Synthesis, Structural Characterization, and Bonding Analysis of (η⁴-1-azadiene)Fe(CO)₃Complexes:  Consequences of the Electronic Structure on Molecular and Crystal Properties

A series of 1-azadiene ligands as well as the corresponding (eta(4)-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 Huckel calculations. The electronic origin of the out-of-plane position of the 1-azadiene hydrogen atom I-I(C3) has been justified in terms of a better pi*-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 accepters. Hydrogen bonding of the azadiene moiety is always realized by the hydrogen atom at the C-N double bond and the hydrogen atom in alpha-position with respect to the imine group.