Intramolecular (sp3-hybridized) C-H activation: Yttrium alkyls versus transient yttrium hydrides

Article abstract of DOI:10.1021/om700668m

Intramolecular sp³-hybridized C-H activation of novel yttrium alkyl and hydrido complexes supported by bulky aminopyridinato ligands derived from deprotonated 2,6-(diisopropylphenyl)-[6-(2,6-dimethylphenyl)pyridin-2-yl] amine (Ap′-H) is reported. Reaction of YCl₃ with a 2-fold molar excess of Ap′K in THF afforded complex [Ap′₂YCl(thf)] (1). Alkylation of 1 with an equimolar amount of LiCH₂SiMe ₃ in hexane allowed isolation of the alkylyttrium derivative [Ap′₂YCH₂-SiMe₃(thf)] (2) in 65% yield. Unexpectedly treatment of complex 2 with PhSiH₃ (toluene, 20 °C) afforded the product of intramolecular sp³-hybridized C-H bond activation, Ap′(Ap′-H)Y(THF) (3). Most likely a hydrid species is formed in the course of this reaction, which undergoes rapid C-H activation since 3 is formed from 2 directly about 500 times slower than in the presence of 1 equiv of PhSiH₃. Molecular structures of complexes 2 and 3 have been confirmed by X-ray crystal structure analysis.

Full text of DOI:10.1021/om700668m

5770
Organometallics 2007, 26, 5770-5773
Intramolecular (sp3-hybridized) C-H Activation: Yttrium Alkyls
versus Transient Yttrium Hydrides
Grigorii G. Skvortsov,† Georgii K. Fukin,† Alexander A. Trifonov,*,† Awal Noor,§
Christian Do¨ring,§ and Rhett Kempe*,§
G.A. RazuVaeV Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49,
GSP-445, 603950 Nizhny NoVgorod, Russian Federation, and Lehrstuhl Anorganische Chemie II,
UniVersita¨t Bayreuth, 95440 Bayreuth, Germany
ReceiVed July 3, 2007
Summary: Intramolecular sp3-hybridized C-H actiVation of
noVel yttrium alkyl and hydrido complexes supported by bulky
aminopyridinato ligands deriVed from deprotonated 2,6-(diiso-
propylphenyl)-[6-(2,6-dimethylphenyl)pyridin-2-yl]amine (Ap′-
H) is reported. Reaction of YCl3 with a 2-fold molar excess of
Ap′K in THF afforded complex [Ap′2YCl(thf)] (1). Alkylation
of 1 with an equimolar amount of LiCH2SiMe3 in hexane
allowed isolation of the alkylyttrium deriVatiVe [Ap′2YCH2-
SiMe3(thf)] (2) in 65% yield. Unexpectedly treatment of com-
plex 2 with PhSiH3 (toluene, 20 °C) afforded the product
of intramolecular sp3-hybridized C-H bond actiVation,
Ap′(Ap′-H)Y(THF) (3). Most likely a hydrid species is formed
in the course of this reaction, which undergoes rapid C-H
actiVation since 3 is formed from 2 directly about 500 times
slower than in the presence of 1 equiV of PhSiH3. Molecular
structures of complexes 2 and 3 haVe been confirmed by X-ray
crystal structure analysis.
were represented exclusively by sandwich-2 and half-sandwich-
type (“constrained geometry”)5 complexes. Remarkable progress
has been made in this field, and the formerly challenging
monomeric4 and dihydrido6 species became a reality. The
hydride supported by benzamidinate ligands {[PhC(NSiMe3)2]2Y-
7
(µ-H)}2 synthesized by Teuben and co-workers in 1993 was
the first and for a long time the sole example of a cyclopenta-
dienyl-free rare earth metal hydrido complex. The recent
advances in rare earth metal hydrido chemistry8 are linked to
application of novel types of coordination environments: bis-
(silylamido)biphenyl,9a calyx-tetrapyrrol,9b,c salicylaldimine,9d,e
tris(pyrazolil)borate.9f However, to date, “post-metallocene”
hydrides still remain a rarity. Recently we reported the synthesis
of the series of hydrido lanthanide complexes supported by
bulky tetrasubstituted guanidinate ligands, which have demon-
strated high catalytic activity in olefin polymerization.10 Steri-
cally demanding aminopyridinato ligands11 were also success-
fully used as a suitable coordination environment for stabilization
of monomeric lanthanide species. Both guanidinate and ami-
nopyridinate frameworks have a common feature: a chelating
monoanionic planar NCN moiety. In order to investigate the
influence of ancillary ligation on the reactivity of the Y-H and
Y-C bonds, we decided to employ the aminopyridinate ligation
system for synthesis of alkyl and hydrido complexes. Herein
we report on the exiting outcomes of the attempts to synthesize
alkyl and hydrido yttrium complexes supported by the bulky
aminopyridinate ligand system.
