Intramolecular C-H and C-O bond activation of dimethylplatinum(II) complexes having the PN ligand

Article abstract of DOI:10.1246/cl.2003.66

Reaction of (cod)PtMe₂ with 1 equiv and 2 equiv of the PN ligand (PN = o-Ph₂PC₆H₄CH₂OCH₂C ₅H₄N-2) (1) gave cis-(PNκP:κN)PtMe₂ (2) and cis-(PN-κP)₂

Full text of DOI:10.1246/cl.2003.66

66
Chemistry Letters Vol.32, No.1 (2003)
Intramolecular C–H and C–O Bond Activation of Dimethylplatinum(II)
Complexes Having the PNLigand
Yasutaka Kataoka,ꢀ Tatsuya Nakamura, and Kazuhide Taniꢀy
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
yHigashiosaka Junior College, Higashiosaka, Osaka 577-8567
(Received September 17, 2002;CL-020793)
Reaction of (cod)PtMe2 with 1 equiv and 2 equiv of the PN
ligand (PN = o-Ph2PC6H4CH2OCH2C5H4N-2) (1) gave cis-(PN-
ꢀP:ꢀN)PtMe2 (2) and cis-(PN-ꢀP)2PtMe2 (3) respectively. The
complex 2 underwent activation of the benzylic C–H bond in
toluene at 110 ꢁC to afford (PCN-ꢀP:ꢀC:ꢀN)PtMe (4). Toluene
solution of 3 was refluxed for 42 h in the presence of 2 equiv of the
PN ligand to afford the C–O bond cleavage product cis-(PN-
ꢀP)(Ph2PC6H4CH2-ꢀP:ꢀC)PtMe (cis-5) along with cis-(PN-
ꢀP)(PCN-ꢀP:ꢀC)PtMe (cis-6).
Scheme 1.
Transition metal coordination chemistry of hemilabile
ligands has been an extremely active research area due to their
unusual switching functions for supplying an open coordination
site at the metal center or closing an unnecessary site under an
appropriate condition.1 Different coordination ability between
the coordination atoms of the hemilabile ligands is also important
in a stereo-discriminating step in asymmetric reactions;espe-
cially some PN chelate ligands are efficiently used as chiral
ligands in palladium catalyzed reactions.2 We have paid much
attention to the hemilabile ligands and prepared a unique hetero-
chelate PN hybrid ligand (1), which is able to act as a P–N
bidentate, a P–O–N tridentate or a P–C–N tridentate ligand.3
Through our iridium chemistry of 1, the flexibility inherent in the
PN ligand is found to play an important role for stabilizing
reactive species: we have succeeded in isolations of both Ir(I) and
Ir(III) complexes before and after C–H bond activation by
employing the PN ligand.3a
center with the cis-configuration. The 31P NMR spectrum in
CDCl3 showed a singlet at ꢁ 22.7 with 195Pt satellites
(1JPt-P ¼ 1949 Hz), which was typical of a phosphorus atom cis
to a nitrogen atom in platinum complexes.7 The 1H NMR signal
for the proton at the 6-position of the pyridine ring in the PN
ligand showed a characteristic shape for that of the cis-
coordinated PN ligand.3b Reaction of (cod)PtMe2 with 2 equiv
of 1 in CH2Cl2 at room temperature for 10 days gave another
dimethyl complex, cis-(PN-ꢀP)2PtMe2 (3), as white powders in
83% isolated yield (Scheme 1).8 The signal shape of the 1H NMR
for the proton at the 6-position of the pyridine ring in the PN
ligand was similar to that of the free PN ligand.3b All spectral data
indicated that the complex 3 had the cis-configuration, in which
the two PN ligands act as a P-coordinated monodentate ligand.
The H and 31P NMR analyses showed that the complex 3 was
1
stable in toluene at room temperature but above 40 ꢁC dissociated
one of the PN ligands easily to give an equilibrium mixture with 2.
