DFT-calculation of the structural isomers of (1-methyl-4-phenyl-1-azabuta-1,3-diene)tetracarbonyliron(0) and (4-phenyl-1-oxabuta-1,3-diene)tetracarbonyliron(0).

DFT calculations (B3LYP/LANL2DZ/6-31 G*) were used to investigate the ways in which 1-methyl-4-phenyl-1-azabuta-1,3-diene and 4-phenyl-1-oxabuta-1,3-diene bind to a Fe(CO)(4) moiety. As possible coordination modes, eta(2)-coordination across the C=C or C=N/C=O bond, sigma-coordination to the lone pair of the heteroatom, or eta(3)-coordination through the C=C-C or the N=C-C/O=C-C moiety were considered. The latter forms involve coupling of the non-coordinated atom of the heterodiene with one of the carbonyl ligands to an acyl species. The calculated geometric parameters of all structures compare well with X-ray crystallographic data of similar complexes. The species in which the ligand is transoid and sigma-coordinated is lowest in energy, for both compounds studied. However, the eta(2)-alkene bound 1-oxabuta-1,3-diene complex is practically equal in energy to the sigma-transoid form and thus competes. This agrees with experimental observations that the heterodiene is sigma-bonded in Fe(CO)(4)(1-methyl-4-phenyl-1-azabuta-1,3-diene) but eta(2)-coordinated in Fe(CO)(4)(4-phenyl-1-oxabuta-1,3-diene). The solvent dependence was estimated from single point PCM calculations, for CH(2)Cl(2) as solvent. For the 1-azabuta-1,3-diene complexes, the relative energies of eta(2)-olefin and eta(3)-allyl forms are inverted, with the eta(3)-allyl form being more stable in polar solvents. The 1-oxabuta-1,3-diene complexes in their eta(2)-olefin and sigma-O forms change order of relative energy, and conversion to the sigma-O form is expected in a polar medium for these complexes. Calculated IR vibrational stretching frequencies of the carbonyl ligands and the C[double bond, length as m-dash]N/C[double bond, length as m-dash]O bond were compared with experimental data, to produce the best fits for the sigma-transoid form of Fe(CO)(4)(1-methyl-4-phenyl-1-azabuta-1,3-diene) and eta(2)-olefin bonded Fe(CO)(4)(4-phenyl-1-oxabuta-1,3-diene). These results are again consistent with the experiment and show that the DFT method applied in this work can be used as an aid for structural validation.