CORRELATION OF STRUCTURE AND INTERNAL DYNAMICS FOR (TRIS(2,6-DIMETHOXYPHENYL)METHYL)TIN TRIHALIDES - A HOMOLOGOUS SERIES OF 7-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 P2(1)/c: a = 9.8087(5) angstrom, b = 16.081(3) angstrom, c = 15.864(2) angstrom, beta = 106.120(6)-degrees, Z = 4. Crystals of 2 are orthorhombic, space group P2(1)2(1)2(1), a = 11.622(2) angstrom, b = 15.192(2) angstrom, c = 33.379(3) angstrom, Z = 8. The asymmetric unit contains two independent, heterochiral molecules and one molecule of diethyl ether. Crystals of 3 are triclinic, space group P1BAR: a = 10.049(l) angstrom, b = 10.7709(9) angstrom, c = 14.996(3) angstrom, a = 93.18(l)-degrees, beta = 104.77(l)-degrees, gamma = 116.263(9)-degrees, 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: AG*: 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-0 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 Sn-119 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.