Mechanistic study of the photofragmentation of the clusters [Os3(CO)10(diene)] (diene = cis- 1,3-butadiene, 1,3-cyclohexadiene): Direct observation of the open-triangle primary photoproduct with nanosecond time-resolved infrared and UV-Vis spectroscopy

Article abstract of DOI:10.1016/S0020-1693(99)00560-5

The photochemistry of the clusters [Os₃(CO)₁₀(diene)] (diene = cis-1,3-butadiene, 1,3-cyclohexadiene) was studied in detail. Upon near-UV irradiation photofragmentation occurs to give mononuclear [Os(CO)₃(diene)]. The second fragmentation product [Os₂(CO)₇] was trapped by reaction with CO, or with alkenes at low temperatures, to produce the dinuclear complexes [Os₂(CO)₇(L)₂] (L = CO, ethene, 1-octene). The fragmentation was studied further with nanosecond time-resolved UV-Vis and IR spectroscopy and was found to proceed via the initial formation of a triosmium photoproduct with a lifetime of about 100 ns in hexane. From the spectroscopic data we infer that this intermediate has a coordinatively unsaturated open-triangle structure with two bridging carbonyl groups, one bridging the split (CO)₄Os-Os(CO)₂(diene) bond. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00560-5

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Inorganica Chimica Acta 300–302 (2000) 597–603
Mechanistic study of the photofragmentation of the clusters
[Os3(CO)10(diene)] (diene=cis-1,3-butadiene, 1,3-cyclohexadiene):
direct observation of the open-triangle primary photoproduct with
nanosecond time-resolved infrared and UV–Vis spectroscopy
Maarten J. Bakker a, Frank W. Vergeer a, Frantisˇek Hartl a,*, Omar S. Jina b,
Xue-Zhong Sun b, Michael W. George b
a Institute of Molecular Chemistry, Uni6ersiteit 6an Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
b School of Chemistry, Uni6ersity of Nottingham, Nottingham NG7 2RD, UK
Received 13 September 1999; accepted 3 December 1999
Abstract
The photochemistry of the clusters [Os3(CO)10(diene)] (diene=cis-1,3-butadiene, 1,3-cyclohexadiene) was studied in detail.
Upon near-UV irradiation photofragmentation occurs to give mononuclear [Os(CO)3(diene)]. The second fragmentation product
[Os2(CO)7] was trapped by reaction with CO, or with alkenes at low temperatures, to produce the dinuclear complexes
[Os2(CO)7(L)2] (L=CO, ethene, 1-octene). The fragmentation was studied further with nanosecond time-resolved UV–Vis and IR
spectroscopy and was found to proceed via the initial formation of a triosmium photoproduct with a lifetime of about 100 ns in
hexane. From the spectroscopic data we infer that this intermediate has a coordinatively unsaturated open-triangle structure with
two bridging carbonyl groups, one bridging the split (CO)4OsꢀOs(CO)2(diene) bond. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Photochemistry; Osmium complexes; Diene complexes; Triosmium complexes
1. Introduction
triangle cluster [M3(CO)11(m-CO)] as the primary pho-
toproduct; although, to date this species has never been
directly observed. Recently, we showed that substitu-
tion of two carbonyl ligands in [Os3(CO)12] by a chelat-
ing a-diimine ligand (e.g. 2,2%-bipyridine) drastically
changed the photochemical behaviour [3,4,10]. Visible
irradiation of [Os3(CO)10(a-diimine)] results in metal–
metal bond cleavage yielding reactive biradical or zwit-
terionic photoproducts, depending on the reaction
conditions and the nature of the a-diimine.
In this paper we report on the mechanistic investiga-
tion of the photochemistry of another class of
[Os3(CO)10(L)] clusters, with L=1,3-diene coordinated
to one of the osmium atoms. The structure of these
clusters with diene=cis-1,3-butadiene (1a) [11] and 1,3-
cyclohexadiene (1b) [12] is depicted schematically in
Fig. 1. We were able to detect and characterize the
primary photoproducts of 1 with nanosecond time-re-
solved IR (TRIR) and UV–Vis absorption (TA) spec-
Thermal reactions of transition metal carbonyl clus-
ters are frequently performed at elevated temperature
and/or pressure, which often induces undesired side
reactions. An increasing number of examples demon-
strate that the photochemical activation of triruthenium
and triosmium clusters may provide an alternative
pathway to selective product formation under mild
conditions [1–9]. The photochemical behaviour of the
clusters [M3(CO)12] (M=Ru, Os) has been studied
most extensively [1,2]. These clusters show a rich photo-
chemistry which is strongly dependent on the irradia-
tion wavelength. Irradiation into the two lowest-energy
absorption bands has been proposed to give the open-
* Corresponding author. Tel.: +31-20-525 6450; fax: +31-20-525
6456.
E-mail address: hartl@anorg.chem.uva.nl (F. Hartl)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00560-5

598
M.J. Bakker et al. / Inorganica Chimica Acta 300–302 (2000) 597–603
(hexane) u (m in M−1 cm−1): 246 (sh), 331 (6.7×103),
ca. 400 (sh) nm.
2.1.1.2. Synthesis of [Os3(CO)10(1,3-cyclohexadiene)]
(1b). 1,3-Cyclohexadiene (1.8 ml) was added to a solu-
tion of [Os3(CO)10(MeCN)2] (330 mg) in CH2Cl2 (100
ml). The reaction mixture was stirred for 1 h. After this
period the solvent was evaporated in vacuo. Purifica-
tion of the crude product by column chromatography
over silica using 1:5 dichloromethane/hexane as eluent
yielded 1b as a yellow powder in 70% yield. IR (hexane)
w(CO): 2111 (m), 2062 (s), 2032 (s), 2023 (vs), 2009 (s),
Fig. 1. Schematic structure of [Os3(CO)10(1,3-cis-butadiene)] (1a).
1
troscopy. The results are compared with the photoreac-
1991 (w), 1982 (m), 1972 (w), 1938 (w) cm−1. H NMR
tivity of the clusters [M3(CO)12] (M=Ru, Os).
(CDCl3) l: 5.59 (2H, dd), 3.79 (2H, d), 1.88 (4H, s)
ppm. UV–Vis (hexane) u (m in M−1 cm−1): 244 (sh),
342 (10.1×103), ca. 400 (sh) nm.
2. Experimental
2.2. Spectroscopic measurements
2.1. Materials and preparations
UV–Vis absorption spectra were recorded on a
Varian Cary 4E and a Hewlett–Packard 84533 spec-
trophotometer, and FT IR spectra on a Bio-Rad FTS-7
or a Bio-Rad FTS-60A spectrometer (16 scans at 2
cm−1 resolution). Low temperature IR measurements
were performed using an Oxford Instruments DN 1704/
54 liquid nitrogen cryostat with CaF2 and quartz win-
[Os3(CO)12] (ABCR), 1,3-butadiene (UCAR), 1,3-cy-
clohexadiene (Acros), ethene (Hoek Loos) and CO
(99.5%, Hoek Loos) were used as purchased. Trimethyl-
amine-N-oxide dihydrate, Me3NO·2H2O (Alfa), was
dehydrated before use by vacuum sublimation. 1-
Octene (Sigma) was passed over alumina and saturated
with N2 before use. Solvents of analytical (Acros: n-
hexane, dichloromethane, THF, 2-methyltetrahydro-
furan (2-MeTHF); Sigma–Aldrich: n-heptane) and
spectroscopic (Merck: CCl4) grade quality were dried
over sodium wire (n-hexane, THF, 2-MeTHF) or CaH2
(n-heptane, dichloromethane, CCl4) and freshly distilled
under nitrogen. CDCl3 and CD2Cl2 (CIL) for NMR
experiments were used as purchased.
1
dows. H NMR spectra were recorded with a Bruker
AMX 300 spectrometer.
