CAPÍTULO III EL APOYO DE SERVICIO DE
C. El Batallón de Transportes en Apoyo
In order to demonstrate phosphine loss and the formation of the 16 electron fragment, [CpRu(PPh3)Me] upon UV irradiation, a reaction was undertaken in the presence of an excess of triethylphosphine. The triethylphosphine ligand should efficiently “trap” the intermediate, [CpRu(PPh3)Me], as it is formed, and therefore allow characterisation of the new mono-substituted phosphine complex, CpRu(PPh3)(PEt3)Me. This approach was described in Chapter 2 for CpRu(PPh3)2Cl.
3.4.1.1 Thermal reactions of CpRu(PPh3)2Me with PEt3
A J-Y NMR tube was charged with 5 mg of CpRu(PPh3)2Me and dissolved in d6- benzene, to a depth of 4 cm. Using a micro syringe 9 μl of PEt3 was added to the sample. Initial 1H and 31P{1H} NMR spectra were recorded prior to the thermal reaction. The sample tube was heated to 323 K in a silicone oil bath for 24 hours.
Comparison of the 31P{1H} NMR recorded prior to thermal reaction and after, reveal the presence of four new peaks, with the 31P resonance for CpRu(PPh3)2Me (δ 55.3) being absent. These signals appeared at δ -6.4, 35.2, 62.9 and 40.5. The peak at δ -6.4 corresponds to the resonance of liberated triphenylphosphine, suggesting that one or more of the new complexes has exchanged phosphine ligands. The peaks at δ 35.2 and 62.9 possess a mutual coupling of 39.3 Hz suggesting that two distinct phosphine ligands are coordinated to the metal centre in this complex. Owing to the small intensity of the signal, full characterisation of this complex was not possible at this point (see the subsequent reaction for full characterisation of this product). The following experiments employing photochemical irradiation at low temperature, led to greater conversion to this species, CpRu(PPh3)(PEt3)Me, and allowed its full NMR characterisation.
The dominant product of the thermal reaction yields the 31P signal at δ 40.5. This signal appears as a singlet, which indicates that the complex either contains only one phosphine ligand, or if two, both are the same. A 1H/31P HMQC experiment connected this phosphine resonance with signals in the 1H NMR spectrum at δ 0.15 (t, 5.8), 1.48,
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1.90, 0.92 and 4.54. The lack of any connections to signals corresponding to aryl protons clearly demonstrates that triphenylphosphine is not present in this complex. The signal at δ 4.54 is consistent with the protons of a ruthenium bound η5
-cyclopentadienyl moiety. The resonance at δ 0.15 appears as a triplet with a 5.8 Hz splitting. Integration of this signal with the cyclopentadienyl proton resonance at δ 4.54 reveals a ratio of 3:5 (respectively), confirming that this signal belongs to a ruthenium-bound methyl group. The triplet splitting is characteristic of a bis-phosphine complex (as discussed earlier in this chapter for CpRu(PPh3)2Me), suggesting that the identity of this complex as CpRu(PEt3)2Me. Confirmation of this identity was made through the investigation of the remaining signals at δ 1.48, 1.90 and 0.92, which were found to mutually couple through a 1H COSY NMR experiment. The signals at δ 1.48 and 1.90 were observed to mutually couple to the same carbon resonance at δ 31.8 in a 1H/13C HMQC experiment. This result clearly shows that these proton resonances correspond to the diastereotopic protons on the methylene carbon of the ruthenium-bound PEt3 ligand. The resonance at δ 0.92 may similarly be assigned as belonging to the protons of the terminal methyl of the ethyl chain of the PEt3 ligand. In the corresponding ESI spectrum obtained after reaction, peaks at m/z M+ 418 and [M+ - PEt3] 299, were found, which also confirms the identity as CpRu(PEt3)2Me. This suggests that CpRu(PPh3)(PEt3)Me reacts readily thermally to form CpRu(PEt3)2Me at 323 K.
