Hemilabile bonding of 1-oxa-4,7-dithiacyclononane in cyclometallated palladium(ii) complexes†
The synthesis and characterization of a series of cyclometallated complexes of Pd(II) incorporating the mixed donor ligand 1-oxa-4,7-dithiacyclononane (aneS2O) are presented in this study. Complexes of the form [Pd(C^N)(aneS2O)](PF6) (C^N = 2-phenylpyridine (ppy) 1b, 4-(2-pyridyl)benzaldehyde (ppyCHO) 2b, 7,8-benzoquinoline (bzq) 3b, 2-benzothienylpyridine (btp) 4b, 2-phenylbenzothiazole (pbt) 5b), were obtained in high-yield from a simple two-step synthetic scheme. All of these complexes were fully characterized by NMR, ESI-MS, IR, combustion analyses, and most (1b, 2b, 4b, 5b) by X-ray crystallography. Solution 1H and 13C NMR studies of [Pd(C^N)(aneS2O)](PF6) complexes demonstrate complicated aneS2O behavior at room temperature. Variable temperature NMR reveals dynamic bonding of the aneS2O ligand consistent with the presence of both endodentate and exodentate bonding modes. This is in stark contrast to the related aneS3 (1,4,7-trithiacyclononane) cogeners that demonstrate fluxional endodentate bonding only in solution. X-ray structures reveal only exodenate aneS2O bonding in this series, unlike the related aneS3 complexes that show endodenate bonding with an axial Pd⋯S interaction. DFT calculations performed on endo and exo aneS2O bonding forms of 4b, as well as a transition state calculation for interconversion, suggest reasonable access to both bonding forms based on the energy barrier. Natural bond order calculations provide further evidence for a weak axial Pd⋯O interaction in the endo form of 4b.