2.3. Photochemistry
All photochemical samples were prepared under a
nitrogen atmosphere using standard inert gas tech-
niques. Typical concentrations used were 10−3–10−4
M cluster and 0.1–1.0 M alkene. The solution for the
TRIR experiment was pump-degassed and overpres-
sured with argon.
2.1.1. Synthetic procedures
The syntheses of the clusters 1a and 1b using
[H2Os3(CO)10] as a starting material have been reported
by Tachikawa et al. [11]. We have followed a similar
synthetic procedure as employed by Braga et al. for the
synthesis of 1b [12], using [Os3(CO)10(MeCN)2] [13]. All
syntheses were performed under an inert atmosphere of
dry nitrogen, using standard Schlenk techniques.
For continuous-wave photochemistry a Philips HPK
125 W high-pressure mercury lamp served as a light
source, equipped with the appropriate cut-off filters.
TA spectra were obtained by irradiating the samples
with 7 ns pulses (fwhm) of a 355 nm line (5 mJ
pulse−1), obtained by frequency doubling of the 1064
nm fundamental of a Spectra Physics GCR-3 Nd:YAG
laser. As the compounds under study were not photo-
stable, a home-made flow cell was used. The probe light
from a low-pressure, high-power EG&G FX-504 Xe
lamp was passed through the sample cell and dispersed
by an Acton SpectraPro-150 spectrograph equipped
with 150 or 600 g mm−1 grating and a tunable slit
(1–500 mm), resulting in a 6 or 1.2 nm maximal resolu-
tion, respectively. The data collection system consisted
of a gated intensified CCD detector (Princeton Instru-
ments ICCD-576EMG/RB), a programmable pulse
generator (PG-200), and an EG&G Princeton Applied
Research Model 9650 digital delay generator. With this
2.1.1.1. Synthesis of [Os3(CO)10(1,3-butadiene)] (1a).
1,3-Butadiene was bubbled for 1 min through a solu-
tion of [Os3(CO)10(MeCN)2] (400 mg) in THF (100 ml).
The reaction mixture was stirred for 30 min, followed
by solvent evaporation in vacuo. The crude product
was purified by column chromatography over silica
using dichloromethane/hexane 1:4 as eluent. The
product was obtained as a yellow powder in 65% yield.
IR (hexane) w(CO): 2113 (m), 2064 (s), 2033 (s), 2029
(s), 2025 (s), 2011 (s), 1999 (vw), 1992 (w,sh), 1985 (m),
1
1982 (sh), 1945 (w), 1935 (w) cm−1. H NMR (CDCl3)
l: 5.39 (2H, t), 2.41 (2H, d), 0.56 (2H, d) ppm. UV–Vis

M.J. Bakker et al. / Inorganica Chimica Acta 300–302 (2000) 597–603
599
absorption at a selected wavenumber were monitored
with a photovoltaic 77 K MCT detector (Laser Moni-
toring S-100) with a rise-time of approximately 100 ns.
The kinetic traces obtained at different wavenumbers
were used to construct the transient IR spectra point by
point. The solution in the TRIR cell was replaced after
each laser shot. An accurate time profile for the kinetic
traces were recorded using a faster photovoltaic 77 K
MCT detector (Kolmar Technologies) with a rise-time
of approximately 7 ns.
3. Results and discussion
3.1. UV–Vis spectra
The UV–Vis spectra of 1a, 1b and [Os3(CO)12] are
presented in Fig. 2 and summarized in Table 1.
[Os3(CO)12] possesses a lowest-energy absorption band
at 384 nm, visible as a shoulder of the more intense
second band at 325 nm. Similar absorption bands are
also present in the UV–Vis spectra of 1a and 1b, being
slightly shifted to longer wavelengths.
Fig. 2. UV–Vis spectra of the clusters [Os3(CO)12] (· · ·), 1a (—) and
1b (- - -) in hexane at 293 K.
