Synthesis, Characterization, and Reactivity of Furan- and Thiophene-Functionalized Bis(N-heterocyclic carbene) Complexes of Iron(II)

The synthesis of iron­(II) complexes bearing new heteroatom-functionalized methylene-bridged bis­(N-heterocyclic carbene) ligands is reported. All complexes are characterized by single-crystal X-ray diffraction (SC-XRD), nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis. Tetrakis­(acetonitrile)-<i>cis</i>-[bis­(<i>o</i>-imidazol-2-ylidenefuran)­methane]­iron­(II) hexafluorophosphate (<b>2a</b>) and tetrakis­(acetonitrile)-<i>cis</i>-[bis­(<i>o</i>-imidazol-2-ylidenethiophene)­methane]­iron­(II) hexafluorophosphate (<b>2b</b>) were obtained by aminolysis of [Fe­{N­(SiMe₃)₂}₂(THF)] with furan- and thiophene-functionalized bis­(imidazolium) salts <b>1a</b> and <b>1b</b> in acetonitrile. The SC-XRD structures of <b>2a</b> and <b>2b</b> show coordination of the bis­(carbene) ligand in a bidentate fashion instead of a possible tetradentate coordination. The four other coordination sites of these distorted octahedral complexes are occupied by acetonitrile ligands. Crystallization of <b>2a</b> in an acetone solution by the slow diffusion of Et₂O led to the formation of <i>cis</i>-diacetonitriledi­[bis­(<i>o</i>-imidazol-2-ylidenefuran)­methane]­iron­(II) hexafluorophosphate (<b>3a</b>) with two bis­(carbene) ligands coordinated in a bidentate manner and two cis-positioned acetonitrile molecules. Compounds <b>2a</b> and <b>2b</b> are the first reported iron­(II) carbene complexes with four coordination sites occupied by solvent molecules, and it was demonstrated that those solvent ligands can undergo ligand-exchange reactions