Intramolecular C–H bond activation at the benzylic position
of the PN ligand occurred by heating a toluene solution of 2 at
110 ꢁC for 15 h to produce the monomethyl Pt(II) complex, (PCN-
ꢀP:ꢀC:ꢀN)PtMe (4), as whitish yellow powders in 79% isolated
yield (equation 1).9 The 31P NMR spectrum of 4 showed a singlet
at ꢁ 29.9 with 195Pt satellites (1JPt-P ¼ 4400 Hz), which was a
typical value for the presence of a weak trans donor ligand. The
1H NMR signal for the proton at the 6-position of the pyridine ring
displayed a characteristic shape for that of the trans-coordinated
PN ligand.3b The 1H NMR signal of the benzylic proton of the PN
ligand at ꢁ 5.62 has Pt satellites, indicating that the benzylic
carbon is attached to the platinum metal directly. Only one methyl
signal was observed at ꢁ 0.17 with 195Pt satellites. Generation of
Chart 1.
Redox systems between Pt(II) and Pt(IV) have been of
interest in recent years, e.g., activation of hydrocarbon at a
cationic Pt(II) species and reductive elimination from a Pt(IV)
species.4 Compared to remarkable results on the C–H bond
activation, there are only few examples on Pt-mediated C–O bond
activation.5 Herein we report the preparation of several Pt(II)
complexes having our PN ligand and show not only intramole-
cular C–H bond activation but also novel C–O bond activation of
the Pt(II) complexes.
Reaction of (cod)PtMe2 with a little excess of PN ligand 1
(1.1 equiv) in CH2Cl2 at room temperature for 2 h gave the
dimethyl complex, cis-(PN-ꢀP:ꢀN)PtMe2 (2), as white powders
in 75% isolated yield (Scheme 1). The structure of 2 was fully
characterized by elemental analysis as well as spectral data.6 The
PN ligand acts as a P–N bidentate ligand and coordinates to the Pt
1
CH4 was confirmed by the H NMR (ꢁ 0.18). All experimental
data were consistent with the conclusion that the complex 4 had a
square planar geometry and the PN ligand acted as a P–C–N
1
tridentate ligand. From the kinetic studies using H NMR, the
activation parameters for the C–H bond activation were deter-
mined;at 110 ꢁC ÁGz ¼ 126 kJ/mol, ÁHz ¼ 111 kJ/mol,
ÁSz ¼ À39 J/Kmol. The negative activation entropy indicates
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.1 (2003)
67
that the approach of the benzylic C–H bond to the Pt center is the
rate-determining step, which is similar to that obtained in the
intramolecular C–H bond activation of [Ir(cod)(PN-
ꢀP:ꢀN)][PF6].3a
ꢀP)3PtMe2, but at present we could not observe the evidence in
the NMR analyses.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
References and Notes
1
2
3
4
For recent reviews: a) C. S. Slone, D. A. Weinberger, and C. A.
Mirkin, Prog. Inorg. Chem., 48, 233 (1999). b) P. Braunstein and F.
Naud, Angew. Chem., Int. Ed., 40, 680 (2001).
‘‘Comprehensive Asymmetric Catalysis,’’ ed. by E. N. Jacobsen, A.
Pfaltz, and H. Yamamoto, Springer-Verlag, Berlin Heidelberg New
York (1999).
a) Y. Kataoka, Y. Imanishi, T. Yamagata, and K. Tani, Organo-
metallics, 18, 3563 (1999). b) Y. Kataoka, K. Shizuma, T.
Yamagata, and K. Tani, Chem. Lett., 2001, 300.
For recent examples: a) R. A. Periana, D. J. Taube, S. Gamble, H.
Taube, T. Satoh, and H. Fujii, Science, 280, 560 (1998). b) K. L.
Bartlett, K. I. Goldberg, and W. T. Borden, J. Am. Chem. Soc., 122,
1456 (2000). c) H. Heiberg, L. Johansson, O. Gropen, O. B. Ryan,
O. Swang, and M. Tilset, J. Am. Chem. Soc., 122, 10831 (2000). d)
L. Johansson, M. Tilset, J. A. Labinger, and J. E. Bercaw, J. Am.
Chem. Soc., 122, 10846 (2000). e) L. Johansson and M. Tilset, J.
