Experimental and computational study of auxiliary molecular effects on the mechanism of the addition of hydrazines to a low-valent germanium complex

Article abstract of DOI:10.1021/om300271c

Reaction of the diarylgermylene Ge(ArMe₆)₂ (ArMe ₆ = C₆H₃-2,6-(C₆H ₂-2,4,6-(CH₃)₃)₂) with hydrazine and methylhydrazine gave the first structurally characterized Ge(IV) hydrazide complexes, (ArMe₆)₂Ge(H)NHNHR (R = H, Me). A detailed computational study of the mechanism of this N-H bond activation showed that the reaction proceeds via an intermolecular proton transfer, which is mediated by a second equivalent of hydrazine that interacts with the hydrazine-Ge(ArMe ₆)₂ adduct intermediate through N- - -H hydrogen bonding. This auxiliary effect is blocked when the substituted derivative N,N-dimethylhydrazine is used, and a Lewis adduct complex of Ge(ArMe ₆)₂ is the only product obtained.

Full text of DOI:10.1021/om300271c

Article
pubs.acs.org/Organometallics
Experimental and Computational Study of Auxiliary Molecular
Effects on the Mechanism of the Addition of Hydrazines to a Low-
Valent Germanium Complex
Zachary D. Brown,† Jing-Dong Guo,‡ Shigeru Nagase,‡ and Philip P. Power*,†
†Department of Chemistry, University of California, Davis, 1 Shields Avenue, Davis, California 95616, United States
‡Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
S
* Supporting Information
ABSTRACT: Reaction of the diarylgermylene Ge(ArMe
) =
(ArMe
2
6
6
C6H3-2,6-(C6H2-2,4,6-(CH3)3)2) with hydrazine and methylhydrazine
gave the first structurally characterized Ge(IV) hydrazide complexes,
(ArMe )2Ge(H)NHNHR (R = H, Me). A detailed computational study of
6
the mechanism of this N−H bond activation showed that the reaction
proceeds via an intermolecular proton transfer, which is mediated by a
second equivalent of hydrazine that interacts with the hydrazine−
Ge(ArMe )2 adduct intermediate through N- - -H hydrogen bonding. This
6
auxiliary effect is blocked when the substituted derivative N,N-
dimethylhydrazine is used, and a Lewis adduct complex of Ge(ArMe )2 is the only product obtained.
6
ince 2005, the activation of small molecules by main-group
complexes under ambient conditions has been attracting
fragment coordinated by six gallium atoms.19 Roesky and co-
S
workers have reported that silylenes, LSiII (L= β-diketiminate),
undergo N−H bond insertion to yield a Si(IV) hydrazide
complex,20 as well as the isolation of a Ge(II) hydrazide which
undergoes proton insertion from the hydrazine molecule onto
the unsaturated backbone of the diketiminate.21
increasing interest.1 The first example involved the cleavage of
dihydrogen by a germanium analogue of an alkyne.2 In
addition, Stephan and Bertrand also demonstrated the room-
temperature activation of dihydrogen and ammonia by
frustrated Lewis pairs3 and singlet carbenes.4 Since these initial
discoveries, the reactivity of certain open-shell main-group
complexes has been shown to often mimic that of transition
metals5 through the activation of several small molecules such
as olefins,6 P4,7 and CO2.8 Investigations of the activation of
N−H bonds by main-group-metal complexes4,9,10 have been
prominent because of their importance as potential precursors
to main-group-metal nitrides,11 and because the transition-
metal-mediated activation of N−H bonds in processes such as
hydroamination is rare since they tend to favor the formation of
Lewis base (Werner) complexes.12 Hydrazine and its
substituted congeners have proven to be an interesting class
of compounds for reactivity studies because of their extensive
use in industry as fuels,13 in organic synthesis,14 and as
intermediates in the biological reduction of dinitrogen to
ammonia.15 Although numerous examples of main-group
hydrazides have been reported in the literature, mechanistic
insights into their formation and reactivity are rare, and direct
comparisons with related amido complex formation have been
limited.16 To date, the reactivity of main-group-metal
complexes toward hydrazine and related substrates has largely
focused on reactions of substituted hydrazines with metal alkyls
or hydrides which, via alkane or H2 elimination, yield numerous
interesting cage structures.17,18 Fox example, Uhl and co-
workers have isolated several unusual group 13 metal hydrazide
cage species and their thermolysis products, including the
notable gallium “hydrazinetetraide”, which contains a [N−N]4−
Herein we describe the process by which hydrazines react
Me6
with the heavier element carbene analogue Ge(ArMe )2 (Ar
=
6
C6H3-2,6-(C6H2-2,4,6-(CH3)3)2)22 to yield the first
germanium(IV) hydrazides, (ArMe )2Ge(H)NHNHR (R = H
6
(1), Me (2)). Density functional theory (DFT) calculations on
the formation of 1 show that there is a transition state in which
a second molecule of hydrazine is also involved in N−H bond
activation. This occurs via N- - -H hydrogen bonding of the
second hydrazine to a hydrazine molecule that is already
coordinated to germanium via a dative N- - -Ge bond.
