptotropic migration even if gas-phase calculation or solid-
state structure determination showed a dihedral angle greater
6
◦
6
series of substituted compounds [(h -C6H5)C6H4X]Cr(CO)3 (X =
than 22 . Here we report that the para-amino compound [(h -
C6H5)(C6H4-4-NH2)]Cr(CO)3 (1), which has an arene–arene dihe-
dral angle of 38.01(6)◦ in the solid state, nevertheless undergoes
Br, D, CH3, SnMe3) showed that near coplanarity of the arene rings
in the biphenyl ligand is required for haptotropic rearrangement,
6
thermal haptotropic rearrangement in solution to form [(h -
aDepartment of Chemistry, University of Wisconsin – La Crosse, 1725 State
Street, La Crosse, WI, 54601, USA. E-mail: czerwins.curt@uwlax.edu
C6H4-4-NH2)(C6H5)]Cr(CO)3 (2). Furthermore, we found that
6
the ortho-amino compound [(h -C6H5)(C6H4-2-NH2)]Cr(CO)3 (3),
bDepartment, of Chemistry, University of Wisconsin – Madison, 1101
University Avenue, Madison, WI, 53706, USA
despite its larger arene–arene dihedral angle of 67.70(7)◦, surpris-
6
† Electronic supplementary information (ESI) available: 1H NMR spectra
and X-ray crystallographic data for compounds 1–4 and figures giving rate
data for conversion of 1 to 2 and for conversion of 3 to 4. CCDC reference
numbers 778546–778549. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10994d
ingly undergoes haptotropic rearrangement to form [(h -C6H4-
2-NH2)(C6H5)]Cr(CO)3 (4) with a rate that is approximately
five times faster than that observed for the conversion of 1
to 2.
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structures of 1 and 2 (Fig. 1 and 2) definitively showed the bonding
arrangements for each compound and the arene–arene dihedral
angles, which may be an important consideration for haptotropic
rearrangements.
Results and discussion
When 4-aminobiphenyl (5) was heated with chromium hexacar-
bonyl in 9 : 1 dibutyl ether : tetrahydrofuran, the product obtained
was the expected isomer [(h -C6H4-4-NH2)(C6H5)]Cr(CO)3 (2),
6
which has a chromium tricarbonyl fragment coordinated to the
para-amino-substituted arene ring (Scheme 1).7 This expected
predominance of Cr(CO)3 coordination to the electron-rich
amino-substituted ring, rather than the unsubstituted ring, has
been previously observed in aminobiphenyl chromium tricarbonyl
syntheses.8–10
6
Fig. 1 X-ray crystal structure of [(h -C6H5)(C6H4-4-NH2)]Cr(CO)3 (1).
Scheme 1 Synthesis of isomer 2 by direct heating.
The 1H NMR spectrum of 2 in acetone-d6 shows characteristic
signals for the para-amino-substituted arene ring shifted markedly
upfield (5.24–6.25 ppm) compared to free 5, whereas signals for
protons of the other ring appear in the free phenyl region of the
spectrum (7.34–7.55 ppm). This chemical shift pattern implies
that the amino-substituted ring is coordinated to the Cr(CO)3
fragment.
6
The analog [(h -C6H5)(C6H4-4-NH2)]Cr(CO)3 (1), is a known
compound that has a para-amino group attached to the uncoordi-
nated ring. It was synthesized in 18% overall yield from Cr(CO)6,
by repeating the method of Gubin and coworkers (Scheme 2).7,11
In this scheme, 4-acetamidobiphenyl (6) reacts with Cr(CO)6 to
6
Fig. 2 X-ray crystal structure of [(h -C6H4-4-NH2)(C6H5)]Cr(CO)3 (2).
6
produce a mixture of [(h -C6H5)(C6H4-4-NHCOCH3)]Cr(CO)3
6
(7) and [(h -C6H4-4-NHCOCH3)(C6H5)]Cr(CO)3 (8) which can
The crystal structure of 2 shows some notable deviations from
the ideal three-legged piano-stool geometry. The coordinated ring
is slightly tilted away from the carbonyl atom C13, which makes the
Cr1–C2 and Cr1–C4 distances longer (av. 2.25 A) than the Cr1–
C1 and Cr–C5 distances (av. 2.20 A). The Cr1–C3 bond length
of 2.3541(14) A substantially exceeds the Cr1–C6 distance of
2.2444(14) A. The substituted atoms C3 and C6 are displaced from
the plane of the arene ring away from the chromium, rendering
a slight boat distortion to the ring. The dihedral angles between
the plane defined by atoms C1, C2, C4, and C5 and planes C2–
C3–C4 and C1–C5–C6 are calculated to be 6.51(19)◦ and 3.8(2)◦,
be separated by column chromatography. Since an amide is less
electron donating and more sterically demanding than an amino
group, it is not surprising that the major isomer formed is 7, which
is then hydrolyzed under basic conditions to form 1.
˚
˚
The 1H NMR spectrum of 1 in acetone-d6 shows characteristic
signals for the para-amino-substituted ring in the free aromatic re-
gion of the spectrum (6.71–7.38 ppm), while signals for the metal-
bound phenyl ring are shifted upfield (5.51–5.95 ppm). While 1 is
a known compound, its crystallographic characterization had not
been previously reported. Obtaining single-crystal X-ray crystal
˚
˚
Scheme 2 Synthesis and thermal rearrangement of isomer 1.
