Synthesis and kaleidoscopic reactivities of bis(tritolylgermyl) bis(dimethylphenylphosphine)platinum(II)

Article abstract of DOI:10.1246/cl.2006.810

A series of tritolylgermylplatinum complexes Pt(GeAr₃) ₂-(PMe₂Ph)₂ (Ar = o-tol, m-tol, and p-tol) were prepared and kaleidoscopic reactivities were observed by only changing the substituents at the germyl tolyl groups: (1) cis-trans isomerization at platinum, (2) C-H bond activation of ortho methyl group, and (3) reductive elimination at Pt giving Ge-Ge bond. Copyright

Full text of DOI:10.1246/cl.2006.810

810
Chemistry Letters Vol.35, No.7 (2006)
Synthesis and Kaleidoscopic Reactivities of
Bis(tritolylgermyl)bis(dimethylphenylphosphine)platinum(II)
Yoko Usui,1 Takashi Fukushima,1 Masato Nanjo,1 Kunio Mochida,ꢀ1 Kuniyoshi Akasaka,2
Takako Kudo,2 and Sanshiro Komiya3
1Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588
2Department of Fundamental Studies, Faculty of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8588
3Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588
(Received May 16, 2006; CL-060573; E-mail: kunio.mochida@gakushuin.ac.jp)
A series of tritolylgermylplatinum complexes Pt(GeAr3)2-
(PMe2Ph)2 (Ar ¼ o-tol, m-tol, and p-tol) were prepared and
kaleidoscopic reactivities were observed by only changing the
substituents at the germyl tolyl groups: (1) cis–trans isomeriza-
tion at platinum, (2) C–H bond activation of ortho methyl group,
and (3) reductive elimination at Pt giving Ge–Ge bond.
Ge2
Pt
Ge1
P1
P1
Ge1'
P1'
Pt
Ge1
P2
Group 14 element compounds have attracted growing
interest not only as possible synthetic tools in organic chemistry
but also for their potential use as new materials. In particular,
considerable effort has been devoted to syntheses of group 14
element compounds binding late transition metals, as these
complexes are regarded as intermediates in a number of transi-
tion-metal-catalyzed transformations of group 14 element com-
pounds. Disilyl– and distannyl–platinum complexes have been
intimately studied because it was considered to be an important
key intermediate in Pt-catalyzed disilylation or stannylation of
unsaturated compounds.1 Transition-metal-catalyzed germyla-
tion in organic synthesis have been also reported.2 Furthermore,
germanium-containing polymers were synthesized by late transi-
tion-metal catalysis and their applications for material technolo-
gy such as conductivity, thermochromism, photoconductivity,
and nonlinear optical effect have been investigated.3 Previously,
we have isolated divalent germylplatinum complexes as an inter-
mediate in the digermylation of alkynes catalyzed by zero-valent
Pt complexes.2,4 Herein, we describe the syntheses of a series
of tritolylgermylplatinum complexes Pt(GeAr3)2(PMe2Ph)2
(Ar ¼ o-tol, m-tol, and p-tol) and their kaleidoscopic reactivi-
ties.
1-trans
2-trans
Figure 1. Molecular structures of 1-trans and 2-trans.
Pt1–P2 and Ge2–Pt–P2, whereas 2-trans has a square-planar
structure.
Syntheses and structures of cis- and trans-digermylplatinum
complexes have been reported.2,6 Preferential cis geometry of
Pt(SiR3)2(PR03)2 was attributed to a large trans influence of
organosilyl ligand,7 while Kim et al. reported the isolation of
trans-Pt(SiHPh3)2(PMe3)2.8 In addition, bulky germyl groups,
in comparison with the corresponding silyl groups, presumably
disfavor the sterically congested cis form. Three different types
of selective reactions were observed by only changing the sub-
stituents at the germyl tolyl groups: (1) cis–trans isomerization
at platinum, (2) C–H bond activation of ortho methyl group,
and (3) reductive elimination at Pt giving Ge–Ge bond.
Firstly, photoinduced and thermal isomerization of diger-
mylplatinum complexes were observed. Irradiation of 2-trans
(ꢀmax ¼ 310 nm in CH2Cl2) with a Xenon lamp (hꢁ > 450
nm) in 1,2-dichloroethane-d4, (CD2Cl)2, at room temperature
for 30 min induced the smooth isomerization to give the
corresponding cis isomer, cis-Pt[Ge(m-tol3)]2(PMe2Ph)2 (2-cis)
(2-cis/2-trans = 98/2).
