Christe Research Group
Our interests are very broad and range from very basic studies to applied research of academic, Government or industrial interest. We are particularly interested in high energy density materials (HEDM), inorganic main group chemistry, polynitrogen and nitramine chemistry, high-oxygen carriers for the replacement of the presently used toxic ammonium perchlorate and hydrazine-based propellants by higher-performing green compounds, energetic ionic liquids, chemistry at the limits of oxidation and coordination, the synthesis and characterization of novel carbocation and fluorocarbon compounds, and the development of quantitative scales for fundamental properties, such as Lewis acidity, oxidizer strength and the “nakedness” of fluoride ion sources. Our work exploits the synergism of theory and synthesis and greatly benefits from close collaborations with several groups of theoreticians. The main goal of our research is to advance the state of the art and strive for spectacular breakthroughs, rather than settling for small incremental improvements.
In the following section, some typical examples of our work in different areas are highlighted. Because of space limitations, we have omitted a vast body of work in the areas of halogen fluorides, inorganic halogen oxidizers and singlet delta oxygen gas generators. The pertinent references to these data can be found in the publications list in the bio of Karl Christe by clicking on his picture.
Novel High-Energy Density Matter (HEDM)
Nitryl Cyanide, NCNO2
Martin Rahm, Guillaume Belanger-Chabot, Ralf Haiges, and Karl O. Christe, Angew. Chem. Int. Ed., 2014, 53, 6893.
Cover page in Angewandte Chemie
[BH3C(NO2)3]-, the First Room Temperature Stable Trinitromethylborate
Guillaume Bélanger-Chabot, Martin Rahm, Ralf Haiges, and Karl O. Christe, Angew. Chem. Int. Ed., 2013, 52, 11002.
The BH3 group in [BH3C(NO2)3]- provides a significant increase instability compared to the previously studied BCl3-analog. The [Na(glyme)2], PNP+ and PPh4+ salts of [BH3C(NO2)3]- were successfully isolated and characterized by NMR and vibrational spectroscopy. The PNP+ salt was structurally characterized, confirming the theoretically predicted B-C connectivity. In addition, evidence was obtained for the slower formation of the disubstituted [BH2(C(NO2)3)2]- anion. While the dissociation barrier of [BH3C(NO2)3]- is too low for practical applications, we demonstrated both experimentally and theoretically for the first time that kinetically room temperature stable boron-trinitromethyl compounds can exist. The most remarkable feature of the [BH3C(NO2)3]- salts is the coexistence of a strongly oxidizing trinitromethyl and a strongly reducing BH3 moiety in the same anion, a marriage between fire and water.
Karl O. Christe, William W. Wilson, Guillaume Bélanger-Chabot, Ralf Haiges, Jerry A. Boatz, Martin Rahm, Surya Prakash, Thomas Saal, and Mathias Hopfinger, Angew. Chem. Int. Ed., 2014, 53, in press.
Whereas NF3 and N(NO2)3 are known, the mixed fluoronitroamines, FN(NO2)2 and F2NNO2, had been unknown. One of these, FN(NO2)2, has now been prepared and characterized by multinuclear NMR and Raman spectroscopy. FN(NO2)2 is the first known example of an inorganic fluoronitroamine. It is a thermally unstable highly energetic material formed by the fluorination of the dinitramide anion using NF4+ salts as the preferred fluorinating agent.
Energetic Bis(3,5-dinitro‑1H‑1,2,4-triazolyl)dihydro- and dichloroborates and Bis(5-nitro‑2H‑tetrazolyl)‑, Bis(5-(trinitromethyl)‑2H‑tetrazolyl)‑, and Bis(5-(fluorodinitromethyl)‑2H‑tetrazolyl)dihydroborate
Ralf Haiges, C. Bigler Jones, and Karl O. Christe, Inorg. Chem., 2013, 52, 5551.