Even after a lapse of twenty-five years since the pioneering
works on the synthesis of the first molecular rare earth metal
hydrido complexes,1 these compounds still attract considerable
attention2 and remain one of the most promising classes of
compounds for various catalytic applications.3 Enhanced reac-
tivity of hydrido complexes, which allows even C-F bond
activation,4 gives evidence of their high potential in stoichio-
metric reactions. Until very recently rare earth metal hydrides
* To whom correspondence should be addressed. E-mail: (A.A.T.)
trif@iomc.ras.ru; (R.K.) kempe@uni-bayreuth.de.
Bulky 2,6-(diisopropylphenyl)-[6-(2,6-dimethylphenyl)pyri-
† G.A. Razuvaev Institute of Organometallic Chemistry of Russian
Academy of Sciences.
din-2-yl]amine (Ap′-H) (Scheme 1)11a,b was used as the ligand
§ Universita¨t Bayreuth.
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G.; Li, Y.; Yasuda, H.; Maruo, T.; Kanehisha, N.; Kai, Y. Organometallics
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10.1021/om700668m CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/27/2007

Communications
Scheme 1. Bulky Amidopyridinato Ligand (Ap′H)
Organometallics, Vol. 26, No. 24, 2007 5771
Scheme 2. Synthesis of 1
precursor for preparation of the yttrium bis(aminopyridinato)
chloride, alkyl, and hydride complexes.
Figure 1. ORTEP diagram (30% probability thermal ellipsoids)
of [Ap′2YCH2SiMe3(thf)] (2) [a ) 12.6840(6) Å, b ) 12.8250(8)
Å, c ) 19.2775(11) Å, R ) 105.398(5)°, â ) 91.182(4)°, γ )
91.712(5)°, R1 ) 0.0706, wR2 (all data) ) 0.1522] showing the
numbering scheme. Hydrogen atoms and carbon atoms of THF and
methyl radicals of isopropyl groups are omitted for clarity. Selected
bond distances [Å] and angles [deg]: C(1)-Y(1) 2.342(5), N(1)-
Y(1) 2.554(3), N(2)-Y(1) 2.301(3), N(3)-Y(1) 2.487(3), N(4)-
Y(1) 2.344(3), O(1)-Y(1) 2.343(3), N(2)-Y(1)-C(1) 97.17(15),
N(2)-Y(1)-N(4) 101.80(12), C(1)-Y(1)-N(4) 101.27(15), C(1)-
Y(1)-O(1) 93.27(15), N(4)-Y(1)-O(1) 145.32(11), N(2)-Y(1)-
N(3) 154.30(11), C(1)-Y(1)-N(3) 99.94(14), N(4)-Y(1)-N(3)
56.10(11), O(1)-Y(1)-N(3) 90.62(11), N(2)-Y(1)-N(1) 55.77-
(11), C(1)-Y(1)-N(1) 150.88(14), N(4)-Y(1)-N(1) 95.40(11).
Reaction of anhydrous YCl3 with a 2-fold molar excess of
Ap′K in THF at 50 °C afforded complex [Ap′2YCl(thf)] (1),
which was isolated after recrystallization from THF-hexane
mixtures in 87% yield as yellow crystals (Scheme 2). No ate-
complex formation was observed.
Complex 1 is well soluble in THF and toluene and sparingly
soluble in hexane. Clear, yellow crystals of complex 1 suitable
for X-ray crystal structure investigation were obtained by slow
condensation of hexane into a THF solution at room tempera-
1
ture. The H and 13C{1H} NMR data of complex 1 in C6D6 at
20 °C show the expected sets of resonances due to the
aminopyridinate fragment and the coordinated THF molecule.
