Benzene C-H activation by platinum(II) complexes of bis(2- diphenylphosphinophenyl)amide

Article abstract of DOI:10.1039/b501520k

The amido diphosphine complexes [PNP]PtMe and [PNP]PtOTf, where [PNP] ^- is bis(2-diphenylphosphinophenyl)-amide, effectively activate the benzene C-H bond in the presence of an appropriate Lewis acid or base, leading to the formation of [PNP]Pt

Full text of DOI:10.1039/b501520k

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Benzene C–H activation by platinum(II) complexes of
bis(2-diphenylphosphinophenyl)amide{
Lan-Chang Liang,* Jia-Ming Lin and Wei-Ying Lee
Received (in Cambridge, UK) 3rd February 2005, Accepted 11th March 2005
First published as an Advance Article on the web 23rd March 2005
DOI: 10.1039/b501520k
Subsequent methylation of [PNP]PtCl with MeMgCl afforded
The amido diphosphine complexes [PNP]PtMe and
[PNP]PtOTf, where [PNP]2 is bis(2-diphenylphosphinophenyl)-
amide, effectively activate the benzene C–H bond in the
presence of an appropriate Lewis acid or base, leading to the
formation of [PNP]PtPh quantitatively.
[PNP]PtMe cleanly. The methyl complex may be alternatively
prepared by addition of H[PNP]6 to an ethereal solution of
[PtMe2(m-SMe2)]2 at 235 uC. Similarly to their nickel and
palladium analogues,6,7 both [PNP]PtCl and [PNP]PtMe are
thermally stable at high temperatures. For instance, no decom-
position was observed when [PNP]PtCl (13.0 mM in benzene) or
[PNP]PtMe (12.7 mM in benzene) was heated to 150 uC under
aerobic conditions for three days as indicated by 31P{1H} NMR
spectroscopy.
The search for appropriate methods to facilitate intermolecular
C–H bond activation of hydrocarbons continues to constitute an
active area of exploratory research.1 Much recent attention has
been paid to systems involving platinum, due largely to the
pioneering work of Shilov and co-workers who successfully
demonstrated selective alkane oxidation by aqueous solutions of
chloroplatinate salts.2 In this regard, multidentate chelating ligands
incorporating exclusively nitrogen donor atoms have been
examined extensively.3 In contrast, studies involving platinum
complexes supported by phosphine-derived chelating ligands
remain relatively undeveloped.4 This is somewhat surprising in
view of the facile accessibility of 31P NMR handle to the
investigation of mechanistic C–H activation processes. We have
recently prepared a series of o-phenylene derived amido phosphine
ligands and demonstrated the versatility of these hybrid
compounds in reactivity involving a variety of metals.5 For
instance, nickel and palladium complexes of bis(2-diphenylphos-
phinophenyl)amide ([PNP]2)6,7 are found to be markedly
thermally stable, even at elevated temperatures, therefore leading
to differing reaction chemistry relative to the closely related
[N(SiMe2CH2PR2)2]2 system.8 In this contribution, we describe
the preparation and characterization of divalent platinum com-
plexes of [PNP]2, and their reactivity with respect to intermo-
lecular benzene C–H bond activation.
