Oligomerisation of isopropenylacetylene in the presence of Fe3(CO)12.: Crystal and molecular structure of the open-cluster isomers Fe3(CO)10[H2CC(CH3)CC(H)C(H)C(CO)C(CH3)CH2], Fe3(CO)10[HCC(CH3)C(H)C(H)C(H)C(CO)C(CH3)CH2] and of the binuclear complex Fe2(CO)5[C15H18(CO)]

The reaction of 2-methyl-1-buten-3-yne (isopropenylacetylene, IPA) with triiron dodecacarbonyl in refluxing benzene leads to medium yields of four main derivatives. Two of them are the trinuclear open-cluster isomers Fe-3(CO)(10)[H2CC(CH3)CC(H)C(H)C(CO)C(CH3)CH2] (complex 1a) and Fe-3(Co)(10)[HCC(CH3)C(H)C(H)C(H)C(CO)C(CH3)CH2] (complex 1b) containing an organic ligand formed upon tail-to-tail dimerization of the alkyne. In both complexes one isolated iron atom is part of a metallacyclic five-membered ring formed by three carbon atoms of one alkyne molecule and a carbon belonging to a CO. The other two iron atoms are linked together; however their bonding to the organic moiety is different in complexes 1a and 1b. This type of isomerism and the coordination mode of the unique iron atom are unprecedented in alkyne-cluster chemistry. The other two complexes are binuclear and have been identified, respectively, as Fe-2(CO)(5)[C15H18(CO)] (2) and Fe-2(CO)(6)L-3 [L = IPA] (3). Complex 2 contains an unprecedented organic ligand formed by three alkyne molecules and one CO; this is different from other L3CO (troponic) ligands found in the literature. The structures of complexes 1a, 1b and 2 have been studied by X-ray crystallography. Complex 3 did not give crystals suitable for X-ray analysis. Reaction pathways leading to new complexes are hypothesized.