A germanium isocyanide complex featuring (n → π*) back-bonding and its conversion to a hydride/cyanide product via C-H bond activation under mild conditions

Article abstract of DOI:10.1021/ja211874u

Reaction of the diarylgermylene Ge(Ar^Me6)₂ [Ar ^Me6 = C₆H₃-2,6-(C₆H ₂-2,4,6-(CH₃)₃)₂] with tert-butyl isocyanide gave the Lewis adduct species (Ar

Full text of DOI:10.1021/ja211874u

Communication
pubs.acs.org/JACS
A Germanium Isocyanide Complex Featuring (n → π*) Back-Bonding
and Its Conversion to a Hydride/Cyanide Product via C−H Bond
Activation under Mild Conditions
Zachary D. Brown,† Petra Vasko,‡ James C. Fettinger,† Heikki M. Tuononen,‡ and Philip P. Power*,†
†Department of Chemistry, University of California, 1 Shields Avenue, Davis, California 95616, United States
‡Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
lower than that of the free ligand have been isolated.5 Although
the possibility of π back-bonding in these adducts has not been
explored, there have been numerous recent reports of the
activation of small molecules such as H2, NH3, or C2H4 via
synergistic pathways in which a main-group atom(s) behaves as
both an electron donor and acceptor.6−8 We now report that
ABSTRACT: Reaction of the diarylgermylene Ge(ArMe
)
6
2
[ArMe = C6H3-2,6-(C6H2-2,4,6-(CH3)3)2] with tert-butyl
6
isocyanide gave the Lewis adduct species
t
(ArMe )2GeCNBu , in which the isocyanide ligand displays
6
a decreased C−N stretching frequency consistent with an
n → π* back-bonding interaction. Density functional
theory confirmed that the HOMO is a Ge−C bonding
combination between the lone pair of electrons on the
germanium atom and the C−N π* orbital of the
isocyanide ligand. The complex undergoes facile C−H
bond activation to produce a new diarylgermanium
hydride/cyanide species and isobutene via heterolytic
cleavage of the N−But bond.
the addition of tert-butyl isocyanide to the diarylgermylene
Me6
Ge(ArMe )2 [Ar = C6H3-2,6-(C6H2-2,4,6-(CH3)3)2] leads to
6
the formation of the Lewis adduct (ArMe )2GeCNBu (1).
t
6
Further analysis of the metal−isocyanide bonding mode in 1 by
density functional theory (DFT) indicated that 1 displays back-
bonding from the lone pair on germanium to the π* orbital of
the isocyanide ligand.9 Adduct 1 also undergoes C−H
activation upon mild heating of its hexane solutions to produce
the Ge(IV) hydride/cyanide complex (ArMe )2Ge(H)CN (2)
6
and isobutene in essentially quantitative yield (Scheme 1).
ynergism is a central concept for the understanding of
S
bonding in organometallic compounds, most notably in
transition-metal complexes of σ-donor/π-acceptor ligands.1
Carbonyl (CO) and isocyanide (CNR) molecules are among
the most widely studied ligands of this type, and the latter
generally display stronger σ-donor and weaker π-acceptor
character than carbonyls.1,2 In addition, whereas the C−O
stretching frequency is always lowered upon complexation to a
transition metal, the C−N frequency of an isocyanide can shift
to either higher or lower frequency. The shifts to higher
frequency generally occur in transition-metal complexes
wherein the π-donating tendency toward the isocyanide is
weak and the M−C bond is dominated by σ effects.2,3
Scheme 1. C−H Bond Activation of tert-Butyl Isocyanide by
Diarylgermylene
Complex 1 was initially obtained by the addition of 1.1 equiv
10
2
of tert-butyl isocyanide to a stirred solution of Ge(ArMe
)
in
6
toluene. Concentration of the solution and overnight storage at
Tetrylene−isocyanide complexes (isocyanide complexes of
divalent diorgano derivatives of the group 14 (tetrel) elements)
may be divided into two categories that are defined by two
bonding extremes. The first involves multiple bonding between
the isocyanide carbon and the main-group element, in which
the isocyanide is subsumed into a heterocumulene structure.4
The other extreme involves a simple donor−acceptor
interaction between the carbon and the main-group atom
(i.e., a single σ bond). To date, the limited number of
spectroscopically characterized stannylene− and plumbylene−
isocyanide complexes are Lewis adducts and display higher
ν(CN) stretching frequencies than the free ligand, indicating
primarily a σ-donor bond with little or no π back-bonding to
the isocyanide. Silicon−isocyanide complexes tend to form
heterocumulenes, however, when very bulky isocyanides are
used, Lewis adducts that exhibit ν(CN) stretching frequencies
ca. −20 °C afforded a crystalline mixture of 1 and unreacted
Ge(ArMe )2. UV−vis spectroscopy showed that 1 was in
6
t
equilibrium with freely dissociated Ge(ArMe )2 and CNBu in
6
solution. However, 1 was isolated pure by reaction of a 5-fold
excess of tert-butyl isocyanide and Ge(ArMe )2 in pentane at
6
room temperature. The reaction was rapid, and its completion
was indicated by the fading of the characteristic deep-purple
color of the germylene to yellow. The removal of all volatile
fractions afforded 1 as a yellow powder in quantitative yield.
Crystals of 1 suitable for single-crystal X-ray diffraction (XRD)
were grown from a concentrated hexane solution of 1 in the
presence of excess tert-butyl isocyanide by slow cooling to ca.
Received: December 20, 2011
Published: February 13, 2012
© 2012 American Chemical Society
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Communication
−20 °C. X-ray crystallographic analysis of 1 showed that the
germanium is three-coordinate and is bound to two aryls and
an isocyanide ligand (Figure 1). The complexation of the
Figure 2. Highest occupied Kohn−Sham orbital of 1 (contour value
0.04). H atoms have been omitted for clarity.
bonding interaction. Consequently, the bonding was examined
in more detail with energy decomposition analysis (EDA) and
by investigation of the corresponding natural orbitals of
chemical valence (NOCVs) (see the SI). The results showed
that the instantaneous interaction energy between the tert-butyl
isocyanide and diarylgermylene fragments is −78 kJ mol−1. This
consists of Pauli repulsion (716 kJ mol−1), quasi-classical
electrostatic interaction (−437 kJ mol−1), and orbital
interactions (−357 kJ mol−1). Because of the lack of symmetry
in adduct 1, the orbital interaction term cannot easily be
expressed as a sum of σ and π contributions. However, an
estimate of the relative importance of the bonding and back-
bonding components to the total orbital interaction was
obtained using the constrained space orbital variation
procedure. When the unoccupied orbitals of the diary-
lgermylene fragment were removed from the EDA calculation,
only back-bonding interactions were possible, and the total
orbital interaction term decreased to 148 kJ mol−1. Similarly,
removing the unoccupied orbitals from tert-butyl isocyanide
rendered the back-bonding interaction impossible, and the
calculated orbital interaction energy decreased to 244 kJ mol−1.
These results indicate that the back-bonding interaction
constitutes ca. 40% of the total orbital interaction. Qualitatively
similar results were obtained by analyzing the NOCVs
calculated for 1: the two most important orbitals corresponded
to the relevant σ and π interactions, with energy contributions
of 225 and 84 kJ mol−1 (24%), respectively. Consequently,
these data indicate that the back-bonding interaction represents
roughly one-third of the total orbital interaction.
Figure 1. Thermal ellipsoid plot (30%) of 1. H atoms and solvent
molecules are not shown. Selected bond distances (Å) and angles
(deg) [calculated values]: Ge(1)−C(Ar) (ave) = 2.052(3) [2.051],
Ge(1)−C(49) = 2.075(3) [2.041], C(49)−N(49) = 1.159(4) [1.164],
N(49)−C(50) = 1.455(9) [1.438], C(Ar)−Ge(1)−C(49) (ave) =
93.8(1) [95.1], Ge(1)−C(49)−N(49) = 156.9(2) [155.7], C(49)−
N(49)−C(50) = 175.1(6) [174.4], C(1)−Ge(1)−C(25) = 118.7(2)
[123.0].
isocyanide ligand is almost perpendicular to the Ge−C(ipso)2
plane, with an angle of ca. 103° between the Ge(1)−C(49)
bond and the Ge(1)−[C(ipso)]2 plane. In the isocyanide
moiety, the Ge(1)−C(49) and C(49)−N(49) bond distances
are 2.075(3) and 1.159(4) Å, and the C(49)−N(49)−C(50)
moiety is almost linear, with a bond angle of 175.0(2)°.