Experimental and computational data show that this hydro-
gen-bonding interaction is essential to the N−H bond
activation, as the proton transfer to germanium becomes
blocked by the use of N,N-dimethylhydrazine. In effect, the
terminal methyl groups on the coordinated N,N-dimethylhy-
drazine ligand do not engage in N- - -H hydrogen bonding to
the second molecule of NH2NMe2 and the adduct intermediate
(ArMe )2GeNH2NMe2 (3) is the only product formed (Scheme
6
1).
When the germylene Ge(ArMe )2 was treated with an excess
6
of anhydrous hydrazine or methylhydrazine (ca. 3 mol equiv),
the characteristic deep purple color of the solution faded, and
slow cooling of the concentrated solution to ca. −20 °C
Received: April 3, 2012
Published: May 2, 2012
© 2012 American Chemical Society
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Scheme 1. Reactions of the Diarylgermylene Ge(ArMe )2 with
N2H4, N2H3Me, and NH2NMe2
6
Figure 3. Thermal ellipsoid (30%) drawing of 3. Carbon-bound
hydrogen atoms and a flanking mesityl ring (C40−C48) are not
shown for clarity. Selected bond lengths (Å) and angles (deg):
Ge(1)−N(1) = 2.110(4), N(1)−N(2) = 1.439(5), C(Ar)−Ge(1)−
N(1) (av) = 94.3(1), Ge(1)−N(1)−N(2) = 118.7(3).
produced suitable single crystals of 1 or 2 for X-ray diffraction
study. Complex 1 (Figure 1) was found to have a monomeric
structure with distorted-tetrahedral coordination geometry at
the germanium. Consistent with the higher germanium
oxidation state, the Ge(1)−C(Ar) distances (1.981(2) and
1.984(2) Å) are shortened slightly with respect to those in
Ge(ArMe )2 (cf. 2.033(4) Å), and the C(Ar)−Ge(1)−C(Ar)
6
bond angle (113.0(1)°) is slightly narrower than the 114.4° in
22
Ge(ArMe )2. The Ge(1)−H(1) and Ge(1)−N(1) bond
6
Figure 1. Thermal ellipsoid (30%) drawings of (left) 1 and (right) 2.
Carbon-bound hydrogen atoms are not shown for clarity. Selected
bond lengths (Å) and angles (deg): 1, Ge(1)−N(1) = 1.832(2),
Ge(1)−H(1) = 1.52(2), N(1)−N(2) = 1.435(2), Ge(1)−N(1)−N(2)
= 117.1(1); 2, Ge(1)−N(1) = 1.832(3), Ge(1)−H(1) = 1.48(2),
N(1)−N(2) = 1.435(4), Ge(1)−N(1)−N(2) = 116.6(2).
distances are within the expected range for germanium(IV)
hydrides and amides (1.52(2) and 1.832(2) Å, respectively),10
and the N−N distance in the hydrazido moiety is also
shortened slightly to 1.435(2) Å from 1.45 Å.23 The shortening
of the N−N bond in 1 in comparison to that in free hydrazine
and the higher degree of pyramidalization of the germanium-
bound nitrogen (∑° = 342.9° for N(1) in comparison to ∑° =
Figure 2. Calculated energy (kcal mol−1) and drawings of intermediates and transition states with selected distances (Å) for the reaction of
Ge(ArMe )2 with N2H4 via inter- and intramolecular proton transfer at the B3PW91 level.
6
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Figure 4. Calculated energy (kcal mol−1) and drawings of intermediates and transition states with selected distances (Å) for the reaction of
Ge(ArMe )2 with NH2NMe2 at the B3PW91 level.
6
354.9° for N(2)) is consistent with a more covalent Ge−N
bond and little to no interaction between the electron lone pair
on nitrogen and the germanium atom. The FT-IR spectrum of
1 shows three weak bands at 3393, 3334, and 3259 cm−1,
assigned to the three N−H stretching modes of hydrazine. The
germanium-bound hydrogen in 1 was located and freely refined
from the X-ray data, and the presence of the hydride was
further confirmed by absorptions in the FT-IR (2105 cm−1)
second hydrazine molecule to the electron-rich germanium
atom with simultaneous transfer of H1 from the bound
hydrazine to the second molecule of hydrazine. This transition
state is related to that for the activation for ammonia by an
analogous germanium species, and the total activation energy
for the formation of 1 is similar at 23.0 kcal mol−1 (cf. 17.0 kcal
10
mol−1 for (ArMe )2Ge(H)NH2). We presume that 2 is
6
obtained by a route that is similar to that of 1 to give the
methyl-substituted species.
