Aromatic PCN palladium pincer complexes. Probing the hemilability through reactions with nucleophiles

A series of unsymmetrical PCN pincer ligands (1-(3-((di-tert-butylphosphino)methyl)phenyl)-N,N-dialkylmethanamine) were cyclometalated with palladium to generate a series of new PCN supported Pd(II) chloro complexes, (PCN)PdCl (4-6), where alkyl = methyl, ethyl, and n-propyl, which were fully characterized by NMR spectroscopy and X-ray crystallography. The N,N-dimethyl complex 4 reacts with methyl lithium to give the corresponding methyl and dimethyl complexes (PCN)PdMe (12) and Li[(PCN)PdMe₂] (13), which could not be isolated but were characterized in solution. The substitution reactions of (PCN)PdCl (4-6) with iodide to form the corresponding iodo complexes (PCN)PdI (7-9) were investigated by use of UV-vis stopped-flow spectrophotometry. The experiments were performed in methanol over a temperature range from 293 to 325 K. The reactions are reversible and were shown to proceed exclusively via the solvento complex in two reversible consecutive steps. Activation parameters for both the forward and reverse reactions were determined, and they, together with reactivity trends, support an associative pathway. No displacement of the nitrogen donor was detected, and overall this points to a limited hemilability of the ligands on palladium.