The synthesis of a series of coordinatively unsaturated high-spin Fe(II) complexes of the new dianionic pincer ligand H2MesPDPPh (H2MesPDPPh = 2,6-bis(5-(2,4,6-trimethylphenyl)-3- phenyl-1H-pyrrol-2-yl)pyridine) has been accomplished. (MesPDPPh)Fe(THF), (MesPDPPh)Fe(OEt2), and (MesPDPPh)Fe(py)2 have been characterized via 1H NMR, Mössbauer spectroscopy, X-ray crystallography, and magnetic susceptibility measurements. Cyclic voltammetry measurements indicate multiple oxidative events for (MesPDPPh)Fe(THF) and (MesPDPPh)Fe(py)2, tentatively assigned as Fe(II)/Fe(III) and Fe(III)/Fe(IV) redox couples. One electron chemical oxidation of (MesPDPPh)Fe(py)2 was performed with either triphenylmethyl chloride or iodine, generating the synthetically versatile high-spin Fe(III) complexes (MesPDPPh)Fe(Cl)(py) and (MesPDPPh)Fe(I)(py), respectively. Azidation of (MesPDPPh)Fe(Cl)(py) with sodium azide in THF cleanly generated the corresponding high-spin Fe(III) azido analogue, (MesPDPPh)Fe(N3)(py)(THF). Alternatively, in an attempt to access the Fe(IV) oxidation state, aryl azides were employed as two-electron oxidants. However, treatment of (MesPDPPh)Fe(THF) with para-tolyl azide afforded the diamagnetic tetrazene complex (MesPDPPh)Fe(pTolNNNNpTol), a low-spin Fe(III) center antiferromagnetically coupled to a monoanionic tetrazene ligand. Cyclic voltammetry measurements of (MesPDPPh)Fe(pTolNNNNpTol) indicated multiple redox events of 0.34, -0.93, and -1.94 V vs. Fc/Fc+. Reduction of (MesPDPPh)Fe(pTolNNNNpTol) with CoCp2 in dichloromethane furnished the ion pair {[(MesPDPPh)Fe(pTolNNNNpTol)]-[CoCp]+}. High resolution X-ray crystallography, Mössbauer spectroscopy, and full molecule density functional theory calculations at the B3LYP level suggest the nature of this reduction to be ligand-based, accounting for the observed high-spin ground state of this Fe(III) species.

Redox chemistry of pyridine bispyrrolide iron complexes

256th National Meeting (2018)

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