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

The preparation and characterisation of iron(II) eta(3)-but-1-en-3-yn-2-yl complexes [Fe(eta(3)-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(H-2)L](+) 7 wi th phenylacetylene in alcohol solvent. The coordinated but-1-en-3-yn-2-yl f ragment is bound as a sigma-vinyl/pi-acetylenic ligand. In solution, comple x 1 exists as a pair of equilibrating isomers (1a and 1b) which differ in t he anchoring mode of the butenynyl ligand i.e. depending on whether the pi- 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 p eaks in the NOESY spectrum also provided information on the mechanism of ex change between 1a and 1b. The crystal structure of 1 showed the solid state structure to be that of the major solution state isomer 1a (pi-bound acety lenic group is cis to the apical phosphorus of L). Complex 1 catalyses the stereospecific head-to-head dimerisation of phenylacetylene to Z-1,4-diphen ylbut-1-en-3-yne.