Salts of bis(3,5-dinitro-1H-1,2,4-triazolyl)dihydro- and dichloroborate and bis(5-nitro-2H-tetrazolyl)-, bis(5-
trinitromethyl-2H-tetrazolyl)-, and bis(5-fluorodinitromethyl-2H-tetrazolyl)dihydroborate anions have been synthesized by the treatment of hydroborates or chloroborates with the corresponding nitroazoles or nitroazolates, respectively. Alkali-metal salts of these dihydroborates are energetic and can be shock-sensitive, while salts with larger organic cations, such as NMe4+, PPh4+, or (Ph3P)2N+, are less sensitive. Poly(nitroazolyl)borates are promising candidates for a new class of environmentally benign energetic materials and high-oxygen carriers.
Energetic High-Nitrogen Compounds: 5‑(Trinitromethyl)‑2H‑tetrazole and -tetrazolates, Preparation, Characterization, and Conversion into 5‑(Dinitromethyl)tetrazoles
Ralf Haiges* and Karl O. Christe, Inorg. Chem., 2013, 52, 7249.
A convenient access to 5-(trinitromethyl)-2H-tetrazole (HTNTz) has been developed, based on the exhaustive nitration of 1H-tetrazole-5-acetic acid, which was prepared from ethyl cyanoacetate and HN3 in a 1,3-dipolar cycloaddition reaction, followed by basic hydrolysis. HTNTz was converted into the ammonium, guanidinium, rubidium, cesium, copper, and silver 5-(trinitromethyl)-2H-tetrazolates. In addition, the ammonia adducts of the copper and silver salts were isolated. The reaction of HTNTz with hydrazine and hydroxylamine resulted in the formation of hydrazinium 5- (dinitromethyl)tetrazolate and hydroxylammonium 5-(dinitromethyl)-1H-tetrazolate,
respectively. Acid treatment of both 5-(dinitromethyl)tetrazolates resulted in the isolation of 5-(dinitromethylene)-4,5-dihydro-1H-tetrazole, which was converted into potassium 5-(dinitromethyl)-1H-tetrazolate by reaction with K2CO3. All prepared compounds were fully characterized by 1H, 13C, 14N, and 15N NMR spectroscopy and X-ray crystal structure determination. Initial safety testing (impact, friction, and electrostatic sensitivity) and thermal stability measurements (differential thermal analysis, DTA) were also carried out. The 5-(trinitromethyl) and 5-(dinitromethyl)tetrazoles are highly energetic materials that explode upon impact or heating.
oxygen-balanced, energetic ionic liquids of interest for liquid
Polynitrogen Chemistry: One of the discoveries, which has received coverage even in the New York Times (Feb 2, 1999) and London Times (Feb 10, 1999) and was selected by Chem. & Eng. News as one of chemistry’s top five achievements of 1999, is the single-step synthesis of the N5+ cation in essentially quantitative yield and the determination of its structure. The N5+ cation is only the second homonuclear polynitrogen species that has been isolated in bulk and may provide the basis for new high-energy-density materials.
K. O. Christe, R. Haiges, W. W. Wilson, and J. A. Boatz, Inorg. Chem., 2010, 49,1245.
Tao Yu, Bo Wu, Ying-Zhe Liu, Wei-Peng Lai, Ralf Haiges, and Karl O. Christe, RSC Adv. 2014, 4, 28377.
The reaction of NOF2+ with 2 HN3 results in the formation of an unstable 1-oxo-N7O+ cation which decomposes to N5+ and N2O. The mechanism of this reaction was established by N-NMR experiments using 15N substitution and extensive computational studies. It also represents a more facile synthesis of N5+ salts because the NOF2+ starting material is more readily available than N2F+.
Distribution of 15N labels (marked by an asterisk) in the reaction of unlabeled N4FO+ and 50% a- and g-labeled HN3. Pathway A involves a cyclic intermediate containing a 1-oxo-N7O+ intermediate and results in unlabeled N2O and a- and g-labeled N5+, while Pathway B, involving 4-oxo-N7O+ as an intermediate, results in N2O labeled on the central N and N5+ labeled in the a-position.
14N- and 15N-NMR spectra of the products from the reaction of unlabeled N4FO+ and 50%
a- and g-labeled HN3.
Polyazide Chemistry: During the past seven years, Ralf Haiges from our group has prepared and characterized a very large number of polyazido compounds. Typical examples are shown below.
Why are [P(C6H5)4]+N3- and [As(C6H5)4]+N3- Ionic Salts and Sb(C6H5)4N3 and Bi(C6H5)4N3 are Covalent Solids? A Theoretical Study Provides an Unexpected Answer.
