Cyclometalation of substrates containing imine and pyridyl anchoring groups by iron and cobalt complexes

Article abstract of DOI:10.1002/anie.200460978

Steric control dictates the cyclometalation of aromatic substrates containing imine anchoring groups upon reaction with basic iron or cobalt complexes. As structural isomers, the azadiene systems 1 and 2 undergo regiospecific activation of the (vinylic or

Full text of DOI:10.1002/anie.200460978

Angewandte
Chemie
Table 1: Isoelectronic compounds with exchange of N for CH (A, B),
structural isomers (A, C) with azadiene topology, and reagents to effect
cyclometalation.
Substrate
Reagent
[MnCH3(CO)4][3]
[Fe(PMe3)4]
[Fe(CH3)2(PMe3)4]
[CoCH3(PMe3)4]
[CoCH3(PMe3)4]
conditions[5] as well as their isoelectronic structural isomers
having an azadiene topology, the 2-vinylpyridines (C,
Table 1). The new, stable, and structurally characterized
compounds containing N,C-metallacycles can be viewed as
models for the organometallic intermediates in ruthenium-
À
À
C H Activation
and rhodium-catalyzed C C coupling reactions between
aromatic imines and olefins utilized in organic synthesis.[6]
In these reactions typically conducted in refluxing toluene the
metalated species[7] are not isolated but are converted into
metal-free products in a catalytic process.
Cyclometalation of Substrates Containing Imine
and Pyridyl Anchoring Groups by Iron and Cobalt
Complexes
After simply combining the phenyl ketimines, benzaldi-
mines, or 2-vinylpyridine with cobalt or iron complexes at
À708C, we observed a color change indicating that the
reaction was already occurring; for methyl complexes this was
accompanied by evolution of gas (methane, GC). The mixture
was warmed to 208C to give complete conversion and
moderate to high yields of crystalline product (Table 2).
Thus benzophenone imine was smoothly ortho-metalated
with methyltetrakis(trimethylphosphane)cobalt [Eq. (1)].
Hans-Friedrich Klein,* Sebnem Camadanli,
Robert Beck, Diana Leukel, and Ulrich Flꢀrke
Cyclometalations transform barely activated or nonactivated
À
C H bonds of coordinated ligands into C–metal bonds.
Metallacyclic species are known for almost all transition
elements, and the majority of them contain five-membered
rings.[1] The first examples of cyclometalation were observed
for azobenzene (B) with nickel, palladium, and platinum.[2]
Bruce et al. applied the reaction to Schiff bases of benzalde-
hyde (A) and also described examples with manganese,
rhenium, ruthenium, and rhodium (Table 1).[3] If, in the series
azobenzene, benzalaniline, and stilbene, N atoms are replaced
by CH groups,[4] then elevated temperatures are required for
cyclometalation to occur. The reaction with benzalaniline
must be run in THF or toluene at reflux, and stilbene does not
react.
We have recently been able to effect cyclometalation
reactions for the first time at basic cobalt and iron centers by
using imine and pyridyl anchoring groups. Phenyl ketimines
and benzaldimines react smoothly under particularly mild
The green crystals of 1 obtained from pentane decompose
at 1068C under argon; at 208C in air the crystal surface
remains unchanged for at least 15 minutes. NMR spectra
obtained from [D8]THF solutions are compatible with a
trigonal-bipyramidal configuration around cobalt and a C-
axial/N-equatorial coordination of the (2-iminobenzoyl)-
phenyl ligand. In the IR spectrum the n(NH) band is observed
with a hypsochromic shift of 45 cmÀ1 indicating coordination
through the N atom (Table 2).
[*] Prof. Dr. H.-F. Klein, S. Camadanli, Dr.-Ing. R. Beck,
Dipl.-Ing. D. Leukel
Eduard-Zintl-Institut fꢀr Anorganische und Physikalische Chemie
Technische Universitꢁt Darmstadt
Petersenstrasse 18, 64287 Darmstadt (Germany)
Fax : (+49)6151-16-4173
The X-ray crystal structure of 1[9] confirms this config-
uration at cobalt. Bond lengths and angles in the molecule
remain within the range of expected values. The sum of the
internal bond angles of the chelate ring (542.68) indicates little
strain (planar five-membered ring: 5408). This is also reflected
by the positions of the next-neighbor atoms at the metal
E-mail: hfklein@ac.chemie.tu-darmstadt.de
Dr. U. Flꢂrke
Anorganische und Analytische Chemie
Universitꢁt Paderborn
Warburger Strasse 100, 33098 Paderborn (Germany)
Angew. Chem. Int. Ed. 2005, 44, 975 –977
DOI: 10.1002/anie.200460978
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
975

