Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments

Two
novel tripodal ligands, (BIMPNMes,Ad,Me)− and (MIMPNMes,Ad,Me)2–, combining two
types of donor atoms, namely, NHC and phenolate donors, were synthesized
to complete the series of N-anchored ligands, ranging from chelating
species with tris­(carbene) to tris­(phenolate) chelating
arms. The complete ligand series offers a convenient way of tuning
the electronic and steric environment around the metal center, thus,
allowing for control of the complex’s reactivity. This series
of divalent complexes of Mn, Fe, and Co was synthesized and characterized
by 1H NMR, IR, and UV/vis spectroscopy as well as by single-crystal
X-ray diffraction studies. Variable-temperature SQUID magnetization
measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of
increasing zero-field splitting |D| from Mn­(II),
to Fe­(II), to Co­(II) complexes. Zero-field 57Fe Mössbauer
spectroscopy of the Fe­(II) complexes 3, 4, 8, and 11 shows isomer shifts δ
that increase gradually as carbenes are substituted for phenolates
in the series of ligands. From the single-crystal structure determinations
of the complexes, the different steric demand of the ligands is evident.
Particularly, the molecular structure of 1in
which a pyridine molecule is situated next to the Mn–Cl bondand
those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives,
which, in contrast to the corresponding symmetrical TIMENR ligands, allow for side access of, e.g., organic substrates, to
the reactive metal center.