The role of group 14 element hydrides in the activation of C-H bonds in cyclic olefins

Article abstract of DOI:10.1021/ja305853d

Formally, triple-bonded dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C₆H₃-2,6-(C₆H₃-2,6- ⁱPr₂)₂] have been previously shown to activate aliphatic, allylic C-H bonds in cyclic olefins, cyclopentadiene (CpH), cyclopentene (c-C₅H₈) and 1,4-cyclohexadiene, with intriguing selectivity. In the case of the five-membered carbocycles, cyclopentadienyl species ArECp [E = Ge (3), Sn (4)] are formed. In this study, we examine the mechanisms for activation of CpH and c-C₅H₈ using experimental methods and describe a new product found from the reaction between 1 and c-C₅H₈, an asymmetrically substituted digermene ArGe(H)Ge(c-C₅H₉)Ar (5), crystallized in 46% yield. This compound contains a hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with 3, explaining the fate of the liberated H atoms following triple C-H activation. We show that when these C-H activation reactions are carried out in the presence of tert-butyl ethylene (excess), compounds {ArE(CH₂CH₂tBu)}₂ [E = Ge(8), Sn(9)] are obtained in addition to ArECp; in the case of CpH, the neohexyl complexes replace the production of H₂ gas, and for c-C₅H ₈ they displace cyclopentyl product 5 and account for all the hydrogen removed in the dehydroaromatization reactions. To confirm the source of 8 and 9, it was demonstrated that these molecules are formed cleanly between the reaction of (ArEH)₂ [E = Ge(6), Sn(7)] and tert-butyl ethylene, new examples of noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type 6 and 7 can be surmised to be reactive intermediates in the production of 3 and 4, along with H₂, from 1 and 2 and CpH (respectively), or the formation of 3 and 5 from 1. The reaction of 6 or 7 with CpH gave 3 or 4, respectively, with concomitant H ₂ evolution, demonstrating the basic nature of these low-valent group 14 element hydrides and their key role in the 'cascade' of C-H activation steps. Additionally, during the course of these studies a new polycyclic compound (ArGe)₂(C₇H₁₂) (10) was obtained in 60% yield from the reaction of 1,6-heptadiene and 1 via double [2 + 2] cycloaddition and gives evidence for a nonradical mechanism for these types of reactions.

Full text of DOI:10.1021/ja305853d

Article
pubs.acs.org/JACS
The Role of Group 14 Element Hydrides in the Activation of C−H
Bonds in Cyclic Olefins
Owen T. Summerscales,† Christine A. Caputo, Caroline E. Knapp,‡ James C. Fettinger,
and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: Formally, triple-bonded dimetallynes ArEEAr [E =
Ge (1), Sn (2); Ar = C6H3-2,6-(C6H3-2,6-iPr2)2] have been
previously shown to activate aliphatic, allylic C−H bonds in cyclic
olefins, cyclopentadiene (CpH), cyclopentene (c-C5H8) and 1,4-
cyclohexadiene, with intriguing selectivity. In the case of the five-
membered carbocycles, cyclopentadienyl species ArECp [E = Ge
(3), Sn (4)] are formed. In this study, we examine the mechanisms
for activation of CpH and c-C5H8 using experimental methods and
describe a new product found from the reaction between 1 and c-
C5H8, an asymmetrically substituted digermene ArGe(H)Ge(c-C5H9)Ar (5), crystallized in 46% yield. This compound contains a
hydrogenated cyclopentyl moiety and is found to be produced in a 3:2 ratio with 3, explaining the fate of the liberated H atoms
following triple C−H activation. We show that when these C−H activation reactions are carried out in the presence of tert-butyl
ethylene (excess), compounds {ArE(CH2CH2tBu)}2 [E = Ge(8), Sn(9)] are obtained in addition to ArECp; in the case of CpH,
the neohexyl complexes replace the production of H2 gas, and for c-C5H8 they displace cyclopentyl product 5 and account for all
the hydrogen removed in the dehydroaromatization reactions. To confirm the source of 8 and 9, it was demonstrated that these
molecules are formed cleanly between the reaction of (ArEH)2 [E = Ge(6), Sn(7)] and tert-butyl ethylene, new examples of
noncatalyzed hydro-germylation and -stannylation. Therefore, the presence of transient hydrides of the type 6 and 7 can be
surmised to be reactive intermediates in the production of 3 and 4, along with H2, from 1 and 2 and CpH (respectively), or the
formation of 3 and 5 from 1. The reaction of 6 or 7 with CpH gave 3 or 4, respectively, with concomitant H2 evolution,
demonstrating the basic nature of these low-valent group 14 element hydrides and their key role in the ‘cascade’ of C−H
activation steps. Additionally, during the course of these studies a new polycyclic compound (ArGe)2(C7H12) (10) was obtained
in 60% yield from the reaction of 1,6-heptadiene and 1 via double [2 + 2] cycloaddition and gives evidence for a nonradical
mechanism for these types of reactions.
