Photochemistry of platinum phosphine complexes. Perspective in C-H bond activation

Article abstract of DOI:10.1016/S0020-1693(01)00709-5

The photochemistry of the diphosphino Pt(II) hydrides [LPtH₂] (L=(t-Bu)₂P(CH₂)₂P(t-Bu)₂ (7); L=(t-Bu)₂P(CH₂)₃P(t-Bu)₂ (8);L=(t-Bu)(Ph)P(CH₂)₂P(Ph)(t-Bu) (9)) is reported. The primary photoevent is the dissociation of H₂ and formation of the 14-e [LPt] species. These coordinatively unsaturated intermediates provide a versatile entry point into the C-H bond activation of hydrocarbons. [LPt] reacts with benzene in an oxidative addition reaction to yield [LPt(H)(C₆H₅)] complexes. The importance of the metal centre and ancillary ligation in the C-H bond activation is discussed.

Full text of DOI:10.1016/S0020-1693(01)00709-5

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Inorganica Chimica Acta 330 (2002) 59–62
Photochemistry of platinum phosphine complexes. Perspective in
CꢀH bond activation
Rita Boaretto, Silvana Sostero, Orazio Traverso *
Department of Chemistry, Photochemistry Research Center of CNR, Uni6ersity of Ferrara Via Borsari 46, I-44100 Ferrara, Italy
Received 9 June 2001; accepted 2 July 2001
This paper is respectfully dedicated to the memory of Professor Luigi M. Venanzi
Abstract
The photochemistry of the diphosphino Pt(II) hydrides [LPtH2] (L=(t-Bu)2P(CH2)2P(t-Bu)2 (7); L=(t-Bu)2P(CH2)3P(t-Bu)2
(8);L=(t-Bu)(Ph)P(CH2)2P(Ph)(t-Bu) (9)) is reported. The primary photoevent is the dissociation of H2 and formation of the 14-e
[LPt] species. These coordinatively unsaturated intermediates provide a versatile entry point into the CꢀH bond activation of
hydrocarbons. [LPt] reacts with benzene in an oxidative addition reaction to yield [LPt(H)(C6H5)] complexes. The importance of
the metal centre and ancillary ligation in the CꢀH bond activation is discussed. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Photolysis; Diphosphino Pt(II) hydride complexes; CꢀH bond activation
producing a coordinatively unsaturated metal centre
capable of oxidatively adding a carbon–hydrogen bond
of alkane or alkene. Phosphines are important ligands
because they are among the few in which electronic and
steric properties can be systematically manipulated to
stabilise the metal fragment LnPt (L=PPh3, n=2;
L=PꢀCH2ꢀCH2ꢀP, n=1) produced in situ by
photodissociation of the one two electron (CO,
CH2ꢁCH2) or two one electron (H, alkyl, h1-allyl)
ligands [4].
In this paper, we have chosen to focus on the reac-
tions of CꢀH bond activation with soluble synthetic
Platinum complexes in homogeneous solution. We also
focus primarily on photochemical production of reac-
tive unsaturated fragments generated by loss of
CH2ꢁCH2 and H2 ligands from Platinum phosphine
complexes.
1. Introduction
The field of CꢀH activation developed from an inter-
est in the homogeneous catalysis of a variety of alkane
and alkene conversions by phosphine complexes of
transition metals [1,2]. These are believed to involve an
oxidative addition as the CꢀH bond breaking step, and
may be either thermal or photochemical.
Metal olefine, metal hydride and metal carbonyl
complexes with phosphines as ancillary ligands play a
key role in the CꢀH bond activation sequences of a
variety of important photochemical processes leading to
alkane and alkene functionalisations [3,4]. As such,
reach photochemistry of carbonyl, hydride and olefine
complexes with phosphine ligands bound to Platinu-
m(II) constitutes a critical entry point for the develop-
ment of efficient CꢀH bond activation processes [4–6].
Indeed, extensive photochemical studies have been car-
ried out on the above type of complexes proceeding via
the even numbered, 14- and 16-electron Pt intermedi-
ates. The photochemical dissociation of ligands such as
CO, H2 or alkene has proved an excellent way of
2. Experimental details
2.1. Photochemistry of platinum phosphine complexes
During the past decade our research group [3] has
reported that irradiation of [(PPh3)2PtC2H4] (1) with
280 nm light induces ethylene dissociation (Eq. (1)).
* Corresponding author. Tel.: +39-532-291 158; fax: +39-532-
240 709.
E-mail address: tr2@unife.it (O. Traverso).
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00709-5

