Photochemical or electrochemical bond breaking-exploring the chemistry of (μ2-alkyne)Co2(CO)6 complexes using time-resolved infrared spectroscopy, spectro-electrochemical and density functional methods

Article abstract of DOI:10.1039/c9dt03006a

The photochemistry of (μ₂-CRCR′)Co₂(CO)₆ complexes (R = pyrenyl, R′ = H; R = pyrenyl, R′ = ferrocenyl; R = ferrocenyl, R = H) was investigated by ps-time-resolved infrared spectroscopy at room temperature in dichloromethane solution. The main focus of these studies was to determine the primary photoprocess relevant to the light assisted Pauson-Khand reaction. These studies were supported by spectro-electrochemical investigations and density functional calculations which suggest that the primary process to initiate the Pauson-Khand reaction involves a homolytic cleavage of the Co-Co bond forming a high-spin diradical species and not CO-loss as previously thought.

Full text of DOI:10.1039/c9dt03006a

View Article Online
View Journal
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: F. J.R. Cerpentier,
J. Manton, E. Harvey, I. Clark, G. M. Greetham, C. Long and M. Pryce, Dalton Trans., 2019, DOI:
10.1039/C9DT03006A.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
rsc.li/dalton
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
ISSN 0306-0012
COMMUNICATION
Douglas W. Stephen et al.
shall the Royal Society of Chemistry be held responsible for any errors
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH+, X = S, Se, Te)
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
rsc.li/dalton

Please do not adjust margins
Page 1 of 13
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT03006A
ARTICLE
Photochemical or Electrochemical Bond Breaking – Exploring the
Chemistry of (µ2-alkyne)Co2(CO)6 Complexes using Time-Resolved
Infrared Spectroscopy, Spectro-electrochemical and Density
Functional Methods
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Jennifer C. Manton,a Florian J.R. Cerpentier,a Emma C. Harvey,a Ian P. Clark,b Gregory M. Greetham,b Conor Long,a* and Mary
T. Prycea*
The phorochemistry of (2-CRCR’)Co2(CO)6 complexes (R=pyrenyl, R’=H; R=pyrenyl, R’=ferrocenyl; R=ferrocenyl,R=H) was
investigated by ps-time-resolved infrared spectroscopy at room temperature in dichloromethane solution. The main focus
of these studies was to determine the primary photoprocess relevant to the light assisted Pauson-Khand reaction. These
studies were supported by spectro-electrochemical investigations and density functional calculations which suggest that the
primary process to initiate the Pauson-Khand reaction involves a homolytic cleavage of the Co-Co bond forming a high-spin
diradical species and not CO-loss as previously thought.
features: firstly the photochemical reaction occurred under
conditions where the thermal energy is scarce, suggesting that
the photo-expulsion occurs from an excited state which
presents an accelerating potential with respect to CO-loss.
Secondly the resulting 16-electron Cr(CO)5 fragment interacts
with the isolating medium, indicating exceptional reactivity.12
These two features were subsequently confirmed by time-
resolved spectroscopic techniques which showed that
photoinduced CO-loss from Cr(CO)6 occurs very rapidly, on the
Introduction
Soon after the emergence of organometallic chemistry as a
discrete topic in chemistry, photochemical methods were used
to probe the unique properties of this molecular class.1, 2 In
particular the photoinduced heterolytic cleavage of metal-
ligand bonds, for example a metal carbonyl bond, where the
bonding electrons leave with the ligand, provides the
opportunity to investigate the chemistry of highly electron
deficient and reactive metal complex fragments. The newly
developed time-resolved (flash photolysis)3 and low-
temperature techniques such as matrix isolation4 were ideally
suited to study these systems. Homolytic cleavage, of a bond to
produce radical species, is also an important process in many
catalytic systems including polymerisations.5, 6
The Group 6 metal hexacarbonyl complexes, and in particular
Cr(CO)6, emerged as a prototypical system, mainly because of
the exceptionally high quantum yield (CO) for CO loss, and also
the fact that the photochemical reaction was observed even at
very low temperatures (10 K). In addition, the spectroscopic
properties of the photoproducts Cr(CO)5 and molecular CO are
quite distinct from those of the starting complex, making
detection and identification of the photoproduct relatively
unambiguous. The pioneering work of Turner and Perutz7-14
who investigated the low-temperature matrix photochemistry
of Cr(CO)6 in a variety of matrix media revealed two important
timescale of molecular vibrations,
a so-called ultrafast
process.15-18 The Cr(CO)5 photofragment is also very reactive
interacting even with alkanes, hitherto considered chemically
inert.19 Thus a general consensus developed that photoinduced
CO-loss from all homoleptic metal carbonyl complexes are
ultrafast.
More recently the development of ab initio quantum chemical
methods provided a theoretical basis to explain the ultrafast
nature of CO-loss from homoleptic metal carbonyl complexes.
Both wavefunctional20, 21 and Density Functional Theory (DFT)22,
23
were used to model the development of the lowest energy
singlet excited state of Cr(CO)6 along productive vibrational
modes, which explained both the high quantum yield and the
time-scale of the CO-loss processes. Of these methods, DFT is
the more computationally reasonable for most organometallic
systems.
The application of Time-Dependent Density
Functional Theory (TDDFT) to heteroleptic metal carbonyl
complexes such as (6-benzene)Cr(CO)3, revealed “bound”
excited states on the singlet surface.24, 25 These excited states
can be detected using Time-Resolved InfraRed spectroscopy
with pico-second time-resolution (ps-TRIR) as they exist for
many tens of pico-seconds. CO-loss is achieved by traversing a
small thermal barrier, a process called “arrested” CO-loss. The
timescale of CO-loss can therefore range from tens of femto-
a. School of Chemical Sciences, Dublin City University, Dublin 9, Ireland.
b. Central Laser Facility, Science & Technology Facilities Council, Research Complex
at Harwell, Rutherford Appleton Laboratory, Didcot, OX11 0QX, United Kingdom.
†*Corresponding authors Mary Pryce 353 1 7008005 mary.pryce@dcu.ie; conor
long@dcu.ie
Electronic Supplementary Information (ESI) available: 51 Pages including calculated
atomic coordinates See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 2 of 13
ARTICLE
Journal Name
seconds to hundreds of pico-seconds depending on the in Figure 1. In addition, spectro-electrochemical methods have
View Article Online
electronic structure of accessible exited states, and CO-loss can been used to determine the stabilityDoOfI: 1t0h.1e03o9x/Cid9iDzTe0d30a0n6dA
also compete with the formation of electronic excited states.26 reduced complexes.
