Polynitrogen & High Nitrogen Chemistry: A New World of Challenges March 25, 2004 Cal State University, Fullerton Ashwani Vij Research Scientist AFRL/PRSP Air Force Research Laboratory Distribution Statement A: Public Release, Distribution Unlimited Form Approved OMB No. 0704-0188 Report Documentation Page Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2. REPORT TYPE 23 FEB 2004 - 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Polynitrogen and High Nitrogen Chemistry: A New World of Challenges 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER Ashwani Vij DARP 5e. TASK NUMBER A205 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Air Force Research Laboratory (AFMC),AFRL/PRS,5 Pollux Drive,Edwards AFB,CA,93524-7048 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES The original document contains color images. 14. ABSTRACT Polynitrogen compounds contain only nitrogen atoms and are expected to have unusual properties. Most important among these are High endothermicity šGreenŠ propellant šcombustionŠ product is only gaseous N2 High density High Isp values when compared to other monopropropellants or bipropellants High detonation velocity 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT a. REPORT b. ABSTRACT c. THIS PAGE unclassified unclassified unclassified 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON 84 Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Why Polynitrogen Compounds ? • Polynitrogen compounds contain only nitrogen atoms and are expected to have unusual properties. Most important among these are: • High endothermicity • “Green” propellant “combustion” product is only gaseous N 2 March 25, 2004 • High density • High Isp values when compared to other monopropropellants or bipropellants • High detonation velocity Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 2 Predicted Specific Impulse (s) Values for Neutral Polynitrogen Compounds 500 Monopropellants Specific Impulse (s) 450 400 350 Bipropellants Blended Monopropellants 300 Standard Propellants AFRL In Development AFRL In Research 250 200 Po Hy NT Pe Nit Nit rox dra rat rat O/M lyn e eb i itro zin de MH ble len e /MM ge n d d2 n H 1 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 3 Polynitrogen Program Objectives Discover, synthesize, characterize, and scale-up novel, highly energetic polynitrogen compounds Technical Approach: • Exploit synergism between theory and synthesis w Use computational expertise to identify the most promising candidates and predict their properties w Use experimental expertise to design synthesis approaches, prepare novel compounds, and characterize products + N5 cation March 25, 2004 − N5 anion + − N10 (C 3) N7 anion N11 cation • ^^'^X Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 4 Challenge of Polynitrogen HEDM Synthesis • All the energy must come from endothermicity, and sensitivity typically increases with endothermicity • Basis for high energy content is the large differences in bond energies Carbon bond enthalpies C−C 85 kcal/mol C=C 143 kcal/mol C≡C 194 kcal/mol (−HC=CH)n− +34 85 + 143 HC ≡CH 194 Nitrogen bond enthalpies N− N 38 kcal/mol N= N N≡ N 100 kcal/mol 226 kcal/mol (− N= N)n− -88 38 + 100 stable polymers, unstable monomers N ≡N 226 unstable polymers, stable monomer This is the reason why N-N polymers March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 5 Research Philosophy and Technical Approach Initially we preferred catenated over cyclic or polycyclic compounds A + o^ • Although polycyclic compounds are more energetic due to strain energy, and some of them have large barriers to decomposition (tetrahedral N 4), synthetic routes for their preparation are much more difficult March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 6 Polynitrogen for Dummies What has Thermodynamics and Kinetics got to do with it ?? Kinetics Thermodynamics It is an uphill battle !! Low Barrier towards catastrophic downfall Pumping in energy into a polynitrogen species is like pushing a boulder uphill March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 7 Polynitrogen for Dummies • Metastability requires a delicate balancing act !! March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 8 Polynitrogen for Dummies Avoid a domino effect !!! Assembling a polynitrogen chain is like assembling metastable dominos