Remote Supergroup for Chemistry Undergraduates: An Inclusive Scientific Community for Primarily Undergraduate Institutions

The Remote Supergroup for Chemistry Undergraduates (RSCU) is a community of students and faculty from primarily undergraduate institutions that aims to (1) engage students in discussions of chemical research, (2) inform students of further educational and career pathways, (3) increase awareness and discourse of equity issues in science, and (4) foster scientific community across institutions. RSCU engaged participants in impactful virtual activities during the summer of 2020 when the COVID-19 pandemic precluded in-person undergraduate research experiences, and the program continued in 2021 as in-person research resumed. Results from self-reported surveys show that RSCU successfully achieved its aims both years, and both students and faculty research mentors benefited from participation. The diverse activities and scientific network cultivated by RSCU complement undergraduate research experiences and could be adapted to other disciplines

Crystal structure of 1-(2,6-diisopropylphenyl)-1H-imidazole

The crystal structure of the title compound, C15H20N2 or DippIm, is reported. At 106 (2) K, the molecule has monoclinic P21/c symmetry with four molecules in the unit cell. The imidazole ring is rotated 80.7 (1)° relative to the phenyl ring. Intermolecular stabilization primarily results from close contacts between the N atom at the 3-position on the imidazole ring and the C—H bond at the 4-position on the neighboring DippIm, with aryl–aryl distances outside of the accepted distance of 5 Å for π-stacking

C(sp3)-H Fluorination with a Copper(II)/(III) Redox Couple

Despite the growing interest in the synthesis of fluorinated organic compounds, few methods are able to incorporate fluoride ion directly into alkyl C-H bonds. Here, we report the C(sp3)-H fluorination reactivity of a formally copper(III) fluoride complex. The C-H fluorination intermediate, LCuF, along with its chloride and bromide analogs, LCuCl and LCuBr, were prepared directly from halide sources with a chemical oxidant and fully characterized. While all three copper(III) halide complexes capture carbon radicals efficiently to afford C(sp3)-halogen bonds, LCuF is two orders of magnitude more efficient at hydrogen atom abstraction (HAA) than LCuCl and LCuBr. Alongside reported kinetic data for other LCu(III) species, we established a positive correlation between ligand basicity and the rate of HAA. The capability of LCuF to perform both hydrogen atom abstraction and radical capture was leveraged to enable fluorination of allylic and benzylic C-H bonds and α-C-H bonds of ethers at room temperature.</p

Prediction of high-valent iron K-edge absorption spectra by time-dependent Density Functional Theory

In recent years a number of high-valent iron intermediates have been identified as reactive species in iron-containing metalloproteins. Inspired by the interest in these highly reactive species, chemists have synthesized Fe(IV) and Fe(V) model complexes with terminal oxo or nitrido groups, as well as a rare example of an Fe(VI)-nitrido species. In all these cases, X-ray absorption spectroscopy has played a key role in the identification and characterization of these species, with both the energy and intensity of the pre-edge features providing spectroscopic signatures for both the oxidation state and the local site geometry. Here we build on a time-dependent DFT methodology for the prediction of Fe K- pre-edge features, previously applied to ferrous and ferric complexes, and extend it to a range of Fe(IV), Fe(V) and Fe(VI) complexes. The contributions of oxidation state, coordination environment and spin state to the spectral features are discussed. These methods are then extended to calculate the spectra of the heme active site of P450 Compound II and the non-heme active site of TauD. The potential for using these methods in a predictive manner is highlighted

Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((CNC)-C-iPr)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 degrees C and 4 atm of H-2 pressure, ((CNC)-C-iPr)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, ((CNC)-C-iPr)CoH, was accomplished by hydrogenation of ((CNC)-C-iPr)CoCH3. Over the course of 3 h at 22 degrees C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H-2-(CNC)-C-iPr)CoN2. Similar alkyl migration was observed upon treatment of ((CNC)-C-iPr)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active

Bis(imino)pyridine Iron Dinitrogen Compounds Revisited: Differences in Electronic Structure Between Four- and Five-Coordinate Derivatives.

