Final Report Summary - INDEN (New pincer-type complexes derived from 2-indenylidene ligands)
The aim of the project was the development of a new pincer ligand platform for transition metals based on an indene skeleton, and the investigation of the reactivity of the ensuing Pd complexes. In particular, the project aimed at the development of metal-ligand cooperative catalytic transformations. Such metal-ligand cooperation might enable the activation / formation of chemical bonds under mild conditions and in the absence of a co-catalyst, which is in agreement with an environmentally friendly and sustainable chemistry.
During the first year of the grant, Dr Nebra developed the synthesis of a new family of 2-indenylidene palladium pincer complexes. The experimental and computational investigations carried out for these 2-indenylidene palladium pincer complexes have evidenced a non-innocent character of the indenylidene pincer ligand (D. Bourissou et al. Organometallics 2011, 30, 6416). In particular, the non-innocent character of the indenylidene ligand was illustrated by the preparation of original polymetallic species (D. Bourissou et al. Inorg. Chem. 2011, 50, 6378 and Chem. Eur. J. 2012, 18, 8474). The nucleophilic behaviour of the 2-indenylidene moiety and the bridging ability of the sulphur atoms in the side arms support the coordination of several metal fragments, such as (PdCl2(PPh3))2, (RhCl(nbd))2 and (IrCl(coe)2)2 to the chloropalladate 1a, leading to the formation of original homo- and hetero-polymetallic complexes. The hetero-bimetallic complex (PdCl(Ph2P=S)2Rh(nbd)) (4) was isolated in very good yield (94 %) and X-ray crystallography substantiated its structural analogy with the related Pd / Pd and Pd / Ir complexes. In addition, variable-temperature nuclear magnetic resonance (VT-NMR) experiments evidenced the shift of the pendant (Rh(nbd)) fragment between the two edges of the pincer ligand (activation barrier of 12.7+/-0.5 kcal / mol at Tc = 15 degrees of Celsius, in agreement with the calculated activation barrier 16.2 kcal / mol, determined in collaboration with prof. L Maron). We have also prepared the related monometallic complex ((IndH(Ph2P=S)2)Rh(nbd)) (6), for which no evidence of metal shift could be observed even under heating at 85 degrees of Celsiu (358 K) and for which calculations predicted an activation barrier as high as 26 kcal / mol, showing therefore the relevant role of the presence of the Pd atom for such metal migration.
These results have opened the way for a new project initiated very recently on the group, and focused on bimetallic catalysis. The objective is to take advantage of the presence of the two metals on the same scaffold to promote cooperative or tandem catalysis.
Inspired by the non-innocent behaviour of the indenylidene Pd pincer complexes in the stoichiometric tests, the research interest was then focused on the development of a new metal-ligand cooperative mode of action in catalysis. In order to illustrate this interesting concept of cooperation between Pd and the indenylidene moiety, a reaction model was chosen, namely the cycloisomerisation of alkynoic acids. This reaction is very interesting as it leads to the formation of lactones (a type of compounds that can show biological activities) in only one step with high atom economy. We have compared the catalytic activity of a family of indenylidene Pd pincer complexes 1a-c vs an analogous family of 2-indenyl Pd pincer complexes 7-H and 7-Me, protonated or methylated in the C1 of the indene moiety. Indenylidene Pd pincer complexes (1a-c) stand as very efficient all-in-one catalysts for the cycloisomerisation of alkynoic acids. They are competent towards a broad range of alkynoic acids (almost 30 substrates were cyclised), including functionalised and internal ones, and give access to 5- as well as 6- and 7-membered lactones with very high selectivities and in excellent yields (most often > 95 %). It has to be noted that, on the contrary to other Pd catalyst, complexes 1a-c do not need the presence of a base to promote the reaction. In addition, they are robust catalysts that can be recycled (catalyst 1b has been used for ten cycles without losing activity).
The research performed during these 24 months has thus let us to prepare an indenyl-based pincer platform. This ligand platform acts as a non-innocent ligand, leading to the synthesis of hetero-bimetallic complexes presenting an original metallic shift between C1 and C3 of the ligand backbone. This is the first reported example of a metal migration induced by the presence of a secondary metallic atom in the molecule. Finally, the metal-ligand cooperation in catalysis was illustrated for the cyclisation of alkynoic acids to yield vinyl-lactones, where metal and ligand act in concert to efficiently promote the substrate activation. Despite the prominent role of Pd in homogeneous catalysis, Pd complexes have not been involved in cooperative catalysis until very recently and the results described herein represent only the second example in this area. All these new achievements will contribute to get a better understanding in very challenging topics as bio-catalysis, cooperative catalysis and / or the role of poly-metallic species in macromolecular structures.