Karl O. Christe, Ralf Haiges, Jerry A. Boatz, H. Donald Brooke Jenkins, Edward B. Garner, David A. Dixon, Inorg. Chem., 2011, 50, 3752.
A recent crystallographic study has shown that, in the solid state, P(C6H5)4N3 and As(C6H5)4N3 have ionic [M(C6H5)4]+N3- type structures, whereas Sb(C6H5)4N3 exists as a pentacoordinated covalent solid. Using the results from density functional theory, lattice energy (VBT) calculations, sublimation energy estimates, and Born-Fajans-Haber cycles, it is shown that the maximum coordination numbers of the central atom M, the lattice energies of the ionic solids, and the sublimation energies of the covalent solids have no or little influence on the nature of the solids. Unexpectedly, the main factor determining whether the covalent or the ionic structures are energetically favored, is the first ionization potential of [M(C6H5)4]. The calculations show that at ambient temperature the ionic structure is favored for P(C6H5)4N3 and covalent structures are favored for Sb(C6H5)4N3 and Bi(C6H5)4N3, while As(C6H5)4N3 presents a borderline case.
Preparation and Characterization of the Binary Group 13 Azides M(N3)3 and M(N3)3·CH3CN (M=Ga, In, Tl), [Ga(N3)5]2-, and [M(N3)6]3- (M=In, Tl)
Ralf Haiges,* Jerry A. Boatz, Jodi M. Williams, and Karl O. Christe, Angew. Chem. Int. Ed., 2011, 50, 8828.
Cover Page in Angew. Chemie
Binary Group 15 Polyazides. Structural Characterization of [Bi(N3)4]-, [Bi(N3)5]2-, [bipy·Bi(N3)5]2-, [Bi(N3)6]3-, bipy·As(N3)3, bipy·Sb(N3)3 and [(bipy)2·Bi(N3)3]2, and on the Lone Pair Activation of Valence Electrons
Ralf Haiges, Martin Rahm, David A. Dixon, Edward B. Garner III, and Karl O. Christe, Inorg. Chem., 2012, 51, 1127.
The binary group 15 polyazides As(N3)3, Sb(N3)3, and Bi(N3)3 were stabilized by either anion or donor−acceptor adduct formation. Crystal structures are reported for [Bi(N3)4]−, [Bi(N3)5]2−, [bipy·Bi(N3)5]2−, [Bi(N3)6]3−, bipy·As(N3)3, bipy·Sb(N3)3, and [(bipy)2·Bi(N3)3]2. The lone valence electron pair on the central atom of these pnictogen(+III) compounds can be either sterically active or inactive. The [Bi(N3)5]2− anion possesses a sterically active lone pair and a monomeric pseudo-octahedral structure with a coordination number of 6, whereas its 2,2′-bipyridine adduct exhibits a pseudo-monocapped trigonal prismatic structure with CN 7 and a sterically inactive lone pair. Because of the high oxidizing power of Bi(+V), reactions aimed at Bi(N3)5 and [Bi(N3)6]− resulted in the reduction to bismuth(+III) compounds by [N3]−. The powder X-ray diffraction pattern of Bi(N3)3 was recorded at 298 K and is distinct from that calculated for Sb(N3)3 from its single-crystal data at 223 K. The [(bipy)2·Bi(N3)3]2 adduct is dimeric and derived from two BiN8 square antiprisms sharing an edge consisting of two μ1,1-bridging N3 ligands and with bismuth having CN 8 and a sterically inactive lone pair. The novel bipy·As(N3)3 and bipy·Sb(N3)3 adducts are monomeric and isostructural and contain a sterically active lone pair on their central atom and a CN of 6. A systematic quantum chemical analysis of the structures of these polyazides suggests that the M06-2X density functional is well suited for the prediction of the steric activity of lone pairs in main-group chemistry. Furthermore, it was found that the solid-state structures can strongly differ from those of the free gas-phase species or those in solutions and that lone pairs that are sterically inactive in a chemical surrounding can become activated in the free isolated species.
