A new twist on an old molecule: a rotameric isomer of bis(pentafluorophenyl)mercury

Suspending crystals of bis(pentafluorophenyl)mercury [Hg(C6F5)(2)] (1a) in a saturated n-hexane solution for a prolonged period results in transformation into the new polymorph [Hg(C6F5)(2)] (1 b) monoclinic P2(1)/n, a = 11.7060(3) angstrom, b = 7.8531(2) angstrom, c = 13.5429(4) angstrom, beta = 107.247(3)degrees, V = 1189.00(6) angstrom(3). The C-Hg-C angle in 1 b is 177.10(11)degrees and the pentafluorophenyl rings are rotated 74.6(1)degrees to each other in comparison to 1a, the form known for 50+ years, which has a rotation angle of 58.0(2)degrees. Polymorph 1 b could also be obtained by crystallization from chloroform, or a CHCl3/n-hexane (1:1) mixture at -20 degrees C, or by seeding with a crystal of 1 b. Theoretical calculations suggest that the barrier to rotation is low.

Automated summary

Suspending crystals of bis(pentafluorophenyl)mercury [Hg(C6F5)(2)] in a saturated n-hexane solution for a prolonged period results in a new polymorph with a low barrier to rotation. The theoretical calculations suggest a low rotation barrier for the transformation.