Halogen and Sulfur Oxidation of Germanium and Tin Dications

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.

Utilization of a Tris(carbene)borate Ligand for Umpolung Reactivity of a Nucleophilic Tin(II) Cation Salt

We show that a tris(carbene)borate (TCB) ligand, namely [PhB(^tBuIm)₃]^- ([PhB(^tBuIm)₃]^- = phenyltris(3-tert-butylimidazol-2-ylidene)borato), is capable of stabilizing an unprecedented nucleophilic Sn(II) cation salt. Unlike known Sn(II) cations, the strong electron-donating ability of [PhB(^tBuIm)₃]^- makes the cationic tin atom electron-rich, σ-donating yet slightly π-accepting, which allows for the ensuing facile oxidation with o-chloranil and S₈ as well as coordination with coinage metals. The former oxidations give the Sn(IV) cation salts, while the latter reactions produce the metal complexes. The electronic structures of these species are thoroughly probed by quantum chemical computations. These results uncover an added role for TCB ligands in isolating unprecedented p-block species.

Structural Diversity in Divalent Group 14 Triflate Complexes Involving Endocyclic Thia-Macrocyclic Coordination

A highly unusual series of M(II) (M = Ge, Sn, Pb) complexes with endocyclic thioether macrocyclic coordination and with coordination numbers ranging from three to nine have been prepared by the reaction of [9]aneS₃ (1,4,7-trithiacyclononane), [12]aneS₄ (1,4,7,10-tetrathiacyclododecane), or [24]aneS₈ (1,4,7,10,13,16,19,22-octathiacyclotetracosane) with M(OTf)₂ (M = Sn and Pb; OTf = CF₃SO₃^-) or with GeCl₂·dioxane and 2 mol equiv of TMSOTf (Me₃SiO₃SCF₃) in a mixture of anhydrous CH₂Cl₂ and MeCN. The isolated bulk products are characterized by ¹H, ¹³C{¹H}, ¹⁹F{¹H}, and ¹¹⁹Sn{¹H} NMR and IR spectroscopy, high-resolution ESI⁺ MS, and microanalytical data. Crystal structures are also reported for [M(L)][OTf]₂ (M = Ge, Sn, Pb; L = [9]aneS₃, [12]aneS₄) and for [M([24]aneS₈)][OTf]₂ (M = Sn, Pb). In all cases, the ligand is bound in an endocyclic fashion, but the coordination environment and number are highly dependent on the group 14 ion, the macrocyclic ring size, and the number of S-donor atoms it presents. Solution NMR spectroscopic data suggest that the metal-macrocycle coordination is retained in solution but that the triflate anions are extensively dissociated on the NMR timescale. Density functional theory calculations on the [M([9]aneS₃)]²⁺ and [M([12]aneS₄)]²⁺ (M = Ge, Sn, Pb) dications reveal that the HOMO is centered on the group 14 atom as a directional "lone pair"; it also retains a significant amount of positive charge.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe₂CH₂C₆H₄)(mesityl)Sb]⁺ (A) and [(NMe₂C₆H₄)(mesityl)Sb]⁺ (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.

Novel Heteroleptic Tin(II) Complexes Capable of Forming SnO and SnO2 Thin Films Depending on Conditions Using Chemical Solution Deposition

