Photochemical studies of (η5-C5H 5)Ru(PPh3)2Cl and (η5-C 5H5)Ru(PPh3)2Me: Formation of Si-H and C-H bond activation products

Article abstract of DOI:10.1039/c3dt52069b

Studies examining the photochemical reactivity of CpRu(PPh ₃)₂Cl and CpRu(PPh₃)₂Me towards the two electron donor ligands PEt₃, C₂H₄, DMSO, the CH bond activatable reagents tetrahydrofuran, toluene, and pyridine, and the SiH bond activatable reagents HSiEt₃ and HSi(Me) ₂CHCH₂) are presented. Broadband UV irradiation of CpRu(PPh₃)₂Cl leads to the formation of mono-substitution products such as CpRu(PPh₃)(PEt₃)Cl which are inert to further photochemical reaction, although thermally bis-substituted products such as CpRu(PEt₃)₂Cl can be formed. Room temperature irradiation of the related complex CpRu(PPh₃)₂Me with L = PEt₃, C₂H₄, and DMSO also produces CpRu(PPh₃)(L)Me. However, when these reactions are followed by in situ laser irradiation (325 nm source) at low temperature, three solvent activated isomers (ortho, meta and para) of CpRu(PPh₃) ₂(C₆H₄Me) are detected in toluene in addition to η¹- and η³-coordinated benzyl species. Furthermore, photolysis in THF leads to both the C-D bond activation product CpRu(PPh₃)₂(OC₄D₇) and the labile coordination complex CpRu(PPh₃)(THF)Me. Now CH₄ rather than CH₃D is liberated which suggests the involvement of an orthometallated species. The photochemically driven reaction of CpRu(PPh ₃)₂Me with HSiEt₃ at 198 K generates CpRu(κ²-2-C₆H₄PPh₂)(SiEt ₃)H and thereby confirms a role for an orthometallated complex is this process. Irradiation in cyclohexane produces the known orthometallated complex, CpRu(κ²-2-C₆H₄PPh ₂)(PPh₃), and CH₄ in accordance with this reactivity. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3dt52069b

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Photochemical studies of (η5-C5H5)Ru(PPh3)2Cl
and (η5-C5H5)Ru(PPh3)2Me: formation of Si–H and
C–H bond activation products†
Cite this: Dalton Trans., 2014, 43,
1162
Johnathan L. Clark and Simon B. Duckett*
Studies examining the photochemical reactivity of CpRu(PPh3)2Cl and CpRu(PPh3)2Me towards the two
electron donor ligands PEt3, C2H4, DMSO, the CH bond activatable reagents tetrahydrofuran, toluene, and
pyridine, and the SiH bond activatable reagents HSiEt3 and HSi(Me)2CHvCH2) are presented. Broadband UV
irradiation of CpRu(PPh3)2Cl leads to the formation of mono-substitution products such as CpRu(PPh3)-
(PEt3)Cl which are inert to further photochemical reaction, although thermally bis-substituted products
such as CpRu(PEt3)2Cl can be formed. Room temperature irradiation of the related complex CpRu-
(PPh3)2Me with L = PEt3, C2H4, and DMSO also produces CpRu(PPh3)(L)Me. However, when these reactions
are followed by in situ laser irradiation (325 nm source) at low temperature, three solvent activated isomers
(ortho, meta and para) of CpRu(PPh3)2(C6H4Me) are detected in toluene in addition to η1- and η3-co-
ordinated benzyl species. Furthermore, photolysis in THF leads to both the C–D bond activation product
CpRu(PPh3)2(OC4D7) and the labile coordination complex CpRu(PPh3)(THF)Me. Now CH4 rather than CH3D
is liberated which suggests the involvement of an orthometallated species. The photochemically driven
reaction of CpRu(PPh3)2Me with HSiEt3 at 198 K generates CpRu(κ2-2-C6H4PPh2)(SiEt3)H and thereby
confirms a role for an orthometallated complex is this process. Irradiation in cyclohexane produces the
known orthometallated complex, CpRu(κ2-2-C6H4PPh2)(PPh3), and CH4 in accordance with this reactivity.
Received 31st July 2013,
Accepted 20th October 2013
DOI: 10.1039/c3dt52069b
www.rsc.org/dalton
intramolecular C–H bond activation at 363 K in decalin to
form the orthometallation product CpRu(κ2-2-C6H4PPh2)(PPh3)
Introduction
The bis-phosphine complex, CpRu(PPh3)2Cl (1), was first dis- (3).10,11 Similarly, the C–H bond activation of benzene by
covered by Wilkinson et al.1 as a side product of the reaction 2 or 3 produces CpRu(PPh3)2Ph while toluene leads to
of RuCl2(PPh3)3 with cyclopentadiene. This was confirmed by CpRu(PPh3)2(CH2C6H5) and meta and para tolyl derivatives
Stone et al. who presented data on its characterisation and CpRu(PPh3)2(C6H4Me).12,13
reactivity.2 The method by which CpRu(PPh3)2Cl can be syn-
While thermal reactivity studies of these types of ruthenium
thesised has been revised a number of times over the past complex have been widely explored, examples of their photo-
thirty years,3,4 and it is now prepared most commonly by chemical reactivity are not common. Cyclopentadienyl iron
the high yielding route of Bruce et al.5 The resulting avail- derivatives of these types of complex have however been widely
ability of CpRu(PPh3)2Cl makes it an ideal starting point in the examined, both thermally and photochemically.14–17 Notable
synthesis of other cyclopentadienyl ruthenium complexes. examples of such work include studies by Pannell et al.,18–22
Over the last half century, this class of complex has been used who over several decades probed the photochemical activity of
to study the activation of small molecules and to facilitate a bis-carbonyl cyclopentadienyl iron complexes towards silanes.
