Consumo total de nutrientes

This type of fluxionality has previously been noted in other vinyl complexes®® and has been attributed to a windshield wiper effect. It is best described as a systematic ’swapping’ from one metal centre to another of the Tc-interaction.

c^— c

= 5 = = ^

This type of exchange can be effected by two possible mechanisms. The first involves an intermediate in which both of the vinyl carbons remain bound in some manner to the metal atoms. As shown diagrammatically below, this allows no rearrangement and hence retention of stereochemistry is observed.

— M ^ - M—

I

Ï

Figure 3.5 q-tt Fluxionality with Retention of Stereochemistry

The alternative mechanism considers exchange via a zwitterionic type intermediate in which only one vinylic carbon is metal bound and rotation about the C„-Cp axis can be effected allowing either retention or inversion of the stereochemistry.

\ \ .sH \ ,.R

c +c c

:M ^ M— M;

h

k

F ig u re 3.6 G - n Fluxionality with Inversion o f Stereochemistry

3.7.2 [Fe2(CO)4(M-HC=CH2)(M-PCy2)(M-dppm)]

On warming a d®-toluene solution of 2.Cy, a change occurs in the spectrum. At approximately 60°C the signals begin to broaden, and

above 108°C coalescence occurs suggesting a fluxional process (Figure 3.7).

From the temperature at which coalescence occurred, and the frequency separation (in Hz) of the two coalescing signals (which for 2.C y is 1652Hz), it is possible to calculate the approximate free energy of activation for the fluxional process, AG*, using the equation in Figure 3.8. For 2.C y the approximate value for AG* is 68±2 KJmol'\

Av = freq. sep. (Hz) Tg = coel. temp. (K) h = 6.62620x10'^ (Js) k = 1.38062x10'^^(JK'') R = 8.3143 (JK-'mol ') AG^ = - RTr In

71

Figure 3.8 Equation for calculating AG*

Although nmr spectroscopy is very informative in determining some aspects of fluxionality such as the temperature of coalescence and the symmetry of intermediates, it gives one little insight into the details of the transformation.

I'ig u ic 3.7 Vaiiabic Ic m p . 'P N M R Spectrum ol |Fc2(C0)^(p-CH-CH2)(p-PCy2)(p-dppm)] 2.Cy, showing coalescence o f the diphosphine ends

108°C

100°C

80°C

6 0 X

40°C

2 0 X

For example, in the case of 2.Cy whether the hydrogens on the p-carbon interconvert. This is generally not the case,®^ ®^ but it has been noted in a few cases®^’®"^’®® and so the fluxionality was monitored by nmr spectroscopy. Very little change, other than a slight sharpening, occurred in the resonances due to the vinyl protons upon raising the temperature to 100°C, whilst the other proton resonances in the phenyl and cyclohexyl regions broadened considerably, indicating that and Hp^^ans do not interconvert and hence there is no rotation about the vinylic C„-Cp axis during the windshield-wiper fluxionality. This therefore suggests that the first proposed mechanism (Figure 3.5) is most likely, as retention of stereochemistry is observed.

3.7.3 [Fe2(CO)4(|j-MeC=CH2)(M-PCy2)(|J-dppm)]

The a-substituted propyne insertion product, 3.Cy, gave rise to a nmr spectrum at room temperature which showed two distinct diphosphine phosphorus resonances indicating a static vinyl moiety at this temperature. In order to investigate this unusually high barrier to exchange via the "windshield wiper" process, a variable temperature nmr study was carried out on 3.Cy. Similar to 2.Cy, warming a d®-toluene solution of 3.Cy resulted in a gradual broadening of the diphosphine resonances. However, at 70°C, signals due to a new species are observed before coalescence occurs. Interpolation of the size of the resonances against the temperature gave a very approximate coalescence temperature of 80°C (353K). The thermal transformation is irreversible as cooling back to room temperature affords only the thermal product.

