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)-cis-[bis­(o-imidazol-2-ylidenefuran)­methane]­iron­(II) hexafluorophosphate
(2a) and tetrakis­(acetonitrile)-cis-[bis­(o-imidazol-2-ylidenethiophene)­methane]­iron­(II) hexafluorophosphate
(2b) were obtained by aminolysis of [Fe­{N­(SiMe3)2}2(THF)] with furan- and thiophene-functionalized
bis­(imidazolium) salts 1a and 1b in acetonitrile.
The SC-XRD structures of 2a and 2b 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 2a in an acetone solution
by the slow diffusion of Et2O led to the formation of cis-diacetonitriledi­[bis­(o-imidazol-2-ylidenefuran)­methane]­iron­(II)
hexafluorophosphate (3a) with two bis­(carbene) ligands
coordinated in a bidentate manner and two cis-positioned acetonitrile
molecules. Compounds 2a and 2b 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.