There are many organometallic systems where the role of light
in promoting chemical process remains poorly understood. One
of these is the Pauson-Khand Reaction (PKR) which involves a
[2+2+1] cycloaddition of an alkene, an alkyne and CO to form a
cyclopentenone.27 It is generally accepted that the reaction
mechanism involves the initial reaction of the reagent alkyne
with Co2(CO)8, to form (µ2-alkyne)Co2(CO)6. The Co2(CO)6
fragment supplies the CO required in the subsequent cyclisation
reaction. Typically the PKR is initiated thermally,28 however
some examples of visible light/thermal or light-assisted
reactions are known in which the reaction mixture is irradiated
with little regard to which step in the overall reaction is
Figure 1. The (μ2-R,R’-alkyne)Co2(CO)6 complexes in this investigation, 1 R=pyrenyl,R’=H;
2 R=pyrenyl, R’=ferrocenyl; 3 R=ferrocenyl; R’=H
promoted by photon absorption. There are few reports of
purely light-driven PKR.29 An intriguing feature of these
investigations, is the fact that experiments have revealed little
about the chemical or indeed photochemical behaviour of (µ2-
Results
As a starting point, the UV/visible characteristics of the
unsubstituted (μ2-C2H2)Co2(CO)6 complex was modelled using
DFT and TDDFT methods. Dichloromethane (DCM) was selected
as solvent for most calculations.47 The lowest energy, and very
weak transition centred at 500 nm, consists mainly of a cobalt-
centred  to * transition of the Co-Co bond with minor
contributions from a Co to CO(axial) charge-transfer, forming
the lowest energy singlet Excited State (ES1).33, 48 By following
this excited state along the axial CO-loss reaction coordinate it
is clear that ES1 is bound with respect to this reaction (see
supporting information Figure SI1). The second singlet excited
state (ES2) is mainly ligand-field in character and is unbound
with respect to axial CO loss, but this excited state cannot be
36 directly populated from the ground-state as the transition is
formally symmetry forbidden. The more intense absorption at
350 nm (ES8) is also mainly Co-based but with greater
contributions from a Co to alkyne charge-transfer transition
which has the effect of increasing the oscillator strength. This
excited state is bound with respect to axial CO loss but offers
the possibility of crossing to the lower ligand field state in a non-
adiabatic description which can result in axial CO-loss. This
process will be ultrafast as it is achieved by population of an
excited state which presents an accelerating energy profile
towards CO-loss but the efficiency of the process is low because
alkyne)Co2(CO)6 complexes. Once the (µ2-alkyne)Co2(CO)6
complex is formed it proceeds to product without trace of
observable intermediates apart from one example where a
pendant alkene is present on the alkyne ligand.30, 31 Matrix
isolation experiments indicated that CO-loss occurs upon
photolysis but at photon energies far in excess of those used in
the light-assisted PKR.32, 33 Light activation of the PKR can be
useful as it facilitates the synthesis of thermally sensitive
cyclopentenones.34 Consequently a better understanding of
the electronic structure of the (2-alkyne)Co2(CO)6 complexes
and accessible excited states is important.
Simple wave functional methods such as Hartree-Fock
calculations cannot accurately model the ground state
electronic structure of (2-alkyne)Co2(CO)6 complexes.35,
Multi-configuration methods suggest that the ground state of
(2-alkyne)Co2(CO)6 complexes are best described as singlet (or
low-spin) diradical species with less than optimal overlap of the
cobalt d orbitals involved in the Co-Co bond. This results in a
so-called “bent” Co-Co bond where the volume of maximum
electron density does not lie on the vector joining the two
cobalt nuclei. However multi-configuration calculations are
computationally expensive, and their use quickly becomes
unrealistic when many calculations are required to model
excited state behaviour along specific reaction coordinates.
38
of crossing to lower energy bound excited states in particular
ES1. Examination of the energetics of equatorial CO-loss
confirmed that no optically accessible excited states leads to
this reaction. This conclusion is consistent with published
results where only axial CO loss was observed following
photolysis.32, 33
The effect of varying the Co-Co distance on the excited state
energies of (2-HCCH)Co2(CO)6 was then calculated to
determine if population of the accessible singlet excited states
result in a lengthening of the Co-Co bond. The results of these
calculations are presented in Figure 2 and show that population
of the lowest energy singlet state will tend to lengthen the
Cobalt-Cobalt distance to 2.56397 Å (downward arrow in Figure
2(a)).
Consequently both Density Functional (BLYP)37,
or Hybrid
Density Functional (B3LYP)39 methods can be used to provide
reasonable models for the bonding in this complex at moderate
computational cost. These methods when coupled with Time
Dependent (TD)40-46 calculations can then model the dynamic
behaviour of the ground and excited states along chosen
reaction coordinates. Energy profiles which steadily reduce are
described as “unbound” states with respect to the chosen
reaction coordinate, while those exhibiting a local minimum are
dubbed “bound” states.
In addition to quantum chemical calculations, we have used
time-resolved spectroscopic techniques to gain
a fuller
understanding of the mechanism of light-assisted PKR. Three
alkyne-substituted complexes were synthesized as represented
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 13
Dalton Transactions
Please do not adjust margins
Journal Name
ARTICLE
The experimental and simulated UV/visible spectra of the This is reassuring, as it confirms that the constituent transitions,
View Article Online
DOI: 10.1039/C9DT03006A
alkyne-substituted complexes which are the subject of this derived from the TDDFT calculations, are realistic descriptions
study were then investigated. The UV/visible spectra of 1, 2, of the optically accessible excited states.
and 3 complexes are presented as Figure SI2 with extinction
parameters in Table SI1 of the supporting information.
Figure 2. (a) The adiabatic behaviour of the five lowest energy singlet excited states
Figure 4. (a) The behaviour of ES1 to ES5 for (μ2-1-pyrenylethyne)Co2(CO)6 (1) along the
relative to the singlet ground-state energy (blue) as the Co-Co distance is varied in (2-
Cobalt-Cobalt stretch reaction coordinate showing the non-bonding character of all low-
HCCH)Co2(CO)6. The data interval is indicated by the data markers on the singlet ground-
lying singlet excited states; (b) the behaviour of the five lowest energy singlet excited
state profile but are omitted from the excited state profiles for clarity, (b) The adiabatic
states with respect to the axial Co-CO bond length, the dashed curve represents the non-
behaviour of the lowest energy triplet excited state (red) and the singlet ground state
adiabatic description showing the crossing to lower states, with the branching region
highlighted with a circle.
(blue) indicating the accelerating profile of the lowest energy triplet excited state
towards a high spin di-radical species (red plot).
The lowest energy absorption at approximately 590 nm is
predominantly pyrene to cobalt charge-transfer in character
with some electron density moving to the carbonyl ligands but
also with substantial  to * character. A similar description
Both pyrenyl-substituted complexes (1 and 2) exhibit an
absorbance maximum close to 400 nm, the excitation
wavelength used in the ps-TRIR experiments. In the case of 1
there is excellent agreement between the simulated and
applies to the absorption at 433 nm. The intense feature at 387
experimental spectrum (Figure 3) particularly in terms of the
nm is mainly a ligand-field transition which is non-bonding with
lowest energy absorption maximum.
respect to the Co-Co interaction (Figure 4(a)), but ES3, ES4 and
ES5 singlet excited states present an accelerating profile
towards axial CO loss (Figure 4(b)). This process must compete
with crossings to lower energy bound states reducing the
efficiency of the CO-loss process (see dashed curve in Figure
4(b)). These calculations suggest that population of, for
example, ES5 will result in ultrafast axial CO-loss but will also
populate lower energy excited states.
