A complex with nitrogen single, double, and triple bonds to the same chromium atom: synthesis, structure, and reactivity

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.

Synthesis, Characterization, and Reactivity of Tris(imido)chromium(VI) Complexes

Multiple tris(imido)chromium(VI) complexes, including neutral and ionic compounds, have been synthesized and characterized. (tBuN)2Cr(NHtBu)Cl can be deprotonated by KN(SiMe3)2, yielding K[(tBuN)3CrCl]. This tris(imido) anion undergoes nucleophilic substitution by PPh3 and tBuNH2 to form (tBuN)3Cr(PPh3) and (tBuN)2Cr(NHtBu)2, respectively. (tBuN)2Cr(NHtBu)2 loses one amido proton to form K[(tBuN)3Cr(NHtBu)] upon reaction with KN(SiMe3)2. The imido ligands of K[(tBuN)3CrCl] and (tBuN)3Cr(PPh3) are attacked by the electrophile MeI to produce (tBuN)2Cr(NMetBu)Cl and (tBuN)2Cr(NMetBu)I, respectively. An alternate way to make tris(imido) anions is deprotonation of (tBuN)2Cr(NHtBu)Cl by an alkyl lithium reagent, e.g., Me3SiCH2Li. The resulting Li[(tBuN)3CrCl] was alkylated by a second equivalent of Me3SiCH2Li to form Li[(tBuN)3Cr(CH2SiMe3)]. Reactivity studies of tris(imido) complexes show cycloaddition with PhNCO or CO2 to form metallacycles.

Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity

Serendipitous discovery of bida (i.e., N1-Ar-N2-((1-Ar-1-benzo[d]imidazol-2-yl)methyl)benzene-1,2-diamide; Ar = 2,6-iPr-C6H3), a potentially redox noninnocent, hemilabile pincer ligand with a methylene group that may facilitate proton/H atom reactivity, prompted its investigation. Chromium was chosen for study due to its multiple stable oxidation states. Disodium salt (bida)Na2(THF)n was prepared by thermal rearrangement of (dadi)Na2(THF)4 (i.e., (N,N'-di-2-(2,6-diisopropylphenylamine)phenylglyoxaldiimine)-Na2(THF)4). Salt metathesis of (bida)Na2(THF)n (generated in situ) with CrCl3(THF)3 or Cl3V═NAr (Ar = 2,6-iPr2C6H3) afforded (bida)CrCl(THF) (1-THF) and (bida)ClV═NAr, respectively. Substitutions provided (bida)CrCl(PMe2Ph) (1-PMe2Ph) and (bida)CrR(THF) (2-R, where R = Me, CH2CMe2Ph (Nph)). Oxidation of 1-THF with ArN3 (Ar = 2,6-iPr2C6H3) or AdN3 (Ad = 1-adamantyl) generated (bida)ClCr═NAr (3═NAr) and (bida)ClCr═NAd (3═NAd) and subsequent alkylation converted these to (bida)R'Cr═NR (R' = Me, R = Ad, Ar, 5═NR; R' = CH2CMe2Ph (Nph), R = Ad, Ar, 6═NR). In contrast, the addition of AdN3 to 2-Nph gave the insertion product (bida)Cr(κ2-N,N-ArN3Nph) (7). Addition of N-chlorosuccinimide to 1-THF produced (bia)CrCl2(THF) (8), where bia is the pincer derived via hydrogen atom loss from bida methylene. A similar HAT afforded (bia)ClCr(CNAr')2 (9, Ar' = 2,6-Me2C6H3) when 3═NAd was exposed to Ar'NC. An empirical equation of charge was applied to each bida species, whose metric parameters are unchanging despite formal oxidation state conversions from Cr(III) to Cr(V). Calculations and Mulliken spin density assessments reveal several situations in which antiferromagnetic (AF) coupling and admixtures of integer ground states (GSs) describe a complicated electronic structure.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

Oxidation of a series of Cr^V nitride salen complexes (Cr^VNSal^R) with different para-phenolate substituents (R = CF₃, tBu, NMe₂) was investigated to determine how the locus of oxidation (either metal or ligand) dictates reactivity at the nitride. Para-phenolate substituents were chosen to provide maximum variation in the electron-donating ability of the tetradentate ligand at a site remote from the metal coordination sphere. We show that one-electron oxidation affords Cr^VI nitrides ([Cr^VINSal^R]⁺; R = CF₃, tBu) and a localized Cr^V nitride phenoxyl radical for the more electron-donating NMe₂ substituent ([Cr^VNSal^NMe2]^•+). The facile nitride homocoupling observed for the Mn^VI analogues was significantly attenuated for the Cr^VI complexes due to a smaller increase in nitride character in the M≡N π* orbitals for Cr relative to Mn. Upon oxidation, both the calculated nitride natural population analysis (NPA) charge and energy of molecular orbitals associated with the {Cr≡N} unit change to a lesser extent for the Cr^V ligand radical derivative ([Cr^VNSal^NMe2]^•+) in comparison to the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu). As a result, [Cr^VNSal^NMe2]^•+ reacts with B(C₆F₅)₃, thus exhibiting similar nucleophilic reactivity to the neutral Cr^V nitride derivatives. In contrast, the Cr^VI derivatives ([Cr^VINSal^R]⁺; R = CF₃, tBu) act as electrophiles, displaying facile reactivity with PPh₃ and no reaction with B(C₆F₅)₃. Thus, while oxidation to the ligand radical does not change the reactivity profile, metal-based oxidation to Cr^VI results in umpolung, a switch from nucleophilic to electrophilic reactivity at the terminal nitride.