Table 1
UV–Vis absorption maxima of [Os3(CO)12] and the diene clusters 1a
and 1b in hexane
Cluster
umax (nm)
3.2. Continuous-wa6e photochemistry
[Os3(CO)12
1a
1b
]
243, 325, 384 (sh)
246 (sh), 331 (6.7 103), ca. 400 (sh, br)
244 (sh), 342 (10.1 103), ca. 400 (sh, br)
In order to initiate photoreactions from the low-lying
excited states, solutions of 1a and 1b were irradiated
with a high-pressure mercury lamp using cut-off filters
transparent at uirr\420, 375 or 335 nm. For compari-
son, the unsubstituted cluster [Os3(CO)12] has been
found to be photoreactive only in the presence of Lewis
base ligands (e.g. alkenes, phosphines) upon irradiation
into its two lowest absorption bands in hydrocarbon
solvents [1,2]. In contrast to [Os3(CO)12], irradiation of
1a and 1b in pure hexane at room temperature already
resulted in fragmentation of the clusters according to
Scheme 1. The products were characterized by IR and
1H NMR spectroscopy.
OMA-4 setup, I and I0 are measured simultaneously
using a double 8 kernel 200 mm optical fiber. This setup
is programmed and accessed using WINSPEC (v 1.6.1,
Princeton Instruments) under Windows.
Time-resolved IR (TRIR) experiments were per-
formed using the set-up at the University of Notting-
ham [14]. A Nd:YAG laser (Quanta-Ray GCR-12; 355
nm) was used to initiate reactions and the change in
infrared absorption was determined using an infrared
diode laser (Mu¨tek MSD 1100). The changes in IR
Scheme 1. Reaction mechanism for the photofragmentation of the triosmium diene clusters 1a and 1b.

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M.J. Bakker et al. / Inorganica Chimica Acta 300–302 (2000) 597–603
Table 2
IR w(CO) wavenumbers of the photoproducts of the clusters 1a and 1b
Cluster
T (K)
Solvent
pentane
Photoproduct
w(CO) (cm−1
)
1a a
298
3a (Ref. [15])
2068, 1995, 1985
[Os3(CO)12
not assigned
]
2068, 2036, 2015, 2004
2118, 2086, 2055, 2047, 2021
1b b
1a c
298
298
hexane
3b (Ref. [16])
2063, 1987,1981
2068, 2036, 2015, 2004
2082, 2057, 2043, 2024, 1997
[Os3(CO)12
]
not assigned
hexane/CO
3a (Ref. [15])
2069, 1994, 1984
[Os2(CO)9] (Ref. [17])
2079, 2037, 2023, 2011, 1776
2069, 2036, 2015, 2003
2085, 2045
d
[Os3(CO)12
]
not assigned
1a c
213
hexane/CO
3a (Ref. [15])
2070, 1994, 1983
[Os2(CO)9] (Ref. [17])
2081, 2038, 2024, 2011, 1773
2037, 1994
[Os(CO)5] d
1a c
1a c
213
213
hexane/ethene
3a (Ref. [15])
4
2070, 1994, 1983
2060, 2037, 2020, 2012, 1768
hexane/1-octene
3a (Ref. [15])
2069, 1993, 1983
5
2054, 2036, 2015, 2008, 1764
a uirr\375 nm.
b uirr\420 nm.
c uirr\335 nm.
d Secondary photoproducts formed upon prolonged irradiation. Ref. [18].
[Os2(CO)9] photodecomposed into [Os(CO)5], in agree-
ment with the observations of Turner and co-workers
[18].
