Vibrational and solid-state phosphorus-31 nuclear magnetic resonance spectroscopic studies of 1:1 complexes of PPh₃ with gold(I) halides; crystal structure of [AuBr(PMe₃)]

The crystal structure of [AuBr(PMe₃)], like that of the corresponding chloride, was found to consist of three crystallographically independent molecules linked through weak Au ⋯ Au contacts to form helical chains. The Au ⋯ Au distances [3.648(1), 3.980(2), 3.548(2) Å] are, however, significantly longer than in the chloro- and iodo-complexes, implying that the Au ⋯ Au interaction is weakest in the bromide. The far-IR and Raman spectra of [AuX(PMe₃)] (X = Cl, Br or I) have been measured, and assignments made for the bands observed. The spectra were analysed to see whether they show any effects of the weak Au ⋯ Au bonding evident in the crystal structures. In particular, the low-wavenumber limit of previously reported Raman spectra for these species has been extended in order to search for v(Au₂) bands. The X = Cl or I compounds show a strong Raman band which can be assigned to the δ(PAuX) deformation, but no v(Au₂) bands were found in the region predicted on the basis of recent assignments for such modes in other complexes which display Au ⋯ Au contacts of similar length. Possible reasons, including an interaction between v(Au₂) and δ(PAuX) modes, are discussed. The solid-state cross polarization magic angle spinning ³¹P NMR spectra of the chloride and iodide complexes consist of doublets due to the presence of ¹J(¹⁹⁷Au-³¹P) coupling. This is a rare observation of spin-spin coupling between the spin I = 1/2 ³¹P nucleus and the strongly quadrupole coupled ¹⁹⁷Au nucleus, and permits estimation of the ¹J(¹⁹⁷Au-³¹P) coupling constants for these complexes.