Structural Characterization of Thermochromic and Spin Equilibria in Solid-State Ni(detu)₄Cl₂ (detu = <i>N</i>,<i>N</i>′‑Diethylthiourea)

Consecutive thermochromic lattice distortional and spin crossover equilibria in solid-state Ni­(detu)₄Cl₂ (detu = <i>N</i>,<i>N</i>′-diethylthiourea) are investigated by variable-temperature X-ray crystallography (173–333 K), DFT calculations, and differential scanning calorimetry. Thermochromism and anomalous magnetism were reported previously (S. L. Holt, Jr., et al. <i>J. Am. Chem. Soc.</i> <b>1964</b>, <i>86</i>, 519–520); the latter was attributed to equilibration of a singlet ground state and a thermally accessible triplet state, but structural data were not obtained. A crystal structure at 173(2) K revealed [Ni­(detu)₄]²⁺ centers with distorted planar ligation of nickel­(II) to the four sulfur atoms, with an average Ni–S bond length of 2.226(3) Å. The nickel ion was displaced out-of-plane by 0.334 Å toward a proximal apical chloride at a nonbonding distance of 3.134(1) Å. Asymmetry in the <i>trans</i> S–Ni–S angles was coupled to a monoclinic ↔ tetragonal lattice distortion (<i>T</i>_1/2 = 254 ± 11 K), resulting in thermochromism. Spin crossover occurs by tetragonal modulation of nickel­(II) with approach of the proximal chloride at higher temperatures (<i>T</i>_1/2 = 383 ± 18 K), which is consistent with a contraction of −0.096(4) Å in the Ni···Cl separation observed at 293 K. A high-spin (<i>S</i> = 1) square-pyramidal [Ni­(dmtu)₄Cl]⁺ model (dmtu = <i>N</i>,<i>N</i>′-dimethylthiourea) was optimized by DFT calculations, which estimated limiting equatorial Ni–S bond lengths of 2.45 Å and an apical Ni–Cl bond of 2.43 Å. Electronic spectra of the spin isomers were calculated by TD-DFT methods. Assignment of the FTIR spectrum was assisted by frequency calculations and isotope substitution

Aerobic and Hydrolytic Decomposition of Pseudotetrahedral Nickel Phenolate Complexes

Pseudotetrahedral nickel­(II) phenolate complexes Tp^R,MeNi-OAr (Tp^R,Me = hydrotris­(3-R-5-methylpyrazol-1-yl)­borate; R = Ph {<b>1a</b>}, Me {<b>1b</b>}; OAr = O-2,6-^<i>i</i>Pr₂C₆H₃) were synthesized as models for nickel-substituted copper amine oxidase apoenzyme, which utilizes an N₃O (i.e., His₃Tyr) donor set to activate O₂ within its active site for oxidative modification of the tyrosine residue. The bioinspired synthetic complexes <b>1a</b>,<b>b</b> are stable in dilute CH₂Cl₂ solutions under dry anaerobic conditions, but they decompose readily upon exposure to O₂ and H₂O. Aerobic decomposition of <b>1a</b> yields a range of organic products consistent with formation of phenoxyl radical, including 2,6-diisopropyl-1,4-benzoquinone, 3,5,3′,5′-tetraisopropyl-4,4′-diphenodihydroquinone, and 3,5,3′,5′-tetraisopropyl-4,4′-diphenoquinone, which requires concurrent O₂ reduction. The dimeric product complex di­[hydro­{bis­(3-phenyl-5-methylpyrazol-1-yl)­(3-<i>ortho</i>-phenolato-5-methylpyrazol-1-yl)­borato}­nickel­(II)] (<b>2</b>) was obtained by <i>ortho</i> C–H bond hydroxylation of a 3-phenyl ligand substituent on <b>1a</b>. In contrast, aerobic decomposition of <b>1b</b> yields a dimeric complex [Tp^Me,MeNi]₂(μ-CO₃) (<b>3</b>) with unmodified ligands. However, a unique organic product was recovered, assigned as 3,4-dihydro-3,4-dihydroxy-2,6-diisopropylcyclohex-5-enone on the basis of ¹H NMR spectroscopy, which is consistent with dihydroxylation (i.e., addition of H₂O₂) across the <i>meta</i> and <i>para</i> positions of the phenol ring. Initial hydrolysis of <b>1b</b> yields free phenol and the known complex [Tp^Me,MeNi­(μ-OH)]₂, while hydrolysis of <b>1a</b> yields an uncharacterized intermediate, which subsequently rearranges to the new sandwich complex [(Tp^Ph,Me)₂Ni] (<b>4</b>). Autoxidation of the released phenol under O₂ was observed, but the reaction was slow and incomplete. However, both <b>4</b> and the <i>in situ</i> hydrolysis intermediate derived from <b>1a</b> react with added H₂O₂ to form <b>2</b>. A mechanistic scheme is proposed to account for the observed product formation by convergent oxygenation and hydrolytic autoxidation pathways, and hypothetical complex intermediates along the former were modeled by DFT calculations. All new complexes (i.e., <b>1a</b>,<b>b</b> and <b>2</b>–<b>4</b>) were fully characterized by FTIR, ¹H NMR, and UV–vis–NIR spectroscopy and by X-ray crystallography