with perfect spacing, without prematurely triggering an unwanted collapse March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 9 Recipe for Synthesizing Neutral Polynitrogen Compounds • Combine a polynitrogen cation with a polynitrogen anion to form a neutral polynitrogen compound. N x+ + Ny - N x+y ONLY TWO STABLE POLYNITOGEN IONS KNOWN TO EXIST IN BULK Cation Anion + March 25, 2004 - N5+ cation N3- anion (discovered in 1999, AFRL, Christe) (discovered in 1890, Curtius) Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 10 Selection of Suitable Starting Materials for N5 + Synthesis • Requirements: Ø Starting fragments must have relatively weak bonds Ø Must have formal positive charge (first IP of N2 = 359 kcal/mol) Ø Coupling reaction must be endothermic Ø Suitable solvent must be used as a heat sink and for stabilization • Ideal candidate system: F N N AsF6− March 25, 2004 + H + N N N -HF N N N N AsF6 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton + − N 11 Synthesis of the N 2F+SbF6− Precursor • Reduction of N2F4 to N2F2 · Graphite + AsF5 C12 AsF5 · 2 C12+AsF6− + trans-N2F2 C12 AsF5 + N 2F4 • trans-cis isomerization of N 2F 2: trans-N2F2 + AsF5 + − N2F AsF6 + Na F T/P HF + N2F AsF6 − NaAsF6 + cis-N2F2 • Formation of N2F+SbF6−: cis-N2F2 + SbF5 HF + N2F SbF6 − Aim: Can we cut any steps and decrease the synthesis time? March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 12 trans-cis Isomerization of N 2F2 ü Improved process (only ~10% SbF 5 needed as a catalyst) room temperature trans-N2F2 + N2F+Sb2F11- SbF5 + cis-N2F2 4 days ü Other process: trans-N2F 2 AlF 3, 45 °C cis-N2F2 15 hours ü Catalyst is not consumed and can be reused ü Gives pure cis-N 2F2 in high yield. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 13 Actual Synthesis of N5 +AsF6 − • Reaction system worked as planned: + N2F AsF6 − HF + − + HN3 → N5 AsF 6 + HF -78° C Ø High yield Ø Only byproducts were 20-40% H 2N3+AsF6− Ø 2 mmol (0.5 g) scale • Properties of N5+AsF6 −: Ø White solid Ø Sparingly soluble in HF Ø Marginally stable at 22°C Ø Highly energetic Ø Reacts violently with water and organics Ø Calculated ∆Hf (298°C) = 351 kcal/mol March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 14 Characterization of N 5+AsF 6− • 14N and 15N NMR spectroscopy • Low-temperature Raman and IR spectroscopy of normal and isotopically labeled N 5+ • Normal coordinate analysis • Mass spectrometry • Calculations: Ø Electronic structure and geometry Ø Vibrational spectra, including isotopic shifts Ø NMR chemical shifts Ø Heat of formation March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 15 Vacuum Line Synthesis March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 16 Safer replacements for HN 3 in the N 5+ synthesis • HN3 is very shock sensitive and frequently explodes in the presence of fluorinating agents (possible formation of FN 3) • HN3 can be replaced by insensitive, commercially available (CH3)3SiN3 (TMS azide) + − N2F MF6 + (CH3)3SiN3 SO2 + − N5 MF6 + (CH3)3SiF (M = As, Sb) • HF solutions of HN 3 generated from NaN 3 and HF are another alternative to handling HN3 directly • Use of FEP-double U-tube apparatus to generate HN3 in situ. AVIOD METAL VALVES AND CONNECTORS • N 5+ formation has been demonstrated for both systems in high yield, and N5SbF6 is now routinely prepared on a 5 g scale March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 17 . 1,.. . ..: t- ^m^ I March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton , 18 Background in Nitrogen Chemistry Ø HN(SO2F)2 and HN(SO2CF3)2 : Bis(fluorosulfonyl) and bis(trifluoromethylsulfonayl)imides and their derivatives (Electrophiles) Ø Synthesis and reactivity of Perfluorovinylamines: RfN-CF=CF2 (fire retardants, surfactants etc.) Ø Phosphonitrilic compounds: N3 P3 monomers/prepolymers Ø Triazenes, Mono- and Dicarbaphosphazenes: N3 CxP3-x and P-C-N polymers Ø Sappharenes, Sulfur/Selenium-Nitrogen macrocyclic ring systems. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 19 Oxidizing Power of N5 + • The electron affinity of N5+ was determined by examining its ability to oxidize the following substrates: First IP of substrate (eV) N5+SbF6− + NO NO +SbF6− + 2.5 N2 9.26 N5+SbF6− + NO 2 NO2+SbF6− + 2.5 N2 9.75 N5+SbF6− + Br 2 Br2+SbF6− + 2.5 N2 10.52 N5+SbF6− + Cl 2 Cl2+SbF6− + 2.5 N2 11.48 N5+SbF6− + O 2 O2+SbF6− + 2.5 N2 12.07 Xe2+SbF6− + 2.5 N2 12.13 N5+SbF6− + 2 Xe • N 5+ is a weaker oxidizer than PtF 6, which can oxidize O 2 to O 2+. + also a weaker oxidizer than O 2 , which can oxidize Xe to Xe2 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton + It is 20 Electron Affinity of N 5+ • Electron affinity (EA) of N 5+ needed for stability predictions of new N 5+ salts using Born-Haber cycles • EA of an oxidizer equals the IP of the substrate for gas-phase reactions; when solids are involved, lattice energy changes must be included N5+ SbF6-(s) + UL N5 SbF6 +119 N5+(g) + SbF6-(g) ElAff N5 +(g) N5 (g) Br2(g) ∆Hr ≤ 0 Br 2+SbF6-(s) + N5(g) 0 + 0 + SbF 6-(g) Br 2(g) 1.IP Br2 + +242 UL Br2 +SbF6 - -125 0 Br 2+(g) • The EA of N5+ falls between 236 and 255 kcal/mol (10.24 – 11.05 eV); it is a powerful one-electron oxidizer that neither fluorinates nor oxygenates March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 21 The Taming of N 5+SbF 6− • Desired a more stable N5+ salt • Prepared N5+SbF6−: + HF − N2F SbF 6 + HN3 → N5+SbF6− + HF -78°C to RT • Properties of N5+SbF6−: Ø White solid Ø Stable to 70°C Ø Obtained in high purity Ø Does not explode at 150 kg•cm (impact sensitivity test) Ø Exhibits all the still missing vibrational bands with the predicted frequencies Ø Soluble in SO2, SO2ClF, and HF Ø Are preparing it routinely on a 5 g scale March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 22 Raman Spectrum of N 5+SbF6− ZHKf March 25, 2004 2nn too 1D00 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 23 Infrared Spectrum of N 5+SbF 6− March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 24 Vibrational Assignments for N 5+ Observed Calculated IR Raman B3LYP/ 6-311+G(2d) 2268 2268 2236 2229 ν1 (A1) 2205 2205 2282 2175 ν7 (B1) 1167 1032 872 850 818 ν2 (A1) 672 678 644 ν3 (A1) 478 502 475 ν5 (A2) 424 405 ν6 (B1) 414 436 399 ν9 (B2) 204 193 181 ν4 (A1) 1090 1055 873 Fermi Resonance 425 412 March 25, 2004 CCSD(T)/ 6-311+G(2d) Assignment Distribution A: Public Release, Distribution Unlimted California State University, Fullerton ν3 (A1) + ν9 (B2) ν8 (B2) 25 Geometry of the N 5+ Cation 112º N 167º 1.11Å Calculated Structure N (+) (+) (-) ..N (-) N N N.. (+) N .. N .. N .. (+) .. 1.30 Å .. N .. V-Shaped Geometry Resonance Structure 1.302 Å 111.2º 168.1º 1.107Å Experimental Structure Vij, Wilson, Vij, Tham, Sheehy & Christe, J. Am. Chem. Soc., 2001,123, 6308-6313 N2 makes contacts at 2.723 and 2.768 Å N4 contacts are at 2.887 and 2.814 Å C&E News, 2000, 78, 41 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 26 Synthesis of New, More Energetic N5+ Salts • Salts with Energetic Counterions – N5+N3− ØDesired Metathesis: N5SbF6 + CsN3 SO2 -64°C N5N3 + CsSbF6 ØObtained Products CsSbF6 + 4 N2 + ØBorn-Haber Cycle Shows that Stabilization of N 5+N3− Requires a Minimum Lattice Energy of 183 ± 20 kcal/mol, but Estimated U L for N5+N3− Is only 130 kcal/mol March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 27 Unexpected complexation of SO2 with the azide ion in CsN3 Structure shows novel SO2N3 as well as SO3N3 groups March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 28 Synthesis of New, More Energetic N5+ Salts • Salts with Energetic Counterions – N5+NO 3− ØDesired Metathesis: SO2 N5SbF6 + CsNO3 N5NO 3 + CsSbF6 -64 to 20°C ØDid Not Proceed because CsNO 3 Is Less Soluble in SO 2 than CsSbF6 ØUL Required for Stabilization Is 154 kcal/mol; Estimate for N 5NO3 Is 129 kcal/mol • Salts with Energetic Counterions – N5+ClO 4− ØDesired Metathesis Resulted in: N5SbF6 + CsClO4 HF -78°C NO +ClO4− + CsSbF6 + N 2 ØUL Required for Stabilization Is 138 kcal/mol; Estimate for N 5ClO4 Is 125 kcal/mol March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 29 Synthesis of more energetic N5+ salts, and estimated energy content of N 5+N3− • Heat of Formation of N 5+N 3− Ø∆Hf (298) of N 5+(g) = 351 kcal/mol (Calculated Value) Ø∆Hf (298) of N 3−(g) = 43.2 kcal/mol (NBS Tables) ØLattice Energy of N5+N3− ≈ 130 ± 20 kcal/mol (Christe Estimate) So ∆H f (298) of N 5+N 3− = 351 + 43 – 130 = 264 ± 25 kcal/mol • Energy Density of N 5+N 3−(s) = 2.36 kcal/g • Comparison with Other Molecular Systems (kcal/g): O3 C(N 3)3+N(NO 2) 2− HN 3 N 5+N3− H 2/O 2 0.71 1.42 1.63 2.36 3.21 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 30 Synthesis of new N 5+ salts N5B(CF3)4 • N 5SbF 6 successfully converted to N 5B(CF3)4 by metathesis in SO 2 solution N5SbF 6 + KB(CF3)4 • SO2 -64°C N5B(CF3)4 + KSbF6 N 5B(CF3)4 Is a white solid, stable at room temperature Ø Characterized by mass balance Ø Characterized by vibrational spectroscopy Ø Characterized by 14N, 11B, and 13C NMR Ø Indefinitely stable in HF solution at room temperature with no decomposition products nor any unidentified species March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 31 (In)Compatability of N5 + Conclusion……Attempts to couple N 5+ with energetic anions may result in explosive reactions !!! March 25, 2004 N5+N3- N5+ClO4- N5+NO3- N5+N(NO2 )2- Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 32 Syntheses of new N5 + salts (N5)2 SnF6 and N5 SnF5 • Salt with higher N 5+ content (2:1 Cation/Anion Ratio) 2 N 5SbF6 + Cs2SnF 6 • HF -78°C (N5)2SnF6 + 2 CsSbF6 (N 5)2SnF6 marginally stable, but Friction Sensitive with explosive decomposition ØWhite solid with double the N 5+ content of N 5SbF6 ØImportant step toward synthesis of salts with “touching” Polynitrogen ions Intensity (N 5)2SnF6 _L_Jl ]}: ztoa Frequency, cm-1 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 33 Thermal Generation of N 5SnF 5 from (N5)2SnF 6 • Thermolysis of (N5)2SnF6 above room temperature (N5)2SnF6 • > 20°C N5SnF5 + "N5F" Properties of N5SnF 5 Ø White solid Ø Stable up to 50-60ºC Ø Characterized by vibrational and multi-Nuclear Magnetic Resonance spectroscopy Ø Contains Sn2F102- and Sn4 F204- anions • “N 5F” Unstable Ø Only decomposition products observed by FTIR and noncondensible measurements: N 2, trans-N2F2 and NF3 Ø J. Phys. Chem. 2003, . March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 34 w 119 ^±p Sn NMR Spectrum of N5 SnF5 [Sn 4F20 ] 2- Fax Fax Fbr Feq Feq Fax Fax Fax Fbr Fax . t. F eq F eq F ax Feq Feq Fax F br F eq Feq Fbr Fax [Sn2 F10 ] 4- Fbr Feq Fbr F eq Feq Feq F ax F ax F ax Simulated .1 r^'*5[ir^'«Swr*'^«iE7''TE»*'*«iC^ M._k Experimental March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 35 Attempted Preparation of the N 7O+ Cation • Another promising Polynitrogen target ion Is N7 O+ cation. Ø Reaction of NOF2SbF6 with HN 3 studied in HF at –78 °C Ø N3NOF+SbF6- isolated as white solid stable up to ~ -20 °C Ø N3NOF+ exists as both a z- and e-isomer NOF2+SbF6 - + HN3 - HF O N N F N3NOF+SbF6- + HN 3 O N N SbF6- E-isomer - HF N F Z-isomer N N N O x N3 N N3 SbF6- Ø Characterized by vibrational and multi-nuclear resonance spectroscopy and calculations March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 36 The NOF2+ Cation Case Due to their similar space requirements and electronic configurations, oxygen and fluorine ligands in oxofluorides are frequently disordered, particularly when the central atom lies on an intramolecular rotation axis. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 37 The NOF2+ Cation Case…. The Structure of NOF 2+AsF 6- ü The crystals grown from HF ü Monoclinic space group P21/n ü Cell constants: a = 7.513(2) Å , b = % 8.083(2) Å, c = 10.314(2) Å; β = 107.46(2)º Y What is wrong with this structure ?? N-O = 1.190(4) Å …long! N-F = 1.245(4), 1.246(4) Å …short Angle O-N-F = 122º …wider Angle F-N-F = 116º …narrower March 25, 2004 ü Z=4 ü R = 0.0372 ü Refined oxygen occupancy in NOF 2+ cation is 55% Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 38 The disordered NOF2+ cation case…. Extracting the “true” geometry Bond Length (Å) 1.3 Plot of N-O and N-F distances versus %F occupancy 1.2835 1.25 1.2 .0 y=0 1.15 1.1 RN-O 135 + 1.1 017x 1. Sums of partial occupancies for O/F at any site is restricted to ONE. 1.1135 0 20 40 60 80 % F Occupancy 2. The total O occupancy equals 100 ONE and total F occupancies equals TWO. Plot of F-N-O and F-N-F versus %F occupancy 130 Bond Angle (degrees) Rules for refining occupancies RN-F 125 126.0 ∠ F-N-F 120 y= ∠ F-N-O 115 -0.3 618 Refined Occupancies x+ 144 .12 110 107.9 F/O = 77 and 78% O/F = 45% 105 50 March 25, 2004 60 70 80 90 100 %F Distribution Occupancy A: Public Release, Distribution Unlimted California State University, Fullerton 39 Results of Geometric “Extraction” Calculated Experimental B3YLP/ CCSD(T)/ 631+G(2d)*, VTZ “Apparent” “Extraction” Method N-O (Å) 1.129 1.137 1.190(4) 1.114 N-F (Å) 1.312 1.305 1.245(4), 1.246(4) 1.284 O-N-F (º) 125.8 125.6 122.0(3), 122.1(3) 126.0 F-N-F (º) 108.4 108.8 115.9(3) 107.9 3.17, 7.33 3.03, 6.68 R (wR2) (%) ----- * Gillespie, R. J. et al., Inorg. Chem., 1998, 37, 6884 The analysis demonstrates that the crystal structure of F 2NO+AsF6 -, extracted from an oxygen/fluorine disordered structure, is in very good agreement with the theoretical predictions March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 40 Factors influencing the stability of Polynitrogen compounds ? Thermodynamic Factors 1. Electron Affinity of the Cation ?A fixed value, if we aim for a N 5+ salt , i.e., 10.5-11.5 eV 2. First Ionization Potential of the Anion ?The azide ion has a very low value of about 2.1 eV, which is the main + - reason for the instability of N5 N3 ?New polynitrogen anions are needed with higher first IP values. N5 - and N7 anions are most promising candidates 3. Lattice Energy of the Crystal ?UL fixed by the molar volumes of cation and anion. Born -Haber cycle calculations for the lattice energy estimated for N 5+N3- are 50 kcal/mole lower than the requirement for the stabilization of an ionic salt ? Kinetic Factors ?Low activation energy towards decomposition! These energy values determine the stability of the individual ions March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 41 Polynitrogen Anions Identification and Synthesis of Polynitrogen Anions March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 42 New Polynitrogen Anions as Counterparts for N 5+ Heptanitrogen anion (N7-) • Theoreticians predict reasonable stability • No reports have been published on attempts to prepare this anion. Work is in progess! R H R Si N R H R R Si N R Cl M N3 March 25, 2004 Cl TMSN 3 N R N3 R Si N R N3 +M F -R3SiF N3 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 43 New Polynitrogen Anions as Counterparts for N 5+ Pentazole anion (N5-) • Theoretical calculations show that this anion has a 28 kcal/mole activation energy barrier for decomposition and its decomposition to N 3- and N2 is only 11 kcal/mol exothermic N 5Energy • Free pentazole has not been isolated or characterized to date. Only aryl substituted pentazoles can be isolated and stabilized at low temperatures. These compounds rapidly decompose above 273K to form aryl azides and N 2 gas Distribution A: Public Release, Distribution Unlimted March 25, 2004 California State University, Fullerton 28 kcal 11 kcal N 3- + N 2 44 Identifying Potential Polynitrogen Precursors This ion has been suggested as a useful precursor to new polynitrogen molecules... ... but calculations predict it to be unstable. + NN C March 25, 2004 3.21 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 45 Synthetic Challenge – How do we make These New Anions?? Synthesis of Substituted Pentazoles Sources for the Pentazole Anion (N 5-) Silyl Diazonium Salts Aryl Diazonium Salts R R Si N2+ R +N3R N R = electron releasing group I. Ugi, Angew Chem., 1961, 73, 172 +N3- N R Si N N R N Unknown March 25, 2004 N2+ R N R Distribution A: Public Release, Distribution Unlimted Known California State University, Fullerton N N N N 46 Formation and Stability of Silyl Diazonium Salts • Attempts to synthesize silyl diazonium salts + N2F SbF6 - + Me3SiSiMe3 -Me3SiF Me3SiN2+SbF6- OR R3SiNH2 • + NO+ BF4- -H2O R3SiN2+ BF 4- R3SiN2+ salts are unstable and spontaneously lose N2 R3SiN2 + X- -N2 R3Si+ X- Theoretical calculations support this experimental observation March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 47 Use of Aryl Diazonium Salts – A Better Bet! N 2+ R X - + N3 - N R N N N + M+ XN M+ N5- + R • R must be an electron releasing group, i.e., -NMe2, -OH, -OCH 3, -OC6H5,-O -, etc. • Some of these substituted arylpentazoles have been known for about four decades but no success had been achieved to cleave the N 5 ring from the aryl group X Aryl Pentazoles can rapidly lose N 2 at room temperature N R N N March 25, 2004 N N -N2 R Distribution A: Public Release, Distribution Unlimted California State University, Fullerton N N N 48 Synthesis of Aryldiazonium Salts Aqueous Media R NH2 NaNO2/HCl R < 0 °C N2+Cl- NaBF4 R N2+BF4- R = H, OH, OCH3, OC 6H5 , OC 6 H4 N2+, N(CH 3 )2 mo March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 49 Synthesis of Aryldiazonium Salts…Nonaqueous synthesis Non-aqueous Media isoamyl nitrite R Ø NH2 CF3COOH CH2 Cl2 R Colas and Goeldner reported that the