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R. Boaretto et al. / Inorganica Chimica Acta 330 (2002) 59–62
hw
[(PPh3)2PtC2H4] ꢀꢀꢂ [(PPh3)2Pt]+C2H4
(1)
It was encouraging to find that photolysis of 7–9
complexes in benzene led to dissociation of H2 and the
formation of [LPt(H)(C6H5)] complexes 10–12 [(10)
L=(t-Bu)2P(CH2)2P(t-Bu)2; (11) L=(t-Bu)2P(CH2)3-
P(t-Bu)2; (12) L=(t-Bu)(Ph)P(CH2)2P(Ph)(t-Bu)] as
products of intermolecular CꢀH activation of C6H6. No
orthometallation products were observed.
Hence, the formation of 10–12 must arise from
highly reactive species of Pt(0) initially formed as a
result of photoreductive elimination of H2 from the
Pt(II) diphosphinehydride (Eq. (3)).
1
280 nm
2
The 14 valence electron [(PPh3)2Pt] (2) intermediate is
quite reactive. In CH2Cl2 solution, the final product
obtained was the complex [(PPh3)2Pt(Cl)(CH2Cl)], re-
sulting from oxidative addition of CH2Cl2 to the 14
electron intermediate 2 [7]. However, irradiation of 1
with 254 nm light did not induce ethylene loss but
rather orthometallation of one of the PPh3 ligands
forming the ethylene–hydride complex 3 (Eq. (2)).
hw
[(PPh3)2PtC2H4] ꢀꢀꢀꢂ [(PPh3)(Ph2PC6H4)Pt(H)(C2H4)]
1
254 nm
3
hw
C H
6
[LPtH2] “ [LPt] “6[LPt(H)(C6H5)]
(3)
(2)
7–9
−H
10–12
2
The hydrido complex 3 undergoes a secondary pho-
toreaction forming the ethyl complex [(Ph3P)(Ph2PC6-
H4)Pt(C2H5)] (4) as a result of photoinduced CH2ꢁCH2
insertion in the PtꢀH bond of complex 3. At 280 nm,
photodissociation of CH2ꢁCH2 from 1 occurs at conve-
nient rates in dichloromethane and ethanol.
The intermediate, [(PPh3)2Pt] (2), does not react with
CꢀH bonds of saturated or unsaturated hydrocarbons.
Photolysis (u=280 nm) of 1 in mixed benzene–cyclo-
hexane or benzene–cyclopentane solutions leads only
to [(PPh3)2Pt]2 as the platinum-containing reaction
product. This result raises questions about whether a
very suitable metal centre is required in order to induce
intermolecular CꢀH activation. Independent work in
Whitesides laboratory [8,9] demonstrates that thermal
reductive elimination of neopentane from [cis-hydri-
The [LPt] species are mononuclear chelate intermedi-
ates which are highly unsaturated, both coordinatively
and electronically, and which could promptly abstract
hydrogen from C6H6.
Elimination of H2 proved to proceed in a concerted
fashion since irradiation of an equimolecular mixture of
7 and [LPtD2] 7% gave only H2 and D2, no HD being
detected.
These crossover experiments indicate that reductive
photoelimination of H2 is strictly intramolecular. The
electronic absorption spectra of 7–9 complexes revealed
little regarding the nature of the photoreactive excited
states since no well-resolved bands were observed in the
250–350 nm region.
The use of state correlation diagrams, which point to
possible pathways interconnecting reactants and prod-
ucts [4,10,11], appears useful in rationalising the pho-
toreactivity of the [LPtH2] considered. It is usually
assumed that photodissociation is the consequence of
exciting an electron from a bonding orbital to the
corresponding antibonding orbital. In a qualitative en-
ergy-level scheme for the system [LPtH2] (assuming a
pseudo-square planar coordination) the highest orbital
doneopentyl(bis-dicyclohexylphosphinoethane)Pt]
(5)
produces the reactive intermediate [bis-dicyclo-
hexylphosphinoethane]Pt and that this reacts with CꢀH
bonds in saturated and unsaturated hydrocarbons. In
somewhat related experiments, we observed that pho-
tolysis of 5 in cyclopentane gave the corresponding
[LPt(cyclopentyl)H] complex
6
(L=Cy2Pꢀ(CH2)2ꢀ
PCy2). Perhaps the most important discovery differenti-
ating the photochemical activation and the thermal
analog is its greater yield of oxidative addition product,
most likely due to the fact that photolysis was carried
out at a lower temperature.
sx
is the principal bonding orbital between H2 and
2
2
–y
Pt, and the lowest unoccupied orbital s*2
is strongly
2
antibonding between the metal and H2.xThus an active
–y
excited state involved in either depopulation of sx
2
2
–y
This evidence along with molecular orbital consider-
ations [1,10] suggest that electronic and steric properties
of bent diphosphines are important factors that should
increase intermolecular reactivity of Pt(0) species of the
[(PꢀP)Pt] type (PꢀP=diphosphine), enabling access to
CꢀH bond activation of hydrocarbons.
With this information in hand, we focused our atten-
tion on the photochemistry of dihydridediphosphine
complexes of Pt(II): [(t-Bu)2P(CH2)2P(t-Bu)2PtH2] (7),
[(t-Bu)2P(CH2)3P(t-Bu)2PtH2] (8), and [(t-Bu)(Ph)-
P(CH2)2P(Ph)(t-Bu)PtH2] (9).
or population of s*2
can be suggested since either
2
x
–y
should greatly weaken the PtꢀH2 bonding.
In an attempt to extend the chemistry of [LPtH2]
species to alkane CꢀH bond activation, we carried out
the photolysis of the complexes 7–9 in cyclopentane
solvent. Although photodissociation of the H2 does
take place, the [LPt] species does not react with the
alkane under similar conditions used in C6H6. The
major Platinum-containing products obtained are the
dinuclear complexes [LPtꢀPtL] (13).
The results seem to indicate that the intermediate
species [LPt] are fairly strong nucleophiles inert to an
attack by alkane, thus enabling the formation of the
dimer products.
The cis-dihydrides 7–9 were thermally stable even in
solution at 90 °C and show no tendency to dissociation
of the coordinate hydride.