Aerobic and Hydrolytic Decomposition of Pseudotetrahedral Nickel Phenolate Complexes

Pseudotetrahedral nickel­(II) phenolate complexes Tp^R,MeNi-OAr (Tp^R,Me = hydrotris­(3-R-5-methylpyrazol-1-yl)­borate; R = Ph {<b>1a</b>}, Me {<b>1b</b>}; OAr = O-2,6-^<i>i</i>Pr₂C₆H₃) were synthesized as models for nickel-substituted copper amine oxidase apoenzyme, which utilizes an N₃O (i.e., His₃Tyr) donor set to activate O₂ within its active site for oxidative modification of the tyrosine residue. The bioinspired synthetic complexes <b>1a</b>,<b>b</b> are stable in dilute CH₂Cl₂ solutions under dry anaerobic conditions, but they decompose readily upon exposure to O₂ and H₂O. Aerobic decomposition of <b>1a</b> yields a range of organic products consistent with formation of phenoxyl radical, including 2,6-diisopropyl-1,4-benzoquinone, 3,5,3′,5′-tetraisopropyl-4,4′-diphenodihydroquinone, and 3,5,3′,5′-tetraisopropyl-4,4′-diphenoquinone, which requires concurrent O₂ reduction. The dimeric product complex di­[hydro­{bis­(3-phenyl-5-methylpyrazol-1-yl)­(3-<i>ortho</i>-phenolato-5-methylpyrazol-1-yl)­borato}­nickel­(II)] (<b>2</b>) was obtained by <i>ortho</i> C–H bond hydroxylation of a 3-phenyl ligand substituent on <b>1a</b>. In contrast, aerobic decomposition of <b>1b</b> yields a dimeric complex [Tp^Me,MeNi]₂(μ-CO₃) (<b>3</b>) with unmodified ligands. However, a unique organic product was recovered, assigned as 3,4-dihydro-3,4-dihydroxy-2,6-diisopropylcyclohex-5-enone on the basis of ¹H NMR spectroscopy, which is consistent with dihydroxylation (i.e., addition of H₂O₂) across the <i>meta</i> and <i>para</i> positions of the phenol ring. Initial hydrolysis of <b>1b</b> yields free phenol and the known complex [Tp^Me,MeNi­(μ-OH)]₂, while hydrolysis of <b>1a</b> yields an uncharacterized intermediate, which subsequently rearranges to the new sandwich complex [(Tp^Ph,Me)₂Ni] (<b>4</b>). Autoxidation of the released phenol under O₂ was observed, but the reaction was slow and incomplete. However, both <b>4</b> and the <i>in situ</i> hydrolysis intermediate derived from <b>1a</b> react with added H₂O₂ to form <b>2</b>. A mechanistic scheme is proposed to account for the observed product formation by convergent oxygenation and hydrolytic autoxidation pathways, and hypothetical complex intermediates along the former were modeled by DFT calculations. All new complexes (i.e., <b>1a</b>,<b>b</b> and <b>2</b>–<b>4</b>) were fully characterized by FTIR, ¹H NMR, and UV–vis–NIR spectroscopy and by X-ray crystallography

Eight-Coordinate, Stable Fe(II) Complex as a Dual ¹⁹F and CEST Contrast Agent for Ratiometric pH Imaging

