Alkene metatheses in transition metal coordination spheres: dimacrocyclizations that join trans positions of square-planar platinum complexes to give topologically novel diphosphine ligands

Abstract

The alkene-containing phosphines PPh((CH₂)_nCHCH₂)₂)₂ (4) are prepared from PPhH₂, n-BuLi, and the corresponding bromoalkenes (1 ∶ 2 ∶ 2), and combined with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C₆F₅)(S(CH₂CH₂–)₂)]₂ (12) to give the square-planar adducts trans-(Cl)(C₆F₅)Pt(PPh((CH₂)_nCHCH₂)₂)₂ (11, 93–73%; n = a, 2; b, 3; c, 4; d, 5; e, 6; f, 8). Ring-closing metatheses with Grubbs' catalyst (2) are studied. With 11e, two isomers of trans-(Cl)(C₆F₅)Pt(PPh(CH₂)₁₄P(CH₂)₁₄Ph) (15e) are isolated after hydrogenation. Both form via dimacrocyclization between the trans-phosphine ligands, but differ in the dispositions of the PPh rings (syn, 31%; anti, 7%). The alternative intraligand metathesis product trans-(Cl)(C₆F₅)Pt(PPh(CH₂)₁₄)₂ (16e) is independently prepared by (i) protecting 4e as a borane adduct, H₃B·PPh((CH₂)₆CHCH₂)₂, (ii) cyclization with 2 and hydrogenation to give H₃B·PPh(CH₂)₁₄, (iii) deprotection and reaction with 12. The sample derived from 11e contains ≤2% 16e; mass spectra suggest that the other products are dimers or oligomers. The structures of syn-15e, anti-15e and 16e are verified crystallographically, and the macrocycle conformations analyzed. As expected from the (CH₂)_n segment length, 11a undergoes intraligand metathesis to give (Z,Z)-trans-(Cl)(C₆F₅)Pt(PPh(CH₂)₂CHCH(CH₂)₂)₂ (86%), as confirmed by a crystal structure of the hydrogenation product. Although 11b does not yield tractable products, 11c gives syn-(E,E)-trans-(Cl)(C₆F₅)Pt(PPh(CH₂)₄CHCH(CH₂)₄P(CH₂)₄CHCH(CH₂)₄Ph) (21%). This structure, and that of the hydrogenation product (syn-15c; 95%), are verified crystallographically. Analogous sequences with 11d,f give syn-15d,f (5 and 14% overall).