Bond formation and coupling between germyl and bridging germylene ligands in dinuclear palladium(I) complexes

Article abstract of DOI:10.1002/anie.201411041

The dinuclear palladium(I) complexes [L-(Ar₂HGe)Pd(μ-GeAr₂)₂Pd(GeHAr₂)L] (Ar = Ph, p-Tol; L= PMe₃, tBuNC) contain terminal germyl and bridging germylene ligands with the experimentally observed Ge...Ge bond lengths of 2.8263(4) ? (L = PMe₃) and 2.928(1) ? (L = tBuNC), which are close to the longest Ge-Ge bond reported to date [2.714(1) ?]. Significant Ge...Ge interactions between the germylene and germyl ligands (PMe₃ complexes > tBuNC complexes) are supported by DFT calculations, Wiberg bond indices (WBI), and natural bond orbital (NBO) analyses. Exchanging tBuNC for PMe₃ ligands increases the Ge...Ge interaction, and simultaneously activates two Pd-Ge bonds. Adding the chelating diphosphine 1,2-bis(diethylphosphino)-ethane (depe) to the PMe₃ complexes results in the intramolecular coupling of germyl and germylene ligands followed by extrusion of a digermane.

Full text of DOI:10.1002/anie.201411041

Angewandte
Chemie
DOI: 10.1002/anie.201411041
Reaction Mechanisms
Bond Formation and Coupling between Germyl and Bridging
Germylene Ligands in Dinuclear Palladium(I) Complexes**
Makoto Tanabe, Shumpei Omine, Naoko Ishikawa, Kohtaro Osakada,* Yoshihiro Hayashi, and
Susumu Kawauchi
Abstract: The dinuclear palladium(I) complexes [L-
(Ar2HGe)Pd(m-GeAr2)2Pd(GeHAr2)L] (Ar= Ph, p-Tol; L =
PMe3, tBuNC) contain terminal germyl and bridging germy-
lene ligands with the experimentally observed Ge···Ge bond
lengths of 2.8263(4) ꢀ (L = PMe3) and 2.928(1) ꢀ (L =
À
tBuNC), which are close to the longest Ge Ge bond reported
to date [2.714(1) ꢀ]. Significant Ge···Ge interactions between
the germylene and germyl ligands (PMe3 complexes > tBuNC
complexes) are supported by DFT calculations, Wiberg bond
indices (WBI), and natural bond orbital (NBO) analyses.
Exchanging tBuNC for PMe3 ligands increases the Ge···Ge
À
interaction, and simultaneously activates two Pd Ge bonds.
Adding the chelating diphosphine 1,2-bis(diethylphosphino)-
ethane (depe) to the PMe3 complexes results in the intra-
molecular coupling of germyl and germylene ligands followed
by extrusion of a digermane.
F
or various transition-metal-catalyzed reactions, such as the
polymerization of methylene precursors by palladium cata-
lysts,[1] Fischer–Tropsch reactions on the surface of iron-based
catalysts,[2] and the oligomerization of disilanes, trisubstituted
germanes, and dialkyl stannanes promoted by platinum,
nickel, ruthenium, rhodium, and palladium complexes,[3]
bond-formation reactions were proposed between two
carbon-, silicon-, germanium-, or tin-containing ligands coor-
dinated to the metal by either a single or double bond
(Scheme 1a). Although many research groups have docu-
mented and discussed similar reactions for transition-metal
complexes with silicon and germanium ligands,[4] only few
examples including isolated complexes, for example, a 1,2-
silyl migration on an iron complex (Scheme 1b), have been
reported.[5] The conversion of in situ prepared mononuclear
Scheme 1. Representative examples for 1,1-insertion of carbene ana-
À
logues into M E (E=C, Si, Ge, Sn) bonds.