The 1H NMR signals of the THF methylene protons in 1 appear
as broad singlets, reflecting labile coordination.
Figure 1. An X-ray diffraction study has revealed that complex
2 is monomeric. The coordination environment of the yttrium
atom is set up by four nitrogen atoms of two chelating
aminopyridinato ligands, one carbon atom of the alkyl group,
and one oxygen atom of the THF molecule and can be
considered as a strongly distorted octahedron. The yttrium atom
coordination number in complex 2 is 6.
The Y-C bond length in complex 2 (2.342(5) Å) is
comparable to the values reported for related compounds
[[t-BuC6H3(CHdN)C6H3-i-Pr2]2YCH2SiMe2Ph] (2.384(2) Å),13a
[(Me3C6H2NdPPh(C6H4)(C6H4)NC6H3-iPr2)YCH2SiMe3(thf)]
(2.418(5) Å),13b [Me2Si(C5H3-t-Bu)2YCH2SiMe3(thf)] (2.399-
(2) Å),13c and [O{(CH2)2C9H6}2YCH2SiMe3] (2.376(8), 2.35-
(1) Å)13d taking into account the difference of yttrium ionic radii
at different coordination numbers.13e The metric parameters
within the Ap′2Y fragment are indicative of a localization of
the anionic function of the ligand at the amido N atom.
The most common synthetic route to hydrido complexes is
σ-bond metathesis reaction of a parent alkyl on treatment with
dihydrogen3d,f or phenylsilane.14 In order to prepare a hydrido
yttrium complex supported by amidopyridinato ligands, we have
carried out reaction of equimolar amounts of complex 2 and
PhSiH3 in toluene at 20 °C.
Alkylation of complex 1 with LiCH2SiMe3 was carried out
in hexane at 20 °C and resulted in the formation of the
alkylyttrium derivative [Ap′2YCH2SiMe3(thf)] (2) (Scheme 2),
which was isolated after recrystallization from hexane as yellow,
moisture- and air-sensitive crystals in 65% yield.
1
In the H NMR spectrum of complex 2 at 20 °C (C6D6) the
hydrogen atoms of the methylene group attached to the yttrium
atom appear as a multiplet ([AB system], δ ) -0.39 ppm)
indicative of two diastereotopic CH2 protons. In the 13C{1H}
NMR spectrum the appropriate carbon gives rise to a doublet
1
at 43.3 ppm (2JYC ) 43 Hz). A singlet at 0.20 ppm in the H
NMR spectrum corresponds to the methyl protons of the SiMe3
group. It is worthy of note that the signal sets corresponding to
the aminopyridinate fragments in the 1H NMR spectra of 1 and
1
2 are essentially different. Thus in the H NMR spectrum of
complex 1 the protons of the methyl substituents appear as a
singlet at 2.20 ppm and the methyl protons of isopropyl groups
appear as a doublet (3JH-H ) 7.0 Hz) at 1.10 ppm. On the other
1
hand, the same groups of protons in the H NMR spectrum of
2 give rise to two singlets (1.57 and 2.24 ppm) and a complex
multiplet in the region 0.97-1.37 ppm, respectively. Introduc-
tion of the sterically demanding CH2SiMe3 substituent into the
coordination sphere of the yttrium results in slowing of ligand
exchange processes and phenyl group rotation of the amidopy-
ridinato ligands due to increased hindrance. The 1H NMR signals
of the THF methylene protons in 1 appear as broad singlets,
reflecting labile coordination.
Crystallization of 2 by slow cooling of the concentrated
toluene-hexane solution to -20 °C resulted in single crystals
containing one molecule of toluene per one molecule of [Ap′2-
YCH2SiMe3(thf)]. The molecular structure of 2 is shown in
(12) Trifonov, A. A.; Lyubov, D. M.; Fedorova, E. A.; Fukin, G. K.;
Schumann, H.; Muehle, S.; Hummert, M.; Bochkarev, M. N. Eur. J. Inorg.
Chem. 2006, 747-756.
