DFT calculation and AIM-analysis of the substituent influence on the structure of (1-azabuta-1,3-diene)tetracarbonyliron(0) complexes

1-Azabuta-1,3-dienes can coordinate to the tetracarbonyliron(0) moiety in four ways, to form (1-azabuta-1,3-diene)tetracarbonyliron(0) complexes with the ligand bonded in an η2 fashion through the alkene, η2 coordinated through its CN bond, σ-bonded to the lone pair of the nitrogen atom, or η3 coordinated through the CC–C moiety under concomitant coupling of the imine nitrogen with one of the carbonyl ligands to a carbamoyl species. In the experiment, the equilibrium between these species strongly depends on factors such as the nature of the substituents at the ligand, the solvent and the temperature. In this work, DFT calculations (B3LYP/LANL2DZ/6-31G* and 6-311++G**) and an AIM-analysis of the topology of the charge density were used to investigate the influence of the substituents at the 1-azabuta-1,3-diene ligand on the structural, electronic and energetic properties of these constitutional isomers. In most cases, the calculations correctly predict the observed structure, even in situations where the energy differences between related species are rather small. Substituents larger than CH3 at N and H at C2 disfavour the structures with an η2 coordination to the CN bond to such an extent that they cease to exist as minimum energy structures. Also the σ-N forms distort significantly with the introduction of substituents at N or C2 and become energetically less favourable. The geometries of the η2-alkene form do not change much upon substitution, whereas the η3 form tolerates steric strain best and becomes most favourable when the substituent at C2 is large. The activation barrier between the η2-alkene and the η3-allyl form is low (7.5–1.4 kcal/mol) and allows for an equilibration between these species. The conversion of the η2-alkene into the σ-N form requires almost complete dissociation of the ligand from the Fe(CO)4 moiety. Accordingly, its activation barrier is higher (approx. 14 kcal/mol) and fairly independent of the nature of the substituents at the azabutadiene ligand.

Substituents in 1- and 2-position of the 1-azabuta-1,3-diene ligand in Fe(CO)4(1-azabuta-1,3-diene) have a strong influence on the relative energies of the complexes of different coordination modes and thus determine whether the compound exists as η2-alkene, η3-allyl, or σ-N complex.Figure optionsDownload as PowerPoint slideResearch highlights
► The potential energy surface of the title complexes consists of various local minima.
► These are interconnected by relatively low activation barriers.
► The relative energies depend strongly on minor changes in the substitution pattern.
► Mechanisms for their interconversions are proposed.