Tetrapalladium complex with bridging germylene ligands. Structural change of the planar Pd4Ge3 core

Article abstract of DOI:10.1021/ja208565q

A complex with a planar hexagonal Pd₄Ge₃ core, [Pd{Pd(dmpe)}₃(μ ₃-GePh₂)₃], was synthesized and characterized by X-ray and NMR measurements as well as by DFT calculations. 4-tert-Butylbenzenethiol converted the Pd₄ complex into ahexapalladium complex, [{Pd₃(μ-GePh₂)_2- (μ-H)(μ₃-GePh₂(SC₆H₄ ^tBu-4))}₂(μ-dmpe)], composed of two Pd ₃Ge₃ units bridged by a dmpe ligand. The addition of CuI or AgI to the Pd₄ complex yielded [Pd(μ-MI){Pd-(dmpe)} ₃(μ₃-GePh₂)₃] (M = Cu, Ag), in which Cu or Ag bridges a Pd-Pdbond of the Pd₄Ge₃ core. The CuI adducts in solution undergo a pivot motion of the CuI on the surface of the Pd₄Ge₃ plane on the NMR time scale.

Full text of DOI:10.1021/ja208565q

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Tetrapalladium Complex with Bridging Germylene Ligands. Structural
Change of the Planar Pd4Ge3 Core
Makoto Tanabe,† Naoko Ishikawa,† Mai Chiba,† Tomohito Ide,† Kohtaro Osakada,*,† and Tomoaki Tanase‡
†Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-3 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
‡Department of Chemistry, Faculty of Science, Nara Woman’s University, Kitauoya-higashi-machi, Nara 630-8506, Japan
S Supporting Information
b
equimolar dmpe afforded a tetrapalladium complex with bridging
germylene ligands, [Pd{Pd(dmpe)}3(μ3-GePh2)3] (2):
ABSTRACT: A complex with a planar hexagonal Pd4Ge3
core, [Pd{Pd(dmpe)}3(μ3-GePh2)3], was synthesized and
characterized by X-ray and NMR measurements as well as
by DFT calculations. 4-tert-Butylbenzenethiol converted the
Pd4 complex into a hexapalladium complex, [{Pd3(μ-GePh2)2-
t
(μ-H)(μ3-GePh2(SC6H4 Bu-4))}2(μ-dmpe)], composed
of two Pd3Ge3 units bridged by a dmpe ligand. The addition
of CuI or AgI to the Pd4 complex yielded [Pd(μ-MI){Pd-
(dmpe)}3(μ3-GePh2)3 ] (M = Cu, Ag), in which Cu or Ag
bridges a PdÀPd bond of the Pd4Ge3 core. The CuI adducts
in solution undergo a pivot motion of the CuI on the surface
of the Pd4Ge3 plane on the NMR time scale.
The reaction product contains only 2, PCy3, and [Pd(dmpe)2]n
(n = 1, 2),13 as revealed by 31P{1H} NMR spectroscopy. Heating
a mixture of the isolated 2 and dmpe (1:3) at 80 °C regenerates
[{Pd(dmpe)}2(μ-Ge2Ph4)(μ-GePh2)], accompanied by the for-
mation of [Pd(dmpe)2]n. Although [Pd(SiHPh2)2(dmpe)] re-
acted with [Pd(PCy3)2] to produce 1, the analogous reaction of
[Pd(GeHPh2)2(dmpe)] with [Pd(PCy3)2] yielded a dipalla-
dium complex with bridging germyl ligands [{Pd(PCy3)}2
(μ-GeHPh2)2]14 rather than 2. Thus, the use of the dinuclear
complex [{Pd(dmpe)}2(μ-Ge2Ph4)(μ-GePh2)] as one of the start-
ing materials enabled a smooth and selective formation of the Pd4Ge3
complex 2. The reaction in eq 1 is reversible, and proper choice of the
reaction conditions resulted in the formation of 2 in high yield.