ruthenium, iridium, and tungsten complexes, containing
methylene, methyl, or phenyl ligands, into the corresponding
complexes with ethyl or benzyl ligands, was tentatively
À
assigned to C C bond-formation reactions between methyl-
ene and alkyl ligands.[6] The Fischer–Tropsch reaction was
proposed to proceed either by an insertion of a methylene,
bridging two metal centers, to afford metal–alkyl (M-CH2-
[*] Dr. M. Tanabe, S. Omine, N. Ishikawa, Prof. Dr. K. Osakada
Chemical Resources Laboratory
[8]
R)[7] or metal–alkenyl (M-CH CHR) bonds (Scheme 1c),
=
or by coupling between the methylene and alkylidene ligands
and the alkyl ligand.[9] The former pathway was investigated
by Maitlis and co-workers, who used dinuclear rhodium and
iridium model complexes containing methyl and bridging
methylene ligands.[10] Chemical oxidation of these complexes
with [IrCl6]2À resulted in the formation of propylene by
insertion of the bridging methylene ligand (Scheme 1d).[11]
Bond formation between two different ligands within a bimet-
allic framework remains a highly important, yet still obscure
area in the field of organotransition-metal chemistry. Herein,
we report the structure of two bimetallic palladium(I)
complexes, each containing a germyl and a bridging germy-
Tokyo Institute of Technology
4259-R1-3 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: kosakada@res.titech.ac.jp
Y. Hayashi, Prof. Dr. S. Kawauchi
Department of Organic and Polymeric Materials
Tokyo Institute of Technology, Tokyo 152-8552 (Japan)
[**] This work was financially supported by Grants-in-Aid for Scientific
Research (No. 24350027) and (No. 25410061) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan. The
numerical calculations were carried out on the TSUBAME 2.5
supercomputer at the Tokyo Institute of Technology, Tokyo (Japan),
and on the supercomputer at the Research Center for Computa-
tional Science, Okazaki (Japan).
À
lene ligand, as well as their Ge Ge bond-forming reactions
through an expected reaction mechanism.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411041.
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Dipalladium
GeAr2)2Pd(GeHAr2)(CNtBu)] (1a: Ar= Ph; 1b: Ar= p-
Tol) and [(Me3P)(Ar2HGe)Pd(m-GeAr2)2Pd(GeHAr2)-
complexes
[(tBuNC)(Ar2HGe)Pd(m-
latter. Important metric parameters of the [Pd2Ge4] moiety in
À
1a and 2a are summarized in Figure 1b. The Pd Ge1 bond
lengths in 1a [2.3600(9) ꢀ] and 2a [2.3975(4) ꢀ] are com-
=
(PMe3)] (2a: Ar= Ph; 2b: Ar= p-Tol) were prepared from
cis-[PdMe2(PMe3)2], H2GeAr2, and tBuNC. Single crystals,
suitable for X-ray diffraction analysis,[12] were obtained from
parable to the Pd Ge bond lengths in mononuclear palladium
=
complexes with germylene ligands, such as [(R3P)2Pd Ge{N-
(SiMe3)2}2] [R = Et: 2.330(5) ꢀ; R = Ph: 2.3281(4) ꢀ],[17] and
À
shorter than in other complexes with Pd Ge bonds
[2.4259(2)–2.5024(4) ꢀ].[4c] The complex 2a contains a shorter
À
À
Pd* Ge1 bond [2.4445(4) ꢀ], and longer Pd Ge1
À
[2.3975(4) ꢀ] and Pd Ge2 [2.5002(5) ꢀ] bonds compared to
those of 1a [2.4634(9) ꢀ, 2.3600(9) ꢀ, and 2.4631(8) ꢀ,
respectively].
To examine the stability of these bonds and the interaction
in the complexes, density functional theory (DFT) calcula-
tions were carried out for 1a and 2a at the wB97X-D/6-
31G(d) and SDD (for Pd) level of theory using Gaussian09.
The Ge···Ge bond lengths in 1a (2.931 ꢀ) and 2a (2.811 ꢀ)
obtained from DFT calculations are in good agreement with
the metric parameters obtained from the X-ray diffraction
experiments [2.928(1) ꢀ and 2.8263(4) ꢀ, respectively]. The
HOMO of both complexes was found to be localized on the
the diffusion of n-hexane into toluene (1a, 1b) or THF (2a)
solutions containing the respective target compound.