Turning to the ferrocenyl-substituted derivative (3), the
simulated UV/vis spectrum is compared to the experimental
spectrum in Figure 5. Again, the simulated spectrum agrees
well with the experimental one despite the weakness of the
features in the visible region. The lowest energy transitions can
be classified as ferrocenyl to cobalt carbonyl charge-transfer in
nature and have small oscillator strengths, i.e. they are very
weak. Plotting the behaviour of these states along the Co-CO
(axial) reaction coordinate, (supporting information Figure SI3)
confirmed that these states are bound with respect to axial CO-
loss. It is only when the 10th excited state is reached that an
accelerating profile towards CO-loss is predicted. This state
Figure 3. The simulated (dashed) and experimental (solid line) spectra of
1 in
dichloromethane (1.2  10-2 M) with the principal vertical transition energies and
associated electron density difference maps for the resulting excited states, blue
volumes indicating regions where the electron density is lower in the excited state
compared to the ground-state, and red volumes regions where the electron density is
greater in the excited state compared to the ground state, the inset is a ball-and-stick
diagram for 1 showing the orientation of the molecule
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 4 of 13
ARTICLE
Journal Name
must cross nine lower energy states in progressing to CO-loss approximately 14% of the initial bleach. In addition, persistent
View Article Online
-1
DOI: 10.1039/C9DT03006A
and each of these lower energy states are bound with respect (> 500 ps) features are evident at 1975, 2005, and 2071 cm and
to CO-loss. Consequently, the ferrocenyl derivative is not a a further band at 2028 cm-1 is implied because the intensities of
good candidate for ultrafast CO loss and this reaction is likely to the bleach signals do not correspond to the CO band intensities
be very inefficient. This is very surprising given the observation of the parent. These new features are indicated by asterisks in
of high yielding visible light induced PKR when 3 is irradiated in the inset in Figure 6, and agree well with the calculated CO
the presence of excess norbornene. This calls into question positions (B3LYP/6-31G(d)) of the CO-loss species, (μ2-
whether the proposal that photoinduced CO-loss is the primary pyrenylethyne)Co2(CO)5 (the red spectrum) and the band
step in the PKR is correct.29
positions observed in matrix isolation studies for the initially
produced axial CO-loss species in related systems.32, 33 The ps-
TRIR data were analysed using a global analysis approach, and
this confirmed the presence of the CO-loss features in the early
timescale data (blue spectrum in the insert of Figure 6).
Excitation of 1 at 400 nm produces both a hot ground-state
species with a lifetime of 40 ps and a CO-loss species formed in
an ultrafast process.
Figure 5. A comparison of the simulated and experimental UV/vis spectrum of (μ2-1-
ferrocenylethyne)Co2(CO)6 (3) (1.6  10-2 M) showing the weak visible absorptions at 580
(yellow transition) and 480 nm (green transition) which are mainly iron to Co2(CO)6
charge transfer in character and a mainly Cobalt-based ligand field transition at 380 nm
(red transition) corresponding to population of ES10.
ps-TRIR studies
Quantum chemical calculations described above indicated that
the most likely system to observe photoinduced CO loss was the
pyrene substituted complex 1. This complex absorbs strongly
Figure 6. The ps-TRIR spectra recorded following pulsed photolysis of 1 (exc. = 400 nm)
in hexane solution at 19C, spectra were recorded at delay times of 7.64, 11.8, 16.22,
25.98, 37.34, 50.9, 67.72, 89.94, 104.42, 122.68, 147.46, 186.16, 280.0 and 500.0 ps after
the excitation pulse with arrows indicating the time-dependent behaviour of the
features, and the insert shows the spectrum obtained at 500 ps with an expanded 
absorbance scale with persistent features marked with asterisks at 1975, 2005, ca 2028,
and 2071 cm-1 (black spectrum in insert), the lower red spectrum, in both the main plot
and in the inset is the calculated spectrum at the B3LYP/6-31g(d) level of 1 and the CO-
loss species (μ2-pyrenylethyne)Co2(CO)5 respectively, while the blue spectrum was
extracted by global analysis of the time-dependent data (see text).
at excitation wavelength used in the ps-TRIR apparatus (exc.
=
400 nm) and progression of the accessible excited states (Figure
4(b)) would lead, at least in part, to ultrafast CO loss. The
spectral changes observed in the CO region following pulsed
photolysis of 1 in hexane solution at 19C are presented in
Figure 6 (IR spectral features are presented in the supporting
information Table SI2). These are presented as difference
spectra ( absorbance) where the negative absorbances
indicate bleaching of the parent hexacarbonyl absorptions
following excitation. Product bands are evident as positive
absorbances, and the spectrum measured within 7.64 ps of the
excitation pulse exhibits three broad features, each
approximately 10 cm-1 to the low energy side of each parent
band. Their broadness, relative to the width of the parent
bands, suggesting excess vibrational energy which is dissipated
in approximately 10 ps. The features decay over the
subsequent 40 ps with concomitant recovery of the parent
absorptions, as indicated by the arrows in Figure 6. It is
reasonable to assign the three broad product features to a
“hot” species but whether this species is electronically excited
or on the ground-state will be discussed later. A residual bleach
of the parent features persists for over 1 ns, and the largest
residual bleach is observed for the 2053 cm-1 feature at
A ps-TRIR study of the spectral changes observed following 400
nm excitation of 3 showed similar features to those obtained
for 1, however features which could be associated with the CO-
loss product are considerably weaker (Figure 7). The residual
depletion of the 2050 cm-1 band, measured 500 ps after the
excitation pulse, is approximately 5% of the initial depletion.
This represents the maximum yield of any CO-loss species. It is
clear therefore that photoinduced CO-loss from 3 is less
efficient than in the case of 1. Global analysis of the of the
spectral data revealed a long-lived species which is formed
within the excitation pulse (blue spectrum in the inset in Figure
7) which has characteristics like the CO-loss species obtained
following photolysis of 1, for example the band indicated with
an asterisk. However, the global analysis indicated that the
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 13
Dalton Transactions
Please do not adjust margins
Journal Name
ARTICLE
lifetime of this species was considerably shorter than the Spectro electrochemistry with monitoring in botVhiewthAerticIlRe Oanlninde
DOI: 10.1039/C9DT03006A
lifetime of the CO-loss species obtained from species 1 (220 ps UV-Vis regions was used to detect products that may form
vs 1200 ps supporting information Figure SI3). In addition, the following either reduction or oxidation of the complexes. These
calculated IR spectrum of the CO-loss species at its optimum studies were undertaken to gauge the stability of these
geometry (red spectrum in the inset in Figure 7) shows the complexes with respect to a one electron change to the overall
presence of a bridging carbonyl band just below 1900 cm-1 configuration.