Rossi have demonstrated 1,2-addition of a variety of allylic C−
H bonds across a Rh−Rh single bond in a porphyrin dimer,
resulting in cleavage of the metal−metal bond, although it is
thought that this bond dissociates to give a pair of
metalloradicals prior to C−H activation.11 Heterolytic cleavage
of an allylic C−H bond has also been recently found to occur in
a pair of polar frustrated Lewis pair systems based on group 13
and 15 element centers.12
We recently reported that the formally triple-bonded
dimetallynes ArEEAr [E = Ge (1), Sn (2); Ar = C6H3-2,6-
(C6H3-2,6-iPr2)2]13 react with the cyclic olefins cyclopentadiene
(CpH), cyclopentene, and 1,4-cyclohexadiene (CHD) to afford
C−H activation products at room temperature.14 The reaction
of 1 and 2 with CpH gave ArECp [E = Ge (3), Sn (4)],
containing an aromatized cyclopentadienyl anion, along with
evolution of hydrogen gas. 1 reacted with cyclopentene to give
the same dehydroaromatization product in lower yield, formed
from triple C−H activation and was also found to react with
INTRODUCTION
■
C−H activation of inert hydrocarbons is a challenging and
important chemical transformation, having potential application
in both fuel production and organic synthesis.1 In typical metal-
mediated processes, activation is achieved via inner- or outer-
sphere mechanisms, indicating either oxidative addition directly
at a metal center or homolytic reaction at an activated ligand,
such as an oxo group.2,3 A variation of inner-sphere-type
reactivity is heterolytic 1,2-oxidative addition across a polar
multiple metal−nonmetal bond, e.g., metal−imido double
bond.4 Activations using inorganic main group species
meanwhile are usually understood to operate via homolytic
radical-initiated reactions;5−8 inner-sphere 1,2-addition path-
ways in which a transition metal is absent have yet to be
delineated for these types of compounds, although outer-sphere
reactivity has been recently described for tin and aluminum
oxides.9 Kempe and co-workers reported an intramolecular C−
H cleavage of Cp− induced by polar Lu−Re metal−metal single
bonds in the presence of alkyl ligand as base, and a complex
series of intramolecular C−H and C−N activations at a WW
bond has been described.10 Additionally, Wayland and Del
Received: June 15, 2012
Published: August 23, 2012
© 2012 American Chemical Society
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Article
materials (ArGe)2,13b (ArSn)2,13c (ArGeH)2, and (ArSnH)2 were
prepared according to previously published procedures.16,17 Cyclo-
pentene, 1,6-heptadiene, and tert-butyl ethylene were purchased from
Aldrich and distilled over sodium prior to use. Cyclopentadiene was
freshly cracked immediately before use.
CHD to give a mixture of (ArGeH)2, trace amounts of benzene,
and a 7-germanorbornadiene species bound to a cyclohex-2-
enyl fragment (Scheme 1). These transformations are unique to
Scheme 1. Summary of Previously Reported C−H Activation
Reactions between Dimetallynes and Olefins14
Synthesis of ArGe(Cp) (3) and ArGe(H)Ge(c-C5H9)Ar (5). To a
solution of (ArGe)2 (1) (0.400 g, 0.42 mmol) in 50 mL pentane,
cyclopentene (50 μL, 0.54 mmol) was added via syringe, resulting in a
color change from dark red to emerald green over a period of 12 h.
Recrystallization at −20 °C overnight gave green crystals of 5 (0.195 g,
0.19 mmol, 46% yield), mp 120 °C (dec). 1H NMR (C6D6, 298 K): δ
= 1.00 (d, 12H, o-CH(CH3)2, J = 6.9 Hz), 1.03 (d, 12H, o-CH(CH3)2,
J = 6.9 Hz), 1.05 (d, 12H, o-CH(CH3)2, J = 6.9 Hz), 1.12 (d, 12H, o-
CH(CH3)2, J = 6.9 Hz), 1.14−1.37 (m, 5H, c-C5H9), 1.52 (br, 2H, c-
C5H9), 1.69 (br, 2H, c-C5H9), 2.92 (sept, 4H, CH(CH3)2, J = 6.9 Hz),
3.00 (sept, 4H, CH(CH3)2, J = 6.9 Hz), 5.87 (s, 1H, Ge-H) 6.96−7.30
(m, 18H, m-C6H3, p-C6H3, m-Dipp and p-Dipp; Dipp = 2,6-iPr2-
C6H3). 13C{1H} NMR (150 MHz, C6D6, 298 K): δ = 0.4, 21.8, 22.8,
25.6, 26.5, 29.8, 44.3, 45.9, 47.3, 121.9, 123.1, 124.7, 126.8, 131.6,
134.7, 141.3, 142.7, 145.1, 146.3, 148.6; λmax (ε in mol−1 L cm−1) =
406 nm (1500). IR (NaCl, Nujol; selected): Ge−H 2010 cm−1.
Cooling of the mother-liquor to −80 °C for 4 days gave colorless
crystals of 3 (0.142 g, 0.26 mmol, 31% yield), mp 155 °C;
characterizing data reported previously.14
t
Synthesis of {ArGe(CH2CH2 Bu)}2 (8) from (ArGeH)2 and tert-Butyl
Ethylene. To a green solution of (ArGeH)2 (0.020 g, 0.021 mmol) in
C6D6 (0.6 mL), tert-butyl ethylene was added via syringe. After 48 h,
1H NMR spectroscopy gave a spectrum consistent with {ArGe-
(CH2CH2 Bu)}2 (major) and ArGeH3 (minor).14 1H NMR (300
t
MHz, C6D6, 298 K): δ = 0.62 (d, J = 6.8 Hz, 12H), 0.77 (d, J = 6.7 Hz,
12H), 0.83 (s, 18H), 0.87 (m, 4H), overlapping doublets 1.11 (d, J =
6.7 Hz, 12H), 1.13 (d, J = 6.7 Hz, 12H) 1.21 (m, 4H), 2.57 (sept, J =
6.7 Hz, 4H, CH(CH3)2), 2.90 (sept, J = 6.7 Hz, 4H, CH(CH3)2),
6.82−7.35 (m, 18H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp).
t
Synthesis of {ArSn(CH2CH2 Bu)}2 (9) from (ArSnH)2 and tert-Butyl
the field of C−H activation in that they constitute 1,2-additions
across a multiple metal−metal bond.15 Furthermore, both the
products obtained and the nonpolarity of the multiple bonds
are consistent with homolytic cleavage, providing a striking
counterpoint to the known reactivity at polar metal−element
multiple bonds. As a new motif in the field of C−H activation,
these reactions demand further study.
