Synthesis of stable cis-dichloro- and cis-dimethylplatinum(II) complexes bearing bulky primary phosphines and introduction of an alkyl group on the primary phosphine ligand

Article abstract of DOI:10.1246/cl.2008.1192

Stable cis-dichloro- and cis-dimethylplatinum(II) complexes bearing bulky primary phosphines, such as BbtPH₂ (Bbt = 2,6-bis[bis(trimethylsilyl) methyl]-4-[tris(trimethylsilyl)-rnethyljphenyl) and TbtPH₂ (Tbt = 2,4,6-tris[bis(trimethy

Full text of DOI:10.1246/cl.2008.1192

1192
Chemistry Letters Vol.37, No.12 (2008)
Synthesis of Stable cis-Dichloro- and cis-Dimethylplatinum(II) Complexes Bearing Bulky
Primary Phosphines and Introduction of an Alkyl Group on the Primary Phosphine Ligand
Takahiro Sasamori, Masahiro Kawai, Nobuhiro Takeda,y and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received August 28, 2008; CL-080820; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp)
Stable cis-dichloro- and cis-dimethylplatinum(II) com-
plexes bearing bulky primary phosphines, such as BbtPH2
(Bbt = 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)-
methyl]phenyl) and TbtPH2 (Tbt = 2,4,6-tris[bis(trimethylsi-
lyl)methyl]phenyl), were synthesized and isolated as stable com-
pounds. The treatment of cis-[PtMe2(PH2Bbt)2] with lithium
diisopropylamide (LDA) followed by the addition of an alkyl
halide afforded the corresponding alkylated complexes, cis-
[PtMe2(PHRBbt)(PH2Bbt)] (R ¼ Me, Et, Bu, etc.). The spectro-
scopic observation of the intermediacy of the lithiated complex,
cis-[PtMe2(PHLiBbt)(PH2Bbt)], has been accomplished.
plexes 1a and 1b are stable even under reflux conditions in
CHCl3 in contrast to the case of cis-[PtCl2(PH2Mes)2],4a which
undergoes facile dimerization at room temperature, reflecting
the steric protection ability of Tbt and Bbt groups. X-ray crystal-
lographic analysis of 1a7 (Figure 1) showed that the P–Pt
˚
[2.3329(12)–2.3503(12) A] and Pt–Cl [2.2155(11)–2.2194(12)
˚
A] bonds of 1a were somewhat longer and shorter than those
of the known dichloroplatinum(II) complex bearing secondary
phosphines, cis-[PtCl2(PHMes2)2] (3)8 [P–Pt: 2.2315(9)–
˚
˚
2.2481(9) A, Pt–Cl: 2.3385(9)–2.3445(10) A]. Although one
can guess that such structural features should be due to the elec-
tronic properties of a primary phosphine, the longer P–Pt bonds
of 1a, which make the Pt–Cl bonds of 1a shorter for electronic
reasons (such as trans influence),1 should be due to the steric
bulkiness of the Tbt group. Indeed, the theoretical calculations
showed that the optimized structure of dichloroplatinum(II)
complex bearing less hindered primary phosphines, cis-
[PtCl2(PH2Ph)2], is similar to that of cis-[PtCl2(PHPh2)2] bear-
ing secondary phosphines with respect to the Pt–P and Pt–Cl
bond lengths (Table S1).7 That is, the inherent coordinating abil-
ity of a primary phosphine should be similar to that of a secon-
dary phosphine from the viewpoint of electronic properties. The
NMR data (Table S2, ꢁP, ꢁH(ArPH2), ꢁPt, 1JPPt, 1JHP) of 1a and
1b were in good agreement with those of the previously reported
cis-[PtCl2(PH2Mes)2] (4),8 indicating that the perturbation based
Several types of phosphines are frequently used as ligands
toward transition metals and play important roles in catalytic
chemistry.1 While the ligand chemistry of tertiary phosphines
(R3P) has been well established,1 that of secondary phosphines
(R2PH) has attracted much attention from the viewpoints of their
lower steric hindrance as compared with tertiary phosphines and
unique reactivity of the P–H bond, which can potentially under-
go deprotonation followed by introduction of another functional
group or oligomerization leading to novel multinuclear com-
plexes.2 Although primary phosphines (RPH2) should exhibit re-
activities characteristic of the PH2 moiety and work as a unique
ligand as well as secondary phosphines, primary phosphines