9440 | Dalton Trans., 2011, 40, 9439–9446 This journal is
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respectively. The arene–phenyl dihedral angle spans 38.01(6)◦. In
contrast, in 1 the dihedral angle between planes C1–C2–C4–C5
and C1–C5–C6 is only 1.24(18)◦, but the arene–phenyl dihedral
angle is very similar at 38.28(4)◦. The Cr1–C(Ph) distances in 6
fall within the expected range.
When 1 was heated in dibutyl ether at 140 ◦C, thin-layer chro-
matography showed gradual disappearance of 1 and formation of
2. Continued heating overnight led to the formation and isolation
of 2 in 85% yield, with no 1 detectable by 1H NMR spectroscopy
(Scheme 2). To determine the rate of rearrangement of 1 to 2, a
degassed diglyme-d14 solution of 1 was sealed in an NMR tube and
the solution was heated at 140 ◦C. Periodic collection of 1H NMR
spectra led to the determination of a rate constant of 5.0 ¥ 10-5 s-1
(half-life ª 230 min) for the rearrangement of 1 to 2. Comparison
to the internal standard showed that a small amount (~10%) of
decomposition to 5 occurred during this rearrangement.
Scheme 3 Synthesis of isomer 4 by direct heating.
free aromatic region of the spectrum (7.4–7.6 ppm). This indicates
that the amino-substituted ring is coordinated to the Cr(CO)3
fragment.
Analog 3 was synthesized by first reacting 2-acetamidobiphenyl
(10) with chromium hexacarbonyl under reflux with 9 : 1 dibutyl
6
ether : tetrahydrofuran, which led to isolation of [(h -C6H5)(C6H4-
2-NHCOCH3)]Cr(CO)3 (11) in 38% yield.
6
The other isomer, [(h -C6H4-2-(NHCOCH3)-C6H5)]Cr(CO)3
(12), though formed and observed by 1H NMR, was not isolated.
Basic hydrolysis of 11 gave 3 in 14% overall yield, based on the
starting amount of Cr(CO)6 (Scheme 4).
The observed rearrangement of 1 to 2 was especially interesting
in light of the 38.01(6)◦ arene–phenyl dihedral angle in 1. It
suggests that the strong electron donating effect of a substituent
group, amino in this case, can predominate over steric effects,
promoting haptotropic rearrangement even when the observed
arene–phenyl dihedral angle is greater than 22◦. In an attempt to
further examine this predominance of electronic effects, we wanted
The 1H NMR spectrum of 3 in acetone-d6 shows signals for the
ortho-amino-substituted ring in the free aromatic region (6.70–
7.24 ppm), while signals for the protons of the unsubstituted ring
are shifted upfield (5.68–5.87 ppm) into the metal-bound aryl
region of the spectrum, indicating that the unsubstituted ring is
coordinated to the Cr(CO)3 fragment. The bonding arrangements
in 3 and 4 are clearly shown in their single-crystal X-ray crystal
structures (Fig. 3 and 4).
6
to synthesize the ortho-substituted analog [(h -C6H5)(C6H4-2-
NH2)]Cr(CO)3 (3), since we expected it to have an arene–phenyl
dihedral angle even larger than that observed in 1. Our DFT
geometry optimization of 1 performed at the B3LYP/6-31G* level
of theory produced a 34.7(11)◦ arene–phenyl dihedral angle.12 The
equivalent angle in the theoretically-optimized isomer 3 measured
51.9(10)◦.
In the single crystal X-ray crystal structure of 4, the phenyl
and amino substituents of the coordinated ring are each bent
slightly upwards, away from the chromium. Atoms C1 and C6
are displaced from the plane defined by atoms C2–C3–C4–C5
˚
When 2-aminobiphenyl (9) was heated with chromium hexacar-
bonyl in 9 : 1 dibutyl ether : tetrahydrofuran, the single product
obtained was [(h -C6H4-2-NH2)(C6H5)]Cr(CO)3 (4), which has a
by 0.062(3) and 0.008(4) A, respectively, rendering a very slight
1.69(9)◦ butterfly folding of the arene ring along the C2–C5
6
vector. The six Cr–C(arene) bond distances range from 2.2129(1)
to 2.3077(18) A, with the Cr1–C1 bond length being the longest.
These features, which may result from steric interactions within
˚
chromium tricarbonyl fragment coordinated to the ortho-amino-
1
substituted arene ring (Scheme 3). The H NMR spectrum of 4
in acetone-d6 shows signals for the ortho-amino-substituted arene
ring shifted upfield (4.02–5.90 ppm) compared to free 9, whereas
signals for the protons for the unsubstituted ring appear in the
4, are not evident in the structure of 3, where all six Cr–C(arene)
bond lengths are within 0.012 A, with an average of 2.216(8) A. The
C1–C7 bond that connects the two arene rings is nearly coplanar
˚
˚
Scheme 4 Synthesis and thermal rearrangement of isomer 3.
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A degassed ortho-dichlorobenzene-d4 solution of 1 was sealed
in an NMR tube and the solution was heated at 140 ◦C. Periodic
collection of 1H NMR spectra led to the determination of a rate
constant of 1.7 ¥ 10-5 s-1 (half-life ª 680 min) for the rearrangement
of 1 to 2, with less than 10% of decomposition to 5 occurring
during this rearrangement. This is approximately three times
slower than the rate measured in diglyme-d14. Similarly, a degassed
ortho-dichlorobenzene-d4 solution of 3 was sealed in an NMR
tube and the solution was heated at 140 ◦C. Periodic collection
1
of H NMR spectra led to the determination of a rate constant
of 2.1 ¥ 10-4 s-1 (half-life ª 54 min) for the rearrangement of
3 to 4, showing that the rearrangement is only slightly slower
in ortho-dichlorobenzene-d4 than in diglyme-d14. During this
rearrangement, decomposition to 9 accounted for less than 10%
of material at the end of the reaction.