number of catalytic transformations.6–9
They discovered that isomerisation of the silyl moiety in CpFe-
Studies of the thermal reactivity of CpRu(PPh3)2Me (2) have (CO)2(CH2(SiMe2)2SiMe3) proceeds through the silene inter-
demonstrated that it is more reactive than 1, undergoing mediate CpFe(CO)(SiMe2SiMe3)(η2-CH2vSiMe2).18 Ruthenium
bimetallic carbonyl complexes have been examined and com-
pared to their iron analogues.23,24 Here Farrow and Knox25
showed how (Cp*Ru(CO)2)2 photolysis in CHCl3 led to Cp*Ru-
Department of Chemistry, University of York, YO10 5DD, UK.
(CO)2Cl. In contrast, photochemical reaction with ethene and
hydrogen retains a bimetallic product structure, with mono-
carbonyl substitution resulting, while with H2 the bridging
E-mail: Simon.Duckett@york.ac.uk; Fax: +44 (0)1904 322516;
Tel: +44 (0)1904 322564
†Electronic supplementary information (ESI) available: Synthetic and character-
carbonyls are replaced by bridging hydrides and a ruthenium–
ruthenium double bond.
isation details, example NMR traces, and further product/irradiation time evol-
ution plots. See DOI: 10.1039/c3dt52069b
1162 | Dalton Trans., 2014, 43, 1162–1171
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Indeed, one of the major benefits of photochemically 200 W Hg–Xe arc lamp or a Kimmon 63 mW He–Cd laser
induced reactions, over thermal studies, is the opportunity to (operating at 325 nm).
introduce product selectivity. For example, while the thermal
CpRu(PPh3)Me(THF) (11): 1H NMR 0.16 (d, 3H, Ru–CH3),
substitution of a carbonyl ligand in CpMn(CO)3 by THF 4.59 (s, 5H, η5-C5H5), 7.08 (m, p-P(C6H5)3), 7.12 (m, m-P-
requires high temperatures, the same reaction proceeds photo- (C6H5)3), 7.53 (m, |3JPH| = 5.9 Hz, o-P(C6H5)3). 31P{1H} NMR
chemically at room temperature.26 The product of this reac- 61.3 (s, P(C6H5)3).
tion, CpMn(CO)2THF, is relatively labile27 and can be used as a
precursor to many other CpMn(CO)2L type complexes (where
for example L = N2, η1 P–P (dmpm = bis(dimethylphosphino-
methane) or dmpp
= 1,3-bis(dimethylphosphinopropane),
piperidine or 4-acetylpyridine).28–32
Low temperature photolysis may also be employed to
generate and stabilise 16-electron intermediates, which
subsequently undergo coordination of an incoming ligand to
form a coordinatively saturated 18-electron complex. The net
effect of this process is to promote ligand exchange at the
metal centre which is useful in cases where the parent com-
plexes do not undergo ready thermal ligand loss. Haddleton
and Perutz used this method to generate [CpRh(C2H4)] from
the parent complex CpRh(C2H4)2 in an argon/methane matrix
at 12 K through λ > 200 nm irradiation.33–35 They subsequently
showed that in a CO containing matrix the formation of CpRh-
(C2H4)(CO) was possible. This photochemical route has since
been used to form a range of novel and previously inaccessible
CpRh complexes.
CpRu(PPh3)2(2-C4H7O) (12): 1H NMR 1.75 & 1.94 (t, |3JHH| =
6.2 Hz, Hc), 2.19 & 2.41 (q, 2, |3JHH| = 5.4 Hz, Hb), 3.70 & 3.86
(q, |3JHH| = 5.4 Hz, Hd), 4.23 (s, 5H, η5-C5H5), 5.37 (dt, 1, |3JHH
|
= 1.7 |3JPH| = 16.3 Hz, Ha), 7.09 (m, p-P(C6H5)3), 7.16 (m,
m-P(C6H5)3), 7.38 (m, 12H, o-P(C6H5)3). 31P{1H} NMR 41.9
(s, P(C6H5)3).
Photochemistry has also been used in conjunction with
irradiation within an NMR probe to generate unstable species
at low temperature and then characterise them. Geftakis and
Ball first demonstrated this method using a fibre optic cable36
and successfully formed CpRe(CO)2(cyclopentane) in situ.37–39
At York, a similar in situ irradiation apparatus uses a laser
beam, provided by a He–Cd laser in this study.40–42 This in situ
technique allowed the monitoring of the photochemical reac-
tivity of the N-heterocyclic carbene complex, Ru(IEt2Me2)-
(PPh3)2(CO)H2 (IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-
ylidene) with the result that H2 and PPh3 loss was demon-
strated.43 It was also used to follow the formation of CpCo-
(C2H4)(SiEt3)H from CpCo(C2H4)2.44
Results and discussion
Thermal reaction of CpRu(PPh3)2Cl (1) with PEt3
In order to investigate the thermal reactivity of 1, a d8-toluene
solution of the complex was first heated to 353 K with a 7 fold
excess of PEt3 for 24 hours. Subsequent NMR monitoring
revealed the complete conversion of 1 into CpRu(PEt3)2Cl (4)
which yields a 31P NMR signal at δ 46.8, and CpRu(PPh3)(PEt3)-
Cl (5) which yields a pair of doublets at δ 27.8 and 45.5 that are
indicative of mono-substitution. The ratio of 4 : 5 proved to be
2 : 1 at this point. Upon further heating at 353 K for 24 h the
complete conversion to 4 was achieved.