The complexes 1a and 2a adopt similar structures with
a planar [Pd2Ge4] core exhibiting a close contact between the
Ge atoms of the germyl and germylene ligands (Figure 1a).
The observed Ge···Ge bond lengths [1a: 2.928(1) ꢀ; 2a:
À
Ge···Ge moieties and the Pd* Ge1 bonds, thus corroborating
À
2.8263(4) ꢀ] are close to the longest Ge Ge s bond reported
the presence of bonding interactions between the Ge atoms.
The Wiberg bond indices (WBI) of 1a (0.397) and 2a (0.487)
suggest Ge···Ge interactions in both complexes, whereby the
interaction in 2a is significantly more pronounced than that in
1a.
Natural bond orbital (NBO) second-order perturbation
energy analysis of 1a and 2a revealed that the dominant
interaction corresponds to a donation of electron density from
the Ge···Ge s orbital to an empty 5s orbital of the Pd center
(Figure 2a).[18] The calculated NBO interaction energies
(193.6 kcalmolÀ1) and the back-donation from the occupied
dx2Ày2 orbital of the Pd center to the Ge···Ge s* orbital of 2a
(22.8 kcalmolÀ1; Figure 2b) are significantly smaller than
those of 1a (315.1 and 35.9 kcalmolÀ1, respectively), while the
Figure 1. a) ORTEP representation of 1a and 2a with thermal ellip-
soids set at 50% probability. Carbon-bound hydrogen atoms are
omitted for clarity. 1a and 2a contain a crystallographic center of
À
symmetry in the middle of the Pd Pd* bond. Atoms with asterisks are
crystallographically equivalent to those having the same number with-
out asterisks. b) Selected bond lengths [ꢀ] and angles [8] for 1a and
2a.
so far [2.714(1) ꢀ],[13,14] and much shorter than the corre-
sponding bond lengths in other dinuclear transition-metal
complexes,
such
as
[{(OC)(Ph3Ge)Pt}(m-GePh2)2{Pt-
(GePh3)(CO)}] [3.1803(5) ꢀ][15] or [{(OC)2(Ph2HGe)HIr}(m-
GePh2)2{IrH(GeHPh2)(CO)2}] [3.179(1) ꢀ].[16] For the com-
plex 2a, containing PMe3 ligands, a shorter Ge···Ge distance
and a smaller Pd*-Pd-Ge2 angle [124.17(1)8] were observed
relative to those in 1a which has tBuNC ligands [130.88(3)8],
thus suggesting significantly higher levels of interaction
between the Ge atoms of the former complex relative to the
Figure 2. Results of the NBO analysis for 2a. a) Electron donation
from the filled Ge···Ge s orbital into the empty 5s orbital on palladium.
b) Electron back-donation from a filled dx2Ày2 orbital on palladium into
the empty s* orbital of the Ge···Ge bond.
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electron occupancy for the Ge···Ge s orbital of 1a (1.552) is
smaller than that of 2a (1.621). Thus, electron localization in
the Ge···Ge s orbital is significantly higher for 2a relative to
1a. All these results suggest that the increase of Ge···Ge
interaction is directly correlated to the activation of both the
À
À
Pd Ge1 and Pd Ge2 bonds. They also suggest that these
changes are closely associated to their bond lengths and
strengths, and that the interaction occurs in a concerted
À
manner. The formation of a three-center-two-electron Ge
Scheme 2. The effect of the auxiliary ligand L (PMe3 or tBuNC) on the
bond lengths in 1a and 2a. Exchanging tBuNC with PMe3 results in
a contraction of the red bonds and an elongation of the blue bonds.
H···Ge bond as an explanation for the observed close contact
of the Ge atoms can be excluded based on the analysis of the
X-ray diffraction experiments [Pd-Ge2-C angles for 1a:
106.1(2)8/104.8(2)8; 2a: 111.56(7)8/108.95(8)8], the chemical
À
shift of the Ge H hydrogen atoms (dH = 5.28–5.37 ppm) in
the H NMR spectra, and the results of the NBO analysis
digermane
into
1,1,2,2,3,3-hexaphenyltrigermane
H-
1
(GePh2)3H, and heating the reaction mixture to 708C
completed the reaction, thus affording H(GePh2)3H and
[Pd(depe)2] as the final products (Scheme 3a). A crossover
experiment using a mixture of 2a and 2b revealed that the
À
between the Ge H bonding orbital and the orbitals of the
bridging germylene ligands.