6
Fig. 3 X-ray crystal structure of [(h -C6H5)(C6H4-2-NH2)]Cr(CO)3 (3).
The results of these measurements show that in diglyme-d14
the rearrangement of 3 to 4 is nearly five times faster than
the rearrangement of 1 to 2, and in ortho-dichlorobenzene-d4
the rate enhancement is twelve-fold. This was surprising since
the ortho-substituted isomer 3 has an arene–phenyl dihedral
angle that is nearly 30 degrees greater than that found in
6
the para-substituted isomer 1. Furthermore, ortho-bromo [(h -
C6H5)C6H4Br]Cr(CO)3 has been previously reported to rearrange
more slowly than its para-substituted isomer, and ortho-methyl
6
[(h -C6H5)C6H4CH3]Cr(CO)3 does not undergo rearrangement,
presumably due to a large 62.01◦ dihedral angle.5 It was likewise
expected that the extreme conformational twisting of rings, shown
both crystallographically and by calculation, would prevent or at
least somewhat decrease the observed rate of rearrangement of
3 to 4. One possible explanation for the observed rate increase
is that the ortho-amino group in 3 may cause a reduction of
the arene–phenyl dihedral angle during the early steps of the
mechanism. The amino group could donate electron density to
chromium, providing intramolecular stabilization of unsaturated
intermediates that would be formed during conversion to 4
(Scheme 5). Though it was not directly observed by NMR, the type
6
Fig. 4 X-ray crystal structure of [(h -CH-2-NH)(CH)]Cr(CO) (4).
with the coordinated ring. The structures of both 3 and 4
exhibit large arene–phenyl dihedral angles spanning 67.70(7)◦ and
63.05(5)◦, respectively. These large dihedral angles, much larger
than those observed in the para isomers 1 and 2, presumably result
from steric interactions of the amino group with ortho-hydrogen
atoms of the other ring.6
Heating 3 at 140 ◦C caused it to convert to 4. TLC monitoring of
a refluxing solution of 3 in dibutyl ether showed that the conversion
of 3 to 4 was complete in four hours, leading to isolation of 4 in
90% yield, with no 3 detectable by 1H NMR. To determine the rate
of rearrangement of 3 to 4, a degassed diglyme-d14 solution of 3
was sealed in an NMR tube and the solution was heated at 140 ◦C.
of intermediate formed would be similar to (h -carbazole)Cr(CO)3
6
6
or (h -pyrrole)Cr(CO)3 compounds, which are known.13,14 By
comparison, the para-amino group in 1 would not be capable of
providing such intramolecular stabilization during the conversion
of 1 to 2.
1
Periodic collection of H NMR spectra led to the determination
of an observed rate constant of 2.6 ¥ 10-4 s-1 (half-life ª 44 min) for
the rearrangement of 3 to 4. Comparison to the internal standard
showed that less than 10% of decomposition to 9 had occurred
during the rearrangement.
Scheme 5 Proposed rearrangement via a carbazole-like intermediate.
NMR rate measurements for conversion of 1 to 2 and 3 to 4
were repeated in ortho-dichlorobenzene-d4. While the potential
role of diglyme or ortho-dichlorobenzene in the mechanisms of
these rearrangements cannot be completely discounted, poly-
halogenated arenes tend not to coordinate to Cr(CO)3 to form
stable compounds. Heating separate samples of 1 and 3 in ortho-
dichlorobenzene (non-deuterated) led to a rearrangement to 2
in 92% yield and 4 in 89% yield, respectively, with no observed
(ortho-dichlorobenzene)Cr(CO)3 formed in either case. NMR rate
measurements were not performed in hydrocarbon solvents such
as decane or decalin since compounds 1–4 are insoluble in these
hydrocarbons.
For both 1 to 2 and 3 to 4 the rates of rearrangement appear
to be independent of concentration. For conversion of 1 to 2,
when NMR rate measurements in ortho-dichlorobenzene-d4 were
repeated with initial concentrations of 1 ranging between 0.03 M
and 0.12 M, the observed rate constants varied from 1.3 ¥ 10-5
s-1 to 1.7 x10-5 s-1. For conversion of 3 to 4, when NMR rate
measurements in ortho-dichlorobenzene-d4 were repeated with
concentrations ranging from 0.03 M to 0.12 M, the observed
rate constants varied from 1.8 ¥ 10-4 s-1 to 2.2 ¥ 10-4 s-1,
and higher initial concentrations of 1 or 3 did not necessarily
9442 | Dalton Trans., 2011, 40, 9439–9446
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Table 1 Crystal data and data collection parameters for 1–4
Identification code
1
2
3
4
Empirical formula
Formula weight
Temperature
C15H11CrNO3
305.25
C15H11CrNO3
305.25
C15H11CrNO3
305.25
C15H11CrNO3
305.25
100(2) K
100(2) K
100(2) K
100(2) K
˚
˚
˚
˚
Wavelength
0.71073 A
1.54178 A
0.71073 A
0.71073 A
Crystal system
Monoclinic
P21/n
6.3121(6)
12.3958(12)
16.2900(15)
90
Triclinic
P1
Monoclinic
P21/c
Orthorhombic
Pbca
7.5427(5)
12.7303(9)
26.7851(19)
90
¯
Space group
˚
a (A)
8.4228(3)
8.8378(3)
9.1761(4)
96.810(3)