Ethylene. To a red solution of (ArSnH)2 (0.019 g, 0.019 mmol) in
C6D6 (0.6 mL), tert-butyl ethylene was added via syringe. After 48 h,
1H NMR spectroscopy gave a spectrum consistent with the clean
t
synthesis of {ArSn(CH2CH2 Bu)}2 in essentially quantitative yield. 1H
NMR (600 MHz, C6D6, 298 K): δ = 0.54 (br, 4H, 3J = 4.8 Hz,
3
CH2CH2C(CH3)3, 0.82 (s, 18H, CH2CH2C(CH3)3, 1.07, (d, 24H, J
= 24 Hz, CH(CH3)2), 1.31 (d, 24H, 3J = 6.8 Hz, CH(CH3)2), 1.51 (m,
4H, CH2CH2C(CH3)3), 3.23 (sept, 8H, 3J = 6.8 Hz, CH(CH3)2), 7.12
(d, 6H, 3J = 7.8 Hz, Ar), 7.16−7.18 (m, 6H, Ar), 7.29 (t, 6H, 3J = 7.8
Hz, Ar).
In this paper we use experimental methods to examine these
reactions in more detail in order to gain mechanistic insight and
discover both the implicit and explicit role of hydrides as
intermediary species and products in these C−H activation
reactions. We delineate the mechanism of activation of CpH
and continue by describing similar efforts to find a likely
pathway for the oxidation of c-C5H8. In particular we have
isolated a new side product from the reaction between 1 and
cyclopentene, a multiple-bonded asymmetric digermene
featuring hydride and hydrogenated cyclopentyl substituents
ArGe(H)Ge(c-C5H9)Ar (5) which explains the fate of the
liberated hydrogen atoms in the transformation of c-C5H8 to
Synthesis of {ArGe}2(C7H12) (10). To a solution of (ArGe)2 (1)
(0.400 g, 0.42 mmol) in 40 mL pentane, 1,6-heptadiene (62 μL, 0.46
mmol) was added via microliter syringe giving a pale-yellow solution
after six hours. Recrystallization at −20 °C overnight gave colorless
crystals of 10 (0.262 g, 0.25 mmol, 60% yield), mp 195−198 °C
1
(dec.). H NMR (600 MHz, C6D6, 298 K): δ = 1.13 (d, J = 6.8 Hz,
6H), 1.23 (d, J = 6.7 Hz, 6H), 1.34 (d, J = 6.8 Hz, 12H), 1.40−1.47
(m, 2H), 1.49−1.56 (m, 2H), 1.63−1.33 (m, 2H), 1.89 (pq, 2H), 3.04
(br, 8H), 4.41 (m, 4H), 7.00−7.04 (m, 6H), 7.22 (d, 8H), 7.32 (pt,
4H). 13C{1H} NMR (150 MHz, C6D6, 298 K): δ = 10.7. 13.9, 18.5,
22.0, 22.3, 22.4, 22.6, 22.9, 23.0, 23.7, 23.9, 24.1, 25.2, 25.3, 25.4, 25.6,
25.8, 25.9, 28.9, 30.3, 30.4, 30.5, 30.30.6, 30.7, 30.9, 33.3, 33.7, 35.4,
38.8, 67.6, 122.4, 122.6, 122.7, 122.8, 123.2 (2 C), 128.1, 128.2, 128.7,
130.4, 131.0, 131.0, 140.7, 140.8, 141.2, 141.6, 145.4, 146.3.
X-ray Crystallographic Studies of 5·Hexane and 10·Pentane.
The crystal structures were solved by direct methods using the SHELX
version 6.1 program package, corrected for Lorentz and polarization
effects with SAINT18 and absorption using Blessing’s method as
incorporated into the program SADABS.19,20 The SHELXTL21
program package was implemented to determine the probable space
groups and set up the initial files. The structures were determined by
direct methods with the successful location of a majority of the
nonhydrogen atoms using the program XS.21 The structure was
refined with XL.22 Noteworthy parameters are summarized in Table 1.
In 5·hexane, 5 was found to possess two different central cores, one a
−
C5H5 . Finally we describe reactions with other olefins,
including the reaction of 1 with 1,6-heptadiene to give a
tricyclo species (ArGe)2(C7H12) (10), the result of cyclo-
addition rather than C−H activation and which gives
compelling evidence for a nonradical pathway for these
competing reactions between olefins and dimetallynes.