R. Boaretto et al. / Inorganica Chimica Acta 330 (2002) 59–62
61
It should be emphasised, however, that oxidative
addition product [LPt(H)(R)] (R=alkyl)-resulting from
H abstraction from the alkane solvent-would itself be
expected to undergo a secondary photolysis of reduc-
tive elimination type. It would, therefore appear that
the phosphine-stabilised platinum species [LPt] does not
react intermolecularly with unactivated CꢀH bonds of
alkane although it react easily with benzene. Recently
Bercaw and co-workers [12] reported that sufficiently
electrophilic cationic Pt(II) centres containing the
N,N,N%,N%-tetramethylendiamine (TMEDA) ligand
should be able to activate alkane CꢀH bonds.
Analysis for H2 by Gaschromatography was carried
out by using a Perkin–Elmer F17 Gaschromatograph
,
with a 5 A molecular sieves column.
The NMR spectra were recorder on a Bruker AC 200
instrument and in part on an AMX 500 operating for
1H at 200.13 (500.13) MHz and for 31P at 81.0 (202.4)
MHz. The chemical shift scales are relative to internal
1
TMS for H and to external H3PO4 for 31P.
The FT-IR spectra were recorded on a Bruker
IFSS88 FT-IR spectrometer.
UV–vis spectra were recorded with a JASCO-Uvidec
650 spectrophotometer.
4.2. Photolysis of [(PPh3)2PtC2H4] (1)
3. Conclusion and perspectives
Compound 1 is a white compound with maximum
absorption in benzene–cyclohexane(1:1) solution at 285
nm. Photolysis (u=280 nm) of 1 (10−2 M in benzene–
cyclohexane) under argon results in the formation of a
dark red solution. Irradiation was stopped after 50 min.
Removal of the solvent under vacuum yielded a brown
A high metal centre electrophilicity is clearly an
essential requisite to make CꢀH bond activation of
alkane feasible. Electrophilicity is a consequence not
only of the chemical nature of the central metal and its
oxidation state, but it is also tuned somewhat by the
ancillary ligands bonded to it. In addition, it is also
important that frontier orbital of correct symmetry in
the central metal be available to accept the electronden-
sity of the CꢀH bond to be activated [13]. The general
approach to CꢀH activation by oxidative addition com-
monly involves a low-valent transition metal with good
donor ligands (hydride [1], Cp* [1,5], Tp*(hydro-
trispyrazolylborate). This now seems to be a general
reaction for hydride complexes with these ligands, in-
deed it has already been observed for compounds of Ir,
Rh, and Pt [5,14,15].
1
residue identified as [(PPh3)2Pt]2 by H- and 31P-NMR
[3].
1H-NMR spectrum of the distillate showed the reso-
nance of free C2H4. The same result was obtained in the
photolysis of 1 in benzene–cyclopentane solution.
4.3. Photolysis of
[Cy2Pꢀ(CH2)2ꢀPCy2Pt(H)(CH2ꢀCMe3)] (5)
The electronic spectrum of 5 in cyclopentane shows
an absorption maximum at 235 nm and two shoulders
at 278 and 300 nm.
A solution of 5 (0.15 mmol) in cyclopentane was
irradiated for 1 h at 280 nm. During irradiation, a
white precipitate formed. The solvent was removed in
vacuo and the white product identified as
[Cy2Pꢀ(CH2)2ꢀPCy2Pt(H)(C5H9)] (6) by FT-IR and
NMR (1H and 31P) spectra [8,9].
4. Experimental
4.1. General data
All preparations and photochemical experiments
were carried out under a dry argon atmosphere using
standard Schlenk techniques. The solvents were dried
by refluxing over the appropriate substrates and were
distilled under argon prior to use.
The yield of neopentane was quantitative as deter-
mined by GC. The yield of 6 was 75.8%.
4.4. Photolysis of dihydride diphosphine Pt(II) 7–9, in
The starting materials [(PPh3)2PtC2H4] (1),
[Cy2Pꢀ(CH2)2ꢀPCy2Pt(H)R] (5) (Cy=cyclohexyl; R=
neopentyl), [(t-Bu)2P(CH2)nP(t-Bu)2PtH2] (7, n=2; 8,
n=3) and [(t-Bu)2(Ph)P(CH2)2P(Ph)(t-Bu)2PtH2] (9)
were prepared as described in the literature [8,16].
Elemental analysis was performed by the Microana-
lytical Laboratory of the Department of Chemistry,
University of Ferrara, Italy.
Photochemical reactions were performed with the
output of a 500W Oriel Hg lamp fitted with an IR
blocking filter and a water filter to remove excess heat.
The appropriate wavelength of irradiation was obtained
with an Applied Photophysics f 3/4 monochromator.
benzene
A solution of complex 7 (0.20 mmol) in benzene was
irradiated for 1 h at 280 nm. Analysis of the gases
above the irradiated solution showed the presence of
H2. The solvent was removed in vacuo and the colour-
less residue (yield 72%) was identified as [(t-Bu)2Pꢀ
(CH2)2ꢀP(t-Bu)2Pt(H)(C6H5)] (10) by elemental analysis
[C, calc. 44.67, Found: 44.52; H, calc. 7.78, Found:
7.65], FT-IR and 1H-NMR spectra. FT-IR (nujol,
cm−1): w(PtꢀH) 1992. H-NMR (CD2Cl2): l 7.5 (t with
Pt satellites, JPꢀH=7.5 Hz, JPtꢀH=58.2 Hz, 2H), 7.02
(t, JPꢀH=7.5 Hz, 2H), 6.70 (t, JPꢀH=7.5 Hz, 1H),
1