The bonds between the PdI centers in these complexes
exhibit smaller WBI indices (1a: 0.174, 2a: 0.149) and are
longer [1a: 2.8037(8) ꢀ, 2a: 2.9013(4) ꢀ] compared to other
dinuclear palladium(I) complexes such as [(PCy3)Pd(m-
À
formation of the digermane involved an intramolecular Ge
Ge bond-forming process as outlined below (Scheme 3b;
Tol = p-Tol). The addition of an excess of depe (8 equiv) to an
equimolar mixture of 2a and 2b produced homodigermanes
H(GePh2)2H and H(GeTol2)2H as the major products of the
initial period (t < 30 min), and two additional minor 1H NMR
signals (dH = 5.617 and 5.622 ppm) were assigned to the Ge
H hydrogen atoms of the heterodigermane 1,1-diphenyl-2,2-
ditolyldigermane H(GePh2GeTol2)H (Figure 3).
The product mixture obtained after heating to 708C
contained the four trigermanes [H(GePh2)3 H], [H(GePh2)2-
(GeTol2)H] (dH = 5.719 and 5.727 ppm), [H(GePh2)-
(GeTol2)2H] (dH = 5.744 and 5.750 ppm), and [H(GeTol2)3H]
in a 1.0:1.1:1.3:1.3 ratio. Symmetrically mixed trigermanes
[H(GePh2)(GeTol2)(GePh2)H] and [H(GeTol2)(GePh2)-
GeHPh2)2Pd(PCy3)]
[2.752(1) ꢀ][19]
or
[(Me3P)Pd(m-
SiHPh2)2Pd(PMe3)n] [n = 1, 2.691(1) ꢀ; n = 2, 2.740(1) ꢀ].[20]
Based on X-ray and DFT results, the metal–metal bonds in
the PMe3-containing complexes (2a, b) are significantly more
destabilized compared to those in 1a, b. Thus, it can be
concluded that all the bond distances between the Pd and
Pd(Ge) atoms are influenced by the auxiliary ligands. A color-
coded scheme based on the X-ray crystallography and DFT
analyses (Scheme 2) shows which bonds are elongated and
shortened as a result of exchanging tBuNC for PMe3 ligands.
In comparison with the electron-accepting tBuNC ligand, the
À
À
À
electron-donating PMe3 ligand activates the Pd Ge1, Pd
Ge2, and Pd Pd* bonds, whereas it stabilizes the Pd* Ge1
bond and the Ge1···Ge2 interaction.
Upon addition of the bidentate diphosphine 1,2-bis(die-
thylphosphino)ethane (depe), 2a released 1,1,2,2-tetraphe-
nyldigermane H(GePh2)2H, which was observed in the
1H NMR spectrum of the reaction mixture after 30 minutes.
Further reaction slowly converted a proportion of this
À
À
(GeTol2)H],
which
could
be
formed
from
[H(GePh2GeTol2)H], were obtained in much smaller
amounts. Scheme 4 shows a possible pathway for the for-
mation of the digermanes. Initial coordination of the chelat-
ing diphosphine to a palladium center enhances the bond
formation between the germyl and germylene ligands (A).
The resulting digermyl ligand and a hydride, formed by a-
À
Scheme 3. Ge Ge bond-formation reactions in dipalladium complexes with terminal germyl and bridging germylene ligands. a) Reaction of 2a
and 2b with depe at room temperature. b) Crossover experiment between 2a and 2b and depe.
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cooperative interaction of the metal centers in di- or multi-
nuclear transition-metal complexes, new reactions and reac-
tion pathways should be possible and expected for these
complexes.
Received: November 13, 2014
Published online: January 13, 2015
Keywords: bridging ligands · density functional calculations ·
.
germanium · ligand effects · P ligands
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germylene ligands to form a digermyl ligand occurs initially,
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À
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