CAPÍTULO V. EMPLEO DEL BATALLÓN DE COMUNICA-
B. EMPLEO DEL BATALLÓN DE COMUNICACIONES
3.4.2.1 Conversion of diphenylacetylene into stilbene through
Cycle 1
The first hydride transfer reaction proceeds through transition state 11a’ where the hydride ligand is 1.53 Å from diphenylacetylene. This reaction leads to Ru(H)(CO)(dhae)(η1- CPh=CPhH), as 15a, with the remaining hydride ligand trans to the vacant site on the metal and the vinyl group trans to dhae. The barrier for from this reaction (11a’) is 36.7 kJ mol-1 and the overall reaction is favourable by -34.1 kJ mol-1. The second hydride transfer
reaction can then proceed through transition state 15a’ which has a significant barrier of 74.6 kJ mol-1. This transition state has the hydride ligand at a distance of 1.51 Å from the vinyl α-carbon, with the hydride adding to the vinyl group on the same face as the first hydride, forming cis-stilbene. The reaction then proceeds through a rearrangement where the CO ligand moves cis to both ends of the dhae ligand. This rearrangement leads to a more stable geometry and a further stabilisation occurs via interaction with the phenyl ring of the vinyl ligand. This forms species 16a from 15a, with an enthalpy change of -69.6 kJ mol-1. This difference in stability with position of the CO ligand was previously reflected in the formation of two isomers of Ru(CO)(dhae)(η2-diphenyacetylene) as 13a and 13b.
The addition of dihydrogen to 16-electron 16a proceeds via a barrierless oxidative addition forming Ru(H)2(CO)(dhae)(η2-cis-stilbene) (17b, 17a is discussed shortly), with an enthalpy
change of -28.2 kJ mol-1. This complex has a different arrangement of the hydride and CO ligands, that is more akin to 11b, where both hydrides are cis to cis-stilbene. This change results in the rearrangement of the ligand sphere during the transition from 15a’ to 16a. The dissociation of cis-stilbene from 17b then forms 5-coordinate Ru(H)2(CO)(dhae)
(previously identified 6c) with an enthalpy change of +120.4 kJ mol-1. The free energy change from this loss is more favourable at +63.1 kJ mol-1 but this value will be larger than the true value due to the over estimation of the entropic effects from forming two species from one in the gas phase. The free energy profile is revealed to be similar to that of the enthalpy profile, with the coordination of dihydrogen to 16a being slightly unfavourable due to the loss of entropy when two molecules combine to form one. Detailed illustrations of the key geometries identified are illustrated in Figure 3.12.
141
Figure 3.12: Illustrations of key geometries involved in the beginning of Cycle 1
The incorporation of dihydrogen into Ru(H)(CO)(dhae)(η1-CPh=CPhH) (15a) prior to the second hydride transfer reaction was examined. This addition was found to be favourable by -26.7 kJ mol-1 and leads to Ru(H)(CO)(dhae)(η2-H2)(η1-CPh=CPhH) (as 16b). This modest
stabilisation reflects the coordination of dihydrogen and results in an 18-electron complex without oxidative addition. The second hydride transfer reaction can then proceed through transition state 16b’ to form coordinated cis-stilbene. The dihydrogen ligand is simultaneously oxidised and the dihydride complex 17a forms. The barrier to this migration is 44.2 kJ mol-1, and is lower than the barrier from Ru(H)(CO)(dhae)(η1-CPh=CPhH) as 15a. The resultant complex Ru(H)2(CO)(dhae)(η2-cis-stilbene), as 17a, is analogous to the starting
complex 11a, with one hydride cis to cis-stilbene and the second hydride trans to it. The loss of cis-stilbene reforms Ru(H)2(CO)(dhae) as 6a which can then recoordinate
diphenylacetylene and reform 11a and start the cycle again. This loss is unfavourable in terms of enthalpy and free energy, with changes of 98.4 and 38.9 kJ mol-1 respectively. The geometry of transition state 16b’ and the final geometries of Ru(H)2(CO)(dhae)(η2-cis-
stilbene) as isomers 17a and 17b are illustrated in Figure 3.13.
It is also feasible that a second molecule of diphenylacetylene can coordinate to the complex whenever a vacant site is created; the coordination to 15a is discussed in Section 3.5. Coordination to Ru(CO)(dhae)(η2-cis-stilbene) (17b) is significantly unfavourable; the steric repulsion of the phenyl rings in the coordinated cis-stilbene effectively shields the metal and makes the approach of an incoming molecule of diphenylacetylene unlikely. This
15a’
142 contrasts the approach of dihydrogen which is significantly smaller and able to approach the metal centre successfully. The thermodynamic values for the reactions identified here are listed in Table 3.4. The profiles for these thermodynamic changes are illustrated in Figure 3.14.