In order to investigate the mechanism for the photoisomeri-
zation of 2-trans without dissociation or association of PMe2Ph,
ab initio molecular orbital calculations at the CASSCF level
were performed for the model complex, Pt(GeH3)2(PH3)2. The
results suggest that the photoisomerizaion is possible to proceed
via a conical intersection (the crossing point of the ground and
excited potential energy surfaces) with the tetrahedral structure.9
2-cis was thermally converted to the trans isomer in
(CD2Cl)2 at 50 ꢁC for 5 h (2-trans/2-cis = 80/20) (eq 2). The
thermal isomerization of 2-cis to 2-trans at various temperature
(40–60 ꢁC) was followed by observing the change in the two
methyl signals of the PMe2Ph ligands by means of 1H NMR
spectroscopy. The isomerization of 2-cis showed the first-order
kinetics. Rate constant in (CD2Cl)2 at 50 ꢁC was 1:32 ꢃ
10ꢂ4 sꢂ1 with apparent activation energy of 99.0 kJ molꢂ1 from
Arrhenius plot. Activation enthalpy and entropy from Eyring
Bis(tritolylgermyl)bis(dimethylphenylphophine)platinum (II),
trans-Pt(GeAr3)2(PMe2Ph)2 (Ar ¼ o-tol (1-trans), m-tol (2-
trans)) were prepared by the treatment of cis-PtCl2(PMe2Ph)2
with two molar amount of LiGeAr3 in THF at 23 ꢁC for 2 h
(eq 1). These complexes were characterized by NMR spectros-
copy and elemental analysis.
GeAr3
PhMe2P
Cl
PhMe2P
Ar3Ge
+
Pt
Pt
2 LiGeAr3
(1)
THF
Ar =
PhMe2P
Cl
PMe2Ph
o-tol 1-trans (20%)
m-tol 2-trans (87%)
p-tol 3 (cis:trans = 3:7)
A mixture of cis- and trans-Pt[Ge(p-tol)3]2(PMe2Ph)2 (3)
was formed from the reaction of cis-PtCl2(PMe2Ph)2 with
LiGe(p-tol)3 in THF-d8 at ꢂ40 ꢁC in situ. Molecular structures
of 1-trans and 2-trans were determined by X-ray structure anal-
ysis as shown in Figure 1.5 1-trans has a distorted square-planar
geometry with a dihedral angle of 34.8ꢁ between the plane Ge1–
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
811
References and Notes
1
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P2
Ge
Pt
C2
P1
C1
C3
2
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Figure 2. Molecular structure of 4.
plot were 96.3 kJ molꢂ1 and ꢂ20:7 J molꢂ1 Kꢂ1, respectively.
Addition of PMe2Ph caused acceleration of the isomerization
of 2-cis. This result suggests that the isomerization of 2-cis pro-
ceeds through an associative pathway involving a five coordinate
intermediate formed upon association of PMe2Ph to 2-cis.
3
T. Hayashi, Y. Uchimaru, N. P. Reddy, M. Tanaka, Chem. Lett.
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hν (> 450 nm)
Ge(m-tol)3
PhMe2P
PhMe2P
PhMe2P
Ge(m-tol)3
Ge(m-tol)3
Pt
Pt
(2)
Ge(m-tol)3
PMe2Ph
∆ (40−60 °C)
2-trans
2-cis
We have also carried out ab initio molecular orbital calcula-
tions to confirm such an associative five-coordinated mechanism
with a Berry pseudorotation and located the transition state for
the model complex Pt(GeH3)2(PH3)2.10 There has been little
study about precise determination of relative thermodynamic
stability between cis and trans isomers for Pt(ER3)2L2 (E ¼
Si, Ge, and Sn; L = PR03), except for ref 8, while the intramo-
lecular twist-rotational motion between two ER3 group was
observed with cis-Pt(ER3)2L2 (E ¼ Si and Sn; L = PR03).1
Secondly, irradiation (hꢁ > 450 nm, room temperature) or
heating (100 ꢁC) of 1-trans caused C–H bond activation of o-tol-
yl group to give a germaplatinacycle complex Pt[Ge(o-tol)2(o-
C6H4CH2)](PMe2Ph)2 (4) and hydrogermane HGe(o-tol)3 quan-
titatively (eq 3). Molecular structure of 4 was determined by
X-ray structure analysis as depicted in Figure 2.11 An ORTEP
drawing of 4 clearly shows formation of the new Pt–CH2 bond
by C–H activation of the methyl group at o-tolyl moiety. This
is probably enhanced by close proximity of the Me group to Pt
or another germyl group leading to C–H bond cleavage.