Top-Down N-Doped Carbon Quantum Dots for Multiple Purposes: Heavy Metal Detection and Intracellular Fluorescence

In the present study, we successfully synthesized N-doped carbon quantum dots (N-CQDs) using a top-down approach, i.e., hydroxyl radical opening of fullerene with hydrogen peroxide, in basic ambient using ammonia for two different reaction times. The ensuing characterization via dynamic light scattering, SEM, and IR spectroscopy revealed a size control that was dependent on the reaction time, as well as a more pronounced -NH2 functionalization. The N-CQDs were probed for metal ion detection in aqueous solutions and during bioimaging and displayed a Cr3+ and Cu2+ selectivity shift at a higher degree of -NH2 functionalization, as well as HEK-293 cell nuclei marking.

Thermochemistry of Tungsten-3p Elements for Density Functional Theory, Caveat Lector!

There are two primary foci in this research on WE (E = Si, P, and S) bonds: prediction of their bond dissociation enthalpies (BDEs), including σ- and π-bond energy components, and assessing the uncertainty of these BDE predictions for levels of theory commonly used in the literature. The internal standards for computational accuracy include metal-element bond lengths (mean absolute error = 1.8 ± 1.2%), main group homolog BDEs versus higher levels of ab initio theory (W1U and G4 BDEs, R² = 0.98), and DLPNO-CCSD(T)/def2-QZVPP calculations for metal-ligand BDEs (R² = 0.88). The W═Si first π-bond is underreported for density functional theory (DFT)/MP2 methods versus DLPNO-CCSD(T), while the latter shows negligible strength for the W;Si second π-bond, consistent with the literature. This research highlights clear issues with the underlying assumptions required for the use of perturbation theory methods for the fragments derived from W-P homolysis. The difficulties associated with modeling the metal thermochemistry with DFT (and MP2) levels of theory are manifest in the broad standard deviations observed. However, the average BDEs found using 48 popular DFT and MP2 levels of theory are reliable, 10.8 ± 6.8% mean absolute error (with W-P removed) versus DLPNO-CCSD(T), with the caveat that the individual basis set/pseudopotential/valence basis set combination can vary wildly. Analysis of the absolute error percentages with respect to the level of theory indicates little benefit to going higher on Jacob's Ladder, as simpler methods have lower error versus high-level ab initio techniques such as G4 and DLPNO-CCSD(T).

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]^Ph (H₂[OO]^Ph = [1,1′:4′,1′’-terphenyl]-2,2′’-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]^Ph(THF)₂ (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr₂([OO]^Ph)₂ (in CH₂Cl₂/THF). The electronic structure and bonding in Cr[OO]^Ph(THF)₂ were probed by density functional theory calculations. Isolated Cr₂([OO]^Ph)₂ undergoes facile reaction with 4-MeC₆H₄N₃, 4-MeOC₆H₄N₃, or 3,5-Me₂C₆H₃N₃ to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]^Ph(N(4-MeC₆H₄))₂ was confirmed by X-ray crystallography. The reaction of Cr₂([OO]^Ph)₂ with bulkier azides N₃R (MesN₃, AdN₃) forms paramagnetic products, formulated as Cr[OO]^Ph(NR). The attempted formation of a Cr–alkylidene complex (using N₂CPh₂) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]^Ph(NNCPh₂)₂. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr₂([OO]^Ph)₂ or H₂[OO]^Ph with NO⁺ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.

Tungsten-Ligand Bond Strengths for 2p Elements Including σ- and π-Bond Strength Components, A Density Functional Theory and ab Initio Study

Three W^VI crystal structures with multifarious metal-ligand bond types are used to theoretically predict homolytic metal-element bond enthalpies with 11 popular DFT functionals, MP2 wave function methods, and four common valence basis set/pseudopotentials in order to evaluate the accuracy and precision of the resultant bond enthalpy data. To our knowledge, for the first time, estimates of component metal-ligand σ- and π-bond strengths are computed. The WE (E = C, N, O) bond enthalpies have the consistent trend σ > second π > first π. In contrast, the element-element BDE trend for the 2p homologues is second π > first π > σ for nitrogen and oxygen, and σ > first π > second π for carbon. These differences may underpin the differences in stability trends and thus reactivity behavior for metal-element multiple bonds as compared to the element-element multiple bonds, and metal-element triple bonds versus their corresponding double bonded counterparts. For example, Odom et al. show that MeI nucleophilically attacks at the imide (M═N) rather than the nitride (M ≡ N) ligand; the relative π-bond strengths derived herein provide a thermodynamic rationalization for this site preference. In this study, it is deduced from the calculated thermodynamics that the W-oxo ligand is more congruous with a triple bond than a double bond, consistent with the bonding model set forth in the seminal 1961 Ballhausen-Gray paper.