have been less studied as a ligand toward transition-metal com-
plexes most probably owing to their inherent instability under
ambient conditions and their extremely high reactivity toward
transition-metal complexes.3 Attempted synthesis of cis-dichlo-
roplatinum complex, cis-[PtCl2(MesPH2)2], by the reaction of
[PtCl2(cod)] with MesPH2 resulted in the formation of a bridged
dimer, [PtCl(PH2Mes)(ꢀ-PHMes)]2, generated by the intermo-
lecular dehydrochlorination reaction of the intermediary cis-
[PtCl2(PH2Mes)2].4 On the other hand, the isolation of MesꢀPH2
(Mesꢀ = 2,4,6-tri-t-butylphenyl) demonstrated that a primary
phosphine bearing a bulky aryl substituent can be easily handled
as a stable compound even in the open air.5 We have already
reported the synthesis and isolation of new, stable primary phos-
phines, BbtPH2 and TbtPH2, which have the effective steric pro-
tection groups, Bbt and Tbt, respectively.6 We describe here
the synthesis of the stable cis-dichloro- and cis-dimethylplati-
num(II) complexes bearing ArPH2 (Ar ¼ Tbt and Bbt) ligands
and the alkylation of the PH moiety of the cis-dimethylplati-
num(II) complexes leading to the formation of novel plati-
num(II) complexes bearing both primary and secondary phos-
phines.
Scheme 1.
(a)
(b)
P
P*
C*
Pt
Pt
P
P
Cl
C
Cl
The reaction of cis-[PtCl2(cod)] with ArPH2 (Ar ¼ Tbt and
Bbt) in CHCl3 at 72 ꢁC gave the corresponding cis-dichloro-
platinum(II) complexes 1a and 1b bearing bulky primary phos-
phines in high yields (1a, Ar ¼ Tbt, 90%; 1b, Ar ¼ Bbt, 81%,
Scheme 1).7 Of particular note is that cis-dichloroplatinum com-
Figure 1. Molecular structures of (a) 1a and (b) 2. Thermal ellipsoids are
drawn at 50% (for 1a) and 30% (for 2) probability levels. Hydrogen atoms
other than those on the P atoms are omitted for clarity. One of the two in-
dependent molecules of 1a in the unit cell was shown.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.12 (2008)
1193
on the steric bulkiness of Tbt and Bbt groups in 1a and 1b should
be negligible in solution.
In summary, a variety of cis-dichloro- and cis-dimethyl-
platinum(II) complexes bearing primary phosphine ligands have
been successfully synthesized as stable compounds. It was
found that cis-[PtMe2(PH2Bbt)2] (2) could be deprotonated by
the addition of an excess amount of LDA to give cis-
[PtMe2(PH2Bbt)(PHLiBbt)] (6), which was trapped by alkyl
halides to afford the corresponding alkylated complexes 5, cis-
[PtMe2(PH2Bbt)(PHRBbt)].
On the other hand, the treatment of cis-[PtMe2(cod)] with
BbtPH2 in benzene at room temperature afforded cis-[PtMe2-
(PH2Bbt)2] (2) as stable colorless crystals in 73% yield. Di-
methylplatinum(II) complex 2 is stable under ambient conditions
in the solid and even in solution, though it underwent decompo-
sition at 60 ꢁC in CHCl3 to afford a complicated mixture. To the
best of our knowledge, no report has appeared on the synthesis
and generation of a dimethylplatinum(II) complex bearing pri-
mary phosphines,9 and hence the spectroscopic properties of 2
should be of great importance. In the NMR spectra, characteristic
signals corresponding to BbtPH2 units were observed at ꢁP
ꢂ73:7, ꢁH 5.10, and ꢁPt ꢂ4504 with coupling constants of
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 17GS0207 and 20036024) and the Global COE
Program (B09) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
References and Notes
1
1JHP ¼ 323 Hz and the satellite of JPPt ¼ 1472, where the ꢁP
y
Present address: Department of Chemistry and Chemical Biology,
Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho,
Kiryu 376-8515.
and ꢁH values were shifted to a lower field region than those of
1
the free ligand (BbtPH2, ꢁP ꢂ145:2, ꢁH 3.62, JPH ¼ 203). X-
1
2
R. H. Crabtree, in The Organometallic Chemistry of the Transition
Metals, 4th ed., John Wiley & Sons, Inc., Hoboken, New Jersey, 2005.
