| dc.description.abstract | Carbon dioxide is a major anthropogenic greenhouse gas and one of the biggest causes of global climate change. Utilization of CO2 to produce chemicals is an interesting proposition to mitigate its negative impact. However, the chemical inertness of CO2 presents a challenge to its transformation into useful chemicals. Transition metal complexes supported by pincer ligands have proved successful previously for the activation of small molecules. Here, we report our investigations into the use of a Pd^I –Pd^I dimer [(^FPNP)Pd]2, previously utilized by our group to activate molecules such as H2O, H2, NH3, O2, and to transform CO2 into CO, a more widely used C1-feedstock for chemicals. Use of trimethylsilyl reagents proved vital for O-atom abstraction to produce CO, and an in-house CO trap, (^MePNP)Ir(Ph)(H) was used to quantify the amount of CO evolved.
Transition metal catalyzed cross coupling reactions have made synthesis of previously unattainable substrates possible, thus opening new avenues into new specialty chemicals, pharmaceuticals and agro chemicals. Group 10 metals, especially Pd have been used almost exclusively for these reactions in the past. The reasons for the popularity of these Pd catalysts are the versatility, functional group tolerance and wide substrate scope exhibited by them. More recently, however, metals outside group 10 such as Cu, Ir, Rh have found uses in these reactions. Especially, (POCOP)Rh systems previously reported by our group have been very useful in catalysis and mechanistic studies. Based on the success of these Rh catalysts and high natural abundance and low prices of cobalt, we envisioned the use of analogous Co systems as catalysts for catalytic cross coupling. With that as our primary goal, Co^III complexes such as (POCOP)Co(Ar)(X) and (POCOP)Co(Ar)(SAr’) presumed to be part of the catalytic C-S couplingcycle have been isolated and their interchange has been probed. However, contrary to the observations in Rh, C-S RE is not observed in the thermolysis of (POCOP)Co(Ar)(SAr’); instead CArCligand is the predominant reaction. A shift to the PNP ligands was made and indeed C-S RE is observed although accompanied by decomposition to (PNP)Co^II species.
A relatively unexplored way of incorporating new functionality into pincer transition metal complexes is having multi-nuclear pincers that can be connected by different linkers to influence the degree of proximity and in turn the cooperativity between multiple metal sites. Such cooperativity can prove beneficial for reactions where one metal center is not enough. Here we have expanded the library of binucelar pincer ligands and complexes by reporting ligands of different steric bulk, linkers with incorporated functionality, and also metalation of these pincers with Ni and Pd. | en |
