Generation of trans-spanning diphosphine ligands via alkene metathesis: Synthesis, structure, and dynamic behavior of a missing link in a series of square-planar platinum complexes

Reaction of KPPh2 and Br(CH2)5CHCH2 gives the phosphine PPh2(CH2)5CHCH2 (89%), which is treated with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C6F5)(S(CH2CH2–)2)]2 to yield trans-(Cl)(C6F5)Pt(PPh2(CH2)5CHCH2)2 (4b, 80%). Ring-closing alkene metathesis (Grubbs’ catalyst) gives (5b, 84%), which features a trans-spanning diphosphine ligand. The Z/E CC mixture is hydrogenated (1 atm, 10% Pd/C) to give (6b, 99%). The crystal structures of 4b and (Z)-5b are determined. In the former, both (CH2)5CHCH2 moieties are directed on the same side of the platinum square plane. Low temperature 13C NMR spectra of 6b show two sets of signals for the diastereotopic PPh2 groups. These coalesce upon warming, which requires the passage of the chloride ligand through the macrocycle. Analysis by the complete bandwidth method gives ΔH‡ and ΔS‡ values of 6.0 ± 0.4 kcal mol−1 and −13.9 ± 2.6 eu. The 31P, 13C, and 2H CP/MAS NMR spectra of polycrystalline 6b and 6b-d2 are studied, and indicate appreciable conformational mobility of the methylene chain in the solid state.

Complex 4b undergoes ring-closing alkene metathesis to give the 15-membered macrocycle 5b, the Z CC isomer of which is crystallographically characterized; hydrogenation yields 6b, for which NMR data show the chloride ligand can pass through the macrocycle (ΔH‡ = 6.0 ± 0.4 kcal mol−1, ΔS‡ = −13.9 ± 2.6 eu). Figure optionsDownload as PowerPoint slide