Diastereopure cationic NCN-pincer palladium complexes with square planar η4-N,C,N,O coordination

Neutral NCN-pincer palladium bromide complex 2 containing the monoanionic, enantiopure pincer ligand 2,6-bis{[(S)-2-hydroxymethyl-1-pyrrolidinyl]methyl}phenyl bromide (1) with bis-ortho-(S)-prolinol substituents was synthesised and isolated as a mixture of three stereoisomers[(S-N, S-N, S-C, S-C), (R-N, S-N, S-C, S-C), and (R-N, R-N, S-C, S-C)] in a 1:1:1 ratio. Upon abstraction of the bromide ion from the unresolved mixture of 2, single diastereomers of the cationic complexes [3]BF4 and [3]PF6, respectively, were formed with a unique eta(4)-N,C,N,O coordination mode of ligand 1. X-ray crystal structure determination established the intramolecular eta(4)-N,C,N,O coordination of 1 to palladium where the typical mer eta(3)-N,C,N pincer coordination is accompanied by coordination of one of the hydroxy groups of the (S)-prolinol moieties. The water molecule that was cocrystallised in the crystal strycture of [3]PF6 does not coordinate to palladium, but instead is involved in a hydrogen bonding network. The catalytic potential of both cationic complexes, [3]BF4 and [3]PF6, was tested in an aldol reaction of aldehydes with methyl iso-cyanoacetate to yield the oxazoline products as racemic mixtures.