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Intramolecular CЦH Activation in Alkaline-Earth Metal Complexes.

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Communications
C?H Activation in Ca Complexes
Intramolecular C?H Activation in Alkaline-Earth
Metal Complexes**
Sjoerd Harder*
In the last decade the number of s- and p-bonded organometallic complexes of the heavier alkaline-earth metals (Ca,
Sr, Ba) has increased tremendously. Although studies on the
reactivities of this group of organometallic compounds are
still scarce, it is clear that they are more than just ?heavy
[*] Priv.-Doz. Dr. S. Harder
Fachbereich Chemie, Universitt Konstanz
Postfach 5560, M738, 78457 Konstanz (Germany)
Fax: (+ 49) 7531-883137
E-mail: harder@chemie.uni-konstanz.de
[**] This work was generously supported by the Deutsche Forschungsgemeinschaft and BASF-AG (Ludwigshafen, Germany). Prof. Dr.
H.-H. Brintzinger (Universitt Konstanz) is kindly acknowledged for
stimulating discussions. I also would like to thank Dr. M. H.
Prosenc and the Rechenzentrum Hamburg for ?long-distance? CPU
time.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200351055
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Angewandte
Chemie
Grignard complexes?.[1?3] Their structural and chemical
behavior has often been compared to that of early d-block
and especially lanthanide organometallic compounds.[4] The
herein described observation of C?H activation in a benzylcalcium complex represents another analogy between the
chemistry of alkaline-earth and early d- and f-block metals.
We recently introduced the heteroleptic benzylcalcium compound 1 as an
initiator for the living syndiotactic polymerization of styrene.[5] In order to
improve the syndioselectivity we are
currently varying the spectator ligand.
The success of the b-diketiminate
(nacnac) ligand in polymerization catalysis[6] prompted us to explore its use in
heteroleptic benzylcalcium initiators.
Heteroleptic alkaline-earth complexes can be prepared by
ligand-exchange between two homoleptic compounds: R2M
+ R02M Q 2 RMR?.[7] Such Schlenk equilibria are controlled
by steric as well as electronic effects (for 1 the Schlenk
equilibrium is completely on the side of the heteroleptic
complex).[5] On the basis of the steric bulk of the dipp-nacnac
ligand (dipp = N,N?-bis(2,6-diisopropylphenyl)), we anticipated the Schlenk equilibrium between complex 2 and the
sterically crowded [Ca(dipp-nacnac)2] (3) to be on the side of
the heteroleptic complex (Scheme 1). However, at ambient
temperature a mixture of 2 and 3 showed no ligand exchange
at all. Structural analyses of 3 showed that this complex is
effectively stabilized by CHиииp interactions between isopropyl groups and aryl rings.[8] These stabilizing interactions
might be responsible for the unexpected behavior of 3 in
ligand-exchange reactions. When the reaction mixture of 2
and 3 was heated at 50 8C, the heteroleptic species 4[9] formed
first. One day later large light-yellow crystalline blocks
precipitated, which crystal structure analysis revealed to be
the dimeric calcium complex 5 with bridging dipp-nacnac
dianions (Scheme 1). This complex was formed by deprotonation of a methyl group in the backbone of the anionic dippnacnac ligand by the benzyl anion.
The crystal structure of the centrosymmetric dimer 5
(Figure 1) shows a dipp-nacnac ligand in an unusual N,N-trans
conformation. The dianionic ligand coordinates to two different Ca2+ ions in unequal modes. At first sight it seems to serve
as an 1-azapentadienyl ligand as well as an 1-azaallyl ligand
(Figure 2). The coordination sphere of calcium is completed
with one thf ligand.
Figure 1. Crystal structure of dimer 5.
Figure 2. Two views of the partial structure of the nacnac2 ion in 5.
Only the hydrogen atoms at C1 are shown. Selected bond lengths [D]:
Ca1-C1 2.594(3), Ca1-C2 2.824(2), Ca1-C3 2.777(3), Ca1-C4 2.832(4),
Ca1-N2 2.418(2), Ca1?-C1 2.662(3), Ca1?-C2 2.793(2), Ca1?-N1 2.361(2),
C1-C2 1.418(4), C2-C3 1.447(3), C2-N1 1.364(3), C3-C4 1.404(3), C4N2 1.340(3).