EXPERIMENTAL
■
General Procedures. All operations were carried out by using
modified Schlenk techniques under an atmosphere of dry argon or
nitrogen. Solvents were dried over an alumina column and degassed
1
prior to use. The H and 13C NMR spectroscopic data were recorded
on Varian INOVA 300 or 400 MHz spectrometers. The H and 13C
1
NMR spectra were referenced to the deuterated solvent. The starting
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Table 1. Selected X-ray Crystallographic Data for 5·hexane
and 10·pentane
Scheme 2. Proposed Pathway for C−H Activation of CpH by
ArEEAr (E = Ge, Sn)
compound
5·hexane
10·pentane
empirical formula
formula weight, gmol−1
T/λ
C71H98.12Ge2
1096.79
C69.50H92Ge2
1072.61
90(2) K/1.54178 Å
monoclinic
C2/c/8
90(2) K/1.54178 Å
monoclinic
P21/c/4
crystal system
space group/Z
a, Å
21.587(1)
14.923(1)
39.688(2)
90.48(1)
20.5795(16)
13.4167(10)
22.7242(17)
109.826(2)
5902.5(8)
1.207
b, Å
c, Å
β, °
V, Å3
ρ, mg/m3
abs. coeff., mm−1
F(000)
12784.7(6)
1.140
1.428
1.538
4705
2292
crystal size, mm
θ range, °
reflns collected
ind. reflns
0.23 × 0.14 × 0.11
4.10−68.50
18113
0.37 × 0.30 × 0.22
7.72−67.75
27667
10 458
10 462
R(int) = 0.0305
9593
R(int) = 0.0246
10 390
obs. reflns (I > 2σ(I))
completeness to 2θ
max, min trans.
89.00%
97.90%
0.8552, 0.7375
1.054
0.7236, 0.6030
1.071
goodness-of-fit F2
final R [I > 2σ(I)]
species.16,17,24a,25 Subsequently, the production of hydrogen
results from deprotonation of another equivalent of CpH (pKa
= 18) by (ArEH)x.
R1 = 0.0486,
wR2 = 0.1342
R1 = 0.0521
wR2 = 0.1368
R1 = 0.0282
wR2 = 0.0737
R1 = 0.0283
wR2 = 0.0738
R (all data)
To test this proposed route, a number of reactions with
various olefins were undertaken in order to verify the presence
of transient group 14 element hydrides (ArEH)x as
intermediary products. By running the C−H activation
reactions in the presence of an olefin that can insert into
these main group hydride bonds, we can form an alkyl species
which should not participate in further chemistry and can be
short Ge(1)Ge(2) bond 2.3098(5) Å with a lone hydride 94% of
the time and also a 6% core composed of Ge(1b)−Ge(2b) = 2.614 Å
and three hydrides requiring partial rotations of three methyl groups,
namely, C(26), C(44), and C(76) to accommodate the various
disorders. Three orientations of a full occupancy hexane molecule were
also optimized. Hydrogen atoms were idealized, except for H(61)
which was allowed to refine freely throughout the final refinement
stages. For 10·pentane, the solvent molecule was found to lie on a
special position and therefore C(71) was found to be half occupied
such that C(72) is also half the time of a terminal methyl group. The
final difference Fourier maps were featureless indicating that the
structures are both correct and complete.
1
easily detected by H NMR. Insertion of olefins into E−H
bonds usually requires a transition-metal catalyst (e.g., in
hydrosilylation),26 however for subvalent group 14 element
species, it has been recently found that insertion is a facile
process in the absence of external catalysts or elevated
temperatures.27 Presumably, this occurs via cooperative
interaction of the olefin σ and π bonds with a vacant p-orbital
and occupied s-orbital present in the low-coordinate species.28
It was supposed that if a subvalent hydride was present in the
reaction of dimetallyne with CpH, it could be detected by the
addition of an excess of olefin, as a competitive substrate versus
CpH toward the hydride. This hypothesis assumes that the
insertion reaction is faster than deprotonation, a factor which
can be managed by using an excess of the insertion substrate. A
possible complication is that the dimetallyne species 1 and 2
undergo both reversible and irreversible cycloaddition reactions
with various olefins.13,29 However, although these reactions are
comparatively fast, they are highly selective, and using the Ar
ligand, they typically only occur with small or activated olefins,
e.g., ethylene and norbornadiene, but not with substituted
olefins, such as propene or butenes (see later for a greater
discussion of this phenomenon).29a
RESULTS AND DISCUSSION
■
First we consider the simplest of the reactions, the formation of
Cp products 3 and 4 from CpH and 1 and 2, respectively, along
with hydrogen gas (Scheme 1). This type of reactivity is known
for highly reducing species, e.g., alkali metals, for which electron
−
transfer is found to initiate the formation of C5H6 and
proceeds to rapidly form an aromatized Cp− and a H• radical.23
We propose an alternate mechanism for the dimetallyne
reaction, based on oxidative addition followed by a simple
acid−base reaction. First, there is an oxidative addition of the
C−H bond across the triple EE bond to give an unsymmetric
digermyne Ar(H)GeGe(C5H5)Ar intermediate, which then
dissociates the two different metallanediyl fragments due to the
weak EE bond (Scheme 2). This results in the formation of a
16
divalent group 14 element hydride of the type (ArEH)x and
Screening 1 and 2 with a variety of linear and branched
olefins using NMR techniques showed that tert-butyl ethylene
is unreactive with these dimetallynes, even in excess with
heating to 100 °C for several days. Furthermore, this substrate
has been previously shown to function as an effective acceptor
for incipient H• radicals, giving neohexane as the hydrogenated
the metallanediyl product ArGeCp, which can be considered to
be, in effect, an overall σ bond metathesis. Hydride species of
this type have been isolated and reported previously, cf.,
(ArGeH)2 (6) and (ArSnH)2 (7), synthesized from the direct
reaction of H2 and ArEEAr or reduction of (ArECl)2
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product30 and so may function as an indicator of two different
mechanisms.
