High-Frequency and -Field EPR (HFEPR) Investigation of a Pseudotetrahedral Cr^IV Siloxide Complex and Computational Studies of Related Cr^IVL₄ Systems

Abstract

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 SiO2 and is used to produce polyethylene. Among the catalytically relevant oxidation states of chromium is CrIV. Families of neutral, homoleptic, four-coordinate complexes, CrL4, with a variety of monoanionic, monodentate ligands, such as L = alkyls, aryls, amides, ketimides (R2C = N–), alkoxides, and siloxides, are available and can provide information regarding Cr sites in heterogeneous materials. For example, the previously reported siloxide, Cr­(DTBMS)4, where DTBMS = –OSiMe t Bu2 (di-tert-butylmethylsiloxide), may be considered a molecular analogue of CrIV supported by SiO2. Such CrL4 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 d2 S = 1 complex would exhibit no zero-field splitting (zfs), and the zfs is indeed small and observable by X-band EPR for several CrR4 and Cr­(OR)4 complexes. In contrast, Cr­(DTBMS)4 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)4 in the solid state shows the presence of three very similar triplet species with the major component having D = +0.556 cm–1. 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)4, and other complexes of formula Cr­(ER n )4, 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 CrL4 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.

Tunable Cr4+ Molecular Color Centers

“…Indeed, multiple magnetically inequivalent, S = 1 species have been previously observed for frozen solutions of similar samples, such as Cr(1-norbornyl) 4 , Cr(neopentyl) 4 , and Cr((trimethylsilyl)methyl) 4 , and solid state samples of Cr(di-tert-butylmethylsiloxide) 4 . 44,45,58 As a result, all subsequent pulsed EPR data for 6′ refer to these two sites as site 1, with |D| = 0.76 GHz and E = 0.22 GHz, and site 2, with |D| = 1.56 GHz and E = 0.37 GHz. These results suggest that both lattice matching and crystallographic site symmetry may influence the spin properties, such that tuning the host material offers a pathway to readily modulate the ground state ZFS values.…”

“…40,79 Alternatively, pseudo-T d tetrasiloxide or tetraalkoxide Cr 4+ complexes yield relatively small ZFS values for microwave addressability, but the ligand fields are likely too weak to boost the energy of the spin-triplet excited states above the energy of the spin-singlet excited state. 45,101 Thus, the appropriate design of these Cr 4+ compounds, or related spin qubits, necessitates simultaneous consideration of ground state spin properties and excited state structure to achieve the desired optical-spin interface.…”

“…While numerous homoleptic CrR 4 compounds have been previously investigated, such as Cr(di-tert-butyl-methylsiloxide) 4 , Cr(tert-butoxide) 4 , and Cr(dimethylamido) 4 , the inherently poor metal−ligand orbital overlap of the tetrahedral ligand field often proves too weak to achieve the electronic structure in Figure 1. 45 To circumvent this issue, we focused on Cr(alkyl) 4 systems, where the strong field alkyl donors result in higher energy spin-triplet excited states than the corresponding aryl compounds (Figure 1) along with suitably small ZFS values for spin manipulation. Additionally, as the energy of the emissive 1 ES is dictated by interelectronic repulsion, 46,47 we chose these σ-only alkyl donors to reduce electron delocalization onto the ligand relative to the Cr−aryl systems.…”

Deposition of Tetracoordinate Co(II) Complex with Chalcone Ligands on Graphene

“…The magnetic properties of magnetic molecules can be precisely measured by high-frequency electron spin resonance (HF-ESR) both in bulk [11][12][13][14][15][16][17][18][19][20] and on a surface [21][22][23]. Primarily, the Zeeman and zero-field-splitting (ZFS) contributions to the spin Hamiltonian with information about the intrinsic magnetic properties of a molecule can be determined.…”

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

“…It is noteworthy to mention that EPR spectroscopy is much more sensitive to small perturbations in zero-field splitting than Mössbauer spectroscopy. [61][62][63][64] suggesting [L(Fe)-SAr*] 2À had a high-spin electronic configuration and [L(Fe)] À was low-spin. Reduction of [L(Fe)-SAr*] À by 2.4 equiv.…”