The studies were performed at applied
which is not observed in the ps-TRIR data. The green spectrum potentials of -1.5 V (vs Ag/Ag+) for complexes 1, 2, and 3 where
in the inset in Figure 7 shows the calculated spectrum of the CO- the radical anion is formed. Two oxidising potentials were
loss species at a local minimum on the potential energy applied, a 1 V potential for complexes 2 and 3 to observe the
hypersurface close to the initially produced CO-loss species, and oxidation of the ferrocenyl substituent, and at 1.7 V for 1, 2, and
this spectrum correlates well with both the final spectrum 3 where the radical cations based on the cobalt centres are
obtained in the ps-TRIR experiment (500 ps after excitation, formed.
black spectrum) and the initially produced component
extracted from the global analysis treatment of the ps-TRIR data
IR Spectro-electrochemistry
(blue spectrum). The features indicated with asterisks are When a potential of -1.5 V was applied to solutions of
common to all three spectra. This suggests that the initially complexes 1, 2, or 3, a decrease in the absorbance of the all
produced CO-loss species may then isomerise over the cobalt-carbonyl CO bands (down arrows in Figure 8(a)) was
subsequent 200 ps but it is not clear from the ps-TRIR data what observed. A broad feature at 1900 cm-1 is produced indicating
-
is the ultimate product of this isomerisation.
the formation of Co(CO)4 as the only observable metal carbonyl
fragment.49 Evidence for the release of the alkyne ligand was
obtained by monitoring in the UV/vis region (Figure 8(b)) where
the structured features of the free ligand are produced (up
arrows).
Figure 7. The ps-TRIR spectra recorded following pulsed photolysis of 3 (exc. = 400 nm)
in hexane solution at 19C, spectra were recorded at delay times of 7.64, 11.8, 16.22,
20.92, 31.42, 43.78, 67.72, 89.94, 122.68, 186.16, and 500 ps after the excitation pulse
with arrows indicating the time-dependent behaviour of the features, the insert shows
the final spectrum obtained 500 ps after the excitation pulse (black) the decay adjusted
spectrum of the long-lived species (blue) and the calculated spectrum (red) of the
optimized CO-loss species and (green) the calculated spectrum of the initially produced
CO-loss species.
The calculated structure indicates an asymmetric coordination
of the alkyne ligand to the two Co centres. Bitterwolf and co-
workers suggested a structural rearrangement of the carbonyl
ligands from the initially produced axial CO-loss species to the
equatorial CO-loss to explain the spectroscopic changes
observed upon annealing a Nujol matrix to 140 K.33 It is clear
from the global analysis data that the axial CO-loss species
produced from 1 and especially 3 are labile (Figure SI4).
However, in no case did the calculations indicate the formation
of a CO-loss photoproduct in which the vacant coordination site
is in the equatorial position. The dominant product following
400 nm photolysis of 3 is a vibrationally excited species and that
the very low yield of the CO-loss species could only have a minor
role in promoting the PKR.
Figure 8. (a) the spectral changes observed in the IR region following reduction (-1.5 V)
of 2 in DCM showing the loss of metal carbonyl features (down arrows) and formation of
Co(CO)4- (* up arrow); (b) The UV/vis absorption changes observed following reduction
(-1.5 V) of 2 in DCM showing the decay of 2 (down arrows) and the liberation of
uncoordinated pyrene ligand (up arrows)
At an applied potential of 1 V, 1 appeared to be stable because
this potential was insufficient to form the cobalt-centred radical
cation. In the case of complexes 2 or 3, both of which contain
Spectro-electrochemical Studies
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 6 of 13
ARTICLE
Journal Name
a ferrocene moiety, a decrease in the parent bands in the This process is ultrafast but very inefficient. In adVdieitwioArnti,clge lOonblinael
carbonyl region was observed with concomitant formation of analysis of the ps-TRIR data suggests thDatOIt:h1e0.1C0O39-/lCo9ssDTs0p3e0c0i6eAs
new bands at 2097, 2065, and circa 2040 cm-1 for 2 and 2033 are themselves labile. Photolysis of 3 at 400 nm also produced
and 2002 cm-1 for 3 (Figure 9) The product bands appear at the axial CO-loss product but with a greatly reduced efficiency
slightly higher wavenumbers to those of the parent cobalt- even compared to 1 or 2. This contrasts with the observation
carbonyl complexes and these new stretching vibrations result that 3, in particular, catalyses the PKR when irradiated with
from the oxidation of the ferrocenyl substituent (FeII/FeIII).
visible light.29 This would suggest, contrary to the common
belief, that CO-loss is not the primary process in the photo-
assisted PKR. The spectro-electrochemical studies indicate that
a single electron reduction of all complexes in this study is
sufficient to decompose the di-cobalt complexes, producing
uncoordinated alkyne ligand and a rupture of the Co-Co bond.
Clearly a relatively minor change to the electron configuration
can have a dramatic effect on the overall stability of these
complexes particularly with respect to weakening of the Co-Co
bond. The ps-TRIR studies indicate that the dominant
photochemical pathway following photolysis of 1 at 400 nm is
the formation of a vibrationally hot species as indicated by the
broadness of the CO bands (30 cm-1 Full Width at Half
Maximum) when compared to the parent (7 cm-1 FWHM). The
nature of this species is worthy of discussion, in particular
whether it is an electronically excited or a ground state species.
Time-resolved investigations by Harris and co-workers,50 using
both IR pump;IR probe and UV pump;IR probe methods on a
related diiron system, (2-benzene-1,2-dithiolate)Fe2(CO)6,
produced similar transient features to those observed here. It
was concluded that the transient IR features were the result of
processes on the electronic ground state. In common with our
results, the initially produced broad IR features relaxed to the
parent in two stages, the first a cooling process where excess
energy is dissipated to the solvent occurs quickly, within 20 ps.
A second slower process involving the vibrational relaxation of
the isolated metal carbonyl modes takes place over 80 to 300
ps. With our systems, and using only UV-pump;IR-probe, we
Figure 9. The IR spectral changes following electrolysis at 1V for (a) 1 and (b) 2 in DCM also observed two relaxation processes that occur on similar, if
solution
slightly shorter, timescales of 10 ps and 57 ps. We propose a
similar explanation for the faster decay process, i.e. vibrational
It would appear, at least qualitatively, that these complexes are
more stable with respect to oxidation than reduction.
Reduction of all complexes results in ligand loss and the
formation of inorganic cobalt species and free alkyne ligand.
The structure and reactivity of the singly reduced 1 (i.e. 1•-)
were modelled. In the reduced species the Co-Co bond length
is greater than in the parent (2.755 and 2.442 Å respectively).