108.190(3)
90.924(3)
7.2106(9)
28.140(3)
7.1535(8)
90
118.175(2)
90
˚
b (A)
˚
c (A)
a (◦)
b (◦)
96.9940(10)
90
90
g (◦)
90
3
3
3
3
˚
˚
˚
˚
Volume
1265.1(2) A
643.32(4) A
2
1279.5(3) A
2571.9(3) A
Z
4
4
8
Density (calculated)
Absorption coefficient
F(000)
Crystal size/mm
Theta range for data
collection
1.603 Mg m-3
0.909 mm-1
624
1.576 Mg m-3
7.381 mm-1
312
1.585 Mg m-3
0.899 mm-1
624
1.577 Mg m-3
0.895 mm-1
1248
0.41 ¥ 0.39 ¥ 0.35
0.44 ¥ 0.29 ¥ 0.26
0.37 ¥ 0.36 ¥ 0.20
0.47 ¥ 0.42 ¥ 0.19
2.52 to 29.97◦
5.05 to 71.55◦
2.90 to 26.47◦
3.04 to 26.39◦
Index ranges
-8 £ h £ 8 -17 £ k £ 16 -22
£ l £ 22
-8 £ h £ 9 -10 £ k £ 10 -11
£ l £ 10
-9 £ h 8 -35 £ k £ 34 -8
£ l £ 8
-9 £ h £ 9, -15 £ k £ 15 -33
£ l £ 33
Reflections collected
Independent
reflections
17223
12763
10331
19756
3465 [R(int) = 0.0242]
2365 [R(int) = 0.0168]
2615 [R(int) = 0.0254]
2629 [R(int) = 0.0288]
Completeness to
resolution of 0.84 A
99.1%
94.2%
99.3%
99.7%
˚
Absorption correction
Max. and min.
Multi-scan with SADABS
0.7414 and 0.7068
Empirical with SADABS
0.2489 and 0.1412
Multi-scan with SADABS
0.8406 and 0.7320
Multi-scan with SADABS
0.8484 and 0.6785
transmission
Refinement method
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Full-matrix least-squares
on F2
Data / restraints /
parameters
3465 / 0 / 187
2365 / 0 / 189
2615 / 0 / 187
2629 / 1 / 187
Goodness-of-fit on F2
Final R indices
[I > 2s(I)]
1.042
1.013
1.060
1.033
R1 = 0.0275 wR2 = 0.0770
R1 = 0.0214 wR2 = 0.0586
R1 = 0.0292 wR2 = 0.0804
R1 = 0.0350 wR2 = 0.0980
R indices (all data)
Largest diff peak and
hole
R1 = 0.0305 wR2 = 0.0795
R1 = 0.0216 wR2 = 0.0587
R1 = 0.0304 wR2 = 0.0813
R1 = 0.0391 wR2 = 0.1011
-3
-3
-3
-3
˚
˚
˚
˚
0.488 and -0.237 e.A
0.288 and -0.238 e.A
0.410 and -0.307 e.A
0.746 and -0.392 e.A
correspond to a faster observed rate of rearrangement. The results
of these observations at various concentrations are consistent
with a unimolecular reaction and intramolecular haptotropic
rearrangement for both 1 and 3. It is worth noting that the
observed rate constants for the para isomer 1 are within an order
of magnitude of the rate constants previously reported for the
approximately 10% formation of 2 after four half-lives. The results
of these crossover experiments indicate that while the observed
rearrangements of 1 and 3 may involve some intermolecular
exchange of ligands, they are predominantly intramolecular.
Conclusions
6
series [(h -C6H5)C6H4X]Cr(CO)3 (X = Br, D, CH3, SnMe3), while
the observed rate constants for the ortho isomer 3 are an order of
The observed thermal rearrangements of 1 and 3 demonstrate
that haptotropic rearrangements can occur in substituted (h -
magnitude greater.5
6
To further examine the intermolecular versus intramolecular
nature of the rearrangements of 1 to 2 and 3 to 4, the NMR rate de-
terminations were repeated in the presence of free aminobiphenyl
ligand, with the reaction progress monitored for signs of crossover
products. When a solution of 1 in ortho-dichlorobenzene-d4 was
heated at 140 ◦C in the presence of an equimolar amount of
9, the expected rearrangement to 2 occurred, with an observed
rate constant of 1.8 ¥ 10-4 s-1, along with approximately 12%
formation of 4 after four half-lives. Similarly, when 3 in ortho-
dichlorobenzene-d4 was heated at 140 ◦C in the presence of an
equimolar amount of 5, the expected rearrangement to 4 occurred,
with an observed rate constant of 2.2 ¥ 10-5 s-1, along with
biphenyl)Cr(CO)3 compounds even if calculations or single crystal
X-ray crystallography show a large canting of arene rings. At least
in the case of the strongly electron-donating amino substituent,
electronic factors can predominate over steric effects. These results
emphasize that while single crystal X-ray crystallography and
DFT calculations can indicate the presence of strong steric effects
or unfavorable conformations within a compound, they might
not be the ultimate predictor of the possibility of haptotropic
rearrangement in solution, where near coplanarity of arene rings
can be readily achieved through C–C single bond rotation.6 At the
very least, these results suggest that any upper limit on arene–arene
dihedral angle should likely be higher than 22◦. The unexpectedly
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accelerated rearrangement of 3 to 4 indicates that in specific cases,
where the ring substituent could also coordinate to the metal
as a donor ligand, the predicted solid state or calculated arene–
arene dihedral angle of the starting compound may be even less
important in predicting the possibility of inter-ring haptotropic
rearrangement. These results may be especially important for the
atmosphere. Solvents were removed by vacuum distillation.