Although complexes with azaallyl ligands are well-precedented,[10] the structural information on azapentadienyl?
metal coordination is rather scarce.[11] The 2,4-disubstituted
azapentadienyl moiety in 5 deviates significantly from planarity (Figure 2 b). Average and maximum deviations from
the least-squares plane N1-C1-C2-C3-C4-C5-N2 are 0.132 ?
and 0.233 ?, respectively. Similar deviations have been
observed in the monoanionic dipp-nacnac ligand.[8] The
bond lengths within the dipp-nacnac2 framework, however,
suggest an electronic situation different from that in 6. The
long C2C3 bond (1.447(3) ?) indicates that the ligand
should not be regarded as a delocalized eight-electron
azapentadienyl/azaallyl dianion, but rather as a combination
Scheme 1. Ligand exchange between 2 and 3 to give heteroleptic complex 4 and formation of dimer 5.
Angew. Chem. Int. Ed. 2003, 42, 3430 ? 3434
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Communications
of two four-electron azaallyl anions linked by the C2C3 bond
(7). Both 2-substituted azaallyl substructures, N1/C1/C2/C3
and N2/C3/C4/C5, are largely planar and are tilted with
respect to each other around the long C2C3 bond with an
interplanar angle of 20.6(2)8. The C?C and C?N distances
within the azaallyl units compare very well to those observed
in a homoleptic [Mg(azaallyl)2] complex (d(CC) =
1.406(6) ?, d(CN) = 1.347(5) ?).[10b] The aryl substituents
on N are turned slightly out of the azaallyl planes, as is often
observed in this class of ligands. The carbon atoms that bridge
the Ca2+ ions display hybdridization between sp2 and sp3 (the
sum of the valence angles is 345(1)8).
Geometry optimization of a nacnac2 model ion by ab
initio methods[12] results in a planar structure (Figure 3) in
which most bond lengths compare well to those in the crystal
Scheme 2. Equilibrium between dimer 5 and monomer 8 in THF.
Ar = 2,6-diisopropylphenyl.
The deprotonation reaction described in Scheme 1 can be
classified as an intramolecular C?H activation or cyclometalation, a reaction type that is common for early transitionmetal complexes.[15] The formation of ?tuck-in? complexes
(Scheme 3) are the best documented examples of intra-
Scheme 3. Formation of ?tuck-in? complexes.
molecular C?H activation and are well-known for metal
complexes of groups 3b (including the lanthanides),[15c,d]
4b,[15e?g] and 5b.[15h] However, the reaction in Scheme 1 can
also be described as a directed metalation reaction. The orthodirected lithiation of methoxybenzene[16] (Scheme 4) is an
3432
Figure 3. MP2/6-31 + G*-optimized structures of the nacnac2 ion.
NIMAG = Number of imaginary frequencies. The N,N-trans-conformation (left) corresponds to the energy minimum (NIMAG = 0). Values in
parentheses are the bond lengths [D] in the crystal structure. Values in
square brackets are the group charges (sum of the charges of the
heavy atom and the attached hydrogen atoms; NPA method).
Scheme 4. ortho-Directed lithiation of methoxybenzene.
structure. Only the CC bond connecting the azaallyl units
appears longer in the calculated structure of the free nacnac2
ion. The N,N-trans conformation is 5.4 kcal mol1 lower in
energy than the planar N,N-cis conformation. This is presumably due to electrostatic repulsion between the negatively
charged nitrogen atoms, which bear most of the negative
charge.
The extremely air-sensitive complex 5 is insoluble in
aromatic solvents but dissolves slightly in THF. The 1H NMR
spectrum of a freshly prepared solution of 5 in [D8]THF shows
slow conversion of one (dipp-nacnac)Ca species into another;
equilibrium is reached after 24 h. All NMR data point to an
equilibrium between species containing the N,N-trans and
N,N-cis conformations,[13] which is strongly temperature- and
concentration-dependent (see Experimental Section). It is
therefore likely that the dimer 5 is in equilibrium with a
monomeric complex 8 in N,N-cis conformation (Scheme 2).[14]
The preference for the N,N-cis conformation in 8 could be due
to the advantageous bridging of Ca2+ between the most
electronegative nitrogen atoms.
example of regioselective deprotonation via an intermediate
coordination complex. Such complex-induced proximity
effects (CIPEs) are widely used in stereo- and regioselective
organic syntheses.[17] The unique position of calcium between
the alkali metals and the early transition metals apparently
allows a comparison of its chemistry to that of both groups.
The fact that intramolecular C?H activation was initially
termed ortho-metalation[15a,b] illustrates its relation to orthodirected lithiation.
It should be stressed, however, that the second deprotonation of the negatively charged dipp-nacnac ligand in good
yield (61 %) is an unusual reaction.[18] Whereas, twofold
deprotonation of the acetylacetonato ligand (acac) is wellknown,[19] attempted double deprotonation of dipp-nacnacH
with four equivalents (a threefold excess!) of the superbase
mixture nBuLi/KOtBu in THF and a subsequent quench with
deuterated methanol resulted only in limited deuteration of
the methyl group (< 5 %).[20] Also, reaction of (o-Me2NC6H4)CHSiMe3K+ with dipp-nacnacK+ does not result in
formation of the dipp-nacnac2 ion. For these reasons the
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2003, 42, 3430 ? 3434
Angewandte
Chemie
expression C?H activation for the reaction in Scheme 1 is
certainly not out of place.
The observation of C-H activation in benzylcalcium
complexes is another pillar in the bridge that closes the gap
between the chemistry of alkaline-earth and d(f)-block
metals.
Experimental Section
Crystalline [Ca{(o-Me2N-C6H4)CHSiMe3}2(thf)2][5a] (2, 3.70 g,
6.19 mmol) and [Ca(dipp-nacnac)2][8] (3, 5.30 g, 6.06 mmol) were
dissolved in benzene (35 mL) and heated at 50 8C. After one night at
this temperature light-yellow crystals of 5 started to form. Large
crystalline blocks of 5 were isolated after a reaction time of two days:
3.98 g, 3.77 mmol, 61 %. The reaction also proceeds at room temperature (the formation of crystals starts after one month).
1
H NMR (600 MHz, [D8]THF, 20 8C). Dimer 5 with an N,N-trans
conformation: d = 0.91 (d, 3J(H,H) = 6.8 Hz, 3 H; iPr-Me), 0.96 (d,
3
J(H,H) = 6.9 Hz, 3 H; iPr-Me), 1.07 (d, 3J(H,H) = 6.9 Hz, 3 H; iPrMe), 1.11 (d, 3J(H,H) = 6.7 Hz, 3 H; iPr-Me), 1.13 (d, 3J(H,H) =
6.8 Hz, 3 H; iPr-Me), 1.19 (d, 3J(H,H) = 6.9 Hz, 3 H; iPr-Me), 1.20
(d, 3J(H,H) = 6.8 Hz, 3 H; iPr-Me), 1.21 (d, 3J(H,H) = 6.7 Hz, 3 H; iPrMe), 1.49 (s, 3 H; MeC5), 1.77 (m, thf), 2.06 (dd, 2J(H,H) = 4.7 Hz and
4
J(H,H) = 1.3 Hz, 1 H; CH2), 3.09 (h, 3J(H,H) = 6.8 Hz, 1 H; iPr-CH),
3.23 (h, 3J(H,H) = 6.7 Hz, 1 H; iPr-CH), 3.46 (h, 3J(H,H) = 6.9 Hz,
1 H; iPr-CH), 3.52 (h, 3J(H,H) = 6.8 Hz, 1 H; iPr-CH), 3.58 (m, thf),
4.03 (d, 2J(H,H) = 4.7 Hz, 1 H; CH2), 4.11 (d, 4J(H,H) = 1.3 Hz, 1 H;
CHC3), 6.69?7.12 ppm (m, 6 H; aryl). Monomer 8 with an N,N-cis
conformation: d = 1.11 (d br, 3J(H,H) = 6.8 Hz, 6 H; iPr-Me), 1.16
(d br, 3J(H,H) = 6.9 Hz, 6 H; iPr-Me), 1.21 (d br, 3J(H,H) = 6.8 Hz,
