Pseudohalides. Part III. Infrared, ultraviolet, and proton magnetic resonance spectra of boron isocyanates and isothiocyanates

Abstract

The infrared spectra of thirty-seven iso- and isothio-cyanatoboranes, having one, two, or three isocyanate or iso-thiocyanate groups attached directly to the boron atom, have been recorded in the region 5000–400 cm.^–1. From the frequency, intensity, and shape of the absorption band associated with the asymmetric stretching motion of the –NCX (X = O or S) functions, the iso-structure (B–NCX) is proposed for all the compounds. The -NCO and –NCS asymmetric stretching vibrations fall at 2273 ± 30 and 2089 ± 31 cm.^–1, respectively, for the two classes of compound. From the frequencies of other vibrational modes [v(N–H), v(B–N), v(B–O), and v(C–N)], conclusions are drawn as to the relative electron-attraction (-l effect) and effectiveness in π-bond formation (+M effect) to boron of halogens and pseudohalogens (–NCO and –NCS). Full assignments are proposed for (OCNBNH)₃, (SCNBNH)₃, (OCNBNMe)₃, (SCNBNMe)₃, (MeO)₂BNCO, and (MeO)₂BNCS. Ultraviolet spectra of isothiocyanatoboranes show absorption maxima 250 ± 5 mµ(log ε= 2·7 ± 0·5), whilst the isocyanatoborane chromophore does not absorb in the range 220–350 mµ. Proton magnetic resonance spectra on the series (XBNEt)₃ and PhB(X)NMe₂ show that the relative ease of BX π-bonding is F > NCO > NCS > Cl > Br.