Variable coordination behaviour of pyrazole-containing N,P and N,P(O) ligands towards palladium(ii)

Abstract

Three bidentate, mixed-donor ligands based on a triphenylphosphine unit bearing a pyrazole group in the ortho-position of one phenyl ring have been synthesised; the N,P ligand [2-(3-pyrazolyl)phenyl]diphenylphosphine pzphos has been synthesised and transformed into new N,P(O) and N,P(S) derivatives, [2-(3-pyrazolyl)phenyl]diphenylphosphine oxide pzphos(O) and [2-(3-pyrazolyl)phenyl]diphenylphosphine sulfide pzphos(S), respectively. The coordination chemistry of pzphos and pzphos(O) towards palladium(II) has been investigated. Depending on the ligand to metal molar ratio employed in the reactions of palladium(II) with pzphos , either the 1 : 1 chelate [Pd(pzphos)Cl₂] 1a or the 2 : 1 N,P chelate [Pd(pzphos)₂]Cl₂1b was obtained. 1b contains two six-membered chelate rings in which the chlorides have been displaced from the inner coordination sphere of palladium. Exchange of the chloride anions in 1b for perchlorate anions was achieved using AgClO₄ to give [Pd(pzphos)₂][ClO₄]₂1c. Reaction of pzphos(O) under the same conditions forms the 2 : 1 adduct [Pd(pzphos(O))₂Cl₂] 2b regardless of the metal to ligand ratio or the order of addition of reactants. Unlike the N,P chelate 1b, the N,P(O) ligands in complex 2b bind in a monodentate fashion through the N-donor atoms of the pyrazole rings. Abstraction of the chloro ligands in compound 2b using AgClO₄ gave the 2 : 1 N,P(O) chelate [Pd{pzphos(O)}₂][ClO₄]₂2c, in which entropically unfavourable 7-membered chelate rings are formed. X-Ray diffraction has been used to confirm the solid-state structures of the pzphos(O) ligand and the complexes 1b, 1c, 2b and 2c.