Communications
Table 2: Syntheses[8] and properties of metallacyclic compounds of cobalt and iron.
an octahedral coordination environment
made up of three PMe3 groups in meri-
dional positions, the N,C-chelate ligand,
and a hydrido ligand trans to the imine
donor of the chelate. The methyliron
compound 3 shows the same configuration
À
in solution, except that the Fe H group of
À
5 is replaced by a Fe CH3 group. Com-
R1
R2
M
Yield [%]
Color
Decomp. [8C]
n˜ [cmÀ1
]
pounds 5 and 4 are isoelectronic by the
analogy of Co and Fe-H.
1
2
3
4
5
6
7
H
H
CH3
H
H
C6H5
C6H5
H
C(CH3)3
C(CH3)3
H
Co
76
78
65
42
48
41
62
green
violet
violet
green
violet
green
green
<106
<105
<115
<112
<118
<78
3301 (N-H)
3281 (N-H)
1180 d(FeCH3)
3358 (N-H)
3335 (N-H),1795 (Fe-H)
–
–
Combining 2-vinylpyridine with meth-
yltetrakis(trimethylphosphane)cobalt in
THF resulted in evolution of gas and a
color change from orange–red to green.
According to Equation (3) the metallacy-
clic complex 6 formed.
FeH
FeCH3
Co
FeH
Co
H
H
C6H5
Co
<97
Dark green crystals of 6 were grown from pentane at
À208C; their surface planes reflect incident daylight as red
light. At 788C under argon these dichroitic properties were
lost, and thermal decomposition commenced. When air was
admitted, the crystals rapidly deliquesced and underwent
oxidative decomposition already at 208C. Both the NMR
spectra in [D8]THF and the crystal structure of 6[12] (Figure 2)
Figure 1. X-ray crystal structure of 1. Selected distances [ꢃ] and angles
[8]: Co1–N1 1.882(5), Co1–C12 1.959(6), Co1–P1 2.2178(18), Co1–P2
2.1858(19), Co1–P3 2.189(2), N1–C10 1.337(8); N1-Co1-C12 80.6(2),
C12-Co1-P1 168.15(19), C10-N1-Co1 121.5(4).
center, which show no gross deviation from ideal trigonal-
bipyramid coordination (Figure 1).
When an equimolar mixture of tetrakis(trimethylphos-
phane)iron and (2,2-dimethyl-1-phenyl)propylidenimine[10] in
pentane was warmed from À708C to 208C, the initial orange
mixture turned dark red. Through oxidative addition the
hydridoiron(ii) complex 5 formed [Eq. (2)].
Figure 2. X-ray crystal structure of 2. Selected distances [ꢃ] and angles
[8]: Co1–C7 1.913(2), Co1–N1 2.006(2), Co1–P1 2.1526(7), Co1–P2
2.2030(7), Co1–P3 2.1575(7), C6–C7 1.337(3); N1-Co1-C7 80.5(1), P2-
Co1-C7 175.50(8).
show molecular complex entities in which the C-donor
function is axial and the N-donor function is equatorial. The
chelate and the metal center form a planar five-membered
ring with a typical bite angle at the cobalt atom (N1-Co-C7
80.5(1)8).
The IR spectra of by-products give no evidence for
Violet crystals of 5 are stable up to 1188C under argon.
isomers. Also, in the course of the analogous formation of 7
According to NMR spectra in [D8]THF[11] the iron center has
neither of the two aromatic C H groups available for the
À
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Angewandte
Chemie
removed in vacuo, and the residue was extracted with fresh
formation of six-membered metallacycles were activated in
competition with the vinyl group.
This result demonstrates that in structural isomers con-
taining a 1,4-interchanged azadiene topology (Table 1) acti-
pentane over a glass-sinter disc (G3). When the solution was
cooled, crystals formed. The solution was decanted off, and the
crystals were washed with cold pentane and dried in vacuo to
afford analytically pure materials 1–7 (Table 2).
À
vation occurs at the formally interchanged C H group, which
[9] Crystal data for 1: C22H37CoNP3, Mr = 467.4; crystal size: 0.20 ꢁ