Figure 3.13: Detailed illustrations of transition state 16b’ and later-stage complexes of Cycle 1
Table 3.4: Summary of the thermodynamic values for diphenylacetylene hydrogenation according to
Cycle 1. Values are in kJ mol-1
Label Formed from Reaction Relative Enthalpy Relative free energy 11a’ 11a Hydride migration barrier 157.0 166.5
15a 11a Hydride migration 86.1 93.7
15a’ 15a Hydride migration barrier 160.7 165.5
16a 15a Hydride migration 16.5 32.9
17b 16a Oxidative addition of H2 -11.7 37.3
16b 15a Coordination of H2 59.4 103.2
16b’ 16b Hydride migration barrier 103.6 153.8
17a 16b Hydride migration -31.4 20.0
143
Figure 3.14: Relative enthalpy profile for the catalytic hydrogenation of diphenylacetylene starting
from Ru(H)2(CO)(dhae)(η2-diphenylacetylene) as 11a, forming cis-stilbene (Cycle 1). The free energy
profile is shown in red.
The loss of cis-stilbene from Ru(H)2(CO)(dhae)(η2-cis-stilbene) as isomers 17a and 17b
results in the formation of Ru(H)2(CO)(dhae), as intermediates 6a and 6c respectively; the
reaction of 6a has been described in Section 3.3.2.1 and forms 11a. 11b can result from the favourable addition of diphenylacetylene to 6c, with relative enthalpy and free energy changes of -124.3 and -66.7 kJ mol-1 respectively. It should be noted that the feasible reaction to form 17b means that the minor starting complex 11b is likely to become the dominant species in the hydrogenation of diphenylacetylene once hydrogenation has been initiated. This is due to the reaction with the stable complex 16a being more likely than the intermediate 15a, as 16a requires the approach of another ligand whereas 15a can react further without involving another molecule.
11a
11a’
15a
15a’
16a
17b
6c
+cis-
stilbene
120.2
157.0
86.1
160.7
16.5
-11.7
108.9
67.0
6a
103.6
16b’
16b
17a
59.4
-31.4
144
3.4.2.1 Conversion of diphenylacetylene into stilbene through
Cycle 2
Cycle 2 starts with Ru(H)2(CO)(dhae)(η2-diphenylacetylene) as 11b, where
diphenylacetylene is trans to one end of the dhae ligand and cis to both hydride ligands. Diphenylacetylene is not aligned with either plane formed by the other ligands and forms 45° dihedral angles with both hydride ligands, illustrated in Figure 3.10 in 11b. This alignment reduces the steric interaction between the phenyl rings and any other ligand in the complex. Both hydride ligands are capable of undergoing hydride transfer to diphenylacetylene but only the hydride trans to CO is considered, as this will create less steric repulsion in the resulting transition state. If the models used included the phenyl rings on dhae, this steric interaction would be more significant.
Hydride migration proceeds through transition state 11b’ which creates a low barrier of 15.3 kJ mol-1 to the formation of Ru(H)(CO)(dhae)η1-CPh=CPhH) as 15b. The remaining hydride ligand is cis to the vinyl group and so the second hydride migration reaction is possible. This migration proceeds through transition state 15b’ which forms a barrier of 62.2 kJ mol-1. This transition state leads to cis-stilbene, and these transition states are illustrated in Figure 3.15.
A subtle rearrangement in 15b was identified which changes the pathway of the second hydride transfer, which involves a minor barrier of 2.1 kJ mol-1 as 15c’. This rotation, involving the vinyl ligand, changes the orientation of the transferred hydrogen on the β- carbon and the phenyl ring on the α-carbon such that it is directed towards the arsenic centre rather than the hydride ligand and leads to the formation of 15c, which is 6.0 kJ mol-1 more stable than 15b.
145
Figure 3.15: Illustrations of selected intermediates identified at the beginning of Cycle 2
Hydride transfer in 15c has a low barrier of 9.7 kJ mol-1 and therefore likely to dominate. Importantly, this hydride migration proceeds to the opposite face of the alkene bond and forms trans-stilbene. Relaxed constrained scans showed that the transition state 15d’ leads to the trans isomer regardless of the initial vinyl alignment. Additionally, a similar scan with the full model (where dhae was replaced by dpae) revealed trans-stilbene formation to still occur. The resulting complex, 16c, is formed favourably, with an enthalpy change of 110.1 kJ mol-1 and is stabilised by an interaction with a phenyl ring of stilbene as seen with 16a. Addition of dihydrogen to 16c forms 17c, which is of similar geometry to 17b but contains trans-stilbene.