4
5
Preliminary crystallographic data for 1-trans: C74H96O4-
Ge2P2Pt, Mr ¼ 1451:72, triclinic, P1, a ¼ 13:2860ð15Þ,
˚
b ¼ 14:1620ð14Þ, c ¼ 17:7320ð14Þ A, ꢂ ¼ 96:177ð5Þ, ꢃ ¼
103:711ð6Þ, ꢄ ¼ 111:638ð5Þꢁ, V ¼ 2941:8ð5Þ A , Z ¼ 2,
3
˚
Dcalcd ¼ 1:639 g cmꢂ3
,
Fð000Þ ¼ 1488, ꢅ ¼ 3:496 mmꢂ1
,
R ¼ 0:0584 (I > 2ꢆðIÞ), wR2 ¼ 0:1634 (all data), GOF ¼
1:206. Crystallographic data for 2-trans: C58H64Ge2P2Pt,
ꢀ
Mr ¼ 1163:30,
triclinic,
13:8290ð10Þ, c ¼ 17:4500ð17Þ A, ꢂ ¼ 104:025ð5Þ, ꢃ ¼
P1,
˚
a ¼ 12:5600ð8Þ,
b ¼
102:670ð5Þ, ꢄ ¼ 112:854ð4Þꢁ, V ¼ 2537:7ð3Þ A , Z ¼ 2,
3
˚
Dcalcd ¼ 1:522 g cmꢂ3
,
Fð000Þ ¼ 1168, ꢅ ¼ 4:026 mmꢂ1
,
R ¼ 0:0545 (I > 2ꢆðIÞ), wR2 ¼ 0:1526 (all data), GOF ¼
0:990.
6
7
F. Glockling, K. A. Hooton, J. Chem. Soc. A 1967, 1066; 1968,
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The conical intersection is found to be 218 kJ molꢂ1 less stable
than the trans isomer in the ground state at the CASSCF(6,6)/
LANL2DZ level.
(o-tol)2
Ge
Pt
PhMe2P
1-trans
+
HGe(o-tol)3
(3)
1) hν (> 450 nm)
PhMe2P
rt, 30 min
4
or
2) 100 °C, 1 h
1) 93%, 2) 97%
1) 91%, 2) 92%
Lastly, clean Ge–Ge bond formation at Pt was observed for
tris(p-tol)germylplatinum derivative 3. A mixture of cis and
trans isomers 3 was immediately generated from the reaction
of cis-PtCl2(PMe2Ph)2 with LiGe(p-tol)3 at ꢂ40 ꢁC in situ and
characterized by multinuclear NMR spectra. Interestingly, on
warming to room temperature, reductive elimination proceeded
to form (p-tol)3Ge–Ge(p-tol)3 in quantitative yield (eq 4).
8
9
PhMe2P
Ge(p-tol)3
Pt
(4)
(p-tol)3Ge Ge(p-tol)3
10 The energy barrier from the trans isomer is estimated to be
120 kJ molꢂ1 at the HF/LANL2DZ+6-31G(d) level.
11 Crystallographic data for 4: C49H54GeP2Pt, Mr ¼ 972:54, tri-
THF-d8
-40 °C−rt
(p-tol)3Ge
PMe2Ph
91%
3, cis:trans = 3:7
ꢀ
clinic, P1, a ¼ 10:4030ð2Þ, b ¼ 12:7170ð4Þ, c ¼ 16:ꢁ8210ð6Þ
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas (No. 16033256, Reaction Control
of Dynamic Complexes) from Ministry of Education, Culture,
Sports, Science and Technology, Japan.
˚
A, ꢂ ¼ 80:411ð2Þ, ꢃ ¼ 79:010ð2Þ, ꢄ ¼ 87:075ð2Þ , V ¼
2153:57ð11Þ A , Z ¼ 2, Dcalcd ¼ 1:500 g cmꢂ3
,
Fð000Þ ¼
3
˚
976, ꢅ ¼ 4:049 mmꢂ1
0:1531 (all data), GOF ¼ 1:369.
,
R ¼ 0:0481 (I > 2ꢆðIÞ), wR2 ¼
Published on the web (Advance View) June 17, 2006; doi:10.1246/cl.2006.810