Correlation of Structure and Internal Dynamics for (Tris(2,6-dimethoxyphenyl)methyl)tin Trihalides: A Homologous Series of Seven-Coordinate Tin Compounds

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: ΔG^ǂ: 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 SnO 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.