There is also evidence for a slight asymmetric binding of the
alkyne ligand to the Co, Co-C1 2.01617 compared to Co-C2
1.99427 Å. Several possible reaction coordinates were selected
to model decomposition routes for this complex and the energy
changes along these reaction coordinates are presented in the
supporting information Figure SI5. The Co-Co bond stretching
is the lowest energy pathway which perhaps explains the
cooling by dissipation of thermal energy to the solvent.
However, we can offer an alternative explanation for the slower
process namely the formation of a ground-state species in
which the Co-Co bond distance is longer than in the parent,
possibly a high-spin diradical species (Figure 2(b). Such a
proposal is not without precedent; a transient lengthening of
the Fe-Fe bond in (μ2-S2C3H6)Fe2(CO)6 was proposed following
visible light excitation.51
The fundamental problem remains, how does visible light
absorption promote the PKR? By populating the lowest energy
excited state either directly or through internal conversion from
higher optically accessible excited states, the Co-Co bond length
increases. This provides a pathway to move from the initially
formed low-spin diradical excited state to a high-spin diradical
such as that described in Figure 2(b). This would have a lifetime
-
formation of the Co(CO)4 decomposition fragment.
sufficiently long to engage in bimolecular reactions with alkene
53
substrates.52,
To confirm that a high spin diradical is
Discussion
consistent with the spectral changes in the IR, the ps-TRIR
spectrum was obtained 90 ps after 532 nm excitation of 1 in
hexane solution. This spectrum is presented in Figure 10 which
The ps-TRIR studies supported by TDDFT calculations on 1 or 2
showed that axial CO-loss occurs upon photolysis at 400 nm.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 13
Dalton Transactions
Please do not adjust margins
Journal Name
ARTICLE
compares the experimental spectrum to the calculated mixture defined at zero kJ mol-1. These results areVpierweAsreticnleteOdnlinine
DOI: 10.1039/C9DT03006A
difference spectrum obtained by subtracting the simulated Figure 11.
spectrum of the parent complex from that of the high spin
diradical.
Despite the significant overlap of features,
particularly the middle feature, the 90 ps difference spectrum is
an excellent fit to the simulated spectrum. It is worth noting
also, that the broadness of the low energy feature is the result
of the overlap of four fundamental modes and not an indication
of vibrational relaxation. The symmetry of the high-spin
diradical is reduced to C1 which exhibits in six CO bands only
two of which (the two high energy bands) are fully resolved.
Figure 10. The experimental difference spectrum obtained 78 ps after 532 nm excitation
of 1 in hexane (red) and the simulated difference spectrum obtained by subtracting the
parent spectrum from that of the high-spin diradical species (assuming 90% conversion)
(blue)
Scheme. The proposed reaction scheme for the light assisted Pauson-Khand reaction
(metal carbonyl ligands are represented as simple lines for clarity), the Co to Co
interaction indicated with
a single bond for low-spin and two radicals high-spin
configurations, the shading indicates the principal intermediate species in the catalytic
cycle.
We can propose a slight modification to the accepted catalytic
cycle for the thermal PKR to explain the visible light promoted
PKR.30, 54 This scheme does not involve photoinduced CO-loss
as the primary step (Scheme)
The initial photochemical step involving cleavage of the Co-Co
bond forming the high-spin diradical. This is followed by an
insertion of the alkene into one Co-alkyne bond. The reaction
then proceeds in the conventional manner involving CO
insertion uptake of CO, release of the product cyclopentenone
and regeneration of the di-cobalt alkyne complex. It is
noteworthy that this scheme involves insertion of CO into a CO-
alkyl bond from a coordinatively saturated cobalt centre. The
inserted CO originates on the original Co2(CO)6 fragment. This
is consistent with mass spectrometry experiments using 13CO
enrichment which confirmed that the carbonyl unit in the
resulting cyclopentenone originated in the Co2(CO)6 unit.55
Notwithstanding the primary focus of this work to elucidate the
photochemical initiation of the PKR, at the suggestion of a
reviewer, the proposed catalytic cycle in the Scheme was
modelled using B3LYP/6-31G(d) in acetonitrile. The species
modelled were the alkene insertion, CO-insertion, and
cyclopentenone formation when bound to a Co2(CO)6 fragment
starting with complex 1 in the presence of prop-1-ene and CO.
Both high-spin and low-spin configurations were examined, and
their energies expressed relative to the energy of the starting
Figure 11. The energies of 1 (low-spin blue; high-spin red) and the alkene-insertion, CO-
insertion and cyclopentenone species as indicated in the Scheme, showing an energy
cascade from the high-spin starting complex through to the cyclopentanone product
species, the shading colour indicates the corresponding intermediate steps in the
Scheme.
These calculations indicate that once the high-spin species is
produced in the initiation step, the insertion reactions and the
product formation occur in
a cascade of energetically
favourable intermediates. No activation parameters were
calculated for these steps, as these will be the subject of future
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 8 of 13
ARTICLE
Journal Name
work. These results also suggest that the displacement of the molecular geometry at a convenient model chemistry (in terms
View Article Online
DOI: 10.1039/C9DT03006A
cyclopentenone from the coordination sphere of the cobalt is of computational cost) in order to simulate the infrared
likely to be the rate determining step in the catalytic cycle, and absorptions particularly in the CO region. In these calculations
that the relaxation from the high-spin to low-spin surface the ground state structure was optimized to tight convergence
happens at the CO-insertion step. A full description of the limits using a pruned (99,590) grid. Both the BLYP and B3LYP
structures and energies of these intermediates is available in functionals were used with a 6-311G basis set,61, 62 augmented
the supporting information as .mol files.
with diffuse and polarisation functions (6-311++G(3df,3pd)).63,
64
Geometry optimizations using unrestricted BLYP or B3LYP
functionals yielded ground state singlet species with no
Experimental
evidence for significant spin contamination.
For both
functionals, the lowest energy triplet state occurs at higher
energy than the singlet at the optimized geometry of the lowest
energy singlet state (1.82 eV for BLYP and 1.58 eV for B3LYP).
