Two-Coordinate Iron(I) Complexes on the Edge of Stability: Influence of Dispersion and Steric Effects

Sequential treatment of Fe­[N­(SiMe3)­DIPP]2 (2) with Et3NHCl, IAd, and KC8 (IAd = 1,3-diadamantyl-imidazolin-2-ylidene, DIPP = 2,6-diisopropylphenyl) results in the formation of two new complexes, (IAd)­Fe­[N­(SiMe3)­DIPP]2 (3) and {(IAd)­Fe­[N­(SiMe3)­DIPP]}2(μ-N2) (4), via disproportionation and N2 trapping of the intermediate species (IAd)­Fe­[N­(SiMe3)­DIPP] (A), respectively. Independent syntheses of complexes 3 and 4 were also developed. These results differ from those observed with the IPr-supported analogue (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene); a similar treatment of 2 with Et3NHCl, IPr, and KC8 results exclusively in the formation of the two-coordinate complex (IPr)­Fe­[N­(SiMe3)­DIPP] (1). DFT calculations show that the IAd–Fe­[N­(SiMe3)­DIPP] interaction is weaker than the analogous IPr–Fe­[N­(SiMe3)­DIPP] interaction by 10.6 kJ/mol. This difference appears to stem largely from greater steric repulsion and diminished dispersion stabilization in the IAd-containing transient species A in comparison to 1.