2.1 Materials

DMA, LiCl, Gd(acac)₃ and Er(acac)₃ were obtained from Aldrich and used as received.

5-Parahydroxyphenyl 10,15,20-tritolylporphyrin (TTPOH) was synthesised according to the Little method [11] and purified on silica gel column using a CHCl₃ petroleum ether gradient and then by preparative thin layer chromatography. The course of the reaction was monitored on a Lambda 25 Perkin Elmer UV–vis Spectrophotometer. Reactions were activated by using a Synthewave 402 Prolabo microwave apparatus.

2.2 Kinetic studies

A stock solution of porphyrin was made at 7.4 × 10⁻⁴ mol L⁻¹ and to five samples (10 mL) was added Er(acac)₃ or Gd(acac)₃ at a concentration of 2 × 10⁻³ mol L⁻¹ in dimethylacetamide. In each sample the required quantities of lithium ions were introduced from a stock solution in DMA (0.118 mol L⁻¹). The actual concentration in monohydroxytetratolylporphyrin was monitored by UV–vis spectroscopy at 514 and 552 nm. Reactions were run under pseudo-first order kinetic conditions, with the lanthanide metal ion in a large excess over the free base porphyrin.

2.3 Classical heating

The five samples were immersed in an oil bath thermostated at 150 ± 1 °C, removed at increasing times and then quickly cooled and analysed by UV–vis spectrophotometry.

2.4 Microwave heating

Each sample was put in the reactor and activated for the stipulated time at a constant power input. In all cases the final temperature was 150 °C and verified by measuring it at the end of the experiment with a thermometer. After removing the reactor, the solution was quickly cooled and analysed by UV–vis spectrophotometry.

3.1 Influence of the lithium ions

The great enhancement of the rate of porphyrin metallation by Cu²⁺ in the presence of Li⁺ observed previously in dichloromethane [9] encouraged us to extend the method to lanthanide ions which are much more difficult to bind to porphyrins. The use of dimethylacetamide as a solvent permitted us to reach larger values of Li⁺, up to 25 mM.

In these series of experiments the metallation reaction of the porphyrin by erbium (Scheme 1) was conducted in the presence of increasing quantities of lithium ions. Two methods of activation were used for comparison: microwave activation and classical heating.

In the case of microwave irradiation, the reaction time was fixed at 2 min which is sufficient to reach a significant reaction. The power input was maintained constant at 165 W during the reaction time. In all cases the final temperature never exceeded 150 ± 1 °C (Fig. 1).

When classical heating was employed the temperature was the same and the reaction time was 1 h in order to have a comparable yield.

The percentages of complex formed as a function of lithium concentration are plotted in Fig. 2 for microwave irradiation and for classical heating. During microwave irradiation complex formation is hardly detectable in the absence of Li⁺ in contrast to experiments conducted under classical heating. It should be recalled that in this case the reaction time is only 2 min instead of 1 h for classical heating. When lithium is added, in the two cases we observe a linear relationship between the percentage of complex formed and the lithium concentration. The slope of the straight line is steeper in the case of the microwave activation by approximately 2-fold. So the catalytic effect of lithium is much more pronounced in the case of microwave activation. It should be noted that classical heating needs a much longer reaction time (30×) in order to reach the same yield.

These results demonstrate a considerable effect of both the lithium ions and the microwave activation on the metallation rate and yield.

3.2 Kinetics

The metallation of TTPOH was then studied at a constant concentration in LiCl in order to determine the rate constant of the reaction with the modes of activation: microwave and classical heating.

For comparison the temperature conditions were the same with classical heating and microwave activation. The oil bath was maintained at 150 ± 1 °C and the power input of the microwave reaction was chosen as to reach 150 °C (Fig. 1).

The rate of the formation of metallated porphyrin is first order with respect to porphyrin concentration at constant concentration of Li⁺ and Er(acac)₃.

3.2.1 Erbium

Erbium metallation was conducted in the presence of a constant lithium concentration (2.7 mM) in order to have sufficient reaction time (10 min) for a kinetic study (Fig. 3).

Linear relationship is observed for the plot of ln[TTPOH] versus time in the case of classical heating as well as for microwave activation.

The k_a value obtained for microwave activation, 1.25 × 10⁻³ s⁻¹, was ca. 35-fold the value for classical heating, 3.5 × 10⁻⁵ s⁻¹ that is a considerable enhancement of the reaction rate. It is to be noticed that the experimental conditions, temperature and pressure, are the same.

3.2.2 Gadolinium

In the case of gadolinium we increased the lithium concentration to 5.4 mM in order to have reaction times comparable to the previous experiments. As for erbium the k_a for microwave irradiation (1.2 × 10⁻² s⁻¹) was higher than for classical heating (8.8 × 10⁻⁴ s⁻¹) but only by 14-fold (Fig. 4).

Microwave irradiation exerts a strong effect on the rate constant when compared to classical heating in the same conditions of temperature and pressure. This effect is more pronounced for erbium (35-fold) than for gadolinium (14-fold). This observation could be explained by the higher hardness of Er³⁺ [12]. In this case the transition state of metallation reaction would be more polar and it has been shown previously [10] that the more polar the transition state, the more important is the microwave effect. These new findings are in accordance with the thermodynamic explanation of the so-called microwave effect.