Figure 1 shows the molecular structure of 2 with its Pd4Ge3
core having one central Pd atom (Pdcent), three edge Pd atoms
(Pdedge), and three bridging Ge atoms. These Pd and Ge atoms
form a plane, as shown in Figure 1b. The distances between the
Pdcent and Ge atoms (2.3207(7)À2.3344(7) Å) are close to the
lengths of the PddGe bonds in mononuclear palladium germy-
lenecomplexes,[(R3P)2PddGe{N(SiMe3)2}2] (R=Et,2.330(5) Å;
R = Ph, 2.3281(4) Å).15 The Pd and Ge atoms of 2 form a
distorted hexagonal core with PdedgeÀGe distances (2.5545(7)À
2.6037(7) Å) and PdÀPd distances (2.7568(6)À2.7904(6) Å)
that are longer than the PdcentÀGe distances. The Pd4Ge3 core
of 2 is larger than the Pd4Si3 core of 1 (PdcentÀSi bond of 1 =
2.2521(8)À2.2674(8) Å). The three PdedgeÀGeÀPdedge
bond angles of 2 (136.68(2)À137.78(2)°) are close to the
PdedgeÀSiÀPdedge angles of 1 (138.00(3)À138.73(3)°), and
the hexagonal cores of 1 and 2 have similar shapes.
ultinuclear transition metal complexes containing a planar
M
arrangement of metal centers linked via metalÀmetal bonds
are rare among transition metal clusters.1 Bridging or semibrid-
ging CO or CNR ligands,2 a pair of planar polyaromatic ligands,3
and bulky organic ligands were employed as the auxiliary ligands
of planar multimetallic complexes.4 The π-acceptor character of
the first two ligands and the steric hindrance of the last one
stabilize the respective complexes. Bridging Si ligands5,6 having
electron-releasing character have recently been found to form
complexes having a planar multimetallic core.7À9 We also reported
a tetrapalladium complex, [Pd{Pd(dmpe)}3(μ3-SiPh2)3] (1),
which contains a central Pd4Si3 unit composed of four Pd centers
and three bridging silylene ligands.10
Although these complexes are expected to show unique chemical
properties owing to their planar multimetallic core and reactive
metalÀsilicon bonds, there have been few systematic studies of
their reactions. In this paper, we present the synthesis and
structure of a complex with a similar structure to 1, but with a
heavier group 14 element, Ge, as the coordinating atom.11 The
skeletal change reactions of this complex also form unprece-
dented multinuclear complexes.
Density functional theory (DFT) calculations of complex 2
were conducted with the Gaussian 09 quantum chemistry program
package (MPWB95/6-31G(d,p)).16 The geometry of the com-
plex was optimized using the actual atom positions obtained by
The treatment of a dipalladium complex with bridging digermene
and germylene ligands, [{Pd(dmpe)}2(μ-Ge2Ph4)(μ-GePh2)],12
with twice the molar amount of [Pd(PCy3)2] and then with
Received: September 10, 2011
Published: October 17, 2011
r
2011 American Chemical Society
18598
dx.doi.org/10.1021/ja208565q J. Am. Chem. Soc. 2011, 133, 18598–18601
|

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Figure 1. (a) Thermal ellipsoids (50%) of 2 and (b) its Pd4Ge3 plane.
Two crystallographically independent molecules exist. Selected bond dis-
tances (Å) and angles (deg) of one of the molecules: Pd1ÀPd2 2.7824(6),
Pd1ÀPd3 2.7585(6), Pd1ÀPd4 2.7836(6), Pd1ÀGe1 2.3319(7), Pd1À
Ge2 2.3207(7), Pd1ÀGe3 2.3338(6), Pd2ÀGe2 2.5921(6), Pd2ÀGe3
2.5767(7), Pd3ÀGe1 2.5637(7), Pd3ÀGe3 2.5852(7), Pd4ÀGe1 2.5895(7),
Pd4ÀGe2 2.5788(7), Pd3ÀGe1ÀPd4 137.05(2), Pd2ÀGe2ÀPd4
137.67(2), Pd2ÀGe3ÀPd3 136.71(2).
Figure 2. Thermal ellipsoids (30%) of 3. The molecule has a crystal-
lographic point of symmetry at the midpoint of P3 and P3*. Selected
bond distances (Å) and angles (deg): Pd1ÀPd2 2.799(2), Pd2ÀPd3
2.767(2), Pd3ÀS 2.501(3), Pd1ÀGe1 2.436(2), Pd2ÀGe1 2.386(2),
Pd2ÀGe2 2.411(2), Pd2ÀGe3 2.380(2), Pd3ÀGe2 2.582(2), Pd3À
Ge3 2.407(2), Ge2ÀS 2.466(3), Pd1ÀH1 2.2(1), Pd2ÀH1 2.0(2),
Pd1ÀPd2ÀPd3 168.54(4), Pd2ÀPd3ÀS 110.71(7), Pd1ÀGe1ÀPd2
70.94(5), Pd2ÀGe2ÀPd3 67.20(5), Pd2ÀGe3ÀPd3 70.63(5).
