Molecular complexes of early transition metals are of interest as catalysts for the production of fine chemicals or polymers. These earth abundant metals (e.g., chromium) are attractive as replacements for precious metals and are often less toxic and environmentally more benign. One important feature of many such complexes is their paramagnetism, which can be studied by nuclear and electronic paramagnetic resonance (i.e., NPR and EPR) spectroscopy. In cases where there is more than one unpaired electron (S &gt; 1/2), zero-field splitting (ZFS) can result. This phenomenon may lead to "silence" in conventional (i.e., X-band) EPR, making measurements at high magnetic field and frequency (i.e., HFEPR) necessary. We describe spectroscopic studies on high-spin (S = 3/2) chromium(III) supported by a rigid, quinolone donor-functionalized cyclopentadienyl ligand (Figure 1). These complexes, where R = Me, Ph and X = halides, in combination with a co-catalyst, serve as excellent catalysts for alkene polymerization. They have been structurally characterized by XRD and exhibit approximate Cs symmetry. They give relatively well resolved NMR spectra that are dominated by the Fermi-contact shift contribution to the overall NMR shift, however, a reliable experimental analysis of the minor pseudocontact shift is possible, only if the g-tensor and ZFS parameters have been obtained by other methods, namely EPR. HFEPR spectroscopic investigation of several of these organochromium(III) complexes, both in the solid state and in toluene frozen solution, indeed provide definitive values for their spin Hamiltonian (g–tensor and ZFS parameters) for analysis of the NMR spectra. These results are also correlated to recent theoretical studies on the electronic structure of related systems. The effectiveness of applying paramagnetic resonance techniques to high-spin organometallic complexes is a potentially broadly applicable result of these studies.

Paramagnetic resonance spectroscopy of catalytically relevant organochromium(III) complexes

255th National Meeting (2018)

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