Cationic complexes

Earlier in this manuscript, we introduced the generation of neutral copper(I) nucleophiles catalysts and their efficiency in organic chemistry. However, this system can have several disadvantages such as a weak chirality induction and addition of a base to continue the catalytic cycle. Therefore the use of cationic complexes as a new class of catalysts for metal catalysis has emerged as a great alternative to solve this problem. In this part, we are going to describe the important characteristics of such complexes as well as their synthesis and development in our laboratory.

Weakly or non-coordinating anions (WCAs) are of considerable interest as counterions for the synthesis of novel ionic compounds as well as counterions for cationic catalysts, due to their potential in enhancing reactivity of metal complexes.13 _{Moreover, little attention has been paid} to the development of Cu(I) complexes with WCAs and to their potential catalytic applications. The [Cu(MeCN)₄]+ _{cation coordinated} with some couterions such as BF4-, ClO4- and PF6- is well known both

13 _{For reviews on WCAs see: (a) Strauss, S. H., Chem. Rev. 1993, 93, 927-942. (b)}

Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D., Inorg.

Chem. 2002, 41, 2209-2212. (c) Krossing, I.; Raabe, I., Angew. Chem. Int. Ed.

2004, 43, 2066-2090. (d) Zhang, Y.; Sun, W.; Freund, C.; Santos, A. M.;

Herdtweck, E.; Mink, J.; Kühn, F. E., Inorganica Chimica Acta 2006, 359, 4723- 4729.

with respect to its properties, structure and catalytic activity.14 _{Salts of} tetrakis(acetonitrile)copper(I), [Cu(MeCN)4]+, are exceedingly useful starting materials for the synthesis of copper(I) complexes possessing polydentate ligands. To extend the utility of such chemistry from the generally used solvents dichloromethane, acetonitrile… to relatively low- dielectric solvents, the group of Karlin have employed fluorinated tetraarylborate anions such as tetrakis(3,5-bis- trifluoromethylphenyl)borate.15 _{They also used B(C}

6F5)4- as the counteranion since it affords excellent solubility. Another advantage is that these cationic complexes are easily crystallized and characterized by X-ray crystallography.16 _{However, our group was interested in using} hydrogen bifluoride as counteranion. In that manner, a new pathway to unprecedented (NHC)copper(I) bifluoride complexes was developed, which proved to be excellent catalysts for nucleophilic transfer onto

14 _{(a) Knaust, J.; Knight, D.; Keller, S., J. Chem. Cryst. 2003, 33, 813-823. (b)}

D az-Requejo, M., M.; Pérez, P. J., J. Organomet. Chem. 2001, 617, 110-118. (c) Borriello, C.; Cucciolito, M. E.; Panunzi, A.; Ruffo, F., Tetrahedron Asymmetry

2001, 12, 2467-2471. (d) Storhoff, B., N.; Lewis, C., H., Coord. Chem. Rev. 1977,

23, 1-29.

15 _{Kopf, M.-A.; Neuhold, Y.-M.; Zuberbühler, A. D.; Karlin, K. D., Inorg. Chem.}

1999, 38, 3093-3102.

16 _{Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D., Inorg.} Chem. 2002, 41, 2209-2212.

electrophilic double bonds, such as aldehydes and chiral imines (Scheme 16).17

Scheme 16. Synthesis of (NHC)CuFHF.

These small and nucleophilic fluorinated anions have several attractive features. Hydrogen fluoride anions are of significant structural and theoretical interest as classical examples of the strongest hydrogen bonds.18 _{In addition the structures with bulky cations represent special} cases of weak cation-anion interactions, thus contributing to the study of “isolated” hydrogen fluoride anions.19 _{This prompted us to design a new} family of well defined diphosphine copper(I) complexes bearing a bifluoride counteranion. First experiments were performed by Dr. Julien Petrignet and Dr. Alexandre Welle (Scheme 17). It should be mentioned that ligandless copper(I) fluoride is not isolable. Preparation of the in situ LCuCl was carried out in THF with copper(I) chloride and a diphosphine ligand. Addition of sodium tert-butoxide allowed the

17 _{Vergote, T.; Nahra, F.; Welle, A.; Luhmer, M.; Wouters, J.; Mager, N.; Riant,}

O.; Leyssens, T., Chem. Eur. J. 2012, 18, 793-798.

18 _{Hibbert, F.; Emsley, J., Adv. Phys. Org. Chem. 1991, 26, 255-379.}

19 _{Troyanov, S. I.; Morozov, I. V.; Kemnitz, E., Z. Anorg. Allg. Chem. 2005, 631,}

formation of the copper-alkoxide intermediate 37. Removal of NaCl by filtration under argon and then reaction of 37 with Et3N.3HF yielded the new copper complex. However, they found that the counteranion was H2F3- instead of HF2-.

Scheme 17. First generation of cationic complexes.

This complex was purified by crystallization in acetonitrile to afford white, air stable crystals in 65% yield when MeOBiphep 38 was used as ligand. 19_{F NMR analysis showed the presence of fluoride ions and X-ray} analysis revealed a copper(I) cationic complex bearing the diphosphine ligand and two molecules of acetonitrile. Counterion was well identified as H₂F₃- _{(Figure 5).}

With this complex readily prepared, they wanted to know if interesting catalytic activity can be achieved. Despite encouraging results, problems of reproducibility were observed. This is due to the presence of several species. In addition, in some cases, the nature of the conteranion was very difficult to confirm in spiteof X-ray analysis (H2F3- versus HF2-). To avoid these problems Dr. Thomas Hermant decided to improve this methodology and developed another strategy for the synthesis of new cationic complexes (Scheme 18).

Scheme 18. Second generation of cationic complexes.

Copper(I) chloride and a diphosphine ligand were stirred in THF for 30 minutes then the commercially available source of bifluoride, AgHF2, was added to the reaction mixture. Filtration under inert atmosphere allowed the elimination of salts and after removal of the solvent, a white powder was obtained. A large number of ligands and differents solvents were tested. These complexes have several advantages: they are air stable, highly active in catalysis and easily prepared. Nevertheless a rapid degradation was observed in solution and some silver impurities seemed to be always present. Moreover, no crystals were obtained with any of the ligands attempted. Therefore they were not able to confirm the structure of both cationic complex and counteranion. Finally a solution was found to avoid some of these drawbacks. A novel strategy was

developed and applied by our group as reported in scheme 19. This methodology will be described in more details later on in this chapter.