Accurate mapping of small changes in pH is essential to the diagnosis of diseases such as cancer. The difficulty in mapping pH accurately <i>in vivo</i> resides in the need for the probe to have a ratiometric response so as to be able to independently determine the concentration of the probe in the body independently from its response to pH. The complex Fe^II-DOTAm-F12 behaves as an MRI contrast agent with dual ¹⁹F and CEST modality. The magnitude of its CEST response is dependent both on the concentration of the complex and on the pH, with a significant increase in saturation transfer between pH 6.9 and 7.4, a pH range that is relevant to cancer diagnosis. The signal-to-noise ratio of the ¹⁹F signal of the probe, on the other hand, depends only on the concentration of the contrast agent and is independent of pH. As a result, the complex can ratiometrically map pH and accurately distinguish between pH 6.9 and 7.4. Moreover, the iron­(II) complex is stable in air at room temperature and adopts a rare 8-coordinate geometry

Structural Properties of the Acidification Products of Scandium Hydroxy Chloride Hydrate

The structural properties of a series of scandium inorganic acid derivatives were determined. The reaction of Sc⁰ with concentrated aqueous hydrochloric acid led to the isolation of [(H₂O)₅Sc­(μ-OH)]₂4Cl·2H₂O (<b>1</b>). Compound <b>1</b> was modified with a series of inorganic acids (i.e., HNO₃, H₃PO₄, and H₂SO₄) at room temperature and found to form {[(H₂O)₄Sc­(κ²-NO₃)­(μ-OH)]­NO₃}₂ (<b>2a</b>), [(H₂O)₄Sc­(κ²-NO₃)₂]­NO₃·H₂O (<b>2b</b>) (at reflux temperatures), {6­[H]­[Sc­(μ-PO₄)­(PO₄)]₆}_<i>n</i> (<b>3</b>), and [H]­[Sc­(μ₃-SO₄)₂]·2H₂O (<b>4a</b>). Additional organosulfonic acid derivatives were investigated, including tosylic acid (H-OTs) to yield {[(H₂O)₄Sc­(OTs)₂]­OTs}·2H₂O (<b>4b</b>) in H₂O and [(DMSO)₃Sc­(OTs)₃] (<b>4c</b>) in dimethyl sulfoxide and triflic acid (H-OTf) to form [Sc­(H₂O)₈]­OTf₃ (<b>4d</b>). Other organic acid modifications of <b>1</b> were also investigated, and the final structures were determined to be {([(H₂O)₂Sc­(μ-OAc)₂]­Cl)₆}_<i>n</i> (<b>5</b>) from acetic acid (H-OAc) and [Sc­(μ-TFA)₃Sc­(μ-TFA)₃]_<i>n</i> (<b>6</b>) from trifluoroacetic acid (H-TFA). In addition to single-crystal X-ray structures, the compounds were identified by solid-state and solution-state ⁴⁵Sc nuclear magnetic resonance spectroscopic studies

Structural Properties of the Acidification Products of Scandium Hydroxy Chloride Hydrate

The structural properties of a series of scandium inorganic acid derivatives were determined. The reaction of Sc⁰ with concentrated aqueous hydrochloric acid led to the isolation of [(H₂O)₅Sc­(μ-OH)]₂4Cl·2H₂O (<b>1</b>). Compound <b>1</b> was modified with a series of inorganic acids (i.e., HNO₃, H₃PO₄, and H₂SO₄) at room temperature and found to form {[(H₂O)₄Sc­(κ²-NO₃)­(μ-OH)]­NO₃}₂ (<b>2a</b>), [(H₂O)₄Sc­(κ²-NO₃)₂]­NO₃·H₂O (<b>2b</b>) (at reflux temperatures), {6­[H]­[Sc­(μ-PO₄)­(PO₄)]₆}_<i>n</i> (<b>3</b>), and [H]­[Sc­(μ₃-SO₄)₂]·2H₂O (<b>4a</b>). Additional organosulfonic acid derivatives were investigated, including tosylic acid (H-OTs) to yield {[(H₂O)₄Sc­(OTs)₂]­OTs}·2H₂O (<b>4b</b>) in H₂O and [(DMSO)₃Sc­(OTs)₃] (<b>4c</b>) in dimethyl sulfoxide and triflic acid (H-OTf) to form [Sc­(H₂O)₈]­OTf₃ (<b>4d</b>). Other organic acid modifications of <b>1</b> were also investigated, and the final structures were determined to be {([(H₂O)₂Sc­(μ-OAc)₂]­Cl)₆}_<i>n</i> (<b>5</b>) from acetic acid (H-OAc) and [Sc­(μ-TFA)₃Sc­(μ-TFA)₃]_<i>n</i> (<b>6</b>) from trifluoroacetic acid (H-TFA). In addition to single-crystal X-ray structures, the compounds were identified by solid-state and solution-state ⁴⁵Sc nuclear magnetic resonance spectroscopic studies

