Synthesis and Characterization of Novel Iron(II) Complexes with Tetradentate Bis(N-heterocyclic carbene)–Bis(pyridine) (NCCN) Ligands

Three novel iron­(II) complexes bearing tetradentate ligands
of
the type pyridine–bis­(N-heterocyclic carbene)–pyridine
(NCCN) have been synthesized. The compounds trans-diacetonitrile­[bis­(o-imidazol-2-ylidenepyridine)­alkane]­iron­(II)
hexafluorophosphate (alkane = methane (2a), ethane (2b)) and cis-diacetonitrile­[1,3-bis­(o-imidazol-2-ylidenepyridine)­propane]­iron­(II) hexafluorophosphate
(2c) have been characterized by single-crystal X-ray
diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR),
and infrared spectroscopy (IR). Cyclic voltammetry (CV) measurements
show reversible oxidation of Fe­(II) to Fe­(III). The rotational barrier
of the bridge has been determined via variable-temperature NMR (VT-NMR)
studies of 2b. In all complexes, the Fe centers coordinatein
addition to the NCCN ligandstwo acetonitrile ligands in the
solid state as well as in solution. The alkylene bridge connecting
the two NHCs has an influence on the coordination mode of the NCCN
ligands. Whereas the methylene- and ethylene-bridged NHC moieties
lead to a nearly planar geometry of the NCCN ligand and two trans-positioned
acetonitrile ligands, the propylene-bridged complex 2c exhibits a sawhorse-type coordination mode, with two cis-oriented
acetonitrile ligands. The reactivity of the synthesized complexes
toward substitution of the solvent ligands was investigated, showing
that acetonitrile is readily substituted by benzonitrile. Upon addition
of carbon monoxide, one acetonitrile ligand is replaced by CO to yield
complexes 3a–c, as shown by NMR and
IR spectroscopy, as well as by XRD in the case of compound 3c.