Here we describe a series of studies on the photochemical
reactivity of CpRu(PPh3)2R (where = Cl or Me) using broadband
UV irradiation, and/or the laser in situ irradiation method.
This approach allows access to both C–H and Si–H bond acti-
vation pathways, while producing definitive mechanistic infor-
Photochemical reaction of 1 with PEt3
mation. In this case, we hoped to answer the question of As a starting point, the photochemical reactivity of CpRu-
whether direct CH bond activation or orthometallation play a (PPh3)2Cl was examined at 198 K using the in situ laser
defining role in the reaction chemistry of 2.
irradiation method. No evidence for any reaction was evident
in the associated NMR spectra after 18 h irradiation using the
325 nm source. A 200 W He–Xe arc lamp (broad-band UV) was
then employed. This method required the sample to be irra-
diated outside the NMR spectrometer. The sample was then
transferred into the probe of the NMR spectrometer, precooled
Experimental
Details of the synthetic and NMR procedures used can be as required, to record the 1H and 31P{1H} NMR spectra that
found in the ESI.† Full NMR data for species 1–23 can also be were required to follow the reaction.
found there. Partial data is presented in this section for com-
An analogous solution of 1 containing PEt3 was therefore
plexes 11 and 12 for illustration. Throughout this work, UV prepared and irradiated at 198 K using this ex situ setup. Only
photochemical studies were carried out using either a Oriel the mono-substituted reaction product 5 was formed and after
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18 hours of UV irradiation full conversion to it was achieved. of 6, the temperature of the solution was raised to 298 K in
The sample was then washed with ethanol to remove the order to appraise the thermal stability of 6. The corresponding
excess PEt3 and liberated PPh3 and re-dissolved in d8-toluene. 1H and 31P{1H} NMR spectra revealed that 6 reverted back to 1
A 5-fold excess of PMe3 was then added and the resulting before measurement could be made. No reaction was evident
sample re-photolysed using ex situ at 233 K. No change in when thermal initiation was employed.
sample composition was observed. The product complex 5 is
These results demonstrate that 1 does not activate the C–H
therefore photoinsensitive under these conditions. It should bonds of these reagents, nor the solvent C–D bonds under
be noted that when 5 is warmed with PMe3, the previously irradiation in the presence of ethene or PEt3. More impor-
characterised complex CpRu(PMe3)2Cl45 is formed. We there- tantly, no evidence was found for any orthometallation pro-
fore conclude that the preparation of such mono-substituted ducts that might result from involvement of the putative 16-
complexes can be achieved selectively photochemically.
electron intermediate CpRu(PPh3)Cl. This situation contrasts
with that found for the methyl analogue of 1, CpRu(PPh3)2Me
(2) which has been shown previously to activate the C–H
bonds of benzene and to undergo orthometallation of the aryl
ring of the phosphine.11 These reported observations suggest
that 2 possesses a significantly different reactivity that of 1.
Studies where the photochemical reactivity of 2 is examined
are now presented. In this case, 325 nm irradiation using the
in situ method was employed. Scheme 1 details the main reac-
tion products that are detected and characterised in this work.
Photochemical reaction of 1 with ethane
A second substitution reaction was tested in order to assess
whether the photolysis of 1 with ethene (pressurised to
1.5 bar) in d8-toluene led to the formation of the η2-ethene
coordination complex CpRu(PPh3)(η2-C2H4)Cl (6) or the corres-
ponding C–H activation product CpRu(PPh3)(C2H3)(H)Cl. This
photolysis reaction was undertaken ex situ at 198 K. The result-
ing 1H NMR spectrum recorded after 12 hours of photolysis
contained a new resonance at δ 4.23 which is consistent with
the formation of a η5-Cp containing complex. Four new peaks,
broad multiplets at δ 0.56, 1.18, 2.23 and 2.64 were also
Thermal and photochemical reactions of CpRu(PPh3)2Me (2)
with PEt3
detected which correspond to the inequivalent protons of an These studies were completed to assess the ability of 2 to
η2-bound ethene ligand. Following the NMR characterisation undergo a substitution reaction with a suitable 2-electron
Scheme 1 Reaction products formed when 2 is subject to UV irradiation as specified.
1164 | Dalton Trans., 2014, 43, 1162–1171
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donor. To achieve this, a d8-THF solution of 2 containing a time. The time taken to achieve 50% conversion of 2 is 7.4 h.
7-fold excess of PEt3 was prepared and heated to 353 K for The corresponding 1H NMR spectra contained characteristic
24 hours. Related spectral changes to those described earlier for C5H5 resonances which vary in intensity in a similar way.
the reaction of 1 with PEt3 were found. For example, the result- There are no signals in the high field region (δ 0 to −20 range)
1
ing 31P{1H} NMR spectra now contain a major signal at δ 40.5 of the H NMR spectrum where metal-hydride resonances are
due to CpRu(PEt3)2Me (7) and minor signals for CpRu(PPh3)- commonly observed.