1H NMR spectroscopy of 1 at room temperature in the
presence of an excess of tert-butyl isocyanide showed only one
But signal, indicating rapid exchange of complexed and
uncomplexed isocyanide ligands, but two signals were observed
at temperatures below −25 °C. Dissolving pure 1 in hexane at
room temperature afforded a green solution that changed to
yellow upon cooling and deep-purple upon warming. Van’t
Hoff analysis of the UV−vis spectra of 1 recorded at different
temperatures indicated that ΔHdiss = 53(5) kJ mol−1 for 1. The
IR spectrum of 1 displayed an absorption at 2132 cm−1, which
is slightly lower than the stretching frequency of the free
isocyanide (2134 cm−1). The dissociation enthalpy determined
for 1, coupled with the shift to lower frequency of the C−N
stretching band, suggested the existence of back-bonding
between the lone pair on germanium and the π* orbital of
the isocyanide.
The formation and bonding of 1 was investigated computa-
tionally using DFT [see the Supporting Information (SI)]. The
optimized structure of the adduct was in good agreement with
the X-ray data (Figure 1), and the calculated dissociation
energy was 47 kJ mol−1. Similarly, the calculated ν(CN)
stretching band of 1 (2234 cm−1) was shifted to lower
frequency relative to the stretching frequency of the free
isocyanide (2247 cm−1). The HOMO of 1 is a Ge−C bonding
combination between the lone pair of electrons on the
germanium atom and the C−N π* orbital of the isocyanide
ligand (Figure 2), and is indicative of the presence of a back-
The bonding in 1 may be compared to that in the few known
congeners containing heavier group-14 elements, including
[(CH2)C(SiMe3)]2SiCNR (R = C6H3-2,6-Pri2, 1-adamantyl),11
ArF SnCNMes (ArF = C6H2-2,4,6-(CF3)3; Mes = mesityl),12
2
and [Si(SiMe3)3]2ECNBut (E = Sn, Pb).13 The silicon species
have bonding similar to that in the carbon-based cumulenes, as
illustrated by A in Scheme 2.14 Unlike the silicon species, the
tin and lead complexes form Lewis adducts with the isocyanide
ligands (C). The tin−isocyanide adduct ArF SnCNMes exhibits
2
weak binding [ΔHdiss = 29.6(4) kJ mol−1 vs 53(5) kJ mol−1 in
1],12 and the C−N frequency is 48 cm−1 higher than in the free
isocyanide, indicating that the Sn−C bonding is primarily of the
σ-donor type.15 Although the C−N stretching frequency in 1 is
only slightly less than that in the free ligand, it stands in sharp
contrast to other σ-bonded main-group isocyanide adducts,
where the C−N stretching frequencies increase proportionally
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Journal of the American Chemical Society
Communication
formation of isobutene with no other side products as well as
bleaching of the yellow color of the solution.
Scheme 2. Tetrylene−Isocyanide Bonding Modes
Until now, mechanistic insight into the dealkylation and
insertion products of metal−isocyanide complexes has been
limited. It was reported that the activation of tert-butyl
isocyanide by the iron(0) phosphine complex Fe-
(PMe3)2(CNBut)3 gave Fe(PMe3)2(CNBut)2(H)(CN) via
homolytic cleavage of the N−But bond, with the subsequent
But radical acting as a hydrogen donor to yield isobutene.17
Silylenes have been reported to insert into the N−R bond of
cyclohexyl isocyanide, and the rearrangement was attributed to
an alkyl shift from the isocyanide ligand to silicon upon
formation of the adduct.18 Tokitoh and co-workers also
to the strength of the E−C bond, typically by several tens of
wavenumbers.15 Thus, the comparison of the experimental data
for 1 and its group-14-element analogues indicates that the
back-donation of electrons (and the degree of multiple-bond
formation) from the group-14 element to the isocyanide ligand
decreases very rapidly in the heavier atoms.
