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

Abstract

Reaction of KPPh 2 and Br(CH 2) 5CH CH 2 gives the phosphine PPh 2(CH 2) 5CH CH 2 (89%), which is treated with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C 6F 5)(S(CH 2CH 2–) 2)] 2 to yield trans-(Cl)(C 6F 5)Pt(PPh 2(CH 2) 5CH CH 2) 2 ( 4b, 80%). Ring-closing alkene metathesis (Grubbs’ catalyst) gives ▪ ( 5b, 84%), which features a trans-spanning diphosphine ligand. The Z/ E C C 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 (CH 2) 5CH CH 2 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 P Ph 2 groups. These coalesce upon warming, which requires the passage of the chloride ligand through the macrocycle. Analysis by the complete bandwidth method gives Δ H ‡ and Δ S ‡ 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- d 2 are studied, and indicate appreciable conformational mobility of the methylene chain in the solid state.