Understanding the lability of a trans bound pyridine ligand in a saturated six-membered N-heterocyclic carbene based (NHC)PdCl2(pyridine) type complex: A case study

The lability of a trans “throwaway” pyridine ligand in a PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) themed saturated six-membered N-heterocyclic carbene complex namely, (1,3-difurfuryl tetrahydropyrimidin-2-ylidene)PdCl2(pyridine) (2), was investigated with the aid of a combined experimental and computational studies. In this regard, a new saturated six-membered N-heterocyclic carbene complex 2 was synthesized from the reaction of 1,3-difurfuryl tetrahydropyrimidin-1-ium chloride (1) with PdCl2 in the presence of KN(SiMe3)2 as a base in pyridine. Significantly enough, the complex 2 represents the first structurally characterized example of a PEPPSI themed trans-(NHC)PdX2(pyridine) (X = halide) type of a complex supported over a saturated six-membered N-heterocyclic carbene ligand. The lability of a trans “throwaway” pyridine ligand in 2 was probed by a displacement reaction with PPh3 to yield the corresponding phosphine analog, (1,3-difurfuryl tetrahydropyrimidin-2-ylidene)PdCl2(PPh3) (3), at room temperature under ambient condition. Detailed density functional theory (DFT) studies aimed at understanding the lability of the trans “throwaway” pyridine ligand in 2, against a PPh3 ligand yielding 3, points towards a kinetic factor rather than a thermodynamic one as a reason for the lability of the pyridine in 2.

A combined experimental and computational study reveal that the lability of a trans “throwaway” pyridine ligand against a PPh3 ligand in a PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) themed saturated six-membered N-heterocyclic carbene palladium complex is primarily due to a kinetic factor rather than a thermodynamic one.Figure optionsDownload as PowerPoint slide