X-ray crystallography. The average PdcentÀPdedge and PdcentÀGe
bond distances of 2.796 and 2.363 Å are close to those deter-
mined from the X-ray data (2.776 and 2.329 Å, respectively).
The Wiberg bondindex of 2 (PdcentÀPdedge = 0.194, PdcentÀGe=
0.541, and PdedgeÀGe = 0.401À0.442) suggests that the
PdcentÀGe bond is stronger than the PdedgeÀGe bond and is
consistent with the X-ray crystallographic results, which also
show PdcentÀGe bonds shorter than the PdedgeÀGe bonds.
The charge population analysis of the Pd4Ge3 core indicates
a higher negative charge of the Pdcent atom (À0.56) than that
of the Pdedge atoms (À0.47). The molecular orbitals obtained
from the above calculations suggest a large contribution of the
PdcentÀGe and PdedgeÀGe bonds to stabilize the planar Pd4Ge3
core (Supporting Information).17
(2.799(2) Å) is longer than that between Pd2 and Pd3 (2.767(2)
Å), in spite of the bridging coordination of the hydride to
Pd1ÀPd2. The thiolato is attached to Ge2 and Pd3, forming
t
the GePh2(SC6H4 Bu-4) ligand. An analogous MÀGeÀS trian-
gular bonding was reported in the thiolatogermyl iron complex.19
The dmpe bridges the two Pd3Ge3 units to form the hexanuclear
molecule, similar to [{Ni(SiH2C6H4SiH2)(dmpe)}2(μ-dmpe)]20
and [{Pd{Pd(dmpe)}3(μ3-SiHPh)3}2(μ-dmpe)].10 The 1H NMR
spectrum of 3shows the bridging hydrido signal at δÀ2.48, with the
coupling of two P nuclei of the chelating dmpe ligand (J= 13, 94 Hz).
The 31P{1H} NMR spectrum of 3 displays three characteristic
signals at δ 23.8, 16.3, and À11.9. The first two signals, with a large
coupling constant (JPÀP = 33 Hz), are assigned to the chelating
dmpe ligand, while the last signal is assigned to the bridging dmpe.
The reaction forming the hexapalladium complex 3 in the
presence of the thiol involves the reorganization of the Pd4Ge3
core. Scheme 1 summarizes the possible reaction pathway.21
Protonation of the central Pd atom of 2 by thiol forms the
cationic intermediate A, which undergoes rearrangement of
the Pd4Ge3 core and elimination of Pd(dmpe) to produce the
tripalladium intermediate B. The partial dissociation of a dmpe
ligand and coordination of the thiolato forms C, which then under-
goesdimerization with loss of a dmpe molecule. The formation of
t
The addition of an equimolar amount of HSC6H4 Bu-4 to a
hexane solution of 2 caused the separation of [{Pd3(μ-GePh2)2-
t
(μ-H)(μ3-GePh2(SC6H4 Bu-4))}2(μ-dmpe)] (3) as a yellow
solid, as shown in eq 2. The 31P{1H} NMR spectrum of the
reaction mixture indicated the accompanying formation of
t
[Pd(dmpe)2]n and [Pd(SC6H4 Bu-4)2(dmpe)].18
t
[Pd(dmpe)2]n and [Pd(SC6H4 Bu-4)2(dmpe)] in the reaction is
attributed to further reaction of the Pd(dmpe) intermediate with
dmpe and thiol, respectively. The isolated hexanuclear complex 3
is thermally stable and does not change its structure in solution,
even at 60 °C. The addition of dmpe to a benzene-d6 solution of 3
([dmpe]/[3] = 0.5/1), however, formed a mixture of the tetra-
nuclear complex 2 and [Pd(dmpe)2]n, which were identified from
the 31P{1H} NMR peaks of the solution.22 Adams et al. recently
reported a reversible structural change of their multinuclear com-
plex. The complex [Pd{Re2(CO)8(μ-SbPh2)(μ-H)}2] releases
its central Pd upon treatment with PCy3, while the resulting
[{Re2(CO)8(μ-SbPh2)(μ-H)}2] stores the Pd of Pd(dba)2 and
regenerates the Re4Pd complex.23
Complex 3 is composed of two Pd3(μ-GePh2)2(μ-H)(μ3-
GePh2(SC6H4 Bu-4)) units and a bridging dmpe ligand, as
revealed by X-ray crystallography (Figure 2). Three Pd atoms
t
of each unit are aligned almost linearly (Pd1ÀPd2ÀPd3 168.54(4)°)
t
and are bridged by two GePh2 ligands and a GePh2(SC6H4 Bu-4)
ligand. A hydride ligand derived from the thiol bridges two
Pd centers, Pd1 and Pd2. The distance between Pd1 and Pd2
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COMMUNICATION
Scheme 1
Figure 3. (a) Thermal ellipsoids (50%) of (a) 4 and (b) its Pd4Ge3
plane. Selected bond distances (Å) and angles (deg): CuÀPd1 2.632(1),
CuÀPd2 2.462(1), Pd1ÀPd2 2.7367(8), Pd1ÀPd3 2.7964(8), Pd1À
Pd4 2.7738(8), Pd1ÀGe1 2.358(1), Pd1ÀGe2 2.384(1), Pd1ÀGe3
2.347(1), Pd2ÀGe2 2.597(1), Pd2ÀGe3 2.887(1), Pd3ÀGe1 2.585(1),
Pd3ÀGe3 2.532(1), Pd4ÀGe1 2.606(1), Pd4ÀGe2 2.530(1), IÀCuÀ
Pd2 159.15(5), Pd3ÀGe1ÀPd4 136.39(4), Pd2ÀGe2ÀPd4 128.24(3),
Pd2ÀGe3ÀPd3 131.34(3).
Equimolar CuI or AgI reacts with complex 2 to cause their
addition to a PdÀPd bond to afford the pentanuclear complexes
[Pd(μ-MI){Pd(dmpe)}3(μ3-GePh2)3] (4, M = Cu; 5, M = Ag):
Figure 4. 1HNMRspectraof4at À90, À60, À30, and 25 °Cintoluene-d8.
core. Although X-ray crystallography showed a clear difference
between PdÀPd bonds with bridging CuI and those without
bridging CuI, three phenyl groups on the same face of the Pd4Ge3
core are equivalent in the NMR spectrum. The spectra at low
temperature show changes in the aromatic hydrogen peaks of the
GePh2 ligands. Two ortho hydrogen signals of the GePh2 ligands
are observed at δ 8.31 and 7.74 with equal intensities at 25 °C
(Figure 4). Meta hydrogen signals corresponding to three
hydrogens are observed at δ 7.33, although the others overlap
with the signals of other aromatic hydrogens. On cooling the
solution to À90 °C, a set of ortho hydrogen signals (δ 8.31) and
the meta hydrogen signal are extremely broadened; these are
assigned to the Ph groups on the Cu-coordinated side of the Pd4Ge3
plane. The peak width of the other ortho hydrogen signal at δ
7.74 changes to a much less significant degree.
Figure 3 shows the X-ray structure of 4. Both 4 and 5 retain the
bonds between Pd and Ge atoms of the Pd4Ge3 core but lose
planarity owing to the bridging of Cu or Ag to a PdÀPd bond.
Two Pd atoms and one Cu or Ag atom form a triangle composed of
three d10 metals. The Pd1ÀPd2 bond (2.7367(8) Å) of 4 is shorter
than the Pd1ÀPd3 (2.7964(8) Å) and Pd1ÀPd4 (2.7738(8) Å)
bonds of 4 and the PdcentÀPdedge bonds of 2 (2.7568(6)À
2.7904(6) Å). The presence of a CuÀPd2 bond (2.462(1) Å)
shorter than the CuÀPd1 bond (2.632(1) Å) and of a large
IÀCuÀPd2 bond angle (159.15(5)°) suggests a more significant
interaction of Pd2 with the Cu atom than that of Pd1. The four
Pd atoms of 4 are included in a plane, while one of the Ge atoms
(Ge2) deviates from the plane by 0.95 Å because of the
elongation of the Pd1ÀGe2 bond (2.384(1) Å) compared with
the other PdcentÀGe bonds of 4 (Pd1ÀGe1 2.358(1), Pd1ÀGe3
2.347(1) Å). Trinuclear Pt(0) complexes, [Pt3(μ-CO)3(PR3)3À4],
were reported to react with Cu(I), Ag(I), and Au(I) to form
heterometallic complexes with ML (M = Cu(I), Ag(I); L = phosphine,
halides) capping over the Pt3 face. The complexes 4 and 5 show
the interaction of Cu and Ag with only the two Pd centers.24
The NMR results of 4 below room temperature show the
dynamic behavior of the molecule on the NMR time scale.