Unprecedented Conformational Variability in Main Group Inorganic Chemistry: the Tetraazido-Arsenite and -Antimonite Salts A+[M(N3)4]- (A = NMe4, PPh4, PNP, M = As, Sb), Five Similar Salts, Five Different Anion Structures
Ralf Haiges, Martin Rahm, and Karl O. Christe, Inorg. Chem., 2013, 52, 402.
A unique example for conformational variability in inorganic main group chemistry has been discovered. The arrangement of the azido ligands in the pseudo-trigonal bipyramidal [As(N3)4]- and [Sb(N3)4]- anions theoretically can give rise to seven different conformers which have identical MN4 skeletons but different azido ligand arrangements and very similar energies. We have synthesized and structurally characterized five of these conformers by subtle variations in the nature of the counter ion. Whereas conformational variability is common in organic chemistry, it is rare in inorganic main group chemistry and is usually limited to two. To our best knowledge, the experimental observation of five distinct single conformers for the same type of anion is unprecedented. Theoretical calculations at the M06-ZX/cc-pwCVTZ-PP level for all seven possible basic conformers show that (1) the energy differences between the five experimentally observed conformers are about 1 kcal/mol or less, and (2) the free monomeric anions are the energetically favored species in the gas phase and also for [As(N3)4]- in the solid state, whereas for [Sb(N3)4]- associated anions are energetically favored in the solid state and in solutions. Raman spectroscopy shows that in the azide antisymmetric stretching region, the solid-sate spectra are are distinct for the different conformers and permit their identification. The spectra of solutions are solvent dependent and differ from those of the solids indicating the presence of rapidly exchanging equilibria of different conformers. The only compound for which a solid with a single well-ordered conformer could not be isolated was [N(CH3)4][As(N3)4] which formed a room-temperature, viscous, ionic liquid. Its Raman spectrum was identical to that of its CH3CN solution indicating the presence of an equilibrium of multiple conformers.
Preparation of the first Manganese(III) and Manganese(IV) Azides Mn(N3)3·CH3CN, (bipy)Mn(N3)4, and [Mn(N3)6]2-, as well as Mn(N3)2,
[Mn(N3)4]2-, and (bipy)2Mn(N3)2
Ralf Haiges, Robert J. Buszek, Jerry A. Boatz, and Karl O. Christe, Angew. Chem. Int. Ed., 2014, 53, 8200.
Fluoride-azide exchange reactions of Me3SiN3 with MnF2 and MnF3 in acetonitrile resulted in the isolation of
Mn(N3)2 and Mn(N3)3·CH3CN, respectively. While Mn(N3)2 forms [PPh4]2[Mn(N3)4] and (bipy)2Mn(N3)2 upon reaction with PPh4N3 and 2,2’-bipyridine (bipy), respectively, the manganese(III) azide undergoes disproportionation and forms mixtures of [PPh4]2[Mn(N3)4] and [PPh4]2[Mn(N3)6], as well as (bipy)2Mn(N3)2 and (bipy)Mn(N3)4. Neat and highly sensitive Cs2[Mn(N3)6] was obtained through the reaction of Cs2MnF6 with Me3SiN3 in CH3CN.
"Naked Fluoride” and High Coordination Number Chemistry:
Chemical Synthesis of Elemental Fluorine:
The CF3- Anion:
The Long-Lived Trifluoromethide Anion: A Key Intermediate in Nucleophilic Trifluoromethylations
G. K. Surya Prakash,* Fang Wang, Zhe Zhang, Ralf Haiges, Martin Rahm, Karl O. Christe, Thomas
Mathew, George A. Olah, Angew. Chem. Int. Ed., 2014, 53, 11575.
The trifluoromethide anion is the postulated key intermediate in nucleophilic trifluoromethylation reactions. However, for more than six decades, the trifluoromethide anion was widely believed to exist only as a short-lived transient species in the condensed phase. It has now been prepared in bulk for the first time in THF solution. The trifluoromethide anion with [K(18-crown-6)]+ as a counter cation was unequivocally characterized by low-temperature 19F and 13C NMR spectroscopy. Its intermediacy in nucleophilic trifluoromethylation reactions was directly evident by its reaction chemistry with various electrophilic substrates. Variable low-temperature NMR spectroscopy along with quantum mechanical calculations support the persistence of the trifluoromethide anion.