(13) (a) Emslie, D. J. H.; Piers, W. E.; MacDonald, R. J. Chem. Soc.,
Dalton Trans. 2002, 293-294. (b) Liu, B.; Cui, D.; Ma, J.; Chen, X.; Jing,
X. Chem.-Eur. J. 2006, 13, 834-845. (c) Molander, G. A.; Dowdy, E.
D.; Noll, B. C. Organometallics 1998, 17, 3754-3758. (d) Qian, C.; Zou,
G.; Sun, J. J. Chem. Soc., Dalton Trans. 1999, 519-520. (e) Schannon, R.
D. Acta Crystallogr. 1976, A32, 751-767.
(14) Voskoboinikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin,
K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics
1997, 16, 4041-4055.

5772 Organometallics, Vol. 26, No. 24, 2007
Communications
Scheme 3. Synthesis of 2
Scheme 4. Synthesis of the Intramolecular C-H Bond
Activated Product 3 via Hydride Formation (above) and
Directly from the Alkyl (below)
Figure 2. ORTEP diagram (30% probability thermal ellipsoids)
of [Ap′(Ap-H′)Y(thf)] (3) [a ) 13.2877(10) Å, b ) 18.5068(12)
Å, c ) 20.1473(16) Å, â ) 105.196(6)°, R1 ) 0.0646, wR2 (all
data) ) 0.1219] showing the numbering scheme. Hydrogen atoms,
carbon atoms of THF, and methyl radicals of isopropyl groups are
omitted for clarity. Selected bond distances [Å] and angles [deg]:
C(1)-Y(1) 2.420(11), N(1)-Y(1) 2.338(7), N(3)-Y(1) 2.430(7),
N(2)-Y(1) 2.431(8), N(4)-Y(1) 2.289(6), O(1)-Y(1) 2.321(6),
N(4)-Y(1)-N(1) 144.7(2), N(4)-Y(1)-N(3) 57.3(2), N(1)-
Y(1)-N(3) 154.7(2), N(4)-Y(1)-N(2) 100.1(2), N(1)-Y(1)-N(2)
56.0(2), N(3)-Y(1)-N(2) 147.6(2), Y(1)-C(1)-C(2)-112.2(7).
Unexpectedly, instead of the product of σ-bond metathesis
reaction, the complex resulting from intramolecular C-H bond
activation was isolated, [Ap′(Ap-H′)Y(thf)] (3) (Scheme 4), in
72% yield. Complex 3 is also formed from 2 via standing in,
for instance, a benzene solution (Scheme 4). This process needs
a couple of weeks at room temperature. The yellow, crystalline
compound 3 is highly moisture- and air-sensitive. Complex 3
is soluble in THF and toluene and sparingly soluble in aliphatic
hydrocarbons. Transparent yellow crystals suitable for single-
crystal X-ray diffraction study of 3 were obtained by slow
cooling of a concentrated heptane solution to -20 °C. The
molecular structure of 3 is depicted in Figure 2 and has revealed
that complex 3 is monomeric, having the coordination number
6. The yttrium atom coordination environment can be classified
as a strongly distorted octahedron. Unlike the parent alkyl
compound 2, one aminopyridinato ligand of complex 3 is
tridentate due to metalation of the methyl group of one of the
Me2C6H3 fragments with formation of the new Y-C σ-bond.
The bond length between yttrium and the “benzylic” carbon
atom in 3 (2.420(11) Å) is longer than that in parent alkyl
complex 2 (2.342(5) Å), but shorter than the values previously
reported for Y-C bonds in the benzylic complexes [(C5Me5)2-
YCH2Ph(thf)] (2.484(6) Å)15a and [{PhC(NSiMe3)N(CH2)3-
NMe2}2YCH2Ph] (2.487(6)) Å.15b The environment of the C(1)
is slightly distorted; the value of the angle Y(1)-C(1)-C(2) is
112.2(7)°. The aminopyridinato ligands in complex 3 are
coordinated to the yttrium atom in different ways. The type of
coordination of the bidentate Ap′ ligand is similar to that
observed in related complexes 2: one short Y-N bond with
the amido nitrogen atom (2.289(6) Å) and one long with the
nitrogen atom of the pyridine fragment (2.430(7) Å). In the
tridentate Ap-H′ ligand, formation of the Y-C bond dramatically
influences the bonding situation. The covalent bond between
yttrium and the amido nitrogen atom (2.432(8) Å) becomes
longer than the coordination bond between yttrium and the
pyridine nitrogen atom (2.338(7) Å); hence a switch from the
amidopyridine to the aminopyridinate form is observed.16
Moreover Y-C bond formation influences mutual orientation
of the 2,6-dimethylphenyl and pyridyl fragments. In the biden-
tate ligand the planes of these fragments adopt a near-orthogonal
orientation (the value of the dihedral angle is 96.0°), while in
the tridentate ligand the dihedral angle between these planes
decreases to 47.4°.
