Search

Energetic plasticizers are being sought for their use in energetic formulations when combined with explosives. An energetic plasticizer based on the insensitive highly explosive 3-amino-5-nitro-l,2,4-triazole (ANTA) was synthesized and characterized by spectroscopy, X-ray crystallography, thermal analyses, and safety testing. Lastly, density functional theory calculations were employed to examine the observed selectivity among the three nucleophilic ring nitrogen atoms of ANTA toward electrophiles such as ANTA acrylate; this selectivity was found to be a combination of steric, electronic, and hydrogen bonding effects.

We present a new integrated experimental and modeling effort that assesses the intrinsic sensitivity of energetic materials based on their reaction rates. The High Explosive Initiation Time (HEIT) experiment has been developed to provide a rapid assessment of the high-temperature reaction kinetics for the chemical decomposition of explosive materials. This effort is supported theoretically by quantum molecular dynamics (QMD) simulations that depict how different explosives can have vastly different adiabatic induction times at the same temperature. In this work, the ranking of explosive initiation properties between the HEIT experiment and QMD simulations is identical for six different energetic materials, even though they contain a variety of functional groups. We have also determined that the Arrhenius kinetics obtained by QMD simulations for homogeneous explosions connect remarkably well with those obtained from much longer duration one-dimensional time-to-explosion (ODTX) measurements. Kinetic Monte Carlo simulations have been developed to model the coupled heat transport and chemistry of the HEIT experiment to explicitly connect the experimental results with the Arrhenius rates for homogeneous explosions. These results confirm that ignition in the HEIT experiment is heterogeneous, where reactions start at the needle wall and propagate inward at a rate controlled by the thermal diffusivity and energy release. Overall, this work provides the first cohesive experimental and first-principles modeling effort to assess reaction kinetics of explosive chemical decomposition in the subshock regime and will be useful in predictive models needed for safety assessments.

Here, we report Rh and Ir complexes containing a triaryl SBS pincer ligand flanked by neutral thiomethyl donor groups. Oxidative addition of the B–H bond in the diazaborole proligand H(MeSBSMe) to [Ir(COD)Cl]2or [Rh(COE)Cl]2yielded the chloride-bridged dimers [(MeSBSMe)MH(μ-Cl)]2, where M = Ir (1) or Rh (2). Addition of CO to 1and 2yielded unstable products that were difficult to isolate, but crystals of monomeric (MeSBSMe)IrH(CO)Cl (3) recovered from the reactions with 1revealed a change in the MeSBSMecoordination mode from merto fac. Treating 1and 2with LiN(SiMe3)2yielded mer-(MeSBSMe)IrH[N(SiMe3)2] (4) and the unusual RhI–RhIIIdimer (MeSBSMe)Rh(μ-H)[(MeS(μ-B)(μ-SMe)]Rh[N(SiMe3)2] (5) with both merand facMeSBSMe. 4and 5did not exhibit any alkane transfer hydrogenation reactivity when tested with tert-butylethylene and cyclooctane, but they are highly active for alkene isomerization with 1-hexene. Optimized isomerization reactions showed the highest turnover number (TON) with 4at 60 °C after 16 h (TON = 10 000), and both catalysts are effective even when tested at room temperature with similar loadings (TON = 600). Collectively, these data highlight the reactivity and inherent coordinative flexibility of the MeSBSMeligand for comparison to more well-established PBP complexes.

