Neutral and cationic phosphine and arsine complexes of tin(iv) halides: synthesis, properties, structures and anion influence

From Sn(II) to Sn(IV): Enhancing Lewis Acidity Via Oxidation

We demonstrate the increased Lewis acidity on going from Sn(II) to Sn(IV) by oxidizing TpMe2SnOTf (OTf = SO3CF3) to TpMe2SnF(OTf)2. Replacement of the fluoride ion in TpMe2SnF(OTf)2 by a triflate, resulting in TpMe2Sn(OTf)3 further enhances the Lewis acidity at tin. 119Sn NMR spectroscopy, modified Gutmann-Beckett test, computational analysis, and catalytic phosphine oxide deoxygenation support the claims.

Synthesis, spectroscopic and structural properties of Sn(II) and Pb(II) triflate complexes with soft phosphine and arsine coordination

Reaction of the divalent M(OTf)₂ (M = Sn, Pb; OTf = CF₃SO₃) with soft phosphine and arsine ligands, L, where L = o-C₆H₄(ER₂)₂ (E = P, R = Me or Ph; E = As, R = Me), MeC(CH₂ER₂)₃ (E = P, R = Ph; E = As, R = Me), PhP(CH₂CH₂PPh₂)₂ or P(CH₂CH₂PPh₂)₃, affords complexes of stoichiometry M(L)(OTf)₂ as white powders, which have been characterised via elemental analysis, ¹H, ¹⁹F{¹H}, ³¹P{¹H} and ¹¹⁹Sn NMR spectroscopy, with the expected ³¹P-¹¹⁹Sn and ³¹P-²⁰⁷Pb couplings clearly evident. The crystal structures of nine of these pnictine complexes are reported, in each case revealing retention of one or both OTf anions, which gives rise to a diverse range of coordination environments including monomers, as well as varying degrees of oligomerisation to form weakly associated (OTf-bridged) dimers, trimers and polymers. ¹⁹F{¹H} NMR spectra indicate that the OTf is essentially anionic (dissociated) in solution. Anion metathesis of [M(OTf)₂{MeC(CH₂PPh₂)₃}] with Na[BAr^F] (BAr^F = B{3,5-(CF₃)₂C₆H₃}₄) yields the corresponding [M{MeC(CH₂PPh₂)₃}][BAr^F]₂ salts, the crystal structures of all three (M = Ge, Sn, Pb) reveal pyramidal dications with discrete [BAr^F]^- anions providing charge balance. Density functional theory (DFT) calculations on these [M{MeC(CH₂PPh₂)₃}]²⁺ (M = Ge, Sn, Pb) dications using the B3LYP-D3 functional show the presence of a directional lone pair, which is a mixture of valence s and p_z character, with the valence p-orbital character decreasing down group 14. Natural bond orbital (NBO) analysis also shows that the natural charge at the metal centre increases and the charge on the P centre decreases upon going down group 14.

Neutral and Cationic Complexes of Silicon(IV) Halides with Phosphine Ligands

The reaction of SiI₄ and PMe₃ in n-hexane produced the yellow salt, [SiI₃(PMe₃)₂]I, confirmed from its X-ray structure, containing a trigonal bipyramidal cation with trans-phosphines. This contrasts with the six-coordination found in (the known) trans-[SiX₄(PMe₃)₂] (X = Cl, Br) complexes. The diphosphines o-C₆H₄(PMe₂)₂ and Et₂P(CH₂)₂PEt₂ form six-coordinate cis-[SiI₄(diphosphine)], which were also characterized by X-ray crystallography, multinuclear NMR, and IR spectroscopy. Reaction of trans-[SiX₄(PMe₃)₂] (X = Cl, Br) with Na[BAr^F] (BAr^F = [B{3,5-(CF₃)₂C₆H₃}₄]) produced five-coordinate [SiX₃(PMe₃)₂][BAr^F], but while Me₃SiO₃SCF₃ also abstracted chloride from trans-[SiCl₄(PMe₃)₂], the reaction products were six-coordinate complexes [SiCl₃(PMe₃)₂(OTf)] and [SiCl₂(PMe₃)₂(OTf)₂] with the triflate coordinated. X-ray crystal structures were obtained for [SiCl₃(PMe₃)₂][BAr^F] and [SiCl₂(PMe₃)₂(OTf)₂]. The charge distribution across the silicon species was also examined by natural bond orbital (NBO) analyses of the computed density functional theory (DFT) wavefunctions. For the [SiX₄(PMe₃)₂] and [SiX₃(PMe₃)₂]⁺ complexes, the positive charge on Si decreases and the negative charge on X decreases going from X = F to X = I. Upon going from [SiX₄(PMe₃)₂] to [SiX₃(PMe₃)₂]⁺, i.e., removal of X^–, there is an increase in positive charge on Si and a decrease in negative charge on the X centers (except for the case X = F). The positive charge on P shows a slight decrease.

Unusual neutral and monocationic silicon(IV) iodide complexes with soft phosphine coordination are formed in hydrocarbon solvents, while treatment of [SiCl₄(PMe₃)₂] with NaBAr^F forms the [SiCl₃(PMe₃)₂]⁺ cation; the electronic structures of [SiX₄(PMe₃)₂] and [SiX₃(PMe₃)₂]⁺ (X = F, Cl, Br, I) are probed using DFT calculations.

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

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.

Neutral and cationic germanium(IV) fluoride complexes with phosphine coordination - synthesis, spectroscopy and structures

The neutral complexes trans-[GeF₄(PⁱPr₃)₂] and [GeF₄(κ²-L)] (L = CH₃C(CH₂PPh₂)₃ or P(CH₂CH₂PPh₂)₃) are obtained from [GeF₄(MeCN)₂] and the ligand in CH₂Cl₂. Treatment of [GeF₄(PMe₃)₂] with n equivalents of TMSOTf (Me₃SiO₃SCF₃) leads to formation of the series [GeF_4-n(PMe₃)₂(OTf)_n] (n = 1, 2, 3), each of which contains six-coordinate Ge(IV) with trans PMe₃ ligands and X-ray structural data confirm that the OTf groups interact with Ge(IV) to varying degrees. Unexpectedly, [GeF₃(PMe₃)₂(OTf)] undergoes reductive defluorination in solution, forming the Ge(II) complex, [Ge(PMe₃)₃][OTf]₂ (and [FPMe₃]⁺). The bulkier PⁱPr₃ leads to formation of the ionic [GeF₃(ⁱPr₃P)₂][OTf], containing a [GeF₃(ⁱPr₃P)₂]⁺ cation. [GeF₄{o-C₆H₄(PMe₂)₂}], containing the cis-chelating diphosphine, also reacts with n equivalents of TMSOTf to generate [GeF_4-n{o-C₆H₄(PMe₂)₂}(OTf)_n] (n = 1, 2, 3). As for the PMe₃ system, the trifluoride, [GeF₃{o-C₆H₄(PMe₂)₂}(OTf)], is unstable to reductive defluorination in solution, producing the pyramidal Ge(II) complex [Ge{(o-C₆H₄(PMe₂)₂}(OTf)][OTf], whose crystal structure has been determined. The [GeF₃{Ph₂P(CH₂)₂PPh₂}(OTf)] and [GeF₂{Ph₂P(CH₂)₂PPh₂}(OTf)₂], obtained similarly from the parent tetrafluoride complex, are poorly soluble, however their structures were confirmed crystallographically. The complexes in this work have been characterised via variable temperature ¹H, ¹⁹F{¹H} and ³¹P{¹H} NMR studies in solution, IR spectroscopy and microanalysis and through single crystal X-ray analysis of representative examples across each series. Trends in the NMR and structural parameters are also discussed.

Tin(IV) fluoride complexes with neutral phosphine coordination and comparisons with hard N- and O-donor ligands

The reactions of trans-[SnF₄(PMe₃)₂] with one, two or three equivalents of Me₃SiO₃SCF₃ (TMSOTF), respectively, in anhydrous CH₂Cl₂ form six-coordinate [SnF_4-n(PMe₃)₂(OTf)_n] (n = 1-3), which have been characterised by microanalysis, IR and multinuclear NMR (¹H, ¹⁹F{¹H}, ³¹P{¹H} and ¹¹⁹Sn) spectroscopy. The crystal structure of [SnF₃(PMe₃)₂(OTf)] reveals the three fluorines are in a mer-arrangement with mutually trans PMe₃ ligands. The multinuclear NMR spectra confirm this structure is retained in solution, and show that [SnF₂(PMe₃)₂(OTf)₂] has trans-phosphines, while [SnF(PMe₃)₂(OTf)₃] has trans PMe₃ groups and hence mer-triflate ligands. The [SnF_4-n(PMe₃)₂(OTf)_n] are unstable in solution and the decomposition products include [Me₃PF]⁺ and the tin(II) complexes [Sn(PMe₃)₂(OTf)₂] and [Sn₃F₅(OTf)], both of the latter identified by their crystal structures. The reaction of trans-[SnF₄(PⁱPr₃)₂] containing the bulkier phosphine, with one and two equivalents of TMSOTf produced unstable mono- and bis-triflates, which the NMR data also suggest contain weakly coordinated triflate, [SnF₃(PⁱPr₃)₂(OTf)] and [SnF₂(PⁱPr₃)₂(OTf)₂], again with axial phosphines, although some OTf dissociation from the former to give [SnF₃(PⁱPr₃)₂]⁺ may occur in solution at room temperature. The new phosphine complexes of SnF₄, trans-[SnF₄(PⁱPr₃)₂] and (cis) [SnF₄(κ²-triphos)] (triphos = CH₃C(CH₂PPh₂)₃) have also been fully characterised, including the crystal structure of [SnF₄(κ²-triphos)]. Attempts to promote P₃-coordination by further treatment of this complex with TMSOTf were unsuccessful. The [SnF₄(L)₂] (L = dmso, py, pyNO, DMF, OPPh₃) complexes, which exist as mixtures of cis and trans isomers, react with one equivalent of TMSOTf, followed by addition of one equivalent of L, to form the ionic [SnF₃(L)₃][OTf] complexes, which were characterised by microanalysis, IR and multinuclear NMR spectroscopy. In nitromethane solution they are a mixture of mer and fac isomers based upon multinuclear NMR data (¹H, ¹⁹F{¹H}, ¹¹⁹Sn). Reaction of [SnF₄(OPPh₃)₂] with two equivalents of TMSOTf and further OPPh₃ produced [SnF₂(OPPh₃)₄][OTf]₂, which is a mixture of cis and trans isomers in solution. The crystal structure of [SnF₂(OPPh₃)₄][OTf]₂ confirms the trans isomer in the solid state, with the triflate ionic. These complexes are rare examples of fluorotin(IV) cations with neutral monodentate ligands.