The 13C{1H} NMR spectrum of 4 at room temperature exhibits
two ipso carbon signals at close positions (δ 156.5 and 155.8) due
to the presence of CuI-bonded and CuI-free sides of the Pd4Ge3
Scheme 2 shows a plausible mechanism to account for the
results: the cleavage and formation of a CuÀPdedge bond occur
rapidly and reversibly to cause the apparent pivot motion of CuI
on the surface of the Pd4Ge3 plane. The dissociation of CuI from
4 can be excluded on the basis of the two 1H and 13C{1H} NMR
1
signals of the phenyl groups. The H NMR spectrum of 5
at À90 °C shows the broadening of the two ortho hydrogen
peaks to a similar extent. Thus, CuI and AgI produce the
complexes 4 and 5, respectively, with similar structures in the
solid state, but the behaviors of the two adducts are different in
solution.25
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Scheme 2
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In summary, we succeeded in isolating 2, characterizing its
structure, and carrying out its DFT analysis. The striking findings
in this study are the formation of the tetrapalladium complex 2 from
the mono- and dinuclear Pd complexes and the skeletal change of
the planar Pd4Ge3 core caused by thiol and CuI, which have the
character of Brønsted and Lewis acids, respectively. The combina-
tion of low-valent Pd and the Ge ligand enabled the formation of the
multinuclear complexes with various structures and their smooth
conversion involving the skeletal change.
(12) Tanabe, M.; Ishikawa, N.; Hanzawa, M.; Osakada, K. Organo-
metallics 2008, 27, 5152–5158.
(13) Tanabe, M.; Hanzawa, M.; Osakada, K. Organometallics 2010,
29, 3535–3540.
(14) Tanabe, M.; Ishikawa, N.; Osakada, K. Organometallics 2006,
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(15) Cygan, Z. T.; Bender, J. E., IV; Litz, K. E.; Kampf, J. W.;
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(16) Frisch, M. J.; et al. Gaussian 09, revision B.01; Gaussian, Inc.:
Wallingford, CT, 2009.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and charac-
b
terization data; MO interaction diagram for 2; and complete ref 16. This
material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(17) Theoretical calculation of planar [Ni(SiH2)6] was reported to
form the planar NiSi6 core stabilized by the NiÀSi and SiÀSi bonds. See:
Tang, H.; Hoffman, D. M.; Albright, T. A.; Deng, H.; Hoffmann, R.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1616–1618.
Corresponding Author
kosakada@res.titech.ac.jp
t
’ ACKNOWLEDGMENT
(18) We characterized [Pd(SC6H4 Bu-4)2(dmpe)] by comparison
of its NMR data with those of the complex which was isolated from the
reaction of [Pd(SiHPh2)2(dmpe)] with excess HSC6H4 Bu-4. See
Supporting Information.
(19) Okazaki, M.; Kimura, H.; Komuro, T.; Okada, H.; Tobita, H.
Chem. Lett. 2007, 36, 990–991.
This workwasfinancially supported by Grants-in-Aid for Scientific
Research for Scientific Research (No. 19205008) and for Scientific
Research on Priority Areas (No. 19027018) from the Ministry of
Education, Culture, Sport, Science, and Technology, Japan.
t
(20) Shimada, S.; Rao, M. L. N.; Tanaka, M. Organometallics 1999,
18, 291–293.
(21) Several other pathways also lead to the formation of 3. For
example, the oxidative addition of thiol to a PdÀPd bond of 2 and the
subsequent rearrangement of the complex would result in formation of
3 via the neutral intermediates corresponding to AÀC.
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in solution because no coalescence signals were observed above À90 °C.
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involving a rapid pivot motion over the NMR time scale, monodentate
coordination of AgI to the Pd center, or the smooth dissociation/
recoordination of AgI to the Pd4Ge3 core.
18601
dx.doi.org/10.1021/ja208565q |J. Am. Chem. Soc. 2011, 133, 18598–18601