Fluoride-Ion Donor Properties of AsF5.

Arsenic pentafluoride undergoes ligand-induced autoionization in the presence of 1,10-phenanthroline (phen) in a SO2ClF solution to form the donor-stabilized [AsF4(phen)][AsF6] salt. Reacting [AsF4(phen)][AsF6] with the strong Lewis acid SbF5·SO2 yields the mixed arsenic-antimony salt [AsF4(phen)][Sb2F11]. These salts are the first examples of crystallographically characterized donor-stabilized [AsF4]+ cations. The analogous reaction of AsF5 and 2,2'-bipyridine (bipy) does not result in autoionization but leads to the formation of the neutral 2:1 adduct (AsF5)2·bipy. The gas-phase and solution fluoride-ion affinities of [AsF4]+ and [SbF4]+ were calculated, revealing them to be incredibly strong Lewis acids. Density functional theory calculations and natural bond orbital analysis show that significant electron-pair donation from phen to the As center in [AsF4(phen)]+ occurs and quenches the extreme electrophilicity of the [AsF4]+ cation.

Interaction Modes Between SF4 and Ketones; Study of Intermediates in Deoxofluorination Reactions.

Interactions between ketones and SF4 are studied for the simplest ketone, acetone, and the bulky polycyclic 2-adamantanone. Acetone forms the 1 : 2 adduct SF4 ⋅ [O=C(CH3 )2 ]2, as well as the dimeric 1 : 1 adducts [SF4 ⋅ O=C(CH3 )2 ]2 as identified by low-temperature Raman spectroscopy and, for the latter, X-ray crystallography. In both adducts, SF4 acts as a double chalcogen-bond donor to two keto groups. In contrast 2-adamantanone does not form an isolable solid adduct with SF4 ; in the presence of HF, however, it forms SF4 ⋅ O=C10 H14 O ⋅ HF, which comprises chains with weak S-O and S-FH chalcogen bonds in the crystal structure. Sulfur tetrafluoride in this compound is readily lost at -85 °C, leading to the isolation of C10 H14 O ⋅ HF at low temperature. Density functional theory (DFT) calculations aid in vibrational assignments and serve to describe the interactions of the keto group with SF4 and HF, as well as interactions between SF4 with HF. It is found that separate and combined CO-HF and CO-SF4 chalcogen bonds do not polarize the C=O group to any significant degree.

Donor-Stabilised [SbF4 ]+ : SbF5 as a Fluoride-Ion Donor.

Antimony pentafluoride is a strong Lewis acid and fluoride-ion acceptor that has not previously demonstrated any discreet fluoride-ion donor properties. The first donor-stabilised [SbF4 ]+ cations were prepared from the autoionisation of SbF5 in the presence of bidentate N-donor ligands 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) as their [SbF6 ]- salts. The [SbF4 (N-N)][Sb2 F11 ] (N-N=bipy, phen) salts were synthesised by the addition of one equivalent of SbF5 ⋅SO2 to [SbF4 (N-N)][SbF6 ] in liquid SO2. The salts show remarkable stability and were characterised by Raman spectroscopy and multinuclear NMR spectroscopy. The crystal structures of [SbF4 (phen)][SbF6 ] ⋅ 3CH3 CN and [SbF4 (phen)][SbF6 ] ⋅ 2SO2 were determined, showing distorted octahedral cations. DFT calculations and NBO analyses reveal that significant degree of electron-pair donation from N to Sb stabilizes [SbF4 ]+ with the Sb-N bond strength being approximately two thirds of that of the Sb-F bonds in these cations and the cationic charge being primarily ligand-centred.

Correlating axial and equatorial ligand field effects to the single-molecule magnet performances of a family of dysprosium bis-methanediide complexes.

Treatment of the new methanediide-methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh2S)2}2-) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)2][Dy(SCS)2(K(DME)2)2] (2Dy), [Dy(SCS)2][Na(DME)3] (3Dy) and [Dy(SCS)2][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)2][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh2NSiMe3)2}2-, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy-5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s-1). Fitting the alternating current magnetic susceptibility data for 2Dy-5Dy gives energy barriers to magnetic relaxation (U eff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy-4Dy join a privileged group of SMMs with U eff values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy-4Dy, the Dy-components can be grouped into 2Dy-cation/4Dy and 2Dy-anion/3Dy, where the former have almost linear C[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with short average Dy[double bond, length as m-dash]C distances, and the latter have more bent C[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with longer average Dy[double bond, length as m-dash]C bonds. Both U eff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M-L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between U eff barrier and axial linearity in structurally comparable systems.