(PEt3)Me (8). Upon further heating for 16 h, the complete con-
version of 2 into 7 was demonstrated by NMR spectroscopy.
The observation of 8 suggests that the 16-electron reaction
intermediate [CpRu(PPh3)Me] can be formed under photolysis
When a second sample was prepared and the corres- and that rapid reaction with PEt3 ensues. Under thermal con-
ponding in situ photochemical reaction monitored at 198 K ditions, the dominance of CpRu(PEt3)2Me suggests complete
over 18 h, only 8 was formed. The progress of this reaction was phosphine exchange is possible. Under irradiation however,
1
followed during this period by H and 31P{1H} NMR methods. the selective formation of 8 is achieved even after 30 hours of
New proton resonances at δ 0.39, 0.83, 1.39, 1.50 and 4.49 are photolysis with the 325 nm source.
diagnostic of 8. Specifically, the peak at δ 4.49 is consistent
Photochemical reaction of 2 with DMSO
with that expected for a cyclopentadienyl ring while the triplet
at δ 0.39 (JPH = 5.6 Hz) is consistent with the proton resonance In view of the fact that PEt3 is highly reactive, a further reac-
of a ruthenium-bound methyl group. The triplet splitting indi- tion with the milder reagent dimethylsulfoxide was under-
cates that two phosphine ligands couple to this group and are taken. Previous studies have shown that DMSO may interact
hence are bound to the metal. Two doublets of equal intensity, with metal centres, as a two electron donor, through either M–S
that share a common splitting of 39.3 Hz, are observed in the (e.g. [Ru(NH3)5(DMSO)](PF6)2)46 or M–O (e.g. [CuBr2(DMSO)3)47
corresponding 31P{1H} NMR spectrum at δ 35.2 and 62.9. coordination modes. The coordination mode that DMSO
These resonances represent the two 31P nuclei of the phos- adopts may be determined by X-ray crystallography,48 obser-
phines of 8. A large singlet resonance at δ −6.4 which is vation of the change of the v(OvS) relative to free DMSO using
characteristic of free triphenylphosphine is also detected.
IR methods,49 or as confirmed more recently, through the use
2D 1H/31P HMQC, 1H/13C HMQC and 1H-COSY spectra were of NMR techniques.50 Here we describe the formation of the
then used to fully characterise this product. Specifically, the products for reaction between 2 with DMSO, and determine
two 31P resonances connect to common 1H resonances at δ the coordination mode of the bound DMSO moiety. Accord-
0.39, 1.39, 1.50, 4.49 and 7.61. While the 31P resonance at δ ingly, a sample of 2 was prepared in d8-THF that contained 5
1
62.9 couples strongly to H resonances at δ 7.61, 7.06 and 6.97 µl of DMSO. UV irradiation led to the loss of PPh3 and a new
of the ortho, meta and para phenyl protons of the bound tri- 31P signal was observed at δ 59.0 in addition to notable 1H
phenylphosphine ligand of 8. Signals at 1H δ 0.83, 1.39 and methyl signals at δ 0.02, 2.57 and 2.37. These data confirm
1.50 were found to couple strongly to the 31P resonance at that the selective formation of the mono-substituted complex
δ 35.2. The proton resonances at δ 1.39 and 1.50 correspond to CpRu(PPh3)(DMSO)Me (9) was achieved. The CH3 moieties of
the inequivalent CH2 protons of the triethylphosphine ligand the coordinated DMSO ligand are inequivalent and show a
(confirmed by their mutual coupling to a common carbon at strong connection to the Cp ring protons according to nOe
1
δ 23.4 in the corresponding H/13C HMQC experiment) while data. The DMSO ligands coordination mode was confirmed as
that at δ 0.83 corresponds to the Me group.
arising through a Ru–S link by the downfield shift of the
The plot shown in Fig. 1 illustrates the change in relative CH3–13C signal relative to that of free DMSO.50 These results
intensities of these 31P signals as a function of irradiation also confirm that the reaction is selective to mono substitution
with no hydride or orthometallation products being indicated.
The plot shown in ESI Fig. 1† presents the change in relative
intensities of the species in solution as a function of
irradiation time. The time taken to achieve 50% conversion of
2 into 9 was 15.5 h.
Photochemical reaction of 2 with ethane
An analogous reaction to that described earlier between 1 and
ethene was then monitored; CpRu(PPh3)(η2-C2H4)Me (10), has
been previously synthesised under forcing conditions (50–150
bar of ethene at 373 K).51 It was intended that this photo-
chemically initiated reaction would yield 10 under mild con-
ditions thereby allowing us to probe its reactivity in a rigorous
way. In order to achieve this, a sample of 2 in d8-THF was
pressurized with 1.5 bar of ethene gas. This sample was then
cooled to 198 K and subjected to 325 nm irradiation for
18 hours using the in situ laser set-up. The reactions progress
Fig. 1 Plot of the relative 31P signal intensities as
irradiation time (325 nm, in d8-THF) during the reaction of 2 ( ) with
PEt3 at 198 K leading to the formation of 8 ( ).