Inter- and intramolecular activation of sp2- and sp3-hybridized
C-H bonds by cyclopentadienyl17,5b and cyclopentadienyl-
free7a,9e,17f,18 lanthanide alkyl and hydride complexes is de-
scribed. Intermolecular activation of C-H bonds of methyl
groups of permethylated cyclopentadienyl rings was observed
under thermolysis conditions, for instance, of the dimeric
hydrido complex [(C5Me5)2YH]2.19 The formation of complex
3 can result from three different reactions pathways: First, direct
decomposition of complex 2 (Scheme 5, (I)) can occure. The
second possible pathway is toluene C-H activation and
(17) (a) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276-277.
(b) Watson, P. L.; Parshall, G. B. Acc. Chem. Res. 1985, 18, 51-56. (c)
Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.;
Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987,
109, 203-219. (d) den Haan, K. H.; Wiestra, Y.; Teuben, J. H. Organo-
metallics 1987, 6, 2053-2060. (e) Watson, P. L. J. Am. Chem. Soc. 1983,
105, 6491-6493. (f) Duchateau, R.; van Wee, C. T.; Teuben, J. H.
Organometallics 1996, 15, 2291-2302. (g) Mu, Y.; Piers, W. E.; Mac-
Quarrie, D. C.; Zaworotko, M. J.; Young, V. G. Organometallcs 1996, 15,
2720-2726. (h) Evans, W. J.; Champagne, T. M.; Ziller, J. W. J. Am. Chem.
Soc. 2006, 128, 14270-14271. (i) Evans, W. J.; Perotti, J. M.; Ziller, J. W.
J. Am. Chem. Soc. 2005, 127, 1068-1069. (j) Booij, M.; Kiers, N. H.;
Meetsma, A.; Teuben, J. H.; Smeets, W. J. J.; Spek, A. L. Organometallics
1989, 8, 2454-2461.
(18) (a) Sigiyama, H.; Gambarotta, S.; Yap, G. P. A.; Wilson, D. R.;
Thiele, S. K.-H. Organometallics 2004, 23, 5054-5061. (b) Duchateau,
R.; Tuinstra, T.; Brussee, E. A. C.; Meetsma, A.; Van Duijnen, P. T.;
Teuben, J. H. Organometallics 1997, 16, 3511-3522. (c) Fryzuk, M. D.;
Haddad, T. S.; Rettig, S. J. Organometallics 1991, 10, 2026-2036. (d)
Emslie, D. J. H.; Piers, W. E.; Parvez, M. Dalton Trans. 2003, 2615-
2620.
(15) (a) Mandel, A.; Magull, J. Z. Anorg. Allg. Chem. 1996, 622, 1913-
1919. (b) Bambirra, S.; Brandsma, M. J. R.; Brusse, E. A. C.; Meetsma,
A.; Hessen, B.; Teuben, J. H. Organometallics 2000, 19, 3197-3204.
(16) Deeken, S.; Motz, G.; Kempe, R. Z. Anorg. Allg. Chem. 2007, 633,
320-325.
(19) Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma,
A.; Teuben, J. H. Organometallics 1993, 12, 3531-3540.

