Reaction of tris(2,6-dimethoxyphenyl)methyl cation and the stannous halides produced a series of SnX3 (1, X = F; 2, X = Cl; 3, X = Br; 4, X = I) derivatives of the triarylmethyl. These species constitute a homologous series of heptacoordinate tin compounds. Crystals of 1 are monocliniC., space group P21/c: a = 9.8087(5) Å, b = 16.081(3) Å, c = 15.864(2) Å, β = 106.120(6)°, Z = 4. Crystals of 2 are orthorhombic, space group P212121, a = 11.622(2) Å, b = 15.192(2) Å, c = 33.379(3) Å, Z = 8. The asymmetric unit contains two independent, heterochiral molecules and one molecule of diethyl ether. Crystals of 3 are triclinic, space group P1̅: a = 10.049(1) Å, b = 10.7709(9) Å, c = 14.996(3) Å, α = 93.18(1)°, β = 104.77(1)°, γ = 116.263(9)°, Z = 2. The tin atoms are covalently bound to three halogens and the central carbon of the triarylmethyl unit; their association with the three near methoxy groups raises their coordination number to seven. Methoxy group site exchange barriers were determined for 1–4 by variable temperature NMR spectroscopy and/or by the saturation spin transfer method: ΔGǂ: 1, 83 kJ mol−1; 2, 71 kJ mol−1; 3, 65/64 kJ mol−1 (both methods); 4, 59 kJ mol−1. Since aryl ring rotation requires concomitant SnO bond breaking, the activation energies serve to measure the relative strengths of the Sn ligation by the ether oxygens. The trend observed (F > Cl > Br > I) correlates well with progressive molecular structure changes among 1, 2, and 3 and with expectations from the relative Lewis acidities of the tin halides. The order of the 119Sn NMR chemical shifts for 2–4 also conforms to expectations based on halide electronegativity, but shows a strong leveling effect which is attributed to coordination by the ether tripods.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry