The previous experiment demonstrated that the 16 electron complex, CpRu(PPh3)Me, can be trapped by coordination of the two electron donor PEt3 prior to orthometallation. This further demonstrated that CpRu(PPh3)2Me is photosensitive on a reasonable timescale at 198 K, where T1/2 is achieved upon 240 minutes of irradiation.
The preparation of CpRu(κ2-2-C6H4PPh2)(PPh3) from CpRu(PPh3)2Me proceeds initially also via the loss of triphenylphosphine, which is known to occur thermally 196 and has also been shown through our own work to occur via UV photolysis in cyclohexane (described in Chapter 7).
A series of photolysis experiments were therefore undertaken with CpRu(PPh3)2Me at low temperature (198 K) in order to generate, stabilise and ultimately characterise the observable reaction intermediates that are formed in such a reaction in the absence of a strongly ligating species such as PEt3. Four reaction outcomes might be envisaged:
- Detection of CpRu(κ2-2-C6H4PPh2)(PPh3) as a single product
- Loss of PPh3 and detection of CpRu(PPh3)Me which may be solvated - Detection of CpRu(κ2-2-C6H4PPh2)HMe
- Detection of CpRu(κ2-2-C6H4PPh2) which may be solvated or undergo CH bond activation
Based on literature precedent, stabilisation of the sixteen-electron intermediate, [CpRu(PPh3)Me], by phosphine loss, would require a coordinating solvent such as THF (coordination through oxygen) or toluene (η2
-arene coordination). 201 Through previous photolysis experiments, THF was found to be less likely to undergo CH activation when compared to toluene, which is consistent with the use of THF as a solvent for many C-H activation reactions. 202 Using THF would therefore provide an increased chance of forming a solvent “trapped” derivative of [CpRu(PPh3)Me]. Additionally, THF can act as a σ-donor thereby stabilising the ruthenium centre. For these reasons, it was decided to use THF (deuterated) initially in the low temperature photolysis experiments with
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CpRu(PPh3)2Me.
A J-Y NMR tube was therefore charged with CpRu(PPh3)2Me, and dry degassed d8- THF alone. The photolysis of this sample was conducted to see if any new products would form without any other reagents being present. The sample was irradiated for 18 hours using the in situ 325 nm laser at 198 K.
Four possible outcomes, indicated below, were identified as being possible in THF:
- Detection of CpRu(κ2-2-C6H4PPh2)(PPh3) as a single product
- Loss of PPh3 and detection of either CpRu(PPh3)(THF)Me or the THF C-D bond activation product CpRu(PPh3)Me(D)(OC4D7)
- Detection of CpRu(κ2-2-C6H4PPh2)HMe
- Detection of CpRu(κ2-2-C6H4PPh2) which again might be solvated or have undergone THF C-D bond activation
After four hours irradiation, a 1H NMR peak at δ 4.59 corresponding to the cyclopentadienyl ring of a new complex was observed. This resonance coupled to a 31P signal at δ 61.3 in a 1
H/31P HMQC experiment. Further coupling to a 1H peak at δ 0.16 due to a set of methyl protons was evident. This signal showed a doublet splitting of 5.91 Hz thereby confirming the formation of a mono phosphine based product. In agreement with this suggestion, a singlet was observed in the 31P NMR spectrum at δ - 6.4 due to free triphenylphosphine (Figure 3.11). The product giving rise to this signal decomposed when the temperature was raised above 218 K. A second minor product CpRu(PPh3)2(C4H7O) (which is discussed in more detail later in this Section) was also produced.
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Figure 3.11 31P{1H} NMR spectrum showing the 31P signals corresponding to CpRu(PPh3)(THF)Me and CpRu(PPh3)2(C4H7O), following 5 hours of photolysis
A 16-electron complex would be far too unstable to be observed at 193 K (typically these observations are made using IR techniques, while low temperature matrices are employed to stabilise the products). 203 The lack of a hydride resonance confirms that orthometallation of the phosphine has not occurred. There are therefore two possible reaction outcomes that need to be considered, the first being that THF has coordinated to the ruthenium centre via the oxygen lone pair. The second situation corresponds to where an interaction with the C-D bond occurs which could either break it or weakly interact with it. It should be noted that there are two types of C-D bond in d8-THF.
Similar half-sandwich rhodium complexes which possess a σ coordinated THF moiety have been previously reported in the literature. 204, 205 Such complexes have also been shown to be stable at temperatures approaching 223 K. Peaks belonging to coordinated THF cannot be seen in this 1H NMR spectrum, since d8-THF is used.
CpRu(PPh3)2Me
CpRu(PPh3)2(C4H7O)
CpRu(PPh3)(THF)Me
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Figure 3.12 A plot of the relative 31P resonances over time, for the photochemically formed products of the reaction between CpRu(PPh3)2Me and THF, at 193 K (original
illustration appears in colour)
a. CpRu(PPh3)2Me b. CpRu(PPh3)(THF)Me c. CpRu(PPh3)2(C4H7O)
Figure 3.12 illustrates the time course of this reaction, simulation of these data suggest that CpRu(PPh3)2Me reacts photochemically to form CpRu(PPh3)(THF)Me (Figure 3.13, A) and CpRu(PPh3)2(C4H7O) (Figure 3.13, B) in two competing pathways. The kobs for these processes are 0.050 s-1 (for CpRu(PPh3)2Me → CpRu(PPh3)(THF)Me) and 0.030 s-1 (for CpRu(PPh3)2Me → CpRu(PPh3)2(C4H7O), with no evidence of interconversion between CpRu(PPh3)(THF)Me and CpRu(PPh3)2(C4H7O). The presence of both of these complexes shows that THF is a weakly coordinating ligand, as THF can coordinate to the metal centre prior to orthometallation, but also associates after orthometallation, to for the C-H activation product, CpRu(PPh3)2(C4H7O). No evidence was obtained in these spectra for CpRu(PPh3)(C4H7O)(H)(Me) which must be unstable and rapidly eliminate MeH.
