Many ligand substitution reactions involving d8 transition metals employ a five-coordinate transition state which is relatively unstable. The goal of our research is to access a series of pseudo five-coordinate cyclometallated model complexes which closely resemble these transition states. Characterizing model systems that mirror these transition states will aid in the design of better catalysts for a range of organic transformations. Our target complexes were of the forms [Pd(C^N)([9]aneS3)](PF6) and [Pd(C^N)([9]aneS2O)](PF6) ([9]aneS3 = 1,4,7-trithiacyclononane, [9]aneS2O = 1-oxa-4,7-dithiacyclononane, C^N = 2-(2’-thienyl)pyridine, 2-(3’-thienyl)pyridine, 2-(2’-pyridyl)benzothiophene, or 2-phenylbenzothiazole). The synthesis of cyclometallated [9]aneS3 complexes was achieved through a two-step procedure involving stable [Pd(C^N)(O2CCH3)]2 intermediates. The [9]aneS3 complexes were characterized through a variety of methods including X-ray crystallography, 1H and 13C NMR, IR, electrochemistry, and elemental analysis. NMR spectroscopy indicates the bound [9]aneS3 is fluxional in solution as a single 13C resonance is observed for all six carbons of the ligand. X-ray crystallography shows two genuine Pd-S bonds (2.2-2.4 Å), and one longer Pd...S interaction (2.86-2.91Å) in each structure. Electrochemical measurements revealed PdII/III and PdIII/IV reversible oxidations are present in each complex of the series. Recent investigations focus on [9]aneS2O analogs, employing mixed oxygen/sulfur donors. Though [9]aneS3 complexes exhibit endodentate coordination at PdII, the [9]aneS2O ligand adopts exodentate conformations when ligated. DFT calculations to predict and confirm experimental results will also be presented. Our studies of these series of cyclometallated palladium(II) complexes indicated they indeed are five-coordinate species in solution and in the solid-state as well as emulate the redox chemistry of catalytic systems.

Palladium(II) thiacrown and oxathiacrown complexes with cyclometallating ligands

251st National Meeting (2016)

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