The solution NMR data of [PNP]PtCl and [PNP]PtMe are
consistent with a square-planar geometry for these molecules,
reminiscent of the corresponding nickel and palladium
chemistry.6,7 The [PNP]2 ligand is in a meridional coordination
mode, as evidenced by the presence of diagnostic virtual triplet9
resonances observed for the o-phenylene carbon atoms in the
13C{1H} NMR spectrum. The phosphorus donors exhibit only
one singlet resonance flanked by 195Pt satellite signals with 1JPtP of
2751 and 2983 Hz for [PNP]PtCl and [PNP]PtMe, respectively
(Table 1). The methyl ligand in [PNP]PtMe is unambiguously
observed as a triplet resonance at 1.37 ppm with 3JHP of 5.5 Hz in
the 1H NMR spectrum and a triplet at 222.71 ppm with 2JCP of
5.7 Hz in the 13C{1H} NMR spectrum.
Single crystals of [PNP]PtCl and [PNP]PtMe suitable for X-ray
diffraction studies were grown by slow evaporation of a
concentrated benzene solution at room temperature. The X-ray
structures of [PNP]PtCl{ and [PNP]PtMe,{ as illustrated in Figs. 1
and 2, respectively, are in good agreement with the solution
structures determined by NMR spectroscopy. The platinum center
in both molecules lies perfectly on the square plane defined by the
four donor atoms with the chloride or methyl ligand being trans to
the amido nitrogen atom. Similarly to that found in [PNP]PdCl,7
the two o-phenylene rings in [PNP]PtCl and [PNP]PtMe are tilted
with respect to the coordination plane due to the steric repulsion
between the two CH groups ortho to the amido nitrogen atom.
The remaining parameters are unexceptional.
6
The reaction of [PNP]Li(THF)2 with PtCl2(SMe2)2 in THF
at 235 uC produced [PNP]PtCl quantitatively (Scheme 1).
Alkyl abstraction mediated by Lewis acidic B(C6F5)3 from
transition metal complexes has been well-documented.10
Scheme 1 Synthesis of platinum complexes supported by the [PNP]2
ligand. Reagents and conditions: i, PtCl2(SMe2)2, THF, 235 uC; ii,
MeMgCl, THF, 235 uC; iii, AgOTf, C6H6, RT; iv, HOTf, C6H6, RT; v,
PhMgCl, THF, 235 uC; vi, NEt3 or MeNCy2 or DABCO, C6H6, 110 or
150 uC; vii, B(C6F5)3, C6H6, RT; viii, 0.5 equiv. of [PtMe2(m-SMe2)]2,
THF–Et2O, 235 uC; ix, cis-PtPh2(SMe2)2, THF, 235 uC.
Table 1 31P NMR data for [PNP]PtXa
X
dP
1JPtP
Cl
25.43
30.57
27.76
27.90
2751
2983
2809
2967
Me
OTf
Ph
a
{ Electronic supplementary information (ESI) available: preparation and
characterization of compounds. See http://www.rsc.org/suppdata/cc/b5/
b501520k/
All spectra were recorded in C6D6; chemical shifts in ppm,
coupling constants in Hz.
*lcliang@mail.nsysu.edu.tw
2462 | Chem. Commun., 2005, 2462–2464
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reaction of [PNP]PtCl with AgOTf quantitatively on the basis of
31P{1H} NMR investigation. The lability of the triflate ligand in
[PNP]PtOTf is demonstrated by its facile displacement by pyridine
(py) and acetonitrile.11 Fig. 3 depicts the X-ray structure of
{[PNP]Pt(py)}OTf,{ in which the fourth coordination site of the
square-planar platinum center is occupied by a pyridine ligand.
Nevertheless, the triflate complex [PNP]PtOTf is thermally stable
in benzene (6.8 mM) at 150 uC for 20 h as indicated by 31P{1H}
NMR spectroscopy, as is its pyridine adduct {[PNP]Pt(py)}OTf
(8.4 mM).
Interestingly, heating a benzene solution of [PNP]PtOTf to
110 uC or above in the presence of a variety of aliphatic amines
such as triethylamine, N-methyldicyclohexylamine (MeNCy2), or
1,4-diazabicyclo[2.2.2]octane (DABCO) yielded [PNP]PtPh quan-
titatively. This reaction is complete in 2.5 h at 150 uC as indicated
by 31P{1H} NMR spectroscopy. Attempts to employ an excess
amount of amines did not increase the rate of formation of
[PNP]PtPh. In contrast, no reaction was found for [PNP]PtCl
under similar conditions, suggesting that prior dissociation or
displacement of the labile triflate ligand in [PNP]PtOTf is essential
for intermolecular benzene C–H bond activation in this process. A
similar phenomenon was also reported recently for platinum
complexes supported by a bis(8-quinolinyl)amide ligand.12
In summary, we have prepared a series of platinum complexes
supported by the tridentate [PNP]2 ligand and demonstrated
intermolecular benzene C–H bond activation promoted by these
molecules in the presence of an appropriate Lewis acid or base.