a function of
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Table 1 Rate constant for ethene rotation in 10
T/K
k/s−1
253
243
233
223
213
203
2.96 × 10−2
8.24 × 10−3
1.66 × 10−3
1.96 × 10−4
4.30 × 10−5
4.80 × 10−6
Fig. 2 Plot of the relative proportions of 2 ( ), 10 ( ) and 11 ( ) as a
function of irradiation time (325 nm, in d8-THF) for the reaction of 2
with ethene at 198 K.
was monitored during this period by recording 1H and 31P{1H}
NMR spectra every 12 minutes. In the 31P NMR spectra, the
singlet at δ 55.3 for 2 reduced in intensity while three new
peaks grew in at δ −6.4, δ 67.7 and δ 61.3. The latter of these
31P resonances is due to the THF adduct CpRu(PPh3)(THF)Me
(11) which will be described later in this manuscript, while the
former is due to free PPh3. Fig. 2 shows how the proportion of
these products varies over the 18 hour irradiation period.
Under these conditions, 14.2 h was taken to achieve 50% con-
version of 2.
Fig. 3 Eyring plot as used to determine the barrier to ethene rotation in
10.
Alkene rotation was then examined over the temperature
range 203–253 K through a series of EXSY measurements and
appropriate rate constant data obtained. These data are pre-
sented in Table 1 while Fig. 3 illustrates the corresponding
Comparison of the initial 1H NMR spectrum with that
recorded after 4 hours of photolysis shows the formation of a
new resonance at δ 4.63 in the “Cp region”. Four new peaks at
Eyring plot which yielded ΔH‡ = 72.9 3.0 kJ mol−1, ΔS‡
=
21.7 4.0 J K−1 mol−1 and ΔG3‡00 = 66.4 0.8 kJ mol−1. It
should be noted that the Eyring plot used rate constants which
were multiplied by two in order to account for the possibility
of the equally likely reverse-reaction.52 Similar ΔG‡ values of
ethene rotation have been reported for cyclopentadienyl
1
δ 0.67, 1.28, 2.54 and 2.81 (multiplets) were also found. A H
COSY experiment demonstrated coupling between these four
proton resonances. A final proton resonance was found at
δ 0.43 (d, 6.7 Hz) due to the methyl protons of this product.
These six resonances are consistent with those expected for 10
if alkene rotation is restricted by the low temperature.
rhodium complexes: CpRh(CH2vCH(CO2 Bu))2 ΔG‡295 = 72.5
t
0.1 kJ mol−1 53 and Cp*Rh(CH2vCH(Me)) (SiEt3)(H) ΔG‡300
=
72.7 0.1 kJ mol−1 54
It should be noted that there was no evi-
.
1
Data collected in a H/31P HMQC experiment revealed con-
dence for any CH bond activation of the ethene ligand and
upon warming to room temperature 2 was reformed.
nections between the δ 67.7 31P signal and the four ethene
protons resonances, the methyl resonance, the Cp signal, and
1
a new ortho phenyl proton signal at δ 133.2. A H/13C HMQC
experiment located the corresponding 13C resonances while
examination of COSY data revealed the remaining the phenyl
group signals.
Photochemical reaction of 2 in cyclohexane
The introduction has already detailed how the orthometalla-
tion product CpRu(κ2-2-C6H4PPh2)(PPh3) (3) and the C–H
activation product CpRu(PPh3)2Ph can be formed from 2. Fur-
thermore, the reactions with the added ligands, DMSO, PEt3
and ethene have established that the corresponding substi-
tution products can be formed. However, no evidence for any
CH bond activation was observed in the reactions presented so
far. In order to monitor more clearly the C–H/C–D bond acti-
vation potential of 2, a d12-cyclohexane solution was prepared
where the only source of ligation for a photoproduct is the
solvent itself or a C–H bond within the initial photoproduct.
When this sample was irradiated for 12 h a reaction was
evident according to NMR. In this case the 31P{1H} NMR signal
of 2 was replaced by two doublets at δ −16.9 (JPP = 33.8 Hz)
and 63.5 (JPP = 33.8 Hz). The absence of the expected 31P{1H}
When this sample was warmed to 233 K, the ethene proton
signals started to broaden, indicating that the two trans proton
pairs undergo site exchange due to η2-ethene ligand rotation.
In order to confirm this, a series of selective nOe measure-
ments were undertaken. In such an experiment, when the
signal at
δ 0.67 is probed, magnetisation transfer into
the signal at δ 2.54 is observed. These data confirm that the
protons giving rise to these resonances are trans to each other
and while no link to the Cp resonance was observed, one to
the RuMe resonance was seen.
This suggests that the δ 0.67 yielding proton points away from
the Cp ring and is on the same side as the RuMe in 10. No evi-
dence for products resulting from methyl migration was found.
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NMR signal at δ −6.4 for free phosphine in this reaction indi- resonance, further ortho-, meta- and para-PPh3 resonances
1
cates that both phosphine ligands remain bound to the ruthe- were located at δ 7.38, 7.16 and 7.09. The H signal at δ 5.37
nium centre. The lack of a ruthenium–methyl moiety and was confirmed as arising from the α-proton of the ruthenium-
observation of methane were also evident in these spectra. bound tetrahydrofuryl group through the observation of a one-
These data confirm that the known product 3 is readily bond carbon coupling to a 13C signal at δ 145.7 which exhibi-
formed, and confirm the existence of a CH bond activation ted a triplet splitting to phosphorus of 17.8 Hz. The presence
pathway in the photochemistry of 2. Owing to the relatively of two triphenylphosphine ligands in 12 is therefore confirmed
high freezing point of cyclohexane (280 K), low temperate and these data identify it as CpRu(PPh3)2(2-C4H7O). No
irradiation of such a sample was not feasible. Additionally, no hydride signals were observed during these studies.
activation of the solvent was indicated. The presented NMR
The minor product 11 which yielded the 31P signal at δ 61.3
proved to connect with a H signal at δ 4.59 due to a Cp ring.