Column chromatography (silica gel, 75 : 25 diethyl ether : hexane)
of the remaining oily residue gave two yellow bands. Collection of
the second yellow band led to isolation of 7 as an air-stable yellow
1
solid (1.60 g, 48%). H NMR (acetone-d6, 300 MHz) d 8.95 (s,
1H, NH), 7.94 (d, J = 6.9 Hz, 2H), 7.29 (d, J = 6.9 Hz, 2H), 5.90
(d, J = 6.5 Hz, 2H), 5.73 (t, J = 6.5 Hz, 2H), 5.44 (t, J = 6.5 Hz,
1H), 2.80 (s, 3H, CH3).
6
further design of (h -arene)Cr(CO)3 compounds that can act as
molecular switches.
[(g6-C6H5)(C6H4-4-NH2)]Cr(CO)3 (1). This was prepared ac-
cording to the method described by Gubin.7 A mixture of 7 (1.10 g,
3.17 mmol) and KOH (3.5 g, 62.4 mmol) in 3 : 1 ethanol : water
(25 mL) was degassed and heated at 80 ◦C for 7 h under a
nitrogen atmosphere. The solution was extracted with CH2Cl2 (3 ¥
30 mL) and the combined organic extracts were dried (MgSO4),
filtered, concentrated, and purified by column chromatography
(silica gel, 50 : 50 hexane : diethyl ether) t◦o give 1 as a yellow air-
Experimental
General methods
Reactions and manipulations of air-sensitive materials were
performed under a nitrogen atmosphere using standard glove
box and Schlenk techniques. NMR spectra were obtained on
a Bruker AC300 spectrometer (1H NMR at 300 MHz and 13C
NMR at 75 MHz) and a Bruker Avance III 400 spectrometer
(1H NMR at 400 MHz and 13C NMR at 100 MHz). Solution
infrared spectra were recorded on a MIDAC Prospect-IR PRS-
102 using CsI solution cells. Mass spectra were obtained on an Au-
toSpec magnetic sector mass spectrometer (Waters (Micromass)).
Elemental analyses were performed by Columbia Analytical
Services (Tucson, AZ). Hexane, tetrahydrofuran, and diethyl ether
were purified using a Vacuum Atmospheres Company 103991
Solvent Purifier immediately prior to use. Di-n-butyl ether was
stirred over sodium and vacuum distilled prior to use. Chromium
hexacarbonyl, 2-aminobiphenyl, 4-aminobiphenyl, acetic anhy-
dride, and 2,6-difluorotoluene, obtained from Aldrich Chemical
Company, were used without further purification. CDCl3, acetone-
d6, diglyme-d14, and ortho-dichlorobenzene-d4 were obtained from
Cambridge Isotope Laboratories, Inc. and were used without
further purification.
◦
1
stable solid (0.354 g, 37%), mp 150–152 C (lit. 156–157 C). H
NMR (acetone-d6, 300 MHz) d 7.39 (d, J = 6.9 Hz, 2H), 6.71 (d,
J = 6.9 Hz, 2H), 5.95 (d, J = 6.5 Hz, 2H), 5.78 (t, J = 6.5 Hz, 2H),
5.50 (t, J = 6.5 Hz, 1H), 4.96 (s, 2H).
[(g6-C6H4-2-NH2)(C6H5)]Cr(CO)3
(4).
A
mixture
of
chromium hexacarbonyl (1.050 g, 4.77 mmol) and 2-
aminobiphenyl (0.810 g, 4.79 mmol) in di-n-butyl ether
(27 mL) and tetrahydrofuran (3 mL) was degassed and heated
under reflux for 24 h under a nitrogen atmosphere. Solvents
were removed by vacuum distillation. Column chromatography
(silica gel, 50 : 50 hexane : diethyl ether) of the remaining yellow
solid gave a yellow band from which 4 was isolated as a yellow,
◦
1
air-stable solid (1.28 g, 88% yield), mp 153–156 C. H NMR
(acetone-d6, 300 MHz) d 7.4–7.6 (m, 5H, aryl), 5.90 (m, 2H,
metal-bound aryl), 5.22 (d, J = 6.9 Hz, 1H, metal-bound aryl),
4.02 (m, 3H, NH2 overlapping with metal-bound aryl). 1H NMR
(CDCl3, 300 MHz) d 7.4–7.6 (m, 5H, aryl), 5.72 (d, J = 6.9 Hz,
1H, metal-bound aryl), 5.65 (t, J = 6.9 Hz, 1H, metal-bound
aryl), 4.89 (d, J = 6.9 Hz, 1H, metal-bound aryl), 4.87 (t, J =
Syntheses
[(g6-C6H4-4-NH2)(C6H5)]Cr(CO)3
(2).
A
mixture
of
1
6.9 Hz, 1H, metal-bound aryl), 3.64 (s, 2H, NH2). 13C{ H} NMR
chromium hexacarbonyl (1.00 g, 4.54 mmol) and 4-aminobiphenyl
(0.780 g, 4.61 mmol) in di-n-butyl ether (27 mL) and
tetrahydrofuran (3 mL) was degassed and heated under
reflux for 24 h under a nitrogen atmosphere. Solvents were
removed by vacuum distillation. Column chromatography (silica
gel, 80 : 20 hexane : diethyl ether) of the remaining yellow solid
gave a fast-moving yellow band from which 2 was isolated as
a yellow, air-stable solid (0.640 g, 46% yield), mp 173–175 ◦C.