In the course of the irradiation, the IR spectra
showed the disappearance of the w(CO) bands of the
starting compounds with simultaneous appearance of
w(CO) bands of the mononuclear complexes
[Os(CO)3(diene)] (diene=cis-1,3-butadiene (3a) [15],
1,3-cyclohexadiene (3b) [16]). Additional new w(CO)
bands showed up due to the formation of [Os3(CO)12]
and one or more unassigned carbonyl compounds, see
Another revealing result was obtained upon irradia-
tion of 1a (uirr\335 nm) dissolved in (a) ethene-satu-
rated, and (b) 0.1 M 1-octene-containing hexane at 213
K. Besides formation of [Os(CO)3(cis-1,3-butadiene)]
(3a), a new product was observed, possessing an IR
w(CO) band at 1768 cm−1 (ethene) or 1764 cm−1
(1-octene), which is characteristic for the presence of a
bridging CO ligand. In analogy with [Os2(CO)9] (w(CO)
at 1776 cm−1) and [Os2(CO)8(ethene)] (w(CO) at 1780
cm−1) [18], the latter product is proposed to be the
hitherto unobserved [Os2(CO)7(L)2], (L=ethene, 4, 1-
octene, 5) with one carbonyl group occupying a bridg-
ing position, as illustrated in Fig. 3. The lower w(CO)
frequencies of the CO bridge in 4 and 5 are consistent
with the substitution of two and one terminal CO
ligands in [Os2(CO)9] and [Os2(CO)8(ethene)], respec-
tively. In accordance with the more electron-donating
character of 1-octene, the complex [Os2(CO)7(1-
octene)2] (5) has all its w(CO) bands slightly shifted to
lower frequency compared with the ethene-containing
1
Table 2. H NMR spectra were recorded before and
after irradiation of 1a in CD2Cl2 in the NMR tube.
New signals of 3a were observed at 0.28 (2H, m), 1.89
(2H, m) and 5.39 (2H, m) ppm, in good agreement with
the literature values [15]. In the ‘aromatic’ region two
multiplets were observed at 6.93 and 8.63 ppm, with an
overall intensity lower than 5% compared to that of the
3a signals. The corresponding minor product(s) were
not characterized further.
During the photofragmentation of 1a and 1b, mono-
and/or dinuclear intermediates are presumably formed,
that are coordinatively unsaturated and therefore
highly reactive. In order to trap and characterize such
transient species, the photoreactions of 1a were also
performed in the presence of suitable donor molecules.
During near-UV irradiation of 1a in CO-saturated hex-
ane (uirr\335 nm) at room temperature a neat conver-
sion of the cluster into [Os(CO)3(cis-1,3-butadiene)]
(3a) and [Os2(CO)9] [17] was monitored by IR spec-
troscopy. Prolonged irradiation led to decomposition of
[Os2(CO)9] and formation of [Os3(CO)12]. When the
experiment was repeated at 213 K, initially produced
Fig. 3. Proposed structure for [Os2(CO)6(m-CO)(L)2] with L=ethene
(4), 1-octene (5).

M.J. Bakker et al. / Inorganica Chimica Acta 300–302 (2000) 597–603
601
derivative 4. It is noteworthy that neither
[Os(CO)4(alkene)] nor [Os2(CO)8(m-alkene)], that are
formed upon irradiation of [Os3(CO)12] in the presence
of alkene [19–21], were observed as the photoproducts
of 1a and 1b.
3.3. Nanosecond time-resol6ed measurements
Summarizing the results described above, the diene
cluster
mononuclear complex [Os(CO)3(diene)] and a dios-
mium complex. Assuming that (CO)4OsꢀOs-
1 undergoes photofragmentation into the
a
(CO)2(diene) bond in 1 becomes significantly weakened
in the excited state, the initial step on the reaction path
is therefore metal–metal bond cleavage and formation
of a biradical or [Os3(CO)9(m-CO)(diene)], the structural
analogue of the proposed primary photoproduct
formed upon near-UV irradiation of [Os3(CO)12] [1,2].
In order to detect and assign such transient species, we
recorded nanosecond time-resolved absorption (TA)
spectra under different conditions.
Transient absorption spectra were recorded at vari-
ous time delays after excitation with the laser pulse.
Directly after excitation of 1a in hexane, bleaching was
observed below 445 nm due to depletion of the ground
state, while a broad and intense transient absorption
arose between 445 and 750 nm with a maximum at 632
nm. This transient disappeared completely and the
bleaching of the absorption of 1a recovered (approxi-
mately 70%) at the same rate (lifetime ~=105 ns). The
partial recovery of 1a agrees with the fragmentation of
the transient to the final photoproducts. Similar results
were obtained for the cluster 1b (see Fig. 4). In this
case, the long-wavelength transient species (umax=636
nm) lived 94 ns.