Metal–ligand cooperativity (MLC), a phenomenon that leverages reactive ligands to promote synergistic reactions with metals, has proven to be a powerful approach to achieving new and unprecedented chemical transformations with metal complexes. While many examples of MLC are known with a wide range of substrates, experimentally quantifying how ligand modifications affect MLC binding strength remains a challenge. Here we describe how cyclic voltammetry (CV) was used to quantify differences in MLC binding strength in a series of square-pyramidal Ru complexes. This method relies on using multifunctional ligands (those capable of both MLC and ligand-centered redox activity) as electrochemical reporters of MLC binding strength. The synthesis and characterization of Ru complexes with three different redox-active tetradentate ligands and two different ancillary phosphines (PPh3and PCy3) are described. Titration CV studies conducted using BH3·THF with BH3as a model MLC substrate allowed ΔGMLCto be quantified for each complex. Compared to our base triaryl ligand, increasing π conjugation in the backbone of the redox-active ligand enhanced MLC binding, whereas increasing π conjugation in the flanking groups decreased the MLC binding strength. Structures and spectroscopic data collected for the isolated MLC complexes are also described along with supporting DFT calculations that were used to illuminate electronic factors that likely account for the observed differences in the MLC binding strength. These results demonstrate how redox-active ligands and CV can be used to quantify subtle differences in the MLC binding strength across a series of structurally related complexes with different ligand modifications.

Metal–ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH3molecules undergo MLC binding with a triaryl noninnocent N2S2ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N2S2ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

The continued development of redox-active ligands requires an understanding as to how ligand modifications and related factors affect the locus of redox activity and spin density in metal complexes. Here we describe the synthesis, characterization, and electronic structure of nickel complexes containing triaryl NNNN (1) and SNNS (2) ligands derived from o-phenylenediamine. The tetradentate ligands in 1and 2were investigated and compared to those in metal complexes with compositionally similar ligands to determine how ligand-centered redox properties change when redox-active flanking groups are replaced with redox-innocent NMe2or SMe. A derivative of 2in which the phenylene backbone was replaced with ethylene (3) was also prepared to interrogate the importance of o-phenylenediamine for ligand-centered redox activity. Cyclic voltammograms collected for 1and 2revealed two fully reversible ligand-centered redox events. Remarkably, several quasi-reversible ligand-centered redox waves were also observed for 3despite the absence of the o-phenylenediamine subunit. Oxidizing 1and 2with silver salts containing different counteranions (BF4–, OTf–, NTf2–) allowed the electrochemically generated complexes to be analyzed as a function of different oxidation states using single-crystal X-ray diffraction (XRD), EPR spectroscopy, and S K-edge X-ray absorption spectroscopy. The experimental data are corroborated by DFT calculations, and together, they reveal how the location of unpaired spin density and electronic structure in singly and doubly oxidized salts of 1and 2varies depending on the coordinating ability of the counteranions and exogenous ligands such as pyridine.

Here we describe the synthesis, structures, and reactivity of Ru complexes containing a triaryl, redox-active S2N2ligand derived from o-phenylenediamine and thioanisole subunits. The coordination chemistry of N,N′-bis[2-(methylthio)phenyl]-1,2-diaminobenzene [H2(MeSNNSMe)] was established by treating RuCl2(PPh3)3with H2(MeSNNSMe) to yield {Ru[H2(MeSNNSMe)]Cl(PPh3)}Cl (1). Coordinated H2(MeSNNSMe) was sequentially deprotonated to form Ru[H(MeSNNSMe)]Cl(PPh3) (2) followed by the five-coordinate, square pyramidal complex Ru(MeSNNSMe)(PPh3) (3). Single-crystal X-ray diffraction (XRD) studies revealed that the ligand structurally rearranged around the metal at each deprotonation step to conjugate the adjacent aryl groups with the o-phenylenediamine backbone. Deprotonation of 2with NaBH4or treatment of 3with BH3·tetrahydrofuran (THF) yielded Ru[(μ-H)BH2](MeSNNSMe)(PPh3) (5) with BH3bound across a Ru–N bond in a metal–ligand cooperative fashion. The cyclic voltammogram of 3in THF revealed three redox events consistent with one-electron oxidations and reductions of the o-phenylenediamine backbone and the metal (Ru3+/Ru2+). Reactions of 3with CO, HBF4, and benzoic acid yielded the new complexes Ru(MeSNNSMe)(CO)(PPh3), {Ru[H(MeSNNSMe)](PPh3)(THF)}BF4, and Ru[H(MeSNNSMe)](PPh3)(PhCO2), indicating broader suitability for small molecule binding and reactivity studies. Subsequent nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry data are reported in addition to molecular structures obtained from single-crystal XRD studies.