Platinum(ii) as an assembly point for carbide and nitride ligands

The sequential treatment of (Cy₃P)₂Cl₂RuC with [PtCl₂(C₂H₄)]₂ and (dbm)₂CrN affords a platinum(ii) center coordinated by both carbide and nitride ligands.

Synthesis and Characterization of Tungsten Nitrido Amido Guanidinato Complexes as Precursors for Chemical Vapor Deposition of WNxCy Thin Films

Tungsten nitrido amido guanidinato complexes of the type WN(NR₂)[(NR')₂C(NR₂)]₂ (R = Me, Et_; R' = ⁱ Pr, Cy) were synthesized as precursors for aerosol-assisted chemical vapor deposition (AACVD) of WN_xC_y thin films. The reaction of tungsten nitrido amido complexes of the type WN(NR₂)₃ (R = Me, Et) with two equivalents of a carbodiimide R'N=C=NR' (R' = ⁱ Pr, Cy) resulted in two insertions of a carbodiimide into W-N(amido) bonds, affording bis(guanidinato) amido nitrido tungsten complexes. These compounds were characterized by ¹⁴N NMR, indicating distinctive chemical shifts for each type of N-bound ligand. Crystallographic structure determination of WN(NMe₂)[(N ⁱ Pr)₂C(NMe₂)]₂ showed the guanidinato ligands to be non-equivalent. The complex WN(NMe₂)[(N ⁱ Pr)₂C(NMe₂)]₂ was demonstrated to serve as a precursor for AACVD of WN_xC_y thin films, resulting in featureless, X-ray amorphous thin films for growth temperatures 200 - 400 °C.

Nitrogen atom transfer mediated by a new PN3P-pincer nickel core via a putative nitrido nickel intermediate

A synthetic cycle for a complete nitrogen atom transfer reaction is achieved by irradiating the (PN³P)Ni(N₃)/RNC mixture and subsequent treatment of the resultant products with alkyl halides.

Two lanthanide metal–organic frameworks as sensitive luminescent sensors for the detection of Cr2+ and Cr2O72− in aqueous solutions

Two lanthanide–organic frameworks [Ln(HPIDC)(m-bdc)·1.5H₂O]_n (Ln = Eu 1 or Tb 2; H₃PIDC = 2-(4-pyridyl)-1H-imidazole-4,5-dicarboxylic acid; m-H₂bdc = 1,3-benzenedicarboxylic acid) were synthesized under hydrothermal conditions.

Quantifying ligand effects in high-oxidation-state metal catalysis

Catalysis by high-valent metals such as titanium(IV) impacts our lives daily through reactions like olefin polymerization. In any catalysis, optimization involves a careful choice of not just the metal but also the ancillary ligands. Because these choices dramatically impact the electronic structure of the system and, in turn, catalyst performance, new tools for catalyst development are needed. Understanding ancillary ligand effects is arguably one of the most critical aspects of catalyst optimization and, while parameters for phosphines have been used for decades with low-valent systems, a comparable system does not exist for high-valent metals. A new electronic parameter for ligand donation, derived from experiments on a high-valent chromium species, is now available. Here, we show that the new parameters enable quantitative determination of ancillary ligand effects on catalysis rate and, in some cases, even provide mechanistic information. Analysing reactions in this way can be used to design better catalyst architectures and paves the way for the use of such parameters in a host of high-valent processes.

Metal Ion Modeling Using Classical Mechanics

Metal ions play significant roles in numerous fields including chemistry, geochemistry, biochemistry, and materials science. With computational tools increasingly becoming important in chemical research, methods have emerged to effectively face the challenge of modeling metal ions in the gas, aqueous, and solid phases. Herein, we review both quantum and classical modeling strategies for metal ion-containing systems that have been developed over the past few decades. This Review focuses on classical metal ion modeling based on unpolarized models (including the nonbonded, bonded, cationic dummy atom, and combined models), polarizable models (e.g., the fluctuating charge, Drude oscillator, and the induced dipole models), the angular overlap model, and valence bond-based models. Quantum mechanical studies of metal ion-containing systems at the semiempirical, ab initio, and density functional levels of theory are reviewed as well with a particular focus on how these methods inform classical modeling efforts. Finally, conclusions and future prospects and directions are offered that will further enhance the classical modeling of metal ion-containing systems.