Am. Chem. Soc., 123, 739 (2001). f) L. Johansson, O. B. Ryan, C.
Rꢀmming, and M. Tilset, J. Am. Chem. Soc., 123, 6579 (2001). g) H.
A. Zhong, J. A. Labinger, and J. E. Bercaw, J. Am. Chem. Soc., 124,
1378 (2002).
Unexpected C–O bond activation was observed in the
reaction of 3. A toluene solution of 3 was refluxed for 42 h in
the presence of 2 equiv of the PN ligand to afford the C–O bond
cleavage product, cis-(PN-ꢀP)(Ph2PC6H4CH2-ꢀP:ꢀC)PtMe
(cis-5) in 95% along with cis-(PN-ꢀP)(PCN-ꢀP:ꢀC)PtMe (cis-
6) in 5% (Scheme 2).10 The complex cis-6 was confirmed to be a
C–H bond activation product by the separate experiment, in
which the reaction of 4 with the PN ligand under reflux conditions
afforded cis-6 quantitatively after isomerization of intermediate
complex trans-(PN-ꢀP)(PCN-ꢀP:ꢀC)PtMe (trans-6).11 When a
similar reaction was conducted in the absence of additional PN
ligand, the products derived from the C–H bond activation (4 and
cis-6) were obtained mainly (4 : cis-5 : cis-6 ¼ 35 : 36 : 29)
(Scheme 2). The structure of cis-5 was fully characterized by
elemental analysis as well as spectral data. The 31P NMR
spectrum displayed one set of two doublet signals
(2JP-P ¼ 7:8 Hz) with 195Pt satellites (1JPt-P ¼ 1988 and
1880 Hz), indicating the two phosphorus atoms were coordinated
to the Pt with the cis-configuration. The 13C NMR spectrum
displayed two kinds of carbon directly bound to the Pt metal at ꢁ
4.8 for Pt–CH3 and 38.0 for the benzyl carbon. In the 1H NMR
spectrum, two kinds of benzyl protons with equal intensity (2H)
appear;one is a singlet signal at ꢁ5.24 and another one is a doublet
signal at ꢁ3.90 with 195Pt satellites. These spectra indicate that the
cleavage of the C(benzyl)–O bond in only one of the two PN
ligands in 3 resulted in the formation of the Pt–C bond.12
5
6
a) S. Komiya and T. Shindo, J. Chem. Soc., Chem. Commun., 1984,
1672. b) A. Yamamoto, Adv. Organomet. Chem., 34, 111 (1992). c)
X. Zhang, E. J. Watson, C. A. Dullaghan, S. M. Gorun, and D. A.
Sweigart, Angew. Chem., Int. Ed., 38, 2206 (1999).
2: IR (KBr tablet): 1604 cmÀ1 (ꢃC=N). 1H NMR (CDCl3): ꢁ 8.57–
8.70 (m, 1H), 6.35–8.10 (m, 17H), 5.77 (d, 2JH-H ¼ 11:8 Hz, 1H),
5.46 (d, 2JH-H ¼ 11:5 Hz, 1H), 4.63 (d, 2JH-H ¼ 11:8 Hz, 1H), 4.22
(d, 2JH-H ¼ 11:8 Hz, 1H), 0.88 (d, 2JPt-H ¼ 94:8 Hz,
2
3
3JP-H ¼ 7:4 Hz, 3H), 0.41 (d, JPt-H ¼ 77:2 Hz, JP-H ¼ 7:4 Hz,
3H). 31P{1Hg NMR (CDCl3): ꢁ 22.7 (s, 1JPt-P ¼ 1949 Hz). Found:
C, 53.01;H, 4.42;N, 2.39%;Calcd for C 27H28NOPPt: C, 53.29;H,
4.64;N, 2.30%.
7
8
P. S. Pregosin and R. W. Kunz, ‘‘31P and 13C NMR of Transition
Metal Complexes,’’ Springer-Verlag, Berlin (1979) pp 94–99.
