The three weak bands of nearly equal intensities character-
istic for fac-[W(CO) (g2-propene) ], 1D, are observed at 1974,
not well-resolved. After UV photolysis of 1 new bands appear
in the visible region ([400 nm) but they are very broad.
3
3
1909 and 1873 cm~1. Such a pattern of bands was detected for
The photochemical behaviour of trans-[W(CO) (g2-
4
alkene) ] complexes in low-temperature hydrocarbon glasses
2
fac-[W(CO) (g4-NBD)(g2-NBD)] (1993.6, 1926.5 and 1900.2
cm~1)22 and for fac-[W(CO) (g4-NBD)(g2-C H )] (1992,
3
or solutions is consistent with previously observed photo-
3
2 4
1929 and 1887 cm~1).22 Over the time of UV photolysis, the
bands for 1D do not change intensity but the three bands for
the mer isomer 1E, at 2020(vw), 1938(vs) and 1928(s) cm~1,
increase very much [Fig. 4(c,d)]. The pattern of IR bands due
to 1E is similar in intensity and position to those of mer-
chemistry in Ar at 16 K.19 The facile loss of the coordinated
alkene after UV (j \ 313 or 367 nm) photolysis produces the
unsaturated alkene transient AO, which after transÈcis isomer-
ization to A and recombination with the alkene gives a cis
isomer of the original compound (Scheme 1). Subsequent pho-
tolysis of C at j [ 420 nm regenerates the trans isomer [Fig.
4(e)]. Similar transÈcis isomerization was also observed for
coordinatively unsaturated species, but in contrast to AO, the
16-electron species AOO formed after CO loss in alkane glass
decayed with a simultaneous increase of AII after visible light
radiation. With visible light photolysis AI also decayed, prob-
ably by isomerization to AII (Scheme 1). D isomerizes to E
under the inÑuence of visible light [Fig. 4(d)]. This obser-
vation explains why C and D could not be detected after
[W(CO) (g2-(Z)-cyclooctene)(g4-NBD)] observed by Grevels
3
et al.12 at 2019.5(w), 1942.5(s) and 1932.5(m) cm~1. The analo-
gous compounds mer-[W(CO) (g4-NBD)(g2-NBD)] and mer-
3
[W(CO) (g4-NBD)(g2-C H )] have bands at 2017.5, 1943.7,
3
2 4
1933.4 cm~1 and 2031, 1940 cm~1,22 respectively.
Subsequent irradiation of the solution with visible light
(j [ 420 nm) destroyed 1C and regenerated the trans isomer 1
[Fig. 4(e)]. The bands due to 1D also decay, whilst those for
1E increase in intensity. It has thus been clearly demonstrated
that selective UV photolysis can isomerize trans- to cis-
bis(alkene) complexes, but visible photolysis has the reverse
e†ect. Also the fac (1D) isomer transforms into the mer (1E)
isomer under the inÑuence of visible light [Fig. 4(e)].
broad-band photolysis of [W(CO) ] and alkene.
6
Among the allyl hydrido compounds, only the species
detected as BII decays under irradiation with light having
j [ 420 nm, whilst B does not change and BI increases in
abundance.
E†ect of temperature
At higher temperatures photolysis with light having j \ 313
nm leads to the formation of [W(CO) (g2-alkene)], F [Fig.
Experiments at di†erent temperatures allowed us to detect
the various coordinatively unsaturated species. The inter-
5
3(b,c)]. However, when subsequent photolysis occurs at
j \ 367 nm, the intensity of bands due to F decreases but
mediate species trans-[W(CO) (g2-alkene)(s)], AO and mer-
those of W(CO) , G, increases. The result of the irradiation of
4
[W(CO) (g2-alkene) (s)], AOO, can be observed only in frozen
6
compounds 1È3 at j \ 367 nm, irrespective of the tem-
3
2
alkanes at 123 K for pentane and 153 K for hexane (Fig. 2). At
higher temperatures (193 and 203 K) these species isomerize
perature, is that F does not appear. It is known from the liter-
ature that the electronic spectrum of 1F exhibits a maximum
at j \ 355 nm.8 Irradiation into this band causes alkene loss
very fast to the IR-detected species cis-[W(CO) (g2-
4
alkene)(s)], A, and fac-[W(CO) (g2-alkene) (s)], AI (Scheme 1),
and the formation of W(CO) , very short-lived at higher tem-
3
2
which are the main products of short-time UV photolysis
[Figs. 1(a) and 3].