Both values are smaller than the singlet-triplet energy gap of
2.284 eV obtained from the computationally more expensive
complete active space calculations using a molecular geometry
derived from X-ray diffraction data. The optimized Co-Co
distance was 2.551 Å for BLYP and 2.488 Å for B3LYP (a value of
2.442 Å was obtained using the B3LYP functional and the
smaller 6-31G(d) basis set). These compare to the Co-Co
distance of 2.463 Å in (2-HCCC6H10OH)Co2(CO)665 36 or 2.476 Å
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenk techniques. Mobile phases for
column chromatography were dried over MgSO4 before use. All
solutions were deoxygenated by purging with nitrogen for ~10
min. Column chromatography was carried out using neutral
silica gel. Silica Gel (Merck) was used as received. 1-
bromopyrene, 1-ethynylferrocene, TMS acetylene, and
cobaltcarbonyl (Co2(CO)8) were obtained from Sigma-Aldrich
and used without further purification. IR spectra were recorded
on a Perkin-Elmer 2000 FTIR spectrophotometer (2 cm-1
resolution) in a 0.1 mm pathlength cell with NaCl plates using
spectrophotometric grade DCM.56-58 1H NMR spectra were
recorded using a Brüker AC 400 spectrophotometer in CDCl3
and were calibrated by the residual proton resonances of the
solvent peak. UV spectra were measured using an Agilent 8453
UV-Vis spectrophotometer in a 1 cm quartz cell using
spectrophotometric grade DCM. ps-TRIR photolysis was carried
out with λexc 400 nm , and τFWHM = 150 fs using the ULTRA facility
at the Rutherford Applelton Laboratory. Details of the
experimental setup have been presented elsewhere.59
Spectrophotometric grade DCM or n-hexane were used for
these experiments. The photochemical quantum yields for CO-
loss were determined using potassium ferrioxalate actinometry
at irradiation wavelengths of 313, 365, 405 or 546 nm in
spectrophotometric grade n-heptane (see supporting
information Table SI3). Triphenylphosphine, present in excess,
was used as the trapping ligand for the pentacarbonyl
photoproduct.
66
in (2-C6H5CCC6H5)Co2(CO)6 both values were obtained by
single crystal X-ray diffraction methods. Because of the
respectable performance of the B3LYP functional in modelling
the Co-Co distance coupled with the smaller 6-31G(d) basis set
this model chemistry was selected to probe the ground and
excited-state behaviour of all the complexes in this study.
Complete descriptions of the optimised structures are available
as .mol files in the supporting information.
Syntheses.
All the ligands used in this study were prepared using the
Sonogashira coupling reaction,67 using the following method. A
25 cm3 aliquot of di-isopropylamine was purged with nitrogen
for 10 minutes, following which 1.77 mmol of the appropriate
bromoalkane was added and the solution was purged with
nitrogen for a further 10 minutes. Pd(PPh3)2Cl2 0.071 mmol
(4%), CuI 0.071 mmol (4%) and PPh3 0.142 mmol (8%) and the
required terminal alkyne 2.66 mmol were then added and the
solution was brought to its reflux temperature (88°C) for 8
hours under a nitrogen atmosphere. The solvent was then
removed under reduced pressure. The product was extracted
into a mixture of DCM:hexane (1 : 5 v/v). The resulting solution
was dried over anhydrous MgSO4, filtered, and the solvent
removed under reduced pressure. The product was purified
using column chromatography on silica using n-hexane:DCM,
25:75 (v/v) as the mobile phase.
All spectro-electrochemical measurements were performed at
room temperature. Solutions were prepared under a nitrogen
flow with a freshly prepared 0.1 M tetrabutylammonium
hexafluorophosphate solution in either dichloromethane or
acetonitrile. A 1 mm omni-cell (Specac) was used equipped with
a platinum mesh working electrode, platinum mesh counter
electrode and silver wire quasi-reference electrode. All
spectroelectric experiments were conducted on a CHI 600E (CH
Instruments) potentiostat. IR measurements used a Spectrum
65 FT-IR spectrometer (Perkin Elmer), while UV-Vis experiments
measurements used an Agilent 8453 UV-Visible spectrometer.
Pyrene-ethynylferrocene. Yield: 96%. 1H NMR: (400 MHz,
CDCl3) 8.62 ppm (d, J= 9.2 Hz, 1H), 7.9 ppm (m, 8H), 4.665 ppm
(dd, Ja= 1.6 Hz, Jb= 1.6 Hz , 2H), 4.33 ppm (s, 7H).68
Removal of the TMS protection groups. 20 cm3 of freshly
distilled methanol was purged with nitrogen for 10 min and 1.3
mmol of 1-trimethylsilylacetylene was then added followed by
0.17 mmol of K2CO3. The solution was stirred under nitrogen
for 4 hr and the solvent was then removed under reduced
Computational Studies
All quantum calculations were conducted using the Gaussian 16
suite of programmes (Revision B.01).60 The computational
studies initially focussed on the simplest alkyne complex (2-
C2H2)Co2(CO)6 before attempting to model the larger pyrenyl
and ferrocenyl substituted derivatives. Our initial calculations
on (2-C2H2)Co2(CO)6 were centred on obtaining an accurate
pressure.
The residue was dissolved in 100 cm3 of
dichloromethane and washed with 4  25 cm3 of 5% (w/v)
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 13
Dalton Transactions
Please do not adjust margins
Journal Name
ARTICLE
aqueous NaHCO3. The organic phase was dried over anhydrous derivative, led to the conclusion that CO-loss wasVnieowtArrteicqleuOinrleinde
DOI: 10.1039/C9DT03006A
MgSO4. The solvent was removed. The product was purified by to promote the reaction.
chromatography using silica and hexane: DCM, 25:75 (v/v) as
the mobile phase.69
1-ethynylpyrene. Yield: 70%. 1H NMR: (400 MHz, CDCl3), 8.5
ppm (d, J = 8.8 Hz, 1H), 8.15 ppm (m, 8H), 3.6 ppm (s, 1H).
Synthesis of (µ2-alkyne)Co2(CO)6 complexes. 25 cm3 of hexane
Conflicts of interest
There are no conflicts to declare.
was purged with N2 for 10 min and 0.45 mmol of the alkyne
ligand was then added and solution was purged with N2 for a
further 10 min. 0.9 mmol of cobaltcarbonyl was added to the
Acknowledgements
reaction mixture and the vessel was sealed. The reaction
The authors gratefully acknowledge funding provided by
mixture was allowed to stir for 8 hr under N2. The solvent was
Environmental Protection Agency (2008-ET-MS-3-52) and the
removed and the pure product was obtained by column
Sustainable Energy Authority of Ireland (18/RDD/282). The
chromatography using a silica solid phase and DCM: hexane, 1:1
authors also thank the STFC Central Laser Facility for granting
as the solvent.
access to the ULTRA system under EU Access Grant No.92004.
(µ2-1-ethynylpyrene)Co2(CO)6 (1. Yield: 78%. 1H NMR: (400
The authors wish to acknowledge the DJEI/DES/SFI/HEA Irish
MHz, CDCl3), 8.55 ppm (d, J= 9.2 Hz 1H), 8.15 ppm (m, 8H), 6.95
Centre for High-End Computing (ICHEC) for the provision of
ppm (s, 1H). UV-Vis: (DCM) 274 nm (20.5 x 103 M-1 cm-1), 392
computational facilities and support.
nm (19.1 x 103 M-1 cm-1), 577 nm (1.1 x 103 M-1 cm-1). IR: (DCM)
2092, 2056, 2030 cm-1.
