Fluoro-Germanium (IV) Cations with Neutral Co-Ligands—Synthesis, Properties and Comparison with Neutral GeF₄ Adducts

Abstract

The reaction of [GeF4L2], L = dmso (Me2SO), dmf (Me2NCHO), py (pyridine), pyNO (pyridine-N-oxide), OPPh3, OPMe3, with Me3SiO3SCF3 (TMSOTf) and monodentate ligands, L, in a 1:1:1 molar ratio in anhydrous CH2Cl2 formed the monocations [GeF3L3][OTf]. These rare trifluoro-germanium (IV) cations were characterised by microanalysis, IR, 1H, 19F{1H} and, where appropriate, 31P{1H} NMR spectroscopy. The 19F{1H} NMR data show that in CH3NO2 solution the complexes exist as a mixture of mer and fac isomers, with the mer isomer invariably having the higher abundance. The X-ray structure of mer-[GeF3(OPPh3)3][OTf] is also reported. The attempts to remove a second fluoride using a further equivalent of TMSOTf and L were mostly unsuccessful, although a mixture of [GeF2(OAsPh3)4][OTf]2 and [GeF3(OAsPh3)3][OTf] was obtained using excess TMSOTf and OAsPh3. The reaction of [GeF4(MeCN)2] with TMSOTf in CH2Cl2 solution, followed by the addition of 2,2′:6′,2”-terpyridine (terpy) formed mer-[GeF3(terpy)][OTf], whilst a similar reaction with 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3-tacn) in MeCN solution produced fac-[GeF3(Me3-tacn)][OTf]. Dicationic complexes bearing the GeF22+ fragment were isolated using the tetra-aza macrocycles, 1,4,7,10-tetramethyl-1,4,7,10-tetra-azacyclododecane (Me4-cyclen) and 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane (Me4-cyclam), which reacted with [GeF4(MeCN)2] and two equivalents of TMSOTf to cleanly form the dicationic difluoride salts, cis-[GeF2(Me4-cyclen)][OTf]2 and trans-[GeF2(Me4-cyclam)][OTf]2. The 19F{1H} NMR spectroscopy shows that in CH3NO2 solution there are four stereoisomers present for trans-[GeF2(Me4-cyclam)][OTf]2, whereas the smaller ring-size of Me4-cyclen accounts for the formation of only cis-[GeF2(Me4-cyclen)][OTf], and is confirmed crystallographically. New spectroscopic data are also reported for [GeF4(L)2] (L = dmso, dmf and pyNO). Density functional theory calculations were used to probe the effect on the bonding as fluoride ligands were sequentially removed from the germanium centre in the OPMe3 complexes.

Keywords

• germanium
• fluoride
• cations
• crystal structures