Communications
Scheme 5. Possible Pathways to Convert 2 into 3
Organometallics, Vol. 26, No. 24, 2007 5773
Figure 3. Top: First-order kinetic plot for the decomposition of
2 (296 K, toluene-d8). Bottom: Plot of concentration of complex
2 versus t (h) in the presence of PhSiH3 (296 K, toluene-d8, C(2)0
) 0.062 mol/L, C(PhSiH3)0 ) 0.062 mol/L).
subsequent decomposition of the yttrium benzyl complex
(Scheme 5, (II)). Third such a process can include a σ-bond
metathesis reaction on treatment of 2 with PhSiH3, formation
of highly reactive hydride species, followed by metalation of a
methyl group (Scheme 5, (III)).
alkyl and for the transient hydride it can be roughly estimated
that a C-H activation involving an Ln-hydride can be more
than 500 times faster than for the corresponding Ln-alkyl
reaction. It might be even faster since the monitoring studies
did not indicate the presence of a hydride species, and thus the
hydride formation seems to be the rate-limiting step, which
means the C-H activation of the hydride is faster than derived
from the half-time period comparison. It only appears as slow
since the hydride formation is slow. Further work on this subject
is being actively pursued as well as investigations of this C-H
activation chemistry at other rare earth or group 3 metals in
order to allow “interlanthanide” comparison.
Several conclusions can be drawn from this study. The bulky
aminopyridinato ligand framework has been shown to be well-
suited for the synthesis of isolable yttrium alkyl species. These
alkyls undergo a very slow intramolecular C-H activation at a
sp3-hybridized carbon at room temperature. Corresponding
hydrides, generated by the reaction of the alkyl with PhSiH3,
undergo an intramolecular metalation reaction very fast at room
temperature. Intramolecular sp3-hybridized C-H activation of
a hydride can be more than 500 times faster than that of an
alkyl.
In order to understand which reaction is responsible for the
formation of complex 3, the decomposition of 2 and disappear-
ance of complex 2 in the presence of PhSiH3 were investigated
by NMR spectroscopy. Both reactions were carried out in
toluene-d8 at 296 K. Complex 2 was rather thermally robust.
Its decomposition was studied by monitoring the decrease of
the integral intensity of the signal corresponding to the methyl
protons of the silylalkyl group attached to the yttrium atom.
The 1H NMR-monitoring detected Me4Si formation in the course
of the reaction. The results are presented in Figure 3 (top.).
A linear decrease of ln(C/C0) with time was observed, which
is consistent with a rate law involving first-order dependence
on 2 (Figure 3, top). The rate constant was found to be k )
0.0038 h-1, and a half-time of reaction of about 181 h was
observed. Moreover, complex [Ap′2YCH2Ph(thf)] (4), the
expected intermediate of the solvent metalation route, was
synthesized independently. Complex 4 turned out to be a stable
alkyl compound since no decomposition was observed in
benzene-d6 solution at room temperature during 2 h. These
studies indicate that the decomposition of complex 2 does not
involve metalation of the solvent (alkyl toloyl exchange) due
to the observed clear reaction order.
Reaction of complex 2 with an equimolar amount of PhSiH3
was studied by monitoring the decrease of the integral intensity
of the signal corresponding to the methyl protons of the
Acknowledgment. This work has been supported by the
Russian Foundation for Basic Research (Grant Nos. 05-03-
32390, 06-03-32728), the RFBR-DFG grant (06-03-04001), and
DFG Schwerpunktprogramm 1166 “Lanthanoidspezifische Funk-
tionalita¨ten in Moleku¨l und Material”. A.A.T. thanks the Russian
Foundation for Science Support and R.K. the Fonds der
Chemischen Industrie.
1
silylalkyl group attached to the yttrium atom in the H NMR
spectrum of 2. The results of these studies are presented in
Figure 3 (bottom). The disappearance of 2 in the presence of
PhSiH3 is much faster than that in the case of decomposition
of 2 at the same temperature without PhSiH3. The data do not
follow a clear reaction order, and the time for consumption of
1
50% of complex 2 is about 22 min. The H NMR spectra did
Supporting Information Available: Detailed information about
the synthesis and characterization of the lanthanide complexes
described here and crystallographic details of the structures de-
termined by X-ray crystal structure analysis are available. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
not indicate the presence of monomeric or dimeric yttrium
hydride species in the reaction mixture. This may serve as
evidence that the intermediate hydride species formed are highly
reactive and rapidly undergo an intramolecular metalation, which
leads to the formation of complex 3. By comparing the half-
time periods of the C-H activation reaction observed for the
OM700668M