Syntheses, Structures, and Bonding of NgF₂⋅CrOF₄, NgF₂⋅2CrOF₄(Ng=Kr, Xe), and (CrOF₄)_∞

Abstract

The noble‐gas difluoride adducts, NgF2⋅CrOF4 and NgF2⋅2CrOF4 (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single‐crystal X‐ray diffraction. The low fluoride ion affinity of CrOF4 renders it incapable of inducing fluoride ion transfer from NgF2 (Ng=Kr and Xe) to form ion‐paired salts of the [NgF]+ cations having either the [CrOF5]− or [Cr2O2F9]− anions. The crystal structures show the NgF2⋅CrOF4 adducts are comprised of Ft−Ng−Fb‐ ‐ ‐Cr(O)F4 structural units in which NgF2 is weakly coordinated to CrOF4 by means of a fluorine bridge, Fb, in which Ng−Fb is elongated relative to the terminal Ng−Ft bond. In contrast with XeF2⋅2MOF4 (M=Mo or W) and KrF2⋅2MoOF4, in which the Lewis acidic, F4(O)M‐ ‐ ‐Fb‐ ‐ ‐M(O)F3 moiety coordinates to Ng through a single M‐ ‐ ‐Fb−Ng bridge, both fluorine ligands of NgF2 coordinate to CrOF4 molecules to form F4(O)Cr‐ ‐ ‐Fb−Ng−Fb‐ ‐ ‐Cr(O)F4 adducts in which both Ng−Fb bonds are only marginally elongated relative to the Ng−F bonds of free NgF2. Quantum‐chemical calculations show that the Cr−Fb bonds of NgF2⋅CrOF4 and NgF2⋅2CrOF4 are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr‐ ‐ ‐Fb−Ng bridge angles and staggered O−Cr‐ ‐ ‐Fb−Ng−Ft dihedral angles. The crystal structures and Raman spectra of two CrOF4 polymorphs have also been obtained. Both are comprised of fluorine‐bridged chains that are cis‐ and trans‐fluorine‐bridged with respect to oxygen.

Chromium Oxide Tetrafluoride and Its Reactions with Xenon Hexafluoride; the [XeF5]+ and [Xe2F11]+ Salts of the [CrVIOF5]−, [CrVOF5]2−, [CrV2O2F8]2−, and [CrIVF6]2− Anions

“…The reaction of equimolar amounts of XeF 6 and CrOF 4 in aHF at −78 °C initially yielded crystalline XeF 6 ⋅ 1.5 HF and α‐CrOF 4 , which were confirmed by unit‐cell determinations. Upon warming to room temperature, α‐CrOF 4 and XeF 6 ⋅ 1.5 HF rapidly dissolved in aHF to give an amber solution.…”

“…The CrOF 4 molecules of [XeF 5 ] 2 [CrF 6 ] ⋅ 2 CrOF 4 ( 1 ) are trans to one another and interact with the [CrF 6 ] 2− anion through Cr‐ ‐ ‐F contacts (Cr(1)‐ ‐ ‐F(1), 2.216(3) Å), and with neighboring [XeF 5 ] + cations through short Xe‐ ‐ ‐F contacts (vide supra). The Cr‐ ‐ ‐F contacts in 1 and 6 are comparable to those of α‐CrOF 4 (2.274(3)–2.333(3) Å), β‐CrOF 4 (2.3659(6) Å), and XeF 2 ⋅ 2 CrOF 4 (Cr(2)‐ ‐ ‐F(1), 2.386(1) Å) . The Cr−O bonds of [XeF 5 ] 2 [CrF 6 ] ⋅ 2 CrOF 4 ( 1 ) (1.571(4) Å) and [XeF 5 ][Xe 2 F 11 ][CrOF 5 ] ⋅ 2 CrOF 4 ( 6 ) (1.547(3) Å, 1.530(3) Å) have significant double‐bond character and are comparable in length to those of α‐CrOF 4 (1.539(3)–1.558(4) Å), β‐CrOF 4 (1.5490(7) Å), and XeF 2 ⋅ 2 CrOF 4 (1.545(2) Å) .…”

“…The geometric parameters of the co‐crystallized CrOF 4 molecules in [XeF 5 ] 2 [CrF 6 ] ⋅ 2 CrOF 4 ( 1 ) (Figure ) and [XeF 5 ][Xe 2 F 11 ][CrOF 5 ] ⋅ 2 CrOF 4 ( 6 ) (Figure ) are very similar to those of α‐ and β‐CrOF 4 and XeF 2 ⋅ 2 CrOF 4 . The co‐crystallized CrOF 4 molecules of both salts interact with a fluorine ligand of the anion through a Cr‐ ‐ ‐F contact that is trans to the Cr−O bond.…”

“…The noble‐gas difluorides, NgF 2 (Ng=Kr, Xe), are sufficiently fluorobasic to form adducts with the Group 6 d 0 metal oxide tetrafluorides, MOF 4 (M=Cr, Mo, W), with the general formulae NgF 2 ⋅ n MOF 4 ( n= 1, 2) . Members of the MOF 4 series are intermediate strength fluoride‐ion acceptors which do not fully transfer fluoride ion to form [NgF][MOF 5 ] or [NgF][M 2 O 2 F 9 ] salts.…”

“…Members of the MOF 4 series are intermediate strength fluoride‐ion acceptors which do not fully transfer fluoride ion to form [NgF][MOF 5 ] or [NgF][M 2 O 2 F 9 ] salts. Instead, NgF 2 coordinates to the metal by means of Ng–F‐ ‐ ‐M bridges . Xenon hexafluoride is more fluorobasic than XeF 2 , and is, therefore, more likely to donate fluoride ion to CrOF 4 to form [XeF 5 ] + and [Xe 2 F 11 ] + salts of [CrOF 5 ] − .…”