Fig. 4. Nanosecond time-resolved transient absorption spectra
recorded during the partial back reaction of the primary photo-
product of 1b in hexane; the time delay between the spectra equals 45
ns. The dotted line represents the UV–Vis spectrum of the parent
cluster 1b.
The presence of the long-wavelength absorption in
the TA spectra supports the hypothesis that the pri-
mary photoproducts 2a and 2b are coordinatively un-
saturated. Related di- or trinuclear metal carbonyls
possessing an absorption band at considerably longer
wavelength than the saturated precursor complexes, are
frequently encountered. As examples may serve
[Re2(CO)9] [22], [Pt2(CO)Cp2] [23], [Os2(CO)8] [24],
[Os3(CO)11] [20] and [H2Os3(CO)10] [25].
Unfortunately, the transient absorption UV–Vis
spectra do not provide sufficient structural information
about the primary photoproducts 2a and 2b. For this
reason we studied the course of the photoreaction of 1b
with nanosecond time-resolved infrared (TRIR) spec-
troscopy. The TRIR spectrum of 1b was recorded in
heptane by monitoring the changes in absorption be-
tween 2130 and 1740 cm−1 directly after excitation
with the 355 nm pulse of a Nd:YAG laser. The result-
ing difference absorption spectrum is shown in Fig. 5.
Within the detector rise-time, strong bleaching of the
absorption bands of 1b was observed. Simultaneously,
w(CO) bands arose at 2118, 2050, 2031, 2017, 2004,
1995, 1951, 1857, 1801 cm−1, which were attributed to
the novel transient product 2b. The most interesting
feature of the TRIR spectrum is the presence of the two
w(CO) bands at 1857 and 1801 cm−1, which can be
attributed to bridging carbonyl ligands. The w(CO)
bands of the transient species completely disappeared
after 200 ns, while only a partial recovery of the parent
cluster was observed, estimated to approximately 70%
For both clusters 1 the experiments were repeated in
CCl4, that has been proven to be an efficient scavenger
for triosmium (bi)radicals [3]. Importantly, in this case
the use of CCl4 hardly affected the TA spectra. For 1a
the lifetime of the transient slightly decreased to 85 ns,
for 1b it slightly increased to 127 ns. These results
exclude the possible generation of a biradical as the
primary photoproduct. In the case of biradical forma-
tion, the reaction with CCl4 should considerably
shorten the transient lifetime as determined in hexane.
The TA spectra of 1a and 1b in a hexane solution
containing the donor ligand 1-octene again revealed the
same absorption changes. In this case, however, the
lifetime of the transient species was reduced to 48 ns
and 32 ns for 1a and 1b, respectively. In addition, no
recovery of the bleach was observed. This result indi-
cates that the primary photoproducts, 2a and 2b, are
coordinatively unsaturated and readily undergo com-
plexation with 1-octene. The latter adduct is very labile
and undergoes fast fragmentation preventing the back-
reaction to the parent clusters 1a and 1b.

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M.J. Bakker et al. / Inorganica Chimica Acta 300–302 (2000) 597–603
Table 3
Lifetimes and absorption maxima of the primary photoproducts of 1a
and 1b obtained from the nanosecond time-resolved absorption spec-
tra
Cluster
Solvent
~ (ns)
umax (nm)
1a
1a
1a
1b
1b
1b
hexane
1.0 M 1-octene in hexane
CCl4
hexane
1.0 M 1-octene in hexane
CCl4
105
48
85
94
32
632
630
632
636
632
635
127
product towards CCl4 excludes its biradical nature.
Another possibility is a structure analogous to that
postulated for the open-triangle transients [M3(CO)11(m-
CO)] (M=Ru, Os) (see Fig. 6(A)). In such a case,
however, a single IR w(CO) band is expected in the
bridging-CO region, as was observed for the closely
related photoproduct [Ru3(CO)12(m-CO)] [26], see Fig.
