The early first row transition metals, i.e, Groups 3 through 6, are commonly known for their low electron count configurations in many organometallic complexes. Examples are 3d0 species such as Cp2TiX2 (3d0) and CpV(=O)X2, along with numerous examples of analogous traditional coordination complexes. Even those with occupied 3d orbitals often have only 3d1 configurations (S = 1/2 spin ground state), so that electron paramagnetic resonance (EPR) spectroscopy can in principle be applied, using conventional microwave frequencies (X-band, ~9 GHz) and corresponding magnetic field ranges (up to ~0.6 T). There are, however, organometallic and related coordination complexes in which the early transition metal ion has a 3d2, 3d3, or even 3d4 electronic configuration, making it possible for the spin ground state to be respectively either S = 1, S = 3/2, and either S = 1 or 2. In these high-spin" cases, the electronic configuration is subject to the effects of zero-field splitting (zfs), which can make the use of conventional EPR less rewarding, or even useless. We will describe the application of EPR spectroscopy at high frequencies (up to ~600 GHz) and correspondingly high magnetic fields (up to 15 T) to study the electronic structure of organometallic and/or coordination complexes of TiII, VIII, and CrIV (3d2, each with S = 1), of VII and CrIII (3d3, each with S = 3/2), and of CrII, 3d4, but with S = 1, rather than the true high-spin, S = 2 ground state. The ligand platforms in these cases will include cyclopentadienide (Cp), β-diketiminates ("NacNac"), diphosphines, diamines, and other such ligands of fundamental interest. The information obtainable from this technique, referred to as HFEPR, will be demonstrated in concert with computational studies on the electronic structure of these complexes.

Paramagnetic resonance studies of early first row transition metal ion complexes: These complexes are not always just 3d0 or 3d1

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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