Studies directed to define the reaction mechanism and scope of
applicable hydrocarbon substrates are currently under way.
We thank the National Science Council of Taiwan for financial
support (NSC 93-2113-M-110-016) of this work and Prof. Michael
Y. Chiang (NSYSU) and Mr Ting-Shen Kuo (National Taiwan
Normal University) for crystallographic assistance.
Fig. 1 Molecular structure of [PNP]PtCl with thermal ellipsoids drawn
at the 35% probability level. The asymmetric unit cell contains five
unbound benzene molecules, which are omitted for clarity. Selected bond
distances (s) and angles (u): Pt(1)–N(1) 2.024(6), Pt(1)–P(2) 2.270(2),
Pt(1)–P(1) 2.284(2), Pt(1)–Cl(1) 2.318(2); N(1)–Pt(1)–P(2) 83.72(19), N(1)–
Pt(1)–P(1) 83.60(19), P(2)–Pt(1)–P(1) 167.30(8), N(1)–Pt(1)–Cl(1)
177.51(19), P(2)–Pt(1)–Cl(1) 94.52(8), P(1)–Pt(1)–Cl(1) 98.18(8).
Fig. 2 Molecular structure of [PNP]PtMe with thermal ellipsoids drawn
at the 35% probability level. Selected bond distances (s) and angles (u):
Pt(1)–N(1) 2.09(2), Pt(1)–C(1) 2.11(3), Pt(1)–P(1A) 2.2737(18), Pt(1)–P(1)
2.2737(18); N(1)–Pt(1)–C(1) 180.0, N(1)–Pt(1)–P(1A) 82.31(7), C(1)–Pt(1)–
P(1A) 97.69(7), N(1)–Pt(1)–P(1) 82.31(7), C(1)–Pt(1)–P(1) 97.69(7), P(1A)–
Pt(1)–P(1) 164.63(13).
Interestingly, treatment of a benzene solution of [PNP]PtMe with a
stoichiometric amount of B(C6F5)3 at room temperature led to the
formation of [PNP]PtPh in quantitative yield, a consequence that
arises from the cleavage of a benzene C–H bond. This reaction is
complete in 31 h as indicated by 31P{1H} NMR spectroscopy. The
identity of [PNP]PtPh was further confirmed by independent
preparation of this molecule from the reactions of either cis-
PtPh2(SMe2)2 with H[PNP]6 or [PNP]PtCl with PhMgCl in
ethereal solutions (Scheme 1). Controlled experiments reveal that
the conversion of [PNP]PtMe to [PNP]PtPh is proportional to the
substoichiometry of B(C6F5)3 employed, indicating a non-catalytic
process for this reaction.
Fig. 3 Molecular structure of {[PNP]Pt(py)}OTf with thermal ellipsoids
drawn at the 35% probability level. One toluene molecule found in the
asymmetric unit cell is omitted for clarity. Selected bond distances (s) and
angles (u): Pt(1)–N(1) 2.025(5), Pt(1)–N(2) 2.056(5), Pt(1)–P(2) 2.2735(17),
Pt(1)–P(1) 2.2941(17); N(1)–Pt(1)–N(2) 175.6(2), N(1)–Pt(1)–P(2)
82.68(16), N(2)–Pt(1)–P(2) 93.60(16), N(1)–Pt(1)–P(1) 84.00(16), N(2)–
Pt(1)–P(1) 99.65(15), P(2)–Pt(1)–P(1) 166.60(6).