6(B), and for the dinuclear species [Os2(CO)7(m-
CO)(ethene)] [18]. The TRIR spectrum of 1b instead
exhibits two apparent small-wavenumber w(CO) bands,
which points to the doubly CO-bridged structure of the
transient [Os3(CO)8(m-CO)2(L)] (2) as depicted schemat-
ically in Fig. 6(C). In this structure, the
(CO)4OsꢀOs(CO)2(diene) bond between Os1 and Os2 is
split and the two osmium centres are connected to-
gether by the bridging carbonyl group, which probably
moved from a terminal position at Os2 in 1. The
resulting electron deficiency at Os2 is partly compen-
sated by donation from the carbonyl ligand bridging
the Os2ꢀOs3 bond. It is important to note that semi-
bridging carbonyls at a cluster core are hardly de-
tectable by IR spectroscopy [27,28]. The primary
photoproduct 2 is still coordinatively unsaturated,
which explains its shorter lifetime in the presence of
added 1-octene (see Table 3).
Fig. 5. TRIR spectrum of 2b, the primary photoproduct of 1b, in
heptane measured 45 ns after laser excitation with 355 nm. For
comparison, the FT IR spectrum of the parent cluster 1b is shown as
a dotted line.
from comparison of the initial and final band intensities
at 2063 and 2024 cm−1. The lifetime of the transient
species 2b was determined by monitoring the intensity
of the w(CO) bands at 1857 and 1801 cm−1 as a
function of time. The calculated value of 68 ns com-
pares reasonably well with the result obtained from the
ns TA spectra of 1b in hexane, considering the quality
of the kinetic traces. The quality of the TRIR spectrum
was not sufficiently high to trace unambiguously w(CO)
bands due to the fragmentation products.
Evaluation of the results of the time-resolved experi-
ments allows one to propose a plausible structure of the
primary photoproduct 2. First of all, its formation
within a few nanoseconds and nearly complete back
reaction to the starting cluster within a few hundred
nanoseconds identifies it as a triosmium cluster, as such
behaviour is very unlikely for products of a photofrag-
mentation process. The inertness of the primary photo-
4. Conclusions
The experimental results document that substitution
of two carbonyls in [Os3(CO)12] by a 1,3-diene ligand
strongly affects the photochemistry of the cluster. The
reaction course is summarized in Scheme 1. The excita-
tion of 1 leads to the short-lived primary photoproduct
2, for which a structure has been proposed on the basis
of the combined spectroscopic data. In the absence of
the donor ligands alkene or CO, 2 mainly reacts back,
but for approximately 30% fragmentation occurs, pro-
ducing [Os(CO)3(diene)] and [Os2(CO)7]. The latter in-
termediate could not be directly detected, as it rapidly
decomposes into [Os3(CO)12] and other, unassigned os-
mium carbonyl products. In contrast to 1, for the
clusters [M3(CO)12] (M=Ru, Os) in hexane, the open-
core primary photoproduct [M3(CO)11(m-CO)] has not
Fig. 6. Proposed structures of the open-core (primary) photoproducts
of: (A) [M3(CO)12] (M=Ru, Os); (B) [Ru3(CO)12] under CO atmo-
sphere; and (C) of the diene cluster 1.

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603
[9] N.E. Leadbeater, J. Lewis, P.R. Raithby, G.P. Ward, Eur. J.
Inorg. Chem. (1998) 1479.
[10] J.W.M. van Outersterp, M.T. Garriga Oostenbrink, H.A.
Nieuwenhuis, D.J. Stufkens, F. Hartl, Inorg. Chem. 34 (1995)
6312.
[11] M. Tachikawa, J.R. Shapley, R.C. Haltiwanger, C.G. Pierpont,
J. Am. Chem. Soc. 98 (1976) 4651.
[12] D. Braga, F. Grepioni, E. Parisini, B.F.G. Johnson, C.M. Mar-
tin, J.G.M. Nairn, J. Lewis, M. Martinelli, J. Chem. Soc.,
Dalton Trans. (1993) 1891.