σ- vs π‑Bonding in Manganese(II) Allyl Complexes

Reaction of two equivalents of K­[1,3-(SiMe₃)₂C₃H₃] (= K­[A′]) with MnCl₂ in THF produces the allyl complex A′₂Mn­(thf)₂; if the reaction is conducted in ether, the solvent-free heterometallic manganate species K₂MnA′₄ is isolated instead. With the related allyl K­[1,1′,3-(SiMe₃)₃C₃H₂] (= K­[A″]), reaction with MnCl₂ in THF/TMEDA produces the corresponding adduct A″₂Mn­(tmeda). In the solid state, both A′₂Mn­(thf)₂ and A″₂Mn­(tmeda) are monomeric complexes with σ-bonded allyl ligands (Mn–C = 2.174(2) and 2.189(2) Å, respectively). In contrast, K₂MnA′₄ is a two-dimensional coordination polymer, in which two of the allyl ligands on the Mn cation are σ-bonded (Mn–C = 2.197(6), 2.232(7) Å) and the third is π-bonded (Mn–C = 2.342(7)–2.477(7) Å). Both σ-allyls are π-coordinated to potassium cations, promoting the polymer in two directions; the π-allyl ligand is terminal. Density functional theory (DFT) calculations indicate that isolated high-spin (C₃R₂H₃)₂Mn (R = H, SiMe₃) complexes would possess π-bound ligands. A mixed hapticity (π-allyl)­(σ-allyl)­MnE structure would result with the addition of either a neutral ligand (e.g., THF, MeCN) or one that is charged (Cl, H). Both allyl ligands in a bis­(allyl)manganese complex are expected to adopt a σ-bonded mode if two THF ligands are added, as is experimentally observed in A′₂Mn­(thf)₂. The geometry of allyl–Mn­(II) bonding is readily modified; DFT results predict that [(C₃H₅)­Mn]⁺ and (C₃H₅)­MnCl should be σ-bonded, but the allyl in (C₃H₅)­MnH is found to exhibit a symmetrical π-bonded arrangement. Some of this behavior is reminiscent of that found in bis­(allyl)magnesium chemistry

σ- vs π‑Bonding in Manganese(II) Allyl Complexes

Reaction of two equivalents of K­[1,3-(SiMe₃)₂C₃H₃] (= K­[A′]) with MnCl₂ in THF produces the allyl complex A′₂Mn­(thf)₂; if the reaction is conducted in ether, the solvent-free heterometallic manganate species K₂MnA′₄ is isolated instead. With the related allyl K­[1,1′,3-(SiMe₃)₃C₃H₂] (= K­[A″]), reaction with MnCl₂ in THF/TMEDA produces the corresponding adduct A″₂Mn­(tmeda). In the solid state, both A′₂Mn­(thf)₂ and A″₂Mn­(tmeda) are monomeric complexes with σ-bonded allyl ligands (Mn–C = 2.174(2) and 2.189(2) Å, respectively). In contrast, K₂MnA′₄ is a two-dimensional coordination polymer, in which two of the allyl ligands on the Mn cation are σ-bonded (Mn–C = 2.197(6), 2.232(7) Å) and the third is π-bonded (Mn–C = 2.342(7)–2.477(7) Å). Both σ-allyls are π-coordinated to potassium cations, promoting the polymer in two directions; the π-allyl ligand is terminal. Density functional theory (DFT) calculations indicate that isolated high-spin (C₃R₂H₃)₂Mn (R = H, SiMe₃) complexes would possess π-bound ligands. A mixed hapticity (π-allyl)­(σ-allyl)­MnE structure would result with the addition of either a neutral ligand (e.g., THF, MeCN) or one that is charged (Cl, H). Both allyl ligands in a bis­(allyl)manganese complex are expected to adopt a σ-bonded mode if two THF ligands are added, as is experimentally observed in A′₂Mn­(thf)₂. The geometry of allyl–Mn­(II) bonding is readily modified; DFT results predict that [(C₃H₅)­Mn]⁺ and (C₃H₅)­MnCl should be σ-bonded, but the allyl in (C₃H₅)­MnH is found to exhibit a symmetrical π-bonded arrangement. Some of this behavior is reminiscent of that found in bis­(allyl)magnesium chemistry