For examples, see: a) W. Radecka-Paryzek, A. J. Mclennan, R. J.
Puddephatt, Inorg. Chem. 1986, 25, 3097. b) R. Glaser, D. J. Kountz,
R. D. Waid, J. C. Gallucci, D. W. Meek, J. Am. Chem. Soc. 1984, 106,
ray crystallographic analysis of 2 showed that 2 possesses a C2
axis passing through the central Pt atom in the crystalline state
˚
and its Pt–P bond length is 2.272(4) A, the value of which is sim-
ilar to those of [PtMe2(PR3)2] (PR3 ¼ PEt3, PMe2Ph, etc.).10
We have examined the modification of the P–H moiety of 2.
The treatment of 2 with an excess amount of LDA in THF at 0 ꢁC
followed by the addition of an alkyl halide (RX) as an electro-
phile afforded the corresponding dimethylplatinum(II) com-
plexes 5a–5e bearing BbtPH2 and BbtPHR in moderate yields
(Scheme 2 and Table 1). The formation of 5a–5e in these reac-
tions should be most likely interpreted in terms of the intermedi-
acy of cis-[PtMe2(PH2Bbt)(PHLiBbt)] (6), the formation of
which was evidenced by the 31P NMR spectral data reasonably
assignable to those of 6 [in THF, ꢁP ꢂ71:0 (1JPPt ¼ 642 Hz,
´
´
´
6324. c) L. R. Falvello, J. Fornies, J. Gomez, E. Lalinde, A. Martın,
M. T. Moreno, J. Sacristan, Chem.—Eur. J. 1999, 5, 474. d) L. R.
´
´
Falvello, J. Fornies, A. Martın, J. Gomez, E. Lalinde, M. T. Moreno,
J. Sacristan, Inorg. Chem. 1999, 38, 3116. e) S. Crementieri, P. Leoni,
´
´
´
F. Marchetti, L. Marchetti, M. Pasquali, Organometallics 2002, 21,
2575. f) M. L. Buil, M. A. Esteruelas, E. On˜ate, N. Ruiz, Organometallics
1998, 17, 3346. g) L.-B. Han, T. D. Tilley, J. Am. Chem. Soc. 2006, 128,
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3
a) J. Ho, Z. Hou, R. J. Drake, D. W. Stephan, Organometallics 1993, 12,
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Soc. 1994, 116, 11159. c) I. V. Kourkine, M. B. Chapman, D. S. Glueck,
K. Eichele, R. E. Wasylishen, G. P. A. Yap, L. M. Liable-Sands, A. L.
Rheingold, Inorg. Chem. 1996, 35, 1478. d) F. A. Cotton, G. Wilkinson,
C. A. Murillo, M. Bochmann, in Advanced Inorganic Chemistry, 6th ed.,
John Wiley & Sons, Canada, 1999.
1
2JPP ¼ 18:5 Hz, JPH ¼ 183 Hz), ꢂ63:4 (1JPPt ¼ 1675 Hz,
1
2JPP ¼ 18:5 Hz, JPH ¼ 311 Hz)].11 Lithiated compound 6 was
4
5
a) I. V. Kourkine, M. B. Chapman, D. S. Glueck, K. Eichele, R. E.
Wasylishen, G. P. A. Yap, L. M. Liable-Sands, A. L. Rheingold, Inorg.