6 H; iPr-Me), 1.27 (d br, 3J(H,H) = 6.8 Hz, 6 H; iPr-Me), 1.55 (s, 3 H;
MeC5), 1.77 (m, thf), 2.04 (d, 2J(H,H) = 2.9 Hz, 1 H; CH2), 2.83 (d,
2
J(H,H) = 2.9 Hz, 1 H; CH2), 3.58 (m, thf), 3.68 (h br, 3J(H,H) =
6.9 Hz, 2 H; iPr-CH), 3.76 (h br, 3J(H,H) = 6.8 Hz, 2 H; iPr-CH), 4.77
(s, 1 H; CHC3), 6.69?7.12 ppm (m, 6 H; aryl). The N,N-trans/N,N-cis
ratio in a 0.028 m solution of 5 in THF at 20 8C is 35:65 (66:34 at 60 8C;
at this temperature no coalescence of the CH2 signals is observed,
indicating slow rotation around the CCH2 bond). This ratio is also
concentration dependent: a 0.014 m solution at 20 8C shows a ratio of
19:81.
Crystal structure determination of 5:[21] Measurement on an
Enraf Nonius CAD4, T = 120 8C, MoKa, 2qmax = 52.28, 6149 independent reflections, 3912 reflections observed with I > 2s(I). Crystal
data: orthorhombic, space group Pbca, a = 13.6171(9), b =
21.2667(19), c = 21.4561(17) ?, V = 6213.5(8) ?3, C66H96Ca2N4O2,
Mr = 1057.63, Z = 4, R1 = 0.0507, wR2 = 0.1386, GOF = 1.00, 1max =
0.40 e ?3, 1min = 0.32 e ?3, m = 0.228 mm1 (no absorption correction applied). The CH2 hydrogens were observed and refined freely.
CCDC-200810 contains 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 Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: (+ 44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
Received: January 30, 2003 [Z51055]
.
Keywords: alkaline-earth metals и calcium и carbanions и
C?H activation и cyclometalation
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[8] S. Harder, Organometallics, 2002, 21, 3782.
[9] The 1H NMR spectrum of the reaction mixture shows the
homoleptic species 2 and 3, as well as a new species (4) which
contains the (o-Me2N-C6H4)CHSiMe3 ion and the dipp-nacnac
ligand and could not be isolated.
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[12] MP2/6-31 + G*-optimized structures with energies corrected for
zero-point vibrations: E(N,N-trans) = 303.533785 Hartree,
E(N,N-cis) = 303.525245 Hartree. Gaussian 98 (Revision A.7),
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
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[13] All signals were assigned by means of two-dimensional NMR
spectroscopic methods. The existence of the N,N-cis conformer
was proven by the presence of a strong NOE signal between the
CH proton in the nacnac bridge and one of the protons in the
CH2 group. The N,N-trans conformer does not show this NOE
signal. The latter species shows a long-range 4J(H,H) coupling of
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3433
Communications
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
3434
1.3 Hz between the CH proton in the nacnac bridge and one of
the protons in the CH2 group. This is typical for H-C-C-C-H
substructures with a W geometry.
An increase in temperature shifts the equilibrium to 5, the side
with the least solvation and highest entropy. A decrease in the
concentration shifts the equilibrium to the least aggregated
species, that is, monomer 8.
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The reaction mixture was concentrated to dryness, CD3OD was
added, and the sample was analyzed immediately by 1H NMR
spectroscopy. Only traces of CH2D could be detected, whereas a
quench of 5 with CD3OD shows the expected 1:1 ratio of CH3/
CH2D. The NMR spectra must be measured immediately after
quenching, because under basic conditions dipp-nacnac in
CD3OD is slowly deuterated in the Me groups.
A. L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Universiteit Utrecht, Utrecht, Nederland, 2000.
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