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.