1
and H NMR (5.45 ppm) spectra. Complex 2 (Figure 1) was
isolated in a manner similar to that of 1, and 1H NMR
In contrast to 1 and 2, treatment of the germylene with an
excess of N,N-dimethylhydrazine affords the Lewis adduct 3
(Figure 3), and no formation of a germanium(IV) hydrazide is
observed. The lack of formation of the germanium(IV)
hydrazide is in sharp contrast to the facile nature of the
spectroscopy indicates that there was no formation of the
structural isomer (ArMe )2Ge(H)NMeNH2. Apparently, steric
6
effects disfavor the formation of a Lewis base complex to the
m e t h y l - s u b s t i t u t e d n i t r o g e n a n d o n l y t h e
(ArMe )2GeNH2NHMe adduct is observed. However, this
reactions of Ge(ArMe )2 with N2H4 and N2H3Me, which cleanly
6
6
species can also hydrogen bond via an N- - -H interaction to
a second NH2NHMe molecule and N−H bond activation is
observed to afford 2.24
yield the Ge(IV) N−H bond activated products. The
coordination of the −NH2 end of N,N-dimethylhydrazine
molecule to the germanium can be described as a simple dative
bond between the lone pair of nitrogen and the empty p orbital
on the germylene. The Ge(1)−N(1) bond (2.103(4) Å) is
more than 0.2 Å longer than those observed in 1 or 2 and is
coordinated at a 97.6° angle with respect to the Ge−C(ipso)2
plane. This bond angle is considerably less than those in 1
(128.8°) and 2 (127.6°) and is consistent with angles observed
in other known neutral σ adducts of germylenes.28
In order to gain a greater understanding of these reactions,
the mechanism for the formation of 1 was studied computa-
tionally by DFT.25 A transition state for intramolecular proton
transfer (Figure 2, TS′) was found on the potential energy
surface (PES) for the reaction of N2H4 with Ge(ArMe )2;
6
however, this pathway was deemed unlikely because its
activation energy was calculated to be greater than 40 kcal
mol−1 above the starting materials. Consequently, we turned
our attention to a mechanism involving intermolecular proton
transfer. Transition state 1 (TS-1)26 involves the initial
The calculated reaction pathway for the reaction of
Ge(ArMe )2 with N,N-dimethylhydrazine (Figure 4) displayed
6
transition states and intermediates, albeit with larger energy
differences, similar to those in the reaction pathway for the
formation of 1. Transition state 1 (TS-1) is higher in energy, ca.
5 kcal mol−1, with respect to the starting materials in
comparison to the reaction pathway for 1 (Figure 2), where
the two are nearly degenerate. As in 1, a hydrogen-bonding
interaction is predicted to stabilize INT-2, but in this case,
steric repulsion between the terminal methyl groups of the
hydrazine ligand and the adjacent molecule of N,N-
dimethylhydrazine significantly increases the energy required
approach of a hydrazine molecule to Ge(ArMe )2, which is
6
immediately followed by the formation of the adduct
intermediate (INT-1). The approach of the second molecule
of hydrazine (INT-2) unexpectedly does not lead to an increase
in energy. The energy required to overcome the conformational
change in the molecule to accommodate the second molecule
of N2H4 is offset by an intermolecular hydrogen bonding
interaction, ca. 12 kcal mol−1.27 Transition state 2 (TS-2)
represents the concerted proton migration of H2 from the
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−1
product, (ArMe )2Ge(H)N(H)NMe2, is only 3.1 kcal mol
6
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higher than that for 1 (26.1 kcal mol−1), but the transition state
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6
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intermediates and transition states in comparison to analogous
reactions between germylenes and ammonia. The intermolec-
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hydrogen bonding between a germylene−hydrazine adduct
intermediate (Figure 2, INT-1) with a second molecule of
hydrazine (Figure 2, INT-2). This auxiliary interaction is
inhibited when the germanium species is treated with N,N-
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Text and figures giving synthesis details and characterization
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crystal data for 1−3. This material is available free of charge via
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
(17) For a recent review of group 13 metal−hydrazine cage
compounds, see: Uhl, W.; Layh, M.; Rezaeirad, B. Inorg. Chem.
2011, 50, 12275−12283.
We are grateful to the U.S. Department of Energy Office of
Basic Energy Sciences (DE-FG02-07ER46475) for financial
support. This work was also financially supported by Grants-in-
Aid for Scientific Research on Priority Area and Next
Generation Super Computing Project (Nanoscience Program)
from the Ministry of Education, Science, Sports, and Culture of
Japan.
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(24) A full account of the structural and spectroscopic details of 2 can
be found in the Supporting Information.
(25) See the Supporting Information for a full account of the
computational methods.
(26) The activation energy was calculated to be very small, which
means that this step would not be rate-determining. See the
Supporting Information for further details.
(27) Single-point calculations using a larger basis set (6-311G(2d) for
germanium and 6-31G(d,p) for all other atoms) were performed to
determine the total energy required to reconfigure the molecule so
that association with the second eqivalent of N2H4 was possible.
(28) We have recently reported a germylene−isocyanide complex
which features similar bond angles with respect to the aryl−
germanium plane. See: Brown, Z. D.; Vasko, P.; Fettinger, J. C.;
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