Lecture I University of Oulu, Finland Main-Group Chemistry Summer School August 27-31, 2012 Neil Bartlett’s Discovery of Noble-Gas Reactivity; Its Aftermath and Significance 1 Beginning of the noble-gas story at King’s College, Newcastle, England • Neil Bartlett, a Ph.D. student, wanted to study the preparation of PtF2 by reduction of PtF4. PtF4 was contaminated by bromine. 3Pt + 4BrF3 3PtF4 + 2Br2 • Bartlett heated impure PtF4 in F2 to oxidize the bromine to BrF5, with the hope that this would liberate BrF5 from the PtF4. The fluorination was done in a stream of diluted fluorine in a shallow Ni boat, placed in a Pyrex glass tube. When heated, the PtF4 became darker. Finally, a deep red vapor emerged from the boat and condensed on the cooler glass downstream. This occurred just as it became clear that the fluorine was attacking the glass tube to liberate oxygen and SiF4. • PtF4, already known in November 1956, was the highest known Pt fluoride. • Bartlett initially concluded that he probably obtained a new oxide fluoride of platinum, PtO2F6. 2 Continuation of the story at the University of British Columbia, Canada • Bartlett continued his research on PtO2F6 at UBC. He found that the solid was paramagnetic, ruling out the possibility of PtX. • The X-ray powder diffraction pattern contained a strong set of lines indicative of a simple cubic Pt-atom sublattice. 2− • Slow hydrolysis of the compound gave PtF6 as a solution species indicating that the compound was a "PtF6" species. + − • Together with its magnetic properties, the formulation, O2 PtF6 , was suggested. • From simple lattice energy considerations, it was concluded that electron affinity of PtF6 should exceed 7 eV. • This led Bartlett to attempt to oxidize xenon. 3 Bartlett’s comments regarding the synthesis of + − O2 PtF6 + − • The discovery of O2 PtF6 was accidental. + − • O2 PtF6 was easily made but its correct characterization probably involved, according to Bartlett, the most difficult work of his entire career. • Both ions were then unknown in chemical compounds. • Oxidation of oxygen required that PtF6 be a one electron oxidizer of unprecedented strength. 4 + − Born-Fajans-Haber Cycle for O2 PtF6 (enthalpies in kJ mol–1) + − O2 (g) + PtF6 (g) 1171 523 (lattice energy) (IP) < 648 (EA) + − O2(g) + PtF6(g) O2 PtF6 (s) 5 Ionization Energies (eV) and Diameters of Noble- Gas Atoms (pm) He+ 24.6 Ne+ 21.6 Ar+ Kr+ 15.8 Xe+ 14.0 Ionization potential for 12.1 eV + Ng(g) Ng (g) + e He Ne Ar Kr Xe Atomic Diameters 260 320 384 396 436 1eV = 96.49 kJ mol–1, 23.06 kcal mol–1 6 Thermal Chemistry of the Reactions of O2 and –1 Xe with PtF6 (kJ mol ) + − O2 (g) O2 (g) + e H = 1171 + − Xe (g) Xe (g) + e H = 1167 − − PtF6 (g) + e PtF6 (g) H = 750 + – Xe(g) + PtF6 (g) Xe (g) + PtF6 (g) H = 417 + – + – Xe (g) + PtF6 (g) "Xe PtF6 (s)" –HL = –460 + – Xe(g) + PtF6 (g) "Xe PtF6 (s)" H ≈ –43 NOTE: Although G may be +ve at room temperature (TS is –ve), the actual structure of XePtF is unknown. 6 7 Neil Bartlett in his laboratory in 1962 8 Oxidation of Xenon with PtF6 20 oC + – Xe + PtF6 "Xe PtF6 " N. Bartlett, Proc. Chem. Soc. 1962, 218. 9 10 From the introduction to the book “The Oxidation of Oxygen and Related Chemistry” by Neil Bartlett: “Genuine new directions in research are unanticipated. They are unlikely to be part of a research proposal. It is the unanticipated event (such as the first observation of O2PtF6) which is so important, and has to be followed up. A new viewpoint then develops. In such a way Noble-Gas Chemistry was born.” World Scientific Series in 20th Century Chemistry – Vol. 9, 2001 11 2006 International Historic Chemical Landmark Dedication University of British Columbia, Vancouver 12 International Historic Chemical Landmark (2006) University of British Columbia, Vancouver 13 “Neil Bartlett and Reactive Noble Gases” “In this building in 1962 Neil Bartlett demonstrated the first reaction of a noble gas. The noble gas family of elements - helium, neon, argon, krypton, xenon, and radon - had previously been regarded as inert. By combining xenon with a platinum fluoride, Bartlett created the first noble gas compound. This reaction began the field of noble gas chemistry, which became fundamental to the scientific understanding of the chemical bond. Noble gas compounds have helped create anti-tumor agents and have been used in lasers.