Addition of one equiv. of triflic acid to a benzene solution of
[PNP]PtMe at room temperature afforded [PNP]PtOTf in 61%
isolated yield. The triflate complex can also be prepared from the
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Lan-Chang Liang,* Jia-Ming Lin and Wei-Ying Lee
K. Angermund, W. C. Kaska, H.-J. Fan and M. B. Hall, Angew.
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Department of Chemistry and Center for Nanoscience &
Nanotechnology, National Sun Yat-sen University, Kaohsiung 80424,
Taiwan E-mail: lcliang@mail.nsysu.edu.tw; Fax: +886-7-5253908
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{ Crystal data: for {[PNP]PtCl?5C6H6: C66H58ClNP2Pt, M 5 1157.61,
monoclinic, space group P21/n, a 5 17.3780(5), b 5 12.7050(4),
c 5 25.5550(7) s, b 5 104.5820(10)u, V 5 5460.5(3) s3, T 5 100(2) K,
Z 5 4, m(Mo-Ka) 5 2.719 mm21, 21551 reflections measured, 9430 unique
(Rint 5 0.1316) which were used in all calculations. Final R1 [I .
2s(I)] 5 0.0781, wR2 [I . 2s(I)] 5 0.2004, R1 (all data) 5 0.0914, wR2 (all
data) 5 0.2174, GOF (on F2) 5 1.032, CCDC 261444. For [PNP]PtMe:
C37H31NP2Pt, M 5 746.66, orthorhombic, space group F2dd, a 5 9.9610(2),
b 5 18.6010(4), c 5 32.7250(9) s, V 5 6063.4(2) s3, T 5 293(2) K, Z 5 8,
m(Mo-Ka) 5 4.761 mm21, 7661 reflections measured, 2559 unique
(Rint 5 0.0536) which were used in all calculations. Final R1 [I .
2s(I)] 5 0.0355, wR2 [I . 2s(I)] 5 0.0990, R1 (all data) 5 0.0438, wR2 (all
data)
5 5 0.975, CCDC 261443. For
0.1123, GOF (on F2)
{[PNP]Pt(py)}OTf?PhMe: C49H41F3N2O3P2PtS, M 5 1051.93, monoclinic,
space group P21/n, a 5 15.1140(2), b 5 17.6950(3), c 5 16.9010(3) s,
b 5 105.9800(10)u, V 5 4345.38(12) s3, T 5 100(2) K, Z 5 4, m(Mo-
Ka) 5 3.409 mm21, 32860 reflections measured, 7654 unique (Rint 5 0.0676)
which were used in all calculations. Final R1 [I . 2s(I)] 5 0.0422, wR2 [I .
2s(I)] 5 0.1023, R1 (all data) 5 0.0609, wR2 (all data) 5 0.1399, GOF (on
F2) 5 1.181, CCDC 261442. The hydrogen atoms in the methyl group of
[PNP]PtMe and the toluene methyl group of {[PNP]Pt(py)}OTf?PhMe can
not be located experimentally. See http://www.rsc.org/suppdata/cc/b5/
b501520k/ for crystallographic data in CIF or other electronic format.
11 We note that [PNP]PtOTf reacts with dichloromethane at room
temperature to give [PNP]PtCl quantitatively, whereas
{[PNP]Pt(NCMe)}OTf is stable under similar conditions.
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1
Spectroscopic data for {[PNP]Pt(NCMe)}OTf: H NMR (C6D6–THF,
199.979 MHz): d 7.880 (m, 2H, Ar), 7.663 (m, 8H, Ar), 7.268 (m, 6H,
Ar), 7.069 (m, 6H, Ar), 6.953 (m, 2H, Ar), 6.793 (t, 2H, Ar), 6.375 (t,
2H, Ar), 1.863 (s, 3H, NCCH3); 31P{1H} NMR (THF, 80.952 MHz):
d 28.990 (1JPPt 5 2555 Hz); 19F NMR (THF, 188.151 MHz): d –80.589.
12 S. B. Harkins and J. C. Peters, Organometallics, 2002, 21, 1753–1755.
2464 | Chem. Commun., 2005, 2462–2464
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