Photochemical initiation of the reaction with Ru(CO)3(dpae) (A1) was observed to lead to
Ru(H)2(CO)2(dpae), A3 as the sole photoproduct. This could involve single carbonyl loss, as
described, or 14-electron Ru(CO)(dpae). Previous studies indicate this double ligand loss product is involved with the phosphine analogues.[142, 143] The loss of two carbonyls from 1 was therefore modelled and three intermediates identified as 5a–b3. Intermediates 5a and 5a3 have similar structures with the remaining carbonyl approximately trans to an arsenic centre whereas in 5b3, the CO ligand lies out of the dhae-metal plane. The geometries of these three structures are in shown in Figure 2.14 and it can be seen that 5a and 5a3 are of similar energy whereas 5b3 is higher in energy.
Figure 2.14: Geometries and thermodynamic values of the identified 14 electron intermediates of
Ru(CO)(dhae)
The thermodynamic values calculated here reveal no significant preference for the formation of one 14-electron species over another. Importantly, the formation of singlet 5a
5a
5a
35b
3397.8
(311.4)
391.8
(298.5)
406.4
(315.4)
77 is potentially favoured, as it requires no spin flip transition during its formation. These three 14-electron species can interact with solvent, dihydrogen or one of the dissociated carbonyls in the next step. Interaction with the toluene solvent is addressed in Section 2.3.6. The recombination of a carbonyl ligand to singlet 5a results in singlet 16-electron 4b. The formation of singlet 4a would require an unfavourable rearrangement and so only 4b is formed. This results in only one 16-electron isomer which reacts with dihydrogen to form 3a. Intermediate 4b can reform 1 by adding a second carbonyl ligand. The addition of CO to the two 14-electron triplet intermediates can also occur, creating 16-electron triplets. In these reactions, no spin-flip is required and so is feasible. It is also worth noting that none of the 14-electron intermediates identified are accessible thermally due to the significant thermodynamic cost of their formation. Due to the significant enhancement from para- hydrogen observed experimentally, any pathway involving a triplet species is minor and so not considered further.
The addition of dihydrogen to singlet 5a results in 16-electron Ru(H)2(CO)(dhae) (6a), where
one hydride ligand is trans to the carbonyl ligand, the other hydride being trans to a vacant site. This geometry is thermodynamically the most favourable as hydride ligands are most stable when no ligand is present trans to them; this maximises the electron density they share with the metal in unsaturated systems. The subsequent binding of CO to 6a results in the formation of 3a as the sole product. The binding of another dihydrogen molecule to 6a results though in dihydride-dihydrogen species, 6b, which is unable to undergo further reaction. Whilst the binding of the dihydrogen ligand is favourable, the free energy change is favourable by only -51 kJ mol-1 compared to the change of -88 kJ mol-1 in terms of enthalpy. This means that the recombination of a carbonyl is preferred to form 3a. These structures and their relative energies are shown in Figure 2.15. The experimental data showed no difference in the reaction products with thermal or photochemical initiation (apart from the intensity increase of 3.4 fold for the signals arising from A3 with photolysis at 333 K), with dihydride 3a observed as the only reaction product. The complexes identified here, with associated thermodynamic values, are consistent with these findings. One significant result shown in Figure 2.15 is that 5a results in the formation of only the starting complex and dihydride 3a in keeping with experimental findings.
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Figure 2.15: Relative enthalpy profile for the reactions of 14-electron Ru(CO)(dhae) as 5a (formed via
photochemical initiation) with CO or H2
When the full ligand system is introduced, the geometries of the three 14-electron states and reaction thermodynamic values remain very similar to those of the simple model. These are summarised in Table 2.3 along with the equivalent simple ligand model energies. As can be seen from the table, the simple model results in values which match the full model to within 10 kJ mol-1, although for only 2 out of 8 values does the simple model produce a lower energy solution. The difference in electronic effects between the dhae and the dpae ligand is therefore relatively small here. Additionally, as the phenyl rings in dpae are directed away from the metal centre, together with the small size of H2 and linear
bonding mode of CO with the metal centre here, little steric difference is encountered.
5a
1
4b
6a
4b
6b
3a
1
406.4
222.8
163.6
140.8
36.4
79
Table 2.3: Comparison of the relative enthalpies of the 14-electron species potentially formed by
photolysis of Ru(dpae)(CO)3 (1) with the simple and full theoretical models. All values are in kJ mol-1
Label Formula Electronic State
Formed from Simple
Model Full model
5a Ru(CO)(dhae) Singlet 2x CO loss from 1 397.8 392.4
5a3 Ru)(CO)(dhae Triplet 2x CO loss from 1 391.8 392.3
5b3 Ru(CO)(dhae) Triplet 2x CO loss from 1 406.4 398.9
6a Ru(H)2(CO)(dhae) Singlet H2 addition to 5a 228.8 226.3
6b Ru(H)2(CO)(dhae)(H2) Singlet H2 addition to 6a 140.8 136.7
4b Ru(CO)2(dhae) Singlet CO addition to 5a 163.6 156.8
4b3 Ru(CO)2(dhae) Triplet CO addition to 5a3 223.7 225.7
4a3 Ru(CO)2(dhae) Triplet CO addition to 5a3 or 5b3 215.8 212.6
2.3.5.1 Summary of the reactions of Ru(CO)3(dpae) with H2
The DFT calculations therefore predict that Ru(CO)3(dhae) or Ru(CO)3(dpae) can produce
Ru(H)2(CO)2(dhae) (3a) and Ru(H)2(CO)2(dpae) upon reaction with H2 via singlet 4b. The
work presented here is therefore consistent with previous experimental and theoretical studies.[142] The triplets of Ru(CO)2(dhae) are higher in energy than the singlet, in agreement
with the observation of PHIP. The formation of 5a is unfeasible with thermal initiation and photochemical initiation would be required. Ultimately, if formed, 14-electron 5a leads to the same stable 18-electron species as through 16-electron 4b.
80