5
peratures, which traps early photoejected CO to form G. Thus
the photolysis of 1È3 with light having j \ 367 nm prevents
the appearance of F.
At 233 K and higher, coordinatively unsaturated species are
too unstable to be detected. The main products observable by
IR at 263 K are W(CO) , G, [W(CO) (g2-alkene)], F, and the
Only selective photolysis allowed us to generate interme-
diates with a high enough yield for detection by IR. UnÐltered
photolysis gave spectra too complicated to allow any system-
atic analysis of the relevant photochemical events.
6
5
allyl hydride BII [Fig. 3(c)]. The formation of G and F at
higher temperatures indicates that alkene loss and the forma-
tion of thermally unstable [W(CO) (g2-alkene)(s)], AO,
4
(Scheme 1) is the main photochemical process after irradiation
E†ect of a coordinated alkene
at j \ 313 nm. AO traps early photoejected CO to form F.
Several possible photogenerated intermediates could arise
from either WwCO or Wwalkene bond breaking or from
isomerization. Such intermediates are not readily identiÐable,
but the identiÐcation of particular intermediates is greatly
simpliÐed by studying the photolysis of a series of related pre-
cursor compounds that yield the same intermediates. The fea-
tures for propene and 1-butene complexes are very similar.
Also the photochemical products of 1 and 2 have almost iden-
tical IR characteristics (Table 1). Di†erent behaviour was
observed in the case of the photochemical reaction of the
cyclopentene complex 3. Irradiation of compound 3 produced
coordinatively unsaturated species but IR spectroscopy
showed that neither of these are transformed into allyl
hydride. Thus, in the case of this cyclic alkene the activation
of the CwH bond has not been observed.
There are two bands at 2083(vw) and 1964(s) cm~1 character-
istic for F. The third strong band predicted for F (C ) is
4v
obscured by an intense band of 2 [Fig. 3(c)]. Three bands at
2082, 1961 and 1944 cm~1 were observed for [W(CO) (g2-
5
C H )] in methylcyclohexane at 77 K.8 The UV photolysis of
3
6
F leads to alkene loss and, after CO trapping, the appearance
of G [Fig. 3(c) and Scheme 1].
Another interesting observation is the unexpected stability
of the photochemically formed cis isomer C, which can be
observed even at 263 K [Fig. 3(c)]. The cis isomer of a
bis(alkene)tetracarbonyltungsten complex was Ðrst observed
as a thermally labile product during the photolysis of
[W(CO) ] and C H or 1-C H in rigid alkane at 77 K.8
6
3
6
5 10
Subsequently, cis-[W(CO) (g2-C H ) ] was observed in a
4
2 4 2
liquid Xe solution.9 More recently, our studies revealed that
cis-[W(CO) (g2-alkene) ] (alkene \ 1-pentene and cyclo-
pentene) can be formed in the photochemical isomerization of
4
2
Conclusions
trans-[W(CO) (g2-alkene) ] in an argon matrix at 16 K.19
The results presented here go some way towards bridging the
gap between low-temperature matrices and the room-
temperature solutions in which most preparative and catalytic
reactions take place. The solution photochemistry of trans-
[W(CO) (g2-alkene) ] complexes is similar to their matrix
4
2
Here we show that a cis isomer can be observed at much
higher temperature, even at 263 K.
E†ect of photolysing radiation wavelength
4
2
The electronic spectrum of trans-[W(CO) (g2-alkene) ] com-
photochemistry.19 The trans-[W(CO) (g2-alkene) ] complexes
4
2
4
2
plexes in an alkane solution exhibit a maximum at about 300
nm.18 However, the electronic spectrum of the photolysis
product of 1 measured in the IR cell applied in these studies is
isomerize to cis complexes upon UV irradiation either in
matrices or in solution. However, in an alkane solution we
can observe also the formation of coordinatively unsaturated
New J. Chem., 1998, Pages 1539È1544
1543
Szymanska-Buzar, Teresa