demonstrates that the insertion reaction, with excess olefin
(respective to CpH), is preferential to CpH deprotonation
under these conditions, which would produce H2. Indeed,
NMR experiments revealed that (ArSnH)2 reacts with CpH to
give 4 and H2, whereas (ArGeH)2 reacts with CpH to give 3
and ArGeH3 (Scheme 5). In the Ge reaction, (ArGeH)2 is
The reactions with (ArEH)2 E = Ge (6) and Sn (7) were
undertaken and demonstrate successful insertion into the E−H
t
bonds to give {ArE(CH2CH2 Bu)}2 E = Ge (8) or Sn (9),
respectively (Scheme 3). These transformations were identified
Scheme 3. Reactions between Hydrides and tert-Butyl
Scheme 5. Reactions between Hydrides and CpH
Ethylene
assumed to act as an acceptor for the H2 produced in situ (see
above). Together, these reactions verify the two steps of the
C−H activation process: First, (ArEH)2 (or monomeric form
thereof) is formed as an intermediate, and second, the species 6
or 7 can function as a base to deprotonate the acidic proton in
CpH. If the electron-transfer route was taken, as proven for
alkali metals, then it would be expected that either H2 or
neohexane should be detected when the reaction was carried
out with dimetallyne in the presence of tert-butyl ethylene.
One inconsistent feature of these studies is our observation
of ArGeH3 with the (ArGeH)2 reactions, whereas for the
original reactions with digermyne 1 we do not find significant
quantities of this side product. We believe that this is the case
because when H2 is produced in the dimetallyne C−H
activation reaction, 1 is in excess with respect to the
intermediary amounts of (ArGeH)x, and therefore any H2
produced is much more likely to react with 1 and form a
monohydrogenated (divalent) species. In the case of the
hydride reactions, 6 is in excess therefore H2 is likely to react
with divalent 6 and form tetravalent ArGeH3, which is not
active as a base or insertion substrate.
1
by H NMR and by the disappearance of the distinctive E−H
stretch in the IR spectra; unfortunately, despite many attempts,
single crystals of these compounds could not be obtained. This
type of reaction has precedent in our laboratory for the closely
t
related species {Ar*Sn(CH2CH2 Bu)}2 (Ar* = C6H3-2,6-
(C6H3-2,4,6-iPr3)2), obtained in good yield from (Ar*SnH)2
and tert-butyl ethylene, and which has been fully characterized,
including an X-ray crystallographic study. The use of tert-butyl
ethylene therefore ought to be a reliable substrate for insertion
into intermediary hydrides generated in C−H activation
reactions.
In the case of the germanium reaction, the additional side
product, ArGeH3, was observed in low yield (ca. 10%), along
with a similar quantity of an unidentified terphenyl-bound
species. Further experimentation revealed that (ArGeH)2 reacts
with H2 to give a mixture of ArGeH3 (15%) and Ar(H)2GeGe-
(H)2Ar (85%), whereas (ArSnH)2 is inert to further reaction;
recent calculations indicate that these reactions proceed via
ArGeGeH2Ar.24c These new observations reveal a higher
reactivity for germanium and demonstrate that the apparently
straightforward insertion reactions between low-valent group
14 element hydrides and olefins likely operate in a complex
manner, perhaps yielding free H2 as a side product. The
reaction of this olefin with E−H bonds meanwhile contrasts
with Ir hydride systems in which reductive elimination occurs
as part of a catalytic cycle to yield neohexane and a reactive Ir
species which is capable of C−H activation.30
The triple C−H activation of cyclopentene by 1 to give
aromatized Cp product 3 (31% recrystallized yield) is clearly a
more demanding reaction than that discussed for CpH, and the
less reactive distannyne 2 is not capable of this trans-
formation.14 In our initial report we could not characterize
any side products, despite the low yield of 3 implying their
presence, however we are happy to now report the full outcome
of this reaction and therefore deduce a likely pathway for the
reaction. It was found, after optimization, that the addition of
1.25 equiv of cyclopentene to red digermyne 1 over 12 h gave a
We found that reaction of 1 or 2 with CpH in the presence
of excess tert-butyl ethylene yields a mixture of Cp compounds
3 or 4 and insertion products 8 or 9, respectively, in a 2:1 molar
ratio, as shown by H NMR spectroscopy (Scheme 4). No
hydrogen was detected in these reactions, although H2 is
1
1
dark-green solution. H NMR spectroscopy (C6D6) revealed
the presence of Cp compound 3 and a new product 5 which
could be separated from 3 by fractional crystallization allowing
full structural characterization of this species.