0 20 40 60 80 100 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Series1 Series2 Series3 a. b. c.
Duration of sample irradiation (hours)
R elative int ens it y of 31 P NM R s ignals ( % )
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Figure 3.13 Structure of CpRu(PPh3)(C4H8O)Me and CpRu(PPh3)2(C4H7O)
Ru O PPh3 Me O Ru PPh3 PPh3 A B
These products were identified through the use of a second sample containing a 2:1 mixture of protio THF and d8-THF. This allows 1H/13C HMQC data to be recorded, and hence the detection of key 1H resonances. The sample was irradiated for a total time of 18 hours at 198 K (to ensure maximum conversion to any potentially new products given the limited liquid N2 source used for cooling) and tracked during the irradiation by recording 1D 1H and 31P{1H} NMR experiments every 12 minutes.
Examination of the NMR spectra after 4 hours revealed the lack of any signals that could be associated with a ruthenium-hydride or a Ru-2-CH signal. 206-210 The δ 61.3 31
P resonance is clearly visible but no protio THF signals could be located. We therefore conclude that this highly unstable product is fluxional even at 198 K and a σ coordinated THF moiety is likely.
As mentioned, this species converted into a new product, CpRu(PPh3)2(C4H7O) (Figure 3.13, B), which was stable at room temperature and yields a resonance at δ 41.9 in the 31
P NMR spectrum. The 31P resonance of CpRu(PPh3)2(C4H7O) appears at δ 41.9 and connects, using a 1H/31P HMQC experiment, to a Cp resonance at δ 4.23 and to other peaks at δ 5.37, 3.86, 3.70, 1.75, 1.94, 2.41 and 2.19 which were subsequently indentified as proton resonances of a tetrahydrofuryl group. Significantly, there was no methyl proton resonance in this product. Bound ortho, meta and para PPh3 resonances were located at δ 7.38, 7.16 and 7.09, using 1H/31P HMQC and 1H COSY NMR experiments. The 1H signal at δ 5.37 was identified as the α-proton of a ruthenium- bound tetrahydrofuryl group, which was determined by the coupling observed by this proton signal to a carbon signal at δ 145.7 (t, 17.8 Hz), found using a 1H/13C HMQC
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NMR experiment. The triplet splitting indicated the presence of two triphenylphosphine within the structure, and the coupling constant of 17.8 Hz, is typical of a |2JCP| splitting, consistent with a ruthenium-bound carbon in this complex. A 1H COSY experiment was employed to show the coupling between the signal at δ 5.37 and the remaining pairs of inequivalent protons, for the tetrahydrofuryl moiety; δ 3.86, 3.70, 1.75, 1.94, 2.41 and 2.19. Complete characterisation data for this complex appears in Table 3.9.
Integration of 1H NMR signals described above confirmed that a complex of the type CpRu(PPh3)2(C4H7O) was formed and the chemical shift data would suggest that the Ru-C bond involves the carbon adjacent to oxygen. This site of activation is consistent with the known acidity of these C-H bonds 211 and studies of similar complexes. 212, 213
Upon re-examination of these 1H NMR spectra, a peak consistent with formation of protio-methane (singlet at δ 0.19) 214 was evident (Figure 3.14). If this were formed from the d8-THF, it would be CH3D. This suggests that the orthometallation product CpRu(κ2
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Figure 3.14 1H spectra depicting the increasing intensity of the signal corresponding to liberated methane (original in colour)
To determine whether CpRu(PPh3)2(C4H7O) can indeed formed via the orthometallated intermediate [CpRu(κ2-2-C6H4PPh2)], a sample of CpRu(κ2-2-C6H4PPh2)(PPh3) was prepared by the UV irradiation of CpRu(PPh3)2Me in cyclohexane within the NMR tube. Following the full conversion to CpRu(κ2-2-C6H4PPh2)(PPh3), the cyclohexane solvent was replaced with d8-THF. Further photolysis of the sample for 16 hours resulted in 70% conversion to CpRu(PPh3)2(C4H7O). Evidence for partial deuteration of the PPh3 ligand in the ortho positionwas now evident in the 1H NMR spectrum (this is further described in Section 3.4.12). This is consistent with the low temperature photochemical formation of CpRu(PPh3)2(C4H7O).
Inc re as ing i rr adi at ion ti m e CH3 triplet of CpRu(PPh3)2Me CH3 doublet of CpRu(PPh3)(THF)Me Free CH4 signal 5.9 Hz
Expansion of the 1H signal for the CH 3 of
CpRu(PPh3)(THF)Me, with the accompanying
CH4 signal. After 20 hours of irradiation.
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