1H NMR (acetone-d6, 300 MHz) d 7.55 (m, 2H, aryl), 7.34 (m,
3H, aryl), 6.25 (d, J = 6.9 Hz, 2H, metal-bound aryl), 5.40 (s,
(CDCl3, 75 MHz) d 234.2, 130.6, 129.8, 129.1, 128.7, 107.4, 100.0,
96.3, 92.0, 82.3, 75.3. IR (CH2Cl2) 3462, 3362, 1958, 1877 cm-1.
HRMS (EI) calcd for C15H11CrNO3 305.0144, found 305.0140.
Anal. calcd for C15H11CrNO3: C, 59.02; H, 3.63; N, 4.59. Found
C, 58.73; H, 3.46; N, 4.54.
[(g6-C6H5)(C6H4-2-NHCOCH3)]Cr(CO)3 (11).
A
mixture
of chromium hexacarbonyl (2.00 g, 9.09 mmol) and 2-
acetamidobiphenyl (2.00 g, 9.48 mmol) in di-n-butyl ether (27 mL)
and tetrahydrofuran (3 mL) was degassed and heated under reflux
for 24 h under a nitrogen atmosphere. Solvents were removed by
vacuum distillation. Column chromatography (silica gel, 75 : 25
diethyl ether : hexane) of the remaining oily residue gave two yellow
bands. Collection of the second yellow band led to isolation of 11
as an air-stable yellow solid (1.20 g, 38%). 1H NMR (acetone-d6,
300 MHz) d 8.79 (s, 1H, NH), 7.65 (d, J = 6.9 Hz, 1H, aryl),
7.47 (d, J = 6.9 Hz, 1H, aryl), 7.36 (t, J = 6.9 Hz, 1H, aryl), 7.28
(t, J = 6.9 Hz, 1H, aryl), 5.89 (dt, J = 6.5, 1.0 Hz, 2H, metal-
bound aryl), 5.73, (m, 3H, metal-bound aryl), 2.87 (s, 3H, CH3).
1H NMR (CDCl3, 300 MHz) d 8.10 (d, J = 6.9 Hz, 1H, aryl), 7.88
(s, 1H, NH), 7.37 (m, 2H, aryl), 7.21 (t, J = 6.9 Hz, 1 H, aryl),
2H, NH2), 5.24 (d, J = 6.9 Hz, 2H, metal-bound aryl). 13C{ H}
1
NMR (acetone-d6, 75 MHz) d 236.1, 129.9, 128.7, 127.4, 126.8,
115.9, 98.3, 78.1, 76.0. IR (CH2Cl2) 3451, 3390, 1949, 1866 cm-1.
HRMS (EI) calcd for C15H11CrNO3 305.0144, found 305.0139.
Anal. calcd for C15H11CrNO3: C, 59.02; H, 3.63; N, 4.59. Found:
C, 59.14; H, 3.60; N, 4.52.
[(g6-C6H5)(C6H4-4-NHCOCH3)]Cr(CO)3 (7). This com-
pound was prepared according to the method described
by Gubin.8 A mixture of chromium hexacarbonyl (2.10 g,
9.55 mmol) and 4-acetamidobiphenyl15 (2.03 g, 9.62 mmol)
in di-n-butyl ether (27 mL) and tetrahydrofuran (3 mL) was
degassed and heated under reflux for 24 h under a nitrogen
1
5.50 (m, 5H, metal-bound aryl), 2.27 (s, 3H, CH3). 13C{ H} NMR
9444 | Dalton Trans., 2011, 40, 9439–9446
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(CDCl3, 75 MHz) d 232.0, 168.8, 134.8, 131.3, 130.2, 125.3, 123.7,
108.6, 107.8, 94.6, 92.9, 92.5, 24.8. IR (CH2Cl2) 3326, 1973, 1896,
1696 cm-1. HRMS (EI) calcd for C17H13CrNO4 347.0250, found
347.0233. Anal. calcd for C17H13CrNO4: C, 58.79; H, 3.77; N, 4.03.
Found C, 58.78; H, 3.57; N, 4.41.
Preparative scale rearrangement of 1 to 2
A solution of 1 (0.200 g, 0.656 mmol) in di-n-butyl ether
(10 mL) in a Schlenk flask equipped with a condenser was
degassed and heated under reflux under a nitrogen atmosphere.
Periodic monitoring by thin layer chromatography showed gradual
disappearance of 1 and formation of 2. After heating for 16 h, the
solvent was removed by vacuum distillation and the remaining
yellow residue was taken up in diethyl ether and filtered through
a pad of silica gel. Evaporation of the solvent gave 2 as a yellow
powder (0.169 g, 85% yield, > 95% pure by 1H NMR).
[(g6-C6H5)(C6H4-2-NH2)]Cr(CO)3 (3). A mixture of 11 (1.00 g,
2.88 mmol) and KOH (3.0 g, 53.5 mmol) in 3 : 1 ethanol : water
(25 mL) was degassed and heated at 80 ◦C for 7 h under a nitrogen
atmosphere. The solution was extracted with CH2Cl2 (3 ¥ 30 mL)
and the combined organic extracts were dried (MgSO4), filtered,
concentrated, and purified by column chromatography (silica gel,
50 : 50 hexane : diethyl ether) to give 3 as a yellow air-stable solid
(0.322 g, 37%), mp 128–130 ◦C. 1H NMR (acetone-d6, 300 MHz)
d 7.24 (d, J = 6.9 Hz, 1H, aryl), 7.09 (t, J = 6.9 Hz, 1H, aryl), 6.81
(d, J = 6.9 Hz, 1H, aryl), 6.70 (t, J = 6.9 Hz, 1H, aryl), 5.87 (d, J =
6.5 Hz, 2H, metal-bound aryl), 5.68, (m, 3H, metal-bound aryl),
Preparative scale rearrangement of 3 to 4
A solution of 3 (0.210 g, 0.688 mmol) in di-n-butyl ether
(10 mL) in a Schlenk flask equipped with a condenser was
degassed and heated under reflux under a nitrogen atmosphere.