1
data for 3 is in agreement with that reported.11
This 31P signal also coupled to a 1H peak at δ 0.16 which
appeared as a doublet of 5.91 Hz thereby confirming 11 pos-
sesses a RuMe group and corresponds to a mono phosphine
complex. The phosphine that is released in this reaction pro-
duced the expected singlet 31P resonance at δ −6.4. These data
suggest that 11 corresponds to the solvent coordination
complex, CpRu(PPh3)Me(THF), although it should be noted
that the THF moiety was too labile to be detected, even at
198 K. Similar THF-coordination complexes are known26,55,56
and in this case 11 is unstable above 218 K. There are two
potential routes to the 12, as outlined in Scheme 2.
Photochemical reaction of 2 with THF
In order to probe this orthometallation reaction, a d8-THF solu-
tion of 2 was prepared and irradiated at 198 K. Two new
species were formed under these conditions which yielded
31P NMR signals at δ 41.9 and 61.3. ESI Fig. 2† shows how the
intensity of these signals varies over the 18 hour irradiation
period. After 18 hours only 30% of 2 was converted into
product and this lower conversion is taken to suggest that 2
must reform during this process (we estimate it would take
1
36.2 h for 50% conversion). H NMR signals at δ 0.1957 due to
methane and at δ 0.16 due to a RuMe are also observed,
although no signals attributable to 3 are seen (see Fig. 4).
In order to confirm the identity of these species an ana-
logous solution consisting of 2 in a 2 : 1 mixture of d8- and
h8-THF was examined. In the d8-THF experiment, only the two
1H NMR signals at δ 4.23 and 7.38 connected to the dominant
31P signal at δ 41.9. This changed for the d8–h8 mixture as con-
nections to signals at δ 5.37, 3.86, 3.70, 1.75, 1.94, 2.41 and
2.19 were also observed. These resonances arise from a tetra-
hydrofuryl group and while there was no methyl proton
Photochemical reaction of 2 with pyridine
Following the demonstration that both 11 and 12 form when 2
is irradiated in THF solution, the reactivity of 2 towards
Fig. 4 Change in 1H NMR intensities for: (a) the methyl resonance of
CpRu(PPh3)2Me (2), (b) the free methane signal and (c) the methyl signal
for CpRu(PPh3)(THF)Me (11) with increasing irradiation time.
Scheme 2 Potential UV initiated reaction pathways leading to CpRu(PPh3)-
Me(THF) (11) and CpRu(PPh3)2(2-C4H7O) (12) from 2 in d8-THF solution.
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pyridine was examined. Irradiation of 2 with pyridine in d8- (η3-CH2Ph) (17) respectively. A small 1H NMR peak at δ 0.17
THF solution led to the conversion of 2 into a single product was observed in these NMR samples due to liberated methane.
13 which yielded a 31P NMR signal at δ 68.3 (17.4 h for 50% No evidence was found for any η2-coordinated toluene com-
conversion of 2). The lack of any signals for 11 confirmed that plexes nor was any evidence found for the orthometallation
the coordination strength of the pyridine is greater than that complex 3 in these spectra. The characterisation of CpRu-
of THF. NMR data for CpRu(PPh3)(pyridine)Me (13) is pre- (PPh3)2(σ-CH2C6H5) (16) and CpRu(PPh3)(η3-CH2C6H5) (17) is
sented in the ESI.†
detailed in the ESI.† Previously the thermal reactivity of CpRu-
Upon warming this solution, the formation of a new (PPh3)2Me with toluene was been explored by Lehmkuhl
product 14, yielding a 31P{1H} NMR resonance at δ 54.7 et al.12,13 who observed 15b, 15c and 16.
resulted. This 31P signal was found to couple to 1H NMR
1
Photochemical reaction of 2 with naphthalene
signals at δ 4.64, 7.00 and 7.51, through a H/31P HMQC NMR
experiment. The signals at δ 4.64 and 7.51 have relative inte- Following the successful reaction with toluene, naphthalene
grals of 5 : 12, which indicates that they arise from a cyclo- was considered as a potential substrate. Surveys of the litera-
pentadienyl group and the ortho-phenyl protons of 14, CpRu- ture show that naphthalene can bind in an η2-fashion to 16-
(PPh3)2(2-pyridyl).
electron fragments through a 1,2 interaction;60–62 the 2,3
coordination mode is less common.63 In the case of CpRh-
(PMe3)(η2-naphthalene), diagnostic proton resonances are seen
at δ 3.49 (qd, J = 5.6, 2.4 Hz) and δ 4.09 (td, J = 6.6, 2.6 Hz) for
the η2-bound olefinic protons.64 A solution containing 8 mg of
naphthalene and 2 was therefore prepared and irradiated
in situ at 198 K.
