Photolysis of Zeise salt in aqueous solution: Photocatalysis of the hydration of olefins to alcohols

Article abstract of DOI:10.1016/j.inoche.2012.07.040

The photolysis of aqueous Zeise salt essentially leads to the release of ethylene, but about 10% undergo a photohydration which is initiated by MLCT excitation:Pt^IICl₃(C₂H₄)] ^- + 2 H₂O - hν→ [Pt^II(H ₂O)Cl₃] ^- + CH₃CH₂OH Since [Pt(H₂O)Cl₃]^- adds again ethylene to regenerate [Pt(C₂H₄)Cl₃]^- the overall reaction proceeds as a photocatalysis converting C₂H ₄ to C₂H₅OH. When ethylene was replaced by 1-hexene the photoaquation to 1-hexanol takes place with TON > 2.30.

Full text of DOI:10.1016/j.inoche.2012.07.040

Inorganic Chemistry Communications 24 (2012) 134–135
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photolysis of Zeise salt in aqueous solution
Photocatalysis of the hydration of olefins to alcohols
⁎
Horst Kunkely, Arnd Vogler
Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The photolysis of aqueous Zeise salt essentially leads to the release of ethylene, but about 10% undergo a
photohydration which is initiated by MLCT excitation:
Received 23 May 2012
Accepted 23 July 2012
Available online 31 July 2012
PtIICl3ðC2H4Þꢀ− þ 2H2O―hν→½PtIIðH2OÞCl3ꢀ− þ CH3CH2OH
Keywords:
Coordination chemistry
Zeise salt
Since [Pt(H2O)Cl3]− adds again ethylene to regenerate [Pt(C2H4)Cl3]− the overall reaction proceeds as a
photocatalysis converting C2H4 to C2H5OH. When ethylene was replaced by 1‐hexene the photoaquation to
1‐hexanol takes place with TON>2.30.
Photocatalysis
© 2012 Published by Elsevier B.V.
Platinum complexes
Olefin complexes
Photochemistry
The hydration of ethylene to ethanol is a very important reaction
in organic chemistry
is indeed formed in this photolysis which can also proceed as a rather
simple photocatalysis for the conversion of olefins to the corresponding
alcohols.
C2H4 þ H2O→CH3CH2OH
ð1Þ
The photolysis of Zeise salt in aqueous solution proceeds essentially in
agreement with previous observations [4]. The spectral changes which
accompany the irradiation indicate the release of ethylene as a ligand.
Upon addition of chloride to the photolyzed solution PtCl42− is formed
from the Pt(II) complex which ejected ethylene while the starting com-
plex [Pt(C2H4)Cl3]− is not affected. Moreover, the photolysis led also to
the formation of ethanol which was detected by an enzyme-based (alco-
hol dehydrogenase or alcohol oxidase) method provided as UV test at
340 nm (Roche) or saliva test (Sanelco), respectively. This measurement
revealed that a stoichiometric ratio of PtCl42− to ethanol of 10. According-
ly, about 10% of released ethylene was converted to ethanol which was
formed with a quantum yield Ø=0.01 at λirr=305 nm. The amount of
8% of the released ethylene which could not be recovered [4] roughly cor-
responds to the amount of ethanol formed in the photolysis.
It is carried out for the commercial production of ethanol in large
scale and proceeds as acid catalysis, for example with phosphoric acid
as catalyst at 300 ° C and a pressure of 60 to 70 atmospheres. Owing
to the high activation energy [1] this hydration does not take place
under ambient condition even in the presence of catalysts. We antic-
ipated that this aquation might occur photochemically. Transition
metal complexes with ethylene as ligands are good candidates for
such a photoreaction. Previous studies of the photochemistry of olefin
complexes [2,3] have not considered this possibility. For example, the
photochemistry of Zeise salt has been examined in quite some detail
by Natarajan and Adamson in 1971 [4]. The most efficient reaction
was found to be the ejection of ethylene as antithermal behavior:
The photochemical release of ethylene leads to the formation of
[Pt(H2O)Cl3]− which is able to add free ethylene again as a ligand. It
follows that the hydration of ethylene or other olefins should proceed
as photocatalysis. We examined this assumption but used 1−hexene
instead of ethylene for experimental convenience. The detection and
determination of 1−hexanol is carried out and by the same procedure
as the analysis of ethanol. 1−Hexene is a liquid which is not miscible
with water. Upon addition of this olefin to a PtCl42− containing aqueous
solution, 1−hexene forms a separate layer on the top. A slow stream of
argon did not only prevent the access of oxygen but as stirring proce-
dure this argon flow provided the saturation of this aqueous solution
with 1−hexene which is then available for coordination to Pt(II). The
photolysis of this solution showed that it takes place as a photocatalysis
½PtIICl3ðC2H4Þꢀ−―hν=H2O→½PtIICl3ðH2OÞꢀ− þ C2H4
ð2Þ
It was assumed that this release is initiated by LF (ligand field) ex-
cited states which generally induce ligand substitutions. However, a
theoretical investigation later led to the conclusion that this photolysis
originates from a PtII→π* ethylene MLCT excited state [5]. This sugges-
tion, in turn, caused our suspicion that the photoejection of ethylene
might not be the only reaction with the participation of the ethylene
ligand. Interestingly, in the original study 8% of the released ethylene
could not be recovered [4]. Now, we report our observation that ethanol
⁎
Corresponding author. Tel.: + 49 941 943 4716.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
1387-7003/$ – see front matter © 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.inoche.2012.07.040