Coordination of dihydrogen to Ru(H)(CO)(dhae)(η1-CPh=CPhH) (as 15b or 15c) prior to the second hydride transfer reaction forms Ru(H)(CO)(dhae)(η2-H2)(η1-CPh=CPhH) as 16d. The
subsequent reaction via 16d’ involves the simultaneous oxidation of dihydrogen and the second hydride transfer. The barrier for this is significant at 63.3 kJ mol-1, and higher than the route without dihydrogen addition. This transition state leads to cis-stilbene, and results in Ru(CO)(dhae)(η2-H2)(η2-cis-stilbene) as isomer 17b. The barrier from 16d’ is in
keeping with that calculated in Cycle 1 as 15b’, leading to the formation of cis-stilbene. The geometries of selected intermediates and transition states are illustrated in Figure 3.16. The exact pathway followed will therefore depend on the concentration of H2. Hence, the
pathways will operate in competition.
146
Figure 3.16: Illustrations of selected major intermediates identified for Cycle 2
Ru(H)2(CO)(dhae)(η2-cis-stilbene), as 17b, can then undergo dissociation of cis-stilbene and
the formation of 16-electron 6c as previously detailed. Loss of trans-stilbene from 17c also leads to the formation of 6c, which involves an enthalpy change of 102.8 kJ mol-1 and a
lower free energy change of 43.2 kJ mol-1. Intermediate 6c can then add another diphenylacetylene ligand and can re-enter the hydrogenation cycle. This addition was also described for Cycle 1. The reaction enthalpies, and free energies changes are detailed in Table 3.5, with the pathways illustrated in Figure 3.17. The rearrangement from 15b to 15c is not included for clarity.
Table 3.5: Summary of the thermodynamic values for diphenylacetylene hydrogenation via Cycle 2.
Values are in kJ mol-1
Label Formed from Reaction Relative Enthalpy Relative free energy 11b’ 11b Hydride migration barrier 161.4 168.8
15c 11b Hydride migration 112.9 125.9
15d’ 15c Hydride migration barrier 128.6 138.9
16c 15c Hydride migration 8.8 25.1
17c 16c Coordination of H2 -14.1 31.2
16d 15c Coordination of H2 66.8 109.3
16d’ 16d Hydride migration barrier 130.1 174.7
17b 16d Hydride migration 4.1 49.9
147
Figure 3.17: Relative enthalpy profile for the catalytic hydrogenation of diphenylacetylene starting
from complex 11b, forming cis and trans-stilbene (Cycle 2). The free energy profiles are shown in red. The terms cisS and transS refer to cis- and trans-stilbene respectively.
The binding of another molecule of diphenylacetylene to intermediate Ru(H)(CO)(dhae)(η1- CPh=CPhH) as 15c is discussed in Section 3.5. The binding of diphenylacetylene to the vacant site in Ru(H)(CO)(dhae)(η2-trans-stilbene), as 16c, is unfavourable due to the steric repulsion from the trans-stilbene ligand; this ligand also interacts with the metal to shield the vacant site.
15d’
16c
16d’
17b
16d
146.1
17c
161.4
112.9
128.6
66.8
130.1
8.8
-14.1
4.1
11b
11b’
15c
6c
+cisS
6c
+transS
148
3.4.2.2 Conversion of diphenylacetylene into stilbene through
Cycle 3
Only one hydride ligand is cis to diphenylacetylene in Ru(H)2(CO)2(κ1-dhae)(η2-
diphenylacetylene), as 12a, and so hydride transfer proceeds through this route (transition state 12a’). This process is downhill with a barrier of 22.1 kJ mol-1 (from 12a’, forming 12c) with a rearrangement to 18a having a barrier of 9.6 kJ mol-1. The complex produced via 12c’ (18a) is 45.5 kJ mol-1 more stable than 12a. The second hydride migration via 18a has a sizable barrier of 86.5 kJ mol-1 via transition state 18a’ and is therefore disfavoured. Alternatively, dihydrogen can add to 18a forming Ru(H)(CO)(η1-dhae)(η2-H2)(η1-CPh=CPhH)
as 19a. The second transfer reaction then involves transition state 19a’ and a barrier of 79.5 kJ mol-1. The most likely pathway is now however recoordination of the free end of the η1- dhae ligand. Recoordination occurs through a small barrier of 7.3 kJ mol-1 (from 18aR’) and forms complex Ru(H)(CO)2(dhae)(η1-CPh=CPhH), as isomer 20a. The second hydride transfer
reaction then occurs through transition state 20a’ with a barrier of 73.8 kJ mol-1 (enthalpy), to form Ru(CO)2(dhae)(η2-cis-stilbene) as 20b. This complex is similar to experimentally
identified A8. Whilst the barrier for this second hydride transfer reaction is the same as those calculated with 18a’ and 19a’, the recoordination of the free end of η1-dhae is likely due to the chelate effect. Selected key transition states are also illustrated in Figure 3.18.