Periodic monitoring by thin layer chromatography showed gradual
disappearance of 3 and formation of 4. After heating for 4 h, the
solvent was removed by vacuum distillation and the remaining
yellow residue was taken up in diethyl ether and filtered through
a pad of silica gel. Evaporation of the solvent gave 4 as a yellow
powder (0.190 g, 90% yield, > 95% pure by 1H NMR).
1
4.82 (s, 2H, NH2). H NMR (CDCl3, 300 MHz) d 7.31 (d, J =
6.9 Hz, 1H, aryl), 7.19 (t, J = 6.9 Hz, 1H, aryl), 6.84 (t, J = 6.9 Hz,
1H, aryl), 6.77 (d, J = 6.9 Hz, 1H, aryl), 5.67 (d, J = 6.5, 2H,
metal-bound aryl), 5.44, (m, 3H, metal-bound aryl), 4.13 (s, 2H,
1
NH2). 13C{ H} NMR (CDCl3, 75 MHz) d = 232.4, 144.4, 131.3,
130.4, 129.5, 119.2, 116.8, 110.3, 94.9, 93.0, 92.5. IR (CH2Cl2)
3429, 3330, 1970, 1892 cm-1. HRMS (EI) calcd for C15H11CrNO3
305.0144, found 305.0140. Anal. calcd for C15H11CrNO3: C, 59.02;
H, 3.63; N, 4.59. Found C, 59.19; H, 3.80; N, 4.53.
Aminobiphenyl crossover in the rearrangement of 1 to 2
In an NMR tube were placed 1 (0.020 g, 0.066 mmol), 2,6-
difluorotoluene (0.010 g, 0.078 mmol), 2-aminobiphenyl (0.012 g,
0.071 mmol), and ortho-dichlorobenzene-d4 (0.550 mL). The
tube was degassed by three freeze-pump-thaw cycles and sealed
Rate constant determinations
In a typical experiment, 1 or 3 (0.010–0.015 g) and the in-
ternal standard 2,6-difluorotoluene (0.005–0.010 g), which does
not coordinate to Cr(CO)3 even upon prolonged heating at
140 ◦C, were placed in an NMR tube. Diglyme-d14 or ortho-
dichlorobenzene-d4 (0.500–0.600 mL) was added to establish
a 0.06 M concentration of the chromium arene complex and
approximately a 0.06 M concentration of 2,6-difluorotoluene. For
concentration dependence studies, the starting masses of 1 or
3 ranged from 0.005–0.008 g for ~0.03 M solutions to 0.020–
0.030 g for ~0.12 M solutions. The solution was subjected to three
freeze-pump-thaw cycles and sealed under vacuum. An initial 1H
NMR spectrum was collected at ambient temperature and the
tube was completely immersed in a silicone oil bath with a Glas-
Col DigiTrol II thermostat controller maintaining the temperature
at 140 0.5 ◦C. Periodically, the rearrangement reaction was
quenched simply by removing the tube from the oil bath, collecting
a spectrum, and returning the tube to the bath for continued
heating. The process was continued for at least four half-lives. For
conversion of 1 to 2, the disappearance of metal-coordinated arene
peaks of 1 was monitored and these peaks were integrated relative
to the CH3 peak of the internal standard. For conversion of 3 to
4, disappearance of the upfield metal-coordinated arene peak of 3
was monitored, since this peak is well-separated from the metal-
coordinated arene peaks of 4. This peak was integrated relative to
the CH3 peak of the internal standard. For each series of spectra,
logarithms (of arene area/initial arene area) were plotted versus
time to yield a straight line from which half-lives and rate constants
were calculated. For each of the NMR-scale rearrangements of 1
to 2 and 3 to 4, final spectra showed that the free ligands 4-
aminobiphenyl and 2-aminobiphenyl, respectively, constituted less
than 10% of the products.
1
under vacuum. An initial H NMR spectrum was collected at
ambient temperature and the tube was completely immersed in
a thermostat-controlled oil bath at 140 ◦C. After 4 half-lives,
1
a H NMR spectrum at ambient temperature showed that the
rearrangement of 1 to 2 had occurred with approximately 12%
formation of the intermolecular crossover product 4.
Aminobiphenyl crossover in the rearrangement of 3 to 4
In an NMR tube were placed 3 (0.018 g, 0.059 mmol), 2,6-
difluorotoluene (0.010 g, 0.078 mmol), 4-aminobiphenyl (0.011 g,
0.065 mmol), and ortho-dichlorobenzene-d4 (0.490 mL). The
tube was degassed by three freeze-pump-thaw cycles and sealed
1
under vacuum. An initial H NMR spectrum was collected at
ambient temperature, and the tube was completely immersed in
a thermostat-controlled oil bath at 140 ◦C. After 4 half-lives,
1
a H NMR spectrum at ambient temperature showed that the
rearrangement of 3 to 4 had occurred with approximately 10%
formation of the intermolecular crossover product 2.