Unexpectedly Stable (Chlorocarbonyl)(<i>N-</i>ethoxycarbonylcarbamoyl)disulfane, and Related Compounds That Model the Zumach–Weiss–Kühle (ZWK) Reaction for Synthesis of 1,2,4-Dithiazolidine-3,5-diones

The Zumach–Weiss–Kühle (ZWK) reaction provides 1,2,4-dithiazolidine-3,5-diones [dithiasuccinoyl (Dts)-amines] by the rapid reaction of <i>O</i>-ethyl thiocarbamates plus (chlorocarbonyl)­sulfenyl chloride, with ethyl chloride and hydrogen chloride being formed as coproducts, and carbamoyl chlorides or isocyanates generated as yield-diminishing byproducts. However, when the ZWK reaction is applied with (<i>N</i>-ethoxythiocarbonyl)­urethane as the starting material, heterocyclization to the putative “Dts-urethane” <i>does not occur</i>. Instead, the reaction directly provides (chlorocarbonyl)­(<i>N-</i>ethoxycarbonylcarbamoyl)­disulfane, a reasonably stable crystalline compound; modified conditions stop at the (chlorocarbonyl)­[1-ethoxy-(<i>N</i>-ethoxycarbonyl)­formimidoyl]­disulfane intermediate. The title (chlorocarbonyl)(carbamoyl)­disulfane <i>cannot</i> be converted to the elusive Dts derivative, but rather gives (<i>N</i>-ethoxycarbonyl)­carbamoyl chloride upon thermolysis, or (<i>N-</i>ethoxycarbonyl)­isocyanate upon treatment with tertiary amines. Additional transformations of these compounds have been discovered, providing entries to both known and novel species. X-ray crystallographic structures are reported for the title (chlorocarbonyl)­(carbamoyl)­disulfane; for (methoxycarbonyl)­(<i>N</i>-ethoxycarbonylcarbamoyl)­disulfane, which is the corresponding adduct after quenching in methanol; for [1-ethoxy-(<i>N</i>-ethoxycarbonyl)­formimidoyl]­(<i>N</i>′-methyl-<i>N</i>′-phenylcarbamoyl)­disulfane, which is obtained by trapping the title intermediate with <i>N</i>-methylaniline; and for (<i>N</i>-ethoxycarbonylcarbamoyl)­(<i>N</i>′-methyl-<i>N</i>′-phenylcarbamoyl)­disulfane, which is a short-lived intermediate in the reaction of the title (chlorocarbonyl)­(carbamoyl)­disulfane with excess <i>N</i>-methylaniline. The new chemistry and structural information reported herein is expected to contribute to accurate modeling of the ZWK reaction trajectory

Solvate Structures and Computational/Spectroscopic Characterization of LiPF₆ Electrolytes

Raman spectroscopy is a powerful method for identifying ion–ion interactions, but only if the vibrational band signatures for the anion coordination modes can be accurately deciphered. The present study characterizes the PF₆^– anion P–F Raman symmetric stretching vibrational band for evaluating the PF₆^–···Li⁺ cation interactions within LiPF₆ crystalline solvates to create a characterization tool for liquid electrolytes. To facilitate this, the crystal structures for two new solvates(G3)₁:LiPF₆ and (DEC)₂:LiPF₆ with triglyme and diethyl carbonate, respectivelyare reported. DFT calculations for Li-PF₆ solvates have been used to aid in the assignments of the spectroscopic signatures. The information obtained from this analysis provides key guidance about the ionic association information which may be obtained from a Raman spectroscopic evaluation of electrolytes containing the LiPF₆ salt and aprotic solvents. Of particular note is the overlap of the Raman bands for both solvent-separated ion pair (SSIP) and contact ion pair (CIP) coordination in which the PF₆^– anions are uncoordinated or coordinated to a single Li⁺ cation, respectively