0.25 ꢁ 0.35 mm, orthorhombic, space group Pna21, a = 16.815(4),
may be aromatic or vinylic. Furthermore these findings are in
accord with the notion[13] that the N-donor function is
coordinated first, followed by activation and regiospecific
b = 9.473(3), c = 15.676(5) ꢂ, V= 2497.0(13) ꢂ3, Z = 4, 1calcd
=
1.243 gcmÀ3, F(000) = 922; 2974 reflections with 2.848 < 2q <
49.988, Stoe-Stadi-4 diffractometer, MoKa radiation, no absorp-
tion correction, graphite monochromator, structure solution
using Patterson and Fourier methods, refinement from 2974
independent reflections (Rint = 0.0925) for 244 parameters based
on F2 with SHELXL97, R = 0.0460, wR2 = 0.1073 (all data).
Max./min. residual electron densities 0.593/À0.373 eꢂÀ3. All
non-hydrogen atoms were refined anisotropically; the hydrogen
atoms were set at calculated positions.
À
À
cleavage of the most suitable ligand C H bond. The M H
function thus formed either remains in the final product, as
observed after reaction at the iron(0) center, or when there is
À
an adjacent M CH3 unit, it is eliminated as methane as in all
the other cyclometalation reactions described here. As the
metals retain their low oxidation states in the products and
since the effects of donor ligands are similar before and after
reaction, except for the chelate effect, it should be possible to
[10] F. J. Weiberth, S. S. Hall, J. Org. Chem. 1987, 52, 3901 – 3904.
[11] 5: 1H NMR (500 MHz, [D8]THF, 258C, TMS): d = À17.5 (dt,
ci2sJ(PAH) = 80 Hz, cisJ(PBH) = 22 Hz, 1H, FeH), 0.88 (br s, 18H,
PACH3), 1.40 (d, 32J(PH) = 4.5 Hz, 9H, PBCH3), 1.46 (s, 9H,
CCH3), 6.70–7.90 (m, 4H, HAr), 9.13 ppm (s, 1H, NH); 31P NMR
(202 MHz, [D8]THF, 258C, H3PO4): d = 18.2 (d, 2J(PP) = 37 Hz,
À
perform a second C H activation. Subsequent elimination
À
accompanied by C C coupling would facilitate a catalytic
reaction mode. Experiments with this aim are currently under
way.
2
2P), 23.7 ppm (t, J(PP) = 37 Hz, 1P).
Received: June 16, 2004
[12] a) Crystal data for 6 (C16H33CoNP3): Mr = 391.3; crystal size:
0.40 ꢁ 0.35 ꢁ 0.25 mm, monoclinic, space group P21/c, a =
14.753(2), b = 14.528(2), c = 9.9920(14) ꢂ, b = 102.975(2)8, V=
2086.9(5) ꢂ3, Z = 4, 1calcd = 1.245 gcmÀ3, F(000) = 832; 23679
reflections with 2.848 < 2q < 56.788, Bruker AXS SMART
APEX CCD[14] diffractometer, MoKa radiation (m =
1.047 mmÀ1), graphite monochromator, semiempirical absorp-
tion correction using equivalent reflections (SADABS[14]).
Structure solution using direct methods,[14] refinement[14] from
5022 independent reflections (Rint = 0.050) for 199 parameters
based on F2, R1 (I > 2s(I)) = 0.038, wR2 (all data) = 0.092. Max./
min. residual electron densities 0.49/À0.22 eꢂÀ3. All non-hydro-
gen atoms were refined anisotropically, hydrogen atoms with
riding model at idealized positions and isotropic parameters
Uiso(H) = 1.2Ueq(C) or 1.5Ueq(-CH3). b) CCDC-240668 and
À239924 (1 and 6) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
Revised: October 18, 2004
Published online: January 11, 2005
À
Keywords: C H activation · cobalt · cyclometalation · iron ·
N ligands
.
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[8] General procedure: Solutions of the iron complexes in pentane
or of [CoCH3(PMe3)4] in THF on a scale of about 3 mmol were
combined with equimolar amounts of the imine in the same
solvent and stirred at À708C. The reaction mixture was allowed
to warm to 208C and stirred for 3 h at 208C. The volatiles were
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