[13] R. Zoet, J.T.B.H. Jastrzebski, G. van Koten, T. Mahabiersing,
K. Vrieze, Organometallics 7 (1988) 2108.
[14] M.W. George, M. Poliakoff, J.J. Turner, Analyst 119 (1994) 551.
[15] S. Zobl-Ruh, W. von Philipsborn, Helv. Chim. Acta 63 (1980)
773.
been detected so far, being probably much shorter-lived
and prone to rapid fragmentation in the presence of
Lewis base ligands. Apparently, the presence of the
diene ligand stabilizes the primary photoproduct 2 with
regard to the regeneration of the parent cluster. At the
same time, the photofragmentation of the cluster be-
comes competitive. In the presence of alkene the forma-
tion of 2 is completely irreversible. At low tempera-
tures, the fragmentation products [Os2(CO)7(L)2] (L=
CO, alkene) could be observed besides [Os(CO)3-
(diene)].
Acknowledgements
[16] E.G. Bryan, A.L. Burrows, B.F.G. Johnson, J. Lewis, G.M.
Schiavon, J. Organomet. Chem. 129 (1977) C19.
[17] J.R. Moss, W.A.G. Graham, J. Chem. Soc., Dalton Trans.
(1977) 95.
[18] A. Haynes, M. Poliakoff, J.J. Turner, B.R. Bender, J.R. Norton,
J. Organomet. Chem. 383 (1990) 497.
Professors D.J. Stufkens and A. Oskam are acknowl-
edged for many fruitful discussions. L. Boomsma is
thanked for typing part of the manuscript.
[19] M.R. Burke, J. Takats, F.-W. Grevels, J.G.A. Reuvers, J. Am.
Chem. Soc. 105 (1983) 4092.
[20] J.G. Bentsen, M.S. Wrighton, J. Am. Chem. Soc. 109 (1987)
4518.
[21] A.J. Poe¨, C.V. Sekhar, J. Am. Chem. Soc. 108 (1986) 3673.
[22] S. Firth, W.E. Klotzbu¨cher, M. Poliakoff, J.J. Turner, Inorg.
Chem. 26 (1987) 3370.
References
[1] P.C. Ford, J. Organomet. Chem. 383 (1990) 339.
[2] N.E. Leadbeater, J. Chem. Soc., Dalton Trans. (1995) 2923.
[3] J. Nijhoff, M.J. Bakker, F. Hartl, D.J. Stufkens, W.-F. Fu, R.
van Eldik, Inorg. Chem. 37 (1998) 661.
[23] A.J. Dixon, S. Firth, A. Haynes, M. Poliakoff, J.J. Turner, N.M.
Boag, J. Chem. Soc., Dalton Trans. (1988) 1501.
[24] F.-W. Grevels, W.E. Klotzbu¨cher, F. Seils, K. Schaffner, J.
Takats, J. Am. Chem. Soc. 112 (1990) 1995.
[4] J. Nijhoff, M.J. Bakker, F. Hartl, D.J. Stufkens, J. Organomet.
Chem. 572 (1999) 271.
[5] J. Nijhoff, F. Hartl, D.J. Stufkens, Organometallics 18 (1999)
4380.
[25] A.J. Deeming, S. Hasso, J. Organomet. Chem. 114 (1976) 313.
[26] F.-W. Grevels, W.E. Klotzbu¨cher, J. Schrickel, K. Schaffner, J.
Am. Chem. Soc. 116 (1994) 6229.
[6] E.W. Ainscough, A.M. Brodie, R.K. Coll, T.G. Kotch, A.J.
Lees, A.J.A. Mair, J.M. Waters, J. Organomet. Chem. 517
(1996) 173.
[27] C.G. Pierpont, Inorg. Chem. 16 (1977) 636.
[28] W.-Y. Yeh, M.-A. Hsu, S.-M. Peng, G.-H. Lee, Organometallics
18 (1999) 880.
[7] N.E. Leadbeater, J. Organomet. Chem. 573 (1999) 211.
[8] A.J. Arce, A.J. Deeming, Y. De Sanctis, D.M. Speel, A. Di
Trapani, J. Organomet. Chem. 580 (1999) 370.
.
.