Structural and Electronic Comparison of 1st Row Transition Metal Complexes of a Tripodal Iminopyridine Ligand

We report the preparation and characterization of a series
of divalent
3d transition metal complexes (Cr to Zn, 1–7), featuring the multidentate, tripodal iminopyridine Schiff-base
ligand trimethyl 6,6′,6″-((1E,1′E,1″E)-((nitrilotris­(ethane-2,1-diyl))­tris­(azanylylidene))­tris-(methanylylidene))­trinicotinate
(L5‑OOMe). X-ray structural
studies carried out on 1–5 and 7 reveal complex geometries ranging from local octahedral
coordination to significant distortion toward trigonal prismatic geometry
to heptacoordinate environments. Regardless of coordination mode,
magnetic and spectroscopic studies show the ligand to provide moderately
strong ligand fields: the Fe complex is low-spin, while the Co and
Mn complexes are high-spin at all temperatures probed. Cyclic voltammograms
exhibit multiple reversible ligand-based reductions, which are relatively
consistent throughout the series; however, the electrochemical behavior
of the Cr complex 1 is fundamentally different from those
of the other complexes. Time-dependent (TD) density functional theory
(DFT) and natural transition
orbital (NTO) computational analyses are presented for the ligand,
its anion, and complexes 1–7: the
computed spectra reproduce the major differential features of the
observed visible absorption spectra, and NTOs provide viable interpretations
for the observed features. The combined studies indicate that all
complexes contain neutral ligands bound to M­(II) ions, except for
the Cr complex 1, which is best described as a Cr­(III)
species bound to a radical anionic ligand.