The hydride/cyanide complex 2 was first isolated and
structurally characterized as an unexpected minor product
(>0.5%) from a synthesis of 1. A more rational synthesis of 2
was performed by heating a stirred solution of 1 in hexane to
ca. 75 °C under static vacuum with a 5-fold excess of tert-butyl
isocyanide in order to support the formation of 1 instead of the
dissociated starting materials. The color of the solution changed
from yellow to colorless overnight. The volatile materials were
removed, and the white residue was washed with a small
amount of pentane and dried under reduced pressure to afford
2 in 91% yield. Colorless single crystals of 2 suitable for XRD
studies were grown by slow cooling of a toluene/pentane
solution (1:2 v/v) to ca. −20 °C overnight, and the structure of
2 (Figure 3) was found to be similar to those of other published
1
reported H NMR characterization of C−H bond activation
resulting from the reaction of a transient silylene with tert-butyl
isocyanide. They proposed a mechanism involving a concerted
proton migration from the tert-butyl group to the silaketimine
accompanied by isobutene elimination.5
The formation of 2 and isobutene from 1 was examined with
DFT calculations to elucidate a possible transition state for the
reaction. Attempts to remove a proton from the tert-butyl
group led to a steep rise in the total energy, inconsistent with
the facile nature of the reaction. Consequently, we turned our
attention to a concerted mechanism related to the reaction
pathway proposed by Tokitoh. The calculations led to the
characterization of a transition state corresponding to the
formation of a formally anionic germanium and a But cation
(i.e., heterolytic bond cleavage), as shown in Scheme 3.
Scheme 3. Calculated Energies for the Formation of 1 and 2
Following the internal reaction coordinate along the reaction
path led to complete dissociation of the But cation coupled with
a simultaneous C−H proton transfer to germanium. The
conversion of 1 to 2 was found to be exothermic by 39 kJ
mol−1, and the calculated activation energy was 108 kJ mol−1.
Despite an extended search, no other transition states could be
located on the potential energy surface, nor did the calculations
give any support for a pathway involving homolytic N−But
bond cleavage, as there was no indication of an internal
instability in the Kohn−Sham determinant.19
The decomposition was studied experimentally by heating
hexane solutions of 1, prepared in the presence of excess tert-
butyl isocyanide, to temperatures of 40−70 °C. The
disappearance of the UV−vis absorptions at 400 and 579 nm
were monitored in order to measure the rate of formation of 2.
An Eyring plot afforded Eact = 74 kJ mol−1 for the formation of
2, which is ca. 30 kJ mol−1 lower than other reported examples
of C−H bond activation of tert-butyl isocyanide by transition-
metal complexes.17 These data confirm the presence of a highly
activated germanium center coupled with the high stability of
the reaction products. A radical mechanism involving homolytic
cleavage of the N−But bond of 1 was deemed unlikely because
heating adduct 1 (with a 5-fold excess of isocyanide) in the
presence of excess 1,4-cyclohexadiene yielded no trace of
Figure 3. Thermal ellipsoid plot (30%) of 2. C-bound H atoms are not
shown. Selected bond distances (Å) and angles (deg) [calculated
values]: Ge(1)−C(Ar) (ave) = 1.972(2) [1.998], Ge(1)−C(49) =
1.952(2) [1.953], Ge(1)−H(1) = 1.48(3) [1.527], C(49)−N(49) =
1.145(3) [1.153], C(Ar)−Ge(1)−C(49) (ave) = 106.01(8) [104.3],
Ge(1)−C(49)−N(49) = 170.1(2) [172.9], C(1)−Ge(1)−C(25) =
124.9(1) [136.0].
germanium(IV) cyanide complexes.16 A broad absorption at
2117 cm−1 in the FT-IR spectrum was predicted to be due to
the terminal Ge−H stretching mode (calculated value 2218
cm−1), and the weak absorption at 2177 cm−1 was attributed to
the terminal C−N stretching mode (calculated value 2326
1
cm−1). Monitoring the H NMR spectrum of a sample of 1
1
during heating with excess isocyanide showed the clean
isobutane by H NMR spectroscopy.
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(15) Trialkylaluminum−isocyanide adducts, which possess an
essentially pure σ-donor interaction, are reported to have ν(CN)
shifts ca. 85 cm−1 higher in frequency. See: Fisher, J. D.; Wei, M. Y.;
Willett, R.; Shapiro, P. J. Organometallics 1994, 13, 3324.