During the first year of the grant, Dr Nebra developed the synthesis of a new family of 2-indenylidene palladium pincer complexes. The experimental and computational investigations carried out for these 2-indenylidene palladium pincer complexes have evidenced a non-innocent character of the indenylidene pincer ligand (D. Bourissou et al. Organometallics 2011, 30, 6416). In particular, the non-innocent character of the indenylidene ligand was illustrated by the preparation of original polymetallic species (D. Bourissou et al. Inorg. Chem. 2011, 50, 6378 and Chem. Eur. J. 2012, 18, 8474). The nucleophilic behaviour of the 2-indenylidene moiety and the bridging ability of the sulphur atoms in the side arms support the coordination of several metal fragments, such as (PdCl2(PPh3))2, (RhCl(nbd))2 and (IrCl(coe)2)2 to the chloropalladate 1a, leading to the formation of original homo- and hetero-polymetallic complexes. The hetero-bimetallic complex (PdCl(Ph2P=S)2Rh(nbd)) (4) was isolated in very good yield (94 %) and X-ray crystallography substantiated its structural analogy with the related Pd / Pd and Pd / Ir complexes. In addition, variable-temperature nuclear magnetic resonance (VT-NMR) experiments evidenced the shift of the pendant (Rh(nbd)) fragment between the two edges of the pincer ligand (activation barrier of 12.7+/-0.5 kcal / mol at Tc = 15 degrees of Celsius, in agreement with the calculated activation barrier 16.2 kcal / mol, determined in collaboration with prof. L Maron). We have also prepared the related monometallic complex ((IndH(Ph2P=S)2)Rh(nbd)) (6), for which no evidence of metal shift could be observed even under heating at 85 degrees of Celsiu (358 K) and for which calculations predicted an activation barrier as high as 26 kcal / mol, showing therefore the relevant role of the presence of the Pd atom for such metal migration.
These results have opened the way for a new project initiated very recently on the group, and focused on bimetallic catalysis. The objective is to take advantage of the presence of the two metals on the same scaffold to promote cooperative or tandem catalysis.
Inspired by the non-innocent behaviour of the indenylidene Pd pincer complexes in the stoichiometric tests, the research interest was then focused on the development of a new metal-ligand cooperative mode of action in catalysis. In order to illustrate this interesting concept of cooperation between Pd and the indenylidene moiety, a reaction model was chosen, namely the cycloisomerisation of alkynoic acids. This reaction is very interesting as it leads to the formation of lactones (a type of compounds that can show biological activities) in only one step with high atom economy. We have compared the catalytic activity of a family of indenylidene Pd pincer complexes 1a-c vs an analogous family of 2-indenyl Pd pincer complexes 7-H and 7-Me, protonated or methylated in the C1 of the indene moiety. Indenylidene Pd pincer complexes (1a-c) stand as very efficient all-in-one catalysts for the cycloisomerisation of alkynoic acids. They are competent towards a broad range of alkynoic acids (almost 30 substrates were cyclised), including functionalised and internal ones, and give access to 5- as well as 6- and 7-membered lactones with very high selectivities and in excellent yields (most often > 95 %). It has to be noted that, on the contrary to other Pd catalyst, complexes 1a-c do not need the presence of a base to promote the reaction. In addition, they are robust catalysts that can be recycled (catalyst 1b has been used for ten cycles without losing activity).
The research performed during these 24 months has thus let us to prepare an indenyl-based pincer platform. This ligand platform acts as a non-innocent ligand, leading to the synthesis of hetero-bimetallic complexes presenting an original metallic shift between C1 and C3 of the ligand backbone. This is the first reported example of a metal migration induced by the presence of a secondary metallic atom in the molecule. Finally, the metal-ligand cooperation in catalysis was illustrated for the cyclisation of alkynoic acids to yield vinyl-lactones, where metal and ligand act in concert to efficiently promote the substrate activation. Despite the prominent role of Pd in homogeneous catalysis, Pd complexes have not been involved in cooperative catalysis until very recently and the results described herein represent only the second example in this area. All these new achievements will contribute to get a better understanding in very challenging topics as bio-catalysis, cooperative catalysis and / or the role of poly-metallic species in macromolecular structures.