evolved in the original reaction of 1 or 2 with CpH. This
Scheme 4. Reaction of CpH with Dimetallynes Carried out
in the Presence of Excess tert-Butyl Ethylene
Compound 5, Ar(H)GeGe(c-C5H9)Ar, was isolated as a
green solid in 46% recrystallized yield. The X-ray structural data
were consistent with a trans-pyramidal digermene core
arrangement with new bonds to terminally bound hydride
and cyclopentyl moieties (Figure 1). A small impurity (6%) of
digermane ArGe(H)2Ge(H)(c-C5H9)Ar was found to cocrys-
tallize in the crystal selected for X-ray studies, which may result
from the addition of H2 to 5 or its unsymmetric isomer ArGe
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[Ge(1)−C(1) 1.976(3) and Ge(2)−C(31) 1.988(3) Å], similar
to that found in the asymmetric distannene compound
(Ar)Sn(μ-Br)Sn(CH2C6H4-4-iPr)(Ar).35
One striking feature of this digermene is that the Ge−Ge
bond does not dissociate to give (ArGeH)2 and {ArGe(c-
C5H9)}n. This is in contrast with the proposed mechanism for
the C−H activation of CpH (Scheme 2), in which we propose
that the formation of a similar asymmetric digermene
intermediate {Ar(H)GeGe(η5-Cp)Ar} is followed by facile
Ge−Ge bond cleavage. The increased coordination number of
the π-bound Cp ligand would be expected to give a greater
driving force toward dissociation than the σ-bonded c-C5H9
moiety, by virtue of a greater steric and electronic stabilization
of the germanediyl form, and a high kinetic barrier for the latter
prevents the formation of symmetrical products, such as
{ArGe(R)}2. Solid samples of 5 are stable indefinitely under
anaerobic conditions, however in solution they decompose over
a period of weeks. No evidence for reductive elimination of the
cyclopentyl and hydride groups, i.e., the formation of
cyclopentane, could be found.
Figure 1. Thermal ellipsoid (50%) plot of 5. H atoms except H(1) and
isopropyl groups are not shown. Selected bond lengths [Å] and bond
angles [°]: Ge(1)−C(1) 1.976(3), Ge(1)−Ge(2) 2.3098(5), Ge(2)−
C(61) 1.988(3), Ge(2)−C(31) 1.988(3), Ge(1)−H(1) 1.460(0),
C(61)−C(62) 1.529(4), C(61)−C(65) 1.541(4), C(62)−C(63)
1.535(5), C(63)−C(64) 1.575(6), C(64)−C(65) 1.498(5); C(1)−
Ge(1)−Ge(2) 117.4(1), C(61)−Ge(2)−C(31) 112.2(1), C(61)−
Ge(2)−Ge(1) 119.1(1), C(31)−Ge(2)−Ge(1) 114.0(1), C(62)−
C(61)−C(65) 105.5(2), C(61)−C(62)−C(63) 103.0(3), C(62)−
C(63)−C(64) 101.6(3), C(65)−C(64)−C(63) 103.1(3), C(64)−
C(65)−C(61) 107.9(3).
The data described above show clearly that C−H activation
of the methylene groups in two molecules of c-C5H8 has given
dehydrogenated c-C5H5 coupled with the partial hydrogenation
of three other molecules of c-C5H8 in 5, i.e., a disproportio-
nation of H atoms. This 3:2 molar ratio of 5 and 3 is observed
1
accurately in the H NMR spectrum of the reaction mixture
without evidence for any other products (see Figure 2 and
Scheme 6).
Ge(H)(c-C5H9)Ar. This is discussed in greater detail in the
Experimental section. Analysis of the 1H NMR spectrum of the
recrystallized product does not show any signals attributable to
this species that are observable above the baseline noise,
therefore it is not a significant side product in the bulk solid.
The C5 ring is bound through one carbon with a Ge−C bond
length of 1.988(3) Å, a normal distance for Ge(II)-alkyl single
bonds and was found to be fully saturated, with average C−C−
C angles (104.2(3)°) and distances (1.536(5) Å) consistent
with sp3 carbon hybridization. The hydrocarbon hydrogen
atoms could be located crystallographically in the structure
which shows a saturated c-C5H9 group. Additionally, the
structural data were best refined with a single terminal
germanium hydride on Ge(1) set at a distance of 1.46(0) Å.
Spectroscopic features for Ge−H were found as a band at 2010
cm−1 in the IR spectrum and as a signal in the H NMR
1
spectrum at 5.87 ppm. These values are similar to those found
in Ar(H)GeGe(H)Ar (2100 and 2060 cm−1 and 5.87
ppm).16a The Ge−Ge distance of 2.3098(5) Å is within the
range found for GeGe double bonds in digermenes.16a The
multiple bonding is further evidenced by an n− → n+
absorption at 406 nm (ε = 1500 mol−1 L cm−1). Significant
pyramidalization is observed at Ge(2), with an out-of-plane
angle of 36.8° consistent with the presence of nonbonding
electron density,32 supporting a Ge(II)/Ge(II) structure, as
opposed to Ge(I)/Ge(IV) with both the hydride and
cyclopentyl substituent bound to Ge(2). The Ge(2)−Ge(1)−
C(Ar) angle of 117.4(1)° also is consistent with this
designation: two-coordinate Ge(II) centers typically bind at
tighter angles of 90−100°.31,32 There are no reported examples
of structurally characterized unsymmetrically substituted
digermenes for comparison with 5.31,33,34 This asymmetric
substitution at the GeGe moiety is not, however, reflected in
Ge−C(Ar) distances, which are found to be within esd values
Figure 2. 1H NMR spectrum (C6D6) of the crude reaction mixture of
1 with c-C5H8 showing clean conversion to 3 and 5 after 12 h.
Overlapping signals at δ 1.05 and 3.00 ppm are noted; the complex
aromatic region has not been delineated.
