Platinum(ii) alkyl complexes of chelating dibridgehead diphosphines P((CH2)n)3P (n = 14, 18, 22); facile cis/trans isomerizations interconverting gyroscope and parachute like adducts

Abstract

The gyroscope like dichloride complexes trans-Pt(Cl)₂(P((CH₂)_n)₃P) (trans-2; n = c, 14; e, 18; g, 22) and MeLi (2 equiv.) react to yield the parachute like dimethyl complexes cis-Pt(Me)₂(P((CH₂)_n)₃P) (cis-4c,e,g, 70–91%). HCl (1 equiv.) and cis-4c react to give cis-Pt(Cl)(Me)(P((CH₂)₁₄)₃P) (cis-5c, 83%), which upon stirring with silica gel or crystallization affords trans-5c (89%). Similar reactions of HCl and cis-4e,g give cis/trans-5e,g mixtures that upon stirring with silica gel yield trans-5e,g. A parallel sequence with trans-2c/EtLi gives cis-Pt(Et)₂(P((CH₂)₁₄)₃P) (cis-6c, 85%) but subsequent reaction with HCl affords trans-Pt(Cl)(Et)(P((CH₂)₁₄)₃P) (trans-7c, 45%) directly. When previously reported cis-Pt(Ph)₂(P((CH₂)₁₄)₃P) is treated with HCl (1 equiv.), cis- and trans-Pt(Cl)(Ph)(P((CH₂)₁₄)₃P) are isolated (44%, 29%), with the former converting to the latter at 100 °C. Reactions of trans-5c and LiBr or NaI afford the halide complexes trans-Pt(X)(Me)(P((CH₂)₁₄)₃P) (trans-9c, 88%; trans-10c, 87%). Thermolyses and DFT calculations that include acyclic model compounds establish trans > cis stabilities for all except the dialkyl complexes, for which energies can be closely spaced. The σ donor strengths of the non-phosphine ligands are assigned key roles in the trends. The crystal structures of cis-4c, trans-5c, trans-7c, and trans-10c are determined and analyzed together with the computed structures.