H. Kunkely, A. Vogler / Inorganic Chemistry Communications 24 (2012) 134–135
135
Scheme 1.
Scheme 2.
for alcohol formation. After irradiation with λ=254 nm for 5 h the TON
(1−hexanol/PtCl42−) amounts to 2.30. This TON can be certainly in-
creased by careful control of the reaction conditions but side reactions,
in particular the photooxidation of Pt(II) to Pt(IV) [4] can interfere with
the desired photocatalysis.
The competing photorelease of C2H4 from [PtIICl3(C2H4)]− does
only reduce the efficiency of the desired photohydration of ethylene
but does not interfere with its occurrence. Side reactions of the pho-
tolysis of [PtIICl3(C2H4)]− may lead to a slow loss of Pt(II) by photoox-
idation to Pt(IV) [4].
Owing to an easier handling of a liquid instead of a gas the
photocatalysis was probed with 1−hexene which is converted to 1−
hexanolupon photolysis of Pt2+ ions inaqueous solution. As expected
the TON exceeds the minimum of 1. At an irradiation time of about 5 h
the TON amounts to 2.30. The long irradiation time is apparently
necessary because of the low light intensity at λirr =254 nm. More-
over, it is probably also caused by the slow regeneration of the Pt(II)
hexene complex which could originate from the small concentration
of hexene in the aqueous solution owing to its low solubility.
MLCT (PtII→π* ethylene) excitation of [PtCl3(C2H4)]− requires ir-
radiation with light shorter than 400 nm [5]. It is associated with a
shift of electron density from Pt(II) to ethylene and in a limiting de-
scription the relaxed MLCT state may be viewed as a Pt(VI) complex.
It is well known that MLCT states can lead to the release of ligands
[6,7], but Scheme 1 suggests that the negative charge of the ligand fa-
cilitates also the addition of a proton and subsequently of hydroxide
regenerating Pt(II) with the formation of ethanol (Scheme 2).
The overall process is then simply a hydration of ethylene to gener-
ate ethanol according to Eq. (1). In the original study [4] this mode was
overlooked since it is not very efficient (φ=0.01) in comparison to the
main reaction, the release of ethylene, which takes place with φ=0.1 at
λ
irr=305 nm. Nevertheless, the formation of ethanol is rather interest-
Acknowledgement
ing not only in its own right but also because of its potential to perform
this photolysis as photocatalysis.
In aqueous solution Pt2+ ions are well known to add olefins as a
ligand. Since the photolysis of [PtCl3(C2H4)]− or most likely of any other
Pt(II) ethylene complex leads to the formation of some ethanol but pre-
serves Pt(II), the ethylene complex can simply be regenerated by a suf-
ficient supply of ethylene. The formation of ethanol proceeds then as a
photocatalysis:
We are grateful for financial support by DFG (grant Vo 211/19-1).
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