BEGIN:VCALENDAR VERSION:2.0 PRODID:-//ZContent.net//ZapCalLib 1.0//EN CALSCALE:GREGORIAN METHOD:PUBLISH BEGIN:VEVENT SUMMARY:Paramagnetic resonance spectroscopy of catalytically relevant chrom ium complexes LOCATION:Chemistry A101 TZID:America/Denver DTSTART:20221129T160000 UID:2026-04-30-15-10-32@natsci.colostate.edu DTSTAMP:20260430T151032 Description:About the Seminar\n\nMolecular complexes of early transition me tals are of interest as catalysts for the production of fine chemicals or polymers. One important feature of some such complexes is their paramagnet ism\, which can be studied by nuclear and electron paramagnetic resonance (i.e.\, NPR and EPR) spectroscopy. In cases where there is more than one u npaired electron (S >\; 1/2)\, zero-field splitting (ZFS) can result. Th is phenomenon may lead to \"silence\" in conventional (i.e.\, X-band) EPR\ , making measurements at high magnetic field and microwave frequency (i.e. \, HFEPR) necessary. We describe spectroscopic\, and associated computatio nal\, studies on two types of molecular complexes of chromium.\n\nThe firs t described is a chromium(IV) complex coordinated by a bulky siloxide liga nd.1 This complex is a model for some aspects of silica-supported Cr-based catalysts\, e.g.\, the Philips catalyst.2 Cr(IV) (3d2\, S = 1) is relativ ely uncommon and detailed computational studies on a series of Cr(ERn)4 sp ecies (E = C\, N\, O\, F\; n = 3\, 2\, 1\, 0)\, as well as HFEPR of the si loxide are described to provide insight into the electronic structure and potentially the catalytic properties of these species.\n\nThe second syste m comprises chromium(III) (3d3\, S = 3/2) supported by a rigid\, quinolone donor-functionalized cyclopentadienyl ligand (see Figure).3 These complex es\, where R = Me\, Ph and X = halides\, serve as catalysts for alkene pol ymerization. They give well resolved NPR spectra that are dominated by the Fermi-contact shift contribution to the overall chemical shift\, however\ , a complete experimental analysis is possible only if the g-tensor and ZF S parameters have been obtained\, namely by EPR. HFEPR spectroscopic inves tigation of several of these organochromium(III) complexes provides defini tive values for these spin Hamiltonian parameters for analysis of the NPR spectra. These results are also correlated to recent theoretical studies o n the electronic structure of related systems. \n\n \;\n\nThe effectiv eness of applying paramagnetic resonance techniques to high-spin early tra nsition metal complexes is a potentially broadly applicable result of thes e studies.\n\n \;\n\nReferences\n\n Marshak\, M. P.\; Nocera\, D. G.\ , Chromium(IV) Siloxide. Inorg. Chem. 2013\, 52\, 1173–1175.\n Fong\, A .\; Vandervelden\, C.\; Scott\, S. L.\; Peters\, B.\, Computational Suppor t for Phillips Catalyst Initiation via Cr–C Bond Homolysis in a Chromacy clopentane Site. ACS Catal. 2018\, 8\, 1728–1733.\n Ronellenfitsch\, M. \; Gehrmann\, T.\; Wadepohl\, H.\; Enders\, M.\, Improving 1-Hexene Incorp oration of Highly Active Cp–Chromium-Based Ethylene Polymerization Catal ysts. Macromolecules 2017\, 50\, 35–43.\n\n \;\n\nAbout the Speaker\ n\n 4:00 pm END:VEVENT END:VCALENDAR