A synthetic, spectroscopic and magnetic susceptibility study of selected main group and transition metal fluoro compounds

• Main Author: Cader, Mohamed Shah Roshan
• Format: Others
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2925

This study was initiated in order to synthesize, and where appropriate, to investigate the magnetic properties of selected main group and transition metal cationic complexes, all stabilized by weakly basic fluoro anions derived either from the Brönsted superacids HSO₃F and HSO₃CF₃,or the Lewis acids SbF₅and AsF₅. Of the preparative reactions, the solvolysis of metal(ll) fluorosulfates in excess SbF₅ according to: [scientific formula – please see full abstract) with M=Ni, Pd, Cu, Ag or Sn, is found to be a useful synthetic route to the corresponding divalent hexafluoro antimonates. The products, as their precursors, are characterized as CdCl₁₂- type layered polymeric compounds. Relevant vibrational (Raman and IR), electronic and ¹¹⁹Sn-Mössbauer spectra as well as magnetic susceptibility measurements and X-ray powder data are reported. Several compounds prepared by this method display unusual features: Pd(SbF₆)₂ is, like its fluorosulfate precursor, paramagnetic with the Pd²+ ion in a ³A₂g ground state. Ag(SbF₆)₂,unlike its paramagnetic blue valence isomer, is diamagnetic and nearly white in color, and is formulated as the mixed valency complex Ag(I)[Ag(III)(SbF₆)₄]. The Cu(SbF₆)₂ compound also contains, in addition to Cu²+ ions, small quantities of Cu(I) and Cu(Ill) ions. Both Ni(SbF₆)₂ and Pd(SbF₆)₂ exhibit temperature dependent low magnetic moments, indicative of antiferromagnetic exchange. Pd(SbF₆)₂ also displays very weak ferro magnetism below -lOK. The Sn(SbF₆)₂ product from the above synthesis, and its precursor Sn(SO₃F)₂,react with excess 1,3,5-trimethylbenzene(mesitylene or mes) to give the it-arene adducts Sn(SbF₆)₂ 2mes and Sn(SO₃F)₂ mesin high yield. The adducts are characterized by elemental analysis and infrared spectra. The adduct formation is followed by ¹¹⁹Sn Mössbauer spectroscopy. It is found that only tin(ll) compounds with large, weakly nucleophilic anions are capable of forming mesitylene complexes, while SnCl₂,SnF₂,and stannocene do not give any indication of adduct formation under similar reaction conditions. The divalent fluorosulfates Ni(SO₃F)₂,Pd(SO₃F)₂ and Ag(SO₃F)₂,precursors to the M(SbF₆)₂ compounds, and the mixed valency Pd(II)[Pd(IV)(SO₃F)₆],as well as their corresponding trifluoromethylsulfate derivatives Ni(SO₃CF₃)₂, Pd(SO₃CF₃)₂ and Ag(SO₃CF₃)₂,investigated for their magnetic behavior by susceptibility studies down to -4 K, show significant magnetic exchange, and except in Ag(SO₃CF₃)₂,the onset of magnetic exchange becomes observable at low temperatures. The fluorosulfates are found to exhibit strong ferromagnetism below -11 K, whereas the trifluoromethylsulfates behave as anti ferromagnets with the spin interactions noted over a wider temperature range. The maximum magnetic susceptibilities of Ni(SO₃F)₂,Pd(SO₃F)₂ and Ag(SO₃F)₂ indicate saturation magnetization, and hence for these compounds field dependent maximum magnetic moments are obtained in the temperature range -5 to 10.5 K. Maxima in the susceptibility vs. tempera ture plots are noted for the antiferromagnets Pd(SO₃CF₃)₂ and Ag(SO₃CF₃)₂ at -4 and -13 K respectively. Unlike in the corresponding divalent antiferromagnetic fluorides, no spin canting is detected in the trifluoromethylsulfates at lower temperatures. Magnetic susceptibility measurements to -4 K are also carried out for the main group molecular cations withinO2[AsF6],Br2[Sb3F16]andI2[Sb2F1i] The data are interpreted utilizing previous results from photoelectron spectroscopy, known crystal structures, magnetic studies on the superoxide ion and the ozonide ion, and in the case of Oj[AsF6],previous ESR studies. The magnetic properties of the three materials are quite different. Br2[Sb3F16]obeys Curie- Weiss law between 80 and 4 K. The magnetic moment decreases slightly from 2.04 B at room temperature to 1.93 B at 4 K. I2[Sb2F11]exhibits relatively strong antiferromagnetic coupling with a maximum in XM observed at -54 K. The magnetic moment (corrected for TIP) decreases from 1.92 B at 124 K to 0.41 B at 4 K. Experimental susceptibilities for this compound over the temperature range 300-4 K have been compared to values calculated using three different theoretical models for extended chains of antiferromagnetically coupled paramagnetic species.02+[AsF6]exhibits Curie-Weiss behavior over the temperature range 60-2 K. The magnetic moment, uncorrected for TIP, varies from 1.63 B at 80 K to 1.17 I.LB at 2 K, and the presence of weak antiferromagnetic coupling in this material is suggested. === Science, Faculty of === Chemistry, Department of === Graduate