Chem. 1996, 35, 1478. b) trans-[PtCl2(PH2Mes)2] can be isolated as a
stable complex, see ref. 4a.
found to be stable in THF at 0 ꢁC, though the complicated mix-
ture was obtained on warming up of the reaction mixture to room
temperature in the sealed tube. When BuCl was used as an elec-
trophile, not the expected complex, 5d, but a complicated mix-
ture was obtained. However, 5d was obtained by using BuBr
as an electrophile, implying the electron-transfer process in the
reaction of 6 with electrophiles.
a) K. Issleib, H. Schmidt, C. Wirkner, Z. Anorg. Allg. Chem. 1982, 488,
´
75. b) G. Bertrand, C. Couret, J. Escudie, S. Majid, J.-P. Majoral,
Tetrahedron Lett. 1982, 23, 3567. c) A. H. Cowley, J. E. Kilduff, T. H.
Newman, M. Pakulski, J. Am. Chem. Soc. 1982, 104, 5820. d) M.
Yoshifuji, K. Shibayama, N. Inamoto, T. Matsushita, K. Nishimoto, J.
Am. Chem. Soc. 1983, 105, 2495.
6
7
a) N. Nagahora, T. Sasamori, N. Takeda, N. Tokitoh, Chem.—Eur. J.
2004, 10, 6146. b) N. Nagahora, T. Sasamori, Y. Watanabe, Y. Furukawa,
N. Tokitoh, Bull. Chem. Soc. Jpn. 2007, 80, 1884.
The experimental procedures, spectral data, and results of X-ray crystal-
lographic analysis (CCDC 699756 (1a), 699757 (2)) and theoretical cal-
culations are shown in Supporting Information. Supporting Information
is also available electronically on the CSJ-Journal Web site, http://www.
csj.jp/journals/chem-lett/index.html.
Scheme 2.
8
9
E. M. Pelczar, E. A. Nytko, M. A. Zhuravel, J. M. Smith, D. S. Glueck, R.
Sommer, C. D. Incarvito, A. L. Rheingold, Polyhedron 2002, 21, 2409.
Such a dimethylplatinum(II) complex with phosphines bearing P–H bond
may undergo ready decomposition along with the elimination of CH4. For
example, see: D. W. Meek, R. Waid, K. D. Tau, R. M. Kirchner,
C. N. Morimoto, Inorg. Chim. Acta 1982, 64, L221.
Table 1. Lithiation of 2 followed by the addition of electrophiles
Electrophile
MeI
Product
Yield/%
69
ꢁP
ꢁPt
1JPPt/Hz
5a
(R ¼ Me)
5b
ꢂ57:9
ꢂ79:5
ꢂ33:8
ꢂ71:0
ꢂ33:8
ꢂ71:9
—
ꢂ39:4
ꢂ70:8
ꢂ39:5
ꢂ70:9
ꢂ4498
1540
1556
1483
1554
1482
1537
—
1495
1553
1493
1551
10 C. M. Haar, S. P. Nolan, W. J. Marshall, K. G. Moloy, A. Prock, W. P.
Giering, Organometallics 1999, 18, 474.
11 The chemical shift and JPPt value of one of the observed signals at ꢁP
EtBr
PrBr
83
81
ꢂ4520
ꢂ4479
(R ¼ Et)
1
5c
1
ꢂ63:4 are similar to those of 2. The JPPt value (642 Hz) of the signal
(R ¼ Pr)
C.M.
5d
(R ¼ Bu)
at ꢁP ꢂ71:0 of 6 should be characteristic of a phosphido complex, since
it is smaller than that of the other signal owing to the high p character of
the P–Pt bond. For example, see: a) D. K. Wicht, S. N. Paisner, B. M.
Lew, D. S. Glueck, G. P. A. Yap, L. M. Liable-Sands, A. L. Rheingold,
C. M. Haar, S. P. Nolan, Organometallics 1998, 17, 652. b) A. Handler,
BuCl
BuBr
—
78
—
ꢂ4490
OctBr
5e
(R ¼ Oct)
63
ꢂ4515
P. Peringer, E. P. Muller, J. Chem. Soc., Dalton Trans. 1990, 3725.
¨
Published on the web (Advance View) November 1, 2008; doi:10.1246/cl.2008.1192