3.7.4 [Fe2(CO)4(p-HC=CHMe)(p-PCy2)(p-dppm)]

In contrast to 3.Cy, the nmr spectrum at room temperature for the p-substituted isomer, 4.Cy was broad and the temperature had to be lowered in order to obtain coalescence, which occurred at -20°C (253K), a difference of 100° between the two isomers. Calculating the approximate free energies of activation for the fluxionality in these two complexes gave values of 68±2

Chapter 3

and 47+2 KJmol'^ for 3.Cy and 4.Cy respectively. The large difference is probably due to primarily steric reasons. Clearly, the activation barrier for the fluxional process is lowered significantly when the methyl group is attached at Op probably due to less steric interactions in the transition state of the process. Examining Figure 3.5 shows that during the fluxionality the a-substituent o c c u p ie s a position of le a s t steric in te rfe re n c e in th e transition state where it is located equidistant from the two diphosphine ends. In comparison to this, the steric interactions during the fluxionality of S.Cy do not change much due to the fact that the a-carbon is fixed in position (Figure 3.5) and hence the activation barrier for the process cannot be lowered. Generally, such a high activation barrier to a,7i-vinyl exchange is not observed and there are only a few other examples of high energy processes.®®

3.7.5 [Fe2(CO)4(p-CPh=CH2)(|J-PPh2)(|J-dppm)]

At room temperature, the major isomer, S.Ph exhibited three well resolved doublets of doublets in the nmr spectrum indicating a static vinyl moiety. In contrast, the minor isomer, S.Ph, gave rise to a triplet and a doublet suggesting rapid a,7t-vinyl fluxionality. Raising the temperature caused broadening of the signals for S.Ph and coalescence occurred at 70°G. Calculating the free energy of activation for the process in each of the two isomers gave values of 63±1 KJmoM for S.Ph and 45+1 KJmol’^ for S.Ph, considerably smaller considering such a small change in geometry. It is surprising that the orientation of the diphosphine can have such a pronounced effect upon the rate of fluxionality. The different behaviour is almost certainly a consequence of steric factors, since while in S.Ph the 1-phenyl substituent is directed well away from all the bulky phenyl rings on phosphorus, in the frans-isomer, S.Ph, this interaction is expected to be considerable.

3.7.5 [F02(GO)4(p-GH=GHPh)(M-PCy2)(M-dppm)]

To obtain distinct, sharp resonances for all three phosphorus

environments in the nmr spectrum required lowering the temperature of the solution of 6.Cy to -70°C. Warming the solution back to room temperature caused a broadening of signals with the onset of fluxionality, and coalescence occurred at -40°C (233K). As before, the free energy of activation for the process was calculated from this temperature and the frequency difference (in Hz) between the coalescing signals (see table below) and was found to be comparable to the value for the ^-substituted methyl complex. The reason for this lower activation barrier, 34±2 KJmol \ is as for 4.Cy only the value is even lower due to the larger steric bulk of the phenyl group, and hence a more pronounced difference in steric interactions between ground and transition states. complex group at C„ group at Op coell. temp. (K) separation (Hz) AG (KJmol'^) 2.Cy H H 378 1652 68±2 S.Cy Me H 353 233 68+2 4.Cy H Me 253 556 47+2 6.Cy H Ph 233 573 34+2

Although phenyl and methyl moieties are known to be electron- withdrawing and electron donating groups respectively, the author does not believe that this is an electronic effect due to the fact that the values for 2.Cy and S.Cy are so similar (identical within experimental error.) Also, it seems unlikely that any electronic factors could lower the activation barrier for complexes containing both substituents on the (3-carbon. The phenyl moiety clearly has more steric bulk and it is suggested that this will interact with phenyl rings of the diphosphine considerably more than either H or Me, thus, affecting the fluxional process.

Chapter 3

3.8 Reaction of [Fe2(CO)4(p-CO)(p-H)(|j-PR2)(M“dppm)] 1 with Propargyl