“ 14 Synthesis of the First Noble-Gas Compound xPtF6 + Xe Xe(PtF6)x 1< x < 2 sticky red-colored solid XRDP of the product Xe(PtF6)X always exhibited the characteristic pattern + − + − of XeF PtF6 (identical to that of XeF RuF6 ). Conclusion: + − XePtF6 + PtF6 XeF PtF6 + PtF5 T < 60 °C + − Xe(PtF6)2 XeF Pt2F11 orange red friable solid + − XRDP shows only XeF PtF6 T < 60 °C XeF+PtF − + PtF XeF+Pt F − analogous and isomorphous 6 5 2 11 + − with XeF Ir2F11 L. Graham, O. Graudejus, N. K. Jha, N. Bartlett Coord. Chem. Rev. 2000, 197, 321-334. 15 The Best Preparation of “XePtF6” PtF6 (diluted 1:6 in SF6) + Xe (in excess) “XePtF6” XePtF6, a mustard-yellow solid, gave neither a Raman spectrum nor XRDP. It neither reacted nor dissolved in aHF, the color suggested that PtF5 was absent. + − It is weakly paramagnetic (small quantities of XeF PtF6 and PtF5 are present even in the best preparations). XePtF6, when pure, may be a relative of diamagnetic XePdF6. L. Graham, O. Graudejus, N. K. Jha, N. Bartlett Coord. Chem. Rev. 2000, 197, 321-334. 16 Reaction of PtF4 and XeF2 in aHF solvent aHF + 2− 2XeF2 (solv) + PtF4 (s) 2XeF (solv) + PtF6 (solv) multi-fold excess of XeF2 yellow solution + 2− removal of aHF + − n(XeF )2PtF6 nXeF2 + (XeF )n(PtF5 )n 2− PtF6 in aHF is probably stabilized by solvation and therefore less strongly polarizes the XeF+ cation, (solv) “XePtF6” which, after removal of aHF, gives the strongly polarizing “naked” XeF+ cation. This is capable of diamagnetic yellow − 2− − solid removing F from PtF6 to yield PtF5 , which could then oligomerize. 17 Can we speculate about the structure of XePtF6? II IV + − Diamagnetic Xe Pt F6 (probably the polymeric salt (XeF )n(PtF5 )n is the thermodynamically preferred form of “XePtF6”. The products of the further + − + − oxidation by PtF6 are Pt(V) derivatives XeF PtF6 , XeF Pt2F11 , and PtF5. The structure of XePtF6 is not yet known. Based on Bartlett’s considerations, the structure should be akin to the structure of the + − polymeric salt (XeF )n(CrF5 )n. 18 + − (XeF )n(CrF5 )n 50°C mCrF5 + nXeF2 mXeF2·CrF4 + 0.5mXeF4 + (n – 1.5m)XeF2 n > 5m K. Lutar, I. Leban, T. Ogrin, B. Žemva, Eur. J. Solid State Inorg. Chem., 1992. 19 20 21 22 23 Rapid Development of Noble-Gas Chemistry Experimental Techniques Then Available Metal vacuum lines and inert fluoroplastics (Kel-F, Teflon). For handling F2 and aggressive fluorine compounds. Synthetic fluorine chemistry expertise at that time and metal hexafluoride chemistry (Manhattan project). Physical Methods for Structural Characterization: Diffraction methods (neutron & X-ray) Vibrational spectroscopy (Raman & Infrared) Nuclear magnetic resonance Mössbauer spectroscopy Mass spectrometry Electron spin resonance 404 pages Thermochemistry 24 Theoretical studies Geometries of Noble-Gas Compounds Predicted by VSEPR R. J. Gillespie In Noble Gas Compounds; H. H. Hyman, Ed.; University of Chicago Press: Chicago, 1963, pp 333−339. 25 The Early Years XeF2 1962 XeF 1962 4 Most noble-gas chemistry precursors XeF 1962 6 were prepared within 2 years of Neil XeO 1962 3 Bartlett’s discovery KrF2 1963 XeOF4 1963 XeO 4– 1963 6 H. H. Hyman’s edited book “Noble-gas Compounds” XeO4 1964 XeO2F2 1967 XeO3F2 1968 Stable compounds of Xe and Kr were formed having the oxidation states: Xe(II), Xe(IV), Xe(VI), Xe(VIII) and Kr(II). Only Xe–F, Xe–O, Kr–F bonds were known.a a 129 129 Transient Xe-Cl bonds were formed by radioactive decay of ICl2 and ICl4 129m 129m 129 to XeCl2 and XeCl4. XeCl2 and XeCl4 were detected by their Xe Mössbauer emission spectra. 26 The Quest for Stablea Bonds with Noble-Gases Xe–N 1974 Xe–Xe 1978 Xe–Cr 1983 Kr–N 1988 Kr–O 1989 Xe–C 1989 Xe–W 1992 Xe–Mo 1996 Ng–M 1996 Ng = Ar, Kr, Xe; M = Cr, Mo, W Xe–Au 2000 Xe–Re 2000 b Ar–H 2000 b Ar–F 2000 Xe–Cl 1999, 2001 Xe–Hg 2003 a Stable in solution and/or the solid state. b Matrix-isolation study. 27 Some Synthetic & Structural Highlights Since Neil Bartlett’s Discovery Xe–N Bond CF2Cl2 XeF2 + HN(SO2F)2 FXeN(SO2F)2 + HF 0 oC, 4 days R. D. LeBlond, D. D. DesMarteau, J. Chem. Soc., Chem. Commun. 1974, 555. • X-ray crystal structure of the first Xe-N bonded compound FXeN(SO2F)2 –55 oC 28 J. F. Sawyer, G. J. Schrobilgen, S. J. Sutherland, Inorg. Chem. 1982, 21, 4064-4072. XeII –C Bonds CH2Cl2 or CH3CN B(C6F5)3 + XeF2 [C6F5Xe][B(C6F5)nF4–n] –50 oC or –40 oC D. Naumann, W. Tyrra, J. Chem. Soc., Chem. Commun. 1989, 47–50. H.-J. Frohn, S. Jakobs, J. Chem. Soc., Chem. Commun. 1989, 625–627. H.-J. Frohn, S. Jakobs, G. Henkel, Angew. Chem., Int. Ed. Engl. 1989, 28, 1506–1507. CH3CN [C6F5Xe][BF4] + M[BY4] [C6F5Xe][BY4] + M[BF4]↓ RT to –40 °C Y = CF3, CN, C6F5 M = K, Cs Xe–C 2.081(3) Å K. Koppe, H.-J. Frohn, H. P. A. Mercier, G. J. Schrobilgen, G. J., Inorg.