Crystal and molecular structure of SnMePh[C10H5(OMe-5)(CH2NMe2-8)]Br and proton nuclear magnetic resonance study of the puckering of the chelate ring in five-co-ordinate triorganotin halides SnRR′-[C10H5(OMe-5)(CH2NMe2-8)]X and SnRR′[CH2C6H4(CH2NMe2-2)]X

Abstract

8-Dimethylaminomethyl-5-methoxynaphthyl- and 2-dimethylaminomethylbenzyl-diorganotin halides have been prepared via the organolithium–diorganotin dihalide route in 50–55% yield. The molecular structure of the title compound (3) has been determined by a single-crystal X-ray diffraction study using standard Patterson and Fourier techniques, and refined by block-diagonal least squares to R= 0.024 for 3 014 observed diffractometer data (Mo-K_α radiation). Crystals are triclinic, space group P with Z= 2 in a unit cell of dimensions a= 7.795(5), b= 11.178(2), c= 12.159(1)Å, α= 106.20(2), β= 91.75(3), and γ= 100.83(3)°. The Sn co-ordination is pseudo-trigonal bipyramidal with N and Br in axial positions [Sn–N 2.401(4), Sn–Br 2.7391(7)Å]. Dynamic ¹H n.m.r. spectroscopy shows that the trigonal-bipyramidal structure is retained in solution. In the slow-exchange limit (3) exists in two diastereoisomeric forms as a result of the puckering of the chelate ring and the chirality at the Sn centre. On raising the temperature two different processes occur, the first of which involves interconversion between the two ring conformations. The second process (either Berry pseudo-rotation processes in the five-coordinate conformer, or Sn–N dissociation followed by combined pyramidal inversion at the N centre and rotation about the C–N bond) is difficult to define because of the coincident CH₂ and OMe resonances. Dynamic ¹H n.m.r. spectra of SnMePh[CH₂C₆H₄(CH₂NMe₂-2)]Br (5) reveal that the six-membered chelate ring containing two sp³-C atoms is very flexible compared with the five-membered ring in SnMePh[C₆H₄(CH₂NMe₂-2)]Br. Accordingly stereoisomerization in the five-co-ordinate conformer of (5) could be detected.