Three-Coordinate Iron(0) Complexes with N-Heterocyclic Carbene and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and Ligand Substitution Reactions

Low-coordinate iron(0) species are implicated as intermediates in a range of iron-catalyzed organic transformations. Isolable iron(0) complexes with coordination numbers of less than four, however, are rarely known. In continuing with our interests in three-coordinate iron(0) complexes with N-heterocyclic carbene (NHC) and alkene ligation, we report herein the synthesis and ligand substitution reactivity of three-coordinate iron(0) complexes featuring monodentate alkene ligands, [(NHC)Fe(eta(2)-vtms)(2)] (vtms = vinyltrimethylsilane, NHC = 1,3-bis(2',6'-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2',6'-diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cylPr), 2; 1,3-bis(2',6'-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1-3 were synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-diisopropylphenyl)-substituted NHC ligands, vtms, and KC8. Reactivity study of 1 revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones, isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(eta(2)-HCCAr)] (Ar = Ph, 5; p-CH3C6H4, 6; 3,5-(CF3)(2)C6H3, 7), [(IPr)Fe(eta(2) -OCPh2)(2)] (8), [(IPr)Fe(CNR)(4)] (R = 2,6-Me2C6H3, 9; Bu-t, 10), and (IPr)Fe(CO)(4) (11) were prepared in good yields. These iron(0) complexes have been characterized by H-1-NMR, solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, Fe-57 Mossbauer spectroscopy, and elemental analysis. Characterization data and computational studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1-3 and 5-8 and S = 0 ground states for 9- 11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from occupied Fe 3d orbitals to the pi orbitals of the C=C, C (math)C, and C=O moieties of the pi ligands. In addition, 1 proved an effective precatalyst for the cyclotrimerization reaction of alkynes.