A new heteroleptic complex series of tin was synthesized by the salt metathesis reaction of SnX₂ (X = Cl, Br, and I) with aminoalkoxide and various N-alkoxy-functionalized carboxamide ligands. The complexes, [ClSn(dmamp)]₂ (1), [BrSn(dmamp)]₂ (2), and [ISn(dmamp)]₂ (3), were prepared from the salt metathesis reaction of SnX₂ with one equivalent of dmamp; [Sn(dmamp)(empa)]₂ (4), [Sn(dmamp)(mdpa)]₂ (5), and [Sn(dmamp)(edpa)]₂ (6) were prepared via the salt metathesis reaction using complex 2 with one equivalent of N-alkoxy-functionalized carboxamide ligand. Complexes 1-5 displayed dimeric molecular structures with tin metal centers interconnected by μ₂-O bonding via the alkoxy oxygen atom. The molecular structures of complexes 1-5 showed distorted trigonal bipyramidal geometries with lone pair electrons in the equatorial position. Using complex 6 as a tin precursor, SnO _x films were deposited by chemical solution deposition (CSD) and subsequent post-deposition annealing (PDA) at high temperatures. SnO and SnO₂ films were selectively obtained under controlled PDA atmospheres of argon and oxygen, respectively. The SnO films featured a tetragonal romarchite structure with high crystallinity and a preferred growth orientation along the (101) plane. They also exhibited a lower transmittance of >52% at 400 nm due to an optical band gap of 2.9 eV. In contrast, the SnO₂ films exhibited a tetragonal cassiterite crystal structure and an extremely high transmittance of >97% at 400 nm was observed with an optical band gap of 3.6 eV.

Pyramidal Dicationic Ge(II) Complexes with Homoleptic Neutral Pnictine Coordination: A Combined Experimental and Density Functional Theory Study

An unusual series of Ge(II) dicationic species with homoleptic phosphine and arsine coordination, [Ge(L)][OTf]₂, L = 3 × PMe₃, triphos (MeC(CH₂PPh₂)₃), triars (MeC(CH₂AsMe₂)₃), or κ³-tetraphos (P(CH₂CH₂PPh₂)₃) (OTf^- = O₃SCF₃^-) have been prepared by reaction of [GeCl₂(dioxane)] with L and 2 mol equiv of Me₃SiOTf in anhydrous CH₂Cl₂ (or MeCN for L = triars, triphos). X-ray crystal structures are reported for [Ge(PMe₃)₃][OTf]₂, [Ge(triars)][OTf]₂, and [Ge(κ³-tetraphos)][OTf]₂, confirming homoleptic P₃- or As₃-coordination at Ge(II) in each case and with the discrete OTf^- anions providing a charge balance. The Ge-P/As bond lengths are significantly shorter than those in neutral germanium(II) dihalide complexes with diphosphine or diarsine coordination. Solution NMR spectroscopic data indicate that the complexes are labile in solution. Using excess AsMe₃ and [GeCl₂(dioxane)] gives only the neutral product, [Ge(AsMe₂)₂(OTf)₂], the crystal structure of which shows four coordination at Ge(II), via two As donor atoms and an O atom from two κ¹-OTf^- ligands; further weak, long-range intermolecular interactions give a chain polymer. The electronic structure of the [Ge(PMe₃)₃]²⁺ dication has been investigated using density functional theory (DFT) calculations. The computed geometrical parameters for this dication are in good agreement with the experimental X-ray crystallographic values in [Ge(PMe₃)₃][OTf]₂. The results also indicate that the pyramidal arrangement of the [Ge(PMe₃)₃]²⁺ (computed P-Ge-P angle 96.8° at the B3LYP-D3 level) arises from a balance between electronic energy (E_elec) contributions, which favor a lower P-Ge-P angle, and nuclear-nuclear contributions (E_nn), which favor a higher P-Ge-P angle, to the total energy (E_TOT). An Atoms in Molecules (AIM) analysis reveals that one reason why E_elec decreases as the P-Ge-P angle decreases is because of C···H and H···H interactions between atoms on different CH₃ groups. The stability of the [Ge(PMe₃)₃]²⁺ dication is enhanced by the distribution of a significant part of the positive charge on Ge²⁺ to the atomic centers of the PMe₃ ligands. Similar results were obtained for [Ge(AsMe₃)₃][OTf]₂, showing the tris-AsMe₃ complex to be less stable compared to the PMe₃ analogue. Related calculations were also performed for the neutral [Ge(PMe₃)₂(OTf)₂] and [Ge(AsMe₃)₂(OTf)₂] complexes.