Triazolic Ligand Systems - Synthesis, Functionalization and Metal Coordination

On the basis of the experiences with the iminophosphorane ligand systems further func-tionalization to the alkyl backbone was performed. The tripodal alkyl backbone derived from the organoazide 1 proved to be a valuable structural building block to synthesize tripodal triazolic ligand systems. With the introduction of 1,2,3-triazole rings to each arm of the organoazide 1 by employ-ing classic click-chemistry it was possible to synthesize and characterize eight triazolic ligand sys-tems 7 to 14 with five of those system previously unknown (7, 10, 11, 13, 14). These ligand syssys-tems are shown in Figure 3.2.1.

Figure 3.2.1: Tripodal triazolic ligand systems discussed in chapter 2.2.

The used catalyzed click-chemistry reaction could be improved upon by changes in catalyst load-ings, reaction times and conditions. Furthermore, in a cooperation with REICHMANN^[99] a new car-bene based copper catalyst could be identified which lead to a further optimization of the reaction and the resulting yields of the target compounds. It was also possible to crystalize some of these ligand systems and determine their solid state structure with an X-ray diffraction experiment giving further insight into the structure of these compounds. All ligand systems differ in their respective substituent at the triazolic rings varying from small less steric demanding substituents to bulkier substituents. With this selection of different ligand systems subsequent metal coordination reac-tions were made possible in order to get a further understanding of the influence of these substit-uents during coordination reactions.

Additionally, it was deemed necessary to try and further functionalize the triazolic ligand systems with an additional coordination site at the ligand backbone. The previously mentioned ligand sys-tems 7 - 14 offer three possible coordination sites with one at each of the ligand arms. With the change of the alkyl backbone to a mesitylene ring a fourth possible coordination site could be

gained by the introduction of a π-system. To accomplish this a new organoazide 16 could be syn-thesized as a starting material for the buildup of the ligands. The compounds could then be synthe-sized by the mentioned click-chemistry approach to give rise to ligand system 17 and a second new ligand system 18 which are shown in Figure 3.2.2.

Figure 3.2.2: Ligand systems 17 and 18 with a bridging π-system discussed in chapter 2.3.

Unfortunately, the employed copper catalyst made the workup of these reactions very difficult low-ering the yields substantially. Some side reactions could also be identified during the reaction fur-ther hindering the initial synthesis. These unexpected difficulties and the low solubility of the target compounds in common solvents lead to a termination of this part of the work and no further reac-tions have been carried out on these two ligand systems.

In order to prepare possible metal coordination reactions with the ligand systems 8 - 14 bearing an alkyl backbone these compounds have been subjected to a methylation reaction with methyl iodide to generate the salt form of a mesoionic carbene. By doing this a methyl group could be introduced at the N3 position of the triazolic rings leading to a decrease of the pks value. This opened up new reaction pathways where milder bases could be used in order to deprotonate the triazolic rings and form in situ the mesoionic carbene. The resulting compounds 20 - 25 are shown in Figure 3.2.3.

Figure 3.2.3: Salts of mesoionic carbenes 20 - 25 discussed in chapter 2.4.1.

The iodide anions present in the salts can influence the reactivity of the ligand systems because of their coordinating properties and their tendency to form bigger anionic clusters during later coor-dination reactions. Therefore two of these ligand systems 20 and 24 were subjected to an ion ex-change reaction with bulky weakly-coordinating anions in order to eliminate the possible influence of the iodide anions. As weakly-coordinating anions [BAr^Cl4]⁻ and [BAr^F4]⁻ have been chosen. This lead to four new compounds 26 - 29 shown in Figure 3.2.4.

Figure 3.2.4: Salts of ligand systems (26 - 29) with weakly-coordinating anions discussed in chapter 2.4.2.

After the successful synthesis of a wide range of ligand systems with varying properties it was then tried to get an understanding of their behavior in metal coordination reactions. A proper synthetic route could be identified to transfer the previously synthesized salt forms of a mesoionic carbene with the help of silver oxide to a metal complex. Initially, by addition of silver oxide to the ligand system, a deprotonation reaction occurs forming the mesoionic carbene which then undergoes a coordination reaction with the provided silver atom. This silver-MIC complex can then react in a transmetalation reaction with an additional metal salt to form the target metal complex. With this reaction pathway two new unprecedented structures could be synthesized. One metal complex 30 with a dimeric binding motif where two ligand systems are bridged by gold atoms on each of the ligand arms and also another complex 31 with a monomeric binding motif consisting of a ligand system where each arm is coordinated to a CuBr moiety. Additionally the copper complex 31 pos-sesses interesting structural properties. The reaction pathway leading to the formation of these complexes could be optimized and a better understanding of the variables involved to selectively form either a monomeric or dimeric complex could be gained. These complexes are shown in Figure 3.2.5.

30 31

Figure 3.2.5: Metal complex 30 with a dimeric binding motif and bridging gold atoms and copper complex 31 discussed in chapter 2.4.3.

Additional metal coordination reactions were carried out with the catalytic relevant metal atoms palladium and nickel. Following the same reaction pathway consisting of a deprotonation/metal coordination step with silver oxide and subsequent transmetalation with a metal salt lead to two new complexes with novel binding motifs. The palladium complex 32 shows a bidentante binding motif with two of the ligand arms while the third arm bears an additional palladium metal. A com-parable binding motif could be found for the nickel complex 33 where two ligand arms bind to the metal in a bidentate fashion while the third remaining arm stayed unreacted. The proposed struc-tures are shown in Figure 3.2.6.

Figure 3.2.6: Synthesized metal complexes 32 and 33 with coordinated palladium or nickel atoms discussed in chapter 2.4.4.

With the two metal complexes 32 and 33 it was not possible to get crystals suitable for X-ray dif-fraction experiments and instead the structure was determined with the help of high resolution TOF mass spectrometry coupled with CID experiments.

Additional work on the topic of the tripodal triazolic ligand systems should focus around further functionalization of the employed ligand systems. One of the draw backs of the bulkier lig-and systems lig-and resulting metal complexes is their low solubility in common solvents making it more difficult to properly analyze these compounds therefore it is deemed necessary to introduce more polar groups to parts of the ligand system in order to help with the solubility in polar solvents.

An increase of solubility would also lead to a better reactivity in subsequent reactions. The intro-duction of an additional coordination site at the connecting backbone of the ligand system is ex-pected to lead to new binding motifs opening up the possibilities to coordinate additional main-group and transition metal atoms for example in a true tripodal binding motif after metal coordi-nation. Further work might also center around the deprotonation and metal coordination reaction when transferring the salts to their mesoionic counterparts. Studies on the formation and type of silver complex which is formed during this synthetic step could lead to a better understanding of the properties of the ligand system in metal coordination. With this in mind a better projection into the type and binding motif of the complexes formed in subsequent transmetalation reactions can be made. The copper-MIC complex 31 is also of special interest. Additional spectroscopic experi-ments and theoretical considerations are needed to truly classify the nature of the Cu – Cu bond which is formed between the complex and a symmetry generated counterpart in the structure.

These proposed adaptations should lead to a further improvement of the ligand systems whereas the results presented in this work already show the huge potential for C3 symmetric tripodal ligand systems and their metal complexes.