Figure 3.7 Structure of CpRu(PEt3)2Me
Ru PEt3
PEt3 Me
3.4.1.2 Photolysis of CpRu(PPh3)2Me with PEt3
In order to plot the formation of the mono and bis products, the in situ-laser setup was employed to monitor the progress of the reaction at room temperature (Figure 3.8). The plot demonstrates that after an irradiation time of 18 hours full conversion of the complex CpRu(PPh3)2Me to CpRu(PEt3)2Me can be achieved. The intermediate
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complex, CpRu(PPh3)(PEt3)Me, achieves a maximum intensity of ~70% after 7.5 hours, prior to the total conversion to CpRu(PEt3)2Me.
Figure 3.8 A plot of the relative 31P resonances over time, for the photochemically formed products of the reaction between CpRu(PPh3)2Me and PEt3, at 298 K (original
illustration appears in colour)
a. CpRu(PPh3)2Me b. CpRu(PPh3)(PEt3)Me c. CpRu(PEt3)2Me
Simulation of these data using a kinetic model that allows CpRu(PPh3)2Me to form CpRu(PPh3)(PEt3)Me, CpRu(PPh3)2Me to form CpRu(PEt3)2Me and CpRu(PPh3)(PEt3)Me to form CpRu(PEt3)2Me. This produced effective photochemical rates of formation (which shall now be referred to as kobs) of 0.311, 0.079 and 0.154 s-1 for each process respectively. This confirms that the pathway involves the stepwise conversion from CpRu(PPh3)2Me to CpRu(PPh3)(PEt3)Me to CpRu(PEt3)2Me. The activity of CpRu(PPh3)(PEt3)Me relative to CpRu(PPh3)2Me is 2:1. This shows that CpRu(PPh3)2Me is more photoactive than CpRu(PPh3)(PEt3)Me.
b.
a.
c.
Duration of sample irradiation (hours)
R elative int ens it y of 31 P NM R s ignals ( % )
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3.4.1.3 Photolysis of CpRu(PPh3)2Me with PEt3 at 198 K
In a second reaction, PEt3 was added to a Young’s tap capped NMR tube containing CpRu(PPh3)2Me in d8-THF. The sample was degassed prior to in situ irradiation using a UV laser for 18 hours at 198 K. The progress of the reaction was tracked during this period by 1D 1H and 31P{1H} NMR methods. A comparison of the 1H NMR spectrum recorded prior to UV irradiation with one that was recorded after 4 hours of irradiation, showed new proton resonances at δ 0.39, 0.83, 1.39, 1.50 and 4.49. The peak at δ 4.49 is consistent with the expected position of the proton resonances of the cyclopentadienyl ring in CpRu(PPh3)(PEt3)Me. A new triplet (5.6 Hz) peak at δ 0.39 is consistent with the proton resonances of a ruthenium-bound methyl group, the apparent triplet splitting indicates that two phosphine ligands couple to this group and are hence bound to the complex.
There are no signals in the high field region (δ 0 to -20 range) of the 1H NMR spectrum where hydride resonances are commonly observed. The lack of a hydride signal and the observed methyl resonance suggest that no cyclometallated phosphine ligands are present within the complex. Nor has cyclometallation been involved in the route to the formation of this complex because as demonstrated later such products should be stable. This is due to the presence of the methyl group within the present complex. Were CH activation to have occurred, rapid elimination of methane would be expected.
