Applications of electron paramagnetic resonance spectroscopy to heavy main-group radicals

The exploration of heavy main-group radicals is rapidly expanding, for which electron paramagnetic resonance (EPR) spectroscopic characterisation plays a key role. EPR spectroscopy has the capacity to deliver information of the radical's electronic, geometric and bonding structure. Herein, foundations of electron-nuclear hyperfine analysis are detailed before reviewing more recent applications of EPR spectroscopy to As, Sb, and Bi centred radicals. Additional diverse examples of the application of EPR spectroscopy to other heavy main group radicals are highlighted

Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals

We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb-. (2-R, R=B[N(Dip)CH](2) 2-B, 2,6-Mes(2)C(6)H(3) 2-C, N(SiMe3)Dip 2-N) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)](2), Dip=2,6-i-Pr2C6H3) and one bulky B- (2-B), C- (2-C), or N-based (2-N) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N. Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH](2)] ([K([2.2.2]crypt)][2-B]) containing the stibanyl anion [2-B](-), which was shown to possess significant Sb-B multiple-bonding character

Valence-to-Core X-ray Emission Spectroscopy as a Probe of O-O Bond Activation in Cu2O2 Complexes

Valence-to-Core (VtC) X-ray emission spectroscopy (XES) was used to directly detect the presence of an O-O bond in a complex comprising the [Cu-2(II)(mu-eta(2):eta(2)-O-2)](2+) core relative to its isomer with a cleaved O-O bond having a [Cu-2(III)(mu-O)(2)](2+) unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu-2(II)(mu-eta(2):eta(2)-O-2)](2+) core corresponds to the copper stabilized in-plane 2p pi peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O-O bond activation in mu-eta(2):eta(2)-O-2 species and highlight the potential of this method for time-resolved studies of reaction mechanisms

Spectroscopic X-ray and Mössbauer Characterization of M-6 and M-5 Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation

The present study employs a suite of spectroscopic techniques to evaluate the electronic and bonding characteristics of the interstitial carbide in a set of iron-carbonyl-carbide clusters, one of which is substituted with a molybdenum atom. The M6C and M5C clusters are the dianions (Et4N)(2)[Fe-6(mu(6)-C)(mu(2)-CO)(2)(CO)(14)] (1), [K-(benzo-18-crown-6)](2)[Fe-5(mu(5)-C)(mu 2-CO)](1)(CO)(13)] (2), and [K(benzo-18-crown-6)](2)[Fe5Mo(mu(6)-C)(mu(2)-CO)(2)(CO)(15)] (3). Because 1 and 2 have the same overall cluster charge (2-) but different numbers of iron sites (1: 6 sites -> 2: 5 sites), the metal atoms of 2 are formally oxidized compared to those in 1. Despite this, Mossbauer studies indicate that the iron sites in 2 possess significantly greater electron density (lower spectroscopic oxidation state) compared with those in 1. Iron K-edge X-ray absorption and valence-to-core X-ray emission spectroscopy measurements, paired with density functional theory spectral calculations, revealed the presence of significant metal-to-metal and carbide 2p-based character in the filled valence and low-lying unfilled electronic manifolds. In all of the above experiments, the presence of the molybdenum atom in 3 (Fe5Mo) results in somewhat unremarkable spectroscopic properties that are essentially a "hybrid" of 1 (Fe-6) and 2 (Fe-5). The overall electronic portrait that emerges illustrates that the central inorganic carbide ligand is essential for distributing charge and maximizing electronic communication throughout the cluster. It is evident that the carbide coordination environment is quite flexible and adaptive: it can drastically modify the covalency of individual Fe-C bonds based on local structural changes and redox manipulation of the clusters. In light of these findings, our data and calculations suggest a potential role for the central carbon atom in FeMoco, which likely performs a similar function in order to maintain cluster integrity through multiple redox and ligand binding events

Redox Activity of Noninnocent 2,2 '-Bipyridine in Zinc Complexes: An Experimental and Theoretical Study

[Image: see text] We report on a systematical reactivity study of β-diketiminate zinc complexes with redox-active 2,2′-bipyridine (bpy). The reaction of LZnI (L = HC[C(Me)N(2,6-iPr(2)C(6)H(3))](2)) with NaB(C(6)F(5))(4) in the presence of bpy yielded [LZn(bpy)][B(C(6)F(5))(4)] (1), with bpy serving as a neutral ligand, whereas reduction reactions of LZnI with 1 or 2 equiv of KC(8) in the presence of bpy gave the radical complex LZn(bpy) (2) and [2.2.2-Cryptand-K][LZn(bpy)] (3), in which bpy either acts as a π-radical anion or a diamagnetic dianion, respectively. The paramagnetic nature of 2 was confirmed via solution magnetic susceptibility measurements, and UV–vis spectroscopy shows that 2 exhibits absorption bands typical for bpy radical species. The EPR spectra of 2 and its deuterated analog 2-d(8) demonstrate that the spin density is localized to the bpy ligand. Density functional theoretical calculations and natural bond orbital analysis were employed to elucidate the electronic structure of complexes 1–3 and accurately reproduced the structural experimental data. It is shown that reduction of the bpy moiety results in a decrease in the β-diketiminate co-ligand bite angle and elongation of the Zn–N(β-diketiminate) bonds, which act cooperatively and in synergy with the bpy ligand by decreasing Zn–N(bpy) bond lengths to stabilize the energy of the LUMO

Synthesis of a Ga-Stabilized As-Centered Radical and a Gallastibene by Tailoring Group 15 Element-Carbon Bond Strengths

A convenient synthetic route to Ga-stabilized pnictogen-centered radicals and gallapnictenes by manipulation of pnictogen-carbon bond strengths is presented. Two equivalents of LGa (L = HC[C(Me)N(Dip)](2), Dip = 2,6-i-Pr2C6H3) react with (CpAsCl2)-As-Ar(Cp-Ar = C-5(4-t-BuC6H4)(5)) with formation of the arsenic-centered radical [L(Cl)Ga](2)As. 1. In contrast, the analogous reaction with TerSbCl(2). (Ter = 2,6-Mes(2)C(6)H(3); Mes = 2,4,6-Me3C6H2) yields the gallastibene LGa=SbTer (2) containing a Ga-Sb double bond, whereas reactions of DipSbCl(2) with one and two equivalents of LGa give the monoinsertion and bisinsertion products L(Cl)GaSb(ClDip) (3) and [L(Cl]Ga](2)SbDip (4), respectively. 1-4 were structurally characterized by single crystal X-ray diffraction. The description of 1 as an arsenic-centered radical is consistent with results of electron paramagnetic resonance and density functional theory (DFT) studies. The pi-bonding in LGa=SbTer (2) is estimated to 10.68 kcal mol(-1) by variable-temperature (VT) NMR spectroscopy, and DFT studies reveal a significant pi-bonding interaction between Sb and Ga