Structure and dynamic behaviour of palladium(II) complexes formed from trans-N2S2 macrocyclic ligands and the crystal structure of [PdL1][PF6]2(L1= 7,8,17,18-tetrahydro-6H, 16H-dibenzo[g,o][1,9,5,13]dithiadiazacyclohexadecine)

Abstract

The sixteen-membered trans-N₂S₂ macrocycle 7,8,17,18-tetrahydro-6H, 16H-dibenzo[g,o][1,9,5,13]dithiadiazacyclohexadecine, L¹, gives a complex with palladium(II) which exists in solution as an equilibrium mixture of two diastereomers, observable at low temperature by ¹³C n.m.r. The two diastereomers are meso and racemic forms of the complex related by inversion at one of the two sulphur stereocentres. They interconvert on the n.m.r. time-scale at elevated temperatures, with ΔG^‡= 63.3 kJ mol^–1 at 288 K. Measurement of the relative intensities of the two sets of resonances gives K= 0.36 (racemic →meso) which is substantially temperature invariant in the range 230–280 K. Crystallisation from an acetonitrile–methanol (4 : 1) mixture, as the PF₆^– salt, yields a single compound which has been shown by X-ray crystallography to be the meso diastereomer, in which the Pd atom lies at an inversion centre and has distorted square-planar geometry with Pd–S 2.307(1)Å, Pd–N 2.047(4)Å, S–Pd–N (benzenoid chelate ring) 84.7(1)°. The PF₆^– anions lie above and below the co-ordination plane with one Pd–F 3.152 Å and other structural features which suggest some Pd–F interaction. The fourteen-membered analogue of L¹,6,7,15,16-tetrahydrodibenzo[f,m][1,8,4,11]dithiadiazacyclotetradecine, L², also forms a complex with palladium(II), but, in contrast, only one of the two diastereomers exists in solution at concentrations detectable by n.m.r. spectroscopy.