The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, ((PDI)-P-iPr)FeN2 and ((PDI)-P-iPr)Fe(N-2)(2) ((TDI)-T-iPr = 2,6-(2,6-Pr-i(2)-C6H3-N=CMe)(2)C5H3N), have been investigated by a combination of spectroscopic techniques (NMR, Mossbauer, X-ray Absorption, and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, ((RPDI)-R-iPr)FeN2 ((RPDI)-R-iPr = 2,6-(2,6-Pr-i(2)-C6H3-N=CR)(2)C5H3N; R = Et, Pr-i), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the ((RPDI)-R-iPr)FeN2 compounds are intermediate spin ferrous derivatives (S-Fe = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (S-PDI = 1). While this ground state description is identical to that previously reported for ((PDI)-P-iPr)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally sigma-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For ((RPDI)-R-iPr)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the ((RPDI)-R-iPr)FeN2 family, one of the iron singly occupied molecular orbitals (SOMOs) is essentially d(z)(2) in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of ((RPDI)-R-iPr)FeN2 and ((PDI)-P-iPr)Fe(DMAP) differ from ((PDI)-P-iPr)Fe(N-2)(2), a highly covalent molecule with a redox noninnocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004

Oxidative Addition of Carbon-Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, ((PDI)-P-iPr)Fe(N-2)(2) and [((PDI)-P-Me)Fe(N-2))(2)(mu(2)-N-2) ((PDI)-P-R = 2,6-(2,6-R-2-C6H3- N=CMe)(2)C5H3N; R = Me, Pr-i), resulted in oxidative addition of a C C bond at ambient temperature to yield the corresponding iron biphenyl compounds, ((PDI)-P-R)Fe(biphenyl). The molecular structures of the resulting bis(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mossbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand

Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe­(NC^tBu₂)₄ (<b>1</b>) for 8 h at 50 °C generates the mixed valent Fe­(III)/Fe­(II) bimetallic complex Fe₂(NC^tBu₂)₅ (<b>2</b>) in moderate yield. Also formed in this reaction are <i>tert</i>-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe⁴⁺ center. Reaction of <b>1</b> with 1 equiv of acetylacetone affords the Fe­(III) complex, Fe­(NC^tBu₂)₂(acac) (<b>3</b>), concomitant with formation of bis­(<i>tert</i>-butyl)­ketimine, <i>tert</i>-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of <b>1</b> and its lower-valent analogues [Li­(12-crown-4)₂]­[Fe­(NC^tBu₂)₄] (<b>5</b>) and [Li­(THF)]₂[Fe­(NC^tBu₂)₄] (<b>6</b>) were recorded. We also revisited the chemistry of Fe­(1-norbornyl)₄ (<b>4</b>) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for <b>1</b>

Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe­(NC^tBu₂)₄ (<b>1</b>) for 8 h at 50 °C generates the mixed valent Fe­(III)/Fe­(II) bimetallic complex Fe₂(NC^tBu₂)₅ (<b>2</b>) in moderate yield. Also formed in this reaction are <i>tert</i>-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe⁴⁺ center. Reaction of <b>1</b> with 1 equiv of acetylacetone affords the Fe­(III) complex, Fe­(NC^tBu₂)₂(acac) (<b>3</b>), concomitant with formation of bis­(<i>tert</i>-butyl)­ketimine, <i>tert</i>-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of <b>1</b> and its lower-valent analogues [Li­(12-crown-4)₂]­[Fe­(NC^tBu₂)₄] (<b>5</b>) and [Li­(THF)]₂[Fe­(NC^tBu₂)₄] (<b>6</b>) were recorded. We also revisited the chemistry of Fe­(1-norbornyl)₄ (<b>4</b>) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for <b>1</b>

Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen Complexes: Evidence for Formation of a Chelate Trianion.

Oxidation and reduction of the bis­(imino)­pyridine iron dinitrogen compound, (^iPrPDI)­FeN₂ (^iPrPDI = 2,6-(2,6-ⁱPr₂–C₆H₃–NCMe)₂C₅H₃N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (^iPrPDI)­FeN₂ with [Cp₂Fe]­[BAr^F₄] (BAr^F₄ = B­(3,5-(CF₃)₂-C₆H₃)₄) in diethyl ether solution resulted in N₂ loss and isolation of [(^iPrPDI)­Fe­(OEt₂)]­[BAr^F₄]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field ⁵⁷Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron­(II) center (<i>S</i>_Fe = 2) antiferromagnetically coupled to a bis­(imino)­pyridine radical anion (<i>S</i>_PDI = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na­(15-crown-5)]­[(^iPrPDI)­FeN₂], was isolated following addition of sodium naphthalenide to (^iPrPDI)­FeN₂ in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis­(imino)­pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (^iPrBPDI)­Fe­(N₂)₂ resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [(^iPrBPDI)­FeN₂]⁻ and [(^iPrBPDI)­FeN₂]^2‑, highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states