p-phenoxydiazonium trifluoroacetate to be a double salt. However, our results show no such behavior. In the case of a double salt, the –OH group can get protonated which prohibits pentazole formation! N2+CF3COO- N2+CF3COO-. CF3COOH HO Colas and Goeldner, Eur. J. Org. Chem. 1999, 1357-1366 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 50 Single or Double Diazonium Salt ? Consequences of Lone Pair Occupation! We DO NOT find any trifluoroacetic acid double salt. In fact, such a double salt would kill the pentazole formation P-1 N2+CF 3COO-. CF3COOH ?? HO 2 ion pairs within the asymmetric unit March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 51 Pentazole Formation… Role of the Substituent Electronic Effects Me 2N NH2 i. xs NaNO 2/HCl ~0 °C, ii. NaN N H Me 2N 3 N N NO3 N N - N2 H Me 2N N3 NO3 NaNO2 + HCl 3 HONO (aq) March 25, 2004 <0 °C NaCl + HONO H3O+ + NO3- + 2NO Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 52 Identification of Arylpentazoles Pentazoles can be characterized by low temperature NMR spectral studies using 15N labeled samples. N4 N3 • 1H NMR: AB-type spectrum with Ha and H b at 8.0 and 7.0 ppm • 14N NMR: N 1 at ~ -80 ppm • 15N NMR: N 2/N 5 at ~ -27 ppm and N 3/N 4 at ~4 ppm N5 N1 N2 Ha Ha Hb Hb R Note: Qualitative evidence for the presence of a pentazole ring: N2 gas evolution in solution March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 53 Cleavage of the Aryl-Pentazole Bond with Retention of the Pentazole Ring • Chemical Methods Ø Ozonolysis does not work! (Ugi, Radziszewski) V. Benin, P. Kszynski and G. J. Radziszewski, J. Org. Chem., 2002, 67, 1354 Ø Nucleophilic substitution using strong nucleophiles such as the OH -, OR-, F- etc. • Collisional Fragmentation (ElectroSpray Ion Mass Spectroscopy – ESIMS) N N N Ø Electrospray is very gentle and produces high concentration of the parent anion which can be mass selected Ø Collisional fragmentation of the mass selected anions with variable collisional energies allow tailoring of fragmentation Ø Negative ion detection eliminates interference from neutral or positively charged species March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton N N Weakest bond Nu R 54 ESIMS of para-Phenoxypentazole Observed peaks in the MSMS of 162 N N N N N N N N Low Collison Voltage -N 2 -CO O- ES MSMS O- N -N2 High Collison Voltage Om/z = 134 m/z = 78 m/z = 106 N N -C6H4O N m/z = 162 N N m/z = 70 March 25, 2004 N Distribution A: Public Release, Distribution Unlimted California State University, Fullerton -N2 N3 m/z = 42 55 15 N N Labeling of the Pentazole Ring N N bN aN N N N bN aN R R Na Nb *N N N R N N N N Na N Net Structure R N N N Na R March 25, 2004 Nb Distribution A: Public Release, Distribution Unlimted California State University, Fullerton N N N Na Nb R 56 Is the Peak at m/z 70 indeed due to the Pentazole Anion? 15N Observed peaks in the MSMS of 163 Labeled Pentazole 2/5 N N4 N3 5N N1 3/4 N Low Collison Voltage N2 ES MSMS m/z = 163 15N N C -N 2* N -CO O- m/z = 135/134 O Om/z = 106 m/z = 78 Labeling experiment shows that CO is lost in the last step High Collison Voltage statistically distributed over N2, N 3, N4 & N5 March 25, 2004 or -N2* N or -N2 -N 2 Unlabeled O- N1 -C6H4O N N N N N m/z = 71 Definitive proof for the pentazole anion Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 57 ESI-MS-MS fragmentation of 4pentazolylphenolate anion at low and high collision voltages. V -10 V -50 V 1 5N Labeled -75 V Irel Negat ive ion, f ull- range CI D mass spect ra of t he mass select ed, 15N labeled (m/ (m/ z 163) and unlabeled (m/ z 162) peaks due t o [OC6 H4 N 5 ]– recorded at collision volt ages of –75, - 50, and –10 Volt s. All spect ra are mult i- channel spect ra and t he t ypical mass resolut ion and noise level are shown f or t he m/ z 70 and 71 peaks in t he insert s. -75 V Unlabeled C&E News, 2002, 80, 8 m/z March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 58 Crystal structure of 4-Hydroxyphenylazide The thermal decomposition of 4hydroxyphenylpentazole (4-HPP) results in the loss of N 2 gas and the formation of 4-hydroxyphenylazide. The “two” hydrogen atoms present on the p-oxygen atom are disordered. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 59 Pentazoles with Heterocyclic Substituents • Tetrazolyl system is unstable above -70 °C and the pentazole ring rapidly decomposes to liberate N2 gas. N N N NH2 N i. NaNO2/HCl N ii. -70 °C, LiN3 * N H • N N N N N N -N2 N N N N N N3 N A. Hammerl and T. M. Klapoetke, Inorg . Chem. 2002, 41, 906-912 In comparison, the pentazole ring derived from 2-amino-4,5dicyanoimidazole shows higher thermal stability (-30 °C) C N NH2 C N March 25, 2004 N H i. NaNO 2/HCl N C ii. -30 °C, NaN3* N N N N N N C N Na N Distribution A: Public Release, Distribution Unlimted California State University, Fullerton -N2 C N N3 N C N N Na 60 15 N NMR of 2-pentazolyl-4,5-dicyanoimidazole 2,5 3,4 N 2 C N 1 N N 3 N C N 15 N March 25, 2004 N N N 5 4 NMR recorded in a mixture of methanol and acetonitrile at -30 °C, nitromethane used as an external reference (0 ppm) Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 61 Crystal structure of 2-Diazo-4,5dicyanoimidazole N-N = 1.096 ? C-N = 1.334, 1.336 ? March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 62 Does the Pentazolate Anion Exist in Solution ? S5 3 is N N 2 C N N 1 N N N 3,4 N N 2,5 C N N N 5 N N 3 4 ,^iM«» ^ImilfiiLWMlii HiHifillltfrtiihilLiMiry T IC ^ 1 I -] -'-I—— -iB \ -iJD 1 -at r—— ^,-1-^—^T -m -35 ITpx #iLUi Ø 15N ion. «HHW NMR shows a peak at -10 ppm (-30 °C), which slowly decomposes to form N 2 and azide Ø This peak is also observed upon adding a base to the solution of arylpentazoles at -30 °C. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 63 High Nitrogen Chemistry Synthesis, Mechanistic Studies and Structural Characterization of Binary Metal Azides March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 64 Reactions of Group 15 halides with Trimethylsilylazide Crystalline binary metal azides were obtained upon reacting the corresponding metal fluorides with TMSN 3. These compounds were reported as either liquids or tacky solids by Klapoetke et al. MF3 + Me3SiN3 -Me3SiF M(N3)3 These solids could be sublimed under vacuum to yield colorless diffraction quality crystals with no incidents of explosion or thermal decomposition March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 65 Structure of As(N3 )3 One of the azide groups N7-N8-N9 destroys the C3 symmetry March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 66 Crystal Structure of Sb(N 3)3 All azide groups oriented in a propeller-like fashion March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 67 Crystal Structure of Sb(N 3)3 View down the three-fold axis, all azide groups equivalent Example of perfect C 3 symmetry March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 68 Crystal Structure of Sb(N 3)3 Sb N “Star of David” Perspective March 25, 2004 “Isle of Man” Perspective Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 69 Reactivity of hexachloroantimonate (VI) with Trimethylsilylazide [Ph4M][SbCl6] + Me3SiN 3 -Me3SiCl CH3CN o 60 C [Ph4M][SbCl6-x(N 3)x] M = P, As; x = 2-6 üThe substitution of all the six chlorine atoms in SbCl 6by the azide groups could not be accomplished in a single step, as reported in literature. The stepwise substitution gives a good insight into the substitution mechanism. ü Total substitution was achieved after four “refreshment” cycles of the reagents. During the intermediate cycles, the azide content gradually increased from two to five. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 70 Episode I…Generation of the starting material Ph 4MCl + SbCl5 1,2-DCE [Ph4M][SbCl6] M = P, As Cl Cl Cl Sb Cl Cl Cl March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 71 Episode II….cis- or trans- disubstitution with azide groups? N3 Cl Cl Cl Cl N3 Cl Sb Sb Cl Cl Cl N3 N3 cis-isomer trans-isomer % % March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 72 Episode III…Substitution of 3 rd chlorine… fac- or mer- isomer ??? Cl N3 Cl Cl Cl N3 N3 Sb Sb Cl Cl N3 N3 N3 • \ • mer-SbCl 3(N 3)3 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton fac-SbCl3(N3)3 73 Episode IV…Capturing the “transition state” during the fourth substitution! Cl N3 Cl Sb N3 N3 i N3 Cis- vs. trans- substitution Cl N3 Sb N3 N3 Cl March 25, 2004 N3 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 74 Chloropentaazidoantimonate(VI) Anion Cl N3 Sb N3 N3 N3 N3 The Structure of Ph 4PSbCl(N 3)5 ü The crystals grown from CH3 CN ü Triclinic space group P-1 ü Cell constants: a = 11.134(3) Å , b = 11.663(3) Å, c = 13.754(4) Å; α = 104.314(5)º; β = 97.914(5)º; γ = 115.807(4)º ü Z=2 ü R = 0.0762 ü All azide distances “normal” except N10-N11-N12 March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 75 Episode VI…Complete substitution of chlorine atoms No crystal structure obtained yet. However, IR and Raman spectroscopy shows that Sb-Cl bonds are absent i.e., complete substitution by the azide groups. March 25, 2004 N3 N3 Sb N3 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton N3 N3 N3 76 Solvent effect in halide substitution reaction with TMSN3 Using tetrahydrofuran (THF) in place of acetonitrile (AN) results in the formation of the Sb2OCl62- anion. This probably results from the ring opening oxidation of THF. [Ph 4P][SbCl6 ] March 25, 2004 + Me 3SiN 3 THF 2[Ph4 P]+ .[Sb 2OCl 6] 2-.SbCl3 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 77 Summary – Polynitrogen Anions Ø Synthesized aryl pentazoles: hydroxy group at the paraposition on the aryl ring gives the best results as observed during this study. Ø Demonstrated selective cleavage of C-N bond by ESIMS with retention of pentazole ring. Results confirmed studying 15N labeled pentazoles. Ø First experimental detection of pentazolate anion. Ø Synthesis of pentazoles with a heterocylic substitutents Ø Addition of OH- ions to a solution of pentazole suggest C-N bond cleavage. Ø Offers potential pathway for bulk synthesis of N 5- salts March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 78 Summary- Polynitrogen Cations Ø Use of AlF 3 as an efficient catalyst for the trans-cis isomerization of N2F2, which is a precursor for the synthesis of N5AsF6 and N5SbF6. Ø Successfully demonstrated conversion of N5SbF 6 into other salts, such as N5B(CF3)4 and N5SnF5 Ø Prepared and characterized (N5)2SnF6, thereby doubling the N5+ content of N5SbF6 Ø Obtained experimental and computational evidence for instability of N5N3, N5NO3, N5N(NO 2)2 and N5ClO4 Ø Ø Ø Prepared and characterized the N3NOF+ cation Attempted the preparation of N2(N3)3 + cation Attempted the preparation of N(N3)4+ cation March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 79 Conclusions Ø Ø AlF3 is the best catalyst for the isomerization of trans-N2F2 Ø Only one fluorine atom in N(O)F 2+ has been replaced with an azide ion to form the N3N(O)F+ cation Ø Ø Ø The N2(N3)3+ cation could not be stabilized and isolated Ø 2-Pentazolyl-4,5-imidazole appears to undergo chemical C-N bond cleavage. Results are under investigation! N5+ cation can be stabilized with anions such as B(CF 3)4-, SnF5-, SnF62-, SbF 6- and Sb2F112- but NOT with N3-, NO3-, ClO 4- and N(NO2)- The N(N3)4+ cation could not be stabilized and isolated Pentazoles with substituents other than the aryl group can be prepared and stabilized at low temperatures. March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 80 AFRL/USC/UC Coworkers and Collaborators Air Force Research Laboratory, Edwards Dr. Karl Christe, Dr. William Wilson, Ms. Vandana Vij University of Southern California Dr. Ralf Haiges University of California, Riverside Dr. Fook Tham University of California, Santa Barbara Dr. James Pavlovich March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 81 Acknowledgments Dr. Robert Corley, Tech Dr. Timothy Haddad, NMR Dr. Rusty Blanski, NMR Dr. James Pavlovich, UCSB-MS Dr. Fook Tham, UCR-X-ray March 25, 2004 Mr. Michael Huggins Dr. Ronald Channell Mr. Wayne Kalliomaa Dr. Don Woodbury Dr. Arthur Morrish Dr. Michael Berman Dr. David Campbell Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 82 BACKUP March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 83 General approach for treating disorder Bond Length (Arbitrary Units) Plot of O-X and X-F distances versus %F occupancy b 1/3(RO-X + 2RX-F) RX-F a ½(RO-X + RX-F) RO-X 0 25 50 75 100 % F Occupancy Plot of F-X-O and F-X-F angles versus %F occupancy ∠ F-X-O Bond Angle (degrees) 360/3º c ∠ F-X-F 50 60 70 80 90 • For a linear C • v structure, FXO, midpoint is at 50% occupancy (plot a) • For a trigonal C2v species F2XO, equilibrium point is weighted for the two types of atoms i.e., 2F and 1O (plot b) • Plot c shows equilibrium bond angles for equal occupancies for two Fs (2/3) and O (1/3) i.e., 120 º. Also angle F-X-O = (1/2)(360angle F-X-F) 100 %F Occupancy March 25, 2004 Distribution A: Public Release, Distribution Unlimted California State University, Fullerton 84