Figure 3.18: Detailed illustrations of key transition states identified in Cycle 3
The loss of cis-stilbene from 20b is unfavourable by +92.1 kJ mol-1 and forms Ru(CO)2(dhae),
as 4b, as identified previously. The free energy change is more favourable at 31.1 kJ mol-1. This 16-electron intermediate can then react with diphenylacetylene or hydrogen as previously described. These geometries and thermodynamic profiles are illustrated in Figure 3.19, with the reaction enthalpies and free energies shown in Table 3.6.
149
Table 3.6: Summary of thermodynamic values for diphenylacetylene hydrogenation via Cycle 3.
Values are in kJ mol-1
Label Formed from Reaction Relative Enthalpy Relative free energy 12a' 12a Hydride migration barrier 92.4 127.6
12d 12a Hydride migration 64.0 101.9
12d’ 12d Rearrangement barrier 73.6 115.3
18a 12d Rearrangement 18.5 58.9
18a’ 18a Hydride migration barrier 86.5 138.4
19a 18a Coordination of H2 0.3 69.8
19a’ 19a Hydride migration barrier 79.8 177.5 18aR’ 18a Recoordination of η1-dhae
barrier
25.8 72.6
20a 18a Recoordination of η1-dhae -51.2 4.7 20a’ 20a Hydride migration barrier 22.6 87.0
20b 20a Hydride migration -92.5 -34.3
Figure 3.19: Catalytic cycle for the hydrogenation of diphenylacetylene from complex 17a (Cycle 3).
The pathway shown in blue is for the recoordination of the free end of η1-dhae with 18a. The free energy profile is illustrated in red
18aR’
20a
20a’
20b
12a
12a’
12d
19a
18a
12d’
18a’
70.3
4b
+cisS
19a’
150 It should be noted that the barriers for recoordination of the chelate could be significantly lower here than in the real system, due to the simplification of dpae to dhae.
It is possible that de-chelation can occur with 20b in competition with the loss of cis- stilbene to form 4b. However the analysis of the thermodynamic changes for this reaction revealed that cis-stilbene dissociation was favoured. Formation of 4b allows the reformation of Ru(H)2(CO)2(dhae), as 3a, which was previously determined to be a stable
species in Chapter 2. Hence, the hydrogenation of either isomer of stilbene though a de- chelation route can be considered to be negligible.
3.4.2.3 Summary of diphenylacetylene hydrogenation pathways
The pathways identified for the three starting species all lead to cis-stilbene according to the calculations performed here. It has been found that the formation of trans-stilbene is also possible, but this appears to require the correct distribution of inner sphere ligands to allow rotation of the C=C bond of the vinyl ligand. This rotation allows the second hydride transfer reaction to take place on the opposite face to that of the β-CH group formed during the first hydride transfer pathway.The first hydride transfer reaction was found to proceed through relatively low barriers, with the second transfer proceeding through significantly higher barriers, except that of 15d’ in Cycle 2. These low barriers are consistent with the failure to experimentally detect any complexes of the type Ru(H)2(CO)(dpae)(η2-diphenylacetylene). The sizeable barriers
predicted for the second hydride transfer reaction mean that the lifetime of the 16-electron intermediates of the type Ru(H)(CO)(dpae)(η1-CPh=CPhH) can be significant enough for a successful collision to occur with another ligand, potentially changing the products formed. The steric crowding of the metal centre could still allow the coordination of another ligand, indicated by the barriers predicted for the recoordination of the free end of the η1-dhae ligand with Cycle 3.
The role of the alkene in these reactions was not innocent. The interaction of the phenyl groups in cis or trans-stilbene with the metal centre did not prevent coordination of dihydrogen to form a dihydride complex capable of further hydrogenation, although, it should be noted that such complexes were not seen experimentally. The ability of these
151 complexes to further react could account for this observation. The steric bulk of cis-stilbene was found to aid in its dissociation from the metal complex more than the dissociation of trans-stilbene. This is consistent with the alignment of the phenyl rings. Crowding of the metal centre by either isomer of stilbene provides a barrier to the coordination of a second molecule of diphenylacetylene in contrast to the approach of dihydrogen.