High-Frequency and -Field EPR (HFEPR) Investigation of a Pseudotetrahedral Cr Siloxide Complex and Computational Studies of Related CrL Systems.

Chromium species are the active sites in a variety of heterogeneous catalysts, such as the Phillips catalyst, which is composed of Cr ions supported by SiO and is used to produce polyethylene. Among the catalytically relevant oxidation states of chromium is Cr. Families of neutral, homoleptic, four-coordinate complexes, CrL, with a variety of monoanionic, monodentate ligands, such as L = alkyls, aryls, amides, ketimides (RC = N), alkoxides, and siloxides, are available and can provide information regarding Cr sites in heterogeneous materials. For example, the previously reported siloxide, Cr(DTBMS), where DTBMS = OSiMe Bu (di- tert-butylmethylsiloxide), may be considered a molecular analogue of Cr supported by SiO. Such CrL complexes can have either a singlet ( S = 0) or triplet ( S = 1) spin ground state, and the spin state preferences of such complexes are not fully understood. A truly tetrahedral d S = 1 complex would exhibit no zero-field splitting (zfs), and the zfs is indeed small and observable by X-band EPR for several CrR and Cr(OR) complexes. In contrast, Cr(DTBMS) has zfs beyond the range amenable to X-band EPR so that high-frequency and high-field EPR (HFEPR) is appropriate. HFEPR of Cr(DTBMS) in the solid state shows the presence of three very similar triplet species with the major component having D = +0.556 cm. Classical ligand-field theory (LFT) and quantum chemical theory (QCT), including ab initio methods, use EPR and electronic absorption spectra to give a complete picture of the electronic structure of Cr(DTBMS), and other complexes of formula Cr(ER ), E = C, n = 3; E = N, n = 2; E = O, n = 1; E = F, n = 0. Computations show the importance of ligand steric bulk and of π-bonding in controlling the subtleties of electronic structure of CrL species. These electronic structure results, including zfs, which is a measure of excited state accessibility, for both triplet and singlet excited states, might be related to the catalytic activity of paramagnetic Cr species.