1
(µ2-Pyrene-ethynylferrocene)Co2(CO)6 (2). Yield: 59%. H NMR:
References
(400 MHz, CDCl3), 8.7 ppm (d, J= 8.08 Hz, 1H), 8.24 ppm (m, 8H),
4.6 ppm (dd, Ja= 1.84 Hz, Jb= 1.84 Hz, 2H), 4.48 ppm (dd, Ja= 1.84
1.
2.
P. L. Pauson, J. Am. Chem. Soc., 1954, 76, 2187-2191.
T. E. Bitterwolf, J. Organomet. Chem., 2004, 689, 3939-
3952.
R. G. W. Norrish and G. Porter, Nature, 1949, 164, 658-658.
E. Whittle, D. A. Dows and G. C. Pimentel, J. Chem. Phys.,
1954, 22, 1943-1943.
L. E. N. Allan, M. R. Perry and M. P. Shaver, Prog. Polym.
Sci., 2012, 37, 127-156.
M. Hurtgen, C. Detrembleur, C. Jerome and A. Debuigne,
Polymer Reviews, 2011, 51, 188-213.
J. K. Burdett, J. M. Grzybowski, R. N. Perutz, M. Poliakoff, J.
J. Turner and R. F. Turner, Inorg. Chem., 1978, 17, 147-154.
J. J. Turner, J. K. Burdett, R. N. Perutz and M. Poliakoff, Pure
Appl. Chem., 1977, 49, 271-285.
Hz, Jb= 1.84 Hz 2H), 4.32 ppm (s, 5H). UV-Vis: (DCM) 279 nm
(18.7 x 103 M-1 cm-1), 389 nm (11.7 x 103 M-1 cm-1), 574 nm (1.3
x 103 M-1 cm-1). IR: (DCM) 2085, 2048, 2023 cm-1.
3.
4.
1
(µ2-1-ethynylferrocene)Co2(CO)6 (3). Yield: 98%, H NMR: (400
MHz, CDCl3), 6.28 ppm (s, 1H), 4.33 ppm (d, Ja= 1.84 Hz, Jb= 1.84
Hz 4H), 4.15 ppm (s, 5H). UV-Vis: (DCM) 290 nm (17.1 x 103 M-
1 cm-1), 361 nm (4.4 x 103 M-1 cm-1), 430 nm (1.8 x 103 M-1 cm-1),
614 nm (1.1 x 103 M-1 cm-1). IR: (DCM) 2090, 2051, 2023 cm-1.70
Complexes 1, 2 and 3 have been synthesised previously and all
spectroscopic data obtained is in agreement with previous
reports.29, 71
5.
6.
7.
8.
9.
J. K. Burdett, R. N. Perutz, M. Poliakoff and J. J. Turner, J.
Chem. Soc., Chem. Commun., 1975, 157-159.
J. K. Burdett, M. A. Graham, R. N. Perutz, M. Poliakoff, A. J.
Rest, J. J. Turner and R. F. Turner, J. Am. Chem. Soc., 1975,
97, 4805-4808.
J. K. Burdett, R. N. Perutz, M. Poliakoff and J. J. Turner,
Inorg. Chem., 1976, 15, 1245-1245.
R. N. Perutz and J. J. Turner, J. Am. Chem. Soc., 1975, 97,
4791-4800.
R. N. Perutz and J. J. Turner, J. Chem. Soc., Faraday Trans.,
1973, 452-461.
M. A. Graham, J. J. Turner, Poliakof.M and R. N. Perutz, J.
Organomet. Chem., 1972, 34, C34-&.
X. L. Xie and J. D. Simon, J. Phys. Chem., 1989, 93, 4401-
4404.
J. D. Simon and X. L. Xie, J. Phys. Chem., 1989, 93, 291-293.
J. D. Simon and X. L. Xie, J. Phys. Chem., 1987, 91, 5538-
5540.
J. D. Simon and X. L. Xie, J. Phys. Chem., 1986, 90, 6751-
6753.
Supporting Information.
10.
Supporting information includes details of actinometric
quantum yield experiment, TRIR spectra of 1b, 2b and 3b,
electrochemical data and details of quantum chemical
calculations including atomic coordinates of all optimises
structures in the form of embedded .mol files.
11.
12.
13.
14.
15.
Conclusions
Contrary to previous proposals that photoinduced CO-loss from
(2-alkyne)Co2(CO)6 complexes was the primary photochemical
step in the visible light assisted Pauson-Khand reaction, our
investigations implicate a high-spin diradical species formed by
homolytic cleavage of the Co-Co bond. Attempts to maximise
the quantum efficiency of CO-loss by tuning the absorption
profile of a series of alkyne-substituted complexes to the
excitation wavelength in time-resolved spectroscopic studies
failed to significantly increase the CO-loss efficiency. This,
despite the increased efficiency of the visible light assisted
Pauson-Khand reaction for the ferrocenyl substituted
16.
17.
18.
19.
J. M. Kelly, C. Long and R. Bonneau, J. Phys. Chem., 1983,
87, 3344-3349.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 10 of 13
ARTICLE
20.
Journal Name
S. Villaume, A. Strich, C. Daniel, S. A. Perera and R. J. 49.
Bartlett, PCCP, 2007, 9, 6115-6122.
R. L. Hollingsworth, J. W. Beattie, A. Grass, P. D. Martin, S.
View Article Online
Groysman and R. L. Lord, J. Chem. Soc., Dalton Trans., 2018,
DOI: 10.1039/C9DT03006A
21.
N. Ben Arnor, S. Villaume, D. Maynau and C. Daniel, Chem.
Phys. Lett., 2006, 421, 378-382.
A. Rosa, E. J. Baerends, S. J. A. van Gisbergen, E. van Lenthe,
47, 15353-15363.
B. W. Caplins, J. P. Lomont, S. C. Nguyen and C. B. Harris, J.
Phys. Chem. A, 2014, 118, 11529−11540.
50.
22.
J. A. Groeneveld and J. G. Snijders, J. Am. Chem. Soc., 1999, 51.
121, 10356-10365.
C. Pollak, A. Rosa and E. J. Baerends, J. Am. Chem. Soc.,
1997, 119, 7324-7329.
J. L. Bingaman, C. L. Kohnhorst, G. A. Van Meter, B. A.
McElroy, E. A. Rakowski, B. W. Caplins, T. A. Gutowski, C. J.
Stromberg, C. E. Webster and E. J. Heilweil, J. Phys. Chem.
A, 2012, 116, 7261-7271.
23.
24.
25.
C. Long, in Photophysics of Organometallics, ed. A. J. Lees, 52.
Springer, Berlin/Heidelberg, 2010, vol. 29, ch. 2, pp. 37-71. 53.
I. P. Clark, M. W. George, G. M. Greetham, E. C. Harvey, C.
Long, J. C. Manton and M. T. Pryce, J. Phys. Chem. A, 2010, 54.
114, 11425 - 11431.