Cover page in Angewandte Chemie
Convenient Access to Trifluoromethanol:
Quantitative Scales for Oxidizing Power, Lewis Acidity and “Nakedness of Fluoride Ion Sources:
Quantifying the Nature of Lone Pair Domains
Martin Rahm and Karl. O. Christe, ChemPhysChem., 2013, 14, 3714.
The lone pair lies at the heart of chemistry, and is often in a conceptual respect the site of chemical reactivity. Here we show how the analysis of the electron localization function
(ELF) can be improved upon by introducing intra-basin partitioning. The high-ELF localization domain population (HELP), is presented as a probe of the more chemically relevant and electron-localized regions of the ELF lone pair basin, and shown to correlate with a range of physical properties and quantum mechanical constructs, such as the ESP, e and H+ affinity, IP, HOMO/LUMO energies, atomic charge, NBO lone pair orbitals, and molecular geometry. A strong connection between topological density and orbital-based analysis of chemistry is intuitively expected, yet this is the first time that an average electron population, obtained by any localization method, can be linked to properties of molecules. The application of HELP as a descriptor of lone pair domains is predicted to be of general use.
Structural and Energetic Properties of Closed Shell XFn (X = Cl, Br, and I; n = 1−7) and XOnFm (X = Cl, Br, and I; n = 1−3; m = 0−6) Molecules and Ions Leading to Stability Predictions for Yet Unknown Compounds
K. Sahan Thanthiriwatte, Monica Vasiliu, David A. Dixon, and Karl O. Christe Inorg. Chem., 2012, 51, 10966.
Atomization energies at 0 K and heats of formation at 0 and 298 K were predicted for the closed shell compounds XF, XF2−, XF2+, XF3, XF4−, XF4+, XF5, XF6−, XF6+ (X = Cl and Br) and XO+,
XOF, XOF2−, XOF2+, XOF3, XOF4−, XOF4+, XOF5, XOF6−, XO2+, XO2F, XO2F2−, XO2F2+, XO2F3, XO2F4−, XO3+, XO3F, XO3F2− (X =Cl, Br, and I) using a composite electronic structure approach based on coupled cluster CCSD(T) calculations extrapolated to the complete basis set limit with additional corrections. The calculated heats of formation are in good agreement with the available experimental data. The calculated heats of formation were used to predict fluoride affinities, fluorine cation affinities, and F2 binding energies. On the basis of our results, BrOF5 and BrO2F3 are predicted to be stable against spontaneous loss of F2 and should be able to be synthesized, whereas BrF7, ClF7, BrOF6−, and ClOF6− are unstable by a very wide margin. The stability of ClOF5 is a borderline case. Although its F2 loss is predicted to be exothermic by 4.4 kcal/mol, it may have a sufficiently large barrier toward decomposition and be preparable. This situation would resemble ClO2F3 which was successfully synthesized in spite of being unstable toward F2 loss by 3.3 kcal/mol. On the other hand, the ClOF4+ and BrOF4+ cations are less likely to be preparable with F2 loss exothermicities of −17.5 and −9.3 kcal/mol, respectively. On the basis of the F− affinities of ClOF (45.4 kcal/mol), BrOF (58.7 kcal/mol), and BrO2F3 (65.7 kcal/mol) and their predicted stabilities against loss of F2, the ClOF2−, BrOF2−, and BrO2F4− anions are excellent targets for synthesis. Our previous failure to prepare the ClO2F4− anion can be rationalized by the predicted high exothermicity of −17.4 kcal/mol for the loss of F2.
The F+ and F- Affinities of Simple NxFy and OxFy Compounds
Daniel J. Grant, Tsang-Hsiu Wang, Monica Vasiliu, David A. Dixon, and Karl O. Christe, Inorg. Chem., 2011, 50, 1914.
Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for the neutral and ionic NxFy and OxFy systems using coupled cluster theory with single and double excitations and including a perturbative triples correction (CCSD(T)) method with correlation consistent basis sets extrapolated to the complete basis set (CBS) limit. To achieve near chemical accuracy ((1 kcal/mol), three corrections to the electronic energy were added to the frozen core CCSD(T)/CBS binding energies: corrections for core-valence, scalar relativistic, and first order atomic spin-orbit effects. Vibrational zero point energies were computed at the CCSD(T) level of theory where possible. The calculated heats of formation are in good agreement with the available experimental values, except for FOOF because of the neglect of higher order correlation corrections. The F+ affinity in the NxFy series increases from N2 to N2F4 by 63 kcal/mol, while that in the O2Fy series decreases by 18 kcal/mol from O2 to O2F2. Neither N2 nor N2F4 is predicted to bind F-, and N2F2 is a very weak Lewis acid with an F- affinity of about 10 kcal/mol for either the cis or trans isomer. The low F- affinities of the nitrogen fluorides explain why, in spite of the fact that many stable nitrogen fluoride cations are known, no nitrogen fluoride anions have been isolated so far. For example, the F- affinity of NF is predicted to be only 12.5 kcal/mol which explains the numerous experimental failures to prepare NF2 - salts from the well-known strong acid HNF2. The F- affinity of O2 is predicted to have a small positive value and increases for O2F2 by 23 kcal/ mol, indicating that the O2F3 - anion might be marginally stable at subambient temperatures. The calculated adiabatic ionization potentials and electron affinities are in good agreement with experiment considering that many of the experimental values are for vertical processes.
Electron Affinities, Fluoride Affinities, and Heats of Formation of the Second Row Transition Metal Hexafluorides: MF6 (M = Mo, Tc, Ru, Rh, Pd, Ag)
Raluca Craciun, Rebecca T. Long, David A. Dixon, and Karl O. Christe, J. Phys. Chem. A, 2010, 114, 7571.
High-level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the second row transition metal hexafluorides. The electron affinities, heats of formation, first (MF6 ® MF5 + F) and average M-F bond dissociation energies, and fluoride affinities of MF6 (MF6 + F-® MF7 -) and MF5 (MF5 + F- ® MF6 -) were calculated. The electron affinities are higher than those of the corresponding third row hexafluorides, making them stronger one-electron oxidizers. The calculated electron affinities, in good agreement with the available experimental values, are 4.23 eV for MoF6, 5.89 eV for TcF6, 7.01 eV for RuF6, 6.80 eV for RhF6, 7.95 eV for PdF6, and 8.89 eV for AgF6. The corresponding pentafluorides are also very strong Lewis acids, although their acidities on the pF- scale are about one unit lower than those of the third row pentafluorides. The performance of a wide range of DFT exchange-correlation functionals was benchmarked by comparing them to our more accurate CCSD(T) results.
Third Row Transition Metal Hexafluorides, Extraordinary Oxidizers, and Lewis Acids: Electron Affinities, Fluoride Affinities, and Heats of Formation of WF6, ReF6, OsF6, IrF6, PtF6, and AuF6
Raluca Craciun, Desiree Picone, Rebecca T. Long, Shenggang Li, David A. Dixon, Kirk A. Peterson, and Karl O. Christe
High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the third row transition metal hexafluorides. The electron affinities, heats of formation, first (MF6 → MF5 + F) and average M-F bond dissociation energies, and fluoride affinities of MF6 (MF6 + F- → MF7-) and MF5 (MF5 + F- → MF6-) were calculated. The electron affinities which are a direct measure for the oxidizer strength increase monotonically from WF6 to AuF6, with PtF6 and AuF6 being extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values, and are as follows: WF6 (3.15 eV), ReF6 (4.58 eV), OsF6 (5.92 eV), IrF6 (5.99 eV), PtF6 (7.09 eV), and AuF6 (8.20 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated, and only three gave satisfactory results. The corresponding pentafluorides are extremely strong Lewis acids, with OsF5, IrF5, PtF5, and AuF5 significantly exceeding the acidity of SbF5. The optimized geometries of the corresponding MF7- anions for W through Ir are classical MF7- anions with M-F bonds; however, for PtF7- and AuF7- non-classical anions were found with a very weak external F-F bond between an MF6- fragment and a fluorine atom. These two anions are text book examples for “superhalogens” and can serve as F atom sources under very mild conditions, explaining the ability of PtF6 to convert NF3 to NF4+, ClF5 to ClF6+, and Xe to XeF+ and why Bartlett failed to observe XePtF6 as the reaction product of the PtF6/Xe reaction.