Xantphos-Capped Pd(II) and Pt(II) Macrocycles of Aryldithiolates: Structural Variation and Catalysis in C-C Coupling...

The self-assembly of Xantphos-capped M(OTf)₂ (M = cis-[M'(Xantphos)]²⁺; M' = Pd, Pt) with bridging ligands 1,4-benezenedithiol or 4,4'-biphenyldithiol has been investigated. The reactions have yielded complexes [M{S(C₆H₄) _nSH}]₂(OTf)₂ (I) and [M₂{S(C₆H₄) _nS}]₂(OTf)₄ (II) ( n = 1 or 2). The equilibrium between I and II has been established in platinum complexes for n = 2, whereas the analogous Pd complex exclusively exist as II. These results are different from our previously reported dppe or triethyl phosphine-capped complexes which showed only type II. The same reaction with 1,3-benezenedithiol lead to the complex [M₂(SC₆H₄SSC₆H₄S)](OTf)₂ (III), containing a S-S bond between two thiolate ligands, formed via a complex of type I in solution. Characterization of the complexes was accomplished by NMR spectroscopy, UV-vis spectroscopy and mass spectrometry, and X-ray crystallography. Density functional calculations were performed to estimate the relative stability of three types of complexes. The palladium complexes are excellent catalysts in Suzuki C-C cross coupling reactions under mild conditions, and can be reused eight times without losing significant yield. The activity of the Pd catalysts derived from three dithiol ligand follows opposite trend of the stability as III > II > I. The comparative catalytic activity of the tetranuclear Pd complexes (II) of bis-phosphines of varied bite angles, including the structurally characterized [Pd₂(dppf)₂(SC₁₂H₈S)]₂(OTf)₄ has also been demonstrated.