El precio

3.3.1 The presence of two N-Hpeaks in the NMR spectra of 1_PF6and

6_2PF6

By1H NMR the presence of 2 non-equivalent NH protons was observed. This is believed to be a result of the ethylenediamine-ruthenium 5-membered ring not being in a symmetrical magnetic field. In a magnetic field a ring current is induced in the overlying biphenyl ring which can influence the magnetic field around the amine and ethyl protons on the ethylenediamine. Protons which occupy the space close to the arene will be affected differently than those further away. This phenomenon has been noted many times before.28The observstoion of NOESY peaks for the pyridine adduct (6) allowed NOE crosspeaks to be detected between the ortho-proton on the pyridine (o-Hpyr) and the downward pointing

protons on the amine (N-Hd) to determine the identity of each N-H peak in the

NMR spectrum. Assuming1would be subject to similar effects from the ring

current, conclusive assignments of δ (N-Hu) = 6.03ppm and δ (N-Hd) = 3.99ppm

for solutions of1in 154 mM D2O were made.

3.3.2 Exchange rates of NHuand NHd

Studies were carried out in 154 mM NaCl solutions to monitor exchange rates of1H for2H in D2O as a method of studying the mechanism for inversion of

the stereochemistry on [(η6

-bip)Ru(Et-en)Cl]+. There were two reasons for the design of this experiment. Firstly, the NaCl suppresses hydrolysis38and ensures that only NH/D exchange of the chlorido species would be investigated, without

studying the non-chiral complex was similar and it was to prevent the kinetics being complicated by the interconversion between diastereoisomers.

These initial studies on the exchange rates of NHuand NHdshowed two

distinct rates of exchange. The reason for this difference in exchange rates became the main area of focus of this work. A similar effect was noted on

ethylenediamine Co(III) complexes33,39and was believed to be a result ofcisand

transeffects. However, in the complex studied here (1) both ethylenediamine nitrogens appear to be exposed to the samecisandtranseffects, so there must be other factors leading to the difference in rates.

3.3.3 Exchange rates at various temperatures

Studies at various temperatures have shown a continuous trend with NHd

undergoing exchange substantially faster than the NHuprotons. Using the rate

constants for exchange at various temperatures the activation parameters for the exchange process were determined. The activation parameters of both NHdand

NHuexchange were almost identical to within 3%, with average activation energy

of 111.3 kJ mol-1.This clearly implies that both proceed via the same transition

state. Of further interest is the entropy of activation of 43.4 J mol-1, the positive value being indicative of a dissociative pathway as entropy increases when an atom dissociates. The activation energy is less than the dissociation energy of free N-H bonds (319 kJ mol-1)40; this is to be expected as the lone pair of electrons on the nitrogen is coordinated to the metal, weakening the N-H bond on the

e-_{induced flow}

Increase lability of protons

e-_{induced flow}

Increase lability of protons

Electron density flows towards the Ru making the protons more labile Arene

Cl

e-_{induced flow}

Increase lability of protons

e-_{induced flow}

Increase lability of protons

Electron density flows towards the Ru making the protons more labile Arene

Cl

Figure 3.24.A representation of the electron flow from the nitrogen to the ruthenium weakening the N-H bond.

3.3.4 Effects of ionic strength on exchange rates

To confirm the mechanism is indeed dissociative, the effects of ionic strength were investigated. It was found that ionic strength had an effect on the exchange rates of NHdand NHu. The rates of exchange for NHdincreased with

ionic strength, but decreased for NHu. The increase in the rate of reaction with

increasing ionic strength is indicative of a dissociative mechanism, adding further weight to evidence from§3.3.3(above). The reduction in the exchange rates of the NHuprotons may be a result of the increased ionic strength stabilising an

intermediate in the exchange mechanism.

3.3.5 Exchange rates of NDuand NDd

When the reverse reaction was studied (2H/1H exchange), the exchange rates were reduced (when effects of ionic strength were taken into account). As with the exchange of protons for deuterons, there is an observable difference between the exchange rates of the up and downward pointing deuterons. This

3.3.6 Effects of pH on exchange rates

The proportionality between the rates of exchange and the pH* (Figure 3.20) of the sample provides insight into the mechanism. The exchange of protons for deuterons on1at low pH* is almost negligible, with no exchange is observed at pH* < 3 over 48 h. This observation further supports the conclusion that the initial step in the reaction must be proton abstraction. Unlike exchange of protons for deuterons on proteins which are both acid and base catalsysed41, the exchange of the ligand amino protons appears to only be base-catalysed, given that at low pH* values, exchange was not observed. This is in agreement with observation of the rates of interconversion of stereoisomers of the ethyl-en complexes5Aand5B

which appear to be base catalysed.8These results are also in agreement with much of the previous work by Fujita,31,42where a dependence of [-OH] on the rate of proton exchange was also observed.