Photochemical reaction of 2 with toluene
The activation of the CH bond of an arene is now a relatively
common reaction in inorganic chemistry.58,59 Evidence has
been already presented here for both orthometallation and
THF and pyridine CH bond activation products during studies
on the reactivity of 2.
When a d8-toluene solution of 2 was irradiated for 6 hours
the formation of five products was revealed by NMR methods.
These data include the detection of new 31P resonances at δ
The resulting 31P{1H} NMR showed the formation of two
product signals at δ 63.2 and δ 62.8 in the ratio of 20 : 8. In the
corresponding 1H/31P HMQC NMR experiment, the δ 63.2
1
resonance connected to H NMR signals at δ 4.55 and 0.95 (d,
5.8 Hz) which are consistent with the resonances expected for
protons of a coordinated cyclopentadienyl ring and a ruthe-
nium bound methyl group (respectively). Further connections
to diagnostic η2-bound olefinic protons were visible at δ 3.72
and 4.13. The smaller 31P resonance at δ 62.8 was observed to
couple to proton signals at δ 4.51, 4.06, 3.67 and 0.89 (d,
5.8 Hz). The nature of the doublet splitting for the two methyl
resonances reveals that each of these complexes possesses only
one coordinated triphenylphosphine ligand.
1
51.6, 52.0, 52.3, 52.6 and 60.3. However, the corresponding H
NMR spectra revealed little information that could be used to
identify these species.
In order to fully characterise them, a second sample was
prepared containing protio toluene alone and it was then irra-
diated for 20 hours. At this point, the protio toluene was
removed and the sample dissolved in d8-toluene and the
resulting 31P NMR spectrum contained the five original signals
observed in the d8-toluene reaction. Using a combination of
1
Analysis of the recorded H NMR spectra during the photo-
1H/31P HMQC and H COSY NMR experiments, it proved poss-
1
chemical reaction also revealed the growth of a series of new
peaks in the range δ 6.4 and δ 6.9 that are also diagnostic of a
bound naphthalene ring. However, the overlap of these signals
with those of the triphenylphosphine and free naphthalene
resonances made the characterisation of these complexes
using 1H COSY and 2D NOE experiments difficult. 2D NOE
spectra did, however, reveal two small, connected, signals
between δ 4.13 and δ 4.55 resonances that are consistent with
an interaction between the Cp ring and an η2-bound naphthal-
ene proton.
ible to characterise these species, 15a, 15b, 15c, 16 and 17.
Scheme 3 reveals the site at which C–H bond activation has
occurred in order to form these species. Notably, no hydride
signals were present in any of these 1H NMR spectra. The
NMR characteristics of these products are presented in the
ESI.†
It should be noted that the 31P signals at δ 51.6 and
60.3 arise from CpRu(PPh3)2(CH2Ph) (16) and CpRu(PPh3)-
It has been shown in a number of instances that when an
arene binds to a metal centre to form a complex such as CpRh-
(PMe3)(η2-arene), the pi face of the aromatic ring can point
either up or down. Two rotamers therefore often exist which can
interconvert.64 Warming this solution above 230 K led to the
decomposition of the resultant complexes rather than further
reaction to form the corresponding naphthyl product.
Photochemical reaction of 2 with HSiEt3
Until now, the reactions discussed have focused on exploring
Scheme 3 Ortho, meta, para and benzyl C–H activation products with
their relative composition where R = RuCp(PPh3)2.
the potential for C–H bond activation. The following sections
1168 | Dalton Trans., 2014, 43, 1162–1171
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outline how 2 reacts with a silane in a further attempt to
observe RuH-bond containing species. Photolysis of a d8-
toluene solution of 2 containing HSiEt3 at room temperature
resulted in extremely complicated 1H and 31P{1H} NMR
spectra. A number of hydride resonances were seen in the
1
range δ −11.5 and −13.0. Notably, H NMR signals were also
seen for the Cp and ethyl groups of the known complex
65
CpRu(PPh3)2SiEt3 (19) at δ 4.53, 1.19 and 1.31. Furthermore,
CpRu(PPh3)(SiEt3)2H (20) provided a hydride resonance at
δ −11.89 that coupled to a single 31P centre with JPH = 9.7 Hz.
Upon prolonged UV photolysis of this solution, CpRu(PPh3)2H
was formed and found to increase in concentration until after
4 hours it became dominant.
Photochemical reaction of 2 with HSiEt3 at 198 K
The reaction between 2 and HSiEt3 was then initiated by in situ
laser irradiation in d8-THF at 198 K. The resulting 31P{1H}
NMR spectrum now clearly showed the presence of a new
1
species (21) which yields a singlet at δ −16.2. A H–31P HMQC
experiment connected this resonance to further proton signals
at δ 7.07, 7.37, 7.74, 7.55, 7.62, 7.74, 8.09, 4.82 and δ −9.13 (d,
23.0 Hz). The δ 4.78 signal is characteristic of a cyclopentadi-
enyl ring while those at δ 7.09, 7.07 and 7.01 are due to ortho,
meta and para protons of a phenyl ring. The remaining four
signals, at δ 7.55, 7.62, 7.74, 7.18, were also observed to couple
Scheme 4 Reaction pathways for the formation of the orthometallation
product 21. Deuterium labelling indicates CpRu(κ2-2-C6H4PPh2)(SiEt3)D is
formed via route 2.
