Insertion reactions into palladium–carbon bonds of complexes containing new rigid bidentate nitrogen ligands

Abstract

The neutral complexes Pd(Me)Cl(DPIA) ( 3) and Pd(Me)Cl(DQIA) ( 4), containing the novel rigid bidentate nitrogen ligands (5,6-dihydro-[1]pyrindin-7-ylidene)-isopropylamine (DPIA) and (6,7-dihydro-5 H-quinolin-8-ylidene)-isopropylamine (DQIA), respectively, have been synthesized. Complexes 3 and 4 react quantitatively with CO to give the neutral acylpalladium complexes Pd(C(O)Me)Cl(DPIA) ( 6) and Pd(C(O)Me)Cl(DQIA) ( 7), respectively. Complexes 3, 4, 6, and 7, which were present as mixtures of the cis and trans isomers, were fully characterized, and in the case of complexes 6 and 7 single crystal X-ray structures have been determined. The molecular structure of 6 and 7 show a square planar geometry with the α-diimine ligands coordinated in a bidentate fashion with comparable bite angles of about 78°. The acylpalladium complexes 6, 7, and Pd(C(O)Me)Cl( iPr–PyCa) ( 5), containing the nitrogen ligand 2-( N-2-propanecarbaldimino)pyridine ( iPr–PyCa), which is the flexible analogue of DPIA and DQIA, react with norbornadiene to yield the ionic alkyl complexes [Pd(C 7H 8C(O)Me)(DPIA)]Cl ( 9a), [Pd(C 7H 8C(O)Me)(DQIA)]Cl ( 10a), and [Pd(C 7H 8C(O)Me)( iPr–PyCa)]Cl ( 8a), respectively. Interestingly, the nature of the α-diimine ligand influences the reaction rate of the norbornadiene insertion in the order N–N=DQIA≪ iPr–PyCa<DPIA. Competition experiments and comparison of the crystal data from 6 and 7 indicate that the complexation strength of the α-diimine ligand is exactly opposite to the reactivity of the corresponding acylpalladium complexes toward norbornadiene, which suggest a mechanism via intermediates containing a unidentate coordinated α-diimine ligand.