(48e) New Bis(pyridyl)Siloxane-Pd(OAc)2 Catalytic Complexes for the Selective Oxidation of Alcohols to Aldehydes: A DFT Study

The selective oxidation of alcohols to ketones or aldehydes is a process of tremendous industrial importance. As such, the rational development of highly efficient catalytic systems has become a priority for both industry and academia. In the past decade, the Pd(OAc)₂ catalytic complex has generated significant interest because it allows for molecular oxygen to replace oxidants that are environmentally unfriendly in the selective oxidation of alcohols to carbonyl compounds. However, a major concern for this system is catalyst deactivation via agglomeration of Pd atoms to form particles. It has been previously shown that the effect of agglomeration is reduced in the presence of pyridine ligands with the highest catalytic activity achieved at Pd:pyridine = 1:2. The concept of stabilization of the catalytic system via the introduction of ligands has been taken a step further towards active site design by incorporating an inorganic chain composed of a different number of monomeric siloxane units which connect two pyridine molecules. Such a novel system has the promise of not only improving the catalyst stability but also allowing for the rational incorporation of functional groups which, in a biomimetic fashion, would potentially enhance catalytic activity under conditions of low temperature and pressure. To date, experimental evidence suggests that the single most important factor in achieving this goal using bis(pyridyl)siloxane-Pd(OAc)₂ catalytic complexes is the length of the oligomeric tether. As a first step towards understanding the experimental results and being able to rationally modify bis(pyridyl)siloxane-Pd(OAc)₂ based catalysts, we employed high-level density functional theory (DFT) calculations as well as standard methods from statistical thermodynamics to explore the electronic, strain and entropic effects associated with the formation of the catalytic complex when the oligomer length and position of bonding with the pyridine moieties were systematically varied. Insights into the origins of the effect of the binding properties of the different complexes on the catalytic activity will be discussed.