Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity

The possibility to alter properties
of metal complexes without significant steric changes is a useful
tool to tailor the reactivity of the complexes. Herein we present
the synthesis of iron complexes with the tripodal phosphane ligands
Triphos and TriphosSi and report on their different coordination
properties. Whereas reaction of TriphosSi and FeX2 (X = Cl, Br) exclusively afforded (TriphosSi)­FeX2 with a κ2-coordinated ligand, the homologous
C-derived Fe complexes show rapid conversion in solution to afford
[(Triphos)­Fe­(CH3CN)3]­[Fe2Cl6] or [(Triphos)­Fe­(CH3CN)3]­[FeBr4], respectively. The structural conversion
was found to be temperature- and solvent-dependent and was accompanied
by a linear change of the overall magnetization. The different ligand
influence was shown to have a significant effect on the ability of
(TriphosSi)­FeCl2 and (Triphos)­FeCl2 to perform the Sonogashira cross-coupling reaction of 4-iodotoluene
and phenyl acetylene as well as the hydrosilylation of acetophenone.
The results presented herein show the different coordination properties
of two structurally homologous tripodal ligands and demonstrate the
importance of geometrically controlled ligand field splitting on the
stability and reactivity of metal complexes. The C/Si exchange therefore
provides a simple and straightforward tool to manipulate properties
and reactivity of metal complexes.