3: IR (KBr tablet): 1590, 1571 cmÀ1 (ꢃC=N). 1H NMR (CDCl3): ꢁ
8.36–8.50 (m, 2H), 6.80–7.95 (m, 34H), 4.85 (s, 4H), 4.41 (s, 4H),
2
3
0.41 (dd, JPt-H ¼ 69:8 Hz, JP-H ¼ 8:5 Hz, 8.2 Hz, 6H). 31P{1Hg
1
NMR (CDCl3): ꢁ 24.2 (s, JPt-P ¼ 1818 Hz). FABMS: m=z 994
(Mþ+2), 977 (Mþ-Me), 962 (Mþ-2Me).
9
4: IR (KBr tablet): 1605 cmÀ1 (ꢃC=N). 1H NMR (CDCl3): ꢁ 8.88–
3
9.06 (m, JPt-H ¼ 35:2 Hz, 1H), 6.85–8.00 (m, 17H), 5.62 (s,
2JPt-H ¼ 73:1 Hz, 1H), 4.93 (d, JH-H ¼ 12:4 Hz, 1H), 4.67 (d,
2
2JH-H ¼ 12:6 Hz, 1H), 0.17 (d, JPt-H ¼ 45:9 Hz, JP-H ¼ 1:9 Hz,
3H). 31P{1Hg NMR (CDCl3): ꢁ 29.9 (s, 1JPt-P ¼ 4400 Hz). Found:
C, 52.66;H, 4.00;N, 2.34%;Calcd for C 26H24NOPPt: C, 52.70;H,
4.08;N, 2.36%.
2
3
Scheme 2.
The C–H bond activation would proceed through a 14-
electron species such as (PN-ꢀP)PtMe2 after dissociation of the
nitrogen donor from 2 or the PN ligand from 3.4a The fact that the
C–H bond activation of 3 in the presence of the PN ligand was
suppressed compared to that in the absence of the ligand strongly
suggests that the formation of the 14-electron species is an
important step for the C–H bond activation. On the contrary, the
intramolecular C–O bond of 3 has been accelerated by addition of
excess ligands;the fact is inconsistent with the presence of the 14-
electron species. Prof. Komiya reported the C–O bond cleavage
via ꢂ-aryloxy elimination from an aryloxyethylplatinum(II)
complex and suggested an associative mechanism involving a
five-coordinated intermediate.5a Our system also has a possibility
that contains a similar five-coordinated intermediate such as (PN-
10 cis-5: IR (KBr tablet): 1590, 1571 cmÀ1 (ꢃC=N). 1H NMR (C6D6): ꢁ
8.36–8.48 (m, 1H), 6.55–8.05 (m, 31H), 5.24 (s, 2H), 4.40 (s, 2H),
2
3
3.90 (d, JPt-H ¼ 78:4 Hz, JP-H ¼ 9:1 Hz, 2H), 1.02 (dd,
3
2JPt-H ¼ 67:3 Hz, JP-H ¼ 8:5 Hz, 8.2 Hz, 3H). 31P{1Hg NMR
1
2
(C6D6): ꢁ 44.6 (d, JPt-P ¼ 1988 Hz JP-P ¼ 7:8 Hz, 1P), 24.1 (d,
1JPt-P ¼ 1880 Hz, 2JP-P ¼ 7:8 Hz, 1P). Found: C, 62.21;H, 4.76;N,
1.61%;Calcd for C 45H41NOP2Pt: C, 62.16;H, 4.70;N, 1.57%.
11 cis-6: 31P{1Hg NMR (C6D6): ꢁ 47.3 (d, JPt-P ¼ 2030 Hz,
1
1
2
2JP-P ¼ 5:6 Hz, 1P), 22.5 (d, JPt-P ¼ 1678 Hz, JP-P ¼ 5:6 Hz,
1P). trans-6: 31P{1Hg NMR (C6D6): ꢁ 47.6 (d, JPt-P ¼ 3343 Hz,
1
1
2
2JP-P ¼ 440 Hz, 1P), 26.8 (d, JPt-P ¼ 3062 Hz, JP-P ¼ 440 Hz,
1P).
12 Generation of 2-pyridylmethanol and CH4 was confirmed by the
GCMS and the 1H NMR respectively.