Vibrational study of ionic association in polar aprotic solvents. Isocyanate complexes of non-transition-metal ions

Abstract

Complexes between alkaline-earth-metal ions M2+ and cyanate OCN− in aprotic solvents are studied by infrared spectrometry in the ν(CN) region (2150–2250 cm−1). In dimethylsulphoxide (DMSO) and hexamethylphosphotriamide (HMPT) isocyanate MNCO ion pairs exhibit a specific ν(CN) band while M(NCO)2 species usually have two ν(CN) bands. Pure alkaline earth cyanates are in part dissociated into MNCO+ and OCN− in DMSO. When they are soluble they are not dissociated in N,N-dimethylformanride (DMF) and in pyridine. The Mg2+-OCN− system in HMPT is the only case where two different ion pairs are observed: p1 at 2205 cm−1 and p2 at 2213 cm−1. The high temperature species p2 is tighter than p1. The reaction p1 → p2 is a desolvation reaction as shown by the large values of ΔH and ΔS (ΔH = 37 kJ.mol−1 and ΔS = 100 J.K−1.mol−1). Similar results are obtained with other non-transition ions (Al3+, Zn2+, Cd2+, Hg2+, Ag+, Tl+). The Δν(CN) wavenumber shift of an ion pair relative to free OCN− (≈ 2137 cm1−) is always positive and roughly proportional to the polarizing power of Mz+. This is in agreement with an isocyanate bonding MNCO. When Mz+ is a soft cation Δν(CN) is smaller than expected. This effect is common to all cases of M…CN bonding. The i.r. spectrum of AgOCN in tetrahydrothiophene (THT) exhibits two bands at 2162 cm' (AgNCO) and 2250 cm−1. The second band is assigned to a bridging group AgOCNAg in (AgNCO)2.