Molecular structure of and restricted internal rotation about the tin-tin bond in [ClSn(CH2CH2CH2)2NCH 3]2 and solution isomerism and isomerizations in [CH3Sn(CH2CH2CH2) 2NCH3]2, two compounds with five-coordinate tin centers bound to each other

Article abstract of DOI:10.1021/om00093a004

The crystal structure of [ClSn(CH₂CH₂CH₂)₂NCH ₃]₂ (1) has been determined by an X-ray study. The crystal data are as follows: triclinic space group P1, a = 10.757 (6), b = 7.673 (3), c = 12.662 (7) ?; α = 83.49 (4), β = 90.26 (4), γ = 72.48 (4)°; V = 989.4 ?³; Z = 2. The structure was refined to R = 0.045. The two five-coordinate tin moieties exhibit approximately trigonal-bipyramidal geometries with the chlorine and nitrogen atoms occupying apical positions and the tin-tin and tin-carbon bonds occupying equatorial positions and are twisted with respect to one another by about 120° along the tin-tin bond. The ¹H, ¹³C, and ¹¹⁹Sn NMR data indicate that in solution 1 exists as only one isomer having the same structure as in the solid state. At low temperature, the ¹³C spectra reveal restricted internal rotation about the tin-tin bond with an activation barrier of about 11.5 ± 0.5 kcal/mol. In contrast, [CH₃Sn(CH₂CH₂CH₂) ₂NCH₃]₂ (2) exists in solution as a mixture of at least three isomers, A, B, and C. NMR data on the major isomer A of 2 are explained in terms of a structure with the two methyl groups and the two intramolecularly coordinating nitrogen atoms in apical positions at each tin and the two ring carbon chains and the tin-tin bond in equatorial positions, in agreement with both the cyclic ligand structure and the polarity rule. For the medium and minor isomers of 2, B and C, respectively, no unambiguous structure can be proposed: B and C either have respectively only one and no methyl group in the apical position, with one and two tin atoms in the apical position, or have one and two four-coordinate tin centers, respectively. An analysis of the cross peaks of a two-dimensional EXSY ¹¹⁹Sn NMR spectrum of 2 reveals that the isomers A and B interconvert through an uncorrelated rearrangement of one tin center at a time. No evidence could be found that the internal rotation about the tin-tin bond is restricted in 2 on the NMR time scale at temperatures at which it is in 1. The differences between the dynamic stereochemical behaviors of 1 and 2 are explained in terms of the apicophilicity differences between a methyl group and a chlorine atom.