J. Mattay, Angew. Chem. Int. Ed., 2007, 46, 663-665.
L. Bertini, P. Fantucci and L. De Gioia, Organometallics,
2011, 30, 487-498.
P. Magnus and L. M. Principe, Tetrahedron Lett., 1985, 26,
4851-4854.
26.
B. J. Aucott, A. K. Duhme-Klair, B. E. Moulton, I. P. Clark, I. 55.
V. Sazanovich, M. Towrie, L. A. Hammarback, I. J. S.
Fairlamb and J. M. Lynam, Organometallics, 2019, 38,
D. Lesage, A. Milet, A. Memboeuf, J. Blu, A. E. Greene, J. C.
Tabet and Y. Gimbert, Angew. Chem. Int. Ed., 2014, 53,
1939-1942.
2391-2401.
56.
I. P. Clark, M. W. George, G. M. Greetham, E. C. Harvey, C.
Long, J. C. Manton and M. T. Pryce, J. Phys. Chem. A, 2010,
114, 11425-11431.
I. P. Clark, M. W. George, G. M. Greetham, E. C. Harvey, C.
Long, J. C. Manton and M. T. Pryce, J. Phys. Chem. A, 2011,
115, 2985-2993.
I. P. Clark, M. W. George, G. M. Greetham, E. C. Harvey, C.
Long, J. C. Manton, H. McArdle and M. T. Pryce, J. Phys.
Chem. A, 2012, 116, 962-969.
G. M. Greetham, Q. Cao, I. P. Clark, P. S. Codd, M. W.
George, R. C. Farrow, T. Matousek, A. W. Parker, M.
Pollard, D. A. Robinson, Z. J. Xin and M. Towrie, Appl.
Spectrosc., 2010, 64, 1311-1319.
27.
28.
29.
P. L. Pauson and I. U. Khand, Ann. N.Y. Acad. Sci., 1977, 295,
2-14.
B. L. Pagenkopf and T. Livinghouse, J. Am. Chem. Soc., 1996, 57.
118, 2285-2286.
A. C. Coleman, C. Long, A. Meetsma, B. L. Feringa, W. R.
Browne and M. T. Pryce, J. Chem. Soc., Dalton Trans., 2009, 58.
7885-7887.
M. Yamanaka and E. Nakamura, J. Am. Chem. Soc., 2001,
123, 1703-1708.
30.
31.
59.
S. A. Brusey, E. V. Banide, S. Dörrich, P. O’Donohue, Y.
Ortin, H. Müller-Bunz, C. Long, P. Paul Evans and M. J.
McGlinchey, Organometallics, 2009, 28, 6308-6319.
32.
33.
34.
35.
36.
C. M. Gordon, M. Kiszka, I. R. Dunkin, W. J. Kerr, J. S. Scott 60.
and J. Gebicki, J. Organomet. Chem., 1998, 554, 147-154.
T. E. Bitterwolf, W. B. Scallorn and C. A. Weiss, J.
Organomet. Chem., 2000, 605, 7-14.
K. Asano, Y. Uesugi and J. Yoshida, Org. Lett., 2013, 15,
2398-2401.
J. Overaard, H. F. Clausen, J. A. Platts and B. B. Iversen, J.
Am. Chem. Soc., 2008, 130, 3834-3843.
J. A. Platts, G. J. S. Evans, M. P. Coogan and J. Overgaard,
Inorg. Chem., 2007, 46, 6291-6298.
A. D. Becke, Phys. Rev. A, 1988, 38, 3098-3100.
C. T. Lee, W. T. Yang and R. G. Parr, Phys. Rev. B, 1988, 37,
785-789.
A. D. Becke, J. Chem. Phys., 1993, 98, 5648-5652.
R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996,
256, 454-464.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A.
Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich,
J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P.
Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D.
Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B.
Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G.
Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K.
Throssell, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. J.
Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N.
Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K.
Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J.
Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R.
Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O.
Farkas, J. B. Foresman and D. J. Fox, Gaussian 16, 2016.
A. D. McLean and G. S. Chandler, J. Chem. Phys., 1980, 72,
5639-5648.
R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem.
Phys., 1980, 72, 650-654.
T. Clark, J. Chandrasekhar, G. W. Spitznagel and P. V.
Schleyer, J. Comput. Chem., 1983, 4, 294-301.
M. J. Frisch, J. A. Pople and J. S. Binkley, J. Chem. Phys.,
1984, 80, 3265-3269.
J. Overgaard, H. F. Clausen, J. A. Platts and B. B. Iversen, J.
Am. Chem. Soc., 2008, 130, 3834-3843.
D. Gregson and J. A. K. Howard, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1983, 39, 1024-1027.
K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron
Lett., 1975, 4467-4470.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub, J.
Chem. Phys., 1998, 108, 4439-4449.
R. E. Stratmann, G. E. Scuseria and M. J. Frisch, J. Chem.
Phys., 1998, 109, 8218-8224.
C. Van Caillie and R. D. Amos, Chem. Phys. Lett., 2000, 317,
159-164.
C. Van Caillie and R. D. Amos, Chem. Phys. Lett., 1999, 308,
249-255.
F. Furche and R. Ahlrichs, J. Chem. Phys., 2002, 117, 7433-
7447.
G. Scalmani, J. Frisch, B. Mennucci, J. Tomasi, R. Cammi and
V. Barone, J. Chem. Phys., 2006, 124, 094107-094104.
M. Cossi, V. Barone, R. Cammi and J. Tomasi, Chem. Phys.
Lett., 1996, 255, 327-335.
61.
62.
63.
64.
65.
66.
67.
J. Marhenke, S. M. Massick and P. C. Ford, Inorg. Chim.
Acta, 2007, 360, 825-836.
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 13
Dalton Transactions
Please do not adjust margins
Journal Name
ARTICLE
68.
L. Cuffe, R. D. A. Hudson, J. F. Gallagher, S. Jennings, C. J.
View Article Online
DOI: 10.1039/C9DT03006A
McAdam, R. B. T. Connelly, A. R. Manning, B. H. Robinson
and J. Simpson, Organometallics, 2005, 24, 2051-2060.
W. B. Austin, N. Bilow, W. J. Kelleghan and K. S. Y. Lau, J.
Org. Chem., 1981, 46, 2280-2286.
N. M. Boyle, A. C. Coleman, C. Long, K. L. Ronayne, W. R.
Browne, B. L. Feringa and M. T. Pryce, Inorg. Chem., 2010,
49, 10214-10216.
69.
70.
71.
A. Coleman, Ph.D. Thesis, Dublin City University, 2007.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
Please do not adjust margins

Dalton Transactions
Page 12 of 13
View Article Online
The photoassisted Pauson-Khand reaction involves the formation of a high-spin diradicaDl OspI:e10c.i1e03s9a/Cn9dDT03006A
not CO loss as previously thought.

Page 13 of 13
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT03006A
80x40mm (200 x 200 DPI)