1
in a H–1H COSY experiment and are characteristic of those in
shows how the proportion of these species varies with photo-
lysis time. Such reactivity is expected.66
an orthometallated ring of triphenylphosphine. This is con-
firmed by the fact the δ −16.2 31P signal of 21 is similar in
position to that of orthometallated 3 which was seen at
δ −16.9. As the δ −9.13 hydride signal of 21 appears as a
doublet, where JPH = 23.0 Hz, it is which established as a
mono-phosphine complex.
Conclusions
The reactivity of 1 and 2 towards PEt3, ethene and DMSO has
been examined. Thermally, for the reaction with PEt3, the bis-
substituted products CpRu(PEt3)2Cl (4) and CpRu(PEt3)2Me (7)
are readily formed via their mono-substituted counterparts
CpRu(PPh3)(PEt3)Cl (5) and CpRu(PPh3)(PEt3)Me (8). In con-
trast, under broad-band UV irradiation, 1 and 2 react to selec-
tively produce 5 and 8. This conversion can be achieved with
325 nm irradiation for 2 but not 1. This suggests that the first
reaction step in this sequence is the loss of phosphine, to
form the corresponding 16-electron intermediate, which is
trapped by PEt3, or DSMO. When ethane is used and the reac-
tion examined at low temperature, CpRu(PPh3)(C2H4)Cl (6)
and CpRu(PPh3)(C2H4)Me (10) are readily formed in a similar
process. Under these conditions, no evidence for CH bond
activation of the alkene was seen, and upon warming the refor-
mation of 1 and 2, respectively, was observed.
1
Connections between the cyclopentadienyl H resonance of
21 and signals at δ 0.38, 0.54 and 0.81 in a 2D NOE experiment
were observed. The protons giving rise to three peaks were
shown to couple and hence they are attributed to CH2 (δ 0.38
and 0.54) and CH3 (δ 0.81) groups in a ruthenium-bound
triethylsilyl ligand; the two CH2 protons couple to a single
carbon resonance at δ 13.8. 21 is therefore identified as CpRu-
(κ2-2-C6H4PPh2)(SiEt3)H and upon warming it decomposes to
form an array of products including those described earlier.
Deuterotriethylsilane (DSiEt3) was then employed to
confirm how 21 forms. With DSiEt3, while signals for 21 were
seen, no hydride signal was visible at δ −9.13 in the corres-
ponding 1H NMR spectrum. This suggests that 21 possesses
a RuD group and that the orthometallation step, which yields
a RuH, must precede SiD bond activation. This sequence is
illustrated in Scheme 4.
In contrast, when 1 was irradiated in C6D12 with no added
ligand no reaction was found. Under the same conditions, 2
was observed to undergo a reaction where the activation of a
Photochemical reaction of 2 with dimethylvinylsilane
A reaction with dimethylvinylsilane was also examined. Low C–H bond of the phosphine led ultimately to the known
temperature in situ irradiation led to the formation of both complex CpRu(κ2-2-C6H4PPh2)(PPh3) (3).10,12 This suggests
CpRu(PPh3)(η3-Si(Me)2–CHvCH2) (22) and CpRu(PPh3)2- that the implicated 16-electron intermediate CpRu(PPh3)Me is
(Si(Me)2–CHvCH2) (23). The reactions progress was again able to react with an internal CH bond as there is no suitable
tracked using 1H and 31P{1H} NMR spectroscopy. ESI Fig. 3† trapping ligand present to form CpRu(κ2-2-C6H4PPh2)(Me)(H).
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The reaction of 2 with HSiEt3 did however enable the low
temperature detection of a hydride containing orthometallated
product in the form of CpRu(κ2-2-C6H4PPh2)(SiEt3)H (21).
Studies with DSiEt3 suggest that the elusive species CpRu(κ2-2-
C6H4PPh2)(Me)H rapidly eliminates MeH before adding HSiEt3
to 16-electron CpRu(κ2-2-C6H4PPh2) because CpRu(κ2-2-
C6H4PPh2)(SiEt3)D and not CpRu(κ2-2-C6H4PPh2)(SiEt3)H is
formed. Furthermore, upon monitoring the analogous reaction
of 2 with HSi(Me2)CHvCH2), the production of a 2 : 1 mixture
of CpRu(PPh3)(η3-Si(Me2)CHvCH2) (22) and CpRu(PPh3)2-
(Si(Me2)CHvCH2) (23) was achieved. This is consistent with
the suggestion that CpRu(κ2-2-C6H4PPh2) is indeed involved in
these reactions, as CpRu(κ2-2-C6H4PPh2)(Si(Me2)CHvCH2)H
can readily form both 22 or 23. These observations are sum-
marised in Scheme 5 and add to the earlier findings of Stone
and Lehmkuhl.4,12,13
We are grateful for financial support from the University of
York and the Spanish MEC Consolider Ingenio 2010-
ORFEO-CSD2007-00006 research programme. We also thank
Professor Robin Perutz for helpful discussions.
Scheme 5 Reaction mechanism as revealed through low temperature
photochemical studies on 2. Species in blue are not detected but
inferred from the observed reaction products.
Notes and references
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Subsequently MeH elimination and phosphine recoordination
to what is CpRu(κ2-2-C6H4PPh2) are expected to lead to 3.
In contrast, when the UV irradiation of 2 is completed in
THF at low temperature, the proposed 16-electron intermediate
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