How Innocent are Potentially Redox Non-Innocent Ligands? Electronic Structure and Metal Oxidation States in Iron-PNN Complexes as a Representative Case Study

Herein
we present a series of new α-iminopyridine-based iron-PNN pincer
complexes [FeBr2LPNN] (1), [Fe­(CO)2­LPNN] (2), [Fe­(CO)2­LPNN]­(BF4) (3), [Fe­(F)­(CO)2­LPNN]­(BF4) (4), and
[Fe­(H)­(CO)2­LPNN]­(BF4) (5) with formal oxidation states ranging from Fe(0) to Fe­(II)
(LPNN = 2-[(di-tert-butylphosphino)­methyl]-6-[1-(2,4,6-mesitylimino)­ethyl]­pyridine).
The complexes were characterized by a variety of methods including 1H, 13C, 15N, and 31P NMR,
IR, Mössbauer, and X-ray photoelectron spectroscopy (XPS) as
well as electron paramagnetic resonance (EPR) and magnetic circular
dichroism (MCD) spectroscopy, SQUID magnetometry, and X-ray crystallography,
focusing on the assignment of the metal oxidation states. Ligand structural
features suggest that the α-iminopyridine ligand behaves as
a redox non-innocent ligand in some of these complexes, particularly
in [Fe­(CO)2LPNN] (2), in which
it appears to adopt the monoanionic form. In addition, the NMR spectroscopic
features (13C, 15N) indicate the accumulation
of charge density on parts of the ligand for 2. However,
a combination of spectroscopic measurements that more directly probe
the iron oxidation state (e.g., XPS), density functional theory (DFT)
calculations, and electronic absorption studies combined with time-dependent
DFT calculations support the description of the metal atom in 2 as Fe(0). We conclude from our studies that ligand structural
features, while useful in many assignments of ligand redox non-innocence,
may not always accurately reflect the ligand charge state and, hence,
the metal oxidation state. For complex 2, the ligand
structural changes are interpreted in terms of strong back-donation
from the metal center to the ligand as opposed to electron transfer.