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A series of 1-azadiene ligands as well as the corresponding (η4-azadiene)Fe(CO)3 complexes have been synthesized and structurally characterized by means of single-crystal X-ray diffraction. At the molecular level, the 1-azadiene assumes a trans conformation as free ligand, while a cis coordination is found in the organometallic complexes. In the coordinated ligand the H(C3) hydrogen is found nonplanar with respect to the C−C−C−N plane. A molecular orbital analysis has been carried out by means of extended Hückel calculations. The electronic origin of the out-of-plane position of the 1-azadiene hydrogen atom H(C3) has been justified in terms of a better π*-back-bonding between the 1-azadiene and Fe(CO)3 molecular fragments. A comparison of cis and trans conformations of the ligand has also been performed. The organometallic crystals show the presence of a large number of hydrogen bond interactions of the C−H- - -O type involving CO-ligand acceptors. Hydrogen bonding of the azadiene moiety is always realized by the hydrogen atom at the C−N double bond and the hydrogen atom in α-position with respect to the imine group.

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.

The reaction of the octahedral complexes cis‐ or trans‐[Ru(C≡N‐tBu)4(CN)2], (Ru(CN)2), with the photochemically active [bis(bipyridyl)bibenzimidazole‐ruthenium(II)], (RubiH2), component via hydrogen bond association results in the formation of supramolecular aggregates with modified photophysical properties. The first structural characterization of a supramolecular adduct is reported, in which the N–H functions of the ruthenium bound bibenzimidazole ligand via hydrogen bonds serve as an interface directly toward a cyanide ligand acting as a bridge to another transition metal. In addition, the maximum stoichiometry of the supramolecular associates is estimated by NMR titration experiments and association constants of the hydrogen bonded adducts are determined by the decay of the emission intensity during the titration experiments.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)