The presence of a Ge−H moiety in 5, together with the
isolation of (ArGeH)2 from the related reaction of 1 and 1,4-
cyclohexadiene, further implicates the likely role of germanium
hydrides in these types of C−H activation processes. Insertion
of 1 equiv of cyclopentene into a Ge−H bond would result in a
cyclopentyl group of the type found in 5. Indeed, Baceiredo and
co-workers have recently reported this type of insertion
reaction between a base-stabilized Si(II) hydride and cyclo-
pentene.27a A further telling characteristic of product 5 is that it
contains a cyclopentyl fragment rather than a cyclopentenyl
group. The latter would be a logical product of C−H activation
α to the unsaturated bond and has been reported by Banaszak-
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Scheme 6. A Balanced Reaction Scheme for the Formation of 5 and 3 from Digermyne 1
Holl in reactions using mixtures of a germylene or stannylene
and aryl halide.7
with cyclopentene was found to be unaffected when carried out
with an excess of dihydroanthracene.36
These results are consistent with the formation of 3 as a
primary C−H activation product and 5 as a secondary product
formed by insertion of cyclopentene into a Ge−H bond. In the
case of the competition experiment, insertion of tert-butyl
ethylene is preferred exclusively, and no evidence for the
formation of hydrogenated cyclopentyl species 5 could be
found. In order to examine the identity of this germanium
hydride intermediate, (ArGeH)2 was reacted with both 1 equiv
Finally, an unusual possibility to consider is that 5 results
from a sterically frustrated [2 + 2] cycloaddition, which would
give a β-digermenylalkyl diradical species shown in Scheme 7.
C−H abstraction by these 1,4-diradical centers would furnish
species 5.
Scheme 7. A Putative 1,4-Diradical Intermediate in the [2 +
2] Cycloaddition of Cyclopentene with 1
1
and excess cyclopentene and after 48 h gave a H NMR
spectrum identical to that obtained for 5 (Scheme 9). We
Scheme 9. Reaction between Hydride 6 and c-C5H8
To probe this mechanism, the olefin, tert-butyl ethylene, was
chosen for a competition experiment to determine the presence
of transient hydrides. When 1 was reacted with cyclopentene in
the presence of excess tert-butyl ethylene a similar color change
from red to green was observed over 12 h. 1H NMR
spectroscopy revealed a clean mixture of C−H activated Cp
product 3 and the insertion product 8 in 2:3 molar ratio with
no other products (Scheme 8). No evidence was found for the
cyclopentyl product 5 or hydrogenated alkanes (neohexane or
cyclopentane). To test for diradical intermediates, the reaction
observe a single insertion event despite the existence of two
hydrides per dimer; presumably 5 is inert to further reaction for
steric reasons. One curious observation is that the independent
insertion experiment, although successful, occurred at a much
slower rate than that observed for the C−H activation reaction:
2 days in contrast with 12 h for the latter. It is possible that a
dissociated form of (ArGeH)2 is formed, which effects this
insertion more rapidly than the dimer.
These data strongly indicate the presence of transient
hydrides as the source of product 5 and contrast with the
Crabtree iridium system which C−H activates cyclopentene in
the presence of tert-butyl ethylene to make a cyclopentadienyl
anion and neohexane, rather than insertion products.1d,e,30 The
question as to how the Ge−C bond is formed in 3 remains
however. A likely route, shown in Scheme 6, forms free CpH as
Scheme 8. Formation of Cp Derivative 3 and Insertion
Product 8 from Digermyne and Cyclopentene in the
Presence of Excess tert-Butyl Ethylene
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an intermediate by partial oxidative dehydrogenation of c-C5H8
and is favorable because the diene would consequently be
expected to be reduced by the mechanism described earlier
(Scheme 2), i.e., the Ge−C bond is formed by a [1, 2]-addition
to the Ge−Ge triple bond of 1. This route could be prefigured
by a μ2-η1:η1-H,H coordination mode followed by a sigmatropic
rearrangement in which both an allylic and an unactivated
secondary C−H bond are broken. Alternatively addition could
occur at a single center to form the asymmetric isomer of 6,
ArGeGe(H)2Ar, which has been described previously.24
Although we do not detect free hydrogen in the cyclopentene
reaction (unlike the CpH reaction), this would be expected; 1
is known to react quickly (within one minute at room
temperature) with H2 to form 6,16a which in turn participates in
insertion chemistry with the excess cyclopentene to yield 5.
One notable feature of this proposed route is the stepwise
nature of the transformations, i.e., that the initial reaction is
simply an abstraction of two H atoms leaving a conjugated
diene as a free organic product, which then in turn participates
in a series of further reactions. Attempts to trap the CpH
intermediate have failed due to the incompatibility of these
trapping agents with 1 (e.g., alkali metals, strong bases,
transition-metal carbonyls).
1,6-heptadiene reacts with 1 in a double set of [2 + 2]
cyclizations to yield a new tricyclic compound (ArGe)2(C7H12)
(10) (Scheme 10).
Scheme 10. Synthesis of 10 from 1 and 1,6-heptadiene
The structure, as determined by X-ray methods, shows a
Ge−Ge single bond at 2.4691(3) Å incorporated into a tricyclic
organogermane architecture with two 1,2-digermacyclobutane
rings and a 1-germacyclohexane moiety in chair conformation
(Figure 3). This central core is a 3,9-digerma analogue of the
Another possibility to consider is that allylic activation may
proceed by oxidative addition of the C−H bond, in a manner
similar to that proposed for the CpH reaction, giving a Ge(II)
cyclopentenyl species, similar to the Banaszak-Holl com-
pounds,7 as intermediate. This seems less likely given the
necessity of further C−H activation of a Ge-bound cyclo-
pentenyl group. If this transformation occurred in an
intramolecular manner, the products obtained would be in
higher oxidation states, i.e., +3 and +4, inconsistent with the
major products that have been characterized. Intermolecular H
abstraction cannot be ruled out, however it seems unlikely
given the previously noted steric effect of the large terphenyl
groups.