X-ray crystallographic analysis
Under a nitrogen atmosphere at ambient temperature, slow
evaporation of diethyl ether solutions of 1–4 led to the formation
of single crystals of each compound suitable for X-ray crystallo-
graphic analysis. Crystallographic and data collection parameters
are provided in Table 1. Molecular drawings of 1–4 showing the
atom numbering schemes are provided in Fig. 1, 2, 3, and 4,
respectively. Additional crystallographic details are available in
the ESI.†
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3 J. Pan, J. W. Kampf, A. J. Ashe and III, Organometallics, 2006, 25,
197; H. C. Jahr, M. Nieger and K. H. Do¨tz, Chem. Commun., 2003,
2866.
4 Y. F. Oprunenkno, O. B. Afanasova, I. A. Shaposhnikova and Y. A.
Ustynyuk, Metalloorg. Khim., 1991, 4, 1377.
Acknowledgements
Support for this research has been provided by the United
States Army Research Office DURIP (Defense University Re-
search Instrumentation Program), the University of Wisconsin-
La Crosse Faculty Research Grant Program, and the University
of Wisconsin-La Crosse Undergraduate Research Grant Program.
We gratefully acknowledge Prof. Robert McGaff (UW-La Crosse)
for his helpful discussions.
5 Y. F. Oprunenko, Russ. Chem. Rev., 2000, 69, 683.
6
6 Low temperature NMR studies of the bromobiphenyl analog ((h -
C6H5)(C6H4-2-Br)Cr(CO)3 gave an upper limit of 9 kcal mol-1 for
rotation about the arene–arene bond. C. J. Czerwinski, I. A. Guzei,
T. J. Cordes, K. M. Czerwinski and N. A. Mlodik, Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun., 2003, 59, m499.
7 C. A. L. Mahaffy and P. L. Pauson, Inorg. Synth., 1979, 19,
154.
8 A. Z. Kreindlin, V. X. Khandkarova and S. P. Gubin, J. Organomet.
Chem., 1975, 92, 197.
Notes and references
1 K. H. Do¨tz and H. C. Jahr, Chem. Rev., 2004, 4, 61; M. J. Morris, in
Comprehensive Organometallic Chemistry II, ed. E. W. Abel, F. G. A.
Stone and G. Wilkinson, Pergamon Press, New York, 1995, 5, 501; R.
Benn, R. Mynott, I. Topalovic and F. Scott, Organometallics, 1989, 8,
2299; A. Stanger, Organometallics, 1991, 10, 2979; K. Nakasuji, M.
Yamaguchi and I. Murata, J. Am. Chem. Soc., 1986, 108, 325; J. Tsuji,
J. Organomet. Chem., 1986, 300, 281; R. U. Kirss and P. M. Treichel,
J. Am. Chem. Soc., 1986, 108, 853; K. H. Do¨tz, J. Stendel, S. Mu¨ller,
M. Nieger, S. Ketrat and M. Dolg, Organometallics, 2005, 24, 3219;
R. H. Crabtree and C. P. Parnell, Organometallics, 1984, 3, 1727; S. D.
9 H. Lumbroso, C. Liegeois, D. A. Brown and N. Fitzpatrick, J.
Organomet. Chem., 1979, 165, 341.
1
10 While the H NMR spectrum of 2 has been previously reported, the
compound has not been previously isolated or fully characterized. Full
details of its synthesis, isolation, and characterization are included in
the experimental section of this paper.
11 In the original report of this synthesis, 1 was synthesized in 19% yield
from Cr(CO)6 (ref. 8).
12 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
et al., Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT,
USA, 2004.
¨
Cunningham, K. Ofele and B. R. Willeford, J. Am. Chem. Soc., 1983,
105, 3724; Y. F. Oprunenkno, O. B. Afanasova, I. A. Shaposhnikova, Y.
A. Ustynyuk, N. A. Ustynyuk, D. N. Kravtsov and E. A. Chernyshev,
Metalloorg. Khim., 1988, 1, 815; A. Ceccon, A. Gambaro, F. Gottardi,
S. Santi and A. Venzo, J. Organomet. Chem., 1991, 412, 85.
2 Haptotropic rearrangement has been the subject of several theoretical
studies. See: T. A. Albright, P. Hormann, R. Horrmann, C. P. Lillya
and P. A. Dobosh, J. Am. Chem. Soc., 1983, 105, 3396; J. Silvestre and
T. A. Albright, J. Am. Chem. Soc., 1985, 107, 6829; S. Ketrat, S. Mu¨ller
and M. Dolg, J. Phys. Chem. A, 2007, 111, 6094; Y. F. Oprunenko,
N. G. Akhmedov, D. N. Laikov, S. G. Malyugina, V. I. Mstislavsky, V.
A. Roznyatovsky, Y. A. Ustynyuk and N. A. Ustynyuk, J. Organomet.
Chem., 1999, 583, 136–145.
13 A. N. Nesmeyanov, N. A. Ustynyuk, T. Thoma, N. S. Prostakov, A. T.
Soldatenkov, V. G. Pleshakov, K. Urga, Y. A. Ustynyuk, O. I. Trifonova
and Y. F. Oprunenko, J. Organomet. Chem., 1982, 231, 5.
¨
14 K. Ofele and E. Dotzauer, J. Organomet. Chem., 1971, 30, 211.
15 The
known
compounds
2-acetamidobiphenyl
and
4-
acetamidobiphenyl were prepared according to literature procedures
by heating the appropriate aminobiphenyl with acetic anhydride and
collecting and recrystallizing the solid acetamidobiphenyl product.
See D. I. Glatzhofer, R. R. Roy and K. N. Cossey, Org. Lett., 2002, 4,
2349.
9446 | Dalton Trans., 2011, 40, 9439–9446
This journal is
The Royal Society of Chemistry 2011
©