In conclusion, structural and spectroscopic characterization
of the first germylene−isocyanide complex indicates the
formation of an adduct with n → π* back-bonding from Ge
to the isocyanide ligand. This species readily undergoes C−H
bond activation to form the Ge(IV) hydride/cyanide product 2.
Although main-group isocyanide species that display lowered
C−N stretching frequencies have been reported,20 the
possibility of back-bonding had not yet been explored. Further
theoretical explorations of these interactions are underway and
will be reported in a future publication.21
(16) Hihara, G.; Hynes, R. C.; Lebuis, A. M.; Riviere-Baudet, M.;
Wharf, I.; Onyszchuk, M. J. Organomet. Chem. 2000, 598, 276.
(17) Tennent, C. L.; Jones, W. D. Can. J. Chem. 2005, 83, 626.
(18) Xiong, Y.; Yao, S.; Driess, M. Chem.Eur. J. 2009, 15, 8542.
(19) As no other mechanism was apparent from the calculations, the
difference between the experimental and calculated values of Eact can
be attributed at least in part to the neglect of multiple polarization and
diffuse functions in the basis set, leading to an inferior description of
the transition state in comparison with the reactant and products. We
note that the choice of the basis set was determined by the size of the
systems in question, and its enlargement would have made the
calculations prohibitively time-consuming.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization of 1 and 2, tables of crystallo-
graphic data, details of kinetic measurements, details of
computational analyses, and CIFs for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
i
iPr4
(20) The digermyne−isocyanide adducts [ArPr GeGeAr ](CNR)n
4
[R = But, n = 1; R = Mes, n = 2; AriPr = C6H3-2,6-(C6H3-2,6-Pri2)2]
4
[see: (a) Cui, C. M.; Olmstead, M. M.; Fettinger, J. C.; Spikes, G. H.;
Power, P. P. J. Am. Chem. Soc. 2005, 127, 17530. (b) Spikes, G. H.;
Power, P. P. Chem. Commun. 2007, 85. ] have been reported to have
lower ν(CN) stretching frequencies than their tin analogues [see:
(c) Peng, Y.; Wang, X.; Fettinger, J. C.; Power, P. P. Chem. Commun.
2010, 46, 943 ]. Explorations of the back-bonding in these complexes
are underway and will be reported in a full accout of this work.
(21) An experimental and computational investigation of a full series
of tetrylene−isocyanide complexes is currently underway in order to
examine the variations in bonding with increased atomic number (cf.
ref 9). We have also prepared other germylene−isonitrile adducts,
AUTHOR INFORMATION
Corresponding Author
pppower@ucdavis.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the U.S. Department of Energy, Office of
Basic Energy Sciences (DE-FG02-07ER46475) and the
Academy of Finland for support of this work. Z.D.B. thanks
A. F. Panasci for assistance with kinetics experiments.
(ArMe )2GeCNR (R = Me, C6H11, Mes), which exhibit ν(CN) shifts
6
similar to that in 1.
REFERENCES
■
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(3) The shift to a higher C−N stretching frequency for σ-bonded
isonitrile complexes is thought to be due to an increase in the effective
positive charge on the carbon atom upon donation of its lone pair.
This leads to an increase in the polarity of the C−N bond and hence
to an increase in its strength and stretching frequency, see: Nakamoto,
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(4) For a review of group-14 heterocumulenes, see: Escudie, J.;
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389.
(9) Initial attempts to isolate a tin or lead analogue of 1 by the
addition of tert-butyl isonitrile to E(ArMe
) (E = Sn, Pb) were
2
6
unsuccessful. There was no apparent reaction between the stannylene
or plumbylene and tert-butyl isonitrile.
(10) Simons, R. S.; Pu, L. H.; Olmstead, M. M.; Power, P. P.
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(11) Abe, T.; Iwamoto, T.; Kabuto, C.; Kira, M. J. Am. Chem. Soc.
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̈
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(14) Furthermore, the nitrogen atoms in these complexes are
distinctly sp2-hybridized and have C−N−R bond angles of 146.3 and
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1, which has a C−N−R bond angle of 175.1(6)°.
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