Figure 3. Thermal ellipsoid (50%) plot of 10. H atoms and isopropyl
groups not shown. Selected bond lengths [Å]: Ge(1)−Ge(2)
2.4691(3), Ge(1)−C(8) 1.9763(14), Ge(1)−C(1) 1.9829(15),
Ge(1)−C(7) 1.9953(15), Ge(2)−C(38) 1.9672(15), Ge(2)−C(6)
1.9988(15), Ge(2)−C(2) 2.0052(15).
Further experiments with dimetallynes 1 and 2 showed no
reactivity with either cyclohexene, norbornene, cyclohepta-
triene, fluorene, 1,5-cyclooctadiene, or triphenyl methane at
temperatures up to 80 °C. This is in contrast with reported
disilynes (stabilized by large silyl substituents) which undergo
[2 + 2] cycloadditions with cyclohexene, although larger
substrates have yet to be reported.37,38 This may be due to the
different steric environment at the dimetallyne cores; this
question will remain until a synthetic route to (ArSi)2 can be
discovered. Given the negligible olefin strain energies of c-C5H8
and c-C6H10, it is likely that the reaction between 1 and
cyclopentene is enabled by reduced steric hindrance (tightening
of C−CC angles from 123.6° to 111.3°) and therefore must
be near the limit tolerated by the ArGeGeAr complex.
Norbornene has similar bond angles (108.6°) and some olefin
strain energy (ca. 2 kcal/mol), but presumably this reaction is
impeded by the bulk imposed by the additional CH2CH2
atoms. Norbornadiene has been found to undergo a [2 + 2]
cycloaddition with 1 and 2 under ambient conditions;29a the
energy required to overcome the kinetic barrier imposed by the
bulky ligands may in this case be attributed to a higher olefin
strain energy of norbornadiene versus norbornene (15.5 kcal/
mol higher).39
hypothetical all-carbon molecule tricyclo[3.3.1.03,9]nonane.
Germacyclohexane moieties are known and display similar
geometry to that found in 10.40 The Ge−Ge bond distance is
normal for a single bond (c f., 2.44 Å in elemental germanium),
indicating less strain than in the digermahypostrophene cage
compound (ArGe)2(C8H8) in which the Ge−Ge distance is
found at 2.566(1) Å and is susceptible to thermolysis under
mild heating.29c Otherwise, bond distances and angles within
the molecule are unexceptional.
This is an insightful result as 1,6-hexadiene has been used as
a means of detecting radical pathways for cycloaddition
chemistry, e.g., through a 1,4-diradical intermediate of the
type shown in Scheme 7, for which the unpaired electron that
develops on the β-carbon is found to undergo rapid (k = 1 ×
105 s−1) internal cyclization with the remaining double bond
giving a cyclopentane moiety which is easily characterized.41
This radical cycloaddition pathway must be considered as the
nonradical [2 + 2] cyclization is symmetry forbidden by the
Woodward−Hoffmann rules. We find here that cycloaddition
has proceeded unimpeded, therefore supporting a mechanism
that proceeds through a [1 + 2] addition followed by
rearrangement, as proposed and calculated by Sekiguchi,
Nagase, and co-workers for a disilyne.38 This is in contrast to
the disilene {(tBuMe2Si)2Si}2 which reacts with the same
substrate to give an ene addition product with allylic C−H
abstraction, i.e., a 1,5 hydrogen shift.41
Although we have yet to find either C−H or cycloaddition
reactions with many of these larger cyclic olefins, during the
process of screening we discovered that, although smaller
olefins such as propene are unreactive to these dimetallynes,
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Although the unsaturated bonds in the cycloalkenes
described provide weakened allylic C−H bonds that drive the
regioselectivity observed in these reactions, it is still unclear
why these reactions occur in preference to cycloaddition
chemistry, which have also been observed for dimetallynes and
would be expected to be particularly favorable with CpH, a
classic Diels−Alder diene.13,29 In the case of transition-metal
systems in which a terminal MX moiety can C−H activate
aliphatic substrates, cycloaddition is found to occur in
preference in the case of olefinic compounds.4 In our case, an
answer to this dichotomous behavior may lie in the partial
diradical character of the EE multiple bonds, which are
intermediate between double and triple bonds and difficult to
model. The distannyne core of 2 is more spatially accessible
than the lighter digermyne analogue 1, evidenced by the longer
EE distance (Ge−Ge 2.2850(6) Å in 1,13b Sn−Sn 2.6675(4)
Å in 2),13a yet we observe a lower reactivity for 2 compared to
1. This has been reported previously for the activation of other
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data, in CIF format for 5 and 10. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
pppower@ucdavis.edu.
■
Present Addresses
†Chemistry Division, Los Alamos National Laboratory, Los
Alamos, New Mexico 87545.
‡Department of Chemistry, Imperial College London, South
Kensington Campus, London SW7 2AZ United Kingdom.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
small molecules13,30 and can be explained by a lower diradical
c
■
We thank the U.S. Department of Energy, DE-FG02-
07ER46475, for support of this work and the Natural Sciences
and Engineering Research Council of Canada for a PDF
(C.A.C.).
character in 2 compared to 1, previously indicated by
calculations.42 This is consistent with the C−H activation
reactions being initiated by a radical-type abstraction
mechanism.
REFERENCES
■
CONCLUSION
■
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