Three-coordinate Bis(N-heterocyclic carbene)iron(0) complexes with alkene and alkyne ligation: Synthesis and characterization

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl₂C₆H₃)₂] (1, Xyl = 2,6-Me₂C₆H₃). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC≡CH]₀/[1]₀), referred to as the low ([RC≡CH]₀/[1]₀ < 40) and high ([RC≡CH]₀/[1]₀ > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

Cyanide-alkene competition in a diiron complex and isolation of a multisite (cyano)alkylidene-alkene species

The μ-(amino)alkylidyne complex [Fe₂Cp₂(CO)₂(μ-CO){μ-CNMe(CH₂CHCH₂)}]CF₃SO₃, [1]CF₃SO₃, reacted with NBu₄CN in dichloromethane affording the μ-(cyano)(amino)alkylidene [Fe₂Cp₂(CO)₂(μ-CO){μ-C(CN)N(Me)(CH₂CHCH₂)}], 2, in 91% yield. Decarbonylation of 2 by using Me₃NO in acetone at room temperature yielded [Fe₂Cp₂(CO)(μ-CO){μ-κ³_C-C(CN)N(Me)(CH₂CHCH₂)}], 3, containing a multidentate alkylidene-alkene ligand occupying both a bridging site and a terminal site, in admixture with the μ-(amino)alkylidyne cyanide product [Fe₂Cp₂(CN)(CO)(μ-CO){μ-CN(Me)(CH₂CHCH₂)}], 4. The reaction of the μ-(amino)alkylidyne imine complex [Fe₂Cp₂(CO)(μ-CO)(NHCPh₂){μ-CN(Me)(CH₂CHCH₂)}]CF₃SO₃, [7]CF₃SO₃, with NBu₄CN gave 3 with an optimized yield of 75% via imine elimination. According to DFT calculations, 3 is less stable than its geometric isomer 4 by 13.4 kcal mol^-1 and quantitative conversion to 4 was achieved by refluxing a THF solution of 3 for 2 hours. No replacement of alkene coordination occurred upon treating 3 with CO or PPh₃. The previously unknown compounds 2, 3, 4 and [7]CF₃SO₃ were fully characterized by analytical and spectroscopic techniques and the structure of 3 was elucidated by single crystal X-ray diffraction.