Butenynyl complexes of iron(II) containing the tripodal tetraphosphine ligand P(CH₂CH₂PMe₂)₃

The preparation and characterisation of iron(II) η³-but-1-en-3-yn-2-yl complexes [Fe(η³-RC≡C-C=CHR)L]+ [L = P(CH₂CH₂PMe₂)₃, R = Ph 1; R = Bu^t 2; R = p-HC≡CC₆H₄ 3] is reported. The phenyl substituted butenynyl complex 1 was prepared by the reaction of FeCl₂L 5, FeH(Cl)L 6 or [FeH(H₂)L]⁺ 7 with phenylacetylene in alcohol solvent. The coordinated but-1-en-3-yn-2-yl fragment is bound as a σ-vinyl/π-acetylenic ligand. In solution, complex 1 exists as a pair of equilibrating isomers (1a and 1b) which differ in the anchoring mode of the butenynyl ligand i.e. depending on whether the π-bound acetylenic group is cis or trans to the apical phosphorus of L in the octahedral coordination sphere. Assignment of the relative stereochemistry of 1a and 1b was achieved by analysis of the 2D NOESY spectrum. Exchange peaks in the NOESY spectrum also provided information on the mechanism of exchange between 1a and 1b. The crystal structure of 1 showed the solid state structure to be that of the major solution state isomer 1a (π-bound acetylenic group is cis to the apical phosphorus of L). Complex 1 catalyses the stereospecific head-to-head dimerisation of phenylacetylene to Z-1,4-diphenylbut-1-en-3-yne.