The corresponding 31P{1H} NMR spectrum, recorded after 4 hours clearly showed the appearance of several new resonances. A large singlet resonance at δ -6.4 is characteristic of free triphenylphosphine, which shows that PPh3 had been lost from the parent complex thereby indicating that phosphine exchange had occurred. The lack of any other resonances with negative chemical shifts means that cyclometallation of either of the phosphine ligands to the ruthenium centre had not occurred. There were two doublet peaks of equal intensity that share a common splitting of 39.3 Hz at δ 35.2 and 62.9 in the 31P{1H} NMR spectrum. These doublet resonances represent the two 31P nuclei in the distinct environments of the two phosphine ligands for the complex CpRu(PPh3)(PEt3)Me. The mutual coupling between the two 31P nuclei results in the 39.3 Hz splitting of these resonances. The chart shown in Figure 3.9 presents the change in relative intensities of these 31P signals as a function of the irradiation time. The
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corresponding 1H spectra contained characteristic C5H5 resonances which also vary in a similar way. The kobs for the process CpRu(PPh3)2Me → CpRu(PPh3)(PEt3)Me is 0.094 s-1, with no evidence in the NMR spectra for the formation of C-H activation products. The corresponding T1/2 is 510 minutes.
Figure 3.9 A plot of the relative 31P resonances over time, for the photochemically formed products of the reaction between CpRu(PPh3)2Me and PEt3, at 193 K (original
illustration appears in colour)
a. CpRu(PPh3)2Me b. CpRu(PPh3)(PEt3)Me
A 1H/31P HMQC of the sample showed that these two 31P resonances connected to common 1H resonances at δ 0.39, 1.39, 1.50, 4.49 and 7.61. The resonance at 31P δ 62.9 couples strongly to 1H resonances at δ 7.61, 7.06 and 6.97, which comprise the ortho,
meta and para phenyl protons of the bound triphenylphosphine ligand. Signals at 1H δ
0.83, 1.39 and 1.50 were found to couple strongly to the 31P resonance at δ 35.2. These peaks represent the proton resonances of the ethyl chains of the triethylphosphine. The proton resonances at δ 1.39 and 1.50 correspond to the inequivalent CH2 protons of the triethylphosphine ligand (confirmed by their mutual coupling to a common carbon at δ 23.4 in the corresponding 1H/13C HMQC experiment).
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 a. b.
Duration of sample irradiation (hours)
R elative int ens it y of 31 P NM R s ignals ( % )
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Figure 3.10 Structure of CpRu(PPh3)(PEt3)Me
Ru
PEt3
PPh3 Me
The observation of this complex as a product of the irradiation of CpRu(PPh3)2Me with triethylphosphine, confirms that the reaction intermediate [CpRu(PPh3)Me] is formed under photolysis, although PEt3 is so reactive no other products are formed. Under thermal conditions, the dominance of CpRu(PEt3)2Me (Figure 3.10) suggests that complete phosphine exchange is possible. Under irradiation however, the selective formation of CpRu(PPh3)(PEt3)Me is achieved even after 30 hours of photolysis with the 325 nm source.
It can therefore be concluded that the reaction of CpRu(PPh3)2Me with PEt3 reacts both thermally and photochemically to form CpRu(PPh3)(PEt3)Me. Subsequent reactivity to form CpRu(PEt3)2Me occurs thermally, but was also observed in photochemical reactions as a consequence of warming effects of the UV lamp. At low temperature, selective photochemistry allowed for the conversion to CpRu(PPh3)(PEt3)Me only. These results are consistent with those found for the analogous reactions conducted with CpRu(PPh3)2Cl in Chapter 2.
The chloride derivative described in the previous chapter reacts to form similar complexes. Neither of these complexes undergo C-H activation with the alkyl chains of PEt3, or the ortho phenyl protons of PPh3, suggesting that association of the phosphine occurs faster than the orthometallation process. The corresponding 31P resonances for the chloride derivative appear to be shifter significantly upfield compared with those observed for the methyl derivative. This is likely attributed to the electron withdrawing effect of the chloride moiety, which may also be responsible for its lack of ability to orthometallate (as discussed in Chapter 1, electron-rich metal centres tend to readily activate C-H bonds). In both cases the JPP coupling constants for the mono substituted product appear similar, and are indicative of a cis-type phosphine arrangement.
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