62
R. Boaretto et al. / Inorganica Chimica Acta 330 (2002) 59–62
[2] B.A. Arndisen, R.G. Bergman, T.A. Mobley, T.H. Peterson,
Acc. Chem. Res. 28 (1995) 162–214 (and references therein).
[3] S. Sostero, O. Traverso, M. Lenarda, M. Graziani, J.
Organomet. Chem. 134 (1977) 259.
[4] G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry,
Academic press, New York, 1979.
[5] S.E. Bromberg, H. Yang, M.C. Asplund, T. Lian, B.K. McNa-
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−1.12 (dd with Pt satellites, JPꢀH=20 Hz, JP%ꢀH=190
Hz, JPtꢀH=1165 Hz, 1H).
The photolysis of complexes 8, 9 were performed as
described for complex 7. H2 was evolved and the
phenylhydrido complexes 11 and 12 were obtained (the
yield was 70%). Apart from the changes due to the
1
presence of different diphosphines, the FT-IR and H-
NMR spectra were as for 10.
Photolysis of an equimolar amount of 7 and 7-d2 in
benzene was carried out as described for 7. Analysis of
the gases above the irradiated solution showed only H2
and D2.
[7] G. Bartocci, A. Maldotti, S. Sostero, O. Traverso, J. Organomet.
Chem. 253 (1983) 253.
[8] M. Hackett, J.A. Ibers, P. Jernakoff, G.M. Whitesides, J. Am.
Chem. Soc. 108 (1986) 8094.
[9] (a) M. Hackett, G.M. Whitesides, J. Am. Chem. Soc. 110 (1988)
1449;
4.5. Photolysis of dihydride diphosphine Pt(II) 7–9, in
cyclohexane or in cyclopentane
(b) M. Hackett, J.A. Ibers, P. Jernakoff, G.M. Whitesides, J.
Am. Chem. Soc. 108 (1986) 8094.
These were performed as described for irradiation in
C6H6. Photoinduced elimination of H2 was obtained.
Removal of the solvent from the irradiated solution
[10] L.H. Pignolet, Homogeneous Catalysis with Metal Phosphine
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[12] M.W. Woltcamp, J.A. Labinger, J.E. Bercaw, J. Am. Chem. Soc.
119 (1997) 848.
1
gave a red compound identified by H- and 31P-NMR
spectra as [(t-Bu)2Pꢀ(CH2)2ꢀP(t-Bu)2Pt]2 [16].
[13] H. Rabaa, J.Y. Saillard, R. Hoffmann, J. Am. Chem. Soc. 108
(1986) 4327.
[14] A. Ferrari, E. Polo, H. Ruegger, S. Sostero, L.M. Venanzi,
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