Chemical synthesis with metal atoms. The preparation and structure of cyclo-octadiene trifluorophosphine complexes of chromium. The crystal structure of (η5-cyclo-octa-1,3-dienyl)hydridotris(trifluorophosphine) chromium

Abstract

The reaction of cyclo-octa-1,5-diene (1,5-cod), PF₃, and chromium atoms produces [Cr(1,5-cod)(PF₃)₄](1) and [Cr(C₈H₁₁)(PF₃)₃H](2). The use of 1,3- rather than 1,5-cod produces only (2) under the same conditions. Complex (1) is readily converted into (2) by heating (90 °C, in toluene). The preparation of [Cr(1,5-cod)(CO)₄](3) from 1,3- or 1,5-cod is also described. The structure of (2) was determined by X-ray crystallography. The compound crystallizes in the orthorhombic space group Pbca with a= 13.452(3), b= 14.419(4), c= 15.268(3)Å, and Z= 8. The final R value was 0.065 9 for 1 699 observed reflections. The structure is retained in solution below –80 °C, but is shown by ¹³C and ¹H n.m.r. spectroscopy to become fluxional upon warming. There are two distinct exchange processes; one results in pairwise coalescence of the ¹³C and ¹H resonances; the other finally leads to a set of only two ¹H signals with an intensity ratio of 8 : 4, thus indicating equivalence of the eight ring positions.